Modulation of sodium currents in rat CA1 neurons by carbamazepine and valproate after kindling epileptogenesis

Understanding mechanisms of drug resistance in epilepsy and strategies for overcoming it

INTRODUCTION

The present evidence indicates that approximately 70% of patients with epilepsy can be successfully treated with antiepileptic drugs (AEDs). A significant proportion of patients are not under sufficient control, and pharmacoresistant epilepsy is clearly associated with poor quality of life and increased morbidity and mortality. There is a great need for newer therapeutic options able to reduce the percentage of drug-resistant patients.

AREAS COVERED

A number of hypotheses trying to explain the development of pharmacoresistance have been put forward. These include: target hypothesis (altered AED targets), transporter (overexpression of brain efflux transporters), pharmacokinetic (overexpression of peripheral efflux transporters in the intestine or kidneys), intrinsic severity (initial high seizure frequency), neural network (aberrant networks), and gene variant hypothesis (genetic polymorphisms).

EXPERT OPINION

A continuous search for newer AEDs or among non-AEDs (blockers of efflux transporters, interleukin antagonists, cyclooxygenase inhibitors, mTOR inhibitors, angiotensin II receptor antagonists) may provide efficacious drugs for the management of drug-resistant epilepsy. Also, combinations of AEDs exerting synergy in preclinical and clinical studies (for instance, lamotrigine + valproate, levetiracetam + valproate, topiramate + carbamazepine) might be of importance in this respect. Preclinically antagonistic combinations must be avoided (lamotrigine + carbamazepine, lamotrigine + oxcarbazepine).

Capsaicin inhibits sodium currents and epileptiform activity in prefrontal cortex pyramidal neurons

Capsaicin, a compound found in chili peppers, causes burning sensations by acting on the peripheral sensory system. However, it has also been reported to exert substantial effects on central neurons. The aim of this patch-clamp study was to test the antiepileptic potential of capsaicin in prefrontal cortical pyramidal neurons. Capsaicin at a concentration of 60 μM inhibited neuronal excitability. Moreover, later spikes in response to 50-s-long current steps were much smaller in amplitude in the presence of 60 μM capsaicin than in control solution. The tested compound did not influence the membrane potential. Voltage-clamp recordings showed that capsaicin markedly enhanced the use-dependent block of sodium channels (sodium currents were evoked at frequencies of 0,5 Hz and 10 Hz). The presence of the compound shifted the steady-state inactivation curve of sodium channels towards hyperpolarization, which suggests greater inactivation of sodium channels at rest in the presence of capsaicin. Moreover, capsaicin inhibited epileptiform events evoked in three different proepileptic solutions. Capsaicin abolished interictal-like events lasting less than 1 s recorded in zero magnesium solution with an increased potassium ion concentration. The drug also abolished long ictal events evoked in zero magnesium solution containing 4-AP. Moreover, ictal events recorded in zero magnesium solution containing picrotoxin were substantially shortened in the presence of capsaicin. We suggest that capsaicin exerts an antiepileptic effect. The important mechanism behind this phenomenon seems to be the inhibition of sodium channels, which is an effect of many antiepileptic drugs.

Prenatal exposure to oxcarbazepine increases hippocampal apoptosis in rat offspring

This study assessed apoptosis in the offspring of rats exposed to oxcarbazepine (OXC) from day 7 to 15 of gestation. Three groups of pregnant Wistar rats were used: 1) Control, treated with saline solution; 2) treated with 100 mg/kg OXC; 3) treated with 100 mg/kg of carbamazepine (CBZ, as a positive control for apoptosis); the route of administration was intragastric. Apoptosis was detected at three postnatal ages using the TUNEL technique in the CA1, and CA3 regions of the hippocampus and in the dentate gyrus (DG); neurogenesis was assessed in the DG using an antibody against doublecortin. The litter characteristics were recorded. OXC increased apoptosis in all regions (p < 0.01) at the three ages evaluated. Lamination disruption occurred in CA1 and CA3 due to the neuron absence and to ectopic neurons; there were also malformations in the dorsal lamina of the DG in 38% and 25% of the pups born from rats treated with OXC and CBZ respectively. CBZ also increased apoptosis. No clear effect on neurogenesis in the DG was observed. The size of the litter was smaller (p < 0.01) in the experimental groups. Nineteen-day OXC fetuses had low weight (p < 0.01), but 21 and 30 postnatal days old CBZ and OXC pups were overweight (p < 0.01). The results demonstrate that OXC administered during gestation is pro-apoptotic, alters the cytoarchitecture of the hippocampus, reduces litter size, and probably influences postnatal weight. We provide evidence of the proapoptotic effect of CBZ when administered early in gestation.

Reduction of spike generation frequency by cooling in brain slices from rats and from patients with epilepsy

This study aimed to understand the mechanism by which brain cooling terminates epileptic discharge. Cortical slices were prepared from rat brains (n = 19) and samples from patients with intractable epilepsy that had undergone temporal lobectomy (n = 7). We performed whole cell current clamp recordings at approximately physiological brain temperature (35℃) and at cooler temperatures (25℃ and 15℃). The firing threshold in human neurons was lower at 25℃ (-32.6 mV) than at 35℃ (-27.0 mV). The resting potential and spike frequency were similar at 25℃ and 35℃. Cooling from 25℃ to 15℃ did not change the firing threshold, but the resting potential increased from -65.5 to -54.0 mV and the waveform broadened from 1.85 to 6.55 ms, due to delayed repolarization. These changes enhanced the initial spike appearance and reduced spike frequency; moreover, spike frequency was insensitive to increased levels of current injections. Similar results were obtained in rat brain studies. We concluded that the reduction in spike frequency at 15℃, due to delayed repolarization, might be a key mechanism by which brain cooling terminates epileptic discharge. On the other hand, spike frequency was not influenced by the reduced firing threshold or the elevated resting potential caused by cooling.

