Antinociceptive Effects of Amiloride and Benzamil in Neuropathic Pain Model Rats

Amiloride and benzamil showed antinocicepitve effects in several pain models through the inhibition of acid sensing ion channels (ASICs). However, their role in neuropathic pain has not been investigated. In this study, we investigated the effect of the intrathecal amiloride and benzamil in neuropathic pain model, and also examined the role of ASICs on modulation of neuropathic pain. Neuropathic pain was induced by L4-5 spinal nerve ligation in male Sprague-Dawley rats weighing 100-120 g, and intrathecal catheterization was performed for drug administration. The effects of amiloride and benzamil were measured by the paw-withdrawal threshold to a mechanical stimulus using the up and down method. The expression of ASICs in the spinal cord dorsal horn was also analyzed by RT-PCR. Intrathecal amiloride and benzamil significantly increased the paw withdrawal threshold in spinal nerve-ligated rats (87%±12% and 76%±14%, P=0.007 and 0.012 vs vehicle, respectively). Spinal nerve ligation increased the expression of ASIC3 in the spinal cord dorsal horn (P=0.01), and this increase was inhibited by both amiloride and benzamil (P<0.001 in both). In conclusion, intrathecal amiloride and benzamil display antinociceptive effects in the rat spinal nerve ligation model suggesting they may present an alternative pharmacological tool in the management of neuropathic pain at the spinal level.     

The effect of intrathecal gabapentin on mechanical and thermal hyperalgesia in neuropathic rats induced by spinal nerve ligation

Gabapentin decreases the level of glutamate and elevates that of alpha-amino-butyric acid in the central nervous system. Gabapentin was shown to have antinociceptive effects in several facilitated pain models. Intrathecal gabapentin was also known to be effective in reducing mechanical allodynia in animals with neuropathic pain. In this study, we investigated to see whether intrathecal gabapentin produces antihyperalgesic effects on thermal and mechanical hyperalgesia in neuropathic rats and whether its effects are associated with motor impairment. To induce neuropathic pain in Sprague-Dawley rats, left L5 and L6 spinal nerves were ligated. After a week, lumbar catheterization into subarachnoid space was performed. Then, paw withdrawal times to thermal stimuli and vocalization thresholds to paw pressure were determined before and up to 2 hr after intrathecal injection of gabapentin. Also, motor functions including performance times on rota-rod were determined. Intrathecal gabapentin attenuated significantly thermal and mechanical hyperalgesia in neuropathic rats, but did not block thermal and mechanical nociception in sham-operated rats. Intrathecal gabapentin of antihyperalgesic doses inhibited motor coordination performance without evident ambulatory dysfunction. This study demonstrates that intrathecal gabapentin is effective against thermal and mechanical hyperalgesia, in spite of moderate impairment of motor coordination.     

Dexmedetomidine decreases hyperalgesia in neuropathic pain by increasing acetylcholine in the spinal cord

The activation of α2-adrenoceptors has attracted attention as a therapeutic target for neuropathic pain, which remains a clinical challenge. In the present study, we examined the interaction between α2-adrenergic and cholinergic signaling in a rat model of neuropathic pain induced by spinal nerve ligation (SNL). Intrathecal administration of dexmedetomidine, which is a selective α2-adrenoceptor agonist (0.1–1.0 μg), dose-dependently suppressed hyperalgesia in SNL rats but did not alter paw withdrawal thresholds in normal rats. The analgesic effect of dexmedetomidine was abolished by intrathecal pretreatment with idazoxan (30 μg) and atropine (30 μg), which antagonize the α2-adrenoreceptor and muscarinic receptor, respectively. In vivo microdialysis in the lumbar spinal dorsal horn revealed that acetylcholine concentrations increased after dexmedetomidine perfusion (1 μM), but only in SNL rats. The combination of an ineffective dose of intrathecal dexmedetomidine with intraperitoneal donepezil, which is a cholinesterase inhibitor, decreased neuropathic hypersensitivity. These results suggest that plasticity of the spinal noradrenergic–cholinergic axis only occurs in neuropathic pain states. Thus, drug combinations that strengthen the noradrenergic–cholinergic interaction may provide therapeutic benefit in neuropathic pain.     

