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Benzodiazepines (BZDs), which enhance γ-aminobutyric acid (GABAA ) receptor-mediated inhibition, are the first-line therapy for treatment of status epilepticus (SE). However, pharmacoresistance to BZDs develops rapidly after SE initiation. This is due to an activity-dependent internalization of BZD-sensitive GABAA receptors during SE. Stiripentol (STP) is a positive allosteric modulator of GABAA receptors with a unique subunit selectivity profile. We report that in a rodent model of SE, STP terminates behavioral seizures and remains effective in established SE when seizures have become BZD resistant. The anticonvulsant effects of STP are age dependent, with greater potency in juvenile animals. Whole cell recordings from dentate granule cells in hippocampal slices reveal that STP potentiates GABAergic inhibitory postsynaptic currents (IPSCs) and tonic GABAergic currents by acting at a site on the GABAA receptor that is separate from the benzodiazepine binding site. This potentiation persists in established SE, whereas potentiation of GABAergic inhibition by BZDs is lost. STP potentiates IPSCs in juvenile animals with greater potency than in adult animals. We suggest that STP, either alone or as add-on therapy, may prove useful in treating established and BZD-resistant status epilepticus. Furthermore, STP may be particularly effective in terminating SE in children when SE is most prevalent., (Wiley Periodicals, Inc. © 2013 International League Against Epilepsy.)

Infantile spasms are the classical seizure type of West syndrome. Infantile spasms often herald a dismal prognosis, due to the high probability to evolve into intractable forms of epilepsies with significant cognitive deficits, especially if not adequately treated. The current therapies-high doses of adrenocorticotropic hormone, steroids, or the gamma-aminobutyric acid (GABA) transaminase inhibitor vigabatrin--are often toxic and may not always be effective. The need to identify new therapies for spasms has led to the generation of a number of rodent models of infantile spasms. These include acute and chronic models of infantile spasms, with cryptogenic or symptomatic origin, many of which are based on specific etiologies. In this review, we summarize the clinical experience with treating infantile spasms and the main features of the new animal models of infantile spasms and discuss their utility in the preclinical development of new therapies for infantile spasms.

Clinically available anticonvulsant drugs fail to control seizures in approximately 30% of epileptic patients. If hippocampal sclerosis is combined with focal dysplasia or similar developmental alterations, the likelihood of incomplete seizure control may reach >90%. Because only a minority of epilepsy patients benefit from epilepsy surgery, we need more research into the mechanisms of drug refractoriness. In this review we analyze different approaches to study pharmacoresistance and underlying mechanisms using in vitro models. Epileptiform discharges after prolonged application of low Mg(2+) artificial cerebrospinal fluid (ACSF) or 4-aminopyridine (4-AP), or combined application of these convulsants with bicuculline in acute hippocampal-entorhinal cortex slices reveal pharmacoresistance and point to loss of gamma-aminobutyric acid (GABA)ergic function, in part due to reduced delivery of GABA from presynaptic terminals. Interestingly, epileptiform activity in immature tissue (organotypic hippocampal slice cultures and acute intact hippocampus) is immediately resistant to available antiepileptic drugs, and preliminary evidence points to a role of alterations in Cl(-) homeostasis. Seizure-like events can also be induced in dissected tissues from human epileptic patients. Future research on human tissue may provide useful information for understanding the mechanisms underlying pharmacoresistance.

Antiepileptic drugs (AEDs) such as phenobarbital, phenytoin, and valproic acid, when given in therapeutic doses to neonatal rats, cause pronounced neuronal apoptotic cell death. This effect is especially pronounced in the striatum and cortex during the second postnatal week, a period corresponding to the "brain growth spurt" (third trimester of gestation and early infancy) in humans. Of particular concern is the fact that phenobarbital is the most frequently used therapy for neonatal epilepsy. If AED-induced neuronal cell death leads to long-term functional impairment, then it becomes crucial to find therapies that avoid this neurotoxicity in the sensitive period. Herein we examine short- and long-term functional effects following exposure of neonatal rat pups to phenobarbital; the functions tested include striatal gamma-aminobutyric acid (GABA)ergic synaptic responses and reflex development in pups, and fear conditioning, emotionality, and sensory-motor gating in adults. In all cases, phenobarbital exposure during the second postnatal week was sufficient to cause significant impairment. In contrast, adult animals exposed as pups to lamotrigine (given in a dose that does not cause apoptotic neuronal death) were not impaired on the tasks we examined. Our data suggest that treatments devoid of proapoptotic actions may be promising therapies for avoiding adverse outcomes after neonatal exposure. In addition, our findings identify early exposure to certain AEDs as an important potential risk factor contributing to psychiatric and neurologic abnormalities later in life.

Purpose: Past work has demonstrated that kainic acid (KA)-induced seizures could cause the enhancement of excitation and lead to neuronal death in rat hippocampus. To counteract such an imbalance between excitation and inhibition, we designed experiments by activating the inhibitory gamma-aminobutyric acid (GABA) receptor to investigate whether such activation suppresses the excitatory glutamate signaling induced by KA and to elucidate the underlying molecular mechanisms., Methods: Muscimol coapplied with baclofen was intraperitoneally administrated to the rats 40 min before KA injection by intracerebroventricular infusion. Subsequently we used a series of methods including immunoprecipitation, immunoblotting, histologic analysis, and immunohistochemistry to analyze the interaction, expression, and phosphorylation of relevant proteins as well as the survival of the CA1/CA3 pyramidal neurons., Results: Coadministration of muscimol and baclofen exerted neuroprotection against neuron death induced by KA; inhibited the increased assembly of the GluR6-PSD-95-MLK3 module induced by KA; and suppressed the activation of MLK3, MKK7, and JNK3., Discussion: Taken together, we demonstrate that coactivation of the inhibitory GABA receptors can attenuate the excitatory JNK3 apoptotic signaling pathway via inhibiting the increased assembly of the GluR6-PSD-95-MLK3 signaling module induced by KA. This provides a new insight into the therapeutic approach to epileptic seizure.

