Induction of thermal and mechanical hypersensitivity by parathyroid hormone-related peptide through upregulation of TRPV1 function and trafficking

Decoding migraine disorders: parathyroid hormone-related peptide receptors as key genetic drivers

Migraine is a complex neurological disorder, and the most common migraine categories are migraine with aura and without aura. The higher prevalence of migraine in related individuals compared to the general population indicates a potential genetic predisposition; however, gene expression, which is influenced by both genetic and environmental factors, can also be a major factor in the migraine susceptibility. Given the high number of Portuguese migraine patients whose diagnosis and treatment have not yet been well established, we decided to carry out a whole transcriptome analysis within a migraine Portuguese cohort. This study aims to identify potential biomarkers that could contribute to improved migraine therapy. We performed total RNA sequencing on whole blood samples from 15 migraine patients and 12 age-matched controls. Differential expression analysis and gene set enrichment analysis were performed in different migraine subgroups. Finally, we performed the protein-protein interaction networks of differentially expressed genes. Gene set enrichment analysis comparing migraine patients with controls highlighted upregulated pathways linked to metabolism, and downregulated immuno-inflammatory pathways. Moreover, the groups of female migraine patients and female migraine without aura patients emphasized significant upregulated pathways, including G protein-coupled receptors signalling pathways, when compared with female controls. Interestingly, we found two important differentially expressed genes related to parathyroid hormone: PTH1R and PTH2. PTH1R was upregulated in female migraine without aura versus female controls, while PTH2 was both upregulated between female migraine patients and female controls, as well as between female migraine without aura and controls. Here, we show, for the first time, the involvement of parathyroid hormone receptors and their associated gene expression patterns in female migraine patients. These molecules stand out as sturdy and promising biomarkers for innovative therapeutic in female migraine patients.

Caldendrin Is a Repressor of PIEZO2 Channels and Touch Sensation in Mice

The sense of touch is crucial for cognitive, emotional, and social development and relies on mechanically activated (MA) ion channels that transduce force into an electrical signal. Despite advances in the molecular characterization of these channels, the physiological factors that control their activity are poorly understood. Here, we used behavioral assays, electrophysiological recordings, and various mouse strains (males and females analyzed separately) to investigate the role of the calmodulin-like Ca2+ sensor, caldendrin, as a key regulator of MA channels and their roles in touch sensation. In mice lacking caldendrin (Cabp1 KO), heightened responses to tactile stimuli correlate with enlarged MA currents with lower mechanical thresholds in dorsal root ganglion neurons (DRGNs). The expression pattern of caldendrin in the DRG parallels that of the major MA channel required for touch sensation, PIEZO2. In transfected cells, caldendrin interacts with and inhibits the activity of PIEZO2 in a manner that requires an alternatively spliced sequence in the N-terminal domain of caldendrin. Moreover, targeted genetic deletion of caldendrin in Piezo2-expressing DRGNs phenocopies the tactile hypersensitivity of complete Cabp1 KO mice. We conclude that caldendrin is an endogenous repressor of PIEZO2 channels and their contributions to touch sensation in DRGNs.

Role of Src kinase in regulating protein kinase C mediated phosphorylation of TRPV1

BACKGROUND

Transient receptor potential vanilloid-1 (TRPV1), activated by heat, acidic pH, endogenous vanilloids and capsaicin, is essential for thermal hyperalgesia. Under inflammatory conditions, phosphorylation of TRPV1 by protein kinase C (PKC) can sensitize the channel and decrease the activation threshold. Src kinase also phosphorylates TRPV1, promoting channel trafficking to the plasma membrane. These post-translational modifications are important for several chronic pain conditions. This study presents a previously undescribed relationship between Src and PKC phosphorylation of TRPV1, influencing the thermal hypersensitivity associated with TRPV1 activation.

METHODS

We assessed TRPV1 channel activity using intracellular calcium imaging and patch-clamp electrophysiology in mouse dorsal root ganglion cultures. Additionally, we used behavioural experiments to evaluate plantar thermal sensitivity following intraplantar injections of activators of known modulators of TRPV1 with and without an Src antagonist.

