Mast cells: versatile gatekeepers of pain

Exploring Localized Provoked Vulvodynia: Insights from Animal Model Research

Provoked vulvodynia represents a challenging chronic pain condition, characterized by its multifactorial origins. The inherent complexities of human-based studies have necessitated the use of animal models to enrich our understanding of vulvodynia's pathophysiology. This review aims to provide an exhaustive examination of the various animal models employed in this research domain. A comprehensive search was conducted on PubMed, utilizing keywords such as "vulvodynia", "chronic vulvar pain", "vulvodynia induction", and "animal models of vulvodynia" to identify pertinent studies. The search yielded three primary animal models for vulvodynia: inflammation-induced, allergy-induced, and hormone-induced. Additionally, six agents capable of triggering the condition through diverse pathways were identified, including factors contributing to hyperinnervation, mast cell proliferation, involvement of other immune cells, inflammatory cytokines, and neurotransmitters. This review systematically outlines the various animal models developed to study the pathogenesis of provoked vulvodynia. Understanding these models is crucial for the exploration of preventative measures, the development of novel treatments, and the overall advancement of research within the field.

Involvement of peripheral mast cells in a fibromyalgia model in mice

Fibromyalgia is a painful disorder of unknown aetiology that presents activation and recruitment of innate immune cells, including mast cells. Efforts have been made to understand its pathogenesis to manage it better. Thus, we explored the involvement of peripheral mast cells in an experimental model of fibromyalgia induced by reserpine. Reserpine (1 mg/kg) was subcutaneously (s.c.) injected once daily in the back of male Swiss mice for three consecutive days. We analysed mechanical and cold allodynia, muscle fatigue and number of mast cell in plantar tissue. The fibromyalgia induction produced mast cell infiltration (i.e., mastocytosis) in the mice's plantar tissue. The depletion of mast cell mediators with the compound 48/80 (0.5-4 mg/kg, intraperitoneal (i.p.)) or the mast cell membrane stabilizer ketotifen fumarate (10 mg/kg, oral route (p.o.) widely (80-90 %) and extensively (from 1 up to 10 days) prevented reserpine-induced mechanical and cold allodynia and muscle fatigue. Compound 48/80 also prevented the reserpine-induced mastocytosis. Finally, we demonstrated that PAR-2, 5-HT2A, 5-HT3, H1, NK1 and MrgprB2 receptors, expressed in neuronal or mast cells, seem crucial to mediate fibromyalgia-related cardinal symptoms since antagonists or inhibitors of these receptors (gabexate (10 mg/kg, s.c.), ENMD-1068 (10 mg/kg, i.p.), ketanserin (1 mg/kg, i.p.), ondansetron (1 mg/kg, p.o.), promethazine (1 mg/kg, i.p.), and L733,060 (5 mg/kg, s.c.), respectively) transiently reversed the reserpine-induced allodynia and fatigue. The results indicate that mast cells mediate painful and fatigue behaviours in this fibromyalgia model, representing potential therapy targets to treat fibromyalgia syndrome.

Mast cell silencing: A novel therapeutic approach for urticaria and other mast cell-mediated diseases

Chronic urticaria (CU) is a mast cell (MC)-dependent disease with limited therapeutic options. Current management strategies are directed at inhibiting IgE-mediated activation of MCs and antagonizing effects of released mediators. Due to the complexity and heterogeneity of CU and other MC diseases and mechanisms of MC activation-including multiple activating receptors and ligands, diverse signaling pathways, and a menagerie of mediators-strategies of MC depletion or MC silencing (i.e., inhibition of MC activation via binding of inhibitory receptors) have been developed to overcome limitations of singularly targeted agents. MC silencers, such as agonist monoclonal antibodies that engage inhibitory receptors (e.g., sialic acid-binding immunoglobulin-like lectin8 -[Siglec-8] [lirentelimab/AK002], Siglec-6 [AK006], and CD200R [LY3454738]), have reached preclinical and clinical stages of development. In this review, we (1) describe the role of MCs in the pathogenesis of CU, highlighting similarities with other MC diseases in disease mechanisms and response to treatment; (2) explore current therapeutic strategies, categorized by nonspecific immunosuppression, targeted inhibition of MC activation or mediators, and targeted modulation of MC activity; and (3) introduce the concept of MC silencing as an emerging strategy that could selectively block activation of MCs without eliciting or exacerbating on- or off-target, immunosuppressive adverse effects.

The role of neuroinflammation in the transition of acute to chronic pain and the opioid-induced hyperalgesia and tolerance

Current evidence suggests that activation of glial and immune cells leads to increased production of proinflammatory mediators, creating a neuroinflammatory state. Neuroinflammation has been proven to be a fundamental mechanism in the genesis of acute pain and its transition to neuropathic and chronic pain. A noxious event that stimulates peripheral afferent nerve fibers may also activate pronociceptive receptors situated at the dorsal root ganglion and dorsal horn of the spinal cord, as well as peripheral glial cells, setting off the so-called peripheral sensitization and spreading neuroinflammation to the brain. Once activated, microglia produce cytokines, chemokines, and neuropeptides that can increase the sensitivity and firing properties of second-order neurons, upregulating the signaling of nociceptive information to the cerebral cortex. This process, known as central sensitization, is crucial for chronification of acute pain. Immune-neuronal interactions are also implicated in the lesser-known complex regulatory relationship between pain and opioids. Current evidence suggests that activated immune and glial cells can alter neuronal function, induce, and maintain pathological pain, and disrupt the analgesic effects of opioid drugs by contributing to the development of tolerance and dependence, even causing paradoxical hyperalgesia. Such alterations may occur when the neuronal environment is impacted by trauma, inflammation, and immune-derived molecules, or when opioids induce proinflammatory glial activation. Hence, understanding these intricate interactions may help in managing pain signaling and opioid efficacy beyond the classical pharmacological approach.

The Association Between Immune-Related Conditions Across the Life-Course and Provoked Vulvodynia

Vulvodynia, impacts up to 8% of women by age 40, and is hypothesized to manifest through an altered immune-inflammatory response. To test this hypothesis, we identified all women born in Sweden between 1973 and 1996 diagnosed with localized provoked vulvodynia (N76.3) and/or vaginismus (N94.2 or F52.5) between 2001 and 2018. We matched each case to two women from the same birth year with no vulvar pain ICD codes. As a proxy for immune dysfunction, we used Swedish Registry data to capture 1) immunodeficiencies, 2) single organ and multiorgan autoimmune conditions, 3) allergy and atopies, and 4) malignancies involving immune cells across the life course. Women with vulvodynia, vaginismus or both were more likely to experience immune deficiencies (OR 1.8, 95% CI, 1.2-2.8), single organ (OR 1.4, 95% CI, 1.2-1.6) and/or multi-organ (OR 1.6, 95% CI, 1.3-1.9) immune disorders, and allergy/atopy conditions (OR 1.7, 95% CI, 1.6-1.8) compared to controls. We observed greater risk with increasing numbers of unique immune related conditions (1 code: OR = 1.6, 95% CI, 1.5-1.7; 2 codes: OR = 2.4, 95% CI, 2.1-2.9; 3 or more codes: OR = 2.9, 1.6-5.4). These findings suggest that women with vulvodynia may have a more compromised immune system either at birth or at points across the life course than women with no vulvar pain history. PERSPECTIVE: Women with vulvodynia are substantially more likely to experience a spectrum of immune related conditions across the life course. These findings lend support to the hypothesis that chronic inflammation initiates the hyperinnervation that causes the debilitating pain in women with vulvodynia.

A model for irritable bowel syndrome and anxiety comorbidities in relation to alcohol use disorders

About 95% of human body serotonin synthesis occurs in the gastrointestinal tract (GI). Lack of sufficient serotonin levels is thought to play a key role in mood disorders, including anxiety disorders. In this study, we looked at a disorder affecting the GI tract, irritable bowel syndrome (IBS), and aimed to determine whether IBS is differentially associated with anxiety disorders in 252 chronic pain patients in the presence of a history of alcohol use disorders (AUD) given that alcohol is an extremely aggressive substance for the GI mucosa. We found that while the prevalence of IBS was not affected by the presence of AUD in chronic pain patients, IBS had significantly higher comorbidity with anxiety disorders in chronic pain patients with comorbid alcohol use disorders. We argue that these findings highlight mechanistic differences in the comorbidity of anxiety disorders with chronic pain and AUD, implicating a central role for GI problems stemming from chronic alcohol use. The findings may have important implications for the treatment of IBS patients with AUD who commonly present with anxiety disorders which could motivate the continuation of problematic drinking and impede recovery success. We propose that addressing GI problems in patients with AUD may help manage AUD and recovery more effectively.

