β2- and β3-adrenergic receptors drive COMT-dependent pain by increasing production of nitric oxide and cytokines

Genetics of ischemic stroke functional outcome

Ischemic stroke, which accounts for 87% of cerebrovascular accidents, is responsible for massive global burden both in terms of economic cost and personal hardship. Many stroke survivors face long-term disability-a phenotype associated with an increasing number of genetic variants. While clinical variables such as stroke severity greatly impact recovery, genetic polymorphisms linked to functional outcome may offer physicians a unique opportunity to deliver personalized care based on their patient's genetic makeup, leading to improved outcomes. A comprehensive catalogue of the variants at play is required for such an approach. In this review, we compile and describe the polymorphisms associated with outcome scores such as modified Rankin Scale and Barthel Index. Our search identified 74 known genetic polymorphisms spread across 48 features associated with various poststroke disability metrics. The known variants span diverse biological systems and are related to inflammation, vascular homeostasis, growth factors, metabolism, the p53 regulatory pathway, and mitochondrial variation. Understanding how these variants influence functional outcome may be helpful in maximizing poststroke recovery.

The association of gene polymorphisms in catechol-O'methyltransferase (COMT) and β2-adrenergic receptor (ADRB2) with temporomandibular joint disorders

OBJECTIVE

Temporomandibular disorder (TMD) has a multifactorial etiology that includes environmental, psychological, and genetic factors. This study aimed to evaluate the possible relationship between polymorphisms in Catechol-O-methyltransferase (COMT) and β2-adrenergic receptor (ADRB2) genes with TMD.

DESIGN

This observational case-control study included 80 patients and 70 healthy controls. The diagnosis of TMD was made using the diagnostic criteria for TMD and the following TMD categories were used for the case group: muscular TMD and articular TMD (disc displacement and arthralgia). A genotyping study of gene polymorphisms in COMT (rs 9332377) and ADRB2 (rs20530449) was performed from genomic DNA isolated from blood. The chi-square test was used to analyze the relationships. P < 0.05 was accepted as a significant difference.

RESULTS

The polymorphic TT and CT genotype for COMT (rs rs9332377) was significantly higher in the articular TMD group while the non-polymorphic CC genotype was significantly lower in the articular TMD group (P < 0.05). Regarding ADRB2 (rs20530449), the polymorphic GG genotype was similarly considerably more common in the articular TMD group (p < 0.05). In addition, the T allele in the COMT (rs rs9332377) gene was found to be significantly higher in the articular TMD group (p < 0.05).

CONCLUSIONS

In the Turkish population, gene polymorphisms in COMT (rs9332377) and ADRB2 (rs2053044) were associated with articular TMD. This study supports the hypothesis that changes in COMT and ADRB2 genes may play a role in temporomandibular joint pain and predisposition to TMD.

Effect of propranolol on temporomandibular joint pain in repeatedly stressed rats

Stress substantially increases the risk of developing painful temporomandibular disorders (TMDs) by influencing the release of endogenous catecholamines. Propranolol, an antagonist of β-adrenergic receptors, has shown potential in alleviating TMD-associated pain, particularly when the level of catecholamines is elevated. The aim of this study was to explore whether intra-articular propranolol administration is effective in diminishing temporomandibular joint (TMJ) pain during repeated stress situations. Additionally, we investigated the effect of repeated stress on the expression of genes encoding β-adrenoceptors in the trigeminal ganglion. In the present study, rats were exposed to a stress protocol induced by sound, then to the administration of formalin in the TMJ (to elicit a nociceptive response), followed immediately afterward by different doses of propranolol, after which the analgesic response to propranolol was evaluated. We also assessed the levels of beta-1 and beta-2 adrenergic receptor mRNAs (Adrb1 and Adrb2, respectively) using reverse transcription-quantitative PCR (RT-qPCR). Our findings revealed that propranolol administration reduces formalin-induced TMJ nociception more effectively in stressed rats than in non-stressed rats. Furthermore, repeated stress decreases the expression of the Adrb2 gene within the trigeminal ganglion. The findings of this study are noteworthy as they suggest that individuals with a chronic stress history might find potential benefits from β-blockers in TMD treatment.

Transforming Growth Factor-β-Activated Kinase 1 (TAK1) Mediates Chronic Pain and Cytokine Production in Mouse Models of Inflammatory, Neuropathic, and Primary Pain

The origin of chronic pain is linked to inflammation, characterized by increased levels of proinflammatory cytokines in local tissues and systemic circulation. Transforming growth factor beta-activated kinase 1 (TAK1) is a key regulator of proinflammatory cytokine signaling that has been well characterized in the context of cancer and autoimmune disorders, yet its role in chronic pain is less clear. Here, we evaluated the ability of our TAK1 small-molecule inhibitor, takinib, to attenuate pain and inflammation in preclinical models of inflammatory, neuropathic, and primary pain. Inflammatory, neuropathic, and primary pain was modeled using intraplantar complete Freund's adjuvant (CFA), chronic constriction injury (CCI), and systemic delivery of the catechol-O-methyltransferase (COMT) inhibitor OR486, respectively. Behavioral responses evoked by mechanical and thermal stimuli were evaluated in separate groups of mice receiving takinib or vehicle prior to pain induction (baseline) and over 12 days following CFA injection, 4 weeks following CCI surgery, and 6 hours following OR486 delivery. Hindpaw edema was also measured prior to and 3 days following CFA injection. Upon termination of behavioral experiments, dorsal root ganglia (DRG) were collected to measure cytokines. We also evaluated the ability of takinib to modulate nociceptor activity via in vitro calcium imaging of neurons isolated from the DRG of Gcamp3 mice. In all 3 models, TAK1 inhibition significantly reduced hypersensitivity to mechanical and thermal stimuli and expression of proinflammatory cytokines in DRG. Furthermore, TAK1 inhibition significantly reduced the activity of tumor necrosis factor (TNF)-primed/capsaicin-evoked DRG nociceptive neurons. Overall, our results support the therapeutic potential of TAK1 as a novel drug target for the treatment of chronic pain syndromes with different etiologies. PERSPECTIVE: This article reports the therapeutic potential of TAK1 inhibitors for the treatment of chronic pain. This new treatment has the potential to provide a greater therapeutic offering to physicians and patients suffering from chronic pain as well as reduce the dependency on opioid-based pain treatments.

Clinical correlates of pain in adults with hard-to-heal leg ulcers: a cross-sectional study

OBJECTIVE

Pain is a complex symptom associated with hard-to-heal (chronic) leg ulcers that is often poorly managed. The objective of this study was to gain greater understanding by investigating relationships between physical and psychosocial factors, and pain severity in adults with hard-to-heal leg ulcers.

METHOD

A secondary analysis of data collected for a longitudinal, observational study of adults with hard-to-heal leg ulcers was undertaken. Data were collected over a 24-week period, including variables relating to sociodemographics, clinical variables, medical status, health, ulcer and vascular histories, and psychosocial measures. Multiple linear regression modelling was used to determine the independent influences of these variables on pain severity, as measured with a Numerical Rating Scale (NRS).

RESULTS

Of 142 participants who were recruited, 109 met the inclusion criteria for this study, of whom: 43.1% had venous ulcers; 41.3% had mixed ulcers; 7.3% had arterial ulcers; and 8.3% had ulcers from some other cause. The final model explained 37% (adjusted r²=0.370) of the variation in the pain NRS scores. Controlling for analgesic use, salbutamol use (p=0.005), clinical signs of infection (p=0.027) and ulcer severity (p=0.001) were significantly associated with increased pain, while the presence of diabetes (p=0.007) was significantly associated with a decrease in pain.

