Ghrelin in the postnatal development of the gastrointestinal tract
Ghrelin is a 28-amino acid peptide first isolated from rat and human stomachs. Together with the recently discovered 23-amino acid obestatin, it is derived from proghrelin by posttranslational processing. Cells immunoreactive to ghrelin are widely distributed in the gastric mucosa in domestic and laboratory animals and in humans. Ghrelin plays an important role in energy homeostasis, body weight control, and food intake, whereas obestatin seems to induce the opposite effects. Ghrelin and ghrelin receptor expression have been found in developing gastrointestinal foetal and neonatal tissues, and substantial amounts of ghrelin are present in colostrum, thereby suggesting its presumable role in perinatal development. Ghrelin was shown to positively influence weight gain, increase GH, insulin and cortisol secretion. It also stimulates gastrointestinal tissue structure and function development in weaned animals. Surprisingly, responses in suckling neonates were found to be opposite to those in weanlings. Ghrelin retarded gastric, intestinal and pancreatic development and showed a tendency to reduce body weight gain. Recent studies suggest that the biphasic effect of ghrelin in young rats on pancreas and stomach growth seems to be related to age-dependent changes of the release of anabolic IGF-1. In the perinatal period, obestatin is detected in the rat stomach, pancreas and blood plasma. The obestatin concentration in rats is abruptly reduced after birth, contrasting with an increase in the concentration of acylated ghrelin. Further progressive reduction in pancreas obestatin is observed until weaning.