The opioid receptor selectivity for trimebutine in isolated tissues experiments and receptor binding studies

Differences of affinity to and selectivity for trimebutine between peripheral and central opioid receptors have been investigated. Trimebutine inhibited electrically induced contraction of guinea-pig ileum (GPI) and mouse vas deferens (MVD) but not of rabbit vas deferens, and the inhibition was antagonized by naloxone and, to lesser extent, by nor-binaltorphimine (nor-BNI). The pA2 values for morphine and trimebutine with naloxone were higher than the values for these compounds with nor-BNI in both GPI and MVD preparations. GPI preparations incubated with a high concentration of morphine or trimebutine developed tolerance; however, there was no cross-tolerance between them, suggesting difference in the underlying mechanisms. In mouse and guinea-pig brain homogenate trimebutine was about 1/13 as potent as morphine to displace the [3H]naloxone binding, while it has no appreciable affinity for kappa-opioid receptors in [3H]U-69593, a selective kappa-receptor agonist. These results suggest that trimebutine, showing its low affinity to opioid receptors, possesses mu-receptor selective properties rather than those of kappa-opioid receptor in the peripheral tissues and in the central brain homogenate.