Release and recycling of the readily releasable vesicle population in a synapse possessing no reserve population.

We previously demonstrated that the tergotrochanteral muscle (TTM) of Drosophila is innervated by unique synapses that possess a small readily releasable/recycling vesicle population (active zone population), but not the larger reserve vesicle population observed at other neuromuscular junctions in this animal. Using light and electron microscopic techniques and intracellular recording from the G1 muscle fiber of the TTM, the release and recycling characteristics of the readily releasable/recycling population were observed without any possible contribution from a reserve population. Our results indicate 1) the total number of vesicles in synapses presynaptic to the G1 fiber correlates with the total number of quanta that can be released onto this fiber; 2) the number of quanta released by a single action potential onto the G1 fiber is about one half the number of morphologically "docked" vesicles in active zones onto the G1, and this ratio decreases in a partially depleted state; 3) the recycling rate at 1-Hz stimulation, a frequency that does not cause any depression, is 0.24 recycled vesicle/active zone/s; and 4) normal-appearing spontaneous release occurs from the active zone vesicle population and, unlike synapses that possess a reserve population, the frequency of this release is reduced after high-frequency evoked activity.