Anti-nociceptive and anti-inflammatory actions of sulforaphane in chronic constriction injury-induced neuropathic pain mice

Neuropathic pain is still considered as incurable disease as current therapies are not ideal in terms of efficacy and tolerability. It is imperative to search for novel drugs to obtain better treatments. Sulforaphane (SFN), a derivative of glucoraphanin present in cruciferous vegetables, exhibits therapeutic effects on inflammation-related diseases. Since inflammation plays an important role in regulating chronic pain, in the present study, we investigated anti-nociceptive effects of SFN and its underlying mechanisms in a neuropathic pain mouse model, sciatic nerve chronic constriction injury (CCI). SFN (0.1–100 mg/kg) was injected intraperitoneally for 7 days when pain behaviors, including mechanical allodynia and thermal hyperalgesia, reached to the maximum in CCI mice. We observed that SFN dose-dependently attenuated CCI-induced pain behavioral hypersensitivity, accompanied by reduction in pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and upregulation of an anti-inflammatory cytokine (IL-10). Moreover, SFN counteracted CCI enhancement of COX2 and iNOS in injured nerves, two key enzymes implicated in inflammation and neuropathic pain. Furthermore, pretreatment of naloxone, an antagonist of opioid receptors, significantly blocked SFN attenuation of behavioral hypersensitivity without affecting SFN modulation of inflammatory cytokines in CCI mice. Interestingly, CCI-induced increase in µ-opioid receptors in injured sciatic nerves was further increased by SFN treatment. Taken together, SFN has both anti-nociceptive and anti-inflammatory actions.     

Behavioral and pharmacological characterization of a distal peripheral nerve injury in the rat

Previous rat neuropathic pain models have utilized peripheral nerve injuries that damage a significant proportion of large nerves such as the sciatic nerve or its divisions. Injuries that lead to neuropathic pain in humans may involve the peripheral extremities. The current study evaluated the behavioral effects of injury to the plantar nerves in the rat (distal nerve injury-DNI). A delayed onset of hypersensitivity to an innocuous mechanical stimulus was observed following cutting of the left plantar nerves, whereas mechanical hypersensitivity developed more rapidly in rats with either an injury near the sciatic nerve trunk (chronic constriction injury (CCI), spared nerve injury (SNI)) or a spinal nerve root (spinal nerve ligation (SNL). Similar to other nerve injury pain models, rats with injured plantar nerves also developed an early onset and persistent sensitivity to a cooling stimulus. The effects of morphine, gabapentin and imipramine on mechanical and cold hypersensitivity were evaluated in rats with a DNI, CCI and SNI. In all three models, morphine dose-dependently suppressed mechanical and cold hypersensitivity, whereas gabapentin only suppressed mechanical hypersensitivity. Imipramine had no effect on either cold or mechanical hypersensitivity in any of the nerve-injured rats. The pharmacological data suggest that the underlying basis of neuropathic pain may be similar irrespective of the site of nerve injury.     

