The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Neuropathic pain is a severe and chronic condition widely found in the general population.The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients.During the processing of pain,the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation.Furthermore,the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies.Here,we will review the complex interplay between cells(satellite glial cells and inflammatory cells)and factors(cytokines,neurotrophic factors and genetic factors)that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain.More importantly,we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

Activation of G-protein-coupled receptor 39 reduces neuropathic pain in a rat model

Activated G-protein-coupled receptor 39(GPR39)has been shown to attenuate inflammation by interacting with sirtuin 1(SIRT1)and peroxisome proliferator-activated receptor-γcoactivator 1α(PGC-1α).However,whether GPR39 attenuates neuropathic pain remains unclear.In this study,we established a Sprague-Dawley rat model of spared nerve injury-induced neuropathic pain and found that GPR39 expression was significantly decreased in neurons and microglia in the spinal dorsal horn compared with sham-operated rats.Intrathecal injection of TC-G 1008,a specific agonist of GPR39,significantly alleviated mechanical allodynia in the rats with spared nerve injury,improved spinal cord mitochondrial biogenesis,and alleviated neuroinflammation.These changes were abolished by GPR39 small interfering RNA(siRNA),Ex-527(SIRT1 inhibitor),and PGC-1αsiRNA.Taken together,these findings show that GPR39 activation ameliorates mechanical allodynia by activating the SIRT1/PGC-1αpathway in rats with spared nerve injury.

Dynamic long-term microstructural and ultrastructural alterations in sensory nerves of rats of paclitaxel-induced neuropathic pain

Background Paclitaxel,as a first line anti-neoplastic compound,frequently produces long-term pain after tumors have been treated.Clinical manifestations are varied and non-specific.Pathology of the nervous system during the development of the neuropathic pain is unclear.Thus,eady diagnosis and treatment is often unsatisfying for patients.This study aimed to promote considerate understanding of the structural alteration of sensory nerves.Methods All rats were simply randomized into 3 groups：paclitaxel group,vehicle group and saline group.An established rat model of paclitaxel-induced peripheral neuropathy （2 mg/kg） was chosen for our research,behavior tests were operated during the procedure of 56 days.All rats were sampled on days 0,3,7,28 and 56.The hind paw plantar skin,sciatic nerves,dorsal root ganglion and attached fibers,and lumbar spinal cord were processed for light and electron microscopy.The differences among 3 groups were analyzed with one-way analysis of vadance （ANOVA）.Results We affirmed that paclitaxel-induced mechano-aliodynia and mechano-hyperalgesia occured after a 3-7-day delay,and this pain peaked at day 28 and persisted to day 56.Paclitaxel and vehicle treatment both evoked thermalhyperalgesia.Paclitaxel-induced axonal and myelin sheath degeneration was evident.At days 3 and 7,significant increases in atypical mitochondria in both myelinated axons and C-fibers of paclitaxel-treated nerves indicated that injured mitochondria correlated to specific paclitaxel-induced neuropathic pain,and the abnormity sustained till day 56.Microtubule was unaffected in myelinated axons or C-fibers in paclitaxel-or vehicle-treated rats.Significant increase of G ratio was evident with paclitaxel injection at days 7 and 28.Conclusion Our research suggests a causal role for axonal degeneration,abnormalities in axonal mitochondria,and structural modification of axonal microtubules in paclitaxel-induced neuropathic pain,and the abnormal mitochondria could be connected to the chronic neuropathic pain.

Epigenetic combined with transcriptomic analysis of the m6A methylome after spared nerve injury-induced neuropathic pain in mice

Epigenetic changes in the spinal cord play a key role in the initiation and maintenance of nerve injury-induced neuro pathic pain.N6-methyladenosine(m6A)is one of the most abundant internal RNA modifications and plays an essential function in gene regulation in many diseases.However,the global m6A modification status of mRNA in the spinal cord at different stages after neuropathic pain is unknown.In this study,we established a neuropathic pain model in mice by preserving the complete sural nerve and only damaging the common peroneal nerve.High-throughput methylated RNA immunoprecipitation sequencing res ults showed that after spared nerve injury,there were 55 m6A methylated and diffe rentially expressed genes in the spinal cord.Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway results showed that m6A modification triggered inflammatory responses and apoptotic processes in the early stages after spared nerve injury.Over time,the diffe rential gene function at postoperative day 7 was enriched in "positive regulation of neurogenesis" and "positive regulation of neural precursor cell prolife ration." These functions suggested that altered synaptic morphological plasticity was a turning point in neuropathic pain formation and maintenance.Results at postoperative day 14 suggested that the persistence of neuropathic pain might be from lipid metabolic processes,such as "very-low-density lipoprotein particle clearance," "negative regulation of choleste rol transport" and "membrane lipid catabolic process." We detected the expression of m6A enzymes and found elevated mRNA expression of Ythdf2 and Ythdf3 after spared nerve injury modeling.We speculate that m6A reader enzymes also have an important role in neuropathic pain.These results provide a global landscape of mRNA m6A modifications in the spinal cord in the spared nerve injury model at diffe rent stages after injury.

