New insights in ghrelin orexigenic effect

Ghrelin, a peptide hormone first discovered as the endogenous ligand of the growth hormone secretagogue receptor (GHS-R), is predominantly produced and released into the circulation by ghrelin cells (X/A-like) of the stomach fundus cells. Ghrelin has multiple actions in multiple tissues. In particular, it is the most potent known endogenous orexigenic peptide, and plays a significant role in glucose homeostasis: deletion of the genes encoding ghrelin and/or its receptor prevents high-fat diet from inducing obesity, increases insulin levels, enhances glucose-stimulated insulin secretion and improves peripheral insulin sensitivity. In addition to its already mentioned roles, ghrelin has other activities including stimulation of pituitary hormones secretion, regulation of gastric and pancreatic activity, modulation of fatty acid metabolism via specific control of AMP-activated protein kinase (AMPK), and cardiovascular and hemodynamic activities. In addition, modulation of cartilage and bone homeostasis, sleep and behavioral influences, and modulation of the immune system, as well as effects on cell proliferation, are other relevant actions of ghrelin. In this review, we summarize several aspects of ghrelin effects at hypothalamic level and their implications in the control of food intake and energy balance.