Many hormones that are classified as neuropeptides are synthesized in vertebrate gonads in addition to the brain. Receptors for these hormones are also expressed in gonadal tissue; thus there is potential for a highly localized autocrine or paracrine effect of these hormones on a variety of gonadal functions. In the present study we focused on gonadotropin-inhibitory hormone (GnIH), a neuropeptide that was first discovered in the hypothalamus of birds. We present different lines of evidence for the synthesis of GnIH and its receptor in the avian reproductive system including gonads and accessory reproductive organs by studies on two orders of birds: Passeriformes and Galliformes. Binding sites for GnIH were initially identified via in vivo and in vitro receptor fluorography, and were localized in ovarian granulosa cells along with the interstitial layer and seminiferous tubules of the testis. Furthermore, species-specific primers produced clear PCR products of GnIH and GnIH receptor (GnIH-R) in songbird and quail gonadal and other reproductive tissues, such as oviduct, epididymis and vas deferens. Sequencing of the PCR products confirmed their identities. Immunocytochemistry detected GnIH peptide in ovarian thecal and granulosa cells, testicular interstitial cells and germ cells and pseudostratified columnar epithelial cells in the epididymis. In situ hybridization of GnIH-R mRNA in testes produced a strong reaction product which was localized to the germ cells and interstitium. In the epididymis, the product was also localized in the pseudostratified columnar epithelial cells. In sum, these results indicate that the avian reproductive system has the capability to synthesize and bind GnIH in several tissues. The distribution of GnIH and its receptor suggest a potential for autocrine/paracrine regulation of gonadal steroid production and germ cell differentiation and maturation.
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