CONCLUSION

Pain is a highly complex and pervasive symptom associated with hard-to-heal leg ulcers. Novel variables were identified as being associated with pain in this population. The model also included wound type as a variable; however, despite being significantly correlated to pain at the bivariate level of analysis, in the final model, the variable did not reach significance. Of the variables included in the model, salbutamol use was the second most significant. This is a unique finding that, to the authors' knowledge, has not been previously reported or studied. Further research is required to better understand these findings and pain in general.

Effect of sound-induced repeated stress on the development of pain and inflammation in the temporomandibular joint of female and male rats

Temporomandibular disorder (TMD) is a common painful condition of the temporomandibular joint (TMJ) and associated structures. Stress is a significant risk factor for developing this painful condition that predominantly affects women. This study aimed to test the hypothesis that stress increases the risk of developing TMJ pain by facilitating inflammatory mechanisms in female and male rats. To test this hypothesis, we evaluated TMJ carrageenan-induced expression of pro-inflammatory cytokines and migration of inflammatory cells and TMJ formalin-induced nociception in female and male rats submitted to a repeated stress protocol induced by sound. We found that sound-induced repeated stress facilitates TMJ inflammation and contributes to TMJ nociception development equally in females and males. We conclude that stress is a risk factor for developing painful TMJ conditions in males and females, at least in part, by favoring the inflammatory process similarly in both sexes.

Catechol-O-Methyltransferase Loss Drives Cell-Specific Nociceptive Signaling via the Enteric Catechol-O-Methyltransferase/microRNA-155/Tumor Necrosis Factor α Axis

BACKGROUND & AIMS

The etiology of abdominal pain in postinfectious, diarrhea-predominant irritable bowel syndrome (PI-IBS-D) is unknown, and few treatment options exist. Catechol-O-methyltransferase (COMT), an enzyme that inactivates and degrades biologically active catecholamines, plays an important role in numerous physiologic processes, including modulation of pain perception. Our objective was to determine the mechanism(s) of how decreased colonic COMT in PI-IBS-D patients contributes to the chronic abdominal pain phenotype after enteric infections.

METHODS

Colon neurons, epithelial cells, and macrophages were procured with laser capture microdissection from PI-IBS-D patients to evaluate cell-specific colonic COMT, microRNA-155 (miR-155), and tumor necrosis factor (TNF) α expression levels compared to recovered patients (infection cleared: did not develop PI-IBS-D) and control individuals. COMT-/-, colon-specific COMT-/-, and miR-155-/- mice and human colonoids were used to model phenotypic expression of COMT in PI-IBS-D patients and to investigate signaling pathways linking abdominal pain. Citrobacter rodentium and trinitrobenzene sulfonic acid animal models were used to model postinflammatory changes seen in PI-IBS-D patients.

RESULTS

Colonic COMT levels were significantly decreased and correlated with increased visual analog scale abdominal pain ratings in PI-IBS-D patients compared to recovered patients and control individuals. Colonic miR-155 and TNF-α were increased in PI-IBS-D patients with diminished colonic COMT. COMT-/- mice had significantly increased expression of miR-155 and TNF-α in both colon tissues and dorsal root ganglia. Introduction of cV1q antibody (anti-TNF-α) into mice reversed visceral hypersensitivity after C rodentium and trinitrobenzene sulfonic acid.

CONCLUSIONS

Decreased colonic COMT in PI-IBS-D patients drives abdominal pain phenotypes via the COMT/miR-155/TNF-α axis. These important findings will allow new treatment paradigms and more targeted and personalized medicine approaches for gastrointestinal disorders after enteric infections.

Genetic and epigenetic regulation of Catechol-O-methyltransferase in relation to inflammation in chronic fatigue syndrome and Fibromyalgia

BACKGROUND

Catechol-O-methyltransferase (COMT) has been shown to influence clinical pain, descending modulation, and exercise-induced symptom worsening. COMT regulates nociceptive processing and inflammation, key pathophysiological features of Chronic Fatigue Syndrome and Fibromyalgia (CFS/FM). We aimed to determine the interactions between genetic and epigenetic mechanisms regulating COMT and its influence on inflammatory markers and symptoms in patients with CFS/FM.

METHODS

A case-control study with repeated-measures design was used to reduce the chance of false positive and increase the power of our findings. Fifty-four participants (28 patients with CFS/FM and 26 controls) were assessed twice within 4 days. The assessment included clinical questionnaires, neurophysiological assessment (pain thresholds, temporal summation, and conditioned pain modulation), and blood withdrawal in order to assess rs4818, rs4633, and rs4680 COMT polymorphisms and perform haplotype estimation, DNA methylation in the COMT gene (both MB-COMT and S-COMT promoters), and cytokine expression (TNF-α, IFN-γ, IL-6, and TGF-β).

RESULTS

COMT haplotypes were associated with DNA methylation in the S-COMT promoter, TGF-β expression, and symptoms. However, this was not specific for one condition. Significant between-group differences were found for increased DNA methylation in the MB-COMT promoter and decreased IFN-γ expression in patients.

DISCUSSION

Our results are consistent with basic and clinical research, providing interesting insights into genetic-epigenetic regulatory mechanisms. MB-COMT DNA methylation might be an independent factor contributing to the pathophysiology of CFS/FM. Further research on DNA methylation in complex conditions such as CFS/FM is warranted. We recommend future research to employ a repeated-measure design to control for biomarkers variability and within-subject changes.

Autoantibodies from patients with complex regional pain syndrome (CRPS) induce pro-inflammatory effects and functional disturbances on endothelial cells in vitro

ABSTRACT

Complex regional pain syndrome (CRPS) is an inadequate local response after a limb trauma, which leads to severe pain and autonomic and trophic changes of the affected limb. Autoantibodies directed against human β2 adrenergic and muscarinic M2-receptors (hβ2AR and hM2R) have been described in CRPS-patients previously.We analyzed sera from CRPS-patients for autoantibodies against hß2AR, hM2R and endothelial cells, and investigated the functional effects of purified IgG, derived from 13 CRPS patients, on endothelial cells. Eleven healthy controls, seven radial fracture patients without CRPS, and 10 patients with peripheral arterial vascular disease served as controls.CRPS-IgG, but not control IgG, bound to the surface of endothelial cells (P < 0.001) and to hβ2AR and hM2R (P < 0.05), the latter being reversed by adding β2AR and M2R antagonists. CRPS-IgG led to an increased cytotoxicity and a reduced proliferation rate of endothelial cells, and by adding specific antagonists, the effect was neutralized. Regarding second messenger pathways, CRPS-IgG induced ERK-1/2-, P38-, and STAT1-phosphorylation, while AKT-phosphorylation was decreased at the protein level. In addition, increased expression of adhesion molecules (ICAM-1, VCAM-1) on the mRNA-level was induced by CRPS-IgG, thus inducing a pro-inflammatory condition of the endothelial cells.Our results show that patients with CRPS not only develop autoantibodies against hβ2AR and hM2R, but these antibodies interfere with endothelial cells, inducing functional effects on these in vitro, and thus might contribute to the pathophysiology of CRPS.

