Opioid-Induced Pronociceptive Signaling in the Gastrointestinal Tract Is Mediated by Delta-Opioid Receptor Signaling

Abstract

Opioid tolerance (OT) leads to dose escalation and serious side effects, including opioid-induced hyperalgesia (OIH). We sought to better understand the mechanisms underlying this event in the gastrointestinal tract. Chronicin vivoadministration of morphine by intraperitoneal injection in male C57BL/6 mice evoked tolerance and evidence of OIH in an assay of colonic afferent nerve mechanosensitivity; this was inhibited by the δ-opioid receptor (DOPr) antagonist naltrindole when intraperitoneally injected in previous morphine administration. Patch-clamp studies of DRG neurons following overnight incubation with high concentrations of morphine, the µ-opioid receptors (MOPr) agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-Enkephalin (DAMGO) or the DOPr agonist [D-Ala2, D-Leu5]-Enkephalin evoked hyperexcitability. The pronociceptive actions of these opioids were blocked by the DOPr antagonist SDM25N but not the MOPr antagonist D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2. The hyperexcitability induced by DAMGO was reversed after a 1 h washout, but reapplication of low concentrations of DAMGO or [D-Ala2, D-Leu5]-Enkephalin restored the hyperexcitability, an effect mediated by protein kinase C. DOPr-dependent DRG neuron hyperexcitability was blocked by the endocytosis inhibitor Pitstop 2, and the weakly internalizing DOPr agonist ARM390 did not cause hyperexcitability. Bioluminescence resonance energy transfer studies in HEK cells showed no evidence of switching of G-protein signaling from Gito a Gspathway in response to either high concentrations or overnight incubation of opioids. Thus, chronic high-dose opioid exposure leads to opioid tolerance and features of OIH in the colon. This action is mediated by DOPr signaling and is dependent on receptor endocytosis and downstream protein kinase C signaling.SIGNIFICANCE STATEMENTOpioids are effective in the treatment of abdominal pain, but escalating doses can lead to opioid tolerance and potentially opioid-induced hyperalgesia. We found that δ-opioid receptor (DOPr) plays a central role in the development of opioid tolerance and opioid-induced hyperalgesia in colonic afferent nociceptors following prolonged exposure to high concentrations of MOPr or DOPr agonists. Furthermore, the role of DOPr was dependent on OPr internalization and activation of a protein kinase C signaling pathway. Thus, targeting DOPr or key components of the downstream signaling pathway could mitigate adverse side effects by opioids.

Evolving acidic microenvironments during colitis provide selective analgesic targets for a pH-sensitive opioid

“…5,15 Furthermore, our previous studies have shown that opioid responses in visceral DRG neurons did not differ between female and male mice. 19,50 Such findings are consistent with the current evidence on sex differences in opioid analgesia, suggesting sex-specific therapeutic interventions are not warranted. 11,12 The cumulative actions of opioids and their metabolites with repeated dosing could amplify potential on-target side effects or could cause tolerance.…”

Effects of the clathrin inhibitor Pitstop-2 on synaptic vesicle recycling at a central synapse in vivo

“…Of note, two biochemical studies have raised doubts about the specificity of Pitstop-2 as a clathrin inhibitor ( Dutta et al, 2012 ; Willox et al, 2014 ). However, there are also numerous studies showing Pitstop-2 to specifically block clathrin-dependent processes in neurons ( von Kleist et al, 2011 ; Merriam et al, 2013 ; Neef et al, 2014 ; Missig et al, 2017 ; DiCello et al, 2019 ; Gomez et al, 2021 ; Zhang et al, 2021 ; Jaramillo-Polanco et al, 2022 ). Nevertheless, potential off target effects of Pitstop-2 need to be considered with respect to our results.…”