Variations in Ca Influx Can Alter Chelator-Based Estimates of Ca Channel-Synaptic Vesicle Coupling Distance.

The timing and probability of synaptic vesicle fusion from presynaptic terminals is governed by the distance between voltage-gated Ca channels (VGCCs) and Ca sensors for exocytosis. This VGCC-sensor coupling distance can be determined from the fractional block of vesicular release by exogenous Ca chelators, which depends on biophysical factors that have not been thoroughly explored. Using numerical simulations of Ca reaction and diffusion, as well as vesicular release, we examined the contributions of conductance, density, and open duration of VGCCs, and the influence of endogenous Ca buffers on the inhibition of exocytosis by EGTA. We found that estimates of coupling distance are critically influenced by the duration and amplitude of Ca influx at active zones, but relatively insensitive to variations of mobile endogenous buffer. High concentrations of EGTA strongly inhibit vesicular release in close proximity (20-30 nm) to VGCCs if the flux duration is brief, but have little influence for longer flux durations that saturate the Ca sensor. Therefore, the diversity in presynaptic action potential duration is sufficient to alter EGTA inhibition, resulting in errors potentially as large as 300% if Ca entry durations are not considered when estimating VGCC-sensor coupling distances. The coupling distance between voltage-gated Ca channels and Ca sensors for exocytosis critically determines the timing and probability of neurotransmitter release. Perfusion of presynaptic terminals with the exogenous Ca chelator EGTA has been widely used for both qualitative and quantitative estimates of this distance. However, other presynaptic terminal parameters such as the amplitude and duration of Ca entry can also influence EGTA inhibition of exocytosis, thus confounding conclusions based on EGTA alone. Here, we performed reaction-diffusion simulations of Ca-driven synaptic vesicle fusion, which delineate the critical parameters influencing an accurate prediction of coupling distance. Our study provides guidelines for characterizing and understanding how variability in coupling distance across chemical synapses could be estimated accurately.