Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Pain Biomarkers in Fibromyalgia Syndrome: Current Understanding and Future Directions

Fibromyalgia is a complex and heterogeneous clinical syndrome, mainly characterized by the presence of widespread pain, possibly associated with a variety of other symptoms. Fibromyalgia can have an extremely negative impact on the psychological, physical and social lives of people affected, sometimes causing patients to experience dramatically impaired quality of life. Nowadays, the diagnosis of fibromyalgia is still clinical, thus favoring diagnostic uncertainties and making its clear identification challenging to establish, especially in primary care centers. These difficulties lead patients to undergo innumerable clinical visits, investigations and specialist consultations, thus increasing their stress, frustration and even dissatisfaction. Unfortunately, research over the last 25 years regarding a specific biomarker for the diagnosis of fibromyalgia has been fruitless. The discovery of a reliable biomarker for fibromyalgia syndrome would be a critical step towards the early identification of this condition, not only reducing patient healthcare utilization and diagnostic test execution but also providing early intervention with guideline-based treatments. This narrative article reviews different metabolite alterations proposed as possible biomarkers for fibromyalgia, focusing on their associations with clinical evidence of pain, and highlights some new, promising areas of research in this context. Nevertheless, none of the analyzed metabolites emerge as sufficiently reliable to be validated as a diagnostic biomarker. Given the complexity of this syndrome, in the future, a panel of biomarkers, including subtype-specific biomarkers, could be considered as an interesting alternative research area.

Analgesic Effect of Passive Range-of-Motion Exercise on the Healthy Side for Pain after Total Knee Arthroplasty: A Prospective Randomized Trial

Background

Exercise can reduce the pain threshold momentarily and induce analgesia, which is called exercise-induced hypoalgesia (EIH). Exercise therapy for inducing EIH may be an effective treatment option for pain. We aimed at investigating whether continuous passive motion (CPM) on both healthy and affected sides could induce EIH and reduce pain in the operated knee in patients after unilateral total knee arthroplasty (TKA). Patients and Methods. In this prospective randomized controlled trial, participants were randomly assigned to two groups: a bilateral group that received bilateral exercise on the operated and healthy sides and a unilateral group that received exercise therapy only on the affected side. We enrolled 40 patients aged ≥60 years who were scheduled to undergo unilateral TKA. Visual analogue scale (VAS) scores and range of motion (ROM) on the operated side were measured immediately before and after CPM on postoperative days 2, 4, 7, and 14. The primary outcome was the difference in the VAS scores before and after CPM on postoperative day 14. The secondary outcome was the difference in the ROM before and after CPM on postoperative day 14.

Results

Comparison of VAS scores before and after CPM showed no significant intergroup differences on all measurement dates. However, there was a significant difference in values on day 14 (P < 0.05). Both groups showed an increase in ROM after CPM, with significant increments observed on days 2 and 4 in the bilateral group and on day 14 in the unilateral group. There was no significant difference in values on postoperative day 14.

Conclusion

Post-TKA pain was reduced by performing the same exercise on the healthy knee during CPM therapy. This could be due to EIH, and the results indicated that EIH can also influence postoperative pain immediately after surgery.

Intravenous Methadone for Perioperative and Chronic Cancer Pain: A Review of the Literature

Intravenous methadone may be useful in acute and chronic pain management compared with other opioids because of its pharmacokinetic and pharmacodynamic characteristics, including the long duration of effect and ability to modulate both pain stimuli propagation and analgesic descending pathways. However, methadone is underused in pain medicine because of several misperceptions. A review of studies was performed to assess data regarding the use of methadone in perioperative pain and chronic cancer pain. The majority of studies have shown that intravenous methadone produces an effective postoperative analgesia and lowers opioid consumption in the postoperative period, without more adverse effects in comparison with other opioid analgesics, and has an interesting potential to prevent persistent postoperative pain. A minority of studies investigated the use of intravenous methadone for cancer pain management. These studies were mostly case series that showed promising activities of intravenous methadone for difficult pain conditions. There is sufficient evidence suggesting that intravenous methadone is effective in perioperative pain, while more studies are needed in patients with cancer pain.

Activation of the microglial P2X7R/NLRP3 inflammasome mediates central sensitization in a mouse model of medication overuse headache

Background

Excessive use of headache treatments often leads to the development, progression and exacerbation of primary headache, which is defined as medication overuse headache (MOH). A significant pathophysiological mechanism of MOH is central sensitization. Recent evidence suggests that central sensitization in chronic headache is a result of inflammatory responses mediated by microglial activation in the trigeminal nucleus caudalis (TNC). However, it is unknown whether microglial activation has an impact on the central sensitization of MOH. Accordingly, the goal of this research was to determine how microglial activation and the P2X7R/NLRP3 inflammasome signaling pathway in the TNC contribute to the pathogenesis of MOH.

Methods

Repeated intraperitoneal injection of sumatriptan (SUMA) was used to establish a mouse model of MOH. Basal mechanical hyperalgesia was evaluated using von Frey filaments. As central sensitization biomarkers, the c-Fos and CGRP expression levels were measured by immunofluorescence analysis. We estimated the expression of microglial biomarkers (Iba1 and iNOS) within the TNC by qRT-PCR, western blotting and immunofluorescence analysis. To elucidate the effect of microglial activation and the P2X7/NLRP3 signaling pathway on central sensitization in MOH, we evaluated whether the microglia-specific inhibitor minocycline, the P2X7R-specific antagonist BBG and the NLRP3-specific inhibitor MCC950 altered SUMA-caused mechanical hyperalgesia. Furthermore, we examined c-Fos and CGRP expression within the TNC following individual injections of these inhibitors.

Results

Repeated SUMA injection induced basal mechanical hyperalgesia, increased c-Fos and CGRP levels, and activated microglia within the TNC. Inhibiting microglial activation with minocycline prevented the emergence of mechanical hyperalgesia and cut down c-Fos and CGRP expression. Immunofluorescence colocalization analysis revealed that P2X7R was predominantly co-localized with microglia. The levels of P2X7R and the NLRP3 inflammasome were elevated by repeated SUMA injection, and blocking P2X7R and NLRP3 inhibited mechanical hyperalgesia and cut down c-Fos and CGRP expression within the TNC.

Conclusion

Based on the current findings, inhibiting microglial activation could reduce central sensitization caused by chronic SUMA treatment via the P2X7R/NLRP3 signaling pathway. The clinical management of MOH may benefit from a novel strategy that inhibits microglial activation.

Depressive Symptoms in Patients with Fibromyalgia: Current Evidence and Preventive Approaches

Fibromyalgia is a chronic inflammatory disease characterized by multifocal pain, fatigue, and cognitive impairment [...].

The microglial innate immune receptors TREM-1 and TREM-2 in the anterior cingulate cortex (ACC) drive visceral hypersensitivity and depressive-like behaviors following DSS-induced colitis

Inflammatory bowel disease (IBD) is a chronic condition with a high recurrence rate. To date, the clinical treatment of IBD mainly focuses on inflammation and gastrointestinal symptoms while ignoring the accompanying visceral pain, anxiety, depression, and other emotional symptoms. Evidence is accumulating that bi-directional communication between the gut and the brain is indispensable in the pathophysiology of IBD and its comorbidities. Increasing efforts have been focused on elucidating the central immune mechanisms in visceral hypersensitivity and depression following colitis. The triggering receptors expressed on myeloid cells-1/2 (TREM-1/2) are newly identified receptors that can be expressed on microglia. In particular, TREM-1 acts as an immune and inflammatory response amplifier, while TREM-2 may function as a molecule with a putative antagonist role to TREM-1. In the present study, using the dextran sulfate sodium (DSS)-induced colitis model, we found that peripheral inflammation induced microglial and glutamatergic neuronal activation in the anterior cingulate cortex (ACC). Microglial ablation mitigated visceral hypersensitivity in the inflammation phase rather than in the remission phase, subsequently preventing the emergence of depressive-like behaviors in the remission phase. Moreover, a further mechanistic study revealed that overexpression of TREM-1 and TREM-2 remarkably aggravated DSS-induced neuropathology. The improved outcome was achieved by modifying the balance of TREM-1 and TREM-2 via genetic and pharmacological means. Specifically, a deficiency of TREM-1 attenuated visceral hyperpathia in the inflammatory phase, and a TREM-2 deficiency improved depression-like symptoms in the remission phase. Taken together, our findings provide insights into mechanism-based therapy for inflammatory disorders and establish that microglial innate immune receptors TREM-1 and TREM-2 may represent a therapeutic target for the treatment of pain and psychological comorbidities associated with chronic inflammatory diseases by modulating neuroinflammatory responses.

PACAP6-38 improves nitroglycerin-induced central sensitization by modulating synaptic plasticity at the trigeminal nucleus caudalis in a male rat model of chronic migraine

AIMS

Chronic migraine (CM) is a common neurological disorder with complex pathogenesis. Evidence suggests that pituitary adenylate cyclase-activating peptide (PACAP) induces migraine-like attacks and may be potential a new target for migraine treatment, but the therapeutic results of targeting PACAP and its receptors are not uniform. Therefore, the aim of this study was to investigate the regulatory effect of PACAP type I receptor (PAC1R) antagonist, PACAP6-38, on nitroglycerin (NTG)-induced central sensitization in a CM model.

