Valproate enhances GABA turnover in the substantia nigra

Clinical Correlation of Altered Molecular Signatures in Epileptic Human Hippocampus and Amygdala

Widespread alterations in the expression of various genes could contribute to the pathogenesis of epilepsy. The expression levels of various genes, including major inhibitory and excitatory receptors, ion channels, cell type-specific markers, and excitatory amino acid transporters, were assessed and compared between the human epileptic hippocampus and amygdala, and findings from autopsy controls. Moreover, the potential correlation between molecular alterations in epileptic brain tissues and the clinical characteristics of patients undergoing epilepsy surgery was evaluated. Our findings revealed significant and complex changes in the expression of several key regulatory genes in both the hippocampus and amygdala of patients with intractable epilepsy. The expression changes in various genes differed considerably between the epileptic hippocampus and amygdala. Different correlation patterns were observed between changes in gene expression and clinical characteristics, depending on whether the patients were considered as a whole or were subdivided. Altered molecular signatures in different groups of epileptic patients, defined within a given category, could be viewed as diagnostic biomarkers. Distinct patterns of molecular changes that distinguish these groups from each other appear to be associated with epilepsy-specific functional consequences.

Long-lasting antiseizure effects of chronic intrasubthalamic convection-enhanced delivery of valproate

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN.

Neuronal imbalance of excitation and inhibition in schizophrenia: a scoping review of gamma-band ASSR findings

Recent empirical findings suggest that altered neural synchronization, which is hypothesized to be associated with an imbalance of excitatory (E) and inhibitory (I) neuronal activities, may underlie a core pathophysiological mechanism in patients with schizophrenia. The auditory steady-state response (ASSR) examined by electroencephalography (EEG) and magnetoencephalography (MEG) has been proposed as a potential biomarker for evaluating altered neural synchronization in schizophrenia. For this review, we performed a comprehensive literature search for papers published between 1999 and 2021 examining ASSRs in patients with schizophrenia. Almost all EEG-ASSR studies reported gamma-band ASSR reductions, especially to 40-Hz stimuli both in power and/or phase synchronization in chronic and first-episode schizophrenia. In addition, similar to EEG-ASSR findings, MEG-ASSR deficits to 80-Hz stimuli (high gamma) have been reported in patients with schizophrenia. Moreover, the 40-Hz ASSR is likely to be a predictor of the onset of schizophrenia. Notably, increased spontaneous (or ongoing) broadband (30-100 Hz) gamma power has been reported during ASSR tasks, which resembles the increased spontaneous gamma activity reported in animal models of E/I imbalance. Further research on ASSRs and evoked and spontaneous gamma oscillations is expected to elucidate the pathophysiology of schizophrenia with translational implications.

Drug-induced stuttering: A comprehensive literature review

Drug-induced stuttering (DIS) is a type of neurogenic stuttering (NS). Although DIS has not been reported as frequently as other cases of NS in the literature, it is not a negligible adverse drug reaction (ADR) which can significantly affect the quality of life if not treated. This literature review aims to evaluate the epidemiological and clinical characteristics of DIS and suggests some pathophysiological mechanisms for this ADR. Relevant English-language reports in Google Scholar, PubMed, Web of Science, and Scopus were identified and assessed without time restriction. Finally, a total of 62 reports were included. Twenty-seven drugs caused 86 episodes of stuttering in 82 cases. The most episodes of DIS were related to antipsychotic drugs (57%), mostly including clozapine, followed by central nervous system agents (11.6%) and anticonvulsant drugs (9.3%). The majority of the cases were male and between the ages of 31 and 40 years. Repetitions were the most frequent core manifestations of DIS. In 55.8% of the episodes of DIS, the offending drug was withdrawn to manage stuttering, which resulted in significant improvement or complete relief of stuttering in all cases. Based on the suggested pathophysiological mechanisms for developmental stuttering and neurotransmitters dysfunctions involved in speech dysfluency, it seems that the abnormalities of several neurotransmitters, especially dopamine and glutamate, in different circuits and areas of the brain, including cortico-basal ganglia-thalamocortical loop and white matter fiber tracts, may be engaged in the pathogenesis of DIS.

Valproate selectively suppresses adolescent anabolic/androgenic steroid-induced aggressive behavior: implications for a role of hypothalamic γ-aminobutyric acid neural signaling

Pubertal male Syrian hamsters (Mesocricetus auratus) treated with anabolic/androgenic steroids (AASs) during adolescence (P27-P56) display a highly intense aggressive phenotype that shares many behavioral similarities with pathological aggression in youth. Anticonvulsant drugs like valproate that enhance the activity of the γ-aminobutyric acid (GABA) neural system in the brain have recently gained acceptance as a primary treatment for pathological aggression. This study examined whether valproate would selectively suppress adolescent AAS-induced aggressive behavior and whether GABA neural signaling through GABAA subtype receptors in the latero-anterior hypothalamus (LAH; an area of convergence for developmental and neuroplastic changes that underlie aggression in hamsters) modulate the aggression-suppressing effect of this anticonvulsant medication. Valproate (1.0-10.0 mg/kg, intraperitoneal) selectively suppressed the aggressive phenotype in a dose-dependent fashion, with the effective anti-aggressive effects beginning at 5 mg/kg, intraperitoneally. Microinfusion of the GABAA receptor antagonist bicuculline (7.0-700 ng) into the LAH reversed valproate's suppression of AAS-induced aggression in a dose-dependent fashion. At the 70 ng dose of bicuculline, animals expressed the highly aggressive baseline phenotype normally observed in AAS-treated animals. These studies provide preclinical evidence that the anticonvulsant valproate selectively suppresses adolescent, AAS-induced aggression and that this suppression is modulated, in part, by GABA neural signaling within the LAH.

The Auditory Steady-State Response: Electrophysiological Index for Sensory Processing Dysfunction in Psychiatric Disorders

Sensory processing is disrupted in several psychiatric disorders, including schizophrenia, bipolar disorder, and autism spectrum disorder. In this review, we focus on the electrophysiological auditory steady-state response (ASSR) driven by high-frequency stimulus trains as an index for disease-associated sensory processing deficits. The ASSR amplitude is suppressed within the gamma band (≥30 Hz) among these patients, suggesting an imbalance between GABAergic and N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission. The reduced power and synchronization of the 40-Hz ASSR are robust in patients with schizophrenia. In recent years, similar ASSR deficits at gamma frequencies have also been reported in patients with bipolar disorder and autism spectrum disorder. We summarize ASSR abnormalities in each of these psychiatric disorders and suggest that the observed commonalities reflect shared pathophysiological mechanisms. We reviewed studies on phase resetting in which a salient sensory stimulus affects ASSR. Phase resetting induces the reduction of both the amplitude and phase of ASSR. Moreover, phase resetting is also affected by rare auditory stimulus patterns or superimposed stimuli of other modalities. Thus, sensory memory and multisensory integration can be investigated using phase resetting of ASSR. Here, we propose that ASSR amplitude, phase, and resetting responses are sensitive indices for investigating sensory processing dysfunction in psychiatric disorders.

