Motilin

Overview

Motilin is a 22-amino acid peptide hormone produced by specialized enteroendocrine M cells (sometimes called Mo cells) located primarily in the duodenum and proximal jejunum. It was isolated and named in 1973 by John Brown and colleagues at the University of British Columbia during efforts to purify gastrointestinal peptides from porcine intestinal extracts — the name reflects its observed motility-stimulating activity on the stomach and small intestine.

Motilin's defining physiological role is driving Phase III of the interdigestive migrating motor complex (MMC) — the cyclical, coordinated wave of contractions that sweeps unabsorbed material through the stomach and small intestine during fasting, often called the "intestinal housekeeper." Plasma motilin levels oscillate cyclically during the fasted state, rising sharply before each Phase III burst of contractions and falling during the quiescent Phases I and II. Eating suppresses motilin release and terminates the MMC cycle until the next interdigestive interval.

The discovery that the macrolide antibiotic erythromycin binds and activates the motilin receptor has made motilin biology clinically relevant beyond its native physiology. Erythromycin and derivatives (motilides) developed as motilin receptor agonists have been extensively investigated as prokinetic agents, though issues with tachyphylaxis and cardiac QT effects have complicated development.

Motilin is also closely related evolutionarily to ghrelin: the two peptides share sequence and structural features, their receptors (GPR38 for motilin, GHS-R1a for ghrelin) are paralogs, and they have overlapping but distinct physiology. Interestingly, mice and rats have a non-functional motilin gene — they synthesize motilin only vestigially or not at all — meaning that much motilin research has been conducted in species (dog, human, rabbit) that retain a functional motilin-motilin receptor axis.

Structure/Sequence

Human Motilin: FVPIFTYGELQRMQEKERNKGQ (22 aa)

• Length: 22 amino acids
• Molecular weight: ~2,698 g/mol
• Gene: MLN (chromosome 6p21.31)
• Preproprotein: 114-aa precursor; mature peptide excised from center
• Free N- and C-termini: Not amidated
• N-terminal Phe: Critical for receptor binding
• C-terminal conformation: Helical region important for activity

Structural Features

• N-terminal hexapeptide (FVPIFT): Contains the "message" for motilin receptor activation
• Central region: Secondary structure important for receptor binding
• Truncations: Motilin(1-14) retains partial activity; shorter fragments inactive

Species Differences

Mice and rats have pseudogenes or non-functional motilin genes — they do not produce bioactive motilin. Dogs, pigs, rabbits, and humans have functional motilin. This species distribution has important research implications, as rodent models are not useful for motilin physiology.

Evolutionary Relationship to Ghrelin

Motilin and ghrelin share:

• Similar N-terminal hydrophobic residues (Phe-Val/Leu motif)
• Paralogous receptors (motilin receptor and GHS-R1a)
• Overlap in regulation of gastrointestinal motility and food intake
• Evolutionary origin from a common ancestral gene

Mechanism of Action

Motilin Receptor (GPR38 / MLNR)

Motilin binds and activates the motilin receptor:

• G-protein coupling: Gq-dominant with some Gs
• Activates phospholipase C → IP3/DAG → calcium mobilization
• Expressed in: Gastrointestinal smooth muscle, enteric neurons, interstitial cells of Cajal, pituitary, cardiovascular tissue
• Paralog of GHS-R1a: Shares ~52% identity with ghrelin receptor

Migrating Motor Complex (MMC)

• Motilin is the principal humoral driver of Phase III of the MMC
• Plasma motilin rises sharply (3-10 fold) before Phase III contractions
• Phase III originates in the antrum/proximal duodenum and propagates distally
• Eating immediately suppresses motilin release and disrupts the MMC cycle
• The MMC clears unabsorbed material ("housekeeping" function)

Direct Smooth Muscle Effects

• Stimulates antral and duodenal smooth muscle contraction
• Effects on LES (lower esophageal sphincter) tone
• Activates enteric cholinergic neurons
• Coordinates propagative contractions through enteric nervous system

Erythromycin and Motilide Pharmacology

• Erythromycin binds the motilin receptor with micromolar affinity and acts as a non-peptide agonist
• Prokinetic doses (much lower than antibiotic doses) stimulate gastric emptying
• Chronic use leads to receptor desensitization (tachyphylaxis)
• Non-antibiotic motilides have been developed to separate prokinetic from antimicrobial activity

Interaction with Feeding

• Eating suppresses motilin release
• The meal-to-meal motilin pattern is a key readout of fasted state
• Motilin and ghrelin may be coordinated: both fluctuate with meal timing, though with different patterns

Appetite Effects

• ICV motilin has been reported to stimulate food intake in some studies
• Effects are modest compared to ghrelin
• Peripheral role in hunger signaling, possibly through MMC-induced gastric contractions interpreted as hunger sensations

Research Summary

Common Discussion Topics

1. Interdigestive housekeeping — Motilin is the archetypal driver of the MMC, the coordinated fasting-state motility cycle that clears the small intestine of debris and bacterial overgrowth. Disorders of MMC function (e.g., small intestinal bacterial overgrowth, SIBO) implicate motilin signaling pathways.

2. Erythromycin as motilin agonist — The discovery that the macrolide antibiotic erythromycin activates the motilin receptor revealed how antibiotic-induced gastrointestinal side effects can overlap with prokinetic pharmacology. Non-antibiotic motilides retain the prokinetic profile while avoiding microbiome effects.

3. Rodent research limitation — The absence of functional motilin in mice and rats is an important species-specific limitation for translational research. Motilin pharmacology must be studied in dogs, rabbits, humans, or other species, and results from rodent feeding studies with exogenous motilin must be interpreted with the absence of endogenous motilin in mind.

4. Motilin-ghrelin paralogy — Motilin and ghrelin represent paralogous peptide-receptor systems with overlapping but specialized functions: motilin dominates MMC regulation while ghrelin dominates hunger signaling. The evolutionary relationship is a valuable model for studying gene duplication and divergence in peptide systems.

5. Tachyphylaxis and chronic dosing — Chronic motilin agonism (e.g., with erythromycin) produces tolerance via receptor internalization and downregulation. This has been a persistent challenge in developing motilin-based prokinetics for chronic gastroparesis.

Related Compounds

• Ghrelin — paralogous peptide with overlapping biology
• Cholecystokinin — post-prandial satiety gut peptide
• Secretin — related duodenal peptide with distinct receptor
• Obestatin — same-preprohormone counterpart of ghrelin