Cloning and characterization of chicken galanin type I receptors

Abstract

In the present study, we report the identification of 2 chicken genes with considerable homology to galanin receptor type 1 (GalR1). Galanin, a 29- to 30- amino acid neuropeptide with diverse physiological effects, has been reported to be widely distributed in mammalian nervous systems and peripheral tissues. Through the interactions with the 3 known distinct G protein-coupled receptors (GPCRs), i.e. GalR1, 2 and 3, galanin was found to be involved in a broad spectrum of biological functions including modulation of hormone release, nociception, cognitive and feeding behaviour in mammalian species. To our knowledge, galanin receptors have yet to be cloned and characterized in any avian species. Using reverse-transcription polymerase chain reaction (RTPCR), 2 full-length cDNAs of GalR1 homologues, which we termed GalR1a and GalR1b, were cloned from chicken brain and intestine tissue respectively. GalR1a encodes a 357- amino acid precursor protein that shares 84%, 82% and 84% sequence identities to the human, mouse and rat homologues respectively. On the other hand, GalR1b is 363 amino acids in length with comparatively lower homologies to the mammalian homologues (human, 53%; mouse, 53%; rat, 52%). Using RT-PCR, we also examined the expression of the two receptors in adult chicken tissues. Both GalR1a and GalR1b were found to be expressed in most of the tissues examined with similar patterns. Using a pGL3-CRE-luciferase reporter system, forskolin-stimulated luciferase activity in Chinese hamster ovary (CHO) cells expressing GalR1a was inhibited by galanin in a dose-dependent manner, confirming the functional coupling of Gi protein to GalR1a. The same functional assay is under way for GalR1b. Cloning and characterization of chicken galanin receptors would provide a better insight into the physiological functions of galanin in the avian species.