The number and organization of Ca²⁺ channels in the active zone shapes neurotransmitter release from schaffer collateral synapses

Fast synaptic transmission requires tight colocalization of Ca²⁺channels and neurotransmitter vesicles. It is generally thought that Ca²⁺ channels are expressed abundantly in presynaptic active zones, that vesicles within the same active zone have similar release properties, and that significant vesicle depletion only occurs at synapses with high release probability. Here we show, at excitatory CA3→CA1 synapses in mouse hippocampus, that release from individual vesicles is generally triggered by only one Ca²⁺ channel and that only few functional Ca²⁺ channels may be spread in the active zone at variable distances to neighboring neurotransmitter vesicles. Using morphologically realistic Monte Carlo simulations, we show that this arrangement leads to a widely heterogeneous distribution of release probability across the vesicles docked at the active zone, and that depletion of the vesicles closest to Ca²⁺ channels can account for the Ca²⁺ dependence of short-term plasticity at these synapses. These findings challenge the prevailing view that efficient synaptic transmission requires numerous presynaptic Ca²⁺ channels in the active zone, and indicate that the relative arrangement of Ca²⁺ channels and vesicles contributes to the heterogeneity of release probability within and across synapses and to vesicle depletion at small central synapses with low average release probability.