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

Genetic polymorphisms on temporomandibular disorders: Network meta-analysis

OBJECTIVE

The aim of this systematic review and network meta-analysis (NMA) is to compare and rank the effects of different genetic polymorphisms on the susceptibility of temporomandibular disorders (TMDs) occurrence.

DESIGN

The central question formulated was: "Are genetic polymorphisms involved in the etiology of TMDs?" Following PROSPERO registration (CRD42024507886), electronic searches were conducted in five databases for publications up to November 2024.

RESULTS

Sixty-three studies were included in the systematic review and 7 composed the NMA. The qualitative analysis summarized the association between 120 genes (and 206 polymorphisms) and TMDs. Thirty-two polymorphisms (in 24 genes) were linked to overall TMDs, while 22 polymorphisms (in 22 genes) with degenerative bone changes in the temporomandibular joint (TMJ). Additionally, 17 polymorphisms were identified in cases of painful chronic TMD, while 12 polymorphisms in intra-articular disorders. These polymorphisms were in genes related to neurotransmission (COMT, ADRB2, DRD2, ANKK1, SLC6A4 and HTR2A), inflammatory mediators (TNFα, IL10 and MMP1), sex hormones (ESR1and ESRRB), oxidative stress (GSTM1) and bone metabolism (VDR). A protective effect for myalgia occurrence with the COMT_rs165774 polymorphism compared to the wild-type genotype was found in the pairwise meta-analysis (AG genotype: OR: 0.33; 95 %CI: 0.14, 0.76; p < 0.01 and GG genotype: OR: 0.32; 95 %CI: 0.14, 0.74; p < 0.01) and this polymorphism showed the highest probability of being associated with the myalgia (97 %) and arthralgia (93 %) conditions.

CONCLUSIONS

Genetic polymorphisms in genes related to neurotransmission, inflammatory response, and sex hormones seem to be risk factors related to the TMDs pathogenesis.

miR-374 family is a key regulator of chronic primary pain onset

Introduction

Chronic primary pain conditions (CPPCs) are linked to catecholamine activation of peripheral adrenergic receptors. Yet, catecholamine-dependent epigenetic mechanisms, such as microRNA (miRNA) regulation of mRNA transcripts, remain largely unknown.

Objectives

We sought to identify RNA species correlated with case status in 3 pain cohorts, to validate RNAs found to be dysregulated in a mouse model of CPPC onset, and to directly test the role of adrenergic receptors in miRNA regulation. Furthermore, we tested antinociceptive effects of miR-374 overexpression.

Methods

We used RNA-seq and quantitative reverse transcription polymerase chain reaction to measure RNA expression in 3 pain cohorts. Next, we validated identified RNAs with quantitative reverse transcription polymerase chain reaction in a mouse model of CPPC onset, measuring expression in plasma, peripheral (adipose, muscle, dorsal root ganglia [DRG]), and central (spinal cord) tissues. Then, we stimulated adrenergic receptors in primary adipocyte and DRG cultures to directly test regulation of microRNAs by adrenergic signaling. Furthermore, we used in vitro calcium imaging to measure the antinociceptive effects of miR-374 overexpression.

Results

We found that one miRNA family, miR-374, was downregulated in the plasma of individuals with temporomandibular disorder, fibromyalgia syndrome, or widespread pain following a motor vehicle collision. miR-374 was also downregulated in plasma, white adipose tissue, and spinal cord from mice with multisite mechanical sensitivity. miR-374 downregulation in plasma and spinal cord was female specific. Norepinephrine stimulation of primary adipocytes, but not DRG, led to decreased miR-374 expression. Furthermore, we identified tissue-specific and sex-specific changes in the expression of predicted miR-374 mRNA targets, including known (HIF1A, NUMB, TGFBR2) and new (ATXN7, CRK-II) pain targets. Finally, we demonstrated that miR-374 overexpression in DRG neurons reduced capsaicin-induced nociceptor activity.

Conclusions

Downregulation of miR-374 occurs between adrenergic receptor activation and mechanical hypersensitivity, and its adipocyte source implicates adipose signaling in nociception. Further study of miR-374 may inform therapeutic strategies for the millions worldwide who experience CPPCs.

