IL-1 beta potentiates heat-activated currents in rat sensory neurons: involvement of IL-1RI, tyrosine kinase, and protein kinase C

Sphingosine-1-phosphate-induced nociceptor excitation and ongoing pain behavior in mice and humans is largely mediated by S1P3 receptor

The biolipid sphingosine-1-phosphate (S1P) is an essential modulator of innate immunity, cell migration, and wound healing. It is released locally upon acute tissue injury from endothelial cells and activated thrombocytes and, therefore, may give rise to acute post-traumatic pain sensation via a yet elusive molecular mechanism. We have used an interdisciplinary approach to address this question, and we find that intradermal injection of S1P induced significant licking and flinching behavior in wild-type mice and a dose-dependent flare reaction in human skin as a sign of acute activation of nociceptive nerve terminals. Notably, S1P evoked a small excitatory ionic current that resulted in nociceptor depolarization and action potential firing. This ionic current was preserved in "cation-free" solution and blocked by the nonspecific Cl(-) channel inhibitor niflumic acid and by preincubation with the G-protein inhibitor GDP-β-S. Notably, S1P(3) receptor was detected in virtually all neurons in human and mouse DRG. In line with this finding, S1P-induced neuronal responses and spontaneous pain behavior in vivo were substantially reduced in S1P(3)(-/-) mice, whereas in control S1P(1) floxed (S1P(1)(fl/fl)) mice and mice with a nociceptor-specific deletion of S1P(1)(-/-) receptor (SNS-S1P(1)(-/-)), neither the S1P-induced responses in vitro nor the S1P-evoked pain-like behavior was altered. Therefore, these findings indicate that S1P evokes significant nociception via G-protein-dependent activation of an excitatory Cl(-) conductance that is largely mediated by S1P(3) receptors present in nociceptors, and point to these receptors as valuable therapeutic targets for post-traumatic pain.

Enhancement by interleukin-1β of AMPA and NMDA receptor-mediated currents in adult rat spinal superficial dorsal horn neurons

Background

Proinflammatory cytokine interleukin-1β (IL-1β) released from spinal microglia plays an important role in the maintenance of acute and chronic pain states. However, the cellular basis of this action remains poorly understood. Using whole-cell patch-clamp recordings, we examined the action of IL-1β on AMPA- and NMDA-receptor-mediated currents recorded from substantia gelatinosa (SG) neurons of adult rat spinal cord slices which are key sites for regulating nociceptive transmission from the periphery.

Results

AMPA- and NMDA-induced currents were increased in peak amplitude by IL-1β in a manner different from each other in SG neurons. These facilitatory actions of IL-1β were abolished by IL-1 receptor (IL-1R) antagonist (IL-1ra), which by itself had no detectable effects on AMPA- and NMDA-induced currents. The AMPA- but not NMDA-induced current facilitated by IL-1β was recovered to control level 30 min after IL-1β washout and largely depressed in Na⁺-channel blocker tetrodotoxin-containing or nominally Ca²⁺-free Krebs solution. Minocycline, a microglia inhibitor, blocked the facilitatory effect of IL-1β on AMPA- but not NMDA-induced currents, where minocycline itself depressed NMDA- but had not any effects on AMPA-induced currents.

Conclusions

IL-1β enhances AMPA and NMDA responses in SG neurons through IL-1R activation; the former but not latter action is reversible and due to an increase in neuronal activity in a manner dependent on extracellular Ca²⁺ and minocycline. It is suggested that AMPA and NMDA receptors are positively modulated by IL-1β in a manner different from each other; the former but not latter is mediated by a neurotransmitter released as a result of an increase in neuronal activity. Since IL-1β contributes to nociceptive behavior induced by peripheral nerve or tissue injury, the present findings also reveal an important cellular link between neuronal and glial cells in the spinal dorsal horn.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Cytokine expression in the epidural space: a model of noncompressive disc herniation-induced inflammation

STUDY DESIGN

Animal study.

OBJECTIVE

Development of an animal model for the study of biochemical changes that occur in the epidural space after intervertebral disc herniation.

SUMMARY OF BACKGROUND DATA

Although strong evidence for an inflammatory component exists, the biochemical processes underlying pain after disc herniation remain unknown.

METHODS

Epidural lavage was performed in 48 rats after L5 dorsal root ganglion exposure at baseline and 3, 6, or 24 hours after placement of autologous nucleus pulposus (NP) (N = 15), saline (N = 15), or NP + an interferon-γ antibody (anti-IFN-γ; N = 18) directly onto the dorsal root ganglion. Multiplex assays quantifying interleukin (IL)-1α, IL-1β, IL-2, IL-4, IL-6, IL-10, tumor necrosis factor α (TNF-α), IFN-γ, and granulocyte-macrophage colony-stimulating factor (GM-CSF) were performed. NP (N = 7) was also analyzed for these cytokines by placing NP into saline and measuring the relative concentration.

RESULTS

Cytokines measured low at baseline (0-100 pg/mL) in all groups. Compared with saline, NP application caused IL-6 elevation, peaking at T = 3 hours, that was prevented by anti-IFN-γ. NP induced elevation of TNF-α, peaking at T = 24 hours and was prevented by anti-IFN-γ. IFN-γ was elevated after NP at T = 3 hours and T = 24 hours. IL-1α was similar after saline versus NP. The concentrations of IL-1β and IL-10 were elevated at T = 3 hours, 6 hours, and 24 hours in all groups without between-groups difference. The level of IL-4 peaked at T = 3 hours in the NP group and was different than saline and NP + anti-IFN-γ groups, but the time effect was insignificant. There was no change for GM-CSF. The concentration of cytokines measured in normal NP was less than 2 pg/mL for all cytokines except TNF-α.

CONCLUSION

In this model of acute noncompressive disc herniation, NP caused the elevation of epidural IL-6, TNF-α, and IFN-γ--all attenuated by IFN-γ blockade. IL-1β and IL-10 were both significantly elevated by saline alone and their response was not prevented by IFN-γ blockade. This model may prove useful for the study of the biochemical processes by which NP induces inflammation-induced nerve root irritation and radiculopathic pain.

Role of inflammation and cytokines in peripheral nerve regeneration

This chapter provides a review of immune reactions involved in classic as well as alternative methods of peripheral nerve regeneration, and mainly with a view to understanding their beneficial effects. Axonal degeneration distal to nerve damage triggers a cascade of inflammatory events alongside injured nerve fibers known as Wallerian degeneration (WD). The early inflammatory reactions of WD comprise the complement system, arachidonic acid metabolites, and inflammatory mediators that are related to myelin fragmentation and activation of Schwann cells. Fine-tuned upregulation of the cytokine/chemokine network by Schwann cells activates resident and hematogenous macrophages to complete the clearance of axonal and myelin debris and stimulate regrowth of axonal sprouts. In addition to local effects, immune reactions of neuronal bodies and glial cells are also implicated in the survival and conditioning of neurons to regenerate severed nerves. Understanding of the cellular and molecular interactions between the immune system and peripheral nerve injury opens new possibilities for targeting inflammatory mediators to improve functional reinnervation.

Construction of a global pain systems network highlights phospholipid signaling as a regulator of heat nociception

The ability to perceive noxious stimuli is critical for an animal's survival in the face of environmental danger, and thus pain perception is likely to be under stringent evolutionary pressure. Using a neuronal-specific RNAi knock-down strategy in adult Drosophila, we recently completed a genome-wide functional annotation of heat nociception that allowed us to identify α2δ3 as a novel pain gene. Here we report construction of an evolutionary-conserved, system-level, global molecular pain network map. Our systems map is markedly enriched for multiple genes associated with human pain and predicts a plethora of novel candidate pain pathways. One central node of this pain network is phospholipid signaling, which has been implicated before in pain processing. To further investigate the role of phospholipid signaling in mammalian heat pain perception, we analysed the phenotype of PIP5Kα and PI3Kγ mutant mice. Intriguingly, both of these mice exhibit pronounced hypersensitivity to noxious heat and capsaicin-induced pain, which directly mapped through PI3Kγ kinase-dead knock-in mice to PI3Kγ lipid kinase activity. Using single primary sensory neuron recording, PI3Kγ function was mechanistically linked to a negative regulation of TRPV1 channel transduction. Our data provide a systems map for heat nociception and reinforces the extraordinary conservation of molecular mechanisms of nociception across different species.

