Sensitization Of Peripheral Nociceptors Research Articles

Inflammation in Older Poles with Localized and Widespread Chronic Pain—Results from a Population-Based PolSenior Study

Background: Inflammation leads to a decrease in the excitation threshold and the sensitization of peripheral nociceptors. However, little is known about the effect of inflammation on the sensing of regional (CRegP) and widespread chronic pain (CWP) in older adults. This analysis aimed to characterize the prevalence and associates of both types of chronic pain in a population-based cohort. Methods: Our analysis was based on the Polish nationwide PolSenior study database. We excluded participants with moderate-to-severe dementia. Respondents answered questions concerning the occurrence of pain in 10 regions. CWP was defined as chronic pain present in the axial region (neck, upper back, lower back) and any part of both the lower (lower leg, hip, knee, foot) and upper (shoulder, hand) extremities. Inflammatory status was divided into three subgroups: no inflammation (CRP < 3 mg/dL), mild inflammation (CPR 3–10 mg/dL and IL-6 < 10 ng/mL), and significant inflammation (CRP ≥ 10 mg/dL or IL-6 ≥ 10 ng/mL). Results: CRegP was more frequent (33.9%) than CWP (8.8%). The occurrence of CWP was more frequent in subgroups with significant inflammation than in both subgroups with mild or no inflammation (11.4% vs. both 8.4%). Women (OR 3.67; 95% CI: 2.58–5.21) and subjects with major depression symptoms were more likely to experience CWP (OR 2.85; 95% CI: 1.68–4.82), while, malnourished participants were more likely to report CRegP (OR 2.00; 95% CI: 1.52–2.62). Conclusions: Significant inflammation is associated with increased occurrence of CWP in older adults. Female sex and major depression are the most significant associates of CWP, while malnutrition is the most significant associate of CRegP.

Nociceptors are functionally male or female: from mouse to monkey to man.

The prevalence of many pain conditions often differs between sexes. In addition to such quantitative distinctions, sexual dimorphism may also be qualitative reflecting differences in mechanisms that promote pain in men and women. A major factor that influences the likelihood of pain perception is the threshold for activation of nociceptors. Peripheral nociceptor sensitization has been demonstrated to be clinically relevant in many pain conditions. Whether peripheral nociceptor sensitization can occur in a sexually dimorphic fashion, however, has not been extensively studied. To address this fundamental knowledge gap, we used patch clamp electrophysiology to evaluate the excitability of dorsal root ganglion neurones from male or female rodents, non-human primates, and humans following exposure to putative sensitizing agents. Previous studies from our laboratory, and others, have shown that prolactin promotes female-selective pain responses in rodents. Consistent with these observations, dorsal root ganglion neurones from female, but not male, mice were selectively sensitized by exposure to prolactin. The sensitizing action of prolactin was also confirmed in dorsal root ganglion neurones from a female macaque monkey. Critically, neurones recovered from female, but not male, human donors were also selectively sensitized by prolactin. In the course of studies of sleep and pain, we unexpectedly observed that an orexin antagonist could normalize pain responses in male animals. We found that orexin B produced sensitization of male, but not female, mouse, macaque, and human dorsal root ganglion neurones. Consistent with functional responses, increased prolactin receptor and orexin receptor 2 expression was observed in female and male mouse dorsal root ganglia, respectively. Immunohistochemical interrogation of cultured human sensory neurones and whole dorsal root ganglia also suggested increased prolactin receptor expression in females and orexin receptor 2 expression in males. These data reveal a functional double dissociation of nociceptor sensitization by sex, which is conserved across species and is likely directly relevant to human pain conditions. To our knowledge, this is the first demonstration of functional sexual dimorphism in human sensory neurones. Patient sex is currently not a common consideration for the choice of pain therapy. Precision medicine, based on patient sex could improve therapeutic outcomes by selectively targeting mechanisms promoting pain in women or men. Additional implications of these findings are that the design of clinical trials for pain therapies should consider the proportions of male or female patients enrolled. Lastly, re-examination of selected past failed clinical trials with subgroup analysis by sex may be warranted.