METHODS

Sprague-Dawley (SD) rats received repeated injections of NTG to construct a CM model. Mechanical and thermal thresholds were measured using Von Frey filaments and hot plate tests. C-Fos expression was measured by western blotting and immunofluorescence staining to assess the central sensitization. PACAP6-38 was intracerebrally injected into the trigeminal nucleus caudalis (TNC), and then the changes in c-Fos, the synaptic-associated proteins, phospho-ERK1/2 (p-ERK1/2), phosphorylation of cyclic adenosine monophosphate response element-binding protein (p-CREB) and brain-derived neurotrophic factor (BDNF) were detected. Transmission electron microscopy (TEM) and Golgi-Cox staining were used to observe the ultrastructure of synapses and dendritic structures of TNC neurons.

RESULTS

The results showed that PACAP and PAC1R expression were significantly raised in the TNC after repeated NTG injections. Additionally, PACAP6-38 treatment alleviated nociceptive sensitization, inhibited NTG-induced overexpression of c-Fos and synaptic-associated proteins in the TNC of CM rat, restored aberrant synaptic structures. Furthermore, the expression of ERK/CREB/BDNF pathway was depressed by PACAP6-38.

CONCLUSIONS

Our results demonstrated that abnormal synaptic structure in the TNC of CM, which could be reversed by inhibition of PAC1R via down-regulating the ERK/CREB/BDNF signaling pathway. PACAP6-38 improves NTG-induced central sensitization by regulating synaptic plasticity in the TNC of CM rat, which may provide new insights into the treatments targeting PACAP/PAC1R in migraine.

Pain Mechanisms Associated With Disease Activity in Patients With Rheumatoid Arthritis Treated With Disease-Modifying Antirheumatic Drugs: A Regression Tree Analysis

OBJECTIVE

Although pain affects the assessment of disease activity in patients with rheumatoid arthritis (RA), pain is not always directly related to peripheral joint inflammation. Peripheral and central nervous system regulatory mechanisms also affect pain perception. We used regression tree methodology to identify mechanisms most predictive of disease activity after disease-modifying antirheumatic drug (DMARD) treatment.

METHODS

Disease activity was evaluated using the Disease Activity Score in 28 joints (DAS28) in 176 patients with RA, before and after starting a DMARD. Quantitative sensory testing (QST), including pressure pain thresholds (PPTs), temporal summation, and conditioned pain modulation (CPM), were used to assess pain mechanisms. Regression tree methodology was used to determine the QST modalities most predictive of DAS28 after DMARD treatment.

RESULTS

This analysis identified 4 groups defined by baseline DAS28 category and either knee PPT (a combined measure of peripheral and central nervous system dysregulation) or CPM (a measure of descending pain inhibition). Among patients starting with low/moderate disease activity, lower knee PPT (PPT ≤ 4.65 kgf) most strongly predicted higher posttreatment disease activity (group 1 mean DAS28 2.8 [SD 1.0] vs group 2 mean DAS28 3.5 [SD 1.0]). Among patients starting with high baseline disease activity, less efficient descending pain modulation (CPM ≤ 1.55) most strongly predicted higher posttreatment disease activity (group 3 mean DAS28 3.4 [SD 1.4] vs group 4 mean DAS28 4.6 [SD 1.1]).

CONCLUSION

These results highlight the importance of identifying and treating aberrant peripheral and central pain regulation in patients with RA starting or switching DMARD therapy.

The major pain source of rotator cuff-related shoulder pain: A narrative review on current evidence

BACKGROUND

Rotator cuff-related shoulder pain (RCRSP) was proposed to have a complex pain mechanism, but the exact aetiology is still unclear. A recent review summarised the updated research to analyse the traditional concept of shoulder impingement which may not be accurate. Current studies have demonstrated that mechanical factors including a reduction in subacromial space, scapular dyskinesia and different acromial shapes are unlikely directly contributing to RCRSP.

AIMS

Since the precise RCRSP pain mechanism remains unclear, the aim of this narrative review is to discuss possible sources of pain contributing to RCRSP according to the mechanisms-based pain classifications.

RESULTS AND DISCUSSION

Research findings on potential mechanical nociceptive factors of RCRSP are conflicting; investigations of neuropathic and central pain mechanisms of RCRSP are limited and inconclusive. Overall, available evidence has indicated moderate to strong correlations between RCRSP and chemical nociceptive sources of pain.

CONCLUSION

Results from current research may provide new directions for future studies on the aetiology of RCRSP and its clinical management towards a biochemical view instead of the traditional mechanical hypothesis.

Reliability and minimal detectable difference of pressure pain thresholds in a pain-free population

The objective of this work was to evaluate the inter-rater and intra-rater reliability and minimal detectable difference (MDD) of pressure pain thresholds (PPTs) in pain-free participants with two examiners over two consecutive days in a cross-sectional study design. Examiners used a standardized method to measure and locate a specific testing site over tibialis anterior for PPT testing with a hand-held algometer. The mean of each examiner's three PPT measurements was used to calculate the intraclass correlation coefficient, inter-rater reliability, and intra-rater reliability. The minimal detectable difference (MDD) was calculated. Eighteen participants were recruited (11 female). The inter-rater reliability was 0.94 and 0.96 on day 1 and day 2, respectively. Intra-rater reliability for the examiners was 0.96 and 0.92 on day 1 and day 2, respectively. The MDD on day 1 was 1.24 kg/cm2 (CI: 0.76-2.03) and the MDD on day 2 was 0.88 kg/cm2 (CI: 0.54-1.43). This study demonstrates high inter- and intra-rater reliability and the MDD values for this method of pressure algometry.

Novel proresolving lipid mediator mimetic 3-oxa-PD1n-3 docosapentaenoic acid reduces acute and chronic itch by modulating excitatory and inhibitory synaptic transmission and astroglial secretion of lipocalin-2 in mice

ABSTRACT

Specialized proresolving mediators (SPMs) have demonstrated potent analgesic actions in animal models of pathological pain. The actions of SPMs in acute and chronic itch are currently unknown. Recently, n-3 docosapentaenoic acid (DPA) was found to be a substrate for the biosynthesis of several novel families of SPMs and 3-oxa-PD1 n-3 DPA (3-oxa-PD1) is an oxidation-resistant metabolic stable analogue of the n-3 DPA-derived protectin D1 (PD1). In this article, we demonstrate that 3-oxa-PD1 effectively reduces both acute and chronic itch in mouse models. Intrathecal injection of 3-oxa-PD1 (100 ng) reduced acute itch induced by histamine, chloroquine, or morphine. Furthermore, intrathecal 3-oxa-PD1 effectively reduced chronic itch, induced by cutaneous T-cell lymphoma (CTCL), allergic contact dermatitis with dinitrofluorobenzene, and psoriasis by imiquimod. Intratumoral injection of 3-oxa-PD1 also suppressed CTCL-induced chronic itch. Strikingly, the antipruritic effect lasted for several weeks after 1-week intrathecal 3-oxa-PD1 treatment. Whole-cell recordings revealed significant increase in excitatory postsynaptic currents in spinal dorsal horn (SDH) neurons of CTCL mice, but this increase was blocked by 3-oxa-PD1. 3-oxa-PD1 further increased inhibitory postsynaptic currents in SDH neurons of CTCL mice. Cutaneous T-cell lymphoma increased the spinal levels of lipocalin-2 (LCN2), an itch mediator produced by astrocytes. 3-oxa-PD1 suppressed LCN2 production in CTCL mice and LCN2 secretion in astrocytes. Finally, CTCL-induced anxiety was alleviated by intrathecal 3-oxa-PD1. Our findings suggest that 3-oxa-PD1 potently inhibits acute and chronic itch through the regulation of excitatory or inhibitory synaptic transmission and astroglial LCN2 production. Therefore, stable SPM analogs such as 3-oxa-PD1 could be useful to treat pruritus associated with different skin injuries.

The Neurophysiological Lesion: A Scoping Review

Objective

The purpose of this study was to examine the extent of the literature on the neurophysiological lesion as referenced in functional neurology.

Methods

A literature search was performed within the period from 2010 to March 2021. Search terms included central sensitization, central sensitivity syndrome, nociplastic pain, cold hyperalgesia, heat hyperalgesia, mechanical hyperalgesia, dynamic mechanical allodynia, temporal summation, spatial summation, and descending inhibition. A qualitative synthesis summarized the research findings, including clinical conditions and effect of spinal manipulation.

Results

There were 30 studies, which included 7 high-level studies (meta-analysis or systematic reviews), 22 randomized controlled studies, and 1 scoping review. The findings suggest the existence of the changes in the central integrated state of a population of neurons with various disorders, experimentally induced stimulation, and treatment. The current literature suggests plasticity of the central integrative state (CIS) with the onset of pathologies and the changes in the CIS with different conservative nonpharmacologic treatments.

Conclusions

This review suggests changes in the resting state of the CIS of a population of neurons that exist in the physiologic lesion may change in response to various therapies, including manipulative therapy. The findings from this review provide support of the hypothesis that nonpharmacologic conservative care may affect the neurophysiological lesion. However, studies were heterogeneous and evidence was lacking in the translation of targeting the therapies to distinct neuronal areas for clinical outcomes to treat specific disease states.

