Muscarinic Agonist-Mediated Increases in Serum Corticosterone Levels Are Abolished in M2 Muscarinic Acetylcholine Receptor Knockout Mice

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Abstract
Muscarinic acetylcholine receptors (M1–M5) regulate many key functions in the central and peripheral nervous system. Due to the lack of receptor subtype-selective ligands, however, the physiological roles of individual muscarinic receptor subtypes remain to be determined. In this study, we examined the effects of the muscarinic M2/M4receptor-preferring agonist [5R-(exo)]-6-[4-butylthio-1,2,5-thiadiazol-3-yl]-1-azabicyclo-[3.2.1]-octane (BuTAC) on serum corticosterone levels in M2 and M4 receptor single knockout (KO) and M2,4receptor double KO mice. Responses were compared with those obtained with the corresponding wild-type (WT) mice. BuTAC (0.03–0.3 mg/kg s.c.) dose dependently and significantly increased serum corticosterone concentrations in WT mice to 5-fold or greater levels compared with vehicle controls. In muscarinic M2 and M2,4 KO mice, however, BuTAC had no significant effect on corticosterone concentrations at doses of 0.1, 0.3, and 1 mg/kg s.c. In both WT and muscarinic M4 KO mice increases in serum corticosterone concentrations induced by BuTAC (0.1 and 0.3 mg/kg) were not significantly different and were blocked by scopolamine. In summary, the muscarinic M2,4-preferring agonist BuTAC had no effect on corticosterone levels in mice lacking functional muscarinic M2 receptors. These data suggest that the muscarinic M2 receptor subtype mediates muscarinic agonist-induced activation of the hypothalamic-pituitary-adrenocortical axis in mice.