TRPV1 Antagonists and Chronic Pain: Beyond Thermal Perception

Lipid nanoparticle delivery of siRNA to dorsal root ganglion neurons to treat pain

Sensory neurons within the dorsal root ganglion (DRG) are the primary trigger of pain, relaying activity about noxious stimuli from the periphery to the central nervous system; however, targeting DRG neurons for pain management has remained a clinical challenge. Here, we demonstrate the use of lipid nanoparticles (LNPs) for effective intrathecal delivery of small interfering RNA (siRNA) to DRG neurons, achieving potent silencing of the transient receptor potential vanilloid 1 (TRPV1) ion channel that is predominantly expressed in nociceptor sensory neurons. This leads to a reversible interruption of heat-, capsaicin-, and inflammation-induced nociceptive conduction, as observed by behavioral outputs. Our work provides a proof-of-concept for intrathecal siRNA therapy as a novel and selective analgesic modality.

Advancing Pain Understanding and Drug Discovery: Insights from Preclinical Models and Recent Research Findings

Despite major advancements in our understanding of its fundamental causes, pain-both acute and chronic-remains a serious health concern. Various preclinical investigations utilizing diverse animal, cellular, and alternative models are required and frequently demanded by regulatory approval bodies to bridge the gap between the lab and the clinic. Investigating naturally occurring painful disorders can speed up medication development at the preclinical and clinical levels by illuminating molecular pathways. A wide range of animal models related to pain have been developed to elucidate pathophysiological mechanisms and aid in identifying novel targets for treatment. Pain sometimes drugs fail clinically, causing high translational costs due to poor selection and the use of preclinical tools and reporting. To improve the study of pain in a clinical context, researchers have been creating innovative models over the past few decades that better represent pathological pain conditions. In this paper, we provide a summary of traditional animal models, including rodents, cellular models, human volunteers, and alternative models, as well as the specific characteristics of pain diseases they model. However, a more rigorous approach to preclinical research and cutting-edge analgesic technologies may be necessary to successfully create novel analgesics. The research highlights from this review emphasize new opportunities to develop research that includes animals and non-animals using proven methods pertinent to comprehending and treating human suffering. This review highlights the value of using a variety of modern pain models in animals before human trials. These models can help us understand the different mechanisms behind various pain types. This will ultimately lead to the development of more effective pain medications.

A Local Heating Profile to Manage Lower Back Pain in an Automotive Seat: A Pilot Study

Lower back pain (LBP) is one of the most prevalent health losses in adults worldwide. Historically, heat has been successfully used for treating pain and relieving tight muscles. Given the effective contact with the occupant's back and proximity to the heat source, coupled with increasing commute times, automotive seats offer an opportunity to intervene. Fifteen adults (nine female) who experienced acute, subacute, and chronic lower back pain were recruited to examine the effectiveness of heat delivered to the lower back in providing temporary pain relief. Participants sat in a car seat for 38 min on two days, which included a 5-min baseline followed by a 33-min intervention; control, or localized. For the control condition, participants sat for 33 min without any thermal devices on, while the localized condition heated and maintained the seat surface temperature of the lower seat back area to ~45 °C. Over the 33-min control condition, the back skin temperature increased by ~1-2 °C and did not impact the subjective LBP. Heating the lower back for 33 min to ~39 °C reduced the subjective LBP by 10%. We demonstrated that lower back pain can be alleviated from an automotive seat providing heat to the lower back within normal commute times in those with lower back pain.

The dual role of TRPV1 in peripheral neuropathic pain: pain switches caused by its sensitization or desensitization

The transient receptor potential vanilloid 1 (TRPV1) channel plays a dual role in peripheral neuropathic pain (NeuP) by acting as a "pain switch" through its sensitization and desensitization. Hyperalgesia, commonly resulting from tissue injury or inflammation, involves the sensitization of TRPV1 channels, which modulates sensory transmission from primary afferent nociceptors to spinal dorsal horn neurons. In chemotherapy-induced peripheral neuropathy (CIPN), TRPV1 is implicated in neuropathic pain mechanisms due to its interaction with ion channels, neurotransmitter signaling, and oxidative stress. Sensitization of TRPV1 in dorsal root ganglion neurons contributes to CIPN development, and inhibition of TRPV1 channels can reduce chemotherapy-induced mechanical hypersensitivity. In diabetic peripheral neuropathy (DPN), TRPV1 is involved in pain modulation through pathways including reactive oxygen species and cytokine production. TRPV1's interaction with TRPA1 channels further influences chronic pain onset and progression. Therapeutically, capsaicin, a TRPV1 agonist, can induce analgesia through receptor desensitization, while TRPV1 antagonists and siRNA targeting TRPV1 show promise in preclinical studies. Cannabinoid modulation of TRPV1 provides another potential pathway for alleviating neuropathic pain. This review summarizes recent preclinical research on TRPV1 in association with peripheral NeuP.

