Involvement of subtype 1 metabotropic glutamate receptors in apoptosis and caspase-7 over-expression in spinal cord of neuropathic rats.

The effect of the non-selective, 1-aminoindan-1,5-dicarboxylic acid (AIDA), and selective (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl) methanone (JNJ16259685), metabotropic glutamate subtype 1 (mGlu1) receptor antagonists, on rat sciatic nerve chronic constrictive injury (CCI)-induced hyperalgesia, allodynia, spinal dorsal horn apoptosis, and gliosis was examined at 3 and 7 days post-injury. RT-PCR analysis showed increased expression of bax, apoptotic protease-activating factor-1 (apaf-1), nestin, GFAP, and caspase-7 mRNA in the dorsal horn spinal cord by 3 days post-CCI. At 7 days post-CCI, only over-expression of bcl-2, nestin and GFAP mRNA was observed. Administration of AIDA reduced thermal hyperalgesia and mechanical allodynia at 3 and 7 days post-CCI; administration of JNJ16259685 reduced thermal hyperalgesia at 3 and 7 days post-CCI, but not mechanical allodynia. AIDA decreased the mRNA levels of bax, apaf-1, GFAP and caspase-7 genes. JNJ16259685 increased the mRNA levels of bcl-2 and GFAP gene, and decreased APAF-1 and caspases-7 genes. Inhibiting mGlu1 receptors also reduced TUNEL-positive profiles and immunohistochemical reactivity for caspase-7. We report here that despite inhibiting CCI-induced over-expression of pro-apoptotic genes in the spinal cord dorsal horn, the selective mGlu1 receptor antagonist JNJ16259685 exerted only a slight and transient allodynic effect. Moreover, JNJ16259685, but not the non-selective AIDA, increased astrogliosis which may account for its decreased analgesic efficacy. This study provides evidence that the contemporary and partial blockade of group I and likely ionotropic glutamate receptors may be a more suitable therapy than selective blockade of mGlu1 subtype receptors condition to decrease neuropathic pain symptoms.     

坐骨神经慢性压迫损伤性疼痛与脊髓背角P物质关系研究

目的观察大鼠坐骨神经慢性压迫性损伤（CCI）后脊髓背角P物质表达的变化,探讨P物质在疼痛发生机制中的作用。方法SD雄性大鼠60只,随机分为：A组：CCI组（30只）;B组：对照组（30只）。术前及术后3、7、14、28 d分别测定大鼠热痛阈值、机械痛阈值和行为学评分。术后3、7、14、28d每组取4只,麻醉后用4%多聚甲醛灌注固定,取L4-6段脊髓,以备免疫组化,测定SP的变化。结果所有CCI动物从术后第3天起,出现明显的疼痛行为学改变和热痛阈值、机械痛阈值的降低,与对照组比较差异有统计学意义（P〈0.05或P〈0.01）。免疫组织化学结果表明,A组术后术侧明显高于B组（P〈0.05或P〈0.01）;A组术侧明显高于健侧（P〈0.05或P〈0.01）;而B组仅在第4天术侧高于健侧（P〈0.05）。结论慢性坐骨神经损伤后,脊髓背角SP的表达增加,而且表达增加与CCI大鼠的痛觉过敏、行为变化在时相上基本一致,说明CCI大鼠痛觉过敏与脊髓背角SP的表达增加有关。     

Effects of intrathecal epigallocatechin gallate, an inhibitor of Toll-like receptor 4, on chronic neuropathic pain in rats

