Interleukin‐6 induces microglial CX3CR1 expression in the spinal cord after peripheral nerve injury through the activation of p38 MAPK

Peripheral nerve injury leading to neuropathic pain induces the upregulation of interleukin (IL)‐6 and microglial CX3CR1 expression, and activation of p38 mitogen‐activated protein kinase (MAPK) in the spinal cord. Here, we investigated whether IL‐6 regulates CX3CR1 expression through p38 MAPK activation in the spinal cord in rats with chronic constriction injury (CCI) of the sciatic nerve. Similar temporal changes in the expression of IL‐6, phosphorylated p38 MAPK and CX3CR1 were observed following CCI. The increases in CX3CR1 expression, p38 MAPK activation and pain behavior after CCI were suppressed by blocking IL‐6 action with a neutralizing antibody, while they were enhanced by supplying exogenous recombinant rat IL‐6 (rrIL‐6). rrIL‐6 also induced increases in spinal CX3CR1 expression, p38 MAPK activation and pain behavior in naïve rats without nerve injury. Furthermore, treatment with the p38 MAPK‐specific inhibitor, SB203580, suppressed the increase in CX3CR1 expression induced by CCI or rrIL‐6 treatment. Finally, blocking CX3CR1 or p38 MAPK activation prevented the development of mechanical allodynia and thermal hyperalgesia induced by CCI or rrIL‐6 treatment. These results suggest a new mechanism of neuropathic pain, in which IL‐6 induces microglial CX3CR1 expression in the spinal cord through p38 MAPK activation, enhancing the responsiveness of microglia to fractalkine in the spinal cord, thus playing an important role in neuropathic pain after peripheral nerve injury.     

Amelioration of chronic neuropathic pain after partial nerve injury by adeno-associated viral (AAV) vector-mediated over-expression of BDNF in the rat spinal cord

Changing the levels of neurotrophins in the spinal cord micro-environment after nervous system injury has been proposed to recover normal function, such that behavioral response to peripheral stimuli does not lead to chronic pain. We have investigated the effects of recombinant adeno-associated viral (rAAV)-mediated over-expression of brain-derived neurotrophic factor (BDNF) in the spinal cord on chronic neuropathic pain after unilateral chronic constriction injury (CCI) of the sciatic nerve. The rAAV-BDNF vector was injected into the dorsal horn at the thirteenth thoracic spinal cord vertebra (L(1) level) 1 week after CCI. Allodynia and hyperalgesia induced by CCI in the hindpaws were permanently reversed, beginning 1 week after vector injection, compared with a similar injection of a control rAAV-GFP vector (green fluorescent protein) or saline. In situ hybridization for BDNF demonstrated that both dorsal and ventral lumbar spinal neurons contained an intense signal for BDNF mRNA, at 1 to 8 weeks after vector injection. There was no similar BDNF mRNA over-expression associated with either injections of saline or rAAV-GFP. These data suggest that chronic neuropathic pain is sensitive to early spinal BDNF levels after partial nerve injury and that rAAV-mediated gene transfer could potentially be used to reverse chronic pain after nervous system injuries in humans.     

Downregulation of spinal glutamate transporter EAAC1 following nerve injury is regulated by central glucocorticoid receptors in rats

Previous studies have shown that glucocorticoid receptors (GR) were upregulated, whereas glutamate transporters were downregulated, within the spinal cord dorsal horn after peripheral nerve injury. However, the relationship between the expression of spinal GR and glutamate transporter after nerve injury remains unknown. In the present study, we examined the hypothesis that central GR would regulate the expression of spinal glutamate transporter EAAC1 following chronic constriction nerve injury (CCI) in rats. CCI induced a significant downregulation of EAAC1 expression primarily within the ipsilateral spinal cord dorsal horn when examined on postoperative day 7 using both Western blot and immunohistochemistry. The downregulation of EAAC1 was significantly diminished after either the GR antagonist RU38486 (4 > 2 = 0.5 microg = vehicle) or a GR antisense oligonucleotide was administered intrathecally twice daily for postoperative day 1-6. Moreover, CCI induced a significant downregulation of nuclear factor kappaB (NF-kappaB) within the ipsilateral spinal cord dorsal horn, which also was attenuated by either RU38486 (4 > 2 = 0.5 microg = vehicle) or a GR antisense oligonucleotide. The immunohistochemical data indicated a pattern of colocalization between GR and EAAC1 as well as GR and NF-kappaB within the spinal cord dorsal horn. Since, NF-kappaB has been shown to regulate the expression of those cellular elements linked to inflammation and tissue injury and its activity can be negatively regulated by GR activation, these results suggest that spinal GR through NF-kappaB may play a significant role in the regulation of EAAC1 expression after peripheral nerve injury, a cellular pathway that may contribute to the development of neuropathic pain behaviors in rats.     

