CHARACTERIZATION OF PROGESTERONE-3-[I-125-BSA] BINDING-SITES IN THE MEDIAL PREOPTIC AREA AND ANTERIOR HYPOTHALAMUS

In this study we utilized radiolabeled progesterone (P) conjugated to bovine serum albumin (BSA) at position 3 (P-3-[I-125-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-[I-125-BSA] was linear across a tissue concent ration range of 0.005 to 0.02 mg protein/0.1 mi 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-[I-125-BSA] at 0 degrees C. Analysis of data from competition binding experiments using P-3-BSA re vealed high- and low-affinity binding sites in the MPOA-AH. Involvemen t of MPOA-AH binding sites with a G-protein was suggested by a reducti on of P-3-[I-125-BSA] binding in the presence of the non-hydrolyzable GTP analog GTP gamma S but not ATP gamma S. In addition, if homogenate s from the MPOA-AH were preincubated with 10(-5) M of the G-protein an tagonist cholera toxin for 30 min at 37 degrees C, competition binding data indicated only high-affinity binding sites. Once daily injection s of OVXed rats with 4 mg P for 12 days significantly increased the de nsity of P-3-[I-125-BSA] binding sites in the MPOA-AH. This treatment did not affect P-3-[I-125-BSA] binding in the dorsal tectum, medial ba sal hypothalamus, ventral tegmental area or the thymus.