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The perspective of GABA replenishment therapy in the epilepsies: A critical evaluation of hopes and concerns
Life Sciences, Vol. 33, pp. Printed in the U.S.A.
1629-1640
Pergamon Press
MINIREVIEW THE PERSPECTIVE OF GABA REPLENISHMENT THEXAPY IN THE EPILRPSIES:
A CRITICAL EVALUATIONOF HOPES AND CONC&RNS Ruggero G. Fariello and Maharaj K. Ticku Department of Neurology, Jefferson Medical College Philadelphia, Pennsylvania 19107 and Division of Molecular Pharmacology, Department of Pharmacology University of Texas Health Science Center at San Antonio San Antonio. Texas 70284
Summary Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Ma1 type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative. Certain diseases are characterized by neuronal degeneration or abiotrophy of specific neurochemical systems resulting in depletion of natural substances in the brain. Replenishment therapy aims to correct the neurochemical imbalance by administration of the substance in question, a percursor or an agonist. Biochemical, physiological and pharmacological studies have suggested that GABA is involved in many cerebral and extracerebral functions. Thus, "GABA replenishment" treatment has been advocated for seizures (25,28,59,60), psychosis (5,94), movement disorders (51,86,95), pain (16), hypertension (85) and sleep disturbances (12). Biochemical evidence impaired GABA-mediated inhibition is available for few of these conditions (Table 1). This review will evaluate the evidence in favor of the use of GABA agonists to treat some convulsive states, and will analyze merits and flaws of the possible therapeutic strategies. The areas in need of further exploration for safe clinical use of such therapy will be outlined. Present Knowledge of the Morphology and Pharmacology of the GABA Receptor Complex GABA appears to be the major inhibitory neurotransmitter in the mammalian CNS, producing both pre- and postsynaptic inhibition (47). In presynaptic inhibition, a nerve ending makes contact close to an excitatory synaptic bouton (FIG. 1). Postsynaptic inhibition is mediated through contact between the released transmitter and the postsynaptic neuronal membrane which causes opening of the Cl- ionophore gate. Depolarization or
0024-3205/83$3.00 + .OO Copyright (c) 1983 Pergamon Press Ltd.
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TABLE I
GABA
LA D ~~=IIII~IIIIIIILII~II=DIIII1IIIPIIIIIIID~~~~~~~~~~~~~~~~~~~~
CAT!A RCCEPTORS
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4
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SPASTICITY
7
1
?
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1
PROGABIDE VALPROIC
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N. ACCUMB L THMANUS.
76
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1
CENERMIZED SEIZURES
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7
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ACID
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ALCOHOLISM ? t? 96,102 ? ____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_____~
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hyperpolarization of the membrane will be mostly a function of membrane potential and Cl- gradient. Most inhibitory (hyperpolarizing) GABA responses occur near soma sites where excitatory (depolarizing) responses have been described in the dendrites (1,2), in the olfactory bulb (14) and sensory and biochemical cervical ganglia (9). Neurophysiological, pharmacologic and studies suggest that the postsynaptic apparatus with which GABA combines to produce its response is composed of at least three interacting components (FIG. 1): (a) (b) (c)
GABA receptor (recognition) sites Benzodiazepine binding sites Picrotoxin binding sites
The three components of the benzodiazepine-GABA receptor-ionophore complex have been characterized in vitro by radioligand binding assays using 3H GABA and 3H muscimol for GABA receptor sites (7,23,70); 3&benzodiazepines for the benzodiazepine binding site (64,89); -(3H) dihydropicrotoxinin (DHP) for the picrotoxin site (97,101) and a newly found radioligand: (35s) t- butylbycycylophosphorothionate (BBPT) (90). Binding of BBPT is also and anticonvulsants which inhibited by several classes of convulsants The in vitro binding appear to modulate GABAergic transmission (100). studies have demonstrated that these three ligands bind to three distinct The exact sites and can be used as markers for characterizing these sites. in vivo location of the binding sites in the postsynpatic membrane is unknown, and the interactions among these three components of the benzodiazPharmacological epine-GABA receptor-ionore complex are far from clear. studies have suggested the existence of various subtypes of GABA receptors A low affinity sodium dependent site, two sodium independent (10,11,61). sites (one high and one low affinity) and a calcium dependent, bicuculline These subtypes of GABA reinsensitive binding site have been identified. ceptors have different regional distribution in the encephalon (for an For instance, receptors in the extensive review of this subject, see 22). cerebellum have different properties from GABA receptors in the brainstem Extrapolation of the "in vitro" to the "in viva" and neostriatum (13).
