Effects of low temperature and terminal membrane potential on quantal size at frog neuromuscular junction

Abstract

Two mechanisms proposed for the quantal release of acetylcholine (ACh) are as follows: the quanta are pre-packaged in vesicles and released by exocytosis and the ACh is released from the cytoplasm of the nerve terminal by the opening of an ACh channel. Miniature end-plate currents (mepcs) were reversibly decreased in amplitude and increased in duration as the temperature was decreased between 15.degree. and 6.degree. C. The amplitude decreased with a Q10 of 2.4 between 15.degree. and 11.degree. C, and then with a Q10 of 3 between 11.degree. and 6.degree. C. The half-decay time increased with a Q10 between 4.degree. and 5.degree. C over the entire temperature range. The effect of temperature on the end-plate current (epc) in response to ionophoretically applied ACh was also studied. The epc in response to a set, sustained dose of ACh was 30% larger at 11.degree. than at 15.degree. C, and about 10% larger at 6.degree. than at 15.degree. C. The difference in the end-plate response to brief pulses of ACh (mepcs) and to sustained application of ACh was analyzed by Dionne and Stevens. 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 the temperature data are analyzed in this way, the amount of ACh/quanta acting on the end-plate is altered by < 25% over the temperature range 15.degree.-6.degree. C. In the experiments in which the nerve terminal membrane potential was shifted by external currents, miniature end-plate potential amplitude was independent of the terminal membrane potential. This conclusion was confirmed by measuring mepcs in low Ca2+ Ringer solution containing 2.0 and 22.0 mM-KCl; there was no consistent change in mepcs amplitude. 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 the ACh+ through the terminal membrane should be potential-dependent.