The role of ultrasonographic synovial assessment in rheumatoid arthritis patients with concomitant fibromyalgia

Objectives

This study aimed to compare the prevalence and musculoskeletal ultrasonography (US) findings of rheumatoid arthritis (RA) patients with concomitant fibromyalgia (FM) according to the 1990 American College of Rheumatology (ACR) FM classification criteria or the 2016 ACR FM diagnostic criteria.

Patients and methods

This cross-sectional study included 63 patients (17 males, 46 females; mean age: 48.2±7.1 years; range, 18 to 62 years) with RA. Medical history and laboratory data were obtained from electronic records. Clinical examination, composite disease activity measures, functional status, and the German 7-joint ultrasound score were assessed to evaluate disease activity and synovial inflammation. The patients were divided into three groups: patients who met only the 2016 ACR criteria, patients who met only the 1990 ACR criteria, and patients who met both criteria.

Results

In patients with RA, concomitant FM prevalence was 34.9% according to the 2016 ACR FM diagnostic criteria versus 23.8% according to the 1990 ACR FM classification criteria. Rheumatoid arthritis patients with FM had high tender joint count and disease activity scores, while musculoskeletal US findings were similar. Patients who met only the 2016 ACR FM diagnostic criteria had significantly higher gray-scale US and power Doppler US synovitis scores than patients who satisfied only ACR 1990 FM classification criteria (p=0.03 and p=0.02, respectively).

Conclusion

Synovial inflammation is a prominent sign in RA patients diagnosed with FM according to the 2016 ACR FM diagnostic criteria. The higher disease activity seen in the presence of FM in RA patients is associated with FM rather than synovitis.

Do "central sensitization" questionnaires reflect measures of nociceptive sensitization or psychological constructs? A systematic review and meta-analyses

ABSTRACT

Central sensitization (CS) is defined as an increased nociceptive responsiveness due to sensitization of neurons in the central nervous system, usually the result of prolonged nociceptive input or a disease state associated with noxious inputs (eg, polyarthritis). The concept of CS has recently been adopted in clinical assessments of chronic pain, but its diagnosis in humans may now include a wide range of hypervigilant responses. The purpose of this review is to ascertain whether self-report questionnaires linked with CS are associated with enhanced nociceptive responses or whether they measure sensitivity in a broader sense (ie, emotional responses). According to our published, PROSPERO-registered review protocol (CRD42021208731), a predefined search of studies that involve the Central Sensitization Inventory (CSI) or Pain Sensitivity Questionnaire (PSQ), correlated with either nociceptive sensory tests or emotional hypervigilance was conducted on MEDLINE, PsycINFO, and Web of Science. Correlations between the CSI or PSQ with our primary outcomes were extracted and meta-analysed. A review of 66 studies totalling 13,284 participants found that the CSI (but not the PSQ) strongly correlated with psychological constructs: depression, anxiety, stress, pain catastrophising, sleep, and kinesiophobia. The CSI and PSQ showed weak or no correlations with experimental measures of nociceptive sensitivity: pain thresholds, temporal summation, or conditioned pain modulation. The PSQ did, however, correlate strongly with phasic heat and tonic cold pain tests. The studies reviewed did not provide sufficient evidence that self-report measures reflect a canonical understanding of CS. The CSI more closely reflects psychological hypervigilance than increased responsiveness of nociceptive neurons.

Exploring Non-Invasive Salivary Biomarkers for Acute Pain Diagnostics: A Comprehensive Review

Pain is one of the most common complaints leading to a pediatric emergency department visit and is associated with various painful procedures, leading to increased anxiety and stress. Assessing and treating pain in children can be challenging, so it is crucial to investigate new methods for pain diagnosis. The review aims to summarize the literature on non-invasive salivary biomarkers, such as proteins and hormones, for pain assessment in urgent pediatric care settings. Eligible studies were those that included novel protein and hormone biomarkers in acute pain diagnostics and were not older than 10 years. Chronic pain studies were excluded. Further, articles were divided into two groups: studies in adults and studies in children (<18 years). The following characteristics were extracted and summarized: study author, enrollment date, study location, patient age, study type, number of cases and groups, as well as tested biomarkers. Salivary biomarkers, such as cortisol, salivary α-amylase, and immunoglobulins, among others, could be appropriate for children as saliva collection is painless. However, hormonal levels can differ among children in different developmental stages and with various health conditions, with no predetermined levels of saliva. Thus, further exploration of biomarkers in pain diagnostics is still necessary.

Involvement of Mrgprd-expressing nociceptors-recruited spinal mechanisms in nerve injury-induced mechanical allodynia

Mechanical allodynia and hyperalgesia are intractable symptoms lacking effective clinical treatments in patients with neuropathic pain. However, whether and how mechanically responsive non-peptidergic nociceptors are involved remains elusive. Here, we showed that von Frey-evoked static allodynia and aversion, along with mechanical hyperalgesia after spared nerve injury (SNI) were reduced by ablation of Mrgprd^CreERT2-marked neurons. Electrophysiological recordings revealed that SNI-opened Aβ-fiber inputs to laminae I-II_o and _vII_i, as well as C-fiber inputs to _vII_i, were all attenuated in Mrgprd-ablated mice. In addition, priming chemogenetic or optogenetic activation of Mrgprd⁺ neurons drove mechanical allodynia and aversion to low-threshold mechanical stimuli, along with mechanical hyperalgesia. Mechanistically, gated Aβ and C inputs to _vII_i were opened, potentially via central sensitization by dampening potassium currents. Altogether, we uncovered the involvement of Mrgprd⁺ nociceptors in nerve injury-induced mechanical pain and dissected the underlying spinal mechanisms, thus providing insights into potential therapeutic targets for pain management.

Thermal Grill Illusion in Chronic Lower Back Pain: A Case-Control Study

Purpose

This study aimed to use thermal grill illusion (TGI), an experimental model of pain processing and central mechanisms, to evaluate the perception of TGI-related sensations or pain in patients with chronic lower back pain (CLBP).

Patients and Methods

The perception of TGI (warmth/heat, cold, unpleasantness, pain, burning, stinging, and prickling) was examined in 66 patients with CLBP and compared with that in 22 healthy participants. The visual analog scale (VAS) scores for CLBP, Oswestry Disability Index (ODI), and 12-Item Short Form Survey (SF-12) scores were obtained from the included patients with CLBP.

Results

The CLBP group showed a less intense perception of TGI for sensations of warmth/heat, unpleasantness, and pain than the control group. The CLBP group felt burning sensations lesser than the control (2.77 vs 4.55, P=0.016). In the CLBP group, there were significant correlations between the ODI and the degree of unpleasantness (r=0.381, P=0.002) and prickling sensation (r=0.263, P=0.033). There were also significant correlations between the mental component score of the SF-12 and the degree of warmth/heat (r=-0.246, P=0.046), unpleasantness (r=-0.292, P=0.017), pain (r=-0.292, P=0.017), and burning sensations (r=-0.280, P=0.023).

Conclusion

Our results may be useful for clinicians to evaluate the effectiveness of drugs or interventions to manage centralized LBP.

Neurophysiological and Psychosocial Mechanisms of Fibromyalgia: A Comprehensive Review and Call for An Integrative Model

Research into the neurobiological and psychosocial mechanisms involved in fibromyalgia has progressed remarkably in recent years. Despite this, current accounts of fibromyalgia fail to capture the complex, dynamic, and mutual crosstalk between neurophysiological and psychosocial domains. We conducted a comprehensive review of the existing literature in order to: a) synthesize current knowledge on fibromyalgia; b) explore and highlight multi-level links and pathways between different systems; and c) build bridges connecting disparate perspectives. An extensive panel of international experts in neurophysiological and psychosocial aspects of fibromyalgia discussed the collected evidence and progressively refined and conceptualized its interpretation. This work constitutes an essential step towards the development of a model capable of integrating the main factors implicated in fibromyalgia into a single, unified construct which appears indispensable to foster the understanding, assessment, and intervention for fibromyalgia.

Glial Glutamate Transporter Modulation Prevents Development of Complete Freund's Adjuvant-Induced Hyperalgesia and Allodynia in Mice

Glial glutamate transporter (GLT-1) modulation in the hippocampus and anterior cingulate cortex (ACC) is critically involved in nociceptive pain. The objective of the study was to investigate the effects of 3-[[(2-methylphenyl) methyl] thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, against microglial activation induced by complete Freund's adjuvant (CFA) in a mouse model of inflammatory pain. Furthermore, the effects of LDN-212320 on the protein expression of glial markers, such as ionized calcium-binding adaptor molecule 1 (Iba1), cluster of differentiation molecule 11b (CD11b), mitogen-activated protein kinases (p38), astroglial GLT-1, and connexin 43 (CX43), were measured in the hippocampus and ACC following CFA injection using the Western blot analysis and immunofluorescence assay. The effects of LDN-212320 on the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus and ACC were also assessed using an enzyme-linked immunosorbent assay. Pretreatment with LDN-212320 (20 mg/kg) significantly reduced the CFA-induced tactile allodynia and thermal hyperalgesia. The anti-hyperalgesic and anti-allodynic effects of LDN-212320 were reversed by the GLT-1 antagonist DHK (10 mg/kg). Pretreatment with LDN-212320 significantly reduced CFA-induced microglial Iba1, CD11b, and p38 expression in the hippocampus and ACC. LDN-212320 markedly modulated astroglial GLT-1, CX43, and, IL-1β expression in the hippocampus and ACC. Overall, these results suggest that LDN-212320 prevents CFA-induced allodynia and hyperalgesia by upregulating astroglial GLT-1 and CX43 expression and decreasing microglial activation in the hippocampus and ACC. Therefore, LDN-212320 could be developed as a novel therapeutic drug candidate for chronic inflammatory pain.

