N-methyl-d-aspartate receptor subunit 2B on keratinocyte mediates peripheral and central sensitization in chronic post-ischemic pain in male rats

ALKBH5 modulates bone cancer pain in a rat model by suppressing NR2B expression

Currently, the clinical treatment of bone cancer pain (BCP) is mainly related to its pathogenesis. The aim of the present study was to elucidate the potential role of N6-methyladenosine (m6A) in BCP in the spinal cord dorsal root ganglia (DRG) of BCP rats and its specific regulatory mechanism in N-methyl-d-aspartate receptor subunit 2B (NR2B). A rat model of BCP was constructed by tibial injection of Walker256 cells, and ALKBH5 and NR2B expression in the spinal cord DRG was detected. ALKBH5 was silenced or overexpressed in PC12 cells to verify the regulatory effect of ALKBH5 on NR2B. The specific mechanism underlying the interaction between ALKBH5 and NR2B was investigated using methylated RNA immunoprecipitation and dual-luciferase reporter gene assays. The results showed increased expression of m6A, decreased expression of ALKBH5, and increased expression of NR2B in the DRG of the BCP rat model. Overexpression of ALKBH5 inhibited NR2B expression, whereas interference with ALKBH5 caused an increase in NR2B expression. In NR2B, interference with ALKBH5 caused an increase in m6A modification, which caused an increase in NR2B. Taken together, ALKBH5 affected the expression of NR2B by influencing the stability of the m6A modification site of central NR2B, revealing that ALKBH5 is a therapeutic target for BCP.

Effects of swimming exercise on neuropathic pain in a rat model: role of glutamate

OBJECTIVE

The pain-reducing effects of the exercise were exerted through different mechanisms. Knowing more clear mechanisms helps to find more approach that is therapeutic. The objective of the present study is the evaluation of cerebrospinal fluid (CSF) glutamate level alteration in neuropathic pain rats and whether physical activity could modulate it.

METHODS

In the present study 104 male Wistar rats weighing 180-220 g were randomly divided into 4 groups (Sham, Sham + Exe, Neuropathy, and Neuropathy + Exe) which in turn each group subdivided into 4 groups according to time points for behavioral testing and CSF sampling (Baseline, 2 weeks, 3 weeks, and 4 weeks). To induction of neuropathy (by chronic constriction injury,), after anesthetizing with a mixture of ketamine (80 mg/kg) and xylazine (10 mg/kg), the animal's right sciatic nerve was exposed and was ligated using four movable catgut chromic suture 4/0. The exercise protocol included 25 min of daily swimming, 5 days a week for 4 weeks. Thermal hyperalgesia and mechanical tactile threshold were detected using the plantar test and Von Frey filaments, respectively. CSF glutamate level was determined using high-performance liquid chromatography.

RESULTS

Findings indicated that mechanical and thermal thresholds significantly (p < 0.01, p < 0.05 respectively) decreased in the neuropathy group against that in sham groups. On the other hand, exercise significantly increased mechanical tactile threshold (p < 0.0012) and thermal threshold (p < 0.05) compared to the neuropathy group. Moreover, CSF glutamate level prominently (p < 0.01) was increased in the neuropathy group compared to the sham group, and swimming exercise significantly (p < 0.001) reduced it.

IN CONCLUSION

The present findings provide new evidence showing that medium-intensity swimming exercise attenuates pain-like behaviors in neuropathic pain animals, which is possibly due to decreasing CSF glutamate level and its neurotransmission.

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

NMDA Receptors Regulate Oxidative Damage in Keratinocytes during Complex Regional Pain Syndrome in HaCaT Cells and Male Rats

Complex regional pain syndrome (CRPS), a type of primary chronic pain, occurs following trauma or systemic disease and typically affects the limbs. CRPS-induced pain responses result in vascular, cutaneous, and autonomic nerve alterations, seriously impacting the quality of life of affected individuals. We previously identified the involvement of keratinocyte N-methyl-d-asparagic acid (NMDA) receptor subunit 2 B (NR2B) in both peripheral and central sensitizations in CRPS, although the mechanisms whereby NR2B functions following activation remain unclear. Using an in vivo male rat model of chronic post-ischemia pain (CPIP) and an in vitro oxygen-glucose deprivation/reoxygenation (OGD/R) cell model, we discovered that oxidative injury occurs in rat keratinocytes and HaCaT cells, resulting in reduced cell viability, mitochondrial damage, oxidative damage of nucleotides, and increased apoptosis. In HaCaT cells, OGD/R induced increases in intracellular reactive oxygen species levels and disrupted the balance between oxidation and antioxidation by regulating a series of antioxidant genes. The activation of NMDA receptors via NMDA exacerbated these changes, whereas the inhibition of the NR2B subunit alleviated them. Co-administration of ifenprodil (an NR2B antagonist) and NMDA (an NMDA receptor agonist) during the reoxygenation stage did not result in any significant alterations. Furthermore, intraplantar injection of ifenprodil effectively reversed the altered gene expression that was observed in male CPIP rats, thereby revealing the potential mechanisms underlying the therapeutic effects of peripheral ifenprodil administration in CRPS. Collectively, our findings indicate that keratinocytes undergo oxidative injury in CRPS, with NMDA receptors playing regulatory roles.

