T-type Ca2+ channels in absence epilepsy

The role of HCN channels on the effects of T-type calcium channels and GABAA receptors in the absence epilepsy model of WAG/Rij rats

In this study we used ivabradine (IVA), a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker, to identify its effect on spike-wave discharges (SWDs); and aimed to determine the role of IVA on the effects of T-type calcium channel blocker NNC 55-0396, GABAA receptor agonist muscimol and antagonist bicuculline in male WAG/Rij rats. After tripolar electrodes for electrocorticogram (ECoG) recordings were placed on the WAG/Rij rats' skulls, 5, 10, and 20 mg/kg IVA were intraperitoneally administered for 7 consecutive days and ECoG recordings were obtained on days 0th, 3rd, 6th, and 7th for three hours before and after injections. While acute injection of 5, 10, and 20 mg/kg IVA did not affect the total number and the mean duration of SWDs, subacute administration (7 days) of IVA decreased the SWDs parameters 24 hours after the 7th injection. Interestingly, when IVA was administered again 24 hours after the 6th IVA injection, it increased the SWDs parameters. Western-blot analyses showed that HCN1 and HCN2 expressions decreased and HCN4 increased in the 5-month-old WAG/Rij rats compared to the 1-month-old WAG/Rij and 5-month-old native Wistar rats, while subacute IVA administration increased the levels of HCN1 and HCN2 channels, except HCN4. Subacute administration of IVA reduced the antiepileptic activity of NNC, while the proepileptic activity of muscimol and the antiepileptic activity of bicuculline were abolished. It might be suggested that subacute IVA administration reduces absence seizures by changing the HCN channel expressions in WAG/Rij rats, and this affects the T-type calcium channels and GABAA receptors.

Genetic mechanisms in generalized epilepsies

The genetic generalized epilepsies (GGEs) have been proved to generate from genetic impact by twin studies and family studies. The genetic mechanisms of generalized epilepsies are always updating over time. Although the genetics of GGE is complex, there are always new susceptibility genes coming up as well as copy number variations which can lead to important breakthroughs in exploring the problem. At the same time, the development of ClinGen fades out some of the candidate genes. This means we have to figure out what accounts for a reliable gene for GGE, in another word, which gene has sufficient evidence for GGE. This will improve our understanding of the genetic mechanisms of GGE. In this review, important up-to-date genetic mechanisms of GGE were discussed.

Calcium Channels in Retinal Function and Disease

Voltage-gated Ca2+ (Cav) channels play pivotal roles in regulating gene transcription, neuronal excitability, and neurotransmitter release. To meet the spatial and temporal demands of visual signaling, Cav channels exhibit unusual properties in the retina compared to their counterparts in other areas of the nervous system. In this article, we review current concepts regarding the specific subtypes of Cav channels expressed in the retina, their intrinsic properties and forms of modulation, and how their dysregulation could lead to retinal disease.

Hippocampus-related cognitive disorders develop in the absence of epilepsy and ataxia in the heterozygous Cacna1a mutant mice tottering

CACNA1A-associated epilepsy and ataxia frequently accompany cognitive impairments as devastating co-morbidities. However, it is unclear whether the cognitive deficits are consequences secondary to the neurological symptoms elicited by CACNA1A mutations. To address this issue, Cacna1a mutant mice tottering (tg), and in particular tg/+ heterozygotes, serve as a suitable model system, given that tg/+ heterozygotes fail to display spontaneous absence epilepsy and ataxia typically observed in tg/tg homozygotes. Here, we examined hippocampus-dependent behaviors and hippocampal learning-related synaptic plasticity in tg mice. In behavioral analyses of tg/+ and tg/tg, acquisition and retention of spatial reference memory were characteristically impaired in the Morris water maze task, while working memory was intact in the eight-arm radial maze and T-maze tasks. tg/+ heterozygotes showed normal motor function in contrast to tg/tg homozygotes. In electrophysiological analyses, Schaffer collateral–CA1 synapses showed a deficit in the maintenance of long-term potentiation in tg/+ and tg/tg mice and an increased paired-pulse facilitation induced by paired pulses with 100 ms in tg/tg mice. Our results indicate that the tg mutation causes a dominant disorder of the hippocampus-related memory and synaptic plasticity, raising the possibility that in CACNA1A-associated human diseases, functionally aberrant Ca_V2.1 Ca²⁺ channels actively induce the observed cognitive deficits independently of the neurological symptoms.

