Bicuculline-induced rhythmic EEG episodes: gender differences and the effects of ethosuximide and baclofen treatment

Continuous spike-waves during slow-wave sleep in a mouse model of focal cortical dysplasia

OBJECTIVE

To examine if mice with focal cortical dysplasia (FCD) develop spontaneous epileptic seizures and, if so, determine the key electroencephalography (EEG) features.

METHODS

Unilateral single freeze lesions to the S1 region (SFLS1R) were made in postnatal day 0-1 pups to induce a neocortical microgyrus in the right cortical hemisphere. Continuous 24-h recordings with intracranial EEG electrodes and behavioral tests were performed in adult SFLS1R and sham-control mice to assess neurologic status.

RESULTS

A high percentage of adult SFLS1R animals (89%, 40/45) exhibited at least one or more spontaneous nonconvulsive seizure events over the course of 24 h. Of these animals, 60% (27/45) presented with a chronic seizure state that was persistent throughout the recording session, consisting of bursts of rhythmic high-amplitude spike-wave activities and primarily occurring during periods of slow-wave sleep. In comparison, none of the control, age-matched, mice (0/12) developed seizures. The epileptic discharge pattern closely resembled a pattern of continuous spike-waves during slow-wave sleep (CSWS) of the human syndrome described as an electrical status epilepticus during slow-wave sleep (ESES). Key findings in the SFLS1R model indicated that the observed CSWS (1) were more prevalent in female (18/23) versus male (9/22, p < 0.05), (2) were strongest in the right S1 region although generalized to other brain regions, (3) were associated with significant cognitive and behavioral deficits, (4) were temporarily alleviated by ethosuximide treatment or optogenetic activation of cortical γ-aminobutyric acid (GABA)ergic neurons, and (5) theta and alpha band rhythms may play a key role in the generalization of spike-wave activities.

SIGNIFICANCE

This is the first report of an in vivo animal FCD model that induces chronic spontaneous electrographic brain seizures. Further characterization of the abnormal oscillations in this mouse model may lead to a better understanding of the mechanisms of CSWS/ESES.

Opioid receptor-dependent sex differences in synaptic plasticity in the hippocampal mossy fiber pathway of the adult rat

The mossy fiber (MF) pathway is critical to hippocampal function and influenced by gonadal hormones. Physiological data are limited, so we asked whether basal transmission and long-term potentiation (LTP) differed in slices of adult male and female rats. The results showed small sex differences in basal transmission but striking sex differences in opioid receptor sensitivity and LTP. When slices were made from females on proestrous morning, when serum levels of 17β-estradiol peak, the nonspecific opioid receptor antagonist naloxone (1 μm) enhanced MF transmission but there was no effect in males, suggesting preferential opioid receptor-dependent inhibition in females when 17β-estradiol levels are elevated. The μ-opioid receptor (MOR) antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP; 300 nm) had a similar effect but the δ-opioid receptor (DOR) antagonist naltrindole (NTI; 1 μm) did not, implicating MORs in female MF transmission. The GABAB receptor antagonist saclofen (200 μm) occluded effects of CTOP but the GABAA receptor antagonist bicuculline (10 μm) did not. For LTP, a low-frequency (LF) protocol was used because higher frequencies elicited hyperexcitability in females. Proestrous females exhibited LF-LTP but males did not, suggesting a lower threshold for synaptic plasticity when 17β-estradiol is elevated. NTI blocked LF-LTP in proestrous females, but CTOP did not. Electron microscopy revealed more DOR-labeled spines of pyramidal cells in proestrous females than males. Therefore, we suggest that increased postsynaptic DORs mediate LF-LTP in proestrous females. The results show strong MOR regulation of MF transmission only in females and identify a novel DOR-dependent form of MF LTP specific to proestrus.

Animal models of absence epilepsies: what do they model and do sex and sex hormones matter?

