Neurotransmitter release and its facilitation in crayfish muscle - VI. Release determined by both, intracellular calcium concentration and depolarization of the nerve terminal

In excitatory neuromuscular junctions of crayfish quantal synaptic currents were recorded focally by means of a macro-patch-clamp electrode. Through the same electrode the nerve terminal was depolarized by current pulses which elicited quantal postsynaptic currents (pEPSCs). The terminals were electrically inexcitable and the quantum content (m) of pEPSCs increased gradually to a saturation level with rising pulse amplitude. A test-pEPSC was elicited by constant current pulses, and its facilitation (F_c) by a preceding pEPSC of varying amplitude was studied. Amplitude and duration of F_c are measures of the amount of Ca entry during the prepulse. These values had a maximum consistently at a much lower prepulse amplitude than necessary to reach maximum release during the prepulse. The potential dependence of F_c is as expected for a potential dependent Ca-entry into the terminal. The fact that release during the prepulse rose for large depolarizations while F_c and thus Ca-entry decreased, indicates a direct promotion of release by depolarization. In another type of experiment the Ca concentration in the terminal ([Ca]_i) was increased greatly by series of depolarizations. During a following test-pEPSC thus [Ca]_i was at an approximately constant high level. However, variations of the amplitude of the test depolarization pulse caused changes of the test-pEPSC by several orders of magnitude, which must be attributed to a direct control of quantal release by depolarization. The mechanism of this direct effect of membrane potential is discussed, extending our model of synaptic release which only contained control by [Ca]_i. The decisive role of control of release by membrane potential for the termination of release after Ca entry is emphasized.