Pituitary-adrenal responses to cholinergic stimulation and acute mild stress are differentially elevated in male and female M(2) muscarinic receptor knockout mice

Adrenocorticotrophic hormone (ACTH) and corticosterone responses to cholinergic stimulation are greater in male rats and mice than in females. To explore the role of M(2) muscarinic receptors in this sex difference, we administered the nonselective muscarinic agonist, oxotremorine, the acetylcholinesterase inhibitor, physostigmine, and saline (a mild stressor) to male and female M(2) receptor knockout (KO) and wild-type (WT) mice of the same genetic background. Because M(2) receptors function primarily as presynaptic autoreceptors, we hypothesized that their absence in M(2) KO mice would increase the sensitivity of hormone responses to cholinergic stimulation in these groups. Both male and female M(2) KO mice were significantly more responsive to the stress of saline injection than were their WT counterparts. Oxotremorine and physostigmine increased ACTH and corticosterone in all four groups, but to a significantly greater degree in KO males compared to WT males, KO females, and WT females. The increase in ACTH also was significantly greater in WT males compared to WT females. By contrast, the increase in corticosterone was significantly more in females compared to males, independent of genotype. Following pretreatment with the nonselective muscarinic antagonist, scopolamine, ACTH and corticosterone responses to oxotremorine and to saline in the M(2) KO mice were comparable with those of their WT counterparts. These findings suggest that the M(2) muscarinic receptor subtype influences male and female pituitary-adrenal responses following stimulation by both mild stress and cholinergic drugs. The M(2) receptor appears to regulate ACTH responses to cholinergic stimulation in males but not in females; however, other muscarinic receptors may be involved because corticosterone responses were higher in females compared to males. Because ACTH and corticosterone responses were greater in male and female M(2) KO mice, the M(2) receptor appears to dampen the stress response.