The fortuitous discovery of the benzodiazepines and the subsequent application of these agents to the treatment of status epilepticus (SE) heralds in the modern age of treating this neurologic emergency. More than 50 years after their discovery, the benzodiazepines remain the drugs of first choice in the treatment of SE. However, the benzodiazepines can be ineffective, especially in those patients whose seizures are the most prolonged. The benzodiazepines act by increasing the affinity of gamma-aminobutyric acid (GABA) for GABAA receptors. A receptor's subunit composition affects its functional and pharmacologic properties, trafficking, and cellular localization. The GABAA receptors that mediate synaptic inhibition typically contain a gamma2 subunit and are diazepam-sensitive. Among the GABAA receptors that mediate tonic inhibition are the benzodiazepine-insensitive delta subunit-containing receptors. The initial studies investigating the pathogenesis of SE demonstrated that a reduction in GABA-mediated inhibition within the hippocampus was important in maintenance of SE, and this reduction correlated with a rapid modification in the postsynaptic GABAA receptor population expressed on the surface of the hippocampal principal neurons. Subsequent studies found that this rapid modification is, in part, mediated by an activity-dependent, subunit-specific trafficking of the receptors that resulted in the reduction in the surface expression of the benzodiazepine-sensitive gamma2 subunit-containing receptors and the preserved surface expression of the benzodiazepine-insensitive delta subunit-containing receptors. This improved understanding of the changes in the trafficking of GABAA receptors during SE partially accounts for the development of benzodiazepine-pharmacoresistance and has implications for the current and future treatment of benzodiazepine-refractory SE.

Purpose: To explore the cellular mechanisms of cobalt-induced epileptiform discharges in mouse hippocampal slices., Methods: Hippocampal slices were prepared from adult mice and briefly exposed to a CoCl(2)-containing external solution. Population and single cell activities were examined via extracellular and whole-cell patch recordings., Results: Brief cobalt exposure induced spontaneous, ictal-like discharges originating from the CA3 area. These discharges were suppressed by anticonvulsants, gap junction blockers, or by raising extracellular Ca(2+), but their generation was not associated with overall hyperexcitability or impairment in GABAergic inhibition in the CA3 circuit. Electroencephalographic ictal discharges of similar waveforms were observed in behaving rats following intrahippocampal cobalt infusion., Discussion: Mechanisms involving activity-dependent facilitation of gap junctional communication may play a major role in cobalt-induced epileptiform discharges.

Development of tolerance (i.e., the reduction in response to a drug after repeated administration) is an adaptive response of the body to prolonged exposure to the drug, and tolerance to antiepileptic drugs (AEDs) is no exception. Tolerance develops to some drug effects much more rapidly than to others. The extent of tolerance depends on the drug and individual (genetic?) factors. Tolerance may lead to attenuation of side effects but also to loss of efficacy of AEDs and is reversible after discontinuation of drug treatment. Different experimental approaches are used to study tolerance in laboratory animals. Development of tolerance depends on the experimental model, drug, drug dosage, and duration of treatment, so that a battery of experimental protocols is needed to evaluate fully whether tolerance to effect occurs. Two major types of tolerance are known. Pharmacokinetic (metabolic) tolerance, due to induction of AED-metabolizing enzymes has been shown for most first-generation AEDs, and is easy to overcome by increasing dosage. Pharmacodynamic (functional) tolerance is due to "adaptation" of AED targets (e.g., by loss of receptor sensitivity) and has been shown experimentally for all AEDs that lose activity during prolonged treatment. Functional tolerance may lead to complete loss of AED activity and cross-tolerance to other AEDs. Convincing experimental evidence indicates that almost all first-, second-, and third-generation AEDs lose their antiepileptic activity during prolonged treatment, although to a different extent. Because of diverse confounding factors, detecting tolerance in patients with epilepsy is more difficult but can be done with careful assessment of decline during long-term individual patient response. After excluding confounding factors, tolerance to antiepileptic effect for most modern and old AEDs can be shown in small subgroups of responders by assessing individual or group response. Development of tolerance to the antiepileptic activity of an AED may be an important reason for failure of drug treatment. Knowledge of tolerance to AED effects as a mechanism of drug resistance in previous responders is important for patients, physicians, and scientists.

Purpose: The mechanisms that mediate the acute anticonvulsant effects of deoxycorticosterone (DOC) were investigated in young rats., Methods: Fifteen-day-old rats were pretreated with a variety of compounds, including (a) agonists of the receptors that bind DOC (mineralocorticoid receptors); (b) the DOC 5alpha- and 5alpha-3alpha-reduced metabolites, plus agonists that bind the receptors of the 5alpha-reduced metabolite of DOC (progesterone receptors); and (c) DOC itself in the presence and absence of metabolism and receptor blockers. Fifteen minutes later, pentylenetetrazol (PTZ) was administered, and maximal pentylenetetrazol (MMT) seizure responses were scored., Results: Agonists of mineralocorticoid receptors increased the latency to forelimb flexion in PTZ seizures and sometimes suppressed the seizures completely. At low, nonconvulsant doses, spironolactone (a mineralocorticoid-receptor antagonist) blocked the anticonvulsant effects of a nonsedating, but not a sedating, dose of DOC. These data suggest the possible direct involvement of mineralocorticoid receptors in the anticonvulsant effects of DOC. At low, nonconvulsant doses, finasteride (which blocks the metabolism of DOC) partially blocked the protective effects of DOC, suggesting the contribution of metabolites to the anticonvulsant actions of DOC. Dihydrodeoxycorticosterone (DHDOC)-the first metabolite of DOC, an agonist at progesterone receptors, and an allosteric modulator of the gamma-aminobutyric acid (GABA)(A) receptor-and tetrahydrodeoxycorticosterone, a secondary metabolite of DOC and an allosteric modulator of the GABA(A) receptor, both blocked MMT seizures., Conclusions: These findings suggest that both DOC and its metabolites may contribute to the anticonvulsant effects seen in young rats, perhaps acting via interactions with several different receptors.

Drug-resistant epilepsy with uncontrolled severe seizures despite state-of-the-art medical treatment continues to be a major clinical problem for up to one in three patients with epilepsy. Although drug resistance may emerge or remit in the course of epilepsy or its treatment, in most patients, drug resistance seems to be continuous and to occur de novo. Unfortunately, current antiepileptic drugs (AEDs) do not seem to prevent or to reverse drug resistance in most patients, but add-on therapy with novel AEDs is able to exert a modest seizure reduction in as many as 50% of patients in short-term clinical trials, and a few become seizure free during the trial. It is not known why and how epilepsy becomes drug resistant, while other patients with seemingly identical seizure types can achieve seizure control with medication. Several putative mechanisms underlying drug resistance in epilepsy have been identified in recent years. Based on experimental and clinical studies, two major neurobiologic theories have been put forward: (a) removal of AEDs from the epileptogenic tissue through excessive expression of multidrug transporters, and (b) reduced drug-target sensitivity in epileptogenic brain tissue. On the clinical side, genetic and clinical features and structural brain lesions have been associated with drug resistance in epilepsy. In this article, we review the laboratory and clinical evidence to date supporting the drug-transport and the drug-target hypotheses and provide directions for future research, to define more clearly the role of these hypotheses in the clinical spectrum of drug-resistant epilepsy.