氯胺酮鞘内给药对神经病理性痛大鼠脊髓背角小胶质细胞活化、TNF-α和IL-1β产生的影响

目的:探究鞘内注射氯胺酮(ketamine,KTM)对坐骨神经结扎(CCI)神经病理性疼痛大鼠模型行为、脊髓背角肿瘤坏死因子-α(TNF-α)和白介素1β(IL-1β)以及小胶质细胞标记蛋白OX42表达的影响。方法:(1)建立CCI模型大鼠,鞘内注射KTM或MI(米诺四环素)进行干预,测定大鼠机械缩足阈值(MWT)和热缩足潜伏期(TWL)。(2)运用ELISA法检测TNF-α和IL-1β表达水平。(3)使用Western Blot法检测OX42的表达情况。结果:(1)KTM组和MI组大鼠MWT和TWL值较CCI组显著升高;KTM组大鼠MWT和TWL值高于CCI组(P0.05)。(2)KTM组和MI组TNF-α和IL-1β表达水平低于CCI组(P0.05);其中KTM组TNF-α和IL-1β表达水平显著高于MI组(P0.05)。(3)KTM组和MI组OX42表达水平显著低于CCI组;KTM组OX42表达水平高于MI组(P0.05)。结论:鞘内KTM可以抑制脊髓背角小胶质细胞激活,减少TNF-α和IL-1β表达,显著改善坐骨神经结扎引起的神经病理性疼痛。     

曲古抑菌素A通过抑制脊髓背角内的炎症反应减轻大鼠神经病理性痛

目的:观察曲古抑菌素A (TSA)对脊神经结扎(SNL)大鼠镇痛效果及分子机制。方法:40只健康雄性Sprague Dawley(SD)大鼠随机分为假手术组(sham)、曲古抑菌素A处理组(TSA)、脊神经结扎组(SNL)和SNL+TSA组(SNL+TSA)。采用L5脊神经结扎(SNL)的方法建立神经病理性痛模型,鞘内注射TSA进行干预,通过von Frey丝和热板实验检测大鼠的痛敏,应用免疫荧光染色方法观察大鼠脊髓背角内HDAC1的表达情况;应用Western Blot方法观察大鼠脊髓背角内胶质纤维酸性蛋白(GFAP)和离子钙接头蛋白分子1(Iba-1)的表达水平;应用real time RT-PCR方法检测大鼠脊髓背角内TNF-α、IL-1β和IL-6的mRNA表达水平。结果:SNL模型大鼠术后机械性痛阈值和热痛阈值均显著降低(P <0.05),鞘内给予TSA能够明显缓解大鼠患侧后足机械性痛敏和热痛敏; SNL模型大鼠脊髓背角内HDAC1的表达较对照组明显增加,而鞘内注射TSA可显著抑制其表达; SNL术后脊髓背角内GFAP和Iba-1的表达显著升高(P <0.05),...     

大鼠坐骨神经受压和解压后背根节和脊髓内神经元nNOS表达的变化

目的 :观察坐骨神经受压及解压后大鼠腰段背根节和脊髓内神经元型一氧化氮合酶 (nNOS)表达的变化 ,借以探讨外周神经源性痛的发病和影响机制。方法 :大鼠随机分为压迫组、解压组和对照组 ,采用聚乙烯管压迫坐骨神经的动物模型 ,用免疫细胞化学方法并结合计算机图像分析进行研究。结果 :与对照组比较 ,压迫组和解压组腰4～ 6背根节中nNOS的表达显著增加 ,相应节段脊髓背角的表达则明显降低 ;解压组与压迫组比较 ,背根节中nNOS的表达明显减少 ,而脊髓背角的已经下调的nNOS表达则回升 ,但仍然低于对照组水平。结论 :NO可能与神经源性痛时在中枢和外周的痛觉敏感性形成和神经系统长时程改变有关。     

