Curcumin attenuates thermal hyperalgesia in a diabetic mouse model of neuropathic pain

Diabetic neuropathic pain, an important microvascular complication in diabetes mellitus is recognised as one of the most difficult types of pain to treat. A lack of the understanding of its aetiology, inadequate relief, development of tolerance and potential toxicity of classical antinociceptives warrant the investigation of the newer agents to relieve this pain. The aim of the present study was to explore the antinociceptive effect of curcumin and its effect on tumour necrosis factor-alpha (TNF-alpha) and nitric oxide (NO) release in streptozotocin induced diabetic mice. Four weeks after a single intraperitoneal injection of streptozotocin (200 mg/kg), mice were tested in the tail immersion and hot-plate assays. Diabetic mice exhibited significant hyperalgesia along with increased plasma glucose and decreased body weights as compared with control mice. Chronic treatment with curcumin (15, 30 and 60 mg/kg body weight; p.o.) for 4 weeks starting from the 4th week of streptozotocin injection significantly attenuated thermal hyperalgesia and the hot-plate latencies. Curcumin also inhibited the TNF-alpha and NO release in a dose dependent manner. These results indicate an antinociceptive activity of curcumin possibly through its inhibitory action on NO and TNF-alpha release and point towards its potential to attenuate diabetic neuropathic pain.     

Development and expression of neuropathic pain in CB1 knockout mice

Neuropathic pain is a clinical manifestation characterized by the presence of spontaneous pain, allodynia and hyperalgesia. Here, we have evaluated the involvement of CB1 cannabinoid receptors in the development and expression of neuropathic pain. For this purpose, partial ligation of the sciatic nerve was performed in CB1 cannabinoid receptor knockout mice and their wild-type littermates. The development of mechanical and thermal allodynia, and thermal hyperalgesia was evaluated by using the von Frey filaments, cold-plate and plantar tests, respectively. Pre-surgical tactile and thermal withdrawal thresholds were similar in both genotypes. In wild-type mice, sciatic nerve injury led to a neuropathic pain syndrome characterized by a marked and long-lasting reduction of the paw withdrawal thresholds to mechanical and thermal stimuli. These manifestations developed similarly in mice lacking CB1 cannabinoid receptors. We have also investigated the consequences of gabapentin administration in these animals. Gabapentin (50 mg/kg/day, i.p.) induced a similar suppression of mechanical and thermal allodynia in both wild-type and CB1 knockout mice. Mild differences between genotypes were observed concerning the effect of gabapentin in the expression of thermal hyperalgesia. Taken together, our results indicate that CB1 cannabinoid receptors are not critically implicated in the development of neuropathic pain nor in the anti-allodynic and anti-hyperalgesic effects of gabapentin in this model.     

The synthetic cannabinoids attenuate allodynia and hyperalgesia in a rat model of trigeminal neuropathic pain

Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge. At present there are few effective drugs. Here we have evaluated the effects of the synthetic cannabinoid WIN 55,212-2 on mechanical allodynia and thermal hyperalgesia in a rat model of trigeminal neuropathic pain produced by a chronic constriction injury (CCI) of the infraorbital branch of the trigeminal nerve (ION). Relative to sham operation controls, rats with the CCI-ION consistently displayed hyperresponsiveness to von Frey filament and heat stimulation of the vibrissal pad. Both mechanical allodynia and thermal hyperalgesia are seen both ipsilateral and contralateral to the side of nerve injury, but is significantly more severe ipsilaterally. Administration of WIN 55,212-2 (0.3-5 mg/kg i.p.) dose-dependently increased the mechanical and heat withdrawal thresholds. WIN 55,212-2 (0.3-3 mg/kg i.p.) produced no significant motor deficits in animals using the rotarod test. The effect of WIN 55,212-2 was mimicked by cannabinoid CB1 receptor agonist HU 210 and was antagonized by CB1 receptor antagonist AM 251, but not by CB2 receptor antagonist AM 630 or vanilloid receptor 1 antagonist capsazepine, suggesting the involvement of CB1 receptors. CCI-ION also induced a time-dependent upregulation of CB1 receptors primarily within the ipsilateral superficial laminae of the trigeminal caudal nucleus revealed by both Western blot and immunohistochemistry. Taken together, these results suggest that cannabinoids may be a useful therapeutic approach for the clinical management of trigeminal neuropathic pain disorders.     

