Morphine activates neuroinflammation in a manner parallel to endotoxin

Abstract

Opioids create a neuroinflammatory response within the CNS, compromising opioid-induced analgesia and contributing to various unwanted actions. How this occurs is unknown but has been assumed to be via classic opioid receptors. Herein, we provide direct evidence that morphine creates neuroinflammation via the activation of an innate immune receptor and not via classic opioid receptors. We demonstrate that morphine binds to an accessory protein of Toll-like receptor 4 (TLR4), myeloid differentiation protein 2 (MD-2), thereby inducing TLR4 oligomerization and triggering proinflammation. Small-molecule inhibitors, RNA interference, and genetic knockout validate the TLR4/MD-2 complex as a feasible target for beneficially modifying morphine actions. Disrupting TLR4/MD-2 protein-protein association potentiated morphine analgesia in vivo and abolished morphine-induced proinflammation in vitro, the latter demonstrating that morphine-induced proinflammation only depends on TLR4, despite the presence of opioid receptors. These results provide an exciting, nonconventional avenue to improving the clinical efficacy of opioids.protein-protein interaction | pain management therapy | drug discovery

Nor-Binaltorphimine Blocks the Adverse Effects of Morphine after Spinal Cord Injury

“…In vitro and in vivo evidence suggests that opioid administration results in the activation of glial cells and the release of pro-inflammatory cytokines. [53][54][55][56]65,66 Although non-classic opioid receptor signaling has been implicated in these opioid-immune interactions, 8,67 our findings point to a previously overlooked role of KOR. Although not tested in this study, we posit that morphine's adverse effects may result from activation of KORs on glial cells.…”

“…6 Although morphine is a prototypical opioid agonist that binds with high affinity to the l-opioid receptor (MOR), it also binds with lower affinity to the d-opioid receptor (DOR) and KORs, as well as to non-classic receptors, such as the toll-like receptors (TLRs). 7,8 Using selective agonists for each opioid receptor subtype, we found that only administration of GR89696 (the KOR agonist) was sufficient to undermine recovery after SCI. 6 These data concur with previous literature.…”

Glial and Neuroimmune Mechanisms as Critical Modulators of Drug Use and Abuse

“…In silico docking simulations predicted that opioids would bind to the LPS-binding pocket of MD-2, the co-receptor for TLR4 , which was later confirmed in vitro (Wang et al, 2012a). In an extensive series of experiments, morphine was determined to induce conformational changes at the MD-2 structure to induce TLR4 oligomerization, resulting in pro-inflammatory interleukin production that was blocked by pharmacological or RNA interference at the TLR4/MD-2 complex (Wang et al, 2012a). Likewise, (+)-naloxone and (+)-naltrexone, which are stereoisomers of traditional antagonists to opioid receptors, have been characterized as selective antagonists to TLR4 signaling with no appreciable function at opioid receptors Wang et al, 2016).…”

“…Opioid-induced activation through non-classical binding has experimental precedent in the context of neuropathic pain (Watkins et al, 2009); opioidinduced hyperalgesia was reported in opioid receptor triple knockout mice, suggesting alternative sites of action for altering nociception (Juni et al, 2007). In silico docking simulations predicted that opioids would bind to the LPS-binding pocket of MD-2, the co-receptor for TLR4 , which was later confirmed in vitro (Wang et al, 2012a). In an extensive series of experiments, morphine was determined to induce conformational changes at the MD-2 structure to induce TLR4 oligomerization, resulting in pro-inflammatory interleukin production that was blocked by pharmacological or RNA interference at the TLR4/MD-2 complex (Wang et al, 2012a).…”

“…For instance, the morphine metabolite morphine-3-glucuronide exhibits strong affinity for the TLR4 receptor, while morphine-6-glucuronide exhibits no TLR4 activation (Lewis et al, 2010;Wang et al, 2012a). Alternative synthetic opioids would produce different metabolites, which could be an important factor when considering similarities and differences in behavioral outcomes to these opioid drugs.…”