Post-injury administration of minocycline: an effective treatment for nerve-injury induced neuropathic pain

Neuropathic pain is an intractable clinical problem, affecting millions of people worldwide. Preemptive administration of minocycline has been confirmed useful for treating neuropathic pain by inhibiting spinal microglia activation and consequently lowering proinflammatory cytokine expression. However, most patients with neuropathic pain have no chance to receive preemptive treatment and it remains unclear whether there is a therapeutic time window for post treatment with minocycline. The present study is to confirm the effect and the therapeutic time window of intrathecal minocycline on spinal nerve ligation (SNL)-induced neuropathic pain after lesion. Behavioral test and immunohistochemistry are utilized to determine the variation of mechanical allodynia and microglia phosphorylated-p38 (p-p38) expression respectively after intrathecal minocycline. Results showed that post-injury intrathecal minocycline attenuated mechanical allodynia effectively together with inhibiting spinal microglia p-p38 expression on post operative day (POD) 1, POD 3 and POD 7. Additionally, results from POD 10 and POD 21 showed that intrathecal minocycline suppressed spinal microglia p-p38 expression but without any effects on reversing mechanical allodynia. It is concluded that post-injury intrathecal minocycline is an effective therapeutic intervention for treating SNL-induced neuropathic pain by inhibiting spinal microglia activation. Accordingly, there is indeed a therapeutic time window for post-injury intrathecal minocycline, which is the initiation stage of neuropathic pain development.     

大鼠桡神经钳夹伤后背根节和脊髓nNOS免疫阳性神经元的变化

目的研究大鼠桡神经钳夹伤后背根神经节(DRG)和脊髓nNOS免疫阳性神经元的变化,探讨NO是否参与大鼠桡神经钳夹伤后的DRG和脊髓水平的痛觉调制。方法用辣根过氧化物酶追踪大鼠桡神经的由来;大鼠桡神经钳夹伤结合免疫组化法,研究桡神经钳夹伤后DRG鄄和脊髓的nNOS免疫阳性结构变化。结果(1)大鼠桡神经的组成范围在C5～T1;(2)大鼠桡神经钳夹伤后,DRG内nNOS免疫阳性神经元的变化难以分析:脊髓后角nNOS免疫阳性结构数量减少、免疫强度下降。结论大鼠桡神经钳夹伤后,脊髓nNOS免疫阳性结构发生可塑性变化,NO在桡神经钳夹伤后的痛觉调制中有一定的作用。     

Intrathecal S-nitroso-N-acetylpenicillamine and L-cysteine attenuate nerve injury-induced allodynia through noradrenergic activation in rats

Spinal norepinephrine release and activation of spinal alpha(2)-adrenergic receptors represent important components of descending control of nociception. Recent studies have shown that nitric oxide is capable of stimulating neuronal norepinephrine release in the presence of thiol-containing compounds such as L-cysteine. In the present study, we tested a hypothesis in a rodent model of neuropathic pain that intrathecal injection of the nitric oxide donor S-nitroso-N-acetylpenicillamine and L-cysteine produces an antiallodynic action mediated by the spinal alpha(2)-adrenergic receptors. Allodynia was induced in rats by ligation of the left lumbar L5/L6 spinal nerves. Mechanical allodynia was quantified by application of von Frey filaments to the left hindpaw. Intrathecal injection of 20-100microg of S-nitroso-N-acetylpenicillamine in the presence of 200microg of L-cysteine, but not D-cysteine, dose-dependently attenuated the allodynia. Intrathecal injection of a combination of 100microg of S-nitroso-N-acetylpenicillamine and 50-200microg of L-cysteine also inhibited the allodynia in a dose-dependent manner. Pretreatment with a nitric oxide scavenger, carboxy-PTIO, or depletion of norepinephrine with a specific neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, prevented the antiallodynic action of intrathecal S-nitroso-N-acetylpenicillamine and L-cysteine. Furthermore, the antiallodynic effect produced by intrathecal injection of a combination of S-nitroso-N-acetylpenicillamine and L-cysteine was abolished by pretreatment with intrathecal injection of a non-specific alpha-adrenergic receptor antagonist, phentolamine, or an alpha(2) receptor antagonist, idazoxan.This study provides the first functional evidence that spinal nitric oxide interacts with the thiol-containing compounds to produce an antiallodynic effect in neuropathic pain. We propose that such an action is mediated by endogenous norepinephrine and spinal alpha(2)-adrenergic receptors.     

