European Journal of Pharmacolo~', 213 (1992) 343-349 ~ 1992 Elsevier Science Publishers B.V. All rights reserved (11114-2999/92/$05.00

EJP 52371

Evidence for GABA B-mediated mechanisms in experimental generalized absence seizures O. C a r t e r S n e a d , I I I Dit'ision ¢~fNeurology, ('hihlrens Hospital Los Angeles. Department of Neuroh~gy, Unicersity of Southern California. School of Medicine, Los Angeles. CA 9002Z U.S.A.

Received 31 October 1991, revised MS received 20 December 1991, accepted 7 January 1992

Experimental absence seizures are characterized by the fact that they are exacerbated by both direct and indirect G,a agonists. To date most of the studies that have examined this phenomenon have utilized GABA A agonists. We assessed effect of a GABA~ agonist, baclofen and a specific GABA)~ antagonist in two pharmacologic models of absence seizure rodent after using either y-hydroxybutyrate or pentylenetetrazole to induce the bilaterally synchronous spike wave discha that typify absence seizures in rodent. Baclofen markedly prolonged and the GABA~ antagonist attenuated or blocked experimental absence seizures in both models. These data suggest a role for GABA n-related mechanisms in the pathogenes generalized absence seizures and raise the possibility that GABA B antagonists may have therapeutic potential as antiabs~ drugs. Absence seizures; Epilepsy; GABA A receptors; GABAr~ receptors: y-[tydroxybutyric acid: Pentylcnetetrazole

1. Introduction Petit mal or generalized absence seizures differ clinically and experimentally from any other seizure type (Berkovic et al., 1987). Clinically, absence seizures occur in children and have the classic E E G abnormality of 3 / s spike wave discharge which is associated with behavioral arrest and occasional automatisms, but no aura or postictal state (Penry et al., 1975; [x)ckman, 1989). Pharmacologically, absence seizures respond to ethosuximide, valproate and trimethadione (Sherwin, 19891, and are worsened by phenytoin and carbamazepine (Roseman, 1961; Snead and Hosey, 1985). One of the defining features of generalized absence seizures is their potentiation by increased y-aminobutyric acid (GABA)ergic activity in the brain. Enhancement of GABAergic activity potentiates clinical (MysIobodsky, 1976; Van der Linden et al., 1981) and all experimental forms of generalized absence seizure activity (Gloor, 1984; Vergnes et al., 1984; Peeters et al., 1990; Smith and Bierkamper, 1990; Snead, 1984, 1990a) and may even be sufficient to produce bilaterally synchronous spike wave discharges (SWD) under certain

conditions (Fariello and Golden, 1987; Gloor Fariello, 1988). This effect is opposite to that see~ animal models of generalized convulsive and pal seizures where G A B A is anticonvulsant (Snead, 19 Although G A B A A agonists such as muscimol 4,5,6,7-tetrahydroisooxazolo(5,4-c)pyridin-3-ol (TF exacerbate experimental absence seizures, G A B A A tagonists do not block experimental absence (Sn~ 1984) nor does the macromolecular G A B A A recel complex appear to bc primarily involved in the gen of bilaterally synchronous spike wave discharges 1 characterize absence seizures (Snead, 1990b). Th( fore, we set out to test the hypothesis that GAB, mediated mechanisms are involved in this 1: nomenon. Here we demonstrate that the E E G behavioral abnormalities in two pharmacologic mo( of experimental absence seizures in rats were mark( exacerbated by the G A B A B agonist baclofen, blocked or attenuated by a specific GABAH ant~ nist.