While epidemiological data suggest a female prevalence in human childhood- and adolescence-onset typical absence epilepsy syndromes, the sex difference is less clear in adult-onset syndromes. In addition, although there are more females than males diagnosed with typical absence epilepsy syndromes, there is a paucity of studies on sex differences in seizure frequency and semiology in patients diagnosed with any absence epilepsy syndrome. Moreover, it is unknown if there are sex differences in the prevalence or expression of atypical absence epilepsy syndromes. Surprisingly, most studies of animal models of absence epilepsy either did not investigate sex differences, or failed to find sex-dependent effects. However, various rodent models for atypical syndromes such as the AY9944 model (prepubertal females show a higher incidence than prepubertal males), BN model (also with a higher prevalence in males) and the Gabra1 deletion mouse in the C57BL/6J strain offer unique possibilities for the investigation of the mechanisms involved in sex differences. Although the mechanistic bases for the sex differences in humans or these three models are not yet known, studies of the effects of sex hormones on seizures have offered some possibilities. The sex hormones progesterone, estradiol and testosterone exert diametrically opposite effects in genetic absence epilepsy and pharmacologically-evoked convulsive types of epilepsy models. In addition, acute pharmacological effects of progesterone on absence seizures during proestrus are opposite to those seen during pregnancy. 17β-Estradiol has anti-absence seizure effects, but it is only active in atypical absence models. It is speculated that the pro-absence action of progesterone, and perhaps also the delayed pro-absence action of testosterone, are mediated through the neurosteroid allopregnanolone and its structural and functional homolog, androstanediol. These two steroids increase extrasynaptic thalamic tonic GABAergic inhibition by selectively targeting neurosteroid-selective subunits of GABAA receptors (GABAARs). Neurosteroids also modulate the expression of GABAAR containing the γ2, α4, and δ subunits. It is hypothesized that differences in subunit expression during pregnancy and ovarian cycle contribute to the opposite effects of progesterone in these two hormonal states.

GABA-A receptors play a minor role in cortical epileptic afterdischarges in immature rats

The role of inhibitory γ-aminobutyric acid-A (GABA-A) system in the cortical epileptic afterdischarges (ADs) was studied at three different developmental stages of rats. Animals 12, 18 and 25days old with implanted epidural electrodes were pretreated with bicuculline (1 and 2mg/kgi.p.) and 15min later repeatedly stimulated with low frequency trains with stepwise increasing current intensity. Bicuculline only exceptionally decreased threshold current intensities necessary for elicitation of movements directly bound to stimulation, spike-and wave ADs, clonic seizures and transition into a limbic type of ADs. Duration of ADs was not systematically affected by either dose of bicuculline. In contrast, transcallosal evoked potentials exhibited under the influence of bicuculline steeper curve expressing relation between intensity of stimuli and amplitude of responses. In contrast to GABA-B receptors, GABA-A receptors do not play an important role in generation and arrest of cortical epileptic ADs in immature rats.

NITRIC OXIDE INVOLVEMENT IN SEIZURES ELICITED BY PENTYLENTETRAZOL AND SEX DEPENDENCE

It has been known that susceptibility to some types of epilepsy is affected by sex. In addition, the role of NO in epileptogenesis is still unclear; NO has been suggested to be either an anticonvulsive or a proconvulsive agent. In an attempt to elucidate both the role of NO and sex differences in sensitivity to seizures, male and female Wistar rats were treated intraperitoneally (i.p.) by pentylentetrazol (PTZ)(80 mg/kg) and by a nitric oxide synthase(NOS) inhibitor N-omega-nitro-L-arginine-mthylester(L-NAME)(50mg/kg) and a NO precursor sodium-nitroprusside(SNP)(2.5mg/kg)- applied 15 min. before PTZ injection. Latency, frequency, severity, and duration of generalized clonic and clonic-tonic convulsions were recorded. Furthermore, alterations in severity, latency, frequency, and duration of convulsions were observed to correlate with NO. Both sexes, injected with PTZ, showed repetitive seizure patterns. Seizures were found to be more severe in females. L-NAME and SNP pretreatment produced paradoxical effects on PTZ-induced seizures in both sexes. L-NAME completely prevented PTZ-induced seizures in male rats, whereas increased severity, frequency, duration, and significantly shortened the latency in female rats. Unexpectedly, SNP increased convulsion severity, frequency, duration, and shortened latencies in male, whereas it decreased convulsion severity, frequency, and duration and prolonged latency in females. These results indicate that endogenous NO is involved in the regulation of convulsive action suggesting a role depending on sex.

