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
P. Igelmund et al., 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, Experimental Brain Research, 109(1), 1996, pp. 22-32
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.