Valproic acid potently inhibits interictal-like epileptiform activity in prefrontal cortex pyramidal neurons

Valproic acid has a long-standing reputation of effectively treating the symptoms of not only epilepsy but also psychiatric conditions. In the latter, the exact mechanism by which valproate exerts its effect remains unclear. In this study, epileptiform bursts were recorded from pyramidal neurons in the prefrontal cortex (the brain region thought to be involved in psychiatric disorders) using the patch-clamp technique. An extracellular solution with no magnesium ions and elevated potassium levels that is known to induce epileptiform activity in vitro was used. Because of their short durations, the epileptiform bursts were regarded as interictal-like epileptiform activity, which is believed to be involved in cognitive impairment. Interictal discharges occur in many neuropsychiatric disorders as well as in healthy population. Epileptic activity in prefrontal cortex pyramidal neurons was potently inhibited by two therapeutic concentrations of valproic acid (20 μM and 200 μM). Moreover, valproate suppressed spontaneous excitatory postsynaptic potentials. Epileptiform bursts were fully inhibited by NMDA receptor antagonist, which suggests that epileptiform activity is driven by NMDA receptors. The inhibition of excitability in prefrontal cortex pyramidal neurons by valproate was also shown. This study shows that it is possible to evoke NMDA-dependent epileptiform activity in prefrontal cortex pyramidal neurons in vitro. We suggest that the prefrontal cortex is a good region for studying the influence of drugs on interictal epileptiform activity.

[Epigenetic modifications of chromatin in epilepsy: a potential mechanism of pharmacoresistance?]

Pharmacoresistance in epilepsy is an important problem from both clinical and fundamental perspectives. The existent hypotheses of pharmacoresistance are based on long term plastic rebuilding of the epileptic brain. One of potential mechanisms mediating such protracted changes are alterations of gene expression induced by epigenetic modifications of chromatin in brain cells of epileptic patients. Recently, changes in DNA methylation and histone post-translational modifications were reported in brain tissues of patients with pharmacoresistant epilepsy. Unfortunately, these data remain fragmentary and contradictory, therefore the results of animal models can partially fill this gap. The authors present a short review of the data concerning a potential role of epigenetic modifications in epilepsy.

Activity Clamp Provides Insights into Paradoxical Effects of the Anti-Seizure Drug Carbamazepine

A major challenge in experimental epilepsy research is to reconcile the effects of anti-epileptic drugs (AEDs) on individual neurons with their network-level actions. Highlighting this difficulty, it is unclear why carbamazepine (CBZ), a frontline AED with a known molecular mechanism, has been reported to increase epileptiform activity in several clinical and experimental studies. We confirmed in an in vitro mouse model (in both sexes) that the frequency of interictal bursts increased after CBZ perfusion. To address the underlying mechanisms, we developed a method, activity clamp, to distinguish the response of individual neurons from network-level actions of CBZ. We first recorded barrages of synaptic conductances from neurons during epileptiform activity and then replayed them in pharmacologically isolated neurons under control conditions and in the presence of CBZ. CBZ consistently decreased the reliability of the second action potential in each burst of activity. Conventional current-clamp recordings using excitatory ramp or square-step current injections failed to reveal this effect. Network modeling showed that a CBZ-induced decrease of neuron recruitment during epileptic bursts can lead to an increase in burst frequency at the network level by reducing the refractoriness of excitatory transmission. By combining activity clamp with computer simulations, the present study provides a potential explanation for the paradoxical effects of CBZ on epileptiform activity.

SIGNIFICANCE STATEMENT The effects of anti-epileptic drugs on individual neurons are difficult to separate from their network-level actions. Although carbamazepine (CBZ) has a known anti-epileptic mechanism, paradoxically, it has also been reported to increase epileptiform activity in clinical and experimental studies. To investigate this paradox during realistic neuronal epileptiform activity, we developed a method, activity clamp, to distinguish the effects of CBZ on individual neurons from network-level actions. We demonstrate that CBZ consistently decreases the reliability of the second action potential in each burst of epileptiform activity. Network modeling shows that this effect on individual neuronal responses could explain the paradoxical effect of CBZ at the network level.

Does valproate therapy in epileptic patients contribute to changing atherosclerosis risk factors? The role of lipids and free fatty acids

BACKGROUND

We aimed to demonstrate the relationship between the valproate (VPA) treatment versus lipid and serum free fatty acids (FFAs) profiles to be the potential atherosclerosis risk factor in epileptic patients.

METHODS

Fasting blood samples were taken from 21 adult VPA-treated patients and 21 controls. The profiles of lipids, FFAs, clinical parameters and body mass index (BMI) were evaluated.