HO-1/CO 信号通路对切口痛大鼠炎症因子的调控作用

目的 观察脊髓血红素氧合酶-1（HO-1）/CO 信号通路对切口痛大鼠炎症因子的调节作用及可能机制。方法 切口痛模型大鼠 36 只在实验即刻（0 h）, 术后 1、4、8、12 及 24 h 各处死 6 只, 取患侧脊髓腰膨大处, Western blot 法检测 HO-1 蛋白表达, 酶联免疫吸附试验（ELISA）法检测炎症因子肿瘤坏死因子-α（TNF-α）、白细胞介素（IL）-1β、IL-6 和高迁移率族蛋白 1（HMGB1）。 另外, 对照（C）组 36 只大鼠不做切口痛模型;切口痛模型大鼠 144 只按处理方式不同分为切口痛（IP）组, 切口痛+HO-1 诱导剂（IP+hemin）组, 术前给予腹腔注射大鼠 100 mg/kg 血红素（hemin）; 切口痛+HO-1 抑制剂（IP+Znpp-IX）组, 术前给予腹腔注射大鼠 45 μmoL/kg 锌原卟啉（Znpp）-IX; 切口痛+ CO 释放剂（IP+CORM-2）组, 于术前经腹腔注射给予 10 mg/kg 一氧化碳释放分子（CORM）-2。 各组于实验即刻（0 h）, 术后 1、4、8、12 及 24 h 行机械缩足阈值（PWMT）和热缩足潜伏期（PWTL）的检测, 应用 ELISA 法检测各组 TNF- α、IL-1β、IL-6 和 HMGB1 的表达情况。 结果 与 0 h 比较, 术后 1、4、8、12 及 24 h HO-1 蛋白表达水平和 TNF-α、 IL-1β、IL-6 和 HMGB1 均增高（P＜ 0.05）。 与 C 组比较, 其余组大鼠各时间点 PWMT 值和 PWTL 值减少, TNF-α、 IL-1β、IL-6 和 HMGB1 表达增加（P＜ 0.05）; 与 IP 组比较, IP+hemin 组和 IP+CORM-2 组大鼠 1、4、8、12 及 24 h 时 PWMT 值和 PWTL 值均增高, 而 TNF-α、IL-1β、IL-6 和 HMGB1 表达减少, IP+Znpp-IX 组 PWMT 值和 PWTL 值降低, 但 TNF-α、IL-1β、IL-6 和 HMGB1 表达增加（P＜ 0.05）。 结论 切口痛可诱发大鼠 HO-1 表达增加, 而 HO-1/CO 信号通路在调控切口痛大鼠炎症因子的释放方面发挥重要作用。     

CO from enhanced HO activity or from CORM-2 inhibits both O2- and NO production and downregulates HO-1 expression in LPS-stimulated macrophages

Carbon monoxide (CO) arising from heme degradation, catalyzed particularly by the stress-inducible heme oxygenase-1 (HO-1), has recently been demonstrated to provide cytoprotection against cell death in macrophages stimulated with bacterial lipopolysaccharide (LPS). In the present study, we determined the effects of CO on the production of reactive oxygen species (ROS) and nitric oxide (NO) by the LPS-stimulated RAW 264.7 macrophages. In addition, effect of CO-exposure on the production of superoxide (O(2)(-)) in the phorbol myristate acetate (PMA)-stimulated PLB-985 neutrophils was determined. Production of ROS by the LPS-stimulated macrophages pretreated with 50microM [Ru(CO)(3)Cl(2)](2), a CO-releasing molecule (CORM-2), was abolished and the production of O(2)(-) by the PMA-stimulated neutrophils pretreated with the CORM-2 was decreased markedly. The CORM-2 (50microM) was not cytotoxic to both the unstimulated and LPS-stimulated macrophages when determined by employing mitochondrial reductase function test (MTT assay). In macrophages pretreated with increasing doses of CORM-2, both the LPS-derived upregulations of iNOS (NO production) and HO-1 expression (CO production) were suppressed in a dose-dependent manner. Alternatively, when the macrophages were treated with LPS and CO-donor together, the LPS-derived increase in NO production was decreased. Conversely, when the control and LPS-stimulated macrophages were treated with zinc protoporphyrin IX (ZnPP) to inhibit the HO activity blocking endogenous production of CO (basal and enhanced), macrophages died extensively. Interestingly, production of NO in the LPS-stimulated macrophages increased significantly following the ZnPP treatment. Addition of CORM-2 to the LPS-treated cells that were being treated additionally with ZnPP did not prevent the cell death. However, endogenous overproduction of CO by super-induction of HO-1 (obtained by pretreatment of macrophages with either buthionine sulfoximine or hemin) decreased the LPS-derived iNOS expression without affecting cell survival. Combined, these results indicated that enhanced HO activity is essential for the survival of LPS-stimulated macrophages. Thus, upregulation of HO-1 and overproduction of CO may allow the survival of LPS-stimulated macrophages; first, by eliminating the free heme to prevent Fenton reaction, second, by limiting the availability of free heme required for induction of NO-producing heme enzyme (i.e., iNOS), third, by limiting additional production of O(2)(-) and NO via CO-derived inhibition on the activities of heme enzymes like NADPH oxidase and iNOS, respectively. CO may allow the LPS-activated macrophages to return back to the normal quiet state insensitive to additional stimuli causing oxidative stress.     

