Progression Of Neuropathic Pain Research Articles

Dorsal root ganglion-derived exosomes deteriorate neuropathic pain by activating microglia via the microRNA-16-5p/HECTD1/HSP90 axis

BackgroundThe activated microglia have been reported as pillar factors in neuropathic pain (NP) pathology, but the molecules driving pain-inducible microglial activation require further exploration. In this study, we investigated the effect of dorsal root ganglion (DRG)-derived exosomes (Exo) on microglial activation and the related mechanism.MethodsA mouse model of NP was generated by spinal nerve ligation (SNL), and DRG-derived Exo were extracted. The effects of DRG-Exo on NP and microglial activation in SNL mice were evaluated using behavioral tests, HE staining, immunofluorescence, and western blot. Next, the differentially enriched microRNAs (miRNAs) in DRG-Exo-treated microglia were analyzed using microarrays. RT-qPCR, RNA pull-down, dual-luciferase reporter assay, and immunofluorescence were conducted to verify the binding relation between miR-16-5p and HECTD1. Finally, the effects of ubiquitination modification of HSP90 by HECTD1 on NP progression and microglial activation were investigated by Co-IP, western blot, immunofluorescence assays, and rescue experiments.ResultsDRG-Exo aggravated NP resulting from SNL in mice, promoted the activation of microglia in DRG, and increased neuroinflammation. miR-16-5p knockdown in DRG-Exo alleviated the stimulating effects of DRG-Exo on NP and microglial activation. DRG-Exo regulated the ubiquitination of HSP90 through the interaction between miR-16-5p and HECTD1. Ubiquitination alteration of HSP90 was involved in microglial activation during NP.ConclusionsmiR-16-5p shuttled by DRG-Exo regulated the ubiquitination of HSP90 by interacting with HECTD1, thereby contributing to the microglial activation in NP.

Histone deacetylase as emerging pharmacological therapeutic target for neuropathic pain: From epigenetic to selective drugs.

Neuropathic pain remains a formidable challenge for modern medicine. The first-line pharmacological therapies exhibit limited efficacy and unfavorable side effect profiles, highlighting an unmet need for effective therapeutic medications. The past decades have witnessed an explosion in efforts to translate epigenetic concepts into pain therapy and shed light on epigenetics as a promising avenue for pain research. Recently, the aberrant activity of histone deacetylase (HDAC) has emerged as a key mechanism contributing to the development and maintenance of neuropathic pain. In this review, we highlight the distinctive role of specific HDAC subtypes in a cell-specific manner in pain nociception, and outline the recent experimental evidence supporting the therapeutic potential of HDACi in neuropathic pain. We have summarized studies of HDAC in neuropathic pain in Pubmed. HDACs, widely distributed in the neuronal and non-neuronal cells of the dorsal root ganglion and spinal cord, regulate gene expression by deacetylation of histone or non-histone proteins and involving in increased neuronal excitability and neuroinflammation, thus promoting peripheral and central sensitization. Importantly, pharmacological manipulation of aberrant acetylation using HDAC-targeted inhibitors (HDACi) has shown promising pain-relieving properties in various preclinical models of neuropathic pain. Yet, many of which exhibit low-specificity that may induce off-target toxicities, underscoring the necessity for the development of isoform-selective HDACi in pain management. Abnormally elevated HDACs promote neuronal excitability and neuroinflammation by epigenetically modulating pivotal gene expression in neuronal and immune cells, contributing to peripheral and central sensitization in the progression of neuropathic pain, and HDACi showed significant efficacy and great potential for alleviating neuropathic pain.

Circular RNA-GRIN2B Suppresses Neuropathic Pain by Targeting the NF-κB/SLICK Pathway.

