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Interaction of cortex and thalamus in spike and wave discharges of feline generalized penicillin epilepsy
EXPERIMENTAL
NEUROLOGY
76, 196-217
(1982)
Interaction of Cortex and Thalamus in Spike and Wave Discharges of Feline Generalized Penicillin Epilepsy MASSIMO AVOLI AND PIERRE GLOOR' Department
of Neurology and Neurosurgery, Neurological Institute, Montreal, Received
August
McGill University and Montreal Quebec, H3A 284, Canada 31. 1981
The transition from spindles to spike and wave (SW) discharges of feline generalized penicillin epilepsy was studied using simultaneous EEG recordings from mutually related cortical and thalamic sites after i.m. injection (350,000 IU/kg) or diffuse cortical application of a weak solution (100-300 IU/hemisphere) of penicillin. Both procedures induced similar changes at cortical and thalamic levels, those in the thalamus developing at the same time or slightly later but never earlier than in the cortex. These changes consisted of: (i) amplitude increase of spindles, development of positive phases, and decrease in amplitude, followed by disappearance of every second spindle wave as SW discharges developed, (ii) facilitation, progressive amplitude increase, and increase or development of positive phases of recruiting responses to midline thalamic stimulation. Once SW had developed, a decrease in cortical excitability by cortical application of 15% KC1 caused the cortical and thalamic SW discharges to disappear and to be replaced by spindles. These results demonstrate that important changes in thalamic activity occur during the development of cortical SW discharge whether induced by i.m. penicillin or by diffusecortical application of a weak penicillin solution. Changes in thalamic activity appear to he secondary to changes in cortical activity. Thus, although cortical SWs are triggered by thalamocortical inputs which originally were spindle-inducing, these inputs change after penicillin, and reflect an alteration in thalamic activity imposed by the cortex through corticothalamic volleys. In their turn, they modify the cortical response. Abbreviations: SW-spike and wave; FGPE-feline generalized penicillin epilepsy; EEGelectroencephalogram; MSS-middle suprasylvian gyrus; RRs-recruiting responses; LPnucleus lateralis posterior; VL, VPL-nucleus ventrolateralis, ventropostero-lateralis; NCMnucleus centralis medialis; AS-anterior sigmoid gyrus. ’ This work was supported by grant MT-3140 awarded by the Medical Research Council of Canada to Dr. Gloor. Dr. Avoli was a Killam Scholar. The authors thank Dr. G. Kostopoulos for helpful criticism, Mrs. S. Schiller and Mrs. T. De La Fosse for technical assistance, as well as Miss G. Robillard and Mrs. K. Douglas for secretarial assistance. 196 0014-4886/82/040196-22$02.00/O Copyright All rights
(d 1982 by Academic Press. Inc. of reproduction in any form rcserwd
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INTRODUCTION Feline generalized penicillin epilepsy (FGPE), produced by i.m. injection of a large dose of penicillin, is an experimental model which bears many electroencephalographic (EEG), behavioral, and pharmacologic similarities to human generalized corticoreticular (“centrencephalic”) epilepsy (5, 8, 10, 12, 13, 19, 23). Since the publication of the classic paper by Jasper and DroogleeverFortuyn ( 17) thalamocortical connections have been known to play an important role in the genesis of bilaterally synchronous spike and wave (SW) discharges. The study of the role of these connections has been the focus of interest of our previous research on the mechanism of SW discharges of FGPE (8). To date, the main emphasis has been on the cortical mechanisms of epileptic discharge in this model. It was suggested that the generalized SW discharges of FGPE represent the response of hyperexcitable cortex to thalamocortical volleys, normally evoking spindles and/or recruiting responses (10, 11, 24, 25). Furthermore, a transition from spindles to SW discharges after the i.m. injection of penicillin was demonstrated both at A
Medial
Row
Lateral
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FIG. I. Determination of mutually related sites in nucleus lateralis posterior (LP) and the middle suprasylvian gyrus (MSS). A-averaged potentials (20 samples) evoked by single shock (0.2 ms, 0.8 mA) delivered at two LP sites (A and 0) and recorded by a row of silver ball electrodes placed in two different (medial and lateral) areas of the MSS. B--location of the two electrode rows in the MSS. C-sites of stimulation in the LP. Note that in this experiment the short-latency component of the evoked field potential displays its largest amplitude in lead 2 of the medial row.
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the EEG and unitary level (19, 20). The fundamental change in this transition appears to be an increased firing of cortical neurons in association with individual spindle waves as the latter were transformed into the “spike” of SW discharges (20). There was some evidence that changes in excitability at the thalamic level were not required for producing the epileptic bursts in this model (11, 24) although SW discharges are known to be crucially dependent on thalamic inputs to the cortex (3, 22). However, this view may be simplistic and deserves reexamination, because little work has been done so far on thalamic mechanisms in this model by using direct-thalamic recordings. It has, however, been known that the thalamic EEG assumes a pattern similar to that seen in the cortex when the SW bursts of FGPE have fully developed (5, 25, 27). How this came about remained unknown. The present paper is an EEG study dealing with this issue. A microphysiologic study of the same topic is presently in progress. METHODS Anesthesia and Surgical Procedures. Acute experiments were carried out in cats of either sex weighing between 2.6 and 4.2 kg. The surgical, analgesic, and neuroleptic procedures were described previously (19). Experiments were carried out in awake animals immobilized with gallamine triethiodide (Flaxedil) and restrained by a painless method. Wound edges were locally anesthetized with novocaine. The CO2 content of the expiratory air was monitored by a Beckman gas analyzer and maintained at 3.5 to 4%. Body temperature was maintained at 38.0 & 0.5”C by an electric blanket controlled through a rectal probe. Recording and Stimulation. The EEG was recorded directly from the cortex using an 8-channel Elema-Schoenander Mingograf. Bipolar and monopolar recordings were taken bilaterally from the middle suprasylvian gyrus (MSS) and the anterior sigmoid gyrus (AS). The reference electrode
FIG. 2. Transition from spindles to spike and wave (SW) potentials in a cat with continuous i.v. infusion of sodium thiopental. Spindles and SW bursts were triggered by single-shock stimulation to NCM. In this and the following figures, abbreviations are as follows: ASanterior sigmoid gyrus, MSS-middle suprasylvian gyrus, LP-nucleus lateralis posterior, VL-nucleus ventralis lateralis, VPL-nucleus ventralis posterolateralis, NCM-nucleus centralis medialis, R-right, L-left, (A)-anterior, (P&-posterior. Single-shock stimulation delivered to the NCM: 0.5 ms, 8 mA, 1 every 10 s. Samples are taken before (control) and at different times (numbers on top indicate minutes) after i.m. injection of penicillin. Enlarged samples show EEG activity of the two functionally related sites in R MSS (A) and R LP, before as well as 50 and 75 min after penicillin, respectively.
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was attached to the neck muscles. Concentric bipolar electrodes (interelectrode distance 0.8 to 1 mm, resistance 40 to 50 Kfi) were placed in the nucleus centralis medialis (NCM) (A, 10.5; V, 1; L, 0), nucleus lateralis posterior (LP) (A, 6.5 to 8; V, 3 to 6; L, 4.5), nucleus ventrolateralis (VL) (A, 10.5; V, 1 to 2; L, 4.5), and nucleus ventroposterolateralis (VPL) (A, 9; V, 1; L, 7.5) of the thalamus. The atlas of Jasper and Ajmone-Marsan ( 16) was used for establishing stereotaxic coordinates. Bipolar recordings from depth structures were used in all experiments to minimize the danger of picking up distant electrical activity (e.g., of cortex or hippocampus) by volume conduction (2). Single shock or low-frequency repetitive stimulation (5 to 10 Hz) was delivered through the depth electrodes using a constant-current Nuclear Chicago or WPI stimulator (0.2 to 0.5 ms, 0.5 to 10 mA). The evoked responses were averaged by a Fabri-Tek 1052 (1024 point-signal) averager. Electrode placements were verified in gross postmortem or histologic preparations. Determination of Functionally
Related Thalamic and Cortical Sites.
