Effect of Methylprednisolone on Neuropathic Pain and Spinal Glial Activation in Rats

Background: Basic data are lacking regarding the efficacy and mechanisms of action of corticosteroids in neuropathic pain. Because recent studies indicate that spinal glial activation mediates the pathologic pain states, the authors sought to determine the effects of systemic and intrathecal methylprednisolone on the development and maintenance of neuropathic pain and spinal glial activation in a rat model.

Methods: Rats were anesthetized, and L5 and L6 spinal nerves were tightly ligated. Then, continuous infusion of systemic (4 mg [middle dot] kg-1 [middle dot] day-1) or intrathecal (80 [mu]g [middle dot] kg-1 [middle dot] day-1) methylprednisolone or saline was started. Mechanical allodynia and thermal hyperalgesia were evaluated on days 4 and 7 postoperatively with von Frey and Hargreaves tests, respectively. Spinal astrocytic activation was evaluated with glial fibrillary acidic protein immunoreactivity on day 7. In other groups of rats, continuous 3-day treatment with intrathecal methylprednisolone or saline was started 7 days after spinal nerve ligation, when neuropathic pain had already developed. Behavioral tests and immunostaining were performed up to 3 weeks after the treatment.

Results: Spinal nerve ligation induced mechanical allodynia and thermal hyperalgesia on days 4 and 7 postoperatively. Glial fibrillary acidic protein immunoreactivity was remarkably enhanced on day 7. Both systemic and intrathecal methylprednisolone inhibited the development of neuropathic pain states and glial activation. Three-day treatment with intrathecal methylprednisolone reversed existing neuropathic pain state and glial activation up to 3 weeks after the treatment.     

Role for cyclooxygenase 2 in the development and maintenance of neuropathic pain and spinal glial activation

BACKGROUND: Lines of evidence have indicated that cyclooxygenase 2 plays a role in the pathophysiology of neuropathic pain. However, the site and mechanism of its action are still unclear. Spinal glia has also been reported to mediate pathologic pain states. The authors evaluated the effect of continuous intrathecal or systemic cyclooxygenase-2 inhibitor on the development and maintenance of neuropathic pain and glial activation in a spinal nerve ligation model of rats. METHODS: Continuous intrathecal infusion of meloxicam (32 or 320 mug . kg . day) or saline was started immediately after L5-L6 spinal nerve ligation. Mechanical allodynia and thermal hyperalgesia were evaluated on days 4 and 7 postoperatively. Spinal astrocytic activation was evaluated with glial fibrially acidic protein immunoreactivity on day 7. In other groups of rats, continuous intrathecal meloxicam was started 7 days after spinal nerve ligation, and effects on established neuropathic pain and glial activation were evaluated. Last, effects of continuous systemic meloxicam (16 mg . kg . day) on existing neuropathic pain and glial activation were examined. RESULTS: Intrathecal meloxicam prevented the development of mechanical allodynia and thermal hyperalgesia induced by spinal nerve ligation. It also inhibited spinal glial activation responses. In contrast, when started 7 days after the nerve ligation, intrathecal meloxicam did not reverse established neuropathic pain and glial activation. Systemic meloxicam started 7 days after ligation partially reversed neuropathic behaviors but not glial activation. CONCLUSIONS: Spinal cyclooxygenase 2 mediates the development but not the maintenance of neuropathic pain and glial activation in rats. Peripheral cyclooxygenase 2 plays a part in the maintenance of neuropathic pain.     

Role for Cyclooxygenase 2 in the Development and Maintenance of Neuropathic Pain and Spinal Glial Activation

Background: Lines of evidence have indicated that cyclooxygenase 2 plays a role in the pathophysiology of neuropathic pain. However, the site and mechanism of its action are still unclear. Spinal glia has also been reported to mediate pathologic pain states. The authors evaluated the effect of continuous intrathecal or systemic cyclooxygenase-2 inhibitor on the development and maintenance of neuropathic pain and glial activation in a spinal nerve ligation model of rats.

Methods: Continuous intrathecal infusion of meloxicam (32 or 320 [mu]g [middle dot] kg-1 [middle dot] day-1) or saline was started immediately after L5-L6 spinal nerve ligation. Mechanical allodynia and thermal hyperalgesia were evaluated on days 4 and 7 postoperatively. Spinal astrocytic activation was evaluated with glial fibrially acidic protein immunoreactivity on day 7. In other groups of rats, continuous intrathecal meloxicam was started 7 days after spinal nerve ligation, and effects on established neuropathic pain and glial activation were evaluated. Last, effects of continuous systemic meloxicam (16 mg [middle dot] kg-1 [middle dot] day-1) on existing neuropathic pain and glial activation were examined.

