SDF1-CXCR4 Signaling Contributes to the Transition from Acute to Chronic Pain State

Research Progress on the Mechanisms of Central Post-Stroke Pain: A Review

Central Post-Stroke Pain (CPSP) is a primary sequelae of stroke that can develop in the body part corresponding to the cerebrovascular lesion after stroke, most typically after ischemic stroke but also after hemorrhagic stroke. The pathogenesis of CPSP is currently unknown, and research into its mechanism is ongoing. To summarize current research on the CPSP mechanism and provide guidance for future studies. Use "central post-stroke pain," "stroke AND thalamic pain," "stroke AND neuropathic pain," "post-stroke thalamic pain" as the search term. The search was conducted in the PubMed and China National Knowledge Infrastructure databases, summarizing and classifying the retrieved mechanism studies. The mechanistic studies on CPSP are extensive, and we categorized the included mechanistic studies and summarized them in terms of relevant pathway studies, relevant signals and receptors, relevant neural tissues, and described endoplasmic reticulum stress and other relevant studies, as well as summarized the mechanisms of acupuncture treatment. Studies have shown that the pathogenesis of CPSP involves the entire spinal-thalamo-cortical pathway and that multiple substances in the nervous system are involved in the formation and development of CPSP. Among them, the relevant receptors and signals are the hotspot of research, and the discovery and exploration of different receptors and signals have provided a wide range of therapeutic ideas for CPSP. As a very effective treatment, acupuncture is less studied regarding the analgesic mechanism of CPSP, and further experimental studies are still needed.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

EphA1 aggravates neuropathic pain by activating CXCR4/RhoA/ROCK2 pathway in mice

Neuropathic pain is a refractory disease with limited treatment options due to its complex mechanisms. Whereas erythropoietin-producing hepatocyte A1 (EphA1) mediates the production of inflammatory factors that are important in the progression of neurological diseases, its role and molecular mechanisms in neuropathic pain remain unclear. In the present study, we established a mouse model of chronic constriction injury (CCI). EphA1 expression was observed to be progressively upregulated at the mRNA and protein levels with the progression of the disease. Subsequently, knockdown of EphA1 expression levels using adenovirus short hairpin RNA (AAV-shEphA1) revealed an increase in mechanical stimulation withdrawal threshold (PWT) and withdrawal latency (PWL) when EphA1 expression was decreased, accompanied by improved dorsal root ganglion injury, increased leukocytosis, decreased microglia, and decreased levels of pro-inflammatory factors. For the underlying mechanism, it was found that EphA1 regulates the activity of the RhoA/ROCK2 pathway by modulating the level of CXCR4. Inhibition of CXCR4 and RhoA/ROCK2 could effectively alleviate the promoting effect of EphA1 upregulation on neuropathic pain. In conclusion, our study suggests that depletion of EphA1 ameliorates neuropathic pain by modulating the CXCR4/RhoA/ROCK2 signaling pathway, and targeting EphA1 may be a potential clinical treatment for neuropathic pain.

Identification of Epigenetic Interactions between MicroRNA-30c-5p and DNA Methyltransferases in Neuropathic Pain

Neuropathic pain is a prevalent and severe chronic syndrome, often refractory to treatment, whose development and maintenance may involve epigenetic mechanisms. We previously demonstrated a causal relationship between miR-30c-5p upregulation in nociception-related neural structures and neuropathic pain in rats subjected to sciatic nerve injury. Furthermore, a short course of an miR-30c-5p inhibitor administered into the cisterna magna exerts long-lasting antiallodynic effects via a TGF-β1-mediated mechanism. Herein, we show that miR-30c-5p inhibition leads to global DNA hyper-methylation of neurons in the lumbar dorsal root ganglia and spinal dorsal horn in rats subjected to sciatic nerve injury. Specifically, the inhibition of miR-30-5p significantly increased the expression of the novo DNA methyltransferases DNMT3a and DNMT3b in those structures. Furthermore, we identified the mechanism and found that miR-30c-5p targets the mRNAs of DNMT3a and DNMT3b. Quantitative methylation analysis revealed that the promoter region of the antiallodynic cytokine TGF-β1 was hypomethylated in the spinal dorsal horn of nerve-injured rats treated with the miR-30c-5p inhibitor, while the promoter of Nfyc, the host gene of miR-30c-5p, was hypermethylated. These results are consistent with long-term protection against neuropathic pain development after nerve injury. Altogether, our results highlight the key role of miR-30c-5p in the epigenetic mechanisms' underlying neuropathic pain and provide the basis for miR-30c-5p as a therapeutic target.

Secondary damage and neuroinflammation in the spinal dorsal horn mediate post-thalamic hemorrhagic stroke pain hypersensitivity: SDF1-CXCR4 signaling mediation

