Ca(2+) channels and transmitter release at the active zone

Ca(2+)-dependent transmitter release is the most important signaling mechanism for fast information transfer between neurons. Transmitter release takes places at highly specialized active zones with sub-micrometer dimension, which contain the molecular machinery for vesicle docking and -fusion, as well as a high density of voltage-gated Ca(2+) channels. In the absence of direct evidence for the ultrastructural localization of Ca(2+) channels at CNS synapses, important insights into Ca(2+) channel-vesicle coupling has come from functional experiments relating presynaptic Ca(2+) current and transmitter release, at large and accessible synapses like the calyx of Held. First, high slope values in log-log plots of transmitter release versus presynaptic Ca(2+) current indicate that multiple Ca(2+) channels are involved in release control of a single vesicle. Second, release kinetics in response to step-like depolarizations revealed fast- and slowly releasable sub-pools of vesicles, FRP and SRP, which, according to the "positional" model, are distinguished by a differential proximity to Ca(2+) channels. Considering recent evidence for a rapid conversion of SRP- to FRP vesicles, however, we highlight that multivesicular release events and clearance of vesicle membrane from the active zone must be taken into account when interpreting kinetic release data. We conclude that the careful kinetic analysis of transmitter release at presynaptically accessible and molecularly targeted synapses has the potential to yield important insights into the molecular physiology of transmitter release.