Endogenous interleukin-1β in neuropathic rats enhances glutamate release from the primary afferents in the spinal dorsal horn through coupling with presynaptic N-methyl-D-aspartic acid receptors

Euchromatin histone-lysine N-methyltransferase 2 regulates the expression of potassium-sodium-activated channel subfamily T member 1 in primary sensory neurons and contributes to remifentanil-induced pain sensitivity

Intraoperative remifentanil administration has been linked to increased postoperative pain sensitivity. Recent studies have identified the involvement of euchromatic histone-lysine N-methyltransferase 2 (Ehmt2/G9a) in neuropathic pain associated with the transcriptional silencing of many potassium ion channel genes. This study investigates whether G9a regulates the potassium sodium-activated channel subfamily T member 1 (Slo2.2) in remifentanil-induced post-incisional hyperalgesia (RIH) in rodents. We performed remifentanil infusion (1 μg·kg-1·min-1 for 60 min) followed by plantar incision to induce RIH in rodents. Our results showed that RIH was accompanied by increased G9a and H3K9me2 production and decreased Slo2.2 expression 48 h postoperatively. Deletion of G9a rescued Slo2.2 expression in DRG and reduced RIH intensity. Slo2.2 overexpression also reversed this hyperalgesia phenotype. G9a overexpression decreased Slo2.2-mediated leak current and increased excitability in the small-diameter DRG neurons and laminal II small-diameter neurons in the spinal dorsal horn, which was implicated in peripheral and central sensitization. These results suggest that G9a contributes to the development of RIH by epigenetically silencing Slo2.2 in DRG neurons, leading to decreased central sensitization in the spinal cord. The findings may have implications for the development of novel therapeutic targets for the treatment of postoperative pain.

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.

Dysregulation of sphingolipid metabolism in pain

Pain is a clinical condition that is currently of great concern and is often caused by tissue or nerve damage or occurs as a concomitant symptom of a variety of diseases such as cancer. Severe pain seriously affects the functional status of the body. However, existing pain management programs are not fully satisfactory. Therefore, there is a need to delve deeper into the pathological mechanisms underlying pain generation and to find new targets for drug therapy. Sphingolipids (SLs), as a major component of the bilayer structure of eukaryotic cell membranes, also have powerful signal transduction functions. Sphingolipids are abundant, and their intracellular metabolism constitutes a huge network. Sphingolipids and their various metabolites play significant roles in cell proliferation, differentiation, apoptosis, etc., and have powerful biological activities. The molecules related to sphingolipid metabolism, mainly the core molecule ceramide and the downstream metabolism molecule sphingosine-1-phosphate (S1P), are involved in the specific mechanisms of neurological disorders as well as the onset and progression of various types of pain, and are closely related to a variety of pain-related diseases. Therefore, sphingolipid metabolism can be the focus of research on pain regulation and provide new drug targets and ideas for pain.

NLRs and inflammasome signaling in opioid-induced hyperalgesia and tolerance

We investigated the role that innate immunological signaling pathways, principally nod-like receptors (NLRs) and inflammasomes, in the manifestation of the contradictory outcomes associated with opioids, namely hyperalgesia, and tolerance. The utilization of opioids for pain management is prevalent; nonetheless, it frequently leads to an increased sensitivity to pain (hyperalgesia) and reduced efficacy of the medication (tolerance) over an extended period. This, therefore, represents a major challenge in the area of chronic pain treatment. Recent studies indicate that the aforementioned negative consequences are partially influenced by the stimulation of NLRs, specifically the NLRP3 inflammasome, and the subsequent assembly of the inflammasome. This process ultimately results in the generation of inflammatory cytokines and the occurrence of neuroinflammation and the pathogenesis of hyperalgesia. We also explored the putative downstream signaling cascades activated by NOD-like receptors (NLRs) and inflammasomes in response to opioid stimuli. Furthermore, we probed potential therapeutic targets for modifying opioid-induced hyperalgesia, with explicit emphasis on the activation of the NLRP3 inflammasome. Ultimately, our findings underscore the significance of conducting additional research in this area that includes an examination of the involvement of various NLRs, immune cells, and genetic variables in the development of opioid-induced hyperalgesia and tolerance. The present review provides substantial insight into the possible pathways contributing to the occurrence of hyperalgesia and tolerance in individuals taking opioids.

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

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

Chinese Tuina remodels the synaptic structure in neuropathic pain rats by downregulating the expression of N-methyl D-aspartate receptor subtype 2B and postsynaptic density protein-95 in the spinal cord dorsal horn

OBJECTIVE

To investigate whether the Chinese massage system, Tuina, exerts analgesic effects in a rat model of chronic constriction injury (CCI) by remodeling the synaptic structure in the spinal cord dorsal horn (SCDH).

METHODS

Sixty-nine male Sprague-Dawley rats were randomly and evenly divided into the normal group, sham group, CCI group, CCI + Tuina group, CCI + MK-801 [an -methyl D-aspartate receptor subtype 2B (NR2B) antagonist] group, and CCI + MK-801 + Tuina group. The neuropathic pain model was established using CCI with right sciatic nerve ligation. Tuina was administered 4 d after CCI surgery, using pressing manipulation for 10 min, once daily. Motor function was observed with the inclined plate test, and pain behaviors were observed by the Von Frey test and acetone spray test. At 19 d after surgery, the L3-L5 spinal cord segments were removed. Glutamate, interleukin 1β (IL-1β), and tumor necrosis factor-α (TNF-α) levels were detected by enzyme-linked immunosorbent assay. The protein expression levels of NR2B and postsynaptic density protein-95 (PSD-95) were detected by Western blot, and the synaptic structure was observed by transmission electron microscopy (TEM).

RESULTS

CCI reduced motor function and caused mechanical and cold allodynia in rats, increased glutamate concentration and TNF-α and IL-1β levels, and increased expression of synapse-related proteins NR2B and PSD-95 in the SCDH. TEM revealed that the synaptic structure of SCDH neurons was altered. Most of these disease-induced changes were reversed by Tuina and intrathecal injection of MK-801 ( < 0.05 or < 0.01). For the majority of experiments, no significant differences were found between the CCI + MK-801 and CCI + MK-801 + Tuina groups.

CONCLUSIONS

Chinese Tuina can alleviate pain by remodeling the synaptic structure, and NR2B and PSD-95 receptors in the SCDH may be among its targets.

Soluble epoxide hydrolase inhibitor blockage microglial cell activation in subnucleus caudalis in a persistent model of arthritis

Rheumatoid arthritis (RA) is a chronic condition characterized by pain and infiltration of immune cells into the joint. Immune cells can be activated, producing inflammatory cytokines, leading to continuously degenerative and inflammatory reactions and the temporomandibular joint (TMJ) can be affected by RA. In this scenario, novel targets are needed to increase treatment efficacy with minimized side effects. The epoxy-eicosatrienoic acids (EETs), are endogenous signaling molecules, playing important roles in diminishing inflammation and pain but are promptly metabolized by soluble epoxide hydrolase (sEH), generating less-bioactive acids.Therefore, sEH inhibitors is an interest therapeutic target to enhance the beneficial effect of natural EETs. TPPU is a potent sEH inhibitor that is capable of dampening EETs hydrolysis. Thus, we aimed to assess the impact of pharmacological sEH inhibition on a persistent model of albumin-induced arthritis in the TMJ, in two scenarios: first, as post-treatment, in an installed arthritic condition, and second, the protective role, in preventing the development of an arthritic condition. In addition, we investigate the influence of sEH inhibition on microglia cell activation in the trigeminal subnucleus caudalis (TSC) and in vitro experiments. Finally, we examined the astrocyte phenotype. Oral administration of TPPU, acts in multiple pathways, in a protective and reparative post-treatment, ameliorating the preservation of the TMJ morphology, reducing the hypernociception, with an immunosuppressive action reducing neutrophil and lymphocytes and pro-inflammatory cytokines in the TMJ of rats. In TSC, TPPU reduces the cytokine storm and attenuates the microglia activated P2X7/Cathepsin S/Fractalkine pathway and reduces the astrocyte activation and glutamate levels. Collectively, our findings revealed that sEH inhibition mitigates hypersensitive nociception through the regulation of microglia activation and astrocyte modulation, demonstrating the potential use of sEH inhibitors as immunoresolvents in the treatment of autoimmune disorders.

