Progesterone Receptor in the Hypothalamic Cytosol of Female Rats*

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Abstract
A progesterone (P)-binding component with a sedimentation coefficient of 8S has been demonstrated in hypothalamic cytosol from ovariectomized, estrogen-primed rats using both [3H]P-containing sucrose-glycerol gradient analysis and dextran-coated charcoal adsorption of the preisolated 8S component. The association rate constant (K+1) was determined to be 1.90 ± 0.38 (SD) × 106 M-1 min-1 at 0–2 C. The dissociation rate constant (K-1) was 1.86 × 10-2 min”1, as calculated from the half-dissociation time [37.0 ± 7.3 (SD) min]. The apparent dissociation constant (Kd) at 0–2 C was determined to be 6–10 nM by Scatchard plot analysis of data obtained from either direct [3H]P binding or competition of the [3H]P binding by nonradioactive P and by calculating from K-1/K+1. The 8S binding component was protein in nature, and the concentration of binding sites was 12 fmol/mg cytosol protein. On a per U cytos6l protein basis, the relative capacities of the specific 8S binding components were: uterus » pituitary > hypothalamus > hippocampus/ amygdala > cerebral cortex. Competition studies showed a high specificity for P and 5α-dihydroprogesterone. Corticosterone (C), although competing for the binding, had an affinity 8-fold less than P. Implantation of C in adrenalectomized, ovariectomized, estrogen-implanted rats suppressed the 8S binding of [3H]C without affecting the [3H]P binding. The binding of [3H]P to the cytoplasmic 8S component of hypothalamus was greater than that of combined hippocampus and amygdala, while the reverse was observed for the binding of [3H]dexamethasone. These results demonstrate in rat hypothalamic cytosol a tissueand hormone-specific, high affinity, 8S progesterone-binding protein which has many of the properties expected of a hormone receptor.