Cyclooxygenase-1 is a marker for a subpopulation of putative nociceptive neurons in rat dorsal root ganglia

Peripheral macrophages contribute to nociceptor priming in mice with chronic intermittent hypoxia

Obstructive sleep apnea (OSA) is a prevalent sleep disorder that is associated with increased incidence of chronic musculoskeletal pain. We investigated the mechanism of this association in a mouse model of chronic intermittent hypoxia (CIH) that mimics the repetitive hypoxemias of OSA. After 14 days of CIH, both male and female mice exhibited behaviors indicative of persistent pain, with biochemical markers in the spinal cord dorsal horn and sensory neurons of the dorsal root ganglia consistent with hyperalgesic priming. CIH, but not sleep fragmentation alone, induced an increase in macrophage recruitment to peripheral sensory tissues (sciatic nerve and dorsal root ganglia), an increase in inflammatory cytokines in the circulation, and nociceptor sensitization. Peripheral macrophage ablation blocked CIH-induced hyperalgesic priming. The findings suggest that correcting the hypoxia or targeting macrophage signaling might suppress persistent pain in patients with OSA.

Profiling of fatty acid metabolism in the dorsal root ganglion after peripheral nerve injury

Peripheral nerve injury (PNI) induces neuronal hyperexcitability, which underlies neuropathic pain. The emergence of RNA sequencing technologies has enabled profiling of transcriptional changes in pathological conditions. However, these approaches do not provide information regarding metabolites such as lipids that are not directly encoded by genes. Fatty acids (FAs) are some of the essential lipids in mammalian organisms and are mainly stored as membrane phospholipids. In response to various biological stimuli, FAs are rapidly released and converted into several mediators, such as eicosanoids and docosanoids. FAs themselves or their metabolites play important roles in physiology and pathology. In this study, using a comprehensive lipidomic analysis of FA metabolites, 152 species were measured in the dorsal root ganglia of mice at multiple time points after PNI. We found that PNI increased the ω-6 FA metabolites produced by cyclooxygenases but not those produced by lipoxygenases or cytochrome P450 enzymes in the dorsal root ganglia. In contrast, ω-3 FA metabolites biosynthesized by any enzyme transiently increased after nerve injury. Overall, these findings provide a new resource and valuable insights into PNI pathologies, including pain and nerve regeneration.

Involvement of the tuberomammillary nucleus of the hypothalamus in the modulation of nociception and joint edema in a model of monoarthritis

AIMS

Investigate the involvement of the histaminergic projections from tuberomammillary nucleus (TMN) to the spinal cord in the modulation of nociception and peripheral edema in a model of monoarthritis.

MAIN METHODS

Subacute monoarthritis was induced by an intraarticular injection of carrageenan followed by LPS 72 h later. Disability and joint edema were assessed at the 3rd hour after LPS and at every hour up to 6 h.

KEY FINDINGS

Intrathecal administration of histamine potentiated joint incapacitation and edema, while the H1R antagonist cetirizine decreased both. The H3R agonist immepip decreased both incapacitation and edema, while the H3R antagonist thioperamide had the opposite effect. The microinjection of glutamate into the ventral TMN (vTMN) caused an increase of incapacitation and articular edema, whereas the blockade of this nucleus by cobalt chloride inhibited both parameters. Intrathecal administration of cetirizine prevented the increase of incapacitation and joint edema caused by glutamate microinjection into the vTMN. Similarly, an intrathecal injection of the NKCC1 cotransporter inhibitor bumetanide prevented the effects of glutamate microinjection into the vTMN, whereas coadministration of histamine with bumetanide only inhibited the potentiation of joint edema. A microinjection of orexin B into the vTMN potentiated incapacitation and joint edema, while coadministration of the OX1/2 receptor antagonist almorexant with orexin B did not.

SIGNIFICANCE

These data support the notion that TMN participates in the modulation of a peripheral inflammatory process by means of histaminergic projections to the spinal cord, and the hypothalamus may trigger TMN activation by means of glutamate and orexin.

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.

Contribution of large-sized primary sensory neuronal sensitization to mechanical allodynia by upregulation of hyperpolarization-activated cyclic nucleotide gated channels via cyclooxygenase 1 cascade

Under physiological state, small- and medium-sized dorsal root ganglia (DRG) neurons are believed to mediate nociceptive behavioral responses to painful stimuli. However, recently it has been found that a number of large-sized neurons are also involved in nociceptive transmission under neuropathic conditions. Nonetheless, the underlying mechanisms that large-sized DRG neurons mediate nociception are poorly understood. In the present study, the role of large-sized neurons in bee venom (BV)-induced mechanical allodynia and the underlying mechanisms were investigated. Behaviorally, it was found that mechanical allodynia was still evoked by BV injection in rats in which the transient receptor potential vanilloid 1-positive DRG neurons were chemically deleted. Electrophysiologically, in vitro patch clamp recordings of large-sized neurons showed hyperexcitability in these neurons. Interestingly, the firing pattern of these neurons was changed from phasic to tonic under BV-inflamed state. It has been suggested that hyperpolarization-activated cyclic nucleotide gated channels (HCN) expressed in large-sized DRG neurons contribute importantly to repeatedly firing. So we examined the roles of HCNs in BV-induced mechanical allodynia. Consistent with the overexpression of HCN1/2 detected by immunofluorescence, HCNs-mediated hyperpolarization activated cation current (I_h) was significantly increased in the BV treated samples. Pharmacological experiments demonstrated that the hyperexcitability and upregulation of I_h in large-sized neurons were mediated by cyclooxygenase-1 (COX-1)-prostaglandin E2 pathway. This is evident by the fact that the COX-1 inhibitor significantly attenuated the BV-induced mechanical allodynia. These results suggest that BV can excite the large-sized DRG neurons at least in part by increasing I_h through activation of COX-1.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSION

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

Effects of NSAIDs and paracetamol (acetaminophen) on protein kinase C epsilon translocation and on substance P synthesis and release in cultured sensory neurons

Celecoxib, diclofenac, ibuprofen, and nimesulide are nonsteroidal anti-inflammatory drugs (NSAIDs) very commonly used for the treatment of moderate to mild pain, together with paracetamol (acetaminophen), a very widely used analgesic with a lesser anti-inflammatory effect. In the study reported here, we tested the efficacy of celecoxib, diclofenac, and ibuprofen on preprotachykinin mRNA synthesis, substance P (SP) release, prostaglandin E₂ (PGE₂) release, and protein kinase C epsilon (PKCɛ) translocation in rat cultured sensory neurons from dorsal root ganglia (DRGs). The efficacy of these NSAIDs was compared with the efficacy of paracetamol and nimesulide in in vitro models of hyperalgesia (investigated previously). While nimesulide and paracetamol, as in previous experiments, decreased the percentage of cultured DRG neurons showing translocation of PKCɛ caused by 100 nM thrombin or 1 μM bradykinin in a dose-dependent manner, the other NSAIDs tested did not have a significant effect. The amount of SP released by peptidergic neurons and the expression level of preprotachykinin mRNA were assessed in basal conditions and after 70 minutes or 36 hours of stimulation with an inflammatory soup (IS) containing potassium chloride, thrombin, bradykinin, and endothelin-1. The release of SP at 70 minutes was inhibited only by nimesulide, while celecoxib and diclofenac were effective at 36 hours. The mRNA basal level of the SP precursor preprotachykinin expressed in DRG neurons was reduced only by nimesulide, while the increased levels expressed during treatment with the IS were significantly reduced by all drugs tested, with the exception of ibuprofen. All drugs were able to decrease basal and IS-stimulated PGE₂ release. Our study demonstrates novel mechanisms of action of commonly used NSAIDS.