Capsaicin: Emerging Pharmacological and Therapeutic Insights

Capsaicin, the most prominent pungent compound of chilli peppers, has been used in traditional medicine systems for centuries; it already has a number of established clinical and industrial applications. Capsaicin is known to act through the TRPV1 receptor, which exists in various tissues; capsaicin is hepatically metabolised, having a half-life correlated with the method of application. Research on various applications of capsaicin in different formulations is still ongoing. Thus, local capsaicin applications have a pronounced anti-inflammatory effect, while systemic applications have a multitude of different effects because their increased lipophilic character ensures their augmented bioavailability. Furthermore, various teams have documented capsaicin's anti-cancer effects, proven both in vivo and in vitro designs. A notable constraint in the therapeutic effects of capsaicin is its increased toxicity, especially in sensitive tissues. Regarding the traditional applications of capsaicin, apart from all the effects recorded as medicinal effects, the application of capsaicin in acupuncture points has been demonstrated to be effective and the combination of acupuncture and capsaicin warrants further research. Finally, capsaicin has demonstrated antimicrobial effects, which can supplement its anti-inflammatory and anti-carcinogenic actions.

Painful diabetic neuropathy: The role of ion channels

Painful diabetic neuropathy (PDN) is a common chronic complication of diabetes that causes neuropathic pain and negatively affects the quality of life. The management of PDN is far from satisfactory. At present, interventions are primarily focused on symptomatic treatment. Ion channel disorders are a major cause of PDN, and a complete understanding of their roles and mechanisms may provide better options for the clinical treatment of PDN. Therefore, this review summarizes the important role of ion channels in PDN and the current drug development targeting these ion channels.

IUPHAR review: Navigating the role of preclinical models in pain research

Chronic pain is a complex and challenging medical condition that affects millions of people worldwide. Understanding the underlying mechanisms of chronic pain is a key goal of preclinical pain research so that more effective treatment strategies can be developed. In this review, we explore nociception, pain, and the multifaceted factors that lead to chronic pain by focusing on preclinical models. We provide a detailed look into inflammatory and neuropathic pain models and discuss the most used animal models for studying the mechanisms behind these conditions. Additionally, we emphasize the vital role of these preclinical models in developing new pain-relief drugs, focusing on biologics and the therapeutic potential of NMDA and cannabinoid receptor antagonists. We also discuss the challenges of TRPV1 modulation for pain treatment, the clinical failures of neurokinin (NK)- 1 receptor antagonists, and the partial success story of Ziconotide to provide valuable lessons for preclinical pain models. Finally, we highlight the overall success and limitations of current treatments for chronic pain while providing critical insights into the development of more effective therapies to alleviate the burden of chronic pain.

Novel marine natural products as effective TRPV1 channel blockers

Chronic pain management poses a formidable challenge to healthcare, exacerbated by current analgesic options' limitations and adverse effects. Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel, has emerged as a promising target for novel analgesics. However, safety and tolerability concerns have constrained the development of TRPV1 modulators. In this study, we explored marine-derived natural products as a source of potential TRPV1 modulators using high-throughput dye-uptake assays. We identified chrexanthomycins, a family of hexacyclic xanthones, exhibited potent TRPV1 inhibitory effects, with compounds cC and cF demonstrating the most significant activity. High-resolution patch-clamp assays confirmed the direct action of these compounds on the TRPV1 channel. Furthermore, in vivo assays revealed that cC and cF effectively suppressed capsaicin-induced pain sensation in mice, comparable to the known TRPV1 inhibitor, capsazepine. Structural-activity relationship analysis highlighted the importance of specific functional groups in modulating TRPV1 activity. Our findings underscore the therapeutic potential of chrexanthomycins and pave the way for further investigations into marine-derived TRPV1 modulators for pain management.

Antinociceptive activity of doliroside B

CONTEXT

Dolichos trilobus Linn (Leguminosae) is often used in Yi ethnic medicine to treat pain, fracture, and rheumatism.

OBJECTIVE

To explore the therapeutic potential of doliroside B (DB) from D. trilobus and its disodium salt (DBDS) and the underlying mechanism in pain.

MATERIALS AND METHODS

In the writhing test, Kunming mice were orally treated with DB and DBDS at doses of 0.31, 0.62, 1.25, 2.5, and 5 mg/kg. Vehicle, morphine, indomethacin, and acetylsalicylic acid were used as negative and positive control on the nociception-induced models, respectively. In the hot plate test, mice were orally treated with DB and DBDS at doses of 2.5, 5, 10, and 20 mg/kg. In the formalin test, mice were orally treated with DB and DBDS at doses of 2.5, 5, 10, and 20 mg/kg. In the meanwhile, lipopolysaccharide-induced inflammatory model in RAW264.7 macrophages was adopted to study the mechanism of pain alleviation for DBDS.