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FIG. 1 Schematic representation of GABA action and cestiny at synaptic terminals. Presynaptic inhibition is here illustrated as if occurring on a GABAergic terminal which is an artificial conPresynaptic inhibition reduces the excitatory transstruction. mitter release by reducing the amplitude of the arriving action potential achieved through membrane depolarization and outward Binding of GABA to the postulated autoreceptor movement of Cl-. A BZP-linked and a may reduce release or synthesis of GABA. nonBZP-linked receptor are illustrated. The latter corresponds to the CA+t dependent bicuculline insensitive site: baclofen Its activation is thought to decrease monobinds to.this site. The location of this receptor is unknown aminergic activity. (?presynaptic) and its representation in the figure is arbitrary. Solid bars crossing the arrow indicated antagonism to the reaction Amino oxyacetic acid and y-acetylenic GABA or to GABA transport. For simpliantagonize both GABA synthesis and GABA catabolism. city, GABA uptake and catabolism have been illustrated only within a glial cell. 6 ALA= aminolevelinic acid; AOAA=aminoxyacetic acid; 3APS=3 aminopropanesulfonic acid; BICUC=Bicuculline; BZP=benzodiazepines; DA=dopamine; DH=dehydrogenase; GABA T=GABA transaminase; GAD=glutamic acid decarboxylase; GHB=hydroxybutirate; 5HT= serotonin; MUSC=muscimol; NA=noradenaline; PTX=picrotoxin; SSA= succinic semialdehyde. condition cannot be made since many physio-chemical variables (temperature, potential difference and ionic concentration) are remarkably different in As a consequence, it is yet unknown which one of the the two situations. many subtypes of GABA receptors is functionally important, although it has
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been suggested that the benzodiazepine-linked, low affinity binding site may be the one (22). Evidence for Involvement for GABA in Seizure Disorders Drugs which depress GABAergic transmission produce convulsions. In contrast, several of the currently used anticonvulsants influence indirectly GABAergic transmission. These facts do not provide direct evidence for an involvement of GABA in seizure disorders and have been reviewed extensively (59,60). Epilepsy is a nonhomogenous group of disorders. It is misleading to lump all epilepsies into a single category. Separating epilepsies into Focal and Primary Generalized types is also an oversimplification, but it does serve the purpose of distinguishing between two entities that, despite their common denomination may have more dissimilarities than affinities. In particular, Primary Generalized Epilepsy accompanied by EEG bilaterally synchronous spike and waves (Petit Ma1 type) differs from Focal Epilepsy in its basic pathophysiologic abnormalities (33,78), in pharmacoloWe shall, therefore, deal gical (27,38,39) and in clinical aspects. independently with Focal Epilepsy and Primary Generalized Epilepsy of Petit Ma1 type. Focal Epilepsy Most of the present knowledge of cellular epileptogenesis derives from studies of acute Focal Epilepsy. Neurons belonging to an acute epileptic focus show paroxysmal depolarization shifts (PDS's) in a sufficiently large population of neurons seems to be responsible for the appearance of the electrographic interictal spike. Interictal spikes are considered as the cornerstone upon which all epileptic activity is built on in cases of Focal Contributing factors to the generation of PDS's include enhanceEpilepsy. ment of excitatory phenomena (35,68) and impaired inhibitory function, particularly, the one mediated by GABA (3,19,21,43). Ictal discharges would originate from a breakdown of self limiting inhibitory mechanisms in the focus or projected into it through terminals originating at a distance. It has been suggested that extrasynaptic events such as ectopic action potentials contribute to the generation of interictal epileptic events (see 79 and 80 for review). Such ectopic potentials are calcium-dependent spikes that have been demonstrated also in nonepileptic neurons such as cerebellar Purkinje's cells (92). It is not clear if and how an enhancement of GABA synaptic function could affect this extrasynaptic activity. A concordance of experimental and clinical findings has strengthened the hypothesis of a pathogenetic role of GABA deficiency in Focal Epilepsy. Ribak et al. (81) have shown a decreased number of synaptic boutons containing glutamic acid decarboxylase (GAD) in alumina foci in monkeys; Bakay and Harris (5) in the same animal model have detected decreased GABA concentration, reduced GABA receptor binding and reduced GAD activity over the epileptic site; Lloyd et al. (53) have shown that some human epilaptic foci contain subnormal amounts of GAD, reduced GABA receptors and above normal GABA catabolizing enzyme (GABA trasaminase). There was great variability among various subjects, but patients with a gliotic lesion in the spiking cortex had more consistently diminished GABA receptors and GAD activity. Lloyd's data indicates that in a selected group of patients affected by Temporal Lobe Epilepsy, there is a deficiency in the GABA system which may It is not known if any be partly corrected by exogenous supply of GABA. subtype of GABA receptor is selectively involved.
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Primary Generalized Epilepsy of Petit Ma1 Type The major pathophysiological difference between Focal and Primary Generalized Epilepsy is the absence of PDS's in neurons (33,78) and therefore, the impossibility of distinguishing between epileptic and nonepileptic Most direct and all the indirect GABA agonists do not ameliorate neurons. experimental models of Primary Generalized Epilepsy with bilateral spike and On the contrary, there are several studies showing the wave discharges. Since drugs that may partly act through opposite (27,34,37,46,62,66,74). indirect potentiation of GABA activity such as dipropylacetic acid and benzodiaaepines are clinically useful in the treatment of Primary Generalized Epilepsy of the Petit Ma1 type, further studies are necessary for a better understanding of the role of inhibition in this form of epilepsy. In summary, in Focal Epilepsy, GABA deficiency: 1)
2) 3)
May play a co-factor role in the genesis of the cellular basic abnormalities leading to the production of PDS's and EEG spikes Has been detected morphologically and pharmacologically in chronic experimental foci Has been demonstrated in the foci of a small group of patients, but not in others
On the other hand, experimental work suggests that nonsynaptic phenomena participate in the genesis of basic epileptiform activity. Such calcium mediated spikes probably will not be modified by treatments aimed to act on synaptic mechanisms. In Primary Generalized Epilepsy with EEG, expression of bilaterally synchronous spike and wave experimental and clinical data disagree. The former points to a role of GABA in potentiating such epileptic activity, whereas in clinical practice, some drugs with indirect GABAmimetic action show satisfactory therapeutic efficacy. However, the relevance of the GABAmimetic property to the anticonvulsant action is still controversial. Indeed, evidence is now accumulating which demonstrates how some drugs which have been hastily called GABA agonists such as valproic acid, might rather act through a glutamatelaspartate antagonism (15). General Consideration of the Meaning of Biochemical Findings The biochemical data on the GABA system in neuropsychiatric disorders must be interpreted bearing in mind some important consideratons: I) A decreased amount of transmitters does not necessarily mean decreased function of that transmitter unless it is demonstrated that its synaptic availability is diminished 2) Knowledge of the state of the GABA receptor system is pivotal to the assessment of GABA function. In fact, if neurons carrying GABA receptors degenerate, the GABA function will result severely impaired in spite of increased amounts of nonutilized regional GABA. Furthermore, in such a case, exogenous supply or endogenous increase of GABA cannot result in increased GABA function. Three major conditions of diminished GABA function may be recognized: A) Degeneration of presynaptic elements or of the synaptic vesicles containing GAD B) Loss of postsynaptic GABA cells or of GABA receptor sites in the postsynaptic membrane. This occur either: may desensitization) or a) Reversibly (i.e., pharmacological
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b) Irreversibly neuronal (systemic degeneration) Increased GABA uptake (as it might be hypothesized in case of glial proliferation) and increased GABA metabolism.
