Calcium-dependent, sustained enhancement of excitability during washout of aconitine in rat hippocampal slices.

Extracellular recording of the stimulus-evoked population spike was performed in the CA1 area of rat hippocampal slices in order to investigate delayed effects of the plant alkaloids aconitine and veratridine. Veratridine (1 microM and 10 microM) suppressed the orthodromic and antidromic population spike. After washout of the drug, only a partial recovery was obtained. Aconitine (1 microM) exerted the same inhibitory action as veratridine. However, after washout, the spike amplitude was enhanced compared with the control. This enhancement of the spike amplitude was dependent on the concentration of aconitine and was maintained during the observation period of at least 2 h. Lowering the Ca2+ concentration of the bathing medium from 2.5 mM to 1.25 mM during application of aconitine attenuated recovery and prevented the enhancement observed during washout of the drug. Application of aconitine in the presence of CdCl2 as well as in the presence of inhibitors of protein kinase C and Ca2+/calmodulin-dependent protein kinase II prevented the increase in spike amplitude during washout with standard artificial cerebrospinal fluid. In contrast, the N-methyl-D-aspartate (NMDA) receptor antagonists D-AP5 and MK-801 as well as the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione were ineffective in abolishing the aconitine-induced enhancement. These data support the conclusion that different modes of action are involved in the effects of aconitine but not veratridine. It is concluded that the aconitine-induced increase in neuronal activity is mediated by intracellular Ca2+-dependent mechanisms leading to an activation of Ca2+-dependent protein kinases. This effect is independent of Ca2+ entrance through NMDA and non-NMDA receptors.