β2-and β3-Adrenergic Receptors Contribute to Cancer-Evoked Pain in a Mouse Model of Osteosarcoma via Modulation of Neural Macrophages

The mechanisms involved in the development and maintenance of cancer pain remain largely unidentified. Recently, it has been reported that β-adrenergic receptors (β-ARs), mainly β2-and β3-ARs, contribute to tumor proliferation and progression and may favor cancer-associated pain and neuroinflammation. However, the mechanism underlying β-ARs in cancer pain is still unknown. Here, we investigated the role of β1-, β2-and β3-ARs in a mouse model of cancer pain generated by the para-tibial injection of K7M2 osteosarcoma cells. Results showed a rapid tumor growth in the soft tissue associated with the development of mechanical allodynia in the hind paw ipsilateral to the injected site. In addition to reduce tumor growth, both propranolol and SR59230A, β1-/β2-and β3-AR antagonists, respectively, attenuated mechanical allodynia, the number of macrophages and an oxidative stress by-product accumulated in the ipsilateral tibial nerve. The selective β1-AR antagonist atenolol was able to slightly reduce the tumor growth but showed no effect in reducing the development of mechanical allodynia. Results suggest that the development of the mechanical allodynia in K7M2 osteosarcoma-bearing mice is mediated by oxidative stress associated with the recruitment of neural macrophages, and that antagonism of β2-and β3-ARs contribute not solely to the reduction of tumor growth, but also in cancer pain. Thus, the targeting of the β2-and β3-ARs signaling may be a promising therapeutic strategy against both tumor progression and the development of cancer-evoke pain in osteosarcoma.

Contribution of COMT and BDNF Genotype and Expression to the Risk of Transition From Acute to Chronic Low Back Pain

OBJECTIVES

A number of factors, including heritability and the environment, contribute to risk of transition from acute low back pain to chronic low back pain (CLBP). The aim of this study was to (1) compare somatosensory function and pain ratings at low back pain (LBP) onset between the acute low back pain and CLBP conditions and (2) evaluate associations between BDNF and COMT polymorphisms and expression levels at LBP onset to acute and chronic pain burden and risk for transition to the chronic pain state.

METHODS

In this longitudinal study, 220 participants were enrolled following recent onset of LBP and data were collected until the LBP resolved or until the end of the study at 6 months. Forty-two participants' pain resolved before 6 weeks from onset and 42 participants continued to have pain at 6 months. Patient-reported pain burden, somatosensory function (quantitative sensory testing), and blood samples were collected at each study visit.

RESULTS

CLBP is associated with greater pain burden and somatosensory hypersensitivity at the time of LBP onset. COMT rs4680 genotype (GG) was associated with acute cold pain sensitivity and with the risk for transition to CLBP while COMT expression was independently associated with risk for transition.

DISCUSSION

CLBP was characterized by higher reported pain burden and augmented hypersensitivity at LBP onset. COMT expression and genotype were associated with acute pain burden and likelihood of transition to CLBP.

Systems and Circuits Linking Chronic Pain and Circadian Rhythms

Research over the last 20 years regarding the link between circadian rhythms and chronic pain pathology has suggested interconnected mechanisms that are not fully understood. Strong evidence for a bidirectional relationship between circadian function and pain has been revealed through inflammatory and immune studies as well as neuropathic ones. However, one limitation of many of these studies is a focus on only a few molecules or cell types, often within only one region of the brain or spinal cord, rather than systems-level interactions. To address this, our review will examine the circadian system as a whole, from the intracellular genetic machinery that controls its timing mechanism to its input and output circuits, and how chronic pain, whether inflammatory or neuropathic, may mediate or be driven by changes in these processes. We will investigate how rhythms of circadian clock gene expression and behavior, immune cells, cytokines, chemokines, intracellular signaling, and glial cells affect and are affected by chronic pain in animal models and human pathologies. We will also discuss key areas in both circadian rhythms and chronic pain that are sexually dimorphic. Understanding the overlapping mechanisms and complex interplay between pain and circadian mediators, the various nuclei they affect, and how they differ between sexes, will be crucial to move forward in developing treatments for chronic pain and for determining how and when they will achieve their maximum efficacy.

Efficacy and safety of propranolol for treatment of temporomandibular disorder pain: a randomized, placebo-controlled clinical trial

Propranolol is a nonselective beta-adrenergic receptor antagonist. A multicenter, randomized, double-blind, placebo-controlled, parallel-group, phase 2b trial enrolled participants aged 18 to 65 years with temporomandibular disorder myalgia to evaluate efficacy and safety of propranolol compared with placebo in reducing facial pain. Participants were randomized 1:1 to either extended-release propranolol hydrochloride (60 mg, BID) or placebo. The primary endpoint was change in facial pain index (FPI = facial pain intensity multiplied by facial pain duration, divided by 100). Efficacy was analyzed as a mean change in FPI from randomization to week 9 and as the proportion of participants with ≥30% or ≥50% reductions in FPI at week 9. Regression models tested for treatment-group differences adjusting for study site, sex, race, and FPI at randomization. Of 299 participants screened, 200 were randomized; 199 had at least one postrandomization FPI measurement and were included in intention-to-treat analysis. At week 9, model-adjusted reductions in mean FPI did not differ significantly between treatment groups (-1.8, 95% CL: -6.2, 2.6; P = 0.41). However, the proportion with a ≥30% reduction in FPI was significantly greater for propranolol (69.0%) than placebo (52.6%), and the associated number-needed-to-treat was 6.1 (P = 0.03). Propranolol was likewise efficacious for a ≥50% reduction in FPI (number-needed-to-treat = 6.1, P = 0.03). Adverse event rates were similar between treatment groups, except for more frequent fatigue, dizziness, and sleep disorder in the propranolol group. Propranolol was not different from placebo in reducing mean FPI but was efficacious in achieving ≥30% and ≥50% FPI reductions after 9 weeks of treatment among temporomandibular disorder participants.

Adrenergic signalling in osteoarthritis

Adrenoceptors (ARs) mediate the effects of the sympathetic neurotransmitters norepinephrine (NE) and epinephrine (E) in the human body and play a central role in physiologic and pathologic processes. Therefore, ARs have long been recognized as targets for therapeutic agents, especially in the field of cardiovascular medicine. During the past decades, the contribution of the sympathetic nervous system (SNS) and particularly of its major peripheral catecholamine NE to the pathogenesis of osteoarthritis (OA) attracted growing interest. OA is the most common degenerative joint disorder worldwide and a disease of the whole joint. It is characterized by progressive degradation of articular cartilage, synovial inflammation, osteophyte formation, and subchondral bone sclerosis mostly resulting in chronic pain. The subchondral bone marrow, the periosteum, the synovium, the vascular meniscus and numerous tendons and ligaments are innervated by tyrosine hydroxylase-positive (TH+) sympathetic nerve fibers that release NE into the synovial fluid and cells of all abovementioned joint tissues express at least one out of nine AR subtypes. During the past decades, several in vitro studies explored the AR-mediated effects of NE on different cell types in the joint. So far, only a few studies used animal OA models to investigate the contribution of distinct AR subtypes to OA pathogenesis in vivo. This narrative review shortly summarizes the current background knowledge about ARs and their signalling pathways at first. In the second part, we focus on recent findings in the field of NE-induced AR-mediated signalling in different joint tissues during OA pathogenesis and at the end, we will delineate the potential of targeting the adrenergic signalling for OA prevention or treatment. We used the PubMed bibliographic database to search for keywords such as 'joint' or 'cartilage' or 'synovium' or 'bone' and 'osteoarthritis' and/or 'trauma' and 'sympathetic nerve fibers' and/or 'norepinephrine' and 'adrenergic receptors / adrenoceptors' as well as 'adrenergic therapy'.