Central glucocorticoid receptors regulate the upregulation of spinal cannabinoid-1 receptors after peripheral nerve injury in rats

Previous studies have shown that peripheral nerve injury upregulated both glucocorticoid receptors (GR) and cannabinoid-1 receptors (CB1R) within the spinal cord dorsal horn in rats. However, the relationship between the expression of spinal GR and CB1R after nerve injury remains unclear. Here, we examined the hypothesis that the upregulation of spinal CB1R induced by chronic constriction nerve injury (CCI) in rats would be regulated by spinal GR. CCI induced the upregulation of spinal CB1R primarily within the ipsilateral spinal cord dorsal horn as revealed by Western blot and immunohistochemistry. The expression of CB1R in CCI rats was substantially attenuated by intrathecal treatment with either the GR antagonist RU38486 or a GR antisense oligonucleotide given twice daily for postoperative day 1-6, whereas the expression of spinal CB1R was enhanced following intrathecal administration of a GR sense oligonucleotide twice daily for postoperative day 1-6. Furthermore, the upregulation of spinal CB1R after nerve injury was prevented in adrenalectomized rats, which was at least partially restored with the intrathecal administration of an exogenous GR agonist dexamethasone, indicating that corticosteroids (endogenous GR agonists) were critical to spinal GR actions. Since the development of neuropathic pain behaviors in CCI rats was attenuated by either RU38486 or a GR antisense oligonucleotide, these results suggest that CB1R is a downstream target for spinal GR actions contributory to the mechanisms of neuropathic pain.     

Electroporation-mediated pain-killer gene therapy for mononeuropathic rats

The relatively low expression levels achieved from transferred genes have limited the application of nonviral vectors for gene transfer into the spinal cord in vivo. Thus, the aim of this study was to evaluate the efficacy of electroporation-mediated pro-opiomelanocortin (POMC) gene therapy for neuropathic pain using an animal model of chronic constrictive injury (CCI). Firstly, the optimal pulse characteristics (voltage, pulse duration, number of shocks) were investigated for in vivo electroporation-mediated gene transfer into the spinal cord. The electroporation process makes use of plasmid DNA, which expresses the POMC gene. Expression levels were evaluated in this study by Western blot. We conclude that the optimal conditions for electroporation are a pulse voltage of 200 V, 75-ms duration, 925-ms interval, for five iterations. Secondly, electroporation treatment for neuropathic pain was attempted on CCI rats using plasmid DNA that expresses the POMC gene. Intrathecal administrations of the POMC vector elevated spinal beta-endorphin levels, as manifested in a significantly elevated pain threshold for the CCI limbs. This result suggests that gene therapy for neuropathic pain using this novel technique is very efficacious, and thus shows promise for further clinical trials.     

Gene array analysis to determine the components of neuropathic pain signaling

The molecular modification of the pain pathway represents one of the major mechanisms underlying neuropathic pain. Recently, gene array studies have been carried out to identify the genes that are regulated at the spinal cord level after peripheral nerve injury. These studies demonstrate that peripheral nerve injury causes marked changes in gene expression in both the dorsal root ganglion (DRG) and the dorsal spinal cord. The markedly regulated molecules include, for example, neuropeptides, receptors, ion channels, signal transduction molecules and synaptic vesicle proteins. Upregulation of the Ca2+ channel alpha2/delta1 subunit, gamma-aminobutyric acid A receptor alpha5 subunit, Na+ channels and nicotinic acetylcholine receptors in the DRG and dorsal spinal cord indicates their potential roles in neuropathic pain control.     

