Nerve terminals are unique among cellular secretory systems in that they can sustain vesicular release at a high rate. Although little is known about the mechanisms that account for the distinctive features of neurotransmitter release, it can be assumed that neuron-specific proteins are involved. One such protein family, the synapsins, are believed to regulate neurotransmitter release through phosphorylation-dependent interactions with synaptic vesicles and cytoskeletal elements. Here we show that clusters of vesicles at synaptic release sites are composed of two pools, a distal pool containing synapsin and a proximal pool devoid of synapsin and located adjacent to the presynaptic membrane. Presynaptic injection of synapsin antibodies resulted in the loss of the distal pool, without any apparent effect on the proximal pool. Depletion of this distal pool was associated with a marked depression of neurotransmitter release evoked by high-frequency (18-20 Hz) but not by low-frequency (0.2 Hz) stimulation. Thus the availability of the synapsin-associated pool of vesicles seems to be required to sustain release of neurotransmitter in response to high-frequency bursts of impulses.

The proline-rich COOH-terminal region of dynamin binds various Src homology 3 (SH3) domain-containing proteins, but the physiological role of these interactions is unknown. In living nerve terminals, the function of the interaction with SH3 domains was examined. Amphiphysin contains an SH3 domain and is a major dynamin binding partner at the synapse. Microinjection of amphiphysin's SH3 domain or of a dynamin peptide containing the SH3 binding site inhibited synaptic vesicle endocytosis at the stage of invaginated clathrin-coated pits, which resulted in an activity-dependent distortion of the synaptic architecture and a depression of transmitter release. These findings demonstrate that SH3-mediated interactions are required for dynamin function and support an essential role of clathrin-mediated endocytosis in synaptic vesicle recycling.

Coordination between sequential steps in synaptic vesicle endocytosis, including clathrin coat formation, fission, and uncoating, appears to involve proteinprotein interactions. Here, we show that compounds that disrupt interactions of the SH3 domain of endophilin with dynamin and synaptojanin impair synaptic vesicle endocytosis in a living synapse. Two distinct endocytic intermediates accumulated. Free clathrin-coated vesicles were induced by a peptide-blocking endophilin's SH3 domain and by antibodies to the proline-rich domain (PRD) of synaptojanin. Invaginated clathrin-coated pits were induced by the same peptide and by the SH3 domain of endophilin. We suggest that the SH3 domain of endophilin participates in both fission and uncoating and that it may be a key component of a molecular switch that couples the fission reaction to uncoating.