STINGing away the pain: the role of interferon-stimulated genes.

Pain and inflammation are biologically intertwined responses that warn the body of potential danger. In this issue of the JCI, Defaye, Bradaia, and colleagues identified a functional link between inflammation and pain, demonstrating that inflammation-induced activation of stimulator of IFN genes (STING) in dorsal root ganglia nociceptors reduced pain-like behaviors in a rodent model of inflammatory pain. Utilizing mice with a gain-of-function STING mutation, Defaye, Bradaia, and colleagues identified type I IFN regulation of voltage-gated potassium channels as the mechanism of this pain relief. Further investigation into mechanisms by which proinflammatory pathways can reduce pain may reveal druggable targets and insights into new approaches for treating persistent pain.

Different pain phenotypes are associated with anti-Caspr2 autoantibodies.

Autoantibodies against contactin-associated protein 2 (Caspr2) not only induce limbic autoimmune encephalitis but are also associated with pain conditions. Here, we analyzed clinical data on pain in a large cohort of patients included into the German Network for Research in Autoimmune Encephalitis. Out of 102 patients in our cohort, pain was a frequent symptom (36% of all patients), often severe (63.6% of the patients with pain) and/or even the major symptom (55.6% of the patients with pain). Pain phenotypes differed between patients. Cluster analysis revealed two major phenotypes including mostly distal-symmetric burning pain and widespread pain with myalgia and cramps. Almost all patients had IgG4 autoantibodies and some additional IgG1, 2, and/or 3 autoantibodies, but IgG subclasses, titers, and presence or absence of intrathecal synthesis were not associated with the occurrence of pain. However, certain pre-existing risk factors for chronic pain like diabetes mellitus, peripheral neuropathy, or preexisting chronic back pain tended to occur more frequently in patients with anti-Caspr2 autoantibodies and pain. Our data show that pain is a relevant symptom in patients with anti-Caspr2 autoantibodies and support the idea of decreased algesic thresholds leading to pain. Testing for anti-Caspr2 autoantibodies needs to be considered in patients with various pain phenotypes.

Immersion in Water Between 20-30oC Mediated Inflammations Marker to Reduced Pain Among Indonesian With Gout Arthritis: A Community-Based Randomized Controlled Trial.

BACKGROUND: Gout is triggered by high urate levels and causes inflammation, pain, and an impaired quality of life. Immersion in water at 20-30°C reduces inflammation and pain in arthritis. Yet, relationships of immersion in water at 20-30°C with urate levels and the nucleotide-binding domain (NOD)-like receptor protein 1 (NLRP1) inflammasome have never been clarified. OBJECTIVES: We aimed to investigate the effects of immersion in water at 20-30°C on urate levels, the NLRP1 inflammasome, pain, and quality of life among acute gout patients. METHODS: A community-based randomized control trial design was used with 2 parallel-intervention groups: immersion in water at 20-30°C (20 min/day for 4 weeks) group and a control group. In total, 76 eligible participants in Tomohon City, Indonesia, were assigned using block randomization. We analyze the results (coef. ß) and 95% confidence intervals (CIs) using a generalized estimating equation model. We analyzed mediating effects using a path analysis. RESULTS: Significant pain alleviation (ß = -2.06 [95% CI = -2.67∼-1.45]; ß = -2.42 [95% CI = -2.97∼-1.87]) and improved quality of life (ß = 5.34 [95% CI = 3.12-7.57]; ß = 9.93 [95% CI = 7.02-12.83]) were detected at 2 and 4 weeks of follow-up compared to the pre-test and control group. Urate levels (ß = -0.34 [95% CI = -0.52∼-0.16]) were reduced at the 2-week follow-up, but there was no significant change in the NLRP1 inflammasome compared to the pre-test and control group after immersion in water at 20-30°C. Both the NLRP1 inflammasome (ß = -0.48 [95% CI = -0.63∼-0.34]); water 0.01) and urate levels (ß = -0.11 [95% CI = -0.24∼-0.03]; p < 0.01) had partial indirect (mediating) effects on the link between immersion in water at 20-30°C and pain at the 4-week follow-up. CONCLUSIONS: Immersion in water at 20-30°C significantly decreased pain and increased the quality of life. Immersion in water at 20-30°C mediated NLRP1 and urate levels to decrease pain, although it had no significant effect on the NLRP1 inflammasome concentration after 4 weeks of follow-up and reduced urate levels only at 2 weeks after immersion in water at 20-30°C.

