Molecular cloning, tissue distribution, and ontogenetic expression of ghrelin and regulation of expression by fasting and refeeding in the grass carp (Ctenopharyngodon idellus).

Ghrelin is a recently discovered brain-gut peptide in fish and mammals. It has two main physiological functions-stimulating growth hormone secretion and regulating appetite. To investigate the biological function of ghrelin in appetite regulation in grass carp Ctenopharyngodon idellus, the full-length cDNA sequence of the ghrelin gene was obtained by RT-PCR and rapid amplification of cDNA ends methods. Homology analysis indicated that ghrelin cDNA sequence is conserved in teleosts. The grass carp ghrelin gene consists of four exons and three introns, which is similar to that of common carp, zebrafish and humans. Real-time quantitative PCR was used to detect ghrelin mRNA expression in the study. In adult tissues, high levels of gene expression were found in the foregut; moderate levels in the muscle, liver, hypothalamus, white adipose tissue, midgut, heart, and pituitary; and lower levels in the three other examined tissues. During embryonic development, ghrelin mRNA expression could be detected as early as fertilized egg stage and displayed an increase in expression until heart appearance stage. After hatching, the level of ghrelin mRNA expression decreased sharply to the lowest level at 1 day post-hatching (dph), then increased to a higher level at 7 dph and subsequently decreased to a relative stabilized level from 15 to 35 dph. The ghrelin mRNA expression was significantly upregulated in the brain and intestine during fasting and was downregulated after refeeding. The results suggest that ghrelin mRNA expression has obvious tissue specificity and may have a role in appetite regulation in grass carp.