Pharmacological evidence for a 7-benzylidenenaltrexone-preferring opioid receptor mediating the inhibitory actions of peptidic δ7 and μ-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 mediating the actions of opioids on ion transport was characterized in mucosa-submucosa sheets from porcine ileum. Electrical transmural stimulation evoked transient increases in short-circuit current, an electrical measure of neurogenic ion transport, in this preparation. After serosal addition, the peptidic δ-opioid agonists [D-Ala²]-deltorphin II (plC₅₀ = 8.4 ± 0.7), [D-Ala²,D-Leu⁵]enkephalin (DADLE), [D-Pen²,D-Pen⁵]-enkephalin (DPDPE), and [D-Ser²,Leu⁵,Thr⁶]-enkephalin (DSLET), and the μ-opioid agonists [D-Ala²,N-methyl-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) (plC₅₀ = 8.0 ± 0.1), endomorphin I, and PL-017 inhibited shortcircuit current elevations. Nonpeptidic μ- or δ-opioid agonists (morphine, loperamide, and SNC80) and κ-opioid agonists (U-50,488H and U-69,593) were <360-fold less potent than deltorphin II. At 100 nM, the δ₁-opioid antagonist 7-benzylidenenaltrexone reduced the potencies of DPDPE and DAMGO by 13.5- and 15.5-fold, respectively; at an identical concentration naltriben, a δ₂-opioid antagonist, or the μ-opioid antagonist D-Phe-Cys-TyrD-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 primary antisera directed toward cloned opioid receptors, δ-opioid receptor immunoreactivity was immunohistochemically localized in submucosal neurons and nerve fibers, but immunoreactivities to κ- or μ-opioid receptors were not detected in the mucosa-submucosa. These results suggest that a novel 7-benzylidenenaltrexone-sensitive opioid receptor is expressed in submucosal neurons of the porcine ileum, which mediates the inhibitory effects of peptidic μ- and δ-opioid agonists on neurogenic ion transport.