Combination chemotherapy in rodents: a model for chemotherapy-induced neuropathic pain and pharmacological screening

Chemotherapy-induced neuropathic pain (CINP) remains a therapeutic challenge, with no US-FDA approved drugs or effective treatments available. Despite significant progress in unravelling the pathophysiology of CINP, the clinical translation of this knowledge into tangible outcome remains elusive. Here, we employed behavioural and pharmacological approaches to establish and validate a novel combination-based chemotherapeutic model of peripheral neuropathy. Male Sprague Dawley rats were subjected to chemotherapy administration followed by assessment of pain behaviour at different time-points post-chemotherapy. Paclitaxel-treated animals displayed an enhanced thermal and mechanical hypersensitivity from day four onwards which continued till day thirty-five post last paclitaxel injection. Notably, rats subjected to combination chemotherapy, displayed prolonged hypersensitivity that emerged on day four and persisted until day fifty-six. RT-PCR analysis revealed significant upregulation in DRG and spinal mRNA expressions of TRP channels (TRPA1, TRPV1, & TRPM8), pro-inflammatory cytokines (TNF-α & IL-1β) and neuropeptides, Substance P and CGRP in both the pain models. Interestingly, the combination chemotherapy model demonstrated a significant increase in DRG and spinal NR2B expressions compared to rats solely treated with paclitaxel. Pharmacological investigations revealed that gabapentin treatment substantially mitigates pain hypersensitivity in both the combined chemotherapy and paclitaxel-administered groups, with the simultaneous reversal of cellular and molecular changes observed in the lumbar DRG and spinal cord of rats. The findings from this study suggests that combination chemotherapy model exhibits heightened and prolonged hypersensitivity in comparison to the conventional paclitaxel-induced neuropathic pain model. This model not only recapitulates clinical biomarkers of neuropathy but also presents a potential alternative platform for screening analgesic drugs targeted at CINP.

Complement Receptor C3aR1 Contributes to Paclitaxel-Induced Peripheral Neuropathic Pain in Mice and Rats

Cancer chemotherapy-induced neuropathic pain is a devastating pain syndrome without effective therapies. We previously reported that rats deficient in complement C3, the central component of complement activation cascade, showed a reduced degree of paclitaxel-induced mechanical allodynia (PIMA), suggesting that complement is integrally involved in the pathogenesis of this model. However, the underlying mechanism was unclear. Complement activation leads to the production of C3a, which mediates inflammation through its receptor C3aR1. In this article, we report that the administration of paclitaxel induced a significantly higher expression level of C3aR1 on dorsal root ganglion (DRG) macrophages and expansion of these macrophages in DRGs in wild-type (WT) compared with in C3aR1 knockout (KO) mice. We also found that paclitaxel induced less severe PIMA, along with a reduced DRG expression of transient receptor potential channels of the vanilloid subtype 4 (TRPV4), an essential mediator for PIMA, in C3aR1 KO than in WT mice. Treating WT mice or rats with a C3aR1 antagonist markedly attenuated PIMA in association with downregulated DRG TRPV4 expression, reduced DRG macrophages expansion, suppressed DRG neuron hyperexcitability, and alleviated peripheral intraepidermal nerve fiber loss. Administration of C3aR1 antagonist to TRPV4 KO mice further protected them from PIMA. These results suggest that complement regulates PIMA development through C3aR1 to upregulate TRPV4 on DRG neurons and promote DRG macrophage expansion. Targeting C3aR1 could be a novel therapeutic approach to alleviate this debilitating pain syndrome.

