Enhanced Sensitivity of  3 4 Nicotinic Receptors in Enteric Neurons after Long-Term Morphine: Implication for Opioid-Induced Constipation

Abstract

Opioid-induced constipation is a major side effect that persists with long-term opioid use. Previous studies demonstrated that nicotine-induced contractions are enhanced after long-term morphine exposure in guinea pig ileum. In the present study, we examined whether the increased sensitivity to nicotine could be observed in single enteric neurons after long-term morphine exposure, 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 a holding potential of 260 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 receiving long-term (16-24 h) but not short-term (10 min) exposure to morphine. Quantitative mRNA analysis showed that nicotinic acetylcholine receptor (nAChR) subunit expression in the mouse ileum was a3 $ b2 . b4 . a5 . a4 . b3 . a6. Nicotine-induced currents were obtained in neurons from a7, b2, a5, and a6 knockout mice. The currents were, however, inhibited by mecamylamine (10 mM) and the a3b4 blocker a-conotoxin AuIB (3 mM), suggesting that nicotineinduced currents were mediated by the a3b4 subtype of nAChRs on enteric neurons. Conversely, NS3861, a partial agonist at a3b4 nAChR, enhanced fecal pellet expulsion in a dose-dependent manner in mice that received long-term, but not short-term, morphine treatment. Overall, our findings suggest that the efficacy of nAChR agonists on enteric neurons is enhanced after long-term morphine exposure, and activation of the a3b4 subtype of nAChR reverses chronic, but not acute, morphine-induced constipation.

Insights into the Role of Opioid Receptors in the GI Tract: Experimental Evidence and Therapeutic Relevance

“…An α3β4 nicotinic receptor agonist increased fecal pellet output in mice treated chronically but not acutely with morphine. These data suggest that tolerance to the inhibitory effects of morphine on gut motility may be due in part to upregulation of α3β4 nicotinic receptors on small intestinal myenteric neurons (Gade et al , 2016). …”

“…This effect would suppress interneuronal and neuromuscular transmission (Smith et al , 2012). Myenteric neurons in the guinea pig and mouse small intestine express nicotinic receptors composed of α3 and β4 subunits and these receptors mediate most fast excitatory postsynaptic potentials in the myenteric plexus (Zhou et al, 2002; Gade et al, 2016). Studies of mouse small intestinal myenteric neurons maintained in primary culture showed that nicotine-induced inward currents were larger in neurons exposed to morphine for 16–24 hours compared to neurons exposed to morphine for 1 hour.…”

The role of morphine‐ and fentanyl‐induced impairment of intestinal epithelial antibacterial activity in dysbiosis and its impact on the microbiota‐gut‐brain axis

“…There is increasing evidence that enteric neurons and gut‐innervating extrinsic neurons can modulate the response of the intestinal epithelial barrier to gut microbiota 46–48 . Morphine is known to modulate the excitability of these neurons by altering the activity of voltage‐gated ion channels 12,49–52 . Thus, altered activity of neurons might form the basis for opioid‐induced intestinal barrier dysfunction and the resultant dysbiosis.…”

The Role of Morphine-Induced Impairment of Intestinal Epithelial Antibacterial Activity in Dysbiosis and its Impact on the Microbiota-Gut-Brain Axis

“…VIPergic neurons have been shown to regulate the activity of intestinal epithelial cells via crosstalk with innate lymphoid cells, ILC2, and ILC3 [36][37][38] . Morphine is known to modulate the excitability of enteric neurons by altering the activity of voltage-gated ion channels [39][40][41][42] . Morphine can also alter the expression of IL-18 in enteric neurons via opioid receptors 25 .…”