The Red Nucleus Interleukin-6 Participates in the Maintenance of Neuropathic Pain Induced by Spared Nerve Injury

Red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain through stimulating the expressions of TNF-α and IL-1β

Our previous study has identified that glutamate in the red nucleus (RN) facilitates the development of neuropathic pain through metabotropic glutamate receptors (mGluR). Here, we further explored the actions and possible molecular mechanisms of red nucleus mGluR Ⅰ (mGluR1 and mGluR5) in the development of neuropathic pain induced by spared nerve injury (SNI). Our data indicated that both mGluR1 and mGluR5 were constitutively expressed in the RN of normal rats. Two weeks after SNI, the expressions of mGluR1 and mGluR5 were significantly boosted in the RN contralateral to the nerve injury. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN contralateral to the nerve injury at 2 weeks post-SNI significantly ameliorated SNI-induced neuropathic pain. However, unilateral administration of mGluRⅠ agonist DHPG to the RN of normal rats provoked a significant mechanical allodynia, this effect could be blocked by LY367385 or MTEP. Further studies indicated that the expressions of TNF-α and IL-1β in the RN were also elevated at 2 weeks post-SNI. Administration of mGluR1 antagonist LY367385 or mGluR5 antagonist MTEP to the RN at 2 weeks post-SNI significantly inhibited the elevations of TNF-α and IL-1β. However, administration of mGluR Ⅰ agonist DHPG to the RN of normal rats significantly enhanced the expressions of TNF-α and IL-1β, these effects were blocked by LY367385 or MTEP. These results suggest that activation of red nucleus mGluR1 and mGluR5 facilitate the development of neuropathic pain by stimulating the expressions of TNF-α and IL-1β. mGluR Ⅰ maybe potential targets for drug development and clinical treatment of neuropathic pain.

Red nucleus mGluR2 but not mGluR3 mediates inhibitory effect in the development of SNI-induced neuropathological pain by suppressing the expressions of TNF-α and IL-1β

Our previous study has verified that activation of group Ⅰ metabotropic glutamate receptors (mGluRⅠ) in the red nucleus (RN) facilitate the development of neuropathological pain. Here, we further discussed the functions and possible molecular mechanisms of red nucleus mGluR Ⅱ (mGluR2 and mGluR3) in the development of neuropathological pain induced by spared nerve injury (SNI). Our results showed that mGluR2 and mGluR3 both were constitutively expressed in the RN of normal rats. At 2 weeks post-SNI, the protein expression of mGluR2 rather than mGluR3 was significantly reduced in the RN contralateral to the nerve lesion. Injection of mGluR2/3 agonist LY379268 into the RN contralateral to the nerve injury at 2 weeks post-SNI significantly attenuated SNI-induced neuropathological pain, this effect was reversed by mGluR2/3 antagonist EGLU instead of selective mGluR3 antagonist β-NAAG. Intrarubral injection of LY379268 did not alter the PWT of contralateral hindpaw in normal rats, while intrarubral injection of EGLU rather than β-NAAG provoked a significant mechanical allodynia. Further studies indicated that the expressions of nociceptive factors TNF-α and IL-1β in the RN were enhanced at 2 weeks post-SNI. Intrarubral injection of LY379268 at 2 weeks post-SNI significantly suppressed the overexpressions of TNF-α and IL-1β, these effects were reversed by EGLU instead of β-NAAG. Intrarubral injection of LY379268 did not influence the protein expressions of TNF-α and IL-1β in normal rats, while intrarubral injection of EGLU rather than β-NAAG significantly boosted the expressions of TNF-α and IL-1β. These findings suggest that red nucleus mGluR2 but not mGluR3 mediates inhibitory effect in the development of SNI-induced neuropathological pain by suppressing the expressions of TNF-α and IL-1β. mGluR Ⅱ may be potential targets for drug development and clinical treatment of neuropathological pain.

