Ionotropic and metabotropic receptors, protein kinase A, protein kinase C, and Src contribute to C-fiber-induced ERK activation and cAMP response element-binding protein phosphorylation in dorsal horn neurons, leading to central sensitization

Duration of the preemptive analgesic effects of low- and high-frequency transcutaneous electrical nerve stimulation in rats with acute inflammatory pain

Transcutaneous electrical nerve stimulation (TENS) activates various pathways to induce antinociceptive effects, based on the frequencies used. This study evaluates the preemptive analgesic effects and their duration of low- (LT: 4 Hz) and high-frequency TENS (HT: 100 Hz) using a rat model of acute inflammatory pain. Acute inflammation was induced by injecting 1% formalin into the hind paws of rats. LT or HT was applied for 30 min before formalin injection. Pain-related behaviors, such as licking, flinching, and lifting, were recorded for 60 min postinjection. Immunohistochemistry was used to assess the number of phosphorylated extracellular signal-regulated kinase (pERK)- and c-fos-positive cells in the spinal cord. Naloxone, a μ-opioid receptors (MORs) antagonist, and naltrindole, a δ-opioid receptors (DORs) antagonist, were administered before TENS application. Pain behavior duration and pERK- and c-fos-positive cell expression were then measured. LT and HT pretreatment significantly reduced both pain behaviors and the number of pERK- and c-fos-positive cells postformalin injection. Naloxone and naltrindole partially reversed the effects of LT and HT, respectively. Notably, HT's analgesic effect lasted up to 120 min whereas that of LT persisted for 90 min. LT and HT effectively exerted their preemptive analgesic effects on acute inflammatory pain by inhibiting pERK and c-fos expression in the spinal cord. HT presented a longer-lasting effect compared to LT. MOR and DOR activation may contribute to LT and HT's analgesic mechanisms, respectively.

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

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

Sensory neuron-specific long noncoding RNA in small non-peptidergic dorsal root ganglion neurons selectively impairs nerve injury-induced mechanical hypersensitivity

AIMS

Nerve injury-induced mechanical hypersensitivity is one of major clinical symptoms in neuropathic pain patients. Understanding molecular mechanisms underlying this symptom is crucial for developing effective therapies. The present study was to investigate whether sensory neuron-specific long noncoding RNA (SS-lncRNA) predominantly expressed in small non-peptidergic dorsal root ganglion (DRG) neurons repaired nerve injury-induced mechanical hypersensitivity.

MATERIALS AND METHODS

SS-lncRNA downregulation in the mas-related G protein-coupled receptor member D (Mrgprd)-expressed DRG neurons was rescued and mimicked by crossbreeding MrgprdCreERT2/+ lines with Rosa26SS-lncRNA knock-in mice and SS-lncRNAfl/fl mice, respectively, followed by tamoxifen injection.

KEY FINDINGS

Rescuing SS-lncRNA downregulation in the Mrgprd-expressed DRG neurons significantly reversed the spinal nerve ligation (SNL)-induced reduction of the calcium-activated potassium channel subfamily N member 1 (KCNN1) in these DRG neurons and alleviated the SNL-induced mechanical hypersensitivity, without affecting the SNL-induced heat and cold nociceptive hypersensitivities, on the ipsilateral side. Conversely, mimicking SS-lncRNA downregulation in the Mrgprd-expressed DRG neurons reduced basal KCNN1 expression in these DRG neurons and produced the enhanced response to mechanical stimulation, but not thermal and cold stimuli, on bilateral sides. Mechanistically, SS-lncRNA downregulation caused a reduction in its binding to lysine-specific demethylase 6B (KDM6B) and consequent recruitment of less KDM6B to Kcnn1 promoter and an increase of H3K27me3 enrichment in this promoter in injured DRG.

SIGNIFICANCE

Our findings suggest that SS-lncRNA downregulation in small non-peptidergic sensory neurons is required specifically for nerve injury-induced mechanical hypersensitivity likely through silencing KCNN1 expression caused by KDM6B-gated increase of H3K27me3 enrichment in Kcnn1 promoter in these neurons.

The acid-sensing nociceptor TRPV1 controls breast cancer progression in bone via regulating HGF secretion from sensory neurons

Cancers showing excessive innervation of sensory neurons (SN) in their microenvironments are associated with poor outcomes due to promoted growth, increased tumor recurrence, metastasis, and cancer pain, suggesting SNs play a regulatory role in cancer aggressiveness. Using a preclinical model in which mouse 4T1 breast cancer (BC) cells were injected into the bone marrow of tibiae, we found 4T1 BC cells aggressively colonized bone with bone destruction and subsequently spread to the lung. Of note, 4T1 BC colonization induced the acidic tumor microenvironment in bone in which SNs showed increased innervation and excitation with elevated expression of the acid-sensing nociceptor transient receptor potential vanilloid-1 (TRPV1), eliciting bone pain (BP) assessed by mechanical hypersensitivity. Further, these excited SNs produced increased hepatocyte growth factor (HGF). Importantly, the administration of synthetic and natural TRPV1 antagonists and genetic deletion of TRPV1 decreased HGF production in SNs and inhibited 4T1 BC colonization in bone, pulmonary metastasis from bone, and BP induction. Our results suggest the TRPV1 of SNs promotes BC colonization in bone and lung metastasis via up-regulating HGF production in SNs. The SN TRPV1 may be a novel therapeutic target for BC growing in the acidic bone microenvironment and for BP.

Effects of Peripheral Nerve Injury on the Induction of c-Fos and Phosphorylated ERK in the Brainstem Trigeminal Sensory Nuclear Complex

Background

Sequential changes in brainstem and spinal cord neurons after traumatic injury to peripheral nerves are related to neuropathic pain symptoms.

Purpose

This study was conducted to elucidate the influence of nerve insult on stimulus-induced c-Fos expression and ERK phosphorylation by brainstem neurons.

Methods

The brainstem trigeminal sensory nuclear complex (BTSNC) was examined for neuronal profiles immunolabeled with c-Fos and phosphorylated ERK (p-ERK) antibodies elicited by stimulation of the tongue with capsaicin after lingual or inferior alveolar nerve (IAN) injury.

Results

Abundant neuronal profiles immunolabeled for c-Fos and p-ERK elicited by capsaicin were distributed in the spinal trigeminal nucleus caudalis (Vc) without nerve injury. The spinal trigeminal nucleus oralis (Vo) contained limited numbers of these neuronal profiles after stimulation of the tongue. A significant reduction of these neuronal profiles in the ipsilateral Vc was detected after lingual nerve injury. After IAN injury, an increased number of neuronal profiles immunolabeled for c-Fos elicited by capsaicin was noted, while that of p-ERK was left unchanged in the ipsilateral Vc. On the both sides of the Vo, an increased number of capsaicin-induced neuronal profiles immunolabeled for c-Fos and p-ERK was detected after lingual or IAN injury.

Conclusion

Differential effects of lingual or IAN injury on stimulus-induced c-Fos expression and ERK phosphorylation by Vo and Vc neurons may be involved in the complex nature of symptoms of trigeminal neuralgia.

Disentangling the enigmatic role of ephrin signaling in chronic pain: Moving towards future anti-pain therapeutics

Chronic pain is a common and debilitating condition with a huge social and economic burden worldwide. Currently, available drugs in clinics are not adequately effective and possess a variety of severe side effects leading to treatment withdrawal and poor quality of life. The ongoing search for new therapeutics with minimal side effects for chronic pain management remains a high research priority. Erythropoietin-producing human hepatocellular carcinoma cell receptor (Eph) is a tyrosine kinase receptor that is involved in neurodegenerative disorders, including pain. The Eph receptor interacts with several molecular switches, such as N methyl d-aspartate receptor (NMDAR), mitogen-activated protein kinase (MAPK), calpain 1, caspase 3, protein kinase a (PKA), and protein kinase Cy (PKCy), which in turn regulates pathophysiology of chronic pain. Here we highlight the emerging evidence of the Ephs/ephrin system as a possible near-future therapeutic target for the treatment of chronic pain and discuss the various mechanism of its involvement. We critically analyse the present status of Eph receptor system and conclude that extrapolating the pharmacological and genetic approaches using a strong therapeutic development framework could serve as next-generation analgesics for the management of chronic pain.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

Lactate secreted via MCT4 from bone‑colonizing breast cancer excites sensory neurons via GPR81

Breast cancer (BC) bone metastasis causes bone pain (BP), which detrimentally damages the quality of life and outcome of patients with BC. However, the mechanism of BC‑BP is poorly understood, and effective treatments are limited. The present study demonstrated a novel mechanism of BC‑BP using a mouse model of bone pain, in which mouse (EO771) and human (MDA‑MB‑231) BC cells were injected in the bone marrow cavity of tibiae. Western blot analysis using sensory nerves, in vivo assessment of cancer pain and in vitro calcium flux analysis were performed. These mice developed progressive BC‑BP in tibiae in conjunction with an upregulation of phosphorylated pERK1/2 and cAMP‑response element‑binding protein (pCREB), which are molecular indicators of neuron excitation, in the dorsal root ganglia (DRG) of sensory nerves. Importantly, mice injected with BC cells, in which the expression of the lactic acid transporter monocarboxylate transporter 4 (MCT4) was silenced, exhibited decreased BC‑BP with downregulated expression of pERK1/2 and pCREB in the DRG and reduced circulating levels of lactate compared with mice injected with parental BC cells. Further, silencing of the cell‑surface orphan receptor for lactate, G protein‑coupled receptor 81 (GPR81), in the F11 sensory neuron cells decreased lactate‑promoted upregulation of pERK1/2 and Ca²⁺ influx, suggesting that the sensory neuron excitation was inhibited. These results suggested that lactate released from BC cells via MCT4 induced BC‑BP through the activation of GPR81 of sensory neurons. In conclusion, the activation of GPR81 of sensory neurons by lactate released via MCT4 from BC was demonstrated to contribute to the induction of BC‑BP, and disruption of the interactions among lactate, MCT4 and GPR81 may be a novel approach to control BC‑BP.

