Limited Numbers of Recycling Vesicles in Small CNS Nerve Terminals: Implications for Neural Signaling and Vesicular Cycling

The tiny nerve terminals of central synapses contain far fewer vesicles than preparations commonly used for analysis of neurosecretion. Photoconversion of vesicles rendered fluorescent with the dye FM1-43 directly identified vesicles capable of engaging in exo-endocytotic recycling following stimulated Ca(2+) entry. This recycling pool typically contained 30-45 vesicles, only a minority fraction (15-20% on average) of the total vesicle population. The smallness of the recycling pool would severely constrain rates of quantal neurotransmission if classical pathways were solely responsible for vesicle recycling. Fortunately, vesicles can undergo rapid retrieval and reuse in addition to conventional slow recycling, to the benefit of synaptic information flow and neuronal signaling.