Rapid endocytosis does not recycle vesicles within the readily releasable pool

Endocytosis is essential in maintaining exocytosis at secretory cells. Rapid endocytosis with a time course less than a few seconds is widely observed at nerve terminals and non-neuronal secretory cells. It is generally assumed that rapid endocytosis recycles vesicles within the readily releasable pool (RRP) via a kiss-and-run mechanism that involves rapid opening and closure of a fusion pore at the release site. The present work suggests that both rapid (tau less than approximately 2 s) and slow (tau = approximately 10-20 s) endocytosis do not recycle vesicles to the RRP but to a recycling pool at least a few times larger than the RRP at a nerve terminal, the calyx of Held in rat brainstem. Challenging the long-held view that rapid endocytosis offers a rapid, local vesicle recycling within the RRP, our finding calls for reconsideration of the function and the underlying mechanism of rapid endocytosis. We suggest that rapid endocytosis provides the nerve terminal the ability to recycle vesicles rapidly via the recycling pool and to maintain the normal morphology of the nerve terminal, including the release site, by rapidly clearing the fused vesicle membrane from the release site during intense firing.