Numerous studies revealed that spinal inflammation and immune response play an important role in neuropathic pain. In this study, we investigated the effects of intrathecal injection of a Toll-like receptor (TLR4) inhibitor epigallocatechin gallate (EGCG) on neuropathic pain induced by chronic constriction injury of the sciatic nerve (CCI). A total of 120 rats were randomly assigned into 4 groups: sham-operated group, CCI group, CCI plus normal saline group and CCI plus EGCG group. CCI and sham surgeries were performed and both thermal hyperalgesia and mechanical allodynia were tested. Lumbar spinal cord was sampled and the mRNA and protein expressions of TLR4 and High Mobility Group 1 protein (HMGB1) were detected, the contents of tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and interleukin-10 (IL-10) were measured by ELISA, and immunohistochemistry for nuclear factor kappa B (NF-κB) was also carried out. When compared with the sham group, both mechanical and heat pain thresholds were significantly decreased, and the mRNA and protein expressions of TLR4 and HMGB1, the contents of TNF-α, IL-1β and IL-10 in the spinal cords and NF-κB expression in the spinal dorsal horn were markedly increased in CCI rats (P<0.05). After intrathecal injection of EGCG (1mg/kg) once daily from 1day before to 3days after CCI surgery, the expressions of TLR4, NF-κB, HMGB1, TNF-α and IL-1β were markedly decreased while the content of IL-10 in the spinal cord increased significantly accompanied by dramatical improvement of pain behaviors in CCI rats (P<0.05). These results show that the TLR4 signaling pathway plays an important role in the occurrence and development of neuropathic pain, and the therapy targeting TLR4 might be a novel strategy in the treatment of neuropathic pain.     

Activation of ERK／CREB pathway in spinal cord contributes to chronic constrictive injury-induced neuropathic pain in rats

Aim: To investigate whether activation and translocation of extracellular signalregulated kinase (ERK) is involved in the induction and maintenance of neuropathic pain, and effects of activation and translocation of ERK on expression of pCREB and Fos in the chronic neuropathic pain. Methods: Lumbar intrathecal catheters were chronically implanted in male Sprague-Dawley rats. The left sciatic nerve was loosely ligated proximal to the sciatica‘s trifurcation at approximately 1.0 mm intervals with 4-0 silk sutures. The mitogen-activated protein kinase kinase (MEK) inhibitor U0126 or phosphorothioate-modified antisense oligonucleotides (ODN) were intrathecally administered every 12 h, 1 d pre-chronic constriction injury (CCI) and 3 d post-CCI. Thermal and mechanical nociceptive thresholds were assessed with the paw withdrawal latency (PWL) to radiant heat and von Frey filaments. The expression of pERK, pCREB, and Fos were assessed by both Western blotting and immunohistochemical analysis. Results: Intrathecal injection of U0126 or ERK antisense ODN significantly attenuated CCI-induced mechanical allodynia and thermal hyperalgesia. CCI significantly increased the expression of p-ERK-IR neurons in the ipsilateral spinal dorsal horn to injury, not in the contralateral spinal dorsal horn. The time courses of pERK expression showed that the levels of both cytosol and nuclear pERK, but not total ERK, were increased at all points after CCI and reached a peak level on postoperative d 5. CCI also significantly increased the expression of pCREB and Fos. Phospho-CREB-positive neurons were distributed in all laminae of the bilateral spinal cord and Fos was expressed in laminae I and II of the ipsilateral spinal dorsal horn. Intrathecal injection of U0126 or ERK antisense ODN markedly suppressed the increase of CCI-induced pERK, pCREB and c-Fos expression in the spinal cord. Conclusion:The activation of ERK pathways contributes to neuropathic pain in CCI rats, and the function of pERK may partly be accomplished via the cAMP response element binding protein (CREB)-dependent gene expression.     

Changes in spinal and supraspinal endocannabinoid levels in neuropathic rats

Recent studies have shown that activation of the cannabinoid CB(1) receptor by synthetic agonists, and pharmacological elevation of endocannabinoid levels, suppress hyperalgesia and allodynia in animal models of neuropathic pain. However, the concentrations of endocannabinoids in the nervous tissues involved in pain transmission during neuropathic pain have never been measured. Here we have determined the levels of anandamide and 2-arachidonoylglycerol (2-AG), as well as of the analgesic anandamide congener, palmitoylethanolamide (PEA), in three brain areas involved in nociception, i.e. the dorsal raphe (DR), periaqueductal grey (PAG) and rostral ventral medulla (RVM), as well as in the spinal cord (SC), following chronic constriction injury (CCI) of the sciatic nerve in the rat, in comparison with sham-operated rats. After 3 days from CCI, anandamide or 2-AG levels were significantly enhanced only in the SC or PAG, respectively. After 7 days from CCI, when thermal hyperalgesia and mechanical allodynia are maximal, a strong (1.3-3-fold) increase of both anandamide and 2-AG levels was observed in the PAG, RVM and SC. At this time point, anandamide, but not 2-AG, levels were also enhanced in the DR. PEA levels were significantly decreased in the SC after 3 days, and in the DR and RVM after 7 days from CCI. These data indicate that anandamide and 2-AG, operating at both spinal and supra-spinal levels, are up-regulated during CCI of the sciatic nerve, possibly to inhibit pain. Yet to be developed substances that inhibit both endocannabinoid and PEA inactivation might be useful for the treatment of neuropathic pain.     