Results: Intrathecal meloxicam prevented the development of mechanical allodynia and thermal hyperalgesia induced by spinal nerve ligation. It also inhibited spinal glial activation responses. In contrast, when started 7 days after the nerve ligation, intrathecal meloxicam did not reverse established neuropathic pain and glial activation. Systemic meloxicam started 7 days after ligation partially reversed neuropathic behaviors but not glial activation.     

Chronic intrathecal infusion of minocycline prevents the development of spinal-nerve ligation-induced pain in rats

BACKGROUND AND OBJECTIVES: Minocycline is a second-generation tetracycline with multiple biological effects, including inhibition of microglial activation. Recently, microglial activation has been implicated in the development of nerve injury-induced neuropathic pain. In this study, the authors examined the effects of continuous intrathecal minocycline on the development of neuropathic pain and microglial activation induced by L5/6 spinal-nerve ligation in rats. METHODS: Under isoflurane anesthesia, male Sprague-Dawley rats (200-250 g) received right L5/6 spinal-nerve ligation and intrathecal catheters connected to an infusion pump. Intrathecal saline or minocycline (2 and 6 microg/h) was given continuously after surgery for 7 days (n = 8 per group). The rat right hind paw withdrawal threshold to von Frey filament stimuli and withdrawal latency to radiant heat were determined before surgery and on days 1 to 7 after surgery. Spinal microglial activation was evaluated with OX-42 immunoreactivity on day 7 after surgery. RESULTS: Spinal-nerve ligation induced mechanical allodynia and thermal hyperalgesia on the affected hind paw of saline-treated rats. Intrathecal minocycline (2 and 6 microg/h) prevented the development of mechanical allodynia and thermal hyperalgesia induced by nerve ligation. It also inhibited nerve ligation-induced microglial activation, as evidenced by decreased OX-42 staining. No obvious histopathologic change was noted after intrathecal minocycline (6 microg/h) infusion. CONCLUSIONS: In this study, the authors demonstrate the preventive effect of continuous intrathecal minocycline on the development of nociceptive behaviors induced by L5/6 spinal-nerve ligation in rats. Further studies are required to examine if continuous intrathecal minocycline could be used safely in the clinical setting.     

The antiallodynic and antihyperalgesic effects of neurotropin in mice with spinal nerve ligation

Although Neurotropin(R) (NTP) has been used clinically as an analgesic in Japan for many years, its effect on neuropathic pain in animal models has not been examined in detail. Its main effect has been indicated to be activation of the descending monoaminergic pain inhibitory systems. To study the effect of NTP on neuropathic pain, we subjected mice to spinal nerve ligation. NTP inhibited both tactile allodynia and mechanical and thermal hyperalgesia in a dose-dependent manner. When the effect of NTP was examined after depletion of monoamines in the spinal cord by intrathecal neurotoxins, the antiallodynic and antihyperalgesic effects were still observed after serotonergic denervation, but not after noradrenergic denervation. In addition, intracerebroventricular NTP increased withdrawal threshold and latency although intrathecal or local administration of NTP did not. These results suggest that the antiallodynic and antihyperalgesic effect of NTP on neuropathic pain induced by spinal nerve ligation is mediated principally through the action at supraspinal sites and through activation of spinal noradrenergic systems, possibly via the descending inhibitory pathway.     

Antiallodynic effects of systemic and intrathecal morphine in the spared nerve injury model of neuropathic pain in rats

BACKGROUND: The efficacy of opioids for neuropathic pain remains controversial. The effects of morphine on pain behavior were investigated in two animal models of neuropathic pain: the spared nerve injury (SNI) model and the spinal nerve ligation (SNL) model. METHODS: Nerve injuries were created in rats either by tight ligation and section of the left tibial and common peroneal nerves (SNI) or by unilateral ligation of L5 and L6 spinal nerves (SNL). Paw withdrawal threshold to mechanical stimuli was measured using the up-down method in the hairy and glabrous skin territories of the sural nerve for SNI rats or in the mid-plantar paw of SNL rats. RESULTS: Before SNI, the median paw withdrawal thresholds in hairy and glabrous skin were similar (26 g [25%, 75% quartiles: 26, 26 g]). The paw withdrawal threshold decreased after SNI in both hairy and glabrous skin (P < 0.001). Thirty days after the SNI, the threshold in hairy skin (0.3 g) was significantly lower than in glabrous skin (1.9 g; P < 0.001). In blinded experiments, both subcutaneous and intrathecal morphine (0.1-10 microg) dose-dependently attenuated mechanical allodynia induced by SNI measured in the hairy skin, an effect that was naloxone reversible. The ED50 for the intrathecal morphine was 0.52 microg (95% confidence interval, 0.31-0.90 microg). Morphine (1 microg intrathecal) attenuated SNI-induced mechanical allodynia in glabrous skin with potency similar to that in hairy skin. In SNL rats, morphine (30 microg intrathecal) almost completely reversed the SNL-induced mechanical allodynia. CONCLUSIONS: (1) SNI-induced mechanical allodynia is characterized by a lower paw withdrawal threshold in hairy versus glabrous skin; (2) systemic and intrathecal morphine reverse SNI-induced mechanical allodynia in a dose-dependent fashion; and (3) intrathecal morphine also reverses SNL-induced mechanical allodynia. These results suggest that intrathecal opioids are likely to be effective in the treatment of neuropathic pain.     

