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Animal models of the epilepsies
Brain Research Reviews, 14 (1989) 245-278
245
Elsevier BRESR 90101
Animal models of the epilepsies Robert S. Fisher Departments of Neurology and Neurosurgery,
The Johns Hopkins University School of Medicine, Baltimore,
MD 21205 (U.S.A.)
(Accepted 13 June 1989) Key words: Epilepsy; Animal model; Rodent; Cat; Monkey; Electroencephalography
CONTENTS ............................................................................................................................................ Introduction 1.1. Background ...................................................................................................................................... 1.2. Clinical epilepsy ................................................................................................................................. ................................................................................................................... 1.3. The electroencephalogram
246 246 247 249
Models for simple partial seizures, acute ...................................................................................................... 2.1. Topical convulsants ............................................................................................................................. 2.1.1. Focal penicillin ......................................................................................................................... 2.1.2. Other focal chemical convulsants ................................................................................................. 2.2. Acute electrical stimulation .................................................................................................................. ............................................................................................................................. 2.3. GABA-withdrawal 2.4. Neocortical brain slices ....................................................................................................................... 2.5. Disadvantages of acute models .............................................................................................................
251 251 251 252 252 252 252 253
Models for simple partial seizures, chronic .................................................................................................... 3.1. Cortically implanted metals .................................................................................................................. 3.1.1. Alumina hydroxide .................................................................................................................... 3.1.2. Other metals: cobalt, tungsten, zinc, iron ...................................................................................... 3.2. Cryogenic injury ................................................................................................................................ 3.3. Ganglioside antibody injection ............................................................................................................. .............................................................................................................. 3.4. Systemic focal epileptogenesis
253 253 253 253 253 253 254
Models for complex partial seizures ............................................................................................................. 4.1. Kainic acid ....................................................................................................................................... 4.2. Tetanus toxin .................................................................................................................................... 4.3. Injections into area tempesta ............................................................................................................... 4.4. Kindling ........................................................................................................................................... 4.5. Brain slices ....................................................................................................................................... 4.5.1. Rodent in vitro hippocampal slices ............................................................................................... 4.5.2. Isolated cell preparations ............................................................................................................ 4.5.3. Human neurosurgical tissue ........................................................................................................
254 254 254 255 255 256 256 257 257
Generalized tonic-clonic seizures ................................................................................................................. 5.1. Genetic ............................................................................................................................................ 5.1.1. Photosensitive baboons .............................................................................................................. 5.1.2. Audiogenic seizures in mice ........................................................................................................ 5.1.3. Totterer mice and other seizure-prone mouse strains ....................................................................... 5.1.4. Genetically epilepsy-prone rats .................................................................................................... 5.1.5. Mongolian gerbil ....................................................................................................................... 5.1.6. Drosophila shakers ....................................................................................................................
258 258 258 258 259 259 260 260
Correspondence: R.S. Fisher, Departments of Neurology and Neurosurgery, The Johns Hopkins Hospital, Meyer l-130,600 North Wolfe Street, Baltimore, MD 21205, U.S.A.
0006-8993/89/$03.50 0
1989 Elsevier Science Publishers B.V. (Biomedical Division)
246
5.1.7. Miscellaneous genetically seizure-prone animals .............................................................................. 5.2. Maximal electroshock ......................................................................................................................... 5.3. Systemic convulsants ........................................................................................................................... 5.3.1. Pentylenetetrazol ....................................................................................................................... 5.3.2. Systemic penicillin as a tonic-clonic model ..................................................................................... 5.3.3. Other inhibitory antagonists ........................................................................................................ 5.4. Metabolic derangements ......................................................................................................................
260 260 261 261 262 263 264
6. Models for generalized absence seizures .......................... ... .............. ...... . ‘.. ..,. ..,... . .. .____________....... .,... 6.1. Thalamic stimulation .......................................................................................................................... 6.2. Bilaterai cortical foci .......................................................................................................................... 6.3. Systemic penicillin .............................................................................................................................. 6.4. ~-Hydroxybutyrate .................. ........................................................................................................... 6.5. Intraventricular opiates ....................................................................................................................... 6.6. THIP ............................................................................................................................................... 6.7. Genetic rodent models of absence .._., .... .. ... ... ... .._...._...........................................................
264 264 265 265 266 266 266 266
7. Animal models of status epilepticus
267
8. Conclusion 9. Summary
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267
,..........___._....._.._.,..,..,,,,,_,................._.,.__..__.._......... 269
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269
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269
The study of the epilepsies has been dependent upon use of mode1 systems. Ethical considerations rule out use of the neuroscientist’s modern tools intracellular recording, microchemical analysis and anatomical tracer techniques in intact human brain. Screening of thousands of compounds for possible anticonvulsant activity cannot be performed in the clinic. Much of what is known about the epilepsies is derived directly or indirectly from animal models. It is thus germane to consider the value and limitations of our animal models for this disorder. Many species of animals do develop spontaneous seizures”“; however, these cases are sporadic, and not usually in animals suitable for experimentation. More reliable preparations have therefore been developed7’ (see Table I). The large number of mode1 systems derives from two reasons. First, none of the models is fully trustworthy as an imitation of clinical epilepsy. Important findings therefore require validation in several models. Second, there are multiple types of the epilepsies to model. Simple partial epilepsy resulting from a frontal lobe tumor is a very different disorder from familial petit ma1
epilepsy. Table I categorizes the models by seizure type. Some models serve for more than one type. There are many more models of seizures than of the epilepsies, i.e. of the condition of chronically recurrent spontaneous seizures. Nevertheless, the phrase ‘animal model of epilepsy’ is often used in place of the more precise phrase ‘animal model of a seizure disorder’. A seizure (‘ictus’) is a clinical and behavioral event. In vitro brain slices cannot seize, and certainly cannot suffer epilepsy. This does not detract from the insight into mechanisms of the epilepsies than can be gained from use of mode1 systems. In this area terminology is difficult; however, clarity is required. Researchers tend to employ the mode1 of greatest familiarity and convenience; rarely, if ever, would an experiment be performed both in tissue culture and in photosensitive Papio papio baboons. Nevertheless, a more logical way to choose a model would be first to choose the experimental question. Questions regarding influence of anticonvulsants on membrane ion channels can be answered in tissue culture. If the investigator is asking whether surgical interruption of certain neuronal pathways can block seizure spread, then experiments must be done in intact mammals, and most usefully in primates. Use of inappropriate model systems may lead to waste of
Acknowledgements References
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. . .. .. .. .
.. .
. . . . . . . . . .__... ._. _..___._.. . . . .
1. INTRODUCTION
247 TABLE I Animal models of the epilepsies
Simplepartial, acute
Generalized tonic-clonic
Complex partial
Topical convulsants Penicillin Bicuculline Picrotoxin Strychnine Cholinergics Anticholinergics Other Acute electrical stimulation GABA-withdrawal Neocortical brain slices
Genetic Photosensitive baboons Audiogenic seizures in mice Totterer and El mice Genetically prone rats Mongolian gerbil Drosophila shakers Maximal electroshock Chemical convulsants Pentylenetetrazol Bemegride Picrotoxin Bicuculline Methionine sulfoximide Penicillin Other Metabolic derangements Hypoxia Hypoglycemia Hyperbaric oxygen Hypercarbia Uremia Drug withdrawal High temperature
Kainic acid Tetanus toxin Injection into area tempesta Kindling Brain slices Rodent hippocampal slices Isolated cell preparations Human neurosurgical tissue
Simple partial,
chronic
Cortically implanted metals Aluminum hydroxide Cobalt Tungsten Zinc Iron Cryogenic injury Ganglioside antibody injections Systemic focal epileptogenesis
time, money and animal lives, and furthermore may promulgate irrelevant results. Advantages and disadvantages of each of the major model systems are outlined below. The discussion is designed to be conceptual, and not a full recipe for actual use of the model. More extensive descriptions of laboratory techniques may be found in an old, but still very useful, book by Purpura and colleagues242, entitled Experimental Models of Epilepsy - A Manual for the Laboratory Worker. Discussion of models pertaining to anticonvulsant drug development may be found in a thorough review by Ldscher and Schmidt163. Basic models of the epilepsies are used to explore questions about seizures and the electrical activity of the brain. Table II lists some of the key questions that have been addressed. These questions pertain to the underlying EEG generators of electrical potentials associated with seizures; the nature and identity of neuronal systems able to produce epilepsy; issues of why seizures start, spread and stop, why seizures occur when they do; what type of pathologies in brain give rise to seizures; whether seizures cause damage to brain;
Generalized absence
Thalamic stimulation Bilateral cortical foci Systemic penicillin y-Hydroxy-butyrate Intraventricular opiates General rat models
Status epilepticus
Lithium-pilocarpine Cobalt-homocystine Recurrent stimulation
and to the mechanism of action of anticonvulsants. As we explore each of the main models for the epilepsies, we will consider the ability of the model to address these questions. 1.2. Clinical epilepsy Epilepsy is not a disease, but a collection of diverse syndromes, some of which are secondary to other brain derangements, and some of which are seemingly primary. Johns Hughlings Jackson, one of the founders of modern neurology, portrayed a seizure as a ‘sudden, excessive and temporary nervous discharge . . . of a few cells which have got far above the rest of the cortical cells in degree of tension and instability . . .‘126. Epilepsy is considered to be a disorder intrinsic to the brain: either ‘hereditary tendency’ or a prior insult has rendered a portion of the brain electrically unstable. Any serious injury to the brain can lead to epilepsy; common ones include: major head trauma, stroke, hemorrhage, infection (meningitis, encephalitis or abscess), vascular malformations, and benign or malignant tumors. Although a seizure can occur at the time of the initial insult, there may also be a
248 TABLE II
Each of the seizures listed so far is referable to abnormal electrical activity in a relatively localized
Questions aboutseizures
part of the brain. There are in addition several varieties of seizures which lack apparent focality, and are therefore called ‘primary generalized’ sei-
What generates the EEG? What is the neuronal basis of epilepsy? Where is the ‘pacemaker’? Why do seizures spread? Why do seizures stop? Why do seizures occur when they do? Why are seizures varied? What is the pathology of epilepsy? Why is there a latent period? Are seizures bad for the brain? Anticonvulsant mechanisms?
latent period of months or years to development epilepsy lo. The reason for this latent period
zures. The two prototypes generalized tonic-clonic’)
are grand ma1 (‘primary and petit ma1 (‘primary
generalized absence’). Grand ma1 seizures begin with an initial loss of consciousness, followed by generalized extensor rigidity referred to as the tonic phase, followed after some seconds by rhythmical 4-limb and face jerking, called the clonic phase. Petit of is
unknown, but it can be replicated in animal models of the epilepsies (see below). Manifestations of a seizure depend in part upon the age and behavioral state (asleep, awake, active) of the subject, anticonvulsant drug therapy, and location in the brain at which the electrical abnormality occurs207. A discharge in the motor area can induce rhythmical uncontrolled motor activity in the face or limbs. Seizure activity in the somatosensory regions of brain causes sensations of tingling or numbness. Occipital and posterior parietal discharges generate primary visual phenomena: jagged lines, light flashes, colors; familiar to sufferers of the non-epileptiform condition, classic migraine. Stereotyped odors or tastes at the onset of a seizure suggest origin in the uncinate region of the inferior frontal lobe, responsible for gustatory sensation. Seizures arising in association regions of cortex tend to give rise to more complex phenomena, or simply disruption of ongoing normal function. The most common type of seizure in adults arises from limbic structures in the temporal lobes93. These seizures are preceded in over 50% of cases by auras: actually themselves localized seizures, causing visceral discomfort, flushing, tingling, cognitive-perceptual distortions such as deja vu, micropsia or vertigo141. Awareness and memory acquisition are decreased during many limbic seizures. Patients appear to be in a dream-like or fugue ‘state for periods lasting seconds to several minutes. Automatic actions: fumbling, lip-smacking, chewing, repeating stock phrases, undressing, walking in circles are common during seizures originating in the limbic system.
ma1 seizures
have
usual
onset
in childhood,
and
present as a few seconds of staring with rapid return to awareness. There has been great debate over the past decades about the actual site of origin of these primary generalized seizuresx4. A definitive classification of seizures will be elusive until the pathophysiology of seizures is better understood; nevertheless, efforts have been devoted to classification of seizures”‘. For the clinician the importance of even such an interim scheme is in its correlation with underlying brain pathology, with prognosis, and with likely responsiveness to particular medications or surgical therapies. For the researcher the classification serves as a reminder not to draw excessively broad conclusions about the multiple types of clinical epilepsy from limited animal models. Seizures are classified as to whether their onset is partial (focal) or generalized (Table III). Partial seizures are subcategorized into simple (without loss of awareness) and complex (with loss or blunting of awareness). The simple partial seizures may be motor, sensory, sensory-motor, autonomic-visceral or cognitive. The latter two categories of simple partial seizures usually occur as an ‘aura’ to complex partial seizures. Complex partial seizures were described above. The name is a synonym for the older terms: ‘temporal lobe seizure’, ‘psychomotor seizure’, ‘limbic seizure’. Any partial seizure may spread to wide regions of brain and become generalized. If this generalization is rapid and unobserved, a secondarily generalized seizure may be mistaken for a primary generalized tonic-clonic seizure, thereby leading to suboptimal diagnosis and therapy. Generalized absence seizures (petit mal) may present with only a brief blank stare or with a stare plus brief
249 automatisms fluttering.
and
motor
In instances
activities, where
such as eyelid
automatisms
are ex-
tensive, it can be difficult to differentiate absence from complex partial seizures. Generalized tonicclonic (grand mal) seizures were detailed above, and constitute the most popularly held picture of epilepsy, with falling, stiffening and jerking, accompanied by loss of consciousness, and sometimes bladder
incontinence.
lightening-like
Myoclonic jerking
seizures
consist
of
of the face or extremities,
once or a few times. Prolonged rhythmical jerking does not occur in myoclonic seizures. Tonic seizures, with stiffening, and clonic seizures with rhythmical jerking, may occasionally occur in isolation from the more common tonic-clonic, grand ma1 pattern. Atonit seizures, previously called ‘akinetic’ or ‘astatic’ seizures, present with sudden loss of muscle tone, and a pitch forward to the ground, with loss of awareness lasting for an almost imperceptibly short period of time. From 10 to 25% of seizures may remain unclassified, because they fail to fit into an existing category, or because descriptive details are lacking. 1.3.
The electroencephalogram
The electroencephalogram (EEG) is the most useful test in diagnosis and classification of epilepsy,
TABLE III Internationalclassification of the epilepsies Adapted from Dreifuss et al.“. Partial Simple Motor Sensory Sensory-motor Autonomic-visceral Cognitive Complex (psychomotor) With and without aura With and without automatism Secondarily generalized Generalized Absence (petit mal) Tonic-clonic (grand mal) Myoclonic Tonic Clonic Atonic Unclassified
second
only
time-varying
to the clinical record
history*l’.
of electrical
potential
EEG
is a
(voltage)
differences at paired points of the scalp. Traditionally, 8 or 16 channels are recorded, although the number of channels is arbitrary. Basic research4 has shown EEG voltages to result from summed, synchronous synaptic activity in the cortical regions under
the surface
recording
electrodes.
Neuronal
action potentials make little direct contribution to the EEG since they are so brief as to be relatively asynchronous.
Clinical
electroencephalographers
study records for overall slowing of normal frequencies - correlated with general cerebral dysfunction from numerous causes, and for local slowing suggestive of a local cerebral disturbance, possibly in the wake of local ischemia, hemorrhage, trauma, tumor, infection or a recent partial seizure. Seizure discharges are an example of extreme synchronization of the EEG. Several EEG patterns have been recognized for the past half-century as a ‘signature’ of epileptiform activity. During absence (petit mal) seizures the EEG shows 3-4/s rhythmical spike-wave complexes (Fig. 1). Generalized tonic-clonic (grand mal) seizures correspond to a different pattern (Fig. 2). The initial EEG change in a seizure may be a paradoxical low-voltage desynchronization of the EEG73. With tonic-clonic seizures this is followed by rhythmical 15-25 per second sharp activity, decelerating to slower frequencies, resolving usually to spikes (or polyspikes) and slow waves. After the seizure, the background EEG flattens and slows relative to the baseline for several minutes. Seizures are sometimes referred to as an ‘ictus’; the period after a seizure as the ‘postictal’ period; and the between-seizure period as the ‘interictal’ period. Interictally, the EEG can be normal. Alternatively, subclinical interictal seizure discharges may be observed. Such discharges may be too brief to affect behavior. Interictal discharges are known to be a useful marker of a site in a patient’s brain from which seizures may arise*r*. Care must be taken both in research and clinical settings to distinguish normal sharp discharges, such as vertex waves of sleep (Fig. 3A) from epileptiform sharp waves (Fig. 3B). The term ‘spike’ to connote an interictal EEG discharge is regrettable since it promotes confusion with ‘spike’ as a descriptor of a single neuron action potential. Many animal model systems for the study
250 of the epilepsies are actually models of the interictal spike. It is therefore relevant to our discussion of animal models of the epilepsies to note that interictal seizure patterns have been a source of confusion for both clinicians and epilepsy researchers. Interpretation of the meaning of interictal spikes clinically or in animal experiments requires understanding of a few key points. First, interictal spikes or sharp waves are diagnosed by pattern re#gnitjon, but the context in which these patterns occur is often very important. Several eon-epiie~tiform patterns may look similar to sharp waves or spike$“; for example, the fully normal vertex wave of sleep (Fig. 3A). Second, interictal seizure patterns can appear in individuals with no history of epilepsyru2. Third, lack of interictal discharges does not exclude epilepsy: interi~tal EEGs may be negative in a large number of people with epifepsy’i*. Diagnostic yield of the EEG can be increased by repeat recordings, recordings during sleep, recordings after activating procedures such as h~ervent~lation~ photic stimulation (flashing lights) and sleep deprivation, and by prolonged continuous monitoring in diagnostically difficult cases. Fourth, the site of interictal EEG discharge only approximately correlates with the origin of clinical seizures. Occasional patients evidence interictal spikes maximal on the scalp at one site; yet, ictal events (clinical seizures) appear electroencephalographically to arise from another site. It is uncertain whether this discrepancy results from a site sampling error in the clinical EEG recordings, or a fundamental difference in the meaning and mechanism of interictal and ictal discharges m . Neither clinicians nor researchers can assume that a spike-like pattern in an EEG indicates epilepsy, unless there is supporting behavioral evi-
Terms describing seizure-related EEG activity are often confusing, In the clinical literature, ‘spike’ connotes an EEG pattern consistent with abnormai synchronous firing of a group of neurons. Associated with the event is depolarization, with a transient net flux of positive ions into neuron?. This renders the extracellular space negative in the region of the discharging neurons, and progressively less negative as voltages are measured at increasing distances from the synchronously firing neurons. At standard EEG amplifjcatiol~s a spike should look ‘pointy’, which is variably defined as having a biphasic or triphasic shape with a duration at haif~height of Iess than 70 ms (although some use 50 ms). ‘Sharp waves’ (e.g. Fig. 3B) are similar in appearance to ‘spikes’, but not as pointed, with durations of 70-200 ms, measured at half the peak amplitude. Spikes and sharp waves have similar connotations to a clinical electroencephalngrapher: each is an interictal pattern that may be associated with seizure activity, within the constraints noted above. Numerous clinical EEG terms ‘I2 have arisen which sound similar to spike and sharp wave, but do not connote epilepsy. These include such descriptive phrases as “minor sharp transients’, ‘minor sharp activity’, ‘rhythmic sharp activity‘, ‘blunted sharp waves’, ‘wicket spikes’ and the somewhat controversial pattern ‘small sharp
dence.
SPIKE-WAVE MORPHOLOGY
‘ax
]55*lv
FPZ-A2
Fig. 1. Scalp EEG recording showing spontaneous 3-4/s spike-wave discharges in a patient with absence epilepsy. For clarity only 3 EEG channels (of the original 16) are displayed.
TONIC - CLONIC SEIZURE ONSET LEFT TEMPORAL SUBDURAL GRID
200 uv
l__...1 see
Fig. 2. Onset of a tonic-clonic generalized seizure in a patient undergoing evaluation for resective surgery. Recordings were obtained from a grid of 64 stainless steef recording disks (0.s mm diameter, 10 mm spacing) inserted into the subdural space over left frontal and temporal lobes. Each channel recorded from adjacent pairs of electrodes.Montage is unlinked, e.g. no electrode was common to two channels. Rhythmic sharp activity was evident at the start of the episode.
2.51 spikes’. Nomenclature is even less clear with recordings directly from brain tissue, as is often the case with experimental models. Spikes and sharp waves, singly and in rhythmic runs, are plentiful in corticograms, since direct cortical recordings circumvent the high frequency filter usually provided in scalp recordings by CSF, skull and skin. Electroencephalographers usually employ a stricter standard of sharpness for corticographic spikes and sharp waves in comparison with potentials recorded from the scalp. These standards are subjective, and may differ among different experienced readers. Researchers in the animal models of the epilepsies have no clear standard for terminology. Discharges have been called spikes, sharp waves, epileptiform firing, ictaform discharge, burst discharge, afterdischarge, ringing and countless other ambiguous terms. In reviewing literature on animal models of the epilepsies it is important for the reader to evaluate critically whether an ‘epilepsy’-related electrical event has been defined with clarity. 2. MODELS
FOR
SIMPLE
PARTIAL
SEIZURES,
ACUTE
spike neurons in the region of the focus tend to fire synchronously. The penicillin model has been one of the most important models for answering questions about the neuronal basis of epilepsy. Initial observations about the paroxysmal depolarization shift were based upon intracellular recordings from penicillin foci in cat neocortex178,235,328.This model is also suitable for analysis of spread of seizure activity. Radiolabeling of penicillin215 shows it to be largely restricted to a zone of a few square millimeters. Neurons surrounding this focus manifest substantial inhibitory activity in an attempt to restrain the spread of seizure activity238. The ionic micro-environment near the penicillin focus has been analyzed
A.
VERTEX
WAVES
=4-F8 Al-T3 T3-c3 c3-cz cz-c4
Models in this category are analogues of acute cortical injury leading to seizure discharges, such as might occur with an intracranial abscess, trauma or hematoma. in clinical injuries such as these there may be immediate seizures, then a latent period to development of chronically-recurrent, apparently spontaneous seizures. Experiments in the acute models may, however, be accomplished in a single sitting.
C4-T4 T4-A2 T5-P3
B. SHARP
WAVES
2.1. Topical convulsants 2.1.1.
Focal penicillin. The most popular method
to study simple partial (focal) seizures has been by application of a topical convulsant. The common antibiotic, penicillin, was discovered to be such a topical convulsant during neurosurgical procedures in which it was applied to brain to prevent infection” ll. When a cottonoid pledget soaked in 1.7-3.4 mM penicillin is placed on exposed rat or cat cortex, regionally placed electrodes record recurring interictal spikes within a few minutes. These discharges resemble human interictal spikes recorded from cortex at corticography. During the interictal
FP2-F6
yLM
T6-02
Fig. 3. EEG tracings from human scalp to illustrate normal vertex waves (A), and abnormal sharp waves (B) with phase reversals over the left temporal region. Although vertex waves and epileptiform sharp waves are similar superficially, they are distinguishable by location and accompanying EEG activity.
252 in great detai17x*““~““. At low dose of penicillin tissue culture’“‘, anesthetized animal cortex”’
in
be produced
or
hippocampal
y-
into motor cortex of baboons for 7 days29. Upon cessation of the infusion focal EEG spikes and
aminobutyric postsynaptic
slice
penicillin
selectively
blocks
acid (GABA)-mediated inhibitory potentials (IPSPS)~‘.““. At higher doses
actions are less specific15. 2.1.2. Other focal chemical convulsants. Other drugs have been used to produce an acute focus, including bicuculline34, picrotoxin**, stry~hnine142, cholinergics69.29”.2’” and anti~holinergics~‘. The first 3 of these, along with penicillin, the action of the inhibitory
are antagonists to neurotransmitter,
GABA.
to the
This
has
contributed
view
that
epilepsy results from disinhibition. Further support for this view has come from recognition of the importance of benzodiazepine receptors in relation to inhibitory processes and epilepsylx”, since the benzodiazepine receptor is closely linked to the GABA receptor. 2.2. Acute electrical stimulation Another popular model of simple partial seizures can be generated by direct electrical stimulation of cortical tissue. As was shown decades ago by Adrian”, repetitive electrical pulses can lead to rhythmic sharp discharges (‘afterdischarges’) that persist for seconds or minutes after the electric stimuli cease. Paradigms usually involve bipolar steel ball electrodes resting on cortex, applying l-5 s long trains of alternating square-wave pulses, 0.5-5 mA amplitude, OS-10 ms individual pulse duration, at frequencies of lo-100 Hz3. The response is similar in animal cortex or in human seizure patients in whom stimuli are used to map functional regions of neocortex (Fig. 4). The discharge appears to be primarily a tonic electrical discharge with frequencies of 18-25 Hz. Afterdischarges can lead to seizures which are indistinguishable from partial simple or secondarily generalized tonic-clonic seizures. Studies of afterdischarges are useful in analysis of pathways of seizure spread, since the site of origin is well-defined. They are a less useful model for the disorder epilepsy, since seizures do not occur spontaneously, and originate only at the site of electrical stimulation. 2.3. GABA-withdrawal An interesting subacute
focal seizure
model can
spike-waves
by infusing
GABA
arise from motor
by automatisms
and hind-limb
via micro-syringe
cortex,
accompanied
myoclonus.
2.4. Neocortical brain slices Slices of rat, mouse,
rabbit or guinea pig neocor-
tex may be maintained alive in vitro and exposed to various chemical convulsant~~.‘~’ as a model of acute partial seizures. Discussion of the slice model will be deferred until later, when the hippocampal slice mode1 system is reviewed. Surgical
ablation
of epileptic
foci
in patients
provides material which can be studied by the slice technique. Neocortex from such foci contains cells which show orthodromically evoked analogues of paroxysmal depolarization shifts*“, Another study260, however, emphasizes the limitations of in vitro study of surgical specimens. In surgically obtained specimens of seizure foci cellular spontaneous bursting is rare or absent. Evoked bursting is seen in normal monkey cortical brain s1ices260, as well as excised human foci. Comparison of ‘epileptic’ and ‘normal’ tissue is hampered by lack of suitable controls. The true utility of the brain slice technique has so far derived from controlled animal experi-
AFTER~ISCHARGES NE~ORTICAL
STIMULATION
Fig. 4. Electrica afterdischarges recorded from a subdurally implanted grid of electrodes in a human patient. For clarity, only 3 tracings are shown from a 64-channel recording array. The left half of the figure shows baseline electrocorticog~phic activity from the lateral left temporal lobe. Bipolar alternating square-wave electrical stimulation was applied at 50 Hz, 5 mA for 1 s to electrodes 1 and 2, each 3 mm diameter, with 10 mm center-to-center separation. Following transient amplifier blocking (flat line) repetitive focal afterdischarges were seen in the region of electrodes 1 and 2. Discharges persisted from seconds to minutes, and could progress to generalized tonicclonic seizures.
253 teral to the aluminum
lesion,
specimens.
gression
generalized
2.5. Disadvantages of acute models
zures. Interictal and ictal EEGs appear similar to clinical studies. Neuropathological specimens obtained from biopsies in the region of an established
ments
(see below),
rather
than analysis
of human
There are general disadvantages common to the acute models. First, each of the drugs may have idiosyncratic effects which have little to do with epilepsy. Penicillinase, to take an extreme example, is effective against lepsy”&, but against
the penicillin model of epino other model. Second, the
to secondarily
and occasional tonic-clonic
prosei-
alumina focus in monkeys show gliosis and distortion of dendritic neuronal trees, similar to the picture seen in human neocortical foci317. When this scar is excised abnormal EEG activity remote from the foci resolves”‘.
Response
to standard
anticonvulsants
acute models appear to be very intense; many cells in the focus participate in epileptiform activity. This degree of participation is not matched by neurons in
parallels those of patients with focal epilepsy16r. 3.1.2. Other metals: cobalt, tungsten, zinc, iron. Cortical implantations of other metals such as
human foci, studied by extracellular recordings at neurosurgery”26.327. Third, a focus is very wellcircumscribed; much more so than occurs in clinical epilepsy, where the border between ‘involved’ epileptogenic tissue and ‘normal’ tissue is usually indistinct. Fourth, anesthesia is required to establish the animal preparation, and effects of anesthesia may confound the experiment. Fifth, these models last only minutes-to-hours, and do not result in recurrent seizures. The models do not afford time for development of morphological changes in neurons, dendrites, glia and regional circulation known to be a feature of clinical epileptic tissue.
cobalt”,
3. MODELS CHRONIC
FOR
SIMPLE
PARTIAL
SEIZURES,
3.1. Cortically implanted metals 3.1.1. Alumina hydroxide, The best validated and most realistic models for the epilepsies are those employing implantation of metals in brain to generate a state of ‘spontaneously’ recurrent simple partial seizures. The prototype of this group of models is the alumina hydroxide gel model, discovered by the Kopeloffs143, and elaborated by investigators at the University of Washington at Seattle, under the direction of Arthur Ward314. In a typical preparation 4% aluminum hydroxide will be injected into surgically exposed monkey neocortex at a few adjacent sites. A similar model can be produced in the cat”. Spontaneous and recurrent seizures generally begin one to two months after the injection, and persist for as long as several years. The seizures themselves are similar to simple partial seizures in humans, with rhythmic jerking of an extremity or face contrala-
tungsten2’,
and zinc223 can produce
chronic
or subacute models of recurrent seizures in animals. None of these models is documented as extensively as the aluminum model in monkeys. GABA receptors have, however, been found to be decreased in the region of cobalt foci of rat motor cortex, 2-3 weeks after establishment of the focus230. Iron, in the form of either ferrous or ferric sulfate, can also induce recurrent seizures when injected into mammalian cortex151.246. This phenomenon may have direct bearing upon mechanisms of post-traumatic epilepsy3r9, since trauma may inject iron-containing hemoglobin complexes into cortex. The metal-deposition models are good models of chronic simple partial seizures, but they are laborious and expensive to prepare, especially in the case of the monkey aluminum hydroxide model. Additionally, they do involve injection of irritating compounds into cortex, which may have effects not seen with clinical seizure foci. 3.2. Cryogenic injury One model that does not require injection of exogenous drugs into brain is the cryogenic or freeze lesion model for partial simple seizures1105162. Ethylchloride lesions or cold-trauma from a liquid nitrogen probe produces a highly epileptogenic lesion, giving rise to seizures within a few hours of the lesion and persisting for a few days. Substantial cerebral edema generally accompanies the lesion. 3.3. Ganglioside antibody injection Karpiak and associates 135~136have produced seizures by injection of antibodies to brain gangliosides into rat cortex. After a delay of 24 h focal spiking
2.54 appears charges
in the region of the injection. then become more generalized,
EEG disand may
recur after a single injection for more than 90 days. Clinical manifestations in the animals are rare. This
interest in KA has derived from its ability to produce relatively selective lesions of cell bodies in brain, while sparing
axons of passage41. For reasons
that
model is not a convenient nor well-characterized preparation, but it does raise the interesting question
are still incompletely understood, KA has an especially prominent toxic effect on hippocampus, even when injected systemically, or at brain sites remote
of whether certain epileptogenic immunologica1ly mediated.
from hippocampus*~‘. In doses required to produce cell injury,
lesions
may be
3.4. Systemic focal epileptogenesis A model, mixing features of focal and generalized epilepsy is the preparation referred to by Remler and co-workers’@ as ‘systemic focal epileptogenesis’. Rats
receive
radiation
to a volume
of cerebrum
equivalent to 0.25 ml. At a time 3-6 months later, when the blood-brain barrier in that region is locally disrupted, bicuculline methiodide (2 mg/kg) is injected systemically. A seizure focus will be produced, with recurrent EEG spikes and focal seizures, enduring for several weeks after a single injection. These spikes are abated by phenytoin, phenobarbital, chlordiazepoxide and valproic acidz5”. The chronic models provide opportunities to study serial changes in the period between an insult to brain and the development of ongoing epilepsy. The mechanisms of these latent periods are presently unknown. 4. MODELS FOR COMPLEX PARTIAL SEIZURES
Complex partial seizures usually arise from the limbic lobe, including amygdala, hippocampus, and less often, temporal neocortex or extratemporal structures’~. Separation of models for complex partial seizures and simple partial seizures is artificial, since the distinction depends more upon where in brain the model is applied than upon intrinsic differences in the model. Nevertheless, models for complex partial seizures have engendered considerable recent attention. The 4 primary models to be discussed: kainic acid, tetanus toxin, kindling and the hippocampal in vitro slice, have each emerged since Purpura and colleagues’ 1972 compendium of experimental models of the epilepsies242. 4.1. Kainic acid Kainic acid (KA) is a rigid analog of the putative excitatory neurotransmitter, glutamate’82. Primary
less KA
than those can induce
seizures in hippocampus. Animals given KA 4 mg/kg i.v. lh4, or 0.8-2.0 pg intra-hippocampally3’, will show periodic arrest of activity, masticatory movements, complex motor tension to generalized
activity, and sometimes extonic-clonic activity. Ste-
reoencephalography shows major spike activity originating in the limbic systemzOs. KA is a prototype of an excitotoxic compound. Nevertheless, at threshold convulsant doses in region CA1 of the hippocampal slice, the convulsant effect results from disinhibition7’.r5”, probably reflecting a functional inactivation of the inhibitory interneuronal cells prior to the principal neurons. KA produces an acute or subacute mode1 of seizures, lasting hours to days. The accompanying hippocampal lesions may be considered to confound the model, or alternatively, to portray the pattern of limbic cell damage which can occur with clinical status epilepticus2”‘. Tetanus toxin A model of recurrent, chronic partial seizures can be produced by injection of tetanus toxin into rat or cat hippocampus’92. Categorization of the model with complex partial seizure models results from the location of the usual injection site in limbic structures, rather than the properties of the toxin itself. Tetanus is a disease316 produced by a 145,000 Da toxin from the gram-positive bacteria, Clostridium tetani. in the disease state toxin is transported from the periphery to the spinal cord, where it is believed to interfere with presynaptic release of inhibitory neurotransmitter 234. In contrast, injection into hippocampus of a dose of toxin 3-6 times the mouse LD,, probably produces effects only locallyr9*. Seizures may occur within a day after injection and then on a chronically recurrent basis over weeks. A seizure in a rat typically begins with arrest of activity, followed by myoclonic jerks of the front limbs, and in some animals generalized tonic-clonic seizures19*. 4.2.
255
Whether
or not
the
upon several factors,
seizure including
generalizes
depends
spread to the cingu-
model long-term
plastic changes in brain excitability.
Such plastic changes
are believed
to participate
not
but also in memory model is conceptually
and re-
late area113. The EEG shows concurrent 3-20 Hz spiking or spike-wave activity. For approximately
only in epileptogenesis, learning9’. The kindling
one month
lated to models for long-term potentiation24, although kindling paradigms tend to be more chronic
animals
have an average
of about
100
seizures per day. Thereafter, the seizures decrease, such that the animals are seizure-free by several months
after injection
of tetanus.
The mechanisms
of tetanus-induced recurrent seizures are unknown. The tetanus toxin model resembles those produced by injection
of other seizurogenic
substances
into hippocampus, but it has some intriguing idiosyncrasies. Visible neuronal destruction is absent’93 and the epileptogenic activity is time-limited. 4.3. Injections into area tempesta While examining regions near prepyriform cortex for antinociceptive activity, Piredda, Gale and colleagues made note of a discrete convulsion-prone site228*229. At this site, now referred to as ‘area tempesta’, unilateral injection of picomole quantities of bicuculline, carbachol, kainic acid, glutamate, aspartate or N-methyl-D-aspartate (NMDA) produce bilateral clonic motor seizures. At this site the anticonvulsant GABA agonist muscimol, and the anticonvulsant glutamate-antagonist, 2-amino-phosphonoheptanoic acid, are powerfully inhibitory to seizures induced in rat by i.v. bicuculline228.229. It is difficult to know where to place this model in a classification scheme. Clonic motor seizures and EEG discharges are apparently generalized, even though the chemical convulsant is limited to the limbic region of one hemisphere. Seizures from the area tempesta are most probably analogous to rapidly secondarily generalized complex partial seizures from a temporal or inferior frontal focus. The model raises the issue of whether certain apparently primary generalized seizures actually are secondarily generalized seizures arising from an occult focus. 4.4. Kindling Kindling is a phenomenon by which repeated shocks to various parts of brain result in enhanced electrical excitability of brain. The effect was discovered by Delgado and colleagues’, elaborated by Goddard”, and subsequently studied in detail by literally hundreds of laboratories91~‘85*227~245~308. Kindling has become one of the most popular ways to
than those for LTP, and focus more on epileptic changes than on enhanced evoked electrical responses. Kindling is usually tion of the amygdala,
initiated by electrical stimulabut most regions of forebrain
can be kindled98,‘53. Every species so far studied is subject to kindling308, from frogs to primates, and probably man as wel1304. To produce the model, bipolar stimulating wires are implanted in amygdala309 or elsewhere in brain. The animal recovers from the surgery, then daily electrical stimulus trains are applied via the electrodes, typically using parameters such as 0.2-1.0 mA at 60 Hz for 2-s trains’53. A fairly wide range of stimulation parameters may be effective in induction of kindling. After a few days of stimulation a train of shocks begins to induce electrical afterdischarges, which become progressively more complex and prolonged with each kindling stimulus. At this time, the animal is said to be ‘kindled’. If continued for a few weeks, rodents exhibit ‘spontaneous’ epileptic seizures227 even in the absence of their priming shocks; presumably, brain excitability has at this point reached a state in which normal afferent activity can trigger epileptiform afterdischarges. Racine has classified the behavioral response to kindling into 5 stages of epileptiform activity: Class I = facial clonus; Class II = facial clonus and rhythmic head nodding; Class III = facial clonus, head nodding and forelimb clonus; Class IV = facial clonus, head nodding, forelimb clonus and rearing; Class V = facial clonus, head nodding, forelimb clonus, rearing and falling244. Classes IV and V may be considered as models of secondarily generalized complex partial seizures. Rapid kindling paradigms able to model status epilepticus in rodents, within a few hours or days of kindling have been described165’166. Repeated stimulation by excitatory chemicals can also produce kindling’~,“‘. After several decades of work the mechanisms of kindling remain unknown. Anatomical studies with light and electron microscopy have been unre-
256 vealing”. interactions
Several between
studies
have
kindling
shown
transient
have been especially
and neurotransmitter
productive
ing of the epilepsies7’3’s4,
for our understand-
because
of the important
systems30.y6, including acetylcholine, norepinephrine, serotonin, glutamate, GABA and cyclic nu-
role of the hippocampus in complex partial seizures. Several recent texts’43’“H.262 have reviewed these
cleotides.
preparations
None of these,
ciently consistent nor mechanism for kindling.
however, long-lasting
has been suffito
explain
a
Pharmacological prophylaxis of kindling has been reviewed by Wada”“‘. Phenobarbital, diazepam and valproate block kindled seizures and block the development
of the kindling
process. Phenytoin
and
carbamazepine block seizures once kindling has occurred, but do not reliably block the establishment of kindled
seizures.