Nociception is the perception of noxious, potentially damaging stimuli; and this pain or its equivalent behavioral readout is evolutionarily conserved from fruit flies to humans. Using genetic techniques in the fruit fly, we have been able to evaluate the potential functional contribution of every gene in the fruit fly genome for a role in avoidance of high noxious temperatures (heat pain-like responses). Using this functional genomics data set, we have developed a conserved network map of heat pain/nociception that predicts numerous conserved genes and pathways as novel pain pathways, including phospholipid signaling. Studies in multiple mutant mice confirmed a role for lipid signaling in pain perception, and more specifically we identify the critical lipid kinase (PI3Kγ) as a negative regulator of TRPV1 (receptor for noxious heat and capsaicin, the active component in chili peppers) signaling. This finding shows that our fly-based genetic pain network map is a valuable tool for the discovery of novel “nociception genes” in mammals.

Antihypernociceptive activity of anethole in experimental inflammatory pain

Anethole has been reported to have antioxidant, antibacterial, antifungal, antiinflammatory, and anesthetic properties. In the present study, we evaluated the effects of anethole in two pain models of inflammatory origin: acute inflammation induced by carrageenan and persistent inflammation induced by Complete Freund's adjuvant. We evaluated the effects of anethole (125, 250, and 500 mg/kg) on the development of paw oedema and mechanical hypernociception. The liver was collected for histological analysis. Paw skin was collected to determine the levels of the cytokines tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-17 (IL-17), and myeloperoxidase activity. Blood was collected to assess alanine transaminase (ALT) and aspartate transaminase (AST). The chemical composition of star anise oil was determined by gas chromatography/mass spectrometry (GC/MS), showing a presence of anethole of 98.1%. Oral pretreatment with anethole in mice inhibited paw oedema, mechanical pernociception, myelopewroxidase activity, TNF-α, IL-1β and IL-17 levels in acute and persistent inflammation models. Additionally, anethole treatment did not alter prostaglandin E2-induced mechanical hypernociception. Possible side effects were also examined. Seven-day anethole treatment did not alter plasma AST and ALT levels, and the histological profile of liver tissue was normal. The present study provides evidence of the antiinflammatory and analgesic activities of anethole in acute and persistent inflammation models.

The role of the immune system in the generation of neuropathic pain

Persistent pain is a sequela of several neurological conditions with a primary immune basis, such as Guillain-Barré syndrome and multiple sclerosis. Additionally, diverse forms of injury to the peripheral or the central nervous systems--whether traumatic, metabolic, or toxic--result in substantial recruitment and activation of immune cells. This response involves the innate immune system, but evidence also exists of T-lymphocyte recruitment, and in some patient cohorts antibodies to neuronal antigens have been reported. Mediators released by immune cells, such as cytokines, sensitise nociceptive signalling in the peripheral and central nervous systems. Preclinical data suggest an immune pathogenesis of neuropathic pain, but clinical evidence of a central role of the immune system is less clear. An important challenge for the future is to establish to what extent this immune response initiates or maintains neuropathic pain in patients and thus whether it is amenable to therapy.

Gene expression in skin, muscle, and dorsal root ganglion after plantar incision in the rat

BACKGROUND

Treating postoperative pain remains a significant challenge for perioperative medicine. Recent studies have shown that nerve growth factor is up-regulated and contributes to incisional pain. To date, few studies have examined expression of other neurotrophin-related mediators that may contribute to the development and/or maintenance of incisional pain.

METHODS

Male Sprague-Dawley rats underwent a plantar incision, and pain behaviors were examined (n = 6). In a separate group of rats, expression of neurotrophic factors were studied. At various times after incision (n = 4) or sham surgery (n = 4), the skin, muscle, and dorsal root ganglia were harvested and total RNA isolated. Real-time reverse transcription polymerase chain reaction was performed and the fold change in gene expression was analyzed using significance analysis of microarrays.

RESULTS

Several genes were changed (P < 0.05) as early as 1 h after incision. Expression of artemin and nerve growth factor were increased in both incised skin and muscle. Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-5 were all down-regulated in the skin but up-regulated in the muscle 48 h after incision. Few genes changed in the dorsal root ganglion. Most changes in expression occurred in the first 48 h after incision, a timeframe when pain behavior was the greatest.

CONCLUSION

Surgical incision is associated with pain-related gene expression changes in skin, muscle, and, to a lesser extent, dorsal root ganglion. The gene expression profile provides clues as to mediators that are involved in peripheral sensitization and pain transmission after surgical incision and also suggest mechanisms for resolution of postoperative pain when more persistent pain syndromes like neuropathic pain continue.

G protein-coupled receptor kinase 6 acts as a critical regulator of cytokine-induced hyperalgesia by promoting phosphatidylinositol 3-kinase and inhibiting p38 signaling

The molecular mechanisms determining magnitude and duration of inflammatory pain are still unclear. We assessed the contribution of G protein-coupled receptor kinase (GRK)-6 to inflammatory hyperalgesia in mice. We showed that GRK6 is a critical regulator of severity and duration of cytokine-induced hyperalgesia. In GRK6⁻/⁻ mice, a significantly lower dose (100 times lower) of intraplantar interleukin (IL)-1β was sufficient to induce hyperalgesia compared with wild-type (WT) mice. In addition, IL-1β hyperalgesia lasted much longer in GRK6⁻/⁻ mice than in WT mice (8 d in GRK6⁻/⁻ versus 6 h in WT mice). Tumor necrosis factor (TNF)-α-induced hyperalgesia was also enhanced and prolonged in GRK6⁻/⁻ mice. In vitro, IL-1β-induced p38 phosphorylation in GRK6⁻/⁻ dorsal root ganglion (DRG) neurons was increased compared with WT neurons. In contrast, IL-1β only induced activation of the phosphatidylinositol (PI) 3-kinase/Akt pathway in WT neurons, but not in GRK6⁻/⁻ neurons. In vivo, p38 inhibition attenuated IL-1β- and TNF-α-induced hyperalgesia in both genotypes. Notably, however, whereas PI 3-kinase inhibition enhanced and prolonged hyperalgesia in WT mice, it did not have any effect in GRK6-deficient mice. The capacity of GRK6 to regulate pain responses was also apparent in carrageenan-induced hyperalgesia, since thermal and mechanical hypersensitivity was significantly prolonged in GRK6⁻/⁻ mice. Finally, GRK6 expression was reduced in DRGs of mice with chronic neuropathic or inflammatory pain. Collectively, these findings underline the potential role of GRK6 in pathological pain. We propose the novel concept that GRK6 acts as a kinase that constrains neuronal responsiveness to IL-1β and TNF-α and cytokine-induced hyperalgesia via biased cytokine-induced p38 and PI 3-kinase/Akt activation.

Oncostatin M induces heat hypersensitivity by gp130-dependent sensitization of TRPV1 in sensory neurons

Oncostatin M (OSM) is a member of the interleukin-6 cytokine family and regulates eg. gene activation, cell survival, proliferation and differentiation. OSM binds to a receptor complex consisting of the ubiquitously expressed signal transducer gp130 and the ligand binding OSM receptor subunit, which is expressed on a specific subset of primary afferent neurons. In the present study, the effect of OSM on heat nociception was investigated in nociceptor-specific gp130 knock-out (SNS-gp130^-/-) and gp130 floxed (gp130^fl/fl) mice.

