Pharmacology of absence epilepsy

Serotonin and epilepsy

In recent years, there has been increasing evidence that serotonergic neurotransmission modulates a wide variety of experimentally induced seizures. Generally, agents that elevate extracellular serotonin (5-HT) levels, such as 5-hydroxytryptophan and serotonin reuptake blockers, inhibit both focal and generalized seizures, although exceptions have been described, too. Conversely, depletion of brain 5-HT lowers the threshold to audiogenically, chemically and electrically evoked convulsions. Furthermore, it has been shown that several anti-epileptic drugs increase endogenous extracellular 5-HT concentration. 5-HT receptors are expressed in almost all networks involved in epilepsies. Currently, the role of at least 5-HT(1A), 5-HT(2C), 5-HT(3) and 5-HT(7) receptor subtypes in epileptogenesis and/or propagation has been described. Mutant mice lacking 5-HT(1A) or 5-HT(2C) receptors show increased seizure activity and/or lower threshold. In general, hyperpolarization of glutamatergic neurons by 5-HT(1A) receptors and depolarization of GABAergic neurons by 5-HT(2C) receptors as well as antagonists of 5-HT(3) and 5-HT(7) receptors decrease the excitability in most, but not all, networks involved in epilepsies. Imaging data and analysis of resected tissue of epileptic patients, and studies in animal models all provide evidence that endogenous 5-HT, the activity of its receptors, and pharmaceuticals with serotonin agonist and/or antagonist properties play a significant role in the pathogenesis of epilepsies.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

On-off intermittency in time series of spontaneous paroxysmal activity in rats with genetic absence epilepsy

In the present paper we consider the on-off intermittency phenomena observed in time series of spontaneous paroxysmal activity in rats with genetic absence epilepsy. The method to register and analyze the electroencephalogram with the help of continuous wavelet transform is also suggested.

Are absences truly generalized seizures or partial seizures originating from or predominantly involving the pre-motor areas?

In both the current (1981) ILAE Classification of Epileptic Seizures and the recently Proposed Diagnostic Scheme for People with Epilepsy and Epileptic Seizures, typical absence seizures are defined as generalized seizures, implying widespread subcortical and cortical neuronal involvement from onset with impairment of consciousness as the clinical hallmark. Clinical observations from three patients and clinical and experimental data from the literature suggest, however, that: (1) consciousness is retained in many typical absences; (2) the true hallmark of these seizures is arrest of motor initiation due to disturbance of pre-motor area frontal-lobe function; (3) typical absences and partial seizures from these areas may show similar clinical and EEG features and involve the same neuronal circuits. The neuronal system primarily involved in these seizures consists of a relatively limited cortico-thalamo-cortical circuit, including the reticular thalamic nucleus, the thalamocortical relay and the predominantly anterior and mesial frontal cerebral cortex, with the cortex probably acting as the primary driving site. Typical absences thus should not be classified or defined as generalized seizures, particularly since neuropathological and imaging studies increasingly argue for localized structural abnormalities, even in idiopathic or primary generalized epilepsy. These observations further highlight the intrinsic weaknesses of the current classification system for seizures and support further adaptations of the diagnostic system currently under development.

Pharmacological treatment of childhood absence epilepsy

This review discusses current pharmacological treatment of childhood absence epilepsy (CAE). The key to successful treatment is the correct diagnosis of the epileptic syndrome, hence the initial part of the paper discusses the definition, diagnostic criteria and epidemiology. This is followed by a detailed analysis of pharmacological agents used in the treatment of CAE. The characteristics of old and new anticonvulsants used in the treatment of CAE are also reviewed. For each of the drugs, the mechanism of action, usual dose, common side effects and recommendations for treatment are also discussed. A separate section focuses on instances when anticonvulsants may exacerbate seizures. Particular emphasis is given to the evidence currently available, on which clinical practice needs to be based.