The relevance of kindling to clinical epilepsy has been debated 1.91. Can an ongoing epileptic focus induce an independent epileptic focus at remote, synaptically linked regions of brain? Such a secondary epileptogenic lesion has been called a ‘mirror focus’, and has been shown to occur in experimental rodent and frog models204,31”. If such secondary epileptogenesis resulted from clinical seizure foci, then there would be reason to consider early surgical removal of seizure foci to prevent their ‘spread’. Goldensohn’“’ has argued that evidence for kindling or mirror foci resulting from clinical epileptogenic lesions is scanty, and that management of epilepsy should not be based upon the animal model of the mirror focus. This is not to say that human brain cannot be kindled; only that clinical epileptic discharges probably are not of an optimal nature to invoke the kindling phenomenon. 4.5. Brain slices 4.5.1. Rodent in vitro hippocanapal slices The pioneering investigations of neurophysiological mechanisms of the epilepsies were performed primarily in anesthetized cat and rodent models of epilepsy222.236. Work in these models is technically difficult, because of confounding effects of anesthesia, cardiorespiratory pulsations, presence of dura, subarachnoid membranes and the blood-brain barrier, and relative inaccessibility of neurons to recording probes. In the last 15 years, investigators have increasingly turned to in vivo brain slice systems. These slices were first prepared for analysis of and later adapted for the cerebral metabolismnr3, study of physiology. 329. Studies of hippocampal slices
in detail.
Brain slices have the advan-
tages of mechanical stability (which greatly facilitates intracellular recording), absence of a bloodbrain barrier for applied drugs, and absence of anesthetics. Disadvantages include uncertainty about presence or absence of relevant circuitry for a phenomenon under study, and partial mechanical injury and hypoxia of tissue. Despite these disadvantages,
brain slices can exhibit an impressive
range
of neuronal behaviors, including synaptic plasticity’, 26’ and epileptiform bursting328. To prepare hippocampal slices, a rodent (rat, mouse, rabbit or guinea pig} is decapitated, the brain is removed and the hippocampus rapidly dissected free. Slices of about 0.5 mm thickness are made with a dropped-razor tissue cutter or a vibrotome. Cuts approximately perpendicular to the long axis of the hippocampus preserve a three-neuron synaptic circuit and associated recurrent circuitry”. After cutting, slices are then incubated in a holding chamber in which they can be kept moist and oxygenated. Slices will remain healthy for more than 18 h when properly handled. Recording is done in a special chamber, which either oxygenates the slice by mist or by submersion in liquid artificial cerebrospinal fluid. Intracellular recordings from pyramidal neurons in the slice document relative preservation of passive and active membrane properties and of synaptic interactions2s6.257. Since brain slices do not move, epileptiform activity is a strictly electrical phenomenon. The usual index of epilepsy in the hippocampal slice is development either of spontaneous or shock-evoked repetitive firing of neurons. An example of epileptiform firing in hippocampus exposed to penicillin f1.7 mM) is shown in Fig. 5. In the control state. a shock to the afferents of region CA1 pyramidal neurons produces a single population discharge (Fig. 33, top), reflecting nearly simultaneous discharge of a few hundred pyramidal neurons”‘. After addition of a convulsant drug to the slice medium - in this case, penicillin - the same single shock produces longer bursts of neuronal firing (Fig. 5B, bottom). Intracellular recordings
257 show a large, long depolarization the paroxysmal
depolarization
(Fig. 5A) similar to shift seen in anesthe-
tized, epileptiform cat cortex3**. Spontaneous repetitive firing of hippocampal neurons occurs after addition of penicillin to slice perfusing medium, and
the epilepsies 15*. Ionic and electrophysiologic anisms
of the epileptiform
paroxysmal
tion shift have been elucidated
mech-
depolariza-
in the slice1’5~222~236.
The concept of pacemakers driving apparently synchronous discharges has been analyzed in region CA3 and CA1 of the hippocampal slice259. Recur-
is paced from region CA2-3259. Very recently, hippocampal slice models of electrographic tonicclonic seizure-like (‘ictaform’) events have been
rent excitatory circuitry in region CA3 has been identified as a major contributor to the pacemaker
developed, using low-magnesium baths**‘. Data from hippocampal slice experiments have been exported to a computerized model of this
properties of region CA3293. The role of disinhibition in seizure origin has been studied in hippocampal slice<‘. Slices have given important informa-
region
tion on the nature of the interictal-ictal
of brain,
developed
by Traub
The model has made several regarding conditions sufficient
and Wong293.
elegant predictions to induce synchro-
nous bursting in synaptically related networks of neurons. The slice model is also very useful for screening actions of putative anticonvulsant drugs*l’, although there is not necessarily a correlation with efficacy or toxicity in the whole animal. The hippocampal slice has been one of the most useful models for the study of basic mechanisms of
A
PEN 1.7 mM
IIIIII
B -
1 PENICILLIN
I
Y
-10
msec
Fig. 5. Epileptiform bursting in rat in vitro hippocampal slice. A: an intracellular recording from a CA1 region pyramidal neuron. In the control condition a shock to afferent Shaffer collateral fibers (50 V, 50 ,w) produced an excitatory postsynaptic potential (EPSP). Overlayed is a recording to the same stimulus after addition of penicillin G, 1.7 mM, to the slice perfusate. The response changed to a paroxysmal depolarizing shift (PDS) with 3 action potentials. B: extracellular recording from CA1 hippocampal pyramidal cell layer showing evoked population fields in response to an electrical shock of afferent fibers. In control perfusate neurons discharged synchronously only once, producing one field response. After addition of penicillin G, 1.7 mM, to the bath the same stimulus produced 5 population responses. A and B were recorded from separate slices. Stimulus artifacts were deleted. Data were obtained by Aryanpur and Fisher (1989).
transition’**.
Plastic properties of the brain, such as long-term potentiation24 have successfully been modelled and studied in hippocampal slices9,261. The slice system has been very useful to neuropharmacologists, since the blood-brain barrier is circumvented217. This has permitted examination of mechanisms of convulsant*r 9” and anticonvulsant6,38 drugs. 4.5.2. Isolated cell preparations. A recent trend. among neurophysiologists has been a move away from brain slices to isolated neuronal preparations, prepared either by growing cells in tissue culture4* or by mechanical and enzymatic dissociation of brain slices onto Petri dishes13’. These preparations permit direct visualization and impalement of single neurons. Glass pipettes may be opposed directly to membrane in order to record currents through membrane in response to voltage, ionic or chemical changes’58. This so-called ‘patch-clamp’ technique is extremely powerful in documentation of membrane channel properties. Through this technique neurophysiologists have explored voltage-sensitive calcium and potassium channels, membrane responses to neurotransmitters and basic mechanisms of antiepileptic drug action. Neuronal monolayers will form synapses, and it is therefore possible to study cell-cell communication in tissue culture; however, there is no assurance that the pharmacology of the circuitry resembles that of the native parent tissue. In general, isolated cell preparations are best suited for study of fundamental properties of membranes and synapses. 4.5.3. Human neurosurgical tissue. During neurosurgery to treat epilepsy, epileptogenic tissue becomes available for neurohistologic’6, neurophysiologic258 and neuropharmacologic10’~301 study. This approach surpasses those of models, in that it
258 explores the clinical disorder of epilepsy, rather than an animal model. Unfortunately, study of neurosurgical material also has its drawbacks; in particular tissue may be damaged by intra-operative trauma, hemorrhage and anoxia-ischemia2h6. There is also the substantial problem of controls: if a measurement is made on the tissue, to what should it be compared? Ethical constraints rule out ablation of normal surgical
brain, field.
except Normal
when tissue
unavoidable in the can be obtained at
EEG activity of Papio papio has been reviewed by Naquet and Meldrum 2w Visual information travels to occipital regions, where an occipital driving response follows the frequency of the light flashes. Nonetheless, the epileptological process shows a predominantly frontal topography, similar to distribution of epileptiform discharges in clinical absence epilepsy. Bilateral fronto-central sharp waves may be found with the baboons in a resting state or in light
sleep.
ILS produces
spikes
or 3/s bilateral
autopsy, but here postmortem changes in chemistry, structure and certainly in physiology complicate
spike-waves. Depth electroencephalography shows spread of spike activity from cortex to deep struc-
interpretation.
tures, including thalamus (especially centrum medianum), pons and reticular formation. If a seizure progresses to a tonic-clonic stage there are variable frontal rhythmical rapid spike discharges, then synchronous bursts of polyspikes and slow waves with the clonic stage. Postictally, EEG voltage is depressed. This EEG sequence is similar to that seen in human tonic-clonic epilepsy. Photosensitive seizures in Pupio pupio are inhibited by drugs useful against clinical tonic-clonic and myoclonic epilepsy, including benzodiazepines, barbiturates and valproic acid. Less favorable therapeutic effects are found with phenytoin, carbamazepine and trimethadione”‘. 5.1.2. Audiogenic seizures in mice. The breeding of inbred homogeneous strains of mice with particular phenotypes has become a service industry at Jackson Laboratories, Bar Harbor, MN, where catalogs offer dozens of strains of mice with seizure disorders. Noebels213 has listed 12 mouse strains
5. GENERALIZED
5.1.
TONIC-CLONIC
SEIZURES
Genetic
There are no good animal models for primary generalized, spontaneously recurrent tonic-clonic (grand mal) seizures. Because idiopathic grand ma1 epilepsy shows a genetic component”“, investigators have attempted to develop models from genetically aberrant strains of animals; including baboons, beagles, Mongolian gerbils, mice, rats and fruit-flies. Each of these models has distinctions from clinical grand mal, either in the requirement for certain types of precipitating stimuli, or other associated non-epileptic deficits. 5.1.1. Photosensitive baboons. The model of primary generalized epilepsy closest to that seen in man is photosensitive epilepsy in the Senegalese baboon, Papio pupio. This model is, unfortunately, very expensive and difficult to maintain. In 1966 Killam and associates13y,‘4” discovered that Senegalese baboons could suffer generalized tonic-clonic seizures in response to intermittent light stimulation (ILS) at frequencies close to 2.5 flashes per second. The Papio pupio model has been reviewed in detail by Naquet and Meldrum2”‘. ILS first produces eyelid, then face and body clonus. Tonic spasms or full tonic-clonic convulsions may then ensue and persist beyond termination of the light flash, Seizure threshold of the baboons can fluctuate week-to-week”“‘. Rarely, animals may show apparently spontaneous seizures. In distinction to this pattern, patients with primary generalized tonic-clonic seizures usually evidence apparently spontaneous seizures, and are photoprecipitated only in a minority of case?“‘.
with spontaneous seizures and 5 with seizures evoked by sensory stimuli. each due to a single locus mutation. Particularly well-studied strains include the DBAI2J mice with audiogenic seizures, the totterer and the El mouse. The most popular rodent model of generalized epilepsy is the mouse with sound-induced seizures. Several strains of mice have been bred for a propensity for audiogenic seizures4”, including the DBAI2J and the SJLiJ strains. DBA/2J mice almost universally exhibit severe sound-induced seizures between age 2 and 4 weeks, after which susceptibility gradually declines26’. At 8 weeks of age, when DBAIU mice are free from audiogenic seizures, they still show a low threshold to seizures induced by maximal electroshock or excitatory amino acidsh2.
The SJL/J strain, seizures,
in contrast,
develops
less severe
only after a few days of ‘priming’ exposure
to sounds”. The nature of the inducing noise is not critical, but should be loud, sudden and contain frequency components in the 12-16 kHz range. A susceptible mouse will first startle, then begin to run and leap. In some cases progression will continue to clonic jerks, and prolonged tonic spasms. Repetitive seizures can be fatal to the DBA/2J mouse. Good EEG recordings are scarce in this model because of the startle and running phases. can be prevented by phenytoin or valproic
Audiogenic seizures or phenobarbital’s4
acid2”‘.
The audiogenic
mouse seizure model has no direct
counterpart in clinical epilepsy. Nevertheless, the true utility of the model lies in its ability to permit analysis of genetic factors leading to seizure-like events. It has recently been observed that seizuresusceptible strains of mice are deficient in a calciumdependent ATPase 26s. This type of observation may have relevance for our understanding of basic mechanisms of seizure susceptibility. 5.1.3. Totterer mice and other seizure-prone mouse strains. Hereditary mouse models of primary generalized epilepsy can be classified under models for generalized tonic-clonic seizures or for absence seizures. The totterer mouse (tg/tg strain) was first studied because of its hereditary ataxia1”3; however, it was noted to also suffer myoclonus, frequent partial and absence seizures’34.216, with accompanying 6-7/s spike-wave EEG changes214. Transmission to progeny is recessive. Seizures appear at about 4 weeks of age. They begin with bilateral hind leg spasms, followed by unilateral, clonic jerking. The clonus may spread, but it does not generalize. Variant strains of the totterer mice may present various different phenotypic seizure characteristics. The tg/tgla variant strain, for example, seizes for prolonged periods265. Totterer mouse strains provide fascinating models for study of neurotransmitters and epilepsy. Noradrenergic innervation from the locus coeruleus to the limbic forebrain is crucial for development of seizures and ataxia’56. Totterers have increased numbers of noradrenergic terminals and elevated norepinephrine levels in hippocampus, cerebellum and locus coeruleus’56. Lesions of noradrenergic terminals by the selective neurotoxin 6-hydroxydopamine
partially protects animals against seizures, myoclonus214.
Confirmation
noradrenergic
axons from the coeruleus
ataxia and
of the role of increased comes from
study of another C57BU6J mutant called the quaking mouse, with myelin deficits, tremor and spontaneous or stimulus-evoked tonic-clonic seizures’79. How these findings relate to a suspected inhibitory role for norepinephrine in several regions of brain2@‘, remains to be determined. The ‘epilepsy’ (abbreviated ‘El’) mouse was first discovered by Imaizumi and colleagues in 1959125. Seizures are best induced as tossing or spinning are not effective.
by vestibular
stimuli,
the mice. Audiogenic
Manifestations
of seizures
such
stimuli in this
strain may include limb and face automatisms such as chewing and salivating279. Electrical discharges originate in deep limbic structures279. These features are analogous to clinical complex partial epilepsy. Like human complex partial epilepsy, seizures may generalize to tonic-clonic activity. Heritability of the vestibulogenic seizure tendency in El is dominant, but the gene locus or loci and neurochemical defects are unknown. 5.1.4. Genetically epilepsy-prone rats. To electrophysiologists and experimental psychologists, the rat is more familiar than the mouse as a laboratory animal. Fortunately, rats also exhibit seizure discharges. Jobe, Dailey, Laird and colleagues at the University of Illinois College of Medicine have studied extensively a strain of genetically epilepsyprone rats (GEPR), susceptible to seizures induced by sound, hyperthermia and various electrical or chemical stimuli247. A GEPR-9 colony is severely seizure-prone and a GEPR3 colony only moderately seizure-prone, allowing for controlled comparisons. Animals from both colonies run about wildly, and fall from clonic jerks when stimulated by sounds, but GEPR-9 rats also suffer tonic extensor convulsions. As with other rodent reflex epilepsy models, seizures are most prominent at a certain stage of development, with increasing seizure tendencies appearing over the period 2-4 weeks after birth12”. The standard clinical anticonvulsants exhibit efficacy in blocking audiogenic seizures in GEPRs~~. The seizure prone character of these rats relates in some way to abnormalities in the auditory pathways, including impaired cochlear function226. Injection of amino acid antagonists into the inferior
260 colliculus is highly effective in blocking audiogenic seizures in GEPRs~~, presumably through a block of afferent
impulses.
Efferent
block can be produced
sence seizures,
and phenytoin,
valproate
and phe-
nobarbital inhibit tonic seizures in the SER rat2’3. 5.1.5. Mongolian gerbil. Another rodent studied
by injecting amino acid antagonists into substantia nigra and brainstem reticular formation”‘.
for its seizure tendency is the Mongolian Gerbils exhibit generalized tonic-clonic
Neurotransmitter alterations have been examined in the GEPR148. Most impressive have been alterations in amines. Deficits of norepinephrine con-
when handled or agitated. This preparation has been used to screen anticonvulsants’8. Unfortunately, as reviewed by Majkowski and Kaplan”‘, the utility of
tent4’ and turnover13’
the gerbil model is limited by variability of seizure threshold and a progressively longer refractory pe-
have been identified
in brain-
stem, telencephalon and cerebellum of both GEPR9 and GEPR-3 rats, and binding sites for prazosin, a selective al-adrenergic ligand, are significantly decreased in frontal cortex of GEPRs versus normal Sprague-Dawley rats. Decreased content or abnormal functional systems for other neurotransmitters may also play a role in the GEPR’s seizures. Abnormalities so far identified have included decreased brain serotonin”‘, possibly increased cholinergic content in thalamus and striatum14”, decreased responsiveness of inferior colliculus neurons to applied GABA”“, and lower threshold for ‘wetdog shakes’ from intraventricular morphinez4s. A specific abnormality of excitatory amino acid-mediated neurotransmission has not been reported; however, audiogenic seizures similar to those in the GEPR may be produced in normal rats by infusing N-methyl-o-aspartate into inferior cohicuhis’yx. Of these neurotransmitter abnormalities, it is uncertain which are contributors to the seizures and which are consequences of the seizures - a problem common to analysis of neurotransmitters in clinical epilepsy. One way to clarify this issue with animal models is to examine neurotransmitters in the young (4day-old) GEPR prior to onset of seizures. Using this approach, Roberts and Ribak251 have shown a significant increase in glutamic acid decarboxylasepositive neurons in inferior colliculus of the young GEPR, suggesting that disinhibition may play a role in seizure susceptibility. A strain of rats has been described recently, with apparently spontaneous absence-like and tonic seizures. These rats are progeny of cross-matings between the so-called ‘tremor’ homozygous rat and the ‘zitter’ homozygous rat263. This spontaneously epileptic rat (SER) exhibits 5-7/s spike-wave-like complexes in cortex and hippocampus during periods of abnormal immobility. Ethosuximide, trimethadione, valproate and phenobarbital inhibit ab-
gerbi1287. seizures
riod for seizures. 5.1.6. Drosophila shakers. Formal genetic analysis has long been performed upon Drosophila melanogaster (the fruit-fly). Recently, this field has become of interest to epileptologists. A mutant fly, named ‘Shaker’, has been observed to shake and flutter when exposed to ether28”. Intracellular recordings from the flight muscles indicate deficits of the so-called ‘A-current’. This current is carried by potassium ions and used to repolarize membrane after excitation-induced depolarization. Its absence would be expected to result in prolonged firing of excitable tissue. The full power of molecular genetics is now being applied to the analysis of the Shaker”3, 220~28y,giving an opportunity to link a seizure-like behavior with a characterized physiologic and genetic deficit. Whether such a genetic abnormality of potassium channels occurs in humans is unknown. 5.1.7. Miscellaneous genetically seizure-prone animals. Several other less-studied genetic models of epilepsy have been reviewed by Seyfried and Glase?, Maxson et al.‘“” and Loscher and Schmidtn’“. Studies have been performed on photosensitive epileptic chickens43, rabbits163, hamsters330 and dogs’14. These genetic model systems are not like human epilepsy, but they are contributory to our understanding of how a gene mutation can lead to seizures*‘“. It is important to note that a ‘single mutation (e.g. totterer) can lead to seizures, and that such seizures may be accompanied by a variety of neurological abnormalities. In other models (e.g. GEPR) the genetic deficit is associated with complex and multifactorial neurotransmitter abnormalities. 5.2. Maximal electroshock Maximal electroshock (MES) is arguably the best-studied and most useful animal model of seizures. Since 1870 it has been known that electrical
261 shocks
to animals
Spiegel
quantified
zures
can produce
seizuress3.
the technique
by skull-shocks
In 1937
of producing
to animals274.
Merritt
constant-current
research are listed in Table IV. Chemical convulsants are convenient for screening of putative anticonvulsants281,283.
or constant-voltage
delivery systems. Typical stimulation parameters in drug-screening protocols233 are 50 mA in mice and 150 mA in rats, 60 Hertz delivered electrodes for 0.2 s. Several variations
systemically.
and
techniques of the MES animal mode1282. Electricity may be applied via ear-clip or cornea1 electrodes, either
seizures when administered
A few of those
PutnamlY4 applied MES screening to series of barbiturate derivatives, resulting in the development of phenytoin. Swinyard has reviewed in detail the modern
using
generalized
sei-
via cornea1 exist among
of greatest
interest
for epilepsy
5.3.1. Pentylenetetrazol. The prototype 276,283in the class of systemic convulsants enetetrazol derivative*” mice, rats, parenteral
(PTZ,
metrazol).
PTZ
agent**l, is pentyl-
is a tetrazol
with consistent convulsive actions in cats and primates, when given by the route.
PTZ initially
produces
myoclonic
jerks, which then become sustained, and may lead to a generalized tonic-clonic seizure. EEGs show spikewaves or polyspikes124. An example of an EEG from
different laboratories. Tonic-seizures may be produced in mice with ear-clip electrodes delivering 50/s alternating square-wave pulses at currents of approximately 15 mA for 1 s146. A distinction is made between minimal and maximal seizures282. Minimal seizures are characterized by a ‘stun reaction’ and clonic movements of the face and forelimbs. A minimal electroshock seizure threshold is sometimes defined by a stimulus that produces 5 s of sustained clonus. Maximal seizures show tonic hind-limb extension and flexion, followed by clonus. A MES seizure meets criteria if there is tonic hind-limb extension. Studies may choose to evaluate either minimal or maximal electroshock seizures. Furthermore, they may measure the threshold for inducing seizures in 50% or 97% of the animals, or they may count the percent of animals exhibiting MES at a shock current 4-5 times higher than the threshold current. Statistical methods, related to probit analysis, have been used to compare MES thresholds between control and test groups159. Drugs able to inhibit MES seizures in mice and rats are considered to be candidate therapies for primary and secondarily generalized tonic-clonic epilepsies233,281 (see Table IV). Phenytoin remains one of the most effective drugs at inhibition of MES seizures. Carbamazepine is also effective; ethosuximide is not. Valproate and benzodiazepines can inhibit MES seizures, but at poor therapeutic/toxic ratios.
a mouse given PTZ 85 mg/kg i.p. is shown in Fig. 6146. Drug screening protocols generally set 5 s of continuous clonus as a threshold for scoring presence of a clonic seizure, even though this criterium can be imprecise. With i.v. administration of PTZ 1% solution to mice, the threshold dose for clonic seizures is about 50 mg/kg and for tonic-clonic seizures 90 mg/kglm. Subcutaneous administration of PTZ results in clonic seizures in 97% of animals
5.3. Systemic convulsants Numerous chemical compounds
N-methyl-or-asp
may
produce
TABLE IV Systemicchemical convulsants Drug
Species
Dosage @g/kg)
Ref.
FTZ
Mouse
85 S.C. 50-75 i.p. 50 i.v. 70 S.C. 40i.p. 20i.v. 600,OOOi.m. 300,OOOi.m. 21 i.p. 3.2s.~. 2i.v. 2.7 S.C. 0.3 i.v. 1.5 S.C. 200 i.p.
283 277 160 283 177 303 67 79 108 233 107 233 189 20 292 112 190 52
Rat
Strychnine MS0
Cat Rat Cat Rat Mouse Dog Mouse Monkey Mouse Rat
Allylglycine Flurothyl
Baboon Mouse
Homocysteine
Mouse Rat Mouse Mouse
Penicillin Bemegride Picrotoxin Bicuculline
550 i.v. 1.5ml total inhaled 875 i.p. 1000 i.p. 34os.c. 105 i.v.
82 7 47 47
262 tested
(CD97)
with dosages
of 85 mg/kg in mouse
and 70 mglkg in rat. The mechanism of action of PTZ is only partially understood. Early studies suggested it exerted effects primarily on cortex4.23; however, later workers3’j3 argued that mesencephalic non-specific reticular formation neurons were activated by systemMirski and ic PTZ before cortical neurons. Ferrendelli2’X’~201 recently role of the mammillary mammillothalamic zures in mouse vinyl-GABA, into reticular
have shown the important
bodies, anterior
thalami and
tracts in mediation of PTZ seiand guinea pig. Injection of y-
an inhibitor of GABA degradation. formation, anterior media1 hypothala-
mus and caudal hypothalamus blocks PTZ-induced seizures in rats 199. In cat, however, the clonic phase of seizures appears mediated by forebrain”“. At a synaptic level PTZ appears to interact with the {GABA receptor benzodiazepine chloride ionophore} complex*“, presumably in some way decreasing the potency of inhibition and leading to seizures. Wilson and Escueta”” suggested that PTZ blocked GABA-mediated inhibition. Pharmacologic antagonists to benzodiazepines do not appear able to block seizures from PTZ36, so other mechanisms for PTZ-induced seizures must also be important. In region CA3 of guinea pig hippocampal slice (see above for discussion of the hippocampal slice mode1 system) PTZ 2-10 mM results in alternating depolarizing and hyperpolarizing neuronal bursts”‘.
BASELINE
PTZ 85 mg/kg
Fig. 6. Seizure induced in a mouse by S.C. injection of pentylenetetrazol (PTZ, 85 mg/kg). Recordings were obtained from needle electrodes in the scalp, configured as shown in the diagram to the right. Baseline EEG is shown on the left. The middle panel shows two segments of EEG, separated in time by 45 s, after PTZ injection. PTZ resulted in recurrent runs of rhythmical sharp activity with shifting phase reversals. During the times of EEG discharge the mouse exhibited twitching of vibrissae and mild clonus of the front limbs. Tracings have been darkened by hand for purposes of reproduction. Data were obtained by Krauss and Fisher (1989).
TABLE Inhibition
V
ofMES
and PTZ seizures in mouse
PTZ is administered s.c., and test drug i.p. The dosage is the ED50%, i.e. the dosage in mg/kg needed to inhibit seizures in 50% of the tested animals. M-MES, mouse maximal electroshock seizure; R-MES, rat maximal electroshock seizure; M-PTZ, mouse pentylenetetrazol seizure; R-PTZ, rat pentylenetetrazol seizure. Anticonvulsant
M-MES
R-MES
M-PTZ
R-PTZ
Phenytoin Carbamazepine Phenobarbital Valproic acid Ethosuximide Diazepam
9.5 8.8 21.8 272 N.E. 19.1
14 4 12 100 N.E. 5
N.E. N.E. 13 149 130 0.2
N.E. N.E. 7 74 54” 0.3
N.E., not effective;
a given p.0.
These bursts are ‘paced’ more by calcium currents than by GABA-mediated inhibition. PTZ was adopted as a screening test for anticonvulsants in part in response to the discovery that the anti-absence drug, ethosuximide, failed to alter maxima1 electroshock seizure thresholds. In contrast, some drugs effective against MES seizures, such as phenytoin and carbamazepine, are ineffective against PTZ seizures233. Common practice among drug companies testing prototype anticonvulsants has been to presume that drugs effective against PTZ seizures would be good anti-absence (anti-petit mal) therapies, and drugs active against MES would be good anti tonic-clonic (anti-grand mal) therapie@‘. This assumption has been called into question’6’, and may be an excessive simplification. Nevertheless, PTZ does appear to have a preferentially subcortical locus of action, and may identify anticonvulsants not marked by screening against MES. Table V lists inhibitory mg/kg dosages of several important clinical anticonvulsants against MES and PTZ seizures”7,‘6”.233,?97. 5.3.2. Systemic penicillin as a tonic-clonic model. Penicillin was discussed above as an agent able to produce acute focal seizures, when placed on cortex. Clinical experience has indicated that high systemic doses of penicillin in humans can produce myoclonus, generalized tonic-clonic seizures and encepha10pathy8’. In the hospital setting encephalopathy occurs most commonly with i.v. dosages above 20 million units per day, especially if concurrent renal
263
failure brain
maintains barrierz6.
high levels and alters the bloodPrince
and Farre11237 showed
that
parenteral penicillin could produce generalized seizures in cats. Farriello67 later extended the model to rats. The model was elaborated by Testa and Gloor;?s6, Fisher and Prince79s0, Quesney and associates243 and then studied extensively by Gloor, Quesney, Avoli and colleagues at Montreal’*. Parenteral penicillin in the feline serves as a model for several different types of seizures, including absence seizures, myoclonic seizures and absence seizures. The animal tends to pass through each of
against picrotoxin,
but not bicuculline.
ineffective
both types of seizure;
against
Phenytoin
is
whereas,
diazepam is highly effective against both types. Phenobarbital has intermediate effectiveness, and the anti-absence agents, valproate and ethosuximide, have low but detectable effectiveness. Several antagonists of GABA synthesis have been available for decades, and still find occasional use as inducers
of seizure activity. These agents include thiosemicarbazide324 and methionine allylglycine’, sulfoximine89. Meldrum et a1.19’ used allylglycine to
these stages after a dose of at least 300,000 units per kilogram of i.m. penicillin. Since the model is
generate prolonged status epilepticus in monkeys, in a classic study of the effect of status on brain histology. Allylglycine’21 and thiosemicarbazide are
probably most useful as an absence model, discussion is deferred to that section. 5.3.3. Other inhibitory antagonists. Other popular systemic convulsants include bemegride, picrotoxin, bicuculline, methionine sulfoximine, allylglycine, strychnine and certain general anesthetics. Each of these has slightly different actions; however, the choice of one model over another usually depends primarily upon familiarity to the investigator. Bemegride (Megimide) is a glutarimide derivative similar in action to PTZ’07*252. It has been used to produce clonic or tonic-clonic seizures, or to activate focal epilepsy278. Several of the drugs used to produce partial seizures when focally applied, for example picrotoxin and bicuculline, will produce generalized clonic and tonic-clonic seizures when given systemically. Cellular mechanisms of these convulsive drugs are incompletely understood. As noted above, both are GABA antagonists, and will block physiologic inhibitory postsynaptic potentials88,118. Picrotoxin and bicuculline do not antagonize GABA by the same mechanism: bicuculline’s antagonism is pharmacologically competitive, but picrotoxin’s is not17, possibly because picrotoxin interacts with the chloride ionophore of the GABA receptor complex, rather than with the GABA binding site. Brain metabolism early in bicucullineinduced generalized tonic-clonic seizures is greatest in neocortex and synaptically linked regions, as opposed to brainstem59y60. Clinically useful anticonvulsants tend to have parallel effectiveness against picrotoxin- and bicuculline-induced seizures233, with the unexplained exception of carbamazepine, which has some benefit
believed to inhibit glutamic acid decarboxylase, the synthetic enzyme for GABA. Seizures induced with allylglycine reduce immunoreactivity for GABA in hippocampus and cerebellum’91. Actions of methionine sulfoximine (MSO) are complex. Onset of seizures may be delayed hours to days, depending upon the species89,‘86 and the dosage241,278. MSOappears to alter synthesis of amino acids in the tricarboxylic acid cycle300, perhaps altering balance between GABA and glutamate content. A potent generalized seizure model can be produced by i.v. injection of strychnine167. Strychnine interacts with GABA-benzodiazepine receptors28, but a more important action of strychnine is probably against glycine45. Glycine is an important inhibitory neurotransmitter in brainstem and spinal cord240 with structural homology to the much larger strychnine molecule”. Strychnine serves as a noncompetitive inhibitor of the glycine receptor’76. Resulting seizures differ in character from those produced by primary GABA antagonists63 in that they are mainly extensor tonic, with little cortical EEG seizure activity 278. These seizures are not fully relieved by reasonable doses of any of the commonly used anticonvulsants233, including benzodiazepines46. Certain inhalation anesthetics are known to be convulsant in man 72. Flurothyl is a hexafluorinated non-flammable ether which is a useful experimental convulsant in mice22. Advantages of flurothyl include ease of testing, a clear endpoint of a clonictonic seizure with loss of righting reflex, and a threshold independent of body weight5*. A typical paradigm involves injection of 1.5 ml of flurothyl
264 into a 2-gallon
sealed
and stirred
jar containing
a
mouse. The dependent variable is latency to clonictonic seizure after injection of the anesthetic. Some convulsants apparently act by mimicry of excitatory neurotransmission. It is often difficult to induce seizures by systemic administration of glutamateZ5’, penetrate
although
monosodium
glutamate
can
to
brain and produce convulsions in rats13’. More potent or more blood-brain
a progressively
higher
incidence
of clonic or tonic
seizures. Protection against NMDA-induced seizures and lethality is provided by valproate, diazepam and specific NMDA bital, phenytoin
antagonists, but not by phenobaror ethosuximide47~155.
5.4. ~et~hoZic dera~geme~f~ In clinical practice many metabolic derangements can lead to seizures including hypoxia, hypoglyce-
lo-day-old barrier-permeable analogues of glutamate are usually used. One such agent, kainic acid, was discussed
mia, uremia, drug withdrawal and high temperature74,211. These conditions have not in general been
above in models of complex partial seizures,
useful
since it
has a predilection for hmbic structures. Homocysteine thiolactone has been used as a model of generalized convulsions in rodents109.275. Homocysteine is a sulfur-containing amino acid, structurally similar to glutamine, and also to the glutamate analog, homocysteic acid*‘. Humans with deficiencies of cystathione /3-synthetase will accumulate homocysteine and suffer from seizure?‘. Mites’ or rats’ given approximately 1000 mg/kg homocysteine thiolactone i.p. induce clonic and whole-body clonictonic convulsions, with latencies ranging from 10 to 60 min. Repetitive seizures, status epilepticus and death are all common with this model. Mechanism of epileptogenicity with this agent is complex. Extracellular administration of homocysteine by pressure to cortical neurons excites the neurons, in concentration similar to those required by glutamate32s. Homocysteine may, however, impinge upon multiple neurotransmitter systems, at several sites in the brain’. Glutamate receptors have been categorized by their sensitivity to the non-endogenous physiological probes, NMDA, quisqualate and kainat?. Evidence suggests that quisqualate and kainate receptors are involved in conventional synaptic transmission; whereas NMDA receptors are implicated in potentiated synaptic processes, including seizures. Administration of 340 mgikg racemic N-methylaspartate i.p. or 105 mgikg i.v. to mice produces clonic convulsions in mice4’. The dextro form of racemic N-methyl-aspartate is the biologically active form, such that 200 mg/kg of NMDA leads to lethal status epilepticus in 100% of treated mice within 10 min’““. Behavioral effects of NMDA are profound”“. Doses of 100 mgikg i.p. in mice induce scratching and tail-biting, and higher doses produce
for studying
mechanisms
of the
epiIepsies
because they usually produce other CNS disturbances peculiar to the model employed. Three metabolic disturbances have, however, been employed in a few studies of generalized seizures: hyperbaric oxygen323, hypercarbia3” and drug withdrawalti.“. 6. MODELS
FOR
GENERALIZED
ABSENCE
SEI-
ZURES
Clinical absence is characterized by arrest of ongoing activity, partial or full decrease of awareness and concurrent 3-4/s spike-waves in the EEG”. 2’2. No animal model mimics this condition precisely, but 5 have provided useful approximations: thalamic stimulation, bilateral cortical foci, systemic penicillin, ~-hydroxybutyrate, and genetic rodent models. 6.1. Thalamic stimulation Classic studies by Morison and Dempsey203 defined the concept of a thalamic reticular formation, able to influence wide areas of cortex. Jasper and Droogleever-Fortuyn”’ and Hunter and Jasper’23 showed that stimulation of midline and intralaminar thalamus could produce absence and EEG spikewaves. The behavioral and EEG abnormalities did not long outlast the stimulation, and occurred only at certain states of arousal. Recent evidence in patients suggests that certain thalamic stimulation parameters may actually be anticonvulsant3”4. Pollen and associates’” I .232investigated the cellular basis for the spike-wave discharges in the thalamic stimulation model. EEG spikes were associated with depolarizing shifts intraceliularly and synchronous cell firing. Waves appeared linked to inhibitory neuronal processes.
265
The role of thalamus
versus cortex in the gener-
of course
that
clinical
correlates of EEG spike-wave (SW) discharges?; and (2) what is the role of cortex versus subcortical structures in generation of SW bursts?
6.2. Bilateral cortical foci
Intracellular studies of neurons during the spike of SW feline penicillin epilepsy are in agreement about
dysfunction”‘. Applisuch as estrogen or
pentylenetetrazol”” or penicillin79.94 to widespread regions of cat cortex produces bursts of 2.5-3/s spike-wave discharges synchronous to within 15 ms. A similar model can be produced in rhesus monkeys I’3 These studies were all performed in anesthetized animals, so behavior could not be analyzed. Marcus and associates”” therefore developed a bilateral cobalt implantation model in rhesus monkeys, with chronic and behavioral features. Cobalt powder, 60 mg, was impregnated into a 1 x 1 cm gelfoam pledget, and placed bilaterally on premotor cortex. Bilateral EEG spike discharges could be recorded 80-120 min after placement of the cobalt. Subsequent EEG findings included wellformed 3/s bilaterally symmetric spike-wave discharges. By 2-3 h behavioral absence could be observed, with eyelid opening and upward movement of the eyes. A few animals had recurrent seizures for up to 72 h. This model is of considerable theoretical interest. Obviously, it is not suited for anticonvulsant screening procedures. For pragmatic reasons, a much more popular model of multifocal cortical epilepsy is the model produced by high systemic doses of penicillin. 6.3. Systemic penicillin Intramuscular injection of 300,000 units/kg of penicillin G into a cat285 results in recurrent episodes of arrested activity, staring, myoclonus, facial-oral twitching and occasional progression to generalized tonic-clonic seizures (see above). Seizure activity begins about 1 h after injection of drug and continues intermittently for 6-8 h. The EEG shows a variety of spike-wave morphologies79, emerging from a relatively normal background. These features are similar to those seen with clinical absence, except
The
feline
apparently
because of the need for chronic electrode implantation, and for ongoing stimulation during testing.
is a result of diffuse cortical cation of dilute convulsants
years.
recurs
ation of absence epilepsy remains a subject of great interest and controversyY2. The thalamic stimulation model for petit ma1 is, however, infrequently used
Models of petit ma1 produced by bilateral cortical foci derive from the hypothesis that absence epilepsy
for
absence
spontaneously
generalized
penicillin model has been particularly useful in the study of two questions: (1) what are the cellular
the correlates many cortical with action
of the EEG spike. During the spike neurons exhibit large depolarizations potential
generationgO.
