Five switches
The five receptors
MC1R through MC5R — five G-protein-coupled receptors that read one signal in five different tissues. Learn these five and the whole field falls into place: pigment, cortisol, appetite, inflammation, and secretion.
The genius of the system is thrift — one signal, read five ways, so a single family of hormones can watch over skin, stress, hunger, and repair at once.
Everything in this field routes through five receptors. Same messenger, five different rooms — and in each room a different job: the colour switch in skin, the cortisol trigger in the adrenal gland, the appetite brake in the brain. It is the shortest path into the whole system, because every drug and every disease here comes back to one of these five.
What the five have in common
The melanocortin receptors are close cousins — a tight subfamily of class-A (rhodopsin-like) G-protein-coupled receptors. Three features are shared across all five, and they are worth holding in mind before the differences:[1]
- One activation key. The melanocortin peptides cut from POMC all carry the same central His-Phe-Arg-Trp motif, and that four-residue core is what physically switches the receptor on. It is why a single hormone family can address five receptors — and why drugs copy that exact sequence.[1]
- One main wire. Activated, each receptor couples to the stimulatory G-protein Gs, raising the second messengercyclic AMP inside the cell. The downstream effect differs only because different cells are listening — a melanocyte makes pigment, an adrenal cell makes cortisol.[1]
- A recurring need for help. Several of these receptors depend on small accessory proteins (MRAP and MRAP2) to fold, reach the surface, or tune their output. MC2R cannot function at all without MRAP; MRAP2 modulates the appetite receptors.[2]
The system also builds in its own off switches. Where most receptor families are pushed only by agonists, the melanocortins have dedicated endogenous antagonists: agouti signalling protein (ASIP) blocks MC1R to lighten skin and coat, and agouti-related peptide (AgRP) opposes MC3R and MC4R to drive hunger — the latter acting as a true inverse agonist that pushes the receptor below its resting activity.[1]
The five at a glance
Same chemistry, five destinations. This is the whole family on one screen — each receptor links to its full reference page.
| Receptor | Where it acts | Main job | Key ligands | When it fails | Approved drug |
|---|---|---|---|---|---|
| MC1R | Skin & hair (melanocytes); immune cells | Sets pigment — protective eumelanin vs. pale pheomelanin | α-MSH, ACTH; ASIP blocks | Red hair, fair skin, ↑ melanoma risk | Afamelanotide |
| MC2R | Adrenal cortex | Cortisol output at the bottom of the stress axis | ACTH only | Familial glucocorticoid deficiency | — (needs MRAP to work) |
| MC3R | Hypothalamus; gut, heart, immune cells | Energy partitioning; timing of growth & puberty | α-, β-, γ-MSH, ACTH; AgRP blocks | Later puberty, reduced lean mass | — |
| MC4R | Hypothalamus & wider CNS | The appetite & body-weight rheostat | α-MSH; AgRP is an inverse agonist | Commonest single-gene cause of severe obesity | Setmelanotide, bremelanotide |
| MC5R | Exocrine glands (sebaceous, lacrimal); immune cells | Exocrine secretion — sebum and lipids | α-MSH, ACTH | Defective sebum (mouse); human role emerging | — |
Explore each receptor
Each has a full reference page — the mechanism, the human genetics, and the drugs that target it. Or trace which ligands hit which receptor in the interactive explorer.
Switches melanocytes from red/yellow pheomelanin toward brown/black eumelanin — the master control of pigmentation and UV response.
The ACTH receptor — drives adrenal steroidogenesis and cortisol output at the bottom of the HPA stress axis.
Tunes energy partitioning, the timing of puberty and growth, and natriuresis; a brake complementing MC4R.
The central rheostat of appetite and body weight — and the target of the anti-obesity agent setmelanotide.
Regulates exocrine secretion — notably sebum and lipid production — and contributes to immune modulation.
Common questions
How many melanocortin receptors are there?
Five: MC1R, MC2R, MC3R, MC4R and MC5R. All are G-protein-coupled receptors that respond to melanocortin peptides derived from POMC, but each is expressed in different tissues and controls a different job — pigment, cortisol, energy partitioning, appetite and exocrine secretion respectively.
What do the melanocortin receptors have in common?
They share a common activation mechanism: melanocortin peptides carry the same core His-Phe-Arg-Trp sequence that switches the receptors on, and all five couple mainly to the Gs protein and raise the second messenger cyclic AMP. What differs is where each receptor sits and what that cAMP signal is wired to do.
Which melanocortin receptors are drug targets?
Two are targeted by approved drugs today: MC1R (afamelanotide, for a rare light-sensitivity disorder) and MC4R (setmelanotide for rare obesity, and bremelanotide for low sexual desire). MC3R and MC5R are studied but have no approved drug, and MC2R is targeted only indirectly through the ACTH stress axis.
Why does one hormone control such different things?
Because the same messenger is read by five different receptors in five different places. Evolution reused one signalling chemistry — a POMC-derived peptide activating a Gs-coupled receptor — and wired it to pigment in the skin, cortisol in the adrenal gland, and appetite in the brain. That is why a drug aimed at one receptor can have effects that look unrelated, such as skin darkening from an appetite agent.
References
- 1.Cone RD. Studies on the physiological functions of the melanocortin system. Endocr Rev. 27(7):736–49. 2006. link ↗
- 2.Ramachandrappa S, Gorrigan RJ, Clark AJL, Chan LF. The melanocortin receptors and their accessory proteins. Front Endocrinol (Lausanne). 4:9. 2013. link ↗
- 3.Harno E, Gali Ramamoorthy T, Coll AP, White A. POMC: the physiological power of hormone processing. Physiol Rev. 98(4):2381–2430. 2018. link ↗