Low catechol-O-methyltransferase and stress potentiate functional pain and depressive behavior, especially in female mice

Low levels of catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines, and stress, which potentiates catecholamine release from sympathetic nerves, are fundamental to chronic functional pain syndromes and comorbid depression, which predominantly affect females. Here, we sought to examine the independent and joint contributions of low COMT and stress to chronic functional pain and depression at the behavioral and molecular level. Male and female C57BL/6 mice received sustained systemic delivery of the COMT inhibitor OR486 over 14 days and underwent a swim stress paradigm on days 8 to 10. Pain and depressive-like behavior were measured over 14 days, and brain-derived neurotrophic factor (BDNF; a factor involved in nociception and depression) and glucocorticoid receptor (GR; a stress-related receptor) expression were measured on day 14. We found that stress potentiates the effect of low COMT on functional pain and low COMT potentiates the effect of stress on depressive-like behavior. The joint effects of low COMT and stress on functional pain and depressive-like behavior were significantly greater in females vs males. Consistent with behavioral data, we found that stress potentiates COMT-dependent increases in spinal BDNF and low COMT potentiates stress-dependent decreases in hippocampal BDNF in females, but not males. Although low COMT increases spinal GR and stress increases hippocampal GR expression, these increases are not potentiated in the OR486 + stress group and are not sex-specific. These results suggest that genetic and environmental factors that enhance catecholamine bioavailability cause abnormalities in BDNF signaling and increase risk of comorbid functional pain and depression, especially among females.

VAL158MET catechol O-methyltransferase polymorphism contributes to the development of preeclampsia

Objectives: Establishment of association between: (a) Val158Met COMT (G1947A) polymorphism and preeclampsia; (b) cytokines gene expression and COMT genotypes. Methods: 50 preeclampsia and 50 healthy pregnant women were enrolled. COMT genotyping was done by PCR/RFLP. TNF-α, IL-1β, and IL-6 mRNA levels were determined by Real-time PCR. Results: Variant (AA) homozygotes carried 3.7-fold increased preeclampsia odds, especially for severe (OR = 9.0, 95%CI (2.09-38.799)) and early forms (OR = 6.6, 95%CI (1.62-26.87)). AA homozygotes with PE had higher TNF-α levels compared to controls (P = 0.012). Conclusions: Val158Met COMT polymorphism increases preeclampsia risk. TNF-α expression and Val158Met COMT polymorphism have concomitant roles in PE pathogenesis.

Plasma Concentrations of Select Inflammatory Cytokines Predicts Pain Intensity 48 Hours Post-Shoulder Muscle Injury

OBJECTIVES

The relationship between elevated inflammatory cytokine levels and peak pain intensity following acute musculoskeletal injury has not been fully elucidated in high risk subgroups. Identifying the role that these cytokines have on pain responses may help with developing tailored therapeutic approaches.

METHODS

Data were collected from 54 participants who were vulnerable to a robust pain response and delayed recovery following musculoskeletal injury. Participants completed baseline active and resting pain measurements and a blood draw before an exercised induced shoulder muscle injury. Participants returned at 24 and 48 hours postinjury for follow-up pain measurements and blood draws. Blood plasma was analyzed for interleukin (IL)-1β, IL-6, IL-8, IL-10, and tumor necrosis factor α. Pearson bivariate correlations were performed between cytokines and pain measurements to identify candidate variables for stepwise multiple linear regression predicting pain intensity reports.

RESULTS

Pearson bivariate correlation identified 13/45 correlations between inflammatory cytokines and resting pain intensity and 9/45 between inflammatory cytokines and active pain (P<0.05, r≥0.3 or r≤-0.3). This led to 5 stepwise multiple linear regression models, of which 4 met the statistical criterion (P<0.0167); including IL-10 baseline plasma concentrations predicting active pain (r=0.19) and resting pain (r=0.15) intensity 48 hours postinjury. IL-6 and IL-10 plasma concentrations at 48 hours were respectively associated with active and resting pain at 48 hours.

DISCUSSION

These findings suggest that elevated concentrations of inflammatory cytokines, specifically IL-10 (at baseline and 48 h) and IL-6 (at 48 h), may play a role in heightened pain responses following exercise-induced muscle injury.

Central Nervous System Targets: Glial Cell Mechanisms in Chronic Pain

Interactions between central glial cells and neurons in the pain circuitry are critical contributors to the pathogenesis of chronic pain. In the central nervous system (CNS), two major glial cell types predominate: astrocytes and microglia. Injuries or pathological conditions which evoke pain are concurrently associated with the presence of a reactive microglia or astrocyte state, which is characterized by a variety of changes in the morphological, molecular, and functional properties of these cells. In this review, we highlight the changes that reactive microglia and astrocytes undergo following painful injuries and insults and discuss the critical and interactive role these two cell types play in the initiation and maintenance of chronic pain. Additionally, we focus on several crucial mechanisms by which microglia and astrocytes contribute to chronic pain and provide commentary on the therapeutic promise of targeting these pathways. In particular, we discuss how the inflammasome in activated microglia drives maturation and release of key pro-inflammatory cytokines, which drive pain through neuronal- and glial regulations. Moreover, we highlight several potentially-druggable hemichannels and proteases produced by reactive microglia and astrocytes in pain states and discuss how these pathways regulate distinct phases during pain pathogenesis. We also review two emerging areas in chronic pain research: 1) sexually dimorphic glial cell signaling and 2) the role of oligodendrocytes. Finally, we highlight important considerations for potential pain therapeutics targeting glial cell mediators as well as questions that remain in our conceptual understanding of glial cell activation in pain states.

Catecholaminergic and opioidergic system mediated effects of reboxetine on diabetic neuropathic pain

RATIONALE

Current data indicate that the noradrenergic system plays a critical role in neuropathic pain treatment. Notably, drugs that directly affect this system may have curative potential in neuropathy-associated pain.

OBJECTIVES

The aim of this study was to evaluate the potential therapeutic efficacy of reboxetine, a potent and selective noradrenaline reuptake inhibitor, on hyperalgesia and allodynia responses in rats with experimental diabetes. Furthermore, mechanistic studies were performed to elucidate the possible mode of actions.

METHODS

Experimental diabetes was induced by a single dose of streptozotocin. Mechanical hyperalgesia, mechanical allodynia, thermal hyperalgesia, and thermal allodynia responses in diabetic rats were evaluated by Randall-Selitto, dynamic plantar, Hargreaves, and warm plate tests, respectively.

RESULTS

Reboxetine treatment (8 and 16 mg/kg for 2 weeks) demonstrated an effect comparable to that of the reference drug, pregabalin, improving the hyperalgesic and allodynic responses secondary to diabetes mellitus. Pretreatment with phentolamine, metoprolol, SR 59230A, and atropine did not alter the abovementioned effects of reboxetine; however, the administration of α-methyl-para-tyrosine methyl ester, propranolol, ICI-118,551, SCH-23390, sulpiride, and naltrindole significantly inhibited these effects. Moreover, reboxetine did not induce a significant difference in the rat plasma glucose levels.