RESULTS

Using calcium imaging and patch-clamp techniques, we demonstrated that pharmacological inhibition of Src kinase or mutation of the Src phosphorylation site on TRPV1 prevented PKC but not PKA-mediated sensitization of TRPV1 in vitro. We found that intraplantar injection of the PKC activator phorbol 12-myristate 13-acetate (PMA) or bradykinin produces thermal hypersensitivity that can be attenuated by pharmacological inhibition of Src. Additionally, complete Freund's Adjuvant (CFA)-induced inflammatory hypersensitivity could also be attenuated by local Src kinase inhibition.

CONCLUSIONS

Our data demonstrate that Src phosphorylation is critical for PKC-mediated sensitization of TRPV1. Further, in a model of inflammatory pain, CFA, Src kinase inhibition could reduce thermal hypersensitivity. Targeting of Src kinase may have analgesic benefits in inflammatory pain conditions.

SIGNIFICANCE

Src kinase-mediated phosphorylation of TRPV1 is a critical regulator of the PKC-induced sensitization induced by multiple inflammatory mediators. This suggest a new regulatory mechanism governing TRPV1 function and a potential therapeutic target for inflammatory type pain, including cancer pain where Src antagonists are currently utilized.

Anterior insular cortex mediates hyperalgesia induced by chronic pancreatitis in rats

Central sensitization plays a pivotal role in the maintenance of chronic pain induced by chronic pancreatitis (CP), but cortical modulation of painful CP remains elusive. This study was designed to examine the role of anterior insular cortex (aIC) in the pathogenesis of hyperalgesia in a rat model of CP. CP was induced by intraductal administration of trinitrobenzene sulfonic acid (TNBS). Abdomen hyperalgesia and anxiety were assessed by von Frey filament and open field tests, respectively. Two weeks after surgery, the activation of aIC was indicated by FOS immunohistochemical staining and electrophysiological recordings. Expressions of VGluT1, NMDAR subunit NR2B and AMPAR subunit GluR1 were analyzed by immunoblottings. The regulatory roles of aIC in hyperalgesia and pain-related anxiety were detected via pharmacological approach and chemogenetics in CP rats. Our results showed that TNBS treatment resulted in long-term hyperalgesia and anxiety-like behavior in rats. CP rats exhibited increased FOS expression and potentiated excitatory synaptic transmission within aIC. CP rats also showed up-regulated expression of VGluT1, and increased membrane trafficking and phosphorylation of NR2B and GluR1 within aIC. Blocking excitatory synaptic transmission significantly attenuated abdomen mechanical hyperalgesia. Specifically inhibiting the excitability of insular pyramidal cells reduced both abdomen hyperalgesia and pain-related anxiety. In conclusion, our findings emphasize a key role for aIC in hyperalgesia and anxiety of painful CP, providing a novel insight into cortical modulation of painful CP and shedding light on aIC as a potential target for neuromodulation interventions in the treatment of CP.

Regulation of the firing activity by PKA-PKC-Src family kinases in cultured neurons of hypothalamic arcuate nucleus

The cAMP‐dependent protein kinase A family (PKAs), protein kinase C family (PKCs), and Src family kinases (SFKs) are found to play important roles in pain hypersensitivity. However, more detailed investigations are still needed in order to understand the mechanisms underlying the actions of PKAs, PKCs, and SFKs. Neurons in the hypothalamic arcuate nucleus (ARC) are found to be involved in the regulation of pain hypersensitivity. Here we report that the action potential (AP) firing activity of ARC neurons in culture was up‐regulated by application of the adenylate cyclase activator forskolin or the PKC activator PMA, and that the forskolin or PMA application‐induced up‐regulation of AP firing activity could be blocked by pre‐application of the SFK inhibitor PP2. SFK activation also up‐regulated the AP firing activity and this effect could be prevented by pre‐application of the inhibitors of PKCs, but not of PKAs. Furthermore, we identified that forskolin or PMA application caused increases in the phosphorylation not only in PKAs at T197 or PKCs at S660 and PKCα/βII at T638/641, but also in SFKs at Y416. The forskolin or PMA application‐induced increase in the phosphorylation of PKAs or PKCs was not affected by pre‐treatment with PP2. The regulations of the SFK and AP firing activities by PKCs were independent upon the translocation of either PKCα or PKCβII. Thus, it is demonstrated that PKAs may act as an upstream factor(s) to enhance SFKs while PKCs and SFKs interact reciprocally, and thereby up‐regulate the AP firing activity in hypothalamic ARC neurons.

