The effects of leptin in the brain. Leptin is an adipokine secreted by adipose tissue in direct relation to fat mass. (A) When leptin levels fall, as in fasting, appetite-enhancing neuropeptides NPY and AgRP are secreted, whereas the secretion of appetite-suppressing signals such as MSH is inhibited. (B) When fat mass increases, leptin inhibits NPY and AgRP secretion while stimulating the release of appetite-suppressing hormone MSH.
[Information from M. H. Stipanuk, Biochemical, Physiological, & Molecular Aspects of Human Nutrition, 2d ed. (Saunders/Elsevier, 2006), Fig. 22-2.]