The neurobiology of antiepileptic drugs

Identification of aquaporin 4 inhibitors using in vitro and in silico methods

The in vitro inhibitory effects and in silico docking energies of 18 compounds with respect to aquaporin 4 (AQP4) were investigated. More than half of the compounds tested showed inhibitory activity in the in vitro functional assay and included the 5-HT(1B/1D) agonists sumatriptan, and rizatriptan. Moreover, the observed inhibitory activity of the compounds used in this study at 20 microM showed a strong correlation with their in silico docking energies, r(2)=0.64, which was consistent with that found in previous studies. The AQP4 inhibitory IC(50) values of three compounds, 2-(nicotinamido)-1,3,4-thiadiazole, sumatriptan and rizatriptan, were subsequently found to be 3, 11, and 2 microM, respectively.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

SV2A protein is a broad-spectrum anticonvulsant target: functional correlation between protein binding and seizure protection in models of both partial and generalized epilepsy

SV2A, a synaptic vesicle protein, has been recently identified as a binding target for levetiracetam (Keppra). The specific mechanism by which SV2A binding leads to seizure protection has not yet been fully elucidated. However, a functional correlation between SV2A binding affinity and anticonvulsant potency has been observed in the mouse audiogenic seizure model. The present study was undertaken to test whether similar correlations exist in rodent models of partial and generalized epilepsies. As expected, there was a high degree of correlation between anticonvulsant potency and SV2A binding affinity in the mouse audiogenic seizure model (r(2)=0.77; p<0.001). A similar correlation was also observed in the mouse corneal kindling (r(2)=0.80; p<0.01) and in the rat model of generalized absence epilepsy (GAERS) (r(2)=0.72; p<0.01). Moreover, there were no significant differences between the slopes and intercepts of regression lines in these models. Interestingly, the protective potencies in these three epilepsy models were also well correlated with each other. As such, protective doses of a given SV2A ligand in one model could be easily predicted based on the data obtained in another model. Taken together, these results support the concept that SV2A protein is an important target for both partial and generalized epilepsies and thereby relevant for the generation of new antiepileptic drugs with potential broad-spectrum efficacy.

Early treatment suppresses the development of spike-wave epilepsy in a rat model

PURPOSE

Current treatments for epilepsy may control seizures, but have no known effects on the underlying disease. We sought to determine whether early treatment in a model of genetic epilepsy would reduce the severity of the epilepsy phenotype in adulthood.

METHODS

We used Wistar albino Glaxo rats of Rijswijk (WAG/Rij) rats, an established model of human absence epilepsy. Oral ethosuximide was given from age p21 to 5 months, covering the usual period in which seizures develop in this model (age approximately 3 months). Two experiments were performed: (1) cortical expression of ion channels Nav1.1, Nav1.6, and HCN1 (previously shown to be dysregulated in WAG/Rij) measured by immunocytochemistry in adult treated rats; and (2) electroencephalogram (EEG) recordings to measure seizure severity at serial time points after stopping the treatment.

RESULTS

Early treatment with ethosuximide blocked changes in the expression of ion channels Nav1.1, Nav1.6, and HCN1 normally associated with epilepsy in this model. In addition, the treatment led to a persistent suppression of seizures, even after therapy was discontinued. Thus, animals treated with ethosuximide from age p21 to 5 months still had a marked suppression of seizures at age 8 months.

DISCUSSION

These findings suggest that early treatment during development may provide a new strategy for preventing epilepsy in susceptible individuals. If confirmed with other drugs and epilepsy paradigms, the availability of a model in which epileptogenesis can be controlled has important implications both for future basic studies, and human therapeutic trials.

Felbamate but not phenytoin or gabapentin reduces glutamate release by blocking presynaptic NMDA receptors in the entorhinal cortex

We have shown that a number of anticonvulsant drugs can reduce glutamate release at synapses in the rat entorhinal cortex (EC) in vitro. We have also shown that presynaptic NMDA receptors (NMDAr) tonically facilitate glutamate release at these synapses. In the present study we determined whether, phenytoin, gabapentin and felbamate may reduce glutamate release by blocking the presynaptic NMDAr. Whole cell patch clamp recordings of spontaneous excitatory postsynaptic currents (sEPSCs) were used as a monitor of presynaptic glutamate release. Postsynaptic NMDAr were blocked with internal dialysis with an NMDAr channel blocker. The antagonist, 2-AP5, reduced the frequency of sEPSCs by blocking the presynaptic facilitatory NMDAr, but did not occlude a reduction in sEPSC frequency by gabapentin or phenytoin. Felbamate also reduced sEPSC frequency, but this effect was occluded by prior application of 2-AP5. Thus, whilst all three drugs can reduce glutamate release, only the action of felbamate seems to be due to interaction with presynaptic NMDAr.

Postnatal concerns in children born to women with epilepsy

Infants born to mothers with epilepsy are at substantial risk for neurocognitive and behavioral disorders. Although exposure of the child to antiepileptic drugs (AEDs) during pregnancy and postnatally through breast milk has been implicated in disorders of higher cortical function, there have been relatively few clinical or animal studies examining the long-term effects of AEDs on cognition in the developing brain. In the limited animal studies done thus far, drug-specific effects on cognitive function have been identified. Phenobarbital, in particular, has been found to lead to adverse cognitive outcomes, whereas the newer AEDs have generally had more favorable outcomes. Although the pathophysiological mechanisms responsible for these deficits remain largely unknown, there is evidence that AEDs can adversely effect neuronal proliferation and migration, and increase apoptosis. While animal studies can provide valuable information regarding mechanism of AED-induced developmental pathology, they do not provide insight into cortical functions unique to humans, such as speech and language. Understanding the full spectrum of AED-induced effects on the developing brain will require both rigorous basic science and clinical studies.