Because of earlier work indicating that the SW discharges of FGPE appear first and usually predominate in the MSS (21), particular attention was given to recording from functionally related sites in the MSS and LP (15, 18). By stimulating with single shocks (0.1 to 0.2 ms, 0.5 to 0.8 mA) at various depths in the LP we determined in each experiment the MSS area where the short-latency field potential (5 to 9 ms peak latency) evoked by the LP displayed maximal amplitude (Fig. 1). We assumed that this indicated that the two sites were closely interconnected. During the entire experiment the EEG activity was recorded at these cortical and thalamic sites. Pharmacologic Procedures. Generalized penicillin epilepsy was induced (i) in 20 cats by the i.m. injection of 350,000 II-J/kg sodium penicillin G and (ii) in seven animals by applying to large areas of cerebral cortex of each hemisphere a filter paper soaked in a weak aqueous solution of penicillin G (100 to 300 II-J/hemisphere). Very small i.v. doses of sodium thiopental (0.5 to 1 mg/kg/h) were continuously given throughout the experiment to seven cats which were injected with penicillin and three cats with epicortical penicillin application. This was done because spindles, either spontaneous or evoked, were not consistently present in undrugged cats, as the animals’ EEGs showed an alternation of sleep and waking patterns. The use of thiopental infusion created a condition under which single shocks to the NCM or LP consistently elicited spindle bursts so that the transition from spindles to SW could be continuously studied (19, 20). This small amount of barbiturate was insufficient to produce an appreciable increase in spontaneous spindling.
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Cortical spreading depression was induced in seven cats by applying to the cerebral cortex a small, square (2 X 2 mm) filter paper soaked in a 15% KC1 solution. RESULTS The data collected from cats with or without continuous infusion of very low doses of thiopental will be described together, because they were similar. Spindles recorded from the cortex (both in the MSS and AS) consisted mainly of negative waves (100 to 250 pV, 7 to 14 Hz) which resembled the “recruiting” or type II spindles of Spencer and Brookhart (28). However, additional doses of barbiturate induced the appearance of biphasic negative-positive waves [“augmenting” or type I spindles of Spencer and Brookhart (28)]. Synchronously with the cortical spindles there occurred thalamic spindles with frequencies identical to those recorded in the cortex. Rarely spindle bursts of type II were recorded only in the cortex and not in the corresponding thalamic structures (7). Parenteral
Administration
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Transition from Spindles to Spike and Wave. As previously reported by Kostopoulos et al. (19, 20) the first change observed in the cortex 15 to 40 min after the i.m. injection of penicillin, was a progressive increase in the amplitude of spindle waves associated with the development or amplitude increase of positive phases which either preceded or followed the negative ones (Fig. 2, +32 min; Fig. 3, +42 min). Coincident with this change, one of every two cortical spindle waves started to decrease in amplitude and ultimately disappeared, the remaining spindle wave gradually developing into the spike of the SW discharge (Fig. 2, +50 and 58
FIG. 4. Recruiting responses (RRs) during transition from spindles to spike and wave (SW) potentials induced by i.m. injection of penicillin. A-superimposed recordings from the MSS and LP of RRs induced by low-frequency (S/s) repetitive stimulation (0.4 ms. 2 mA) of the NCM (0); results taken before (control) and at different times (numbers on the left indicate minutes) after i.m. penicillin; time calibration: 200 ms. B-averaged RRs (30 traces) recorded in another cat during transition to SW after i.m. penicillin; low frequency (7/s) repetitive stimulation (0.2 ms, 1.5 mA) of NCM (0); explanation as in A; time calibration: 400 ms. Stimulation parameters remained constant throughout the experiment in A and B. C-amplitudes (ordinate, millivolts) of positive and negative phases of RRs recorded in MSS and LP at different times (in minutes, abscissa) after penicillin in five different experiments. Only the first detectable evoked potential (e.g., the fourth in A and the sixth in B) was plotted at each time interval after penicillin. The different symbols (0, A, n , 0, +) refer to different animals.
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FIG. 5. Percentage increase (ordinates) of both positive and negative phases of recruiting responses (RRs) in cortex (MSS, -) and thalamus (LP, - - -) at different times (in minutes, abscissa) after penicillin. Same results and symbols as shown in Fig. 4C. Note that
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min; Fig. 3, +63 and +75 mm). The slow wave of the SW complex, which ultimately replaced the reduced spindle wave, was poorly developed as had often been observed previously in acute experiments ( 10). In addition, whereas spindles before penicillin in undrugged cats were usually recorded from one or two of the cortical areas investigated, the modified spindle bursts or incompletely developed SW bursts were often present bilaterally and synchronously in all cortical areas from which records were taken (compare control and +63 min in Fig. 3). Thalamic spindles often showed similar changes after penicillin, though these were less clear and consistent than in the cortex (Figs. 2, 3). At other times thalamic spindles were decreased in amplitude, unidentifiable, or even missing during the transition period. In a few cases it was possible to record cortical spindles displaying the above-described changes while the thalamus was still producing normal spindles. Penicillin-induced changes were never observed in the thalamus before they were seen in the cortex. Changes in Recruiting Responses (RRs) during the Transition to Spike and Wave. In five experiments, RRs recorded from the MSS were analyzed
during the transition from spindles to SW after i.m. penicillin. The RRs were evoked by repetitive stimulation of the NCM (5 to 8 Hz) using pulses of an intensity of 0.2 to 1 mA (200 to 500 ps) which, before penicillin, evoked responses only with the third to fifth stimulus of a train. These responses consisted mainly of negative or negative-positive cortical waves. The subsequent stimuli in the train produced cortical potentials with recruiting features. Throughout the entire experiment the stimulation parameters remained unchanged. In agreement with previous data from our laboratory (Kostopoulos, submitted for publication) 15 to 20 min after the i.m. injection of penicillin, three changes were observed: (a) a facilitation of the cortical recruiting response, i.e., the first stimulus was capable of eliciting the first cortical recruiting wave; (b) an increase in amplitude of the recruiting waves, and (c) the appearance, or further increase, if already present, of positive phases in the cortical recruiting waves (Fig. 4A and B). Similar changes in the amplitude and in the shape of the recruiting waves were seen in the thalamus when the cortical RRs exhibited these changes after penicillin (Fig. 4A and B). When the amplitudes of the negative and positive phase of the first recruiting wave elicited before and after penicillin were plotted throughout the experiment at various in E, the transitory percentage increase of negative phases of the RRs recorded in the LP 30 min after penicillin might be explained by the fact that in this experiment negative phases of thalamically recorded RRs were very low in amplitude (less than 0.03 mV) in control conditions (see Fig. 4C).
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stages of the transition period, it was noted that thalamic changes always followed those in the cortex (Fig. 4C). This was also evident when these changes were expressed as percentage increase (Fig. 5). The time course of these changes was similar to that of the alterations spindles underwent during this period. Stage of Fully Developed Spike and Wave Activity. As previously reported (10, 12, 13) the fully developed cortical SW discharges of FGPE were almost always generalized and bilaterally synchronous. In some cases, the SWs were typical with prominent negative slow waves. For such SWs the components described by Weir (30) for human SW complexes could clearly be identified (Fig. 2, MSS (A), +75 min; Fig. 6, left MSS). However, more often, SW complexes of FGPE were characterized by a prominent biphasic spike followed by only a small negative or even positive wave, or the wave was missing or merged into the descending slope of the negative phase of the spike. (Fig. 2, MSS (P), +75 min; Fig. 3, + 100; Fig. 6A, MSS (A); Fig. 10, control). The underlying intracortical excitatory and inhibitory processes are, however, identical in all SW bursts regardless of the variations of their surface EEG morphology (20). Also atypical SWs can be converted into typical ones by merely depressing the excitability of the most superficial cortical layers ( 10). Cortical SW discharges were accompanied by well represented thalamic SW bursts of similar wave morphology and frequency. These were at times absent or of very low amplitude during the initial or final parts of cortical SW discharges. A characteristic feature of thalamic bursts was the smooth domelike shape of the wave component, the spike component being often small or absent (Fig. 2, t75; Fig. 3, +lOO). The thalamic SWs varied in parallel to those simultaneously recorded in the cortex. Thus, as shown in Fig. 6, the well formed cortical SW discharges had well developed thalamic counterparts, whereas cortical bursts with either a less well defined SW shape or low amplitude were poorly represented at the thalamic level (compare SW discharges in Fig. 6B, 1 and 2). We never observed modified spindles in the cortex, characteristic of the transition period, with simultaneous, well developed SW activity in the thalamus. In accordance with earlier evidence (11, 19,25), single-shock stimulation delivered to the thalamus evoked SW bursts represented in both cortex and thalamus. However, it was noted in this series of experiments that singleshock thalamic stimulation had a high probability of evoking SWs only if the stimulus was delivered more than 4 s after a preceding burst. When the stimulus was applied 0 to 2 s after the end of the preceding SW discharge (Fig. 6A), the probability of evoking a SW burst was low.
