Model Of Peripheral Neuropathic Pain Research Articles

Involvement of TRPM2 in a wide range of inflammatory and neuropathic pain mouse models

Recent evidence suggests a role of transient receptor potential melastatin 2 (TRPM2) in immune and inflammatory responses. We previously reported that TRPM2 deficiency attenuated inflammatory and neuropathic pain in some pain mouse models, including formalin- or carrageenan-induced inflammatory pain, and peripheral nerve injury-induced neuropathic pain models, while it had no effect on the basal mechanical and thermal nociceptive sensitivities. In this study, we further explored the involvement of TRPM2 in various pain models using TRPM2-knockout mice. There were no differences in the chemonociceptive behaviors evoked by intraplantar injection of capsaicin or hydrogen peroxide between wildtype and TRPM2-knockout mice, while acetic acid-induced writhing behavior was significantly attenuated in TRPM2-knockout mice. In the postoperative incisional pain model, no difference in mechanical allodynia was observed between the two genotypes. By contrast, mechanical allodynia in the monosodium iodoacetate-induced osteoarthritis pain model and the experimental autoimmune encephalomyelitis model were significantly attenuated in TRPM2-knockout mice. Furthermore, mechanical allodynia in paclitaxel-induced peripheral neuropathy and streptozotocin-induced painful diabetic neuropathy models were significantly attenuated in TRPM2-knockout mice. Taken together, these results suggest that TRPM2 plays roles in a wide range of pathological pain models based on peripheral and central neuroinflammation, rather than physiological nociceptive pain.

Angiotensin II type 2-receptor: new clinically validated target in the treatment of neuropathic pain.

Neuropathic pain is a large unmet medical need. The angiotensin II type 2 (AT2 ) receptor is a target with promising data in rodent models of peripheral neuropathic pain. The AT2 receptor has attracted attention on the basis of human data from a proof-of-concept clinical trial showing that oral EMA401, a highly selective, peripherally restricted, small molecule AT2 receptor antagonist, at 100 mg twice-daily for 4 weeks, alleviated postherpetic neuralgia, an often intractable type of peripheral neuropathic pain.

A Genome Wide Association Study Reveals Genetic Predisposition for Bortezomib-Induced Peripheral Neuropathy in Multiple Myeloma By Variation in the PREP1-Cbs Locus

Introduction: Bortezomib has become an important part of myeloma therapy, despite the occurrence of toxicities such as bortezomib induced peripheral neuropathy (BiPN). Since effective prophylactic treatment is lacking, onset of BiPN can only be remedied by dose reduction or stop of treatment. Here, using a genome-wide genotyping method, we investigated the potential genetic predisposition to BiPN in MM patients who received bortezomib-dexamethasone (VD) induction therapy prior to autologous stem-cell transplantation (ASCT).Methods: We performed a genome-wide association study using the Affymetrix SNP 6.0 platform. In total 469 cases from the IFM 2005-01, IFM2007-02 clinical trials or routine diagnostic were included as discovery cohort. Another 114 samples from the HOVON-65/GMMG-HD4 trial were used as validation. Patients with BiPN grade 2 or higher after initiation of bortezomib treatment were assigned as cases (n=155 in discovery, n=40 in validation) and the remaining patients that did not developed BiPN were considered controls (n=314 in discovery, n=74 in validation). Additional exclusion criteria were a minor allele frequency ≤ 5%, genotype frequency < 95% or Hardy Weinberg equilibrium p-value <1 x 10-5; 371,075 tagging SNPs were thus included for analysis. Association of SNPs to BiPN was tested using a Cochran-Armitage trend test. Six SNPs were found with parametric p-value < 1 x 10-5. These SNPs were validated using the validation cohort.Results: Of three loci identified by six SNPs in the discovery cohort, one previously unreported gene locus (rs2839629) remained associated to BiPN in the validation data set. This locus at 21q22.3 had odd ratios of 1.89 (p<1x10-6) and 2.02 (p = 0.02) in the discovery and validation cohorts, respectively. It is localized in the 3’ UTR of PBX/knotted 1 homeobox 1 (PKNOX1; alias PREP1), which encodes for a homeodomain transcription factor. Amongst others, PKNOX1 may modulate levels of chemokine monocyte chemoattractant protein-1 (MCP-1). MCP-1 is universally increased in different models of peripheral neuropathic pain and may be considered as a biomarker of chronic pain (Zhang and de Koninck, J. Neurochem. 97:772-783 (2006)). Haplotype analysis revealed a strong linkage disequilibrium (LD, r2 = 0.87) to the neighbouring gene CBS which encodes an endogenous H2S-producing enzyme. The CBS-H2S signalling pathway is implicated in the pathogenesis of a variety of neurodegenerative and inflammatory disorders, and specifically in neuropathy models (Takahashi et al., Pain, 150, 183-191, 2010).Conclusions: Our data provides evidence for susceptibility to BiPN in MM by variation in the PREP1-CBS locus, and suggests a new potential target in neuro-protective strategies of treatment. Validation of this finding may allow for the identification of patients at increased risk of BiPN which may benefit alternative treatments such as carfilzomib and better clinical management of this toxicity. DisclosuresSonneveld:Celgene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Onyx: Honoraria, Research Funding; Millenium: Honoraria, Research Funding. Moreau:Celgene Corporation: Honoraria, Membership on an entity’s Board of Directors or advisory committees.

