Dynorphin-(1-13) restores the potency of morphine on the tolerant guinea pig ileum.

Since it has been reported that dynorphin (1-13) enhances the analgetic effects of morphine in mice tolerant either to morphine or beta-endorphin and modulates opioid agonist binding in the tolerant state, experiments were initiated to determine whether comparable phenomenon could be obtained in vitro with dynorphin-(1-13) on the isolated longitudinal muscle of the guinea pig ileum. In the guinea pig ileum rendered tolerant to morphine dynorphin-(1-13) was found to cause a dose-dependent restoration in sensitivity to the inhibitory action of morphine. This phenomenon appeared specific for dynorphin-(1-13) since [Leu5]enkephalin, beta-endorphin, and ethylketocyclazocine did not change significantly the IC50 of morphine in the tolerant preparation. Furthermore, dynorphin-(1-13) attenuated the contraction elicited by naloxone in the morphine tolerant-dependent tissue. It is concluded, that dynorphin-(1-13) restores sensitivity to morphine in the tolerant-dependent state by a modulatory effect probably on the mu receptor that is distinct from its kappa action.