Mechanisms underlying mechanical sensitization induced by complement C5a: the roles of macrophages, TRPV1, and calcitonin gene-related peptide receptors

The complement system significantly contributes to the development of inflammatory and neuropathic pain, but the underlying mechanisms are poorly understood. Recently, we identified the signaling pathway responsible for thermal hypersensitivity induced by the complement system component C5a. Here, we examine the mechanisms of another important action of C5a, induction of mechanical hypersensitivity. We found that intraplantar injection of C5a produced a dose-dependent mechanical sensitization and that this effect was blocked by chemogenetic ablation of macrophages in both male and female mice. Knockout of TRPV1 or pretreatment with the TRPV1 antagonists, AMG9810 or 5'-iodoresiniferatoxin (5'-IRTX), significantly reduced C5a-induced mechanical sensitization. Notably, local administration of 5'-IRTX 90 minutes after C5a injection resulted in a slow, but complete, reversal of mechanical sensitization, indicating that TRPV1 activity was required for maintaining C5a-induced mechanical hypersensitivity. This slow reversal suggests that neurogenic inflammation and neuropeptide release may be involved. Indeed, pretreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist (but not an antagonist of the neurokinin 1 receptor) prevented C5a-induced mechanical sensitization. Furthermore, intraplantar injection of CGRP produced significant mechanical sensitization in both wild-type and TRPV1 knockout mice. Taken together, these findings suggest that C5a produces mechanical sensitization by initiating macrophage-to-sensory-neuron signaling cascade that involves activation of TRPV1 and CGRP receptor as critical steps in this process.

TRPV1 channel contributes to remifentanil-induced postoperative hyperalgesia via regulation of NMDA receptor trafficking in dorsal root ganglion

Background

Remifentanil is widely used in general anesthesia due to its reliability and rapid onset. However, remifentanil-induced postoperative hyperalgesia might be a challenge nowadays. Accumulating evidence suggests that the transient receptor potential vanilloid 1 (TRPV1) was involved in the development of neuropathic pain and hyperalgesia. However, the contribution of TRPV1 in modulating remifentanil-induced postoperative hyperalgesia is still unknown. The aim of this study is the contribution of TRPV1 to the surface expression of N-methyl-d-aspartate (NMDA) receptors in remifentanil-induced postoperative hyperalgesia.

Methods

The hot plate test and the Von Frey test were performed to evaluate thermal and mechanical hyperalgesia. Capsazepine (CPZ) was administrated intrathecally to confirm our results. TRPV1, NMDA receptors, CaMKII (calcium/calmodulin-dependent kinase II), and protein kinase C (PKC) in the dorsal root ganglion (DRG) were detected by Western blotting. Immunofluorescence assay was applied to analyze the distribution of TRPV1 and the relationship between TRPV1 and NMDA receptor subunit 1 (NR1).

Results

Remifentanil-induced both thermal and mechanical postoperative hyperalgesia. Here, we found the membrane trafficking of NR1, possibly due to the activation of TRPV1 in DRG neurons after remifentanil infusion. Furthermore, intrathecal injection of CPZ was able to relieve remifentanil-induced postoperative hyperalgesia according to a behavioral test and CPZ confirmed that TRPV1 is involved in NR1 trafficking. In addition, CaMKII/PKC but not protein kinase A (PKA) contributed to remifentanil-induced postoperative hyperalgesia.

Conclusion

Our study demonstrates that TRPV1 receptors are involved in remifentanil-induced postoperative hyperalgesia. TRPV1 contributes to the persistence of remifentanil-induced postoperative hyperalgesia through the trafficking of NMDA receptors via the activation of CaMKII-PKC signaling pathways in DRG neurons.

