Pain control in inflammation governed by selectins

[Opioids in musculoskeletal pain]

Opioids are the most potent analgesics available and are well established for the treatment of severe acute, surgical and cancer pain. Due to their high effectiveness, their use in chronic non-cancer pain (CNCP) is being propagated. However, the use of opioids is still controversial due to their side effects, such as tolerance, addiction or withdrawal, and administrative difficulties associated with their prescription. Chronic rheumatic diseases, in particular low back pain and arthritis, are the leading causes of CNCP. The present article provides a brief overview of the role of opioids in chronic rheumatic diseases, pointing out that a national guideline for opioid use in CNCP is expected at the end of 2008. Furthermore, the peripheral effects of opioids on pain and inflammation in rheumatic diseases will be outlined.

DREAM is reduced in synovial fibroblasts of patients with chronic arthritic pain: is it a suitable target for peripheral pain management?

Introduction

The endogenous pain-relieving system depends in part on the regulation of nociceptive signals through binding of opioids to the corresponding opioid receptor. Interfering with the trans-repression effect of downstream regulatory element antagonist modulator (DREAM) on the transcription of the opioid dynorphin-encoding prodynorphin (pdyn) gene might enhance pain relief in the periphery.

Methods

Expression levels were measured in osteoarthritis (OA) synovial fibroblast-like cells (SFLCs) (n = 8) and in peripheral blood mononuclear cells (PBMCs) from OA patients (n = 53) and healthy controls (n = 26) by real-time polymerase chain reaction. Lysed OA SFLCs were analyzed by immunoprecipitation. Translation of DREAM mRNA was inhibited by small interfering RNAs (siRNAs). Expressions of DREAM, pdyn, and c-fos mRNAs were measured at 24, 48, and 72 hours after transfection.

Results

The expression of DREAM mRNA was shown in both healthy and OA SFLCs as well as PBMCs. Inhibiting transcription using siRNAs led to a marked reduction in DREAM expression after 24, 48, and 72 hours. However, no significant changes in c-fos and pdyn expression occurred. In addition, DREAM mRNA expression was significantly reduced in OA patients with chronic pain (pain intensity as measured by a visual analog scale scale of greater than 40), but no pdyn expression was detectable.

Conclusion

To our knowledge, this is the first report showing the expression of DREAM in SFLCs and PBMCs on the mRNA level. However, DREAM protein was not detectable. Since repression of pdyn transcription persists after inhibiting DREAM translation, DREAM appears to play no functional role in the kappa opioid receptor system in OA SFLCs. Therefore, our data suggest that DREAM appears not to qualify as a target in peripheral pain management.

GaAlAs (830 nm) low-level laser enhances peripheral endogenous opioid analgesia in rats

BACKGROUND

Low-level laser therapy (LLLT) has been reported to relieve pain with minimal side effects. Recent studies have demonstrated that opioid-containing immune cells migrate to inflamed sites and release beta-endorphins to inhibit pain as a mode of peripheral endogenous opioid analgesia. The present study investigates whether LLLT may enhance peripheral endogenous opioid analgesia.

METHODS

The effect of LLLT on opioid analgesia and production was evaluated in vivo in a rat model of inflammation as well as in vitro in Jurkat cells, a human T-cell leukemia cell line. mRNA expression of the beta-endorphin precursors proopiomelanocortin and corticotrophin releasing factor was assessed by reverse transcription polymerase chain reaction.

RESULTS

LLLT produced an analgesic effect in inflamed peripheral tissue which was transiently antagonized by naloxone. Beta-endorphin precursor mRNA expression increased with LLLT, both in vivo and in vitro.

CONCLUSION

This study demonstrates that LLLT produces analgesic effects in a rat model of peripheral inflammation. We further revealed an additional mechanism of LLLT-mediated analgesia via enhancement of peripheral endogenous opioids. These findings suggest that LLLT induces analgesia in rats by enhancing peripheral endogenous opioid production in addition to previously reported mechanisms.

Targeting of opioid-producing leukocytes for pain control

It is accepted that inflammatory mediators released from leukocytes contribute to the generation of pain. However, it is less well known that immune cells also produce mediators that can effectively counteract pain. These include anti-inflammatory cytokines and opioid peptides. This article concentrates on recent evidence that interactions between leukocyte-derived opioid peptides and their receptors on peripheral sensory neurons can result in potent, clinically relevant inhibition of pathological pain. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G-protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, all these changes lead to an enhanced peripheral analgesic efficacy of opioids. The major source of local endogenous opioid ligands (beta-endorphin, enkephalins, endomorphins and dynorphin) are leukocytes. These cells contain and upregulate signal-sequence encoding mRNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor-triggered release of Ca(2+) from endoplasmic reticulum. Once secreted opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of excitatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness or addiction. Future aims include the selective targeting of opioid-containing leukocytes to sites of painful injury and the augmentation of opioid peptide and receptor synthesis.

Influence of pain treatment by epidural fentanyl and bupivacaine on homing of opioid-containing leukocytes to surgical wounds

Endogenous opioids released from leukocytes extravasating into injured tissue can interact with peripheral opioid receptors to inhibit nociception. Animal studies have shown that the homing of opioid-producing leukocytes to the injured site is modulated by spinal blockade of noxious input. This study investigated whether epidural analgesia (EDA) influences the migration of beta-endorphin (END) and/or met-enkephalin (ENK)-containing leukocytes into the subcutaneous wound tissue of patients undergoing abdominal surgery. In part I patients received general anesthesia combined either with intra- and postoperative EDA (with bupivacaine and fentanyl) or with postoperative patient controlled intravenous analgesia (PCIA; with the opioid piritramide). In part II patients received general anesthesia combined with either epidural fentanyl or bupivacaine which was continued postoperatively. Samples of cutanous and subcutanous tissue were taken from the wound site at the beginning, at the end and at various times after surgery, and were examined by immunohistochemistry for the presence of END and ENK. We found that (i) epidural bupivacaine, fentanyl and PCIA provided similar and clinically acceptable postoperative pain relief; (ii) compared to PCIA, epidural bupivacaine or fentanyl did not change the gross inflammatory reaction within the surgical wound; (iii) opioid-containing leukocytes were almost absent in normal subcutaneous tissue but migrated to the inflamed wound tissue in ascending numbers within a few hours, reaching a peak at about 24 h after surgery; (iv) compared to PCIA, EDA resulted in significantly decreased homing of END-containing leukocytes to the injured site at 24 h after surgery; and (v) the magnitude of this decrease was similar regardless of the epidural medication. These findings suggest that nociceptive but not sympathetic neurons are primarily involved in the attraction of opioid-containing leukocytes during early stages of inflammation.

