Frog cutaneous pectoris nerve muscle preparations were studied by the freeze-fracture technique under the following conditions: (a) during repetitive indirect stimulation for 20 min, 10/s; (b) during recovery from this stimulation; and (c) during treatment with 20 mM K+. Indirect stimulation causes numerous dimples or protuberances to appear on the presynaptic membrane of nerve terminal, and most are located near the active zones. Deep infoldings of the axolemma often develop between the active zones. Neither the number nor the distribution of dimples, protuberances, of infoldings changes markedly during the first minute of recovery. The number of dimples, protuberances, and infoldings is greatly reduced after 10 min of recovery. Since endocytosis proceeds vigorously during the recovery periods, we conclude that endocytosis occurs mostly at the active zones, close to the sites of exocytosis. 20 mM K+ also causes many dimples or protuberances to appear on the axolemma of the nerve terminal but they are distributed almost uniformly along the presynaptic membrane. Experiments with horseradish peroxidase (HRP) show that recycling of synaptic vesicles occurs in 20 mM K+. This recycling is not accompanied by changes in the number of coated vesicles. Since both exocytosis and endocytosis occur in 20 mM K+, it is difficult to account for this unique distribution. However, we suggest that K+ causes dimples or protuberances to appear between the active zones because it activates latent sites of exocytosis specified by small numbers of large intramembrane particles located between active zones. The activation of latent release sites may be related to the complex effects that K+ has on the quantal release of neurotransmitter.

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