Sensitivity to movement-evoked pain, central sensitivity symptoms, and pro-nociceptive profiles in people with chronic shoulder pain: A parallel-group cross-sectional investigation.

OBJECTIVE: To investigate whether sensitivity to movement-evoked pain (SMEP), central sensitivity symptom burden, and quantitative sensory testing (QST) outcomes differ between healthy controls and people with chronic shoulder pain. METHODS: People with chronic shoulder pain (n = 39) and healthy controls (n = 26) completed validated questionnaires measuring demographic, pain characteristics, psychological factors, social support, sleep quality, central sensitivity inventory (CSI), and physical activity levels. A blinded assessor administered QST measuring pressure pain threshold, temporal summation, conditioned pain modulation, and cold hyperalgesia. All participants performed repeated lifting of weighted canisters and reported severity of pain over successive lifts of the weighted canisters. Between-group differences in the QST, SMEP and CSI scores were investigated. Demographic and psychosocial variables were adjusted in the analyses. RESULTS: Dynamic mechanical allodynia, mechanical temporal summation, movement-evoked pain scores, SMEP index, and CSI scores were significantly (p ≤ 0.05) higher in the chronic shoulder pain group than in healthy controls. A significant proportion of people with chronic shoulder pain presented with pro-nociceptive profiles and experienced higher pain severity, interference, and disability. CONCLUSIONS: People with chronic shoulder pain displayed symptoms and signs of central sensitization. Future research should investigate the predictive role of central sensitization on clinical outcomes in shoulder pain.

Dynamic Change of Endocannabinoid Signaling in the Medial Prefrontal Cortex Controls the Development of Depression After Neuropathic Pain.

Many patients with chronic pain conditions suffer from depression. The mechanisms underlying pain-induced depression are still unclear. There are critical links of medial prefrontal cortex (mPFC) synaptic function to depression, with signaling through the endocannabinoid (eCB) system as an important contributor. We hypothesized that afferent noxious inputs after injury compromise activity-dependent eCB signaling in the mPFC, resulting in depression. Depression-like behaviors were tested in male and female rats with traumatic neuropathy [spared nerve injury (SNI)], and neuronal activity in the mPFC was monitored using the immediate early gene c-fos and in vivo electrophysiological recordings. mPFC eCB Concentrations were determined using mass spectrometry, and behavioral and electrophysiological experiments were used to evaluate the role of alterations in eCB signaling in depression after pain. SNI-induced pain induced the development of depression phenotypes in both male and female rats. Pyramidal neurons in mPFC showed increased excitability followed by reduced excitability in the onset and prolonged phases of pain, respectively. Concentrations of the eCBs, 2-arachidonoylglycerol (2-AG) in the mPFC, were elevated initially after SNI, and our results indicate that this resulted in a loss of CB1R function on GABAergic interneurons in the mPFC. These data suggest that excessive release of 2-AG as a result of noxious stimuli triggers use-dependent loss of function of eCB signaling leading to excessive GABA release in the mPFC, with the final result being behavioral depression.SIGNIFICANCE STATEMENT Pain has both somatosensory and affective components, so the complexity of mechanisms underlying chronic pain is best represented by a biopsychosocial model that includes widespread CNS dysfunction. Many patients with chronic pain conditions develop depression. The mechanism by which pain causes depression is unclear. Although manipulation of the eCB signaling system as an avenue for providing analgesia per se has not shown much promise in previous studies. An important limitation of past research has been inadequate consideration of the dynamic nature of the connection between pain and depression as they develop. Here, we show that activity-dependent synthesis of eCBs during the initial onset of persistent pain is the critical link leading to depression when pain is persistent.

Gait analysis as a robust pain behavioural endpoint in the chronic phase of the monoiodoacetate-induced knee joint pain in the rat.