In the event of a pathological process in which the mechanism B(a) operates, it is obvious that GABA replenishment treatment will not achieve therapeutic goals, given the absence of the effector organ. Thus, the single most important fact for applicability of the replenishment idea is the existence of a sufficiently spared receptor system which seems to occur in the above exemplified conditions A, B(a) and C. Several cases of Focal Epilepsy seem to fall in group A (53). On the Approaches to Correct GABA Dysfunction The anticonvulsant effect of GABA may be specific or nonspecific. Nonspecifically, GABA indiscriminately depresses the activity of neurons, epileptic or not, at most levels of the central nervous system. Earlier findings suggesting a pivotal role of enhanced GABA function in a small area of the brainstem, centered around the substantia nigra in suppressing generalized seizures of Grand Ma1 type, (29) have received further substantiation (40). GABA also blocks glutamate- and aspartate-induced neuronal excitation. GABA antagonistic effects toward the excess release of glutamate and aspartate (which has been proposed as one pathogenetic mechanism of some seizures) is also another possible, nonspecific, but nevertheless effective, anticonvulsant mechanism (18,103). Specifically, GABA could correct a GABA imbalance within epileptic foci. Different therapeutic strategies may be suitable in each one of the conditions of impaired GABA function described in the previous paragraph. They can be reduced to three major categories: 1) 2) 3)
Prodrug Direct GABA receptor agonist GABA uptake and/or metabolism inhibitors
GABA Compounds acting through mechanism No. 3 are called indirect agonists; compounds acting through No. 2 are called direct GABA agonists. A prodrug is defined as an inactive substance necessary to bypass insurmountable hurdles otherwise impeding the therapeutic action as in the case of of L-DOPA for dopamine replenishment. The desired pharmacological action is expleted then by a metabolite.
1)
In the case of GABA, a prodrug must bypass the excitatory glutamate step, since glutamate is a powerful agent. If the glutamate step is bypassed, a prodrug could be used rationally even when GAD activity is severely impaired. Otherwise, intact GAD function is necessary in addition to a relatively spared receptor system. Prodrugs could be used in association with uptake and/or catabolic inhibitors. Potential drawbacks with prodrug therapy include the development of suband lack of receptor downregulation) sensitivity (or specificity.
2)
Direct Agonists. The only precondition for the use of direct GABA agonists is the presence of a sufficient number of GABA receptors. Direct GABA receptor agonists may offer the advantage of possessing specific affinity for various subtypes of selectGABA receptors and therefore, a better therapeutic ivity. The major anticipated drawback of direct GABA agonists induction of receptor the possibility of consists in desensitization.
Prodrug.
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3)
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Indirect Agonists. For the achievement of successful therapeutic benefits from uptake and metabolic inhibitors, it is essential to have sufficient amounts of GAD and GABA and a They will not provide relatively spared receptor system. specificity; development of desensitization would be quite interfering possible and they may have the disadvantage of with other similar enzymatic systems and producing undesired effects related to their complex molecular structure and their metabolism. The recent discovery of "suicidal enzymes" 0Wl-l offers the hope of better selectivity for a specific enzymatic system. Substrates activate and irreversibly block a specific enzymatic system. Reviews of the ralative advantages and disadvantaees,of the use of suicidal substrates such as gammavinylgaba have been recently published (49,55,63).
What is the Best GABAmimetic Treatment for Epilepsies? From the arguments discussed in the preceding paragraphs, it seems that GABA replenishment therapy in certain types of epilepsies could be best attempted by administration of direct GABA agonists, i.e., those that act at the GABA receptor site. However, a major problem with this approach is that some GABA agonists do not cross the blood brain barrier, while others have psychoactive properties or produce metabolites with unknown effects. Alternative approaches to be explored would include synthesis of drugs which may act at the benzodiazepine or picrotoxin receptor site in a fashion which results in facilitation of GABAergic transmission. There is evidence that several anticonvulsant and depressant drugs (e.g. benzodiazepine, anxiolytic-like nonbenzodiazepine valproic acid, diphenylhydantoines, etazolate and barbiturates, etc.) facilitate GABAergic transmission by acting at these sites (6,20,36,42,56,64,67,70,77,87,93,98,99). Foreseeable Problems and Limitations With the GABA Replenishment Approach GABA replenishment The most serious anticipated problems with the approach arise from the fact that GABA deficiency may be restricted to local areas. GABA systems are ubiquitous in the CNS and systemically administered GABA has been implidrugs could induce their indiscriminate activation. autonomic and cated in motor, sensory, psychic, higher intellectual endocrine functions. Flooding the brain with GABAmimetic compounds may induce effects on all these activities. Particularly, indirect GABA agonists and prodrugs will not exert a selective action on any given GABA "subsystem". Direct GABA agonists may have different selectivity for subpopulation of GABA receptors and a relatively more targeted action may be sought through their employment. An intriguing finding is the epileptogenic action of many indirect and some direct GABA agonists. Gamma-vinyl-GABA, beta-hydroxybutyrate, muscimol and THIP induce frank, epileptiform abnormalities in the EEG and occasional myoclonus in various animal species (24,83,84,88). Higher doses induce burst-suppression on the EEG (26). An initial report of similar properties of Progabide (24) has not been confirmed using ethanol or DMSO as vehicles. Some of the GABA transaminase inhibitors block pyridoxal dependent enzymes and may cause seizures because of their antagonism to GAD, which is also a B6 dependent enzyme (73), thus diminishing GABA availability. For other compounds, the reasons for their epileptogenic actions are unclear. Also unclear is the basis for the epileptogenic action of direct GABA agonists.
