Hyperalgesic properties of 15-lipoxygenase products of arachidonic acid

12/15-Lipoxygenases mediate neuropathic-like pain hypersensitivity in female mice

It is estimated that chronic neuropathic pain conditions exhibit up to 10% prevalence in the general population, with increased incidence in females. However, nonsteroidal inflammatory drugs (NSAIDs) are ineffective, and currently indicated prescription treatments such as opioids, anticonvulsants, and antidepressants provide only limited therapeutic benefit. In the current work, we extended previous studies in male rats utilizing a paradigm of central Toll-like receptor 4 (TLR4)-dependent, NSAID-unresponsive neuropathic-like pain hypersensitivity to male and female C57BL/6N mice, uncovering an unexpected hyperalgesic phenotype in female mice following intrathecal (IT) LPS. In contrast to previous reports in female C57BL/6J mice, female C57BL/6N mice displayed tactile and cold allodynia, grip force deficits, and locomotor hyperactivity in response to IT LPS. Congruent with our previous observations in male rats, systemic inhibition of 12/15-Lipoxygenases (12/15-LOX) in female B6N mice with selective inhibitors - ML355 (targeting 12-LOX-p) and ML351 (targeting 15-LOX-1) - completely reversed allodynia and grip force deficits. We demonstrate here that 12/15-LOX enzymes also are expressed in mouse spinal cord and that 12/15-LOX metabolites produce tactile allodynia when administered spinally (IT) or peripherally (intraplantar in the paw, IPLT) in a hyperalgesic priming model, similar to others observations with the cyclooxygenase (COX) metabolite Prostaglandin E 2 (PGE 2 ). Surprisingly, we did not detect hyperalgesic priming following IT administration of LPS, indicating that this phenomenon likely requires peripheral activation of nociceptors. Collectively, these data suggest that 12/15-LOX enzymes contribute to neuropathic-like pain hypersensitivity in rodents, with potential translatability as druggable targets across sexes and species using multiple reflexive and non-reflexive outcome measures.

Associations between Plasma Lipid Mediators and Chronic Daily Headache Outcomes in Patients Randomized to a Low Linoleic Acid Diet with or without Added Omega-3 Fatty Acids

A previous report showed that 12-week lowering of dietary omega-6 linoleic acid (LA) coupled with increased omega-3 polyunsaturated fatty acid (PUFA) intake (H3-L6 diet) reduced headache frequency and improved quality of life in patients with chronic daily headaches (CDHs) compared to dietary LA reduction alone (L6 diet). The trial also showed that targeted dietary manipulation alters PUFA-derived lipid mediators and endocannabinoids. However, several additional classes of lipid mediators associated with pain in preclinical models were not measured. The current secondary analysis investigated whether the clinical benefits of the H3-L6 diet were related to changes in plasma unesterified PUFA-derived lipid mediators known to be involved in nociception, including prostanoids. Lipid mediators were measured by ultra-high-pressure liquid chromatography coupled with tandem mass-spectrometry. Compared to baseline, dietary LA lowering with or without added omega-3 fatty acids did not alter unesterified n-6 PUFA-derived lipid mediators, although several species derived from LA, di-homo-gamma-linolenic acid, and arachidonic acid were positively associated with headache frequency and intensity, as well as mental health burden. Alpha-linolenic acid (ALA)-derived metabolites were also associated with increased headache frequency and intensity, although they did not change from the baseline in either dietary group. Compared to baseline, docosahexaenoic acid (DHA)-derived epoxides were more elevated in the H3-L6 group compared to the L6 group. Diet-induced elevations in plasma DHA-epoxides were associated with reduced headache frequency, better physical and mental health, and improved quality of life (p < 0.05). Prostanoids were not detected, except for PGF2-alpha, which was not associated with any outcomes. This study demonstrates that diet-induced changes in DHA-epoxides were associated with pain reduction in patients with chronic headaches, whereas n-6 PUFA and ALA metabolites were associated with nociception. Lipid mediator associations with mental health and quality of life paralleled pain management outcomes in this population. The findings point to a network of multiple diet-modifiable lipid mediator targets for pain management in individuals with CDHs.

Knock-In Mice Expressing a 15-Lipoxygenating Alox5 Mutant Respond Differently to Experimental Inflammation Than Reported Alox5-/- Mice

Arachidonic acid 5-lipoxygenase (ALOX5) is the key enzyme in the biosynthesis of pro-inflammatory leukotrienes. We recently created knock-in mice (Alox5-KI) which express an arachidonic acid 15-lipoxygenating Alox5 mutant instead of the 5-lipoxygenating wildtype enzyme. These mice were leukotriene deficient but exhibited an elevated linoleic acid oxygenase activity. Here we characterized the polyenoic fatty acid metabolism of these mice in more detail and tested the animals in three different experimental inflammation models. In experimental autoimmune encephalomyelitis (EAE), Alox5-KI mice displayed an earlier disease onset and a significantly higher cumulative incidence rate than wildtype controls but the clinical score kinetics were not significantly different. In dextran sodium sulfate-induced colitis (DSS) and in the chronic constriction nerve injury model (CCI), Alox5-KI mice performed like wildtype controls with similar genetic background. These results were somewhat surprising since in previous loss-of-function studies targeting leukotriene biosynthesis (Alox5^−/− mice, inhibitor studies), more severe inflammatory symptoms were observed in the EAE model but the degree of inflammation in DSS colitis was attenuated. Taken together, our data indicate that these mutant Alox5-KI mice respond differently in two models of experimental inflammation than Alox5^−/− animals tested previously in similar experimental setups.

Human lipoxygenase isoforms form complex patterns of double and triple oxygenated compounds from eicosapentaenoic acid

Lipoxygenases (ALOX) are lipid peroxidizing enzymes that catalyze the biosynthesis of pro- and anti-inflammatory lipid mediators and have been implicated in (patho-)physiological processes. In humans, six functional ALOX isoforms exist and their arachidonic acid oxygenation products have been characterized. Products include leukotrienes and lipoxins which are involved in the regulation of inflammation and resolution. Oxygenation of n3-polyunsaturated fatty acids gives rise to specialized pro-resolving mediators, e.g. resolvins. However, the catalytic activity of different ALOX isoforms can lead to a multitude of potentially bioactive products. Here, we characterized the patterns of oxygenation products formed by human recombinant ALOX5, ALOX15, ALOX15B and ALOX12 from eicosapentaenoic acid (EPA) and its 18-hydroxy derivative 18-HEPE with particular emphasis on double and triple oxygenation products. ALOX15 and ALOX5 formed a complex mixture of various double oxygenation products from EPA, which include 5,15-diHEPE and various 8,15-diHEPE isomers. Their biosynthetic mechanisms were explored using heavy oxygen isotopes (H₂¹⁸O, ¹⁸O₂ gas) and three catalytic activities contributed to product formation: i) fatty acid oxygenase activity, ii) leukotriene synthase activity, iii) lipohydroperoxidase activity. For ALOX15B and ALOX12 more specific product patterns were identified, which was also the case when these enzymes reacted in concert with ALOX5. Several double oxygenated compounds were formed from 18-HEPE by ALOX5, ALOX15B and ALOX12 including previously identified resolvins (RvE2, RvE3), while formation of triple oxygenation products, e.g. 5,17,18-triHEPE, required ALOX5. Taken together our data show that EPA can be converted by human ALOX isoforms to a large number of secondary oxygenation products, which might exhibit bioactivity.

