Black widow spider venom (BWSV) was applied to frog nerve-muscle preparations bathed in Ca2+-containing, or Ca2+-free, solutions and the neuromuscular junctions were studied by the freeze-fracture technique. When BWSV was applied for short periods (10-15 min) in the presence of Ca2+, numerous dimples (P face) or protuberances (E face) appeared on the presynaptive membrane and approximately 86% were located immediately adjacent to the double rows of large intramembrane particles that line the active zones. When BWSV was applied for 1 h in the presence of Ca2+, the nerve terminals were depleted of vesicles, few dimples or protuberances were seen, and the active zones were almost completely disorganized. The P face of the presynaptic membrane still contained large intramembrane particles. When muscles were soaked for 2-3 h in Ca2+-free solutions, the active zones became disorganized, and isolated remnants of the double rows of particles were found scattered over the P face of the presynaptic membrane. When BWSV was applied to these preparations, dimples or protuberances occurred almost exclusively alongside disorganized active zones or alongside dispersed fragments of the active zones. The loss of synaptic vesicles from terminals treated with BWSV probably occurs because BWSV interferes with the endocytosis of vesicle membrane. Therefore, we assume that the dimples or protuberances seen on these terminals identify the sites of exocytosis, and we conclude that exocytosis can occur mostly in the immediate vicinity of the large intramembrane particles. Extracellular Ca2+ seems to be required to maintain the grouping of the large particles into double rows at the active zones, but is not required for these particles to specify the sites of exocytosis.

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