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FIG. 6. A-spontaneous and thalamic (NCM, single shock of 0.5 ms, 8 mA)-induced spike and wave (SW) discharges 2 h after i.m. penicillin. Note the failure of the second single-shock stimulus to induce a SW burst. B-relationship between cortical and thalamic activity during NCM-induced (1) and spontaneous (2) SW bursts.
Topical Application of Penicillin to Cortex
Cortical spindles in cats in which a weak solution of penicillin (100 to 300 II-l/hemisphere) had been applied diffusely to the cortical surface showed changes similar to those observed after i.m. injection of penicillin. As shown in Figs. 7 (+ 18) and 8 (+20, +32), cortical spindle bursts induced by NCM stimulation acquired positive phases, and increased in amplitude 10 to 35 min after penicillin application to the cortex. With diffuse cortical application of penicillin, the changes observed during the transition period
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were initially more localized, involving only one area of cortex; only later was there involvement of other cortical areas. With diffuse cortical application, too, the first changes appeared in the MSS (Fig. 8). Thalamic spindles showed only slight changes during the transition period (Figs. 7, 8). However, the appearance of cortical SW bursts coincided with that of synchronous SW rhythms in the thalamus (Figs. 7, 8). The well developed SW bursts induced by cortical application of penicillin were sometimes not bilaterally synchronous and at other times were localized to a circumscribed area of cortex. Well formed cortical SW bursts had well developed thalamic SW counterparts as had been the case for SW bursts induced by i.m. injection of penicillin, whereas the less well defined cortical SW bursts induced by cortical penicillin application had no thalamic counterparts. During the transition from spindles to SW discharges after topical application of penicillin, RRs recorded in the MSS underwent changes very similar to those observed after i.m. injection of penicillin: the RRs were facilitated and showed an increase in the amplitude of both positive and negative phases (Fig. 9). An increase in amplitude of thalamic recruiting waves also was observed in these experiments. These changes began to appear 10 to 20 min after the topical application of penicillin. Effects Induced upon Cortical and Thalamic Spike and Wave Discharges by a Selective Decrease of Cortical Excitability
As shown in Fig. 10, unilateral application of 15% KC1 to a small area of cortex induced a decrease in amplitude of both background activity and SW bursts recorded from the cortex on the side of KC1 application. Cortical SW discharges were replaced by predominantly negative spindle-like bursts which appeared to be mainly synchronous with the epileptogenic bursts remaining on the intact side. Although a transient decrease in amplitude of these contralateral SW discharges was observed at times, they remained otherwise unaffected throughout the cortical spreading depression involving the other hemisphere. Thalamic activity recorded from the hemisphere where KC1 was applied decreased in amplitude during unilateral cortical spreading depression. Thalamic SW discharges disappeared, but rhythmic oscillations of low amplitude (40 to 70 pV) at the SW frequency at times FIG. 7. Transition from spindles to SW in a cat with continuous i.v. infusion of sodium thiopental. Single-shock stimulation to NCM: 0.5 ms, 5 mA. Samples are taken before (control) and at different times after diffuse cortical application of a weak solution of penicillin (200 IU/hemisphere). Enlarged recordings show the EEG activity of R MSS (P) and R LP before (A), 18 min (B), and 45 min (C and D) after penicillin application.
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were recorded synchronously with the contralateral SW discharges (Fig. 10, 5 and 12 min after KCl). Subsequent application of KC1 to the second hemisphere or bilateral application of KC1 simultaneously to both hemispheres caused a marked depression of the background activity over the entire cortex bilaterally. During this depression, SW discharges disappeared completely from both the cortex and thalamus and were replaced by spindles which could be either spontaneous or induced by single-shock stimulation to thalamic nuclei. During the recovery from cortical spreading depression, cortical spindles increased in amplitude and later were replaced by SW discharges both in the thalamus and cortex as the voltage of the background activity fully recovered. In accordance with previous studies (4, lo), the ratio between the frequency of the spindle wave and that of the SW complexes was about 2:l. DISCUSSION Feline generalized penicillin epilepsy was used as a tool in furthering our understanding of the physiopathogenetic mechanisms underlying generalized SW bursts associated with absence attacks. The justification for using this model for this purpose is that the bilateral synchronous SW discharges in the cat, as in man, are associated with blinking, bilateral facial myoclonus, staring (12, 23), and behavioral unresponsiveness to extraneous stimuli (Taylor and Gloor, in preparation). Evidence for Primary
Role of Cortex
in Spike and Wave Discharges
Previous observations from this laboratory (8, 10, 11) had suggested that in transforming spindles into SW discharges, penicillin acts primarily on the cortex. Our data are in accordance with this, as the following evidence shows. (i) Changes in shape, amplitude, and frequency of spindles (either thalamically induced or spontaneous), during transition to SW after i.m. injection of penicillin, first occur in the cortex and only later in the thalamus, or they occur simultaneously at both levels; they never occur in the thalamus first. (ii) After i.m. penicillin, RRs evoked by repetitive stimulation of the NCM exhibit similar changes: both the increase of positive and negative phases of RRs during transition to SW discharges consistently appear first in the cortex and only later in the thalamus. (iii) After the application of a weak solution of penicillin to widespread cortical areas of both hemispheres, SW discharges, practically indistinguishable from those observed after i.m. injection, are seen to develop.
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Thalamic spindles during the transition to SW discharges, under these circumstances change little, until full-blown SW discharges appear in the cortex. The changes in cortical and thalamic RRs are very similar to those seen after i.m. penicillin. (iv) A selective decrease of cortical excitability induced by topical application of KC1 to the cortex causes the SW discharges to revert to spindles both in the cortex and thalamus. This change is quite different from that obtained with intrathalamic application of KC1 which induces a functional depression of the thalamus rather than of the cortex and causes both SWs and spindles to disappear from the cortical EEG (3). Thalamic
Participation in Spike and Wave Discharge Generalized Penicillin Epilepsy
of Feline
Even though microinjections of penicillin into the thalamus did not produce generalized SW discharges (11, 24) or any other epileptic activity (1, 9), the integrity of the thalamus and of thalamocortical connections is necessary for triggering the cortical SW discharges of FGPE (3, 22). The present findings demonstrate, however, that the thalamus, though probably unable to produce SW bursts by itself, becomes intimately involved in the SW discharges of FGPE as is shown by the fact that it exhibits well formed SW bursts occurring synchronously with those in the cortex, whenever the latter are fully developed. The thalamic counterparts of less well developed cortical SW discharges are poorly developed. These thalamic changes occur after i.m. injection of penicillin as well as after its diffuse cortical application in a dilute solution. The occurrence of thalamic changes under the latter circumstances had not been noted ( 11,24) in earlier studies for reasons that are not quite clear. Presumptive
Mechanisms of Thalamocorticothalamic Interactions in Generalized Spike and Wave Discharges
Because SW discharges of FGPE can be recorded in the thalamus (VL, VPL, and LP nuclei) after widespread topical application of a weak penFIG. 9. Recruiting responses (RRs) during transition to spike and wave discharges after cortical application of a weak penicillin solution (300 IU/hemisphere). A and B-averaged RRs (40 traces) recorded from the MSS and LP in two different experiments. Samples taken before (controls) and at different times (numbers on the left indicate minutes after cortical penicillin application). Repetitive stimulation to the NCM: 5 Hz, 0.2 ms, 1 mA in A and 0.8 mA in B. Time calibration 400 ms in A, 1000 ms in B. C-amplitudes (ordinate, millivolts) of positive and negative phases of RRs recorded in the MSS and LP at different times (in minutes, abscissa) after cortical application of penicillin in three experiments. Only the first detectable evoked potential (e.g., the third in A and fifth in B) was plotted at each time interval after penicillin application. The stimulation parameters remained constant throughout the experiments.
