EVIDENCE FOR FUNCTIONAL DELTA-OPIATE RECEPTORS IN THE RAT INTESTINE

The selective delta-opiate agonists D-Ser(2), Leu(5) Thr(6)-enkephalin (DSLET), n-Ala(2), D-Leu(5)-enkephalin and D-Pen(2), D-Pen(5)-enkephalin caused inhibition of the cholinergic contraction produced by transmural stimulation of the rat isolated jejunum. Dynorphin A, which is an agonist at both kappa- and delta-opioid receptors also inhibited the cholinergic contraction, as did leu- and met-enkephalin. The selective mu-receptor agonist D-Ala(2)-NMe-Phe(4), Gly-ol(5)-enkephalin was the least potent of all peptides tested. In general, the order of potency of the peptides was similar to that reported for the delta-receptor-rich mouse vas deferens with potency values similar to those recorded previously for the hamster vas deferens. The selective delta-opioid antagonist naltrindole caused parallel shifts to the concentration-effect curve to DSLET giving a pA(2) value of 10.15. The results indicate that the previously identified delta-binding sites in the rat jejunum may correspond to functional delta-opiate receptors involved in attenuating acetylcholine release.