A mouse model of chronic primary pain that integrates clinically relevant genetic vulnerability, stress, and minor injury

Chronic primary pain conditions (CPPCs) affect over 100 million Americans, predominantly women. They remain ineffectively treated, in large part because of a lack of valid animal models with translational relevance. Here, we characterized a CPPC mouse model that integrated clinically relevant genetic (catechol-O-methyltransferase; COMT knockdown) and environmental (stress and injury) factors. Compared with wild-type mice, Comt+/- mice undergoing repeated swim stress and molar extraction surgery intervention exhibited pronounced multisite body pain and depressive-like behavior lasting >3 months. Comt+/- mice undergoing the intervention also exhibited enhanced activity of primary afferent nociceptors innervating hindpaw and low back sites and increased plasma concentrations of norepinephrine and pro-inflammatory cytokines interleukin-6 (IL-6) and IL-17A. The pain and depressive-like behavior were of greater magnitude and longer duration (≥12 months) in females versus males. Furthermore, increases in anxiety-like behavior and IL-6 were female-specific. The effect of COMT genotype × stress interactions on pain, IL-6, and IL-17A was validated in a cohort of 549 patients with CPPCs, demonstrating clinical relevance. Last, we assessed the predictive validity of the model for analgesic screening and found that it successfully predicted the lack of efficacy of minocycline and the CB2 agonist GW842166X, which were effective in spared nerve injury and complete Freund's adjuvant models, respectively, but failed in clinical trials. Yet, pain in the CPPC model was alleviated by the beta-3 adrenergic antagonist SR59230A. Thus, the CPPC mouse model reliably recapitulates clinically and biologically relevant features of CPPCs and may be implemented to test underlying mechanisms and find new therapeutics.

Blocking brown adipocyte β3-adrenoceptor attenuates blood-spinal cord barrier impairment and chronic postsurgical pain in a rat model of preoperative stress

Preoperative stress has been recognized as an independent risk factor for chronic postsurgical pain (CPSP). However, the underlying mechanisms of CPSP influenced by preoperative stress remain elusive. Previous studies indicated that excessive stress could induce disruption of the blood-spinal cord barrier (BSCB). We wondered whether and how BSCB involves in CPSP by using a single prolonged stress (SPS) combining plantar incision model in male rats to mimic preoperative stress-related postsurgical pain. Here, we observed that preoperative SPS-exposed rats exhibited relentless incisional pain, which was accompanied by impairment of BSCB and persistent elevation of serum IL-6. Intraperitoneal injections of Tocilizumab (an IL-6 receptor monoclonal antibody) not only mitigated BSCB breakdown but also alleviated pain behaviors. In addition, intervening β3-adrenoceptor (ADRB3) signaling in brown adipocytes by SR59230a (a specific ADRB3 antagonist) treatment or removal of brown adipose tissues could effectively decrease serum IL-6 levels, ameliorate BSCB disruption, and alleviate incisional pain. Further results displayed that SI-exposed rats also showed markedly spinal microglia activation. And exogenous His-tagged IL-6 could pass through the disrupted BSCB, which might contribute to microglia activation. Injection of SR59230a or ablation of brown adipose tissues could effectively reduce the activation of spinal microglia. Thus, our findings suggest that serum IL-6 induced by brown adipocyte ADRB3 signaling contributed to BSCB disruption and spinal microglia activation, which might be involved in preoperative stress mediated CPSP. This work indicates a promising treatment strategy for preoperative stress induced CPSP by blocking ADRB3.

Compromised trigemino-coerulean coupling in migraine sensitization can be prevented by blocking beta-receptors in the locus coeruleus

BACKGROUND

Migraine is a disabling neurological disorder, characterized by recurrent headaches. During migraine attacks, individuals often experience sensory symptoms such as cutaneous allodynia which indicates the presence of central sensitization. This sensitization is prevented by oral administration of propranolol, a common first-line medication for migraine prophylaxis, that also normalized the activation of the locus coeruleus (LC), considered as the main origin of descending noradrenergic pain controls. We hypothesized that the basal modulation of trigeminal sensory processing by the locus coeruleus is shifted towards more facilitation in migraineurs and that prophylactic action of propranolol may be attributed to a direct action in LC through beta-adrenergic receptors.

