Anticonvulsant actions of lamotrigine on spontaneous thalamocortical rhythms

Validated animal models for antiseizure drug (ASD) discovery: Advantages and potential pitfalls in ASD screening

Since 1993, over 20 new anti-seizure drugs (ASDs) have been identified in well-established animal seizure and epilepsy models and subsequently demonstrated to be clinically effective in double-blinded, placebo-controlled clinical trials in patients with focal onset seizures. All clinically-available ASDs on the market today are effective in at least one of only three preclinical seizure and epilepsy models: the acute maximal electroshock (MES), the acute subcutaneous pentylenetetrazol (scPTZ) test, or the kindled rodent with chronic evoked seizures. Thus, it reasons that preclinical ASD discovery does not need significant revision to successfully identify ASDs for the symptomatic treatment of epilepsy. Unfortunately, a significant need still persists for more efficacious and better tolerated ASDs. This is particularly true for those patients whose seizures remain drug resistant. This review will focus on the continued utility of the acute MES and scPTZ tests, as well as the kindled rodent for current and future ASD discovery. These are the only "clinically validated" rodent models to date and been heavily used in the search for novel and more efficacious ASDs. This is to say that promising ASDs have been brought to the clinic on the basis of efficacy in these particular seizure and epilepsy models alone. This review also discusses some of the inherent advantages and limitations of these models relative to existing and emerging preclinical models. It then offers insight into future efforts to develop a preclinical model that will advance a truly transformative therapy for the symptomatic treatment of difficult to treat focal onset epilepsy. This article is part of the special issue entitled 'New Epilepsy Therapies for the 21st Century - From Antiseizure Drugs to Prevention, Modification and Cure of Epilepsy'.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

CNS Adverse Effects: From Functional Observation Battery/Irwin Tests to Electrophysiology

This chapter describes various approaches for the preclinical assessment of drug-induced central nervous system (CNS) adverse effects. Traditionally, methods to evaluate CNS effects have consisted of observing and scoring behavioral responses of animals after drug is administered. Among several behavioral testing paradigms, the Irwin and the functional observational battery (FOB) are the most commonly used assays for the assessment of CNS effects. The Irwin and FOB are considered good first-tier assays to satisfy the ICH S7A guidance for the preclinical evaluation of new chemical entities (NCE) intended for humans. However, experts have expressed concern about the subjectivity and lack of quantitation that is derived from behavioral testing. More importantly, it is difficult to gain insight into potential mechanisms of toxicity by assessing behavioral outcomes. As a complement to behavioral testing, we propose using electrophysiology-based assays, both in vivo and in vitro, such as electroencephalograms and brain slice field-potential recordings. To better illustrate these approaches, we discuss the implementation of electrophysiology-based techniques in drug-induced assessment of seizure risk, sleep disruption, and cognitive impairment.

T-type Ca2+ channels in normal and abnormal brain functions

Low-voltage-activated T-type Ca(2+) channels are widely expressed in various types of neurons. Once deinactivated by hyperpolarization, T-type channels are ready to be activated by a small depolarization near the resting membrane potential and, therefore, are optimal for regulating the excitability and electroresponsiveness of neurons under physiological conditions near resting states. Ca(2+) influx through T-type channels engenders low-threshold Ca(2+) spikes, which in turn trigger a burst of action potentials. Low-threshold burst firing has been implicated in the synchronization of the thalamocortical circuit during sleep and in absence seizures. It also has been suggested that T-type channels play an important role in pain signal transmission, based on their abundant expression in pain-processing pathways in peripheral and central neurons. In this review, we will describe studies on the role of T-type Ca(2+) channels in the physiological as well as pathological generation of brain rhythms in sleep, absence epilepsy, and pain signal transmission. Recent advances in studies of T-type channels in the control of cognition will also be briefly discussed.