Antinociceptive effects of sinomenine in a rat model of neuropathic pain.

Sinomenine is a principal ingredient of traditional Chinese medicine, Sinomenium Acutum, which has been reported to have various pharmacological effects including anti-rheumatism and immunomodulation. This study examined the effects of sinomenine in rats that received chronic constriction injury (CCI), a model of peripheral neuropathic pain. CCI injury on the right sciatic nerve led to long-lasting mechanical hyperalgesia. Acute sinomenine treatment (10–40 mg/kg, i.p.) significantly and dose-dependently reversed mechanical hyperalgesia. In addition, the antinociceptive effects of sinomenine remained stable during repeated daily treatment for up to 2 weeks. Although sinomenine did not alter the duration of immobility in the forced swimming test in healthy animals, it dose-dependently reversed the increased immobility time in rats receiving CCI, suggesting that sinomenine attenuated chronic pain-induced depressive-like behavior. The antinociceptive effects of sinomenine were blocked by the GABAa receptor antagonist bicuculine. The doses of sinomenine studied here did not significantly alter the spontaneous locomotor activity. Together, these results suggested that sinomenine exerts significant antinociceptive effects for neuropathic pain via GABAa-mediated mechanism, which suggests that sinomenine may be useful for the management of chronic painful conditions such as neuropathic pain.

Optimization and pharmacological characterization of a refined cisplatin-induced rat model of peripheral neuropathic pain.

Chemotherapy-induced peripheral neuropathy (CIPN) is the major dose-limiting side-effect of many front-line anticancer drugs. This study was designed to establish and pharmacologically characterize a refined rat model of cisplatin-induced CIPN. Adult male Sprague-Dawley rats received four (n=18) or five (n=18) single intraperitoneal bolus doses of cisplatin at 3 mg/kg, or saline (control group), once-weekly. Body weight and general health were assessed over a 49-day study period. von Frey filaments and the Hargreaves test were used to define the time course for the development of mechanical allodynia and thermal hypoalgesia in the hindpaws and for efficacy assessment of analgesic/adjuvant agents. The general health of rats administered four cisplatin doses was superior to that of rats administered five doses. Mechanical allodynia was fully developed (paw withdrawal thresholds≤6 g) in the bilateral hindpaws from day 32 to 49 for both cisplatin dosing regimens. They also showed significant thermal hypoalgesia in the bilateral hindpaws. In cisplatin-treated rats with paw withdrawal thresholds of up to 6 g, single bolus doses of gabapentin and morphine produced dose-dependent analgesia, whereas meloxicam and amitriptyline lacked efficacy. We have established and pharmacologically characterized a refined rat model of CIPN that is suitable for efficacy profiling of compounds from analgesic discovery programmes.

