Epilepsy-Related Voltage-Gated Sodium Channelopathies: A Review

Epilepsy is a disease characterized by abnormal brain activity and a predisposition to generate epileptic seizures, leading to neurobiological, cognitive, psychological, social, and economic impacts for the patient. There are several known causes for epilepsy; one of them is the malfunction of ion channels, resulting from mutations. Voltage-gated sodium channels (NaV) play an essential role in the generation and propagation of action potential, and malfunction caused by mutations can induce irregular neuronal activity. That said, several genetic variations in NaV channels have been described and associated with epilepsy. These mutations can affect channel kinetics, modifying channel activation, inactivation, recovery from inactivation, and/or the current window. Among the NaV subtypes related to epilepsy, NaV1.1 is doubtless the most relevant, with more than 1500 mutations described. Truncation and missense mutations are the most observed alterations. In addition, several studies have already related mutated NaV channels with the electrophysiological functioning of the channel, aiming to correlate with the epilepsy phenotype. The present review provides an overview of studies on epilepsy-associated mutated human NaV1.1, NaV1.2, NaV1.3, NaV1.6, and NaV1.7.

Epileptogenic Channelopathies: Experimental Models of Human Pathologies

The discovery of genetically determined epileptic syndromes associated with specific mutations of genes codifying for subunits of voltage or ligand-activated ion channels highlights the role of ion channels in epileptogenesis. In vitro and in vivo models of channel pathology have been used to define the functional consequence of the mutations identified in human epilepsies. The evaluation of gene-channel mutations based on molecular and physiological techniques have provided significant knowledge on the cellular mechanisms leading to inherited human epilepsies, and possibly to nongenetic human epilepsies due to "acquired" channel pathologies. We review some of the studies that have explored human epileptic disorders through experimental manipulations of these channels, highlighting some of the difficulties that have arisen using "in vitro" preparations or rodent models. These findings underscore the need for further studies to address the mechanisms involved in mutated-channel dysfunctions.

Carbamazepine and topiramate modulation of transient and persistent sodium currents studied in HEK293 cells expressing the Na(v)1.3 alpha-subunit

PURPOSE

The transient and the persistent Na(+) current play a distinct role in neuronal excitability. Several antiepileptic drugs (AEDs) modulate the transient Na(+) current and block the persistent Na(+) current; both effects contribute to their antiepileptic properties. The interactions of the AEDs carbamazepine (CBZ) and topiramate (TPM) with the persistent and transient Na(+) current were investigated.

METHODS

HEK293 cells stably expressing the alpha-subunit of the Na(+) channel Na(V)1.3 were used to record Na(+) currents under voltage-clamp by using the patch-clamp technique in whole-cell configuration and to investigate the effects of CBZ and TPM.

RESULTS

The persistent Na(+) current was present in all cells and constituted 10.3 +/- 3.8% of the total current. CBZ partially blocked the persistent Na(+) current in a concentration-dependent manner [median effective concentration (EC(50)), 16 +/- 4 microM]. CBZ also shifted the steady-state inactivation of the transient Na(+) current to negative potentials (EC(50), 14 +/- 11 microM). TPM partially blocked the persistent Na(+) current with a much higher affinity (EC(50), 61 +/- 37 nM) than it affected the steady-state inactivation of the transient Na(+) current (EC(50), 3.2 +/- 1.8 microM). For the latter effect, TPM was at most half as effective as CBZ.

CONCLUSIONS

The persistent Na(+) current flowing through the alpha-subunit of the Na(V)1.3 channel is partially blocked by CBZ at about the same therapeutic concentrations at which it modulates the transient Na(+) current, adding a distinct aspect to its anticonvulsant profile. The TPM-induced partial block of the persistent Na(+) current, already effective at low concentrations, could be the dominant action of this drug on the Na(+) current.

The role of the persistent Na(+) current during cardiac ischemia and hypoxia

INTRODUCTION

Inadequate or zero cardiac perfusion, if prolonged beyond about 10 minutes, can result in irreversible cell damage. Paradoxically, much of this damage occurs when perfusion is restored, and this appears to be linked to an uncontrolled rise in intracellular calcium. This article reviews the causes of this rise in calcium.

METHODS AND RESULTS

Data that have arisen from a variety of techniques to measure intracellular ion concentrations in cardiac cells are reviewed. Fluorescence measurements in intact hearts and isolated cells show that the rise in [Ca(2+)](i) is preceded by a rise in [Na(+)](i), a finding that has led to the coupled exchanger theory, which postulates that the high activity of the Na-H exchanger, as a result of intracellular acidification, increases [Na(+)](i) and that this slows or reverses the Na-Ca exchanger. However, the [Na(+)](i) appears to come from several sources: the Na-H exchanger, the Na-HCO(3) symporter, and the persistent Na(+) current, I(Na(p)). The latter appears to be important because blockers of Na-H exchange (e.g., Cariporide) have been shown to be only partially protective against reperfusion damage, whereas TTX and other Na(+) channel blockers offer equal or better protection. Patch clamp experiments in isolated cells have shown that I(Na(p)) is increased by hypoxia, although the mechanisms are not known.

CONCLUSION

Blockers of I(Na(p)) may provide an alternative strategy for preventing reperfusion damage in myocardium, either alone or in combination with Na-H exchange blockers.