Reprint of “Induction of heme oxygenase-1 ameliorates vascular dysfunction in streptozotocin-induced type 2 diabetic rats”

AimsTo explore the effects of heme oxygenase-1 (HO-1) on vascular dysfunction in high fat diet streptozotocin-induced type 2 diabetic (T2D) rats.MethodsRats received a high-fat diet followed by a low dose of streptozotocin (30 mg/kg) to induce T2D. T2D rats were treated with hemin (1, 5, or 25 mg/kg) or carbon monoxide-releasing molecule-2 (CORM-2, 5 mg/kg) for 4 weeks. Isometric contractions of aortic rings were measured. The expression of cyclooxygenase-2 (COX-2) and activities of HO, SOD, and MDA were evaluated.ResultsThe fasting blood glucose, blood insulin levels, and IR index in T2D rats were higher than those in the control group, which were ameliorated by HO-1 inducer hemin. The antidiabetic effect was accompanied by enhanced HO activity. The vascular relaxation response to ACh was decreased in T2D rats, while treatment with hemin could prevent such decrease in vasorelaxation. An increase in COX-2 expression was found in the aortas of T2D rats. Treatment of T2D rats with COX-2 inhibitor NS398 restored ACh-induced vasodilation. COX-2 overexpression in T2D rats was inhibited by hemin. Hemin treatment also inhibited the decline of SOD activity and the increase of MDA content in the aorta of T2D rats. CORM-2, an agent which releases the HO-1 product CO, could mimic the beneficial effect of hemin.ConclusionInduction of HO-1 with hemin ameliorates the abnormality of endothelium-dependent vascular relaxation in T2D rats. A possible mechanism involves suppression of reactive oxygen species production and inhibition of COX-2 up-regulation induced by diabetes mellitus.     

The RS504393 Influences the Level of Nociceptive Factors and Enhances Opioid Analgesic Potency in Neuropathic Rats

Increasing evidence has indicated that activated glial cells releasing nociceptive factors, such as interleukins and chemokines, are of key importance for neuropathic pain. Significant changes in the production of nociceptive factors are associated with the low effectiveness of opioids in neuropathic pain. Recently, it has been suggested that CCL2/CCR2 signaling is important for nociception. Here, we studied the time course changes in the mRNA/protein level of CD40/Iba-1, CCL2 and CCR2 in the spinal cord/dorsal root ganglia (DRG) in rats following chronic constriction injury (CCI) of the sciatic nerve. Moreover, we examined the influence of intrathecal preemptive and repeated (daily for 7 days) administration of RS504393, CCR2 antagonist, on pain-related behavior and the associated biochemical changes of some nociceptive factors as well as its influence on opioid effectiveness. We observed simultaneous upregulation of Iba-1, CCL2, CCR2 in the spinal cord on 7th day after CCI. Additionally, we demonstrated that repeated administration of RS504393 not only attenuated tactile/thermal hypersensitivity but also enhanced the analgesic properties of morphine and buprenorphine under neuropathy. Our results proof that repeated administration of RS504393 reduced the mRNA and/or protein levels of pronociceptive factors, such as IL-1beta, IL-18, IL-6 and inducible nitric oxide synthase (iNOS), and some of their receptors in the spinal cord and/or DRG. Furthermore, RS504393 elevated the spinal protein level of antinociceptive IL-1alpha and IL-18 binding protein. Our data provide new evidence that CCR2 is a promising target for diminishing neuropathic pain and enhancing the opioid analgesic effects.     