Clinical pharmacology and mechanism of action of zonisamide

Antiepileptic drugs (AEDs) suppress seizures by selectively modifying the excitability of neurons and blocking seizure firing with minimal disturbance of nonepileptic activity. All AEDs have been shown to work by at least one of 3 main mechanisms of action: through modulation of voltage-gated ion channels, enhancement of synaptic inhibition, and inhibition of synaptic excitation. Zonisamide is a novel AED that has a broad combination of complementary mechanisms of action, which may offer a clinical advantage over other antiepileptic agents. By altering the fast inactivation threshold of voltage-dependent sodium channels, zonisamide reduces sustained high-frequency repetitive firing of action potentials. Zonisamide also inhibits low-threshold T-type calcium channels in neurons, which may prevent the spread of seizure discharge across cells. In addition, zonisamide is a weak inhibitor of carbonic anhydrase. However, this mechanism is not believed to contribute to the antiepileptic activity of zonisamide. Although zonisamide also seems to alter dopamine, serotonin, and acetylcholine metabolism, it is not clear to what extent these effects on neurotransmitters are involved in the clinical actions of the drug. In addition to these actions, recent evidence suggests that zonisamide may exert neuroprotective actions, independent of its antiepileptic activity. These potential effects may be important in preventing neuronal damage caused by recurrent seizures. Therefore, it seems that the multiple pharmacological actions of zonisamide may contribute to the seizure reductions observed in a wide range of epilepsies and may help to preserve efficacy in individual patients despite possible changes in electrophysiological status.

Emerging drugs for epilepsy

Epilepsy affects < or = 1% of the world's population. Antiepileptic drugs (AEDs) are the mainstay of treatment, although more than a third of patients are not rendered seizure free with existing medications. Uncontrolled epilepsy is associated with increased mortality and physical injuries, and a range of psychosocial morbidities, posing a substantial economic burden on individuals and society. Limitations of the present AEDs include suboptimal efficacy and their association with a host of adverse reactions. Continued efforts are being made in drug development to overcome these shortcomings employing a range of strategies, including modification of the structure of existing drugs, targeting novel molecular substrates and non-mechanism-based drug screening of compounds in traditional and newer animal models. This article reviews the need for new treatments and discusses some of the emerging compounds that have entered clinical development. The ultimate goal is to develop novel agents that can prevent the occurrence of seizures and the progression of epilepsy in at risk individuals.

T-current related effects of antiepileptic drugs and a Ca2+ channel antagonist on thalamic relay and local circuit interneurons in a rat model of absence epilepsy

Channel blocking, anti-oscillatory, and anti-epileptic effects of clinically used anti-absence substances (ethosuximide, valproate) and the T-type Ca2+ current (IT) blocker mibefradil were tested by analyzing membrane currents in acutely isolated local circuit interneurons and thalamocortical relay (TC) neurons, slow intrathalamic oscillations in brain slices, and spike and wave discharges (SWDs) occurring in vivo in Wistar Albino Glaxo rats from Rijswijk (WAG/Rij). Substance effects in vitro were compared between WAG/Rij and a non-epileptic control strain, the ACI rats. Ethosuximide (ETX) and valproate were found to block IT in acutely isolated thalamic neurons. Block of IT by therapeutically relevant ETX concentrations (0.25-0.75 mM) was stronger in WAG/Rij, although the maximal effect at saturating concentrations (>or=10 mM) was stronger in ACI. Ethosuximide delayed the onset of the low threshold Ca2+ spike (LTS) of neurons recorded in slice preparations. Mibefradil (>or=2 microM) completely blocked IT and the LTS, dampened evoked thalamic oscillations, and attenuated SWDs in vivo. Computational modeling demonstrated that the complete effect of ETX can be replicated by a sole reduction of IT. However, the necessary degree of IT reduction was not induced by therapeutically relevant ETX concentrations. A combined reduction of IT, the persistent sodium current, and the Ca2+ activated K+ current resulted in an LTS alteration resembling the experimental observations. In summary, these results support the hypothesis of IT reduction as part of the mechanism of action of anti-absence drugs and demonstrate the ability of a specific IT antagonist to attenuate rhythmic burst firing and SWDs.

Clinical use of antiepileptic drugs at a referral centre for epilepsy

PURPOSE

The National Centre for Epilepsy in Norway admits patients with refractory epilepsy from the whole country. The purpose of this study was to investigate how antiepileptic drugs (AEDs) are used at the centre and compare it with the total consumption in the country and international guidelines regarding clinical use of AEDs.

MATERIAL AND METHODS

A prevalence study was carried out from patient records from 264 patients (136 adults and 128 children). The use of AEDs, gender, polytherapy, common drug combinations, serum concentration measurements, concomitant medication and comorbid conditions were investigated.

RESULTS

Of the 15 AEDs in use, valproate, lamotrigine and levetiracetam were most frequently used. In the country at large, carbamazepine, valproate and lamotrigine were used the most. Valproate and lamotrigine occurred most frequently in combination. In adults, oxcarbazepine and topiramate were used more frequently in women than in men. Children used benzodiazepines three times as often as adults. Newer AEDs were mostly used for partial seizures, in accordance with international guidelines. Thirty-five percent of adults and 20% of children suffered from comorbid CNS-related conditions. The use of concomitant medication was widespread. Serum concentrations were in accordance to recommended therapeutic ranges.

CONCLUSION

The results demonstrate that newer AEDs were much more frequently used at the epilepsy centre than in the country as a whole. The use of polytherapy was common, and therapeutic drug monitoring was used for optimal individualized drug treatment. AEDs were used in accordance to international guidelines and recommendations, which can serve as a reference for other prescribers.

Extracellular proton-modulated pore-blocking effect of the anticonvulsant felbamate on NMDA channels

Felbamate (FBM) is a potent nonsedative anticonvulsant whose clinical effect is chiefly related to gating modification (and thus use-dependent inhibition) rather than pore block of N-methyl-D-aspartate (NMDA) channels at pH 7.4. Using whole-cell recording in rat hippocampal neurons, we examined the effect of extracellular pH on FBM action. In sharp contrast to the findings at pH 7.4, the inhibitory effect of FBM on NMDA currents shows much weakened use-dependence at pH 8.4. Moreover, FBM neither accelerates the activation kinetics of the NMDA channel, nor enhances the currents elicited by very low concentrations of NMDA at pH 8.4. These differential effects of FBM between pH 7.4 and 8.4 are abolished in the mutant NMDA channels which lack proton sensitivity. Most interestingly, the inhibitory effect of FBM becomes flow-dependent and is evidently stronger in inward than in outward NMDA currents at pH 8.4. These findings indicate that FBM has a significantly more manifest pore-blocking effect on the NMDA channel at pH 8.4 than at pH 7.4. FBM therefore acts as an opportunistic pore blocker modulated by extracellular proton, suggesting that the FBM binding site is located at the junction of a widened and a narrow part of the ion conduction pathway. Also, we find that the inhibitory effect of FBM on NMDA currents is antagonized by external but not internal Na+, and that increase of external Na+ decreases the binding rate without altering the unbinding rate of FBM. These findings indicate that the FBM binding site faces the extracellular rather than the intracellular solution, and coincides with the outmost ionic (e.g., Na+) site in the NMDA channel pore. We conclude that the FBM binding site very likely is located in the external pore mouth, where extracellular proton, Na+, FBM, and NMDA channel gating have an orchestrating effect.

