Prostaglandin E(2) and 6-keto-prostaglandin F(1alpha) production is elevated following traumatic injury to sciatic nerve

Dietary Concentrate-to-Forage Ratio Affects Rumen Bacterial Community Composition and Metabolome of Yaks

Changes in dietary composition affect the rumen microbiota in ruminants. However, information on the effects of dietary concentrate-to-forage ratio changes on yak rumen bacteria and metabolites is limited. This study characterized the effect of three different dietary concentrate-to-forage ratios (50:50, C50 group; 65:35, C65 group; 80:20, C80 group) on yak rumen fluid microbiota and metabolites using 16S rRNA gene sequencing and liquid chromatography-mass spectrometry (LC-MS) analyses. Rumen fermentation parameters and the abundance of rumen bacteria were affected by changes in the dietary concentrate-to-forage ratio, and there was a strong correlation between them. At the genus level, higher relative abundances of norank_f__F082, NK4A214_group, Lachnospiraceae_NK3A20_group, Acetitomaculum, and norank_f__norank_o__Clostridia_UCG-014 were observed with a high dietary concentrate-to-forage ratio (P < 0.05). Combined metabolomic and enrichment analyses showed that changes in the dietary concentrate-to-forage ratio significantly affected rumen metabolites related to amino acid metabolism, protein digestion and absorption, carbohydrate metabolism, lipid metabolism, and purine metabolism. Compared with the C50 group, 3-methylindole, pantothenic acid, D-pantothenic acid, and 20-hydroxy-leukotriene E4 were downregulated in the C65 group, while spermine and ribose 1-phosphate were upregulated. Compared to the C50 group, Xanthurenic acid, tyramine, ascorbic acid, D-glucuronic acid, 6-keto-prostaglandin F1a, lipoxin B4, and deoxyadenosine monophosphate were upregulated in the C80 group, while 3-methylindole and 20-hydroxy-leukotriene E4 were downregulated. All metabolites (Xanthurenic acid, L-Valine, N-Acetyl-L-glutamate 5-semialdehyde, N-Acetyl-L-glutamic acid, Tyramine, 6-Keto-prostaglandin F1a, Lipoxin B4, Xanthosine, Thymine, Deoxyinosine, and Uric acid) were upregulated in the C80 group compared with the C65 group. Correlation analysis of microorganisms and metabolites provided new insights into the function of rumen bacteria, as well as a theoretical basis for formulating more scientifically appropriate feeding strategies for yak.

LXR agonist modifies neuronal lipid homeostasis and decreases PGD2 in the dorsal root ganglia in western diet-fed mice

The prevalence of peripheral neuropathy is high in diabetic and overweight populations. Chronic neuropathic pain, a symptom of peripheral neuropathy, is a major disabling symptom that leads to a poor quality of life. Glucose management for diabetic and prediabetic individuals often fail to reduce or improve pain symptoms, therefore, exploring other mechanisms is necessary to identify effective treatments. A large body of evidence suggest that lipid signaling may be a viable target for management of peripheral neuropathy in obese individuals. The nuclear transcription factors, Liver X Receptors (LXR), are known regulators of lipid homeostasis, phospholipid remodeling, and inflammation. Notably, the activation of LXR using the synthetic agonist GW3965, delayed western diet (WD)-induced allodynia in rodents. To further understand the neurobiology underlying the effect of LXR, we used translating ribosome affinity purification and evaluated translatomic changes in the sensory neurons of WD-fed mice treated with the LXR agonist GW3965. We also observed that GW3965 decreased prostaglandin levels and decreased free fatty acid content, while increasing lysophosphatidylcholine, phosphatidylcholine, and cholesterol ester species in the sensory neurons of the dorsal root ganglia (DRG). These data suggest novel downstream interplaying mechanisms that modifies DRG neuronal lipid following GW3965 treatment.