T-type Ca²⁺ channels in absence epilepsy

Low-voltage-activated T-type Ca²⁺ channels are highly expressed in the thalamocortical circuit, suggesting that they play a role in this brain circuit. Indeed, low-threshold burst firing mediated by T-type Ca²⁺ channels has long been implicated in the synchronization of the thalamocortical circuit. Over the past few decades, the conventional view has been that rhythmic burst firing mediated by T-type channels in both thalamic reticular nuclie (TRN) and thalamocortical (TC) neurons are equally critical in the generation of thalamocortical oscillations during sleep rhythms and spike-wave-discharges (SWDs). This review broadly investigates recent studies indicating that even though both TRN and TC nuclei are required for thalamocortical oscillations, the contributions of T-type channels to TRN and TC neurons are not equal in the genesis of sleep spindles and SWDs. T-type channels in TC neurons are an essential component of SWD generation, whereas the requirement for TRN T-type channels in SWD generation remains controversial at least in the GBL model of absence seizures. Therefore, a deeper understanding of the functional consequences of modulating each T-type channel subtype could guide the development of therapeutic tools for absence seizures while minimizing side effects on physiological thalamocortical oscillations. This article is part of a Special Issue entitled: Calcium channels.

Cross-reactivity of ryanodine receptors with plasma membrane ion channel modulators

Various pharmacological agents designed to modulate plasma membrane ion channels seem to significantly affect intracellular Ca²⁺ signaling when acting on their target receptor. Some agents could also cross-react (modulate channels or receptors beyond their putative target) with intracellular Ca²⁺ transporters. This study investigated the potential of thirty putative modulators of either plasma membrane K⁺, Na⁺, or transient receptor potential (TRP) channels to cross-react with intracellular Ca²⁺ release channels [i.e., ryanodine receptors (RyRs)] from skeletal muscle sarcoplasmic reticulum (SR). Screening for cross-reactivity of these various agents was performed by measuring the rate of spontaneous Ca²⁺ leak or caffeine-induced Ca²⁺ release from SR microsomes. Four of the agents displayed a strong cross-reactivity and were further evaluated with skeletal RyR (RyR1) reconstituted into planar bilayers. 6,12,19,20,25,26-Hexahydro-5,27:13,18:21,24-trietheno-11,7-metheno-7H-dibenzo [b,n][1,5,12,16]tetraazacyclotricosine-5, 13-diium dibromide (UCL 1684; K⁺ channel antagonist) and lamotrigine (Na⁺ channel antagonist) were found to significantly inhibit the RyR1-mediated caffeine-induced Ca²⁺ release. TRP channel agonists anandamide and (-)menthol were found to inhibit and activate RyR1, respectively. High concentrations of nine other agents produced partial inhibition of RyR1-mediated Ca²⁺ release from SR microsomes. Various pharmacological agents, especially TRP modulators, also inhibited a minor RyR1-independent component of the SR Ca²⁺ leak. Overall, ∼43% of the agents selected cross-reacted with RyR1-mediated and/or RyR1-independent Ca²⁺ leak from intracellular stores. Thus, cross-reactivity should be considered when using these classes of pharmacological agents to determine the role of plasmalemmal channels in Ca²⁺ homeostasis.

Epilepsy in Angelman syndrome

Angelman syndrome is a neurogenetic disorder caused by lack of UBE3A gene expression from the maternally inherited chromosome 15 due to various 15q11-q13 abnormalities. In addition to severe developmental delay, virtual absence of speech, motor impairment, a behavioural phenotype that includes happy demeanor, and distinctive rhythmic electroencephalographic features, over 90% of patients have epilepsy. Many different seizure types may occur, atypical absences and myoclonic seizures being particularly prevalent. Non-convulsive status epilepticus is common, sometimes in the context of the epileptic syndrome referred to as myoclonic status in non-progressive encephalopathies. Epilepsy predominates in childhood, but may persist or reappear in adulthood. Management is difficult in a proportion of patients. It might be improved by better understanding of pathophysiology. Current hypotheses involve abnormal inhibitory transmission due to impaired regulation of GABAA receptors related to functional absence of UBE3A and abnormal hippocampal CaMKII activity.

Design, synthesis, and anticonvulsant activity of N-phenylamino derivatives of 3,3-dialkyl-pyrrolidine-2,5-diones and hexahydro-isoindole-1,3-diones

In the present study on the development of new anticonvulsants, the library of differently substituted N-phenylamino pyrrolidine-2,5-dione and hexahydro-isoindole-1,3-dione derivatives was synthesized. The anticonvulsant activity of all the compounds was evaluated using the maximal electroshock (MES) and pentylenetetrazole (scPTZ) screens, which are the most widely employed seizure models for early identification of candidate anticonvulsants. Their neurotoxicity was determined applying the rotorod test. The pharmacological results revealed that the majority of compounds were effective in electrical (MES) and/or pentylenetetrazole induced seizure (scPTZ) models. The quantitative in vivo anticonvulsant evaluation of N-phenylamino-3,3-dimethyl-pyrrolidine-2,5-dione (15), conducted at the time of peak pharmacodynamic activity (TPE), showed the MES ED(50) value of 69.89mg/kg in rats. The median toxic dose (TD(50)) was 500mg/kg, providing compound 15 with a protective index (TD(50)/ED(50)) of 7.15 in the MES test.