Diabetic neuropathic pain:Physiopathology and treatment

Diabetic neuropathy is a common complication of both type 1 and type 2 diabetes,which affects over 90% of the diabetic patients.Although pain is one of the main symptoms of diabetic neuropathy,its pathophysiological mechanisms are not yet fully known.It is widely accepted that the toxic effects of hyperglycemia play an important role in the development of this complication,but several other hypotheses have been postulated.The management of diabetic neuropathic pain consists basically in excluding other causes of painful peripheral neuropathy,improving glycemic control as a prophylactic therapy and using medications to alleviate pain.First line drugs for pain relief include anticonvulsants,such as pregabalin and gabapentin and antidepressants,especial y those that act to inhibit the reuptake of serotonin and noradrenaline.In addition,there is experimental and clinical evidence that opioids can be helpful in pain control,mainly if associated with first line drugs.Other agents,including for topical application,such as capsaicin cream and lidocaine patches,have also been proposed to be useful as adjuvants in the control of diabetic neuropathic pain,but the clinical evidence is insufficient to support their use.In conclusion,a better understanding of the mechanisms underlying diabetic neuropathic pain will contribute to the search of new therapies,but also to the improvement of the guidelines to optimize pain control with the drugs currently available.

Electroacupuncture attenuates neuropathic pain after brachial plexus injury

Electroacupuncture has traditionally been used to treat pain, but its effect on pain following brachial plexus injury is still unknown. In this study, rat models of an avulsion injury to the left brachial plexus root （associated with upper-limb chronic neuropathic pain） were given electroacupuncture stimulation at bilateral Quchi （LIll）, Hegu （LI04）, Zusanli （ST36） and Yanglingquan （GB34）. After electroacupuncture therapy, chronic neuropathic pain in the rats＇ upper limbs was significantly attenuated. Immunofluorescence staining showed that the expression of β-endorphins in the arcuate nucleus was significantly increased after therapy. Thus, experimental findings indi- cate that electroacupuncture can attenuate neuropathic pain after brachial plexus injury through upregulating β-endorphin expression.

Rapamycin Ameliorates Neuropathic Pain by Activating Autophagy and Inhibiting Interleukin-1β in the Rat Spinal Cord

Autophagy acts as an important homoeostatic mechanism by degradation of cytosolic con- stituents and plays roles in many physiological processes. Recent studies demonstrated that autophagy can also regulate the production and secretion of the proinflammatory cytokine interleukin-1β （IL-1β）, which plays a critical role in the development and maintenance ofneuropathic pain. In the present study, the paw withdrawal threshold （PWT） and paw withdrawal latency （PWL） were significantly decreased after spinal nerve ligation （SNL）, and the changes were accompanied by inhibited autophagy in the spi- nal microglia and increased mR.NA and protein levels of IL-1β in the ipsilateral spinal cord. We then investigated the antinociceptive effect of rapamycin, a widely used autopahgy inducer, on SNL-induced neuropathic pain in rats and found that treatment with intrathecal rapamycin significantly attenuated the mechanical allodynia and thermal hyperalgesia. Moreover, rapamycin significantly enhanced autophagy in the spinal microglia, whereas it reduced the mRNA and protein levels of IL-1β in the ipsilateral spinal cord. Our results showed that rapamycin could ameliorate neuropathic pain by activating autophagy and inhibiting IL-1β in the spinal cord.