Cellular
corre-
lates of the wave are less clear. Most cells appear inhibited during the wave, although occasional cortical units can be seen to fire rapidly during the wave79. These presumably are interneurons mediating inhibition. Possible mechanisms of inhibition during the wave include chloride-dependent GABAergic inhibition, and also non-GABA dependent processes such as calcium-dependent potassium efflux after cell excitation. It has not yet been possible to demonstrate that disinhibition by penicillin, shown for simple model systems (see above), leads to SWs in anesthetized cat cortex. Several studies have shown no major change in GABA-mediated inhibition in the feline penicillin mode151,9”,145. During the early phase of SW bursts from parenteral penicillin, cat neocortical neurons show normal responsiveness to applied GABA or glutamate144. These findings have implications for the discussion of GABA’s role in the epilepsies. Failure to show a clear change in GABAergic inhibition in this model does not rule out subtle, but important, decreases in the balance between excitation and inhibition, nor decreases in inhibition at distal dendritic branches. Clinical absence (petit mal) epilepsy has been hypothesized to originate subcortically, with participation of brainstem and thalamic reticular formation84.225. Absence seizures in the feline penicillin model have been difficult to reconcile with this hypothesis92. SW discharges appear in cortex before they appear in mesial thalamus or reticular formation 14,79.Application of penicillin to wide regions of cortex, but not to thalamus, can produce SW EEG discharges92. Discharges in this model probably originate cortically, but are maintained and elaborated by recurrent thalamo-cortical circuitry13%92. Response of feline generalized penicillin epilepsy
266 to anticonvulsant
drugs
appears
to parallel
the
clinical response in absence epilepsy: ethosuximide and valproate are more effective than phenytoin’os*224. Parenteral penicillin in rats can produce seizures, but the model
bears
little
resemblance
to clinical
absence. 6.4. y-~ydroxybutyrate y-Hydroxybutyrate (GHB) is a metabolite of GABA that can produce absence-like episodes in a variety of species 271. This model has been developed in detail by 0. Carter Sneed. Treatment of rats with loo-150 mglkg GHB produces rapid spiking and 4-6/s spike-waves in the EEG and absence-like behavior. Analogues of clinical absence can also be seen in monkeys2hy. The model shows a developmental*‘” and pharmacologic profile26x similar to petit ma1 epilepsy. This is a promising chemical model of absence, which may supplement the more familiar feline parenteral penicillin model. 6.5. Zntraventricular opiates Endogenous and synthetic opiates can induce a wide range of neurophysiologic and behavioral states in experimental animals25. Low-dose morphine sulfate is believed to be anticonvulsant, but high-dose parenteral morphine can induce clonic convulsions Opiate peptides do not appear to in rodents”‘. penetrate significantly into the CNS after systemic administration; however, Urea and associatesz9s demonstrated that methionine enkephalin produced seizures in rats when given intraventricularly. The endogenous opiate /3-endorphin, given intraventricularly, can also produce seizuresri7. These seizures are difficult to classify. Rodents exhibit arrest of activity, staring, rearing and a type of trembling given the descriptive appellation, ‘wet-dog shakes’. The EEG shows repetitive spikes followed by runs of slow waves in hippocampus and related structures, perhaps because these structures are periventricular, or possibly because of a specific susceptibility of the limbic system to opiates. The behavioral-EEG pattern after intraventricular opiates can be classified either with the complex partial”’ or the absence273 models of the epilepsies. Relation to petit mat epilepsy has been supported by ontogenetic studies of opiate-induced seizures273 and by the relatively specific responsiveness of these seizures to anti-petit
ma1 agents”*. A different view was taken by Caldecott-Hazard, Engel, Chugani and associates33 and Torteila and colleagues’% who argued that opiate-related
phenomena,
with
behavioral
arrest
and slow waves, were more analogous to the postictal state. The intraventricular opiate model is not experimentally convenient, since ventricles must be penetrated and consideration given to controls for nonspecific CNS trauma. The primary advantage of this model is in its elucidation of actual opiate-related mechanisms for seizures. Several lines of evidence suggest such mechanisms may be important. In animals, seizures generate a compound in the CSF that is anticonvulsant in other animals, and whose effects are antagonized by naloxone29’. Seizureprone GEPR-9 rats are much more susceptible to mo~bine-induced wet-dog shakes and clonic convulsions than are GEPR3 rats or controls248. In patients, the opiate antagonist naloxone can increase frequency of interictal spiking in patients with complex partial seizures202, and binding of radiolabeled opiate receptor hgands is increased in vivo in the region of unilateral temporal lobe seizure focis’. 6.6. THIP THIP (4,5,6,7-tetrahydroxyisoxazolo-4,5c-pyridine3-01) is a bicyclic partial GABA-A receptor agonist that inhibits benzodi~epine binding to the GABAbenzodiazepine receptor complex’22~33*, probably by blocking effects of endogenous GABA. High doses of THIP can also interact with choline& systems332. Intraperitoneal injection of THIP 5-10 mg/kg in rats repIi~bly produces bilaterally synchronous EEG spike-waves and absence-like behaviors enduring for several hour@‘. 6.7. Genetic rodent models of absence Up to 30% of Sprague-Dawley3’ and Wistar’75*30’ rats exhibit spontaneous seizures3’ accompanied by behavioral arrest, face twitching and 7-11/s EEG spike-wave discharges. This trait for spontaneous seizures has not been bred true, although a Wistar WAGiRij strain of rats from Glaxo Laboratories, U.K. has been shown to exhibit spontaneous absences and EEG spike-waves3y~“02.
267 zures. Seizures may be interrupted
7. ANIMAL MODELS OF STATUS EPILEPTICUS
Certain
research
questions
require
models
of
by administration
of phenytoin, phenobarbital or benzodiazepines soon after injection of homocysteine.
recurrent seizures or status epilepticus. Many of the chemical convulsants able to produce seizures - for example, kainic acid, NMDA, flurothyl, bicuculline
Several electrical paradigms have been suggested as a recipe for producing status epilepticus in rats. In one experimental series196 continuous sine wave 40
and
PA limbic electrical
pentylenetetrazol
-
can also produce
status
stimulation
produced
convulsive
epilepticus when administered in large quantities to rodents. Unless treated, these administrations are
status in 4 of 18 rats. Three additional demonstrated electrical status epilepticus
often rapidly lethal. One recently popularized
mini and Nadler306 pulse trains (0.3 ms monophasic
ticus is the lithium-pilocarpine
tonic-clonic model of status epilepmode132~‘32,205,312.In
this model rats are pretreated with lithium chloride, in doses approximating 3 mEq/kg i.p. At least 20 h later the cholinergic agent pilocarpine is given S.C. at 25-30 mg/kg. Generalized clonic or tonic-clonic seizure activity begins about 30 min after administration of pilocarpine, and continues for several hours. The EEG pattern displays a progression very similar to the stages seen in human status epilepticus312. Chronic pretreatment for one month with daily lithium reduces the convulsant threshold of of status epipilocarpine 26-fold 206. Development lepticus with the lithium-pilocarpine treatment can be inhibited by atropine, diazepam, valproate, phenytoin, carbamazepine, phenobarbital, and paraldehyde2”‘. Pilocarpine (20 mg/kg) can produce florid motor status epilepticus in the absence of pretreatment with lithium, provided rats have been partially kindled by amygdalar stimulation32. Another drug combination able to induce status epilepticus in animals is focal cobalt in conjunction with systemic homocysteine”‘. Homocysteine was discussed above as an agent able on its own to produce powerful tonic-clonic seizures. Walton and Treiman suggest the following protoco1313. Naive rats undergo implantation of powdered cobalt 25 mg on the dura via a burr hole in the skull. Animals recover for about 5 days, by which time EEG polyspikes or behavioral motor seizures are evident. Homocysteine thiolactone 5.5 mmol/kg is then given i.p. Latency from injection to the first seizure is about 30 min, to the second 8 min more. Tonic-clonic seizures then recur every 5-10 mitt, with an EEG-clinical evolution over up to 24 h similar to that seen in clinical status epilepticus294. This model also has the advantage of a ‘focus’ (presumably the site of prior cobalt implantation) for frequently recurring sei-
movements.
In a paradigm
animals without
by Vicedo-
square-wave pulses at 20 Hz, 10-s duration, 30-s inter-train intervals) were delivered to fimbria and adjusted to the amperage needed to give maximal synaptic response in CA3. Status epilepticus occurred in 85% of subjects within less than 7 h. Status can also result from stimulation of a kindled hippocampal focus lg4. These electrical models are more labor-intensive than the chemical models, but avoid introduction of exogenous pharmacology into the preparation. 8. CONCLUSION
This review has enumerated over 50 animal models for the epilepsies. What common lessons may be learned from these models? First, there is no one model suitable to all questions. Some circumstances, for example screening of putative anticonvulsant drugs, require low cost and convenience. Epilepsy is a heterogeneous disorder and epilepsy research a heterogeneous enterprise; one size model does not fit all. Models show mechanisms of models; not necessarily mechanisms of epilepsy. Because the EEG and behavioral picture of a model looks similar to a clinical seizure type does not mean the pathophysiology is the same. To illustrate with an extreme: sleep has much in common with epilepsy - it is a spontaneously recurring event associated with blunting of consciousness and EEG hypersynchrony, including EEG potentials that can look very similar to epileptiform sharp waves. But sleep is not epilepsy. Some drugs can produce many models of epilepsy. Penicillin, administered in various forms can imitate simple partial epilepsy, generalized myoclonic, generalized tonic-clonic and generalized absence epi-
268 lepsy. It is unclear what this tells us about the continuum of clinical epilepsy. For the experimenter, caution is required. Careful behavioral and
spikes or seizures. In these models there is opportunity to consider prophylaxis of epilepsy3i9. These
EEG
possible idiosyncratic effects of the irritating metal, not specifically analogous to clinical epilepsy.
model
is needed
to
discern what type of seizure disorder a particular experiment.
is modelled
in
The
analysis
review
of an animal
has not
attempted
to discuss
the
models are laborious,
expensive,
Maximal electroshock sant models, exemplified
and also subject to
and the chemical convulby systemic pentylenetet-
models in order of popularity. It is, however, a useful exercise to list the most widely used models, and the key questions about epilepsy that have been
razol, are best for screening possible anticonvulsant drugs 163 Anatomy of seizure origin and spread have been studied in these simple models’9,201. Such
addressed by each. The half-dozen most productive models arguable have been the acute penicillin focal
studies have suggested originate in neocortex,
model,
seizures
the chronic aluminum
hydroxide
model, the
systemic pentylenetetrazol generalized tonic-clonic model, the maximal electroshock tonic-clonic seizure model, the kindling model of complex partial and secondarily generalized seizures, and the hippocampal in vitro slice. Work on the acute penicillin focus in cat cortex has illustrated the cellular electrophysiology of focal of the epilepsy’7x, and the basic phenomenology paroxysmal depolarization shift235. Because penicillin initiates spikes and seizures from a known locality, the spread of seizures from that region and mechanisms to inhibit spread can be studied2”*. The penicillin focus is not useful for questions on pathology of epilepsy, since it has onset within minutes of drug application to cortex. It is one of several experimental and clinical examples of the principle that chronic neuropathologic changes (e.g. gliosis and cortical reorganization) are not a necessary condition for seizures. The aluminum hydroxide and related metal deposition models of chronically recurrent simple partial and secondarily generalized seizures have served as some of the most realistic laboratory models of partial epilepsy. The models have been useful for studying development of pathologic changes occurring with epilepsy 3’7. Issues of seizure spread, and also controversies about establishment of mirror seizure foci, have been explored with the aluminum model”‘. Firing patterns of neurons in and near a metal deposition focus have been analyzed, contributing to our understanding of EEG synchrony and mechanisms of interictal spike generation151. Metal deposition models should be ideal for study of largely unexplored questions about the latent period between cortical injury and emergence of EEG
originate
that electroshock seizures whereas pentylenetetrazol
in diencephalon.
Findings
about
the functional anatomy of the epilepsies will require confirmation in more realistic models, for example, photogenic seizures in Papio papio baboons, and ultimately in human patients. The kindling model of epilepsy has probably been more subject to study than any other model. Kindling studies are especially useful for investigations of changes that occur in brain over time. The large number of investigations into kindling24s,308 have begun to answer questions about the neurochemical changes that occur before and after establishment of an epileptiform area of the brain. Kindling avoids introduction of exogenous chemicals into a system. It permits study both of seizures, and the propensity of brain regions to develop seizures. The kindling model of epilepsy is not yet well-understood. Its elucidation will advance our understanding of epilepsy and of all plastic changes in brain. Hippocampal slices, and other in vitro simple neuronal systems, have been productive models for exploration of basic mechanisms of epilepsy and EEGs4.138,262. This system has allowed detailed study of circuit properties leading to synchronous firing of neurons259,2y3. Regional pacemakers for synchronous cell firing can be isolated in hippocampal slices, and studied in relation to portions of the slice that are not spontaneously active2”9. The main benefit of the slice may, however, be the ease with which it permits study of actions of putative neurotransmitters or drugs on neurons”. Despite recent dicussion of seizure-like events in the slice*“‘, the hippocampal slice cannot really model the complex electrographic and behavioral manifestations of epileptic seizures. The strength of the slice and culture systems is in the opportunity to perform
269 basic work,
pharmacology for example,
and physiology. Important on anticonvulsant mechanisms
has been performed in tissue culture models’69. Popularity of particular animal models for the epilepsies will wax and wane with developments in research.
The recent
wave of investigations
on the
genetic bases of the epilepsies will, for example, undoubtedly increase studies of mouse and rat mutant strains with epilepsy265. This review cannot catalog the insights gained from models of epilepsy, without going too far afield. Most of what we know about the science of epilepsy has derived from animal models. The reader may refer to the proceedings of a recent conference on basic mechanisms of the epilepsies for a review of this fruitful area53. A pragmatic use for animal models of the epilepsies has been anticonvulsant drug development. A variety of models are useful for anticonvulsant drug screening but it has been difficult to predict efficacy and safety of drugs for particular seizure types with the models. Hundreds of drugs have been found effective against one or more animal models’63 in the past decade, but no novel agent has reached the market for 10 years. This observation underscores the complexity of the epilepsies, the need for better models, and for better understanding of our current models.
sants,
acute
electrical
stimulation,
cortically
im-
planted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone and baboon,
strains of mouse, rat, gerbil, fruitfly maximal electroshock seizures, sys-
temic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, y-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model’s strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.
9. SUMMARY ACKNOWLEDGEMENTS
The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulREFERENCES 1 Adamec, R.E., Stark-Adamec, C., Perrin, R. and Livingston, K.E., What is the relevance of kindling for human temporal lobe epilepsy? In J.A. Wada (Ed.), Kindling II, Raven, New York, 1981. 2 Adrian, E.D., The spread of activity in the cerebral cortex, J. Physiol. (Lund.), 88 (1936) 127-161. 3 Ajmone-Marsan, C., Focal electrical stimulation. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 147-172.
During the preparation of this review the author was supported by National Institutes of Health grants ROl-NS25128-02 and POl-NS07226-18 and the Klingenstein Fund. The secretarial services of Mrs. Mildred Weininger are gratefully acknowledged. 4 Ajmone-Marsan,
C. and Marossero, F., Electrocorticographic and electrochordographic study of the convulsions induced by cardiazol, Electroencephalogr. Clin. Neuro-
physiol., 2 (1950) 133-142. 5 Alberici, M., Rodriques de Lores Arnaiz, G. and DeRo-
bertis, E., Glutamic acid decarboxylase inhibition and ultrastructural changes by the convulsant drug allylglytine, Biochem. Pharmacol., 18 (1969) 137-143. 6 Alger, B.E. and Nicoll, R.A., Pharmacological evidence for two kinds of GABA receptor in rat hippocampal pyramidal cells studied in vitro, J. Physiol. (Lond.), 328 (1982) 125-141. 7 Allen, I.C., Grieve,
A. and Griffiths,
R., Differential
270 changes in the content of amino acid neurotransmitters in discrete regions of rat brain prior to the onset and during the course of homocysteine-induced seizures, 1. Neurothem., 46 (1986) 1582-1592. 8 Alonso-DeFlorida, F. and Delgado, J.M.R., Lasting behavioral and EEG changes in cats induced by prolonged stimulation of the amygdala, Am. J. Physiol., 193 (1958) 223-229. 9 Andersen, P., Sundberg, S.H., Sveen, 0. and Wigstrom, H., Specific long-lasting potentiation of synaptic transmission in hippocampal slices, Nufure (Land.), 266 (1977) 736-737. IO Annegers, J.F., Grabow, J.D., Groover, R.V., Laws, Jr., E.R., Elveback L.R. and Kurland, L.T., Seizures after head trauma: a population study, Neurology, 30 (1980) 683-689. 11 Aprison, M.H., Lipkowitz, K.B. and Simon, J.R., Identification of a glycine-like fragment on the strychnine molecule, J. Neurosci. Res., 17 (1987) 209-213. I2 Avoli, M., Mechanisms of generalized epilepsy with spike and wave discharges, The London Symposium, 39th Edition, Elsevier, 1987, pp. 184-190. 13 Avoli, M. and Gloor, P., Role of the thalamus in generalized penicillin epilepsy: observations on decorticated cats, Exp. Neurol., 77 (1982) 386-402. 14 Avoli, M., Gloor, P., Kostopoulos, G. and Gotman, J., An analysis of penicillin induced generalized spike and wave discharges using simultaneous recording of cortical and thalamic single units, .I. Neurophysiol., 50 (1983) 819-837. 15 Ayala, G.F. and Vasconsetto,
C., Penicillin as an epileptogenic agent: its effect on an isolated neuron. Science, 167 (1970) 1257-1260. 16 Babb, T.L., Metabolic, morphologic and electrophysiologic profiles of human temporal lobe foci: an attempt at correlation, Adv. Exp. Med. Biol., 203 (1986) 115-125. I7 Barolet, A.W., Li, A., Liske, S. and Morris, M.E., Antagonist actions of bicuculline methiodide and picrotoxin on extrasynaptic gamma-aminobutyric acid receptors, Can. J. Physiol. Pharmacol., 63 (1985) 1465-1470. 18 Bartoszyk, G.D. and Hamer, M., The genetic animal model for reflex epilepsy in the Mongolian gerbil: differential efficacy of new anticonvulsive drugs and prototype antiepileptics, Pharmacol. Res. Commun., 19 (1987) 429-440. I9 Ben-Ari, Y., Tremblay, E., Riche, D., Ghilini, G. and
Naquet, R.. Electrographic, clinical and pathological alterations following systemic administration of kainic acid, bicuculline or pentetrazole: metabolic mapping using the deoxyglucose method with special reference to the pathology of epilepsy, Neuroscience, 6 (1981) 13611391. 20 Bigler, E.D., Comparison of effects of bicuculline, strych-
nine, and picrotoxin and those of pentylenetetrazol on photically evoked after discharges, Epilepsia. 18 (1977) 465-470. 21 Bingmann,
D. and Speckmann, E.J., Actions of pentylenetetrazol (PTZ) on CA3 neurons in hippocampal slices of guinea pigs, Exp. Brain Rex, 64 (1986) 94-104. 22 Biossier, J.R., Simon, P., Villeneuve, A. and Larouse, C., Flurothyl in mice: seizure, post-convulsive behavior. and interactions with psychotropic drugs, Can. J. Physiol. Pharmacol.. 46 (1968) 93-100. 23 Bircher. R.P.. Kanai, T. and Wang. S.C.. Intravenous
cortical and intraventricular dose-effect relationship of pentylenetetrazol, picrotoxin and deslanoside in dogs, Electroencephalogr. C/in. Neurophysiol., 14 (1962) 256-267. 24 Bliss, T. and Lomo, T., Long-lasting potentiation of
synaptic transmission in the dentate area of the anesthetized rabbit following stimulation of the perforant path, J. Physiol. (Land.), 232 (1973) 311-356. 25 Bloom, F., Segal, D., Ling, N. and
Guillemin, R., Endorphins: profound behavioral effects in rats suggest new etiological factors in mental illness, Science, 194
(1976) 630-632. 26 Bloomer, H.A.,
Penicillin-induced
Barton, L.J. and Maddock, Jr., R.J., encephalography in uremic patients, J.
Am. Med. Assoc., 200 (1967) 121-123. 27 Blum, B. and Liban, E., Experimental
28
29
30
31
baso-temporal epilepsy in the cat. Discrete epileptogenic lesions produced in the hippocampus or amygdaloid nucleus by tungstic acid, Neurology, 10 (1960) 546-554. Braestrup, C. and Neilsen, M., Strychnine as a potent inhibitor of the brain GABA/benzodiazepine complex, Brain Res. Bull., 5 (Suppl. 2) (1980) 681-684. Brailowsky, S., Menini, C., Silva-Barrat, C. and Naquet, R., Epileptogenic gamma-aminobutyric acid-withdrawal syndrome after chronic, intracortical infusion in baboons, Neurosci. Lett., 74 (1987) 75-80. Burchfiel, J.L., Kindling in the rat hippocampus: studies of neurotransmitter mechanisms and transfer mechanisms. In J.A. Wada (Ed.), Kindling II, 2nd edn. Raven, New York, 1981, pp. 295-301. Burton, N.R., Smith, D.A.S. and Stone, T.W., The mouse neocortical slice: preparation and responses to excitatory amino acids, Comp. Biochem. Physiol., 88C
(1987) 47-55. 32 Buterbaugh, G.G.,
Michelson, H.B. and Keyser, D.O., Status epilepticus facilitated by pilocarpine in amygdalakindled rats, Exp. Neural., 94 (1986) 91-102. 33 Caldecott-Hazard, S. and Engel, Jr., J., Limbic postictal events: anatomical substrates and opioid receptor involvement, Prog. Nemo-Psychopharmacol. Biol. Psychiatry, 11 (1987) 389-418. 34 Campbell, A.M. and Holmes, O., Bicuculline epileptogenesis in the rat, Bruin Res., 323 (1984) 239-246. 35 Cavalheiro, E.A., Riche, D.A. and La Salle, G.L.G., Long-term effects of intrahippocampal kainic acid injection in rats: a method for inducing spontaneous recurrent seizures, Electroencephalogr. Clin. Neurophysiol., 53 (1982) 581-589. 36 Chavoix, C., Hantraye,
P.. Brouillet, E., Guibert, B., Fukuda, H., De la Sayette, V., Fournier, D., Naquet, R. and Maziere, M., Status epilepticus induced by pentylenetetrazole modulates in vivo (11C) Ro 16-1788 binding to benzodiazepine receptors. Effects of ligands acting at the supramolecular receptor complex, Eur. J. Pharma-
col., 146 (1988) 207-214. 37 Clark. C.R., Comparative
anticonvulsant activity and neurotoxicity of 4-amino-N-(2,6-dimethylphenyl)benzamide and prototype antiepileptic drugs in mice and rats,
Epilepsiu, 29 (1988) 198-203. 38 Coan, E.J.. Saywood, W. and Collingridge,
G.L., MK801 blocks NMDA receptor-mediated synaptic transmission and long term potentiation in rat hippocampal slices, Neurosci. Left., 80 (1987) 111-l 14. 39 Coenen, A.M.L. and Van Luijtelaar, E.L.J.M., The WAGiRij rat model for absence epilepsy: age and sex
271 factors, 40 Collins, Penry, (Eds.).
Epilepsy Res., 1 (1987) 297-301. R.L., Audiogenic seizures. In D.P. Purpura, J.K. D.M. Woodbury, D.B. Tower and R.D. Walter
Experimental Models of Epilepsy -A Manual for the Laboratory Worker, Raven, New York, 1972, pp.
347-372. 41 Coyle, J.T., Molliver, M.E. and Kuhar, M.J., In situ injection of kainic acid: a new method for selectively lesioning neuronal cell bodies while sparing axons of passage, J. Comp. Neurol., 180 (1978) 301-323. 42 Crain. S.M.. Tissue culture models of epileptiform activity. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York. 1972, pp. 291-316. 43 Crawford. R.D., A new mutant causing epileptic seizures in domestic fowls, Poultry Sci.. 48 (1969) 1799. 44 Creutzfeldt. 0. and Houchin. J., Neuronal basis of EEG waves. In A. Remond (Ed.), Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 2, Part C, Elsevier. Amsterdam, 174, pp. 5-5.5. 45 Curtis, D.R., Duggan, A. W. and Johnston, G.A.R., The specificity of strychnine as a glycine antagonist in the mammalian spinal cord, Exp. Brain Res., 12 (1971) 547-565.
46 Curtis, D.R., Game, C. and Lodge, D., Benzodiazepine and central glycine receptors, Br. J. Pharmacol., 56 (1976) 307-311. 47 Czuczwar, S.J.. Frey, H.H. and Loscher, W., Antagonism
of N-methyl-n.r-aspartic acid-induced convulsions by antiepileptic drugs and other agents, Eur. J. Pharmacol., 108 (1985) 273-280. 48 Dailey, J.W. and Jobe, P.C., Anticonvulsant drugs and the genetically epilepsy-prone rat, Fed. Proc., 44 (1985) 2640-2644. 49 Dailey. J.W. and Jobe, P.C., Indices of noradrenergic
function in the central nervous system of seizure-naive genetically epilepsy-prone rats, Epilepsia, 27 (1986) 665670. 50 Daniels, J.C. and Spehlman, R., The convulsant effect of topically applied atropine, Electroenceph. Chn. Neurophysiol., 34 (1973) 83-87. 51 Davenport. J., Schwindt, P.C. and Grill, W.E., Epilep-
togenic doses of penicillin do not reduce a monosynaptic GABA-mediated postsynaptic inhibition in the intact anesthetized cat, Exp. Neurol., 65 (1979) 552-572. 52 Dawson, Jr., R. and Bierkamper, G., Flurothyl seizure thresholds in mice treated neionatally with a single injection of monosodium glutamate (MSG): evaluation of experimental parameters in fluorothyl seizure testing, Pharmacol. Biochem. Behav., 28 (1987) 165-169. 53 Delgado-Escueta, A.V. (Ed.), Basic Mechanisms of the Epilepsies: Molecular and Cellular Approaches, Adv. Neurol. Vol 44, Raven, New York, 1986, pp. l-1096. 54 Dingledine, R., Brain Slices, Plenum, New York, 1984,
442 pp. 55 Dingledine, R. and Gjerstad, L., Reduced inhibition during epileptiform activity in the in vitro hippocampal slice, J. Physiol. (Land.), 305 (1980) 297-313. 56 Dow, R.S., Fernandez-Guardiola, A. and Manni, E., The production of experimental cobalt epilepsy in the rat, Electroenceph. Clin. Neurophysiol., 14 (1962) 399-407. 57 Dreifuss, F.E. and the Commission on Classification and
Terminology
of the International
League against Epi-
lepsy, Proposal for revised clinical and electroencephalographic classification of epileptic seizures, Epilepsia, 22 (1981) 489-501. 58 Driver, M.V. and McGillivray, B.B., Electroencephalography. In J. Laidlaw and A. Richens (Eds.), A Textbook of Epilepsy, Churchill Livingstone, New York, 1982, pp. 155-194. 59 Dworsky, S. and McCandless, D.W., Regional cerebral energy metabolism in bicuculline induced seizures, Neurochem. Res., 12 (1987) 237-240. 60 Dwyer, B.E., Fujikawa, D.G. and Waterlain, C.G., Metabolic anatomy of generalized bicuculline seizures in the newborn marmoset monkey, Exp. Neurol., 94 (1986) 213-217.
61 Eadie, M.J., Clinical use of antiepileptic drugs. In H.H. Frey and D. Janz (Eds.), Antiepileptic Drugs. Handbook of Experimental Pharmacology, Springer, Berlin, 1985, pp. 765-790. 62 Engstrom, EL. and Woodbury, D.M., Seizure susceptibility in DBA and C57 mice: the effects of various convulsants, Epilepsia, 29 (1988) 389-395. 63 Esplin, D.W. and Zablocka-Esphn, B., Mechanisms of action of convulsants. In H.H. Jasper, A.A. Ward Jr. and A. Pope (Eds.), Basic Mechanisms of the Epilepsies, Little, Brown, Boston, 1969, pp. 167-183. 64 Essig, C.F., Drug-withdrawal convulsions in animals. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 495-508. 65 Faingold, C.L., Gehlbach, G. and Caspary, D.M., Decreased effectiveness of GABA-mediated inhibition in the inferior colliculus of the genetically epilepsy-prone rat, Exp. Neural., 93 (1986) 145-159. 66 Faingold, CL., Millan, M.H., Boersma, C.A. and Meldrum, B.S., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway, Exp. Neurol., 99 (1988) 678-686. 67 Fariello, R.G., Parenteral penicillin in rats: an experimental model for multifocal epilepsy, Epilepsia, 16 (1976) 217-222.
68 Fariello, R.G. and Golden, G.T., The THIP-induced model of bilateral synchronous spike and wave in rodents, Neuropharmacology,
26 (1987) 161-165.
69 Ferguson, J.H.and Jasper, H.H., Laminar DC studies of acetylcholine activated epileptiform discharge in cerebral cortex, Electroencep. Clin. Neurophysiol., 30 (1971) 377-390. 70 Feria-Velasco,
A., Olivares, N., Rivas, F., Velasco, M. and Velasco, F., Alumina cream-induced focal motor epilepsy in cats. IV. Thickness and cellularity of layers in the peri-lesional motor cortex, Arch. Neurol., 37 (1980) 287-290.
71 Fialip, J., Aumaitre, O., Eschalier, A., Maradeix, B., Dordain, G. and Laverenne, J., Benzodiazepine withdrawal seizures: analysis of 48 case reports, Clin. Neuropharmacol., 10 (1987) 538-544. 72 Fink, M., Kahn, R.L., Karp, E., Pollack, M., Green, M.A.. Alan, B. and Lefkowits, H.J., Inhalant-induced convulsions, Arch. Gen. Psychiatry, 4 (1961) 259-266. 73 Fisch, B.J. and Pedley, T.A., Generalized tonic-clonic epilepsies. In H. Liiders and R.P. Lesser (Eds.), Epilepsy: Electroclinical Syndromes, Springer, New York, 1987, pp. 151-185.
272 74 Fisher, R.S.. Seizure Disorders. In P.D. Zieve, J.R. Burton and L.R. Barker (Eds.), Principles of Ambulatory Medicine, Williams and Wilkins, Baltimore, 1982, pp. 824-840. 75 Fisher, R.S., The hippocampal slice. Am. I. EEG Tech., 27 (1987) 1-14. 76 Fisher. R.S.. Epilepsy. In M.M. Swindle and R.J. Adams (Eds.), Experimental Surgery and Physiology: Induced Animal Models of Disease, Williams and Wilkins, Baltimore, 1988, pp. 272-274. 77 Fisher, R.S. and Alger, B.E., Electrophysiological mechanisms of kainic acid induced epileptiform activity in the rat hippocampal slice, J. Neurosci., 4 (1984) 1312-1323. 78 Fisher, R.S., Pedley, T.A., Moody. W.J. and Prince. D.A., The role of extracellular potassium in hippocampal epilepsy, Arch. Neurol., 33 (1976) 76-83. 79 Fisher, R.S. and Prince, D.A., Spike-wave rhvthms in cat cortex induced by parenteral penicillin. I. Electroencephalographic features, Electroenceph. Clin. Neurophysiol., 42 (1977) 608-624. 80 Fisher, R.S. and Prince, D.A., Spike-wave rhythms in cat cortex induced by parenteral penicillin. II. Cellular features. Electroenceph. Clin. Neurophysiol., 42 (1977) 625-639. 81 Fossieck. Jr., B. and Parker, R.H., Neurotoxicity during intravenous infusion of penicillin: a review, J. Clin. Pharmacol., 14 (1974) 504-512. 82 Freed, W.J., Selective inhibition of homocysteine-induced seizures by glutamic acid diethyl ester and other glutamate esters, Epilepsia, 26 (1985) 30-36. 83 Fritsch, G. and Hitzig. E.. Ueber die elektrische Erregbarkeit der Grosshirns, Arch. Anat. Physiol. Wissensch. Med., 37 (1870) 300. Cited in Swinyard (1972). 84 Fromm, G.H., Faingold. C.L., Browning, R.A. and Burnham, W.M., Epilepsy and the Reticular Formation: The Role of Reticular Core in Convulsive Seizures, Liss. New York, 1987, pp. l-224. 85 Frost, J., Mayberg, H.S., Fisher, R.S., Douglas, K.H., Dannals, R.F., Links, J.M., Wilson, A.A., Ravert, H.T., Rosenbaum, A.E., Snyder, S.H. and Wagner, H., Muopiate receptors measured by positron emission tomography are increased in temporal lobe epilepsy. Ann. Neurol., 23 (1988) 231-237. 86 Fuller, J.L. and Collins, R.L., Temporal parameters of sensitization of audiogenic seizures in SJLiJ mice, Dev. Psychobiol., 1 (1968) 185-188. 87 Gaull, G.E., Homocysteinuria and other disorders of sulfur metabolism. In E. Goldensohn and S. Appel (Eds.), Scientific Approaches to Clinical Neurology. Lea and Febiger, Philadelphia, PA, 1977, pp. 46-54. 88 Gean, P.W. and Shinnick-Gallagher, P., Picrotoxin induced epileptiform activity in amygdaloid neurons, Neurosci. Lett., 73 (1987) 149-154. 89 Gershoff, S.N. and Elvehjem, C.A., The relative effect of methionine sulfoximine on different species, J. Nutr., 45 (1951) 451-458. 90 Giaretta. D.. Avoli, M. and Gloor, P., Intracellular recordings in pericruciate neurons during spike and wave discharges of feline generalized penicillin epilepsy, Brain Res., 405 (1987) 68-79. 91 Girgis, M., Kindling as a model of limbic epilepsy, Neuroscience, 6 (1981) 1695-1706. 92 Gloor, P., Generalized epilepsy with spike-and-wave discharge: a reinterpretation of its electrographic and
clinical manifestations, Epilepsia. 20 (1979) 571-588. 93 Gloor, P., Olivier, A., Quesney, L.F., Andermann, F. and Horowitz, S.. The role of the limbic system in experiential phenomena of temporal lobe epilepsy, Ann. Neural., 12 (1982) 129-144. 94 Gloor. P., Quesney. L.F. and Zumstein, H., Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. II. Topical application of penicillin to the cerebral cortex and to subcortical structures, Electroencephalogr. Clin. Neurophysiol., 43 (1977) 79-94. 95 Goddard, G.V., Development of epileptic seizures through brain stimulation at low intensity, Nature (Land.), 214 (1967) 1020-1021. .. . 96 Goddard, G.V., The kindling model of epttepsy, trends Neurosci., 6 (1983) 275-279.97 Goddard, G.V. and Douglas, R.M., Does the engram of kindling model the engram of normal long term daily memory?. Can J. Neural. Sci., 2 (1975) 385-394. 98 Goddard, G.V., McIntyre, D.C. and Leech, C.K.. A permanent change in brain function resulting from daily electrical stimulation, Exp. Neural., 25 (1969) 295-330. 99 Golden, G.T. and Fariello, R.G., Penicillin spikes in rat: limitations of a single model for the study of anticonvulsants, Neuropharmacology, 23 (1984) 1001-1007. 100 Goldensohn, ES., The relevance of secondary epileptogenesis to the treatment of epilepsy: kindling and the mirror focus, Epilepsia, 25 (Suppl. 2) (1984) S156-S168. 101 Goldstein, D.S., Nadi, N.S., Stull, R., Wyler, A.R. and Porter, R.J., Levels of catechols in epileptogenic and nonepileptogenic regions of the human brain, J. Neurothem., 50 (1988) 225-229. 102 Goodin, D.S. and Aminoff. M.J., Does the interictal EEG have a role in the diagnosis of epilepsy?, Lancer, 1 (1984) 837-838. 103 Green. M.C. and Sidman, R.L., Tottering - a neuromuscular mutation in the mouse, J. Hered., 53 (1962) 233-237. .^ 104 Grossman, S.P., Chemically induced epileptiform setzures in the cat, Science, 142 (1963) 409-411. . 105 Guberman, A., Gloor, P. and Sherwin, A.L., Response of generalized penicillin epilepsy in the cat to ethosuximide and diphenylhydantoin, Neurology, 25 (1975) 785764. 106 Gutnick. M.J. and Prince, D.A., Penicillinase and the convulsant action of penicillin, Neurology, 21 (1971) 759-764. 107 Hahn, F.. Analeptics. Phurmacol. Rev., 12 (1960) 447530. 108 Hahn, F. and Oberdorf, A., Vergleichende Untersuchungen uber die Krampfwirkung von bemegrid, pentetrazol und pikrotoxin, Arch. Int. Pharmacodyn. Ther., 135 (1962) 9-30. 109 Hammond. E.J., Hurd, R.W., Wilder, B.J. and Thompson, F.J., Focal and generalized experimental seizures induced by homocysteine, Electroencephalogr. Clin. Neurophysiol., 49 (1980) 184-186. 110 Hanna, G.R. and Stalmaster, R.M., Cortical epileptic lesions produced by freezing, Neurology, 23 (1973) 918-925. 111 Harris, A.B. and Lockard, J.S., Absence of seizures or mirror foci in experimental epilepsy after excision of alumina and astrogliotic scar, Epilepsia, 22 (1981) 107122.
273 112 Harris, B., Cortical alterations due to methionine sulfoximine, Arch. Neural., 11 (1964) 388-407. 113 Hawkins, CA. and Mellanby, J.H., Limbic epilepsy induced by tetanus toxin: a longitudinal electroencephalographic study, Epilepsiu, 28 (1987) 431-444. 114 Hegreberg, G.A. and Padgett, G.A., Inherited progressive epilepsy of the dog with comparison to Lafora’s disease of man, Fed. Proc., 35 (1976) 1202-1205. 115 Heinemann, U., Changes in the neuronal micro-environment and epileptiform activity. In H.G. Wieser, E.-J. Speckmann and J. Engel Jr. (Eds.), Current Problems in Epilepsy. 3. The Epileptic Focus, Libbey, London, 1987, pp. 27-44. 116 Heinemann, U., Lux, H.D. and Gutnick, M.J., Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat, Exp. Bruin Res., 27 (1977) 237-243. 117 Henriksen, S.J., Bloom, F.E., McCoy, F., Ling, N. and Guillemin, R., Beta-endorphin induces nonconvulsive limbic seizures, Proc. Natl. Acad. Sci. H.S.A., 75 (1978) 5221-5225. 118 Heyer, E.K., Nowak, L.M. and MacDonald, R.L., Bicuculline: a convulsant with synaptic and non-synaptic actions, Neurology, 31 (1981) 1381-1390. 119 Hill, R.G., Simmonds, M.A. and Straughn, D.W., A comparative study of some convulsant substances as gamma-aminobutyric acid antagonists in the feline cerebral cortex, Br. J. Pharmacol., 49 (1973) 37-51. 120 Hjeresen, D.L., Franck, J.E. and Amend, D.L., Ontogeny of seizure incidence, latency, and severity in genetically epilepsy prone rats, Dev. Psychobiol., 20 (1987) 355-363.
121 Horton, R.W. and Meldrum, B.S., Seizures induced by allyglycine, 3-mercaptoprioprionic acid and 4-deoxypyridine in mice and photosensitive baboons, and different modes of inhibition of cerebral glutamic acid decarboxylase, Br. J. Pharmacol., 49 (1973) 52-63. 122 Hosli, E., Krogsgaard-Larsen, P. and Hosli, L., Autoradiographic localization of binding sites for the gammaaminobutyric acid analogues 4,5,6,7-tetrahydroisoxazolo5,4-c-pyridin-3-01 (THIP) isoguvacine and baclofen on cultured neurons of rat cerebellum and spinal cord, Neurosci.
Lett., 61 (1985) 153-157.
123 Hunter, J. and Jasper, H.H., Effects of thalamic stimulation in unanesthetized animals, Electroencephalogr. Clin. Neurophysiol., 1 (1949) 305-324. 124 Huot, J., Radouco-Thomas, S. and Raduoco-Thomas, C., Qualitative and quantitative evaluation of experimentally induced seizures. In J. Mercier (Ed.), Anticonvulsant Drugs, Vol. 1, Pergamon, Oxford, 1973, pp. 123-185. 125 Imaizumi, K., Ito, S., Kutsukake, G., Takazawa, T., Fujiwara, K. and Tutikawa, K., Epilepsy-like anomoly of mice, Exp. Anim. (Tokyo), 8 (1959) 6-10. 126 Jackson, J.H., On convulsive seizures (Lumleian Lectures) in J. Taylor (Ed.), Selected Writings of John Hughlings Jackson, Vol. 1 (1931). Hodder and Stoughton, London. 127 Jasper, H.H. and Droogleever-Fortuyn, J., Experimental studies on the functional anatomy of petit ma1 epilepsy, Res. Publ. 272-278. 128 Jensen,
Assoc.