The monoiodoacetate-induced rat model of osteoarthritis knee pain is widely used. However, there are between-study differences in the pain behavioural endpoints assessed and in the dose of intraarticular monoiodoacetate administered. This study evaluated the robustness of gait analysis as a pain behavioural endpoint in the chronic phase of this model, in comparison with mechanical hyperalgesia in the injected (ipsilateral) joint and development of mechanical allodynia in the ipsilateral hind paws. Groups of Sprague-Dawley rats received a single intraarticular injection of monoiodoacetate at 0.5, 1, 2 or 3 mg or vehicle (saline) into the left (ipsilateral) knee joint. An additional group of rats were not injected (naïve group). The pain behavioural methods used were gait analysis, measurement of pressure algometry thresholds in the ipsilateral knee joints, and assessment of mechanical allodynia in the ipsilateral hind paws using von Frey filaments. These pain behavioural endpoints were assessed premonoiodoacetate injection and for up to 42-days postmonoiodoacetate injection in a blinded manner. Body weights were also assessed as a measure of general health. Good general health was maintained as all rats gained weight at a similar rate for the 42-day study period. In the chronic phase of the model (days 9-42), intraarticular monoiodoacetate at 3 mg evoked robust alterations in multiple gait parameters as well as persistent mechanical allodynia in the ipsilateral hind paws. For the chronic phase of the monoiodoacetate-induced rat model of osteoarthritis knee pain, gait analysis, such as mechanical allodynia in the ipsilateral hind paws, is a robust pain behavioural measure.

Extracellular signal-regulated kinases 2 (Erk2) and Erk5 in the central nervous system differentially contribute to central sensitization in male mice.

Nervous systems are designed to become extra sensitive to afferent nociceptive stimuli under certain circumstances such as inflammation and nerve injury. How pain hypersensitivity comes about is key issue in the field since it ultimately results in chronic pain. Central sensitization represents enhanced pain sensitivity due to increased neural signaling within the central nervous system (CNS). Particularly, much evidence indicates that underlying mechanism of central sensitization is associated with the change of spinal neurons. Extracellular signal-regulated kinases have received attention as key molecules in central sensitization. Previously, we revealed the isoform-specific function of extracellular signal-regulated kinase 2 (Erk2) in spinal neurons for central sensitization using mice with Cre-loxP-mediated deletion of Erk2 in the CNS. Still, how extracellular signal-regulated kinase 5 (Erk5) in spinal neurons contributes to central sensitization has not been directly tested, nor is the functional relevance of Erk5 and Erk2 known. Here, we show that Erk5 and Erk2 in the CNS play redundant and/or distinct roles in central sensitization, depending on the plasticity context (cell types, pain types, time, etc.). We used male mice with Erk5 deletion specifically in the CNS and found that Erk5 plays important roles in central sensitization in a formalin-induced inflammatory pain model. Deletion of both Erk2 and Erk5 leads to greater attenuation of central sensitization in this model, compared to deletion of either isoform alone. Conversely, Erk2 but not Erk5 plays important roles in central sensitization in neuropathic pain, a type of chronic pain caused by nerve damage. Our results suggest the elaborate mechanisms of Erk signaling in central sensitization.

Exacerbated Headache-Related Pain in the Single Prolonged Stress Preclinical Model of Post-traumatic Stress Disorder.

Chronic headache pain is one of the most commonly reported comorbid pain conditions with post-traumatic stress disorder (PTSD) patients and resistant to effective treatment, yet no combined preclinical model of the two disorders has been reported. Here, we used a modified chronic headache pain model to investigate the contribution of single prolonged stress (SPS) model of PTSD with sodium nitroprusside (SNP)-induced hyperalgesia. Injection of SNP (2 mg/kg, i.p.) occurred every other day from day 7 to day 15 after initiation of SPS in rats. Paw withdrawal threshold (PWT) to von Frey stimuli and tail flick latencies (TFL) dramatically decreased as early as 7 days after SPS and lasted until at least day 21. Basal PWT and TFL also significantly decreased during the SNP treatment period. The lower nociceptive thresholds recovered in 6 days following the final SNP injection in SNP group, but not in SPS + SNP group. Elevated nociceptin/OFQ (N/OFQ) levels observed in cerebrospinal fluid of SPS rats were even higher in SPS + SNP group. Glial fibrillary acidic protein (GFAP) and N/OFQ peptide (NOP) receptor mRNA expression increased in dorsal root ganglia (DRG) 21 days after SPS exposure; mRNA increases in the SPS/SNP group was more pronounced than SPS or SNP alone. GFAP protein expression was upregulated in trigeminal ganglia by SPS. Our results indicate that traumatic stress exaggerated chronic SNP-induced nociceptive hypersensitivity, and that N/OFQ and activated satellite glia cells may play an important role in the interaction between both conditions.

Photophobia and allodynia in persistent post-traumatic headache are associated with higher disease burden.