Purpose: Low-frequency (6 Hz), long-duration (3 s) electrical stimulation in mice produces seizures characterized by immobility, focal clonus, and automatic behaviors reminiscent of human limbic epilepsy. Renewed interest has been expressed in this seizure model with the recognition that it is sensitive to a broad spectrum of anticonvulsants (AEDs) and may have distinct pharmacologic responsiveness from other in vivo tests of AED efficacy. Here we sought to determine whether the progesterone-derived neuroactive steroid allopregnanolone (5alpha,3alpha-P) and several structural analogues with varying degrees of activity as positive allosteric modulators of gamma-aminobutyric acid (GABA)A receptors are protective in the 6-Hz seizure model., Methods: Mice were pretreated with neuroactive steroids (15 min before) or clonazepam (CZP; 30 min before) to 6-Hz corneal stimulation (32 mA). Animals that failed to exhibit immobility were considered protected., Results: The neuroactive steroids prevented 6-Hz seizures with rank order of potencies (ED50 values): ganaxolone (6.3 mg/kg) > 5alpha,3alpha-P (14.2 mg/kg) > or = 5beta,3alpha-P (14.4 mg/kg) > 5alpha,3beta-P (>100 mg/kg). CZP also was protective (ED(50) value, 0.075 mg/kg). The potencies of the neuroactive steroids and CZP are similar to their previously reported activities in the pentylenetetrazol (PTZ) seizure model., Conclusions: Neuroactive steroids have comparable potencies in the 6-Hz and PTZ models. Their structural specificity in both models corresponds with their activities as positive allosteric modulators of GABAA receptors, although ganaxolone is more potent than expected, probably because it has greater bioavailability. The 6-Hz model may be a valuable tool in drug development for the identification of GABAergic AEDs., (Copyright 2004 International League Against Epilepsy)

Purpose: Cortical dysplasia (CD) is associated with epilepsy in both the pediatric and adult populations. The mechanism underlying seizures with cortical malformations is still poorly understood. To study the physiology of dysplastic cortex, we developed an experimental model of CD., Methods: Pregnant rats were given intraperitoneal injections of carmustine (1-3-bis-chloroethyl-nitrosourea; BCNU) on embryonic day 15 (E15). Cortical histology was examined in the resulting pups at P0, P28, and P60. In addition, evoked and spontaneous field potential recordings were obtained in cortical slices from adult control and BCNU-exposed rats. Finally, we used whole-cell recordings to compare physiologic properties of pyramidal neurons and gamma-aminobutyric acid (GABA) responses in control and BCNU-treated animals., Results: Features characteristic of CD were found in the offspring, including laminar disorganization, cytomegalic neurons, and neuronal heterotopias. Dysplastic cortex also contained abnormal clusters of Cajal-Retzius (CR) cells and disruption of radial glial fibers, as demonstrated with immunohistochemistry. Under conditions of partial GABAA-receptor blockade with 10 microM bicuculline methiodide (BMI), slices of dysplastic cortex demonstrated a significant increase in the number of spontaneous and evoked epileptiform discharges. Individual pyramidal neurons in dysplastic cortex were less sensitive to application of GABA compared with controls., Conclusions: BCNU exposure in utero produces histologic alterations suggestive of CD in rat offspring. Dysplastic cortex from this model demonstrates features of hyperexcitability and decreased neuronal sensitivity to GABA. Such physiologic alterations may underlie the increased epileptogenicity of dysplastic cortex.

Purpose: Perimenstrual catamenial epilepsy, the increase in seizure frequency that some women with epilepsy experience near the time of menstruation, may in part be related to withdrawal of the progesterone metabolite allopregnanolone, an endogenous anticonvulsant neurosteroid that is a potent positive allosteric gamma-aminobutyric acidA (GABA(A)) receptor modulator. The objective of this study was to develop an animal model of perimenstrual catamenial epilepsy for use in evaluating drug-treatment strategies., Methods: A state of prolonged high serum progesterone (pseudopregnancy) was induced in 26-day-old female rats by sequential injection of pregnant mares' serum gonadotropin and human chorionic gonadotropin. Neurosteroid withdrawal was induced by treatment with finasteride (100 mg/kg, i.p.), a 5alpha-reductase inhibitor that blocks the conversion of progesterone to allopregnanolone. Plasma progesterone and allopregnanolone levels were measured by gas chromatography/electron capture negative chemical ionization mass spectrometry. Seizure susceptibility was evaluated with the convulsant pentylenetetrazol (PTZ)., Results: Plasma allopregnanolone levels were markedly increased during pseudopregnancy (peak level, 55.1 vs. control diestrous level, 9.3 ng/mL) and were reduced by 86% 24 h after finasteride treatment (6.4 ng/mL). Progesterone levels were unaffected by finasteride. After finasteride-induced withdrawal, rats showed increased susceptibility to PTZ seizures. There was a significant increase in the number of animals exhibiting clonic seizures when challenged with subcutaneous PTZ (60 mg/kg) compared with control pseudopregnant animals not undergoing withdrawal and nonpseudopregnant diestrous females. The CD50 (50% convulsant dose) was 46 mg/kg, compared with 73 mg/kg in nonwithdrawn pseudopregnant animals and 60 mg/kg in diestrous controls. The threshold doses for induction of various seizure signs, measured by constant intravenous infusion of PTZ, were reduced by 30-35% in neurosteroid-withdrawing animals compared with control diestrous females. No change in threshold was observed in pseudopregnant rats treated from days 7 to 11 with finasteride, demonstrating that high levels of progesterone alone do not alter seizure reactivity., Conclusions: Neurosteroid withdrawal in pseudopregnant rats results in enhanced seizure susceptibility, providing an animal model of perimenstrual catamenial epilepsy that can be used for the evaluation of new therapeutic approaches.