Validation of High Throughput Screening Assays Against Three Subtypes of Cav3 T-Type Channels Using Molecular and Pharmacologic Approaches

T-type Ca(2+) channels encoded by voltage-gated Ca(2+) channel (Ca(v)) 3.1, 3.2, and 3.3 genes play important physiological roles and serve as therapeutic targets for neurological and cardiovascular disorders. Currently there is no selective T-channel blocker. To screen for such a blocker, we developed three stable cell lines expressing human recombinant Ca(v)3.1, 3.2, or 3.3 channels and then examined their usefulness in high throughput screens. All three cell lines displayed an increase in intracellular Ca(2+) in response to changes in extracellular Ca(2+) as detected with Ca(2+)-sensitive dyes using a fluorometric imaging plate reader (FLIPR [Molecular Devices, Sunnyvale, CA] or FlexStation [Molecular Devices]). The signal-to-noise ratio was 2-4. Co-expression of Ca(v)3.2 with a mouse leak K(+) channel, which by virtue of being open at rest hyperpolarizes the cell membrane, blocked the fluorescent signal. Co-addition of KCl to these cells induced a Ca(2+) signal that was similar to that observed in the cell line expressing Ca(v)3.2 alone. These results confirm that the detection of intracellular Ca(2+) increase in cells expressing Ca(v)3.2 alone results from Ca(2+) entry through channels that are open at the resting membrane potential of each cell line (i.e., window currents). Testing known drugs on Ca(v)3 channels showed that block could be reliably detected using the FlexStation assay, FLIPR assay, or voltage clamp recordings using the IonWorks HT system (Molecular Devices). These results support the use of the FLIPR window current assay for primary drug screening and high throughput patch recordings for secondary screening of novel T-channel blockers.

Lamotrigine decreases EEG synchronization in a use-dependent manner in patients with idiopathic generalized epilepsy

OBJECTIVE

To investigate the quantitative EEG effects of lamotrigine (LTG) monotherapy.

HYPOTHESIS

LTG was predicted to decrease thalamo-cortical neuronal synchronization in idiopathic generalized epilepsy (IGE).

METHODS

Waking EEG background activity of 19 IGE patients was investigated before treatment and in the course of LTG monotherapy. Raw absolute power (RAP), raw percent power (RRP), and raw mean frequency (RMF) were computed for 19 electrodes and four frequency bands (delta=1.5-3.5Hz, theta=3.5-7.5Hz, alpha=7.5-12.5Hz, and beta=12.5-25.0Hz). Inter- and intrahemispheric coherence was computed for eight electrode pairs and the four frequency bands. In addition, scalp-averages were calculated for each variable. Group differences were computed by means of nonparametric statistics including correction for multiple comparisons.

RESULTS

Main results were decreased delta and theta RAP (p<0.05 for scalp-averages). LTG compressed the delta, theta, and alpha RAP datasets, reducing the upper limit of the scatter in particular. Spearman r-values indicated marked correlation between the starting values (RAPuntreated) and the LTG-related decrease (RAPtreated-RAPuntreated) in three bands: delta (r=-0.72; p=0.0005), theta (r=-0.59; p=0.007), and alpha (r=-0.61; p=0.006). Thus, the greater the baseline neuronal synchronization, the marked the dampening effect of LTG on it. The remaining findings were decreased theta RRP, theta RMF, and increased alpha RMF (p<0.05 for scalp-averages). The electrode-related changes were small but topographically consistent across the 19 electrode sites. LTG did not affect coherence.

CONCLUSIONS

1. LTG partially normalized the spectral composition of EEG background activity. LTG decreased pathological thalamo-cortical synchronization in use-dependent manner. 2. LTG did not cause quantitative EEG alterations suggesting worsening of the physiological brain functions. Instead, its profile suggested a mild psychostimulant effect.

