Selective blockade of N-type calcium channels by levetiracetam

Purpose: We investigated the effect of the new antiepileptic drug (AED) levetiracetam (LEV) on different types of high-voltage-activated (HVA) Ca2+ channels in freshly isolated CA I hippocampal neurons of rats. Methods: Patch-clamp recordings of HVA Ca2+ channel activity were obtained from isolated hippocampal CAI neurons. LEV was applied by gravity flow from a pipette placed near the cell, and solution changes were made by electromicrovalves. Ca2+ channel blockers were used for separation of the channel subtypes. Results: The currents were measured in controls and after application of 1-200 muM LEV. LEV irreversibly inhibited the HVA calcium current by similar to18% on the average. With a prepullse stimulation protocol, which can eliminate direct inhibition of Ca2+ channels by G proteins, we found that G proteins were not involved in the pathways underlying the LEV inhibitory effect. This suggested that the inhibitory effect arises from a direct action of LEV on the channel molecule. The blocking mechanism of LEV was not related to changes in steady-state activation or inactivation of Ca2+ channels. LEV also did not influence the rundown of the HVA Ca2+ current during experimental protocols lasting similar to10 min. Finally, LEV at the highest concentration used (200 muM) did not influence the activity of L-, P- or Q-type Ca2+ channels in CAI neurons, while selectively influencing the activity of N-type calcium channels. The maximal effect on these channels separated from other channel types was -similar to37%. Conclusions: Our results provide evidence that LEV selectively inhibits N-type Ca2+ channels of CAI pyramidal hippocampal neurons. These data suggest the existence of a subtype of N-type channels sensitive to LEV, which might be involved in the molecular basis of its antiepileptic action.