To sustain high rates of transmitter release, synaptic terminals must rapidly resupply vesicles to release sites and prime them for exocytosis. Here we describe imaging of single synaptic vesicles near the plasma membrane of live ribbon synaptic terminals. Vesicles were captured at small, discrete active zones near the terminal surface. An electric stimulus caused them to undergo rapid exocytosis, seen as the release of a fluorescent lipid from the vesicles into the plasma membrane. Next, vesicles held in reserve about 20 nm from the plasma membrane advanced to exocytic sites, and became release-ready 250 ms later. Apparently a specific structure holds vesicles at an active zone to bring v-SNAREs and t-SNAREs, the proteins that mediate vesicle fusion, within striking distance of each other, and then allows the triggered movement of such vesicles to the plasma membrane.
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