Allodynia In Rats Research Articles

A partial L5 spinal nerve ligation induces a limited prolongation of mechanical allodynia in rats: An efficient model for studying mechanisms of neuropathic pain

The relationship between pain severity and the extent of injury to a peripheral nerve remains elusive. In this study, we compared the pain behavior resulting from partial (1/3–1/2 thickness) and full L5 spinal nerve ligation (SNL) in rats. The decrease in paw withdrawal threshold (PWT) to mechanical stimuli in the hindpaw ipsilateral to the injury was comparable in the two groups on days 3–21 post-injury. However, the decreased PWT recovered earlier in the partial SNL group than in the full SNL group. These observations suggest that the duration of neuropathic pain behavior, but not the early development of mechanical allodynia, is dependent on the extent of nerve injury. On days 6 and 15 post-injury, when the mechanical allodynia was similar in the two groups, systemic morphine induced a greater reduction of mechanical allodynia in the partial SNL group than in the full SNL group. Furthermore, in partial SNL rats, at post-injury time points when they had largely recovered from the neuropathic pain state, systemic administration of naloxone hydrochloride (day 53) or naloxone methiodide (a non-selective peripherally acting opioid receptor antagonist; day 64) or intra-plantar injection of naloxone methiodide rekindled mechanical pain hypersensitivity in the ipsilateral hindpaw, suggesting a prolonged activation of endogenous opioidergic pain-inhibition. Therefore, partial SNL in rats may represent an efficient model for studying the mechanisms of neuropathic pain, testing effects of analgesic/antihyperalgesic drugs, and understanding endogenous pain-inhibitory mechanisms that lead to reversal of the pain behavior with time.

Nociceptor-Expressed Ephrin-B2 Regulates Inflammatory and Neuropathic Pain

BackgroundEphB receptors and their ephrin-B ligands play an important role in nervous system development, as well as synapse formation and plasticity in the adult brain. Recent studies show that intrathecal treatment with EphB-receptor activator ephrinB2-Fc induced thermal hyperalgesia and mechanical allodynia in rat, indicating that ephrin-B2 in small dorsal root ganglia (DRG) neurons and EphB receptors in the spinal cord modulate pain processing. To examine the role of ephrin-B2 in peripheral pain pathways, we deleted ephrin-B2 in Nav1.8+ nociceptive sensory neurons with the Cre-loxP system. Sensory neuron numbers and terminals were examined using neuronal makers. Pain behavior in acute, inflammatory and neuropathic pain models was assessed in the ephrin-B2 conditional knockout (CKO) mice. We also investigated the c-Fos expression and NMDA receptor NR2B phosphorylation in ephrin-B2 CKO mice and littermate controls.ResultsThe ephrin-B2 CKO mice were healthy with no sensory neuron loss. However, pain-related behavior was substantially altered. Although acute pain behavior and motor co-ordination were normal, inflammatory pain was attenuated in ephrin-B2 mutant mice. Complete Freund's adjuvant (CFA)-induced mechanical hyperalgesia was halved. Formalin-induced pain behavior was attenuated in the second phase, and this correlated with diminished tyrosine phosphorylation of N-methyl-D-aspartic acid (NMDA) receptor subunit NR2B in the dorsal horn. Thermal hyperalgesia and mechanical allodynia were significantly reduced in the Seltzer model of neuropathic pain.ConclusionsPresynaptic ephrin-B2 expression thus plays an important role in regulating inflammatory pain through the regulation of synaptic plasticity in the dorsal horn and is also involved in the pathogenesis of some types of neuropathic pain.

Complete Freund’s Adjuvant-Induced Intervertebral Discitis as an Animal Model for Discogenic Low Back Pain