The anaesthetic and intensive care of status epilepticus

PURPOSE OF REVIEW

Status epilepticus refractory to first and second-line anticonvulsants represents an outstanding challenge to the medical team. This review covers new insights into the anaesthetic and intensive care of status epilepticus.

RECENT FINDINGS

In the anaesthetic treatment of status epilepticus, barbiturates, midazolam, or propofol are the drugs of choice, but the lack of controlled prospective data as yet does not allow the recommendation of a preference for one of the substances, unequivocally. Recent observational studies reported on inhalational anaesthetics and supplementary nonanaesthetics such as levetiracetam or topiramate that may be administered if status epilepticus is not controlled by intravenous anaesthetics. Nonpharmacological approaches including resective surgery, brain stimulation, and hypothermia may be applied in selected patients, only. Psychogenic nonepileptic status epilepticus commonly mimics refractory generalized convulsive status epilepticus, and clinical features discerning the two conditions are presented. The underlying cause is the major contributor for a difficult-to-treat 'malignant' course of status epilepticus and together with age determines mortality and functional outcome.

SUMMARY

The common substances administered for the anaesthetic treatment of status epilepticus require assessment in a prospective randomized controlled trial. Alternative pharmacological or nonpharmacological approaches need further studies as well before they can be recommended as part of the therapeutic regime.

Refractory seizures associated with an organic aciduria in a dog

A 6-month-old, female Cavalier King Charles spaniel exhibited seizures that were difficult to control with standard anticonvulsants over a 12-month period. The diagnosis of an organic aciduria with excessive excretion of hexanoylglycine was determined when the dog was 20 months old. Recurrent and cluster seizures were eventually controlled with the addition of levetiracetam to potassium bromide and phenobarbital.

Formulation of Spray-Dried Phenytoin Loaded Poly(ε-Caprolactone) Microcarrier Intended for Brain Delivery to Treat Epilepsy

This study evaluates the efficacy of the spray-drying technique in the bioengineering of phenytoin (PHT) containing poly(epsilon-caprolactone) (PCL) microcarrier intended for brain delivery for long-term treatment of epilepsy. Through orthogonally designed experiments, the optimal formulation and process variables for the preparation of PCL-microcarriers containing PHT were obtained. The produced microcarriers were characterized by coulter counter, scanning electron, scanning transmission electron microscopies, differential scanning calorimetry, powder X-ray diffraction, and in vitro release. The results showed that the produced microcarriers have a spherical structure, uniform size distribution, and a particle mean diameter of about 4.0 microm, which is suitable for brain delivery. The PHT was loaded as dispersed microcrystals within the PCL-microcarriers. From this system, PHT was released slowly into a buffer solution for approximately 14 days without any burst effect. These data suggested that PHT containing spray-dried PCL-microcarrier may be a promising drug delivery system for local brain delivery and long-term treatment of pharmacoresistant epilepsy.

Rat epileptic seizures evoked by BmK αIV and its possible mechanisms involved in sodium channels

This study showed that rat unilateral intracerebroventricular injection of BmK alphaIV, a sodium channel modulator derived from scorpion Buthus martensi Karsch, induced clusters of spikes, epileptic discharges and convulsion-related behavioral changes. BmK alphaIV potently promoted the release of endogenous glutamate from rat cerebrocortical synaptosomes. In vitro examination of the effect of BmK alphaIV on intrasynaptosomal free calcium concentration [Ca(2+)](i) and sodium concentration [Na(+)](i) revealed that BmK alphaIV-evoked glutamate release from synaptosomes was associated with an increase in Ca(2+) and Na(+) influx. Moreover, BmK alphaIV-mediated glutamate release and ion influx was completely blocked by tetrodotoxin, a blocker of sodium channel. Together, these results suggest that the induction of BmK alphaIV-evoked epileptic seizures may be involved in the modulation of BmK alphaIV on tetrodotoxin-sensitive sodium channels located on the nerve terminal, which subsequently enhances the Ca(2+) influx to cause an increase of glutamate release. These findings may provide some insight regarding the mechanism of neuronal action of BmK alphaIV in the central nervous system for understanding epileptogenesis involved in sodium channels.

On Demand Up-regulation of Therapeutic Genes in the Brain: Fiction or Reality?

Enhancing GABAAReceptor α 1 Subunit Levels in Hippocampal Dentate Gyrus Inhibits Epilepsy Development in an Animal Model of Temporal Lobe Epilepsy Raol YH, Lund IV, Bandyopadhyay S, Zhang G, Roberts DS, Wolfe JH, Russek SJ, Brooks-Kayal AR. J Neurosci 2006;26(44):11342–11346. Differential expression of GABAA receptor (GABR) subunits has been demonstrated in hippocampus from patients and animals with temporal lobe epilepsy (TLE), but whether these changes are important for epileptogenesis remains unknown. Previous studies in the adult rat pilocarpine model of TLE found reduced expression of GABR α1 subunits and increased expression of α4 subunits in dentate gyrus (DG) of epileptic rats compared with controls. To investigate whether this altered subunit expression is a critical determinant of spontaneous seizure development, we used adeno-associated virus type 2 containing the α4 subunit gene (GABRA4) promoter to drive transgene expression in DG after status epilepticus (SE). This novel use of a condition-dependent promoter upregulated after SE successfully increased expression of GABR α1 subunit mRNA and protein in DG at 1–2 weeks after SE. Enhanced α1 expression in DG resulted in a threefold increase in mean seizure-free time after SE and a 60% decrease in the number of rats developing epilepsy (recurrent spontaneous seizures) in the first 4 weeks after SE. These findings provide the first direct evidence that altering GABR subunit expression can affect the development of epilepsy and suggest that α1 subunit levels are important determinants of inhibitory function in hippocampus.