Stromal cell-derived CCL2 drives neuropathic pain states through myeloid cell infiltration in injured nerve.

Neuropathic pain resulting from peripheral nerve injury involves many persistent neuroinflammatory processes including inflammatory chemokines that control leukocyte trafficking and activate resident cells. Several studies have shown that CCL2 chemokine, a potent attractant of monocytes, and its cognate receptor, CCR2, play a critical role in regulating nociceptive processes during neuropathic pain. However, the role of CCL2 in peripheral leukocyte infiltration-associated neuropathic pain remains poorly understood. In particular, the contribution of individual CCL2-expressing cell populations (i.e. stromal and leukocytes) to immune cell recruitment into the injured nerve has not been established. Here, in preclinical model of peripheral neuropathic pain (i.e. chronic constriction injury of the sciatic nerve), we have demonstrated that, CCL2 content was increased specifically in nerve fibers. This upregulation of CCL2 correlated with local monocyte/macrophage infiltration and pain processing. Furthermore, sciatic intraneural microinjection of CCL2 in naïve animals triggered long-lasting pain behavior associated with local monocyte/macrophage recruitment. Using a specific CCR2 antagonist and mice with a CCL2 genetic deletion, we have also established that the CCL2/CCR2 axis drives monocyte/macrophage infiltration and pain hypersensitivity in the CCI model. Finally, specific deletion of CCL2 in stromal or immune cells respectively using irradiated bone marrow-chimeric CCI mice demonstrated that stromal cell-derived CCL2 (in contrast to CCL2 immune cell-derived) tightly controls monocyte/macrophage recruitment into the lesion and plays a major role in the development of neuropathic pain. These findings demonstrate that in chronic pain states, CCL2 expressed by sciatic nerve cells predominantly drove local neuro-immune interactions and pain-related behavior through CCR2 signaling.

Central administration of C‐X‐C chemokine receptor type 4 antagonist alleviates the development and maintenance of peripheral neuropathic pain in mice (LB618)

C‐X‐C chemokine receptor type 4 (CXCR4) is an emerging area of pain hypersensitivity in the peripheral nervous system; however, its role in the central nervous system (CNS) for pain processing remains unclear. This study aimed at exploring the function of central CXCR4 in pain processing in vivo using its specific antagonist AMD3100 and partial sciatic nerve ligation (pSNL) model of peripheral neuropathic pain in C57BL/6 mice. A single intrathecal (central) administration of AMD3100 (intrathecal AMD3100, 5 μg) was found to prevent the development of pSNL‐induced allodynia. Intrathecal AMD3100 (1 μg, 5 μg and 25 μg) also reversed the established pSNL‐induced mechanical allodynia in a dose‐dependent way on post‐operative day 7 (POD 7). In rotarod test, intrathecal AMD3100 of different dosages (1 μg, 5 μg and 25 μg) did not impair the motor function of mice. Among mitogen‐activated protein kinases (MAPKs) pathways, intrathecal AMD3100 was found to downregulate the activation of JNK1 and p38 pathway signaling proteins in the L3‐L5 spinal cord segment as assessed by western blotting on POD 7. Our results firstly suggest that central (spinal) CXCR4 appears to be involved in the development and maintenance of peripheral neuropathic pain, implicating that CXCR4 would potentially be a therapeutic target for chronic neuropathic pain.Grant Funding Source: Supported by the departmental fund, Department of Anaesthesiology, The University of Hong Kong

Antihyperalgesic effects of imidazoline I2 receptor ligands in rat models of inflammatory and neuropathic pain

