We analyzed synaptic connections between granule cells (GCs) and basket cells (BCs) in acute cerebellar slices from wild-type mice (WT) and mice deficient of Munc13-3, a cerebellar specific isoform of Munc13 (Munc-KO). Postsynaptic BCs were voltage clamped in the whole-cell patch clamp configuration. GCs inputs were stimulated in paired recordings for which individual GCs were stimulated in the cell-attached configuration. GC-mediated EPSCs in BCs were characterized by i) rapid kinetics, ii) rather asynchronous release and iii) marked paired-pulse facilitation. In the WT, the paired-pulse ratio (PPR) at 100 Hz was 2.7 (n=14). Deletion of Munc13-3, resulted in significantly increased PPR of 4.3 ± 0.74 (n=4; cf. Bao et al. 2010, J Neurosci). In order to identify the Munc13-3 dependent step in the release cascade that induced the change in PPR, we performed multiple-probability fluctuation analyses, which were based on the charge of EPSCs recorded at different extracellular Ca²⁺ concentrations. Control experiments (in kyurenic acid) indicated that postsynaptic receptor saturation did not distort the analysis. For the WT we found a quantal size (q) of 151 ± 30.8 fC, a binomial parameter N of 4.2 ± 2.6, and an average release probability (p) of 0.29 ± 0.074 (mean ± SE, n=5). In Munc-KO, p was reduced to ~0.22. We conclude that Munc13-3 is involved in the final steps of vesicle priming.

Supported by the DFG (GRK Interneuro, 1097).