Central post-stroke pain (CPSP) is an intractable neuropathic pain, which can be caused by primary lesion of central somatosensory system. It is also a common sequelae of the thalamic hemorrhagic stroke (THS). So far, the underlying mechanisms of CPSP remain largely unknown. Our previous studies have demonstrated that SDF1-CXCR4 signaling in the hemorrhagic region contributes to the maintenance of the THS pain hypersensitivity via mediation of the thalamic neuroinflammation. But whether the spinal dorsal horn, an initial point of spinothalamic tract (STT), suffers from retrograde axonal degeneration from the THS region is still unknown. In this study, neuronal degeneration and loss in the spinal dorsal horn were detected 7 days after the THS caused by intra-thalamic collagenase (ITC) injection by immunohistochemistry, TUNEL staining, electron microscopy, and extracellular multi-electrode array (MEA) recordings, suggesting the occurrence of secondary apoptosis and death of the STT projecting neuronal cell bodies following primary THS via retrograde axonal degeneration. This retrograde degeneration was accompanied by secondary neuroinflammation characterized by an activation of microglial and astrocytic cells and upregulation of SDF1-CXCR4 signaling in the spinal dorsal horn. As a consequence, central sensitization was detected by extracellular MEA recordings of the spinal dorsal horn neurons, characterized by hyperexcitability of both wide dynamic range and nociceptive specific neurons to suprathreshold mechanical stimuli. Finally, it was shown that suppression of spinal neuroinflammation by intrathecal administration of inhibitors of microglia (minocycline) and astrocytes (fluorocitrate) and antagonist of CXCR4 (AMD3100) could block the increase in expression levels of Iba-1, GFAP, SDF1, and CXCR4 proteins in the dorsal spinal cord and ameliorate the THS-induced bilateral mechanical pain hypersensitivity, implicating that, besides the primary damage at the thalamus, spinal secondary damage and neuroinflammation also play the important roles in maintaining the central post-THS pain hypersensitivity. In conclusion, secondary neuronal death and neuroinflammation in the spinal dorsal horn can be induced by primary thalamic neural damage via retrograde axonal degeneration process. SDF1-CXCR4 signaling is involved in the mediation of secondary spinal neuroinflammation and THS pain hypersensitivity. This finding would provide a new therapeutic target for treatment of CPSP at the spinal level.

Mediators of Neuropathic Pain; Focus on Spinal Microglia, CSF-1, BDNF, CCL21, TNF-α, Wnt Ligands, and Interleukin 1β

Intractable neuropathic pain is a frequent consequence of nerve injury or disease. When peripheral nerves are injured, damaged axons undergo Wallerian degeneration. Schwann cells, mast cells, fibroblasts, keratinocytes and epithelial cells are activated leading to the generation of an “inflammatory soup” containing cytokines, chemokines and growth factors. These primary mediators sensitize sensory nerve endings, attract macrophages, neutrophils and lymphocytes, alter gene expression, promote post-translational modification of proteins, and alter ion channel function in primary afferent neurons. This leads to increased excitability and spontaneous activity and the generation of secondary mediators including colony stimulating factor 1 (CSF-1), chemokine C-C motif ligand 21 (CCL-21), Wnt3a, and Wnt5a. Release of these mediators from primary afferent neurons alters the properties of spinal microglial cells causing them to release tertiary mediators, in many situations via ATP-dependent mechanisms. Tertiary mediators such as BDNF, tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and other Wnt ligands facilitate the generation and transmission of nociceptive information by increasing excitatory glutamatergic transmission and attenuating inhibitory GABA and glycinergic transmission in the spinal dorsal horn. This review focusses on activation of microglia by secondary mediators, release of tertiary mediators from microglia and a description of their actions in the spinal dorsal horn. Attention is drawn to the substantial differences in the precise roles of various mediators in males compared to females. At least 25 different mediators have been identified but the similarity of their actions at sensory nerve endings, in the dorsal root ganglia and in the spinal cord means there is considerable redundancy in the available mechanisms. Despite this, behavioral studies show that interruption of the actions of any single mediator can relieve signs of pain in experimental animals. We draw attention this paradox. It is difficult to explain how inactivation of one mediator can relieve pain when so many parallel pathways are available.

Inhibition of CXCR4 in Spinal Cord and DRG with AMD3100 Attenuates Colon-Bladder Cross-Organ Sensitization

Background

Cross-sensitization of pelvic organs is one theory for why symptoms of gut sickness and interstitial cystitis/bladder pain syndrome overlap. Experimental colitis has been shown to trigger bladder hyperactivity and hyperalgesia in rats. The chemokine receptor CXCR4 plays a key role in bladder function and central sensitization. We aim to study the role of CXCR4 and its inhibitor AMD3100 in colon-bladder cross-organ sensitization.

Methods

The colitis model was established by rectal infusion of trinitrobenzene sulfonic acid. Western blot and immunofluorescence were used to assess the expression and distribution of CXCR4. Intrathecal injection of AMD3100 (a CXCR4 inhibitor) and PD98059 (an ERK inhibitor) were used to inhibit CXCR4 and downstream extracellular signal-regulated kinase (ERK) in the spinal cord and dorsal root ganglion (DRG). Intravesical perfusion of resiniferatoxin was performed to measure the pain behavior counts of rats, and continuous cystometry was performed to evaluate bladder voiding function.

Results

Compared to the control group, CXCR4 was expressed more in bladder mucosa and colon mucosa, L6-S1 dorsal root ganglion (DRG), and the corresponding segment of the spinal dorsal horn (SDH) in rats with colitis. Moreover, intrathecal injection of the AMD3100 suppressed bladder overactivity, bladder hyperalgesia, and mastocytosis symptoms caused by colitis. Furthermore, AMD3100 effectively inhibited ERK activation in the spinal cord induced by experimental colitis. Finally, treatment with PD98059 alleviated bladder overactivity and hyperalgesia caused by colitis.

Conclusion

Increased CXCR4 in the DRG and SDH contributes to colon inflammation-induced bladder overactivity and hyperalgesia partly via the phosphorylation of spinal ERK. Treatment targeting the CXCR4/ERK pathway might provide a potential new approach for the comorbidity between the digestive system and the urinary system.