RESULTS

DBDS (5 mg/kg) inhibited the writhing number by 80.2%, which exhibited the highest antinociceptive activity in pain models. DBDS could selectively inhibite the activity of COX-1. Meanwhile, it also reduced the production of NO, iNOS, and IL-6 by 55.8%, 69.0%, and 49.9% inhibition, respectively. It was found that DBDS also positively modulated the function of GABAA1 receptor.

DISCUSSION AND CONCLUSIONS

DBDS displayed antinociceptive activity by acting on both the peripheral and central nervous systems, which may act on multitargets. Further work is warranted for developing DBDS into a potential drug for the treatment of pain.

Mechanisms involved in the antinociceptive and anti-inflammatory effects of xanthotoxin

Xanthotoxin (XAT) is a natural furanocoumarin clinically used in the treatment of skin diseases such as vitiligo and psoriasis. Recent studies have also investigated its effects on anti-inflammatory, anti-cognitive dysfunction, and anti-amnesia as a guideline for clinic application. However, little is known about its effects on pain relief. Here, we tested the analgesic effects of XAT in serious acute pain and chronic pain models. For acute pain, we used hot-, capsaicin- and formalin-induced paw licking. Nociceptive threshold was measured by mechanical stimuli with von Frey filaments. For chronic pain, we injected complete Freund's adjuvant (CFA) into the mice's plantar surface of the hind paw to induce inflammatory pain. Heat and mechanical hyperalgesia were evaluated by radiant heat and von Frey filament tests, respectively. To investigate the mechanisms underlying the analgesic effect of XAT, we used calcium imaging and western blot to assess transient receptor potential vanilloid 1 (TRPV1) activity and expression in isolated L4-L6 dorsal root ganglion (DRG) neurons. Haematoxylin and eosin (HE) staining, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA) were used to examine immune cell recruitment and proinflammatory factor release from skin tissue from paw injection sites. Our results demonstrated that XAT not only reduced acute pain behaviors generated by hot, capsaicin, and formalin but also attenuated CFA-induced heat and mechanical hyperalgesia. The analgesic activity of XAT may be achieved by controlling peripheral inflammation, lowering immune cell infiltration at the site of inflammatory tissue, reducing inflammatory factor production, and therefore inhibiting TRPV1 channel sensitization and expression.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

Nociception and pain in humans lacking a functional TRPV1 channel

BACKGROUNDChronic pain is a debilitating illness with currently limited therapy, in part due to difficulties in translating treatments derived from animal models to patients. The transient receptor potential vanilloid 1 (TRPV1) channel is associated with noxious heat detection and inflammatory pain, and reports of adverse effects in human trials have hindered extensive efforts in the clinical development of TRPV1 antagonists as novel pain relievers.METHODSWe examined 2 affected individuals (A1 and A2) carrying a homozygous missense mutation in TRPV1, rendering the channel nonfunctional. Biochemical and functional assays were used to analyze the mutant channel. To identify possible phenotypes of the affected individuals, we performed psychophysical and medical examinations.RESULTSWe demonstrated that diverse TRPV1 activators, acting at different sites of the channel protein, were unable to open the cloned mutant channel. This finding was not a consequence of impairment in the expression, cellular trafficking, or assembly of protein subunits. The affected individuals were insensitive to application of capsaicin to the mouth and skin and did not demonstrate aversive behavior toward capsaicin. Furthermore, quantitative sensory testing of A1 revealed an elevated heat-pain threshold but also, surprisingly, an elevated cold-pain threshold and extensive neurogenic inflammatory, flare, and pain responses following application of the TRPA1 channel activator mustard oil.CONCLUSIONOur study provides direct evidence in humans for pain-related functional changes linked to TRPV1, which is a prime target in the development of pain relievers.FUNDINGSupported by the Israel Science Foundation (368/19); Teva's National Network of Excellence in Neuroscience grant (no. 0394886) and Teva's National Network of Excellence in Neuroscience postdoctoral fellowship.