Reorganization of dorsal root ganglion neurons following chronic sciatic nerve constriction injury: correlation with morphine and lidocaine analgesia

Chronic constriction injury of the sciatic nerve is an animal model for neuropathic pain. In this model, the analgesic potency of systemic morphine was significantly diminished in nerve-injured mice (ED(50) 19.4 mg/kg) compared with sham-operated mice (ED(50) 3.3 mg/kg) using a unilateral hot plate withdrawal test, with a similar reduction in sensitivity of intrathecal morphine. The sciatic nerve injury resulted in a reorganization of the dorsal root ganglion (DRG) neurons. Immunohistochemically, the chronic constriction injury triggered a withdrawal of C-fibers from the ipsilateral dorsal horn of the spinal cord. Although A-beta terminals centrally sprouted into Lamina II of the dorsal horn of the spinal cord, the peripheral A-beta fibers in the skin retracted from the epidermis to deeper layers of the dermis. To explore the functional significance of these dermal changes, we examined the topical morphine and lidocaine analgesia following chronic sciatic nerve constriction. Both morphine and lidocaine retained topical activity following chronic sciatic nerve injury, but their analgesic dose-response curves were shifted to the right when compared to sham-operated mice. Thus, the chronic nerve constriction injury model is associated with pathological changes in distribution of the central and peripheral axons of the dorsal root ganglion neurons that correspond to a decreased pharmacological sensitivity to topical analgesic agents.     

Down-regulation of GFRalpha-1 expression by antisense oligodeoxynucleotide aggravates thermal hyperalgesia in a rat model of neuropathic pain

Glial cell line-derived neurotrophic factor (GDNF) has been hypothesized to play an important role in the modulation of nociceptive signals especially during neuropathic pain. The present study examined the expression of GDNF and GFRalpha-1 (the high-affinity receptor of GDNF) in dorsal root ganglions (DRG) in a rat model of neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve. In order to address the role of GDNF and GFRalpha-1 in neuropathic pain, antisense oligodeoxynucleotide (ODN) specifically against GFRalpha-1 was intrathecally administered to result in down-regulation of GFRalpha-1 expression. The results showed that both the protein and mRNA levels of GDNF and GFRalpha-1 were significantly increased after CCI, while the thermal hyperalgesia of neuropathic pain rats could be significantly aggravated by antisense ODN treatment, but not by normal saline (NS) or mismatch ODN treatment. The present study demonstrated that endogenous GDNF and GFRalpha-1 might play an anti-hyperalgesic role in neuropathic pain of rats. In addition, we found a down-regulation of somatostatin (SOM) in DRG and spinal dorsal horn after expression of GFRalpha-1 was knocked down, which suggested the possible relationship between the anti-hyperalgesic effect of GDNF and GFRalpha-1 on neuropathic pain and endogenous SOM.     