Although numerous animal models for low back pain associated with intervertebral disk (IVD) degeneration have been proposed, insufficient data have been provided to make any conclusions regarding pain. Our aim in this study was to determine the reliability of complete Freund's adjuvant (CFA) injection into the rat spine as an animal model representing human discogenic pain. We studied IVD degenerative changes with pain development after a 10-microL CFA injection into the L5-6 IVD of adult rats using behavioral, histologic, and biochemical studies. Serial histologic changes were analyzed to detect degenerative changes. Expression of calcitonin gene-related peptide (CGRP), prostaglandin E (PGE), and inducible nitric oxide synthase (iNOS) were determined using immunohistochemistry or real-time polymerase chain reaction as support data for pain development. In addition, CGRP immunoreactivity (ir) at the IVD was considered indirect evidence of neural ingrowth into the IVD. There was a significant increase of the hindpaw withdrawal response in the CFA group until 7 wk postoperatively (P < 0.05). Histologic analyses revealed progressive degenerative changes of the disks without any damage in adjacent structures, including nerve roots. In the CGRP-ir staining study, the bilateral dorsal horns and IVD had positive ir after intradiscal CFA injection. CGRP mRNA expression was increased in the dorsal root ganglion (DRG) at 2 and 4 wk, whereas PGE and iNOS mRNAs were markedly increased at 2 wk. The increment of CGRP expression was higher in allodynic rats compared with nonallodynic rats. Intradiscal CFA injection led to chronic disk degeneration with allodynia, which was suggested by pain behavior and expression of pain-related mediators. The increment of CGRP, PGE, and iNOS also suggest pain-related signal processing between the IVD and the neural pathway in this animal model. This animal model may be useful for future research related to the pathophysiology and development of novel treatment for spine-related pain.

Chronic restraint stress induces mechanical and cold allodynia, and enhances inflammatory pain in rat: Relevance to human stress-associated painful pathologies

Whereas acute stress often results in analgesia, chronic stress can trigger hyperalgesia/allodynia. This influence of long-term stress on nociception is relevant to numerous painful pathologies, such as fibromyalgia (FM), characterized by diffuse muscular pain (hyperalgesia) and/or tenderness (allodynia). Hence, there is a need for pre-clinical models integrating a chronic-stress dimension to the study of pain. Here, we assessed the effects of protracted/intermittent stress produced by daily, 1 h restraint periods in cylinders, 4 days/week over 5 weeks, on eight models of hyperalgesia and allodynia in rats. This type of stress potentiated chemical hyperalgesia in the formalin model (160 and 76% increase of pain score above controls, during the early and late phases, respectively). It also produced thermal allodynia in response to cold (paw acetone test: 200% increase of allodynia score during week 3–5) and heat (42 °C tail immersion test: 15% decrease of withdrawal threshold, from week 2 onward). This stress also resulted in mechanical allodynia in the von Frey filaments model (60% decrease in threshold during week 2–5). However, such a stress regimen had no influence in the Randall–Selitto test of mechanical hyperalgesia, and in the tail immersion models of cold (4 °C) or hot (48 °C) thermal hyperalgesia, as well as cold (15 °C) allodynia. This model of prolonged/intermittent restraint stress may be useful in investigating the mechanisms linking stress and pain, and provide an assay to assess the potential therapeutic efficacy of drugs targeted against painful pathologies with a strong stress component, including but not restricted to FM.

Pharmacological characterization of standard analgesics on mechanical allodynia in streptozotocin-induced diabetic rats

The present study was designed to investigate the anti-allodynic effects of current analgesic agents, such as pregabalin, amitriptyline, mexiletine, morphine, and diclofenac, in a rat model of streptozotocin (STZ)-induced diabetic neuropathy. Diabetic rats developed a sustained decrease in withdrawal threshold response to the von Frey test within 8 weeks after a single injection of STZ (45 mg/kg, i.v.). The anti-allodynic effects of analgesic agents were examined after a single oral or subcutaneous administration at 3 and 7 weeks after beginning of STZ-treatment. Pregabalin (3–30 mg/kg, p.o.), an antiepileptic agent, dose-dependently blocked the mechanical allodynia in rats treated both at 3 and 7 weeks. Mexiletine (10–100 mg/kg, p.o.), a sodium channel blocker, dose-dependently ameliorated mechanical allodynia in rats treated at 3 weeks; however, the efficacy was diminished at 7 weeks. Morphine (1–10 mg/kg, s.c.) was effective in rats treated at 3 weeks; however, it was ineffective at 7 weeks. Conversely, an antidepressant amitriptyline (0.3–3 mg/kg, p.o.) improved mechanical allodynia in rats treated at 7 weeks, whereas it was ineffective at 3 weeks. Diclofenac, a non-steroidal anti-inflammatory drug, was ineffective at both time points. These results demonstrate that, except for diclofenac, the standard analgesic agents tested can effectively alleviate the mechanical allodynia seen in STZ-induced diabetic neuropathy. Their efficacies varied depending on the duration of the diabetic condition, suggesting that temporal changes in pharmacodynamic factors could affect the responsiveness of this model to analgesic agents.