Lithium attenuates pain-related behavior in a rat model of neuropathic pain: possible involvement of opioid system

Lithium is a major drug for bipolar disorder and mania. Recently, many studies have shown the neuroprotective effect of lithium in different models of neurodegenerative diseases. The present study was carried out to examine the effect of lithium in a rat model of neuropathic pain induced by partial sciatic nerve ligation and the possible role of opioid system in this effect. To do so, animals received acute injection of saline, lithium (5, 10 and 15 mg/kg,) and naloxone (1 mg/kg) or the combination of naloxone (1 mg/kg) with lithium (10 mg/kg) intraperitoneally on the testing days. Thermal hyperalgesia, mechanical and cold allodynia were measured on the days 3, 5, 7, 10 and 14 after surgery. Lithium decreased thermal hyperalgesia scores with dose of 5, 10 and 15 mg/kg and cold and mechanical allodynia scores with dose of 10 and 15 mg/kg, significantly. The opioid antagonist naloxone prevented the effect of lithium on thermal hyperalgesia and mechanical allodynia while it did not show any effect on the acetone-induced cold allodynia. Our results suggest that lithium can be considered as a therapeutic potential for the treatment of some aspects of neuropathic pain and that the opioid system may be involved in the lithium-induced attenuation of thermal hyperalgesia and mechanical allodynia.     

虎杖提取物对神经病理性疼痛模型大鼠的镇痛作用研究

目的：研究虎杖提取物对神经病理性疼痛模型大鼠的镇痛作用,并探讨其作用机制。方法雄性SD大鼠,随机分为对照组和坐骨神经缩狭模型（chronic construction injury, CCI）组。CCI术后14 d,应用Von Frey法测定机械刺激缩足反射阈值（mechanical withdrawal threshold, MWT）,观察口服不同剂量的虎杖提取物对CCI大鼠机械性痛觉超敏的影响。并考察不同剂量虎杖提取物对CCI大鼠脊髓p-ERK、p-P38水平及小胶质细胞活化的影响。结果虎杖提取物可明显升高CCI大鼠术侧的MWT值（P〈0.05）,而对正常大鼠的MWT值没有显著影响（P〉0.05）。同时虎杖提取物可显著抑制CCI模型大鼠脊髓ERK和p38的磷酸化,并显著抑制IBA-1表达增加。结论虎杖提取物对CCI诱导的机械性痛觉超敏具有显著性的抑制作用,且镇痛作用维持时间长于吗啡,其机制可能是通过抑制脊髓水平ERK和p38的磷酸化,及通过抑制小胶质细胞活化而实现。     

Actions of N-arachidonyl-glycine in a rat neuropathic pain model

While cannabinoid receptor agonists reduce the abnormal pain sensations associated with animal models of neuropathic pain states they also produce CB(1) receptor mediated side effects. Recently, a number of arachidonic acid-amino acid conjugates, including N-arachidonyl-glycine (NAGly), have been identified which are structurally related to the endocannabinoid arachidonyl ethanolamide (anandamide). In the present study we examined the effect of NAGly in a rat model of neuropathic pain. Intrathecal administration of NAGly (700 nmol) and the pan-cannabinoid receptor agonist HU-210 (30 nmol) reduced the mechanical allodynia induced by partial ligation of the sciatic nerve. The NAGly induced anti-allodynia was dose dependent and, unlike HU-210, was unaffected by the cannabinoid CB(1) and CB(2) receptor antagonists, AM251 and SR144528 (30 nmol). The NAGly degradation products, arachidonic acid and glycine (700 nmol), did not reduce allodynia. HU-210, but not NAGly produced a reduction in rotarod latency. These findings suggest that NAGly may provide a novel analgesic approach to alleviate neuropathic pain.     