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

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

The Interactions of Magnesium Sulfate and Cromoglycate in a Rat Model of Orofacial Pain; The Role of Magnesium on Mast Cell Degranulation in Neuroinflammation

Mast cell degranulation impacts the development of pain and inflammation during tissue injury. We investigated the antinociceptive effect of a combination of cromoglycate and magnesium in the orofacial model of pain and the histological profile of the effect of magnesium in orofacial pain. In male Wistar rats, formalin (1.5%, 100 µL) was injected subcutaneously into the right upper lip of rats after cromoglycate and/or magnesium. Pain was measured as the total time spent on pain-related behavior. Toluidine blue staining was used to visualize mast cells under the light microscope. In the formalin test, in phase 1, magnesium antagonized the antinociceptive effect of cromoglycate, while in phase 2, it potentiated or inhibited its effect. Magnesium significantly reduced mast cell degranulation in the acute phase by about 23% and in the second phase by about 40%. Pearson’s coefficient did not show a significant correlation between mast cell degranulation and pain under treatment with magnesium. The cromoglycate–magnesium sulfate combination may prevent the development of inflammatory orofacial pain. The effect of a combination of cromoglycate–magnesium sulfate depends on the nature of the pain and the individual effects of the drugs. Magnesium reduced orofacial inflammation in the periphery, and this effect did not significantly contribute to its analgesic effect.

Mechanisms of Transmission and Processing of Pain: A Narrative Review

Knowledge about the mechanisms of transmission and the processing of nociceptive information, both in healthy and pathological states, has greatly expanded in recent years. This rapid progress is due to a multidisciplinary approach involving the simultaneous use of different branches of study, such as systems neurobiology, behavioral analysis, genetics, and cell and molecular techniques. This narrative review aims to clarify the mechanisms of transmission and the processing of pain while also taking into account the characteristics and properties of nociceptors and how the immune system influences pain perception. Moreover, several important aspects of this crucial theme of human life will be discussed. Nociceptor neurons and the immune system play a key role in pain and inflammation. The interactions between the immune system and nociceptors occur within peripheral sites of injury and the central nervous system. The modulation of nociceptor activity or chemical mediators may provide promising novel approaches to the treatment of pain and chronic inflammatory disease. The sensory nervous system is fundamental in the modulation of the host's protective response, and understanding its interactions is pivotal in the process of revealing new strategies for the treatment of pain.

The multifaceted role of mast cells in joint inflammation and arthritis

OBJECTIVE

To review current knowledge surrounding the role of mast cells in joint inflammation and arthritis.

METHOD

Narrative review.

RESULTS

Mast cells (MCs) are commonly observed in the synovium of the joint, particularly surrounding blood vessels and nerve endings. Some studies have reported increased MC number and degranulation in patients with osteoarthritis (OA). In two studies, MCs were the only immune cell type found in higher concentrations in synovium of OA patients compared to rheumatoid arthritis patients. Activation of MCs in OA includes signaling pathways such as immunoglobulin E/Fc epsilon Receptor 1 (IgE/FcεR1), immunoglobulin G/Fc gamma receptor (IgG/FcγR), complement, and toll-like cell surface receptor-mediated signaling, resulting in context-dependent release of either pro-inflammatory and/or anti-inflammatory mediators within the joint. Activation of MCs results in the release of pro-inflammatory mediators that ultimately contribute to inflammation of the synovium, bone remodeling, and cartilage damage. However, some studies have proposed that MCs can also exhibit anti-inflammatory effects by secreting mediators that inactivate pro-inflammatory cytokines such as interleukin 6 (IL-6).

CONCLUSIONS

MCs may play a role in mediating synovial inflammation and OA progression. However, the mechanisms governing MC activation, the downstream pro- and/or anti-inflammatory effects, and their impact on osteoarthritis pathogenesis remains to be elucidated and requires extensive further study. Furthermore, it is important to establish the pathways of MC activation in OA to determine whether MCs exhibit varying phenotypes as a function of disease stage. Ultimately, such research is needed before understanding whether MCs could be targeted in OA treatments.

The effects of alpha-lipoic acid (ALA) on the urinary bladder injury in rats exposed to chronic stress: A histochemical study

Objective: In the present study, we aimed to investigate the morphological and biochemical effects of alpha-lipoic acid (ALA) on bladder injury caused by water avoidance stress (WAS) and to show its effect on the number of degranulated mast cells, which increase after stress. Materials and Methods: Wistar albino rats were subjected to WAS and the animals in the treatment group were injected ALA. After the urinary bladder tissues were subjected to routine tissue processing, hematoxylin-eosin staining and periodic acid-Schiff reaction were applied to observe general morphology and acidic toluidine blue method to investigate mast cells. Biochemical assessments of malondialdehyde (MDA) and glutathione (GSH) were also obtained. Transmission electron microscope was used for the ultrastructural, and scanning electron microscope for the topographical analyses. Results: The experiments showed that chronic stress caused injury in the bladder, increased degranulated and total number of mast cells and decreased GSH and increased MDA levels. ALA treatment after WAS ameliorated bladder injury in most areas, decreased degranulated and total mast cell number and increased GSH and decreased MDA levels. Conclusion: It was concluded that ALA can be a useful agent in the treatment of interstitial cystitis.

How "Neuronal" Are Human Skin Mast Cells?

Mast cells are evolutionarily old cells and the principal effectors in allergic responses and inflammation. They are seeded from the yolk sac during embryogenesis or are derived from hematopoietic progenitors and are therefore related to other leukocyte subsets, even though they form a separate clade in the hematopoietic system. Herein, we systematically bundle information from several recent high-throughput endeavors, especially those comparing MCs with other cell types, and combine such information with knowledge on the genes’ functions to reveal groups of neuronal markers specifically expressed by MCs. We focus on recent advances made regarding human tissue MCs, but also refer to studies in mice. In broad terms, genes hyper-expressed in MCs, but largely inactive in other myelocytes, can be classified into subcategories such as traffic/lysosomes (MLPH and RAB27B), the dopamine system (MAOB, DRD2, SLC6A3, and SLC18A2), Ca²⁺-related entities (CALB2), adhesion molecules (L1CAM and NTM) and, as an overall principle, the transcription factors and modulators of transcriptional activity (LMO4, PBX1, MEIS2, and EHMT2). Their function in MCs is generally unknown but may tentatively be deduced by comparison with other systems. MCs share functions with the nervous system, as they express typical neurotransmitters (histamine and serotonin) and a degranulation machinery that shares features with the neuronal apparatus at the synapse. Therefore, selective overlaps are plausible, and they further highlight the uniqueness of MCs within the myeloid system, as well as when compared with basophils. Apart from investigating their functional implications in MCs, a key question is whether their expression in the lineage is due to the specific reactivation of genes normally silenced in leukocytes or whether the genes are not switched off during mastocytic development from early progenitors.

Dimethyl Fumarate (DMF) Alleviated Post-Operative (PO) Pain through the N-Methyl-d-Aspartate (NMDA) Receptors

The management of post-operative (PO) pain has generally been shown to be inadequate; therefore, acquiring a novel understanding of PO pain mechanisms would increase the therapeutic options available. There is accumulating evidence to implicate N-methyl-d-aspartate (NMDA) receptors in the induction and maintenance of central sensitization during pain states by reinforcing glutamate sensory transmission. It is known that DMF protects from oxidative glutamate toxicity. Therefore, NMDA receptor antagonists have been implicated in peri-operative pain management. Recent advances demonstrated that dimethyl fumarate (DMF), a non-opioid and orally bioavailable drug, is able to resolve neuroinflammation through mechanisms that drive nociceptive hypersensitivity. Therefore, in this study, we evaluated the role of DMF on pain and neuroinflammation in a mouse model of PO pain. An incision of the hind paw was performed, and DMF at two different doses (30 and 100 mg/kg) was administered by oral gavage for five consecutive days. Mechanical allodynia, thermal hyperalgesia and locomotor dysfunction were evaluated daily for five days after surgery. Mice were sacrificed at day 7 following PO pain induction, and hind paw and lumbar spinal cord samples were collected for histological and molecular studies. DMF administration significantly reduced hyperalgesia and allodynia, alleviating motor disfunction. Treatment with DMF significantly reduced histological damage, counteracted mast cell activation and reduced the nuclear factor kappa-light-chain-enhancer of the activated B cell (NF-κB) inflammatory pathway, in addition to downregulating tumor necrosis factor-α (TNF-α), Interleukin-1β (Il-1β) and Il-4 expression. Interestingly, DMF treatment lowered the activation of NMDA receptor subtypes (NR2B and NR1) and the NMDA-receptor-interacting PDZ proteins, including PSD93 and PSD95. Furthermore, DMF interfered with calcium ion release, modulating nociception. Thus, DMF administration modulated PO pain, managing NMDA signaling pathways. The results suggest that DMF positively modulated persistent nociception related to PO pain, through predominantly NMDA-receptor-operated calcium channels.