Sexually dimorphic expression of KCC2 and GABA function

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate depolarizing effects, which result in activation of calcium-sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors acquire their classical hyperpolarizing signaling. The switch from depolarizing to hyperpolarizing GABA(A)-ergic signaling is triggered through the developmental shift in the balance of chloride cotransporters that either increase (i.e. NKCC1) or decrease (i.e. KCC2) intracellular chloride. The maturation of GABA(A) signaling follows sex-specific patterns, which correlate with the developmental expression profiles of chloride cotransporters. This has first been demonstrated in the substantia nigra, where the switch occurs earlier in females than in males. As a result, there are sensitive periods during development when drugs or conditions that activate GABA(A) receptors mediate different transcriptional effects in males and females. Furthermore, neurons with depolarizing or hyperpolarizing GABA(A)-ergic signaling respond differently to neurotrophic factors like estrogens. Consequently, during sensitive developmental periods, GABA(A) receptors may act as broadcasters of sexually differentiating signals, promoting gender-appropriate brain development. This has particular implications in epilepsy, where both the pathophysiology and treatment of epileptic seizures involve GABA(A) receptor activation. It is important therefore to study separately the effects of these factors not only on the course of epilepsy but also design new treatments that may not necessarily disturb the gender-appropriate brain development.

Evaluation of gabapentin and ethosuximide for treatment of acute nonconvulsive seizures following ischemic brain injury in rats

Acute seizures following brain injury have been associated with a worsening of patient outcome, but they are often undiagnosed and untreated when they occur without motor convulsions. Here, we sought to compare the antiseizure profile of ethosuximide (EXM; 125-312.5 mg/kg i.v.) and gabapentin (GBP; 0.3-50 mg/kg. i.v.) in a rat model of nonconvulsive seizures (NCS) induced by brain ischemia. Seizures were detected by continuous electroencephalographic monitoring for 24 h following permanent middle cerebral artery occlusion (MCAo). Both "preseizure" and "postseizure" treatment effects were evaluated. Control rats experienced a 91% incidence of NCS (averaging 10-11 NCS/rat), which was significantly reduced following preseizure treatment (delivered 20 min post-MCAo) with either EXM (ED(50) = 161 mg/kg) or GBP (ED(50) = 10.5 mg/kg). In contrast to preseizure treatment effects, only GBP reduced NCS when given after the first seizure event. A further, albeit nonsignificant, 20% reduction in NCS incidence was measured when given in combination postseizure. Drug treatment also reduced infarct volume, which was positively correlated to the number of NCS events (r = 0.475; P < 0.001). EXM and GBP treatment of cultured neurons exposed to neurotoxic or ischemic insults showed no neuroprotective effects, suggesting that in vivo neuroprotection can be attributed to anti-seizure effects. We conclude that EXM and GBP significantly attenuate NCS in a dose-related manner and may help to improve patient outcome from brain ischemia-induced seizure activity.