METHODS

We used simultaneous in vivo extracellular recordings from the trigeminocervical complex (TCC) and LC of male Sprague-Dawley rats to characterize the relationship between these two areas following repeated meningeal inflammatory soup infusions. Von Frey Hairs and air-puff were used to test periorbital mechanical allodynia. RNAscope and patch-clamp recordings allowed us to examine the action mechanism of propranolol.

RESULTS

We found a strong synchronization between TCC and LC spontaneous activities, with a precession of the LC, suggesting the LC drives TCC excitability. Following repeated dural-evoked trigeminal activations, we observed a disruption in coupling of activity within LC and TCC. This suggested an involvement of the two regions' interactions in the development of sensitization. Furthermore, we showed the co-expression of alpha-2A and beta-2 adrenergic receptors within LC neurons. Finally propranolol microinjections into the LC prevented trigeminal sensitization by desynchronizing and decreasing LC neuronal activity.

CONCLUSIONS

Altogether these results suggest that trigemino-coerulean coupling plays a pivotal role in migraine progression, and that propranolol's prophylactic effects involve, to some extent, the modulation of LC activity through beta-2 adrenergic receptors. This insight reveals new mechanistic aspects of LC control over sensory processing.

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.

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.

Beta 2-adrenergic receptor mediates noradrenergic action to induce cyclic adenosine monophosphate response element-binding protein phosphorylation in satellite glial cells of dorsal root ganglia to regulate visceral hypersensitivity

ABSTRACT

Sympathoneuronal outflow into dorsal root ganglia (DRG) is suggested to be involved in sympathetically maintained chronic pain, which is mediated by norepinephrine (NE) action on DRG cells. This study combined in vitro and in vivo approaches to identify the cell types of DRG that received NE action and examined cell type-specific expression of adrenergic receptors (ARs) in DRG. Using DRG explants, we identified that NE acted on satellite glial cells (SGCs) to induce the phosphorylation of cAMP response element-binding protein (CREB). Using primarily cultured SGCs, we identified that beta (β)2-adrenergic receptor but not alpha (α)adrenergic receptor nor other βAR isoforms mediated NE-induced CREB phosphorylation and CRE-promoted luciferase transcriptional activity. Using fluorescence in situ hybridization and affinity purification of mRNA from specific cell types, we identified that β2AR was expressed by SGCs but not DRG neurons. We further examined β2AR expression and CREB phosphorylation in vivo in a model of colitis in which sympathetic nerve sprouting in DRG was observed. We found that β2AR expression and CREB phosphorylation were increased in SGCs of thoracolumbar DRG on day 7 after colitis induction. Inhibition but not augmentation of β2AR reduced colitis-induced calcitonin gene-related peptide release into the spinal cord dorsal horn and colonic pain responses to colorectal distention. Prolonged activation of β2AR in naive DRG increased calcitonin gene-related peptide expression in DRG neurons. These findings provide molecular basis of sympathetic modulation of sensory activity and chronic pain that involves β2AR-mediated signaling in SGCs of DRG.

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.

Intracerebroventricular injection of propranolol blocked analgesic and neuroprotective effects of resveratrol following L5 spinal nerve ligation in rat

OBJECTIVES

Resveratrol as a natural polyphenolic agent can alleviate neuropathic pain symptoms. The mechanism of analgesic activity of resveratrol is far from clear. The current study examine whether analgesic activity of resveratrol is mediated by its neuroprotective and anti-oxidant activity in the neuropathic pain. We further examine whether analgesic activity of resveratrol is mediated by β-adrenoceptors in the brain.

METHODS

Neuropathic pain induced by L₅ spinal nerve ligation (SNL). Male Wistar rats assigned into sham, SNL, SNL + resveratrol (40 μg/5 μL), and SNL + resveratrol + propranolol (a non-selective β-adrenoceptor antagonist, 30 μg/5 μL) groups. Drugs injected intracerebroventricular (ICV) at day SNL surgery and daily for 6 days following SNL. Thermal allodynia and anxiety examined on days of -1, 2, 4, and 6 following SNL. Electrophysiological study performed on day 6 following SNL for evaluation of resveratrol effects on sciatic nerve conduction velocity (NCV). The activity of catalase (Cat) and superoxide dismutase (SOD) enzymes in the brain assessed on days 6 following SNL.

RESULTS

Resveratrol significantly decreased thermal allodynia (and not anxiety) in all experimental days. Additionally, resveratrol significantly increased NCV, and also normalized the disrupted Cat and SOD activities following neuropathic pain. Furthermore, propranolol significantly blocked the analgesic and neuroprotective effects of resveratrol.

