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The effects of 4-aminopyridine (4-AP) on electrophysiological post-synaptic responses evoked by field stimulation or evoked focally using a loose patch-clamp technique, and on radiolabelled transmitter release were studied in the Torpedo electric organ. In this preparation, 4-AP had three major effects: it greatly potentiated the amount of acetylcholine (ACh) released by a nerve impulse, it prolonged the duration of the post-synaptic electroplaque current (e.c.) by several hundreds of milliseconds, and it increased the delay of responses triggered by a presynaptic action potential. Noise analysis performed at different times during the focally recorded giant response showed that it was made of a sustained release of ACh quanta. The maximum synchronous release of transmitter, expressed as the maximum number of quanta simultaneously delivered/micron2 of presynaptic membrane, was apparently not modified by 4-AP. A slightly different dose dependence was found for the effects of 4-AP on the potentiation of transmitter release and on the prolongation of the synaptic delay. The effects of tetraethylammonium (TEA) and other K+ channel blockers on these parameters were similar to those of 4-AP. Strong depolarizing pulses applied focally to a nerve ending were able to evoke a giant response even in the presence of 1 microM-tetrodotoxin (TTX). The prolongation of the discharge by 4-AP was therefore not caused by repetitive re-excitation of the nerve branches. Both the amplitude and the time course of the giant response were Ca2+ dependent. At a low Mg2+ concentration, the Ca2+ dependence of transmitter release was identical in the presence or absence of 4-AP. Paradoxically, in the presence of 4-AP, addition of 4 mM-Mg2+ considerably increased the Ca2+ dependence of release, whereas in the absence of 4-AP, Mg2+ blocked transmitter release by decreasing its sensitivity to Ca2+. This potentiating interaction between Mg2+ and 4-AP was not seen with TEA or guanidine. In conclusion, 4-AP potentiates ACh release in two different ways in the Torpedo electric organ: it promotes a sustained quantal release of transmitter during several hundreds of milliseconds without any significant change in the maximal synchronous release, it interacts with Mg2+ in such a manner that the sensitivity to Ca2+ of the nerve terminals is increased.