Ca²⁺-binding proteins are thought to induce paired-pulse facilitation by saturating in response to the first action potential^[1]. However, facilitation at parallel fiber (PF) to Purkinje cell synapses has been reported to be unaffected by loss of calretinin (CR)^[2], the major calcium binding protein present in PFs. We performed paired electrophysiological recordings and presynaptic calcium imaging in acute slices from wild-type (WT) and calretinin-deficient mice (CR-KO)^[2], followed by quantal analysis and numerical Ca²⁺ simulations. We find that PF-synapses in WT and CR-KO mice indeed show a similar facilitation that declines from ~2.4 at 5 ms interstimulus interval to unity within 200 ms. Facilitation was caused by a higher failure rate in responses to the first compared to the second action potential. However, while the initial failure rate was 0.60 ± 0.04 in the WT, it was reduced to 0.40 ± 0.06 in CR-KOs. Fluctuation analysis revealed a strong increase in the initial release probability (p) from 0.20 ± 0.06 in WT to 0.40 ± 0.08 in mutants (at 2 mM [Ca²⁺]_o), while the quantal size (q) and the binominal parameter N were equal (~-10 pA and 2.0, respectively). Presynaptic calcium signals were 30% larger in the mutant but nonetheless did not saturate CR. Thus, during brief bursts of activity, CR linearly scales down the release probability at PF synapses. Supported by the DFG (EI 342/4-1)

¹Blatow et al. Neuron 2003

^[2]Schiffmann et al. PNAS 1999