Multiple molecular forms of gonadotropin-releasing hormone in teleost fish brain

Abstract

Gonadotropin-releasing hormone (GnRH) immunoreactive peptides in extracts of hake ( Merluccius capensis) and tilapia ( Tilapia sparrmanii) brain were investigated by high performance liquid chromatography (HPLC) and radioimmunoassay with region-specific antisera. In hake brain, content and concentration of GnRH was higher in the pituitary gland than in the hypothalamic lobes or extrahypothalamic brain. Hake pituitary gland GnRH was purified by six consecutive HPLC systems. The major GnRH molecular form co-eluted with salmon brain GnRH (Trp 7, Leu 8-GnRH) in four different HPLC systems which were specifically designed to separate the four natural vertebrate GnRHs (mammalian, salmon, chicken I and II). The immunoreactive peak in the final purification step had a retention time identical to that of Trp 7, Leu 8-GnRH and an UV absorbance (280 nm) peak appropriate for two tryptophan residues in the peptide, as in Trp 7, Leu 8-GnRH. Six additional less hydrophobic forms of GnRH were detected. Tilapia brain extract contained two major GnRH molecular forms which had identical retention times to chicken GnRH I (Gln 8-GnRH) and Trp 7, Leu 8-GnRH in an HPLC system which separates the natural vertebrate GnRHs. The immunological properties of these two immunoreactive peaks, determined by relative interaction with four region-specific GnRH antisera raised against vertebrate GnRHs, were identical to those of Gln 8-GnRH and Trp 7, Leu 8-GnRH. Additional GnRH molecular forms were also detected. In summary, these findings indicate that a major GnRH molecule in hake pituitary gland is Trp 7,Leu 8-GnRH, while tilapia brain contains both Trp 7,Leu 8-GnRH and Gln 8-GnRH. Additional GnRH molecular forms were detected in both species. The presence of multiple GnRH-related molecules in teleost fish brain suggests that the GnRH molecule has experienced gene duplication during evolution and the different structures co-opted for as yet undetermined physiological functions.