Muscarinic receptor subtypes and sexual behavior in female rats

Cholinergic muscarinic systems are involved in the regulation of female sexual behavior in rats and hamsters. This series of experiments was designed to determine whether sexual behavior in female rats is controlled preferentially by one of the traditional muscarinic receptor subtypes. Intraventricular infusion of the muscarinic antagonist scopolamine (10 micrograms bilaterally) which binds with high affinity to both M1 and M2 subtypes inhibited sexual behavior, as indicated by the incidence of lordosis, in ovariectomized rats treated with estrogen and progesterone. In contrast, the M1-selective antagonist pirenzepine failed to reduce the incidence of lordosis following intraventricular infusion (10 to 80 micrograms bilaterally). Biochemical analyses revealed that intraventricular infusion of scopolamine (10 micrograms bilaterally) inhibited both M1 and M2 binding in brain tissues while intraventricular infusion of pirenzepine (10 micrograms bilaterally) completely inhibited M1 binding without affecting M2 binding. Intraventricular infusions of the acetylcholinesterase inhibitor physostigmine (10 micrograms bilaterally), the cholinergic agonist carbachol (1 microgram bilaterally), and the muscarinic agonist oxotremorine-M (0.1 micrograms bilaterally) activated lordosis in ovariectomized females primed with low doses of estrogen. In contrast, the putative M1 agonist McN-A-343 failed to significantly increase lordosis following intraventricular infusions (1, 10, 20 micrograms bilaterally). According to biochemical results, the ability of these agents to activate lordosis in female rats was related to their affinities for M2 binding sites not M1 binding sites. In a final experiment, estrogen treatment of ovariectomized rats did not alter muscarinic subtype binding in several brain areas as measured by the M1-selective ligand [3H] pirenzepine and the M2-selective ligand [3H] oxotremorine-M. The results of these experiments confirm that muscarinic systems contribute to the regulation of lordosis in female rats and indicate that M2 binding sites rather than M1 binding sites may be a critical component of this regulation.