The efficacy and tolerability of levetiracetam in pharmacoresistant epileptic dogs

Twenty-two dogs with idiopathic epilepsy which were pharmacoresistant to phenobarbitone and bromide were treated with levetiracetam as an add-on medication. Records of eight dogs were used retrospectively to determine a safe, efficient levetiracetam dosage. Fourteen dogs were entered into a prospective, open label, non-comparative study. After 2 months of levetiracetam oral treatment (10 mg/kg TID), 8/14 dogs responded significantly to the treatment and seizure frequency was reduced by 50%. In dogs that remained refractory, the dosage was increased to 20 mg/kg TID for 2 months. One further dog responded to levetiracetam treatment. Levetiracetam responders had a significant decrease in seizure frequency of 77% (7.9+/-5.2 to 1.8+/-1.7 seizures/month) and a decrease in seizure days per month of 68% (3.8+/-1.7 to 1.2+/-1.1 seizure days/month). However, 6/9 responders experienced an increase in seizure frequency and seizure days after 4-8 months continuing with the levetiracetam treatment at the last effective dosage. Levetiracetam was well tolerated by all dogs and sedation was the only side-effect reported in just one of the 14 dogs.

Characterization of the gating conformational changes in the felbamate binding site in NMDA channels

The anticonvulsant effect of felbamate (FBM) is ascribable to inhibition of N-methyl-d-aspartate (NMDA) currents. Using electrophysiological studies in rat hippocampal neurons to examine the kinetics of FBM binding to and unbinding from the NMDA channel, we show that FBM modifies NMDA channel gating via a one-to-one binding stoichiometry and has quantitatively the same enhancement effect on NMDA and glycine binding to the NMDA channel. Moreover, the binding rates of FBM to the closed and the open/desensitized NMDA channels are 187.5 and 4.6 x 10(4) M(-1) s(-1), respectively. The unbinding rates of FBM from the closed and the open/desensitized NMDA channels are approximately 6.2 x 10(-2) and approximately 3.1 s(-1), respectively. From the binding and unbinding rate constants, apparent dissociation constants of approximately 300 and approximately 70 microM could be calculated for FBM binding to the closed and the open/desensitized NMDA channels, respectively. The slight (approximately fourfold) difference in FBM binding affinity to the closed and to the open/desensitized NMDA channels thus is composed of much larger differences in the binding and unbinding kinetics (approximately 250- and approximately 60-fold difference, respectively). These findings suggest that the effects of NMDA and glycine binding coalesce or are interrelated before or at the actual activation gate, and FBM binding seems to modulate NMDA channel gating at or after this coalescing point. Moreover, the entrance zone of the FBM binding site very likely undergoes a much larger conformational change along the gating process than that in the binding region(s) of the binding site. In other words, the FBM binding site becomes much more accessible to FBM with NMDA channel activation, although the spatial configurations of the binding ligand(s) for FBM themselves are not altered so much along the gating process.

Voltage-gated Na+channels in neuropathic pain

Neuropathic pain occurs as a result of some form of injury to the nervous system. Although the basis of the disease remains to be fully elucidated, numerous studies have suggested a major role for ion channels in the pathogenesis of neuropathic pain. As Na+ channels play a fundamental role in not only the generation but also in the conduction of an action potential, they have received considerable attention in the aetiology of pain sensation and have become important pharmacological targets. In this review, the authors discuss the importance of specific Na+ channel isoforms in the pathophysiology of neuropathic pain and the present use of Na+ channel antagonists in the treatment of neuropathic pain.

Antiepileptic drugs in migraine: from clinical aspects to cellular mechanisms

Migraine and epilepsy share several clinical features, and epilepsy is a comorbid condition of migraine. Clinical studies have shown that some antiepileptic drugs are effective at preventing migraine attacks. A rationale for their use in migraine prophylaxis is the hypothesis that migraine and epilepsy share several common pathogenetic mechanisms. An imbalance between excitatory glutamate-mediated transmission and GABA-mediated inhibition in specific brain areas has been postulated in these two pathological conditions. Moreover, abnormal activation of voltage-operated ionic channels has been implicated in both migraine and epilepsy. Cortical spreading depression has been found to be involved in the pathophysiology of epilepsy, in addition to the generation of migraine aura.

Channelopathies in idiopathic epilepsy

Approximately 70% of all patients with epilepsy lack an obvious extraneous cause and are presumed to have a predominantly genetic basis. Both familial and de novo mutations in neuronal voltage-gated and ligand-gated ion channel subunit genes have been identified in autosomal dominant epilepsies. However, patients with dominant familial mutations are rare and the majority of idiopathic epilepsy is likely to be the result of polygenic susceptibility alleles (complex epilepsy). Data on the identity of the genes involved in complex epilepsy is currently sparse but again points to neuronal ion channels. The number of genes and gene families associated with epilepsy is rapidly increasing and this increase is likely to escalate over the coming years with advances in mutation detection technologies. The genetic heterogeneity underlying idiopathic epilepsy presents challenges for the rational selection of therapies targeting particular ion channels. Too little is currently known about the genetic architecture of the epilepsies, and genetic testing for the known epilepsy genes remains costly. Pharmacogenetic studies have yet to explain why 30% of patients do not respond to the usual antiepileptic drugs. Despite this, the recognition that the idiopathic epilepsies are a group of channelopathies has, to a limited extent, explained the therapeutic action of the common antiepileptic drugs and has assisted clinical diagnosis of some epilepsy syndromes.

Pharmacokinetics and Drug Interactions with Zonisamide

Polypharmacy is a widely employed treatment strategy in epilepsy, particularly for individuals with poorly controlled seizures. Drug combinations should be carefully considered to minimize the potential for unfavorable interactions. Older-generation antiepileptic drugs (AEDs) are well known for their pharmacokinetic interaction potential, which generally results from alterations in the metabolism of concomitant drugs due to effects on the cytochrome P450 (CYP) and uridine glucuronyl transferase enzyme systems. Newer agents, such as zonisamide, are less likely to cause adverse drug interactions. A series of interaction studies has revealed zonisamide to be without effect on the steady-state pharmacokinetics of carbamazepine, phenytoin, sodium valproate, or lamotrigine. However, zonisamide is principally inactivate by CY3A4-dependent reduction. Consequently, carbamazepine, phenytoin, and phenobarbital all increase its clearance, an interaction that may necessitate a dosage increase, but which will also permit more rapid attainment of steady-state zonisamide concentrations. Otherwise, zonisamide is essentially devoid of clinically significant interactions with other AEDs, oral contraceptives and, indeed, all other classes of therapeutic agents investigated to date. As a result, it is reasonable to conclude that zonisamide has a favorable pharmacokinetic profile and that it may be a useful and uncomplicated agent when employed as adjunctive therapy in refractory epilepsy.

