Cyclooxygenase 2 expression in the spared nerve injury model of neuropathic pain

Endocannabinoid biosynthetic enzymes regulate pain response via LKB1-AMPK signaling

Diacylglycerol lipase-beta (DAGLβ) serves as a principal 2-arachidonoylglycerol (2-AG) biosynthetic enzyme regulating endocannabinoid and eicosanoid metabolism in immune cells including macrophages and dendritic cells. Genetic or pharmacological inactivation of DAGLβ ameliorates inflammation and hyper-nociception in preclinical models of pathogenic pain. These beneficial effects have been assigned principally to reductions in downstream proinflammatory lipid signaling, leaving alternative mechanisms of regulation largely underexplored. Here, we apply quantitative chemical- and phospho-proteomics to find that disruption of DAGLβ in primary macrophages leads to LKB1-AMPK signaling activation, resulting in reprogramming of the phosphoproteome and bioenergetics. Notably, AMPK inhibition reversed the antinociceptive effects of DAGLβ blockade, thereby directly supporting DAGLβ-AMPK crosstalk in vivo. Our findings uncover signaling between endocannabinoid biosynthetic enzymes and ancient energy-sensing kinases to mediate cell biological and pain responses.

Cathepsin B Gene Knockout Improves Behavioral Deficits and Reduces Pathology in Models of Neurologic Disorders

Cathepsin B (CTSB) is a powerful lysosomal protease. This review evaluated CTSB gene knockout (KO) outcomes for amelioration of brain dysfunctions in neurologic diseases and aging animal models. Deletion of the CTSB gene resulted in significant improvements in behavioral deficits, neuropathology, and/or biomarkers in traumatic brain injury, ischemia, inflammatory pain, opiate tolerance, epilepsy, aging, transgenic Alzheimer's disease (AD), and periodontitis AD models as shown in 12 studies. One study found beneficial effects for double CTSB and cathepsin S KO mice in a multiple sclerosis model. Transgenic AD models using amyloid precursor protein (APP) mimicking common sporadic AD in three studies showed that CTSB KO improved memory, neuropathology, and biomarkers; two studies used APP representing rare familial AD and found no CTSB KO effect, and two studies used highly engineered APP constructs and reported slight increases in a biomarker. In clinical studies, all reports found that CTSB enzyme was upregulated in diverse neurologic disorders, including AD in which elevated CTSB was positively correlated with cognitive dysfunction. In a wide range of neurologic animal models, CTSB was also upregulated and not downregulated. Further, human genetic mutation data provided precedence for CTSB upregulation causing disease. Thus, the consilience of data is that CTSB gene KO results in improved brain dysfunction and reduced pathology through blockade of CTSB enzyme upregulation that causes human neurologic disease phenotypes. The overall findings provide strong support for CTSB as a rational drug target and for CTSB inhibitors as therapeutic candidates for a wide range of neurologic disorders. SIGNIFICANCE STATEMENT: This review provides a comprehensive compilation of the extensive data on the effects of deleting the cathepsin B (CTSB) gene in neurological and aging mouse models of brain disorders. Mice lacking the CTSB gene display improved neurobehavioral deficits, reduced neuropathology, and amelioration of neuronal cell death and inflammatory biomarkers. The significance of the compelling CTSB evidence is that the data consilience validates CTSB as a drug target for discovery of CTSB inhibitors as potential therapeutics for treating numerous neurological diseases.

Peroxisome Metabolism Contributes to PIEZO2-Mediated Mechanical Allodynia

Mutations in the peroxisomal half-transporter ABCD1 cause X-linked adrenoleukodystrophy, resulting in elevated very long-chain fatty acids (VLCFA), progressive neurodegeneration and an associated pain syndrome that is poorly understood. In the nervous system of mice, we found ABCD1 expression to be highest in dorsal root ganglia (DRG), with satellite glial cells (SGCs) displaying higher expression than neurons. We subsequently examined sensory behavior and DRG pathophysiology in mice deficient in ABCD1 compared to wild-type mice. Beginning at 8 months of age, Abcd1-/y mice developed persistent mechanical allodynia. DRG had a greater number of IB4-positive nociceptive neurons expressing PIEZO2, the mechanosensitive ion channel. Blocking PIEZO2 partially rescued the mechanical allodynia. Beyond affecting neurons, ABCD1 deficiency impacted SGCs, as demonstrated by high levels of VLCFA, increased glial fibrillary acidic protein (GFAP), as well as genes disrupting neuron-SGC connectivity. These findings suggest that lack of the peroxisomal half-transporter ABCD1 leads to PIEZO2-mediated mechanical allodynia as well as SGC dysfunction. Given the known supportive role of SGCs to neurons, this elucidates a novel mechanism underlying pain in X-linked adrenoleukodystrophy.

Nuclear Factor κB-COX2 Pathway Activation in Non-myelinating Schwann Cells Is Necessary for the Maintenance of Neuropathic Pain in vivo

Chronic neuropathic pain leads to long-term changes in the sensitivity of both peripheral and central nociceptive neurons. Glial fibrillary acidic protein (GFAP)-positive glial cells are closely associated with the nociceptive neurons including astrocytes in the central nervous system (CNS), satellite glial cells (SGCs) in the sensory ganglia, and non-myelinating Schwann cells (NMSCs) in the peripheral nerves. Central and peripheral GFAP-positive cells are involved in the maintenance of chronic pain through a host of inflammatory cytokines, many of which are under control of the transcription factor nuclear factor κB (NFκB) and the enzyme cyclooxygenase 2 (COX2). To test the hypothesis that inhibiting GFAP-positive glial signaling alleviates chronic pain, we used (1) a conditional knockout (cKO) mouse expressing Cre recombinase under the hGFAP promoter and a floxed COX2 gene to inactivate the COX2 gene specifically in GFAP-positive cells; and (2) a tet-Off tetracycline transactivator system to suppress NFκB activation in GFAP-positive cells. We found that neuropathic pain behavior following spared nerve injury (SNI) significantly decreased in COX2 cKO mice as well as in mice with decreased glial NFκB signaling. Additionally, experiments were performed to determine whether central or peripheral glial NFκB signaling contributes to the maintenance of chronic pain behavior following nerve injury. Oxytetracycline (Oxy), a blood-brain barrier impermeable analog of doxycycline was employed to restrict transgene expression to CNS glia only, leaving peripheral glial signaling intact. Signaling inactivation in central GFAP-positive glia alone failed to exhibit the same analgesic effects as previously observed in animals with both central and peripheral glial signaling inhibition. These data suggest that the NFκB-COX2 signaling pathway in NMSCs is necessary for the maintenance of neuropathic pain in vivo.

