Abstract
At the calyx of the Held synapse in the auditory brainstem, P/Q-type presynaptic Ca2+ currents undergo an activity-dependent facilitation during repetitive activation mediated by neuronal calcium sensor 1 (NCS-1). Of all 4 Ca2+-binding sites of NCS-1, we primarily focused on third EF-hand motif because E120Q mutant has a phenotype in synaptic transmission or exocytosis in other preparation. Unlike the wild-type NCS-1, direct loading of NCS-1 (E120Q) that loses Ca2+-biniding capability, into the nerve terminal did not induce the facilitation of calcium current in the basal stimulation. When NCS-1 (E120Q) was included in the presynaptic pipette solution, the rise time of presynaptic Ca2+ current was not significantly faster than rise times in the absence of NCS-1. The current-voltage (I-V) relationship of presynaptic calcium current measured at 1 ms after the onset of the command pulse had a similar peak at 0 mV in the presence of NCS-1 (E120Q) and in the absence of NCS-1. To evaluate the activity-dependence further, we tested if the NCS-1 (E120Q) may inhibit the calcium current facilitation caused by a short burst of depolarization (100 Hz by 10 stimuli). In control, calcium current caused 60% facilitation, whereas NCS-1 (E120Q) caused only 20% facilitation. These results suggest that the third EF hand motif (calcium binding site) performs as a sensor detecting the alteration of residual calcium concentration at nerve terminals. [J Physiol Sci. 2007;57 Suppl:S153]