Activation of Protease-Activated Receptor-1 Causes Chronic Pain in Lupus-Prone Mice Via Suppressing Spinal Glial Glutamate Transporter Function and Enhancing Glutamatergic Synaptic Activity

Systemic lupus erythematosus (SLE) is an unpredictable autoimmune disease where the body's immune system mistakenly attacks healthy tissues in many parts of the body. Chronic pain is one of the most frequently reported symptoms among SLE patients. We previously reported that MRL lupus prone (MRL/lpr) mice develop hypersensitivity to mechanical and heat stimulation. In the present study, we found that the spinal protease-activated receptor-1(PAR1) plays an important role in the genesis of chronic pain in MRL/lpr mice. Female MRL/lpr mice with chronic pain had activation of astrocytes, over-expression of thrombin and PAR1, enhanced glutamatergic synaptic activity, as well as suppressed activity of adenosine monophosphate-activated protein kinase (AMPK) and glial glutamate transport function in the spinal cord. Intrathecal injection of either the PAR1 antagonist, or AMPK activator attenuated heat hyperalgesia and mechanical allodynia in MRL/lpr mice. Furthermore, we also identified that the enhanced glutamatergic synaptic activity and suppressed activity of glial glutamate transporters in the spinal dorsal horn of MRL/lpr mice are caused by activation of the PAR1 and suppression of AMPK signaling pathways. These findings suggest that targeting the PAR1 and AMPK signaling pathways in the spinal cord may be a useful approach for treating chronic pain caused by SLE. PERSPECTIVE: Our study provides evidence suggesting activation of PAR1 and suppression of AMPK in the spinal cord induces thermal hyperalgesia and mechanical allodynia in a lupus mouse model. Targeting signaling pathways regulating the PAR1 and AMPK could potentially provide a novel approach to the management of chronic pain caused by SLE.

Glial Glutamate Transporter Modulation Prevents Development of Complete Freund's Adjuvant-Induced Hyperalgesia and Allodynia in Mice

Glial glutamate transporter (GLT-1) modulation in the hippocampus and anterior cingulate cortex (ACC) is critically involved in nociceptive pain. The objective of the study was to investigate the effects of 3-[[(2-methylphenyl) methyl] thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, against microglial activation induced by complete Freund's adjuvant (CFA) in a mouse model of inflammatory pain. Furthermore, the effects of LDN-212320 on the protein expression of glial markers, such as ionized calcium-binding adaptor molecule 1 (Iba1), cluster of differentiation molecule 11b (CD11b), mitogen-activated protein kinases (p38), astroglial GLT-1, and connexin 43 (CX43), were measured in the hippocampus and ACC following CFA injection using the Western blot analysis and immunofluorescence assay. The effects of LDN-212320 on the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus and ACC were also assessed using an enzyme-linked immunosorbent assay. Pretreatment with LDN-212320 (20 mg/kg) significantly reduced the CFA-induced tactile allodynia and thermal hyperalgesia. The anti-hyperalgesic and anti-allodynic effects of LDN-212320 were reversed by the GLT-1 antagonist DHK (10 mg/kg). Pretreatment with LDN-212320 significantly reduced CFA-induced microglial Iba1, CD11b, and p38 expression in the hippocampus and ACC. LDN-212320 markedly modulated astroglial GLT-1, CX43, and, IL-1β expression in the hippocampus and ACC. Overall, these results suggest that LDN-212320 prevents CFA-induced allodynia and hyperalgesia by upregulating astroglial GLT-1 and CX43 expression and decreasing microglial activation in the hippocampus and ACC. Therefore, LDN-212320 could be developed as a novel therapeutic drug candidate for chronic inflammatory pain.

Emerging roles of GPR109A in regulation of neuroinflammation in neurological diseases and pain

Neuroinflammation plays a critical role in the pathological process of multiple neurological disorders and pathological pain conditions. GPR109A, a Gi protein-coupled receptor, has emerged as an important therapeutic target for controlling inflammation in various tissues and organs. In this review, we summarized current data about the role of GPR109A in neuroinflammation. Specifically, we focused on the pharmacological features of GPR109A and signaling pathways used by GPR109A to ameliorate neuroinflammation and symptoms in Alzheimer's disease, Parkinson's disease, multiple sclerosis, stroke, and pathological pain conditions.

EZH2 Methyltransferase Regulates Neuroinflammation and Neuropathic Pain

Recent studies by us and others have shown that enhancer of zeste homolog-2 (EZH2), a histone methyltransferase, in glial cells regulates the genesis of neuropathic pain by modulating the production of proinflammatory cytokines and chemokines. In this review, we summarize recent advances in this research area. EZH2 is a subunit of polycomb repressive complex 2 (PRC2), which primarily serves as a histone methyltransferase to catalyze methylation of histone 3 on lysine 27 (H3K27), ultimately resulting in transcriptional repression. Animals with neuropathic pain exhibit increased EZH2 activity and neuroinflammation of the injured nerve, spinal cord, and anterior cingulate cortex. Inhibition of EZH2 with DZNep or GSK-126 ameliorates neuroinflammation and neuropathic pain. EZH2 protein expression increases upon activation of Toll-like receptor 4 and calcitonin gene-related peptide receptors, downregulation of miR-124-3p and miR-378 microRNAs, or upregulation of Lncenc1 and MALAT1 long noncoding RNAs. Genes suppressed by EZH2 include suppressor of cytokine signaling 3 (SOCS3), nuclear factor (erythroid-derived 2)-like-2 factor (NrF2), miR-29b-3p, miR-146a-5p, and brain-specific angiogenesis inhibitor 1 (BAI1). Pro-inflammatory mediators facilitate neuronal activation along pain-signaling pathways by sensitizing nociceptors in the periphery, as well as enhancing excitatory synaptic activities and suppressing inhibitory synaptic activities in the CNS. These studies collectively reveal that EZH2 is implicated in signaling pathways known to be key players in the process of neuroinflammation and genesis of neuropathic pain. Therefore, targeting the EZH2 signaling pathway may open a new avenue to mitigate neuroinflammation and neuropathic pain.

Therapeutic potential of progesterone in spinal cord injury-induced neuropathic pain: At the crossroads between neuroinflammation and N-methyl-D-aspartate receptor

In recent decades, an area of active research has supported the notion that progesterone promotes a wide range of remarkable protective actions in experimental models of nervous system trauma or disease, and has also provided a strong basis for considering this steroid as a promising molecule for modulating the complex maladaptive changes that lead to neuropathic pain, especially after spinal cord injury. In this review, we intend to give the readers a brief appraisal of the main mechanisms underlying the increased excitability of the spinal circuit in the pain pathway after trauma, with particular emphasis on those mediated by the activation of resident glial cells, the subsequent release of proinflammatory cytokines and their impact on N-methyl-D-aspartate receptor function. We then summarize the available preclinical data pointing to progesterone as a valuable repurposing molecule for blocking critical cellular and molecular events that occur in the dorsal horn of the injured spinal cord and are related to the development of chronic pain. Since the treatment and management of neuropathic pain after spinal injury remains challenging, the potential therapeutic value of progesterone opens new traslational perspectives to prevent central pain.