MNK1 and MNK2 Expression in the Human Dorsal Root and Trigeminal Ganglion

Mitogen activated protein kinase interacting kinases (MNK) 1 and 2 are serine/threonine protein kinases that play an important role in translation of mRNAs through their phosphorylation of the RNA 5′-cap binding protein, eukaryotic translation initiation factor (eIF) 4E. These kinases are downstream targets for mitogen activated protein kinases (MAPKs), extracellular activity regulated protein kinase (ERK) and p38. MNKs have been implicated in the sensitization of peripheral nociceptors of the dorsal root and trigeminal ganglion (DRG and TG) using transgenic mouse lines and through the use of specific inhibitors of MNK1 and MNK2. While specific knockout of the Mknk1 gene suggests that it is the key isoform for regulation of nociceptor excitability and nociceptive behaviors in mice, both MKNK1 and MKNK2 genes are expressed in the DRG and TG of mice and humans based on RNA sequencing experiments. Single cell sequencing in mice suggests that Mknk1 and Mknk2 may be expressed in different populations of nociceptors. We sought to characterize mRNA expression in human DRG and TG (N = 3 ganglia for both DRG and TG) for both MNK1 and MNK2. Our results show that both genes are expressed by nearly all neurons in both human ganglia with expression in other cell types as well. Our findings provide evidence that MNK1 and MNK2 are expressed by human nociceptors of males and females and suggest that efforts to pharmacologically target MNKs for pain would likely be translatable due its conserved expression in both species.

Identification and Characterization of a New Splicing Variant of Prokineticin 2.

Prokineticin 2 (PROK2) is a secreted bioactive peptide that regulates a variety of biological responses via two GPCRs, the prokineticin receptors (PROKRs). The aim of this study was to characterize a new alternatively spliced product of the prok2 gene consisting of four exons. The 40-amino acid peptide, designated PROK2C, is encoded by exon 1 and exon 4, and its expression was detected in the hippocampus and spinal cord of mice. PROK2C was expressed in a heterologous system, Pichia pastoris, and its binding specificity to the amino-terminal regions of PROKR1 and PROKR2 was investigated by GST pull-down experiments. In addition, the introduction of the unnatural amino acid p-benzoyl-L-phenylalanine using amber codon suppression technology demonstrated the role of tryptophan at position 212 of PROKR2 for PROK2C binding by photoactivatable cross-linking. The functional significance of this new isoform was determined in vivo by nociceptive experiments, which showed that PROK2C elicits strong sensitization of peripheral nociceptors to painful stimuli. In order to analyze the induction of PROK2C signal transduction, STAT3 and ERK phosphorylation levels were determined in mammalian CHO cells expressing PROKR1 and PROKR2. Our data show by in vivo and in vitro experiments that PROK2C can bind and activate both prokineticin receptors.

The efficacy of botulinum toxin A treatment for tension-type or cervicogenic headache: a systematic review and meta-analysis of randomized, placebo-controlled trials.

