It sits in central circuitry regulating energy homeostasis: the system matching intake to expenditure and defending body weight over time.

Why is it the best characterised central melanocortin receptor?

Defects in it cause severe early-onset obesity, giving an unambiguous human phenotype and eventually an approved medicine for that population.

Does a non-selective alpha-MSH compound reach it?

Yes. MC4R is one of the four receptors that recognise alpha-MSH, so engagement follows from the scaffold.

MC4R and Energy Homeostasis

The short answer

The melanocortin-4 receptor is a central regulator of energy homeostasis, reviewed by Krashes and colleagues in Nature Neuroscience in 2016. It is one of the four receptors alpha-MSH recognises, so a non-selective agonist engages it.

Key facts

Receptor: MC4R
Location: Central nervous system
Governs: Energy homeostasis
Key review: Krashes 2016, Nat Neurosci (PMID 26814590)
Circuit work: Ju 2018 (PMID 29329999)
Recognises alpha-MSH: Yes, one of four that do

What energy homeostasis means here

The regulation matching intake to expenditure over long periods. It is not appetite in the moment but the system defending a body weight, the reason weight is stable across years despite enormous variation in daily eating. MC4R sits within that regulatory circuitry.

How the pathway is organised

Neurons producing POMC release alpha-MSH within the brain, and that signal acts on MC4R-expressing neurons downstream. It is a central melanocortin circuit using the same hormone as the peripheral pigmentation system, which is why one hormone family reaches such unrelated physiology.

Research material referenced

MT-2 10mg, third-party HPLC tested

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Why MC4R is the best characterised of the central receptors

Because defects in it cause a defined phenotype. MC4R pathway variants produce severe early-onset obesity, which gave the field an unambiguous human readout and eventually an approved medicine, setmelanotide, developed for that population.

The consequence for a non-selective compound

MC4R recognises alpha-MSH. A compound built on an alpha-MSH scaffold and described as non-selective is therefore engaging a receptor that governs energy balance, whether or not that was intended. Effects at that receptor are not side reactions in a chemical sense. They are the same pharmacology occurring in different circuitry.

Why the circuit detail matters

Ju and colleagues reviewed MC4R control of neuronal circuits in Pharmacological Research in 2018, framed toward therapeutics. Knowing which neurons express a receptor and what they connect to is what allows an effect to be predicted rather than discovered, and it is why selectivity work depends on circuit-level understanding.

The boundary

This describes published receptor neurobiology. MT-2 holds no marketing authorisation anywhere, is prohibited nowhere on this site as anything other than laboratory research material, and no claim is made about appetite, weight or any effect in any person.

Frequently asked questions

What does MC4R do?: It sits in central circuitry regulating energy homeostasis: the system matching intake to expenditure and defending body weight over time.
Why is it the best characterised central melanocortin receptor?: Defects in it cause severe early-onset obesity, giving an unambiguous human phenotype and eventually an approved medicine for that population.
Does a non-selective alpha-MSH compound reach it?: Yes. MC4R is one of the four receptors that recognise alpha-MSH, so engagement follows from the scaffold.

Extended research context

The MT-2 (Melanotan II) deep dive

Deep dive: the identifier collision that points the wrong way

This site has now found several plausible-looking identifiers that resolve to the wrong molecule. A PubChem record for tirzepatide cited as retatrutide. Thymosin beta-4's CAS quoted as TB-500's. An unrelated organic acid returned for KPV. This one is the most consequential of the set, and the reason is direction. Searching PubChem for melanotan returns CID 16197727: afamelanotide, at 1,646.8 Da for C78H111N21O19. That is Melanotan I, a linear thirteen-residue peptide, and it is an APPROVED MEDICINE, marketed as Scenesse for erythropoietic protoporphyria. Melanotan II is CID 92432: cyclic, seven residues, 1024.2 Da, 622 Da lighter, and assessed by no regulator anywhere. So the wrong record does not merely mislead about mass. It resolves from an unlicensed compound toward a licensed one, and the sequential naming (I and II) makes them look like versions of a single product rather than the structurally distinct compounds they are. Anyone verifying a certificate by searching the bare name lands on an approved drug's record and may never notice which compound it names.