Spinal GABA(A) and GABA(B) receptor pharmacology in a rat model of neuropathic pain

BACKGROUND: This study tests the hypothesis that loss of spinal activity of gamma-aminobutyric acid (GABA) contributes to the allodynia and hyperalgesia observed after peripheral nerve injury. METHODS: Intrathecal catheters were implanted in male Sprague-Dawley rats. Antinociception was assessed by measuring withdrawal latency to immersion of the tail in a 52 degrees C water bath. Nerve injury was produced by ligation of the L5 and L6 spinal nerves. Testing was performed 4-14 days after spinal nerve ligation, when tactile allodynia and thermal hyperalgesia were established. Tactile allodynia was quantitated using the threshold to withdrawal of the hind paw on probing with von Frey filaments. Thermal hyperalgesia was quantitated using the latency to withdrawal of the hind paw from radiant heat. Motor function was tested using a rotarod apparatus. RESULTS: Spinal administration of the GABAA receptor antagonist bicuculline or the GABAB receptor antagonist phaclofen produced tactile allodynia and thermal hyperalgesia in normal rats. The GABAB receptor agonist baclofen, administered spinally, produced antinociception in the tail-flick test, whereas the GABAA receptor agonist isoguvacine did not. Isoguvacine and baclofen each reversed tactile allodynia and thermal hyperalgesia produced by spinal nerve ligation. Baclofen but not isoguvacine prolonged thermal withdrawal latency in nerve-injured rats beyond preoperative values. Baclofen but not isoguvacine impaired motor function. CONCLUSIONS: Pharmacologic inhibition of intrinsic GABA tone in normal rats resulted in tactile allodynia and thermal hyperalgesia, consistent with the hypothesis being tested. Exogenous administration of GABA agonists reversed spinal nerve ligation-induced allodynia and hyperalgesia, also consistent with this hypothesis. Isoguvacine produced specific antihyperalgesic and antiallodynic effects, whereas assessment of the effects of baclofen was complicated by motor dysfunction. Spinal GABAA agonists may provide a specific therapy for neuropathic pain.     

Preemptive intrathecal ketamine injection produces a long-lasting decrease in neuropathic pain behaviors in a rat model.

BACKGROUND AND OBJECTIVES: Ketamine is an N-Methyl-D-Aspartate (NMDA) receptor antagonist, which has been found to effectively treat somatic and neuropathic pain. This study examines the effect (on neuropathic pain) of preemptive ketamine using different routes of administration (intrathecal versus intraperitoneal). METHODS: The Institutional Animal Care and Use Committee approved the study. Thirty male Sprague-Dawley rats (250-275 g) were divided into three treatment groups [intrathecal saline/intraperitoneal saline or Control (CTL), intrathecal ketamine/intraperitoneal saline (ITK), and intrathecal saline/intraperitoneal ketamine (IPK)] prior to undergoing surgery to induce neuropathic pain by tight ligation of the left L5 and L6 spinal nerves. All drugs were given 15 minutes before nerve ligation. The ITK group received intrathecal ketamine (0.5% solution, 1 mg/kg), the IPK group received intraperitoneal ketamine (0.5% solution, 1 mg/kg), saline was given in equal volume (approximately 0.05 mL). Mechanical allodynia, cold allodynia, and ongoing pain behaviors indicative of neuropathic pain were assessed on postoperative days 1, 3, 7, and 14 using validated methods. RESULTS: Compared with the CTL group, the ITK group showed a state of decreased mechanical allodynia, cold allodynia, and ongoing pain as revealed by the von Frey hair, acetone, and cold plate testing, respectively. Further, this decrease was sustained for at least 2 weeks. The IPK group showed intermediate results between the CTL and ITK. CONCLUSIONS: Neuropathic pain behaviors were significantly reduced for at least 2 weeks after intrathecal ketamine was preemptively administered to animals undergoing surgery to induce neuropathic pain. The mechanism of action is thought to be prevention of spinal cord sensitization.     