IL-10/β-Endorphin-Mediated Neuroimmune Modulation on Microglia during Antinociception

Microglia are glial cells centrally related to pathophysiology and neuroimmunological regulation of pain through microglia-neuron crosstalk mechanisms. In contrast, anti-inflammatory mechanisms guided by immunological effectors such as IL-10 trigger the secretion of analgesic substances, culminating in the differential expression of genes encoding endogenous opioid peptides, especially β-endorphin. Thus, when β-endorphin binds to the µ-opioid receptor, it generates neuronal hyperpolarization, inhibiting nociceptive stimuli. This review aimed to summarize the recent advances in understanding the mechanism by which IL-10/β-endorphin can reduce pain. For this, databases were searched for articles from their inception up until November 2022. Two independent reviewers extracted the data and assessed the methodological quality of the included studies, and seventeen studies were considered eligible for this review. Several studies have demonstrated the impact of IL-10/β-endorphin in reducing pain, where IL-10 can stimulate GLP-1R, GRP40, and α7nAChR receptors, as well as intracellular signaling pathways, such as STAT3, resulting in increased β-endorphin expression and secretion. In addition, molecules such as gabapentinoids, thalidomide, cynandione A, morroniside, lemairamin, and cinobufagin, as well as non-pharmacological treatments such as electroacupuncture, reduce pain through IL-10 mediated mechanisms, reflecting a microglia-dependent β-endorphin differential increase. This process represents a cornerstone in pain neuroimmunology knowledge, and the results obtained by different studies about the theme are presented in this review.

Multiple Chemical Sensitivity: It's time to catch up to the science

Multiple chemical sensitivity (MCS) is a complex medical condition associated with low dose chemical exposures. MCS is characterized by diverse features and common comorbidities, including fibromyalgia, cough hypersensitivity, asthma, and migraine, and stress/anxiety, with which the syndrome shares numerous neurobiological processes and altered functioning within diverse brain regions. Predictive factors linked to MCS comprise genetic influences, gene-environment interactions, oxidative stress, systemic inflammation, cell dysfunction, and psychosocial influences. The development of MCS may be attributed to the sensitization of transient receptor potential (TRP) receptors, notably TRPV1 and TRPA1. Capsaicin inhalation challenge studies demonstrated that TRPV1 sensitization is manifested in MCS, and functional brain imaging studies revealed that TRPV1 and TRPA1 agonists promote brain-region specific neuronal variations. Unfortunately, MCS has often been inappropriately viewed as stemming exclusively from psychological disturbances, which has fostered patients being stigmatized and ostracized, and often being denied accommodation for their disability. Evidence-based education is essential to provide appropriate support and advocacy. Greater recognition of receptor-mediated biological mechanisms should be incorporated in laws, and regulation of environmental exposures.

Vertical 0.6 V sub-10 nm oxide-homojunction transistor gated by a silk fibroin/sodium alginate crosslinking hydrogel for pain-sensitization enhancement emulation

The sensory nervous system of humans mainly depends on continuous training and memory to improve the pain-perceptional abilities for the complex noxious information in the real world and make appropriate responses. Unfortunately, the solid-state device for emulating this pain recognition with ultralow voltage operation still remains to be a great challenge. Herein, a vertical transistor with an ultrashort channel of ∼9.6 nm and ultralow voltage of ∼0.6 V based on protonic silk fibroin/sodium alginate crosslinking hydrogel electrolyte is successfully demonstrated. Such a hydrogel electrolyte with high ionic conductivity allows the transistor to work in an ultralow voltage, while the vertical transistor structure makes it have an ultrashort channel. Pain perception, memory, and sensitization can be integrated into this vertical transistor. Furthermore, using the photogating effect of light stimulus, the device displays multi-state pain-sensitization enhancement abilities through Pavlovian training. Most importantly, the cortical reorganization that reveals a close relationship among the pain stimulus, memory, and sensitization is finally realized. Therefore, this device can provide a great opportunity for multi-dimensional pain assessment, which is of great significance for the new generation of bio-inspired intelligent electronics, such as bionic robots, and smart medical equipment.

Reduced Spinal Cord Gray Matter in Patients with Fibromyalgia Using Opioids Long-term

Objective

Chronic pain involves alterations in brain gray matter volume (GMV). Moreover, opioid medications are known to reduce GMV in numerous brain regions involved in pain processing. However, no research has evaluated (1) chronic pain-related GMV alterations in the spinal cord or (2) the effect of opioids on spinal cord GMV. Accordingly, this study evaluated spinal cord GMV in health controls and patients with fibromyalgia who were using and not using opioids long-term.

Methods

We analyzed average C5 - C7 GMV of the spinal cord dorsal and ventral horns in separate female cohorts of healthy controls (HC, n = 30), fibromyalgia patients not using opioids (FMN, n = 31), and fibromyalgia patients using opioids long-term (FMO, n = 27). To assess the effect of group on average dorsal and ventral horn GMV, we conducted a one-way multivariate analysis of covariance.

Results

After controlling for age, we observed a significant effect of group on ventral horn GMV (p = 0.03, η2 = 0.09), and on dorsal horn GMV (p = 0.05, η2 = 0.08). Tukey's posthoc comparisons showed that, compared to HC participants, FMOs had significantly lower ventral (p = 0.01) and dorsal (p = 0.02) GMVs. Among FMOs only, ventral horn GMV was significantly positively associated with pain severity and interference, and both dorsal and ventral GMVs were significantly positively associated with cold pain tolerance.

Conclusion

Long-term opioid use may impact sensory processing in fibromyalgia via gray matter changes within the cervical spinal cord.

Characterization of Fragile X Mental Retardation Protein expression in human nociceptors and their axonal projections to the spinal dorsal horn

Fragile X Mental Retardation Protein (FMRP) regulates activity-dependent RNA localization and local translation to modulate synaptic plasticity throughout the central nervous system. Mutations in the FMR1 gene that hinder or ablate FMRP function cause Fragile X Syndrome (FXS), a disorder associated with sensory processing dysfunction. FXS premutations are associated with increased FMRP expression and neurological impairments including sex dimorphic presentations of chronic pain. In mice, FMRP ablation causes dysregulated dorsal root ganglion (DRG) neuron excitability and synaptic vesicle exocytosis, spinal circuit activity, and decreased translation-dependent nociceptive sensitization. Activity-dependent, local translation is a key mechanism for enhancing primary nociceptor excitability that promotes pain in animals and humans. These works indicate that FMRP likely regulates nociception and pain at the level of the primary nociceptor or spinal cord. Therefore, we sought to better understand FMRP expression in the human DRG and spinal cord using immunostaining in organ donor tissues. We find that FMRP is highly expressed in DRG and spinal neuron subsets with substantia gelatinosa exhibiting the most abundant immunoreactivity in spinal synaptic fields. Here, it is expressed in nociceptor axons. FMRP puncta colocalized with Nav1.7 and TRPV1 receptor signals suggesting a pool of axoplasmic FMRP localizes to plasma membrane-associated loci in these branches. Interestingly, FMRP puncta exhibited notable colocalization with calcitonin gene-related peptide (CGRP) immunoreactivity selectively in female spinal cord. Our results support a regulatory role for FMRP in human nociceptor axons of the dorsal horn and implicate it in the sex dimorphic actions of CGRP signaling in nociceptive sensitization and chronic pain.

The Biology of Pain: Through the Rheumatology Lens

Chronic pain is a major socioeconomic burden globally. The most frequent origin of chronic pain is musculoskeletal. In inflammatory musculoskeletal diseases such as rheumatoid arthritis (RA), chronic pain is a primary determinant of deleterious quality of life. The pivotal role of peripheral inflammation in the initiation and perpetuation of nociceptive pain is well-established among patients with musculoskeletal diseases. However, the persistence of pain, even after the apparent resolution of peripheral inflammation, alludes to the coexistence of different pain states. Recent advances in neurobiology have highlighted the importance of nociplastic pain mechanisms. In this review we aimed to explore the biology of pain with a particular focus on nociplastic pain in RA.