Peripheral inflammatory hyperalgesia depends on the COX increase in the dorsal root ganglion

It is well established that dorsal root ganglion (DRG) cells synthesize prostaglandin. However, the role that prostaglandin plays in the inflammatory hyperalgesia of peripheral tissue has not been established. Recently, we have successfully established a technique to inject drugs (3 μL) directly into the L5-DRG of rats, allowing in vivo identification of the role that DRG cell-derived COX-1 and COX-2 play in the development of inflammatory hyperalgesia of peripheral tissue. IL-1β (0.5 pg) or carrageenan (100 ng) was administered in the L5-peripheral field of rat hindpaw and mechanical hyperalgesia was evaluated after 3 h. Administration of a nonselective COX inhibitor (indomethacin), selective COX-1 (valeryl salicylate), or selective COX-2 (SC-236) inhibitors into the L5-DRG prevented the hyperalgesia induced by IL-1β. Similarly, oligodeoxynucleotide-antisense against COX-1 or COX-2, but not oligodeoxynucleotide-mismatch, decreased their respective expressions in the L5-DRG and prevented the hyperalgesia induced by IL-1β in the hindpaw. Immunofluorescence analysis demonstrated that the amount of COX-1 and COX-2, constitutively expressed in TRPV-1(+) cells of the DRG, significantly increased after carrageenan or IL-1β administration. In addition, indomethacin administered into the L5-DRG prevented the increase of PKCε expression in DRG membrane cells induced by carrageenan. Finally, the administration of EP1/EP2 (7.5 ng) or EP4 (10 µg) receptor antagonists into L5-DRG prevented the hyperalgesia induced by IL-1β in the hindpaw. In conclusion, the results of this study suggest that the inflammatory hyperalgesia in peripheral tissue depends on activation of COX-1 and COX-2 in C-fibers, which contribute to the induction and maintenance of sensitization of primary sensory neurons.

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.

Immunohistochemical evidence for the relationship between microglia and GnRH neurons in the preoptic area of ovariectomized rats with and without steroid replacement

Prostaglandins (PGs), whose synthesis is catalyzed by the rate-limiting enzyme cyclooxygenase (COX) including COX-1 and COX-2, are among the important mediators involved in the regulation of gonadotropin-releasing hormone (GnRH) secretion. However, the cellular origin of PGs remains obscure in terms of its relationship to GnRH neurons. The present study was therefore aimed to clarify the anatomical relationship between COX-1-producing microglia and GnRH neurons in the preoptic area (POA), and to examine possible influence of ovarian steroids. We performed a triple labeled immunofluorescent histochemistry of COX-1, CD11b (a specific marker for microglia) and GnRH in the POA of ovarian steroid-primed and non-primed ovariectomized rats. The result confirmed our previous study suggesting COX-1 immunoreactivity in the vicinity of, but not within, GnRH neurons in the POA. COX-1 around GnRH cells was entirely (100%) localized in cells containing CD11b regardless of steroid replacement in ovariectomized rats. These CD11b-immunoreactive cells had small cell bodies and highly branched fibers characteristic of ramified microglia. Three-dimensional reconstruction of confocal images revealed close proximity of some COX-1-containing microglia and GnRH neurons. These results showed selective and constitutive expression of COX-1 in ramified microglia in the vicinity of GnRH neurons, providing evidence for intercellular communication, mediated by PGs, from microglia to GnRH cells.

Neuronal IL-17 receptor upregulates TRPV4 but not TRPV1 receptors in DRG neurons and mediates mechanical but not thermal hyperalgesia

In addition to the proinflammatory cytokines tumor necrosis factor-α, interleukin-6 and interleukin-1ß, the cytokine interleukin-17 (IL-17) is considered an important mediator of autoimmune diseases such as rheumatoid arthritis. Because tumor necrosis factor-α and interleukin-1ß have the potential to influence the expression of transduction molecules such as transient receptor potential vanilloid 1 (TRPV1) in dorsal root ganglion (DRG) neurons and thus to contribute to pain we explored in the present study whether IL-17A activates DRG neurons and influences the expression of TRPV1. The IL-17A receptor was visualized in most neurons in dorsal root ganglion (DRG) sections as well as in cultured DRG neurons. Upon long-term exposure to IL-17A, isolated and cultured rat DRG neurons showed a significant upregulation of extracellular-regulated kinase (ERK) and nuclear factor κB (NFκB). Long-term exposure of neurons to IL-17A did not upregulate the expression of TRPV1. However, we found a pronounced upregulation of transient receptor potential vanilloid 4 (TRPV4) which is considered a candidate transduction molecule for mechanical hyperalgesia. Upon the injection of zymosan into the paw, IL-17A-deficient mice showed less mechanical hyperalgesia than wild type mice but thermal hyperalgesia was not attenuated in IL-17A-deficient mice. These data show, therefore, a particular role of IL-17 in mechanical hyperalgesia, and they suggest that this effect is linked to an activation and upregulation of TRPV4.

Evidence that LPS-reactive arthritis in rats depends on the glial activity and the fractalkine-TNF-α signaling in the spinal cord

It is known that primary afferent central terminal sensitization can influence peripheral inflammation, however, it remains to be understood whether spinal cord glia can also contribute to this process. Our aim was to investigate the effect of spinal cord glia inhibition on the pathogenesis of LPS-induced knee-joint monoarthritis in rats and also to investigate the role of fractalkine and TNF-α. LPS was injected into the knee-joint previously primed with carrageenan to cause articular incapacitation, edema, synovial leukocyte infiltration, and GFAP and CD11b/c spinal immunoreactivity (glia-IR) increase. Articular edema was more sensitive to the inhibition by intrathecal fluorocitrate and minocycline than nociception and synovial leukocyte content. The higher doses of both drugs were ineffective when given by intraperitoneal route. Corticosteroid synthesis inhibition by aminoglutethimide did not change the glia inhibitors effect. The inhibitory effect of the dorsal root potential inhibitor, furosemide, was not additive to that caused by fluorocitrate and minocycline. Intrathecal anti-fractalkine and anti-TNF-α inhibited edema, nociception, and synovial leukocytes, while fractalkine caused the opposite effects. The fractalkine effect was inhibited by fluorocitrate and anti-TNF-α. Finally, fluorocitrate, minocycline and anti-fractalkine attenuated, but fractalkine increased, GFAP and CD11b/c IR. The evidence reported herein supports the hypothesis that spinal fractalkine release is involved in glia activation, which via the spinal release of TNF-α, seems to be involved in the development and maintenance of this arthritis model. A possible modulation of the dorsal root reflexes is discussed. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Nimesulide inhibits protein kinase C epsilon and substance P in sensory neurons - comparison with paracetamol

