Revisiting the molecular basis of synaptic transmission.

Measurements of the time elapsed during synaptic transmission has shown that synaptic vesicle (SV) fusion lags behind Ca-influx by approximately 60 microseconds (µsec). The conventional model cannot explain this extreme rapidity of the release event. Synaptic transmission occurs at the active zone (AZ), which comprises of two pools of SV, non-releasable "tethered" vesicles, and a readily-releasable pool of channel-associated Ca-primed vesicles, "RRP". A recent TIRF study at cerebellar-mossy fiber-terminal, showed that subsequent to an action potential, newly "tethered" vesicles, became fusion-competent in a Ca-dependent manner, 300-400 ms after tethering, but were not fused. This time resolution may correspond to priming of tethered vesicles through Ca-binding to Syt1/Munc13-1/complexin. It confirms that Ca-priming and Ca-influx-independent fusion, are two distinct events. Notably, we have established that Ca channel signals evoked-release in an ion flux-independent manner, demonstrated by Ca-impermeable channel, or by substitution of Ca with channel -impermeable La. Thus, conformational changes in a channel coupled to RRP appear to directly activate the release machinery and account for a µsec Ca-influx-independent vesicle fusion. Rapid vesicle fusion driven by non-ionotropic channel signaling strengthens a conformational-coupling mechanism of synaptic transmission, and contributes to better understanding of neuronal communication vital for brain function.