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Oestrogen‐Insensitive Progestin Receptors in the Central Nervous System: Physicochemical and Immunoreactive Characteristics

The intracellular effects of progesterone in the central nervous system are exerted via two distinct receptors: The classical oestrogen‐regulated progestin receptor and the non‐oestrogen‐inducible progestin receptor. To assess whether the oestrogen‐insensitive receptor is related to the oestrogen‐dependent receptor or whether it is a different binding macromolecule, its physicochemical and immunoreactive characteristics (immunoprecipitation with polyclonal anti‐uterine progesterone receptor antisera) were studied in neural tissues of the female rat. The results disclosed that the dissociation constant (Kd, 0.4–0.5 ± 10−9 M), stereospecificity and sedimentation coefficient (7–8 S) in linear sucrose gradients of the oestrogen‐insensitive progestin receptor were identical to those reported for the oestrogen‐regulated progestin receptor, although its saturation binding capacity was significantly lower. Results of in vitro nuclear acceptor assays revealed that the progestin receptor complexes from cerebellum and cerebral cortex, were able to specifically bind to cell nuclei preparations in a fashion similar to that observed with the uterine progestin receptor, although to a lesser extent. Interestingly, a similar nuclear uptake of receptor complexes was noticed when standardized cerebellum and uterus cytosol preparations with a similar receptor content were used. The anti‐uterine progesterone receptor immunoglobulins used in the immunoprecipitation studies were able to specifically recognize the progestin receptor populations of the anterior pituitary and hypothalamus (oestrogen‐regulated receptors) as well as those of the cerebellum and cerebral cortex (oestrogen‐insensitive receptors). The results presented show that both the oestrogen‐sensitive and the oestrogen‐insensitive cytosol progestin receptors in brain bind the same progesterone‐like molecules used as radioligands and also react with the same antibody when tested in an immunoprecipitation radioassay. The striking similarities found in binding kinetics, physicochemical characteristics and immunoreactive behaviour in the two progestin receptors studied demonstrated that both macromolecules belong to the same family of proteins in spite of their different sensitivity to oestrogens. The overall data seem to suggest a common origin of the two progestin receptor populations in brain but with different mechanisms of hormonal regulation.