Seasonal Regulation and Functional Significance of GnIH in Goldfish

Abstract

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide named for its ability to inhibit gonadotropin (GTH) production in birds and mammals. Subsequently, variants of GnIH peptides have been discovered in different vertebrate species, including teleosts. In amphibians, GnIH was found to stimulate growth hormone (GH) release, suggesting a role in the regulation of somatic growth. These findings indicate that GnIH is an important component of the multifactorial control of reproduction and growth. However, information on physiological significance of GnIH is rather limited, and much less is known regarding the functions of these peptides in teleosts. The main purpose of my thesis was to study the physiological role of goldfish GnIH (gGnIH) in the regulation of basal and native gonadotropin-releasing hormone-induced GTH and GH expression and secretion, in vivo and in vitro. The findings highlight the importance of seasonality and stage of gonadal recrudescence, and provide evidence for both direct and indirect actions of gGnIH, which resulted in both stimulation and inhibition of GTH and GH production. In goldfish, gGnIH can uncouple GTH and GH synthesis and release, as well as differentially regulate FSH- and LH-β mRNA expression in a seasonally-dependent manner. Additionally, the presence of gGnIH and gGnIH receptor (gGnIH-R) in the goldfish ovary and testis was the basis for studying its direct action on cultured gonads. Treatment with gGnIH altered the expression of genes involved in gonadal functions, providing a strong support for the hypothesis that gGnIH plays a role in the paracrine/autocrine regulation of gonadal functions in goldfish. The findings in this thesis indicate that gGnIH plays a complex role in multifactorial control of growth and reproduction in goldfish, as compared to a dominant inhibitory role in the regulation of the gonadotropic axis as observed in higher vertebrates. Furthermore, the results presented provide novel information on the physiological functions of gGnIH in goldfish at the level of multiple target tissues, and provide a framework for understanding the role of gGnIH in the multifactorial control of reproduction and growth in teleosts.