A new imidazoline I2 receptor ligand, CR4056, is effective for chronic inflammatory pain and diabetic neuropathy. However, it is unclear whether other I2 receptor ligands have similar effects and whether antinociceptive tolerance develops with repeated treatment. The Von Frey filament test was used to measure mechanical hyperalgesia and the plantar test to measure thermal hyperalgesia in rats injected with complete Freund's adjuvant (CFA) treatment or had undergone surgery to induce chronic constriction injury (CCI), models of inflammatory pain and peripheral neuropathic pain respectively. The effects of morphine and I2 receptor ligands, 2-BFI, BU224, tracizoline and CR4056, 3.2-32 mg·kg(-1) , i.p., on hyperalgesia or affective pain (as measured by a place escape/avoidance paradigm) were studied in separate experiments. Morphine and the I2 receptor ligands (2-BFI, BU224 and tracizoline) all dose-dependently attenuated mechanical and thermal hyperalgesia in CFA-treated rats. The anti-hyperalgesic effects of 2-BFI in CFA-treated and CCI rats were attenuated by the I2 receptor antagonist idazoxan. The combination of 2-BFI and morphine produced additive effects against mechanical hyperalgesia in CFA-treated rats. Repeated treatment (daily for 7-9 days) with 2-BFI or CR4056 did not produce antinociceptive tolerance in CFA-treated or CCI rats. Morphine and the I2 receptor ligands (2-BFI, BU224 and CR4056) were all effective at attenuating place escape/avoidance behaviour in CFA-treated rats. Imidazoline I2 receptor ligands have antihyperalgesic effects in rat models of inflammatory and neuropathic pain and may represent a new class of pharmacotherapeutics for the management of chronic pain.

Roles of Transient Receptor Potential Melastatin 2 Expressed on Immune Cells in Neuropathic Pain

Neuropathic pain is a pathological pain condition that often results from peripheral nerve injury. Several lines of evidence suggest that neuroinflammation mediated by the interaction between immune cells and neurons plays an important role in the pathogenesis of neuropathic pain. Transient receptor potential melastatin 2 (TRPM2) is a nonselective Ca(2+)-permeable cation channel that acts as a sensor for reactive oxygen species. Recent evidence suggests that TRPM2 expressed on immune cells plays an important role in immune and inflammatory responses. In this study, we examined the roles of TRPM2 expressed on immune and glial cells in neuropathic pain. TRPM2 deficiency attenuated pain behaviors (mechanical allodynia, thermal hyperalgesia and spontaneous pain behaviors) in various kinds of inflammatory and neuropathic pain, but not in nociceptive pain models. In peripheral nerve injury-induced neuropathic pain models, TRPM2 deficiency diminished infiltration of neutrophils mediated through CXCL2 production from macrophages around the injured peripheral nerve and activation of spinal microglia, suggesting that TRPM2 expressed on macrophages and microglia aggravates peripheral and spinal pronociceptive inflammatory responses. Furthermore, we examined the infiltration of peripheral immune cells into the injured nerve and spinal cord using bone marrow chimeric mice by crossing wildtype and TRPM2-knockout mice. The results suggest that TRPM2 plays an important role in the infiltration of peripheral immune cells, particularly macrophages, into the spinal cord, rather than into the injured nerves. The spinal infiltration of macrophages mediated by TRPM2 may contribute to the pathogenesis of neuropathic pain.

Ion channels and pain: Important steps towards validating a new therapeutic target for neuropathic pain