Res.
Nerv.
Ment.
Dis.,
26 (1946)
M.S. and Yaari, Y., The relationship between interictal and ictal paroxysms in an in vitro model of focal hippocampal epilepsy, Ann. Neurol., 24 (1988) 591-598.
129 Jobe, P.C., Dailey, J. W. and Reigel, C.E., Noradrenergic and serotonergic determinants of seizure susceptibility and severity in genetically epilepsy-prone rats, Life Sci., 39 (1986) 775-782.
130 Jobe, P.C., Ko, K.H. and Daily, J.W., Abnormalities in norepinephrine turnover rate in the central nervous system of the genetically epilepsy-prone rat, Brain Res., 290 (1984) 357-360. 131 Johnston, G.A.R., Convulsions induced in lo-day-old rats by intraperitoneal injection of monosodium glutamate and related excitant amino acids, Biochem. Pharmacol., 22 (1973) 137-140. 132 Jope, R.S., Morrisett, R.A. and Snead III, O.C., Characterization of lithium potentiation of pilocarpine-induced status epilepticus in rats, Exp. Neural., 91 (1986) 471-480. 133 Kamb, A., Iverson, L.E. and Tanouye, M.A., Molecular characterization of Shaker, a Drosophila gene that encodes a potassium channel, Cell, 50 (1987) 405-413. 134 Kaplan, B.J., Seyfried, T.N. and Glaser, G.H., Spontaneous poly-spike discharges in an epileptic mutant mouse (tottering), Exp. Neural., 66 (1979) 577-586. 135 Karpiak, S.E., Graf, L. and Rapport, M.M., Antiserum to brain gangliosides produces recurrent epileptiform activity, Science, 194 (1976) 735-737. 136 Karpiak, S.E., Mahadik, S.P., Graf, L. and Rapport, M.M., An immunological model of epilepsy: seizures induced by antibodies to G.M.l ganglioside, Epilepsia, 22 (1981) 189-196.
137 Kay, A.R. and Wong, R.K., Isolation of neurons suitable for patch-clamping from adult mammalian central nervous system, J. Neurosci. Meth., 16 (1986) 227-238. 138 Kerkut, G.A. and Wheal, H.V., Electrophysiology of Isolated Mammalian CNS Preparations, Academic, New York, 1981. 139 Killam, K.F., Killam, E.K. and Naquet, R., An animal model of light-sensitive epilepsy, Electroenceph. Clin. Neurophysiol.,
22 (1967) 497-513.
140 Killam, K.F., Naquet, R. and Bert, J., Paroxysmal responses to intermittent light stimulation in a population of baboons (Papio papio), Epilepsia, 7 (1966) 215-219. 141 King, D.W. and Ajmone-Marsan, C., Clinical features and ictal patterns in epileptic patients with EEG temporal lobe foci, Ann. Neural., 2 (1977) 138-147. 142 Knopman, D.S., Acute strychnine-induced seizures in cats: a Golgi study, Epilepsia, 791-792 (1975) 523-526. 143 Kopeloff, L.M., Chusid, J.G. and Kopeloff, N., Epilepsy in Macacca muluttu after cortical or intracerebral alumina, Arch. Neural. Psychiatry, 74 (1955) 523-526. 144 Kostopoulos, G., Neuronal sensitivity to GABA
and glutamate in generalized epilepsy with spike and wave discharges, Exp. Neural., 92 (1986) 20-36. 145 Kostopoulos, G., Avoli, M. and Gloor, P., Participation of cortical recurrent inhibition in the genesis of spike and wave discharges in feline generalized penicillin epilepsy, Bruin Res., 267 (1983) 101-112. 146 Krauss, G.L., Kaplan, P. and Fisher,
R.S., Parenteral magnesium sulfate fails to control electroshock and pentylenetetrazol seizures in mice, Epilepsy Res., in press. 147 La Salle, G.L.G., Amygdaloid kindling in the rat: regional differences and general properties. In J.A. Wada (Ed.), Kindling H, Raven, New York, 1981, pp. 31-47. 148 Laird, H.E., Dailey, J.W. and Jobe, P.C., Neurotransmitter abnormalities in genetically epileptic rodents, Fed. Proc., 43 (1984) 2505-2509.
274 149 Land, H.E, Hadjiconstantinou, M. and Neff, N.H., Abnormalities in the central cholinergic transmitter system of the genetically epilepsy-prone rat, Life Sci., 39 (1986) 783-787. 150 Lancaster, B. and Wheal, H.V., Chronic failure of inhibition of the CA1 area of the hippocampus following kainic acid lesions of the CA314 area, Brain Res., 295 (1984) 317-324. 151 Lange, SC., Neafsey, E.J. and Wyler, A.R., Neuronal activity in chronic ferric chloride epileptic foci in cats and monkey, Epilepsia, 21 (1980) 251-254. 1.52 Langmoen, LA. and Andersen, P., The hippocampal slice in vitro. A description of the technique and some examples of the opportunities it offers. In G.A. Kerkut and H.V. Wheal (Eds.), Electrophysiology of Isolated Mammalian CNS Preparations, Academic, New York, 1981, pp. 52-105. 153 LaSalle, G.L.G., Amygdaloid kindling in the rat: regional differences and general properties. In J.A. Wada (Ed.), Kindling II, Raven, New York, 1981, pp. 31-47. 154 Laursen, A.M., The contribution of in vitro studies to the understanding of epilepsy, Acta Neural. &and., 69 (1984) 367-375. 155 Leander, J.D., Lawson, R.R., Ornstein, P.L. and Zimmerman, D.M., N-methyl-o-aspartic acid-induced lethality in mice: selective antagonism by phencyclidine-like drugs, Brain Res., 448 (1988) 115-120. 156 Levitt, P. and Noebels, J., Mutant mouse tottering: selective increase of locus ceruleus axons in a defined single-locus mutation, Proc. Natl. Acad. Sci. U.S.A., 78 (1981) 4630-4643.
157 Lieb, J.P., Engel Jr., J., Gevins, A. and Crandall, P.H., Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy, Epilepsia, 22 (1981) 515-538. 158 Lindau, M. and Neher, E., Patch-clamp techniques for time-resolved capacitance measurements in single cells, Eur. J. Physiol., 411 (1988) 137-146.
159 Litchfield, J .T. and Wilcoxon, F., A simplified method of evaluating dose-effect experiments, J. Pharmacol. Exp. Ther., 96 (1949) 99-113. 160 Little, H.J., Nutt, D.J. and Taylor, S.C., Optimizing the
161
162
163
164
165
pentelenetetrazol infusion method for measuring seizure thresholds, Br. J. Pharmacol., (Suppl.), 88 (1986) 326P. Lockard, J.S., Uhlir, V., DuCharme, L.L., Farquhar, J.A. and Huntsman, B.J., Efficacy of standard anticonvulsants in monkey model with spontaneous motor seizures, Epilepsia, 16 (1975) 301-317. Loiseau, H., Averet, N., Arrigoni and Cohadon, F., The early phase of cryogenic lesions: an experimental model of seizures updated, Epilepsia, 28 (1987) 251-258. Loscher, W.L. and Schmidt, D., Which animal models should be used in the search for new antiepileptic drug? A proposal based on experimental and clinical considerations, Epilepsy Res., 2 (1988) 145-181. Lothman, E. W., Collins, R.C. and Ferrendelli, J.A., Kainic acid-induced limbic seizures: electrophysiologic studies, Neurology, 31 (1981) 806-812. Lothman, E.W., Hatlelid, J.M., Zorumski, C.F., Conry, J.A., Moon, P.F. and Perlin, J.B., Kindling with rapidly recurring hippocampal seizures, Brain Res., 360 (1985) 83-91.
166 Lothman, E. W., Perlin, J.B. and Salerno, R.A., Response properties of rapidly recurring hippocampal seizures in rats, Epilepsy Res., 2 (1988) 356-366.
167 Luse, S.A., Goldring, S. and O’Leary, J.L., Seizure activity due to intravenous strychnine, Arch. Neural., 11 (1964) 296-302. 168 MacDonald, R.L. and Barker, J.L., Pentelenetetrazol and penicillin are selective antagonists of GABA-mediated post-synaptic inhibition in cultured mammalian neurons, Nature (Lond.), 267 (1977) 720-721. 169 MacDonald, R.L. and McLean. M.J., Mechanisms of anticonvulsant drug action, Electroencephalogr. Clin. Neurophysiol., (Suppl.), 39 (1987) 200-208. 170 Magistris, M.R., Mouradian, MS. and Gloor, P., Generalized convulsions induced by pentylenetetrazol in the cat: participation of forebrain, brainstem and spinal cord, Epilepsia, 29 (1988) 379-388.
171 Majkowski, J. and Kaplan, H., Value of Mongolian gerbils in antiepileptic drug evaluation, Epilepsia, 24 (1983) 609-615.
172 Marcus, E.M., Experimental models of petit ma1 epilepsy. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 113-146.
173 Marcus, E.M. and Watson, C.W., Bilateral symmetrical epileptonic foci in monkey cerebral cortex: mechanisms of interactions and regional variations in capacity for synchronous spike slow wave discharges, Arch. Neurol., 19 (1968) 99-116.
174 Marcus, E.M., Watson, C. W. and Goldman, P.L., Effects of steroids on cerebral electrical activity: epileptogenic effects of conjugated estrogens and related compounds in the cat and rabbit, Arch. Neurol., 15 (1966) 521-532. 175 Marescaux, C., Micheletti, G., Vergnes, M., Depaulis, A., Rumbach, L. and Warter, J.M., A model of chronic spontaneous petit-ma] like seizures in the rat: comparison with pentylenetetrazol-induced seizures, Epilepsia, 25 (1984) 326-331.
176 Marvizon, J.C., Vazquez, J., Garcia Calvo, M., *Mayor Jr., F., Ruiz Gomez, A., Valdivieso, F. and Benavides, J., The glycine receptor: pharmacological studies and mathematical modeling of the allosteric interaction between the glycine- and strychnine-binding sites, Mol. Pharmacol., 30 (1986) 590-597.
177 Mason, CR. and Cooper, R.M., A permanent change in convulsive threshold in normal and brain-damaged rats with repeated small doses of pentelenetetrazole, Epilepsia, 13 (1972) 663-674.
178 Matsumoto, H. and Ajmone-Marsan, C., Cortical cellular phenomona in experimental epilepsy: interictal manifestations, Exp. Neurol., 9 (1964) 286-304. 179 Maurin, Y., Berger, B., LeSaux, F., Gay, M. and Baumann, N., Increased number of locus ceruleus noradrenergic neurons in the convulsive mutant quaking mouse, Neurosci. Lett., 57 (1985) 313-318. 180 Maxson, S.C., Fine, M.D., Ginsburg, B.E. and Koniecki, D.L., A mutant for spontaneous seizures in C57BLIlOBg mice, Epilepsia, 24 (1983) 15-24. 181 McCormick, D.A., Connors, B.W., Lighthall, J.W. and Prince, D.A., Comparative electrophysiology of pyramidal and sparsely spiny stellate neurons of the neocortex, J. Neurophysiol., 54 (1985) 782-806. 182 McGeer, E.G., Olney, J.W. and McGeer, P.L., The allosteric interaction between the glycine- and strychninebinding sites, Kainic Acid as a Tool in Neurobiology, Raven, New York, 1978.
275
183 McIlwain, H., Metabolic response in vitro to electrical stimulation of sections of mammalian brain, Biochem. J., 49 (1951) 382-393. 184 McIntyre, D.C., Stokes, K.A. and Edson, N., Status epilepticus following stimulation of a kindled hippocampal focus in intact and commissurotomized rats, Exp. Neurol., 94 (1986) 554-570. 185 McNamara, J., Kindling: an animal model of complex partial epilepsy, Ann. Neural., 16 (Suppl.) (1984) 72-76. 186 McQueen, J.K. and Woodbury, D.M., Attempts to produce spike and wave complexes in the electrocorticogram of the rat, Epilepsia, 16 (1975) 295-299. 187 Meldrum, B.S., Preclinical test systems for evaluation of novel compounds. In B.S. Meldrum and R.J. Porter (Eds.), Current Problems in Epilepsy. 4. New Anticonvulsant Drugs, Libbey, London, 1986, pp. 31-48. 188 Meldrum, B.S. and Chapman, A.G., Benzodiazepine receptors and their relationship to epilepsy, Epilepsia, 27 (Suppl. 1) (1986) S3-S13. 189 Meldrum, B.S. and Horton, R.W., Convulsive effects of 4-deoxypyridoxine and of bicuculline in photosensitive baboons (Papio papio) and in rhesus monkeys (Macaca mulatta), Brain Res., 35 (1971) 419-436. 190 Meldrum, B.S., Horton, R. W. and Brierly, J.B., Epilep-
tic brain damage in adolescent baboons following seizures induced by allylglycine, Brain, 97 (1974) 407-418. 191 Meldrum, B.S., Swan, J.H., Ottersen, O.P. and StormMathisen, J., Redistribution of transmitter amino acids in rat hippocampus and cerebellum during seizures induced by r-allylglycine and bicuculline: and immunocytochemical study with antisera against conjugated GABA, glutamate and aspartate, Neuroscience, 22 (1987) 17-27. 192 Mellanby, J., Hawkins, C., Mellanby, H., Rawhns, J.N.P. and Impey, M.E., Tetanus toxin as a tool for studying epilepsy, J. Physiol. (Paris), 79 (1984) 207-215. 193 Mellanby, J., George, G., Robinson, A. and Thompson, P.A., Epileptiform syndrome in rats produced by injecting tetanus toxin into the hippocampus, J. Neural. Neurosurg. Psychiatry, 40 (1977) 404-414. 194 Merritt, H.H. and Putnam,
anticonvulsant
T.J., A new series of drugs tested by experiments on animals,
Arch.
Psychiatry, 39 (1938) 1003-1015.
Neural.
195 Metrakos,
J.D. and Metrakos, K., Genetic studies in clinical epilepsy. In H.H. Jasper, A.A. Ward and A. Pope (Eds.), Basic Mechanisms of the Epilepsies, 1969, pp. 700-708. 196 Milgram, N.W., Green, I., Liberman, M., Riexinger, K. and Petit, T.L., Establishment of status epilepticus by limbic system stimulation in previously unstimulated rats, Exp. Neural., 88 (1985) 553-564. 197 Millan, M.H., Meldrum, B.S.,
Boersma, C.A. and Faingold, C.L., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. II. Efferent seizure propagating pathways, Exp. Neural., 99 (1988)
687-698. 198 Millan, M.H., Meldrum, B.S. and Faingold, C.L., Induc-
tion of audiogenic seizure susceptibility by focal infusion of excitant amino acid or bicuculline into the inferior colliculus of normal rats, Exp. Neural., 91 (1986) 634639. 199 Miller, J.W., McKeon, A.C. and Ferrendelli, J.A., Functional anatomy of pentylenetetrazol and electroshock seizures in the rat brainstem, Ann. Neural., 22 (1987) 615-621.
200 Mirski, M.A. and Ferrendelli, J.A., Anterior mediation of generalized pentylenetetrazol
thalamic seizures,
Brain Res., 399 (1986) 212-223. 201 Mirski, M.A. and Ferrendelli, J.A.,
Interruption of the connections of the mammillary bodies protects against generalized pentylenetetrazol seizures in guinea pigs, J.
Neurosci., 7 (1987) 662-670. 202 Molaie, M. and Kadzielawa,
203 204
205
206
207
208 209
K., Effect of naloxone infusion on rate of epileptiform discharges in patients with complex partial seizures, Epilepsia, 29 (1988) 698. Morison, R.S. and Dempsey, E.W., A study of thalamocortical relations, Am. J. Physiol., 135 (1942) 281-292. Morrell, F., Secondary epileptogenic lesions, Epilepsia, 1 (1960) 538-560. Morrisett, R.A., Jope, R.S. and Snead III O.C., Effects of drugs on the initiation and maintenance of status epilepticus induced by administration of pilocarpine to lithium-pretreated rats, Exp. Neural., 97 (1987) 193-200. Morrisett, R.A., Jope, R.S. and Snead III, O.C., Status epilepticus is produced by administration of cholinergic agonists to lithium-treated rats: comparison with kainic acid, Exp. Neural., 98 (1987) 594-605. Moses III, H.M. and Fisher, R.S., Syncope, seizures and other episodic disorders. In A.M. Harvey, R.T. Johns, V.A. McKusick, A.H. Owens Jr. and R.S. Ross (Eds.), The Principles and Practice of Medicine, 22nd edn., Appleton & Lange, Norwalk, CT, 1988, pp. 1025-1033. Nadler, V., Kainic acid as a tool for the study of temporal lobe epilepsy, Life Sci., 29 (1981) 2031-2042. Naquet, R. and Meldrum, B.S., Photogenic seizures in baboon. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental
Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 373-406. 210 Newmark, M.E. and Penry, J.K., Photosensitivity and Epilepsy: A Review, Raven, New York, 1979.
211 Niedermeyer,
E. and Charles, C. Thomas (Eds.), ComSpringfield, 1974, pp. l-334. 212 Niedermeyer, E., Epileptic seizure disorder. In E. Niedermeyer and F. Lopes da Silva (Eds.), Electroencephapendium
of the Epilepsies,
lography: Basic Releated Fields,
Principles,
Clinical
Applications
and
2nd edn., Urban & Schwarzenberg, Baltimore, 1987, pp. 405-510. 213 Noebels, J.L., Analysis of inherited epilepsy using single locus mutations in mice, Fed. Proc., 38 (1979) 2405-2410. 214 Noebels, J.L., A single gene error of noradrenergic axon growth synchronizes central neurones, Nature (Land.), 310 (1984) 409-411. 215 Noebels, J.L. and Pedley, T.A., Anatomic localization of
topically applied [“‘C]peniciIlin during experimental focal epilepsy in cat neocortex, Brain Res., 125 (1977) 293-303. 216 Noebels, J.L. and Sidman, R.J., Inherited epilepsy: spike-wave and focal motor seizures in the mutant mouse tottering, Science, 204 (1979) 1334-1336. 217 Oliver, A.P., Hoffer, B.J. and Wyatt, R.J., The hippocampal slice: a model system for studying the pharmacology of seizures and for screening anticonvulsant drugs, Epilepsia, 18 (1977) 543-548. 218 Olney, J.W., Rhee, V. and Ho, O.L.,
Kainic acid: a powerful neurotoxic analog of glutamate, Brain Res., 77
(1974) 501-512. 219 Olsen R.W., The GABA postsynaptic
tor-ionophore anticonvulsant
membrane recepcomplex. Site of action of convulsant and drugs, Mol. Cell. Biochem., 39 (1981)
276 261-279. 220 Papazian, D.M., Schwarz, T.L., Tempel, B.L., Jan, Y.N. and Yan, L.Y., Cloning of genomic and complementary DNA from Shaker, a putative potassium channel gene from Drosophila, Science, 237 (1987) 749-753. 221 Papy, J.J. and Naquet, R., Cerebral blood flow and seizures induced by MetrazolR in the baboon, Clin. Dev. Med., 39 (1971) 283-296. 222 Pedley, T.A., The pathophysiology of focal epilepsy, Ann. Neural., 3 (1978) 2-9. 223 Pei, Y., Zhao, D., Huang, J. and Cao, L., Zinc-induced seizures: a new experimental model of epilepsy, Epilepsia, 24 (1983) 169-176. 224 Pellegrini, A., Gloor,
225
226
227
228
229
230
231 232
P. and Sherwin, A.L., Effect of valproate sodium on generalized penicillin epilepsy in the cat, Epilepsia, 19 (1978) 351-360. Penfield, W. and Jasper, H.H., Epilepsy and the Functional Anatomy of the Human Brain, Little Brown, Boston, 1954, p. 713. Penny, J.E., Brown, R.D., Wallace, M.S. and Henley, C.M.. Auditory aspects of seizure in the genetically epilepsy prone rat, Life Sci., 39 (1986) 887-895. Pinel, J.P.J. and Rovner, L.I., Experimental epileptogenesis: kindling - induced epilepsy in rats, Exp. Neurol., 58 (1978) 190-202. Piredda, S. and Gale, K., Role of excitatory amino acid transmission in the genesis of seizures elicited from the deep prepiriform cortex, Brain Res., 377 (1986) 205-210. Piredda, S., Lim, S.R. and Gale, K., Intracerebral site of convulsant action of bicuculline, Life Sci., 36 (1985) 1295-1298. Pitkanen, A., Saano, V., Hyvonen, K., Airaksinen, M.M. and Riekkinen, P.J., Decreased GABA benzodiazepine, and picrotoxinin receptor binding in brains of rats after cobalt-induced epilepsy, Epilepsia, 28 (1987) 1l- 16. Pollen, D.A., Experimental spike and wave responses in petit mal epilepsy, Epilepsia, 9 (1968) 221-232. Pollen, D.A., Reid, K. and Perot, P., Micro-electrode studies of experimental 3isec wave and spike in the cat,
Electroencephalogr.
Clin. Neurophysiol.,
17 (1964) 57-67.
233 Porter,
R.J., Cereghino, J.J., Gladding, G.D., Hessie. B.J., Kupferberg, H.J., Scoville, B. and White, B.G.. Antiepileptic drug development program, Cleve. Clin. Q.,
51 (1984) 293-305. 234 Price, D.L., Griffin, J., Young, A., Peck, K. and Stocks.
235 236
237
238
239 240
241
A., Tetanus toxin: direct evidence for retrograde intraaxonal transport, Science, 188 (1975) 945-947. Prince, D.A., The depolarization shift in ‘epileptic’ neurons, Exp. Neurol., 21 (1968) 467-485. Prince, D.A., Neurophysiology of epilepsy, Annu. Rev. Neurosci., 1 (1978) 395-415. Prince, D.A. and Farrell, D., ‘Centrencephalic’ spikewave discharges following parenteral penicillin injection in the cat, Neurology, 19 (1969) 309-310. Prince, D.A. and Wilder, B.J., Control mechanisms in cortical epileptogenic foci, Arch. Neurol., 16 (1967) 194-202. Prince, C.A. and Wong, R.K.S., Human epileptic neurons studied in vitro, Brain Res., 210 (1981) 323-333. Probst, A., Cortes, R. and Palacios, J.M., The distribution of glycine receptors in the human brain. A light microscopic autoradiographic study using [3H]strychnine. Neuroscience, 17 (1986) 11-35. Proler, M. and Kellaway. P.. The methionine sulfoximine
syndrome in the cat, Epilepsia, 3 (1965) 117-130. D.P., Penry, J.K., Woodbury, D.M., Tower, D.B. and Walter, R.D., Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, p. 615. 243 Quesney, L.F., Gloor, P., Kratzenberg, E. and Zumstein, H., Pathophysiology of generalized penicillin epilepsy in the cat: role of cortical and subcortical structures. I. Systematic application of penicillin, Electroenceph. Clin.
242 Purpura,
Neurophysiol., 42 (1977) 640-655. 244 Racine, R., Modification of seizure activity by electrical stimulation. I. After-discharge threshold, Electroenceph. Clin. Neurophysiol., 32 (1972) 269-279. 245 Racine, R., Kindling: the first decade, J. Neurosurg., 3 (1978) 234-252. 246 Reid, S.A., Sypert, G.W. and Boggs, W.M., Histopathol-
ogy of the ferric-induced chronic epileptic focus in cats and monkey, Exp. Neural., 66 (1979) 205-210. 247 Reigel, C.E., Dailey, J.W. and Jobe, P.C., The genetically epilepsy-prone rat: an overview of seizure-prone characteristics and responsiveness to anticonvulsant drugs, Life Sci., 39 (1986) 763-774. 248 Reigel, C.E., Jobe, P.C., Dailey, J.W. and Stewart, J.J., Responsiveness of genetically epilepsy-prone rats to intracerebroventricular morphine-induced convulsions, Life Sci., 42 (1988) 1743-1749. 249 Remler, M.P. and Marcussen, W.H., Systemic focal epileptogenesis, Epilepsia, 27 (1986) 35-42. 250 Remler, M.P., Sigvardt, K. and Marcussen, W.H., Phar-
macological response of systemically derived focal epileptic lesions, Epilepsia, 27 (1986) 671-677. 251 Roberts, R.C. and Ribak, C.E., Anatomical changes of the GABAergic system in the inferior colliculus of the genetically epilepsy-prone rat, Life Sci., 39 (1986) 789798. 252 Rodin, E.A.,
Rutledge, L.T. and Calhoun, H.D., Megimide and metrazol: a comparison of their convulsant properties in man and cat, Electroencephalogr. Clin.
Neurophysiol., 10 (1958) 719-723. 253 Sasa, M.. Ohno. Y., Ujihara, H., Fujita, Y., Yoshimura,
M., Takaori, S., Serikawa, T. and Yamada, J., Effects of antiepileptic drugs on absence-like and tonic seizures in the spontaneously epileptic rat, a double mutant rat, Epilepsia, 29 (1988) 505-513. 254 Sato, T., Mori, N., Kurokochi,
A., Tojo, Y. and Kumashiro, H.. A new model of epileptic seizure utilizing excitatory amino acids and dimethyl sulfoxide, Epilepsia,
27 (1986) 620. 255 Schecter. P.J.,
Tranier, Y. and Grove, J., Effect of n-dipropylacetate on amino acid concentrations in the mouse brain: correlations with anticonvulsant activity, J.
Neurochem., 31 (1978) 1325-1327. 256 Schwartzkroin, P.A., Characteristics
recorded intracellularly Res., 85 (1975) 257 Schwartzkroin, in vitro, Brain 258 Schwartzkroin,
of CA1 neurons in the hippocampal slice, Brain
423-435.
P.A., Further characteristics of CA1 cells Res., 128 (1977) 53-68.
P.A. and Haglund, M.M., Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe. Epilepsia, 27 (1986)
523-533. 259 Schwartzkroin,
P.A. and Prince, D.A., Cellular and field potential properties of epileptogenic hippocampal slices, Brain Res., 147 (1978) 117-130.
277
260 Schwartzkroin, P.A., Turner, D.A., Knowles, W.D. and Wyler, A.R.. Studies of human and monkey ‘epileptic’ neocortex in the in vitro slice preparation, Ann. Neurol., 13 (1983) 249-257. 261 Schwartzkroin, P.A. and Wester, K., Long-lasting facilitation of a synaptic potential following tetanization in the in vitro hippocampal slice, Brain Res., 89 (1975) lO7- 119. 262 Schwartzkroin. P.A. and Wheal. H.V.. Elecrrophysiology of Epilepsy, Academic, New York, 1984, 420 pp. 263 Serikawa, T. and Yamada, J.. Epileptic seizures in rats homozygous for two mutations, zitter and tremor, J. Hered..
77 (1986) 441-444.
264 Sessler, EM..
Cheng, J.T. and Waterhouse, B.D., Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters, Bruin Res., 446 (1988)
77-89. 265 Seyfried,
mutants
279 Suzuki, J. and Nakamoto, Y., Seizure patterns and electroencephalograms of El mouse, Electroencephalogr. Clin. Neurophysiol.,
201-204. 282 Swinyard, E.A., Electrically induced convulsions. In D.P.
Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy A Munuul for the Laboratory Worker, Raven, New York, 1972, pp. 435-458. 283 Swinyard, E.A., Brown, W.C. and Goodman, L.S., Comparative assay of antiepileptic drugs in mice and rats, J. Phurmacol. Exp. Ther., 106 (1952) 319-330. 284 Swinyard, E.A., Castellion, A. W., Fink, G.B. and Good-
T.N. and Glaser, G.H.. A review of mouse as genetic models of epilepsy, Epilepsiu, 26
(1985) 143-150. 266 Sherwin, A.L.,
surgical specimens
Guide to neurochemical analysis of of human brain, Epilepsy Res., 2
285
(1988) 281-288. 267 Sloviter. R.S.. Decreased
286
268
287
269
270
271
hippocampal inhibition and a selective loss of interneurons in experimental epilepsy, Science, 235 (1987) 73-76. Snead III, O.C., Gamma-hydroxybutyrate in the monkey. I. Electroencephalographic. behavioral and pharmacokinetic studies, Neurology, 28 (1978) 636-642. Snead III, O.C., Gamma-hydroxybutyrate in the monkey. II. Effect of chronic oral anticonvulsant drugs, Neurology, 28 (1978) 643-648. Snead III, O.C.. Ontogeny of gamma-hydroxybutyric acid. II. Electroencephalographic effects, Dev. Bruin Res., IS (1984) 89-96. Snead III, O.C.. Gamma-hydroxybutyrate model of generalized absence seizures: further characterization and comparison with other absence models, Epilepsia, 29
(1988) 361-368. 272 Snead III, O.C.
and Bearden, L.J., Anticonvulsants specific for petit mal antagonize epileptogenic effect of leucine enkephalin, Science, 210 (1980) 1031-1033. 273 Snead III, O.C. and Stephens, H., The ontogeny of seizures induced by leucine-enkephalin and P-endorphin, Ann. Nemo/., 15 (1984) 594-598. 274 Spiegel, E.A., Quantitative determination of the convulsive reactivity by electrical stimulation of the brain with the skull intact, J. Lab. Clin. Med., 22 (1937) 1274-1276. 275 Sprince, H., Parker, C.M., Josephs, J.A. and Magazino, J., Convulsant activity of homocysteine and other shortchain mercapto acids: protection therefrom, Ann. N.Y.
288
289
290
291
292
Neural.,
294
295
12 (1953) 262-276.
277 Stone. W.E., Convulsant actions of tetrazole derivatives, Pharmacology, 3 (1970) 367-370. 278 Stone, W.E., Systemic chemical convulsants and meta-
296
bolic derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 407-432.
man, L.S., Some neurophysiological and neuropharmacological characteristics of audiogenic seizure susceptible mice, J. Pharmucol. Exp. Ther., 140 (1963) 375-384. Taylor-Courval, D. and Gloor, P., Behavioral alterations associated with generalized spike and wave discharges in the EEG of the cat, Exp. Neural., 83 (1984) 167-186. Testa, G. and Gloor, P., Generalized penicillin epilepsy in the cat: effect of midbrain cooling, Electroencephulogr. Clin. Neurophysiol., 36 (1974) 517-524. Thiessen, D.D., Lindzey, G. and Friend, H.C., Spontaneous seizures in the Mongolian gerbil (Meriones unguicuhzms), Psychonom. Sci., 11 (1968) 227-228. Timpe, L.C. and Jan, L.Y., Gene dosage and complementation analysis of the Shaker locus in Drosophila, J. Neurosci., 7 (1987) 1307-1317. Timpe, L.C., Schwarz, T.L., Tempel, B.L., Papazian, D.M., Jan, Y.N. and Jan, L.Y., Expression of functional potassium channels from Shaker cDNA in Xenopus oocytes, Nature (Lond.), 331 (1988) 143-145. Tortella, EC. and Long, J.B., Endogenous anticonvulsant substance in rat cerebral spinal fluid following a generalized seizure, Science, 228 (1985) 1106-1107. Tortella, EC., Long, J.B. and Holaday, J.W., Endogenous opioid systems: physiological role in the selflimitation of seizures, Bruin Res., 15 (1985) 174-178. Toussi, H.R., Schatz, R.A. and Waszczak, B.L., Suppression of methionine sulfoximine seizures by intranigral gamma-vinyl GABA injection, Eur. J. Phurmacol., 137
(1987) 261-264. 293 Traub, R.D. and Wong, R.K.S.,
Acud. Sci., 166 (1969) 323-325. 276 Starzl, T.E., Niemer, W.T., Dell, M. and Forgave, P.R.,
Cortical and subcortical electrical activity in experimental seizures induced by metrazol, J. Neuroputhol. Exp.
43 (1977) 299-311.
280 Swartzwelder, H.S., Lewis, D.V., Anderson, W.W. and Wilson, W.A.. Seizure-like events in brain slices: suppression by interictal activity, Bruin Res., 410 (1987) 362-366. 281 Swinyard, E.A., Laboratory assay of clinically effective antiepileptic drugs, J. Am. Phurm. Assoc., 38 (1949)
297
Cellular mechanisms of neuronal synchronization in epilepsy, Science, 216 (1982) 745-747. Treiman, D.M., Walton, N.Y., DeGiorgio, CM., Wickbolt, C.L. and Salisbury, S.M., Sequential electro-clinical patterns in generalized convulsive status epilepticus in man and three experimental models of status epilepticus in the rat, Neurology, 37, Suppl. 1 (1987) 244-245. Turski, W.A., Cavalheiro, E.A., Coimbra, C., da Penha Berzaghi, M., Ikonomidou-Turski, C. and Turski, L., Only certain antiepileptic drugs prevent seizures induced by pilocarpine, Bruin Res. Rev., 12 (1987) 281-305. Turski, W.A., Cruczwar, S.J., Kleinrok, 2. and Turski, L., Cholinomimetics produce seizures and brain damage in rats, Experiemia (Basel), 39 (1983) 1408-1411. Ulloque, R.A., Chweh, A.Y. and Swinyard, E.A., Effects of gamma-aminobutyric acid (GABA) receptor
278 agonists on the neurotoxicity and anticonvulsant activity of barbiturates in mice, J. Pharmacol. Exp. Ther., 237 (1986) 468-489. 298 Urea. G., Frenk, H., Liebeskind, J.C. and Taylor, A.M., Morphine and enkephalin: analgesic and epileptic properties, Science, 197 (1977) 83-86. 299 Usunoff, G., Atsev, E. and Tchavdarov, D.. On the mechanisms of picrotoxin epileptic seizures (macro- and micro-electrode investigations), Electroencephalogr. Chn. Neurophysioi..
315 Watkins, J.C. and Evans, R.H., Excitatory amino acid transmitters, Annu. Rev. Pharmacof. Toxicol., 21 (1981) 165-204. 316 Weinstein,
317
318
27 (1969) 444.
300 Van den Berg. C.J. and Van den Velden, J., The effect
of methion~ne sulphoximine on the incorporation of labetled glucose, acetate, phenylalanine and prohne into glutamate and related amino acids in the brains of mice, .I. Neurochem., 17 (1970) 985-991. 301 van Gelder, N.M., Contributions of basic neurochemistry towards a novel concept of epilepsy, Neurochem. Ref., 12 (1987) 111-119. 302 Van Luijtelaar,
303
304
305
306
307 308 309
310
E.L.J.M. and Coenen, A.M.L., Two types of electrocortical payroxysms in an inbred strain of rats, Neurosci. Left., 70 (1986) 393-397. Velasco, E, Velasco, M., Estrada-Villanueva, E and Machado, J.P., Specific and nonspecific multiple unit activities during the onset of pentylenetetrazol seizures. I. Intact animals, Epilepsia. 16 (1975) 207-214. Velasco. F., Velasco, M., Ogarrio, C. and Fanghanel, G., Electrical stimulation of the centromedjan thalamic nucleus in the treatment of convulsive seizures: a preliminary report, Epilepsia, 28 (1987) 421-430. Vergnes, M., Marescaux, C., Depauiis, A., Micheletti, G. and Warter, J.M., Spontaneous spike and wave discharges in thalamus and cortex in a rat model of genetic petit-ma1 like seizures, Exp. Neural., 96 (1987) 127-136. Vicedomini, J.P. and Nadler, J.V., A model of status epilepticus based on electrical stimulation of hippocampal afferent pathways, Exp. Neural., 96 (1987) 681-691. Wada, J.A., Pharmacological prophylaxis in the kindling model of epilepsy, Arch. Neurol., 34 (1977) 389-395. Wada, J.A., Kindling il. Raven, New York, 1981. Wada, J.A., Mizoguchi, T. and Osawa, T, Secondary generalized convulsive seizures induced by daily amygdaloid stimuIation in rhesus monkeys, .~e~~o~ogy, 2X (1978) 1026-1036. Wada, J.A., Terao, A. and Booker, HE., Longitudinal correlative analysis of epileptic baboon, Papio papio,
Neurology, 22 (1972) 1272-1285. 311 Walker, A.E. and Johnson, H.C., Convulsive factor in commercial penicillin, Arch. Surg., 50 (1945) 69-73.
312 Walton, N.Y. and Treiman, D.M., Response of status epilepticus induced by lithium and pilocarpine to treatment with diazepam, Exp. Neural., 101 (1988a) 267-275. 313 Walton, N.Y. and Treiman, D.M., Experimental secondarily generalized convulsive status epilepticus induced by o.t_-homocysteine thiolactone, Epilepsy Res., 2 (1988b) 79-86. 314 Ward, A.A., Topical convulsant metals. In D.P. Purpura,
J.K. Penry. D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimenial Modefs of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 13-35.
319
320
321
322
323
324
325
326
327
328
329
L., Tetanus. New Engl. J. Med., 289 (1973) 1293-1296. Westrum, L.E., White. L.E. and Ward, A.A., Morphology of experimental epileptic focus, J. Neurosurg., 21 (1964) 1033-1046. Wilder, B.J. and Morrell. F., Cellular behavior in secondary epileptogenic lesions, Neurology. 17 (1967) 11931204. Willmore, L.J., Recurrent seizure induced by cortical iron injection, a model of post-traumatic epilepsy, Ann. Neural.. 4 (1978) 329-333. Wilson, W.A. and Escueta, A.V.. Common synaptic effects of pentylenetetrazol and penicillin, Brain Res., 116 (1974) 217-249. Withrow, CD., Systemic carbon dioxide derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 477-494. Wong, R.K.S. and Prince, D.A., Dendritic mechanisms underlying penicillin-induced epileptiform activity, Science, 204 (1979) 1228-1231. Wood. J.D., Systemic oxygen derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of epilepsy A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 4.59-476. Wood. J.D. and Abrahams, D.E., The comparative effects of various hydrazides on gamma-aminobutyric acid and its metabolism. J. Neurochem., 18 (1971) 1017-1025. Wuerthele, SE., Yasuda, R.P., Freed, W.J. and Hoffer, B.J., The effects of local application of homocysteine on neuronal activity in the central nervous system of the rat, Life Sci., 31 (1982) 2683-2691. Wyler, A.R., Gjemann, G.A. and Ward Jr., A.A., Neurons in human epileptic cortex: correlation between unit and EEG activity, Ann. Neurol., 11 (1982) 301-30X. Wyler, A.R. and Ward Jr.. A.A., Neurons in human epileptic cortex: response to direct cortical stimulation, J. Neurosurg., 55 (1981) 904-908. Yamamoto, C., Intracellular study of seizure-like after discharges elicited in thin hippocampal sections in vitro. Exp. Neurol., 35 (1972) 154-164. Yamamoto, C. and McIlwain, H., Electrical activities in thin sections from the mammalian brain maintained in chemically-defined media in vitro, J. Neurochem., 13
(1966) 1333-1343. 330 Yoon, C.H.. Peterson, J.S. and Corrow, D., Spontaneous
seizures: a new mutation in Syrian golden hamsters, 1. 67 (1976) 115-116. 331 Zarkovsky, A.M., Bicucuiline-sensitive and -insensitive effects of THIP on the binding of ~3H]~unitrazepan, Hered.,
~europharmaco~og~~, 26 (1987) 737-741. 332 Zorn, S.H. and Enna, S.J., The GABA
agonist THIP, attenuates antinociception in the mouse by modifying central cholinergic transmission, Neuropharmacology, 26 (1987) 433-437.
245
Elsevier BRESR 90101
Animal models of the epilepsies Robert S. Fisher Departments of Neurology and Neurosurgery,
The Johns Hopkins University School of Medicine, Baltimore,
MD 21205 (U.S.A.)
