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

Differences of affinity ot 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 κ-opioid receptors in [3H]U-69593, a selective κ-receptor agonist. These results suggest that trimebutine, showing its low affinity to opioid receptors, possesses μ-receptor selective properties rather than those of κ-opioid receptor in the peripheral tissues and in the central brain homogenate