Preparation and characterization of a high-affinity monoclonal antibody against nerve growth factor.

Nerve growth factor (NGF) is produced and released in injured tissues or chronic pain tissues caused by other diseases. Studies have shown that monoclonal antibodies targeting NGF have a good efficacy in the treatment of osteoarthritis (OA), low back pain and chronic pain, which may be a promising therapy. In this study, DNA sequences of NGF-his and NGF-hFc were synthesized using eukaryotic expression system and subcloned into pTT5 expression vector. After that, NGF proteins were expressed by transient expression in HEK293E cells. We immunized mice with NGF-hFc protein and fused mouse spleen cells to prepare hybridomas. NGF-His protein was used to screen out the hybridoma supernatant that could directly bind to NGF. Antibodies were purified from hybridioma supernatant. Futhermore, via surface plasmon resonance (SPR) screening, six anti-NGF mAbs were screened to block the binding of NGF and TrkA receptor in the treatment of chronic pain. Among them, 58F10G10H showed high affinity (KD = 1.03 × 10-9 M) and even better than that of positive control antibody Tanezumab (KD = 1.53 × 10-9 M). Moreover, the specific reactivity of 58F10G10H was demonstrated by TF-1 cell proliferation activity experiments, competitive binding Enzyme-linked immunosorbent assay (ELISA) and the arthritis animal models in mice, respectively. In conclusion, in this study, a method for the preparation of high-yield NGF-HFC and NGF-His proteins was designed, and a high-affinity monoclonal antibody against NGF with potential for basic research and clinical application was prepared.

Neuro-Immunity and Gut Dysbiosis Drive Parkinson's Disease-Induced Pain.

Parkinson's disease (PD) is the second most common neurodegenerative disorder, affecting 1-2% of the population aged 65 and over. Additionally, non-motor symptoms such as pain and gastrointestinal dysregulation are also common in PD. These impairments might stem from a dysregulation within the gut-brain axis that alters immunity and the inflammatory state and subsequently drives neurodegeneration. There is increasing evidence linking gut dysbiosis to the severity of PD's motor symptoms as well as to somatosensory hypersensitivities. Altogether, these interdependent features highlight the urgency of reviewing the links between the onset of PD's non-motor symptoms and gut immunity and whether such interplays drive the progression of PD. This review will shed light on maladaptive neuro-immune crosstalk in the context of gut dysbiosis and will posit that such deleterious interplays lead to PD-induced pain hypersensitivity.

Neuroimmune and Mu-Opioid Receptor Alterations in the Mesocorticolimbic System in a Sex-Dependent Inflammatory Pain-Induced Alcohol Relapse-Like Rat Model.

