Characterization of opioid receptors modulating neurogenic contractions of circular muscle from porcine ileum and evidence that δ- and κ-opioid receptors are coexpressed in myenteric neurons

Opioid receptors (ORs) in the myenteric plexus mediate the antimotility actions of opioids in the small intestine. In this study, ORs modulating neurogenic circular muscle contractions in the porcine ileum were characterized by pharmacological and immunohistochemical approaches. Circular muscle-myenteric plexus strips manifested tetrodotoxin- and atropine-sensitive contractions during (ON) and after (OFF) electrical field stimulation. The κ-OR agonists U-50,488H and U-69,593 inhibited ON contractions (plC₅₀ = 7.61 and 8.22, respectively). U-69,593 action was inhibited by the κ-OR antagonist norbinaltorphimine with an antagonist equilibrium constant (K_e) of 4.2 nM. Selective δ-OR agonists [D-Ala²]-deltorphin II, DSLET, DADLE, SNC80, and DPDPE inhibited OFF contractions (plC₅₀ = 9.17, 8.63, 8.50, 8.26, and 7.47, respectively). The selective δ-OR antagonist naltriben reduced the inhibitory actions of SNC80 and DSLET with respective K_e values of 2.3 and 3.0 nM. In addition, norbinaltorphimine inhibited the actions of these agonists with respective K_e values of 0.7 and 4.2 nM. The μ-OR agonists DAMGO, loperamide, or morphine exhibited relatively low activities in inhibiting ON and OFF contractions. Using primary antisera directed toward cloned opioid receptors, δ-OR immunoreactivity was observed to be localized alone or in combination with κ-OR immunoreactivity in myenteric neurons; μ-OR immunoreactivity was absent. The results suggest that myenteric δ- and κ-opioid receptors mediate the antitransit effects of opioids in the porcine small intestine. These receptors may be functionally coupled in a subpopulation of myenteric neurons.