NOX4-derived ROS are neuroprotective by balancing intracellular calcium stores

Hyperexcitability is associated with neuronal dysfunction, cellular death, and consequently neurodegeneration. Redox disbalance can contribute to hyperexcitation and increased reactive oxygen species (ROS) levels are observed in various neurological diseases. NOX4 is an NADPH oxidase known to produce ROS and might have a regulating function during oxidative stress. We, therefore, aimed to determine the role of NOX4 on neuronal firing, hyperexcitability, and hyperexcitability-induced changes in neural network function. Using a multidimensional approach of an in vivo model of hyperexcitability, proteomic analysis, and cellular function analysis of ROS, mitochondrial integrity, and calcium levels, we demonstrate that NOX4 is neuroprotective by regulating ROS and calcium homeostasis and thereby preventing hyperexcitability and consequently neuronal death. These results implicate NOX4 as a potential redox regulator that is beneficial in hyperexcitability and thereby might have an important role in neurodegeneration.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

MMG22 Potently Blocks Hyperalgesia in Cisplatin-treated Mice

MMG22 is a bivalent ligand containing MOR agonist and mGluR5 antagonist pharmacophores connected by a 22-atom linker. Intrathecal (i.t.) administration of MMG22 to inflamed mice has been reported to produce fmol-range antinociception in the reversal of LPS-induced hyperalgesia. MMG22 reduced hyperalgesia in the spared nerve injury (SNI) model of neuropathic pain at 10 days after injury but not at 30 days after injury, perhaps related to the inflammation that occurs early after injury but subsequently subsides. The present study determined the efficacy of MMG22 in cisplatin-treated male mice in order to provide data relating to the efficacy of MMG22 in the treatment of neuropathic pain that is associated with inflammation. Groups of eight mice each received daily intraperitoneal (i.p.) injections of cisplatin for seven days to produce robust mechanical allodynia defined by the decrease in withdrawal threshold using an electronic von Frey applied to the plantar surface of the hind paw. Intrathecal administration of MMG22 potently reduced mechanical hyperalgesia (ED50 0.04 fmol/mouse) without tolerance, whereas MMG10 was essentially inactive. Morphine was less potent than MMG22 by >5-orders of magnitude and displayed tolerance. Subcutaneous MMG22 was effective (ED50 = 2.41 mg/kg) and devoid of chronic tolerance. We propose that MMG22 induces the formation of a MOR-mGluR5 heteromer through selective interaction with the upregulated NR2B subunit of activated NMDAR, in view of the 4600-fold reduction of i.t. MMG22 antinociception by the selective NR2B antagonist, Ro25-6981. A possible explanation for the substantially reduced potency for MMG22 in the SNI model is discussed.

Orofacial Pain Management: An Overview of the Potential Benefits of Palmitoylethanolamide and Other Natural Agents

Pain is the most common symptom that dentists are confronted with, whether acute (pulpitis, acute periodontitis, post-surgery, etc.) or chronic diseases, such as periodontitis, muscle pain, temporomandibular joint (TMJ) disorders, burning mouth syndrome (BMS), oral lichen planus (OLP) and others. The success of therapy depends on the reduction in and management of pain through specific drugs, hence the need to analyze new pain medications with specific activity, which are suitable for long-term use, with a low risk of side effects and interactions with other drugs, and capable of leading to a reduction in orofacial pain. Palmitoylethanolamide (PEA) is a bioactive lipid mediator, which is synthesized in all tissues of the body as a protective pro-homeostatic response to tissue damage and has aroused considerable interest in the dental field due to its anti-inflammatory, analgesic, antimicrobial, antipyretic, antiepileptic, immunomodulatory and neuroprotective activities. It has been observed that PEA could play a role in the management of the pain of orofacial origin, including BMS, OLP, periodontal disease, tongue a la carte and temporomandibular disorders (TMDs), as well as in the treatment of postoperative pain. However, actual clinical data on the use of PEA in the clinical management of patients with orofacial pain are still lacking. Therefore, the main objective of the present study is to provide an overview of orofacial pain in its many manifestations and an updated analysis of the molecular pain-relieving and anti-inflammatory properties of PEA to understand its beneficial effects in the management of patients with orofacial pain, both neuropathic and nociceptive in nature. The aim is also to direct research toward the testing and use of other natural agents that have already been shown to have anti-inflammatory, antioxidant and pain-relieving actions and could offer important support in the treatment of orofacial pain.

Efficacy of Low-Dose Naltrexone and Predictors of Treatment Success or Discontinuation in Fibromyalgia and Other Chronic Pain Conditions: A Fourteen-Year, Enterprise-Wide Retrospective Analysis

Current pharmacologic treatments may provide limited analgesia in fibromyalgia and other chronic pain disorders. Low-dose naltrexone (LDN) has emerged as a potential analgesic option that has been minimally explored. This study aims to describe current real-world prescribing practices of LDN, to investigate if patients have a perceived benefit of LDN in treating pain symptoms and to identify predictors associated with a perceived benefit or discontinuation of LDN. We evaluated all outpatient prescriptions for LDN prescribed for any pain indication in the Mayo Clinic Enterprise from 1 January 2009 to 10 September 2022. A total of 115 patients were included in the final analysis. The patients were 86% female, had a mean age of 48 ± 16 years, and 61% of prescriptions were for fibromyalgia-related pain. The final daily dose of oral LDN ranged from 0.8 to 9.0 mg, while the most common dose was 4.5 mg once daily. Of patients who reported follow-up data, 65% reported benefit in their pain symptoms while taking LDN. Adverse effects were reported in 11 (11%) patients and 36% discontinued taking LDN by the most recent follow-up. Concomitant analgesic medications were used by 60% of patients and were not associated with perceived benefit nor discontinuation of LDN, including concomitant opioids. LDN is a relatively safe pharmacologic option that may benefit patients with chronic pain conditions and warrants further investigation in a prospective, controlled, and well-powered randomized clinical trial.

The analgesic effect of total intravenous anaesthesia with propofol versus inhalational anaesthesia for acute postoperative pain after hepatectomy: a randomized controlled trial

BACKGROUND

Postoperative pain control can be challenging in patients undergoing hepatectomy. A previous retrospective study on hepatobiliary/ pancreatic surgeries showed better postoperative pain control in patients who received propofol TIVA. The aim of this study was to determine the analgesic effect of propofol TIVA for hepatectomy. This clinical study has been registered at ClinicalTrials.gov (NCT03597997).

METHODS

A prospective randomized controlled trial was performed to compare the analgesic effect of propofol TIVA versus inhalational anaesthesia. Patients aged between 18 and 80 years old with an American Society of Anesthesiologist (ASA) physical status of I-III scheduled for elective hepatectomy were recruited. Ninety patients were randomly allocated to receive either propofol TIVA (TIVA group) or inhalational anaesthesia with sevoflurane (SEVO group). Perioperative anaesthetic/analgesic management was the same for both groups. Numerical rating scale (NRS) pain scores, postoperative morphine consumption, quality of recovery, patient satisfaction and adverse effects were evaluated during the acute postoperative period and at 3 and 6 months after surgery.

RESULTS

No significant differences were found for acute postoperative pain scores (both at rest and during coughing) and postoperative morphine consumption between TIVA and SEVO groups. Patients given TIVA had lower pain scores with coughing at 3 months after surgery (p = 0.014, and FDR < 0.1). TIVA group was associated with better quality of recovery on postoperative day (POD) 3 (p = 0.038, and FDR < 0.1), less nausea (p = 0.011, and FDR < 0.1 on POD 2; p = 0.013, and FDR < 0.1 on POD 3) and constipation (p = 0.013, and FDR < 0.1 on POD 3).

CONCLUSION

Propofol TIVA did not improve acute postoperative pain control compared to inhalational anaesthesia in patients who underwent hepatectomy. Our results do not support the use of propofol TIVA for reducing acute postoperative pain after hepatectomy.

Dual contribution of ASIC1a channels in the spinal processing of pain information by deep projection neurons revealed by computational modeling

Dorsal horn of the spinal cord is an important crossroad of pain neuraxis, especially for the neuronal plasticity mechanisms that can lead to chronic pain states. Windup is a well-known spinal pain facilitation process initially described several decades ago, but its exact mechanism is still not fully understood. Here, we combine both ex vivo and in vivo electrophysiological recordings of rat spinal neurons with computational modeling to demonstrate a role for ASIC1a-containing channels in the windup process. Spinal application of the ASIC1a inhibitory venom peptides mambalgin-1 and psalmotoxin-1 (PcTx1) significantly reduces the ability of deep wide dynamic range (WDR) neurons to develop windup in vivo. All deep WDR-like neurons recorded from spinal slices exhibit an ASIC current with biophysical and pharmacological characteristics consistent with functional expression of ASIC1a homomeric channels. A computational model of WDR neuron supplemented with different ASIC1a channel parameters accurately reproduces the experimental data, further supporting a positive contribution of these channels to windup. It also predicts a calcium-dependent windup decrease for elevated ASIC conductances, a phenomenon that was experimentally validated using the Texas coral snake ASIC-activating toxin (MitTx) and calcium-activated potassium channel inhibitory peptides (apamin and iberiotoxin). This study supports a dual contribution to windup of calcium permeable ASIC1a channels in deep laminae projecting neurons, promoting it upon moderate channel activity, but ultimately leading to calcium-dependent windup inhibition associated to potassium channels when activity increases.

Barriers and facilitators associated with musculoskeletal complaints in individuals with upper limb absence - focus group results and a scoping review

PURPOSE

To guide better prevention and treatment and to develop research priorities, this study aims to create an overview of facilitators and barriers for the development and persistence of musculoskeletal complaints (MSCs) in individuals with upper limb absence (ULA).