NFAT1 Orchestrates Spinal Microglial Transcription and Promotes Microglial Proliferation via c-MYC Contributing to Nerve Injury-Induced Neuropathic Pain

Peripheral nerve injury-induced spinal microglial proliferation plays a pivotal role in neuropathic pain. So far, key intracellular druggable molecules involved in this process are not identified. The nuclear factor of activated T-cells (NFAT1) is a master regulator of immune cell proliferation. Whether and how NFAT1 modulates spinal microglial proliferation during neuropathic pain remain unknown. Here it is reported that NFAT1 is persistently upregulated in microglia after spinal nerve ligation (SNL), which is regulated by TET2-mediated DNA demethylation. Global or microglia-specific deletion of Nfat1 attenuates SNL-induced pain and decreases excitatory synaptic transmission of lamina II neurons. Furthermore, deletion of Nfat1 decreases microglial proliferation and the expression of multiple microglia-related genes, such as cytokines, transmembrane signaling receptors, and transcription factors. Particularly, SNL increases the binding of NFAT1 with the promoter of Itgam, Tnf, Il-1b, and c-Myc in the spinal cord. Microglia-specific overexpression of c-MYC induces pain hypersensitivity and microglial proliferation. Finally, inhibiting NFAT1 and c-MYC by intrathecal injection of inhibitor or siRNA alleviates SNL-induced neuropathic pain. Collectively, NFAT1 is a hub transcription factor that regulates microglial proliferation via c-MYC and guides the expression of the activated microglia genome. Thus, NFAT1 may be an effective target for treating neuropathic pain.

Gama-aminobutyric acid (GABA) alleviates hepatic inflammation via GABA receptors/TLR4/NF-κB pathways in growing-finishing pigs generated by super-multiparous sows

The offspring of super-multiparous sows face problems such as decreased growth performance, poor meat quality and even diseases in animal husbandry. Gama-aminobutyric acid (GABA) has long been known to promote growth and suppress inflammation, but little is known about the mechanisms. A total of 72 growing-finishing pigs from the 8th generation were randomly allotted to 2 groups with 6 replicates per treatment to receive a corn–soybean basal diet or the basal diet supplemented 20 mg/kg GABA for 60 d. After the animal-trial period, samples of serum and liver were collected for further analysis. Additionally, a lipopolysaccharide (LPS)-induced inflammatory model using HepG2 cells was established to explore the role of GABA on regulating hepatic inflammation. The results indicated that inflammatory cell infiltration occurs in the liver of progeny of super-multiparous sows, and dietary supplementation with GABA influenced liver morphology, increased activities of antioxidant enzymes and decreased the expression abundance of pro-inflammatory cytokines, including tumor necrosis factor-α (TNFα) and interleukin (IL)-1β, in the liver of growing-finishing pigs (P < 0.05). In addition, GABA supplementation increased mRNA expressions of peroxisome proliferator-activated receptor γ (PPARγ) and GABA receptors (GABARs), and reduced the expression of toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling (P < 0.05). Additionally, an in vitro experiment demonstrated that GABA decreased the expressions of hepatic TLR4/NF-κB signaling via activating GABARs under LPS-stress (P < 0.05). In summary, liver injury may affect the growth performance of growing-finishing pigs by changing hepatic mitochondrial metabolism, the expression of pro-inflammatory cytokines and TLR4/NF-κB pathway and that GABA supplementation has a restorative effect by acting on GABARs.

Phytohormone abscisic acid ameliorates neuropathic pain via regulating LANCL2 protein abundance and glial activation at the spinal cord

Spinal neuroinflammation plays a critical role in the genesis of neuropathic pain. Accumulating data suggest that abscisic acid (ABA), a phytohormone, regulates inflammatory processes in mammals. In this study, we found that reduction of the LANCL2 receptor protein but not the agonist ABA in the spinal cord is associated with the genesis of neuropathic pain. Systemic or intrathecal administration of ABA ameliorates the development and pre-existence of mechanical allodynia and heat hyperalgesia in animals with partial sciatic nerve ligation (pSNL). LANCL2 is expressed only in microglia in the spinal dorsal horn. Pre-emptive treatment with ABA attenuates activation of microglia and astrocytes, ERK activity, and TNFα protein abundance in the dorsal horn in rats with pSNL. These are accompanied by restoration of spinal LANCL2 protein abundance. Spinal knockdown of LANCL2 gene with siRNA recapitulates the behavioral and spinal molecular changes induced by pSNL. Activation of spinal toll-like receptor 4 (TLR4) with lipopolysaccharide leads to activation of microglia, and over production of TNFα, which are concurrently accompanied by suppression of protein levels of LANCL2 and peroxisome proliferator activated-receptor γ. These changes are ameliorated when ABA is added with LPS. The anti-inflammatory effects induced by ABA do not requires G_i protein activity. Our study reveals that the ABA/LANCL2 system is a powerful endogenous system regulating spinal neuroinflammation and nociceptive processing, suggesting the potential utility of ABA as the management of neuropathic pain.

Effective Oriental Magic for Analgesia: Acupuncture

Pain is a kind of complex physiological and psychological symptom, which makes the person debilitated and uncomfortable. Some persistent pain is unbearable for the patients, reducing the quality of life and bringing considerable pressure to the individuals and society. Pain killers seem to be effective in analgesia for patients, but their safety and addiction are crucial issues. From the theory of traditional Chinese medicine (TCM), the blocked meridian is the main cause of pain, and effective acupuncture can play a positive analgesic effect. Acupuncture that can date back thousands of years is one of the ancient medical practices in China. Its safety and effectiveness are respected. Based on its superior safety and inferior side effects, it has been gradually recognized as a therapeutic intervention method for complementary medicine, which is also generally used to treat multiple pain diseases. It is shown by modern medical studies that neurotransmitters are the material basis for the acupuncture effect, and the effect of acupuncture analgesia is related to changes in neurotransmitters. However, the specific mechanism has not been elucidated. This review aims to comprehensively discuss the historical evolution of acupuncture analgesia, clinical research of acupuncture analgesia, comparison of acupuncture and drug therapy, the neurotransmitter mechanism of acupuncture analgesia, the effect of acupuncture manipulation on analgesia, and bibliometric analysis of acupuncture treatment for pain, to explore the superiority and related mechanism of acupuncture analgesia from different aspects, and to provide a more effective treatment for alleviating patients' pain.

Fractalkine/CX3CR1 Pathway in Neuropathic Pain: An Update

Injuries to the nervous system can result in a debilitating neuropathic pain state that is often resistant to treatment with available analgesics, which are commonly associated with several side-effects. Growing pre-clinical and clinical evidence over the last two decades indicates that immune cell-mediated mechanisms both in the periphery and in the Central Nervous System (CNS) play significant roles in the establishment and maintenance of neuropathic pain. Specifically, following peripheral nerve injury, microglia, which are CNS resident immune cells, respond to the activity of the first pain synapse in the dorsal horn of spinal cord and also to neuronal activity in higher centres in the brain. This microglial response leads to the production and release of several proinflammatory mediators which contribute to neuronal sensitisation under neuropathic pain states. In this review, we collect evidence demonstrating the critical role played by the Fractalkine/CX₃CR₁ signalling pathway in neuron-to-microglia communication in neuropathic pain states and explore how strategies that include components of this pathway offer opportunities for innovative targets for neuropathic pain.

Effects of co-administration of vitamin E and lithium chloride on chronic constriction injury-induced neuropathy in male Wistar rats: Focus on antioxidant and anti-inflammatory mechanisms

OBJECTIVES

This study investigated the antinociceptive effects of co-administration of lithium chloride (LiCl) and vitamin E (Vit E) on chronic constriction injury (CCI)-induced peripheral neuropathy in male Wistar rats. It further explored the anti-inflammatory and neuroprotective properties of LiCl and Vit E, which may be complementary to the antinociceptive effects of the two substances.

METHODS

Thirty-six male Wistar rats, 190.00 ± 10.00 g of body weight were randomly assigned to 6 experimental groups and administered with normal saline, Vit E, LiCl, or their combination, once daily for 21 days. CCI was used to induce neuropathic pain (NP) and mechanical allodynia was assessed using von Frey filaments and pinprick test. Open field maze (OFM) was used to assess the exploratory behavior. Biochemical parameters were assessed in the dorsal root ganglion after 21 days of treatment.

