Suppressing HMGB1-TLR4-Mediated Neuroinflammation Alleviates Morphine Tolerance Via Inhibiting AMPK-HO-1 Pathway in the Spinal Cord of Mice

Background and objectives: A major unresolved issue in treating pain is the analgesic tolerance produced by opioids. Neuroinflammation was thought to be important in the development of morphine tolerance. The role of High mobility group box-1 (HMGB1) in morphine tolerance is elusive. Methods: ICR mice were used for tail-flick test to evaluate morphine tolerance. SD rats were used to collect the CSF to investigate whether morphine could induce the efflux of HMGB1 into extracellular environment. The neural cell line SH-SY5Y and the microglial cell line BV-2 were used to investigate the pharmacological effects and the mechanism of morphine-induced neuroinflammation. The activation of microglia was assessed by immunofluorescence staining. Neuroinflammation-related cytokines were measured by western blot and real-time PCR. The level of HMGB1 and related signaling pathway were evaluated by western blot and immunofluorescence staining.Results: Morphine induces the release of HMGB1 from neurons. The released HMGB1 activated microglia and triggered TLR4-mediated inflammatory response, leading to the phosphorylation of nuclear factor-κB (NF-κB) p65 and the upregulation of IL-1β. The secretion of HMGB1 was under the control of AMPK-HO-1 pathway. AMPK inhibitor and HO-1 inhibitor inhibited the release of HMGB1 and suppressed HMGB1-TLR4 mediated neuroinflammation.Conclusion: Our study indicated that morphine-induced extracellular HMGB1 was critical for morphine tolerance. The release of HMGB1 was regulated by AMPK-HO-1 pathway. Our findings may represent a bright prospect for the improvement of morphine tolerance with HMGB1 inhibitor and/or the inhibitors for AMPK-HO-1 axis.