Spontaneous release of multiquantal miniature excitatory junction potentials induced by a Drosophila mutant.

1. Intracellular recordings were made from muscle fibre No. 6 of the dorsal longitudinal flight muscle (DLM) of Drosophila melanogaster in both wild-type flies and the temperature-sensitive paralytic mutant, shibirets-1 (shi). 2. Continuous recordings of the miniature excitatory junction potentials (MEJPs) in this fibre were made as the temperature was changed from 19 to 29 degrees C, and back to 19 degrees C. In shi flies, synapses become depleted of vesicles at 29 degrees C due to a temperature-dependent blockage in the recycling process, while transmitter release proceeds normally. When the temperature is lowered to 19 degrees C, recycling is allowed to proceed and recovery of the full complement of synaptic vesicles gradually occurs in about 20 min. 3. It was observed that the MEJP amplitude distribution in shi flies was unimodal at 19 degrees C prior to heating (as was wild-type), but during recovery from 8 min exposure to 29 degrees C became multimodal, with peaks at roughly integral multiples of the original peak prior to heating. This effect was never seen in wild-type flies. 4. Also, during recovery, the MEJP did not occur randomly, but rather occurred in a clustered fashion. 5. It is concluded that during recovery from depletion in shi neuromuscular junctions, a condition exists which causes the synchronization of spontaneous release, causing multiquantal MEJPs or clustering of MEJPs, depending on the degree of synchronization. 6. The possible role of Ca2+ in this phenomenon is discussed.