RESULTS

Mechanical allodynia was developed in rats following CCI. Co-administration of LiCl and Vit E synergistically reduced mechanical hyperalgesia in rats which were significantly different compared with the single administration of either Vit E or LiCl. Combined doses of Vit E and LiCl significantly increases the explorative behavior in the OFM. CCI increased malondialdehyde (MDA), tumor necrotic factor-alpha (TNF-α), calcitonin gene-related polypeptide, calcium ion (Ca2+ ), and reduced superoxide dismutase (SOD) activities. Co-administration of LiCl and Vit E significantly reduced MDA, TNF-α, but increased SOD compared with ligated control.

DISCUSSION

The findings revealed that the synergistic effects of the co-administration of Vit E and LiCl in ameliorating NP are mediated by their anti-inflammatory and antioxidant properties.

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

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

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.

Simultaneous hyperbaric oxygen therapy during systemic chemotherapy reverses chemotherapy-induced peripheral neuropathy by inhibiting TLR4 and TRPV1 activation in the central and peripheral nervous system

BACKGROUND AND OBJECTIVES

Chemotherapy-induced peripheral neuropathy (CIPN) is considered one of the most common sequelae in patients with cancer who experience consistent abnormal sensations or pain symptoms during or after paclitaxel (PAC) chemotherapy. Transient receptor potential vanilloid 1 (TRPV1) and toll-like receptor 4 (TLR4) have been reported to interact in the nervous system in patients with CIPN. The antinociceptive effects of hyperbaric oxygen therapy (HBOT) on CIPN was demonstrated in this study through behavior tests. Using a CIPN rat model, we examined the effects of simultaneous HBOT (SHBOT) administration during chemotherapy and discovered that SHBOT achieved better reversal effects than chemotherapy alone.

MATERIALS AND METHODS

Twenty-four rats were randomly allocated to four groups: control, PAC, SHBOT, and HBOT after PAC groups. Behavior tests were performed to evaluate mechanical allodynia and thermal hyperalgesia status. Tissues from the spinal cord and dorsal root ganglions were collected, and TLR4 and TRPV1 expression and microglial activation were investigated through immunofluorescence (IF) staining.

RESULTS

The mechanical and thermal behavior tests revealed that HBOT intervention during PAC treatment led to the early alleviation of CIPN symptoms and inhibited CIPN deterioration. IF staining revealed that TLR4, TRPV1, and microglial activation were all upregulated in PAC-injected rats and exhibited early and significant downregulation in SHBOT-treated rats.

CONCLUSION

This study is the first to demonstrate that the use of SHBOT during PAC treatment has potential for the early suppression of CIPN initiation and deterioration, indicating that it can alleviate CIPN symptoms and may reverse CIPN in patients undergoing systemic chemotherapy.

Novel Mechanism for Memantine in Attenuating Diabetic Neuropathic Pain in Mice via Downregulating the Spinal HMGB1/TRL4/NF-kB Inflammatory Axis

Diabetic neuropathic pain (DNP) is a common diabetic complication that currently lacks an efficient therapy. The aim of the current work was to uncover the anti-allodynic and neuroprotective effects of memantine in a model of mouse diabetic neuropathy and its ameliorative effect on the high-mobility group box-1 (HMGB1)/toll-like receptor 4 (TLR4)/nuclear factor-k B (NF-kB) inflammatory axis. Diabetes was prompted by an alloxan injection (180 mg/kg) to albino mice. On the ninth week after diabetes induction, DNP was confirmed. Diabetic mice were randomly allocated to two groups (six mice each); a diabetes mellitus (DM) group and DM+memantine group (10 mg/kg, daily) for five weeks. DNP-related behaviors were assessed in terms of thermal hyperalgesia and mechanical allodynia by hot-plate and von Frey filaments. Enzyme-linked immunosorbent assay (ELISA) kits were used to measure the spinal glutamate, interleukin-1 beta (IL-1β), and tumor necrosis factor-α (TNF-α). The spinal levels of N-methyl-D-aspartate type 1 receptor (NMDAR1), HMGB1, TLR4, and phosphorylated NF-kB were assessed using Western blotting. Histopathological investigation of the spinal cord and sciatic nerves, together with the spinal cord ultrastructure, was employed for assessment of the neuroprotective effect. Memantine alleviated pain indicators in diabetic mice and suppressed excessive NMDAR1 activation, glutamate, and pro-inflammatory cytokine release in the spinal cord. The current study validated the ability of memantine to combat the HMGB1/TLR4/NF-kB axis and modulate overactive glutamate spinal transmission, corroborating memantine as an appealing therapeutic target in DNP.

Characterization of proinflammatory markers in the ventral tegmental area across mouse models of chronic stress

Despite the high prevalence of major depressive disorder (MDD), understanding of the biological underpinnings remains limited. Rodent models suggest that changes in activity and output of dopamine (DA) neurons in the ventral tegmental area (VTA) are important for depressive-like phenotypes. Additionally, brain inflammatory processes are thought to contribute to MDD pathology and inflammation in the VTA has been linked to changes in VTA DA neuronal activity. Thus, we sought to determine whether there is increased inflammatory signaling in the VTA following forms of chronic stress that induce depressive-like symptoms. First, we subjected male mice to either physical or vicarious chronic social defeat stress (CSDS), paradigms known to induce long-term depressive-like behavior and changes in VTA signaling. Second, we subjected male and female mice to subchronic variable stress (SCVS), a paradigm that induces depressive-like behavior only in female mice. We then isolated mRNA from the VTA and assessed proinflammatory gene regulation via RT-PCR. Our results show that physical, but not vicarious, CSDS increases interleukin 1β (IL-1β) mRNA expression and this inversely correlates with social interaction score. In contrast, IL-1β expression was unchanged in male or female mice following SCVS. No significant increases in VTA ionized calcium binding adapter molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) immunochemistry were detected following CSDS that would be indicative of a robust inflammatory response. In conclusion, we show that chronic stressors distinctively alter expression of proinflammatory genes in the VTA and changes may depend on the severity and time-course of the stress exposure.

Neuronal interleukin-1 receptors mediate pain in chronic inflammatory diseases

Chronic pain is a major comorbidity of chronic inflammatory diseases. Here, we report that the cytokine IL-1β, which is abundantly produced during multiple sclerosis (MS), arthritis (RA), and osteoarthritis (OA) both in humans and in animal models, drives pain associated with these diseases. We found that the type 1 IL-1 receptor (IL-1R1) is highly expressed in the mouse and human by a subpopulation of TRPV1⁺ dorsal root ganglion neurons specialized in detecting painful stimuli, termed nociceptors. Strikingly, deletion of the Il1r1 gene specifically in TRPV1⁺ nociceptors prevented the development of mechanical allodynia without affecting clinical signs and disease progression in mice with experimental autoimmune encephalomyelitis and K/BxN serum transfer–induced RA. Conditional restoration of IL-1R1 expression in nociceptors of IL-1R1–knockout mice induced pain behavior but did not affect joint damage in monosodium iodoacetate–induced OA. Collectively, these data reveal that neuronal IL-1R1 signaling mediates pain, uncovering the potential benefit of anti–IL-1 therapies for pain management in patients with chronic inflammatory diseases.