A GABRB3 promoter haplotype associated with childhood absence epilepsy impairs transcriptional activity

Childhood absence epilepsy (CAE) is considered to exhibit a complex non-Mendelian pattern of inheritance. So far, only few CAE susceptibility genes have been identified. In a previous study of our group, an association between the GABA(A) receptor beta3 subunit (GABRB3) gene and CAE was shown. To further investigate this association, we screened 45 CAE patients of the first study for mutations in the 10 exons, the exon-intron boundaries and the regulatory sequences of GABRB3. Although we found no functionally relevant mutation, we did identify 13 single nucleotide polymorphisms (SNPs) in the GABRB3 gene region from the exon 1a promoter to the beginning of intron 3. Using these SNPs we defined four haplotypes for the respective GABRB3 gene region. A transmission disequilibrium test in the same 45 CAE patients and their parents indicated a significant association of this region and CAE (P=0.007075). Reporter gene assays in NT2 cells using exon 1a promoter constructs indicated that the disease-associated haplotype 2 promoter causes a significantly lower transcriptional activity than the haplotype 1 promoter that is over-represented in the controls. In silico analysis suggested that an exchange from T (haplotype 1) to C (haplotype 2) within this promoter impairs binding of the neuron-specific transcriptional activator N-Oct-3. Electrophoretic mobility shift assays demonstrated that the respective polymorphism reduces the nuclear protein binding affinity, thus explaining the results of the reporter gene assays. Reduced expression of the GABRB3 gene could therefore be one potential cause for the development of CAE, pathogenetically relevant in our patient group.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Absence epilepsy associated with moyamoya disease. Case report

The authors present the case of a 6-year-old girl with typical absence epilepsy induced by hyperventilation associated with moyamoya disease (MMD). A diffuse 3-Hz spike-and-wave complex induced by hyperventilation was apparent on an electroencephalogram, and her seizures were intractable to medication. Significant ischemia in the bilateral frontal lobes was present. The epilepsy disappeared after superficial temporal artery-middle cerebral artery anastomosis with encephalomyosynangiosis on both sides. In the treatment of children with intractable absence epilepsy, the possibility of underlying MMD and indications that revascularization surgery may be needed should be taken into consideration.

NMDA-NR1 and AMPA-GluR4 receptor subunit immunoreactivities in the absence epileptic WAG/Rij rat

From an age of 2-3 months onwards, the WAG/Rij rat, a genetic model for absence epilepsy, develops spike-wave discharges (SWD). SWD start in the peri-oral somatosensory cortex (POsc), whereas the rostral reticular thalamic nucleus (rRTN) contributes to synchronizing the thalamo-cortical oscillations. We hypothesize that N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptors in the POsc and rRTN are involved in, respectively, the initiation and synchronization of SWD activity. As a first step to test this hypothesis, 3 months old non-epileptic and 6 months old absence epileptic WAG/Rij rats were compared with age-matched non-epileptic ACI control rats. The presence of NMDA and AMPA receptors was assessed by quantifying immunostaining for the NMDA-NR1 subunit and the AMPA-GluR4 subunit, respectively. In the POsc, WAG/Rij rats of both ages showed less NMDA-NR1 (-14.7%) and AMPA-GluR4 (-8.7%) subunit staining than ACI rats. From 3 to 6 months, AMPA-GluR4 subunit staining more strongly increased in the rRTN of WAG/Rij rats than of ACI rats. Further studies should support our assumption that in the POsc of the WAG/Rij rat, SWD start as a result of reduced NMDA- and AMPA-mediated glutamatergic stimulation, and that AMPA-GluR4 containing neurons in the rRTN of this rat strain contribute to synchronization of thalamic and cortical neurons.

Impaired feedforward inhibition of the thalamocortical projection in epileptic Ca2+ channel mutant mice, tottering

