Chronic morphine-mediated upregulation of high mobility group box 1 in the spinal cord contributes to analgesic tolerance and hyperalgesia in rats

Abstract

Analgesic tolerance and hyperalgesia hinder the long-term utility of opioids. We examined whether spinal high mobility group box 1 (HMGB1) is involved in morphine tolerance and its underlying mechanisms by using a model of repeated intrathecal (i.t.) injections of morphine. The results showed that chronic i.t. morphine exposure led to increased expression of HMGB1, Toll-like receptor 4 (TLR4), and receptor for advanced glycation end products (RAGE) and their mRNAs in the dorsal horn. Morphine challenge also promoted HMGB1 expression and release in cultured spinal neurons, but these effects were inhibited by TAK-242, naloxone (antagonists of TLR4), and TLR4 siRNA. Intrathecal coadministration of morphine with TAK-242 or PDTC (inhibitor of NF-κB activation) also reduced HMGB1 expression in the spinal cord. Repeated i.t. coinjections of morphine with glycyrrhizin (GL, an HMGB1 inhibitor) or HMGB1 siRNA prevented reduction of the maximal possible analgesic effect (MPAE) of morphine and alleviated morphine withdrawal-induced hyperalgesia. The established morphine tolerance and hyperalgesia were partially reversed when i.t. injections of GL or HMGB1 antibody started at day 7 of morphine injection. Repeated i.t. injections of morphine with HMGB1 siRNA inhibited the activation of NF-κB, but not that of JNK and p38. A single i.t. injection of HMGB1 in naïve rats caused pain-related hypersensitivity and reduction in MPAE. Moreover, phosphorylated NF-κB p65, TNF-α, and IL-1β levels in the dorsal horn were upregulated following this treatment, but this upregulation was prevented by coinjection with TAK-242. Together, these results suggest that morphine-mediated upregulation of spinal HMGB1 contributes to analgesic tolerance and hyperalgesia via activation of TLR4/NF-κB signaling, and the HMGB1 inhibitor might be a promising adjuvant to morphine in the treatment of intractable pain in the clinic.

Toll-Like Receptor 4 (TLR4)/Opioid Receptor Pathway Crosstalk and Impact on Opioid Analgesia, Immune Function, and Gastrointestinal Motility

“…In these two studies of morphine, only extracellular HMGB1, acting as a pro-inflammatory mediator, HMGB1 in the media also increased ( 66 , 67 ). Taken together, these findings indicate that morphine increases the expression and release of HMGB ( 66 , 67 ). Studies investigating the underlying mechanism demonstrated that TLR4, P2X7R, caspase-1 antagonists, and TLR4 siRNA inhibited the increased levels of HMGB1, while opioid receptor antagonists did not ( 66 ).…”

“…In macrophages and monocytes, HMGB1 was found to be released after stimulation with LPS, TNF-α, or IL-1β ( 64 , 65 ). Notably, a study of chronic intrathecal injection of morphine showed that the expression of HMGB1, TLR4, and RAGE in the rat spinal dorsal horn increased ( 66 ), while another study of a neuropathic pain model showed that subcutaneous administration of morphine increased HMGB1 expression even at 5 weeks after morphine was ceased ( 67 ). In these two studies of morphine, only extracellular HMGB1, acting as a pro-inflammatory mediator, HMGB1 in the media also increased ( 66 , 67 ).…”

“…Studies investigating the underlying mechanism demonstrated that TLR4, P2X7R, caspase-1 antagonists, and TLR4 siRNA inhibited the increased levels of HMGB1, while opioid receptor antagonists did not ( 66 ). Therefore, TLR4 may partially mediate morphine-induced HMGB1 production ( 66 , 67 ).…”

<p>Spinal TLR4/P2X7 Receptor-Dependent NLRP3 Inflammasome Activation Contributes to the Development of Tolerance to Morphine-Induced Antinociception</p>

“…In previous research, TLR4 signaling has been reported to contribute to the development of morphine tolerance in the spinal cord 37 and midbrain ventrolateral periaqueductal gray. 17 Morphine binds to MD-2, the accessory protein of TLR4, thereby activating TLR4 and triggering proinflammatory signaling, 18 such as by the release of IL-1β 38 and soluble TNF, 17 subsequently increasing robust neuroinflammatory responses, and ultimately increasing neuroexcitation.…”

Role of high‐mobility group box 1 and its modulation by thrombomodulin/thrombin axis in neuropathic and inflammatory pain

“…The emerging evidence for the role of HMGB1 thus opens a novel avenue for a deeper understanding of the known mechanisms for pathological pain, as HMGB1 directly activates RAGE and TLR4, and facilitates CXCR4 signalling through its dimerization by binding to two molecules of CXCL12 (see Figure 1b). There is also evidence that HMGB1 is involved in morphine tolerance and morphine withdrawal‐induced hyperalgesia/allodynia (Grace et al, 2018; Qian et al, 2019). Thus, it is necessary to clarify the relationship between HMGB1 and the known pain regulation systems.…”