Epinephrine produces a beta-adrenergic receptor-mediated mechanical hyperalgesia and in vitro sensitization of rat nociceptors

Inflammatory signals enhance piezo2-mediated mechanosensitive currents

Heightened nociceptor function caused by inflammatory mediators such as bradykinin (BK) contributes to increased pain sensitivity (hyperalgesia) to noxious mechanical and thermal stimuli. Although it is known that sensitization of the heat transducer TRPV1 largely subserves thermal hyperalgesia, the cellular mechanisms underlying mechanical hyperalgesia have been elusive. The role of the mechanically activated (MA) channel piezo2 (known as FAM38B) present in mammalian sensory neurons is unknown. We test the hypothesis that piezo2 activity is enhanced by BK, an algogenic peptide that induces mechanical hyperalgesia within minutes. Piezo2 current amplitude is increased and inactivation is slowed by bradykinin receptor beta 2 (BDKRB2) activation in heterologous expression systems. Protein kinase A (PKA) and protein kinase C (PKC) agonists enhance piezo2 activity. BDKRB2-mediated effects are abolished by PKA and PKC inhibitors. Finally, piezo2-dependent MA currents in a class of native sensory neurons are enhanced 8-fold by BK via PKA and PKC. Thus, piezo2 sensitization may contribute to PKA- and PKC-mediated mechanical hyperalgesia.

Potential autonomic risk factors for chronic TMD: descriptive data and empirically identified domains from the OPPERA case-control study

Several case-control studies have been conducted that examine the association between autonomic variables and persistent pain conditions; however, there is a surprising dearth of published studies in this area that have focused on temporomandibular disorders (TMD). The current study presents autonomic findings from the baseline case-control study of the OPPERA (Orofacial Pain: Prospective Evaluation and Risk Assessment) cooperative agreement. Measures of arterial blood pressure, heart rate, heart rate variability, and indirect measures of baroreflex sensitivity were assessed at rest and in response to a physical (orthostatic) and psychological (Stroop) stressors in 1,633 TMD-free controls and 185 TMD cases. In bivariate and demographically adjusted analyses, greater odds of TMD case status were associated with elevated heart rates, reduced heart rate variability, and reduced surrogate measures of baroreflex sensitivity across all experimental procedures. Principal component analysis was undertaken to identify latent constructs revealing 5 components. These findings provide evidence of associations between autonomic factors and TMD. Future prospective analyses in the OPPERA cohort will determine if the presence of these autonomic factors predicts increased risk for developing new onset TMD.

PERSPECTIVE

This article reports autonomic findings from the OPPERA Study, a large prospective cohort study designed to discover causal determinants of TMD pain. Findings indicate statistically significant differences between TMD cases and controls across multiple autonomic constructs at rest and during both physical and psychologically challenging conditions. Future analyses will determine whether these autonomic factors increase risk for new onset TMD.

Genetic contribution of catechol-O-methyltransferase variants in treatment outcome of low back pain: a prospective genetic association study

Background

Treatment outcome of low back pain (LBP) is associated with inter-individual variations in pain relief and functional disability. Genetic variants of catechol-O-methyltransferase (COMT) gene have previously been shown to be associated with pain sensitivity and pain medication. This study examines the association between COMT polymorphisms and 7–11 year change in Oswestry Disability Index (ODI) and Visual Analog Score (VAS) for LBP as clinical outcome variables in patients treated with surgical instrumented lumbar fusion or cognitive intervention and exercise.

Methods

93 unrelated patients with chronic LBP for duration of >1 year and lumbar disc degeneration (LDD) were treated with lumbar fusion (N = 60) or cognitive therapy and exercises (N = 33). Standardised questionnaires assessing the ODI, VAS LBP, psychological factors and use of analgesics, were answered by patients both at baseline and at 7–11 years follow-up. Four SNPs in the COMT gene were successfully genotyped. Single marker as well as haplotype association with change in ODI and VAS LBP, were analyzed using Haploview, linear regression and R-package Haplostats. P-values were not formally corrected for multiple testing as this was an explorative study.

Results

Association analysis of individual SNPs adjusted for covariates revealed association of rs4633 and rs4680 with post treatment improvement in VAS LBP (p = 0.02, mean difference (β) = 13.5 and p = 0.02, β = 14.2 respectively). SNPs, rs4633 and rs4680 were found to be genotypically similar and in strong linkage disequilibrium (LD). A significant association was found with covariates, analgesics (p = 0.001, β = 18.6); anxiety and depression (p = 0.008, β = 15.4) and age (p = 0.03, mean difference per year (β) = 0.7) at follow-up. There was a tendency for better improvement among heterozygous patients compared to the homozygous. No association was observed for the analysis of the common haplotypes, these SNPs were situated on.

Conclusions

Results suggest an influence of genetic variants of COMT gene in describing the variation in pain after treatment for low back pain. Replication in large samples with testing for other pain related genes is warranted.

Participation of the NO/cGMP/K+ATP pathway in the antinociception induced by Walker tumor bearing in rats

Implantation of Walker 256 tumor decreases acute systemic inflammation in rats. Inflammatory hyperalgesia is one of the most important events of acute inflammation. The L-arginine/NO/cGMP/K⁺ATP pathway has been proposed as the mechanism of peripheral antinociception mediated by several drugs and physical exercise. The objective of this study was to investigate a possible involvement of the NO/cGMP/K⁺ATP pathway in antinociception induced in Walker 256 tumor-bearing male Wistar rats (180-220 g). The groups consisted of 5-6 animals. Mechanical inflammatory hypernociception was evaluated using an electronic version of the von Frey test. Walker tumor (4th and 7th day post-implantation) reduced prostaglandin E₂- (PGE₂, 400 ng/paw; 50 µL; intraplantar injection) and carrageenan-induced hypernociception (500 µg/paw; 100 µL; intraplantar injection). Walker tumor-induced analgesia was reversed (99.3% for carrageenan and 77.2% for PGE₂) by a selective inhibitor of nitric oxide synthase (L-NAME; 90 mg/kg, ip) and L-arginine (200 mg/kg, ip), which prevented (80% for carrageenan and 65% for PGE₂) the effect of L-NAME. Treatment with the soluble guanylyl cyclase inhibitor ODQ (100% for carrageenan and 95% for PGE₂; 8 µg/paw) and the ATP-sensitive K⁺ channel (KATP) blocker glibenclamide (87.5% for carrageenan and 100% for PGE₂; 160 µg/paw) reversed the antinociceptive effect of tumor bearing in a statistically significant manner (P < 0.05). The present study confirmed an intrinsic peripheral antinociceptive effect of Walker tumor bearing in rats. This antinociceptive effect seemed to be mediated by activation of the NO/cGMP pathway followed by the opening of KATP channels.

The effect of noise-cancelling headphones or music on pain perception and anxiety in men undergoing transrectal prostate biopsy

OBJECTIVE

To assess the effect of noise-cancelling headphones with or without music on patient pain and anxiety associated with routine, office-based transrectal ultrasound (TRUS)-guided prostate biopsy in a prospective randomized study.

METHODS

Patients scheduled for prostate biopsy as a result of elevated prostate-specific antigen and/or abnormal digital rectal examination were prospectively enrolled and randomized into a control, noise-cancelling headphones, or music-headphones group. Patients completed pain and anxiety questionnaires and had their physiological parameters assessed before and after the procedure and compared across groups.