Ultramicronized Palmitoylethanolamide and Paracetamol, a New Association to Relieve Hyperalgesia and Pain in a Sciatic Nerve Injury Model in Rat

Inflammation is known to be an essential trigger of the pathological changes that have a critical impact on nerve repair and regeneration; moreover, damage to peripheral nerves can cause a loss of sensory function and produces persistent neuropathic pain. To date, various potential approaches for neuropathic pain have focused on controlling neuroinflammation. The aim of this study was to investigate the neuroprotective effects of a new association of ultramicronized Palmitoylethanolamide (PEAum), an Autacoid Local Injury Antagonist Amide (ALIAmide) with analgesic and anti-inflammatory properties, with Paracetamol, a common analgesic, in a rat model of sciatic nerve injury (SNI). The association of PEAum-Paracetamol, in a low dose (5 mg/kg + 30 mg/kg), was given by oral gavage daily for 14 days after SNI. PEAum-Paracetamol association was able to reduce hyperalgesia, mast cell activation, c-Fos and nerve growth factor (NGF) expression, neural histological damage, cytokine release, and apoptosis. Furthermore, the analgesic action of PEAum-Paracetamol could act in a synergistic manner through the inhibition of the NF-κB pathway, which leads to a decrease of cyclooxygenase 2-dependent prostaglandin E₂ (COX-2/PGE₂) release. In conclusion, we demonstrated that PEAum associated with Paracetamol was able to relieve pain and neuroinflammation after SNI in a synergistic manner, and this therapeutic approach could be relevant to decrease the demand of analgesic drugs.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

The prostaglandin E2 receptor EP3 controls CC-chemokine ligand 2-mediated neuropathic pain induced by mechanical nerve damage

Prostaglandin (PG) E₂ is an important lipid mediator that is involved in several pathophysiological processes contributing to fever, inflammation, and pain. Previous studies have shown that early and continuous application of nonsteroidal anti-inflammatory drugs significantly reduces pain behavior in the spared nerve injury (SNI) model for trauma-induced neuropathic pain. However, the role of PGE₂ and its receptors in the development and maintenance of neuropathic pain is incompletely understood but may help inform strategies for pain management. Here, we sought to define the nociceptive roles of the individual PGE₂ receptors (EP1-4) in the SNI model using EP knockout mice. We found that PGE₂ levels at the site of injury were increased and that the expression of the terminal synthase for PGE₂, cytosolic PGE synthase was up-regulated in resident positive macrophages located within the damaged nerve. Only genetic deletion of the EP3 receptor affected nociceptive behavior and reduced the development of late-stage mechanical allodynia as well as recruitment of immune cells to the injured nerve. Importantly, EP3 activation induced the release of CC-chemokine ligand 2 (CCL2), and antagonists against the CCL2 receptor reduced mechanical allodynia in WT but not in EP3 knockout mice. We conclude that selective inhibition of EP3 might present a potential approach for reducing chronic neuropathic pain.

WWL70 protects against chronic constriction injury-induced neuropathic pain in mice by cannabinoid receptor-independent mechanisms

BACKGROUND

Targeting the endocannabinoid system has emerged as an effective strategy for the treatment of inflammatory and neurological diseases. Unlike the inhibition of the principal 2-arachidonyl glycerol (2-AG) hydrolytic enzyme monoacylglycerol lipase (MAGL), which leads to 2-AG overload and cannabinoid receptor desensitization, selective inhibition of the minor 2-AG hydrolytic enzyme alpha, beta-hydrolase domain 6 (ABHD6) can provide therapeutic benefits without producing cannabimimetic side effects. We have shown that inhibition of ABHD6 significantly reduces neuroinflammation and exerts neuroprotection in animal models of traumatic brain injury and multiple sclerosis. However, the role of ABHD6 inhibition on neuropathic pain has not been explored.

METHODS

Neuropathic pain was induced by chronic constriction injury (CCI) of the mouse sciatic nerve and examined by Hargreaves and Von Frey tests. Activation of inflammatory cells and the production of cytokines and chemokines in the spinal cord dorsal horn, dorsal root ganglion (DRG), and sciatic nerve were assessed by qRT-PCR, enzyme-linked immunosorbent assay (ELISA), and immunohistochemistry. The levels of 2-AG and arachidonic acid (AA) in sciatic nerve were quantified by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).

