Supplemental Progesterone Delays Heat Termination and the Loss of Progestin Receptors from Hypothalamic Cell Nuclei in Female Guinea Pigs

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Abstract
Previous studies indicate that the retention of hypothalamic nuclear progestin receptors parallels the expression of sexual receptivity in ovariectomized, hormonally treated guinea pigs. In this study, the effect of a supplementai progesterone injection on heat termination and retention of nuclear progestin receptors was examined. Ovariectomized guinea pigs were injected with 50 µg progesterone 48 h after receiving 4 µg estradiol benzoate and were tested hourly for lordosis. 8 h after progesterone treatment, animals received either 500 µg progesterone or the oil vehicle. The supplementai progesterone injection delayed the termination of heat by more than 2 h compared to oil-injected animals. The effect of supplemental progesterone on the retention of nuclear progestin receptors was then determined. Animals were killed 8, 10, or 14 h after the initial injection of progesterone, and cytosol and nuclear progestin receptor concentrations in mediobasai hypothalamus-preoptic area were determined. Supplemental progesterone further increased the level of nuclear progestin receptors measured at 10 h, and at 14 h these levels were still elevated above baseline. In contrast, nuclear progestin receptor levels in animals which received only the initial progesterone injection had returned to baseline by 14 h, a time when similarly treated animals were no longer receptive. These results suggest that the mechanism by which heat termination occurs may involve loss of nuclear progestin receptors.