Synchronous Spike-wave Discharges Research Articles

Parvalbumin neurons in the anterior nucleus of thalamus control absence seizures.

Anterior nucleus of thalamus (ANT) has been widely accepted as a potential therapeutic target for drug-resistant epilepsy. Although atrophy of the ANT was also reported in patients with absence epilepsy, the relationship between the ANT and absence epilepsy has been barely illustrated. Using chemogenetics, we evaluated the effect of ANT parvalbumin (PV) neurons on pentylenetetrazole (PTZ)-induced absence seizures in mice. We found that intraperitoneal injection of PTZ (30 mg/kg) can stably induce absence-like seizures characterized by bilaterally synchronous spike-wave discharges (SWDs). Selective activation of PV neurons in the ANT by chemogenetics could aggravate the severity of absence seizures, whereas selective inhibition of that cannot reverse this condition and even promote absence seizures as well. Moreover, chemogenetic inhibition of ANT PV neurons without administration of PTZ was also sufficient to generate SWDs. Analysis of background EEG showed that chemogenetic activation or inhibition of ANT PV neurons could both significantly increase the EEG power of delta oscillation in the frontal cortex, which might mediate the pro-seizure effect of ANT PV neurons. Our findings indicated that either activation or inhibition of ANT PV neurons might disturb the intrinsic delta rhythms in the cortex and worsen absence seizures, which highlighted the importance of maintaining the activity of ANT PV neurons in absence seizure.

Isolated P/Q Calcium Channel Deletion in Layer VI Corticothalamic Neurons Generates Absence Epilepsy.

This study dissects a critical component of the corticothalamic circuit in spike-wave epilepsy and identifies the developmental importance of P/Q-type calcium channel-mediated presynaptic glutamate release at layer VI pyramidal neuron terminals. Genetic ablation of Cacna1a in layer VI neurons produced synchronous spike-wave discharges in the cortex and thalamus that were inhibited by ethosuximide. These mice also displayed N-type calcium channel compensation at descending thalamic synapses, and consistent with other spike-wave models increased low-threshold T-type calcium currents within postsynaptic thalamic relay and reticular neurons. These results demonstrate, for the first time, that preventing the developmental homeostatic switch from loose to tightly coupled synaptic release at a single class of deep layer cortical excitatory output neurons results in generalized spike-wave epilepsy.

LONG TERM FOLLOW-UP OF LESIONAL AND NON-LESIONAL PATIENTS WITH ELECTRICAL STATUS EPILEPTICUS IN SLOW WAVE SLEEP.

A retrospective study has been done at the Bethesda Children's Hospital Epilepsy Center with those patients whose EEG records fulfilled in one or more records the criteria of electrical status epilepticus in slow wave sleep (ESES) pattern, occupying at least 75% of NREM sleep with bilateral discharges, and had detailed disease history and long term follow-up data, between 2000 and 2012. PATIENTS AND METHODS--Thirty-three patients (mean 11.1 +/- 4.2 years of age) were studied by 171 sleep EEG records. Sleep was recorded after sleep deprivation or during spontaneous sleep at least for one hour length of NREM. From the 492 EEGs, 171 sleep records were performed (average five/patient). Average follow-up time was 7.5 years. Eighty-two ESES records have been analyzed in 15 non-lesional and 18 lesional (11 with dysgenetic and seven with perinatal-asphyxic or vascular origin) patients. Variability of seizure types, seizure frequency and frequency of status epilepticus was higher in the lesional group. Impairment of the cognitive functions was moderate and partial in the non-lesional, while severely damaged in the lesional group. EEG records of 29 patients shawed unihemispherial spike fields with a perpendicular axis (in anterior, medial and posterior variants) to the Sylvian fissure, regardless their lesional or non-lesional origin. Only three (lone nonlesional and two lesional) patients had bilateral synchronous spike-wave discharges with bilateral symmetric frontocentral spike fields. The individual discharges of the sleep EEG pattern were very similar to the awake interictal records except their extension in time and field, their increased number, amplitude, and continuity of them and furthermore in the increased trans-hemispheral propagation and their synchronity. Assumed circuits involved in the pathomechanism of discharges during NREM sleep in ESES are discussed based on our findings.