The pathogeneses of chronic tension-type headache (CTTH) and cervicogenic headache (CEH) are not well established. Peripheral activation or sensitization of myofascial nociceptors is suggested as a potential mechanism and injections of botulinum toxin (BONTA) have thus been used in the treatment for both headache conditions. BONTA inhibits the release of acetylcholine at the neuromuscular junction and inhibits contraction of skeletal muscles. If the pain is precipitated by increased tone in cervical muscles, local injections of BONTA could represent a prophylactic measure. However, the treatment is still controversial, and a thorough assessment of the current evidence is required. This review aims to assess the evidence of BONTA injection as a prophylactic treatment for CTTH and CEH by reviewing and examining the quality of placebo-controlled, randomized trials. Data sources: we searched in the following databases: PubMed (including Medline), Embase, Cochrane Central register of Controlled Trials, Cinahl, Amed, SCOPUS and Google Scholar including other repository sources. Both MeSH and free keywords were used in conducting the systematic search in the databases. The search covered publications from the root of the databases to November 2020. The review included RCTs, comparing single treatment of BONTA with placebo on patients with CTTH or CEH above 18 years of age, by measuring pain severity/relief or headache frequency. The following data were extracted: year of publication, country, setting, trial design, number of participants, injection procedure, BONTA dosages, and clinical outcome measures. To assess validity and quality, and risk of bias, the Oxford Pain Validity Scale, Modified Jadad Scale, last version of Cochrane Collaboration's tool for assessing risk of bias (RoB 2), and the CONSORT 2010 Checklist were used. The trials were assessed, and quality scored independently by two of the reviewers. A quantitative synthesis and meta-analyses of headache frequency and intensity were performed. We extracted 16 trials, 12 on prophylactic BONTA treatment for CTTH and four on CEH. Of these 12 trials (8 on CTTH and 4 on CEH) were included in the quantitative synthesis. A majority of the trials found no significant difference on the primary outcome measure when BONTA treatment was compared with placebo. Three "positive" trials, reporting significant difference in favor of BONTA treatment, but two of these were hampered by low validity and quality scores and high risk of bias. There is no clear clinical evidence supporting prophylactic treatment with BONTA for CTTH or CEH.

Locus revealed: Painlessness via loss of NaV1.7 at central terminals of sensory neurons

How genetic loss of the sodium channel NaV1.7 results in painlessness is puzzling. MacDonald etal. (2021) demonstrate that instead of impairing peripheral excitability, NaV1.7 channels at central terminals of pain-sensing afferents play a pivotal role in the balance between pain and analgesia.

Mechanistic insights into the role of the chemokine CCL2/CCR2 axis in dorsal root ganglia to peripheral inflammation and pain hypersensitivity

BackgroundPain is reported as the leading cause of disability in the common forms of inflammatory arthritis conditions. Acting as a key player in nociceptive processing, neuroinflammation, and neuron-glia communication, the chemokine CCL2/CCR2 axis holds great promise for controlling chronic painful arthritis. Here, we investigated how the CCL2/CCR2 system in the dorsal root ganglion (DRG) contributes to the peripheral inflammatory pain sensitization.MethodsRepeated intrathecal (i.t.) administration of the CCR2 antagonist, INCB3344 was tested for its ability to reverse the nociceptive-related behaviors in the tonic formalin and complete Freund’s adjuvant (CFA) inflammatory models. We further determined by qPCR the expression of CCL2/CCR2, SP and CGRP in DRG neurons from CFA-treated rats. Using DRG explants, acutely dissociated primary sensory neurons and calcium mobilization assay, we also assessed the release of CCL2 and sensitization of nociceptors. Finally, we examined by immunohistochemistry following nerve ligation the axonal transport of CCL2, SP, and CGRP from the sciatic nerve of CFA-treated rats.ResultsWe first found that CFA-induced paw edema provoked an increase in CCL2/CCR2 and SP expression in ipsilateral DRGs, which was decreased after INCB3344 treatment. This upregulation in pronociceptive neuromodulators was accompanied by an enhanced nociceptive neuron excitability on days 3 and 10 post-CFA, as revealed by the CCR2-dependent increase in intracellular calcium mobilization following CCL2 stimulation. In DRG explants, we further demonstrated that the release of CCL2 was increased following peripheral inflammation. Finally, the excitation of nociceptors following peripheral inflammation stimulated the anterograde transport of SP at their peripheral nerve terminals. Importantly, blockade of CCR2 reduced sensory neuron excitability by limiting the calcium mobilization and subsequently decreased peripheral transport of SP towards the periphery. Finally, pharmacological inhibition of CCR2 reversed the pronociceptive action of CCL2 in rats receiving formalin injection and significantly reduced the neurogenic inflammation as well as the stimuli-evoked and movement-evoked nociceptive behaviors in CFA-treated rats.ConclusionsOur results provide significant mechanistic insights into the role of CCL2/CCR2 within the DRG in the development of peripheral inflammation, nociceptor sensitization, and pain hypersensitivity. We further unveil the therapeutic potential of targeting CCR2 for the treatment of painful inflammatory disorders.

