Reversible depletion of synaptic vesicles induced by application of high external potassium to the frog neuromuscular junction.

1. Reversible depletion of synaptic vesicles from frog cutaneous pectoris neuromuscular junctions was studied by application of a Ringer solution containing 115 mM-K propionate.2. During the release of transmitter, the synaptic vesicle membrane is added to the axolemmal membrane. Under the conditions of high K(+)-induced release, the synaptic vesicle membrane accumulates as folds formed in the region of the axolemmal membrane between the active zones. In depleted terminals, large vesicular structures appear and the evidence shows that some of them (possibly all) are formed as axolemmal infoldings. During formation of such infoldings the active zones remain fixed in position with respect to the post-junctional membrane.3. During recovery in normal Ringer solution, which followed 30 min depolarization in high K(+) Ringer solution, spontaneous m.e.p.p.s were detected as early as 9 min after the start of the recovery period and the average time for their reappearance was 17 min.4. At the end of a 20 min recovery period which followed K(+) depolarization, small accumulations of synaptic vesicles were again found within the terminal close to the active zones. At this time coated vesicles and coated pits were seen associated with the prejunctional axolemma and its infoldings. It appears that synaptic vesicles are re-formed directly from these coated vesicles.5. After 60 min recovery from K(+) depolarization, at which time stimulation of the motor nerve induced a muscle twitch, the structure of the terminals closely resembled that of control preparations.6. The entire synaptic vesicle recycling process can take place in the absence of the neurone soma.