Inhibition of motility in isolated horse small intestine is mediated by κ but not µ opioid receptors.
Authors: Menozzi A, Pozzoli C, Zullian C, Poli E, Serventi P, Bertini S
Journal: Equine veterinary journal
Summary
# Editorial Summary: Opioid Receptor Involvement in Equine Intestinal Motility Opioid medications are commonly used in equine practice for analgesia, yet their effects on gastrointestinal function remain incompletely understood—a particularly important consideration given the species' susceptibility to ileus and other motility disorders. Menozzi and colleagues investigated which opioid receptor subtypes mediate changes in small intestinal contractility by exposing isolated equine jejunal tissue to various opioid agonists (morphine, fentanyl, and U69593, a selective κ-receptor agonist) and antagonists in organ bath preparations, measuring effects on both spontaneous and electrically stimulated muscle contractions. The κ-receptor agonist U69593 significantly inhibited electrically induced contractions—an effect that was blocked by naloxone, confirming κ-receptor mediation—whilst morphine showed only marginal effects; fentanyl also reduced contractions, but this effect persisted despite opioid receptor antagonism, suggesting an ancillary anticholinergic mechanism rather than opioid-receptor-specific action. These findings indicate that κ-receptor activation represents the primary opioid-mediated inhibitory pathway in equine small intestinal motility, whilst μ-receptor involvement appears minimal. For clinical practitioners, this research suggests that drugs with selective κ-receptor activity carry the greatest risk of motility complications, and that the gastrointestinal effects of fentanyl may operate through non-opioid mechanisms deserving of separate consideration when selecting analgesics for colic cases or perioperative pain management.
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Practical Takeaways
- •κ-selective opioids may have greater inhibitory effects on equine intestinal motility than μ-selective agents, which could be relevant when selecting analgesics in horses with motility concerns
- •Fentanyl's anti-motility effects in horse small intestine appear independent of classical opioid receptors, suggesting its anticholinergic properties may be the primary mechanism affecting gut function
- •Further research is needed to characterize μ opioid receptor involvement in equine intestinal function before making clinical recommendations for opioid selection in cases of ileus or reduced motility
Key Findings
- •κ opioid receptor agonist (U69593) reduced electrically induced contractions in equine jejunal muscle, demonstrating κ receptor-mediated inhibition of intestinal motility
- •μ opioid receptor agonists (morphine and fentanyl) showed minimal or no effect on spontaneous contractions, indicating marginal role of μ receptors in equine small intestinal motility
- •Fentanyl-induced inhibition of contractions was not competitively antagonized by naloxone or naloxonazine, suggesting an anticholinergic rather than opioid receptor-mediated mechanism
- •Naloxone competitively antagonized U69593, confirming κ receptor specificity for the observed inhibitory effect on electrically evoked contractions