Reactive oxygen species affect spinal cell type-specific synaptic plasticity in a model of neuropathic pain

TRPA1 agonist-responsive afferents contribute to central sensitization by suppressing spinal GABAergic interneurons through Somatostatin 2A receptors

Altered nociception, a key feature of nociplastic pain, often involves central sensitization. We previously found that central sensitization underlying a nociplastic pain state in female mice depends on the ongoing activity of TRPA1 agonist-responsive afferents. Here we investigated how the activity of these afferents induces and maintains central sensitization at the spinal level. We hypothesized that in the superficial dorsal horn where somatostatin (SST) is expressed in excitatory interneurons and the SST2A receptor (SST2A-R) in GABAergic inhibitory interneurons (GABAn), TRPA1 agonist-responsive afferents stimulate SST-expressing excitatory interneurons (SSTn), leading to GABAn suppression through SST2A-R and resulting in altered nociception. We tested this hypothesis using ex vivo Ca2+ imaging of dorsal root-attached spinal cord slices expressing GCaMP6f in either SSTn or GABAn and in vivo assessment of mechanical hypersensitivity. The dorsal root was chemically (with allyl isothiocyanate, AITC) and electrically stimulated to activate TRPA1-expressing nociceptors and all afferents, respectively. The stimulation of dorsal root with AITC excited SSTn. During activation of AITC-responsive afferents, a subset of SSTn showed potentiated responses to both low- and high-threshold afferent inputs, whereas a subset of GABAn showed suppressed responses to those afferents in an SST2A-R-dependent manner. Intrathecally administered SST2A-R antagonist inhibited the development of mechanical hypersensitivity by intraplantar AITC injection and alleviated persistent mechanical hypersensitivity in the murine model of nociplastic pain. These results suggest that the activity of TRPA1 agonist-responsive afferents induces and maintains central sensitization by activating dorsal horn SSTn and suppressing GABAn via SST2A-R, resulting in altered nociception that manifests as mechanical hypersensitivity. PERSPECTIVES: This article presents experimental evidence that TRPA1 agonist-responsive afferents induce and maintain central sensitization at the spinal level by activating somatostatin (SST)-expressing excitatory interneurons and suppressing GABAergic inhibitory interneurons via SST2A receptors. Spinal SST2A-R may represent a promising target for treating mechanical pain hypersensitivity due to central sensitization by TRPA1 agonist-responsive afferents.

SIRT1-regulated ROS generation activates NMDAR2B phosphorylation to promote central sensitization and allodynia in a male chronic migraine rat model

Background

Central sensitization is one of the pivotal pathological mechanisms in chronic migraine (CM). Silent information regulator 1 (SIRT1) was shown to be involved in CM, but its specific mechanism is unclear. Reactive oxygen species (ROS) are increasingly regarded as important signaling molecules in several models of pain. However, studies about the role of ROS in the central sensitization of CM model are rare. We thus explored the specific process of SIRT1 involvement in the central sensitization of CM, focusing on the ROS pathway.

Methods

Inflammatory soup was repeatedly administered to male Sprague-Dawley rats to establish a CM model. The SIRT1 expression level in trigeminal nucleus caudalis (TNC) tissues was assessed by qRT-PCR and Western blotting analysis. The levels of ROS were detected by a Tissue Reactive Oxygen Detection Kit, DHE staining, and the fluorescence signal intensity of 8-OHdG. A ROS scavenger (tempol), a SIRT1 activator (SRT1720), a SIRT1 inhibitor (EX527), and a mitochondrial fission inhibitor (Mdivi-1) were used to investigate the specific molecular mechanisms involved. NMDAR2B, CGRP, ERK, and mitochondrial fission-related protein were evaluated by Western blotting, and the CGRP level in frozen sections of the TNC was detected via immunofluorescence staining.

Results

After repeated inflammatory soup infusion and successful establishment of the CM rat model, SIRT1 expression was found to be significantly reduced, accompanied by elevated ROS levels. Treatment with Tempol, SRT1720, or Mdivi-1 alleviated allodynia and reduced the increase in NMDAR2B phosphorylation and CGRP and ERK phosphorylation in the CM rat. In contrast, EX527 had the opposite effect in CM rat. SRT1720 and EX527 decreased and increased ROS levels, respectively, in CM rats, and tempol reversed the aggravating effect of EX527 in CM rats. Furthermore, the regulatory effect of SIRT1 on ROS may include the involvement of the mitochondrial fission protein DRP1.

Conclusion

The results indicate the importance of SIRT1 in CM may be due to its role in regulating the production of ROS, which are involved in modulating central sensitization in CM. These findings could lead to new ideas for CM treatment with the use of SIRT1 agonists and antioxidants.

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

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

The analgesic effects of botulinum neurotoxin by modulating pain-related receptors; A literature review

Botulinum neurotoxins (BoNTs), produced by Clostridium botulinum, have been used for the treatment of various central and peripheral neurological conditions. Recent studies have suggested that BoNTs may also have a beneficial effect on pain conditions. It has been hypothesized that one of the mechanisms underlying BoNTs' analgesic effects is the inhibition of pain-related receptors' transmission to the neuronal cell membrane. BoNT application disrupts the integration of synaptic vesicles with the cellular membrane, which is responsible for transporting various receptors, including pain receptors such as TRP channels, calcium channels, sodium channels, purinergic receptors, neurokinin-1 receptors, and glutamate receptors. BoNT also modulates the opioidergic system and the GABAergic system, both of which are involved in the pain process. Understanding the cellular and molecular mechanisms underlying these effects can provide valuable insights for the development of novel therapeutic approaches for pain management. This review aims to summarize the experimental evidence of the analgesic functions of BoNTs and discuss the cellular and molecular mechanisms by which they can act on pain conditions by inhibiting the transmission of pain-related receptors.

Targeting the Main Sources of Reactive Oxygen Species Production: Possible Therapeutic Implications in Chronic Pain

Humans have long been combating chronic pain. In clinical practice, opioids are firstchoice analgesics, but long-term use of these drugs can lead to serious adverse reactions. Finding new, safe and effective pain relievers that are useful treatments for chronic pain is an urgent medical need. Based on accumulating evidence from numerous studies, excess reactive oxygen species (ROS) contribute to the development and maintenance of chronic pain. Some antioxidants are potentially beneficial analgesics in the clinic, but ROS-dependent pathways are completely inhibited only by scavenging ROS directly targeting cellular or subcellular sites. Unfortunately, current antioxidant treatments do not achieve this effect. Furthermore, some antioxidants interfere with physiological redox signaling pathways and fail to reverse oxidative damage. Therefore, the key upstream processes and mechanisms of ROS production that lead to chronic pain in vivo must be identified to discover potential therapeutic targets related to the pathways that control ROS production in vivo. In this review, we summarize the sites and pathways involved in analgesia based on the three main mechanisms by which ROS are generated in vivo, discuss the preclinical evidence for the therapeutic potential of targeting these pathways in chronic pain, note the shortcomings of current research and highlight possible future research directions to provide new targets and evidence for the development of clinical analgesics.