RNA regulation of inflammatory responses in glia and its potential as a therapeutic target in central nervous system disorders

A major hallmark of neuroinflammation is the activation of microglia and astrocytes with the induction of inflammatory mediators such as IL-1β, TNF-α, iNOS, and IL-6. Neuroinflammation contributes to disease progression in a plethora of neurological disorders ranging from acute CNS trauma to chronic neurodegenerative disease. Posttranscriptional pathways of mRNA stability and translational efficiency are major drivers for the expression of these inflammatory mediators. A common element in this level of regulation centers around the adenine- and uridine-rich element (ARE) which is present in the 3' untranslated region (UTR) of the mRNAs encoding these inflammatory mediators. (ARE)-binding proteins (AUBPs) such as Human antigen R (HuR), Tristetraprolin (TTP) and KH- type splicing regulatory protein (KSRP) are key nodes for directing these posttranscriptional pathways and either promote (HuR) or suppress (TTP and KSRP) glial production of inflammatory mediators. This review will discuss basic concepts of ARE-mediated RNA regulation and its impact on glial-driven neuroinflammatory diseases. We will discuss strategies to target this novel level of gene regulation for therapeutic effect and review exciting preliminary studies that underscore its potential for treating neurological disorders.

Inhibition of the RNA Regulator HuR by SRI-42127 Attenuates Neuropathic Pain After Nerve Injury Through Suppression of Neuroinflammatory Responses

Microglial activation with the production of pro-inflammatory mediators such as IL-6, TNF-α, and IL-1β, is a major driver of neuropathic pain (NP) following peripheral nerve injury. We have previously shown that the RNA binding protein, HuR, is a positive node of regulation for many of these inflammatory mediators in glia and that its chemical inhibition or genetic deletion attenuates their production. In this report, we show that systemic administration of SRI-42127, a novel small molecule HuR inhibitor, attenuates mechanical allodynia, a hallmark of NP, in the early and chronic phases after spared nerve injury in male and female mice. Flow cytometry of lumbar spinal cords in SRI-42127-treated mice shows a reduction in infiltrating macrophages and a concomitant decrease in microglial populations expressing IL-6, TNF-α, IL-1β, and CCL2. Immunohistochemistry, ELISA, and qPCR of lumbar spinal cord tissue indicate suppression of these cytokines and other inflammatory mediators. ELISA of plasma samples in the acute phase also shows attenuation of inflammatory responses. In summary, inhibition of HuR by SRI-42127 leads to the suppression of neuroinflammatory responses and allodynia after nerve injury and represents a promising new direction in the treatment of NP.

Red Nucleus Interleukin-6 Evokes Tactile Allodynia in Male Rats Through Modulating Spinal Pro-inflammatory and Anti-inflammatory Cytokines

Our previous studies have clarified that red nucleus (RN) interleukin (IL)-6 is involved in the maintenance of neuropathic pain and produces a facilitatory effect by activating JAK2/STAT3 and ERK pathways. In this study, we further explored the immune molecular mechanisms of rubral IL-6-mediated descending facilitation at the spinal cord level. IL-6-evoked tactile allodynia was established by injecting recombinant IL-6 into the unilateral RN of naive male rats. Following intrarubral administration of IL-6, obvious tactile allodynia was evoked in the contralateral hindpaw of rats. Meanwhile, the expressions of pro-inflammatory cytokines tumor necrosis factor-α (TNF-α), IL-1β, and IL-6 were elevated in the contralateral spinal dorsal horn (L4–L6), blocking spinal TNF-α, IL-1β, or IL-6 with neutralizing antibodies relieved IL-6-evoked tactile allodynia. Conversely, the levels of anti-inflammatory cytokines transforming growth factor-β (TGF-β) and IL-10 were reduced in the contralateral spinal dorsal horn (L4–L6), an intrathecal supplement of exogenous TGF-β, or IL-10 attenuated IL-6-evoked tactile allodynia. Further studies demonstrated that intrarubral pretreatment with JAK2/STAT3 inhibitor AG490 suppressed the elevations of spinal TNF-α, IL-1β, and IL-6 and promoted the expressions of TGF-β and IL-10 in IL-6-evoked tactile allodynia rats. However, intrarubral pretreatment with ERK inhibitor PD98059 only restrained the increase in spinal TNF-α and enhanced the expression of spinal IL-10. These findings imply that rubral IL-6 plays descending facilitation and produces algesic effect through upregulating the expressions of spinal pro-inflammatory cytokines TNF-α, IL-1β, and IL-6 and downregulating the expressions of spinal anti-inflammatory cytokines TGF-β and IL-10 by activating JAK2/STAT3 and/or ERK pathways, which provides potential therapeutic targets for the treatment of pathological pain.