Valproic acid-mediated neuroprotection in intracerebral hemorrhage via histone deacetylase inhibition and transcriptional activation

The modification of histone N-terminal tails by acetylation or deacetylation can alter the interaction between histones and DNA, and thus regulate gene expression. Recent experiments have demonstrated that valproic acid (VPA), a well-known anti-epileptic drug, can directly inhibit histone deacetylase (HDAC) activity and cause the hyperacetylation of histones. Moreover, VPA has been shown to mediate neuronal protection by activating signal transduction pathways and by inhibiting proapoptotic factors. In this study, we attempted to determine whether VPA alleviates cerebral inflammation and perihematomal cell death after intracerebral hemorrhage (ICH). Adult male rats received intraperitoneal injections of 300 mg/kg VPA or PBS twice a day after ICH induction. VPA treatment inhibited hematoma expansion, perihematomal cell death, caspase activities, and inflammatory cell infiltration. In addition, VPA treatment had the following expressional effects; it activated the translations of acetylated histone H3, pERK, pAKT, pCREB, and HSP70; up-regulated bcl-2 and bcl-xl but down-regulated bax; and down-regulated the mRNAs of Fas-L, IL-6, MMP-9, MIP-1, MCP-1, and tPA. VPA-treated rats also showed better functional recovery from 1 day to 4 weeks after ICH. Here we show that VPA induces neuroprotection in a murine ICH model and that its neuroprotective effects are mediated by transcriptional activation following HDAC inhibition.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Combination therapy in epilepsy: when and what to use

After being regarded as a last resort for over two decades, the role of combination therapy as a treatment strategy for epilepsy is undergoing re-evaluation. This is a result of the growing appreciation that all seizures cannot be controlled by monotherapy in a substantial proportion of patients, and of the development of a range of modern antiepileptic drugs (AEDs), some of which are better tolerated and less prone to complex pharmacokinetic drug interactions than their older counterparts.Robust evidence to guide clinicians on when and how to combine AEDs is lacking, and current practice recommendations are largely empirical. Monotherapy should remain the treatment of choice for newly diagnosed epilepsy. A combination of two AEDs can be considered after failure, resulting from lack of efficacy, of one or two different monotherapy regimens. A few patients will become seizure-free with a combination of three AEDs, but treatment with a combination of four or more is unlikely to be successful. There is some evidence to support a pharmacomechanistic approach to AED combination. Care should be taken to avoid excessive drug load, which is associated with increased toxicity. Bigger and better randomised, controlled studies are needed to determine the optimal time and way to combine AEDs.

Antiepileptic drugs and muscarinic receptor-dependent excitation in the rat subiculum

Field and intracellular recordings were made in an in vitro slice preparation to establish whether the antiepileptic drugs topiramate and lamotrigine modulate cholinergic excitation in the rat subiculum. Bath application of carbachol (CCh, 70-100microM) induced: (i) spontaneous and synchronous field oscillations (duration=up to 7s) that were mirrored by intracellular depolarizations with rhythmic action potential bursts; and (ii) depolarizing plateau potentials (DPPs, duration=up to 2.5s) associated with action potential discharge in response to brief (50-100ms) intracellular depolarizing current pulses. Ionotropic glutamatergic receptor antagonists abolished the field oscillations without influencing DPPs, while atropine (1microM) markedly reduced both types of activity. Topiramate (10-100microM, n=8-13 slices) or lamotrigine (50-400microM, n=3-12) decreased in a dose-dependent manner, and eventually abolished, CCh-induced field oscillations. During topiramate application, these effects were accompanied by marked DPP reduction. When these antiepileptic drugs were tested on DPPs recorded in the presence of CCh+ionotropic glutamatergic and GABA receptor antagonists, only topiramate reduced DPPs (n=5-19/dose; IC(50)=18microM, n=48). Similar effects were induced by topiramate during metabotropic glutamate receptor antagonism (n=5), which did not influence DPPs. Thus, topiramate and lamotrigine reduce CCh-induced epileptiform synchronization in the rat subiculum but only topiramate is effective in controlling DPPs. We propose that muscarinic receptor-mediated excitation represents a target for the action of some antiepileptic drugs such as topiramate.

Ca2+ channel alpha2delta ligands: novel modulators of neurotransmission

The term 'Ca2+ channel alpha2delta ligands' has recently been applied to an evolving drug class that includes gabapentin (Neurontin) and pregabalin (Lyrica), and reflects significant progress over the past decade in elucidating the mechanism of action of these drugs: a novel, specific action at one of the subunits constituting voltage-sensitive Ca2+ channels. Binding of these ligands to the alpha2delta subunit is considered to explain their usefulness in treating several clinical disorders, including epilepsy, pain from diabetic neuropathy, postherpetic neuralgia and fibromyalgia, and generalized anxiety disorder. The evidence indicates a relationship between alpha2delta subunit binding and the modulation of processes that subserve neurotransmission. This modulation is characterized by a reduction of the excessive neurotransmitter release that is observed in certain neurological and psychiatric disorders.

Lacosamide: a review of preclinical properties

Lacosamide (LCM), (SPM 927, (R)-2-acetamido-N-benzyl-3-methoxypropionamide, previously referred to as harkoseride or ADD 234037) is a member of a series of functionalized amino acids that were specifically synthesized as anticonvulsive drug candidates. LCM has demonstrated antiepileptic effectiveness in different rodent seizure models and antinociceptive potential in experimental animal models that reflect distinct types and symptoms of neuropathic as well as chronic inflammatory pain. Recent results suggest that LCM has a dual mode of action underlying its anticonvulsant and analgesic activity. It was found that LCM selectively enhances slow inactivation of voltage-gated sodium channels without affecting fast inactivation. Furthermore, employing proteomic affinity-labeling techniques, collapsin-response mediator protein 2 (CRMP-2 alias DRP-2) was identified as a binding partner. Follow-up experiments confirmed a functional interaction of LCM with CRMP-2 in vitro. LCM did not inhibit or induce a wide variety of cytochrome P450 enzymes at therapeutic concentrations. In safety pharmacology and toxicology studies conducted in mice, rats, rabbits, and dogs, LCM was well tolerated. Either none or only minor side effects were observed in safety studies involving the central nervous, respiratory, gastrointestinal, and renal systems and there is no indication of abuse liability. Repeated dose toxicity studies demonstrated that after either intravenous or oral administration of LCM the adverse events were reversible and consisted mostly of exaggerated pharmacodynamic effects on the CNS. No genotoxic or carcinogenic effects were observed in vivo, and LCM showed a favorable profile in reproductive and developmental animal studies. Currently, LCM is in a late stage of clinical development as an adjunctive treatment for patients with uncontrolled partial-onset seizures, and it is being assessed as monotherapy in patients with painful diabetic neuropathy. Further trials to identify LCM's potential in pain and for other indications have been initiated.