Effect and underlying mechanisms of spirocyclopiperazinium salt compound DXL-A-24 in rats following spinal nerve ligation

PURPOSE

Neuropathic pain represents a significant public health problem and its effective management remains a challenge. The present study is designed to evaluate the analgesic effect of the spirocyclopiperazinium salt compound DXL-A-24 in spinal nerve ligation (SNL) model, and further to explore the possible molecular mechanisms.

METHODS

SNL model was established on rats, and mechanical allodynia and thermal hyperalgesia were estimated with the von Frey and hot plate tests; the expression of CaMKIIα, CREB, JAK2, STAT3 and c-fos was determined by western blotting; the protein level of TNF-α was analysed by ELISA; the mRNA expression of TNF-α and c-fos was detected using qRT-PCR analysis and the receptor blocking test was used for target searching.

RESULTS

Administration of DXL-A-24 (1, 0.5, 0.25 mg/kg, i.g.) obviously relieved SNL-induced mechanical allodynia and thermal hyperalgesia in rats (P < 0.01), with the percentage of pain threshold elevation (PTE%) was 103 %, 68 % and 47 %, respectively, in mechanical allodynia; the percentage of maximal possible effect (MPE%) was 56 %, 34 % and 21 %, respectively, in thermal hyperalgesia on day 7 after SNL. Pretreatment with peripheral α7 nicotinic or M4 muscarinic receptor antagonist, the effect of DXL-A-24 was completely blocked (P > 0.05). DXL-A-24 significantly reduced the upregulated pCaMKIIα, pCREB, pJAK2, pSTAT3 and TNF-α protein (P < 0.01), which could be blocked by α7 nicotinic receptor or M4 muscarinic receptor antagonist. In addition, administration of DXL-A-24 attenuated the mRNA and protein expression of c-fos and TNF-α mRNA in DRG of SNL rat. We did not observe significant acute toxicity and chronic hepatorenal impairment at effective dose and high dose.

CONCLUSIONS

We report firstly that administration of DXL-A-24 displays obvious antineuropathic pain effects in SNL rats. The underlying mechanism may involve the reduction of the CaMKIIα/CREB and JAK2/STAT3 signalling pathways, and the suppression of TNF-α and c-fos expression, which may be mediated by activating peripheral α7 nicotinic and M4 muscarinic receptors. This study may provide a new perspective for developing new antineuralgic drug.

An LPAR5-antagonist that reduces nociception and increases pruriception

Introduction

The G-protein coupled receptor LPAR₅ plays a prominent role in LPA-mediated pain and itch signaling. In this study we focus on the LPAR₅-antagonist compound 3 (cpd3) and its ability to affect pain and itch signaling, both in vitro and in vivo.

Methods

Nociceptive behavior in wild type mice was induced by formalin, carrageenan or prostaglandin E2 (PGE₂) injection in the hind paw, and the effect of oral cpd3 administration was measured. Scratch activity was measured after oral administration of cpd3, in mice overexpressing phospholipase A2 (sPLA2tg), in wild type mice (WT) and in TRPA1-deficient mice (Trpa1 KO). In vitro effects of cpd3 were assessed by measuring intracellular calcium release in HMC-1 and HEK-TRPA1 cells.

Results

As expected, nociceptive behavior (induced by formalin, carrageenan or PGE₂) was reduced after treatment with cpd3. Unexpectedly, cpd3 induced scratch activity in mice. In vitro addition of cpd3 to HEK-TRPA1 cells induced an intracellular calcium wave that could be inhibited by the TRPA1-antagonist A-967079. In Trpa1 KO mice, however, the increase in scratch activity after cpd3 administration was not reduced.

Conclusions

Cpd3 has in vivo antinociceptive effects but induces scratch activity in mice, probably by activation of multiple pruriceptors, including TRPA1. These results urge screening of antinociceptive candidate drugs for activity with pruriceptors.

Modulating the tachykinin: Role of substance P and neurokinin receptor expression in ocular surface disorders

Substance P (SP) is a tachykinin expressed by various cells in the nervous and immune systems. SP is predominantly released by neurons and exerts its biological and immunological effects through the neurokinin receptors, primarily the neurokinin-1 receptor (NK1R). SP is essential for maintaining ocular surface homeostasis, and its reduced levels in disorders like diabetic neuropathy disrupt the corneal tissue. It also plays an essential role in promoting corneal wound healing by promoting the migration of keratocytes. In this review, we briefly discuss the structure, expression, and function of SP and its principal receptor NK1R. In addition, SP induces pro-inflammatory effects through autocrine or paracrine action on the immune cells in various ocular surface pathologies, including dry eye disease, herpes simplex virus keratitis, and Pseudomonas keratitis. We provide an in-depth review of the pathogenic role of SP in various ocular surface diseases and several new approaches developed to counter the immune-mediated effects of SP either through modulating its production or blocking its target receptor.

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.

Neuropathic Pain in Multiple Sclerosis and Its Animal Models: Focus on Mechanisms, Knowledge Gaps and Future Directions

Multiple sclerosis (MS) is a multifaceted, complex and chronic neurological disease that leads to motor, sensory and cognitive deficits. MS symptoms are unpredictable and exceedingly variable. Pain is a frequent symptom of MS and manifests as nociceptive or neuropathic pain, even at early disease stages. Neuropathic pain is one of the most debilitating symptoms that reduces quality of life and interferes with daily activities, particularly because conventional pharmacotherapies do not adequately alleviate neuropathic pain. Despite advances, the mechanisms underlying neuropathic pain in MS remain elusive. The majority of the studies investigating the pathophysiology of MS-associated neuropathic pain have been performed in animal models that replicate some of the clinical and neuropathological features of MS. Experimental autoimmune encephalomyelitis (EAE) is one of the best-characterized and most commonly used animal models of MS. As in the case of individuals with MS, rodents affected by EAE manifest increased sensitivity to pain which can be assessed by well-established assays. Investigations on EAE provided valuable insights into the pathophysiology of neuropathic pain. Nevertheless, additional investigations are warranted to better understand the events that lead to the onset and maintenance of neuropathic pain in order to identify targets that can facilitate the development of more effective therapeutic interventions. The goal of the present review is to provide an overview of several mechanisms implicated in neuropathic pain in EAE by summarizing published reports. We discuss current knowledge gaps and future research directions, especially based on information obtained by use of other animal models of neuropathic pain such as nerve injury.

Cytohesin-2 mediates group I metabotropic glutamate receptor-dependent mechanical allodynia through the activation of ADP ribosylation factor 6 in the spinal cord

Group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, in the spinal cord are implicated in nociceptive transmission and plasticity through G protein-mediated second messenger cascades leading to the activation of various protein kinases such as extracellular signal-regulated kinase (ERK). In this study, we demonstrated that cytohesin-2, a guanine nucleotide exchange factor for ADP ribosylation factors (Arfs), is abundantly expressed in subsets of excitatory interneurons and projection neurons in the superficial dorsal horn. Cytohesin-2 is enriched in the perisynapse on the postsynaptic membrane of dorsal horn neurons and forms a protein complex with mGluR5 in the spinal cord. Central nervous system-specific cytohesin-2 conditional knockout mice exhibited reduced mechanical allodynia in inflammatory and neuropathic pain models. Pharmacological blockade of cytohesin catalytic activity with SecinH3 similarly reduced mechanical allodynia and inhibited the spinal activation of Arf6, but not Arf1, in both pain models. Furthermore, cytohesin-2 conditional knockout mice exhibited reduced mechanical allodynia and ERK1/2 activation following the pharmacological activation of spinal mGluR1/5 with 3,5-dihydroxylphenylglycine (DHPG). The present study suggests that cytothesin-2 is functionally associated with mGluR5 during the development of mechanical allodynia through the activation of Arf6 in spinal dorsal horn neurons.

