Pain and immunity: implications for host defence

Tumor Neurobiology in the Pathogenesis and Therapy of Head and Neck Cancer

The neurobiology of tumors has attracted considerable interest from clinicians and scientists and has become a multidisciplinary area of research. Neural components not only interact with tumor cells but also influence other elements within the TME, such as immune cells and vascular components, forming a polygonal relationship to synergistically facilitate tumor growth and progression. This review comprehensively summarizes the current state of the knowledge on nerve-tumor crosstalk in head and neck cancer and discusses the potential underlying mechanisms. Several mechanisms facilitating nerve-tumor crosstalk are covered, such as perineural invasion, axonogenesis, neurogenesis, neural reprogramming, and transdifferentiation, and the reciprocal interactions between the nervous and immune systems in the TME are also discussed in this review. Further understanding of the nerve-tumor crosstalk in the TME of head and neck cancer may provide new nerve-targeted treatment options and help improve clinical outcomes for patients.

Calcium Carbonate/Polydopamine Composite Nanoplatform Based on TGF-β Blockade for Comfortable Cancer Immunotherapy

Cancer pain seriously reduces the quality of life of cancer patients. However, most research about cancer focuses solely on inhibiting tumor growth, neglecting the issue of cancer pain. Therefore, the development of therapeutic agents with both tumor suppression and cancer pain relief is crucial to achieve human-centered treatment. Here, the work reports curcumin (CUR) and ropivacaine (Ropi) coincorporating CaCO3/PDA nanoparticles (CaPNMCUR+Ropi) that realized efficient tumor immunotherapy and cancer pain suppression. The therapeutic efficiency and mechanism are revealed in vitro and in vivo. The results indicate that CaPNMCUR+Ropi underwent tumor microenvironment-responsive degradation and realized rapid release of calcium ions, Ropi, and CUR. The excessive intracellular calcium triggered the apoptosis of tumor cells, and the transient pain caused by the tumor injection was relieved by Ropi. Simultaneously, CUR reduced the levels of immunosuppressive factor (TGF-β) and inflammatory factor (IL-6, IL-1β, and TNF-α) in the tumor microenvironment, thereby continuously augmenting the immune response and alleviating inflammatory pain of cancer animals. Meanwhile, the decrease of TGF-β leads to the reduction of transient receptor potential vanilloid 1 (TRPV1) expression, thereby alleviating hyperalgesia and achieving long-lasting analgesic effects. The design of the nanosystem provides a novel idea for human-centered tumor treatment in the future.

Alcohol use and the pain system

The World Health Organization's epidemiological data from 2016 revealed that while 57% of the global population aged 15 years or older had abstained from drinking alcohol in the previous year, more than half of the population in the Americas, Europe, and Western Pacific consumed alcohol. The spectrum of alcohol use behavior is broad: low-risk use (sensible and in moderation), at-risk use (e.g., binge drinking), harmful use (misuse) and dependence (alcoholism; addiction; alcohol use disorder). The at-risk use and misuse of alcohol is associated with the transition to dependence, as well as many damaging health outcomes and preventable causes of premature death. Recent conceptualizations of alcohol dependence posit that the subjective experience of pain may be a significant contributing factor in the transition across the spectrum of alcohol use behavior. This narrative review summarizes the effects of alcohol at all levels of the pain system. The pain system includes nociceptors as sensory indicators of potentially dangerous stimuli and tissue damage (nociception), spinal circuits mediating defensive reflexes, and most importantly, the supraspinal circuits mediating nocifensive behaviors and the perception of pain. Although the functional importance of pain is to protect from injury and further or future damage, chronic pain may emerge despite the recovery from, and absence of, biological damage (i.e., in the absence of nociception). Like other biological perceptual systems, pain is a construction contingent on sensory information and a history of individual experiences (i.e., learning and memory). Neuroadaptations and brain plasticity underlying learning and memory and other basic physiological functions can also result in pathological conditions such as chronic pain and addiction. Moreover, the negative affective/emotional aspect of pain perception provides embodied and motivational components that may play a substantial role in the transition from alcohol use to dependence.

Pain and the biochemistry of fibromyalgia: patterns of peripheral cytokines and chemokines contribute to the differentiation between fibromyalgia and controls and are associated with pain, fat infiltration and content

Objectives

This explorative study analyses interrelationships between peripheral compounds in saliva, plasma, and muscles together with body composition variables in healthy subjects and in fibromyalgia patients (FM). There is a need to better understand the extent cytokines and chemokines are associated with body composition and which cytokines and chemokines differentiate FM from healthy controls.

Methods

Here, 32 female FM patients and 30 age-matched female healthy controls underwent a clinical examination that included blood sample, saliva samples, and pain threshold tests. In addition, the subjects completed a health questionnaire. From these blood and saliva samples, a panel of 68 mainly cytokines and chemokines were determined. Microdialysis of trapezius and erector spinae muscles, phosphorus-31 magnetic resonance spectroscopy of erector spinae muscle, and whole-body magnetic resonance imaging for determination of body composition (BC)-i.e., muscle volume, fat content and infiltration-were also performed.

Results

After standardizing BC measurements to remove the confounding effect of Body Mass Index, fat infiltration and content are generally increased, and fat-free muscle volume is decreased in FM. Mainly saliva proteins differentiated FM from controls. When including all investigated compounds and BC variables, fat infiltration and content variables were most important, followed by muscle compounds and cytokines and chemokines from saliva and plasma. Various plasma proteins correlated positively with pain intensity in FM and negatively with pain thresholds in all subjects taken together. A mix of increased plasma cytokines and chemokines correlated with an index covering fat infiltration and content in different tissues. When muscle compounds were included in the analysis, several of these were identified as the most important regressors, although many plasma and saliva proteins remained significant.

Discussion

Peripheral factors were important for group differentiation between FM and controls. In saliva (but not plasma), cytokines and chemokines were significantly associated with group membership as saliva compounds were increased in FM. The importance of peripheral factors for group differentiation increased when muscle compounds and body composition variables were also included. Plasma proteins were important for pain intensity and sensitivity. Cytokines and chemokines mainly from plasma were also significantly and positively associated with a fat infiltration and content index.

Conclusion

Our findings of associations between cytokines and chemokines and fat infiltration and content in different tissues confirm that inflammation and immune factors are secreted from adipose tissue. FM is clearly characterized by complex interactions between peripheral tissues and the peripheral and central nervous systems, including nociceptive, immune, and neuroendocrine processes.

BCG induced lower urinary tract symptoms during treatment for NMIBC-Mechanisms and management strategies

Non-muscle invasive bladder cancer (NMIBC) accounts for ~70-75% of total bladder cancer tumors and requires effective early intervention to avert progression. The cornerstone of high-risk NMIBC treatment involves trans-urethral resection of the tumor followed by intravesical Bacillus Calmette-Guerin (BCG) immunotherapy. However, BCG therapy is commonly accompanied by significant lower urinary tract symptoms (LUTS) including urinary urgency, urinary frequency, dysuria, and pelvic pain which can undermine treatment adherence and clinical outcomes. Despite this burden, the mechanisms underlying the development of BCG-induced LUTS have yet to be characterized. This review provides a unique perspective on the mechanisms thought to be responsible for the development of BCG-induced LUTS by focussing on the sensory nerves responsible for bladder sensory transduction. This review focuses on how the physiological response to BCG, including inflammation, urothelial permeability, and direct interactions between BCG and sensory nerves could drive bladder afferent sensitization leading to the development of LUTS. Additionally, this review provides an up-to-date summary of the latest clinical data exploring interventions to relieve BCG-induced LUTS, including therapeutic targeting of bladder contractions, inflammation, increased bladder permeability, and direct inhibition of bladder sensory signaling. Addressing the clinical burden of BCG-induced LUTS holds significant potential to enhance patient quality of life, treatment compliance, and overall outcomes in NMIBC management. However, the lack of knowledge on the pathophysiological mechanisms that drive BCG-induced LUTS has limited the development of novel and efficacious therapeutic options. Further research is urgently required to unravel the mechanisms that drive BCG-induced LUTS.

