STAT1 as a downstream mediator of ERK signaling contributes to bone cancer pain by regulating MHC II expression in spinal microglia

Battling pain from osteoarthritis: causing novel cell death

Osteoarthritis (OA) is a significant contributor to pain and disability worldwide. Pain is the main complaint of OA patients attending the clinic and has a large impact on their quality of life and economic standards. However, existing treatments for OA-related pain have not been shown to achieve good relief. The main focus is on preventing and slowing the progression of OA so that the problem of OA pain can be resolved. Pain caused by OA is complex, with the nature, location, duration, and intensity of pain changing as the disease progresses. Previous research has highlighted the role of various forms of cell death, such as apoptosis and necrosis, in the progression of pain in OA. Emerging studies have identified additional forms of novel cell death, such as pyroptosis, ferroptosis, and necroptosis that are linked to pain in OA. Different types of cell death contribute to tissue damage in OA by impacting inflammatory responses, reactive oxygen species (ROS) production, and calcium ion levels, ultimately leading to the development of pain. Evidence suggests that targeting novel types of cell death could help alleviate pain in OA patients. This review delves into the complex mechanisms of OA pain, explores the relationship between different modes of novel cell death and pain, and proposes novel cell death as a viable strategy for the treatment of these conditions, with the goal of providing scientific references for the development of future OA pain treatments and drugs.

Valproate attenuates somatic hyperalgesia induced by orofacial inflammation combined with stress through inhibiting spinal IL-6 and STAT1 phosphorylation

Temporomandibular disorder (TMD) and fibromyalgia syndrome (FMS) may present as comorbid conditions, but treatment options are ineffective. The purpose of this study was to investigate whether valproate (VPA) attenuates somatic hyperalgesia induced by orofacial inflammation combined with stress, which represents a model of pain associated with TMD and FMS comorbidity, and to explore the potential mechanisms. The results showed that VPA inhibited somatic hyperalgesia induced by orofacial inflammation combined with stress, and down-regulated the interleukin-6 (IL-6) expression in the L4-L5 spinal dorsal horn of female rats. The anti-nociceptive effect of VPA was blocked by single or 5 consecutive day intrathecal administration of recombinant rat IL-6. Orofacial inflammation combined with stress up-regulated the ratio of phosphorylated signal transducer and activator of transcription 1 (p-STAT1) to STAT1 (p-STAT1/STAT1) in the spinal cord. VPA did not affect the STAT1 expression, while it down-regulated the ratio of p-STAT1/STAT1. The expression of STAT3 and the ratio of p-STAT3/STAT3 were not affected by orofacial inflammation combined with stress and VPA treatment. Intrathecal administration of exogenous IL-6 up-regulated the ratio of p-STAT1/STAT1. These data indicate that VPA attenuated somatic hyperalgesia induced by orofacial inflammation combined with stress via inhibiting spinal IL-6 in female rats, and the mechanism may involve the alteration of activation status of spinal STAT1. Thus, VPA may be a new candidate analgesic that targets IL-6 and STAT1 for the treatment of pain associated with the comorbidity of TMD and FMS.

Ropivacaine-loaded hydrogels for prolonged relief of chemotherapy-induced peripheral neuropathic pain and potentiated chemotherapy

Chemotherapy can cause severe pain for patients, but there are currently no satisfactory methods of pain relief. Enhancing the efficacy of chemotherapy to reduce the side effects of high-dose chemotherapeutic drugs remains a major challenge. Moreover, the treatment of chemotherapy-induced peripheral neuropathic pain (CIPNP) is separate from chemotherapy in the clinical setting, causing inconvenience to cancer patients. In view of the many obstacles mentioned above, we developed a strategy to incorporate local anesthetic (LA) into a cisplatin-loaded PF127 hydrogel for painless potentiated chemotherapy. We found that multiple administrations of cisplatin-loaded PF127 hydrogels (PFC) evoked severe CIPNP, which correlated with increased pERK-positive neurons in the dorsal root ganglion (DRG). However, incorporating ropivacaine into the PFC relieved PFC-induced CIPNP for more than ten hours and decreased the number of pERK-positive neurons in the DRG. Moreover, incorporating ropivacaine into the PFC for chemotherapy is found to upregulate major histocompatibility complex class I (MHC-I) expression in tumor cells and promote the infiltration of cytotoxic T lymphocytes (CD8+ T cells) in tumors, thereby potentiating chemotherapy efficacy. This study proposes that LA can be used as an immunemodulator to enhance the effectiveness of chemotherapy, providing new ideas for painless cancer treatment.

The impact of sex and physical activity on the local immune response to muscle pain

Induction of muscle pain triggers a local immune response to produce pain and this mechanism may be sex and activity level dependent. The purpose of this study was to measure the immune system response in the muscle following induction of pain in sedentary and physically active mice. Muscle pain was produced via an activity-induced pain model using acidic saline combined with fatiguing muscle contractions. Prior to induction of muscle pain, mice (C57/BL6) were sedentary or physically active (24hr access to running wheel) for 8 weeks. The ipsilateral gastrocnemius was harvested 24hr after induction of muscle pain for RNA sequencing or flow cytometry. RNA sequencing revealed activation of several immune pathways in both sexes after induction of muscle pain, and these pathways were attenuated in physically active females. Uniquely in females, the antigen processing and presentation pathway with MHC II signaling was activated after induction of muscle pain; activation of this pathway was blocked by physical activity. Blockade of MHC II attenuated development of muscle hyperalgesia exclusively in females. Induction of muscle pain increased the number of macrophages and T-cells in the muscle in both sexes, measured by flow cytometry. In both sexes, the phenotype of macrophages shifted toward a pro-inflammatory state after induction of muscle pain in sedentary mice (M1 + M1/2) but toward an anti-inflammatory state in physically active mice (M2 + M0). Thus, induction of muscle pain activates the immune system with sex-specific differences in the transcriptome while physical activity attenuates immune response in females and alters macrophage phenotype in both sexes.

