Is ghrelin a signal for the development of metabolic systems?

Ghrelin, produced in the stomach, acts on growth hormone secretagogue receptors (GHSRs) in hypothalamic neurons to potently increase food intake. However, male mice with deletions of ghrelin (Ghrl^-/- mice) or GHSR (Ghsr^-/- mice) display normal growth and regulation of food intake. Furthermore, adult Ghrl^-/- mice display a normal sensitivity to high-fat diet-induced obesity. These findings from early studies raised the question as to whether the ghrelin system is an essential component for the regulation of food intake and body weight homeostasis. However, recent studies by Wortley et al. and Zigman et al. demonstrate that Ghrl^-/- and Ghsr^-/- mice are resistant to diet-induced obesity when fed a high-fat diet during the early post-weaning period (see the related articles beginning on pages 3564 and 3573). This commentary highlights 3 key issues raised by these 2 reports: (a) the impact of ghrelin on the development of metabolic systems; (b) the constitutive activity of GHSR; and (c) gender differences in the sensitivity to deletion of the ghrelin signaling system.