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Primary sensory neurons are a heterogeneous population of cells able to respond to both innocuous and noxious stimuli. Like most neurons they are highly compartmentalised, allowing them to detect, convey and transfer sensory information. These compartments include specialised sensory endings in the skin, the nodes of Ranvier in myelinated axons, the cell soma and their central terminals in the spinal cord. In this review, we will highlight the importance of these compartments to primary afferent function, describe how these structures are compromised following nerve damage and how this relates to neuropathic pain.

Although sex dimorphism is increasingly recognized as an important factor in pain, female-specific pain signaling is not well-studied. Here we report that administration of IL-23 produces mechanical pain (mechanical allodynia) in female, but not male mice, and chemotherapy-induced mechanical pain is selectively impaired in female mice lacking Il23 or Il23r. IL-23-induced pain is promoted by estrogen but suppressed by androgen, suggesting an involvement of sex hormones. IL-23 requires C-fiber nociceptors and TRPV1 to produce pain but does not directly activate nociceptor neurons. Notably, IL-23 requires IL-17A release from macrophages to evoke mechanical pain in females. Low dose IL-17A directly activates nociceptors and induces mechanical pain only in females. Finally, deletion of estrogen receptor subunit α (ERα) in TRPV1(+) nociceptors abolishes IL-23- and IL-17-induced pain in females. These findings demonstrate that the IL-23/IL-17A/TRPV1 axis regulates female-specific mechanical pain via neuro-immune interactions. Our study also reveals sex dimorphism at both immune and neuronal levels.

Pain is a under-recognized association of leucine-rich glioma-inactivated 1 (LGI1) and contactin-associated protein-like 2 (CASPR2) antibodies. Of 147 patients with these autoantibodies, pain was experienced by 17 of 33 (52%) with CASPR2- versus 20 of 108 (19%) with LGI1 antibodies (p = 0.0005), and identified as neuropathic in 89% versus 58% of these, respectively. Typically, in both cohorts, normal nerve conduction studies and reduced intraepidermal nerve fiber densities were observed in the sampled patient subsets. In LGI1 antibody patients, pain responded to immunotherapy (p = 0.008), often rapidly, with greater residual patient-rated impairment observed in CASPR2 antibody patients (p = 0.019). Serum CASPR2 antibodies, but not LGI1 antibodies, bound in vitro to unmyelinated human sensory neurons and rodent dorsal root ganglia, suggesting pathophysiological differences that may underlie our clinical observations. ANN NEUROL 2021;90:683-690.

Objective We evaluated pain in leucine-rich glioma inactivated1(LGI1) and contactin-associated protein2(CASPR2)-antibody positive(Ab+) patients, to evaluate clinical associations and pathophysiology of treatable pain syndromes. Methods 108 LGI-Ab+, 33 CASPR2-Ab+, and 6 LGI1/CASPR-Ab+patients were phenotyped. Pain questionnaires were undertaken to identify neuropathic pain using the Douleur Neuropathique(DN4), patient reported outcome measurement information system(PROMIS), and quality of life(EQ5D). Skin biopsies, and serum binding to cell-based assays, sensory neuronal cocultures, and dorsal root ganglion(DRG) cultures were undertaken. Results 39/147 patients described pain, including 17/33 CASPR2-Ab+(52%), 20/108 LGI1-Ab+(19%), and 2 LGI/CASPR2-Ab+patients. Questionnaires completed in 23/39(59%) revealed comparable DN4 scores(p=0.319) with 58% of LGI1-Ab+ and 67% of CASPR2-Ab+patients having neuropathic pain. Patients rated >50% response in 8/30(27%) analgesia trials, versus 20/40(55%) immunotherapy trials(p=0.045). PROMIS ratings were similar between LGI1-Ab+ and CASPR2-Ab+patients at nadir(p=0.662), but showed more improvement following immunotherapy in LGI1-Ab+(p=0.008) than CASPR2-Ab+patients(p=0.125). At follow-up(median 57 months) CASPR2-Ab+patients showed more impairment in mobility(p=0.014), daily activities(p=0.019), and anxiety/depression(p=0.043); and lower overall health(p=0.019) on the EQ5D compared to LGI1-Ab+patients. Intraepidermal nerve fibre density was reduced in 2 LGI1-Ab+ and 1 CASPR2-Ab+patients. Serum immunoglobulin G(IgG) from 6/16 CASPR2-Ab+patients bound to sensory neuronal cocultures compared to 0/14 LGI1-Ab+patients(p=0.019) and 0/12 healthy controls. Serum IgG from 10/16 CASPR2-Ab+patients bound to DRG cultures compared to 1/14 LGI1-Ab+patients(p=0.0024) and 1/12 healthy controls. Conclusion Neuropathic pain may be present in both LGI-Ab+ and CASPR2-Ab+patients, and is immunotherapy responsive. Serum IgG from CASPR2-Ab+patients more frequently bound sensory neurons and dorsal root ganglia, suggesting pathophysiological differences which may underlie the more severe pain in these patients.