Synergistic antiallodynic interaction of the metamizol‐gabapentin combination

AbstractThis study was designed to evaluate the possible antiallodynic interaction between metamizol and gabapentin in rats submitted to L5/L6 spinal nerve ligation. Metamizol, gabapentin, or a combination of both drugs were assessed after oral and intrathecal administration in neuropathic rats. Metamizol partially reduced tactile allodynia after intrathecal, but not oral, administration. Conversely, gabapentin reduced tactile allodynia in a dose‐dependent manner after both administration routes. Oral administration of a constant dose of metamizol (600 mg/kg) significantly increased the gabapentin‐induced antiallodynic effect. Moreover, the gabapentin ED50 value was lower in the presence than in the absence of metamizol. Intrathecal co‐administration of metamizol and gabapentin in a dose‐fixed ratio (0.5:0.5) reduced tactile allodynia in rats. The theoretical ED30 value for the spinal combination estimated from the isobologram was 118.4±12 µg, whereas that experimental ED30 value was 66.2±10.1 µg indicating a synergistic interaction. Results indicate that metamizol, a cyclo‐oxygenase 2 inhibitor, is able to reduce tactile allodynia as well to increase the antiallodynic effect of gabapentin in the neuropathic rat. This combination could be useful to treat neuropathic pain in humans. Drug Dev Res 2009. © 2009 Wiley‐Liss, Inc.

A nitro-arginine derivative of trimebutine (NO 2-Arg-Trim) attenuates pain induced by colorectal distension in conscious rats

Irritable bowel syndrome (IBS) is characterized by dysfunction of the afferent pathways that may lead to visceral hypersensitivity. Trimebutine is a weak μ opioid receptor agonist used in the treatment of IBS. We report on the effects of a novel derivative in which trimebutine has been salified with nitro-arginine (NO 2-Arg-Trim), in modulating nociception to colorectal distension (CRD) in intact and post-colitis rats, an animal model that mimics some features of IBS. Colorectal sensitivity and pain were assessed by measuring the abdominal withdrawal score (AWR) during CRD. Healthy rats were treated with vehicle, trimebutine (10 mg/kg i.p.) or NO 2-Arg-Trim (4, 8 and 16 mg/kg i.p.). Post-colitis, allodynic rats were investigated 4 weeks after colitis induction. Treating healthy rats with NO 2-Arg-Trim resulted in a dose-dependent attenuation of CRD-induced nociception and in an inhibition of CRD-induced overexpression of spinal cFOS mRNA. NO 2-Arg-Trim-induced antinociception was reversed by the opioid receptor antagonist naloxone and by the NO synthase–cGMP pathway inhibitor methylene blue, while L-NAME had no effect. The antinociceptive effect of NO 2-Arg-Trim was maintained in a rodent model of post-inflammatory allodynia. In this setting, NO 2-Arg-Trim but not trimebutine, significantly down-regulated the spinal cFOS mRNA expression and increased blood concentrations of NO 2 + NO 3. Moreover, the expression of several genes involved in inflammation and pain, as IL-1β, TNFα, COX2 and iNOS, was up-regulated in colonic tissue from post-colitis rats and NO 2-Arg-Trim, but not trimebutine, effectively reversed this effect. In summary, these data suggest that NO 2-Arg-Trim inhibits nociception induced by CRD in both healthy and post-colitis, allodynic rats. The NO 2–arginine moiety interacts with the opioid agonist trimebutine to potentiate its analgesic activity. This study provides evidence that NO 2–arginine derivative of trimebutine might have beneficial effect in the treatment of painful intestinal disorders.

