The gastrin-releasing peptide (GRP) system in the lumbosacral spinal cord controls male sexual function in rats. In contrast, in female rats, GRP neurons could scarcely be detected around puberty when circulating ovarian steroid hormones such as estradiol and progesterone levels are increasing. However, little information is available on feminizing or demasculinizing effects of ovarian steroids on the central nervous system in female puberty and adulthood. In this study, to visualize the spinal GRP neurons in vivo, we generated a GRP-promoter-Venus transgenic (Tg) rat line and studied the effects of the sex steroid hormones on GRP expression in the rat lumbar cord by examining the Venus fluorescence. In these Tg rats, the sexually dimorphic spinal GRP neurons controlling male sexual function were clearly labeled with Venus fluorescence. As expected, Venus fluorescence in the male lumbar cord was markedly decreased after castration and restored by chronic androgen replacement. Furthermore, androgen-induced Venus expression in the spinal cord of adult Tg males was significantly attenuated by chronic treatment with progesterone but not with estradiol. A luciferase assay using a human GRP-promoter construct showed that androgens enhance the spinal GRP system, and more strikingly, that progesterone acts to inhibit the GRP system via an androgen receptor–mediated mechanism. These results demonstrate that circulating androgens may play an important role in the spinal GRP system controlling male sexual function not only in rats but also in humans and that progesterone could be an important feminizing factor in the spinal GRP system in females during pubertal development.
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