RESULTS

Treatment with the selective ABHD6 inhibitor WWL70 significantly alleviated CCI-induced thermal hyperalgesia and mechanical allodynia. Microglia activation, macrophage infiltration, and the production of nociceptive mediators were reduced in the ipsilateral lumbar spinal cord dorsal horn, DRG, and sciatic nerve of WWL70-treated animals. The diminished cytokine and chemokine production is likely due to the inhibitory effect of WWL70 on NF-κB phosphorylation. Surprisingly, the anti-nociceptive and anti-inflammatory effects of WWL70 were not reversed by addition of the cannabinoid receptor antagonists. Treatment with WWL70 did not alter the levels of 2-AG, AA, and the phosphorylation of cytosolic phospholipase A₂ (cPLA₂), but significantly reduced the production of prostaglandin E₂ (PGE₂) and the expression of cyclooxygenase-2 (COX-2) and prostaglandin E synthase-2 (PGES2) in the injured sciatic nerve.

CONCLUSIONS

This study reveals a novel mechanism for the antinociceptive effect of the 2-AG catabolic enzyme ABHD6 inhibitor WWL70. Understanding the interaction between endocannabinoid and eicosanoid pathways might provide a new avenue for the treatment of inflammatory and neuropathic pain.

Peripheral nerve and diclofenac sodium: Molecular and clinical approaches

Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most frequently prescribed medications worldwide. Diclofenac sodium (DS), one of these NSAIDs, has a high specificity for arachidonic acid-degrading cyclooxygenase (COX)-2 enzymes. This drug can be used to relieve neuropathic pain. In this review, we examine the relevant researches, including in vivo, animal, and clinical human studies, with the aim of understanding the effect of DS on the peripheral nerves. In injured nerves, COX-2 is potently upregulated around the injury site. When a nerve is damaged, both COX-1 and COX-2 expression is increased in macrophages and Schwann cells. In addition, COX inhibitors can promote axonal outgrowth in cultured neurons. Neuropathic pain occurs after injury and leads to dysfunction of the peripheral nervous system. NSAIDs can modulate the nociceptive and inflammatory pain pathways and control neuropathic pain. DS may accelerate nerve regeneration and its effects on healing, as well as causing deleterious effects in the developing nerves. DS teratogenicity disrupts myelin sheath thickness and axon structure. Understanding the possible benefits and limitations of DS and specific conditions such as prenatal use will be of benefit in clinical practice.

Antihyperalgesic and anti-inflammatory effects of atorvastatin in chronic constriction injury-induced neuropathic pain in rats

Atorvastatin is a 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase inhibitor used in treatment of hypercholesterolemia and prevention of coronary heart disease. The aim of this study is to investigate the antihyperalgesic and anti-inflammatory effects of atorvastatin (3, 10, and 30 mg/kg by oral gavages for 14 days) in chronic constriction injury (CCI) model of neuropathic pain in rats. CCI caused significant increase in tumor necrosis factor-α, interleukin 1 beta, prostaglandin E2, along with matrix metalloproteases (MMP-2) and nerve growth factor (NGF) levels in sciatic nerve and spinal cord concomitant with mechanical and thermal hyperalgesia, which were significantly reduced by oral administration of atorvastatin for 14 days as compared to CCI rats. Our study demonstrated that atorvastatin attenuates neuropathic pain through inhibition of cytokines, MMP-2, and NGF in sciatic nerve and spinal cord suggesting that atorvastatin could be an additional therapeutic strategy in management of neuropathic pain.

Epigenetic modification of DRG neuronal gene expression subsequent to nerve injury: etiological contribution to complex regional pain syndromes (Part I)

DRG is of importance in relaying painful stimulation to the higher pain centers and therefore could be a crucial target for early intervention aimed at suppressing primary afferent stimulation. Complex regional pain syndrome (CRPS) is a common pain condition with an unknown etiology. Recently added new information enriches our understanding of CRPS pathophysiology. Researches on genetics, biogenic amines, neurotransmitters, and mechanisms of pain modulation, central sensitization, and autonomic functions in CRPS revealed various abnormalities indicating that multiple factors and mechanisms are involved in the pathogenesis of CRPS. Epigenetics refers to mitotically and meiotically heritable changes in gene expression that do not affect the DNA sequence. As epigenetic modifications potentially play an important role in inflammatory cytokine metabolism, neurotransmitter responsiveness, and analgesic sensitivity, they are likely key factors in the development of chronic pain. In this dyad review series, we systematically examine the nerve injury-related changes in the neurological system and their contribution to CRPS. In this part, we first reviewed and summarized the role of neural sensitization in DRG neurons in performing function in the context of pain processing. Particular emphasis is placed on the cellular and molecular changes after nerve injury as well as different models of inflammatory and neuropathic pain. These were considered as the potential molecular bases that underlie nerve injury-associated pathogenesis of CRPS.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Arachidonic acid derivatives and their role in peripheral nerve degeneration and regeneration