General Pathways of Pain Sensation and the Major Neurotransmitters Involved in Pain Regulation

Pain has been considered as a concept of sensation that we feel as a reaction to the stimulus of our surrounding, putting us in harm's way and acting as a form of defense mechanism that our body has permanently installed into its system. However, pain leads to a huge chunk of finances within the healthcare system with continuous rehabilitation of patients with adverse pain sensations, which might reduce not only their quality of life but also their productivity at work setting back the pace of our economy. It may not look like a huge deal but factor in pain as an issue for majority of us, it becomes an economical burden. Although pain has been researched into and understood by numerous researches, from its definition, mechanism of action to its inhibition in hopes of finding an absolute solution for victims of pain, the pathways of pain sensation, neurotransmitters involved in producing such a sensation are not comprehensively reviewed. Therefore, this review article aims to put in place a thorough understanding of major pain conditions that we experience-nociceptive, inflammatory and physiologically dysfunction, such as neuropathic pain and its modulation and feedback systems. Moreover, the complete mechanism of conduction is compiled within this article, elucidating understandings from various researches and breakthroughs.

Tackling Pain Associated with Rheumatoid Arthritis: Proton-Sensing Receptors

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. Chronic pain that comes with RA turns independent, essentially becoming its own disease. It could partly explain that a significant number (50%) of RA patients fail to respond to current RA therapies that focus mainly on suppression of joint inflammation. The acute phase of pain seems to associate with joint inflammation in early RA. In established RA, the chronic phase of pain could be linked to inflammatory components of neuron-immune interactions and noninflammatory components. Accumulating evidence suggests that the initial inflammation and autoimmunity in RA (preclinical RA) begin outside of the joint and may originate at mucosal sites and alterations in the composition of microbiota located at mucosal sites could be essential for mucosal inflammation, triggering joint inflammation. Fibroblast-like synoviocytes in the inflamed joint respond to cytokines to release acidic components, lowering pH in synovial fluid. Extracellular proton binds to proton-sensing ion channels, and G-protein-coupled receptors in joint nociceptive fibers may contribute to sensory transduction and release of neurotransmitters, leading to pain and hyperalgesia. Activation of peripheral sensory neurons or nociceptors further modulates inflammation, resulting in neuroinflammation or neurogenic inflammation. Peripheral and central nerves work with non-neuronal cells (such as immune cells, glial cells) in concert to contribute to the chronic phase of RA-associated pain. This review will discuss actions of proton-sensing receptors on neurons or non-neuronal cells that modulate RA pathology and associated chronic pain, and it will be beneficial for the development of future therapeutic treatments.

Anti-Inflammatory and Antinociceptive Activities of Anthraquinone-2-Carboxylic Acid

Anthraquinone compounds are one of the abundant polyphenols found in fruits, vegetables, and herbs. However, the in vivo anti-inflammatory activity and molecular mechanisms of anthraquinones have not been fully elucidated. We investigated the activity of anthraquinones using acute inflammatory and nociceptive experimental conditions. Anthraquinone-2-carboxylic acid (9,10-dihydro-9,10-dioxo-2-anthracenecarboxylic acid, AQCA), one of the major anthraquinones identified from Brazilian taheebo, ameliorated various inflammatory and algesic symptoms in EtOH/HCl- and acetylsalicylic acid- (ASA-) induced gastritis, arachidonic acid-induced edema, and acetic acid-induced abdominal writhing without displaying toxic profiles in body and organ weight, gastric irritation, or serum parameters. In addition, AQCA suppressed the expression of inflammatory genes such as cyclooxygenase- (COX-) 2 in stomach tissues and lipopolysaccharide- (LPS-) treated RAW264.7 cells. According to reporter gene assay and immunoblotting analyses, AQCA inhibited activation of the nuclear factor- (NF-) κB and activator protein- (AP-) 1 pathways by suppression of upstream signaling involving interleukin-1 receptor-associated kinase 4 (IRAK1), p38, Src, and spleen tyrosine kinase (Syk). Our data strongly suggest that anthraquinones such as AQCA act as potent anti-inflammatory and antinociceptive components in vivo, thus contributing to the immune regulatory role of fruits and herbs.

Pharmacology of the capsaicin receptor, transient receptor potential vanilloid type-1 ion channel

The capsaicin receptor, transient receptor potential vanilloid type 1 ion channel (TRPV1), has been identified as a polymodal transducer molecule on a sub-set of primary sensory neurons which responds to various stimuli including noxious heat (> -42 degrees C), protons and vanilloids such as capsaicin, the hot ingredient of chilli peppers. Subsequently, TRPV1 has been found indispensable for the development of burning pain and reflex hyperactivity associated with inflammation of peripheral tissues and viscera, respectively. Therefore, TRPV1 is regarded as a major target for the development of novel agents for the control of pain and visceral hyperreflexia in inflammatory conditions. Initial efforts to introduce agents acting on TRPV1 into clinics have been hampered by unexpected side-effects due to wider than expected expression in various tissues, as well as by the complex pharmacology, of TRPV1. However, it is believed that better understanding of the pharmacological properties of TRPV1 and specific targeting of tissues may eventually lead to the development of clinically useful agents. In order to assist better understanding of TRPV1 pharmacology, here we are giving a comprehensive account on the activation and inactivation mechanisms and the structure-function relationship of TRPV1.

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.