The bidirectional roles of the cGAS-STING pathway in pain processing: Cellular and molecular mechanisms

Pain is a common clinical condition. However, the mechanisms underlying pain are not yet fully understood. It is known that the neuroimmune system plays a critical role in the pathogenesis of pain. Recent studies indicated that the cyclic-GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway can activate the innate immune system by sensing both extrinsic and intrinsic double-stranded DNA in the cytoplasm, which is involved in pain processing. In this review, we summarise (1) the roles of the cGAS-STING pathway in different pain models, (2) the effect of the cGAS-STING pathway in different cells during pain regulation, and (3) the downstream molecular mechanisms of the cGAS-STING pathway in pain regulation. This review provides evidence that the cGAS-STING pathway has pro- and anti-nociceptive effects in pain models. It has different functions in neuron, microglia, macrophage, and T cells. Its downstream molecules include IFN-I, NF-κB, NLRP3, and eIF2α. The bidirectional roles of the cGAS-STING pathway in pain processing are mediated by regulating nociceptive neuronal sensitivity and neuroinflammatory responses. However, their effects in special brain regions, activation of astrocytes, and the different phases of pain require further exploration.

Spinal voltage-gated potassium channel Kv1.3 contributes to neuropathic pain via the promotion of microglial M1 polarization and activation of the NLRP3 inflammasome

BACKGROUD

Studies have shown that the activation of microglia is the main mechanism of neuropathic pain. Kv1.3 channel is a novel therapeutic target for treating neuroinflammatory disorders due to its crucial role in subsets of microglial cells. As such, it may be involved in the processes of neuropathic pain, however, whether Kv1.3 plays a role in neuroinflammation following peripheral nerve injury is unclear.

METHOD

The spared nerve injury model (SNI) was used to establish neuropathic pain. Western blot and immunofluorescence were used to examine the effect of Kv1.3 in the SNI rats. PAP-1, a Kv1.3 specific blocker was administered to alleviate neuropathic pain in the SNI rats.

RESULTS

Neuropathic pain and allodynia occurred after SNI, the levels of M1 (CD68, iNos) and M2 (CD206, Arg-1) phenotypes were up-regulated in the spinal cord, and the protein levels of NLRP3, caspase-1 and IL-1β were also increased. Pharmacological blocking of Kv1.3 with PAP-1 alleviated hyperpathia induced by SNI. Meanwhile, intrathecal injection of PAP-1 reduced M1 polarization and decreased NLRP3, caspase-1 and IL-1β expressions of protein levels.

CONCLUSION

Our research indicates that the Kv1.3 channel in the spinal cord contributes to neuropathic pain by promoting microglial M1 polarization and activating the NLRP3 inflammasome.

Downregulation of nuclear STAT2 protein in the spinal dorsal horn is involved in neuropathic pain following chronic constriction injury of the rat sciatic nerve

Regulation of gene transcription in the spinal dorsal horn (SDH) plays a critical role in the pathophysiology of neuropathic pain. In this study, we investigated whether the transcription factor STAT2 affects neuropathic pain and evaluated its possible mechanisms. A proteomic analysis showed that the nuclear fraction of STAT2 protein in the SDH was downregulated after chronic constriction injury of the rat sciatic nerve, which was associated with the development of neuropathic pain. Similarly, siRNA-induced downregulation of STAT2 in the SDH of naïve rats also resulted in pain hypersensitivity. Using RNA-sequencing analysis, we showed that reduction of nuclear STAT2 after chronic constriction injury was associated with increased expression of microglial activation markers, including the class II transactivator and major histocompatibility complex class II proteins. In addition, siRNA-induced downregulation of STAT2 promoted microglial activation and pro-inflammatory cytokine expression in the SDH. Taken together, these results showed that chronic constriction injury caused downregulation of nuclear STAT2 in the SDH, which may result in microglial activation and development of neuropathic pain. Our findings indicate that restoration of nuclear expression of STAT2 could be a potential pathway for the treatment of neuropathic pain.

JAK/STAT pathway: Extracellular signals, diseases, immunity, and therapeutic regimens

Janus kinase/signal transduction and transcription activation (JAK/STAT) pathways were originally thought to be intracellular signaling pathways that mediate cytokine signals in mammals. Existing studies show that the JAK/STAT pathway regulates the downstream signaling of numerous membrane proteins such as such as G-protein-associated receptors, integrins and so on. Mounting evidence shows that the JAK/STAT pathways play an important role in human disease pathology and pharmacological mechanism. The JAK/STAT pathways are related to aspects of all aspects of the immune system function, such as fighting infection, maintaining immune tolerance, strengthening barrier function, and cancer prevention, which are all important factors involved in immune response. In addition, the JAK/STAT pathways play an important role in extracellular mechanistic signaling and might be an important mediator of mechanistic signals that influence disease progression, immune environment. Therefore, it is important to understand the mechanism of the JAK/STAT pathways, which provides ideas for us to design more drugs targeting diseases based on the JAK/STAT pathway. In this review, we discuss the role of the JAK/STAT pathway in mechanistic signaling, disease progression, immune environment, and therapeutic targets.

Preoperative Acute Sleep Deprivation Causes Postoperative Pain Hypersensitivity and Abnormal Cerebral Function

Preoperative sleep loss can amplify post-operative mechanical hyperalgesia. However, the underlying mechanisms are still largely unknown. In the current study, rats were randomly allocated to a control group and an acute sleep deprivation (ASD) group which experienced 6 h ASD before surgery. Then the variations in cerebral function and activity were investigated with multi-modal techniques, such as nuclear magnetic resonance, functional magnetic resonance imaging, c-Fos immunofluorescence, and electrophysiology. The results indicated that ASD induced hyperalgesia, and the metabolic kinetics were remarkably decreased in the striatum and midbrain. The functional connectivity (FC) between the nucleus accumbens (NAc, a subregion of the ventral striatum) and the ventrolateral periaqueductal gray (vLPAG) was significantly reduced, and the c-Fos expression in the NAc and the vLPAG was suppressed. Furthermore, the electrophysiological recordings demonstrated that both the neuronal activity in the NAc and the vLPAG, and the coherence of the NAc-vLPAG were suppressed in both resting and task states. This study showed that neuronal activity in the NAc and the vLPAG were weakened and the FC between the NAc and the vLPAG was also suppressed in rats with ASD-induced hyperalgesia. This study highlights the importance of preoperative sleep management for surgical patients.