Purpose: Perimenstrual catamenial epilepsy may in part be due to withdrawal of the endogenous progesterone-derived neurosteroid allopregnanolone that potentiates gamma-aminobutyric acidA (GABA(A)) receptor-mediated inhibition. Here we sought to determine whether the anticonvulsant potencies of neuroactive steroids, benzodiazepines, phenobarbital (PB), and valproate (VPA) are altered during the heightened seizure susceptibility accompanying neurosteroid withdrawal in a rat model of perimenstrual catamenial epilepsy., Methods: Test drugs were evaluated for their ability to alter the convulsant activity of pentylenetetrazol (PTZ) in young adult female rats, in pseudopregnant rats with prolonged exposure to high levels of progesterone (and its neurosteroid metabolites), and in pseudopregnant rats 24 h after acute withdrawal of neurosteroids by treatment with the 5alpha-reductase inhibitor finasteride. Test drugs were administered at doses equivalent to twice their ED50 values for protection against PTZ-induced clonic seizures in naive young adult female rats., Results: The anticonvulsant activity of allopregnanolone (5 mg/kg, s.c.), pregnanolone (5 mg/kg, s.c.), allotetrahydrodeoxycorticosterone (15 mg/kg, s.c.), and tetrahydrodeoxycorticosterone (10 mg/kg, s.c.) were enhanced by 34-127% after neurosteroid withdrawal. The anticonvulsant activity of PB (65 mg/kg, i.p.) was also enhanced by 24% in neurosteroid-withdrawn animals. In contrast, the anticonvulsant activity of diazepam (4 mg/kg, i.p.), bretazenil (0.106 mg/kg, i.p.), and VPA (560 mg/kg, i.p.) were reduced or unchanged in neurosteroid-withdrawn animals., Conclusions: The anticonvulsant activity of neuroactive steroids is potentiated after neurosteroid withdrawal, supporting the use of such agents in the treatment of perimenstrual catamenial epilepsy.

Purpose: Identification of changes in neurotransmitter function in animal models of epilepsy provides a basis for rational drug development and an understanding of the mechanisms underlying epileptogenesis. We investigated changes in the efficacy of the benzodiazepine type I agonist zolpidem and the polyamine site N-methyl-D-aspartate receptor antagonist ifenprodil in a rat model of microgyria., Methods: Neonatal freeze lesions were used to produce a microsulcus in the normally lissencephalic rat neocortex with anatomical similarities to human polymicrogyria. Whole-cell voltage-clamp recordings were made from visually identified layer 2/3 pyramidal cells in acutely prepared brain slices from nonlesioned and lesioned rats., Results: The effect of 20 nmol/L zolpidem on the decay time constant of inhibitory postsynaptic currents was significantly less in neurons from brain slices containing the freeze lesion. A higher concentration (100 nmol/L) of zolpidem was equally efficacious in lesioned and nonlesioned cortex. In lesioned cortex, the threshold for evoking epileptiform discharges was significantly increased in the presence of 10 micromol/L ifenprodil. This effect was significant in both intrinsic hyperexcitability and partial disinhibition with 2 micromol/L bicuculline in lesioned cortex. Ifenprodil had significantly less effect on the threshold of discharges evoked in control cortex in the partial disinhibition model., Conclusions: The decreased sensitivity of gamma-aminobutyric acid A receptors to 20 nmol/L zolpidem in the freeze-lesion model is consistent with a delayed or arrested maturation in this animal model. These data support a delay in the developmental switch from alpha2 to alpha1 subunits in gamma-aminobutyric acid A receptors of neocortical pyramidal cells in lesioned cortex. The increased ifenprodil sensitivity of the threshold for evoking epileptiform discharges in both control and disinhibited slices containing the microsulcus is explained by a delay in the expression of the 2A (NR2A) N-methyl-D-aspartate receptor subunit. Delayed development may be a hallmark of this type of cortical dysplasia.

Purpose: These studies further investigate the ability of topiramate (TPM) to enhance gamma-aminobutyric acid (GABA)-mediated inhibition through a benzodiazepine-insensitive pathway., Methods: Topiramate (30 and 100 microM) enhancement of GABA (1 microM)-evoked currents in primary cultures of mouse cortical neurons was studied by using whole-cell electrophysiologic techniques. Results obtained with TPM (30 microM) were compared with those obtained with clonazepam (CZP; 1 microM)., Results: Topiramate enhanced GABA currents in a subset of cortical neurons tested. In addition, TPM enhanced GABA-evoked currents in CZP-insensitive neurons, and CZP was effective in a subset of TPM-insensitive neurons. Related studies in vivo demonstrated that intraperitoneal (i.p.) administration of either TPM (25 mg/kg) or CZP (0.012 mg/kg) increases pentylenetetrazol (PTZ) seizure threshold. This effect of CZP, but not TPM, was reversed by the benzodiazepine (BZD) antagonist flumazenil (FMZ; 40 mg/kg, i.p.)., Conclusions: These results indicate that GABA(A)-receptor sensitivity to TPM is not dependent on the presence of BZD sensitivity. Enhancement of GABA-mediated inhibition through a BZD-insensitive pathway may represent one mechanism through which TPM exerts its anticonvulsant action.

In this overview, we discuss the discovery and development of topiramate (TPM) as an anticonvulsant, including notable aspects of its chemical, biologic, and pharmacokinetic properties. In particular, we highlight its anticonvulsant profile in traditional seizure tests and animal models of epilepsy and the results of recent electrophysiological and biochemical studies using cultured neurons that have revealed a unique combination of pharmacologic properties of TPM. Finally, we present a hypothesis for the mechanistic basis of the anticonvulsant activity of TPM, which proposes that TPM binds to certain membrane ion channel proteins at phosphorylation sites and thereby allosterically modulates channel conductance and secondarily inhibits protein phosphorylation.

Purpose: To evaluate effects of vigabatrin (VGB) by using [123I]iomazenil single-photon emission computed tomography (SPECT) to estimate central gamma-aminobutyric acid (GABA(A))/benzodiazepine receptors (BZRs), and magnetic resonance spectroscopy (MRS) to assess tissue GABA levels., Methods: Six patients with partial seizures had both SPECT and MRS before and 25-84 days after starting VGB (3 g p.o., q.d.). SPECT was acquired by using the constant-infusion method and, after nonuniform attenuation correction, coregistered with T1-weighted MR Imaging (MRI) A volume of interest (VOI) of 3 x 2 x 2 cc over the occipital cortex, used for MRS acquisition, was positioned on both MRI and coregistered SPECT. Occipital activity was divided by either total plasma activity or plasma [123I]iomazenil concentration to estimate BZR distribution volume (V(T)-p and V'(T), respectively). Wilcoxon's test was used for VOI differences in GABA levels, BZR V(T)-p or V'(T). SPM96 (either no global normalization or proportional scaling) was used to compare BZR V(T)-p changes in the patients with and without VGB with test-retest data in eight healthy age-matched controls., Results: Occipital GABA levels were increased threefold (without VGB, 1.1+/-0.1 micromol/g; with VGB, 2.9+/-0.5 micromol/g; p = 0.027). BZR distribution volumes showed no change, when estimated by either V(T)-p (without VGB, 6.00+/-0.91 ml/g; with VGB, 5.86+/-0.44 ml/g; p = 0.92) or V(T) (without VGB, 41.1+/-11.2 ml/g; with VGB, 41.2+/-9.9 ml/g; p = 0.75). No significant changes were detected by SPM96., Conclusions: A clinically effective dose of VGB caused a threefold increase in tissue GABA levels but was not associated with a substantial BZR downregulation.