Chronic administration of amitriptyline and caffeine in a rat model of neuropathic pain: multiple interactions

This study was designed to determine (1) whether chronic amitriptyline administration was effective in alleviating symptoms of neuropathic pain in a rat model of spinal nerve injury, and (2) whether the effect of amitriptyline involved manipulation of endogenous adenosine, by determining the effect of caffeine, a non-selective adenosine A₁ and A₂ receptor antagonist, on its actions. Nerve injury was produced by unilateral spinal nerve ligation of the fifth and sixth lumbar nerves distal to the dorsal root ganglion, and this resulted in stimulus-evoked thermal hyperalgesia and static tactile mechanical allodynia. Animals received pre- and post-surgical intraperitoneal doses of amitriptyline (10 mg/kg) and caffeine (7.5 mg/kg), alone or in combination, and following surgery, were administered amitriptyline (15–18 mg/kg/day) and caffeine (6–8 mg/kg/day), alone or in combination, in the drinking water. Rats were tested for thermal reaction latencies and static tactile thresholds at 7, 14 and 21 days following surgery. In the paw ipsilateral to the nerve ligation, chronic amitriptyline administration consistently decreased the thermal hyperalgesia produced by spinal nerve ligation over a 3-week period, and this effect was blocked by concomitant caffeine administration at all time intervals. In the contralateral paw, thermal withdrawal latencies were more variable, with the most reproducible finding being a reduction in thermal thresholds in the amitriptyline–caffeine combination group. There was no effect by either drug or the drug combination on the static tactile allodynia produced by spinal nerve ligation in the ipsilateral paw. However, chronic amitriptyline administration induced a tactile hyperaesthesia in the contralateral paw at all time intervals, and this effect was exacerbated by concomitant chronic caffeine administration. The results of this study indicate that chronic administration of amitriptyline is effective in alleviating thermal hyperalgesia, but not static tactile allodynia, in the hindpaw ipsilateral to nerve injury, and the block of this effect by caffeine suggests that this effect is partially achieved through manipulation of endogenous adenosine systems. Additionally, chronic amitriptyline administration induces contralateral hyperaesthetic responses that are augmented by caffeine. Both the symptom-specific effect, and adenosine involvement in amitriptyline action may be important considerations governing its use in neuropathic pain.     

Sildenafil induces hyperalgesia via activation of the NO-cGMP pathway in the rat neuropathic pain model

Persistent stimulation of nociceptors and C-fibers by tissue injury causes hyperalgesia and allodynia by sensitization of nociceptors and facilitation of synaptic transmission in the spinal cord. The important participant in the inflammatory response of injured peripheral nerve may be nitric oxide (NO). The aim of the present study was to test the sensitivity of PDE5 inhibitor sildenafil in chronic constriction injury (CCI) model a rat model of neuropathic pain. Sciatic nerve injury is associated with development of hyperalgesia 14 days after the nerve ligation. Sildenafil (100 and 200 μg/rat, i.t.) produced a significant decrease in pain threshold, which in lower dose did not alter the nociceptive threshold. The hyperalgesic effect of sildenafil was blocked by L-NAME and methylene blue (MB), which on per se treatment showed antinociceptive effect in nerve ligated rats. The results from the present study indicated that the major activation of NO-cGMP pathway in the chronic constriction injury model of neuropathic pain. The aggravation of hyperalgesic response might be due to the increased cGMP levels resulting in PKG-I activation and its upregulation.     

Minocycline and pentoxifylline attenuate allodynia and hyperalgesia and potentiate the effects of morphine in rat and mouse models of neuropathic pain

Recent research has shown that microglial cells which are strongly activated in neuropathy can influence development of allodynia and hyperalgesia. Here we demonstrated that preemptive and repeated i.p., administration (16 h and 1 h before injury and then after nerve ligation twice daily for 7 days) of minocycline (15; 30; 50 mg/kg), a potent inhibitor of microglial activation, significantly attenuated the allodynia (von Frey test) and hyperalgesia (cold plate test) measured on day 3, 5, 7 after chronic constriction injury (CCI) in rats. Moreover, the 40% improvement of motor function was observed. In mice, i.p., administration of minocycline (30 mg/kg) or pentoxifylline (20 mg/kg) according to the same schedule also significantly decreased allodynia and hyperalgesia on day 7 after CCI. Antiallodynic and antihyperalgesic effect of morphine (10 mg/kg; i.p.) was significantly potentiated in groups preemptively and repeatedly injected with minocycline (von Frey test, 18 g versus 22 g; cold plate test, 13 s versus 20 s in rats and 1.2 g versus 2.2 g; 7.5 s versus 10 s in mice; respectively) or pentoxifylline (1.3 g versus 3 g; 7.6 s versus 15 s in mice; respectively). Antiallodynic and antihyperalgesic effect of morphine (30 μg; i.t.) given by lumbar puncture in mice was also significantly potentiated in minocycline-treated group (1.2 g versus 2.2 g; 7.5 s versus 11 s; respectively). These findings indicate that preemptive and repeated administration of glial inhibitors suppresses development of allodynia and hyperalgesia and potentiates effects of morphine in rat and mouse models of neuropathic pain.     