RESULTS

Eighty-eight patients were enrolled. Pain scores increased from baseline across all study groups, with the lowest mean score in the music group. No appreciable change was noted in anxiety scores after the procedure between groups (P>.05). Although postbiopsy systolic blood pressure values remained comparable with baseline levels in all groups, postbiopsy diastolic blood pressure increased in the control and headphones groups (P=.062 and .088, respectively) but remained stable in the music group (P=.552) after biopsy, indicating lesser physiological response to anxiety and pain in this group.

CONCLUSION

Music-induced attention shift during prostate biopsy may have a beneficial impact on procedural anxiety and pain perception, but no apparent effect was noted for use of headphones alone. Further studies are necessary to explore strategies to reduce perceived anxiety and pain in men undergoing prostate biopsy.

Potential processes involved in the initiation and maintenance of whiplash-associated disorders: discussion paper 3

STUDY DESIGN

Nonsystematic review and discussion of the etiological processes involved in whiplash-associated disorders (WAD).

OBJECTIVE

To summarize the research and identify priorities for future research.

SUMMARY OF BACKGROUND DATA

Although there is convergent evidence of a peripheral lesion in some individuals after whiplash injury, in the majority of injured people, a lesion cannot be established with current imaging technology. Therefore, it is important to consider processes that underlie the initiation and maintenance of whiplash pain as this may allow for the development and testing of interventions to target these processes and improve outcomes.

METHODS

A nonsystematic review was performed to summarize current knowledge regarding potential etiological processes involved in the initiation and maintenance of WAD and to identify future research priorities.

RESULTS

There are several etiological processes potentially involved in the initiation and maintenance of WAD. These include augmented nociceptive processing, stress system responses, and psychosocial and sociocultural factors. Recent findings also indicate that morphological changes in the neck muscles of injured people show some association with poor recovery, but the mechanisms underlying these changes are not clear. Preliminary evidence indicates associations between these processes. Future research priorities include the following: more sophisticated investigation and analysis of interactions between the various processes; whether the modification of these processes is achievable and if modification can improve health outcomes; and to clarify factors involved in the initiation of whiplash pain versus those involved in symptom maintenance.

CONCLUSION

Research to date indicates that there are several physiological and psychological etiological processes that may underlie the initiation and maintenance of whiplash-related pain and disability. Further research is required to determine relationships and interactions between these factors and to determine whether their modification is possible and will improve outcomes after injury.

The potential contribution of stress systems to the transition to chronic whiplash-associated disorders

STUDY DESIGN

A narrative description highlighting preclinical and clinical evidence that physiologic stress systems contribute to whiplash-associated disorders (WAD) pathogenesis.

OBJECTIVE

To present several lines of evidence supporting the hypothesis that physiologic stress systems contribute to WAD pathogenesis.

SUMMARY OF BACKGROUND DATA

In addition to subjecting soft tissue to biomechanical strain, a motor vehicle collision (MVC) event is also an acute stressor which activates physiologic stress systems. Increasing data from animal and human studies suggest that the activation of these stress systems may contribute to long-lasting changes in pain sensitivity after tissue injury.

METHODS

Nonsystematic review of several lines of evidence that together suggest that physiologic systems involved in the stress response may contribute to the development of WAD.

RESULTS

Stress systems which appear capable of producing hyperalgesia and allodynia include catecholaminergic systems, serotonin systems, and the hypothalamic-pituitary-adrenocortical system. Evidence for the role of these systems comes, in part, from studies examining the association between genetic variants and chronic pain outcomes. For example, in a recent study of acute neck pain after MVC, patients with certain genotypes of an enzyme involved in catecholamine metabolism were more than twice as likely to report moderate or severe neck pain in the emergency department. Such pain vulnerability because of stress system function may interact with the effects of biomechanical injury and psychobehavioral responses to influence the development of WAD.

CONCLUSION

More research examining the influence of stress systems on WAD are needed. If these systems do influence WAD outcomes, then treatments which diminish the adverse effects of stress systems may be a useful component of multimodal therapeutic interventions for individuals at risk of chronic pain development after MVC.

Nerve growth factor enhances the excitability of rat sensory neurons through activation of the atypical protein kinase C isoform, PKMζ

Our previous work showed that nerve growth factor (NGF) increased the excitability of small-diameter capsaicin-sensitive sensory neurons by activating the p75 neurotrophin receptor and releasing sphingolipid-derived second messengers. Whole cell patch-clamp recordings were used to establish the signaling pathways whereby NGF augments action potential (AP) firing (i.e., sensitization). Inhibition of MEK1/2 (PD-98059), PLC (U-73122, neomycin), or conventional/novel isoforms of PKC (bisindolylmaleimide I) had no effect on the sensitization produced by NGF. Pretreatment with a membrane-permeable, myristoylated pseudosubstrate inhibitor of atypical PKCs (aPKCs: PKMζ, PKCζ, and PKCλ/ι) blocked the NGF-induced increase in AP firing. Inhibitors of phosphatidylinositol 3-kinase (PI3K) also blocked the sensitization produced by NGF. Isolated sensory neurons were also treated with small interfering RNA (siRNA) targeted to PKCζ. Both Western blots and quantitative real-time PCR established that PKMζ, but neither full-length PKCζ nor PKCλ/ι, was significantly reduced after siRNA exposure. Treatment with these labeled siRNA prevented the NGF-induced enhancement of excitability. Furthermore, consistent with the high degree of catalytic homology for aPKCs, internal perfusion with active recombinant PKCζ or PKCι augmented excitability, recapitulating the sensitization produced by NGF. Internal perfusion with recombinant PKCζ suppressed the total potassium current and enhanced the tetrodotoxin-resistant sodium current. Pretreatment with the myristoylated pseudosubstrate inhibitor blocked the increased excitability produced by ceramide or internal perfusion with recombinant PKCζ. These results demonstrate that NGF leads to the activation of PKMζ that ultimately enhances the capacity of small-diameter capsaicin-sensitive sensory neurons to fire APs through a PI3K-dependent signaling cascade.

Asians differ from non-Hispanic Whites in experimental pain sensitivity

This study examined differences between Asians and non-Hispanic Whites (Whites) in pain sensitivity, and its relationship to mean arterial pressure (MAP) and heart rate (HR). In 30 Whites (50% female) and 30 Asians (50% female), experimental pain sensitivity was assessed with a hand cold pressor task, yielding measures of pain threshold, tolerance, intensity, and unpleasantness. Mean arterial pressure and HR measurements taken at rest and in response to speech stress were assessed. Perceived stress, anxiety, perfectionism, parental criticism, parental expectations and depressive symptoms were also measured. The results indicated that for the cold pain test, Asians demonstrated significantly lower pain threshold and tolerance levels than Whites. Although no ethnic differences were seen for MAP or HR responses to stress, for Whites higher stress MAP levels were correlated with reduced pain sensitivity, while for Asians higher baseline and stress HR levels were correlated with reduced pain sensitivity. Asians reported higher parental expectations and greater parental criticism than Whites. For Asians only, higher levels of perfectionism were related to more depressive symptoms, anxiety and perceived stress. These results indicate that Asian Americans are more sensitive to experimental pain than Whites and suggest ethnic differences in endogenous pain regulatory mechanisms (e.g. MAP and HR). The results may also have implications for understanding ethnic differences in clinical pain.

