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Endogenous opioid peptides and selective opioid \u03b4-, \u03ba- and \u03bc-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 \u03b4-receptors. 5-HT<jats:sub>4<\/jats:sub>agonists (HTF-919 and R-093877), which also increase the velocity of propulsion, acted synergistically with the \u03b4-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-HT<jats:sub>4<\/jats:sub>agonists caused significant increases in the velocity of propulsion (50 \u00b1 7 to 77 \u00b1 8%). We conclude that 5-HT<jats:sub>4<\/jats:sub>agonists and opioid \u03b4-receptor antagonists act synergistically to facilitate propulsive activity in isolated colonic segments.<\/jats:p>","DOI":"10.1152\/ajpgi.1998.275.5.g979","type":"journal-article","created":{"date-parts":[[2017,12,24]],"date-time":"2017-12-24T09:27:41Z","timestamp":1514107661000},"page":"G979-G983","source":"Crossref","is-referenced-by-count":20,"title":["5-HT<sub>4<\/sub>receptor agonists and \u03b4-opioid receptor antagonists act synergistically to stimulate colonic propulsion"],"prefix":"10.1152","volume":"275","author":[{"given":"A. 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