The role of circular RNAs (circRNAs) in neuropathic pain is linked to the fundamental physiological mechanisms involved. However, the exact function of circRNAs in the context of neuropathic pain is still not fully understood. The functional impact of circGRIN2B on the excitability of dorsal root ganglion (DRG) neurons was investigated using siRNA or overexpression technology in conjunction with fluorescence in situ hybridization and whole-cell patch-clamp technology. The therapeutic efficacy of circGRIN2B in treating neuropathic pain was confirmed by assessing the pain threshold in a chronic constrictive injury (CCI) model. The interaction between circGRIN2B and NF-κB was examined through RNA pulldown, RIP, and mass spectrometry assays. CircGRIN2B knockdown significantly affected the action potential discharge frequency and the sodium-dependent potassium current flux (SLICK) in DRG neurons. Furthermore, knockdown of circGRIN2B dramatically reduced the SLICK channel protein and mRNA expression in vivo and in vitro. Our research confirmed the interaction between circGRIN2B and NF-κB. These findings demonstrated that circGRIN2B promotes the transcription of the SLICK gene by binding to NF-κB. In CCI rat models, the overexpression of circGRIN2B has been shown to hinder the progression of neuropathic pain, particularly by reducing mechanical and thermal hyperalgesia. Additionally, this upregulation significantly diminished the levels of the inflammatory cytokines IL-1β, IL-6, and TNF-α in the DRG. Upon reviewing these findings, it was determined that circGRIN2B may mitigate the onset of neuropathic pain by modulating the NF-κB/SLICK pathway.

Huangqin decoction attenuates spared nerve injury (SNI)-induced neuropathic pain by modulating microglial M1/M2 polarization partially mediated by intestinal nicotinamide metabolism

BackgroundThe incidence of neuropathic pain is progressively increasing over time. The activation of M1-type microglia plays a crucial role in the initiation and progression of neuropathic pain. Huangqin Decoction (HQD) is traditionally used to alleviate dysentery and abdominal pain. However, it remains unclear whether HQD can effectively mitigate neuropathic pain and the underlying mechanisms. PurposeThe present study aims to investigate the impact of HQD on neuropathic pain induced by spared nerve injury (SNI) in mice, and to elucidate whether the analgesic effect of HQD is associated with microglia polarization. MethodsThe analgesic effect of HQD on SNI mice was investigated through assessments of mechanical pain threshold, thermal pain threshold, cold pain threshold, and motor ability. We elucidated the molecular mechanisms of HQD in alleviating SNI-induced neuropathic pain by focusing on microglia polarization and intestinal metabolite abnormalities. The expression levels of markers associated with microglia polarization (Iba-1, CD68, CD206, iNOS) was detected by immunofluorescence and Western blot, and the levels of inflammatory factors (IL-4, IL-10, IL-6, TNF-α) were assessed by ELISA. UPLC-QTOF-MS metabolomics was utilized to identify differential metabolites in the intestines of SNI mice. We screened the differential metabolites related to microglial polarization by correlation analysis, subsequently nicotinamide was selected for validation in LPS-induced BV-2 cells. ResultsOur findings demonstrated that HQD (20 g/kg) significantly enhanced the mechanical pain threshold, thermal pain threshold, and cold pain threshold, and protected the injured DRG neurons of SNI mice. Moreover, HQD (20 g/kg) obviously suppressed the expression of microglia M1 polarization markers (Iba-1, CD68, iNOS, IL-6, TNF-α), and promoted the expression of microglia M2 polarization markers (CD206, IL-10, IL-4) in the spinal cord of SNI mice. Additionally, HQD (20 g/kg) prominently ameliorated intestinal barrier damage by upregulating Claudin 1 and Occludin expression in the colon of SNI mice. Furthermore, HQD (20 g/kg) rectified 19 metabolite abnormalities in the intestine. Notably, nicotinamide (100 μM), an amide derivative with anti-inflammatory property, effectively suppresses microglia activation and polarization in LPS-induced BV-2 cells by downregulating IL-6 level and CD68 expression while upregulating IL-4 level and CD206 expression. ConclusionIn summary, HQD alleviates neuropathic pain in SNI mice by regulating the activation and polarization of microglia, partially mediated through intestinal nicotinamide metabolism.

LPAR6 Participates in Neuropathic Pain by Mediating Astrocyte Cells via ROCK2/NF-κB Signal Pathway.

