5-HT4 receptor agonists and δ-opioid receptor antagonists act synergistically to stimulate colonic propulsion

Opioid neurons exert a tonic restraint on inhibitory VIP/PACAP/NOS motoneurons of the enteric nervous system. A decrease in opioid peptide release during the descending phase of the peristaltic reflex, which underlies propulsive activity, leads to an increase in vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating polypeptide (PACAP), and nitric oxide (NO) release and circular muscle relaxation. These effects are accentuated by opioid receptor antagonists. Endogenous opioid peptides and selective opioid delta-, kappa- and mu-receptor agonists decreased the velocity of pellet propulsion in isolated segments of guinea pig colon, whereas selective antagonists increased velocity in a concentration-dependent fashion with an order of potency indicating preferential involvement of delta-receptors. 5-HT4 agonists (HTF-919 and R-093877), which also increase the velocity of propulsion, acted synergistically with the delta-receptor antagonist naltrindole; a threshold concentration of naltrindole (10 nM) shifted the concentration-response curve to HTF-919 to the left by 70-fold. A combination of 10 nM naltrindole with threshold concentrations of the 5-HT4 agonists caused significant increases in the velocity of propulsion (50 +/- 7 to 77 +/- 8%). We conclude that 5-HT4 agonists and opioid delta-receptor antagonists act synergistically to facilitate propulsive activity in isolated colonic segments.