Spinal leptin contributes to the pathogenesis of neuropathic pain in rodents

Pain after nerve injury, a phenomenon referred to as neuropathic pain, is a debilitating clinical condition, but the underlying mechanisms remain unclear. As leptin, an adipocytokine produced mainly by nonneuronal tissue, has been implicated in the regulation of neuronal functions, we examined the role of leptin in neuropathic pain using a rat model of the condition chronic constriction sciatic nerve injury (CCI). We report that leptin critically contributed to pain behaviors following CCI. Specifically, spinal administration of a leptin antagonist prevented and reversed neuropathic pain behaviors in rats. Further examination revealed that levels of both leptin and the long form of the leptin receptor (Ob-Rb) were substantially increased within the ipsilateral spinal cord dorsal horn after peripheral nerve injury. Mechanistic studies showed that leptin upregulated the expression of both the spinal NMDA receptor and IL-1β through the JAK/STAT pathway. Furthermore, these CCI-induced behavioral and cellular responses were diminished in leptin-deficient mice and mimicked by spinal administration of exogenous leptin in naive rats. Our findings reveal a critical role for spinal leptin in the pathogenesis of neuropathic pain and suggest what we believe to be a novel form of nonneuronal and neuronal interactions in the mechanisms of pathological pain.     

Repeated intrathecal injections of plasmid DNA encoding interleukin-10 produce prolonged reversal of neuropathic pain

Neuropathic pain is a major clinical problem unresolved by available therapeutics. Spinal cord glia play a pivotal role in neuropathic pain, via the release of proinflammatory cytokines. Anti-inflammatory cytokines, like interleukin-10 (IL-10), suppress proinflammatory cytokines. Thus, IL-10 may provide a means for controlling glial amplification of pain. We recently documented that intrathecal IL-10 protein resolves neuropathic pain, albeit briefly (approximately 2-3 h), given its short half-life. Intrathecal gene therapy using viruses encoding IL-10 can also resolve neuropathic pain, but for only approximately 2 weeks. Here, we report a novel approach that dramatically increases the efficacy of intrathecal IL-10 gene therapy. Repeated intrathecal delivery of plasmid DNA vectors encoding IL-10 (pDNA-IL-10) abolished neuropathic pain for greater than 40 days. Naked pDNA-IL-10 reversed chronic constriction injury (CCI)-induced allodynia both shortly after nerve injury as well as 2 months later. This supports that spinal proinflammatory cytokines are important in both the initiation and maintenance of neuropathic pain. Importantly, pDNA-IL-10 gene therapy reversed mechanical allodynia induced by CCI, returning rats to normal pain responsiveness, without additional analgesia. Together, these data suggest that intrathecal IL-10 gene therapy may provide a novel approach for prolonged clinical pain control.     

Upregulation of spinal cannabinoid-1-receptors following nerve injury enhances the effects of Win 55,212-2 on neuropathic pain behaviors in rats

Exogenous cannabinoids are effective in attenuating neuropathic pain behaviors induced by peripheral nerve injury, but the mechanisms of their effectiveness remain unclear. Here we examined the expression of spinal cannabinoid-1-receptors (CB1Rs) following chronic constriction sciatic nerve injury (CCI) and its relation to the effects of a CBR agonist (Win 55,212-2) on neuropathic pain in rats. CCI induced a time-dependent upregulation of spinal CB1Rs primarily within the ipsilateral superficial spinal cord dorsal horn as revealed by both Western blot and immunohistochemistry. This CCI-induced CB1R upregulation was at least in part mediated through tyrosine kinase receptors (Trk), because intrathecal treatment with the Trk inhibitor K252a (1 microg) for postoperative days 1-6 significantly reduced the CB1R upregulation in CCI rats. At the intracellular level, the mitogen-activated protein kinase (ERK-MAPK) inhibitor PD98059 (1 microg) prevented, while the protein kinase C inhibitor chelerythrine (10 microg) partially reduced, the CCI-induced CB1R upregulation when each agent was administered intrathecally for postoperative days 1-6. Importantly, the CCI-induced upregulation of spinal CB1Rs enhanced the effects of Win 55,212-2 on both thermal hyperalgesia and mechanical allodynia, since inhibition of the CB1R upregulation by PD98059 resulted in a significant reduction of the effects of Win 55,212-2 in CCI rats. These results indicate that upregulation of spinal CB1Rs following peripheral nerve injury may contribute to the therapeutic effects of exogenous cannabinoids on neuropathic pain.     