Neuronal transient receptor potential (TRP) channels and noxious sensory detection in sickle cell disease

Pain is the leading cause for hospitalization in patients with sickle cell disease (SCD). While the characteristics of SCD pain can vary widely between patients and between phases of the disease (e.g. vasoocclusive crisis pain vs. chronic pain), similar neuronal mechanisms likely underlie the various aspects of nociceptive processing. In the peripheral nervous system, small unmyelinated C fibers and lightly-myelinated Aδ fibers detect and transmit noxious stimuli. Both classes of neurons express members of the transient receptor potential (TRP) family, a group of ligand gated ion-channels that are activated by thermal, chemical, and mechanical stimuli. Promiscuous TRP channel family members are activated by a wide range of stimuli, many of which are dysregulated in patients with SCD and transgenic SCD mouse models. In 2011, our lab published the first report of TRP channel contributions to rodent SCD pain. Since that time, additional basic and clinical research efforts have investigated the genetic and biochemical status of TRP channels in SCD, placing particular focus on TRPV1. This review will discuss these advances and highlight the clinical SCD presentations that have not yet been studied, but which may be mediated by TRP channel activity.

Macrophage angiotensin II type 2 receptor triggers neuropathic pain

Peripheral nerve damage initiates a complex series of structural and cellular processes that culminate in chronic neuropathic pain. The recent success of a type 2 angiotensin II (Ang II) receptor (AT2R) antagonist in a phase II clinical trial for the treatment of postherpetic neuralgia suggests angiotensin signaling is involved in neuropathic pain. However, transcriptome analysis indicates a lack of AT2R gene (Agtr2) expression in human and rodent sensory ganglia, raising questions regarding the tissue/cell target underlying the analgesic effect of AT2R antagonism. We show that selective antagonism of AT2R attenuates neuropathic but not inflammatory mechanical and cold pain hypersensitivity behaviors in mice. Agtr2-expressing macrophages (MΦs) constitute the predominant immune cells that infiltrate the site of nerve injury. Interestingly, neuropathic mechanical and cold pain hypersensitivity can be attenuated by chemogenetic depletion of peripheral MΦs and AT2R-null hematopoietic cell transplantation. Our study identifies AT2R on peripheral MΦs as a critical trigger for pain sensitization at the site of nerve injury, and therefore proposes a translatable peripheral mechanism underlying chronic neuropathic pain.

Examining the Reversibility of Long-Term Behavioral Disruptions in Progeny of Maternal SSRI Exposure

Serotonergic dysregulation is implicated in numerous psychiatric disorders. Serotonin plays widespread trophic roles during neurodevelopment; thus perturbations to this system during development may increase risk for neurodevelopmental disorders. Epidemiological studies have examined association between selective serotonin reuptake inhibitor (SSRI) treatment during pregnancy and increased autism spectrum disorder (ASD) risk in offspring. It is unclear from these studies whether ASD susceptibility is purely related to maternal psychiatric diagnosis, or if treatment poses additional risk. We sought to determine whether maternal SSRI treatment alone or in combination with genetically vulnerable background was sufficient to induce offspring behavior disruptions relevant to ASD. We exposed C57BL/6J or Celf6^+/- mouse dams to fluoxetine (FLX) during different periods of gestation and lactation and characterized offspring on tasks assessing social communicative interaction and repetitive behavior patterns including sensory sensitivities. We demonstrate robust reductions in pup ultrasonic vocalizations (USVs) and alterations in social hierarchy behaviors, as well as perseverative behaviors and tactile hypersensitivity. Celf6 mutant mice demonstrate social communicative deficits and perseverative behaviors, without further interaction with FLX. FLX re-exposure in adulthood ameliorates the tactile hypersensitivity yet exacerbates the dominance phenotype. This suggests acute deficiencies in serotonin levels likely underlie the abnormal responses to sensory stimuli, while the social alterations are instead due to altered development of social circuits. These findings indicate maternal FLX treatment, independent of maternal stress, can induce behavioral disruptions in mammalian offspring, thus contributing to our understanding of the developmental role of the serotonin system and the possible risks to offspring of SSRI treatment during pregnancy.

Angiotensin II Triggers Peripheral Macrophage-to-Sensory Neuron Redox Crosstalk to Elicit Pain