Intrathecal Lamotrigine Attenuates Mechanical Allodynia and Suppresses Microglial and Astrocytic Activation in a Rat Model of Spinal Nerve Ligation

Purpose

Lamotrigine, a novel anticonvulsant, is a sodium channel blocker that is efficacious in certain forms of neuropathic pain. Recently, microglial and astrocytic activation has been implicated in the development of nerve injury-induced neuropathic pain. We have assessed the effects of continuous intrathecal administration of lamotrigine on the development of neuropathic pain and glial activation induced by L5/6 spinal-nerve ligation in rats.

Materials and Methods

Following left L5/6 spinal nerve ligation (SNL), Sprague-Dawley male rats were intrathecally administered lamotrigine (24, 72, or 240 µg/day) or saline continuously for 7 days. Mechanical allodynia of the left hind paw to von Frey filament stimuli was determined before surgery (baseline) and once daily for 7 days postoperatively. On day 7, spinal activation of microglia and astrocytes was evaluated immunohistochemically, using antibodies to the microglial marker OX-42 and the astrocyte marker glial fibrillary acidic protein (GFAP).

Results

Spinal-nerve ligation induced mechanical allodynia in saline-treated rats, with OX-42 and GFAP immunoreactivity being significantly increased on the ipsilateral side of the spinal cord. Continuously administered intrathecal lamotrigine (240 µg/day) prevented the development of mechanical allodynia, and lower dose of lamotrigine (72 µg/day) ameliorated allodynia. Intrathecal lamotrigine (72 and 240 µg/day) inhibited nerve ligation-induced microglial and astrocytic activation, as evidenced by reduced numbers of cells positive for OX-42 and GFAP.

Conclusion

Continuously administered intrathecal lamotrigine blocked the development of mechanical allodynia induced by SNL with suppression of microglial and astrocytic activation. Continuous intrathecal administration of lamotrigine may be a promising therapeutic intervention to prevent neuropathy.     

ClC-3在神经病理性痛大鼠脊髓背角及背根神经节的表达及其参与痛行为调控的研究

目的:观察电压门控性氯通道(voltage-gated chloride channel,ClC)3型在腓总神经结扎神经病理性痛模型大鼠脊髓背角(spinal dorsal horn,SDH)和背根神经节(dorsal root ganglion,DRG)内的表达变化及阻断氯离子通道后痛行为的改变。方法:应用免疫组织化学染色法、蛋白印迹法以及痛行为检测观察ClC-3在神经病理性痛大鼠SDH和DRG的变化和作用。结果:在正常大鼠,ClC-3主要位于DRG神经元胞膜;在SDH,ClC-3阳性纤维主要位于Ⅰ层。在腓总神经结扎大鼠,1周内结扎侧背角Ⅰ层及DRG的ClC-3表达增加,2～4周表达逐渐减少,在DRG也观察到相同的现象。给予氯离子通道阻断剂后,腓总神经结扎大鼠的痛阈下降。结论:ClC-3在神经病理性痛早期表达上调,随病程发展逐渐下降;阻断ClC-3可使大鼠痛阈下降。     

