Anticonvulsant and proconvulsant effects of inhibitors of GABA degradation in the amygdala-kindling model

An update on the novel methods for the discovery of antiseizure and antiepileptogenic medications: where are we in 2024?

INTRODUCTION

Despite the availability of around 30 antiseizure medications, 1/3 of patients with epilepsy fail to become seizure-free upon pharmacological treatment. Available medications provide adequate symptomatic control in two-thirds of patients, but disease-modifying drugs are still scarce. Recently, though, new paradigms have been explored.

AREAS COVERED

Three areas are reviewed in which a high degree of innovation in the search for novel antiseizure and antiepileptogenic medications has been implemented: development of novel screening approaches, search for novel therapeutic targets, and adoption of new drug discovery paradigms aligned with a systems pharmacology perspective.

EXPERT OPINION

In the past, worldwide leaders in epilepsy have reiteratively stated that the lack of progress in the field may be explained by the recurrent use of the same molecular targets and screening procedures to identify novel medications. This landscape has changed recently, as reflected by the new Epilepsy Therapy Screening Program and the introduction of many in vitro and in vivo models that could possibly improve our chances of identifying first-in-class medications that may control drug-resistant epilepsy or modify the course of disease. Other milestones include the study of new molecular targets for disease-modifying drugs and exploration of a systems pharmacology perspective to design new drugs.

OV329, a novel highly potent γ-aminobutyric acid aminotransferase inactivator, induces pronounced anticonvulsant effects in the pentylenetetrazole seizure threshold test and in amygdala-kindled rats

OBJECTIVE

An attractive target to interfere with epileptic brain hyperexcitability is the enhancement of γ-aminobutyric acidergic (GABAergic) inhibition by inactivation of the GABA-metabolizing enzyme GABA aminotransferase (GABA-AT). GABA-AT inactivators were designed to control seizures by raising brain GABA levels. OV329, a novel drug candidate for the treatment of epilepsy and addiction, has been shown in vitro to be substantially more potent as a GABA-AT inactivator than vigabatrin, an antiseizure drug approved as an add-on therapy for adult patients with refractory complex partial seizures and monotherapy for pediatric patients with infantile spasms. Thus, we hypothesized that OV329 should produce pronounced anticonvulsant effects in two different rat seizure models.

METHODS

We therefore examined the effects of OV329 (5, 20, and 40 mg/kg ip) on the seizure threshold of female Wistar Unilever rats, using the timed intravenous pentylenetetrazole (ivPTZ) seizure threshold model as a seizure test particularly sensitive to GABA-potentiating manipulations, and amygdala-kindled rats as a model of difficult-to-treat temporal lobe epilepsy.

RESULTS

GABA-AT inactivation by OV329 clearly increased the threshold of both ivPTZ-induced and amygdala-kindled seizures. OV329 further showed a 30-fold greater anticonvulsant potency on ivPTZ-induced myoclonic jerks and clonic seizures compared to vigabatrin investigated previously. Notably, all rats were responsive to OV329 in both seizure models.

SIGNIFICANCE

These results reveal an anticonvulsant profile of OV329 that appears to be superior in both potency and efficacy to vigabatrin and highlight OV329 as a highly promising candidate for the treatment of seizures and pharmacoresistant epilepsies.

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

Mechanisms of Action of Antiseizure Drugs and the Ketogenic Diet

Antiseizure drugs (ASDs), also termed antiepileptic drugs, are the main form of symptomatic treatment for people with epilepsy, but not all patients become free of seizures. The ketogenic diet is one treatment option for drug-resistant patients. Both types of therapy exert their clinical effects through interactions with one or more of a diverse set of molecular targets in the brain. ASDs act by modulation of voltage-gated ion channels, including sodium, calcium, and potassium channels; by enhancement of γ-aminobutyric acid (GABA)-mediated inhibition through effects on GABAA receptors, the GABA transporter 1 (GAT1) GABA uptake transporter, or GABA transaminase; through interactions with elements of the synaptic release machinery, including synaptic vesicle 2A (SV2A) and α2δ; or by blockade of ionotropic glutamate receptors, including α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors. The ketogenic diet leads to increases in circulating ketones, which may contribute to the efficacy in treating pharmacoresistant seizures. Production in the brain of inhibitory mediators, such as adenosine, or ion channel modulators, such as polyunsaturated fatty acids, may also play a role. Metabolic effects, including diversion from glycolysis, are a further postulated mechanism. For some ASDs and the ketogenic diet, effects on multiple targets may contribute to activity. Better understanding of the ketogenic diet will inform the development of improved drug therapies to treat refractory seizures.

Human fetal brain-derived neural stem/progenitor cells grafted into the adult epileptic brain restrain seizures in rat models of temporal lobe epilepsy

Cell transplantation has been suggested as an alternative therapy for temporal lobe epilepsy (TLE) because this can suppress spontaneous recurrent seizures in animal models. To evaluate the therapeutic potential of human neural stem/progenitor cells (huNSPCs) for treating TLE, we transplanted huNSPCs, derived from an aborted fetal telencephalon at 13 weeks of gestation and expanded in culture as neurospheres over a long time period, into the epileptic hippocampus of fully kindled and pilocarpine-treated adult rats exhibiting TLE. In vitro, huNSPCs not only produced all three central nervous system neural cell types, but also differentiated into ganglionic eminences-derived γ-aminobutyric acid (GABA)-ergic interneurons and released GABA in response to the depolarization induced by a high K⁺ medium. NSPC grafting reduced behavioral seizure duration, afterdischarge duration on electroencephalograms, and seizure stage in the kindling model, as well as the frequency and the duration of spontaneous recurrent motor seizures in pilocarpine-induced animals. However, NSPC grafting neither improved spatial learning or memory function in pilocarpine-treated animals. Following transplantation, grafted cells showed extensive migration around the injection site, robust engraftment, and long-term survival, along with differentiation into β-tubulin III⁺ neurons (∼34%), APC-CC1⁺ oligodendrocytes (∼28%), and GFAP⁺ astrocytes (∼8%). Furthermore, among donor-derived cells, ∼24% produced GABA. Additionally, to explain the effect of seizure suppression after NSPC grafting, we examined the anticonvulsant glial cell-derived neurotrophic factor (GDNF) levels in host hippocampal astrocytes and mossy fiber sprouting into the supragranular layer of the dentate gyrus in the epileptic brain. Grafted cells restored the expression of GDNF in host astrocytes but did not reverse the mossy fiber sprouting, eliminating the latter as potential mechanism. These results suggest that human fetal brain-derived NSPCs possess some therapeutic effect for TLE treatments although further studies to both increase the yield of NSPC grafts-derived functionally integrated GABAergic neurons and improve cognitive deficits are still needed.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

GABA metabolism and transport: effects on synaptic efficacy

GABAergic inhibition is an important regulator of excitability in neuronal networks. In addition, inhibitory synaptic signals contribute crucially to the organization of spatiotemporal patterns of network activity, especially during coherent oscillations. In order to maintain stable network states, the release of GABA by interneurons must be plastic in timing and amount. This homeostatic regulation is achieved by several pre- and postsynaptic mechanisms and is triggered by various activity-dependent local signals such as excitatory input or ambient levels of neurotransmitters. Here, we review findings on the availability of GABA for release at presynaptic terminals of interneurons. Presynaptic GABA content seems to be an important determinant of inhibitory efficacy and can be differentially regulated by changing synthesis, transport, and degradation of GABA or related molecules. We will discuss the functional impact of such regulations on neuronal network patterns and, finally, point towards pharmacological approaches targeting these processes.