A Glra3 phosphodeficient mouse mutant establishes the critical role of protein kinase A-dependent phosphorylation and inhibition of glycine receptors in spinal inflammatory hyperalgesia

ABSTRACT

Glycinergic neurons and glycine receptors (GlyRs) exert a critical control over spinal nociception. Prostaglandin E2 (PGE2), a key inflammatory mediator produced in the spinal cord in response to peripheral inflammation, inhibits a certain subtype of GlyRs (α3GlyR) that is defined by the inclusion of α3 subunits and distinctly expressed in the lamina II of the spinal dorsal horn, ie, at the site where most nociceptive nerve fibers terminate. Previous work has shown that the hyperalgesic effect of spinal PGE2 is lost in mice lacking α3GlyRs and suggested that this phenotype results from the prevention of PGE2-evoked protein kinase A (PKA)-dependent phosphorylation and inhibition of α3GlyRs. However, direct proof for a contribution of this phosphorylation event to inflammatory hyperalgesia was still lacking. To address this knowledge gap, a phospho-deficient mouse line was generated that carries a serine to alanine point mutation at a strong consensus site for PKA-dependent phosphorylation in the long intracellular loop of the GlyR α3 subunit. These mice showed unaltered spinal expression of GlyR α3 subunits. In behavioral experiments, they showed no alterations in baseline nociception, but were protected from the hyperalgesic effects of intrathecally injected PGE2 and exhibited markedly reduced inflammatory hyperalgesia. These behavioral phenotypes closely recapitulate those found previously in GlyR α3-deficient mice. Our results thus firmly establish the crucial role of PKA-dependent phosphorylation of α3GlyRs in inflammatory hyperalgesia.

Manganese Oxide Nanozymes Ameliorate Mechanical Allodynia in a Rat Model of Partial Sciatic Nerve-Transection Induced Neuropathic Pain

BACKGROUND

Reactive oxygen species (ROS) induced oxidative stress is linked to numerous neurological diseases, including neuropathic pain. Natural ROS scavenging enzymes like superoxide dismutase (SOD) and catalase have been found to be efficient in alleviating neuropathic pain. However, their sensitivity towards extreme pH and a short half-life limit their efficacy in vivo. Manganese oxide nanoparticles (MONPs) are recently known to possess ROS scavenging properties. In this study, MONPs were examined for their therapeutic effect on neuropathic pain.

METHODS

The MONPs were synthesized by a hydrothermal method. The synthesized MONPs were characterized by UV/Vis, TEM, SEM, FTIR, NTA and XRD. The biocompatibility of the nanoparticles is evaluated in neural cells using LDH assay. MONPs were evaluated for their antioxidant activity by DPPH assay. In addition, in vitro ROS scavenging properties were examined in bone marrow-derived macrophage (BMDM) cells using 2',7'-dichlorofluorescin diacetate (DCFDA) assay. To evaluate the in vivo efficacy of nanoparticles, neuropathic pain was induced in Wistar rats by partial sciatic nerve transection (PSNT). On post-transection days 14 to 18, rats were intrathecally injected with MONPs and paw withdrawal threshold was measured. The spinal cords were collected and processed for Western blotting and histological analysis.

RESULTS

The synthesized MONPs were biocompatible and showed effective antioxidant activity against DPPH free radical scavenging. Further, the nanoparticles scavenged ROS efficiently in vitro in BMDM and their intrathecal administration significantly reduced mechanical allodynia as well as the expression of cyclooxygenase-2 (COX-2), an important mediator of chronic and inflammatory pain in the spinal dorsal horns of PSNT rats.

CONCLUSION

As ROS play a significant role in neuropathic pain, we expect that MONPs could be a promising tool for the treatment of various inflammatory diseases and might also serve as a potential nanocarrier for the delivery of analgesics.

Neuroprotective effects of hyperbaric oxygen (HBO) therapy on neuronal death induced by sciatic nerve transection in rat

Background

Recent studies shows that hyperbaric oxygen (HBO) therapy exerts some protective effects against neural injuries. The purpose of this study was to determine the neuroprotective effects of HBO following sciatic nerve transection (SNT).

Methods

Rats were randomly divided into five groups (n = 14 per group): Sham-operated (SH) group, SH + HBO group, SNT group, and SNT + pre- and SNT + post-HBO groups (100% oxygen at 2.0 atm absolute, 60 min/day for five consecutive days beginning on 1 day before and immediately after nerve transaction, respectively). Spinal cord segments of the sciatic nerve and related dorsal root ganglions (DRGs) were removed 4 weeks after nerve transection for biochemical assessment of malodialdehyde (MDA) levels in spinal cord, biochemical assessment of superoxide dismutase (SOD) and catalse (CAT) activities in spinal cord, immunohistochemistry of caspase-3, cyclooxigenase-2 (COX-2), S100beta (S100ß), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) in spinal cord and DRG.

Results

The results revealed that MDA levels were significantly decreased in the SNT + pre-HBO group, while SOD and CAT activities were significantly increased in SNT + pre- and SNT + post-HBO treated rats. Attenuated caspase-3 and COX-2 expression, and TUNEL reaction could be significantly detected in the HBO-treated rats after nerve transection. Also, HBO significantly increased S100ß expression.

Conclusions

Based on these results, we can conclude that pre- and post-HBO therapy had neuroprotective effects against sciatic nerve transection-induced degeneration.

Is there a reasonable excuse for not providing post-operative analgesia when using animal models of peripheral neuropathic pain for research purposes?

Introduction

The induction of neuropathic pain-like behaviors in rodents often requires surgical intervention. This engages acute nociceptive signaling events that contribute to pain and stress post-operatively that from a welfare perspective demands peri-operative analgesic treatment. However, a large number of researchers avoid providing such care based largely on anecdotal opinions that it might interfere with model pathophysiology in the longer term.

Objectives

To investigate effects of various peri-operative analgesic regimens encapsulating different mechanisms and duration of action, on the development of post-operative stress/welfare and pain-like behaviors in the Spared Nerve Injury (SNI)-model of neuropathic pain.

Methods

Starting on the day of surgery, male Sprague-Dawley rats were administered either vehicle (s.c.), carprofen (5.0mg/kg, s.c.), buprenorphine (0.1mg/kg s.c. or 1.0mg/kg p.o. in Nutella^®), lidocaine/bupivacaine mixture (local irrigation) or a combination of all analgesics, with coverage from a single administration, and up to 72 hours. Post-operative stress and recovery were assessed using welfare parameters, bodyweight, food-consumption, and fecal corticosterone, and hindpaw mechanical allodynia was tested for assessing development of neuropathic pain for 28 days.

Results

None of the analgesic regimes compromised the development of mechanical allodynia. Unexpectedly, the combined treatment with 0.1mg/kg s.c. buprenorphine and carprofen for 72 hours and local irrigation with lidocaine/bupivacaine, caused severe adverse effects with peritonitis. This was not observed when the combination included a lower dose of buprenorphine (0.05mg/kg, s.c.), or when buprenorphine was administered alone (0.1mg/kg s.c. or 1.0mg/kg p.o.) for 72 hours. An elevated rate of wound dehiscence was observed especially in the combined treatment groups, underlining the need for balanced analgesia. Repeated buprenorphine injections had positive effects on body weight the first day after surgery, but depressive effects on food intake and body weight later during the first week.

Conclusion

Post-operative analgesia does not appear to affect established neuropathic hypersensitivity outcome in the SNI model.