GABAB receptor antagonism abolishes the learning impairments in rats with chronic atypical absence seizures

Chronic atypical absence seizures are a component of the Lennox-Gastaut syndrome, a disorder invariably associated with severe cognitive impairment in children. However, the cause of this intellectual delay remains unclear. The AY9944 model of chronic atypical absence seizures in rats reliably reproduces the electrographic, behavioral, pharmacological and cognitive features of clinical atypical absence. Using this model, we tested the hypothesis that the cognitive impairment associated with this disorder involves a gamma-aminobutyric acid B (GABA(B)) receptor-mediated mechanism. Therefore, we examined the effect of a specific, high affinity GABA(B) receptor antagonist, CGP35348, on the atypical absence seizures, the working memory deficits, and the altered long-term potentiation that we have observed in the AY9944 model. CGP35348 blocked atypical absence seizures, restored long-term potentiation to normal level, and reversed the cognitive deficit in the AY9944-treated animals. However, dose-response studies showed that lower doses of CGP35348 that failed to influence atypical absence seizure activity, completely reversed the spatial working memory deficit. These data suggest that GABA(B) receptor-mediated mechanisms are responsible for the cognitive dysfunction in the AY9944 model of chronic atypical absence seizures and further, that their cognitive impairment is independent of the seizure activity. The data raise the possibility that GABA(B) receptor antagonists may have therapeutic potential for the treatment of cognitive impairment in epilepsy syndromes where atypical absence seizures are a component.

Role of NMDA, nicotinic, and GABA receptors in the steady-state visual-evoked potential in rats

Agonists and antagonists at the NMDA, GABA, and nicotinic acetylcholine receptors were administered to adult male rats to evaluate the contribution of these pathways to the visual-evoked potential (VEP). Rats were presented with an onset/offset pattern at a temporal frequency (4.55 Hz) resulting in a steady-state VEP. Averaged VEPs were Fourier transformed and VEP amplitudes were calculated at 1x stimulus frequency (F1) and 2x stimulus frequency (F2). About 30 min after administration, NMDA (10 mg/kg, i.p.; n = 9) increased F1 amplitude by 350% and decreased F2 amplitude by 48%. Memantine (4.5 mg/kg, i.p.; n = 10) increased F1 amplitude by 50%, 10 min post-injection. Similarly, nicotine (0.1 mg/kg, s.c.; n = 9) increased F1 amplitude by 55%, 20 min after drug administration. Muscimol (1 mg/kg, i.p.; n = 10) increased F1 amplitude significantly from 20 to 45 min post-injection. Mecamylamine (6 mg/kg, i.p.; n = 10) decreased F2 amplitude by 70% during the 60-min testing session. Bicuculline (0-0.5 mg/kg, i.p.; n = 8-10 rats/dose) did not significantly alter either F1 or F2 amplitudes. Results indicate important roles for glutamate and nicotinic acetylcholine receptors in both F1 and F2, while GABA receptors contribute to F1.

Altered glutamate and GABA release within thalamocortical circuitry in metabotropic glutamate receptor 4 knockout mice

The metabotropic glutamate receptor 4 is highly expressed presynaptically on thalamocortical neurons that are involved in the pathogenesis of generalized absence seizures. Mutant mice devoid of metabotropic glutamate receptor 4 are completely resistant to absence seizures induced by low doses of GABA type A receptor antagonists. The purpose of this study was to test the hypothesis that there is altered glutamate and GABA release within thalamocortical circuitry in mice devoid of metabotropic glutamate receptor 4. Extracellular GABA and glutamate release were determined in ventrobasal thalamus, the nucleus reticularis thalami and laminae I-III, and IV-VI of cerebral cortex (laminae I-III of cerebral cortex, and laminae IV-VI of cerebral cortex) using in vivo microdialysis techniques on awake, free moving mice. A significant increase of both basal and K(+)-evoked glutamate release was detected in the ventrobasal thalamus, the nucleus reticularis thalami and laminae IV-VI of cerebral cortex of mice devoid of metabotropic glutamate receptor 4 mice. There also was a significant increase in both basal and K(+)-evoked GABA release in the mice devoid of metabotropic glutamate receptor 4, but a significant decrease of GABA release in laminae IV-VI of cerebral cortex. However, there was no alteration of either GABA or glutamate release in laminae I-III of cerebral cortex, cortical laminae that are not involved in absence seizures. These data indicate that deletion of the metabotropic glutamate receptor 4 gene results in a selective perturbation of glutamate and GABA release within the thalamocortical circuitry involved in the pathogenesis of absence seizures.

