Characterization of the receptors for chicken GHRH and GHRH-related peptides: Identification of a novel receptor for GHRH and the receptor for GHRH-LP (PRP)

Growth hormone–releasing hormone and its structurally related peptides, GHRH-like peptide (GHRH-LP) (also called PRP), peptide histidine-isoleucine (PHI), vasoactive intestinal polypeptide (VIP), and pituitary adenylate cyclase-activating polypeptide (PACAP), have been reported to play important physiological roles in pituitary and extrapituitary tissues of vertebrates; however, little is known about the identity of these GHRH-related peptide receptors in birds. In this study, 6 receptors for GHRH and GHRH-related peptides (cGHRHR1, cGHRHR2, cGHRH-LPR, cPAC1, cVPAC1, and cVPAC2) were cloned from chicken brain or pituitary, and their functionalities were examined in Chinese hamster ovary (CHO) cells using a pGL3-CRE-luciferase reporter system. Results showed that: (1) all receptors are G protein–coupled receptors functionally coupled to the intracellular PKA signaling pathway; (2) 2 GHRH receptors (cGHRHR1 and cGHRHR2) were identified, and both receptors could be potently activated by cGHRH; (3) cGHRH-LP could activate its specific receptor cGHRH-LPR (cPRP-R), and it also activated cGHRHR1 and cGHRHR2; and (4) PACAP could potently activate its receptors cPAC1, cVPAC1 and cVPAC2; however, cVPAC1 and cVPAC2 could also be effectively activated by cVIP and tPHI, indicating that they can serve as VIP receptors and potential PHI receptors. Using a reverse transcription polymerase chain reaction assay, we further examined the mRNA expression of these receptors in adult chicken tissues. The expressions of cGHRHR1, cGHRHR2, and cGHRH-LPR are restricted mainly to the pituitary and/or brain, whereas cPAC1, cVPAC1, and cVPAC2 are expressed in most of the tissues examined. Collectively, our study identified the receptors for chicken GHRH and GHRH-related peptides, including a novel GHRH receptor (cGHRHR2), and established a basis to elucidate the roles of these peptides in target tissues.