Gonadotropin-inhibitory hormone (GnlH): Biosynthesis, mode of action and functional significance in birds

Probing undiscovered hypothalamic neuropeptides that play important roles in the regulation of pituitary function in birds is essential for the progress of avian neuroendocrinology. The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion. Gonadal sex steroids and inhibin inhibit gonadotropin secretion via feedback from the gonads, but a neuropeptide inhibitor of gonadotropin secretion was, until recently, unknown in birds as well as other vertebrates. In 2000, Tsutsui and co-workers discovered a novel hypothalamic dodecapeptide that inhibits gonadotropin release in quail and termed it gonadotropin- inhibitory hormone (GnlH). This was the first demonstration of a hypothalamic neuropeptide inhibiting gonadotropin release in any vertebrate. From the past 8 years of research, we now know that GnlH exists in several avian species and acts as a new key neurohormone for the regulation of avian reproduction. GnlH acts on the pituitary and GnRH neurons in the hypothalamus via a novel G protein-coupled receptor for GnlH to inhibit gonadal development and maintenance by decreasing gonadotropin release and synthesis. GnlH neurons express melatonin receptor and melatonin stimulates the expression of GnlH. Thus, GnlH is capable of transducing photoperiodic information via changes in the melatonin signal, thereby influencing the reproductive axis. This review summarises the advances made in our understanding of the biosynthesis, mode of action and functional significance of GnlH in birds.