In this paper we describe new actions of nimesulide and paracetamol in cultured peripheral neurons isolated from rat dorsal root ganglia (DRG). Both drugs were able to decrease in a dose-dependent fashion the number of cultured DRG neurons showing translocation of protein kinase C epsilon (PKCɛ) caused by exposure to 1 μM bradykinin or 100 nM thrombin. In addition, the level of substance P (SP) released by DRG neurons and the level of preprotachykinin mRNA expression were measured in basal conditions and after 70 minutes or 36 hours of stimulation with nerve growth factor (NGF) or with an inflammatory soup containing bradykinin, thrombin, endothelin-1, and KCl. Nimesulide (10 μM) significantly decreased the mRNA levels of the SP precursor preprotachykinin in basal and in stimulated conditions, and decreased the amount of SP released in the medium during stimulation of neurons with NGF or with the inflammatory soup. The effects of paracetamol (10 μM) on such response was lower. Nimesulide completely inhibited the release of prostaglandin E2 (PGE2) from DRG neurons, either basal or induced by NGF and by inflammatory soup, while paracetamol decreased PGE2 release only partially. Our data demonstrate, for the first time, a direct effect of two drugs largely used as analgesics on DRG neurons. The present results suggest that PKCɛ might be a target for the effect of nimesulide and paracetamol, while inhibition of SP synthesis and release is clearly more relevant for nimesulide than for paracetamol mechanism of action.

Significance of brain tissue oxygenation and the arachidonic acid cascade in stroke

The significance of the hypoxia component of stroke injury is highlighted by hypermetabolic brain tissue enriched with arachidonic acid (AA), a 22:6n-3 polyunsaturated fatty acid. In an ischemic stroke environment in which cerebral blood flow is arrested, oxygen-starved brain tissue initiates the rapid cleavage of AA from the membrane phospholipid bilayer. Once free, AA undergoes both enzyme-independent and enzyme-mediated oxidative metabolism, resulting in the formation of number of biologically active metabolites which themselves contribute to pathological stroke outcomes. This review is intended to examine two divergent roles of molecular dioxygen in brain tissue as (1) a substrate for life-sustaining homeostatic metabolism of glucose and (2) a substrate for pathogenic metabolism of AA under conditions of stroke. Recent developments in research concerning supplemental oxygen therapy as an intervention to correct the hypoxic component of stroke injury are discussed.

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

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

Protease activated receptors 1 and 4 sensitize TRPV1 in nociceptive neurones

Protease-activated receptors (PAR1-4) are activated by proteases released by cell damage or blood clotting, and are known to be involved in promoting pain and hyperalgesia. Previous studies have shown that PAR2 receptors enhance activation of TRPV1 but the role of other PARs is less clear. In this paper we investigate the expression and function of the PAR1, 3 and 4 thrombin-activated receptors in sensory neurones. Immunocytochemistry and in situ hybridization show that PAR1 and PAR4 are expressed in 10 - 15% of neurons, distributed across all size classes. Thrombin or a specific PAR1 or PAR4 activating peptide (PAR1/4-AP) caused functional effects characteristic of activation of the PLCβ/PKC pathway: intracellular calcium release, sensitisation of TRPV1, and translocation of the epsilon isoform of PKC (PKCε) to the neuronal cell membrane. Sensitisation of TRPV1 was significantly reduced by PKC inhibitors. Neurons responding to thrombin or PAR1-AP were either small nociceptive neurones of the peptidergic subclass, or larger neurones which expressed markers for myelinated fibres. Sequential application of PAR1-AP and PAR4-AP showed that PAR4 is expressed in a subset of the PAR1-expressing neurons. Calcium responses to PAR2-AP were by contrast seen in a distinct population of small IB4+ nociceptive neurones. PAR3 appears to be non-functional in sensory neurones. In a skin-nerve preparation the release of the neuropeptide CGRP by heat was potentiated by PAR1-AP. Culture with nerve growth factor (NGF) increased the proportion of thrombin-responsive neurons in the IB4- population, while glial-derived neurotropic factor (GDNF) and neurturin upregulated the proportion of thrombin-responsive neurons in the IB4+ population. We conclude that PAR1 and PAR4 are functionally expressed in large myelinated fibre neurons, and are also expressed in small nociceptors of the peptidergic subclass, where they are able to potentiate TRPV1 activity.

Modulators of pain: Bv8 and prokineticins

Bv8 is a small protein secreted by frog skin. Mammalian homologues of Bv8, the prokineticins PK1 and PK2, and their G-protein coupled receptors PKR1 and PKR2 have been identified and linked to several biological effects. Bv8 elicits a dose-dependent reduction in nociceptive threshold to thermal and mechanical stimuli applied to the skin of tail and paw of rats and mice and increases the sensitivity to nociceptive mediators as capsaicin and prostaglandins. The receptors for Bv8/PKs are present in a fraction of peptidergic population of C-fibre neurons, and in a fraction of A myelinated-fibre neurons. In mouse and rat dorsal root ganglia, PKR-expressing neurons also express TRPV1 and the activation of PKRs sensitises TPRV1 to the action of capsaicin. Mice lacking PKR1 gene exhibit impaired Bv8-induced hyperalgesia, develop deficient responses to noxious heat, capsaicin and protons and show reduced thermal and mechanical hypersensitivity to paw inflammation, indicating a requirement for PKR1 signalling associated with activation and sensitisation of primary afferent fibres. PKs are highly expressed by neutrophils and other inflammatory cells and must be considered as new pronociceptive mediators in inflammatory tissues. Bv8-like hyperalgesic activity was demonstrated in extracts of rat inflammatory granulocytes. Bv8 stimulate macrophage and T lymphocyte to differentiate between an inflammatory and Th1 profile indicating that Bv8/PK proteins play a role in immuno-inflammatory responses. Blockade of PKRs may represent a novel therapeutic strategy in acute and inflammatory pain conditions.

Role of PAF receptor in proinflammatory cytokine expression in the dorsal root ganglion and tactile allodynia in a rodent model of neuropathic pain

BACKGROUND

Neuropathic pain is a highly debilitating chronic pain following damage to peripheral sensory neurons and is often resistant to all treatments currently available, including opioids. We have previously shown that peripheral nerve injury induces activation of cytosolic phospholipase A(2) (cPLA(2)) in injured dorsal root ganglion (DRG) neurons that contribute to tactile allodynia, a hallmark of neuropathic pain. However, lipid mediators downstream of cPLA(2) activation to produce tactile allodynia remain to be determined.

PRINCIPAL FINDINGS

Here we provide evidence that platelet-activating factor (PAF) is a potential candidate. Pharmacological blockade of PAF receptors (PAFRs) reduced the development and expression of tactile allodynia following nerve injury. The expression of PAFR mRNA was increased in the DRG ipsilateral to nerve injury, which was seen mainly in macrophages. Furthermore, mice lacking PAFRs showed a reduction of nerve injury-induced tactile allodynia and, interestingly, a marked suppression of upregulation of tumor necrosis factor alpha (TNFalpha) and interleukin-1beta (IL-1beta) expression in the injured DRG, crucial proinflammatory cytokines involved in pain hypersensitivity. Conversely, a single injection of PAF near the DRG of naïve rats caused a decrease in the paw withdrawal threshold to mechanical stimulation in a dose-dependent manner and an increase in the expression of mRNAs for TNFalpha and IL-1beta, both of which were inhibited by pretreatment with a PAFR antagonist.