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

The Role of Neuroinflammation in Complex Regional Pain Syndrome: A Comprehensive Review

Complex Regional Pain Syndrome (CRPS) is an excess and/or prolonged pain and inflammation condition that follows an injury to a limb. The pathogenesis of CRPS is multifaceted that remains incompletely understood. Neuroinflammation is an inflammatory response in the peripheral and central nervous systems. Dysregulated neuroinflammation plays a crucial role in the initiation and maintenance of pain and nociceptive neuronal sensitization, which may contribute to the transition from acute to chronic pain and the perpetuation of chronic pain in CRPS. The key features of neuroinflammation encompass infiltration and activation of inflammatory cells and the production of inflammatory mediators in both the central and peripheral nervous systems. This article reviews the role of neuroinflammation in the onset and progression of CRPS from six perspectives: neurogenic inflammation, neuropeptides, glial cells, immune cells, cytokines, and keratinocytes. The objective is to provide insights that can inform future research and development of therapeutic targets for CRPS.

Animal models of complex regional pain syndrome: A scoping review

BACKGROUND

complex regional pain syndrome (CRPS) leads to a debilitating chronic pain condition. The lack of cause, etiology, and treatment for CRPS has been widely explored in animal models.

OBJECTIVE

Provide a comprehensive framework of the animal models used for investigating CRPS.

ELIGIBILITY CRITERIA

Preclinical studies to induce the characteristics of CRPS, with a control group, in any language or publication date.

SOURCES OF EVIDENCE

The search was performed in the Medline (PubMed) and ScienceDirect databases.

RESULTS

93 studies are included. The main objective of the included studies was to understand the CRPS model. Rats, males and adults, exposed to ischemia/reperfusion of the paw or fracture of the tibia were the most common characteristics. Nociceptive evaluation using von Frey monofilaments was the most widely adopted in the studies.

CONCLUSIONS

For the best translational science between the animal models and individuals with CRPS, future studies should include more heterogeneous animals, and multiple assessment tools, in addition to improving the description and performance of measures that reduce the risk of bias.

Differential roles of NMDAR subunits 2A and 2B in mediating peripheral and central sensitization contributing to orofacial neuropathic pain

The spinal N-methyl-d-aspartate receptor (NMDAR), particularly their subtypes NR2A and NR2B, plays pivotal roles in neuropathic and inflammatory pain. However, the roles of NR2A and NR2B in orofacial pain and the exact molecular and cellular mechanisms mediating nervous system sensitization are still poorly understood. Here, we exhaustively assessed the regulatory effect of NMDAR in mediating peripheral and central sensitization in orofacial neuropathic pain. Von-Frey filament tests showed that the inferior alveolar nerve transection (IANX) induced ectopic allodynia behavior in the whisker pad of mice. Interestingly, mechanical allodynia was reversed in mice lacking NR2A and NR2B. IANX also promoted the production of peripheral sensitization-related molecules, such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, brain-derived neurotrophic factor (BDNF), and chemokine upregulation (CC motif) ligand 2 (CCL2), and decreased the inward potassium channel (Kir) 4.1 on glial cells in the trigeminal ganglion, but NR2A conditional knockout (CKO) mice prevented these alterations. In contrast, NR2B CKO only blocked the changes of Kir4.1, IL-1β, and TNF-α and further promoted the production of CCL2. Central sensitization-related c-fos, glial fibrillary acidic protein (GFAP), and ionized calcium-binding adaptor molecule 1 (Iba-1) were promoted and Kir4.1 was reduced in the spinal trigeminal caudate nucleus by IANX. Differential actions of NR2A and NR2B in mediating central sensitization were also observed. Silencing of NR2B was effective in reducing c-fos, GFAP, and Iba-1 but did not affect Kir4.1. In contrast, NR2A CKO only altered Iba-1 and Kir4.1 and further increased c-fos and GFAP. Gain-of-function and loss-of-function approaches provided insight into the differential roles of NR2A and NR2B in mediating peripheral and central nociceptive sensitization induced by IANX, which may be a fundamental basis for advancing knowledge of the neural mechanisms' reaction to nerve injury.

Identification of a Glutamatergic Claustrum-Anterior Cingulate Cortex Circuit for Visceral Pain Processing