Activation of p38 Mitogen-activated Protein Kinase in Spinal Microglia Contributes to Incision-induced Mechanical Allodynia

Recent studies have implicated the activation of stress-activated mitogen-activated protein kinase (MAPK) p38 in spinal microglial cells for development of neuropathic and inflammatory pain. The aim of the present study was to investigate whether phosphorylation of p38 (p-p38) also mediates mechanical allodynia and thermal hyperalgesia induced by plantar incision. After rats received a plantar incision surgery, mechanical allodynia and thermal hyperalgesia were determined by von Frey filaments and radiant heat, respectively, and the number of p-p38 immunoreactive cells in the dorsal horn was quantified to determine p38 activation at different time points after incision. The p38 inhibitor FR167653 was administered intrathecally 30 min before hind paw plantar incision to determine the role of p38 in postoperative pain. A significant increase in number of p-p38 immunoreactive cells was observed in the ipsilateral L4-5 spinal dorsal horn from 1 h to 3 days after the incision. p-p38 was found predominantly in microglia. However, microglial activation (assessed by OX-42 upregulation) was not evident until 3 days after plantar incision. Intrathecal pretreatment of FR167653 attenuated incision-induced mechanical allodynia from 1 h to day 2 and significantly reduced activation of p38 in the dorsal horn 1 day after plantar incision. However, FR167653 only inhibited heat hyperalgesia at an early time point. Plantar incision-induced mechanical allodynia can be prevented by the p38 inhibitor. Our results suggest that p38 activation in spinal microglia play a role in incision-induced mechanical allodynia in rats. Therefore, p38 inhibition may be useful in treating postsurgical pain.

P80. Resiniferatoxin Induced Alteration of Neurotrophic Factors in the Dorsal Root Ganglia of Allodynic Rats

P80. Resiniferatoxin Induced Alteration of Neurotrophic Factors in the Dorsal Root Ganglia of Allodynic Rats

Resiniferatoxin Induced Alteration of Neurotrophic Factors in the Dorsal Root Ganglia of Allodynic Rats

Resiniferatoxin Induced Alteration of Neurotrophic Factors in the Dorsal Root Ganglia of Allodynic Rats

GAIT ABNORMALITIES AND SENSORY CHANGES ACCOMPANY NEURO-INFLAMMATION IN ANIMAL MODELS OF DISC HERNIATION RADICULOPATHY

Objective: The goal of this study was to evaluate gait and behavioral changes in an animal model of disc-herniation induced radiculopathy. A second objective was to correlate these functional changes to evidence of neuroinflammation and autoreactive lymphocyte immune activation. Methods: The animal model of radiculopathy involved harvesting autologous nucleus pulposus (NP) from a tail intervertebral disc, and then exposing the L5 dorsal root ganglion (DRG) by hemilaminectomy and partial facetectomy. Experimental animals (n = 16) received NP placement onto the DRG andcontrol animals (n = 16) underwent exposure only. At weekly time points, animals were evaluated for mechanical allodynia by Von Frey testing and for gait symmetry by digitized video analysis. At sacrifice, serum was evaluated for inflammatory cytokine content. The L5 DRGs were then evaluated by immunohistochemistry for mediators of inflammation and immune activation. Statistical analyses were at the ? = 0.05 level of significance, with Bonferroni corrections for multiple comparisons when appropriate. Results: Sensory testing revealed persistent mechanical allodynia in rats subjected to NP stimulus compared with the sham surgery group (Von Frey testing, p &lt; 0.01). Gait analysis reflected the functional consequence of this altered sensation revealing marked asymmetry and apreference to place load on the contralateral limb (symmetry index, p &lt; 0.01). Serum cytokine expressionwas equivalent between groups, reaffirming that the sensation and behavioral changes observed in these animals results primarily from local inflammatory changes. Immunohistochemicalanalysis of the sectioned DRGs after sacrifice revealed equivalent post-surgical inflammatory activation (IL23, p = 0.47), but substantial immuneactivation in the NP group (IL17, p = 0.01). Conclusion: This model of radiculopathy provides the first evidence of altered gait symmetry and locomotor ability in animals subjected to non-compressive placement of NP tissue. Systemic inflammation was absent as expected, but mechanical allodynia, local inflammation, and autoreactive immune activation were observed. Future work involves therapeutic interventions targeting these changes to rescue animals from the phenotype of inflammatory radiculopathy.