Spinal glial activation and cytokine expression after lumbar root injury in the rat

STUDY DESIGN: This study was designed to examine the behaviorial immunohistochemical changes of spinal glial cells and spinal Interleukin (IL)-1beta expression after various nerve root injuries used as models of lumbar radiculopathy. OBJECTIVES: In order to better understand the role of central inflammation in the pathophysiologic mechanisms that give rise to pain associated with lumbar radiculopathy, this research studied the relationship between pain-related behavior associated with spinal glial activation and IL-1beta expression generated by three types of nerve root injury: loose ligation with chromic gut, loose ligation with silk, and tight ligation with silk. SUMMARY OF BACKGROUND DATA: An animal model of lumbar radiculopathy originally described by Kawakami and Weinstein involved loose ligation of unilateral L4-L6 nerve roots with chromic gut. Characterization and establishment of such an animal model of low back pain enables further investigation of the nature of the pathophysiologic mechanisms associated with lumbar radiculopathy in humans. METHODS: Seventy-three rats were divided into four treatment groups. Chromic group (n = 25): The L5 nerve roots (dorsal and ventral) were exposed by hemilaminectomy and loosely ligated with chromic gut. Tight silk group (n = 18): The exposed L5 nerve roots were tightly ligated extradurally with 5-0 silk suture. Loose silk group (n = 15): two loose ligatures of 5-0 silk were placed around the exposed L5 nerve roots. Sham group (n = 15): the rats were subjected to laminectomy alone for exposing nerve roots. Following surgery, thermal hyperalgesia and mechanical allodynia was assessed time-dependently up to 42 days post operatively. At 1, 3, 7, 14, and 42 days postoperatively, the rats in each group were perfused with fixative. The L5 spinal cord segments was harvested and cryosectioned for glial and cytokine immunohistochemistry. RESULTS: In the chromic and the tight silk group, an immediate and sustained mechanical allodynia was observed in the ipsilateral hind paw up to 35 days postoperatively. The loose silk group also showed an immediate mechanical allodynia that subsided by 14 days postoperatively. Sham-treated animals exhibited mild mechanicalallodynia for the initial 7 days after the surgery. Thermalhyperalgesia was evident in the three primary treatment groups, but not in the sham-treated rats. OX-42 expression was elevated in the gray matter of the L5 spinal section by 3 days in the chromic, the tight silk, and the loose silk groups as compared to the sham group. Astrocytic activation increased over time in all groups except the sham group. There was no direct correlation between degree of microglial response and severity of pain behaviors. In contrast, astrocytic activation demonstrated a direct relationship with the elevation of mechanical allodynia for the first 7 days. In addition, spinal IL-1beta protein expression was increased bilaterally in the superficial layer of the dorsal horn and cell nuclei of the ventral horns in the ligature treated groups as compared with the sham group. CONCLUSION: Direct mechanical and/or chemical injury to lumbar roots in the rat gives rise to pain behavior suggestive of lumbar radiculopathy. The finding that glial activation and enhanced IL-1beta expression are observed in the spinal cord after root injury supports a central, neuroimmune component in the generation of lumbar radiculopathy. A further understanding of the immunologic consequences of root injury may lead to further development and the novel use of selective cytokine-inflammatory inhibitors for the treatment of low back pain associated with radiculopathy.     

Effects of topical application of clonidine cream on pain behaviors and spinal Fos protein expression in rat models of neuropathic pain, postoperative pain, and inflammatory pain

BACKGROUND: Clonidine can effectively reduce pain and/or hypersensitivity. However, the antihypersensitivity effects of clonidine topically applied in cream (CC) have not been investigated. The authors evaluated effects of topical application of CC on pain behaviors and spinal Fos-like immunoreactivity in rats with hypersensitivity. METHODS: Clonidine (30, 100, and 300 microg/g) was prepared in a cream base. In rat models of neuropathic pain, inflammatory pain, and postoperative pain, the authors evaluated effects of CC (0.1 g), topically applied onto the plantar surface of the injured or uninjured paw, on thermal hyperalgesia and mechanical allodynia to von Frey filaments. The authors also evaluated effects of CC on lumbar spinal Fos-like immunoreactivity. RESULTS: In neuropathic rats, CC applied onto the injured paw reduced thermal hyperalgesia and mechanical allodynia dose dependently, whereas CC applied onto the uninjured paw had no effect. The antihypersensitivity effects of CC were antagonized by intraperitoneal yohimbine (10 mg/kg). Further, CC reduced Fos-like immunoreactivity in neuropathic rats. In contrast, CC in a single dose had no effects on hyperalgesia, allodynia, or Fos-like immunoreactivity in rats with inflammatory or postoperative pain. In rats with postoperative pain, CC repeatedly applied for 6 days reduced thermal hyperalgesia, but not mechanical allodynia, in the postoperative days, whereas it had no effects on hyperalgesia or allodynia in those with inflammatory pain. CONCLUSIONS: Topical CC in concentrations examined significantly reduced hypersensitivity and lumbar spinal Fos-like immunoreactivity in rats with neuropathic pain, probably through activation of peripherally located alpha2 adrenoceptors. However, CC was only partially effective and totally ineffective in rats with postoperative pain and inflammatory pain, respectively.     

