Molecular characterization of cholecystokinin in grass carp (Ctenopharyngodon idellus): cloning, localization, developmental profile, and effect of fasting and refeeding on expression in the brain and intestine.

Cholecystokinin (CCK) is a multi-functional brain-gut peptide in fish and mammals. To investigate the role of CCK in appetite regulation in fish, a 770-bp full-length cDNA sequence of CCK gene was obtained by RT-PCR and rapid amplification of cDNA ends methods in grass carp Ctenopharyngodon idellus. Homology analysis showed that the CCK cDNA sequence of grass carp had the highest similarity (90 %) to that of goldfish Carassius auratus and a higher similarity (>70 %) to those of other teleosts than to mammals. The PCR amplification using genomic DNA identified that the CCK gene of grass carp was comprised of three exons and two introns. Real-time quantitative PCR was used to detect CCK mRNA expression in adult tissues. High levels of gene expression were found in the hypothalamus and pituitary; moderate levels in the intestine, muscle and white adipose tissue; and low levels in other tissues. During early development (i.e., fertilized eggs to 35-day post-hatching larvae) the levels of CCK mRNA expression were higher during embryonic developmental stages than during post-hatch larval stages. Fasting decreased CCK mRNA expression levels in the brain and intestine, whereas refeeding resulted in an increase of expression. The results suggest that CCK mRNA expression has obvious tissue specificity and may have a role in feed intake regulation in grass carp.