One shape, four receptors

The melanocortin family has five receptors: MC1R on melanocytes for pigment, MC2R on the adrenal gland for cortisol, MC3R and MC4R in the brain for appetite, energy use and sexual arousal, and MC5R on glands. A cyclic analogue holds the key shape rigidly and fits most of them, so one injection pulls several levers at once. This is the central fact about Melanotan II and the reason its reported effects read as a list rather than a single action. The ring is also what makes it survive long enough to do all of it.