Vigabatrin (VGB) is a structural analogue of the inhibitory neurotransmitter gamma-amino butyric acid (GABA), which produces its antiepileptic effect by irreversibly inhibiting the degradative enzyme GABA-transaminase. This produces an increase in central nervous system (CNS) GABA levels. VGB is among the few antiepileptic drugs (AEDs) that was synthesized with a specific targeted mechanism in mind and was subsequently demonstrated to function by that mechanism. Tiagabine, a GABA reuptake blocker, is the only other "designer drug" among the currently available AEDs. Therefore, VGB is among the few AEDs for which the mechanism of action is well understood. Recently, safety issues have been raised with regard to the use of vigabatrin. This article reviews the mechanism of action, pharmacokinetics, safety, and efficacy of VGB.

Since 1993, several new antiepileptic drugs (AEDs) have been introduced for management of partial seizures. Like the established AEDs, the new drugs are believed to exert their anticonvulsant action through enhancement of inhibitory-mediated neurotransmission, or reduction of excitatory-mediated neurotransmission, or by a combination of both. Among the new drugs, vigabatrin (VGB) and tiagabine (TGB) are unique in that they were derived from mechanistic-based drug discovery programs designed to identify effective AEDs that inhibit the metabolism and reuptake of the inhibitory neurotransmitter GABA, respectively. For many of the newer AEDs, several molecular mechanisms of action have been identified. For example, felbamate (FBM), lamotrigine (LTG), zonisamide (ZNS), topiramate (TPM), oxcarbazepine (OCBZ), and possibly gabapentin (GBP) share a similar mechanism with that defined for phenytoin (PHT) and carbamazepine (CBZ), i.e., a voltage- and use-dependent block of voltage-sensitive sodium (Na+) channels. In addition to their effects on Na+ currents, TPM, ZNS, and FBM also appear to act as allosteric modulators of the GABA(A) receptor, whereas GBP appears to increase brain GABA levels. GBP, ZNS, FBM, LTG, and OCBZ attenuate voltage-sensitive calcium (Ca2+) channels, albeit through different mechanisms and with different classes of Ca2+ channels. FBM and TPM differ from both the established and newer AEDs in their ability to modulate NMDA- and AMPA/kainate-mediated excitatory neurotransmission, respectively. The multiple mechanisms of action associated with FBM, TPM, ZNS, GBP, and perhaps LTG, and the unique modulation of GABA levels by VGB and TGB, are likely to account for the anticonvulsant efficacy of these newer AEDs in patients with epilepsy. For each of the new drugs, their proposed mechanisms of action are discussed in relationship to their preclinical and clinical anticonvulsant profiles.

New antiepileptic drugs designed for enhancing GABAergic inhibition, such as vigabatrin (VGB) may be effective in Angelman syndrome (AS), because associated convulsions could be related to a reduced GABA-receptor density or receptor abnormality. From our preliminary experiences in four children with AS treated with VGB, we conclude that it may induce and increase seizures in patients with AS.

Purpose: Studies were conducted to establish the safety, tolerability, and pharmacokinetics of the antiepileptic drug (AED) ganaxolone. Ganaxolone belongs to a novel class of neuroactive steroids called epalons, which specifically modulate the gamma-aminobutyric acid type A (GABA[A]) receptor in the central nervous system (CNS). Chemically related to progesterone but devoid of any hormonal activity, the epalons have potent antiepileptic, anxiolytic, sedative, and hypnotic activities in animals., Methods: Ninety-six healthy male and female volunteers received ganaxolone in a variety of formulations, doses, and dosing regimens. The pharmacokinetics of ganaxolone were systematically characterized, and adverse events associated with drug use were documented., Results: Ganaxolone was well tolerated after single doses (< or =1,500 mg) and after multiple doses (< or =300 mg b.i.d. for 10 days). Steady-state plasma levels (trough) occurred after approximately 7 days of dosing, with mean steady-state plasma concentrations (Cmax) in multiple dose studies of between 32 ng/ml (50-mg doses) and 376 ng/ml (500-mg doses). No serious or life-threatening adverse events attributed to the drug were observed. The majority of adverse events reported were mild (82%) to moderate (14%) and were limited to headache, dizziness, somnolence, gastrointestinal disturbances, and malaise., Conclusions: Ganaxolone alone or formulated with pharmaceutical-grade excipients is rapidly absorbed from the gastrointestinal tract after oral administration in doses ranging from 50 to 1,500 mg. Pharmacokinetic analysis revealed a linear and proportional increase in the area under the curve (AUC) and Cmax values with increasing dose within the expected therapeutic dose range. Safety and tolerability in the clinical program were unremarkable.

More than 50 million persons worldwide suffer from epilepsy, many of whom are refractory to treatment with standard antiepileptic drugs (AEDs). Fortunately, new AEDs commercialized since 1990 are improving the clinical outlook for many patients. Our growing understanding of anticonvulsant mechanisms and the relevance of preclinical animal studies to clinical antiepileptic activity have already contributed to the design of several new AEDs and should be increasingly beneficial to further efforts at drug development. Mechanisms have been identified for older AEDs [phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), barbiturates, benzodiazepines (BZDs), ethosuximide (ESM)] and newer AEDs [vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP) tiagabine (TGB), felbamate (FBM), topiramate (TPM)]. Several novel anticonvulsant mechanisms have recently been discovered. FBM appears to be active at the strychnine-insensitive glycine binding site of the NMDA receptor. TPM is active on the kainate/AMPA subtype of glutamate receptor and at a potentially novel site on the GABA(A) receptor. For several reasons, availability of a single AED with multiple mechanisms of action may be preferred over availability of multiple AEDs with single mechanisms of action. These reasons include ease of titration, lack of drug-drug interactions, and reduced potential for pharmacodynamic tolerance.

Although a wide variety of drugs are available for treatment of epilepsy, many patients with epilepsy still experience uncontrolled seizures. In addition, there is a need for new drugs that can halt epileptogenesis after brain injury. Mechanisms that underlie seizure processes constitute potential target areas for the development of new antiepileptic drugs (AEDs). An understanding of the underlying mechanisms of interictal spike discharge and seizure spread is critical for the development of AEDs for treatment of partial seizures. Suppression of specific forms of voltage-dependent calcium currents and inhibition of GABA(B) receptor-mediated inhibition are two key target areas for new AEDs to treat primary generalized seizures. As researchers gain more understanding of the cellular, molecular, and genetic mechanisms underlying seizure propagation, we should be better able to develop therapeutic agents designed to suppress seizure-provoking mechanisms and to enhance the brain's natural protective mechanisms.