Functional neuroimaging abnormalities in idiopathic generalized epilepsy.

Magnetic resonance imaging (MRI) techniques have been used to quantitatively assess focal and network abnormalities. Idiopathic generalized epilepsy (IGE) is characterized by bilateral synchronous spike–wave discharges on electroencephalography (EEG) but normal clinical MRI. Dysfunctions involving the neocortex, particularly the prefrontal cortex, and thalamus likely contribute to seizure activity. To identify possible morphometric and functional differences in the brains of IGE patients and normal controls, we employed measures of thalamic volumes, cortical thickness, gray–white blurring, fractional anisotropy (FA) measures from diffusion tensor imaging (DTI) and fractional amplitude of low frequency fluctuations (fALFF) in thalamic subregions from resting state functional MRI. Data from 27 patients with IGE and 27 age- and sex-matched controls showed similar thalamic volumes, cortical thickness and gray–white contrast. There were no differences in FA values on DTI in tracts connecting the thalamus and prefrontal cortex. Functional analysis revealed decreased fALFF in the prefrontal cortex (PFC) subregion of the thalamus in patients with IGE. We provide minimum detectable effect sizes for each measure used in the study. Our analysis indicates that fMRI-based methods are more sensitive than quantitative structural techniques for characterizing brain abnormalities in IGE.

System epilepsy: Reply to Lüders et al., 2009

To the Editors: In an article recently published in Epilepsia, Lüders et al. (2009) propose a diagrammatic representation of their views on region-specific multifactorial epileptogenicity. In the same article, Lüders et al. comment on the report of a workshop—“Conceptual dichotomies in classifying epilepsies: Partial versus generalized and idiopathic versus symptomatic” (Capovilla et al., 2009)—also published in Epilepsia. In particular, Lüders et al. express their disagreement about the proposed definition of “system epilepsy” by saying that “simply coining a term and talking about system epilepsy will not be necessarily of any help” for understanding the pathophysiologic bases of the classification of epilepsy. I would like to point out that “system epilepsy” is not proposed as a mere linguistic expression but as a concept to designate those types of epilepsy in which seizures are accounted for by discharges involving a specific system, defined according to anatomic and physiologic criteria. The concept implies a specific susceptibility of the involved system as a whole, although within that system some trigger areas may be identified. Examples are childhood absence epilepsy (CAE) and benign epilepsy with centrotemporal spikes (BECTS). These epilepsy syndromes are presently classified into two separate categories (generalized and partial epilepsies, respectively), although many epileptologists agree that BECTS shares more characteristics with CAE than with other focal epilepsies due to known external causative factors. Quite interestingly, recent information from experimental studies suggests that bilateral synchronous spike–wave discharges of CAE actually have a localized origin within the involved thalamocortical circuitry at the somatosensory cortex (Meeren et al., 2002; Polack et al., 2007). Until further evidence clarifies this matter, everyone is obviously free to keep his/her own view. Personally, however, I believe that the concept of “system epilepsy” could generate more productive testable hypotheses than the waving diagrams proposed by Lüders et al. (2009). I confirm that I have read the Journal’s position on issues involved in ethical publication and affirm that this letter is consistent with those guidelines. I have no conflicts of interest to disclose.