METHODS

Exploratory mixed methods design. A focus group (FG) was organized with individuals with ULA about MSCs and associated factors. An inductive approach was employed to the transcript and the studies. A scoping review was performed to systematically identify barriers and facilitators. The International Classification of Functioning, Disability, and Health was used to create an integrated overview of the results.

RESULTS

Eleven participants participated in the FG, eight of them currently sustained or had sustained MSCs in the last year. Ten studies were included in the scoping review. The final overview consisted of 67 associated factors. Participants of the FG predominantly mentioned psychosocial factors, whereas the literature dominantly reported biomechanical factors.

CONCLUSIONS

The extensive overview of 67 factors showed that facilitators and barriers for MSCs are heterogeneous and aids in a better understanding of the complex nature of MSCs. Several biomechanical and psychosocial factors contribute to MSCs, but the association with a prosthesis remains unclear. Implications for rehabilitationMusculoskeletal complaints (MSCs) are highly prevalent in the population with upper limb absence (ULA) and the overview of 67 factors could help in the prevention and treatment of MSCs.Psychosocial factors in the development and persistence of MSCs are underreported in literature, but are important contributors to MSCs according to patients.Wearing a prosthesis does not seem to be protective for the development or persistence of MSCs.Social support, especially from significant others and employers, is essential to help protect MSCs in those with ULA.

TFOS Lifestyle: Impact of lifestyle challenges on the ocular surface

Many factors in the domains of mental, physical, and social health have been associated with various ocular surface diseases, with most of the focus centered on aspects of dry eye disease (DED). Regarding mental health factors, several cross-sectional studies have noted associations between depression and anxiety, and medications used to treat these disorders, and DED symptoms. Sleep disorders (both involving quality and quantity of sleep) have also been associated with DED symptoms. Under the domain of physical health, several factors have been linked to meibomian gland abnormalities, including obesity and face mask wear. Cross-sectional studies have also linked chronic pain conditions, specifically migraine, chronic pain syndrome and fibromyalgia, to DED, principally focusing on DED symptoms. A systematic review and meta-analysis reviewed available data and concluded that various chronic pain conditions increased the risk of DED (variably defined), with odds ratios ranging from 1.60 to 2.16. However, heterogeneity was noted, highlighting the need for additional studies examining the impact of chronic pain on DED signs and subtype (evaporative versus aqueous deficient). With respect to societal factors, tobacco use has been most closely linked to tear instability, cocaine to decreased corneal sensitivity, and alcohol to tear film disturbances and DED symptoms.

Prolactin and oxytocin: potential targets for migraine treatment

Migraine is a severe neurovascular disorder of which the pathophysiology is not yet fully understood. Besides the role of inflammatory mediators that interact with the trigeminovascular system, cyclic fluctuations in sex steroid hormones are involved in the sex dimorphism of migraine attacks. In addition, the pituitary-derived hormone prolactin and the hypothalamic neuropeptide oxytocin have been reported to play a modulating role in migraine and contribute to its sex-dependent differences. The current narrative review explores the relationship between these two hormones and the pathophysiology of migraine. We describe the physiological role of prolactin and oxytocin, its relationship to migraine and pain, and potential therapies targeting these hormones or their receptors.In summary, oxytocin and prolactin are involved in nociception in opposite ways. Both operate at peripheral and central levels, however, prolactin has a pronociceptive effect, while oxytocin appears to have an antinociceptive effect. Therefore, migraine treatment targeting prolactin should aim to block its effects using prolactin receptor antagonists or monoclonal antibodies specifically acting at migraine-pain related structures. This action should be local in order to avoid a decrease in prolactin levels throughout the body and associated adverse effects. In contrast, treatment targeting oxytocin should enhance its signalling and antinociceptive effects, for example using intranasal administration of oxytocin, or possibly other oxytocin receptor agonists. Interestingly, the prolactin receptor and oxytocin receptor are co-localized with estrogen receptors as well as calcitonin gene-related peptide and its receptor, providing a positive perspective on the possibilities for an adequate pharmacological treatment of these nociceptive pathways. Nevertheless, many questions remain to be answered. More particularly, there is insufficient data on the role of sex hormones in men and the correct dosing according to sex differences, hormonal changes and comorbidities. The above remains a major challenge for future development.

Electroacupuncture at Fengchi(GB20) and Yanglingquan(GB34) Ameliorates Paralgesia through Microglia-Mediated Neuroinflammation in a Rat Model of Migraine

Background: Multiple studies have suggested that paralgesia (hyperalgesia and cutaneous allodynia) in migraine reflects the activation and sensitisation of the trigeminovascular system (TGVS). In particular, it reflects the second-order and higher nerve centre sensitisation, which is caused and maintained by neuroinflammation. Microglia activation leads to the release of proinflammatory cytokines involved in inflammatory responses. Accumulating evidence indicates that electroacupuncture (EA) is effective in ameliorating paralgesia, but the underlying mechanisms of EA in migraine attacks caused by microglia and microglia-mediated inflammatory responses are still unclear. The purpose of this study was to explore whether EA could ameliorate the dysregulation of pain sensation by suppressing microglial activation and the resulting neuroinflammatory response, and to evaluate whether this response was regulated by Toll-like receptor 4 (TLR4)/nuclear factor-kappa B(NF-κB) in the trigeminal nucleus caudalis (TNC) in a rat model of migraine. Methods: Repeated Inflammatory Soup (IS) was infused into the dura for seven sessions to establish a recurrent migraine-like rat model, and EA treatment was administered at Fengchi (GB20) and Yanglingquan (GB34) after daily IS infusion. Facial mechanical withdrawal thresholds were measured to evaluate the change in pain perception, and plasma samples and the TNC tissues of rats were collected to examine the changes in calcitonin gene-related peptide (CGRP), the Ibal-1-labelled microglial activation, and the resulting inflammatory response, including interleukin-1β (IL-1β), tumour necrosis factor-α (TNF-α), interleukin-6 (IL-6), and their regulatory molecules TLR4/NF-κB, via enzyme-linked immunosorbent assay (ELISA), real-time polymerase chain reaction (RT-PCR), immunohistochemistry (IHC) and Western blot analysis. Results: Repeated IS injections into the dura induced facial mechanical paralgesia, which is the manifestation of migraine attacks, and increased the expression of CGRP, Ibal-1, microglial mediated inflammatory cytokines (IL-1β, TNF-α, IL-6), and regulatory molecules TLR4/NF-κB. EA at GB20/34 significantly attenuated repetitive IS-induced pain hypersensitivity. This effect was consistent with decreased levels of CGRP and inflammatory cytokines in the plasma and the TNC via the inhibition of microglia activation, and this response may be regulated by TLR4/NF-κB. Conclusions: EA ameliorated paralgesia in repetitive IS-induced migraine-like rats, which was mainly mediated by a reduction in microglial activation and microglial-mediated inflammatory responses that could be regulated by TLR4/NF-κB.

Nerve injury disrupts temporal processing in the spinal cord dorsal horn through alterations in PV + interneurons

How mechanical allodynia following nerve injury is encoded in patterns of neural activity in the spinal cord dorsal horn (DH) is not known. We addressed this using the spared nerve injury model of neuropathic pain and in vivo electrophysiological recordings. Surprisingly, despite dramatic behavioral over-reactivity to mechanical stimuli following nerve injury, an overall increase in sensitivity or reactivity of DH neurons was not observed. We did, however, observe a marked decrease in correlated neural firing patterns, including the synchrony of mechanical stimulus-evoked firing, across the DH. Alterations in DH temporal firing patterns were recapitulated by silencing DH parvalbumin + (PV + ) inhibitory interneurons, previously implicated in mechanical allodynia, as were allodynic pain-like behaviors in mice. These findings reveal decorrelated DH network activity, driven by alterations in PV + interneurons, as a prominent feature of neuropathic pain, and suggest that restoration of proper temporal activity is a potential treatment for chronic neuropathic pain.

Laryngeal sensory neuropathy caused by COVID-19: findings using laryngeal electromyography

PURPOSE

Laryngeal sensory neuropathy (LSN) is caused by a disorder of the superior laryngeal nerve or the recurrent laryngeal nerve. A diagnosis of LSN should include laryngeal electromyography (LEMG) and laryngovideostroboscopy (LVS). The aim of this study was to characterize the physical and subjective symptoms of neuropathy in patients diagnosed with LSN following COVID-19.

MATERIAL AND METHODS

Since the beginning of the COVID-19 pandemic, 6 patients who had recovered from the disease presented to us with LSN symptoms. All patients underwent laryngological and phoniatric examination, objective and subjective voice assessment, and LEMG.

RESULTS

The most common LSN symptom reported by patients was periodic hoarseness of varying severity. Other common symptoms were the sensation of a foreign body in the throat and voice fatigue. Endoscopy often showed functional abnormalities. The LSN patients could be characterized by LEMG recordings, and all showed abnormal activity of the cricothyroid (CT) muscle. The degree of EMG changes in the CT correlated moderately with the severity of dysphonia.

CONCLUSIONS

Sensory neuropathy of the larynx may be a long-lasting complication of SARS-COV-2 infection. The severity of EMG neuropathic changes in the CT muscle broadly corresponds to the severity of dysphonia.

