DIFFERENTIAL PHYSIOLOGY AND MORPHOLOGY OF MOTOR AXONS TO VENTRAL LONGITUDINAL MUSCLES IN LARVAL DROSOPHILA

Morphological and physiological characteristics of the two major motor axons supplying the commonly studied ventral longitudinal muscle fibe rs (6 and 7) of third-instar Drosophila melanogaster larvae were inves tigated. The innervating terminals of the two motor axons differ in th e size of their synapse-bearing varicosities. The terminal with the la rger varicosities also fluoresces more brightly when stained with the vital fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodi de (4-Di-2-Asp) and occupies a larger total contact area on the muscle fiber. Through selective simultaneous recording of synaptic currents from identified boutons in living preparations during elicitation of s ynaptic potentials, it was shown that the axon with the smaller varico sities generates a large excitatory junction potential (EJP) in muscle 6 and that the axon with the larger varicosities generates a smaller EJP. Short-term facilitation is more pronounced for the smaller EJP. I n preparations treated with 4-Di-2-Asp, the fluorescence of smaller va ricosities increases with stimulation that elicits the large EJPs, ind icating an activity-dependent entry of calcium that enhances mitochond rial fluorescence. The differences in morphology and physiology of the two axons are similar to, though less pronounced than, those observed in ''phasic'' and ''tonic'' motor axons of crustaceans. (C) 1994 Wile y-Liss, Inc.