Cortical Focus Drives Widespread Corticothalamic Networks during Spontaneous Absence Seizures in Rats

Absence seizures are the most pure form of generalized epilepsy. They are characterized in the electroencephalogram by widespread bilaterally synchronous spike-wave discharges (SWDs), which are the reflections of highly synchronized oscillations in thalamocortical networks. To reveal network mechanisms responsible for the initiation and generalization of the discharges, we studied the interrelationships between multisite cortical and thalamic field potentials recorded during spontaneous SWDs in the freely moving WAG/Rij rat, a genetic model of absence epilepsy. Nonlinear association analysis revealed a consistent cortical "focus" within the peri-oral region of the somatosensory cortex. The SWDs recorded at other cortical sites consistently lagged this focal site, with time delays that increased with electrode distance (corresponding to a mean propagation velocity of 1.4 m/sec). Intra-thalamic relationships were more complex and could not account for the observed cortical propagation pattern. Cortical and thalamic sites interacted bi-directionally, whereas the direction of this coupling could vary throughout one seizure. However, during the first 500 msec, the cortical focus was consistently found to lead the thalamus. These findings argue against the existence of one common subcortical pacemaker for the generation of generalized spike-wave discharges characteristic for absence seizures in the rat. Instead, the results suggest that a cortical focus is the dominant factor in initiating the paroxysmal oscillation within the corticothalamic loops, and that the large-scale synchronization is mediated by ways of an extremely fast intracortical spread of seizure activity. Analogous mechanisms may underlie the pathophysiology of human absence epilepsy.

Dopamine characteristics in different rat genotypes: the relation to absence epilepsy.

Dopaminergic neurotransmission has been shown to participate in the control of absence epilepsy. This type of epilepsy, a generalized non-convulsive form, is associated with bursts of bilateral synchronous spike wave discharges (SWDs) recorded in the EEG. In a previous study, it was suggested that two features of the apomorphine-susceptible (APO-SUS) rat genotype, a relatively low dopaminergic reactivity of the nigrostriatal system and relatively high dopaminergic reactivity of the mesolimbic system, contribute to the high incidence of SWDs. Indeed, apomorphine-unsusceptible (APO-UNSUS) rats, characterized by opposite dopaminergic features, show considerably less SWDs than APO-SUS rats. The first goal of the present study was to assess the baseline SWD incidence in four rat genotypes (WAG/Rij, ACI, APO-SUS and APO-UNSUS) in order to replicate previous findings. It was expected that both the APO-SUS and WAG/Rij rats would show a considerably higher SWD incidence in comparison to the APO-UNSUS and ACI rats. For this purpose, rats were registered for a 19 hour period. Assuming that haloperidol decreases dopaminergic transmission in the nigrostriatal system via inhibition of the dopamine receptors and enhances dopaminergic transmission in the mesolimbic system via inhibition of the noradrenergic receptors, it was postulated that haloperidol would enhance the difference in dopaminergic reactivity between both systems in favor of the accumbens. Therefore, the second purpose in the present study was to investigate whether haloperidol (2 mg/kg, IP) could further potentiate SWD incidence when injected in the APO-SUS rats, already characterized by a relatively low dopaminergic reactivity of the nigrostriatal system and relatively high dopaminergic reactivity of the mesolimbic system, in comparison to the APO-UNSUS rat genotype. Finally, the third aim was to study if another epileptic rat genotype, the WAG/Rij, would show similar increases in SWD incidence following an injection with haloperidol as expected for the APO-SUS. First, previous findings were replicated: the value of the hourly number of SWDs decreased in the following order: APO-SUS > WAG/Rij > APO-UNSUS and ACI. Secondly, earlier data were extended by the fact that the APO-SUS responded to a systemic injection of haloperidol with an increase in SWD number and duration, in contrast to the APO-UNSUS rats. The hypothesis that the SWD incidence would be mostly affected by haloperidol in the APO-SUS rats, was confirmed by these findings. It is suggested that haloperidol increases the SWD incidence in APO-SUS rats by enhancing the difference between the dopaminergic reactivity in the nigrostriatal and mesolimbic system. Finally, further research is required to provide evidence in favor of the hypothesis that the relative dominance of the dopaminergic mesolimbic system is smaller in WAG/Rij than in APO-SUS.

Ganaxolone, a selective, high-affinity steroid modulator of the gamma-aminobutyric acid-A receptor, exacerbates seizures in animal models of absence.