Purpose: To determine whether seizures have age-specific features, we studied the role of gamma-aminobutyric acidB (GABAB) transmission in rats of various ages (9, 15, 30, and 60 postnatal days)., Methods: We used a GABAB receptor agonist baclofen (2 or 5 mg/kg intraperitoneally, i.p.) and a GABAB receptor antagonist CGP 35348 (100 or 600 mg/kg i.p.) in the pentylenetetrazol (PTZ)-induced model of clonic and tonic-clonic seizures (100 mg/kg subcutaneously, s.c.)., Results: Whereas baclofen was anticonvulsant and CGP 35348 proconvulsant in most animals, there were distinct age-related differences in the effectiveness of these drugs and the antagonist had some anticonvulsant activity in adults. Furthermore, the two drugs acting at GABAB receptors had a different profile of action in clonic seizures as compared with tonic-clonic seizures., Conclusions: The differences in the age-specific action of the GABAB agonist and antagonist suggest the different GABAB receptor subsets may mediate the drug effects. The results indicate that putative antiepileptic drugs (AEDs) must be tested during development because it may not be possible to extrapolate age-specific anticonvulsant effects from studies in adult animals.

Purpose: The antiepileptic effects of clobazam, a 1,5-benzodiazepine, have been well documented in animal experiments and clinical trials. However, the drug's mechanisms of antiepileptic actions are still undetermined. The purpose of this study was to learn how clobazam and its active metabolite modulate gamma-aminobutyric acid (GABA)-activated currents in rat cerebral neurons in culture., Methods: Whole-cell voltage-clamp recordings were performed on cultured cerebral neurons of the rat. Clobazam or its metabolite N-desmethylclobazam was dissolved in the extracellular solution and applied for 2 s by pressure ejection from a micropipette. To maintain GABA-activated currents, 2 mM Mg adenosine triphosphate (ATP) was added to the intracellular solution., Results: GABA elicited outward currents that were mediated by GABAA receptor-coupled Cl- channels. Applying clobazam with 10 microM GABA elicited enhanced outward currents. Flumazenil, an antagonist of the benzodiazepine receptor, inhibited the enhancing effect of clobazam. The enhancement ratio increased as much as 2.28-fold in a dose-dependent manner at a concentration of 3 microM clobazam. However, it started to decrease at a concentration of 10 microM clobazam. The metabolite N-desmethylclobazam was tested in the same manner, and exhibited an identical dose-dependent enhancement of GABA-activated currents., Conclusions: The antiepileptic effects of the 1,5-benzodiazepines are attributed to the enhancement of GABAergic inhibitory neurotransmission. The antiepileptic effects of clobazam are thought to depend mainly on its active metabolite N-desmethylclobazam, which is present in high concentrations in patients who receive long-term clobazam. Clobazam's enhancement of GABA-activated currents was most marked on weaker GABA currents. We therefore infer that clobazam acts more efficiently on tissues in which the release of GABA is diminished.

We examined the effect of chronic treatment with abecarnil, a selective agonist at gamma-aminobutyric acid(A) (GABA(A)) receptors, on the development of tolerance to its anticonvulsant effect in pentylenetetrazole (PTZ)-kindled rats. We used two different experimental protocols to differentiate between pharmacological (noncontingent) and contingent tolerance. In one group of animals, kindling was suspended and abercarnil (1mg/kg intraperitoneally, i.p.) was administered three times daily for 15 days. In a second group of rats, PTZ-kindling was continued during chronic treatment with abecarnil. Tolerance to the anticonvulsant effect of a subsequent challenge dose of abecarnil (0.5 mg/kg i.p.) did not develop in either experimental group.

We review the neurochemical and behavioral profile of the selective gamma-aminobutyric acid (GABA) uptake inhibitor, (R)-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl) nipecotic acid hydrochloride [tiagabine (TGB), previously termed NNC 05-0328, NO 05-0328, and NO-328], which is currently in phase III clinical trials for epilepsy. TGB is a potent, and specific GABA uptake inhibitor. TGB lacks significant affinity for other neurotransmitter receptor binding sites and/or uptake sites. In electrophysiological experiments in hippocampal slices in culture, TGB prolonged the inhibitory postsynaptic potentials (IPSP) and inhibitory postsynaptic currents (IPSC) in the CA1 and CA3 produced by the addition of exogenous GABA. In vivo microdialysis shows that TGB also increases extracellular GABA overflow in a dose-dependent manner. Together these biochemical data suggest that the in vitro and in vivo mechanism of action of TGB is to inhibit GABA uptake specifically, resulting in an increase in GABAergic mediated inhibition in the brain. TGB is a potent anticonvulsant agent against methyl-6,7-dimethyoxy-4-ethyl-B-carboline-3-carboxylate (DMCM)-induced clonic convulsions (mice), subcutaneous pentylenetetrazol (PTZ)-induced tonic convulsions (mice and rats), sound-induced convulsions in DBA/2 mice and genetically epilepsy-prone rats (GEPR), and electrically induced convulsions in kindled rats. TGB is partially efficacious, against subcutaneous PTZ-induced clonic convulsions, and photically induced myoclonus in Papio papio. TGB is weakly efficacious in the intravenous PTZ seizure threshold test and the maximal electroshock seizure (MES) test and produces only partial protection against bicuculline (BIC)-induced convulsions in rats. The overall biochemical and anticonvulsant profile of TGB suggests potential utility in the treatment of chronic seizure disorders such as generalized clonic-tonic epilepsy (GTCS), photomyoclonic seizures, myoclonic petit mal epilepsy, and complex partial epilepsy.

The neurochemical basis of absence seizures and the mechanism of their suppression by valproate (VPA) are uncertain. We used positron emission tomography (PET) to determine whether an abnormality of [11C]flumazenil binding to benzodiazepine (BZD)-GABAA receptors exists in patients with childhood and juvenile absence epilepsy and to examine the effects of VPA on [11C]flumazenil binding. The regional cerebral volume of distribution (Vd) of [11C]flumazenil in patients not treated with VPA was not different from that in normal controls; Vd was lower in patients treated with VPA, and the number of receptors available for binding was significantly reduced in such patients as compared with normal controls. There was no evidence of a primary abnormality of the BZD-GABAA receptor in childhood and juvenile absence epilepsy (CAE/JAE), but the data suggest that treatment with VPA is associated with a reduction in [11C]flumazenil binding that may be relevant to its mode of action in CAE/JAE.

