Progestin Receptors in Rat Brain: Distribution and Properties of Cytoplasmic Progestin-Binding Sites*

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Abstract
Putative progestin receptors have been characterized in brain and pituitary tissue from untreated and estrogenprimed ovariectomized-adrenalectomized rats. The properties of these sites appear indistinguishable from those of cytoplasmic progestin receptors from the uterus: 1) sedimentation coefficient of 7S, which is reduced by half in the presence of 0.3 m KC1; 2) specificity of binding which strongly favors synthetic and natural progestins as opposed to glucocorticoids, androgens, and estrogens; 3) a dissociation constant for binding the synthetic progestin [3H]R5020 (17α,21-dimethyl-19-norpregna-4, 9-diene-3,20- dione) of 0.3 nm; and 4) similar rates of formation and dissociation of the [3H]R5020-receptor complexes. In these respects, the estrogen-inducible and noninducible receptors of the brain also appear to be indistinguishable from each other. Estrogen induction of progestin receptors is apparent in uterus (6-fold), pituitary (8-fold), mediobasal hypothalamus (4-fold), and preoptic area (4-fold), all estrogen receptor-containing areas. The corticomedial amygdala does not show an estrogen effect on progestin receptor levels even though it contains estrogen receptor sites. The midbrain of the rat does not show an estrogen induction of progestin receptors. The concentration of estrogen-insensitive receptors in the brain is relatively low and of the same order of magnitude as in nonstimulated hypothalamus, preoptic area, and pituitary, yet variations are seen among the estrogen-insensitive structures, with lowest levels occurring in cerebellum (6–7 fmol/mg protein) and highest levels occurring in cerebral cortex (≈25 fmol/mg protein). These findings are discussed in relation to the actions of progesterone which do and do not require estrogen priming and in relation to intracranial progesterone implantation studies. (Endocrinology106: 192, 1980)