Intrathecal spinal progenitor cell transplantation for the treatment of neuropathic pain

Injury to, or dysfunction of, the nervous system can lead to spontaneous pain, hyperalgesia, and/or allodynia. It is believed that the number and activity of GABAergic neurons gradually decreases over the dorsal horn. Glutamic acid decarboxylase (GAD) immunocompetence has been demonstrated on spinal progenitor cells (SPCs) cultivated in vitro. The intrathecal implantation of these cultivated progenitor cells may provide a means of alleviating neuropathic pain. Chronic constriction injury (CCI) of the sciatic nerve was used to induce chronic neuropathic pain in the hind paw of rats. SPCs (1 x 10(6)) were implanted intrathecally on the third day after the CCI surgery. The behavioral response to thermal hyperalgesia was observed and recorded during the 14 days postsurgery. Various techniques were utilized to trace the progenitor cells, confirm the differentiation, and identify the neurotransmitters involved. GAD immunoactivity was revealed for 65% of the cultivated spinal progenitor cells in our study. We also determined that transplanted cells could survive more than 3 weeks postintrathecal implantation. Significant reductions were demonstrated for responses to thermal stimuli for the CCI rats that had received intrathecal SPC transplantation. A novel intrathecal delivery with SPCs reduced CCI-induced neuropathic pain.     

坐骨神经细胞外信号调节激酶的激活引起小鼠坐骨神经部分结扎后的神经病理性疼痛

背景丝裂原活化蛋白激酶家族在几种疼痛模型中都起着重要的作用,但是,磷酸化细胞外信号调节激酶（phosphorylatedextracellularsignal-regulatedkinase,pERK）在周围神经损伤区域的具体作用尚不清楚。这项研究中,我们研究了受损的坐骨神经中pERK能否引起坐骨神经部分结31,术（partialsciaticnerveligation,PSL）的小鼠产生神经病理性疼痛。方法小鼠接受PSL,用蛋白印迹和免疫组化法检测坐骨神经中的pERKl／2（p44／42）。分两次注射U0126（一种ERK抑制剂）,即PSL术前30分钟神经内注射（20nmol／2μl）,PSL术后第1天神经周围注射（20nmol／10μl）,分别用热刺激缩足实验和机械刺激缩足实验对PSL导致的热痛觉过敏和异常触痛进行评估。结果经蛋白质印迹法检测,从第1天至第14天,实施模拟手术的小鼠坐骨神经中pERK1／2水平恒定并且与未经实验的小鼠相同;而在行PSL手术的小鼠中,PSL术后第1天就发现pERK1／2明显增加,一直持续至术后第3天。经免疫组化法检测,行PSL手术组与行模拟手术组相比,术后第1天坐骨神经中pERK1／2的免疫活性显著升高。双重免疫染色法的结果证实,PSL术后第1天坐骨神经中pERK1／2的免疫活性增强区被Schwann细胞的标志物——神经胶质纤维酸性蛋白（glialfibrillaryacidicprotein,GFAP）所确定,而巨噬细胞的标志物——F4／80却没有发现这种免疫活性的增强区。经U0126处理后,PSL术后第3天、第7天和第14天PSL诱发的热痛觉过敏明显减轻。经U0126处理后,在PSL术后第7天和第14天PSL诱发的异常触痛也明显减轻。结论受损周围神经系统的Schwann细胞中ERK激活可能对神经病理性疼痛的产生起了重要的作用。我们的研究结果说明pERK本身和ERK相关介质是神经病理性疼痛的潜在治疗靶点。     

Nerve injury induces a tonic bilateral mu-opioid receptor-mediated inhibitory effect on mechanical allodynia in mice