Cyclin D1 mediates pain behaviour in a rat model of breast cancer-induced bone pain by a mechanism involving regulation of the proliferation of spinal microglia

Aims

The involvement of cyclin D1 in the proliferation of microglia, and the generation and maintenance of bone cancer pain (BCP), have not yet been clarified. We investigated the expression of microglia and cyclin D1, and the influences of cyclin D1 on pain threshold.

Methods

Female Sprague Dawley (SD) rats were used to establish a rat model of BCP, and the messenger RNA (mRNA) and protein expression of ionized calcium binding adaptor molecule 1 (IBA1) and cyclin D1 were detected by reverse transcription-polymerase chain reaction (RT-PCR) and western blot, respectively. The proliferation of spinal microglia was detected by immunohistochemistry. The pain behaviour test was assessed by quantification of spontaneous flinches, limb use, and guarding during forced ambulation, mechanical paw withdrawal threshold, and thermal paw withdrawal latency.

Results

IBA1 and cyclin D1 in the ipsilateral spinal horn increased in a time-dependent fashion. Spinal microglia proliferated in BCP rats. The microglia inhibitor minocycline attenuated the pain behaviour in BCP rats. The cyclin-dependent kinase inhibitor flavopiridol inhibited the proliferation of spinal microglia, and was associated with an improvement in pain behaviour in BCP rats.

Conclusion

Our results revealed that the inhibition of spinal microglial proliferation was associated with a decrease in pain behaviour in a rat model of BCP. Cyclin D1 acts as a key regulator of the proliferation of spinal microglia in a rat model of BCP. Disruption of cyclin D1, the restriction-point control of cell cycle, inhibited the proliferation of microglia and attenuated the pain behaviours in BCP rats. Cyclin D1 and the proliferation of spinal microglia may be potential targets for the clinical treatment of BCP.

Cite this article: Bone Joint Res 2022;11(11):803–813.

Neuroinflammation in the medial prefrontal cortex exerts a crucial role in bone cancer pain

Bone cancer pain (BCP) is one of the most common types of pain in cancer patients which compromises the patient's functional status, quality of life, and survival. Central hyperalgesia has increasingly been identified as a crucial factor of BCP, especially in the medial prefrontal cortex (mPFC) which is the main cortical area involved in the process of pain and consequent negative emotion. To explore the genetic changes in the mPFC during BCP occurrence and find possible targets for prediction, we performed transcriptome sequencing of mPFC in the BCP rat model and found a total of 147 differentially expressed mRNAs (DEmRNAs). A protein-protein interaction (PPI) network revealed that the DEmRNAs mainly participate in the inflammatory response. Meanwhile, microglia and astrocytes were activated in the mPFC of BCP rats, further confirming the presence of neuroinflammation. In addition, Gene Ontology (GO) analysis showed that DEmRNAs in the mPFC are mainly involved in antigen processing, presentation of peptide antigen, and immune response, occurring in the MHC protein complex. Besides, the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that DEmRNAs are mainly enriched in the pathways of phagosome, staphylococcus aureus infection, and antigen processing, in which MHCII participate. Furthermore, immunostaining showed that MHCII is mainly located in the microglia. Microglia are believed to be involved in antigen processing, a key cause of BCP. In vivo, minocycline (MC) treatment inhibits the activation of microglia and reduces the expression of MHCII and proinflammatory cytokines, thereby alleviating BCP and pain-related anxiety. Taken together, our study identified differentially expressed genes in the BCP process and demonstrated that the activation of microglia participates in the inflammatory response and antigen process, which may contribute to BCP.

Astrocytic ERK/STAT1 signaling contributes to maintenance of stress-related visceral hypersensitivity in rats

The rostral anterior cingulate cortex (rACC) has been found to be an important brain region in mediating visceral hypersensitivity. However, the underlying mechanisms remain unclear. This study aimed to explore the role of astrocytes in the maintenance of visceral hypersensitivity induced by chronic water avoidance stress (WAS) as well as the potential signaling pathway that activates astrocytes in the rACC. We found that ACC-reactive astrogliosis resulted in the overexpression of c-fos, TSP-1, and BDNF in stress-related visceral hypersensitivity rats. Visceral hypersensitivity was reversed by pharmacological inhibition of astrocytic activation after WAS, as were the overexpression of c-fos, TSP-1 and BDNF. Activation of the astrocytic Gi-pathway increased the visceral sensitivity and expression of c-fos, TSP-1, and BDNF. Visceral hypersensitivity was also ameliorated by the pharmacological inhibition of ERK and STAT1 phosphorylation after WAS. Furthermore, inhibition of the ERK-STAT1 cascade reduced astrocytic activation. These findings suggest that astrocytic ERK/STAT1 signaling in the rACC contributes to the maintenance of stress-related visceral hypersensitivity. PERSPECTIVE: Visceral hypersensitivity is a key factor in the pathophysiology of irritable bowel syndrome. This study highlights the important role of astrocytic ERK/STAT1 signaling in activating astrocytes in the rostral anterior cingulate cortex, which contributes to visceral hypersensitivity.

Sestrin2 overexpression attenuates osteoarthritis pain via induction of AMPK/PGC-1α-mediated mitochondrial biogenesis and suppression of neuroinflammation

BACKGROUND

Our previous study indicated that reactive oxygen species (ROS) are critically involved in chronic pain. Sestrin2 (Sesn2), a novel stress-inducible protein, is evidenced to reduce the generation of ROS. The study examined the role of Sesn2 in osteoarthritis (OA) pain and delineated the underlying molecular mechanisms.

METHODS

In the present study, we investigated the impact of Sesn2 on mitochondrial biogenesis in a rat model of OA pain. After adeno-associated viral (AAV)-Sesn2EGFP was injected for 14 days, OA was induced by intra-articular injection of monosodium iodoacetate (MIA). We assessed pain behaviors (weight-bearing asymmetry and paw withdrawal threshold) and explored possible mechanisms in the L4-6 spinal cord.

RESULTS

Our results showed that overexpression of Sesn2 in the spinal cord alleviated pain behaviors in OA rats. Moreover, overexpression of Sesn2 increased the activity of AMP-activated protein kinase (AMPK) signaling and significantly restored mitochondrial biogenesis. Besides, Sesn2 overexpression inhibited the activation of astrocytes and microglia, and decreased the production of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in the spinal cord of the OA pain rats. These effects were significantly reversed by an AMPK inhibitor.

