INSULIN-LIKE RECEPTOR AND INSULIN-LIKE PEPTIDE ARE LOCALIZED AT NEUROMUSCULAR-JUNCTIONS IN DROSOPHILA

Insulin and insulin-like growth factor (IGF) receptors are members of the tyrosine kinase family of receptors, and are thought to play an im portant role in the development and differentiation of neurons. Here w e report the presence of an insulin-like peptide and an insulin recept or (dInsR) at the body wall neuromuscular junction of developing Droso phila larvae. dInsR-like immunoreactivity was found in all body wall m uscles at the motor nerve branching regions, where it surrounded synap tic boutons. The identity of this immunoreactivity as a dInsR was conf irmed by two additional schemes, in vivo binding of labeled insulin an d immunolocalization of phosphotyrosine. Both methods produced stainin g patterns markedly similar to dInsR-like immunoreactivity. The presen ce of a dInsR in whole larvae was also shown by receptor binding assay s. This receptor was more specific for insulin (>25-fold) than for IGF II, and did not appear to bind IGF I. Among the 30 muscle fibers per hemisegment, insulin-like immunoreactivity was found only on one fiber , and was localized to a subset of morphologically distinct synaptic b outons. Staining in the CNS was limited to several cell bodies in the brain lobes and in a segmental pattern throughout most of the abdomina l ganglia, as well as in varicosities along the neuropil areas of the ventral ganglion and brain lobes. Insulin-like peptide and dInsR were first detected by early larval development, well after neuromuscular t ransmission begins. The developmental significance of an insulin-like peptide and its receptor at the neuromuscular junction is discussed.