Characterization of Early Inflammatory Events Leading to Provoked Vulvodynia Development in Rats

Background

Provoked vulvodynia (PV) is the main cause of vulvar pain and dyspareunia. The etiology of PV has not yet been elucidated. However, PV is associated with a history of recurrent inflammation, and its often accompanied by increases in the numbers of mast cells (MCs) and sensory hyperinnervation in the vulva. Therefore, this study aimed to examine the role of MCs and the early inflammatory events in the development of chronic vulvar pain in a rat model of PV.

Methods

Mechanical and thermal vulvar sensitivity was measured for 5 months following zymosan vulvar challenges. Vulvar changes in glutamate and nerve growth factor (NGF) were analyzed using ELISA. Immunofluorescence (IF) staining of the vulvar section after 20, 81, and 160 days of the zymosan challenge were performed to test MCs accumulation, hyperinnervation, and expression of pain channels (transient receptor potential vanilloid/ankyrin-1-TRPV1 & TRPA1) in vulvar neurons. Changes in the development of vulvar pain were evaluated following the administration of the MCs stabilizer ketotifen fumarate (KF) during zymosan vulvar challenges.

Results

Zymosan-challenged rats developed significant mechanical and thermal vulvar sensitivity that persisted for over 160 days after the zymosan challenge. During inflammation, increased local concentrations of NGF and glutamate and a robust increase in MCs degranulation were observed in zymosan-challenged rats. In addition, zymosan-challenged rats displayed sensory hyperinnervation and an increase in the expression of TRPV1 and TRPA1. Treatment with KF attenuated the upregulated level of NGF during inflammation, modulated the neuronal modifications, reduced MCs accumulation, and enhanced mechanical hypersensitivity after repeated inflammation challenges.

Conclusion

The present findings suggest that vulvar hypersensitivity is mediated by MCs accumulation, nerve growth, and neuromodulation of TRPV1 and TRPA1. Hence, KF treatment during the critical period of inflammation contributes to preventing chronic vulvar pain development.

Potential Neuroimmune Interaction in Chronic Pain: A Review on Immune Cells in Peripheral and Central Sensitization

Chronic pain is a long-standing unpleasant sensory and emotional feeling that has a tremendous impact on the physiological functions of the body, manifesting itself as a dysfunction of the nervous system, which can occur with peripheral and central sensitization. Many recent studies have shown that a variety of common immune cells in the immune system are involved in chronic pain by acting on the peripheral or central nervous system, especially in the autoimmune diseases. This article reviews the mechanisms of regulation of the sensory nervous system by neutrophils, macrophages, mast cells, B cells, T cells, and central glial cells. In addition, we discuss in more detail the influence of each immune cell on the initiation, maintenance, and resolution of chronic pain. Neutrophils, macrophages, and mast cells as intrinsic immune cells can induce the transition from acute to chronic pain and its maintenance; B cells and T cells as adaptive immune cells are mainly involved in the initiation of chronic pain, and T cells also contribute to the resolution of it; the role of glial cells in the nervous system can be extended to the beginning and end of chronic pain. This article aims to promote the understanding of the neuroimmune mechanisms of chronic pain, and to provide new therapeutic ideas and strategies for the control of chronic pain at the immune cellular level.

Advancing the Understanding of Acupoint Sensitization and Plasticity Through Cutaneous C-Nociceptors

Acupoint is the key area for needling treatment, but its physiology is not yet understood. Nociceptors, one of the responders in acupoints, are responsible for acupuncture manipulation and delivering acupuncture signals to the spinal or supraspinal level. Recent evidence has shown that various diseases led to sensory hypersensitivity and functional plasticity in sensitized acupoints, namely, acupoint sensitization. Neurogenic inflammation is the predominant pathological characteristic for sensitized acupoints; however, the underlying mechanism in acupoint sensitization remains unclear. Recent studies have reported that silent C-nociceptors (SNs), a subtype of C nociceptors, can be “awakened” by inflammatory substances released by sensory terminals and immune cells under tissue injury or visceral dysfunction. SNs can transform from mechano-insensitive nociceptors in a healthy state to mechanosensitive nociceptors. Activated SNs play a vital role in sensory and pain modulation and can amplify sensory inputs from the injured tissue and then mediate sensory hyperalgesia. Whether activated SNs is involved in the mechanism of acupoint sensitization and contributes to the delivery of mechanical signals from needling manipulation remains unclear? In this review, we discuss the known functions of cutaneous C nociceptors and SNs and focus on recent studies highlighting the role of activated SNs in acupoint functional plasticity.

Skin biomarkers associated with complex regional pain syndrome (CRPS) Type I: a systematic review

Despite increasing research, the pathophysiology of Complex Regional Pain Syndrome (CRPS) remains poorly understood. Due to its easy accessibility, the skin represents an ideal approach to gain a better understanding of the underlying processes. We conducted a systematic review of original studies investigating potential biomarkers cutaneous biomarkers in CRPS. Original articles with a minimum level IV of evidence were screened using the following databases: Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE and Web of Science Core Collection. Quality assessment was performed using the Methodological Index for Non-Randomized Studies criteria. A total of 11 studies exploring cutaneous biomarkers in 299 CRPS Type I patients were included. The biomarkers identified revealed implications of the following pathophysiological processes: inflammation via interleukins and TNF-a, vascular dysregulation (ET-1/NOx disturbances and hypoxia-high lactate), small fiber neuropathy and hypersensitivity. In terms of skin morphology, evidence suggests: neurite loss, increased expression and disturbed migration of mast cells, as well as an increased expression of α1-adrenoceptors on keratinocytes. The data supporting hypersensitivity had a high risk of bias on quality assessment. The current review has emphasized the current state of knowledge regarding the cutaneous biomarkers in patients suffering from CRPS Type I. Our results serve as a basis for future developments of techniques that would either facilitate diagnosis or may represent therapeutic targets. Trial registration PROSPERO: CRD42020203405. Level of evidence: IV (Systematic Review).

Prior Methamphetamine Use Disorder History Does Not Impair Interoceptive Processing of Soft Touch in HIV Infection

Introduction: Interoception, defined as the sense of the internal state of one’s body, helps motivate goal-directed behavior. Prior work has shown that methamphetamine (METH) use disorder is associated with altered interoception, and that this may contribute to risky behavior. As people with HIV (PWH) may also experience disrupted bodily sensations (e.g., neuropathy), an important question is whether PWH with a history of METH use disorder might exhibit greater impairment of interoceptive processing. Methods: Eighty-three participants stratified by HIV infection and a past history of methamphetamine use disorder experienced a soft touch paradigm that included slow brush strokes on the left forearm and palm during blood-oxygen level-dependent functional MRI acquisition. To assess differences in interoception and reward, voxelwise analyses were constrained to the insula, a hub for the evaluation of interoceptive cues, and the striatum, which is engaged in reward processing. Results: Overall, individuals with a history of METH use disorder had an attenuated neural response to pleasant touch in both the insula and striatum. Longer abstinence was associated with greater neural response to touch in the insula, suggesting some improvement in responsivity. However, only PWH with no METH use disorder history had lower brain activation in the insula relative to non-using seronegative controls. Conclusions: Our findings suggest that while METH use disorder history and HIV infection independently disrupt the neural processes associated with interoception, PWH with METH use disorder histories do not show significant differences relative to non-using seronegative controls. These findings suggest that the effects of HIV infection and past methamphetamine use might not be additive with respect to interoceptive processing impairment.

The role of peripheral nerve signaling in endometriosis

A hallmark of endometriosis - a chronic debilitating condition whose causes are poorly understood - is neuronal innervation of lesions. Recent evidence demonstrates that the peripheral nervous system plays an important role in the pathophysiology of this disease. Sensory nerves, which surround and innervate endometriotic lesions, not only drive the chronic and debilitating pain associated with endometriosis but also contribute to a pro-growth phenotype by secreting neurotrophic factors and interacting with surrounding immune cells. The diverse array of contributions that neurons play in endometriosis indicate that it should be considered as a nerve-centric disease. This review is focused on the emerging field of exoneural biology and how it applies to the field of endometriosis, in particular the role that peripheral nerves play in driving and maintaining endometriotic lesions. A better understanding of the mechanisms of neuronal contribution to endometriosis, as well as their interactions with accompanying stromal and immune cells, will unearth novel disease-relevant pathways and targets, providing additional, more selective therapeutic horizons.

