Rapid reuse of readily releasable pool vesicles at hippocampal synapses

Functional presynaptic vesicles have been subdivided into readily releasabl e (RRP) and reserve (RP) pools. We studied recycling properties of RRP vesi cles through differential retention of FM1-43 and FM2-10 and by varying the time window for FM dye uptake. Both approaches indicated that vesicles res iding in the RRP underwent rapid endocytosis (tau approximate to 1 s), wher eas newly recruited RP vesicles were recycled slowly (tau approximate to s) . With repeated challenges (hypertonic or electrical stimuli), the ability to release neurotransmitter recovered 10-fold more rapidly than restoration of FM2-10 destaining. Finding neurotransmission in the absence of destaini ng implied that rapidly endocytosed RRP Vesicles were capable of reuse, a p rocess distinct from repopulation from the RP. Reuse would greatly expand t he functional capabilities of a limited number of vesicles in CNS terminals , particularly during intermittent bursts of activity.