Differential effects of calcium entry blockers on pre- and postsynaptic influx of calcium in the rat hippocampus in vitro.

A decrease in extracellular free Ca ([Ca2+]o) in response to stimulation of Schaffer collaterals could be recorded in or near the stratum pyramidale even when synaptic transmission was completely blocked. Under the same conditions, alvear stimulation also evoked a decrease in [Ca2+]o at the same site. We attributed the former to influx of Ca2+ into presynaptic terminals and the latter to influx into postsynaptic (pyramidal) cells. Both pre- and postsynaptic Ca2+ influx were completely blocked by Ni2+ (2.5 mM). Nifedipine (5-10 microM). verapamil (50-100 microM) and fendiline (100-200 microM) reduced the postsynaptic influx of Ca2+ but did not alter Ca2+ loss from the extracellular space into presynaptic terminals. The calcium channel activators, BAY-K 8644 and CGP 28,392, had no consistent effect on either pre- or postsynaptic influx. Occasional enhancement of both pre- and postsynaptic responses was seen. In most studies the agents were without effect and on occasions a reduction in both responses was seen. The results could indicate that Ca-channels at pre- and postsynaptic sites in CA1 may be of different types.