Progress in Pathological and Therapeutic Research of HIV-Related Neuropathic Pain

HIV-related neuropathic pain (HRNP) is a neurodegeneration that gradually develops during the long-term course of acquired immune deficiency syndrome (AIDS) and manifests as abnormal sock/sleeve-like symmetrical pain and nociceptive hyperalgesia in the extremities, which seriously reduces patient quality of life. To date, the pathogenesis of HRNP is not completely clear. There is a lack of effective clinical treatment for HRNP and it is becoming a challenge and hot spot for medical research. In this study, we conducted a systematic review of the progress of HRNP research in recent years including (1) the etiology, classification and clinical symptoms of HRNP, (2) the establishment of HRNP pathological models, (3) the pathological mechanisms underlying HRNP from three aspects: molecules, signaling pathways and cells, (4) the therapeutic strategies for HRNP, and (5) the limitations of recent HRNP research and the future research directions and prospects of HRNP. This detailed review provides new and systematic insight into the pathological mechanism of HRNP, which establishes a theoretical basis for the future exploitation of novel target drugs. HIV infection, antiretroviral therapy and opioid abuse contribute to the etiology of HRNP with symmetrical pain in both hands and feet, allodynia and hyperalgesia. The pathogenesis involves changes in cytokine expression, activation of signaling pathways and neuronal cell states. The therapy for HRNP should be patient-centered, integrating pharmacologic and nonpharmacologic treatments into multimodal intervention.

Effects of Corticosterone on the Excitability of Glutamatergic and GABAergic Neurons of the Adolescent Mouse Superficial Dorsal Horn

Stress evokes age-dependent effects on pain sensitivity and commonly occurs during adolescence. However, the mechanisms linking adolescent stress and pain remain poorly understood, in part due to a lack of information regarding how stress hormones modulate the function of nociceptive circuits in the adolescent CNS. Here we investigate the short- and long-term effects of corticosterone (CORT) on the excitability of GABAergic and presumed glutamatergic neurons of the spinal superficial dorsal horn (SDH) in Gad1-GFP mice at postnatal days (P)21-P34. In situ hybridization revealed that glutamatergic SDH neurons expressed significantly higher mRNA levels of both glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) compared to adjacent GABAergic neurons. The incubation of spinal cord slices with CORT (90 min) evoked select long-term changes in spontaneous synaptic transmission across both cell types in a sex-dependent manner, without altering the intrinsic firing of either Gad1-GFP+ or GFP- neurons. Meanwhile, the acute bath application of CORT significantly decreased the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs), as well as the frequency of miniature inhibitory postsynaptic currents (mIPSCs), in both cell types leading to a net reduction in the balance of spontaneous excitation vs. inhibition (E:I ratio). This CORT-induced reduction in the E:I ratio was not prevented by selective antagonists of either GR (mifepristone) or MR (eplerenone), although eplerenone blocked the effect on mEPSC amplitude. Collectively, these data suggest that corticosterone modulates synaptic function within the adolescent SDH which could influence the overall excitability and output of the spinal nociceptive network.

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.

Up-regulation of LCN2 in the anterior cingulate cortex contributes to neural injury-induced chronic pain

Chronic pain caused by disease or injury affects more than 30% of the general population. The molecular and cellular mechanisms underpinning the development of chronic pain remain unclear, resulting in scant effective treatments. Here, we combined electrophysiological recording, in vivo two-photon (2P) calcium imaging, fiber photometry, Western blotting, and chemogenetic methods to define a role for the secreted pro-inflammatory factor, Lipocalin-2 (LCN2), in chronic pain development in mice with spared nerve injury (SNI). We found that LCN2 expression was upregulated in the anterior cingulate cortex (ACC) at 14 days after SNI, resulting in hyperactivity of ACC glutamatergic neurons (ACC^Glu) and pain sensitization. By contrast, suppressing LCN2 protein levels in the ACC with viral constructs or exogenous application of neutralizing antibodies leads to significant attenuation of chronic pain by preventing ACC^Glu neuronal hyperactivity in SNI 2W mice. In addition, administering purified recombinant LCN2 protein in the ACC could induce pain sensitization by inducing ACC^Glu neuronal hyperactivity in naïve mice. This study provides a mechanism by which LCN2-mediated hyperactivity of ACC^Glu neurons contributes to pain sensitization, and reveals a new potential target for treating chronic pain.

Dysfunction of inhibitory interneurons contributes to synaptic plasticity via GABABR-pNR2B signaling in a chronic migraine rat model

Background

According to our previous study, the loss of inhibitory interneuron function contributes to central sensitization in chronic migraine (CM). Synaptic plasticity is a vital basis for the occurrence of central sensitization. However, whether the decline in interneuron-mediated inhibition promotes central sensitization by regulating synaptic plasticity in CM remains unclear. Therefore, this study aims to explore the role of interneuron-mediated inhibition in the development of synaptic plasticity in CM.

Methods

A CM model was established in rats by repeated dural infusion of inflammatory soup (IS) for 7 days, and the function of inhibitory interneurons was then evaluated. After intraventricular injection of baclofen [a gamma-aminobutyric acid type B receptor (GABABR) agonist] or H89 [a protein kinase A (PKA) inhibitor), behavioral tests were performed. The changes in synaptic plasticity were investigated by determining the levels of the synapse-associated proteins postsynaptic density protein 95 (PSD95), synaptophysin (Syp) and synaptophysin-1(Syt-1)]; evaluating the synaptic ultrastructure by transmission electron microscopy (TEM); and determining the density of synaptic spines via Golgi-Cox staining. Central sensitization was evaluated by measuring calcitonin gene-related peptide (CGRP), brain-derived neurotrophic factor (BDNF), c-Fos and substance P (SP) levels. Finally, the PKA/Fyn kinase (Fyn)/tyrosine-phosphorylated NR2B (pNR2B) pathway and downstream calcium-calmodulin-dependent kinase II (CaMKII)/c-AMP-responsive element binding protein (pCREB) signaling were assessed.

Results

We observed dysfunction of inhibitory interneurons, and found that activation of GABABR ameliorated CM-induced hyperalgesia, repressed the CM-evoked elevation of synapse-associated protein levels and enhancement of synaptic transmission, alleviated the CM-triggered increases in the levels of central sensitization-related proteins, and inhibited CaMKII/pCREB signaling via the PKA/Fyn/pNR2B pathway. The inhibition of PKA suppressed the CM-induced activation of Fyn/pNR2B signaling.

Conclusion

These data reveal that the dysfunction of inhibitory interneurons contributes to central sensitization by regulating synaptic plasticity through the GABABR/PKA/Fyn/pNR2B pathway in the periaqueductal gray (PAG) of CM rats. Blockade of GABABR-pNR2B signaling might have a positive influence on the effects of CM therapy by modulating synaptic plasticity in central sensitization.

Antinociceptive Effects of Aaptamine, a Sponge Component, on Peripheral Neuropathy in Rats

Aaptamine, a natural marine compound isolated from the sea sponge, has various biological activities, including delta-opioid agonist properties. However, the effects of aaptamine in neuropathic pain remain unclear. In the present study, we used a chronic constriction injury (CCI)-induced peripheral neuropathic rat model to explore the analgesic effects of intrathecal aaptamine administration. We also investigated cellular angiogenesis and lactate dehydrogenase A (LDHA) expression in the ipsilateral lumbar spinal cord after aaptamine administration in CCI rats by immunohistofluorescence. The results showed that aaptamine alleviates CCI-induced nociceptive sensitization, allodynia, and hyperalgesia. Moreover, aaptamine significantly downregulated CCI-induced vascular endothelial growth factor (VEGF), cluster of differentiation 31 (CD31), and LDHA expression in the spinal cord. Double immunofluorescent staining showed that the spinal VEGF and LDHA majorly expressed on astrocytes and neurons, respectively, in CCI rats and inhibited by aaptamine. Collectively, our results indicate aaptamine's potential as an analgesic agent for neuropathic pain. Furthermore, inhibition of astrocyte-derived angiogenesis and neuronal LDHA expression might be beneficial in neuropathy.