CONCLUSIONS

Our results indicate that the PAF/PAFR system has an important role in production of TNFalpha and IL-1beta in the DRG and tactile allodynia following peripheral nerve injury and suggest that blocking PAFRs may be a viable therapeutic strategy for treating neuropathic pain.

PGE(2)-induced lasting nociception to heat: evidences for a selective involvement of A-delta fibres in the hyperpathic component of hyperalgesia

Animal models for mechanical pressure or heat nociception usually only measure the threshold response latency. In this study, the effect of typical sensitising treatments on the lasting nocifensive behaviour elicited after a supra-threshold heating stimulus - the hyperpathic component of hypernociception - was assessed. Male Wistar rats received either intra-plantar (i.pl.) injection of 350ng PGE(2) (50microL) or topical application (t.a.) of 100% dimethylsulfoxide (DMSO), and 10mM capsaicin. One hour after the paw treatments the number of nocifensive events (NNE) was scored hourly (6h), for 5min, immediately after a hind paw immersion in hot water (50 degrees C/7s). PGE(2), DMSO and capsaicin increased the NNE -induced by the supra-threshold stimuli. Indomethacin (2.5mg/kg i.p.), given 30min before paw treatments, completely inhibited NNE in all groups (P<0.01). However, indomethacin given 60min after PGE(2) did not reverse this sensitisation. PGE(2) and DMSO did not lower the heat threshold in the paw withdrawal test, although carrageenan and capsaicin were effective (P<0.05). Capsaicin neonatal treatment (CNT) (50mg/kg) reduced the sensitisation induced by DMSO and capsaicin (P<0.01), but not that induced by PGE(2). These data suggest that the heat-induced lasting nociception is probably conveyed by Aeth nociceptors, and PGE(2) seems to be more selective to induce this phenomenon than the thermal threshold lowering. In addition, this hyperpathic effect induced by DMSO and capsaicin seems to be indirectly mediated by PGE(2) and C-fibres.

Localization of COX-1 and COX-2 in the intracranial dura mater of the rat

Primary headaches such as migraine can be aborted by systemic administration of non-steroidal anti-inflammatory drugs (NSAIDs), potentially through the non-selective inhibition of cyclooxygenase (COX) activity in the intracranial meninges. In this study we have used single and double labeling immunohistochemistry to examine the distribution of the COX-1 and COX-2 isoforms in the intracranial dura mater of the rat and identify cell types that express them. COX-1 immunoreactivity was found in medium and small dural blood vessels and was co-expressed with the endothelial cell markers vimentin and the endothelial isoform of nitric oxide synthase (ecNOS). COX-1 was also found to be present in most dural mast cells. COX-2 was mainly expressed in ED2-positive resident dural macrophages. Constitutive COX-2 expression was also found in some axonal profiles, many of which were co-labeled with the nociceptor peptide marker CGRP. The findings suggest that NSAIDs may abort headache, at least in part, by inhibiting either neuronal or non-neuronal COX activity in the dura mater.

COX2 in CNS neural cells mediates mechanical inflammatory pain hypersensitivity in mice

A cardinal feature of peripheral inflammation is pain. The most common way of managing inflammatory pain is to use nonsteroidal antiinflammatory agents (NSAIDs) that reduce prostanoid production, for example, selective inhibitors of COX2. Prostaglandins produced after induction of COX2 in immune cells in inflamed tissue contribute both to the inflammation itself and to pain hypersensitivity, acting on peripheral terminals of nociceptors. COX2 is also induced after peripheral inflammation in neurons in the CNS, where it aids in developing a central component of inflammatory pain hypersensitivity by increasing neuronal excitation and reducing inhibition. We engineered mice with conditional deletion of Cox2 in neurons and glial cells to determine the relative contribution of peripheral and central COX2 to inflammatory pain hypersensitivity. In these mice, basal nociceptive pain was unchanged, as was the extent of peripheral inflammation, inflammatory thermal pain hypersensitivity, and fever induced by lipopolysaccharide. By contrast, peripheral inflammation-induced COX2 expression in the spinal cord was reduced, and mechanical hypersensitivity after both peripheral soft tissue and periarticular inflammation was abolished. Mechanical pain is a major symptom of most inflammatory conditions, such as postoperative pain and arthritis, and induction of COX2 in neural cells in the CNS seems to contribute to this.

Inflammation reduces mechanical thresholds in a population of transient receptor potential channel A1-expressing nociceptors in the rat

Inflammatory hypersensitivity is characterized by behavioural reductions in withdrawal thresholds to noxious stimuli. Although cutaneous primary afferent neurones are known to have lowered thermal thresholds in inflammation, whether their mechanical thresholds are altered remains controversial. The transient receptor potential channel A1 (TRPA1) is a receptor localized to putative nociceptive neurones and is implicated in mechanical and thermal nociception. Herein, we examined changes in the properties of single primary afferents in normal and acutely inflamed rats and determined whether specific nociceptive properties, particularly mechanical thresholds, are altered in the subpopulation of afferents that responded to the TRPA1 agonist cinnamaldehyde (TRPA1-positive afferents). TRPA1-positive afferents in normal animals belonged to the mechanonociceptive populations, many of which also responded to heat or capsaicin but only a few of which responded to cold. In acute inflammation, a greater proportion of afferents responded to cinnamaldehyde and an increased proportion of dorsal root ganglion neurones expressed TRPA1 protein. Functionally, in inflammation, TRPA1-positive afferents showed significantly reduced mechanical thresholds and enhanced activity to agonist stimulation. Inflammation altered thermal thresholds in both TRPA1-positive and TRPA1-negative afferents. Our data show that a subset of afferents is sensitized to mechanical stimulation by inflammation and that these afferents are defined by expression of TRPA1.

Prostaglandin-induced activation of nociceptive neurons via direct interaction with transient receptor potential A1 (TRPA1)

Inflammation contributes to pain hypersensitivity through multiple mechanisms. Among the most well characterized of these is the sensitization of primary nociceptive neurons by arachidonic acid metabolites such as prostaglandins through G protein-coupled receptors. However, in light of the recent discovery that the nociceptor-specific ion channel transient receptor potential A1 (TRPA1) can be activated by exogenous electrophilic irritants through direct covalent modification, we reasoned that electrophilic carbon-containing A- and J-series prostaglandins, metabolites of prostaglandins (PG) E(2) and D(2), respectively, would excite nociceptive neurons through direct activation of TRPA1. Consistent with this prediction, the PGD(2) metabolite 15-deoxy-Delta(12,14)-prostaglandin J(2) (15dPGJ(2)) activated heterologously expressed human TRPA1 (hTRPA1-HEK), as well as a subset of chemosensitive mouse trigeminal neurons. The effects of 15dPGJ(2) on neurons were blocked by both the nonselective TRP channel blocker ruthenium red and the TRPA1 inhibitor (HC-030031), but unaffected by the TRPV1 blocker iodo-resiniferatoxin. In whole-cell patch-clamp studies on hTRPA1-HEK cells, 15dPGJ(2) evoked currents similar to equimolar allyl isothiocyanate (AITC) in the nominal absence of calcium, suggesting a direct mechanism of activation. Consistent with the hypothesis that TRPA1 activation required reactive electrophilic moieties, A- and J-series prostaglandins, and the isoprostane 8-iso-prostaglandin A(2)-evoked calcium influx in hTRPA1-HEK cells with similar potency and efficacy. It is noteworthy that this effect was not mimicked by their nonelectrophilic precursors, PGE(2) and PGD(2), or PGB(2), which differs from PGA(2) only in that its electrophilic carbon is rendered unreactive through steric hindrance. Taken together, these data suggest a novel mechanism through which reactive prostanoids may activate nociceptive neurons independent of prostaglandin receptors.