Sensory satellite glial Gq-GPCR activation alleviates inflammatory pain via peripheral adenosine 1 receptor activation

Glial fibrillary acidic protein expressing (GFAP+) glia modulate nociceptive neuronal activity in both the peripheral nervous system (PNS) and the central nervous system (CNS). Resident GFAP+ glia in dorsal root ganglia (DRG) known as satellite glial cells (SGCs) potentiate neuronal activity by releasing pro-inflammatory cytokines and neuroactive compounds. In this study, we tested the hypothesis that SGC Gq-coupled receptor (Gq-GPCR) signaling modulates pain sensitivity in vivo using Gfap-hM3Dq mice. Complete Freund’s adjuvant (CFA) was used to induce inflammatory pain, and mechanical sensitivity and thermal sensitivity were used to assess the neuromodulatory effect of glial Gq-GPCR activation in awake mice. Pharmacogenetic activation of Gq-GPCR signaling in sensory SGCs decreased heat-induced nociceptive responses and reversed inflammation-induced mechanical allodynia via peripheral adenosine A1 receptor activation. These data reveal a previously unexplored role of sensory SGCs in decreasing afferent excitability. The identified molecular mechanism underlying the analgesic role of SGCs offers new approaches for reversing peripheral nociceptive sensitization.

Trigger point dry needling for the treatment of myofascial pain syndrome: current perspectives within a pain neuroscience paradigm.

Myofascial pain syndrome is a pain condition characterized by the presence of trigger points. Current evidence, mostly experimental studies, clearly supports a role of trigger points on peripheral and central sensitization since they are able to contribute to sensitization of peripheral nociceptors, spinal dorsal horn neurons, and the brainstem. Several interventions are proposed for treating trigger points, dry needling being one of the most commonly used by clinicians. There is no consensus on the clinical application of trigger point dry needling: some authors propose that local twitch responses should be elicited during the needling intervention to be effective, whereas others do not. The application of trigger point dry needling is able to reduce the excitability of the central nervous system by reducing peripheral nociception associated to the trigger point, by reducing dorsal horn neuron activity, and by modulating pain-related brainstem areas. However, the effects are mainly observed in the short-term, and effect sizes are moderateto small. Therefore, the current review proposes that the application of trigger point dry needling should be integrated into current pain neuroscience paradigm by combining its application with pain neuroscience education, graded exercise and manual therapy. Additionally, patient’s expectations, beliefs, previous experiences and patient–clinician interaction should be considered when integrating trigger point dry needling into a comprehensive treatment approach.

Overexpression of P2X3 and P2X7 Receptors and TRPV1 Channels in Adrenomedullary Chromaffin Cells in a Rat Model of Neuropathic Pain.

We have tested the hypothesis that neuropathic pain acting as a stressor drives functional plasticity in the sympathoadrenal system. The relation between neuropathic pain and adrenal medulla function was studied with behavioral, immunohistochemical and electrophysiological techniques in rats subjected to chronic constriction injury of the sciatic nerve. In slices of the adrenal gland from neuropathic animals, we have evidenced increased cholinergic innervation and spontaneous synaptic activity at the splanchnic nerve–chromaffin cell junction. Likewise, adrenomedullary chromaffin cells displayed enlarged acetylcholine-evoked currents with greater sensitivity to α-conotoxin RgIA, a selective blocker of α9 subunit-containing nicotinic acetylcholine receptors, as well as increased exocytosis triggered by voltage-activated Ca2+ entry. Altogether, these adaptations are expected to facilitate catecholamine output into the bloodstream. Last, but most intriguing, functional and immunohistochemical data indicate that P2X3 and P2X7 purinergic receptors and transient receptor potential vanilloid-1 (TRPV1) channels are overexpressed in chromaffin cells from neuropathic animals. These latter observations are reminiscent of molecular changes characteristic of peripheral sensitization of nociceptors following the lesion of a peripheral nerve, and suggest that similar phenomena can occur in other tissues, potentially contributing to behavioral manifestations of neuropathic pain.