Oligonucleotide IMT504 reduces neuropathic pain after peripheral nerve injury

We have recently shown that the administration of bone marrow stromal cells (MSCs) prevents the development of mechanical and thermal allodynia in animals subjected to a sciatic nerve injury. Furthermore, exogenously administered MSCs have been shown to participate in the repair and regeneration of damaged tissues in a variety of animal models. However, some limitations of this therapeutic approach, basically related to the ex vivo cell manipulation procedure, have arisen. IMT504, the prototype of the PyNTTTTGT class of immunostimulatory oligonucleotides, stimulates MSC expansion both in vitro and in vivo. In this study, we evaluated the effect of IMT504 systemic administration on the development of mechanical and thermal allodynia in rats subjected to a sciatic nerve crush. Animals were treated with IMT504, MSCs or saline either immediately after performing the lesion or 4 days after it, and were evaluated using the von Frey and Choi tests at different times after injury. Control animals developed both mechanical and thermal allodynia. Animals receiving either IMT504 or MSCs immediately after injury did not develop mechanical allodynia and presented a significantly lower number of nociceptive responses to cold stimulation as compared to controls. Moreover, injury-induced allodynia was significantly reduced after IMT504 delayed treatment. Our results show that the administration of IMT504 reduces neuropathic pain-associated behaviors, suggesting that IMT504 could represent a possible therapeutic approach for the treatment of neuropathic pain.

Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats

Spinal cord injury (SCI) impairs sensory systems causing chronic allodynia. Mechanisms underlying neuropathic pain have been more extensively studied following peripheral nerve injury (PNI) than after central trauma. Microglial activation, pro-inflammatory cytokine production and activation of p38 MAP kinase pathways may induce at-level allodynia following PNI. We investigated whether midthoracic SCI elicits similar behavioral and cellular responses below the level of injury (lumbar spinal cord; L5). Importantly, we show that anatomical connections between L5 and supraspinal centers remain intact after moderate SCI allowing direct comparison to a well-established model of peripheral nerve injury. We found that SCI elicits below-level allodynia of similar magnitude to at-level pain caused by a peripheral nerve injury. Moreover, the presence of robust microglial activation in L5 cord predicted allodynia in 86% of rats. Also increased phosphorylation of p38 MAP kinase occurred in the L5 dorsal horn of allodynic rats. For below-level allodynia after SCI, TNF-α and IL-1β increased in the L5 dorsal horn by 7 dpo and returned to baseline by 35 dpo. Interestingly, IL-6 remains at normal levels early after SCI and increases at chronic time points. Increased levels of pro-inflammatory cytokines also occurred in the thalamus after SCI-induced allodynia. These data suggest that remote microglial activation is pivotal in the development and maintenance of below-level allodynia after SCI. Fractalkine, a known activator of microglia, and astrocytes were not primary modulators of below-level pain. Although the mechanisms of remote microglial activation are unknown, this response may be a viable target for limiting or preventing neuropathic pain after SCI in humans.

Exacerbated mechanical allodynia in rats with depression-like behavior

Although a clinical connection between pain and depression has long been recognized, how these two conditions interact remains unclear. Here we report that both mechanical allodynia and depression-like behavior were significantly exacerbated after peripheral nerve injury in Wistar-Kyoto (WKY) rats, a genetic variation of Wistar rats with demonstrable depression-like behavior. Administration of melatonin into the anterior cingular cortex contralateral to peripheral nerve injury prevented the exacerbation of mechanical allodynia with a concurrent improvement of depression-like behavior in WKY rats. Moreover, there was a lower plasma melatonin concentration and a lower melatonin receptor expression in the anterior cingular cortex in WKY rats than in Wistar rats. These results suggest that there exists a reciprocal relationship between mechanical allodynia and depression-like behavior and the melatoninergic system in the anterior cingular cortex might play an important role in the interaction between pain and depression.