(Accepted 13 June 1989) Key words: Epilepsy; Animal model; Rodent; Cat; Monkey; Electroencephalography
CONTENTS ............................................................................................................................................ Introduction 1.1. Background ...................................................................................................................................... 1.2. Clinical epilepsy ................................................................................................................................. ................................................................................................................... 1.3. The electroencephalogram
246 246 247 249
Models for simple partial seizures, acute ...................................................................................................... 2.1. Topical convulsants ............................................................................................................................. 2.1.1. Focal penicillin ......................................................................................................................... 2.1.2. Other focal chemical convulsants ................................................................................................. 2.2. Acute electrical stimulation .................................................................................................................. ............................................................................................................................. 2.3. GABA-withdrawal 2.4. Neocortical brain slices ....................................................................................................................... 2.5. Disadvantages of acute models .............................................................................................................
251 251 251 252 252 252 252 253
Models for simple partial seizures, chronic .................................................................................................... 3.1. Cortically implanted metals .................................................................................................................. 3.1.1. Alumina hydroxide .................................................................................................................... 3.1.2. Other metals: cobalt, tungsten, zinc, iron ...................................................................................... 3.2. Cryogenic injury ................................................................................................................................ 3.3. Ganglioside antibody injection ............................................................................................................. .............................................................................................................. 3.4. Systemic focal epileptogenesis
253 253 253 253 253 253 254
Models for complex partial seizures ............................................................................................................. 4.1. Kainic acid ....................................................................................................................................... 4.2. Tetanus toxin .................................................................................................................................... 4.3. Injections into area tempesta ............................................................................................................... 4.4. Kindling ........................................................................................................................................... 4.5. Brain slices ....................................................................................................................................... 4.5.1. Rodent in vitro hippocampal slices ............................................................................................... 4.5.2. Isolated cell preparations ............................................................................................................ 4.5.3. Human neurosurgical tissue ........................................................................................................
254 254 254 255 255 256 256 257 257
Generalized tonic-clonic seizures ................................................................................................................. 5.1. Genetic ............................................................................................................................................ 5.1.1. Photosensitive baboons .............................................................................................................. 5.1.2. Audiogenic seizures in mice ........................................................................................................ 5.1.3. Totterer mice and other seizure-prone mouse strains ....................................................................... 5.1.4. Genetically epilepsy-prone rats .................................................................................................... 5.1.5. Mongolian gerbil ....................................................................................................................... 5.1.6. Drosophila shakers ....................................................................................................................
258 258 258 258 259 259 260 260
Correspondence: R.S. Fisher, Departments of Neurology and Neurosurgery, The Johns Hopkins Hospital, Meyer l-130,600 North Wolfe Street, Baltimore, MD 21205, U.S.A.
0006-8993/89/$03.50 0
1989 Elsevier Science Publishers B.V. (Biomedical Division)
246
5.1.7. Miscellaneous genetically seizure-prone animals .............................................................................. 5.2. Maximal electroshock ......................................................................................................................... 5.3. Systemic convulsants ........................................................................................................................... 5.3.1. Pentylenetetrazol ....................................................................................................................... 5.3.2. Systemic penicillin as a tonic-clonic model ..................................................................................... 5.3.3. Other inhibitory antagonists ........................................................................................................ 5.4. Metabolic derangements ......................................................................................................................
260 260 261 261 262 263 264
6. Models for generalized absence seizures .......................... ... .............. ...... . ‘.. ..,. ..,... . .. .____________....... .,... 6.1. Thalamic stimulation .......................................................................................................................... 6.2. Bilaterai cortical foci .......................................................................................................................... 6.3. Systemic penicillin .............................................................................................................................. 6.4. ~-Hydroxybutyrate .................. ........................................................................................................... 6.5. Intraventricular opiates ....................................................................................................................... 6.6. THIP ............................................................................................................................................... 6.7. Genetic rodent models of absence .._., .... .. ... ... ... .._...._...........................................................
264 264 265 265 266 266 266 266
7. Animal models of status epilepticus
267
8. Conclusion 9. Summary
,.
....
I..........................
_.__.
,, ...... ,, ........
,.
., .,
267
,..........___._....._.._.,..,..,,,,,_,................._.,.__..__.._......... 269
. . . .
269
.
269
The study of the epilepsies has been dependent upon use of mode1 systems. Ethical considerations rule out use of the neuroscientist’s modern tools intracellular recording, microchemical analysis and anatomical tracer techniques in intact human brain. Screening of thousands of compounds for possible anticonvulsant activity cannot be performed in the clinic. Much of what is known about the epilepsies is derived directly or indirectly from animal models. It is thus germane to consider the value and limitations of our animal models for this disorder. Many species of animals do develop spontaneous seizures”“; however, these cases are sporadic, and not usually in animals suitable for experimentation. More reliable preparations have therefore been developed7’ (see Table I). The large number of mode1 systems derives from two reasons. First, none of the models is fully trustworthy as an imitation of clinical epilepsy. Important findings therefore require validation in several models. Second, there are multiple types of the epilepsies to model. Simple partial epilepsy resulting from a frontal lobe tumor is a very different disorder from familial petit ma1
epilepsy. Table I categorizes the models by seizure type. Some models serve for more than one type. There are many more models of seizures than of the epilepsies, i.e. of the condition of chronically recurrent spontaneous seizures. Nevertheless, the phrase ‘animal model of epilepsy’ is often used in place of the more precise phrase ‘animal model of a seizure disorder’. A seizure (‘ictus’) is a clinical and behavioral event. In vitro brain slices cannot seize, and certainly cannot suffer epilepsy. This does not detract from the insight into mechanisms of the epilepsies than can be gained from use of mode1 systems. In this area terminology is difficult; however, clarity is required. Researchers tend to employ the mode1 of greatest familiarity and convenience; rarely, if ever, would an experiment be performed both in tissue culture and in photosensitive Papio papio baboons. Nevertheless, a more logical way to choose a model would be first to choose the experimental question. Questions regarding influence of anticonvulsants on membrane ion channels can be answered in tissue culture. If the investigator is asking whether surgical interruption of certain neuronal pathways can block seizure spread, then experiments must be done in intact mammals, and most usefully in primates. Use of inappropriate model systems may lead to waste of
Acknowledgements References
.. ....
. . .. .. .. .
.. .
. . . . . . . . . .__... ._. _..___._.. . . . .
1. INTRODUCTION
247 TABLE I Animal models of the epilepsies
Simplepartial, acute
Generalized tonic-clonic
Complex partial
Topical convulsants Penicillin Bicuculline Picrotoxin Strychnine Cholinergics Anticholinergics Other Acute electrical stimulation GABA-withdrawal Neocortical brain slices
Genetic Photosensitive baboons Audiogenic seizures in mice Totterer and El mice Genetically prone rats Mongolian gerbil Drosophila shakers Maximal electroshock Chemical convulsants Pentylenetetrazol Bemegride Picrotoxin Bicuculline Methionine sulfoximide Penicillin Other Metabolic derangements Hypoxia Hypoglycemia Hyperbaric oxygen Hypercarbia Uremia Drug withdrawal High temperature
Kainic acid Tetanus toxin Injection into area tempesta Kindling Brain slices Rodent hippocampal slices Isolated cell preparations Human neurosurgical tissue
Simple partial,
chronic
Cortically implanted metals Aluminum hydroxide Cobalt Tungsten Zinc Iron Cryogenic injury Ganglioside antibody injections Systemic focal epileptogenesis
time, money and animal lives, and furthermore may promulgate irrelevant results. Advantages and disadvantages of each of the major model systems are outlined below. The discussion is designed to be conceptual, and not a full recipe for actual use of the model. More extensive descriptions of laboratory techniques may be found in an old, but still very useful, book by Purpura and colleagues242, entitled Experimental Models of Epilepsy - A Manual for the Laboratory Worker. Discussion of models pertaining to anticonvulsant drug development may be found in a thorough review by Ldscher and Schmidt163. Basic models of the epilepsies are used to explore questions about seizures and the electrical activity of the brain. Table II lists some of the key questions that have been addressed. These questions pertain to the underlying EEG generators of electrical potentials associated with seizures; the nature and identity of neuronal systems able to produce epilepsy; issues of why seizures start, spread and stop, why seizures occur when they do; what type of pathologies in brain give rise to seizures; whether seizures cause damage to brain;
Generalized absence
Thalamic stimulation Bilateral cortical foci Systemic penicillin y-Hydroxy-butyrate Intraventricular opiates General rat models
Status epilepticus
Lithium-pilocarpine Cobalt-homocystine Recurrent stimulation
and to the mechanism of action of anticonvulsants. As we explore each of the main models for the epilepsies, we will consider the ability of the model to address these questions. 1.2. Clinical epilepsy Epilepsy is not a disease, but a collection of diverse syndromes, some of which are secondary to other brain derangements, and some of which are seemingly primary. Johns Hughlings Jackson, one of the founders of modern neurology, portrayed a seizure as a ‘sudden, excessive and temporary nervous discharge . . . of a few cells which have got far above the rest of the cortical cells in degree of tension and instability . . .‘126. Epilepsy is considered to be a disorder intrinsic to the brain: either ‘hereditary tendency’ or a prior insult has rendered a portion of the brain electrically unstable. Any serious injury to the brain can lead to epilepsy; common ones include: major head trauma, stroke, hemorrhage, infection (meningitis, encephalitis or abscess), vascular malformations, and benign or malignant tumors. Although a seizure can occur at the time of the initial insult, there may also be a
248 TABLE II
Each of the seizures listed so far is referable to abnormal electrical activity in a relatively localized
Questions aboutseizures
part of the brain. There are in addition several varieties of seizures which lack apparent focality, and are therefore called ‘primary generalized’ sei-
What generates the EEG? What is the neuronal basis of epilepsy? Where is the ‘pacemaker’? Why do seizures spread? Why do seizures stop? Why do seizures occur when they do? Why are seizures varied? What is the pathology of epilepsy? Why is there a latent period? Are seizures bad for the brain? Anticonvulsant mechanisms?
latent period of months or years to development epilepsy lo. The reason for this latent period
zures. The two prototypes generalized tonic-clonic’)
are grand ma1 (‘primary and petit ma1 (‘primary
generalized absence’). Grand ma1 seizures begin with an initial loss of consciousness, followed by generalized extensor rigidity referred to as the tonic phase, followed after some seconds by rhythmical 4-limb and face jerking, called the clonic phase. Petit of is
unknown, but it can be replicated in animal models of the epilepsies (see below). Manifestations of a seizure depend in part upon the age and behavioral state (asleep, awake, active) of the subject, anticonvulsant drug therapy, and location in the brain at which the electrical abnormality occurs207. A discharge in the motor area can induce rhythmical uncontrolled motor activity in the face or limbs. Seizure activity in the somatosensory regions of brain causes sensations of tingling or numbness. Occipital and posterior parietal discharges generate primary visual phenomena: jagged lines, light flashes, colors; familiar to sufferers of the non-epileptiform condition, classic migraine. Stereotyped odors or tastes at the onset of a seizure suggest origin in the uncinate region of the inferior frontal lobe, responsible for gustatory sensation. Seizures arising in association regions of cortex tend to give rise to more complex phenomena, or simply disruption of ongoing normal function. The most common type of seizure in adults arises from limbic structures in the temporal lobes93. These seizures are preceded in over 50% of cases by auras: actually themselves localized seizures, causing visceral discomfort, flushing, tingling, cognitive-perceptual distortions such as deja vu, micropsia or vertigo141. Awareness and memory acquisition are decreased during many limbic seizures. Patients appear to be in a dream-like or fugue ‘state for periods lasting seconds to several minutes. Automatic actions: fumbling, lip-smacking, chewing, repeating stock phrases, undressing, walking in circles are common during seizures originating in the limbic system.
ma1 seizures
have
usual
onset
in childhood,
and
present as a few seconds of staring with rapid return to awareness. There has been great debate over the past decades about the actual site of origin of these primary generalized seizuresx4. A definitive classification of seizures will be elusive until the pathophysiology of seizures is better understood; nevertheless, efforts have been devoted to classification of seizures”‘. For the clinician the importance of even such an interim scheme is in its correlation with underlying brain pathology, with prognosis, and with likely responsiveness to particular medications or surgical therapies. For the researcher the classification serves as a reminder not to draw excessively broad conclusions about the multiple types of clinical epilepsy from limited animal models. Seizures are classified as to whether their onset is partial (focal) or generalized (Table III). Partial seizures are subcategorized into simple (without loss of awareness) and complex (with loss or blunting of awareness). The simple partial seizures may be motor, sensory, sensory-motor, autonomic-visceral or cognitive. The latter two categories of simple partial seizures usually occur as an ‘aura’ to complex partial seizures. Complex partial seizures were described above. The name is a synonym for the older terms: ‘temporal lobe seizure’, ‘psychomotor seizure’, ‘limbic seizure’. Any partial seizure may spread to wide regions of brain and become generalized. If this generalization is rapid and unobserved, a secondarily generalized seizure may be mistaken for a primary generalized tonic-clonic seizure, thereby leading to suboptimal diagnosis and therapy. Generalized absence seizures (petit mal) may present with only a brief blank stare or with a stare plus brief
249 automatisms fluttering.
and
motor
In instances
activities, where
such as eyelid
automatisms
are ex-
tensive, it can be difficult to differentiate absence from complex partial seizures. Generalized tonicclonic (grand mal) seizures were detailed above, and constitute the most popularly held picture of epilepsy, with falling, stiffening and jerking, accompanied by loss of consciousness, and sometimes bladder
incontinence.
lightening-like
Myoclonic jerking
seizures
consist
of
of the face or extremities,
once or a few times. Prolonged rhythmical jerking does not occur in myoclonic seizures. Tonic seizures, with stiffening, and clonic seizures with rhythmical jerking, may occasionally occur in isolation from the more common tonic-clonic, grand ma1 pattern. Atonit seizures, previously called ‘akinetic’ or ‘astatic’ seizures, present with sudden loss of muscle tone, and a pitch forward to the ground, with loss of awareness lasting for an almost imperceptibly short period of time. From 10 to 25% of seizures may remain unclassified, because they fail to fit into an existing category, or because descriptive details are lacking. 1.3.
The electroencephalogram
The electroencephalogram (EEG) is the most useful test in diagnosis and classification of epilepsy,
TABLE III Internationalclassification of the epilepsies Adapted from Dreifuss et al.“. Partial Simple Motor Sensory Sensory-motor Autonomic-visceral Cognitive Complex (psychomotor) With and without aura With and without automatism Secondarily generalized Generalized Absence (petit mal) Tonic-clonic (grand mal) Myoclonic Tonic Clonic Atonic Unclassified
second
only
time-varying
to the clinical record
history*l’.
of electrical
potential
EEG
is a
(voltage)
differences at paired points of the scalp. Traditionally, 8 or 16 channels are recorded, although the number of channels is arbitrary. Basic research4 has shown EEG voltages to result from summed, synchronous synaptic activity in the cortical regions under
the surface
recording
electrodes.
Neuronal
action potentials make little direct contribution to the EEG since they are so brief as to be relatively asynchronous.
Clinical
electroencephalographers
study records for overall slowing of normal frequencies - correlated with general cerebral dysfunction from numerous causes, and for local slowing suggestive of a local cerebral disturbance, possibly in the wake of local ischemia, hemorrhage, trauma, tumor, infection or a recent partial seizure. Seizure discharges are an example of extreme synchronization of the EEG. Several EEG patterns have been recognized for the past half-century as a ‘signature’ of epileptiform activity. During absence (petit mal) seizures the EEG shows 3-4/s rhythmical spike-wave complexes (Fig. 1). Generalized tonic-clonic (grand mal) seizures correspond to a different pattern (Fig. 2). The initial EEG change in a seizure may be a paradoxical low-voltage desynchronization of the EEG73. With tonic-clonic seizures this is followed by rhythmical 15-25 per second sharp activity, decelerating to slower frequencies, resolving usually to spikes (or polyspikes) and slow waves. After the seizure, the background EEG flattens and slows relative to the baseline for several minutes. Seizures are sometimes referred to as an ‘ictus’; the period after a seizure as the ‘postictal’ period; and the between-seizure period as the ‘interictal’ period. Interictally, the EEG can be normal. Alternatively, subclinical interictal seizure discharges may be observed. Such discharges may be too brief to affect behavior. Interictal discharges are known to be a useful marker of a site in a patient’s brain from which seizures may arise*r*. Care must be taken both in research and clinical settings to distinguish normal sharp discharges, such as vertex waves of sleep (Fig. 3A) from epileptiform sharp waves (Fig. 3B). The term ‘spike’ to connote an interictal EEG discharge is regrettable since it promotes confusion with ‘spike’ as a descriptor of a single neuron action potential. Many animal model systems for the study
250 of the epilepsies are actually models of the interictal spike. It is therefore relevant to our discussion of animal models of the epilepsies to note that interictal seizure patterns have been a source of confusion for both clinicians and epilepsy researchers. Interpretation of the meaning of interictal spikes clinically or in animal experiments requires understanding of a few key points. First, interictal spikes or sharp waves are diagnosed by pattern re#gnitjon, but the context in which these patterns occur is often very important. Several eon-epiie~tiform patterns may look similar to sharp waves or spike$“; for example, the fully normal vertex wave of sleep (Fig. 3A). Second, interictal seizure patterns can appear in individuals with no history of epilepsyru2. Third, lack of interictal discharges does not exclude epilepsy: interi~tal EEGs may be negative in a large number of people with epifepsy’i*. Diagnostic yield of the EEG can be increased by repeat recordings, recordings during sleep, recordings after activating procedures such as h~ervent~lation~ photic stimulation (flashing lights) and sleep deprivation, and by prolonged continuous monitoring in diagnostically difficult cases. Fourth, the site of interictal EEG discharge only approximately correlates with the origin of clinical seizures. Occasional patients evidence interictal spikes maximal on the scalp at one site; yet, ictal events (clinical seizures) appear electroencephalographically to arise from another site. It is uncertain whether this discrepancy results from a site sampling error in the clinical EEG recordings, or a fundamental difference in the meaning and mechanism of interictal and ictal discharges m . Neither clinicians nor researchers can assume that a spike-like pattern in an EEG indicates epilepsy, unless there is supporting behavioral evi-
Terms describing seizure-related EEG activity are often confusing, In the clinical literature, ‘spike’ connotes an EEG pattern consistent with abnormai synchronous firing of a group of neurons. Associated with the event is depolarization, with a transient net flux of positive ions into neuron?. This renders the extracellular space negative in the region of the discharging neurons, and progressively less negative as voltages are measured at increasing distances from the synchronously firing neurons. At standard EEG amplifjcatiol~s a spike should look ‘pointy’, which is variably defined as having a biphasic or triphasic shape with a duration at haif~height of Iess than 70 ms (although some use 50 ms). ‘Sharp waves’ (e.g. Fig. 3B) are similar in appearance to ‘spikes’, but not as pointed, with durations of 70-200 ms, measured at half the peak amplitude. Spikes and sharp waves have similar connotations to a clinical electroencephalngrapher: each is an interictal pattern that may be associated with seizure activity, within the constraints noted above. Numerous clinical EEG terms ‘I2 have arisen which sound similar to spike and sharp wave, but do not connote epilepsy. These include such descriptive phrases as “minor sharp transients’, ‘minor sharp activity’, ‘rhythmic sharp activity‘, ‘blunted sharp waves’, ‘wicket spikes’ and the somewhat controversial pattern ‘small sharp
dence.
SPIKE-WAVE MORPHOLOGY
‘ax
]55*lv
FPZ-A2
Fig. 1. Scalp EEG recording showing spontaneous 3-4/s spike-wave discharges in a patient with absence epilepsy. For clarity only 3 EEG channels (of the original 16) are displayed.
TONIC - CLONIC SEIZURE ONSET LEFT TEMPORAL SUBDURAL GRID
200 uv
l__...1 see
Fig. 2. Onset of a tonic-clonic generalized seizure in a patient undergoing evaluation for resective surgery. Recordings were obtained from a grid of 64 stainless steef recording disks (0.s mm diameter, 10 mm spacing) inserted into the subdural space over left frontal and temporal lobes. Each channel recorded from adjacent pairs of electrodes.Montage is unlinked, e.g. no electrode was common to two channels. Rhythmic sharp activity was evident at the start of the episode.
2.51 spikes’. Nomenclature is even less clear with recordings directly from brain tissue, as is often the case with experimental models. Spikes and sharp waves, singly and in rhythmic runs, are plentiful in corticograms, since direct cortical recordings circumvent the high frequency filter usually provided in scalp recordings by CSF, skull and skin. Electroencephalographers usually employ a stricter standard of sharpness for corticographic spikes and sharp waves in comparison with potentials recorded from the scalp. These standards are subjective, and may differ among different experienced readers. Researchers in the animal models of the epilepsies have no clear standard for terminology. Discharges have been called spikes, sharp waves, epileptiform firing, ictaform discharge, burst discharge, afterdischarge, ringing and countless other ambiguous terms. In reviewing literature on animal models of the epilepsies it is important for the reader to evaluate critically whether an ‘epilepsy’-related electrical event has been defined with clarity. 2. MODELS
FOR
SIMPLE
PARTIAL
SEIZURES,
ACUTE
spike neurons in the region of the focus tend to fire synchronously. The penicillin model has been one of the most important models for answering questions about the neuronal basis of epilepsy. Initial observations about the paroxysmal depolarization shift were based upon intracellular recordings from penicillin foci in cat neocortex178,235,328.This model is also suitable for analysis of spread of seizure activity. Radiolabeling of penicillin215 shows it to be largely restricted to a zone of a few square millimeters. Neurons surrounding this focus manifest substantial inhibitory activity in an attempt to restrain the spread of seizure activity238. The ionic micro-environment near the penicillin focus has been analyzed
A.
VERTEX
WAVES
=4-F8 Al-T3 T3-c3 c3-cz cz-c4
Models in this category are analogues of acute cortical injury leading to seizure discharges, such as might occur with an intracranial abscess, trauma or hematoma. in clinical injuries such as these there may be immediate seizures, then a latent period to development of chronically-recurrent, apparently spontaneous seizures. Experiments in the acute models may, however, be accomplished in a single sitting.
C4-T4 T4-A2 T5-P3
B. SHARP
WAVES
2.1. Topical convulsants 2.1.1.
Focal penicillin. The most popular method
to study simple partial (focal) seizures has been by application of a topical convulsant. The common antibiotic, penicillin, was discovered to be such a topical convulsant during neurosurgical procedures in which it was applied to brain to prevent infection” ll. When a cottonoid pledget soaked in 1.7-3.4 mM penicillin is placed on exposed rat or cat cortex, regionally placed electrodes record recurring interictal spikes within a few minutes. These discharges resemble human interictal spikes recorded from cortex at corticography. During the interictal
FP2-F6
yLM
T6-02
Fig. 3. EEG tracings from human scalp to illustrate normal vertex waves (A), and abnormal sharp waves (B) with phase reversals over the left temporal region. Although vertex waves and epileptiform sharp waves are similar superficially, they are distinguishable by location and accompanying EEG activity.
252 in great detai17x*““~““. At low dose of penicillin tissue culture’“‘, anesthetized animal cortex”’
in
be produced
or
hippocampal
y-
into motor cortex of baboons for 7 days29. Upon cessation of the infusion focal EEG spikes and
aminobutyric postsynaptic
slice
penicillin
selectively
blocks
acid (GABA)-mediated inhibitory potentials (IPSPS)~‘.““. At higher doses
actions are less specific15. 2.1.2. Other focal chemical convulsants. Other drugs have been used to produce an acute focus, including bicuculline34, picrotoxin**, stry~hnine142, cholinergics69.29”.2’” and anti~holinergics~‘. The first 3 of these, along with penicillin, the action of the inhibitory
are antagonists to neurotransmitter,
GABA.
to the
This
has
contributed
view
that
epilepsy results from disinhibition. Further support for this view has come from recognition of the importance of benzodiazepine receptors in relation to inhibitory processes and epilepsylx”, since the benzodiazepine receptor is closely linked to the GABA receptor. 2.2. Acute electrical stimulation Another popular model of simple partial seizures can be generated by direct electrical stimulation of cortical tissue. As was shown decades ago by Adrian”, repetitive electrical pulses can lead to rhythmic sharp discharges (‘afterdischarges’) that persist for seconds or minutes after the electric stimuli cease. Paradigms usually involve bipolar steel ball electrodes resting on cortex, applying l-5 s long trains of alternating square-wave pulses, 0.5-5 mA amplitude, OS-10 ms individual pulse duration, at frequencies of lo-100 Hz3. The response is similar in animal cortex or in human seizure patients in whom stimuli are used to map functional regions of neocortex (Fig. 4). The discharge appears to be primarily a tonic electrical discharge with frequencies of 18-25 Hz. Afterdischarges can lead to seizures which are indistinguishable from partial simple or secondarily generalized tonic-clonic seizures. Studies of afterdischarges are useful in analysis of pathways of seizure spread, since the site of origin is well-defined. They are a less useful model for the disorder epilepsy, since seizures do not occur spontaneously, and originate only at the site of electrical stimulation. 2.3. GABA-withdrawal An interesting subacute
focal seizure
model can
spike-waves
by infusing
GABA
arise from motor
by automatisms
and hind-limb
via micro-syringe
cortex,
accompanied
myoclonus.
2.4. Neocortical brain slices Slices of rat, mouse,
rabbit or guinea pig neocor-
tex may be maintained alive in vitro and exposed to various chemical convulsant~~.‘~’ as a model of acute partial seizures. Discussion of the slice model will be deferred until later, when the hippocampal slice mode1 system is reviewed. Surgical
ablation
of epileptic
foci
in patients
provides material which can be studied by the slice technique. Neocortex from such foci contains cells which show orthodromically evoked analogues of paroxysmal depolarization shifts*“, Another study260, however, emphasizes the limitations of in vitro study of surgical specimens. In surgically obtained specimens of seizure foci cellular spontaneous bursting is rare or absent. Evoked bursting is seen in normal monkey cortical brain s1ices260, as well as excised human foci. Comparison of ‘epileptic’ and ‘normal’ tissue is hampered by lack of suitable controls. The true utility of the brain slice technique has so far derived from controlled animal experi-
AFTER~ISCHARGES NE~ORTICAL
STIMULATION
Fig. 4. Electrica afterdischarges recorded from a subdurally implanted grid of electrodes in a human patient. For clarity, only 3 tracings are shown from a 64-channel recording array. The left half of the figure shows baseline electrocorticog~phic activity from the lateral left temporal lobe. Bipolar alternating square-wave electrical stimulation was applied at 50 Hz, 5 mA for 1 s to electrodes 1 and 2, each 3 mm diameter, with 10 mm center-to-center separation. Following transient amplifier blocking (flat line) repetitive focal afterdischarges were seen in the region of electrodes 1 and 2. Discharges persisted from seconds to minutes, and could progress to generalized tonicclonic seizures.
253 teral to the aluminum
lesion,
specimens.
gression
generalized
2.5. Disadvantages of acute models
zures. Interictal and ictal EEGs appear similar to clinical studies. Neuropathological specimens obtained from biopsies in the region of an established
ments
(see below),
rather
than analysis
of human
There are general disadvantages common to the acute models. First, each of the drugs may have idiosyncratic effects which have little to do with epilepsy. Penicillinase, to take an extreme example, is effective against lepsy”&, but against
the penicillin model of epino other model. Second, the
to secondarily
and occasional tonic-clonic
prosei-
alumina focus in monkeys show gliosis and distortion of dendritic neuronal trees, similar to the picture seen in human neocortical foci317. When this scar is excised abnormal EEG activity remote from the foci resolves”‘.
Response
to standard
anticonvulsants
acute models appear to be very intense; many cells in the focus participate in epileptiform activity. This degree of participation is not matched by neurons in
parallels those of patients with focal epilepsy16r. 3.1.2. Other metals: cobalt, tungsten, zinc, iron. Cortical implantations of other metals such as
human foci, studied by extracellular recordings at neurosurgery”26.327. Third, a focus is very wellcircumscribed; much more so than occurs in clinical epilepsy, where the border between ‘involved’ epileptogenic tissue and ‘normal’ tissue is usually indistinct. Fourth, anesthesia is required to establish the animal preparation, and effects of anesthesia may confound the experiment. Fifth, these models last only minutes-to-hours, and do not result in recurrent seizures. The models do not afford time for development of morphological changes in neurons, dendrites, glia and regional circulation known to be a feature of clinical epileptic tissue.
cobalt”,
3. MODELS CHRONIC
FOR
SIMPLE
PARTIAL
SEIZURES,
3.1. Cortically implanted metals 3.1.1. Alumina hydroxide, The best validated and most realistic models for the epilepsies are those employing implantation of metals in brain to generate a state of ‘spontaneously’ recurrent simple partial seizures. The prototype of this group of models is the alumina hydroxide gel model, discovered by the Kopeloffs143, and elaborated by investigators at the University of Washington at Seattle, under the direction of Arthur Ward314. In a typical preparation 4% aluminum hydroxide will be injected into surgically exposed monkey neocortex at a few adjacent sites. A similar model can be produced in the cat”. Spontaneous and recurrent seizures generally begin one to two months after the injection, and persist for as long as several years. The seizures themselves are similar to simple partial seizures in humans, with rhythmic jerking of an extremity or face contrala-
tungsten2’,
and zinc223 can produce
chronic
or subacute models of recurrent seizures in animals. None of these models is documented as extensively as the aluminum model in monkeys. GABA receptors have, however, been found to be decreased in the region of cobalt foci of rat motor cortex, 2-3 weeks after establishment of the focus230. Iron, in the form of either ferrous or ferric sulfate, can also induce recurrent seizures when injected into mammalian cortex151.246. This phenomenon may have direct bearing upon mechanisms of post-traumatic epilepsy3r9, since trauma may inject iron-containing hemoglobin complexes into cortex. The metal-deposition models are good models of chronic simple partial seizures, but they are laborious and expensive to prepare, especially in the case of the monkey aluminum hydroxide model. Additionally, they do involve injection of irritating compounds into cortex, which may have effects not seen with clinical seizure foci. 3.2. Cryogenic injury One model that does not require injection of exogenous drugs into brain is the cryogenic or freeze lesion model for partial simple seizures1105162. Ethylchloride lesions or cold-trauma from a liquid nitrogen probe produces a highly epileptogenic lesion, giving rise to seizures within a few hours of the lesion and persisting for a few days. Substantial cerebral edema generally accompanies the lesion. 3.3. Ganglioside antibody injection Karpiak and associates 135~136have produced seizures by injection of antibodies to brain gangliosides into rat cortex. After a delay of 24 h focal spiking
2.54 appears charges
in the region of the injection. then become more generalized,
EEG disand may
recur after a single injection for more than 90 days. Clinical manifestations in the animals are rare. This
interest in KA has derived from its ability to produce relatively selective lesions of cell bodies in brain, while sparing
axons of passage41. For reasons
that
model is not a convenient nor well-characterized preparation, but it does raise the interesting question
are still incompletely understood, KA has an especially prominent toxic effect on hippocampus, even when injected systemically, or at brain sites remote
of whether certain epileptogenic immunologica1ly mediated.
from hippocampus*~‘. In doses required to produce cell injury,
lesions
may be
3.4. Systemic focal epileptogenesis A model, mixing features of focal and generalized epilepsy is the preparation referred to by Remler and co-workers’@ as ‘systemic focal epileptogenesis’. Rats
receive
radiation
to a volume
of cerebrum
equivalent to 0.25 ml. At a time 3-6 months later, when the blood-brain barrier in that region is locally disrupted, bicuculline methiodide (2 mg/kg) is injected systemically. A seizure focus will be produced, with recurrent EEG spikes and focal seizures, enduring for several weeks after a single injection. These spikes are abated by phenytoin, phenobarbital, chlordiazepoxide and valproic acidz5”. The chronic models provide opportunities to study serial changes in the period between an insult to brain and the development of ongoing epilepsy. The mechanisms of these latent periods are presently unknown. 4. MODELS FOR COMPLEX PARTIAL SEIZURES
Complex partial seizures usually arise from the limbic lobe, including amygdala, hippocampus, and less often, temporal neocortex or extratemporal structures’~. Separation of models for complex partial seizures and simple partial seizures is artificial, since the distinction depends more upon where in brain the model is applied than upon intrinsic differences in the model. Nevertheless, models for complex partial seizures have engendered considerable recent attention. The 4 primary models to be discussed: kainic acid, tetanus toxin, kindling and the hippocampal in vitro slice, have each emerged since Purpura and colleagues’ 1972 compendium of experimental models of the epilepsies242. 4.1. Kainic acid Kainic acid (KA) is a rigid analog of the putative excitatory neurotransmitter, glutamate’82. Primary
less KA
than those can induce
seizures in hippocampus. Animals given KA 4 mg/kg i.v. lh4, or 0.8-2.0 pg intra-hippocampally3’, will show periodic arrest of activity, masticatory movements, complex motor tension to generalized
activity, and sometimes extonic-clonic activity. Ste-
reoencephalography shows major spike activity originating in the limbic systemzOs. KA is a prototype of an excitotoxic compound. Nevertheless, at threshold convulsant doses in region CA1 of the hippocampal slice, the convulsant effect results from disinhibition7’.r5”, probably reflecting a functional inactivation of the inhibitory interneuronal cells prior to the principal neurons. KA produces an acute or subacute mode1 of seizures, lasting hours to days. The accompanying hippocampal lesions may be considered to confound the model, or alternatively, to portray the pattern of limbic cell damage which can occur with clinical status epilepticus2”‘. Tetanus toxin A model of recurrent, chronic partial seizures can be produced by injection of tetanus toxin into rat or cat hippocampus’92. Categorization of the model with complex partial seizure models results from the location of the usual injection site in limbic structures, rather than the properties of the toxin itself. Tetanus is a disease316 produced by a 145,000 Da toxin from the gram-positive bacteria, Clostridium tetani. in the disease state toxin is transported from the periphery to the spinal cord, where it is believed to interfere with presynaptic release of inhibitory neurotransmitter 234. In contrast, injection into hippocampus of a dose of toxin 3-6 times the mouse LD,, probably produces effects only locallyr9*. Seizures may occur within a day after injection and then on a chronically recurrent basis over weeks. A seizure in a rat typically begins with arrest of activity, followed by myoclonic jerks of the front limbs, and in some animals generalized tonic-clonic seizures19*. 4.2.
255
Whether
or not
the
upon several factors,
seizure including
generalizes
depends
spread to the cingu-
model long-term
plastic changes in brain excitability.
Such plastic changes
are believed
to participate
not
but also in memory model is conceptually
and re-
late area113. The EEG shows concurrent 3-20 Hz spiking or spike-wave activity. For approximately
only in epileptogenesis, learning9’. The kindling
one month
lated to models for long-term potentiation24, although kindling paradigms tend to be more chronic
animals
have an average
of about
100
seizures per day. Thereafter, the seizures decrease, such that the animals are seizure-free by several months
after injection
of tetanus.
The mechanisms
of tetanus-induced recurrent seizures are unknown. The tetanus toxin model resembles those produced by injection
of other seizurogenic
substances
into hippocampus, but it has some intriguing idiosyncrasies. Visible neuronal destruction is absent’93 and the epileptogenic activity is time-limited. 4.3. Injections into area tempesta While examining regions near prepyriform cortex for antinociceptive activity, Piredda, Gale and colleagues made note of a discrete convulsion-prone site228*229. At this site, now referred to as ‘area tempesta’, unilateral injection of picomole quantities of bicuculline, carbachol, kainic acid, glutamate, aspartate or N-methyl-D-aspartate (NMDA) produce bilateral clonic motor seizures. At this site the anticonvulsant GABA agonist muscimol, and the anticonvulsant glutamate-antagonist, 2-amino-phosphonoheptanoic acid, are powerfully inhibitory to seizures induced in rat by i.v. bicuculline228.229. It is difficult to know where to place this model in a classification scheme. Clonic motor seizures and EEG discharges are apparently generalized, even though the chemical convulsant is limited to the limbic region of one hemisphere. Seizures from the area tempesta are most probably analogous to rapidly secondarily generalized complex partial seizures from a temporal or inferior frontal focus. The model raises the issue of whether certain apparently primary generalized seizures actually are secondarily generalized seizures arising from an occult focus. 4.4. Kindling Kindling is a phenomenon by which repeated shocks to various parts of brain result in enhanced electrical excitability of brain. The effect was discovered by Delgado and colleagues’, elaborated by Goddard”, and subsequently studied in detail by literally hundreds of laboratories91~‘85*227~245~308. Kindling has become one of the most popular ways to
than those for LTP, and focus more on epileptic changes than on enhanced evoked electrical responses. Kindling is usually tion of the amygdala,
initiated by electrical stimulabut most regions of forebrain
can be kindled98,‘53. Every species so far studied is subject to kindling308, from frogs to primates, and probably man as wel1304. To produce the model, bipolar stimulating wires are implanted in amygdala309 or elsewhere in brain. The animal recovers from the surgery, then daily electrical stimulus trains are applied via the electrodes, typically using parameters such as 0.2-1.0 mA at 60 Hz for 2-s trains’53. A fairly wide range of stimulation parameters may be effective in induction of kindling. After a few days of stimulation a train of shocks begins to induce electrical afterdischarges, which become progressively more complex and prolonged with each kindling stimulus. At this time, the animal is said to be ‘kindled’. If continued for a few weeks, rodents exhibit ‘spontaneous’ epileptic seizures227 even in the absence of their priming shocks; presumably, brain excitability has at this point reached a state in which normal afferent activity can trigger epileptiform afterdischarges. Racine has classified the behavioral response to kindling into 5 stages of epileptiform activity: Class I = facial clonus; Class II = facial clonus and rhythmic head nodding; Class III = facial clonus, head nodding and forelimb clonus; Class IV = facial clonus, head nodding, forelimb clonus and rearing; Class V = facial clonus, head nodding, forelimb clonus, rearing and falling244. Classes IV and V may be considered as models of secondarily generalized complex partial seizures. Rapid kindling paradigms able to model status epilepticus in rodents, within a few hours or days of kindling have been described165’166. Repeated stimulation by excitatory chemicals can also produce kindling’~,“‘. After several decades of work the mechanisms of kindling remain unknown. Anatomical studies with light and electron microscopy have been unre-
256 vealing”. interactions
Several between
studies
have
kindling
shown
transient
have been especially
and neurotransmitter
productive
ing of the epilepsies7’3’s4,
for our understand-
because
of the important
systems30.y6, including acetylcholine, norepinephrine, serotonin, glutamate, GABA and cyclic nu-
role of the hippocampus in complex partial seizures. Several recent texts’43’“H.262 have reviewed these
cleotides.
preparations
None of these,
ciently consistent nor mechanism for kindling.
however, long-lasting
has been suffito
explain
a
Pharmacological prophylaxis of kindling has been reviewed by Wada”“‘. Phenobarbital, diazepam and valproate block kindled seizures and block the development
of the kindling
process. Phenytoin
and
carbamazepine block seizures once kindling has occurred, but do not reliably block the establishment of kindled
seizures.