鞘内注射巴氯酚对神经病理性痛大鼠的镇痛作用及其对脊髓GABA转运体-1的影响

目的研究鞘内注射GABAB受体激动剂巴氯酚(ba-clofen,Bac)对神经病理性痛大鼠的镇痛作用及其对脊髓GABA转运体-1的影响。方法建立坐骨神经结扎致神经病理性痛大鼠模型。在行为学实验部分,将32只大鼠随机分为NS组、Bac1组、Bac2组、Bac3组(n=8),分别鞘内注射生理盐水、0.1、0.3、1.0μg巴氯酚10μl,并分别于给药前、给药后0.5、1、2、4、8、12、24 h测定大鼠机械缩足反射阈值(mechanical withdrawl threshold,MWT)和热缩足反射潜伏期(thermal withdrawl latency,TWL)以及运动功能。在第2部分,将大鼠分为NS组与Bac组,鞘内分别给予0.3μg巴氯酚或生理盐水,分别于给药前、给药后1、4、8 h取大鼠脊髓腰段,免疫组织化学检测脊髓节段GAT-1免疫阳性神经元(n=6);分别于给药前、给药后30 min、1、2、4、8、12和24 h取大鼠脊髓腰段,用Western blot方法测定脊髓节段GAT-1蛋白含量(n=4)。结果鞘内注射巴氯酚后0.5～2 h,Bac1组、Bac2组与Bac3组大鼠MWT和TWL均较NS组明显升高(P<0.01,P<0.05),鞘内给药后4 h,Bac2与Bac3组大鼠MWT和TWL仍较NS组明显升高(P<0.01,P<0.05),给药后8 h,Bac3组MWT与TWL仍高于NS组(P<0.05)。与NS组和给药前比较,鞘内注射0.3μg巴氯酚后0.5～4h,大鼠脊髓节段的GAT-1蛋白含量均明显降低(P<0.01,P<0.05),大鼠脊髓背角浅层GAT-1免疫阳性染色灰度值亦明显降低(P<0.01,P<0.05),至给药后8 h,GAT-1的表达逐渐增多,与NS组和给药前相比差异均无统计学意义(P>0.05)。结论鞘内注射GABAB受体激动剂巴氯酚可减轻坐骨神经结扎致神经病理性痛大鼠的痛觉过敏,其镇痛作用可能与抑制脊髓水平GAT-1的表达有关。     

Increase in hemokinin-1 mRNA in the spinal cord during the early phase of a neuropathic pain state

Background and purpose:

Substance P (SP), a representative member of the tachykinin family, is involved in nociception under physiological and pathological conditions. Recently, hemokinin-1 (HK-1) was identified as a new member of this family. Although HK-1 acts on NK₁ tachykinin receptors that are thought to be innate for SP, the roles of HK-1 in neuropathic pain are still unknown.

Experimental approach:

Using rats that had been subjected to chronic constrictive injury (CCI) of the sciatic nerve as a neuropathic pain model, we examined the changes in expression of SP- and HK-1-encoding genes (TAC1 and TAC4, respectively) in the L4/L5 spinal cord and L4/L5 dorsal root ganglia (DRGs) in association with changes in pain-related behaviours in this neuropathic pain state.

Key results:

The TAC4 mRNA level was increased on the ipsilateral side of the dorsal spinal cord, but not in DRGs, at day 3 after CCI. In contrast, the TAC1 mRNA level was significantly increased in the DRGs at day 3 after CCI without any changes in the dorsal spinal cord. Analysis of a cultured microglial cell line revealed the presence of TAC4 mRNA in microglial cells. Minocycline, an inhibitor of microglial activation, blocked the increased expression of TAC4 mRNA after CCI and inhibited the associated pain-related behaviours and microglial activation in the spinal cord.

Conclusions and implications:

The present results suggest that HK-1 expression is increased at least partly in activated microglial cells after nerve injury and is clearly involved in the early phase of neuropathic pain.     

Blockade of glutamate mGlu5 receptors in a rat model of neuropathic pain prevents early over-expression of pro-apoptotic genes and morphological changes in dorsal horn lamina II

We used rats with a sciatic nerve chronic constrictive injury (CCI) and combined behavioural, molecular and morphological approaches to assess the involvement of mGlu5 receptors in neuropathic pain-associated hyperalgesia and spinal cord neuron apoptosis. Mechanical and thermal hyperalgesia developed 2-3 days after surgery. Morphological changes in the ipsilateral L4-L5 lamina II consisted of: (i) cell loss (38 +/- 5%), (ii) increased TUNEL-positive profiles, (iii) decreased SP-immunoreactive primary afferents, and (iv) reactive gliosis. Molecular expression data suggested a bi-phasic response of bcl-2 family genes in CCI. An early (2-3 days post-CCI) E2F1- and p53-independent apoptosis appeared in the spinal cord as the pro-apoptotic bax gene increased (320 +/- 19%), followed by an increased expression of the anti-apoptotic bcl-2 and bcl-xL genes (60 +/- 11% and 110 +/- 15%, respectively) 7 days from CCI. The selective mGlu5 receptor antagonist, MPEP (2 mg/kg i.p. twice daily), prevented the development of thermal hyperalgesia and transiently reduced mechanical hyperalgesia. Despite the MPEP treatment, which normalised bax/bcl-2 and bcl-xL/bcl-xS ratios at all times post-CCI, mechanical hyperalgesia reappeared by 7 days after CCI. Similarly, MPEP was cytoprotective at 3, but not 7 days post-CCI. This study shows that: (a) spinal cord neuron loss may be triggered by a p53- and E2F1-independent apoptosis in lamina II with the participation of glutamate mGlu5 receptors, (b) these receptors seem to be involved transiently, as their blockade was no longer protective by 7 days CCI, and (c) this delayed cell death occurred in the absence of Bax activation, suggesting the involvement of an alternative death pathway.     