Emerging therapeutic targets for schizophrenia: a framework for novel treatment strategies for psychosis

Introduction: Antipsychotic drugs are central to the treatment of schizophrenia, but their limitations necessitate improved treatment strategies. Multiple lines of research have implicated glutamatergic dysfunction in the hippocampus as an early source of pathophysiology in schizophrenia. Novel compounds have been designed to treat glutamatergic dysfunction, but they have produced inconsistent results in clinical trials. Areas covered: This review discusses how the hippocampus is thought to drive psychotic symptoms through its influence on the dopamine system. It offers the reader an evaluation of proposed treatment strategies including direct modulation of GABA or glutamate neurotransmission or reducing the deleterious impact of stress on circuit development. Finally, we offer a perspective on aspects of future research that will advance our knowledge and may create new therapeutic opportunities. PubMed was searched for relevant literature between 2010 and 2020 and related studies. Expert opinion: Targeting aberrant excitatory-inhibitory neurotransmission in the hippocampus and its related circuits has the potential to alleviate symptoms and reduce the risk of transition to psychosis if implemented as an early intervention. Longitudinal multimodal brain imaging combined with mechanistic theories generated from animal models can be used to better understand the progression of hippocampal-dopamine circuit dysfunction and heterogeneity in treatment response.

Interaction of Sodium Valproate With Low-Frequency Electrical Stimulation During Kindlingn

Introduction

The interaction between antiepileptic drugs and brain electrical stimulation is a potential therapy to control seizures in patients with pharmacoresistance to drugs. So, the present study aimed to design to determine the effect of a subeffective dose of sodium valproate combined with low-frequency electrical stimulation during kindling.

Methods

One tripolar electrode was implanted stereotactically in the CA1 hippocampus of male Wistar rats. One week after surgery, the rats were kindled by electrical stimulation of hippocampus in a rapid manner (12 stimulations/day) for 6 days with sodium valproate alone or combined with low-frequency electrical stimulation (four packages contained 200 monophasic square wave pulses of 0.1-ms duration at 1 Hz, immediately after kindling stimulations). The duration of afterdischarge, maximum latency to stages 4 and 5, and the maximum duration of these stages were recorded by electromadule during kindling.

Results

Application of sodium valproate with low-frequency electrical stimulation caused a reduction in cumulative afterdischarge duration. The maximum latency to the onset of stage 5 seizure increased after sodium valproate application alone, without having a significant effect on the fourth stage. Our findings showed reductions in the seizures duration and increasing in the latency times of both stages after the application of sodium valproate with low-frequency electrical stimulation.

Conclusion

It seems that usage of sodium valproate with low-frequency electrical stimulation during kindling was more effective to suppress the epileptic activity than its administration alone and may have a critical role on the antiepileptic effects of sodium valproate.

From Cannabinoids and Neurosteroids to Statins and the Ketogenic Diet: New Therapeutic Avenues in Rett Syndrome?

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene, being one of the leading causes of mental disability in females. Mutations in the MECP2 gene are responsible for 95% of the diagnosed RTT cases and the mechanisms through which these mutations relate with symptomatology are still elusive. Children with RTT present a period of apparent normal development followed by a rapid regression in speech and behavior and a progressive deterioration of motor abilities. Epilepsy is one of the most common symptoms in RTT, occurring in 60 to 80% of RTT cases, being associated with worsening of other symptoms. At this point, no cure for RTT is available and there is a pressing need for the discovery of new drug candidates to treat its severe symptoms. However, despite being a rare disease, in the last decade research in RTT has grown exponentially. New and exciting evidence has been gathered and the etiopathogenesis of this complex, severe and untreatable disease is slowly being unfolded. Advances in gene editing techniques have prompted cure-oriented research in RTT. Nonetheless, at this point, finding a cure is a distant reality, highlighting the importance of further investigating the basic pathological mechanisms of this disease. In this review, we focus our attention in some of the newest evidence on RTT clinical and preclinical research, evaluating their impact in RTT symptomatology control, and pinpointing possible directions for future research.

Pharmacotherapy for Refractory and Super-Refractory Status Epilepticus in Adults

Patients with prolonged seizures that do not respond to intravenous benzodiazepines and a second-line anticonvulsant suffer from refractory status epilepticus and those with seizures that do not respond to continuous intravenous anesthetic anticonvulsants suffer from super-refractory status epilepticus. Both conditions are associated with significant morbidity and mortality. A strict pharmacological treatment regimen is urgently required, but the level of evidence for the available drugs is very low. Refractory complex focal status epilepticus generally does not require anesthetics, but all intravenous non-anesthetizing anticonvulsants may be used. Most descriptive data are available for levetiracetam, phenytoin and valproate. Refractory generalized convulsive status epilepticus is a life-threatening emergency, and long-term clinical consequences are eminent. Administration of intravenous anesthetics is mandatory, and drugs acting at the inhibitory gamma-aminobutyric acid (GABA)_A receptor such as midazolam, propofol and thiopental/pentobarbital are recommended without preference for one of those. One in five patients with anesthetic treatment does not respond and has super-refractory status epilepticus. With sustained seizure activity, excitatory N-methyl-d-aspartate (NMDA) receptors are increasingly expressed post-synaptically. Ketamine is an antagonist at this receptor and may prove efficient in some patients at later stages. Neurosteroids such as allopregnanolone increase sensitivity at GABA_A receptors; a Phase 1/2 trial demonstrated safety and tolerability, but randomized controlled data failed to demonstrate efficacy. Adjunct ketogenic diet may contribute to termination of difficult-to-treat status epilepticus. Randomized controlled trials are needed to increase evidence for treatment of refractory and super-refractory status epilepticus, but there are multiple obstacles for realization. Hitherto, prospective multicenter registries for pharmacological treatment may help to improve our knowledge.

Defects at the crossroads of GABAergic signaling in generalized genetic epilepsies

Seizure disorders are very common and affect 3% of the general population. The recurrent unprovoked seizures that are also called epilepsies are highly diverse as to both underlying genetic basis and clinic presentations. Recent genetic advances and sequencing technologies indicate that many epilepsies previously thought to be without known causes, or idiopathic generalized epilepsies (IGEs), are virtually genetic epilepsy as they are caused by genetic variations. IGEs are estimated to account for ∼15-20% of all epilepsies. Initially IGEs were primarily considered channelopathies, because the first genetic defects identified in IGEs involved ion channel genes. However, new findings indicate that mutations in many non ion channel genes are also involved in addition to those in ion channel genes. Interestingly, mutations in many genes associated with epilepsy affect GABAergic signaling, a major biological pathway in epilepsy. Additionally, many antiepileptic drugs work via enhancing GABAergic signaling. Hence, the review will focus on the mutations that impair GABAergic signaling and selectively discuss the newly identified STXBP1, PRRT2, and DNM1 in addition to those long-established epilepsy ion channel genes that also impair GABAergic signaling like SCN1A and GABA_A receptor subunit genes. GABAergic signaling includes the pre- and post- synaptic mechanisms. Some mutations, such as STXBP1, PRRT2, DNM1, and SCN1A, impair GABAergic signaling mainly via pre-synaptic mechanisms while those mutations in GABA_A receptor subunit genes impair GABAergic signaling via post-synaptic mechanisms. Nevertheless, these findings suggest impaired GABAergic signaling is a converging pathway of defects for many ion channel or non ion channel mutations associated with genetic epilepsies.

