IKEDA, Kazuo; KOENIG, Jane H; Beckman Research Institute of the City of Hope: Structure and function of two types of neuromuscular junctions in Drosophila

The function of the tergotrochanteral muscle (TTM) is to provide instantaneous leg extension for the lift-off jump at flight initiation, while that of the dorsal longitudinal muscle (DLM) is to provide continuous muscle activity during flight. Our morphological and electrophysiological studies on the synapses of these functionallly distinct muscles have revealed a correlation between the synaptic structure and the synaptic output to these two muscles. Our previous studies using conventional electron microscopy (EM) have demonstrated that the DLM terminal possesses two distinct vesicle populations – a small readily-releasable population located around the dense body at the active zone and a much larger reserve population dispersed throughout the cytoplasm. The TTM terminal, on the other hand, is unique in possessing only the small active zone population. In this study we have characterized the electrophysiological responses of these two neuromuscular junctions by recording intracellularly from individual muscle fibers while stimulating the giant interneuron at various frequencies (1-100Hz). The TTM was observed to respond up to 100 Hz, but depression occurred almost immediately. On the other hand, the DLM was able to respond to sustained, high frequency firing without depression. This observation correlates well with the different functional needs of these two muscle types – a short burst of action potentials for the jumping response of the TTM as opposed to the continuous firing for flight in the DLM. Also, the data suggest that the reserve vesicle population may protect the DLM neuromuscular junctions from depression and provide the capability for the long term firing required during flight. Supported by a grant from NIH, NINDS. RO1-18856.