Relationship of the reserve vesicle population to synaptic depression in the tergotrochanteral and dorsal longitudinal muscles of Drosophila.

We have previously demonstrated that Drosophila synapses possess two vesicle populations-a small active zone population replenished by "fast" recycling and a much larger reserve population replenished by a slower recycling mechanism that includes endosomal intermediates. In this paper, we demonstrate that the synapses onto the tergotrochanteral muscle (TTM) are very unusual in that they possess only the active zone vesicle population but not the reserve population. The depression characteristics to repetitive stimulation of the TTM were compared with those of the dorsal longitudinal muscle (DLM), the synapses of which possess both an active zone and a reserve population. It was observed that the TTM response depressed more quickly than that of the DLM. To further explore the possible contribution of the reserve population to release, using the shibire mutant, DLM synapses were experimentally constructed that possess only the active zone population, and their depression characteristics were compared with those of the same synapses possessing both populations. It was observed that responses from DLM synapses possessing only the active zone population depressed more quickly than the same synapses possessing both populations. These experiments were conducted under conditions of blocked recycling so that the difference in stimulation tolerance represents the contribution of the reserve population to release. Furthermore, the depression curve of the DLM synapses lacking a reserve population now closely approximated that of the TTM synapses. These data suggest that the reserve vesicle population of DLM synapses may contribute to transmitter release during repetitive firing at physiological frequencies (5-10 Hz).