melanocortin system
The melanocortin system is the brain's central balancing scale for appetite, with one neuron type pushing down and another pushing up on the same lever. That shared lever is a single receptor — the melanocortin-4 receptor — and whether it is activated or blocked decides whether you feel like eating less or more.
Concretely, POMC neurons make alpha-MSH, an activating signal, while AgRP neurons make agouti-related peptide, a blocking signal. Both act on the melanocortin-4 receptor (MC4R) in downstream hypothalamic neurons. When alpha-MSH wins, food intake drops and energy expenditure rises; when AgRP wins, hunger rises. Upstream, leptin and insulin tune this balance according to the body's energy stores.
This pathway is the single most important known genetic regulator of human body weight. MC4R mutations are the most common monogenic cause of severe obesity, and the system is a prime drug target — MC4R agonists treat certain rare genetic obesities, illustrating how directly this circuit sets the body's defended weight.
The name comes from the “melanocortin” peptides (like MSH) that also regulate skin pigmentation; the same receptor family does double duty in coloration and energy balance.