Inflammation and osteoarthritis-related pain

Osteoarthritis (OA) is one of the major causes of chronic pain. Although OA has long been considered anon-inflammatory "wear and tear" disease leading to loss of articular cartilage, recent findings provide convincing evidence that inflammatory mechanisms play apivotal role in the pathophysiology of OA. In OA mononuclear cells (e. g. T‑cells and macrophages) infiltrate the synovial membrane and the levels of pro-inflammatory cytokines in peripheral blood and synovial fluid samples are elevated. Increased release of inflammatory mediators including interleukin (IL) IL-1β, IL-6, IL-8, IL-15 und tumor necrosis factoralpha (TNF‑α) induces the expression of proteolytic enzymes such as matrix metalloproteinases resulting in cartilage breakdown. Molecular and cellular interactions between the immune and nervous system are also involved in the development of OA-related pain. Inflammatory mediators including IL-6 und TNF‑α lead to peripheral sensitization of joint nociceptors and growth factors (e. g. NGF) trigger the expression of TRPV1 channels in primary afferents. Moreover, neuropeptides reduce the threshold of nociceptors of OA joints. The current review highlights the role of inflammatory mechanisms in OA-induced joint pain considering clinical signs of inflammation and major inflammatory pathways.

PK2β ligand, a splice variant of prokineticin 2, is able to modulate and drive signaling through PKR1 receptor

Prokineticin-2 (PK2) is a secreted bioactive peptide that signals through two GPCRs, the prokineticin receptors (PKRs), and regulates a variety of biological processes including angiogenesis, immunity and nociception. The PK2 primary transcript has two alternative splice variants, PK2 and PK2L (a Long form) which is cleaved in an active peptide, named PK2β that preferentially binds to PKR1 receptor.The aim of this study was to characterize the PK2β.Using different Saccharomyces cerevisiae strains, we examined the specificity of PKR1 and PKR2 G-protein coupling following PK2β binding. Data obtained in yeast confirmed that PK2 binds both receptors, inducing a comparable response throughout a promiscuous coupling of G protein subtypes. Conversely, we demonstrated, for the first time, that PK2β preferentially binding to PKR1, activates a signaling cascade that not depends on Gαi/o coupling. The binding specificity of PK2β for PKR1 was evaluated by the analysis of PKR mutant in yeast and GST pull-down experiments, suggesting an important role of PKR1 amino-terminal region. We also evaluated the ability of PK2β to differentially activate PKR1 and/or PKR2 by in vivo nociceptive experiments and we showed that PK2β induces intense sensitization of peripheral nociceptors to painful stimuli through the activation of PKR1. To analyze PK2β-induced signal transduction, we demonstrated the inability of PK2β to induce STAT3 protein phosphorylation in organotypic primary explants from mice Dorsal Root Ganglion (DRG), an important pain station. The control of the concentration ratio between PK2β and PK2 could be one of the keys to allow the specificity of the cell response of prokineticin signaling pathway.

Human visceral nociception: findings from translational studies in human tissue.

Peripheral sensitization of nociceptors during disease has long been recognized as a leading cause of inflammatory pain. However, a growing body of data generated over the last decade has led to the increased understanding that peripheral sensitization is also an important mechanism driving abdominal pain in highly prevalent functional bowel disorders, in particular, irritable bowel syndrome (IBS). As such, the development of drugs that target pain-sensing nerves innervating the bowel has the potential to be a successful analgesic strategy for the treatment of abdominal pain in both organic and functional gastrointestinal diseases. Despite the success of recent peripherally restricted approaches for the treatment of IBS, not all drugs that have shown efficacy in animal models of visceral pain have reduced pain end points in clinical trials of IBS patients, suggesting innate differences in the mechanisms of pain processing between rodents and humans and, in particular, how we model disease states. To address this gap in our understanding of peripheral nociception from the viscera and the body in general, several groups have developed experimental systems to study nociception in isolated human tissue and neurons, the findings of which we discuss in this review. Studies of human tissue identify a repertoire of human primary afferent subtypes comparable to rodent models including a nociceptor population, the targeting of which will shape future analgesic development efforts. Detailed mechanistic studies in human sensory neurons combined with unbiased RNA-sequencing approaches have revealed fundamental differences in not only receptor/channel expression but also peripheral pain pathways.

