Developmental regulation of the intracellular Ca 2+ sensitivity of vesicle fusion and Ca 2+ –secretion coupling at the rat calyx of Held

Developmental refinement of synaptic transmission can occur via changes in several pre‐ and postsynaptic factors, but it has been unknown whether the intrinsic Ca 2+ sensitivity of vesicle fusion in the nerve terminal can be regulated during development. Using the calyx of Held, a giant synapse in the auditory pathway, we studied the presynaptic mechanisms underlying the developmental regulation of Ca 2+ –secretion coupling, comparing a time period before, and shortly after the onset of hearing in rats. We found an ∼2‐fold leftward shift in the relationship between EPSC amplitude and presynaptic Ca 2+ current charge ( Q Ca ), indicating that brief presynaptic Ca 2+ currents become significantly more efficient in driving release. Using a Ca 2+ tail current protocol, we also found that the high cooperativity between EPSC amplitude and Q Ca was slightly reduced with development. In contrast, in presynaptic Ca 2+ uncaging experiments, the intrinsic Ca 2+ cooperativity of vesicle fusion was identical, and the intrinsic Ca 2+ sensitivity was slightly reduced with development. This indicates that the significantly enhanced release efficiency of brief Ca 2+ currents must be caused by a tighter co‐localization of Ca 2+ channels and readily releasable vesicles, but not by changes in the intrinsic properties of Ca 2+ ‐dependent release. Using the parameters of the intrinsic Ca 2+ sensitivity measured at each developmental stage, we estimate that during a presynaptic action potential (AP), a given readily releasable vesicle experiences an about 1.3‐fold higher ‘local’ intracellular Ca 2+ concentration ([Ca 2+ ] i ) signal with development. Thus, the data indicate a tightening in the Ca 2+ channel–vesicle co‐localization during development, without a major change in the intrinsic Ca 2+ sensitivity of vesicle fusion.