RESULTS

No significant differences between the study group and controls were found for any of the studied parameters. However, significant differences in the total cholesterol (CHOL), low-density-lipoprotein cholesterol (LDL), triglycerides, the CHOL/HDL (high-density-lipoprotein cholesterol) ratio, and Atherogenic Index of Plasma were observed for overweight patients when compared to those of normal weight. Patients with uncontrolled epilepsy tended to have significantly lower palmitic acid level than seizure-free patients. Oleic acid was found to be positively correlated with VPA concentration for patients with uncontrolled epilepsy, and with the dose corrected VPA concentration for all the patients. The acid was however negatively correlated with stearic acid for both the controls and the patients with uncontrolled epilepsy. PLS method revealed CHOL, LDL, triglycerides and myristic acid to be positively interrelated for the whole group under the study, whereas these parameters were found to be negatively correlated with VPA concentration, and positively with BMI. Furthermore, high sensitivity C-reactive protein was found to be negatively correlated with palmitic acid levels.

CONCLUSION

Overweight VPA-treated patients are exposed to higher risk of atherosclerosis. Alterations in FFAs are likely to depend on seizures control, and on VPA levels.

Physiology and Pathophysiology of Sodium Channel Inactivation

Voltage-gated sodium channels are present in different tissues within the human body, predominantly nerve, muscle, and heart. The sodium channel is composed of four similar domains, each containing six transmembrane segments. Each domain can be functionally organized into a voltage-sensing region and a pore region. The sodium channel may exist in resting, activated, fast inactivated, or slow inactivated states. Upon depolarization, when the channel opens, the fast inactivation gate is in its open state. Within the time frame of milliseconds, this gate closes and blocks the channel pore from conducting any more sodium ions. Repetitive or continuous stimulations of sodium channels result in a rate-dependent decrease of sodium current. This process may continue until the channel fully shuts down. This collapse is known as slow inactivation. This chapter reviews what is known to date regarding, sodium channel inactivation with a focus on various mutations within each NaV subtype and with clinical implications.

Seizures triggered by pentylenetetrazol in marmosets made chronically epileptic with pilocarpine show greater refractoriness to treatment

The efficiency of most of the new antiepileptic drugs (AEDs) on clinical trials still falls short the success reported in pre-clinical studies, possibly because the validity of the animal models is insufficient to fully represent the human pathology. To improve the translational value for testing AEDs, we propose the use of non-human primates. Here, we suggest that triggering limbic seizures with low doses of PTZ in pilocarpine-treated marmosets might provide a more effective basis for the development of AED. Marmosets with epileptic background were more susceptible to seizures induced by PTZ, which were at least 3 times longer and more severe (about 6 times greater frequency of generalized seizures) in comparison to naïve peers. Accordingly, PTZ-induced seizures were remarkably less attenuated by AEDs in epileptic than naïve marmosets. While phenobarbital (40mg/kg) virtually abolished seizures regardless of the animal's background, carbamazepine (120mg/kg) and valproic acid (400mg/kg) could not prevent PTZ-induced seizures in epileptic animals with the same efficiency as observed in naïve peers. VPA was less effective regarding the duration of individual seizures in epileptic animals, as assessed in ECoG (p=0.05). Similarly following CBZ treatment, the behavioral manifestation of generalized seizures lasted longer in epileptic (p<0.05), which were also more frequent than in the naïve group (p<0.05). As expected, epileptic marmosets experiencing stronger seizures showed more NPY- and ΔFosB-immunostained neurons in a number of brain areas associated with the generation and spread of limbic seizures. Our results suggest that PTZ induced seizures over an already existing epileptic background constitutes a reliable and controllable mean for the screening of new AEDs.

Carbamazepine modulates the spatiotemporal activity in the dentate gyrus of rats and pharmacoresistant humans in vitro

INTRODUCTION

Human hippocampal tissue resected from pharmacoresistant epilepsy patients was investigated to study the effect of the antiepileptic drug CBZ (carbamazepine) and was compared to similar experiments in the hippocampus of control rats.

METHODS

The molecular layer of the DG (dentate gyrus) of human epileptic tissue and rat nonepileptic tissue was electrically stimulated and the evoked responses were recorded with voltage-sensitive dye imaging to characterize the spatiotemporal properties.

RESULTS

Bath applied CBZ (100 μmol/L) reduced the amplitude of the evoked responses in the human DG, albeit that no clear use-dependent effects were found at frequencies of 8 or 16 Hz. In nonepileptic control DG from rats, CBZ also reduced the amplitude of the evoked response in the molecular layer of the DG as well as the spatial extent of the response.

CONCLUSIONS

This study demonstrates that CBZ still reduced the activity in the DG, although the patients were clinically diagnosed as pharmacoresistant for CBZ. This suggests that in the human epileptic brain, the targets of CBZ, the voltage-gated Na(+) channels, are still sensitive to CBZ, although we used a relative high concentration and it is not possibility to assess the actual CBZ concentration that reached the target in the patient. We also concluded that the effect of CBZ was found in the activated region of the DG, quite comparable to the observations in the nonepileptic rat.

