Predominance of δ-opioid-binding sites in the porcine enteric nervous system

The antidiarrheal and constipating actions of opioids are mediated in part by enteric neurons, which lie within the wall of the small intestine and colon, but the differential expression of specific, high-affinity opioid-binding sites in ganglionated plexuses within functionally distinct intestinal segments has not been examined. We determined the saturation binding characteristics under Na⁺-free conditions of the nonselective opioid receptor (OPR) ligand [³H][(5α,7α)-17-(cyclopropylmethyl)-4,5-epoxy-18,19- dihydro-3-hydroxy-6-methoxy-α, α-dimethyl-6,14-ethenomorphinan-7-methanol] (diprenorphine) and the respective δ-, κ-, and μ-OPR ligands [³H]naltrindole, D-(5α,7α,8β)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxoaspiro -(4,5)dec-8-yl]benze-neacetamide ([³H]U-69,593), and [³H][D-Ala²,N-Me-Phe⁴,Gly⁵-ol]- enkephalin (DAMGO) in neuronal membranes isolated from myenteric and submucosal plexuses of porcine small intestine and colon. Naloxone-displaceable [³H]diprenorphine-binding sites (K_D values ranging from 0.2-0.5 nM and B_max = 50-95 fmol/mg of protein) were found in both subregions from all gut segments examined. High-affinity [³H]naltrindole sites (K_D = 60-140 pmol) were at highest densities (approximately 60 fmol/mg of protein) in submucosal plexus of the ileum and distal colon myenteric plexus and were at lowest densities (8-9 fmol/mg of protein) in the submucosal plexuses of cecum and distal colon. [³H]U-69,593 sites (KD = 0.3-4 nM) were present only in the myenteric plexuses of all segments examined, with highest densities in cecum and proximal colon (44-47 fmol/mg of protein). [³H]DAMGO-binding sites were expressed at relatively low densities in the enteric plexuses of all gut regions. These results indicate that δ-OPRs predominate in the porcine enteric nervous system with a more circumscribed expression of κ- and μ-OPRs.