SYMPATHETIC NEUROMUSCULAR-TRANSMISSION AT A VARICOSITY IN A SYNCYTIUM

The autonomic neuromuscular junction at a varicosity in the vas defere ns is defined by the localization of the vesicle-associated protein sy ntaxin in high concentrations in the axolemma and a high density of P2 x(1) receptors in a cluster beneath the varicosity. Calcium fluxes hav e been observed in all individual varicosities of a nerve terminal on the arrival of an impulse even though recordings made from these varic osities of the electrical signs of transmission with loose-patch elect rodes over the varicosities show that they have very different probabi lities for the secretion of a quantum. The fact that some varicosities seldom release a quantum on the arrival of an impulse is supported by the observation that antibodies against the N-terminus of synaptotagm in, which uniquely label the inside of synaptic vesicles when they und ergo exocytosis, fail to do so in some varicosities during nerve stimu lation whereas they do in others. It is suggested that the probability for secretion from a varicosity depends on the number of secretosomes that the varicosity possesses, where a secretosome is a complex of sy ntaxin, synaptotagmin, an N-type calcium channel, and a synaptic vesic le. (C) 1998 Wiley-Liss, Inc.