Bv8/Prokineticin proteins and their receptors

The Bv8/Prokineticins (PKs) are a new family of peptides identified in frog, fish, reptiles and mammals that signal through two highly homologous G-protein coupled receptors, PKR1 and PKR2. Bv8/PK proteins possess a unique structural motif comprising five disulfide bonds and a completely conserved N-terminal hexapeptide sequence that is essential for the peptide's biological activities. Over the past few years, several biological functions of Bv8/PK proteins have been elucidated. This review considers all the published data on the action and physiological role of this new biological system implicated in angiogenesis and neurogenesis, in reproduction and cancer and in regulating physiological functions that underly circadian rhythms, such as the sleep/wake cycle, hormone secretion and ingestive behaviors. The high expression level of human Bv8/PK2 in bone marrow, lymphoid organs and leukocytes suggested an involvement of these peptides in hematopoiesis and in inflammatory and immunomodulatory processes. Our review highlights the role of the Bv8/PK and their receptor system in setting the pain threshold under normal and pathological conditions.

Synergistic interaction between meloxicam and aminoguanidine in formalin-induced nociception in mice

OBJECTIVES

The objective of this study was to examine the nature of interaction between cyclooxygenase-2 inhibitor meloxicam and inducible nitric oxide synthase inhibitor aminoguanidine in formalin-induced nociception in mice and the possible therapeutic advantage.

METHODS

Antinociceptive effect of meloxicam (1, 3, 10 and 30 mg/kg, oral) and aminoguanidine (10, 30, 100 and 300 mg/kg, oral) and their combinations was examined in formalin-induced paw licking model in mice. Analysis of variance and isobolographic method were employed to identify the nature of antinociceptive interaction.

RESULTS

Higher doses of meloxicam (10 and 30 mg/kg) and aminoguanidine (100 and 300 mg/kg) produced significant reduction in paw licking time (antinociceptive) in late phase of formalin-induced nociception. Combination of sub-threshold dose of meloxicam (3 mg/kg) with increasing doses of aminoguanidine (10, 30, 100 and 300 mg/kg) resulted in synergistic antinociceptive effect. Similarly, co-administration of sub-threshold dose of aminoguanidine (30 mg/kg) with increasing doses of meloxicam (1, 3, 10 and 30 mg/kg) produced significant reduction in formalin-induced paw licking behaviour. The experimental ED(50) for combination with their confidence limits are below the confidence interval of theoretical line of additive interaction, suggesting synergistic nature of interaction between meloxicam and aminoguanidine in isobolographic analysis.

CONCLUSION

Co-administration of meloxicam and aminoguanidine showed synergistic antinociceptive effect which might possibly reduce gastrointestinal toxicity associated with the use of meloxicam.

Phospholipase Cbeta 3 mediates the scratching response activated by the histamine H1 receptor on C-fiber nociceptive neurons

Phospholipase Cbeta (PLCbeta) isozymes represent a family of molecules that link G protein-coupled receptors (GPCRs) to an intracellular signaling network. Here, we investigated the function of PLCbeta isozymes in sensory neurons by using mutant mice deficient for specific PLCbeta family members. Expression analysis indicated that PLCbeta3, one of the four isoforms, is predominantly expressed in a subpopulation of C-fiber nociceptors. A subset of these neurons expressed the histamine H1 receptor. Ca(2+) imaging studies revealed that PLCbeta3 specifically mediates histamine-induced calcium responses through the histamine H1 receptor in cultured sensory neurons. In line with this, we found that PLCbeta3(-/-) mice showed significant defects in scratching behavior induced by histamine; histamine-trifluoromethyl-toluidine (HTMT), a selective H1 agonist; and compound 48/80, a mast cell activator. These results demonstrate that PLCbeta3 is required to mediate "itch" sensation in response to histamine acting on the histamine H1 receptor in C-fiber nociceptive neurons.

Bradykinin-induced nociceptor sensitisation to heat depends on cox-1 and cox-2 in isolated rat skin

Bradykinin is an important inflammatory mediator that can either activate and/or sensitise nociceptors to heat stimuli applied to the skin. Several studies have suggested that prostaglandins and thus the cyclooxygenase (cox) enzymes are important in the sensitisation process but little is known about the relative involvement of the two cox isoforms, cox-1 and cox-2. Extracellular recordings were made from C-mechanoheat-sensitive fibres in isolated rat skin-saphenous nerve preparations. Bradykinin-mediated sensitisation of heat responses in these afferents was significantly attenuated by the selective cox-1 inhibitor, SC-560, and by the selective cox-2 inhibitor, NS-398. In the same experiments, bradykinin-mediated induction of ongoing activity was reduced by SC-560 but not NS-398. In a second series of experiments, bradykinin-stimulated synthesis and release of prostaglandin E2 (PGE2) was measured in isolated skin-nerve preparations. Although the basal release of PGE2 appeared unaffected by either drug, bradykinin-stimulated PGE2 release from the skin was inhibited by both SC-560 and NS-398. Immunocytochemical evaluation revealed cox-1 immunostaining was present in large cutaneous nerve branches, small intradermal nerve bundles as well as nerve endings within the skin. Cox-1 labelling was also present in non-neuronal cell types such as mast cells. Cox-2 immunoreactivity was weak but where present was located in small nerve bundles, smaller intradermal nerve bundles and nerve endings. This study shows that both cox isoforms are present in skin and that they have an important role in mediating bradykinin-evoked heat sensitisation of C-MH sensitive fibres through cox-1 and cox-2 dependent prostaglandin synthesis.

The role of the cylooxygenase pathway in nociception and pain

Cycloxygenase (COX) pathways have long been targeted for the treatment of inflammatory pain, initially through the use of NSAIDs. With the demonstration of two major COX isoforms, COX-1 and COX-2, involved in the production of prostanoids, but with different distribution and regulation, selective COX-2 inhibitors have been developed. This review covers factors influencing COX enzyme activity, the role of their products in the development and maintenance of pain and discusses recent safety concerns of COX-2 inhibitors.

