How Opioid Peptides Slow Down the Small Intestine by Blocking Relaxation Signals | Research

The takeaway

Met-enkephalin, leu-enkephalin, and dynorphin all reduce inhibitory nerve signals in canine duodenal muscle through delta and mu receptors — explaining how opioids impair normal gut relaxation.

All 3 opioids blocked relaxation signals
Met-enkephalin, leu-enkephalin, and dynorphin all reduced inhibitory junction potentials in duodenal circular muscle

What the researchers found

Met-enkephalin, leu-enkephalin, and dynorphin decreased inhibitory junction potentials in canine duodenal circular muscle via delta and mu receptors.

Why it matters

This study helps explain why opioid drugs cause constipation. Opioid peptides interfere with the nerve signals that tell gut muscles to relax, slowing down normal intestinal movement.

How the study worked

Dog duodenal circular muscle strips were studied in vitro using simultaneous mechanical and electrical recording. Various opioid agonists and receptor-selective drugs were applied.

What this study cannot tell us

Animal study using isolated dog intestinal tissue in vitro. Conditions differ from a living animal. Results may not translate directly to human gut physiology.

How to read the evidence

Preliminary in-vitro study using isolated canine intestinal tissue with electrophysiological recording.

When this study was published

Published in 1991. The opioid mechanism of gut dysmotility has been extensively characterized and informs current GI pharmacology.

The bigger picture

Understanding how opioids block specific nerve signals in the gut helps explain why constipation is the most common and persistent opioid side effect, and guides development of peripherally-acting treatments.

Questions still open

Are delta or mu receptors more important targets for treating opioid constipation?
Do endogenous gut opioid levels change in functional bowel disorders?

Common questions

Why do opioids cause constipation?

Normal gut movement requires cycles of contraction and relaxation. Opioid peptides block the nerve signals that trigger relaxation, so the gut stays contracted and contents can't move through normally.

Can this be treated without affecting pain relief?

Yes — drugs like methylnaltrexone and naloxegol block opioid receptors in the gut without entering the brain, relieving constipation while preserving the central pain-relieving effects.

Read the original research

Effect of opioid peptides on circular muscle of canine duodenum.

The Journal of physiology, 434, 409-22

View on PubMed ↗

Citation

Bauer, A J; Szurszewski, J H. (1991). Effect of opioid peptides on circular muscle of canine duodenum.. The Journal of physiology, 434, 409-22.