OBJECTIVE: To assess photophobia and allodynia in subjects with post-traumatic headache and examine how these sensory hypersensitivities associate with clinical measures of disease burden. BACKGROUND: Post-traumatic headache is the most frequent and disabling long-term consequence of mild traumatic brain injury. There is evidence of sensory dysfunction in acute post-traumatic headache, and it is known from other headache conditions that sensory amplifications correlate with more severe disease. However, systematic studies in post-traumatic headache are surprisingly scarce. METHODS: We tested light and tactile sensitivity, along with measures of disease burden, in 30 persistent post-traumatic headache subjects and 35 controls. RESULTS: In all, 79% of post-traumatic headache subjects exhibited sensory hypersensitivity based on psychophysical assessment. Of those exhibiting hypersensitivity, 54% exhibited both light and tactile sensitivity. Finally, sensory thresholds were correlated across modalities, as well as with headache attack frequency. CONCLUSIONS: In this study, post-traumatic headache subjects with both light and tactile sensitivity had significantly higher headache frequencies and lower sensitivity thresholds to both modalities, compared to those with single or no sensory hypersensitivity. This pattern suggests that hypersensitivity across multiple modalities may be functionally synergistic, reflect a higher disease burden, and may serve as candidate markers of disease.

Promiscuous G-Protein-Coupled Receptor Inhibition of Transient Receptor Potential Melastatin 3 Ion Channels by Gßγ Subunits.

Transient receptor potential melastatin 3 (TRPM3) is a nonselective cation channel that is inhibited by Gßγ subunits liberated following activation of Gαi/o protein-coupled receptors. Here, we demonstrate that TRPM3 channels are also inhibited by Gßγ released from Gαs and Gαq Activation of the Gs-coupled adenosine 2B receptor and the Gq-coupled muscarinic acetylcholine M1 receptor inhibited the activity of TRPM3 heterologously expressed in HEK293 cells. This inhibition was prevented when the Gßγ sink ßARK1-ct (C terminus of ß-adrenergic receptor kinase-1) was coexpressed with TRPM3. In neurons isolated from mouse dorsal root ganglion (DRG), native TRPM3 channels were inhibited by activating Gs-coupled prostaglandin-EP2 and Gq-coupled bradykinin B2 (BK2) receptors. The Gi/o inhibitor pertussis toxin and inhibitors of PKA and PKC had no effect on EP2- and BK2-mediated inhibition of TRPM3, demonstrating that the receptors did not act through Gαi/o or through the major protein kinases activated downstream of G-protein-coupled receptor (GPCR) activation. When DRG neurons were dialyzed with GRK2i, which sequesters free Gßγ protein, TRPM3 inhibition by EP2 and BK2 was significantly reduced. Intraplantar injections of EP2 or BK2 agonists inhibited both the nocifensive response evoked by TRPM3 agonists, and the heat hypersensitivity produced by Freund's Complete Adjuvant (FCA). Furthermore, FCA-induced heat hypersensitivity was completely reversed by the selective TRPM3 antagonist ononetin in WT mice and did not develop in Trpm3-/- mice. Our results demonstrate that TRPM3 is subject to promiscuous inhibition by Gßγ protein in heterologous expression systems, primary neurons and in vivo, and suggest a critical role for this ion channel in inflammatory heat hypersensitivity.SIGNIFICANCE STATEMENT The ion channel TRPM3 is widely expressed in the nervous system. Recent studies showed that Gαi/o-coupled GPCRs inhibit TRPM3 through a direct interaction between Gßγ subunits and TRPM3. Since Gßγ proteins can be liberated from other Gα subunits than Gαi/o, we examined whether activation of Gs- and Gq-coupled receptors also influence TRPM3 via Gßγ. Our results demonstrate that activation of Gs- and Gq-coupled GPCRs in recombinant cells and sensory neurons inhibits TRPM3 via Gßγ liberation. We also demonstrated that Gs- and Gq-coupled receptors inhibit TRPM3 in vivo, thereby reducing pain produced by activation of TRPM3, and inflammatory heat hypersensitivity. Our results identify Gßγ inhibition of TRPM3 as an effector mechanism shared by the major Gα subunits.

Microglia Promote Increased Pain Behavior through Enhanced Inflammation in the Spinal Cord during Repeated Social Defeat Stress.

