Receptor pharmacology
Everything MT-II does, good and bad, follows from a single fact: it is not selective for the pigment receptor.
One key, four locks: the pharmacology of non-selectivity
Everything MT-II does, good and bad, follows from a single fact: it is not selective for the pigment receptor. This unit unpacks what that means, which tissues carry melanocortin receptors, and why this course prints no number for how tightly MT-II holds any of them.
Understanding the receptor map turns MT-II from a mysterious "tanning drug" into a predictable piece of pharmacology, where every effect maps to a tissue. What it does not do is turn it into a measured one.
Key terms
The receptor profile
A dermatology reference sums MT-II up in one line: it mimics melanocortin peptides non-selectively. In skin the relevant receptor is MC1R, the signal on melanocytes that responds to alpha-MSH by switching on eumelanin synthesis. Everything else MT-II does follows from the same message reaching melanocortin receptors elsewhere in the body.
That second column is the honest part of this page. You may meet a tidy grid of MT-II affinities online, one number per receptor, presented as settled pharmacology. Binding at the human MC1, MC3, MC4 and MC5 receptors was measured in a 1997 study this course cites: MT-II bound all four, most tightly at MC1, then MC4, and more weakly at MC3 and MC5. Check any grid you meet against that paper: same molecule, same receptors, and binding rather than activation. What binding cannot tell you is how strongly MT-II switches each receptor on. The molecule is described as non-selective, it acts in skin, and its reported effects reach far beyond skin.
AdvancedWhy no affinity number is reprinted in this unit
Binding of MT-II at cloned human MC1, MC3, MC4 and MC5 receptors was measured in a 1997 study, and it reports that the cyclic lactams, MT-II among them, showed an overall higher affinity for the melanocortin receptors than the disulphide analogues tested alongside them. Its table gives a binding constant for MT-II at each of the four receptors. This course describes that table rather than reprinting it, because the classic trap is a real number attached to the wrong molecule, or copied from a table nobody checked. A binding constant measured in cells also says nothing about how strongly the receptor is switched on, or how much of it a given dose engages in a person: for that, the primary pharmacology has to be read in full, molecule by molecule.