Ganaxolone (3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one) is a novel neurosteroid which has anticonvulsant properties in a number of seizure models as well as the ability to enhance function of the gamma-aminobutyric acid-A (GABA(A)) receptor complex via a neurosteroid binding site. The object of these experiments was to ascertain the efficacy of ganaxolone against absence seizures. Ganaxolone was assessed in the low-dose pentylenetetrazol (PTZ) and the gamma-hydroxybutyric acid (GHB) model of absence seizures in rats. Ganaxolone pretreatment resulted in a significant prolongation of absence seizure in both the PTZ and GHB models. Further, ganaxolone in doses above 20 mg/kg alone produced bilaterally synchronous spike wave discharges (SWDs) associated with behavioral arrest. These data suggest that augmentation of GABA(A) receptor complex function by neurosteroids has the potential to result in or exacerbate absence seizures.

Regional variation in bilateral synchrony of spike-wave discharge before and after corpus callosotomy

In patients undergoing corpus callosotomy (CC), we investigated whether regions with a high degree of bilateral synchrony of spike-wave discharge correspond to the regions with major epileptogenic abnormalities and whether these regions are included within the cortical distribution of callosal fibers bisectioned. We studied 16 patients who underwent anterior (ant 2/3–4/5) CC for intractable symptomatic generalized epilepsies (SGE) with preoperative interictal generalized synchronous spike-wave discharges (GSSW). The degree of bisynchrony and synmorphism (i.e., similarity of wave form) was measured using cross-correlation analysis and the measurement of amplitude differences between homologous regions. Preoperatively, frontocentral regions had higher values of correlation coefficient (R) and interhemispheric synchrony (IS) than temporal and occipital regions. Furthermore, the temporal region, as compared with the other regions, had higher value of interhemispheric time difference (ITD). Postoperatively, the GSSW were disrupted and there was a significant decrease in the levels of R and IS, and a significant increase in amplitude difference in all regions, especially in the frontocentral regions where many of the lateralized spike-wave discharges were observed in routine electroencephalograms (EEGs). In our patients with SGE, the regions with a high degree of bilateral synchrony correspond to the regions with major epileptogenic abnormalities. They are predominantly frontocentral regions, and are included in the extent of the cortical distribution affected by the bisection of callosal fibers. These analyses will provide additional information regarding patient selection, the extent of callosal bisection, and seizure outcome.

Basic mechanisms of generalized absence seizures

Generalized absence seizures are neurophysiologically, pharmacologically, and developmentally unique and comprise the primary seizure type in a number of different absence epilepsy syndromes. Over the last 10 years, the availability of a number of animal models of generalized absence seizures and of sophisticated in vitro electrophysiological techniques that allow investigation of cortical and thalamic networks has begun to shed light on the pathogenesis of this disorder. The basic underlying mechanism appears to involve thalamocortical circuitry and the generation of abnormal oscillatory rhythms from that particular neuronal network. Biochemical mechanisms operative within thalamocortical circuitry during this neuronal oscillation seem to entail phase-locked gamma-aminobutyric acid (GABA)B-mediated inhibition alternating with glutamate-mediated excitation. The basic cellular mechanism operative within this tension between excitation and inhibition appears to involve the T-type calcium current. Local circuitry within the thalamus may influence these oscillatory rhythms by GABAA-mediated inhibition. Pharmacological factors at play external to thalamocortical circuitry include cholinergic, dopaminergic, and noradrenergic mechanisms. Pathways that utilize these various neurotransmitters project onto the thalamus and/or cortex from sites distant to those structures and may modulate the process either up or down. Perturbation of one or more of these neuronal networks may lead to abnormal neuronal oscillatory rhythms within thalamocortical circuitry, with a resultant generation of bilaterally synchronous spike wave discharges that characterize generalized absence seizures. Our increasing understanding of the basic mechanisms that underlie generalized absence seizures promises to allow, for the first time, a rational design of drug treatment for a seizure disorder based on the pathogenesis of that disorder.