Chronic visceral pain is a major challenge for both patients and health providers. Although the central sensitization of the brain is thought to play an important role in the development of visceral pain, the detailed neural circuits remain largely unknown. Using a well-established chronic visceral hypersensitivity model induced by neonatal maternal deprivation (NMD) in male mice, we identified a distinct pathway whereby the claustrum (CL) glutamatergic neuron projecting to the anterior cingulate cortex (ACC) is critical for visceral pain but not for CFA-evoked inflammatory pain. By a combination of in vivo circuit-dissecting extracellular electrophysiological approaches and visceral pain related electromyographic (EMG) recordings, we demonstrated that optogenetic inhibition of CL glutamatergic activity suppressed the ACC neural activity and visceral hypersensitivity of NMD mice whereas selective activation of CL glutamatergic activity enhanced the ACC neural activity and evoked visceral pain of control mice. Further, optogenetic studies demonstrate a causal link between such neuronal activity and visceral pain behaviors. Chemogenetic activation or inhibition of ACC neural activities reversed the effects of optogenetic manipulation of CL neural activities on visceral pain responses. Importantly, molecular detection showed that NMD significantly enhances the expression of NMDA receptors and activated CaMKIIα in the ACC postsynaptic density (PSD) region. Together, our data establish a functional role for CL→ACC glutamatergic neurons in gating visceral pain, thus providing a potential treatment strategy for visceral pain.SIGNIFICANCE STATEMENT Studies have shown that sensitization of anterior cingulate cortex (ACC) plays an important role in chronic pain. However, it is as yet unknown whether there is a specific brain region and a distinct neural circuit that helps the ACC to distinguish visceral and somatic pain. The present study demonstrates that claustrum (CL) glutamatergic neurons maybe responding to colorectal distention (CRD) rather than somatic stimulation and that a CL glutamatergic projection to ACC glutamatergic neuron regulates visceral pain in mice. Furthermore, excessive NMDA receptors and overactive CaMKIIα in the ACC postsynaptic density (PSD) region were observed in mice with chronic visceral pain. Together, these findings reveal a novel neural circuity underlying the central sensitization of chronic visceral pain.

N-Methyl D-aspartate receptor subtype 2B/Ca2+/calmodulin-dependent protein kinase II signaling in the lateral habenula regulates orofacial allodynia and anxiety-like behaviors in a mouse model of trigeminal neuralgia

Trigeminal neuralgia (TN) is a peripheral nerve disorder often accompanied by abnormalities in mood. The lateral habenula (LHb) plays important roles in the modulation of pain and emotion. In the present study, we investigated the involvement of the LHb in the mechanisms underlying allodynia and anxiety induced by partial transection of the infraorbital nerve (pT-ION) in mice. Our results indicated that pT-ION induced persistent orofacial allodynia and anxiety-like behaviors, which were correlated with increased phosphorylation of N-Methyl D-aspartate receptor (NMDAR) subtype 2B (p-NR2B) and Ca²⁺/calmodulin-dependent protein kinase II (p-CaMKII) in LHb neurons. Bilateral inhibition of NMDARs and CaMKII in the LHb attenuated the allodynia and anxiety-like behavior induced by pT-ION. Furthermore, bilateral activation of NMDARs in the LHb increased the expression of p-NR2B and p-CaMKII and induced orofacial allodynia and anxiety-like behaviors in naive mice. Adeno-associated virus (AAV)-mediated expression of hM3D(Gq) in CaMKII⁺ neurons of the bilateral LHb, followed by clozapine-N-oxide (CNO) administration, also triggered orofacial allodynia and anxiety-like behaviors in naïve mice with successful virus infection in LHb neurons (verified based on immunofluorescence). In conclusion, these findings suggest that activation of NMDA/CaMKII signaling in the LHb contributes to the occurrence and development of TN and related anxiety-like behaviors. Therefore, suppressing the activity of CaMKII⁺ neurons in the bilateral LHb by targeting NMDA/CaMKII may represent a novel strategy for treating pain and anxiety associated with TN.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

Chronic Pain after Bone Fracture: Current Insights into Molecular Mechanisms and Therapeutic Strategies

Bone fracture following traumatic injury or due to osteoporosis is characterized by severe pain and motor impairment and is a major cause of global mortality and disability. Fracture pain often originates from mechanical distortion of somatosensory nerve terminals innervating bones and muscles and is maintained by central sensitization. Chronic fracture pain (CFP) after orthopedic repairs is considered one of the most critical contributors to interference with the physical rehabilitation and musculoskeletal functional recovery. Analgesics available for CFP in clinics not only have poor curative potency but also have considerable side effects; therefore, it is important to further explore the pathogenesis of CFP and identify safe and effective therapies. The typical physiopathological characteristics of CFP are a neuroinflammatory response and excitatory synaptic plasticity, but the specific molecular mechanisms involved remain poorly elucidated. Recent progress has deepened our understanding of the emerging properties of chemokine production, proinflammatory mediator secretion, caspase activation, neurotransmitter release, and neuron-glia interaction in initiating and sustaining synaptogenesis, synaptic strength, and signal transduction in central pain sensitization, indicating the possibility of targeting neuroinflammation to prevent and treat CFP. This review summarizes current literature on the excitatory synaptic plasticity, microgliosis, and microglial activation-associated signaling molecules and discusses the unconventional modulation of caspases and stimulator of interferon genes (STING) in the pathophysiology of CFP. We also review the mechanisms of action of analgesics in the clinic and their side effects as well as promising therapeutic candidates (e.g., specialized pro-resolving mediators, a caspase-6 inhibitor, and a STING agonist) for pain relief by the attenuation of neuroinflammation with the aim of better managing patients undergoing CFP in the clinical setting.

