The effects of the selective kappa-opioid agonist MR 2034 on the guinea-pig ileum.

The effects of the selective kappa-opioid agonist MR 2034 on the guinea-pig ileum were compared with those produced by the mu-agonist morphine. MR 2034 induced acute tolerance and inhibited electrically evoked contractions to the same extent as morphine. The slope of the concentration-effect curve to MR 2034 was not significantly different from that of morphine. However, MR 2034 and morphine may act at their respective receptors since naloxone was 6.57 times more effective at inhibiting responses to morphine than MR 2034. MR 2034, like morphine, induced acute dependence as revealed by naloxone-induced contractions of tissues that had been incubated with MR 2034 for 5 h. The functional properties of kappa-opoid receptors are the same as mu-receptors in the guinea-pig ileum.