Inhibition of motility in isolated horse small intestine mediated by κ- but not μ-opioid receptors

The effects of preferential m (morphine), selective m (fentanyl), selective k (compound U69593) opioid receptor agonists, and nonselective (naloxone) and selectivem(naloxonazine) antagonists on equine small intestinal motilitywere evaluated in vitro. Samples of circular muscle fromequine jejunumwere placed in isolated organ baths and drug-induced modifications of both spontaneous and electrically evoked contractile activity were measured. None of the opioid agonists induced a significant change in spontaneous contractions. Fentanyl and U69593 reduced electrically induced contractions, whereas morphine reduced them only slightly. Naloxone competitively antagonised U69593, but both naloxone and naloxonazine were unable to counteract the inhibition of contractions induced by fentanyl. The inhibition of contractions shown by fentanyl is therefore probably not mediated by opioid receptors, but due to an anticholinergic activity of this drug. In summary, these data showed an inhibitory effect exerted by k receptors on equine small intestinal motility, whereas the role of m receptors seemed marginal and would need further characterisation.