Glutamate triggers the expression of functional ionotropic and metabotropic glutamate receptors in mast cells

Mast cells are emerging as players in the communication between peripheral nerve endings and cells of the immune system. However, it is not clear the mechanism by which mast cells communicate with peripheral nerves. We previously found that mast cells located within healing tendons can express glutamate receptors, raising the possibility that mast cells may be sensitive to glutamate signaling. To evaluate this hypothesis, we stimulated primary mast cells with glutamate and showed that glutamate induced the profound upregulation of a panel of glutamate receptors of both the ionotropic type (NMDAR1, NMDAR2A, and NMDAR2B) and the metabotropic type (mGluR2 and mGluR7) at both the mRNA and protein levels. The binding of glutamate to glutamate receptors on the mast cell surface was confirmed. Further, glutamate had extensive effects on gene expression in the mast cells, including the upregulation of pro-inflammatory components such as IL-6 and CCL2. Glutamate also induced the upregulation of transcription factors, including Egr2, Egr3 and, in particular, FosB. The extensive induction of FosB was confirmed by immunofluorescence assessment. Glutamate receptor antagonists abrogated the responses of the mast cells to glutamate, supporting the supposition of a functional glutamate-glutamate receptor axis in mast cells. Finally, we provide in vivo evidence supporting a functional glutamate-glutamate receptor axis in the mast cells of injured tendons. Together, these findings establish glutamate as an effector of mast cell function, thereby introducing a novel principle for how cells in the immune system can communicate with nerve cells.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part I. a literature review

INTRODUCTION

The cellular prion protein (PrP^C) is well known for its pathogenic roles in prion diseases, several other neurodegenerative diseases (such as Alzheimer's disease), and multiple types of cancer, but the beneficial aspects of PrP^C and its cleavage products received much less attention.

AREAS COVERED

Here the authors will systematically review the literatures on the negative as well as protective aspects of PrP^C and its derivatives (especially PrP N-terminal N1 peptide and shed PrP). The authors will dissect the current findings on N1 and shed PrP, including evidence for their neuroprotective effects, the categories of PrP^C cleavage, and numerous cleavage enzymes involved. The authors will also discuss the protective effects and therapeutic potentials of PrP^C-rich exosomes. The cited articles were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.

EXPERT OPINION

PrP and its N-terminal fragments have strong neuroprotective activities that should be explored for therapeutics and prophylactics development against prion disease, Alzheimer's disease and a few other neurodegenerative diseases. The strategies to develop PrP-based therapeutics and prophylactics for these neurodegenerative diseases will be discussed in a companion article (Part II).

Electroacupuncture Relieves Pain and Attenuates Inflammation Progression Through Inducing IL-10 Production in CFA-Induced Mice

The therapeutic effect of electroacupuncture (EA) on inflammatory pain has been well recognized clinically, but the mechanism is unclear. Interleukin-10 (IL-10), which is produced by regulatory T (Treg) cell, is a key anti-inflammatory cytokine for relieving inflammatory pain. Therefore, the aim of this study is to investigate whether EA could inhibit CFA-induced pain and attenuate inflammation progression by regulating the activation of immunocyte and inducing the expression of IL-10. In this study, mice were treated with EA (2/100 Hz, 2 mA) for five consecutive days after 1 day of CFA injection. The behavioral tests were measured and analyzed after the daily EA treatment; then, hind paw, spinal cord, and spleen tissues were prepared for assessment. The results showed that EA treatment significantly increased the mechanical threshold and thermal latency after CFA injection and boosted the expression of IL-10 in paw and spinal cord tissues. EA treatment promoted Treg cells; suppressed macrophage and neutrophils cells; reduced the expression of IL-1β, NLRP3, and TNF-α; and ultimately relieved inflammatory pain. The findings suggested that the analgesic and anti-inflammatory effect of EA treatment could be partially associated with suppression of pro-inflammatory cytokines mediated by induction of IL-10.

Mast cell stabilizer ketotifen fumarate reverses inflammatory but not neuropathic-induced mechanical pain in mice

Supplemental Digital Content is Available in the Text.

Our preclinical findings indicate that ketotifen fumarate's analgesic effects are MC-dependent, and the case series report presented supports its use for the treatment of chronic pain.

Introduction:

Mast cell (MC) activation could establish a positive feedback loop that perpetuates inflammation and maintains pain. Stabilizing MCs with ketotifen fumarate (KF) may disrupt this loop and relieve pain.

Objective:

We aimed to test the effect of treatment with KF in pain assays in mice and in a case series of patients with chronic widespread pain.

Methods:

The analgesic effect of KF was tested in CD-1 mice injected with formalin, complete Freund's adjuvant, or subjected to spared nerve injury. In addition, wild-type (C57BL/6) and MC-deficient (C57BL/6-Kit^W-sh/W-sh) mice were injected with formalin or complete Freund's adjuvant and treated with KF. Patients with chronic widespread pain (n = 5; age: 13–16 years) who failed to respond to standard of care participated in a 16-week treatment trial with KF (6 mg/d). Ketotifen fumarate's therapeutic effect was evaluated using the patient global impression of change.

Results:

In the mouse experiments, KF produced dose- and MC-dependent analgesic effects against mechanical allodynia in the acute and chronic inflammatory pain but not neuropathic pain assays. In the patient case series, 4 patients reported that activity limitations, symptoms, emotions, and overall quality of life related to their pain condition were “better” or “a great deal better” since beginning treatment with KF. This was accompanied by improvements in pain comorbid symptoms.

Conclusion:

Treatment with KF is capable of reducing established inflammatory-induced mechanical nociception in an MC-dependent manner in mice, and it may be beneficial for the treatment of chronic pain conditions.

Sensing the world and its dangers: An evolutionary perspective in neuroimmunology

Detecting danger is key to the survival and success of all species. Animal nervous and immune systems cooperate to optimize danger detection. Preceding studies have highlighted the benefits of bringing neurons into the defense game, including regulation of immune responses, wound healing, pathogen control, and survival. Here, we summarize the body of knowledge in neuroimmune communication and assert that neuronal participation in the immune response is deeply beneficial in each step of combating infection, from inception to resolution. Despite the documented tight association between the immune and nervous systems in mammals or invertebrate model organisms, interdependence of these two systems is largely unexplored across metazoans. This review brings a phylogenetic perspective of the nervous and immune systems in the context of danger detection and advocates for the use of non-model organisms to diversify the field of neuroimmunology. We identify key taxa that are ripe for investigation due to the emergence of key evolutionary innovations in their immune and nervous systems. This novel perspective will help define the primordial principles that govern neuroimmune communication across taxa.

Cutaneous Neuroimmune Interactions in Peripheral Neuropathic Pain States

Bidirectional interplay between the peripheral immune and nervous systems plays a crucial role in maintaining homeostasis and responding to noxious stimuli. This crosstalk is facilitated by a variety of cytokines, inflammatory mediators and neuropeptides. Dysregulation of this delicate physiological balance is implicated in the pathological mechanisms of various skin disorders and peripheral neuropathies. The skin is a highly complex biological structure within which peripheral sensory nerve terminals and immune cells colocalise. Herein, we provide an overview of the sensory innervation of the skin and immune cells resident to the skin. We discuss modulation of cutaneous immune response by sensory neurons and their mediators (e.g., nociceptor-derived neuropeptides), and sensory neuron regulation by cutaneous immune cells (e.g., nociceptor sensitization by immune-derived mediators). In particular, we discuss recent findings concerning neuroimmune communication in skin infections, psoriasis, allergic contact dermatitis and atopic dermatitis. We then summarize evidence of neuroimmune mechanisms in the skin in the context of peripheral neuropathic pain states, including chemotherapy-induced peripheral neuropathy, diabetic polyneuropathy, post-herpetic neuralgia, HIV-induced neuropathy, as well as entrapment and traumatic neuropathies. Finally, we highlight the future promise of emerging therapies associated with skin neuroimmune crosstalk in neuropathic pain.

Unique nasal septal island in dromedary camels may play a role in pain perception: microscopic studies

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The septal island in dromedaries is a distinctive anatomical structure.

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It has a curiously rostral location and innervated by the trigeminal nerve.