Electroacupuncture ameliorates mechanical hypersensitivity by down-regulating spinal Janus kinase 2/signal transducer and activation of transcription 3 and interleukin 6 in rats with spared nerve injury

BACKGROUND

Evidence shows that the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling pathway participates in the pathogenesis of neuropathic pain. Our previous study revealed that electroacupuncture (EA) attenuated neuropathic pain via activation of alpha-7 nicotinic acetylcholine receptor (α7nAChR) in the spinal cord. However, whether 2 Hz EA alleviates neuropathic pain by regulating the downstream molecules JAK2/STAT3 has not been fully clarified.

METHODS

Paw withdrawal threshold (PWT) was used as a marker of mechanical allodynia in rats with spared nerve injury (SNI). After applying 2 Hz EA on day 3, 7, 14 and 21 post-surgery, spinal expression of JAK2, STAT3 and pro-inflammatory cytokine interleukin (IL)-6 was examined using quantitative reverse transcription and real-time polymerase chain reaction (qRT-PCR) and Western blot analysis. Intrathecal injection of the α7nAChR antagonist alpha-bungarotoxin (α-Bgtx) was used to further explore the mechanism underlying the effects of 2 Hz EA on expression of JAK2/STAT3 in SNI rats.

RESULTS

It was found that levels of spinal STAT3 and IL-6 mRNA, as well as levels of phosphorylated (p)-JAK2, p-STAT3 and IL-6 protein, were markedly increased in SNI rats. 2 Hz EA attenuated the SNI-induced up-regulation of p-JAK2, p-STAT3 and IL-6 expression in the spinal cord. Furthermore, intrathecal injection of α-Bgtx (1.0 μg/kg) not only inhibited the effect of 2 Hz EA on mechanical hypersensitivity but also ameliorated the down-regulation of p-JAK2, p-STAT3 and IL-6 expression induced by 2 Hz EA.

CONCLUSION

This study revealed that 2 Hz EA attenuated SNI-induced mechanical hypersensitivity and the concomitant up-regulation of spinal JAK2, STAT3 and IL-6 in SNI rats, suggesting that suppression of the JAK2/STAT3 signaling pathway might be the mechanism underlying the therapeutic effect of 2 Hz EA on neuropathic pain.

Red nucleus IL-33 facilitates the early development of mononeuropathic pain in male rats by inducing TNF-α through activating ERK, p38 MAPK, and JAK2/STAT3

Background

Our recent studies have identified that the red nucleus (RN) dual-directionally modulates the development and maintenance of mononeuropathic pain through secreting proinflammatory and anti-inflammatory cytokines. Here, we further explored the action of red nucleus IL-33 in the early development of mononeuropathic pain.

Methods

In this study, male rats with spared nerve injury (SNI) were used as mononeuropathic pain model. Immunohistochemistry, Western blotting, and behavioral testing were used to assess the expressions, cellular distributions, and actions of red nucleus IL-33 and its related downstream signaling molecules.