Pro-algesic versus analgesic actions of immune cells

PURPOSE OF REVIEW

When tissue is destroyed, pain arises. Tissue destruction is associated with an inflammatory reaction. This leads to activation of nociceptors. The following review will concentrate on pro-algesic and analgesic mediators, which arise from immune cells or resident cells in the periphery or the circulation during inflammation.

RECENT FINDINGS

In early inflammation endogenous hyperalgesic mediators are produced, including cytokines, chemokines, nerve growth factor as well as bradykinin, prostaglandins and ATP. Simultaneously, analgesic mediators are secreted: opioid peptides, somatostatin, endocannabinoids and certain cytokines. Inflammation increases the expression of peripheral opioid receptors on sensory nerve terminals and enhances their signal transduction, as well as the amount of opioid peptides in infiltrating immune cells. Interference with the recruitment of opioid-containing immune cells into inflamed tissue by blockade of adhesion molecules or by intrathecal morphine injection reduces endogenous analgesia.

SUMMARY

Inflammatory pain is the result of the interplay between pro-algesic and analgesic mediators. To avoid central side effects, future analgesic therapy should be targeted at either selectively blocking novel pro-algesic mediators or at enhancing endogenous peripheral analgesic effects.

Treatment of inflamed pancreas with enkephalin encoding HSV-1 recombinant vector reduces inflammatory damage and behavioral sequelae

This study assessed the efficacy of pancreatic surface delivered enkephalin (ENK)-encoding herpes simplex virus type 1 (HSV-1) on spontaneous behaviors and spinal cord and pancreatic enkephalin expression in an experimental pancreatitis model. Replication-defective HSV-1 with proenkephalin complementary DNA (cDNA) (HSV-ENK) or control beta-galactosidase cDNA (HSV-beta-gal), or media vehicle (Veh) was applied to the pancreatic surface of rats with dibutyltin dichloride (DBTC)-induced pancreatitis. Spontaneous exploratory behavioral activity was monitored on days 0 and 6 post DBTC and vector treatments. The pancreas, thoracic dorsal root ganglia (DRG, T9-10), and spinal cord (T9-10) were immunostained for met-enkephalin (met-ENK), beta-gal, and HSV-1 proteins. Spinal cord was also immunostained for c-Fos, and pancreas was stained for the inflammatory marker regulated on activation, normal T-cells expressed and secreted (RANTES), mu-opioid receptor, and hemotoxylin/eosin. On day 6, compared to pancreatitis and vector controls, the DBTC/HSV-ENK treated rats had significantly improved spontaneous exploratory activities, increased met-ENK staining in the pancreas and spinal cord, and normalized c-Fos staining in the dorsal horn. Histopathology of pancreas in DBTC/HSV-ENK treated rats showed preservation of acinar cells and cytoarchitecture with minimal inflammatory cell infiltrates, compared to severe inflammation and acinar cell loss seen in DBTC/HSV-beta-gal and DBTC/Veh treated rats. Targeted transgene delivery and met-ENK expression successfully produced decreased inflammation in experimental pancreatitis.

The role of the hormones of the hypothalamo-hypophyseal-adrenocortical system in the analgesic effect of corticotropin-releasing hormone

Experiments on anesthetized male rats were performed to study the role of the hormones of the hypothalamo-hypophyseal-adrenocortical system (HHACS) in analgesia induced by central or systemic administration of corticoliberin-releasing hormone (CRF). Studies of the contribution of HHACS hormones were performed by blocking HHACS function by administration of hydrocortisone at a pharmacological dose one week before experiments started. Blockade of HHACS function, resulting in the inability of the system to increase hormone levels, resulted in a decrease in the analgesic effect resulting from systemic administration of CRF and completely abolished the analgesic effect after central administration of CRF. These data lead to the conclusion that there are two components involved in increasing the pain sensitivity threshold in response to administration of CRF: 1) a component dependent on HHACS hormones in central and systemic administration of CRF; 2) a component independent of HHACS hormones on systemic administration of CRF.

Beta-endorphin, Met-enkephalin and corresponding opioid receptors within synovium of patients with joint trauma, osteoarthritis and rheumatoid arthritis

OBJECTIVE

Intra-articularly applied opioid agonists or antagonists modulate pain after knee surgery and in chronic arthritis. Therefore, the expression of beta-endorphin (END), Met-enkephalin (ENK), and mu and delta opioid receptors (ORs) within synovium of patients with joint trauma (JT), osteoarthritis (OA) and rheumatoid arthritis (RA) were examined.

METHODS

Synovial samples were subjected to double immunohistochemical analysis of opioid peptides with immune cell markers, and of ORs with the neuronal markers calcitonin gene-related peptide (CGRP) and tyrosine hydroxylase (TH).

RESULTS

END and ENK were expressed by macrophage-like (CD68(+)) and fibroblast-like (CD68(-)) cells within synovial lining layers of all disorders. In the sublining layers, END and ENK were mostly expressed by granulocytes in patients with JT, and by macrophages/monocytes, lymphocytes and plasma cells in those with OA and RA. Overall, END- and ENK-immunoreactive (IR) cells were more abundant in patients with RA than in those with OA and JT. ORs were found on nerve fibres and immune cells in all patients. OR-IR nerve fibres were significantly more abundant in patients with RA than in those with OA and JT. muORs and deltaORs were coexpressed with CGRP but not with TH.

CONCLUSIONS

Parallel to the severity of inflammation, END and ENK in immune cells and their receptors on sensory nerve terminals are more abundant in patients with RA than in those with JT and OA. These findings are consistent with the notion that, with prolonged and enhanced inflammation, the immune and peripheral nervous systems upregulate sensory nerves expressing ORs and their ligands to counterbalance pain and inflammation.

Opioids

Opioids are the most effective and widely used drugs in the treatment of severe pain. They act through G protein-coupled receptors. Four families of endogenous ligands (opioid peptides) are known. The standard exogenous opioid analgesic is morphine. Opioid agonists can activate central and peripheral opioid receptors. Three classes of opioid receptors (mu, delta, kappa) have been identified. Multiple pathways ofopioid receptor signaling (e.g., G(i/o) coupling, cAMP inhibition, Ca++ channel inhibition) have been described. The differential regulation of effectors, preclinical pharmacology, clinical applications, and side effects will be reviewed in this chapter.