Functional, metabolic, and synaptic changes after seizures as potential targets for antiepileptic therapy

Little is known about how the brain limits seizure duration and terminates seizures. Depending on severity and duration, a single seizure is followed by various functional, metabolic, and synaptic changes that may form targets for novel therapeutic strategies. It is long known that most seizures are followed by a period of postictal refractoriness during which the threshold for induction of additional seizures is increased. The endogenous anticonvulsant mechanisms involved in this phenomenon may be relevant for both spontaneous seizure arrest and increase of seizure threshold after seizure arrest. Postictal refractoriness has been extensively studied in various seizure and epilepsy models, including electrically and chemically induced seizures, kindling, and genetic animal models of epilepsy. During kindling development, two antagonistic processes occur simultaneously, one responsible for kindling-like events and the other for terminating ictus and postictal refractoriness. Frequently occurring seizures may lead to an accumulation of postictal refractoriness that may last weeks. The mechanisms involved in seizure termination and postictal refractoriness include changes in ionic microenvironment, in pH, and in various endogenous neuromodulators such as adenosine and neuropeptides. In animal models, the anticonvulsant efficacy of several antiepileptic drugs (AEDs) is increased during postictal refractoriness, which is a logical consequence of the interaction between endogenous anticonvulsant processes and the mechanism of AEDs. As discussed in this review, enhanced understanding of these endogenous processes may lead to novel targets for AED development.

Time–course and dose–response relationships of imperatorin in the mouse maximal electroshock seizure threshold model

This study was designed to evaluate the anticonvulsant effects of imperatorin (a furanocoumarin isolated from fruits of Angelica archangelica) in the mouse maximal electroshock seizure threshold model. The threshold for electroconvulsions in mice was determined at several times: 15, 30, 60 and 120 min after i.p. administration of imperatorin at increasing doses of 10, 20, 30, 40, 50 and 100 mg/kg. The evaluation of time-course relationship for imperatorin in the maximal electroshock seizure threshold test revealed that the agent produced its maximum antielectroshock action at 30 min after its i.p. administration. In this case, imperatorin at doses of 50 and 100 mg/kg significantly raised the threshold for electroconvulsions in mice by 38 and 68% (P<0.05 and P<0.001), respectively. The antiseizure effects produced by imperatorin at 15, 60 and 120 min after its systemic (i.p.) administration were less expressed than those observed for imperatorin injected 30 min before the maximal electroshock seizure threshold test. Based on this study, one can conclude that imperatorin produces the anticonvulsant effect in the maximal electroshock seizure threshold test in a dose-dependent manner.

Isobolographic characterization of interactions between vigabatrin and tiagabine in two experimental models of epilepsy

To characterize the type of interactions between vigabatrin (VGB) and tiagabine (TGB) -- two newer antiepileptic drugs influencing GABA-ergic neurotransmitter system, the isobolographic analysis was used in two experimental models of epilepsy: the maximal electroshock seizure threshold (MEST) test and pentylenetetrazole (PTZ)-induced seizures in mice. Results indicated that VGB and TGB administered separately (i.p.) increased the electroconvulsive threshold in mice, which allowed the calculation of their TID(20) values (threshold increasing doses by 20% over the threshold of control animals) in the MEST test. The TID(20) for VGB was 226.2 mg/kg and that for TGB was 4.4 mg/kg. With isobolography, the combinations of VGB with TGB (at fixed-ratios of 1:3, 1:1 and 3:1) exerted additive interactions in the MEST test in mice. Similarly, VGB and TGB injected separately (i.p.) suppressed the PTZ-induced seizures, and their ED(50) values (median effective doses, protecting 50% of the animals tested against clonic convulsions) for VGB and TGB were 622.5 mg/kg and 0.8 mg/kg, respectively. Isobolographic analysis of interactions revealed that the combinations of VGB with TGB at the fixed-ratios of 1:3 and 1:1 produced supra-additive (synergistic) interactions against PTZ-induced seizures. Only the combination of VGB with TGB at the fixed-ratio of 3:1 was additive in the PTZ test. The evaluation of acute adverse-effect potential for all fixed-ratio combinations of VGB with TGB (administered at their TID(20) and ED(50) values from the MEST and PTZ tests) revealed that none of the examined combinations affected motor coordination in the chimney test and altered neuromuscular tone in the grip-strength test in mice. In contrast, VGB in combinations with TGB produced the antinociceptive effects with respect to suppression of acute thermal pain in animals subjected to the hot-plate test. Based on this preclinical study, one can ascertain that the combination of VGB with TGB would provide an adequate seizure control in epileptic patients.

Resistance to phenobarbital extends to phenytoin in a rat model of temporal lobe epilepsy

PURPOSE

Most patients who are resistant to the first antiepileptic drug (AED) treatment are also resistant to a treatment with a second or third AED, indicating that patients who have an inadequate response to initial treatment with AEDs are likely to have refractory epilepsy. Animal models of refractory epilepsy are important tools to study mechanisms of AED resistance and develop new treatment strategies for counteracting resistance. We have recently described a rat model of temporal lobe epilepsy (TLE), in which spontaneous recurrent seizures (SRS) develop after a status epilepticus induced by sustained electrical stimulation of the basolateral amygdala. Prolonged treatment of epileptic rats with phenobarbital (PB) resulted in two subgroups, PB responders and PB nonresponders.

METHODS

In the present study we examined if rats with PB-resistant seizures are also resistant to phenytoin (PHT), using continuous EEG/video recording of spontaneous seizures.

RESULTS

First, a new group of 15 epileptic rats was produced and selected by treatment with PB into responders (8 rats) and nonresponders (6 rats), respectively. During subsequent treatment with PHT, the doses of PHT had to be individually adjusted for each rat to avoid toxicity. Treatment with PHT led to complete seizure control in two animals and a >50% reduction of seizure frequency in three other rats, which were considered PHT responders. In nine of the remaining rats, PHT did not exert any clear anticonvulsant effect, so that these rats were considered nonresponders. Plasma levels of PHT did not differ significantly between responders and nonresponders. When comparing the PB and PHT nonresponder groups, five of the six PB-resistant rats (83%) were also resistant to PHT, demonstrating that rats that have an inadequate response to initial treatment with PB are likely to be also resistant to treatment with a second AED.

CONCLUSIONS

The AED-resistant rats of our model meet the definition of pharmacoresistance in animal models, that is, persistent seizure activity not responding to at least two AEDs at maximum tolerated doses. This new model of pharmacoresistant TLE may be useful in the targeted development of new therapies for refractory epilepsy.

Characterization of the Anticonvulsant, Behavioral and Pharmacokinetic Interaction Profiles of Stiripentol in Combination with Clonazepam, Ethosuximide, Phenobarbital, and Valproate Using Isobolographic Analysis

PURPOSE

Isobolographic analysis was used to characterize the interactions between stiripentol (STP) and clonazepam (CZP), ethosuximide (ETS), phenobarbital (PB), and valproate (VPA) in suppressing pentylenetetrazole (PTZ)-induced clonic seizures in mice.

METHODS

The anticonvulsant and acute adverse (neurotoxic) effects of STP in combination with the various conventional antiepileptic drugs (AEDs), at fixed ratios of 1:3, 1:1, and 3:1, were evaluated in the PTZ and chimney tests in mice using the isobolographic analysis. Additionally, protective indices (PI) and benefit indices (BI) were calculated to identify their pharmacological profiles so that a ranking in relation to advantageous combination could be established. Moreover, adverse-effect paradigms were determined by use of the step-through passive avoidance task (long-term memory), threshold for the first pain reaction, grip-strength test (neuromuscular tone), and the hot plate test (acute thermal pain). Brain AED concentrations were also measured so as to ascertain any pharmacokinetic contribution to the pharmacodynamic interactions.