Clinical studies indicate that psychosocial stress contributes to adverse chronic pain outcomes in patients, but it is unclear how this is initiated or amplified by stress. Repeated social defeat (RSD) is a mouse model of psychosocial stress that activates microglia, increases neuroinflammatory signaling, and augments pain and anxiety-like behaviors. We hypothesized that activated microglia within the spinal cord facilitate increased pain sensitivity following RSD. Here we show that mechanical allodynia in male mice was increased with exposure to RSD. This stress-induced behavior corresponded with increased mRNA expression of several inflammatory genes, including IL-1ß, TNF-α, CCL2, and TLR4 in the lumbar spinal cord. While there were several adhesion and chemokine-related genes increased in the lumbar spinal cord after RSD, there was no accumulation of monocytes or neutrophils. Notably, there was evidence of microglial activation selectively within the nociceptive neurocircuitry of the dorsal horn of the lumbar cord. Elimination of microglia using the colony stimulating factor 1 receptor antagonist PLX5622 from the brain and spinal cord prevented the development of mechanical allodynia in RSD-exposed mice. Microglial elimination also attenuated RSD-induced IL-1ß, CCR2, and TLR4 mRNA expression in the lumbar spinal cord. Together, RSD-induced allodynia was associated with microglia-mediated inflammation within the dorsal horn of the lumbar spinal cord.SIGNIFICANCE STATEMENT Mounting evidence indicates that psychological stress contributes to the onset and progression of adverse nociceptive conditions. We show here that repeated social defeat stress causes increased pain sensitivity due to inflammatory signaling within the nociceptive circuits of the spinal cord. Studies here mechanistically tested the role of microglia in the development of pain by stress. Pharmacological ablation of microglia prevented stress-induced pain sensitivity. These findings demonstrate that microglia are critical mediators in the induction of pain conditions by stress. Moreover, these studies provide a proof of principle that microglia can be targeted as a therapeutic strategy to mitigate adverse pain conditions.

Cumulative Effects of Psychologic Distress, Visceral Hypersensitivity, and Abnormal Transit on Patient-reported Outcomes in Irritable Bowel Syndrome.

BACKGROUND & AIMS: Little is known about the link between pathophysiologic factors and symptoms of irritable bowel syndrome (IBS), or whether these factors have cumulative effects on patient-reported outcomes (PROs). We investigated whether pathophysiologic alterations associated with IBS have cumulative or independent effects on PROs. METHODS: We performed a retrospective analysis of data from 3 cohorts of patients with IBS (n = 407; 74% female; mean age, 36 ± 12 years), based on Rome II or Rome III criteria, seen at a specialized unit for functional gastrointestinal disorders in Sweden from 2002 through 2014. All patients underwent assessments of colonic transit time (radiopaque markers); compliance, allodynia, and hyperalgesia (rectal barostat); anxiety and depression (Hospital Anxiety and Depression scale), as pathophysiologic factors. Dysfunction was defined by available normal values. PROs included IBS symptom severity, somatic symptom severity, and disease-specific quality of life. RESULTS: Allodynia was observed in 36% of patients, hyperalgesia in 22%, accelerated colonic transit in 18%, delayed transit in 7%, anxiety in 52%, and depression in 24%: each of these factors was associated with severity of at least 1 symptom of IBS. Rectal compliance was not associated with more severe symptoms of IBS. At least 3 pathophysiologic factors were present in 20% of patients, 2 in 30%, 1 in 31%, and none in 18%. With increasing number of pathophysiologic abnormalities, there was a gradual increase in IBS symptom severity (P < .0001) and somatic symptom severity (P < .0001), and a gradual reduction in quality of life (P < .0001). CONCLUSION: Visceral hypersensitivity, including allodynia and hyperalgesia, abnormal colonic transit, and psychologic factors are all associated with IBS symptoms. These factors have a cumulative effect on gastrointestinal and nongastrointestinal symptoms, as well as on quality of life, in patients with IBS and are therefore relevant treatment targets.

Activation of acid-sensing ion channel 3 by lysophosphatidylcholine 16:0 mediates psychological stress-induced fibromyalgia-like pain.

