IDENTIFICATION AND CHARACTERIZATION OF A RECEPTOR FROM GOLDFISH SPECIFIC FOR A TELEOST GROWTH HORMONE-RELEASING HORMONE-LIKE PEPTIDE

In mammals, growth hormone-releasing hormone (GHRH), acting via the GH RH receptor, plays an important role in the regulation of growth hormo ne (GH) synthesis and secretion as well as the proliferation and diffe rentiation of somatotropes in the pituitary, In fishes, information co ncerning the functional role of the characterized GHRHs is limited. Fo r that reason, a putative goldfish GHRH receptor cDNA was characterize d in this study. The receptor cDNA is 2,243 bp in length, encoding a 4 38-amino-acid-long polypeptide with 7 putative transmembrane-spanning regions, which is a characteristic of G-protein-coupled receptors. The receptor, when expressed in COS-7 cells, showed minimal responses (2- fold cAMP responses) when stimulated with 100 nM of human GHRH, pituit ary adenylate cyclase-activating polypeptide (PACAP) and vasoactive in testinal polypeptide (VIP). However, this receptor was found to be spe cific for a carp GHRH-like peptide isolated from the brain of common c arp (Cyprinus carpio); there was a significant and dose-dependent incr ease in intracellular cAMP (a maximum response of 22-fold increase wit h an EC50 of 0.1 nM) when the transfected cells were stimulated with t his peptide. As a preliminary study to investigate the functional role of this receptor, the tissue distribution of the mRNA was analyzed by reverse-transcription-polymerase chain reaction. The receptor mRNA wa s found to be present in the brain, pituitary, gut, gill, heart, liver , skeletal muscle, spleen, ovary and testis. Together with a goldfish PACAP type I receptor and a VIP1 receptor recently isolated in our lab oratory, characterization of this putative GHRH receptor provides the molecular basis for the future understanding of the neuroendocrine con trol of growth and reproduction by these neuropeptides in goldfish as well as other teleosts.