Role of Peripheral Mu, Delta and Kappa Opioid Receptors in Oploid-Induced Inhibition of Gastrointestinal Transit in Rats1

The roles of various types of opioid receptors in opioid-induced local inhibition of gastrointestinal transit were studied in rats 5 mm after a charcoal meal, injecting i.p. relatively selective ago-nists and antagonists. The proposed mu agonists, morphine and [D-Ala2, MePhe4,Gly-ol5jenkephalin (DAMGO), and the nonselec-tive delta agonist, [D-Ala2,D-Leu5]enkephalin (DADLE), produced a dose-related inhibition of gastrointestinal transit at the peak time and the i.p. doses producing a 50 % reduction of the control values (Aro) were 0.01 5, 0.006 and 0.023 mg/kg, respectively, for morphine, DAMGO and DADLE. The effect of the other non-selective delta agonist, [D-Ser,L-Leu5]enkephalyl-Thr (DSLET), was not linearly related with the dose. The proposed selective delta agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), and the Se-lective kappa agonist, U-69,593, up to 2 and 1 5 mg/kg i.p., respectively, did not delay gastrointestinal transit. Naloxone (0.0