Modulation of Second Messengers in the Nervous System of Larval Manduca sexta by Muscarinic Receptors

: Measurements were made of the effects of muscarinic agents on endogenous levels of cyclic AMP and cyclic GMP, and the turnover of radiolabeled inositol phosphates in the abdominal nervous system of larval Manduca sexta. Cyclic AMP levels were increased by treatment with 3‐isobutyl‐1‐methylxanthine or tetrodotoxin, but the muscarinic agonist oxotremorine‐M and the muscarinic antagonist scopolamine had no consistent effects. In contrast, cyclic GMP levels were significantly increased by oxotremorine‐M and by oxotremorine‐M in the presence of 3‐isobutyl‐1‐methylxanthine and tetrodotoxin but not in the presence of scopolamine. Using lithium to inhibit the recycling of inositol phospholipid metabolites in isolated nerve cords, we detected a small but consistent increase in inositol phosphate production by oxotremorine‐M. The primary inositol metabolite generated during a 5‐min exposure to oxotremorine‐M co‐eluted 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 tetrodotoxin but inhibited by scopolamine, suggesting that it is directly mediated by muscarinic receptors. Further evidence for coupling between muscarinic receptors and inositol metabolism was obtained using a cell‐free preparation of nerve cord membranes labeled with [3H]inositol. Incubation with oxotremorine‐M evoked a significant increase in labeled inositol bisphosphate, consistent with muscarinic receptors coupling to phosphatidylinositol metabolism. The accumulation of inositol bisphosphate in cell‐free preparations suggests that the normal breakdown to inositol monophosphate requires cytosolic components. Together, these results indicate that muscarinic acetylcholine receptors in Manduca couple predominantly to the inositol phospholipid signaling system, although some receptors may modulate cyclic GMP.