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Speculative hypotheses to explain this phenomenon include:
1)
2)
3)
4) 5)
6)
At a given concentration in vivo, the drug might have higher affinity for the postulated presynaptic autoreceptor rather than for the postsynaptic binding site. This will result in a GABA antagonistic action. Delta ALA, a possible presynaptic binding agent, is a convulsant (11). Sustained activation of the Cl- ionophore may change the chloride gradient and the membrane resting potential and subsequent opening of those channels may cause cellular efflux of Cl- with reversal of GABA action. Disinhibition phenomena may occur when GABA systems impinge on The downstream neuron5 will then other inhibitory systems. become excited The drug might generate metabolites with GABA antagonistic action or acting as convulsants through other systems Acute desensitization. Adding glycine (an inhibitory neurotransmitter) to the bathing medium of spinal cultures produces paroxysmal epileptiform activity which is believed to be due to rapid desensitization (58). Predominant action of GABA agonists on the dendritic excitaGiven the high drug tory (depolarizing) receptor sites. an concentration required to evoke epileptiform activity, effect on dendritic receptors cannot be excluded in spite of the report that THIP has much lower affinity for the dendritic as compared to the somatic GABA receptor (2).
Given the absence of experimental data regarding this problem, the Neverspeculative nature of the above explanations should be emphasized. theless, we must be concerned by the fact that some GABAmimetics may cause effects similar to the abnormalities that are supposed to antagonize. the problem of the development of tolerance related to Finally, receptor downregulation or desensitization should be carefully considered. Krnjevic has recently GABA receptors do desensitize in acute condition. reviewed the probable mechanisms underlying this phenomenon (47). In acute experiments, fading effects and even reversal of action was observed after systemic repetitive administration of both GABA and 3 APS, a powerful specific GABA receptor binding agent (30). The reversing effect was not seen after multiple iontophores. Repetitive administration of indirect GABA agonists and single administration of a direct GABA receptor binding agent induce reversible alteration in the cholinergic and dopaminergic systems other (16,31,32). Given the complex interrelationships between GABA and neurotransmitters, the desensitization phenomena may induce a chain reaction involving the function of other transmitter systems. Theoretically, among GABAmimetic, direct GABA agonists have the highest probability of inducing receptor alterations. Conclusions the GABA replenishment treatment may offer a rational approach to treatment of seizure disorders in a selective subpopulation of patients with Other patients affected by Primary or Secondary Generalpartial seizures. ized Epilepsy of Grand Ma1 type may benefit, for nonspecific reasons, from The experimental evidence now available suggests GABAmimetic treatment. that in cases of Primary Generalized Epilepsy with bilaterally synchronous the spike and wave discharges, GABA agonists should be ineffective. All above outlined possible drawbacks indicate areas in need of further research and should not discourage the pursuit of this novel approach. At the
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present time, this is still the best logical alternative to a current pharmacological armamentarium essentially based on empirical experience alone. No one of the presently available anticonvulsants can convincingly test the GABA hypothesis. Progabide has been recently introduced in various countries for treatment of partial and generalized human epilepsies. The preliminary results are very encouraging (65). The scientific importance of the outcome of these studies is quite remarkable. The thesis of a pathogenetic role of GABA in at least some of the human epilepsies awaits a crucial test that can be provided only by a drug, such as Progabide, acting through unquestionable and predominant GABAmimetic properties.
Acknowledgement The skillful acknowledged.
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C.A. TAMMINGA, J.W. CRAYTON and T.N. CHASE, Arch. Gen. Psychiat. -36 595-598 (1979). 96. Br. J. Pharmacol. 70 M.K. TICKU, 403-410 (1980). M.K. TICKU, M. BAN and R.W. OLSON, Mol. Pharmazl. 14 391-402. (1978). 97. 218-222 (1981). 98. M.K. TICKU, and W.C. DAVIS, Brain Res. 22399. M.K. TICKU and R.W. OLSEN, Life Sci. 22 1643-1651 (1978). Neurotransmitters 100. M.K. TICKU, R. THYAGARAJAN and R. RAMANJANEYfi, Seizures and Epilepsy II. Edited by R.G. FARIELLO, J. ENGEL, JR., P.L. MORSELLI and L.F. QUESNEY, Raven Press, New York, (In Press). 101. M.K. TICKU, P. VANNESS, J.W. HAYCOCK, W.B. LEVY and R.W. OLSEN, Brain Res. 150 642-647 (1978). 102. V. T&, S.H. SNYDER, F.L. MAJOR and R.J. HAWLEY, Ann. Neurol. 2 289292 (1981). 103. N.M. VAN Edited GELDER, Aminoacids Neurotransmitters. by DEFEUDIS and P. MANDEL, pp. 115-125, Press, New York F.V. Raven (1981).