RESULTS

All AED combinations comprising of STP and CZP, ETS, PB, and VPA (at the fixed ratios of 1:3, 1:1 and 3:1) were additive in terms of clonic seizure suppression in the PTZ test. However, these interactions were complicated by changes in brain AED concentrations consequent to pharmacokinetic interactions. Thus STP significantly increased total brain ETS and PB concentrations, and decreased VPA concentrations, but was without effect on CZP concentrations. In contrast, PB significantly decreased and VPA increased total brain STP concentrations while CZP and ETS were without effect. Furthermore, while isobolographic analysis revealed that STP and CZP in combination, at the fixed ratios of 1:1 and 3:1, were supraadditive (synergistic; p < 0.05), the combinations of STP with CZP (1:3), ETS, PB, or VPA (at all fixed ratios of 1:3, 1:1, and 3:1) were barely additivity in terms of acute neurotoxic adverse effects in the chimney test. Additionally, none of the examined combinations of STP with conventional AEDs (CZP, ETS, PB, VPA--at their median effective doses from the PTZ-test) affected long-term memory, threshold for the first pain reaction, neuromuscular tone, and acute thermal pain.

CONCLUSIONS

Based on BI values, the combination of STP with PB at the fixed ratio of 1:3 appears to be a particularly favourable combination. In contrast, STP and CZP or ETS (at the fixed ratios of 1:1 and 3:1) were unfavorable combinations. However, these conclusions are confounded by the fact that STP is associated with significant pharmacokinetic interactions. The remaining combinations of STP with PB (1:1 and 3:1), CZP (1:3), ETS (1:3), and VPA (at all fixed ratios of 1:3, 1:1, and 3:1) do not appear to be potential favorable AED combinations.

Experience-Dependent Formation and Recruitment of Large Vesicles from Reserve Pool

The sizes and contents of transmitter-filled vesicles have been shown to vary depending on experimental manipulations resulting in altered quantal sizes. However, whether such a presynaptic regulation of quantal size can be induced under physiological conditions as a potential alternative mechanism to alter the strength of synaptic transmission is unknown. Here we show that presynaptic vesicles of glutamatergic synapses of Drosophila neuromuscular junctions increase in size as a result of high natural crawling activities of larvae, leading to larger quantal sizes and enhanced evoked synaptic transmission. We further show that these larger vesicles are formed during a period of enhanced replenishment of the reserve pool of vesicles, from which they are recruited via a PKA- and actin-dependent mechanism. Our results demonstrate that natural behavior can induce the formation, recruitment, and release of larger vesicles in an experience-dependent manner and hence provide evidence for an additional mechanism of synaptic potentiation.

Various GABA-mimetic drugs differently affect cocaine-evoked hyperlocomotion and sensitization

To substantiate the notion that cocaine behavioral effects may be influenced by gamma-aminobutyric acid (GABA) neurotransmission male Wistar rats were injected with gabapentin (a cyclic GABA analogue), tiagabine (a GABA reuptake inhibitor), or vigabatrin (a GABA transaminase inhibitor) before acute or repeated treatment with cocaine evoking either locomotor hyperactivation or sensitization. Gabapentin (1-30 mg/kg), tiagabine (2.5-10 mg/kg) or vigabatrin (75-250 mg/kg) attenuated the cocaine (10 mg/kg)-induced hyperactivation and in the highest doses they also decreased basal locomotor activation. Vigabatrin (75-250 mg/kg) dose-dependently reduced the development of cocaine sensitization in rats treated repeatedly (days 1-5) with cocaine (10 mg/kg) and then challenged with cocaine (10 mg/kg) following 5-day withdrawal; the remaining drugs were ineffective. When injected acutely with a cocaine challenge dose, gabapentin (3-10 mg/kg) or vigabatrin (150 mg/kg), but not tiagabine, significantly attenuated the expression of cocaine sensitization. The present results show that enhanced GABA-ergic neurotransmission exerted inhibitory actions on acute responses to cocaine, however, only in a case of vigabatrin the inhibition seems to be unrelated to the inhibitory effect of the drugs on basal locomotor activity. The finding that vigabatrin protected against the development and the expression of cocaine sensitization further supports its therapeutic potential in the treatment of cocaine dependence.

Regulation of the expression of GABAA receptor subunits by an antiepileptic drug QYS

It has been reported that the antiepileptic drug qingyangshenylycosides (QYS) modulated the function of GABAergic system. However, little is known about the effects of QYS on the gene expression of GABA receptors in the central nervous system (CNS). In the present study, we examined the effects of QYS on the expression of GABAA receptor subunits in different regions of the mouse brain. The results showed that treatment of QYS significantly increased the expressions of Gabra1, Gabra2 and Gabr4 and decreased the expression of Gabrg2 in inferior colliculus. Moreover, Gabrb2 expression was up-regulated and Gabra5 was down-regulated in hippocampus, while the expressions of Gabra1 and Gabrb2 were induced in cortex after QYS treatment. These data indicated that QYS had different effects on the expression of GABAA receptor subunits in different brain regions. These results may help to reveal the molecular mechanism of anticonvulsant action of QYS.

Involvement of Scn1b and Kcna1 ion channels in audiogenic seizures and PTZ-induced epilepsy

We have undertaken chemical genetic approach using Qingyangshenylycosides (QYS), a natural product compound, to explore the molecular mechanisms underlying different types of epilepsy models. Two animal models were used for these studies, i.e., audiogenic seizure (AGS) and pentylenetetrazol (PTZ)-induced generalized epilepsy in DBA/2J mice. We show that the latency of AGS is prolonged and the severity of seizures (the percentages of the tonus, Tonus_%) is reduced in the QYS-treated animals. These results indicate that QYS has anticonvulsant effect on the AGS model. However, we find that administration of QYS has an opposite effects on PTZ-induced generalized epilepsy. Both the latency of the generalized epilepsy and the latency of death are decreased after QYS treatment in PTZ-induced epilepsy. We examine the molecular basis of the distinct roles of QYS in these two epilepsy models by using gene expression data. Our results show that a voltage-gated sodium channel (Scn1b) and a voltage-gated potassium channel (Kcna1) are differentially expressed in AGS and PTZ-induced epilepsy models as well as in QYS-treated animals. Our results demonstrate that a chemical genetic approach may help to reveal both the molecular mechanisms of different epilepsies and the mechanism of action of the antiepileptic drugs.

Vigabatrin in Low Doses Selectively Suppresses the Clonic Component of Audiogenically Kindled Seizures in Rats

PURPOSE

The effect of systemic administration of the gamma-aminobutyric acid (GABA)-transaminase inhibitor vigabatrin (VGB) on different components of convulsions was tested in the model of audiogenically kindled seizures, which consist of brainstem (running, tonus) and forebrain (clonus) elements.

METHODS

Audiogenically susceptible rats of Krushinsky-Molodkina (KM), Wistar, and WAG/Rij strains received repeated sound stimulation (60 dB, 10-80 kHz) until kindled audiogenic seizures were reliably elicited. Kindled audiogenic seizures consisted of running, tonic, and generalized clonic phases in KM rats (severe audiogenic seizures) and of running and Racine stage 5 facial/forelimb clonus in Wistar and WAG/Rij rats (moderate seizures). Vehicle, 100, or 200 mg/kg of VGB was intraperitoneally injected 2, 4 and 24 h before the induction of kindled audiogenic seizures.