Immune Homeostasis: A Novel Example of Teamwork

All living organisms must maintain homeostasis to survive, reproduce, and pass their traits on to the next generation. If homeostasis is not maintained, it can result in various diseases and ultimately lead to death. Physiologists have coined the term "homeostasis" to describe this process. With the emergence of immunology as a separate branch of medicine, the concept of immune homeostasis has been introduced. Maintaining immune homeostasis is crucial to support overall homeostasis through different immunological and non-immunological routes. Any changes in the immune system can lead to chronic inflammatory or autoimmune diseases, immunodeficiency diseases, frequent infections, and cancers. Ongoing scientific advances are exploring new avenues in immunology and immune homeostasis maintenance. This chapter introduces the concept of immune homeostasis and its maintenance through different mechanisms.

Biodegradable nanoparticles targeting circulating immune cells reduce central and peripheral sensitization to alleviate neuropathic pain following spinal cord injury

ABSTRACT

Neuropathic pain is a critical source of comorbidity following spinal cord injury (SCI) that can be exacerbated by immune-mediated pathologies in the central and peripheral nervous systems. In this article, we investigate whether drug-free, biodegradable, poly(lactide- co -glycolide) (PLG) nanoparticle treatment mitigates the development of post-SCI neuropathic pain in female mice. Our results show that acute treatment with PLG nanoparticles following thoracic SCI significantly reduces tactile and cold hypersensitivity scores in a durable fashion. Nanoparticles primarily reduce peripheral immune-mediated mechanisms of neuropathic pain, including neuropathic pain-associated gene transcript frequency, transient receptor potential ankyrin 1 nociceptor expression, and MCP-1 (CCL2) chemokine production in the subacute period after injury. Altered central neuropathic pain mechanisms during this period are limited to reduced innate immune cell cytokine expression. However, in the chronic phase of SCI, nanoparticle treatment induces changes in both central and peripheral neuropathic pain signaling, driving reductions in cytokine production and other immune-relevant markers. This research suggests that drug-free PLG nanoparticles reprogram peripheral proalgesic pathways subacutely after SCI to reduce neuropathic pain outcomes and improve chronic central pain signaling.

Biphasic Hormetic-like Effect of Lebecetin, a C-type Lectin of Snake Venom, on Formalin-induced Inflammation in Mice

BACKGROUND

Integrins, important extracellular matrix (ECM) receptor proteins, are affected by inflammation and can participate in the maintenance of many painful conditions. Although they are ubiquitous and changeable across all cell types, the roles of these cell adhesion molecules in pathological pain have not been fully explored.

OBJECTIVE

We evaluated the effects of the subcutaneous injection of lebecetin, a C-type lectin isolated from Macrovipera lebetina snake venom, previously reported to inhibit α5β1 and αv integrin activity, on different components of inflammation induced by the formalin administration in the hind paw of mice.

METHODS

The formalin-induced nocifensive behavior, edema, and histopathological changes in the hind paw associated with cytokine, iNOS, and COX2 expression, nociceptive-specific neuron activity, and microglial activation analysis in the spinal cord were evaluated in mice receiving vehicle or lebecetin pretreatment.

RESULTS

Lebecetin inhibited the nocifensive responses in the formalin test, related edema, and cell infiltration in the injected paw in a biphasic, hormetic-like, and dose-dependent way. According to that hormetic trend, a reduction in pro-inflammatory cytokines IL-6, IL-8, and TNF-alpha and upregulation of the anti-inflammatory cytokine IL-10 in the spinal cord were found with the lowest doses of lebecetin. Moreover, COX2 and iNOS expression in serum and spinal cord followed the same biphasic pattern of cytokines. Finally, nociceptive neurons sensitization and activated microglia were normalized in the dorsal horn of the spinal cord by lebecetin.

CONCLUSION

These findings implicate specific roles of integrins in inflammation and tonic pain, as well as in the related central nervous system sequelae.

Neuro-bone tissue engineering: emerging mechanisms, potential strategies, and current challenges

The skeleton is a highly innervated organ in which nerve fibers interact with various skeletal cells. Peripheral nerve endings release neurogenic factors and sense skeletal signals, which mediate bone metabolism and skeletal pain. In recent years, bone tissue engineering has increasingly focused on the effects of the nervous system on bone regeneration. Simultaneous regeneration of bone and nerves through the use of materials or by the enhancement of endogenous neurogenic repair signals has been proven to promote functional bone regeneration. Additionally, emerging information on the mechanisms of skeletal interoception and the central nervous system regulation of bone homeostasis provide an opportunity for advancing biomaterials. However, comprehensive reviews of this topic are lacking. Therefore, this review provides an overview of the relationship between nerves and bone regeneration, focusing on tissue engineering applications. We discuss novel regulatory mechanisms and explore innovative approaches based on nerve-bone interactions for bone regeneration. Finally, the challenges and future prospects of this field are briefly discussed.

Potential uses of auditory nerve stimulation to modulate immune responses in the inner ear and auditory brainstem

Bioelectronic medicine uses electrical stimulation of the nervous system to improve health outcomes throughout the body primarily by regulating immune responses. This concept, however, has yet to be applied systematically to the auditory system. There is growing interest in how cochlear damage and associated neuroinflammation may contribute to hearing loss. In conjunction with recent findings, we propose here a new perspective, which could be applied alongside advancing technologies, to use auditory nerve (AN) stimulation to modulate immune responses in hearing health disorders and following surgeries for auditory implants. In this article we will: (1) review the mechanisms of inflammation in the auditory system in relation to various forms of hearing loss, (2) explore nerve stimulation to reduce inflammation throughout the body and how similar neural-immune circuits likely exist in the auditory system (3) summarize current methods for stimulating the auditory system, particularly the AN, and (4) propose future directions to use bioelectronic medicine to ameliorate harmful immune responses in the inner ear and auditory brainstem to treat refractory conditions. We will illustrate how current knowledge from bioelectronic medicine can be applied to AN stimulation to resolve inflammation associated with implantation and disease. Further, we suggest the necessary steps to get discoveries in this emerging field from bench to bedside. Our vision is a future for AN stimulation that includes additional protocols as well as advances in devices to target and engage neural-immune circuitry for therapeutic benefits.

Docking for EP4R antagonists active against inflammatory pain

The lipid prostaglandin E2 (PGE2) mediates inflammatory pain by activating G protein-coupled receptors, including the prostaglandin E2 receptor 4 (EP4R). Nonsteroidal anti-inflammatory drugs (NSAIDs) reduce nociception by inhibiting prostaglandin synthesis, however, the disruption of upstream prostanoid biosynthesis can lead to pleiotropic effects including gastrointestinal bleeding and cardiac complications. In contrast, by acting downstream, EP4R antagonists may act specifically as anti-inflammatory agents and, to date, no selective EP4R antagonists have been approved for human use. In this work, seeking to diversify EP4R antagonist scaffolds, we computationally dock over 400 million compounds against an EP4R crystal structure and experimentally validate 71 highly ranked, de novo synthesized molecules. Further, we show how structure-based optimization of initial docking hits identifies a potent and selective antagonist with 16 nanomolar potency. Finally, we demonstrate favorable pharmacokinetics for the discovered compound as well as anti-allodynic and anti-inflammatory activity in several preclinical pain models in mice.

TissueGene-C induces long-term analgesic effects through regulation of pain mediators and neuronal sensitization in a rat monoiodoacetate-induced model of osteoarthritis pain

OBJECTIVE

TissueGene-C (TG-C), a combination of human allogeneic chondrocytes and irradiated GP2-293 cells engineered to overexpress transforming growth factor-β1 (TGF-β1), has been developed as a novel cell-based gene therapy and a candidate for disease modifying osteoarthritis drug (DMOAD). We aim to investigate analgesic mechanism of TG-C in a pre-clinical animal model with monoiodoacetate (MIA)-induced pain.