Subcutaneous injection of pathophysiologically relevant concentrations of OSM into the hind-paw of C57BL6J wild type mice significantly reduced paw withdrawal latencies to heat stimulation. In contrast to gp130^fl/fl mice, OSM did not induce heat hypersensitivity in vivo in SNS-gp130^-/- mice. OSM applied at the receptive fields of sensory neurons in in vitro skin-nerve preparations showed that OSM significantly increased the discharge rate during a standard ramp-shaped heat stimulus. The capsaicin- and heat-sensitive ion channel TRPV1, expressed on a subpopulation of nociceptive neurons, has been shown to play an important role in inflammation-induced heat hypersensitivity. Stimulation of cultured dorsal root ganglion neurons with OSM resulted in potentiation of capsaicin induced ionic currents. In line with these recordings, mice with a null mutation of the TRPV1 gene did not show any signs of OSM-induced heat hypersensitivity in vivo.

The present data suggest that OSM induces thermal hypersensitivity by directly sensitizing nociceptors via OSMR-gp130 receptor mediated potentiation of TRPV1.

The Central Role of Glia in Pathological Pain and the Potential of Targeting the Cannabinoid 2 Receptor for Pain Relief

Under normal conditions, acute pain processing consists of well-characterized neuronal signaling events. When dysfunctional pain signaling occurs, pathological pain ensues. Glial activation and their released factors participate in the mediation of pathological pain. The use of cannabinoid compounds for pain relief is currently an area of great interest for both basic scientists and physicians. These compounds, bind mainly either the cannabinoid receptor subtype 1 (CB(1)R) or cannabinoid receptor subtype 2 (CB(2)R) and are able to modulate pain. Although cannabinoids were initially only thought to modulate pain via neuronal mechanisms within the central nervous system, strong evidence now supports that CB(2)R cannabinoid compounds are capable of modulating glia, (e.g. astrocytes and microglia) for pain relief. However, the mechanisms underlying cannabinoid receptor-mediated pain relief remain largely unknown. An emerging body of evidence supports that CB(2)R agonist compounds may prove to be powerful novel therapeutic candidates for the treatment of chronic pain.

Local action of the proinflammatory cytokines IL-1β and IL-6 on intracranial meningeal nociceptors

BACKGROUND

Peripheral nociceptive action of the proinflammatory cytokines IL-1β and IL-6 has been implicated in the pathogenesis of numerous pain syndromes. An increase in the level of these cytokines in jugular venous blood has been reported during migraine attacks, suggesting their potential involvement in mediating the intracranial headache of migraine.

METHODS

In this work we examined, using in vivo single-unit recording of meningeal nociceptors in the trigeminal ganglion of anesthetized rats, whether the peripheral actions of IL-1β and IL-6 can promote the activation and sensitization of nociceptors that innervate the intracranial meninges, two neural processes that are believed to play a key role in promoting the intracranial throbbing pain of migraine.

RESULTS

We found that meningeal application of IL-1β leads to the activation and mechanical sensitization of about 70% and 45% of the nociceptors respectively. In contrast, IL-6 was a very poor modulator of meningeal nociceptors' response properties affecting overall only about 20% of the nociceptors.

CONCLUSIONS

Our study provides for the first time in vivo electrophysiological evidence that meningeal action of IL-1β can promote the activation and increased mechanosensitivity of intracranial meningeal nociceptors and that IL-6 generally lacks these properties. Future studies are required to examine the mechanism that plays a role in mediating the nociceptive effects of IL-1β on meningeal nociceptors, which may serve as a target for migraine therapy.

Role of interleukins, IGF and stem cells in BPH

The condition known as benign prostatic hyperplasia may be defined as a benign enlargement of the prostate gland resulting from a proliferation of both benign epithelial and stromal elements. It might also be defined clinically as a constellation of lower urinary tract symptoms (LUTSs) in aging men. The purpose of this review is to consider the ways in which inflammatory cytokines belonging to the interleukin family, members of the IFG family, and stem cells may contribute to the development and progression of BPH-LUTS. This might occur in three mechanisms: One, interleukin signaling, IFG signaling and stem cells may contribute to reactivation of developmental growth mechanisms in the adult prostate leading to tissue growth. Two, given that epidemiologic studies indicate an increased incidence of BPH-LUTS in association with obesity and diabetes, IFG signaling may provide the mechanistic basis for the effect of diabetes and obesity on prostate growth. Three, expression of interleukins in association with inflammation in the prostate may induce sensitization of afferent fibers innervating the prostate and result in increased sensitivity to pain and noxious sensations in the prostate and bladder and heightened sensitivity to bladder filling.

Antinociceptive and gastroprotective actions of ethanolic extract from Pluchea sagittalis (Lam.) Cabrera

ETHNOPHARMACOLOGICAL RELEVANCE

Pluchea sagittalis, an herbaceous plant widely distributed in South America, is used in folk medicine for the treatment of digestive diseases and inflammation.

AIM OF THE STUDY

This study was designed to investigate the antinociceptive and gastroprotective effects of the ethanolic extract (EE) of aerial parts from Pluchea sagittalis in rodents.

MATERIALS AND METHODS

The antinociceptive effects of EE was evaluated in mice after oral administration in chemical tests (acetic-acid, glutamate and formalin) or by biting behavior following intrathecal administration of cytokines such as interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in mice. Furthermore, rats were treated with EE and subsequently exposed to acute gastric lesions induced by 80% ethanol. Afterwards the gastric lesion extension and the mucus levels of gastric mucosa were measured.

RESULTS

The oral administration of EE showed a dose-dependent inhibition of acetic acid-induced abdominal constrictions and glutamate-induced pain in mice, with ID(50) values of 624.0 (523.0-746.0) mg/kg and 368.0 (216.0-628.0) mg/kg, respectively. In the formalin test, the EE also produced significant inhibition of the inflammatory phase, with an ID(50) value of 411.0 (183.0-721.0) mg/kg; however, it was ineffective in the neurogenic phase caused by formalin. In addition, oral treatment with EE caused a significant inhibition of biting behavior induced by i.t. injection of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). The antinociception caused by the EE (300 mg/kg, p.o.) was not reversed by naloxone (1 mg/kg, i.p.) when assessed in the acetic acid writhing test. The EE (300-1000 mg/kg, p.o.) did not affect the motor coordination of animals in an open-field model. Oral treatment with the EE protected rats against gastric lesions induced by ethanol, with an ID(50) value of 55.0 (46.6-64.9) mg/kg, and increased the mucus levels of gastric mucosa to levels found in the non-lesioned group.

CONCLUSIONS

The mechanism by which the extract produced antinociception still remains unclear, but this effect seems to be primarily related to the modulation or inhibition of the action of pro-inflammatory mediators. Furthermore, these data support, at least in part, the ethnomedical use of Pluchea sagittalis.

Role of the transient receptor potential vanilloid 1 in inflammation and sepsis

The transient receptor potential vanilloid 1 (TRPV1) is a thermoreceptor that responds to noxious temperatures, as well as to chemical agonists, such as vanilloids and protons. In addition, its channel activity is notably potentiated by proinflammatory mediators released upon tissue damage. The TRPV1 contribution to sensory neuron sensitization by proalgesic agents has signaled this receptor as a prime target for analgesic and anti-inflammatory drug intervention. However, TRPV1 antagonists have notably failed in clinical and preclinical studies because of their unwanted side effects. Recent reports have unveiled previously unrecognized anti-inflammatory and protective functions of TRPV1 in several diseases. For instance, this channel has been suggested to play an anti-inflammatory role in sepsis. Therefore, the use of potent TRPV1 antagonists as a general strategy to treat inflammation must be cautiously considered, given the deleterious effects that may arise from inhibiting the population of channels that have a protective function. The use of TRPV1 antagonists may be limited to treating those pathologies where enhanced receptor activity contributes to the inflamed state. Alternatively, therapeutic paradigms, such as reduction of inflammatory-mediated increase of receptor expression in the cell surface, may be a better strategy to prevent abrogation of the TRPV1 subpopulation involved in anti-inflammatory and protective processes.