BACKGROUND: Mice lacking the mu-opioid receptor gene have been used to characterize the role of mu-opioid receptors in nociception and the analgesic actions of opioid agonists. In this study, the authors determined the role of mu-opioid receptors in neuropathic pain behaviors and the effectiveness of mu- and kappa-opioid receptor agonists on this behavior in mice. METHODS: The authors studied the behavioral responses of mu-opioid receptor knockout and wild-type mice to thermal and mechanical stimuli before and after neuropathic pain induced by unilateral ligation and section of the L5 spinal nerve. Response to mechanical stimuli was evaluated by determining the frequency of hind paw withdrawal to repetitive stimulation using a series of von Frey monofilaments. Thermal hyperalgesia was assessed by determining the paw withdrawal latencies to radiant heat and frequency of hind paw withdrawal to cooling stimuli. The effects of systemic morphine, the kappa-opioid agonist U50488H, and naloxone on responses to mechanical and thermal stimuli were also studied in spinal nerve-injured mice. RESULTS: After spinal nerve injury, wild-type mice developed increased responsiveness to mechanical, heat, and cooling stimuli ipsilateral to nerve injury. mu-Opioid receptor knockout mice not only had more prominent mechanical allodynia in the nerve-injured paw, but also expressed contralateral allodynia to mechanical stimuli. Hyperalgesia to thermal stimuli was similar between mu-opioid knockout and wild-type animals. Morphine decreased mechanical allodynia dose dependently (3-30 mg/kg subcutaneous) in wild-type mice--an effect that was attenuated in the heterozygous mice and absent in the homozygous mu-opioid knockout mice. The kappa-opioid agonist U50488H (3-10 mg/kg subcutaneous) attenuated mechanical allodynia in wild-type, heterozygous, and homozygous mu-opioid mice. Naloxone in wild-type mice resulted in enhanced ipsilateral and contralateral allodynia to mechanical stimuli that resembled the pain behavior observed in mu-opioid receptor knockout mice. CONCLUSIONS: The authors' observations indicate that (1) unilateral nerve injury induces a bilateral tonic activation of endogenous mu-opioid receptor-mediated inhibition that attenuates mechanical allodynia but not thermal hyperalgesia, (2) both mu- and kappa-opioid agonists attenuate neuropathic pain in mice, and (3) the antihyperalgesic actions of morphine are mediated primarily via mu-opioid receptors.     

A prostaglandin E2 receptor subtype EP1 receptor antagonist (ONO-8711) reduces hyperalgesia, allodynia, and c-fos gene expression in rats with chronic nerve constriction

Chronic constriction injury (CCI) of the sciatic nerve in rats induces persistent mechanical hyperalgesia and allodynia. CCI is widely known as a model of neuropathic pain, and many studies using this model have been reported. Recently, c-fos has been used as a neural marker of pain, and various studies have assessed the relationship between hyperalgesia and c-fos expression in the lumbar spinal cord. In this study, we examined the role of a prostaglandin E2 receptor subtype EP1 receptor antagonist (ONO-8711) in a rat CCI model. EP1 receptor antagonist (EP1-ra) oral administration from day 8 to day 14 significantly reduced hyperalgesia and allodynia in the three pain tests on day 15. EP1-ra treatment from day 8 to 14 also reduced c-fos-positive cells in laminae I-II, III-IV, and V-X compared with saline treatment. A single dose of EP1-ra treatment on day 8 significantly reduced hyperalgesia and allodynia at 1 h and 2 h after administration, but the efficacy was not observed at 24 h. We conclude that EP1-ra treatment may be useful for hyperalgesia and allodynia and that EP1 receptor mechanisms are involved in the maintenance of c-fos gene expression induced by nerve injury. IMPLICATIONS: We examined whether a prostaglandin E2 receptor subtype EP1 receptor antagonist abrogates neuropathic pain induced by chronic constriction injury model in rats. The EP1 receptor antagonist significantly reduced hyperalgesia, allodynia, and c-fos positive cells. These findings suggested that EP1 receptor antagonists may have a role in treatment of neuropathic pain.     

Regulation der spinalen Expression von Nociceptin unter neuropathischen Schmerzen

Nociceptin is the endogenous ligand of a new opioid receptor, the opioid receptor-like-1 (ORL1) receptor. Chronic inflammatory pain causes an increase in the expression of nociceptin and the ORL1 receptor in the dorsal horn of rat spinal cord, thus indicating an involvement of the endogenous nociceptin/ORL1 system in mechanisms of pathological pain. This study investigates the influence of neuropathic pain on the expression of nociceptin using immunohistochemistry. To induce neuropathic pain, a ligation of the sciatic nerve was performed in 12 rats under general anesthesia. A sham operation was performed in 12 rats of the control group. Nerve ligation caused a significant ipsilateral thermal hyperalgesia, a typical sign of neuropathic pain. The paw withdrawal latency was decreased by 45.7+/-4.9% ( p<0.05) at day 5 and by 37.3+/-1.8% ( p<0.05) at day 10. Although hyperalgesia was fully present after 5 days, no changes in nociceptin immunoreactivity in the lumbar spinal cord were detected at this time point. Ten days after nerve ligation, there was a 2.46+/-0.38 fold ( p<0.05) bilateral increase in nociceptin immunoreactivity in the lamina superficiales (I and II), with a notable increase in the inner lamina II at the level of L4. Further investigations are necessary to elucidate the relationship between neuropathic pain, the nociceptin-ORL1 receptor system and potential therapeutic options.     

