Calcium/synaptotagmin-evoked compound fusion increases quantal size and synaptic strength

Exocytosis at synapses generally refers to fusion between vesicles and the plasma membrane1. Although compound fusion between vesicles2,3 was proposed at ribbon-type synapses4,5, whether it exists, how it is mediated, and what role it plays at conventional synapses remain unclear. Here we addressed this issue at a nerve terminal containing conventional active zones. High potassium application and high frequency firing induced giant capacitance up-steps reflecting exocytosis of vesicles larger than regular ones, followed by giant down-steps reflecting bulk endocytosis. They also induced giant vesicle-like structures, as observed with electron microscopy, and giant miniature EPSCs (mEPSCs) reflecting more transmitter release. Calcium and its sensor for vesicle fusion, synaptotagmin, were required for these giant events. After high frequency firing, calcium/synaptotagmin-dependent mEPSC size increase was paralleled by calcium/synaptotagmin-dependent post-tetanic potentiation (PTP). These results suggest that calcium/synaptotagmin mediates compound fusion between vesicles, that exocytosis of compound vesicles increases quantal size which enhances synaptic strength and thus contributes to the generation of PTP, and that exocytosed compound vesicles may be retrieved via bulk endocytosis. We suggest to include a new vesicle cycling route, compound exocytosis followed by bulk endocytosis, into models of synapses, where currently only vesicle fusion with the plasma membrane is considered (Fig. S1)1.