Peripheral distribution of presynaptic sites of abdominal motor and modulatory neurons in Manduca sexta larvae.

Abstract

Insect muscle fibers are commonly innervated by multiple motor neurons and efferent unpaired median (UM) neurons. The role of UM neurons in the modulation rather than rapid activation of muscle contraction (Evans and O'Shea [1977] Nature 270:257-259) suggests that their terminal varicosities may differ structurally and functionally from the presynaptic terminals of motor neurons. Furthermore, differences in the characteristics of UM neuron terminal varicosities may be correlated with functional differences among their diverse target muscles. Larval abdominal body wall muscles in the hawkmoth, Manduca sexta, consist of large, elongated fibers that are multiterminally innervated by one and occasionally two motor neurons (Levine and Truman [1985] J. Neurosci. 5:2424-2431). The fibers are also innervated by one of two efferent UM neurons that bifurcate to innervate targets on both sides of the abdomen (Pflüger et al. [1993] J. Comp. Neurol. 335:508-522). In this study, the intracellular tracer biocytin was used to identify the targets of the UM neurons and to distinguish their terminal axonal varicosities on the muscle fibers. An antiserum to the synaptic vesicle protein, synaptotagmin, was used to label synaptic vesicles, and the styryl dye FM1-43 was used to demonstrate release and recycling. Most of the abdominal muscles in a given hemisegment were found to be supplied by one of the two UM neurons. Terminal varicosities of the excitatory motor neurons were large (3-7 pm) and were found in rows of rosettes that extended to every aspect of the muscle fiber; these varicosities were designated as type I terminals. The UM neuron terminal varicosities also occupied every aspect of the fiber but were smaller (1-3 microm) and more separated from each other; these were designated as type II terminals. Both type I and type II terminals are synaptotagmin immunoreactive and, as shown by FM1-43 staining, are sites of synaptic vesicle recycling. The excitatory motor neuron terminals (type I) could easily be loaded and unloaded with FM1-43, which indicates their capacity for repeated vesicular exocytosis and recycling. In contrast, the dye could not as readily be unloaded from UM neuron terminals (type II), which may indicate that they have a slower turnover of synaptic vesicles.