Results

IL-33 and its receptor ST2 were constitutively expressed in the RN in naive rats. After SNI, both IL-33 and ST2 were upregulated significantly at 3 days and peaked at 1 week post-injury, especially in RN neurons, oligodendrocytes, and microglia. Blockade of red nucleus IL-33 with anti-IL-33 neutralizing antibody attenuated SNI-induced mononeuropathic pain, while intrarubral administration of exogenous IL-33 evoked mechanical hypersensitivity in naive rats. Red nucleus IL-33 generated an algesic effect in the early development of SNI-induced mononeuropathic pain through activating NF-κB, ERK, p38 MAPK, and JAK2/STAT3, suppression of NF-κB, ERK, p38 MAPK, and JAK2/STAT3 with corresponding inhibitors markedly attenuated SNI-induced mononeuropathic pain or IL-33-evoked mechanical hypersensitivity in naive rats. Red nucleus IL-33 contributed to SNI-induced mononeuropathic pain by stimulating TNF-α expression, which could be abolished by administration of inhibitors against ERK, p38 MAPK, and JAK2/STAT3, but not NF-κB.

Conclusions

These results suggest that red nucleus IL-33 facilitates the early development of mononeuropathic pain through activating NF-κB, ERK, p38 MAPK, and JAK2/STAT3. IL-33 mediates algesic effect partly by inducing TNF-α through activating ERK, p38 MAPK and JAK2/STAT3.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02198-9.

Role of IL-6 in the regulation of neuronal development, survival and function

The pleiotropic cytokine interleukin-6 (IL-6) is emerging as a molecule with both beneficial and destructive potentials. It can exert opposing actions triggering either neuron survival after injury or causing neurodegeneration and cell death in neurodegenerative or neuropathic disorders. Importantly, neurons respond differently to IL-6 and this critically depends on their environment and whether they are located in the peripheral or the central nervous system. In addition to its hub regulator role in inflammation, IL-6 is recently emerging as an important regulator of neuron function in health and disease, offering exciting possibilities for more mechanistic insight into the pathogenesis of mental, neurodegenerative and pain disorders and for developing novel therapies for diseases with neuroimmune and neurogenic pathogenic components.

Red nucleus IL-6 mediates the maintenance of neuropathic pain by inducing the productions of TNF-α and IL-1β through the JAK2/STAT3 and ERK signaling pathways

We previously reported that interleukin (IL)-6 in the red nucleus (RN) is involved in the maintenance of neuropathic pain induced by spared nerve injury (SNI), and exerts a facilitatory effect via Janus-activated kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) and extracellular signal-regulated kinase (ERK) signal transduction pathways. The present study aimed at investigating the roles of tumor necrosis factor-α (TNF-α) and IL-1β in RN IL-6-mediated maintenance of neuropathic pain and related signal transduction pathways. Being similar to the elevation of RN IL-6 three weeks after SNI, increased protein levels of both TNF-α and IL-1β were also observed in the contralateral RN three weeks after the nerve injury. The upregulations of TNF-α and IL-1β were closely correlative with IL-6 and suppressed by intrarubral injection of a neutralizing antibody against IL-6. Administration of either the JAK2 antagonist AG490 or the ERK antagonist PD98059 to the RN of rats with SNI remarkably increased the paw withdrawal threshold (PWT) and inhibited the up-regulations of local TNF-α and IL-1β. Further experiments indicated that intrarubral injection of exogenous IL-6 in naive rats apparently lowered the PWT of the contralateral hindpaw and boosted the local expressions of TNF-α and IL-1β. Pretreatment with AG490 could block IL-6-induced tactile hypersensitivity and suppress the up-regulations of both TNF-α and IL-1β. However, injection of PD98059 in advance only inhibited the upregulation of IL-1β, but not TNF-α. These findings indicate that RN IL-6 mediates the maintenance of neuropathic pain by inducing the productions of TNF-α and IL-1β. IL-6 induces the expression of TNF-α through the JAK2/STAT3 pathway, and the production of IL-1β through the JAK2/STAT3 and ERK pathways.

