Figure 4. Estimation of initial Prsite at GC–PC synapses. A1, Time course of A1 as [Ca2+]e was varied: 2.5, 4, and 1.5 mm. Insets at right, Averages of 10 traces at 2.5, 4, and 1.5 mm [Ca2+]e. Numbers refer to the time points identified in the time course. A2, Average of mean A1 for all experiments as a function of [Ca2+]e, normalized to the value at 2.5 mm [Ca2+]e, in control conditions (black circles; n = 9) and in the presence of γDGG (red circles; n = 5) fitted by a sigmoid using the following formula: base + max/(1 + exp (([Ca2+]half − [Ca2+]e)/rate))). In control conditions, base = 0.02, max = 5.17, [Ca2+]half = 3.38, rate = 0.76; in γDGG, base = 0, max = 5.28, [Ca2+]half = 3.64, rate = 0.77. B1, Mean PPRA2/A1 at different interstimulus intervals (from black line to light gray line: 5, 20, 200, and 2000 ms), as a function of [Ca2+]e (n = 10) fitted by a single exponential. Inset, Same plot for mean PPRA3/A2. B2, Mean PPRA2/A1 as a function of interstimulus interval at 1.5 mm (blue, n = 4), 2.5 mm (purple, n = 10), 3 mm (green, n = 4), and 4 mm (red, n = 9) [Ca2+]e fitted by a single exponential. Inset, Same plot for mean PPRA3/A2. C1, Variance–mean analysis: example of variance of A1 at 1.5, 2.5, and 4 mm [Ca2+]e for one experiment plotted against mean A1 (same cell as in A1) and same color code as in C1. Each experiment was fitted by a parabola using a binomial model (dotted line) or a multinomial model with CVI = 0.2 and CVII = 0.34 (solid line). Error bars are an evaluation of the theoretical error of the sample variance (σsample) for n observations. Inset, Individual values of Q and N (n = 9) obtained using the multinomial model. C2, Parabolae for all cells (n = 9 cells) normalized at the maximum variance (varmax) and maximum amplitude (N · Q). Color-filled circles represent the mean variance of A1 for all experiments and the corresponding initial Prsite for 1.5 mm (blue, Prsite = 0.08 ± 0.08), 2.5 mm (purple, Prsite = 0.25 ± 0.16), and 4 mm (red, Prsite = 0.67 ± 0.17) [Ca2+]e. Inset, Individual values of Prsite at 2.5 mm Ca2+ (n = 9). D1, PPRA2/A1 of each experiment as a function of initial Prsite estimated in Figure 4C2 and fitted by a line (n = 9 cells) at 5, 20, 50, and 200 Hz. D2, Initial Prsite of GC–PC paired recordings estimated by comparison of their PPR with the data in D1. The estimated Prsite was 0.31 ± 0.26 (red triangle) at 2.5 mm [Ca2+]e.