Injury, inflammation, and nerve damage initiate a wide variety of cellular and molecular processes that culminate in hyperexcitation of sensory nerves, which underlies chronic inflammatory and neuropathic pain. Using behavioral readouts of pain hypersensitivity induced by angiotensin II (Ang II) injection into mouse hindpaws, our study shows that activation of the type 2 Ang II receptor (AT2R) and the cell-damage-sensing ion channel TRPA1 are required for peripheral mechanical pain sensitization induced by Ang II in male and female mice. However, we show that AT2R is not expressed in mouse and human dorsal root ganglia (DRG) sensory neurons. Instead, expression/activation of AT2R on peripheral/skin macrophages (MΦs) constitutes a critical trigger of mouse and human DRG sensory neuron excitation. Ang II-induced peripheral mechanical pain hypersensitivity can be attenuated by chemogenetic depletion of peripheral MΦs. Furthermore, AT2R activation in MΦs triggers production of reactive oxygen/nitrogen species, which trans-activate TRPA1 on mouse and human DRG sensory neurons via cysteine modification of the channel. Our study thus identifies a translatable immune cell-to-sensory neuron signaling crosstalk underlying peripheral nociceptor sensitization. This form of cell-to-cell signaling represents a critical peripheral mechanism for chronic pain and thus identifies multiple druggable analgesic targets.SIGNIFICANCE STATEMENT Pain is a widespread health problem that is undermanaged by currently available analgesics. Findings from a recent clinical trial on a type II angiotensin II receptor (AT2R) antagonist showed effective analgesia for neuropathic pain. AT2R antagonists have been shown to reduce neuropathy-, inflammation- and bone cancer-associated pain in rodents. We report that activation of AT2R in macrophages (MΦs) that infiltrate the site of injury, but not in sensory neurons, triggers an intercellular redox communication with sensory neurons via activation of the cell damage/pain-sensing ion channel TRPA1. This MΦ-to-sensory neuron crosstalk results in peripheral pain sensitization. Our findings provide an evidence-based mechanism underlying the analgesic action of AT2R antagonists, which could accelerate the development of efficacious non-opioid analgesic drugs for multiple pain conditions.

Deficits in Burrowing Behaviors Are Associated With Mouse Models of Neuropathic but Not Inflammatory Pain or Migraine

Burrowing, or the removal of material from an enclosed tube, is emerging as a prominent means of testing changes in a voluntary behavior in rodent models of various pain states. Here, we report no significant differences between male and female mice in terms of burrowing performance, in a substantially shorter time frame than previous reports. We found that the color of the burrow tube affects the variability of burrowing performance when tested in a lit room, suggesting that light aversion is at least a partial driver of this behavior. Spared nerve injury (SNI; as a model of neuropathy) impairs burrowing performance and correlates with enhanced mechanical sensitivity as assessed by von Frey filaments, as well as being pharmacologically reversed by an analgesic, gabapentin. Loss of the SNI-induced burrowing deficit was observed with daily testing post-surgery, but not when the testing interval was increased to 5 days, suggesting a confounding effect of daily repeat testing in this paradigm. Intraplantar complete Freund’s adjuvant (as a model of inflammatory pain) and systemic nitroglycerin (as a model of migraine-like symptoms) administration did not induce any burrowing deficit, indicating that assessment of burrowing behavior may not be universally suitable for the detection of behavioral changes across all rodent pain models.

Parathyroid hormone-related peptide activates and modulates TRPV1 channel in human DRG neurons

Parathyroid hormone-related peptide (PTHrP) is associated with advanced tumor growth and metastasis, especially in breast, prostate and myeloma cancers that metastasize to bones, resulting in debilitating chronic pain conditions. Our recent studies revealed that the receptor for PTHrP, PTH1R, is expressed in mouse DRG sensory neurons, and its activation leads to flow-activation and modulation of TRPV1 channel function, resulting in peripheral heat and mechanical hypersensitivity. In order to verify the translatability of our findings in rodents to humans, we explored whether this signalling axis operates in primary human DRG sensory neurons. Analysis of gene expression data from recently reported RNA deep sequencing experiments performed on mouse and human DRGs reveals that PTH1R is expressed in DRG and tibial nerve. Furthermore, exposure of cultured human DRG neurons to PTHrP leads to slow-sustained activation of TRPV1 and modulation of capsaicin-induced channel activation. Both activation and modulation of TRPV1 by PTHrP were dependent on PKC activity. Our findings suggest that functional PTHrP/PTH1R-TRPV1 signalling exists in human DRG neurons, which could contribute to local nociceptor excitation in the vicinity of metastatic bone tumor microenvironment.

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.