GRK2 in sensory neurons regulates epinephrine-induced signalling and duration of mechanical hyperalgesia

Epinephrine (EPI) contributes to hyperalgesia in inflammatory and stress conditions. EPI signals via adrenoceptors, which are regulated by G protein-coupled receptor kinase 2 (GRK2). We previously reported that GRK2 is decreased in nociceptors during chronic inflammation. Herein, we investigated whether GRK2 modulates EPI-induced mechanical and thermal hyperalgesia by using GRK2(+/-) mice, which express 50% of the GRK2 protein. We demonstrate for the first time that EPI-induced mechanical as well as thermal hyperalgesia is prolonged to approximately 21 days in GRK2(+/-) mice, whereas it lasts only 3 to 4 days in wild-type mice. Using cell- specific GRK2-deficient mice, we further show that a low level of GRK2 in primary sensory neurons is critical for this prolongation of EPI-induced hyperalgesia. Low GRK2 in microglia had only a small effect on EPI-induced hyperalgesia. Low GRK2 in astrocytes did not alter EPI-induced hyperalgesia. EPI-induced hyperalgesia was prolonged similarly in mice with tamoxifen-induced homozygous or heterozygous deletion of GRK2. In terms of EPI signalling pathways, the protein kinase A (PKA) inhibitor H-89 inhibited EPI-induced mechanical hyperalgesia in wild-type mice, whereas H-89 had no effect in mice with low GRK2 in sensory neurons (SNS-GRK2(+/-) mice). Conversely, intraplantar injection of the protein kinase Cε PKCε inhibitor TAT-PKC(εv1-2) inhibited hyperalgesia in sensory neuron specific (SNS)-GRK2(+/-) mice and not in wild-type mice. These results indicate that low GRK2 in primary sensory neurons switches EPI-induced signalling from a protein kinase A-dependent toward a PKCε-dependent pathway that ultimately mediates prolonged EPI-induced hyperalgesia.

The role of protein kinase C epsilon in neural signal transduction and neurogenic diseases

Protein kinase C epsilon (PKC ɛ) is one of major isoforms in novel PKC family. Although it has been extensively characterized in the past decade, the role of PKC ɛ in neuron is still not well understood. Advances in molecular biology have now removed significant barriers to the direct investigation of PKC ɛ functions in vivo, and PKC ɛ has been increasingly implicated in the neural biological functions and associated neurogenic diseases. Recent studies have provided important insights into the influence of PKC ɛ on cortical processing at both the single cell level and network level. These studies provide compelling evidence that PKC ɛ could regulate distinct aspects of neural signal transduction and suggest that the coordinated actions of a number of molecular signals contribute to the specification and differentiation of PKC ɛ signal pathway in the developing brain.

Interleukin-13 reduces hyperalgesia and the level of interleukin-1β in BALB/c mice infected with Leishmania major with an up-regulation of interleukin-6

The anti-inflammatory cytokines interleukin-10 (IL-10) and interleukin-13 (IL-13) were shown to reduce hyperalgesia in some models such as rats exposed to UV rays. In addition, IL-10 was also shown to reduce hyperalgesia in high dose of Leishmania major-induced inflammation in BALB/c mice accompanied by a significant decrease in the levels of interleukin-1β (IL-1β) in the paws of infected mice, while no effect on the levels of IL-6 was observed. In this study, we injected BALB/c mice with a high dose of L. major and treated them with IL-13 (15 ng/animal) for twelve days (excluding the weekends) and hyperalgesia was assessed using thermal pain tests. Furthermore, the levels of IL-1β and IL-6 were also assessed at different post-infection days. Our results show that IL-6 and more importantly IL-1β don't play a direct role in the L. major-induced hyperalgesia and that IL-13 induces this hyperalgesia through the down-regulation of IL-1β and another proinflammatory cytokine (most probably TNF-α). Furthermore, our data show that IL-13 leads to the upregulation of the level IL-6 which initially seems to have no direct role in the induced hyperalgesia. Therefore, we suggest that the L. major-induced hyperalgesia is mainly mediated by the cytokine cascade leading to the production of sympathetic amines.

When does acute pain become chronic?

The transition from acute to chronic pain appears to occur in discrete pathophysiological and histopathological steps. Stimuli initiating a nociceptive response vary, but receptors and endogenous defence mechanisms in the periphery interact in a similar manner regardless of the insult. Chemical, mechanical, and thermal receptors, along with leucocytes and macrophages, determine the intensity, location, and duration of noxious events. Noxious stimuli are transduced to the dorsal horn of the spinal cord, where amino acid and peptide transmitters activate second-order neurones. Spinal neurones then transmit signals to the brain. The resultant actions by the individual involve sensory-discriminative, motivational-affective, and modulatory processes in an attempt to limit or stop the painful process. Under normal conditions, noxious stimuli diminish as healing progresses and pain sensation lessens until minimal or no pain is detected. Persistent, intense pain, however, activates secondary mechanisms both at the periphery and within the central nervous system that cause allodynia, hyperalgesia, and hyperpathia that can diminish normal functioning. These changes begin in the periphery with upregulation of cyclo-oxygenase-2 and interleukin-1β-sensitizing first-order neurones, which eventually sensitize second-order spinal neurones by activating N-methyl-d-aspartic acid channels and signalling microglia to alter neuronal cytoarchitecture. Throughout these processes, prostaglandins, endocannabinoids, ion-specific channels, and scavenger cells all play a key role in the transformation of acute to chronic pain. A better understanding of the interplay among these substances will assist in the development of agents designed to ameliorate or reverse chronic pain.

Catechol O-methyltransferase haplotype predicts immediate musculoskeletal neck pain and psychological symptoms after motor vehicle collision

Genetic variations in the catechol-O-methyltransferase (COMT) gene have been associated with experimental pain and risk of chronic pain development, but no studies have examined genetic predictors of neck pain intensity and other patient characteristics after motor vehicle collision (MVC). We evaluated the association between COMT genotype and acute neck pain intensity and other patient characteristics in 89 Caucasian individuals presenting to the emergency department (ED) after MVC. In the ED in the hours after MVC, individuals with a COMT pain vulnerable genotype were more likely to report moderate-to-severe musculoskeletal neck pain (76 versus 41%, RR = 2.11 (1.33-3.37)), moderate or severe headache (61 versus 33%, RR = 3.15 (1.05-9.42)), and moderate or severe dizziness (26 versus 12%, RR = 1.97 (1.19-3.21)). Individuals with a pain vulnerable genotype also experienced more dissociative symptoms in the ED, and estimated a longer time to physical recovery (median 14 versus 7 days, P = .002) and emotional recovery (median 8.5 versus 7 days, P = .038). These findings suggest that genetic variations affecting stress response system function influence the somatic and psychological response to MVC, and provide the first evidence of genetic risk for clinical symptoms after MVC.

PERSPECTIVE

The association of COMT genotype with pain symptoms, psychological symptoms, and recovery beliefs exemplifies the pleiotropic effects of stress-related genes, which may provide the biological substrate for the biopsychosocial model of post-MVC pain. The identification of genes associated with post-MVC symptoms may also provide new insights into pathophysiology.

Quantitative automated microscopy (QuAM) elucidates growth factor specific signalling in pain sensitization

Background

Dorsal root ganglia (DRG)-neurons are commonly characterized immunocytochemically. Cells are mostly grouped by the experimenter's eye as "marker-positive" and "marker-negative" according to their immunofluorescence intensity. Classification criteria remain largely undefined. Overcoming this shortfall, we established a quantitative automated microscopy (QuAM) for a defined and multiparametric analysis of adherent heterogeneous primary neurons on a single cell base.

