Can presynaptic depolarization release transmitter without calcium influx?
Zucker, R.S.; Landò, L.; Fogelson, A.
Journal de Physiologie 81(4): 237-245
1986
ISSN/ISBN: 0021-7948 PMID: 2883310 Document Number: 272284
1.degree. Recent experimental evidence suggesting that presynaptic depolarization can evoke transmitter release without calcium influx has been re-examined. 2.degree. The presynaptic terminal of the squid giant synapse can be depolarized by variable amounts while recording presynaptic calcium current under voltage clamp and postsynaptic responses. Small depolarizations open few calcium channels with large single channel currents. Large depolarizations approaching the calcium equilibrium potential open many channels with small single channel currents. When responses to small and large depolarizations eliciting similar total macroscopic calcium currents are compared, the large pulses evoke more transmitter release. 3.degree. This apparent voltage-dependence of transmitter release may be explained by the greater overlap of calcium concentration domains surrounding single open calcium channels when many closely apposed channels open at large depolarizations. This channel domain overlap leads to higher calcium concentrations at transmitter release sites and more release for large depolarizations than for small depolarizations which open few widely dispersed channels. 4.degree. At neuromuscular junctions, a subthreshold depolarizing pulse to motor nerve terminals may release over a thousand times as much transmitter if it follows a brief train of presynaptic action potentials than if it occurs in isolation. This huge synaptic facilitation has been taken as indicative of a direct effect of voltage which is manifest only when prior activity raises presynaptic resting calcium levels. 5.degree. This large facilitation is actually due to a post-tetanic supernormal excitability in motor nerve terminals, causing the previously subthreshold test pulse to become suprathreshold and elicit a presynaptic action potential. 6.degree. When motor nerve terminals are depolarized by two pulses, as the first pulse increases above a certain level it evokes more transmitter release but less facilitation of the response to the second pulse. This was believed to indicate that a large depolarization can release more transmitter by a direct effect, even while calcium influx, and therefore residual calcium and facilitation, drop as the calcium equilibrium potential is approached. 7.degree. This result may be explained by the nonuniform depolarization of nerve terminals under a macro patch electrode. As the first pulse increases, it recruits release from terminals under the rim of the electrode, causing release to grow, while central terminals depolarized to the calcium equilibrium potential release little transmitter. A small test pulse excites only central terminals under the electrode opening, where there is no residual calcium, and so shows no facilitation. On the contrary, large test pulses, which excite terminals under the electrode rim, evince a facilitation which does grow as release to the first pulse grows. Similar geometrical problems exist when the prepulse is delivered to an intracellular microelectrode in a motor neuron brance. 8.degree. Computer simulations using models of calcium diffusion in nerve terminals show that the time course of transmitter release will be preserved under conditions of different external calcium concentrations and states of facilitation, if calcium influx alone is responsible for triggering transmitter release. 9.degree. Depolarization of nerve terminals by raising potassium in a calcium-free medium fails to increase transmitter release even if intracellular calcium is elevated by hyperosmotic treatment. When calcium channels are opened in this reverse calcium gradient situation, MEPSP frequency drops. This effect is blocked by cobalt. 10.degree. Depolarizing pulses and action potentials fail to release transmitter in a calcium-free medium, even when mitochondrial uncouplers are used to raise intracellular calcium. 11.degree. Presynaptic depolarization does not appear to be capable of evoking transmitter release in the absence of calcium influx, even if presynaptic calcium is elevated by some other m.