Sexual dimorphism in endothelin-1 induced mechanical hyperalgesia in the rat

While the onset of mechanical hyperalgesia induced by endothelin-1 was delayed in female rats, compared to males, the duration was much longer. Given that the repeated test stimulus used to assess nociceptive threshold enhances hyperalgesia, a phenomenon we have referred to as stimulus-induced enhancement of hyperalgesia, we also evaluated for sexual dimorphism in the impact of repeated application of the mechanical test stimulus on endothelin-1 hyperalgesia. In male and female rats, endothelin-1 induced hyperalgesia is already maximal at 30 min. At this time stimulus-induced enhancement of hyperalgesia, which is observed only in male rats, persisted for 3-4h. In contrast, in females, it develops only after a very long (15 day) delay, and is still present, without attenuation, at 45 days. Ovariectomy eliminated these differences between male and female rats. These findings suggest marked, ovarian-dependent sexual dimorphism in endothelin-1 induced mechanical hyperalgesia and its enhancement by repeated mechanical stimulation.

Soluble epoxide hydrolase inhibition, epoxygenated fatty acids and nociception

The soluble epoxide hydrolase (sEH) enzyme regulates the levels of endogenous epoxygenated fatty acid (EFA) lipid metabolites by rapidly degrading these molecules. The EFAs have pleiotropic biological activities including the modulation of nociceptive signaling. Recent findings indicate that the EFAs, in particular the arachidonic acid (AA) derived epoxyeicosatrienoic acids (EETs), the docosahexaenoic acid (DHA) derived epoxydocosapentaenoic acids (EpDPEs) and eicosapentaenoic acid (EPA) derived epoxyeicosatetraenoic acids (EpETEs) are natural signaling molecules. The tight regulation of these metabolites speaks to their importance in regulating biological functions. In the past several years work on EFAs in regard to their activities in the nervous system evolved to demonstrate that these molecules are anti-inflammatory and anti-nociceptive. Here we focus on the recent advances in understanding the effects of sEH inhibition and increased EFAs on the nociceptive system and their ability to reduce pain. Evidence of their role in modulating pain signaling is given by their direct application and by inhibiting their degradation in various models of pain. Moreover, there is mounting evidence of EFAs role in the crosstalk between major nociceptive and anti-nociceptive systems which is reviewed herein. Overall the fundamental knowledge generated within the past decade indicates that orally bioavailable small molecule inhibitors of sEH may find a place in the treatment of a number of diverse painful conditions including inflammatory and neuropathic pain.

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.

Lipoxygenase inhibitors suppressed carrageenan-induced Fos-expression and inflammatory pain responses in the rat

Lipoxygenase (LO) metabolites are generated in inflamed tissues. However, it is unclear whether the inhibition of the LO activity regulates the expression of c-Fos protein, a pain marker in the spinal cord. Here we used a carrageenan-induced inflammation model to examine the role of LO in the development of c-Fos expression. Intradermally injected carrageenan caused elevated number of cells exhibiting Fos-like immunoreactivity (Fos-LI) in the spinal dorsal horn, and decreased the thermal and mechanical threshold in Hargreaves and von Frey tests. Pretreatment with an inhibitor of phospholipase A2, that generates the LO substrate, prior to the carrageenan injection significantly reduced the number of Fos-(+) cells. A general LO inhibitor NDGA, a 5-LO inhibitor AA-861 and a 12-LO inhibitor baicalein also exhibited the similar effects. Moreover, the LO inhibitors suppressed carrageenan-induced thermal and mechanical hyperalgesic behaviors, which inidcates that the changes in Fos expression correlates with those in the nociceptive behaviors in the inflamed rats. LO products are endogenous TRPV1 activators and pretreatment with BCTC, a TRPV1 antagonist inhibited the thermal but not the mechanical hypersensitivity. Overall, our results from the Fos-LI and behavior tests suggest that LO products released from inflamed tissues contribute to nociception during carrageenan-induced inflammation, indicating that the LO pathway is a possible target for modulating inflammatory pain.

Inhibitory activity of salicylic acid on lipoxygenase-dependent lipid peroxidation

BACKGROUND

Since iron is essential for lipoxygenase activity and salicylic acid (SA) can interact with the metal, possible lipoxygenase inhibition by SA was investigated.

METHODS

Kinetic spectrophotometric evaluation of enzymatic lipid peroxidation catalyzed by soybean lipoxygenase (SLO), rabbit reticulocyte 15-lipoxygenase (RR15-LOX), porcine leukocyte 12-lipoxygenase (PL12-LOX) and human recombinant 5-lipoxygenase (HR5-LOX) with and without SA.

RESULTS

SA inhibited linoleic, arachidonic and docosahexaenoic acid or human lipoprotein peroxidation catalyzed by SLO with IC50 of, respectively, 107, 153, 47 and 108 microM. Using the same substrates, SA inhibited RR15-LOX with IC50 of, respectively, 49, 63, 27 and 51 microM. Further, arachidonic acid peroxidation catalyzed by PL12-LOX and HR5-LOX was inhibited by SA with IC50 of 101 and 168 microM, respectively. Enzymatic inhibition was complete, reversible and non-competitive. Conceivably due to its lower hydrophobicity, aspirin was less effective, indicating acetylation-independent enzyme inhibition. SA and aspirin were ineffective peroxyl radical scavengers but readily reduced Fe3+, i.e. FeCl3, to Fe2+, suggesting their capacity to reduce Fe3+ at the enzyme active site. Indeed, similar to the catecholic redox inhibitor nordihydroguaiaretic acid, SA inhibited with the same efficiency both ferric and the native ferrous SLO form, indicating that these compounds reduce the active ferric enzyme leading to its inactivation.

GENERAL SIGNIFICANCE

SA can inhibit lipoxygenase-catalyzed lipid peroxidation at therapeutic concentrations, suggesting its possible inhibitory activity against enzymatic lipid peroxidation in the clinical setting.

Involvement of LTB4 in zymosan-induced joint nociception in mice: participation of neutrophils and PGE2

Leukotriene B4 (LTB4) mediates different inflammatory events such as neutrophil migration and pain. The present study addressed the mechanisms of LTB4-mediated joint inflammation-induced hypernociception. It was observed that zymosan-induced articular hypernociception and neutrophil migration were reduced dose-dependently by the pretreatment with MK886 (1-9 mg/kg; LT synthesis inhibitor) as well as in 5-lypoxygenase-deficient mice (5LO(-/-)) or by the selective antagonist of the LTB(4) receptor (CP105696; 3 mg/kg). Histological analysis showed reduced zymosan-induced articular inflammatory damage in 5LO(-/-) mice. The hypernociceptive role of LTB4 was confirmed further by the demonstration that joint injection of LTB4 induces a dose (8.3, 25, and 75 ng)-dependent articular hypernociception. Furthermore, zymosan induced an increase in joint LTB4 production. Investigating the mechanism underlying LTB4 mediation of zymosan-induced hypernociception, LTB4-induced hypernociception was reduced by indomethacin (5 mg/kg), MK886 (3 mg/kg), celecoxib (10 mg/kg), antineutrophil antibody (100 mug, two doses), and fucoidan (20 mg/kg) treatments as well as in 5LO(-/-) mice. The production of LTB4 induced by zymosan in the joint was reduced by the pretreatment with fucoidan or antineutrophil antibody as well as the production of PGE2 induced by LTB4. Therefore, besides reinforcing the role of endogenous LTB4 as an important mediator of inflamed joint hypernociception, these results also suggested that the mechanism of LTB4-induced articular hypernociception depends on prostanoid and neutrophil recruitment. Furthermore, the results also demonstrated clearly that LTB4-induced hypernociception depends on the additional release of endogenous LTs. Concluding, targeting LTB4 synthesis/action might constitute useful therapeutic approaches to inhibit articular inflammatory hypernociception.

