Cloning of the chicken pituitary receptor for growth hormone-releasing hormone

Details of the regulation of GH in birds are unclear. In this report, a receptor was cloned from chicken pituitary cDNA with 61% amino acid sequence identity to the human pituitary GHRH receptor. Phylogenies inferred from sequence alignments support that this is the chicken counterpart of the GHRH receptor known in mammals. Northern blotting shows that this receptor message is expressed in chicken pituitary, with lesser amounts seen in hypothalamus and brain but not in liver. The recombinant chicken receptor binds human GHRH with high affinity and specificity and signals cAMP accumulation. Surprisingly, available peptides synthesized to the published sequence for chicken GHRH-like peptide (cGHRH-LP) were inactive at this receptor. To address this we recloned the cDNA for this cGHRH-LP from chicken hypothalami. The revised sequence encodes lysine at position 21, which is consistent with all reported GHRH sequences from other species but different from the originally published chicken sequence. When this revised cGHRH-LP sequence was synthesized, it had improved but still weak potency at the cloned receptor. Consistent with the activity at the cloned receptor, human GHRH was potent when assayed in live chickens or on chicken pituitary membranes, but cGHRH-LP was not. We conclude that we have cloned a putative GHRH receptor that is homologous to mammalian GHRH receptors and functionally expressed in chicken pituitary, but that the identity of the endogenous ligand remains unclear. The chicken GHRH receptor cloned in this study can serve as a tool to identify its ligand and to clarify the evolutionary development of the regulation of GH.