Zonisamide blocks T-type calcium channel in cultured neurons of rat cerebral cortex.

We investigated the effect of zonisamide, a new antiepileptic drug, on voltage-dependent Ca2+ currents in cultured neurons of rat cerebral cortex. Whole-cell voltage-clamp recordings demonstrated at least two distinct voltage-dependent Ca2+ currents: (1) a low-threshold, rapidly inactivating component, T-type Ca2+ current, which is sensitive to 100 microM Ni2+, and (2) a high-threshold, slowly inactivating (long-lasting) component, L-type Ca2+ current. Zonisamide, a new anticonvulsant effective against maximal electroshock (MES) seizures in mice reduced T-type Ca2+ current in a dose-dependent manner. The mean percentage of reduction was 59.5 +/- 7.2% at 500 microM, but zonisamide had no effect on L-type Ca2+ current. A methylated analog of zonisamide, which is ineffective against MES seizures in mice, was tested at a concentration of 500 microM, and reduced neither T-type nor L-type Ca2+ current. These findings suggest that the effects of zonisamide against MES seizures might occur through the reduction of T-type Ca2+ current. Because drugs that are effective against MES seizures are thought to prevent seizure discharge spread, T-type Ca2+ channels could underlie a cellular mechanism of spreading activity in epileptic seizures.