Munc13-3 is a presynaptic protein implicated in vesicle priming that is strongly expressed in cerebellar granule cells (GCs) [1]. Mouse deficient of Munc13-3 (KO) showed an increased paired-pulse ratio (PPR), which led to the hypothesis that Munc13-3 increases the release probability (p_r) of vesicles [2]. We analyzed unitary synaptic connections between GCs and basket cells (BCs) in acute cerebellar slices from wild-type (WT) and KO mice. Postsynaptic BCs were voltage clamped in the whole-cell patch-clamp configuration, presynaptic GCs were stimulated in loose-patch configuration. Unitary EPSCs recorded from KO GCs showed normal kinetics and synaptic latency, but an increased fraction of synaptic failures (p=0.043, n>=12) and an increased paired-pulse ratio (p=0.04, n=12; cf. [2]). Using a multiple-probability fluctuation analysis we found that neither the charge of single quanta (100-150 fC) nor the binominal parameter N (~2) were affected by loss of Munc13-3 but that p_r dropped from 0.37 (±0.53, n=7) in the WT to 0.21 (±0.04, n=7) in the KO (p=0.02). Bath application of 100 µM EGTA-AM (10 min) lead to a reduction in EPSCs that was significantly (p=0.01; n>=10) stronger in the KO. In the WT as well as in the KO, EGTA affected the phasic and asynchronous release components [3] to a similar extent. We conclude that Munc13-3 is involved in the final steps of vesicle priming, tightening influx-release coupling [4].

Supported by the DFG (GRK Interneuro, 1097)

References:

1. Augustin, Korte, Rickmann, Kretzschmar, Südhof, Herms & Brose (2001) J. Neurosci. 21: 10-7

2. Bao, Reim & Sakaba (2010) J. Neurosci. 30:8171-9

3. Atluri, & Regehr (1998) J. Neurosci. 18: 8214-27.

4. Schmidt, Brachtendorf, Arendt, Hallermann, Ishiyama, Bornschein, Gall, Schiffmann, Heckmann & Eilers (in press) Curr. Biol.