After peripheral nerve injury, a process of axonal degradation, debris clearance, and subsequent regeneration is initiated by complex local signaling, called Wallerian degeneration (WD). This process is in part mediated by neuroglia as well as infiltrating inflammatory cells and regulated by inflammatory mediators such as cytokines, chemokines, and the activation of transcription factors also related to the inflammatory response. Part of this neuroimmune signaling is mediated by the innate immune system, including arachidonic acid (AA) derivatives such as prostaglandins and leukotrienes. The enzymes responsible for their production, cyclooxygenases and lipooxygenases, also participate in nerve degeneration and regeneration. The interactions between signals for nerve regeneration and neuroinflammation go all the way down to the molecular level. In this paper, we discuss the role that AA derivatives might play during WD and nerve regeneration, and the therapeutic possibilities that arise.

Prostaglandin E2 contributes to the synthesis of brain-derived neurotrophic factor in primary sensory neuron in ganglion explant cultures and in a neuropathic pain model

Brain-derived neurotrophic factor (BDNF) exists in small to medium size neurons in adult rat dorsal root ganglion (DRG) and serves as a modulator at the first synapse of the pain transmission pathway in the spinal dorsal horn. Peripheral nerve injury increases BDNF expression in DRG neurons, an event involved in the genesis of neuropathic pain. In the present study, we tested the hypothesis that prostaglandin E2 (PGE2) over-produced in injured nerves contributes to the up-regulation of BDNF in DRG neurons. Two weeks after partial sciatic nerve ligation (PSNL), BDNF levels in the ipsilateral L4-L6 DRG of injured rats were significantly increased compared to the contralateral side. Perineural injection of a selective cyclooxygenase (COX2) inhibitor or a PGE2 EP4 receptor antagonist not only dose-dependently relieved PSNL elicited mechanical hypersensitivity, but also suppressed the increased BDNF levels in DRG neurons. PSNL shifted BDNF expression in the ipsilateral DRG from small to medium and larger size injured neurons. BDNF is mainly co-expressed with the EP1 and EP4 while moderately with the EP2 and EP3 receptor subtypes in naïve and PSNL rats. PSNL also shifted the expression of EP1-4 receptors to a larger size population of DRG neurons. In DRG explant cultures, a stabilized PGE2 analog 16,16 dimethyl PGE2 (dmPGE2) or the agonists of EP1 and EP4 receptors significantly increased BDNF levels and the phosphorylated protein kinase A (PKA), extracellular signal-regulated kinase (ERK)/mitogen activated protein kinase (MAPK) and cAMP response element binding protein (CREB). The EP1 and EP4 antagonists, a sequester of nerve growth factor (NGF), the inhibitors of PKA and MEK as well as CREB small interfering RNA suppressed dmPGE2-induced BDNF. Taken together, EP1 and EP4 receptor subtypes, PKA, ERK/MAPK and CREB signaling pathways as well as NGF are involved in PGE2-induced BDNF synthesis in DRG neurons. Injured nerve derived-PGE2 contributes to BDNF up-regulation in DRG neurons following nerve injury. Facilitating the synthesis of BDNF in primary sensory neurons is a novel mechanism underlying the role of PGE2 in the genesis of neuropathic pain.