Recent advances in acute pain management: understanding the mechanisms of acute pain, the prescription of opioids, and the role of multimodal pain therapy

In this review, we discuss advances in acute pain management, including the recent report of the joint American Pain Society and American Academy of Pain Medicine task force on the classification of acute pain, the role of psychosocial factors, multimodal pain management, new non-opioid therapy, and the effect of the “opioid epidemic”. In this regard, we propose that a fundamental principle in acute pain management is identifying patients who are most at risk and providing an “opioid free anesthesia and postoperative analgesia”. This can be achieved by using a multimodal approach that includes regional anesthesia and minimizing the dose and the duration of opioid prescription. This allows prescribing medications that work through different mechanisms. We shall also look at the recent pharmacologic and treatment advances made in acute pain and regional anesthesia.

Pharmacological validation of voluntary gait and mechanical sensitivity assays associated with inflammatory and neuropathic pain in mice

The urgent need for more effective analgesic treatment options has prompted a re-evaluation of the behavioral tests used to assess pain in pre-clinical research, with an emphasis on inclusion of more voluntary, un-evoked behavioral assessments of pain. In order to validate voluntary gait analysis and a voluntary mechanical conflict-avoidance assay, we tested mouse models of neuropathy (spared nerve injury) and inflammation (complete Freund's adjuvant) alongside reflexive measures of mechanical and thermal hypersensitivity. To establish whether the observed changes in behavioral responses were pain-related, known analgesics (buprenorphine, gabapentin, carprofen) were also administered. Spared nerve injury persistently altered several gait indices, whereas complete Freund's adjuvant caused only transient changes. Furthermore, known analgesics could not reverse these gait changes, despite demonstrating their previously established efficacy in reflexive measures of mechanical and thermal hypersensitivity. In contrast, the mechanical conflict-avoidance assay demonstrated aversion in mice with neuropathy and inflammation-induced hypersensitivity, which could both be reversed by analgesics. We conclude that voluntary gait changes in rodent neuropathic and inflammatory pain models are not necessarily indicative of pain-related adaptations. On the other hand, mechanical conflict-avoidance represents a valid operant assay for quantifying pain-related behaviors in mice that can be reversed by known analgesics.

Parathyroid hormone related protein concentration in human serum and CSF correlates with age

BACKGROUND

Parathyroid Hormone-Related Protein (PTHrP) is involved in intracellular calcium (Ca) regulation, and has been demonstrated to participate in regulation of Ca in brain cells, activation of neurons, and modulation of pain. However, there are conflicting reports regarding the presence of PTHrP in CSF.

DESIGN AND METHODS

PTHrP and Ca were quantified in paired CSF and serum samples using mass spectrometry-based methods. Associations between PTHrP and Ca concentrations with age, sex and concentrations of nine CSF diagnostic markers in a set of 140 paired serum and CSF patient samples were evaluated.

RESULTS

The observed median PTHrP concentration in CSF was 51 times higher than in serum; the median concentration of Ca in CSF was 1.8 times lower than in serum. We observed positive correlation between concentrations of PTHrP in CSF and serum (p=0.013). Distribution of PTHrP concentrations in serum was associated with age (p=0.0068) and the concentrations were higher in women. In samples with serum calcium concentrations within the reference intervals (n=118), central 95% distribution of concentrations for Ca-CSF, PTHrP-serum and PTHrP-CSF were 5.4 (4.5-6.1) mg/dL, 1.2 (0.5-2.5) pmol/L, 62 (22-125) pmol/L, respectively.

CONCLUSIONS

Our data demonstrate that PTHrP is a normal constituent of human CSF with median concentrations 51 fold higher than in serum. Elevated serum PTHrP concentrations were positively correlated with age and significantly higher in women. Our data suggest that CSF could be a significant source of circulating PTHrP.

The Complement System Component C5a Produces Thermal Hyperalgesia via Macrophage-to-Nociceptor Signaling That Requires NGF and TRPV1