Microglia control the glycinergic but not the GABAergic synapses via prostaglandin E2 in the spinal cord

Microglia can influence the excitatory responses of neurons, but less is known about how these immune cells in the brain may influence inhibitory neurotransmitters. Cantaut-Belarif et al. report that prostaglandin production by Toll-like receptor–stimulated microglia can influence the glycinergic but not GABAergic responses of neurons by altering the lateral diffusion of glycine receptors specifically within the synaptic membrane.

Microglia control excitatory synapses, but their role in inhibitory neurotransmission has been less well characterized. Herein, we show that microglia control the strength of glycinergic but not GABAergic synapses via modulation of the diffusion dynamics and synaptic trapping of glycine (GlyR) but not GABA_A receptors. We further demonstrate that microglia regulate the activity-dependent plasticity of glycinergic synapses by tuning the GlyR diffusion trap. This microglia–synapse cross talk requires production of prostaglandin E2 by microglia, leading to the activation of neuronal EP2 receptors and cyclic adenosine monophosphate–dependent protein kinase. Thus, we now provide a link between microglial activation and synaptic dysfunctions, which are common early features of many brain diseases.

Microglial TNFα Induces COX2 and PGI2 Synthase Expression in Spinal Endothelial Cells during Neuropathic Pain

Prostaglandins (PGs) are typical lipid mediators that play a role in homeostasis and disease. They are synthesized from arachidonic acid by cyclooxygenase 1 (COX1) and COX2. Although COX2 has been reported to be upregulated in the spinal cord after nerve injury, its expression and functional roles in neuropathic pain remain unclear. In this study, we investigated the expression of Cox2, PGI2 synthase (Pgis), and prostaglandin I2 receptor (IP receptor) mRNA in the rat spinal cord after spared nerve injury (SNI). Levels of Cox2 and Pgis mRNA increased in endothelial cells from 24 to 48 h after nerve injury. IP receptor mRNA was constitutively expressed in dorsal horn neurons. A COX2 inhibitor and IP receptor antagonists attenuated pain behavior in the early phase of neuropathic pain. Furthermore, we examined the relationship between COX2 and tumor necrosis factor-α (TNFα) in the spinal cord of a rat SNI model. Levels of TNFα mRNA transiently increased in the spinal microglia 24 h after SNI. The TNF receptors Tnfr1 and Tnfr2 mRNA were colocalized with COX2. Intrathecal injection of TNFα induced Cox2 and Pgis mRNA expression in endothelial cells. These results revealed that microglia-derived TNFα induced COX2 and PGIS expression in spinal endothelial cells and that endothelial PGI2 played a critical role in neuropathic pain via neuronal IP receptor. These findings further suggest that the glia–endothelial cell interaction of the neurovascular unit via transient TNFα is involved in the generation of neuropathic pain.

Redoxins in peripheral neurons after sciatic nerve injury

Peripheral nerve injury causes redox stress in injured neurons by upregulations of pro-oxidative enzymes, but most neurons survive suggesting an activation of endogenous defense against the imbalance. As potential candidates we assessed thioredoxin-fold proteins, called redoxins, which maintain redox homeostasis by reduction of hydrogen peroxide or protein dithiol-disulfide exchange. Using a histologic approach, we show that the peroxiredoxins (Prdx1-6), the glutaredoxins (Glrx1, 2, 3 and 5), thioredoxin (Txn1 and 2) and their reductases (Txnrd1 and 2) are expressed in neurons, glial and/or vascular cells of the dorsal root ganglia (DRGs) and in the spinal cord. They show distinct cellular and subcellular locations in agreement with the GO terms for "cellular component". The expression and localization of Glrx, Txn and Txnrd proteins was not affected by sciatic nerve injury but peroxiredoxins were upregulated in the DRGs, Prdx1 and Prdx6 mainly in non-neuronal cells and Prdx4 and Prdx5 in DRG neurons, the latter associated with an increase of respective mRNAs and protein accumulation in peripheral and/or central fibers. The upregulation of Prdx4 and Prdx5 in DRG neurons was reduced in mice with a cre-loxP mediated deficiency of hypoxia inducible factor 1 alpha (HIF1α) in these neurons. The results identify Prdx4 and Prdx5 as endogenous HIF1α-dependent, transcriptionally regulated defenders of nerve injury evoked redox stress that may be important for neuronal survival and regeneration.

Cellular sources of cyclooxygenase-1 and -2 up-regulation in the spinal dorsal horn after spinal nerve ligation

AIMS

Recent studies suggested that the development of neuropathic pain associated with neural injury may be partly due to up-regulation of cyclooxygenase (COX) in the central nervous system. However, the cellular sources of COX-1 and COX-2 up-regulation following nerve injury are unclear.

METHODS

We investigated the spinal cellular sources of COX-1 and COX-2 in association with allodynia following L5 spinal nerve ligation (SNL).

RESULTS

Post-SNL pain-related behaviour was shown by increased sensitivity to mechanical stimulation. There was a significant increase in both COX-1 and COX-2 immunoreactivity (P < 0.01) on the ipsilateral side of spinal dorsal horn. Double immunofluorescence labelling demonstrated that COX-1 immunoreactive cells colocalized chiefly with dorsal horn neuronal nuclei and microglia, whereas COX-2 was expressed in neuronal cytoplasm.

CONCLUSION

These findings demonstrate that while spinal dorsal horn neurones are important source of COX-1 and COX-2 after nerve injury, microglia also contribute to the pathogenesis of neuropathic pain, partly by producing additional COX-1.

The relationship between opioids and immune signalling in the spinal cord

Opioids are considered the gold standard for the treatment of moderate to severe pain. However, heterogeneity in analgesic efficacy, poor potency and side effects are associated with opioid use, resulting in dose limitations and suboptimal pain management. Traditionally thought to exhibit their analgesic actions via the activation of the neuronal G-protein-coupled opioid receptors, it is now widely accepted that neuronal activity of opioids cannot fully explain the initiation and maintenance of opioid tolerance, hyperalgesia and allodynia. In this review we will highlight the evidence supporting the role of non-neuronal mechanisms in opioid signalling, paying particular attention to the relationship of opioids and immune signalling.

Prostacyclin mediates neuropathic pain through interleukin 1β-expressing resident macrophages

Prostacyclin is an important mediator of peripheral pain sensation. Here, we investigated its potential participation in mediating neuropathic pain and found that prostacyclin receptor (IP) knockout mice exhibited markedly decreased pain behavior. Application of an IP antagonist to the injury site or selective IP deficiency in myeloid cells mimicked the antinociceptive effect observed in IP knockout mice. At the site of nerve injury, IP was expressed in interleukin (IL) 1β-containing resident macrophages, which were less common in IP knockout mice. Local administration of the IP agonist cicaprost inhibited macrophage migration in vitro and promoted accumulation of IP- and IL1β-expressing cells as well as an increase of IL1β concentrations at the application site in vivo. Fittingly, the IL1-receptor antagonist anakinra (IL-1ra) decreased neuropathic pain behavior in wild-type mice but not in IP knockout mice. Finally, continuous, but not single administration, of the cyclooxygenase inhibitor meloxicam early after nerve injury decreased pain behavior and the number of resident macrophages. Thus, early synthesis of prostacyclin at the site of injury causes accumulation of IL1β-expressing macrophages as a key step in neuropathic pain after traumatic injury.