Neuropathic pain (NPP) is a common type of chronic pain. Glial cells, including astrocytes (AS), are believed to play an important role in the progression of NPP. AS cells can be divided into various types based on their expression profiles, among which A1 and A2 types have clear functions. A1-type AS cells are neurotoxic, while A2-type AS cells exert neuroprotective functions. Some types of lysophosphatidic acid receptors (LPAR) have been shown to play a role in NPP. However, it remains unclear how AS cells and LPAR6 affect the occurrence and progression of NPP. In this study, we established a mouse model of chronic constriction injury (CCI) to simulate NPP. It was found that the expression of LPAR6 in AS cells of the spinal dorsal horn was increased in the CCI model, and the thresholds of mechanical and thermal pain were elevated after knocking out LPAR6, indicating that LPAR6 and AS cells participated in the occurrence of NPP. The experiment involved culturing primary AS cells and knocking down LPAR6 by Lentivirus. The results showed that the NF-κB signal pathway was activated and the number of A1-type AS cells increased in the CCI model. However, LPAR6 knockdown inhibited the NF-κB signal pathway and A1-type AS cells. The results of the mRNA sequencing and immunoprecipitation test indicate an interaction between LPAR6 and ROCK2. Inhibiting ROCK2 by Y-27632 increased mechanical and thermal pain thresholds and alleviated NPP at the molecular level. The study presents evidence that LPAR6 activates the NF-κB pathway through ROCK2 and contributes to the progression of NPP by increasing A1-type AS and decreasing A2-type AS. This suggests that LPAR6 could be a potential therapeutic target for alleviating NPP. Clinical applications that are successful can offer new therapeutic options, enhance the quality of life for patients, and potentially uncover new mechanisms for pain modulation.

Reduced circ_lrrc49 in trigeminal ganglion contributes to neuropathic pain in mice by downregulating Ist1 and impairing autophagy.

Orofacial neuropathic pain is a common symptom induced by orofacial nerve injury caused by a range of trauma or dental and maxillofacial procedures but lacks effective treatment. Circular RNAs (circRNAs) participate in the regulatory processes of neuropathic pain. Nevertheless, the biological roles of circRNAs in orofacial neuropathic pain remain unexplored. In this study, circRNA sequencing and Real-time quantitative polymerase chain reaction (RT-qPCR) were carried out. Notably, a novel circRNA named circ_lrrc49 was identified to be downregulated following chronic constriction injury of the infraorbital nerve (CCI-ION) in mice on day 14. Subsequent RNA Antisense Purification (RAP)-mass spectrometry and RNA immunoprecipitation found a direct interaction between circ_lrrc49 and increased sodium tolerance 1 homolog (Ist1). Western blot (WB) identified decreased expression of Ist1 on day 14 post-CCI-ION. Considering the known relationship between Ist1 and autophagy, LC3-II and p62 were detected to be upregulated, and an accumulation of autophagosomes were observed at the same time point. Besides, the knockdown of circ_lrrc49 by small interfering RNA (siRNA) reduced Ist1 expression, increased LC3-II, p62 levels and autophagosomes amount, and evoked orofacial mechanical hypersensitivity, which could be counteracted by the Ist1 overexpression. Similarly, the knockdown of Ist1 by siRNA also increased LC3-II and p62 levels and evoked orofacial mechanical hypersensitivity without influence on circ_lrrc49. Moreover, autophagy activation by rapamycin alleviated orofacial mechanical hypersensitivity evoked by CCI-ION or circ_lrrc49 knockdown. In conclusion, our data revealed the existence of a circ_lrrc49/Ist1/autophagy signaling axis contributing to the progression of orofacial neuropathic pain. These discoveries reveal the intricate molecular processes that drive orofacial neuropathic pain and identify circ_lrrc49 as a promising target for potential therapeutic interventions.

NT5DC2 knockdown suppresses progression, glycolysis, and neuropathic pain in triple-negative breast cancer by blocking the EGFR pathway.

Triple-negative breast cancer (TNBC) is an exceptionally aggressive breast cancer subtype associated with neuropathic pain. This study explores the effects of 5'-nucleotidase domain-containing protein 2 (NT5DC2) on the progression of TNBC and neuropathic pain. Microarray analysis was conducted to identify differentially expressed genes in TNBC and the pathways involved. Gain- and loss-of-function assays of NT5DC2 were performed in TNBC cells, followed by detection of the extracellular acidification rate, adenosine triphosphate (ATP) levels, lactic acid production, glucose uptake, proliferation, migration, and invasion in TNBC cells. Macrophages were co-cultured with TNBC cells to examine the release of polarization-related factors and cytokines. A xenograft tumor model was established for in vivo validation. In addition, a mouse model of neuropathic pain was established through subepineural injection of TNBC cells, followed by measurement of the sciatic functional index and behavioral analysis to assess neuropathic pain. NT5DC2 was upregulated in TNBC and was positively correlated with epidermal growth factor receptor (EGFR). NT5DC2 interacted with EGFR to promote downstream signal transduction in TNBC cells. NT5DC2 knockdown diminished proliferation, migration, invasion, the extracellular acidification rate, ATP levels, lactic acid production, and glucose uptake in TNBC cells. Co-culture with NT5DC2-knockdown TNBC cells alleviated the M2 polarization of macrophages. Furthermore, NT5DC2 knockdown reduced tumor growth and neuropathic pain in mice. Importantly, activation of the EGFR pathway counteracted the effects of NT5DC2 knockdown. NT5DC2 knockdown protected against TNBC progression and neuropathic pain by inactivating the EGFR pathway.

