Endocytic active zones: Hot spots for endocytosis in vertebrate neuromuscular terminals

We have used a sensitive activity-dependent probe, sulforhodamine 101 (SR10 1), to view endocytic events within snake motor nerve terminals. After very brief neural stimulation at reduced temperature, SR101 is visualized exclu sively at punctate sites located just inside the presynaptic membrane of ea ch terminal bouton, The number of sites (similar to 26 sites/bouton) and th eir location tin register with postsynaptic folds) are similar to the numbe r and location of active zones in snake motor terminals, suggesting a spati al association between exocytosis and endocytosis under these stimulus cond itions. With more prolonged stimulation, larger SR101-containing structures appear at the bouton margins. Thus endocytosis occurs initially at distinc t sites, which we call "endocytic active zones," whereas further stimulatio n recruits a second endocytic paradigm.