The complement cascade is a principal component of innate immunity. Recent studies have underscored the importance of C5a and other components of the complement system in inflammatory and neuropathic pain, although the underlying mechanisms are largely unknown. In particular, it is unclear how the complement system communicates with nociceptors and which ion channels and receptors are involved. Here we demonstrate that inflammatory thermal and mechanical hyperalgesia induced by complete Freund's adjuvant was accompanied by C5a upregulation and was markedly reduced by C5a receptor (C5aR1) knock-out or treatment with the C5aR1 antagonist PMX53. Direct administration of C5a into the mouse hindpaw produced strong thermal hyperalgesia, an effect that was absent in TRPV1 knock-out mice, and was blocked by the TRPV1 antagonist AMG9810. Immunohistochemistry of mouse plantar skin showed prominent expression of C5aR1 in macrophages. Additionally, C5a evoked strong Ca(2+) mobilization in macrophages. Macrophage depletion in transgenic macrophage Fas-induced apoptosis mice abolished C5a-dependent thermal hyperalgesia. Examination of inflammatory mediators following C5a injection revealed a rapid upregulation of NGF, a mediator known to sensitize TRPV1. Preinjection of an NGF-neutralizing antibody or Trk inhibitor GNF-5837 prevented C5a-induced thermal hyperalgesia. Notably, NGF-induced thermal hyperalgesia was unaffected by macrophage depletion. Collectively, these results suggest that complement fragment C5a induces thermal hyperalgesia by triggering macrophage-dependent signaling that involves mobilization of NGF and NGF-dependent sensitization of TRPV1. Our findings highlight the importance of macrophage-to-neuron signaling in pain processing and identify C5a, NGF, and TRPV1 as key players in this cross-cellular communication.

SIGNIFICANCE STATEMENT

This study provides mechanistic insight into how the complement system, a key component of innate immunity, regulates the development of pain hypersensitivity. We demonstrate a crucial role of the C5a receptor, C5aR1, in the development of inflammatory thermal and mechanical sensitization. By focusing on the mechanisms of C5a-induced thermal hyperalgesia, we show that this process requires recruitment of macrophages and initiation of macrophage-to-nociceptor signaling. At the molecular level, we demonstrate that this signaling depends on NGF and is mediated by the heat-sensitive nociceptive channel TRPV1. This deeper understanding of how immune cells and neurons interact to regulate pain processing is expected to facilitate mechanism-based approaches in the development of new analgesics.

Anti-vascular endothelial growth factor treatment decreases bladder pain in cyclophosphamide cystitis: a Multidisciplinary Approach to the Study of Chronic Pelvic Pain (MAPP) Research Network animal model study

OBJECTIVE

To investigate whether treatment with anti-vascular endothelial growth factor (VEGF)-neutralizing antibodies can reduce pain and voiding dysfunction in the cyclophosphamide (CYP) cystitis model of bladder pain in mice.

MATERIALS AND METHODS

Adult female mice received anti-VEGF-neutralizing antibodies (10 mg/kg i.p. B20-4.1.1 VEGF mAb) or saline (control) pre-treatment, followed by CYP (150 mg/kg i.p.) to induce acute cystitis. Pelvic nociceptive responses were assessed by applying von Frey filaments to the pelvic area. Spontaneous micturition was assessed using the void spot assay.

RESULTS

Systemic anti-VEGF-neutralizing antibody treatment significantly reduced the pelvic nociceptive response to CYP cystitis compared with control (saline). In the anti-VEGF pre-treatment group, there was a significant increase in pelvic hypersensitivity, measured by the area under the curve (AUC) using von Frey filaments at 5 h post-CYP administration (P = 0.004); however, by 48 h and 96 h post-CYP administration, pelvic hypersensitivity had reduced by 54% and 47%, respectively, compared with the 5 h post-CYP administration time point, and were no longer significantly different from baseline (P = 0.22 and 0.17, respectively). There was no difference in urinary frequency and mean voided volume between the two pre-treatment groups.

CONCLUSION

Systemic blockade of VEGF signalling with anti-VEGF-neutralizing antibodies was effective in reducing pelvic/bladder pain in the CYP cystitis model of bladder pain. Our data support the further investigation of the use of anti-VEGF antibodies to manage bladder pain or visceral pain.