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icillin solution to the cortex, it is possible to conclude that, at least in these experiments, thalamic SW bursts are not due to a local direct epileptogenic action of penicillin on the thalamus, especially since Quesney and Gloor (24) showed that under these circumstances the thalamic concentration of penicillin is negligibly small. The appearance of SW discharge in the thalamus, therefore, is due most likely to a secondary activation of the thalamus by volleys arising from the cortex and mediated through corticothalamic connections. These pathways, which are thought to reciprocate the thalamocortical ones (15) were demonstrated in the cat brain both anatomically (26) and electrophysiologically [( 14, 29); Avoli and Kostopoulos, submitted for publication]. Further evidence that SW discharges recorded in the thalamus are induced by corticothalamic volleys is derived from the fact that both the cortical and thalamic SW bursts of FGPE induced by the i.m. injection of penicillin disappear when one selectively decreases the excitability of the cortex by means of cortical spreading depression. It was previously hypothesized that SW bursts of FGPE represent an abnormal response of the cortex to normal volleys arising from the thalamus (8, 10, 11, 19, 20. 24, 25). Implicit in this view was the notion that thalamic activity need not change in the course of cortical SW development, The present findings, however, suggest that this view must be modified: changes in thalamic activity, as they develop during the transition period from spindles to SW bursts in response to altered corticothalamic inputs, in their turn induce changes in the projected thalamocortical activity. This, in turn, alters the cortical response. This view may explain the reduction and gradual disappearance of every second cortical spindle wave during the transition from spindle to SW (19, 20) which could not be entirely attributed to the shunting of cortical neuronal membranes by the inhibitory postsynaptic potentials underlying the slow wave component of the SW complexes (6) which develops in the time slot formerly occupied by the eliminated spindle wave. Although crucially dependent on a change in cortical neuronal excitability, the full-blown SW discharges of FGPE probably represent an abnormal oscillatory phenomenon involving both cortex and thalamus, the two intimately interacting in a closed loop. Such a close coupling of thalamic and cortical activity might in some way contribute to the tight temporal organization and maintenance of SW bursts at both levels. Furthermore, the abnormal interaction between cortex and thalamus during SW discharges might possibly also contribute to the behavioral expression of this type of epileptic discharge. FIG. 10. Effects of unilateral (top) and bilateral (bottom) cortical spreading depression induced by topical application of KC1 upon spike and wave discharges of feline generalized penicillin epilepsy more than 2 h after i.m. injection of penicillin. Solid triangles indicate single-shock stimulation (0.5 ms, 5 mA) in the NCM.
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Using EEG techniques alone, however, it is not possible to draw any firm conclusions about the specific role of the thalamus in the elaboration of SW bursts. Microphysiological studies presently in progress in our laboratory are more promising in this regard. REFERENCES 1. AVOLI, M. 1980. Electroencephalographic and pathophysiologic features of rat parenteral penicillin epilepsy. Exp. Neural. 69: 373-382. 2. AVOLI, M., AND P. GLOOR. 1980. Danger of false localization of field potentials. Electroenceph. Clin. Neurophysiol. 50: 2 17P. 3. AVOLI, M., AND P. GLOOR. 1981. The effects of transient functional depression of the thalamus on spindles and on bilateral synchronous epileptic discharges of feline generalized penicillin epilepsy. Epilepsia 22: 443-452. 4. AVOLI, M., I. SIATITSAS, G. KOSTOPOULOS, AND P. GLOOR. 1981. Effects of post-ictal depression on experimental spike and wave discharges. Elecrroenceph. Clin. Neurophysiol. 52: 372-374. 5. FISHER, R. S., AND D. A. PRINCE. 1977. Spike-wave rhythms in cat cortex induced by parenteral penicillin. I. Electroencephalographic features. Elecfroenceph. Clin. Neurophysiol. 42: 608-624. 6. FISHER, R. S., AND D. A. PRINCE. 1977. Spike-wave rhythms in cat cortex induced by parenteral penicillin. II. Cellular features. Electroenceph. Clin. Neurophysiol. 42: 625639. 7. GANES, T., AND P. ANDERSEN. 1975. Barbiturate spindle activity in functionally corresponding thalamic and cortical somatosensory areas in the cat. Brain Res. 98: 457472. 8. GLOOR, P. 1979. Generalized epilepsy with spike-and-wave discharge: a reinterpretation of its electrographic and clinical manifestations. Epilepsia 20: 571-588. 9. GLOOR, P., G. HALL, AND F. COCEANI. 1966. Differential sensitivity of various brain structures to the epileptogenic action of penicillin. Exp. Neural. 16: 333-348. 10. GLOOR, P., A. PELLEGRINI, AND G. KOS~OPOULOS. 1979. Effects of changes in cortical excitability upon the epileptic bursts in generalized penicillin epilepsy of the cat. Electroenceph. Clin. Neurophysiol. 46: 274-289. 11. GLOOR, P., L. F. QUESNEY, AND H. ZUMSTEIN. 1977. 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. Electroenceph. Clin. Neurophysiol. 43: 79-94. 12. GLOOR, P., AND G. TESTA. 1974. Generalized penicillin epilepsy in the cat: effects of intracarotid and intravertebral pentylenetetrazol and amobarbital injections. Elecrroenceph. Clin. Neurophysiol. 36: 499-5 15. 13. GUBERMAN, A., AND P. GLOOR. 1974. Cholinergic drug studies of generalized penicillin epilepsy in the cat. Bruin Res. 78: 203-222. 14. GUTNICK, M. J., AND D. A. PRINCE. 1974. Effects of projected cortical epileptiform discharges on neuronal activities in cat VPL. I. Interictal discharge, J. Neurophysiol. 37: 1310-1327. 15. HEATH, C. J., AND E. G. JONES. 1971. The anatomical organization of the suprasylvian gyrus of the cat. Ergeb. And Entwicklungsgesch. 45: l-64. 16. JASPER, H. H., AND C. AJMONE-MARSAN. 1954. A stereotaxic atlas of the diencephalon of the cat. National Research Council of Canada, Ottawa.
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17. JASPER, H. H., AND J. DROOGLEEVER-FORTUYN. 1946. Experimental studies of the functional anatomy of petit ma1 epilepsy. Res. Publ. Assoc. Res. Nerv. Merit. Dis. 26: 272-298. 18. KITSIKIS, A., AND M. STERIADE. 1975. Thalamic, callosal and reticular converging inputs to parietal association cortex in cat. Brain Res. 93: 516-524. 19. KOSTOPOULOS, G., P. GLOOR, A. PELLEGRINI, AND I. SIATITSAS. 1981. A study of the transition from spindles to spike and wave discharge in feline generalized penicillin epilepsy: EEG features. Exp. Neurol. 73: 43-54. 20. KOSTOPOULOS, G., P. GLOOR, A. PELLEGRINI, AND J. GOTMAN. 1981. A study of the transition from spindles to spike and wave discharge in feline generalized penicillin epilepsy: microphysiological features. Exp. Neurol. 73: 55-77. 21. PELLEGRINI, A., AND P. GLOOR. 1979. Effects of bilateral partial diencephalic lesions on cortical epileptic activity in generalized penicillin epilepsy in the cat. Exp. Neural. 66: 285-308. 22. PELLEGRINI, A., J. MUSGRAVE, ANDP. GLOOR. 1979. Role of afferent input of subcortical origin in the genesis of bilaterally synchronous epileptic discharges of feline generalized penicillin epilepsy. Exp. Neurol. 64: 155- 173. 23. PRINCE, D., AND D. FARRELL. 1969. “Centrencephalic” spike-wave discharges following parenteral penicillin injection in the cat. Neurology (New York) 19: 309-310. 24. QUESNEY. L. F., AND P. GLOOR. 1978. Generalized penicillin epilepsy in the cat: correlation between electrophysiological data and distribution of “C-penicillin in the brain. Epilepsia 19: 35-45. 25. QUESNEY, L. F., P. GLOOR, E. KRATZENBERG, AND H. ZUMSTEIN. 1977. Pathophysiology of generalized penicillin epilepsy in the cat: the role of cortical and subcortical structures. I. Systemic application of penicillin. Electroenceph. Clin. Neurophysiol. 42: 640655. 26. RINVIK, E. 1968. The corticothalamic projection from pericruciate and coronal gyri in the cat. An experimental study with silver-impregnation methods. Brain Res. 10: 79119. 27. RODIN, E., H. KITANO, B. NAGAO, AND M. RODIN. 1977. The results of penicillin G administration on chronic unrestrained cats: electrographic and behavioral observations. Electroenceph.