Controlled-Release Oxycodone Alone or Combined with Gabapentin for Management of Malignant Neuropathic Pain

Objective： To evaluate the analgesic efficacy of controlled-release （CR） oxycodone and gabapentin in malignant neuropathic pain （NP）. Methods： Patients with malignant NP were enrolled and baseline pain intensity （PI） was recorded. They initially took one week CR oxycodone and were allocated to two different groups at day 8 by reevaluated PI. Patients with mild pain went to CR oxycodone mono-therapy group （OO group） and took another two weeks CR oxycodone. Others went to （CR oxycodone combined gabapentin group （OG group） and received additional gabapentin. Daily doses and side effects were recorded. Results： Fifty-eight （92.06%） of the 63 enrolled patients completed the initial week＇s therapy. Twenty-two （37.93%） went to OO group and PI significantly reduced at day 15 （2.00 vs. 2.62, P=0.004）, but not improved at day 22 （1.90 vs. 2.00, P=0.54）. Thirty-six （62.07%） patients went to OG group and PI was significantly reduced at day 15 （4,47 vs. 2.94, P〈0.001）, but not improved at day 22 （2.94 vs. 2.75, P=0.136）. Mean daily dose （MDD） of CR oxycodone at day 8 was 62.64 mg. It was significantly increased at days 15 and 22 （71.43 mg vs. 62.64 rag, P=0.021; 81.90 mg vs. 71.43 mg, P=0.004） in OO group. MDD of gabapentin was significantly increased at day 22 compared to day 15 （862.50 mg vs. 993.75 mg, P〈0.001）. Constipation was occurred in 13.64% of the patients in OO group and 14.26 % in OG group. Conclusion： Malignant NP may be well controlled by oxycodone mono-therapy. Early combination with gabapentin is sensible when pain is not satisfactory relieved by oxycodone alone. The side effects of them are manageable.

Molecular mechanisms underlying the effects of acupuncture on neuropathic pain

Acupuncture has been used to treat neuropathic pain for a long time, but its mechanisms of action remain unknown. In this study, we observed the effects of electroacupuncture and manual acu- puncture on neuropathic pain and on ephrin-B/EphB signaling in rats models of chronic constriction injury-induced neuropathic pain. The results showed that manual acupuncture and electroacu- puncture significantly reduced mechanical hypersensitivity following chronic constriction injury, es- pecially electroacupuncture treatment. Real-time PCR results revealed that ephrin-B1/B3 and EphB1/B2 mRNA expression levels were significantly increased in the spinal dorsal horns of chronic constriction injury rats. Electroacupuncture and manual acupuncture suppressed the high expres- sion of ephrin-B1 mRNA, and elevated EphB3/B4 mRNA expression. Electroacupuncture signifi- cantly enhanced the mRNA expression of ephrin-B3 and EphB3/B6 in the dorsal horns of neuro- pathic pain rats. Western blot results revealed that electroacupuncture in particular, and manual acupuncture, significantly up-regulated ephrin-B3 protein levels in rat spinal dorsal horns. The re- sults of this study suggest that acupuncture could activate ephrin-B/EphB signaling in neuropathic pain rats and improve neurological function.

Persistent post-surgical pain and neuropathic pain after total knee replacement

AIM: To study the prevalence of persistent post-surgical pain(PPSP) and neuropathic pain(NP) after total knee replacement(TKR).METHODS: MEDLINE and Embase databases were searched for articles published until December 2014 in English language. Published articles were included if they referred to pain that lasts at least 3 mo after primary TKR for knee osteoarthritis, and measured pain with pain specific instruments. Studies that referred to pain caused by septic reasons and implant malalignment were excluded. Both prospective and retrospective studies were included and only 14 studies that match the inclusion criteria were selected for this review.RESULTS: The included studies were characterized by the heterogeneity on the scales used to measure pain and pre-operative factors related to PPSP and NP. The reported prevalence of PPSP and NP seems to be relatively high, but it varies among different studies. There is also evidence that the prevalence of post-surgical pain is related to the scale used for pain measurement. The prevalence of PPSP is ranging at 6 mo from 16% to 39% and at 12 mo from 13.1% to 23% and even 38% of the patients. The prevalence of NP at 6 mo post-operatively is ranging from 5.2% to13%. Pre-operative factors related to the development of PPSP also differ, including emotional functioning, such as depression and pain catastrophizing, number of comorbidities, pain problems elsewhere and operations in knees with early grade of osteoarthritis.CONCLUSION: No firm conclusions can be reached regarding the prevalence of PPSP and NP and the related factors due to the heterogeneity of the studies.

