A specific membrane binding protein for progesterone in rat brain: sex differences and induction by estrogen

Progesterone conjugated to bovine serum albumin (BSA) was used as a probe to study sex differences and the effects of hormonal status on binding of progesterone to crude synaptosomal membrane preparations (P2) derived from the mediobasal hypothalamic-anterior hypothalamic-preoptic area or the corpus striatum. Binding of 125I-labeled BSA linked to progesterone at the 11 position of the steroid (P-11-BSA) was decreased by competition with unlabeled P-11-BSA or P-3-BSA (in which progesterone is bound to BSA at the 3 position). P-3-BSA displayed higher affinity than P-11-BSA. Hypothalamic and striatal preparations from adult females show high specific binding (60-80%) to the progesterone-BSA conjugate. Specific binding was reduced more than 80% 14 days after ovariectomy. Estrogen treatment (10 micrograms per rat for 4 days) of 14-day ovariectomized rats restored specific binding to levels equivalent to intact females. In contrast, adult males displayed drastically reduced or no specific binding in either tissue. No specific binding was detected after orchidectomy. Estrogen treatment of orchidectomized animals induced specific binding sites similar to those in intact females. Additionally, an affinity probe was developed by linking primary amines on the P-3-BSA conjugate to agarose activated aldehydes in an AminoLink column. A digitoxin-solubilized fraction from female rat P2 cerebellum preparations yielded a single major band after affinity purification with an estimated molecular mass of 40-50 kDa in an SDS/PAGE system after silver stain. These results show a reversible sex difference in the specific binding of progesterone to synaptosomal membrane sites in the central nervous system of male and female rats which is dependent on estrogen.