Autism spectrum disorders (ASDs) are a heterogeneous group of neurodevelopmental disorders of genetic and environmental etiologies. Some ASD cases are syndromic: associated with clinically defined patterns of somatic abnormalities and a neurobehavioral phenotype (e.g., Fragile X syndrome). Many cases, however, are idiopathic or non-syndromic. Such disorders present themselves during the early postnatal period when language, speech, and personality start to develop. ASDs manifest by deficits in social communication and interaction, restricted and repetitive patterns of behavior across multiple contexts, sensory abnormalities across multiple modalities and comorbidities, such as epilepsy among many others. ASDs are disorders of connectivity, as synaptic dysfunction is common to both syndromic and idiopathic forms. While multiple theories have been proposed, particularly in idiopathic ASDs, none address why certain brain areas (e.g., frontotemporal) appear more vulnerable than others or identify factors that may affect phenotypic specificity. In this hypothesis article, we identify possible routes leading to, and the consequences of, altered connectivity and review the evidence of central and peripheral synaptic dysfunction in ASDs. We postulate that phenotypic specificity could arise from aberrant experience-dependent plasticity mechanisms in frontal brain areas and peripheral sensory networks and propose why the vulnerability of these areas could be part of a model to unify preexisting pathophysiological theories.

A number of clinical studies indicated an association between autoantibodies and neuropathic pain. This is supported by the observation that immunotherapies that reduce antibody levels alleviate pain in patients and suggests that autoantibodies are not a byproduct of pathology but instead important drivers of neuropathic pain. These autoantibodies can target both neuronal and nonneuronal antigens within the sensory nervous system. Possible pathogenic mechanisms include nerve damage and inflammation as well as disruption of ion channel function. Whether autoantibodies are truly causal to neuropathic pain and exactly what their prevalence is in such pain conditions are important questions that are being addressed with the use of passive transfer in preclinical models and the screening of patient sera. Such studies support the idea that autoantibodies are a mechanism to cause neuropathic pain and provide insight into the molecular components regulating pain sensitivity in a pathological setting. Therefore, this work not only will be applicable to the treatment of patients with autoantibody-mediated pain, but also will facilitate the development of therapies to treat neuropathic pain in the more general context.

Purpose of review Over the last 20 years, several neurological conditions have been identified which appear to be caused directly by autoantibodies targeting receptors, ion channels and related proteins on neuronal or glial cells. Neuroimmune interactions are now accepted contributors to chronic pain conditions. Autoantibodies might be one such cause and here we highlight their potential role in pathological pain. Recent findings Recent studies have given more weight to the idea that autoantibodies can be directly related to pain; this is suggested by the success of immunotherapy in patients and preclinical studies in animal models. For example, in complex regional pain syndrome, plasma exchange or intravenous immunoglobulins have been successful in reducing pain scores. Similarly, immunotherapies reduce autoantibody levels and pain in neuromyelitis optica and voltage-gated potassium channel complex antibody positive patients. Furthermore, animal studies show that IgG autoantibodies from patients with rheumatoid arthritis or complex regional pain syndrome can recapitulate pain phenotypes in mice. Summary There is growing evidence that some pain syndromes may be caused by autoantibodies to proteins that modify or exacerbate pain sensation. This has potentially direct therapeutic advantages for these patients and possible wider implications for sufferers of chronic pain more generally.