A Systematic Review of the Biological Effects of Cordycepin

We conducted a systematic review of the literature on the effects of cordycepin on cell survival and proliferation, inflammation, signal transduction and animal models. A total of 1204 publications on cordycepin were found by the cut-off date of 1 February 2021. After application of the exclusion criteria, 791 papers remained. These were read and data on the chosen subjects were extracted. We found 192 papers on the effects of cordycepin on cell survival and proliferation and calculated a median inhibitory concentration (IC50) of 135 µM. Cordycepin consistently repressed cell migration (26 papers) and cellular inflammation (53 papers). Evaluation of 76 papers on signal transduction indicated consistently reduced PI3K/mTOR/AKT and ERK signalling and activation of AMPK. In contrast, the effects of cordycepin on the p38 and Jun kinases were variable, as were the effects on cell cycle arrest (53 papers), suggesting these are cell-specific responses. The examination of 150 animal studies indicated that purified cordycepin has many potential therapeutic effects, including the reduction of tumour growth (37 papers), repression of pain and inflammation (9 papers), protecting brain function (11 papers), improvement of respiratory and cardiac conditions (8 and 19 papers) and amelioration of metabolic disorders (8 papers). Nearly all these data are consistent with cordycepin mediating its therapeutic effects through activating AMPK, inhibiting PI3K/mTOR/AKT and repressing the inflammatory response. We conclude that cordycepin has excellent potential as a lead for drug development, especially for age-related diseases. In addition, we discuss the remaining issues around the mechanism of action, toxicity and biodistribution of cordycepin.

Intervertebral foramen injection of plerixafor attenuates neuropathic pain after chronic compression of the dorsal root ganglion: Possible involvement of the down-regulation of Nav1.8 and Nav1.9

Neuropathic pain is a common chronic pain condition with major impact on quality of life. However, its physiopathologic mechanism remains unknown and pain management is still a challenge. Accumulating evidence indicated that C-X-C chemokine receptor type 4 (CXCR4) played a critical role in the process of pain. Thus, the present study aimed to investigate whether intervertebral foramen injection of CXCR4 antagonist, plerixafor, was able to relieve neuropathic pain and explore the possible underlying mechanism. Chronic compression of the dorsal root ganglion (CCD) was established as a typical model of neuropathic pain. The results indicated that CCD induced multiple pain-related behaviors and the expression of CXCR4, Nav1.8 and Nav1.9 was significantly increased in compressed dorsal root ganglion (DRG) neurons. Knocking down CXCR4 expression could significantly reduce neuropathic pain and intervertebral foramen plerixafor injection (IVFP) dramatically decreased the up-regulation of Nav1.8 and Nav1.9 and attenuated neuropathic pain. The analgesic duration of IVFP was maintained at least for 24 h which was much longer than intervertebral foramen injection of Nav1.8 blocker and local anesthetics. Therefore, our study provided evidence that IVFP could reduce the expression of Nav1.8 and Nav1.9 in DRG neurons which might contribute to, at least in part, the analgesic effect of plerixafor on CCD-induced neuropathic pain. It is concluded that IVFP was an effective and applicable treatment approach for neuropathic pain.

Role of GABAAR in the Transition From Acute to Chronic Pain and the Analgesic Effect of Electroacupuncture on Hyperalgesic Priming Model Rats

Chronic pain is a costly health problem that impairs health-related quality of life when not effectively treated. Regulating the transition from acute to chronic pain is a new therapeutic strategy for chronic pain that presents a major clinical challenge. The underlying mechanisms of pain transition are not entirely understood, and strategies for preventing this transition are lacking. Here, a hyperalgesic priming model was used to study the potential mechanism by which γ-aminobutyric acid receptor type A (GABAAR) in the dorsal root ganglion (DRG) contributes to pain transition. Furthermore, electroacupuncture (EA), a modern method of acupuncture, was administered to regulate pain transition, and the mechanism underlying EA’s regulatory effect was investigated. Hyperalgesic priming was induced by intraplanar injection of carrageenan (Car)/prostaglandin E₂ (PGE₂). The decrease in mechanical withdrawal threshold (MWT) induced by PGE₂ returned to baseline 4 h after injection in NS + PGE₂ group, and still persisted 24 h after injection in Car + PGE₂ group. Lower expression of GABAAR in the lumbar DRG was observed in the model rats. Furthermore, activating or blocking GABAAR could reversed the long-lasting hyperalgesia induced by Car/PGE₂ injection or produced a persistent hyperalgesia. In addition, GABAAR may be involved in Protein Kinase C epsilon (PKCε) activation in the DRG, a mark molecular of pain transition. EA considerably increased the mechanical pain thresholds of hyperalgesic priming model mammals in both the acute and chronic phases. Furthermore, EA upregulated the expression of GABAAR and inhibited the activation of PKCε in the DRG. In addition, peripheral administration of picrotoxin blocked the analgesic effect of EA on the model rats and abolished the regulatory effect of EA on PKCε activation. These findings suggested that GABAAR plays a key role in both the transition from acute to chronic pain and the analgesic effect of EA on hyperalgesic priming.