Effectiveness of Low Level Laser Therapy and Low Intensity Pulsed Ultrasound on Neuropathic Pain among the Experimentally Induced Peripheral Nerve Injury in Rat Model

Background: Peripheral nerve injury creates an impact on the inflammatory process, leading to an elevation inHypoxia-inducible factor-1 alpha levels, which contributes to the development and progression of neuropathicpain. An innovative approach to alleviating neuropathic pain involves targeting inflammation.Purpose: This study aims to compare the effect of Low-Level Laser Therapy (LLLT) and Low-Intensity PulsedUltrasound (LIPUS) on neuropathic pain among the experimentally induced peripheral nerve injury in rat model.Methods: Rats with induced sciatic nerve crush injury were randomly divided into three groups: control, LIPUStreatment, and LLLT treatment. Mechanical paw withdrawal threshold analysis was performed before the injuryand from POD 2nd, 7th, 14th, and 21st to assess effectiveness of the treatment.Result: Low-Level Laser Therapy (LLLT) group showing statistically greater improvement on POD 14 and 21compared to both the LIPUS group and the control group in terms of paw withdrawal threshold measurementsamong rats with crush-induced neuropathic pain (P < 0.001).Conclusion: This study found that the use of LLLT has a beneficial effect in the management of neuropathic painand control of analgesia compared to LIPUS by inhibiting the upregulation of HIF-1α synthesis during ischemia,hypoxia, and inflammation. However further studies are recommended to analyze long term results in largersample to control neuropathic pain.

Newly discovered functions of miRNAs in neuropathic pain: Transitioning from recent discoveries to innovative underlying mechanisms.

Neuropathic pain is a widespread clinical issue caused by somatosensory nervous system damage, affecting numerous individuals. It poses considerable economic and public health challenges, and managing it can be challenging due to unclear underlying mechanisms. Nevertheless, emerging evidence suggests that neurogenic inflammation and neuroinflammation play a role in developing pain patterns. Emerging evidence suggests that neurogenic inflammation and neuroinflammation play significant roles in developing neuropathic pain within the nervous system. Increased/decreased miRNA expression patterns could affect the progression of neuropathic and inflammatory pain by controlling nerve regeneration, neuroinflammation, and the expression of abnormal ion channels. However, our limited knowledge of miRNA targets hinders a complete grasp of miRNA's functions. Meanwhile, exploring exosomal miRNA, a recently uncovered role, has significantly advanced our comprehension of neuropathic pain's pathophysiology in recent times. In this review, we present a comprehensive overview of the latest miRNA studies and explore the possible ways miRNAs might play a role in the development of neuropathic pain.

The Role of Machine Learning in Predicting the Onset and Progression of Neuropathic Pain After Spinal Cord Injury: A Literature Review

Introduction: Developing a diagnostic tool that can determine whether a patient will develop neuropathic pain following a spinal cord injury can aid clinicians in treatment procedures and improve patient outcomes. Developing new detection technology can take years, thus finding a way to use existing diagnostic tools would be optimal. Machine learning can be leveraged to incorporate existing data and classify patient outcomes when there are obvious patterns for classification. Methods: A review of full reports published in English was conducted through PubMed. The relevant keywords used in this search included “neuropathic pain”, “spinal cord injury”, machine learning, and “predict” among others. Eight relevant citations were retrieved and reviewed. Results: A decision tree regressor model using clinical measures for neuropathic pain and level of spinal cord injury found that BMI and anxiety scores were the most influential variables in predicting outcomes. A similar tree for functional magnetic resonance imaging (fMRI) data found ventral and dorsal tissue bridges to be predictors of neuropathic pain. Another fMRI study pointed to a strong correlation between changes in perioperative blood oxygen levels at the ipsilateral frontal lobe and neuropathic pain outcomes. Magnetic resonance spectroscopy (MRS) implicated a lower glutamate-glutamine/myoinositol ratio in high neuropathic pain. Various machine learning algorithms were evaluated in building an EEG classifier in two separate studies, and classification accuracies greater than 80% were reached in both. A classifier built using positron emission tomography data attained classification accuracies of 87.5%. Discussion: The most common machine learning algorithm used in building classifiers was support vector machines, linear discriminant analysis and neural net. Regression trees were also used, but they were used to elucidate the variables influencing predictions. Each study has its limitations, either due to limitations of the study method, classification method or data type. Conclusion: There exist many methods to study neuropathic pain and spinal cord injury and each method provides different information regarding the mechanism of pain, influential variables, and physiological changes that occur with pain. Classification can be done using any of these methods to achieve acceptable accuracies, but these accuracies are not enough for a clinical prognostic classifier.

