Calcium/synaptotagmin-mediated Compound Fusion Increases Quantal Size And Causes Post-tetanic Potentiation At Synapses

Abstract

Exocytosis at synapses generally refers to fusion between vesicles and the plasma membrane. Although fusion between vesicles, known as compound fusion, occurs in non-neuronal secretory cells and has recently been proposed at ribbon-type synapses, it remains unclear whether it exists, how it is mediated, and what role it plays at the vast majority of synapses, where release occurs at conventional active zones. Here we addressed this issue in rats and mice at a large nerve terminal containing conventional active zones. High potassium application induced giant capacitance up-steps at the release face of nerve terminals, which were larger than the membrane capacitance of regular vesicles. These giant up-steps were not comprised of several smaller steps, nor were they bulk endocytic vesicles that had re-fused. High potassium application also induced giant vesicle-like structures in nerve terminals and giant miniature EPSCs (mEPSCs) that reflected release of a large amount of transmitter. The giant up-steps, giant vesicle-like structures, and giant mEPSCs were abolished by removing the extracellular calcium or by knocking out synaptotagmin II, the calcium sensor mediating fusion at calyces. These results suggest that calcium binding with synaptotagmin II mediates compound fusion and increases quantal size. Compound fusion significantly contributed to the generation of a widely observed synaptic plasticity, post-tetanic potentiation (PTP) of the EPSC, because 1) action potential trains that generated PTP also evoked giant up-steps and increased the mEPSC amplitude, 2) the time course and the degree of the mEPSC amplitude increase paralleled those of PTP, and 3) both the mEPSC amplitude increase and PTP were abolished by the calcium buffer EGTA or synaptotagmin II knockout. Our finding may be of wide application because intense nerve activity, PTP, and giant miniature currents occur in physiological conditions at many synapses.