Identification of Differentially Expressed Genes and Key Pathways in the Dorsal Root Ganglion After Chronic Compression

Neuropathic pain (NP) is caused by primary or secondary impairment of the peripheral or central nervous systems. Its etiology is complex and involves abnormal patterns of gene expression and pathway activation. Using bioinformatics analysis, we aimed to identify NP-associated changes in genes and pathways in L4 and L5 dorsal root ganglia (DRG) in a rat model of NP induced by chronic compression of the DRG (CCD). Genome-wide transcriptional analyses were used to elucidate the molecular mechanisms underlying NP. We screened differentially expressed genes (DEGs) 7 days after CCD in comparison with sham-operated controls. Quantitative real-time polymerase chain reaction (RT-qPCR) and western blotting were used to confirm the presence of key DEGs. Kyoto Encyclopedia of Genes and Genomes (KEGG)-pathway analysis of DEGs and global signal transduction network analysis of DEGs were also conducted. The CCD group developed clear mechanical and thermal allodynia in the ipsilateral hind paw compared with the sham group. This comparison identified 1,887 DEGs, with 1156 upregulated and 731 downregulated DEGs, and 123 DEG-enriched pathways. We identified the key candidate genes that might play a role in the development of NP, namely syndecan 1 (Sdc1), phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit gamma (Pi3k), Janus kinase 2 (Jak2), jun proto-oncogene, AP-1 transcription factor subunit (Jun), and interleukin 6 (IL-6) by analyzing the global signal transduction network. RT-qPCR and western blot analysis confirmed the microarray results. The DEGs Sdc1, Pi3k, Jak2, Jun, and IL-6, and the cytokine signaling pathway, the neuroactive ligand-receptor interaction, the toll-like receptor signaling pathway, and the PI3K-Akt signaling pathway may have decisive modulatory roles in both nerve regeneration and NP. These results provide deeper insight into the mechanism underlying NP and promising therapeutic targets for its treatment.

Gabexate mesilate ameliorates the neuropathic pain in a rat model by inhibition of proinflammatory cytokines and nitric oxide pathway via suppression of nuclear factor-κB

BACKGROUND

This study examined the effects of gabexate mesilate on spinal nerve ligation (SNL)-induced neuropathic pain. To confirm the involvement of gabexate mesilate on neuroinflammation, we focused on the activation of nuclear factor-κB (NF-κB) and consequent the expression of proinflammatory cytokines and inducible nitric oxide synthase (iNOS).

METHODS

Male Sprague-Dawley rats were used for the study. After randomization into three groups: the sham-operation group, vehicle-treated group (administered normal saline as a control), and the gabexate group (administered gabexate mesilate 20 mg/kg), SNL was performed. At the 3rd day, mechanical allodynia was confirmed using von Frey filaments, and drugs were administered intraperitoneally daily according to the group. The paw withdrawal threshold (PWT) was examined on the 3rd, 7th, and 14th day. The expressions of p65 subunit of NF-κB, interleukin (IL)-1, IL-6, tumor necrosis factor-α, and iNOS were evaluated on the 7th and 14th day following SNL.

RESULTS

The PWT was significantly higher in the gabexate group compared with the vehicle-treated group (P < 0.05). The expressions of p65, proinflammatory cytokines, and iNOS significantly decreased in the gabexate group compared with the vehicle-treated group (P < 0.05) on the 7th day. On the 14th day, the expressions of p65 and iNOS showed lower levels, but those of the proinflammatory cytokines showed no significant differences.

CONCLUSIONS

Gabexate mesilate increased PWT after SNL and attenuate the progress of mechanical allodynia. These results seem to be involved with the anti-inflammatory effect of gabexate mesilate via inhibition of NF-κB, proinflammatory cytokines, and nitric oxide.