Chemokine receptor CXCR4 activates the RhoA/ROCK2 pathway in spinal neurons that induces bone cancer pain

Background

Chemokine receptor CXCR4 has been found to be associated with spinal neuron and glial cell activation during bone cancer pain. However, the underlying mechanism remains unknown. Furthermore, the RhoA/ROCK2 pathway serves as a downstream pathway activated by CXCR4 during bone cancer pain. We first validated the increase in the expressions of CXCR4, p-RhoA, and p-ROCK2 in the spinal dorsal horn of a well-characterized tumor cell implantation-induced cancer pain rat model and how these expressions contributed to the pain behavior in tumor cell implantation rats. We hypothesized that spinal blockade of the CXCR4-RhoA/ROCK2 pathway is a potential analgesic therapy for cancer pain management.

Methods

Adult female Sprague–Dawley rats (body weight of 180–220 g) and six- to seven-week old female Sprague–Dawley rats (body weight of 80–90 g) were taken. Ascitic cancer cells were extracted from the rats (body weight of 80–90 g) with intraperitoneally implanted Walker 256 mammary gland carcinoma cells. Walker 256 rat mammary gland carcinoma cells were then injected (tumor cell implantation) into the intramedullary space of the tibia to establish a rat model of bone cancer pain.

Results

We found increased expressions of CXCR4, p-RhoA, and p-ROCK2 in the neurons in the spinal cord. p-RhoA and p-ROCK2 were co-expressed in the neurons and promoted by overexpressed CXCR4. Intrathecal delivery of CXCR4 inhibitor Plerixafor (AMD3100) or ROCK2 inhibitor Fasudil abrogated tumor cell implantation-induced pain hypersensitivity and tumor cell implantation-induced increase in p-RhoA and p-ROCK2 expressions. Intrathecal injection of stromal-derived factor-1, the principal ligand for CXCR4, accelerated p-RhoA expression in naive rats, which was prevented by postadministration of CXCR4 inhibitor Plerixafor (AMD3100) or ROCK2 inhibitor Fasudil.

Conclusions

Collectively, the spinal RhoA/ROCK2 pathway could be a critical downstream target for CXCR4-mediated neuronal sensitization and pain hypersensitivity in bone cancer pain, and it may serve as a potent therapeutic target for pain treatment.

Upregulating miR-130a-5p relieves astrocyte over activation-induced neuropathic pain through targeting C-X-C motif chemokine receptor 12/C-X-C motif chemokine receptor 4 axis

OBJECTIVES

This study intends to explore the role and specific mechanism of miR-130a-5p in neuropathic pain through regulating the C-X-C motif chemokine receptor 12 (CXCL12)-C-X-C motif chemokine receptor 4 (CXCR4) pathway.

METHODS

First, mouse neuropathic pain model was constructed by spinal nerve ligation. MiR-130a-5p mimics were used to upregulate miR-130a-5p in vivo. The behaviour and pain scores of the spinal cord injury (SCI) mice were assessed. In addition, astrocytic activation as well as inflammatory response in the spinal lesions was determined.

RESULTS

The results manifested miR-130a-5p was notably downregulated in neuropathic pain model and reached the lowest point at 3 days after injury. Besides, tail vein injection of miR-130a-5p mimics inhibited the activation and inflammatory response of astrocytes, thus alleviating chronic constriction injury-induced neuropathic pain. Moreover, miR-130a-5p inactivated CXCR4 and its downstream Rac1, nuclear factor-κB (NF-κB) and extracellular regulated protein kinases signalling pathways by attenuating CXCL12.

CONCLUSION

MiR-130a-5p inactivated astrocytes by targeting CXCL12/CXCR4, thus alleviating SCI-induced neuropathic pain.

Longitudinal Transcriptomic Profiling in Carrageenan-Induced Rat Hind Paw Peripheral Inflammation and Hyperalgesia Reveals Progressive Recruitment of Innate Immune System Components

Pain is a common but potentially debilitating symptom, often requiring complex management strategies. To understand the molecular dynamics of peripheral inflammation and nociceptive pain, we investigated longitudinal changes in behavior, tissue structure, and transcriptomic profiles in the rat carrageenan-induced peripheral inflammation model. Sequential changes in the number of differentially expressed genes are consistent with temporal recruitment of key leukocyte populations, mainly neutrophils and macrophages with each wave being preceded by upregulation of the cell-specific chemoattractants, Cxcl1 and Cxcl2, and Ccl2 and Ccl7, respectively. We defined 12 temporal gene clusters based on expression pattern. Within the patterns we extracted genes comprising the inflammatory secretome and others related to nociceptive tissue remodeling and to sensory perception of pain. Structural tissue changes, involving upregulation of multiple collagens occurred as soon as 1-hour postinjection, consistent with inflammatory tissue remodeling. Inflammatory expression profiling revealed a broad-spectrum, temporally orchestrated molecular and cellular recruitment process. The results provide numerous potential targets for modulation of pain and inflammation. PERSPECTIVE: This study investigates the highly orchestrated biological response during tissue inflammation with precise assessment of molecular dynamics at the transcriptional level. The results identify transcriptional changes that define an evolving inflammatory state in rats. This study provides foundational data for identifying markers of, and potential treatments for, inflammation and pain in patients.