Emerging drugs for the prevention of migraine

INTRODUCTION

Migraine is a common and disabling neurological disorder. A greater understanding of the pathophysiological mechanisms underlying migraine has led to the availability of specific new drugs targeting calcitonin gene-related peptide (CGRP). The success of the CGRP inhibitors validates research efforts into migraine-specific therapies.

AREAS COVERED

There are additional promising therapeutic targets that will be covered in this paper, focusing on the pain phase. They include pituitary adenylate cyclase-activating polypeptide (PACAP), the orexinergic system, the nitric oxide signaling pathway specifically neuronal nitric oxide synthase inhibitors (nNOSi), and metabotropic glutamate receptor 5 (mGluR5).

EXPERT OPINION

Based on currently available research; the targets discussed in this paper are all on equal footing with each other in terms of their potential as effective novel migraine therapies. There is a need for more clinical trials to pinpoint which of these potential drug targets will be effective for migraine preventio.

Activation of SK3 channel plays a pivotal role in modulation of trigeminal neuralgia

Objective: To investigate whether small conductance Ca²⁺ activated^K+ channels; Trigeminal ganglion; Trigeminal neuralgia. (SK3) exists in normal rats' trigeminal ganglions (TG) and its effect on their pain thresholds.Methods: In total, 110 healthy adult male Sprague-Dawley (SD) rats were involved in this study. Ten rats were dissected to collect their liver tissues, TG and DRG. The rest of the rats were randomly assigned to either the experimental group or the control group. The animal model of trigeminal neuralgia (TN) was established by infraorbital nerve ligation. The expression of SK3 channels in their livers, TG and dorsal root ganglions (DRG) were detected. And different doses of SK3 channel activator and inhibitor were administered to the rats in both groups 15 days after the operation; meanwhile, their pain thresholds were also measured.Results: The expression of SK3 channel was found in TG. In the experimental group, the pain threshold was significantly decreased and there was a decreased level of SK3 than that in the control group at 15 days after operation. The administration of SK3 channel agonist (CyPPA) could significantly improve the pain threshold, while, the pain threshold decreased after administration of SK3 channel antagonist (Apamin).Conclusion: The SK3 channel may play a pivotal role in the pathogenesis of trigeminal neuralgia, and it may be one of the potential targets for the treatment of trigeminal neuralgia.

The nonreceptor protein tyrosine kinase Src participates in every step of cancer-induced bone pain

Cancer-induced bone pain (CIBP) is a refractory form of pain that has a high incidence in advanced tumors. Src protein tyrosine kinase is mainly composed of six domains, with two states of automatic inhibition and activation. The modular domain allows Src to conveniently regulate by and communicate with a variety of proteins, directly or indirectly participate in each step of the CIBP process. Src is beneficial to the growth and proliferation of tumor cells, and it can promote the metastases of primary tumors to bone. In the microenvironment of bone metastasis, it mainly mediates bone resorption, activates related peripheral receptors to participate in the formation of pain signals, and may promote the generation of pathological sensory nerve fibers. In the process of pain signal transmission, it mainly mediates NMDAR and central glial cells to regulate pain signal intensity and central sensitization, but it is not limited to these two aspects. Both basic experimentation and clinical research have shown encouraging potential, providing new ideas and inspiration for the prevention and treatment of CIBP.

The role of cyclin-dependent kinase 5 in neuropathic pain

The chronification of pain can be attributed to changes in membrane receptors and channels underlying neuronal plasticity and signal transduction largely within nociceptive neurons that initiate and maintain pathological pain states. These proteins are subject to dynamic modification by posttranslational modifications, creating a code that controls protein function in time and space. Phosphorylation is an important posttranslational modification that affects ∼30% of proteins in vivo. Increased phosphorylation of various nociceptive ion channels and of their modulators underlies sensitization of different pain states. Cyclin-dependent kinases are proline-directed serine/threonine kinases that impact various biological and cellular systems. Cyclin-dependent kinase 5 (Cdk5), one member of this kinase family, and its activators p35 and p39 are expressed in spinal nerves, dorsal root ganglia, and the dorsal horn of the spinal cord. In neuropathic pain conditions, expression and/or activity of Cdk5 is increased, implicating Cdk5 in nociception. Experimental evidence suggests that Cdk5 is regulated through its own phosphorylation, through increasing p35's interaction with Cdk5, and through cleavage of p35 into p25. This narrative review discusses the molecular mechanisms of Cdk5-mediated regulation of target proteins involved in neuropathic pain. We focus on Cdk5 substrates that have been linked to nociceptive pathways, including channels (eg, transient receptor potential cation channel and voltage-gated calcium channel), proteins involved in neurotransmitter release (eg, synaptophysin and collapsin response mediator protein 2), and receptors (eg, glutamate, purinergic, and opioid). By altering the phosphoregulatory "set point" of proteins involved in pain signaling, Cdk5 thus appears to be an attractive target for treating neuropathic pain conditions.

Systems Biology Guided Gene Enrichment Approaches Improve Prediction of Chronic Post-surgical Pain After Spine Fusion

Objectives

Incorporation of genetic factors in psychosocial/perioperative models for predicting chronic postsurgical pain (CPSP) is key for personalization of analgesia. However, single variant associations with CPSP have small effect sizes, making polygenic risk assessment important. Unfortunately, pediatric CPSP studies are not sufficiently powered for unbiased genome wide association (GWAS). We previously leveraged systems biology to identify candidate genes associated with CPSP. The goal of this study was to use systems biology prioritized gene enrichment to generate polygenic risk scores (PRS) for improved prediction of CPSP in a prospectively enrolled clinical cohort.

Methods

In a prospectively recruited cohort of 171 adolescents (14.5 ± 1.8 years, 75.4% female) undergoing spine fusion, we collected data about anesthesia/surgical factors, childhood anxiety sensitivity (CASI), acute pain/opioid use, pain outcomes 6-12 months post-surgery and blood (for DNA extraction/genotyping). We previously prioritized candidate genes using computational approaches based on similarity for functional annotations with a literature-derived "training set." In this study, we tested ranked deciles of 1336 prioritized genes for increased representation of variants associated with CPSP, compared to 10,000 randomly selected control sets. Penalized regression (LASSO) was used to select final variants from enriched variant sets for calculation of PRS. PRS incorporated regression models were compared with previously published non-genetic models for predictive accuracy.

Results

Incidence of CPSP in the prospective cohort was 40.4%. 33,104 case and 252,590 control variants were included for association analyses. The smallest gene set enriched for CPSP had 80/1010 variants associated with CPSP (p < 0.05), significantly higher than in 10,000 randomly selected control sets (p = 0.0004). LASSO selected 20 variants for calculating weighted PRS. Model adjusted for covariates including PRS had AUROC of 0.96 (95% CI: 0.92-0.99) for CPSP prediction, compared to 0.70 (95% CI: 0.59-0.82) for non-genetic model (p < 0.001). Odds ratios and positive regression coefficients for the final model were internally validated using bootstrapping: PRS [OR 1.98 (95% CI: 1.21-3.22); β 0.68 (95% CI: 0.19-0.74)] and CASI [OR 1.33 (95% CI: 1.03-1.72); β 0.29 (0.03-0.38)].

Discussion

Systems biology guided PRS improved predictive accuracy of CPSP risk in a pediatric cohort. They have potential to serve as biomarkers to guide risk stratification and tailored prevention. Findings highlight systems biology approaches for deriving PRS for phenotypes in cohorts less amenable to large scale GWAS.

Extracellular signal-regulated kinases 2 (Erk2) and Erk5 in the central nervous system differentially contribute to central sensitization in male mice

Nervous systems are designed to become extra sensitive to afferent nociceptive stimuli under certain circumstances such as inflammation and nerve injury. How pain hypersensitivity comes about is key issue in the field since it ultimately results in chronic pain. Central sensitization represents enhanced pain sensitivity due to increased neural signaling within the central nervous system (CNS). Particularly, much evidence indicates that underlying mechanism of central sensitization is associated with the change of spinal neurons. Extracellular signal-regulated kinases have received attention as key molecules in central sensitization. Previously, we revealed the isoform-specific function of extracellular signal-regulated kinase 2 (Erk2) in spinal neurons for central sensitization using mice with Cre-loxP-mediated deletion of Erk2 in the CNS. Still, how extracellular signal-regulated kinase 5 (Erk5) in spinal neurons contributes to central sensitization has not been directly tested, nor is the functional relevance of Erk5 and Erk2 known. Here, we show that Erk5 and Erk2 in the CNS play redundant and/or distinct roles in central sensitization, depending on the plasticity context (cell types, pain types, time, etc.). We used male mice with Erk5 deletion specifically in the CNS and found that Erk5 plays important roles in central sensitization in a formalin-induced inflammatory pain model. Deletion of both Erk2 and Erk5 leads to greater attenuation of central sensitization in this model, compared to deletion of either isoform alone. Conversely, Erk2 but not Erk5 plays important roles in central sensitization in neuropathic pain, a type of chronic pain caused by nerve damage. Our results suggest the elaborate mechanisms of Erk signaling in central sensitization.