Polymorphisms of ABAT, SCN2A and ALDH5A1 may affect valproic acid responses in the treatment of epilepsy in Chinese

Aim: The clinical efficacy of valproic acid (VPA) varies greatly among epileptic patients. To find the potential genetic factors related to VPA responses, the pharmacogenetics study was conducted. Methods: Two hundred and one Chinese Han epileptic patients who were treated by VPA for at least 1 year were recruited. Up to 24 SNPs in 11 candidate genes that correlate with the metabolism, transport or target of VPA were genotyped. Results: Three SNPs, rs1731017 (ABAT), rs2304016 (SCN2A) and rs1054899 (ALDH5A1) were found associated with VPA responses with the p-values of 0.003, 0.007 and 0.048, respectively. Further interaction analysis showed that the interaction between rs17183814 (ABAT) and rs1641022 (SCN2A) was also correlated with the response of VPA (p = 0.006). Conclusion: This study found three SNPs and one interaction among ABAT, SCN2A and ALDH5A1 were significantly associated with VPA response, which indicated that these genes may play important roles in the pharmacological mechanism of VPA.

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.

Neuroinformatics analyses reveal GABAt and SSADH as major proteins involved in anticonvulsant activity of valproic acid

The unequivocal hypotheses about anticonvulsant activity of valproic acid (VPA) have always been a basic hurdle in designing next generation neurotherapeutics, particularly the anti-epileptic drugs. The present study reports about a comprehensive in-silico investigation into qualitative and quantitative binding of VPA and corresponding natural ligands of four major enzymes involved in neurotransmissions, namely-GABA transaminase (GABAt), α-keto glutarate dehydrogenase (α-KGDH), Succinate Semialdehyde dehydrogenase (SSADH) and Glutamate Decarboxylase (GAD), respectively. The molecular docking analyses revealed that VPA inhibits GABAt and α-KGDH through allosteric while SSADH through competitive mode of binding. There is an observed elevation in binding of glutamate over GAD in the presence of VPA. The docking inhibition constant (Ki) of VPA to all the studied enzymatic receptors were observed to be well below the therapeutic concentration of VPA in blood, except for α-KGDH, thus favouring GABAergic over glutamatergic mode of anticonvulsant activity of VPA. The report is probably the first comprehensive in-silico molecular study about VPA action.

Molecular mechanisms of antiseizure drug activity at GABAA receptors

The GABAA receptor (GABAAR) is a major target of antiseizure drugs (ASDs). A variety of agents that act at GABAARs s are used to terminate or prevent seizures. Many act at distinct receptor sites determined by the subunit composition of the holoreceptor. For the benzodiazepines, barbiturates, and loreclezole, actions at the GABAAR are the primary or only known mechanism of antiseizure action. For topiramate, felbamate, retigabine, losigamone and stiripentol, GABAAR modulation is one of several possible antiseizure mechanisms. Allopregnanolone, a progesterone metabolite that enhances GABAAR function, led to the development of ganaxolone. Other agents modulate GABAergic "tone" by regulating the synthesis, transport or breakdown of GABA. GABAAR efficacy is also affected by the transmembrane chloride gradient, which changes during development and in chronic epilepsy. This may provide an additional target for "GABAergic" ASDs. GABAAR subunit changes occur both acutely during status epilepticus and in chronic epilepsy, which alter both intrinsic GABAAR function and the response to GABAAR-acting ASDs. Manipulation of subunit expression patterns or novel ASDs targeting the altered receptors may provide a novel approach for seizure prevention.

Gamma band neural synchronization deficits for auditory steady state responses in bipolar disorder patients

Periodic auditory click stimulation has been reported to elicit an auditory steady state response (ASSR). The ASSR has been suggested to reflect the efficiency of γ-amino butyric acid (GABA) inhibitory interneuronal activity. Although a potential role for GABAergic dysfunction has been previously proposed, the role of neural synchronization in the ASSR in people with bipolar disorder (BD) has received little attention. In the current study, we investigated ASSRs to 20 Hz, 30 Hz, 40 Hz and 80 Hz click trains in BD patients. A total of 14 (4 males) BD patients and 25 (10 males) healthy controls participated in this study. ASSRs were obtained using whole-head 306-channel magnetoencephalography to calculate, ASSR power values and phase locking factors (PLF). BD patients exhibited significantly reduced mean ASSR power and PLF values bilaterally at frequencies of 30, 40, and 80 Hz (p<0.05 for these frequencies). At 20 Hz, bipolar patients showed no significant reduction in mean ASSR power and PLF values. There was a significant negative correlation between 80 Hz-ASSR-power values obtained from the right hemisphere and scores on the Hamilton Depression Rating Scale (rho = −0.86, p = 0.0003). The current study showed reduced low and high gamma band ASSR power and PLF bilaterally with no significant beta band ASSR reduction in BD patients. BD patients are characterized by deficits in gamma band oscillations, which may be associated with GABA inhibitory interneuronal activity dysfunction.

The anticonvulsant response to valproate in kindled rats is correlated with its effect on neuronal firing in the substantia nigra pars reticulata: a new mechanism of pharmacoresistance

Resistance to antiepileptic drugs (AEDs) is a major problem in epilepsy treatment. However, mechanisms of resistance are only incompletely understood. We have recently shown that repeated administration of the AED phenytoin allows selecting resistant and responsive rats from the amygdala kindling model of epilepsy, providing a tool to study mechanisms of AED resistance. We now tested whether individual amygdala-kindled rats also differ in their anticonvulsant response to the major AED valproate (VPA) and which mechanism may underlie the different response to VPA. VPA has been proposed to act, at least in part, by reducing spontaneous activity in the substantia nigra pars reticulata (SNr), a main basal ganglia output structure involved in seizure propagation, seizure control, and epilepsy-induced neuroplasticity. Thus, we evaluated whether poor anticonvulsant response to VPA is correlated with low efficacy of VPA on SNr firing rate and pattern in kindled rats. We found (1) that good and poor VPA responders can be selected in kindled rats by repeatedly determining the effect of VPA on the electrographic seizure threshold, and (2) a significant correlation between the anticonvulsant response to VPA in kindled rats and its effect on SNr firing rate and pattern. The less VPA was able to raise seizure threshold, the lower was the VPA-induced reduction of SNr firing rate and the VPA-induced regularity of SNr firing. The data demonstrate for the first time an involvement of the SNr in pharmacoresistant experimental epilepsy and emphasize the relevance of the basal ganglia as target structures for new treatment options.

Protective effects of valproic acid on the nigrostriatal dopamine system in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mouse model of Parkinson's disease

The use of animal models (including the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine [MPTP] mouse model) to mimic dopaminergic (DAergic) cell loss and striatal dopamine (DA) depletion, as seen in Parkinson's disease (PD), has implicated a multitude of factors that might be associated with DAergic cell death in PD including excitotoxicity, inflammation, and oxidative stress. All of these factors have been shown to be reduced by administration of histone deacetylase (HDAC) inhibitors (HDACis) resulting in some degree of neuroprotection in various models of neurodegenerative disease including in Huntington's disease and amyotrophic lateral sclerosis. However, there is limited information of effects of HDACis in PD models. We have previously shown HDACis to be partially protective against 1-methyl-4-phenylpyridinium (MPP(+))-mediated cell loss in vitro. The present study was conducted to extend these findings to an in vivo PD model. The HDACi valproic acid (VPA) was co-administered with MPTP for 5 days to male FVBn mice and continued for an additional 2 weeks, throughout the period of active neurodegeneration associated with MPTP-mediated DAergic cell loss. VPA was able to partially prevent striatal dopamine depletion and almost completely protect against substantia nigra DAergic cell loss. These results suggest that VPA may be a potential disease-modifying therapy for PD.

