Review The zebrafish as a model system for forebrain GnRH neuronal development Eytan Abraham a , Ori Palevitch b , Yoav Gothilf b , Yonathan Zohar a, * a Center of Marine Biotechnology, University of Maryland Biotechnology Institute, Baltimore, MD, USA b Faculty of Life Sciences, Tel-Aviv University, Tel-Aviv, Israel article info Article history: Received 8 October 2008 Revised 31 December 2008 Accepted 20 January 2009 Available online 31 January 2009 Keywords: Zebrafish GnRH Development Transgenic CNS abstract Development and function of the forebrain gonadotropin-releasing hormone (GnRH) neuronal system has long been the focus of study in various vertebrate species. This system is crucial for reproduction and an important model for studying tangential neuronal migration. In addition, the finding that multiple forms of GnRH exist in the CNS as well as in non-CNS tissues, coupled with the fact that GnRH fibers pro- ject to many CNS regions, implies that GnRH has a variety of functions in addition to its classic reproduc- tive role. The study of the GnRH system and its functions is, however, limited by available model systems and methodologies. The transgenic (Tg) GnRH3:EGFP zebrafish line, in which GnRH3 neurons express EGFP, allows in vivo study of the GnRH3 system in the context of the entire animal. Coupling the use of this line with the attributes and molecular tools available in zebrafish has expanded our ability to study the forebrain GnRH system. Herein, we discuss the use of the Tg(GnRH3:EGFP) zebrafish line as a model for studying forebrain GnRH neurons, both in developing larvae and in sexually mature animals. We also discuss the potential use of this line to study regulation of GnRH3 system development. Published by Elsevier Inc. 1. Introduction GnRH is a decapeptide neurohormone that is crucial for reproduction. To date, 15 variants of GnRH have been described in vertebrates (Lethimonier et al., 2004; Kavanaugh et al., 2008). All vertebrate species studied have two or three paralogous forms of GnRH. The hypophysiotropic GnRH neurons are located primarily in the preoptic area (POA)–hypothalamus, but in most species are spread in a loose continuum from the olfactory re- gion to the POA–hpothalamus (Campbell, 2007). Axons extend from the POA–hypothalamic GnRH neurons to innervate the hypophyseal blood portal system in higher vertebrates or pitui- tary in fish. GnRH decapeptide acts in the pituitary to induce the biosynthesis and release of gonadotropins from the adenohy- pophysis into the blood stream. The midbrain GnRH neuron pop- ulation is located in the midbrain tegmentum (Herbison, 2006; Kah et al., 2007). Over the years, several GnRH nomenclature schemes have been developed. The recent consensus is the GnRH1/2/3 termi- nology (White et al., 1998). However this terminology is diver- gently used, either based on GnRH function/location, or based on GnRH gene phylogeny. The function/location methodology may however be challenging to use given that additional forms of GnRH will probably be found and the fact that not all func- tions of each GnRH are known. Herein the phylogenetic-based GnRH1/2/3 terminology is used. According to this nomenclature, the zebrafish hypothalamic form is GnRH3. However in sea bream (Sparus aurata), for example, the hypophysiotropic form is GnRH1, and the terminal nerve (TN) form is GnRH3 (Lethim- onier et al., 2004; Kuo et al., 2005). In mammals, the hypophys- iotropic form is GnRH1. Recently, a fourth branch of the GnRH phylogenetic tree has been proposed, this branch includes lam- prey GnRHs (Kavanaugh et al., 2008). In teleost species that have two forms of GnRH, the hypotha- lamic form is located in the olfactory bulbs (OB)–TN and POA– hypothalamus (Amano et al., 1991; Steven et al., 2003; Lethim- onier et al., 2004; Abraham et al., 2008). In these species, GnRH3 has been shown to elicit gonadotropin-releasing activity and is thought to assimilate non-redundant functions of GnRH1 (Amano et al., 1995; Okubo and Nagahama, 2008). In fish spe- cies that have three forms of GnRH, the third form is located primarily in the olfactory and terminal nerve (TN) regions, although some overlap between the two forebrain forms exists (Gonzalez-Martinez et al., 2004; Okubo et al., 2006). GnRH neu- rons, either GnRH3, GnRH1, or both, depending on the species, extend a complex network of fibers that innervates many CNS regions. The multiple forms and locations of GnRH in the CNS as well as in extra-CNS regions, and the extensive innervation of the CNS by GnRH neurons, suggests that GnRH plays numerous roles through- out development and maturity. The classical view of GnRH as a gonadotropin-releasing hormone continues expanding to encom- pass additional roles. 0016-6480/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.ygcen.2009.01.012 * Corresponding author. E-mail address: zohar@umbi.umd.edu (Y. Zohar). General and Comparative Endocrinology 164 (2009) 151–160 Contents lists available at ScienceDirect General and Comparative Endocrinology journal homepage: www.elsevier.com/locate/ygcen