Evidence concerning the role of alcohol-induced neuroinflammation in alcohol intake and relapse has increased in the last few years. It is also proven that mu-opioid receptors (MORs) mediate the reinforcing properties of alcohol and, interestingly, previous research suggests that neuroinflammation and MORs could be related. Our objective is to study neuroinflammatory states and microglial activation, together with adaptations on MOR expression in the mesocorticolimbic system (MCLS) during the abstinence and relapse phases. To do so, we have used a sex-dependent rat model of complete Freund's adjuvant (CFA)-induced alcohol deprivation effect (ADE). Firstly, our results confirm that only CFA-treated female rats, the only experimental group that showed relapse-like behavior, exhibited specific alterations in the expression of phosphorylated NFκB, iNOS, and COX2 in the PFC and VTA. More interestingly, the analysis of the IBA1 expression revealed a decrease of the microglial activation in PFC during abstinence and an increase of its expression in the relapse phase, together with an augmentation of this activation in the NAc in both phases that only occur in female CFA-treated rats. Additionally, the expression of IL1ß also evidenced these dynamic changes through these two phases following similar expression patterns in both areas. Furthermore, the expression of the cytokine IL10 showed a different profile than that of IL1ß, indicating anti-inflammatory processes occurring only during abstinence in the PFC of CFA-female rats but neither during the reintroduction phase in PFC nor in the NAc. These data indicate a downregulation of microglial activation and pro-inflammatory processes during abstinence in the PFC, whereas an upregulation can be observed in the NAc during abstinence that is maintained during the reintroduction phase only in CFA-female rats. Secondly, our data reveal a correlation between the alterations observed in IL1ß, IBA1 levels, and MOR levels in the PFC and NAc of CFA-treated female rats. Although premature, our data suggest that neuroinflammatory processes, together with neural adaptations involving MOR, might play an important role in alcohol relapse in female rats, so further investigations are warranted.

Macrophage as a Peripheral Pain Regulator.

A neuroimmune crosstalk is involved in somatic and visceral pathological pain including inflammatory and neuropathic components. Apart from microglia essential for spinal and supraspinal pain processing, the interaction of bone marrow-derived infiltrating macrophages and/or tissue-resident macrophages with the primary afferent neurons regulates pain signals in the peripheral tissue. Recent studies have uncovered previously unknown characteristics of tissue-resident macrophages, such as their origins and association with regulation of pain signals. Peripheral nerve macrophages and intestinal resident macrophages, in addition to adult monocyte-derived infiltrating macrophages, secrete a variety of mediators, such as tumor necrosis factor-α, interleukin (IL)-1ß, IL-6, high mobility group box 1 and bone morphogenic protein 2 (BMP2), that regulate the excitability of the primary afferents. Neuron-derived mediators including neuropeptides, ATP and macrophage-colony stimulating factor regulate the activity or polarization of diverse macrophages. Thus, macrophages have multitasks in homeostatic conditions and participate in somatic and visceral pathological pain by interacting with neurons.

The complement cascade in the regulation of neuroinflammation, nociceptive sensitization, and pain.

The complement cascade is a key component of the innate immune system that is rapidly recruited through a cascade of enzymatic reactions to enable the recognition and clearance of pathogens and promote tissue repair. Despite its well-understood role in immunology, recent studies have highlighted new and unexpected roles of the complement cascade in neuroimmune interaction and in the regulation of neuronal processes during development, aging, and in disease states. Complement signaling is particularly important in directing neuronal responses to tissue injury, neurotrauma, and nerve lesions. Under physiological conditions, complement-dependent changes in neuronal excitability, synaptic strength, and neurite remodeling promote nerve regeneration, tissue repair, and healing. However, in a variety of pathologies, dysregulation of the complement cascade leads to chronic inflammation, persistent pain, and neural dysfunction. This review describes recent advances in our understanding of the multifaceted cross-communication that takes place between the complement system and neurons. In particular, we focus on the molecular and cellular mechanisms through which complement signaling regulates neuronal excitability and synaptic plasticity in the nociceptive pathways involved in pain processing in both health and disease. Finally, we discuss the future of this rapidly growing field and what we believe to be the significant knowledge gaps that need to be addressed.

Neuron-Glia-Immune Triad and Cortico-Limbic System in Pathology of Pain.

Pain is an unpleasant sensation that alerts one to the presence of obnoxious stimuli or sensations. These stimuli are transferred by sensory neurons to the dorsal root ganglia-spinal cord and finally to the brain. Glial cells in the peripheral nervous system, astrocytes in the brain, dorsal root ganglia, and immune cells all contribute to the development, maintenance, and resolution of pain. Both innate and adaptive immune responses modulate pain perception and behavior. Neutrophils, microglial, and T cell activation, essential components of the innate and adaptive immune responses, can play both excitatory and inhibitory roles and are involved in the transition from acute to chronic pain. Immune responses may also exacerbate pain perception by modulating the function of the cortical-limbic brain regions involved in behavioral and emotional responses. The link between an emotional state and pain perception is larger than what is widely acknowledged. In positive psychological states, perception of pain along with other somatic symptoms decreases, whereas in negative psychological states, these symptoms may worsen. Sex differences in mechanisms of pain perception are not well studied. In this review, we highlight what is known, controversies, and the gaps in this field.