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W.W.
TOURTELLOTTE and
1629-1640
Pergamon Press
MINIREVIEW THE PERSPECTIVE OF GABA REPLENISHMENT THEXAPY IN THE EPILRPSIES:
A CRITICAL EVALUATIONOF HOPES AND CONC&RNS Ruggero G. Fariello and Maharaj K. Ticku Department of Neurology, Jefferson Medical College Philadelphia, Pennsylvania 19107 and Division of Molecular Pharmacology, Department of Pharmacology University of Texas Health Science Center at San Antonio San Antonio. Texas 70284
Summary Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Ma1 type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative. Certain diseases are characterized by neuronal degeneration or abiotrophy of specific neurochemical systems resulting in depletion of natural substances in the brain. Replenishment therapy aims to correct the neurochemical imbalance by administration of the substance in question, a percursor or an agonist. Biochemical, physiological and pharmacological studies have suggested that GABA is involved in many cerebral and extracerebral functions. Thus, "GABA replenishment" treatment has been advocated for seizures (25,28,59,60), psychosis (5,94), movement disorders (51,86,95), pain (16), hypertension (85) and sleep disturbances (12). Biochemical evidence impaired GABA-mediated inhibition is available for few of these conditions (Table 1). This review will evaluate the evidence in favor of the use of GABA agonists to treat some convulsive states, and will analyze merits and flaws of the possible therapeutic strategies. The areas in need of further exploration for safe clinical use of such therapy will be outlined. Present Knowledge of the Morphology and Pharmacology of the GABA Receptor Complex GABA appears to be the major inhibitory neurotransmitter in the mammalian CNS, producing both pre- and postsynaptic inhibition (47). In presynaptic inhibition, a nerve ending makes contact close to an excitatory synaptic bouton (FIG. 1). Postsynaptic inhibition is mediated through contact between the released transmitter and the postsynaptic neuronal membrane which causes opening of the Cl- ionophore gate. Depolarization or
0024-3205/83$3.00 + .OO Copyright (c) 1983 Pergamon Press Ltd.
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TABLE I
GABA
LA D ~~=IIII~IIIIIIILII~II=DIIII1IIIPIIIIIIID~~~~~~~~~~~~~~~~~~~~
CAT!A RCCEPTORS
54,OP
PARKINSONN 52 1 ;;,;yN, 4 1% ____________________~~~~_~~~_~~~~~~~~~~~~~~~~~~~~~~~-----~~~ HUNTINGTOM CHOREA
7’3 I
NONE
a,s7,9i 1
1 ::;“’
4
CI
cLItJIc*L. TRIALS
4
,“%:~:~,,,A,,-I VINYLGAHA
*_‘I
____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_~~___
SPASTICITY
7
1
?
BACLOFEN
+
llUsCIMOL
-
:zc!:F,cID ---_------__~
1
PROGABIDE VALPROIC
, +
____________________~_~~_~~~~~~~~~~~~~~~~~~~~~~~~~~_~~__~___ SCHIZOPHREWIA
N. ACCUMB L THMANUS.
76
1 ____________________~~~~~~~~~~~~~~~~~~~~~~~~__~_____________ PARTIAL S,S3,81 SEIZURES c s i ____________________~~~~~~~~~~~~~~~~~~~~~__~___
1
CENERMIZED SEIZURES
?
?
7
OR
N 5, 53
ACID
____________________~---~~-~~-~~~~~~~~~~~--~~~~~~~~~~~~~~~~~
ALCOHOLISM ? t? 96,102 ? ____________________~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~_____~
NONE
PAIN
?
THIP
+
7
THIP rlusc IWOI. PROCIBIDE
c_ 7
t
?
____________________~---~~-~-~~~~~~~~~~~~~~~~~~~~~~~~~__~___ TARDIVE DYSKINESIA
?
?
____________________~~~~~~~_~~~__~__~___~_~~________________
hyperpolarization of the membrane will be mostly a function of membrane potential and Cl- gradient. Most inhibitory (hyperpolarizing) GABA responses occur near soma sites where excitatory (depolarizing) responses have been described in the dendrites (1,2), in the olfactory bulb (14) and sensory and biochemical cervical ganglia (9). Neurophysiological, pharmacologic and studies suggest that the postsynaptic apparatus with which GABA combines to produce its response is composed of at least three interacting components (FIG. 1): (a) (b) (c)
GABA receptor (recognition) sites Benzodiazepine binding sites Picrotoxin binding sites
The three components of the benzodiazepine-GABA receptor-ionophore complex have been characterized in vitro by radioligand binding assays using 3H GABA and 3H muscimol for GABA receptor sites (7,23,70); 3&benzodiazepines for the benzodiazepine binding site (64,89); -(3H) dihydropicrotoxinin (DHP) for the picrotoxin site (97,101) and a newly found radioligand: (35s) t- butylbycycylophosphorothionate (BBPT) (90). Binding of BBPT is also and anticonvulsants which inhibited by several classes of convulsants The in vitro binding appear to modulate GABAergic transmission (100). studies have demonstrated that these three ligands bind to three distinct The exact sites and can be used as markers for characterizing these sites. in vivo location of the binding sites in the postsynpatic membrane is unknown, and the interactions among these three components of the benzodiazPharmacological epine-GABA receptor-ionore complex are far from clear. studies have suggested the existence of various subtypes of GABA receptors A low affinity sodium dependent site, two sodium independent (10,11,61). sites (one high and one low affinity) and a calcium dependent, bicuculline These subtypes of GABA reinsensitive binding site have been identified. ceptors have different regional distribution in the encephalon (for an For instance, receptors in the extensive review of this subject, see 22). cerebellum have different properties from GABA receptors in the brainstem Extrapolation of the "in vitro" to the "in viva" and neostriatum (13).
