Differential expression of three different prepro-GnRH (gonadotrophin-releasing hormone) messengers in the brain of the European sea bass (Dicentrarchus labrax)

The expression sites of three prepro-gonadotrophin-releasing hormones (GnRP Is), corresponding to seabream GnRH (sbGnRH: Ser(8)-mGnRH, mammalian GnRH), salmon GnRH (sGnRH: Trp(7)Leu(8)-mGnRH), and chicken GnRH-II (cGnRH-II: Hi s(5)Trp(7)Tyr(8)-mGnRH) forms were studied in the brain of a perciform fish , the European sea bass (Dicentrarchus labrax) by means of in situ hybridiz ation. The riboprobes used in this study correspond to the three GnRH-assoc iated peptide (GAP)-coding regions of the prepro-GnRH cDNAs cloned from the same species (salmon GAP: sGAP; seabream GAP: sbGAP; chicken GAP-II: cIIGA P), which show little oligonucleotide sequence identity (sGAP versus sbGAP: 42%; cIlGAP versus sbGAP: 36%; sGAP versus cIIGAP: 41%). Adjacent paraffin sections (6 mm) throughout the entire brain were treated in parallel with each of the three anti-sense probes and the corresponding sense probes, dem onstrating the high specificity of the hybridization signal. The results sh owed that both sGAP and sbGAP mRNAs had a broader expression in the olfacto ry bulbs, ventral telencephalon, and preoptic region, whereas cIIGAP mRNA e xpression was confined to large cells of the nucleus of the medial longitud inal fascicle. In the olfactory bulbs, both the signal intensity and the nu mber of positive cells were higher with the sGAP probe, whereas sbGAP mRNA- expressing cells were more numerous and intensely stained in the preoptic r egion. Additional isolated sbGAP-positive cells were detected in the ventro lateral hypothalamus. These results demonstrate a clear overlapping of sGAP - and sbGAP-expressing cells in the forebrain of the European sea bass, in contrast to previous reports in other perciforms showing a clear segregatio n of these two cell populations. (C) 2001 Wiley-Liss, Inc.