Acute pain is caused by noxious stimuli, and by stimuli that threaten to or cause tissue damage. Pain serves a crucial physiological function as a rapid warning system that can help prevent injury or limit the extent of damage. However, chronic pain, which is pain that persists after the initial noxious stimulus, tissue damage, and subsequent healing periods have passed, and is elicited by stimulus types or levels that do not normally elicit pain, is a major clinical challenge. Most chronic pain patients complain of a lack of complete relief from currently available drugs, and many of these drugs have adverse side effects. As such, there are intensive basic research and drug discovery efforts focused on developing better treatments for chronic pain (http://www.ninds.nih.gov/disorders/chronic_pain/chronic_pain.htm). One major form of chronic pain is neuropathic, in that it is based in the neurons involved in perception of pain, and the transduction, or processing of the resultant pain signals to the brain. Peripheral neuropathic pain is due to altered function and sensitization of neurons within the peripheral nociceptive system (i.e., nociceptive neurons), the sensory system responsible for the perception of pain and the transduction of pain signals to the spinal cord (http://www.iasppainorg/AM/Template.cfm?Section=Pain_Definitions.). A recent paper published in Experimental Neurology (Tsantoulas et al., 2014) identifies a novel contributor to peripheral neuropathic pain that may represent an attractive target for future drug discovery efforts aimed at ameliorating this form of chronic pain. Sensory ganglia such as trigeminal ganglia, and dorsal root ganglia (DRG), contain a variety of primary sensory neurons, with distinct morphologies and functions. Sensory neurons with medium and large diameter cell bodies have myelinated axons or A fibers that transduce signals from a variety of sensory modalities, and while those with small diameter cell bodies give rise to unmyelinated C fibers that transduce painful stimuli [reviewed in (Lawson, 2002)]. Within these major classes of neurons are those with a diversity of morphologies and functions, allowing for the selective discrimination of a wide variety of painful and non-painful stimuli. Within these neurons are the ion channels that underlie the fundamental steps of sensory transduction, namely the initiation of a propagating electrical signal at the sensory nerve endings in tissues, its conduction along the axon, and its transmission from the central terminals to neurons in the spinal cord [reviewed in (Gold and Gebhart, 2010)]. There are also a number of regulatory ion channels that shape diverse aspects of each of these events. When dysregulated, these ion channels that underlie the normal function of nociceptive neurons can contribute to the pathological states of neuropathic pain [reviewed in (Gold and Gebhart, 2010)]. The recent paper by Tsantoulas et al., (Tsantoulas et al., 2014) focuses on the role of a specific family of ion channels that serves as a critical component of the sensory transduction machinery in DRG neurons. The expression of these channels is reduced in an animal model of peripheral neuropathic pain. The authors were able to link this downregulation to a crucial aspect of peripheral neuropathic pain, the hyperexcitability of the nociceptive neurons that leads to ectopic transduction of pain signals to the spinal cord in the absence of a painful stimulus. This study not only identifies an important role for this particular class of ion channels, but also suggests that enhancing the activity of the remaining channels represents an attractive approach for future drug discovery efforts targeting neuropathic pain.

Discovery of tetrahydropyrido[4,3-d]pyrimidine derivatives for the treatment of neuropathic pain

A series of tetrahydropyridopyrimidine derivatives were synthesized and evaluated for neurotoxicity and peripheral analgesic activity followed by assessment of antiallodynic and antihyperalgesic potential in two peripheral neuropathic pain models, the chronic constriction injury (CCI) and partial sciatic nerve ligation (PSNL). Compounds (4b and 4d) exhibiting promising efficacies in four behavioral assays of allodynia and hyperalgesia (spontaneous pain, tactile allodynia, cold allodynia and mechanical hyperalgesia) were quantified for their ED50 values (15.12–65.10mg/kg). Studies carried out to assess the underlying mechanism revealed that the compounds suppressed the inflammatory component of the neuropathic pain and prevented oxidative and nitrosative stress.

Exploration of supraspinal mechanisms in effects of spinal cord stimulation: Role of the locus coeruleus

The neurobiological mechanisms of spinal cord stimulation (SCS) when applied for neuropathic pain are still incompletely known. Previous research indicates that brainstem circuitry is pivotal for the SCS effect. The present study aims at exploring the possible contribution to the SCS effects of the pain controlling system emanating from the locus coeruleus (LC) in the brain stem. Experiments were performed on the rat-spared nerve injury pain model. After evaluation of the attenuation of mechanical hypersensitivity induced by SCS, the effects of SCS on neuronal activity in the LC and on the noradrenaline (NA) content in the dorsal spinal cord were analyzed. SCS produced a significant increase in the discharge rate of LC neurons only in rats behaviorally responding to SCS as compared to non-responding and control animals. The NA content in the dorsal quadrant of the spinal cord ipsilateral to the nerve injury was analyzed using enzyme-linked immunosorbent assay in responding, non-responding and intact control rats both immediately following SCS and without SCS. No differences were found between these groups. In awake animals, lidocaine silencing of the ipsilateral LC or blocking of spinal noradrenergic system by intrathecal administration of α1,2 adrenoceptor antagonists failed to influence the antihypersensitivity effect of SCS. The present results indicate that the SCS-induced control of hypersensitivity in an experimental animal model of peripheral neuropathic pain may not be explained by the activation of direct spinal projections of noradrenergic LC neurons, while supraspinal projections of LC neurons still may play a role in the SCS effect.

