Neighboring Purkinje neurons, connected via axon collaterals, allow for studying transmitter release and short term plasticity at an inhibitory, central synapse. We addressed the hypothesis that saturation of calbindin¹ underlies paired pulse facilitation (PPF) using paired electrophysiological recordings and presynaptic Ca²⁺ imaging from wild-type (WT) and CB mutants, with subsequent quantal analysis and numerical simulations. We find PPF evident in juvenile WT mice (P8-12) with a paired pulse ratio of ~ 1.4 at 5 ms interstimulus intervals and no significant alteration in mutants. In both genotypes, synapses operate at low release probability and a large number of release sites with no significant difference in quantal size (q, WT: -60 ± 21 pA, CB mutants: -80 ± 22 in 2 mM [Ca²⁺]_o). WT recordings showed a high failure rate of 0.35 ± 0.09 in response to the first action potential (AP) compared to 0.10 ± 0.07 for the second AP. CB mutants showed a similar first failure rate (0.41 ± 0.16), but, compared to WT, a significantly larger second failure rate (0.36 ± 0.15). Ca²⁺ imaging experiments combined with numerical simulations indicate that AP-mediated calcium transients increase the CB occupancy only by 5 %. Long-term whole-cell recordings (2 h) for wash-out of CB and/or infusion of the slow Ca²⁺ buffer EGTA (10 mM) indicate that the Ca²⁺ nanodomain coupling is tighter in the WT than in CB mutants. We hypothesize that CB physically links Ca²⁺ channels to the release machinery. Supported by the DFG (EI 342/4-1)

¹Orduz & Llano, PNAS, 2007; Blatow et al., Neuron, 2003