Genetic evidence for involvement of neuronally expressed S1P₁ receptor in nociceptor sensitization and inflammatory pain

Sphingosine-1-phosphate (S1P) is a key regulator of immune response. Immune cells, epithelia and blood cells generate high levels of S1P in inflamed tissue. However, it is not known if S1P acts on the endings of nociceptive neurons, thereby contributing to the generation of inflammatory pain. We found that the S1P₁ receptor for S1P is expressed in subpopulations of sensory neurons including nociceptors. Both S1P and agonists at the S1P₁ receptor induced hypersensitivity to noxious thermal stimulation in vitro and in vivo. S1P-induced hypersensitivity was strongly attenuated in mice lacking TRPV1 channels. S1P and inflammation-induced hypersensitivity was significantly reduced in mice with a conditional nociceptor-specific deletion of the S1P₁ receptor. Our data show that neuronally expressed S1P₁ receptors play a significant role in regulating nociceptor function and that S1P/S1P₁ signaling may be a key player in the onset of thermal hypersensitivity and hyperalgesia associated with inflammation.

Ion channels in inflammation

Most physical illness in vertebrates involves inflammation. Inflammation causes disease by fluid shifts across cell membranes and cell layers, changes in muscle function and generation of pain. These disease processes can be explained by changes in numbers or function of ion channels. Changes in ion channels have been detected in diarrhoeal illnesses, pyelonephritis, allergy, acute lung injury and systemic inflammatory response syndromes involving septic shock. The key role played by changes in ion transport is directly evident in inflammation-induced pain. Expression or function of all major categories of ion channels like sodium, chloride, calcium, potassium, transient receptor potential, purinergic receptor and acid-sensing ion channels can be influenced by cyto- and chemokines, prostaglandins, leukotrienes, histamine, ATP, reactive oxygen species and protons released in inflammation. Key pathways in this interaction are cyclic nucleotide, phosphoinositide and mitogen-activated protein kinase-mediated signalling, direct modification by reactive oxygen species like nitric oxide, ATP or protons and disruption of the cytoskeleton. Therapeutic interventions to modulate the adverse and overlapping effects of the numerous different inflammatory mediators on each ion transport system need to target adversely affected ion transport systems directly and locally.

A genome-wide Drosophila screen for heat nociception identifies α2δ3 as an evolutionarily conserved pain gene

Worldwide, acute, and chronic pain affects 20% of the adult population and represents an enormous financial and emotional burden. Using genome-wide neuronal-specific RNAi knockdown in Drosophila, we report a global screen for an innate behavior and identify hundreds of genes implicated in heat nociception, including the α2δ family calcium channel subunit straightjacket (stj). Mice mutant for the stj ortholog CACNA2D3 (α2δ3) also exhibit impaired behavioral heat pain sensitivity. In addition, in humans, α2δ3 SNP variants associate with reduced sensitivity to acute noxious heat and chronic back pain. Functional imaging in α2δ3 mutant mice revealed impaired transmission of thermal pain-evoked signals from the thalamus to higher-order pain centers. Intriguingly, in α2δ3 mutant mice, thermal pain and tactile stimulation triggered strong cross-activation, or synesthesia, of brain regions involved in vision, olfaction, and hearing.

Involvement of lysophosphatidic acid in bone cancer pain by potentiation of TRPV1 via PKCε pathway in dorsal root ganglion neurons

Background

It has been demonstrated that lysophosphatidic acid (LPA) released from injury tissue and transient receptor potential vanilloid 1 (TRPV1) receptor are implicated in the induction of chronic pain. In the present study we examined whether an interaction between LPA receptor LPA₁ and TRPV1 in dorsal root ganglion (DRG) neurons contributes to the development of bone cancer pain.

Results

Bone cancer was established by injection of mammary gland carcinoma cells into the rat tibia. Following the development of bone cancer pain, the TRPV1 expression and capsaicin-evoked currents were up-regulated in rat DRG neurons at L_4-6 segments. Immunohistochemistry staining revealed a high co-localization of LPA₁ with TRPV1 in DRG neurons. In isolated DRG neurons, whole-cell patch recording showed that capsaicin-induced currents were potentiated by LPA in a dose-dependent manner. The potentiation was blocked by either LPA₁ antagonist, protein kinase C (PKC) inhibitor or PKCϵ inhibitor, but not by protein kinase A (PKA) inhibitor or Rho inhibitor. In the behavioral tests, both mechanical allodynia and thermal hyperalgesia in bone cancer rats were attenuated by LPA₁ antagonist.

Conclusion

LPA potentiates TRPV1 current via a PKCϵ-dependent pathway in DRG neurons of rats with bone cancer, which may be a novel peripheral mechanism underlying the induction of bone cancer pain.

Molecular effects of interleukin-1β on dorsal root ganglion neurons: prevention of ligand-induced internalization of the bradykinin 2 receptor and downregulation of G protein-coupled receptor kinase 2

In dorsal root ganglion sections numerous small-to medium-sized neurons were found to exhibit extensive colocalization of the bradykinin receptor 2, the interleukin-1 receptor 1 and G protein-coupled receptor kinase 2. Application of bradykinin to cultured DRG neurons caused substantial internalization of the bradykinin 2 receptor which significantly reduced the responsiveness of DRG neurons to a second application of bradykinin. Such an internalization was not observed in DRG neurons which were exposed to long-term pretreatment with interleukin-1β. The long-term incubation with interleukin-1β on its own did neither change the proportion of neurons which expressed the bradykinin 2 receptor in the cytoplasma nor the proportion of neurons expressing the bradykinin 2 receptor in the membrane but it reduced the proportion of neurons expressing G protein-coupled receptor kinase 2, an enzyme which facilitates the internalization of G protein-coupled receptors. These results show that interleukin-1β maintains the responsiveness of DRG neurons to bradykinin in the long-term range, and they suggest that the downregulation of G protein-coupled receptor kinase 2 could be a cellular mechanism involved in this interleukin-1β effect.

Insights into the regulation of chemokine receptors by molecular signaling pathways: functional roles in neuropathic pain

Inflammation plays a central role in the manner that the nervous system responds to injury. These effects include vasodilatation, increased vascular permeability, plasma extravasation, cell migration, and pain. Extracellular signals associated with inflammation may also lead to increased levels of pro-nociceptive chemokines/receptors that directly contribute to persistent or chronic pain behavior. To date, research focused on improving the treatment of chronic pain has largely ignored the role of inflammation-associated transcription factors such as nuclear transcription factor in activated T cells (NFAT). Herein we discuss the idea that activation of this transcription factor may be responsible for the production of chemokines receptors in both neuronal and non-neuronal cells of the peripheral nervous system. Taken together, a better understanding of the transcription of these pro-nociceptive genes may lead to the development of novel analgesic targets.

The effect of morphine on glial cells as a potential therapeutic target for pharmacological development of analgesic drugs

Opioids have played a critical role in achieving pain relief in both modern and ancient medicine. Yet, their clinical use can be limited secondary to unwanted side effects such as tolerance, dependence, reward, and behavioral changes. Identification of glial-mediated mechanisms inducing opioid side effects include cytokine receptors, kappa-opioid receptors, N-methyl-D-aspartate receptors, and the recently elucidated Toll-like receptors. Newer agents targeting these receptors such as AV411, MK-801, AV333, and SLC022, and older agents used outside the United States or for other disease conditions, such as minocycline, pentoxifylline, and UV50488H, all show varied but promising profiles for providing significant relief from opioid side effects, while simultaneously potentiating opioid analgesia.