The tottering (tg) mice have a mutation in the CaV2.1 (P/Q-type) voltage-dependent Ca2+ channel alpha(1)2.1 subunit gene. tg mice show not only cerebellar ataxia but also absence epilepsy, which begins at approximately 3 weeks of age and persists throughout life. Similarities in EEG and sensitivity to antiepileptic drugs suggest that tg mice are a good model for human absence epilepsy. Although imbalance between excitatory and inhibitory activity in the thalamocortical network is thought to contribute to the pathogenesis of absence epilepsy, the effect of the mutation on thalamocortical synaptic responses remains unknown. Here we showed imbalanced impairment of inhibitory synaptic responses in tg mice using brain slice preparations. Somatosensory thalamocortical projection makes not only monosynaptic glutamatergic connections but also disynaptic GABAergic connections, which mediate feedforward inhibition, onto layer IV neurons. In tg mice, IPSC amplitudes recorded from layer IV pyramidal cells of the somatosensory cortex in response to thalamic stimulation became disproportionately reduced compared with EPSC amplitudes at later developmental stages (postnatal days 21-30). Similar results were obtained by local stimulation of layer IV pyramidal neurons. However, IPSC reduction was not seen in layer V pyramidal neurons of epileptic tg mice or in layer IV pyramidal neurons of younger tg mice before the onset of epilepsy (postnatal days 14-16). These results showed that the feedforward inhibition from the thalamus to layer IV neurons of the somatosensory cortex was severely impaired in tg mice and that the impairment of the inhibitory synaptic transmission was correlated to the onset of absence epilepsy.

Starting and stopping mechanisms of absence epileptic seizures are revealed by hazard functions

We show that the hazard function provides useful information about the starting and the stopping mechanisms of absence epileptic seizures. The hazard function quantifies changes in the probability that an event (respectively, the starting and the stopping of a seizure) occurs in some small time interval given that it has not occurred yet. It informs us about changes in the concentration of endogenous substances that modulate the neuronal signalling properties of (parts of) the brain. In a pharmacological experiment, we used the hazard function to study the effect of a GABA-transaminase inhibitor (vigabatrin) on the starting and the stopping mechanisms of absence epileptic seizures in a genetic rat model of absence epilepsy (the WAG/Rij rat). This experiment showed that a high GABA level changed the stopping mechanism of the absence epileptic seizures, creating much better conditions for very long seizures to develop. With respect to the starting mechanism, it was found that both with a high and a low GABA level, there was evidence for a recovery mechanism that decreases the probability that a new seizure starts. Initially, this probability is larger with a high GABA level, but gradually it converges to the same constant baseline probability as in the condition with a low GABA level.

The convulsive and electroencephalographic changes produced by nonpeptidic delta-opioid agonists in rats: comparison with pentylenetetrazol

delta-Opioid agonists produce convulsions and antidepressant-like effects in rats. It has been suggested that the antidepressant-like effects are produced through a convulsant mechanism of action either through overt convulsions or nonconvulsive seizures. This study evaluated the convulsive and seizurogenic effects of nonpeptidic delta-opioid agonists at doses that previously were reported to produce antidepressant-like effects. In addition, delta-opioid agonist-induced electroencephalographic (EEG) and behavioral changes were compared with those produced by the chemical convulsant pentylenetetrazol (PTZ). For these studies, EEG changes were recorded using a telemetry system before and after injections of the delta-opioid agonists [(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-methoxyphenyl)methyl]-N,N-diethylbenz (SNC80) and [(+)-4-[alpha(R)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-hydroxyphenyl)methyl]-N,N-diethylbenzamide [(+)-BW373U86]. Acute administration of nonpeptidic delta-opioid agonists produced bilateral ictal and paroxysmal spike and/or sharp wave discharges. delta-Opioid agonists produced brief changes in EEG recordings, and tolerance rapidly developed to these effects; however, PTZ produced longer-lasting EEG changes that were exacerbated after repeated administration. Studies with antiepileptic drugs demonstrated that compounds used to treat absence epilepsy blocked the convulsive effects of nonpeptidic delta-opioid agonists. Overall, these data suggest that delta-opioid agonist-induced EEG changes are not required for the antidepressant-like effects of these compounds and that neural circuitry involved in absence epilepsy may be related to delta-opioid agonist-induced convulsions. In terms of therapeutic development, these data suggest that it may be possible to develop delta-opioid agonists devoid of convulsive properties.

Beta and Gamma Range EEG Power-Spectrum Correlation with Spiking Discharges in DBA/2J Mice Absence Model: Role of GABAB Receptors

PURPOSE

To describe the correlations between spiking pattern and EEG power spectrum frequency in DBA/2J mice, a model for murine absence seizures, after gamma-aminobutyric acid (GABA)(B) modulation.

