Progestin binding in vitro by the brain cell nuclei of ovariectomized oestrogen-primed rats

The uptake and binding of 17 alpha, 21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione, a synthetic progestin, by the hypothalamus and cerebral cortex of ovariectomized oestrogen-primed rats was examined in vitro. Uptake of this steroid by the medial basal hypothalamus was higher than that by the remaining hypothalamus and cerebral cortex. The component in the cytosol from whole hypothalami which bound the radioactive progestin sedimented in the 7S region when centrifuged in a sucrose density gradient. The tritiated progestin was displaced by incubation with non-radioactive progestin or progesterone but not by oestradiol-17 beta, corticosterone or 5 alpha-dihydrotestosterone (1 mumol/l). No 7S binding component was detected in a similar preparation from the cerebral cortex. The nuclear fraction from whole hypothalami extracted by KCl (0.4 mol/l) contained a progestin-binding complex which sedimented at 9S and which was heat-labile and protein in nature. It was concluded that the hypothalamus of ovariectomized oestrogen-primed rats contains progestin-binding material in the cytoplasm and progestin, bound to such material, is transported from the cytoplasm to the nucleus.