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。     

Induction of heme oxygenase-1 with hemin attenuates hippocampal injury in rats after acute carbon monoxide poisoning

Carbon monoxide (CO) poisoning is a major cause of brain injury and mortality; delayed neurological syndrome (DNS) is encountered in survivors of acute CO exposure. The toxic effects of CO have been attributed to oxidative stress induced by hypoxia. Heme oxygenase-1 (HO-1) is the inducible heme oxygenase isoform, and its induction acts as an important cellular defense mechanism against oxidative stress, cellular injury and disease. In this study, we examined the functional roles of HO-1 induction in a rat model of CO-exposured hippocampal injury. We report that acute CO exposure produces severe hippocampal injury in rats. However, hemin pretreatment reduced both the CO-induced rise in hippocampal water content and levels of neuronal damage in the hippocampus; survival rates at 24 h were significantly improved. Upregulation of HO-1 by hemin pretreatment resulted in a significant decrease in hippocampal levels of malondialdehyde (MDA), a marker of oxidative stress; levels of pro-apoptotic caspase-3 were also reduced. In contrast, inhibition of HO activity by administration of tin protoporphyrin IX (SnPP, a specific inhibitor of HO) abolished the neuroprotective effects of HO-1 induction. These data suggested that the upregulation of endogenous HO-1 expression therefore plays a pivotal protective role in CO neurotoxicity. Though the precise mechanisms underlying hemin-mediated HO-1 induction and neuroprotection are not known, these may involve the anti-oxidant and anti-apoptotic effects of HO-1 enzyme activity.     

Carbon monoxide-releasing molecules (CO-RMs) attenuate the inflammatory response elicited by lipopolysaccharide in RAW264.7 murine macrophages

The enzyme heme oxygenase-1 (HO-1) is a cytoprotective and anti-inflammatory protein that degrades heme to produce biliverdin/bilirubin, ferrous iron and carbon monoxide (CO). The anti-inflammatory properties of HO-1 are related to inhibition of adhesion molecule expression and reduction of oxidative stress, while exogenous CO gas treatment decreases the production of inflammatory mediators such as cytokines and nitric oxide (NO). CO-releasing molecules (CO-RMs) are a novel group of substances identified by our group that are capable of modulating physiological functions via the liberation of CO. We aimed in this study to examine the potential anti-inflammatory characteristics of CORM-2 and CORM-3 in an in vitro model of lipopolysaccharide (LPS)-stimulated murine macrophages. Stimulation of RAW264.7 macrophages with LPS resulted in increased expression of inducible NO synthase (iNOS) and production of nitrite. CORM-2 or CORM-3 (10-100 microM) reduced nitrite generation in a concentration-dependent manner but did not affect the protein levels of iNOS. CORM-3 also decreased nitrite levels when added 3 or 6 h after LPS exposure. CORM-2 or CORM-3 did not cause any evident cytotoxicity and produced an increase in HO-1 expression and heme oxygenase activity; this effect was completely prevented by the thiol donor N-acetylcysteine. CORM-3 also considerably reduced the levels of tumor necrosis factor-alpha, another mediator of the inflammatory response. The inhibitory effects of CORM-2 and CORM-3 were not observed when the inactive compounds, which do not release CO, were coincubated with LPS. These results indicate that CO liberated by CORM-2 and CORM-3 significantly suppresses the inflammatory response elicited by LPS in cultured macrophages and suggest that CO carriers can be used as an effective strategy to modulate inflammation.     

Treatment with a carbon monoxide-releasing molecule inhibits chronic inflammatory pain in mice: nitric oxide contribution

Rationale

Carbon monoxide synthetized by inducible heme oxygenase (HO-1) exerts potent anti-inflammatory and antinociceptive effects during acute and neuropathic pain, but its role in the modulation of chronic inflammatory pain and the possible involvement of nitric oxide in this action remain unknown.