SIX1 induced HULC modulates neuropathic pain and Schwann cell oxidative stress after sciatic nerve injury

Neuropathic pain is a severe and debilitating condition caused by damage to the peripheral nerve or central nervous system. Although several mechanisms have been identified, the underlying pathophysiology of neuropathic pain is still not fully understood. Unfortunately, few effective therapies are available for this condition. Therefore, there is an urgent need to investigate the underlying mechanisms of neuropathic pain to develop more effective treatments. Long non-coding RNAs (lncRNAs) have recently gained attention due to their potential to modulate protein expression through various mechanisms. LncRNAs have been implicated in many diseases, including neuropathic pain. This study aimed to identify a novel lncRNA involved in neuropathic pain progression. The lncRNA microarray analysis showed that lncRNA Upregulated in Liver Cancer (HULC) was significantly upregulated in spinal cord tissue of sciatic nerve injury (SNI) rats. Further experiments confirmed that HULC promoted neuropathic pain progression and aggravated H2O2-induced Schwann cell injury. Mechanistically, Sine Oculis Homeobox 1 (SIX1) regulated the transcriptional expression of HULC, and both SIX1 and HULC were involved in neuropathic pain and Schwann cell injury. The results of our research indicate the existence of a previously unknown SIX1/HULC axis that plays a significant role in the development and progression of neuropathic pain, shedding light on the complex mechanisms that underlie this debilitating condition. These findings offer novel insights into the molecular pathways involved in neuropathic pain. This study underscores the potential of targeting lncRNAs as a viable approach to alleviate the suffering of patients with neuropathic pain.

Electroacupuncture alleviates mechanical allodynia and anxiety-like behaviors induced by chronic neuropathic pain via regulating rostral anterior cingulate cortex-dorsal raphe nucleus neural circuit.

Epidemiological studies in patients with neuropathic pain have demonstrated a strong association between neuropathic pain and psychiatric conditions such as anxiety. Preclinical and clinical work has demonstrated that electroacupuncture (EA) effectively alleviates anxiety-like behaviors induced by chronic neuropathic pain. In this study, a potential neural circuitry underlying the therapeutic action of EA was investigated. The effects of EA stimulation on mechanical allodynia and anxiety-like behaviors in animal models of spared nerve injury (SNI) were examined. EA plus chemogenetic manipulation of glutamatergic (Glu) neurons projecting from the rostral anterior cingulate cortex (rACCGlu ) to the dorsal raphe nucleus (DRN) was used to explore the changes of mechanical allodynia and anxiety-like behaviors in SNI mice. Electroacupuncture significantly alleviated both mechanical allodynia and anxiety-like behaviors with increased activities of glutamatergic neurons in the rACC and serotoninergic neurons in the DRN. Chemogenetic activation of the rACCGlu -DRN projections attenuated both mechanical allodynia and anxiety-like behaviors in mice at day 14 after SNI. Chemogenetic inhibition of the rACCGlu -DRN pathway did not induce mechanical allodynia and anxiety-like behaviors under physiological conditions, but inhibiting this pathway produced anxiety-like behaviors in mice at day 7 after SNI; this effect was reversed by EA. EA plus activation of the rACCGlu -DRN circuit did not produce a synergistic effect on mechanical allodynia and anxiety-like behaviors. The analgesic and anxiolytic effects of EA could be blocked by inhibiting the rACCGlu -DRN pathway. The role of rACCGlu -DRN circuit may be different during the progression of chronic neuropathic pain and these changes may be related to the serotoninergic neurons in the DRN. These findings describe a novel rACCGlu -DRN pathway through which EA exerts analgesic and anxiolytic effects in SNI mice exhibiting anxiety-like behaviors.