2. Materials and methods 2. l. Dn~gs

Correspondence to: O.C. Snead, III, Childrens tlospital [x)s Angeles, Box 42. 465(I Sunset Boulevard, Los Angeles, CA 90027, U . S . A . Tel. 1.213.669 2498, fax 1.213.667 2019.

y-nutyrolactone (GBL), and pentylenetetra~ (PTZ) were obtained from Sigma (St.l_x)uis, MO). "

344 specific G A B A B agonist, baclofen and the G A B A ~ antagonist C G P 35348 (Olpe et al., 1990; Olpe and Karlsson, 1990) were a gift of Dr. R. Bernasconi (Ciba Geigy, Basel, Switzerland). All drugs were given i.p. G B L was given in a dose of 1(10 m g / k g as the pure drug. In previous work standardizing the y-hydroxy butyric acid ( G H B ) model of generalized seizures, G B L has been used to induce the absence-like seizure because G B L produces exactly the same progression of E E G and behavioral events in the rat as G H B (Scotti de Darolis and Messotti, 1979; Snead ct al., 1980), but with a more rapid onset of action and predictable dose response (Bearden et al., 1980). GBI, is converted rapidly and irreversibly to G H B after parenteral administration (Lettieri and Fung, 1978a,b) and is biologically inactive (Snead, 1991). P T Z was given in a dose of 20 m g / k g . This dose produces E E G and behavioral changes that are more similar to those observed in human absence seizures than higher doses of P T Z which result in clonic seizures and spike trains (Marescaux et al,, 1984; Depaulis et al., 1989). PTZ, baclofen and C G P 35348 were each dissolved in normal saline to make a dosage volume of 1 m l / k g . All control animals received normal saline i.p. in a dose of 1 m l / k g instead of the G A B A ~ agonists or antagonists.

2.2. Animals Male Spraguc-Dawley rats (Charles River, DE, USA) weighing 250-300 g were used for all experiments involving adult animals. These animals were housed singly, with ad libitum access to food and water, and water, and maintained on a 12-h light dark cycle. All animals were drug naive,

administration the experimental absence seizure , quantitated as described below. Paired, drug n~ controls were used in all experiments using saline lieu of baclofen. In another group of experiments clofen was given alone in the same dose to determin this drug produced any ECoG or behavioral change

2.4.2. Effect of CGP 35348 on experimental absence C G P 35348 was given i.p. in a dose range of 100m g / k g 30 rain prior to either P T Z or GBL. Follo~ G H B or P T Z administration the experimental absc seizure was quantitated as described below. Pai~ drug naive controls were used in all experiments u~ saline in lieu of CGP 35348. In another group experiments C G P 35348 was given alone in a dose 400 m g / k g to determine if this drug produced ECoG or behavioral changes.

2.5. Data analysis The experimental absence seizures induced by b G H B and P T Z may be objectively quantitated by m suring the duration of the bilaterally synchronous S~ induced by these compounds. G H B and PTZ-indu SWD duration was compared in baclofen- and C 35348-treated animals vs. paired saline controls. Me and S.E. were calculated for all SWD data. The d were expressed in latency of SWD onset (rain) ; cumulated SWD duration (s) for 20-min periods (1 paulis et al., 1989). Within a specified period, comp son between the different drug treatments was can out using a non-parametric analysis of variance related samples. When significance occurred, M a n n - W h i t n e y test was used for comparison betw. two treatments (Dcpaulis et al., 1989). There were f to six animals in each group for all experiments.

2.3. Surgeries and recordings Permanent epidural electrodes were implanted in control and drug treated animals under halothane anesthesia to allow continuous recording of the electrocorticogram (ECoG). Seven days were allowed for recovery before c o m m e n c e m e n t of experiments. All ECoG recordings were made with animals in the freely moving state in shielded, heated, clear plexiglass containers so that the behavioral response to drug could be observed and correlated with any drug-induced ECoG event. The E C o G was recorded continuously for 60 min prior to administration of any drug and for 3 h after administration of G B E or PTZ.

Z4. Experimental design 2.4.1. Effect of baclofen on experimental absence Baclofen was given i.p. in a dose of 5 m g / k g 60 min prior to cither P T Z or GBL. Following G H B or P T Z

3. Results

3.1. Effect ofbaclofen in pharmacologic models" ofgeJ alized absence seizures In control experiments systemic administration 100 m g / k g G B L resulted in bilaterally synchron SWD associated with behavioral arrest, vibrissal twil ing, and facial myoclonus, all within 4 - 5 min of adn istration. The SWD had a frequency of 4 - 6 c / s witl7 amplitude of 200-400 /xV. Within 4 min of onset SWD became continuous. The response to P T Z ' identical to G B L in terms of behavior and E E G fi ings except that the SWD never became continuou: the PTZ-treatcd animals, but occurred in bursts lasl 1-5 s. Baclofen alone produced no discernable E E G behavioral changes, nor did baclofen treatment re