The Involvement of Caspases in Neuroinflammation and Neuronal Apoptosis in Chronic Pain and Potential Therapeutic Targets

Chronic pain is a common, complex and unpleasant sensation following nerve injury, tissue trauma, inflammatory diseases, infection and cancer. It affects up to 25% of adults and is increasingly recognized as the leading cause of distress, disability and disease burden globally. Chronic pain is often refractory to most current analgesics, thus emphasizing the requirement for improved therapeutic medications. It is of great importance to elucidate the specific pathogenesis of chronic pain with different etiologies. Recent progress has advanced our understanding in the contribution of neuroinflammation and glial cells (microglia and astrocyte) activation in the plasticity of excitatory nociceptive synapses and the development of chronic pain phenotypes. Oxidative stress-associated neuronal apoptosis is also identified to be a pivotal step for central pain sensitization. The family of cysteine aspartate specific proteases (Caspases) has been well known to be key signaling molecules for inflammation and apoptosis in several neurological conditions. Recent studies have highlighted the unconventional and emerging role of caspases in microgliosis, astrocytes morphogenesis, chemokines release, cytokines secretion and neuronal apoptosis in initiating and maintaining synaptogenesis, synaptic strength and signal transduction in persistent pain hypersensitivity, suggesting the possibility of targeting caspases pathway for prevention and treatment of chronic pain. In this review, we will discuss and summarize the advances in the distinctive properties of caspases family in the pathophysiology of chronic pain, especially in neuropathic pain, inflammatory pain, cancer pain and musculoskeletal pain, with the aim to find the promising therapeutic candidates for the resolution of chronic pain to better manage patients undergoing chronic pain in clinics.

An Injectable Hydrogel for Treatment of Chronic Neuropathic Pain

Current treatments for chronic neuropathic pain often fall short. A small-molecular compound ZL006 can suppress N-Methyl-D-aspartate receptor (NMDAR)-mediated neuropathic pain behaviors without blocking essential NMDAR function and brings new hope for neuropathic pain therapy. The persistent nature of neuropathic pain mandates the long-term treatment. However, similar to existing analgesics, ZL006 has only a short duration of action. To unleash the therapeutic potential of ZL006, the stability of ZL006 in aqueous solutions is investigated, and a ZL006-incorporated P407-based thermo-responsive injectable hydrogel is developed. The computational analysis is performed to help achieve the desired ZL006-loaded hydrogel system and elucidate the gelation mechanism. The hydrogel matrix can be loaded with ZL006 in an aqueous phase at room temperature without costly specialized equipment and no organic solvent, where the sol is formed and injectable. On subcutaneous administration and subsequent rapid warming to physiological temperature, the sol is converted to a gel. The thermo-responsive hydrogel at body temperature enables the extended release of encapsulated ZL006, and therefore a single subcutaneous injection of ZL006-hydrogel produces a prolonged and stable analgesic action in mice with spinal nerve ligation. Our study provides a practical chronic neuropathic pain therapy and a new perspective on future applications of ZL006. This article is protected by copyright. All rights reserved.

Inhibition of pan-Aurora kinase attenuates evoked and ongoing pain in nerve injured rats via regulating KIF17-NR2B mediated signaling

Kinesins (KIF's) are the motor proteins which are recently reported to be involved in the trafficking of nociceptors leading to chronic pain. Aurora kinases are known to be involved in the regulation of KIF proteins which are associated with the activation of N-methyl-D-aspartate (NMDA) receptors. Here, we investigated the effect of tozasertib, a pan-Aurora kinase inhibitor, on nerve injury-induced evoked and chronic ongoing pain in rats and the involvement of kinesin family member 17 (KIF17) and NMDA receptor subtype 2B (NR2B) crosstalk in the same. Rats with chronic constriction injury showed a significantly decreased pain threshold in a battery of pain behavioural assays. We found that tozasertib [10, 20, and 40 mg/kg intraperitoneally (i.p.)] treatment showed a significant and dose-dependent inhibition of both evoked and chronic ongoing pain in rats with nerve injury. Tozasertib (40 mg/kg i.p.) and gabapentin (30 mg/kg i.p.) treatment significantly inhibits spontaneous ongoing pain in nerve injured rats but did not produce any place preference behaviour in healthy naïve rats pointing towards their non-addictive analgesic potential. Moreover, tozasertib (10, 20, and 40 mg/kg i.p.) and gabapentin (30 mg/kg i.p.) treatment did not altered the normal pain threshold in healthy naïve rats and didn't produce central nervous system associated side effects as well. Western blotting and reverse transcription polymerase chain reaction studies suggested enhanced expressions of NR2B and KIF-17 along with increased nuclear factor kappa β (NFkβ), tumour necrosis factor-α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6) levels in dorsal root ganglion (DRG) and spinal cord of nerve injured rats which was significantly attenuated on treatment with different does of Tozasertib. Findings from the current study suggests that inhibition of pan-Aurora kinase decreased KIF-17 mediated NR2B activation which further leads to significant inhibition of evoked and chronic ongoing pain in nerve-injured rats.

