neuropeptide Y (NPY), a natural substance that acts on the brain to stimulate eating. Laboratory animals injected with NPY greatly overeat. Substances that block the NPY receptor reduce the appetite. Leptin, a hormone that stimulates weight loss, reduces the output of NPY from the hypothalamus, a major production center. Another natural substance that stimulates the urge to eat is peptide YY. In animal experiments it has appeared to be at least as potent as NPY.