Physiology and pharmacology of the vanilloid receptor

The identification and cloning of the vanilloid receptor 1 (TRPV1) represented a significant step for the understanding of the molecular mechanisms underlying the transduction of noxious chemical and thermal stimuli by peripheral nociceptors. TRPV1 is a non-selective cation channel gated by noxious heat, vanilloids and extracellular protons. TRPV1 channel activity is remarkably potentiated by pro-inflammatory agents, a phenomenon that is thought to underlie the peripheral sensitisation of nociceptors that leads to thermal hyperalgesia. Cumulative evidence is building a strong case for the involvement of this receptor in the etiology of both peripheral and visceral inflammatory pain, such as inflammatory bowel disease, bladder inflammation and cancer pain. The validation of TRPV1 receptor as a key therapeutic target for pain management has thrust intensive drug discovery programs aimed at developing orally active antagonists of the receptor protein. Nonetheless, the real challenge of these drug discovery platforms is to develop antagonists that preserve the physiological activity of TRPV1 receptors while correcting over-active channels. This is a condition to ensure normal pro-prioceptive and nociceptive responses that represent a safety mechanism to prevent tissue injury. Recent and exciting advances in the function, dysfunction and modulation of this receptor will be the focus of this review.

Inflammatory pain: the cellular basis of heat hyperalgesia

Injury or inflammation release a range of inflammatory mediators that increase the sensitivity of sensory neurons to noxious thermal or mechanical stimuli. The heat- and capsaicin-gated channel TRPV1, which is an important detector of multiple noxious stimuli, plays a critical role in the development of thermal hyperalgesia induced by a wide range of inflammatory mediators. We review here recent findings on the molecular mechanisms of sensitisation of TRPV1 by inflammatory mediators, including bradykinin, ATP, NGF and prostaglandins. We describe the signalling pathways believed to be involved in the potentiation of TRPV1, and our current understanding of how inflammatory mediators couple to these pathways.

Anti-inflammatory and antinociceptive effects of Baccharis dracunculifolia DC (Asteraceae) in different experimental models

AIM OF THE STUDY

The aerial parts of Baccharis dracunculifolia D.C., popularly known as "alecrim do campo", are used in folk medicine as anti-inflammatory. The aim of the present study was to evaluate the anti-inflammatory and antinociceptive activities of the crude hydroalcoholic extract obtained from leaves of Baccharis dracunculifolia (BdE), which have not been reported.

MATERIALS AND METHODS

BdE was analyzed by HPLC and in vivo evaluated (doses ranging from 50 to 400mg/kg, p.o.) by using the acetic acid-induced abdominal constrictions, paw oedema induced by carrageenan or histamine, overt nociception models using capsaicin, glutamate or phorbol myristate acetate (PMA), formalin-induced nociception and mechanical hypernociception induced by carrageenan or complete Freund adjuvant (CFA). As positive controls it was used paracetamol in both acetic acid and formalin tests; dipyrone in capsaicin, glutamate and PMA-induced nociception; indomethacin in CFA and carrageenan-induced hypernociception models. In addition, the in vitro effects of BdE on COX-2 activity and on the activation of NF-kappaB were also evaluated.

RESULTS

BdE (50-400mg/kg, p.o.) significantly diminished the abdominal constrictions induced by acetic acid, glutamate and CFA. Furthermore, BdE also inhibited the nociceptive responses in both phases of formalin-induced nociception. BdE, administered orally, also produced a long-lasting anti-hypernociceptive effect in the acute model of inflammatory pain induced by carrageenan. It was also observed the inhibition of COX-2 activity by BdE.

CONCLUSION

In summary, the data reported in this work confirmed the traditional anti-inflammatory indications of Baccharis dracunculifolia leaves and provided biological evidences that Baccharis dracunculifolia, like Brazilian green propolis, possess antinociceptive and anti-inflammatory activities.

Neurodevelopmental effects of chronic exposure to elevated levels of pro-inflammatory cytokines in a developing visual system

Background

Imbalances in the regulation of pro-inflammatory cytokines have been increasingly correlated with a number of severe and prevalent neurodevelopmental disorders, including autism spectrum disorder, schizophrenia and Down syndrome. Although several studies have shown that cytokines have potent effects on neural function, their role in neural development is still poorly understood. In this study, we investigated the link between abnormal cytokine levels and neural development using the Xenopus laevis tadpole visual system, a model frequently used to examine the anatomical and functional development of neural circuits.

Results

Using a test for a visually guided behavior that requires normal visual system development, we examined the long-term effects of prolonged developmental exposure to three pro-inflammatory cytokines with known neural functions: interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α. We found that all cytokines affected the development of normal visually guided behavior. Neuroanatomical imaging of the visual projection showed that none of the cytokines caused any gross abnormalities in the anatomical organization of this projection, suggesting that they may be acting at the level of neuronal microcircuits. We further tested the effects of TNF-α on the electrophysiological properties of the retinotectal circuit and found that long-term developmental exposure to TNF-α resulted in enhanced spontaneous excitatory synaptic transmission in tectal neurons, increased AMPA/NMDA ratios of retinotectal synapses, and a decrease in the number of immature synapses containing only NMDA receptors, consistent with premature maturation and stabilization of these synapses. Local interconnectivity within the tectum also appeared to remain widespread, as shown by increased recurrent polysynaptic activity, and was similar to what is seen in more immature, less refined tectal circuits. TNF-α treatment also enhanced the overall growth of tectal cell dendrites. Finally, we found that TNF-α-reared tadpoles had increased susceptibility to pentylenetetrazol-induced seizures.

Conclusions

Taken together our data are consistent with a model in which TNF-α causes premature stabilization of developing synapses within the tectum, therefore preventing normal refinement and synapse elimination that occurs during development, leading to increased local connectivity and epilepsy. This experimental model also provides an integrative approach to understanding the effects of cytokines on the development of neural circuits and may provide novel insights into the etiology underlying some neurodevelopmental disorders.

Neurobiology of inflammation-associated anorexia

Compelling data demonstrate that inflammation-associated anorexia directly results from the action of pro-inflammatory factors, primarily cytokines and prostaglandins E2, on the nervous system. For instance, the aforementioned pro-inflammatory factors can stimulate the activity of peripheral sensory neurons, and induce their own de novo synthesis and release into the brain parenchyma and cerebrospinal fluid. Ultimately, it results in the mobilization of a specific neural circuit that shuts down appetite. The present article describes the different cell groups and neurotransmitters involved in inflammation-associated anorexia and examines how they interact with neural systems regulating feeding such as the melanocortin system. A better understanding of the neurobiological mechanisms underlying inflammation-associated anorexia will help to develop appetite stimulants for cancer and AIDS patients.

N-Methyl-D-aspartate receptor (NMDAR) independent maintenance of inflammatory pain

Following peripheral inflammation, NMDA receptor (NMDAR) activation in spinal cord dorsal horn neurons facilitates the generation of pain in response to low threshold inputs (allodynia) and signals the phosphorylation of protein kinase C (pPKC) and extracellular signal-regulated kinase 2 (pERK2). Intraplantar complete Freund's adjuvant (CFA) induces inflammatory nociception (allodynic pain) at 24 hours (h) with a concurrent increase in neuronal pPKCgamma and pERK2 but not glial pERK2. These effects are attenuated in a spatial knockout of the NMDAR (NR1 KO) confined to SCDH neurons. Although glia and proinflammatory cytokines are implicated in the maintenance of inflammatory pain and neuronal activation, the role of NMDARs and neuronal-glial-cytokine interactions that initiate and maintain inflammatory pain are not well defined. In the maintenance phase of inflammatory pain at 96h after CFA the NR1 KO mice are no longer protected from allodynia and the SCDH expression of pPKCgamma and pERK2 are increased. At 96h the expression of the proinflammatory cytokine, IL-1beta, and pERK2 are increased in astrocytes. Intrathecal IL-1 receptor antagonist (IL-1ra), acting on neuronal IL-1 receptors, completely reverses the allodynia at 96h after CFA. Deletion of NMDAR-dependent signaling in neurons protects against early CFA-induced allodynia. Subsequent NMDAR-independent signaling that involves neuronal expression of pPKCgamma and the induction of pERK2 and IL-1beta in activated astrocytes contributes to the emergence of NMDAR-independent inflammatory pain behavior at 96h after CFA. Effective reduction of the initiation and maintenance of inflammatory pain requires targeting the neuron-astrocyte-cytokine interactions revealed in these studies.