CONCLUSIONS

It is suggested that the analgesic effects of resveratrol is mediated by its neuroprotective and antioxidant activities in the neuropathic rats. Furthermore, propranolol blocked the analgesic and neuroprotective effects of resveratrol.

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.

Systemic administration of a β2-adrenergic receptor agonist reduces mechanical allodynia and suppresses the immune response to surgery in a rat model of persistent post-incisional hypersensitivity

Beta 2 adrenergic receptor (β2 AR) activation in the central and peripheral nervous system has been implicated in nociceptive processing in acute and chronic pain settings with anti-inflammatory and anti-allodynic effects of β2-AR mimetics reported in several pain states. In the current study, we examined the therapeutic efficacy of the β2-AR agonist clenbuterol in a rat model of persistent postsurgical hypersensitivity induced by disruption of descending noradrenergic signaling in rats with plantar incision. We used growth curve modeling of ipsilateral mechanical paw withdrawal thresholds following incision to examine effects of treatment on postoperative trajectories. Depletion of spinal noradrenergic neurons delayed recovery of hypersensitivity following incision evident as a flattened slope compared to non-depleted rats (-1.8 g/day with 95% CI -2.4 to -1.085, p < 0.0001). Chronic administration of clenbuterol reduced mechanical hypersensitivity evident as a greater initial intercept in noradrenergic depleted (6.2 g with 95% CI 1.6 to 10.8, p = 0.013) and non-depleted rats (5.4 g with 95% CI 1.2 to 9.6, p = 0.018) with plantar incision compared to vehicle treated rats. Despite a persistent reduction in mechanical hypersensitivity, clenbuterol did not alter the slope of recovery when modeled over several days (p = 0.053) or five weeks in depleted rats (p = 0.64). Systemic clenbuterol suppressed the enhanced microglial activation in depleted rats and reduced the density of macrophage at the site of incision. Direct spinal infusion of clenbuterol failed to reduce mechanical hypersensitivity in depleted rats with incision suggesting that beneficial effects of β2-AR stimulation in this model are largely peripherally mediated. Lastly, we examined β2-AR distribution in the spinal cord and skin using in-situ hybridization and IHC. These data add to our understanding of the role of β2-ARs in the nervous system on hypersensitivity after surgical incision and extend previously observed anti-inflammatory actions of β2-AR agonists to models of surgical injury.

Effect of comorbid migraine on propranolol efficacy for painful TMD in a randomized controlled trial

Introduction

The migraine-preventive drug propranolol is efficacious in reducing pain from temporomandibular disorder, suggesting potential modifying or mediating effects of comorbid migraine.

Methods

In this randomized controlled trial, myofascial temporomandibular disorder patients were treated with propranolol or placebo for 9 weeks. The primary endpoint was change in a facial pain index derived from daily symptom diaries. Linear and logistic regression models tested for a migraine × treatment-group interaction in reducing facial pain index. Counterfactual models explored changes in headache impact and heart rate as mediators of propranolol's efficacy.

Results

Propranolol's efficacy in reducing facial pain index was greater among the 104 migraineurs than the 95 non-migraineurs: For example, for the binary ≥ 30% reduction in facial pain index, odds ratios were 3.3 (95% confidence limits: 1.4, 8.1) versus 1.3 (0.5, 3.2), respectively, although the interaction was statistically non-significant (p = 0.139). Cumulative response curves confirmed greater efficacy for migraineurs than non-migraineurs (differences in area under the curve 26% and 6%, respectively; p = 0.081). While 9% of the treatment effect was mediated by reduced headache impact, 46% was mediated by reduced heart rate.

Conclusions

Propranolol was more efficacious in reducing temporomandibular disorder pain among migraineurs than non-migraineurs, with more of the effect mediated by reduced heart rate than by reduced headache impact.

Study identification and registration

SOPPRANO; NCT02437383; https://clinicaltrials.gov/ct2/show/NCT02437383

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.