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.

Novel anticonvulsant drugs

Principles of complex mechanisms of action of anticonvulsants including latest reports concerning new antiepileptic drugs (AED) are considered. Different aspects of new anticonvulsant drugs (2nd generation) from preclinical and clinical testing, pharmacokinetics, and mono or combination therapy in children and adults are summarized. In the following condensed synopsis pharmacological and clinical characteristics of gabapentin (GBP), lamotrigine (LTG), levetiracetam (LEV), oxcarbazepine (OXC), pregabalin (PGB) and tiagabine (TGB) as well as topiramate (TPM) and zonisamide (ZNS) are discussed. In addition to the mechanisms of action, pharmacokinetics, interactions, indications and dosages as well as side effects are considered. Important data concerning the effect and tolerability of anticonvulsant drugs can be obtained from controlled studies. In comparison to drugs of the first generation (phenobarbital [PB], primidon [PRD], phenytoin [PHT], carbamazepine [CBZ] and valproic acid [VPA]) the potential for interactions and side effects due to enzyme induction or inhibition is reduced by most of the anticonvulsant drugs of the second generation. New anticonvulsant drugs increase the spectrum of treatment and represent further steps with regard to the optimization of an individual therapy of the epilepsies.

Lacosamide

Lacosamide is a new chemical entity being investigated as an adjunctive treatment for epilepsy, as well as monotherapy for diabetic neuropathic pain. Lacosamide appears to have a dual mode of action: selective enhancement of sodium channel inactivation and modulation of collapsin response mediator protein-2. Rapidly and completely absorbed after oral administration, lacosamide has an elimination half-life of approximately 13 hours and a low potential for drug interactions. Additionally, lacosamide exhibits linear, dose-proportional pharmacokinetics with low intra- and interpatient variability. Randomized controlled trials of adjunctive lacosamide (200, 400, and 600 mg/day) have demonstrated statistically significant reduction in median seizure frequency compared with placebo. In addition, 50% responder rates for lacosamide (400 and 600 mg/day) were statistically superior to placebo. The most frequently reported adverse events (> or =10% of lacosamide-treated patients) included dizziness, headache, and nausea. A double-blind, double-dummy randomized trial of intravenous lacosamide (30- and 60-minute infusion) as replacement for oral lacosamide showed that the safety and tolerability profiles were comparable for intravenous and oral lacosamide. The efficacy and safety results from completed clinical trials, as well as the favorable pharmacokinetic profile, suggest that lacosamide may represent a significant advance in antiepileptic drug therapy.

A pharmacophore derived phenytoin analogue with increased affinity for slow inactivated sodium channels exhibits a desired anticonvulsant profile

Phenytoin (DPH) is a clinically useful sodium (Na) channel blocker with efficacy against partial and generalized seizures. We have developed a novel hydantoin compound (HA) using comparative molecular field analysis (CoMFA) and evaluated its effects on hNa(v)1.2 channels. Both DPH and HA demonstrated affinity for resting (K(r)=13.9microM for HA, K(r)=464microM for DPH) and slow inactivated channels (K(I)=975nM for HA, K(I)=20.6microM for DPH). However, HA also exhibited an affinity for fast inactivated channels (K(I)=2.5microM) and shifted the V(1/2) for activation in the depolarizing direction. Furthermore, HA exhibited profound use dependent block at both 5 and 10Hz stimulation frequencies. In the 6Hz seizure model (32mA) HA had an ED(50) of 47.1mg/kg and a TD(50) of 131mg/kg (protective index (PI)=2.8). In comparison, the ED(50) for DPH was approximately 27.5mg/kg with a TD(50) of 35.6mg/kg (PI approximately 1.3). These findings provide evidence for the utility of CoMFA in the design of novel anticonvulsants and support the hypothesis that states selectivity plays an important role in achieving optimal protection with minimal side effects.

Evaluation of anticonvulsant and analgesic effects of benzyl- and benzhydryl ureides

The anticonvulsant effects of benzyl- and benzhydryl ureides in mice models of seizures (maximal electroshock seizure test, pentylenetetrazol test, picrotoxin-induced seizure test) and the influence on spontaneous locomotor activity has been assessed. Furthermore, the analgesic effect of ureide derivatives was studied in the hot-plate test in mice. Selected compounds were investigated for their in vitro interaction with adenosine receptors as well as the benzodiazepine binding site of GABA(A) receptors. This study demonstrated the strong anticonvulsant activity of several ureides in electrically or chemically induced seizure models, and structure-activity relationships were discussed. 1-Benzyl-3-butyrylurea (9) was found to be equipotent to ethosuximide in the pentylenetetrazol test with regard to the number of attacks as well as the time of the onset of seizures. The ureide 9 also revealed the highest protective activity against seizures in the other models, maximal electroshock seizure and picrotoxin test. Moreover, 1-benzyl-3-butyrylurea was not neurotoxic at doses up to 200 mg/kg. Benzylureides 8-10 showed affinity to the adenosine A1 receptors at low micromolar concentrations. However, the apparent anticonvulsant activity in different seizure models does not appear to result from direct activation of adenosine A1 receptors or GABA(A) receptors, respectively. In the hot-plate test, the majority of investigated compounds exhibited analgesic activity. Again, compound 9 was superior to the other substances investigated, suggesting a potential therapeutic value of that ureide derivative.

Antiepileptic drugs in development

BACKGROUND

Despite the success of several new antiepileptic drugs, about one third of patients with epilepsy are not seizure free on medication. Improvement in this situation might lie in drugs that are currently in development.