Effects of developmental alcohol and valproic acid exposure on play behavior of ferrets

Exposure to alcohol and valproic acid (VPA) during pregnancy can lead to fetal alcohol spectrum disorders and fetal valproate syndrome, respectively. Altered social behavior is a hallmark of both these conditions and there is ample evidence showing that developmental exposure to alcohol and VPA affect social behavior in rodents. However, results from rodent models are somewhat difficult to translate to humans owing to the substantial differences in brain development, morphology, and connectivity. Since the cortex folding pattern is closely related to its specialization and that social behavior is strongly influenced by cortical structures, here we studied the effects of developmental alcohol and VPA exposure on the play behavior of the ferret, a gyrencephalic animal known for its playful nature. Animals were injected with alcohol (3.5g/kg, i.p.), VPA (200mg/kg, i.p.) or saline (i.p) every other day during the brain growth spurt period, between postnatal days 10 and 30. The play behavior of pairs of the same experimental group was evaluated 3 weeks later. Both treatments induced significant behavioral differences compared to controls. Alcohol and VPA exposed ferrets played less than saline treated ones, but while animals from the alcohol group displayed a delay in start playing with each other, VPA treated ones spent most of the time close to one another without playing. These findings not only extend previous results on the effects of developmental exposure to alcohol and VPA on social behavior, but make the ferret a great model to study the underlying mechanisms of social interaction.

Managing drug-resistant epilepsy: challenges and solutions

Despite the development of new antiepileptic drugs (AEDs), ~20%-30% of people with epilepsy remain refractory to treatment and are said to have drug-resistant epilepsy (DRE). This multifaceted condition comprises intractable seizures, neurobiochemical changes, cognitive decline, and psychosocial dysfunction. An ongoing challenge to both researchers and clinicians alike, DRE management is complicated by the heterogeneity among this patient group. The underlying mechanism of DRE is not completely understood. Many hypotheses exist, and relate to both the intrinsic characteristics of the particular epilepsy (associated syndrome/lesion, initial response to AED, and the number and type of seizures prior to diagnosis) and other pharmacological mechanisms of resistance. The four current hypotheses behind pharmacological resistance are the "transporter", "target", "network", and "intrinsic severity" hypotheses, and these are reviewed in this paper. Of equal challenge is managing patients with DRE, and this requires a multidisciplinary approach, involving physicians, surgeons, psychiatrists, neuropsychologists, pharmacists, dietitians, and specialist nurses. Attention to comorbid psychiatric and other diseases is paramount, given the higher prevalence in this cohort and associated poorer health outcomes. Treatment options need to consider the economic burden to the patient and the likelihood of AED compliance and tolerability. Most importantly, higher mortality rates, due to comorbidities, suicide, and sudden death, emphasize the importance of seizure control in reducing this risk. Overall, resective surgery offers the best rates of seizure control. It is not an option for all patients, and there is often a significant delay in referring to epilepsy surgery centers. Optimization of AEDs, identification and treatment of comorbidities, patient education to promote adherence to treatment, and avoidance of triggers should be periodically performed until further insights regarding causative pathology can guide better therapies.

Reciprocal changes in phosphorylation and methylation of mammalian brain sodium channels in response to seizures

Voltage-gated sodium (Nav) channels initiate action potentials in brain neurons and are primary therapeutic targets for anti-epileptic drugs controlling neuronal hyperexcitability in epilepsy. The molecular mechanisms underlying abnormal Nav channel expression, localization, and function during development of epilepsy are poorly understood but can potentially result from altered posttranslational modifications (PTMs). For example, phosphorylation regulates Nav channel gating, and has been proposed to contribute to acquired insensitivity to anti-epileptic drugs exhibited by Nav channels in epileptic neurons. However, whether changes in specific brain Nav channel PTMs occur acutely in response to seizures has not been established. Here, we show changes in PTMs of the major brain Nav channel, Nav1.2, after acute kainate-induced seizures. Mass spectrometry-based proteomic analyses of Nav1.2 purified from the brains of control and seizure animals revealed a significant down-regulation of phosphorylation at nine sites, primarily located in the interdomain I-II linker, the region of Nav1.2 crucial for phosphorylation-dependent regulation of activity. Interestingly, Nav1.2 in the seizure samples contained methylated arginine (MeArg) at three sites. These MeArgs were adjacent to down-regulated sites of phosphorylation, and Nav1.2 methylation increased after seizure. Phosphorylation and MeArg were not found together on the same tryptic peptide, suggesting reciprocal regulation of these two PTMs. Coexpression of Nav1.2 with the primary brain arginine methyltransferase PRMT8 led to a surprising 3-fold increase in Nav1.2 current. Reciprocal regulation of phosphorylation and MeArg of Nav1.2 may underlie changes in neuronal Nav channel function in response to seizures and also contribute to physiological modulation of neuronal excitability.

Ion channels as drug targets in central nervous system disorders

Ion channel targeted drugs have always been related with either the central nervous system (CNS), the peripheral nervous system, or the cardiovascular system. Within the CNS, basic indications of drugs are: sleep disorders, anxiety, epilepsy, pain, etc. However, traditional channel blockers have multiple adverse events, mainly due to low specificity of mechanism of action. Lately, novel ion channel subtypes have been discovered, which gives premises to drug discovery process led towards specific channel subtypes. An example is Na(+) channels, whose subtypes 1.3 and 1.7-1.9 are responsible for pain, and 1.1 and 1.2 - for epilepsy. Moreover, new drug candidates have been recognized. This review is focusing on ion channels subtypes, which play a significant role in current drug discovery and development process. The knowledge on channel subtypes has developed rapidly, giving new nomenclatures of ion channels. For example, Ca(2+)s channels are not any more divided to T, L, N, P/Q, and R, but they are described as Ca(v)1.1-Ca(v)3.3, with even newer nomenclature α1A-α1I and α1S. Moreover, new channels such as P2X1-P2X7, as well as TRPA1-TRPV1 have been discovered, giving premises for new types of analgesic drugs.