RESULTS

At both doses, VGB did not change the seizure latency and the duration of running and tonic convulsions, but suppressed clonic ones in all rat strains. In KM rats, the mean duration of posttonic clonus was significantly reduced at 24 h after 100 mg/kg and from 4 h after 200 mg/kg. In Wistar and WAG/Rij rats, the mean duration of facial/forelimb clonus was reduced from 4 and 2 h after 100- and 200-mg/kg administration, respectively; 24 h after the high-dose injection, clonus was completely blocked in all rats of both strains. No difference in efficacy of VGB between Wistar and WAG/Rij rats was observed.

CONCLUSIONS

VGB more effectively suppresses clonic convulsions than running and tonic ones in audiogenically kindled rats. It is supposed that this selective anticonvulsive effect of VGB results from different sensitivities of forebrain and brainstem epileptic networks to the presumed GABA enhancement.

Pharmacological and behavioral characteristics of interactions between vigabatrin and conventional antiepileptic drugs in pentylenetetrazole-induced seizures in mice: an isobolographic analysis

To characterize the anticonvulsant effects and types of interactions exerted by mixtures of vigabatrin (VGB) and conventional antiepileptic drugs (valproate (VPA), ethosuximide (ESM), phenobarbital (PB), and clonazepam (CZP)) in pentylenetetrazole (PTZ)-induced seizures in mice, the isobolographic analysis for three fixed-ratio combinations of 1 : 3, 1 : 1, and 3 : 1 was used. The adverse-effect profile of the combinations tested, at the doses corresponding to their median effective doses (ED(50)) at the fixed-ratio of 1 : 1 against PTZ-induced seizures, was determined by the chimney (motor performance), step-through passive avoidance (long-term memory), pain threshold (pain sensitivity), and Y-maze (general explorative locomotor activity) tests in mice. Additionally, the observed isobolographic interactions were verified in terms of a pharmacokinetic interaction existence. VGB combined with PB or ESM exerted supra-additive (synergistic) interactions against the clonic phase of PTZ-induced seizures, which was associated with the increment of PB or ESM concentrations in the brains of examined animals. The remaining combinations tested (ie VGB+VPA and VGB+CZP) occurred additive in the PTZ test, which was associated with no significant changes in the brain concentrations of VPA and CZP. None of the examined combinations exerted motor impairment in the chimney test in mice. In the standard variant of passive avoidance task (current of 0.6 mA; 2 s of stimulus duration), the combinations of VGB+CZP and VGB+VPA significantly affected long-term memory in mice. Moreover, VGB in a dose-dependent manner lengthened the latency to the first pain reaction in the pain threshold test in mice. The modified variant of step-through passive avoidance task (current of 0.6 mA; stimulus duration based on the latency from the pain threshold test) revealed no significant changes in the long-term memory of animals for the combinations of VGB+VPA and VGB+CZP; so the observed effects in the standard variant of passive avoidance task were a result of the antinociceptive effects produced by VGB. In the Y-maze test, VGB also, in a dose-dependent manner, increased the general explorative locomotor activity of the animals tested. Similarly, the total number of arm entries in the Y-maze was significantly increased for the combinations of VGB+CZP and VGB+ESM, but not for VGB+PB and VGB+VPA. The application of VGB in combination with PB, ESM, CZP, and VPA suppressed the clonic phase of PTZ-induced seizures, having no harmful or deleterious effects on behavioral functioning of the animals tested, which might be advantageous in further clinical practice.

Inhibition of GABA release by presynaptic ionotropic GABA receptors in hippocampal CA3

Vesicular transmitter release can be regulated by transmitter-gated ion channels at presynaptic axon terminals. The central inhibitory transmitter GABA acts on such presynaptic ionotropic receptors in various cells, including inhibitory interneurons. Here we report that GABA-mediated postsynaptic inhibitory currents in CA3 pyramidal cells of rat hippocampal slices are suppressed by agonists of GABAA receptors. The effect is present for both stimulus-induced and miniature IPSCs, indicating a reduction in the probability of vesicular release by presynaptic, action-potential-independent mechanisms. We conclude that the release of GABA from hippocampal CA3 interneurons is regulated by a negative feedback via presynaptic ionotropic GABA autoreceptors.

Antiepileptic drugs in neuroprotection

Antiepileptic drugs (AEDs) are designed to prevent and suppress seizure activity. Their effects on calcium influx and molecular cascades contributing to necrotic and apoptotic neuronal death, however, suggests that they have functions other than just suppression of excitability. The neuroprotective effects of 20 AEDs currently in use or being investigated in Phase II - III clinical trials for treatment of epilepsy are reviewed. Data analyses is complicated by several factors. Firstly, the available data on the neuroprotective effects of different AEDs varies largely. Secondly, most of the evidence demonstrating neuroprotective effects comes from stroke models and it is uncertain whether these data can be extrapolated to other conditions, such as status epilepticus (SE) or traumatic brain injury. Thirdly, data obtained in adult animals cannot be extrapolated to young animals without caution. For example, AEDs protecting adult brain from stroke or SE-induced injury can cause apoptosis in immature brain. Finally, data comparison is complicated by the variability in study designs and methodologies between studies. With these caveats in mind, an analysis of the available data suggests that AEDs with different mechanisms of action can have mild-to-moderate neuroprotective effects. It is difficult, however, to associate the neuroprotective effects with a favourable functional outcome. For example, it is difficult to conclude that administration of AEDs during the latency phase would have an effect on the molecular cascades underlying epileptogenesis. The few favourable data demonstrating a decrease in the incidence of epilepsy after SE are probably related to the administration of AEDs during SE, which resulted in modification/alleviation of the insult itself and consequently, reduced its epileptogenecity. These experimental data, however, are clinically important because they show that early intervention of SE has an effect on long-term functional outcome. These observations emphasise the need to use additional outcome measures, such as markers of normal development or cognitive performance, when the benefits of neuroprotection achieved by the use of neuroprotective AEDs are assessed.

Biphasic action of vigabatrin on cortical epileptic after-discharges in rats

The time course of the anticonvulsant effect of vigabatrin against cortically induced epileptic after-discharges (ADs) was studied in freely moving rats with implanted electrodes. Adult rats (n=30) were exposed to five stimulation sessions each consisting of six stimulation series at 20-min intervals. The first session was a control one, then two groups of animals (n=10 each) were given vigabatrin (600 or 1,200 mg/kg i.p.), the control animals received physiological saline. Stimulation sessions were repeated 1, 24, 48, and 96 hours after the injection. Control animals exhibited an increased transition from the spike-and-wave type of AD to the second, "limbic" type and an increased intensity of movements accompanying stimulation. ADs in the second and subsequent sessions were, however, shorter than in the first session. Vigabatrin facilitated the transition to the second type of AD 1 h after administration but suppressed this transition as well as decreased the number of stimulations eliciting ADs 48 h later. AD duration and the severity of clonic seizures accompanying spike-and-wave ADs were influenced similarly. The effects of the lower dose of vigabatrin were more marked than those of the higher dose. The biphasic action of vigabatrin in our model might be due either to uneven changes of GABA concentration in different brain structures or to an additional mechanism of action. Our results in a cortical model of seizure demonstrate that the sequence of pro- and anticonvulsant actions of vigabatrin is not restricted to seizures of limbic origin and might represent a general phenomenon.

