Developmental regulation of the intracellular Ca2+ sensitivity of vesicle fusion and Ca2+–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.