Expression of sodium channel α subunits 1.1, 1.2 and 1.6 in rat hippocampus after kainic acid-induced epilepsy

Voltage-gated Na(+) channels control neuronal excitability and are the primary target for the majority of anti-epileptic drugs. This study investigates the (sub)cellular expression patterns of three important brain-associated Na(+) channel α subunits: NaV1.1, NaV1.2 and NaV1.6 during epileptogenesis (induced by kainic acid) using time points that cover the period from induction to the chronic phase of epilepsy. NaV1.1 immunoreactivity was persistently reduced at 1 day, 3 weeks and 2 months after SE in CA1 and CA3. About 50% of the NaV1.1-positive interneurons was lost at one day after SE in all regions investigated. In the hilus a similar reduction in NeuN-positive neurons was found, while in the CA1 and CA3 region the loss in NeuN-positive neurons only reached 15% in the chronic phase of epilepsy. This implies a stronger shift in the balance between excitation and inhibition toward excitation in the CA1 and CA3 region than in the hilus. NaV1.2 immunoreactivity in the inner molecular layer of the dentate gyrus was lower than control at 1 day after SE. It increased at 3 weeks and 2 months after SE in the inner molecular layer and overlapped with sprouted mossy fibers. NaV1.6 immunoreactivity in the dendritic region of CA1 and CA3 was persistently reduced at all time-points during epileptogenesis. Some astrocytes expressed NaV1.1 and NaV1.6 at 3 weeks after SE. Expression data alone are not sufficient to explain changes in network stability, or infer causality in epileptogenesis. These results demonstrate that hippocampal sub-regional expression of NaV1.1, NaV1.2 and NaV1.6 Na(+) channel α subunits is altered during epileptogenesis in a time and location specific way. This implies that understanding epileptogenesis has to take into account several distinct and type-specific changes in sodium channel expression.

The intrinsic severity hypothesis of pharmacoresistance to antiepileptic drugs

Pharmacoresistance to antiepileptic drugs (AEDs) is a barrier to seizure freedom for many persons with epilepsy. For nearly two decades, pharmacoresistance has been framed in terms of factors affecting the access of AEDs to their molecular targets in the brain or the actions of the drugs on these targets. Shortcomings in this prevailing view led to the formulation of the intrinsic severity hypothesis of pharmacoresistance to AEDs, which is based on the recognition that there are neurobiologic factors that confer phenotypic variation among individuals with etiologically similar forms of epilepsy and postulates that more severe epilepsy is more difficult to treat with AEDs. In recent years, progress has been made identifying potential genetic mechanisms of variation in epilepsy severity, including subclinical mutations in ion channels that increase or reduce epilepsy severity in mice. Efforts are underway to identify clinically important genetic modifiers. If it can be demonstrated that such severity factors play a role in pharmacoresistance, treatments could be devised to reverse severity mechanisms. By overcoming pharmacoresistance, this new approach to epilepsy therapy may allow drug refractory patients to achieve seizure freedom without side effects.

Rapid loss of efficacy to the antiseizure drugs lamotrigine and carbamazepine: a novel experimental model of pharmacoresistant epilepsy

PURPOSE

Kindling is a well-established model of secondarily generalized partial seizures that is widely employed in the search for novel antiseizure drugs. During the kindling and postkindling acquisition phase, an active process of neuronal remodeling occurs. We tested the hypothesis that exposure to the voltage-gated sodium channel blockers lamotrigine (LTG) and carbamazepine (CBZ) during the period of active remodeling will lead to a diminished therapeutic effect.

METHODS

Two days after the last kindling stimulation, fully kindled rats were randomized to receive either 0.5% methyl cellulose (MC), LTG (30 mg/kg), or CBZ (40 mg/kg). The effect of LTG and CBZ on behavioral seizure severity and electrographic afterdischarge duration (ADD) was recorded. One week after this treatment, rats in both groups were rechallenged with LTG 30 or CBZ 40 mg/kg and their seizure score and ADD recorded. In vitro efficacy of LTG on neuronal action potentials was also evaluated using whole cell current clamp recording in hippocampal brain slices obtained from kindled control rats, LTG-sensitive kindled rats, and LTG-resistant kindled rats.

KEY FINDINGS

When acutely administered 48 h after the last kindling stimulation, LTG and CBZ blocked the expression of behavioral seizures and reduced the ADD. In contrast, a second challenge dose of LTG or CBZ administered after a 7-day "no drug, no stimulation" period did not result in reduction of either the seizure score or the ADD. Interestingly, the potassium channel opener, ezogabine, also known as retigabine (EZG; 40 mg/kg), blocked the expression of behavioral seizures at both time points evaluated (i.e., 2 days and 9 days after last stimulation). In vivo resistance to LTG was associated with a similar reduction in the ability of LTG to limit action potential firing in CA1 neurons. LTG (50 μm) significantly decreased the number of action potentials generated by a depolarizing current pulse in neurons recorded from slices obtained from kindled control and LTG-sensitive rats, but not in slices obtained from LTG-resistant rats.

