Biochemical and radioautographic analysis of estrogen-inducible progestin receptors in female ferret brain and pituitary: correlations with effects of progesterone on sexual behavior and gonadotropin-releasing hormone-stimulated secretion of luteinizing hormone.

Cytosolic binding molecules for the synthetic progestin [ 3 H]R5020, were isolated in vitro from several brain regions including preoptic area/anterior hypothalamus, mediobasal hypothalamus, medial amygdala and parietal cortex as well as the pituitary gland of ovohysterectomized female ferrets. Binding of [ 3 H]R5020 to cortical cytosols was saturable, of high affinity (apparent dissociation constant of 2.0 nM), and was steroid specific. Pretreatment of ferrets with a Silastic capsule containing estradiol caused significant increments in the concentration of cytosolic R5020 binding sites in hypothalamus and pituitary gland, but not in the other brain regions studied. Brains of additional ovohysterectomized ferrets, which had been primed with estradiol prior to receiving an i.v. injection of [ 3 H]R5020, were processed radioautographically. Intense labeling of cells was seen in the medial and lateral preoptic area, in the lateral hypothalamus, and in the ventromedial and arcuate nuclei of the hypothalamus. Radioautograms from the brains of additional ovohysterectomized ferrets given an i.v. injection of [ 3 H]estradiol revealed labeled cells in all of the above regions, in addition to the basolateral portion of the septum, the bed nucleus of the stria terminalis, and in the anterior amygdaloid area as well as the medial and cortical nuclei of the amygdala. This distribution of neural progesting and estrogen binding sites resembles those previously reported for these steroids in the rat, guinea pig, hamster and macaque. Functional studies showed that acute s.c. implantation of a Silastic capsule containing progesterone to ovohysterectomized ferrets, which had been primed with a low dosage of estradiol, failed to augment their sexual receptivity in limited tests with stimulus males given 4 and 8 h after progesterone treatment. This result contrasts with the well-established facilitatory effect of progesterone on sexual receptivity in rat and guinea pig. Chronic exposure to a progesterone capsule caused significant reductions in sexual receptivity in ovohysterectomized ferrets which were implanted concurrently with a second Silastic capsule containing a high dosage of estradiol. Similar effects of progesterone have been reported in rat and guinea pig, but not in the rhesus monkey. Thus species differences in the ability of progesterone to facilitate or inhibit sexual receptivity are not readily explained by species differences in the neural distribution of estrogen-induced progestin receptors. The ability of a single i.v. injection of 4 μg of gonadotropin releasing hormone (GnRH) to stimulate luteinizing hormone (LH) release was significantly lower in ovohysterectomized females implanted s.c. with Silastic capsules containing estradiol and progesterone than in ferrets exposed only to estradiol. Administration of progesterone alone to ovohysterectomized females had no effect on the ability of GnRH to stimulate LH release. These gonadotropin release patterns were correlated with the concentration of cytosolic progestin receptors in the pituitary gland.