Ba. Trimmer et S. Qazi, MODULATION OF 2ND MESSENGERS IN THE NERVOUS-SYSTEM OF LARVAL MANDUCA-SEXTA BY MUSCARINIC RECEPTORS, Journal of neurochemistry, 66(5), 1996, pp. 1903-1913
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.