Abstract

Opioid-induced constipation is a major side-effect that persists with chronic opioid use. Previous studies demonstrated that nicotine-induced contractions are enhanced following chronic morphine in guinea-pig ileum. In the present study we examined whether the increased sensitivity to nicotine could be observed in single enteric neurons following chronic morphine, determined the subunits in mouse enteric neurons and examined the effect of nicotine in reversing opioid-induced constipation. Nicotine (0.03 - 1 mM) dose-dependently induced inward currents from holding potential of -60 mV in isolated single enteric neurons from the mouse ileum. The amplitude of the currents, but not the potency to nicotine, was significantly increased in neurons exposed to long-term (16-24 hrs) but not short-term (10 mins) morphine. Quantitative mRNA analysis showed that nAchR subunit expression in the mouse ileum was α3 ≥ β2 > β4 > α5 > α4 > β3 > α6. Nicotine induced currents were obtained in neurons from α7, β2, α5 and α6 knock-out mice. The currents were however inhibited by mecamylamine (10 μM) and the α3β4 blocker, α-conotoxin AUIB (3 μM) suggesting that nicotine-indcued currents were mediated by α3β4 subtype of nAChRs on enteric neurons. Conversely, NS3861, a partial agonist at α3β4 nAChR enhanced fecal pellet expulsion in a dose-dependent manner in chronic but not acute morphine treated mice. Overall, our findings suggest that the efficacy of nAChR agonists on enteric neurons is enhanced after chronic morphine exposure and activation of α3β4 subtype of nAChR reverses chronic but not acute morphine induced constipation.