SIGNIFICANCE

Collectively, results obtained from both in vivo and in vitro studies demonstrate that even a single exposure to the sodium channel blockers LTG, or CBZ, during the postkindling remodeling phase leads to an altered pharmacologic response to these two ASDs, but not to EZG. The LTG- and CBZ-resistant amygdala kindled rats may serve as a useful model of therapy-resistant epilepsy.

Effect of valproate, lamotrigine and levetiracetam on excitability and firing properties of CA1 neurons in rat brain slices

The purpose of this study was to analyze the rapid effects of the antiepileptic drugs valproate, lamotrigine, and levetiracetam on excitability and firing properties of hippocampal neurons. The drug effects on resting potential, action potential, and repetitive firing properties were studied in whole-cell current-clamp recordings of CA1 neurons in rat brain slices. Lamotrigine changed action potential rising slope by -24 ± 38 V/s (mean ± SD), peak amplitude by -6.8 ± 5.0 mV, and maximum firing frequency by -60 ± 13%. Lamotrigine thereto increased the voltage threshold by 4.3 ± 4.2 mV and augmented the action potential attenuation during repetitive firing. All effects were significant (P < 0.01 to P < 0.0002) compared to control cells. Valproate and levetiracetam showed no significant effects on these parameters. None of the tested drugs had a significant effect on the resting potential. The lamotrigine effects are consistent with sodium channel blocking which may explain or contribute to the antiepileptic mode of action. Valproate and levetiracetam did not show these effects and the mechanism of their antiepileptic action need to be different. These findings (valproate) differ in some respects from findings reported in cultured or dissociated neurons. In a slice where the neurons have largely preserved connections, drug effects are likely to be more similar to the therapeutic action in the brain.

Study of epileptiform activity in cerebral ganglion of mud crab Scylla serrata

An attempt is made to induce in mud crab (Scylla serrata) epileptiform activities that resemble the generalized epileptic seizures. Cerebral ganglion of crab was exposed in situ, to a convulsant drug pentylenetetrazole (PTZ) 100 mM, for induction of seizures. Also, crabs were pretreated with antiepileptic drug viz sodium valproate (120 μmol/l) to inhibit epileptiform activities. The surface electrical discharges of cerebral ganglion were recorded using Unkelscope (MIT, USA) in control as well as experimental animals. The cerebral ganglion of crab showed a pattern of high cerebral electrical discharges after PTZ treatment compared to control. The sodium valproate promoted sedative action in control and prevented PTZ-mediated epileptiform discharges. Glutamate and GABA contents in cerebral ganglion were assayed. Glutamate level increased (31.45%) during PTZ treatment with concomitant decrease (43.93%) in GABA. Sodium valproate had no effect on glutamate concentration, but it decreased GABA by 24.75%. The present study shows that epileptiform activities can be induced in crabs.

Advances in understanding basic mechanisms of epilepsy and seizures

Sixty years ago the clinical neurophysiology of epilepsy had progressed to the stage that it posed questions that could be addressed by major advances in cellular electrophysiology made around the that time. However, it took about 25-30 years to build up serious momentum in understanding the mechanisms of epileptic discharges. Over the past 2-3 decades developments in pharmacology and molecular biology have substantially increased the depth and complexity of our insights into the nervous system in general and the epileptic brain in particular. One of the biggest advances in our understanding of the brain is in its plasticity in the adult - that is its ability to modify its structure and function. The current state of play is that for most chronic epileptic foci it is possible to identify multiple differences from normal brain tissue in both the structure and function of neurons, neuronal networks and glia. This review will chart some of this progress to give an idea of the pace of advances over the decades.

Management of the patient with medically refractory epilepsy

Epilepsy imposes a significant clinical, epidemiologic and economic burden on societies throughout the world. Despite the development of more than ten new antiepileptic drugs over the past 15 years, approximately a third of patients with epilepsy remain resistant to pharmacotherapy. Individuals who fail to respond, or respond only partially, continue to have incapacitating seizures. Managing patients with medically refractory epilepsy is challenging and requires a structured multidisciplinary approach in specialized clinics. If the problems related to drug resistance could be resolved, even in part, by improving the pharmacokinetic profile of existing drugs, the economic savings would be remarkable and the time required to design drugs that achieve seizure control would be shorter than the discovery of new targets and molecules was required. A promising approach is the use of corticosteroids that may have a dual beneficial effect. Resective brain surgery remains the ultimate and highly successful approach to multiple drug resistance in epileptic patients.

Gain-of-function mutation of Nav1.5 in atrial fibrillation enhances cellular excitability and lowers the threshold for action potential firing

Genetic mutations of the cardiac sodium channel (SCN5A) specific only to the phenotype of atrial fibrillation have recently been described. However, data on the biophysical properties of SCN5A variants associated with atrial fibrillation are scarce. In a mother and son with lone atrial fibrillation, we identified a novel SCN5A coding variant, K1493R, which altered a highly conserved residue in the DIII-IV linker and was located six amino acids downstream from the fast inactivation motif of sodium channels. Biophysical studies of K1493R in tsA201 cells demonstrated a significant positive shift in voltage-dependence of inactivation and a large ramp current near resting membrane potential, indicating a gain-of-function. Enhanced cellular excitability was observed in transfected HL-1 atrial cardiomyocytes, including spontaneous action potential depolarizations and a lower threshold for action potential firing. These novel biophysical observations provide molecular evidence linking cellular "hyperexcitability" as a mechanism inducing vulnerability to this common arrhythmia.

