A use-dependent increase in release sites drives facilitation at calretinin-deficient cerebellar parallel-fiber synapses

Endogenous Ca2+-binding proteins affect synaptic transmitter release and short-term plasticity (SIP) by buffering presynaptic Ca2+ signals. At parallel fiber (PF)-to-Purkinje neuron (PN) synapses in the cerebellar cortex loss of calretinin (CR), the major buffer at PF terminals, results in increased presynaptic Ca2+ transients and an almost doubling of the initial vesicular releases probability (p(r)). Surprisingly, however, it has been reported that loss of CR from PF synapses does not alter paired-pulse facilitation (PPF), while it affects presynaptic Ca2+ signals as well as p(r). Here, we addressed this puzzling observation by analyzing the frequency-and Ca2+-dependence of PPF at unitary PF-to-PN synapses of wild-type (WT) and CR-deficient (CR-/-) mice using paired recordings and computer simulations. Our analysis revealed that PPF in CR-/- is indeed smaller than in the WT, to a degree, however, that indicates that rapid vesicle replenishment and recruitment of additional release sites dominate the synaptic efficacy of the second response. These Ca2+-driven processes operate more effectively in the absence of CR, thereby, explaining the preservation of robust PPF in the mutants.