LncRNA UCA1 could regulate the progression of neuropathic pain by regulating miR-135a-5p

BackgroundNeuropathic pain (NPP) is known as a common neurological disease with high incidence rate. The present work focused on the roles of long non-coding RNA urothelial carcinoma antigen 1(LncRNA UCA1) in NPP and the possible underlying mechanism. MethodsNPP rat model has been established and the levels of UCA1 NPP as well as the group has been determined by RT-PCR method. Next, NPP rats were treated by UCA1 over-expression plasmid and the behaviors, as well as expression of inflammatory cytokines have been examined. Furthermore, target miRNA of UCA1, miR-135a-5p, has been predicted by bioinformatic method, and further verified with the dual-luciferase reporter assay. Finally, the effects of UCA1/ miR-135a-5p axis have been further evaluated. ResultsExpressions of UCA1 were markedly decreased and miR-135a-5p were significantly increased in NPP rats in comparison with the control rats. Over-expression of UCA1 alleviated the inflammatory condition in NPP model by decreasing expression of inflammatory cytokines. miR-135a-5p was confirmed to be a target microRNA of UCA1, and UCA1 may regulate the progress of NPP via targeting miR-135a-5p. ConclusionUCA1 could regulate NPP via affecting miR-135a-5p expression.

DEAD-box helicase 54 regulates microglial inflammatory response in rats with chronic constriction injuries through NF-κB/NLRP3 signaling axis.

Neuropathic pain (NP) is caused by damage to or disease of the somatosensory nervous system, but its mechanism is still not fully understood. In this study, DEAD-box helicase 54 (DDX54) was targeted, and its regulatory role was explored in a chronic constriction injury (CCI) rat model. Microglia and HMC3 cells were stimulated with LPS. The interaction between DDX54 and myeloid differentiation factor-88 adapter protein (MYD88) was verified. A CCI of sciatic nerve model in rats was established. Behavioral testing was performed before and after the CCI. The expressions of IL-1β, TNF-α, and IL-6 were upregulated, and those of DDX54, MYD88, NF-κB, and NOD-like receptor 3 (NLRP3) were upregulated in microglia and HMC3 cells after LPS induction. DDX54 knockdown in microglia and HMC3 cells inhibited IL-1β, TNF-α, and IL-6 expressions and downregulated the protein levels of MYD88, p-NF-κB p65 (p-p65), and NLRP3. DDX54 overexpression promoted the stability of MYD88 mRNA. DDX54 binds to the MYD88-3'-untranslated region (UTR). DDX54 interference in rats could alleviate the decrease of paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) induced by CCI, inhibit Iba1 expression, and reduce inflammatory factors as well as MYD88 and NF-κB expressions. DDX54 promotes the activation of NF-κB/NLRP3 signaling by regulating MYD88 mRNA stability, thereby affecting inflammatory response and NP progression in CCI rats.NEW & NOTEWORTHY The role of DDX54 protein in LPS-induced microglia and a chronic constriction injury (CCI) rat model was investigated for the first time. DDX54 interference can inhibit microglial activation and reduce the secretion of inflammatory factors. The interaction between DDX54 protein and MYD88 mRNA was explored for the first time. DDX54 promotes NF-κB/NLRP3 signaling activation by regulating MYD88 transcription in a CCI rat model.

Mitochondrial-Derived Peptide MOTS-c Ameliorates Spared Nerve Injury-Induced Neuropathic Pain in Mice by Inhibiting Microglia Activation and Neuronal Oxidative Damage in the Spinal Cord via the AMPK Pathway.

