Zonisamide block of cloned human T-type voltage-gated calcium channels

SummaryZonisamide (ZNS) is a multi-target antiepileptic drug reported to be efficient in the treatment of both partial and generalized seizures, with T-type Ca2+ channel blockade being one of its proposed mechanisms of action. In this study, we systematically investigated electrophysiological effects of ZNS on cloned human Cav3.1–3.3 Ca2+ channels in a heterologous HEK-293 expression system using whole cell patch-clamp technique. Concentration-response studies were performed in the range from 5 μM to 2 mM for Cav3.2 Ca2+ channels exhibiting a 15.4–30.8% reduction of Ca2+ influx within the maximum therapeutic plasma range (50–200 μM ZNS). The other T-type Ca2+ channel entities, Cav3.1 and Cav3.3, were even less sensitive to ZNS. Both voltage- and concentration-dependence of inactivation kinetics remained unchanged for Cav3.2 VGCC, whereas Cav3.1 and Cav3.3 exhibited minor, though significant reduction of inactivation-tau. Interestingly, ZNS block of Cav3.2 VGCCs was not use-dependent and remained unaffected by changes in the holding potential. Steady-state inactivation studies did not display a significant shift in steady-state availability of Cav3.2 channels at 100 μM ZNS (ΔV1/2 = 3.1 mV, p = 0.071). Our studies indicate that ZNS is a moderate blocker of human Cav3 T-type Ca2+ channels with little or no effect on Cav3.2 Ca2+ channel inactivation kinetics, use- and state-dependence of blockade. These results suggest that T-type Ca2+ channel inhibition only partially contributes to the anti-absence activity of ZNS antiepileptic drug.