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It has an unusual ultrastructure and may be specialized for nociception.

The septal organs are islands or patches of sensory epithelium, located in the ventral parts of the nasal septum and innervated by the olfactory nerve. The septal island in dromedaries (Camelus dromedarius) was unusually located in the rostro-dorsal part of the nasal septum, where the ethmoidal branch of the trigeminal nerve provides innervation to the island mucosa. Therefore, the objectives of this study were to reveal the microscopic and ultrastructure of this island and to explain the probable functions. Twelve septal islands from 12 healthy male camels were used. Unlike the olfactory epithelium, which has a pseudostratified structure, the island neuroepithelium had a true neural lamination. Furthermore, in electron micrographs, the receptor, bipolar, and basal cells were connected with an orderly, organized network of cell–cell communication, which had some spine synapses. This network substituted the absence of supporting cells, maintained the shape of the tissue, and held the cells together. Moreover, the receptor cells were not similar to any of the different types of olfactory sensory neurons. Instead, they possessed the apical domain that might be specialized for the detection of chemical stimuli. Interestingly, a resident population of immune cells, namely mast cells and macrophages, was observed. The probable functions were discussed based on the cellular context and architecture. The nasal septal island in dromedaries may have a role in pain perception. The receptor cells most probably work as nociceptive cells that interact with the resident immune cells to coordinate pain signaling with immune response.

Immune Actions on the Peripheral Nervous System in Pain

Pain can be induced by tissue injuries, diseases and infections. The interactions between the peripheral nervous system (PNS) and immune system are primary actions in pain sensitizations. In response to stimuli, nociceptors release various mediators from their terminals that potently activate and recruit immune cells, whereas infiltrated immune cells further promote sensitization of nociceptors and the transition from acute to chronic pain by producing cytokines, chemokines, lipid mediators and growth factors. Immune cells not only play roles in pain production but also contribute to PNS repair and pain resolution by secreting anti-inflammatory or analgesic effectors. Here, we discuss the distinct roles of four major types of immune cells (monocyte/macrophage, neutrophil, mast cell, and T cell) acting on the PNS during pain process. Integration of this current knowledge will enhance our understanding of cellular changes and molecular mechanisms underlying pain pathogenies, providing insights for developing new therapeutic strategies.

Nociceptive Sensitization by Activation of Protease-Activated Receptor 2 in a Rat Model of Incisional Pain

Postoperative pain and consequent inflammatory responses after tissue incision adversely affects many surgical patients due to complicated mechanisms. In this study, we examined whether activation of protease-activated receptor 2 (PAR-2), which is stimulated by tryptase from mast cells, elicits nociception and whether the PAR-2 antagonist could reduce incisional nociceptive responses in vivo and in vitro. The effects of a selective PAR-2 antagonist, N3-methylbutyryl-N-6-aminohexanoyl-piperazine (ENMD-1068), pretreatment on pain behaviors were assessed after plantar incision in rats. The effects of a PAR-2 agonist, SLIGRL-NH₂, on nociception was assessed after the injection into the hind paw. Furthermore, the responses of C-mechanosensitive nociceptors to the PAR-2 agonist were observed using an in vitro skin–nerve preparation as well. Intraplantar injection of SLIGRL-NH₂ elicited spontaneous nociceptive behavior and hyperalgesia. Local administration of ENMD-1068 suppressed guarding behaviors, mechanical and heat hyperalgesia only within the first few hours after incision. SLIGRL-NH₂ caused ongoing activity in 47% of C-mechanonociceptors in vitro. This study suggests that PAR-2 may support early nociception after incision by direct or indirect sensitization of C-fibers in rats. Moreover, PAR-2 may play a regulatory role in the early period of postoperative pain together with other co-factors to that contribute to postoperative pain.

Impaired Duodenal Palmitoylethanolamide Release Underlies Acid-Induced Mast Cell Activation in Functional Dyspepsia

BACKGROUND & AIMS

Acid hypersensitivity is claimed to be a symptomatic trigger in functional dyspepsia (FD); however, the neuroimmune pathway(s) and the mediators involved in this process have not been investigated systematically. Palmitoylethanolamide (PEA) is an endogenous compound, able to modulate nociception and inflammation, but its role in FD has not been assessed.

METHODS

Duodenal biopsy specimens from FD and control subjects, and peroxisome proliferator-activated receptor-α (PPARα) null mice were cultured at a pH of 3.0 and 7.4. Mast cell (MC) number, the release of their mediators, and the expression of transient receptor potential vanilloid receptor (TRPV)1 and TRPV4, were evaluated. All measurements also were performed in the presence of a selective blocker of neuronal action potential (tetradotoxin). FD and control biopsy specimens in acidified medium also were incubated in the presence of different PEA concentrations, alone or combined with a selective PPARα or PPAR-γ antagonist.

RESULTS

An acid-induced increase in MC density and the release of their mediators were observed in both dyspeptic patients and controls; however, this response was amplified significantly in FD. This effect was mediated by submucosal nerve fibers and up-regulation of TRPV1 and TRPV4 receptors because pretreatment with tetradotoxin significantly reduced MC infiltration. The acid-induced endogenous release of PEA was impaired in FD and its exogenous administration counteracts MC activation and TRPV up-regulation.

CONCLUSIONS

Duodenal acid exposure initiates a cascade of neuronal-mediated events culminating in MC activation and TRPV overexpression. These phenomena are consequences of an impaired release of endogenous PEA. PEA might be regarded as an attractive therapeutic strategy for the treatment of FD.

Perinatal androgens organize sex differences in mast cells and attenuate anaphylaxis severity into adulthood

Mast cell (MC)-associated diseases, including allergy/anaphylaxis and neuroinflammatory pain disorders, exhibit a sex bias, with females at increase risk. While much attention has been directed toward adult sex hormones as drivers of sex differences, that female sex bias in MC-associated diseases is evident in prepubertal children, suggesting early-life origins of sex differences which have yet to be explored. Utilizing rodent models of MC-mediated anaphylaxis, our data here reveal that, 1) compared with females, males exhibit significantly reduced severity of MC-mediated anaphylactic responses that emerge prior to puberty and persist into adulthood, 2) reduced severity of MC-mediated anaphylaxis in males is linked with the naturally high level of perinatal androgens and can be recapitulated in females by perinatal exposure to testosterone proprionate, 3) perinatal androgen exposure guides bone marrow MC progenitors toward a masculinized tissue MC phenotype characterized by decreased concentration of prestored MC granule mediators (e.g., histamine, serotonin, and proteases) and reduced mediator release upon degranulation, and 4) engraftment of MC-deficient Kit W-sh/W-sh mice with adult male, female, or perinatally androgenized female MCs results in MC-mediated anaphylaxis response that reflects the MC sex and not host sex. Together, these data present evidence that sex differences in MC phenotype and resulting disease severity are established in early life by perinatal androgens. Thus, factors affecting levels of perinatal androgens could have a significant impact on MC development and MC-associated disease risk across the life span.

Assessment of nociception and related quality-of-life measures in a porcine model of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is an autosomal dominant genetic disorder resulting from germline mutations in the NF1 gene, which encodes neurofibromin. Patients experience a variety of symptoms, but pain in the context of NF1 remains largely underrecognized. Here, we characterize nociceptive signaling and pain behaviors in a miniswine harboring a disruptive NF1 mutation (exon 42 deletion). We present the first characterization of pain-related behaviors in a pig model of NF1, identifying unchanged agitation scores, lower tactile thresholds (allodynia), and decreased response latencies to thermal laser stimulation (hyperalgesia) in NF1 (females only) pigs. Male NF1 pigs with tumors showed reduced sleep quality and increased resting, 2 health-related quality-of-life symptoms found to be comorbid in people with NF1 pain. We explore these phenotypes in relationship to suppression of the increased activity of the N-type voltage-gated calcium (CaV2.2) channel by pharmacological antagonism of phosphorylation of a regulatory protein-the collapsin response mediator protein 2 (CRMP2), a known interactor of neurofibromin, and by targeting the interface between the α subunit of CaV2.2 and the accessory β-subunits with small molecules. Our data support the use of NF1 pigs as a large animal model for studying NF1-associated pain and for understanding the pathophysiology of NF1. Our findings demonstrate the translational potential of 2 small molecules in reversing ion channel remodeling seen in NF1. Interfering with CaV2.2, a clinically validated target for pain management, might also be a promising therapeutic strategy for NF1-related pain management.