METHODS

The animals were first tested with the GABA(B) agonist l-baclofen followed by the GABA(B) antagonist SCH 50911. Moreover, digital EEGs recorded under experimental conditions were processed at baseline and 10 and 20 min after l-baclofen injection. This procedure was followed by injection of the GABA(B) antagonist SCH50911 and by an additional EEG evaluation at 10 and 20 min from drug administration. The power spectra analysis of signals was obtained for delta (0.5-3 Hz), theta (3.5-7.5 Hz), alpha (8-12 Hz), beta (13-20 Hz), and gamma (21-50 Hz) frequencies.

RESULTS

The spiking pattern and power spectrum of beta activity was increased by <or=80% after administration of 5 mg/kg l-baclofen, whereas gamma power frequency decreased to the same extent. After administration of 50 mg/kg SCH 50911, spiking activity and beta power frequencies were markedly reduced (>80%), whereas gamma power increased (correlation, 0.92; p < 0.001). The remaining frequency bands were unaffected.

CONCLUSIONS

This study confirms the potential of GABA(B) antagonists in contrasting seizure absence in rodent models and suggests the application of drugs with a similar mechanism in humans. In addition, because GABA(B) antagonists not only contrast seizure in rodent models of absence but also improve "cognitive" performance, it could be hypothesized that gamma increase, correlated with optimized cortical binding during coherent percepts, may produce potential cognition-enhancing effects.

Functional contribution of specific brain areas to absence seizures: role of thalamic gap-junctional coupling

The synchronized discharges typical of seizures have a multifactorial origin at molecular, cellular and network levels. During recent years, the functional role of gap-junctional coupling has received increased attention as a mechanism that may participate in seizure generation. We have investigated the possible functional roles of thalamic and hippocampal gap-junctional communication (GJC) in the generation of spike-and-wave discharges in a rodent model of atypical absence seizures. Seizures in this model spread throughout limbic, thalamic and neocortical areas. Rats were chronically implanted with cannulae to deliver drugs or saline, and local field potentials recordings were performed using intracerebral electrodes positioned in distinct brain areas. Initially, the effects on synaptic transmission of the gap-junctional blockers used in this study were determined. Neither carbenoxolone (CBX) nor 18-alpha-glycyrrhetinic acid altered chemical synaptic transmission at the concentrations tested. These two compounds, when injected via cannulae into the reticular nucleus of the thalamus (NRT), decreased significantly the duration of seizures as compared with saline injections or injections of the CBX inactive derivative glycyrrhizic acid. CBX injections into the hippocampus resulted in diminished seizure activity as well. NRT injections of trimethylamine, which presumably causes intracellular alkalinization (thereby promoting gap-junctional opening), enhanced seizures and spindle activity. These observations suggest that, in this rodent model, thalamic and limbic areas are involved in the synchronous paroxysmal activity and that GJC contributes to the spike-and-wave discharges.

The Effects of Ethosuximide on Amino Acids in Genetic Absence Epilepsy Rat Model

Genetic absence epilepsy rats from Strasbourg (GAERS), a selectively inbred strain of Wistar rats, has been validated as an experimental model for human absence epilepsy. In this model, systemic administration of ethosuximide (ETX) was shown to reduce the spike and wave discharges (SWD). In this study, gamma-aminobutyric acid (GABA) and L-glutamic acid levels in response to ETX injections (i.p., 100 mg/kg) were measured in the microdialysis samples collected from the ventrolateral thalamus (VLT) and the primary motor cortex (M1) area of Wistar rats and GAERS by using HPLC with fluorescent detection. Throughout the microdialysis procedure, continuous EEG recording was performed where ETX was shown to suppress the SWD activity. We demonstrated increased basal GABA levels in the M1 and VLT of GAERS, and ETX treatment did not produce any effect on higher GABA levels in the VLT, but suppressed the increased GABA levels significantly in the M1 of GAERS. All these findings denote the importance of corticothalamic circuitry and the role of increased GABA tonus in primary motor cortex and thalamus of GAERS. The primary motor cortex also seems to be involved in the SWD activity and ETX exerts, at least partially, its neurotransmitter effects through it.

