A study of gastrointestinal opiate receptors: The role of the Mu receptor on gastric emptying: concise communication

Animal and in vitro experiments suggest that opiates exert their actions by interaction with possibly five different subtypes of opiate receptors, identified as mu (μ), kappa (κ), sigma (σ), delta (δ), and epsilon (ε). As yet there is no conclusive evidence for their existence in man. Our experiments on morphine and the enkephalin analog DAMME have suggested at least two types of opiate receptors involved in gastric secretion. In this study we have used the very powerful and nonselective opiate agonist etorphine to stimulate as many of the different opiate receptors as possible. We have then attempted to block selectively the μ receptor by administering a small dose of naloxone. Etorphine delayed gastric emptying whereas naloxone alone had no effect. In combination, the inhibitory effect of etorphine on gastric emptying was incompletely prevented while the subjective effects of etorphine were completely abolished. These results may indicate that μ receptors are important in the regulation of gastric emptying, but that other (non-μ) receptors are also involved. The radionuclide study of gastric emptying, as used here, is a potentially powerful tool in physiological research on the gastrointestinal tract.