CONCLUSIONS

Our findings indicate that the antihyperalgesic and antiallodynic effects of reboxetine are mediated by the catecholaminergic system; β₂-adrenoceptors; D₁-, D₂/D₃-dopaminergic receptors; and δ-opioid receptors. The results suggest that this analgesic effect of reboxetine, besides its neutral profile on glycemic control, may be advantageous in the pharmacotherapy of diabetic neuropathy-induced pain.

Differential Expression of Neuroinflammatory mRNAs in the Rat Sciatic Nerve Following Chronic Constriction Injury and Pain-Relieving Nanoemulsion NSAID Delivery to Infiltrating Macrophages

The neuroinflammatory response to peripheral nerve injury is associated with chronic pain and significant changes in the molecular expression profiles of mRNAs in neurons, glia and infiltrating immune cells. Chronic constriction injury (CCI) of the rat sciatic nerve provides an opportunity to mimic neuropathic injury and quantitatively assess behavior and differential gene expression in individual animals. Previously, we have shown that a single intravenous injection of nanoemulsion containing celecoxib (0.24 mg/kg) reduces inflammation of the sciatic nerve and relieves pain-like behavior for up to 6 days. Here, we use this targeted therapy to explore the impact on mRNA expression changes in both pain and pain-relieved states. Sciatic nerve tissue recovered from CCI animals is used to evaluate the mRNA expression profiles utilizing quantitative PCR. We observe mRNA changes consistent with the reduced recruitment of macrophages evident by a reduction in chemokine and cytokine expression. Furthermore, genes associated with adhesion of macrophages, as well as changes in the neuronal and glial mRNAs are observed. Moreover, genes associated with neuropathic pain including Maob, Grin2b/NMDAR2b, TrpV3, IL-6, Cacna1b/Ca_v2.2, Itgam/Cd11b, Scn9a/Na_v1.7, and Tac1 were all found to respond to the celecoxib loaded nanoemulsion during pain relief as compared to those animals that received drug-free vehicle. These results demonstrate that by targeting macrophage production of PGE₂ at the site of injury, pain relief includes partial reversal of the gene expression profiles associated with chronic pain.

Polymorphisms in COMT, ADRB2 and HTR1A genes are associated with temporomandibular disorders in individuals with other arthralgias

Objective: To evaluate the association between polymorphisms in genes and comorbid presence of arthralgias and TMD.Methods: This is a case-control study. The groups formed were individuals with chronic arthralgia and 1) myofascial pain (n = 42); 2) articular (n = 16); 3) multiple diagnoses (n = 69); 4) with TMD and without some other arthralgia (n = 16); 5) without TMD but with pain in other joints (n = 82); and 6) a control group (n = 72). SNPs in COMT, ADRB2, and HTR1A genes were investigated.Results: The CT genotype for the COMT (rs9332377) gene was associated with the absence of myofascial pain (p = .05). In the ADRB2 (rs1042713) gene, the AA genotype was associated with the absence of myofascial pain (p = .03).Discussion: This study supports the hypothesis that alterations in the COMT, ADRB2, and HTR1A genes influence the presence of chronic pain and TMD.

Association of a Functional Polymorphism in the CHRFAM7A Gene with Inflammatory Response Mediators and Neuropathic Pain after Spinal Cord Injury

The alpha 7 nicotinic acetylcholine receptor, α7 nAChR, plays a central role in regulating inflammatory responses. Previous studies showed that pharmacological inhibitors of α7nAChR have a pro-inflammatory effect, increasing the circulating levels of cytokines such as tumor necrosis factor alpha (TNFα). This study focused on how genetic polymorphisms of the partially duplicated α7nAChR gene (CHRFAM7A), which is highly expressed in peripheral blood cells, contribute to functional outcome after spinal cord injury (SCI). In a cohort of 27 SCI patients and 25 emergency room consented controls (% F/M: 15/85, 24/76; mean ± SE age: 35 ± 1.38 and 35 ± 2.0 respectively), a panel of circulating cytokines, noradrenergic metabolite (normetanephrine [NMN]) levels, and clinical data were available within the first 7 days post-injury (DPI) up to 90 DPI, and were investigated in the acute/subacute (DPI 1-21) and intermediate (DPI 22-90) temporal periods. Cytokine and NMN plasma levels on different DPI were analyzed as a function of CHRFAM7A genotype. TNFα levels, as a representative of some elevated inflammatory mediators, were nearly threefold higher in individuals carrying the del-2bp variant of the CHRFAM7A gene compared with that in the no-deletion genotype (p = 0.001 analysis of variance [ANOVA]) 3 weeks DPI, and twofold higher than genotype-matched acute/subacute non-SCI injury controls within 7 days DPI. In contrast, NMN levels were initially unchanged, although after 3 weeks, NMN levels were significantly decreased in SCI individuals carrying the del-2bp variant compared with non-carriers (p = 0.011 ANOVA). Numerical pain scores over this same period post-injury were significantly elevated in SCI patients carrying the del-2bp variant relative to non-carriers (p = 0.001 ANOVA). Taken together, these data reveal that pro-inflammatory responses associated with CHRFAM7A gene variation may also be associated with differences in pain experience in patients following SCI, at least during the intermediate phase.

Acupuncture Resolves Persistent Pain and Neuroinflammation in a Mouse Model of Chronic Overlapping Pain Conditions

Patients with chronic overlapping pain conditions have decreased levels of catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines. Consistent with clinical syndromes, we previously demonstrated that COMT inhibition in rodents produces persistent pain and heightened immune responses. Here, we sought to determine the efficacy of manual acupuncture in resolving persistent pain and neuroinflammation in the classic inbred C57BL/6 strain and the rapid-wound healing MRL/MpJ strain. Mice received subcutaneous osmotic minipumps to deliver the COMT inhibitor OR486 or vehicle for 13 days. On day 7 after pump implantation, acupuncture was performed at the Zusanli (ST36) point or a non-acupoint for 6 consecutive days. Behavioral responses to mechanical stimuli were measured throughout the experiment. Immunohistochemical analysis of spinal phosphorylated p38 mitogen-activated protein kinase, a marker of inflammation, and glial fibrillary acidic protein, a marker of astrogliosis, was performed on day 13. Results demonstrated that ST36, but not sham, acupuncture resolved mechanical hypersensitivity and reduced OR486-dependent increases in phosphorylated p38 and glial fibrillary acidic protein in both strains. The magnitude of the analgesic response was greater in MRL/MpJ mice. These findings indicate acupuncture as an effective treatment for persistent pain linked to abnormalities in catecholamine signaling and, furthermore, that analgesic efficacy may be influenced by genetic differences. PERSPECTIVE: Chronic overlapping pain conditions remain ineffectively managed by conventional pharmacotherapies. Here, we demonstrate that acupuncture alleviates persistent pain and neuroinflammation linked to heightened catecholaminergic tone. Mice with superior healing capacity exhibit greater analgesic efficacy. Findings indicate acupuncture as an effective treatment for chronic overlapping pain conditions and provide insight into treatment response variability.