BACKGROUND: Neuroimmune interactions play a vital role in many of the most common pain conditions, such as arthritis. There have been many attempts to derive clinically predictive information from an individual's inflammatory response in order to gauge subsequent pain perception.OBJECTIVES: Here, we wanted to test whether this effort could be enhanced and complemented by the use of a model system which takes into account the function of not just circulating, but also tissue-resident immune cells: ultraviolet B (UVB) irradiation of the skin.METHODS: We conducted psychophysical and transcriptional analysis of hyperalgesia arising as a result of UVB-induced inflammation in patients before total knee arthroplasty (TKA, n = 23). Levels of acute postoperative pain were assessed and correlated with preoperative data.RESULTS: Cytokine and chemokine responses after UVB irradiation were found to be inversely correlated with the level of pain experienced after surgery (Spearman's ρ = -0.498).CONCLUSION: It may be possible to use this simple model to study and predict the nature of neuro-immune responses at more remote, clinically relevant sites.SIGNIFICANCE: A simple model of UVB inflammation in the skin might predict the degree of a patient's neuro-immune response and the extent of their postoperative pain after total knee arthroplasty.

Multiple lines of evidence support the notion that much if not most chronic pain is dependent on on-going peripheral activity in nociceptors. This is not to say that central changes are unimportant, only that much of the central change is supported by a peripheral drive. This begs the question of what causes this peripheral drive. In some instances, particularly in association with peripheral nerve injury, nociceptors may become spontaneously active because of alterations in ion channel function or expression. But in most cases nociceptor activity arises because of the actions of peripheral mediators released by injured or damaged tissue. Some of these mediators are well known, such as the prostanoids. Others have more recently been identified, such as nerve growth factor (NGF). However, the limited efficacy of existing analgesic therapies strongly suggests that other important pain mediators exist. Here we discuss the evidence that a family of secreted proteins, the chemokines - well known for their actions in regulating immune cell migration - also play an important role in sustaining abnormal nociceptor activity in persistent pain states.

Charcot-Marie-Tooth disease type 2D (CMT2D) is a peripheral nerve disorder caused by dominant, toxic, gain-of-function mutations in the widely expressed, housekeeping gene,GARS. The mechanisms underlying selective nerve pathology in CMT2D remain unresolved, as does the cause of the mild-to-moderate sensory involvement that distinguishes CMT2D from the allelic disorder distal spinal muscular atrophy type V. To elucidate the mechanism responsible for the underlying afferent nerve pathology, we examined the sensory nervous system in CMT2D mice. We show that the equilibrium between functional subtypes of sensory neuron in dorsal root ganglia is distorted byGarsmutations, leading to sensory defects in peripheral tissues and correlating with overall disease severity. CMT2D mice display changes in sensory behaviour concordant with the afferent imbalance, which is present at birth and non-progressive, indicating that sensory neuron identity is prenatally perturbed and that a critical developmental insult is key to the afferent pathology. This suggests that both neurodevelopmental and neurodegenerative mechanisms contribute to CMT2D pathogenesis, and thus has profound implications for the timing of future therapeutic treatments.Significance StatementCharcot-Marie-Tooth disease (CMT) is a collection of genetically diverse inherited nerve disorders with the unifying feature of peripheral neuron degeneration. The mechanisms triggering this motor and sensory nerve dysfunction remain unresolved, as does the reason for the lack of sensory pathology observed in distal hereditary motor neuropathies, which can be associated with CMT genes. To unravel the mechanisms leading to afferent deterioration, we have studied the sensory nervous system of CMT Type 2D mice. Our work indicates that the specific cellular identity of sensory nerves is perturbed in mutant mice pre-natally. CMT therefore manifests through the complex interplay between malfunctioning developmental, maturation, and survival programs, which has important ramifications for therapeutic timing.

Summary Persistent pain is a sequela of several neurological conditions with a primary immune basis, such as Guillain-Barre syndrome and multiple sclerosis. Additionally, diverse forms of injury to the peripheral or the central nervous systems—whether traumatic, metabolic, or toxic—result in substantial recruitment and activation of immune cells. This response involves the innate immune system, but evidence also exists of T-lymphocyte recruitment, and in some patient cohorts antibodies to neuronal antigens have been reported. Mediators released by immune cells, such as cytokines, sensitise nociceptive signalling in the peripheral and central nervous systems. Preclinical data suggest an immune pathogenesis of neuropathic pain, but clinical evidence of a central role of the immune system is less clear. An important challenge for the future is to establish to what extent this immune response initiates or maintains neuropathic pain in patients and thus whether it is amenable to therapy.