Research progress on the mechanism of chronic neuropathic pain

Chronic neuropathic pain (CNP) refers to pain that lasts for more than three months due to a disease or an injury to the somatosensory nervous system. The incidence of CNP has been increasing in the world, causing it to become a global concern and patients often experience spontaneous pain, hyperalgesia, abnormal pain or even abnormal sensation as some of its main symptoms. In addition to serious pain and poor physical health, CNP also negatively affects patients' mental health, thus impacting the overall quality of their lives. The pathogenesis of CNP is not clear, but some studies have proved that central sensitization, peripheral sensitization, neuroinflammation, dysfunction in descending nociceptive modulatory systems, oxidative stress reaction, activation of glial cells and psychological factors play an important role in the occurrence and development of CNP. In this context, this article summarizes the current research progress on the mechanism of CNP to provide a basis for further research in preventing and treating the disease.

Apoptosis and (in) Pain—Potential Clinical Implications

The deregulation of apoptosis is involved in the development of several pathologies, and recent evidence suggests that apoptosis may be involved in chronic pain, namely in neuropathic pain. Neuropathic pain is a chronic pain state caused by primary damage or dysfunction of the nervous system; however, the details of the molecular mechanisms have not yet been fully elucidated. Recently, it was found that nerve endings contain transient receptor potential (TRP) channels that sense and detect signals released by injured tissues and respond to these damage signals. TRP channels are similar to the voltage-gated potassium channels or nucleotide-gated channels that participate in calcium and magnesium homeostasis. TRP channels allowing calcium to penetrate into nerve terminals can activate apoptosis, leading to nerve terminal destruction. Further, some TRPs are activated by acid and reactive oxygen species (ROS). ROS are mainly produced in the mitochondrial respiratory chain, and an increase in ROS production and/or a decrease in the antioxidant network may induce oxidative stress (OS). Depending on the OS levels, they can promote cellular proliferation and/or cell degeneration or death. Previous studies have indicated that proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), play an important role in the peripheral mediation of neuropathic pain. This article aims to perform a review of the involvement of apoptosis in pain, particularly the role of OS and neuroinflammation, and the clinical relevance of this knowledge. The potential discovery of new biomarkers and therapeutic targets can result in the development of more effective and targeted drugs to treat chronic pain, namely neuropathic pain. Highlights: Oxidative stress and neuroinflammation can activate cell signaling pathways that can lead to nerve terminal destruction by apoptosis. These could constitute potential new pain biomarkers and targets for therapy in neuropathic pain.

TRPM3-mediated dynamic mitochondrial activity in NGF-induced latent sensitization of chronic low back pain

ABSTRACT

The transient receptor potential ion channel TRPM3 is highly prevalent on nociceptive dorsal root ganglion (DRG) neurons, but its functions in neuronal plasticity of chronic pain remain obscure. In an animal model of nonspecific low back pain (LBP), latent spinal sensitization known as nociceptive priming is induced by nerve growth factor (NGF) injection. Here we address the TRPM3-associated molecular basis of NGF-induced latent spinal sensitization at presynaptic level by studying TRPM3-mediated calcium transients in DRG neurons. By investigating TRPM3-expressing HEK cells, we further show the dynamic mitochondrial activity downstream of TRPM3 activation. NGF enhances TRPM3 function, attenuates TRPM3 tachyphylaxis, and slows intracellular calcium clearance; TRPM3 activation triggers more mitochondrial calcium loading than depolarization does, causing a steady-state mitochondrial calcium elevation and a delayed recovery of cytosolic calcium; mitochondrial calcium buffering accounts for approximately 40% of calcium influx subsequent to TRPM3 activation. TRPM3 activation provokes an outbreak of pulsatile superoxide production (mitoflash) that comes in the form of a surge in frequency being tunable. We suggest that mitoflash pulsations downstream of TRPM3 activation might be an early signaling event initiating pain sensitization. Tuning of mitoflash activity would be a novel bottom-up therapeutic strategy for chronic pain conditions such as LBP and beyond.

Nrf2 Activation Mediates Antiallodynic Effect of Electroacupuncture on a Rat Model of Complex Regional Pain Syndrome Type-I through Reducing Local Oxidative Stress and Inflammation

Complex regional pain syndrome type-I (CRPS-I) represents a type of neurovascular condition featured by severe pain in affected extremities. Few treatments have proven effective for CRPS-I. Electroacupuncture (EA) is an effective therapy for pain relief. We explored the mechanism through which EA ameliorates pain in a rat CRPS-I model. The chronic postischemic pain (CPIP) model was established using Sprague-Dawley rats to mimic CRPS-I. We found that oxidative stress-related biological process was among the predominant biological processes in affected hindpaw of CPIP rats. Oxidative stress occurred primarily in local hindpaw but not in the spinal cord or serum of model rats. Antioxidant N-acetyl cysteine (NAC) attenuated mechanical allodynia and spinal glia overactivation in CPIP model rats, whereas locally increasing oxidative stress is sufficient to induce chronic pain and spinal glia overactivation in naive rats. EA exerted remarkable antiallodynia on CPIP rats by reducing local oxidative stress via enhancing nuclear factor erythroid 2-related factor 2 (Nrf2) expression. Pharmacological blocking Nrf2 abolished antioxidative and antiallodynic effects of EA. EA reduced spinal glia overactivation, attenuated the upregulation of inflammatory cytokines, reduced the enhanced TRPA1 channel activity in dorsal root ganglion neurons innervating the hindpaws, and improved blood flow dysfunction in hindpaws of CPIP rats, all of which were mimicked by NAC treatment. Thus, we identified local oxidative injury as an important contributor to pathogenesis of animal CRPS-I model. EA targets local oxidative injury by enhancing endogenous Nrf2-mediated antioxidative mechanism to relieve pain and inflammation. Our study indicates EA can be an alternative option for CRPS-I management.

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.

Painful diabetic peripheral neuropathy: Role of oxidative stress and central sensitisation

AIMS

Diabetic peripheral neuropathy (DPN) occurs in about half of people with diabetes, of whom a quarter may develop chronic pain. Pain may remain for years yet be difficult to treat because the underlying mechanisms remain unclear. There is consensus that processing excessive glucose leads to oxidative stress, interfering with normal metabolism. In this narrative review, we argue that oxidative stress may also contribute to pain.

METHODS

We reviewed literature in PubMed published between January 2005 and August 2021.

RESULTS AND CONCLUSIONS

In diabetes, hyperglycaemia and associated production of reactive species can directly increase pain signalling and activate sensory neurons; or the effects can be indirect, mediated by mitochondrial damage and enhanced inflammation. Furthermore, pain processing in the central nervous system is compromised in painful DPN. This is implicated in central sensitisation and dysfunctional pain modulation. However, central pain modulatory function is understudied in diabetes. Future research is required to clarify whether central sensitisation and/or disturbances in central pain modulation contribute to painful DPN. Positive results would facilitate early detection and future treatment.

Neuron Type-Dependent Synaptic Activity in the Spinal Dorsal Horn of Opioid-Induced Hyperalgesia Mouse Model

Opioids are widely used for pain relief; however, chronic opioid use causes a paradoxical state of enhanced pain sensitivity, termed “Opioid-induced hyperalgesia (OIH).” Despite the clinical importance of OIH, the detailed mechanism by which it enhances pain sensitivity remains unclear. In this study, we tested whether repeated morphine induces a neuronal circuit polarization in the mouse spinal dorsal horn (SDH). Transgenic mice expressing GFP to neurokinin 1 receptor-expressing neurons (sNK1Rn) and GABAergic interneurons (sGABAn) that received morphine [20 mg/kg, once daily for four consecutive days (i.p.)] developed mechanical hypersensitivity. Repeated morphine altered synaptic strengths in the SDH as a specific cell-type but not in a gender-dependent manner. In sNK1Rn and non-tonic firing neurons, repeated morphine treatment significantly increased frequency of spontaneous excitatory postsynaptic current (sEPSC) and evoked EPSC (eEPSC). In addition, repeated morphine treatment significantly decreased evoked inhibitory postsynaptic current (eIPSC) in sNK1Rn. Conversely, in sGABAn and tonic firing neurons, repeated morphine treatment significantly decreased sEPSC frequency and eEPSC, but had no change of eIPSC in sGABAn. Interestingly, repeated morphine treatment significantly decreased neuronal rheobase of sNK1Rn but had no effect on sGABAn. These findings suggest that spinal neuronal circuit polarization maybe the mechanism of OIH and identify a potential therapeutic mechanism to prevent or treat opioid-induced pain.

