| Research Abstract |
Synaptotagmin I (Syt I) project ; Syt I is widely considered to be a major Ca^<2+> sensor for fast transmitter release and has two putative Ca^<2+> binding domains, C2A and C2B. To elucidate its roles in synaptic transmission in situ, we studied synaptic currents at the neuromuscular junction (NMJ) of Drosophila embryos that have mutations in syt I. Synaptic currents were induced by nerve-stimulation in external solutions containing various concentrations of Ca^<2+> or by high K^+ solutions. In a null allele, syt I^<AD4>, synchronized synaptic currents were rarely evoked but not abolished. Its quantal content increased with [Ca^<2+>] with a slope of 0.7 in double logarithmic plot, in contrast to 2.7 in control. The slope of 0.9 in an allele that lacks entire C2B, syt I^<AD1>, was not different than in syt I^<AD4>, whereas in another allele, syt I^<AD3>, with one amino-acid substitution in C2B, it was 1.6. In normal saline, the miniature synaptic current (mini) frequency in syt I^<AD4> was not different than in other syt I alleles or in controls. Considering much smaller hypertonicity and Ca^<2+>-ionophore responses in syt I^<AD4>, this finding suggests that the release probability of docked/primed vesicles for spontaneous release is higher in the absence of Syt I. In high K^+ saline, the dependency of mini frequency on [Ca^<2+>] in syt I^<AD4> was less than that in controls, while that in syt I^<AD3> was even lower than in syt I^<AD4>. We conclude that Drosophila Syt I is a Ca^<2+> sensor for synchronized release but inhibits spontaneous quantal release.
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