Evidence that fast exocytosis can be predominantly mediated by vesicles not docked at active zones in frog saccular hair cells

The prevailing model of neurotransmitter release stipulates that Ca2+ influx triggers the rapid fusion of vesicles that are docked at presynaptic active zones. Under this model, slower tonic release is supported by vesicles clustered nearby that have to translocate to the release sites before fusion. We have examined this hypothesis at the afferent synapse of saccular hair cells of the leopard frog, Rana pipiens. Detailed morphological measurements at this ribbon synapse show that on average 32 vesicles are docked at each active zone. We show that at this 'graded' synapse, depolarization produces an exocytotic 'burst' that is largely complete within 20 ms after fusion of 280 vesicles per active zone, almost an order of magnitude more than expected. Recovery from paired pulse depression occurs with a time constant of 29 ms, indicating that replenishment of this fast-fusing pool of vesicles is also fast. Our results suggest that non-docked vesicles are capable of fast fusion and that these vesicles constitute the vast majority of the fast-fusing pool. The view that the population of fast-fusing presynaptic vesicles is limited to docked vesicles therefore requires re-evaluation. We propose that compound fusion, i.e. the fusion of vesicles with each other before and/or after they fuse with the membrane can explain multivesicular release at this synapse.