Role of TRPV receptors in respiratory diseases

Transient receptor potential vanilloid type channels (TRPVs) are expressed in several cell types in human and animal lungs. Increasing evidence has demonstrated important roles of these cation channels, particularly TRPV1 and TRPV4, in the regulation of airway function. These TRPVs can be activated by a number of endogenous substances (hydrogen ion, certain lipoxygenase products, etc.) and changes in physiological conditions (e.g., temperature, osmolarity, etc.). Activation of these channels can evoke Ca(2+) influx and excitation of the neuron. TRPV1 channels are generally expressed in non-myelinated afferents innervating the airways and lungs, which also contain sensory neuropeptides such as tachykinins. Upon stimulation, these sensory nerves elicit centrally-mediated reflex responses as well as local release of tachykinins, and result in cough, airway irritation, reflex bronchoconstriction and neurogenic inflammation in the airways. Recent studies clearly demonstrated that the excitability of TRPV1 channels is up-regulated by certain autacoids (e.g., prostaglandin E(2), bradykinin) released during airway inflammatory reaction. Under these conditions, the TRPV1 can be activated by a slight increase in airway temperature or tissue acidity. Indirect evidence also suggests that TRPV channels may play a part in the pathogenesis of certain respiratory diseases such as asthma and chronic cough. Therefore, the potential use of TRPV antagonists as a novel therapy for these diseases certainly merits further investigation.

Pain, mast cells, and nerves in peritoneal, ovarian, and deep infiltrating endometriosis

OBJECTIVE

To detect and quantify mast cells in peritoneal, ovarian, and deep infiltrating endometriosis and to study the relationship between mast cells and nerves in endometriosis.

DESIGN

Prospective histological and immunohistochemical study.

SETTING

University of Brussels, Belgium.

PATIENT(S)

Sixty-nine women undergoing laparoscopic excision of endometriosis for pain. Thirty-seven biopsies of normal tissue were obtained from women without endometriosis.

INTERVENTION(S)

Excision of endometriosis from different anatomical locations.

MAIN OUTCOME MEASURE(S)

Immunohistochemistry with chymase and tryptase to confirm the presence of mast cells and activated mast cells, respectively, in endometriotic lesions. Quantification of mast cells, activated mast cells, and degranulating mast cells in the different locations of endometriosis. Study of the relationship between mast cells and nerves by quantifying mast cells located less than 25 mum from nerves immunohistochemically stained with S-100 protein. Preoperative pain score evaluation by visual analogue scales.

RESULT(S)

Patients with deeply infiltrating lesions had significantly higher preoperative pain scores than patients with peritoneal or ovarian endometriosis. Mast cells and degranulating mast cells are significantly more abundant in endometriotic lesions than in nonaffected tissues. Deep infiltrating lesions show a significantly higher number of mast cells, activated mast cells, and mast cells located <25 microm from nerves than peritoneal and ovarian lesions. We found significantly more degranulating mast cells in deep infiltrating lesions than in peritoneal lesions.

CONCLUSION(S)

The presence of increased activated and degranulating mast cells in deeply infiltrating endometriosis, which are the most painful lesions, and the close histological relationship between mast cells and nerves strongly suggest that mast cells could contribute to the development of pain and hyperalgesia in endometriosis, possibly by a direct effect on nerve structures.

Hypernociception elicited by tibio-tarsal joint flexion in mice: a novel experimental arthritis model for pharmacological screening

Mice have been used as animal model to study mechanisms underlying inflammatory and immune diseases. The present study describes a model of joint inflammation-induced hypernociception to discriminate pharmacological tests in mice. A polypropylene tip probe with a large area (4.15 mm2) applied on the plantar surface of the hind paw was used to produce a dorsal flexion of tibio-tarsal joint. Experiments were performed to demonstrate that the probe application did not provoke cutaneous nociception. The decrease in the withdrawal threshold of inflamed joint was used as nociceptive parameter. Administration of zymosan in the tibio-tarsal joint induced a dose and time-dependent hypernociception elicited by articular dorsal flexion movement. Maximal joint hypernociception was detected between 7 and 24 h after zymosan injection, and matched maximal inflammation score as determined by histopathology and neutrophil migration assay. In agreement with the inflammatory hypernociceptive paradigm, flexion-elicited hypernociception induced by zymosan was dose-dependently inhibited by morphine (2-8 mg/kg) and by an effective dose of indomethacin (5 mg/kg). The present study demonstrated that the tibio-tarsal flexion reflex is a behavioral nociceptive model that allows a quantitative evaluation of inflammatory joint hypernociception in mice and its pharmacological modulation.

Selective local PMN recruitment by CXCL1 or CXCL2/3 injection does not cause inflammatory pain

Polymorphonuclear cells (PMN) are recruited in early inflammation and are believed to contribute to inflammatory pain. However, studies demonstrating a hyperalgesic role of PMN did not examine selective PMN recruitment or did not document effective PMN recruitment. We hypothesized that hyperalgesia does not develop after chemokine-induced PMN selective recruitment and is independent of PMN infiltration in complete Freund's adjuvant (CFA)-induced, local inflammation. PMN were recruited by intraplantar injection of CXC chemokine ligand 1 (CXCL1; keratinocyte-derived chemokine), CXCL2/3 (macrophage inflammatory protein-2), or CFA, with or without preceding systemic PMN depletion. Chemokine inoculation resulted in dose (0-30 microg)- and time (0-12 h)-dependent, selective recruitment of PMN as quantified by flow cytometry. CXCL2/3, but not CXCL1, was less effective at high doses, probably as a result of significant down-regulation of CXC chemokine receptor 2 expression on blood PMN. Neither chemokine caused mechanical or thermal hyperalgesia as determined by the Randall-Selitto and Hargreaves test, respectively, despite comparable expression of activation markers (i.e., CD11b, CD18, and L-selectin) on infiltrating PMN. In contrast, CFA injection induced hyperalgesia, independent of PMN recruitment. c-Fos mRNA and immunoreactivity in the spinal cord were increased significantly after inoculation of CFA-independent of PMN-migration but not of CXCL2/3. Measurement of potential hyperalgesic mediators showed that hyperalgesia correlated with local prostaglandin E2 (PGE2) but not with interleukin-1beta production. In summary, hyperalgesia, local PGE2 production, and spinal c-Fos expression occur after CFA-induced inflammation but not after CXCL1- or CXCL2/3-induced, selective PMN recruitment. Thus, PMN seem to be less important in inflammatory hyperalgesia than previously thought.

