Molecular regulation of receptors: interaction of beta-estradiol and progesterone with the muscarinic system.

The effects of various substrates on the binding of agonists to muscarinic receptors were studied in the rat hypothalamus and adenohypophysis by competition experiments using the highly specific tritiated muscarinic antagonist N-methyl-4-piperidyl benzilate. It was found that agonist binding properties were affected only by the steroid sex hormones (beta-estradiol and progesterone), both of which resulted in a decrease in the proportion of high-affinity binding sites and a decrease in the dissociation constant. This suggests a link between the muscarinic system and the mechanism by which these steroids exert their gonadotropin-releasing effect on the adenohypophysis. We propose a model to depict the putative relationship between the muscarinic system and other receptor systems, including that which controls the steroid sex hormones.