Motor cortical epileptic foci in vivo: actions of a calcium channel blocker on paroxysmal neuronal depolarizations.

Focal epileptiform activity was induced by local application of penicillin to the surface of the rat motor cortex. Neurons located within the epileptic focus displayed typical paroxysmal depolarization shifts (PDS). The participation of membrane calcium currents in the generation of PDS was examined by injecting the quaternized calcium entry blocker D890 into single neurons by iontophoresis or by pressure pulses. After intracellular injections of D890, PDS were depressed in amplitude by up to 55%. In a few cases the depression of PDS following intracellular application of D890 was preceded by a transient increase. Similar increases of PDS amplitude were obtained by injections of the calcium chelator EGTA. Control experiments in preparations without epileptic activity revealed that excitatory potentials elicited by thalamic stimulation and Cl(-)-dependent inhibitory postsynaptic potentials evoked by epicortical stimulation were not affected by intracellular D890. In these experiments successful intracellular drug application was verified by monitoring the transient shift of the Cl(-)-equilibrium potential induced by injection of KCl together with D890. It is concluded that in the penicillin-induced epileptic focus of the motor cortex Ca2+ inward currents participate in the generation of neuronal PDS.