SIGNIFICANCE

The results contribute to the understanding of the effect of LTG at the network level.

Recurrent absence status epilepticus (spike-and-wave stupor) associated with lamotrigine therapy

We report on the aggravation of absence seizures by lamotrigine leading to absence status epilepticus in a child. The patient is a 10-year-old girl with a history of absence seizures, learning disabilities, and hereditary neuropathy with liability to pressure palsies. Lamotrigine (up to 12 mg/kg/day) was prescribed after a brief course of valproate was associated with restlessness. Long-acting methylphenidate was also administered. The initial response to lamotrigine appeared to be excellent. The first episode of absence status epilepticus occurred during a febrile illness while lamotrigine was being gradually discontinued. Following this event, lamotrigine dose was increased to 10 mg/kg/day and methylphenidate was continued. Six weeks later, a second absence status epilepticus episode ensued without fever. Sulthiame and clonazepam were substituted for lamotrigine, whereas methylphenidate therapy was continued. A psychiatrist prescribed risperidone 1 month later owing to obsessive-compulsive behavior. Nine months later, she remained free of absence seizures. Whereas the first absence status epilepticus event could have been triggered by fever, the second episode occurred while the daily lamotrigine dose was being increased. Moreover, the patient is seizure free following lamotrigine discontinuation. Hence, it is quite possible that lamotrigine caused seizure aggravation and absence status epilepticus in this child.

Forced normalisation precipitated by lamotrigine

PURPOSE

To report two patients with lamotrigine-induced forced normalization (FN).

METHODS

Evaluation of the patient files, EEG, and video-EEG records, with special reference to the parallel clinical and EEG changes before, during, and after FN.

RESULTS

This is the first documented report of lamotrigine-induced FN. The two epileptic patients (one of them was a 10-year-old girl) were successfully treated with lamotrigine. Their seizures ceased and interictal epileptiform events disappeared from the EEG record. Simultaneously, the patients displayed de novo occurrence of psychopathologic manifestations and disturbed behaviour. Reduction of the daily dose of LTG led to disappearance of the psychopathological symptoms and reappearance of the spikes but not the seizures.

CONCLUSIONS

Lamotrigine may precipitate FN in adults and children. Analysis of the cases showed that lamotrigine-induced FN is a dose-dependent phenomenon and can be treated by reduction of the daily dose of the drug.

From Electrophysiology to Chromatin: A Bottom-Up Approach to Angelman Syndrome

Angelman syndrome is one of the most studied human diseases related to a gene that is expressed on the maternal chromosome only in at least some brain cells. It is caused by inactivation of the UBE3A gene in the brain due to various abnormalities of the 15q11-q13 chromosome inherited from the mother. It is characterized by severe developmental delay, seizures, virtual absence of speech, motor impairment, and a particular behavioral phenotype. Studies of cortical, electromyographic and cerebellar electrophysiology in patients with Angelman syndrome and a mouse model revealed unique rhythmic neurophysiological activities in the cerebral cortex, cerebellar cortex, and muscles. The oscillatory patterns may be linked to molecular pathophysiology of the syndrome involving dysregulation of synaptic neurotransmission through UBE3A-related modulation of functional GABAA receptor complexes.

Topiramate Effective in the Treatment of Intractable Glossodynia

Objective:

To report effective treatment with topiramate in 2 cases of glossodynia (burning tongue and mouth syndrome).

Case Summaries:

Two women, 52 and 53 years old, suffered from intractable unilateral burning mouth syndrome. In both cases, the activities of daily living were severely restricted. Previous treatment with diverse medications had failed. Administration of topiramate 200–350 mg/day stopped the attacks completely in one patient and reduced the frequency by 50–60% in the second.

Discussion:

Glossodynia may present as unilateral intractable pain syndrome with characteristics of neuralgic pain. It is defined as a burning pain in the tongue and mucous membranes of the mouth. There is some research on possible effective medications for treatment, but few data on the beneficial effects of this therapy have been reported. Topiramate has been previously shown to have an antineuralgic effect in trigeminal neuralgia. In the cases of intractable glossodynia presented here, this medication led to significant improvement in the symptoms of this uncommon neuralgic pain.

Conclusions:

Topiramate has a beneficial effect in intractable burning mouth and tongue syndrome. Taking these positive results into consideration, the practitioner can give hope to disabled patients for returning to acceptable activities of daily living.