Neuroimmune interactions in painful TMD: Mechanisms and treatment implications.

The underlying mechanisms and treatment of painful temporomandibular disorders (TMDs) are important but understudied topics in craniofacial research. As a group of musculoskeletal diseases, the onset of painful TMD is proved to be a result of disturbance of multiple systems. Recently, emerging evidence has revealed the involvement of neuroimmune interactions in painful TMD. Inflammatory factors play an important role in peripheral sensitization of temporomandibular joint (TMJ), and neurogenic inflammation in turn enhances TMJs dysfunction in TMD. Furthermore, centralized neuroimmune communications contribute to neuron excitability amplification, leading to pain sensitization, and is also responsible for chronic TMD pain and other CNS symptoms. Therapeutics targeting neuroimmune interactions may shed light on new approaches for treating TMD. In this review, we will discuss the role of neuroimmune interactions in the onset of painful TMD from the peripheral and centralized perspectives, and how understanding this mechanism could provide new treatment options. Insights into the neuroimmune interactions within TMJs and painful TMD would broaden the knowledge of mechanisms and treatments of this multifactorial disease.

Daphnetin inhibits spinal glial activation via Nrf2/HO-1/NF-κB signaling pathway and attenuates CFA-induced inflammatory pain.

Daphnetin (7, 8-dihydroxycoumarin, DAPH), a coumarin derivative isolated from Daphne odora var., recently draws much more attention as a promising drug candidate to treat neuroinflammatory diseases due to its protective effects against neuroinflammation. However, itscontribution to chronic inflammatory pain is largely unknown. In the current work, we investigated the effects of DAPH in a murine model of inflammatory pain induced by complete Freund's adjuvant (CFA) and its possible underlying mechanisms. Our results showed that DAPH treatment significantly attenuated mechanical allodynia provoked by CFA. A profound inhibition of spinal glial activation, followed by attenuated expression levels of spinal pro-inflammatory cytokines, was observed in DAPH-treated inflammatory pain mice. Further study demonstrated that DAPH mediated negative regulation of spinal NF-κB pathway, as well as its preferential activation of Nrf2/HO-1 signaling pathway in inflammatory pain mice. This study, for the first time, indicated that DAPH might preventthe development of mechanical allodynia in mice with inflammatory pain. And more importantly, these data provide evidence for the potential application of DAPH in the treatment of chronic inflammatory pain.

Sensory Neurons, Neuroimmunity, and Pain Modulation by Sex Hormones.

The inclusion of women in preclinical pain studies has become more commonplace in the last decade as the National Institutes of Health (NIH) released its "Sex as a Biological Variable" mandate. Presumably, basic researchers have not had a comprehensive understanding about neuroimmune interactions in half of the population and how hormones play a role in this. To date, we have learned that sex hormones contribute to sexual differentiation of the nervous system and sex differences in behavior throughout the lifespan; however, the cycling of sex hormones does not always explain these differences. Here, we highlight recent advances in our understanding of sex differences and how hormones and immune interactions influence sensory neuron activity to contribute to physiology and pain. Neuroimmune mechanisms may be mediated by different cell types in each sex, as the actions of immune cells are sexually dimorphic. Unfortunately, the majority of studies assessing neuronal contributions to immune function have been limited to males, so it is unclear if the mechanisms are similar in females. Finally, pathways that control cellular metabolism, like nuclear receptors, have been shown to play a regulatory role both in pain and inflammation. Overall, communication between the neuroimmune and endocrine systems modulate pain signaling in a sex-dependent manner, but more research is needed to reveal nuances of these mechanisms.