The preferential mGlu2/3 receptor antagonist, LY341495, reduces the frequency of spike-wave discharges in the WAG/Rij rat model of absence epilepsy

We examined the expression and function of group-II metabotropic glutamate (mGlu) receptors in an animal model of absence seizures using genetically epileptic WAG/Rij rats, which develop spontaneous non-convulsive seizures after 2-3 months of age. Six-month-old WAG/Rij rats showed an increased expression of mGlu2/3 receptors in the ventrolateral regions of the somatosensory cortex, ventrobasal thalamic nuclei, and hippocampus, but not in the reticular thalamic nucleus and in the corpus striatum, as assessed by immunohistochemistry and Western blotting. In contrast, mGlu2/3 receptor signalling was reduced in slices prepared from the somatosensory cortex of 6-month-old WAG/Rij rats, as assessed by the ability of the agonist, LY379268, to inhibit forskolin-stimulated cAMP formation. None of these changes was found in "pre-symptomatic" 2-month-old WAG/Rij rats. To examine whether pharmacological activation or inhibition of mGlu2/3 receptors affects absence seizures, we recorded spontaneous spike-wave discharges (SWDs) in 6-month-old WAG/Rij rats systemically injected with saline, the mGlu2/3 receptor agonist LY379268 (0.33 or 1 mg/kg, i.p.), or with the preferential mGlu2/3 receptor antagonist, LY341495 (0.33, 1 or 5 mg/kg, i.p.). Injection of 1mg/kg of LY379268 (1 mg/kg, i.p.) increased the number of SWDs during 3-7 h post-treatment, whereas injection with LY341495 reduced the number of seizures in a dose-dependent manner. It can be concluded that mGlu2/3 receptors are involved in the generation of SWDs and that an upregulation of these receptors in the somatosensory cortex might be involved in the pathogenesis of absence epilepsy.

Levetiracetam in the Treatment of Idiopathic Generalized Epilepsies

Since its introduction into clinical practice in 1999, levetiracetam, the S enantiomer of piracetam, has rapidly found a secure place, initially in the therapy of partial onset seizures and subsequently in the treatment of idiopathic generalized epilepsies (IGE). It has many of the properties of an "ideal" antiepileptic drug, including rapid absorption, linear pharmokinetics, and sparse drug interactions. Tolerabiliy is generally excellent in both adults and children, although tiredness is a common dose-limiting adverse effect. Occasionally the drug can precipitate behavioral abnormalities, especially in patients with learning disability. There is a wide safety margin in overdose. In common with most antiepileptic drugs its mode of action remains uncertain. Levetiracetam binds to a specific site in the brain, influences intracellular calcium currents and reverses negative allosteric modulators of GABA- and glycine-gated currents in vitro. Its effectiveness has been demonstrated in animal models of epilepsy and in clinical trials of partial onset and IGE. Treatment of IGEs may be straightforward, with many patients demonstrating an excellent and robust response to valproate monotherapy. However, there remains a significant minority of patients for whom valproate is unsuitable, including those who experience unacceptable adverse effects (e.g., weight gain or hair loss) and women of childbearing age in whom the teratogenic potential of valproate is unacceptable. Therapeutic response to lamotrigine in this group is often disappointing, and many clinicians now are turning to the choice of levetiracetam. Efficacy in generalized tonic-clonic seizures and myoclonus is usually apparent and some patients experience improvement in typical absences. Experience of combinations of levetiracetam with other antiepileptic drugs is limited in IGE and the responses are largely anecdotal. In our hands, patients with refractory IGEs may respond to combinations of levetiracetam with valproate, lamotrigine, and phenobarbital, and adverse effects when they occur are usually limited to tiredness. Levetiracetam does not interact with the oral contraceptive pill, simplifying treatment in women of childbearing age. Although animal data look encouraging, questions over levetiracetam's teratogenic potential and overall safety in pregnancy will remain for many years to come.

