Figure 1. Variability of release probability.

Release probability was measured by labeling synapses with FM-dye and monitoring destaining rates upon action potential stimulation, delivered by field stimulation (A), or during single (B) and paired (E) whole-cell recordings. (A) Left, dendrite (orange) with FM4-64 labeled synapses (red). Right, destaining curve fits with 95% confidence interval (shaded areas) for the numbered synapses show a wide range of release probabilities for synapses in the same dendritic branch. (B) Left, axon (blue) with several synapses (red), and respective destaining curve fits (right), also show very different p_rs for synapses along single axons. (C) Summary data of similarity comparisons for all synapses in a branch of dendrite or axon, showing that the mean p_r difference is not significantly different from the average p_r difference expected by chance (1.13 SDs, Wilcoxon rank sum test for axon P = 0.4241, dendrite P = 0.2079). (D) Epifluorescence image of axon (blue) making multiple synaptic contacts (red, FM4-64) with a postsynaptic cell (orange). Inset: representative traces of AP (blue) and evoked EPSC (orange). Scale bars, 15 μm; inset, 2 ms, 20 mV (top), 100 pA (bottom). (E) Stimulation of the presynaptic cell selectively destains FM4-64 fluorescence from the synapse belonging to the labeled axon (3), and not from those originating from unlabeled axons (1,2). Red line is a single exponential fit. (F, G) Destaining traces from 19 synapses from one connection (F) and corresponding release probability frequency histogram (G). Solid line is gamma function fit (λ = 5.8, n = 3).