DESIGN

We used a rat MIA model of osteoarthritis (OA) pain. We examined that TG-C can regulate pain by inhibiting the upregulation of various pain mediators in both knee joint tissue and dorsal root ganglia (DRG) (n = 112) and alleviating pain behavior (n = 41) and neuronal hyperexcitability in DRG (n = 60), afferent nerve fiber (n = 24), and spinal cord (n = 35).

RESULTS

TG-C significantly alleviated pain-related behavior by restoring altered dynamic weight bearing and reduced mechanical threshold of the affected hindlimb. TG-C significantly suppressed the expression of nerve growth factor (NGF) and calcitonin gene-related peptide (CGRP) in inflamed joint tissue. TG-C significantly suppressed the upregulation of tropomyosin receptor kinase A (TrkA) and nerve injury/regeneration protein (GAP43) and activation of Iba1-positive microglial cells in DRG. TG-C significantly recovered neuronal hyperexcitability by restoring RMP and firing threshold and frequency of DRG neurons, attenuating firing rates of mechanosensitive C- or Aδ-nerve fiber innervating knee joint, and lowering increased miniature and evoked excitatory postsynaptic currents (mEPSCs and eEPSCs) in the spinal cord.

CONCLUSION

Our results demonstrated that TG-C exerted potent analgesic effects in a rat MIA model of OA pain by inhibiting the upregulation of pain mediators and modulating neuronal sensitization.

S. aureus drives itch and scratch-induced skin damage through a V8 protease-PAR1 axis

Itch is an unpleasant sensation that evokes a desire to scratch. The skin barrier is constantly exposed to microbes and their products. However, the role of microbes in itch generation is unknown. Here, we show that Staphylococcus aureus, a bacterial pathogen associated with itchy skin diseases, directly activates pruriceptor sensory neurons to drive itch. Epicutaneous S. aureus exposure causes robust itch and scratch-induced damage. By testing multiple isogenic bacterial mutants for virulence factors, we identify the S. aureus serine protease V8 as a critical mediator in evoking spontaneous itch and alloknesis. V8 cleaves proteinase-activated receptor 1 (PAR1) on mouse and human sensory neurons. Targeting PAR1 through genetic deficiency, small interfering RNA (siRNA) knockdown, or pharmacological blockade decreases itch and skin damage caused by V8 and S. aureus exposure. Thus, we identify a mechanism of action for a pruritogenic bacterial factor and demonstrate the potential of inhibiting V8-PAR1 signaling to treat itch.

Antinociceptive effect of Nephelium lappaceum L. fruit peel and the participation of nitric oxide, opioid receptors, and ATP-sensitive potassium channels

Introduction: Nephelium lappaceum L. (Sapindaceae) is a plant known as rambutan. It is used for various purposes in traditional medicine. Objective: We aimed to evaluate the antinociceptive effects of the ethanol extract of the fruit peel of N. lappaceum (EENL), the mechanisms involved in these effects, and the acute toxicity in zebrafish. Methods: We performed chromatography coupled to mass spectrometry, acute toxicity assay in zebrafish, and evaluation in mice submitted to models of nociception and locomotor activity. Results: We identified (epi)-catechin, procyanidin B, and ellagic acid and its derivatives in EENL. We did not find any toxicity in zebrafish embryos incubated with EENL. The locomotor activity of mice submitted to oral pretreatment with EENL was not changed, but it reduced the abdominal constrictions induced by acetic acid, the licking/biting time in both the first and second phase of formalin testing and capsaicin testing, and carrageenan-induced paw mechanical allodynia. Oral pretreatment with EENL increased latency time in the hot plate test. This antinociceptive effect was significantly reversed by naloxone, L-arginine, and glibenclamide respectively showing the participation of opioid receptors, nitric oxide, and KATP channels as mediators of EENL-induced antinociception. Conclusion: EENL causes antinociception with the participation of opioid receptors, nitric oxide, and KATP channels, and is not toxic to zebrafish.

Neanderthal introgression in SCN9A impacts mechanical pain sensitivity

The Nav1.7 voltage-gated sodium channel plays a key role in nociception. Three functional variants in the SCN9A gene (encoding M932L, V991L, and D1908G in Nav1.7), have recently been identified as stemming from Neanderthal introgression and to associate with pain symptomatology in UK BioBank data. In 1000 genomes data, these variants are absent in Europeans but common in Latin Americans. Analysing high-density genotype data from 7594 Latin Americans, we characterized Neanderthal introgression in SCN9A. We find that tracts of introgression occur on a Native American genomic background, have an average length of ~123 kb and overlap the M932L, V991L, and D1908G coding positions. Furthermore, we measured experimentally six pain thresholds in 1623 healthy Colombians. We found that Neanderthal ancestry in SCN9A is significantly associated with a lower mechanical pain threshold after sensitization with mustard oil and evidence of additivity of effects across Nav1.7 variants. Our findings support the reported association of Neanderthal Nav1.7 variants with clinical pain, define a specific sensory modality affected by archaic introgression in SCN9A and are consistent with independent effects of the Neanderthal variants on Nav1.7 function.

Pain science and practice as a 'threshold concept' within undergraduate and pre-registration physiotherapy education: a jewel of the curriculum?

BACKGROUND

Threshold concepts describe learning experiences that transform our understanding of a concept. Threshold concepts are variously: troublesome, transformative, irreversible, integrative and bounded.

PURPOSE

The aim of this narrative review is to consider the case for characterising pain science and practice as a threshold concept within undergraduate and pre-registration physiotherapy education. This article considers the underlying tenets of threshold concepts as they relate to teaching and learning and the relative merits and limitations of characterising pain science and practice as a threshold concept within undergraduate and pre-registration physiotherapy education from both pedagogical and epidemiological perspectives. By evaluating pain, as it relates to physiotherapy education and practice, according to the five defining characteristics of a threshold concept then presenting data related to the epidemiology and impact of pain, the worthiness of characterising pain science and practice as a threshold concept will be discussed and further debate invited.

Anti-Inflammatory Activity of N'-(3-(1H-indol-3-yl)benzylidene)-2-cyanoacetohydrazide Derivative via sGC-NO/Cytokine Pathway

The N-acylhydrazone function has been reported as a pharmacophore group of molecules with diverse pharmacological activities, including anti-inflammatory effects. Therefore, this study was designed to evaluate the anti-inflammatory potential of the compound N'-(3-(1H-indol-3-yl)benzylidene)-2-cyanoacetohydrazide (JR19) in vivo. The study started with the carrageenan-induced peritonitis model, followed by an investigation of leukocyte migration using the subcutaneous air pouch test and an assessment of the antinociceptive profile using formalin-induced pain. A preliminary molecular docking study focusing on the crystallographic structures of NFκB, iNOS, and sGC was performed to determine the likely mechanism of action. The computational study revealed satisfactory interaction energies with the selected targets, and the same peritonitis model was used to validate the involvement of the nitric oxide pathway and cytokine expression in the peritoneal exudate of mice pretreated with L-NAME or methylene blue. In the peritonitis assay, JR19 (10 and 20 mg/kg) reduced leukocyte migration by 59% and 52%, respectively, compared to the vehicle group, with the 10 mg/kg dose used in subsequent assays. In the subcutaneous air pouch assay, the reduction in cell migration was 66%, and the response to intraplantar formalin was reduced by 39%, particularly during the inflammatory phase, suggesting that the compound lacks central analgesic activity. In addition, a reversal of the anti-inflammatory effect was observed in mice pretreated with L-NAME or methylene blue, indicating the involvement of iNOS and sGC in the anti-inflammatory response of JR19. The compound effectively and significantly decreased the levels of IL-6, TNF-α, IL-17, and IFN-γ, and this effect was reversed in animals pretreated with L-NAME, supporting a NO-dependent anti-inflammatory effect. In contrast, pretreatment with methylene blue only reversed the reduction in TNF-α levels. Therefore, these results demonstrate the pharmacological potential of the novel N-acylhydrazone derivative, which acts through the nitric oxide pathway and cytokine signaling, making it a strong candidate as an anti-inflammatory and immunomodulatory agent.