Zika virus promotes CCN1 expression via the CaMKIIα-CREB pathway in astrocytes

Zika virus (ZIKV) infection in the human central nervous system (CNS) causes Guillain–Barre syndrome, cerebellum deformity, and other diseases. Astrocytes are immune response cells in the CNS and an important component of the blood–brain barrier. Consequently, any damage to astrocytes facilitates the spread of ZIKV in the CNS. Connective tissue growth factor/Nephroblastoma overexpressed gene family 1 (CCN1), an important inflammatory factor secreted by astrocytes, is reported to regulate innate immunity and viral infection. However, the mechanism by which astrocyte viral infection affects CCN1 expression remains undefined. In this study, we demonstrate that ZIKV infection up-regulates CCN1 expression in astrocytes, thus promoting intracellular viral replication. Other studies revealed that the cAMP response element (CRE) in the CCN1 promoter is activated by the ZIKV NS3 protein. The cAMP-responsive element-binding protein (CREB), a transacting factor of the CRE, is also activated by NS3 or ZIKV. Furthermore,a specific inhibitor of CREB, i.e. SGC-CBP30, reduced ZIKV-induced CCN1 up-regulation and ZIKV replication. Moreover, co-immunoprecipitation, overexpression, and knockdown studies confirmed that the interaction between NS3 and the regulatory domain of CaMKIIα could activate the CREB pathway, thus resulting in the up-regulation of CCN1 expression and enhancement of virus replication. In conclusion, the findings of our investigations on the NS3-CaMKIIα-CREB-CCN1 pathway provide a foundation for understanding the infection mechanism of ZIKV in the CNS.

Opioid-Induced Hyperalgesic Priming in Single Nociceptors

Clinical µ-opioid receptor (MOR) agonists produce hyperalgesic priming, a form of maladaptive nociceptor neuroplasticity, resulting in pain chronification. We have established an in vitro model of opioid-induced hyperalgesic priming (OIHP), in male rats, to identify nociceptor populations involved and its maintenance mechanisms. OIHP was induced in vivo by systemic administration of fentanyl and confirmed by prolongation of prostaglandin E₂ (PGE₂) hyperalgesia. Intrathecal cordycepin, which reverses Type I priming, or the combination of Src and mitogen-activated protein kinase (MAPK) inhibitors, which reverses Type II priming, both partially attenuated OIHP. Parallel in vitro experiments were performed on small-diameter (<30 µm) dorsal root ganglion (DRG) neurons, cultured from fentanyl-primed rats, and rats with OIHP treated with agents that reverse Type I or Type II priming. Enhancement of the sensitizing effect of a low concentration of PGE₂ (10 nm), another characteristic feature of priming, measured as reduction in action potential (AP) rheobase, was found in weakly isolectin B4 (IB4)-positive and IB4-negative (IB4-) neurons. In strongly IB4-positive (IB4+) neurons, only the response to a higher concentration of PGE₂ (100 nm) was enhanced. The sensitizing effect of 10 nm PGE₂ was attenuated in weakly IB4+ and IB4- neurons cultured from rats whose OIHP was reversed in vivo Thus, in vivo administration of fentanyl induces neuroplasticity in weakly IB4+ and IB4- nociceptors that persists in vitro and has properties of Type I and Type II priming. The mechanism underlying the enhanced sensitizing effect of 100 nm PGE₂ in strongly IB4+ nociceptors, not attenuated by inhibitors of Type I and Type II priming, remains to be elucidated.SIGNIFICANCE STATEMENT Commonly used clinical opioid analgesics, such as fentanyl and morphine, can produce hyperalgesia and chronification of pain. To uncover the nociceptor population mediating opioid-induced hyperalgesic priming (OIHP), a model of pain chronification, and elucidate its underlying mechanism, at the cellular level, we established an in vitro model of OIHP. In dorsal root ganglion (DRG) neurons cultured from rats primed with fentanyl, robust nociceptor population-specific changes in sensitization by prostaglandin E₂ (PGE₂) were observed, when compared with nociceptors from opioid naive rats. In DRG neurons cultured from rats with OIHP, enhanced PGE₂-induced sensitization was observed in vitro, with differences identified in non-peptidergic [strongly isolectin B4 (IB4)-positive] and peptidergic [weakly IB4-positive (IB4+) and IB4-negative (IB4-)] nociceptors.

The HMGB1/RAGE axis induces bone pain associated with colonization of 4T1 mouse breast cancer in bone

•

The 4T1 mouse breast cancer injected in tibiae induced bone pain.

•

The 4T1 breast cancer secreted high mobility group box 1 (HMGB1) that promotes axogenesis of sensory neurons.

•

Bone pain was reduced by HMGB1 antibody and an antagonist for the receptor for advanced glycation end products.

Bone pain is a common complication of breast cancer (BC) bone metastasis and is a major cause of increased morbidity and mortality. Although the mechanism of BC-associated bone pain (BCABP) remains poorly understood, involvement of BC products in the pathophysiology of BCABP has been proposed. Aggressive cancers secrete damage-associated molecular patterns (DAMPs) that bind to specific DAMP receptors and modulate cancer microenvironment. A prototypic DAMP, high mobility group box 1 (HMGB1), which acts as a ligand for the receptor for advanced glycation end products (RAGE) and toll-like receptors (TLRs), is increased in its expression in BC patients with poor outcomes. Here we show that 4T1 mouse BC cells colonizing bone up-regulate the expression of molecular pain markers, phosphorylated ERK1/2 (pERK) and pCREB, in the dorsal root ganglia (DRGs) innervating bone and induced BCABP as evaluated by hind-paw mechanical hypersensitivity. Importantly, silencing HMGB1 in 4T1 BC cells by shRNA reduced pERK and pCREB and BCABP with decreased HMGB1 levels in bone. Further, administration of a neutralizing antibody to HMGB1 or an antagonist for RAGE, FPS-ZM1, ameliorated pERK, pCREB and BCABP, while a TLR4 antagonist, TAK242, showed no effects. Consistent with these in vivo results, co-cultures of F11 sensory neuron-like cells with 4T1 BC cells in microfluidic culture platforms increased neurite outgrowth of F11 cells, which was blocked by HMGB1 antibody. Our results show that HMGB1 secreted by BC cells induces BCABP via binding to RAGE of sensory neurons and suggest that the HMGB1/RAGE axis may be a potential novel therapeutic target for BCABP.

NMDA Receptor Modulates Spinal Iron Accumulation Via Activating DMT1(-)IRE in Remifentanil-Induced Hyperalgesia

N-methyl-D-aspartate (NMDA) receptor activation is known to be critical in remifentanil-induced hyperalgesia. Evidence indicates that iron accumulation participates in NMDA neurotoxicity. This study aims to investigate the role of iron accumulation in remifentanil-induced hyperalgesia. Remifentanil was delivered intravenously in rats to induce hyperalgesia. The NMDA receptor antagonist MK-801 was intrathecally administrated. The levels of divalent metal transporter 1 without iron-responsive element [DMT1(-)IRE] and iron were detected. Behavior testing was performed in DMT1(-)IRE knockdown rats and rats treated with iron chelator DFO. Meanwhile, the spinal dorsal horn neurons were cultured and transfected with DMT1(-)IRE siRNA, and then respectively incubated with remifentanil and MK-801. The levels of intracellular Ca²⁺ and iron were assessed by fluorescence imaging. Our data revealed that spinal DMT1(-)IRE and iron content significantly increased in remifentanil-treated rats, and MK-801 inhibited the enhancements. DMT1(-)IRE knockdown and DFO prevented against remifentanil-induced hyperalgesia. Notably, the levels of Ca²⁺ and iron increased in remifentanil-incubated neurons, and these growths can be blocked by MK-801. DMT1(-)IRE knockdown attenuated iron accumulation but did not influence Ca²⁺ influx. This study suggests that DMT1(-)IRE-mediated iron accumulation is likely to be the downstream event following NMDA receptor activation and Ca²⁺ influx, contributing to remifentanil-induced hyperalgesia. PERSPECTIVE: Remifentanil-induced hyperalgesia is common even when used within clinical accepted doses. This study presents that aberrant iron accumulation is involved in the development of remifentanil-induced hyperalgesia in vivo and in vitro. Iron chelation may be a potential therapeutic strategy for the prevention of hyperalgesia in populations at high risk.