Effects of nordihydroguaiaretic acid on Na+ currents in rat dorsal root ganglion neurons

Nordihydroguaiaretic acid (NDGA) is a lipoxygenase (LO) inhibitor with a strong antioxidant activity. It attenuates nociceptive responses produced by various stimuli, which has been ascribed to its LO inhibition. Primary sensory neurons express multiple Na+ channels that are important in processing normal and abnormal nociception. We examined the effects of NDGA on tetrodotoxin-sensitive and tetrodotoxin-resistant Na+ currents in rat dorsal root ganglion neurons. NDGA inhibited both types of Na+ currents concentration dependently and reversibly. Both activation and inactivation time courses were slowed by NDGA, which were not reversible. NDGA produced a hyperpolarizing shift of the steady-state inactivation curves and reduced the maximal availability of both Na+ currents, indicating that it blocks both inactivated and resting Na+ channels. NDGA shifted the conductance-voltage curves of both Na+ currents toward a depolarizing direction and increased the slope factors of the curves. The recovery of Na+ channels from inactivation was retarded by NDGA. All these effects will reduce the excitability of sensory neurons and should be taken into account when it comes to the antinociceptive effects of NDGA.

Immune and inflammatory mechanisms in neuropathic pain

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

Double dioxygenation by mouse 8S-lipoxygenase: Specific formation of a potent peroxisome proliferator-activated receptor α agonist

Mouse 8S-lipoxygenase (8-LOX) metabolizes arachidonic acid (AA) specifically to 8S-hydroperoxyeicosatetraenoic acid (8S-HPETE), which will be readily reduced under physiological circumstances to 8S-hydroxyeicosatetraenoic acid (8S-HETE), a natural agonist of peroxisome proliferator-activated receptor alpha (PPAR alpha). Here, we investigated whether 8-LOX could further oxygenate AA and whether the products could activate PPARs. The purified recombinant 8-LOX converted AA exclusively to 8S-HPETE and then to (8S,15S)-dihydroperoxy-5Z,9E,11Z,13E-eicosatetraenoic acid (8S,15S-diHPETE). The kcat/Km values for 8S-HPETE and AA were 3.3x10(3) and 2.7x10(4) M(-1) s(-1), respectively. 8-LOX also dioxygenated 8S-HETE and 15S-H(P)ETE specifically to the corresponding 8S,15S-disubstituted derivatives. By contrast, 15-LOX-2, a human homologue of 8-LOX, produced 8S,15S-diH(P)ETE from 8S-H(P)ETE but not from AA nor 15S-H(P)ETE. 8S,15S-diHETE activated PPAR alpha more strongly than 8S-HETE did. The present results suggest that 8S,15S-diH(P)ETE as well as 8S-H(P)ETE would contribute to the physiological function of 8-LOX and also that 8-LOX can function as a potential 15-LOX.

Antiinflammatory Activity of Astilbic Acid from Astilbe chinensis

This study examined the effect of astilbic acid (3beta,6beta-dihydroxyolean-12-en-27-oic acid), which is a herbal medicine isolated from Astilbe chinensis. Astilbic acid inhibited 5-lipoxygenase (5-LOX)-dependent leukotriene C4 (LTC4) generation in bone marrow-derived mast cells in a concentration-dependent manner with an IC50 value of 2.1 microM. In addition, astilbic acid was tested in a rat passive cutaneous anaphylaxis (PCA) reaction assay by administering 10 to 50 mg/kg i.p. Astilbic acid dose dependently inhibited the PCA reaction, which was activated by anti-dinitrophenyl (DNP) IgE. Furthermore, this compound inhibited mouse acetic acid-induced writhing (47-62% inhibition at 0.4-50 mg/kg) after being administered orally, while aspirin (200 mg/kg) showed 62% inhibition. These results suggest that astilbic acid may be beneficial in regulating various inflammatory processes.

Involvement of spinal lipoxygenase metabolites in hyperalgesia and opioid tolerance

This study investigated role of spinal lipoxygenase metabolites in induction of hyperalgesia and development of opioid analgesic tolerance. In the rat, nociception was measured using formalin and tail-flick tests. Intrathecal administration of leukotriene receptor agonist (LTB4) augmented the second phase of the formalin response and marginally increased sensitivity to acute thermal stimulation in the tail-flick test, responses suppressed by 6-(6-(3R-hydroxy-1E,5Z-undecadien-1-yl)-2-pyridinyl)-1,5S-hexanediol (U75302), a leukotriene BLT receptor antagonist. Treatment with 15-hydroxyperoxyeicosatetranoic acid (HPETE) increased phase II formalin activity, but had no effect on tail-flick responses. 12-HPETE failed to produce an effect in either nociceptive test. In the second part of this study, chronic spinal morphine for 5 days produced progressive decline in morphine antinociception and loss in analgesic potency. These effects were attenuated by co-administration of morphine with selective and nonselective lipoxygenase inhibitors. These results suggest involvement of lipoxygenase metabolites in both pain modulation and induction of opioid tolerance at the spinal level.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non-neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

Analgesia induced by local plantar injections of opiates in the formalin test in infant rats

Morphine injected locally to the paw of an adult or an infant rat is analgesic. Opiates specific to micro and kappa opioid receptors, and less consistently to delta opioid receptors, given locally to the site of injury in adult animals are also analgesic in a variety of models of inflammatory pain. To determine which opioid receptor(s) are involved in local analgesia in the immature animal, agonists specific for micro, kappa, and delta opioid receptors were injected into the intraplantar pad in infant rats and the resultant nociceptive behavior and Fos expression assayed in the formalin test. The kappa opioid receptor agonist U50,488 reduced nociceptive behavior in both phases of the formalin test and reduced Fos expression in the dorsal horn of the lumbar spinal cord, at 3 and 21 days of age. Morphiceptin (micro opioid agonist) was analgesic in the 21-day-old pups, but not the 3-day-old pups, measured behaviorally or by Fos expression. DPDPE (delta opioid agonist) was not analgesic at either age. We also tested the effects of opioid receptor antagonists on morphine's local analgesic action. Naltrexone, and to a lesser extent the micro opioid antagonist CTOP, antagonized morphine's analgesic effect. Kappa and delta opioid receptor blockers were inactive. The results demonstrate the ability of the kappa opioid system to mediate analgesia in the neonate at the site of injury in acute and chronic pain models, that the micro opioid agonists are active later in development, but that morphine is analgesic in part through micro opioid receptors.