RECENT DEVELOPMENTS

Some new antiepileptic drugs are modifications of those already available, referred to in this Rapid Review as evolutionary drugs. These modifications of existing drugs are developed to improve effectiveness, often by increasing tolerability. Other drugs work by new mechanisms and are usually discovered through screening of animal models. WHERE NEXT? The large number of drugs currently in clinical trials provides a measure of hope for patients whose epilepsy is not controlled with currently available medication. In the future, this range of antiepileptic drugs will probably increase because of the use of new animal models, discovery of new basic mechanisms of epileptogenesis, acceleration of proof of principle studies in people, and development of new methods of drug delivery.

Pharmacology and mechanism of action of pregabalin: the calcium channel alpha2-delta (alpha2-delta) subunit as a target for antiepileptic drug discovery

Pregabalin (Lyrica) is a new antiepileptic drug that is active in animal seizure models. Pregabalin is approved in US and Europe for adjunctive therapy of partial seizures in adults, and also has been approved for the treatment of pain from diabetic neuropathy or post-herpetic neuralgia in adults. Recently, it has been approved for treatment of anxiety disorders in Europe. Pregabalin is structurally related to the antiepileptic drug gabapentin and the site of action of both drugs is similar, the alpha2-delta (alpha2-delta) protein, an auxiliary subunit of voltage-gated calcium channels. Pregabalin subtly reduces the synaptic release of several neurotransmitters, apparently by binding to alpha2-delta subunits, and possibly accounting for its actions in vivo to reduce neuronal excitability and seizures. Several studies indicate that the pharmacology of pregabalin requires binding to alpha2-delta subunits, including structure-activity analyses of compounds binding to alpha2-delta subunits and pharmacology in mice deficient in binding at the alpha2-delta Type 1 protein. The preclinical findings to date are consistent with a mechanism that may entail reduction of abnormal neuronal excitability through reduced neurotransmitter release. This review addresses the preclinical pharmacology of pregabalin, and also the biology of the high affinity binding site, and presumed site of action.

Hydroxyamide Analogs of Propofol Exhibit State-Dependent Block of Sodium Channels in Hippocampal Neurons: Implications for Anticonvulsant Activity

Although propofol is most commonly known for its general anesthetic properties, at subanesthetic doses, propofol has been effectively used to suppress seizures during refractory status epilepticus, a mechanism, in part, attributed to the inhibition of neuronal sodium channels. In this study, we have designed and synthesized two novel analogs of propofol, HS245 [2-(3-ethyl-4-hydroxy-5-isopropyl-phenyl)-3,3,3-trifluoro-2-hydroxy-propionamide] and HS357 [2-hydroxy-8-(4-hydroxy-3,5-diisopropyl-phenyl)-2-trifluoromethyl-octanoic acid amide], and determined their effects on sodium currents recorded from cultured hippocampal neurons. HS357 had greater affinity for the inactivated state of the sodium channel than propofol and HS245 (0.22 versus 0.74 and 1.2 microM, respectively) and exhibited the greatest ratio of affinity for the resting over the inactivated state. HS357 also demonstrated greater use-dependent block and delayed recovery from inactivation in comparison with propofol and HS245. Under current-clamp conditions, action potentials from hippocampal CA1 neurons in slices were evoked by current injection, or following perfusion with a zero Mg(2+)/7 mM K(+) artificial cerebrospinal fluid solution. Propofol and HS357 reduced the number of current-induced action potentials; however, HS357 caused a greater reduction in the number of spontaneous action potentials. Consistent with these electrophysiology studies, propofol and HS357 protected mice against acute seizures in the 6-Hz (22-mA) partial psychomotor model. Efficacious doses of propofol were associated with an impairment of motor coordination as assessed in the rotorod toxicity assay. In contrast, HS357 demonstrated a 2-fold greater protective index than propofol. Thus, propofol analogs represent an important structural class from which not only effective, but also safer, anti-convulsants may be developed.

Functional changes in astroglial cells in epilepsy

Epilepsy comprises a group of disorders characterized by the periodic occurrence of seizures, and pathologic specimens from patients with temporal lobe epilepsy demonstrate marked reactive gliosis. Since recent studies have implicated glial cells in novel physiological roles in the CNS, such as modulation of synaptic transmission, it is plausible that glial cells may have a functional role in the hyperexcitability characteristic of epilepsy. Indeed, alterations in distinct astrocyte membrane channels, receptors and transporters have all been associated with the epileptic state. This review integrates the current evidence regarding astroglial dysfunction in epilepsy and the potential underlying mechanisms of hyperexcitability. Functional understanding of the cellular and molecular alterations of astroglia-dependent hyperexcitability will help to clarify the physiological role of astrocytes in neural function as well as lead to the identification of novel therapeutic targets.

Experimental and clinical evidence for loss of effect (tolerance) during prolonged treatment with antiepileptic drugs

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

In vivo characterisation of the small-conductance KCa (SK) channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) as a potential anticonvulsant

Owing to their activation by increased intracellular Ca(2+) levels following burst firing, and the resultant hyperpolarisation and dampening of neuronal excitability, the small-conductance Ca(2+)-activated K(+) (SK(Ca)) channels have been proposed as a potential target for novel antiepileptic drugs. Indeed, the channel activator 1-ethyl-2-benzimidazolinone (1-EBIO) has been shown to reduce epileptiform activity in vitro. Accordingly, this study has investigated the therapeutic potential of 1-EBIO using a range of in vivo seizure models, and assessed the adverse effect liability with the rotarod and locomotor activity paradigms. To aid benchmarking of 1-EBIO's therapeutic and adverse effect potential, it was tested alongside two currently marketed antiepileptic drugs, phenytoin and levetiracetam. 1-EBIO was found to be effective at reducing seizure incidence in mice following maximal electroshock (ED(50) 36.0 mg/kg) as well as increasing the threshold to electrically- and pentylenetetrazole-induced seizures (TID(10)s 7.3 and 21.5 mg/kg, respectively). However, results from the mouse rotarod test revealed a strong adverse effect potential within the therapeutic dose range (ID(50) 35.6 mg/kg), implying a significantly inferior therapeutic index with respect to the comparator compounds. These results, therefore, support the in vitro data detailing 1-EBIO's reduction of epileptiform activity. However, the use of in vivo models has revealed a significant adverse effect potential within the therapeutic dose range. Nevertheless, given the multiplicity of SK(Ca) channel subunits and that 1-EBIO has been shown to enhance additional, non-SK(Ca) carried currents, these findings do not preclude the possibility that more selective enhancers of SK(Ca) function could prove to be effective as antiepileptic medications.

