Size of vesicle pools, rates of mobilization, and recycling at neuromuscular synapses of a Drosophila mutant, shibire

Two vesicle pools, readily releasable (RRP) and reserve (RP) pools, are pre sent at Drosophila neuromuscular junctions. Using a temperature-sensitive m utant, shibire(ts), we studied pool sizes and vesicle mobilization rates. I n shibire(ts), due to lack of endocytosis at nonpermissive temperatures, sy naptic currents continuously declined during tetanic stimulation until they ceased as the result of vesicle depletion. By then, similar to 84,000 quan ta were released. Vesicles were mobilized from RP at a rate 1/7-1/10 of RRP . Cytochalasin D inhibited mobilization of vesicles from RP, allowing us to estimate the size of RRP as 14%-19% of all vesicles. Vesicle recycling sup ports synaptic transmission during prolonged tetanic stimulation and the ma ximum recycling rate was 1000 vesicles/s.