Membrane bound catechol-O-methytransferase is the dominant isoform for dopamine metabolism in PC12 cells and rat brain

Impaired dopamine activity in the dorsolateral prefrontal cortex (DLPFC) is thought to contribute to cognitive deficits in diseases such as schizophrenia, attention deficit hyperactivity disorder (ADHD) and traumatic brain injury. Catechol-O-methyltransfease (COMT) metabolizes dopamine and is an important regulator of dopamine signaling in the DLPFC. In mammalian species, two isoforms of COMT protein, membrane-bound COMT (MB-COMT) and soluble COMT (S-COMT), are encoded by one COMT gene and expressed widely. While S-COMT is thought to play a dominant role in the peripheral tissues, MB-COMT is suggested to have a greater role in dopamine metabolism in the brain. However, whether a selective inhibitor for MB-COMT may effectively block dopamine metabolism remains unknown. We generated a knockout of MB-COMT in PC12 cells using CRISPR-cas9 technology to evaluate the effect of both MB and S-COMT on dopamine metabolism. Deletion of MB-COMT in PC12 cells significantly decreased homovanillic acid (HVA), completely depleted 3-methyoxytyramine (3-MT), and significantly increased 3,4-dihydroxyphenylacetic acid (DOPAC) levels. Comparison of the effect of a MB-COMT selective inhibitor LI-1141 on dopamine metabolism in wild type and MB-COMT knockout PC12 cells allowed us to confirm the selectivity of LI-1141 with respect to MB-COMT in cells. Under conditions in which LI-1141 was shown to inhibit only MB-COMT but not S-COMT, it effectively changed dopamine metabolites similar to the effect induced by tolcapone, a non-selective COMT inhibitor, suggesting that selective inhibition of MB-COMT will be effective in blocking dopamine metabolism, providing an attractive therapeutic approach in improving cognition for patients.

Mechanisms of microbial-neuronal interactions in pain and nociception

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Molecular mechanisms of how microorganisms communicate with sensory afferent neurons.

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How pathogenic microorganisms directly communicate with nociceptor neurons to inflict pain on the host.

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Symbiotic bacterial communication with gut-extrinsic sensory afferent neurons.

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Plausible roles on how gut symbionts directly mediate pain and nociception.

Nociceptor sensory neurons innervate barrier tissues that are constantly exposed to microbial stimuli. During infection, pathogenic microorganisms can breach barrier surfaces and produce pain by directly activating nociceptors. Microorganisms that live in symbiotic relationships with their hosts, commensals and mutualists, have also been associated with pain, but the molecular mechanisms of how symbionts act on nociceptor neurons to modulate pain remain largely unknown. In this review, we will discuss the known molecular mechanisms of how microbes directly interact with sensory afferent neurons affecting nociception in the gut, skin and lungs. We will touch on how bacterial, viral and fungal pathogens signal to the host to inflict or suppress pain. We will also discuss recent studies examining how gut symbionts affect pain. Specifically, we will discuss how gut symbionts may interact with sensory afferent neurons either directly, through secretion of metabolites or neurotransmitters, or indirectly,through first signaling to epithelial cells or immune cells, to regulate visceral, neuropathic and inflammatory pain. While this area of research is still in its infancy, more mechanistic studies to examine microbial-sensory neuron crosstalk in nociception may allow us to develop new therapies for the treatment of acute and chronic pain.

Beyond Amitriptyline: A Pediatric and Adolescent Oriented Narrative Review of the Analgesic Properties of Psychotropic Medications for the Treatment of Complex Pain and Headache Disorders

Children and adolescents with recurrent or chronic pain and headache are a complex and heterogenous population. Patients are best served by multi-specialty, multidisciplinary teams to assess and create tailored, individualized pain treatment and rehabilitation plans. Due to the complex nature of pain, generalizing pharmacologic treatment recommendations in children with recurrent or chronic pains is challenging. This is particularly true of complicated patients with co-existing painful and psychiatric conditions. There is an unfortunate dearth of evidence to support many pharmacologic therapies to treat children with chronic pain and headache. This narrative review hopes to supplement the available treatment options for this complex population by reviewing the pediatric and adult literature for analgesic properties of medications that also have psychiatric indication. The medications reviewed belong to medication classes typically described as antidepressants, alpha 2 delta ligands, mood stabilizers, anti-psychotics, anti-sympathetic agents, and stimulants.