Anticonvulsant properties of hypothermia in experimental status epilepticus

Status epilepticus in patients often does not respond to first-line anticonvulsants, and subsequent treatment escalation with continuous intravenous anesthetics may be associated with significant side-effects. Therefore, alternative treatment regimens are urgently needed. Hypothermia has been shown to reduce excitatory transmission and may thus serve as an interesting adjunct in the management of status epilepticus. In the current experiments, three treatment groups were compared. Animals with self-sustaining status epilepticus were treated with external cooling for 3 h, with low-dose diazepam, or with a combination of both. The effect of these regimens on epileptic activity was compared with untreated controls. Animals that underwent cooling were rewarmed, and all animals were monitored for 5 h to assess occurrence and severity of motor seizures and frequency and amplitude of spontaneous epileptic discharges. Cooling alone significantly reduced number and severity of motor seizures but did not alter epileptic discharges. Cooling in addition to low-dose diazepam significantly diminished amplitudes and frequencies of epileptic discharges, while diazepam alone had only a minor reducing effect on discharge amplitudes. However, at later stages of status epilepticus, diazepam significantly reduced motor seizures. Following rewarming, the discharge frequency tended to increase again, suggesting partial reversibility. The current experiments show that in status epilepticus hypothermia exhibits anticonvulsant effects which are most pronounced if co-administered with low-dose diazepam. The results still require confirmation in other animal models and also clinical studies are urgently needed. However, our data indicate that cooling could well become a future adjunct in the treatment of status epilepticus in patients.

A molecular study of hippocampus in dogs with convulsion during canine distemper virus encephalitis

In this study, we have investigated the expression of the nuclear transcription factor (c-Fos, NFkB), growth factors (nerve growth factor--NGF, brain-derived neurotrophic factor--BDNF), peptides (enkephalin, galanin) and glutamate transporter (AA 504-523 rat EAAC1) in 6 dogs sacrificed immediately after seizure attack during encephalomyelitis due to canine distemper virus (CDV) (as assessed by clinical examination, RT-PCR and viral RNA detection either in blood or brain tissue and CDV immunohistochemistry in brain slices). In all these CDV affected dogs, the observed neurological signs included untreatable seizures, leading to cluster seizure activity and status epilepticus. In the inter-ictal phase abnormal mentation, postural and gait deficits and sometimes involuntary movements such as myoclonus were recorded. The same investigation was carried out in 5 control dogs affected by different disorders, all characterized by the absence of seizures. Brains were dissected out immediately after euthanasia and fixed; sections collected from the dorsal hippocampus were processed for immunohistochemistry. By comparing hippocampus sections obtained from dog with and without seizure, the following regulations were observed. A strong up-regulation of glutamate transporter throughout the cell layers was found together with the onset of nuclear Fos and NFkB-IR in the pyramidal cell layer X. Among the investigated peptides, we observed a slight increase in enkephalinergic fibers and a strong up-regulation of mu-opioid receptors, whereas galanin-IR seemed to be weaker. Finally, both NGF and BDNF expression was strongly up-regulated. BDNF-IR was mainly localized in the apical dendrite in pyramidal neurons. To our knowledge, these data offer the first indication that molecular events described in experimental kindling also occur during spontaneous pathology in animal species sharing close similarities to human neuropathology.

Enhancement of brain kynurenic acid production by anticonvulsants—Novel mechanism of antiepileptic activity?

In this study, we describe the effect of antiepileptic drugs on the production of kynurenic acid in rat cortical slices, and on the activity of kynurenic acid biosynthetic enzymes, kynurenine aminotransferases (KATs I and II) in the brain tissue. Phenobarbital, felbamate, phenytoin and lamotrigine (all at 0.5-3.0 mM) enhanced kynurenic acid production in vitro, and stimulated the activity of KAT I. In contrast, vigabatrin, gabapentin and tiagabine inhibited kynurenic acid synthesis in cortical slices with IC(50) of 3.9 (2.8-7.9), 3.7 (2.5-5.4) and 7.5 (3.5-14.3) mM, respectively. Vigabatrin, gabapentin and tiagabine reduced also the activity of KAT I with IC(50) of 1.6 (1.1-2.4), 0.1 (0.01-0.15), 0.9 (0.7-1.2) mM, and the activity of KAT II with IC(50) values of 6.0 (4.8-7.5), 0.2 (0.1-0.3) and 2.0 (1.5-2.6) mM, respectively. In conclusion, the enhancement of kynurenic acid formation displayed by carbamazepine, phenytoin, phenobarbital, felbamate and lamotrigine seems to be a novel mechanism, synergistic with other actions of these drugs, and potentially valuable in terms of better control of epilepsy.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

Refractory epilepsy: mechanisms and solutions

Pharmacoresistance remains a major challenge in epilepsy management. The availability of ten new antiepileptic drugs since the late 1980s has not dramatically improved the outcome of refractory epilepsy. This article provides an overview of the contemporary understanding of epilepsy and the limitation of current treatment modalities, discusses putative biological mechanisms of medical intractability and reviews some of the novel strategies under investigation to overcome the challenge of pharmacoresistance.

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.

Distinct cognitive neurophysiologic profiles for lamotrigine and topiramate

PURPOSE

To contrast the effects of lamotrigine (LTG) and topiramate (TPM) on cognitive task-related and resting-state EEG and evoked potential (EP) measures.

METHODS

We used a double-blind, randomized, crossover design. Healthy adults (N = 29) had two 8-week periods of dose escalation, 4 weeks of drug maintenance (300 mg daily), and 4 weeks of washout. EEG was recorded during working memory (WM) tasks and resting conditions at baseline, at the end of each maintenance phase, and after final washout. RESULTS. LTG did not affect overt performance on the tasks, although it reduced EEG power in both resting and WM task conditions, most prominently in the 6- to 12-Hz frequency range, and attenuated P300 evoked-potential amplitude equally in both WM task loads. TPM slowed responses and increased errors. It also increased EEG power below 6 Hz in all conditions, and reduced the amplitude of a slow wave observed in a difficult version of the WM task.

CONCLUSIONS

The drugs produced both task-independent and task-related alterations in neurophysiologic measures. The EEG and EP changes produced by TPM are consistent with an impairment of WM, as evidenced by overt performance deficits on the behavioral tasks. By contrast, the reduction in synchronous cortical activity produced by LTG was not accompanied by cognitive impairment. It is unknown whether such effects would also be observed at lower doses, such as those that often are used in monotherapy for newly diagnosed patients.