Established antiepileptic drugs (AEDs) decrease membrane excitability by interacting with neurotransmitter receptors or ion channels. AEDs developed before 1980 appear to act on sodium channels, gamma-aminobutyric acid type A (GABAA) receptors, or calcium channels. Benzodiazepines and barbiturates enhance GABAA receptor-mediated inhibition. Phenytoin (PHT), carbamazepine (CBZ), and possibly valproate (VPA) decrease high-frequency repetitive firing of action potentials by enhancing sodium-channel inactivation. Ethosuximide (ESM) and VPA reduce a low threshold (T-type) calcium-channel current. The mechanisms of action of the new AEDs are not fully established. Gabapentin (GBP) binds to a high-affinity site on neuronal membranes in a restricted regional distribution of the central nervous system. This binding site may be related to a possible active transport process of GBP into neurons; however, this has not been proven, and the mechanism of action of GBP remains uncertain. Lamotrigine (LTG) decreases sustained high-frequency repetitive firing of voltage-dependent sodium action potentials that may result in a preferential decreased release of presynaptic glutamate. The mechanism of action of oxcarbazepine (OCBZ) is not known; however, its similarity in structure and clinical efficacy to CBZ suggests that its mechanism of action may involve inhibition of sustained high-frequency repetitive firing of voltage-dependent sodium action potentials. Vigabatrin (VGB) irreversibly inhibits GABA transaminase, the enzyme that degrades GABA, thereby producing greater available pools of presynaptic GABA for release in central synapses. Increased activity of GABA at postsynaptic receptors may underline the clinical efficacy of VGB.

gamma-Aminobutyric acid (GABA) was first proposed as a putative inhibitory neurotransmitter by Elliot and van Gelder in 1958. Since then, numerous efforts have been made to find ways to increase GABA at its receptor sites, based on the findings that decreased GABA results in convulsions in animals and that agents enhancing GABA-mediated functions can have antiepileptic effects. However, the relationship between GABA levels and seizures is not simple. Seizures can occur even in the presence of elevated GABA levels. Indeed, it is possible that regional biochemical differences in the brain can be important. The antiepileptic effects of GABA depend on the mechanism whereby GABA-mediated inhibition is enhanced. Since the 1970s, several compounds have been developed that are designed to act in some manner on the GABA system. These compounds affect GABA-mediated inhibition at different levels and appear to have varied effects, depending on their mechanism of action. To date, specific antiepileptic drugs (AEDs) with potential GABA-inhibitory effects have been designed either to have GABA agonist properties, to inhibit GABA catabolism, to inhibit GABA uptake, or to facilitate GABA release or facilitate GABAA receptor activity. Vigabatrin (VGB) was designed specifically to inhibit GABA transaminase and thereby increase the availability of GABA in the brain. Study data and clinical experience over the past 14 years have demonstrated VGB to be an effective AED.

Tiagabine (TGB) hydrochloride, a nipecotic acid derivative linked to a lipophilic anchor, potently and specifically inhibits uptake of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) into astrocytes and neurons. With microdialysis, TGB has been shown to increase extracellular overflow of GABA in the midbrain of awake rats. TGB administration prolongs neuronal depolarization induced by iontophoretically applied GABA in hippocampal slices. TGB is effective in a wide range of seizure models, including pentylenetetrazol-induced, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM)-induced tonic, amygdala-kindled and picrotoxin-induced convulsions, and maximal electroshock seizures in rodents. In humans, TGB absorption is rapid and complete. It is metabolized in the liver, largely by isoform 3A of the cytochrome P450 family of enzymes. The process of elimination is linear, with a half-life of 5-8 h. TGB does not induce or inhibit metabolic processes, although it provides a target for enzyme inducers that can lower its elimination half-life to 2-3 h. Accordingly, TGB does not alter the concentrations of other antiepileptic drugs (AEDs), with the possible exception of a small decrease in valproate levels. A controlled-release formulation of TGB would offset any potential clinical disadvantage of the short elimination half-life, particularly in patients receiving concurrent treatment with enzyme-induced AEDs.

Some of the properties of 4-bromobenzaldehyde semicarbazone (compound IV), a prototype molecule of a new class of anticonvulsants, aryl semicarbazones, are described. Compound IV demonstrated activity in the maximal electroshock (MES) and subcutaneous pentylenetetrazol (scPTZ) tests in mice, with low neurotoxicity. When given orally to rats, it displayed high potency in the MES test and very low neurotoxicity, resulting in a high protective index (PI). Compound IV displayed no proconvulsant properties, and development of rapid tolerance was not noted. When administered intraperitoneally (i.p.) at doses of 100, 300, or 600 mg/kg to peritoneally (i.p.) at doses of 100, 300, or 600 mg/kg to rats, compound IV had no effect on levels of gamma-aminobutyric acid (GABA) or on GABA-T activity in whole brain. When tested in vitro, compound IV had no effect on rat brain GABA-T at a drug concentration of 100 microM. Although the activities of certain drug-metabolizing enzymes were increased after oral administration of compound IV to rats, these effects were less prominent than those of phenytoin (PHT) and carbamazepine (CBZ). The principal mode of action of compound IV does not appear to be an interaction with the GABAA receptor complex, and other mechanisms, involving excitatory amino acid neurotransmission, will have to be considered in future investigations of the anticonvulsant activity of this compound.

We observed transient parkinsonism in 2 young epileptic patients with valproate (VPA) therapy. Complete recovery from extrapyramidal disorder occurred spontaneously in a few weeks. The lack of apparent susceptibility related to age and to VPA dosage, the rapid recovery from the extrapyramidal reaction, and the prevalence of negative signs such as bradykinesia and bradyphrenia can be considered the main clinical findings of this disease process. Pathophysiologic mechanisms of this rare "toxic" reaction remain unknown, although a transient imbalance between functionally reciprocal subgroups of GABA pathways leading to remediable dopamine inhibition might be hypothesized.

To study further the previously demonstrated suppressive in vivo effect of sodium valproate (VPA) on gamma-aminobutyric acid (GABA) release in the preoptic area, we examined GABA neurotransmission in substantia nigra (SN), using the push-pull cannula technique in freely moving ovariectomized rats. To clarify whether the area in the substantia nigra that is actually perfused is pars reticulata, known to receive rich GABAergic input from striatum, we used the retrograde fluorescence tracer fast blue (FB) after each perfusion experiment, applying the tracer through the push-pull cannula. Nigral perfusion with VPA caused significant suppression of local GABA release. This effect was more marked in a subgroup of animals showing retrograde labeled cell bodies in striatum, i.e., animals with a tracer application site and therefore also a perfusion site precisely in pars reticulata. Our data suggest that VPA inhibits GABA release in rat SN as it does in the preoptic area, which may be in agreement with our hypothesis of enhanced GABAergic neurotransmission by VPA, causing suppression of presynaptic GABA release through negative feedback actions on the GABA autoreceptor complex. Furthermore, the combination of push-pull cannula technique and retrograde fluorescence tracer application appears to be an important tool to prove afferent connections of the area actually perfused in neuronal networks.