CYP1B1-derived epoxides modulate the TRPA1 channel in chronic pain

Pain is often debilitating, and current treatments are neither universally efficacious nor without risks. Transient receptor potential (TRP) ion channels offer alternative targets for pain relief, but little is known about the regulation or identities of endogenous TRP ligands that affect inflammation and pain. Here, transcriptomic and targeted lipidomic analysis of damaged tissue from the mouse spinal nerve ligation (SNL)-induced chronic pain model revealed a time-dependent increase in Cyp1b1 mRNA and a concurrent accumulation of 8,9-epoxyeicosatrienoic acid (EET) and 19,20-EpDPA post injury. Production of 8,9-EET and 19,20-EpDPA by human/mouse CYP1B1 was confirmed in vitro, and 8,9-EET and 19,20-EpDPA selectively and dose-dependently sensitized and activated TRPA1 in overexpressing HEK-293 cells and Trpa1-expressing/AITC-responsive cultured mouse peptidergic dorsal root ganglia (DRG) neurons. TRPA1 activation by 8,9-EET and 19,20-EpDPA was attenuated by the antagonist A967079, and mouse TRPA1 was more responsive to 8,9-EET and 19,20-EpDPA than human TRPA1. This latter effect mapped to residues Y933, G939, and S921 of TRPA1. Intra-plantar injection of 19,20-EpDPA induced acute mechanical, but not thermal hypersensitivity in mice, which was also blocked by A967079. Similarly, Cyp1b1-knockout mice displayed a reduced chronic pain phenotype following SNL injury. These data suggest that manipulation of the CYP1B1-oxylipin-TRPA1 axis might have therapeutic benefit.

Diosgenin Exerts Analgesic Effects by Antagonizing the Selective Inhibition of Transient Receptor Potential Vanilloid 1 in a Mouse Model of Neuropathic Pain

Diosgenin is a botanical steroidal saponin with immunomodulatory, anti-inflammatory, anti-oxidative, anti-thrombotic, anti-apoptotic, anti-depressant, and anti-nociceptive effects. However, the effects of diosgenin on anti-nociception are unclear. Transient receptor potential vanilloid 1 (TRPV1) plays an important role in nociception. Therefore, we investigated whether TRPV1 antagonism mediates the anti-nociceptive effects of diosgenin. In vivo mouse experiments were performed to examine nociception-related behavior, while in vitro experiments were performed to examine calcium currents in dorsal root ganglion (DRG) and Chinese hamster ovary (CHO) cells. The duration of capsaicin-induced licking (pain behavior) was significantly reduced following oral and intraplantar administration of diosgenin, approaching levels observed in mice treated with the TRPV1 antagonist N-(4-tertiarybutylphenyl)-4-(3-cholorphyridin-2-yl) tetrahydropyrazine-1(2H)-carbox-amide. Additionally, oral administration of diosgenin blocked capsaicin-induced thermal hyperalgesia. Further, diosgenin reduced capsaicin-induced Ca²⁺ currents in a dose-dependent manner in both DRG and CHO cells. Oral administration of diosgenin also improved thermal and mechanical hyperalgesia in the sciatic nerve constriction injury-induced chronic pain model by reducing the expression of TRPV1 and inflammatory cytokines in DRG cells. Collectively, our results suggest that diosgenin exerts analgesic effects via antagonism of TRPV1 and suppression of inflammation in the DRG in a mouse model of neuropathic pain.

Transient Receptor Potential Vanilloid 1 Signaling Is Independent on Protein Kinase A Phosphorylation of Ankyrin-Rich Membrane Spanning Protein

The sensory ion channel transient receptor potential vanilloid 1 (TRPV1) is mainly expressed in small to medium sized dorsal root ganglion neurons, which are involved in the transfer of acute noxious thermal and chemical stimuli. The Ankyrin-rich membrane spanning protein (ARMS) interaction with TRPV1 is modulated by protein kinase A (PKA) mediating sensitization. Here, we hypothesize that PKA phosphorylation sites of ARMS are crucial for the modulation of TRPV1 function, and that the phosphorylation of ARMS is facilitated by the A-kinase anchoring protein 79 (AKAP79). We used transfected HEK293 cells, immunoprecipitation, calcium flux, and patch clamp experiments to investigate potential PKA phosphorylation sites in ARMS and in ARMS-related peptides. Additionally, experiments were done to discriminate between PKA and protein kinase D (PKD) phosphorylation. We found different interaction ratios for TRPV1 and ARMS mutants lacking PKA phosphorylation sites. The degree of TRPV1 sensitization by ARMS mutants is independent on PKA phosphorylation. AKAP79 was also involved in the TRPV1/ARMS/PKA signaling complex. These data show that ARMS is a PKA substrate via AKAP79 in the TRPV1 signaling complex and that all four proteins interact physically, regulating TRPV1 sensitization in transfected HEK293 cells. To assess the physiological and/or therapeutic significance of these findings, similar investigations need to be performed in native neurons and/or in vivo.