Seizure susceptibility in intact and ovariectomized female rats treated with the convulsant pilocarpine

Despite numerous neuroendocrinological studies of seizures, the influence of estrogen and progesterone on seizures and epilepsy remains unclear. This may be due to the fact that previous studies have not systematically compared distinct endocrine conditions and included all relevant controls. The goal of the present study was to conduct such a study using pilocarpine as chemoconvulsant. Thus, age and weight-matched, intact or ovariectomized rats were tested to determine incidence of status epilepticus and to study events leading to status. Intact female rats were sampled at each cycle stage (proestrus, estrus, metestrus, or diestrus 2). Convulsant was administered at the same time of day, 10:00-10:30 a.m. Statistical analysis showed that there was a significantly lower incidence of status on the morning of estrus, but differences were attenuated in older animals. Ovariectomized rats were distinct in their rapid progression to status. These results show that the incidence of status in female rats following pilocarpine injection, and the progression to pilocarpine-induced status, are influenced by reproductive state as well as age. The hormonal milieu present specifically on the morning of estrus appears to decrease susceptibility to pilocarpine-induced status, particularly at young ages. In contrast, the chronic absence of reproductive steroids that characterizes the ovariectomized rat leads to a more rapid progression to status. This dissociation between incidence vs. progression provides new insight into the influence of estrogen and progesterone on seizures.

GABAA receptors modulate cannabinoid-evoked hypothermia

Cannabinoids evoke hypothermia by stimulating central CB(1) receptors. GABA induces hypothermia via GABA(A) or GABA(B) receptor activation. CB(1) receptor activation increases GABA release in the hypothalamus, a central locus for thermoregulation, suggesting that cannabinoid and GABA systems may be functionally linked in body temperature regulation. We investigated whether GABA receptors modulate the hypothermic actions of [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one] (WIN 55212-2), a selective cannabinoid agonist, in male Sprague-Dawley rats. WIN 55212-2 (2.5 mg/kg im) produced a rapid hypothermia that peaked 45-90 min postinjection. The hypothermia was attenuated by bicuculline (2 mg/kg ip), a GABA(A) antagonist. However, SCH 50911 (1-10 mg/kg ip), a GABA(B) blocker, did not antagonize the hypothermia. Neither bicuculline (2 mg/kg) nor SCH 50911 (10 mg/kg) by itself altered body temperature. We also investigated a possible role for CB(1) receptors in GABA-generated hypothermia. Muscimol (2.5 mg/kg ip), a GABA(A) agonist, or baclofen (5 mg/kg ip), a GABA(B) agonist, evoked a significant hypothermia. Blockade of CB(1) receptors with SR141716A (2.5 mg/kg im) did not antagonize muscimol- or baclofen-induced hypothermia, indicating that GABA-evoked hypothermia does not contain a CB(1)-sensitive component. Our results implicate GABA(A) receptors in the hypothermic actions of cannabinoids and provide further evidence of a functional link between cannabinoid and GABA systems.