An underlying mechanism behind interventional pulmonology techniques for refractory asthma treatment: Neuro-immunity crosstalk

Asthma is a common disease that seriously threatens public health. With significant developments in bronchoscopy, different interventional pulmonology techniques for refractory asthma treatment have been developed. These technologies achieve therapeutic purposes by targeting diverse aspects of asthma pathophysiology. However, even though these newer techniques have shown appreciable clinical effects, their differences in mechanisms and mutual commonalities still deserve to be carefully explored. Therefore, in this review, we summarized the potential mechanisms of bronchial thermoplasty, targeted lung denervation, and cryoablation, and analyzed the relationship between these different methods. Based on available evidence, we speculated that the main pathway of chronic airway inflammation and other pathophysiologic processes in asthma is sensory nerve-related neurotransmitter release that forms a "neuro-immunity crosstalk" and amplifies airway neurogenic inflammation. The mechanism of completely blocking neuro-immunity crosstalk through dual-ablation of both efferent and afferent fibers may have a leading role in the clinical efficacy of interventional pulmonology in the treatment of asthma and deserves further investigation.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Mechanisms involved in the antinociceptive and anti-inflammatory effects of xanthotoxin

Xanthotoxin (XAT) is a natural furanocoumarin clinically used in the treatment of skin diseases such as vitiligo and psoriasis. Recent studies have also investigated its effects on anti-inflammatory, anti-cognitive dysfunction, and anti-amnesia as a guideline for clinic application. However, little is known about its effects on pain relief. Here, we tested the analgesic effects of XAT in serious acute pain and chronic pain models. For acute pain, we used hot-, capsaicin- and formalin-induced paw licking. Nociceptive threshold was measured by mechanical stimuli with von Frey filaments. For chronic pain, we injected complete Freund's adjuvant (CFA) into the mice's plantar surface of the hind paw to induce inflammatory pain. Heat and mechanical hyperalgesia were evaluated by radiant heat and von Frey filament tests, respectively. To investigate the mechanisms underlying the analgesic effect of XAT, we used calcium imaging and western blot to assess transient receptor potential vanilloid 1 (TRPV1) activity and expression in isolated L4-L6 dorsal root ganglion (DRG) neurons. Haematoxylin and eosin (HE) staining, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA) were used to examine immune cell recruitment and proinflammatory factor release from skin tissue from paw injection sites. Our results demonstrated that XAT not only reduced acute pain behaviors generated by hot, capsaicin, and formalin but also attenuated CFA-induced heat and mechanical hyperalgesia. The analgesic activity of XAT may be achieved by controlling peripheral inflammation, lowering immune cell infiltration at the site of inflammatory tissue, reducing inflammatory factor production, and therefore inhibiting TRPV1 channel sensitization and expression.

Peripheral Activation of Formyl Peptide Receptor 2/ALX by Electroacupuncture Alleviates Inflammatory Pain by Increasing Interleukin-10 Levels and Catalase Activity in Mice

In the context of the electroacupuncture (EA) neurobiological mechanisms, we have previously demonstrated the involvement of formyl peptide receptor 2 (FPR2/ALX) in the antihyperalgesic effect of EA. The present study investigated the involvement of peripheral FPR2/ALX in the antihyperalgesic effect of EA on inflammatory cytokines levels, oxidative stress markers and antioxidant enzymes in an animal model of persistent inflammatory pain. Male Swiss mice underwent intraplantar (i.pl.) injection with complete Freund's adjuvant (CFA). Mechanical hyperalgesia was assessed with von Frey monofilaments. Animals were treated with EA (2/10 Hz, ST36-SP6, 20 minutes) for 4 consecutive days. From the first to the fourth day after CFA injection, animals received i.pl. WRW4 (FPR2/ALX antagonist) or saline before EA. Levels of inflammatory cytokines (TNF, IL-6, IL-4 and IL-10), antioxidant enzymes (catalase and superoxide dismutase), oxidative stress markers (TBARS, protein carbonyl, nitrite/nitrate ratio), and myeloperoxidase activity were measured in paw tissue samples. As previously demonstrated, i.pl. injection of the FPR2/ALX antagonist prevented the antihyperalgesic effect induced by EA. Furthermore, animals treated with EA showed higher levels of IL-10 and catalase activity in the inflamed paw, and these effects were prevented by the antagonist WRW4. EA did not change levels of TNF and IL-6, SOD and MPO activity, and oxidative stress markers. Our work demonstrates that the antihyperalgesic effect of EA on CFA-induced inflammatory pain could be partially associated with higher IL-10 levels and catalase activity, and that these effects may be dependent, at least in part, on the activation of peripheral FPR2/ALX.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

Senso-immunology: the past, present, and future

Pain and mechanical stimulation are thought to be alarm systems that alert the brain to physical abnormalities. When we experience unpleasant feelings in infected or traumatized tissues, our awareness is directed to the afflicted region, prompting activities such as resting or licking the tissue. Despite extensive research into the molecular biology of nociceptors, it was unclear whether their role was limited to the generation and transmission of unpleasant feelings or whether they actively modulate the pathogenesis of infected or traumatized tissues. Recently, it has become clear how the sensory and immune systems interact with one another and share similar receptors and ligands to modify the pathogenesis of various diseases. In this paper, we summarize the mechanisms of crosstalk between the sensory and immune systems and the impact of this new interdisciplinary field, which should be dubbed 'senso-immunology,' on medical science.

Senso-immunology: The Emerging Connection between Pain and Immunity

The sensory and immune systems have been studied independently for a long time, whereas the interaction between the two has received little attention. We have carried out research to understand the interaction between the sensory and immune systems and have found that inflammation and bone destruction caused by fungal infection are suppressed by nociceptors. Furthermore, we have elucidated the molecular mechanism whereby fungal receptors are expressed on nociceptors and skin epithelium, how they cooperate to generate fungal pain, and how colitis and bone metabolism are regulated by mechanosensors expressed on the gut epithelium. Recently, we found that nociceptors prevent septic death by inhibiting microglia via nociceptor-derived hormones. This review summarizes our current state of knowledge on pain biology and outlines the mechanisms whereby pain and immunity interact. Our findings indicate that the sensory and immune systems share a variety of molecules and interact with each other to regulate our pathological and homeostatic conditions. This prompted us to advocate the interdisciplinary science named "senso-immunology," and this emerging field is expected to generate new ideas in both physiology and immunology, leading to the development of novel drugs to treat pain and inflammation.

Quantitative Dynamic Allodynograph-A Standardized Measure for Testing Dynamic Mechanical Allodynia in Chronic Limb Pain

BACKGROUND

Dynamic mechanical allodynia (DMA) is both a symptom and a central sensitization sign, yet no standardized method for quantifying the DMA area has been reported. This study aimed to establish psychometric properties for Quantitative Dynamic Allodynography (QDA), a newly developed protocol measuring the DMA area as a percentage of the body surface.

METHODS

Seventy-eight patients aged 18-65 diagnosed with chronic complex regional pain syndrome (CRPS) participated in this study. Test-retest reliability was conducted twice, one week apart (N = 20), and inter-rater (N = 3) reliability was conducted on 10 participants. Disease severity (CRPS Severity Score, CSS), pain intensity (VAS), and quality of life (SF-36) measures were utilized to test construct validity.

RESULTS

High inter-rater reliability (intraclass correlation coefficient (ICC) = 0.96, p < 0.001) and test-retest reliability (r = 0.98, p < 0.001) were found. Furthermore, the QDA score was found to be correlated with the CSS (r = 0.47, p < 0.001), VAS (r = 0.37, p < 0.001), and the SF-36 physical health total (r = -0.47, p < 0.001) scores.

CONCLUSION

The QDA is the first developed reliable and valid protocol for measuring DMA in a clinical setting and may be used as a diagnostic and prognostic measure in clinics and in research, advancing the pain precision medicine approach.