Deep dive: four stability features in seven residues

PubChem's IUPAC name for CID 92432 is unusually complete and reads as a full specification: N-acetyl-L-norleucyl-L-alpha-aspartyl-L-histidyl-D-phenylalanyl-L-arginyl-L-tryptophyl-L-lysinamide, cyclic (2-7)-peptide. Unpack it and there are four independent protease-resistance features in a molecule of seven residues. A lactam bridge joins the aspartate side chain at position 2 to the lysine side chain at position 7, closing the ring, which both locks conformation for receptor engagement and eliminates the free termini exopeptidases require. Position 4 is D-phenylalanine, the mirror image of the natural form, and proteases are stereospecific enough that they largely cannot process it. The N-terminus is acetylated and the C-terminus amidated, capping both ends. And position 1 is norleucine, which is not among the twenty proteinogenic amino acids at all. So even setting aside the cyclisation and the D residue, this peptide could never have been a gene product. It is a fully synthetic construction rather than a modified natural sequence, and it is among the most chemically robust compounds in this catalogue as a direct result.

Deep dive: why the safety literature is included rather than omitted

There is a published dermatological literature here, and the editorial choice was to state it. Reid and colleagues documented atypical melanocytic naevi following melanotan injection in the Irish Medical Journal in 2013. Eijmael and colleagues published a risk review in the Nederlands Tijdschrift voor Geneeskunde in 2022, a national medical journal considering the topic worth addressing directly. Case reports have a real limitation: they establish an observation without a denominator, so they cannot establish causation or incidence, which is the same constraint that applies to pharmacovigilance disproportionality signals. But that limitation cuts both ways. A case report is not proof of harm and it is not dismissible either. What makes the absence of better data significant here is that no regulator has assessed this compound, so there is no label, no contraindications, no adverse event reporting requirement and no monitoring framework. The information that would normally exist for an injectable product simply does not, and what is available instead is what individual clinicians chose to publish after seeing something. Presenting the compound without that context would give an incomplete picture of what is actually known.

Research applications

▸Melanocortin receptor pharmacology and selectivity research
▸Cyclic peptide and lactam bridge design studies
▸D-amino acid substitution and protease resistance research
▸Structure-activity work on constrained peptide analogues
▸Comparative work on alpha-MSH derivatives
▸Analytical method development for cyclic peptides

Handling checklist

✓Verify against CID 92432, 1024.2 Da, C50H69N15O9, CAS 121062-08-6
✓Never verify by searching 'melanotan' alone: that returns afamelanotide at 1,646.8 Da
✓Require stereochemistry on the certificate, as D-Phe4 is not detectable by mass
✓Protect from light; tryptophan at position 6 is photochemically reactive
✓Store lyophilised, cold, dry and dark
✓Expect no disulfide or oxidation satellites (no cysteine, no methionine)
✓Quantification at 280 nm is available thanks to the tryptophan

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Searching PubChem for 'melanotan' to verify identity

Fix: That returns CID 16197727, afamelanotide, at 1,646.8 Da, an approved medicine and a different molecule. Use CID 92432 or search Melanotan II with the numeral.

✗ Treating Melanotan I and II as versions of one compound

Fix: They differ by 622 Da, by six residues, and by whether the peptide is cyclic or linear. Only Melanotan I has been assessed by regulators.

✗ Reading afamelanotide's approval as covering MT-2

Fix: Different molecule, different structure, different evidence, different assessment. Nothing transfers.

✗ Accepting a sequence written without stereochemistry

Fix: L- and D-phenylalanine have identical mass. A peptide with the L form at position 4 is a different compound that mass spectrometry cannot distinguish.

✗ Assuming a non-selective agonist affects only its intended receptor

Fix: Five melanocortin receptors share binding surfaces across different tissues. Engagement elsewhere is the same pharmacology in another place, not a side reaction.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

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The UK Peptides Editorial Team · Research library, UK Peptides

The editorial team is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate. Corrections are made in place and the review date updated.

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The Receptor a Non-Selective Agonist Also Reaches