Is levetiracetam different from other antiepileptic drugs? Levetiracetam and its cellular mechanism of action in epilepsy revisited

Levetiracetam (LEV) is a new antiepileptic drug that is clinically effective in generalized and partial epilepsy syndromes as sole or add-on medication. Nevertheless, its underlying mechanism of action is poorly understood. It has a unique preclinical profile; unlike other antiepileptic drugs (AEDs), it modulates seizure-activity in animal models of chronic epilepsy with no effect in most animal models of acute seizures. Yet it is effective in acute in-vitro 'seizure' models. A possible explanation for these dichotomous findings is that LEV has different mechanisms of actions, whether given acutely or chronically and in 'epileptic' and control tissue. Here we review the general mechanism of action of AEDs, give an updated and critical overview about the experimental findings of LEV's cellular targets (in particular the synaptic vesicular protein SV2A) and ask whether LEV represents a new class of AED.

Epigenetic regulation of nervous system development by DNA methylation and histone deacetylation

Alterations in the epigenetic modulation of gene expression have been implicated in several developmental disorders, cancer, and recently, in a variety of mental retardation and complex psychiatric disorders. A great deal of effort is now being focused on why the nervous system may be susceptible to shifts in activity of epigenetic modifiers. The answer may simply be that the mammalian nervous system must first produce the most complex degree of developmental patterning in biology and hardwire cells functionally in place postnatally, while still allowing for significant plasticity in order for the brain to respond to a rapidly changing environment. DNA methylation and histone deacetylation are two major epigenetic modifications that contribute to the stability of gene expression states. Perturbing DNA methylation, or disrupting the downstream response to DNA methylation - methyl-CpG-binding domain proteins (MBDs) and histone deacetylases (HDACs) - by genetic or pharmacological means, has revealed a critical requirement for epigenetic regulation in brain development, learning, and mature nervous system stability, and has identified the first distinct gene sets that are epigenetically regulated within the nervous system. Epigenetically modifying chromatin structure in response to different stimuli appears to be an ideal mechanism to generate continuous cellular diversity and coordinate shifts in gene expression at successive stages of brain development - all the way from deciding which kind of a neuron to generate, through to how many synapses a neuron can support. Here, we review the evidence supporting a role for DNA methylation and histone deacetylation in nervous system development and mature function, and present a basis from which to understand how the clinical use of HDAC inhibitors may impact nervous system function.

Comparison of the efficacy of two anticonvulsants, phenytoin and valproate to improve PCP and d-amphetamine induced deficits in a reversal learning task in the rat

Recent studies in our laboratory have shown that PCP (phencyclidine) and d-amphetamine induce a cognitive deficit in rats, in a paradigm of potential relevance for the pathology of schizophrenia. Atypical, but not classical antipsychotics and the anticonvulsant, lamotrigine have been shown to prevent a selective reversal learning deficit induced by PCP. In contrast, only haloperidol reversed the d-amphetamine-induced deficit. The present study aimed to explore the ability of two anticonvulsants with differing mechanism of action, valproate and phenytoin to attenuate the cognitive deficits induced by PCP and d-amphetamine in the reversal learning paradigm. PCP at 1.5 mg/kg and d-amphetamine at 0.5 mg/kg both produced a selective and significant reduction in performance of the reversal phase with no effect on the initial phase of the task in female-hooded Lister rats. Valproate (25-200 mg/kg) and phenytoin (25-50 mg/kg) had no effect on performance when administered alone. Valproate (100-200 mg/kg), whose principle action is thought to be the enhancement of GABA transmission, was unable to prevent the cognitive deficit induced by either PCP or d-amphetamine. Conversely, phenytoin (50 mg/kg), a use-dependent sodium channel inhibitor, significantly prevented the deficit induced by PCP, but not d-amphetamine. These results add to our earlier work with lamotrigine, and suggest that sodium channel blockade may be a mechanism by which some anticonvulsant drugs can prevent the PCP-induced deficit. These data have implications for the use of anticonvulsant drugs in the treatment of cognitive or psychotic disorders.

Standard antiepileptic drugs fail to block epileptiform activity in rat organotypic hippocampal slice cultures

BACKGROUND AND PURPOSE

Earlier studies had demonstrated that tonic-clonic seizure-like events (SLEs) resembling electrographic correlates of limbic seizures in animals and humans can be induced in organotypic hippocampal slice cultures (OHSCs). We have explored OHSCs for their suitability to serve as in vitro models of limbic seizures for studying seizure mechanisms and screening new antiepileptic compounds.

EXPERIMENTAL APPROACH

OHSCs were cultivated according to the interface method. Neuronal activity and extracellular potassium concentration were recorded under submerged conditions. SLEs were induced by lowering magnesium concentration or by applying the potassium channel blocker 4-aminopyridine. The effects of standard antiepileptic drugs (AEDs), carbamazepine, phenytoin, valproic acid, clonazepam, diazepam and phenobarbital sodium on SLEs were analysed.

KEY RESULTS

In more than 93% of OHSCs, AEDs did not prevent the induction of SLEs or stop ongoing seizure activity even when toxic concentrations were applied. This pharmacoresistance was independent of the method of seizure provocation, postnatal age at explantation (P2-P10) and cultivation time in vitro (2 months). SLEs were reversibly blocked by glutamate antagonists or the GABA(A)-agonist muscimol.

CONCLUSIONS AND IMPLICATIONS

We present a simple to establish in vitro model of tonic-clonic SLEs that is a priori pharmacoresistant and thus has an advantage over animal models of pharmacoresistant seizures in which responders and non-responders can be sorted out only after an experiment. OHSCs could be suitable for exploring mechanisms of pharmacoresistant seizures and be used for the identification of new anticonvulsive compounds eventually effective in drug refractory epilepsy.

Animal models for the development of new neuropharmacological therapeutics in the status epilepticus

Status epilepticus (SE) is a major medical emergency associated with significant morbidity and mortality. SE is best defined as a continuous, generalized, convulsive seizure lasting > 5 min, or two or more seizures during which the patient does not return to baseline consciousness. The relative efficacy and safety of different drugs in the treatment of human SE should be determined in a prospective, randomized, blinded study. However, complementary animal models of SE are required to answer important questions concerning the treatment of SE because of the obvious difficulties of setting up such studies in clinical emergency conditions. This review offers an overview of the implementation and characteristics of some of the most prevalent animal models of SE currently in use. A description is also provide about how animal models of SE may facilitate the use of neurobiological techniques to successfully address critical questions in the drug treatment of SE. In particular, the experience with recently introduced drugs such as intravenous valproate will be addressed. Finally, the importance of some animal models and pharmacological approaches is explained and we discuss their impact in the development of therapeutic strategies to improve pharmacological treatment for SE is discussed.