Reversal of behavioural and electrophysiological correlates of experimental peripheral neuropathy by the NK1 receptor antagonist GR205171 in rats

In adult rats response latencies to innocuous mechanical stimuli were found to be reduced and, in electrophysiological studies, the receptive fields of dorsal horn neurones were enlarged 7–14 days after chronic constriction injury of the sciatic nerve. The NK₁ receptor antagonist GR205171 at 3 mg kg⁻¹ blocked responses to NK₁ agonist evoked activity and reversed the mechanical hypersensitivity following nerve ligation in behavioural assays. GR205171 also reversed the receptive field expansion of spinal dorsal horn neurones caused by loose ligation of the sciatic nerve in an electrophysiological assay in anaesthetised rats. The less active enantiomer L-796,325 did not block NK₁ agonist evoked activity at up to 10 mg kg⁻¹ and had no effect on behavioural or electrophysiological changes following nerve injury, indicating that the effects of GR205171 were attributable to selective NK₁ receptor blockade. These data suggest that NK₁ receptor antagonists may be useful for the treatment of certain types of neuropathic pain.     

Cytokine activin C ameliorates chronic neuropathic pain in peripheral nerve injury rodents by modulating the TRPV1 channel

Background and PurposeThe cytokine activin C is mainly expressed in small‐diameter dorsal root ganglion (DRG) neurons and suppresses inflammatory pain. However, the effects of activin C in neuropathic pain remain elusive.Experimental ApproachMale rats and wild‐type and TRPV1 knockout mice with peripheral nerve injury ‐ sciatic nerve axotomy and spinal nerve ligation in rats; chronic constriction injury (CCI) in mice – provided models of chronic neuropathic pain. Ipsilateral lumbar (L)4–5 DRGs were assayed for activin C expression. Chronic neuropathic pain animals were treated with intrathecal or locally pre‐administered activin C or the vehicle. Nociceptive behaviours and pain‐related markers in L4–5 DRGs and spinal cord were evaluated. TRPV1 channel modulation by activin C was measured.Key ResultsFollowing peripheral nerve injury, expression of activin βC subunit mRNA and activin C protein was markedly up‐regulated in L4–5 DRGs of animals with axotomy, SNL or CCI. [Correction added on 26 November 2020, after first online publication: The preceding sentence has been corrected in this current version.] Intrathecal activin C dose‐dependently inhibited neuropathic pain in spinal nerve ligated rats. Local pre‐administration of activin C decreased neuropathic pain, macrophage infiltration into ipsilateral L4–5 DRGs and microglial reaction in L4–5 spinal cords of mice with CCI. In rat DRG neurons, activin C enhanced capsaicin‐induced TRPV1 currents. Pre‐treatment with activin C reduced capsaicin‐evoked acute hyperalgesia and normalized capsaicin‐evoked persistent hypothermia in mice. Finally, the analgesic effect of activin C was abolished in TRPV1 knockout mice with CCI.Conclusion and ImplicationsActivin C inhibits neuropathic pain by modulating TRPV1 channels, revealing potential analgesic applications in chronic neuropathic pain therapy.     

The role of VIP/PACAP receptor subtypes in spinal somatosensory processing in rats with an experimental peripheral mononeuropathy

Peripheral nerve damage often results in the development of chronic pain states, resistant to classical analgesics. Since vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are up-regulated in dorsal root ganglion cells following peripheral nerve injury, we investigated the expression and influence of VPAC1, VPAC2 and PAC1 receptors in rat spinal dorsal horn following a chronic constriction injury (CCI). Electrophysiological studies revealed that selective antagonists of VPAC1, VPAC2 and PAC1 receptors inhibit mustard oil-, but not brush-induced activity of dorsal horn neurones in CCI animals, while cold-induced neuronal activity was attenuated by VPAC1 and PAC1, but not VPAC2 receptor antagonists. Ionophoresis of selective agonists for the receptor subtypes revealed that the VPAC2 receptor agonist excited twice as many cells in CCI compared to normal animals, while the number of cells excited by the VPAC1 receptor agonist decreased and responses to PACAP-38 remained unchanged. In situ hybridisation histochemistry (ISHH) confirmed an increase in the expression of VPAC2 receptor mRNA within the ipsilateral dorsal horn following neuropathy, while VPAC1 receptor mRNA was seen to decrease and that for PAC1 receptors remained unchanged. These data indicate that VIP/PACAP receptors may be important regulatory factors in neuropathic pain states.     