Natural α,β-unsaturated lactones inhibit neuropeptide-induced mast cell activation in an in vitro model of neurogenic inflammation

INTRODUCTION

Mast cells are involved in not only inducing, but also maintaining neurogenic inflammation and neuropathic pain. In previous work, we have demonstrated that dehydroleucodine, xanthatin and 3-benzyloxymethyl-5H-furan-2-one inhibit rat peritoneal and human LAD2 mast cell degranulation induced by compound 48/80 and calcium ionophore A23187. However, the effect of these molecules on neuropeptide-induced mast cell activation has not been studied so far.

OBJECTIVE

The aim of this study was to determine whether dehydroleucodine, xanthatin, and 3-benzyloxymethyl-5H-furan-2-one inhibit neuropeptide-induced mast cell activation.

METHODS

This work is based on in vitro simulation of a neurogenic inflammation scenario involving neuropeptides and mast cells, to subsequently analyze potential therapeutic strategies for neuropathic pain.

RESULTS

Neuromedin-N did not stimulate mast cell serotonin release but substance P and neurotensin did induce serotonin release from peritoneal mast cells in a dose-dependent manner. Mast cell serotonin release induced by substance P and neurotensin was inhibited by dehydroleucodine and xanthatin, but not by 3-benzyloxymethyl-5H-furan-2-one. The inhibitory potency of dehydroleucodine and xanthatin was higher than that obtained with the reference compounds, ketotifen and sodium chromoglycate, when mast cells were preincubated with dehydroleucodine before substance P incubation, and with dehydroleucodine or xanthatin before neurotensin incubation.

CONCLUSIONS

These results are the first strong evidence supporting the hypothesis that dehydroleucodine and xanthatin inhibit substance P- and neurotensin-induced serotonin release from rat peritoneal mast cells. Our findings suggest, additionally, that these α,β-unsaturated lactones could be of value in future pharmacological research related to inappropriate mast cell activation conditions such as neurogenic inflammation and neuropathic pain.

Do Mast Cells Have a Role in Tendon Healing and Inflammation?

Understanding the links between the tendon healing process, inflammatory mechanisms, and tendon homeostasis/pain after tissue damage is crucial in developing novel therapeutics for human tendon disorders. The inflammatory mechanisms that are operative in response to tendon injury are not fully understood, but it has been suggested that inflammation occurring in response to nerve signaling, i.e., neurogenic inflammation, has a pathogenic role. The mechanisms driving such neurogenic inflammation are presently not clear. However, it has recently been demonstrated that mast cells present within the injured tendon can express glutamate receptors, raising the possibility that mast cells may be sensitive to glutamate signaling and thereby modulate neurogenic inflammation following tissue injury. In this review, we discuss the role of mast cells in the communication with peripheral nerves, and their emerging role in tendon healing and inflammation after injury.

T Cell Subpopulations in the Physiopathology of Fibromyalgia: Evidence and Perspectives

Fibromyalgia is one of the most important “rheumatic” disorders, after osteoarthritis. The etiology of the disease is still not clear. At the moment, the most defined pathological mechanism is the alteration of central pain pathways, and emotional conditions can trigger or worsen symptoms. Increasing evidence supports the role of mast cells in maintaining pain conditions such as musculoskeletal pain and central sensitization. Importantly, mast cells can mediate microglia activation through the production of proinflammatory cytokines such as IL-1β, IL-6, and TNFα. In addition, levels of chemokines and proinflammatory cytokines are enhanced in serum and could contribute to inflammation at systemic level. Despite the well-characterized relationship between the nervous system and inflammation, the mechanism that links the different pathological features of fibromyalgia, including stress-related manifestations, central sensitization, and dysregulation of the innate and adaptive immune responses is largely unknown. This review aims to provide an overview of the current understanding of the role of adaptive immune cells, in particular T cells, in the physiopathology of fibromyalgia. It also aims at linking the latest advances emerging from basic science to envisage new perspectives to explain the role of T cells in interconnecting the psychological, neurological, and inflammatory symptoms of fibromyalgia.

Associations of Sleep Disturbance, Atopy, and Other Health Measures with Chronic Overlapping Pain Conditions

AIMS

To quantify the contributions of atopic disorders, sleep disturbance, and other health conditions to five common pain conditions.

METHODS

This cross-sectional analysis used data from 655 participants in the OPPERA study. The authors investigated the individual and collective associations of five chronic overlapping pain conditions (COPCs) with medically diagnosed atopic disorders and self-reported sleep disturbance, fatigue, and symptoms of obstructive sleep apnea. Atopic disorders were allergies, allergic rhinitis, atopic dermatitis, allergic asthma, urticaria, allergic conjunctivitis, and food allergy. Logistic regression models estimated odds ratios as measures of association with temporomandibular disorders, headache, irritable bowel syndrome, low back pain, and fibromyalgia. Measures of sleep and atopy disorders were standardized to z scores to determine the relative strength of their associations with each COPC. Sociodemographic characteristics and body mass index were covariates. Random forest regression analyzed all variables simultaneously, computing importance metrics to determine which variables best differentiated pain cases from controls.

RESULTS

Fatigue and sleep disturbance were strongly associated with each COPC and with the total number of COPCs. An increase of one standard deviation in fatigue or sleep disturbance score was associated with approximately two-fold greater odds of having a COPC. In random forest models, atopic disorders contributed more than other health measures to differentiating between cases and controls of headache, whereas other COPCs were best differentiated by measures of fatigue or sleep.

CONCLUSION

Atopic disorders, previously recognized as predictors of poor sleep, are associated with COPCs after accounting for sleep problems.

Melatonin Plus Folic Acid Treatment Ameliorates Reserpine-Induced Fibromyalgia: An Evaluation of Pain, Oxidative Stress, and Inflammation

Background: Fibromyalgia is a chronic condition characterized by increased sensory perception of pain, neuropathic/neurodegenerative modifications, oxidative, and nitrosative stress. An appropriate therapy is hard to find, and the currently used treatments are able to target only one of these aspects. Methods: The aim of this study is to investigate the beneficial effects of melatonin plus folic acid administration in a rat model of reserpine-induced fibromyalgia. Sprague–Dawley male rats were injected with 1 mg/kg of reserpine for three consecutive days and later administered with melatonin, folic acid, or both for twenty-one days. Results: Administration of reserpine led to a significant decrease in the nociceptive threshold as well as a significant increase in depressive-like symptoms. These behavioral changes were accompanied by increased oxidative and nitrosative stress. Lipid peroxidation was significantly increased, as well as nitrotyrosine and PARP expression, while superoxide dismutase, nonprotein thiols, and catalase were significantly decreased. Endogenously produced oxidants species are responsible for mast cell infiltration, increased expression pro-inflammatory mediators, and microglia activation. Conclusion: Melatonin plus acid folic administration is able to ameliorate the behavioral defects, oxidative and nitrosative stress, mast cell infiltration, inflammatory mediators overexpression, and microglia activation induced by reserpine injection with more efficacy than their separate administration.

Injury-Induced Effectors of Neuropathic Pain

Injuries typically result in the development of neuropathic pain, which decreases in parallel with wound healing. However, the pain may remain after the injury appears to have healed, which is generally associated with an ongoing underlying pro-inflammatory state. Injury induces many cells to release factors that contribute to the development of a pro-inflammatory state, which is considered an essential first step towards wound healing. However, pain elimination requires a transition of the injury site from pro- to anti-inflammatory. Therefore, developing techniques that eliminate chronic pain require an understanding of the cells resident at and recruited to injury sites, the factors they release, that promote a pro-inflammatory state, and promote the subsequent transition of that site to be anti-inflammatory. Although a relatively large number of cells, factors, and gene expression changes are involved in these processes, it may be possible to control a relatively small number of them leading to the reduction and elimination of chronic neuropathic pain. This first of two papers examines the roles of the most salient cells and mediators associated with the development and maintenance of chronic neuropathic pain. The following paper examines the cells and mediators involved in reducing and eliminating chronic neuropathic pain.

Colonic mast cell numbers, symptom profile, and mucosal expression of elements of the epithelial barrier in irritable bowel syndrome

BACKGROUND

This study aimed to determine whether patients with IBS displayed altered mucosal mast cell (MC) numbers and proportions of MCs co-localizing with nerves compared with healthy subjects (HS) and whether these MC characteristics correlated with IBS symptoms, elements of the epithelial barrier, or visceral sensitivity.

METHODS

Mucosal MC characteristics were determined using immunoassay. IBS symptoms, gene expression of elements of the epithelial barrier, fecal serine protease activity, and visceral sensitivity were assessed.

