Presynaptic involvement in frequency facilitation in the hippocampal slice.

Orthodromic stimulation with frequencies from 1 to 30 Hz causes facilitation of extracellularly recorded population spikes and a decrease of extracellular Ca2+ concentration ( [Ca2+]o ) monitored with ion selective microelectrodes at the cell body layer of area CA1 of the hippocampal slice. Reducing the Ca2+ content of the perfusion medium impairs synaptic transmission. However, even under these conditions, weak stimulation evokes a significant decrease of [Ca2+]o, which is ascribed to a presynaptic Ca2+ entry. Stimulation with high intensities and/or frequencies initially induces an accelerated decrease in [Ca2+]o followed by the re-establishment of synaptic transmission, indicating a contribution of the presynaptic site to frequency facilitation.