Transmitter release triggered by a local depolarization in motor nerve terminals of the frog: role of calcium entry and of depolarization.

Quantal synaptic currents (EPSCs) were elicited at neuromuscular junctions of frogs, applying current pulses through the recording current-clamp electrode. Facilitation of a test-EPSC by a preceding EPSC of variable amplitude was determined. This facilitation after a prepulse and release during the prepulse were found to have different dependences on depolarizing current amplitude. In addition, if intraterminal calcium [( Ca]i) was raised by a train of EPSCs, small depolarizations with little additional Ca entry elicited release which depended strongly on pulse amplitude. It is concluded that, in addition to [Ca]i, depolarization of the terminal directly controls quantal release. This depolarization dependence of release is shown to be the probable cause for termination of release after a large Ca entry.