Step 2: Analyze the function of each peptide mentioned in the problem. For example, Peptide YY (PYY) is known to reduce appetite, not increase it, by acting on the hypothalamus and brainstem. Cholecystokinin (CCK) reduces hunger by stimulating the vagus nerve and signaling satiety, not promoting hunger. Leptin, secreted by adipose tissue, suppresses appetite by acting on the hypothalamus, not stimulating it via the vagus nerve.