The relevance of kindling to clinical epilepsy has been debated 1.91. Can an ongoing epileptic focus induce an independent epileptic focus at remote, synaptically linked regions of brain? Such a secondary epileptogenic lesion has been called a ‘mirror focus’, and has been shown to occur in experimental rodent and frog models204,31”. If such secondary epileptogenesis resulted from clinical seizure foci, then there would be reason to consider early surgical removal of seizure foci to prevent their ‘spread’. Goldensohn’“’ has argued that evidence for kindling or mirror foci resulting from clinical epileptogenic lesions is scanty, and that management of epilepsy should not be based upon the animal model of the mirror focus. This is not to say that human brain cannot be kindled; only that clinical epileptic discharges probably are not of an optimal nature to invoke the kindling phenomenon. 4.5. Brain slices 4.5.1. Rodent in vitro hippocanapal slices The pioneering investigations of neurophysiological mechanisms of the epilepsies were performed primarily in anesthetized cat and rodent models of epilepsy222.236. Work in these models is technically difficult, because of confounding effects of anesthesia, cardiorespiratory pulsations, presence of dura, subarachnoid membranes and the blood-brain barrier, and relative inaccessibility of neurons to recording probes. In the last 15 years, investigators have increasingly turned to in vivo brain slice systems. These slices were first prepared for analysis of and later adapted for the cerebral metabolismnr3, study of physiology. 329. Studies of hippocampal slices
in detail.
Brain slices have the advan-
tages of mechanical stability (which greatly facilitates intracellular recording), absence of a bloodbrain barrier for applied drugs, and absence of anesthetics. Disadvantages include uncertainty about presence or absence of relevant circuitry for a phenomenon under study, and partial mechanical injury and hypoxia of tissue. Despite these disadvantages,
brain slices can exhibit an impressive
range
of neuronal behaviors, including synaptic plasticity’, 26’ and epileptiform bursting328. To prepare hippocampal slices, a rodent (rat, mouse, rabbit or guinea pig} is decapitated, the brain is removed and the hippocampus rapidly dissected free. Slices of about 0.5 mm thickness are made with a dropped-razor tissue cutter or a vibrotome. Cuts approximately perpendicular to the long axis of the hippocampus preserve a three-neuron synaptic circuit and associated recurrent circuitry”. After cutting, slices are then incubated in a holding chamber in which they can be kept moist and oxygenated. Slices will remain healthy for more than 18 h when properly handled. Recording is done in a special chamber, which either oxygenates the slice by mist or by submersion in liquid artificial cerebrospinal fluid. Intracellular recordings from pyramidal neurons in the slice document relative preservation of passive and active membrane properties and of synaptic interactions2s6.257. Since brain slices do not move, epileptiform activity is a strictly electrical phenomenon. The usual index of epilepsy in the hippocampal slice is development either of spontaneous or shock-evoked repetitive firing of neurons. An example of epileptiform firing in hippocampus exposed to penicillin f1.7 mM) is shown in Fig. 5. In the control state. a shock to the afferents of region CA1 pyramidal neurons produces a single population discharge (Fig. 33, top), reflecting nearly simultaneous discharge of a few hundred pyramidal neurons”‘. After addition of a convulsant drug to the slice medium - in this case, penicillin - the same single shock produces longer bursts of neuronal firing (Fig. 5B, bottom). Intracellular recordings
257 show a large, long depolarization the paroxysmal
depolarization
(Fig. 5A) similar to shift seen in anesthe-
tized, epileptiform cat cortex3**. Spontaneous repetitive firing of hippocampal neurons occurs after addition of penicillin to slice perfusing medium, and
the epilepsies 15*. Ionic and electrophysiologic anisms
of the epileptiform
paroxysmal
tion shift have been elucidated
mech-
depolariza-
in the slice1’5~222~236.
The concept of pacemakers driving apparently synchronous discharges has been analyzed in region CA3 and CA1 of the hippocampal slice259. Recur-
is paced from region CA2-3259. Very recently, hippocampal slice models of electrographic tonicclonic seizure-like (‘ictaform’) events have been
rent excitatory circuitry in region CA3 has been identified as a major contributor to the pacemaker
developed, using low-magnesium baths**‘. Data from hippocampal slice experiments have been exported to a computerized model of this
properties of region CA3293. The role of disinhibition in seizure origin has been studied in hippocampal slice<‘. Slices have given important informa-
region
tion on the nature of the interictal-ictal
of brain,
developed
by Traub
The model has made several regarding conditions sufficient
and Wong293.
elegant predictions to induce synchro-
nous bursting in synaptically related networks of neurons. The slice model is also very useful for screening actions of putative anticonvulsant drugs*l’, although there is not necessarily a correlation with efficacy or toxicity in the whole animal. The hippocampal slice has been one of the most useful models for the study of basic mechanisms of
A
PEN 1.7 mM
IIIIII
B -
1 PENICILLIN
I
Y
-10
msec
Fig. 5. Epileptiform bursting in rat in vitro hippocampal slice. A: an intracellular recording from a CA1 region pyramidal neuron. In the control condition a shock to afferent Shaffer collateral fibers (50 V, 50 ,w) produced an excitatory postsynaptic potential (EPSP). Overlayed is a recording to the same stimulus after addition of penicillin G, 1.7 mM, to the slice perfusate. The response changed to a paroxysmal depolarizing shift (PDS) with 3 action potentials. B: extracellular recording from CA1 hippocampal pyramidal cell layer showing evoked population fields in response to an electrical shock of afferent fibers. In control perfusate neurons discharged synchronously only once, producing one field response. After addition of penicillin G, 1.7 mM, to the bath the same stimulus produced 5 population responses. A and B were recorded from separate slices. Stimulus artifacts were deleted. Data were obtained by Aryanpur and Fisher (1989).
transition’**.
Plastic properties of the brain, such as long-term potentiation24 have successfully been modelled and studied in hippocampal slices9,261. The slice system has been very useful to neuropharmacologists, since the blood-brain barrier is circumvented217. This has permitted examination of mechanisms of convulsant*r 9” and anticonvulsant6,38 drugs. 4.5.2. Isolated cell preparations. A recent trend. among neurophysiologists has been a move away from brain slices to isolated neuronal preparations, prepared either by growing cells in tissue culture4* or by mechanical and enzymatic dissociation of brain slices onto Petri dishes13’. These preparations permit direct visualization and impalement of single neurons. Glass pipettes may be opposed directly to membrane in order to record currents through membrane in response to voltage, ionic or chemical changes’58. This so-called ‘patch-clamp’ technique is extremely powerful in documentation of membrane channel properties. Through this technique neurophysiologists have explored voltage-sensitive calcium and potassium channels, membrane responses to neurotransmitters and basic mechanisms of antiepileptic drug action. Neuronal monolayers will form synapses, and it is therefore possible to study cell-cell communication in tissue culture; however, there is no assurance that the pharmacology of the circuitry resembles that of the native parent tissue. In general, isolated cell preparations are best suited for study of fundamental properties of membranes and synapses. 4.5.3. Human neurosurgical tissue. During neurosurgery to treat epilepsy, epileptogenic tissue becomes available for neurohistologic’6, neurophysiologic258 and neuropharmacologic10’~301 study. This approach surpasses those of models, in that it
258 explores the clinical disorder of epilepsy, rather than an animal model. Unfortunately, study of neurosurgical material also has its drawbacks; in particular tissue may be damaged by intra-operative trauma, hemorrhage and anoxia-ischemia2h6. There is also the substantial problem of controls: if a measurement is made on the tissue, to what should it be compared? Ethical constraints rule out ablation of normal surgical
brain, field.
except Normal
when tissue
unavoidable in the can be obtained at
EEG activity of Papio papio has been reviewed by Naquet and Meldrum 2w Visual information travels to occipital regions, where an occipital driving response follows the frequency of the light flashes. Nonetheless, the epileptological process shows a predominantly frontal topography, similar to distribution of epileptiform discharges in clinical absence epilepsy. Bilateral fronto-central sharp waves may be found with the baboons in a resting state or in light
sleep.
ILS produces
spikes
or 3/s bilateral
autopsy, but here postmortem changes in chemistry, structure and certainly in physiology complicate
spike-waves. Depth electroencephalography shows spread of spike activity from cortex to deep struc-
interpretation.
tures, including thalamus (especially centrum medianum), pons and reticular formation. If a seizure progresses to a tonic-clonic stage there are variable frontal rhythmical rapid spike discharges, then synchronous bursts of polyspikes and slow waves with the clonic stage. Postictally, EEG voltage is depressed. This EEG sequence is similar to that seen in human tonic-clonic epilepsy. Photosensitive seizures in Pupio pupio are inhibited by drugs useful against clinical tonic-clonic and myoclonic epilepsy, including benzodiazepines, barbiturates and valproic acid. Less favorable therapeutic effects are found with phenytoin, carbamazepine and trimethadione”‘. 5.1.2. Audiogenic seizures in mice. The breeding of inbred homogeneous strains of mice with particular phenotypes has become a service industry at Jackson Laboratories, Bar Harbor, MN, where catalogs offer dozens of strains of mice with seizure disorders. Noebels213 has listed 12 mouse strains
5. GENERALIZED
5.1.
TONIC-CLONIC
SEIZURES
Genetic
There are no good animal models for primary generalized, spontaneously recurrent tonic-clonic (grand mal) seizures. Because idiopathic grand ma1 epilepsy shows a genetic component”“, investigators have attempted to develop models from genetically aberrant strains of animals; including baboons, beagles, Mongolian gerbils, mice, rats and fruit-flies. Each of these models has distinctions from clinical grand mal, either in the requirement for certain types of precipitating stimuli, or other associated non-epileptic deficits. 5.1.1. Photosensitive baboons. The model of primary generalized epilepsy closest to that seen in man is photosensitive epilepsy in the Senegalese baboon, Papio pupio. This model is, unfortunately, very expensive and difficult to maintain. In 1966 Killam and associates13y,‘4” discovered that Senegalese baboons could suffer generalized tonic-clonic seizures in response to intermittent light stimulation (ILS) at frequencies close to 2.5 flashes per second. The Papio pupio model has been reviewed in detail by Naquet and Meldrum2”‘. ILS first produces eyelid, then face and body clonus. Tonic spasms or full tonic-clonic convulsions may then ensue and persist beyond termination of the light flash, Seizure threshold of the baboons can fluctuate week-to-week”“‘. Rarely, animals may show apparently spontaneous seizures. In distinction to this pattern, patients with primary generalized tonic-clonic seizures usually evidence apparently spontaneous seizures, and are photoprecipitated only in a minority of case?“‘.
with spontaneous seizures and 5 with seizures evoked by sensory stimuli. each due to a single locus mutation. Particularly well-studied strains include the DBAI2J mice with audiogenic seizures, the totterer and the El mouse. The most popular rodent model of generalized epilepsy is the mouse with sound-induced seizures. Several strains of mice have been bred for a propensity for audiogenic seizures4”, including the DBAI2J and the SJLiJ strains. DBA/2J mice almost universally exhibit severe sound-induced seizures between age 2 and 4 weeks, after which susceptibility gradually declines26’. At 8 weeks of age, when DBAIU mice are free from audiogenic seizures, they still show a low threshold to seizures induced by maximal electroshock or excitatory amino acidsh2.
The SJL/J strain, seizures,
in contrast,
develops
less severe
only after a few days of ‘priming’ exposure
to sounds”. The nature of the inducing noise is not critical, but should be loud, sudden and contain frequency components in the 12-16 kHz range. A susceptible mouse will first startle, then begin to run and leap. In some cases progression will continue to clonic jerks, and prolonged tonic spasms. Repetitive seizures can be fatal to the DBA/2J mouse. Good EEG recordings are scarce in this model because of the startle and running phases. can be prevented by phenytoin or valproic
Audiogenic seizures or phenobarbital’s4
acid2”‘.
The audiogenic
mouse seizure model has no direct
counterpart in clinical epilepsy. Nevertheless, the true utility of the model lies in its ability to permit analysis of genetic factors leading to seizure-like events. It has recently been observed that seizuresusceptible strains of mice are deficient in a calciumdependent ATPase 26s. This type of observation may have relevance for our understanding of basic mechanisms of seizure susceptibility. 5.1.3. Totterer mice and other seizure-prone mouse strains. Hereditary mouse models of primary generalized epilepsy can be classified under models for generalized tonic-clonic seizures or for absence seizures. The totterer mouse (tg/tg strain) was first studied because of its hereditary ataxia1”3; however, it was noted to also suffer myoclonus, frequent partial and absence seizures’34.216, with accompanying 6-7/s spike-wave EEG changes214. Transmission to progeny is recessive. Seizures appear at about 4 weeks of age. They begin with bilateral hind leg spasms, followed by unilateral, clonic jerking. The clonus may spread, but it does not generalize. Variant strains of the totterer mice may present various different phenotypic seizure characteristics. The tg/tgla variant strain, for example, seizes for prolonged periods265. Totterer mouse strains provide fascinating models for study of neurotransmitters and epilepsy. Noradrenergic innervation from the locus coeruleus to the limbic forebrain is crucial for development of seizures and ataxia’56. Totterers have increased numbers of noradrenergic terminals and elevated norepinephrine levels in hippocampus, cerebellum and locus coeruleus’56. Lesions of noradrenergic terminals by the selective neurotoxin 6-hydroxydopamine
partially protects animals against seizures, myoclonus214.
Confirmation
noradrenergic
axons from the coeruleus
ataxia and
of the role of increased comes from
study of another C57BU6J mutant called the quaking mouse, with myelin deficits, tremor and spontaneous or stimulus-evoked tonic-clonic seizures’79. How these findings relate to a suspected inhibitory role for norepinephrine in several regions of brain2@‘, remains to be determined. The ‘epilepsy’ (abbreviated ‘El’) mouse was first discovered by Imaizumi and colleagues in 1959125. Seizures are best induced as tossing or spinning are not effective.
by vestibular
stimuli,
the mice. Audiogenic
Manifestations
of seizures
such
stimuli in this
strain may include limb and face automatisms such as chewing and salivating279. Electrical discharges originate in deep limbic structures279. These features are analogous to clinical complex partial epilepsy. Like human complex partial epilepsy, seizures may generalize to tonic-clonic activity. Heritability of the vestibulogenic seizure tendency in El is dominant, but the gene locus or loci and neurochemical defects are unknown. 5.1.4. Genetically epilepsy-prone rats. To electrophysiologists and experimental psychologists, the rat is more familiar than the mouse as a laboratory animal. Fortunately, rats also exhibit seizure discharges. Jobe, Dailey, Laird and colleagues at the University of Illinois College of Medicine have studied extensively a strain of genetically epilepsyprone rats (GEPR), susceptible to seizures induced by sound, hyperthermia and various electrical or chemical stimuli247. A GEPR-9 colony is severely seizure-prone and a GEPR3 colony only moderately seizure-prone, allowing for controlled comparisons. Animals from both colonies run about wildly, and fall from clonic jerks when stimulated by sounds, but GEPR-9 rats also suffer tonic extensor convulsions. As with other rodent reflex epilepsy models, seizures are most prominent at a certain stage of development, with increasing seizure tendencies appearing over the period 2-4 weeks after birth12”. The standard clinical anticonvulsants exhibit efficacy in blocking audiogenic seizures in GEPRs~~. The seizure prone character of these rats relates in some way to abnormalities in the auditory pathways, including impaired cochlear function226. Injection of amino acid antagonists into the inferior
260 colliculus is highly effective in blocking audiogenic seizures in GEPRs~~, presumably through a block of afferent
impulses.
Efferent
block can be produced
sence seizures,
and phenytoin,
valproate
and phe-
nobarbital inhibit tonic seizures in the SER rat2’3. 5.1.5. Mongolian gerbil. Another rodent studied
by injecting amino acid antagonists into substantia nigra and brainstem reticular formation”‘.
for its seizure tendency is the Mongolian Gerbils exhibit generalized tonic-clonic
Neurotransmitter alterations have been examined in the GEPR148. Most impressive have been alterations in amines. Deficits of norepinephrine con-
when handled or agitated. This preparation has been used to screen anticonvulsants’8. Unfortunately, as reviewed by Majkowski and Kaplan”‘, the utility of
tent4’ and turnover13’
the gerbil model is limited by variability of seizure threshold and a progressively longer refractory pe-
have been identified
in brain-
stem, telencephalon and cerebellum of both GEPR9 and GEPR-3 rats, and binding sites for prazosin, a selective al-adrenergic ligand, are significantly decreased in frontal cortex of GEPRs versus normal Sprague-Dawley rats. Decreased content or abnormal functional systems for other neurotransmitters may also play a role in the GEPR’s seizures. Abnormalities so far identified have included decreased brain serotonin”‘, possibly increased cholinergic content in thalamus and striatum14”, decreased responsiveness of inferior colliculus neurons to applied GABA”“, and lower threshold for ‘wetdog shakes’ from intraventricular morphinez4s. A specific abnormality of excitatory amino acid-mediated neurotransmission has not been reported; however, audiogenic seizures similar to those in the GEPR may be produced in normal rats by infusing N-methyl-o-aspartate into inferior cohicuhis’yx. Of these neurotransmitter abnormalities, it is uncertain which are contributors to the seizures and which are consequences of the seizures - a problem common to analysis of neurotransmitters in clinical epilepsy. One way to clarify this issue with animal models is to examine neurotransmitters in the young (4day-old) GEPR prior to onset of seizures. Using this approach, Roberts and Ribak251 have shown a significant increase in glutamic acid decarboxylasepositive neurons in inferior colliculus of the young GEPR, suggesting that disinhibition may play a role in seizure susceptibility. A strain of rats has been described recently, with apparently spontaneous absence-like and tonic seizures. These rats are progeny of cross-matings between the so-called ‘tremor’ homozygous rat and the ‘zitter’ homozygous rat263. This spontaneously epileptic rat (SER) exhibits 5-7/s spike-wave-like complexes in cortex and hippocampus during periods of abnormal immobility. Ethosuximide, trimethadione, valproate and phenobarbital inhibit ab-
gerbi1287. seizures
riod for seizures. 5.1.6. Drosophila shakers. Formal genetic analysis has long been performed upon Drosophila melanogaster (the fruit-fly). Recently, this field has become of interest to epileptologists. A mutant fly, named ‘Shaker’, has been observed to shake and flutter when exposed to ether28”. Intracellular recordings from the flight muscles indicate deficits of the so-called ‘A-current’. This current is carried by potassium ions and used to repolarize membrane after excitation-induced depolarization. Its absence would be expected to result in prolonged firing of excitable tissue. The full power of molecular genetics is now being applied to the analysis of the Shaker”3, 220~28y,giving an opportunity to link a seizure-like behavior with a characterized physiologic and genetic deficit. Whether such a genetic abnormality of potassium channels occurs in humans is unknown. 5.1.7. Miscellaneous genetically seizure-prone animals. Several other less-studied genetic models of epilepsy have been reviewed by Seyfried and Glase?, Maxson et al.‘“” and Loscher and Schmidtn’“. Studies have been performed on photosensitive epileptic chickens43, rabbits163, hamsters330 and dogs’14. These genetic model systems are not like human epilepsy, but they are contributory to our understanding of how a gene mutation can lead to seizures*‘“. It is important to note that a ‘single mutation (e.g. totterer) can lead to seizures, and that such seizures may be accompanied by a variety of neurological abnormalities. In other models (e.g. GEPR) the genetic deficit is associated with complex and multifactorial neurotransmitter abnormalities. 5.2. Maximal electroshock Maximal electroshock (MES) is arguably the best-studied and most useful animal model of seizures. Since 1870 it has been known that electrical
261 shocks
to animals
Spiegel
quantified
zures
can produce
seizuress3.
the technique
by skull-shocks
In 1937
of producing
to animals274.
Merritt
constant-current
research are listed in Table IV. Chemical convulsants are convenient for screening of putative anticonvulsants281,283.
or constant-voltage
delivery systems. Typical stimulation parameters in drug-screening protocols233 are 50 mA in mice and 150 mA in rats, 60 Hertz delivered electrodes for 0.2 s. Several variations
systemically.
and
techniques of the MES animal mode1282. Electricity may be applied via ear-clip or cornea1 electrodes, either
seizures when administered
A few of those
PutnamlY4 applied MES screening to series of barbiturate derivatives, resulting in the development of phenytoin. Swinyard has reviewed in detail the modern
using
generalized
sei-
via cornea1 exist among
of greatest
interest
for epilepsy
5.3.1. Pentylenetetrazol. The prototype 276,283in the class of systemic convulsants enetetrazol derivative*” mice, rats, parenteral
(PTZ,
metrazol).
PTZ
agent**l, is pentyl-
is a tetrazol
with consistent convulsive actions in cats and primates, when given by the route.
PTZ initially
produces
myoclonic
jerks, which then become sustained, and may lead to a generalized tonic-clonic seizure. EEGs show spikewaves or polyspikes124. An example of an EEG from
different laboratories. Tonic-seizures may be produced in mice with ear-clip electrodes delivering 50/s alternating square-wave pulses at currents of approximately 15 mA for 1 s146. A distinction is made between minimal and maximal seizures282. Minimal seizures are characterized by a ‘stun reaction’ and clonic movements of the face and forelimbs. A minimal electroshock seizure threshold is sometimes defined by a stimulus that produces 5 s of sustained clonus. Maximal seizures show tonic hind-limb extension and flexion, followed by clonus. A MES seizure meets criteria if there is tonic hind-limb extension. Studies may choose to evaluate either minimal or maximal electroshock seizures. Furthermore, they may measure the threshold for inducing seizures in 50% or 97% of the animals, or they may count the percent of animals exhibiting MES at a shock current 4-5 times higher than the threshold current. Statistical methods, related to probit analysis, have been used to compare MES thresholds between control and test groups159. Drugs able to inhibit MES seizures in mice and rats are considered to be candidate therapies for primary and secondarily generalized tonic-clonic epilepsies233,281 (see Table IV). Phenytoin remains one of the most effective drugs at inhibition of MES seizures. Carbamazepine is also effective; ethosuximide is not. Valproate and benzodiazepines can inhibit MES seizures, but at poor therapeutic/toxic ratios.
a mouse given PTZ 85 mg/kg i.p. is shown in Fig. 6146. Drug screening protocols generally set 5 s of continuous clonus as a threshold for scoring presence of a clonic seizure, even though this criterium can be imprecise. With i.v. administration of PTZ 1% solution to mice, the threshold dose for clonic seizures is about 50 mg/kg and for tonic-clonic seizures 90 mg/kglm. Subcutaneous administration of PTZ results in clonic seizures in 97% of animals
5.3. Systemic convulsants Numerous chemical compounds
N-methyl-or-asp
may
produce
TABLE IV Systemicchemical convulsants Drug
Species
Dosage @g/kg)
Ref.
FTZ
Mouse
85 S.C. 50-75 i.p. 50 i.v. 70 S.C. 40i.p. 20i.v. 600,OOOi.m. 300,OOOi.m. 21 i.p. 3.2s.~. 2i.v. 2.7 S.C. 0.3 i.v. 1.5 S.C. 200 i.p.
283 277 160 283 177 303 67 79 108 233 107 233 189 20 292 112 190 52
Rat
Strychnine MS0
Cat Rat Cat Rat Mouse Dog Mouse Monkey Mouse Rat
Allylglycine Flurothyl
Baboon Mouse
Homocysteine
Mouse Rat Mouse Mouse
Penicillin Bemegride Picrotoxin Bicuculline
550 i.v. 1.5ml total inhaled 875 i.p. 1000 i.p. 34os.c. 105 i.v.
82 7 47 47
262 tested
(CD97)
with dosages
of 85 mg/kg in mouse
and 70 mglkg in rat. The mechanism of action of PTZ is only partially understood. Early studies suggested it exerted effects primarily on cortex4.23; however, later workers3’j3 argued that mesencephalic non-specific reticular formation neurons were activated by systemMirski and ic PTZ before cortical neurons. Ferrendelli2’X’~201 recently role of the mammillary mammillothalamic zures in mouse vinyl-GABA, into reticular
have shown the important
bodies, anterior
thalami and
tracts in mediation of PTZ seiand guinea pig. Injection of y-
an inhibitor of GABA degradation. formation, anterior media1 hypothala-
mus and caudal hypothalamus blocks PTZ-induced seizures in rats 199. In cat, however, the clonic phase of seizures appears mediated by forebrain”“. At a synaptic level PTZ appears to interact with the {GABA receptor benzodiazepine chloride ionophore} complex*“, presumably in some way decreasing the potency of inhibition and leading to seizures. Wilson and Escueta”” suggested that PTZ blocked GABA-mediated inhibition. Pharmacologic antagonists to benzodiazepines do not appear able to block seizures from PTZ36, so other mechanisms for PTZ-induced seizures must also be important. In region CA3 of guinea pig hippocampal slice (see above for discussion of the hippocampal slice mode1 system) PTZ 2-10 mM results in alternating depolarizing and hyperpolarizing neuronal bursts”‘.
BASELINE
PTZ 85 mg/kg
Fig. 6. Seizure induced in a mouse by S.C. injection of pentylenetetrazol (PTZ, 85 mg/kg). Recordings were obtained from needle electrodes in the scalp, configured as shown in the diagram to the right. Baseline EEG is shown on the left. The middle panel shows two segments of EEG, separated in time by 45 s, after PTZ injection. PTZ resulted in recurrent runs of rhythmical sharp activity with shifting phase reversals. During the times of EEG discharge the mouse exhibited twitching of vibrissae and mild clonus of the front limbs. Tracings have been darkened by hand for purposes of reproduction. Data were obtained by Krauss and Fisher (1989).
TABLE Inhibition
V
ofMES
and PTZ seizures in mouse
PTZ is administered s.c., and test drug i.p. The dosage is the ED50%, i.e. the dosage in mg/kg needed to inhibit seizures in 50% of the tested animals. M-MES, mouse maximal electroshock seizure; R-MES, rat maximal electroshock seizure; M-PTZ, mouse pentylenetetrazol seizure; R-PTZ, rat pentylenetetrazol seizure. Anticonvulsant
M-MES
R-MES
M-PTZ
R-PTZ
Phenytoin Carbamazepine Phenobarbital Valproic acid Ethosuximide Diazepam
9.5 8.8 21.8 272 N.E. 19.1
14 4 12 100 N.E. 5
N.E. N.E. 13 149 130 0.2
N.E. N.E. 7 74 54” 0.3
N.E., not effective;
a given p.0.
These bursts are ‘paced’ more by calcium currents than by GABA-mediated inhibition. PTZ was adopted as a screening test for anticonvulsants in part in response to the discovery that the anti-absence drug, ethosuximide, failed to alter maxima1 electroshock seizure thresholds. In contrast, some drugs effective against MES seizures, such as phenytoin and carbamazepine, are ineffective against PTZ seizures233. Common practice among drug companies testing prototype anticonvulsants has been to presume that drugs effective against PTZ seizures would be good anti-absence (anti-petit mal) therapies, and drugs active against MES would be good anti tonic-clonic (anti-grand mal) therapie@‘. This assumption has been called into question’6’, and may be an excessive simplification. Nevertheless, PTZ does appear to have a preferentially subcortical locus of action, and may identify anticonvulsants not marked by screening against MES. Table V lists inhibitory mg/kg dosages of several important clinical anticonvulsants against MES and PTZ seizures”7,‘6”.233,?97. 5.3.2. Systemic penicillin as a tonic-clonic model. Penicillin was discussed above as an agent able to produce acute focal seizures, when placed on cortex. Clinical experience has indicated that high systemic doses of penicillin in humans can produce myoclonus, generalized tonic-clonic seizures and encepha10pathy8’. In the hospital setting encephalopathy occurs most commonly with i.v. dosages above 20 million units per day, especially if concurrent renal
263
failure brain
maintains barrierz6.
high levels and alters the bloodPrince
and Farre11237 showed
that
parenteral penicillin could produce generalized seizures in cats. Farriello67 later extended the model to rats. The model was elaborated by Testa and Gloor;?s6, Fisher and Prince79s0, Quesney and associates243 and then studied extensively by Gloor, Quesney, Avoli and colleagues at Montreal’*. Parenteral penicillin in the feline serves as a model for several different types of seizures, including absence seizures, myoclonic seizures and absence seizures. The animal tends to pass through each of
against picrotoxin,
but not bicuculline.
ineffective
both types of seizure;
against
Phenytoin
is
whereas,
diazepam is highly effective against both types. Phenobarbital has intermediate effectiveness, and the anti-absence agents, valproate and ethosuximide, have low but detectable effectiveness. Several antagonists of GABA synthesis have been available for decades, and still find occasional use as inducers
of seizure activity. These agents include thiosemicarbazide324 and methionine allylglycine’, sulfoximine89. Meldrum et a1.19’ used allylglycine to
these stages after a dose of at least 300,000 units per kilogram of i.m. penicillin. Since the model is
generate prolonged status epilepticus in monkeys, in a classic study of the effect of status on brain histology. Allylglycine’21 and thiosemicarbazide are
probably most useful as an absence model, discussion is deferred to that section. 5.3.3. Other inhibitory antagonists. Other popular systemic convulsants include bemegride, picrotoxin, bicuculline, methionine sulfoximine, allylglycine, strychnine and certain general anesthetics. Each of these has slightly different actions; however, the choice of one model over another usually depends primarily upon familiarity to the investigator. Bemegride (Megimide) is a glutarimide derivative similar in action to PTZ’07*252. It has been used to produce clonic or tonic-clonic seizures, or to activate focal epilepsy278. Several of the drugs used to produce partial seizures when focally applied, for example picrotoxin and bicuculline, will produce generalized clonic and tonic-clonic seizures when given systemically. Cellular mechanisms of these convulsive drugs are incompletely understood. As noted above, both are GABA antagonists, and will block physiologic inhibitory postsynaptic potentials88,118. Picrotoxin and bicuculline do not antagonize GABA by the same mechanism: bicuculline’s antagonism is pharmacologically competitive, but picrotoxin’s is not17, possibly because picrotoxin interacts with the chloride ionophore of the GABA receptor complex, rather than with the GABA binding site. Brain metabolism early in bicucullineinduced generalized tonic-clonic seizures is greatest in neocortex and synaptically linked regions, as opposed to brainstem59y60. Clinically useful anticonvulsants tend to have parallel effectiveness against picrotoxin- and bicuculline-induced seizures233, with the unexplained exception of carbamazepine, which has some benefit
believed to inhibit glutamic acid decarboxylase, the synthetic enzyme for GABA. Seizures induced with allylglycine reduce immunoreactivity for GABA in hippocampus and cerebellum’91. Actions of methionine sulfoximine (MSO) are complex. Onset of seizures may be delayed hours to days, depending upon the species89,‘86 and the dosage241,278. MSOappears to alter synthesis of amino acids in the tricarboxylic acid cycle300, perhaps altering balance between GABA and glutamate content. A potent generalized seizure model can be produced by i.v. injection of strychnine167. Strychnine interacts with GABA-benzodiazepine receptors28, but a more important action of strychnine is probably against glycine45. Glycine is an important inhibitory neurotransmitter in brainstem and spinal cord240 with structural homology to the much larger strychnine molecule”. Strychnine serves as a noncompetitive inhibitor of the glycine receptor’76. Resulting seizures differ in character from those produced by primary GABA antagonists63 in that they are mainly extensor tonic, with little cortical EEG seizure activity 278. These seizures are not fully relieved by reasonable doses of any of the commonly used anticonvulsants233, including benzodiazepines46. Certain inhalation anesthetics are known to be convulsant in man 72. Flurothyl is a hexafluorinated non-flammable ether which is a useful experimental convulsant in mice22. Advantages of flurothyl include ease of testing, a clear endpoint of a clonictonic seizure with loss of righting reflex, and a threshold independent of body weight5*. A typical paradigm involves injection of 1.5 ml of flurothyl
264 into a 2-gallon
sealed
and stirred
jar containing
a
mouse. The dependent variable is latency to clonictonic seizure after injection of the anesthetic. Some convulsants apparently act by mimicry of excitatory neurotransmission. It is often difficult to induce seizures by systemic administration of glutamateZ5’, penetrate
although
monosodium
glutamate
can
to
brain and produce convulsions in rats13’. More potent or more blood-brain
a progressively
higher
incidence
of clonic or tonic
seizures. Protection against NMDA-induced seizures and lethality is provided by valproate, diazepam and specific NMDA bital, phenytoin
antagonists, but not by phenobaror ethosuximide47~155.
5.4. ~et~hoZic dera~geme~f~ In clinical practice many metabolic derangements can lead to seizures including hypoxia, hypoglyce-
lo-day-old barrier-permeable analogues of glutamate are usually used. One such agent, kainic acid, was discussed
mia, uremia, drug withdrawal and high temperature74,211. These conditions have not in general been
above in models of complex partial seizures,
useful
since it
has a predilection for hmbic structures. Homocysteine thiolactone has been used as a model of generalized convulsions in rodents109.275. Homocysteine is a sulfur-containing amino acid, structurally similar to glutamine, and also to the glutamate analog, homocysteic acid*‘. Humans with deficiencies of cystathione /3-synthetase will accumulate homocysteine and suffer from seizure?‘. Mites’ or rats’ given approximately 1000 mg/kg homocysteine thiolactone i.p. induce clonic and whole-body clonictonic convulsions, with latencies ranging from 10 to 60 min. Repetitive seizures, status epilepticus and death are all common with this model. Mechanism of epileptogenicity with this agent is complex. Extracellular administration of homocysteine by pressure to cortical neurons excites the neurons, in concentration similar to those required by glutamate32s. Homocysteine may, however, impinge upon multiple neurotransmitter systems, at several sites in the brain’. Glutamate receptors have been categorized by their sensitivity to the non-endogenous physiological probes, NMDA, quisqualate and kainat?. Evidence suggests that quisqualate and kainate receptors are involved in conventional synaptic transmission; whereas NMDA receptors are implicated in potentiated synaptic processes, including seizures. Administration of 340 mgikg racemic N-methylaspartate i.p. or 105 mgikg i.v. to mice produces clonic convulsions in mice4’. The dextro form of racemic N-methyl-aspartate is the biologically active form, such that 200 mg/kg of NMDA leads to lethal status epilepticus in 100% of treated mice within 10 min’““. Behavioral effects of NMDA are profound”“. Doses of 100 mgikg i.p. in mice induce scratching and tail-biting, and higher doses produce
for studying
mechanisms
of the
epiIepsies
because they usually produce other CNS disturbances peculiar to the model employed. Three metabolic disturbances have, however, been employed in a few studies of generalized seizures: hyperbaric oxygen323, hypercarbia3” and drug withdrawalti.“. 6. MODELS
FOR
GENERALIZED
ABSENCE
SEI-
ZURES
Clinical absence is characterized by arrest of ongoing activity, partial or full decrease of awareness and concurrent 3-4/s spike-waves in the EEG”. 2’2. No animal model mimics this condition precisely, but 5 have provided useful approximations: thalamic stimulation, bilateral cortical foci, systemic penicillin, ~-hydroxybutyrate, and genetic rodent models. 6.1. Thalamic stimulation Classic studies by Morison and Dempsey203 defined the concept of a thalamic reticular formation, able to influence wide areas of cortex. Jasper and Droogleever-Fortuyn”’ and Hunter and Jasper’23 showed that stimulation of midline and intralaminar thalamus could produce absence and EEG spikewaves. The behavioral and EEG abnormalities did not long outlast the stimulation, and occurred only at certain states of arousal. Recent evidence in patients suggests that certain thalamic stimulation parameters may actually be anticonvulsant3”4. Pollen and associates’” I .232investigated the cellular basis for the spike-wave discharges in the thalamic stimulation model. EEG spikes were associated with depolarizing shifts intraceliularly and synchronous cell firing. Waves appeared linked to inhibitory neuronal processes.
265
The role of thalamus
versus cortex in the gener-
of course
that
clinical
correlates of EEG spike-wave (SW) discharges?; and (2) what is the role of cortex versus subcortical structures in generation of SW bursts?
6.2. Bilateral cortical foci
Intracellular studies of neurons during the spike of SW feline penicillin epilepsy are in agreement about
dysfunction”‘. Applisuch as estrogen or
pentylenetetrazol”” or penicillin79.94 to widespread regions of cat cortex produces bursts of 2.5-3/s spike-wave discharges synchronous to within 15 ms. A similar model can be produced in rhesus monkeys I’3 These studies were all performed in anesthetized animals, so behavior could not be analyzed. Marcus and associates”” therefore developed a bilateral cobalt implantation model in rhesus monkeys, with chronic and behavioral features. Cobalt powder, 60 mg, was impregnated into a 1 x 1 cm gelfoam pledget, and placed bilaterally on premotor cortex. Bilateral EEG spike discharges could be recorded 80-120 min after placement of the cobalt. Subsequent EEG findings included wellformed 3/s bilaterally symmetric spike-wave discharges. By 2-3 h behavioral absence could be observed, with eyelid opening and upward movement of the eyes. A few animals had recurrent seizures for up to 72 h. This model is of considerable theoretical interest. Obviously, it is not suited for anticonvulsant screening procedures. For pragmatic reasons, a much more popular model of multifocal cortical epilepsy is the model produced by high systemic doses of penicillin. 6.3. Systemic penicillin Intramuscular injection of 300,000 units/kg of penicillin G into a cat285 results in recurrent episodes of arrested activity, staring, myoclonus, facial-oral twitching and occasional progression to generalized tonic-clonic seizures (see above). Seizure activity begins about 1 h after injection of drug and continues intermittently for 6-8 h. The EEG shows a variety of spike-wave morphologies79, emerging from a relatively normal background. These features are similar to those seen with clinical absence, except
The
feline
apparently
because of the need for chronic electrode implantation, and for ongoing stimulation during testing.
is a result of diffuse cortical cation of dilute convulsants
years.
recurs
ation of absence epilepsy remains a subject of great interest and controversyY2. The thalamic stimulation model for petit ma1 is, however, infrequently used
Models of petit ma1 produced by bilateral cortical foci derive from the hypothesis that absence epilepsy
for
absence
spontaneously
generalized
penicillin model has been particularly useful in the study of two questions: (1) what are the cellular
the correlates many cortical with action
of the EEG spike. During the spike neurons exhibit large depolarizations potential
generationgO.