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Fig. 1. (/.eft) Effect of the GABAI~ agonist, baclofen, on the duration of absence seizures induced by I(X) mg/kg GBL In the this an, absence model studies shown, each I~)int represents the mean cumulated SWD duration for a 20-rain period (n = 4-6 for each point). Bec the S.E.M. was only 10% or less in all groups of experiments. S.E. bars arc not shown. Baclofen (5 mg/kg) resulted in a highly signifl (* P < 0.001 ) prolongation ()f GliB-induced absence seizure activity. (Right) Effect of the GABA B agonist, baclofen, on the duration of abs seizures induced by 20 mg/kg PTZ. Baclofen (5 mg/kg) resulted in a highly significant (* P < 0.001) prolongation of GHB-induced abs seizure activity.

in any significant c h a n g e in latency from e i t h e r G B L or P T Z a d m i n i s t r a t i o n to o n s e t o f S W D . H o w e v e r baclofcn in a dose o f 5 m g / k g r e s u l t e d in a highly significant (P < 0.001) p r o l o n g a t i o n o f b o t h G H B - a n d P T Z - i n d u c e d a b s e n c e seizure activity (fig. 1).

3.2. L~fect of CGP 35348 on experimental absence seizure,s" P r c t r e a t m c n t with the G A B A B a n t a g o n i s t , C G P 35348, r e s u l t e d in a d o s e - d e p e n d e n t d e c r e a s e in the d u r a t i o n of b o t h G H B (figs. 2,3) a n d P T Z - i n d u c e d (figs. 4,5) a b s e n c e seizures. C G P 35348 in a d o s e of 200 m g / k g p r o d u c e d a significant (P < 0.05) d e c r e a s e in S W D d u r a t i o n , while 400 m g / k g r e s u l t e d in c o m p l e t e suprcssion of both EEG and behavioral manifestation

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.- .- ,- .- .- .= ,0 8o 12o ,6o TIME(MIN)AFTERGBLIOOMG/KGIP Fig. 2. Effect of a GABA B antagonist on GliB-induced absence seizures. Three doses of CGP35348 were utilized, If)(), 200 and 400 mg/kg. The GABA B antagonist, CGP 35348, resulted in a dose-dependent decrease in the duration of GHB -induced absence seizures. Treatment with 200 mg/kg produced a significant (* P < 0.05) decrease in spike wave discharge duration while a dose of 400 mg/kg resulted in comnletc suoression of GHB seizures,

of G H B - i n d u c c d s e i z u r e s (fig. 3) and a signifk d e c r e a s e in S W D d u r a t i o n in P T Z - i n d u c c d seizL (fig. 5). C G P 35348 a l o n e p r o d u c e d no c h a n g e in E o r b e h a v i o r from baseline.