Objectives and methods

The antiallodynic and antihyperalgesic effects of a carbon monoxide releasing molecule, tricarbonyldichlororuthenium(II) dimer (CORM-2), daily administered from days 4 to 14 after complete Freund's adjuvant (CFA) injection in wild-type (WT), neuronal (NOS1-KO), and inducible (NOS2-KO) nitric oxide synthases knockout mice, were evaluated using von Frey filaments and plantar tests. Effects of CORM-2 treatment on the expression of HO-1, NOS1, and NOS2 at 14 days after inflammation induction were assessed by Western blot.

Results

Main inflammatory pain symptoms induced by CFA in WT, NOS1-KO, and NOS2-KO mice were significantly reduced in a time-dependent manner by CORM-2 treatment. In all genotypes, inflammation increased the dorsal root ganglia and paw expression of HO-1, but CORM-2 treatment only over-expressed this enzyme in the paw of all genotypes. The increased NOS1 expression induced by inflammation in WT mice was abolished by CORM-2 treatment, while there was no effect of the inflammation in neither CORM-2 treatment in the expression of NOS2 in WT and NOS1-KO mice.

Conclusions

CORM-2 treatment inhibits inflammatory pain through enhancing HO-1 paw expression in all genotypes and reducing NOS1 over-expression in WT mice. An interaction between HO-1/carbon monoxide and NOS1/nitric oxide systems was also demonstrated. CORM-2 treatment may represent a new approach for management chronic inflammatory pain.     

Role of reactive oxygen species and spinal cord apoptotic genes in the development of neuropathic pain.

A mouse model of neuropathic pain consisting of chronic constriction injury (CCI) of the sciatic nerve was used to examine the involvement of reactive oxygen species (ROS) in early spinal cord pro-apoptotic gene over-expression during the development of neuropathic pain. RT-PCR analysis showed increased expression of bax, apoptotic protease-activating factor-1 (apaf-1), and caspase-9 in the dorsal horn spinal cord 3 days after chronic constriction injury of sciatic nerve. Consistent with biomolecular data, a marked increase in TUNEL-positive and caspase-3 active form was observed by 3 days CCI. Administration of phenyl-N-tert-butylnitrone (PBN), a potent ROS scavenger, reduced the development of thermal hyperalgesia and mechanical allodynia at 1 and 3 days post-CCI, and decreased the mRNA levels of bax, apaf-1, and caspase-9. PBN also reduced apoptotic and active Caspase-3 positive profiles in the superficial laminae (I-III) of the spinal cord. This study provides evidence that PBN inhibits over-expression of pro-apoptotic genes and neural apoptosis in the spinal cord dorsal horn induced by early-CCI of the sciatic nerve. These findings suggest that ROS regulate expression of some apoptotic genes which might play a role in the onset of neuropathic pain.     

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.     

Functional role of exogenous administration of substance P in chronic constriction injury model of neuropathic pain in gerbils

Substance P (SP) acts as a transmitter of nociception in both the peripheral and the central nervous system. Because the NK-1 receptors in gerbils are comparable to those in humans, gerbil models could be used to study the role of SP in neuropathic pain. A modification of the rat chronic constriction injury (CCI) model of neuropathic pain was produced in male gerbils by placing four loose chromic catgut ligatures around the sciatic nerve. This procedure clearly resulted in mechanical hypersensitivity. Intraplantar injections of SP and the selective NK-1 receptor agonist, [Sar(9)-Met(O(2))(11)]-substance P (Sar-SP), to the paw ipsilateral to the nerve injury and intrathecal administration of these peptides produced paw-lifting behavior in the CCI gerbils in thermoneutral conditions. In sham-operated and nonoperated controls, no such effects were observed. Systemic administration of the NK-1 antagonist R116301 attenuated the SP and the Sar-SP-induced paw-lifting behavior in the CCI gerbils indicating the role of NK-1 receptors in these effects. Intraplantar injection of the highest dose of SP (200 ng) to the paw contralateral to the CCI produced lifting of the paw ipsilateral to the injury, indicative for spinal mechanisms especially since administration of SP to the ipsilateral front paw or even intracardially did not have any effect at all. The SP-induced responses were not antagonized by the NMDA antagonist MK801. These results indicate that the peripheral and spinal SP reveal an increased reactivity in a neuropathic pain model. This increased pain sensitivity seems to involve spinal NK-1 mechanisms.     