Neuronal CA2+/calmodulin-dependent protein kinase II: the role of structure and autoregulation in cellular function

Highly enriched in brain tissue and present throughout the body, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is central to the coordination and execution of Ca(2+) signal transduction. The substrates phosphorylated by CaMKII are implicated in homeostatic regulation of the cell, as well as in activity-dependent changes in neuronal function that appear to underlie complex cognitive and behavioral responses, including learning and memory. The architecture of CaMKII holoenzymes is unique in nature. The kinase functional domains (12 per holoenzyme) are attached by stalklike appendages to a gear-shaped core, grouped into two clusters of six. Each subunit contains a catalytic, an autoregulatory, and an association domain. Ca(2+)/calmodulin (CaM) binding disinhibits the autoregulatory domain, allowing autophosphorylation and complex changes in the enzyme's sensitivity to Ca(2+)/CaM, including the generation of Ca(2+)/CaM-independent activity, CaM trapping, and CaM capping. These processes confer a type of molecular memory to the autoregulation and activity of CaMKII. Its function is intimately shaped by its multimeric structure, autoregulation, isozymic type, and subcellular localization; these features and processes are discussed as they relate to known and potential cellular functions of this multifunctional protein kinase.

Peripheral group II metabotropic glutamate receptors (mGluR2/3) regulate prostaglandin E2-mediated sensitization of capsaicin responses and thermal nociception

Previous studies have shown that group II metabotropic glutamate receptors (mGluRs) are present on the peripheral terminals of primary sensory neurons, suggesting that they might be involved in nociception. In this study, we investigated the modulation of nociception by peripheral group II mGluRs and the molecular basis of this modulation. Subcutaneous injection of a group II mGluR agonist, 2R,4R 4-aminopyrrolidine-2,4-dicarboxylate (APDC), did not alter thermal sensitivity but blocked prostaglandin E2 (PGE2)-induced thermal hyperalgesia. This effect was blocked by (2s)-2-amino-2-[(1s,2s)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid, a selective group II mGluR antagonist. In cultured primary sensory neurons, APDC blocked PGE2-induced potentiation of capsaicin responses, which was abolished when neurons were pretreated with pertussis toxin. Similar potentiating effects induced by forskolin but not 8-bromo-cAMP were also blocked by the activation of group II mGluRs. These results indicate that peripheral group II mGluRs act via inhibition of adenylyl cyclase to reverse the sensitization of capsaicin receptors and the thermal hyperalgesia induced by PGE2, and suggest that peripheral group II mGluRs might be targeted for therapeutic intervention in inflammatory pain states.

Biology of mucosal pain

Pain is experienced when injury to mucosal tissues occurs. Although the neurobiology of mucosal pain has not been fully elucidated, research has demonstrated that the oral mucosa contains primary afferent nociceptors that respond to thermal, mechanical, and chemical stimuli. Inflammation occurs during the initial phase of mucosal injury caused by stomatotoxic chemotherapy or radiation therapy. This article reviews the mechanisms that underlie acute pain in inflamed cutaneous tissue and summarizes the major mediators that activate and sensitize primary afferent nociceptors. Recommendations for future research to elucidate the neurobiology of mucosal pain throughout the gastrointestinal tract are presented.

Neuroimmune alterations in the complex regional pain syndrome

This review focuses on some clinical aspects of the complex regional pain syndrome, such as oedema, local temperature changes and chronic pain, as a result of supposed neurogenic inflammation. Involvement of the immune system could imply the subsequent release of neuropeptides, pro-inflammatory cytokines and eicosanoids, which in turn leads to a complex cross-talk of primary and secondary generated mediators of inflammation. The development and application of drugs that act through selective receptor antagonism or enzymatic synthesis inhibition to prevent further stimulation of this cascade that could inevitably lead to chronicity of this disease are extensively discussed.

Hyperalgesia due to nerve injury: role of neutrophils

The hypothesis that the early inflammatory cell, the neutrophil, contributes to the hyperalgesia resulting from peripheral nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. The extent and time-course of neutrophilic infiltration of the sciatic nerve and innervated paw skin after partial nerve damage was characterized using immunocytochemistry. The number of endoneurial neutrophils was significantly elevated in sections of operated nerve compared to sections of sham-operated nerve for the entire period studied, i.e. up to seven days post-surgery. This considerable elevation in endoneurial neutrophil numbers was only observed at the site of nerve injury. Depletion of circulating neutrophils at the time of nerve injury significantly attenuated the induction of hyperalgesia. However, depletion of circulating neutrophils at day 8 post-injury did not alleviate hyperalgesia after its normal induction. It is concluded that endoneurial accumulation of neutrophils at the site of peripheral nerve injury is important in the early genesis of the resultant hyperalgesia. The findings support the notion that a neuroimmune interaction occurs as a result of peripheral nerve injury and is important in the subsequent development of neuropathic pain.

Direct activation of capsaicin receptors by products of lipoxygenases: endogenous capsaicin-like substances

Capsaicin, a pungent ingredient of hot peppers, causes excitation of small sensory neurons, and thereby produces severe pain. A nonselective cation channel activated by capsaicin has been identified in sensory neurons and a cDNA encoding the channel has been cloned recently. However, an endogenous activator of the receptor has not yet been found. In this study, we show that several products of lipoxygenases directly activate the capsaicin-activated channel in isolated membrane patches of sensory neurons. Among them, 12- and 15-(S)-hydroperoxyeicosatetraenoic acids, 5- and 15-(S)-hydroxyeicosatetraenoic acids, and leukotriene B(4) possessed the highest potency. The eicosanoids also activated the cloned capsaicin receptor (VR1) expressed in HEK cells. Prostaglandins and unsaturated fatty acids failed to activate the channel. These results suggest a novel signaling mechanism underlying the pain sensory transduction.