CXCL10/CXCR3 Signaling in the DRG Exacerbates Neuropathic Pain in Mice

Chemokines and receptors have been implicated in the pathogenesis of chronic pain. Here, we report that spinal nerve ligation (SNL) increased CXCR3 expression in dorsal root ganglion (DRG) neurons, and intra-DRG injection of Cxcr3 shRNA attenuated the SNL-induced mechanical allodynia and heat hyperalgesia. SNL also increased the mRNA levels of CXCL9, CXCL10, and CXCL11, whereas only CXCL10 increased the number of action potentials (APs) in DRG neurons. Furthermore, in Cxcr3^-/- mice, CXCL10 did not increase the number of APs, and the SNL-induced increase of the numbers of APs in DRG neurons was reduced. Finally, CXCL10 induced the activation of p38 and ERK in ND7-23 neuronal cells and DRG neurons. Pretreatment of DRG neurons with the P38 inhibitor SB203580 decreased the number of APs induced by CXCL10. Our data indicate that CXCR3, activated by CXCL10, mediates p38 and ERK activation in DRG neurons and enhances neuronal excitability, which contributes to the maintenance of neuropathic pain.

Peripheral FGFR1 Regulates Myofascial Pain in Rats via the PI3K/AKT Pathway

Myofascial pain syndrome (MPS) is a type of skeletal pain identified by myofascial trigger points (MTrPs). The formation of MTrPs is linked to muscle damage. The fibroblast growth factor receptor (FGFR1) has been found to cause pain sensitivity while repairing tissue damage. The aim of the current study was to explore the mechanism of FGFR1 in MTrPs. We used a RayBio human phosphorylation array kit to measure p-FGFR1 levels in human control subjects and patients with MTrPs. P-FGFR1 was upregulated in the patients with MTrPs. Then a rat model of MPS was established by a blunt strike on the left gastrocnemius muscles (GM) and eccentric-exercise for 8 weeks with 4 weeks of recovery. After establishing the MPS model, the morphology of the GM changed, and the differently augmented sizes of round fibers (contracture knots) in the transverse section and fusiform shapes in the longitudinal section were clearly seen in the rats with myofascial pain. The expression of p-FGFR1 was upregulated on the peripheral nerves and dorsal root ganglion neurons in the MTrPs group. The spinal Fos protein expression was increased in the MTrPs group. Additionally, the mechanical pain threshold was reduced, and the expression of FGF2, p-FGFR1, PI3K-p110γ, and p-AKT increased in the MTrPs group. PD173074 increased the mechanical pain threshold of the MTrPs group, and inhibited the expression of p-FGFR1, PI3K-p110γ, and p-AKT. Moreover, LY294002 increased the mechanical pain threshold of the MTrPs group. These findings suggest that FGFR1 may regulate myofascial pain in rats through the PI3K/AKT pathway.

Changes in inflammatory plasma proteins from patients with chronic pain associated with treatment in an interdisciplinary multimodal rehabilitation program – an explorative multivariate pilot study

It has been suggested that alterations in inflammation molecules maintain chronic pain although little is known about how these factors influence homeostatic and inflammatory events in common chronic pain conditions. Nonpharmacological interventions might be associated with alterations in inflammation markers in blood. This study of patients with chronic pain investigates whether an interdisciplinary multimodal rehabilitation program (IMMRP) was associated with significant alterations in the plasma pattern of 68 cytokines/chemokines 1 year after rehabilitation and whether such changes were associated with clinical changes. Blood samples and self-reports of pain, psychological distress, and physical activity of 25 complex chronic pain patients were collected pre-IMMRP and at 12-month follow-up. Analyses of inflammatory proteins (cytokines/chemokines/growth factors) were performed directly in plasma using the multiplex immunoassay technology Meso Scale Discovery. This explorative pilot study found that 12 substances, mainly pro-inflammatory, decreased after IMMRP. In two other relatively small IMMRP studies, four of these proinflammatory markers were also associated with decreases. The pattern of cytokines/chemokines pre-IMMRP was associated with changes in psychological distress but not with pain or physical activity. The present study cannot impute cause and effect. These results together with the results of the two previous IMMRP studies suggest that there is a need for larger and more strictly controlled studies of IMMRP with respect to inflammatory markers in blood. Such studies need to consider responders/non-responders, additional therapies, involved pain mechanisms and diagnoses. This and the two other studies open up for developing biologically measurable outcomes from plasma. Such biomarkers will be an important tool for further development of IMMRP and possibly other treatments for patients w ith chronic pain.

RNA control in pain: Blame it on the messenger

mRNA function is meticulously controlled. We provide an overview of the integral role that posttranscriptional controls play in the perception of painful stimuli by sensory neurons. These specialized cells, termed nociceptors, precisely regulate mRNA polarity, translation, and stability. A growing body of evidence has revealed that targeted disruption of mRNAs and RNA-binding proteins robustly diminishes pain-associated behaviors. We propose that the use of multiple independent regulatory paradigms facilitates robust temporal and spatial precision of protein expression in response to a range of pain-promoting stimuli. This article is categorized under: RNA in Disease and Development > RNA in Disease Translation > Translation Regulation RNA Turnover and Surveillance > Regulation of RNA Stability.

