MODULATION OF 2ND MESSENGERS IN THE NERVOUS-SYSTEM OF LARVAL MANDUCA-SEXTA BY MUSCARINIC RECEPTORS

Measurements were made of the effects of muscarinic agents on endogeno us levels of cyclic AMP and cyclic GMP, and the turnover of radiolabel ed inositol phosphates in the abdominal nervous system of larval Mandu ca serta. Cyclic AMP levels were increased by treatment with 3-isobuty l-1-methylxanthine or tetrodo-toxin, but the muscarinic agonist oxotre morine-M and the muscarinic antagonist scopolamine had no consistent e ffects. In contrast, cyclic GMP levels were significantly increased by oxotremorine-M and by oxotremorine-M in the presence of 5-isobutyl-1- methylxanthine and tetrodo-toxin but not in the presence of scopolamin e. Using lithium to inhibit the recycling of inositol phospholipid met abolites in isolated nerve cords, we detected a small but consistent i ncrease in inositol phosphate production by oxotremorine-M. The primar y inositol metabolite generated during a 5-min exposure to oxotremorin e-M coeluted from ion;exchange columns with inositol-1-monophosphate, although other more polar metabolites were also detected. This agonist -evoked increase in inositol phosphate production was unaffected by te trodotoxin but inhibited by scopolamine, suggesting that it is directl y mediated by muscarinic receptors. Further evidence for coupling betw een muscarinic receptors and inositol metabolism was obtained using a cell-free preparation of nerve cord membranes labeled with [H-3]inosit ol, Incubation with oxotremorine-M evoked a significant increase in la beled inositol bisphosphate, consistent with muscarinic receptors coup ling to phosphatidylinositol metabolism. The accumulation of inositol bisphosphate in cell-free preparations suggests that the normal breakd own to inositol monophosphate requires cytosolic components, Together, these results indicate that muscarinic acetylcholine receptors in Man duca couple predominantly to the inositol phospholipid signaling syste m, although some receptors may modulate cyclic GMP.