Sex differences in inflammation and inflammatory pain in cyclooxygenase-deficient mice

There are two cyclooxygenase (COX) genes encoding characterized enzymes, COX-1 and COX-2. Nonsteroidal anti-inflammatory drugs are commonly used as analgesics in inflammatory arthritis, and these often inhibit both cyclooxygenases. Recently, inhibitors of COX-2 have been used in the treatment of inflammatory arthritis, as this isoform is thought to be critical in inflammation and pain. The objective of this study was to determine the effect of COX-1 or COX-2 gene disruption on the development of chronic Freund's adjuvant-induced arthritis and inflammatory pain in male and female mice. The effect of COX-1 or COX-2 gene disruption on inflammatory hyperalgesia, allodynia, inflammatory edema, and arthritic joint destruction was studied. COX-2 knockout mice (COX-2-/-) showed reduced edema and joint destruction in female, but not male, animals. In addition, neither male nor female COX-2-/- mice developed thermal hyperalgesia or mechanical allodynia, either ipsilateral or contralateral to the inflammation. COX-1 gene disruption also reduced inflammatory edema and joint destruction in female, but not male mice, although females of both COX-/- lines did show some bony destruction. There was no difference in ipsilateral allodynia between COX-1 knockout and wild-type animals, but female COX-1-/- mice showed reduced contralateral allodynia compared with male COX-1-/- or wild-type mice. These data show that the gene products of both COX genes contribute to pain and local inflammation in inflammatory arthritis. There are sex differences in some of these effects, and this suggests that the effects of COX inhibitors may be sex dependent.

Neonatal immune challenge alters nociception in the adult rat

Intense pain or intense peripheral inflammation experienced during development can have pronounced effects upon adult pain sensation. However, little is known about the more commonly encountered mild systemic inflammation, such as that experienced with mild illness. Neonatal exposure to lipopolysaccharide (LPS), an established model of immune system activation, has been shown to affect febrile and cyclooxygenase-2 (COX-2) responses to a similar exposure in adulthood. Adult LPS also elicits a range of sickness behaviours, including enhanced responses to painful stimuli. We, therefore, hypothesized that adult sensation and pain responses could be affected by a neonatal LPS challenge. Male and female Sprague-Dawley rats were administered LPS at postnatal day 14 and were tested in adulthood for nociceptive responses to thermal and mechanical stimuli using, respectively, a plantar test apparatus and von Frey filaments, before and after adult LPS. Expression of dorsal root ganglion and lumbar spinal cord COX-2 was also examined. Animals treated as neonates with saline showed the expected hypersensitivity to painful stimuli after adult LPS as well as enhanced spinal cord COX-2. Neonatally LPS-treated rats, however, showed a significantly different profile. They displayed enhanced baseline nociception and elevated basal spinal cord COX-2 compared with neonatally saline-treated rats. Also, rather than the expected hyperalgesia after adult LPS, no changes in nociceptive responses and a reduction in spinal cord COX-2 expression were observed. These findings have important implications for the understanding of pain and its management and highlight the importance of the neonatal period in the development of pain pathways.

The majority of bladder sensory afferents to the rat lumbosacral spinal cord are both IB4- and CGRP-positive

The rat urinary bladder is innervated by neurons in dorsal root ganglia (DRG) that express the neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP), and a fraction of bladder afferents can bind the non-peptidergic marker isolectin B4 (IB4). We used histochemical binding and axonal tracing to identify the bladder afferents, and immunocytochemistry to determine the degree of colocalization of CGRP with IB4 in their cell bodies in DRG and in their central axons in the spinal cord. In the L6 DRG, about 60% of CGRP-positive neurons were also positive for IB4. In the spinal cord, IB4 and CGRP colocalized in fibers and terminals in the inner part of lamina II, the lateral collateral path, and the sacral parasympathetic nucleus (SPN). In SPN, the majority of IB4-positive fibers and terminals were also CGRP-positive. After injection of IB4 into the bladder wall, immunoreaction for IB4 was detected in SPN, but not in lamina II. These results suggest that most IB4-positive afferents from the bladder are also CGRP-positive, and that the distinction between peptidergic and non-peptidergic bladder afferents based on IB4 binding is of limited validity.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

The distribution of small and intermediate conductance calcium-activated potassium channels in the rat sensory nervous system

Small (SK) and intermediate (IK) conductance calcium-activated potassium channels are candidate ion channels for the regulation of excitability in nociceptive neurones. We have used unique peptide-directed antisera to describe the immunocytochemical distribution of the known isoforms of these ion channels in dorsal root ganglia (DRG) and spinal cord of the rat. These investigations sought to characterize further the phenotype and hence possible functions of nociceptive neurone subpopulations in the rat. In addition, using Western blotting, we sought to determine the level of protein expression of SK and IK channels in sensory nervous tissues following induction of inflammation (Freund's Complete Adjuvant (FCA) arthritis model) or nerve injury (chronic constriction injury model). We show that SK1, SK2, SK3 and IK1 are all expressed in DRG and spinal cord. Morphometric analysis revealed that SK1, SK2 and IK1 were preferentially localized to neurones having cell bodies <1000 microm2 (putative nociceptors) in DRG. Dual labeling immunocytochemistry showed that these ion channels co-localize with both CGRP and IB4, known markers of nociceptor sub-populations. SK2 was localized almost exclusively in the superficial laminae of the spinal cord dorsal horn, the region in which many sensory afferents terminate; the distribution of SK1 and IK1 was more widespread in spinal cord, although some preferential labeling within the dorsal horn was observed in the case of IK1. Here we show evidence for a distinctive pattern of expression for certain members of the calcium-activated potassium channel family in the rat DRG.

A role for the anandamide membrane transporter in TRPV1-mediated neurosecretion from trigeminal sensory neurons

Many n-acylethanolamines utilize the anandamide membrane transporter (AMT) to gain facilitated access to the intracellular compartment, hence, we hypothesized that this mechanism might be important for anandamide (AEA)- and N-arachidonoyl-dopamine (NADA)-evoked CGRP release from cultured trigeminal ganglion (TG) neurons. Using [14C]AEA we demonstrated that TG neurons transported AEA in a FAAH- and AMT-inhibitable fashion. Although TRPV1-positive TG neurons were found to express fatty acid amide hydrolase, the application of FAAH inhibitors had no effect on AEA-evoked CGRP release. In contrast, application of the AMT inhibitors OMDM-2 or VDM-11 significantly reduced the potency and efficacy of AEA-, NADA- and capsaicin-evoked CGRP release. Moreover OMDM-2 (IC50 values ranging from 6.4-9.6 microM) and VDM-11 (IC50 values ranging from 5.3-11 microM) inhibited CGRP release evoked by EC80 concentrations of AEA, NADA and CAP and these values were consistent with IC50s obtained for inhibition of uptake. OMDM-2 had no effect on CGRP release per se while VDM-11 evoked CGRP release on its own (EC50 approximately 35 microM) in a CPZ-insensitive, but ruthenium red (RR)-sensitive fashion. This is the first demonstration that TG sensory neurons possess an AMT-like mechanism suggesting that this mechanism is important for the pharmacological action of AEA and NADA at native TRPV1 channels.

