Neuroexcitatory effects of morphine-3-glucuronide are dependent on Toll-like receptor 4 signaling

Abstract

BackgroundMultiple adverse events are associated with the use of morphine for the treatment of chronic non-cancer pain, including opioid-induced hyperalgesia (OIH). Mechanisms of OIH are independent of opioid tolerance and may involve the morphine metabolite morphine-3-glucuronide (M3G). M3G exhibits limited affinity for opioid receptors and no analgesic effect. Previous reports suggest that M3G can act via the Toll-like receptor 4 (TLR4)/myeloid differentiation protein-2 (MD-2) heterodimer in the central nervous system to elicit pain.MethodsImmunoblot and immunocytochemistry methods were used to characterize the protein expression of TLR4 present in lumbar dorsal root ganglion (DRG). Using in vitro intracellular calcium and current clamp techniques, we determined whether TLR4 activation as elicited by the prototypical agonists of TLR4, lipopolysaccharide (LPS) and M3G, contributed to changes in intracellular calcium and increased excitation. Rodents were also injected with M3G to determine the degree to which M3G-induced tactile hyperalgesia could be diminished using either a small molecule inhibitor of the MD-2/TLR4 complex in rats or TLR4 knockout mice. Whole cell voltage-clamp recordings were made from small- and medium-diameter DRG neurons (25 μm < DRG diameter <45 μm) for both control and M3G-treated neurons to determine the potential influence on voltage-gated sodium channels (NaVs).ResultsWe observed that TLR4 immunoreactivity was present in peptidergic and non-peptidergic sensory neurons in the DRG. Non-neuronal cells in the DRG lacked evidence of TLR4 expression. Approximately 15% of assayed small- and medium-diameter sensory neurons exhibited a change in intracellular calcium following LPS administration. Both nociceptive and non-nociceptive neurons were observed to respond, and approximately 40% of these cells were capsaicin-insensitive. Increased excitability observed in sensory neurons following LPS or M3G could be eliminated using Compound 15, a small molecule inhibitor of the TLR4/MD-2 complex. Likewise, systemic injection of M3G induced rapid tactile, but not thermal, nociceptive behavioral changes in the rat, which were prevented by pre-treating animals with Compound 15. Unlike TLR4 wild-type mice, TLR4 knockout mice did not exhibit M3G-induced hyperalgesia. As abnormal pain sensitivity is often associated with NaVs, we predicted that M3G acting via the MD-2/TLR4 complex may affect the density and gating of NaVs in sensory neurons. We show that M3G increases tetrodotoxin-sensitive and tetrodotoxin-resistant (NaV1.9) current densities.ConclusionsThese outcomes provide evidence that M3G may play a role in OIH via the TLR4/MD-2 heterodimer complex and biophysical properties of tetrodotoxin-sensitive and tetrodotoxin-resistant NaV currents.

Imaging Biomarkers of the Neuroimmune System among Substance Use Disorders: A Systematic Review

“…First, opioids elicit an acute inflammatory response likely via TLR4s [77, 78] and/or MORs [224-226]. There is reliable preclinical [227-229] and non-human primate [223] evidence indicating acute opioid administration is proinflammatory in the periphery and brain.…”

“…For example, opioids exert neuroimmune effects via directly binding receptors on microglia, e.g. TLR4s [77, 78] and MORs [224-226]. Other substances, e.g.…”

“…However, abused substances that exert neuroimmune effects via mechanisms unrelated to BBB permeability, i.e. drugs that directly bind receptors on immune cells (e.g., opioids [77, 78]), may not be detected by BBB markers. Finally, imaging morphology can provide valuable metrics for tracking structural changes, but are not specific to neuroimmune signaling per se.…”

Mechanistic Basis of Altered Morphine Disposition in Nonalcoholic Steatohepatitis

“…Although M3G lacks analgesic activity, it has been shown to target other pathways, such as the Toll-like receptor 4 pathway, to elicit neuroexcitatory effects (Due et al, 2012). Despite differing pharmacologic properties, the pharmacokinetic properties of these metabolites are comparable (Handal et al, 2002).…”

Medication-overuse headache and opioid-induced hyperalgesia: A review of mechanisms, a neuroimmune hypothesis and a novel approach to treatment

“…115 It has recently been demonstrated that morphine-3-glucuronide has TLR-4 agonist activity, indicating that codeine and morphine metabolites may contribute to this action. 119 For a detailed discussion regarding the role of TLR-4 in opioid-induced glial activation see review by Watkins at al. 92 Despite the established body of pre-clinical evidence human trials are yet to conclusively demonstrate the impact of central immune signalling on the action of opioids in patient groups or healthy volunteers.…”