The growth factors NGF, GDNF and EGF activate the MAP-kinase Erk1/2 via receptor tyrosine kinase signalling. NGF and GDNF are established factors in regeneration and sensitization of nociceptive neurons. If also the tissue regenerating growth factor, EGF, influences nociceptors is so far unknown. We asked, if EGF can act on nociceptors, and if QuAM can elucidate differences between NGF, GDNF and EGF induced Erk1/2 activation kinetics. Finally, we evaluated, if the investigation of one signalling component allows prediction of the behavioral response to a reagent not tested on nociceptors such as EGF.

Results

We established a software-based neuron identification, described quantitatively DRG-neuron heterogeneity and correlated measured sample sizes and corresponding assay sensitivity. Analysing more than 70,000 individual neurons we defined neuronal subgroups based on differential Erk1/2 activation status in sensory neurons. Baseline activity levels varied strongly already in untreated neurons. NGF and GDNF subgroup responsiveness correlated with their subgroup specificity on IB4(+)- and IB4(-)-neurons, respectively. We confirmed expression of EGF-receptors in all sensory neurons. EGF treatment induced STAT3 translocation into the nucleus. Nevertheless, we could not detect any EGF induced Erk1/2 phosphorylation. Accordingly, intradermal injection of EGF resulted in a fundamentally different outcome than NGF/GDNF. EGF did not induce mechanical hyperalgesia, but blocked PGE₂-induced sensitization.

Conclusions

QuAM is a suitable if not necessary tool to analyze activation of endogenous signalling in heterogeneous cultures. NGF, GDNF and EGF stimulation of DRG-neurons shows differential Erk1/2 activation responses and a corresponding differential behavioral phenotype. Thus, in addition to expression-markers also signalling-activity can be taken for functional subgroup differentiation and as predictor of behavioral outcome. The anti-nociceptive function of EGF is an intriguing result in the context of tissue damage but also for understanding pain resulting from EGF-receptor block during cancer therapy.

Caspase-1 is involved in the genesis of inflammatory hypernociception by contributing to peripheral IL-1β maturation

Background

Caspase-1 is a cysteine protease responsible for the processing and secretion of IL-1β and IL-18, which are closely related to the induction of inflammation. However, limited evidence addresses the participation of caspase-1 in inflammatory pain. Here, we investigated the role of caspase-1 in inflammatory hypernociception (a decrease in the nociceptive threshold) using caspase-1 deficient mice (casp1-/-).

Results

Mechanical inflammatory hypernociception was evaluated using an electronic version of the von Frey test. The production of cytokines, PGE₂ and neutrophil migration were evaluated by ELISA, radioimmunoassay and myeloperoxidase activity, respectively. The interleukin (IL)-1β and cyclooxygenase (COX)-2 protein expression were evaluated by western blotting. The mechanical hypernociception induced by intraplantar injection of carrageenin, tumour necrosis factor (TNF)α and CXCL1/KC was reduced in casp1-/- mice compared with WT mice. However, the hypernociception induced by IL-1β and PGE₂ did not differ in WT and casp1-/- mice. Carrageenin-induced TNF-α and CXCL1/KC production and neutrophil recruitment in the paws of WT mice were not different from casp1-/- mice, while the maturation of IL-1β was reduced in casp1-/- mice. Furthermore, carrageenin induced an increase in the expression of COX-2 and PGE₂ production in the paw of WT mice, but was reduced in casp1-/- mice.

Conclusion

These results suggest that caspase-1 plays a critical role in the cascade of events involved in the genesis of inflammatory hypernociception by promoting IL-1β maturation. Because caspase-1 is involved in the induction of COX-2 expression and PGE₂ production, our data support the assertion that caspase-1 is a key target to control inflammatory pain.

The val158met polymorphism of human catechol-O-methyltransferase (COMT) affects anterior cingulate cortex activation in response to painful laser stimulation

Background

Pain is a complex experience with sensory, emotional and cognitive aspects. Genetic and environmental factors contribute to pain-related phenotypes such as chronic pain states. Genetic variations in the gene coding for catechol-O-methyltransferase (COMT) have been suggested to affect clinical and experimental pain-related phenotypes including regional μ-opioid system responses to painful stimulation as measured by ligand-PET (positron emission tomography). The functional val¹⁵⁸met single nucleotide polymorphism has been most widely studied. However, apart from its impact on pain-induced opioid release the effect of this genetic variation on cerebral pain processing has not been studied with activation measures such as functional magnetic resonance imaging (fMRI), PET or electroencephalography. In the present fMRI study we therefore sought to investigate the impact of the COMT val¹⁵⁸met polymorphism on the blood oxygen level-dependent (BOLD) response to painful laser stimulation.

Results

57 subjects were studied. We found that subjects homozygous for the met158 allele exhibit a higher BOLD response in the anterior cingulate cortex (ACC), foremost in the mid-cingulate cortex, than carriers of the val158 allele.

Conclusion

This result is in line with previous studies that reported higher pain sensitivity in homozygous met carriers. It adds to the current literature in suggesting that this behavioral phenotype may be mediated by, or is at least associated with, increased ACC activity. More generally, apart from one report that focused on pain-induced opioid release, this is the first functional neuroimaging study showing an effect of the COMT val¹⁵⁸met polymorphism on cerebral pain processing.

Effect of catechol-O-methyltransferase polymorphism on response to propranolol therapy in chronic musculoskeletal pain: a randomized, double-blind, placebo-controlled, crossover pilot study

INTRODUCTION

Three common haplotypes in the gene encoding catechol-O-methyltransferase (COMT) have been associated with pain modulation and the risk of developing chronic musculoskeletal pain, namely temporomandibular disorder (TMD). Haplotypes coding for higher enzymatic activity were correlated with lower pain perception. Rodent studies showed that COMT inhibition increases pain sensitivity through beta2/3-adrenergic receptors. We hypothesized that the nonselective beta-adrenergic antagonist propranolol will reduce clinical and experimental pain in TMD patients in a manner dependent on the individuals' COMT diplotype.

METHODS

Forty Caucasian female participants meeting the Research Diagnostic Criteria for TMD were genotyped for COMT polymorphisms and completed a randomized, double-blind, placebo-controlled, two-period crossover pilot study. Each period consisted of a baseline assessment week followed by an intervention week (propranolol or placebo). Changes in clinical pain ratings, psychological status, and responses to heat and pressure stimuli between baseline and intervention weeks were compared across periods.

RESULTS

The number of patients reporting a reduction in pain intensity rating was greater during propranolol treatment (P=0.014) compared with placebo. Propranolol significantly reduced a composite pain index (P=0.02) but did not decrease other clinical and experimental pain ratings. When stratified by the COMT high activity haplotype, a beneficial effect of propranolol on pain perception was noted in patients not carrying this haplotype, a diminished benefit was observed in the heterozygotes, and no benefit was noted in the homozygotes.

CONCLUSION

COMT haplotypes may serve as genetic predictors of propranolol treatment outcome, identifying a subgroup of TMD patients who will benefit from propranolol therapy.