GABA receptors as broadcasters of sexually differentiating signals in the brain

Epileptic seizures are more common in males than in females. One of the areas that has recently been implicated in the higher susceptibility of males to seizures is the substantia nigra reticulata (SNR). Several studies support the existence of phenotypic differences between male and female infantile SNR neurons, and particularly in several aspects of the GABAergic system, including its ability to control seizures. We have recently found that at postnatal day 14-17 (PN14-17) rats, which are equivalent to infants, activation of GABA(A) receptors has different physiological effects in male and female SNR neurons. This is likely due to the differences in the expression of the neuronal-specific potassium-chloride co-transporter KCC2, which regulates the intracellular chloride concentration. In male PN14-17 SNR neurons, GABA(A)-receptor activation with muscimol causes depolarization and increments in intracellular calcium concentration and the expression of calcium regulated genes, such as KCC2. Blockade of L-type voltage-sensitive calcium channels (L-VSCC) by nifedipine decreases KCC2 mRNA expression. However, in PN14-17 females, muscimol hyperpolarizes the SNR neurons, does not increase intracellular calcium, and decreases KCC2 mRNA expression. In PN15 females, nifedipine has no effect on KCC2 mRNA expression in the SNR. This sexually dimorphic function of GABA(A) receptors also creates divergent patterns of estradiol signaling. In male PN15 rats, estradiol decreases KCC2 mRNA expression in SNR neurons. Pretreatment with the GABA(A)-receptor antagonist bicuculline or with nifedipine, prevents the appearance of estradiol-mediated downregulation of KCC2 mRNA expression. In contrast, in PN15 females, estradiol does not influence KCC2 expression. These findings show that, in infantile rats, drugs or conditions that modulate the activity of GABA(A) receptors or L-VSCCs have different effects on the differentiation of the SNR. As a result, they have the potency of causing long-term changes in the function of the SNR in the control of seizures, movement, and the susceptibility to and course of epilepsy and movement disorders.

The influence of gender on the aggravation of absence seizures by carbamazepine in the low-dose pentylenetetrazol rat model

OBJECTIVES

To determine whether carbamazepine (CBZ) aggravates absence seizures in the low-dose pentylenetetrazol (PTZ) rat model in both male and female animals, and investigate for gender differences.

METHODS

Inbred Sprague-Dawley rats were implanted with EEG electrodes. Seven days later PTZ (20 mg/kg, i.p.) was administered following pre-treatment with vehicle or CBZ (20 mg/kg, i.p.) and the occurrence of spike-and-wave discharges (SWDs) on the EEG quantified.

RESULTS

The cumulative SWD for 90-minute post-PTZ was higher in the CBZ versus vehicle pre-treatment arm for both female (mean 110 seconds vs. 62 seconds; P = 0.03) and male (mean 89 seconds vs. 60 seconds; P = 0.03) rats. The increase in SWD duration in the CBZ arm was greater in female rats for the first five 15-minute intervals, but none attained statistical significance (P > 0.05). CBZ pre-treatment resulted in reductions in both SWD frequency (Hz) (male, P = 0.003; female, P < 0.0001) and latency to onset of SWD (male, P = 0.002). The frequency of SWD in CBZ pre-treated rats was lower in females (5.8 Hz vs. 6.1 Hz, P = 0.002) as was the decrease in the SWD burst duration following CBZ versus vehicle pre-treatment (-0.05 seconds vs. -0.25 seconds, P = 0.046).

CONCLUSIONS

CBZ consistently aggravates absence seizures in the low-dose PTZ model in both female and male rats. However, while some gender differences were found, the results failed to support the hypothesis that females are significantly more susceptible to aggravation of the number or duration of absence seizures by CBZ.

Estrogen modulation of NMDA-induced seizures in ovariectomized and non-ovariectomized rats

The objective of this study was to examine the neuroprotective effects of estrogen in response to N-methyl-D-aspartate (NMDA)-induced seizures in both male and female rats. Thirty-eight Long-Evans rats were divided into five groups: ovariectomized females, non-ovariectomized females, ovariectomized females with estrogen replacement (10 microg 17beta-estradiol in 100 microl sesame oil), males given exogenous estrogen and males receiving no estrogen. Using stereotaxic surgery, a cannula was placed in the lateral ventricle for convulsant agent administration (20 microg of NMDA), while an electrode was placed into the hippocampus for seizure recording. Seizure activity was monitored for 20 min. Onset to first seizure, first seizure duration, seizure frequency and total duration of seizures were determined. Rats were pretreated with either sesame oil (vehicle) or estrogen given subcutaneously for 4 days prior to seizure induction on the fourth day. Rats were euthanized 72 h later and the brains removed for histological processing. Electrode and cannula placement were verified microscopically and neuronal integrity was assessed via hematoxylin and eosin staining. Total seizure number was significantly higher in the ovariectomized females compared to the non-ovariectomized females and the ovariectomized females receiving estrogen (P<0.05). Moreover, hippocampal neuronal damage following seizure induction was significant in the ovariectomized rats compared to the non-ovariectomized rats (P<0.05). Pretreatment with estrogen did not affect any of the seizure parameters measured in the male rats. We conclude that estrogen appears to be neuroprotective against NMDA-induced seizures in female ovariectomized rats.

