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Melanocortin Receptors MC1R to MC5R: A Map for Reading Melanotan Research
Papers on melanotan-type peptides can look contradictory until you remember that they act on a family of receptors, not a single one. Which receptor a compound prefers decides where in the body its effects are studied.
Key takeaways
- There are five melanocortin receptors, MC1R through MC5R, all G protein-coupled receptors.
- They are activated by peptides cut from the same precursor, POMC.
- MC1R is the pigmentation receptor, MC2R handles ACTH signaling, and MC3R and MC4R work in the brain.
- Selectivity differences explain why related analogs give different results.
Where the ligands come from
The body makes a large precursor called proopiomelanocortin (POMC) and cuts it into several peptides: ACTH and alpha-, beta- and gamma-MSH. Together they are the melanocortins. Their targets are the five receptors, which couple mainly to the Gs protein and raise intracellular cAMP.
The five receptors
- MC1R. Found on melanocytes, it regulates the type of pigment made. Variants of MC1R are linked to red hair in genetic studies.
- MC2R. The ACTH receptor, in the adrenal cortex, requires an accessory protein (MRAP) to reach the cell surface and is activated only by ACTH.
- MC3R. Expressed in the brain and elsewhere, studied in energy balance and inflammation.
- MC4R. Expressed in the hypothalamus and other brain areas, a central node in the control of energy homeostasis and appetite in animal studies.
- MC5R. Found in exocrine glands and other tissues, linked to secretion.
Natural brakes
The system also has endogenous antagonists. Agouti signaling protein and agouti-related protein (AgRP) bind melanocortin receptors and block or reduce their signaling. In mice, the agouti gene famously changes coat color, and AgRP in the brain opposes MC4R signaling. A ligand-receptor system with both accelerators and brakes is why selectivity is such a focus.
How two analogs differ
- Melanotan-1 (afamelanotide, NDP-alpha-MSH) is a 13-residue linear analog with substitutions that protect it from enzymes, and it is reported to be relatively selective for MC1R.
- Melanotan-2 is a smaller cyclic lactam analog, reported to be non-selective, with activity at MC1R, MC3R, MC4R and MC5R.
This is why the two are not interchangeable in the literature. The design strategies for lasting activity are covered in how researchers extend peptide half-life, and a side-by-side comparison appears in our post on melanotan-1 versus melanotan-2.
Reading these papers carefully
- Check which receptor the assay used. Results in cells expressing MC1R say little about MC4R.
- Compare EC50 values across receptors (see dose-response curves) to understand selectivity.
- Keep species in mind, since receptor distribution and signaling differ between rodents and primates.
- Do not carry findings from one analog over to another without data.
The bottom line
The receptor map is the key to making sense of melanocortin research. Sold for laboratory research use only, these compounds are best understood through the receptors they engage.
Related reading
- Melanotan-1 vs Melanotan-2: How Receptor Selectivity Differs in Research
- Dose-Response Curves in Plain Language: EC50, Efficacy and Hill Slope
- How Researchers Make Peptides Last Longer: D-Amino Acids, Acylation and More
- Browse the full Research Library
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.