TIMING OF SYNAPTIC VESICLE MOBILIZATION CORRELATES STRONGLY WITH RELEASE PROBABILITY BUT NOT RRP SIZE, SUPPORTING A MODULAR MODEL OF PRESYNAPTIC VESICLE TRAFFICKING

Vesicles in synaptic terminals are thought to be segregated into a readily releasable pool (RRP) and multiple reserve pools. The nature of the reserve pools and mechanisms underlying segregation are not known. Previous studies have shown that the size of the readily releasable pool (RRP) varies over an order of magnitude among hippocampal synapses. Here, we use a vGlut-pHluorin construct to show that the probability of release (pv) of vesicles within the RRP also varies by an order of magnitude. The two types of variation are independent, and the combination produces a surprisingly large range of synaptic strengths during low frequency stimulation. Some synapses were extremely weak, but we found no evidence for bona fide silent synapses. Critically, pv correlates tightly with the timing of FM-dye de-staining from recycling vesicles during low frequency stimulation, but RRP size does not. The absence of a correlation between RRP size and FM-dye de-staining suggests that reserve and readily releasable pool sizes are regulated together, by a common mechanism. The findings support a previously proposed modular architecture where each release site is supplied with vesicles from a separate reserve module.