Melanin-Concentrating Hormone

Overview

Melanin-Concentrating Hormone (MCH) is a cyclic 19-amino acid neuropeptide with an interesting evolutionary history. It was first isolated in 1983 by Kawauchi and colleagues from chum salmon pituitary, where it acts on skin melanophores to aggregate melanin pigment — producing skin lightening opposite to the darkening effect of α-MSH. In teleost fish, MCH is a major skin-color-regulating hormone released from the pituitary. The "melanin-concentrating" name reflects this original fish-skin activity.

In mammals, MCH has been evolutionarily repurposed. The gene is expressed primarily in a discrete population of neurons in the lateral hypothalamic area and the zona incerta — the same general region that hosts the orexin-A/hypocretin neurons, though in a separate population. These "MCH neurons" project widely throughout the brain and are now understood as playing central roles in feeding behavior, reward, arousal (particularly REM sleep), emotional regulation, and water balance.

MCH is generated from a 165-amino acid precursor (prepro-MCH) that also encodes two additional bioactive peptides — NEI (neuropeptide EI) and NGE (neuropeptide GE) — through paired basic amino acid cleavage. The three peptides are released in parallel, but MCH has been far more extensively studied than its smaller siblings.

Physiologically, central MCH administration promotes food intake (though less potently than AgRP or NPY), promotes REM sleep, and modulates mood-related and reward-related behaviors. MCH knockout mice are lean, hypophagic, and have altered sleep architecture, confirming its role as a promoter of positive energy balance. This MCH-orexin-A duality — opposing peptides expressed in neighboring but distinct lateral hypothalamic populations — has made the lateral hypothalamus a fascinating model for studying how adjacent neuropeptide systems coordinate behavior.

Structure/Sequence

Mammalian MCH (cyclic): DFDMLRCMLGRVYRPCWQV

With a disulfide bond between Cys7 and Cys16, forming a cyclic structure.

• Length: 19 amino acids
• Molecular weight: ~2,387 g/mol
• Gene: PMCH (chromosome 12q23.2)
• Disulfide bond: Cys7-Cys16 creates a cyclic loop
• Species conservation: Mammalian sequence is highly conserved; fish MCH differs in length and sequence
• Free N- and C-termini: Not amidated

Structural Features

• Cyclic core: The Cys-Cys disulfide creates a compact receptor-binding loop
• DFDMLR N-terminal "tail": Flexible region contributing to receptor interactions
• C-terminal WQV: Contributes to receptor binding
• Truncated forms: MCH lacking the N-terminal hexapeptide retains significant activity

Prepro-MCH Products

The 165-aa precursor generates three peptides:

• MCH (19 aa): Principal bioactive product
• NEI (13 aa): Neuropeptide EI (with a different physiological profile than MCH)
• NGE (19 aa): Neuropeptide GE (roles less well defined)

All three are released in parallel from MCH-expressing neurons.

Mechanism of Action

MCH Receptors

MCH signals through two GPCRs:

MCH1 Receptor (MCHR1, SLC-1, GPR24):

• Expressed in mammals (humans, rodents)
• Couples to Gi/o and Gq
• Primary MCH receptor in mammals
• Widely expressed in brain: nucleus accumbens, cortex, hippocampus, hypothalamus

MCH2 Receptor (MCHR2, SLT):

• Expressed in humans but pseudogene in rodents (non-functional in mice and rats)
• Couples to Gq
• Expression pattern differs from MCHR1
• Less well characterized functionally due to rodent model limitation

Feeding Behavior

• ICV MCH stimulates food intake (modest effect)
• MCH KO mice: lean, hypophagic, metabolically protected
• MCHR1 KO mice: lean, hyperactive, partially hypophagic
• Integration with other hypothalamic feeding peptides

Sleep Regulation

• MCH neurons fire selectively during REM sleep
• MCH promotes REM sleep when administered centrally
• Counterpart to orexin-A neurons, which fire during wakefulness
• Neighboring but non-overlapping lateral hypothalamic populations

Reward and Addiction

• MCH modulates dopaminergic reward signaling
• MCHR1 expressed in nucleus accumbens
• Role in food reward and drug reward
• MCH1R antagonists investigated for obesity and addiction

Mood and Anxiety

• MCH signaling influences anxiety-like and depression-like behaviors
• MCH1R antagonists have antidepressant-like effects in rodent models
• Effects on hippocampal function

Fish Skin Color

• In teleost fish, MCH aggregates melanin in melanophores
• Opposite effect to α-MSH, which disperses melanin (darkening)
• This ancestral function is absent in mammals

Research Summary

Common Discussion Topics

1. Evolutionary repurposing — MCH is a striking example of how a peptide with one function in ancestral species (skin pigmentation in fish) can be retained and repurposed for a completely different function in derived species (feeding and sleep in mammals). The sequence is highly conserved but the physiological context has shifted entirely.

2. Lateral hypothalamic duality — MCH and orexin-A neurons populate the same lateral hypothalamic area but form separate populations with opposing effects on sleep-wake (orexin promotes wake, MCH promotes REM) and complementary effects on feeding. Studying these side-by-side populations has been informative for understanding how adjacent neuropeptide systems coordinate behavior.

3. MCHR2 pseudogene problem — The fact that MCHR2 is functional in humans but a pseudogene in mice and rats is a significant translational limitation. Rodent MCH research addresses only MCHR1 biology, and human MCHR2 pharmacology has required alternative model systems.

4. Antagonists as antidepressant leads — MCHR1 antagonists have shown antidepressant-like and anxiolytic effects in rodent models, opening a research line for small-molecule MCHR1 antagonist development. This adds to the evidence that neuropeptide systems have roles in mood beyond their canonical endocrine/feeding functions.

5. Dual-peptide precursor — Prepro-MCH produces MCH plus NEI and NGE. The existence of co-released peptides from a single precursor is common in neuropeptide biology (POMC is the prototypical example), but the specific functional roles of NEI and NGE remain less well defined than MCH itself.

Related Compounds

• α-MSH — functional opposite in fish pigmentation; partially overlapping mammalian feeding circuits
• Orexin-A — lateral hypothalamic counterpart with opposite sleep-wake effects
• Neuropeptide Y — major orexigenic peptide
• Agouti-Related Peptide — arcuate orexigenic peptide