Clin.
Neurophysiol.
42: 5 18-521.
28. SPENCER, W. A., AND J. M. BROOKHART. 1961. A study of spontaneous spindle waves in sensorimotor cortex of the cat. J. Neurophysiol. 24: 50-65. 29. STERIADE, M., P. WYZINSKY, AND V. APOSTOL. 1972. Corticofugal projections governing rhythmic thalamic activity. Pages 221-272 in T. FRYGIESI, E. RINVIK, AND M. D. YAHZI, Eds., Corticothalamic Projections and Sensorimotor Activities. Raven Press, New York. 30. WEIR, B. 1965. The morphology of the spike-wave complex. Electroenceph. C/in. Neurophysiol. 19: 284-290.
NEUROLOGY
76, 196-217
(1982)
Interaction of Cortex and Thalamus in Spike and Wave Discharges of Feline Generalized Penicillin Epilepsy MASSIMO AVOLI AND PIERRE GLOOR' Department
of Neurology and Neurosurgery, Neurological Institute, Montreal, Received
August
McGill University and Montreal Quebec, H3A 284, Canada 31. 1981
The transition from spindles to spike and wave (SW) discharges of feline generalized penicillin epilepsy was studied using simultaneous EEG recordings from mutually related cortical and thalamic sites after i.m. injection (350,000 IU/kg) or diffuse cortical application of a weak solution (100-300 IU/hemisphere) of penicillin. Both procedures induced similar changes at cortical and thalamic levels, those in the thalamus developing at the same time or slightly later but never earlier than in the cortex. These changes consisted of: (i) amplitude increase of spindles, development of positive phases, and decrease in amplitude, followed by disappearance of every second spindle wave as SW discharges developed, (ii) facilitation, progressive amplitude increase, and increase or development of positive phases of recruiting responses to midline thalamic stimulation. Once SW had developed, a decrease in cortical excitability by cortical application of 15% KC1 caused the cortical and thalamic SW discharges to disappear and to be replaced by spindles. These results demonstrate that important changes in thalamic activity occur during the development of cortical SW discharge whether induced by i.m. penicillin or by diffusecortical application of a weak penicillin solution. Changes in thalamic activity appear to he secondary to changes in cortical activity. Thus, although cortical SWs are triggered by thalamocortical inputs which originally were spindle-inducing, these inputs change after penicillin, and reflect an alteration in thalamic activity imposed by the cortex through corticothalamic volleys. In their turn, they modify the cortical response. Abbreviations: SW-spike and wave; FGPE-feline generalized penicillin epilepsy; EEGelectroencephalogram; MSS-middle suprasylvian gyrus; RRs-recruiting responses; LPnucleus lateralis posterior; VL, VPL-nucleus ventrolateralis, ventropostero-lateralis; NCMnucleus centralis medialis; AS-anterior sigmoid gyrus. ’ This work was supported by grant MT-3140 awarded by the Medical Research Council of Canada to Dr. Gloor. Dr. Avoli was a Killam Scholar. The authors thank Dr. G. Kostopoulos for helpful criticism, Mrs. S. Schiller and Mrs. T. De La Fosse for technical assistance, as well as Miss G. Robillard and Mrs. K. Douglas for secretarial assistance. 196 0014-4886/82/040196-22$02.00/O Copyright All rights
(d 1982 by Academic Press. Inc. of reproduction in any form rcserwd
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INTRODUCTION Feline generalized penicillin epilepsy (FGPE), produced by i.m. injection of a large dose of penicillin, is an experimental model which bears many electroencephalographic (EEG), behavioral, and pharmacologic similarities to human generalized corticoreticular (“centrencephalic”) epilepsy (5, 8, 10, 12, 13, 19, 23). Since the publication of the classic paper by Jasper and DroogleeverFortuyn ( 17) thalamocortical connections have been known to play an important role in the genesis of bilaterally synchronous spike and wave (SW) discharges. The study of the role of these connections has been the focus of interest of our previous research on the mechanism of SW discharges of FGPE (8). To date, the main emphasis has been on the cortical mechanisms of epileptic discharge in this model. It was suggested that the generalized SW discharges of FGPE represent the response of hyperexcitable cortex to thalamocortical volleys, normally evoking spindles and/or recruiting responses (10, 11, 24, 25). Furthermore, a transition from spindles to SW discharges after the i.m. injection of penicillin was demonstrated both at A
Medial
Row
Lateral
Row
MedIal
Row
FIG. I. Determination of mutually related sites in nucleus lateralis posterior (LP) and the middle suprasylvian gyrus (MSS). A-averaged potentials (20 samples) evoked by single shock (0.2 ms, 0.8 mA) delivered at two LP sites (A and 0) and recorded by a row of silver ball electrodes placed in two different (medial and lateral) areas of the MSS. B--location of the two electrode rows in the MSS. C-sites of stimulation in the LP. Note that in this experiment the short-latency component of the evoked field potential displays its largest amplitude in lead 2 of the medial row.
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the EEG and unitary level (19, 20). The fundamental change in this transition appears to be an increased firing of cortical neurons in association with individual spindle waves as the latter were transformed into the “spike” of SW discharges (20). There was some evidence that changes in excitability at the thalamic level were not required for producing the epileptic bursts in this model (11, 24) although SW discharges are known to be crucially dependent on thalamic inputs to the cortex (3, 22). However, this view may be simplistic and deserves reexamination, because little work has been done so far on thalamic mechanisms in this model by using direct-thalamic recordings. It has, however, been known that the thalamic EEG assumes a pattern similar to that seen in the cortex when the SW bursts of FGPE have fully developed (5, 25, 27). How this came about remained unknown. The present paper is an EEG study dealing with this issue. A microphysiologic study of the same topic is presently in progress. METHODS Anesthesia and Surgical Procedures. Acute experiments were carried out in cats of either sex weighing between 2.6 and 4.2 kg. The surgical, analgesic, and neuroleptic procedures were described previously (19). Experiments were carried out in awake animals immobilized with gallamine triethiodide (Flaxedil) and restrained by a painless method. Wound edges were locally anesthetized with novocaine. The CO2 content of the expiratory air was monitored by a Beckman gas analyzer and maintained at 3.5 to 4%. Body temperature was maintained at 38.0 & 0.5”C by an electric blanket controlled through a rectal probe. Recording and Stimulation. The EEG was recorded directly from the cortex using an 8-channel Elema-Schoenander Mingograf. Bipolar and monopolar recordings were taken bilaterally from the middle suprasylvian gyrus (MSS) and the anterior sigmoid gyrus (AS). The reference electrode
FIG. 2. Transition from spindles to spike and wave (SW) potentials in a cat with continuous i.v. infusion of sodium thiopental. Spindles and SW bursts were triggered by single-shock stimulation to NCM. In this and the following figures, abbreviations are as follows: ASanterior sigmoid gyrus, MSS-middle suprasylvian gyrus, LP-nucleus lateralis posterior, VL-nucleus ventralis lateralis, VPL-nucleus ventralis posterolateralis, NCM-nucleus centralis medialis, R-right, L-left, (A)-anterior, (P&-posterior. Single-shock stimulation delivered to the NCM: 0.5 ms, 8 mA, 1 every 10 s. Samples are taken before (control) and at different times (numbers on top indicate minutes) after i.m. injection of penicillin. Enlarged samples show EEG activity of the two functionally related sites in R MSS (A) and R LP, before as well as 50 and 75 min after penicillin, respectively.