Morphometric Golgi study of cortical locations in WAG/Rij rats: the cortical focus theory

Recently it was demonstrated that for the absence epilepsy characteristic spike-wave discharges initially emerge from the somatosensory cortex and quickly involve the rest of the cortex and cortico-thalamic network. This has led to the development of the focal theory of absence epilepsy. In this experiment, this theory was further investigated by studying the neuronal organization of the cortical focal zone, a non-focal zone in genetic epileptic WAG/Rij rats and functional related areas in non-epileptic age matched control rats. A classical Golgi staining technique was used to visualize whole cortical neurons with dendritic and axon arborisation. Apical dendrites of pyramidal cells in epileptic rats were often split, declined and were running in non-perpendicular directions. Quantitative differences between the strains were found for the length of neurons, between focal and control areas mainly for dendritic arborization. A significant "strain-zone" interaction was found for the maximal distance between two points of dendritic arborization, the mean length of a dendritic segment and the number of free terminations of apical dendrites. All this demonstrates that properties of dendrites in the cortical focal area of WAG/Rij rats were at variance with dendritic characteristics outside the focal area and with functional similar areas in non-epileptic controls. These features might reflect the hyperexcitability of somatosensory neurons, which underlie the initiation and spreading of spike-wave discharges in WAG/Rij rats. Finally, these results are in line with the cortical focus theory of absence epilepsy.

Mixed forms of epilepsy in a subpopulation of WAG/Rij rats

Mixed forms of epilepsy in patients are often refractory. Therefore, animal models of comorbid convulsive and nonconvulsive seizure are needed for experimental research. Susceptibility to audiogenic convulsions was studied in a large group of young and adult WAG/Rij rats with inherited absence epilepsy. In 30% of adult rats, sound stimulation provoked audiogenic seizures of moderate intensity. The seizures had two excitation periods separated by a remarkably stable "arrest" of paroxysmal movements. Up to 20% of young WAG/Rij rats were also susceptible to audiogenic seizures, with a longer latency, lower intensity, and more simple seizure patterns. No difference in manifestations of spike-wave discharges was observed between the WAG/Rij rats with and without audiogenic seizures. This subpopulation of WAG/Rij rats genetically predisposed to absence and audiogenic seizures is proposed as an animal model suitable for investigation of basal mechanisms and pharmacological profiles of this mixed form of epilepsy.

Cortical control of generalized absence seizures: effect of lidocaine applied to the somatosensory cortex in WAG/Rij rats

The role of the somatosensory cortex (SmI) in the incidence of spike-wave discharges (SWDs) was studied in a genetic model of absence epilepsy, WAG/Rij rats. SWDs were recently shown to initiate at the perioral area of the SmI and spread over the cortex and thalamus within a few milliseconds [J. Neurosci. 22 (2002) 1480]. It was hypothesized that functional deactivation of the SmI might reduce the appearance of SWDs. This was tested using unilateral microinjections (1 microl) of 2% lidocaine into the SmI in 13 WAG/Rij rats. Electrocorticogram (ECoG) was recorded in free moving animals from four cortical sites after lidocaine and control (saline) injections. Lidocaine effectively diminished the power of the ECoG spectra mostly in the area surrounding the injection site. Deactivation of the perioral region of the SmI reduced the incidence of SWDs at the entire cortex in both hemispheres. The number of SWDs gradually reached control level at the end of the second hour after injections of lidocaine. These data show that proper functioning of SmI is important for the occurrence of SWDs, supporting the idea that absence seizures might have a focal cortical origin.

Is the persistent sodium current a specific target of anti-absence drugs?

The persistent Na+ current (INaP) has been proposed as the putative target of the anti-absence antiepileptic drugs. Accordingly, the effect of reference anti-absence drugs ethosuximide (ESM) and valproate (VPA), and of the new antiepileptic drug levetiracetam (LEV), on INaP have been tested in CA1 hippocampal neurons and compared to the classic anticonvulsant phenytoin (PHT) and the neuroprotective agent riluzole (RIL). Whole-cell patch-clamp recordings of the slowly inactivating current, fully characterized as INaP, were performed with a standard voltage-step protocol on thin hippocampal slices prepared from rat brain. Both PHT (100 microM) and RIL (10 microM) strongly depressed INaP, whereas ESM (1 mM) induced a slight decrease of INaP and VPA (1 mM) had no effect. Likewise, 60-min perfusion with relevant concentrations of LEV (10, 32 or 100 microM) did not modify INaP. In conclusion, these data question the impact of INaP depression as an anti-absence mechanism, and also disclaim the involvement of INaP in the antiepileptic mechanism of LEV.