Differences in the characteristics of tolerance to μ-opioid receptor agonists in the colon from wild type and β-arrestin2 knockout mice

Abstract

Drawbacks to opioid use include development of analgesic tolerance and persistent constipation. We previously reported that tolerance to morphine develops upon repeated exposure in the isolated ileum but not the isolated colon. The cellular mechanisms of antinociceptive tolerance vary among μ-opioid receptor agonists. In this study, we assess β-arrestin2 deletion on the development of tolerance to different opioids in ileum and colon circular muscle. Tolerance was determined by assessing the ability of repeated in-vitro opioid exposure to induce contraction of the circular muscle from C57BL/6 wild type (WT) and β-arrestin2 knockout (KO) mice. Repeated exposure every 30 min with in-between washes resulted in tolerance to all agonists in the ileum of both WT and KO mice. However, in the colon of WT mice, comparison of the contractions between the 4th exposure and 1st response was similar to DAMGO (100 ± 10%; N = 5) but reduced to fentanyl (62 ± 13%; N = 8) and etorphine (38 ± 4%; N = 7) indicative of tolerance to fentanyl and etorphine but not DAMGO. In contrast, all agonists produced tolerance in the colon of KO: DAMGO response at the 4th exposure decreased to 52 ± 10% (N = 5), fentanyl to 20 ± 5% (N = 6) and etorphine 33 ± 7% (N = 6). Differences in tolerance among opioid agonists in the colon suggest ligand bias. The deletion of β-arrestin2 in colon appears to be necessary for tolerance to DAMGO but not fentanyl or etorphine. β-arrestin2 potentially represents an important target for treating opioid-induced bowel dysfunction and warrants further exploration of its ligand bias.

Allostatic Mechanisms of Opioid Tolerance Beyond Desensitization and Downregulation

“…For example, morphine analgesic tolerance is attenuated in the absence of arrestin 3, whereas that of fentanyl is not, despite both agonists being able to recruit arrestin 3 (Figure 1). Arrestin 3 in the colon appears to be necessary for tolerance to the MOP agonist DAMGO, but not fentanyl or etorphine, despite all three agonists effectively inducing MOP internalization [23]. It was also found that JNK2 can regulate MOP tolerance, and recruitment of this molecule can be arrestin-dependent or -independent, depending on the MOP agonist used [24].…”

Biased Agonism of Endogenous Opioid Peptides at theμ-Opioid Receptor

“…Evidence of the involvement of b-arrestin 2 (b-arr2) in the mediation of the adverse effects of morphine has prompted the search for opioids that do not induce b-arr2 recruitment. Indeed, enhanced morphine analgesia with reduced respiratory and gastrointestinal side effects was reported in b-arr2 knockout mice (Bohn et al, 1999;Raehal et al, 2005;Maguma et al, 2012). Recently, ligands with impaired b-arr2 recruitment have been reported to provide potent analgesia with less severe side effects (Groer et al, 2007;DeWire et al, 2013;Soergel et al, 2014).…”

Specific Localization of β-Arrestin2 in Myenteric Plexus of Mouse Gastrointestinal Tract

“…Previous studies from our lab strongly suggest a direct interaction between MOR activation and β-arrestin2 since downregulation of β-arrestin2 is associated with the development of tolerance to opioids in the intestinal tract [8] [7]. β-arrestin2, is a key molecule involved in desensitization of activated G protein-coupled receptors.…”

“…In addition, the selective expression of MOR 1 in neurons confirms our previous functional findings and support the school of thought that MOR 1 receptors are selectively expressed in the neurons and not in the smooth muscle. Our discovery that β-arrestin2 is exclusively expressed in neuronal cells suggests that the opioid tolerance observed following β-arrestin2 deletion is likely due to plasticity within the myenteric plexus [7]. Based on the current findings showing co-localization between MOR 1 and β-arrestin2, and on previously published data showing morphine induced downregulation of β-arrestin2 [5], [8], we postulate that morphine tolerance triggered by sustained activation of MOR 1 receptors is, at least partially, imparted in β-arrestin2 expressing neurons and potentially represents an important therapeutic target.…”