Epigenetic Changes within the Annulus Fibrosus by DNA Methylation in Rat Intervertebral Disc Degeneration Model

Intervertebral disc degeneration (IDD) is an age-dependent progressive spinal disease that causes chronic back or neck pain. Although aging has long been presented as the main risk factor, the exact cause is not fully known. DNA methylation is associated with chronic pain, suggesting that epigenetic modulation may ameliorate disc degeneration. We examined histological changes in the DNA methylation within the discs and their association with pain-related transient receptor potential vanilloid subtype 1 (TrpV1) expression in rats subjected to IDD. Epigenetic markers (5-hydroxymethylcytosine (5hmC), 5-methylcytosine (5Mc)), DNA methyltransferases (DNMTs), and Ten-eleven translocations (Tets) were analyzed using immunohistochemistry, real-time PCR, and DNA dot-blot following IDD. Results revealed high 5mC levels in the annulus fibrosus (AF) region within the disc after IDD and an association with TrpV1 expression. DNMT1 is mainly involved in 5mC conversion in degenerated discs. However, 5hmC levels did not differ between groups. A degenerated disc can lead to locomotor defects as assessed by ladder and tail suspension tests, no pain signals in the von Frey test, upregulated matrix metalloproteinase-3, and downregulated aggrecan levels within the disc. Thus, we found that the DNA methylation status in the AF region of the disc was mainly changed after IDD and associated with aberrant TrpV1 expression in degenerated discs.

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.

Angelica dahurica Extracts Attenuate CFA-Induced Inflammatory Pain via TRPV1 in Mice

Angelica dahurica, belonging to the family Apiaceae, is a well-known herbal medicine. The roots of Angelica dahurica are commonly used for the treatment of headache, toothache, abscess, furunculosis, and acne. However, little is known about their analgesic molecular mechanism underlying pain relief. In this study, we used behavioral tests to assess the analgesic effect of the ADE (Angelica dahurica extracts) on CFA (complete Freund's adjuvant)-induced inflammatory pain mice models. TRPV1 (Transient Receptor Potential Cation Channel Subfamily V Member 1) protein activity in dorsal root ganglion (DRG) was assessed with a calcium imaging assay. TRPV1 expression was detected with western blot and immunohistochemistry. Then, we examined the constituents of ADE using combined ultra-performance liquid chromatography-quadrupole time-of-light mass spectrometry (UPLC/Q-TOF-MS). Our results showed that ADE effectively attenuated mechanical and thermal hypersensitivities in CFA-induced inflammatory pain model in mice. ADE also significantly reduced the activity and the protein expression of TRPV1 in DRG from CFA mice. Therefore, ADE might be an attractive and suitable analgesic agent for the management of chronic inflammatory pain.

Membrane Interactivity of Capsaicin Antagonized by Capsazepine

Although the pharmacological activity of capsaicin has been explained by its specific binding to transient receptor potential vanilloid type 1, the amphiphilic structure of capsaicin may enable it to act on lipid bilayers. From a mechanistic point of view, we investigated whether capsaicin and its antagonist capsazepine interact with biomimetic membranes, and how capsazepine influences the membrane effect of capsaicin. Liposomal phospholipid membranes and neuro-mimetic membranes were prepared with 1,2-dipalmitoylphosphatidylcholine and with 1-palmitoyl-2-oleoylphosphatidylcholine and sphingomyelin plus cholesterol, respectively. These membrane preparations were subjected to reactions with capsaicin and capsazepine at 0.5–250 μM, followed by measuring fluorescence polarization to determine the membrane interactivity to modify the fluidity of membranes. Both compounds acted on 1,2-dipalmitoylphosphatidylcholine bilayers and changed membrane fluidity. Capsaicin concentration-dependently interacted with neuro-mimetic membranes to increase their fluidity at low micromolar concentrations, whereas capsazepine inversely decreased the membrane fluidity. When used in combination, capsazepine inhibited the effect of capsaicin on neuro-mimetic membranes. In addition to the direct action on transmembrane ion channels, capsaicin and capsazepine share membrane interactivity, but capsazepine is likely to competitively antagonize capsaicin’s interaction with neuro-mimetic membranes at pharmacokinetically-relevant concentrations. The structure-specific membrane interactivity may be partly responsible for the analgesic effect of capsaicin.

Effects of Pulsed Radiofrequency with Different Temperature on Model Rats of Chronic Constriction Injury

OBJECTIVES

The treatment for neuropathic pain is still a big challenge. Pulsed radiofrequency technique has been widely used to relieve neuropathic pain in recent years. The purpose of this study is to optimize the temperature for pulsed radiofrequency therapy.

DESIGN

Animal, experimental study.

METHODS

Seventy-five male SD rats were randomly divided into five groups: Sham operation group (Sham group), chronic constriction injury group (CCI group), PRF 42°C group (P42 group), PRF 50°C group (P50 group), and PRF 60°C group (P60 group). The hindpaw withdrawal threshold (HWT), paw thermal withdrawal latency (PTWL), sciatic nerve structure, and the concentration of spinal methionine enkephalin(M-ENK) were detected to identify which temperature is the best for PRF treatment.