Distinct burst properties contribute to the functional diversity of thalamic nuclei

Thalamic neurons fire spikes in two modes, burst and tonic. The function of burst firing is unclear, but the evidence suggests that bursts are more effective at activating cortical cells, and that postinhibition rebound bursting contributes to thalamocortical oscillations during sleep. Bursts are considered stereotyped signals; however, there is limited evidence regarding how burst properties compare across thalamic nuclei of different functional or anatomical organization. Here, we used whole-cell patch clamp recordings and compartmental modeling to investigate the properties of bursts in six sensory thalamic nuclei, to study the mechanisms that can lead to different burst properties, and to assess the implications of different burst properties for thalamocortical transmission and oscillatory functions. We found that bursts in higher-order cells on average had higher number of spikes and longer latency to the first spike. Additionally, burst features in first-order neurons were determined by sensory modality. Shifting the voltage-dependence and density of the T-channel conductance in a compartmental model replicates the burst properties from the intracellular recordings, pointing to molecular mechanisms that can generate burst diversity. Furthermore, the model predicts that bursts with higher number of spikes will drastically reduce the effectiveness of thalamocortical transmission. In addition, the latency to burst limited the rebound oscillatory frequency in modeled cells. These results demonstrate that burst properties vary according to the thalamocortical hierarchy and with sensory modality. The findings imply that, while in burst mode, thalamocortical transmission and firing frequency will be determined by the number of spikes and latency to burst.

Neuronal Cav3 channelopathies: recent progress and perspectives

T-type, low-voltage activated, calcium channels, now designated Cav3 channels, are involved in a wide variety of physiological functions, especially in nervous systems. Their unique electrophysiological properties allow them to finely regulate neuronal excitability and to contribute to sensory processing, sleep, and hormone and neurotransmitter release. In the last two decades, genetic studies, including exploration of knock-out mouse models, have greatly contributed to elucidate the role of Cav3 channels in normal physiology, their regulation, and their implication in diseases. Mutations in genes encoding Cav3 channels (CACNA1G, CACNA1H, and CACNA1I) have been linked to a variety of neurodevelopmental, neurological, and psychiatric diseases designated here as neuronal Cav3 channelopathies. In this review, we describe and discuss the clinical findings and supporting in vitro and in vivo studies of the mutant channels, with a focus on de novo, gain-of-function missense mutations recently discovered in CACNA1G and CACNA1H. Overall, the studies of the Cav3 channelopathies help deciphering the pathogenic mechanisms of corresponding diseases and better delineate the properties and physiological roles Cav3 channels.

Multiple-Ascending Dose Study in Healthy Subjects to Assess the Pharmacokinetics, Tolerability, and CYP3A4 Interaction Potential of the T-Type Calcium Channel Blocker ACT-709478, A Potential New Antiepileptic Drug

BACKGROUND

ACT-709478 is a selective, orally available T-type calcium channel blocker being studied as a potential new treatment in epilepsy. ACT-709478 had previously been investigated in a single-ascending dose study up to a dose of 400 mg.

OBJECTIVES

The aim of this study was to investigate the safety, tolerability, pharmacokinetics, and pharmacodynamics of multiple doses of ACT-709478. In addition, the drug-drug interaction potential of multiple doses of ACT-709478 with the cytochrome P450 3A4 substrate midazolam was investigated.

METHODS

This double-blind, placebo-controlled, randomized study included 46 healthy male and female subjects. Ascending multiple oral doses of ACT-709478 were administered to 10 (cohorts 1-2) or 12 (cohorts 3-4) subjects (two taking placebo per cohort). In cohorts 1-2, 30 or 10 mg ACT-709478 was administered once daily for 12 days. An up-titration regimen was used in cohorts 3-4 with administration of 10, 30, and 60 mg for 7 days each in both cohorts and an additional dose level of 100 mg ACT-709478 once daily for 8 days in cohort 4. Single doses of midazolam were administered at baseline and concomitantly to 60 mg and 100 mg ACT-709478 in cohort 4. Blood sampling for pharmacokinetic evaluations and safety assessments (clinical laboratory, vital signs, adverse events, and electrocardiogram) were performed regularly. Holter electrocardiograms were recorded at baseline and for 24 h at steady state and central nervous system effects were assessed with pharmacodynamic tests at baseline and steady state.