Progress in pharmaceutical development presentation with improved pharmacokinetics: a new formulation for valproate

Successful long-term treatment of patients with epilepsy requires selection of an appropriate antiepileptic regimen, optimal dosing and patient compliance. Recent advances in our understanding of the biological basis of epilepsy and in the choice of treatment options are transforming the global management of these patients. If the achievement of seizure freedom remains the primary goal of any antiepileptic treatment, issues associated with drug acceptability and tolerability, and with quality of life of patients, have gained increasing attention as major determinants of ultimate therapeutic success. Sustained-release formulations of antiepileptic drugs can be very helpful in achieving treatment objectives. Stable serum levels without marked peak-to-trough fluctuations, reduced frequency of dosing and the possibility of dosing flexibility may all improve compliance, patient satisfaction and ultimately quality of life. The efficacy of sodium valproate for the treatment of most types of epilepsy has been demonstrated extensively and this drug remains the mainstay of treatment for many clinical situations. Among the various valproate formulations, extended-release tablets have shown improved patient compliance and satisfaction. However, the tablet size and the limited dosing flexibility could be unsuitable for individualized treatment in special populations such as children, the elderly and patients with swallowing difficulties. A new sustained-release formulation of sodium valproate consisting of tasteless microspheres that can be sprinkled on semi-solid food such as yoghurt or jam has been developed. A stick pack presentation allows individualized dosing and greater convenience.

Molecular and cellular mechanisms of pharmacoresistance in epilepsy

Epilepsy is a common and devastating neurological disorder. In many patients with epilepsy, seizures are well-controlled with currently available anti-epileptic drugs (AEDs), but a substantial (approximately 30%) proportion of patients continue to have seizures despite carefully optimized drug treatment. Two concepts have been put forward to explain the development of pharmacoresistance. The transporter hypothesis contends that the expression or function of multidrug transporters in the brain is augmented, leading to impaired access of AEDs to CNS targets. The target hypothesis holds that epilepsy-related changes in the properties of the drug targets themselves may result in reduced drug sensitivity. Recent studies have started to dissect the molecular underpinnings of both transporter- and target-mediated mechanisms of pharmacoresistance in human and experimental epilepsy. An emerging understanding of these underlying molecular and cellular mechanisms is likely to provide important impetus for the development of new pharmacological treatment strategies.

Valproate reduced synaptic activity increase induced by 4-aminopyridine at the hippocampal CA3-CA1 synapse

PURPOSE

We investigated the effects of valproate (VPA) on excitatory synaptic transmission changes induced by 4-aminopyridine (4-AP) to determine whether the antiepileptic effects shown by VPA can be ascribed to a modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in the CA3-CA1 synapse.

METHODS

Rat hippocampal slices were prepared and maintained in vitro with standard methods. Whole-cell current and voltage-clamp recordings were obtained from CA1 pyramidal neurons by using the "blind" patch-clamp technique in an immersion recording chamber. Increase in the spontaneous excitatory synaptic activity was induced by addition of 4-AP to the medium.

RESULTS

Perfusion with VPA significantly counteracted the increase of frequency and amplitude of the sEPSCs induced by application of 4-AP and suppressed the epileptiform activity.

CONCLUSIONS

We conclude that VPA decreases the 4-AP-induced enhancement of excitatory synaptic activity at the CA3-CA1 synapse, and that this reduction of excitation input to CA1 contributes to the anticonvulsant effects of VPA.

Valproate reduced excitatory postsynaptic currents in hippocampal CA1 pyramidal neurons

Valproate (VPA) is one of the most widely used antiepileptic drugs, and it is also increasingly used for the treatment of neuropsychological disorders and neuropathic pain, as well as migraine prophylaxis. However, the underlying cellular mechanisms of VPA on the synaptic physiology remain unclear. We investigated the effects of VPA on synaptic transmission using the in vitro rat hippocampal slice technique and whole-cell patch clamp recordings from CA1 pyramidal neurons. Perfusion with VPA, at therapeutically attainable concentrations, decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by Schaffer collateral stimulation, without modifying inhibitory postsynaptic currents (IPSCs). Furthermore, VPA induced a significant reduction of the non-NMDA EPSC (non-NMDA(EPSC)) component, without modifying the NMDA EPSC (NMDA(EPSC)) component. Paired pulse facilitation and EPSC variance were not significantly affected by VPA, indicating that VPA did not decrease transmitter release probability, which suggests a postsynaptic mechanism of action. We therefore conclude that VPA decreases excitatory synaptic activity through the modulation of postsynaptic non-NMDA receptors, without modifying synaptic inhibition, and that this reduction of excitation is, at least in part, responsible for the effects of VPA.

Valproate suppresses status epilepticus induced by 4-aminopyridine in CA1 hippocampus region

PURPOSE

We investigated the effects of valproate (VPA) on an in vivo model of status epilepticus (SE) induced by intrahippocampal application of 4-aminopyridine (4-AP).

METHODS

To induce continuous epileptiform activity without a clinical component, 4-AP (100 mM) was slowly injected in the hippocampus of adult rats. Extracellular field potential from the CA1 region of the rat hippocampus was recorded to assess abnormal epileptiform activity. Once the SE seizures were induced by 4-AP, the test drug was injected. In some experiments to test the ability of a drug to prevent the induction of SE, the drug was administered before 4-AP injection.

RESULTS

Intrahippocampal injection of 4-AP induced continuous epileptic activity without a clinical component that lasted >60 min. The intravenous injection of 400-600 mg/kg VPA rapidly (approximately 100 s) abolished the SE, and this effect persisted for >/=4 h in our experimental model. The intravenous injection of 100-300 mg/kg VPA did not abolish previously induced SE, but prevented the appearance of SE when applied before the induction of SE. The intravenous injection of 80 mg/kg phenytoin or carbamazepine did not abolish or prevent SE.

CONCLUSIONS

We conclude that 4-AP-induced SE was suppressed by VPA at 400-600 mg/kg, whereas minor doses (100-300 mg/kg) only prevent the 4-AP-induced SE. Present results suggest the revisiting of VPA as a useful drug for the treatment of SE.

GABAergic dysfunction in mood disorders

The authors review the available literature on the preclinical and clinical studies involving GABAergic neurotransmission in mood disorders. Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter present almost exclusively in the central nervous system (CNS), distributed across almost all brain regions, and expressed in interneurons modulating local circuits. The role of GABAergic dysfunction in mood disorders was first proposed 20 years ago. Preclinical studies have suggested that GABA levels may be decreased in animal models of depression, and clinical studies reported low plasma and CSF GABA levels in mood disorder patients. Also, antidepressants, mood stabilizers, electroconvulsive therapy, and GABA agonists have been shown to reverse the depression-like behavior in animal models and to be effective in unipolar and bipolar patients by increasing brain GABAergic activity. The hypothesis of reduced GABAergic activity in mood disorders may complement the monoaminergic and serotonergic theories, proposing that the balance between multiple neurotransmitter systems may be altered in these disorders. However, low GABAergic cortical function may probably be a feature of a subset of mood disorder patients, representing a genetic susceptibility. In this paper, we discuss the status of GABAergic hypothesis of mood disorders and suggest possible directions for future preclinical and clinical research in this area.