The GABA(A) Receptor: Subunit-Dependent Functions and Absence Seizures

GABA(A) receptors on thalamic relay and reticular (nRT) neurons play a critical role in thalamocortical mechanisms underlying absence seizures. Studies with absence seizure-prone rats and transgenic mice have taken advantage of differences in the subunit compositions of GABA(A) receptors in the two thalamic cell populations to clarify thalamocortical rhythm generating mechanisms and explain the antiabsence activity of benzodiazepines. The relevance of this work is highlighted by the recent finding of a mutation in the GABA(A) receptor gamma2 subunit in a family with childhood absence seizures.

Sex difference in susceptibility to epileptic seizures in rats: importance of estrous cycle

Sex difference in seizure susceptibility is one of the unresolved issues of epilepsy. It is known that estrogen is excitatory and progesterone is inhibitory to the central nervous system. Therefore, it is to be expected that seizure susceptibility may be associated with the estrous cycle, which should be tested in epilepsy research. Otherwise, different results in epilepsy studies could be an artifact of the estrous cycle. Reports in the literature are inconsistent about testosterone effects on seizures. In light of these considerations, sex differences in seizure susceptibility were restudied in rats. There was no significant sex difference in mean latencies to picrotoxin-induced seizures; prestrous-females had the shortest latencies to epileptic seizures compared to males and estrousfemales. With testosterone-injected rats, there was either no sex difference in latencies (to akinetic and focal seizures) or females had significantly shorter latencies than males (to status epilepticus, generalized tonic-clonic seizures, and myoclonic seizures). Testosterone-treated male rats had a significantly longer mean latency than controls for status epilepticus only; otherwise, these males showed no significant differences between mean latencies before and after testosterone (to focal, myoclonic, or generalized tonic-clonic seizures). In females, mean latencies to myoclonic seizures and status epilepticus were significantly shorter after testosterone than before. It was concluded that there is a sex difference in susceptibility to epileptic seizures in rats, provided that the estrous cycle is taken into account. Testosterone may increase and decrease seizure susceptibility in females and males, respectively. These effects may be important for understanding the mechanisms of epileptic phenomena and may provide some important clues to epilepsy treatment.

Modulation of absence seizures by the GABA(A) receptor: a critical rolefor metabotropic glutamate receptor 4 (mGluR4)

Experimental absence seizures are associated with perturbations in the presynaptic release of GABA and glutamate within thalamocortical circuitry. The release of both glutamate and GABA is regulated by group III metabotropic glutamate receptors (mGluRs). Therefore, we examined the susceptibility of mice lacking the mGluR4 subtype of mGluR (mGluR4(-/-)) versus their wild-type controls (mGluR4(+/+)) to absence seizures induced either by gamma-hydroxybutyrate (GHB) or the GABA(B) agonist (-) baclofen or by low doses of the GABA(A) receptor (GABA(A)R) antagonists pentylenetetrazole, bicuculline, or picrotoxin. There was no difference between mGluR4(-/-) and mGluR4(+/+) mice in threshold to absence seizures induced by either GHB or (-) baclofen. In contrast, the mGluR4(-/-) mice were markedly resistant to absence seizures induced by low doses of GABA(A)R antagonists. No differences were observed between mGluR4(-/-) and mGluR4(+/+) mice in threshold to clonic or tonic seizures induced by higher doses of GABA(A)R antagonists, strychnine, or electroshock, indicating that seizure resistance in the mGluR4(-/-) mice was restricted solely to absence seizures. The resistance of mGluR4(-/-) mice to absence seizures induced by GABA(A)R antagonists was mimicked by bilateral administration of a mGluR4 antagonist into the nucleus reticularis thalami (nRT) of mGluR4(+/+) mice. Conversely, intra-nRT administration of a mGluR4 agonist in mGluR4(+/+) mice exacerbated GABA(A)R-induced absence seizures. These data indicate that the presence of mGluR4 within nRT is critical to GABAergic modulation of thalamocortical synchronization in normal and pathological states, such as generalized absence epilepsy.