Spinal NF-kB upregulation contributes to hyperalgesia in a rat model of advanced osteoarthritis

Knee osteoarthritis (OA) pain is the most common joint pain. Currently, dysfunction in the central nervous system rather than knee joint degeneration is considered to be the major cause of chronic knee OA pain; however, the underlying mechanism remains unknown. The aim of this study was to explore whether spinal NF-κB plays a critical role in chronic knee OA pain. In this study, we used a model induced by the intra-articular injection of monosodium iodoacetate. Spinal NF-κB and the phosphorylation and activation status of NF-κB p65/RelA (p-p65) were inhibited by the intrathecal injection of the inhibitor pyrrolidine dithiocarbamate in this model. After behavioral assessment, the knee was dissected for histopathology, and the spinal cord was dissected and examined for NF-κB, p-p65, and cytokine expression. Furthermore, the quantity and activity of neurons, astrocytes, and microglial cells and their colocalization with p-p65 in the spinal dorsal horn were investigated. Our findings included the following: (1) histology, the pathological changes in the joints of the knee OA model were basically consistent with knee OA patients; (2) the protein and transcription levels of NF-κB/p65 and p-p65 increased before day 14, appeared to decrease on day 21 and increased again on day 28, and the tendency of weight bearing was similar; (3) on days 21 and 28, the intrathecal injection of pyrrolidine dithiocarbamate markedly prevented the monosodium iodoacetate-induced reduction in the paw withdrawal threshold; (4) real-time polymerase chain reaction demonstrated that the expression of TNF-α and IL-33 was suppressed in the knee OA model by the intrathecal injection of pyrrolidine dithiocarbamate; and (5) immunofluorescence revealed that astrocytes were activated and that p-p65 was mainly increased in astrocytes. Our findings indicate that the spinal NF-κB/p65 pathway in astrocytes modulates neuroimmunity in rat model of intra-articular monosodium iodoacetate-induced advanced OA.

The Role of Neuroglial Crosstalk and Synaptic Plasticity-Mediated Central Sensitization in Acupuncture Analgesia

Although pain is regarded as a global public health priority, analgesic therapy remains a significant challenge. Pain is a hypersensitivity state caused by peripheral and central sensitization, with the latter considered the culprit for chronic pain. This study summarizes the pathogenesis of central sensitization from the perspective of neuroglial crosstalk and synaptic plasticity and underlines the related analgesic mechanisms of acupuncture. Central sensitization is modulated by the neurotransmitters and neuropeptides involved in the ascending excitatory pathway and the descending pain modulatory system. Acupuncture analgesia is associated with downregulating glutamate in the ascending excitatory pathway and upregulating opioids, 𝛾-aminobutyric acid, norepinephrine, and 5-hydroxytryptamine in the descending pain modulatory system. Furthermore, it is increasingly appreciated that neurotransmitters, cytokines, and chemokines are implicated in neuroglial crosstalk and associated plasticity, thus contributing to central sensitization. Acupuncture produces its analgesic action by inhibiting cytokines, such as interleukin-1β, interleukin-6, and tumor necrosis factor-α, and upregulating interleukin-10, as well as modulating chemokines and their receptors such as CX3CL1/CX3CR1, CXCL12/CXCR4, CCL2/CCR2, and CXCL1/CXCR2. These factors are regulated by acupuncture through the activation of multiple signaling pathways, including mitogen-activated protein kinase signaling (e.g., the p38, extracellular signal-regulated kinases, and c-Jun-N-terminal kinase pathways), which contribute to the activation of nociceptive neurons. However, the responses of chemokines to acupuncture vary among the types of pain models, acupuncture methods, and stimulation parameters. Thus, the exact mechanisms require future clarification. Taken together, inhibition of central sensitization modulated by neuroglial plasticity is central in acupuncture analgesia, providing a novel insight for the clinical application of acupuncture analgesia.

Bromelain reduced pro-inflammatory mediators as a common pathway that mediate antinociceptive and anti-anxiety effects in sciatic nerve ligated Wistar rats

The involvement of pro-inflammatory mediators complicates the complex mechanism in neuropathic pain (NP). This study investigated the roles of bromelain against pro-inflammatory mediators as a mechanism that underpins its antinociceptive and anti-anxiety effects in the peripheral model of NP. Sixty-four male Wistar rats randomly divided into eight groups, were used for the study. A chronic constriction injury model of peripheral neuropathy was used to induce NP. Tail-immersion and von Frey filaments tests were used to assess hyperalgesia while open field and elevated plus mazes were used to assess anxiety-like behaviour. NF-кB, iNOS, nitrate, and pro-inflammatory cytokines were investigated in the plasma, sciatic nerve, and brain tissues using ELISA, spectrophotometer, and immunohistochemistry techniques after twenty-one days of treatment. Bromelain significantly (p < 0.05) improved the cardinal signs of NP and inhibited anxiety-like behaviours in ligated Wistar rats. It mitigated the increases in cerebral cortex interleukin (IL) -1β, IL-6, and PGE₂ levels. Bromelain reduced NF-кB, IL-1β, IL-6, TNF-α, PGE_2, and nitrate concentrations as well as the expression of iNOS in the sciatic nerve. Hence, the antinociceptive and anxiolytic effects of bromelain in the sciatic nerve ligation model of NP is in part due to its ability to reduce nitrosative and inflammatory activities.

Antinociceptive effects of lead acetate in sciatic nerve chronic constriction injury model of peripheral neuropathy in male Wistar rats

The toxicological effects of lead and its compounds have overshadowed its possible health beneficial effects. Currently, the success rate for treating neuropathic pain has been very low. This study investigated the antinociceptive effects of orally administered low dose lead acetate in sciatic nerve ligated Wistar rats. Thirty Wistar rats randomly divided into five groups were used for this study. Chronic constriction injury (CCI) was used to induce neuropathic pain in Wistar rats. Allodynic and hyperalgesic signs were investigated using von Frey filaments and hotplate, respectively. Morris water maze test was used to assess the memory functions of the rats. The study revealed that oral administration of low-dose lead acetate significantly (p < 0.05) increased pain thresholds of ligated rats. CCI enhanced memory function in Wistar rats which was significantly decreased following lead acetate administration. The findings suggest that lead acetate possesses antinociceptive effects in peripherally induced neuropathic pain model in Wistar rats.

Sphingosine-1-phosphate receptor subtype 1 activation in the central nervous system contributes to morphine withdrawal in rodents

Opioid therapies for chronic pain are undermined by many adverse side effects that reduce their efficacy and lead to dependence, abuse, reduced quality of life, and even death. We have recently reported that sphingosine-1-phosphate (S1P) 1 receptor (S1PR1) antagonists block the development of morphine-induced hyperalgesia and analgesic tolerance. However, the impact of S1PR1 antagonists on other undesirable side effects of opioids, such as opioid-induced dependence, remains unknown. Here, we demonstrate that naloxone-precipitated morphine withdrawal in mice altered de novo sphingolipid metabolism in the dorsal horn of the spinal cord and increased S1P that accompanied the manifestation of several withdrawal behaviors. Blocking de novo sphingolipid metabolism with intrathecal administration of myriocin, an inhibitor of serine palmitoyltransferase, blocked naloxone-precipitated withdrawal. Noteworthy, we found that competitive (NIBR-15) and functional (FTY720) S1PR1 antagonists attenuated withdrawal behaviors in mice. Mechanistically, at the level of the spinal cord, naloxone-precipitated withdrawal was associated with increased glial activity and formation of the potent inflammatory/neuroexcitatory cytokine interleukin-1β (IL-1β); these events were attenuated by S1PR1 antagonists. These results provide the first molecular insight for the role of the S1P/S1PR1 axis during opioid withdrawal. Our data identify S1PR1 antagonists as potential therapeutics to mitigate opioid-induced dependence and support repurposing the S1PR1 functional antagonist FTY720, which is FDA-approved for multiple sclerosis, as an opioid adjunct.

Targeting the Sphingosine-1-Phosphate Axis for Developing Non-narcotic Pain Therapeutics

Chronic pain is a life-altering condition affecting millions of people. Current treatments are inadequate and prolonged therapies come with severe side effects, especially dependence and addiction to opiates. Identification of non-narcotic analgesics is of paramount importance. Preclinical and clinical studies suggest that sphingolipid metabolism alterations contribute to neuropathic pain development. Functional sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) antagonists, such as FTY720/fingolimod, used clinically for non-pain conditions, are emerging as non-narcotic analgesics, supporting the repurposing of fingolimod for chronic pain treatment and energizing drug discovery focused on S1P signaling. Here, we summarize the role of S1P in pain to highlight the potential of targeting the S1P axis towards development of non-narcotic therapeutics, which, in turn, will hopefully help lessen misuse of opioid pain medications and address the ongoing opioid epidemic.