CONCLUSIONS

Collectively, these results suggest that Sesn2 overexpression ameliorates mechanical allodynia and weight-bearing asymmetry in OA rats via activation of AMPK/PGC-1α-mediated mitochondrial biogenesis in the spinal cord. Moreover, Sesn2 overexpression attenuates OA-induced neuroinflammation at least partly by activating AMPK signaling. Sesn2 may become an encouraging therapeutic strategy for OA pain relief and other disorders.

Breast cancer metastasis to brain results in recruitment and activation of microglia through annexin-A1/formyl peptide receptor signaling

Background

Despite advancements in therapies, brain metastasis in patients with triple negative subtype of breast cancer remains a therapeutic challenge. Activated microglia are often observed in close proximity to, or within, malignant tumor masses, suggesting a critical role that microglia play in brain tumor progression. Annexin-A1 (ANXA1), a glucocorticoid-regulated protein with immune-regulatory properties, has been implicated in the growth and metastasis of many cancers. Its role in breast cancer-microglia signaling crosstalk is not known.

Methods

The importance of microglia proliferation and activation in breast cancer to brain metastasis was evaluated in MMTV-Wnt1 spontaneous mammary tumor mice and BALBc mice injected with 4T1 murine breast cancer cells into the carotid artery using flow cytometry. 4T1 induced-proliferation and migration of primary microglia and BV2 microglia cells were evaluated using 2D and coculture transwell assays. The requirement of ANXA1 in these functions was examined using a Crispr/Cas9 deletion mutant of ANXA1 in 4T1 breast cancer cells as well as BV2 microglia. Small molecule inhibition of the ANXA1 receptor FPR1 and FPR2 were also examined. The signaling pathways involved in these interactions were assessed using western blotting. The association between lymph node positive recurrence-free patient survival and distant metastasis-free patient survival and ANXA1 and FPR1 and FPR2 expression was examined using TCGA datasets.

Results

Microglia activation is observed prior to brain metastasis in MMTV-Wnt1 mice with primary and secondary metastasis in the periphery. Metastatic 4T1 mammary cancer cells secrete ANXA1 to promote microglial migration, which in turn, enhances tumor cell migration. Silencing of ANXA1 in 4T1 cells by Crispr/Cas9 deletion, or using inhibitors of FPR1 or FPR2 inhibits microglia migration and leads to reduced activation of STAT3. Finally, elevated ANXA1, FPR1 and FPR2 is significantly associated with poor outcome in lymph node positive patients, particularly, for distant metastasis free patient survival.

Conclusions

The present study uncovered a network encompassing autocrine/paracrine ANXA1 signaling between metastatic mammary cancer cells and microglia that drives microglial recruitment and activation. Inhibition of ANXA1 and/or its receptor may be therapeutically rewarding in the treatment of breast cancer and secondary metastasis to the brain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-022-01514-2.

5-HT1F Receptor Agonist Ameliorates Mechanical Allodynia in Neuropathic Pain via Induction of Mitochondrial Biogenesis and Suppression of Neuroinflammation

Neuropathic pain is a devastating disease that affects millions of people worldwide. Serotonin (5-hydroxytryptamine, 5-HT) is involved in pain modulation. Several lines of evidence have indicated that 5-HT1F receptor agonists are potent inducers of mitochondrial biogenesis. In this study, we tested the hypothesis that 5-HT1F receptor agonists ameliorate mechanical allodynia in neuropathic pain via the induction of mitochondrial biogenesis and suppression of neuroinflammation. Male Sprague–Dawley rats were used to establish a neuropathic pain model via spared nerve injury (SNI). The paw withdrawal threshold (PWT) was used to evaluate mechanical allodynia. Real-time polymerase chain reaction was used to examine the mitochondrial DNA (mtDNA) copy number. Western blotting and immunofluorescence were used to examine the expression of target proteins. Our results showed that mitochondrial biogenesis was impaired in the spinal cord of rats with SNI. Moreover, activation of PGC-1α, the master regulator of mitochondrial biogenesis, attenuates established mechanical allodynia in rats with neuropathic pain. In addition, the neuronal 5-HT1F receptor is significantly downregulated in the spinal cord of rats with neuropathic pain. Furthermore, the selective 5-HT1F receptor agonist lasmiditan attenuated established mechanical allodynia in rats with neuropathic pain. Finally, lasmiditan (Las) treatment restored mitochondrial biogenesis and suppressed neuroinflammation in the spinal cord of rats with SNI. These results provide the first evidence that lasmiditan ameliorates mechanical allodynia in neuropathic pain by inducing mitochondrial biogenesis and suppressing neuroinflammation in the spinal cord. Inducers of mitochondrial biogenesis may be an encouraging therapeutic option for the management of neuropathic pain.

Activation of microglial GPR109A alleviates thermal hyperalgesia in female lupus mice by suppressing IL-18 and glutamatergic synaptic activity

Many patients with systemic lupus erythematosus (SLE) live with chronic pain despite advances in medical management in reducing mortality related to SLE. Few animal studies have addressed mechanisms and treatment for chronic pain caused by SLE. In this study, we provide the first evidence for the analgesic effects of a GPR109A specific agonist (MK1903) and its action mechanisms in thermal hyperalgesia in female MRL/lpr mice, an SLE mouse model. Specifically, we show that MRL/lpr mice had a higher sensitivity to thermal stimuli at age 11-16 weeks, which was accompanied with significantly microglial and astrocytic activation, increases in p38 MAPK and glutamatergic synaptic activities in the spinal dorsal horn. We demonstrate that thermal hyperalgesia in MRL/lpr mice was significantly attenuated by intrathecal injection of MK1903. GPR109A was expressed in spinal microglia but not astrocytes or neurons. Its expression was significantly increased in MRL/lpr mice with thermal hyperalgesia. Activation of GPR109A receptors in microglia attenuated glutamatergic synaptic activity via suppressing production of interleukin-18 (IL-18). We provide evidence that activation of GPR109A attenuated thermal hyperalgesia in the SLE animal model via suppressing p38 MAPK activity and production of IL-18. Our study suggests that targeting the microglial GPR109A is a potent approach for reversing spinal neuroinflammation, abnormal excitatory synaptic activity, and management of thermal hyperalgesia caused by SLE.