Nicotine inhibits rapamycin-induced pain through activating mTORC1/S6K/IRS-1-related feedback inhibition loop

Mammalian target of rapamycin complex 1 (mTORC1) inhibitors increase the incidence of pain in patients, and this finding has been replicated in animal models. However, reports on possible analgesics for this condition are scant. Accumulating evidence finds that nicotinic acetylcholine receptors (nAChRs) are involved in mediating pain. However, whether nicotine, a full agonist of nAChRs, alleviates mTORC1 inhibition-induced pain and its underlying mechanisms remain unknown. In this study, pain was induced in naïve male C57BL/6J mice by intraperitoneally injecting rapamycin acutely or repeatedly. Subsequently, pain thresholds, including mechanical and thermal pain, were measured. The involving signaling pathway was tested using western blot analysis and immunofluorescent assay. Changes in neuronal excitability caused by different treatments were also analyzed using whole-cell recording. Microinjection into the anterior cingulate cortex (ACC) was used to test the role of nAChRs containing the α4β2 or α7 subtype in this brain region in pain modulation. Our results showed that nicotine significantly reduced hyperalgesia in mice that received acute or repeated rapamycin injections, and reversed the effects of rapamycin on the phosphorylation of S6K, 4E-BP1, insulin receptor substrate-1 (IRS-1) at Ser636/639, AKT at Ser473, and ERK at Thr202/Tyr204. Whole-cell recording results showed that nicotine reduced the firing rates of pyramidal neurons in the ACC, and a pharmacological blockade of nAChRs containing the α4β2 or α7 subtype in ACC inhibited the antinociceptive effects of nicotine in mice with rapamycin-induced pain. Our findings indicate that analgesics targeting nAChRs can be developed to help patients with rapamycin-induced pain.

CXCL12/CXCR4 signaling contributes to neuropathic pain via central sensitization mechanisms in a rat spinal nerve ligation model

Background

Previous studies have demonstrated that the CXCL12/CXCR4 signaling axis is involved in the regulation of neuropathic pain (NP). Here, we performed experiments to test whether the CXCL12/CXCR4 signaling pathway contributes to the pathogenesis of neuropathic pain after spinal nerve ligation (SNL) via central sensitization mechanisms.

Methods

Neuropathic pain was induced and assessed in a SNL rat model. The expression and distribution of CXCL12 or CXCR4 were examined by immunofluorescence staining and western blot. The effects of CXCL12 rat peptide, CXCL12 neutralizing antibody, CXCR4 antagonist, and astrocyte metabolic inhibitor on pain hypersensitivity were explored by behavioral tests in naive or SNL rats. We measured the expression level of c‐Fos and CGRP to evaluate the sensitization of neurons by RT‐PCR. The activation of astrocyte and microglia was analyzed by measuring the level of GFAP and iba‐1. The mRNA levels of the pro‐inflammatory cytokines such as TNF‐α, IL‐1β, and IL‐6 and Connexin 30, Connexin 43, EAAT 1, EAAT 2 were also detected by RT‐PCR.

Results

First, we found that the expression of CXCL12 and CXCR4 was upregulated after SNL. CXCL12 was mainly expressed in the neurons while CXCR4 was expressed both in astrocytes and neurons in the spinal dorsal horn after SNL. Moreover, intrathecal administration of rat peptide, CXCL12, induced hypersensitivity in naive rats, which was partly reversed by fluorocitrate. In addition, the CXCL12 rat peptide increased mRNA levels of c‐Fos, GFAP, and iba‐1. A single intrathecal injection of CXCL12 neutralizing antibody transiently reversed neuropathic pain in the SNL rat model. Consecutive use of CXCL12 neutralizing antibody led to significant delay in the induction of neuropathic pain, and reduced the expression of GFAP and iba‐1 in the spinal dorsal horn. Finally, repeated intrathecal administration of the CXCR4 antagonist, AMD3100, significantly suppressed the initiation and duration of neuropathic pain. The mRNA levels of c‐Fos, CGRP, GFAP, iba‐1, and pro‐inflammatory cytokines, also including Connexin 30 and Connexin 43 were decreased after injection of AMD3100, while EAAT 1 and EAAT 2 mRNAs were increased.

Conclusion

We demonstrate that the CXCL12/CXCR4 signaling pathway contributes to the development and maintenance of neuropathic pain via central sensitization mechanisms. Importantly, intervening with CXCL12/CXCR4 presents an effective therapeutic approach to treat the neuropathic pain.

Hyperbaric oxygen relieves neuropathic pain through AKT/TSC2/mTOR pathway activity to induce autophagy

Background

Our previous study suggested that HBO treatment attenuated neuropathic pain by inhibiting mTOR to induce autophagy in SNL neuropathic pain model. The aim of this study was to evaluate the role of AKT/TSC2/mTOR pathway in SNL and autophagy and determine whether HBO treatment could relieve neuropathic pain via modulating AKT/TSC2/mTOR pathway.

Materials and methods

Rats were randomly divided into sham, SNL, SNL + HBO treatment, SNL + vehicle, and SNL + AKT inhibitor groups. Neuropathic pain was induced following SNL procedure. Rats in the SNL + HBO group received HBO treatment for 7 consecutive days beginning on postoperative day 1. The SNL + vehicle group received 10 µL of 3% dimethyl sulfoxide in saline. SNL + AKT inhibitor group received 10 µL AKT inhibitor IV intrathecally. Mechanical withdrawal threshold tests were performed to evaluate mechanical hypersensitivity. AKT, p-AKT, TSC2, mTOR, p-mTOR, and LC3-II protein expressions were examined by Western blot analysis.

Results

HBO reversed AKT/TSC2/mTOR upregulation induced by SNL and attenuated neuropathic pain. Intrathecal injection of AKT inhibitor IV decreased the activity of AKT/TSC2/mTOR pathway and increased LC3-II expression accompanied by analgesic effect in SNL rats.

Conclusion

Taken together, our findings demonstrated AKT/TSC2/mTOR pathway was activated in SNL-induced neuropathic pain, and HBO treatment attenuated neuropathic pain via neutralizing AKT/TSC2/mTOR pathway activation.