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Extracellular signal-regulated kinases (ERK) 1 and 2 as a key molecule in pain research

Pain can be divided into nociceptive, inflammatory, and neuropathic pain. It is important to understanding the molecular mechanism of pain signaling in the development of pain relief therapies. Twenty years ago, extracellular signal-regulated kinases (ERK) 1 and 2, which are members of the mitogen-activated protein kinase superfamily, were identified as molecules activated in neurons by the exposure of peripheral tissues to noxious stimuli. Further studies have revealed that peripheral nerve injury induces ERK activation in glial cells, sensory neurons, and second-order neurons, albeit at different time points. Moreover, inhibition of ERK suppresses pathological pain in animals with peripheral nerve injury. Therefore, ERK is currently recognized as an important molecule in pain signaling and a potential novel target for pain treatment. This review introduces recent advances in revealing the regulation of ERK in pain research.

HCN2 contributes to oxaliplatin-induced neuropathic pain by inducing spinal long-term potentiation via activation of NMDA receptor-mediated CaMKII signaling

Our previous findings indicate that HCN2 contributes to oxaliplatin-induced neuropathic pain, but the mechanisms underlying the development of neuropathic pain are still unclear. Here, we found that the rat HCN2 levels significantly increased after high-frequency stimulation-induced long-term potentiation (LTP). Spinal local application of ZD7288 (a cyclic-nucleotide-gated-channel-specific inhibitor) prevented LTP induction after intraperitoneal injection of oxaliplatin. In addition, oxaliplatin administration induced spinal LTP via activation of the CaMKII-CREB cascade in the rat spinal dorsal horn. Moreover, we found that administration of oxaliplatin significantly increased the amplitude of excitatory postsynaptic currents and the number of action potentials, but these effects were attenuated by pretreatment with either CaMKII inhibitor KN-93 or NR2B antagonist Ro 25-6981. An increase in the phosphorylation of spinal N-methyl-d-aspartate (NMDA) receptor subunit 1 (NR1) after oxaliplatin administration was weakened by ZD7288 pretreatment. Administration of noncompetitive NMDA receptor antagonist MK-801 blocked oxaliplatin-evoked CaMKII-CREB cascade activation and prevented HCN2-mediated spinal-LTP induction in vitro and suppressed neuropathic-pain behaviors of rats. All these data suggest that HCN2 contributes to the development of neuropathic pain by inducing spinal LTP via activation of NMDA receptor-mediated CaMKII signaling.

Electroacupuncture reduces fibromyalgia pain by downregulating the TRPV1-pERK signalling pathway in the mouse brain

BACKGROUND

Acupuncture has been clinically recommended as a method of pain relief by the World Health Organization and is widely used by medical doctors. Fibromyalgia (FM) pain has a complex physiological and psychological origin and can be pharmacologically treated with duloxetine, milnacipran and pregabalin. However, these drugs produce undesirable side effects, such as headaches, nausea and diarrhoea. Acupuncture may serve as an effective alternative treatment for pain relief with few side effects.

AIMS

We hypothesised that acupuncture would reduce FM pain by influencing transient receptor potential cation channel subfamily V member 1 (TRPV1) and the downstream phosphorylated extracellular signal-regulated kinases (pERK), which are located in the central thalamus, amygdala and cortex.

METHODS

A FM mouse model was established by injecting two doses of acid saline into 32 female C57/B6 mice. The mice were then assigned to different subgroups (n=8 each) and treated with electroacupuncture (EA) or EA sham control. TRPV1 and pERK expression levels were measured using Western blotting and immunohistochemistry.

RESULTS

Our results demonstrated that the expression of TRPV1 and pERK in the thalamus, amygdala and somatosensory cortex was normal in the control mice, but significantly increased in FM mice; these FM-induced changes in expression were attenuated by EA.

CONCLUSION

Our data suggest that EA can reverse the central sensitisation of the TRPV1-ERK signalling pathway in the mouse brain. Thus, our findings provide mechanistic evidence supporting the potential therapeutic efficacy of EA for treating FM pain.

NADPH oxidase1 inhibition leads to regression of central sensitization during formalin induced acute nociception via attenuation of ERK1/2-NFκB signaling and glial activation

Role of NADPH oxidase1 in the development of inflammatory pain has been demonstrated by gene knockout studies. Nevertheless, pharmacological inhibition of NOX1 is a requisite approach for therapeutic utility. Recently, we have reported the anti-nociceptive effect of newly identified NOX1 specific inhibitor ML171 (2-acetylphenothiazine). Inhibition of NOX1 resulted in attenuation of nociceptive sensitization during acute inflammatory pain via inhibition of ROS generation and its downstream ERK1/2 activation. However, glial activation accompanying inflammation is closely related to the initiation and maintenance of pain. Peripheral nociceptive inputs activate the primary afferents via release of various chemical mediators which are potentially capable of mediating signals from neuron to glia in DRG and subsequently in spinal cord dorsal horn. The subsequent interactions between neuron and glia contribute to pain hypersensitivity. Thus, the present study was focused to investigate the effect of ML171 on ERK1/2 signaling, glial activation, and crosstalk between neuron and glia in a mouse model of formalin induced acute nociception. Thus, the present study was focused to investigate the effect of ML171 on ERK1/2 signaling, glial activation, and crosstalk between neuron and glia in DRG and dorsal horn of the spinal cord of lumbar region (L3-L5) in a mouse model of formalin induced acute nociception. Intraperitoneal administration of ML171 decreased nociceptive behavioral responses, i.e. the flinch and lick counts, in formalin induced nociceptive mice. Immunofluorescence and Western blot analysis demonstrated decreased levels of nociceptive mediators like p-ERK1/2, p-NFκB p65, Iba1 and GFAP in DRG as well as in spinal cord dorsal horn; supporting anti-nociceptive potential of ML171. Further, co-localization studies showed the neuron-glia crosstalk in tissue dependent manner. ERK1/2 was found to be activated in glia and NFκB in neurons in DRG; whereas in case of spinal cord ERK1/2 was activated in neurons and NFκB in astrocytes. Decrease in nociceptive behavioral response and activation of nociceptive mediators after intraperitoneal administration of ML171 strongly advocate anti-nociceptive potential of ML171. This is the first report demonstrating modulation of ERK1/2-NFκB signaling pathway, glial activation and regulation of neuron-glia crosstalk by NADPH oxidase1 inhibition towards its anti-nociceptive action.

Activation of the RAS/B-RAF-MEK-ERK pathway in satellite glial cells contributes to substance p-mediated orofacial pain

The cross talk between trigeminal ganglion (TG) neurons and satellite glial cells (SGCs) is crucial for the regulation of inflammatory orofacial pain. Substance P (SP) plays an important role by activating neurokinin (NK)-I receptors in this cross talk. The activation of extracellular signal-regulated kinase (ERK) 1/2, protein kinase A (PKA) and protein kinase C (PKC) in neurons and SGCs of peripheral ganglions by peripheral inflammation is associated with inflammatory hypersensitivity. This study tested the hypothesis that SP evoked SP-NK-I receptor positive feedback via the Renin-Angiotensin System/B-Protein Kinase A-Rapidly Accelerates Fibrosarcoma-MEK-Extracellular Signal-Regulated Kinase (RAS/PKA-RAF-MEK-ERK) pathway, which is involved in pain hypersensitivity. Inflammatory models were induced in vivo by injecting Complete Freund's adjuvant (CFA) into the whisker pad of rats. SP was administrated to SGCs in vitro for investigating, whether SP regulates the expression of NK-I receptor in the SGC nucleus. The effects of RAS-RAF-MEK, PKA and PKC pathways in this process were measured by co-incubating SGCs with respective Raf, PKA, PKC and MEK inhibitors in vitro and by pre-injecting these inhibitors into the TG in vivo. SP significantly upregulated NK-I receptor, p-ERK1/2, Ras, B-Raf, PKA and PKC in SGCs under inflammatory conditions. In addition, L703,606 (NK-I receptor antagonist), U0126 (MEK inhibitor), Sorafenib (Raf inhibitor) and H892HCL (PKA inhibitor) but not chelerythrine chloride (PKC inhibitor) significantly decreased NK-I mRNA and protein levels induced by SP. The allodynia-related behavior evoked by CFA was inhibited by pre-injection of L703,606, U0126, Sorafenib and H892HCL into the TG. Overall, SP upregulates NK-I receptor in TG SGCs via PKA/RAS-RAF-MEK-ERK pathway activation, contributing to a positive feedback of SP-NK-I receptor in inflammatory orofacial pain.

Moxibustion Eases Chronic Inflammatory Visceral Pain In Rats Via MAPK Signaling Pathway In The Spinal Cord

Purpose

The purpose of this study was to explore the central analgesia mechanism of moxibustion for chronic inflammatory visceral pain (CIVP).

