A resting pool of vesicles is responsible for spontaneous vesicle fusion at the synapse

Synapses relay information through the release of neurotransmitters stored in presynaptic vesicles. The identity, kinetics and location of vesicle pools mobilized by neuronal activity have been studied using a variety of techniques. Here, we describe a novel genetically-encoded probe, biosyn, which consists of a biotinylated VAMP-2 expressed at presynaptic terminals. We exploit the high affinity interaction between streptavidin and biotin to label biosyn with fluorescent streptavidin during vesicle fusion. This approach allows tagging of vesicles sequentially, to visualize and establish the identity of presynaptic pools. Using this technique we were able to distinguish between two different pools of vesicles in rat hippocampal neurons: one that is released in response to presynaptic activity and another, distinct vesicle pool that spontaneously fuses with the plasma membrane. We further established that the spontaneous vesicles belong to a ‘resting pool’ that is normally not mobilized by neuronal activity and whose function is mostly unknown.