血红素氧化酶功能和调节及其在肝病发生中的研究进展

血红素氧化酶是血红素降解过程中的一种限速酶,有3种亚型:HO-1、HO-2和HO-3,其中HO-1的研究较多。而HO-1所降解的产物是CO、铁和胆红素,它们具有重要的抗过氧化特性而保护细胞的功能;同时,在细胞内调节HO-1基因表达的信号通路以及HO-1基因启动因子均可诱导HO-1基因的抗过氧化应激能力,这就为因过氧化应激或非应激性所致的HO-1相关肝病的发病和治疗提供了新的方法。     

Tramadol has a better potency ratio relative to morphine in neuropathic than in nociceptive pain models

BACKGROUND AND OBJECTIVE: Treatment of neuropathic pain remains a challenge and the role of various analgesics in this setting is still debated. The effects of tramadol, an atypically acting analgesic with a combined opioid and monoaminergic mechanism of action, and morphine, a prototypical opioid, were tested in rat models of neuropathic and nociceptive pain. METHODS: Cold allodynia and mechanical hypersensitivity, symptoms of neuropathic pain, were studied in rat models of mononeuropathic pain. Cold allodynia was analyzed in the chronic constriction injury (CCI) model and mechanical hypersensitivity was analyzed in the spinal nerve ligation (SNL) model. Heat-induced rat tail-flick latencies were determined as measure for nociceptive pain. RESULTS: Cold allodynia and mechanical hypersensitivity were strongly attenuated with similar absolute potency after intravenous administration of tramadol and morphine. The doses of drug that were calculated to result in 50% pain inhibition (ED(50)) for tramadol and morphine were 2.1 and 0.9 mg/kg, respectively, in CCI rats and 4.3 and 3.7 mg/kg, respectively, in SNL rats. In the tail-flick assay of acute nociception, the potency of the two drugs differed markedly, as seen by ED(50) values of 5.5 and 0.7 mg/kg intravenously for tramadol and morphine, respectively. Accordingly, the analgesic potency ratio (ED(50) tramadol/ED(50) morphine) of both compounds differed in neuropathic (potency ratio 2.3 in CCI and 1.2 in SNL) and nociceptive pain models (potency ratio 7.8), suggesting a relative increase in potency of tramadol in neuropathic pain compared with nociceptive pain. CONCLUSION: The results of this study are consistent with clinical data supporting the efficacy of opioids in neuropathic pain conditions, and furthermore suggest an additional contribution of the monoaminergic mechanism of tramadol in the treatment of neuropathic pain states.     

Carbon monoxide releasing molecule-2 suppresses stretch-activated atrial natriuretic peptide secretion by activating large-conductance calcium-activated potassium channels

Carbon monoxide (CO) is a known gaseous bioactive substance found across a wide array of body systems. The administration of low concentrations of CO has been found to exert an anti-inflammatory, anti-apoptotic, anti-hypertensive, and vaso-dilatory effect. To date, however, it has remained unknown whether CO influences atrial natriuretic peptide (ANP) secretion. This study explores the effect of CO on ANP secretion and its associated signaling pathway using isolated beating rat atria. Atrial perfusate was collected for 10 min for use as a control, after which high atrial stretch was induced by increasing the height of the outflow catheter. Carbon monoxide releasing molecule-2 (CORM-2; 10, 50, 100 µM) and hemin (HO-1 inducer; 0.1, 1, 50 µM), but not CORM-3 (10, 50, 100 µM), decreased high stretch-induced ANP secretion. However, zinc porphyrin (HO-1 inhibitor) did not affect ANP secretion. The order of potency for the suppression of ANP secretion was found to be hemin > CORM-2 >> CORM-3. The suppression of ANP secretion by CORM-2 was attenuated by pretreatment with 5-hydroxydecanoic acid, paxilline, and 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one, but not by diltiazem, wortmannin, LY-294002, or NG-nitro-L-arginine methyl ester. Hypoxic conditions attenuated the suppressive effect of CORM-2 on ANP secretion. In sum, these results suggest that CORM-2 suppresses ANP secretion via mitochondrial K_ATP channels and large conductance Ca²⁺-activated K⁺ channels.     