Intervention of electroacupuncture on spinal p38 MAPK/ATF-2/VR-1 pathway in treating inflammatory pain induced by CFA in rats

Background

Previous studies have demonstrated that p38 MAPK signal transduction pathway plays an important role in the development and maintenance of inflammatory pain. Electroacupuncture (EA) can suppress the inflammatory pain. However, the relationship between EA effect and p38 MAPK signal transduction pathway in inflammatory pain remains poorly understood. It is our hypothesis that p38 MAPK/ATF-2/VR-1 and/or p38 MAPK/ATF-2/COX-2 signal transduction pathway should be activated by inflammatory pain in CFA-injected model. Meanwhile, EA may inhibit the activation of p38 MAPK signal transduction pathway. The present study aims to investigate that anti-inflammatory and analgesic effect of EA and its intervention on the p38 MAPK signal transduction pathway in a rat model of inflammatory pain.

Results

EA had a pronounced anti-inflammatory and analgesic effect on CFA-induced chronic inflammatory pain in rats. EA could quickly raise CFA-rat’s paw withdrawal thresholds (PWTs) and maintain good and long analgesic effect, while it subdued the ankle swelling of CFA rats only at postinjection day 14. EA could down-regulate the protein expressions of p-p38 MAPK and p-ATF-2, reduced the numbers of p-p38 MAPK-IR cells and p-ATF-2-IR cells in spinal dorsal horn in CFA rats, inhibited the expressions of both protein and mRNA of VR-1, but had no effect on the COX-2 mRNA expression.

Conclusions

The present study indicates that inhibiting the activation of spinal p38 MAPK/ATF-2/VR-1 pathway may be one of the main mechanisms via central signal transduction pathway in the process of anti-inflammatory pain by EA in CFA rats.

COX-1-dependent prostaglandin D2 in microglia contributes to neuropathic pain via DP2 receptor in spinal neurons

Cyclooxygenase (COX) enzyme synthesizes prostaglandins (PGs) from arachidonic acid and exists as two major isozymes, COX-1 and COX-2. The crucial role of prostaglandins in the pathogenesis of inflammatory pain in peripheral tissue and the spinal cord has been established; however its expression dynamics after peripheral nerve injury and its role in neuropathic pain are not clear. In this study, we examined the detailed expression patterns of genes for COX, PGD2 and thromboxane A2 synthases and their receptors in the spinal cord. Furthermore, we explored the altered gene expression of these molecules using the spared nerve injury (SNI) model. We also examined whether these molecules have a role in the development or maintenance of neuropathic pain. We found a number of interesting results in this study, the first was that COX-1 was constitutively expressed in the spinal cord and up-regulated in microglia located in laminae I-II after nerve injury. Second, COX-2 mRNA expression was induced in blood vessels after nerve injury. Third, TXA2 synthase and hematopoietic PGD synthase mRNAs were dramatically increased in the microglia after nerve injury. Finally, we found that intrathecal injection of a COX-1 inhibitor and DP2 receptor antagonist significantly attenuated the mechanical allodynia. Our findings indicate that PGD2 produced by microglia is COX-1 dependent, and that neurons in the spinal cord can receive PGD2 from microglia following peripheral nerve injury. We believe that PGD2 signaling via DP2 signaling pathway from microglia to neurons is one of the triggering factors for mechanical allodynia in this neuropathic pain model.

Microglial cathepsin B contributes to the initiation of peripheral inflammation-induced chronic pain

Interleukin (IL)-1β and IL-18 play critical roles in the induction of chronic pain hypersensitivity. Their inactive forms are activated by caspase-1. However, little is known about the mechanism underlying the activation of pro-caspase-1. There is increasing evidence that cathepsin B (CatB), a typical lysosomal cysteine protease, is involved in the pro-caspase-1 activation and the subsequent maturation of IL-1β and IL-18. In this context, CatB is considered to be an important molecular target to control chronic pain. However, no information is currently available about the role of CatB in chronic pain hypersensitivity. We herein show that CatB deficiency or the intrathecal administration of CA-074Me, a specific CatB inhibitor, significantly inhibited the induction of complete Freund's adjuvant-induced tactile allodynia in mice without affecting peripheral inflammation. In contrast, CatB deficiency did not affect the nerve injury-induced tactile allodynia. Furthermore, CatB deficiency or CA-074Me treatment significantly inhibited the maturation and secretion of IL-1β and IL-18 by cultured microglia following treatment with the neuroactive glycoprotein chromogranin A (CGA), but not with ATP. Moreover, the IL-1β expression in spinal microglia and the induction of tactile allodynia following the intrathecal administration of CGA depended on CatB, whereas those induced by the intrathecal administration of ATP or lysophosphatidic acid were CatB independent. These results strongly suggest that CatB is an essential enzyme for the induction of chronic inflammatory pain through its activation of pro-caspase-1, which subsequently induces the maturation and secretion of IL-1β and IL-18 by spinal microglia. Therefore, CatB-specific inhibitors may represent a useful new strategy for treating inflammation-associated pain.

Prolonged nerve blockade delays the onset of neuropathic pain

Aberrant neuronal activity in injured peripheral nerves is believed to be an important factor in the development of neuropathic pain. Pharmacological blockade of that activity has been shown to mitigate the onset of associated molecular events in the nervous system. However, results in preventing onset of pain behaviors by providing prolonged nerve blockade have been mixed. Furthermore, the experimental techniques used to date to provide that blockade were limited in clinical potential in that they would require surgical implantation. To address these issues, we have used liposomes (SDLs) containing saxitoxin (STX), a site 1 sodium channel blocker, and the glucocorticoid agonist dexamethasone to provide nerve blocks lasting ~1 wk from a single injection. This formulation is easily injected percutaneously. Animals undergoing spared nerve injury (SNI) developed mechanical allodynia in 1 wk; nerve blockade with a single dose of SDLs (duration of block 6.9 ± 1.2 d) delayed the onset of allodynia by 2 d. Treatment with three sequential SDL injections resulting in a nerve block duration of 18.1 ± 3.4 d delayed the onset of allodynia by 1 mo. This very prolonged blockade decreased activation of astrocytes in the lumbar dorsal horn of the spinal cord due to SNI. Changes in expression of injury-related genes due to SNI in the dorsal root ganglia were not affected by SDLs. These findings suggest that formulations of this kind, which could be easy to apply clinically, can mitigate the development of neuropathic pain.

Constitutive cyclo-oxygenase-2 does not contribute to the development of human visceral pain hypersensitivity

BACKGROUND AND AIMS

Central sensitisation (CS), contributes to the development and maintenance of gastrointestinal pain hypersensitivity. Constitutive cyclo-oxygenase-2 (COX-2) contributes to central sensitisation in somatic pain hypersensitivity but its role in mediating visceral pain hypersensitivity is unknown. We therefore conducted a study to determine if COX-2 inhibition with Valdecoxib attenuates the development or early maintenance of CS in a validated human oesophageal pain hypersensitivity model.