Role of prostaglandin E2 in the synthesis of the pro-inflammatory cytokine interleukin-6 in primary sensory neurons: an in vivo and in vitro study

Following various types of nerve injury, cyclooxygenase 2 and prostaglandin E2 (PGE2) are universally and chronically up-regulated in injured nerves and contribute to the genesis of neuropathic pain. Persistent high levels of PGE2 likely exert chronic effects on nociceptive dorsal root ganglion (DRG) neurons. In the present study, we tested the hypothesis that injured nerve-derived PGE2 contributes to the up-regulation of the pro-inflammatory cytokine interleukin-6 (IL-6) in DRG neurons following partial sciatic nerve ligation. In naive adult rats, IL-6 was expressed in only a few small size DRG neurons which all co-expressed EP4 receptors. Partial sciatic nerve ligation increased and shifted IL-6 expression from small to medium and large size damaged DRG neurons. Perineural injection of a selective cyclooxygenase 2 inhibitor or a selective EP4 receptor antagonist significantly suppressed the up-regulation of IL-6 in DRG, suggesting that injured nerve derived PGE2 contributes to the de novo synthesis of IL-6 in DRG neurons through EP4 receptors. In cultured sensory ganglion explants, a stabilized PGE2 analog increased IL-6 mRNA and protein levels through the activation of EP4, protein kinase A, protein kinase C, extracellular regulated protein kinase/MAPK, cAMP response element binding protein and NFκB signalling pathways. Taken together, these data indicate that facilitating the de novo synthesis of pain-related cytokines in injured medium and large size DRG neurons is a novel mechanism underlying the role of injured nerve derived PGE2 in the genesis of neuropathic pain.

Chronic prostaglandin E2 treatment induces the synthesis of the pain-related peptide substance P and calcitonin gene-related peptide in cultured sensory ganglion explants

Prostaglandin E2 (PGE2) is a well known pain and pro-inflammatory mediator abundantly produced in inflamed tissue. It causes pain by directly exciting nociceptive primary sensory neurons (nociceptors) and indirectly stimulating the release of pain-related peptide substance P (SP) and calcitonin gene-related peptide (CGRP). In an ex vivo culture of sensory ganglion explants, we tested the hypothesis that PGE2 could induce the synthesis of SP and CGRP in nociceptors. A stabilized PGE2 analog, 16,16-dimethyl PGE2, in a concentration- and time-dependent manner, significantly increased mRNA and peptide levels of SP and CGRP. The agonists of EP1 and EP4 receptors also significantly increased SP and CGRP levels. Moreover, 16,16-dimethyl PGE2-induced SP and CGRP were blocked by EP1 and EP4 antagonists as well as the inhibitors of both protein kinase A and protein kinase C. Nerve growth factor was partially involved in PGE2-induced SP and CGRP synthesis. Taken together, these results indicate that PGE2 contributes to the synthesis of SP and CGRP in nociceptors, an event mediated by EP1 and EP4 receptors, nerve growth factor and protein kinase A and protein kinase C signalling pathways. We thus conclude that facilitating the synthesis of pain-related peptides in nociceptors is a novel mechanism underlying the role of PGE2 in nociception and chronic pain states.

Celecoxib accelerates functional recovery after sciatic nerve crush in the rat

The inflammatory response appears to be essential in the modulation of the degeneration and regeneration process after peripheral nerve injury. In injured nerves, cyclooxygenase-2 (COX-2) is strongly upregulated around the injury site, possibly playing a role in the regulation of the inflammatory response. In this study we investigated the effect of celecoxib, a COX-2 inhibitor, on functional recovery after sciatic nerve crush in rats. Unilateral sciatic nerve crush injury was performed on 10 male Wistar rats. Animals on the experimental group (n = 5) received celecoxib (10 mg/kg ip) immediately before the crush injury and daily for 7 days after the injury. Control group (n = 5) received normal saline at equal regimen. A sham group (n = 5), where sciatic nerve was exposed but not crushed, was also evaluated. Functional recovery was then assessed by calculating the sciatic functional index (SFI) on days 0,1,7,14 and 21 in all groups, and registering the day of motor and walking onset. In comparison with control group, celecoxib treatment (experimental group) had significant beneficial effects on SFI, with a significantly better score on day 7. Anti-inflammatory drug celecoxib should be considered in the treatment of peripheral nerve injuries, but further studies are needed to explain the mechanism of its neuroprotective effects.