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FIG. 1 Schematic representation of GABA action and cestiny at synaptic terminals. Presynaptic inhibition is here illustrated as if occurring on a GABAergic terminal which is an artificial conPresynaptic inhibition reduces the excitatory transstruction. mitter release by reducing the amplitude of the arriving action potential achieved through membrane depolarization and outward Binding of GABA to the postulated autoreceptor movement of Cl-. A BZP-linked and a may reduce release or synthesis of GABA. nonBZP-linked receptor are illustrated. The latter corresponds to the CA+t dependent bicuculline insensitive site: baclofen Its activation is thought to decrease monobinds to.this site. The location of this receptor is unknown aminergic activity. (?presynaptic) and its representation in the figure is arbitrary. Solid bars crossing the arrow indicated antagonism to the reaction Amino oxyacetic acid and y-acetylenic GABA or to GABA transport. For simpliantagonize both GABA synthesis and GABA catabolism. city, GABA uptake and catabolism have been illustrated only within a glial cell. 6 ALA= aminolevelinic acid; AOAA=aminoxyacetic acid; 3APS=3 aminopropanesulfonic acid; BICUC=Bicuculline; BZP=benzodiazepines; DA=dopamine; DH=dehydrogenase; GABA T=GABA transaminase; GAD=glutamic acid decarboxylase; GHB=hydroxybutirate; 5HT= serotonin; MUSC=muscimol; NA=noradenaline; PTX=picrotoxin; SSA= succinic semialdehyde. condition cannot be made since many physio-chemical variables (temperature, potential difference and ionic concentration) are remarkably different in As a consequence, it is yet unknown which one of the the two situations. many subtypes of GABA receptors is functionally important, although it has
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been suggested that the benzodiazepine-linked, low affinity binding site may be the one (22). Evidence for Involvement for GABA in Seizure Disorders Drugs which depress GABAergic transmission produce convulsions. In contrast, several of the currently used anticonvulsants influence indirectly GABAergic transmission. These facts do not provide direct evidence for an involvement of GABA in seizure disorders and have been reviewed extensively (59,60). Epilepsy is a nonhomogenous group of disorders. It is misleading to lump all epilepsies into a single category. Separating epilepsies into Focal and Primary Generalized types is also an oversimplification, but it does serve the purpose of distinguishing between two entities that, despite their common denomination may have more dissimilarities than affinities. In particular, Primary Generalized Epilepsy accompanied by EEG bilaterally synchronous spike and waves (Petit Ma1 type) differs from Focal Epilepsy in its basic pathophysiologic abnormalities (33,78), in pharmacoloWe shall, therefore, deal gical (27,38,39) and in clinical aspects. independently with Focal Epilepsy and Primary Generalized Epilepsy of Petit Ma1 type. Focal Epilepsy Most of the present knowledge of cellular epileptogenesis derives from studies of acute Focal Epilepsy. Neurons belonging to an acute epileptic focus show paroxysmal depolarization shifts (PDS's) in a sufficiently large population of neurons seems to be responsible for the appearance of the electrographic interictal spike. Interictal spikes are considered as the cornerstone upon which all epileptic activity is built on in cases of Focal Contributing factors to the generation of PDS's include enhanceEpilepsy. ment of excitatory phenomena (35,68) and impaired inhibitory function, particularly, the one mediated by GABA (3,19,21,43). Ictal discharges would originate from a breakdown of self limiting inhibitory mechanisms in the focus or projected into it through terminals originating at a distance. It has been suggested that extrasynaptic events such as ectopic action potentials contribute to the generation of interictal epileptic events (see 79 and 80 for review). Such ectopic potentials are calcium-dependent spikes that have been demonstrated also in nonepileptic neurons such as cerebellar Purkinje's cells (92). It is not clear if and how an enhancement of GABA synaptic function could affect this extrasynaptic activity. A concordance of experimental and clinical findings has strengthened the hypothesis of a pathogenetic role of GABA deficiency in Focal Epilepsy. Ribak et al. (81) have shown a decreased number of synaptic boutons containing glutamic acid decarboxylase (GAD) in alumina foci in monkeys; Bakay and Harris (5) in the same animal model have detected decreased GABA concentration, reduced GABA receptor binding and reduced GAD activity over the epileptic site; Lloyd et al. (53) have shown that some human epilaptic foci contain subnormal amounts of GAD, reduced GABA receptors and above normal GABA catabolizing enzyme (GABA trasaminase). There was great variability among various subjects, but patients with a gliotic lesion in the spiking cortex had more consistently diminished GABA receptors and GAD activity. Lloyd's data indicates that in a selected group of patients affected by Temporal Lobe Epilepsy, there is a deficiency in the GABA system which may It is not known if any be partly corrected by exogenous supply of GABA. subtype of GABA receptor is selectively involved.