RESULTS

ACT-709478 was absorbed with a time to reach the maximum plasma concentration of 3.5-4.0 h and eliminated with a half-life of 45-53 h. Steady state was reached after 5-7 days of dosing and exposure increased dose-proportionally. An accumulation index of approximately three fold was observed in cohorts 1 and 2. Exposure to midazolam was lower upon concomitant administration of 60 and 100 mg ACT-709478 compared to midazolam alone while the half-life and time to reach the maximum plasma concentration of midazolam remained unchanged, suggesting a weak induction at the gastrointestinal but not hepatic level. Pharmacokinetic parameters of 1-hydroxymidazolam were not affected by ACT-709478 administration. The most frequent adverse events were dizziness, somnolence, and headache. A tolerability signal was detected in cohort 1 (30 mg once daily); therefore, the dose was decreased to 10 mg once daily in cohort 2. The subsequently established up-titration regimen, starting with 10 mg once daily, considerably improved tolerability. Multiple doses up to 100 mg once daily were well tolerated. No treatment-related effects were detected on vital signs, clinical laboratory tests, Holter electrocardiogram variables, or in the pharmacodynamic tests.

CONCLUSIONS

ACT-709478 exhibits good tolerability up to 100 mg once daily using an up-titration regimen and pharmacokinetic properties that support further clinical investigations. A weak induction of gastrointestinal cytochrome P450 3A4 activity was observed, unlikely to be of clinical relevance. CLINICALTRIALS.

GOV IDENTIFIER

NCT03165097.

Pharmacological characterization of nicotine-induced tremor: Responses to anti-tremor and anti-epileptic agents

We previously showed that nicotine evoked kinetic tremor by activating the inferior olive, which is implicated in the pathogenesis of essential tremor, via α7 nicotinic acetylcholine receptors. Here, we evaluated the effects of various anti-tremor and anti-epileptic agents on nicotine-induced tremor in mice to clarify the pharmacological characteristics of nicotine tremor. Drugs effective for essential tremor, propranolol, diazepam and phenobarbital, all significantly inhibited kinetic tremor induced by an intraperitoneal (i.p.) injection of nicotine (1 mg/kg). In contrast, none of the medications for Parkinson's disease, l-DOPA, bromocriptine or trihexyphenidyl, affected the nicotine tremor. Among the anti-epileptic agents examined, valproate, carbamazepine and ethosuximide, significantly inhibited nicotine-induced tremor. In addition, a selective T-type Ca²⁺ channel blocker, TTA-A2, also suppressed the nicotine tremor. However, neither gabapentin, topiramate, zonisamide nor levetiracetam significantly affected nicotine-induced tremor. The present results show that nicotine-induced tremor resembles essential tremor not only on the neural basis, but also in terms of the pharmacological responses to anti-tremor agents, implying that nicotine-induced tremor can serve as a model for essential tremor. In addition, it is suggested that anti-epileptic agents, which have stimulant actions on the GABAergic system or blocking actions on voltage-gated Na⁺ channels and T-type Ca²⁺ channels, can alleviate essential tremor.

Out of thin air: Hyperventilation-triggered seizures

Voluntary hyperventilation triggers seizures in the vast majority of people with absence epilepsy. The mechanisms that underlie this phenomenon remain unknown. Herein, we review observations - many made long ago - that provide insight into the relationship between breathing and absence seizures.

Thalamocortical neurons display suppressed burst-firing due to an enhanced Ih current in a genetic model of absence epilepsy

Burst-firing in distinct subsets of thalamic relay (TR) neurons is thought to be a key requirement for the propagation of absence seizures. However, in the well-regarded Genetic Absence Epilepsy Rats from Strasbourg (GAERS) model as yet there has been no link described between burst-firing in TR neurons and spike-and-wave discharges (SWDs). GAERS ventrobasal (VB) neurons are a specific subset of TR neurons that do not normally display burst-firing during absence seizures in the GAERS model, and here, we assessed the underlying relationship of VB burst-firing with Ih and T-type calcium currents between GAERS and non-epileptic control (NEC) animals. In response to 200-ms hyperpolarizing current injections, adult epileptic but not pre-epileptic GAERS VB neurons displayed suppressed burst-firing compared to NEC. In response to longer duration 1,000-ms hyperpolarizing current injections, both pre-epileptic and epileptic GAERS VB neurons required significantly more hyperpolarizing current injection to burst-fire than those of NEC animals. The current density of the Hyperpolarization and Cyclic Nucleotide-activated (HCN) current (Ih) was found to be increased in GAERS VB neurons, and the blockade of Ih relieved the suppressed burst-firing in both pre-epileptic P15-P20 and adult animals. In support, levels of HCN-1 and HCN-3 isoform channel proteins were increased in GAERS VB thalamic tissue. T-type calcium channel whole-cell currents were found to be decreased in P7-P9 GAERS VB neurons, and also noted was a decrease in CaV3.1 mRNA and protein levels in adults. Z944, a potent T-type calcium channel blocker with anti-epileptic properties, completely abolished hyperpolarization-induced VB burst-firing in both NEC and GAERS VB neurons.