Adrenergic nerves regulate intestinal regeneration through IL-22 signaling from type 3 innate lymphoid cells

The intestinal epithelium has high intrinsic turnover rate, and the precise renewal of the epithelium is dependent on the microenvironment. The intestine is innervated by a dense network of peripheral nerves that controls various aspects of intestinal physiology. However, the role of neurons in regulating epithelial cell regeneration remains largely unknown. Here, we investigated the effects of gut-innervating adrenergic nerves on epithelial cell repair following irradiation (IR)-induced injury. We observed that adrenergic nerve density in the small intestine increased post IR, while chemical adrenergic denervation impaired epithelial regeneration. Single-cell RNA sequencing experiments revealed a decrease in IL-22 signaling post IR in denervated animals. Combining pharmacologic and genetic tools, we demonstrate that β-adrenergic receptor signaling drives IL-22 production from type 3 innate lymphoid cells (ILC3s) post IR, which in turn promotes epithelial regeneration. These results define an adrenergic-ILC3 axis important for intestinal regeneration.

IL-33/ST2 signaling in pain and itch: Cellular and molecular mechanisms and therapeutic potentials

Pain is a cardinal feature of many diseases. Chronic pain poses heavy burdens to the suffering patients, both physically and mentally. However, current mainstream medications for chronic pain, including opioids, antidepressants and non-steroid anti-inflammatory drugs are sometimes inefficient for chronic pain management and may cause side effects that limit long term usage. IL-33 belongs to IL-1 cytokine family and it exerts biological activities through binding to its specific receptor ST2. IL-33/ST2 signaling is very important in both innate and adaptive immunity. Emerging evidence indicates IL-33/ST2 signaling regulates pain in both immune and somatosensory systems through promoting neuro-immune or neuron-glia crosstalk, neuroinflammation and neuronal hyperexcitability. Some very latest studies indicate a vital part of IL-33/ST2 in mediating chronic itch. This work aims to overview the existing knowledge regarding the mechanisms of IL-33/ST2 involvement in pain and itch conditions, considering their potential similarities. We also summarized some key findings obtained from clinical studies. The targeting of IL-33/ST2 signaling holds promise for the development of novel therapeutic modalities in the management of pain and itch.

M2 macrophage polarization: a potential target in pain relief

Pain imposes a significant urden on patients, affecting them physically, psychologically, and economically. Despite numerous studies on the pathogenesis of pain, its clinical management remains suboptimal, leading to the under-treatment of many pain patients. Recently, research on the role of macrophages in pain processes has been increasing, offering potential for novel therapeutic approaches. Macrophages, being indispensable immune cells in the innate immune system, exhibit remarkable diversity and plasticity. However, the majority of research has primarily focused on the contributions of M1 macrophages in promoting pain. During the late stage of tissue damage or inflammatory invasion, M1 macrophages typically transition into M2 macrophages. In recent years, growing evidence has highlighted the role of M2 macrophages in pain relief. In this review, we summarize the mechanisms involved in M2 macrophage polarization and discuss their emerging roles in pain relief. Notably, M2 macrophages appear to be key players in multiple endogenous pathways that promote pain relief. We further analyze potential pathways through which M2 macrophages may alleviate pain.

Anti-Pyretic, Analgesic, and Anti-Inflammatory Activities of Meloxicam and Curcumin Co-Encapsulated PLGA Nanoparticles in Acute Experimental Models

Herein, we evaluated the in vivo effects of meloxicam and curcumin co-encapsulated PLGA nanoparticles in experimental acute models of pyrexia, nociception, and inflammation. Seven groups (n = 6) were designed for each investigation and pretreated intraperitoneally (i.p.): the control group, meloxicam (4 mg/kg b.w.), curcumin (15 mg/kg b.w.), and equivalent content containing PLGA capped nanoparticles of meloxicam (Mlx-NP) and curcumin (Cur-NP) alone and in combination (Mlx-Cur-NP; at two doses). The results showed that PLGA encapsulation significantly (p ≤ 0.05) improved the in vivo activities of each compound. Furthermore, co-encapsulation of meloxicam and curcumin potentiated the anti-pyretic effect on yeast-induced pyretic rats, anti-nociceptive effect on nociception induced in rats by formalin and heat, and anti-edematogenic activity in xylene-induced ear edema in rats in a dose-dependent manner. In carrageenan-induced paw inflammation in rats, meloxicam and curcumin co-loading (Mlx-Cur-NP) resulted in significant (p ≤ 0.05) inhibition of paw inflammation, reduction in TNF-α and PGE2 levels, downregulation of expressions of pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6), as well as a decrease in histopathological changes and TNF-α immunoexpression in paw tissues. Moreover, Mlx-Cur-NP demonstrated noteworthy potentiation in pharmacological effects compared to free compounds and mono-compound-loaded nanoparticles. Thus, the association of meloxicam with curcumin in a biodegradable nanocarrier system could provide a promising anti-pyretic, anti-nociceptive, and anti-inflammatory therapeutic approach for acute conditions.

Neuro-immune interactions and immuno-oncology

The nervous system is an important component of the tumor microenvironment (TME), driving tumorigenesis and tumor progression. Neuronal cues (e.g., neurotransmitters and neuropeptides) in the TME cause phenotypic changes in immune cells, such as increased exhaustion and inhibition of effector cells, which promote immune evasion and cancer progression. Two types of immune regulation by tumor-associated nerves are discussed in this review: regulation via neuronal stimuli (i.e., by neural transmission) and checkpoint-mediated neuronal immune regulation. The latter occurs via the expression of immune checkpoints on the membranes of intratumoral nerves and glial cells. Here, we summarize novel findings regarding the neuroimmune circuits in the tumor milieu, while emphasizing the potential targets of new and affordable anticancer therapeutic approaches.

Role of SIRT5 in the analgesic effectiveness of moxibustion at ST36 in mice with inflammatory pain

Sirtuine5 (SIRT5) is an important molecule involved in the pathology of inflammatory diseases. To investigate the impact of SIRT5 on the analgesic effectiveness of moxibustion, we established a complete Freund's adjuvant- (CFA-) induced inflammatory pain in mice model. Moxibustion was applied at the Zusanli (ST36) acupoint in mice with inflammatory pain. The analgesic effectiveness was evaluated by thermal hyperalgesia and mechanical allodynia tests in the right paws after CFA injection. The expression of inflammatory cytokines, including the pro-inflammatory factors IL-1β and TNF-α, and the anti-inflammatory factors IL-4 and TGF-β expressions, was evaluated using by ELISA. Furthermore, SIRT5 was evaluated by immunofluorescence and western blotting. The results showed that, compared with the CFA group, both thermal and mechanical pain thresholds increased with moxibustion and the SIRT5 inhibitor MC3482 intervention at ST36. Additionally, compared to the CFA-induced group, the inflammatory mediators, including IL-1β and TNF-α, decreased, while the anti-inflammatory cytokines IL-4 and TGF-β increased with moxibustion and MC3482 ST36 acupoint injection. Western blot results showed a decreased expression of SIRT5 at the ST36 site with moxibustion and MC3482 injection, compared to the CFA-induced group. SIRT5 expression in the right paw of mice injected with moxibustion and MC3482 was higher than that in the CFA-induced group. This study revealed that SIRT5 expression is involved in moxibustion analgesia and may be a potential mediator in the regulation of analgesia.

Cutaneous immune-related adverse events to immune checkpoint inhibitors: from underlying immunological mechanisms to multi-omics prediction

The development of immune checkpoint inhibitors (ICIs) has dramatically altered the landscape of therapy for multiple malignancies, including urothelial carcinoma, non-small cell lung cancer, melanoma and gastric cancer. As part of their anti-tumor properties, ICIs can enhance susceptibility to inflammatory side effects known as immune-related adverse events (irAEs), in which the skin is one of the most commonly and rapidly affected organs. Although numerous questions still remain unanswered, multi-omics technologies have shed light into immunological mechanisms, as well as the correlation between ICI-induced activation of immune systems and the incidence of cirAE (cutaneous irAEs). Therefore, we reviewed integrated biological layers of omics studies combined with clinical data for the prediction biomarkers of cirAEs based on skin pathogenesis. Here, we provide an overview of a spectrum of dermatological irAEs, discuss the pathogenesis of this "off-tumor toxicity" during ICI treatment, and summarize recently investigated biomarkers that may have predictive value for cirAEs via multi-omics approach. Finally, we demonstrate the prognostic significance of cirAEs for immune checkpoint blockades.

