Diversity of postsynaptic amplitude and failure probability of unitary excitatory synapses between CA3 and CA1 cells in the rat hippocampus.

Different CA3 cells may have dissimilar effects on a CA1 pyramidal cell. In order to test this idea, we studied the amplitude distribution of excitatory postsynaptic currents (EPSCs) in response to weak electrical stimulation of presynaptic axons in the rat hippocampal slice. We accepted the response populations as representative for the effect of, in most cases, a single axon when the EPSCs appeared at a certain threshold stimulation strength, with the subsequent lack of increase in amplitude with further stimulation increase. By comparing the EPSC amplitude distributions obtained from different synaptic inputs to the same CA1 cell, we found differences in the failure probability and the EPSC amplitude, each of which contributed to differences in the mean response amplitude. We conclude that not only the number but also the specific subset of active CA3 cells is important for the synaptically driven discharge of a given CA1 cell.