Abstract
Endogenous Ca2+-binding proteins affect synaptic transmitter release and short-term plasticity (STP) 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 (pr). 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 pr. 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.
Highlights
Ca2+ regulates use-dependent presynaptic short-term plasticity (STP) by controlling the initial vesicular releases probability, the facilitation status of the release apparatus, and by regulating the size and restoration of vesicle pools (Zucker and Regehr, 2002; Regehr, 2012)
IDENTIFICATION OF granule cells (GCs)-TO-Purkinje neuron (PN) PAIRS The GC to PN connectivity is rather low in slice preparations (Isope and Barbour, 2002) and we used the following procedure to establish paired recordings (Figure 1): After the wholecell configuration had been established on a PN, potassium containing pipette solution was puffed from a second patchpipette to regions of the GC layer at a distance of > 100 μm from the soma of the PN
If cells or fibers connecting to the PN were present in this region, Excitatory postsynaptic currents (EPSCs) were apparent in the recording from the PN soma (Figure 1A)
Summary
Ca2+ regulates use-dependent presynaptic short-term plasticity (STP) by controlling the initial vesicular releases probability (pr), the facilitation status of the release apparatus, and by regulating the size and restoration of vesicle pools (Zucker and Regehr, 2002; Regehr, 2012). Endogenous Ca2+ buffers (CaBs) are important regulators of presynaptic Ca2+ signals (Eggermann et al, 2012; Schmidt, 2012) Due to their diffusional mobility (Schmidt et al, 2003, 2005; Arendt et al, 2013), they get close to the site of Ca2+ entry and buffer the release triggering Ca2+ signal even if the diffusional distance between Ca2+ channel and release sensor is a few tens of nanometers only (Eggermann and Jonas, 2012; Bornschein et al, 2013; Schmidt et al, 2013). Contrary to the previous report (Schiffmann et al, 1999), we found that loss of CR resulted in a significant reduction in PPF This reduction, was less prominent than would have been expected for the high pr at CR−/− synapses. Constrained computer simulations combined with an analysis of successes and failures, and multiple probability fluctuation analysis (MPFA) suggest the involvement of a Ca2+-driven mechanism in PPF (Millar et al, 2005; Sakaba, 2008; Valera et al, 2012), which restores and overfills the RP more effectively in CR−/− than in WT, thereby, essentially preserving PPF in the mutants
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