The anticonvulsant valproate increases the turnover rate of gamma-aminobutyric acid transporters

Valproate is an important anticonvulsant currently in clinical use for the treatment of seizures. We used electrophysiological and tracer uptake methods to examine the effect of valproate on a gamma-aminobutyric acid (GABA) transporter (mouse GAT3) expressed in Xenopus laevis oocytes. In the absence of GABA, valproate (up to 50 mm) had no noticeable effect on the steady-state electrogenic properties of mGAT3. In the presence of GABA, however, valproate enhanced the GABA-evoked steady-state inward current in a dose-dependent manner with a half-maximal concentration of 4.6 +/- 0.5 mm. Maximal enhancement of the GABA-evoked current was 275 +/- 10%. Qualitatively similar observations were obtained for human GAT1 and mouse GAT4. The valproate enhancement did not alter the Na(+) or Cl(-) dependence of the steady-state GABA-evoked currents. Uptake experiments under voltage clamp suggested that the valproate enhancement of the GABA-evoked current was matched by an enhancement in GABA uptake. Thus, despite the increase in GABA-evoked current, ion/GABA co-transport remained tightly coupled. Uptake experiments indicated that valproate is not transported by mouse GAT3 in the absence or presence of GABA. Valproate also enhanced the rate of the partial steps involved in transporter presteady-state charge movements. We propose that valproate increases the turnover rate of GABA transporters by an allosteric mechanism. The data suggest that at its therapeutic concentration, valproate may enhance the activity of neuronal and glial GABA transporters by up to 10%.

Basic pharmacology of valproate: a review after 35 years of clinical use for the treatment of epilepsy

Since its first marketing as an antiepileptic drug (AED) 35 years ago in France, valproate has become established worldwide as one of the most widely used AEDs in the treatment of both generalised and partial seizures in adults and children. The broad spectrum of antiepileptic efficacy of valproate is reflected in preclinical in vivo and in vitro models, including a variety of animal models of seizures or epilepsy. There is no single mechanism of action of valproate that can completely account for the numerous effects of the drug on neuronal tissue and its broad clinical activity in epilepsy and other brain diseases. In view of the diverse molecular and cellular events that underlie different seizure types, the combination of several neurochemical and neurophysiological mechanisms in a single drug molecule might explain the broad antiepileptic efficacy of valproate. Furthermore, by acting on diverse regional targets thought to be involved in the generation and propagation of seizures, valproate may antagonise epileptic activity at several steps of its organisation. There is now ample experimental evidence that valproate increases turnover of gamma-aminobutyric acid (GABA) and thereby potentiates GABAergic functions in some specific brain regions thought to be involved in the control of seizure generation and propagation. Furthermore, the effect of valproate on neuronal excitation mediated by the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors might be important for its anticonvulsant effects. Acting to alter the balance of inhibition and excitation through multiple mechanisms is clearly an advantage for valproate and probably contributes to its broad spectrum of clinical effects. Although the GABAergic potentiation and glutamate/NMDA inhibition could be a likely explanation for the anticonvulsant action on focal and generalised convulsive seizures, they do not explain the effect of valproate on nonconvulsive seizures, such as absences. In this respect, the reduction of gamma-hydroxybutyrate (GHB) release reported for valproate could be of interest, because GHB has been suggested to play a critical role in the modulation of absence seizures. Although it is often proposed that blockade of voltage-dependent sodium currents is an important mechanism of antiepileptic action of valproate, the exact role played by this mechanism of action at therapeutically relevant concentrations in the mammalian brain is not clearly elucidated. By the experimental observations summarised in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels about the mechanisms of action of valproate. In view of the advances in molecular neurobiology and neuroscience, future studies will undoubtedly further our understanding of the mechanisms of action of valproate.

Valproate prevents the induction and the expression of MK-801 sensitization

Repeated administration of psychostimulants such as amphetamine, cocaine, and methylphenidate has been shown to induce behavioral sensitization. Sodium valproate, an anticonvulsant agent that enhances GABA activity, and dizocilpine (MK-801), a non-competitive NMDA receptor antagonist, can block the sensitization elicited by psychostimulants. MK-801 also has been demonstrated to sensitize to itself. The objective of the present study was to determine whether valproate disrupts the behavioral sensitization elicited by MK-801. Male Sprague-Dawley rats were given a regimen of repeated MK-801 injections (0.3 mg/kg, i.p.) that produced behavioral sensitization. They were also given valproate, at a dosage (50 mg/kg, i.p.) that prevented behavioral sensitization to stimulants, either during or after multiple MK-801 injections. After the washout period, animals were then re-challenged with MK-801 to determine whether valproate disrupted the behavioral sensitization elicited by MK-801. An activity monitoring system recorded horizontal activity, total distance, and vertical activity of the animals following drug treatment. Results of their locomotor responses demonstrated that valproate disrupted the development/induction and the expression of sensitization to MK-801, as it did to methylphenidate.

Effect of depth electrode implantation with or without subsequent kindling on GABA turnover in various rat brain regions

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied electrical stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that impairment of GABAergic inhibition may be involved. In the present experiments, GABA turnover was determined in vivo by the GABA aminotransferase (GABA-T) inhibition method in 13 brain regions in three groups of rats: (1) a group which was kindled via electrical stimulation of intra-amygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (2) a group of implanted but non-stimulated rats (sham control group) in which neurochemical measurements were done at the same time after electrode implantation as in the kindled group; and (3) a group of non-implanted, naive control rats. Regional GABA levels were determined after vehicle injection as well as 30 and 90 min after administration of aminooxyacetic acid (AOAA) at a dose which completely inhibits GABA-T. Compared to naive controls, prolonged electrode implantation in the amygdala induced a significant reduction of AOAA-induced GABA accumulation in amygdala, hippocampus, piriform cortex, olfactory bulb, frontal cortex, striatum, hypothalamus, tectum, and cerebellar cortex. In view of the GABA hypothesis of kindling, reduced GABA turnover in response to electrode implantation would suggest that the implantation per se exerts a pro-kindling effect, which was recently demonstrated in rats with intraamygdala electrodes. However, amygdala kindling itself appeared to antagonize the effect of electrode implantation in most regions. Thus, although, compared to naive controls, the predominant change in kindled rats was a decrease in GABA turnover, this decrease was less marked than in sham controls. In thalamus and brainstem kindling markedly increased GABA turnover above the levels determined in both naive and sham controls, possibly in response to impaired postsynaptic GABAergic function. The data indicate that both electrode implantation and kindling significantly alter regional GABA turnover, which might contribute to the pathophysiology of the kindling phenomenon. Furthermore, the data substantiate that the choice of adequate controls is critical in neurochemical and functional studies on the kindling phenomenon.

Inhibition of GABA system involved in cyclosporine-induced convulsions

In this study, we attempted to clarify the mechanisms mediating cyclosporine-evoked convulsions. Cyclosporine (50 mg/kg, i.p.) significantly enhanced the intensity of convulsions induced by bicuculline (GABA receptor antagonist), but not those induced by strychnine (glycine receptor antagonist), N-methyl-D-aspartic acid, quisqualic acid or kainic acid (glutamate receptor agonists). Bicuculline plus cyclosporine-induced convulsions were significantly suppressed by an activation of GABAergic transmission with diazepam, phenobarbital and valproate. The GABA turnover estimated by measuring aminooxyacetic acid-induced GABA accumulation in the mouse brain was significantly inhibited by cyclosporine (50 mg/kg, i.p.). When cultured rat cerebellar granule cells were exposed to 1 microM cyclosporine for 24 hr, the specific [3H]muscimol (10 nM) binding to intact granule cells decreased to 53% of vehicle controls. The present study provides the first evidence suggesting that cyclosporine inhibits GABAergic neural activity and binding properties of the GABAA receptor. These events are closely related to the occurrence of adverse central effects including tremors, convulsions, coma and encephalopathy under cyclosporine therapy.