An LPA Species (18:1 LPA) Plays Key Roles in the Self-Amplification of Spinal LPA Production in the Peripheral Neuropathic Pain Model

BackgroundWe previously reported that nerve injury-induced neuropathic pain is initiated by newly produced lysophosphatidic acid (LPA).ResultsIn this study, we developed a quantitative mass spectrometry for detecting LPA species by using Phos-tag. Following nerve injury, the levels of 18:1, 16:0 and 18:0 LPA in the spinal dorsal horn significantly increased at 3 h and declined at 6 h. Among them, 18:1 LPA level was the most abundant. In the same preparation, there were significant elevations in the activities of cytosolic phospholipase A2 (cPLA2) and calcium-independent phospholipase A2 (iPLA2), key enzymes for LPA synthesis, at 1 h, while there was no significant change in phospholipase A1 activity. Pharmacological studies revealed that NMDA and neurokinin 1 receptors, cPLA2, iPLA2 and microglial activation, as well as LPA1 and LPA3 receptors were all involved in the nerve injury-induced LPA production, and underlying cPLA2 and iPLA2 activations. In the cells expressing LPA1 or LPA3 receptor, the receptor-mediated calcium mobilization was most potent with 18:1 LPA, compared with 16:0 or 18:0 LPA. Moreover, the intrathecal injection of 18:1 LPA, but not 16:0 or 18:0 LPA, caused a spinal LPA production and neuropathic pain-like behavior.ConclusionThese results suggest that 18:1 LPA is the predominant ligand responsible for LPA1 and LPA3 receptors-mediated amplification of LPA production through microglial activation.

Antihyperalgesic activity of chlorogenic acid in experimental neuropathic pain

Chlorogenic acid (CGA) is a natural organic phenolic compound that is found in many plants, fruits and vegetables. CGA has beneficial bioactivities and strong therapeutic effects in inflammatory processes. CGA-rich fractions have analgesic activity but CGA has not been tested previously in neuropathic pain, which results from tissue damage, inflammation or injury of the nervous system. Chronic constrictive nerve injury (CCI) is a peripheral neuropathic pain model which initiates an inflammatory cascade. We aimed to determine possible antihyperalgesic effects of CGA in neuropathic pain. Our study showed for the first time that CGA [50, 100 and 200 mg/kg; intraperitoneally (i.p.)] produced significant dose- and time-dependent antihyperalgesic activity in CCI-induced neuropathic pain. In addition, chronic administration of CGA (100 mg/kg/day; i.p. for 14 days) prevented the development of mechanical hyperalgesia and attenuated CCI-induced histopathological changes. On the other hand, CGA (200 mg/kg) did not affect falling latencies of rats in the rota rod test. Hence, CGA might represent a novel potential therapeutic option for the management of neuropathic pain.

Discovery of Fused Triazolo-thiadiazoles as Inhibitors of TNF-alpha: Pharmacophore Hybridization for Treatment of Neuropathic Pain.

