OVEREXPRESSION OF SYNAPTOTAGMIN MODULATES SHORT-TERM SYNAPTIC PLASTICITY AT DEVELOPING NEUROMUSCULAR-JUNCTIONS

The level of synaptotagmin I or II in developing spinal neurons was in creased by injection of synaptotagmin messenger RNA into early blastom eres of Xenopus embryos. The effect of overexpression of synaptotagmin on synaptic function was assayed in Xenopus nerve-muscle cultures wit hin two days after injection. At neuromuscular synapses made by synapt otagmin-overexpressing neurons, the frequency of miniature postsynapti c currents was markedly reduced, while their mean amplitude was unchan ged, as compared to those of control neurons in the same culture. The amplitude of evoked postsynaptic currents elicited by low-frequency te st stimuli was not affected by overexpression. However, synapses made by synaptotagmin-overexpressing neurons exhibited significantly higher paired-pulse facilitation and reduced tetanus-induced depression of t he synaptic response, and there was also an increased number of synapt ic vesicles at regions 100-300 nm from the plasmalemma at such synapse s. These results show that synaptotagmins can exert an inhibitory acti on on the spontaneous exocytosis of synaptic vesicles. The effects on short-term plasticity suggest that synaptotagmin may facilitate vesicu lar supply for the evoked release during higher frequency transmission . (C) 1997 IBRO. Published by Elsevier Science Ltd.