Lacosamide: a new approach to target voltage-gated sodium currents in epileptic disorders

The mechanism of action of several antiepileptic drugs (AEDs) rests on their ability to modulate the activity of voltage-gated sodium currents that are responsible for fast action potential generation. Recent data indicate that lacosamide (a compound with analgesic and anticonvulsant effects in animal models) shares a similar mechanism. When compared with other AEDs, lacosamide has the unique ability to interact with sodium channel slow inactivation without affecting fast inactivation. This article reviews these findings and discusses their relevance within the context of neuronal activity seen during epileptiform discharges generated by limbic neuronal networks in the presence of chemical convulsants. These seizure-like events are characterized by sustained discharges of sodium-dependent action potentials supported by robust depolarizations, thus providing synchronization within neuronal networks. Generally, AEDs such as phenytoin, carbamazepine and lamotrigine block sodium channels when activated. In contrast, lacosamide facilitates slow inactivation of sodium channels both in terms of kinetics and voltage dependency. This effect may be relatively selective for repeatedly depolarized neurons, such as those participating in seizure activity in which the persistence of sodium currents is more pronounced and promotes neuronal excitation. The clinical effectiveness of lacosamide has been demonstrated in randomized, double-blind, parallel-group, placebo-controlled, adjunctive-therapy trials in patients with refractory partial seizures. Further studies should determine whether the effects of lacosamide in animal models and in clinical settings are fully explained by its selective action on sodium current slow inactivation or whether other effects (e.g. interactions with the collapsin-response mediator protein-2) play a contributory role.

[Drug resistance in partial epilepsy: epidemiology, mechanisms, pharmacogenetics and therapeutical aspects]

It has been established that 20-30% of epilepsies are not controlled by antiepileptic drugs. Drug resistance is associated with several major problems, including prognosis, cognitive function, behavior, mortality, cost and quality of life. Apart from classic risk factors for drug resistance, such as neurological, psychiatric, imaging, EEG abnormalities, a high frequency of seizures before medical therapy and complex febrile convulsions, the potential role of multidrug transporters as well as their genetic control and the altered sensitivity of neuronal drug receptors has gained growing attention. In the future, pharmaceutical engineering may bypass these factors. To a certain extent, drug resistance may develop progressively in a neurobiological process and the control of this process could limit its development.

Carbamazepine reduces the behavioural manifestations of tinnitus following salicylate treatment in rats

CONCLUSION

The results are consistent with the hypothesis that carbarmazepine (CBZ) has efficacy against tinnitus in humans.

OBJECTIVE

CBZ is an anti-epileptic drug that is widely used for the treatment of tinnitus. Despite this, there are relatively few clinical trials or preclinical studies supporting its efficacy. In an effort to increase the amount of information available on CBZ, the aim of this study was to investigate the efficacy of CBZ in salicylate-induced tinnitus in rats

MATERIALS AND METHODS

We investigated the effects of CBZ in an animal model of tinnitus induced by the injection of salicylate using a conditioned lick suppression paradigm.

RESULTS

We found that CBZ, at a dose of 15 mg/kg i.p., but not at 5 mg/kg or 30 mg/ kg, significantly suppressed the behavioural manifestations of tinnitus.

Plasticity of antiepileptic drug targets

The mechanisms underlying the development of drug resistance in epilepsy are complex and, at this time, not fully understood. It is widely accepted that the efficacy of an antiepileptic drug (AED) is determined by its ability to cross the blood-brain barrier (BBB) and bind to intraparenchymal target sites. According to the target hypothesis, pharmacoresistance occurs when target sites are structurally and/or functionally modified in such a way that they become less sensitive to AEDs. A large number of targets for AEDs have been identified in the brain, many of which undergo molecular changes during chronic epilepsy. So far, a reduced sensitivity of drug targets to AEDs in chronic human and experimental epilepsy has been suggested for the voltage-gated Na(+) channel and the GABA(A) receptor. At the molecular level, altered drug targets may arise due to the transcriptional regulation of ion channel subunit genes. Alternatively, ion channel subunits may be modified by redox modulation or phosphorylation, a set of mechanisms that may be invoked much more rapidly than transcriptional changes. In addition to functional experiments, genetic studies are currently attempting to identify polymorphisms that are specifically associated with the functional phenotypes of drug resistance and drug responsiveness in the epileptic population. It is hoped that our increased understanding of the target mechanisms underlying pharmacoresistance will lead to the development of novel therapeutic strategies that will improve treatment outcomes in individuals with refractory epilepsy.