The neural cell adhesion molecule antibody blocks cold water swim stress-induced analgesia and cell adhesion between lymphocytes and cultured dorsal root ganglion neurons

BACKGROUND

Opioid-containing immune cells migrate in a site-directed manner into inflamed tissue and adhere to sensory nerve fibers. These cells release opioid peptides in close proximity to these fibers, thereby avoiding localized degradation by peptidases, and delivering opioid peptides proximal to opioid receptors to provide antinociception.

METHODS

The effects of the anti-neural-cell-adhesion molecule (anti-NCAM) were assessed on cold water swim stress-induced antinociception in Wistar rats with Freund's adjuvant-induced inflammation of one hindpaw. Algesiometry was assessed for both thermal and mechanical stimuli. Cell adhesion experiments examining the effects of beta-endorphin and antibodies to NCAM and intercellular cell adhesion molecule-1 and were performed on cultured dorsal root ganglion neurons and isolated lymphocytes. Lymphocyte binding was determined by fluorescence using calcein AM loaded into freshly isolated lymphocytes.

RESULTS

The direct adhesion between lymphocytes and cultured sensory neurons was inhibited by anti-NCAM. This adhesion was also demonstrated to be opioid dependent, with lymphocyte adhesion to cultured sensory neurons reduced in the presence of 1 microM beta-endorphin, which was reversed by 100 microM naloxone. Moreover, anti-NCAM blocked cold-water-swim-induced analgesia in inflamed paws both to thermal and mechanical stimuli. However, anti-NCAM did not affect fentanyl-induced antinociception.

CONCLUSIONS

This study provides insight into the role of cell adhesion molecules in lymphocyte adhesion to sensory neurons and a link to immune-derived antinociception.

Pain control by CXCR2 ligands through Ca2+-regulated release of opioid peptides from polymorphonuclear cells

Leukocytes counteract inflammatory pain by releasing opioid peptides, which bind to opioid receptors on peripheral sensory neurons. In the early phase of inflammation, polymorphonuclear cells (PMN) are the major source of opioids. Their recruitment is governed by ligands at the chemokine receptor CXCR2. Here, we examined whether chemokines can also induce opioid peptide secretion from PMN and thus inhibit inflammatory pain. In rats with hindpaw inflammation, intraplantar injection of CXCL2/3, but not of the CXCR4 ligand CXCL12, elicited naloxone-reversible (i.e., opioid receptor mediated) mechanical and thermal analgesia, which was abolished by systemic PMN depletion. Both CXCR1/2- and CXCR4-ligands induced PMN chemotaxis, but only CXCR1/2 ligands triggered opioid release from human and rat PMN in vitro. This release was unaltered by extracellular Ca2+ chelation, was mimicked by thapsigargin and was blocked by inhibitors of the inositol 1,4,5-triphosphate receptor (IP3) and by intracellular Ca2+ chelation, indicating that it required Ca2+ from intracellular but not extracellular sources. Furthermore, release was partially reduced by phosphoinositol-3-kinase (PI3K) inhibitors. Adoptive transfer of allogenic PMN into PMN-depleted rats reconstituted CXCL2/3-induced analgesia, which was inhibited by prior ex vivo chelation of intracellular Ca2+. These findings demonstrate that, beyond cell recruitment, CXCR2 ligands induce Ca2+-regulated opioid release from PMN and thereby inhibit inflammatory pain in vivo.

Beta-endorphin differentially affects inflammation in two inbred rat strains

It has been shown that inflammation of rat paws elicits accumulation of opioid peptide beta-endorphin-containing immune cells in the inflamed subcutaneous tissue, contributing to immunocyte-produced pain suppression. However, the possible mechanisms involved in the pharmacological application of beta-endorphin in rat paw inflammation have not been investigated. The present study was set up to explore the effects of intraplantar injection of beta-endorphin on Concanavalin A-induced paw edema in two inbred rat strains, Albino Oxford (AO) and Dark Agouti (DA). Both high dose-induced suppression and low dose-induced potentiation of edema development in AO and DA rats, respectively, were blocked with antagonists specific for delta (naltrindole) and kappa (nor-binaltorphimine) opioid receptors. beta-endorphin in vitro decreased phagocytosis and increased nitric oxide (NO) production in air pouch granulocytes obtained from AO rats. However, in cells from DA rat strain beta-endorphin modulated both phagocytosis and NO production in a concentration-dependent manner. It could be concluded that the strain-dependent opposing effects of beta-endorphin on paw inflammation are mediated through delta and kappa opioid receptors and probably involve changes in the production of reactive oxygen species by inflammatory cells. Our results point to the importance of genotype for pharmacological manipulations and the development of inflammation.

Peripheral antinociceptive effects of exogenous and immune cell-derived endomorphins in prolonged inflammatory pain

Endomorphins (EMs) are endogenous selective mu-opioid receptor agonists. Their role in inflammatory pain has not been fully elucidated. Here we examine peripheral antinociception elicited by exogenously applied EM-1 and EM-2 and the contribution of EM-containing leukocytes to stress- and corticotropin-releasing factor (CRF)-induced antinociception. To this end, we applied behavioral (paw pressure) testing, radioligand binding, immunohistochemistry, and flow cytometry in rats with unilateral hindpaw inflammation induced with Freund's adjuvant. EMs injected directly into both hindpaws produced antinociception exclusively in inflamed paws. This was blocked by locally applied mu-receptor-selective (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) but not kappa-receptor-selective (nor-binaltorphimine) antagonists. Delta-receptor antagonists (naltrindole and N,N-diallyl-Tyr-Aib-Aib-Phe-Leu) did not influence EM-1-induced but dose-dependently decreased EM-2-induced antinociception. Antibodies against beta-endorphin, methionine-enkephalin, or leucine-enkephalin did not significantly change EM-2-induced antinociception. Both EMs displaced binding of [3H]-[D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin to mu-receptors in dorsal root ganglia (DRG). Using [3H]-naltrindole or [(125)I]-[D-Pen2,5]-enkephalin, no detectable delta-binding was found in DRG of inflamed hindlimbs. Numerous beta-endorphin-containing and fewer EM-1- and EM-2-containing leukocytes were detected in subcutaneous tissue of inflamed paws. Leukocyte-depleting serum decreased the number of immigrating opioid-containing immune cells and attenuated swim stress- and CRF-induced antinociception in inflamed paws. Both forms of antinociception were strongly attenuated by anti-beta-endorphin and to a lesser degree by anti-EM-1 and anti-EM-2 antibodies injected into inflamed paws. Together, exogenously applied and immune cell-derived EMs alleviate prolonged inflammatory pain through selective activation of peripheral opioid receptors. Exogenous EM-2 in addition to mu-receptors also activates peripheral delta-receptors, which does not involve actions via other opioid peptides.