Nociception and the differential expression of cyclooxygenase-1 (COX-1), the COX-1 variant retaining intron-1 (COX-1v), and COX-2 in mouse dorsal root ganglia (DRG)

Prostaglandins (PGs) formed via the cyclooxygenase (COX) pathway mediate hyperalgesia in sensory nerve endings. To investigate the role of the COX isoforms in pain transmission we recently studied nociception in COX-isozyme-deficient mice using models of "sharp" rapidly transmitted pain (hot-plate) and slowly developing, diffuse pain (writhing) [Ballou L, Botting RM, Goorha S, Zhang J, Vane JR. Nociception in cyclooxygenase isozyme-deficient mice. Proc Natl Acad Sci USA 2000;97:10272]. Our results demonstrated that COX-1 (and not COX-2) was the primary isoform involved in nociception in both model systems. Given the importance of dorsal root ganglia (DRG) in pain transmission we examined the expression patterns of COX-1, -2 and the recently described variant of COX-1 retaining intron-1, originally referred to as "COX-3" but hereafter referred to as COX-1 variant (COX-1v), in mouse L4 or L5 DRG taken from normal and COX-isozyme-deficient mice. Messenger RNA and protein for COX isoforms from DRG, spinal cord as well as, heart, brain, kidney, spleen and skin of adult mice were isolated and analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. Patterns of COX-isoform expression were determined using immunohistochemical techniques. We found that COX-1 and COX-1v were both expressed in neurons while COX-2 expression was completely undetectable in the DRG. Immunohistochemical analysis of COX expression in DRG of mice exhibiting the chronic pain and inflammation associated with collagen-induced arthritis (CIA) expressed COX-1 and COX-1v while no COX-2 could be detected. For purposes of comparison, COX-1v mRNA was also expressed in heart, brain, spinal cord, kidney, spleen and skin. Together, these data support a role for COX-1 and perhaps COX-1v, not COX-2, as the primary producers of PGs in mouse DRG in normal and in mice subject to chronic pain and inflammation. These data also suggest potential alternative analgesic mechanisms of action for the newly developed, COX-2 selective inhibitors and the nonsteroidal anti-inflammatory drugs (NSAIDs) in pain transmission in the peripheral nervous system.

Activation of EP4 receptors contributes to prostaglandin E2-mediated stimulation of renal sensory nerves

Induction of cyclooxygenase-2 (COX-2) in the renal pelvic wall increases prostaglandin E(2) (PGE(2)) leading to stimulation of cAMP production, which results in substance P (SP) release and activation of renal mechanosensory nerves. The subtype of PGE receptors involved, EP2 and/or EP4, was studied by immunohistochemistry and renal pelvic administration of agonists and antagonists of EP2 and EP4 receptors. EP4 receptor-like immunoreactivity (LI) was colocalized with calcitonin gene-related peptide (CGRP)-LI in dorsal root ganglia (DRGs) at Th(9)-L(1) and in nerve terminals in the renal pelvic wall. Th(9)-L(1) DRG neurons also contained EP3 receptor-LI and COX-2-LI, each of which was colocalized with CGRP-LI in some neurons. No renal pelvic nerves contained EP3 receptor-LI and only very few nerves COX-2-LI. The EP1/EP2 receptor antagonist AH-6809 (20 microM) had no effect on SP release produced by PGE(2) (0.14 microM) from an isolated rat renal pelvic wall preparation. However, the EP4 receptor antagonist L-161,982 (10 microM) blocked the SP release produced by the EP2/EP4 receptor agonist butaprost (10 microM) 12 +/- 2 vs. 2 +/- 1 and PGE(2), 9 +/- 1 vs. 1 +/- 0 pg/min. The SP release by butaprost and PGE(2) was similarly blocked by the EP4 receptor antagonist AH-23848 (30 microM). In anesthetized rats, the afferent renal nerve activity (ARNA) responses to butaprost 700 +/- 100 and PGE(2).780 +/- 100%.s (area under the curve of ARNA vs. time) were unaffected by renal pelvic perfusion with AH-6809. However, 1 microM L-161,982 and 10 microM AH-23848 blocked the ARNA responses to butaprost by 94 +/- 5 and 78 +/- 10%, respectively, and to PGE(2) by 74 +/- 16 and 74 +/- 11%, respectively. L-161,982 also blocked the ARNA response to increasing renal pelvic pressure 10 mmHg, 85 +/- 5%. In conclusion, PGE(2) increases renal pelvic release of SP and ARNA by activating EP4 receptors on renal sensory nerve fibers.

Characterization of bradykinin-induced prostaglandin E2 release from cultured rat trigeminal ganglion neurones

Bradykinin and prostaglandins are both local mediators strongly implicated in pain and inflammation. Here, we have investigated the effects of bradykinin on the release of prostaglandin E(2) from cultured neurones derived from adult rat trigeminal ganglia. Bradykinin was a potent inducer of prostaglandin E(2) release, an effect that was likely mediated by bradykinin B(2) receptors, as the bradykinin-induced prostaglandin E(2) release was attenuated by the bradykinin B(2) receptor-selective antagonist, arginyl-L-prolyl-trans-4-hydroxy-L-prolylglycyl-3-(2-thienyl)-L-alanyl-L-seryl-D-1,2,3,4-tetrahydro-3-isoquinolinecarbonyl-L-(2 alpha, 3 beta, 7a beta)-octahydro-1H-indole-2-carbonyl-L-arginine (HOE 140), but not by the bradykinin B(1) receptor-selective antagonist, des-Arg(9),[Leu(8)]-bradykinin. Furthermore, bradykinin-induced prostaglandin E(2) release was inhibited following treatment with the phospholipase A(2) inhibitor, arachidonyltrifluoromethyl ketone (AACOCF(3)), the nonselective cyclooxygenase inhibitor, piroxicam, the mitogen-activated protein kinase kinase-1 (MEK1) inhibitor, 2'-amino-3'-methoxyflavone (PD98059), and the protein kinase C inhibitor, bisindolylmaleimide XI (Ro320432). Taken together, these data suggest that bradykinin, acting via bradykinin B(2) receptors, induces prostaglandin E(2) release from trigeminal neurones through the protein kinase C and mitogen-activated protein kinase-dependent activation of phospholipase A(2) and consequent stimulation of cyclooxygenases.

Effect of endotoxin treatment on the expression of cyclooxygenase-2 and prostaglandin synthases in spinal cord, dorsal root ganglia, and skin of rats

