Block of T-Type Ca2+ Channels Is an Important Action of Succinimide Antiabsence Drugs

Genetic variants in absence epilepsy: a contextual consideration of calcium current kinetics

Functional Characterization and Neuronal Modeling of the Effects of Childhood Absence Epilepsy Variants of Cacna1h, a T-Type Calcium Channel

Vitko I, Chen Y, Arias JM, Shen Y, Wu XR, Perez-Reyes E

J Neurosci 2005;25(19):4844-4855

Sequencing of the T-type Ca2+ channel gene CACNA1H revealed 12 nonsynonymous single nucleotide polymorphisms (SNPs) that were found only in childhood absence epilepsy (CAE) patients. One SNP, G773D, was found in two patients. The present study reports the finding of a third patient with this SNP, as well as analysis of their parents. Because of the role of T-channels in determining the intrinsic firing patterns of neurons involved in absence seizures, it was suggested that these SNPs might alter channel function. The goal of the present study was to test this hypothesis by introducing these polymorphisms into a human Cav3.2a cDNA and then study alterations in channel behavior using whole-cell patch-clamp recording. Eleven SNPs altered some aspect of channel gating. Computer simulations predict that seven of the SNPs would increase firing of neurons, with three of them inducing oscillations at similar frequencies, as observed during absence seizures. Three SNPs were predicted to decrease firing. Some CAE-specific SNPs (e.g., G773D) coexist with SNPs also found in controls (R788C); therefore, the effect of these polymorphisms were studied. The R788C SNP altered activity in a manner that would also lead to enhanced burst firing of neurons. The G773D–R788C combination displayed different behavior than either single SNP. Therefore, common polymorphisms can alter the effect of CAE-specific SNPs, highlighting the importance of sequence background. These results suggest that CACNA1H is a susceptibility gene that contributes to the development of polygenic disorders characterized by thalamocortical dysrhythmia, such as CAE.

The Epileptic Neuron Redux

Upregulation of a T-Type Ca2+ Channel Causes a Long-Lasting Modification of Neuronal Firing Mode after Status Epilepticus

Su H, Sochivko D, Becker A, Chen J, Jiang Y, Yaari Y, Beck H

J Neurosci 2002;22:3645–3655

A single episode of status epilepticus (SE) causes numerous structural and functional changes in the brain that can lead to the development of a chronic epileptic condition. Most studies of this plasticity have focused on changes in excitatory and inhibitory synaptic properties. However, the intrinsic firing properties that shape the output of the neuron to a given synaptic input also may be persistently affected by SE. Thus 54% of CA1 pyramidal cells, which normally fire in a regular mode, are persistently converted to a bursting mode after an episode of SE induced by the convulsant pilocarpine. In this model, intrinsic bursts evoked by threshold-straddling depolarizations, and their underlying spike after depolarizations (ADPs), were resistant to antagonists of N-, P/Q-, or L-type Ca2+ channels but were readily suppressed by low (30–100 μ M) concentrations of Ni2+ known to block T- and R-type Ca2+ channels. The density of T-type Ca2+ currents, but not of other pharmacologically isolated Ca2+ current types, was upregulated in CA1 pyramidal neurons after SE. The augmented T-type currents were sensitive to Ni2+ in the same concentration range that blocked the novel intrinsic bursting in these neurons (IC50 = 27 (μ M). These data suggest that SE may persistently convert regular-firing cells to intrinsic bursters by selectively increasing the density of a Ni2+-sensitive T-type Ca2+ current. This nonsynaptic plasticity considerably amplifies the output of CA1 pyramidal neurons to synaptic inputs and most probably contributes to the development and expression of an epileptic condition after SE.