KEY RESULTS

The MC numbers per mm² were 2.0 (0.0-6.0) in patients with IBS (n = 43) and 3.5 (1.1-9.1) in HS (n = 20, P = .26). Of these, MCs were 0.0 (0.0-20) % vs 3.1 (0.0-18) % (P = .76) in IBS and HS, respectively, in co-localization with nerve fibers. MC characteristics were equivalent in the different IBS subtypes. Hierarchical cluster analysis identified two distinct groups among patients with IBS: MC high (higher MC numbers and proportions of MCs co-localizing with nerves) and MC low (lower MC numbers and proportions of MCs co-localizing with nerves). The MC high and MC low groups could not be discriminated with regard to IBS symptoms, parameters of visceral sensitivity, gene expression of elements of the epithelial barrier, and fecal protease activity.

CONCLUSION AND INFERENCES

There was no evidence of increased infiltration or altered localization of MCs in the colonic mucosa of patients with IBS. These MC characteristics were not linked to global IBS symptoms or mucosal expression of elements of the epithelial barrier. These findings indicate that quantity and location of mucosal MCs are factors not involved in the pathophysiology of IBS.

Neutrophils promote CXCR3-dependent itch in the development of atopic dermatitis

Chronic itch remains a highly prevalent disorder with limited treatment options. Most chronic itch diseases are thought to be driven by both the nervous and immune systems, but the fundamental molecular and cellular interactions that trigger the development of itch and the acute-to-chronic itch transition remain unknown. Here, we show that skin-infiltrating neutrophils are key initiators of itch in atopic dermatitis, the most prevalent chronic itch disorder. Neutrophil depletion significantly attenuated itch-evoked scratching in a mouse model of atopic dermatitis. Neutrophils were also required for several key hallmarks of chronic itch, including skin hyperinnervation, enhanced expression of itch signaling molecules, and upregulation of inflammatory cytokines, activity-induced genes, and markers of neuropathic itch. Finally, we demonstrate that neutrophils are required for induction of CXCL10, a ligand of the CXCR3 receptor that promotes itch via activation of sensory neurons, and we find that that CXCR3 antagonism attenuates chronic itch.

Chronic itch is a debilitating disorder that can last for months or years. Eczema, or atopic dermatitis, is the most common cause for chronic itch, affecting one in ten people worldwide. Many treatments for the condition are ineffective, and the exact cause of the disease is unknown, but many different types of cells are likely involved. These include skin cells and inflammation-promoting immune cells, as well as nerve cells that detect inflammation, relay itch and pain information to the brain, and regulate the immune system.

Learning more about how these cells interact in eczema may help scientists find better treatments for the condition. So far, a lot of research has focused on static ‘snapshots’ of mature eczema lesions from human skin or animal models. These studies have identified abnormalities in genes or cells, but have not revealed how these genes and cells interact over time to cause chronic itch and inflammation.

Now, Walsh et al. reveal that immune cells called neutrophils trigger chronic itch in eczema. The experiments involved mice with a condition that mimics eczema, and showed that removing the neutrophils in these mice alleviated their itching. They also showed that dramatic and rapid changes occur in the nervous system of mice suffering from the eczema-like condition. For example, excess nerves grow in the animals’ damaged skin, genes in the nerves that detect sensations become hyperactive, and changes occur in the spinal cord that have been linked to nerve pain. When neutrophils are absent, these changes do not take place.

These findings show that neutrophils play a key role in chronic itch and inflammation in eczema. Drugs that target neutrophils, which are already used to treat other diseases, might help with chronic itch, but they would need to be tested before they can be used on people with eczema.

Mast Cells, Neuroinflammation and Pain in Fibromyalgia Syndrome

Fibromyalgia Syndrome (FMS) is a disorder of chronic, generalized muscular pain, accompanied by sleep disturbances, fatigue and cognitive dysfunction. There is no definitive pathogenesis except for altered central pain pathways. We previously reported increased serum levels of the neuropeptides substance P (SP) and its structural analogue hemokinin-1 (HK-1) together with the pro-inflammatory cytokines IL-6 and TNF in FMS patients as compared to sedentary controls. We hypothesize that thalamic mast cells contribute to inflammation and pain, by releasing neuro-sensitizing molecules that include histamine, IL-1β, IL-6 and TNF, as well as calcitonin-gene related peptide (CGRP), HK-1 and SP. These molecules could either stimulate thalamic nociceptive neurons directly, or via stimulation of microglia in the diencephalon. As a result, inhibiting mast cell stimulation could be used as a novel approach for reducing pain and the symptoms of FMS.

Nanomedicine-driven neuropathic pain relief in a rat model is associated with macrophage polarity and mast cell activation

We explored the immune neuropathology underlying multi-day relief from neuropathic pain in a rat model initiated at the sciatic nerve, by using a nanoemulsion-based nanomedicine as a biological probe. The nanomedicine is theranostic: both therapeutic (containing celecoxib drug) and diagnostic (containing near-infrared fluorescent (NIRF) dye) and is small enough to be phagocytosed by circulating monocytes. We show that pain-like behavior reaches a plateau of maximum hypersensitivity 8 days post-surgery, and is the rationale for intravenous delivery at this time-point. Pain relief is evident within 24 h, lasting approximately 6 days. The ipsilateral sciatic nerve and associated L4 and L5 dorsal root ganglia (DRG) tissue of both nanomedicine and control (nanoemulsion without drug) treated animals was investigated by immunofluorescence and confocal microscopy at the peak of pain relief (day-12 post-surgery), and when pain-like hypersensitivity returns (day-18 post-surgery). At day-12, a significant reduction of infiltrating macrophages, mast cells and mast cell degranulation was observed at the sciatic nerve following treatment. In the DRG, there was no effect of treatment at both day-12 and day-18. Conversely, at the DRG, there is a significant increase in macrophage infiltration and mast cell degranulation at day-18. The treatment effect on immune pathology in the sciatic nerve was investigated further by assessing the expression of macrophage cyclooxygenase-2 (COX-2)-the drug target-and extracellular prostaglandin E2 (PGE2), as well as the proportion of M1 (pro-inflammatory) and M2 (anti-inflammatory) macrophages. At day-12, there is a significant reduction of COX-2 positive macrophages, extracellular PGE2, and a striking reversal of macrophage polarity. At day-18, these measures revert to levels observed in control-treated animals. Here we present a new paradigm of immune neuropathology research, by employing a nanomedicine to target a mechanism of neuropathic pain-resulting in long-lasting pain relief--whilst revealing novel immune pathology at the injured nerve and associated DRG.

Tetrahydrocannabinol Reduces Hapten-Driven Mast Cell Accumulation and Persistent Tactile Sensitivity in Mouse Model of Allergen-Provoked Localized Vulvodynia

Vulvodynia is a remarkably prevalent chronic pain condition of unknown etiology. An increase in numbers of vulvar mast cells often accompanies a clinical diagnosis of vulvodynia and a history of allergies amplifies the risk of developing this condition. We previously showed that repeated exposures to oxazolone dissolved in ethanol on the labiar skin of mice led to persistent genital sensitivity to pressure and a sustained increase in labiar mast cells. Here we sensitized female mice to the hapten dinitrofluorobenzene (DNFB) dissolved in saline on their flanks, and subsequently challenged them with the same hapten or saline vehicle alone for ten consecutive days either on labiar skin or in the vaginal canal. We evaluated tactile ano-genital sensitivity, and tissue inflammation at serial timepoints. DNFB-challenged mice developed significant, persistent tactile sensitivity. Allergic sites showed mast cell accumulation, infiltration of resident memory CD8⁺CD103⁺ T cells, early, localized increases in eosinophils and neutrophils, and sustained elevation of serum Immunoglobulin E (IgE). Therapeutic intra-vaginal administration of Δ⁹-tetrahydrocannabinol (THC) reduced mast cell accumulation and tactile sensitivity. Mast cell-targeted therapeutic strategies may therefore provide new ways to manage and treat vulvar pain potentially instigated by repeated allergenic exposures.

γδ T Cells Modulate Myeloid Cell Recruitment but Not Pain During Peripheral Inflammation

Circulating immune cells, which are recruited to the site of injury/disease, secrete various inflammatory mediators that are critical to nociception and pain. The role of tissue-resident immune cells, however, remains poorly characterized. One of the first cells to be activated in peripheral tissues following injury are γδT cells, which serve important roles in infection, disease, and wound healing. Using a mouse line lacking these cells, we sought to identify their contribution to inflammatory pain. Three distinct models of peripheral inflammatory pain were used: intraplantar injection of formalin (spontaneous inflammatory pain), incisional wound (acute inflammatory pain), and intraplantar injection of complete Freund's adjuvant (chronic inflammatory pain). Our results show that absence of γδT cells does not alter baseline sensitivity, nor does it result in changes to mechanical or thermal hypersensitivity after tissue injury. Myeloid cell recruitment did show differential changes between models of acute and chronic inflammatory pain. These results were consistent in both male and female mice, suggesting that there are no sex differences in these outcomes. This comprehensive characterization suggests that γδT cells do not contribute to basal sensitivity or the development and maintenance of inflammatory pain.