Peripheral and spinal antihyperalgesic activity of najanalgesin isolated from Naja naja atra in a rat experimental model of neuropathic pain

Snake venoms are a rich source of various compounds that have applications in medicine and biochemistry. Recently, it has been demonstrated that najanalgesin isolated from the venom of Naja naja atra exerts analgesic effects on acute pain in mice. The objective of this study was to evaluate the antinociceptive effect of najanalgesin in a rat model of neuropathic pain, induced by L5 spinal nerve ligation and transaction. We observed that intraperitoneal (i.p.) administration of najanalgesin produced significant increase in hind paw withdrawal latency (HWL) in response to both mechanical and thermal stimulation. Moreover, a single dose of najanalgesin was able to induce antinociceptive activity that lasted for 1 week. Intrathecal injection of najanalgesin increased the HWL in response to mechanical stimuli. The antinociceptive effect of najanalgesin administered intrathecally was partly inhibited by intrathecal injection of naloxone or atropine. These results demonstrate that najanalgesin has antinociceptive effects on the central and peripheral system in the rat neuropathic pain model. The opioid receptor and muscatinic receptor are involved in najanalgesin-induced antinociception in the spinal cord. This research supports the possibility of using najanalgesin as a novel pharmacotherapeutic agent for neuropathic pain.     

Lack of the nitric oxide-cyclic GMP-potassium channel pathway for the antinociceptive effect of intrathecal zaprinast in a rat formalin test

Zaprinast is a phosphodiesterase inhibitor that is active in various models of pain when administered locally. In addition, the antinociception of zaprinast is involved in the nitric oxide (NO)-cGMP pathway. However, the effect of zaprinast administered spinally has not been examined. Therefore, this study examined the effect of zaprinast on the formalin-induced nociception at the spinal level. Next, the role of the NO-cGMP-potassium channel pathway on the effect of zaprinast was further clarified. Catheters were inserted into the intrathecal space of male Sprague-Dawley (SD) rats. Pain was induced by applying 50 microl of a 5% formalin solution to the hindpaw. The change in the zaprinast-induced effect was examined after an intrathecal pretreatment with a NO synthase inhibitor (l-NMMA), a guanylyl cyclase inhibitor (ODQ) or a potassium channel blocker (glibenclamide). Zaprinast produced an antinociceptive effect during phase 1 and phase 2 in the formalin test. Intrathecal l-NMMA, ODQ and glibenclamide did not reverse the antinociception of zaprinast in either phase of the formalin test. These results suggest that zaprinast is effective against both acute pain and the facilitated pain state at the spinal level. However, the NO-sensitive cGMP-potassium channel pathway is not contributable to the antinociceptive mechanism of zaprinast in the spinal cord.     

c-fos反义寡核苷酸探针对大鼠炎性痛和神经病理性痛的影响

研究c-fos反义寡核苷酸探针(antisense oligodeoxynucleotides,ASO)对大鼠炎性痛和神经病理性痛的作用效果,为临床慢性疼痛的治疗提供理论依据。以大鼠足底注射福尔马林(formalin)致炎性痛模型和腰5脊神经结扎(spinal nerve ligation,SNL)诱导神经病理性痛模型为基础,分别观察鞘内给予c-fosASO对两种模型大鼠的行为学及腰5脊髓背角Fos阳性神经元表达的影响。行为学检测结果表明:和正义探针(SO)组以及生理盐水(NS)对照组相比,鞘内预先给予c-fosASO后,大鼠足底注射福尔马林引起自发缩足反射次数和SNL诱导的机械性缩足反射阈值(mechanical withdrawal threshold,MWT)和热缩足反射潜伏期(thermal withdrawal latency,TWL)均有明显缓解(P<0.05),而c-fosASO不能逆转SNL诱导的神经病理性痛。免疫荧光组织化学染色结果发现:和对照组相比,鞘内给予c-fosASO能显著抑制福尔马林和SNL诱导的大鼠腰髓背角Fos阳性神经元的增加。上述结果提示,脊髓背角c-fos的活化对于炎性痛和神经病理性痛的发展都有显著意义,而抑制其活化在预防阶段比治疗阶段对慢性疼痛的缓解作用更为显著。     