Joint disease in haemophilia: Pathophysiology, pain and imaging.

Haemarthroses cause major morbidity in patients with haemophilia. Blood has devastating effects on all joint components, resulting in synovitis, osteochondral degeneration and ultimately end-stage haemophilic arthropathy. Key players in this process are iron and inflammation. Preventing joint bleeds is of utmost importance to maintain joint health as targeted therapies directed against blood-induced inflammation and iron-mediated processes are lacking. Joint bleeds result in acute pain as well as chronic pain due to synovitis or arthropathy. Acute pain originates from nociceptors activated by tissue damage. In chronic inflammation, central and peripheral sensitization of nociceptors might occur resulting in chronic pain. This also triggers a series of brain disorders such as emotional fear, anxiety, mood depression and impairment of cognitive functions. Treatment of haemophilia-related pain not only consists of analgesics, but also of exercise, education and in selected cases antidepressants and anticonvulsants. For objective assessment of joint structural outcome and detecting earlier changes of haemophilic arthropathy, both ultrasound (US) and magnetic resonance (MR) imaging have shown valuable. Both can be considered equally able to reveal signs of disease activity. MR imaging is able to visualize haemosiderin deposition and is more comprehensive in depicting osteochondral changes. Disadvantages of MR imaging are the duration of the examination, evaluation of a single joint at a time, costs and may require sedation, and it may need intraarticular contrast injection to depict initial osteochondral changes with accuracy. As such, US is a more useful screening tool and can be used for repeated follow-up examinations.

Targeting pain at its source in sickle cell disease.

Sickle cell disease (SCD) is a genetic disorder associated with hemolytic anemia, end-organ damage, reduced survival, and pain. One of the unique features of SCD is recurrent and unpredictable episodes of acute pain due to vasoocclusive crisis requiring hospitalization. Additionally, patients with SCD often develop chronic persistent pain. Currently, sickle cell pain is treated with opioids, an approach limited by adverse effects. Because pain can start at infancy and continue throughout life, preventing the genesis of pain may be relatively better than treating the pain once it has been evoked. Therefore, we provide insights into the cellular and molecular mechanisms of sickle cell pain that contribute to the activation of the somatosensory system in the peripheral and central nervous systems. These mechanisms include mast cell activation and neurogenic inflammation, peripheral nociceptor sensitization, maladaptation of spinal signals, central sensitization, and modulation of neural circuits in the brain. In this review, we describe potential preventive/therapeutic targets and their targeting with novel pharmacologic and/or integrative approaches to ameliorate sickle cell pain.

Neuronal Transforming Growth Factor beta Signaling via SMAD3 Contributes to Pain in Animal Models of Chronic Pancreatitis