Beta2-adrenoceptor agonists alleviate neuropathic allodynia in mice after chronic treatment

BACKGROUND AND PURPOSE

Antidepressants are a first-line treatment against neuropathic pain. We previously demonstrated that beta(2)-adrenoceptors are necessary for antidepressants to exert their anti-allodynic action. The aim of the present study was to assess whether beta(2)-adrenoceptor agonists could be sufficient to alleviate neuropathic allodynia.

EXPERIMENTAL APPROACH

We used a murine model of neuropathy induced by unilateral sciatic nerve cuffing in C57BL/6J mice. We previously demonstrated that this animal model is sensitive to chronic, but not to acute, treatment with antidepressant drugs, which is clinically relevant. The mechanical allodynia was evaluated using the von Frey filaments.

KEY RESULTS

We showed that chronic but not acute treatment with the beta-adrenoceptor agonists, bambuterol, isoprenaline, fenoterol, salbutamol, salmeterol, terbutaline or ritodrine suppressed mechanical allodynia. We confirmed that the action of these beta-adrenoceptor agonists was mediated through beta(2)-adrenoceptors by blocking it with intraperitoneal or intrathecal, but not intracerebroventricular or intraplantar, injections of the antagonist ICI118551. We also showed that chronic treatments with the beta-adrenoceptor antagonists, propranolol or ICI118551 did not suppress the allodynia.

CONCLUSIONS AND IMPLICATIONS

Our data show that chronic treatment with beta-adrenoceptor agonists has the same antiallodynic properties as treatments with antidepressant drugs. This study was, however, conducted in an animal model, and a clinical validation will be required to confirm the value of the present findings in patients.

Morphine peripheral analgesia depends on activation of the PI3Kgamma/AKT/nNOS/NO/KATP signaling pathway

Morphine is one of the most prescribed and effective drugs used for the treatment of acute and chronic pain conditions. In addition to its central effects, morphine can also produce peripheral analgesia. However, the mechanisms underlying this peripheral action of morphine have not yet been fully elucidated. Here, we show that the peripheral antinociceptive effect of morphine is lost in neuronal nitric-oxide synthase null mice and that morphine induces the production of nitric oxide in primary nociceptive neurons. The activation of the nitric-oxide pathway by morphine was dependent on an initial stimulation of PI3Kgamma/AKT protein kinase B (AKT) and culminated in increased activation of K(ATP) channels. In the latter, this intracellular signaling pathway might cause a hyperpolarization of nociceptive neurons, and it is fundamental for the direct blockade of inflammatory pain by morphine. This understanding offers new targets for analgesic drug development.

Neurobiological mechanisms of pain in sickle cell disease

Pain is a frequent complaint of people living with sickle cell disease (SCD); however, the neurobiology of pain in SCD remains poorly understood. Whereas this pain has been thought to be primarily related to visceral and somatic tissue injury subsequent to vaso-occlusion events, emerging evidence from human and animal studies has suggested that a component of SCD pain may be related to neuropathic processes. Significant knowledge has been obtained from studies of molecular and neurobiological mechanisms leading to and maintaining neuropathic pain. Some of the most promising evidence has implicated major roles of protein kinase C and Ca2+/calmodulin-dependent protein kinase II, and their interaction with the N-methyl-D-aspartate receptors and the transient receptor potential vanilloid 1 receptor in the development of neuropathic pain. The latest evidence from our studies suggests that these pathways are important for SCD pain as well. Coupled with emerging animal models of SCD pain, we can now start to elucidate neurobiological mechanisms underlying pain in SCD, which may lead to better understanding and effective therapies.

Adrenergic stimulation mediates visceral hypersensitivity to colorectal distension following heterotypic chronic stress

BACKGROUND & AIMS

Chronic stress exacerbates or causes relapse of symptoms such as abdominal pain and cramping in patients with irritable bowel syndrome. We investigated whether chronic stress increases plasma norepinephrine and sensitizes colon-specific dorsal root ganglion (DRG) neurons by increasing expression of nerve growth factor (NGF) in the colon wall.

METHODS

Heterotypic chronic stress (HeCS) was applied to male Wistar rats and neurologic and molecular responses were analyzed. Tissues were analyzed for NGF expression.

RESULTS

HeCS significantly increased visceromoter response to colorectal distension; expression of NGF increased in colonic muscularis externa and mucosa/submucosa. Rheobase decreased, resting membrane potential was depolarized, and electrogenesis of action potentials increased in colon-specific thoracolumbar DRG neurons. Luminal administration of resiniferatoxin in distal colon, systemic administration of anti-NGF antibody, or inhibition of the NGF receptor trkA by k252a or antisense oligonucleotides in thoracolumbar DRG blocked the chronic stress-induced visceral hypersensitivity to colorectal distension. Blockade of alpha1/alpha2- and beta1/beta2-adrenergic receptors prevented the stress-induced visceral hypersensitivity and increased expression of NGF in the colon wall. HeCS did not induce any inflammatory response in the colon wall.

CONCLUSIONS

The peripheral stress mediator norepinephrine induces visceral hypersensitivity to colorectal distension in response to HeCS by increasing the expression of NGF in the colon wall, which sensitizes primary afferents in the absence of an inflammatory response.

IL-17 mediates articular hypernociception in antigen-induced arthritis in mice

IL-17 is an important cytokine in the physiopathology of rheumatoid arthritis (RA). However, its participation in the genesis of nociception during RA remains undetermined. In this study, we evaluated the role of IL-17 in the genesis of articular nociception in a model of antigen (mBSA)-induced arthritis. We found that mBSA challenge in the femur-tibial joint of immunized mice induced a dose- and time-dependent mechanical hypernociception. The local IL-17 concentration within the mBSA-injected joints increased significantly over time. Moreover, co-treatment of mBSA challenged mice with an antibody against IL-17 inhibited hypernociception and neutrophil recruitment. In agreement, intraarticular injection of IL-17 induced hypernociception and neutrophil migration, which were reduced by the pre-treatment with fucoidin, a leukocyte adhesion inhibitor. The hypernociceptive effect of IL-17 was also reduced in TNFR1(-/-) mice and by pre-treatment with infliximab (anti-TNF antibody), a CXCR1/2 antagonist or by an IL-1 receptor antagonist. Consistent with these findings, we found that IL-17 injection into joints increased the production of TNF-alpha, IL-1beta and CXCL1/KC. Treatment with doxycycline (non-specific MMPs inhibitor), bosentan (ET(A)/ET(B) antagonist), indomethacin (COX inhibitor) or guanethidine (sympathetic blocker) inhibited IL-17-induced hypernociception. IL-17 injection also increased PGE(2) production, MMP-9 activity and COX-2, MMP-9 and PPET-1 mRNA expression in synovial membrane. These results suggest that IL-17 is a novel pro-nociceptive cytokine in mBSA-induced arthritis, whose effect depends on both neutrophil migration and various pro-inflammatory mediators, as TNF-alpha, IL-1beta, CXCR1/2 chemokines ligands, MMPs, endothelins, prostaglandins and sympathetic amines. Therefore, it is reasonable to propose IL-17 targeting therapies to control this important RA symptom.

Acute and persistent nociceptive paw sensitisation in mice: the involvement of distinct signalling pathways

AIMS

Many fundamental pharmacological studies in pain and inflammation have been performed on rats. However, the pharmacological findings were generally not extended to other species in order to increase their predictive therapeutic value. We studied acute and chronic inflammatory nociceptive sensitisation of mouse hind paws by prostaglandin E(2) (PGE(2)) or dopamine (DA), as previously described in rats. We also investigated the participation of the signalling pathways in acute and persistent sensitisation.

