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n all vertebrates, gonadotropin releasing hormone (GnRH) is a key component of the hypothalamo-pituitary-gonadal axis. In birds, two distinct forms of GnRH (cGnRH-I and cGnRH-II) have been characterized, and a third immunoreactive form similar to lamprey GnRH-III has been reported. Each GnRH is produced by a different population of neurons. cGnRH-I is predominantly synthesized in cells located in the preoptic and septal areas, whereas cGnRH-II and -III are produced in neurons located mainly in the mesencephalon and around the third ventricle, respectively. Although GnRH neurons have been shown to project fibers to the median eminence, it is still unclear whether all three peptides are released in the hypothalamo-pituitary portal vascular system. Nonetheless, in vitro as well as in vivo experiments confirmed that they all have the ability to induce the release of gonadotropins. In the anterior pituitary gland, GnRHs mediate their effects by binding to specific G-coupled protein receptors, and in chickens, two different receptors have been characterized (cGnRHR-I and cGnRHR-II). In addition to the full-length receptors, several splice variants have also been detected. Although the physiological relevance of these variants is not known, they may participate in the regulation of intracellular signaling. While the expression of the full-length cGnRHR-I appears to fluctuate during an ovulatory cycle, levels remain constant between reproductive stages in chickens. In contrast, pituitary levels of cGnRHR-II mRNA are directly correlated with the reproductive status of the bird. Whether these two GnRHRs are expressed by the same cells in the anterior pituitary gland is not known. However, since luteinizing hormone (LH) and follicle stimulating hormone (FSH) are produced by two different cell populations in chickens, it is possible that different combinations of GnRHs and GnRHRs differentially mediate the synthesis and release of these gonadotropins

Gonadotropin Releasing Hormones and their Receptors in Avian Species