Thalamic NMDA receptors in the γ-hydroxybutyrate model of absence seizures: A cerebral microinjection study in rats

The possible role of thalamic NMDA receptors in the generation of experimental absence-like seizures was studied in rats. Bilaterally synchronous spike wave discharges were induced by γ-hydroxybutyric acid (GHB) and were recorded simultaneously from different thalamic nuclei and the layers I–IV of frontoparietal cortex. Bilateral infusions of NMDA into thalamic mediodorsal nucleus, the intralaminar central lateral/paracentral nucleus, ventroposterolateral, or reticular nucleus of the thalamus in conscious rats, prior to GHB administration suppressed GHB-induced SWD in a dose dependent manner. However, no such suppression of GHB-induced SWD was observed when NMDA infusions were made into the above thalamic sites after the onset or development of GHB-induced SWD. Pretreatment with high doses of competitive (CGP 43487) or non-competitive NMDA receptor antagonists (MK-801 and ketamine) also dose dependently suppressed GHB-induced SWD. Both MK-801 and CGP 43487 dose dependently antagonized NMDA-mediated inhibition of GHB-induced SWD activity but at lower doses did not produce significant inhibition of GHB-induced SWD. The anti-SWD effects of NMDA, MK-801 and ketamine but not CGP 43487 were more pronounced in the mediodorsal and intralaminar thalamic nuclei than in the ventroposterolateral or reticular nucleus of thalamus. Because low doses of NMDA antagonists failed to disrupt the generation of seizures in the GHB model, these findings do not support a role for thalamic NMDA receptors in the pathogenesis of absence-like seizures induced by γ-hydroxybutyric acid.

Effects of cholinergic drugs on genetic absence seizures in rats.

Wistar rats of a selected strain show spontaneous generalized non-convulsive seizures with bilateral synchronous spike-wave discharges on the cortical electroencephalograph (EEG). The 7 to 9 c/s spike-wave discharges occur predominantly in waking states of inactivity. The effects of cholinergic drugs on the cumulated duration of spike-wave discharges were investigated in this rat model of absence epilepsy. I.p. injections of drugs which potentiate cholinergic neurotransmission, namely the acetylcholinesterase inhibitor, physostigmine (0.1-0.5 mg/kg), the muscarinic receptor agonists, oxotremorine (0.25-1 mg/kg) and pilocarpine (0.125-2 mg/kg), and the nicotinic receptor agonist, nicotine (0.062-2 mg/kg), suppressed discharges in a dose-dependent manner and induced an arousal-like cortical EEG. The muscarinic receptor antagonist, scopolamine, increased the spike-wave discharges at doses below 0.05 mg/kg; at higher doses (0.05-1 mg/kg) it decreased discharges and induced a sleep-like EEG. The nicotinic receptor antagonist, mecamylamine (0.5-6 mg/kg), had no effect on spike-wave discharges or the EEG. These results suggest that cholinergic activity accounts for the preferential occurrence of absence seizures in states of reduced arousal.

Electroclinical seizures in Lennox-Gastaut syndrome.

We analyzed electroclinical seizures observed by long video split-screen recording in 21 patients with Lennox-Gastaut syndrome (LGS). All patients had atypical absence seizures, 18 (81%) had tonic seizures, and 4 (21%) had myoclonic-atonic seizures. Tonic seizures were axial with flexion or extension of the head or trunk, or global with generalized tonic spasm mimicking infantile spasm, or involved the eyeballs only (either brief, with upward deviation of eyeballs or long, with oscillatory nystagmus). EEG showed either a bilateral 10-13-Hz rhythm or generalized synchronous spike wave at 3 Hz. Myoclonic-atonic seizures involving limbs, trunk, or neck were either brief or massive; the discharges were 2-3.5-Hz spike wave. Atypical absence seizures evolved gradually, terminated abruptly, and manifested alone or with subtle motor activity or oral automatism. EEG discharges were variable and of different types: (a) Diffuse irregular spike wave at 2-2.5 Hz with or without fragmentation (consciousness was regained during fragmentation or when spike wave discharges were < 2 Hz), (b) irregular diffuse fast activity at 10-13 Hz, or (c) a combination of fast spike wave or sharp waves of increasing amplitude followed by synchronous spike wave discharges at 3 Hz.

