Gonadotropin-Releasing Hormones in Fishes

Abstract

Most vertebrates appear to synthesize in their brains a factor(s) which controls certain aspects of reproduction. Of the vertebrates studied to date, only the primitive hagfishes appear to lack this factor, gonadotropin-releasing hormone (GnRH). The most studied, but not the sole, function of GnRH is stimulation of the synthesis and release of pituitary gonadotropin(s) which subsequently affects the gonads. In this role GnRH is released into the brain-pituitary portal vessels in tetrapods, but in fishes GnRH is released in various ways from nerve terminals: near the pituitary gonadotropic cells, near connective tissue layers proximal to the anterior pituitary, or into a primitive portal system (see Bal15). The primary structure of four GnRH molecules is known. In each case, GnRH is a peptide with 10 amino acids and common Nand C terminal residues. Considerable homology exists among these molecules, suggesting the GnRH peptides in different vertebrates are members of a phylogenetic family.