Effects of vitamin D administration on nociception and spinal cord pro-oxidant and antioxidant markers in a rat model of neuropathic pain

Reactive oxygen species (ROS) are involved in neuropathic pain, a complicated condition after nerve tissue lesion. Vitamin D appears to improve symptoms of pain and exhibits antioxidant properties. We investigated the effects of oral administration of vitamin D3, the active form of vitamin D, on nociception, the sciatic functional index (SFI), and spinal cord pro-oxidant and antioxidant markers in rats with chronic constriction injury (CCI) of the sciatic nerve, a model of neuropathic pain. Vitamin D3 (500 IU/kg per day) attenuated the CCI-induced decrease in mechanical withdrawal threshold and thermal withdrawal latency (indicators of antinociception) and SFI. The vitamin prevented increased lipid hydroperoxide levels in injured sciatic nerve without change to total antioxidant capacity (TAC). Vitamin D3 prevented increased lipid hydroperoxide, superoxide anion generation (SAG), and hydrogen peroxide (H2O2) levels in the spinal cord, which were found in rats without treatment at 7 and 28 days post-CCI. A significant negative correlation was found between mechanical threshold and SAG and between mechanical threshold and H2O2 at day 7. Vitamin D3 also prevented decreased spinal cord total thiols content. There was an increase in TAC in the spinal cord of vitamin-treated CCI rats, compared to CCI rats without treatment only at 28 days. No significant changes were found in body weight and blood parameters of hepatic and renal function. These findings demonstrated, for first time, that vitamin D modulated pro-oxidant and antioxidant markers in the spinal cord. Since antinociception occurred in parallel with oxidative changes in the spinal cord, the oxidative changes may have contributed to vitamin D-induced antinociception.

Spine adjusting instrument (Impulse®) attenuates nociception and modulates oxidative stress markers in the spinal cord and sciatic nerve of a rat model of neuropathic pain

OBJECTIVE

Oxidative stress plays an important role in neuropathic pain. Spinal manipulative therapy (SMT) can exert beneficial effects in pain outcomes in humans and animal models. SMT can also modulate oxidative stress markers in both humans and animals. We aimed to determine the effect of Impulse®-assisted SMT (ISMT) on nociception and oxidative stress biomarkers in the spinal cord and sciatic nerve of rats with neuropathic pain (NP).

METHODS

NP was induced by chronic constriction injury (CCI) of the sciatic nerve. Animals were randomly assigned to naive, sham (rats with sciatic nerve exposure but without ligatures) and CCI, with and without ISMT. ISMT was applied onto the skin area corresponding to the spinous process of L4-L5, 3 times/week, for 2 weeks. Mechanical threshold, latency to paw withdrawal to thermal stimulus and oxidative stress biomarkers in spinal cord and sciatic nerve were the main outcomes evaluated.

RESULTS

ISMT significantly increased mechanical threshold and withdrawal latency after CCI. In the spinal cord, ISMT prevented the increase of pro-oxidative superoxide anion generation and hydrogen peroxide levels. Lipid hydroperoxide levels both in the spinal cord and in the sciatic nerve were attenuated by ISMT. Total antioxidant capacity increased in the spinal cord and sciatic nerve of CCI rats with and without ISMT. CCI and ISMT did not significantly change the total thiol content of the spinal cord.

CONCLUSIONS

Our findings suggest reduced oxidative stress in the spinal cord and/or nerve may be an important mechanism underlying a therapeutic effect of SMT to manage NP non-pharmacologically.

Occupational exposure of rural workers to pesticides in a vegetable-producing region in Brazil

The health of family farmers is at risk due to occupational exposure to pesticides. The aims of the current study were to investigate the level of farmers' perception of risks associated with pesticide use and to assess their health condition based on biochemical and immunological tests. Family farmers living in a vegetable-producing region in Southern Brazil were selected to participate in the study. More than 70% of the family farmers were often exposed to more than one type of pesticides; 41.2% were intensively using several pesticides for more than one decade and 74.4% were not using personal protective equipment (PPE) at the time of pesticide handling due to low perception of the risks posed by these chemicals. Enzymatic analysis performed in participants' blood samples showed changes in catalase (CAT) and glutathione reductase (GR) activity, in lipid peroxidation (TBARS) and carbonylated protein levels, as well as in chemoattractant (IL-8) and anti-inflammatory (IL-10) interleukin expression. Low perception of health-related risks posed by pesticides can be attributed to factors such as low schooling and lack of information, which put farmers' health at risk, as evidenced by blood biochemical and immunological changes.

Nicotinamide adenine dinucleotide phosphate oxidase 2-derived reactive oxygen species contribute to long-term potentiation of C-fiber-evoked field potentials in spinal dorsal horn and persistent mirror-image pain following high-frequency stimulus of the sciatic nerve

High-frequency stimulation (HFS) of the sciatic nerve has been reported to produce long-term potentiation (LTP) and long-lasting pain hypersensitivity in rats. However, the central underlying mechanism remains unclear. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) belongs to a group of electron-transporting transmembrane enzymes that produce reactive oxygen species (ROS). Here, we found that NOX2 was upregulated in the lumbar spinal dorsal horn after HFS of the left sciatic nerve, which induced bilateral pain and spinal LTP in both male and female rats. Blocking NOX2 with blocking peptide or shRNA prevented the development of bilateral mechanical allodynia, the induction of spinal LTP, and the phosphorylation of N-methyl-d-aspartate (NMDA) receptor 2B (GluN2B) and nuclear factor kappa-B (NF-κB) p65 after HFS. Moreover, NOX2 shRNA reduced the frequency and amplitude of both spontaneous excitatory postsynaptic currents and miniature excitatory postsynaptic currents in laminar II neurons. Furthermore, 8-hydroxyguanine (8-OHG), an oxidative stress marker, was increased in the spinal dorsal horn. Spinal application of ROS scavenger, Phenyl-N-tert-butylnitrone (PBN), depressed the already established spinal LTP. Spinal application of H2O2, one ROS, induced LTP and bilateral mechanical allodynia, increased the frequency and amplitude of spontaneous excitatory postsynaptic currents in laminar II neurons, and phosphorylated GluN2B and p65 in the dorsal horn. This study provided electrophysiological and behavioral evidence that NOX2-derived ROS in the spinal cord contributed to persistent mirror-image pain by enhancing the synaptic transmission, which was mediated by increasing presynaptic glutamate release and activation of NMDA receptor and NF-κB in the spinal dorsal horn.

Combination of autophagy and NFE2L2/NRF2 activation as a treatment approach for neuropathic pain

Macroautophagy/autophagy, an evolutionarily conserved process, plays an important role in the regulation of immune inflammation and nervous system homeostasis. However, the exact role and mechanism of autophagy in pain is still unclear. Here, we showed that impaired autophagy flux mainly occurred in astrocytes during the maintenance of neuropathic pain. No matter the stage of neuropathic pain induction or maintenance, activation of autophagy relieved the level of pain, whereas inhibition of autophagy aggravated pain. Moreover, the levels of neuroinflammation and reactive oxygen species (ROS) were increased or decreased following autophagy inhibition or activation. Further study showed that inhibition of autophagy slowed the induction, but increased the maintenance of neuroinflammatory responses, which could be achieved by promoting the binding of TRAF6 (TNF receptor-associated factor 6) to K63 ubiquitinated protein, and increasing the levels of p-MAPK8/JNK (mitogen-activated protein kinase 8) and nuclear factor of kappa light polypeptide gene enhancer in B cells (NFKB/NF-κB). Impaired autophagy also reduced the protective effect of astrocytes on neurons against ROS stress because of the decrease in the level of glutathione released by astrocytes, which could be improved by activating the NFE2L2/NRF2 (nuclear factor, erythroid derived 2, like 2) pathway. We also demonstrated that simultaneous activation of autophagy and the NFE2L2 pathway further relieved pain, compared to activating autophagy alone. Our study provides an underlying mechanism by which autophagy participates in the regulation of neuropathic pain, and a combination of autophagy and NFE2L2 activation may be a new treatment approach for neuropathic pain.