Opioids trigger alpha 5 beta 1 integrin-mediated monocyte adhesion

Inflammatory reactions involve a network of chemical and molecular signals that initiate and maintain host response. In inflamed tissue, immune system cells generate opioid peptides that contribute to potent analgesia by acting on specific peripheral sensory neurons. In this study, we show that opioids also modulate immune cell function in vitro and in vivo. By binding to its specific receptor, the opioid receptor-specific ligand DPDPE triggers monocyte adhesion. Integrins have a key role in this process, as adhesion is abrogated in cells treated with specific neutralizing anti-alpha5beta1 integrin mAb. We found that DPDPE-triggered monocyte adhesion requires PI3Kgamma activation and involves Src kinases, the guanine nucleotide exchange factor Vav-1, and the small GTPase Rac1. DPDPE also induces adhesion of pertussis toxin-treated cells, indicating involvement of G proteins other than Gi. These data show that opioids have important implications in regulating leukocyte trafficking, adding a new function to their known effects as immune response modulators.

[Potential applications and significance of peripheral opioid analgesia]

Recently, the local application of analgesic substances has attracted much interest and has become increasingly relevant in daily clinical practice. The lack of the well-known therapy-limiting central side effects of systemically administered opioids is a clear advantage of "peripheral opioid analgesia". Important prerequisites for the occurrence of peripheral analgesic effects of opioids are good accessibility of the painful site, presence of clinically relevant pain, signs of a local inflammation, exclusion of a fast systemic absorption, and adequately potent analgesic substances. This review gives an outline of recent experience with various clinical applications of "peripheral opioid analgesia".

Endomorphins as Agents for the Treatment of Chronic Inflammatory Disease

Endomorphin (EM)-1 and EM-2 are tetrapeptides located within the mammalian central nervous system and immune tissues, with high affinity and specificity for micro-opioid receptors. Most of the literature has focused on the analgesic properties of EM-1 and EM-2 in animal models of neuropathic or neurogenic pain, but there is persuasive evidence emerging that EMs can also exert potent anti-inflammatory effects in both acute and chronic peripheral inflammation. The purpose of this review is to present and evaluate the evidence for anti-inflammatory properties of EM-1 and EM-2 with a view to their potential for use in chronic human inflammatory disease. Distribution of EMs within the immune system and functional roles as immunomodulatory agents are summarized and discussed. Possible milestones to be met revolve around issues of peptide stability, biodegradability problems and optimal route and method of delivery. The potential for delivery of a low-cost drug with both peripheral anti-inflammatory and analgesic properties, effective in low doses, and targeted to the site of inflammation, should focus our attention on further development of EMs as potent therapeutic agents in chronic inflammation.

Involvement of neuropeptide Y and Y1 receptor in antinociception in the arcuate nucleus of hypothalamus, an immunohistochemical and pharmacological study in intact rats and rats with inflammation

Neuropeptide Y (NPY) plays an important role in pain modulation at different levels in the central nervous system. In the brain, NPY and NPY receptors distribute abundantly in the arcuate nucleus of hypothalamus (ARC), a structure involved in pain processing. The present study was undertaken to investigate the role of NPY in nociceptive modulation in the ARC of intact rats and rats with carrageenan-induced inflammation. Intra-ARC administration of NPY induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulation in intact rats, which was attenuated by the Y1 receptor antagonist NPY28-36. Intra-ARC administration of NPY also induced dose-dependent increases in HWLs to noxious stimulation in rats with inflammation. Furthermore, intra-ARC injection of either the antiserum against NPY or NPY28-36 induced decreases in HWLs in rats with inflammation, while both of them produced no effects in intact ones. Additionally, there were marked increases of Y1 receptor in the bilateral ARC of rats with inflammation tested by immunohistochemistry, while no significant changes of NPY were observed, implicating that the increased Y1 receptor has an important effect in the NPY-induced antinociception. We also found that intra-ARC injection of Y2 receptor agonist NPY3-36 produced no significant antinociception in either intact rats or rats with inflammation. Together, we demonstrate that NPY exerts an antinociceptive effect in the ARC of intact rats and rats with inflammation. Both Y1 receptor and endogenous released NPY in the ARC are involved in the nociceptive modulation during inflammation.

Nociceptive scores and endorphin-containing cells reduced by low-level laser therapy (LLLT) in inflamed paws of Wistar rat

OBJECTIVE

This study aimed to investigate how local pain relief is mediated by laser therapy and how dose affects the relationship.

METHODS

Inflammation was induced in the hind-paws of Wistar rats. Two groups of rats received 780-nm laser therapy (Spectra-Medics Pty Ltd.) at one of two doses (2.5 and 1 J/cm(2)). One group acted as a control. Scores of nociceptive threshold were recorded using paw pressure and paw thermal threshold measures.

RESULTS

A dose of 1 J/cm(2) had no statistically significant effect on antinociceptive responses. A dose of 2.5 J/cm(2) demonstrated a statistically significant effect on paw pressure threshold ( p < 0.029) compared to controls. There was no difference in paw thermal threshold responses and paw volumes at either dose. Immunohistochemistry in control animals demonstrated normal beta-endorphin containing lymphocytes in control inflamed paws but no beta-endorphin containing lymphocytes in rats that received laser at 2.5 J/cm(2).

CONCLUSION

The results confirm previous findings that the effect of laser therapy is dose-related. The mechanism of effect may occur via a differentiated pressure-sensitive neural pathway rather than a thermal-sensitive neural pathway. The significance of the immunohistochemistry findings remains unknown.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Peripheral opioid analgesia: Clinical applications

Peripheral opioid analgesia is undoubtedly of clinical relevance, especially considering that systemic opioid therapy often is hampered by central side effects. Despite some clinical studies that do not show peripheral opioid-mediated pain control, mostly because of methodologic shortcomings, studies evaluating inflammatory pain conditions show clear evidence and the number and the sites of applications are increasing. The intention of this paper is to give insight into the recent experience with the clinical applications of peripheral opioid analgesia.