METHODS

Healthy volunteers were studied in two randomised, double blind, crossover studies in which pain thresholds (PT) to electrical stimulation were assessed in the proximal oesophagus, chest wall and foot, prior to and following a distal oesophageal acid infusion. Protocol 1: Valdecoxib, (40 mg) or matching placebo was given orally for 4 days prior to oesophageal acid infusion. Protocol 2: IV Parecoxib (40 mg) or saline was given 120 min after oesophageal acid infusion.

RESULTS

Valdecoxib did not prevent the induction of secondary allodynia in the proximal oesophagus nor did it attenuate it following its establishment. Chest wall PT fell following oesophageal acid but foot PT remained unchanged; highlighting the development viscero-somatic convergence due to CS. Valdecoxib had no analgesic or anti-hyperalgesic effect on chest wall or foot PT.

CONCLUSIONS

Neither the induction nor initial maintenance of acid induced oesophageal pain hypersensitivity is prevented by Valdecoxib, suggesting that constitutive spinal COX-2 does not contribute to the development or early maintenance of acute visceral central sensitisation.

Ameliorative effect of combined administration of inducible nitric oxide synthase inhibitor with cyclooxygenase-2 inhibitors in neuropathic pain in rats

OBJECTIVES

The objective of this study was to examine the effects of rofecoxib, meloxicam, both cyclooxygenase-2 (COX-2) inhibitors and aminoguanidine hydrochloride, an inducible nitric oxide synthase (iNOS) inhibitor and their combinations in neuropathic pain in rats.

METHODS

Neuropathy was induced by chronic constriction injury (CCI) of right sciatic nerve under ketamine anesthesia in rats. Effect of ED(50) of aminoguanidine hydrochloride, rofecoxib and meloxicam administered orally was investigated using behavioral tests. Effect of combinations of aminoguanidine hydrochloride with rofecoxib and meloxicam was also investigated in neuropathic pain employing behavioral tests.

RESULTS

Behavioral tests, mechanical, thermal and cold stimuli confirmed the development of neuropathic pain after CCI. Aminoguanidine hydrochloride, rofecoxib and meloxicam when administered alone, produced significant increase in paw withdrawal threshold to mechanical stimuli at 6 h in ipsilateral hind paw after CCI. Co-administration of aminoguanidine hydrochloride (30 mg/kg) with rofecoxib (1.31 mg/kg) and meloxicam (1.34 mg/kg) was also found to produce significant increase in paw withdrawal latencies to mechanical stimuli at 6 h. Combined administration of aminoguanidine hydrochloride with meloxicam and rofecoxib produced significant rise in pain threshold for mechanical hyperalgesia in ipsilateral hind paw when compared with the groups treated with aminoguanidine hydrochloride, meloxicam and rofecoxib alone.

CONCLUSION

Co-administration of meloxicam and rofecoxib with aminoguanidine hydrochloride may be an alternative approach for the treatment of neuropathic pain.

Reactive astrocytes inhibit the survival and differentiation of oligodendrocyte precursor cells by secreted TNF-α

Axonal demyelination is a consistent pathological characteristic of spinal cord injury (SCI). Although an increased number of oligodendrocyte progenitor cells (OPCs) is observed in the injured spinal cord, they fail to convert into mature oligodendrocytes. However, little is known about the underlying mechanism. In our study, we identified a link between inhibition of OPC survival and differentiation and reactive astrocytes in glial scar that was mediated by tumor necrosis factor-α (TNF-α). Initially, both glial scar tissue and reactive astrocyte-conditioned medium were shown to inhibit OPC differentiation. Reverse transcriptase polymerase chain reaction (RT-PCR) and immunochemistry revealed that OPCs expressed type 1 TNF-α receptor (TNF-R1). When TNF-α or TNF-R1 was neutralized with antibody, the effect of reactive astrocyte-conditioned medium or recombinant TNF-α protein on OPC differentiation was markedly attenuated. In addition, reactive astrocyte-conditioned medium was also shown to induce OPC apoptosis. All these findings provide the first evidence that reactive astrocytes release TNF-α to inhibit OPC survival and prevent them from differentiating into mature oligodendrocytes, suggesting a mechanism for the failure of remyelination after SCI.

A novel role of prostaglandin E2 in neuropathic pain: blockade of microglial migration in the spinal cord

Neuropathic pain produced by damage to or dysfunction of the nervous system is a common and severely disabling state that affects millions of people worldwide. Recent evidence indicates that activated microglia are key cellular intermediaries in the pathogenesis of neuropathic pain and that ATP serves as the mediator. However, the in vivo mechanism underlying the retention of activated microglia in the injured region has not yet been completely elucidated. Prostaglandin E(2) (PGE(2)) is the principal proinflammatory prostanoid and plays versatile roles by acting via four PGE receptor subtypes, EP1-EP4. In the present study, we investigated the role of PGE(2) in spinal microglial activation in relation to neuropathic pain by using genetic and pharmacological methods. Mice deficient in microsomal prostaglandin E synthase-1 impaired the activation of microglia and the NMDA-nitric oxide (NO) cascade in spinal neurons in the dorsal horn and did not exhibit mechanical allodynia after peripheral nerve injury. The intrathecal injection of indomethacin, a nonsteroidal anti-inflammatory drug, ONO-8713, a selective EP1 antagonist, or 7-nitroindole, a neuronal NO synthase inhibitor, attenuated mechanical allodynia and the increase in activated microglia observed in the established neuropathic-pain state. We further demonstrated that ATP-induced microglial migration was blocked in vitro by PGE(2) via EP2 and by S-nitrosoglutathione, an NO donor. Taken together, the present study suggests that PGE(2) participated in the maintenance of neuropathic pain in vivo not only by activating spinal neurons, but also by retaining microglia in the central terminals of primary afferent fibers via EP2 subtype and via EP1-mediated NO production.

Interactions of estradiol and NSAIDS on carrageenan-induced hyperalgesia

How exogenous estrogen affects inflammatory responses is poorly understood despite the large numbers of women receiving estrogen-alone hormone therapy. The aim of this study was to determine if estradiol alters injury- or inflammation-induced nociceptive responses after carrageenan administration in females and whether its effects are mediated through cyclo-oxygenase (COX) and prostaglandins (PG). To this end, paw withdrawal latencies and serum levels of PGE2 and PGD2 were measured in rats treated with estradiol (0, 10, 20, and 30%) and/or SC560 (COX-1 inhibitor) or NS398 (COX-2 inhibitor) after intraplantar carrageenan administration. Estradiol significantly increased withdrawal latencies before (baseline condition) and after carrageenan administration to one hindpaw. NS398 was anti-nociceptive only in carrageenan treated animals. SC560 increased withdrawal latencies in both paws at 1 and 5hours after carrageenan administration. Co-administration of estradiol and NS398, but not SC560, was additive except for a prolonged anti-nociceptive effects of estradiol combined with NS398. The anti-nociceptive effect extended beyond that observed with either drug or estradiol alone at the 5-hour time point. Estradiol had no significant effect on PGE2 serum levels, but both COX antagonists decreased them. Although neither estradiol nor the COX inhibitors alone had an effect on PGD2 serum levels, co-administration of NS398 and estradiol significantly elevated PGD2 levels. Taken together, our results suggest that estradiol is anti-nociceptive in the thermal test and reduces carrageenan-induced hyperalgesia. These effects are minimally altered through PG-mediated mechanisms.