Effects of Repetitive Transcranial Magnetic Stimulation on Astrocytes Proliferation and nNOS Expression in Neuropathic Pain Rats

This study investigated the effects of different frequencies of repetitive transcranial magnetic stimulation （rTMS） on chronic neuropathic pain in rats. The behavior of rats with experimental chronic neuropathic pain was observed, and the expression of neuronal nitric oxide synthase （nNOS） in the ipsilateral dorsal root ganglions （DRGs） and the activation and proliferation of astrocytes in the ipsilateral spinal dorsal horn were detected. Thirty-two male Sprague-Dawley rats were randomly divided into four groups： sham-operated group, sham-rTMS group, 1 Hz group and 20 Hz group （8 rats in each group）. Chronic constriction nerve injury induced by sciatic nerve ligation was made to establish the models of the chronic neuropathic pain in rats except those in the sham-operated group. Then we applied different frequencies of rTMS to the primary motor cortex （M1） contralateral to the pain side once daily for 10 consecutive days. Pain behavior scores were observed before and after treatment. Western blot analysis was used to detect the expression of nNOS in ipsilateral L4-6 DRGs. Double immunofluorescent labeling for glial fibrillary acidic protein （GFAP） and 5-bromo-2- deoxyuridine （BrdU） was employed to observe the activation and proliferation of astrocytes in the ipsilateral L4-6 spinal dorsal horn. After rTMS treatment, the spontaneous pain behavior scores were significantly lower in the 20 Hz group than those in the sham-rTMS group （P〈0.05）. Moreover, the brush-evoked pain behavior scores were significantly lower in the 20 Hz group than those in the sham-rTMS and 1 Hz group （P〈0.05）, suggesting that the spontaneous pain and brush-evoked pain in the 20 Hz group were significantly alleviated. Western blot analysis revealed that the expression of nNOS in ipsilateral L4-6 DRGs was significantly decreased in the 20 Hz group as compared with the sham-rTMS group and the 1 Hz group （P〈0.01） after rTMS treatment. Double immunofluorescence suggested that the expression of GFAP and the co-localization with BrdU in astrocytes were less in the sham-operated group than those in the sham-rTMS group and the 1 Hz group in L4-6 spinal dorsal horn ipsilateral to the neuropathic pain. After rTMS treatment, the expression of GFAP and the co-localization with BrdU decreased in the 20 Hz group as compared with the sham-rTMS group and the 1 Hz group （P〈0.05）. In addition, the alleviation degree of spontaneous pain and brush-evoked pain in the 20 Hz group was negatively correlated with the expression of nNOS in ipsilateral DRGs and the number of GFAP/BrdU co-labelled astrocytes in L4-6 spinal dorsal horn ipsilateral to the neuropathic pain （P〈0.05）. It was suggested that high-frequency rTMS may relieve neuropathic pain through down-regulating the overexpression of nNOS in ipsilateral DRGs and inhibiting the activity and proliferation of astrocytes in L4-6 spinal dorsal horn ipsilateral to the neuropathic pain.

Estrogen affects neuropathic pain through upregulating N-methyl-D-aspartate acid receptor 1 expression in the dorsal root ganglion of rats

Estrogen affects the generation and transmission of neuropathic pain,but the specific regulatory mechanism is still unclear.Activation of the N-methyl-D-aspartate acid receptor 1（NMDAR1） plays an important role in the production and maintenance of hyperalgesia and allodynia.The present study was conducted to determine whether a relationship exists between estrogen and NMDAR1 in peripheral nerve pain.A chronic sciatic nerve constriction injury model of chronic neuropathic pain was established in rats.These rats were then subcutaneously injected with 17β-estradiol,the NMDAR1 antagonist D（-）-2-amino-5-phosphonopentanoic acid（AP-5）,or both once daily for 15 days.Compared with injured drug na？ve rats,rats with chronic sciatic nerve injury that were administered estradiol showed a lower paw withdrawal mechanical threshold and a shorter paw withdrawal thermal latency,indicating increased sensitivity to mechanical and thermal pain.Estrogen administration was also associated with increased expression of NMDAR1 immunoreactivity（as assessed by immunohistochemistry） and protein（as determined by western blot assay） in spinal dorsal root ganglia.This 17β-estradiol-induced increase in NMDAR1 expression was blocked by co-administration with AP-5,whereas AP-5 alone did not affect NMDAR1 expression.These results suggest that 17β-estradiol administration significantly reduced mechanical and thermal pain thresholds in rats with chronic constriction of the sciatic nerve,and that the mechanism for this increased sensitivity may be related to the upregulation of NMDAR1 expression in dorsal root ganglia.