Management of chronic discogenic low back pain with a thermal intradiscal catheter. A preliminary report

STUDY DESIGN

A prospective nonrandomized clinical trial.

OBJECTIVE

To determine the outcome in a group of patients with chronic, function-limiting low back pain who met the criteria for interbody fusion surgery, but were instead treated with an intradiscal thermal catheter (SpineCath, Oratec Interventions, Inc., Menlo Park, CA).

SUMMARY OF BACKGROUND DATA

This study represents the first reported trial of treatment for chronic discogenic low back pain with a novel thermal intradiscal catheter.

METHODS

The authors evaluated 25 consecutive patients. The minimum duration of nonoperative care with the authors was 6 months. The visual analog pain scores, sitting tolerance times, and SF-36 summary scores were tabulated.

RESULTS

The mean follow-up period was 7 months, and the mean duration of symptoms 58.5 months. Of the 25 patients, 20 (80%) reported a reduction of at least 2 points in visual analog pain scores, and 18 (72%) reported an improvement in sitting tolerance as well as reduction or discontinuance of analgesic medication. Visual analog pain scores improved by a mean reduction of 3.74, a 51% change (P = 0.0001). On the SF-36 physical function subscale, 72% of the patients improved by a mean increase of 15 points (P = 0.001), a mean change of 38%, and by a mean increase of 14 points on the bodily pain subscale (P = 0.004), a mean change of 48%.

CONCLUSIONS

A statistically significant improvement in functional outcome was obtained in patients with chronic discogenic low back pain treated thermally by the SpineCath.

Involvement of resident macrophages and mast cells in the writhing nociceptive response induced by zymosan and acetic acid in mice

Intraperitoneal administration of zymosan and acetic acid induced a dose-dependent nociceptive writhing response in mice. Lavage of the peritoneal cavities with saline reduced the number of total resident peritoneal cells and caused a proportional decrease in the nociceptive responses induced by these stimuli. Furthermore, the specific reduction of the peritoneal mast cell population by intraperitoneal administration of compound 48/80 also reduced the nociceptive responses induced by zymosan and acetic acid. In contrast, enhancement of the peritoneal macrophage population by pretreatment of the cavities with thioglycollate caused an increase in the number of writhes induced by both stimuli. These data suggest that the nociceptive responses induced by zymosan and acetic acid are dependent upon the peritoneal resident macrophages and mast cells. These cells modulate the nociceptive response induced by zymosan and acetic acid via release of tumour necrosis factor alpha (TNF-alpha), interleukin 1beta and interleukin 8. This suggestion is supported by the following observations: (a) pretreatment of the peritoneal cavities with antisera against these cytokines reduced the nociceptive responses induced by these stimuli; (b) peritoneal cells harvested from cavities injected with zymosan or acetic acid released both interleukin 1beta and TNF-alpha; (c) although individual injection of TNF-alpha, interleukin 1beta or interleukin 8 did not induce the nociceptive effect, intraperitoneal injection of a mixture of these three recombinant cytokines caused a significant nociceptive writhing response. In conclusion, our results suggest that the nociceptive activity of zymosan and acetic acid in the writhing model is due to the release of TNF-alpha, interleukin 1beta and interleukin 8 by resident peritoneal macrophages and mast cells.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

The role of steroids and their effects on phospholipase A2. An animal model of radiculopathy

STUDY DESIGN

The possible role of phospholipase A2 in an animal model for lumbar radiculopathy and mechanisms of epidural steroid injections were studied.

OBJECTIVES

To clarify the pathophysiologic mechanism of the recently proved animal model for lumbar radiculopathy and to characterize further the mechanisms of action of steroids.

SUMMARY OF BACKGROUND DATA

There have been several reported animal models of peripheral neuropathy. Recently an animal model that shows reliable behavioral and neurochemical changes was proposed, and epidural steroid injections in this model were effective in the reduction of thermal hyperalgesia and allodynia.

METHOD

In a behavioral study, 24 rats were divided into 4 groups: Group I, loose ligature of the left L4 and L5 nerve roots with 4-0 chromic gut sutures and an epidural injection of 0.1 mL of saline at 3 days after surgery; Group II, same as Group I but with an epidural injection of 0.1 mL of betamethasone on the day before the operation; Group II, same as Group II except injection at 1 day after surgery; Group IV, same as Group II except injection at 3 days after surgery. To test the phospholipase A2 activity in the nerve roots and dorsal root ganglia after the operation, eight rats were killed at given intervals. Analysis of variance techniques were used to test behavioral pattern changes and phospholipase A2 activity across time in each group.

RESULTS

Thermal hyperalgesia reached its maximal point at 3 weeks after surgery in Group I, but in steroid injection groups, the recovery from hyperalgesia was faster than in Group I. However, there was no significant difference in recovery time among steroid injection groups. The level of phospholipase A2 activity was at its maximum at 1 week after surgery in Groups I and IV. It showed a steady reduction in the steroid group, whereas it remained relatively high and dropped rapidly after 3 weeks in the saline-treated group, and returned to the level of a normal nerve root at 6 weeks after surgery.

CONCLUSION

These results suggest that the behavioral pattern changes observed in the irritated nerve root model are caused in part by a high level of phospholipase A2 activity initiated by inflammation, and that the mechanism of action of epidural steroid injection in this model is inhibition of phospholipase A2 activity.

High-dose methylprednisolone prevents extensive sick leave after whiplash injury. A prospective, randomized, double-blind study

STUDY DESIGN

A prospective, randomized, double-blind study comparing high-dose methylprednisolone with placebo.

OBJECTIVES

To evaluate the efficacy of high-dose methylprednisolone when administered within 8 hours after whiplash injury.

SUMMARY OF BACKGROUND DATA

Whiplash injury often results in chronic symptoms. The management of whiplash injuries is controversial, and pharmacologic therapy has received little evaluation. In recent reports, dysfunction of the central nervous system has been indicated in several cases. Methylprednisolone administered within 8 hours after the injury to patients with acute spinal cord injury has been demonstrated to improve the outcome. This procedure was also adopted in a randomized study of cases of whiplash injury in car accidents.

METHODS

Forty patients, 22 men and 18 women with a mean age of 35 years (range, 19-65), were included in the study, 20 in each of two groups. They were treated for whiplash injury, which they had sustained in car accidents. The patients were enrolled if their diagnoses were complete and treatment had begun within 8 hours after injury. Disabling symptoms severe enough to prevent the patient from returning to work, number of sick days before and after injury, and sick-leave profile after injury were used as parameters for the evaluation of the effects of the treatment. Baseline demographic data were controlled for when statistical analysis had been performed.