A key role for gp130 expressed on peripheral sensory nerves in pathological pain

Interleukin-6 (IL-6) is a key mediator of inflammation. Inhibitors of IL-6 or of its signal transducing receptor gp130 constitute a novel class of anti-inflammatory drugs, which raise great hopes for improved treatments of painful inflammatory diseases such as rheumatoid arthritis. IL-6 and gp130 may enhance pain not only indirectly through their proinflammatory actions but also through a direct action on nociceptors (i.e., on neurons activated by painful stimuli). We found indeed that the IL-6/gp130 ligand-receptor complex induced heat hypersensitivity both in vitro and in vivo. This process was mediated by activation of PKC-delta via Gab1/2/PI(3)K and subsequent regulation of TRPV1, a member of the transient receptor potential (TRP) family of ion channels. To assess the relevance of this direct pain promoting effect of IL-6, we generated conditional knock-out mice, which lack gp130 specifically in nociceptors, and tested them in models of inflammatory and tumor-induced pain. These mice showed significantly reduced levels of inflammatory and tumor-induced pain but no changes in immune reactions or tumor growth. Our results uncover the significance of gp130 expressed in peripheral pain sensing neurons in the pathophysiology of major clinical pain disorders and suggest their use as novel pain relieving agents in inflammatory and tumor pain.

Inhibitor kappaB Kinase beta deficiency in primary nociceptive neurons increases TRP channel sensitivity

Inhibitor kappaB kinase (IKK) regulates the activity of the transcription factor nuclear factor-kappa B that normally protects neurons against excitotoxicity. Constitutively active IKK is enriched at axon initial segments and nodes of Ranvier (NR). We used mice with a Cre-loxP-mediated specific deletion of IKKbeta in sensory neurons of the dorsal root ganglion (SNS-IKKbeta(-/-)) to evaluate whether IKK plays a role in sensory neuron excitability and nociception. We observed increased sensitivity to mechanical, cold, noxious heat and chemical stimulation in SNS-IKKbeta(-/-) mice, with normal proprioceptive and motor functions as revealed by gait analysis. This was associated with increased calcium influx and increased inward currents in small- and medium-sized primary sensory neurons of SNS-IKKbeta(-/-) mice during stimulation with capsaicin or Formalin, specific activators of transient receptor potentials TRPV1 and TRPA1 calcium channels, respectively. In vitro stimulation of saphenous nerve preparations of SNS-IKKbeta(-/-) mice showed increased neuronal excitability of A- and C-fibers but unchanged A- and C-fiber conduction velocities, normal voltage-gated sodium channel currents, and normal accumulation of ankyrin G and the sodium channels Nav1.6 at NR. The results suggest that IKKbeta functions as a negative modulator of sensory neuron excitability, mediated at least in part by modulation of TRP channel sensitivity.

Cytokine and chemokine regulation of sensory neuron function

Pain normally subserves a vital role in the survival of the organism, prompting the avoidance of situations associated with tissue damage. However, the sensation of pain can become dissociated from its normal physiological role. In conditions of neuropathic pain, spontaneous or hypersensitive pain behavior occurs in the absence of the appropriate stimuli. Our incomplete understanding of the mechanisms underlying chronic pain hypersensitivity accounts for the general ineffectiveness of currently available options for the treatment of chronic pain syndromes. Despite its complex pathophysiological nature, it is clear that neuropathic pain is associated with short- and long-term changes in the excitability of sensory neurons in the dorsal root ganglia (DRG) as well as their central connections. Recent evidence suggests that the upregulated expression of inflammatory cytokines in association with tissue damage or infection triggers the observed hyperexcitability of pain sensory neurons. The actions of inflammatory cytokines synthesized by DRG neurons and associated glial cells, as well as by astrocytes and microglia in the spinal cord, can produce changes in the excitability of nociceptive sensory neurons. These changes include rapid alterations in the properties of ion channels expressed by these neurons, as well as longer-term changes resulting from new gene transcription. In this chapter we review the diverse changes produced by inflammatory cytokines in the behavior of sensory neurons in the context of chronic pain syndromes.

Sensory pulpal nerve fibres and trigeminal ganglion neurons express IL-1RI: a potential mechanism for development of inflammatory hyperalgesia

AIM

To localize interleukin-1 receptor type I (IL-1RI) in rat dental pulp and trigeminal ganglion (TG) and to test the hypothesis that pulpal inflammation increases neuronal expression of IL-1RI.

METHODOLOGY

Female Wistar rats were subjected to unilateral pulp exposures in the maxillary and mandibular first molars, whereas the contralateral jaws served as untreated controls. Seven days later the animals were transcardiacally perfused and the jaws and the TGs were removed and prepared for immunohistochemistry. Immunoreactivity for IL-1RI was examined alone (DAB) and together with calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), CD31 or CD34 by multiple-labelling immunofluorescence. Quantification of IL-1RI-immunoreactive (-IR) cells in the maxillary and mandibular division of the ganglion was performed in parasagittal immunoreacted sections of the right and left TGs. Data were analysed with Mann-Whitney Rank Sum test (P < 0.05).

RESULTS

Interleukin-1 receptor type I was found on sensory (CGRP-IR) and sympathetic (NPY-IR) nerve fibres and on blood vessels (CD31- and CD34-IR) in the dental pulp. It was also localized on sensory neurons and axons in the TG. Pulpal inflammation significantly increased the expression of IL-1RI in the TG (P < 0.001).

CONCLUSIONS

The localization of IL-1RI on sensory nerve fibres and its up-regulation in TG neurons during pulpal inflammation may imply a direct effect of IL-1 in pulpal nociception. The presence of IL-1RI on sympathetic nerve fibres and on blood vessels may indicate a vasoactive role of the same cytokine in the pulp.

Differential contribution of SNARE-dependent exocytosis to inflammatory potentiation of TRPV1 in nociceptors

Potentiation of the pain-integrator ion channel transient receptor potential vanilloid type 1 (TRPV1) underlies thermal hyperalgesia mediated by a variety of proinflammatory factors. Two complementary mechanisms of TRPV1 inflammatory sensitization have been proposed, namely a decrease of its activation threshold and an increment of its surface expression in nociceptors. Here we investigated the involvement of regulated exocytosis to the inflammatory sensitization of TRPV1 in rat neonatal dorsal root ganglion neurons by proalgesic agents. The contribution of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent exocytosis was evaluated using a small peptide patterned after the synaptosomal-associated protein of 25 kDa (SNAP-25) protein that acts as a specific and potent inhibitor of neuronal exocytosis. We found that TRPV1 sensitization mediated by nerve growth factor, ATP, and IGF-I was accompanied by a higher channel expression in the neuronal plasma membrane, which was prevented by blockade of regulated exocytosis. In contrast, TRPV1 sensitization caused by bradykinin, IL-1beta, and artemin was insensitive to inhibition of SNARE-dependent vesicular fusion and was not due to an increase in TRPV1 surface expression. Therefore, it appears that some, but not all, proinflammatory agents sensitize rat nociceptors by promoting the recruitment of TRPV1 channels to the neuronal surface. These findings support the tenet that SNARE complex-mediated exocytosis of TRPV1 may be a valid therapeutic target to treat inflammatory pain.