Resolvin E1 Inhibits Neuropathic Pain and Spinal Cord Microglial Activation Following Peripheral Nerve Injury

Accumulating evidence indicates that activation of spinal cord microglia plays an important role in the genesis of neuropathic pain. Resolvin E1 (E1) is derived from omega-3 polyunsaturated fatty acid and exhibits potent anti-inflammatory, pro-resolution, and anti-nociceptive effects. We further examined whether RvE1 could reduce neuropathic pain and modulate spinal cord microglial activation. Intrathecal pre-treatment of RvE1 (100 ng) daily for 3 days partially prevented the development of nerve injury-induced mechanical allodynia and up-regulation of IBA-1 (microglial marker) and TNF-α in the spinal cord dorsal horn. Furthermore, intrathecal post-treatment of RvE1 (100 ng), 3 weeks after nerve injury, transiently reduced mechanical allodynia and heat hyperalgesia. Finally, RvE1 blocked lipopolisaccharide-induced microgliosis and TNF-α release in primary micoglial cultures. Our data suggest that RvE1 may attenuate neuropathic pain via inhibiting microglial signaling. Targeting the anti-inflammatory and pro-resolution lipid mediators may offer new options for preventing and treating neuropathic pain.     

星形胶质细胞在坐骨神经分支选择性结扎神经病理性痛中的作用

目的 探讨星形胶质细胞在坐骨神经分支选择性结扎(spared nerve injury,SNI)神经病理性痛中的作用.方法 24只SD大鼠随机分为4组(6只/组):SNI组(建立SNI动物模型同时行鞘内置管术,术后13 d鞘内给予生理盐水);假手术组(处理方式同SNI组,但不损伤坐骨神经及其分支);L-α-aminoadipate(LAA)组(制作SNI动物模型同时行鞘内置管,术后13 d鞘内给予LAA);对照组(不给予任何处理因素).术前2 d及术后14 d检测机械痛和神经病理性痛阈值,术后14 d以Real-time PCR法检测脊髓背角GFAP mRNA变化.结果 术前2 d各组大鼠机械痛域及热痛阈无明显差异(P>0.05).术后14 d假手术组机械痛域、热痛阈及脊髓背角GFAP mRNA较对照组均无明显变化(P>0.05);与对照组相比,SNI组机械痛域及热痛阈明显降低(P<0.05),脊髓背角GFAP mRNA明显增高(P<0.05);与SNI组相比,LAA组机械痛域及热痛阈均明显增高(P<0.05),脊髓背角GFAP mRNA明显降低(P<0.05).结论 LAA特异性抑制脊髓背角星形胶质细胞活性可缓解SNI大鼠神经病理性痛,提示脊髓背角星形胶质细胞活化是SNI大鼠神经病理性的重要机制.     

Effects of milnacipran, a 5-HT and noradrenaline reuptake inhibitor, on C-fibre-evoked field potentials in spinal long-term potentiation and neuropathic pain

BACKGROUND AND PURPOSE

The analgesic action of 5-HT and noradrenaline reuptake inhibitors (SNRIs) on nociceptive synaptic transmission in the spinal cord is poorly understood. We investigated the effects of milnacipran, an SNRI, on C-fibre-evoked field potentials (FPs) in spinal long-term potentiation (LTP), a proposed synaptic mechanism of hypersensitivity, and on the FPs in a neuropathic pain model.

EXPERIMENTAL APPROACH

C-fibre-evoked FPs by electrical stimulation of the sciatic nerve fibres were recorded in the spinal dorsal horn of anaesthetized adult rats, and LTP was induced by high-frequency stimulation of the sciatic nerve fibres. A rat model of neuropathic pain was produced by L5 spinal nerve ligation and transection.