Evidence for G protein modulation of experimental-generalized absence seizures in rat

Pertussis toxin is known to inhibit G proteins via ADP-ribosylation, and a response to pertussis toxin is presumptive evidence of G protein modulation of the activity being studied. Therefore, in order to test the hypothesis that G protein-mediated mechanisms are involved in the pathogenesis of generalized absence seizures, the effect of pertussis toxin in two pharmacological models of generalized absence seizures in rat was investigated. The experimental absence seizure models used were the γ-hydroxybutyrate (GHB) model and low dose pentylenetetrazole (PTZ) model in rat. Pretreatment with pertussis toxin administered intracerebroventricularly (i.c.v.) resulted in a significant decrease in duration of seizure in both models. These data suggest that G-protein-mediated mechanisms may be involved in the pathogenesis of the bilaterally synchronous spike wave discharges (SWD) that characterize experimental absence seizures.

Absence epilepsy: early prognostic signs

We have studied 124 children with typical absence epilepsy. The onset of symptoms was in 12% under 4 years, in 51% between 4–8 years and in 37% above 8 years. The F:M ratio was 2:1 in children under 4 years versus 1:1 above 8 years. Absences alone occurred in 82% and absences followed or preceded by generalized tonic—clonic seizures (GTCS) in 6.5% and 11%, respectively. Simple absences were not seen in children under 4 years and were more frequent (14%) in the 4–8 years age group. Family history was positive for epilepsy in 20% and febrile convulsion in 7%. Sixteen percent had a positive past history of febrile convulsions. All patients showed bilateral, synchronous spike-wave discharges from 2.5 to 4 c/s. Lateralized spikes, spike-slow wave complexes were found in 27%. Photosensitivity was present in 18% and was marked in 12%. Monotherapy with sodium valproate or ethosuximide (91% SV) was successful in 85% of patients with absences alone and 68% of the absences with GTCS. Only 2% were not fully controlled either on monotherapy or polytherapy. Treatment was withdrawn in 41 patients and 13 relapsed. We have identified four factors associated with relapses: (a) poor initial response to treatment, (b) lateralized focal EEG abnormality and/or marked photosensitivity, (c) the evolution to myoclonic epilepsy, and (d) early withdrawal of AED (<3 years).

Involvement of excitatory amino acid mechanisms in γ-hydroxybutyrate model of generalized absence seizures in rats

γ-Hydroxybutyric acid (GHB), a naturally occurring compound which is synthesized from γ-aminobutyric acid (GABA), induces bilaterally synchronous spike wave discharges, associated with behavioral changes, reminiscent of petit mal or generalized absence seizures in rats. In the present study, possible involvement of excitatory amino acids (EAAs) in GHB-induced spike wave discharges was investigated. The noncompetitive antagonist of NMDA receptors, MK-801, attenuated GHB-induced spike wave discharges at all doses tested (0.025–1.0 mg/kg) but dose-dependently induced suppression of EEG bursts in GHB-treated animals. The suppression of bursts was never observed with GHB in control experiments. N- Methyl- D-aspartate (NMDA) had a similar effect on GHB-induced spike wave discharges, when it was administered prior to GHB. This effect of NMDA was partially reversed by MK-801. The competitive antagonists of NMDA receptors, (±)CPP and CGP 43487 and the antagonist at the strychnine-insensitive glycine site, HA-966, also suppressed GHB-induced spike wave discharges with the EEG progressing to suppression of bursts but were weaker in this regard than MK-801 or NMDA. These data raise the possibility of involvement of excitatory amino acids in the GHB model of absence seizures.

