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 space(1-3). The pore can either dilate fully so that the vesicle collapses completely, or close rapidly to generate 'kiss-and-run' fusion(1,2,4-7). The size of the pore determines the release rate(2). At synapses, the size of the fusion pore is unclear, 'kiss-and-run' remains controversial(8-15), and the ability of 'kiss-and-run' fusion to generate rapid synaptic currents(16,17) is questionable(18). 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 (G(p)) was usually > 375 pS and increased rapidly at >= 299 pS ms(-1). 'Kiss-and-run' fusion was seen as a brief capacitance flicker (< 2 s) with G(p) > 288 pS for most flickers, but within 15 - 288 pS for the remaining flickers. Large G(p) (> 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.