The role of mu- and delta- opioid receptors on the intestinal propulsion in rats.

The agonist effects on intestinal transit of relatively selective mu- and delta- ligands, administered intraperitoneally, and their antagonism by the preferentially mu- and delta- antagonists naloxone and ICI 174,864 were studied in rats 5 min after a charcoal meal. The dose-response curves of the preferential mu- ligands morphine and [D-Ala2, MePhe4, Gly-ol5] enkephalin (DAGO) were shifted by naloxone at low doses but not by ICI 174,864. The preferential delta-peptide [D-Pen2, D-Pen5] enkephalin (DPDPE) had no significant agonist activity. [D-Ala2, D-Leu5] enkephalin (DADLE) induced dose-related effects that were weakly antagonized by ICI 174,864 and partly by low-dose naloxone. Thus the inhibition of intestinal transit induced by opioids may depend mainly on the interaction of the agonists at the mu-receptors, while the delta- receptors may play only a secondary role. DADLE agonist effects probably depend on interaction at mu-, delta- and at non-opioid receptors.