Peripheral Nerve Denervation in Streptozotocin-Induced Diabetic Rats Is Reduced by Cilostazol

Background and Objective: Our previous study demonstrated that consistent treatment of oral cilostazol was effective in reducing levels of painful peripheral neuropathy in streptozotocin-induced type I diabetic rats. As diabetic neuropathy is characterized by hyperglycemia-induced nerve damage in the periphery, this study aims to examine the neuropathology as well as the effects of cilostazol treatments on the integrity of peripheral small nerve fibers in type I diabetic rats. Materials and Methods: A total of ninety adult male Sprague-Dawley rats were divided into the following groups: (1) naïve (control) group; (2) diabetic rats (DM) group for 8 weeks; DM rats receiving either (3) 10 mg/kg oral cilostazol (Cilo10), (4) 30 mg/kg oral cilostazol (Cilo30), or (5) 100 mg/kg oral cilostazol (Cilo100) for 6 weeks. Pain tolerance thresholds of hind paws toward thermal and mechanical stimuli were assessed. Expressions of PGP9.5, P2X3, CGRP, and TRPV-1 targeting afferent nerve fibers in hind paw skin and glial cells in the spinal dorsal horn were examined via immunohistochemistry and immunofluorescence. Results: Oral cilostazol ameliorated the symptoms of mechanical allodynia but not thermal analgesia in DM rats. Significant reductions in PGP9.5-, P2X3-, CGRP, and TRPV-1-labeled penetrating nerve fibers in the epidermal layer indicated denervation of sensory nerves in the hind paw epidermis of DM rats. Denervation significantly improved in groups that received Cilo30 and Cilo100 in a dose-dependent manner. Cilostazol administration also suppressed microglial hyperactivation and increased astrocyte expressions in spinal dorsal horns. Conclusions: Oral cilostazol ameliorated hyperglycemia-induced peripheral small nerve fiber damage in the periphery of diabetic rats and effectively mitigated diabetic neuropathic pain via a central sensitization mechanism. Our findings present cilostazol not only as an effective option for managing symptoms of neuropathy but also for deterring the development of diabetic neuropathy in the early phase of type I diabetes.

Effect of duloxetine on pain relief after total knee arthroplasty: A meta-analysis of randomized controlled trials

BACKGROUND

Postoperative pain is one of the most feared complications of total knee arthroplasty. Recently, randomized controlled trials have compared the efficacy of duloxetine in patients undergoing total knee arthroplasty. However, there is no definite answer as to the efficacy and safety of duloxetine.

METHODS

Randomized controlled trials about relevant studies were searched from PubMed (1996 to July 2022), Embase (1996 to July 2022), and Cochrane Library (CENTRAL, July 2022).

RESULTS

Six high-quality studies containing 532 patients met the inclusion criteria. Results show patients in the duloxetine group had better performance in terms of visual analog scale (P < .05), equivalent morphine consumption (P < .05), and length of stay (P < .05).

CONCLUSION

Duloxetine can be used to reduce pain after knee arthroplasty in selected patients.

Analysis of the Influence of IL-6 and the Activation of the Jak/Stat3 Pathway in Fibromyalgia

Background: Fibromyalgia is a medical condition that affects a small percentage of the population, with no known effective treatment. There is evidence to suggest that inflammation is a key factor in the nerve sensitization that characterizes the disorder. Therefore, this paper concentrates on the role of IL-6 in fibromyalgia and the related pain-like symptoms. Methods: This work aimed to evaluate Sprague–Dawley rats, which were injected for three consecutive days with 1 mg/kg of reserpine; IL-6-R Ab was intraperitoneally injected at 1.5 mg/kg seven days after the first reserpine injection. Behavioral analyses were conducted at the beginning of the experiment and at seven and twenty-one days from the first reserpine injection. At this timepoint, the animals were sacrificed, and tissues were collected for molecular and histological analysis. Results: Our data showed the analgesic effect of IL-6-R-Ab administration on mechanical allodynia and thermal hyperalgesia. Additionally, the reserpine + IL-6-R-Ab group showed a reduced expression of the pain-related mediators cFOS and NFG and reduced levels of pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) and chemokines (Cxcl5, Cxcl10 and Cx3cl1). From the molecular point of view, the IL-6-R-Ab administration reduced the gp130 phosphorylation and the activation of the Jak/STAT3 pathway. Additionally, the IL-6-R Ab reduced the activation of neuroinflammatory cells. Conclusions: Our study showed that IL-6 plays a crucial role in fibromyalgia by triggering the Jak/STAT3 pathway, leading to an increase in chemokine levels and activating glial cells.

Exploratory Study on Central Sensitization and Bioelectrical Activity of the Selected Masticatory Muscles in Subjects with Myopia

BACKGROUND

Myopia is one of the most common eye disorders involving abnormal focusing of light rays. The studies recognize the association between the stomatognathic and visual systems. This compound may have a neurological basis associated with disorders such as central sensitization. The main aim of this study was to evaluate the influence of central sensitization on the bioelectrical activity of selected muscles of the masticatory organ in subjects with myopia.

METHODS

Selected masticatory and cervical spine muscles were analyzed using an eight-channel BioEMG III electromyograph. Central sensitization was analyzed using the central sensitization inventory.

RESULTS

Statistical analysis revealed significantly higher scores on the central sensitization inventory in subjects with axial myopia compared to subjects without refractive error. Repeated positive correlations were observed in the sternocleidomastoid muscle activity and negative correlations in the digastric muscle activity during open and closed eyes in myopic subjects.

CONCLUSIONS

Subjects with myopia have an increased score in the central sensitization inventory. The increase in the central sensitization inventory score is connected with the changes within the electromyographic activity of the masticatory and neck muscles. The effect of central sensitization on masticatory muscle activity in myopic subjects requires further study.

Noninvasive Brain Stimulation for Cancer Pain Management in Nonbrain Malignancy: A Meta-Analysis

Purpose. Noninvasive brain stimulation (NIBS) has been reported to have analgesic effects on fibromyalgia and chronic neuropathic pain; however, its effects on cancer pain have yet to be determined. The present study aimed to evaluate the effects of NIBS on patients with pain secondary to nonbrain malignancy. Methods. Electronic databases including PubMed, Embase, Cochrane Library, and Web of Science were searched from inception through June 5th, 2022. Parallel, randomized, placebo-controlled studies were included that enrolled adult patients with cancer pain, except for that caused by brain tumors, compared NIBS with placebo stimulation, and reported sufficient data for performing meta-analysis. Results. Four parallel, randomized, sham-controlled studies were included: two of repetitive transcranial magnetic stimulation (rTMS), one of transcranial direct current stimulation (tDCS), and one of cranial electrical stimulation (CES). rTMS significantly improved pain in the subgroup analysis (standardized mean difference (SMD): −1.148, 95% confidence interval (CI): −1.660 to −0.637, ( $p<0.001$ )), while NIBS was not benefited in reducing pain intensity (SMD: −0.632, 95% CI: −1.356 to 0.092, p = 0.087). Also, NIBS significantly improved depressive symptoms (SMD: −0.665, 95% CI: −1.178 to −0.153, p = 0.011), especially in the form of rTMS (SMD: −0.875, 95% CI: −1.356 to −0.395, $p<0.001$ ) and tDCS (SMD: −1.082, 95% CI: −1.746 to −0.418, p = 0.001). Conclusion. rTMS significantly improved pain secondary to nonbrain malignancy apart from other forms of NIBS without major adverse events.

Investigation of pain sensitivity following 3 nights of disrupted sleep in healthy individuals

BACKGROUND

Poor quality sleep is a common complaint among people with chronic pain. The co-occurrence of poor sleep quality and chronic pain often comes with increased pain intensity, more disability and a higher cost of healthcare. Poor sleep has been suggested to affect measures of peripheral and central pain mechanisms. To date, sleep provocations are the only models proven to affect measures of central pain mechanisms in healthy subjects. However, there are limited studies investigating the effect of several nights of sleep disruption on measures of central pain mechanisms.

METHODS

The current study implemented three nights of sleep disruption with three planned awakenings per night in 30 healthy subjects sleeping at home. Pain testing was conducted at the same time of day at baseline and follow-up for each subject. Pressure pain thresholds were assessed bilaterally on the infraspinatus and gastrocnemius muscles. Using handheld pressure algometry, suprathreshold pressure pain sensitivity and area were also investigated on the dominant infraspinatus muscle. Cuff-pressure pain detection and tolerance thresholds, temporal summation of pain and conditioned pain modulation were investigated using cuff-pressure algometry.

RESULTS

Temporal summation of pain was significantly facilitated (p = 0.022), suprathreshold pain areas (p = 0.005) and intensities (p < 0.05) were significantly increased, and all pressure pain thresholds were decreased (p < 0.005) after sleep disruption compared to baseline.

CONCLUSIONS

The current study found that three consecutive nights of sleep disruption at home induced pressure hyperalgesia and increased measures of pain facilitation in healthy subjects, which is consistent with previous findings.

SIGNIFICANCE

Poor quality of sleep is often experienced by patients with chronic pain, with the most common complaint being nightly awakenings. This exploratory study is the first to investigate changes in measures of central and peripheral pain sensitivity in healthy subjects after sleep disruptions for three consecutive nights without any restrictions on total sleep time. The findings suggest that disruptions to sleep continuity in healthy individuals can induce increased sensitivity to measures of central and peripheral pain sensitization.