Post-inflammatory modification of colonic afferent mechanosensitivity

1. The present review discusses interactions between the immune and nervous systems in post-infectious irritable bowel syndrome (PI-IBS). 2. Visceral pain is the single symptom that most affects the quality of life of patients with irritable bowel syndrome (IBS), yet it is the least successfully managed. An underlying hypersensitivity of colonic afferents to mechanical stimuli has long been implicated in visceral pain in IBS, but little more is known of the physiological aetiology. 3. The PI-IBS patients are a cohort of IBS patients who attribute their symptoms to a preceding gastrointestinal infection by pathogens such as Campylobacter or Salmonella. Current evidence suggests that the immune system remains activated in these patients and contributes to their visceral hypersensitivity. This is characterized by a shift in the phenotype of circulating immune cells towards a Type 1 (Th1 predominating) state. Products from these immune cells sensitize colonic afferents to mechanical stimuli. 4. Rectal instillation of trinitrobenzene sulphonic acid induces a Th1-mediated inflammatory response, consistent with clinical observations in PI-IBS. The visceral hypersensitivity observed in this model is biphasic, with an initial onset characterized by visceral hypersensitivity correlating with histological damage followed by a delayed phase that occurs after histological recovery. Interestingly, this chronic visceral hypersensitivity is mediated by afferents in closest apposition to blood vessels, but furthest from the initial site of damage. 5. Both clinical and experimental evidence indicates that chronic dysregulation of the immune system induces visceral afferent hypersensitivity and, therefore, may be the central mechanism underlying PI-IBS.

Nociceptors are interleukin-1beta sensors

A cardinal feature of inflammation is heightened pain sensitivity at the site of the inflamed tissue. This results from the local release by immune and injured cells of nociceptor sensitizers, including prostaglandin E(2), bradykinin, and nerve growth factor, that reduce the threshold and increase the excitability of the peripheral terminals of nociceptors so that they now respond to innocuous stimuli: the phenomenon of peripheral sensitization. We show here that the proinflammatory cytokine interleukin-1beta (IL-1beta), in addition to producing inflammation and inducing synthesis of several nociceptor sensitizers, also rapidly and directly activates nociceptors to generate action potentials and induce pain hypersensitivity. IL-1beta acts in a p38 mitogen-activated protein kinase (p38 MAP kinase)-dependent manner, to increase the excitability of nociceptors by relieving resting slow inactivation of tetrodotoxin-resistant voltage-gated sodium channels and also enhances persistent TTX-resistant current near threshold. By acting as an IL-1beta sensor, nociceptors can directly signal the presence of ongoing tissue inflammation.

Tumour necrosis factor alpha mediates transient receptor potential vanilloid 1-dependent bilateral thermal hyperalgesia with distinct peripheral roles of interleukin-1beta, protein kinase C and cyclooxygenase-2 signalling

TNFalpha plays a pivotal role in rheumatoid arthritis (RA) but little is known of the mechanisms that link the inflammatory and nociceptive effects of TNFalpha. We have established a murine model of TNFalpha-induced TRPV1-dependent bilateral thermal hyperalgesia that then allowed us to identify distinct peripheral mechanisms involved in mediating TNFalpha-induced ipsilateral and contralateral hyperalgesia. Thermal hyperalgesia and inflammation were assessed in both hindpaws following unilateral intraplantar (i.pl.) TNFalpha. The hyperalgesic mechanisms were analysed through pharmacogenetic approaches involving TRPV1(-/-) mice and TRPV1 antagonists. To study the mediators downstream of TNFalpha, cyclooxygenase (COX) and PKC inhibitors were utilised and cytokine and prostaglandin levels assessed. The role of neutrophils was determined through use of the selectin inhibitor, fucoidan. We show that TNFalpha (10pmol) causes thermal hyperalgesia (1-4h) in the ipsilateral inflamed and contralateral uninjured hindpaws, which is TRPV1-dependent. GF109203X, a PKC inhibitor, suppressed the hyperalgesia indicating that PKC is involved in TRPV1 sensitisation. Ipsilateral COX-2-derived prostaglandins were also crucial to the development of the bilateral hyperalgesia. The prevention of neutrophil accumulation with fucoidan attenuated hyperalgesia at 4 but not at 1h, indicating a role in the maintenance but not in the induction of bilateral hyperalgesia. However, TNFalpha-induced IL-1beta generation in both paws and the presence of local IL-1beta in the contralateral paw were essential for the development of bilateral hyperalgesia. These results identify a series of peripheral events through which TNFalpha triggers and maintains bilateral inflammatory pain. This potentially allows a better understanding of mechanisms involved in TNFalpha-dependent pain pathways in symmetrical diseases such as arthritis.

Mode of action of cytokines on nociceptive neurons

Cytokines are pluripotent soluble proteins secreted by immune and glial cells and are key elements in the induction and maintenance of pain. They are categorized as pro-inflammatory cytokines, which are mostly algesic, and anti-inflammatory cytokines, which have analgesic properties. Progress has been made in understanding the mechanisms underlying the action of cytokines in pain. To date, several direct and indirect pathways are known that link cytokines with nociception or hyperalgesia. Cytokines may act via specific cytokine receptors inducing downstream signal transduction cascades, which then modulate the function of other receptors like the ionotropic glutamate receptor, the transient vanilloid receptors, or sodium channels. This receptor activation, either through amplification of the inflammatory reaction, or through direct modulation of ion channel currents, then results in pain sensation. Following up on results from animal experiments, cytokine profiles have recently been investigated in human pain states. An imbalance of pro- and anti-inflammatory cytokine expression may be of importance for individual pain susceptibility. Individual cytokine profiles may be of diagnostic importance in chronic pain states, and, in the future, might guide the choice of treatment.

The effects of the selective and non-peptide CXCR2 receptor antagonist SB225002 on acute and long-lasting models of nociception in mice

This study evaluated the antinociceptive effects of the selective and non-peptide CXCR2 antagonist SB225002 in mouse models of pain. As assessed in different tests of spontaneous nociception, intraperitoneal (i.p.) administration of SB225002 caused consistent and dose-related reduction of acetic acid-induced abdominal constrictions, whereas it did not significantly affect the nociception evoked by formalin, capsaicin, glutamate or phorbol ester acetate (PMA). Systemic treatment with SB225002 strikingly reduced the spontaneous nociception induced by 8-bromo-cAMP (8-Br-cAMP), or mechanical hypernociception induced by prostaglandin E(2) (PGE(2)), epinephrine, or the keratinocyte-derived chemokine (KC). In the carrageenan model, SB225002 markedly reduced mechanical hypernociception when administered by i.p., intrathecal (i.t.) or intracerebroventricular (i.c.v.) routes, or even when co-administered with carrageenan into the mouse paw, indicating peripheral and central sites of action for SB225002. In addition, i.p. treatment with SB225002 significantly attenuated the increase in MPO activity or the elevation of IL-1beta, TNFalpha or KC levels following carrageenan injection. In the persistent models of pain evoked by complete Freund's adjuvant (CFA) or by the partial ligation of the sciatic nerve (PLSN), the repeated administration of SB225002 displayed prominent and long-lasting antinociceptive effects. Notably, SB225002 did not evoke unspecific central effects, as evaluated in the open-field and rota-rod tests, or even in the latency responses for thermal stimuli. Our data confirm the previous notion on the critical role exerted by chemokines in pain, indicating that selective CXCR2 antagonists, such as SB225002, might well represent interesting and innovative alternatives for the management of both acute and chronic pain.