Controlling Pain by Influencing Neurogenic Pathways

Neurogenic inflammation and ensuing pain can be modulated by inhibiting the function of primary afferent neurons. The best studied mechanism to accomplish such inhibition is the opioid system. Under inflammatory conditions, the anterograde axonal transport of opioid receptors from dorsal root ganglia toward the peripheral sensory nerve endings is augmented. The increased number of opioid receptors (among other mechanisms) leads to improved analgesic effects of exogenously administered ligands (eg, morphine) and of endogenous leukocyte-derived opioid peptides (eg, beta-endorphin). A current concept proposes that during inflammatory processes endogenous opioid peptides can be secreted from immunocytes, occupy peripheral opioid receptors on sensory nerve endings, and produce analgesia by inhibiting the excitability of these nerves or the release of proinflammatory neuropeptides. This article focuses on the role of peripheral opioid receptors in pain control and on novel pharmaceutical concepts for the treatment of patients who suffer from rheumatoid arthritis and other inflammatory pain.

Control of inflammatory pain by chemokine-mediated recruitment of opioid-containing polymorphonuclear cells

Opioid-containing leukocytes can counteract inflammatory hyperalgesia. Under stress or after local injection of corticotropin releasing factor (CRF), opioid peptides are released from leukocytes, bind to opioid receptors on peripheral sensory neurons and mediate antinociception. Since polymorphonuclear cells (PMN) are the predominant opioid-containing leukocyte subpopulation in early inflammation, we hypothesized that PMN and their recruitment by chemokines are important for peripheral opioid-mediated antinociception at this stage. Rats were intraplantarly injected with complete Freund's adjuvant (CFA). Using flow cytometry, immunohistochemistry, and ELISA, leukocyte subpopulations, chemokine receptor (CXCR2) expression on opioid-containing leukocytes and the CXCR2 ligands keratinocyte-derived chemokine (KC), macrophage inflammatory protein-2 (MIP-2) and cytokine-induced neutrophil chemoattractant-2 (CINC-2) were quantified. Paw pressure threshold (PPT) was determined before and after intraplantar and subcutaneous injection of CRF with or without naloxone. PMN depletion was achieved by intravenous injection of an antiserum. Chemokines were blocked by intraplantar injection of anti-MIP-2 and/or anti-KC antiserum. We found that at 2 h post CFA (i) intraplantar but not subcutaneous injection of CRF produced dose-dependent and naloxone-reversible antinociception (P<0.05, ANOVA). (ii) Opioid-containing leukocytes in the paw and CRF-induced antinociception were reduced after PMN depletion (P<0.05, t-test). (iii) Opioid-containing leukocytes mostly expressed CXCR2. MIP-2 and KC, but not CINC-2 were detectable in inflamed but not in noninflamed tissue (P<0.05, ANOVA). (iv) Combined but not single blockade of MIP-2 and KC reduced the number of opioid-containing leukocytes and peripheral opioid-mediated antinociception (P<0.05, t-test; P>0.05, ANOVA). In summary, in early inflammation peripheral opioid-mediated antinociception is critically dependent on PMN and their recruitment by CXCR2 chemokines.

Sympathetic activation triggers endogenous opioid release and analgesia within peripheral inflamed tissue

Stress induces analgesia by mechanisms within and outside the brain. Here we show that the sympathetic nervous system is an essential trigger of intrinsic opioid analgesia within peripheral injured tissue. Noradrenaline, injected directly into inflamed hind paws of male Wistar rats, produced dose-dependent antinociception, reversible by alpha(1)-, alpha(2)- and beta(2)-antagonists. alpha(1)-, alpha(2)- and beta(2)-adrenergic receptors were demonstrated on beta-endorphin-containing immune cells and noradrenaline induced adrenergic receptor-specific release of beta-endorphin from immune cell suspensions. This antinociceptive effect of noradrenaline was reversed by micro - and delta-opioid antagonists as well as by anti-beta-endorphin. Stress-induced peripheral analgesia was abolished by chemical sympathectomy and by adrenergic antagonists. These findings indicate that sympathetic neuron-derived noradrenaline stimulates adrenergic receptors on inflammatory cells to release beta-endorphin, which induces analgesia via activation of peripheral opioid receptors.

Endogenous peripheral antinociception in early inflammation is not limited by the number of opioid-containing leukocytes but by opioid receptor expression

Endogenous inhibition of inflammatory pain is mediated by leukocytes that secrete opioid peptides upon exposure to stress (cold water swim stress, CWS) or after local injection of corticotropin releasing factor (CRF). Since in early inflammation few opioid-containing leukocytes are detected and since peripheral opioid-mediated antinociception is low we examined whether antinociception could be augmented by increased recruitment of opioid-containing polymorphonuclear cells (PMN). Rats were intraplantarly (i.pl.) injected with Freund's complete adjuvant (FCA) and with the PMN-recruiting chemokine macrophage inflammatory protein-2 (MIP-2, 1-10 microg; control: saline) for 2 h. Intraplantar leukocytes were quantified by flow cytometry. Paw pressure threshold (PPT) was determined before and after exposure to CWS, i.pl. injection of CRF and opioid peptides. Opioid receptors (OR) were measured by binding studies in dorsal root ganglia (DRG) and by immunohistochemistry in the paw. Our studies showed that (i) MIP-2 injection dose-dependently augmented recruitment of PMN and opioid-containing leukocytes (5-fold increase in cells/paw, P < 0.05), (ii) PPT was not different between groups at baseline and after CWS or CRF (maximum MPE: 20+/-2.3-29+/-7.2%, P < 0.05), (iii) injection of opioid peptides dose-dependently increased the PPT (P < 0.05, maximum MPE: and 18+/-2.6-21+/-3.6%), (iv) MOR (micro OR, MOP) binding sites in the ipsilateral DRG were unchanged (24+/-2-22+/-1.2 fmol/mg protein, P < 0.05, ANOVA) and (v) the number of MOR and DOR (delta OR, DOP) stained nerve fibers in peripheral tissue were unaltered (both P > 0.05, t-test). In summary, antinociception during early inflammation is apparently not limited by the number of opioid-containing leukocytes but by OR availability.

