Isolation, characterization and expression of cDNAs encoding the catfish‐type and chicken‐II‐type gonadotropin‐releasing‐hormone precursors in the African catfish

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Abstract
The cDNAs encoding the catfish prepro-gonadotropin-releasing hormone and the chicken pre-pro-gonadotropin-releasing hormone II of the African catfish (Clarias gariepinus) have been isolated and sequenced. The catfish gonadotropin-releasing-hormone precursor and the chicken gona-dotropin-releasing-hormone-II precursor have the same overall architecture as other gonadotropin-releasing-hormone precursors identified so far; each is composed of a signal peptide, gonadotropin-releasing hormone and a gonadotropin-releasing-hormone-associated peptide which is connected to gonadotropin-releasing hormone by a Gly-Lys-Arg sequence. The amino acid sequences of catfish gonadotropin-releasing hormone and chicken gonadotropin-releasing hormone II, in combination with the Gly-Lys-Arg sequence, are highly conserved during evolution when compared with the corresponding regions of mammalian, avian (chicken gonadotropin-releasing hormone I) and other fish gonadotropin-releasing-hormone precursors. However, the gonadotropin-releasing-hormone-as-sociated peptide regions are markedly divergent. Northern-blot analysis revealed the presence of a single catfish gonadotropin-releasing-hormone mRNA species of about 470 bases, and the presence of a single chicken gonadotropin-releasing-hormone-II mRNA species of about 650 bases in the African catfish brain. In situ hybridization revealed catfish gonadotropin-releasing-hormone cell bodies rostro-caudally scattered in the olfactory nerve, along both sides of the midline of the telencephalon, in the preoptic area of the ventral hypothalamus, and in the infundibular stalk close to the pituitary. Chicken gonadotropin-releasing-hormone-II cell bodies, however, were exclusively found in the midbrain tegmentum.