Cellular
corre-
lates of the wave are less clear. Most cells appear inhibited during the wave, although occasional cortical units can be seen to fire rapidly during the wave79. These presumably are interneurons mediating inhibition. Possible mechanisms of inhibition during the wave include chloride-dependent GABAergic inhibition, and also non-GABA dependent processes such as calcium-dependent potassium efflux after cell excitation. It has not yet been possible to demonstrate that disinhibition by penicillin, shown for simple model systems (see above), leads to SWs in anesthetized cat cortex. Several studies have shown no major change in GABA-mediated inhibition in the feline penicillin mode151,9”,145. During the early phase of SW bursts from parenteral penicillin, cat neocortical neurons show normal responsiveness to applied GABA or glutamate144. These findings have implications for the discussion of GABA’s role in the epilepsies. Failure to show a clear change in GABAergic inhibition in this model does not rule out subtle, but important, decreases in the balance between excitation and inhibition, nor decreases in inhibition at distal dendritic branches. Clinical absence (petit mal) epilepsy has been hypothesized to originate subcortically, with participation of brainstem and thalamic reticular formation84.225. Absence seizures in the feline penicillin model have been difficult to reconcile with this hypothesis92. SW discharges appear in cortex before they appear in mesial thalamus or reticular formation 14,79.Application of penicillin to wide regions of cortex, but not to thalamus, can produce SW EEG discharges92. Discharges in this model probably originate cortically, but are maintained and elaborated by recurrent thalamo-cortical circuitry13%92. Response of feline generalized penicillin epilepsy
266 to anticonvulsant
drugs
appears
to parallel
the
clinical response in absence epilepsy: ethosuximide and valproate are more effective than phenytoin’os*224. Parenteral penicillin in rats can produce seizures, but the model
bears
little
resemblance
to clinical
absence. 6.4. y-~ydroxybutyrate y-Hydroxybutyrate (GHB) is a metabolite of GABA that can produce absence-like episodes in a variety of species 271. This model has been developed in detail by 0. Carter Sneed. Treatment of rats with loo-150 mglkg GHB produces rapid spiking and 4-6/s spike-waves in the EEG and absence-like behavior. Analogues of clinical absence can also be seen in monkeys2hy. The model shows a developmental*‘” and pharmacologic profile26x similar to petit ma1 epilepsy. This is a promising chemical model of absence, which may supplement the more familiar feline parenteral penicillin model. 6.5. Zntraventricular opiates Endogenous and synthetic opiates can induce a wide range of neurophysiologic and behavioral states in experimental animals25. Low-dose morphine sulfate is believed to be anticonvulsant, but high-dose parenteral morphine can induce clonic convulsions Opiate peptides do not appear to in rodents”‘. penetrate significantly into the CNS after systemic administration; however, Urea and associatesz9s demonstrated that methionine enkephalin produced seizures in rats when given intraventricularly. The endogenous opiate /3-endorphin, given intraventricularly, can also produce seizuresri7. These seizures are difficult to classify. Rodents exhibit arrest of activity, staring, rearing and a type of trembling given the descriptive appellation, ‘wet-dog shakes’. The EEG shows repetitive spikes followed by runs of slow waves in hippocampus and related structures, perhaps because these structures are periventricular, or possibly because of a specific susceptibility of the limbic system to opiates. The behavioral-EEG pattern after intraventricular opiates can be classified either with the complex partial”’ or the absence273 models of the epilepsies. Relation to petit mat epilepsy has been supported by ontogenetic studies of opiate-induced seizures273 and by the relatively specific responsiveness of these seizures to anti-petit
ma1 agents”*. A different view was taken by Caldecott-Hazard, Engel, Chugani and associates33 and Torteila and colleagues’% who argued that opiate-related
phenomena,
with
behavioral
arrest
and slow waves, were more analogous to the postictal state. The intraventricular opiate model is not experimentally convenient, since ventricles must be penetrated and consideration given to controls for nonspecific CNS trauma. The primary advantage of this model is in its elucidation of actual opiate-related mechanisms for seizures. Several lines of evidence suggest such mechanisms may be important. In animals, seizures generate a compound in the CSF that is anticonvulsant in other animals, and whose effects are antagonized by naloxone29’. Seizureprone GEPR-9 rats are much more susceptible to mo~bine-induced wet-dog shakes and clonic convulsions than are GEPR3 rats or controls248. In patients, the opiate antagonist naloxone can increase frequency of interictal spiking in patients with complex partial seizures202, and binding of radiolabeled opiate receptor hgands is increased in vivo in the region of unilateral temporal lobe seizure focis’. 6.6. THIP THIP (4,5,6,7-tetrahydroxyisoxazolo-4,5c-pyridine3-01) is a bicyclic partial GABA-A receptor agonist that inhibits benzodi~epine binding to the GABAbenzodiazepine receptor complex’22~33*, probably by blocking effects of endogenous GABA. High doses of THIP can also interact with choline& systems332. Intraperitoneal injection of THIP 5-10 mg/kg in rats repIi~bly produces bilaterally synchronous EEG spike-waves and absence-like behaviors enduring for several hour@‘. 6.7. Genetic rodent models of absence Up to 30% of Sprague-Dawley3’ and Wistar’75*30’ rats exhibit spontaneous seizures3’ accompanied by behavioral arrest, face twitching and 7-11/s EEG spike-wave discharges. This trait for spontaneous seizures has not been bred true, although a Wistar WAGiRij strain of rats from Glaxo Laboratories, U.K. has been shown to exhibit spontaneous absences and EEG spike-waves3y~“02.
267 zures. Seizures may be interrupted
7. ANIMAL MODELS OF STATUS EPILEPTICUS
Certain
research
questions
require
models
of
by administration
of phenytoin, phenobarbital or benzodiazepines soon after injection of homocysteine.
recurrent seizures or status epilepticus. Many of the chemical convulsants able to produce seizures - for example, kainic acid, NMDA, flurothyl, bicuculline
Several electrical paradigms have been suggested as a recipe for producing status epilepticus in rats. In one experimental series196 continuous sine wave 40
and
PA limbic electrical
pentylenetetrazol
-
can also produce
status
stimulation
produced
convulsive
epilepticus when administered in large quantities to rodents. Unless treated, these administrations are
status in 4 of 18 rats. Three additional demonstrated electrical status epilepticus
often rapidly lethal. One recently popularized
mini and Nadler306 pulse trains (0.3 ms monophasic
ticus is the lithium-pilocarpine
tonic-clonic model of status epilepmode132~‘32,205,312.In
this model rats are pretreated with lithium chloride, in doses approximating 3 mEq/kg i.p. At least 20 h later the cholinergic agent pilocarpine is given S.C. at 25-30 mg/kg. Generalized clonic or tonic-clonic seizure activity begins about 30 min after administration of pilocarpine, and continues for several hours. The EEG pattern displays a progression very similar to the stages seen in human status epilepticus312. Chronic pretreatment for one month with daily lithium reduces the convulsant threshold of of status epipilocarpine 26-fold 206. Development lepticus with the lithium-pilocarpine treatment can be inhibited by atropine, diazepam, valproate, phenytoin, carbamazepine, phenobarbital, and paraldehyde2”‘. Pilocarpine (20 mg/kg) can produce florid motor status epilepticus in the absence of pretreatment with lithium, provided rats have been partially kindled by amygdalar stimulation32. Another drug combination able to induce status epilepticus in animals is focal cobalt in conjunction with systemic homocysteine”‘. Homocysteine was discussed above as an agent able on its own to produce powerful tonic-clonic seizures. Walton and Treiman suggest the following protoco1313. Naive rats undergo implantation of powdered cobalt 25 mg on the dura via a burr hole in the skull. Animals recover for about 5 days, by which time EEG polyspikes or behavioral motor seizures are evident. Homocysteine thiolactone 5.5 mmol/kg is then given i.p. Latency from injection to the first seizure is about 30 min, to the second 8 min more. Tonic-clonic seizures then recur every 5-10 mitt, with an EEG-clinical evolution over up to 24 h similar to that seen in clinical status epilepticus294. This model also has the advantage of a ‘focus’ (presumably the site of prior cobalt implantation) for frequently recurring sei-
movements.
In a paradigm
animals without
by Vicedo-
square-wave pulses at 20 Hz, 10-s duration, 30-s inter-train intervals) were delivered to fimbria and adjusted to the amperage needed to give maximal synaptic response in CA3. Status epilepticus occurred in 85% of subjects within less than 7 h. Status can also result from stimulation of a kindled hippocampal focus lg4. These electrical models are more labor-intensive than the chemical models, but avoid introduction of exogenous pharmacology into the preparation. 8. CONCLUSION
This review has enumerated over 50 animal models for the epilepsies. What common lessons may be learned from these models? First, there is no one model suitable to all questions. Some circumstances, for example screening of putative anticonvulsant drugs, require low cost and convenience. Epilepsy is a heterogeneous disorder and epilepsy research a heterogeneous enterprise; one size model does not fit all. Models show mechanisms of models; not necessarily mechanisms of epilepsy. Because the EEG and behavioral picture of a model looks similar to a clinical seizure type does not mean the pathophysiology is the same. To illustrate with an extreme: sleep has much in common with epilepsy - it is a spontaneously recurring event associated with blunting of consciousness and EEG hypersynchrony, including EEG potentials that can look very similar to epileptiform sharp waves. But sleep is not epilepsy. Some drugs can produce many models of epilepsy. Penicillin, administered in various forms can imitate simple partial epilepsy, generalized myoclonic, generalized tonic-clonic and generalized absence epi-
268 lepsy. It is unclear what this tells us about the continuum of clinical epilepsy. For the experimenter, caution is required. Careful behavioral and
spikes or seizures. In these models there is opportunity to consider prophylaxis of epilepsy3i9. These
EEG
possible idiosyncratic effects of the irritating metal, not specifically analogous to clinical epilepsy.
model
is needed
to
discern what type of seizure disorder a particular experiment.
is modelled
in
The
analysis
review
of an animal
has not
attempted
to discuss
the
models are laborious,
expensive,
Maximal electroshock sant models, exemplified
and also subject to
and the chemical convulby systemic pentylenetet-
models in order of popularity. It is, however, a useful exercise to list the most widely used models, and the key questions about epilepsy that have been
razol, are best for screening possible anticonvulsant drugs 163 Anatomy of seizure origin and spread have been studied in these simple models’9,201. Such
addressed by each. The half-dozen most productive models arguable have been the acute penicillin focal
studies have suggested originate in neocortex,
model,
seizures
the chronic aluminum
hydroxide
model, the
systemic pentylenetetrazol generalized tonic-clonic model, the maximal electroshock tonic-clonic seizure model, the kindling model of complex partial and secondarily generalized seizures, and the hippocampal in vitro slice. Work on the acute penicillin focus in cat cortex has illustrated the cellular electrophysiology of focal of the epilepsy’7x, and the basic phenomenology paroxysmal depolarization shift235. Because penicillin initiates spikes and seizures from a known locality, the spread of seizures from that region and mechanisms to inhibit spread can be studied2”*. The penicillin focus is not useful for questions on pathology of epilepsy, since it has onset within minutes of drug application to cortex. It is one of several experimental and clinical examples of the principle that chronic neuropathologic changes (e.g. gliosis and cortical reorganization) are not a necessary condition for seizures. The aluminum hydroxide and related metal deposition models of chronically recurrent simple partial and secondarily generalized seizures have served as some of the most realistic laboratory models of partial epilepsy. The models have been useful for studying development of pathologic changes occurring with epilepsy 3’7. Issues of seizure spread, and also controversies about establishment of mirror seizure foci, have been explored with the aluminum model”‘. Firing patterns of neurons in and near a metal deposition focus have been analyzed, contributing to our understanding of EEG synchrony and mechanisms of interictal spike generation151. Metal deposition models should be ideal for study of largely unexplored questions about the latent period between cortical injury and emergence of EEG
originate
that electroshock seizures whereas pentylenetetrazol
in diencephalon.
Findings
about
the functional anatomy of the epilepsies will require confirmation in more realistic models, for example, photogenic seizures in Papio papio baboons, and ultimately in human patients. The kindling model of epilepsy has probably been more subject to study than any other model. Kindling studies are especially useful for investigations of changes that occur in brain over time. The large number of investigations into kindling24s,308 have begun to answer questions about the neurochemical changes that occur before and after establishment of an epileptiform area of the brain. Kindling avoids introduction of exogenous chemicals into a system. It permits study both of seizures, and the propensity of brain regions to develop seizures. The kindling model of epilepsy is not yet well-understood. Its elucidation will advance our understanding of epilepsy and of all plastic changes in brain. Hippocampal slices, and other in vitro simple neuronal systems, have been productive models for exploration of basic mechanisms of epilepsy and EEGs4.138,262. This system has allowed detailed study of circuit properties leading to synchronous firing of neurons259,2y3. Regional pacemakers for synchronous cell firing can be isolated in hippocampal slices, and studied in relation to portions of the slice that are not spontaneously active2”9. The main benefit of the slice may, however, be the ease with which it permits study of actions of putative neurotransmitters or drugs on neurons”. Despite recent dicussion of seizure-like events in the slice*“‘, the hippocampal slice cannot really model the complex electrographic and behavioral manifestations of epileptic seizures. The strength of the slice and culture systems is in the opportunity to perform
269 basic work,
pharmacology for example,
and physiology. Important on anticonvulsant mechanisms
has been performed in tissue culture models’69. Popularity of particular animal models for the epilepsies will wax and wane with developments in research.
The recent
wave of investigations
on the
genetic bases of the epilepsies will, for example, undoubtedly increase studies of mouse and rat mutant strains with epilepsy265. This review cannot catalog the insights gained from models of epilepsy, without going too far afield. Most of what we know about the science of epilepsy has derived from animal models. The reader may refer to the proceedings of a recent conference on basic mechanisms of the epilepsies for a review of this fruitful area53. A pragmatic use for animal models of the epilepsies has been anticonvulsant drug development. A variety of models are useful for anticonvulsant drug screening but it has been difficult to predict efficacy and safety of drugs for particular seizure types with the models. Hundreds of drugs have been found effective against one or more animal models’63 in the past decade, but no novel agent has reached the market for 10 years. This observation underscores the complexity of the epilepsies, the need for better models, and for better understanding of our current models.
sants,
acute
electrical
stimulation,
cortically
im-
planted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone and baboon,
strains of mouse, rat, gerbil, fruitfly maximal electroshock seizures, sys-
temic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, y-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model’s strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.
9. SUMMARY ACKNOWLEDGEMENTS
The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulREFERENCES 1 Adamec, R.E., Stark-Adamec, C., Perrin, R. and Livingston, K.E., What is the relevance of kindling for human temporal lobe epilepsy? In J.A. Wada (Ed.), Kindling II, Raven, New York, 1981. 2 Adrian, E.D., The spread of activity in the cerebral cortex, J. Physiol. (Lund.), 88 (1936) 127-161. 3 Ajmone-Marsan, C., Focal electrical stimulation. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 147-172.
During the preparation of this review the author was supported by National Institutes of Health grants ROl-NS25128-02 and POl-NS07226-18 and the Klingenstein Fund. The secretarial services of Mrs. Mildred Weininger are gratefully acknowledged. 4 Ajmone-Marsan,
C. and Marossero, F., Electrocorticographic and electrochordographic study of the convulsions induced by cardiazol, Electroencephalogr. Clin. Neuro-
physiol., 2 (1950) 133-142. 5 Alberici, M., Rodriques de Lores Arnaiz, G. and DeRo-
bertis, E., Glutamic acid decarboxylase inhibition and ultrastructural changes by the convulsant drug allylglytine, Biochem. Pharmacol., 18 (1969) 137-143. 6 Alger, B.E. and Nicoll, R.A., Pharmacological evidence for two kinds of GABA receptor in rat hippocampal pyramidal cells studied in vitro, J. Physiol. (Lond.), 328 (1982) 125-141. 7 Allen, I.C., Grieve,
A. and Griffiths,
R., Differential
270 changes in the content of amino acid neurotransmitters in discrete regions of rat brain prior to the onset and during the course of homocysteine-induced seizures, 1. Neurothem., 46 (1986) 1582-1592. 8 Alonso-DeFlorida, F. and Delgado, J.M.R., Lasting behavioral and EEG changes in cats induced by prolonged stimulation of the amygdala, Am. J. Physiol., 193 (1958) 223-229. 9 Andersen, P., Sundberg, S.H., Sveen, 0. and Wigstrom, H., Specific long-lasting potentiation of synaptic transmission in hippocampal slices, Nufure (Land.), 266 (1977) 736-737. IO Annegers, J.F., Grabow, J.D., Groover, R.V., Laws, Jr., E.R., Elveback L.R. and Kurland, L.T., Seizures after head trauma: a population study, Neurology, 30 (1980) 683-689. 11 Aprison, M.H., Lipkowitz, K.B. and Simon, J.R., Identification of a glycine-like fragment on the strychnine molecule, J. Neurosci. Res., 17 (1987) 209-213. I2 Avoli, M., Mechanisms of generalized epilepsy with spike and wave discharges, The London Symposium, 39th Edition, Elsevier, 1987, pp. 184-190. 13 Avoli, M. and Gloor, P., Role of the thalamus in generalized penicillin epilepsy: observations on decorticated cats, Exp. Neurol., 77 (1982) 386-402. 14 Avoli, M., Gloor, P., Kostopoulos, G. and Gotman, J., An analysis of penicillin induced generalized spike and wave discharges using simultaneous recording of cortical and thalamic single units, .I. Neurophysiol., 50 (1983) 819-837. 15 Ayala, G.F. and Vasconsetto,
C., Penicillin as an epileptogenic agent: its effect on an isolated neuron. Science, 167 (1970) 1257-1260. 16 Babb, T.L., Metabolic, morphologic and electrophysiologic profiles of human temporal lobe foci: an attempt at correlation, Adv. Exp. Med. Biol., 203 (1986) 115-125. I7 Barolet, A.W., Li, A., Liske, S. and Morris, M.E., Antagonist actions of bicuculline methiodide and picrotoxin on extrasynaptic gamma-aminobutyric acid receptors, Can. J. Physiol. Pharmacol., 63 (1985) 1465-1470. 18 Bartoszyk, G.D. and Hamer, M., The genetic animal model for reflex epilepsy in the Mongolian gerbil: differential efficacy of new anticonvulsive drugs and prototype antiepileptics, Pharmacol. Res. Commun., 19 (1987) 429-440. I9 Ben-Ari, Y., Tremblay, E., Riche, D., Ghilini, G. and
Naquet, R.. Electrographic, clinical and pathological alterations following systemic administration of kainic acid, bicuculline or pentetrazole: metabolic mapping using the deoxyglucose method with special reference to the pathology of epilepsy, Neuroscience, 6 (1981) 13611391. 20 Bigler, E.D., Comparison of effects of bicuculline, strych-
nine, and picrotoxin and those of pentylenetetrazol on photically evoked after discharges, Epilepsia. 18 (1977) 465-470. 21 Bingmann,
D. and Speckmann, E.J., Actions of pentylenetetrazol (PTZ) on CA3 neurons in hippocampal slices of guinea pigs, Exp. Brain Rex, 64 (1986) 94-104. 22 Biossier, J.R., Simon, P., Villeneuve, A. and Larouse, C., Flurothyl in mice: seizure, post-convulsive behavior. and interactions with psychotropic drugs, Can. J. Physiol. Pharmacol.. 46 (1968) 93-100. 23 Bircher. R.P.. Kanai, T. and Wang. S.C.. Intravenous
cortical and intraventricular dose-effect relationship of pentylenetetrazol, picrotoxin and deslanoside in dogs, Electroencephalogr. C/in. Neurophysiol., 14 (1962) 256-267. 24 Bliss, T. and Lomo, T., Long-lasting potentiation of
synaptic transmission in the dentate area of the anesthetized rabbit following stimulation of the perforant path, J. Physiol. (Land.), 232 (1973) 311-356. 25 Bloom, F., Segal, D., Ling, N. and
Guillemin, R., Endorphins: profound behavioral effects in rats suggest new etiological factors in mental illness, Science, 194
(1976) 630-632. 26 Bloomer, H.A.,
Penicillin-induced
Barton, L.J. and Maddock, Jr., R.J., encephalography in uremic patients, J.
Am. Med. Assoc., 200 (1967) 121-123. 27 Blum, B. and Liban, E., Experimental
28
29
30
31
baso-temporal epilepsy in the cat. Discrete epileptogenic lesions produced in the hippocampus or amygdaloid nucleus by tungstic acid, Neurology, 10 (1960) 546-554. Braestrup, C. and Neilsen, M., Strychnine as a potent inhibitor of the brain GABA/benzodiazepine complex, Brain Res. Bull., 5 (Suppl. 2) (1980) 681-684. Brailowsky, S., Menini, C., Silva-Barrat, C. and Naquet, R., Epileptogenic gamma-aminobutyric acid-withdrawal syndrome after chronic, intracortical infusion in baboons, Neurosci. Lett., 74 (1987) 75-80. Burchfiel, J.L., Kindling in the rat hippocampus: studies of neurotransmitter mechanisms and transfer mechanisms. In J.A. Wada (Ed.), Kindling II, 2nd edn. Raven, New York, 1981, pp. 295-301. Burton, N.R., Smith, D.A.S. and Stone, T.W., The mouse neocortical slice: preparation and responses to excitatory amino acids, Comp. Biochem. Physiol., 88C
(1987) 47-55. 32 Buterbaugh, G.G.,
Michelson, H.B. and Keyser, D.O., Status epilepticus facilitated by pilocarpine in amygdalakindled rats, Exp. Neural., 94 (1986) 91-102. 33 Caldecott-Hazard, S. and Engel, Jr., J., Limbic postictal events: anatomical substrates and opioid receptor involvement, Prog. Nemo-Psychopharmacol. Biol. Psychiatry, 11 (1987) 389-418. 34 Campbell, A.M. and Holmes, O., Bicuculline epileptogenesis in the rat, Bruin Res., 323 (1984) 239-246. 35 Cavalheiro, E.A., Riche, D.A. and La Salle, G.L.G., Long-term effects of intrahippocampal kainic acid injection in rats: a method for inducing spontaneous recurrent seizures, Electroencephalogr. Clin. Neurophysiol., 53 (1982) 581-589. 36 Chavoix, C., Hantraye,
P.. Brouillet, E., Guibert, B., Fukuda, H., De la Sayette, V., Fournier, D., Naquet, R. and Maziere, M., Status epilepticus induced by pentylenetetrazole modulates in vivo (11C) Ro 16-1788 binding to benzodiazepine receptors. Effects of ligands acting at the supramolecular receptor complex, Eur. J. Pharma-
col., 146 (1988) 207-214. 37 Clark. C.R., Comparative
anticonvulsant activity and neurotoxicity of 4-amino-N-(2,6-dimethylphenyl)benzamide and prototype antiepileptic drugs in mice and rats,
Epilepsiu, 29 (1988) 198-203. 38 Coan, E.J.. Saywood, W. and Collingridge,
G.L., MK801 blocks NMDA receptor-mediated synaptic transmission and long term potentiation in rat hippocampal slices, Neurosci. Left., 80 (1987) 111-l 14. 39 Coenen, A.M.L. and Van Luijtelaar, E.L.J.M., The WAGiRij rat model for absence epilepsy: age and sex
271 factors, 40 Collins, Penry, (Eds.).
Epilepsy Res., 1 (1987) 297-301. R.L., Audiogenic seizures. In D.P. Purpura, J.K. D.M. Woodbury, D.B. Tower and R.D. Walter
Experimental Models of Epilepsy -A Manual for the Laboratory Worker, Raven, New York, 1972, pp.
347-372. 41 Coyle, J.T., Molliver, M.E. and Kuhar, M.J., In situ injection of kainic acid: a new method for selectively lesioning neuronal cell bodies while sparing axons of passage, J. Comp. Neurol., 180 (1978) 301-323. 42 Crain. S.M.. Tissue culture models of epileptiform activity. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York. 1972, pp. 291-316. 43 Crawford. R.D., A new mutant causing epileptic seizures in domestic fowls, Poultry Sci.. 48 (1969) 1799. 44 Creutzfeldt. 0. and Houchin. J., Neuronal basis of EEG waves. In A. Remond (Ed.), Handbook of Electroencephalography and Clinical Neurophysiology, Vol. 2, Part C, Elsevier. Amsterdam, 174, pp. 5-5.5. 45 Curtis, D.R., Duggan, A. W. and Johnston, G.A.R., The specificity of strychnine as a glycine antagonist in the mammalian spinal cord, Exp. Brain Res., 12 (1971) 547-565.
46 Curtis, D.R., Game, C. and Lodge, D., Benzodiazepine and central glycine receptors, Br. J. Pharmacol., 56 (1976) 307-311. 47 Czuczwar, S.J.. Frey, H.H. and Loscher, W., Antagonism
of N-methyl-n.r-aspartic acid-induced convulsions by antiepileptic drugs and other agents, Eur. J. Pharmacol., 108 (1985) 273-280. 48 Dailey, J.W. and Jobe, P.C., Anticonvulsant drugs and the genetically epilepsy-prone rat, Fed. Proc., 44 (1985) 2640-2644. 49 Dailey. J.W. and Jobe, P.C., Indices of noradrenergic
function in the central nervous system of seizure-naive genetically epilepsy-prone rats, Epilepsia, 27 (1986) 665670. 50 Daniels, J.C. and Spehlman, R., The convulsant effect of topically applied atropine, Electroenceph. Chn. Neurophysiol., 34 (1973) 83-87. 51 Davenport. J., Schwindt, P.C. and Grill, W.E., Epilep-
togenic doses of penicillin do not reduce a monosynaptic GABA-mediated postsynaptic inhibition in the intact anesthetized cat, Exp. Neurol., 65 (1979) 552-572. 52 Dawson, Jr., R. and Bierkamper, G., Flurothyl seizure thresholds in mice treated neionatally with a single injection of monosodium glutamate (MSG): evaluation of experimental parameters in fluorothyl seizure testing, Pharmacol. Biochem. Behav., 28 (1987) 165-169. 53 Delgado-Escueta, A.V. (Ed.), Basic Mechanisms of the Epilepsies: Molecular and Cellular Approaches, Adv. Neurol. Vol 44, Raven, New York, 1986, pp. l-1096. 54 Dingledine, R., Brain Slices, Plenum, New York, 1984,
442 pp. 55 Dingledine, R. and Gjerstad, L., Reduced inhibition during epileptiform activity in the in vitro hippocampal slice, J. Physiol. (Land.), 305 (1980) 297-313. 56 Dow, R.S., Fernandez-Guardiola, A. and Manni, E., The production of experimental cobalt epilepsy in the rat, Electroenceph. Clin. Neurophysiol., 14 (1962) 399-407. 57 Dreifuss, F.E. and the Commission on Classification and
Terminology
of the International
League against Epi-
lepsy, Proposal for revised clinical and electroencephalographic classification of epileptic seizures, Epilepsia, 22 (1981) 489-501. 58 Driver, M.V. and McGillivray, B.B., Electroencephalography. In J. Laidlaw and A. Richens (Eds.), A Textbook of Epilepsy, Churchill Livingstone, New York, 1982, pp. 155-194. 59 Dworsky, S. and McCandless, D.W., Regional cerebral energy metabolism in bicuculline induced seizures, Neurochem. Res., 12 (1987) 237-240. 60 Dwyer, B.E., Fujikawa, D.G. and Waterlain, C.G., Metabolic anatomy of generalized bicuculline seizures in the newborn marmoset monkey, Exp. Neurol., 94 (1986) 213-217.
61 Eadie, M.J., Clinical use of antiepileptic drugs. In H.H. Frey and D. Janz (Eds.), Antiepileptic Drugs. Handbook of Experimental Pharmacology, Springer, Berlin, 1985, pp. 765-790. 62 Engstrom, EL. and Woodbury, D.M., Seizure susceptibility in DBA and C57 mice: the effects of various convulsants, Epilepsia, 29 (1988) 389-395. 63 Esplin, D.W. and Zablocka-Esphn, B., Mechanisms of action of convulsants. In H.H. Jasper, A.A. Ward Jr. and A. Pope (Eds.), Basic Mechanisms of the Epilepsies, Little, Brown, Boston, 1969, pp. 167-183. 64 Essig, C.F., Drug-withdrawal convulsions in animals. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 495-508. 65 Faingold, C.L., Gehlbach, G. and Caspary, D.M., Decreased effectiveness of GABA-mediated inhibition in the inferior colliculus of the genetically epilepsy-prone rat, Exp. Neural., 93 (1986) 145-159. 66 Faingold, CL., Millan, M.H., Boersma, C.A. and Meldrum, B.S., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway, Exp. Neurol., 99 (1988) 678-686. 67 Fariello, R.G., Parenteral penicillin in rats: an experimental model for multifocal epilepsy, Epilepsia, 16 (1976) 217-222.
68 Fariello, R.G. and Golden, G.T., The THIP-induced model of bilateral synchronous spike and wave in rodents, Neuropharmacology,
26 (1987) 161-165.
69 Ferguson, J.H.and Jasper, H.H., Laminar DC studies of acetylcholine activated epileptiform discharge in cerebral cortex, Electroencep. Clin. Neurophysiol., 30 (1971) 377-390. 70 Feria-Velasco,
A., Olivares, N., Rivas, F., Velasco, M. and Velasco, F., Alumina cream-induced focal motor epilepsy in cats. IV. Thickness and cellularity of layers in the peri-lesional motor cortex, Arch. Neurol., 37 (1980) 287-290.
71 Fialip, J., Aumaitre, O., Eschalier, A., Maradeix, B., Dordain, G. and Laverenne, J., Benzodiazepine withdrawal seizures: analysis of 48 case reports, Clin. Neuropharmacol., 10 (1987) 538-544. 72 Fink, M., Kahn, R.L., Karp, E., Pollack, M., Green, M.A.. Alan, B. and Lefkowits, H.J., Inhalant-induced convulsions, Arch. Gen. Psychiatry, 4 (1961) 259-266. 73 Fisch, B.J. and Pedley, T.A., Generalized tonic-clonic epilepsies. In H. Liiders and R.P. Lesser (Eds.), Epilepsy: Electroclinical Syndromes, Springer, New York, 1987, pp. 151-185.
272 74 Fisher, R.S.. Seizure Disorders. In P.D. Zieve, J.R. Burton and L.R. Barker (Eds.), Principles of Ambulatory Medicine, Williams and Wilkins, Baltimore, 1982, pp. 824-840. 75 Fisher, R.S., The hippocampal slice. Am. I. EEG Tech., 27 (1987) 1-14. 76 Fisher. R.S.. Epilepsy. In M.M. Swindle and R.J. Adams (Eds.), Experimental Surgery and Physiology: Induced Animal Models of Disease, Williams and Wilkins, Baltimore, 1988, pp. 272-274. 77 Fisher, R.S. and Alger, B.E., Electrophysiological mechanisms of kainic acid induced epileptiform activity in the rat hippocampal slice, J. Neurosci., 4 (1984) 1312-1323. 78 Fisher, R.S., Pedley, T.A., Moody. W.J. and Prince. D.A., The role of extracellular potassium in hippocampal epilepsy, Arch. Neurol., 33 (1976) 76-83. 79 Fisher, R.S. and Prince, D.A., Spike-wave rhvthms in cat cortex induced by parenteral penicillin. I. Electroencephalographic features, Electroenceph. Clin. Neurophysiol., 42 (1977) 608-624. 80 Fisher, R.S. and Prince, D.A., Spike-wave rhythms in cat cortex induced by parenteral penicillin. II. Cellular features. Electroenceph. Clin. Neurophysiol., 42 (1977) 625-639. 81 Fossieck. Jr., B. and Parker, R.H., Neurotoxicity during intravenous infusion of penicillin: a review, J. Clin. Pharmacol., 14 (1974) 504-512. 82 Freed, W.J., Selective inhibition of homocysteine-induced seizures by glutamic acid diethyl ester and other glutamate esters, Epilepsia, 26 (1985) 30-36. 83 Fritsch, G. and Hitzig. E.. Ueber die elektrische Erregbarkeit der Grosshirns, Arch. Anat. Physiol. Wissensch. Med., 37 (1870) 300. Cited in Swinyard (1972). 84 Fromm, G.H., Faingold. C.L., Browning, R.A. and Burnham, W.M., Epilepsy and the Reticular Formation: The Role of Reticular Core in Convulsive Seizures, Liss. New York, 1987, pp. l-224. 85 Frost, J., Mayberg, H.S., Fisher, R.S., Douglas, K.H., Dannals, R.F., Links, J.M., Wilson, A.A., Ravert, H.T., Rosenbaum, A.E., Snyder, S.H. and Wagner, H., Muopiate receptors measured by positron emission tomography are increased in temporal lobe epilepsy. Ann. Neurol., 23 (1988) 231-237. 86 Fuller, J.L. and Collins, R.L., Temporal parameters of sensitization of audiogenic seizures in SJLiJ mice, Dev. Psychobiol., 1 (1968) 185-188. 87 Gaull, G.E., Homocysteinuria and other disorders of sulfur metabolism. In E. Goldensohn and S. Appel (Eds.), Scientific Approaches to Clinical Neurology. Lea and Febiger, Philadelphia, PA, 1977, pp. 46-54. 88 Gean, P.W. and Shinnick-Gallagher, P., Picrotoxin induced epileptiform activity in amygdaloid neurons, Neurosci. Lett., 73 (1987) 149-154. 89 Gershoff, S.N. and Elvehjem, C.A., The relative effect of methionine sulfoximine on different species, J. Nutr., 45 (1951) 451-458. 90 Giaretta. D.. Avoli, M. and Gloor, P., Intracellular recordings in pericruciate neurons during spike and wave discharges of feline generalized penicillin epilepsy, Brain Res., 405 (1987) 68-79. 91 Girgis, M., Kindling as a model of limbic epilepsy, Neuroscience, 6 (1981) 1695-1706. 92 Gloor, P., Generalized epilepsy with spike-and-wave discharge: a reinterpretation of its electrographic and
clinical manifestations, Epilepsia. 20 (1979) 571-588. 93 Gloor, P., Olivier, A., Quesney, L.F., Andermann, F. and Horowitz, S.. The role of the limbic system in experiential phenomena of temporal lobe epilepsy, Ann. Neural., 12 (1982) 129-144. 94 Gloor. P., Quesney. L.F. and Zumstein, H., Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. II. Topical application of penicillin to the cerebral cortex and to subcortical structures, Electroencephalogr. Clin. Neurophysiol., 43 (1977) 79-94. 95 Goddard, G.V., Development of epileptic seizures through brain stimulation at low intensity, Nature (Land.), 214 (1967) 1020-1021. .. . 96 Goddard, G.V., The kindling model of epttepsy, trends Neurosci., 6 (1983) 275-279.97 Goddard, G.V. and Douglas, R.M., Does the engram of kindling model the engram of normal long term daily memory?. Can J. Neural. Sci., 2 (1975) 385-394. 98 Goddard, G.V., McIntyre, D.C. and Leech, C.K.. A permanent change in brain function resulting from daily electrical stimulation, Exp. Neural., 25 (1969) 295-330. 99 Golden, G.T. and Fariello, R.G., Penicillin spikes in rat: limitations of a single model for the study of anticonvulsants, Neuropharmacology, 23 (1984) 1001-1007. 100 Goldensohn, ES., The relevance of secondary epileptogenesis to the treatment of epilepsy: kindling and the mirror focus, Epilepsia, 25 (Suppl. 2) (1984) S156-S168. 101 Goldstein, D.S., Nadi, N.S., Stull, R., Wyler, A.R. and Porter, R.J., Levels of catechols in epileptogenic and nonepileptogenic regions of the human brain, J. Neurothem., 50 (1988) 225-229. 102 Goodin, D.S. and Aminoff. M.J., Does the interictal EEG have a role in the diagnosis of epilepsy?, Lancer, 1 (1984) 837-838. 103 Green. M.C. and Sidman, R.L., Tottering - a neuromuscular mutation in the mouse, J. Hered., 53 (1962) 233-237. .^ 104 Grossman, S.P., Chemically induced epileptiform setzures in the cat, Science, 142 (1963) 409-411. . 105 Guberman, A., Gloor, P. and Sherwin, A.L., Response of generalized penicillin epilepsy in the cat to ethosuximide and diphenylhydantoin, Neurology, 25 (1975) 785764. 106 Gutnick. M.J. and Prince, D.A., Penicillinase and the convulsant action of penicillin, Neurology, 21 (1971) 759-764. 107 Hahn, F.. Analeptics. Phurmacol. Rev., 12 (1960) 447530. 108 Hahn, F. and Oberdorf, A., Vergleichende Untersuchungen uber die Krampfwirkung von bemegrid, pentetrazol und pikrotoxin, Arch. Int. Pharmacodyn. Ther., 135 (1962) 9-30. 109 Hammond. E.J., Hurd, R.W., Wilder, B.J. and Thompson, F.J., Focal and generalized experimental seizures induced by homocysteine, Electroencephalogr. Clin. Neurophysiol., 49 (1980) 184-186. 110 Hanna, G.R. and Stalmaster, R.M., Cortical epileptic lesions produced by freezing, Neurology, 23 (1973) 918-925. 111 Harris, A.B. and Lockard, J.S., Absence of seizures or mirror foci in experimental epilepsy after excision of alumina and astrogliotic scar, Epilepsia, 22 (1981) 107122.
273 112 Harris, B., Cortical alterations due to methionine sulfoximine, Arch. Neural., 11 (1964) 388-407. 113 Hawkins, CA. and Mellanby, J.H., Limbic epilepsy induced by tetanus toxin: a longitudinal electroencephalographic study, Epilepsiu, 28 (1987) 431-444. 114 Hegreberg, G.A. and Padgett, G.A., Inherited progressive epilepsy of the dog with comparison to Lafora’s disease of man, Fed. Proc., 35 (1976) 1202-1205. 115 Heinemann, U., Changes in the neuronal micro-environment and epileptiform activity. In H.G. Wieser, E.-J. Speckmann and J. Engel Jr. (Eds.), Current Problems in Epilepsy. 3. The Epileptic Focus, Libbey, London, 1987, pp. 27-44. 116 Heinemann, U., Lux, H.D. and Gutnick, M.J., Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat, Exp. Bruin Res., 27 (1977) 237-243. 117 Henriksen, S.J., Bloom, F.E., McCoy, F., Ling, N. and Guillemin, R., Beta-endorphin induces nonconvulsive limbic seizures, Proc. Natl. Acad. Sci. H.S.A., 75 (1978) 5221-5225. 118 Heyer, E.K., Nowak, L.M. and MacDonald, R.L., Bicuculline: a convulsant with synaptic and non-synaptic actions, Neurology, 31 (1981) 1381-1390. 119 Hill, R.G., Simmonds, M.A. and Straughn, D.W., A comparative study of some convulsant substances as gamma-aminobutyric acid antagonists in the feline cerebral cortex, Br. J. Pharmacol., 49 (1973) 37-51. 120 Hjeresen, D.L., Franck, J.E. and Amend, D.L., Ontogeny of seizure incidence, latency, and severity in genetically epilepsy prone rats, Dev. Psychobiol., 20 (1987) 355-363.
121 Horton, R.W. and Meldrum, B.S., Seizures induced by allyglycine, 3-mercaptoprioprionic acid and 4-deoxypyridine in mice and photosensitive baboons, and different modes of inhibition of cerebral glutamic acid decarboxylase, Br. J. Pharmacol., 49 (1973) 52-63. 122 Hosli, E., Krogsgaard-Larsen, P. and Hosli, L., Autoradiographic localization of binding sites for the gammaaminobutyric acid analogues 4,5,6,7-tetrahydroisoxazolo5,4-c-pyridin-3-01 (THIP) isoguvacine and baclofen on cultured neurons of rat cerebellum and spinal cord, Neurosci.
Lett., 61 (1985) 153-157.
123 Hunter, J. and Jasper, H.H., Effects of thalamic stimulation in unanesthetized animals, Electroencephalogr. Clin. Neurophysiol., 1 (1949) 305-324. 124 Huot, J., Radouco-Thomas, S. and Raduoco-Thomas, C., Qualitative and quantitative evaluation of experimentally induced seizures. In J. Mercier (Ed.), Anticonvulsant Drugs, Vol. 1, Pergamon, Oxford, 1973, pp. 123-185. 125 Imaizumi, K., Ito, S., Kutsukake, G., Takazawa, T., Fujiwara, K. and Tutikawa, K., Epilepsy-like anomoly of mice, Exp. Anim. (Tokyo), 8 (1959) 6-10. 126 Jackson, J.H., On convulsive seizures (Lumleian Lectures) in J. Taylor (Ed.), Selected Writings of John Hughlings Jackson, Vol. 1 (1931). Hodder and Stoughton, London. 127 Jasper, H.H. and Droogleever-Fortuyn, J., Experimental studies on the functional anatomy of petit ma1 epilepsy, Res. Publ. 272-278. 128 Jensen,
Assoc.
Res.
Nerv.
Ment.
Dis.,
26 (1946)
M.S. and Yaari, Y., The relationship between interictal and ictal paroxysms in an in vitro model of focal hippocampal epilepsy, Ann. Neurol., 24 (1988) 591-598.