The SLC32 transporter, a key protein for the synaptic release of inhibitory amino acids

The SLC32 family comprises a single member: the vesicular inhibitory amino acid transporter (VIAAT) or vesicular GABA transporter (VGAT). It belongs to a eukaryotic-specific superfamily of H(+)-coupled amino acid transporters, which also comprises the mammalian SLC36 and SLC38 transporters. VIAAT exchanges GABA or glycine for protons. It is present on synaptic vesicles of GABAergic and glycinergic neurons, and in some endocrine cells, where it ensures the H(+)-ATPase-driven uptake, and subsequent exocytotic release, of inhibitory amino acids. Despite a similar function in vesicular neurotransmitter loading, VIAAT is not related to the vesicular glutamate transporter (VGLUT, SLC17) or the vesicular monoamine transporter/vesicular acetylcholine transporter (VMAT/VACHT, SLC18) proteins.

The central piriform cortex: anatomical connections and anticonvulsant effect of gaba elevation in the kindling model

The piriform cortex (PC) is thought to be critically involved in the generation and propagation of forebrain (limbic type) seizures in the rat. The PC extends over a large area at the ventrolateral side of the rat brain with an anterior part highly sensitive for bicuculline-induced and a central part most sensitive for electrically induced seizures. Therefore, distinct parts of the PC might be differentially involved in the generation and spread of seizure activity. Since previous studies indicated that a loss of GABAergic inhibition in the PC is involved in the generation of epileptic activity, we microinjected the GABA-transaminase blocker vigabatrin bilaterally in the anterior, central and posterior PC of previously amygdala-kindled rats and repeatedly tested its effect on kindled seizures. Vigabatrin was anticonvulsant in all groups for up to 13 days with a maximal effect 24 h after injection. However, the anticonvulsant effect on seizure generalization was strongest after microinjection in the central PC suggesting that GABAergic synapses in this part are critically involved in the development of generalized seizures. Since differences in anatomical connections of the PC regions may be responsible for differences in seizure susceptibility, we addressed this question by injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin in different PC subregions. Although there were similarities in the projections from different PC subregions, we also found differences between the PC subregions in their projections to structures known to be important in the limbic seizure network, such as the perirhinal cortex, nucleus accumbens, and striatum. These differences in anatomical connectivity between PC subregions may be involved in the differences in seizure susceptibility observed in the present and previous studies.

Transmitter Metabolism as a Mechanism of Synaptic Plasticity: A Modeling Study

The nervous system adapts to experience by changes in synaptic strength. The mechanisms of synaptic plasticity include changes in the probability of transmitter release and in postsynaptic responsiveness. Experimental and neuropharmacological evidence points toward a third variable in synaptic efficacy: changes in presynaptic transmitter concentration. Several groups, including our own, have reported changes in the amplitude and frequency of postsynaptic (miniature) events indicating that alterations in transmitter content cause alterations in vesicular transmitter content and vesicle dynamics. It is, however, not a priori clear how transmitter metabolism will affect vesicular transmitter content and how this in turn will affect pre- and postsynaptic functions. We therefore have constructed a model of the presynaptic terminal incorporating vesicular transmitter loading and the presynaptic vesicle cycle. We hypothesize that the experimentally observed synaptic plasticity after changes in transmitter metabolism puts predictable restrictions on vesicle loading, cytoplasmic–vesicular transmitter concentration gradient, and on vesicular cycling or release. The results of our model depend on the specific mechanism linking presynaptic transmitter concentration to vesicular dynamics, that is, alteration of vesicle maturation or alteration of release. It also makes a difference whether differentially filled vesicles are detected and differentially processed within the terminal or whether vesicle filling acts back onto the terminal by presynaptic autoreceptors. Therefore, the model allows one to decide, at a given synapse, how transmitter metabolism is linked to presynaptic function and efficacy.

Vigabatrin induces tonic inhibition via GABA transporter reversal without increasing vesicular GABA release

Two forms of GABAergic inhibition coexist: fast synaptic neurotransmission and tonic activation of GABA receptors due to ambient GABA. The mechanisms regulating ambient GABA have not been well defined. Here we examined the role of the GABA transporter in the increase in ambient [GABA] induced by the anticonvulsant vigabatrin. Pretreatment of cultured rat hippocampal neurons with vigabatrin (100 microM) for 2-5 days led to a large increase in ambient [GABA] that was measured as the change in holding current induced by bicuculline during patch-clamp recordings. In contrast, there was a decrease in the frequency of spontaneous miniature inhibitory postsynaptic currents mIPSCs with no change in their amplitude distribution, and a decrease in the magnitude of IPSCs evoked by presynaptic stimulation during paired recordings. The increase in ambient [GABA] was not prevented by blockade of vesicular GABA release with tetanus toxin or removal of extracellular calcium. During perforated patch recordings, the increase in ambient [GABA] was prevented by blocking the GABA transporter, indicating that the GABA transporter was continuously operating in reverse and releasing GABA. In contrast, blocking the GABA transporter increased ambient [GABA] during whole cell patch-clamp recordings unless GABA and Na(+) were added to the recording electrode solution, indicating that whole cell recordings can lead to erroneous conclusions about the role of the GABA transporter in control of ambient GABA. We conclude that the equilibrium for the GABA transporter is a major determinant of ambient [GABA] and tonic GABAergic inhibition. We propose that fast GABAergic neurotransmission and tonic inhibition can be independently modified and play complementary roles in control of neuronal excitability.

Efficacy of current antiepileptics to prevent neurodegeneration in epilepsy models

Results of experiments performed in animal epilepsy models and human epilepsy during the past decade indicate that the epileptic brain is not a stable neuronal network, but undergoes modifications caused by the underlying etiology and/or recurrent seizures. In many forms of epilepsy, such as temporal lobe epilepsy, the underlying etiologic factor triggers a cascade of events (epileptogenesis) leading to spontaneous seizures and cognitive decline. In some patients, the condition progresses, due in part to recurrent seizures. The current treatment of epilepsy focuses exclusively on preventing or suppressing seizures, which are symptoms of the underlying disease. Now, however, we are beginning to understand the underlying neurobiology of the epileptic process, as well as factors that might predict the risk of progression in individual patients. Thus, there are new opportunities to develop neuroprotective and antiepileptogenic treatments for patients who, if untreated, would develop drug-refractory epilepsy associated with cognitive decline. These treatments might improve the long-term outcome and quality-of-life of patients with epilepsy. Here we review the available data regarding the neuroprotective effects of antiepileptic drugs (AEDs) at different phases of the epileptic process. Analysis of published data suggests that initial-insult modification and prevention of the progression of seizure-induced damage are candidate indications for treatment with AEDs. An understanding of the molecular mechanisms underlying the progression of epileptic process will eventually show what role AEDs have in the neuroprotective and antiepileptogenic treatment regimen.