OBJECTIVES: Fibromyalgia is commonly considered a stress-related chronic pain disorder, and daily stressors are known triggers. However, the relation between stress and pain development remains poorly defined by clinical approaches. Also, the aetiology remains largely unknown. METHODS: We used a newly developed mouse model and lipidomic approaches to probe the causation and explore the biological plausibility for how perceived stress translates into chronic non-inflammatory pain. Clinical and lipidomic investigations of fibromyalgia were conducted for human validation. RESULTS: Using non-painful sound stimuli as psychological stressors, we demonstrated that mice developed long-lasting non-inflammatory hyperalgesia after repeated and intermittent sound stress exposure. Elevated serum malondialdehyde level in stressed mice indicated excessive oxidative stress and lipid oxidative damage. Lipidomics revealed upregulation of lysophosphatidylcholine 16:0 (LPC16:0), a product of lipid oxidisation, in stressed mice. Intramuscular LPC16:0 injection triggered nociceptive responses and a hyperalgesic priming-like effect that caused long-lasting hypersensitivity. Pharmacological or genetic inhibition of acid-sensing ion channel 3 impeded the development of LPC16:0-induced chronic hyperalgesia. Darapladib and antioxidants could effectively alleviate the stress-induced hyperalgesia by inhibiting LPC16:0 synthesis. Clinical investigations showed that excessive oxidative stress and LPC16:0 expression also exist in patients with fibromyalgia. Moreover, LPC16:0 expression was correlated with pain symptoms in patients with high oxidative stress and disease severity. CONCLUSIONS: Our study provides experimental evidence for the causal effect of psychological stressors on chronic pain development. The findings identify a possible pathophysiological mechanism of stress-induced chronic non-inflammatory pain at molecular, behavioural and clinical levels that might indicate a new therapeutic approach for fibromyalgia.

Early maternal separation accelerates the progression of endometriosis in adult mice.

BACKGROUND: A large body of research highlights the importance of early-life environmental impact on the health outcome in adulthood. However, whether early-life adversity (ELA) has any impact on the development of endometriosis is completely unclear. In this study, we tested the hypothesis that ELA, as manifested by neonatal separation, can accelerate the progression of endometriosis in mouse through activation of the adrenergic receptor ß2 (ADRB2) signaling pathway, leading to increased angiogenesis and progression of endometriotic lesions. METHODS: Eight female Balb/C mice, in late pregnancy, were used used for this study, which later gave birth to 22 female newborn pubs. Eleven additional female Balb/C mice were also used as donors of uterine tissues. The 22 newborn pubs were randomly divided into 2 equal-sized groups, maternal separation (MS) and no separation (NS). Pubs in the MS group were separated from their dams for 3 h/day from postnatal day (PND) 1 to 21, while those in the NS control remained in the home cage with their dams. In adulthood (8-week old), 3 mice in each group were randomly selected to undergo a battery of behavior tests. The remaining 8 mice in each group were induced with endometriosis by intraperitoneal injection of uterine fragments from donor mice. Four weeks after the induction, all mice were sacrificed and their endometriotic lesions were excised for quantification and then prepared for immunohistochemistry analysis. RESULTS: We confirmed that MS during infancy resulted in anxiety and depression-like behaviors as previously reported. We also found that in MS mice the lesion weight was increased by over 2 folds and generalized hyperalgesia was also significantly increased as compared with NS mice. Immunostaining analysis demonstrated that MS accelerated the development of endometriosis likely through decreased dopamine receptor D2 (DRD2) expression and activation of the ADRB2/cAMP-response element binding protein (CREB) signaling pathway, leading to increased angiogenesis and progression of endometriotic lesions. CONCLUSIONS: Exposure of female mouse pups to ELA such as MS during their infancy period accelerates the progression of endometriosis, possibly through altered neuronal wiring and hyperactivity of the hypothalamic-pituitary-adrenal axis.

Nocebo and the contribution of psychosocial factors to the generation of pain.

The biopsychosocial model claims that illness is generated by biological, psychological, and social factors. The nocebo response, particularly nocebo hyperalgesia, is an excellent model and approach to understand these effects and their psychophysiological underpinnings, as nocebos are made of negative psychological and social factors, such as negative expectations and social interactions. There is today experimental evidence that nocebos can create symptoms and illness from nothing, in particular pain, whereby a combination of biological, psychological and social factors interact with each other in the generation of the global painful experience. Several biochemical pathways have been identified, e.g. cholecystokinin and cyclooxygenase, and the activation of these mechanisms has been specifically investigated in the field of pain, analgesia and hyperalgesia. The study of placebo and nocebo oxygen at high-altitude has been crucial to unravel these mechanisms, as reduction of oxygen pressure (hypoxia) leads to headache pain. Indeed, the investigation of oxygen-related conditions, such as hypoxia, represents today an excellent approach to understand how nocebos can contribute to generate illness and pain. In this review we discuss old and new findings that help us better understand the interplay between biology and psychology.

Pain Response to Open Label Placebo in Induced Acute Pain in Healthy Adult Males.