MAIN METHODS

Mechanical sensitisation (hypernociception) induced by intraplantar administrations of PGE(2) or DA was evaluated with an electronic pressure meter. The signalling pathways were pharmacologically investigated with the pre-administration of adenylyl cyclase (AC), cAMP-dependent protein kinase (PKA), protein kinase Cepsilon (PKCepsilon), and the extracellular signal-related kinase (ERK) inhibitors.

KEY FINDINGS

Single or 14days of successive intraplantar injections of PGE(2) or DA-induced acute and persistent hypernociception (lasting for more than 30days), respectively. The involvement of AC, PKA or PKCepsilon was observed in the acute hypernociception induced by PGE(2), while PKA or PKCepsilon were continuously activated during the period of persistent hypernociception. The acute hypernociception induced by DA involves activation of ERK, PKCepsilon, AC or PKA, while persistent hypernociception implicated ERK activation, but not PKA, PKCepsilon or AC.

SIGNIFICANCE

In mice, acute and persistent paw sensitisation involves the different activation of kinases, as previously described for rats. This study opens the possibility of comparing pharmacological approaches in both species to further understand acute and chronic inflammatory sensitisation, and possibly associated genetic manipulations.

Epinephrine, phenylephrine, and methoxamine induce infiltrative anesthesia via alpha1-adrenoceptors in rats

AIM

To assess whether epinephrine, phenylephrine, and methoxamine act via certain subtypes of adrenoceptors to exert their local anesthetic activity.

METHODS

We investigated cutaneous anesthesia from adrenoceptor agonists and/or antagonists in conscious, unanesthetized Sprague-Dawley male rats (weight 200-250 g). Cutaneous anesthesia was evidenced by a block of the cutaneous trunci muscle reflex, which is characterized by reflex movement of the skin over the back produced by twitches of lateral thoracispinal muscles in response to local dorsal cutaneous noxious pinprick.

RESULTS

Local infiltration of epinephrine, L-phenylephrine, or methoxamine alone induces cutaneous anesthesia in rats in a dose-dependent way. Epinephrine is found to be 19 and 29 times more potent than those of methoxamine and L-phenylephrine, respectively. The cutaneous anesthesia induced by epinephrine, phenylephrine, or methoxamine can be significantly reduced by alpha(1)-adrenoceptor antagonists (eg, prazosin), alpha1, alpha2-adrenoceptor antagonist, alpha(1A)-adrenoceptor antagonist (eg, 5-methylurapdil), alpha(1B)-adrenoceptor antagonist (eg, chloroethylclonidine), or alpha(1D)-adrenoceptor antagonist (eg, BMY7873).

CONCLUSION

Our results indicate that epinephrine, phenylephrine and methoxamine all act mainly via mixed subtypes of alpha(1)-adrenoceptors to induce cutaneous anesthesia in the rat.

Variation in the COMT gene: implications for pain perception and pain treatment

Catechol-O-methyltransferase (COMT) is an enzyme that inactivates biologically-active catechols, including the important neurotransmitters dopamine, noradrenaline and adrenaline. These neurotransmitters are involved in numerous physiological processes, including modulation of pain. Genetic variation in the COMT gene has been implicated in variable response to various experimental painful stimuli, variable susceptibility to develop common pain conditions, as well as the variable need for opioids in the treatment of cancer pain. Increased insight into how genetic variants within the COMT locus affect pain perception will contribute to improved understanding of the mechanisms involved in the development of common human pain disorders and may lead to improved strategies for pain treatment. So far, a remarkable complex relationship between COMT genotypes or haplotypes and pain phenotypes has been revealed.

Activation of peripheral kappa/delta opioid receptors mediates 15-deoxy-(Delta12,14)-prostaglandin J2 induced-antinociception in rat temporomandibular joint

This study assessed the effect of the agonist 15d-PGJ(2) administered into the rat temporomandibular joint (TMJ) on nociceptive behavioral and the anti-inflammatory potential of this prostaglandin on TMJ. It was observed that 15-deoxy-(Delta12,14)-prostaglandin J(2) (15d-PGJ(2)) significantly reduced formalin-induced nociceptive behavior in a dose dependent manner, however injection of 15d-PGJ(2) into the contralateral TMJ failed to reduce such effects. This antinociceptive effect is dependent on peroxisome proliferator-activated receptors-gamma (PPAR-gamma) since pre-treatment with GW9662 (PPAR-gamma receptor antagonist) blocked the antinociceptive effect of 15d-PGJ(2) in the TMJ. In addition, the antinociceptive effect of 15d-PGJ(2) was also blocked by naloxone suggesting the involvement of peripheral opioids in the process. Confirming this hypothesis pre-treatment with kappa, delta, but not mu receptor antagonists significantly reduced the antinociceptive effect of 15d-PGJ(2) in the TMJ. Similarly to opioid agonists, the 15d-PGJ(2) antinociceptive action depends on the nitric oxide (NO)/guanilate cyclase (cGMP)/ATP-sensitive potassium channel blocker(K(+)(ATP)) channel pathway since it was prevented by the pre-treatment with the inhibitors of nitric oxide synthase (NOS; aminoguanidine), cGMP (ODQ), or the K(+)(ATP) (glibenclamide). In addition, 15d-PGJ(2) (100 ng/TMJ) inhibits 5-HT-induced TMJ hypernociception. Besides, TMJ treated with 15d-PGJ(2) showed lower vascular permeability, assessed by Evan's Blue extravasation, and also lower neutrophil migration induced by carrageenan administration. Taken together, these results demonstrate that 15d-PGJ(2) has a potential peripheral antinociceptive and anti-inflammatory effect in the TMJ via PPAR-gamma activation. The results also suggest that 15d-PGJ(2) induced-peripheral antinociceptive response in the TMJ is mediated by kappa/delta opioid receptors by the activation of the intracellular l-arginine/NO/cGMP/K(+)(ATP) channel pathway. The pharmacological properties of the peripheral administration of 15d-PGJ(2) highlight the potential use of this PPAR-gamma agonist on TMJ inflammatory pain conditions.

Local antinociception induced by endothelin-1 in the hairy skin of the rat's back

Subcutaneous injection of endothelin-1 (ET-1) into the glabrous skin of the rat's hind paw is known to produce impulses in nociceptors and acute nocifensive behavioral responses, such as hind paw flinching, and to sensitize the skin to mechanical and thermal stimulation. In this report, we show that in contrast to the responses in glabrous skin, ET-1 injected subcutaneously into rat hairy skin causes transient antinociception. Concentrations of 1 to 50 microM ET-1 (in 0.05 mL) depress the local nocifensive response to noxious tactile probing at the injection site with von Frey filaments for 30 to 180 minutes; distant injections have no effect at this site, showing that the response is local. Selective inhibition of ET(A) but not of ET(B) receptors inhibits this antinociception, as does coinjection with nimodipine (40 muM), a blocker of L-type Ca(2+) channels. Local subcutaneous injection of epinephrine (45 microM) also causes antinociception through alpha-1 adrenoreceptors, but such receptors are not involved in the ET-1-induced effect. Both epinephrine and ET-1, at antinociceptive concentrations, reduce blood flow in the skin; the effect from ET-1 is largely prevented by subcutaneous nimodipine. These data suggest that ET-1-induced antinociception in the hairy skin of the rat involves cutaneous vasoconstriction, presumably through neural ischemia, resulting in conduction block.