Effects of Salvia miltiorrhiza Bunge extract and its single components on monosodium urate-induced pain in vivo and lipopolysaccharide-induced inflammation in vitro.

OBJECTIVE: To investigate the possible antinociceptive effects of Salvia (S.) miltiorrhiza Bunge and its single components in monosodium urate (MSU)-induced pain model in mice and lipopolysaccharide (LPS)-induced inflammation model in RAW264.7 cells. METHODS: Pretreatment of S. miltiorrhiza Bunge extract (from 1 to 50 µg/mL) concentration-dependently attenuated LPS-induced nitric oxide (NO) release. The extract of S. miltiorrhiza Bunge (50 or 100 mg/kg) also caused reversals of decreased threshold for pain in the MSU-treated group as measured by Von-Frey test. Furthermore, we assessed the antinociceptive and anti-inflammatory properties of the active single components from S. miltiorrhiza Bunge such as 15, 16-dihydrotanshinone Ⅰ tanshinone Ⅱ cryptotanshinone, miltirone, tanshinone ⅡA, and salvianolic acid B. Some of them showed an anti-inflammatory effect in LPS-induced NO release model and an antinociceptive effect in MSU-treated pain model. RESULTS: Our results suggest that S. miltiorrhiza Bunge extract may exert anti-inflammatory effect by reducing LPS-induced NO release and an antinociceptive property in MSU-treated pain model. Especially, tanshinoneⅡA, miltirone, cryptotanshinone, and 15,16-dihydrotanshinone Ⅰ not only appear to be responsible for LPS-induced NO release induced by S. miltiorrhiza Bunge, but also in the production of S. miltiorrhiza Bunge extract-induced antinociception in MSU-treated pain model. CONCLUSION: Therefore, the analgesic and anti-inflammatory property of S. miltiorrhiza Bunge indicate it as a therapeutic potential candidate for the treatment of pain and inflammation.

Involvement of Macrophages and Spinal Microglia in Osteoarthritis Pain.

PURPOSE OF REVIEW: Chronic pain in osteoarthritis (OA) is characterized by pain sensitization, which involves both peripheral and central mechanisms. Studies suggest synovial macrophage and spinal microglia are implicated in pain sensitization in OA. We, therefore, reviewed the evidence of whether synovial macrophage and spinal microglia facilitated pain sensitization at diverse levels and how this event occurred in OA. RECENT FINDINGS: Peripherally, joint inflammation is now believed to be a source of OA-related pain. Synovial macrophages accumulate in OA inflamed synovium and display a pro-inflammatory phenotype. Abundant macrophage-derived pro-inflammatory cytokines and other pain-causing substance facilitate hyperexcitation of primary sensory neuron in OA-related pain. Thus, activated synovial macrophage was considered a predictor for phenotyping of OA pain clinically. In response to affected joint-derived strong nociception, aberrant neuronal excitability is often associated with the hyperactivity of microglia in the spinal dorsal horn, thereby leading to central sensitization. Hyperactivity of synovial macrophage and spinal microglia underlies the mechanisms of pain sensitization at the peripheral and central level in OA. This concept provides not only a clinically relevant strategy for identifying the phenotype of OA-related pain but also has the potential to develop individualized interventions for OA, particularly in those patients with hyperactivity of macrophage and microglia.

Activation of GPR37 in macrophages confers protection against infection-induced sepsis and pain-like behaviour in mice.

GPR37 was discovered more than two decades ago, but its biological functions remain poorly understood. Here we report a protective role of GPR37 in multiple models of infection and sepsis. Mice lacking Gpr37 exhibited increased death and/or hypothermia following challenge by lipopolysaccharide (LPS), Listeria bacteria, and the mouse malaria parasite Plasmodium berghei. Sepsis induced by LPS and Listeria in wild-type mice is protected by artesunate (ARU) and neuroprotectin D1 (NPD1), but the protective actions of these agents are lost in Gpr37-/- mice. Notably, we found that ARU binds to GPR37 in macrophages and promotes phagocytosis and clearance of pathogens. Moreover, ablation of macrophages potentiated infection, sepsis, and their sequelae, whereas adoptive transfer of NPD1- or ARU-primed macrophages reduced infection, sepsis, and pain-like behaviors. Our findings reveal physiological actions of ARU in host cells by activating macrophages and suggest that GPR37 agonists may help to treat sepsis, bacterial infections, and malaria.

