Localization of the three GnRH types and GnRH receptors in the brain of a cichlid fish: Insights into their neuroendocrine and neuromodulator functions

The cognate receptor for any of the known gonadotropin-releasing hormones (GnRHs) has not been directly demonstrated. In order to establish this and shed light on the functions of GnRH types, we analyzed the neuroanatomical location and time of initial expression of three distinct GnRH receptors (GnRH-Rs) and the three endogenous GnRHs in the brain of developing and sexually mature tilapia Oreochromis niloticus using immunocytochemistry. In all age groups, including males and females, GnRH-RlA was seen specifically in gonadotropes (Parhar et al. [2002] J Neuroendocrinol 14:657-665) but was undetectable in the brain. On day 8 after fertilization, GnRH-RIB was first seen in the periventricular hypothalamus (lateral recess nucleus, posterior recess nucleus, posterior tuberal nucleus) and GnRH-RIII in the olfactory epithelium, olfactory bulb, telencephalon, preoptic region, mediobasal hypothalamus, thalamus, mesencephalon, and in the hindbrain. Double-label immunocytochemistry showed GnRH1 (Ser(8) GnRH)-immunoreactive neuronal processes projecting mainly to the proximal pars distalis of the pituitary, while GnRH2 (His(5), Trp(7), Tyr(8) GnRH) and GnRH3 (Trp(7), Leu(8) GnRH) fibers were observed in close association with cells containing GnRH-RIB and GnRH-RIII in the brain. These results suggest that GnRH-RIA might be hypophysiotropic in nature, whereas GnRH-RIB and GnRH-RIII could have additional neuromodulatory functions. Further, evidence of close proximity of GnRH-R-containing cells and neuronal processes of multiple GnRH types suggests complex cross-talk between several GnRH ligands and GnRH-Rs. J. Comp. Neurol. 487:28-41, 2005. (c) 2005 Wiley-Liss, Inc.