IntroductionNeuropathic pain is a complex, chronic pain state that is usually accompanied by tissue injury. With neuropathic pain, the nerve fibers themselves may be damaged, dysfunctional, or injured.MethodsA series of pharmacophoric hybrids of substituted aryl semicarbazides incorporated into a fused triazolo-thiadiazole nucleus were synthesized and evaluated for neuropathic pain activity. After the assessment of neurotoxicity and peripheral analgesic activity, the compounds were evaluated in two peripheral neuropathic pain models, the chronic constriction injury and partial sciatic nerve ligation, to assess their antiallodynic and antihyperalgesic potential.ResultsSelected compounds exhibiting promising efficacies (4b, 6a, and 7e) revealed median effective dose (ED50) values ranging from 7.62–28.71 mg/kg in four behavioral assays of allodynia and hyperalgesia (spontaneous pain, tactile allodynia, cold allodynia, and mechanical hyperalgesia). Studies carried out to assess the underlying mechanism revealed that compounds suppressed the inflammatory component of the neuropathic pain by inhibiting tumor necrosis factor (TNF)-alpha and preventing oxidative and nitrosative stress.ConclusionUsing a hybrid design approach, the present study identified novel chemical compounds that could be a potential lead for the treatment of neuropathic pain.

Novel Piperazinyl Derivatives with Anti-Hyperalgesic, Anti-Allodynic and Anti-Inflammatory activities Useful for the Treatment of Neuropathic Pain

A series of structurally novel compounds possessing 2-phenylpiperazin-1-yl nicotinamide template was synthesized and evaluated for neuropathic pain activity. After the assessment of neurotoxicity and peripheral analgesic activity, the compounds were evaluated in a peripheral neuropathic pain model, the chronic constriction injury (CCI) to assess their antiallodynic and antihyperalgesic potential. Studies carried out to assess the underlying mechanism revealed that the compounds (5 and 6) suppressed the inflammatory component of the neuropathic pain and inhibited oxidative and nitrosative stress.

Increasing TNF levels solely in the rat hippocampus produces persistent pain-like symptoms

The manifestation of chronic, neuropathic pain includes elevated levels of the cytokine tumor necrosis factor–alpha (TNF). Previously, we have shown that the hippocampus, an area of the brain most notable for its role in learning and memory formation, plays a fundamental role in pain sensation. Using an animal model of peripheral neuropathic pain, we have demonstrated that intracerebroventricular infusion of a TNF antibody adjacent to the hippocampus completely alleviated pain. Furthermore, intracerebroventricular infusion of rTNF adjacent to the hippocampus induced pain behavior in naïve animals similar to that expressed during a model of neuropathic pain. These data support our premise that enhanced production of hippocampal-TNF is integral in pain sensation. In the present study, TNF gene expression was induced exclusively in the hippocampus, eliciting increased local bioactive TNF levels, and animals were assessed for pain behaviors. Male Sprague-Dawley rats received stereotaxic injection of gold nanorod (GNR)-complexed cDNA (control or TNF) plasmids (nanoplasmidexes), and pain responses (i.e., thermal hyperalgesia and mechanical allodynia) were measured. Animals receiving hippocampal microinjection of TNF nanoplasmidexes developed thermal hyperalgesia bilaterally. Sensitivity to mechanical stimulation also developed bilaterally in the rat hind paws. In support of these behavioral findings, immunoreactive staining for TNF, bioactive levels of TNF, and levels of TNF mRNA per polymerase chain reaction analysis were assessed in several brain regions and found to be increased only in the hippocampus. These findings indicate that the specific elevation of TNF in the hippocampus is not a consequence of pain, but in fact induces these behaviors/symptoms.

Analgesic effect of magnetic stimulation on paclitaxel-induced peripheral neuropathic pain in mice

Peripheral neuropathies are common side effects of chemotherapeutic drugs, including taxanes, platinum-based drugs, vinca alkaloids, and thalidomide. The most common symptoms are numbness, tingling and/or burning pain in a stocking-glove distribution. Severe peripheral neuropathies result in dose reductions, a change in chemotherapy regimen, or early cessation of chemotherapy. There are no proven interventions to prevent or treat chemotherapy-induced peripheral neuropathy. We designed and built a unique magnetic stimulator to clarify the effects of magnetic stimulation in the mouse paclitaxel-induced peripheral neuropathic pain model. Magnetic stimulation significantly reversed paclitaxel-induced mechanical hyperalgesia. The analgesic efficacy of magnetic stimulation was inhibited by naloxone, a μ opioid receptor antagonist. These findings indicate that the analgesic effect of magnetic stimulation is likely to be mediated by the endogenous opioid system. Furthermore, a combination of magnetic stimulation and pregabalin, a Ca2+ channel blocker, induced a potent combinational analgesic effect, suggesting that analgesic drug dose reduction might be possible. These findings indicate that there is a potential therapeutic utility for magnetic stimulation in pain relief.

