Specific progestin binding in vitro by anterior pituitary and brain tissues of male rats

Using a highly potent synthetic-progestin, R5020 (17 alpha, 21-dimethyl-19-nor-4, 9-pregnadiene-3, 20-dione), the characterization of progestin binding components was attempted in cytosols from the anterior pituitary and various brain tissues of castrated or castrated-adrenalectomized male rats. In both types of operated rats, estrogen administration increased the number of binding sites of R5020 in the anterior pituitary and hypothalamus-preoptic area, but not in the cerebral cortex, amygdala-hippocampus nor in the cerebellum. After incubation of the anterior pituitary cytosol with [3H]-R5020 with or without various unlabeled steroids, a specific progestin binding was found in the 7-8S region with a binding capacity of 250 fmoles/mg protein and equilibrium dissociation constant of 8 nM. A specific progestin binding was also found in the cytosol from hypothalamus-preoptic area sedimenting in the 7S region with a binding capacity of 150 fmoles/mg protein and a dissociation constant of 6 nM. Specific binding components in these tissues were heat-labile and protein in nature. Little or no 7S binding was found in the other brain tissues. Although 4-5S binding in the cerebral cortex and 4S bind ng in the amygdala-hippocampus and cerebellum were found, binding specificity in those tissues was low. It was concluded that the pituitary gland and hypothalamus-preoptic area of male rats contained progestin receptor in their cytoplasms.