Neurotransmitter release and its facilitation in crayfish muscle. VI. Release determined by both, intracellular calcium concentration and depolarization of the nerve terminal
Dudel, J.; Parnas, I.; Parnas, H.
Pflugers Archiv European Journal of Physiology 399(1): 1-10
1983
ISSN/ISBN: 0031-6768 PMID: 6139784 Document Number: 200988
In excitatory neuromuscular junctions of crayfish quantal synpatic currents were recorded focally by using macro-patch-clamp electrode. Through the same elctrode the nerve terminal was depolarized by current pulses which elicited quantal postsynaptic currents (pEPSC). The terminals were electrically inexcitable and the quantum content (m) of pEPSC increased gradually to a saturation level with rising pulse amplitude. A test-pEPSC was elicited by constant current pulse; its facilitation (Fc) by a preceding pEPSC of varying amplitude was studied. Amplitude and duration of Fc 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 Fc is as expected for a potential dependent Ca-entry into the terminal. The fact that release during the prepulse rose for large depolarizations while Fc 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 appr. constant high level. 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 the 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.