Vigabatrin directed against kindled seizures following cortical insult: impact on epileptogenesis and somatosensory recovery

The anticonvulsant drug vigabatrin has not been found to be detrimental to the recovery process when administered following focal cortical insult. This finding is in contrast to the negative postinjury consequences of other anticonvulsant drugs (e.g., phenobarbital and diazepam) with more direct activation of the GABA/benzodiazepine receptor complex. Moreover, phenobarbital directed against kindled seizures affects functional recovery more adversely than either the drug or subconvulsive seizures alone. The purpose of the present study was to determine whether vigabatrin (150, 200, and 250 mg/kg) directed against kindled seizures would impact recovery from lesion-induced somatosensory deficits. Vigabatrin was coupled with daily electrical kindling of the amygdala during the first week after a unilateral anteromedial cortex (AMC) lesion. Somatosensory recovery was assessed using bilateral tactile stimulation tests. Animals receiving the highest dose of vigabatrin prior to electrical kindling (250 mg/kg vigabatrin/kindled) remained significantly impaired even after two months of testing relative to vehicle/kindled, kindled/250 mg/kg vigabatrin, which received vigabatrin after electrical kindling, and the 150, 200, and 250 mg/kg vigabatrin/nonkindled groups (p < 0.0001). In contrast, neither vigabatrin (at any of the doses tested) nor subconvulsive kindled seizures impacted the recovery process (p > 0.05) when administered alone (i.e., without the drug + seizure interaction). These data add to the accumulating experimental and clinical evidence suggesting that the neurobehavioral consequences of the interaction between anticonvulsant drugs and subclinical seizures after brain insult are detrimental to functional recovery.

Vigabatrin protects against hippocampal damage but is not antiepileptogenic in the lithium-pilocarpine model of temporal lobe epilepsy

In temporal lobe epilepsy (TLE), the nature of the structures involved in the development of the epileptogenic circuit is still not clearly identified. In the lithium-pilocarpine model, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as well as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). In order to determine whether protection of some brain areas could prevent the epileptogenesis induced by status epilepticus (SE) and to identify the cerebral structures involved in the genesis of TLE, we studied the effects of the chronic exposure to Vigabatrin (gamma-vinyl-GABA, GVG) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE. The animals were subjected to SE and GVG treatment (250 mg/kg) was initiated at 10 min after pilocarpine injection and maintained daily for 45 days. These pilo-GVG rats were compared with rats subjected to SE followed by a daily saline treatment (pilo-saline) and to control rats not subjected to SE (saline-saline). GVG treatment induced a marked, almost total neuroprotection in CA3, an efficient protection in CA1 and a moderate one in the hilus of the dentate gyrus while damage in the entorhinal cortex was slightly worsened by the treatment. All pilo-GVG and pilo-saline rats became epileptic after the same latency. Glutamic acid decarboxylase (GAD67) immunoreactivity was restored in pilo-GVG rats compared with pilo-saline rats in all areas of the hippocampus, while it was increased over control levels in the optical layer of the superior colliculus and the substantia nigra pars reticulata. Thus, the present data indicate that neuroprotection of principal cells in the Ammon's horn of the hippocampus is not sufficient to prevent epileptogenesis, suggesting that the hilus and extra-hippocampal structures, that were not protected in this study, may play a role in the genesis of spontaneous recurrent seizures in this model. Furthermore, the study performed in non-epileptic rats indicates that chronic treatment with a GABAmimetic drug upregulates the expression of the protein GAD67 in specific areas of the brain, independently from the seizures.

Single systemic dose of vigabatrin induces early proconvulsant and later anticonvulsant effect in rats

Vigabatrin (VGB), an inhibitor of gamma-aminobutyric acid-aminotransferase, exhibits an antiepileptic effect but several studies indicate that its effect may be biphasic. A time course of an effect of a single injection of VGB on hippocampal epileptic afterdischarges (AD) elicited by an electric stimulation of the angular bundle was examined in adult rats with chronically implanted electrodes. VGB (600 or 1200 mg/kg intraperitoneally) proved to be an efficient anticonvulsant in the intervals of 24 and 48 h--duration of ADs was shortened and behavioral phenomena were less intense. In contrast, ADs were lengthened 4 h after administration. The biphasic effect of VGB was demonstrated, the initial proconvulsant effect might be due to a different onset of VGB action in individual brain structures, but an additional mechanism of action cannot be excluded.

Plasticity of rat central inhibitory synapses through GABA metabolism

1. The production of the central inhibitory transmitter GABA (gamma-aminobutyric acid) varies in response to different patterns of activity. It therefore seems possible that GABA metabolism can determine inhibitory synaptic strength and that presynaptic GABA content is a regulated parameter for synaptic plasticity. 2. We altered presynaptic GABA metabolism in cultured rat hippocampal slices using pharmacological tools. Degradation of GABA by GABA-transaminase (GABA-T) was blocked by gamma-vinyl-GABA (GVG) and synthesis of GABA through glutamate decarboxylase (GAD) was suppressed with 3-mercaptopropionic acid (MPA). We measured miniature GABAergic postsynaptic currents (mIPSCs) in CA3 pyramidal cells using the whole-cell patch clamp technique. 3. Elevated intra-synaptic GABA levels after block of GABA-T resulted in increased mIPSC amplitude and frequency. In addition, tonic GABAergic background noise was enhanced by GVG. Electron micrographs from inhibitory synapses identified by immunogold staining for GABA confirmed the enhanced GABA content but revealed no further morphological alterations. 4. The suppression of GABA synthesis by MPA had opposite functional consequences: mIPSC amplitude and frequency decreased and current noise was reduced compared with control. However, we were unable to demonstrate the decreased GABA content in biochemical analyses of whole slices or in electron micrographs. 5. We conclude that the transmitter content of GABAergic vesicles is variable and that postsynaptic receptors are usually not saturated, leaving room for up-regulation of inhibitory synaptic strength. Our data reveal a new mechanism of plasticity at central inhibitory synapses and provide a rationale for the activity-dependent regulation of GABA synthesis in mammals.

Paradoxical reduction of synaptic inhibition by vigabatrin

GABAergic inhibition, a primary target for pharmacological modulation of excitability in the CNS, can be altered by multiple mechanisms including alteration of GABA metabolism. Gamma-vinyl GABA (vigabatrin, GVG) is an irreversible inhibitor of the GABA catabolic enzyme GABA transaminase, thus its anticonvulsant properties are thought to result from an elevation of brain GABA levels. We examined the effects of GVG on GABAergic synaptic transmission in hippocampal slices. GVG unexpectedly reduced miniature and evoked inhibitory postsynaptic currents (IPSCs) in dentate granule cells. The reduction in synaptic events was accompanied by an increase in tonic GABA(A) receptor-mediated current. These effects developed slowly and persisted following wash out of GVG. The GVG pretreatment reduced sucrose-evoked GABA release as well as postsynaptic sensitivity to exogenous GABA, indicating that both pre- and postsynaptic mechanisms contributed to the reduction in synaptic currents. These results suggest that tonic rather than phasic increases in GABA underlie the anticonvulsant properties of GVG, and that mechanisms that elevate brain neurotransmitter levels do not necessarily correlate with enhanced synaptic release.

The alteration of gamma-aminobutyric acid-transaminase expression in the gerbil hippocampus induced by seizure

It is well established that GABA degradation may play a key role in epileptogenesis. However, whether or not the expression of GABA-transaminase (GABA-T), which catalyzes GABA degradation and participates in the neuronal metabolism via GABA shunt, changes chronologically after on-set of seizure remains to be clarified. To identify the change of GABA-T expression in seizure, GABA-T expression in the gerbil hippocampus, associated with different sequelae of spontaneous seizures, was investigated. The distribution pattern of GABA-T immunoreactive neurons in the hippocampus between the seizure-resistant and pre-seizure group of seizure sensitive gerbils was similar. Interestingly, at 30 min postictal, the enhancement of GABA-T immunoreactivity in the perikarya was apparently observed. This contrasted with the decline in GABA-T immunoreactivity in the granular and pyramidal layer. At 12-24 h postictal, GABA-T immunoreactivity in the hilar neurons had declined significantly. However, the GABA-T immunoreactivity in the granular layer increased. These findings suggest that in the gerbil, the alteration in GABA-T expressions may play an important role in the self-recovery mechanism from seizure attack via both GABA degradation and regulation of neuronal metabolism.