Analgesic effect of AG490, a Janus kinase inhibitor, on oxaliplatin-induced acute neuropathic pain

Neuropathic pain often occurs during chemotherapy with oxaliplatin. AG490 has been shown to exert an antagonistic effect on inflammatory pain, but its effect on oxaliplatin-induced neuropathic pain remains poorly understood. This study sought to observe the analgesic effect of AG490 on acute neuropathic pain induced by a single oxaliplatin treatment and to address the possible mechanism. In this study, we estab- lished a model of oxaliplatin-induced acute neuropathic pain by intraperitoneal injection of 6 mg/kg oxaliplatin. On day 2 after injection, models were intraperitoneally injected with i, 5, or 10 mg/kg AG490. Paw withdrawal threshold to mechanical stimuli and tail withdrawal latency to cold stimuli were determined. Western blot assay was performed to detect the expression of spinal phosphorylated signal transducer and activator of transcription 3 （p-STAT3）. Immunohistochemistry was used to determine the immunoreactivity of p-STAT3 and inter- leukin-6. Results demonstrated that paw withdrawal threshold and tail withdrawal latency were significantly increased by the treatment of AG490 in rats. There was no significant difference in the effect among the different doses of AG490. AG490 10 mg/kg decreased the expression of p-STAT3, the immunoreactivity of p-STAT3 and interleukin-6 in spinal cord of acute neuropathic pain rats. These findings confirm that AG490 can attenuate oxaliplatin-induced acute neuropathic pain and is associated with the inhibition in the JAK/STAT3 signaling pathway.

Proteomic analysis of the dorsal spinal cord in the mouse model of spared nerve injury-induced neuropathic pain

Peripheral nerve injury often causes neuropathic pain and is associated with changes in the expression of numerous proteins in the dorsal horn of the spinal cord. To date, proteomic analysis method has been used to simultaneously analyze hundreds or thousands of proteins differentially expressed in the dorsal horn of the spinal cord in rats or dorsal root ganglion of rats with certain type of peripheral nerve injury. However, a proteomic study using a mouse model of neuropathic pain could be attempted because of abundant protein database and the availability of transgenic mice. In this study, whole proteins were extracted from the ipsilateral dorsal half of the 4th-6th lumbar spinal cord in a mouse model of spared nerve injury（SNI）-induced neuropathic pain. In-gel digests of the proteins size-separated on a polyacrylamide gel were subjected to reverse-phase liquid-chromatography coupled with electrospray ionization ion trap tandem mass spectrometry（MS/MS）. After identifying proteins, the data were analyzed with subtractive proteomics using ProtAn, an in-house analytic program. Consequently, 15 downregulated and 35 upregulated proteins were identified in SNI mice. The identified proteins may contribute to the maintenance of neuropathic pain,and may provide new or valuable information in the discovery of new therapeutic targets for neuropathic pain.

Effects of Electroacupuncture on the Pain Threshold and the NMDA R1 mRNA in DRG on Neuropathic Pain Rats

To observe the effect of multiple electroacupuncture (EA) on the pain threshold and the regulation of N-methyl-D-aspartate (NMDA) receptor in dorsal root ganglia (DRG) of neuropathic pain rats. Rats were prepared with a unilateral chronic constriction injury (CCI) to the sciatic nerve. EA was done in acupoints 'Huan Tiao' and 'Yang Ling Quan' for 30 min every day and the thermal thresholds were detected after EA at 3, 5, 7, 10, 14 days after operation. On day 14 after nerve injury, the in situ hybridization method was used to investigate the change of NMDA R1 mRNA in L4-L5 DRG. The thermal threshold reduced significantly from day 3 after operation in CCI rats. After multiple EA treatment, the ipsilateral thermal hyperalgesia relieved gradually and the thermal threshold had no difference with control side after day 5(P>0. 05). From Day 7 after operation, the thermal threshold at each time point were significantly different compared with CCI group respective-ly(P>0. 05). Moreover the EA had accumulative effect. On Day 14 after operation , the NMDAR1 mRNA positive neurons and the mean optic density in ipsilateral L4-5 DRG were less than that of control side(P<0. 05), mainly in medium and small neurons. After EA treatment, the NMDARl mRNA positive neurons in ipsilateral DRG had no considerable difference comparing with those of control side , significantly increased comparing with CCI group(P<0. 05). It's concluded that the NMDA receptors in DRG relate closely with the generation and development of neuropathic pain. The multiple EA treatment can attenuate the thermal hyperlgesia of neuropathic pain rats and regulate the NMDA receptor.