4. D i s c u s s i o n

T h e e x p e r i m e n t a l p h a r m a c o l o g i c a l m o d e l s o f gel alizcd a b s e n c e s e i z u r e s used in these e x p e r i m e n t s f the criteria which have b e e n set forth for e x p e r i m e a n i m a l m o d e l s o f a b s e n c e ( F a r i e l l o and G o l d e n , 1~ S n e a d , 1988; Mirsky et al., 1986). Both m o d e l s t b e e n s t a n d a r d i z e d in the rat w h e r e i.p. a d m i n i s t r a o f e i t h e r G H B o r low d o s e P T Z results in bilate~ s y n c h r o n o u s S W D at 4 - 6 c / s a s s o c i a t e d with bel ioral arrest, facial clonus, a n d vibrissal twitcl ( M a r c s c a u x et al., 1984; D e p a u l i s ct al., 1989; Sn( 1988). It s h o u l d bc n o t e d that while the e l e c t r o g r a l paroxysms i n d u c e d by these c o m p o u n d s in the differ from the 3 c / s d i s c h a r g e s o b s e r v e d in hut a b s e n c e seizures, this d i f f e r e n c e is of no signific~ from a m o d e l i n g p o i n t o f view since these p h e n o m arc a t t e n u a t e d by a n t i - a b s e n c e drugs ( M a r c s c a u x ct 1984; S n e a d , 1988; G o d s c h a l k et al., 1976) a n d w e n c d by p h c n y t o i n ( S n e a d , 1978). T h e s e two pharm~ logic m o d e l s p a r a l l e l the g e n e t i c m o d e l o f absc s e i z u r e s in rat in t e r m s of p h a r m a c o l o g y , beha~ E E G findings, and i n v o l v e m e n t of t h a l a m o c o r l p a t h w a y s ( M a r e s c a u x et al., 1984; V e r g n e s et al., 19 G A B A e r g i c m e c h a n i s m s have long b e e n h y p o t h e s tO be involved in the p a t h o g e n e s i s o f g e n e r a l i z e d scnce seizures (King, 1979; F r o m m a n d Kohli, 19721 a d d i t i o n to the findin~ that i n c r e a s e d G A B A e

346

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Fig. 3. A representative electrocorticogram of an animal treated with 400 m g / k g CGP 35348 followed in 30 rain by 100 m g / k g (}BL. recording in the control animal illustrates the typical absence seizure where the bilaterally synchronous SWD is associated with behavioral ar vibrissal twitching and facial myoclonus. Neither EEG nor behavioral seizure were observed in the animal treated with CGP 35348.

function in brain results in prolongation of absence seizures, the neurophysiology of GABAergic systems in generalized absence seziures differs from that observed in generalized convulsive or partial seizures. During the SWD in absence seizures, which may bc recorded only from thalamus and cortex (Vcrgnes et al., 1987), there is preservation of GABAcrgic function, sparing of classical inhibitory postsynaptic potentials (IPSPs), and absence of the typical paroxysmal depolarizing shifts seen in focal epileptogenesis (Gloor and Fariello,

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0/.~,~_~, . -x.~ . ~0 ~0 ~0 ~0 ,00 .~0 ,,0 TIME(MIN)AFTERPTZ20MG/KGI.P. Fig. 4. Effect of a GABA B antagonist on PTZ-induced absence seizures. Three doses of ('(,;I)35348 were utilized, ](~l, 2(10 and 400 mg/kg. The GABA n antagonist, CGP 35348, resulted in a dose-dependent significant (* P < 0.05) decrease in the duration of PTZ-in-

duccd absence seizures,

1988; Gloor et al., 1990; Kostopoulos, 1986; AvolJ al., 1990). This preservation of GABAcrgic inhibit may contribute to the synchronization of SWD whic the hallmark of generalized absence seizures, beca the pacing of SWD and bursts of action potential,, absence seizures arc dependent upon rythmically cutting inhibition in both cortex and thalamus (GI and Gariello, 1988). It has thus been postulated t generalized absence seizures could result from an cess of GABAergic inhibition in cortex a n d / o r th~ mus (Fariello and Ticku, 1983; Gloor and Farie 1988; Fariello, 1990). Previous studies concerning the role of GABA the ncurophysiology and neuropharmacology of ge~ alizcd absence seizures have been directed G A B A x-mediated mechanisms because of two line, evidence. First, G A B A A agonists are known t o exa( bate experimental absence (Vergnes et al., 1984; Snc 1984, 1990a). Second, there is evidence that GABA gic neurons in the reticular nucleus of the thala[ and their projections to the specific relay nuclcu., the thalamus may be involved in the elicitation control of experimental absence via a GABA,x-mo ted mechanism (Liu et al., 1991). Mitigating against G A B A ; , hypothesis is the observation that, w G A B A A agonists prolong experimental absence sciz activity, G A B A A antagonists do not attenuate or bl

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Fig. 5. A representative electrocorticogram of an animal treated with 200 m g / k g C G P 35348 followed in 30 rain by 20 m g / k g PTZ. recording in the control animal illustrates the typical absence seizure induced by this dose of PTZ. The behavior during the SWD was the s as in the GBL-treated animal namely behavioral arrest, vibrissal twitching and facial myoclonus.

