Mu-Opioid Receptors in the Brain Reduce Stomach Acid More Than Gut Receptors
Mu-opioid agonists decreased gastric acid secretion primarily through brain receptors, while kappa agonists worked mainly through peripheral gut receptors.
Quick Facts
What This Study Found
Morphine and other mu-opioid agonists decreased gastric acid secretion. They were more potent when injected into the brain (i.c.v.) than into the blood (i.v.), indicating a central mechanism.
The quaternary opioid antagonist naltrexone methylbromide (which cannot cross the blood-brain barrier) blocked brain-injected morphine's effect and partially blocked IV morphine's effect. This confirmed morphine reduces acid through brain receptors, even when given intravenously.
The kappa-selective agonist U-50,488H had the opposite effect: it increased gastric acid when given IV but not when given into the brain. This increase was blocked by naloxone, the muscarinic blocker atropine, and the M1-selective blocker pirenzepine.
The delta-selective agonist DPDPE did not affect acid secretion by either route.
Key Numbers
How They Did This
Pylorus-ligated rats (to collect gastric secretions). Opioid agonists selective for mu, delta, and kappa receptors were given i.c.v. or i.v. Gastric volume and acid output were measured. Antagonists (naloxone, naltrexone methylbromide, atropine, pirenzepine) tested mechanisms. All tested in rats, not people.
Why This Research Matters
This study showed that different opioid receptors have opposite effects on stomach acid. Mu receptors reduce acid (potentially protective) while kappa receptors increase it. This is relevant to understanding stomach ulcers in opioid users and to the gastric effects of different opioid medications.
The Bigger Picture
Understanding which opioid receptors control stomach acid has implications for treating acid reflux, ulcers, and the GI side effects of opioid medications.
What This Study Doesn't Tell Us
Tested in rats with pylorus ligation, an artificial model. Only acute single doses were tested. Some kappa agonists did not reproduce U-50,488H's acid-increasing effect, suggesting the finding may be drug-specific rather than receptor-specific. Species differences may limit human applicability.
Questions This Raises
- ?Could peripheral kappa agonists treat acid-related conditions?
- ?Do opioid pain medications significantly affect stomach acid in patients?
Trust & Context
- Key Stat:
- Location-dependent acid control Brain vs gut opioid receptors have different and sometimes opposite effects on stomach acid
- Evidence Grade:
- Preliminary animal study with systematic comparison of routes and receptor types.
- Study Age:
- Published in 1988 — established the complex receptor map for opioid effects on gastric acid.
- Original Title:
- Roles of central and peripheral mu, delta and kappa opioid receptors in the mediation of gastric acid secretory effects in the rat.
- Published In:
- The Journal of pharmacology and experimental therapeutics, 244(2), 456-62 (1988)
- Authors:
- Fox, D A, Burks, T F(2)
- Database ID:
- RPEP-00071
Evidence Hierarchy
Tests effects in animals (usually mice or rats), not humans.
What do these levels mean? →Frequently Asked Questions
Do opioid pain medications affect stomach acid?
Yes — opioid drugs can reduce stomach acid through central mu-receptor activation. This is usually a minor effect compared to their more prominent gut motility effects.
What is a pylorus-ligated rat?
A surgical model where the stomach outlet is tied off so all acid produced collects inside, allowing precise measurement of gastric acid secretion.
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Cite This Study
https://rethinkpeptides.com/research/RPEP-00071APA
Fox, D A; Burks, T F. (1988). Roles of central and peripheral mu, delta and kappa opioid receptors in the mediation of gastric acid secretory effects in the rat.. The Journal of pharmacology and experimental therapeutics, 244(2), 456-62.
MLA
Fox, D A, et al. "Roles of central and peripheral mu, delta and kappa opioid receptors in the mediation of gastric acid secretory effects in the rat.." The Journal of pharmacology and experimental therapeutics, 1988.
RethinkPeptides
RethinkPeptides Research Database. "Roles of central and peripheral mu, delta and kappa opioid r..." RPEP-00071. Retrieved from https://rethinkpeptides.com/research/fox-1988-roles-of-central-and
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkPeptides research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.