Abstract. The synapsins are a family of four neuronspecific phosphoproteins that have been implicated in the regulation of neurotransmitter release. Greengard. 1995. Nature (Lond.). 375:493-497). Here, using the optical tracer FM 1-43, we characterize the details of synaptic vesicle recycling at individual synaptic boutons in hippocampal cell cultures derived from mice lacking synapsin I or wild-type equivalents. These studies show that both the number of vesicles exocytosed during brief action potential trains and the total recycling vesicle pool are significantly reduced in the synapsin I-deficient mice, while the kinetics of endocytosis and synaptic vesicle repriming appear normal.N 'EURONS use regulated secretion at specialized synaptic contacts to transmit information during patterns of electrical activity. Modulation of the probability of exocytosis during action potential firing is thought to underlie major forms of plasticity necessary for nervous system function. Determining the cellular processes that regulate vesicle exocytosis at presynaptic terminals is thus of central interest to neurobiology.Although much progress has been made in identifying important components that participate in synaptic vesicle trafficking and secretion (for reviews see Scheller, 1995;Sudhof, 1995), the unambiguous assignment of these molecules to specific events in the presynaptic terminal remains a major challenge. Striking similarities have emerged upon comparison of molecular composition of presynaptic terminals with those of more general secretory pathways (Bennett and Scheller, 1993). This molecular homology suggests that many parallels exist between all secretory systems at the level of vesicle delivery, docking, fusion, and retrieval. Chemical synapses differ, however, in distinct ways from their secretory counterparts in other parts of the cell or in nonneural tissues. (a) Secretion is highly regulated, providing a tight coupling between action potential stimulation and neurotransmitter release.

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