Nrf2 Activation Attenuates Chronic Constriction Injury-Induced Neuropathic Pain via Induction of PGC-1α-Mediated Mitochondrial Biogenesis in the Spinal Cord

BACKGROUND

Neuropathic pain is a debilitating disease with few effective treatments. Emerging evidence indicates the involvement of mitochondrial dysfunction and oxidative stress in neuropathic pain. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a potent regulator of the antioxidant response system. In this study, we investigated whether RTA-408 (RTA, a novel synthetic triterpenoid under clinical investigation) could activate Nrf2 and promote mitochondrial biogenesis (MB) to reverse neuropathic pain and the underlying mechanisms.

METHODS

Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve. Pain behaviors were measured via the von Frey test and Hargreaves plantar test. The L4-6 spinal cord was collected to examine the activation of Nrf2 and MB.

RESULTS

RTA-408 treatment significantly reversed mechanical allodynia and thermal hyperalgesia in CCI mice in a dose-dependent manner. Furthermore, RTA-408 increased the activity of Nrf2 and significantly restored MB that was impaired in CCI mice in an Nrf2-dependent manner. Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1α) is the key regulator of MB. We found that the PGC-1α activator also induced a potent analgesic effect in CCI mice. Moreover, the antinociceptive effect of RTA-408 was reversed by the preinjection of the PGC-1α inhibitor.

CONCLUSIONS

Nrf2 activation attenuates chronic constriction injury-induced neuropathic pain via induction of PGC-1α-mediated mitochondrial biogenesis in the spinal cord. Our results indicate that Nrf2 may be a potential therapeutic strategy to ameliorate neuropathic pain and many other disorders with oxidative stress and mitochondrial dysfunction.

Wnt signaling: A prospective therapeutic target for chronic pain

Despite the rapid advance over the past decades to design effective therapeutic pharmacological interventions, chronic pain remains to be an unresolved healthcare concern. Long term use of opioids, the first line analgesics, often causes detrimental side effects. Therefore, a profound understanding of the mechanisms underlying the development and maintenance of chronic pain states is urgently needed for the management of chronic pain. Substantial evidence indicates aberrant activation of Wnt signaling pathways in sciatic nerve, dorsal root ganglia and spinal cord dorsal horn in rodent models of chronic pain. Moreover, growing evidence shows that pharmacological blockage of aberrant activation of Wnt signaling pathways attenuates pain behaviors in animal models of chronic pain. Importantly, both intrathecal injection of Wnt agonists and Wnt ligands to naïve rats lead to the development of mechanical allodynia, which was inhibited by Wnt inhibitors. In this review, we summarized and discussed the therapeutic potential of pharmacological inhibitors of Wnt signaling in chronic pain in preclinical studies. These evidence showed that aberrant activation of Wnt signaling pathways contributed to chronic pain via enhancing neuroinflammation, regulating synaptic plasticity and reducing intraepidermal nerve fiber density. However, these findings raise further questions. Overall, despite the future challenges, these pioneering studies suggest that Wnt signaling is a promising therapeutic target for chronic pain.

Apomorphine Reduces A53T α-Synuclein-Induced Microglial Reactivity Through Activation of NRF2 Signalling Pathway

The chiral molecule, apomorphine, is currently used for the treatment of Parkinson’s disease (PD). As a potent dopamine receptor agonist, this lipophilic compound is especially effective for treating motor fluctuations in advanced PD patients. In addition to its receptor-mediated actions, apomorphine has also antioxidant and free radical scavenger activities. Neuroinflammation, oxidative stress, and microglia reactivity have emerged as central players in PD. Thus, modulating microglia activation in PD may be a valid therapeutic strategy. We previously reported that murine microglia are strongly activated upon exposure to A53T mutant α-synuclein. The present study was designed to investigate whether apomorphine enantiomers could modulate this A53T-induced microglial activation. Taken together, the results provided evidence that apomorphine enantiomers decrease A53T-induced microgliosis, through the activation of the NRF2 signalling pathway, leading to a lower pro-inflammatory state and restoring the phagocytic activity. Suppressing NRF2 recruitment (trigonelline exposure) or silencing specifically Nfe2l2 gene (siRNA treatment) abolished or strongly decreased the anti-inflammatory activity of apomorphine. In conclusion, apomorphine, which is already used in PD patients to mimic dopamine activity, may also be suitable to decrease α-synuclein-induced microglial reactivity.

Hippocampal glutamatergic synapses impairment mediated novel-object recognition dysfunction in rats with neuropathic pain

Cognitive impairment is one of the most common complications associated with chronic pain. Almost 20% of chronic pain patients suffer from cognitive impairment, which may substantially influence their quality of life. Levels of major excitatory neurotransmitters in the central nervous system and alterations in the glutamatergic system may influence cognitive function and the pain sensory pathway. In this study, we adopted the spared nerve injury model to establish the progress of chronic pain and investigated the mechanism underlying the cognitive aspect related to it. At behavioral level, using the novel-object recognition test, mechanical hypersensitivity was observed in peripheral nerve-injured rats because they exhibited recognition deficits. We showed a dramatic decrease in hippocampal glutamate concentration using nuclear magnetic resonance and reduced glutamatergic synaptic transmission using whole-cell recordings. These were associated with deficient hippocampal long-term potentiation induced by high-frequency stimulation of the Schaffer collateral afferent. Ultra-high-performance liquid chromatography revealed lower levels of D-serine in the hippocampus of the spared nerve injury rats and that D-serine treatment could restore synaptic plasticity and cognitive dysfunction. The reduction of excitatory synapses was also increased by administering D-serine. These findings suggest that chronic pain has a critical effect on synaptic plasticity linked to cognitive function and may built up a new target for the development of cognitive impairment under chronic pain conditions.

Disruption of the GABAergic system contributes to the development of perioperative neurocognitive disorders after anesthesia and surgery in aged mice

Aims

Perioperative neurocognitive disorders (PND) are associated with cognitive impairment in the preoperative or postoperative period, and neuroinflammation is thought to be the most important mechanisms especially during the postoperative period. The GABAergic system is easily disrupted by neuroinflammation. This study investigated the impact of the GABAergic system on PND after anesthesia and surgery.