Sustained stimulation of β2- and β3-adrenergic receptors leads to persistent functional pain and neuroinflammation

Functional pain syndromes, such as fibromyalgia and temporomandibular disorder, are associated with enhanced catecholamine tone and decreased levels of catechol-O-methyltransferase (COMT; an enzyme that metabolizes catecholamines). Consistent with clinical syndromes, our lab has shown that sustained 14-day delivery of the COMT inhibitor OR486 in rodents results in pain at multiple body sites and pain-related volitional behaviors. The onset of COMT-dependent functional pain is mediated by peripheral β2- and β3-adrenergic receptors (β2- and β3ARs) through the release of the pro-inflammatory cytokines tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), and interleukin-6 (IL-6). Here, we first sought to investigate the role of β2- and β3ARs and downstream mediators in the maintenance of persistent functional pain. We then aimed to characterize the resulting persistent inflammation in neural tissues (neuroinflammation), characterized by activated glial cells and phosphorylation of the mitogen-activated protein kinases (MAPKs) p38 and extracellular signal-regulated kinase (ERK). Separate groups of rats were implanted with subcutaneous osmotic mini-pumps to deliver OR486 (15 mg/kg/day) or vehicle for 14 days. The β2AR antagonist ICI118551 and β3AR antagonist SR59230A were co-administrated subcutaneously with OR486 or vehicle either on day 0 or day 7. The TNFα inhibitor Etanercept, the p38 inhibitor SB203580, or the ERK inhibitor U0126 were delivered intrathecally following OR486 cessation on day 14. Behavioral responses, pro-inflammatory cytokine levels, glial cell activation, and MAPK phosphorylation were measured over the course of 35 days. Our results demonstrate that systemic delivery of OR486 leads to mechanical hypersensitivity that persists for at least 3 weeks after OR486 cessation. Corresponding increases in spinal TNFα, IL-1β, and IL-6 levels, microglia and astrocyte activation, and neuronal p38 and ERK phosphorylation were observed on days 14-35. Persistent functional pain was alleviated by systemic delivery of ICI118551 and SR59230A beginning on day 0, but not day 7, and by spinal delivery of Etanercept or SB203580 beginning on day 14. These results suggest that peripheral β2- and β3ARs drive persistent COMT-dependent functional pain via increased activation of immune cells and production of pro-inflammatory cytokines, which promote neuroinflammation and nociceptor activation. Thus, therapies that resolve neuroinflammation may prove useful in the management of functional pain syndromes.

Antihyperalgesic Activity of Atomoxetine on Diabetes-Induced Neuropathic Pain: Contribution of Noradrenergic and Dopaminergic Systems

Atomoxetine is a selective noradrenaline reuptake inhibitor drug. Based on the knowledge that agents increasing monoamine levels in the central nervous system have therapeutic potential for neuropathic pain, it is planned to investigate the possible efficacy of atomoxetine on diabetes-induced hyperalgesia, in this study. Randall-Selitto (mechanical noxious stimuli) and Hargreaves (thermal noxious stimuli) tests were used to evaluate nociceptive perception of rats. Obtained data indicated that streptozotocin-induced diabetes causes significant decreases in the paw withdrawal threshold and paw withdrawal latency values of the animals, respectively. However, atomoxetine administered at 3 mg/kg/day for 7 and 14 days improved these diabetes-induced hyperalgesia responses. Furthermore, antihyperalgesic activity was antagonized with α-methyl-para-tyrosine methyl ester, phentolamine, propranolol, and sulpiride pre-treatments. The same effect was not reversed, however, by SCH 23390. These findings demonstrated, for the first time, that atomoxetine possesses significant antihyperalgesic activity on diabetes-induced neuropathic pain and this effect seems to be mediated by α- and β-adrenergic and D₂/D₃ dopaminergic receptors. Results of this present study seem to offer a new indication for an old drug; atomoxetine, but these preclinical data should first be confirmed with further well-designed clinical trials.

A moldable sustained release bupivacaine formulation for tailored treatment of postoperative dental pain

A moldable and biodegradable dental material was designed for customized placement and sustained delivery of bupivacaine (BP) within an extraction cavity. Microparticles comprising poly(lactic-co-glycolic acid) (PLGA) containing BP were generated via solvent-evaporation and combined with absorbable hemostat Gelfoam®. Kinetics of drug release were evaluated by in vitro dialysis assays, showing higher release within the first 24 hours, with subsequent tapering of release kinetics. Formulations of Gelfoam® and BP-PLGA microparticles (GelBP), with three targeted dosing profiles (0.25, 0.5, and 1 mg/kg/day), were evaluated alongside acute subcutaneous BP injections (2 mg/kg) to determine analgesic efficacy in a rat model of tooth extraction pain. Molar extraction resulted in mechanical and thermal cold hyperalgesia in male and female rats. GelBP outperformed acute BP in blocking post-surgical dental pain, with the 0.25 mg/kg GelBP dose preventing hypersensitivity to mechanical (p < 0.01) and thermal cold stimuli (p = 0.05). Molar extraction also resulted in decreased food consumption and weight. Males receiving acute BP and 0.25 mg/kg GelBP maintained normal food consumption (p < 0.002) and weight (p < 0.0001) throughout 7 days. Females, receiving 0.25 mg/kg GelBP maintained weight on days 5-7 (p < 0.04). Customized, sustained release formulation of anesthetic within a tooth extraction cavity holds potential to eliminate post-operative dental pain over several days.

Neuroinflammation and Central Sensitization in Chronic and Widespread Pain

Chronic pain is maintained in part by central sensitization, a phenomenon of synaptic plasticity, and increased neuronal responsiveness in central pain pathways after painful insults. Accumulating evidence suggests that central sensitization is also driven by neuroinflammation in the peripheral and central nervous system. A characteristic feature of neuroinflammation is the activation of glial cells, such as microglia and astrocytes, in the spinal cord and brain, leading to the release of proinflammatory cytokines and chemokines. Recent studies suggest that central cytokines and chemokines are powerful neuromodulators and play a sufficient role in inducing hyperalgesia and allodynia after central nervous system administration. Sustained increase of cytokines and chemokines in the central nervous system also promotes chronic widespread pain that affects multiple body sites. Thus, neuroinflammation drives widespread chronic pain via central sensitization. We also discuss sex-dependent glial/immune signaling in chronic pain and new therapeutic approaches that control neuroinflammation for the resolution of chronic pain.

E3 Ubiquitin Ligase c-cbl Inhibits Microglia Activation After Chronic Constriction Injury

E3 ubiquitin ligase c-Caritas B cell lymphoma (c-cbl) is associated with negative regulation of receptor tyrosine kinases, signal transduction of antigens and cytokine receptors, and immune response. However, the expression and function of c-cbl in the regulation of neuropathic pain after chronic constriction injury (CCI) are unknown. In rat CCI model, c-cbl inhibited the activation of spinal cord microglia and the release of pro-inflammatory factors including tumor necrosis factor alpha (TNF-α), interleukin 1 beta (IL-1β) and interleukin 6 (IL-6), which alleviated mechanical and heat pain through down-regulating extracellular signal-regulated kinase (ERK) pathway. Additionally, exogenous TNF-α inhibited c-cbl protein level vice versa. In the primary microglia transfected with c-cbl siRNA, when treated with TNF-α or TNF-α inhibitor, the corresponding secretion of IL-1β and IL-6 did not change. In summary, CCI down-regulated c-cbl expression and induced the activation of microglia, then activated microglia released inflammatory factors via ERK signaling to cause pain. Our data might supply a novel molecular target for the therapy of CCI-induced neuropathic pain.