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was attached to the neck muscles. Concentric bipolar electrodes (interelectrode distance 0.8 to 1 mm, resistance 40 to 50 Kfi) were placed in the nucleus centralis medialis (NCM) (A, 10.5; V, 1; L, 0), nucleus lateralis posterior (LP) (A, 6.5 to 8; V, 3 to 6; L, 4.5), nucleus ventrolateralis (VL) (A, 10.5; V, 1 to 2; L, 4.5), and nucleus ventroposterolateralis (VPL) (A, 9; V, 1; L, 7.5) of the thalamus. The atlas of Jasper and Ajmone-Marsan ( 16) was used for establishing stereotaxic coordinates. Bipolar recordings from depth structures were used in all experiments to minimize the danger of picking up distant electrical activity (e.g., of cortex or hippocampus) by volume conduction (2). Single shock or low-frequency repetitive stimulation (5 to 10 Hz) was delivered through the depth electrodes using a constant-current Nuclear Chicago or WPI stimulator (0.2 to 0.5 ms, 0.5 to 10 mA). The evoked responses were averaged by a Fabri-Tek 1052 (1024 point-signal) averager. Electrode placements were verified in gross postmortem or histologic preparations. Determination of Functionally
Related Thalamic and Cortical Sites.
Because of earlier work indicating that the SW discharges of FGPE appear first and usually predominate in the MSS (21), particular attention was given to recording from functionally related sites in the MSS and LP (15, 18). By stimulating with single shocks (0.1 to 0.2 ms, 0.5 to 0.8 mA) at various depths in the LP we determined in each experiment the MSS area where the short-latency field potential (5 to 9 ms peak latency) evoked by the LP displayed maximal amplitude (Fig. 1). We assumed that this indicated that the two sites were closely interconnected. During the entire experiment the EEG activity was recorded at these cortical and thalamic sites. Pharmacologic Procedures. Generalized penicillin epilepsy was induced (i) in 20 cats by the i.m. injection of 350,000 II-J/kg sodium penicillin G and (ii) in seven animals by applying to large areas of cerebral cortex of each hemisphere a filter paper soaked in a weak aqueous solution of penicillin G (100 to 300 II-J/hemisphere). Very small i.v. doses of sodium thiopental (0.5 to 1 mg/kg/h) were continuously given throughout the experiment to seven cats which were injected with penicillin and three cats with epicortical penicillin application. This was done because spindles, either spontaneous or evoked, were not consistently present in undrugged cats, as the animals’ EEGs showed an alternation of sleep and waking patterns. The use of thiopental infusion created a condition under which single shocks to the NCM or LP consistently elicited spindle bursts so that the transition from spindles to SW could be continuously studied (19, 20). This small amount of barbiturate was insufficient to produce an appreciable increase in spontaneous spindling.
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Cortical spreading depression was induced in seven cats by applying to the cerebral cortex a small, square (2 X 2 mm) filter paper soaked in a 15% KC1 solution. RESULTS The data collected from cats with or without continuous infusion of very low doses of thiopental will be described together, because they were similar. Spindles recorded from the cortex (both in the MSS and AS) consisted mainly of negative waves (100 to 250 pV, 7 to 14 Hz) which resembled the “recruiting” or type II spindles of Spencer and Brookhart (28). However, additional doses of barbiturate induced the appearance of biphasic negative-positive waves [“augmenting” or type I spindles of Spencer and Brookhart (28)]. Synchronously with the cortical spindles there occurred thalamic spindles with frequencies identical to those recorded in the cortex. Rarely spindle bursts of type II were recorded only in the cortex and not in the corresponding thalamic structures (7). Parenteral
Administration
of Penicillin
Transition from Spindles to Spike and Wave. As previously reported by Kostopoulos et al. (19, 20) the first change observed in the cortex 15 to 40 min after the i.m. injection of penicillin, was a progressive increase in the amplitude of spindle waves associated with the development or amplitude increase of positive phases which either preceded or followed the negative ones (Fig. 2, +32 min; Fig. 3, +42 min). Coincident with this change, one of every two cortical spindle waves started to decrease in amplitude and ultimately disappeared, the remaining spindle wave gradually developing into the spike of the SW discharge (Fig. 2, +50 and 58
FIG. 4. Recruiting responses (RRs) during transition from spindles to spike and wave (SW) potentials induced by i.m. injection of penicillin. A-superimposed recordings from the MSS and LP of RRs induced by low-frequency (S/s) repetitive stimulation (0.4 ms. 2 mA) of the NCM (0); results taken before (control) and at different times (numbers on the left indicate minutes) after i.m. penicillin; time calibration: 200 ms. B-averaged RRs (30 traces) recorded in another cat during transition to SW after i.m. penicillin; low frequency (7/s) repetitive stimulation (0.2 ms, 1.5 mA) of NCM (0); explanation as in A; time calibration: 400 ms. Stimulation parameters remained constant throughout the experiment in A and B. C-amplitudes (ordinate, millivolts) of positive and negative phases of RRs recorded in MSS and LP at different times (in minutes, abscissa) after penicillin in five different experiments. Only the first detectable evoked potential (e.g., the fourth in A and the sixth in B) was plotted at each time interval after penicillin. The different symbols (0, A, n , 0, +) refer to different animals.
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min; Fig. 3, +63 and +75 mm). The slow wave of the SW complex, which ultimately replaced the reduced spindle wave, was poorly developed as had often been observed previously in acute experiments ( 10). In addition, whereas spindles before penicillin in undrugged cats were usually recorded from one or two of the cortical areas investigated, the modified spindle bursts or incompletely developed SW bursts were often present bilaterally and synchronously in all cortical areas from which records were taken (compare control and +63 min in Fig. 3). Thalamic spindles often showed similar changes after penicillin, though these were less clear and consistent than in the cortex (Figs. 2, 3). At other times thalamic spindles were decreased in amplitude, unidentifiable, or even missing during the transition period. In a few cases it was possible to record cortical spindles displaying the above-described changes while the thalamus was still producing normal spindles. Penicillin-induced changes were never observed in the thalamus before they were seen in the cortex. Changes in Recruiting Responses (RRs) during the Transition to Spike and Wave. In five experiments, RRs recorded from the MSS were analyzed
during the transition from spindles to SW after i.m. penicillin. The RRs were evoked by repetitive stimulation of the NCM (5 to 8 Hz) using pulses of an intensity of 0.2 to 1 mA (200 to 500 ps) which, before penicillin, evoked responses only with the third to fifth stimulus of a train. These responses consisted mainly of negative or negative-positive cortical waves. The subsequent stimuli in the train produced cortical potentials with recruiting features. Throughout the entire experiment the stimulation parameters remained unchanged. In agreement with previous data from our laboratory (Kostopoulos, submitted for publication) 15 to 20 min after the i.m. injection of penicillin, three changes were observed: (a) a facilitation of the cortical recruiting response, i.e., the first stimulus was capable of eliciting the first cortical recruiting wave; (b) an increase in amplitude of the recruiting waves, and (c) the appearance, or further increase, if already present, of positive phases in the cortical recruiting waves (Fig. 4A and B). Similar changes in the amplitude and in the shape of the recruiting waves were seen in the thalamus when the cortical RRs exhibited these changes after penicillin (Fig. 4A and B). When the amplitudes of the negative and positive phase of the first recruiting wave elicited before and after penicillin were plotted throughout the experiment at various in E, the transitory percentage increase of negative phases of the RRs recorded in the LP 30 min after penicillin might be explained by the fact that in this experiment negative phases of thalamically recorded RRs were very low in amplitude (less than 0.03 mV) in control conditions (see Fig. 4C).