Injured nerve-derived COX2/PGE2 contributes to the maintenance of neuropathic pain in aged rats

Neuropathic pain (NeP) is a debilitating disease afflicting mostly the aged population. Inflammatory responses in injured nerves play a pivotal role in the pathogenesis of NeP. Injured nerve derived cyclooxygenase 2/prostaglandin E2 (COX2/PGE2) contributes to the genesis of NeP at the early stage in young rats. Here we show that COX2/PGE2 is involved in the maintenance of NeP at a chronic stage in aged rats. Eighteen months after partial sciatic nerve ligation (PSNL), NeP remained prominent in aged rats. COX2 expressing macrophages and PGE2 levels were increased in injured nerves. PGE2 receptors (EP1 and EP4) and pain-related ion channel transient receptor potential vanilloid-1 (TRPV1) were increased in the ipsilateral dorsal root ganglion (DRG) neurons of aged PSNL rats. Perineural injection of a selective COX2 inhibitor NS-398 relieved NeP, reversed PSNL increased expression of EP1, EP4 and TRPV1 and suppressed the levels of pain-related peptide substance P and calcitonin gene-related peptide in DRG neurons. These data suggest that injured nerve-derived PGE2 contributes to the maintenance of NeP at the chronic stage in aged rats. Chronically facilitating the synthesis of pain-related molecules in nociceptive DRG neurons is a novel mechanism underpinning the contribution of PGE2.

Does COX2-dependent PGE2 play a role in neuropathic pain?

Neuropathic pain (NeP) is a common chronic pain state with unmet medical needs. Due to poorly defined underlying mechanisms, current therapies for NeP are far from satisfactory. Mounting evidence suggests that long-term plasticity in pain signaling pathways underpins the pathogenesis of NeP. Inflammatory responses in injured nerves have been recognized as important events initially sensitizing nociceptive neurons and subsequently inducing long-term plasticity in the dorsal root ganglion. Inflammatory cells such as invading macrophages and Schwann cells produce a wide array of inflammatory mediators. Cyclooxygenase 2-dependent prostaglandin E2 (COX2/PGE2) is one of the important mediator abundantly produced in injured nerves and involved in the genesis of NeP. In this mini-review, we highlight possible novel mechanisms underlying the role of COX2/PGE2 in injured nerves in the genesis of NeP. Long lasting COX2/PGE2 in injured nerves may induce chronic effects on nociceptors to facilitate the synthesis of pain-related molecules by stimulating 'en passant' injured or spared axons. COX2/PGE2 may also induce chronic effects on local inflammatory cells in injured nerves to facilitate the synthesis of inflammatory mediators via autocrine and paracrine pathways. COX2/PGE2 in injured nerves and downstream PGE2 EP receptor signaling should be considered as therapeutic targets to more effectively treat chronic NeP.

Schwann cell-derived factors modulate synaptic activities at developing neuromuscular synapses

Glial cells are active participants in the function, formation, and maintenance of the chemical synapse. To investigate the molecular basis of neuron-glia interactions at the peripheral synapse, we examined whether and how Schwann cell-derived factors modulate synaptic function at developing neuromuscular junctions (NMJs). Schwann cell-conditioned medium (SC-CM) from Xenopus Schwann cell cultures was collected and applied to Xenopus nerve-muscle cocultures. We found that SC-CM increased the frequency of spontaneous synaptic currents (SSCs) within 3-15 min by an average of approximately 150-fold at developing neuromuscular synapses. The increase in SSC frequency by SC-CM is a presynaptic effect independent of neuronal excitability and requires the influx of Ca2+. In contrast to its potentiating effect on spontaneous transmitter release, SC-CM suppressed the evoked transmitter release. The SC-CM effect required the presence of motoneuron soma but not protein synthesis. Using molecular weight cutoff filters and dialysis membranes, we found that the molecular weight of functional factor(s) in SC-CM was within 500 and 5000 Da. The SC-CM effect was not attributable to currently known factors that modulate synaptic efficacy, including neurotrophins, glutamate, and ATP. SC-CM also enhanced spontaneous synaptic release at developing NMJs in Xenopus tadpoles in situ. Our results suggest that Schwann cells release small molecules that enhance spontaneous synaptic activities acutely and potently at developing neuromuscular synapses, and the glial cell-enhanced spontaneous neurotransmission may contribute to synaptogenesis.