Selectins and integrins but not platelet-endothelial cell adhesion molecule-1 regulate opioid inhibition of inflammatory pain

1. Control of inflammatory pain can result from activation of opioid receptors on peripheral sensory nerves by opioid peptides secreted from leukocytes in response to stress (e.g. experimental swim stress or surgery). The extravasation of immunocytes to injured tissues involves rolling, adhesion and transmigration through the vessel wall, orchestrated by various adhesion molecules. 2. Here we evaluate the relative contribution of selectins, integrins alpha(4) and beta(2), and platelet-endothelial cell adhesion molecule-1 (PECAM-1) to the opioid-mediated inhibition of inflammatory pain. 3. We use flow cytometry, double immunofluorescence and nociceptive (paw pressure) testing in rats with unilateral hind paw inflammation induced by complete Freund's adjuvant. 4. In inflamed tissue, 43-58% of hematopoietic cells (CD45(+)) expressed opioid peptides. L-selectin and beta(2) were coexpressed by 7 and 98% of opioid-containing leukocytes, respectively. Alpha(4) integrin was expressed in low levels by the majority of leukocytes. Opioid-containing cells, vascular P- and E-selectin and PECAM-1 were simultaneously upregulated. 5. Swim stress produced potent opioid-mediated antinociception in inflamed tissue, unaffected by blockade of PECAM-1. However, blockade of L- and P-selectins by fucoidin, or of alpha(4) and beta(2) by monoclonal antibodies completely abolished peripheral stress-induced antinociception. This coincided with a 40% decrease in the migration of opioid-containing leukocytes to inflamed tissue. 6. These findings establish selectins and integrins alpha(4) and beta(2), but not PECAM-1, as important molecules involved in stress-induced opioid-mediated antinociception in inflammation. They point to a cautious use of anti-inflammatory treatments applying anti-selectin, anti-alpha(4) and anti-beta(2) strategies because they may impair intrinsic pain inhibition.

Expression of adhesion molecules during cadmium hepatotoxicity

Inflammatory processes play a major role in the secondary injury of the liver produced by cadmium (Cd), and infiltration of neutrophils at the site of necrosis is a common observation. Although the infiltration of leukocytes (mainly neutrophils) into sites of injuried tissue within liver during Cd toxicity is mediated by adhesion molecules, little is known about expression of these adhesion molecules during Cd hepatotoxicity. In the present study, the expression of E-, P-selectin, intracellular adhesion molecule-1 (ICAM-1) and platelet-endothelial adhesion molecule-1 (PECAM-1) was analyzed by immunohistochemistry and immunofluoresence during Cd-induced hepatotoxicity in male rats. In contrast to E-, and P-selectin, ICAM-1 and PECAM-1 were constitutively expressed on sinusoidal endothelial cells of control liver. However, P-selectin was not induced within the liver by Cd administration, whereas E-selectin expression was induced in the liver with a marked increase in immunostaining on sinusoidal endothelial cells from 12 h to 7 days. Also, there was an upregulation in ICAM-1 immunostaining on sinusoidal endothelial cells from 12 h to 7 days after Cd administration, whereas there was no obvious change of PECAM-1 immunostaining on sinusoidal endothelial cells until 24 h. However, PECAM-1 expression was markedly decreased at 48 h but significantly increased at 7 days after Cd administration compared to control liver. Taken together, upregulation of E-selectin and ICAM-1 with biphasic changes in PECAM-1 expression within liver after Cd administration suggests an important role for these adhesion molecules during Cd hepatoxicity.

Subcellular pathways of beta-endorphin synthesis, processing, and release from immunocytes in inflammatory pain

The opioid peptide beta-endorphin (END) as well as mRNA for its precursor proopiomelanocortin (POMC) are found not only in the pituitary gland, but also within various types of immune cells infiltrating inflamed sc tissue. During stressful stimuli END is released and interacts with peripheral opioid receptors to inhibit pain. However, the subcellular pathways of POMC processing and END release have not yet been delineated in inflammatory cells. The aim of the present study was to examine the presence of POMC, carboxypeptidase E, the prohormone convertases 1 (PC1), and 2 (PC2), PC2-binding protein 7B2, and the release of END from inflammatory cells in rats. Using immunohistochemistry we detected END and POMC alone or colocalized with PC1, PC2, carboxypeptidase E, and 7B2 in macrophages/monocytes, granulocytes, and lymphocytes of the blood and within inflamed sc paw tissue. Immunoelectron microscopy revealed that END is localized within secretory granules packed in membranous structures in macrophages, monocytes, granulocytes, and lymphocytes. Finally, END is released by noradrenaline from immune cells in vitro. Taken together, our results indicate that immune cells express the entire machinery required for POMC processing into functionally active peptides such as END and are able to release these peptides from secretory granules.

Reduction of ??-Endorphin-Containing Immune Cells in Inflamed Paw Tissue Corresponds with a Reduction in Immune-Derived Antinociception: Reversible by Donor Activated Lymphocytes

The functional integrity of the immune system is essential for peripheral antinociception. Previous studies have demonstrated that immune cells elicit potent antinociception in inflamed tissues and that corticotropin-releasing factor-induced antinociception is significantly inhibited in animals that have undergone cyclosporin A (CsA)-induced immunosuppression. In this study, we examined the effect of a single bolus of CsA on inflammatory nociception. CsA-treated rats had substantially increased nociception compared with nonimmunosuppressed rats, consistent with a reduction in circulating and infiltrating lymphocytes. Furthermore, CsA-treated rats had inhibition of corticotropin-releasing factor-induced immune-derived antinociception, which was dose-dependently reversed by IV injection of concanavalin A-activated donor lymphocytes (1.0-7.0 x 10(6) cells/0.1 mL). In conclusion, our findings provided further evidence that opioid-containing immune cells are essential for peripheral analgesia. It is evident from these findings that control of inflammatory pain relies heavily on a functioning immune system.

IMPLICATIONS

The immune system not only contributes to inflammation, but also provides localized analgesia. A depleted immune system results in a reduction of immune-derived analgesia and a potentiation of inflammatory pain. Donor activated lymphocytes reverse these effects, highlighting the importance of a functional immune system in inflammatory pain.