Methods

An animal model of laparotomy with inhalation anesthesia in 16‐month‐old mice was addressed. Effects of the GABAergic system were assessed using biochemical analysis. Pharmacological blocking of α5GABA_ARs or P38 mitogen‐activated protein kinase (MAPK) were applied to investigate the effects of the GABAergic system.

Results

After laparotomy, the hippocampus‐dependent memory and long‐term potentiation were impaired, the levels of IL‐6, IL‐1β and TNF‐α up‐regulated in the hippocampus, the concentration of GABA decreased, and the protein levels of the surface α5GABA_ARs up‐regulated. Pharmacological blocking of α5GABA_ARs with L655,708 alleviated laparotomy induced cognitive deficits. Further studies found that the P38 MAPK signaling pathway was involved and pharmacological blocking with SB203,580 alleviated memory dysfunctions.

Conclusions

Anesthesia and surgery caused neuroinflammation in the hippocampus, which consequently disrupted the GABAergic system, increased the expressions of surface α5GABA_ARs especially through the P38 MAPK signaling pathway, and eventually led to hippocampus‐dependent memory dysfunctions.

Transcriptomic analysis of long noncoding RNAs and mRNAs expression profiles in the spinal cord of bone cancer pain rats

Bone cancer pain (BCP) is one of the most common types of chronic cancer pain and its pathogenesis has not been fully understood. Long non-coding RNAs (lncRNAs) are new promising targets in the field of pain research, however, their involvements in BCP have not been reported. In the present study, we established the BCP model by implantation of Walker 256 carcinoma cells into rats' tibial medullary cavity and performed transcriptome sequencing of the ipsilateral lumbar spinal cord to explore changes in expression profiles of lncRNA and mRNA. We identified 1220 differently expressed mRNAs (DEmRNAs) (1171 up-regulated and 49 down-regulated) and 323 differently expressed lncRNAs (DElncRNAs) (246 up-regulated and 77 down-regulated) in BCP model, among which 10 DEmRNAs (5 up-regulated and 5 down-regulated) and 10 DElncRNAs (5 up-regulated and 5 down-regulated) were validated the expression by RT-qPCR. Then, we performed Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis on the expression of DEmRNAs and DElncRNAs, showing that they were mainly enriched in inflammatory and immunologic processes/pathways. Finally, we constructed a co-expression network and a ceRNA network of DEmRNAs and DElncRNAs to exhibit a potential regulatory mechanism of DElncRNAs, directly regulating protein coding gene expression in cis or in trans and indirectly regulating protein coding gene expression by sponging miRNA. In conclusion, our study provided a landscape of dysregulated lncRNA and mRNA in spinal cord of bone cancer pain and detected novel potential targets for treatment in the future.

microRNA-329 reduces bone cancer pain through the LPAR1-dependent LPAR1/ERK signal transduction pathway in mice

Background:

Bone cancer pain (BCP) is a common symptom occurring among patients with cancer and has a detrimental effect on their quality of life. Growing evidence has implicated microRNA-329 (miR-329) in the progression of bone diseases. In the present study, we aimed to elucidate the potential effects of miR-329 on BCP in a BCP mouse model via binding to lysophosphatidic acid receptor 1 (LPAR1) through the LPAR1/extracellular signal-regulated kinase (ERK) signaling pathway.

Methods:

Initially, a BCP mouse model was established via injection of 4 × 10⁴ murine breast tumor (4T1 cell) cells (4 μl). The interaction between miR-329 and LPAR1 was identified using a bioinformatics website and dual luciferase reporter gene assay. The modeled mice were subsequently treated with miR-329 mimic, LPAR1 shRNA, or both, in order to examine the effect of miR-329 on the paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) of mice, the expression of LPAR1/ERK signaling pathway-related genes.

Results:

The positive expression rate of LPAR1 protein and extent of ERK1/2 phosphorylation were increased in BCP mouse models. LPAR1 is a target gene of miR-329, which can inhibit the expression of LPAR1. In response to miR-329 overexpression and LPAR1 silencing, BCP mice showed increased PWT and PWL, along with decreased LPAR1 expression and ratio of p-ERK/ERK.

Conclusions:

Altogether, the results obtained indicated that miR-329 can potentially alleviate BCP in mice via the inhibition of LPAR1 and blockade of the LPAR1/ERK signaling pathway, highlighting that upregulation of miR-329 could serve as a therapeutic target for BCP treatment.

The endocannabinoid system: Novel targets for treating cancer induced bone pain

Treating Cancer-induced bone pain (CIBP) continues to be a major clinical challenge and underlying mechanisms of CIBP remain unclear. Recently, emerging body of evidence suggested the endocannabinoid system (ECS) may play essential roles in CIBP. Here, we summarized the current understanding of the antinociceptive mechanisms of endocannabinoids in CIBP and discussed the beneficial effects of endocannabinoid for CIBP treatment. Targeting non-selective cannabinoid 1 receptors or selective cannabinoid 2 receptors, and modulation of peripheral AEA and 2-AG, as well as the inhibition the function of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have produced analgesic effects in animal models of CIBP. Management of ECS therefore appears to be a promising way for the treatment of CIBP in terms of efficacy and safety. Further clinical studies are encouraged to confirm the possible translation to humans of the very promising results already obtained in the preclinical studies.

Abnormalities in Inflammatory Cytokines Confer Susceptible to Chronic Neuropathic Pain-related Anhedonia in a Rat Model of Spared Nerve Injury

Objective

Patients with chronic neuropathic pain (CNP) have a higher incidence to develop depression. However, its pathogenesis has not yet been fully elucidated. Here we aimed to investigate the role of inflammatory cytokines in CNP-related anhedonia, which is a core symptom of depression, and to explore the effects of ketamine and parecoxib on pain and anhedonia.

Methods

A rat model of spared nerve injury (SNI) was constructed to mimic CNP. Hierarchical cluster analysis of sucrose preference test (SPT) was applied to classify the SNI rats into anhedonia susceptible and unsusceptible. Inflammatory cytokines in medial prefrontal cortex (mPFC) of brain, serum and L2–5 spinal cord were measured. Moreover, effects of ketamine or parecoxib on mechanical withdrawal test (MWT) and SPT in anhedonia susceptible rats were detected.