CXCL12/CXCR4 signaling mediated ERK1/2 activation in spinal cord contributes to the pathogenesis of postsurgical pain in rats

Background

It has been demonstrated that upregulation of CXCL12 and CXCR4 in spinal cord involves in the pathogenesis of neuropathic, inflammatory, and cancer pain. However, whether CXCL12/CXCR4 signaling contributes to postsurgical pain remains unknown. The aim of the present study is to investigate the role of CXCL12/CXCR4 signaling in the genesis of postsurgical pain and the underlying mechanism.

Results

Plantar incision in rat hind paw resulted in increased expressions of CXCL12 and CXCR4 in spinal dorsal horn. Double immunofluorescence staining revealed that CXCL12 expressed in neurons and astrocytes, and CXCR4 exclusively co-localized with neuronal cells. Prior administration of AMD3100, a specific antagonist of CXCR4, or CXCL12 neutralizing antibody, intrathecally attenuated plantar incision-induced mechanical allodynia and thermal hyperalgesia. Plantar incision also augmented the phosphorylation of NF-κB p65 in spinal cord. Pre intrathecal (i.t.) injection of PDTC, a specific NF-κB activation inhibitor, alleviated plantar incision-induced postsurgical pain and reduced the expression of CXCL12 in spinal cord. Correlated with the upregulation of CXCL12 and CXCR4, plantar incision also resulted in an increased phosphorylation of extracellular signal-regulated kinase 1/2 and Akt in spinal cord. Prior i.t. administration of AMD3100 prevented extracellular signal-regulated kinase, but not Akt, activation in spinal cord. Rats when given a repetitive i.t. PD98059, a specific extracellular signal-regulated kinase inhibitor, started 30 min before surgery also ameliorate plantar incision-induced mechanical and thermal pain hypersensitivity.

Conclusion

Our results suggests that plantar incision-induced activation of NF-κB signaling may mediate upregulation of CXCL12 in spinal cord, and CXCL12/CXCR4 signaling via extracellular signal-regulated kinase activation contributes to the genesis of postsurgical pain.

Spinal SHP2 Contributes to Exaggerated Incisional Pain in Adult Rats Subjected to Neonatal and Adult Incisions via PI3K

Neonatal injury-induced exaggeration of pain hypersensitivity after adult trauma is a significant clinical challenge. However, the underlying mechanisms remain poorly understood. Growing evidence shows that spinal Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2) contributes to chronic pain in adult rodents. Here we demonstrated that the phosphorylation and expression of SHP2 in synaptosomal fraction of the spinal dorsal horn are elevated in adult rats subjected to neonatal and adult incisions (nIN-IN), and the upregulation of SHP2 is highly correlated with pain hypersensitivity. Intrathecal blockade of SHP2 phosphorylation using a SHP2 protein tyrosine phosphatase inhibitor NSC-87877, or knockdown of SHP2 by intrathecal delivery of small interfering RNA (siRNA), ameliorates mechanical allodynia and heat hyperalgesia in nIN-IN rats. Moreover, the expression of phosphatidylinositol 3-kinase (PI3K) in the spinal dorsal horn is significantly increased in nIN-IN rats. Intrathecal application of PI3K inhibitor, LY294002 or wortmannin, alleviates pain hypersensitivity in nIN-IN rats. Additionally, intrathecal administration of NSC-87877 or SHP2 siRNA attenuates the upregulation of PI3K. Finally, no alternation of SHP2 phosphorylation in the dorsal root ganglion and dorsal root of nIN-IN rats as well as PI3K expression in the dorsal root of nIN-IN rats intrathecally treated with NSC-87877 or SHP2 siRNA is observed. These results suggest that the phosphorylation and expression of SHP2 in the spinal dorsal horn play vital roles in neonatal incision-induced exaggeration of adult incisional pain via PI3K. Thus, SHP2 and PI3K may serve as potential therapeutic targets for exaggerated incisional pain induced by neonatal and adult injuries.

SDF1-CXCR4 Signaling Maintains Central Post-Stroke Pain through Mediation of Glial-Neuronal Interactions

Central post-stroke pain (CPSP) is an intractable central neuropathic pain that has been poorly studied mechanistically. Here we showed that stromal cell-derived factor 1 (SDF1 or CXCL12), a member of the CXC chemokine family, and its receptor CXCR4 played a key role in the development and maintenance of thalamic hemorrhagic CPSP through hypoxia inducible factor 1α (HIF-1α) mediated microglial-astrocytic-neuronal interactions. First, both intra-thalamic collagenase (ITC) and SDF1 injections could induce CPSP that was blockable and reversible by intra-thalamic administration of both AMD3100 (a selective CXCR4 antagonist) and inhibitors of microglial or astrocytic activation. Second, long-term increased-expression of SDF1 and CXCR4 that was accompanied by activations of both microglia and astrocytes following ITC could be blocked by both AMD-3100 and YC-1, a selective inhibitor of HIF-1α. AMD-3100 could also inhibit release of proinflammatory mediators (TNFα, IL1β and IL-6). Increased-expression of HIF-1α, SDF1, CXCR4, Iba1 and GFAP proteins could be induced by both ITC and intra-thalamic CoCl₂, an inducer of HIF-1α that was blockable by both HIF-1α inhibition and CXCR4 antagonism. Finally, inhibition of HIF-1α was only effective in prevention, but not in treatment of ITC-induced CPSP. Taken together, the present study demonstrated that in the initial process of thalamic hemorrhagic state HIF-1α up-regulated SDF1-CXCR4 signaling, while in the late process SDF1-CXCR4 signaling-mediated positive feedback plays more important role in glial-glial and glial-neuronal interactions and might be a novel promising molecular target for treatment of CPSP in clinic.