P2Y6 Receptor-Mediated Spinal Microglial Activation in Neuropathic Pain

Objective

To explore the role of purine family member P2Y6 receptors in regulating neuropathic pain (NP) via neuroinflammation in the spinal cord.

Methods

Chronic constriction injury of the sciatic nerve (CCI) of NP was classic in setting up models on Sprague-Dawley (SD) rats. Experiments were performed on rats with sham surgery, CCI, CCI + MRS2578 (a P2Y6 receptor antagonist), and UDP (a P2Y6 receptor agonist). The hyperalgesia intensity was mirrored by paw withdrawal threshold (PWT) and thermal withdrawal latency (TWL). Immunofluorescence staining and western blot were used to evaluate activated microglial marker Iba-1. Enzyme-linked immunosorbent assay (ELISA) was used to access levels of IL-6. Conventional reverse transcription polymerase chain reaction (RT-PCR) and western blot analysis were used to detect the expression of P2Y6 mRNA and activation of JAK/STAT signaling.

Results

Among all groups, CCI caused decreased PWT and TWL compared to sham surgery, meaning a successful establishment of the NP model. These decreased values of PWT and TWL tests could be prevented by intraperitoneally injected MRS2578 and enhanced by UDP administration. Similarly, CCI induced increase of Iba-1 protein, P2Y6 mRNA expression, and circulating IL-6 secretion, as well as increased JAK2/STAT3 mRNA expression and phosphorylating modification in spinal cord tissues could also be diminished by MRS2578 treatment and exacerbated by UDP.

Conclusions

These findings indicated the crucial role of the P2Y6 receptor in modulating the microglial and inflammatory responses in the process of NP in vivo. Results from this study would provide insights into targeting the P2Y6 receptor to treat NP in the near future.

Red nucleus interleukin-1β evokes tactile allodynia through activation of JAK/STAT3 and JNK signaling pathways

We previously reported that interleukin-1β (IL-1β) in the red nucleus (RN) is involved in pain modulation and exerts a facilitatory effect in the development of neuropathic pain. Here, we explored the actions of signaling pathways, including the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3), c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor-κB (NF-κB) pathways, on RN IL-1β-mediated pain modulation. After a single dose of recombinant rat IL-1β (rrIL-1β, 10 ng) injected into the RN in normal rats, a tactile allodynia was evoked in the contralateral but not ipsilateral hindpaw, commencing 75 min and peaking 120 min postinjection. Up-regulated protein levels of phospho-STAT3 (p-STAT3) and p-JNK were observed in the RN 120 min after rrIL-1β injection, the increases of p-STAT3 and p-JNK were blocked by anti-IL-1β antibody. However, the expression levels of p-ERK, p-p38 MAPK, and NF-κB in the RN were not affected by rrIL-1β injection. RN neurons and astrocytes contributed to IL-1β-evoked up-regulation of p-STAT3 and p-JNK. Further studies demonstrated that injection of the JAK2 antagonist AG490 or JNK antagonist SP600125 into the RN 30 min prior to the administration of rrIL-1β could completely prevent IL-1β-evoked tactile allodynia, while injection of the ERK antagonist PD98059, p38 MAPK antagonist SB203580, or NF-κB antagonist PDTC did not affect IL-1β-evoked tactile allodynia. In conclusion, our data provide additional evidence that RN IL-1β is involved in pain modulation, and that it exerts a facilitatory effect by activating the JAK/STAT3 and JNK signaling pathways.