Results

Tumor necrosis factor (TNF)-α was increased in mPFC, serum and and spinal cord of anhedonia susceptible rats. Furthermore, anhedonia susceptible and unsusceptible rats both increased the interleukin (IL)-1β level in mPFC, serum and spinal cord. IL-6 was altered in serum and spinal cord, but not in mPFC. IL-10 was significantly altered in mPFC and serum, but not in spinal cord. Additionally, ketamine treatment significantly attenuated the decreased results of MWT and SPT in anhedonia susceptible rats, and that parecoxib significantly improved the MWT score, but failed to alter the result of SPT.

Conclusion

These findings suggest that abnormalities in inflammatory cytokines confer susceptible to anhedonia in a rat model of SNI. Ketamine, a fast-acting antidepressant, has pharmacological benefits to alleviate pain and anhedonia symptoms.

The Role of CXCR3 in Neurological Diseases

BACKGROUND

Neurological diseases have become an obvious challenge due to insufficient therapeutic intervention. Therefore, novel drugs for various neurological disorders are in desperate need. Recently, compelling evidence has demonstrated that chemokine receptor CXCR3, which is a G protein-coupled receptor in the CXC chemokine receptor family, may play a pivotal role in the development of neurological diseases. The aim of this review is to provide evidence for the potential of CXCR3 as a therapeutic target for neurological diseases.

METHODS

English journal articles that focused on the invovlement of CXCR3 in neurological diseases were searched via PubMed up to May 2017. Moreover, reference lists from identified articles were included for overviews.

RESULTS

The expression level of CXCR3 in T cells was significantly elevated in several neurological diseases, including multiple sclerosis (MS), glioma, Alzheimer's disease (AD), chronic pain, human T-lymphotropic virus type 1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and bipolar disorder. CXCR3 antagonists showed therapeutic effects in these neurological diseases.

CONCLUSION

These studies provided hard evidence that CXCR3 plays a vital role in the pathogenesis of MS, glioma, AD, chronic pain, HAM/TSP and bipolar disorder. CXCR3 is a crucial molecule in neuroinflammatory and neurodegenerative diseases. It regulates the activation of infiltrating cells and resident immune cells. However, the exact functions of CXCR3 in neurological diseases are inconclusive. Thus, it is important to understand the topic of chemokines and the scope of their activity in neurological diseases.

Interleukin-1beta released by microglia initiates the enhanced glutamatergic activity in the spinal dorsal horn during paclitaxel-associated acute pain syndrome

Patients receiving paclitaxel for cancer treatment often develop an acute pain syndrome (paclitaxel-associated acute pain syndrome, P-APS), which occurs immediately after paclitaxel treatment. Mechanisms underlying P-APS remain largely unknown. We recently reported that rodents receiving paclitaxel develop acute pain and activation of spinal microglial toll like receptor 4 (TLR4) by paclitaxel penetrating into the spinal cord is a critical event in the genesis of P-APS. Our current study dissected cellular and molecular mechanisms underlying the P-APS. We demonstrated that bath-perfusion of paclitaxel, at a concentration similar to that found in the cerebral spinal fluid in animals receiving i.v. paclitaxel (2 mg/kg), resulted in increased calcium activity in microglia instantly, and in astrocytes with 6 min delay. TLR4 activation in microglia by paclitaxel caused microglia to rapidly release interleukin-1β (IL-1β) but not tumor necrosis factor α, IL-6, or interferon-γ. IL-1β release from microglia depended on capthepsin B. IL-1β acted on astrocytes, leading to elevated calcium activity and suppressed glutamate uptake. IL-1β also acted on neurons to increase presynaptic glutamate release and postsynaptic AMPA receptor activity in the spinal dorsal horn. Knockout of IL-1 receptors prevented the development of acute pain induced by paclitaxel in mice. Our study indicates that IL-1β is a crucial molecule used by microglia to alter functions in astrocytes and neurons upon activation of TLR4 in the genesis of P-APS, and targeting the signaling pathways regulating the production and function of IL-1β from microglia is a potential avenue for the development of analgesics for the treatment of P-APS.

Minocycline as a promising therapeutic strategy for chronic pain

Chronic pain remains to be a clinical challenge due to insufficient therapeutic strategies. Minocycline is a member of the tetracycline class of antibiotics, which has been used in clinic for decades. It is frequently reported that minocycline may has many non-antibiotic properties, among which is its anti-nociceptive effect. The results from our lab and others suggest that minocycline exerts strong analgesic effect in animal models of chronic pain including visceral pain, chemotherapy-induced periphery neuropathy, periphery injury induced neuropathic pain, diabetic neuropathic pain, spinal cord injury, inflammatory pain and bone cancer pain. In this review, we summarize the mechanisms underlying the analgesic effect of minocycline in preclinical studies. Due to a good safety record when used chronically, minocycline may become a promising therapeutic strategy for chronic pain in clinic.

Sinomenine attenuates cancer-induced bone pain via suppressing microglial JAK2/STAT3 and neuronal CAMKII/CREB cascades in rat models

Cancer-induced bone pain is one of the most severe types of pathological pain, which often occurs in patients with advanced prostate, breast, and lung cancer. It is of great significance to improve the therapies of cancer-induced bone pain due to the opioids' side effects including addiction, sedation, pruritus, and vomiting. Sinomenine, a traditional Chinese medicine, showed obvious analgesic effects on a rat model of chronic inflammatory pain, but has never been proven to treat cancer-induced bone pain. In the present study, we investigated the analgesic effect of sinomenine after tumor cell implantation and specific cellular mechanisms in cancer-induced bone pain. Our results indicated that single administration of sinomenine significantly and dose-dependently alleviated mechanical allodynia in rats with cancer-induced bone pain and the effect lasted for 4 h. After tumor cell implantation, the protein levels of phosphorylated-Janus family tyrosine kinase 2 (p-JAK2), phosphorylated-signal transducers and activators of transcription 3 (p-STAT3), phosphorylated-Ca2+/calmodulin-dependent protein kinase II (p-CAMKII), and phosphorylated-cyclic adenosine monophosphate response element-binding protein (p-CREB) were persistently up-regulated in the spinal cord horn. Chronic intraperitoneal treatment with sinomenine markedly suppressed the activation of microglia and effectively inhibited the expression of JAK2/STAT3 and CAMKII/CREB signaling pathways. We are the first to reveal that up-regulation of microglial JAK2/STAT3 pathway are involved in the development and maintenance of cancer-induced bone pain. Moreover, our investigation provides the first evidence that sinomenine alleviates cancer-induced bone pain by inhibiting microglial JAK2/STAT3 and neuronal CAMKII/CREB cascades.