Evidence for GABA B-mediated mechanisms in experimental generalized absence seizures

Experimental absence seizures are characterized by the fact that they are exacerbated by both direct and indirect GABA agonists. To date most of the studies that have examined this phenomenon have utilized GABA A agonists. We assessed the effect of a GABA B agonist, baclofen and a specific GABA B antagonist in two pharmacologic models of absence seizures in rodent after using either γ-hydroxybutyrate or pentylenetetrazole to induce the bilaterally synchronous spike wave discharges that typify absence seizures in rodent. Baclofen markedly prolonged and the GABA B antagonist attenuated or blocked the experimental absence seizures in both models. These data suggest a role for GABA B-related mechanisms in the pathogenesis of generalized absence seizures and raise the possibility that GABA B antagonists may have therapeutic potential as antiabsence drugs.

Increased γ-hydroxybutyric acid receptors in thalamus of a genetic animal model of petit mal epilepsy

The distribution and kinetics of specific binding sites for γ-hydroxybutyrate (GHB), a naturally occurring compound known to produce absence-like seizures, was studied in the brains of Wistar rats with spontaneous, bilaterally synchronous spike wave discharges (SWDs), a model of petit mal epilepsy, and non-epileptic controls using [ 3H]GHB autoradiography. [ 3H]GHB receptor binding was increased 40–60% in lateral thalamic nuclei of the epileptic animals. Kinetic analysis showed that the increase in the binding was due to an increase in density of low affinity GHB binding sites in the epileptic animals. Given the ability of GHB to produce petit mal-like seizures when administered to animals, and the fact that the SWDs in the Wistar rat model seem to emanate from lateral thalamus, these data raise the possibility that GHB-mediated mechanisms may play a role in the pathogenesis of petit mal seizures.

Glutathione levels in specific brain regions of genetically epileptic ( tg/tg) mice

The tottering ( tg/tg) mouse is a genetic model of human generalized epilepsy; these mice exhibit spontaneous absence seizures accompanied by bilaterally synchronous spike-wave discharges (6). The mechanism(s) for seizure activity are unknown in these mice. Several recent studies have suggested that membrane lipid peroxidation may be causally involved in some forms of experimentally induced epilepsies (18). Since reduced glutathione (GSH) is the most important free radical scavenging compound in vivo that can prevent membrane lipid peroxidation, the objective of this study was to investigate GSH concentrations in specific central nervous system regions of genetically epileptic, tg/tg, mice as compared to age-matched controls. Three brain regions, cerebellum, hippocampus, and occipital cortex, were dissected, weighed and the concentrations of reduced and oxidized glutathione (GSH and GSSG, respectively) were measured in each of these tissues. GSH content was significantly lower in the occipital cortex of tg/tg mice compared to controls; no differences were observed in the other two brain regions examined. Total GSH content (GSH plus 2 × GSSG) paralleled GSH concentration differences. GSSG content from tg/tg mice was lower in the hippocampus and occipital cortex, compared to controls. This is the first report of an association between decreased central nervous system glutathione concentrations and seizure activity in animals exhibiting generalized seizures.

The Ontogeny of GABAergic Enhancement of the γ‐Hydroxybutyrate Model of Generalized Absence Seizures

The ontogeny of GABAergic enhancement of generalized absence seizures was examined in the gamma-hydroxybutyrate (GHB) model of generalized absence seizures. The GHB seizure was quantitated in developing and adult rats in the presence of varying doses of the GABAa agonist muscimol or intracerebroventricularly (i.c.v.) administered GABA. Both GABA and muscimol potentiated GHB-induced seizures in an age-dependent fashion. The adult dose of 1 mg/kg muscimol was extremely potent in rats less than 28 days of age and resulted in the death of all younger animals tested secondary to profound hypothermia. A dose of 0.1 mg/kg muscimol was associated with a significant prolongation of GHB seizure in rats less than 35 days of age, but had no effect on older animals. The response to GHB was also age dependent, with the greatest sensitivity occurring during the fourth and fifth week of life. The developmental sensitivity of the rat to GHB seizure correlated with enhancement of the seizure by muscimol and GABA, and both phenomena parallel the maturation of thalamocortical recruiting mechanisms thought to play a role in the pathogenesis of the bilaterally synchronous spike wave discharges that characterize generalized absence seizures.