Differential regulation of cutaneous immunity by sensory neuron subsets

The nervous and immune systems have classically been studied as separate entities, but there is now mounting evidence for bidirectional communication between them in various organs, including the skin. The skin is an epithelial tissue with important sensory and immune functions. The skin is highly innervated with specialized subclasses of primary sensory neurons (PSNs) that can be in contact with skin-resident innate and adaptive immune cells. Neuroimmune crosstalk in the skin, through interactions of PSNs with the immune system, has been shown to regulate host cutaneous defense, inflammation, and tissue repair. Here, we review current knowledge about the cellular and molecular mechanisms involved in this crosstalk, as depicted via mouse model studies. We highlight the ways in which different immune challenges engage specialized subsets of PSNs to produce mediators acting on immune cell subsets and modulating their function.

Pregabalin inhibits proinflammatory cytokine release in patients with fibromyalgia syndrome

Objectives

The main goal of the study was to investigate how pregabalin (PGB) affects proinflammatory cytokine release in patients with fibromyalgia syndrome (FMS).

Patients and methods

This experimental research study was conducted with 85 female participants (mean age: 49.6±10.1 years; range, 30 to 73 years) between April 2020 and November 2020. Of the participants, 30 were FMS patients using PGB 150 mg/day for at least three months, 30 were FMS patients not using PGB, and 25 were healthy individuals. The detection of FMS was carried out according to the 2010 American College of Rheumatology diagnostic criteria. Levels of proinflammatory cytokines (interleukin [IL]-2, IL-6, IL-12, IL-17, interferon-gamma, and tumor necrosis factor-alpha) were measured by enzyme-linked immunosorbent assay.

Results

Serum concentrations of proinflammatory cytokines were remarkably decreased in FMS patients using PGB (p<0.001) and were higher in patients with FMS not using PGB than in healthy subjects (p<0.001). The highest values of proinflammatory cytokines were found in the group of FMS patients not using PGB (p<0.001).

Conclusion

These results indicate that PGB inhibits the release of proinflammatory cytokines, suggesting that it can be used as an anti-inflammatory agent in inflammatory cases.

Celastrol-encapsulated microspheres prepared by microfluidic electrospray for alleviating inflammatory pain

Inflammatory pain is induced by trauma, infection, chemical stimulation, etc. It causes severe physical and psychological agony to patients. Celastrol has powerful anti-inflammatory property and has achieved good results in various inflammation-related diseases. However, the low water solubility and multi-system toxicity limit its clinical application. Herein, we proposed alginate microspheres with core-shell structure which encapsulated celastrol by microfluidic electrospray to effectively overcome the shortcomings and improve the therapeutic effect. The microspheres had uniform size and good biocompatibility, and could release the loaded drugs in the gut. The behavioral tests showed that the celastrol-loaded microspheres effectively alleviated inflammatory pain, and the hematoxylin and eosin staining (HE staining), immunofluorescence and detection of inflammatory cytokines showed the anti-inflammatory effect. These results indicated that the microspheres could reduce dose and toxicity without affecting efficacy, and facilitate the application of celastrol in different clinical situations.

Novel neuroendocrine role of γ-aminobutyric acid and gastrin-releasing peptide in the host response to influenza infection

Gastrin-releasing peptide (GRP), an evolutionarily conserved neuropeptide, significantly contributes to influenza-induced lethality and inflammation in rodent models. Because GRP is produced by pulmonary neuroendocrine cells (PNECs) in response to γ-aminobutyric acid (GABA), we hypothesized that influenza infection promotes GABA release from PNECs that activate GABAB receptors on PNECs to secrete GRP. Oxidative stress was increased in the lungs of influenza A/PR/8/34 (PR8)-infected mice, as well as serum glutamate decarboxylase 1, the enzyme that converts L-glutamic acid into GABA. The therapeutic administration of saclofen, a GABAB receptor antagonist, protected PR8-infected mice, reduced lung proinflammatory gene expression of C-C chemokine receptor type 2 (Ccr2), cluster of differentiation 68 (Cd68), and Toll like receptor 4 (Tlr4) and decreased the levels of GRP and high-mobility group box 1 (HMGB1) in sera. Conversely, baclofen, a GABAB receptor agonist, significantly increased the lethality and inflammatory responses. The GRP antagonist, NSC77427, as well as the GABAB antagonist, saclofen, blunted the PR8-induced monocyte infiltration into the lung. Together, these data provide the first report of neuroregulatory control of influenza-induced disease.

Genetic impact of blood C-reactive protein levels on chronic spinal & widespread pain

PURPOSE

Causal mechanisms underlying systemic inflammation in spinal & widespread pain remain an intractable experimental challenge. Here we examined whether: (i) associations between blood C-reactive protein (CRP) and chronic back, neck/shoulder & widespread pain can be explained by shared underlying genetic variants; and (ii) higher CRP levels causally contribute to these conditions.

METHODS

Using genome-wide association studies (GWAS) of chronic back, neck/shoulder & widespread pain (N = 6063-79,089 cases; N = 239,125 controls) and GWAS summary statistics for blood CRP (Pan-UK Biobank N = 400,094 & PAGE consortium N = 28,520), we employed cross-trait bivariate linkage disequilibrium score regression to determine genetic correlations (rG) between these chronic pain phenotypes and CRP levels (FDR < 5%). Latent causal variable (LCV) and generalised summary data-based Mendelian randomisation (GSMR) analyses examined putative causal associations between chronic pain & CRP (FDR < 5%).

RESULTS

Higher CRP levels were genetically correlated with chronic back, neck/shoulder & widespread pain (rG range 0.26-0.36; P ≤ 8.07E-9; 3/6 trait pairs). Although genetic causal proportions (GCP) did not explain this finding (GCP range - 0.32-0.08; P ≥ 0.02), GSMR demonstrated putative causal effects of higher CRP levels contributing to each pain type (beta range 0.027-0.166; P ≤ 9.82E-03; 3 trait pairs) as well as neck/shoulder pain effects on CRP levels (beta [S.E.] 0.030 [0.021]; P = 6.97E-04).

CONCLUSION

This genetic evidence for higher CRP levels in chronic spinal (back, neck/shoulder) & widespread pain warrants further large-scale multimodal & prospective longitudinal studies to accelerate the identification of novel translational targets and more effective therapeutic strategies.

TRPV1 activation in human Langerhans cells and T cells inhibits mucosal HIV-1 infection via CGRP-dependent and independent mechanisms

Upon its mucosal transmission, HIV type 1 (HIV-1) rapidly targets genital antigen-presenting Langerhans cells (LCs), which subsequently transfer infectious virus to CD4+ T cells. We previously described an inhibitory neuroimmune cross talk, whereby calcitonin gene-related peptide (CGRP), a neuropeptide secreted by peripheral pain-sensing nociceptor neurons innervating all mucosal epithelia and associating with LCs, strongly inhibits HIV-1 transfer. As nociceptors secret CGRP following the activation of their Ca2+ ion channel transient receptor potential vanilloid 1 (TRPV1), and as we reported that LCs secret low levels of CGRP, we investigated whether LCs express functional TRPV1. We found that human LCs expressed mRNA and protein of TRPV1, which was functional and induced Ca2+ influx following activation with TRPV1 agonists, including capsaicin (CP). The treatment of LCs with TRPV1 agonists also increased CGRP secretion, reaching its anti-HIV-1 inhibitory concentrations. Accordingly, CP pretreatment significantly inhibited LCs-mediated HIV-1 transfer to CD4+ T cells, which was abrogated by both TRPV1 and CGRP receptor antagonists. Like CGRP, CP-induced inhibition of HIV-1 transfer was mediated via increased CCL3 secretion and HIV-1 degradation. CP also inhibited direct CD4+ T cells HIV-1 infection, but in CGRP-independent manners. Finally, pretreatment of inner foreskin tissue explants with CP markedly increased CGRP and CCL3 secretion, and upon subsequent polarized exposure to HIV-1, inhibited an increase in LC-T cell conjugate formation and consequently T cell infection. Our results reveal that TRPV1 activation in human LCs and CD4+ T cells inhibits mucosal HIV-1 infection, via CGRP-dependent/independent mechanisms. Formulations containing TRPV1 agonists, already approved for pain relief, could hence be useful against HIV-1.