miR-338-5p Levels and Cigarette Smoking are Associated With Neuropathic Pain Severity in Individuals With Spinal Cord Injury: Preliminary Findings From a Genome-Wide microRNA Expression Profiling Screen

ObjectiveTo identify microRNA biomarkers and clinical factors associated with neuropathic pain after spinal cord injury.DesignCross-sectional, secondary analysis of baseline data collected from ongoing clinical studies. Using a genome-wide microRNA screening approach, we studied differential microRNA expression in serum from 43 adults with spinal cord injury enrolled in ongoing clinical studies. Least squares regression was used to identify associations between microRNA expression, clinical factors, and neuropathic pain severity.SettingCommunity-dwelling individuals with spinal cord injury.ParticipantsParticipants (N=43) were at least 18 years old with spinal cord injury, with 28 reporting neuropathic pain and 15 reporting no neuropathic pain.InterventionsNot applicable.Main Outcome MeasuresPain presence, type, and intensity were assessed with the International Spinal Cord Injury Pain Basic Data Set. Serum microRNA normalized deep sequencing counts were quantified from blood samples. Participant demographic factors, injury characteristics, medication use, and health habits were collected via questionnaire.ResultsmiR-338-5p expression and history of cigarette smoking were associated with and explained 37% of the variance in neuropathic pain severity (R²=0.37, F_2,18=5.31, P=.02) independent of other clinical factors. No association was identified between miR-338-5p levels and nociceptive pain severity.ConclusionsOur findings suggest that miR-338-5p and cigarette smoking may both play a role in the development or maintenance of neuropathic pain after spinal cord injury. While additional work is needed to confirm these findings, validated target analysis suggests a neuroprotective role of miR-338-5p in modulating neuroinflammation and neuronal apoptosis and that its downregulation may result in maladaptive neuroplastic mechanisms contributing to neuropathic pain after spinal cord injury.     

Bexarotent Attenuated Chronic Constriction Injury-Induced Spinal Neuroinflammation and Neuropathic Pain by Targeting Mitogen-Activated Protein Kinase Phosphatase-1

It is widely accepted that neuroinflammation in the spinal cord contributes to the development of central sensitization in neuropathic pain. Mitogen-activated protein kinase (MAPK) activation plays a vital role in the development of neuroinflammation in the spinal cord. In this study, we investigated the effect of bexarotene (bex), a retinoid X receptor agonist, on MAPKs activation in chronic constriction injury (CCI)-induced neuropathic pain. The data showed that daily treatment with bex 50 mg/kg significantly alleviated CCI-induced nociceptive hypersensitivity in rats. Bex 50 mg/kg/day inhibited CCI-induced MAPKs (p38MAPK, ERK1/2, and JNK) activation and upregulation of proinflammatory factors (IL-1β, tumor necrosis factor-α and IL-6). Bex also reversed CCI-induced microglia activation in the ipsilateral spinal cord. Furthermore, bex treatment significantly upregulated MKP-1 in the spinal cord. These effects were completely abrogated by MKP-1 inhibitor BCI. These results indicated that bex relieved CCI-induced neuroinflammation and neuropathic pain by targeting MKP-1. Therefore, bex might be a potential agent for the treatment of neuropathic pain.PerspectiveBex could relieve neuropathic pain behaviors in animals by reversing MKP-1 downregulation and MAPKs activation in the spinal cord. Therapeutic applications of bex may be extended beyond cutaneous T-cell lymphoma.     

Intrathecal AAV Serotype 9-mediated Delivery of shRNA Against TRPV1 Attenuates Thermal Hyperalgesia in a Mouse Model of Peripheral Nerve Injury

Gene therapy for neuropathic pain requires efficient gene delivery to both central and peripheral nervous systems. We previously showed that an adenoassociated virus serotype 9 (AAV9) vector expressing short-hairpin RNA (shRNA) could suppress target molecule expression in the dorsal root ganglia (DRG) and spinal cord upon intrathecal injection. To evaluate the therapeutic potential of this approach, we constructed an AAV9 vector encoding shRNA against vanilloid receptor 1 (TRPV1), which is an important target gene for acute pain, but its role in chronic neuropathic pain remains unclear. We intrathecally injected it into the subarachnoid space at the upper lumbar spine of mice 3 weeks after spared nerve injury (SNI). Delivered shTRPV1 effectively suppressed mRNA and protein expression of TRPV1 in the DRG and spinal cord, and it attenuated nerve injury-induced thermal allodynia 10–28 days after treatment. Our study provides important evidence for the contribution of TRPV1 to thermal hypersensitivity in neuropathic pain and thus establishes intrathecal AAV9-mediated gene delivery as an investigative and potentially therapeutic platform for the nervous system.     