Methods

A CIVP rat model was established by 2,4,6-trinitrobenzene sulfonic acid (TNBS) plus 50% ethanol via enema. The analgesic effect of moxibustion was evaluated using the abdominal withdrawal reflex (AWR), mechanical withdrawal threshold (MWT), and thermal withdrawal latency (TWL). The expression profile of phosphorylated proteins of the mitogen-activated protein kinase (MAPK) signaling pathway in the spinal cord was assayed by protein microarray. The differentially expressed proteins were examined by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) for functional clusters and corresponding signaling pathways.

Results

Moxibustion exerted a significant analgesic effect for CIVP rats, mainly presenting as a decrease in the AWR score (all P<0.01) under different levels of distending pressure and an increase in MWT and TWL thresholds (all P<0.05). Compared with the normal group, 76 proteins were upregulated while 15 were downregulated, and MAPK signaling pathway was activated in the model group. Compared with the model group, there were 53 downregulated and 38 upregulated proteins in the moxibustion group, and MAPK signaling pathway was inhibited. Fold change (FC)>1.3 or <0.77 was taken as the screening standard to define the differentially expressed proteins. Fifteen differentially expressed proteins upregulated in the model group were downregulated in the moxibustion group. GO analysis showed that the differentially expressed proteins mainly controlled cellular metabolism regulation, transportation, and stress reactions. KEGG analysis revealed that these differentially expressed proteins were mostly involved in the ERK, JNK, and p38 pathways, and the ERK pathway was predominant.

Conclusion

Moxibustion mitigates CIVP in rats and inhibits the phosphorylation of proteins in the spinal MAPK signaling pathway. The analgesic effect of moxibustion may be associated with the regulation of the spinal MAPK signaling pathway.

RARβ Agonist Drug (C286) Demonstrates Efficacy in a Pre-clinical Neuropathic Pain Model Restoring Multiple Pathways via DNA Repair Mechanisms

Neuropathic pain (NP) is associated with profound gene expression alterations within the nociceptive system. DNA mechanisms, such as epigenetic remodeling and repair pathways have been implicated in NP. Here we have used a rat model of peripheral nerve injury to study the effect of a recently developed RARβ agonist, C286, currently under clinical research, in NP. A 4-week treatment initiated 2 days after the injury normalized pain sensation. Genome-wide and pathway enrichment analysis showed that multiple mechanisms persistently altered in the spinal cord were restored to preinjury levels by the agonist. Concomitant upregulation of DNA repair proteins, ATM and BRCA1, the latter being required for C286-mediated pain modulation, suggests that early DNA repair may be important to prevent phenotypic epigenetic imprints in NP. Thus, C286 is a promising drug candidate for neuropathic pain and DNA repair mechanisms may be useful therapeutic targets to explore.

Intrathecal injection of dexmedetomidine ameliorates chronic neuropathic pain via the modulation of MPK3/ERK1/2 in a mouse model of chronic neuropathic pain

Objective: Despite the application of dexmedetomidine (DEX) as a perioperative adjuvant in local analgesia, the exact analgesic mechanism underpinning chronic neuropathic pain (CNP) awaits our elucidation. Methods: We investigated the molecular mechanisms of the anti-nociceptive effect of DEX on neuropathic pain in a mouse model induced by chronic constriction injury (CCI). Results: DEX administration significantly increased the paw withdrawal latency (PWL) values 0.5 to 2 h post-injection in CCI-induced CNP mice at day 5 to 21 versus dimethyl sulfoxide (DMSO)-treated mice, confirming its analgesic effect. The c-Fos expression was significantly elevated in CCI mice versus the sham-operated group, whereas the elevation was mitigated by DEX injection. Subsequently, the involvement of MKP1 and MKP3 in the pathogenesis of chronic neuropathic pain was evaluated. Western blotting analyses revealed significant decrease in both MKP1 and MKP3 in the spinal cord in CCI group versus the sham group. DEX markedly elevated the MKP3 expression and modestly reduced the MKP1 expression, with insignificant difference in the latter. Co-injection of BCI (an MKP3 inhibitor) and DEX evidently reduced the PWL values in CCI mice. Furthermore, DEX significantly downregulated the phosphorylation of extracellular-signal-regulated kinase (ERK) 1/2, down-stream effector of MKP3 in CCI mice, whereas the downregulation was reversed by BCI. Conclusion: We confirmed that DEX exerts the analgesic effect on chronic neuropathic pain via the regulation of MKP3/ERK1/2 signaling pathway, which may contribute to clarification of the molecular mechanism and novel therapy for chronic neuropathic pain.

Regulation of the firing activity by PKA-PKC-Src family kinases in cultured neurons of hypothalamic arcuate nucleus

The cAMP‐dependent protein kinase A family (PKAs), protein kinase C family (PKCs), and Src family kinases (SFKs) are found to play important roles in pain hypersensitivity. However, more detailed investigations are still needed in order to understand the mechanisms underlying the actions of PKAs, PKCs, and SFKs. Neurons in the hypothalamic arcuate nucleus (ARC) are found to be involved in the regulation of pain hypersensitivity. Here we report that the action potential (AP) firing activity of ARC neurons in culture was up‐regulated by application of the adenylate cyclase activator forskolin or the PKC activator PMA, and that the forskolin or PMA application‐induced up‐regulation of AP firing activity could be blocked by pre‐application of the SFK inhibitor PP2. SFK activation also up‐regulated the AP firing activity and this effect could be prevented by pre‐application of the inhibitors of PKCs, but not of PKAs. Furthermore, we identified that forskolin or PMA application caused increases in the phosphorylation not only in PKAs at T197 or PKCs at S660 and PKCα/βII at T638/641, but also in SFKs at Y416. The forskolin or PMA application‐induced increase in the phosphorylation of PKAs or PKCs was not affected by pre‐treatment with PP2. The regulations of the SFK and AP firing activities by PKCs were independent upon the translocation of either PKCα or PKCβII. Thus, it is demonstrated that PKAs may act as an upstream factor(s) to enhance SFKs while PKCs and SFKs interact reciprocally, and thereby up‐regulate the AP firing activity in hypothalamic ARC neurons.

Roles of Cyclic AMP Response Element Binding Activation in the ERK1/2 and p38 MAPK Signalling Pathway in Central Nervous System, Cardiovascular System, Osteoclast Differentiation and Mucin and Cytokine Production

There are many downstream targets of mitogen-activated protein kinase (MAPK) signalling that are involved in neuronal development, cellular differentiation, cell migration, cancer, cardiovascular dysfunction and inflammation via their functions in promoting apoptosis and cell motility and regulating various cytokines. It has been reported that cyclic AMP response element-binding protein (CREB) is phosphorylated and activated by cyclic AMP signalling and calcium/calmodulin kinase. Recent evidence also points to CREB phosphorylation by the MAPK signalling pathway. However, the specific roles of CREB phosphorylation in MAPK signalling have not yet been reviewed in detail. Here, we describe the recent advances in the study of this MAPK-CREB signalling axis in human diseases. Overall, the crosstalk between extracellular signal-related kinase (ERK) 1/2 and p38 MAPK signalling has been shown to regulate various physiological functions, including central nervous system, cardiac fibrosis, alcoholic cardiac fibrosis, osteoclast differentiation, mucin production in the airway, vascular smooth muscle cell migration, steroidogenesis and asthmatic inflammation. In this review, we focus on ERK1/2 and/or p38 MAPK-dependent CREB activation associated with various diseases to provide insights for basic and clinical researchers.

Ovarian Hormone-dependent and Spinal ERK Activation-regulated Nociceptive Hypersensitivity in Female Rats with Acid Injection-induced Chronic Widespread Muscle Pain

Symptoms of chronic widespread muscle pain (CWP) meet most of the diagnostic criteria for fibromyalgia syndrome, which is prevalent in females. We used an acid injection-induced muscle pain (AIMP) model to mimic CWP. After female rats received an ovariectomy (OVX), acid saline solution was injected into the left gastrocnemius muscle. Time courses of changes in pain behaviours and p-ERK in the spinal cord were compared between groups. Intrathecal injections of oestradiol (E2) to the OVX group before two acid injections and E2 or progesterone (P4) injections in male rats were compared to evaluate hormone effects. We found that repeated acid injections produced mechanical hypersensitivity and enhanced p-ERK expression in the spinal dorsal horn. OVX rats exhibited significantly less tactile allodynia than did the rats in the other groups. The ERK inhibitor U0126 alleviated mechanical allodynia with lower p-ERK expression in the sham females but did not affect the OVX rats. Intrathecal E2 reversed the attenuated mechanical hypersensitivity in the OVX group, and E2 or P4 induced transient hyperalgesia in male rats. Accordingly, our results suggested that ovarian hormones contribute to AIMP through a spinal p-ERK-mediated pathway. These findings may partially explain the higher prevalence of fibromyalgia in females than males.