Schwann cells express IP prostanoid receptors coupled to an elevation in intracellular cyclic AMP

We have shown previously that prostaglandin E(2) (PGE(2)) and prostaglandin I(2) (PGI(2)) are each produced in an explant model of peripheral nerve injury. We report that IP prostanoid receptor mRNA and protein are present in primary rat Schwann cells. IP prostanoid receptor stimulation using prostacyclin produced an elevation in intracellular cyclic AMP concentration ([cAMP](i)) in primary Schwann cells. Peak [cAMP](i) was observed between 5-15 min of stimulation followed by a gradual recovery toward basal level. Phosphorylation of cyclic AMP-response element binding protein (CREB) on Ser(133) was also detected after IP prostanoid receptor stimulation and CREB phosphorylation was inhibited completely by the protein kinase A inhibitor, H-89. Intracellular calcium levels were not affected by IP prostanoid receptor stimulation. Unlike forskolin, IP prostanoid receptor stimulation did not significantly augment Schwann cell proliferation in response to growth factor treatment. However, IP prostanoid receptor stimulation increased the number of Schwann cells that were able to generate a calcium transient in response to P2 purinergic receptor activation. These findings suggest that signaling via the IP prostanoid receptor may by relevant to Schwann cell biology in vivo.

Targeting invading macrophage-derived PGE2, IL-6 and calcitonin gene-related peptide in injured nerve to treat neuropathic pain

Immune and inflammatory responses occurring in an injured nerve have been generally believed to contribute to the generation and maintenance of neuropathic pain. In this review, the authors demonstrate the upregulation of COX-2/prostaglandin E2, IL-6 and calcitonin gene-related peptide in invading macrophages and discuss possible mechanisms involved in their upregulation and how they contribute to the maintenance of neuropathic pain. By acting on nociceptors in dorsal root ganglion and local inflammatory cells via autocrine or paracrine pathways, these inflammatory mediators facilitate spontaneous ectopic activity and sustain nociceptive responses, an important mechanism underlying both ongoing and evoked neuropathic pain state. Targeting these mediators in injured nerve may provide novel therapeutic avenues to more successfully treat nerve injury-associated neuropathic pain.

Altered spinal arachidonic acid turnover after peripheral nerve injury regulates regional glutamate concentration and neuropathic pain behaviors in rats

Spinal glutamate transporters (GT) have been implicated in the mechanisms of neuropathic pain; however, how spinal GT uptake activity is regulated remains unclear. Here we show that alteration of spinal arachidonic acid (AA) turnover after peripheral nerve injury regulated regional GT uptake activity and glutamate homeostasis. Chronic constriction nerve injury (CCI) in rats significantly reduced spinal GT uptake activity ((3)H-glutamate uptake) with an associated increase in extracellular AA and glutamate concentration from spinal microdialysates on postoperative day 8. AACOCF3 (a cytosolic phospholipase A2 inhibitor, 30mug) given intrathecally twice a day for postoperative day 1-7 reversed this CCI-induced spinal AA production, prevented the reduced spinal GT uptake activity and increased extracellular glutamate concentration. Conversely, alteration of spinal AA metabolism by diclofenac (a cyclooxygenase 1/2 inhibitor, 200mug) further reduced spinal GT uptake activity and increased extracellular glutamate concentration in CCI rats. GT uptake activity was also attenuated when AA (10 or 100nM) was directly added into spinal samples of naïve rats in an in vitro(3)H-glutamate uptake assay, indicating a direct inhibitory effect of AA on GT uptake activity. Consistent with these findings, AACOCF3 reduced the development of both thermal hyperalgesia and mechanical allodynia, whereas diclofenac exacerbated thermal hyperalgesia, in CCI rats. Thus, spinal AA turnover may serve as a regulator in CCI-induced changes in regional GT uptake activity, glutamate homeostasis, and neuropathic pain behaviors. These data suggest that regulating spinal AA turnover may be a useful approach to improving the clinical management of neuropathic pain.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.