Abbreviation: 3-MA: 3-methyladenine; 8-OHdG: 8-hydroxydeoxy-guanosine; ACTB: actin, beta; AMPAR: alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor; ATG: autophagy-related; CAMK2/CaMKII: calcium/calmodulin-dependent protein kinase II; CCL7: chemokine (C-C motif) ligand 7; CGAS: cyclic GMP-AMP synthase; CQ: chloroquine; GABA: gamma-aminobutyrate; GCLC: glutamate-cysteine ligase, catalytic subunit; GFAP: glial fibrillary acidic protein; GSH: glutathione; HMOX1/HO-1: heme oxygenase 1; KEAP1: kelch-like ECH-associated protein 1; MAP1LC3/LC3-II: microtubule-associated protein 1 light chain 3 beta (phosphatidylethanolamine-conjugated form); MAPK: mitogen-activated protein kinase; MAPK1/ERK: mitogen-activated protein kinase 1; MMP2: matrix metallopeptidase 2; MAPK8/JNK: mitogen-activated protein kinase 8; MAPK14/p38: mitogen-activated protein kinase 14; NFE2L2/NRF2: nuclear factor, erythroid derived 2, like 2; NFKB/NF-κB: nuclear factor of kappa light polypeptide gene enhancer in B cells; ROS: reactive oxygen species; SLC12A5: solute carrier family 12, member 5; SNL: spinal nerve ligation; TLR4: toll-like receptor 4; TRAF6: TNF receptor-associated factor; TRP: transient receptor potential.

Dexmedetomidine alleviated neuropathic pain in dorsal root ganglion neurons by inhibition of anaerobic glycolysis activity and enhancement of ROS tolerance

Neuropathic pain is a kind of chronic pain that is triggered or caused primarily by damage to the nervous system and neurological dysfunction. It’s known that dexmedetomidine is a new type of highly selective alpha2-adrenoceptor agonist with sedation, anti-anxiety, analgesic and other effects. However, the function and mechanism of dexmedetomidine on neuropathic pain are not clear. Rat DRG neurons were isolated and identified using immunofluorescence assay. Following treatment with H₂O₂, dexmedetomidine or ROS inhibitor (NAC), the apoptosis and ROS levels were examined by flow cytometery; apoptosis- and anaerobic glycolysis-related proteins were determined by Western blot assay; glucose consumption, pyruvic acid, lactic acid and ATP/ADP ratios were also measured. The results revealed that dexmedetomidine inhibited H₂O₂-induced apoptosis and reactive oxygen species (ROS) in rat DRG neurons and in addition, dexmedetomidine down-regulated the expression levels of anaerobic glycolysis-related proteins, significantly reduced glucose, pyruvic acid and lactic acid levels. It also increased the ATP/ADP ratio in H₂O₂-treated rat dorsal root ganglion (DRG) neurons. Moreover, we also demonstrated that ROS inhibitor (NAC) also inhibited H₂O₂-induced apoptosis and anaerobic glycolysis in rat DRG neurons. In conclusion, dexmedetomidine suppressed H₂O₂-induced apoptosis and anaerobic glycolysis activity by inhibiting ROS, in rat DRG neurons. Therefore, dexmedetomidine might play a pivotal role in neuropathic pain by the inhibition of ROS.

Aqueous leaf extract from Luehea divaricata Mart. Modulates oxidative stress markers in the spinal cord of rats with neuropathic pain

ETHNOPHARMACOLOGICAL RELEVANCE

Reactive oxygen species (ROS) play an important role in neuropathic pain (i.e., pain caused by lesion or disease of the somatosensory system). We showed previously that the aqueous extract prepared from Luehea divaricata leaves, a plant explored by native ethnic groups of Brazil to treat different pathologic conditions, exhibits good antioxidant activity and induces analgesia in rats with neuropathic pain (J Ethnopharmacol, 2020; 256:112761. doi: 10.1016/j.jep.2020.112761). The effect was comparable to that of gabapentin, a drug recommended as first-line treatment for neuropathic pain. However, increasing evidence has indicated the need to accurately determine the oxidative stress level of an individual before prescribing supplemental antioxidants.

AIM OF THE STUDY

This study assessed the effects of the oral administration of aqueous extract from leaves of L. divaricata on the sciatic functional index (SFI) and spinal-cord pro-oxidant and antioxidant markers of rats with neuropathic pain.

MATERIALS AND METHODS

Placement of four loose chromic thread ligatures around the sciatic nerve produced chronic constriction injury (CCI) of the sciatic nerve, a commonly employed animal model to study neuropathic pain. Aqueous extract from leaves of L. divaricata (100, 300, 500 and 1000 mg/kg), gabapentin (50 mg/kg) and aqueous extract (500 mg/kg) + gabapentin (30 mg/kg) were administrated per gavage daily for 10 or 35 days post-CCI. Antinociception was assessed using the von Frey test while SFI showed functional recovery post-nerve lesion throughout the experimental period. At days 10 and 35 post-surgery, the lumbosacral spinal cord and a segment of the injured sciatic nerve were dissected out and used to determine lipid hydroperoxide levels and total antioxidant capacity (TAC). The spinal cord was also used to determine superoxide anion generation (SAG), hydrogen peroxide (H₂O₂) levels and total thiol content.

RESULTS

As expected, the extract, gabapentin and extract + gabapentin induced antinociception in CCI rats. While no significant functional recovery was found at 10 days post-CCI, a significant recovery was found in SFI of extract-treated CCI rats at 21 and 35 days post-CCI. A significant functional recovery was found already at day 10 post-CCI in gabapentin and gabapentin + extract-treated CCI rats. The extract treatment prevented increases in lipid hydroperoxides levels and TAC in injured sciatic nerve, which were found in this tissue of vehicle-treated rats at 10 days post-CCI. Extract also prevented an increase in SAG, H₂O₂ and lipid hydroperoxides levels in the spinal cord, which were elevated in this tissue of vehicle-treated rats at 10 and 35 days post-CCI. Extract also prevented a decrease in total thiol content and an increase in TAC in the spinal cord of CCI rats in these same time periods.

CONCLUSIONS

Aqueous extract from L. divaricata leaves was demonstrated, for the first time, to improve SFI and modulate oxidative stress markers in injured sciatic nerve and spinal cord of CCI rats. Thus, the antinociceptive effect of the extract involves modulation of oxidative stress markers in injured sciatic nerve and spinal cord.