Acute effects of gamma-vinyl-GABA on low-magnesium evoked epileptiform activity in vitro

Vigabatrin (gamma-vinyl-GABA, VGB) is a gamma-aminobutyric acid (GABA) derivative designed to boost synaptic inhibition by inhibiting the degradation of GABA in brain tissue. Indeed, VGB shows potent anti-convulsant activity in animal models of epilepsy and in humans with complex partial seizures. However, details of the mechanism of action of VGB are not well understood and the systemic effects include possible pro-convulsant actions. We therefore analysed the effects of VGB in rat brain slices in the low-Mg(2+) model in vitro. VGB at 100 microM-5 mM showed a concentration- and time-dependent reduction of interictal-like events in the hippocampal CA1 region. Likewise, VGB suppressed epileptiform discharges in the medial entorhinal cortex (mEC), which are known to resist conventional anti-convulsants. In contrast, evoked population spikes in CA1 (which became repetitive after washout Mg(2+)) were not altered by VGB. Our data show that VGB is efficient against epileptiform discharges in temporal structures including pharmacoresistant patterns of activity. The waveform of evoked population spikes in this in vitro model is no indicator for the anti-convulsant properties of drugs.

Reversal of the activity-dependent suppression of GABA-mediated inhibition in hippocampal slices from gamma-vinyl GABA (vigabatrin)-pretreated rats

The antiepileptic drug, gamma-vinyl GABA (GVG, vigabatrin), is an irreversible inhibitor of GABA-transaminase, the enzyme responsible for the breakdown of GABA. In hippocampal slices prepared from rats pretreated with either an anticonvulsant dose of GVG (1500 mg/kg) or saline, electrophysiological recordings were performed in order to examine the effects of GVG pretreatment on GABAergic neurotransmission. Although GVG had no effect on the effectiveness of GABA-mediated inhibition when elicited by a single stimulus, it reversed the activity-dependent depression of inhibition which is typically observed when inhibitory pathways are activated repetitively by a train of stimuli delivered at low frequency. Similarly, GVG pretreatment prevented the progressive decline in the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) during low-frequency stimulation of inhibitory interneurons. Thus, in slices from GVG pretreated rats, the amplitudes of both the fast and slow components of the last of a series of IPSPs evoked by a 5 Hz, 4 s train were maintained at 91.5 +/- 6.6% and 87.7 +/- 6.5%, respectively, compared to 61.1 +/- 3.9% and 57.1 +/- 5.0% in control slices. Finally, in slices from GVG pretreated rats, we observed a reduction in the ability of the GABA(B) receptor agonist, baclofen, to decrease the amplitude of monosynaptic inhibitory postsynaptic currents. These results suggest that GVG may produce its frequency-dependent actions by reducing the function of release regulating presynaptic GABA(B) autoreceptors. The frequency-dependent reinforcement of inhibition by GVG may importantly contribute to the anticonvulsant effectiveness of this compound.

Vigabatrin and tiagabine are pharmacologically different drugs. A pre-clinical study

In light of theirclosely related mechanisms of action, and preliminary clinical evidence suggesting that they possess similar efficacies, it has been anecdotally suggested that vigabatrin and tiagabine may prove to be therapeutically indistinguishable. As a result, we have conducted a preclinical comparison of their anticonvulsant profile and mechanism of action. Pentylenetetrazol and maximal electroshock seizures were employed to determine the experimental anticonvulsant profile. Mechanisms of action were investigated using assays of gamma -aminobutyric acid (GABA), GABA-transaminase and glutamic acid decarboxylase in mouse brain and GABA uptake and GABA-transaminase in rat astrocyte cultures. Vigabatrin was without effect on either pentylenetetrazol- or maximal electroshock-induced seizures, whereas tiagabine increased the latency to pentylenetetrazol seizures and reduced the incidence of maximal electroshock seizures. In mouse brain assays, tiagabine was without effect, while vigabatrin increased GABA concentrations and reduced GABA-transaminase and glutamic acid decarboxylase activities. In cortical astrocyte cultures, vigabatrin reduced the activities of both GABA uptake and GABA-transaminase, whereas tiagabine blocked GABA uptake alone. These results suggest that vigabatrin and tiagabine have differing efficacy in experimental seizure models and distinct neurochemical effects. It is possible, then, that these drugs will have different spectra of activity and toxicity profiles in human epilepsy.

Inhibition of uptake, steady-state currents, and transient charge movements generated by the neuronal GABA transporter by various anticonvulsant drugs

1. We have expressed the GABA transporter (GAT1) of mouse brain in Xenopus oocytes and have investigated the effects of four antiepileptic drugs, tiagabine (TGB), vigabatrin (VGB), gabapentin (GBP) and valproate (VAL), on GAT1 transporter function by measurements of 3H-labelled GABA uptake and GAT1-mediated currents. 2. Not only TGB, a well-known inhibitor of GAT1-mediated transport, but also the other drugs efficiently inhibit the uptake of [3H]-GABA by GAT1. Inhibition at 50% is obtained for VGB, TGB, GBP, and VAL at concentrations of about 1 nM, 1 microM, 50 microM and 100 microM, respectively. 3. However, GAT1-mediated steady-state and transient currents are nearly unaffected by VGB, GBP, and VAL at even five times higher concentrations. Only TGB blocks the uptake and steady-state and transient currents at micromolar concentrations. 4. VGB exhibits a complex interaction with GAT1; at concentrations about 1 nM, the inhibition of uptake is released, but at millimolar concentrations the uptake is inhibited again, and also the GAT1-mediated current is finally inhibited at these concentrations with a KI value of 0.5 mM. The concentration dependency of inhibition of uptake can be explained by two interaction sites with different affinities, a blocking site and a transport site. 5. The differences in effects of VAL, GBP, and VGB on uptake and currents can be attributed to the fact that GAT1 has the capability to operate in an electrogenic mode without uptake of GABA. We suggest that inhibition occurs only when GAT1 operates in the GABA-uptake mode. 6. The inhibition of GABA uptake by these four drugs will result in an elevation of the GABA concentration in the synaptic cleft, which will enhance synaptic inhibition and thereby contribute to their antiepileptic effects.

Lamotrigine childhood overdose

A 2-year-old male developed generalized tonic-clonic seizure activity, tremor of limbs, muscle weakness, ataxia, and hypertonia after he swallowed 16 50-mg tablets of lamotrigine. His vital signs were normal, as were electroencephalography and laboratory investigation tests. The urine toxicologic screen revealed no other drugs. Treatment included midazolam and gastric lavage followed by activated charcoal and fluid loads. Symptoms resolved within 24 hours, and the child was discharged without any further complications. Serial blood samples revealed plasma lamotrigine levels at the high adult therapeutic range (3.8 mg/L) but a slow elimination rate. This is the first report of seizure activity reported in a patient receiving an overdose of lamotrigine. However, no evident concentration-effect-side-effect relationship has been established in children. Interestingly in this child, lamotrigine overdose presented exclusively with treatment-emergent neurologic abnormalities, sparing all other systems.