Activation of bitter taste receptors in pulmonary nociceptors sensitizes TRPV1 channels through the PLC and PKC signaling pathway

Bitter taste receptors (T2Rs), a G protein-coupled receptor family capable of detecting numerous bitter-tasting compounds, have recently been shown to be expressed and play diverse roles in many extraoral tissues. Here we report the functional expression of T2Rs in rat pulmonary sensory neurons. In anesthetized spontaneously breathing rats, intratracheal instillation of T2R agonist chloroquine (10 mM, 0.1 ml) significantly augmented chemoreflexes evoked by right-atrial injection of capsaicin, a specific activator for transient receptor potential vanilloid receptor 1 (TRPV1), whereas intravenous infusion of chloroquine failed to significantly affect capsaicin-evoked reflexes. In patch-clamp recordings with isolated rat vagal pulmonary sensory neurons, pretreatment with chloroquine (1-1,000 µM, 90 s) concentration dependently potentiated capsaicin-induced TRPV1-mediated inward currents. Preincubating with diphenitol and denatonium (1 mM, 90 s), two other T2R activators, also enhanced capsaicin currents in these neurons but to a lesser extent. The sensitizing effect of chloroquine was effectively prevented by the phospholipase C inhibitor U73122 (1 µM) or by the protein kinase C inhibitor chelerythrine (10 µM). In summary, our study showed that activation of T2Rs augments capsaicin-evoked TRPV1 responses in rat pulmonary nociceptors through the phospholipase C and protein kinase C signaling pathway.

Nociceptive TRP Channels: Sensory Detectors and Transducers in Multiple Pain Pathologies

Specialized receptors belonging to the transient receptor potential (TRP) family of ligand-gated ion channels constitute the critical detectors and transducers of pain-causing stimuli. Nociceptive TRP channels are predominantly expressed by distinct subsets of sensory neurons of the peripheral nervous system. Several of these TRP channels are also expressed in neurons of the central nervous system, and in non-neuronal cells that communicate with sensory nerves. Nociceptive TRPs are activated by specific physico-chemical stimuli to provide the excitatory trigger in neurons. In addition, decades of research has identified a large number of immune and neuromodulators as mediators of nociceptive TRP channel activation during injury, inflammatory and other pathological conditions. These findings have led to aggressive targeting of TRP channels for the development of new-generation analgesics. This review summarizes the complex activation and/or modulation of nociceptive TRP channels under pathophysiological conditions, and how these changes underlie acute and chronic pain conditions. Furthermore, development of small-molecule antagonists for several TRP channels as analgesics, and the positive and negative outcomes of these drugs in clinical trials are discussed. Understanding the diverse functional and modulatory properties of nociceptive TRP channels is critical to function-based drug targeting for the development of evidence-based and efficacious new generation analgesics.

The stress protein heat shock cognate 70 (Hsc70) inhibits the Transient Receptor Potential Vanilloid type 1 (TRPV1) channel

Background

Specialized cellular defense mechanisms prevent damage from chemical, biological, and physical hazards. The heat shock proteins have been recognized as key chaperones that maintain cell survival against a variety of exogenous and endogenous stress signals including noxious temperature. However, the role of heat shock proteins in nociception remains poorly understood. We carried out an expression analysis of the constitutively expressed 70 kDa heat-shock cognate protein, a member of the stress-induced HSP70 family in lumbar dorsal root ganglia from a mouse model of Complete Freund’s Adjuvant-induced chronic inflammatory pain. We used immunolabeling of dorsal root ganglion neurons, behavioral analysis and patch clamp electrophysiology in both dorsal root ganglion neurons and HEK cells transfected with Hsc70 and Transient Receptor Potential Channels to examine their functional interaction in heat shock stress condition.

Results

We report an increase in protein levels of Hsc70 in mouse dorsal root ganglia, 3 days post Complete Freund’s Adjuvant injection in the hind paw. Immunostaining of Hsc70 was observed in most of the dorsal root ganglion neurons, including the small size nociceptors immunoreactive to the TRPV1 channel. Standard whole-cell patch-clamp technique was used to record Transient Receptor Potential Vanilloid type 1 current after exposure to heat shock. We found that capsaicin-evoked currents are inhibited by heat shock in dorsal root ganglion neurons and transfected HEK cells expressing Hsc70 and TRPV1. Blocking Hsc70 with matrine or spergualin compounds prevented heat shock-induced inhibition of the channel. We also found that, in contrast to TRPV1, both the cold sensor channels TRPA1 and TRPM8 were unresponsive to heat shock stress. Finally, we show that inhibition of TRPV1 depends on the ATPase activity of Hsc70 and involves the rho-associated protein kinase.

Conclusions

Our work identified Hsc70 and its ATPase activity as a central cofactor of TRPV1 channel function and points to the role of this stress protein in pain associated with neurodegenerative and/or metabolic disorders, including aging.