Use of prescription opioid and other drugs among a cohort of persons with Ehlers-Danlos syndrome: A retrospective study

Against the backdrop of increased opioid prescribing in the United States and the associated high rate of side effects, dependence, and addiction, our study examined how opioids and other medications are being used among persons with Ehlers-Danlos syndrome (EDS). EDS is a set of heritable connective tissue disorders with high symptom burden, including chronic pain. Prescription medication use among persons with EDS was compared to a cohort of matched controls using 10 years of administrative claims data from a large database of privately insured patients (2005-2014). Our dataset included 4,294 persons with EDS, ages 5-62 years old. In both adults and children, we found that the percentage of persons with a prescription drug claim was higher in the EDS cohort for all prescription drug classes examined. Among children, opioid use was double in the EDS cohort compared to the control group (27.5% vs. 13.5%); in adults, it was nearly double in EDS patients (62% vs. 34.1%). Among persons who were prescribed opioids, those with EDS had higher cumulative dosages over a 2-year time period versus controls. Our study aids in understanding opioid and prescription drug use patterns in a vulnerable population with high symptom burden and chronic pain that is often severe.

Protease activated receptor 2 (PAR2) activation causes migraine-like pain behaviors in mice

BACKGROUND

Pain is the most debilitating symptom of migraine. The cause of migraine pain likely requires activation of meningeal nociceptors. Mast cell degranulation, with subsequent meningeal nociceptor activation, has been implicated in migraine pathophysiology. Degranulating mast cells release serine proteases that can cleave and activate protease activated receptors. The purpose of these studies was to investigate whether protease activated receptor 2 is a potential generator of nociceptive input from the meninges by using selective pharmacological agents and knockout mice.

METHODS

Ratiometric Ca++ imaging was performed on primary trigeminal and dural cell cultures after application of 2at-LIGRL-NH2, a specific protease activated receptor 2 agonist. Cutaneous hypersensitivity and facial grimace was measured in wild-type and protease activated receptor 2-/- mice after dural application of 2at-LIGRL-NH2 or compound 48-80, a mast cell degranulator. Behavioral experiments were also conducted in mice after dural application of 2at-LIGRL-NH2 (2AT) in the presence of either C391, a selective protease activated receptor 2 antagonist, or sumatriptan.

RESULTS

2at-LIGRL-NH2 evoked Ca2+ signaling in mouse trigeminal neurons, dural fibroblasts and in meningeal afferents. Dural application of 2at-LIGRL-NH2 or 48-80 caused dose-dependent grimace behavior and mechanical allodynia that were attenuated by either local or systemic application of C391 as well as in protease activated receptor 2-/- mice. Nociceptive behavior after dural injection of 2at-LIGRL-NH2 was also attenuated by sumatriptan.

CONCLUSIONS

Functional protease activated receptor 2 receptors are expressed on both dural afferents and fibroblasts and activation of dural protease activated receptor 2 produces migraine-like behavioral responses. Protease activated receptor 2 may link resident immune cells to meningeal nociceptor activation, driving migraine-like pain and implicating protease activated receptor 2 as a therapeutic target for migraine in humans.

Autism, Joint Hypermobility-Related Disorders and Pain

Autism Spectrum Disorder (ASD) and Joint Hypermobility-Related Disorders are blanket terms for two etiologically and clinically heterogeneous groups of pathologies that usually appears in childhood. These conditions are seen by different medical fields, such as psychiatry in the case of ASD, and musculoskeletal disciplines and genetics in the case of hypermobility-related disorders. Thus, a link between them is rarely established in clinical setting, despite a scarce but growing body of research suggesting that both conditions co-occur more often than expected by chance. Hypermobility is a frequent sign of hereditary disorders of connective tissue (e.g., Ehlers-Danlos syndromes, Marfan syndrome), in which the main characteristic is the multisystem fragility that prone to proprioceptive and motor coordination dysfunction and hence to trauma and chronic pain. Considering the high probability that pain remains disregarded and untreated in people with ASD due to communication and methodological difficulties, increasing awareness about the interconnection between ASD and hypermobility-related disorders is relevant, since it may help identify those ASD patients susceptible to chronic pain.

Milonine, an Alkaloid of Cissampelos sympodialis Eichl. (Menispermaceae) Inhibits Histamine Release of Activated Mast Cells

Milonine is an alkaloid of Cissampelos sympodialis Eichl. (Menispermaceae), a plant used in the northeast of Brazil to treat allergies such as asthma, rhinitis, and other conditions. Previously, several alkaloids were isolated from its roots and leaves with pharmacological properties in asthma and acute inflammation models. Therefore, the aim of this study was to evaluate the milonine effect on mast cells degranulation in vivo and in vitro. Swiss mice (n = 8) were used in models of paw edema induced by carrageenan, compound 48/80, or histamine. One hour before challenge, the animals were treated with milonine (at different doses) or standard drugs and, at different time points, the edema formation was measured. In addition, other different methods, such as anaphylactic shock reaction and scratching behavior models both induced by compound 48/80, a mast cell degranulator, were used to assess milonine effect histamine release in vivo. Moreover, milonine effect on mast cell degranulation in vitro was also carried out. Firstly, it was observed that milonine significantly decreased the carrageenan edema formation only at the beginning of the reaction (i.e., up to 2 h after challenge). Furthermore, this alkaloid decreased the edema induced by compound 48/80, maintained the paw tissue integrity, without modulating histamine-induced paw edema. In anaphylactic shock reaction, milonine increased the time of animal survival when compared with compound 48/80 group. Milonine also significantly decreased the scratching behavior induced by compound 48/80 with decreasing of mast cell degranulation in vitro. Therefore, these data indicated that milonine presents anti-allergic properties by decreasing mast cell degranulation rather than acting on histamine effect.

IL-33 mast cell axis is central in LL-37 induced bladder inflammation and pain in a murine interstitial cystitis model

Interstitial cystitis (IC), also known as painful bladder syndrome (PBS), is a debilitating chronic condition that afflicts over 3 million women above the age of 18 in the U.S., and most patients fail to respond to current treatment options. Mast cells have previously been implicated as both a diagnostic and prognostic marker in IC/PBS. Patients with IC/PBS have been shown to have elevated levels of IL-33, a cytokine released in response to tissue insult, in their urine. We hypothesize that mast cell-mediated inflammation induced from IL-33 may play an important role in initiating pain and inflammation in IC/PBS. A human cathelicidin, LL-37, which is found at elevated levels in IC/PBS patients, was used to induce an IC/PBS-like state of inflammation and bladder pain in mast cell deficient C-kit (-/-) and wild type C57Bl/6 (WT) mice. Inflammation was quantified using myeloperoxidase (MPO) expression in bladder tissues measured via ELISA. Response rate to suprapubic stimulation from von Frey filaments was used to assess the relative pain and discomfort. Both types of mice increased IL-33 expression in response to LL-37 exposure. However, mast cell deficient mice demonstrated significantly lower levels of inflammation (p < 0.001) and reduced pain response (p < 0.001) compared to WT mice. These findings implicate an IL-33-mast cell dependent axis with a potential etiology of pain and inflammation in IC/PBS. Future therapeutics aimed at targeting the IL-33 - mast cell axis could potentially serve as useful targets for treating IC/PBS.

Divergent roles of immune cells and their mediators in pain

Chronic pain is a major debilitating condition that is difficult to treat. Although chronic pain may appear to be a disorder of the nervous system, crucial roles for immune cells and their mediators have been identified as important contributors in various types of pain. This review focuses on how the immune system regulates pain and discusses the emerging roles of immune cells in the initiation or maintenance of chronic pain. We highlight which immune cells infiltrate damaged nerves, the dorsal root ganglia, spinal cord and tissues around free nerve endings and discuss through which mechanisms they control pain. Finally we discuss emerging roles of the immune system in resolving pain and how the immune system contributes to the transition from acute to chronic pain. We propose that targeting some of these immune processes may provide novel therapeutic opportunities for the treatment of chronic pain.