Peripheral inflammation causes upregulation of cyclooxygenase in the spinal cord and subsequent increase in prostaglandin biosynthesis. However, prostaglandin synthases, which are downstream of cyclooxygenase control the type of prostaglandin that is formed predominantly. Since there is little known about the regulation of prostaglandin synthases, the present study was conducted in order to determine the effect of endotoxin treatment on the expression of messenger RNA encoding interleukin 1beta, cyclooxygenase-2, and prostaglandin synthases mediating the formation of prostaglandin E(2) (membrane bound prostaglandin E synthase) and prostaglandin D(2) (lipocalin prostaglandin D synthase) in spinal cord, dorsal root ganglia and skin of rats. Endotoxin (2 mg/kg i.p.) induced the expression of interleukin-1beta, cyclooxygenase-2, and membrane bound prostaglandin E synthase messenger RNA in spinal cord, dorsal root ganglia, and skin as determined by reverse transcription polymerase chain reaction. In contrast, basal expression of lipocalin prostaglandin D synthase messenger RNA in spinal cord and dorsal root ganglia was not significantly altered by endotoxin. Dexamethasone (1 mg/kg s.c. at -18 h and -1 h) attenuated the effect endotoxin on the expression of interleukin-1beta, cyclooxygenase-2, and membrane bound prostaglandin E synthase messenger RNA in all tissues investigated, but did not significantly influence expression of lipocalin prostaglandin D synthase mRNA in spinal cord and dorsal root ganglia. In situ hybridisation histochemistry showed endotoxin-induced expression of cyclooxygenase-2 and membrane bound prostaglandin E synthase messenger RNA throughout gray and white matter of spinal cord sections. In dorsal root ganglia, expression of membrane bound prostaglandin E synthase seemed primarily located to non-neuronal cells, while cyclooxygenase-2 messenger RNA was not detectable. The results show that the immune response elicited by endotoxin induced cyclooxygenase-2 and membrane bound prostaglandin E synthase, but not lipocalin prostaglandin D synthase messenger RNA in spinal cord and dorsal root ganglia of rats. The distribution of cyclooxygenase-2 and membrane bound prostaglandin E synthase messenger RNA expressing cells suggests major involvement of non-neuronal cells in spinal prostaglandin biosynthesis. Determination of the regulation of enzymes downstream of cyclooxygenase at the messenger RNA level may represent a valuable tool to investigate effects of analgesic/anti-inflammatory drugs on the regulation of spinal prostaglandin biosynthesis.

COX-3, a cyclooxygenase-1 variant inhibited by acetaminophen and other analgesic/antipyretic drugs: cloning, structure, and expression

Two cyclooxygenase isozymes, COX-1 and -2, are known to catalyze the rate-limiting step of prostaglandin synthesis and are the targets of nonsteroidal antiinflammatory drugs. Here we describe a third distinct COX isozyme, COX-3, as well as two smaller COX-1-derived proteins (partial COX-1 or PCOX-1 proteins). COX-3 and one of the PCOX-1 proteins (PCOX-1a) are made from the COX-1 gene but retain intron 1 in their mRNAs. PCOX-1 proteins additionally contain an in-frame deletion of exons 5-8 of the COX-1 mRNA. COX-3 and PCOX mRNAs are expressed in canine cerebral cortex and in lesser amounts in other tissues analyzed. In human, COX-3 mRNA is expressed as an approximately 5.2-kb transcript and is most abundant in cerebral cortex and heart. Intron 1 is conserved in length and in sequence in mammalian COX-1 genes. This intron contains an ORF that introduces an insertion of 30-34 aa, depending on the mammalian species, into the hydrophobic signal peptide that directs COX-1 into the lumen of the endoplasmic reticulum and nuclear envelope. COX-3 and PCOX-1a are expressed efficiently in insect cells as membrane-bound proteins. The signal peptide is not cleaved from either protein and both proteins are glycosylated. COX-3, but not PCOX-1a, possesses glycosylation-dependent cyclooxygenase activity. Comparison of canine COX-3 activity with murine COX-1 and -2 demonstrates that this enzyme is selectively inhibited by analgesic/antipyretic drugs such as acetaminophen, phenacetin, antipyrine, and dipyrone, and is potently inhibited by some nonsteroidal antiinflammatory drugs. Thus, inhibition of COX-3 could represent a primary central mechanism by which these drugs decrease pain and possibly fever.

Neuropeptides in experimental and degenerative arthritis

Classical symptoms of both inflammatory and degenerative arthritides may contribute to neurogenic responses like wheal, flare, edema, and pain. Rheumatoid arthritis (RA) is an autoimmune disease with an immunogenetic background. Neurogenic inflammation has been considered to play an essential role in RA, in part because of the symmetrical involvement (cross-spinal reflexes) and the predominant involvement of the most heavily innervated small joints of the hands and the feet (highly represented in the hominiculus). In contrast, osteoarthritis (OA) is considered to arise as a result of degeneration of the hyaline articular cartilage, which secondarily results in local inflammation and pain. However, it is possible that the age-related and predominant (compared to nociceptive nerves) degeneration of the proprioceptive, kinesthetic and vasoregulatory nerves can represent the primary pathogenic events. This leads to progressive damage of tissue with extremely poor capacity for self-regeneration. Inflammation, be it primary/autoimmune or secondary/degenerative, leads to peripheral sensitization and stimulation, which may further lead to central sensitization, neurogenic amplification of the inflammatory responses and activation of the neuro-endocrine axis. Neuropeptides serve as messengers, which modulate and mediate the actions in these cascades. Accordingly, many neuropeptides have been used successfully as experimental treatments, most recently VIP, which effectively controlled collagen-induced arthritis in mice. Therefore, it can safely be concluded that better treatment/control of disease activity and pain can be achieved by blocking the cascade leading to initiation and/or amplification of inflammatory process combined with effects on central nociceptive and neuroendocrine responses.

Role for both spinal cord COX-1 and COX-2 in maintenance of mechanical hypersensitivity following peripheral nerve injury

The effectiveness of non-steroidal anti-inflammatory drugs (NSAIDs) in treating neuropathic pain caused by nerve injury has been controversial. In the present study, 4 weeks following partial sciatic nerve ligation, a single intrathecal injection of the cyclooxygenase (COX)-1 preferring inhibitor ketorolac (50 microg) significantly attenuated tactile allodynia for 6 days. The COX-2 preferring inhibitor, NS-398 (60 microg) significantly reversed tactile allodynia 2 h following injection but this anti-allodynic effect did not last greater than 24 h. Surprisingly, the non-selective COX inhibitor, piroxicam (60 microg) was without effect. These data agree with previous studies suggesting that spinal prostaglandin synthesis is important in the maintenance of hypersensitivity states following nerve injury. They differ from results in other models by suggesting that both COX isoenzymes are important in this spinal process, and for the first time demonstrate a remarkably long duration of action from a single intrathecal injection of ketorolac. Inhibition of spinal COX may be an important mechanism of action in treating some patients with neuropathic pain following peripheral nerve injury.

The spinal phospholipase-cyclooxygenase-prostanoid cascade in nociceptive processing

Intrathecal phospholipase A2 (PLA2) and cyclooxygenase-2 (COX-2), but not COX-1, inhibitors attenuate facilitated pain states generated by peripheral injury/inflammation and by direct activation of spinal glutamate and substance P receptors. These results are consistent with the constitutive expression of PLA2 and COX-2 in spinal cord, the spinal release of prostaglandins by persistent afferent input, and the effects of prostaglandins on spinal excitability. Whereas the acute actions of COX-2 inhibitors are clearly mediated by constitutively expressed spinal COX-2, studies of spinal COX-2 expression indicate that it is upregulated by neural input and circulating cytokines. Given the intrathecal potency of COX-2 inhibitors, the comparable efficacy of intrathecal versus systemic COX-2 inhibitors in hyperalgesic states not associated with inflammation, and the onset of antihyperalgesic activity prior to COX-2 upregulation, it is argued that a principal antihyperalgesic mechanism of COX-2 inhibitors lies with modulation of constitutive COX-2 present at the spinal level.