PERSPECTIVE

The pain-inducing effects of ET-1 have been well documented in glabrous skin of the rat, a frequently used test site. The opposite behavioral effect, antinociception, occurs from ET-1 in hairy skin and is correlated with a reduction in blood flow. Vasoactive effects are important in assessing mechanisms of peripherally acting agents.

Adrenergic dysregulation and pain with and without acute beta-blockade in women with fibromyalgia and temporomandibular disorder

In patients with fibromyalgia syndrome (FMS) and temporomandibular disorder (TMD), stress and pain may chronically enhance sympathetic activity, altering cardiovascular responses and worsening pain. This study examined cardiovascular, epinephrine (EPI), norepinephrine (NE), cortisol and clinical pain responses in 54 female patients with these disorders and 34 controls. In a subsample of 10 FMS, 10 TMD patients and 16 controls, using a counterbalanced, double-blind, crossover design, the same responses were assessed after intravenous administration of low dose propranolol vs placebo. Testing included baseline, postural, speech and ischemic pain stressors. FMS patients showed lesser heart rate (HR) increases to posture challenge but greater blood pressure (BP) increases to postural and speech tasks than controls, as well as higher overall BP and greater total vascular resistance (TVR) than TMDs or controls. TMDs showed higher overall cardiac output and lower TVR than controls. Both FMS and TMD groups showed lower baseline NE than controls, and TMDs showed lower overall EPI and NE levels. Group differences in HR, EPI and NE were abolished after propranolol although BP, CO and TVR differences persisted. In both FMS and TMD, the number of painful body sites and ratings of total clinical pain obtained 4 times during each session were significantly lower after beta-blockade vs placebo.

PERSPECTIVE

These findings support the hypothesis that both FMS and TMD may frequently involve dysregulation of beta-adrenergic activity that contributes to altered cardiovascular and catecholamine responses and to severity of clinical pain. Acute treatment with low-dose propranolol led to short-term improvement in all these domains.

The inner ear is involved in the aggravation of nociceptive behavior induced by lowering barometric pressure of nerve injured rats

Patients suffering from neuropathic pain often complain of pain aggravation when the weather is changing. The exact mechanism for weather change-induced pain has not been clarified. We have previously demonstrated that experimentally lowering barometric pressure (LP) intensifies pain-related behaviors in rats with chronic constriction injury (CCI). In the present experiment we examined whether this pain aggravating effect of LP exposure in nerve injured rats is still present after lesioning of the inner ear. We used both CCI and spinal nerve ligation (SNL) models for this study. We injected into the middle ear sodium arsanilate solution (100mg/ml, 50microl/ear), which is known to degenerate vestibular hair cells, under anesthesia the day before surgery. Rats were exposed to LP (27hPa decrease over 8min) 7-9 days after CCI or 5-8 days after SNL surgery, and pain-related behavior (number of paw lifts induced by von Frey hair stimuli) was measured. When the inner ear lesioned SNL or CCI rats were exposed to LP, they showed no augmentation of pain-related behavior. On the other hand, the pain aggravating effect of a temperature decrease (from 24 to 17 degrees C) was maintained in both SNL and CCI rats. These results suggest that the barometric sensor/sensing system influencing nociceptive behavior during LP in rats is located in the inner ear.

The role of PKA and PKCepsilon pathways in prostaglandin E2-mediated hypernociception

BACKGROUND AND PURPOSE

Protein kinase (PK) A and the epsilon isoform of PKC (PKCepsilon) are involved in the development of hypernociception (increased sensitivity to noxious or innocuous stimuli) in several animal models of acute and persistent inflammatory pain. The present study evaluated the contribution of PKA and PKCepsilon to the development of prostaglandin E(2) (PGE(2))-induced mechanical hypernociception.

EXPERIMENTAL APPROACH

Prostaglandin E(2)-induced mechanical hypernociception was assessed by constant pressure rat paw test. The activation of PKA or PKCepsilon was evaluated by radioactive enzymic assay in the dorsal root ganglia (DRG) of sensory neurons from the hind paws.

KEY RESULTS

Hypernociception induced by PGE(2) (100 ng) by intraplantar (i.pl.) injection, was reduced by i.pl. treatment with inhibitors of PKA [A-kinase-anchoring protein St-Ht31 inhibitor peptide (AKAPI)], PKCepsilon (PKCepsilonI) or adenylyl cyclase. PKA activity was essential in the early phase of the induction of hypernociception, whereas PKC activity was involved in the maintenance of the later phase of hypernociception. In the DRG (L4-L5), activity of PKA increased at 30 min after injection of PGE(2) but PKC activity increased only after 180 min. Moreover, i.pl. injection of the catalytic subunit of PKA induced hypernociception which was markedly reduced by pretreatment with an inhibitor of PKCepsilon, while the hypernociception induced by paw injection of PKCepsilon agonist was not affected by an inhibitor of PKA (AKAPI).

CONCLUSIONS AND IMPLICATIONS

Taken together, these findings are consistent with the suggestion that PKA activates PKCepsilon, which is a novel mechanism of interaction between these kinases during the development of PGE(2)-induced mechanical hypernociception.

Nerve growth factor acts with the beta2-adrenoceptor to induce spontaneous nociceptive behavior during temporomandibular joint inflammatory hyperalgesia

AIMS

The aim of this study was to investigate whether the injection of nerve growth factor induces spontaneous nociceptive behavior in the intact or sensitized temporomandibular joint (TMJ) of rats.

MAIN METHODS

NGF was injected into the TMJ 1 h after the TMJ injection of saline or carrageenan and the spontaneous nociceptive behavior was quantified. The mechanism involved in this phenomenon was investigated by the injection of NGF into the carrageenan-sensitized TMJ in the presence of indomethacin or of beta-adrenergic antagonists.

KEY FINDINGS

NGF injected into the TMJ sensitized by a prior TMJ injection of carrageenan but not into the intact TMJ induced a significant nociceptive behavior. Co-injection of the non-specific Trk receptor antagonist k252A with NGF 1 h after the TMJ injection of carrageenan significantly reduced NGF-induced spontaneous nociception supporting the Trk receptor activation in this nociceptive effect. Blockade of prostaglandin synthesis by indomethacin before the TMJ injection of carrageenan did not reduce NGF-induced nociception. Co-administration of carrageenan with the beta2-adrenoceptor antagonist ICI 118.55 but not with the beta1-adrenoceptor antagonist atenolol significantly reduced NGF-induced nociception. The injection of NGF the TMJ sensitized by a previous TMJ injection of epinephrine also induced nociceptive behavior.

SIGNIFICANCE

Taken together, these results indicate that NGF can induce TMJ nociception during TMJ inflammation. Moreover, the expression of this nociceptive response seems to depend on the synergic activity of NGF and sympathetic amines released during TMJ inflammation acting on beta2-adrenergic receptors.