BACKGROUND: Open label placebos with patient education are effective in reducing chronic pain, and recent studies on their effect on pain have established interest in this field. Nevertheless, data on their effect on acute pain are scarce, and on hyperalgesia and allodynia, absent. This study assessed the effect of open label placebos on acute pain in healthy adult males and the influence of placebo education. METHODS: Thirty-two healthy males were included in this prospective, randomized, assessor-blinded crossover, single-center study assessing pain intensities (via numeric rating scale), area of hyperalgesia (von Frey filament), and allodynia (dry cotton swab) in a pain model utilizing intracutaneous electrical stimulation. The authors compared the effect of intravenous open label placebo on pain compared to no treatment. The authors further examined the effect of placebo on hyperalgesia and allodynia, and the influence of education (short vs. detailed) before placebo application. Saliva cortisol concentrations were also measured. RESULTS: Pain ratings (median, first to third quartile) were 21% lower during placebo treatment compared to no treatment, 4.0 (3.2 to 4.9) versus 5.1 (4.7 to 5.4), respectively (P = 0.001). The areas of hyperalgesia and allodynia were lower during placebo treatment compared to no treatment (hyperalgesia, 30 cm [17 to 47] vs. 55 cm [42 to 68], P = 0.003; allodynia, 24 cm [11 to 39] vs. 45 cm [31 to 62], P = 0.007). This corresponds to reductions of 47%. The extent of placebo education had no effect on pain. Saliva cortisol decreased significantly over time and was under the limit of detectability in the majority of participants in postbaseline measurements in both treatment branches. Baseline cortisol was not associated with the placebo effect or strength applied of current to reach defined pain ratings. CONCLUSIONS: Open label placebos might play a role in multimodal analgesic concepts.

Repetitive noxious stimuli during early development affect acute and long-term mechanical sensitivity in rats.

BACKGROUND: Prematurely born infants are frequently exposed to painful procedures in the neonatal intensive care unit, causing changes to the development of the nervous system lasting into adulthood. The current study aims to study acute and long-term consequences of neonatal repetitive noxious stimulation. METHODS: Rat pups received either 4 or 10 unilateral needle pricks per day, while control littermates received 4 or 10 tactile stimuli in the first postnatal week. Behavioural sensitivity was assessed in the neonatal phase, in adulthood, and after re-injury of the same dermatome in adulthood. RESULTS: An increase in the number of repetitive painful stimuli, from 4 to 10 needle pricks per day, resulted in increased mechanical hypersensitivity during the neonatal period. In adulthood, repetitive painful stimuli resulted in hyposensitivity to mechanical stimuli, while thermal sensitivity was unaffected. After re-injury of the same dermatome in adulthood, the number of repetitive noxious stimuli did not affect mechanical hypersensitivity. Both needle prick groups showed an increased duration of postoperative hypersensitivity compared to control. CONCLUSION: This study shows that repetitive noxious stimulation during the early postnatal period affects acute and long-term mechanical sensitivity. Therefore, the amount of nociceptive stimuli should be minimized or adequately treated in a clinical setting.

To Experience or to Be Informed? Classical Conditioning Induces Nocebo Hyperalgesia even when Placebo Analgesia Is Verbally Suggested-Results of a Preliminary Study.

OBJECTIVE: To investigate whether direct experience (i.e., classical conditioning) or verbal suggestion is more important in inducing nocebo hyperalgesia, five groups (total sample size, N = 99) were studied: conditioning, congruent conditioning, incongruent conditioning, verbal suggestion, and control. METHODS: Participants in groups with conditioning experienced more intensive pain stimuli after presentation of a white circle. In the congruent conditioning group, suggestion that the circle would precede more intensive pain stimuli was additionally provided, whereas in the incongruent conditioning group, the opposite suggestion was used. Control and verbal suggestion groups received pain stimuli of one intensity; however, the latter received suggestion that a circle would precede pain stimuli of higher intensity. RESULTS: The nocebo effect was observed in all conditioning groups, regardless of the verbal suggestions used. Moreover, the experience of hyperalgesia was able to nullify the effect of the verbal suggestion of analgesia. Incongruence between verbal suggestion and pain experience produced expectancies that affected nocebo hyperalgesia. CONCLUSIONS: The results of this preliminary study suggest that direct experience seems to be more important than verbal suggestion in inducing nocebo hyperalgesia.

Persons with Complex Regional Pain Syndrome Renegotiate Social Roles and Intimacy: A Qualitative Study.