Electroacupuncture improves neuronal plasticity through the A2AR/cAMP/PKA signaling pathway in SNL rats

Improvements in neuronal plasticity are considered to be conducive to recovery from neuropathic pain. Electroacupuncture (EA) is regarded as an effective rehabilitation method for neuropathic pain. However, the effects and potential mechanism associated with EA-induced repair of hyperesthesia are not fully understood. Evidence has suggested that the adenosine A2A receptor (A2AR) and the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway play an important role in improving neuropathic pain. Here, we examined the function of EA in promoting neuronal plasticity in spinal nerve ligation (SNL) rats. The A2AR antagonist SCH58261, A2AR agonist 2-p-(2-carboxyethyl)phenethylamino-50-N-ethylcarboxamido adenosine HCl (CGS21680) and A2AR siRNA were used to confirm the relationship between A2AR and the cAMP/PKA pathway as well as the effects of A2AR on EA-induced improvements in neurobehavioral state and neuronal plasticity. Mechanical withdrawal threshold (MWT), thermal withdrawal latency (TWL), HE staining, Western blotting, RT-PCR, immunofluorescence, enzyme-linked immunosorbent assay, Nissl staining, silver staining, Golgi-Cox staining and transmission electron microscopy were used to evaluate the changes in neurobehavioral performance, protein expression, neuronal structure and dendrites/synapses. The results showed that EA and CGS21680 improved the behavioral performance, neuronal structure and dendritic/synaptic morphology of SNL rats, consistent with higher expression levels of A2AR, cAMP and PKA. In contrast to the positive effects of EA, SCH58261 inhibited dendritic growth and promoted dendritic spine/synaptic remodeling. In addition, the EA-induced improvement in neuronal plasticity was inhibited by SCH58261 and A2AR siRNA, consistent with lower expression levels of A2AR, cAMP and PKA, and worse behavioral performance. These results indicate that EA suppresses SNL-induced neuropathic pain by improving neuronal plasticity via upregulating the A2AR/cAMP/PKA signaling pathway.

Electroacupuncture may alleviate neuropathic pain via suppressing P2X7R expression

Neuropathic pain is a severe problem that is difficult to treat clinically. Reducing abnormal remodeling of dendritic spines/synapses and increasing the anti-inflammatory effects in the spinal cord dorsal horn are potential methods to treat this disease. Previous studies have reported that electroacupuncture (EA) could increase the pain threshold after peripheral nerve injury. However, the underlying mechanism is unclear. P2X7 receptors (P2X7R) mediate the activation of microglia and participate in the occurrence and development of neuropathic pain. We hypothesized that the effects of EA on relieving pain may be related to the downregulation of the P2X7R. Spinal nerve ligation (SNL) rats were used as a model in this experiment, and 2'(3')-O-(4-benzoyl)benzoyl ATP (BzATP) was used as a P2X7R agonist. We found that EA treatment decreased dendritic spine density, inhibited synaptic reconstruction and reduced inflammatory response, which is consistent with the decrease in P2X7R expression as well as the improved neurobehavioral performance. In contrast to the beneficial effects of EA, BzATP enhanced abnormal remodeling of dendritic spines/synapses and inflammation. Furthermore, the EA-mediated positive effects were reversed by BzATP, which is consistent with the increased P2X7R expression. These findings indicated that EA improves neuropathic pain by reducing abnormal dendritic spine/synaptic reconstruction and inflammation via suppressing P2X7R expression.

Neuroinflammation, oxidative stress and their interplay in neuropathic pain: Focus on specialized pro-resolving mediators and NADPH oxidase inhibitors as potential therapeutic strategies

Neuropathic pain (NP) is a chronic condition that results from a lesion or disease of the nervous system, greatly impacting patients' quality of life. Current pharmacotherapy options deliver inadequate and/or insufficient responses and thus a significant unmet clinical need remains for alternative treatments in NP. Neuroinflammation, oxidative stress and their reciprocal relationship are critically involved in NP pathophysiology. In this context, new pharmacological approaches, aiming at enhancing the resolution phase of inflammation and/or restoring redox balance by targeting specific reactive oxygen species (ROS) sources, are emerging as potential therapeutic strategies for NP, with improved efficacy and safety profiles. Several reports have demonstrated that administration of exogenous specialized pro-resolving mediators (SPMs) ameliorates NP pathophysiology. Likewise, deletion or inhibition of the ROS-generating enzyme NADPH oxidase (NOX), particularly its isoforms 2 and 4, results in beneficial effects in NP models. Notably, SPMs also modulate oxidative stress and NOX also regulates neuroinflammation. By targeting neuroinflammatory and oxidative pathways, both SPMs analogues and isoform-specific NOX inhibitors are promising therapeutic strategies for NP.

Possible implications of animal models for the assessment of visceral pain

Acute pain, provoked generally after the activation of peripheral nociceptors, is an adaptive sensory function that alerts the individual to avoid noxious stimuli. However, uncontrolled acute pain has a maladaptive role in sensory activity leading to development of a chronic pain state which persists even after the damage is resolved, or in some cases, in the absence of an initial local acute injury. Huge numbers of people suffer from visceral pain at least once during their life span, leading to substantial health care costs. Although studies reporting on the mechanism of visceral pain are accumulating, it is still not precisely understood. Therefore, this review aims to elucidate the mechanism of visceral pain through an evaluation of different animal models and their application to develop novel therapeutic approaches for treating visceral pain. To assess the nociceptive responses in viscera, several visceral pain models such as inflammatory, traction, stress and genetic models utilizing different methods of measurement have been devised. Among them, the inflammatory and traction models are widely used for studying the visceral pain mechanism of different disease conditions and post-operative surgery in humans and animals. A hapten, 2,4,6-trinitrobenzene sulfonic acid (TNBS), has been extensively used as an inflammatory agent to induce visceral pain. The traction model seems to cause a strong pain stimulation and autonomic reaction and could thus be the most appropriate model for studying the underlying visceral pain mechanism and for probing the therapeutic efficacies of various anesthetic and analgesics for the treatment of visceral pain and hyperalgesia.

The role of CGRP receptor antagonist (CGRP8-37) and Endomorphin-1 combination therapy on neuropathic pain alleviation and expression of Sigma-1 receptors and antioxidants in rats

BACKGROUND

Spinal cord injury is one of the most common causes of neuropathic pain which is not responsive to common treatments. Owing to the adverse effects of drugs, it seems that the use of Calcitonin Gene-Related Protein (CGRP) receptor antagonist or Morphine and their combination could be an appropriate strategy for pain alleviation.

METHOD

To achieve the objective, fifty six male Wistar rats were divided into seven groups. CGRP8-37 and Endomorphin-1 alone, and in combinated administration, as bolus and continues dose. Both mechanical and cold allodynia, and mechanical hyperalgesia were evaluated before and also15 and 60 min after injection to indicate the efficacy of the therapies in the acute and chronic circumstances on pain induced by spinal cord compression injury. Sigma-1 receptor experssion, oxidant and antioxidant activity after the seven days of the drug adminestration were evaluated.

RESULT

The results showed that Endomorphin-1and CGRP8-37 injections were able to reduce neuropathic pain after spinal cord compression injury. Compared to Endomorphin-1, or CGRP8-37 monotherapy, combination therapy did not show more attenuating effects on the pain threshold. Compared to the continous administration of Endomorphin-1 alone, and CGRP8-37 alone, the continous combination therapy did not reduce the pain further. Molecular studies disclosed the increased expression of the Sigma1 receptor, in the spinal cord after administration of Endomorphin-1, and CGRP8-37 alone, as well as combination therapy. Although, an increase in GPx and SOD activity, and decrease in MDA activity was observed in the combination therapy.

CONCLUSION

Our results demonstrate that either Endomorphin-1 or CGRP receptor antagonist is able to decrease the neuropathic pain after SCI but combination therapy by a CGRP receptor antagonist and Endomorphin-1 did not make any further reduction in pain sensation.

