Control of Neurotransmitter Release Properties by Presynaptic Calcium

Presynaptic terminals of neurons are optimized for neurotransmitter release, which is tightly controlled by presynaptic calcium. Here, we evaluate the role of calcium influx through voltage-gated calcium channels (VGCCs) in regulating the initial vesicular release probability (p) and the number of vesicles available for release by action potentials (effective RRP) at the calyx of Held synapse in mice. Two established methods of estimating effective RRP size and p reveal that both are calcium dependent. Reducing calcium influx by blocking R-type (VGCCs) or P/Q-type VGCCs also reduces EPSC amplitude via p and effective RRP size. Furthermore, activation of gamma-aminobutryic acid class B (GABAB) receptors, which reduces presynaptic calcium by regulating VGCCs without other significant effects on release, also reduces the effective RRP size and p. These findings suggest that the calcium dependence of RRP size may influence the manner in which certain neuromodulators affect neurotransmitter release.