Characterization of progesterone-3-[ 125I-BSA] binding sites in the medial preoptic area and anterior hypothalamus

Abstract

In this study we utilized radiolabeled progesterone (P) conjugated to bovine serum albumin (BSA) at position 3 (P-3-[ 125I-BSA]) to examine steroid receptors in membrane fractions from the medial preoptic area-anterior hypothalamus (MPOA-AH) of ovariectomized (OVXed) rats. In the MPOA-AH binding of P-3-[ 125I-BSA] was linear across a tissue concentration range of 0.005 to 0.02 mg protein/0.1 ml of membrane suspension. Kinetic experiments revealed an association t 1/2 of 51.4 min and a dissociation t 1/2 of 122.5 min for P-3-[ 125I-BSA] at 0°C. Analysis of data from competition binding experiments using P-3-BSA revealed high- and low-affinity binding sites in the MPOA-AH. Involvement of MPOA-AH binding sites with a G-protein was suggested by a reduction of P-3-[ 125I-BSA] binding in the presence of the non-hydrolyzable GTP analog GTPγS but not ATPγS. In addition, if homogenates from the MPOA-AH were preincubated with 10 −5 M of the G-protein antagonist cholera toxin for 30 min at 37°C, competition binding data indicated only high-affinity binding sites. Once daily injections of OVXed rats with 4 mg P for 12 days significantly increased the density of P-3-[ 125I-BSA] binding sites in the MPOA-AH. This treatment did not affect P-3-[ 125I-BSA] binding in the dorsal tectum, medial basal hypothalamus, ventral tegmental area or the thymus.