RESULTS

PRF at 42°C, 50°C and 60°C significantly alleviated the pain in CCI rats. The therapeutic effects of 50°C and 60°C were similar, and both were better than 42°C. In addition, PRF using 42°C, 50°C, and 60°C mediated nerve injury to sciatic nerve were grade 1, 1, and 2, respectively. The concentration of M-ENK in spinal cord increased accompanying with the increasing of the temperature of PRF.

CONCLUSIONS

PRF using 50°C could induce less damage while achieving better improvement of mechanical and thermal pain threshold than 42°C and 60°C in CCI rats, which may be achieved by promoting the expression of M-ENK in spinal cord.

Alloxan as a better option than streptozotocin for studies involving painful diabetic neuropathy

Previous data indicate that the diabetogenic substance streptozotocin might act in nociceptive neurons changing the sensory signal, regardless of hyperglycemia. In the present article the effects of streptozotocin were compared with another diabetogenic drug, alloxan, for diabetes induction in rats. A possible direct effect of these drugs was tested by means of in vivo experiments and in vitro assays using cultured primary nociceptive neurons. Streptozotocin (17.5 and 35 mg/kg), alloxan (15 and 30 mg/kg) or vehicle were injected in adult male rats and the animal groups were separated according to glycemic levels. Body mass, glycemia and paw mechanical sensitivity were evaluated for 5 weeks. Streptozotocin caused an increase in mechanical sensitivity in both hyperglycemic and normoglycemic rats, while alloxan induced mechanical sensitization only in hyperglycemic animals. Injection of both substances induced local inflammation at rat paws; however, only streptozotocin caused significant mechanical sensitization when injected near to sensory neurons at the dorsal root ganglia. Also, streptozotocin treatment induced a reduction in intracellular calcium levels and inhibited capsaicin induced calcium transients and membrane depolarization. Alloxan did not affect calcium levels or membrane potential in primary nociceptive neurons. These findings suggest that alloxan might be a better option for animal studies regarding painful diabetic neuropathy as streptozotocin directly affects nociceptive neurons, probably by modulating TRPV1 channel activation.

Electroacupuncture reduces cold stress-induced pain through microglial inactivation and transient receptor potential V1 in mice

Background

The treatment, and efficacy thereof, is considered to be inadequate with specificity to alleviation of Fibromyalgia and its associated pain. Fibromyalgia patients suffer from chronic and persistent widespread pain and generalized tenderness. Transient receptor potential V1 (TRPV1), which is reported as a Ca²⁺ permeable ion channel that can be activated by inflammation, is reported to be involved in the development of fibromyalgia pain.

Methods

The current study explored the TRPV1 channel functions as a noxious sensory input in mice cold stress model. It remains unknown whether electroacupuncture (EA) attenuates fibromyalgia pain or affects the TRPV1 pathway.

Results

We show that cold stress increases mechanical and thermal pain (day 7: mechanical: 1.69 ± 0.41 g; thermal: 4.68 ± 0.56 s), and that EA and Trpv1 deletion counter this increase. EA and Trpv1 deletion reduced the cold stress-induced increase in inflammatory mediators and TRPV1-related molecules in the hypothalamus, periaqueductal gray (PAG), and cerebellum of mice.

Conclusions

Our results imply that EA has an analgesic effect associated with TRPV1 downregulation. We provide novel evidence that these inflammatory mediators can modulate the TRPV1 signaling pathway and suggest new potential therapeutic targets for fibromyalgia pain.

Mode-selective inhibitory effects of eugenol on the mouse TRPV1 channel

The transient receptor potential vanilloid 1 (TRPV1) channel is a polymodal receptor in sensory nerves and involved in pain sensation. TRPV1 has at least three distinct activation modes that are selectively induced by different stimuli capsaicin, noxious heat, and protons. Although many mode-selective TRPV1 antagonists have been developed for their anticipated analgesic effects, there have been few successful reports because of adverse effects due to burn injuries and hyperthermia. Eugenol is a vanilloid that has been used as an analgesic in the dental treatment, and its TRPV1 activation ability has been reported. However, our knowledge about the underlying mechanisms of the antagonistic effects of eugenol on TRPV1 activation induced by three different modes is limited. Here, we show that eugenol dose-dependently inhibited the capsaicin-activated inward currents of mouse TRPV1 expressed in human embryonic kidney 293 (HEK293) cells. Under low pH conditions, low concentrations of eugenol only enhanced the proton-induced TRPV1 currents, whereas high eugenol concentrations initially potentiated but then immediately abrogated TRPV1 currents. Finally, eugenol had no modulatory effects on heat-activated TRPV1 in electrophysiological and Fura-2-based Ca²⁺ imaging experiments. Our results demonstrate that eugenol is a mode-selective antagonist of TRPV1 and can be evaluated as a lead compound of analgesics targeting TRPV1 without serious side effects.