The etiological contribution of GABAergic plasticity to the pathogenesis of neuropathic pain

Neuropathic pain developing after peripheral or central nerve injury is the result of pathological changes generated through complex mechanisms. Disruption in the homeostasis of excitatory and inhibitory neurons within the central nervous system is a crucial factor in the formation of hyperalgesia or allodynia occurring with neuropathic pain. The central GABAergic pathway has received attention for its extensive distribution and function in neural circuits, including the generation and development of neuropathic pain. GABAergic inhibitory changes that occur in the interneurons along descending modulatory and nociceptive pathways in the central nervous system are believed to generate neuronal plasticity, such as synaptic plasticity or functional plasticity of the related genes or proteins, that is the foundation of persistent neuropathic pain. The primary GABAergic plasticity observed in neuropathic pain includes GABAergic synapse homo- and heterosynaptic plasticity, decreased synthesis of GABA, down-expression of glutamic acid decarboxylase and GABA transporter, abnormal expression of NKCC1 or KCC2, and disturbed function of GABA receptors. In this review, we describe possible mechanisms associated with GABAergic plasticity, such as central sensitization and GABAergic interneuron apoptosis, and the epigenetic etiologies of GABAergic plasticity in neuropathic pain. Moreover, we summarize potential therapeutic targets of GABAergic plasticity that may allow for successful relief of hyperalgesia from nerve injury. Finally, we compare the effects of the GABAergic system in neuropathic pain to other types of chronic pain to understand the contribution of GABAergic plasticity to neuropathic pain.

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.

Ryanodine Receptor to Mitochondrial Reactive Oxygen Species Pathway Plays an Important Role in Chronic Human Immunodeficiency Virus gp120MN-Induced Neuropathic Pain in Rats

BACKGROUND

Chronic pain is one of the most common complaints in patients with human immunodeficiency virus (HIV)-associated sensory neuropathy. Ryanodine receptor (RyR) and mitochondrial oxidative stress are involved in neuropathic pain induced by nerve injury. Here, we investigated the role of RyR and mitochondrial superoxide in neuropathic pain induced by repeated intrathecal HIV glycoprotein 120 (gp120) injection.

METHODS

Recombinant HIV glycoprotein gp120MN was intrathecally administered to induce neuropathic pain. Mechanical threshold was tested using von Frey filaments. Peripheral nerve fiber was assessed by the quantification of the intraepidermal nerve fiber density in the skin of the hindpaw. The expression of spinal RyR was examined using Western blots. Colocalization of RyR with neuronal nuclei (NeuN; neuron marker), glial fibrillary acidic protein (GFAP; astrocyte marker), or ionizing calcium-binding adaptor molecule 1 (Iba1; microglia marker) in the spinal cord was examined using immunohistochemistry. MitoSox-positive profiles (a mitochondrial-targeted fluorescent superoxide indicator) were examined. The antiallodynic effects of intrathecal administration of RyR antagonist, dantrolene (a clinical drug for malignant hyperthermia management), or selective mitochondrial superoxide scavenger, Mito-Tempol, were evaluated in the model.

RESULTS

We found that repeated but not single intrathecal injection of recombinant protein gp120 induced persistent mechanical allodynia. Intraepidermal nerve fibers in repeated gp120 group was lower than that in sham at 2 weeks, and the difference in means (95% confidence interval) was 8.495 (4.79-12.20), P = .0014. Repeated gp120 increased expression of RyR, and the difference in means (95% confidence interval) was 1.50 (0.504-2.495), P = .007. Repeated gp120 also increased mitochondrial superoxide cell number in the spinal cord, and the difference in means (95% confidence interval) was 6.99 (5.99-8.00), P < .0001. Inhibition of spinal RyR or selective mitochondrial superoxide scavenger dose dependently reduced mechanical allodynia induced by repeated gp120 injection. RyR and mitochondrial superoxide were colocalized in the neuron, but not glia. Intrathecal injection of RyR inhibitor lowered mitochondrial superoxide in the spinal cord dorsal horn in the gp120 neuropathic pain model.

CONCLUSIONS

These data suggest that repeated intrathecal HIV gp120 injection induced an acute to chronic pain translation in rats, and that neuronal RyR and mitochondrial superoxide in the spinal cord dorsal horn played an important role in the HIV neuropathic pain model. The current results provide evidence for a novel approach to understanding the molecular mechanisms of HIV chronic pain and treating chronic pain in patients with HIV.

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.

Neonatal Injury Alters Sensory Input and Synaptic Plasticity in GABAergic Interneurons of the Adult Mouse Dorsal Horn

Neonatal tissue injury disrupts the balance between primary afferent-evoked excitation and inhibition onto adult spinal projection neurons. However, whether this reflects cell-type-specific alterations at synapses onto ascending projection neurons, or rather is indicative of global changes in synaptic signaling across the mature superficial dorsal horn (SDH), remains unknown. Therefore the present study investigated the effects of neonatal surgical injury on primary afferent synaptic input to adult mouse SDH interneurons using in vitro patch-clamp techniques. Hindpaw incision at postnatal day (P)3 significantly diminished total primary afferent-evoked glutamatergic drive to adult Gad67-GFP and non-GFP neurons, and reduced their firing in response to sensory input, in both males and females. Early tissue damage also shaped the relative prevalence of monosynaptic A- versus C-fiber-mediated input to mature GABAergic neurons, with an increased prevalence of Aβ- and Aδ-fiber input observed in neonatally-incised mice compared with naive littermate controls. Paired presynaptic and postsynaptic stimulation at an interval that exclusively produced spike timing-dependent long-term potentiation (t-LTP) in projection neurons predominantly evoked NMDAR-dependent long-term depression in naive Gad67-GFP interneurons. Meanwhile, P3 tissue damage enhanced the likelihood of t-LTP generation at sensory synapses onto the mature GABAergic population, and increased the contribution of Ca²⁺-permeable AMPARs to the overall glutamatergic response. Collectively, the results indicate that neonatal injury suppresses sensory drive to multiple subpopulations of interneurons in the adult SDH, which likely represents one mechanism contributing to reduced feedforward inhibition of ascending projection neurons, and the priming of developing pain pathways, following early life trauma.SIGNIFICANCE STATEMENT Mounting clinical and preclinical evidence suggests that neonatal tissue damage can result in long-term changes in nociceptive processing within the CNS. Although recent work has demonstrated that early life injury weakens the ability of sensory afferents to evoke feedforward inhibition of adult spinal projection neurons, the underlying circuit mechanisms remain poorly understood. Here we demonstrate that neonatal surgical injury leads to persistent deficits in primary afferent drive to both GABAergic and presumed glutamatergic neurons in the mature superficial dorsal horn (SDH), and modifies activity-dependent plasticity at sensory synapses onto the GABAergic population. The functional denervation of spinal interneurons within the mature SDH may contribute to the "priming" of developing pain pathways following early life injury.

Pioglitazone, a PPARγ agonist, reduces cisplatin-evoked neuropathic pain by protecting against oxidative stress

Painful peripheral neuropathy is a dose-limiting side effect of cisplatin treatment. Using a murine model of cisplatin-induced hyperalgesia, we determined whether a PPARγ synthetic agonist, pioglitazone, attenuated the development of neuropathic pain and identified underlying mechanisms. Cisplatin produced mechanical and cold hyperalgesia and decreased electrical thresholds of Aδ and C fibers, which were attenuated by coadministration of pioglitazone (10 mg/kg, intraperitoneally [i.p.]) with cisplatin. Antihyperalgesic effects of pioglitazone were blocked by the PPARγ antagonist T0070907 (10 mg/kg, i.p.). We hypothesized that the ability of pioglitazone to reduce the accumulation of reactive oxygen species (ROS) in dorsal root ganglion (DRG) neurons contributed to its antihyperalgesic activity. Effects of cisplatin and pioglitazone on somatosensory neurons were studied on dissociated mouse DRG neurons after 24 hours in vitro. Incubation of DRG neurons with cisplatin (13 µM) for 24 hours increased the occurrence of depolarization-evoked calcium transients, and these were normalized by coincubation with pioglitazone (10 µM). Oxidative stress in DRG neurons was considered a significant contributor to cisplatin-evoked hyperalgesia because a ROS scavenger attenuated hyperalgesia and normalized the evoked calcium responses when cotreated with cisplatin. Pioglitazone increased the expression and activity of ROS-reducing enzymes in DRG and normalized cisplatin-evoked changes in oxidative stress and labeling of mitochondria with the dye MitoTracker Deep Red, indicating that the antihyperalgesic effects of pioglitazone were attributed to its antioxidant properties in DRG neurons. These data demonstrate clear benefits of broadening the use of the antidiabetic drug pioglitazone, or other PPARγ agonists, to minimize the development of cisplatin-induced painful neuropathy.

