Two modes of fusion pore opening revealed by cell-attached recordings at a synapse

Fusion of a vesicle with the cell membrane opens a pore that releases transmitter to the extracellular space1,2,3. The pore can either dilate fully so that the vesicle collapses completely, or close rapidly to generate ‘kiss-and-run’ fusion1,2,4,5,6,7. The size of the pore determines the release rate2. At synapses, the size of the fusion pore is unclear, ‘kiss-and-run’ remains controversial8,9,10,11,12,13,14,15, and the ability of ‘kiss-and-run’ fusion to generate rapid synaptic currents16,17 is questionable18. Here, by recording fusion pore kinetics during single vesicle fusion, we found both full collapse and ‘kiss-and-run’ fusion at calyx-type synapses. For full collapse, the initial fusion pore conductance (Gp) was usually >375 pS and increased rapidly at ≥299 pS ms–1. ‘Kiss-and-run’ fusion was seen as a brief capacitance flicker ( 288 pS for most flickers, but within 15–288 pS for the remaining flickers. Large Gp (>288 pS) might discharge transmitter rapidly and thereby cause rapid synaptic currents, whereas small Gp might generate slow and small synaptic currents. These results show that ‘kiss-and-run’ fusion occurs at synapses and that it can generate rapid postsynaptic currents, and suggest that various fusion pore sizes help to control the kinetics and amplitude of synaptic currents.