KEY RESULTS

Milnacipran produced prolonged inhibition of C-fibre-evoked FPs when applied spinally after the establishment of LTP of C-fibre-evoked FPs in naïve animals. In the neuropathic pain model, spinal administration of milnacipran clearly reduced the basal C-fibre-evoked FPs. These inhibitory effects of milnacipran were blocked by spinal administration of methysergide, a 5-HT_1/2 receptor antagonist, and yohimbine or idazoxan, α₂-adrenoceptor antagonists. However, spinal administration of milnacipran in naïve animals did not affect the basal C-fibre-evoked FPs and the induction of spinal LTP.

CONCLUSION AND IMPLICATIONS

Milnacipran inhibited C-fibre-mediated nociceptive synaptic transmission in the spinal dorsal horn after the establishment of spinal LTP and in the neuropathic pain model, by activating both spinal 5-hydroxytryptaminergic and noradrenergic systems. The condition-dependent inhibition of the C-fibre-mediated transmission by milnacipran could provide novel evidence regarding the analgesic mechanisms of SNRIs in chronic pain.     

Intrathecal lemnalol, a natural marine compound obtained from Formosan soft coral, attenuates nociceptive responses and the activity of spinal glial cells in neuropathic rats

The investigators previously found that the administration of lemnalol, a natural marine compound isolated from the Formosan soft coral Lemnalia cervicorni, produced anti-inflammatory and analgesic effects in carrageenan-injected rats. Recently, several studies have demonstrated that the development and maintenance of neuropathic pain are accompanied by releasing of proinflammatory mediators from activated glial cells in the spinal cord. In this study, we investigated the antinociceptive properties of lemnalol, a potential anti-inflammatory compound, on chronic constriction injury (CCI) in a well-established rat model of neuropathic pain. Our results demonstrated that a single intrathecal administration of lemnalol (0.05-10 μg) significantly attenuated CCI-induced thermal hyperalgesia and mechanical allodynia, 14 days postsurgery. Furthermore, immunohistofluorescence analyses showed that lemnalol (10 μg) also significantly inhibits CCI-induced upregulation of microglial and astrocytic immunohistochemical activation markers in the dorsal horn of the lumbar spinal cord. Double immunofluorescent staining demonstrated that intrathecal injection of lemnalol (10 μg) markedly inhibited spinal proinflammatory mediator tumor necrosis factor-α expression in microglial cells and astrocytes in neuropathic rats. Collectively, our results indicate that lemnalol is a potential therapeutic agent for neuropathic pain, and that further exploration of the effects of lemnalol on glial proinflammatory responses is warranted.     

Glial cell line-derived neurotrophic factormediated enhancement of noradrenergic descending inhibition in the locus coeruleus exerts prolonged analgesia in neuropathic pain

Background and Purpose

The locus coeruleus (LC) is the principal nucleus containing the noradrenergic neurons and is a major endogenous source of pain modulation in the brain. Glial cell line-derived neurotrophic factor (GDNF), a well-established neurotrophic factor for noradrenergic neurons, is a major pain modulator in the spinal cord and primary sensory neurons. However, it is unknown whether GDNF is involved in pain modulation in the LC.

Experimental Approach

Rats with chronic constriction injury (CCI) of the left sciatic nerve were used as a model of neuropathic pain. GDNF was injected into the left LC of rats with CCI for 3 consecutive days and changes in mechanical allodynia and thermal hyperalgesia were assessed. The α₂-adrenoceptor antagonist yohimbine was injected intrathecally to assess the involvement of descending inhibition in GDNF-mediated analgesia. The MEK inhibitor U0126 was used to investigate whether the ERK signalling pathway plays a role in the analgesic effects of GDNF.

Key Results

Both mechanical allodynia and thermal hyperalgesia were attenuated 24 h after the first GDNF injection. GDNF increased the noradrenaline content in the dorsal spinal cord. The analgesic effects continued for at least 3 days after the last injection. Yohimbine abolished these effects of GDNF. The analgesic effects of GDNF were partly, but significantly, inhibited by prior injection of U0126 into the LC.

Conclusions and Implications

GDNF injection into the LC exerts prolonged analgesic effects on neuropathic pain in rats by enhancing descending noradrenergic inhibition.