Effects of Low Temperature and Terminal Membrane Potential on Quantal Size at Frog Neuromuscular Junction

1. Two mechanisms proposed for the quantal release of acetylcholine (ACh) are: (i) that the quanta are pre-packaged in vesicles and released by exocytosis and (ii) that the ACh is released from the cytoplasm of the nerve terminal by the opening of an ACh channel. Our experiments were designed to test aspects of these hypotheses.2. Miniature end-plate currents (m.e.p.c.s) were reversibly decreased in amplitude and increased in duration as the temperature was decreased between 15 and 6 degrees C. The amplitude decreased with a Q(10) of 2.4 between 15 and 11 degrees C, and then with a Q(10) of 3 between 11 and 6 degrees C. The half-decay time increased with a Q(10) between 4 and 5 over the entire temperature range.3. The effect of temperature on the end-plate current (e.p.c.) in response to ionophoretically applied ACh was also studied. The e.p.c. in response to a set, sustained dose of ACh was 30% larger at 11 degrees C than at 15 degrees C, and about 10% larger at 6 degrees C than at 15 degrees C.4. The difference in the end-plate response to brief pulses of ACh (m.e.p.c.s) and to sustained application of ACh was analysed by Dionne & Stevens (1975). The amplitude of the sustained response depends on both the number of channels opened in the end-plate, and the length of time they stay open. When our temperature data are analysed in this way, it appears that the amount of ACh/quanta acting on the end-plate is altered by less than 25% over the temperature range 15-6 degrees C.5. From the experiments in which the nerve terminal membrane potential was shifted by external currents it was concluded that miniature end-plate potential amplitude was independent of the terminal membrane potential (Cooke & Quastel, 1973). This conclusion was confirmed by measuring m.e.p.c.s in low Ca(2+) Ringer solution containing 2.0 and 22.0 mM-KCl; there was no consistent change in m.e.p.c. amplitude.6. The effects of both temperature and terminal membrane potential are more readily interpreted by the vesicle than by the channel hypothesis, since for the channel hypothesis the duration of channel opening should be temperature-sensitive and the efflux of ACh(+) through the terminal membrane should be potential-dependent.