Animal Models of Complex Regional Pain Syndrome Type I

Complex regional pain syndrome (CRPS) is a chronic pain disorder characterized by spontaneous or evoked regionally-confined pain which is out of proportion to the initial trauma event. The disease can seriously affect the quality of the patients’ life, increase the psychological burden, and cause various degrees of disability. Despite the awareness of CRPS among medical practitioners for over a century, its pathogenesis remains unclear, and the available treatment is still unsatisfactory. Effective animal models are the foundation of disease research, which is helpful in understanding the pathogenesis and an in-depth exploration of the appropriate therapeutic approaches. Currently, researchers have established a series of animal models of the disease. There are four main CRPSI animal models: chronic post-ischemic pain (CPIP) model, tibial fracture/cast immobilization model, passive transfer-trauma model, and the needlestick-nerve-injury (NNI) model. The modeling methods of these models are constantly improving over time. In preclinical studies, the interpretation of experimental results and the horizontal comparison between similar studies may be affected by the nature of the experimental animal breeds, sex, diet, and psychology. There is need to facilitate the choice of appropriate animal models and avoid the interference of the factors influencing animal models on the interpretation of research results. The review will provide a basic overview of the influencing factors, modeling methods, and the characteristics of CRPSI animal models.

Gene Expression Profiling of Contralateral Dorsal Root Ganglia Associated with Mirror-Image Pain in a Rat Model of Complex Regional Pain Syndrome Type-I

Background

Mirror-image pain (MIP), which develops from the healthy body region contralateral to the actual injured site, is a mysterious pain phenomenon accompanying many chronic pain conditions, such as complex regional pain syndrome (CRPS). However, the pathogenesis of MIP still remains largely unknown. The purpose of this study is to perform an expression profiling to identify genes related to MIP in an animal model of CRPS-I.

Methods

We established a rat chronic post-ischemic pain (CPIP) model to mimic human CRPS-I. RNA-sequencing (RNA-Seq), bioinformatics, qPCR, immunostaining, and animal behavioral assays were used to screen potential genes in the contralateral dorsal root ganglia (DRG) that may be involved in MIP.

Results

The CPIP model rats developed robust and persistent MIP in contralateral hind paws. Bilateral DRG neurons did not exhibit obvious neuronal damage. RNA-Seq of contralateral DRG from CPIP model rats identified a total 527 differentially expressed genes (DEGs) vs sham rats. The expression changes of several representative DEGs were further verified by qPCR. Bioinformatics analysis indicated that the immune system process, innate immune response, and cell adhesion were among the mostly enriched biological processes, which are important processes involved in pain sensitization, neuroinflammation, and chronic pain. We further identified DEGs potentially involved in pain mechanisms or enriched in small- to medium-sized sensory neurons or TRPV1-lineage nociceptors. By comparing with published datasets summarizing genes enriched in pain mechanisms, we sorted out a core set of genes which might contribute to nociception and the pain mechanism in MIP.

Conclusion

We provided by far the first study to profile gene expression changes and pathway analysis of contralateral DRG for the studying of MIP mechanisms. This work may provide novel insights into understanding the mysterious mechanisms underlying MIP.

Nutritional Supplements for the Treatment of Neuropathic Pain

Neuropathic pain affects 7-10% of the population and is often ineffectively and incompletely treated. Although the gold standard for treatment of neuropathic pain includes tricyclic antidepressants (TCAs), serotonin-noradrenaline reuptake inhibitors, and anticonvulsants, patients suffering from neuropathic pain are increasingly turning to nonpharmacologic treatments, including nutritional supplements for analgesia. So-called "nutraceuticals" have garnered significant interest among patients seeking to self-treat their neuropathic pain with readily available supplements. The supplements most often used by patients include vitamins such as vitamin B and vitamin D, trace minerals zinc and magnesium, and herbal remedies such as curcumin and St. John's Wort. However, evidence surrounding the efficacy and mechanisms of these supplements in neuropathic pain is limited, and the scientific literature consists primarily of preclinical animal models, case studies, and small randomized controlled trials (RCTs). Further exploration into large randomized controlled trials is needed to fully inform patients and physicians on the utility of these supplements in neuropathic pain. In this review, we explore the basis behind using several nutritional supplements commonly used by patients with neuropathic pain seen in rheumatology clinics.

