Characterization of the kisspeptin systems in the brain of european sea bass (D. labrax): Relationships with oestrogen receptors

Abstract

Due to the increasing importance of sea bass (Dicentrarchus labrax) in the European aquaculture, puberty control is the focus of intense research. While the GnRH systems have been well described in this species, very little is known on the kisspeptin systems. Kisspeptins belong to a family of peptides first identified for their capacity to inhibit tumor metastasis through the receptor GPR54 in mammals. It has been largely demonstrated that the kisspeptin system plays an essential role in the neuroendocrine control of puberty and reproduction by stimulating the GnRH neurons and subsequently releasing gonadotropin hormones. Therefore, the kisspeptin system could be the gatekeeper in the onset of reproduction on fish. In sea bass two ligands and two receptors coded by different genes have been identified and molecularly characterized (Felip et al., 2009). Here, we report the neuroanatomical distribution of the cells expressing kiss1 and kiss2 and their cognate receptors in the brain of adult sea bass by in situ hybridization. Our findings (Figure 1) indicate that kiss1-expressing cells are located at the level of the habenular nucleus (Hd) and the mediobasal hypothalamus where a strong expression of kiss1 messengers is observed just above the pituitary stalk which is in line with the kiss1 messenger distribution in the Medaka fish (Kanda et al., 2008). In contrast, kiss2 expressing cells were observed around the nucleus of the lateral recess (NRL) and in the anterior preoptic area. Additionally, kiss1 messengers were also detected in the proximal pars distalis of the pituitary gland. Interestingly, similar to what shown in zebrafish (Servilli et al., 2011), the kiss1r was also expressed in the habenula but, in addition, it was found in the anterior ventral preoptic region. Kiss2r had a much wider expression in the ventral, lateral and dorsal parts of the telencephalon, the entopeduncular nucleus, the preoptic region and the hypothalamus. The relationships between kiss receptors and the GnRH systems were studied using specific antibodies to the GAP portion of the preproGnRH (González-Martínez et al., 2002). Double staining showed that GnRH1 neurons express the kisspeptin receptor 2 mRNA in the anterior ventral preoptic area. In the sea bass, testosterone, 11-ketotestosterone and œstradiol-17ß have been well studied and there is clear evidence of the role of these steroids during the period of reproduction. In sea bass, three estrogen receptors (ERs) have been characterized, named esr1 (ERß), esr2a (ERß2) and esr2b (ERß1) and) their distribution was studied by in situ hybridization (Muriach et al., 2008a; 2008b) in comparison with kiss1 and kiss2 expression. Preliminary data show that all three types of estrogen receptors are strongly expressed in the main hypophysiotropic areas of the fish brain, i.e. the preoptic region and the mediobasal hypothalamus and in the pituitary. In conclusion, this work represents the first neuroanatomical analysis of the kisspeptin systems and their relationship with the three oestrogens receptors in the sea bass brain and suggests a critical role of this system in the control of reproduction.