Chronic pancreatitis (CP) is characterized by pancreatic inflammation and fibrosis, associated with increased pancreatic expression of transforming growth factor beta (TGFB). It is not clear how these might contribute to pain. We investigated whether TGFB signaling via SMAD induces sensitization of pancreatic sensory neurons to increase nociception. CP was induced in Sprague-Dawley rats by infusion of trinitrobenzene sulfonic acid; some rats were given intrathecal infusions of TGFB1. CP was induced in control mice by administration of cerulein; we also studied β1glo/Ptf1acre-ER mice, which on induction overexpress TGFB1 in pancreatic acinar cells, and TGFBr1f/f-CGRPcreER mice, which have inducible disruption of TGFBr1 in calcitonin gene-related peptide-positive neurons. Dominant negative forms of human TGFBR2 and SMAD3 were overexpressed from viral vectors in rat pancreas. Some rats were given the SMAD3 inhibitors SIS3 or halofuginone. After induction of CP, mice were analyzed for pain in behavior tests or electrophysiologic studies of sensory neurons. Pancreatic nociceptor excitability was examined by patch-clamp techniques and nociception was measured by Von Frey Filament tests for referred somatic hyperalgesia and behavioral responses to pancreatic electrical stimulation. Pancreata were collected from mice and rats and analyzed histologically and by enzyme-linked immunosorbent assay and immunohistochemistry. Overexpression of TGFB in pancreatic acinar cells of mice and infusion of TGFB1 into rats resulted in sensory neuron hyperexcitability, SMAD3 activation, and increased nociception. This was accompanied by a reduction in the transient A-type current in pancreas-specific sensory neurons in rats, a characteristic of nociceptive sensitization in animal models of CP. Conversely, pancreata from TGFBr1f/f-CGRPcreER mice, rats with pancreatic expression of dominant negative forms of human TGFBR2 or SMAD3, and rats given small molecule inhibitors of SMAD3 had attenuated neuronal sensitization and pain behavior following induction of CP. In contrast to findings from peripheral administration of TGFB1, intrathecal infusion of TGFB1 reduced hyperalgesia in rats with CP. In pancreata of mice and rats, TGFB promotes peripheral nociceptive sensitization via a direct effect on primary sensory neurons mediated by intra-neuronal SMAD3. This is distinct from the central nervous system, where TGFB reduces nociception. These results provide an explanation for the link between fibrosis and pain in patients with CP. This signaling pathway might be targeted therapeutically to reduce pain in patients with CP.

Parathyroid Hormone-Related Peptide Elicits Peripheral TRPV1-dependent Mechanical Hypersensitivity.

Bone metastasis in breast, prostate and lung cancers often leads to chronic pain, which is poorly managed by existing analgesics. The neurobiological mechanisms that underlie chronic pain associated with bone-metastasized cancers are not well understood, but sensitization of peripheral nociceptors by tumor microenvironment factors has been demonstrated to be important. Parathyroid hormone-related peptide (PTHrP) is highly expressed in bone-metastasized breast and prostate cancers, and is critical to growth and proliferation of these tumors in the bone tumor microenvironment. Previous studies have suggested that PTHrP could sensitize nociceptive sensory neurons, resulting in peripheral pain hypersensitivity. In this study, we found that PTHrP induces both heat and mechanical hypersensitivity, that are dependent on the pain-transducing transient receptor potential channel family vanilloid, member-1 (TRPV1), but not the mechano-transducing TRPV4 and TRPA1 ion channels. Functional ratiometric Ca2+ imaging and voltage-clamp electrophysiological analysis of cultured mouse DRG neurons show significant potentiation of TRPV1, but not TRPA1 or TRPV4 channel activation by PTHrP. Interestingly, PTHrP exposure led to the slow and sustained activation of TRPV1, in the absence of any exogenous channel agonist, and is dependent on the expression of the type-1 parathyroid hormone receptor (PTH1), as well as on downstream phosphorylation of the channel by protein kinase C (PKC). Accordingly, local administration of specific small-molecule antagonists of TRPV1 to mouse hindpaws after the development of PTHrP-induced mechanical hypersensitivity led to its significant attenuation. Collectively, our findings suggest that PTHrP/PTH1-mediated flow activation of TRPV1 channel contributes at least in part to the development and maintenance of peripheral mechanical pain hypersensitivity, and could therefore constitute a mechanism for nociceptor sensitization in the context of metastatic bone cancer pain.

The Absence of Sensory Axon Bifurcation Affects Nociception and Termination Fields of Afferents in the Spinal Cord.

