Transmitter release from nerve terminals evoked by depolarization pulses contains a short phase of repression.

The time course of quantal transmitter release after a depolarization pulse was measured at frog and crayfish motor nerve terminals. Test pulses were arranged to elicit low release, and the delay of first releases and the median of distributions of release times were defined for large (greater than 2000 stimuli) samples. Small, subthreshold depolarizing post-pulses were added directly after the test pulses. Such post-pulses of 1 to 4 ms duration prolonged the delay of first releases and shifted the median of the time course of release by up to 3 ms (at 0 degree C) depending on the duration and on the amplitude of the post-pulses. These 'latency shifts', which have been observed after prolonged depolarizations by other authors, were statistically highly significant. The results of post-pulses were very similar at neuromuscular junctions of frog and crayfish. It is concluded that depolarization in addition to the promotion of release has a short repressing action on release which is partly responsible for synaptic delay.