Allosteric adenosine modulation to reduce allodynia

BACKGROUND: Adenosine and adenosine agonists reduce hypersensitivity following inflammation and peripheral nerve injury models of chronic pain. Because inhibitors of adenosine reuptake or metabolism are also effective at reducing hypersensitivity, it is likely that there is a tonic release of spinal adenosine in these models. One approach to avoid adverse effects from direct agonists is to enhance the effect of the endogenous ligand by administering a positive allosteric modulator of its receptor. METHODS: Rats with mechanical hypersensitivity after spinal nerve ligation received intrathecal injections of adenosine, the allosteric adenosine receptor modulator T62, or their combination, or received systemic T62 alone or with intrathecal injection of a specific A1 adenosine antagonist. RESULTS: Both adenosine and T62 reduced hypersensitivity alone, with 50% maximal doses (ED50) of 14+/-5.9 and 3.7+/-0.8 microg, respectively. They interacted in an additive manner as determined by isobolography. T62 also reduced mechanical hypersensitivity after systemic administration (15 mg/kg), and this effect was blocked by intrathecal injection of 9 microg of the A1-specific adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. CONCLUSIONS: These results add to previous studies that suggest ongoing spinal release of adenosine, which is antiallodynic, in this animal model of neuropathic pain. Positive allosteric modulation of the adenosine receptor reduces hypersensitivity by a spinal mechanism involving A1 adenosine receptor stimulation. Although obvious adverse effects were not observed in this investigation, further study is required to determine the feasibility of the use of such modulators in the treatment of chronic pain associated with hyperalgesia and allodynia.     

Inhibition of spinal prostaglandin synthesis early after L5/L6 nerve ligation prevents the development of prostaglandin-dependent and prostaglandin-independent allodynia in the rat

BACKGROUND: Prostaglandins, synthesized in the spinal cord in response to noxious stimuli, are known to facilitate nociceptive transmission, raising questions about their role in neuropathic pain. The current study tested the hypothesis that spinal nerve ligation-induced allodynia is composed of an early prostaglandin-dependent phase, the disruption of which prevents allodynia. METHODS: Male Sprague-Dawley rats, fitted with intrathecal drug delivery or microdialysis catheters, underwent left L5-L6 spinal nerve ligation or sham surgery. Paw withdrawal threshold, brush-evoked behavior, and the concentration of prostaglandin E2 (PGE2) in spinal cerebrospinal fluid ([PGE2]dialysate) were determined for up to 24 days. PGE2-evoked glutamate release from spinal slices was also determined. RESULTS: Paw withdrawal threshold decreased from at least 15 g (control) to less than 4 g, beginning 1 day after ligation. Brushing the affected hind paw evoked nociceptive-like behavior and increased [PGE2]dialysate (up to 257 +/- 62% of baseline). There was no detectable change in basal [PGE2]dialysate from preligation values. The EC50 of PGE2-evoked glutamate release (2.4 x 10-11 M, control) was significantly decreased in affected spinal segments of allodynic rats (8.9 x 10-15 M). Treatment with intrathecal S(+)-ibuprofen or SC-560, beginning 2 h after ligation, prevented the decrease in paw withdrawal threshold, the brush-evoked increase in [PGE2]dialysate, and the change in EC50 of PGE2-evoked glutamate release. R(-)-ibuprofen or SC-236 had no effect. CONCLUSIONS: The results of this study provide solid evidence that spinal prostaglandins, synthesized by cyclooxygenase-1 in the first 4-8 h after ligation, are critical in the pathogenesis of prostaglandin-dependent and prostaglandin-independent allodynia and that their early pharmacologic disruption affords protection against this neuropathic state in the rat.     