Neuronal interleukin-1 receptors mediate pain in chronic inflammatory diseases

Chronic pain is a major comorbidity of chronic inflammatory diseases. Here, we report that the cytokine IL-1β, which is abundantly produced during multiple sclerosis (MS), arthritis (RA), and osteoarthritis (OA) both in humans and in animal models, drives pain associated with these diseases. We found that the type 1 IL-1 receptor (IL-1R1) is highly expressed in the mouse and human by a subpopulation of TRPV1⁺ dorsal root ganglion neurons specialized in detecting painful stimuli, termed nociceptors. Strikingly, deletion of the Il1r1 gene specifically in TRPV1⁺ nociceptors prevented the development of mechanical allodynia without affecting clinical signs and disease progression in mice with experimental autoimmune encephalomyelitis and K/BxN serum transfer–induced RA. Conditional restoration of IL-1R1 expression in nociceptors of IL-1R1–knockout mice induced pain behavior but did not affect joint damage in monosodium iodoacetate–induced OA. Collectively, these data reveal that neuronal IL-1R1 signaling mediates pain, uncovering the potential benefit of anti–IL-1 therapies for pain management in patients with chronic inflammatory diseases.

SNI and CFA induce similar changes in TRPV1 and P2X3 expressions in the acute phase but not in the chronic phase of pain

Peripheral inflammation and nerve injury usually accompany each other. However, whether inflammatory and neuropathic pain share similar mechanisms at all stages is unknown. TRPV1 and P2X3 are two major ion channels in dorsal root ganglia (DRGs) and are involved in chronic pain. Here, their function and expression in DRGs at different phases of the two types of pain were investigated. Both the paw withdrawal threshold (PWT) and paw withdrawal latency were decreased in rats injected with complete Freud's adjuvant (CFA). However, only the PWT was decreased in rats with spared nerve injury (SNI). CFA increased the magnitude of the TRPV1-mediated Ca²⁺ response but not the P2X3-mediated Ca²⁺ response 14 days after injection. Consistent with this result, the P2X3 expression level in CFA rats was increased only at 3 days after injection. SNI surgery increased the magnitudes of the TRPV1- and P2X3-mediated Ca²⁺ responses and upregulated both TRPV1 and P2X3 expression in lumbar DRGs. The distributions of TRPV1 and P2X3 in DRGs after modeling were observed, and TRPV1 was found to be highly expressed mainly in the L4-L5 DRGs in CFA rats and in the L5-L6 DRGs in SNI rats. P2X3 was highly expressed in the L4-L6 DRGs in CFA rats 3 days after injection but was only highly expressed in the L4 DRG 14 days after modeling. On the other hand, SNI promoted the P2X3 expression L4-L5 DRGs 3 days after surgery, but only L6 DRG 14 days after modeling. All the results indicate that P2X3 and TPRV1 are involved in inflammatory and neuropathic pain by different expression levels and distributions in the lumbar DRG in the chronic stage.

Alterations in Anandamide Synthesis and Degradation during Osteoarthritis Progression in an Animal Model

Osteoarthritis (OA) is a degenerative joint disease manifested by movement limitations and chronic pain. Endocannabinoid system (ECS) may modulate nociception via cannabinoid and TRPV1 receptors. The purpose of our study was to examine alterations in the spinal and joint endocannabinoid system during pain development in an animal model of OA. Wistar rats received intra-articular injection of 3mg of sodium monoiodoacetate (MIA) into the knee joint. Animals were sacrificed on day 2, 7, 14, 21, 28 after injection and lumbar spinal cord, cartilage and synovium were collected. Changes in the transcription levels of the ECS elements were measured. At the spinal level, gene expression levels of the cannabinoid and TRPV1 receptors as well as enzymes involved in anandamide synthesis and degradation were elevated in the advanced OA phase. In the joint, an important role of the synovium was demonstrated, since cartilage degeneration resulted in attenuation of the changes in the gene expression. Enzymes responsible for anandamide synthesis and degradation were upregulated particularly in the early stages of OA, presumably in response to early local joint inflammation. The presented study provides missing information about the MIA-induced OA model and encourages the development of a therapy focused on the molecular role of ECS.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Jedi-1 deficiency increases sensory neuron excitability through a non-cell autonomous mechanism

The dorsal root ganglia (DRG) house the primary afferent neurons responsible for somatosensation, including pain. We previously identified Jedi-1 (PEAR1/MEGF12) as a phagocytic receptor expressed by satellite glia in the DRG involved in clearing apoptotic neurons during development. Here, we further investigated the function of this receptor in vivo using Jedi-1 null mice. In addition to satellite glia, we found Jedi-1 expression in perineurial glia and endothelial cells, but not in sensory neurons. We did not detect any morphological or functional changes in the glial cells or vasculature of Jedi-1 knockout mice. Surprisingly, we did observe changes in DRG neuron activity. In neurons from Jedi-1 knockout (KO) mice, there was an increase in the fraction of capsaicin-sensitive cells relative to wild type (WT) controls. Patch-clamp electrophysiology revealed an increase in excitability, with a shift from phasic to tonic action potential firing patterns in KO neurons. We also found alterations in the properties of voltage-gated sodium channel currents in Jedi-1 null neurons. These results provide new insight into the expression pattern of Jedi-1 in the peripheral nervous system and indicate that loss of Jedi-1 alters DRG neuron activity indirectly through an intercellular interaction between non-neuronal cells and sensory neurons.

