The role of Kisspeptin-GPR54 signaling in the tonic regulation and surge release of gonadotropin-releasing hormone/luteinizing hormone

The Kiss1 gene codes for kisspeptin, which binds to GPR54, a G-protein-coupled receptor. Kisspeptin and GPR54 are expressed in discrete regions of the forebrain, and they have been implicated in the neuroendocrine regulation of reproduction. Kiss1-expressing neurons are thought to regulate the secretion of gonadotropin-releasing hormone ( GnRH) and thus coordinate the estrous cycle in rodents; however, the precise role of kisspeptin-GPR54 signaling in the regulation of gonadotropin secretion is unknown. In this study, we used female mice with deletions in the GPR54 gene [GPR54 knock-outs ( KOs)] to test the hypothesis that kisspeptin-GPR54 signaling provides the drive necessary for tonic GnRH/luteinizing hormone ( LH) release. We predicted that tonic GnRH/LH secretion would be disrupted in GPR54 KOs and that such animals would be incapable of showing a compensatory rise in LH secretion after ovariectomy. As predicted, we found that GPR54 KO mice do not exhibit a postovariectomy rise in LH, suggesting that tonic GnRH secretion is disrupted in the absence of kisspeptin-GPR54 signaling. We also postulated that kisspeptin-GPR54 signaling is critical for the generation of the estradiol ( E)induced GnRH/LH surge and thus E should be incapable of inducing an LH surge in the absence of GPR54. However, we found that E induced Fos expression in GnRH neurons and produced a GnRH-dependent LH surge in GPR54 KOs. Thus, in mice, kisspeptin-GPR54 signaling is required for the tonic stimulation of GnRH/LH secretion but is not required for generating the E-induced GnRH/LH surge.