Glial cell line-derived neurotrophic factor gene transfer exerts protective effect on axons in sciatic nerve following constriction-induced peripheral nerve injury

Damage to peripheral nerves following trauma or neurodegenerative diseases often results in various sensory and motor abnormalities and chronic neuropathic pain. The loss of neurotrophic factor support has been proposed to contribute to the development of peripheral neuropathy. The main objective of this study was to investigate the protective effect of glial cell line-derived neurotrophic factor (GDNF) using peripheral gene delivery in a rat model of constriction-induced peripheral nerve injury. In this study, it was shown that mechanical and thermal hypersensitivity increased on the injured limb at day 7 after chronic constrictive injury (CCI) was induced. The neurological changes were correlated with the structural changes and loss of GDNF/Akt signaling, particularly in the distal stump of the injured sciatic nerve. Subsequently, recombinant adenovirus was employed to evaluate the potential of intramuscular GDNF gene delivery to alleviate the CCI-induced nerve degeneration ad neuropathic pain. After CCI for 3 days, intramuscular injection of adenovirus encoding GDNF (Ad-GDNF) restored the protein level and activity of GDNF/Akt signaling pathway in the sciatic nerve. This was associated with an improved myelination profile and behavioral outcomes in animals with CCI. In conclusion, the present study demonstrates the involvement of GDNF loss in the pathogenesis of CCI-induced neuropathic pain and the therapeutic potential of intramuscular GDNF gene delivery for the treatment of peripheral nerve degeneration.     

Interleukin-1 alpha has antiallodynic and antihyperalgesic activities in a rat neuropathic pain model

Nerve injury and the consequent release of interleukins (ILs) are processes implicated in pain transmission. To study the potential role of IL-1 in the pathogenesis of allodynia and hyperalgesia, IL-1alpha and comparative IL-1beta, IL-6, and IL-10 mRNA levels were quantified using competitive RT-PCR of the lumbar spinal cord and dorsal root ganglia (DRG; L5-L6) three and seven days after chronic constriction injury (CCI) in rats. Microglial and astroglial activation in the ipsilateral spinal cord and DRG were observed after injury. In naive and CCI-exposed rats, IL-1alpha mRNA and protein were not detected in the spinal cord. IL-1beta and IL-6 mRNAs were strongly ipsilaterally elevated on day seven after CCI. In the ipsilateral DRG, IL-1alpha, IL-6, and IL-10 mRNA levels were increased on days three and seven; IL-1beta was elevated only on day seven. Western blot analysis revealed both the presence of IL-1alpha proteins (45 and 31 kDa) in the DRG and the down-regulation of these proteins after CCI. Intrathecal administration of IL-1alpha (50-500 ng) in naive rats did not influence nociceptive transmission, but IL-1beta (50-500 ng) induced hyperalgesia. In rats exposed to CCI, an IL-1alpha or IL-1 receptor antagonist dose-dependently attenuated symptoms of neuropathic pain; however, no effect of IL-1beta was observed. In sum, the first days after CCI showed a high abundance of IL-1alpha in the DRG. Together with the antiallodynic and antihyperalgesic effects observed after IL-1alpha administration, this finding indicates an important role for IL-1alpha in the development of neuropathic pain symptoms.     

