QUANTAL AND NONQUANTAL ACH RELEASE AT DEVELOPING XENOPUS NEUROMUSCULAR-JUNCTIONS IN CULTURE

1. Single acetylcholine receptor (AChR) channel openings, detected by the whole-cell patch clamp technique, were used to monitor quantal and non-quantal ACh release at synapses in 1- and 2-day-old co-cultures o f Xenopus embryonic motoneurons and muscle cells. Release at these syn apses differed from that described earlier for motoneuron growth cones in ways that presumably reflect muscle-nerve inductive influences and the development of neurotransmitter release mechanisms. 2. Miniature endplate currents (MEPCs) occurred at a mean frequency of approximatel y 0.6 s-1 with a skewed distribution and mean amplitude of about twent y channel openings. In addition, occasional brief episodes of rapid de viations in the baseline were observed in some cells, with mean amplit udes of 4-8 pA and durations of a few hundred milliseconds. However, t hese episodes did not closely resemble summated openings of AChR chann els. Moreover, where tested, these episodes were not blocked by curare ; and comparable episodes were seen in an uninnervated myocyte. Thus t hey appear not to reflect ACh release from the nerve terminal. 3. Sing le-channel openings that might have been responses to non-quantal rele ase of ACh were observed at rates of 0.9-12.3 min-1 (mean 3.0 min-1), only 1.5 times the rate of spontaneous AChR channel openings in uninne rvated myocytes (mean 1.4 min-1). 4. We conclude that there is no sign ificant non-quantal ACh leak from the presynaptic contacts in these im mature synapses under these culture conditions. This is in disagreemen t with other, less direct, experimental reports, but consistent with f indings in mature frog motor nerve terminals.