Immunoreactive Gonadotropin-Releasing Hormone (GnRH) Is Detected Only in the Form of Chicken GnRH-II within the Brain of the Green Anole,Anolis carolinensis

Abstract

The presence of multiple forms of gonadotropin-releasing hormone (GnRH) within a single brain is common among vertebrate species. In previous studies of reptiles, two forms of GnRH were isolated from the brain of alligators and the primary structure was determined to be that of chicken (c)GnRH-I and cGnRH-II. GnRH has also been detected by indirect methods in other reptiles including turtles, lizards, and snakes. We used a combination of high-performance liquid chromatography (HPLC) and radioimmunoassay to determine the number and molecular form(s) of GnRH in the brain of a lizard,Anolis carolinensis,that was reported to lack GnRH cells in the forebrain. Immunoreactivity was detected in the same HPLC elution position in which synthetic cGnRH-II elutes, but not in any other position. Detection was based on five antisera that among them detect the 12 known forms of GnRH; these antisera include ones that are specific to cGnRH-I and cGnRH-II. We conclude that the lizardA. carolinensiscontains cGnRH-II, but not cGnRH-I or another known form of GnRH. These data, coupled with our earlier immunocytochemical study, suggest that the lizard studied here lacks cGnRH-I, the form that is found in the terminal nerve, olfactory bulb, and forebrain in nonsquamate reptiles and in birds. Our hypothesis is that the presence of both cGnRH-I and cGnRH-II in the brain is ancestral in the reptilian lineage and retained in the orders that include turtles (Chelonia) or alligators (Crocodilia). However, the pattern in the order Squamata varies: inA. carolinensis,only cGnRH-II is present in the brain and cGnRH-I is absent, whereas in the snakeThamnophilis sirtalis,cGnRH-I is retained and cGnRH-II is absent in the brain, as recently reported. This raises the question of how reproduction is controlled in reptiles that lack one form of GnRH.