The GnRH system in teleosts

Abstract

The decapeptide gonadotropin-releasing hormone (GnRH) was isolated originally from the mammalian hypothalamus and was named for its role as a stimulator of gonadotropin release from the pituitary gland. Multiple molecular forms of GnRH have been found in the brain of individual teleost species. However, the exact number of forms and their respective physiological roles are still uncertain. Our isolation and comparison of GnRH cDNAs in three teleosts, the eel, arowana, and medaka, led to the idea that duplications of an ancestral GnRH gene had occurred to give rise to three paralogous forms predating the emergence of teleosts, and thus all teleosts may possess three GnRH forms. Also, three GnRHs were shown to be expressed from different loci within the medaka brain, supporting the concept that each GnRH has a specific physiological role. In addition, we demonstrated the occurrence of two distinct GnRH receptor (GnRH-R) subtypes in the medaka; these subtypes exhibited remarkably different primary structures, gene organizations, and ligand selectivities. As both these receptors were expressed in the brain and pituitary, however, the difference in their respective physiological function remains uncertain. Intriguingly, although only two GnRH-R subtypes have been found in the teleosts to date, structural and pharmacological analysis does not rule out the possibility that there may be a third subtype of GnRH-R in teleost.