To elucidate the consequences of convulsions, we examined biochemically and electrophysiologically the brains of mice that had sustained two complete tonic-clonic convulsions after administration of pentylenetetrazol (PTZ 50 mg/kg intraperitoneally, i.p.), 48 and 24 h before decapitation. Control mice were injected with saline. Input/output curves of the extracellular synaptic responses in the CA1 area of hippocampal slices showed that PTZ-induced seizures do not establish the persistent change in hippocampal excitability itself that can be detected in vitro. However, use of the paired-pulse stimulation paradigm showed that gamma-aminobutyric acid A (GABAA)-mediated recurrent inhibition was significantly weaker (by 19-25%) in the CA1 area of slices from PTZ-treated mice (PTZ slices) as compared with slices from control mice (control slices). The density of GABAA receptors (high-affinity component) was also lower in hippocampus (by 19%) and cortex (by 14%) of PTZ-treated mice. A GABA-related disinhibitory mechanism underlying PTZ seizures may thus persist for 1 day after the seizure, predisposing the brain to subsequent seizures. On the other hand, the depressant effect of a single dose of adenosine 10 microM on the CA1 synaptic response was stronger (by 35% on population spikes) and longer lasting in PTZ slices as compared with controls. This could be attributed to significantly higher adenosine A1 receptor density in hippocampus (Bmax of [3H]CHA was higher by 34%) as well as cortex and cerebellum of these animals. The phenomenon may reflect an adenosine A1-mediated adaptive mechanism that offers protection from subsequent seizures.

Presently, most strategies for development of antiepileptic drugs (AEDs) center around seizure models that are known to respond to presently marketed AEDs. These strategies do not take into account that epilepsy can be a progressive disease. Moreover, region-specific aspects of epileptogenesis are rarely considered when new AEDs are developed. Seizures in the temporal lobe are often difficult to treat. Animal studies on various seizure models in the hippocampus and the entorhinal cortex (EC) suggest that these structures do not a priori produce seizures that are difficult to treat. However, seizure-like events in the EC tend to progress to a state of status epilepticus-like activity that cannot be suppressed by presently marketed AEDs. Loss of gamma-aminobutyric acid (GABA)ergic neurotransmission and increased excitatory synaptic coupling seem to cooperate for induction of this state. Epilepsy induced alterations in the interaction between the EC and the hippocampus may lead to alterations that facilitate precipitation of seizures. Because of the recurrent interaction between the hippocampus and the EC, these seizures may reach an intensity that is no longer controllable by presently available AEDs. Ontogenetic alterations of the circuitry between the EC and the hippocampus, seizure-induced stabilization of synaptic connections overexpressed during ontogenesis, seizure-induced lesions and subsequent rearrangements of internal cell properties, and synaptic arrangements and kindling-like alterations of nerve cell and glial behavior may all be involved in the generation of a neuronal aggregate whose balance between inhibitory and excitatory processes becomes readily disturbed. Strategies for the development of AEDs treating such seizures should suppress hyperactivity and prevent progression of epileptogenesis. AEDs directed against seizures may be effective if they can be given in sufficient concentrations to suppress very intense local seizures.

We made a comparative study of the anticonvulsant effect of GABA agonists on feline amygdala or hippocampal kindled seizures. Progabide (PGB) [gamma-aminobutyric acid (GABA) receptor agonist 25-100 mg/kg intraperitoneally, i.p.] significantly reduced both the kindled seizure stage and after discharge (AD) duration in a dose-dependent manner. SKF89976A (GABA uptake inhibitor 0.5-2.0 mg/kg i.p.) also significantly reduced the kindled seizure stage. Toxic doses of SKF89976A caused generalized paroxysmal EEG discharges and myoclonus, but AD generation in the kindled focus was suppressed completely. Furthermore, gamma-vinyl GABA (GABA catabolic enzyme inhibitor, GVG 50-200 mg/kg i.p.) significantly reduced the seizure stage, while causing prolongation of the AD duration. In contrast, baclofen (selective GABAB receptor agonist, 1 or 5 mg/kg) did not show anticonvulsant effects on any parameters of kindled seizures. Therefore, these GABA agonists, which potentiate the inhibitory function of the GABAA systems, have potent anticonvulsant effects on partial onset and secondarily generalized limbic seizures.

Anticonvulsant action of a new benzodiazepine, bretazenil (Ro 16-6028), was studied in 240 rats in five age groups: age 7, 12, 18, 25 and 90 days. Motor seizures induced by metrazol (pentamethylenetetrazol, PTZ, 100 mg/kg subcutaneously (s.c.) except for 18-day-old rats which received a dose of 90 mg/kg) served as a model. Animals were pretreated with Ro 16-6028 in doses of 0.001-0.1 mg/kg intraperitoneally (i.p.) 10 min before metrazol. Both types of metrazol-induced seizures, minimal (mMS, predominantly clonic with preserved righting ability) and major (MMS, generalized tonic-clonic), were suppressed by Ro 16-6028 in a dose-dependent manner. Major seizures were always more sensitive to Ro 16-6028 than were minimal seizures. The youngest rats exhibited maximal effects of Ro 16-6028 against major seizures. On the other hand, this drug increased the incidence of minimal seizures in 7- and 12-day-old rats, i.e., in age groups in which this type of seizure is rare under control conditions.

A novel anticonvulsant benzodiazepine bretazenil (Ro 16-6028) was studied electrophysiologically in a model of human absence seizures: rhythmic metrazol activity (RMA) in rats. The effects of Ro 16-6028 pretreatment (0.01, 0.05, or 0.1 mg/kg intraperitoneally, i.p.) were compared with those of clonazepam (CZP, 0.02 or 0.1 mg/kg i.p.), valproate (VPA, 200, 300, or 400 mg/kg) and ethosuximide (ESM, 31.25, 62.5, or 125 mg/kg i.p.) in 45 rats with implanted electrocorticographic electrodes. RMA was elicited by an injection of pentylenetetrazol (metrazol, PTZ) in a dose of 40 or 35 mg/kg i.p. The effects of Ro 16-6028 were similar to those of CZP and VPA, i.e., suppression of RMA episodes, an increase in latency and a decrease in number, and total as well as mean duration. On the other hand, ESM differed from these antiepileptic drugs (AEDs) in inability to shorten the duration of RMA episodes. Based on these results, Ro 16-6028 might be predicted to be efficient against human absence seizures.