Role of interleukin-1beta during pain and inflammation

The cytokine cascade in pain and inflammatory processes is a tremendously complex system, involving glial, immune, and neuronal cell interactions. IL-1beta is a pro-inflammatory cytokine that has been implicated in pain, inflammation and autoimmune conditions. This review will focus on studies that shed light on the critical role of IL-1beta in various pain states, including the role of the intracellular complex, the inflammasome, which regulates IL-1beta production. Evidence will be presented demonstrating the importance of IL-1beta in both the induction of pain and in the maintenance of pain in chronic states, such as after nerve injury. Additionally, the involvement of IL-1beta as a key mediator in the interaction between glia and neurons in pain states will be discussed. Taken together, the evidence presented in the current review showing the importance of IL-1beta in animal and human pain states, suggests that blockade of IL-1beta be considered as a therapeutic opportunity.

Transient receptor potential vanilloid channels in hypertension, inflammation, and end organ damage: an imminent target of therapy for cardiovascular disease?

PURPOSE OF REVIEW

The possible role of several neurohormonal factors in pathogenesis of hypertension has been studied extensively both in humans and in experimental animal models. However, controversial data from some previous studies are indecisive and call for reassessment and development of new targets. This mini-review presents some of the most recent findings about the role of transient receptor potential vanilloid type 1 channels in the development of hypertension and its pathology.

RECENT FINDINGS

The transient receptor potential vanilloid type 1, channel activated by novel endovanilloids or altered pH, temperature, and/or local hemodynamics, may serve as a distinct molecular sensor detecting sodium and water balance and may play a role in preventing salt-induced hypertension and tissue damage. Impairment of the function of the transient receptor potential vanilloid type 1 channels may contribute to increased salt sensitivity, inflammation, and end organ damage.

SUMMARY

Emerging evidence indicates that the transient receptor potential vanilloid type 1 channel plays a key role in cardiovascular health and disease by acting as a sensor and regulator of cardiovascular homeostasis and a protector against cardiovascular injury. Given the huge population who suffers from cardiovascular disease, the study of the transient receptor potential vanilloid channels may improve our understanding of pathogenesis of several common cardiovascular disorders and may lead to the development of therapy for hypertension, inflammation, and organ damage.

Activation of interleukin-1beta receptor suppresses the voltage-gated potassium currents in the small-diameter trigeminal ganglion neurons following peripheral inflammation

The glial cytokine, interleukin-1beta (IL-1beta), potentiates the excitability of nociceptive trigeminal ganglion (TRG) neurons via membrane depolarization following peripheral inflammation. Perforated patch-clamp technique was used this study to show that the mechanism underlying the excitability of small-diameter TRG neurons following inflammation is due to IL-1beta. Inflammation was induced by injection of complete Freund's adjuvant (CFA) into the whisker pad. The TRG neurons innervating the site of inflammation were identified by fluorogold (FG) labeling. The threshold for escape from mechanical stimulation applied to the orofacial area in inflamed rats was significantly lower than observed for control rats. IL-1beta at 1nM suppressed total voltage-gated K(+) currents in most TRG neurons (70%) under voltage-clamp conditions in control and inflamed rats. IL-1beta significantly decreased the total, transient (I(A)) and sustained (I(K)) currents in FG-labeled small-diameter TRG neurons in both groups. The IL-1beta-induced suppression of TRG neuron excitability was abolished by co-administration of ILra, an IL-1beta receptor blocker. The magnitude of inhibition of I(A) and I(K) currents by IL-1beta was significantly greater in inflamed rats than in controls. IL-1beta inhibited I(A) to a significantly greater extent than I(K). These results suggest that the inhibitory effect of I(A) and I(K) currents by IL-1beta in small-diameter TRG neurons potentiates neuronal excitability thereby contributing to trigeminal inflammatory hyperalgesia. These findings provide evidence for the development of voltage-gated K(+) channel openers and IL-1beta antagonists as therapeutic agents for the treatment of trigeminal inflammatory hyperalgesia.

Endogenous tumor necrosis factor alpha (TNFalpha) requires TNF receptor type 2 to generate heat hyperalgesia in a mouse cancer model

To provide a tool to investigate the mechanisms inducing and maintaining cancer-related pain and hyperalgesia, a soft tissue tumor/metastasis model was developed that is applicable in C57BL/6J wild-type and transgenic mice. We show that the experimental tumor-induced heat hyperalgesia and nociceptor sensitization were prevented by systemic treatment with the tumor necrosis factor alpha (TNFalpha) antagonist etanercept. In naive mice, exogenous TNFalpha evoked heat hyperalgesia in vivo and sensitized nociceptive nerve fibers to heat in vitro. TNFalpha enhanced the expression of the nociceptor-specific heat transducer ion channel transient receptor potential vanilloid 1 (TRPV1) and increased the amplitudes of capsaicin and heat-activated ionic currents via p38/MAP (mitogen-activated protein) kinase and PKC (protein kinase C). Deletion of the tumor necrosis factor receptor type 2 (TNFR2) gene attenuated heat hyperalgesia and prevented TRPV1 upregulation in tumor-bearing mice, whereas TNFR1 gene deletion played a minor role. We propose endogenous TNFalpha as a key player in cancer-related heat hyperalgesia and nociceptor sensitization that generates TRPV1 upregulation and sensitization via TNFR2.

Interleukin-1 alpha has antiallodynic and antihyperalgesic activities in a rat neuropathic pain model

Nerve injury and the consequent release of interleukins (ILs) are processes implicated in pain transmission. To study the potential role of IL-1 in the pathogenesis of allodynia and hyperalgesia, IL-1alpha and comparative IL-1beta, IL-6, and IL-10 mRNA levels were quantified using competitive RT-PCR of the lumbar spinal cord and dorsal root ganglia (DRG; L5-L6) three and seven days after chronic constriction injury (CCI) in rats. Microglial and astroglial activation in the ipsilateral spinal cord and DRG were observed after injury. In naive and CCI-exposed rats, IL-1alpha mRNA and protein were not detected in the spinal cord. IL-1beta and IL-6 mRNAs were strongly ipsilaterally elevated on day seven after CCI. In the ipsilateral DRG, IL-1alpha, IL-6, and IL-10 mRNA levels were increased on days three and seven; IL-1beta was elevated only on day seven. Western blot analysis revealed both the presence of IL-1alpha proteins (45 and 31 kDa) in the DRG and the down-regulation of these proteins after CCI. Intrathecal administration of IL-1alpha (50-500 ng) in naive rats did not influence nociceptive transmission, but IL-1beta (50-500 ng) induced hyperalgesia. In rats exposed to CCI, an IL-1alpha or IL-1 receptor antagonist dose-dependently attenuated symptoms of neuropathic pain; however, no effect of IL-1beta was observed. In sum, the first days after CCI showed a high abundance of IL-1alpha in the DRG. Together with the antiallodynic and antihyperalgesic effects observed after IL-1alpha administration, this finding indicates an important role for IL-1alpha in the development of neuropathic pain symptoms.

Spinal glial activation contributes to pathological pain states

Chronic pain, a pathological state, affects millions of people worldwide. Despite decades of study on the neuronal processing of pain, mechanisms underlying the creation and maintenance of enhanced pain states after injury or inflammation remain far from clear. In the last decade, however, the discovery that glial activation amplifies pain has challenged classic neuronal views of "pain". This review focuses on recent developments in understanding that spinal cord glia are involved in pathological pain. We overview the action of spinal glia (both microglia and astrocytes) in several persistent pain models, and provide new evidence that spinal glia activation contributes to the development and maintenance of arthritic pain facilitation. We also attempt to discuss some critical questions, such as how signals are conveyed from primary afferents to spinal glia following peripheral nerve injury and inflammation. What causes glia to become activated after peripheral/central injury/inflammation? And how the activated glia alter neuronal sensitivity and pain processing? Answers to these questions might open a new approach for treatment of pathological pain.