Cardiovascular Brain Circuits

The cardiovascular system is hardwired to the brain via multilayered afferent and efferent polysynaptic axonal connections. Two major anatomically and functionally distinct though closely interacting subcircuits within the cardiovascular system have recently been defined: The artery-brain circuit and the heart-brain circuit. However, how the nervous system impacts cardiovascular disease progression remains poorly understood. Here, we review recent findings on the anatomy, structures, and inner workings of the lesser-known artery-brain circuit and the better-established heart-brain circuit. We explore the evidence that signals from arteries or the heart form a systemic and finely tuned cardiovascular brain circuit: afferent inputs originating in the arterial tree or the heart are conveyed to distinct sensory neurons in the brain. There, primary integration centers act as hubs that receive and integrate artery-brain circuit-derived and heart-brain circuit-derived signals and process them together with axonal connections and humoral cues from distant brain regions. To conclude the cardiovascular brain circuit, integration centers transmit the constantly modified signals to efferent neurons which transfer them back to the cardiovascular system. Importantly, primary integration centers are wired to and receive information from secondary brain centers that control a wide variety of brain traits encoded in engrams including immune memory, stress-regulating hormone release, pain, reward, emotions, and even motivated types of behavior. Finally, we explore the important possibility that brain effector neurons in the cardiovascular brain circuit network connect efferent signals to other peripheral organs including the immune system, the gut, the liver, and adipose tissue. The enormous recent progress vis-à-vis the cardiovascular brain circuit allows us to propose a novel neurobiology-centered cardiovascular disease hypothesis that we term the neuroimmune cardiovascular circuit hypothesis.

Microglial diversity in neuropathic pain

Microglia play pivotal roles in controlling CNS functions in diverse physiological and pathological contexts, including neuropathic pain, a chronic pain condition caused by lesions or diseases of the somatosensory nervous system. In this review article, we summarize evidence primarily from basic research on the role of microglia in the development and remission of neuropathic pain. The identification of a subset of microglia that emerged after pain development and that was necessary for remission of neuropathic pain highlights the highly divergent and dynamic nature of microglia in the course of neuropathic pain. Understanding microglial diversity in terms of gene expression, physiological states, and functional roles could lead to new strategies that aid in the diagnosis and management of neuropathic pain, and that may not have been anticipated from the viewpoint of targeting all microglia uniformly.

The bidirectional roles of the cGAS-STING pathway in pain processing: Cellular and molecular mechanisms

Pain is a common clinical condition. However, the mechanisms underlying pain are not yet fully understood. It is known that the neuroimmune system plays a critical role in the pathogenesis of pain. Recent studies indicated that the cyclic-GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway can activate the innate immune system by sensing both extrinsic and intrinsic double-stranded DNA in the cytoplasm, which is involved in pain processing. In this review, we summarise (1) the roles of the cGAS-STING pathway in different pain models, (2) the effect of the cGAS-STING pathway in different cells during pain regulation, and (3) the downstream molecular mechanisms of the cGAS-STING pathway in pain regulation. This review provides evidence that the cGAS-STING pathway has pro- and anti-nociceptive effects in pain models. It has different functions in neuron, microglia, macrophage, and T cells. Its downstream molecules include IFN-I, NF-κB, NLRP3, and eIF2α. The bidirectional roles of the cGAS-STING pathway in pain processing are mediated by regulating nociceptive neuronal sensitivity and neuroinflammatory responses. However, their effects in special brain regions, activation of astrocytes, and the different phases of pain require further exploration.

Electroacupuncture Alleviates CFA-Induced Inflammatory Pain via PD-L1/PD-1-SHP-1 Pathway

Inflammatory pain is difficult to treat clinically, but electroacupuncture (EA) has been demonstrated to be effective in alleviating inflammatory pain. Programmed cell death ligand-1 (PD-L1) and its downstream signal, Src homology region two domain-containing phosphatase-1 (SHP-1) have a critical role in relieving inflammatory pain. However, whether the PD-L1/PD-1-SHP-1 pathway mediates the analgesic and anti-inflammatory effects of EA in inflammatory pain remains unclear. Here, we observed that EA reversed the complete Freund's adjuvant (CFA)-induced hyperalgesia. EA reduced the expression of IL-6, iNOS, and NF-κB pathway in dorsal root ganglia (DRG) on day 7 after CFA injection but had no effect on the expression of IL-6, iNOS, and NF-κB PP65 on day 21 after CFA injection. Moreover, EA upregulated the protein levels of the PD-L1/PD-1-SHP-1 pathway on day 7 and day 21 after CFA injection. Furthermore, EA upregulated PD-L1 expression in calcitonin gene-related peptide (CGRP)⁺ but not in isohaemagglutinin B4 (IB4)⁺ and NF200⁺ neurons on day 7 and day 21 after CFA injection. Intrathecal injection of the PD-L1/PD-1 inhibitor BMS-1 (50 or 100 µg) blocked the EA-induced analgesic effect, significantly increased IL-6 and iNOS levels, and reduced the levels of PD-L1/PD-1-SHP-1. BMS-1 (50 or 100 µg) significantly reduced the expression of PD-L1 in IB4⁺, CGRP⁺, and NF200⁺ neurons. Our results show that EA's anti-inflammatory and analgesic effects are associated with activating the PD-L1/PD-1-SHP-1 pathway and suppressing its regulated neuroinflammation. This study provides a new potential therapeutic target for treating inflammatory pain.

Differential Gene Expression Among Patients With Heart Failure Experiencing Pain

BACKGROUND

Chronic pain is frequently experienced by patients with heart failure (HF) and is associated with higher mortality, higher symptom burden, and worsened health-related quality of life. However, the genomic mechanisms underlying chronic pain in HF are understudied. Building an understanding of the mechanistic underpinnings of pain may inform novel interventions.

OBJECTIVE

The objective was to identify genes associated with pain from messenger RNA sequence data collected from patients with HF with and without pain.

METHODS

The current study analyzed data from 40 patients with HF previously enrolled in a clinical trial. Pain presence was measured using the Health Utilities Index Mark-3. Genes were tested for differential expression using DESeq2, and differentially expressed genes were analyzed for protein-protein interaction (PPI) and relevant ontological pathways using Metascape. Genes located within the core of the PPI network were considered key in disease-relevant biological pathways. Differentially expressed genes within this PPI network were reviewed in existing literature to narrow down candidate genes of interest. These target genes of interest were reanalyzed in a second sample of 24 patients with HF using validation quantitative polymerase chain reaction.

RESULTS

A total of 334 genes (279 upregulated, 55 downregulated) were differentially expressed between patients with and without pain in the primary sample of 40. These genes were largely aligned with neutrophil degranulation pathways. Seven genes of interest were identified from a core network of 15 co-expressed genes in the PPI network and existing literature. Three of these seven genes, matrix metallopeptidase 8 ( MMP8 ), proprotein convertase subtilisin/kexin type 9 ( PCSK9 ), and neutrophil defensin 3 ( DEFA3 ), were upregulated in patients with pain versus without pain in both the primary and validation samples. All seven genes of interest are involved in immune, inflammatory, and atherosclerotic processes.

DISCUSSION

These results identify potential genes that may play a mechanistic role in chronic pain in HF. Further research is needed to evaluate these potential genes among clearly delineated pain phenotypes.