COMT Genotype and Efficacy of Propranolol for TMD Pain: A Randomized Trial

Propranolol is a nonselective β-adrenergic receptor antagonist that is efficacious in reducing facial pain. There is evidence that its analgesic efficacy might be modified by variants of the catechol-O-methyltransferase (COMT) gene. We tested the hypothesis in a subset of 143 non-Hispanic Whites from a randomized controlled trial of patients with painful temporomandibular disorder (TMD). Patients were genotyped for rs4680, a single nucleotide polymorphism of COMT, and randomly allocated to either propranolol 60 mg twice daily or placebo. During the 9-wk follow-up period, patients recorded daily ratings of facial pain intensity and duration; the product was computed as an index of facial pain. Postbaseline change in the index at week 9 (the primary endpoint) was analyzed as a continuous variable and dichotomized at thresholds of ≥30% and ≥50% reduction. Mixed models for repeated measures tested for the genotype × treatment group interaction and estimated means, odds ratios (ORs), and 95% confidence limits (95% CLs) of efficacy within COMT genotypes assuming an additive genetic model. In secondary analysis, the cumulative response curves were plotted for dichotomized reductions ranging from ≥20% to ≥70%, and genotype differences in area under the curve percentages (%AUC) were calculated to signify efficacy. Mean index reduction did not differ significantly (P = 0.277) according to genotype, whereas the dichotomized ≥30% reduction revealed greater efficacy among G:G homozygotes (OR = 10.9, 95%CL = 2.4, 50.7) than among A:A homozygotes (OR = 0.8, 95%CL = 0.2, 3.2) with statistically significant interaction (P = 0.035). Cumulative response curves confirmed greater (P = 0.003) efficacy for G:G homozygotes (%AUC difference = 43.7, 95%CL = 15.4, 72.1) than for A:A homozygotes (%AUC difference = 6.5, 95%CL = -30.2, 43.2). The observed antagonistic effect of the A allele on propranolol’s efficacy was opposite the synergistic effect hypothesized a priori. This unexpected result highlights the need for better knowledge of COMT’s role in pain pathogenesis if the gene is to be used for precision-medicine treatment of TMD (ClinicalTrials.gov NCT02437383).

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.

Unpredictable stress delays recovery from exercise-induced muscle pain: contribution of the sympathoadrenal axis

Introduction

Although stress is a well-establish risk factor for the development of chronic musculoskeletal pain, the underlying mechanisms, specifically the contribution of neuroendocrine stress axes, remain poorly understood.

Objective

To evaluate the hypothesis that psychological stress-induced activation of the sympathoadrenal stress axis prolongs the muscle pain observed after strenuous exercise.

Methods

Adult male Sprague-Dawley rats were exposed to unpredictable sound stress and eccentric exercise. The involvement of the sympathoadrenal stress axis was evaluated by means of surgical interventions, systemic administration of epinephrine, and intrathecal β₂-adrenergic receptor antisense.

Results

Although sound stress alone did not modify nociceptive threshold, it prolonged eccentric exercise-induced mechanical hyperalgesia. Adrenal medullectomy (ADMdX) attenuated, and administration of stress levels of epinephrine to ADMdX rats mimicked this effect of sound stress. Knockdown of β₂-adrenergic receptors by intrathecal antisense also attenuated sound stress-induced prolongation of eccentric exercise-induced hyperalgesia.

Conclusion

Together, these results indicate that sympathoadrenal activation, by unpredictable sound stress, disrupts the capacity of nociceptors to sense recovery from eccentric exercise, leading to the prolongation of muscle hyperalgesia. This prolonged recovery from ergonomic pain is due, at least in part, to the activation of β₂-adrenergic receptors on muscle nociceptors.

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.

Systems pharmacogenomics - gene, disease, drug and placebo interactions: a case study in COMT

Disease, drugs and the placebos used as comparators are inextricably linked in the methodology of the double-blind, randomized controlled trial. Nonetheless, pharmacogenomics, the study of how individuals respond to drugs based on genetic substrate, focuses primarily on the link between genes and drugs, while the link between genes and disease is often overlooked and the link between genes and placebos is largely ignored. Herein, we use the example of the enzyme catechol-O-methyltransferase to examine the hypothesis that genes can function as pharmacogenomic hubs across system-wide regulatory processes that, if perturbed in andomized controlled trials, can have primary and combinatorial effects on drug and placebo responses.

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.

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.