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stages of the transition period, it was noted that thalamic changes always followed those in the cortex (Fig. 4C). This was also evident when these changes were expressed as percentage increase (Fig. 5). The time course of these changes was similar to that of the alterations spindles underwent during this period. Stage of Fully Developed Spike and Wave Activity. As previously reported (10, 12, 13) the fully developed cortical SW discharges of FGPE were almost always generalized and bilaterally synchronous. In some cases, the SWs were typical with prominent negative slow waves. For such SWs the components described by Weir (30) for human SW complexes could clearly be identified (Fig. 2, MSS (A), +75 min; Fig. 6, left MSS). However, more often, SW complexes of FGPE were characterized by a prominent biphasic spike followed by only a small negative or even positive wave, or the wave was missing or merged into the descending slope of the negative phase of the spike. (Fig. 2, MSS (P), +75 min; Fig. 3, + 100; Fig. 6A, MSS (A); Fig. 10, control). The underlying intracortical excitatory and inhibitory processes are, however, identical in all SW bursts regardless of the variations of their surface EEG morphology (20). Also atypical SWs can be converted into typical ones by merely depressing the excitability of the most superficial cortical layers ( 10). Cortical SW discharges were accompanied by well represented thalamic SW bursts of similar wave morphology and frequency. These were at times absent or of very low amplitude during the initial or final parts of cortical SW discharges. A characteristic feature of thalamic bursts was the smooth domelike shape of the wave component, the spike component being often small or absent (Fig. 2, t75; Fig. 3, +lOO). The thalamic SWs varied in parallel to those simultaneously recorded in the cortex. Thus, as shown in Fig. 6, the well formed cortical SW discharges had well developed thalamic counterparts, whereas cortical bursts with either a less well defined SW shape or low amplitude were poorly represented at the thalamic level (compare SW discharges in Fig. 6B, 1 and 2). We never observed modified spindles in the cortex, characteristic of the transition period, with simultaneous, well developed SW activity in the thalamus. In accordance with earlier evidence (11, 19,25), single-shock stimulation delivered to the thalamus evoked SW bursts represented in both cortex and thalamus. However, it was noted in this series of experiments that singleshock thalamic stimulation had a high probability of evoking SWs only if the stimulus was delivered more than 4 s after a preceding burst. When the stimulus was applied 0 to 2 s after the end of the preceding SW discharge (Fig. 6A), the probability of evoking a SW burst was low.
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Topical Application of Penicillin to Cortex
Cortical spindles in cats in which a weak solution of penicillin (100 to 300 II-l/hemisphere) had been applied diffusely to the cortical surface showed changes similar to those observed after i.m. injection of penicillin. As shown in Figs. 7 (+ 18) and 8 (+20, +32), cortical spindle bursts induced by NCM stimulation acquired positive phases, and increased in amplitude 10 to 35 min after penicillin application to the cortex. With diffuse cortical application of penicillin, the changes observed during the transition period
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were initially more localized, involving only one area of cortex; only later was there involvement of other cortical areas. With diffuse cortical application, too, the first changes appeared in the MSS (Fig. 8). Thalamic spindles showed only slight changes during the transition period (Figs. 7, 8). However, the appearance of cortical SW bursts coincided with that of synchronous SW rhythms in the thalamus (Figs. 7, 8). The well developed SW bursts induced by cortical application of penicillin were sometimes not bilaterally synchronous and at other times were localized to a circumscribed area of cortex. Well formed cortical SW bursts had well developed thalamic SW counterparts as had been the case for SW bursts induced by i.m. injection of penicillin, whereas the less well defined cortical SW bursts induced by cortical penicillin application had no thalamic counterparts. During the transition from spindles to SW discharges after topical application of penicillin, RRs recorded in the MSS underwent changes very similar to those observed after i.m. injection of penicillin: the RRs were facilitated and showed an increase in the amplitude of both positive and negative phases (Fig. 9). An increase in amplitude of thalamic recruiting waves also was observed in these experiments. These changes began to appear 10 to 20 min after the topical application of penicillin. Effects Induced upon Cortical and Thalamic Spike and Wave Discharges by a Selective Decrease of Cortical Excitability
As shown in Fig. 10, unilateral application of 15% KC1 to a small area of cortex induced a decrease in amplitude of both background activity and SW bursts recorded from the cortex on the side of KC1 application. Cortical SW discharges were replaced by predominantly negative spindle-like bursts which appeared to be mainly synchronous with the epileptogenic bursts remaining on the intact side. Although a transient decrease in amplitude of these contralateral SW discharges was observed at times, they remained otherwise unaffected throughout the cortical spreading depression involving the other hemisphere. Thalamic activity recorded from the hemisphere where KC1 was applied decreased in amplitude during unilateral cortical spreading depression. Thalamic SW discharges disappeared, but rhythmic oscillations of low amplitude (40 to 70 pV) at the SW frequency at times FIG. 7. Transition from spindles to SW in a cat with continuous i.v. infusion of sodium thiopental. Single-shock stimulation to NCM: 0.5 ms, 5 mA. Samples are taken before (control) and at different times after diffuse cortical application of a weak solution of penicillin (200 IU/hemisphere). Enlarged recordings show the EEG activity of R MSS (P) and R LP before (A), 18 min (B), and 45 min (C and D) after penicillin application.
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were recorded synchronously with the contralateral SW discharges (Fig. 10, 5 and 12 min after KCl). Subsequent application of KC1 to the second hemisphere or bilateral application of KC1 simultaneously to both hemispheres caused a marked depression of the background activity over the entire cortex bilaterally. During this depression, SW discharges disappeared completely from both the cortex and thalamus and were replaced by spindles which could be either spontaneous or induced by single-shock stimulation to thalamic nuclei. During the recovery from cortical spreading depression, cortical spindles increased in amplitude and later were replaced by SW discharges both in the thalamus and cortex as the voltage of the background activity fully recovered. In accordance with previous studies (4, lo), the ratio between the frequency of the spindle wave and that of the SW complexes was about 2:l. DISCUSSION Feline generalized penicillin epilepsy was used as a tool in furthering our understanding of the physiopathogenetic mechanisms underlying generalized SW bursts associated with absence attacks. The justification for using this model for this purpose is that the bilateral synchronous SW discharges in the cat, as in man, are associated with blinking, bilateral facial myoclonus, staring (12, 23), and behavioral unresponsiveness to extraneous stimuli (Taylor and Gloor, in preparation). Evidence for Primary
Role of Cortex
in Spike and Wave Discharges
Previous observations from this laboratory (8, 10, 11) had suggested that in transforming spindles into SW discharges, penicillin acts primarily on the cortex. Our data are in accordance with this, as the following evidence shows. (i) Changes in shape, amplitude, and frequency of spindles (either thalamically induced or spontaneous), during transition to SW after i.m. injection of penicillin, first occur in the cortex and only later in the thalamus, or they occur simultaneously at both levels; they never occur in the thalamus first. (ii) After i.m. penicillin, RRs evoked by repetitive stimulation of the NCM exhibit similar changes: both the increase of positive and negative phases of RRs during transition to SW discharges consistently appear first in the cortex and only later in the thalamus. (iii) After the application of a weak solution of penicillin to widespread cortical areas of both hemispheres, SW discharges, practically indistinguishable from those observed after i.m. injection, are seen to develop.