T lymphocytes play a role in neuropathic pain following peripheral nerve injury in rats

A catastrophic consequence of peripheral nerve injury is the development of abnormal, chronic neuropathic pain. The inflammatory response at the injury site is believed to contribute to the generation and maintenance of such persistent pain. However, the physiological significance and potential contribution of T cells to neuropathic pain remains unclear. Here we show that T cells infiltrate injured sciatic nerves following chronic constriction injury (CCI), but not uninjured nerves. Congenitally athymic nude rats, which lack mature T cells, developed a significantly reduced mechanical allodynia and thermal hyperalgesia following CCI, compared with their heterozygous littermates. To understand further the role played by different T-cell subsets, we generated polarized populations of type 1 and type 2 T cells, with different cytokine secretion profiles, from spleens of sciatic nerve-injured heterozygous rats. Passive transfer of type 1 T cells, which produce proinflammatory cytokines, into nude rats enhanced the recipients' pain hypersensitivity to a level similar to that of heterozygous donor rats. In contrast, passive transfer of polarized type 2 T cells, which produce anti-inflammatory cytokines, into heterozygous rats modestly though significantly attenuated their pain hypersensitivity. Thus, injection of type 1 and type 2 T-cell subsets produces opposing effects on neuropathic pain. These findings suggest the modulation of the T-cell immune response as a potential target for the treatment of neuropathic pain.

Involvement of corticotropin-releasing hormone receptor subtypes 1 and 2 in peripheral opioid-mediated inhibition of inflammatory pain

In painful inflammation, exogenous as well as endogenous corticotropin-releasing hormone (CRH) can release opioid peptides (mainly beta-endorphin) from various types of immune cells and produce antinociception by activating opioid receptors on peripheral sensory nerve endings. CRH mediates its central effects through two high-affinity membrane receptors, the CRH receptor subtypes 1 and 2. It is unclear at present whether the peripheral antinociceptive effects of CRH are mediated through CRH receptor 1 (CRH R1) or CRH receptor 2 (CRH R2). Employing a double-immunocytochemical technique, this study investigated in Wistar rats with Freund's complete adjuvant-induced hind paw inflammation whether immune cells within blood and inflamed subcutaneous tissue express CRH R1 and/or CRH R2 together with the opioid peptide beta-endorphin (END). Additionally, we examined using selective CRH R1 and CRH R2 antagonists whether peripheral CRH-induced antinociception is mediated by the respective CRH receptor subtypes. We found a high degree of co-expression of END together with both CRH R1 and CRH R2 in macrophage/monocytes, granulocytes and lymphocytes within blood and inflamed subcutaneous tissue. Also we observed a high degree of co-localization of CRH R1 and CRH R2 receptors on circulating and resident immune cells. Both the selective CRH R1 antagonist CP-154,526 and the selective CRH R2 antagonist astressin 2B significantly attenuated peripheral antinociceptive effects of CRH indicating the involvement of both CRH receptor subtypes. Taken together, these findings suggest that in inflammatory pain CRH-induced peripheral antinociception is mediated via both CRH R1 and CRH R2 located on END containing immune cells within inflamed sites.

New treatments for diabetic neuropathy: pathogenetically oriented treatment

Although there is clear evidence from experimental diabetic neuropathy (DN) models that the multiple pathways involved in neuronal degeneration cause overproduction of reactive oxygen species, oxidative stress, and cellular dysfunction, therapeutic approaches addressing these mechanisms have not yet provided a basis for a successful treatment of patients with DN. This review discusses the current knowledge on the pathomechanisms of unchecked reactive oxygen species accumulation, implications for specific treatment, and the need for carefully designed experimental studies and clinical trials closing the gap between promising results in experimental DN and its implementation into a pathogenetically oriented treatment.

Different mechanisms of intrinsic pain inhibition in early and late inflammation

Neuroimmune interactions control pain through activation of opioid receptors on sensory nerves by immune-derived opioid peptides. Here we evaluate mechanisms of intrinsic pain inhibition at different stages of Freund's adjuvant-induced inflammation of the rat paw. We use immunohistochemistry and paw pressure testing. Our data show that in early (6 h) inflammation leukocyte-derived beta-endorphin, met-enkephalin and dynorphin A activate peripheral mu-, delta- and kappa-receptors to inhibit nociception. In addition, central opioid mechanisms seem to contribute significantly to this effect. At later stages (4 days), antinociception is exclusively produced by leukocyte-derived beta-endorphin acting at peripheral mu and delta receptors. Corticotropin-releasing hormone (CRH) is an endogenous trigger of these effects at both stages. These findings indicate that peripheral opioid mechanisms of pain inhibition gain functional relevance with the chronicity of inflammation.

Attacking pain at its source: new perspectives on opioids

The treatment of severe pain with opioids has thus far been limited by their unwanted central side effects. Recent research promises new approaches, including opioid analgesics acting outside the central nervous system, targeting of opioid peptide-containing immune cells to peripheral damaged tissue, and gene transfer to enhance opioid production at sites of injury.

Endothelin-B receptor activation triggers an endogenous analgesic cascade at sites of peripheral injury

Endothelin-1 (ET-1) is a newly described pain mediator that is involved in the pathogenesis of pain states ranging from trauma to cancer. ET-1 is synthesized by keratinocytes in normal skin and is locally released after cutaneous injury. While it is able to trigger pain through its actions on endothelin-A (ET(A)) receptors of local nociceptors, it can coincidentally produce analgesia through endothelin-B (ET(B)) receptors. Here we map a new endogenous analgesic circuit, in which ET(B) receptor activation induces the release of beta-endorphin from keratinocytes and the activation of G-protein-coupled inwardly rectifying potassium channels (GIRKs, also named Kir-3) linked to opioid receptors on nociceptors. These results indicate the existence of an intrinsic feedback mechanism to control peripheral pain in skin, and establish keratinocytes as an ET(B) receptor-operated opioid pool.