Peripheral Mechanisms of Neuropathic Pain-the Role of Neuronal and Non-Neuronal Interactions and Their Implications for Topical Treatment of Neuropathic Pain

Neuropathic pain in humans arises as a consequence of injury or disease of somatosensory nervous system at peripheral or central level. Peripheral neuropathic pain is more common than central neuropathic pain, and is supposed to result from peripheral mechanisms, following nerve injury. The animal models of neuropathic pain show extensive functional and structural changes occurring in neuronal and non-neuronal cells in response to peripheral nerve injury. These pathological changes following damage lead to peripheral sensitization development, and subsequently to central sensitization initiation with spinal and supraspinal mechanism involved. The aim of this narrative review paper is to discuss the mechanisms engaged in peripheral neuropathic pain generation and maintenance, with special focus on the role of glial, immune, and epithelial cells in peripheral nociception. Based on the preclinical and clinical studies, interactions between neuronal and non-neuronal cells have been described, pointing out at the molecular/cellular underlying mechanisms of neuropathic pain, which might be potentially targeted by topical treatments in clinical practice. The modulation of the complex neuro-immuno-cutaneous interactions in the periphery represents a strategy for the development of new topical analgesics and their utilization in clinical settings.

Spinal NR2B phosphorylation at Tyr1472 regulates IRE(-)DMT1-mediated iron accumulation and spine morphogenesis via kalirin-7 in tibial fracture-associated postoperative pain after orthopedic surgery in female mice

BACKGROUND

Prolonged postoperative pain is a major concern and occurs more frequently in women, but mechanisms remain elusive. NR2B-containging N-methyl-d-aspartate (NMDA) receptor is a key component of nociception transduction. Divalent metal transporter 1 (DMT1)-mediated iron overload involves NMDA-induced neurotoxicity in males. Kalirin-7 is vital in synaptic plasticity underlying pathological pain in males. Herein, the requirement for kalirin-7 in NR2B phosphorylation-dependent iron accumulation and spine plasticity in postoperative pain after tibial fracture in female mice has been examined.

METHODS

Pain-related behavior, spinal NR2B phosphorylation at Tyr1472, kalirin-7 expression, DMT1 with/without iron-responsive element (IRE (+) DMT1 and IRE (-) DMT1) level, iron concentration and spine morphology were assessed in females. NR2B antagonist Ro25-6981, iron chelator deferoxamine and kalirin-7 knockdown by short hairpin RNA were employed to assess the potential cascade.

RESULTS

Tibial fracture initiates long-term allodynia lasting at least 21 days postoperatively, and upregulates spinal NR2B phosphorylation, kalirin-7 and IRE (-) DMT1 expression, iron overload and spine density. Ro25-6981 reduces postoperative mechanical and cold allodynia, spinal NR2B phosphorylation, kalirin-7 level and IRE (-) DMT1-mediated iron overload. Kalirin-7 knockdown impairs fracture-associated allodynia, IRE (-) DMT1-mediated iron overload and spine plasticity. Deferoxamine also attenuates behavioral allodynia and spine plasticity. Spinal NMDA application elicits NR2B-dependent mechanical allodynia and iron overload, which is reversed by kalirin-7 knockdown or coadministration of deferoxamine.

CONCLUSION

Spinal NR2B phosphorylation at Tyr1472 upregulates kalirin-7 expression to facilitate IRE (-) DMT1-mediated iron accumulation and spine morphogenesis in the development of fracture-associated postoperative pain in female mice.

The Sigma-2 receptor / transmembrane protein 97 (σ2R/TMEM97) modulator JVW-1034 reduces heavy alcohol drinking and associated pain states in male mice