Neuroimmune interactions in atopic and allergic contact dermatitis

The skin is a barrier organ populated by many types of skin-resident immune cells and sensory neurons. It has become increasingly appreciated that neuroimmune interactions are an important component of inflammatory diseases such as atopic dermatitis and allergic contact dermatitis. Neuropeptides secreted from nerve terminals play an important role in mediating cutaneous immune cell function, and soluble mediators derived from immune cells interact with neurons to induce itch. In this review article, we will explore emerging research describing neuronal effector functions on skin immune cells in mouse models of atopic and contact dermatitis. We will also discuss the contributions of both specific neuronal subsets and secreted immune factors to itch induction and the associated inflammatory processes. Finally, we will explore how treatment strategies have emerged around these findings and discuss the relationship between scratching and dermatitis.

The role of STING signaling in central nervous system infection and neuroinflammatory disease

The cyclic guanosine monophosphate-adenosine monophosphate (GMP-AMP) synthase-Stimulator of Interferon Genes (cGAS-STING) pathway is a critical innate immune mechanism for detecting the presence of double-stranded DNA (dsDNA) and prompting a robust immune response. Canonical cGAS-STING activation occurs when cGAS, a predominantly cytosolic pattern recognition receptor, binds microbial DNA to promote STING activation. Upon STING activation, transcription factors enter the nucleus to cause the production of Type I interferons, inflammatory cytokines whose primary function is to prime the host for viral infection by producing a number of antiviral interferon-stimulated genes. While the pathway was originally described in viral infection, more recent studies have implicated cGAS-STING signaling in a number of different contexts, including autoimmune disease, cancer, injury, and neuroinflammatory disease. This review focuses on how our understanding of the cGAS-STING pathway has evolved over time with an emphasis on the role of STING-mediated neuroinflammation and infection in the nervous system. We discuss recent findings on how STING signaling contributes to the pathology of pain, traumatic brain injury, and stroke, as well as how mitochondrial DNA may promote STING activation in common neurodegenerative diseases. We conclude by commenting on the current knowledge gaps that should be filled before STING can be an effective therapeutic target in neuroinflammatory disease. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology Infectious Diseases > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology.

Glabridin Therapy Reduces Chronic Allodynia, Spinal Microgliosis, and Dendritic Spine Generation by Inhibiting Fractalkine-CX3CR1 Signaling in a Mouse Model of Tibial Fractures

Patients undergoing bone fractures frequently suffer from irritating chronic pain after orthopedic repairs. Chemokine-mediated interactions between neurons and microglia are important steps for neuroinflammation and excitatory synaptic plasticity during the spinal transmission of pathological pain. Recently, glabridin, the main bioactive component of licorice, has been shown to exhibit anti-nociceptive and neuroprotective properties for inflammatory pain. This present study evaluated the therapeutic potential of glabridin and its analgesic mechanisms using a mouse model of tibial fracture-associated chronic pain. Repetitive injections of glabridin were delivered spinally daily for 4 continuous days from days 3 to 6 after the fractures. Herein, we discovered that repeated administrations of glabridin (10 and 50 μg, but not 1 μg) could prevent prolonged cold allodynia and mechanical allodynia following bone fractures. A single intrathecal intervention with glabridin (50 μg) relieved an existing chronic allodynia two weeks following the fracture surgeries. Systemic therapies with glabridin (intraperitoneal; 50 mg kg^-1) were protective against long-lasting allodynia caused by fractures. Furthermore, glabridin restricted the fracture-caused spinal overexpressions of the chemokine fractalkine and its receptor CX3CR1, as well as the elevated number of microglial cells and dendritic spines. Strikingly, glabridin induced the inhibition of pain behaviors, microgliosis, and spine generation, which were abolished with the co-administration of exogenous fractalkine. Meanwhile, the exogenous fractalkine-evoked acute pain was compensated after microglia inhibition. Additionally, spinal neutralization of fractalkine/CX3CR1 signaling alleviated the intensity of postoperative allodynia after tibial fractures. These key findings identify that glabridin therapies confer protection against inducing and sustaining fracture-elicited chronic allodynia by suppressing fractalkine/CX3CR1-dependent spinal microgliosis and spine morphogenesis, suggesting that glabridin is a promising candidate in the translational development of chronic fracture pain control.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

The adaptation model of immunity: A new insight into aetiology and treatment of multiple sclerosis

Current research and drug development for multiple sclerosis (MS) is fully influenced by the self-nonself (SNS) model of immunity, suggesting that breakage of immunological tolerance towards self-antigens expressed in the central nervous system (CNS) is responsible for pathogenesis of MS; thus, immune suppressive drugs are recommended for the management of the disease. However, this model provides incomplete understanding of the causes and pathways involved in the onset and progression of MS and limits our ability to effectively treat this neurological disease. Some pre-clinical and clinical reports have been misunderstood; some others have been underappreciated because of the lack of a theoretical model that can explain them. Also, current immunotherapies are guided according to the models that are not designed to explain the functional outcomes of an immune response. The adaptation model of immunity is proposed to offer a new understanding of the existing data and galvanize a new direction for the treatment of MS. According to this model, the immune system continuously communicates with the CNS through the adaptation receptors (AdRs) and adaptation ligands (AdLs) or co-receptors, signal IV, to support cell growth and neuroplasticity. Alterations in the expression of the neuronal AdRs results in MS by shifting the cerebral inflammatory immune responses from remyelination to demyelination. Therefore, novel therapeutics for MS should be focused on the discovery and targeting of the AdR/AdL axis in the CNS rather than carrying on with immune suppressive interventions.

Multimodal control of dendritic cell functions by nociceptors

It is known that interactions between nociceptors and dendritic cells (DCs) can modulate immune responses in barrier tissues. However, our understanding of the underlying communication frameworks remains rudimentary. Here, we show that nociceptors control DCs in three molecularly distinct ways. First, nociceptors release the calcitonin gene-related peptide that imparts a distinct transcriptional profile on steady-state DCs characterized by expression of pro-interleukin-1β and other genes implicated in DC sentinel functions. Second, nociceptor activation induces contact-dependent calcium fluxes and membrane depolarization in DCs and enhances their production of proinflammatory cytokines when stimulated. Finally, nociceptor-derived chemokine CCL2 contributes to the orchestration of DC-dependent local inflammation and the induction of adaptive responses against skin-acquired antigens. Thus, the combined actions of nociceptor-derived chemokines, neuropeptides, and electrical activity fine-tune DC responses in barrier tissues.

Control of myeloid cell functions by nociceptors

The immune system has evolved to protect the host from infectious agents, parasites, and tumor growth, and to ensure the maintenance of homeostasis. Similarly, the primary function of the somatosensory branch of the peripheral nervous system is to collect and interpret sensory information about the environment, allowing the organism to react to or avoid situations that could otherwise have deleterious effects. Consequently, a teleological argument can be made that it is of advantage for the two systems to cooperate and form an “integrated defense system” that benefits from the unique strengths of both subsystems. Indeed, nociceptors, sensory neurons that detect noxious stimuli and elicit the sensation of pain or itch, exhibit potent immunomodulatory capabilities. Depending on the context and the cellular identity of their communication partners, nociceptors can play both pro- or anti-inflammatory roles, promote tissue repair or aggravate inflammatory damage, improve resistance to pathogens or impair their clearance. In light of such variability, it is not surprising that the full extent of interactions between nociceptors and the immune system remains to be established. Nonetheless, the field of peripheral neuroimmunology is advancing at a rapid pace, and general rules that appear to govern the outcomes of such neuroimmune interactions are beginning to emerge. Thus, in this review, we summarize our current understanding of the interaction between nociceptors and, specifically, the myeloid cells of the innate immune system, while pointing out some of the outstanding questions and unresolved controversies in the field. We focus on such interactions within the densely innervated barrier tissues, which can serve as points of entry for infectious agents and, where known, highlight the molecular mechanisms underlying these interactions.