Peripheral opioid analgesia in experimental human pain models

This placebo-controlled, double-blind crossover study assessed whether exclusive activation of peripheral opioid receptors results in significant pain reduction. To achieve opioid activity restricted to the periphery, we used a short-term (2 h) low dose infusion of morphine-6-beta-glucuronide (M6G) because M6G does not pass the blood-brain barrier during this time in amounts sufficient to induce CNS effects. The lack of central opioid effects of M6G was confirmed by a lack of change of the pupil size and absence of other opioid-related CNS effects. As a positive control, morphine was infused at a dosage that definitely produced CNS effects. This was evident by a rapid decrease of the pupil size and by other typical opioid-related side effects including nausea, vomiting, itchiness, hiccup and sedation. Three different pain models were employed to evaluate the analgesic effects: (i) cutaneous inflammatory hyperalgesia induced by briefly freezing a small skin area to -30 degrees C ('freeze lesion'); (ii) muscle hyperalgesia induced by a series of concentric and eccentric muscle contractions (DOMS model; delayed onset of muscle soreness); and (iii) pain induced by electrical current (5 Hz sinus stimuli of 0-10 mA). M6G significantly reduced cutaneous hyperalgesia in the 'freeze lesion' model as assessed with von Frey hairs. It also reduced muscle hyperalgesia in the DOMS model. Electrical pain, however, was not affected by M6G. Morphine was significantly more active in the 'freeze lesion' and DOMS model, and also significantly increased the electrical pain threshold and tolerance. Subcutaneous tissue concentrations of M6G and morphine as assessed with microdialysis were about half those of the respective plasma concentrations. The results of the study indicate that M6G has antihyperalgesic effects in inflammatory pain through activation of peripheral opioid receptors. Since this occurs at concentrations that do not cause central opioid effects, M6G might be useful as a peripheral opioid analgesic.

Enhanced humoral response in kappa-opioid receptor knockout mice

Opiates are major analgesics and addictive drugs described also as immunomodulators. Here, we investigated the contribution of kappa-opioid receptor (KOR) activity in immunity in vivo by studying immune responses in KOR knockout mice. These animals displayed a modest reduction in thymus cellularity and CD4(+) cell ratio, parallel to a slight increase in immature CD4(+)CD8(+) lymphocytes. In spleen, KOR null animals showed augmented cell number with no change in cell distribution. T and B lymphocyte proliferative capabilities in vitro, Natural Killer activity and steady-state Ig levels were unchanged in KOR-/- mice. We immunized the mice with the antigen keyhole limpet hemocyanin (KLH). Compared to wild-type (WT) mice, KOR-/- animals produced significant higher levels of antigen-specific total Ig, IgM, IgG1 and IgG2a antibodies. This enhancement of humoral activity was not observed in mu-opioid receptor and delta-opioid receptor knockout animals. These results show that endogenous activation of kappa-opioid receptors may exert a tonic inhibition of antibody (Ab) response.

Relief of inflammatory pain in rats by local use of the selective P2X7 ATP receptor inhibitor, oxidized ATP

OBJECTIVE

Oxidized ATP (oATP) is a selective inhibitor of the P2Z/P2X7 ATP receptor for extracellular ATP, which contributes to the antinociceptive effect. This study sought to determine the mechanism by which local administration of oATP is able to relieve inflammatory pain in arthritic rat paws.

METHODS

Arthritis was induced in Wistar rats by injections of Freund's complete adjuvant into one hind paw. Nociceptive thresholds were measured before and after local injection of oATP into the inflamed paws. The influence on pain transmission due to the presence of recruited inflammatory cells at the site of inflammation was determined by inhibiting the initial phase of their migration (by intravenous treatment with fucoidin, which blocks the adhesion molecules of the selectin family). ATP intraplantar content was determined in the different experimental conditions. Histologic features of the hind paws were evaluated by using the anti-P2X7 receptor polyclonal antibody.

RESULTS

Intraplantar administration of oATP into inflamed paws significantly relieved inflammatory pain. The antinociceptive effect of oATP was independent of the immune-cell recruitment. ATP levels in inflamed tissues were significantly reduced by oATP treatment. A variable presence of P2X7 receptors on cutaneous sensory nerves with respect to the different treatments was observed. Following oATP treatment, there was a reduction in P2X7 expression in the endings of peripheral nerves, as well as in endothelial cells.

CONCLUSION

Oxidized ATP inhibits inflammatory pain in arthritic rats by inhibition of the P2X7 receptor for ATP, which is localized on nerve terminals.

Opioid control of inflammatory pain regulated by intercellular adhesion molecule-1

Pain can be effectively controlled by endogenous mechanisms based on neuroimmune interactions. In inflamed tissue immune cell-derived opioid peptides activate opioid receptors on peripheral sensory nerves leading to potent analgesia. This is brought about by a release of opioids from inflammatory cells after stimulation by stress or corticotropin-releasing hormone (CRH). Immunocytes migrate from the circulation to inflamed tissue in multiple steps, including their rolling, adhesion, and transmigration through the vessel wall. This is orchestrated by adhesion molecules on leukocytes and vascular endothelium. Intercellular adhesion molecule-1 [ICAM-1 (or CD54)] is expressed by endothelium and mediates adhesion and extravasation of leukocytes. The goal of this study was to show that ICAM-1 regulates the homing of opioid-producing cells and the subsequent generation of analgesia within sites of painful inflammation. This was accomplished using immunofluorescence, flow cytometry, and behavioral (paw pressure) testing. We found that ICAM-1 is upregulated on the vascular endothelium, simultaneously with an enhanced immigration of opioid-containing immune cells into inflamed paw tissue. The intravenous administration of a monoclonal antibody against ICAM-1 markedly decreased the migration of opioid-containing leukocytes and of granulocytes, monocytes-macrophages, and T cells to the inflamed tissue. At the same time, circulating immunocytes increased in numbers, and macroscopic inflammation (hyperalgesia, paw volume, and paw temperature) remained primarily unchanged. Most importantly, peripheral opioid analgesia elicited either by cold water swim stress or by intraplantar administration of CRH was dramatically reduced. Together, these findings indicate that ICAM-1 expressed on vascular endothelium recruits immunocytes containing opioids to promote the local control of inflammatory pain.

Antinociceptive effect of a new P(2Z)/P2X7 antagonist, oxidized ATP, in arthritic rats

The neurotransmitter adenosine triphosphate (ATP) is released from sensory nerve endings during inflammation and acts at the level of P2X receptors. We used the irreversible inhibitor of P2z/P2X7 receptor, designated oxidized ATP (oATP), to test its possible antinociceptive activity in arthritic rats. We induced unilateral inflammation of the rat hind paw by local injection of Freund's complete adjuvant. Administration of the adjuvant resulted in a significant reduction of paw pressure threshold (PPT). Injection of oATP into inflamed paws significantly increased, in a dose-dependent manner, PPT values to levels comparable with or higher than those evaluated in control uninflamed paws. The data indicate that the P2z/P2X7 receptor system exerts a role in nociception and that oATP, by inhibiting such a receptor, reduces the nociceptive signal in the course of peripheral inflammation.