Mu-opioid agonists decreased gastric acid secretion primarily through brain receptors, while kappa agonists worked mainly through peripheral gut receptors.
Location-dependent acid controlBrain vs gut opioid receptors have different and sometimes opposite effects on stomach acid
What the researchers 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.
Why it 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.
How the study worked
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.
What this study cannot 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.
How to read the evidence
Preliminary animal study with systematic comparison of routes and receptor types.
When this study was published
Published in 1988 — established the complex receptor map for opioid effects on gastric acid.
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.
Questions still open
- Could peripheral kappa agonists treat acid-related conditions?
- Do opioid pain medications significantly affect stomach acid in patients?
Common questions
Do opioid pain medications affect stomach acid?
What is a pylorus-ligated rat?
Read the original research
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
Citation
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.