Gastric cytoprotective effect of morphine is probably not mediated by mu-receptors.

Morphine has a significant protective effect on ethanol-induced gastric lesions. This effect is antagonized by naloxone, suggesting that it is mediated by opioid receptors. In the rat, mu-receptors have been shown to be involved in other gastrointestinal actions of opioids. However, the potent partial agonist at mu-receptors, buprenorphine, not only failed to protect but actually aggravated ethanol-induced lesions at higher doses. The gut-selective mu-agonist, loperamide, also did not have a protective effect, while the mixed opioid, pentazocine, which has an antagonistic action at mu-receptors, by itself protected against ethanol-induced gastric lesions. Our results suggest that mu-receptors are probably not involved in the gastric cytoprotective action of opioids.