Par-4基因在PC12细胞缺氧后的表达及其反义寡核苷酸的抗凋亡作用

目的研究前列腺凋亡反应基因4(Par4)在PC12细胞缺氧后的表达及其反义寡核苷酸(ASODN)的抗凋亡作用。方法在正常或缺氧条件下培养PC12细胞,分组将不同浓度Par4反义寡核苷酸转染PC12细胞,吖啶橙/溴化乙锭荧光染色观察PC12细胞形态,流式细胞分析评价凋亡百分率。Westernblot测定Par4蛋白表达量,比色法检测Caspase3的酶活性。结果缺氧24h,PC12细胞中Par4蛋白表达量上调157.42±7.53,明显高于正常对照组10.51±3.36(P<0.01)。10μmol/L的Par4,ASODN使Par4蛋白降为31.79±3.09,明显低于缺氧组157.42±7.53(P<0.01)。Par4的ASODN抑制缺氧诱导的PC12细胞凋亡,10μmol/LASODN使细胞凋亡百分数降为(34.5±3.5)%,明显低于缺氧组的(69.3±5.7)%(P<0.05)。Par4的ASODN抑制缺氧诱导的PC12细胞中Caspase3的酶活性上调,10μmol/LASODN使其活性降为0.549±0.078,与缺氧组0.917±0.051相比,差别有显著性意义(P<0.05)。结论Par4基因可能参与了缺氧诱导的PC12细胞损害。Par4ASODN可拮抗缺氧诱导的PC12细胞凋亡,其机制可能与Caspase3酶活性下调有关。     

鞘内应用布托啡诺对神经痛大鼠脊髓NMDAR1、MoR mRNA表达的影响

目的：观察鞘内注射布托啡诺对神经源性疼痛大鼠脊髓NMDAR1、MOR mRNA表达的影响,进而探讨布托啡诺治疗神经源性疼痛的可能机制。方法：健康雄性SD大鼠24只,体重（250±20）g,随机分为3组（n=8）,对照组（B组）、生理盐水组（C组）和布托啡诺组（T组）。B组不给予任何干预处理;C组和T组进行鞘内置管并建立大鼠神经病理性疼痛模型（CCI）,C组鞘内每天给予10μl生理盐水,T组鞘内每天输注布托啡诺12μg/10μl（生理盐水稀释）。持续7d后,断头处死大鼠,取出脊髓标本,RT—PCR测定NMDAR1和MORmRNA的表达水平。结果：NMDAR1 mRNA表达,C组与B组相比,NMDAR1 mRNA表达水平显著升高,P〈0．01,T组与C组比较表达水平降低．P〈0．05。MOR mRNA表达,B组、C组、T组各组间相比,差异无统计学意义。结论：鞘内注射布托啡诺可降低CCI模型引起的大鼠脊髓NMDAR1 mRNA表达水平上调,对MOR mRNA表达水平无影响;CCI模型后12d内MOR mRNA的表达水平恢复萨常．     

Efficacy of antiepileptic tetramethylcyclopropyl analogues of valproic acid amides in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are widely utilized in the management of neuropathic pain. The AED valproic acid (VPA) holds out particular promise as it engages a variety of different anticonvulsant mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but potentially life-threatening side effects: teratogenicity and hepatotoxicity. We have synthesized several tetramethylcyclopropyl analogues of VPA amides that are non-teratogenic, and are likely to be non-hepatotoxic, and that exhibit good antiepileptic efficacy. In the present study we have assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation (SNL) model of neuropathic pain. TMCA (2,2,3,3-tetramethylcyclopropanecarboxylic acid, 100–250 mg/kg), TMCD (2,2,3,3-tetramethylcyclopropanecarboxamide, 40–150 mg/kg), MTMCD (N-methyl-TMCD, 20–100 mg/kg), and TMCU (2,2,3,3-tetramethylcyclopropanecarbonylurea, 40–240 mg/kg) all showed dose-related reversal of tactile allodynia, with ED₅₀ values of 181, 85, 41, and 171 mg/kg i.p., respectively. All were more potent than VPA (ED₅₀ = 269 mg/kg). An antiallodynic effect was obtained for TMCD, MTMCD and TMCU at plasma concentrations as low as 23, 6 and 22 mg/L, respectively. MTMCD was found to be non-toxic, non-sedative and equipotent to gabapentin, currently the leading AED in neuropathic pain treatment. Tetramethylcyclopropyl analogues of VPA amides have potential to become a new series of drugs for neuropathic pain treatment.     