Statistical identification of predictors for peripheral neuropathy associated with administration of bortezomib, taxanes, oxaliplatin or vincristine using ordered logistic regression analysis

Chemotherapy-induced peripheral neuropathy (CIPN) is a major drug-induced adverse reaction that becomes a dose-limiting toxicity. However, effective strategies for preventing or treating CIPN are lacking. Accordingly, this study aimed to statistically identify predictors for CIPN. Retrospective analysis was carried out for 190 patients who had been treated with bortezomib (n=28), taxanes (paclitaxel or docetaxel; n=58), oxaliplatin (n=52) or vincristine (n=52) at our hospital between April 2005 and December 2008. The severity of CIPN was assessed at the time of chemotherapy completion, graded as grade 0-5 in accordance with the National Cancer Institute Common Terminology Criteria for Adverse Events v3.0. Multivariate ordered logistic regression analysis was used to investigate predictors for CIPN. Predictors for CIPN in patients that were administered bortezomib were no co-administration of dexamethasone [odds ratio (OR), 0.455; confidence interval (CI), 0.208-0.955; P=0.0376] and sex (male) (OR, 3.035; CI, 1.356-6.793; P=0.0069). For taxanes (paclitaxel or docetaxel), the predictor for CIPN was a large number of chemotherapy cycles (OR, 2.379; CI, 1.035-5.466; P=0.0412). For oxaliplatin, the predictors for CIPN were a large number of chemotherapy cycles (OR, 3.089; CI, 1.598-5.972; P=0.0008) and no co-administration of non-steroidal anti-inflammatory drugs (OR, 0.393; CI, 0.197-0.785; P=0.0082). For vincristine, predictors for CIPN were a large number of chemotherapy cycles (OR, 6.015; CI, 1.880-19.248; P=0.0025) and co-administration of an analgesic adjuvant (OR, 3.907; CI, 1.383-11.031; P=0.0101). In conclusion, our study indicates that CIPN will be alleviated by the co-administration of dexamethasone with bortezomib and non-steroidal anti-inflammatory drugs with oxaliplatin.

Host-pathogen interactions mediating pain of urinary tract infection

BACKGROUND

Pelvic pain is a major component of the morbidity associated with urinary tract infection (UTI), yet the molecular mechanisms underlying UTI-induced pain remain unknown. UTI pain mechanisms probably contrast with the clinical condition of asymptomatic bacteriuria (ASB), characterized by significant bacterial loads without lack symptoms.

METHODS

A murine UTI model was used to compare pelvic pain behavior elicited by infection with uropathogenic Escherichia coli strain NU14 and ASB strain 83972.

RESULTS

NU14-infected mice exhibited pelvic pain, whereas mice infected with 83972 did not exhibit pain, similar to patients infected with 83972. NU14-induced pain was not dependent on mast cells, not correlated with bacterial colonization or urinary neutrophils. UTI pain was not influenced by expression of type 1 pili, the bacterial adhesive appendages that induce urothelial apoptosis. However, purified NU14 lipopolysaccharide (LPS) induced Toll-like receptor 4 (TLR4)-dependent pain, whereas 83972 LPS induced no pain. Indeed, 83972 LPS attenuated the pain of NU14 infection, suggesting therapeutic potential.

CONCLUSIONS

These data suggest a novel mechanism of infection-associated pain that is dependent on TLR4 yet independent of inflammation. Clinically, these findings also provide the rational for probiotic therapies that would minimize the symptoms of infection without reliance on empirical therapies that contribute to antimicrobial resistance.

Overexpression of the wild-type SPT1 subunit lowers desoxysphingolipid levels and rescues the phenotype of HSAN1

Mutations in the SPTLC1 subunit of serine palmitoyltransferase (SPT) cause an adult-onset, hereditary sensory, and autonomic neuropathy type I (HSAN1). We previously reported that mice bearing a transgene-expressing mutant SPTLC1 (tgSPTLC1(C133W)) show a reduction in SPT activity and hyperpathia at 10 months of age. Now analyzed at a later age, we find these mice develop sensory loss with a distal small fiber neuropathy and peripheral myelinopathy. This phenotype is largely reversed when these mice are crossed with transgenic mice overexpressing wild-type SPTLC1 showing that the mutant SPTLC1 protein is not inherently toxic. Simple loss of SPT activity also cannot account for the HSAN1 phenotype, since heterozygous SPTLC1 knock-out mice have reduced SPT activity but are otherwise normal. Rather, the presence of two newly identified, potentially deleterious deoxysphingoid bases in the tgSPTLC1(C133W), but not in the wild-type, double-transgenic tgSPTLC1(WT + C133W) or SPTLC1(+/-) mice, suggests that the HSAN1 mutations alter amino acid selectivity of the SPT enzyme such that palmitate is condensed with alanine and glycine, in addition to serine. This observation is consistent with the hypothesis that HSAN1 is the result of a gain-of-function mutation in SPTLC1 that leads to accumulation of a toxic metabolite.

Reactive astrocytes in glial scar attract olfactory ensheathing cells migration by secreted TNF-alpha in spinal cord lesion of rat

Background

After spinal cord injury (SCI), the formation of glial scar contributes to the failure of injured adult axons to regenerate past the lesion. Increasing evidence indicates that olfactory ensheathing cells (OECs) implanted into spinal cord are found to migrate into the lesion site and induce axons regeneration beyond glial scar and resumption of functions. However, little is known about the mechanisms of OECs migrating from injection site to glial scar/lesion site.

Methods and Findings

In the present study, we identified a link between OECs migration and reactive astrocytes in glial scar that was mediated by the tumor necrosis factor-α (TNF-α). Initially, the Boyden chamber migration assay showed that both glial scar tissue and reactive astrocyte-conditioned medium promoted OECs migration in vitro. Reactive astrocyte-derived TNF-α and its type 1 receptor TNFR1 expressed on OECs were identified to be responsible for the promoting effect on OECs migration. TNF-α-induced OECs migration was demonstrated depending on activation of the extracellular signal-regulated kinase (ERK) signaling cascades. Furthermore, TNF-α secreted by reactive astrocytes in glial scar was also showed to attract OECs migration in a spinal cord hemisection injury model of rat.

Conclusions

These findings showed that TNF-α was released by reactive astrocytes in glial scar and attracted OECs migration by interacting with TNFR1 expressed on OECs via regulation of ERK signaling. This migration-attracting effect of reactive astrocytes on OECs may suggest a mechanism for guiding OECs migration into glial scar, which is crucial for OECs-mediated axons regrowth beyond the spinal cord lesion site.