鞘内注射GDNF对神经病理性痛大鼠脊髓背角p38MAPK蛋白表达的影响

目的 评价鞘内注射胶质细胞源性神经营养因子(GDNF)对神经病理性痛大鼠脊髓背角p38丝裂原活化蛋白激酶(p38MAPK)蛋白表达的影响.方法 取鞘内置管成功的健康雄性SD大鼠120只,周龄6周,体重180～200 g,随机分为4组(n=30):对照组(C组)、假手术组(S组)、神经病理性痛组(P组)和GDNF组.采用结扎L5.6脊神经的方法建立大鼠神经病理性痛模型.C组不给予任何处理;S组只暴露脊神经,但不结扎;P组脊神经结扎后鞘内注射生理盐水10μl,隔日1次,连续14 d;GDNF组脊神经结扎后鞘内注射GDNF 2μg,用生理盐水稀释至10μl,隔日1次,连续14 d.分别于脊神经结扎后3、7和14 d时,取10只大鼠,测定机械痛阈,然后处死,取脊髓背角,分别采用免疫组化法和蛋白质印迹法测定p38MAPK蛋白的表达水平.结果 与S组比较,P组和GDNF组机械痛阈降低,脊髓背角p38MAPK蛋白表达上调(P＜0.05或0.01);与P组比较,GDNF组机械痛阈升高,脊髓背角p38MAPK蛋白表达下调(P＜0.05或0.01).结论 鞘内注射GDNF可通过抑制脊髓背角p38MAPK蛋白的表达减轻大鼠神经病理性痛.     

Intrathecal lidocaine reverses tactile allodynia caused by nerve injuries and potentiates the antiallodynic effect of the COX inhibitor ketorolac

BACKGROUND: Systemic lidocaine and other local anesthetics reduce hypersensitivity states induced by both acute inflammation and peripheral nerve injury in animals and produce analgesia in some patients with chronic pain. The mechanisms underlying the antiallodynic effect of systemic lidocaine are unclear, although most focus is on peripheral mechanisms. Central mechanisms, particularly at the spinal dorsal horn level, are less known. In this study, the authors aimed to determine whether intrathecal lidocaine has an antiallodynic effect on established mechanical allodynia in two well-characterized neuropathic pain rat models: partial sciatic nerve ligation (PSNL) and spinal nerve ligation (SNL). METHODS: Lidocaine (100-300 micro g) was intrathecally injected in PSNL and SNL rats. The withdrawal threshold of both hind paws in response to mechanical stimulation was measured using a series of calibrated von Frey filaments. RESULTS: This single injection reduced ongoing tactile allodynia in PSNL and SNL rats. The antiallodynic effect of intrathecal lidocaine lasted longer in PSNL (> 3 days) than in SNL rats (< 3 days). Intraperitoneal lidocaine (300 micro g) had no effect on tactile allodynia in PSNL rats. In SNL rats, prior intrathecal lidocaine (200 and 300 micro g) potentiated the antiallodynic effect of intrathecal ketorolac, a nonselective cyclooxygenase inhibitor. Intrathecal ketorolac alone had no antiallodynic effect on SNL rats. However, prior intrathecal lidocaine (100 micro g) failed to potentiate the antiallodynic effect of intrathecal ketorolac. CONCLUSION: The authors' data suggest that intrathecal lidocaine possibly suppressed the hyperexcitability of the dorsal horn neurons and likely interacted with eicosanoid systems in the spinal dorsal horn.     

Intrathecal Lidocaine Reverses Tactile Allodynia Caused by Nerve Injuries and Potentiates the Antiallodynic Effect of the COX Inhibitor Ketorolac

Background: Systemic lidocaine and other local anesthetics reduce hypersensitivity states induced by both acute inflammation and peripheral nerve injury in animals and produce analgesia in some patients with chronic pain. The mechanisms underlying the antiallodynic effect of systemic lidocaine are unclear, although most focus is on peripheral mechanisms. Central mechanisms, particularly at the spinal dorsal horn level, are less known. In this study, the authors aimed to determine whether intrathecal lidocaine has an antiallodynic effect on established mechanical allodynia in two well-characterized neuropathic pain rat models: partial sciatic nerve ligation (PSNL) and spinal nerve ligation (SNL).

Methods: Lidocaine (100-300 [mu]g) was intrathecally injected in PSNL and SNL rats. The withdrawal threshold of both hind paws in response to mechanical stimulation was measured using a series of calibrated von Frey filaments.

Results: This single injection reduced ongoing tactile allodynia in PSNL and SNL rats. The antiallodynic effect of intrathecal lidocaine lasted longer in PSNL (> 3 days) than in SNL rats (< 3 days). Intraperitoneal lidocaine (300 [mu]g) had no effect on tactile allodynia in PSNL rats. In SNL rats, prior intrathecal lidocaine (200 and 300 [mu]g) potentiated the antiallodynic effect of intrathecal ketorolac, a nonselective cyclooxygenase inhibitor. Intrathecal ketorolac alone had no antiallodynic effect on SNL rats. However, prior intrathecal lidocaine (100 [mu]g) failed to potentiate the antiallodynic effect of intrathecal ketorolac.