TRPV1 channel contributes to remifentanil-induced postoperative hyperalgesia via regulation of NMDA receptor trafficking in dorsal root ganglion

Background

Remifentanil is widely used in general anesthesia due to its reliability and rapid onset. However, remifentanil-induced postoperative hyperalgesia might be a challenge nowadays. Accumulating evidence suggests that the transient receptor potential vanilloid 1 (TRPV1) was involved in the development of neuropathic pain and hyperalgesia. However, the contribution of TRPV1 in modulating remifentanil-induced postoperative hyperalgesia is still unknown. The aim of this study is the contribution of TRPV1 to the surface expression of N-methyl-d-aspartate (NMDA) receptors in remifentanil-induced postoperative hyperalgesia.

Methods

The hot plate test and the Von Frey test were performed to evaluate thermal and mechanical hyperalgesia. Capsazepine (CPZ) was administrated intrathecally to confirm our results. TRPV1, NMDA receptors, CaMKII (calcium/calmodulin-dependent kinase II), and protein kinase C (PKC) in the dorsal root ganglion (DRG) were detected by Western blotting. Immunofluorescence assay was applied to analyze the distribution of TRPV1 and the relationship between TRPV1 and NMDA receptor subunit 1 (NR1).

Results

Remifentanil-induced both thermal and mechanical postoperative hyperalgesia. Here, we found the membrane trafficking of NR1, possibly due to the activation of TRPV1 in DRG neurons after remifentanil infusion. Furthermore, intrathecal injection of CPZ was able to relieve remifentanil-induced postoperative hyperalgesia according to a behavioral test and CPZ confirmed that TRPV1 is involved in NR1 trafficking. In addition, CaMKII/PKC but not protein kinase A (PKA) contributed to remifentanil-induced postoperative hyperalgesia.

Conclusion

Our study demonstrates that TRPV1 receptors are involved in remifentanil-induced postoperative hyperalgesia. TRPV1 contributes to the persistence of remifentanil-induced postoperative hyperalgesia through the trafficking of NMDA receptors via the activation of CaMKII-PKC signaling pathways in DRG neurons.

Roles of inflammation, neurogenic inflammation, and neuroinflammation in pain

Inflammation is the body's response to injury and infection, involving a complex biological response of the somatosensory, immune, autonomic, and vascular systems. Inflammatory mediators such as prostaglandin, proinflammatory cytokines, and chemokines induce pain via direct activation of nociceptors, the primary sensory neurons that detect noxious stimuli. Neurogenic inflammation is triggered by nerve activation and results in neuropeptide release and rapid plasma extravasation and edema, contributing to pain conditions such as headache. Neuroinflammation is a localized inflammation in the peripheral nervous system (PNS) and central nervous system (CNS). A characteristic feature of neuroinflammation is the activation of glial cells in dorsal root ganglia, spinal cord, and brain which leads to the production of proinflammatory cytokines and chemokines in the PNS and CNS that drives peripheral sensitization and central sensitization. Here, we discuss the distinct roles of inflammation, neurogenic inflammation, and neuroinflammation in the regulation of different types of pain conditions, with a special focus on neuroinflammation in postoperative pain and opioid-induced hyperalgesia.

An integrated review on new targets in the treatment of neuropathic pain

Neuropathic pain is a complex chronic pain state caused by the dysfunction of somatosensory nervous system, and it affects the millions of people worldwide. At present, there are very few medical treatments available for neuropathic pain management and the intolerable side effects of medications may further worsen the symptoms. Despite the presence of profound knowledge that delineates the pathophysiology and mechanisms leading to neuropathic pain, the unmet clinical needs demand more research in this field that would ultimately assist to ameliorate the pain conditions. Efforts are being made globally to explore and understand the basic molecular mechanisms responsible for somatosensory dysfunction in preclinical pain models. The present review highlights some of the novel molecular targets like D-amino acid oxidase, endoplasmic reticulum stress receptors, sigma receptors, hyperpolarization-activated cyclic nucleotide-gated cation channels, histone deacetylase, Wnt/β-catenin and Wnt/Ryk, ephrins and Eph receptor tyrosine kinase, Cdh-1 and mitochondrial ATPase that are implicated in the induction of neuropathic pain. Studies conducted on the different animal models and observed results have been summarized with an aim to facilitate the efforts made in the drug discovery. The diligent analysis and exploitation of these targets may help in the identification of some promising therapies that can better manage neuropathic pain and improve the health of patients.

Phosphorylation of extracellular signal-regulated kinase 1/2 in subepidermal nerve fibers mediates hyperalgesia following diabetic peripheral neuropathy

Peripheral neuropathy, a chronic complication of diabetes mellitus (DM), is often accompanied by the onset of severe pain symptoms that affect quality of life. However, the underlying mechanisms remain elusive. In the present study, we used Sprague-Dawley rats to establish a rodent model of the human type 1 DM by a single intraperitoneal (i.p.) injection with streptozotocin (STZ) (60 mg/kg). Hypersensitivity, including hyperalgesia and allodynia, developed in the STZ-induced diabetic rats. Cutaneous innervation exhibited STZ-induced reductions of protein gene product 9.5-, peripherin-, and neurofilament 200-immunoreactivity (IR) subepidermal nerve fibers (SENFs). Moreover, the decreases of substance P (SP)- and calcitonin gene-related peptide (CGRP)-IR SENFs were distinct gathered from the results of extracellular signal-regulated kinase 1 and 2 (ERK1/2)- and phosphorylated ERK1/2 (pERK1/2)-IR SENFs in STZ-induced diabetic rats. Double immunofluorescence studies demonstrated that STZ-induced pERK1/2-IR was largely increased in SENFs where only a small portion was colocalized with SP- or CGRP-IR. By an intraplantar (i. pl.) injection with a MEK inhibitor, U0126 (1,4-Diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), hyperalgesia was attenuated in a dose-responsive manner. Botulinum toxin serotype A had dose-dependent analgesic effects on STZ-induced hyperalgesia and allodynia, which exhibited equivalent results as the efficacy of transient receptor potential vanilloid (TRPV) channel antagonists. Morphological evidence further confirmed that STZ-induced SP-, CGRP- and pERK1/2-IR were reduced in SENFs after pharmacological interventions. From the results obtained in this study, it is suggested that increases of pERK1/2 in SENFs may participate in the modulation of TRPV channel-mediated neurogenic inflammation that triggers hyperalgesia in STZ-induced diabetic rats. Therefore, ERK1/2 provides a potential therapeutic target and efficient pharmacological strategies to address hyperglycemia-induced neurotoxicity.

Effects of glial glutamate transporter activator in formalin-induced pain behaviour in mice

BACKGROUND

Nociceptive pain remains a prevalent clinical problem and often poorly responsive to the currently available analgesics. Previous studies have shown that astroglial glutamate transporter-1 (GLT-1) in the hippocampus and anterior cingulate cortex (ACC) is critically involved in pain processing and modulation. However, the role of astroglial GLT-1 in nociceptive pain involving the hippocampus and ACC remains unknown. We investigated the role of 3-[[(2-Methylphenyl) methyl]thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, in nociceptive pain model and hippocampal-dependent behavioural tasks in mice.

METHODS

We evaluated the effects of LDN-212320 in formalin-induced nociceptive pain model. In addition, formalin-induced impaired hippocampal-dependent behaviours were measured using Y-maze and object recognition test. Furthermore, GLT-1 expression and extracellular signal-regulated kinase phosphorylation (pERK1/2) were measured in the hippocampus and ACC using Western blot analysis and immunohistochemistry.

RESULTS

The LDN-212320 (10 or 20 mg/kg, i.p) significantly attenuated formalin-evoked nociceptive behaviour. The antinociceptive effects of LDN-212320 were reversed by systemic administration of DHK (10 mg/kg, i.p), a GLT-1 antagonist. Moreover, LDN-212320 (10 or 20 mg/kg, i.p) significantly reversed formalin-induced impaired hippocampal-dependent behaviour. In addition, LDN-212320 (10 or 20 mg/kg, i.p) increased GLT-1 expressions in the hippocampus and ACC. On the other hand, LDN-212320 (20 mg/kg, i.p) significantly reduced formalin induced-ERK phosphorylation, a marker of nociception, in the hippocampus and ACC.

CONCLUSION

These results suggest that the GLT-1 activator LDN-212320 prevents nociceptive pain by upregulating astroglial GLT-1 expression in the hippocampus and ACC. Therefore, GLT-1 activator could be a novel drug candidate for nociceptive pain.

SIGNIFICANCE

The present study provides new insights and evaluates the role of GLT-1 activator in the modulation of nociceptive pain involving hippocampus and ACC. Here, we provide evidence that GLT-1 activator could be a potential therapeutic utility for the treatment of nociceptive pain.