Is it all about sex? Acupuncture for the treatment of pain from a biological and gender perspective

Pain is a unique personal experience showing variability where gender and sex related effects might contribute. The mechanisms underlying the differences between women and men are currently unknown but are likely to be complex and involving interactions between biological, sociocultural and psychological aspects. In women, painful experimental stimuli are generally reported to produce a greater intensity of pain than in men. Clinical pain is often reported with higher severity and frequency, longer duration, and present in a greater number of body regions in women than in men. Women are also more likely to experience a number of painful conditions such as fibromyalgia, temporomandibular dysfunction, migraine, rheumatoid arthritis and irritable bowel syndrome. With regard to biological factors, quantitative as well as qualitative differences in the endogenous pain inhibitory systems have been implicated, as well as an influence of gonadal hormones. Psychosocial factors like sex role beliefs, pain coping strategies, and pain related expectancies may also contribute to the differences. Being exposed to repeated painful visceral events (eg menses, labour) during life may contribute to an increased sensitivity to, and greater prevalence of, pain among women. When assessing the outcome of pharmacological and non-pharmacological therapies in pain treatment, the factors of gender and sex should be taken into account as the response to an intervention may differ. Preferably, treatment recommendations should be based on studies using both women and men as the norm. Due to variability in results, findings from animal studies and experiments in healthy subjects should be interpreted with care.

Stress induces a switch of intracellular signaling in sensory neurons in a model of generalized pain

Stress dramatically exacerbates pain in diseases such as fibromyalgia and rheumatoid arthritis, but the underlying mechanisms are unknown. We tested the hypothesis that stress causes generalized hyperalgesia by enhancing pronociceptive effects of immune mediators. Rats exposed to nonhabituating sound stress exhibited no change in mechanical nociceptive threshold, but showed a marked increase in hyperalgesia evoked by local injections of prostaglandin E(2) or epinephrine. This enhancement, which developed more than a week after exposure to stress, required concerted action of glucocorticoids and catecholamines at receptors located in the periphery on sensory afferents. The altered response to pronociceptive mediators involved a switch in coupling of their receptors from predominantly stimulatory to inhibitory G-proteins (G(s) to G(i)), and for prostaglandin E(2), emergence of novel dependence on protein kinase C epsilon. Thus, an important mechanism in generalized pain syndromes may be stress-induced coactivation of the hypothalamo-pituitary-adrenal and sympathoadrenal axes, causing a long-lasting alteration in intracellular signaling pathways, enabling normally innocuous levels of immune mediators to produce chronic hyperalgesia.

Treatment with DF 2162, a non-competitive allosteric inhibitor of CXCR1/2, diminishes neutrophil influx and inflammatory hypernociception in mice

BACKGROUND AND PURPOSE

Neutrophil migration into tissues is involved in the genesis of inflammatory pain. Here, we addressed the hypothesis that the effect of CXC chemokines on CXCR1/2 is important to induce neutrophil migration and inflammatory hypernociception.

EXPERIMENTAL APPROACH

Mice were treated with a non-competitive allosteric inhibitor of CXCR1/2, DF 2162, and neutrophil influx and inflammatory hypernociception were assessed by myeloperoxidase assay and electronic pressure meter test, respectively, in various models of inflammation.

KEY RESULTS

DF 2162 inhibited neutrophil chemotaxis induced by CXCR1/2 ligands but had no effect on CXCL8 binding to neutrophils. A single mutation of the allosteric site at CXCR1 abrogated the inhibitory effect of DF 2162 on CXCL-8-induced chemotaxis. Treatment with DF 2162 prevented influx of neutrophils and inflammatory hypernociception induced by CXCL1 in a dose-dependent manner. The compound inhibited neutrophil influx and inflammatory hypernociception induced by carrageenan, lipopolysaccharide and zymosan, but not hypernociception induced by dopamine and PGE(2). DF 2162 had a synergistic effect with indomethacin or the absence of TNFR1 to abrogate carrageenan-induced hypernociception. Treatment with DF 2162 diminished neutrophil influx, oedema formation, disease score and hypernociception in collagen-induced arthritis.

CONCLUSIONS AND IMPLICATIONS

CXCR1/2 mediates neutrophil migration and is involved in the cascade of events leading to inflammatory hypernociception. In addition to modifying fundamental pathological processes, non-competitive allosteric inhibitors of CXCR1/2 may have the additional benefit of providing partial relief for pain and, hence, may be a valid therapeutic target for further studies aimed at the development of new drugs for the treatment of rheumatoid arthritis.

Chronic morphine administration enhances nociceptive sensitivity and local cytokine production after incision

BACKGROUND

The chronic use of opioids prior to surgery leads to lowered pain thresholds and exaggerated pain levels after these procedures. Several mechanisms have been proposed to explain this heightened sensitivity commonly termed opioid-induced hyperalgesia (OIH). Most of these proposed mechanisms involve plastic events in the central or peripheral nervous systems. Alterations in the abundance of peripheral mediators of nociception have not previously been explored.

RESULTS

In these experiments mice were treated with saline (control) or ascending daily doses of morphine to generate a state of OIH followed by hind paw incision. In other experiments morphine treatment was initiated at the time of incision. Both mechanical allodynia and peri-incisional skin cytokine levels were measured. Myeloperoxidase (MPO) assays were used to determine neutrophil activity near the wounds. The cytokine production inhibitor pentoxifylline was used to determine the functional significance of the excess cytokines in previously morphine treated animals. Mice treated chronically treated with morphine prior to incision were found to have enhanced skin levels of IL-1beta, IL-6, G-CSF, KC and TNFalpha after incision at one or more time points compared to saline pretreated controls. The time courses of individual cytokines followed different patterns. There was no discernable effect of chronic morphine treatment on wound area neutrophil infiltration. Pentoxifylline reduced cytokine levels and reversed the excess mechanical sensitization caused by chronic morphine administration prior to incision. Morphine treatment initiated at the time of incision did not lead to a generalized enhancement of cytokine production or nociceptive sensitization in excess of the levels observed after incision alone.

CONCLUSION

The enhanced level of nociceptive sensitization seen after incision in animals chronically exposed to morphine is associated with elevated levels of several cytokines previously reported to be relevant to this incisional pain model. The cytokines may be functional in supporting nociceptive sensitization because pentoxifylline reverses both peri-incisional skin cytokine levels and OIH. Opioid administration beginning at the time of incision does not seem to have the same cytokine enhancing effect. Approaches to postoperative pain control involving a reduction of cytokines may be an effective way to control excessive pain in patients chronically using opioids prior to surgical procedures.

Commensal microbiota is fundamental for the development of inflammatory pain

The ability of an individual to sense pain is fundamental for its capacity to adapt to its environment and to avoid damage. The sensation of pain can be enhanced by acute or chronic inflammation. In the present study, we have investigated whether inflammatory pain, as measured by hypernociceptive responses, was modified in the absence of the microbiota. To this end, we evaluated mechanical nociceptive responses induced by a range of inflammatory stimuli in germ-free and conventional mice. Our experiments show that inflammatory hypernociception induced by carrageenan, lipopolysaccharide, TNF-alpha, IL-1beta, and the chemokine CXCL1 was reduced in germ-free mice. In contrast, hypernociception induced by prostaglandins and dopamine was similar in germ-free or conventional mice. Reduction of hypernociception induced by carrageenan was associated with reduced tissue inflammation and could be reversed by reposition of the microbiota or systemic administration of lipopolysaccharide. Significantly, decreased hypernociception in germ-free mice was accompanied by enhanced IL-10 expression upon stimulation and could be reversed by treatment with an anti-IL-10 antibody. Therefore, these results show that contact with commensal microbiota is necessary for mice to develop inflammatory hypernociception. These findings implicate an important role of the interaction between the commensal microbiota and the host in favoring adaptation to environmental stresses, including those that cause pain.