Thermal grill illusion of pain in patients with chronic pain: a clinical marker of central sensitization?

ABSTRACT

The thermal grill illusion of pain (TGIP) is a paradoxical burning pain sensation elicited by the simultaneous application of innocuous cutaneous warm and cold stimuli with a thermode ("thermal grill") consisting of interlaced heated and cooled bars. Its neurophysiological mechanisms are unclear, but TGIP may have some mechanisms in common with pathological pain, including central sensitization in particular, through the involvement of N-methyl- d -aspartate receptors. However, few studies have investigated TGIP in patients with chronic pain and its clinical relevance is uncertain. We hypothesized that the TGIP would be increased in comparison with controls in patients with fibromyalgia or irritable bowel syndrome, which are regarded as typical "nociplastic" primary pain syndromes related to changes in central pain processing. We compared the sensations elicited by a large range of combinations of temperature differentials between the warm and cold bars of a thermal grill applied to the hand between patients with fibromyalgia (n = 30) or irritable bowel syndrome (n= 30) and controls (n = 30). The percentage of TGIP responses and the intensity and unpleasantness of TGIP were significantly greater in patients than controls. Furthermore, positive correlations were found between TGIP intensity and clinical pain intensity and between TGIP intensity and the cold pain threshold measured on the hand. These results are consistent with our working hypothesis of shared mechanisms between TGIP and clinical pain mechanisms in patients with nociplastic chronic pain syndromes and suggest that TGIP might represent a clinical marker of central sensitization in these patients.

Post-surgical contributors to persistent knee pain following knee replacement: The Multicenter Osteoarthritis Study (MOST)

Objective

Pain persistence following knee replacement (KR) occurs in ∼20-30% of patients. Although several studies have identified preoperative risk factors for persistent post-KR pain, few have focused on post-KR contributing factors. We sought to determine whether altered nociceptive signaling and other peripheral nociceptive drivers present post-operatively contribute to post-KR pain.

Design

We included participants from the Multicenter Osteoarthritis Study who were evaluated ∼12 months after KR. We evaluated the relation of measures of pain sensitivity [pressure pain threshold (PPT), temporal summation (TS), and conditioned pain modulation (CPM)] and the number of painful body sites to post-KR WOMAC knee pain, and of the number of painful sites to altered nociceptive signaling using linear or logistic regression models, as appropriate.

Results

171 participants (mean age 69 years, 62% female) were included. TS was associated with worse WOMAC pain post-KR (β ​= ​0.77 95% CI:0.19-1.35) and reduced odds of achieving patient acceptable symptom state (aOR ​= ​0.54 95%CI:0.34-0.88). Inefficient CPM was also associated with worse WOMAC pain post-KR (β ​= ​1.43 95% CI:0.15-2.71). In contrast, PPT was not associated with these outcomes. The number of painful body sites present post-KR was associated with TS (β ​= ​0.05, 95% CI:0.01, 0.05).

Conclusions

Post-KR presence of central sensitization and inefficient descending pain modulation was associated with post-KR pain. We also noted that presence of other painful body sites contributes to altered nociceptive signaling, and this may thus also contribute to the experience of knee pain post-KR. Our findings provide novel insights into central pain mechanisms and other peripheral pain sources contributing to post-KR persistent knee pain.

Differential Cortical Oscillatory Patterns in Amputees With and Without Phantom Limb Pain

Introduction

Phantom limb pain (PLP) as neuropathic pain affects the life of amputees. It is believed an efficient PLP treatment should consider the underlying neurological mechanisms. Hereby, we investigated brain activity in PLP and its relationships to the psychological and cognitive dimensions of chronic pain. We investigate differences in resting brain activities between amputees with and without pain. We hypothesize significant differences in the motor cortex and parietal cortex activity that are related to pain perception. Also, we hypothesize two groups have significant differences in cognitive and psychological components.

Methods

Behavioral assessment (psychological status, life satisfaction, and pain level) and EEG signals of 19 amputees (12 without pain and 7 with pain) were recorded. Data were statistically compared between the two groups. Also, the association between behavioral and neurophysiological data was computed.

Results

The results showed a significant decrease in the pain group for the beta and gamma waves, as well as, for the theta and delta waves in the posterior temporal on both sides, during the eye-open condition. The eyes-closed condition showed that the delta waves were decreased on the right side of the cortex. Also, data showed a significant difference in the correlation of pain features with brain waves between the two groups.

Conclusion

Significant differences were mostly observed in regions related to pain perception rather than the motor cortex. This can be due to the learned strategies to deal with pain and the degree of pain. Results showed maladaptive cognitive processes had a relationship with brain wave activities. According to the result of brain wave activities, it seems that cognitive factors have a role in the experience of PLP rather than neuroplasticity through amputation.

Highlights

Differences found in the parietal and temporal regions of phantom limb pain's (PLP's) suggests cognition's role in the persistence of PLP.Decreased delta power at the posterior temporal cortex in PLP's could be the focus of treatments.Increased activity of the parietal cortex could be helpful in the treatment of PLP's.

Plain Language Summary

PLP is an annoying neurologic pain. A wide range of treatments have focused on this type of pain but couldn't be effective. Recently, researchers suggest BCI-based treatments for better treatment. For this type of treatment, we should know the neurological aspect of PLP. In most studies to investigate or treatment of neurological aspects of PLP, researchers induced pain experimentally or studied acute phantom limb pain. We believed for a better understanding of PLP, should investigate it in a natural and stabilized position. Therefore we studied brain activities in amputees with and without PLP in a resting state to find out differences. Trends in this field express the alpha band differences in the motor cortex. On the contrary, our results showed the most significant difference in high-frequency bandpasses such as beta and gamma. Also, in our study, it seems the parietal and temporal cortex that are related to pain perception is the more relevant to PLP. This study showed a psycho-cognitive aspect of pain such as pain exaggeration has a relation with PLP's brain wave activities. So, we can suggest rather than neuroplasticity through amputation, cognitive factors have a role in the experience of PLP.

Green Light Exposure Elicits Anti-inflammation, Endogenous Opioid Release and Dampens Synaptic Potentiation to Relieve Post-surgical Pain

Light therapy improves multiple conditions such as seasonal affective disorders, circadian rhythm dysregulations, and neurodegenerative diseases. However, little is known about its potential benefits in pain management. While current pharmacologic methods are effective in many cases, the associated side effects can limit their use. Non-pharmacological methods would minimize drug dependence, facilitating a reduction of the opioid burden. Green light therapy has been shown to be effective in reducing chronic pain in humans and rodents. However, its underlying mechanisms remain incompletely defined. In this study, we demonstrate that green light exposure reduced postsurgical hypersensitivity in rats. Moreover, this therapy potentiated the antinociceptive effects of morphine and ibuprofen on mechanical allodynia in male rats. Importantly, in female rats, GLED potentiated the antinociceptive effects of morphine but did not affect that of ibuprofen. We showed that green light increases endogenous opioid levels while lessening synaptic plasticity and neuroinflammation. Importantly, this study reveals new insights into how light exposure can affect neuroinflammation and plasticity in both genders. Clinical translation of these results could provide patients with improved pain control and decrease opioid consumption. Given the noninvasive nature of green light, this innovative therapy would be readily implementable in hospitals. PERSPECTIVE: This study provides a potential additional therapy to decrease postsurgical pain. Given the safety, availability, and the efficacy of green light therapy, there is a significant potential for advancing the green light therapy to clinical trials and eventual translation to clinical settings.

War experiences and relationship problems predict pain sensitivity cross-sectionally among patients with chronic primary pain

BACKGROUND

Most patients suffering from chronic pain are more susceptible to pain and pressure due to higher pain sensitivity. Since psychosocial factors play a central role in developing and maintaining chronic pain, investigating associations between pain sensitivity and psychosocial stressors promises to advance the biopsychosocial understanding of chronic pain.

OBJECTIVES

We aimed to replicate Studer et al.'s (2016) findings about associations of psychosocial stressors with pain sensitivity in a new sample of patients with chronic primary pain (ICD-11, MG30.0).

METHODS

A pain provocation test was used on both middle fingers and earlobes to assess pain sensitivity among 460 inpatients with chronic primary pain. Potentially life-threatening accidents, war experiences, relationship problems, certified inability to work, and adverse childhood experiences were assessed as potential psychosocial stressors. Structural equation modeling was used to investigate associations between psychosocial stressors and pain sensitivity.

RESULTS

We partially replicated Studer et al.'s findings. Similar to the original study, patients with chronic primary pain showed enhanced pain sensitivity values. Within the investigated group, war experiences (β = 0.160, p < .001) and relationship problems (β = 0.096, p = .014) were associated with higher pain sensitivity. In addition, the control variables of age, sex, and pain intensity also showed a predictive value for higher pain sensitivity. Unlike Studer et al., we could not identify a certified inability to work as a predictor of higher pain sensitivity.

CONCLUSIONS

This study showed that beyond age, sex, and pain intensity, the psychosocial stressors of war experiences and relationship problems were associated with higher pain sensitivity.