Neonatal vincristine administration modulates intrinsic neuronal excitability in the rat dorsal root ganglion and spinal dorsal horn during adolescence

Our recent work has shown that the early-life administration of vincristine (VNC), commonly used to treat pediatric cancers, evokes mechanical pain hypersensitivity in rats that emerges during adolescence and persists into adulthood. However, the underlying mechanisms remain unclear, as nothing is known about how neonatal VNC treatment influences peripheral and central nociceptive processing at the cellular level. Here, we used in vitro intracellular microelectrode and whole-cell patch-clamp recordings to evaluate the consequences of early-life VNC administration on the intrinsic membrane properties of adolescent dorsal root ganglion and spinal superficial dorsal horn neurons. The results demonstrate that VNC treatment increased the prevalence and rate of repetitive firing in both large- and medium-diameter sensory neurons, while reducing repetitive firing in small-diameter neurons, in comparison with vehicle-treated littermate controls. By contrast, passive membrane properties and peripheral conduction velocities were similar between experimental groups across all classes of primary afferents. Within the adolescent superficial dorsal horn, neonatal VNC exposure significantly enhanced the intrinsic membrane excitability of lamina I spinoparabrachial neurons, as evidenced by a decrease in rheobase and elevation of repetitive firing frequency compared with controls. Meanwhile, putative interneurons within lamina I exhibited a reduction in repetitive action potential discharge after early-life chemotherapy. Collectively, these findings suggest that neonatal VNC treatment evokes cell type-specific changes in intrinsic excitability at multiple levels of the ascending pain pathway. Overall, this work lays an essential foundation for the future exploration of the ionic mechanisms that drive chemotherapy-induced chronic pain in children and adolescents.

Mitochondrial superoxide increases excitatory synaptic strength in spinal dorsal horn neurons of neuropathic mice

Reactive oxygen species has been suggested as a key player in neuropathic pain, causing central sensitization by changing synaptic strengths in spinal dorsal horn neurons. However, it remains unclear as to what type of reactive oxygen species changes what aspect of synaptic strengths for central sensitization in neuropathic pain conditions. In this study, we investigated whether mitochondrial superoxide affects both excitatory and inhibitory synaptic strengths in spinal dorsal horn neurons after peripheral nerve injury. Upregulation of mitochondrial superoxide level by knockout of superoxide dismutase-2 exacerbated neuropathic mechanical hypersensitivity caused by L5 spinal nerve ligation, whereas downregulation of mitochondrial superoxide level by overexpression of superoxide dismutase-2 alleviated the hypersensitivity. In spinal nerve ligation condition, the frequency of miniature excitatory postsynaptic currents increased, while that of miniature inhibitory postsynaptic currents decreased in spinal dorsal horn neurons. Superoxide dismutase-2-knockout augmented, whereas superoxide dismutase-2-overexpression prevented, the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency. However, superoxide dismutase-2-knockout had no effect on the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency, and superoxide dismutase-2-overexpression unexpectedly decreased miniature inhibitory postsynaptic current frequency in the normal condition. When applied to the spinal cord slice during in vitro recordings, mitoTEMPO, a specific scavenger of mitochondrial superoxide, reduced the spinal nerve ligation-increased miniature excitatory postsynaptic currents frequency but failed to normalize the spinal nerve ligation-decreased miniature inhibitory postsynaptic current frequency. These results suggest that in spinal dorsal horn neurons, high levels of mitochondrial superoxide increase excitatory synaptic strength after peripheral nerve injury and contribute to neuropathic mechanical hypersensitivity. However, mitochondrial superoxide does not seem to be involved in the decreased inhibitory synaptic strength in this neuropathic pain condition.

Cisplatin treatment induces attention deficits and impairs synaptic integrity in the prefrontal cortex in mice

Patients treated for cancer frequently experience chemobrain, characterized by impaired memory and reduced attention. These deficits often persist after treatment, and no preventive or curative interventions exist. In mice, we assessed the effect of cisplatin chemotherapy on attention using the 5-choice serial reaction time task and on synaptic integrity. We also assessed the capacity of mesenchymal stem cells to normalize the characteristics of chemobrain. Mice were trained in the 5-choice serial reaction time task. After reaching advancement criteria at a 4-second stimulus time, they were treated with cisplatin followed by nasal administration of mesenchymal stem cells. Cisplatin reduced the percentage of correct responses due to an increase in omissions, indicating attention deficits. Mesenchymal stem cell treatment reversed these cisplatin-induced deficits in attention. Cisplatin also induced abnormalities in markers of synaptic integrity in the prefrontal cortex. Specifically, cisplatin decreased expression of the global presynaptic marker synaptophysin and the glutamatergic presynaptic marker vGlut2. Expression of the presynaptic GABAergic marker vGAT increased. Nasal mesenchymal stem cell administration normalized these markers of synaptic integrity. In conclusion, cisplatin induces long-lasting attention deficits that are associated with decreased synaptic integrity in the prefrontal cortex. Nasal administration of mesenchymal stem cells reversed these behavioural and structural deficits.

Cell toxicity mechanism and biomarker

Cell toxicity may result in organ dysfunction and cause severe health problem. Recent studies revealed many toxicants may induced the over production of Nitric oxide, reactive oxygen species and the subsequent oxidative stress, cause cell toxicity. Mitochondrion dysfunction maybe the subsequent consequence of oxidative stress and has been recognized as another contributing factor in cell toxicity. Besides, oxidative products induced by some toxicants may also produce the compounds that damage cell DNA, leading to toxicity. Especially, the significance of nanoparticle induced cell toxicity was disclosed recently and attract more concern. The mechanism mainly includes inflammation, oxidative stress and DNA damage. On the other side, some biomarkers of cell toxicity including autophagy, cytokines, miRNA has been identified. The understanding of these phenomenon may enable us to clarify the cell toxicity mechanism then contribute to cell toxicity protection, disease treatment and drug side effect prevention.

Impaired Autophagy of GABAergic Interneurons in Neuropathic Pain

Neuropathic pain (NP) is caused by lesions of the peripheral fibers and central neurons in the somatosensory nervous system and affects 7-10% of the general population. Although the distinct cause of neuropathic pain has been investigated in primary afferent neurons over the years, pain modulation by central sensitization remains controversial. NP is believed to be driven by cell type-specific spinal synaptic plasticity in the dorsal horn. Upon intense afferent stimulation, spinothalamic tract neurons are potentiated, whereas GABAergic interneurons are inhibited leading to long-term depression. Growing evidences suggest that the inhibition of GABAergic neurons plays pivotal roles in the manifestation of neuropathic and inflammatory pain states. Downregulation of GABA transmission and impairment of GABAergic interneurons in the dorsal horn are critical consequences after spinal cord and peripheral nerve injuries. These impairments in GABAergic interneurons may be associated with dysfunctional autophagy, resulting in neuropathic pain. Here, we review an emerging number of investigations that suggest a pivotal role of impaired autophagy of GABAergic interneurons in NP. We discuss relevant research spurring the development of new targets and therapeutic agents of NP and emphasize the need for a multidisciplinary approach to manage NP in the future.