Central Nervous System Targets: Glial Cell Mechanisms in Chronic Pain

Interactions between central glial cells and neurons in the pain circuitry are critical contributors to the pathogenesis of chronic pain. In the central nervous system (CNS), two major glial cell types predominate: astrocytes and microglia. Injuries or pathological conditions which evoke pain are concurrently associated with the presence of a reactive microglia or astrocyte state, which is characterized by a variety of changes in the morphological, molecular, and functional properties of these cells. In this review, we highlight the changes that reactive microglia and astrocytes undergo following painful injuries and insults and discuss the critical and interactive role these two cell types play in the initiation and maintenance of chronic pain. Additionally, we focus on several crucial mechanisms by which microglia and astrocytes contribute to chronic pain and provide commentary on the therapeutic promise of targeting these pathways. In particular, we discuss how the inflammasome in activated microglia drives maturation and release of key pro-inflammatory cytokines, which drive pain through neuronal- and glial regulations. Moreover, we highlight several potentially-druggable hemichannels and proteases produced by reactive microglia and astrocytes in pain states and discuss how these pathways regulate distinct phases during pain pathogenesis. We also review two emerging areas in chronic pain research: 1) sexually dimorphic glial cell signaling and 2) the role of oligodendrocytes. Finally, we highlight important considerations for potential pain therapeutics targeting glial cell mediators as well as questions that remain in our conceptual understanding of glial cell activation in pain states.

Chronic Morphine-Induced Changes in Signaling at the A3 Adenosine Receptor Contribute to Morphine-Induced Hyperalgesia, Tolerance, and Withdrawal

Treating chronic pain by using opioids, such as morphine, is hampered by the development of opioid-induced hyperalgesia (OIH; increased pain sensitivity), antinociceptive tolerance, and withdrawal, which can contribute to dependence and abuse. In the central nervous system, the purine nucleoside adenosine has been implicated in beneficial and detrimental actions of morphine, but the extent of their interaction remains poorly understood. Here, we demonstrate that morphine-induced OIH and antinociceptive tolerance in rats is associated with a twofold increase in adenosine kinase (ADK) expression in the dorsal horn of the spinal cord. Blocking ADK activity in the spinal cord provided greater than 90% attenuation of OIH and antinociceptive tolerance through A₃ adenosine receptor (A₃AR) signaling. Supplementing adenosine signaling with selective A₃AR agonists blocked OIH and antinociceptive tolerance in rodents of both sexes. Engagement of A₃AR in the spinal cord with an ADK inhibitor or A₃AR agonist was associated with reduced dorsal horn of the spinal cord expression of the NOD-like receptor pyrin domain-containing 3 (60%-75%), cleaved caspase 1 (40%-60%), interleukin (IL)-1β (76%-80%), and tumor necrosis factor (50%-60%). In contrast, the neuroinhibitory and anti-inflammatory cytokine IL-10 increased twofold. In mice, A₃AR agonists prevented the development of tolerance in a model of neuropathic pain and reduced naloxone-dependent withdrawal behaviors by greater than 50%. These findings suggest A₃AR-dependent adenosine signaling is compromised during sustained morphine to allow the development of morphine-induced adverse effects. These findings raise the intriguing possibility that A₃AR agonists may be useful adjunct to opioids to manage their unwanted effects. SIGNIFICANCE STATEMENT: The development of hyperalgesia and antinociceptive tolerance during prolonged opioid use are noteworthy opioid-induced adverse effects that reduce opioid efficacy for treating chronic pain and increase the risk of dependence and abuse. We report that in rodents, these adverse effects are due to reduced adenosine signaling at the A₃AR, resulting in NOD-like receptor pyrin domain-containing 3-interleukin-1β neuroinflammation in spinal cord. These effects are attenuated by A₃AR agonists, suggesting that A₃AR may be a target for therapeutic intervention with selective A₃AR agonist as opioid adjuncts.

Physiopathology of pain in systemic erythematosus lupus

Systemic lupus erythematosus is an autoimmune disease that promotes chronic inflammation, with periods of activation and remission. Pain is commonly one of the first symptoms reported by patients with lupus. It interferes with patients' quality of life, leading to a decrease in strength and difficulty in performing daily activities. Given the chronic nature of the disease, the high prevalence of pain and its negative impact on the quality of life of patients with lupus, the present literature review study aims to explain the pathophysiology of pain in systemic lupus erythematosus and the implications of the chronic process and contributing factors. During periods of systemic lupus erythematosus activity, pain is a significant symptom. Despite this, several studies show that severe pain can occur in patients with in mild to moderate disease activity. Also, in the early stages of the disease, the pain may be accentuated by the greater activity of the disease. However, even when advanced disease is under control, there can be comorbidities and accumulated damage that can also cause high levels of pain. This sensitivity is due to the overlap of primary, secondary, and tertiary pain pathologies, which feedback and make this symptom one of the main concerns of patients with lupus. Understanding the pathophysiology of pain in systemic lupus erythematosus, as well as its chronification and contribution factors, is essential to identify effective therapeutic alternatives in these patients at each stage of pain pathology (primary, secondary, and tertiary).

Suppression of Superficial Microglial Activation by Spinal Cord Stimulation Attenuates Neuropathic Pain Following Sciatic Nerve Injury in Rats

We evaluated the mechanisms underlying the spinal cord stimulation (SCS)-induced analgesic effect on neuropathic pain following spared nerve injury (SNI). On day 3 after SNI, SCS was performed for 6 h by using electrodes paraspinally placed on the L4-S1 spinal cord. The effects of SCS and intraperitoneal minocycline administration on plantar mechanical sensitivity, microglial activation, and neuronal excitability in the L4 dorsal horn were assessed on day 3 after SNI. The somatosensory cortical responses to electrical stimulation of the hind paw on day 3 following SNI were examined by using in vivo optical imaging with a voltage-sensitive dye. On day 3 after SNI, plantar mechanical hypersensitivity and enhanced microglial activation were suppressed by minocycline or SCS, and L4 dorsal horn nociceptive neuronal hyperexcitability was suppressed by SCS. In vivo optical imaging also revealed that electrical stimulation of the hind paw-activated areas in the somatosensory cortex was decreased by SCS. The present findings suggest that SCS could suppress plantar SNI-induced neuropathic pain via inhibition of microglial activation in the L4 dorsal horn, which is involved in spinal neuronal hyperexcitability. SCS is likely to be a potential alternative and complementary medicine therapy to alleviate neuropathic pain following nerve injury.

Cannabidiol modulates the expression of neuroinflammatory factors in stress- and drug-induced reinstatement of methamphetamine in extinguished rats

Methamphetamine (METH) is a highly potent and addictive psychostimulant that is frequently abused worldwide. Although the biggest challenge to the efficient treatment of drug dependence is relapse, its mechanism is completely unclear. Plenty of evidence suggests that inflammation contributes to drug-induced reward especially in brain regions that are involved in the reward system, but there is no document about relapse. Cannabidiol (CBD) is a nonpsychoactive cannabinoid that has powerful anti-inflammatory and immunosuppressive properties. A previous research in our laboratory has demonstrated that CBD prevents reinstatement of METH even in 24-hour rapid eye movement (REM) sleep-deprived (RSD) rats. The aim of this study was to assess whether CBD prevents reinstatement of METH through change of gene expression of cytokines such as interleukin-1β, interleukin-6, interleukin-10, and tumor necrosis factor α (TNF-α) in extinguished rats. Real-time polymerase chain reaction (PCR) was used in this research to assay gene expression of cytokines. We found that stress- and drug-induced reinstatement of METH enhanced mRNA expression of cytokines in the prefrontal cortex (PFC) and hippocampus. Furthermore, CBD treatment significantly reduced the mRNA expression of cytokines in the PFC and hippocampus, but CBD treatment in RSD rats increased expression of cytokines in the hippocampus. It seems that enhancement of cytokines leads to change in neurotransmission and so triggers reinstatement of METH.