Anti-inflammatory dopamine- and serotonin-based endocannabinoid epoxides reciprocally regulate cannabinoid receptors and the TRPV1 channel.

The endocannabinoid system is a promising target to mitigate pain as the endocannabinoids are endogenous ligands of the pain-mediating receptors-cannabinoid receptors 1 and 2 (CB1 and CB2) and TRPV1. Herein, we report on a class of lipids formed by the epoxidation of N-arachidonoyl-dopamine (NADA) and N-arachidonoyl-serotonin (NA5HT) by epoxygenases. EpoNADA and epoNA5HT are dual-functional rheostat modulators of the endocannabinoid-TRPV1 axis. EpoNADA and epoNA5HT are stronger modulators of TRPV1 than either NADA or NA5HT, and epoNA5HT displays a significantly stronger inhibition on TRPV1-mediated responses in primary afferent neurons. Moreover, epoNA5HT is a full CB1 agonist. These epoxides reduce the pro-inflammatory biomarkers IL-6, IL-1ß, TNF-α and nitrous oxide and raise anti-inflammatory IL-10 cytokine in activated microglial cells. The epoxides are spontaneously generated by activated microglia cells and their formation is potentiated in the presence of anandamide. Detailed kinetics and molecular dynamics simulation studies provide evidence for this potentiation using the epoxygenase human CYP2J2. Taken together, inflammation leads to an increase in the metabolism of NADA, NA5HT and other eCBs by epoxygenases to form the corresponding epoxides. The epoxide metabolites are bioactive lipids that are potent, multi-faceted molecules, capable of influencing the activity of CB1, CB2 and TRPV1 receptors.

The Role of Neuroglial Crosstalk and Synaptic Plasticity-Mediated Central Sensitization in Acupuncture Analgesia.

Although pain is regarded as a global public health priority, analgesic therapy remains a significant challenge. Pain is a hypersensitivity state caused by peripheral and central sensitization, with the latter considered the culprit for chronic pain. This study summarizes the pathogenesis of central sensitization from the perspective of neuroglial crosstalk and synaptic plasticity and underlines the related analgesic mechanisms of acupuncture. Central sensitization is modulated by the neurotransmitters and neuropeptides involved in the ascending excitatory pathway and the descending pain modulatory system. Acupuncture analgesia is associated with downregulating glutamate in the ascending excitatory pathway and upregulating opioids, 𝛾-aminobutyric acid, norepinephrine, and 5-hydroxytryptamine in the descending pain modulatory system. Furthermore, it is increasingly appreciated that neurotransmitters, cytokines, and chemokines are implicated in neuroglial crosstalk and associated plasticity, thus contributing to central sensitization. Acupuncture produces its analgesic action by inhibiting cytokines, such as interleukin-1ß, interleukin-6, and tumor necrosis factor-α, and upregulating interleukin-10, as well as modulating chemokines and their receptors such as CX3CL1/CX3CR1, CXCL12/CXCR4, CCL2/CCR2, and CXCL1/CXCR2. These factors are regulated by acupuncture through the activation of multiple signaling pathways, including mitogen-activated protein kinase signaling (e.g., the p38, extracellular signal-regulated kinases, and c-Jun-N-terminal kinase pathways), which contribute to the activation of nociceptive neurons. However, the responses of chemokines to acupuncture vary among the types of pain models, acupuncture methods, and stimulation parameters. Thus, the exact mechanisms require future clarification. Taken together, inhibition of central sensitization modulated by neuroglial plasticity is central in acupuncture analgesia, providing a novel insight for the clinical application of acupuncture analgesia.