absence seizures (Snead, 1984). In addition, postsynaptic G A B A A receptor function, as measured by binding to subtypes of this receptor and chloride ion flux, does not seem to be primarily involved in the causation of SWD seizures in the G H B model of absence (Snead, 1990b). Recently, a role for GABAB-mediated mechanisms in the burst firing of thalamocortical cells has been postulated based on the relative density of this receptor subtype in the thalamus and evidence that the GABA B receptor mediates a long lasting potassiumdependent IPSP in the thalamus (Crunelli and Leresche, 1991). Furthermore, i.v. administration of baclofen has been shown to increase the rhythmic burst firing in thalamocortical cells of the ventrobasal complex (Clarke, 1983) a finding commensurate with the current data showing that baclofen is much more effective in potentiating the bilaterally synchronous SWD in experimental absence seizures than the GABA A agonist, muscimol (Snead, 1990a). These data taken in conjunction with the finding that rhythmic SWD in the models utilized in our experiments were significantly attenuated or blocked by a specific GABA a antagonist, lend credence to the hypothesis that the G A B A ~ receptor is involved in the pathogenesis of generalized absence seizures. Increased G A B A a activity within reverberating thalamocortical circuits may accentuate the burst firing of thalamic neurons that is required for the manifestation of absence seizures (Crunelli and Leresche. 1991). Alternatively. a G A B A , antagonist

could dampen these circuits and have an anti-abse effect. The fact that the discharges elicited by PT2 compound that acts at the picrotoxin site of GABA A receptor complex, were attenuated by GABA B antagonist suggests a complicated intcrpla GABA A and G A B A , receptors in eliciting S'~ There is recent evidence for such G A B A A / G A F interaction in the brain. Haehner et al. (1991) 17 demonstrated an influence of G A B A ~ receptor-me, ted activity on the chloride ion channel controllec the GABA A receptor complex. The complete block of GHB-induced SWD by the GABAr~ receptor an onist suggests that G H B may exert its effects upon GABA ~ component of this putative G A B A A / G A F interaction to produce absence-like seizures. In summary these results demonstrate that acti of the GABA B receptor is necessary for expressio~ SWD; however, more experiments are necesar3, prove that this receptor is directly involved in pathogencsis of absence seizures. Also, these data r the possibility that GABA a antagonists may have c cal therapeutic potential as an antiepileptic druj generalized absence epilepsy.

Acknowledgements This work was supported in part by Grant No. RO1 NS 1 from the NINDS. I am grateful to Mr. Chun Che Liu for su

technicalsuor~)rt.

