The nerve network controlling gut movement contains more proenkephalin-derived peptides than prodynorphin-derived ones, with met-enkephalin being the dominant form.
Met-enkephalin dominatesIn the nerve network that controls gut movement
What the researchers found
The myenteric plexus (the nerve network that controls gut movement) of guinea pig small intestine contained a well-defined mix of opioid peptides.
Met-enkephalin dominated at 405 pmol/g of tissue. Three other enkephalin-related peptides were present at 90 to 100 pmol/g each. Two rarer enkephalin forms (BAM-18 and Met-enkephalyl-Arg-Arg-Val-NH2) were present at 24 and 5 pmol/g.
Dynorphin-family peptides (alpha-neoendorphin, beta-neoendorphin, dynorphin A(1-8), and dynorphin B) were all present at 12 to 15 pmol/g. Full-length dynorphin A was very scarce at only 0.8 pmol/g.
No beta-endorphin was detected.
The two rare enkephalin forms (BAM-18 and MERV-NH2) had an unusual receptor profile: they preferred mu and kappa opioid receptors rather than the delta receptors that most enkephalins favor. This means they may have different functional roles in the gut.
Why it matters
This was the most complete map of opioid peptides in gut nerve tissue at its time. The dominance of enkephalins over dynorphins, and the absence of endorphin, shows the gut's opioid system has a very different composition than the brain. The unusual receptor profiles of rare enkephalins suggest specialized roles in gut regulation.
How the study worked
Two-step HPLC separation followed by bioassay on mouse vas deferens tissue, with naloxone confirmation of opioid specificity. Guinea pig small intestine myenteric plexus tissue was extracted and analyzed. Receptor binding profiles were determined for each peptide.
What this study cannot tell us
Tested in guinea pig tissue only. Human gut opioid peptide profiles may differ. The bioassay method measures activity rather than exact mass. Only one segment of the small intestine was analyzed.
How to read the evidence
Preliminary in-vitro study with rigorous HPLC-bioassay methodology in a single species.
When this study was published
Published in 1988 — baseline characterization of gut opioid peptides.
The bigger picture
Understanding which opioid peptides control gut motility is essential for developing targeted treatments for constipation, diarrhea, and irritable bowel syndrome.
Questions still open
- Can selective enkephalin receptor drugs treat gut motility disorders?
- Does the opioid peptide profile change in disease states?
Common questions
What is the myenteric plexus?
Why is met-enkephalin important in the gut?
Read the original research
Tissue content of opioid peptides in the myenteric plexus-longitudinal muscle of guinea-pig small intestine.
Journal of neurochemistry, 51(1), 32-7
Citation
Corbett, A D; McKnight, A T; Kosterlitz, H W. (1988). Tissue content of opioid peptides in the myenteric plexus-longitudinal muscle of guinea-pig small intestine.. Journal of neurochemistry, 51(1), 32-7.