Pentoxifylline decreases allodynia and hyperalgesia in a rat model of neuropathic pain

BACKGROUND AND THE PURPOSE OF THE STUDY: Pentoxifylline (PTX) is a non-specific cytokite pain in several animal models and humans. However, long-term therapeutic effects of PTX on neuropathic pain in a rat model of chronic constriction injury (CCI) are not completely clear. This study was conducted to examine the effect of long-term administration of PTX on neuropathic pain in rats. METHODS : Neuropathic pain was induced by sciatic nerve ligation using of CCI model in rats. Rats were randomly assigned into sham, CCI-saline treated, and CCI-PTX treated (30 or 60 mg/kg ip) groups. PTX or saline administered at 30 min before CCI and daily for 14 days post-CCI. At the days of 3, 7, 11 and 14 following CCI, by using standard methods effects of thermal hyperalgesia, thermal and mechanical allodynia in all groups were examined using the standard methods. RESULTS : The CCI-saline treated group showed a significant increase in mechanical and thermal allodynia, and thermal hyperalgesia as compared with the sham group in the tested days. Administration of the higher dose of PTX (60 mg/kg/day), but not the lower dose (30 mg/kg/day) significantly reduced mechanical and thermal allodynia, as compared with the CCI-saline treated group on days of 3, 7, 11 and 14 (all P values<0.001). Also, both doses of PTX significantly reduced thermal hyperalgesia as compared with the CCI-saline treated group on these days (all P values<0.001). CONCLUSION : Results of this study show that chronic administration of PTX reduces the neuropathic pain in a rat model of CCI. Thus, this drug may have a therapeutic application in the treatment and management of neuropathic pain in humans.     

Broad modulation of neuropathic pain states by a selective estrogen receptor beta agonist

The effects of estrogens on pain perception remain controversial. In animal models, both beneficial and detrimental effects of non-selective estrogens have been reported. ERb-131 a non-steroidal estrogen receptor beta ligand was evaluated in several pain animal models involving nerve injury or sensitization. Using functional and binding assays, ERb-131 was characterized as a potent and selective estrogen receptor beta agonist. In vivo, ERb-131 was devoid of estrogen receptor alpha activity as assessed in a rat uterotrophic assay. ERb-131 alleviated tactile hyperalgesia induced by capsaicin, and reversed tactile allodynia caused by spinal nerve ligation and various chemical insults. Moreover, ERb-131 did not influence the pain threshold of normal healthy animals. Thus, estrogen receptor beta agonism is a critical effector in attenuating a broad range of anti-nociceptive states.     

Characterization of cannabinoid-induced relief of neuropathic pain in rat models of type 1 and type 2 diabetes

Diabetic neuropathy is a frequent complication of diabetes mellitus with a tremendous impact on patients' quality of life, and it remains poorly treated. Cannabinoids relieve the signs of diabetic neuropathy in different experimental models, including streptozotocin- (STZ-) induced type 1 diabetic rodents, and they may also relieve neuropathic signs in type 2 diabetic animals. This study compares the effect of the non-selective cannabinoid agonist WIN 55,212-2 (WIN) in Zucker Diabetic Fatty (ZDF) rats (type 2 diabetes) and in STZ-injected Wistar rats (type 1 diabetes).WIN (or its vehicle) was either systemically administered at a non-psychoactive dose or locally injected. Selective CB1 and CB2 cannabinoid antagonists were used to characterize WIN antineuropathic effects.Both type 1 and type 2 diabetic rats showed mechanical allodynia but not thermal hyperalgesia. WIN alleviated mechanical allodynia in both models of diabetes. In STZ-treated rats, both cannabinoid receptors were involved, whereas in ZDF rats, WIN effects seemed to mainly involve the activation of CB1 receptors. Higher doses of WIN were needed to significantly relieve mechanical allodynia upon intraplantar administration in ZDF vs. STZ-injected rats.Cannabinoids, acting on systemic and/or peripheral receptors, may serve as a new therapeutic alternative for symptom management in painful neuropathy associated with both type 1 and type 2 diabetes. Additionally, our results highlight the need for appropriate selection of diabetic experimental models because the results from studies in STZ-induced diabetic rodents might not be applicable in all diabetic situations.     