348

References Avoli, M., P. Gloor and G. Kostopoulos. 1990, Focal and generalized cpileptiform activity in the cortex, in: Generalized Epilepsy: Neurobiologic Aproachcs, eds. M. Avoli. P. Gloor, (i. Kostopoulos and R. Naquet (Birkhauser. Boston) p. 213. Beardcn, L.J., O.C. Snead, C.T. Hcaly and G.V. Pegram. 1980, Antagonism of gamma-hydroxybutyric acid-induced frequency shifts in the cortical EEG of rats by dipropyl acetate, Electrocncephalogr. Clin. Neurophysiol. 9, 181. Berkovic, S.F., F. Andermann, E. Andermann and P. Gl(x>r, 1987, Concepts of absence epilepsies: discrete syndromes or biologic continuum?, Neurology 37, 993. Clarke, K.A., 1983, Effect of back)fen on sensory transmission through ventrobasal nucleus of rat, Neuropharmacology 22, 1231. Crunelli, V. and N. Lcrcschc, 1991, A role for GABA f) receptors in excitation and inhibition of thalamocortical cells, Trends Neurosci. 14, 16. Depaulis, A., O.C. Snead, C. Marescaux and M. Vergnes. 1989, Suppressive effects of intranigral injection ()f muscimol in three models of generalized non-convulsive epilepsy induced by chcmical agents, Brain Rcs. 498. 64. Faricllo, R.G.. 199(I, Pharmacology of the inhibitory systems in primary generalized epilepsy of 'petit real' type, in: Generalized Epilepsy: Ncurobiologic Aproachcs, cds. M. Avoli. P. GIcxw. G. Kostopouh)s and R. Naquet (Birkhauscr, Boston) p. 232. Faricllo, R.G. and G.T. Golden, 1987, The TlllP-induccd model of bilateral synchronous spike and wave in rodents, Neuropharmacology 26, 161. Faricllo, R.G. and M.K. Ticku, 1983, Minireview: the perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns, Life Sci. 33, 1629. Fromm. G.t'I. and C.M. Kohli. 1972, The role of inhibitory pathways in petit real epilepsy. Ncuroh)gy 22, 1(112. Gloor, P., 1984 Electrophysiology of generalized epilepsy, in: Electrophysiology of Epilepsy, eds. P.A. Schwartzkroin and II. Wheal (Academic Press, New York) p. 1(17. Gloor. P. and R.G. Fariello, 1988, Generalized epilepsy: some of its cellular mechanisms differ from those of focal epilepsy, Trends Ncurosci. 11, 63. Gloor, P.. M. Avoli and G. Kostopoulos, 1990, Thalamocortical relationships in generalized epilepsy with bilaterally synchronous spike and wave discharge, in: Generalized Epilepsy: Neurobiologic Aproachcs, cds. M. Avoli, P. Gloor, G. Kostopoulos and R. Naquct (Birkhauser, Boston) p. 19(I. Godschalk. M., M.R. Dzoljic and I.I,. Bonta, 1976, Antagonism of gamma-hydroxybutyratc-induccd hypersynchronization in the ECoG of rat by anti-petit real drugs. Neurosci. Left. 3. 1173. ttaehncr, I,., S. McQuilkin and R.A. 11arris, 1991. Ccrcbcllar GABA B receptors modulate function of GABA., receptors, FASEB J. 5, 2466. King, (i.A., 1979, Effects of systemically applied GABA agonists and antagonists on v,ave-spike ECoG activity in rat, Neuropharmacology 18. 47. Kostopoulos, G.. 1986, Neuronal sensitivity to GA[3A and glutamate in generalized epilepsy with spike and wave discharges, Exp. Ncurol. 92, 2(1. l.,etticri, J. and H.L. Fung, 1978a, Evaluation and development of gas chromatographic procedures for the determination of y-hydroxybutyric acid and ~-butyrolactonc in plasma, Biochem. Med. 20, 70. Lettieri, J. and H.L. Fung. 1978b, Improved pharmacological activity via pro-drug modification: comparative pharmacokinetics of sodium y-hydroxybutyrate and y-butyrolactonc, Res. Commun. Chem. Pathol. Pharmacol. 22, 107. Liu, Z.. M. Vcrgnes, A. Depaulis and C. Marescaux, 1991. Evidence fl)r a critical role of GABAcrgic transmission within the thalamus