MOTS-c, a recently discovered mitochondrial-derived peptide, plays an important role in many physiological and pathological functions via adenosine monophosphate-activated protein kinase (AMPK) activation. Numerous studies have demonstrated that AMPK is an emerging target for the modulation of neuropathic pain. Meanwhile, microglia-activation-evoked neuroinflammation is known to contribute to the development and progression of neuropathic pain. MOTS-c is also known to inhibit microglia activation, chemokine and cytokine expression, and innate immune responses. Accordingly, in this study, we evaluated the effects of MOTS-c on neuropathic pain and investigated the putative underlying mechanisms. We found that MOTS-c levels in plasma and spinal dorsal horn were significantly lower in mice with spared nerve injury (SNI)-induced neuropathic pain than in control animals. Accordingly, MOTS-c treatment produced pronounced dose-dependent antinociceptive effects in SNI mice; however, these effects were blocked by dorsomorphin, an AMPK inhibitor, but not naloxone, a nonselective opioid receptor antagonist. Moreover, intrathecal (i.t.) injection of MOTS-c significantly enhanced AMPKα1/2 phosphorylation in the lumbar spinal cord of SNI mice. MOTS-c also significantly inhibited proinflammatory cytokine production and microglia activation in the spinal cord. The antinociceptive effects of MOTS-c were retained even when microglia activation in the spinal cord was inhibited by minocycline pretreatment, indicating that spinal cord microglia are dispensable for the antiallodynic effects of MOTS-c. In the spinal dorsal horn, MOTS-c treatment inhibited c-Fos expression and oxidative damage mainly in neurons rather than microglia. Finally, in contrast to morphine, i.t. administration of MOTS-c resulted in limited side effects relating to antinociceptive tolerance, gastrointestinal transit inhibition, locomotor function, and motor coordination. Collectively, the present study is the first to provide evidence that MOTS-c may be a promising therapeutic target for neuropathic pain.

The temporal and spatial signature of microglial transcriptome in neuropathic pain.

Microglial activation following peripheral nerve injury is crucial for neuropathic pain (NP) development; however, studies on time-specific and spatial characteristics of microglial transcriptome are scarce. Firstly, we comparatively analysed microglial transcriptome of different brain regions and multiple timepoints after nerve injury by analysing the gene expression profile of GSE180627 and GSE117320. Then, we performed a mechanical pain hypersensitivity test on 12 rat neuropathic pain models using von Frey fibres at various timepoints after nerve injury. To further explore the key gene clusters closely related to the neuropathic pain phenotype, we conducted a weighted gene co-expression network analysis (WGCNA) on the GSE60670 gene expression profile. Lastly, we performed a single-cell sequencing analysis on GSE162807 for identifying microglia subpopulations. We found that the trend of microglia's transcriptome changes after nerve injury was that mRNA expression changes mainly occur early after injury, which is also consistent with phenotypic changes (NP progression). We also revealed that in addition to spatial specificity, microglia are also temporally specific in NP progression following nerve injury. The WGCNA findings revealed that the functional analysis of the key module genes emphasized the endoplasmic reticulum's (ER's) crucial role in NP. In our single-cell sequencing analysis, microglia were clustered into 18 cell subsets, of which we identified specific subsets of two timepoints (D3/D7) post-injury. Our study further revealed the temporal and spatial gene expression specificity of microglia in neuropathic pain. These results contribute to our comprehensive understanding of the pathogenic mechanism of microglia in neuropathic pain.

Phosphate NIMA-Related Kinase 2-Dependent Epigenetic Pathways in Dorsal Root Ganglion Neurons Mediates Paclitaxel-Induced Neuropathic Pain.

The microtubule-stabilizing drug paclitaxel (PTX) is an important chemotherapeutic agent for cancer treatment and causes peripheral neuropathy as a common side effect that substantially impacts the functional status and quality of life of patients. The mechanistic role for NIMA-related kinase 2 (NEK2) in the progression of PTX-induced neuropathic pain has not been established. Adult male Sprague-Dawley rats intraperitoneally received PTX to induce neuropathic pain. The protein expression levels in the dorsal root ganglion (DRG) of animals were measured by biochemical analyses. Nociceptive behaviors were evaluated by von Frey tests and hot plate tests. PTX increased phosphorylation of the important microtubule dynamics regulator NEK2 in DRG neurons and induced profound neuropathic allodynia. PTX-activated phosphorylated NEK2 (pNEK2) increased jumonji domain-containing 3 (JMJD3) protein, a histone demethylase protein, to specifically catalyze the demethylation of the repressive histone mark H3 lysine 27 trimethylation (H3K27me3) at the Trpv1 gene, thereby enhancing transient receptor potential vanilloid subtype-1 (TRPV1) expression in DRG neurons. Moreover, the pNEK2-dependent PTX response program is regulated by enhancing p90 ribosomal S6 kinase 2 (RSK2) phosphorylation. Conversely, intrathecal injections of kaempferol (a selective RSK2 activation antagonist), NCL 00017509 (a selective NEK2 inhibitor), NEK2-targeted siRNA, GSK-J4 (a selective JMJD3 inhibitor), or capsazepine (an antagonist of TRPV1 receptor) into PTX-treated rats reversed neuropathic allodynia and restored silencing of the Trpv1 gene, suggesting the hierarchy and interaction among phosphorylated RSK2 (pRSK2), pNEK2, JMJD3, H3K27me3, and TRPV1 in the DRG neurons in PTX-induced neuropathic pain. pRSK2/JMJD3/H3K27me3/TRPV1 signaling in the DRG neurons plays as a key regulator for PTX therapeutic approaches.

