Synaptic vesicles: Is kissing a matter of competence?

The `kiss-and-run` model of exocytosis and endocytosis predicts that synapt ic vesicles can undergo fast and efficient recycling, after fusion with the plasmalemma,without intermixing of membranes. Evidence is mounting from se veral new experimental approaches that kiss-and-run occurs at synapses. Dis tinct vesicle pools, which initially were identified in morphological terms , are now being characterized in biochemical and functional terms. In addit ion, at least two functional recycling pathways, operating on different tim e scales (from milliseconds to tens of seconds, have been shown to coexist in the same synaptic system, and the two pathways appear to be differential ly regulated. Taken together, these data suggest that kiss-and-run operates in parallel with the classical, coated-vesicle recycling. Here, we review recent evidence for kiss-and-run recycling and discuss whether it is a dist inct process, dependent on the molecular organization of the fusing vesicle . We propose that vesicles undergo a process of `competence maturation' Acc ording to this view, the specific molecular make-up of the vesicles,their l ocation and their interactions with nerve terminal proteins might determine not only the differential availability of the vesicles for fusion and neur otransmitter release but also the recycling path that they will follow.