Effects of induction/inhibition of endogenous heme oxygenase-1 on lipid metabolism, endothelial function, and atherosclerosis in rabbits on a high fat diet

The heme oxygenase-1 (HO-1) / carbon monoxide (CO) system has been presumed as a therapeutic target for preventing atherosclerosis. However, the exact mechanism(s) underlying this system remains largely undefined. This study aims to examine the influence of induction/inhibition of HO-1 on atherosclerotic plaque using pharmacological approaches and to elucidate potential mechanisms. Rabbits were randomly assigned to receive a standard diet (control group), high fat diet (HFD), HFD plus HO inducer hemin (HFD + H group), and HFD plus an HO inhibitor, zinc protoporphyrin-9 (ZnPP9, HFD + Z group). Atherosclerotic plaque was evaluated using oil red O staining and histological analyses. Immunohistochemistry, western blotting, and RT-PCR were employed to study the expression of HO-1 and endothelin-1 (ET-1). Levels of CO, nitric oxide (NO), eNOS/iNOS activities, NF-κB activity, and TNF-α level were determined. No significant differences of serum lipid levels were observed among the HFD, HFD + Z, and HFD + H groups. In rabbits, HFD induced typical atherosclerotic plaque and increased intima/media thickness ratio, which was markedly reduced in the HFD + H group and further aggravated in the HFD + Z group. Furthermore, hemin increased HO-1 expression, CO levels, and eNOS activity, while decreasing iNOS levels, ET-1 expression, NF-κB activity, and TNF-α level. ZnPP9 caused opposite effects. Induction of the endogenous HO-1/CO system by hemin can prevent atherosclerosis though increasing CO levels, regulating eNOS activity, NF-κB activity, TNF-α levels, and ET-1 levels in rabbits. Our results add new evidence for the importance of HO-1 in the genesis and development of atherosclerosis and provide several possible mechanisms underlying the anti-atherosclerosis effects of HO-1.     

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.     

Antinociceptive effects following intrathecal pretreatment with selective metabotropic glutamate receptor compounds in a rat model of neuropathic pain

In the present study, we examined the effects of intrathecal pretreatment (twice daily injections on postoperative (PO) days 0-3 with the selective Group I (mGluR1a) mGluR antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid ((RS)-AIDA), the selective Group I (mGluR5a) antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), the selective Group II mGluR agonist, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R,4R)-APDC) or the selective Group III mGluR agonist, L-2-amino-4-phosphonobutyrate (L-AP4), on mechanical and cold hypersensitivity associated with chronic constriction injury (CCI) of the sciatic nerve in rats. Mechanical and cold sensitivity was assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. Pretreatment with all of the mGluR agents produced reductions in the development of mechanical hypersensitivity. In addition, all the mGluR agents, except MPEP, were effective in reducing the development of cold hypersensitivity. This study demonstrates that spinal Group I mGluR antagonism, and Group II or III mGluR agonism, can effectively decrease the development of mechanical and cold hypersensitivity associated with CCI in rats. In addition, the results can be interpreted to suggest that activation of spinal Group I mGluRs contributes to spinal plasticity leading to the development of neuropathic pain, and that this effect is offset by activation of groups II and III mGluRs.