Levo-corydalmine attenuates microglia activation and neuropathic pain by suppressing ASK1-p38 MAPK/NF-κB signaling pathways in rat spinal cord

BACKGROUND AND OBJECTIVES

Neuropathic pain is partially refractory to currently available treatments. Although some studies have reported that apoptosis signal-regulating kinase 1 (ASK1) may inhibit chronic pain, the mechanisms underlying this process have not been fully elucidated.

METHODS

Chronic constriction injury (CCI) of the rat sciatic nerve was used to establish a neuropathic pain model. Nociception was assessed using von Frey hair and Hargreaves' methods. Western blot and immunofluorescence were used to detect the cell signaling pathway. BV2 cell line was cultured for in vitro evaluation.

RESULTS

Our results indicated that spinal ASK1 was co-expressed with the microglia marker ionized calcium binding adaptor 1. Additionally, intrathecal administration of ASK1 inhibitor suppressed the activation of spinal microglia and attenuated CCI-induced neuropathic pain. The ASK1 inhibitor also decreased the levels of phosphorylated ASK1 (p-ASK1), p65, p38 mitogen-activated protein kinase (MAPK) and tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) messenger RNA in lipopolysaccharide-stimulated BV2 microglia cells. Intragastric administration of levo-corydalmine (l-CDL) significantly attenuated CCI-induced neuropathic pain and inhibited the expression of p-ASK1 in the spinal cord. l-CDL conspicuously suppressed the activation of spinal microglia in vitro and in vivo. Translocation of nuclearfactor-kappa B (NF-κB) and upregulation of p-p65, TNF-α, IL-1β were inhibited by l-CDL. Further, the analgesic effects of l-CDL were associated with reduced levels of phosphorylated protein kinase C (PKC γ), c-JunNH2-terminal kinase, and extracellular signal-regulated kinase.

CONCLUSIONS

This study showed that the expression of ASK1 in spinal microglia and ASK1 inhibitor suppressed microglia activation via suppression of p38 MAPK/NF-κB, which ultimately attenuated CCI-induced neuropathic pain. l-CDL also inhibited the ASK1-P38 MAPK/NF-κB axis to attenuate CCI-induced neuropathic pain.

Microglial Modulation as a Target for Chronic Pain: From the Bench to the Bedside and Back

With a widespread opioid epidemic and profound biopsychosocial implications, chronic pain is a multifaceted public health issue requiring urgent attention. The treatment of chronic pain is particularly important to anesthesiologists given our unique role as perioperative physicians and pain medicine specialists. The present review details the recent shift from a neuronal theory of chronic pain to one that includes complex neuron-glia interactions. In particular, we highlight microglia, the myeloid-lineage cells of the central nervous system, as initiators of a postinjury neuroimmune response that contributes to the acute to chronic pain transition. We discuss ever-advancing preclinical studies, wherein significant success has been made through pharmacologic and genetic modulation of microglia, and we emphasize where these approaches have made the transition to the clinical realm. Furthermore, we highlight the most current, novel efforts to visualize glial activation in vivo using positron emission tomography and improve the diagnosis of chronic pain through radiotracer binding of specific targets, like the 18 kDa translocator protein in microglia and myeloid-lineage cells. Our rapidly advancing knowledge about microglia and their involvement in pain suggests that the era of glial-targeted therapeutics is just beginning so long as we refocus our attention on optimizing preclinical studies using a clinically informed approach, before translation.

Oxycodone, fentanyl, and morphine amplify established neuropathic pain in male rats

Opioids are widely prescribed for chronic pain, including neuropathic pain, despite growing evidence of long-term harm. Previous preclinical studies have documented exacerbation of nociceptive hypersensitivity, including that induced by peripheral nerve injury, by morphine. The present series of behavioral studies sought to replicate and extend our prior research, which demonstrated a multimonth exacerbation of nociceptive hypersensitivity by a 5-day course of morphine initiated 10 days after nerve injury. The current studies demonstrate that enduring exacerbation of nociceptive hypersensitivity is not restricted to morphine, but rather is also created by the clinically relevant opioids fentanyl and oxycodone when these are likewise administered for 5 days beginning 10 days after nerve injury. Furthermore, enduring exacerbation of nociceptive hypersensitivity is also observed when the same dosing regimen for either morphine, fentanyl, or oxycodone begins 1 month after nerve injury. Finally, a striking result from these studies is that no such exacerbation of nociceptive hypersensitivity occurs when either morphine, fentanyl, or oxycodone dosing begins at the time of nerve injury. These results extend our previous findings that morphine exacerbates nociceptive hypersensitivity to the clinically relevant opioids fentanyl and oxycodone when administered after the development of nociceptive hypersensitivity, while also providing possible clinically relevant insight into when these opioids can be safely administered and not exacerbate neuropathic pain.

Repeated Morphine Prolongs Postoperative Pain in Male Rats

BACKGROUND

Opioids are effective postoperative analgesics. Disturbingly, we have previously reported that opioids such as morphine can worsen inflammatory pain and peripheral and central neuropathic pain. These deleterious effects are mediated by immune mediators that promote neuronal hyperexcitability in the spinal dorsal horn. Herein, we tested whether perioperative morphine could similarly prolong postoperative pain in male rats.

METHODS

Rats were treated with morphine for 7 days, beginning immediately after laparotomy, while the morphine was tapered in a second group. Expression of genes for inflammatory mediators was quantified in the spinal dorsal horn. In the final experiment, morphine was administered before laparotomy for 7 days.

RESULTS

We found that morphine treatment after laparotomy extended postoperative pain by more than 3 weeks (time × treatment: P < .001; time: P < .001; treatment: P < .05). Extension of postoperative pain was not related to morphine withdrawal, as it was not prevented by dose tapering (time × treatment: P = .8; time: P < .001; treatment: P = .9). Prolonged postsurgical pain was associated with increased expression of inflammatory genes, including those encoding Toll-like receptor 4, NOD like receptor protein 3 (NLRP3), nuclear factor kappa B (NFκB), caspase-1, interleukin-1β, and tumor necrosis factor (P < .05). Finally, we showed that of preoperative morphine, concluding immediately before laparotomy, similarly prolonged postoperative pain (time × treatment: P < .001; time: P < .001; treatment: P < .001). There is a critical window for morphine potentiation of pain, as a 7-day course of morphine that concluded 1 week before laparotomy did not prolong postsurgical pain.

CONCLUSIONS

These studies indicate the morphine can have a deleterious effect on postoperative pain. These studies further suggest that longitudinal studies could be performed to test whether opioids similarly prolong postoperative pain in the clinic.