Valproate: a reappraisal of its pharmacodynamic properties and mechanisms of action

Valproate is currently one of the major antiepileptic drugs with efficacy for the treatment of both generalized and partial seizures in adults and children. Furthermore, the drug is increasingly used for therapy of bipolar and schizoaffective disorders, neuropathic pain and for prophylactic treatment of migraine. These various therapeutic effects are reflected in preclinical models, including a variety of animal models of seizures or epilepsy. The incidence of toxicity associated with the clinical use of valproate is low, but two rare toxic effects, idiosyncratic fatal hepatotoxicity and teratogenicity, necessitate precautions in risk patient populations. Studies from animal models on structure-relationships indicate that the mechanisms leading to hepatotoxicity and teratogenicity are distinct and also differ from the mechanisms of anticonvulsant action of valproate. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA synthesis and release and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to attenuate neuronal excitation induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate exerts direct effects on excitable membranes, although the importance of this action is equivocal. Microdialysis data suggest that valproate alters dopaminergic and serotonergic functions. Valproate is metabolized to several pharmacologically active metabolites, but because of the low plasma and brain concentrations of these compounds it is not likely that they contribute significantly to the anticonvulsant and toxic effects of treatment with the parent drug. By the experimental observations summarized in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels of valproate's mechanisms of action.

Enzyme inhibition XI: glutamate decarboxylase activity relationship with the reaction products as determined by the colorimetric and radioisotopic methods

The relationship of glutamate acid decarboxylase (GAD) activity with the reaction products was developed. It was based on incubating sodium glutamate substrate (S) with GAD enzyme (E) when the enzyme-substrate-complex (ES) product was obtained along with gamma aminobutyric acid (GABA) and unreacted sodium glutamate. The reaction products were separated by paper chromatography. The ES, GABA and S products were sprayed with ninhydrin reagent when ninhydrin-colored-complex (NCC) was formed on the paper chromatogram. The products were extracted with 75% ethanol containing 0.5% cupric sulfate. The NCC absorption readings of ES and S products were measured by a spectrophotometer. A standard curve was prepared by plotting absorption readings against different concentrations of sodium glutamate. This curve was the basis of determining GAD activity of E. coli and C. welchii. It was observed that NCC absorption of ES and S products was directly related with the enzyme activity. The qualities of ES and S products in the reaction mixture increased as the enzyme activity increased with the incubation time. On the other hand, some products in the reaction mixture decreased in the presence of an inhibitor of GAD activity. The relationship of reaction products with GAD activity was also established by the radioisotopic method. The results obtained by the chromatographic separation of products followed by the spectrophotometric method of determining GAD activity is a simple, safe and less expensive compared to the other methods.

New visions in the pharmacology of anticonvulsion

Seizures are resistant to treatment with currently available anticonvulsant drugs in about 1 out of 3 patients with epilepsy. Thus, there is a need for new, more effective anticonvulsant drugs for intractable epilepsy. Furthermore, because of the inadequacy of the currently available anticonvulsant armamentarium with respect to safety, newly developed drugs should be less toxic than existing drugs. Previous and current strategies for development of novel anticonvulsants with improved efficacy or safety are critically discussed in this review. 'Old drugs' (or 'first generation' drugs), which were developed and introduced between 1910 and 1970, are compared with new anticonvulsants both in terms of clinical efficacy and safety and in terms of mechanisms of action. The new drugs are referred to as 'second generation' drugs, i.e. anticonvulsants which have been introduced into clinical practice in recent years, or 'third generation' drugs, i.e. compounds in the pipeline of development. In spite of some 30 years of 'modern' neuroscientific epilepsy research, most novel, clinically effective second generation anticonvulsants have been found by screening (i.e. serendipity) or structural variation of known drugs and not by rational strategies based on knowledge of processes involved in generation of seizures or in development of epilepsy. An exception are only the GABA (gamma-aminobutyrate)-mimetic drugs vigabatrin and tiagabine and, to some extent, gabapentin, which have been developed by a rational strategy, i.e. the 'GABA hypothesis' of epilepsy. The fact that preclinical seizure models used for identification and development of novel drugs have been originally validated by old drugs, i.e. conventional anticonvulsants, may explain that several of the new drugs possess mechanisms which do not differ from those of the standard drugs. This may also explain that none of the new drugs seems to offer any marked advantage towards the old, first generation drugs with respect to the ultimate goal of drug treatment of epilepsy, i.e. complete control of seizures, although some of the second generation drugs may have benefits in terms of side effects and tolerability. It is to be hoped that the various novel currently used or planned strategies for drug development produce more effective and safe anticonvulsants than previous strategies. This goal can only be achieved by strengthening our understanding of the fundamental pathophysiology of seizure expression and epileptogenesis as theoretical substrates for new pharmacological strategies, and by devising and refining laboratory models for studying new agents obtained by such strategies.

The influence of established and new antiepileptic drugs on visual perception. 1. A placebo-controlled, double-blind, single-dose study in healthy volunteers

The influence of single oral dosages of carbamazepine (CBZ), valproic acid, vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP), and losigamone (LSG) on visual perception was investigated in ten healthy volunteers according to a double-blind, placebo-controlled, cross-over study design. The test battery comprised visual acuity, the Lanthony-D-15-désaturé colour perception test, increment, postadaptation and transient tritanopia threshold measurements, perception threshold assessment for monochromatic and chromatic gaussian dots, monochromatic gratings and gratings of differing spatial frequency, and critical flicker fusion tests with various stimuli. The only consistent and partly significant effects were seen after VGB and GBP. After VGB, increment, postadaptation and transient tritanopia thresholds and the critical flicker fusion increased, whereas GBP led to a somewhat converse profile. The other tests were not influenced consistently by any antiepileptic drug (AED). We conclude that: (i) gamma-amino-butyric acid-(GABA)-related properties as under the prototype drug VGB result in specific alterations of the transient tritanopia phenomenon which is consistent with the physiological hypothesis for this retinal paradigm based on extracellular recordings in primates. The possible mechanisms why VGB improved critical flicker fusion as the only AED in this trial are discussed. The profile of GBP indicates a unique mechanism of action. We have not observed specific influences on visual perception under AEDs which act mainly via alterations of ion membrane conductance. The transient tritanopia and flicker fusion paradigms we used appear to be promising to investigate antiepileptic drugs with hitherto unknown modes of actions in human noninvasively.

