INTRACELLULAR PROGESTERONE RECEPTORS ARE DIFFERENTIALLY REGULATED BY SEX STEROID-HORMONES IN THE HYPOTHALAMUS AND THE CEREBRAL-CORTEX OF THE RABBIT

The aim of this study was to examine the role of sex steroid hormones in the regulation of intracellular progesterone receptors (PR) in the rabbit central nervous system. We determined PR concentration in cytos ol preparations from the hypothalamus, the frontal, tempo-parietal and occipital cortex, by using the specific binding of the synthetic prog estin [H-3]ORG 2058. PR concentration was higher in the hypothalamus o f intact adult females than in that of adult males and prepubertal fem ales, whereas no significant differences were observed in the cerebral cortex of these animals. PR concentration was similar in the three co rtical regions analyzed, indicating a homogeneous distribution of PR i n the cerebral cortex. The administration of estradiol to ovariectomiz ed animals increased PR concentration in the hypothalamus but not in t he cortex. The administration of progesterone to ovariectomized rabbit s did not modify PR concentration in any region, however when progeste rone was administered after estradiol, it induced a significant diminu tion in hypothalamic PR concentration without effects in the cortex. T hese findings suggest that in the rabbit, PR are estrogen regulated in the hypothalamus but not in the cerebral cortex. In the latter, PR ar e not regulated by progesterone, whereas in the former the estrogen-in duced PR are down-regulated by progesterone. Interestingly, hypothalam ic PR constitutively expressed in ovariectomized animals are progester one-insensitive.