AM404, an anandamide transport inhibitor, reduces plasma extravasation in a model of neuropathic pain in rat: role for cannabinoid receptors

Neuropathic pain consequent to peripheral nerve injury has been associated with local inflammation. Following noxious stimulation afferent fibres release substance P (SP) and calcitonin-gene related peptide (CGRP), which are closely related to oedema formation and plasma leakage. The effect of the anandamide transport blocker AM404 has been studied on plasma extravasation after chronic constriction injury (CCI) which consists in a unilateral loose ligation of the rat sciatic nerve (Bennett and Xie, 1988). AM404 (1-3-10 mg kg(-1)) reduced plasma extravasation in the legated paw, measured as mug of Evans Blue per gram of fresh tissue. A strong effect on vascular permeability was also produced by the synthetic cannabinoid agonist WIN 55,212-2 (0.1-0.3-1 mg kg(-1)). Using specific antagonists or enzyme inhibitors, we demonstrate that cannabinoids act at several levels: data on the 3rd day suggest a strong involvement of substance P (SP) and calcitonin gene-related peptide (CGRP) in the control of vascular tone, whereas at the 7th and 14th days the major role seems to be played by prostaglandins (PGs) and nitric oxide (NO). Capsaicin injection in ligated paws of AM404- or WIN 55,212-2-treated rats resulted in an increase of Evans Blue extravasation, suggesting the involvement of the cannabinergic system in the protective effect of C fibres of ligated paws. Taken together, these data demonstrate the efficacy of cannabinoids in controlling pain behaviour through the modulation of several pain mediators and markers of vascular reactivity, such as SP, CGRP, PGs and NO.     

米诺环素对神经病理性疼痛大鼠脊髓星形胶质细胞激活的影响

目的观察米诺环素对CCI大鼠脊髓水平GFAP表达,探讨脊髓星形胶质细胞在神经病理性疼痛中的不同时程作用。方法将110只SD大鼠随机分成5组:A组:对照组(n=15)、B组:假手术组(n=25)、C组:CCI对照组(n=25)、D组:CCI预给药组(n=25)、E组:CCI治疗组(n=25)。分别测定各组在手术前(以第0天表示)以及手术后第1、4、7、11、14天损伤侧后肢50%机械刺激缩足阈值,然后相应分别取每组大鼠L4-6段脊髓,采用免疫组化染色方法测定脊髓背角星形胶质细胞GFAP表达变化情况。结果①坐骨神经结扎后大鼠出现痛觉过敏且持续增强存在。②B组和D组亦可见星形胶质细胞有轻微的激活,C组脊髓背角星形胶质细胞在术后7d发生明显激活,至术后14d星形胶质细胞被强烈的激活;E组星形胶质细胞术后有较明显的激活但无高峰期。结论脊髓水平星形胶质细胞的激活可能对神经病理性疼痛的产生维持发挥重要作用;米诺环素术前给药抑制星形胶质细胞的激活,减轻神经病理性疼痛。     

HYP-1, a novel diamide compound, relieves inflammatory and neuropathic pain in rats

