Using dual intracellular recordings in slices of adult rat and cat neocortex, the frequency-filtering characteristics of ‘depressing’ synapses made by pyramidal axons at interspike intervals

neocortex, the frequency-filtering characteristics of ‘depressing’ synapses made by pyramidal axons at interspike intervals between 5 and 50 ms were studied. At ‘depressing’ connections from excitatory cells to some inhibitory interneurons (n = 6), recovery from short interspike interval depression was near exponential. Extrapolation of exponentials fitted to this recovery demonstrated a residual 10–20% depression at intervals >50 ms. This slowly decaying component was larger for later excitatory postsynaptic potentials (EPSPs) in trains which were typically more strongly depressed. At >80% of connections between spiny excitatory cells and at pyramid to parvalbumin-immunopositive interneuron connections, however, recovery exhibited a more complex time course. A narrow ‘notch’ (half-width 5 ms), peaking at intervals of 13–25 ms during which the EPSP was depressed further, interrupted recovery from short interval depression. This ‘notch’ was equally apparent for all EPSPs in brief trains and was mediated presynaptically.