Quantitative Sensory Testing, Antihistamines, and Beta-Blockers for Management of Persistent Breast Pain: A Case Series

BACKGROUND

It is not uncommon for mothers to have persistent pain with breastfeeding beyond the first few weeks after birth. Persistent pain can be multifactorial, with neuropathic pain maintained by central sensitization being one dimension. Our knowledge in delineating categories of persistent pain is simple and not very sophisticated.

METHODS

We have developed and tested a Lactation Quantitative Sensory Test (L-QST) to quantify the neuropathic component of persistent breastfeeding pain. We present three case reports of neuropathic breastfeeding pain and treatment, and we discuss the potential role of histamine and catecholamines in persistent breastfeeding-associated pain.

CONCLUSIONS

The L-QST can be a useful tool to quantify neuropathic pain. Further studies are needed to test inter-observer reliability and reproducibility of this tool. Antihistamines can be considered for treating persistent pain in breastfeeding women with a history of allergy or atopy, and beta-blockers may be helpful in women with multiple pain disorders.

Distribution, properties, and inhibitor sensitivity of zebrafish catechol-O-methyl transferases (COMT)

Catechol-O-methyltransferase (COMT; EC 2.1.1.6) is an enzyme with multiple functions in vertebrates. COMT methylates and thus inactivates catecholamine neurotransmitters and metabolizes xenobiotic catechols. Gene polymorphism rs4680 that influences the enzymatic activity of COMT affects cognition and behavior in humans. The zebrafish is widely used as an experimental animal in many areas of biomedical research, but most aspects of COMT function in this species have remained uncharacterized. We hypothesized that both comt genes play essential roles in zebrafish. Both comt-a and comt-b were widely expressed in zebrafish tissues, but their relative abundance varied considerably. Homogenates of zebrafish organs, including the brain, showed enzymatic COMT activity that was the highest in the liver and kidney. Treatment of larval zebrafish with the COMT inhibitor Ro41-0960 shifted the balance of catecholamine metabolic pathways towards increased oxidative metabolism. Whole-body concentrations of dioxyphenylacetic acid (DOPAC), a product of dopamine oxidation, were increased in the inhibitor-treated larvae, although the dopamine levels were unchanged. Thus, COMT is likely to participate in the processing of catecholamine neurotransmitters in the zebrafish, but the inhibition of COMT in larval fish is compensated efficiently and does not have pronounced effects on dopamine levels.

Risk factors and prognosis of pain events during mechanical ventilation: a retrospective study

Background

Pain assessment is highly recommended in patients receiving mechanical ventilation. However, pain intensity and its impact on outcomes in these patients remain obscure. We collected the results of routine pain assessments, utilizing the behavioral pain scale (BPS), from 151 patients receiving mechanical ventilation. Risk factors associated with a pain event, defined as BPS of >5, and its impact on patient outcomes were investigated.

Methods

A total of 151 consecutive adult patients receiving mechanical ventilation for more than 24 h in a single 10-bed ICU were enrolled in this study. The highest BPS within 48 h after the initiation of mechanical ventilation was collected, as well as information about the patients’ characteristics and medication received. We also recorded patient outcomes, including time to successful weaning from mechanical ventilation, time to successful ICU discharge, and 30-day in-hospital mortality. Multivariate logistic regression analysis was used to determine factors independently associated with patients with a BPS of >5. Clinical outcomes were also assessed using multivariate logistic regression analysis, correcting for risk factors.

Results

We analyzed 151 patients. The median highest BPS was 4. The percentage of patients who recorded a BPS of >5 was 19.9% (n = 30). Multivariate logistic regression analysis revealed that the disuse of fentanyl and inotropic support was an independent predictor of pain event. Multivariable Cox regression analysis suggested that the development of a BPS of >5 was associated with increased mortality and a not statistically significant trend towards prolonged mechanical ventilation.

Conclusions

A significant proportion of ventilated patients experienced a BPS of >5 soon after the initiation of mechanical ventilation. Disuse of fentanyl and use of inotropic agents increased the risk of developing a BPS of >5 during mechanical ventilation. An association between adequate analgesia and improved patient outcomes provides a rationale for the assessment of pain during mechanical ventilation, with subsequent intervention if necessary.

Pain events were common among ventilated patients. In critical care settings, appropriate and adequate pain management is warranted, given the association with improved patient outcomes.