Pharmacological evidence for a 7-benzylidenenaltrexone-preferring opioid receptor mediating the inhibitory actions of peptidic delta- and mu-opioid agonists on neurogenic ion transport in porcine ileal mucosa

The antidiarrheal and constipating effects of opiates are partly attributed to reductions in active anion secretion across the intestinal mucosa that are modulated by submucosal neurons. In this study, the opioid receptor med iating the actions of opioids on ion transport was characterized in mucosa- submucosa sheets from porcine ileum. Electrical transmural stimulation evok ed transient increases in short-circuit current, an electrical measure of n eurogenic ion transport, in this preparation. After serosal addition, the p eptidic delta -opioid agonists [D-Ala(2)]-deltorphin II (pIC(50) = 8.4 +/- 0.7), [D-Ala(2),D-Leu(5)]enkephalin (DADLE), [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE), and [D-Ser(2),Leu(5),Thr(6)]-enkephalin (DSLET), and the mu -opio id agonists [D-Ala(2),N-methyl-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) (pIC(50 ) = 8.0 +/- 0.1), endomorphin I, and PL-017 inhibited short-circuit current elevations. Nonpeptidic mu- or delta -opioid agonists (morphine, loperamid e, and SNC80) and kappa -opioid agonists (U-50,488H and U-69,593) were <360 -fold less potent than deltorphin II. At 100 nM, the <delta>(1)-opioid anta gonist 7-benzylidenenaltrexone reduced the potencies of DPDPE and DAMGO by 13.5- and 15.5-fold, respectively; at an identical concentration naltriben, a delta (2)-opioid antagonist, or the mu -opioid antagonist D-Phe-Cys-Tyr- D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) reduced DPDPE potency by 4.1- and 3.4-fold , respectively, but had no significant effect on DAMGO potency. Using prima ry antisera directed toward cloned opioid receptors, delta -opioid receptor immunoreactivity was immunohistochemically localized in submucosal neurons and nerve fibers, but immunoreactivities to kappa -or mu -opioid receptors were not detected in the mucosa-submucosa. These results suggest that a no vel 7-benzylidenenaltrexone-sensitive opioid receptor is expressed in submu cosal neurons of the porcine ileum, which mediates the inhibitory effects o f peptidic mu- and delta -opioid agonists on neurogenic ion transport.