Role of Gonadotropin-Releasing Hormone in Regulation of Gonadotropin Secretion from Amphibian and Reptilian Pituitaries

Abstract

The ubiquity of a brain peptide involved in regulating pituitary gonadotropin (Gn) secretion in vertebrates has been confirmed by a variety of direct and indirect studies. 57 This gonadotropin-releasing hormone (GnRH) [also known as luteinizing hormone-releasing hormone (LHRH)] has been extensively characterized biochemically and physiologically in mammals , but data for nonmammalian species are still very limited. Indeed, our use of the term GnRH, unless specified otherwise, will refer to the mammalian decapeptide: Considerable information has been amassed on GnRH physiology of amphibians in recent years, but the reptiles probably represent the least understood of the tetrapods. The difficulty in measuring directly the levels of Gns in nonmammalian species has certainly contributed to "this problem. However, it is now established that a pair of gonadotropins, similar to the follicle-stimulating hormone (FSH) and luteinizing hormone (LH) of mammals, occurs in at least two orders of amphibians and in croco-dilian and chelonian reptiles; squamates (lizards and snakes) may have only a single Gn whose homologies to FSH and LH remain uncertain (see review32). The present review focuses on recent evidence pertaining to whether and how the synthesis and secretion of these pituitary Gns are regulated by GnRH-like materials in the brain; the chemical nature of GnRHs, and how they modulate secretion of individual pituitary Gns are also considered.