Background Knocking down neuronal LINGO-1 using short hairpin RNAs (shRNAs) might enhance axon regeneration in the central nervous system (CNS). Integration-deficient lentiviral vectors have great potential as a therapeutic delivery system for CNS injuries. However, recent studies have revealed that shRNAs can induce an interferon response resulting in off-target effects and cytotoxicity. Methods CNS neurones were transduced with integration-deficient lentiviral vectors in vitro. The transcriptional effect of shRNA expression was analysed using quantitative real time-polymerase chain reaction and northern blots were used to assess shRNA production. Results Integration-deficient lentiviral vectors efficiently transduced CNS neurones and knocked down LINGO-1 mRNA in vitro. However, an increase in cell death was observed when lentiviral vectors encoding an shRNA were applied or when high vector concentrations were used. We demonstrate that high doses of vector or the use of vectors encoding shRNAs can induce an up-regulation of interferon-stimulated genes (2′,5′-oligoadenylate synthase 1 and protein kinase R although not myxovirus resistance 1) and a down-regulation of off-target genes (including p75NTR and Nogo receptor 1). Furthermore, the northern blot demonstrated that these negative consequences occur even when lentiviral vectors express low levels of shRNAs. Taken together, these results may explain why neurite outgrowth was not enhanced on an inhibitory substrate following transduction with lentiviral vectors encoding an shRNA targeting LINGO-1. Conclusions These findings highlight the importance of including appropriate controls to verify silencing specificity and the requirement to check for an interferon response when conducting RNA interference experiments. However, the potential benefits that RNA interference and viral vectors offer to gene-based therapies to CNS injuries cannot be overlooked and demand further investigation. Copyright © 2012 John Wiley & Sons, Ltd.

Background Bladder pain syndrome (BPS) pathology is poorly understood. Treatment strategies are empirical, with limited efficacy, and affected patients have diminished quality of life. Objective We examined the hypothesis that inflammatory mediators within the bladder contribute to BPS pathology. Design, setting, and participants Fifteen women with BPS and 15 women with stress urinary incontinence without bladder pain were recruited from Cork University Maternity Hospital from October 2011 to October 2012. During cystoscopy, 5-mm bladder biopsies were taken and processed for gene expression analysis. The effect of the identified genes was tested in laboratory animals. Outcome measures and statistical analysis We studied the expression of 96 inflammation-related genes in diseased and healthy bladders. We measured the correlation between genes and patient clinical profiles using the Pearson correlation coefficient. Results and limitations Analysis revealed 15 differentially expressed genes, confirmed in a replication study. FGF7 and CCL21 correlated significantly with clinical outcomes. Intravesical CCL21 instillation in rats caused increased bladder excitability and increased c-fos activity in spinal cord neurons. CCL21 atypical receptor knockout mice showed significantly more c-fos upon bladder stimulation with CCL21 than wild-type littermates. There was no change in FGF7-treated animals. The variability in patient samples presented as the main limitation. We used principal component analysis to identify similarities within the patient group. Conclusions Our study identified two biologically relevant inflammatory mediators in BPS and demonstrated an increase in nociceptive signalling with CCL21. Manipulation of this ligand is a potential new therapeutic strategy for BPS. Patient summary We compared gene expression in bladder biopsies of patients with bladder pain syndrome (BPS) and controls without pain and identified two genes that were increased in BPS patients and correlated with clinical profiles. We tested the effect of these genes in laboratory animals, confirming their role in bladder pain. Manipulating these genes in BPS is a potential treatment strategy., Take Home Message Bladder pain syndrome (BPS) pathology remains poorly understood, and its treatment is difficult. We identified mediators that, from our study of their action in the bladders of laboratory animals, may be implicated in disease pathology. These mediators are unexploited therapeutic targets for BPS treatment.