in the genesis and control of absence seizures in the rat, [3 Res. 545, 1. Lockman. L.A., 1989, Absence, myoclonic and atonic seizures, F atr. Clin. North. Am. 36, 331. Marescaux, C.. G. Micheletti. M. Vcrgnes, A. Depaulis, l,. Ruml and J.M. Warier, 1984, A model of chronic spontaneous I real-like seizures in the rat: comparison with pcntylenetctrai induced seizures, Epilepsia 25. 326. Mirsky, A.F., C.C. Duncan and M.S. Myslobodsky, 1986, Petit epilepsy: a review and integration of recent information, J. ( Ncurophysk)l. 3, 179. Myslobodsky, M., 1976, Petit Mal Epilepsy (Academic Press, i York). Olpc, J.R. and G. Karlsson, 1990, The effects of baclofcn and GABAI) receptor antagonists on k)ng-term potential Naunyn-Schmiedeb. Arch. Pharmacol. 342, 194. Olpe, tI.R.. G. Darlsson, M.F. Pozza, F. Bruggcr, M. Steinm tt.V. Van Riezen, G. Fagg, R.G. ltall, W. Froestl and H. Bitt 1990, C(;P 35348: a centrally active blocker of GABA n re tots, Eur. J. Pharmacol 187, 27. Peeters, B.W.V.M., C.M. Van Rijn, J.M.H. Vossen and A.r Cocnen. 1989, Effects of GABAcrgic agents on spontan( non-convulsive epilepsy, EE(; and behavior in the WAG,/ inbred strain of rats, l,ife Sci. 45, 1171. Pcnry, J.K., R.J. Porter and F.E. Dreifuss, 1975, Simultan, recording of absence seizures with videotape and clectr cephalography. A Study of 374 seizures in 48 patients, Brait 427. Roscman, E., 1961, Dilantin toxicity. Neurology 11. 912. Scotti de Carolis, A. and M. Mcssotti, 1979. Elcctrographic effcc gamma-hydroxybutyrate and gamma-butyrolactonc. Prog. : ropsychopharmacol. 25. 241. Sherwin, A., 1989, Clinical use of ethosuximidc, in: Antiepik drugs, cds. R.tl. l.,evy. R.tt. Mattson, B.S. Meldrum and Pcnry (Raven Press, New York) p. 685. Smith, K.A. and G.G. Bierkampcr, 1990, Paradoxical role of G/ in a chronic model of petit real (absence) like epilepsy in the Eur. J. Pharmacol. 176, 45. Snead, O.('., 1978, Gamma hydroxybutyratc in the monkey II. E of chronic oral anticonvulsant drugs, Neurology 28, 643. Sncad, O.C.. 1983, On the sacred disease: the ncurochemistr epilepsy, Int. Rev. Ncurobiol. 24, 94. Snead, O.C., 1984, GItB. GABA and petit real seizures, in: Ne transmitters, Seizures and Epilepsy, eds R.G. Faricllo, Morsclli, K.G. l,loyd. L.F. Oucsney and J. Engel II CRaven P New York) p. 37. Snead, O.C.. 1988, y-Ilydroxybutyratc model of generalized abs, seizures: further characterization and comparison with othel sence models, Epilepsia 29,361. Snead, O.C., 1990a, The ontogcny of GABAergic cnhancemer the ,/-hydroxybutyratc model of generalized absence sciz, Epilepsia 31. 363. Snead, O.C.. 19906, GABAergic mechanisms in the ,/-hydr butyrate model of absence seizures, Epilepsia 31,675. Sncad, ().C., 1991, The T-hydroxybutyrate model of absence seizl correlation of regional brain levels of "y-hydroxybutyric acid "/-butyrolactonc with spike wave discharges, Neuropharmact 3(1, 161. Snead, ().C. and L.C. Hoscy. 1985, Exacerbation of seizure children by carbamazcpinc, N. Engl. J. Med. 313. 916. Sncad, O.C., L.J. Bearden, C.T. tlealy and V. Pcgram, 198(I, E of acute and chronic anticonvulsant administration on end nous 3,-hydroxybutyratc in rat brain, Neuropharmacology 19 Van der Linden, G.J., H. Mcinardi, S.W.A. Meijer, L. Bossi ('.A. Gomcni. 1981. A double blind cross over trial with gabide (SL76002) against placebo in patients with secon generalized epilepsy, in: Advances in Epileptology: Xllth Epil

International Symposium, eds. M. Dam, l,. Gram and J.K. Penry (Raven Press, New York) p. 141. Vergnes, M., C. Marescaux. G. Micheletli, J. Reis, A. Depaulis, L. Rumbach and J.M. Warter, 1982, Spontaneous paroxysmal electroclinical palterns in rat: a model of generalized non-convulsive epilepsy. Neurosci. I,ett. 33, 99 Vergnes, M., C. Marescaux, G. Michelelti, A. Depaulis, I,. Rumbach

and J.M. Warier, 1984, Enhancement of spike and wave charges by GABAmimctic drugs in rats with sponlaneous mal-likc epilepsy, Neurosci. Lelt. 44, 91. Vergnes, M., C. Marescaux, A. Dcpaulis, G. Micheletti and Warier, 1987, Sponlaneous spike and wave discharges in tl mus and cortex in rat model of genetic petit real-like sei; Exp. Neurol. 96, 127.