Prostaglandin E(2) synthase inhibition as a therapeutic target

BACKGROUND

Most NSAIDs function by inhibiting biosynthesis of PGE(2) by inhibition of COX-1 and/or COX-2. Since COX-1 has a protective function in the gastro-intestinal tract (GIT), non-selective inhibition of both cycloxy genases leads to moderate to severe gastro-intestinal intolerance. Attempts to identify selective inhibitors of COX-2, led to the identification of celecoxib and rofecoxib. However, long-term use of these drugs has serious adverse effects of sudden myocardial infarction and thrombosis. Drug-mediated imbalance in the levels of prostaglandin I(2) (PGI(2)) and thromboxane A(2) (TXA(2)) with a bias towards TXA(2) may be the primary reason for these events. This resulted in the drugs being withdrawn from the market, leaving a need for an effective and safe anti-inflammatory drug.

METHODS

Recently, the focus of research has shifted to enzymes downstream of COX in the prosta glandin biosynthetic pathway such as prostaglandin E(2) synthases. Microsomal prostaglandin E(2) synthase-1 (mPGES-1) specifically isomerizes PGH(2) to PGE(2), under inflammatory conditions. In this review, we examine the biology of mPGES-1 and its role in disease. Progress in designing molecules that can selectively inhibit mPGES-1 is reviewed.

CONCLUSION

mPGES-1 has the potential to be a target for anti-inflammatory therapy, devoid of adverse GIT and cardiac effects and warrants further investigation.

Capnellene, a natural marine compound derived from soft coral, attenuates chronic constriction injury-induced neuropathic pain in rats

BACKGROUND AND PURPOSE

Natural compounds obtained from marine organisms have received considerable attention as potential sources of novel drugs for treatment of human inflammatory diseases. Capnellene, isolated from the marine soft coral Capnella imbricate, 4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalene-2,3a-diol (GB9) exhibited anti-inflammatory actions on activated macrophages in vitro. Here we have assessed the anti-neuroinflammatory properties of GB9 and its acetylated derivative, acetic acid 3a-hydroxy-4,4,6a-trimethyl-3-methylene-decahydro-cyclopenta[]pentalen-2-yl ester (GB10).

EXPERIMENTAL APPROACH

Effects of GB9 or GB10 on the expression of inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) in interferon-gamma (IFN-gamma)-stimulated mouse microglial BV2 cells were measured by Western blot. The in vivo effects of these compounds were examined in the chronic constriction injury (CCI) rat model of neuropathic pain, measuring thermal hyperalgesia, and microglial activation and COX-2 protein in lumbar spinal cord, by immunohistochemistry.

KEY RESULTS

In BV2 cells, GB9 and GB10 inhibited the expression of iNOS and COX-2, stimulated by IFN-gamma. Intrathecal administration of GB9 and GB10 inhibited CCI-induced nociceptive sensitization and thermal hyperalgesia in a dose-dependent manner. Intraperitoneal injection of GB9 inhibited CCI-induced thermal hyperalgesia and also inhibited CCI-induced activation of microglial cells and up-regulation of COX-2 in the dorsal horn of the lumbar spinal cord ipsilateral to the injury.

CONCLUSION AND IMPLICATIONS

Taken together, these data indicate that the marine-derived capnellenes, GB9 and GB10, had anti-neuroinflammatory and anti-nociceptive properties in IFN-gamma-stimulated microglial cells and in neuropathic rats respectively. Therefore, capnellene may serve as a useful lead compound in the search for new therapeutic agents for treatment of neuroinflammatory diseases.

Intracisternal administration of COX inhibitors attenuates mechanical allodynia following compression of the trigeminal ganglion in rats

The purpose of the present study was to investigate the role of central cyclooxygenase (COX) pathways in the modulation of mechanical allodynia following compression of the left trigeminal ganglion. Experiments were carried out on male Sprague-Dawley rats mounted onto a stereotaxic frame under anesthesia. For compression, a 4% agar solution (10 microl) was injected into the trigeminal ganglion. In the control group, rats were sham operated without agar injections. Ipsilateral and contralateral air-puff thresholds significantly decreased following trigeminal ganglion compression. Mechanical allodynia was established within 3 days and lasted beyond postoperative day 30, returning to preoperative levels at approximately 55 days following compression. Intracisternal administration of indomethacin, a non-selective COX inhibitor, SC-560, a selective COX-1 inhibitor, or NS-398, a selective COX-2 inhibitor, significantly inhibited mechanical allodynia. The individual anti-allodynic effects of the three COX inhibitors persisted for 6 h and returned to pretreatment values within 24 h. Based on these results, the blockade of central COX pathways may comprise a potential new therapeutic tool for the treatment of trigeminal ganglion compression-induced nociception.

Periganglionic inflammation elicits a distally radiating pain hypersensitivity by promoting COX-2 induction in the dorsal root ganglion

We have developed a model in which inflammation contiguous to and within a dorsal root ganglion (DRG) was generated by local application of complete Freund's adjuvant (CFA) to the L4 lumbar spinal nerve as it exits from the intervertebral foramen. The periganglionic inflammation (PGI) elicited a marked reduction in withdrawal threshold to mechanical stimuli and an increase in heat pain sensitivity in the ipsilateral hindpaw in the absence of any hindpaw inflammation. The pain sensitivity appeared within hours and lasted for a week. The PGI also induced a prominent increase in IL-1beta and TNF-alpha levels in the DRG and of cyclooxygenase-2 (COX-2) expression in neurons and satellite cells. A selective COX-2 inhibitor reduced the PGI-induced hyperalgesia. We also show that IL-1beta induces COX-2 expression and prostaglandin release in DRG neurons in vitro in a MAP kinase-dependent fashion. The COX-2 induction was prevented by ERK and p38 inhibitors. We conclude that periganglionic inflammation increases cytokine levels, including IL-1beta, leading to the transcription of COX-2 and prostaglandin production in the affected DRG, and thereby to the development of a dermatomally distributed pain hypersensitivity.

Toll-like receptors in peripheral nerve injury and neuropathic pain

Peripheral nerve injury triggers a series of responses in the injured nerve, such as the dissolution of distal axons, the activation of Schwann cells, the production of various proinflammatory mediators, and the infiltration of circulating immune cells. These orchestrated events regulate the degeneration and subsequent regeneration of the injured nerve. In addition, peripheral nerve injury often accompanies chronic pain. Studies in this field have revealed that spinal cord microglia activation plays a critical role in the development of pain hypersensitivity. Recent studies using genetically modified mice indicate that Toll-like receptors (TLRs) are involved in nerve degeneration (Wallerian degeneration) and chronic pain (neuropathic pain) development after nerve injury. Here, we review studies that have implicated TLRs in mediating nerve degeneration/regeneration and neuropathic pain following nerve injury. In addition, we discuss possible mechanisms underlying the roles of TLRs in these neurological disorders.