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

Comparison of the preclinical anticonvulsant profiles of tiagabine, lamotrigine, gabapentin and vigabatrin

Tiagabine is a novel antiepileptic drug which has clinical efficacy against complex refractory and myoclonic seizures. The anticonvulsant mechanism of action of tiagabine results from its blockade of neuronal and glial GABA-uptake, thereby increasing GABA levels in the synaptic cleft. Here we present a comparison of the preclinical anticonvulsant profile of tiagabine with that of lamotrigine, gabapentin and vigabatrin in the following tests (all antiepileptic drugs were administered i.p.): seizures induced by pentylentetrazol (PTZ), 6,7-dimethoxy-4-ethyl-b-carboline-3-carboxylate (DMCM) and maximal electroshock (MES); sound induced seizures in DBA/2 mice and finally acute amygdala kindled seizures. Tiagabine was the most potent drug in antagonizing tonic convulsions induced by PTZ, DMCM and sound induced seizures in DBA/2 mice with ED50 values of 2, 2 and 1 mumol/kg, respectively, followed by lamotrigine with ED50 values of 9, 43 and 6 mumol/kg, respectively. Gabapentin and vigabatrin had ED50 values in the same tests of 185, 452, 66 mumol/kg and 2322, > 7740, 3883 mumol/kg, respectively. Tiagabine was the only drug capable of blocking PTZ-induced clonic convulsions (ED50 = 5 mumol/kg), an effect seen at low but not high doses of tiagabine. Lamotrigine was the only drug which antagonized tonic convulsions in the MES test (ED50 = 36 mumol/kg). Therapeutic index (TI) of antiepileptic drugs in NMRI- and DBA/2-mice ranked with decreasing TI lamotrigine > gabapentin > vigabatrin > tiagabine. All drugs reduced the generalized seizures in amygdala kindled rats, but tiagabine and gabapentin furthermore attenuated afterdischarge duration of amygdala kindled seizures. However, an ED50 value against amygdala kindled focal seizures was only obtained for tiagabine (36 mumol/kg). The data here presented show that tiagabine, lamotrigine, gabapentin and vigabatrin possess different preclinical anticonvulsant profiles which is of relevance to the clinical anticonvulsant profiles of the drugs.

Neurochemical actions of vigabatrin and tiagabine alone and in combination in mouse cortex

1. The effects of repeated administration of the anticonvulsant compounds, vigabatrin (VGB) and tiagabine (TGB), on gamma-aminobutyric acid (GABA) concentration and the activities of GABA-transaminase (GABA-T) and glutamic acid decarboxylase (GAD) were investigated in mouse cortex. 2. VGB alone increased GABA levels and decreased GABA-T and GAD activities. 3. TGB alone was essentially without effect. 4. Low doses of VGB and TGB in combination increased GABA levels when neither drug had such an effect alone. 5. Despite this observation, this study failed to establish any conclusive evidence for an interaction between VGB and TGB that might help to explain their reported clinical synergism.

Effects of tiagabine and vigabatrin on GABA uptake into primary cultures of rat cortical astrocytes

Tiagabine (TGB) and vigabatrin (VGB) are two novel anticonvulsant compounds reported to exert their pharmacological effects via an action on the gamma-aminobutyric acid (GABA) system. We have investigated the effects of acute exposure of these drugs on the uptake of GABA into rat cortical astrocytes in primary culture. Astrocytes were prepared from the cerebral cortices of one day-old rat pups by a mechanical dissociation technique and were assayed for GABA uptake activity after 21 days in culture. Tiagabine (100-300 nM) and VGB (100 microM) reduced GABA uptake when compared to control at four hours post-exposure. GABA uptake was also reduced following eight and 24 hour exposures to 200 nM TGB. A combination of TGB (200 nM) and VGB (100 microM) treatments reduced GABA uptake when compared to both control and VGB treated cultures. These results support the efficacy of TGB as a GABA uptake inhibitor and suggest that VGB may also exert an effect by this mechanism.

The novel antiepileptic drug levetiracetam (ucb L059) appears to act via a specific binding site in CNS membranes

Levetiracetam ((S)-alpha-ethyl-2-oxo-pyrrolidine acetamide, ucb L059) is a novel potential antiepileptic agent presently in clinical development with unknown mechanism of action. The finding that its anticonvulsant activity is highly stereoselective (Gower et al., 1992) led us to investigate the presence of specific binding sites for [3H]levetiracetam in rat central nervous system (CNS). Binding assays, performed on crude membranes, revealed the existence of a reversible, saturable and stereoselective specific binding site. Results obtained in hippocampal membranes suggest that [3H]levetiracetam labels a single class of binding sites (nH = 0.92 +/- 0.06) with modest affinity (Kd = 780 +/- 115 nM) and with a high binding capacity (Bmax = 9.1 +/- 1.2 pmol/mg protein). Similar Kd and Bmax values were obtained in other brain regions (cortex, cerebellum and striatum). ucb L060, the (R) enantiomer of levetiracetam, displayed about 1000 times less affinity for these sites. The binding of [3H]levetiracetam is confined to the synaptic plasma membranes in the central nervous system since no specific binding was observed in a range of peripheral tissues including heart, kidneys, spleen, pancreas, adrenals, lungs and liver. The commonly used antiepileptic drugs carbamazepine, phenytoin, valproate, phenobarbital and clonazepam, as well as the convulsant agent t-butylbicyclophosphorothionate (TBPS), picrotoxin and bicuculline did not displace [3H]levetiracetam binding. However, ethosuximide (pKi = 3.5 +/- 0.1), pentobarbital (pKi = 3.8 +/- 0.1), pentylenetetrazole (pKi = 4.1 +/- 0.1) and bemegride (pKi = 5.0 +/- 0.1) competed with [3H]levetiracetam with pKi values comparable to active drug concentrations observed in vivo. Structurally related compounds, including piracetam and aniracetam, also displaced [3H]levetiracetam binding. (S) Stereoisomer homologues of levetiracetam demonstrated a rank order of affinity for [3H]levetiracetam binding in correlation with their anticonvulsant activity in the audiogenic mouse test (r2 = 0.84, n = 12, P < 0.0001). These results support a possible role of this binding site in the anticonvulsant activity of levetiracetam and substantiate the singular pharmacological profile of this compound. This site remains however to be further characterised.

A review of the preclinical pharmacology of tiagabine: a potent and selective anticonvulsant GABA uptake inhibitor

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.

Acute effects of gamma-vinyl GABA (vigabatrin) on hippocampal GABAergic inhibition in vitro

The acute effects of gamma-vinyl-GABA (GVG) on GABAergic inhibition were investigated in the hippocampal slice preparation using the paired-pulse test of inhibition during extracellular recordings. Superfusion of GVG (100-500 microM) for 60 min resulted in a concentration-dependent decrease in GABAergic inhibition. Slices superfused with higher concentrations of GVG (0.5-1 mM) were hyperexcitable as demonstrated by the appearance of multiple spikes. Binding studies showed that GVG (1 mM) had no effect on the binding of [3H]flunitrazepam or [3H]TBOB and displaced no more than 15% of specific [3H]GABA binding, which indicates that GVG-induced disinhibition is not mediated through an action at the GABAA receptor complex. Consistent with this suggestion is the finding that GVG (500 microM) had little effect on the inhibition of the orthodromically evoked CA1 population spike produced by the GABAA receptor agonist muscimol (10 microM), whereas this inhibition was considerably attenuated by the GABAA receptor antagonist, bicuculline methiodide (5 microM). The results of this study suggest that the acute actions of GVG on the GABAergic neurotransmitter system are not involved in its anticonvulsant effect.