Tizanidine exerts anti-nociceptive effects in spared nerve injury model of neuropathic pain through inhibition of TLR4/NF-κB pathway

Recently, α2-adrenoceptors (α2-AR) agonists have been shown to have anti-nociceptive effects and thus may become a promising therapeutic strategy for neuropathic pain. tizanidine is a highly selective α2-AR agonist, but the effect mechanism of tizanidine in neuropathic pain remains largely unknown. The present study investigated whether tizanidine has anti-nociceptive effects in spared nerve injury (SNI) model of neuropathic pain in rats, as well as explored the underlying molecular mechanism. We found that the rats in SNI group showed significantly higher mechanical and thermal hyperalgesia, accompanied with increased production of proinflammatory cytokines including interleukin-1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α), as well as the activation of Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling. PDTC, an inhibitor of TLR4/NF-κB signaling, could significantly attenuate the SNI-induced mechanical and thermal hyperalgesia and the production of proinflammatory cytokines. Moreover, treatment with tizanidine also attenuated the SNI-induced mechanical and thermal hyperalgesia, suppressed production of the proinflammatory cytokines, and inhibited the activation of TLR4/NF-κB pathway, which could be reversed by pretreatment with BRL44408, a selective α2-AR antagonist. Taken these findings together, we demonstrated that tizanidine has anti-nociceptive effects on neuropathic pain via inhibiting the production of proinflammatory cytokines through suppressing the activation of TLR4/NF-κB p65 signaling pathway.

Red nucleus interleukin-6 participates in the maintenance of neuropathic pain through JAK/STAT3 and ERK signaling pathways

We previously reported that interleukin-6 (IL-6) in the red nucleus (RN) is up-regulated at 3weeks after spared nerve injury (SNI), and plays facilitated role in the later maintenance of neuropathic pain. The current study aimed to reveal the roles of different signaling pathways, including Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK) and phosphatidylinositide 3-kinase/protein kinase B (PI3K/AKT), in RN IL-6-mediated pain modulation. In accord with the increase of IL-6 in the RN following SNI, the protein levels of phospho-STAT3 (p-STAT3), p-ERK and p-JNK were also up-regulated in the RN contralateral to the nerve injury side at 3weeks after SNI. The increases of p-STAT3 and p-ERK (but not p-JNK) were associated with IL-6 and could be blocked by anti-IL-6 antibody. Microinjection of JAK2 inhibitor AG490, ERK inhibitor PD98059 and also JNK inhibitor SP600125 into the RN significantly increased the paw withdrawal threshold (PWT) and alleviated SNI-induced mechanical allodynia. Further studies showed that microinjection of recombinant rat IL-6 (rrIL-6, 20ng) into the RN of normal rats significantly decreased the PWT of rats and increased the local protein levels of p-STAT3 and p-ERK, but not p-JNK. Pre-treatment with AG490 and PD98059 could prevent IL-6-induced mechanical allodynia. Whereas, p-p38 MAPK and p-AKT did not show any expression changes in the RN of rats with SNI or rats treated with rrIL-6. These results suggest that RN IL-6 participates in the later maintenance of SNI-induced neuropathic pain and plays facilitated role through activating JAK/STAT3 and ERK signaling pathways.

Inhibition of interleukin-6 function attenuates the central sensitization and pain behavior induced by osteoarthritis

Chronic pain is the most prominent and disabling symptom in the patients with osteoarthritis (OA), and the underlying mechanism largely remains unclear. Interleukin-6 (IL-6), a proinflammatory cytokine, is critically involved in the development and maintenance of central sensitization in several rodent models of chronic pain. The present study aims to elucidate the IL-6 mediated neurological adaptation in dorsal horn in the rat with monosodium iodoacetate (MIA) - induced OA. Significant upregulation of IL-6 expression was detected in the dorsal horn in the modeled rats. Blockade of IL-6 function by tocilizumab markedly suppressed the activation of astrocytes and microglia in the ipsilateral dorsal horn, reduced c-Fos immunoreactivity in dorsal horn neurons, and attenuated the upregulation of glutamate receptor subunits GluR1 and NR2B in dorsal horn in the rats with MIA-induced OA. It was further reported that administration of tocilizumab significantly improved the performance in weight-bearing test and mitigated the mechanical allodynia in the modeled rats. These data illustrated spinal IL-6 mediated mechanism underlying the chronic pain, and proposed the potential therapeutic effect of tocilizumab on the chronic pain in the setting of OA.