Antinociceptive activity of the S1P-receptor agonist FTY720

FTY720 is a novel immunosuppressive drug that inhibits the egress of lymphocytes from secondary lymphoid tissues and thymus. In its phosphorylated form FTY720 is a potent S1P receptor agonist. Recently it was also shown that FTY720 can reduce prostaglandin synthesis through the direct inhibition of the cytosolic phospholipase A2 (cPLA2). Since prostaglandins are important mediators of nociception, we studied the effects of FTY720 in different models of nociception. We found that intraperitoneal administration of FTY720 reduced dose-dependently the nociceptive behaviour of rats in the formalin assay. Although the antinociceptive doses of FTY720 were too low to alter the lymphocyte count, prostanoid concentrations in the plasma were dramatically reduced. Surprisingly, intrathecally administered FTY720 reduced the nociceptive behaviour in the formalin assay without altering spinal prostaglandin synthesis, indicating that additional antinociceptive mechanisms beside the inhibition of prostaglandin synthesis are involved. Accordingly, FTY720 reduced also the nociceptive behaviour in the spared nerve injury model for neuropathic pain which does not depend on prostaglandin synthesis. In this model the antinociceptive effect of FTY720 was similar to gabapentin, a commonly used drug to treat neuropathic pain. Taken together we show for the first time that FTY720 possesses antinociceptive properties and that FTY720 reduces nociceptive behaviour during neuropathic pain.

Spinal dis-inhibition in inflammatory pain

Inflammatory diseases and neuropathic insults trigger signaling cascades, which frequently lead to intense and long-lasting pain syndromes in affected patients. Such pain syndromes are characterized not only by an increased sensitivity to painful stimuli (hyperalgesia), but also by a qualitative change in the sensory perception of other, tactile stimuli (allodynia) and the occurrence of spontaneous pain in the absence of any sensory input. Long-term potentiation (LTP)-like changes in synaptic transmission between nociceptive C-fibers and spino-periaqueductal grey projection neurons as well as a loss of inhibitory control by GABAergic and glycinergic spinal dorsal horn neurons have repeatedly been proposed as underlying principles. While considerable evidence supports a significant contribution of C-fiber LTP to hyperalgesia, such monosynaptic plasticity cannot explain the occurrence of allodynia and spontaneous pain. In this review, we focus on mechanisms of synaptic dis-inhibition in inflammatory pain and propose that pathologically heightened pain sensitivity can be reversed by restoring synaptic inhibition with drugs that target specific spinal GABAA receptor subtypes.

Pathophysiology and treatment of neuropathic pain associated with syringomyelia

The pain behaviour expressed by dogs with syringomyelia suggests that they experience neuropathic pain, probably due to disordered neural processing in the damaged dorsal horn. As such it is likely that conventional analgesic medication will be ineffective. In this review, physiological and pathological pain processing through the dorsal horn is summarised and mechanisms by which syringomyelia could result in a persistent pain state are discussed. Finally, current knowledge regarding treatment of Chiari malformation and syringomyelia is reviewed and possible drugs which may give improved pain relief in affected dogs are discussed.

Role of the cysteine protease cathepsin S in neuropathic hyperalgesia

Using a gene expression analysis approach we found that the mRNA encoding the lysosomal cysteine protease cathepsin S (CatS) was up-regulated in rat dorsal root ganglia (DRG) following peripheral nerve injury. CatS protein was expressed in infiltrating macrophages in DRG and near the site of injury. At both sites CatS expression progressively increased from day 3 to day 14 after injury. In naïve rats, intraplantar injection of activated rat recombinant (rr) CatS (0.3, 1 microg/rat) induced a mechanical hyperalgesia that developed within half-an-hour, diminished by 3h and was absent after 24h. Activated rrCathepsin B (CatB) and non-activated rrCatS injected intraplantarly at the same or higher doses than activated rrCatS had no effect on rat nociceptive thresholds. In nerve-injured rats, mechanical hyperalgesia, but not allodynia, was significantly reversed for up to 3h by systemic administration of a non-brain penetrant, irreversible CatS inhibitor (LHVS, 3-30 mg/kg s.c.). Depletion of peripheral macrophages by intravenous injection of liposome encapsulate clodronate (1ml, 5 mg/ml) partially reduced established mechanical hyperalgesia but not allodynia, and abolished the anti-hyperalgesic effect of LHVS. Our results demonstrate a pro-nociceptive effect of CatS and indicate that endogenous CatS released by peripheral macrophages contributes to the maintenance of neuropathic hyperalgesia following nerve injury.

A meta-analysis of the analgesic effects of omega-3 polyunsaturated fatty acid supplementation for inflammatory joint pain

Between 40% and 60% of Americans use complementary and alternative medicine to manage medical conditions, prevent disease, and promote health and well-being. Omega-3 polyunsaturated fatty acids (omega-3 PUFAs) have been used to treat joint pain associated with several inflammatory conditions. We conducted a meta-analysis of 17 randomized, controlled trials assessing the pain relieving effects of omega-3 PUFAs in patients with rheumatoid arthritis or joint pain secondary to inflammatory bowel disease and dysmenorrhea. Meta-analysis was conducted with Cochrane Review Manager 4.2.8. for six separate outcomes using standardized mean differences (SMDs) as a measure of effect size: (1) patient assessed pain, (2) physician assessed pain, (3) duration of morning stiffness, (4) number of painful and/or tender joints, (5) Ritchie articular index, and (6) nonselective nonsteroidal anti-inflammatory drug consumption. Supplementation with omega-3 PUFAs for 3-4 months reduces patient reported joint pain intensity (SMD: -0.26; 95% CI: -0.49 to -0.03, p=0.03), minutes of morning stiffness (SMD: -0.43; 95% CI: -0.72 to -0.15, p=0.003), number of painful and/or tender joints (SMD: -0.29; 95% CI: -0.48 to -0.10, p=0.003), and NSAID consumption (SMD: -0.40; 95% CI: -0.72 to -0.08, p=0.01). Significant effects were not detected for physician assessed pain (SMD: -0.14; 95% CI: -0.49 to 0.22, p=0.45) or Ritchie articular index (SMD: 0.15; 95% CI: -0.19 to 0.49, p=0.40) at 3-4 months. The results suggest that omega-3 PUFAs are an attractive adjunctive treatment for joint pain associated with rheumatoid arthritis, inflammatory bowel disease, and dysmenorrhea.

A critical role of toll-like receptor 2 in nerve injury-induced spinal cord glial cell activation and pain hypersensitivity

The activation of spinal cord glial cells has been implicated in the development of neuropathic pain upon peripheral nerve injury. The molecular mechanisms underlying glial cell activation, however, have not been clearly elucidated. In this study, we found that damaged sensory neurons induce the expression of tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and inducible nitric-oxide synthase genes in spinal cord glial cells, which is implicated in the development of neuropathic pain. Studies using primary glial cells isolated from toll-like receptor 2 knock-out mice indicate that damaged sensory neurons activate glial cells via toll-like receptor 2. In addition, behavioral studies using toll-like receptor 2 knock-out mice demonstrate that the expression of toll-like receptor 2 is required for the induction of mechanical allodynia and thermal hyperalgesia due to spinal nerve axotomy. The nerve injury-induced spinal cord microglia and astrocyte activation is reduced in the toll-like receptor 2 knock-out mice. Similarly, the nerve injury-induced pro-inflammatory gene expression in the spinal cord is also reduced in the toll-like receptor 2 knock-out mice. These data demonstrate that toll-like receptor 2 contributes to the nerve injury-induced spinal cord glial cell activation and subsequent pain hypersensitivity.