Synergistic antiallodynic interaction between gabapentin or carbamazepine and either benfotiamine or cyanocobalamin in neuropathic rats

Anticonvulsants, including gabapentin and carbamazepine, have shown activity against several types of neuropathic pain; however, they have limiting side effects that may minimize their use. In this study the possible synergistic interaction between anticonvulsants and benfotiamine or cyanocobalamin on spinal nerve ligation-induced tactile allodynia was assessed. Oral administration of gabapentin (15-300 mg/kg), carbamazepine (10-300 mg/kg), benfotiamine (30-600 mg/kg) or cyanocobalamin (0.3-6.0 mg/kg) significantly reduced tactile allodynia in rats. Maximal antiallodynic effects were reached with gabapentin 300 mg/kg (approximately 70%), carbamazepine 300 mg/kg (approximately 66%), benfotiamine 600 mg/kg (approximately 51%) and cyanocobalamin 6 mg/kg (approximately 59%). At the highest tested doses, gabapentin, but not carbamazepine, benfotiamine or cyanocobalamin, significantly reduced motor coordination. Coadministration of gabapentin or carbamazepine with benfotiamine or cyanocobalamin in a fixed ratio markedly reduced spinal nerve ligation-induced tactile allodynia, showing a synergistic interaction between anticonvulsants and B vitamins. Data indicate that combinations of anticonvulsants with benfotiamine or cyanocobalamin are able to reduce tactile allodynia without affecting motor coordination in rats, and suggest the possible clinical use of these combinations in the treatment of neuropathic pain in humans.

The interaction of morphine and selective serotonin reuptake inhibitors on mechanical allodynia in rats with a spinal nerve ligation

Nerve injury may produce a tactile allodynia. However, there are few reports regarding the interaction of morphine and selective serotonin reuptake inhibitors (SSRIs) during a neuropathic pain state. Therefore, we investigated the antiallodynic interaction between morphine and SSRIs in a rat model of neuropathic pain. Methods: Rats were prepared with a tight ligation of the left fifth and sixth lumbar spinal nerves and chronic intrathecal catheter implantation. Mechanical allodynia was then measured by application of von Frey filaments. Morphine, citalopram and paroxetine were administered intrathecally to obtain the dose-response curves. The 50% effective dose of morphine and citalopram or paroxetine were then coadministered to evaluate the drug interaction. In addition, naloxone and methysergide were administered to examine the reversal of the antiallodynic effect. Results: Intrathecal morphine produced a dose-dependent antagonism of the tactile allodynia, but intrathecal citalopram or paroxetine showed no antiallodynic effects. In addition, a morphine-citalopram or paroxetine combination produced an increase in the withdrawal threshold, but naloxone and methysergide reversed the antiallodynic effect. In addition, no interactions were observed between naloxone and citalopram or paroxetine, or morphine and methysergide. Conclusions: These results suggest that activation of both μ-opioid and serotonin receptors is required for the increased interaction to occur between morphine and SSRIs administered to reduce tactile allodynia. Thus, serotonin receptors take part in the antiallodynic action of morphine at the spinal level.

A Rat Model of Radicular Pain Induced by Chronic Compression of Lumbar Dorsal Root Ganglion with SURGIFLO ™

Radicular pain is a common and debilitating clinical pain condition. To date, the mechanisms of radicular pain remain unclear, partly because of the lack of suitable preclinical models. The authors report a modified rat model of radicular pain that could mimic a subset of clinical radicular pain conditions induced by the soft tissue compression on dorsal root ganglion. A rat model of radicular pain was produced by infiltrating the L5 intervertebral foramen with 60 microl of a hemostatic matrix (SURGIFLO; Johnson & Johnson, Somerville, NJ) resulting in chronic compression of lumbar dorsal root ganglion. Thermal hyperalgesia and mechanical allodynia were measured with or without epidural treatment with triamcinolone. Western blot was used to assess the expression of the NR1 subunit of the N-methyl-D-aspartate receptor and inhibitory factor kappabeta-alpha, an inflammatory marker, within the affected L5 dorsal root ganglion and spinal cord dorsal horn. Chronic compression of lumbar dorsal root ganglion resulted in: (1) persistent mechanical allodynia and thermal hyperalgesia up to 4 or 5 postoperative weeks and (2) up-regulation of the N-methyl-D-aspartate receptor and inhibitory factor kappabeta-alpha within the ipsilateral L5 dorsal root ganglion and spinal cord dorsal horn. Epidural administration of triamcinolone (6.25-100 microg) on postoperative day 3 dose-dependently attenuated both thermal hyperalgesia and mechanical allodynia in rats with chronic compression of lumbar dorsal root ganglion. The data suggest that this modified rat model of chronic compression of lumbar dorsal root ganglion may be a useful tool to explore the mechanisms as well as new therapeutic options of radicular pain.