Region-specific modulation of limbic seizure susceptibility by ovarian steroids

Gonadal steroid hormones can markedly affect seizure susceptibility. Ovariohysterectomized female rats given ovarian steroid hormone supplements were used to evaluate the effects of ovarian steroids on epileptiform activity in hippocampal slices in vitro and on flurothyl-induced seizures in vivo. Seizure susceptibility was compared in the entorhinal cortex (EC) and CA1 regions of the hippocampus perfused with Mg(2+)-free medium, which leads to epileptiform discharges caused by a relief of voltage-dependent NMDA receptor block. After in vivo treatment with 500 microg of progesterone for 2 h prior to slice preparation, the latency to onset of low Mg(2+)-induced epileptiform activity of slices was significantly prolonged compared to slices from controls. In contrast, progesterone replacement accelerated the development of epileptiform activity in the CA1 region. Neither estrogen alone (2 x 2 microg of estradiol benzoate, 48 and 24 h prior to the experiment), nor a combined treatment with estrogen plus progesterone, significantly affected seizure susceptibility in either CA1 or the EC. There were no consistent effects of estrogen or progesterone, alone or in combination, on flurothyl-induced seizures in vivo. The data suggest that in vitro, progesterone alters seizure susceptibility in a site- and seizure model-specific fashion. The differential effects of progesterone may be due to differential expression of progesterone receptor isoforms or metabolites in specific brain areas suggesting that selective modulation of NMDA receptor-dependent epileptiform activity may play a role in hormonal effects on epileptogenesis.

Epileptiform activity generated by endogenous acetylcholine during blockade of GABAergic inhibition in immature and adult rat hippocampus

We tested the effects of the acetylcholinesterase inhibitor eserine (10 microM), an indicator of the activity of endogenous ACh, on the generation of epileptiform discharges during blockade of inhibitory GABA(A)-mediated potentials by bicuculline (10 microM), in the CA3 area of hippocampal slices from postnatal days 4-20 (P4-P20) immature and adult rats. Eserine provoked or significantly increased the frequency of spontaneous synchronous epileptiform discharges, in 6/22 (27%) P4-P10 slices, 34/35 P11-P20 slices and 18/18 adult slices, an epileptogenic effect. In immature slices, spontaneous discharges showed a stable frequency throughout perfusion with eserine, while in 5/11 adult slices an initial fast frequency was followed by a slower steady-state one. The cholinergic agonist carbachol (CCh, 25 microM) provoked only transient or no spontaneous synchronous discharges in adult slices (n=8), thus suggesting that massive activation of cholinergic receptors may lead to suppression of epileptiform activity in adult brain. Stimulus-induced excitatory CA3 responses, were depressed by eserine in approximately half of 20 P4-P10, 45 P11-P20 and 11 adult slices. The depression consisted of a decrease in the amplitude, duration, and number of population spikes of the field potentials by about 30%, a minor neuroprotective effect, which did not change with maturation. The different developmental profiles of the epileptogenic and neuroprotective effects of endogenous ACh suggest that they are mediated by different mechanisms. These experiments demonstrate that, endogenous ACh is sufficient to induce epileptogenesis during a decrease or failure of GABAergic inhibition, in both >/=P10 immature and in adult hippocampus. We therefore suggest that clinical or behavioral conditions which raise the concentration of endogenous ACh may lower the threshold to seizures.