The effects of propranolol on heart rate variability and quantitative, mechanistic, pain profiling: a randomized placebo-controlled crossover study

Background and aims

The autonomic nervous system (ANS) is capable of modulating pain. Aberrations in heart rate variability (HRV), reflective of ANS activity, are associated with experimental pain sensitivity, chronic pain, and more recently, pain modulatory mechanisms but the underlying mechanisms are still unclear. HRV is lowered during experimental pain as well as in chronic pain conditions and HRV can be increased by propranolol, which is a non-selective β-blocker. Sensitization of central pain pathways have been observed in several chronic pain conditions and human mechanistic pain biomarkers for these central pain pathways include temporal summation of pain (TSP) and conditioned pain modulation (CPM). The current study aimed to investigate the effect of the β-blocker propranolol, and subsequently assessing the response to standardized, quantitative, mechanistic pain biomarkers.

Methods

In this placebo-controlled, double-blinded, randomized crossover study, 25 healthy male volunteers (mean age 25.6 years) were randomized to receive 40 mg propranolol and 40 mg placebo. Heart rate, blood pressure, and HRV were assessed before and during experimental pain tests. Cuff pressure pain stimulation was used for assessment of pain detection (cPDTs) and pain tolerance (cPTTs) thresholds, TSP, and CPM. Offset analgesia (OA) was assessed using heat stimulation.

Results

Propranolol significantly reduced heart rate (p<0.001), blood pressure (p<0.02) and increased HRV (p<0.01) compared with placebo. No significant differences were found comparing cPDT (p>0.70), cPTT (p>0.93), TSP (p>0.70), OA-effect (p>0.87) or CPM (p>0.65) between propranolol and placebo.

Conclusions

The current study demonstrated that propranolol increased HRV, but did not affect pressure pain sensitivity or any pain facilitatory or modulatory outcomes.

Implications

Analgesic effects of propranolol have been reported in clinical pain populations and the results from the current study could indicate that increased HRV from propranolol is not associated with peripheral and central pain pathways in healthy male subjects.

Preoperative Norepinephrine Levels in Cerebrospinal Fluid and Plasma Correlate With Pain Intensity After Pediatric Spine Surgery

PURPOSE

Catecholamines were found to be involved in descending pain modulation and associated with perioperative pain. The purpose of the present study was to investigate the relationship between preoperative concentrations of catecholamines and postoperative pain intensity of pediatric patients.

METHODS

Fifty adolescents with idiopathic scoliosis scheduled for elective spinal fusion surgery were enrolled in this prospective cohort study. Preoperative plasma and cerebrospinal fluid (CSF) samples were collected and analyzed by mass spectrometry. Pain intensity was assessed during the acute postoperative period and in the intermediate period.

RESULTS

Preoperative plasma concentrations of norepinephrine (NE) and normetanephrine (NME), as well as the CSF concentration of NE, were significantly correlated with the presence of pain six weeks after surgery (r = 0.48, 0.50, and 0.50, respectively; p < .002). We also found that preoperative NE levels in CSF were significantly higher in patients reporting moderate to severe pain intensity than in patients with mild pain during the first day following surgery (0.268 ± 0.29 ng/mL vs. 0.121 ± 0.074 ng/mL, p = .01), as well as between patients reporting pain and painless patients at 6 weeks postsurgery (0.274 ± 0.282 ng/mL vs. 0.103 ± 0.046 ng/mL respectively, U = 69.5, p = .002).

CONCLUSIONS

These results support the potential role of catecholamine levels in predicting postoperative pain intensity.

Biopsychosocial influence on shoulder pain: Rationale and protocol for a pre-clinical trial

BACKGROUND

Chronic musculoskeletal pain conditions are a prevalent and disabling problem. Preventing chronic musculoskeletal pain requires multifactorial treatment approaches that address its complex etiology. Prior cohort studies identified a high risk subgroup comprised of variation in COMT genotype and pain catastrophizing. This subgroup had increased chance of heightened pain responses (in a pre-clinical model) and higher 12month post-operatives pain intensity ratings (in a clinical model). This pre-clinical trial will test mechanisms and efficacy of personalized pain interventions matched to the genetic and psychological characteristics of the high-risk subgroup.

METHODS

Potential participants will be screened for high risk subgroup membership, appropriateness for exercise-induced muscle injury protocol, and appropriateness for propranolol administration. Eligible participants that consent to the study will then be randomized into one of four treatment groups; 1) personalized pharmaceutical and psychological education; 2) personalized pharmaceutical and general education; 3) placebo pharmaceutical and psychological education; 4) placebo pharmaceutical and psychological education. Over the 5-day study period participants will complete an exercise-induced muscle injury protocol and receive study interventions. Pain and disability assessments will be completed daily, with primary outcomes being duration of shoulder pain (number of days until recovery), peak shoulder pain intensity, and peak shoulder disability. Secondary outcomes include inflammatory markers, psychological mediators, and measures of pain sensitivity regulation.

CONCLUSION

This pre-clinical trial builds on prior cohort studies and its completion will provide foundational data supporting efficacy and mechanisms of personalized interventions for individuals that may be at increased risk for developing chronic shoulder pain.

TRIAL REGISTRATION

ClinicalTrials.gov registry, NCT02620579 (Registered on November 13, 2015).

AG. Persistent Catechol-O-methyltransferase-dependent Pain Is Initiated by Peripheral β-Adrenergic Receptors

BACKGROUND

Patients with chronic pain disorders exhibit increased levels of catecholamines alongside diminished activity of catechol-O-methyltransferase (COMT), an enzyme that metabolizes catecholamines. The authors found that acute pharmacologic inhibition of COMT in rodents produces hypersensitivity to mechanical and thermal stimuli via β-adrenergic receptor (βAR) activation. The contribution of distinct βAR populations to the development of persistent pain linked to abnormalities in catecholamine signaling requires further investigation.

METHODS

Here, the authors sought to determine the contribution of peripheral, spinal, and supraspinal βARs to persistent COMT-dependent pain. They implanted osmotic pumps to deliver the COMT inhibitor OR486 (Tocris, USA) for 2 weeks. Behavioral responses to mechanical and thermal stimuli were evaluated before and every other day after pump implantation. The site of action was evaluated in adrenalectomized rats receiving sustained OR486 or in intact rats receiving sustained βAR antagonists peripherally, spinally, or supraspinally alongside OR486.

RESULTS

The authors found that male (N = 6) and female (N = 6) rats receiving sustained OR486 exhibited decreased paw withdrawal thresholds (control 5.74 ± 0.24 vs. OR486 1.54 ± 0.08, mean ± SEM) and increased paw withdrawal frequency to mechanical stimuli (control 4.80 ± 0.22 vs. OR486 8.10 ± 0.13) and decreased paw withdrawal latency to thermal heat (control 9.69 ± 0.23 vs. OR486 5.91 ± 0.11). In contrast, adrenalectomized rats (N = 12) failed to develop OR486-induced hypersensitivity. Furthermore, peripheral (N = 9), but not spinal (N = 4) or supraspinal (N = 4), administration of the nonselective βAR antagonist propranolol, the β2AR antagonist ICI-118,511, or the β3AR antagonist SR59230A blocked the development of OR486-induced hypersensitivity.

CONCLUSIONS

Peripheral adrenergic input is necessary for the development of persistent COMT-dependent pain, and peripherally-acting βAR antagonists may benefit chronic pain patients.

