Munc13-3 Superprimes Synaptic Vesicles at Granule Cell-to-Basket Cell Synapses in the Mouse Cerebellum

Munc13-3 is a presynaptic protein implicated in vesicle priming that is strongly expressed in cerebellar granule cells (GCs). Mice deficient of Munc13-3 (Munc13-3^–/–) show an increased paired-pulse ratio (PPR), which led to the hypothesis that Munc13-3 increases the release probability (p_r) of vesicles. In the present study, we analyzed unitary synaptic connections between GCs and basket cells in acute cerebellar slices from wild-type and Munc13-3^–/– mice. Unitary EPSCs recorded from Munc13-3^–/– GCs showed normal kinetics and synaptic latency but a significantly increased PPR and fraction of synaptic failures. A quantal analysis revealed that neither the charge of single quanta nor the binominal parameter N were affected by loss of Munc13-3 but that p_r was almost halved in Munc13-3^–/–. Neither presynaptic Ca²⁺ influx was affected by deletion of Munc13-3 nor replenishment of the readily releasable vesicle pool. However, a high concentration of EGTA led to a reduction in EPSCs that was significantly stronger in Munc13-3^–/–. We conclude that Munc13-3 is responsible for an additional step of molecular and/or positional "superpriming" that substantially increases the efficacy of Ca²⁺-triggered release.