MC1R and the switch to dark pigment

Melanocytes carry the melanocortin 1 receptor, MC1R, on their surface. When alpha-MSH binds it, the cell raises cyclic AMP, switches on the master pigment gene MITF, and shifts its production from pheomelanin, the reddish-yellow pigment, to eumelanin, the dark brown one. Afamelanotide binds that same receptor and holds the switch on for far longer than the natural hormone can. The result is more eumelanin, packed into granules and handed to the skin cells above.