Treatment with NADPH oxidase inhibitor apocynin alleviates diabetic neuropathic pain in rats

Increased reactive oxygen species by the activation of NADPH oxidase（NOX） contributes to the development of diabetic complications.Apocynin,a NOX inhibitor,increases sciatic nerve conductance and blood flow in diabetic rats.We investigated potential protective effect of apocynin in rat diabetic neuropathy and its precise mechanism of action at molecular level.Rat models of streptozotocin-induced diabetes were treated with apocynin（30 and 100 mg/kg per day,intragastrically） for 4 weeks.Mechanical hyperalgesia and allodynia were determined weekly using analgesimeter and dynamic plantar aesthesiometer.Western blot analysis and histochemistry/immunohistochemistry were performed in the lumbar spinal cord and sciatic nerve respectively.Streptozotocin injection reduced pain threshold in analgesimeter,but not in aesthesiometer.Apocynin treatment increased pain threshold dose-dependently.Western blot analysis showed an increase in catalase and NOX-p47 phox protein expression in the spinal cord.However,protein expressions of neuronal and inducible nitric oxide synthase（n NOS,i NOS）,superoxide dismutase,glutathion peroxidase,nitrotyrosine,tumor necrosis factor-α,interleukin-6,interleukin-1β,aldose reductase,cyclooxygenase-2 or MAC-1（marker for increased microgliosis） in the spinal cord remained unchanged.Western blot analysis results also demonstrated that apocynin decreased NOX-p47 phox expression at both doses and catalase expression at 100 mg/kg per day.Histochemistry of diabetic sciatic nerve revealed marked degeneration.n NOS and i NOS immunoreactivities were increased,while S-100 immunoreactivity（Schwann cell marker） was decreased in sciatic nerve.Apocynin treatment reversed these changes dose-dependently.In conclusion,decreased pain threshold of diabetic rats was accompanied by increased NOX and catalase expression in the spinal cord and increased degeneration in the sciatic nerve characterized by increased NOS expression and Schwann cell loss.Apocynin treatment attenuates neuropathic pain by decelerating the increased oxidative stress-mediated pathogenesis in diabetic rats.

Muscovite nanoparticles mitigate neuropathic pain by modulating the inflammatory response and neuroglial activation in the spinal cord

Despite numerous efforts to overcome neuropathic pain,various pharmacological drugs often fail to meet the needs and have many side effects.Muscovite is an aluminosilicate mineral that has been reported to have an anti-inflammatory effect,but the efficacy of muscovite for neuropathic pain has not been investigated.Here,we assessed whether muscovite nanoparticles can reduce the symptoms of pain by controlling the inflammatory process observed in neuropathic pain.The analgesic effects of muscovite nanoparticles were explored using partial sciatic nerve ligation model of neuropathic pain,in which one-third to onehalf of the nerve trifurcation of the sciatic nerve was tightly tied to the dorsal side.Muscovite nanoparticles(4 mg/100μL)was given intramuscularly to evaluate its effects on neuropathic pain(3 days per week for 4 weeks).The results showed that the muscovite nanoparticle injections significantly alleviated partial sciatic nerve ligation-induced mechanical and cold allodynia.In the spinal cord,the muscovite nanoparticle injections exhibited inhibitory effects on astrocyte and microglia activation and reduced the expression of pro-inflammatory cytokines,such as interleukin-1β,tumor necrosis factor-α,interleiukin-6 and monocyte chemoattractant protein-1,which were upregulated in the partial sciatic nerve ligation model.Moreover,the muscovite nanoparticle injections resulted in a decrease in activating transcription factor 3,a neuronal injury marker,in the sciatic nerve.These results suggest that the analgesic effects of muscovite nanoparticle on partial sciatic nerve ligation-induced neuropathic pain may result from inhibiting activation of astrocytes and microglia as well as pro-inflammatory cytokines.We propose that muscovite nanoparticle is a potential anti-nociceptive candidate for neuropathic pain.All experimental protocols in this study were approved by the Institutional Animal Ethics Committee(IACUC)at Dongguk University,South Korea(approval No.2017-022-1)on September 28,2017.

