Studies on muscarinic receptors in mouse and rat hypothalamus: a comparison of sex and cyclical differences.

The ligand binding properties of muscarinic receptors present in whole hypothalamus as well as in its three regions (preoptic area, median and posterior regions) were studied in male rats and mice, as well as in female rats during various stages of the estrous cycle, using the tritiated antagonist N-methyl-4-piperidyl benzilate. Kinetic and equilibrium analysis of antagonist binding as well as competition experiments with agonists, were used to probe the nature of the binding sites and possible differences between specific muscarinic sites in the various preparations. We could detect differences in agonist binding parameters between male and female rats in the preoptic area. In female rats binding of agonist to high affinity state is characterized by a lower affinity (27 nM) than in males (3.7 nM). Secondly, the population of agonist high affinity sites at the proestrous stage is much higher than that at other stages of cycle (66 vs. 38%). In addition, the binding properties of muscarinic receptors following intracisternal 6-hydroxydopamine lesion were investigated. This treatment resulted in a decrease of about one sixth of the total muscarinic receptors in the hypothalamus. The data are best interpreted as a degeneration of existing presynaptic muscarinic receptors located on catecholamine terminals in the hypothalamus. The pre- and postsynaptic nature of the muscarinic receptors, their localization in view of their binding properties, and their possible physiological role are discussed.