Sphingosine-1-phosphate receptor 2 modulates pain sensitivity by suppressing the ROS-RUNX3 pathway in a rat model of neuropathy

Neuropathic pain correlates with a lesion or other dysfunction in the nervous system. Sphingosine-1-phosphate receptor 2 (S1P2) is expressed in the central nervous system and modulates synaptic plasticity. The present study aimed to investigate the role of S1P2 in neuropathic pain caused by chronic constriction injury (CCI). Sprague-Dawley rats were allocated into eight groups (n = 15 for each group): sham, CCI, CCI + green fluorescent protein, CCI + S1P2, CCI + Ctrl-short hairpin RNA (shRNA), CCI + S1P2 shRNA, CCI + S1P2 + CYM-5442, and CCI + S1P2 shRNA + CYM-5442. The CCI model was established via sciatic nerve ligation. S1P2 was overexpressed or knocked down by intrathecal injection of adeno-associated virus-S1P2 (AAV-S1P2) or AAV-S1P2 shRNA. The S1P1 agonist, CYM-5442 (1 mg/kg), was injected intraperitoneally after surgery. S1P2 expression, pain thresholds, apoptosis signaling, inflammation, and oxidative stress in rats were then examined. We found that sciatic nerve injury downregulated S1P2 expression in the spinal cords of rats. S1P2 overexpression enhanced pain thresholds. In contrast, S1P2 knockdown decreased pain thresholds in rats exposed to CCI. CCI and S1P2 silencing increased secretion of interleukin-1β (IL-1β), IL-6, and CCL2, whereas S1P2 overexpression decreased. S1P2 impeded CCI-induced reactive oxygen species (ROS) production and runt-related transcription factors 3 (RUNX3) downregulation, and S1P2 knockdown had the opposite effect. S1P2 overexpression suppressed Bax and active caspase 3 expression and promoted Bcl-2 expression, whereas loss of S1P2 reversed their expression. Additionally, S1P1 activation counteracted the effect of S1P2 on pain sensitivity. In conclusion, S1P2 is downregulated in CCI rats and may help modulate neuropathic pain via the ROS/RUNX3 pathway.

Microglia in Pain: Detrimental and Protective Roles in Pathogenesis and Resolution of Pain

The previous decade has seen a rapid increase in microglial studies on pain, with a unique focus on microgliosis in the spinal cord after nerve injury and neuropathic pain. Numerous signaling molecules are altered in microglia and contribute to the pathogenesis of pain. Here, we discuss how microglial signaling regulates spinal cord synaptic plasticity in acute and chronic pain conditions with different degrees and variations of microgliosis. We highlight that microglial mediators such as pro- and anti-inflammatory cytokines are powerful neuromodulators that regulate synaptic transmission and pain via neuron-glial interactions. We also reveal an emerging role of microglia in the resolution of pain, in part via specialized pro-resolving mediators including resolvins, protectins, and maresins. We also discuss a possible role of microglia in chronic itch.

Sphingosine-1-phosphate receptor 1 activation in astrocytes contributes to neuropathic pain

Neuropathic pain afflicts millions of individuals and represents a major health problem for which there is limited effective and safe therapy. Emerging literature links altered sphingolipid metabolism to nociceptive processing. However, the neuropharmacology of sphingolipid signaling in the central nervous system in the context of chronic pain remains largely unexplored and controversial. We now provide evidence that sphingosine-1-phosphate (S1P) generated in the dorsal horn of the spinal cord in response to nerve injury drives neuropathic pain by selectively activating the S1P receptor subtype 1 (S1PR1) in astrocytes. Accordingly, genetic and pharmacological inhibition of S1PR1 with multiple antagonists in distinct chemical classes, but not agonists, attenuated and even reversed neuropathic pain in rodents of both sexes and in two models of traumatic nerve injury. These S1PR1 antagonists retained their ability to inhibit neuropathic pain during sustained drug administration, and their effects were independent of endogenous opioid circuits. Moreover, mice with astrocyte-specific knockout of S1pr1 did not develop neuropathic pain following nerve injury, thereby identifying astrocytes as the primary cellular substrate of S1PR1 activity. On a molecular level, the beneficial reductions in neuropathic pain resulting from S1PR1 inhibition were driven by interleukin 10 (IL-10), a potent neuroprotective and anti-inflammatory cytokine. Collectively, our results provide fundamental neurobiological insights that identify the cellular and molecular mechanisms engaged by the S1PR1 axis in neuropathic pain and establish S1PR1 as a target for therapeutic intervention with S1PR1 antagonists as a class of nonnarcotic analgesics.

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

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

AMPK activation attenuates inflammatory pain through inhibiting NF-κB activation and IL-1β expression

BACKGROUND

Chronic pain is a major clinical problem with limited treatment options. Previous studies have demonstrated that activation of adenosine monophosphate-activated protein kinase (AMPK) can attenuate neuropathic pain. Inflammation/immune response at the site of complete Freund's adjuvant (CFA) injection is known to be a critical trigger of the pathological changes that produce inflammatory pain. However, whether activation of AMPK produces an analgesic effect through inhibiting the proinflammatory cytokines, including interleukin-1β (IL-1β), in inflammatory pain remains unknown.

METHODS

Inflammatory pain was induced in mice injected with CFA. The effects of AICAR (5-aminoimidazole-4-carboxyamide ribonucleoside, an AMPK activator), Compound C (an AMPK inhibitor), and IL-1ra (an IL-1 receptor antagonist) were tested at day 4 after CFA injection. Inflammatory pain was assessed with von Frey filaments and hot plate. Immunoblotting, hematoxylin and eosin (H&E) staining, and immunofluorescence were used to assess inflammation-induced biochemical changes.

RESULTS

The AMPK activator AICAR produced an analgesic effect and inhibited the level of proinflammatory cytokine IL-1β in the inflamed skin in mice. Moreover, activation of AMPK suppressed CFA-induced NF-κB p65 translocation from the cytosol to the nucleus in activated macrophages (CD68⁺ and CX3CR1⁺) of inflamed skin tissues. Subcutaneous injection of IL-1ra attenuated CFA-induced inflammatory pain. The AMPK inhibitor Compound C and AMPKα shRNA reversed the analgesic effect of AICAR and the effects of AICAR on IL-1β and NF-κB activation in inflamed skin tissues.

CONCLUSIONS

Our study provides new information that AMPK activation produces the analgesic effect by inhibiting NF-κB activation and reducing the expression of IL-1β in inflammatory pain.

Investigating the role of interleukin-1 beta and glutamate in inflammatory bowel disease and epilepsy using discovery browsing

BACKGROUND

Structured electronic health records are a rich resource for identifying novel correlations, such as co-morbidities and adverse drug reactions. For drug development and better understanding of biomedical phenomena, such correlations need to be supported by viable hypotheses about the mechanisms involved, which can then form the basis of experimental investigations.

METHODS

In this study, we demonstrate the use of discovery browsing, a literature-based discovery method, to generate plausible hypotheses elucidating correlations identified from structured clinical data. The method is supported by Semantic MEDLINE web application, which pinpoints interesting concepts and relevant MEDLINE citations, which are used to build a coherent hypothesis.

RESULTS

Discovery browsing revealed a plausible explanation for the correlation between epilepsy and inflammatory bowel disease that was found in an earlier population study. The generated hypothesis involves interleukin-1 beta (IL-1 beta) and glutamate, and suggests that IL-1 beta influence on glutamate levels is involved in the etiology of both epilepsy and inflammatory bowel disease.

CONCLUSIONS

The approach presented in this paper can supplement population-based correlation studies by enabling the scientist to identify literature that may justify the novel patterns identified in such studies and can underpin basic biomedical research that can lead to improved treatments and better healthcare outcomes.

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.

Dysregulation of sphingolipid metabolism contributes to bortezomib-induced neuropathic pain

The development of chemotherapy-induced painful peripheral neuropathy is a major dose-limiting side effect of many chemotherapeutics, including bortezomib, but the mechanisms remain poorly understood. We now report that bortezomib causes the dysregulation of de novo sphingolipid metabolism in the spinal cord dorsal horn to increase the levels of sphingosine-1-phosphate (S1P) receptor 1 (S1PR1) ligands, S1P and dihydro-S1P. Accordingly, genetic and pharmacological disruption of S1PR1 with multiple S1PR1 antagonists, including FTY720, blocked and reversed neuropathic pain. Mice with astrocyte-specific alterations of S1pr1 did not develop neuropathic pain and lost their ability to respond to S1PR1 inhibition, strongly implicating astrocytes as a primary cellular substrate for S1PR1 activity. At the molecular level, S1PR1 engaged astrocyte-driven neuroinflammation and altered glutamatergic homeostasis, processes blocked by S1PR1 antagonism. Our findings establish S1PR1 as a target for therapeutic intervention and provide insight into cellular and molecular pathways. As FTY720 also shows promising anticancer potential and is FDA approved, rapid clinical translation of our findings is anticipated.