Parallel processing of quickly and slowly mobilized reserve vesicles in hippocampal synapses

Abstract Vesicles within presynaptic terminals are thought to be segregated into a variety of readily releasable and reserve pools. The nature of the pools and trafficking between them is not well understood, but pools that are slow to mobilize when synapses are active are usually assumed to feed pools that are mobilized more quickly, in a series. However, results from electrophysiological studies of synaptic transmission suggested instead a parallel organization where vesicles within slowly and quickly mobilized reserve pools would separately feed independent reluctant- and fast-releasing subdivisions of the readily releasable pool, without intermixing. We now use FM-dyes to confirm the existence of multiple reserve pools at hippocampal synapses. We then confirm the prediction that slowly and quickly mobilized reserve pools do not intermix, even when mobilized by high frequency stimulation. The result provides a simplifying new constraint on the dynamics of vesicle recycling within presynaptic terminals. The experiments additionally demonstrated extensive heterogeneity among synapses in the relative sizes of slowly and quickly mobilized reserve pools. The heterogeneity suggests equivalent heterogeneity in the probability of release among readily releasable vesicles that may be relevant for understanding information processing and storage.