Attenuating phosphorylation of p38 MAPK in the activated microglia: a new mechanism for intrathecal lidocaine reversing tactile allodynia following chronic constriction injury in rats

Increasing evidences approve the long-term analgesia effects of intrathecal lidocaine in patients with chronic pain and in animal peripheral nerve injury models, but the underlying mechanism remains elusive. Previous evidences suggest that the activation of the p38 MAPK signaling pathway in hyperactive microglia in the dorsal horn of spinal cord involves in nerve injury-induced neuropathic pain. In this study, we demonstrate that attenuating phosphorylation of p38 MAPK in the activated microglia of spinal cord, at least partly, is the mechanism of intrathecal lidocaine reversing established tactile allodynia in chronic constriction injury model of rats. This finding not only provides a new insight into the mechanisms underlying long-term therapeutic effects of lidocaine on neuropathic pain, but also reveals one more specific drug target for analgesia.     

Brain-derived neurotrophic factor–activated astrocytes produce mechanical allodynia in neuropathic pain

Neuropathic pain management is challenging for physicians and a vexing problem for basic researchers. Recent studies reveal that activated spinal astrocytes may play a vital role in nerve injury-induced neuropathic pain, although the mechanisms are not fully understood. We have found increased glial fibrillary acidic protein (GFAP) expression, a hallmark of reactive gliosis, and elevated brain-derived neurotrophic factor (BDNF) expression in the dorsal horn in a rat model of allodynia induced by spinal nerve ligation (SNL). The high GFAP expression and mechanical allodynia that SNL induces were prevented by the intrathecal injection of the BDNF-sequestering fusion protein TrkB/Fc. Additionally, mechanical allodynia and GFAP overexpression was induced by the spinal administration of exogenous BDNF to naive rats, and exogenous BDNF given together with fluorocitrate, an astrocytic metabolism inhibitor, inhibited allodynia and GFAP upregulation. Exogenous BDNF also activated the astrocytes directly when tested in vitro. Furthermore, intrathecal administration of BDNF-stimulated astrocytes also induced mechanical allodynia in naive rats. All of these results indicate that astrocytes activated by BDNF might contribute to mechanical allodynia development in neuropathic pain in rats.     

Analgesic effect of extracts of Chinese medicinal herbs Moutan cortex and Coicis semen on neuropathic pain in mice

Neuropathic pain arising from peripheral nerve injury is a clinical disorder characterized by a combination of spontaneous pain, hyperalgesia and tactile pain (allodynia), and remains a significant clinical problem since it is often poorly relieved by conventional analgesics. To seek an analgesic compound(s) in Chinese herbs, we examined the effect of seven Chinese herbs that are routinely prescribed for pain management in two neuropathic pain models: allodynia induced by intrathecal administration of prostaglandin F₂ (PGF₂) and by selective L5 spinal nerve transection. The extracts of Moutan cortex and Coicis semen dose-dependently alleviated the PGF₂-induced allodynia by oral administration 1 h before intrathecal injection of PGF₂. When orally administrated every day for 7 days, these extracts attenuated neuropathic pain in the ipsilateral side, but not in the contralateral side, day 7 after L5 spinal nerve transection. The increase in NADPH diaphorase activity in the spinal cord associated with neuropathic pain was also blocked by these extracts. These results suggest that Moutan cortex and Coicis semen contain substances effective in neuropathic pain.     