Attenuation of phospholipid signaling provides a novel mechanism for the action of valproic acid

Valproic acid (VPA) is used to treat epilepsy and bipolar disorder and to prevent migraine. It is also undergoing trials for cancer therapy. However, the biochemical and molecular biological actions of VPA are poorly understood. Using the social amoeba Dictyostelium discoideum, we show that an acute effect of VPA is the inhibition of chemotactic cell movement, a process partially dependent upon phospholipid signaling. Analysis of this process shows that VPA attenuates the signal-induced translocation of PH(Crac)-green fluorescent protein from cytosol to membrane, suggesting the inhibition of phosphatidylinositol-(3,4,5)-trisphosphate (PIP(3)) production. Direct labeling of lipids in vivo also shows a reduction in PIP and PIP(2) phosphorylation following VPA treatment. We further show that VPA acutely reduces endocytosis and exocytosis-processes previously shown to be dependent upon PIP(3) production. These results suggest that in Dictyostelium, VPA rapidly attenuates phospholipid signaling to reduce endocytic trafficking. To examine this effect in a mammalian model, we also tested depolarization-dependent neurotransmitter release in rat nerve terminals, and we show that this process is also suppressed upon application of VPA and an inhibitor of phosphatidylinositol 3-kinase. Although a more comprehensive analysis of the effect of VPA on lipid signaling will be necessary in mammalian systems, these results suggest that VPA may function to reduce phospholipid signaling processes and thus may provide a novel therapeutic effect for this drug.

Diminished response of CA1 neurons to antiepileptic drugs in chronic epilepsy

PURPOSE

A substantial proportion of epilepsy patients ( approximately 30%) continue to have seizures despite carefully optimized treatment with antiepileptic drugs (AEDs). One key concept to explain the development of pharmacoresistance is that epilepsy-related changes in the properties of CNS drug targets result in AED-insensitivity of these targets. These changes then contribute to drug-resistance on a clinical level. We have tested this hypothesis in hippocampal CA1 neurons in experimental epilepsy.

METHODS

Using patch-clamp techniques, we thoroughly examined the effects of carbamazepine (CBZ) and phenytoin (PHT) on voltage-gated Na(+) currents (I(Na)) in hippocampal CA1 neurons of sham-control and chronically epileptic rats.

RESULTS

We find that there were significant changes in the effects of PHT, but not CBZ on the voltage-dependence of inactivation, resulting in a significant reduction in voltage-dependent blocking effects in chronically epileptic animals. Conversely, CBZ effects on the time course of recovery from inactivation of I(Na) were significantly less pronounced in epileptic compared to sham-control animals, whereas PHT effects remained unaltered.

CONCLUSIONS

Our findings indicate that AED-sensitivity of Na(+) currents is reduced in chronic epilepsy. The reduction in sensitivity is due to different biophysical mechanisms for CBZ and PHT. Furthermore, comparison to published work suggests that the loss of AED-sensitivity is less pronounced in CA1 neurons than in dentate granule neurons. Thus, these results suggest that target mechanisms of drug resistance are cell type and AED specific. Unraveling these complex mechanisms is likely to be important for a better understanding of the cellular basis of drug-resistant epilepsy.

Novel antiseizure drug mechanisms

The amount of new knowledge being generated regarding brain mechanisms in general, and epileptic mechanisms in particular, is enormous. Anticonvulsant drugs are ineffective in approximately a third of people with epilepsy. To our knowledge, strategies for preventing epilepsy after an initial insult are nonexistent. In this review, we briefly examine some recent novel concepts for preventing seizures, which might lead to enhanced anticonvulsant drug therapy. We start with some known seizure mechanisms that have yet to yield widely used anticonvulsant drugs, including potassium channels, chloride cotransporters, extracellular space constriction, gap junctions and magnesium. Pharmacoresistance is then discussed, focusing on the upregulation of drug-resistance proteins (a concept with significant therapeutic appeal) and the drug-target hypothesis. Two further areas that hold great promise for future therapeutics are sex hormones and inflammatory processes. The genetics of epilepsy are currently being elaborated, providing potential novel anticonvulsant targets. Prevention being better than a cure, we discuss epileptogenesis and its treatment. Given the astounding progress of neuroscience research, one hopes for many new therapeutics for our intractable epileptic patients.

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

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

Molecular and cellular mechanisms of pharmacoresistance in epilepsy

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

Migraine prophylactic drugs work via ion channels

In recent decades, the concept of a vascular origin of migraine has been replaced by theories based on a neuronal pathophysiology. These theories all involve rapid changes in the functioning of the brain, particularly the brain stem, and the trigeminal nerves. While such paroxysmal changes in function could be the result of altered synaptic transmission, or other physiological changes, they could also be due to changes in the function of voltage-regulated sodium and calcium ion channels. Support for this view of migraine as a channelopathy comes from an examination of the likely mechanism of action of migraine prophylactic drugs. It is the present hypothesis that most of the widely used drugs for migraine prevention work by inhibiting the function of one or both of these ion channels. A review of the laboratory research done on most of the commonly used migraine prophylactic drugs, divided into five classes, reveals that they all may work on sodium channels, calcium channels, or both. If this is the common mechanism of action of migraine prophylactics, it should lead toward the development of more effective prophylactic drugs.

A novel epilepsy mutation in the sodium channel SCN1A identifies a cytoplasmic domain for beta subunit interaction

A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel alpha subunit. The mutation decreased modulation of the alpha subunit by beta1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of beta1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-type alpha subunit with the beta1 or beta3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for beta subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the alpha and beta1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.

Drug Resistance in Epilepsy: Putative Neurobiologic and Clinical Mechanisms

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