Charcot-Marie-Tooth disease type 2D (CMT2D) is a peripheral nerve disorder caused by dominant, toxic, gain-of-function mutations in the widely expressed, housekeeping gene, GARS The mechanisms underlying selective nerve pathology in CMT2D remain unresolved, as does the cause of the mild-to-moderate sensory involvement that distinguishes CMT2D from the allelic disorder distal spinal muscular atrophy type V. To elucidate the mechanism responsible for the underlying afferent nerve pathology, we examined the sensory nervous system of CMT2D mice. We show that the equilibrium between functional subtypes of sensory neuron in dorsal root ganglia is distorted by Gars mutations, leading to sensory defects in peripheral tissues and correlating with overall disease severity. CMT2D mice display changes in sensory behavior concordant with the afferent imbalance, which is present at birth and nonprogressive, indicating that sensory neuron identity is prenatally perturbed and that a critical developmental insult is key to the afferent pathology. Through in vitro experiments, mutant, but not wild-type, GlyRS was shown to aberrantly interact with the Trk receptors and cause misactivation of Trk signaling, which is essential for sensory neuron differentiation and development. Together, this work suggests that both neurodevelopmental and neurodegenerative mechanisms contribute to CMT2D pathogenesis, and thus has profound implications for the timing of future therapeutic treatments.

Peripheral sensitization during inflammatory pain is mediated by a variety of endogenous proalgesic mediators including a number of oxidized lipids, some of which serve endogenous modulators of sensory TRP-channels. These lipids are eicosanoids of the arachidonic acid and linoleic acid pathway, as well as lysophophatidic acids (LPAs). However, their regulation pattern during inflammatory pain and their contribution to peripheral sensitization is still unclear. Here, we used the UVB-model for inflammatory pain to investigate alterations of lipid concentrations at the site of inflammation, the dorsal root ganglia (DRGs) as well as the spinal dorsal horn and quantified 21 lipid species from five different lipid families at the peak of inflammation 48 hours post irradiation. We found that known proinflammatory lipids as well as lipids with unknown roles in inflammatory pain to be strongly increased in the skin, whereas surprisingly little changes of lipid levels were seen in DRGs or the dorsal horn. Importantly, although there are profound differences between the number of cytochrome (CYP) genes between mice and rats, CYP-derived lipids were regulated similarly in both species. Since TRPV1 agonists such as LPA 18∶1, 9- and 13-HODE, 5- and 12-HETE were elevated in the skin, they may contribute to thermal hyperalgesia and mechanical allodynia during UVB-induced inflammatory pain. These results may explain why some studies show relatively weak analgesic effects of cyclooxygenase inhibitors in UVB-induced skin inflammation, as they do not inhibit synthesis of other proalgesic lipids such as LPA 18∶1, 9-and 13-HODE and HETEs.

Many persistent pain states (pain lasting for hours, days, or longer) are poorly treated because of the limitations of existing therapies. Analgesics such as nonsteroidal anti-inflammatory drugs and opioids often provide incomplete pain relief and prolonged use results in the development of severe side effects. Identification of the key mediators of various types of pain could improve such therapies. Here, we tested the hypothesis that hitherto unrecognized cytokines and chemokines might act as mediators in inflammatory pain. We used ultraviolet B (UVB) irradiation to induce persistent, abnormal sensitivity to pain in humans and rats. The expression of more than 90 different inflammatory mediators was measured in treated skin at the peak of UVB-induced hypersensitivity with custom-made polymerase chain reaction arrays. There was a significant positive correlation in the overall expression profiles between the two species. The expression of several genes [interleukin-1β (IL-1β), IL-6, and cyclooxygenase-2 (COX-2)], previously shown to contribute to pain hypersensitivity, was significantly increased after UVB exposure, and there was dysregulation of several chemokines (CCL2, CCL3, CCL4, CCL7, CCL11, CXCL1, CXCL2, CXCL4, CXCL7, and CXCL8). Among the genes measured, CXCL5 was induced to the greatest extent by UVB treatment in human skin; when injected into the skin of rats, CXCL5 recapitulated the mechanical hypersensitivity caused by UVB irradiation. This hypersensitivity was associated with the infiltration of neutrophils and macrophages into the dermis, and neutralizing the effects of CXCL5 attenuated the abnormal pain-like behavior. Our findings demonstrate that the chemokine CXCL5 is a peripheral mediator of UVB-induced inflammatory pain, likely in humans as well as rats.