Sequence, genomic organization and expression of two channel catfish, Ictalurus punctatus, ghrelin receptors

Two ghrelin receptor (GHS-R) genes were isolated from channel catfish tissue and a bacterial artificial chromosome (BAC) library. The two receptors were characterized by determining tissue distribution, ontogeny of receptor mRNA expression, and effects of exogenous homologous ghrelin administration on target tissue mRNA expression. Analysis of sequence similarities indicated two genes putatively encoding GHS-R1 and GHS-R2, respectively, which have been known to be present in zebrafish. Organization and tissue expression of the GHS-R1 gene was similar to that reported for other species, and likewise yielded two detectable mRNA products as a result of alternative splicing. Expression of both full-length, GHS-R1a, and splice variant, GHS-R1b, mRNA was highest in the pituitary. Gene organization of GHS-R2 was similar to GHS-R1, but no splice variant was identified. Expression of GHS-R2a mRNA was highest in the Brockmann bodies. GHS-R1a mRNA was detected in unfertilized eggs and throughout embryogenesis, whereas GHR-R2a mRNA was not expressed in unfertilized eggs or early developing embryos and was the highest at the time of hatching. Catfish intraperitoneally injected with catfish ghrelin-Gly had greater mRNA expression of GHS-R1a in pituitaries at 2 h and Brockmann bodies at 4 h, and of GHS-R2a in Brockmann bodies at 6 h post injection. Amidated catfish ghrelin (ghrelin-amide) had no observable effect on expression of either pituitary receptor; however, GHS-R1a and GHS-R2a mRNA expression levels were increased 4 h post injection of ghrelin-amide in Brockmann bodies. This is the first characterization of GHS-R2a and suggests regulatory and functional differences between the two catfish receptors.