The correlation between pain‐related behaviour and spinal microgliosis in four distinct models of peripheral neuropathy

Peripheral nerve injury is associated with a spinal microglial response that has been correlated with the development of behaviours reflective of neuropathic pain. To examine whether this phenomenon is generalizable to neuropathic pain of non-traumatic aetiology, this study investigated the association between spinal microgliosis and behavioural measures of neuropathic hypersensitivity and pain-related anxiety behaviour in four distinct rat models of peripheral neuropathic pain. These were traumatic neuropathy [L5 spinal nerve transection (SNT)], HIV-related neuropathies (either treatment with the antiretroviral drug Zalcitabine (ddC) or combination of perineural exposure to the HIV-gp120 protein and ddC treatment) and varicella zoster virus (VZV) infection. Persistent mechanical hypersensitivity developed in all 'neuropathic' rats. However, spinal microgliosis, as measured by increased CD11b/c immunohistochemical staining and increased numbers of cells expressing CD11b measured by flow cytometry, was evident in the SNT and to a lesser extent in the HIV neuropathy models but not the VZV model. These results suggest that behavioural hypersensitivity and thigmotaxis can only be linked to a microglial response in certain models of neuropathy.

Differential adaptive changes on serotonin and noradrenaline transporters in a rat model of peripheral neuropathic pain

Serotonin and noradrenaline reuptake inhibitors have shown to produce antinociceptive effects in several animal models of neuropathic pain. In the present work, we have analyzed the density of brain and spinal serotonin and noradrenaline transporters (5-HTT and NAT) in a rat model of neuropathic pain, the spinal nerve ligation (SNL). Quantitative autoradiography revealed a significant decrease in the density of 5-HTT ([3H]citalopram binding) at the level of the lumbar spinal cord following 2 weeks of neuropathic surgery (lamina V, −40%: 6.01±0.64nCi/mg tissue in sham-animals vs 3.59±1.56 in SNL-animals; lamina X, −30%: 9.10±2.00 vs 6.40±1.93 and lamina IX, −22%: 12.01±2.41 vs 9.42±1.58). By contrast, NAT density ([3H]nisoxetine binding) was significantly increased (lamina I–II, +34%: 2.20±0.45 vs 2.96±0.65; lamina V, +57%: 1.34±0.28 vs 2.11±0.66; and lamina IX, +58%: 2.39±0.71 vs 3.78±1.10). At supraspinal structures, SNL induced adaptive changes only in the density of 5-HTT (septal nuclei, +33%: 10.18±2.03 vs 13.53±1.14; CA3 field of hippocampus, +18%: 6.94±1.01 vs 8.21±0.81; paraventricular thalamic nucleus, +21%: 15.18±1.88 vs 18.35±2.08; lateral hypothalamic area, +40%: 12.68±1.90 vs 17.8±2.55; ventromedial hypothalamic nuclei, +19%: 7.16±0.92 vs 8.55±0.40; and dorsal raphe nucleus, +15%: 35.22±3.88 vs 40.68±3.11). Thus, we demonstrate, in the SNL model of neuropathic pain, the existence of opposite changes in the spinal expression of 5-HTT (down-regulation) and NAT (up-regulation), and the presence of supraspinal adaptive changes (up-regulation) only on 5-HTT density. These findings may help understanding the pathogeny of neuropathic pain and the differential analgesic action of antidepressants targeting 5-HTT and/or NAT transporters.