A cGMP signaling cascade composed of C-type natriuretic peptide, the guanylyl cyclase receptor Npr2 and cGMP-dependent protein kinase I (cGKI) controls the bifurcation of sensory axons upon entering the spinal cord during embryonic development. However, the impact of axon bifurcation on sensory processing in adulthood remains poorly understood. To investigate the functional consequences of impaired axon bifurcation during adult stages we generated conditional mouse mutants of Npr2 and cGKI (Npr2fl/fl;Wnt1Cre and cGKIKO/fl;Wnt1Cre) that lack sensory axon bifurcation in the absence of additional phenotypes observed in the global knockout mice. Cholera toxin labeling in digits of the hind paw demonstrated an altered shape of sensory neuron termination fields in the spinal cord of conditional Npr2 mouse mutants. Behavioral testing of both sexes indicated that noxious heat sensation and nociception induced by chemical irritants are impaired in the mutants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are not affected. Recordings from C-fiber nociceptors in the hind limb skin showed that Npr2 function was not required to maintain normal heat sensitivity of peripheral nociceptors. Thus, the altered behavioral responses to noxious heat found in Npr2fl/fl;Wnt1Cre mice is not due to an impaired C-fiber function. Overall, these data point to a critical role of axonal bifurcation for the processing of pain induced by heat or chemical stimuli.

Zinc Inhibits TRPV1 to Alleviate Chemotherapy-Induced Neuropathic Pain.

Zinc is a transition metal that has a long history of use as an anti-inflammatory agent. It also soothes pain sensations in a number of animal models. However, the effects and mechanisms of zinc on chemotherapy-induced peripheral neuropathy remain unknown. Here we show that locally injected zinc markedly reduces neuropathic pain in male and female mice induced by paclitaxel, a chemotherapy drug, in a TRPV1-dependent manner. Extracellularly applied zinc also inhibits the function of TRPV1 expressed in HEK293 cells and mouse DRG neurons, which requires the presence of zinc-permeable TRPA1 to mediate entry of zinc into the cytoplasm. Moreover, TRPA1 is required for zinc-induced inhibition of TRPV1-mediated acute nociception. Unexpectedly, zinc transporters, but not TRPA1, are required for zinc-induced inhibition of TRPV1-dependent chronic neuropathic pain produced by paclitaxel. Together, our study demonstrates a novel mechanism underlying the analgesic effect of zinc on paclitaxel-induced neuropathic pain that relies on the function of TRPV1.SIGNIFICANCE STATEMENT The chemotherapy-induced peripheral neuropathy is a major limiting factor affecting the chemotherapy patients. There is no effective treatment available currently. We demonstrate that zinc prevents paclitaxel-induced mechanical hypersensitivity via inhibiting the TRPV1 channel, which is involved in the sensitization of peripheral nociceptors in chemotherapy. Zinc transporters in DRG neurons are required for the entry of zinc into the intracellular side, where it inhibits TRPV1. Our study provides insight into the mechanism underlying the pain-soothing effect of zinc and suggests that zinc could be developed to therapeutics for the treatment of chemotherapy-induced peripheral neuropathy.

Tetrandrine Alleviates Nociception in a Rat Model of Migraine via Suppressing S100B and p-ERK Activation in Satellite Glial Cells of the Trigeminal Ganglia.

Sensitization and activation of the trigeminal ganglia have been implicated in the pathology of migraine. Satellite glial cells (SGCs), a specialized type of glial cells that ensheathe trigeminal neurons, may be critical for peripheral nociceptive sensitization. Tetrandrine (TET), an alkaloid extracted from a traditional Chinese herb, exerts an inhibitory effect on glial activation in vitro and has been used in various neurologic diseases. The current study investigated the effect of TET on nitroglycerin (NTG)-induced trigeminal sensitization and examined potential signaling pathways related to SGC activation in the model of migraine. We measured trigeminal nociceptive thresholds using electronic von Frey rigid tips before and after NTG injection in control rats and rats pretreated with TET, while expression and subcellular location of the inflammatory mediators S100B and activated phosphorylation extracellular signal-regulated kinase (p-ERK) were measured using real-time quantitative polymerase chain reaction, Western blotting, and double immunofluorescence staining. Pretreatment with TET caused a dose-dependent reversal of the trigeminal nociceptive hypersensitivity induced by NTG. In addition, TET pretreatment blocked the activation of S100B and p-ERK in trigeminal ganglion SGCs of NTG-treated rats. Reduced p-ERK activity can suppress the inflammation that leads to hyperexcitability of trigeminal ganglion neurons. Administration of TET may therefore be a safe and effective therapeutic treatment for the hyperalgesic symptoms of migraine.