Complex regional pain syndrome-significant progress in understanding

Research into complex regional pain syndrome (CRPS) has made significant progress. First, there was the implementation of the official IASP "Budapest" diagnostic criteria. It would be desirable to also define exclusion and outcome criteria that should be reported in studies. The next step was to recognize the complex pathophysiology. After trauma, some inflammation is physiological; in acute CRPS, this inflammation persists for months. There is an abundance of inflammatory and a lack of anti-inflammatory mediators. This proinflammatory network (cytokines and probably also other mediators) sensitizes the peripheral and spinal nociceptive system, it facilitates the release of neuropeptides from nociceptors inducing the visible signs of inflammation, and it stimulates bone cell or fibroblast proliferation, and endothelial dysfunction leading to vascular changes. Trauma may also expose nervous system structures to the immune system and triggers autoantibodies binding to adreno- and acetylcholine receptors. In an individual time frame, the pain in this inflammatory phase pushes the transition into "centralized" CRPS, which is dominated by neuronal plasticity and reorganization. Sensory-motor integration becomes disturbed, leading to a loss of motor function; the body representation is distorted leading to numbness and autonomic disturbances. In an attempt to avoid pain, patients neglect their limb and learn maladaptive nonuse. The final step will be to assess large cohorts and to analyze these data together with data from public resources using a bioinformatics approach. We could then develop diagnostic toolboxes for individual pathophysiology and select focused treatments or develop new ones.

Nuclear factor-kappa B regulates pain and COMT expression in a rodent model of inflammation

Nuclear factor-kappa B (NF-κB) is a ubiquitously expressed protein complex regulating the transcription of genes involved in inflammation and pain. Increased NF-κB activity in immune and nervous system cells is linked to several chronic pain conditions in humans as well as inflammation and nerve injury-evoked pain in animals. A recent in vitro study further demonstrates that increased NF-κB activity in astrocytes decreases transcription of catechol-o-methyltransferase (COMT), an enzyme that inactivates catecholamines that cause pain. The purpose of the present study was to examine the relationship between systemic and astrocytic NF-κB activity, pain, and COMT expression in an animal model of inflammation. Results demonstrated that administration of the inflammatory stimulant complete Freund's adjuvant (CFA) led to increased pain and decreased COMT protein expression in an NF-κB-dependent manner. Specifically, we found that rats and mice receiving intraplantar CFA exhibited increased behavioral responses to mechanical and thermal heat stimuli. CFA-evoked pain was blocked in rats receiving a pre-emptive systemic dose of the NF-κB inhibitor MG132 and exacerbated in IKKca mice with constitutive NF-κB activity in astrocytes. Furthermore, we observed NF-κB-linked reductions in COMT expression in midbrain at 6h and 1d following CFA in rats and at 1h and 1d in forebrain and midbrain following CFA in IKKca mice. Collectively, these results demonstrate that systemic and astrocytic NF-κB activity drive inflammatory pain and regulate the expression of COMT in forebrain and midbrain structures.

Alternative Splicing of G Protein-Coupled Receptors: Relevance to Pain Management

Drugs that target G protein-coupled receptors (GPCRs) represent the primary treatment strategy for patients with acute and chronic pain; however, there is substantial individual variability in both the efficacy and adverse effects associated with these drugs. Variability in drug responses is due, in part, to individuals' diversity in alternative splicing of pain-relevant GPCRs. G protein-coupled receptor alternative splice variants often exhibit distinct tissue distribution patterns, drug-binding properties, and signaling characteristics that may impact disease pathology as well as the extent and direction of analgesic effects. We review the importance of GPCRs and their known splice variants to the management of pain.

Fibroblast growth factor 7 is a nociceptive modulator secreted via large dense-core vesicles

Fibroblast growth factor (FGF) 7, a member of FGF family, is initially found to be secreted from mesenchymal cells to repair epithelial tissues. However, its functions in the nervous system are largely unknown. The present study showed that FGF7 was a neuromodulator localized in the large dense-core vesicles (LDCVs) in nociceptive neurons. FGF7 was mainly expressed in small-diameter neurons of the dorsal root ganglion and could be transported to the dorsal spinal cord. Interestingly, FGF7 was mostly stored in LDCVs that did not contain neuropeptide substance P. Electrophysiological recordings in the spinal cord slice showed that buffer-applied FGF7 increased the amplitude of excitatory post-synaptic current evoked by stimulating the sensory afferent fibers. Behavior tests showed that intrathecally applied FGF7 potentiated the formalin-induced acute nociceptive response. Moreover, both acute and inflammatory nociceptive responses were significantly reduced in Fgf7-deficient mice. These results suggest that FGF7 exerts an excitatory modulation of nociceptive afferent transmission.

Catechol-O-methyltransferase inhibition alters pain and anxiety-related volitional behaviors through activation of β-adrenergic receptors in the rat

Reduced catechol-O-methyltransferase (COMT) activity resulting from genetic variation or pharmacological depletion results in enhanced pain perception in humans and nociceptive behaviors in animals. Using phasic mechanical and thermal reflex tests (e.g. von Frey, Hargreaves), recent studies show that acute COMT-dependent pain in rats is mediated by β-adrenergic receptors (βARs). In order to more closely mimic the characteristics of human chronic pain conditions associated with prolonged reductions in COMT, the present study sought to determine volitional pain-related and anxiety-like behavioral responses following sustained as well as acute COMT inhibition using an operant 10-45°C thermal place preference task and a light/dark preference test. In addition, we sought to evaluate the effects of sustained COMT inhibition on generalized body pain by measuring tactile sensory thresholds of the abdominal region. Results demonstrated that acute and sustained administration of the COMT inhibitor OR486 increased pain behavior in response to thermal heat. Further, sustained administration of OR486 increased anxiety behavior in response to bright light, as well as abdominal mechanosensation. Finally, all pain-related behaviors were blocked by the non-selective βAR antagonist propranolol. Collectively, these findings provide the first evidence that stimulation of βARs following acute or chronic COMT inhibition drives cognitive-affective behaviors associated with heightened pain that affects multiple body sites.

Biopsychosocial influence on shoulder pain: risk subgroups translated across preclinical and clinical prospective cohorts

Tailored treatment based on individual risk factors is an area with promise to improve options for pain relief. Musculoskeletal pain has a biopsychosocial nature, and multiple factors should be considered when determining risk for chronic pain. This study investigated whether subgroups comprised genetic and psychological factors predicted outcomes in preclinical and clinical models of shoulder pain. Classification and regression tree analysis was performed for an exercise-induced shoulder injury cohort (n = 190) to identify high-risk subgroups, and a surgical pain cohort (n = 150) was used for risk validation. Questionnaires for fear of pain and pain catastrophizing were administered before injury and preoperatively. DNA collected from saliva was genotyped for a priori selected genes involved with pain modulation (COMT and AVPR1A) and inflammation (IL1B and TNF/LTA). Recovery was operationalized as a brief pain inventory rating of 0/10 for current pain intensity and <2/10 for worst pain intensity. Follow-up for the preclinical cohort was in daily increments, whereas follow-up for the clinical cohort was at 3, 6, and 12 months postoperatively. Risk subgroups comprised the COMT high pain sensitivity variant and either pain catastrophizing or fear of pain were predictive of heightened shoulder pain responses in the preclinical model. Further analysis in the clinical model identified the COMT high pain sensitivity variant and pain catastrophizing subgroup as the better predictor. Future studies will determine whether these findings can be replicated in other anatomical regions and whether personalized medicine strategies can be developed for this risk subgroup.