OBJECTIVE: Persons with complex regional pain syndrome often experience allodynia, where touch is painful. Allodynia is associated with poor prognosis, but the impacts on roles, activities, social relationships, and intimacy remain unclear. There is a need to examine intimacy in complex regional pain syndrome from a lived experience perspective. METHODS: We conducted a secondary analysis of cognitive debriefing interview data from 44 persons with complex regional pain syndrome who completed a patient-reported questionnaire. Using interpretive description and thematic analysis, we analyzed items and responses addressing allodynia, relationships, and intimacy. RESULTS: Two themes were developed to understand intimacy related to the pain experience: a renegotiated social identity and participation and a reinvented intimate self. These themes included elements of a) loss of control, b) loss of shared experiences, c) feeling that their condition was misunderstood, d) a need for self-preservation, e) altered self-concept, and e) the concept of intimacy is broader than sexuality. Our findings suggest that complex regional pain syndrome has pervasive impacts on relationships and intimacy that merit discussion with their health care team. CONCLUSIONS: Persons with persistent pain need to be supported in roles and activities that allow them to express intimacy in their everyday lives.

CaMKIIα Mediates the Effect of IL-17 To Promote Ongoing Spontaneous and Evoked Pain in Multiple Sclerosis.

Pain is a common and severe symptom in multiple sclerosis (MS), a chronic inflammatory and demyelinating disease of the CNS. The neurobiological mechanism underlying MS pain is poorly understood. In this study, we investigated the role of Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) in driving chronic pain in MS using a mouse experimental autoimmune encephalomyelitis (EAE) model. We found that spinal CaMKIIα activity was enhanced in EAE, correlating with the development of ongoing spontaneous pain and evoked hypersensitivity to mechanical and thermal stimuli. Prophylactic or acute administration of KN93, a CaMKIIα inhibitor, significantly reduced the clinical scores of EAE and attenuated mechanical allodynia and thermal hyperalgesia in EAE. siRNA targeting CaMKIIα reversed established mechanical and thermal hypersensitivity in EAE mice. Furthermore, CaMKIIαT286A point mutation mice showed significantly reduced EAE clinical scores, an absence of evoked pain, and ongoing spontaneous pain when compared with littermate wild-type mice. We found that IL-17 is responsible for inducing but not maintaining mechanical and thermal hyperalgesia that is mediated by CaMKIIα signaling in EAE. Together, these data implicate a critical role of CaMKIIα as a cellular mechanism for pain and neuropathy in multiple sclerosis and IL-17 may act upstream of CaMKIIα in the generation of pain.SIGNIFICANCE STATEMENT Pain is highly prevalent in patients with multiple sclerosis (MS), significantly reducing patients' quality of life. Using the experimental autoimmune encephalomyelitis (EAE) model, we were able to study not only evoked hyperalgesia, but also for the first time to demonstrate spontaneous pain that is also experienced by patients. Our study identified a role of spinal CaMKIIα in promoting and maintaining persistent ongoing spontaneous pain and evoked hyperalgesia pain in EAE. We further demonstrated that IL-17 contributes to persistent pain in EAE and functions as an upstream regulator of CaMKIIα signaling. These data for the first time implicated CaMKIIα and IL-17 as critical regulators of persistent pain in EAE, which may ultimately offer new therapeutic targets for mitigating pain in multiple sclerosis.

DRG Voltage-Gated Sodium Channel 1.7 Is Upregulated in Paclitaxel-Induced Neuropathy in Rats and in Humans with Neuropathic Pain.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common adverse effect experienced by cancer patients receiving treatment with paclitaxel. The voltage-gated sodium channel 1.7 (Nav1.7) plays an important role in multiple preclinical models of neuropathic pain and in inherited human pain phenotypes, and its gene expression is increased in dorsal root ganglia (DRGs) of paclitaxel-treated rats. Hence, the potential of change in the expression and function of Nav1.7 protein in DRGs from male rats with paclitaxel-related CIPN and from male and female humans with cancer-related neuropathic pain was tested here. Double immunofluorescence in CIPN rats showed that Nav1.7 was upregulated in small DRG neuron somata, especially those also expressing calcitonin gene-related peptide (CGRP), and in central processes of these cells in the superficial spinal dorsal horn. Whole-cell patch-clamp recordings in rat DRG neurons revealed that paclitaxel induced an enhancement of ProTx II (a selective Nav1.7 channel blocker)-sensitive sodium currents. Bath-applied ProTx II suppressed spontaneous action potentials in DRG neurons occurring in rats with CIPN, while intrathecal injection of ProTx II significantly attenuated behavioral signs of CIPN. Complementarily, DRG neurons isolated from segments where patients had a history of neuropathic pain also showed electrophysiological and immunofluorescence results indicating an increased expression of Nav1.7 associated with spontaneous activity. Nav1.7 was also colocalized in human cells expressing transient receptor potential vanilloid 1 and CGRP. Furthermore, ProTx II decreased firing frequency in human DRGs with spontaneous action potentials. This study suggests that Nav1.7 may provide a potential new target for the treatment of neuropathic pain, including chemotherapy (paclitaxel)-induced neuropathic pain.SIGNIFICANCE STATEMENT This work demonstrates that the expression and function of the voltage-gated sodium channel Nav1.7 are increased in a preclinical model of chemotherapy-induced peripheral neuropathy (CIPN), the most common treatment-limiting side effect of all the most common anticancer therapies. This is key as gain-of-function mutations in human Nav1.7 recapitulate both the distribution and pain percept as shown by CIPN patients. This work also shows that Nav1.7 is increased in human DRG neurons only in dermatomes where patients are experiencing acquired neuropathic pain symptoms. This work therefore has major translational impact, indicating an important novel therapeutic avenue for neuropathic pain as a class.