The influence of established and new antiepileptic drugs on visual perception. II. A controlled study in patients with epilepsy under long-term antiepileptic medication

In this study, we investigated visual performance under chronic antiepileptic drug treatment. Patients were under carbamazepine (CBZ) (n = 18), valproic acid (VPA) (n = 9), CBZ and vigabatrin (VGB) (n = 4), CBZ and gabapentin (GBP) (n = 8), and under CBZ and topiramate (TPR) (n = 6), respectively. Seven untreated patients with epilepsy and 42 healthy volunteers served as controls. The test battery comprised the Lanthony-D15-désaturé colour perception test, increment, postadaptation and transient tritanopia (TT) threshold measurements, visual perception threshold assessments for monochromatic and chromatic gratings and gaussian dots, and critical flicker fusion (CFF) tests. No differences were seen between naive patients and healthy controls. Patients under drug treatment always showed alterations of visual perception. Postadaptation and TT thresholds were altered under each drug regimen after short delays between switching off the adaptation light and switching on the blue test light. Threshold elevations were maximum under the combination of CBZ and TPR and lowest under CBZ and GBP. Consistent impairment of the CFF was seen under combined CBZ and TPR whereas VPA as well as combined CBZ and VGB led to ameliorations the mechanisms of which are discussed. The other tests were less sensitive. In conclusion, alterations of visual function were apparent under chronic antiepileptic drug treatment both with established and new agents. However, it may be difficult to distinguish between effects based on specific modes of action and nonspecific retino- and neurotoxicity.

Effect of valproate on the metabolism of the central nervous system

The effects of valproate on brain energy and lipid metabolism is reviewed. Increasing evidence suggests that valproate uses the monocarboxylic acid carrier in order to cross the blood brain barrier (BBB) and the neural cell plasma membranes. The uptake of valproate into the brain through this mechanism would compete with the uptake of energy precursors, such as the monocarboxylic acids 3-hydroxybutyrate, lactate or pyruvate and with some amino acids, but not with glucose. This could impair brain fuel utilization, specially during the neonatal period or childhood, when lactate or 3-hydroxybutyrate furnishes alternative substrates to glucose for the brain. It is concluded that valproate interference with energy metabolism may have implications for the therapeutic action of the drug, stressing the possibility that valproate-mediated alterations in brain lipid synthesis may contribute to the pharmacological action of the drug.

The anticonvulsant gabapentin decreases firing rates of substantia nigra pars reticulata neurons

Gabapentin is a novel anti-epileptic drug which enhances GABA (gamma-aminobutyric acid) turnover in certain brain regions, including substantia nigra. However, the functional consequences of GABA turnover increases in response to gabapentin and their potential involvement in the anticonvulsant action of this drug are not known. In the present study, we examined the effects of gabapentin on the extracellular, single unit activity of nondopaminergic (presumably GABAergic) neurons of the substantia nigra pars reticulata in rats. During the recordings, the animals were infused with the narcotic opioid analgesic fentanyl, associated with a skeletal muscle relaxant and artificial ventilation. The spontaneous firing of substantia nigra pars reticulata neurons was determined up to about 2 h after i.v. or i.p. administration of gabapentin at doses of 15-30 mg/kg. After both routes of administration, gabapentin markedly reduced neuronal firing when administered at a dose of 20-30 mg/kg, while 15 mg/kg were ineffective in this regard. The suppressive effect of gabapentin was rapid in onset (2 min after i.v. and about 20 min after i.p. injection), reached peak values of about 70% below predrug baseline after about 45-60 min, and remained at this level for at least 2 h. Vehicle administration had no effect on substantia nigra pars reticulata neurons. The ability of gabapentin to alter substantia nigra pars reticulata firing does correlate with its known ability to increase nigral GABA turnover. Since a substantial body of evidence suggests that the substantia nigra pars reticulata is a critical site at which decrease of neuronal firing by potentiation of GABAergic influences results in protection against various seizure types, the suppressive effect of gabapentin on substantia nigra pars reticulata activity may contribute to the anticonvulsant action of this drug.

GABA Levels in the Brain: A Target for New Antiepileptic Drugs

A basic strategy for the pharmacological treatment of epilepsy is to develop drugs that reduce the excitability of CNS neurons at times preceding or during the onset of seizure discharge with minimal effects on normal electrical activity. Several antiepileptic drugs currently in use exert their action by modulating sodium channels or receptors of the abundant inhibitory neurotransmitter, GABA. These approaches, which are often successful in reducing the number or severity of seizures, have some effects that limit their clinical use. More recently, a new class of antiepileptic drugs such as vigabatrin, which blocks GABA degradation enzymes, have been developed as effective antiepileptics and are associated with minimal side effects. Although these drugs do not display agonist or antagonist properties at GABA receptor sites, they do appear to interact with brain GABA systems because NMR spectroscopy studies indicate that subjects given these drugs have elevated brain GABA levels, and in vitro electrophysiological studies on CNS tissue reveal elevated GABA release. The precise cellular mechanisms of antiepileptic action of these GABA metabolic modulators are not clear, but current work on the cellular effects of these drugs suggests a model that may explain their action.

Valproic acid differs in its in vitro effect on glutamic acid decarboxylase activity in neonatal and adult rat brain

1. The in vitro effect of valproic acid (VA) (10(-6) to 10(-3) M) on glutamic acid decarboxylase (GAD) activity in whole brain and cerebral cortex (CC) of neonates and of adult rats was examined. 2. VA did not induce changes on GAD activity either in CC or in the rest of the brain (RB) of adult animals. 3. But at 10(-3) M, VA induced an increase in GAD activity in homogenates of noncortical brain areas of neonates; no increments were found in CC of these animals. This latter increase was detected in the membrane-bound fraction of the enzyme and was not due to physicochemical nonspecific changes related to the potential solvent activity of VA at this high concentration. 4. We may conclude that VA induces changes on GAD activity in neonatal stages of development but not in adult brain. Therefore, although a direct enhancement of GAD activity may play a role in the mechanism of action of VA in pediatric patients, this cannot be verified in the adult population.

Effects of valproate on amino acid and monoamine concentrations in striatum of audiogenic seizure-prone Balb/c mice

The effects of valproate on CNS concentrations of gamma-aminobutyric acid (GABA), glulamate (GLU), glutamine (GLN); dopamine (DA), serotonin (5-HT), and metabolites were examined in tissue extracts of caudate nucleus of genetic substrains of Balb/c mice susceptible (EP) or resistant (ER) to audiogenic seizures. Generalized tonic-clonic seizures observed in EP mice were inhibited by valproate, administered 1 h prior to testing, in a dose-response fashion. Concentrations of GABA, GLU, and GLN, which were lower in EP mice than in ER mice, were significantly increased by valproate at doses of 180 and 360 mg/kg. Concentrations of homovanillic acid (HVA) and hydroxyindoleacetic acid (5-HIAA), metabolites of DA and 5-HT, were substantially increased by valproate at these doses. The in situ activity of tyrosine hydroxylase (TH) was not significantly influenced by valproate, whereas a valproate-induced increase in tryptophan hydroxylase (TPH) activity was observed in both striatum and in midbrain tegmentum. The data are consistent with the interpretation that anti-convulsive doses of valproate influences the intraneuronal metabolism of monoamines, GABA, and glutamate concurrently. Valproate's influence on the metabolism of both major inhibitory (GABA) and excitatory (GLY amino acids in striatum could contribute to its anti-convulsive effects in genetically seizure prone mice, as well as to the accumulation of DA and 5-HT metabolites.