Radioautographic study of the rat brain, uterus and vagina after [ 3H]R-5020 injection

Localization of [ 3H]R-5020, a potent synthetic progestin, was examined by radioautography in the central nervous system and the genital tract of estrogen-primed, ovariectomized rat. In the brain, a concentration of radioactivity was found in the nuclei of neurons localized in different specific areas: the preoptic region and the medio-basal hypothalamus. In the anterior pituitary, cells which concentrated the hormone were found. The uterine tissues (uterine horns and cervix) presented a nuclear concentration of radioactivity in stromal and muscle cells but little or no radioactivity in luminal and glandular epithelia. In the vagina of the same animals, radioactivity was observed in the connective tissue cells, muscle cells and epithelial cells. Hormonal specificity and saturability of radioactivity binding in these target tissues were established by competition with an excess of unlabeled hormones. In the presence of an excess of unlabeled progesterone the nuclear concentration of radioactivity was diminished in all areas, while no inhibition of radioactivity incorporation was observed with unlabeled corticosterone. These results suggest the presence of specific progesterone receptors in the hypothalamus, the pituitary and the genital tract tissues of the rat.