Neuronal correlates of sex/role change in labrid fishes: LHRH-like immunoreactivity.

Gonadotropins have been implicated in the proximate control of socially induced sex and role change in labrid fishes. Since the release of gonadotropins is controlled by Gonadotropin Releasing Hormones, immunocytochemical techniques were used here to determine the qualitative and quantitative distribution of Luteinizing Hormone Releasing Hormone (LHRH, the primary Gonadotropin Releasing Hormone in mammals) in the brain of a sex-reversing labrid fish, Thalassoma bifasciatum. Fibers showing LHRH-like immunoreactivity were distributed throughout the brain, with densest concentrations in the olfactory bulb, the telencephalic area dorsalis and preoptic area of the forebrain, the medial posterior tuber and nucleus of the saccus vasculosus of the hypothalamus, and the tectum and tegmentum of the midbrain. Somata positive for LHRH were found in the anterior and posterior divisions of the ganglion of the nervus terminalis, the preoptic area, and the periventricular rostral midbrain. There were no qualitative differences in the distribution of LHRH-like immunoreactive cells and fibers among the different sexual phases. However, there were quantitative differences in the number of LHRH-like immunoreactive cells in the preoptic area, a brain region known to be involved both in sexual development and in the control of sexual physiology and behavior in adults. Based on this, and results in other teleosts, it seems likely that central control of sex reversal and expression of alternative reproductive behaviors in labrid fishes is more directly related to quantitative changes in the number and/or biosynthetic activity of LHRH-positive somata and their fiber projections.