Presynaptic calcium currents evoking quantal transmission from avian ciliary ganglion neurons

Using whole-cell patch clamp techniques, we simultaneously recorded presynaptic Ca++ current and excitatory postsynaptic currents (EPSCs) from avian neuromuscular junctions in culture. Quantal synaptic transmission was proportional to evoked presynaptic Ca++ current except with large stimuli, which evoked bursts of quanta, reflecting a shift to synchronized release. Synaptic delay, measured from the onset of presynaptic depolarization to the appearance of the first postsynaptic quantal response, was often greater than 100 msec for weak depolarizations but declined as stimulus intensity was increased. Quantal events evoked by Ca++ tail currents had a mean synaptic delay of 1.67 msec. The single type of presynaptic Ca++ current observed displayed an inactivation time constant of greater than 100 msec and tail currents well fit by a single exponential function.