Figure 7. Ca²⁺ uncaging with different pre-flash Ca²⁺ concentrations indicates Ca²⁺-dependent vesicle priming.

(A) Two consecutive recordings from the same cell pair, with the same post-flash Ca²⁺ concentration but different pre-flash Ca²⁺ concentration in the presynaptic terminal. Top: postsynaptic current. Bottom: cumulative release of synaptic vesicles measured by deconvolution analysis of EPSCs superposed with a mono-exponential fit (magenta). Black and blue color represent low and high pre-flash Ca²⁺ concentration, respectively. The pre- and post-flash Ca²⁺ concentrations are indicated in each panel. (B) Comparison of the average post-flash Ca²⁺ concentration between both groups of either low (black) or high (blue) pre-flash Ca²⁺ concentration (n = 18 and 13 pairs, respectively). (C) From left to right: comparisons of the peak amplitude, the number of released vesicles measured as obtained from deconvolution analysis of EPSC, the delay of the release onset, and the release rate. Boxplots show median and 1st/3rd quartiles with whiskers indicating the whole data range. The values above the boxplots represent P-values of Mann-Whitney U tests. (D)Top left: simulated local action potential-evoked Ca²⁺concentrations at 20 nm from a Ca²⁺ channel taken from Delvendahl et al., 2015. Note the almost complete overlap of the two Ca²⁺ concertation traces with low and high basal Ca²⁺ concertation. Bottom left: predicted action potential-evoked EPSCs with low and high basal Ca²⁺ concertations. Right: ratio of the action potential-evoked EPSC amplitude with high and low basal Ca²⁺ concentrations for the experimental data and the model predictions.