Revisiting the molecular basis of synaptic transmission

Measurements of the time elapsed during synaptic transmission has shown that synaptic vesicle (SV) fusion lags behind Ca2+-influx by approximately 60 microseconds (mu sec). 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 Ca2+-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 Ca2+-dependent manner, 300-400 ms after tethering, but were not fused. This time resolution may correspond to priming of tethered vesicles through Ca2+-binding to Syt1/Munc13-1/ complexin. It confirms that Ca2+-priming and Ca2+-influx-independent fusion, are two distinct events. Notably, we have established that Ca2+ channel signals evoked-release in an ion flux-independent manner, demonstrated by Ca2+-impermeable channel, or by substitution of Ca2+ with channel-impermeable La3+. Thus, conformational changes in a channel coupled to RRP appear to directly activate the release machinery and account for a mu sec Ca2+-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 under-standing of neuronal communication vital for brain function.

Automated summary

Measurements have shown that synaptic vesicle fusion lags behind Ca2+-influx by approximately 60 microseconds (mu sec). The conventional model cannot explain this extreme rapidity of the release event. A recent study has found that newly tethered vesicles become fusion-competent in a Ca2+-dependent manner 300-400 ms after tethering. This confirms that Ca2+-priming and Ca2+-influx-independent fusion are two distinct events.