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Thalamic spindles during the transition to SW discharges, under these circumstances change little, until full-blown SW discharges appear in the cortex. The changes in cortical and thalamic RRs are very similar to those seen after i.m. penicillin. (iv) A selective decrease of cortical excitability induced by topical application of KC1 to the cortex causes the SW discharges to revert to spindles both in the cortex and thalamus. This change is quite different from that obtained with intrathalamic application of KC1 which induces a functional depression of the thalamus rather than of the cortex and causes both SWs and spindles to disappear from the cortical EEG (3). Thalamic
Participation in Spike and Wave Discharge Generalized Penicillin Epilepsy
of Feline
Even though microinjections of penicillin into the thalamus did not produce generalized SW discharges (11, 24) or any other epileptic activity (1, 9), the integrity of the thalamus and of thalamocortical connections is necessary for triggering the cortical SW discharges of FGPE (3, 22). The present findings demonstrate, however, that the thalamus, though probably unable to produce SW bursts by itself, becomes intimately involved in the SW discharges of FGPE as is shown by the fact that it exhibits well formed SW bursts occurring synchronously with those in the cortex, whenever the latter are fully developed. The thalamic counterparts of less well developed cortical SW discharges are poorly developed. These thalamic changes occur after i.m. injection of penicillin as well as after its diffuse cortical application in a dilute solution. The occurrence of thalamic changes under the latter circumstances had not been noted ( 11,24) in earlier studies for reasons that are not quite clear. Presumptive
Mechanisms of Thalamocorticothalamic Interactions in Generalized Spike and Wave Discharges
Because SW discharges of FGPE can be recorded in the thalamus (VL, VPL, and LP nuclei) after widespread topical application of a weak penFIG. 9. Recruiting responses (RRs) during transition to spike and wave discharges after cortical application of a weak penicillin solution (300 IU/hemisphere). A and B-averaged RRs (40 traces) recorded from the MSS and LP in two different experiments. Samples taken before (controls) and at different times (numbers on the left indicate minutes after cortical penicillin application). Repetitive stimulation to the NCM: 5 Hz, 0.2 ms, 1 mA in A and 0.8 mA in B. Time calibration 400 ms in A, 1000 ms in B. C-amplitudes (ordinate, millivolts) of positive and negative phases of RRs recorded in the MSS and LP at different times (in minutes, abscissa) after cortical application of penicillin in three experiments. Only the first detectable evoked potential (e.g., the third in A and fifth in B) was plotted at each time interval after penicillin application. The stimulation parameters remained constant throughout the experiments.
12 Minafter
KCI on Right MSS(P)
(P) 40 Min after KCI on RightMSS
5 Min after KCI on Right MSS (P)
16 Min after KCI on Right MSS (P) 20 Min after KCI onRightMSS 4 Min after KCI on left MSS (P)
Control
(P)
N z.
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icillin solution to the cortex, it is possible to conclude that, at least in these experiments, thalamic SW bursts are not due to a local direct epileptogenic action of penicillin on the thalamus, especially since Quesney and Gloor (24) showed that under these circumstances the thalamic concentration of penicillin is negligibly small. The appearance of SW discharge in the thalamus, therefore, is due most likely to a secondary activation of the thalamus by volleys arising from the cortex and mediated through corticothalamic connections. These pathways, which are thought to reciprocate the thalamocortical ones (15) were demonstrated in the cat brain both anatomically (26) and electrophysiologically [( 14, 29); Avoli and Kostopoulos, submitted for publication]. Further evidence that SW discharges recorded in the thalamus are induced by corticothalamic volleys is derived from the fact that both the cortical and thalamic SW bursts of FGPE induced by the i.m. injection of penicillin disappear when one selectively decreases the excitability of the cortex by means of cortical spreading depression. It was previously hypothesized that SW bursts of FGPE represent an abnormal response of the cortex to normal volleys arising from the thalamus (8, 10, 11, 19, 20. 24, 25). Implicit in this view was the notion that thalamic activity need not change in the course of cortical SW development, The present findings, however, suggest that this view must be modified: changes in thalamic activity, as they develop during the transition period from spindles to SW bursts in response to altered corticothalamic inputs, in their turn induce changes in the projected thalamocortical activity. This, in turn, alters the cortical response. This view may explain the reduction and gradual disappearance of every second cortical spindle wave during the transition from spindle to SW (19, 20) which could not be entirely attributed to the shunting of cortical neuronal membranes by the inhibitory postsynaptic potentials underlying the slow wave component of the SW complexes (6) which develops in the time slot formerly occupied by the eliminated spindle wave. Although crucially dependent on a change in cortical neuronal excitability, the full-blown SW discharges of FGPE probably represent an abnormal oscillatory phenomenon involving both cortex and thalamus, the two intimately interacting in a closed loop. Such a close coupling of thalamic and cortical activity might in some way contribute to the tight temporal organization and maintenance of SW bursts at both levels. Furthermore, the abnormal interaction between cortex and thalamus during SW discharges might possibly also contribute to the behavioral expression of this type of epileptic discharge. FIG. 10. Effects of unilateral (top) and bilateral (bottom) cortical spreading depression induced by topical application of KC1 upon spike and wave discharges of feline generalized penicillin epilepsy more than 2 h after i.m. injection of penicillin. Solid triangles indicate single-shock stimulation (0.5 ms, 5 mA) in the NCM.
216
AVOLI
AND
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Using EEG techniques alone, however, it is not possible to draw any firm conclusions about the specific role of the thalamus in the elaboration of SW bursts. Microphysiological studies presently in progress in our laboratory are more promising in this regard. REFERENCES 1. AVOLI, M. 1980. Electroencephalographic and pathophysiologic features of rat parenteral penicillin epilepsy. Exp. Neural. 69: 373-382. 2. AVOLI, M., AND P. GLOOR. 1980. Danger of false localization of field potentials. Electroenceph. Clin. Neurophysiol. 50: 2 17P. 3. AVOLI, M., AND P. GLOOR. 1981. The effects of transient functional depression of the thalamus on spindles and on bilateral synchronous epileptic discharges of feline generalized penicillin epilepsy. Epilepsia 22: 443-452. 4. AVOLI, M., I. SIATITSAS, G. KOSTOPOULOS, AND P. GLOOR. 1981. Effects of post-ictal depression on experimental spike and wave discharges. Elecrroenceph. Clin. Neurophysiol. 52: 372-374. 5. FISHER, R. S., AND D. A. PRINCE. 1977. Spike-wave rhythms in cat cortex induced by parenteral penicillin. I. Electroencephalographic features. Elecfroenceph. Clin. Neurophysiol. 42: 608-624. 6. FISHER, R. S., AND D. A. PRINCE. 1977. Spike-wave rhythms in cat cortex induced by parenteral penicillin. II. Cellular features. Electroenceph. Clin. Neurophysiol. 42: 625639. 7. GANES, T., AND P. ANDERSEN. 1975. Barbiturate spindle activity in functionally corresponding thalamic and cortical somatosensory areas in the cat. Brain Res. 98: 457472. 8. GLOOR, P. 1979. Generalized epilepsy with spike-and-wave discharge: a reinterpretation of its electrographic and clinical manifestations. Epilepsia 20: 571-588. 9. GLOOR, P., G. HALL, AND F. COCEANI. 1966. Differential sensitivity of various brain structures to the epileptogenic action of penicillin. Exp. Neural. 16: 333-348. 10. GLOOR, P., A. PELLEGRINI, AND G. KOS~OPOULOS. 1979. Effects of changes in cortical excitability upon the epileptic bursts in generalized penicillin epilepsy of the cat. Electroenceph. Clin. Neurophysiol. 46: 274-289. 11. GLOOR, P., L. F. QUESNEY, AND H. ZUMSTEIN. 1977. 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. Electroenceph. Clin. Neurophysiol. 43: 79-94. 12. GLOOR, P., AND G. TESTA. 1974. Generalized penicillin epilepsy in the cat: effects of intracarotid and intravertebral pentylenetetrazol and amobarbital injections. Elecrroenceph. Clin. Neurophysiol. 36: 499-5 15. 13. GUBERMAN, A., AND P. GLOOR. 1974. Cholinergic drug studies of generalized penicillin epilepsy in the cat. Bruin Res. 78: 203-222. 14. GUTNICK, M. J., AND D. A. PRINCE. 1974. Effects of projected cortical epileptiform discharges on neuronal activities in cat VPL. I. Interictal discharge, J. Neurophysiol. 37: 1310-1327. 15. HEATH, C. J., AND E. G. JONES. 1971. The anatomical organization of the suprasylvian gyrus of the cat. Ergeb. And Entwicklungsgesch. 45: l-64. 16. JASPER, H. H., AND C. AJMONE-MARSAN. 1954. A stereotaxic atlas of the diencephalon of the cat. National Research Council of Canada, Ottawa.
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