Alcohol Use Disorder (AUD) is a chronic relapsing disorder characterized by compulsive alcohol intake, loss of control over alcohol intake, and a negative emotional state when access to alcohol is prevented. AUD is also closely tied to pain, as repeated alcohol drinking leads to increased pain sensitivity during withdrawal. The sigma-2 receptor, recently identified as transmembrane protein 97 (σ2R/TMEM97), is an integral membrane protein involved in cholesterol homeostasis and lipid metabolism. Selective σ2R/Tmem97 modulators have been recently shown to relieve mechanical hypersensitivity in animal models of neuropathic pain as well as to attenuate alcohol withdrawal signs in C. elegans and to reduce alcohol drinking in rats, suggesting a potential key role for this protein in alcohol-related behaviors. In this study, we tested the effects of a potent and selective σ2R/TMEM97 ligand, JVW-1034, on heavy alcohol drinking and alcohol-induced heightened pain states in mice using an intermittent access model. Administration of JVW-1034 decreased both ethanol intake and preference for ethanol, without affecting water intake, total fluid intake, or food intake. Notably, this effect was specific for alcohol, as JVW-1034 had no effect on sucrose intake. Furthermore, JVW-1034 reduced both thermal hyperalgesia and mechanical hypersensitivity in ethanol withdrawn mice. Our data provide important evidence that modulation of σ2R/TMEM97 with small molecules can mediate heavy alcohol drinking as well as chronic alcohol-induced heightened pain sensitivity, thereby identifying a promising novel pharmacological target for AUD and associated pain states.

Electroacupuncture Alleviates Mechanical Allodynia of a Rat Model of CRPS-I and Modulates Gene Expression Profiles in Dorsal Root Ganglia

Complex regional pain syndrome type-I (CRPS-I) is chronic neurological disorder accompanied with devastating pain. Most conventional medical treatments lack effectiveness, making CRPS-I a challenging clinical condition. Electroacupuncture (EA) showed effectiveness in alleviating the pain symptoms of CRPS-I patients. However, the molecular mechanisms underlying EA's therapeutic effect are still not well-understood. Here, we established the rat chronic post-ischemic pain (CPIP) model to mimic CRPS-I and performed repetitive EA on bilateral hind limbs of the CPIP model rats. We then performed RNA-sequencing (RNA-Seq) to study the differences in gene expression, gene networks, and molecular pathways in ipsilateral DRGs innervating the hind limb of the CPIP model rats with and without repetitive EA treatment. Our results found that repetitive EA treatment significantly alleviated mechanical allodynia in bilateral hind limbs of CPIP model rats. RNA-Seq analysis indicated that EA modulated the expression of multiple genes and gene networks in the DRGs of CPIP model rats. Further bioinformatics analysis identified the up-regulation of an array of genes involved in biological process such as neutrophil chemotaxis and immune response in the DRGs of CPIP model rats after EA treatment. Thus, these results suggest that EA may alleviate pain response in CPIP model rats via regulating multiple genes. Our work may help to further advance the understandings of the molecular mechanisms underlying EA's therapeutic effects on CRPS-I and help to identify novel targets for CRPS-I treatment.

Ifenprodil Reduced Expression of Activated Microglia, BDNF and DREAM Proteins in the Spinal Cord Following Formalin Injection During the Early Stage of Painful Diabetic Neuropathy in Rats

The pharmacological inhibition of glial activation is one of the new approaches for combating neuropathic pain in which the role of glia in the modulation of neuropathic pain has attracted significant interest and attention. Neuron-glial crosstalk is achieved with N-methyl-D-aspartate-2B receptor (NMDAR-2B) activation. This study aims to determine the effect of ifenprodil, a potent noncompetitive NMDAR-2B antagonist, on activated microglia, brain-derived neurotrophic factors (BDNF) and downstream regulatory element antagonist modulator (DREAM) protein expression in the spinal cord of streptozotocin-induced painful diabetic neuropathy (PDN) rats following formalin injection. In this experimentation, 48 Sprague-Dawley male rats were randomly selected and divided into four groups: (n = 12): control, PDN, and ifenprodil-treated PDN rats at 0.5 μg or 1.0 μg for 7 days. Type I diabetes mellitus was then induced by injecting streptozotocin (60 mg/kg, i.p.) into the rats which were then over a 2-week period allowed to progress into the early phase of PDN. Ifenprodil was administered in PDN rats while saline was administered intrathecally in the control group. A formalin test was conducted during the fourth week to induce inflammatory nerve injury, in which the rats were sacrificed at 72 h post-formalin injection. The lumbar enlargement region (L4-L5) of the spinal cord was dissected for immunohistochemistry and western blot analyses. The results demonstrated a significant increase in formalin-induced flinching and licking behavior with an increased spinal expression of activated microglia, BDNF and DREAM proteins. It was also shown that the ifenprodil-treated rats following both doses reduced the extent of their flinching and duration of licking in PDN in a dose-dependent manner. As such, ifenprodil successfully demonstrated inhibition against microglia activation and suppressed the expression of BDNF and DREAM proteins in the spinal cord of PDN rats. In conclusion, ifenprodil may alleviate PDN by suppressing spinal microglia activation, BDNF and DREAM proteins.