The Interaction of Gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on Mechanical Allodynia in Rats with a Spinal Nerve Ligation

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED50). Fractions of ED50s were administered concurrently to establish the ED50 of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 μg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia.

Vanilloid receptor 1-positive neurons mediate thermal hyperalgesia and tactile allodynia

Background context The vanilloid receptor 1 (VR1) is expressed by the type II A-delta and C-fiber neurons, functioning as a molecular integrator for nociception. VR1 can be selectively ablated by resiniferatoxin (RTX), an ultra-potent excitotoxic agonist, when injected into sensory ganglia. Purpose To evaluate the role of the VR1-positive neurons in neuropathic pain. Study design Photochemical injury to rat sciatic nerve (Gazelius model). Methods Two groups of rats underwent the photochemical injury and RTX treatment. RTX was injected in the dorsal root ganglia (DRGs) of the L3, L4, L5, and L6 nerve roots, either after or before the nerve injury. The animals were tested for thermal hyperalgesia (noxious heat stimuli) and mechanical allodynia (von Frey filaments). Immunohistochemical analysis of the DRGs was performed after euthanasia. Results In the tactile allodynic rats, RTX injection in the DRGs improved the average withdrawal threshold from 1.62 g to 5.68 g. Immunohistochemical labeling showed that almost all VR1-positive neurons were eliminated. When RTX was administrated into the ipsilateral DRGs before the nerve injury, this treatment prevented the development of tactile allodynia in 12 out of 14 rats. Immunohistochemical staining revealed that the VR1-positive neurons were eliminated in the rats that did not develop tactile allodynia, whereas they were still present in the allodynic rats. Conclusions VR1-positive neurons are essential for the development of mechanical allodynia. In rats already exhibiting neuropathic pain, the VR1-positive neurons mediate the most sensitive part of mechanical allodynia. RTX injection in sensory ganglia may represent a novel treatment for neuropathic pain.

Characterization of the antinociceptive and anti-inflammatory activities of doxycycline and minocycline in different experimental models

Tetracyclines induce anti-inflammatory effects unrelated to their antimicrobial activities. We investigated the effect induced by minocycline and doxycycline in models of nociceptive and inflammatory pain, edema, fever, cell migration and formation of fibrovascular tissue, as these effects have not been fully investigated. Tetracyclines were administered via intraperitoneal route 1 h before the tests. Minocycline and doxycycline (100 mg/kg) inhibited the second phase of the formalin-induced nociceptive response in mice. Doxycycline (100 mg/kg) also inhibited the first phase. The nociceptive response induced by phorbol 12,13-didecanoate (PDD) in mice was inhibited by doxycycline (100 mg/kg). Furthermore, carrageenan-induced mechanical allodynia in rats was inhibited by doxycycline and minocycline (50 or 100 mg/kg). However, they did not enhance the latency in the hot-plate test. It is unlikely that antinociception resulted from motor incoordination or muscle relaxing effect, as both tetracyclines (100 mg/kg) did not impair the motor activity of mice in the rota-rod test. Doxycycline (50 or 100 mg/kg) or minocycline (50 or 100 mg/kg) inhibited carrageenan-induced paw edema in rats. However, only minocycline (100 mg/kg) inhibited PDD-induced edema. Carrageenan-induced leukocyte migration into the peritoneal cavity of rats was inhibited by both tetracyclines (100 mg/kg). Endotoxin-induced fever in rats was also inhibited by doxycycline (50 or 100 mg/kg) or minocycline (100 mg/kg). Finally, formation of fibrovascular tissue induced by subcutaneous implant of a cotton pellet in mice was inhibited by a 6-day administration of both tetracyclines (50 or 100 mg/kg day). Concluding, this study clearly shows the antinociceptive and anti-inflammatory activities of these second-generation tetracyclines.