In the present study, we investigated whether a novel compound, 2-(2-(4-((4-chlorophenyl)(phenyl)methyl)piperazin-1-yl)-2-oxoethylamino)-N-(3,4,5-trimethoxybenzyl)acetamide (HYP-1), is capable of binding to voltage-gated sodium channels (VGSCs) and evaluated both its inhibitory effect on Na⁺ currents of the rat dorsal root ganglia (DRG) sensory neuron and its in vivo analgesic activity using rat models of inflammatory and neuropathic pain. HYP-1 showed not only high affinity for rat sodium channel (site 2), but also potent inhibitory activity against the TTX-R Na⁺ currents of the rat DRG sensory neuron. HYP-1 co-injected with formalin (5%, 50 μl) under the plantar surface of rat hind paw dose-dependently reduced spontaneous pain behaviors during both the early and late phases. This result was confirmed by c-Fos immunofluorescence in the L4-5 spinal segments. A large number of c-Fos-positive neurons were observed in rat injected with a mixture of formalin and vehicle, but not in rat treated with a mixture of formalin and HYP-1. In addition, the effectiveness of HYP-1 (6 and 60 mg/kg, i.p.) in suppression of neuropathic pain, such as mechanical, cold and warm allodynia, induced by rat tail nerve injury was investigated. HYP-1 showed limited selectivity over hERG, N-type and T-type channels. Our present results indicate that HYP-1, as a VGSC blocker, has potential analgesic activities against nociceptive, inflammatory and neuropathic pain.     

Post-injury repeated administrations of minocycline improve the antinociceptive effect of morphine in chronic constriction injury model of neuropathic pain in rat

It is confirmed that pharmacological attenuation of glial cells can alleviate neuropathic pain by lowering proinflammatory cytokine expression. The present study tries to confirm that post-injury administration of glia inhibitor, minocycline, can attenuate the neuropathic pain symptoms and improves the efficacy of morphine anti-nociception in chronic constriction injury (CCI). Male Wistar rats (230-270 g) underwent surgery for induction CCI model of neuropathy. For assessment of the thermal hyperalgesia and mechanical allodynia after CCI induction, morphine (2.5, 5, 7.5, 10 and 15 mg/kg; s.c.) and saline were administered on post-operative days (PODs) 0, 6 and 14. Hargreaves and Von-Frey tests were performed before and 30 min after morphine administration, respectively. The results showed significant decrease in antinociceptive effect of morphine on POD 6 compared to POD 0 only at the dose of 5 mg/kg. On the other hand, on POD 14 the antinociceptive effect of morphine (5, 7.5, 10 and 15 mg/kg) significantly decreased in comparison with POD 0. In another set of experiments, animals received minocycline (10, 20 and 40 mg/kg; i.p.) for eight days from POD 6 to 13 and then the antinociceptive effect of single dose of morphine 5 mg/kg was tested on POD 14. Behavioral tests showed that minocycline (40 mg/kg) could effectively attenuate the thermal hyperalgesia and mechanical allodynia on POD 13. Moreover, minocycline (40, 20 mg/kg) improved the anti-hyperalgesic, and minocycline (40 mg/kg) improved the anti-allodynic effects of morphine 5 mg/kg on POD 14. It seems that the reduction of antinociceptive effect of morphine after CCI may be mediated through glia activation. Modulation of glial activity by minocycline can attenuate CCI-induced neuropathic pain. It is also shown that repeated post-injury administration of minocycline improves the antinociceptive effect of morphine in neuropathic pain.     

Increased hyperalgesia by 5-nitro-2, 3-(phenylpropylamino)-benzoic acid (NPPB), a chloride channel blocker in crush injury-induced neuropathic pain in rats

Chloride channels belong to diverse group of anion selective channels involved in different signaling processes. The present study was planned to investigate the involvement of chloride channels in crush injury-induced neuropathic pain in rats by using ivermectin, a ligand gated chloride channel opener and NPPB, a CaCC blocker. The effect of ivermectin (5, 10, 20 mg/kg i.p. or 50, 100 and 200 μg/rat by i.c.v. route) and NPPB (10, 20 and 40 mg/kg i.p.) was investigated on pain behavioural thresholds in crush injury-induced neuropathic pain rat model. Reduction in pain threshold by mechanical, thermal and cold stimuli confirmed the development of neuropathic pain in rats after crush injury. Ivermectin administered either by i.p. or i.c.v. route did not alter the pain threshold in mechanical, thermal and, cold allodynia tests in rats. NPPB (20 and 40 mg/kg i.p.) significantly reduced the pain threshold crush injury neuropathic pain model suggesting its hyperalgesic effect. The results showed that NPPB increased significantly the mechanical and thermal hyperalgesia in crush injury-induced neuropathic pain rat model, whereas ivermectin, either by i.p. or i.c.v. route of administration, has no effect on pain symptoms in this model. NPPB hyperalgesic effect is independent of CaCCs inhibition and may be due to blockade of Ca²⁺-activated K⁺ channel.