Effects of intrathecal injection of prednisolone acetate on expression of NR2B subunit and nNOS in spinal cord of rats after chronic compression of dorsal root ganglia

N-methyl-D-aspartate receptor subunit 2B (NR2B) and neuronal nitric oxide synthase (nNOS) play important roles in the mechanism of neuropathic pain. To elucidate how glucocorticoids affect this mechanism, we studied the effects of intrathecal (it) injection of prednisolone acetate (PA) on a nociceptive stimulus and the changes of nNOS and NR2B subunit expression in the spinal dorsal horn of Sprague Dawley rats following chronic compression of the dorsal root ganglia (CCD). Paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were measured for 15 days postoperatively. An it injection of PA (2.0 mg/kg) every 3 days for postoperative days 1 to 15 inhibited the thermal hyperalgesia and tactile allodynia of CCD rats. Chronic compression of the dorsal root ganglia induced time-dependent upregulation of nNOS and NR2B subunits of N-methyl-D-aspartate receptor within the spinal cord dorsal horn ipsilateral to CCD. Both upregulations were significantly diminished by it administration of PA (2.0 mg/kg), but not by lower doses of PA (0.5 or 1.0 mg/kg). The results suggest that PA upregulation of neuronal nitric oxide synthase and NR2B subunit expression in the spinal dorsal horn contributes to PA inhibition of hyperalgesia induced by chronic compression of dorsal root ganglia.     

Involvement of substance P and calcitonin gene-related peptide in development and maintenance of neuropathic pain from spinal nerve injury model of rat

Recently, it has been suggested that uninjured primary sensory neurons contribute to neuropathic pain induced by peripheral nerve injury. However, there is lack of evidences of roles of normal pain transmitting substances such as substance P and calcitonin gene-related peptide (CGRP) in neuropathic pain. Whether substance P and CGRP have a role in spinal nerve-injured neuropathic pain model was tested. Male rats were subjected to L5 and L6 spinal nerve transection (SNT), and mechanical hyperalgesia was evaluated by measuring paw withdrawal threshold (PWT). SNT induced a persistent PWT decrease, a sign of neuropathic pain. Lidocaine was soaked on spinal nerves or intrathecally injected 10 min before SNT to block neuronal discharges caused by the injury, and L703,606 (NK1 receptor antagonist) and CGRP8-37 (CGRP receptor antagonist) were intrathecally injected into the rats to block actions of substance P and CGRP released from central nerve terminals in the spinal cord by injury discharges. The treatments with lidocaine, L703,606 and CGRP8-37 delayed the onset of neuropathic pain by 1-4 days, compared with the saline-treated rats. After neuropathic pain was established, intrathecal injections of L703,606 and CGRP8-37 significantly mitigated mechanical hyperalgesia for 20 min. These results suggest that substance P and CGRP are involved in the development and maintenance of neuropathic pain and that these peptides from the central terminals of intact sensory neurons contribute to the maintenance of peripheral nerve injury-induced neuropathic pain.     

Stage-dependent anti-allodynic effects of intrathecal Toll-like receptor 4 antagonists in a rat model of cancer induced bone pain

It has been reported that Toll-like receptor 4 (TLR4), which plays an important role in glial activation in neuropathic pain, is significantly increased in cancer pain. The present study was designed to assess the role of TLR4 in cancer-induced bone pain (CIBP) by intrathecal administration of TLR4 signaling pathway blocker naloxone or lipopolysaccharide Rhodobacter sphaeroides (LPS-RS). The rats developed significant mechanical allodynia from day 8 after intratibial Walker 256 inoculation. Intrathecal injection of naloxone or LPS-RS at day 8 significantly attenuated mechanical allodynia as shown by increased paw withdrawal thresholds. In contrast, the same pharmacological treatment showed no or slight pain relieving effect at day 16. Our findings demonstrate that the spinal TLR4 signaling pathway contributes to the mechanism underlying CIBP in a stage-dependent manner in rats, and it may be an efficacious target at early stage for the treatment in the future.