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Primary Generalized Epilepsy of Petit Ma1 Type The major pathophysiological difference between Focal and Primary Generalized Epilepsy is the absence of PDS's in neurons (33,78) and therefore, the impossibility of distinguishing between epileptic and nonepileptic Most direct and all the indirect GABA agonists do not ameliorate neurons. experimental models of Primary Generalized Epilepsy with bilateral spike and On the contrary, there are several studies showing the wave discharges. Since drugs that may partly act through opposite (27,34,37,46,62,66,74). indirect potentiation of GABA activity such as dipropylacetic acid and benzodiaaepines are clinically useful in the treatment of Primary Generalized Epilepsy of the Petit Ma1 type, further studies are necessary for a better understanding of the role of inhibition in this form of epilepsy. In summary, in Focal Epilepsy, GABA deficiency: 1)
2) 3)
May play a co-factor role in the genesis of the cellular basic abnormalities leading to the production of PDS's and EEG spikes Has been detected morphologically and pharmacologically in chronic experimental foci Has been demonstrated in the foci of a small group of patients, but not in others
On the other hand, experimental work suggests that nonsynaptic phenomena participate in the genesis of basic epileptiform activity. Such calcium mediated spikes probably will not be modified by treatments aimed to act on synaptic mechanisms. In Primary Generalized Epilepsy with EEG, expression of bilaterally synchronous spike and wave experimental and clinical data disagree. The former points to a role of GABA in potentiating such epileptic activity, whereas in clinical practice, some drugs with indirect GABAmimetic action show satisfactory therapeutic efficacy. However, the relevance of the GABAmimetic property to the anticonvulsant action is still controversial. Indeed, evidence is now accumulating which demonstrates how some drugs which have been hastily called GABA agonists such as valproic acid, might rather act through a glutamatelaspartate antagonism (15). General Consideration of the Meaning of Biochemical Findings The biochemical data on the GABA system in neuropsychiatric disorders must be interpreted bearing in mind some important consideratons: I) A decreased amount of transmitters does not necessarily mean decreased function of that transmitter unless it is demonstrated that its synaptic availability is diminished 2) Knowledge of the state of the GABA receptor system is pivotal to the assessment of GABA function. In fact, if neurons carrying GABA receptors degenerate, the GABA function will result severely impaired in spite of increased amounts of nonutilized regional GABA. Furthermore, in such a case, exogenous supply or endogenous increase of GABA cannot result in increased GABA function. Three major conditions of diminished GABA function may be recognized: A) Degeneration of presynaptic elements or of the synaptic vesicles containing GAD B) Loss of postsynaptic GABA cells or of GABA receptor sites in the postsynaptic membrane. This occur either: may desensitization) or a) Reversibly (i.e., pharmacological
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b) Irreversibly neuronal (systemic degeneration) Increased GABA uptake (as it might be hypothesized in case of glial proliferation) and increased GABA metabolism.
In the event of a pathological process in which the mechanism B(a) operates, it is obvious that GABA replenishment treatment will not achieve therapeutic goals, given the absence of the effector organ. Thus, the single most important fact for applicability of the replenishment idea is the existence of a sufficiently spared receptor system which seems to occur in the above exemplified conditions A, B(a) and C. Several cases of Focal Epilepsy seem to fall in group A (53). On the Approaches to Correct GABA Dysfunction The anticonvulsant effect of GABA may be specific or nonspecific. Nonspecifically, GABA indiscriminately depresses the activity of neurons, epileptic or not, at most levels of the central nervous system. Earlier findings suggesting a pivotal role of enhanced GABA function in a small area of the brainstem, centered around the substantia nigra in suppressing generalized seizures of Grand Ma1 type, (29) have received further substantiation (40). GABA also blocks glutamate- and aspartate-induced neuronal excitation. GABA antagonistic effects toward the excess release of glutamate and aspartate (which has been proposed as one pathogenetic mechanism of some seizures) is also another possible, nonspecific, but nevertheless effective, anticonvulsant mechanism (18,103). Specifically, GABA could correct a GABA imbalance within epileptic foci. Different therapeutic strategies may be suitable in each one of the conditions of impaired GABA function described in the previous paragraph. They can be reduced to three major categories: 1) 2) 3)
Prodrug Direct GABA receptor agonist GABA uptake and/or metabolism inhibitors
GABA Compounds acting through mechanism No. 3 are called indirect agonists; compounds acting through No. 2 are called direct GABA agonists. A prodrug is defined as an inactive substance necessary to bypass insurmountable hurdles otherwise impeding the therapeutic action as in the case of of L-DOPA for dopamine replenishment. The desired pharmacological action is expleted then by a metabolite.
1)
In the case of GABA, a prodrug must bypass the excitatory glutamate step, since glutamate is a powerful agent. If the glutamate step is bypassed, a prodrug could be used rationally even when GAD activity is severely impaired. Otherwise, intact GAD function is necessary in addition to a relatively spared receptor system. Prodrugs could be used in association with uptake and/or catabolic inhibitors. Potential drawbacks with prodrug therapy include the development of suband lack of receptor downregulation) sensitivity (or specificity.
2)
Direct Agonists. The only precondition for the use of direct GABA agonists is the presence of a sufficient number of GABA receptors. Direct GABA receptor agonists may offer the advantage of possessing specific affinity for various subtypes of selectGABA receptors and therefore, a better therapeutic ivity. The major anticipated drawback of direct GABA agonists induction of receptor the possibility of consists in desensitization.
Prodrug.
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3)
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Indirect Agonists. For the achievement of successful therapeutic benefits from uptake and metabolic inhibitors, it is essential to have sufficient amounts of GAD and GABA and a They will not provide relatively spared receptor system. specificity; development of desensitization would be quite interfering possible and they may have the disadvantage of with other similar enzymatic systems and producing undesired effects related to their complex molecular structure and their metabolism. The recent discovery of "suicidal enzymes" 0Wl-l offers the hope of better selectivity for a specific enzymatic system. Substrates activate and irreversibly block a specific enzymatic system. Reviews of the ralative advantages and disadvantaees,of the use of suicidal substrates such as gammavinylgaba have been recently published (49,55,63).