Effect of CXCL12/CXCR4 signaling on neuropathic pain after chronic compression of dorsal root ganglion

Neuropathic pain is a complex, chronic pain state that often accompanies tissue damage, inflammation or injury of the nervous system. However the underlying molecular mechanisms still remain unclear. Here, we showed that CXCL12 and CXCR4 were upregulated in the dorsal root ganglion (DRG) after chronic compression of DRG (CCD), and some CXCR4 immunopositive neurons were also immunopositive for the nociceptive neuronal markers IB4, TRPV1, CGRP, and substance P. The incidence and amplitude of CXCL12-induced Ca²⁺ response in primary sensory neurons from CCD mice was significantly increased compared to those from control animals. CXCL12 depolarized the resting membrane potential, decreased the rheobase, and increased the number of action potentials evoked by a depolarizing current at 2X rheobase in neurons from CCD mice. The mechanical and thermal hypernociception after CCD was attenuated by administration of a CXCR4 antagonist AMD3100. These findings suggest that CXCL12/CXCR4 signaling contributes to hypernociception after CCD, and targeting CXCL12/CXCR4 signaling pathway may alleviate neuropathic pain.

Chemokine receptor CXCR4 regulates CaMKII/CREB pathway in spinal neurons that underlies cancer-induced bone pain

We previously demonstrated that the chemokine receptor CXCR4 plays an important role in cancer-induced bone pain by activating spinal neurons and glial cells. However, the specific neuronal mechanism of CXCR4 signaling is not clear. We further report that CXCR4 contributes to the activation of the neuronal CaMKII/CREB pathway in cancer-induced bone pain. We used a tumor cell implantation (TCI) model and observed that CXCR4, p-CaMKII and p-CREB were persistently up-regulated in spinal neurons. CXCR4 also co-expressed with p-CaMKII and p-CREB, and mediated p-CaMKII and p-CREB expression after TCI. Intrathecal delivery of CXCR4 siRNA or CaMKII inhibitor AIP2 abrogated TCI-induced pain hypersensitivity and TCI-induced increase in p-CaMKII and p-CREB expression. Intrathecal injection of the principal ligand for CXCR4, SDF-1, promoted p-CaMKII and p-CREB expression in naive rats, which was prevented by post-administration of CXCR4 inhibitor Plerixafor or PLC inhibitor U73122. Plerixafor, U73122, or AIP2 also alleviated SDF-1-elicited pain behaviors. Intrathecal injection of CXCR4 siRNA significantly suppressed TCI-induced up-regulation of NMDAR1 mRNA and protein, which is a known gene target of CREB. Collectively, these results suggest that the CaMKII/CREB pathway in spinal neurons mediates CXCR4-facilitated pain hypersensitivity in cancer rats.

Electroacupuncture Attenuates CFA-induced Inflammatory Pain by suppressing Nav1.8 through S100B, TRPV1, Opioid, and Adenosine Pathways in Mice

Pain is associated with several conditions, such as inflammation, that result from altered peripheral nerve properties. Electroacupuncture (EA) is a common Chinese clinical medical technology used for pain management. Using an inflammatory pain mouse model, we investigated the effects of EA on the regulation of neurons, microglia, and related molecules. Complete Freund’s adjuvant (CFA) injections produced a significant mechanical and thermal hyperalgesia that was reversed by EA or a transient receptor potential V1 (TRPV1) gene deletion. The expression of the astrocytic marker glial fibrillary acidic protein (GFAP), the microglial marker Iba-1, S100B, receptor for advanced glycation end-products (RAGE), TRPV1, and other related molecules was dramatically increased in the dorsal root ganglion (DRG) and spinal cord dorsal horn (SCDH) of CFA-treated mice. This effect was reversed by EA and TRPV1 gene deletion. In addition, endomorphin (EM) and N⁶-cyclopentyladenosine (CPA) administration reliably reduced mechanical and thermal hyperalgesia, thereby suggesting the involvement of opioid and adenosine receptors. Furthermore, blocking of opioid and adenosine A1 receptors reversed the analgesic effects of EA. Our study illustrates the substantial therapeutic effects of EA against inflammatory pain and provides a novel and detailed mechanism underlying EA-mediated analgesia via neuronal and non-neuronal pathways.

Animal models of chronic pain: Advances and challenges for clinical translation

Chronic pain is a global problem that has reached epidemic proportions. An estimated 20% of adults suffer from pain, and another 10% are diagnosed with chronic pain each year (Goldberg and McGee, ). Despite the high prevalence of chronic pain (an estimated 1.5 billion people are afflicted worldwide), much remains to be understood about the underlying causes of this condition, and there is an urgent requirement for better pain therapies. The discovery of novel targets and the development of better analgesics rely on an assortment of preclinical animal models; however, there are major challenges to translating discoveries made in animal models to realized pain therapies in humans. This review discusses common animal models used to recapitulate clinical chronic pain conditions (such as neuropathic, inflammatory, and visceral pain) and the methods for assessing the sensory and affective components of pain in animals. We also discuss the advantages and limitations of modeling chronic pain in animals as well as highlighting strategies for improving the predictive validity of preclinical pain studies. © 2016 Wiley Periodicals, Inc.