5-HT2A Receptor-Induced Morphological Reorganization of PKCγ-Expressing Interneurons Gates Inflammatory Mechanical Allodynia in Rat

Mechanical allodynia, a widespread pain symptom that still lacks effective therapy, is associated with the activation of a dorsally directed polysynaptic circuit within the spinal dorsal horn (SDH) or medullary dorsal horn (MDH), whereby tactile inputs into deep SDH/MDH can gain access to superficial SDH/MDH, eliciting pain. Inner lamina II (II_i) interneurons expressing the γ isoform of protein kinase C (PKCγ⁺) are key elements for allodynia circuits, but how they operate is still unclear. Combining behavioral, ex vivo electrophysiological, and morphological approaches in an adult rat model of facial inflammatory pain (complete Freund's adjuvant, CFA), we show that the mechanical allodynia observed 1 h after CFA injection is associated with the following (1) sensitization (using ERK1/2 phosphorylation as a marker) and (2) reduced dendritic arborizations and enhanced spine density in exclusively PKCγ⁺ interneurons, but (3) depolarized resting membrane potential (RMP) in all lamina II_i PKCγ⁺/PKCγ^- interneurons. Blocking MDH 5HT_2A receptors (5-HT_2AR) prevents facial mechanical allodynia and associated changes in the morphology of PKCγ⁺ interneurons, but not depolarized RMP in lamina II_i interneurons. Finally, activation of MDH 5-HT_2AR in naive animals is enough to reproduce the behavioral allodynia and morphological changes in PKCγ⁺ interneurons, but not the electrophysiological changes in lamina II_i interneurons, induced by facial inflammation. This suggests that inflammation-induced mechanical allodynia involves strong morphological reorganization of PKCγ⁺ interneurons via 5-HT_2AR activation that contributes to open the gate for transmission of innocuous mechanical inputs to superficial SDH/MDH pain circuitry. Preventing 5-HT_2AR-induced structural plasticity in PKCγ⁺ interneurons might represent new avenues for the specific treatment of inflammation-induced mechanical hypersensitivity.SIGNIFICANCE STATEMENT Inflammatory or neuropathic pain syndromes are characterized by pain hypersensitivity such as mechanical allodynia (pain induced by innocuous mechanical stimuli). It is generally assumed that mechanisms underlying mechanical allodynia, because they are rapid, must operate at only the level of functional reorganization of spinal or medullary dorsal horn (MDH) circuits. We discovered that facial inflammation-induced mechanical allodynia is associated with rapid and strong structural remodeling of specifically interneurons expressing the γ isoform of protein kinase C (PKCγ) within MDH inner lamina II. Moreover, we elucidated a 5-HT_2A receptor to PKCγ/ERK1/2 pathway leading to the behavioral allodynia and correlated morphological changes in PKCγ interneurons. Therefore, descending 5-HT sensitize PKCγ interneurons, a putative "gate" in allodynia circuits, via 5-HT_2A receptor-induced structural reorganization.

HCN2 contributes to oxaliplatin-induced neuropathic pain through activation of the CaMKII/CREB cascade in spinal neurons

Emerging evidence showed that hyperpolarization-activated cation channels (HCN) participate in the development of inflammatory and neuropathic pain. However, the role of HCN2 in oxaliplatin-induced neuropathic pain remains unknown. Here, we found that HCN2 expression was upregulated in a rat model of oxaliplatin-induced neuropathic pain. Intrathecal injection of ZD7288, an HCN specific inhibitor, decreased the HCN2 level, as well as weakened the neuropathic pain behaviors compared to naive rats. Besides, mechanistic studies revealed that the expression of the spinal N-methyl-D-aspartate receptor subunit 2B was increased after oxaliplatin administration and was reduced by ZD7288 administration. The nociceptive behaviors were reversed by NR2B antagonist Ro 25–6981 in HCN2-overexpression rats. Furthermore, the underlying cellular mechanism demonstrated that ZD7288 administration restrained the enhanced activation of the neuronal calcium–calmodulin-dependent kinase II (CaMKII)/cyclic adenosine monophosphate response element-binding protein cascade after oxaliplatin administration. Moreover, pretreatment of CaMKII inhibitor KN-93 suppressed the nociceptive behaviors, as well as NR2B upregulation induced by overexpression of HCN2. In a word, HCN2 is conducive to oxaliplatin-induced neuropathic pain by activating the neuronal CaMKII/CREB cascade.

Analyses of gene expression profiles in the rat dorsal horn of the spinal cord using RNA sequencing in chronic constriction injury rats

Background

Neuropathic pain is caused by damage to the nervous system, resulting in aberrant pain, which is associated with gene expression changes in the sensory pathway. However, the molecular mechanisms are not fully understood.

Methods

Wistar rats were employed for the establishment of the chronic constriction injury (CCI) models. Using the Illumina HiSeq 4000 platform, we examined differentially expressed genes (DEGs) in the rat dorsal horn by RNA sequencing (RNA-seq) between CCI and control groups. Then, enrichment analyses were performed for these DEGs using Gene Ontology (GO) function, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway, Hierarchical Cluster, and protein-protein interaction (PPI) network.

Results

A total of 63 DEGs were found significantly changed with 56 upregulated (e.g., Cxcl13, C1qc, Fcgr3a) and 7 downregulated (e.g., Dusp1) at 14 days after CCI. Quantitative reverse-transcribed PCR (qRT-PCR) verified changes in 13 randomly selected DEGs. GO and KEGG biological pathway analyses showed that the upregulated DEGs were mostly enriched in immune response-related biological processes, as well as 14 immune- and inflammation-related pathways. The downregulated DEGs were enriched in inactivation of mitogen-activated protein kinase (MAPK) activity. PPI network analysis showed that Cd68, C1qc, C1qa, Laptm5, and Fcgr3a were crucial nodes with high connectivity degrees. Most of these genes which have previously been linked to immune and inflammation-related pathways have not been reported in neuropathic pain (e.g., Laptm5, Fcgr3a).

Conclusions

Our results revealed that immune and defense pathways may contribute to the generation of neuropathic pain after CCI. These mRNAs may represent new therapeutic targets for the treatment of neuropathic pain.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1316-0) contains supplementary material, which is available to authorized users.

Neuroinflammation and Central Sensitization in Chronic and Widespread Pain

Chronic pain is maintained in part by central sensitization, a phenomenon of synaptic plasticity, and increased neuronal responsiveness in central pain pathways after painful insults. Accumulating evidence suggests that central sensitization is also driven by neuroinflammation in the peripheral and central nervous system. A characteristic feature of neuroinflammation is the activation of glial cells, such as microglia and astrocytes, in the spinal cord and brain, leading to the release of proinflammatory cytokines and chemokines. Recent studies suggest that central cytokines and chemokines are powerful neuromodulators and play a sufficient role in inducing hyperalgesia and allodynia after central nervous system administration. Sustained increase of cytokines and chemokines in the central nervous system also promotes chronic widespread pain that affects multiple body sites. Thus, neuroinflammation drives widespread chronic pain via central sensitization. We also discuss sex-dependent glial/immune signaling in chronic pain and new therapeutic approaches that control neuroinflammation for the resolution of chronic pain.

Effects of Different Parameters of Continuous Training and High-Intensity Interval Training in the Chronic Phase of a Mouse Model of Complex Regional Pain Syndrome Type I

This study evaluated the effects of continuous and interval running on a treadmill on mechanical hyperalgesia in an animal model of chronic postischemia pain and analyzed the mechanism of action of this effect. Different groups of male Swiss mice with chronic postischemia pain, induced by 3 hours of paw ischemia followed by reperfusion, ran on the treadmill in different protocols-the speed (10, 13, 16, or 19 m/min), duration (15, 30, or 60 minutes), weekly frequency (3 or 5 times), weekly increase in continuous and interval running speed-were tested. Mechanical hyperalgesia was evaluated by von Frey filament 7, 14, and 21 days after paw ischemia followed by reperfusion. On day 11 after paw ischemia followed by reperfusion and after 5 days of continuous and interval running, concentrations of cytokines, oxidative stress parameters, and extracellular signal-regulated kinase 1/2 and AKT 1/2/3 expression in the spinal cord were measured. The results showed that continuous running has an antihyperalgesic effect that depends on intensity and volume. Interval running has a longer-lasting antihyperalgesic effect than continuous running. The antihyperalgesic effect depends on intensity and volume in continuous running, and increasing speed maintains the antihyperalgesic effect in both protocols. In the spinal cord, both runs decreased tumor necrosis factor-α and interleukin-6 levels and increased interleukin-10. Both running protocols reduced oxidative damage in the spinal cord. Only interval running had lower concentrations of phosphorylated extracellular signal-regulated kinase 1/2 in the spinal cord. Interval running presented a great antihyperalgesic potential with more promising results than continuous running, which may be owing to the fact that the interval running can activate different mechanisms from those activated by continuous running. PERSPECTIVE: A minimum of .5-hour sessions of moderate to high intensity ≥3 times a week are essential parameters for continuous and interval running-induced analgesia. However, interval running was shown to be more effective than continuous running and can be an important adjuvant treatment to chronic pain.