Preoperative anxiety-induced glucocorticoid signaling reduces GABAergic markers in spinal cord and promotes postoperative hyperalgesia by affecting neuronal PAS domain protein 4.

Preoperative anxiety is common in patients undergoing elective surgery and is closely related to postoperative hyperalgesia. In this study, a single prolonged stress model was used to induce preoperative anxiety-like behavior in rats 24 h before the surgery. We found that single prolonged stress exacerbated the postoperative pain and elevated the level of serum corticosterone. Previous studies have shown that glucocorticoid is associated with synaptic plasticity, and decreased spinal GABAergic activity can cause hyperalgesia in rodents. Here, single prolonged stress rats' lumbar spinal cord showed reduced glutamic acid decarboxylase-65, glutamic acid decarboxylase-67, GABA type A receptor alpha 1 subunit, and GABA type A receptor gamma 2 subunit, indicating an impairment of GABAergic system. Furthermore, neuronal PAS domain protein 4 was also reduced in rats after single prolonged stress stimulation, which has been reported to promote GABAergic synapse development. Then, intraperitoneal injection of RU486 (a glucocorticoid receptor antagonist) rather than spironolactone (a mineralocorticoid receptor antagonist) was found to relieve single prolonged stress-induced hyperalgesia and reverse neuronal PAS domain protein 4 reduction and the impairment of GABAergic system. Furthermore, overexpressing neuronal PAS domain protein 4 could also restore the damage of GABAergic system caused by single prolonged stress while interfering with neuronal PAS domain protein 4 caused an opposite effect. Finally, after stimulation of rat primary spinal cord neurons with exogenous corticosterone in vitro, neuronal PAS domain protein 4 and GABAergic markers were also downregulated, and RU486 reversed that. Together, our results demonstrated that preoperative anxiety led to GABAergic system impairment in spinal cord and thus caused hyperalgesia due to glucocorticoid-induced downregulation of neuronal PAS domain protein 4.

Microneedles Coated with Tramadol Exhibit Antinociceptive Effect in a Rat Model of Temporomandibular Hypernociception.

Coated microneedles have emerged as a promising drug delivery system for inflammatory pain treatment. We have previously shown that tramadol injection into the rat temporomandibular joint (TMJ) induces an antinociceptive and anti-inflammatory effect. In this study, microneedles coated with tramadol were investigated as a platform to treat TMJ pain. Male Wistar rats were administered tramadol using an intra-TMJ injection or with microneedles coated with tramadol, followed by 1.5% formalin nociceptive challenge administered 15 minutes later. The nociceptive behavior of rats was evaluated, and their periarticular tissues were removed after euthanasia for analysis. The duration of antinociceptive effect was determined by performing the formalin challenge at different time points extending up to 6 days post tramadol administration. Microneedles coated with tramadol produced an antinociceptive effect similar to injection of tramadol into the rat TMJ. Surprisingly, tramadol delivery using coated microneedles produced a more durable antinociceptive effect lasting as much as 2 days post tramadol delivery as compared with an antinociceptive effect lasting under 2 hours from intra-TMJ injection of tramadol. The proinflammatory cytokines tumor necrosis factor-α and interleukin-1ß (IL-1ß) were found to be reduced, whereas the anti-inflammatory cytokine IL-10 was found to be elevated in tramadol-treated groups. In conclusion, microneedles coated with tramadol can offer a therapeutic option for pain control of inflammatory disorders in the TMJ.