Mechanism of persistent hyperalgesia in neuropathic pain caused by chronic constriction injury

Transmembrane member 16 A(TMEM16 A) is involved in many physiological functions, such as epithelial secretion, sensory conduction, nociception, control of neuronal excitability, and regulation of smooth muscle contraction, and may be important in peripheral pain transmission. To explore the role of TMEM16 A in the persistent hyperalgesia that results from chronic constriction injury-induced neuropathic pain, a rat model of the condition was established by ligating the left sciatic nerve. A TMEM16 A selective antagonist(10 μg T16 Ainh-A01) was intrathecally injected at L5–6. For measurement of thermal hyperalgesia, the drug was administered once at 14 days and thermal withdrawal latency was recorded with an analgesia meter. For measurement of other indexes, the drug was administered at 12 days,once every 6 hours, totally five times. The measurements were performed at 14 days. Western blot assay was conducted to analyze TMEM16 A expression in the L4–6 dorsal root ganglion. Immunofluorescence staining was used to detect the immunoreactivity of TMEM16 A in the L4–6 dorsal root ganglion on the injured side. Patch clamp was used to detect electrophysiological changes in the neurons in the L4–6 dorsal root ganglion. Our results demonstrated that thermal withdrawal latency was shortened in the model rats compared with control rats.Additionally, TMEM16 A expression and the number of TMEM16 A positive cells in the L4–6 dorsal root ganglion were higher in the model rats, which induced excitation of the neurons in the L4–6 dorsal root ganglion. These findings were inhibited by T16 Ainh-A01 and confirm that TMEM16 A plays a key role in persistent chronic constriction injury-induced hyperalgesia. Thus, inhibiting TMEM16 A might be a novel pharmacological intervention for neuropathic pain. All experimental protocols were approved by the Animal Ethics Committee at the First Affiliated Hospital of Shihezi University School of Medicine, China(approval No. A2017-170-01) on February 27, 2017.

Electroacupuncture improves neuropathic pain Adenosine, adenosine 5'-triphosphate disodium and their receptors perhaps change simultaneously

Applying a stimulating current to acupoints through acupuncture needles–known as electroacupuncture–has the potential to produce analgesic effects in human subjects and experimental animals. When acupuncture was applied in a rat model, adenosine 5-triphosphate disodium in the extracellular space was broken down into adenosine, which in turn inhibited pain transmission by means of an adenosine A1 receptor-dependent process. Direct injection of an adenosine A1 receptor agonist enhanced the analgesic effect of acupuncture. The analgesic effect of acupuncture appears to be mediated by activation of A1 receptors located on ascending nerves. In neuropathic pain, there is upregulation of P2X purinoceptor 3 （P2X3） receptor expression in dorsal root ganglion neurons. Conversely, the onset of mechanical hyperalgesia was diminished and established hyperalgesia was significantly reversed when P2X3 receptor expression was downregulated. The pathways upon which electroacupuncture appear to act are interwoven with pain pathways, and electroacupuncture stimuli converge with impulses originating from painful areas. Electroacupuncture may act via purinergic A1 and P2X3 receptors simultaneously to induce an analgesic effect on neuropathic pain.

Poly(ADP-ribose) polymerase inhibition reveals a potential mechanism to promote neuroprotection and treat neuropathic pain

Neuropathic pain is triggered by the lesions to peripheral nerves which alter their structure and function. Neuroprotective approaches that jimit the pathological changes and improve the behavioral outcome have been well explained in different experimental models of neuropathy but translation of such strategies to clinics has been disappointing. Experimental evidences revealed the role of free radicals, especially per- oxynitrite after the nerve injury. They provoke oxidative DNA damage and consequent over-activation of the poly（ADP-ribose） polymerase （PARP） upregulates pro-inflammatory pathways, causing bioenergetic crisis and neuronal death. Along with these changes, it causes mitochondrial dysfunction leading to neu- ronal apoptosis. In related preclinical studies agents that neutralize the free radicals and pharmacological inhibitors of PARP have shown benefits in treating experimental neuropathy. This article reviews the in- volvement of PARP over-activation in trauma induced neuropathy and therapeutic significance of PARP inhibitors in the experimental neuropathy and neuropathic pain.