Decreased long intergenic noncoding RNA P7 predicts unfavorable prognosis and promotes tumor proliferation via the modulation of the STAT1-MAPK pathway in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common neoplasm and is a leading cause of cancer-related death. Despite advances in the diagnosis and management of HCC, its prognosis remain unfavorable. Accumulating evidence has shown that long intergenic noncoding RNAs (lincRNAs) play central roles in the development of HCC. In this study, we identified a long intergenic noncoding RNA referred to as lincRNA P7 in HCC and explored its clinical significance and biological functions in HCC. The expression level of lincRNA P7 was significantly aberrantly deceased in HCC cancer tissues and cells lines. Gain- and loss-of-function experiments revealed that overexpression of lincRNA P7 significantly inhibited the proliferation of HCC-derived cancer cells, whereas lincRNA P7 knockdown promoted cell growth. Mechanistically, lincRNA P7 blocked Erk1/2 signaling and repressed activation of the STAT1 pathway. In nude mouse models, we show that overexpression of lincRNA P7 effectively repressed HCC xenograft tumor growth in vivo. Moreover, a clinical investigation demonstrated that down-regulated lincRNA P7 expression correlated with liver cirrhosis, Hepatitis B virus (HBV) infection, clinical stage of the tumor and recurrence. A Kaplan-Meier survival analysis showed that the expression of lincRNA P7 was significantly related to overall survival (P = 0.003) and recurrence-free survival (P = 0.031). Collectively, our findings suggested that the down-regulation of lincRNA P7 predicts poor clinical outcomes for HCC patients and might be a powerful candidate prognostic biomarker and target in HCC.

Altered expression of differential gene and lncRNA in the lower thoracic spinal cord on different time courses of experimental obstructive jaundice model accompanied with altered peripheral nociception in rats

The spinal origin of jaundice-induced altered peripheral nociceptive response poorly understood. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a jaundice model accompanied by altered peripheral nociceptive response, and then to analyze differential gene and lncRNA expression patterns in the lower thoracic spinal cord on different time courses after BDL operation by using high-throughput RNA sequencing. The differentially expressed genes (DEGs) identified using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis, followed by clustering analysis, Gene Ontology analysis and pathway analysis. As a result, a total of 2033 lncRNAs were differentially expressed 28d after BDL, in which 1545 probe sets were up-regulated and 488 probe sets were down-regulated, whereas a total of 2800 mRNAs were differentially expressed, in which 1548 probe sets were up-regulated and 1252 probe sets were down-regulated. The RNAseq data of select mRNAs and lncRNAs was validated by RT-qPCR. 28d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated whereas the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. 14d after BDL, the expressions of lncRNA NONRATT002335 and NONRATT018085 were significantly up-regulated; the expression of lncRNA NONRATT025415, NONRATT025388 and NONRATT025409 was significantly down-regulated. In conclusion, the present study showed that jaundice accompanied with decreased peripheral nociception involved in the changes of gene and lncRNA expression profiles in spinal cord. These findings extend current understanding of spinal mechanism for obstructive jaundice accompanied by decreased peripheral nociception.

Reactive oxygen species scavengers ameliorate mechanical allodynia in a rat model of cancer-induced bone pain

Cancer-induced bone pain (CIBP) is a frequent complication in patients suffering from bone metastases. Previous studies have demonstrated a pivotal role of reactive oxygen species (ROS) in inflammatory and neuropathic pain, and ROS scavengers exhibited potent antinociceptive effect. However, the role of spinal ROS remains unclear. In this study, we investigated the analgesic effect of two ROS scavengers in a well-established CIBP model. Our results found that intraperitoneal injection of N-tert-Butyl-α-phenylnitrone (PBN, 50 and 100 mg/kg) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol, 100 and 200 mg/kg) significantly suppressed the established mechanical allodynia in CIBP rats. Moreover, repeated injection of PBN and Tempol showed cumulative analgesic effect without tolerance. However, early treatment with PBN and Tempol failed to prevent the development of CIBP. Naive rats received repetitive injection of PBN and Tempol showed no significant change regarding the nociceptive responses. Finally, PBN and Tempol treatment notably suppressed the activation of spinal microglia in CIBP rats. In conclusion, ROS scavengers attenuated established CIBP by suppressing the activation of microglia in the spinal cord.

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PBN and Tempol could suppress established mechanical allodynia in CIBP rats.

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Repeated injection of PBN and Tempol showed cumulative analgesic effect.

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PBN and Tempol failed to prevent the development of CIBP.

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PBN and Tempol could suppress the microglia activation in CIBP rats.

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.

The Role of Spinal GABAB Receptors in Cancer-Induced Bone Pain in Rats

Cancer-induced bone pain (CIBP) remains a major challenge in advanced cancer patients because of our lack of understanding of its mechanisms. Previous studies have shown the vital role of γ-aminobutyric acid B receptors (GABABRs) in regulating nociception and various neuropathic pain models have shown diminished activity of GABABRs. However, the role of spinal GABABRs in CIBP remains largely unknown. In this study, we investigated the specific cellular mechanisms of GABABRs in the development and maintenance of CIBP in rats. Our behavioral results show that acute as well as chronic intrathecal treatment with baclofen, a GABABR agonist, significantly attenuated CIBP-induced mechanical allodynia and ambulatory pain. The expression levels of GABABRs were significantly decreased in a time-dependent manner and colocalized mostly with neurons and a minority with astrocytes and microglia. Chronic treatment with baclofen restored the expression of GABABRs and markedly inhibited the activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase and the cAMP-response element-binding protein signaling pathway.

PERSPECTIVE

Our findings provide, to our knowledge, the first evidence that downregulation of GABABRs contribute to the development and maintenance of CIBP and restored diminished GABABRs attenuate CIBP-induced pain behaviors at least partially by inhibiting the protein kinase/cAMP-response element-binding protein signaling pathway. Therefore, spinal GABABR may become a potential therapeutic target for the management of CIBP.