129 Jobe, P.C., Dailey, J. W. and Reigel, C.E., Noradrenergic and serotonergic determinants of seizure susceptibility and severity in genetically epilepsy-prone rats, Life Sci., 39 (1986) 775-782.
130 Jobe, P.C., Ko, K.H. and Daily, J.W., Abnormalities in norepinephrine turnover rate in the central nervous system of the genetically epilepsy-prone rat, Brain Res., 290 (1984) 357-360. 131 Johnston, G.A.R., Convulsions induced in lo-day-old rats by intraperitoneal injection of monosodium glutamate and related excitant amino acids, Biochem. Pharmacol., 22 (1973) 137-140. 132 Jope, R.S., Morrisett, R.A. and Snead III, O.C., Characterization of lithium potentiation of pilocarpine-induced status epilepticus in rats, Exp. Neural., 91 (1986) 471-480. 133 Kamb, A., Iverson, L.E. and Tanouye, M.A., Molecular characterization of Shaker, a Drosophila gene that encodes a potassium channel, Cell, 50 (1987) 405-413. 134 Kaplan, B.J., Seyfried, T.N. and Glaser, G.H., Spontaneous poly-spike discharges in an epileptic mutant mouse (tottering), Exp. Neural., 66 (1979) 577-586. 135 Karpiak, S.E., Graf, L. and Rapport, M.M., Antiserum to brain gangliosides produces recurrent epileptiform activity, Science, 194 (1976) 735-737. 136 Karpiak, S.E., Mahadik, S.P., Graf, L. and Rapport, M.M., An immunological model of epilepsy: seizures induced by antibodies to G.M.l ganglioside, Epilepsia, 22 (1981) 189-196.
137 Kay, A.R. and Wong, R.K., Isolation of neurons suitable for patch-clamping from adult mammalian central nervous system, J. Neurosci. Meth., 16 (1986) 227-238. 138 Kerkut, G.A. and Wheal, H.V., Electrophysiology of Isolated Mammalian CNS Preparations, Academic, New York, 1981. 139 Killam, K.F., Killam, E.K. and Naquet, R., An animal model of light-sensitive epilepsy, Electroenceph. Clin. Neurophysiol.,
22 (1967) 497-513.
140 Killam, K.F., Naquet, R. and Bert, J., Paroxysmal responses to intermittent light stimulation in a population of baboons (Papio papio), Epilepsia, 7 (1966) 215-219. 141 King, D.W. and Ajmone-Marsan, C., Clinical features and ictal patterns in epileptic patients with EEG temporal lobe foci, Ann. Neural., 2 (1977) 138-147. 142 Knopman, D.S., Acute strychnine-induced seizures in cats: a Golgi study, Epilepsia, 791-792 (1975) 523-526. 143 Kopeloff, L.M., Chusid, J.G. and Kopeloff, N., Epilepsy in Macacca muluttu after cortical or intracerebral alumina, Arch. Neural. Psychiatry, 74 (1955) 523-526. 144 Kostopoulos, G., Neuronal sensitivity to GABA
and glutamate in generalized epilepsy with spike and wave discharges, Exp. Neural., 92 (1986) 20-36. 145 Kostopoulos, G., Avoli, M. and Gloor, P., Participation of cortical recurrent inhibition in the genesis of spike and wave discharges in feline generalized penicillin epilepsy, Bruin Res., 267 (1983) 101-112. 146 Krauss, G.L., Kaplan, P. and Fisher,
R.S., Parenteral magnesium sulfate fails to control electroshock and pentylenetetrazol seizures in mice, Epilepsy Res., in press. 147 La Salle, G.L.G., Amygdaloid kindling in the rat: regional differences and general properties. In J.A. Wada (Ed.), Kindling H, Raven, New York, 1981, pp. 31-47. 148 Laird, H.E., Dailey, J.W. and Jobe, P.C., Neurotransmitter abnormalities in genetically epileptic rodents, Fed. Proc., 43 (1984) 2505-2509.
274 149 Land, H.E, Hadjiconstantinou, M. and Neff, N.H., Abnormalities in the central cholinergic transmitter system of the genetically epilepsy-prone rat, Life Sci., 39 (1986) 783-787. 150 Lancaster, B. and Wheal, H.V., Chronic failure of inhibition of the CA1 area of the hippocampus following kainic acid lesions of the CA314 area, Brain Res., 295 (1984) 317-324. 151 Lange, SC., Neafsey, E.J. and Wyler, A.R., Neuronal activity in chronic ferric chloride epileptic foci in cats and monkey, Epilepsia, 21 (1980) 251-254. 1.52 Langmoen, LA. and Andersen, P., The hippocampal slice in vitro. A description of the technique and some examples of the opportunities it offers. In G.A. Kerkut and H.V. Wheal (Eds.), Electrophysiology of Isolated Mammalian CNS Preparations, Academic, New York, 1981, pp. 52-105. 153 LaSalle, G.L.G., Amygdaloid kindling in the rat: regional differences and general properties. In J.A. Wada (Ed.), Kindling II, Raven, New York, 1981, pp. 31-47. 154 Laursen, A.M., The contribution of in vitro studies to the understanding of epilepsy, Acta Neural. &and., 69 (1984) 367-375. 155 Leander, J.D., Lawson, R.R., Ornstein, P.L. and Zimmerman, D.M., N-methyl-o-aspartic acid-induced lethality in mice: selective antagonism by phencyclidine-like drugs, Brain Res., 448 (1988) 115-120. 156 Levitt, P. and Noebels, J., Mutant mouse tottering: selective increase of locus ceruleus axons in a defined single-locus mutation, Proc. Natl. Acad. Sci. U.S.A., 78 (1981) 4630-4643.
157 Lieb, J.P., Engel Jr., J., Gevins, A. and Crandall, P.H., Surface and deep EEG correlates of surgical outcome in temporal lobe epilepsy, Epilepsia, 22 (1981) 515-538. 158 Lindau, M. and Neher, E., Patch-clamp techniques for time-resolved capacitance measurements in single cells, Eur. J. Physiol., 411 (1988) 137-146.
159 Litchfield, J .T. and Wilcoxon, F., A simplified method of evaluating dose-effect experiments, J. Pharmacol. Exp. Ther., 96 (1949) 99-113. 160 Little, H.J., Nutt, D.J. and Taylor, S.C., Optimizing the
161
162
163
164
165
pentelenetetrazol infusion method for measuring seizure thresholds, Br. J. Pharmacol., (Suppl.), 88 (1986) 326P. Lockard, J.S., Uhlir, V., DuCharme, L.L., Farquhar, J.A. and Huntsman, B.J., Efficacy of standard anticonvulsants in monkey model with spontaneous motor seizures, Epilepsia, 16 (1975) 301-317. Loiseau, H., Averet, N., Arrigoni and Cohadon, F., The early phase of cryogenic lesions: an experimental model of seizures updated, Epilepsia, 28 (1987) 251-258. Loscher, W.L. and Schmidt, D., Which animal models should be used in the search for new antiepileptic drug? A proposal based on experimental and clinical considerations, Epilepsy Res., 2 (1988) 145-181. Lothman, E. W., Collins, R.C. and Ferrendelli, J.A., Kainic acid-induced limbic seizures: electrophysiologic studies, Neurology, 31 (1981) 806-812. Lothman, E.W., Hatlelid, J.M., Zorumski, C.F., Conry, J.A., Moon, P.F. and Perlin, J.B., Kindling with rapidly recurring hippocampal seizures, Brain Res., 360 (1985) 83-91.
166 Lothman, E. W., Perlin, J.B. and Salerno, R.A., Response properties of rapidly recurring hippocampal seizures in rats, Epilepsy Res., 2 (1988) 356-366.
167 Luse, S.A., Goldring, S. and O’Leary, J.L., Seizure activity due to intravenous strychnine, Arch. Neural., 11 (1964) 296-302. 168 MacDonald, R.L. and Barker, J.L., Pentelenetetrazol and penicillin are selective antagonists of GABA-mediated post-synaptic inhibition in cultured mammalian neurons, Nature (Lond.), 267 (1977) 720-721. 169 MacDonald, R.L. and McLean. M.J., Mechanisms of anticonvulsant drug action, Electroencephalogr. Clin. Neurophysiol., (Suppl.), 39 (1987) 200-208. 170 Magistris, M.R., Mouradian, MS. and Gloor, P., Generalized convulsions induced by pentylenetetrazol in the cat: participation of forebrain, brainstem and spinal cord, Epilepsia, 29 (1988) 379-388.
171 Majkowski, J. and Kaplan, H., Value of Mongolian gerbils in antiepileptic drug evaluation, Epilepsia, 24 (1983) 609-615.
172 Marcus, E.M., Experimental models of petit ma1 epilepsy. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 113-146.
173 Marcus, E.M. and Watson, C.W., Bilateral symmetrical epileptonic foci in monkey cerebral cortex: mechanisms of interactions and regional variations in capacity for synchronous spike slow wave discharges, Arch. Neurol., 19 (1968) 99-116.
174 Marcus, E.M., Watson, C. W. and Goldman, P.L., Effects of steroids on cerebral electrical activity: epileptogenic effects of conjugated estrogens and related compounds in the cat and rabbit, Arch. Neurol., 15 (1966) 521-532. 175 Marescaux, C., Micheletti, G., Vergnes, M., Depaulis, A., Rumbach, L. and Warter, J.M., A model of chronic spontaneous petit-ma] like seizures in the rat: comparison with pentylenetetrazol-induced seizures, Epilepsia, 25 (1984) 326-331.
176 Marvizon, J.C., Vazquez, J., Garcia Calvo, M., *Mayor Jr., F., Ruiz Gomez, A., Valdivieso, F. and Benavides, J., The glycine receptor: pharmacological studies and mathematical modeling of the allosteric interaction between the glycine- and strychnine-binding sites, Mol. Pharmacol., 30 (1986) 590-597.
177 Mason, CR. and Cooper, R.M., A permanent change in convulsive threshold in normal and brain-damaged rats with repeated small doses of pentelenetetrazole, Epilepsia, 13 (1972) 663-674.
178 Matsumoto, H. and Ajmone-Marsan, C., Cortical cellular phenomona in experimental epilepsy: interictal manifestations, Exp. Neurol., 9 (1964) 286-304. 179 Maurin, Y., Berger, B., LeSaux, F., Gay, M. and Baumann, N., Increased number of locus ceruleus noradrenergic neurons in the convulsive mutant quaking mouse, Neurosci. Lett., 57 (1985) 313-318. 180 Maxson, S.C., Fine, M.D., Ginsburg, B.E. and Koniecki, D.L., A mutant for spontaneous seizures in C57BLIlOBg mice, Epilepsia, 24 (1983) 15-24. 181 McCormick, D.A., Connors, B.W., Lighthall, J.W. and Prince, D.A., Comparative electrophysiology of pyramidal and sparsely spiny stellate neurons of the neocortex, J. Neurophysiol., 54 (1985) 782-806. 182 McGeer, E.G., Olney, J.W. and McGeer, P.L., The allosteric interaction between the glycine- and strychninebinding sites, Kainic Acid as a Tool in Neurobiology, Raven, New York, 1978.
275
183 McIlwain, H., Metabolic response in vitro to electrical stimulation of sections of mammalian brain, Biochem. J., 49 (1951) 382-393. 184 McIntyre, D.C., Stokes, K.A. and Edson, N., Status epilepticus following stimulation of a kindled hippocampal focus in intact and commissurotomized rats, Exp. Neurol., 94 (1986) 554-570. 185 McNamara, J., Kindling: an animal model of complex partial epilepsy, Ann. Neural., 16 (Suppl.) (1984) 72-76. 186 McQueen, J.K. and Woodbury, D.M., Attempts to produce spike and wave complexes in the electrocorticogram of the rat, Epilepsia, 16 (1975) 295-299. 187 Meldrum, B.S., Preclinical test systems for evaluation of novel compounds. In B.S. Meldrum and R.J. Porter (Eds.), Current Problems in Epilepsy. 4. New Anticonvulsant Drugs, Libbey, London, 1986, pp. 31-48. 188 Meldrum, B.S. and Chapman, A.G., Benzodiazepine receptors and their relationship to epilepsy, Epilepsia, 27 (Suppl. 1) (1986) S3-S13. 189 Meldrum, B.S. and Horton, R.W., Convulsive effects of 4-deoxypyridoxine and of bicuculline in photosensitive baboons (Papio papio) and in rhesus monkeys (Macaca mulatta), Brain Res., 35 (1971) 419-436. 190 Meldrum, B.S., Horton, R. W. and Brierly, J.B., Epilep-
tic brain damage in adolescent baboons following seizures induced by allylglycine, Brain, 97 (1974) 407-418. 191 Meldrum, B.S., Swan, J.H., Ottersen, O.P. and StormMathisen, J., Redistribution of transmitter amino acids in rat hippocampus and cerebellum during seizures induced by r-allylglycine and bicuculline: and immunocytochemical study with antisera against conjugated GABA, glutamate and aspartate, Neuroscience, 22 (1987) 17-27. 192 Mellanby, J., Hawkins, C., Mellanby, H., Rawhns, J.N.P. and Impey, M.E., Tetanus toxin as a tool for studying epilepsy, J. Physiol. (Paris), 79 (1984) 207-215. 193 Mellanby, J., George, G., Robinson, A. and Thompson, P.A., Epileptiform syndrome in rats produced by injecting tetanus toxin into the hippocampus, J. Neural. Neurosurg. Psychiatry, 40 (1977) 404-414. 194 Merritt, H.H. and Putnam,
anticonvulsant
T.J., A new series of drugs tested by experiments on animals,
Arch.
Psychiatry, 39 (1938) 1003-1015.
Neural.
195 Metrakos,
J.D. and Metrakos, K., Genetic studies in clinical epilepsy. In H.H. Jasper, A.A. Ward and A. Pope (Eds.), Basic Mechanisms of the Epilepsies, 1969, pp. 700-708. 196 Milgram, N.W., Green, I., Liberman, M., Riexinger, K. and Petit, T.L., Establishment of status epilepticus by limbic system stimulation in previously unstimulated rats, Exp. Neural., 88 (1985) 553-564. 197 Millan, M.H., Meldrum, B.S.,
Boersma, C.A. and Faingold, C.L., Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. II. Efferent seizure propagating pathways, Exp. Neural., 99 (1988)
687-698. 198 Millan, M.H., Meldrum, B.S. and Faingold, C.L., Induc-
tion of audiogenic seizure susceptibility by focal infusion of excitant amino acid or bicuculline into the inferior colliculus of normal rats, Exp. Neural., 91 (1986) 634639. 199 Miller, J.W., McKeon, A.C. and Ferrendelli, J.A., Functional anatomy of pentylenetetrazol and electroshock seizures in the rat brainstem, Ann. Neural., 22 (1987) 615-621.
200 Mirski, M.A. and Ferrendelli, J.A., Anterior mediation of generalized pentylenetetrazol
thalamic seizures,
Brain Res., 399 (1986) 212-223. 201 Mirski, M.A. and Ferrendelli, J.A.,
Interruption of the connections of the mammillary bodies protects against generalized pentylenetetrazol seizures in guinea pigs, J.
Neurosci., 7 (1987) 662-670. 202 Molaie, M. and Kadzielawa,
203 204
205
206
207
208 209
K., Effect of naloxone infusion on rate of epileptiform discharges in patients with complex partial seizures, Epilepsia, 29 (1988) 698. Morison, R.S. and Dempsey, E.W., A study of thalamocortical relations, Am. J. Physiol., 135 (1942) 281-292. Morrell, F., Secondary epileptogenic lesions, Epilepsia, 1 (1960) 538-560. Morrisett, R.A., Jope, R.S. and Snead III O.C., Effects of drugs on the initiation and maintenance of status epilepticus induced by administration of pilocarpine to lithium-pretreated rats, Exp. Neural., 97 (1987) 193-200. Morrisett, R.A., Jope, R.S. and Snead III, O.C., Status epilepticus is produced by administration of cholinergic agonists to lithium-treated rats: comparison with kainic acid, Exp. Neural., 98 (1987) 594-605. Moses III, H.M. and Fisher, R.S., Syncope, seizures and other episodic disorders. In A.M. Harvey, R.T. Johns, V.A. McKusick, A.H. Owens Jr. and R.S. Ross (Eds.), The Principles and Practice of Medicine, 22nd edn., Appleton & Lange, Norwalk, CT, 1988, pp. 1025-1033. Nadler, V., Kainic acid as a tool for the study of temporal lobe epilepsy, Life Sci., 29 (1981) 2031-2042. Naquet, R. and Meldrum, B.S., Photogenic seizures in baboon. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental
Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 373-406. 210 Newmark, M.E. and Penry, J.K., Photosensitivity and Epilepsy: A Review, Raven, New York, 1979.
211 Niedermeyer,
E. and Charles, C. Thomas (Eds.), ComSpringfield, 1974, pp. l-334. 212 Niedermeyer, E., Epileptic seizure disorder. In E. Niedermeyer and F. Lopes da Silva (Eds.), Electroencephapendium
of the Epilepsies,
lography: Basic Releated Fields,
Principles,
Clinical
Applications
and
2nd edn., Urban & Schwarzenberg, Baltimore, 1987, pp. 405-510. 213 Noebels, J.L., Analysis of inherited epilepsy using single locus mutations in mice, Fed. Proc., 38 (1979) 2405-2410. 214 Noebels, J.L., A single gene error of noradrenergic axon growth synchronizes central neurones, Nature (Land.), 310 (1984) 409-411. 215 Noebels, J.L. and Pedley, T.A., Anatomic localization of
topically applied [“‘C]peniciIlin during experimental focal epilepsy in cat neocortex, Brain Res., 125 (1977) 293-303. 216 Noebels, J.L. and Sidman, R.J., Inherited epilepsy: spike-wave and focal motor seizures in the mutant mouse tottering, Science, 204 (1979) 1334-1336. 217 Oliver, A.P., Hoffer, B.J. and Wyatt, R.J., The hippocampal slice: a model system for studying the pharmacology of seizures and for screening anticonvulsant drugs, Epilepsia, 18 (1977) 543-548. 218 Olney, J.W., Rhee, V. and Ho, O.L.,
Kainic acid: a powerful neurotoxic analog of glutamate, Brain Res., 77
(1974) 501-512. 219 Olsen R.W., The GABA postsynaptic
tor-ionophore anticonvulsant
membrane recepcomplex. Site of action of convulsant and drugs, Mol. Cell. Biochem., 39 (1981)
276 261-279. 220 Papazian, D.M., Schwarz, T.L., Tempel, B.L., Jan, Y.N. and Yan, L.Y., Cloning of genomic and complementary DNA from Shaker, a putative potassium channel gene from Drosophila, Science, 237 (1987) 749-753. 221 Papy, J.J. and Naquet, R., Cerebral blood flow and seizures induced by MetrazolR in the baboon, Clin. Dev. Med., 39 (1971) 283-296. 222 Pedley, T.A., The pathophysiology of focal epilepsy, Ann. Neural., 3 (1978) 2-9. 223 Pei, Y., Zhao, D., Huang, J. and Cao, L., Zinc-induced seizures: a new experimental model of epilepsy, Epilepsia, 24 (1983) 169-176. 224 Pellegrini, A., Gloor,
225
226
227
228
229
230
231 232
P. and Sherwin, A.L., Effect of valproate sodium on generalized penicillin epilepsy in the cat, Epilepsia, 19 (1978) 351-360. Penfield, W. and Jasper, H.H., Epilepsy and the Functional Anatomy of the Human Brain, Little Brown, Boston, 1954, p. 713. Penny, J.E., Brown, R.D., Wallace, M.S. and Henley, C.M.. Auditory aspects of seizure in the genetically epilepsy prone rat, Life Sci., 39 (1986) 887-895. Pinel, J.P.J. and Rovner, L.I., Experimental epileptogenesis: kindling - induced epilepsy in rats, Exp. Neurol., 58 (1978) 190-202. Piredda, S. and Gale, K., Role of excitatory amino acid transmission in the genesis of seizures elicited from the deep prepiriform cortex, Brain Res., 377 (1986) 205-210. Piredda, S., Lim, S.R. and Gale, K., Intracerebral site of convulsant action of bicuculline, Life Sci., 36 (1985) 1295-1298. Pitkanen, A., Saano, V., Hyvonen, K., Airaksinen, M.M. and Riekkinen, P.J., Decreased GABA benzodiazepine, and picrotoxinin receptor binding in brains of rats after cobalt-induced epilepsy, Epilepsia, 28 (1987) 1l- 16. Pollen, D.A., Experimental spike and wave responses in petit mal epilepsy, Epilepsia, 9 (1968) 221-232. Pollen, D.A., Reid, K. and Perot, P., Micro-electrode studies of experimental 3isec wave and spike in the cat,
Electroencephalogr.
Clin. Neurophysiol.,
17 (1964) 57-67.
233 Porter,
R.J., Cereghino, J.J., Gladding, G.D., Hessie. B.J., Kupferberg, H.J., Scoville, B. and White, B.G.. Antiepileptic drug development program, Cleve. Clin. Q.,
51 (1984) 293-305. 234 Price, D.L., Griffin, J., Young, A., Peck, K. and Stocks.
235 236
237
238
239 240
241
A., Tetanus toxin: direct evidence for retrograde intraaxonal transport, Science, 188 (1975) 945-947. Prince, D.A., The depolarization shift in ‘epileptic’ neurons, Exp. Neurol., 21 (1968) 467-485. Prince, D.A., Neurophysiology of epilepsy, Annu. Rev. Neurosci., 1 (1978) 395-415. Prince, D.A. and Farrell, D., ‘Centrencephalic’ spikewave discharges following parenteral penicillin injection in the cat, Neurology, 19 (1969) 309-310. Prince, D.A. and Wilder, B.J., Control mechanisms in cortical epileptogenic foci, Arch. Neurol., 16 (1967) 194-202. Prince, C.A. and Wong, R.K.S., Human epileptic neurons studied in vitro, Brain Res., 210 (1981) 323-333. Probst, A., Cortes, R. and Palacios, J.M., The distribution of glycine receptors in the human brain. A light microscopic autoradiographic study using [3H]strychnine. Neuroscience, 17 (1986) 11-35. Proler, M. and Kellaway. P.. The methionine sulfoximine
syndrome in the cat, Epilepsia, 3 (1965) 117-130. D.P., Penry, J.K., Woodbury, D.M., Tower, D.B. and Walter, R.D., Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, p. 615. 243 Quesney, L.F., Gloor, P., Kratzenberg, E. and Zumstein, H., Pathophysiology of generalized penicillin epilepsy in the cat: role of cortical and subcortical structures. I. Systematic application of penicillin, Electroenceph. Clin.
242 Purpura,
Neurophysiol., 42 (1977) 640-655. 244 Racine, R., Modification of seizure activity by electrical stimulation. I. After-discharge threshold, Electroenceph. Clin. Neurophysiol., 32 (1972) 269-279. 245 Racine, R., Kindling: the first decade, J. Neurosurg., 3 (1978) 234-252. 246 Reid, S.A., Sypert, G.W. and Boggs, W.M., Histopathol-
ogy of the ferric-induced chronic epileptic focus in cats and monkey, Exp. Neural., 66 (1979) 205-210. 247 Reigel, C.E., Dailey, J.W. and Jobe, P.C., The genetically epilepsy-prone rat: an overview of seizure-prone characteristics and responsiveness to anticonvulsant drugs, Life Sci., 39 (1986) 763-774. 248 Reigel, C.E., Jobe, P.C., Dailey, J.W. and Stewart, J.J., Responsiveness of genetically epilepsy-prone rats to intracerebroventricular morphine-induced convulsions, Life Sci., 42 (1988) 1743-1749. 249 Remler, M.P. and Marcussen, W.H., Systemic focal epileptogenesis, Epilepsia, 27 (1986) 35-42. 250 Remler, M.P., Sigvardt, K. and Marcussen, W.H., Phar-
macological response of systemically derived focal epileptic lesions, Epilepsia, 27 (1986) 671-677. 251 Roberts, R.C. and Ribak, C.E., Anatomical changes of the GABAergic system in the inferior colliculus of the genetically epilepsy-prone rat, Life Sci., 39 (1986) 789798. 252 Rodin, E.A.,
Rutledge, L.T. and Calhoun, H.D., Megimide and metrazol: a comparison of their convulsant properties in man and cat, Electroencephalogr. Clin.
Neurophysiol., 10 (1958) 719-723. 253 Sasa, M.. Ohno. Y., Ujihara, H., Fujita, Y., Yoshimura,
M., Takaori, S., Serikawa, T. and Yamada, J., Effects of antiepileptic drugs on absence-like and tonic seizures in the spontaneously epileptic rat, a double mutant rat, Epilepsia, 29 (1988) 505-513. 254 Sato, T., Mori, N., Kurokochi,
A., Tojo, Y. and Kumashiro, H.. A new model of epileptic seizure utilizing excitatory amino acids and dimethyl sulfoxide, Epilepsia,
27 (1986) 620. 255 Schecter. P.J.,
Tranier, Y. and Grove, J., Effect of n-dipropylacetate on amino acid concentrations in the mouse brain: correlations with anticonvulsant activity, J.
Neurochem., 31 (1978) 1325-1327. 256 Schwartzkroin, P.A., Characteristics
recorded intracellularly Res., 85 (1975) 257 Schwartzkroin, in vitro, Brain 258 Schwartzkroin,
of CA1 neurons in the hippocampal slice, Brain
423-435.
P.A., Further characteristics of CA1 cells Res., 128 (1977) 53-68.
P.A. and Haglund, M.M., Spontaneous rhythmic synchronous activity in epileptic human and normal monkey temporal lobe. Epilepsia, 27 (1986)
523-533. 259 Schwartzkroin,
P.A. and Prince, D.A., Cellular and field potential properties of epileptogenic hippocampal slices, Brain Res., 147 (1978) 117-130.
277
260 Schwartzkroin, P.A., Turner, D.A., Knowles, W.D. and Wyler, A.R.. Studies of human and monkey ‘epileptic’ neocortex in the in vitro slice preparation, Ann. Neurol., 13 (1983) 249-257. 261 Schwartzkroin, P.A. and Wester, K., Long-lasting facilitation of a synaptic potential following tetanization in the in vitro hippocampal slice, Brain Res., 89 (1975) lO7- 119. 262 Schwartzkroin. P.A. and Wheal. H.V.. Elecrrophysiology of Epilepsy, Academic, New York, 1984, 420 pp. 263 Serikawa, T. and Yamada, J.. Epileptic seizures in rats homozygous for two mutations, zitter and tremor, J. Hered..
77 (1986) 441-444.
264 Sessler, EM..
Cheng, J.T. and Waterhouse, B.D., Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters, Bruin Res., 446 (1988)
77-89. 265 Seyfried,
mutants
279 Suzuki, J. and Nakamoto, Y., Seizure patterns and electroencephalograms of El mouse, Electroencephalogr. Clin. Neurophysiol.,
201-204. 282 Swinyard, E.A., Electrically induced convulsions. In D.P.
Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy A Munuul for the Laboratory Worker, Raven, New York, 1972, pp. 435-458. 283 Swinyard, E.A., Brown, W.C. and Goodman, L.S., Comparative assay of antiepileptic drugs in mice and rats, J. Phurmacol. Exp. Ther., 106 (1952) 319-330. 284 Swinyard, E.A., Castellion, A. W., Fink, G.B. and Good-
T.N. and Glaser, G.H.. A review of mouse as genetic models of epilepsy, Epilepsiu, 26
(1985) 143-150. 266 Sherwin, A.L.,
surgical specimens
Guide to neurochemical analysis of of human brain, Epilepsy Res., 2
285
(1988) 281-288. 267 Sloviter. R.S.. Decreased
286
268
287
269
270
271
hippocampal inhibition and a selective loss of interneurons in experimental epilepsy, Science, 235 (1987) 73-76. Snead III, O.C., Gamma-hydroxybutyrate in the monkey. I. Electroencephalographic. behavioral and pharmacokinetic studies, Neurology, 28 (1978) 636-642. Snead III, O.C., Gamma-hydroxybutyrate in the monkey. II. Effect of chronic oral anticonvulsant drugs, Neurology, 28 (1978) 643-648. Snead III, O.C.. Ontogeny of gamma-hydroxybutyric acid. II. Electroencephalographic effects, Dev. Bruin Res., IS (1984) 89-96. Snead III, O.C.. Gamma-hydroxybutyrate model of generalized absence seizures: further characterization and comparison with other absence models, Epilepsia, 29
(1988) 361-368. 272 Snead III, O.C.
and Bearden, L.J., Anticonvulsants specific for petit mal antagonize epileptogenic effect of leucine enkephalin, Science, 210 (1980) 1031-1033. 273 Snead III, O.C. and Stephens, H., The ontogeny of seizures induced by leucine-enkephalin and P-endorphin, Ann. Nemo/., 15 (1984) 594-598. 274 Spiegel, E.A., Quantitative determination of the convulsive reactivity by electrical stimulation of the brain with the skull intact, J. Lab. Clin. Med., 22 (1937) 1274-1276. 275 Sprince, H., Parker, C.M., Josephs, J.A. and Magazino, J., Convulsant activity of homocysteine and other shortchain mercapto acids: protection therefrom, Ann. N.Y.
288
289
290
291
292
Neural.,
294
295
12 (1953) 262-276.
277 Stone. W.E., Convulsant actions of tetrazole derivatives, Pharmacology, 3 (1970) 367-370. 278 Stone, W.E., Systemic chemical convulsants and meta-
296
bolic derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 407-432.
man, L.S., Some neurophysiological and neuropharmacological characteristics of audiogenic seizure susceptible mice, J. Pharmucol. Exp. Ther., 140 (1963) 375-384. Taylor-Courval, D. and Gloor, P., Behavioral alterations associated with generalized spike and wave discharges in the EEG of the cat, Exp. Neural., 83 (1984) 167-186. Testa, G. and Gloor, P., Generalized penicillin epilepsy in the cat: effect of midbrain cooling, Electroencephulogr. Clin. Neurophysiol., 36 (1974) 517-524. Thiessen, D.D., Lindzey, G. and Friend, H.C., Spontaneous seizures in the Mongolian gerbil (Meriones unguicuhzms), Psychonom. Sci., 11 (1968) 227-228. Timpe, L.C. and Jan, L.Y., Gene dosage and complementation analysis of the Shaker locus in Drosophila, J. Neurosci., 7 (1987) 1307-1317. Timpe, L.C., Schwarz, T.L., Tempel, B.L., Papazian, D.M., Jan, Y.N. and Jan, L.Y., Expression of functional potassium channels from Shaker cDNA in Xenopus oocytes, Nature (Lond.), 331 (1988) 143-145. Tortella, EC. and Long, J.B., Endogenous anticonvulsant substance in rat cerebral spinal fluid following a generalized seizure, Science, 228 (1985) 1106-1107. Tortella, EC., Long, J.B. and Holaday, J.W., Endogenous opioid systems: physiological role in the selflimitation of seizures, Bruin Res., 15 (1985) 174-178. Toussi, H.R., Schatz, R.A. and Waszczak, B.L., Suppression of methionine sulfoximine seizures by intranigral gamma-vinyl GABA injection, Eur. J. Phurmacol., 137
(1987) 261-264. 293 Traub, R.D. and Wong, R.K.S.,
Acud. Sci., 166 (1969) 323-325. 276 Starzl, T.E., Niemer, W.T., Dell, M. and Forgave, P.R.,
Cortical and subcortical electrical activity in experimental seizures induced by metrazol, J. Neuroputhol. Exp.
43 (1977) 299-311.
280 Swartzwelder, H.S., Lewis, D.V., Anderson, W.W. and Wilson, W.A.. Seizure-like events in brain slices: suppression by interictal activity, Bruin Res., 410 (1987) 362-366. 281 Swinyard, E.A., Laboratory assay of clinically effective antiepileptic drugs, J. Am. Phurm. Assoc., 38 (1949)
297
Cellular mechanisms of neuronal synchronization in epilepsy, Science, 216 (1982) 745-747. Treiman, D.M., Walton, N.Y., DeGiorgio, CM., Wickbolt, C.L. and Salisbury, S.M., Sequential electro-clinical patterns in generalized convulsive status epilepticus in man and three experimental models of status epilepticus in the rat, Neurology, 37, Suppl. 1 (1987) 244-245. Turski, W.A., Cavalheiro, E.A., Coimbra, C., da Penha Berzaghi, M., Ikonomidou-Turski, C. and Turski, L., Only certain antiepileptic drugs prevent seizures induced by pilocarpine, Bruin Res. Rev., 12 (1987) 281-305. Turski, W.A., Cruczwar, S.J., Kleinrok, 2. and Turski, L., Cholinomimetics produce seizures and brain damage in rats, Experiemia (Basel), 39 (1983) 1408-1411. Ulloque, R.A., Chweh, A.Y. and Swinyard, E.A., Effects of gamma-aminobutyric acid (GABA) receptor
278 agonists on the neurotoxicity and anticonvulsant activity of barbiturates in mice, J. Pharmacol. Exp. Ther., 237 (1986) 468-489. 298 Urea. G., Frenk, H., Liebeskind, J.C. and Taylor, A.M., Morphine and enkephalin: analgesic and epileptic properties, Science, 197 (1977) 83-86. 299 Usunoff, G., Atsev, E. and Tchavdarov, D.. On the mechanisms of picrotoxin epileptic seizures (macro- and micro-electrode investigations), Electroencephalogr. Chn. Neurophysioi..
315 Watkins, J.C. and Evans, R.H., Excitatory amino acid transmitters, Annu. Rev. Pharmacof. Toxicol., 21 (1981) 165-204. 316 Weinstein,
317
318
27 (1969) 444.
300 Van den Berg. C.J. and Van den Velden, J., The effect
of methion~ne sulphoximine on the incorporation of labetled glucose, acetate, phenylalanine and prohne into glutamate and related amino acids in the brains of mice, .I. Neurochem., 17 (1970) 985-991. 301 van Gelder, N.M., Contributions of basic neurochemistry towards a novel concept of epilepsy, Neurochem. Ref., 12 (1987) 111-119. 302 Van Luijtelaar,
303
304
305
306
307 308 309
310
E.L.J.M. and Coenen, A.M.L., Two types of electrocortical payroxysms in an inbred strain of rats, Neurosci. Left., 70 (1986) 393-397. Velasco, E, Velasco, M., Estrada-Villanueva, E and Machado, J.P., Specific and nonspecific multiple unit activities during the onset of pentylenetetrazol seizures. I. Intact animals, Epilepsia. 16 (1975) 207-214. Velasco. F., Velasco, M., Ogarrio, C. and Fanghanel, G., Electrical stimulation of the centromedjan thalamic nucleus in the treatment of convulsive seizures: a preliminary report, Epilepsia, 28 (1987) 421-430. Vergnes, M., Marescaux, C., Depauiis, A., Micheletti, G. and Warter, J.M., Spontaneous spike and wave discharges in thalamus and cortex in a rat model of genetic petit-ma1 like seizures, Exp. Neural., 96 (1987) 127-136. Vicedomini, J.P. and Nadler, J.V., A model of status epilepticus based on electrical stimulation of hippocampal afferent pathways, Exp. Neural., 96 (1987) 681-691. Wada, J.A., Pharmacological prophylaxis in the kindling model of epilepsy, Arch. Neurol., 34 (1977) 389-395. Wada, J.A., Kindling il. Raven, New York, 1981. Wada, J.A., Mizoguchi, T. and Osawa, T, Secondary generalized convulsive seizures induced by daily amygdaloid stimuIation in rhesus monkeys, .~e~~o~ogy, 2X (1978) 1026-1036. Wada, J.A., Terao, A. and Booker, HE., Longitudinal correlative analysis of epileptic baboon, Papio papio,
Neurology, 22 (1972) 1272-1285. 311 Walker, A.E. and Johnson, H.C., Convulsive factor in commercial penicillin, Arch. Surg., 50 (1945) 69-73.
312 Walton, N.Y. and Treiman, D.M., Response of status epilepticus induced by lithium and pilocarpine to treatment with diazepam, Exp. Neural., 101 (1988a) 267-275. 313 Walton, N.Y. and Treiman, D.M., Experimental secondarily generalized convulsive status epilepticus induced by o.t_-homocysteine thiolactone, Epilepsy Res., 2 (1988b) 79-86. 314 Ward, A.A., Topical convulsant metals. In D.P. Purpura,
J.K. Penry. D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimenial Modefs of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 13-35.
319
320
321
322
323
324
325
326
327
328
329
L., Tetanus. New Engl. J. Med., 289 (1973) 1293-1296. Westrum, L.E., White. L.E. and Ward, A.A., Morphology of experimental epileptic focus, J. Neurosurg., 21 (1964) 1033-1046. Wilder, B.J. and Morrell. F., Cellular behavior in secondary epileptogenic lesions, Neurology. 17 (1967) 11931204. Willmore, L.J., Recurrent seizure induced by cortical iron injection, a model of post-traumatic epilepsy, Ann. Neural.. 4 (1978) 329-333. Wilson, W.A. and Escueta, A.V.. Common synaptic effects of pentylenetetrazol and penicillin, Brain Res., 116 (1974) 217-249. Withrow, CD., Systemic carbon dioxide derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of Epilepsy - A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 477-494. Wong, R.K.S. and Prince, D.A., Dendritic mechanisms underlying penicillin-induced epileptiform activity, Science, 204 (1979) 1228-1231. Wood. J.D., Systemic oxygen derangements. In D.P. Purpura, J.K. Penry, D.M. Woodbury, D.B. Tower and R.D. Walter (Eds.), Experimental Models of epilepsy A Manual for the Laboratory Worker, Raven, New York, 1972, pp. 4.59-476. Wood. J.D. and Abrahams, D.E., The comparative effects of various hydrazides on gamma-aminobutyric acid and its metabolism. J. Neurochem., 18 (1971) 1017-1025. Wuerthele, SE., Yasuda, R.P., Freed, W.J. and Hoffer, B.J., The effects of local application of homocysteine on neuronal activity in the central nervous system of the rat, Life Sci., 31 (1982) 2683-2691. Wyler, A.R., Gjemann, G.A. and Ward Jr., A.A., Neurons in human epileptic cortex: correlation between unit and EEG activity, Ann. Neurol., 11 (1982) 301-30X. Wyler, A.R. and Ward Jr.. A.A., Neurons in human epileptic cortex: response to direct cortical stimulation, J. Neurosurg., 55 (1981) 904-908. Yamamoto, C., Intracellular study of seizure-like after discharges elicited in thin hippocampal sections in vitro. Exp. Neurol., 35 (1972) 154-164. Yamamoto, C. and McIlwain, H., Electrical activities in thin sections from the mammalian brain maintained in chemically-defined media in vitro, J. Neurochem., 13
(1966) 1333-1343. 330 Yoon, C.H.. Peterson, J.S. and Corrow, D., Spontaneous
seizures: a new mutation in Syrian golden hamsters, 1. 67 (1976) 115-116. 331 Zarkovsky, A.M., Bicucuiline-sensitive and -insensitive effects of THIP on the binding of ~3H]~unitrazepan, Hered.,
~europharmaco~og~~, 26 (1987) 737-741. 332 Zorn, S.H. and Enna, S.J., The GABA
agonist THIP, attenuates antinociception in the mouse by modifying central cholinergic transmission, Neuropharmacology, 26 (1987) 433-437.