RESULTS

At the follow-up examination 6 months after initial treatment, there was a significant difference in disabling symptoms between the actively treated patients and the placebo group (P = 0.047), total number of sick days (P = 0.01), and sick-leave profile (P = 0.003).

CONCLUSIONS

The results of this study indicate that acute treatment with high-dose methylprednisolone may be beneficial in preventing extensive sick leave after whiplash injury. However, the number of patients studied was small, and therefore further prospective, controlled studies are needed.

Role of peridural fibrosis in the failed back: a review

Failed back surgery syndrome (FBSS) is the presence of persistent, disabling pain in the hip, thigh, leg, or lower back of a patient who has undergone a laminectomy or discectomy. Some degree of FBSS is found in approximately 15% of such patients. There may be a direct relationship between the extent of pathology found during the initial surgical procedure and the probability that FBSS will develop. Although FBSS is usually due to improper diagnosis and surgery, another important cause is peridural fibrosis. Part of the answer may have to do with neuromechanics. In a healthy person, pain associated with dural and nerve root movement does not typically occur when performing activities of daily living. In contrast, in a person with peridural scarring, the dura and nerve roots are bound by fibrosis, and putting traction on the nerve roots and dura by back and limb movement produces pain. This pain is aggravated by the presence of inflammation at the surgical site. Research has shown that disc herniation activates the arachidonic acid cascade, resulting in the production of prostaglandins E1 and E2 and leukotriene B, substances that contribute to an inflammatory process that persists after discectomy. In an attempt to inhibit peridural fibrosis, methylprednisolone, polyethylene films, and fat grafts have been applied to the dura after discectomy. These experimental treatments have had limited success. Development of a therapy that reliably prevents peridural adhesive fibrosis will reduce the incidence of FBSS.

Prostaglandin production after experimental discectomy

STUDY DESIGN

This study ascertained the effects of discectomy on prostaglandin synthesis.

OBJECTIVES

The purpose of these novel experiments was to measure the levels of two prostaglandins in lumbar epidural fluid obtained from an area subjected to discectomy. For comparison, lumbar epidural fluid from a site not disturbed by discectomy and fluid from a subcutaneous site were analyzed for the prostaglandins.

SUMMARY OF BACKGROUND DATA

Previous studies have shown that nuclear material obtained from degenerative discs manifests an extraordinarily high level of phospholipase A2 activity. Others have hypothesized that the known inflammatory effects of phospholipase A2 are due to the release of arachidonic acid, which is converted to various eicosanoids, including several algesic prostaglandins (PGI2 and PGE2). No previous study has continuously measured prostaglandin levels in epidural fluid or assessed the effect of discectomy on prostaglandin production.

METHODS

An ultrafiltrate of lumbar epidural fluid of dogs was obtained from indwelling catheters located adjacent to spinal areas that were and were not subjected to discectomy as well as from subcutaneous tissue. The fluid was collected daily for 14 days and analyzed for PGE2 and 6-keto PGF1(alpha) (the stable metabolite of PGI2) by radioimmunoassay.

RESULTS

The concentration of 6-keto PGF1(alpha) and PGE2 in fluid collected during the first 24 hours was significantly higher in the area of discectomy than in the epidural region that was not subjected to discectomy and significantly higher than in fluid obtained from the subcutaneous site. The high level of these prostaglandins at the discectomy site fell rapidly, so that by the end of 48 hours the differences in values between spinal fluid from the discectomy and nondiscectomy regions were not statistically significant. The concentration of the prostaglandins in epidural fluid decreased with time and became minimal within the second week.

CONCLUSION

The removal of normal discs is accompanied for 24 hours by a marked rise in the synthesis of two prostaglandins known to produce pain. Because the concentration of prostaglandins in epidural fluid decreased rapidly thereafter, the initial surge obtained appears to be associated more with chemical factors such as phospholipase A2 than with wound healing.

Pain due to nerve damage: are inflammatory mediators involved?

Damage to peripheral nerves often results in pain and hyperalgesia. We suggest that nerve damage causes an inflammatory response in which cells associated with the nerve release inflammatory mediators such as eicosanoids; these mediators may contribute to the hyperalgesia which results from nerve injury. The cell types most likely to be responsible include macrophages and postganglionic sympathetic neurones. A better understanding of the mechanisms involved should lead to improved therapies for neuropathic pain.

Pathogenesis of degenerative joint disease in the human temporomandibular joint

The wide range of disease prevalences reported in epidemiological studies of temporomandibular degenerative joint disease reflects the fact that diagnoses are frequently guided by the presence or absence of non-specific signs and symptoms. Treatment is aimed at alleviating the disease symptoms rather than being guided by an understanding of the underlying disease processes. Much of our current understanding of disease processes in the temporomandibular joint is based on the study of other articular joints. Although it is likely that the molecular basis of pathogenesis is similar to that of other joints, additional study of the temporomandibular joint is required due to its unique structure and function. This review summarizes the unique structural and molecular features of the temporomandibular joint and the epidemiology of degenerative temporomandibular joint disease. As is discussed in this review, recent research has provided a better understanding of the molecular basis of degenerative joint disease processes, including insights into: the regulation of cytokine expression and activation, arachidonic acid metabolism, neural contributions to inflammation, mechanisms of extracellular matrix degradation, modulation of cell adhesion in inflammatory states, and the roles of free radicals and heat shock proteins in degenerative joint disease. Finally, the multiple cellular and molecular mechanisms involved in disease initiation and progression, along with factors that may modify the adaptive capacity of the joint, are presented as the basis for the rational design of new and more effective therapy.

Neuropathic implications of prosthodontic treatment

Associated with prosthodontic procedures, there may be the development of a chronic pain state caused by injury to peripheral branches of the trigeminal nerve. Neuropathic pains have not been discussed in the prosthodontic literature and yet are becoming increasingly common, especially in relation to the placement of implant prostheses. This article reviews the pathophysiologic and clinical features associated with neuropathic pain after dental procedures (traumatic neuralgia). The prosthodontist should become aware of neuropathic pain as a possible complication of treatment and informed consent should be obtained before procedures are started that might damage nervous tissue.

The role of marine fish oils in the treatment of ulcerative colitis

Recent studies suggest that marine fish-oil supplements, which are rich in n-3 fatty acids, may reduce the inflammation associated with ulcerative colitis. Fish oils may exert their beneficial effects by shifting eicosanoid synthesis to less inflammatory species or by modulating tissue levels of certain cytokines.