What is the Best GABAmimetic Treatment for Epilepsies? From the arguments discussed in the preceding paragraphs, it seems that GABA replenishment therapy in certain types of epilepsies could be best attempted by administration of direct GABA agonists, i.e., those that act at the GABA receptor site. However, a major problem with this approach is that some GABA agonists do not cross the blood brain barrier, while others have psychoactive properties or produce metabolites with unknown effects. Alternative approaches to be explored would include synthesis of drugs which may act at the benzodiazepine or picrotoxin receptor site in a fashion which results in facilitation of GABAergic transmission. There is evidence that several anticonvulsant and depressant drugs (e.g. benzodiazepine, anxiolytic-like nonbenzodiazepine valproic acid, diphenylhydantoines, etazolate and barbiturates, etc.) facilitate GABAergic transmission by acting at these sites (6,20,36,42,56,64,67,70,77,87,93,98,99). Foreseeable Problems and Limitations With the GABA Replenishment Approach GABA replenishment The most serious anticipated problems with the approach arise from the fact that GABA deficiency may be restricted to local areas. GABA systems are ubiquitous in the CNS and systemically administered GABA has been implidrugs could induce their indiscriminate activation. autonomic and cated in motor, sensory, psychic, higher intellectual endocrine functions. Flooding the brain with GABAmimetic compounds may induce effects on all these activities. Particularly, indirect GABA agonists and prodrugs will not exert a selective action on any given GABA "subsystem". Direct GABA agonists may have different selectivity for subpopulation of GABA receptors and a relatively more targeted action may be sought through their employment. An intriguing finding is the epileptogenic action of many indirect and some direct GABA agonists. Gamma-vinyl-GABA, beta-hydroxybutyrate, muscimol and THIP induce frank, epileptiform abnormalities in the EEG and occasional myoclonus in various animal species (24,83,84,88). Higher doses induce burst-suppression on the EEG (26). An initial report of similar properties of Progabide (24) has not been confirmed using ethanol or DMSO as vehicles. Some of the GABA transaminase inhibitors block pyridoxal dependent enzymes and may cause seizures because of their antagonism to GAD, which is also a B6 dependent enzyme (73), thus diminishing GABA availability. For other compounds, the reasons for their epileptogenic actions are unclear. Also unclear is the basis for the epileptogenic action of direct GABA agonists.
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Speculative hypotheses to explain this phenomenon include:
1)
2)
3)
4) 5)
6)
At a given concentration in vivo, the drug might have higher affinity for the postulated presynaptic autoreceptor rather than for the postsynaptic binding site. This will result in a GABA antagonistic action. Delta ALA, a possible presynaptic binding agent, is a convulsant (11). Sustained activation of the Cl- ionophore may change the chloride gradient and the membrane resting potential and subsequent opening of those channels may cause cellular efflux of Cl- with reversal of GABA action. Disinhibition phenomena may occur when GABA systems impinge on The downstream neuron5 will then other inhibitory systems. become excited The drug might generate metabolites with GABA antagonistic action or acting as convulsants through other systems Acute desensitization. Adding glycine (an inhibitory neurotransmitter) to the bathing medium of spinal cultures produces paroxysmal epileptiform activity which is believed to be due to rapid desensitization (58). Predominant action of GABA agonists on the dendritic excitaGiven the high drug tory (depolarizing) receptor sites. an concentration required to evoke epileptiform activity, effect on dendritic receptors cannot be excluded in spite of the report that THIP has much lower affinity for the dendritic as compared to the somatic GABA receptor (2).
Given the absence of experimental data regarding this problem, the Neverspeculative nature of the above explanations should be emphasized. theless, we must be concerned by the fact that some GABAmimetics may cause effects similar to the abnormalities that are supposed to antagonize. the problem of the development of tolerance related to Finally, receptor downregulation or desensitization should be carefully considered. Krnjevic has recently GABA receptors do desensitize in acute condition. reviewed the probable mechanisms underlying this phenomenon (47). In acute experiments, fading effects and even reversal of action was observed after systemic repetitive administration of both GABA and 3 APS, a powerful specific GABA receptor binding agent (30). The reversing effect was not seen after multiple iontophores. Repetitive administration of indirect GABA agonists and single administration of a direct GABA receptor binding agent induce reversible alteration in the cholinergic and dopaminergic systems other (16,31,32). Given the complex interrelationships between GABA and neurotransmitters, the desensitization phenomena may induce a chain reaction involving the function of other transmitter systems. Theoretically, among GABAmimetic, direct GABA agonists have the highest probability of inducing receptor alterations. Conclusions the GABA replenishment treatment may offer a rational approach to treatment of seizure disorders in a selective subpopulation of patients with Other patients affected by Primary or Secondary Generalpartial seizures. ized Epilepsy of Grand Ma1 type may benefit, for nonspecific reasons, from The experimental evidence now available suggests GABAmimetic treatment. that in cases of Primary Generalized Epilepsy with bilaterally synchronous the spike and wave discharges, GABA agonists should be ineffective. All above outlined possible drawbacks indicate areas in need of further research and should not discourage the pursuit of this novel approach. At the
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present time, this is still the best logical alternative to a current pharmacological armamentarium essentially based on empirical experience alone. No one of the presently available anticonvulsants can convincingly test the GABA hypothesis. Progabide has been recently introduced in various countries for treatment of partial and generalized human epilepsies. The preliminary results are very encouraging (65). The scientific importance of the outcome of these studies is quite remarkable. The thesis of a pathogenetic role of GABA in at least some of the human epilepsies awaits a crucial test that can be provided only by a drug, such as Progabide, acting through unquestionable and predominant GABAmimetic properties.
Acknowledgement The skillful acknowledged.
secretarial
assistance
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