MEMBRANE-RECEPTORS FOR ESTROGEN, PROGESTERONE, AND TESTOSTERONE IN THE RAT-BRAIN - FANTASY OR REALITY

1. There are numerous circumstantial evidence supporting the concept t hat steroid hormones control cellular function by means other than the nuclear receptor steroid binding mechanism. It is the intent of this report to present evidence indicating that steroids bind to specific s ites in neuronal membranes. 2. Some of the criteria to define steroid membrane receptors using steroid-BSA conjugates that can be radioiodin ated to desired specific activity have been fulfilled for each of the three sex steroids using crude synaptosomal membrane preparations (P2 fractions) from the CNS of female and male rats. Ligand binding for ea ch of the three steroids indicate high-affinity and high-capacity site s with distinct brain selectivity and stereospecificity. For example, 17 beta-E-6-[I-125]BSA binds hypothalamic P2 fractions (HYP-P2) with a n estimated K-d of about 3 +/- 0.7 nM (X +/- SE; n = 3), whereas the c erebellum P2 (CB-P2) fractions bind the ligand with a K-d of 34 +/- 7 nM and, a B-max of 3 and 42 pmol/mg protein, respectively, Estrogen an d testosterone binding fit best a one-single site, while progesterone binding sites can be best represented by a two-binding site, one high- affinity (K-d = 1-2 nM) and one low affinity (K-d = 62 nM), in CB-P2 f ractions from intact adult female rat brain. Kinetics studies for T-3- [I-125]BSA indicate that the estimated K-d of 30 +/- 2 nM for the olfa ctory bulb P2 fractions (OB-P2) from male rats is in good agreement wi th K-d values computed from Scatchard-derived data using the LIGAND al gorithm. 3. 17 beta-E-6-[I-125]BSA binding sites are stereospecific an d appears to be present as early as 5 days of age in both the OB- and the CB-P2 fractions without changes during development. In contrast, P -6-[I-125]BSA binding sites are practically absent during days 5 and 1 2 and appear by day 22. 4. Finally, membrane receptor. molecules for e strogen and progesterone have been isolated and purified by affinity c hromatography and characterized by PAGE and Western blot. Microsequenc ing of one of the membrane estrogen binding proteins indicates that th e high-affinity site: corresponds to the OSCP subunit of the proton AT P synthase. 5. It remains to be determined if P and T also bind to thi s complex enzyme or if they bind to other subunits of the family of pr oton ATPases. Overall the data indicate that steroid hormones conjugat ed to BSA are important tools to study the ''reality of membrane stero id receptors.''