高压氧对神经病理痛大鼠的NOS系统的影响

目的:观察高压氧对神经病理痛大鼠一氧化氮合酶(nitric oxygen synthase,NOS)系统的影响。方法:40只雄性SD大鼠随机分成五组:即空白对照组(C组);假手术组(S组);坐骨神经慢性缩窄手术组(CCI组);高压氧预处理组(Pre-H组)与高压氧干预组(Post-H组),术后28 d取材观察其脊髓的免疫组化指标,对其NOS各亚型表达进行比较。结果:CCI术后nNOS,iNOS表达明显增加,Pre-H组和Post-H组的nNOS及iNOS含量较CCI组明显降低。eNOS含量在整个过程中变化不大。结论:高压氧可能通过抑制NOS系统表达产生对CCI大鼠疼痛的抑制作用。     

Chemokine contribution to neuropathic pain: Respective induction of CXCL1 and CXCR2 in spinal cord astrocytes and neurons

Recent studies have indicated an important role of chemokines such as CCL2 in the development of chronic pain. However, the distinct roles of different chemokines in the development and maintenance of neuropathic pain and in their interactions with neurons have not been clearly elucidated. We found that spinal nerve ligation (SNL) not only induced persistent neuropathic pain symptoms, including mechanical allodynia and heat hyperalgesia, but also produced sustained CXCL1 upregulation in the spinal cord. Double staining of immunofluorescence and in situ hybridization revealed that CXCL1 was primarily induced in spinal astrocytes. In cultured astrocytes, tumor necrosis factor-α induced robust CXCL1 expression via the activation of the c-jun N-terminal kinase. Intrathecal administration of CXCL1 neutralizing antibody transiently reduced SNL-induced pain hypersensitivity, suggesting an essential role of CXCL1 in neuropathic pain sensitization. In particular, intraspinal delivery of CXCL1 shRNA lentiviral vectors, either before or after SNL, persistently attenuated SNL-induced pain hypersensitivity. Spinal application of CXCL1 not only elicited pain hypersensitivity but also induced rapid neuronal activation, as indicated by the expression of phosphorylated extracellular signal-regulated kinase and cAMP response element binding protein, and c-Fos in spinal cord neurons. Interestingly, CXCR2, the primary receptor of CXCL1, was upregulated in dorsal horn neurons after SNL, and the CXCR2 antagonist SB225002 completely blocked the CXCL1-induced heat hyperalgesia. SB225002 also attenuated SNL-induced pain hypersensitivity. Collectively, our results have demonstrated a novel form of chemokine-mediated glial-neuronal interaction in the spinal cord that can drive neuropathic pain. Inhibition of the CXCL1-CXCR2 signaling may offer a new therapy for neuropathic pain management.     

Spatial patterns of spinal cord [14C]-2-deoxyglucose metabolic activity in a rat model of painful peripheral mononeuropathy.

Spatial patterns of spinal cord glucose metabolic activity were examined in unanesthetized rats with painful peripheral mononeuropathy produced by sciatic nerve ligation (chronic constrictive injury, CCI). Spinal cord metabolic activity was assessed 10 days after nerve ligation by using the fully quantitative [14C]2-deoxyglucose technique. This technique allows simultaneous examination of both neural activity inferred from local glucose utilization and its spatial distribution in multiple spinal regions previously implicated in nociceptive processing. Rats used in the experiment exhibited thermal hyperalgesia to radiant heat applied to the hind paw ipsilateral to nerve ligation and behaviors indicative of spontaneous pain. Sciatic nerve ligation produced a significant increase in spinal cord metabolic activity in four sampling regions (laminae I-IV, V-VI, VII and VIII-IX) of lumbar segments compared to sham-operated rats. The pattern of altered metabolic activity in CCI rats presented 3 distinct features. (1) The spinal cord grey matter both ipsilateral and contralateral to nerve ligation exhibited substantial increases in metabolic activity compared to sham-operated rats. (2) This increase in metabolic activity was somatotopically specific, i.e., higher metabolic rates were observed on the side ipsilateral to nerve ligation than on the contralateral side, and higher metabolic rates were seen in the medial portion of the ipsilateral spinal cord dorsal horn than in the lateral portion. The peak metabolic activity occurred in laminae V-VI of CCI rats, a region involved in nociceptive processing. (3) The increase in spinal cord metabolic activity of CCI rats extended from lumbar segment L1 to L5 in all 4 sampling regions. The substantial increase in metabolic activity in both the ipsilateral and contralateral spinal cord that occurs over an extensive rostro-caudal area in CCI rats may represent a unique pattern of spinal cord metabolic activity distinct from that observed in rats exposed to acute thermal pain. This pattern of spinal cord neural activity in CCI rats may reflect possible radiation of neuropathic pain. In addition, the procedure of curare-induced paralysis in a separate group of CCI rats did not change the extent and patterns of metabolic activity seen in non-paralyzed CCI rats, reflecting a minimal influence of the afferent feedback from flexor motor reflexes on spinal cord metabolic activity following sciatic nerve ligation. This chronic increase in spinal cord neural activity in the absence of overt peripheral stimulation suggests a spinal cord hyperactive state and may account for behaviors suggestive of spontaneous pain in CCI rats.(ABSTRACT TRUNCATED AT 400 WORDS)     

