EFFECTS OF T-TYPE, L-TYPE, N-TYPE, P-TYPE, AND Q-TYPE CALCIUM-CHANNELBLOCKERS ON STIMULUS-INDUCED PRESYNAPTIC AND POSTSYNAPTIC CALCIUM FLUXES IN RAT HIPPOCAMPAL SLICES

The contribution of T-, L-, N-, P-, and Q-type Ca2+ channels to pre- a nd postsynaptic Ca2+ entry during stimulus-induced high neuronal activ ity in area CA1 of mt hippocampal slices was investigated by measuring the effect of specific blockers on stimulus-induced decreases in extr acellular Ca2+ concentration ([Ca2+](o)). [Ca2+](o) was measured with ion-selective electrodes in stratum radiatum (SR) and stratum pyramida le (SP), while Ca2+ entry into neurons was induced with stimulus train s (20 Hz for 10 s) alternately delivered to SR and the alveus, respect ively. The [Ca2+](o) decreases recorded in SR in response to SR stimul ation represented mainly presynaptic Ca2+ entry (Ca-pre), while [Ca2+] (o) decreases recorded in SP in response to alvear stimulation were pr edominantly based on postsynaptic Ca2+ entry (Ca-post). Ethosuximide a nd trimethadione were ineffective In concentrations up to 1 mM. At 10 mM, they reduced Ca-post and, much less, also Ca-pre. Nimodipine (25 m u M) reduced Ca-post and, to a minor extent, Ca-pre. omega-Agatoxin IV A (0.4-1 mu M) and omega-conotoxin MVIIC (1 mu M) also reduced both Ca -pre and Ca-post, but with a stronger action on Ca-pre. omega-Conotoxi n GVIA (3-8 mu M) reduced Ca-post without effect on Ca-pre. We conclud e that during stimulus-induced, high-frequency neuronal activity Ca-po st is carried by P/Q-, N-, and L-type channels and probably a further channel type different from these channels. Ca-pre includes at least P /Q- and possibly L-type channels. N-type channels did not contribute t o Ca-pre in our experiments. Since ethosuximide and trimethadione were only effective in high concentrations, their action may be unspecific . Thus, T-type channels do not seem to play a major part in Ca2+ entry in this situation.