A Review of the Role of Lymphocytes in Chronic Neuropathic Pain: A Clinical Perspective

Chronic neuropathic pain is one of the most common morbidities in the developed world. It has a lifetime incidence of 80% with females being affected more than males. Chronic neuropathic pain pathogenesis and its associated “sickness syndrome” has been poorly understood but recent research has shown a neuroimmune basis. Many studies performed to date have been pre-clinical or animal-based with a lack of human studies in chronic neuropathic pain. Background: We performed a review of literature with emphasis on clinical and human studies. The numbers of such studies are low due to limitations with ethical approval, recruitment and heterogeneity of humans. We aimed to investigate the most recent studies as well as important seminal research in this area. Methods: Literature search was performed using Stella search engine, Pubmed and National Library of Medicine search engines. Pre-clinical, animal and human studies were included. Conclusion: The immune system, in particular the adaptive immune system, regulates the initiation, progression and resolution of chronic neuropathic pain in humans. The exact “switch” which mediates initiation, upregulation and downregulation of the components of the immune system is not known. Further research in this area is needed but the challenge of recruiting patients for CNP studies remains. Study sample sizes tend to be small. However, clinically relevant findings although small can offer important information to further our understanding of CNP.

Zinc deficiency disrupts pain signaling promoting nociceptive but not inflammatory pain in mice.

Zinc (Zn) is an essential micronutrient involved in the physiology of nervous system and pain modulation. There is little evidence for the role of nutritional Zn alternations to the onset and progression of neuropathic (NP) and inflammatory pain. The study investigated the effects of a zinc restricted diet on the development of pain. Weaned mice were submitted to a regular (38 mg/kg of Zn) or Zn deficient (11 mg/kg of Zn) diets for four weeks, pain responses evaluated (mechanical, cold and heat allodynia; formalin- and carrageenan-induced inflammatory hypernociception), plasma and tissues collected for biochemical and metabolomic analysis. Zn deficient diet inhibited animal growth (37%) and changed mice sensitivity pattern, inducing an intense allodynia evoked by mechanical, cold and heat stimulus for four weeks. The inflammatory pain behavior of formalin test was drastically reduced or absent when challenged by an inflammatory stimulus. Zn restriction also reduce plasma TNF, increase neuronal activation, oxidative stress, indicating a disruption of the immune response. Liver metabolomic analyses suggest a downregulation of lipid metabolism of arachidonic acid. Zn restriction since weaned disrupts pain signaling considerably and reduce inflammatory pain. Zn could be considered a predisposing factor for the onset of chronic pain such as painful neuropathies.

Huntington-associated protein 1 inhibition contributes to neuropathic pain by suppressing Cav1.2 activity and attenuating inflammation.

Although pain dysfunction is increasingly observed in Huntington disease, the underlying mechanisms still unknown. As a crucial Huntington-associated protein, Huntington-associated protein 1 (HAP1) is enriched in normal spinal dorsal horn and dorsal root ganglia (DRG) which are regarded as "primary sensory center," indicating its potential functions in pain process. Here, we discovered that HAP1 level was greatly increased in the dorsal horn and DRG under acute and chronic pain conditions. Lack of HAP1 obviously suppressed mechanical allodynia and hyperalgesia in spared nerve injury (SNI)-induced and chronic constriction injury-induced pain. Its deficiency also greatly inhibited the excitability of nociceptive neurons. Interestingly, we found that suppressing HAP1 level diminished the membrane expression of the L-type calcium channel (Cav1.2), which can regulate Ca 2+ influx and then influence brain-derived neurotrophic factor (BDNF) synthesis and release. Furthermore, SNI-induced activation of astrocytes and microglia notably decreased in HAP1-deficient mice. These results indicate that HAP1 deficiency might attenuate pain responses. Collectively, our results suggest that HAP1 in dorsal horn and DRG neurons regulates Cav1.2 surface expression, which in turn reduces neuronal excitability, BDNF secretion, and inflammatory responses and ultimately influences neuropathic pain progression.