Sharp Ca2+ Nanodomains beneath the Ribbon Promote Highly Synchronous Multivesicular Release at Hair Cell Synapses

Hair cell ribbon synapses exhibit several distinguishing features. Structurally, a dense body, or ribbon, is anchored to the presynaptic membrane and tethers synaptic vesicles; functionally, neurotransmitter release is dominated by large EPSC events produced by seemingly synchronous multivesicular release. However, the specific role of the synaptic ribbon in promoting this form of release remains elusive. Using complete ultrastructural reconstructions and capacitance measurements of bullfrog amphibian papilla hair cells dialyzed with high concentrations of a slow Ca2+ buffer (10 mM EGTA), we found that the number of synaptic vesicles at the base of the ribbon correlated closely to those vesicles that released most rapidly and efficiently, while the rest of the ribbon-tethered vesicles correlated to a second, slower pool of vesicles. Combined with the persistence of multivesicular release in extreme Ca2+ buffering conditions (10 mM BAPTA), our data argue against the Ca2+-dependentcompoundfusion of ribbon-tethered vesicles at hair cell synapses. Moreover, during hair cell depolarization, our results suggest that elevated Ca2+ levels enhance vesicle pool replenishment rates. Finally, using Ca2+ diffusion simulations, we propose that the ribbon and its vesicles define a small cytoplasmic volume where Ca2+ buffer is saturated, despite 10 mM BAPTA conditions. This local buffer saturation permits fast and large Ca2+ rises near release sites beneath the synaptic ribbon that can trigger multiquantal EPSCs. We conclude that, by restricting the available presynaptic volume, the ribbon may be creating conditions for the synchronous release of a small cohort of docked vesicles.