Receptor activity
Melanotan 2 mechanism of action
The chemical identity stays the same while the receptor and tissue change the question being asked.
Melanocortin receptors
Melanotan 2 acts as a melanocortin receptor agonist, meaning it activates receptors that respond to melanocortin signals. Alpha-MSH is a natural member of that signaling system. Melanotan II has broad activity, including at MC1R, MC3R and MC4R, as described in the review of melanocortin peptide therapeutics.
Broad activity is the property that most unsettles me. The shorthand tanning peptide makes the chemistry sound as though it has a single destination. The research involves several tissues and several kinds of response.
Pigment production
MC1R activation on melanocytes, the skin’s pigment-making cells, raises the intracellular messenger cAMP. The subsequent signaling increases MITF and tyrosinase activity involved in making melanin, with a shift toward dark eumelanin. This is the pigment pathway described in the melanocortin review. The small human pigmentation trial recorded skin darkening without added UV exposure.
Colour is a visible outcome of that pathway. For me, visible colour answers a pigment question; the long-term safety question remains. The mole and melanoma reports are essential context for the same pigment-cell activity.
Appetite and thermogenesis
Central MC3R and MC4R signaling connects melanocortins with appetite and energy regulation. In male mice given Melanotan II within the nucleus accumbens, food consumption and food-seeking responses fell without conditioned taste aversion or a change in metabolic rate in that experiment. Its location in the brain helps make the question about motivation unusually specific.
Other work examined heat production. Central melanocortin activation in rats reduced body mass and abdominal fat while increasing brown-fat thermogenic capacity. Brown fat burns fuel to produce heat. These are rodent experiments with their own administration routes; the findings establish no human weight-loss treatment. I value those separate findings above the catch-all claim that the compound simply speeds metabolism.
Sexual-function signaling
The controlled study in men with psychogenic erectile dysfunction supports a central melanocortin contribution to erection. Transient nausea, stretching and yawning were also reported. Later work in hormone-primed female rats found increased solicitation behaviour without a change in lordosis, the mating posture measured in that experiment.
An erection, a sexual-motivation measure and a mating reflex are different outcomes. Those separately measured outcomes make the experiment worth a closer look to me. It also makes a sweeping claim about libido look rather careless. The study file gives the human results alongside the animal work.