Altered spinal arachidonic acid turnover after peripheral nerve injury regulates regional glutamate concentration and neuropathic pain behaviors in rats

Spinal glutamate transporters (GT) have been implicated in the mechanisms of neuropathic pain; however, how spinal GT uptake activity is regulated remains unclear. Here we show that alteration of spinal arachidonic acid (AA) turnover after peripheral nerve injury regulated regional GT uptake activity and glutamate homeostasis. Chronic constriction nerve injury (CCI) in rats significantly reduced spinal GT uptake activity ((3)H-glutamate uptake) with an associated increase in extracellular AA and glutamate concentration from spinal microdialysates on postoperative day 8. AACOCF3 (a cytosolic phospholipase A2 inhibitor, 30mug) given intrathecally twice a day for postoperative day 1-7 reversed this CCI-induced spinal AA production, prevented the reduced spinal GT uptake activity and increased extracellular glutamate concentration. Conversely, alteration of spinal AA metabolism by diclofenac (a cyclooxygenase 1/2 inhibitor, 200mug) further reduced spinal GT uptake activity and increased extracellular glutamate concentration in CCI rats. GT uptake activity was also attenuated when AA (10 or 100nM) was directly added into spinal samples of na?ve rats in an in vitro(3)H-glutamate uptake assay, indicating a direct inhibitory effect of AA on GT uptake activity. Consistent with these findings, AACOCF3 reduced the development of both thermal hyperalgesia and mechanical allodynia, whereas diclofenac exacerbated thermal hyperalgesia, in CCI rats. Thus, spinal AA turnover may serve as a regulator in CCI-induced changes in regional GT uptake activity, glutamate homeostasis, and neuropathic pain behaviors. These data suggest that regulating spinal AA turnover may be a useful approach to improving the clinical management of neuropathic pain.     

Acupoint stimulation with diluted bee venom (apipuncture) alleviates thermal hyperalgesia in a rodent neuropathic pain model: involvement of spinal alpha 2-adrenoceptors.

Chemical acupuncture with diluted bee venom (DBV), termed apipuncture, has been traditionally used in oriental medicine to treat several inflammatory diseases and chronic pain conditions. In the present study we investigated the potential antihyperalgesic and antiallodynic effects of apipuncture in a rat neuropathic pain model. DBV (0.25 mg/kg, subcutaneous) was injected into the Zusanli acupoint 2 weeks after chronic constrictive injury (CCI) of the sciatic nerve. Between 5 and 45 minutes after DBV injection, we observed a significant reduction in the thermal hyperalgesia induced by CCI, but apipuncture failed to reduce CCI-induced mechanical allodynia. We subsequently examined whether this antihyperalgesic effect of apipuncture was related to the activation of spinal opioid receptors and/or alpha2-adrenoceptors. Intrathecal pretreatment with naloxone (10 microg/rat), an opioid receptor antagonist, did not reverse the antihyperalgesic effect of apipuncture, whereas pretreatment with idazoxan (40 microg/rat), an alpha2-adrenoceptor antagonist, completely blocked the effect of apipuncture. These results indicate that DBV-induced apipuncture significantly reduces the thermal hyperalgesia generated by CCI and also suggest that this antihyperalgesic effect is dependent on the activation of alpha2-adrenoceptors, but not opioid receptors, in the spinal cord. PERSPECTIVE: The antinociceptive effect of apipuncture was evaluated in a rodent neuropathic pain model. The relieving effect of apipuncture on thermal hyperalgesia was found to be mediated by spinal alpha2-adrenoceptors, but not opioid receptors. These data suggest that apipuncture might be an effective alternative therapy for patients with painful peripheral neuropathy, especially for those who are poorly responsive to opioid analgesics.