The Expanding Therapeutic Potential of Neuronal KCC2

Dysfunctions in GABAergic inhibitory neural transmission occur in neuronal injuries and neurological disorders. The potassium-chloride cotransporter 2 (KCC2, SLC12A5) is a key modulator of inhibitory GABAergic inputs in healthy adult neurons, as its chloride (Cl-) extruding activity underlies the hyperpolarizing reversal potential for GABAA receptor Cl- currents (EGABA). Manipulation of KCC2 levels or activity improve symptoms associated with epilepsy and neuropathy. Recent works have now indicated that pharmacological enhancement of KCC2 function could reactivate dormant relay circuits in an injured mouse's spinal cord, leading to functional recovery and the attenuation of neuronal abnormality and disease phenotype associated with a mouse model of Rett syndrome (RTT). KCC2 interacts with Huntingtin and is downregulated in Huntington's disease (HD), which contributed to GABAergic excitation and memory deficits in the R6/2 mouse HD model. Here, these recent advances are highlighted, which attest to KCC2's growing potential as a therapeutic target for neuropathological conditions resulting from dysfunctional inhibitory input.

Analgesic transient receptor potential vanilloid-1-active compounds inhibit native and recombinant T-type calcium channels

BACKGROUND AND PURPOSE

T-type calcium (Ca_v 3) and transient receptor potential vanilloid-1 (TRPV1) channels play central roles in the control of excitability in the peripheral nervous system and are regarded as potential therapeutic pain targets. Modulators that either activate or inhibit TRPV1-mediated currents display analgesic properties in various pain models despite opposing effects on their connate target, TRPV1. We explored the effects of TRPV1-active compounds on Ca_v 3-mediated currents.

EXPERIMENTAL APPROACH

Whole-cell patch clamp recordings were used to examine the effects of TRPV1-active compounds on rat dorsal root ganglion low voltage-activated calcium currents and recombinant Ca_v 3 isoforms in expression systems.

KEY RESULTS

The classical TRPV1 agonist capsaicin as well as TRPV1 antagonists A-889425, BCTC, and capsazepine directly inhibited Ca_v 3 channels. These compounds altered the voltage-dependence of activation and inactivation of Ca_v 3 channels and delayed their recovery from inactivation, leading to a concomitant decrease in T-type current availability. The TRPV1 antagonist capsazepine potently inhibited Ca_v 3.1 and 3.2 channels (K_D  < 120 nM), as demonstrated by its slow off rate. In contrast, neither the TRPV1 agonists, Palvanil and resiniferatoxin, nor the TRPV1 antagonist AMG9810 modulated Ca_v 3-mediated currents.

CONCLUSIONS AND IMPLICATIONS

Analgesic TRPV1-active compounds inhibit Ca_v 3 currents in native and heterologous systems. Hence, their analgesic effects may not be exclusively attributed to their actions on TRPV1, which has important implications in the current understanding of nociceptive pathways. Importantly, our results highlight the need for attention in the experimental design used to address the analgesic properties of Ca_v 3 channel inhibitors.

Acute inflammation sensitizes knee-innervating sensory neurons and decreases mouse digging behavior in a TRPV1-dependent manner

Ongoing, spontaneous pain is characteristic of inflammatory joint pain and reduces an individual's quality of life. To understand the neural basis of inflammatory joint pain, we made a unilateral knee injection of complete Freund's adjuvant (CFA) in mice, which reduced their natural digging behavior. We hypothesized that sensitization of knee-innervating dorsal root ganglion (DRG) neurons underlies this altered behavior. To test this hypothesis, we performed electrophysiological recordings on retrograde labeled knee-innervating primary DRG neuron cultures and measured their responses to a number of electrical and chemical stimuli. We found that 24-h after CFA-induced knee inflammation, knee neurons show a decreased action potential generation threshold, as well as increased GABA and capsaicin sensitivity, but have unaltered acid sensitivity. The inflammation-induced sensitization of knee neurons persisted for 24-h in culture, but was not observed after 48-h in culture. Through immunohistochemistry, we showed that the increased knee neuron capsaicin sensitivity correlated with enhanced expression of the capsaicin receptor, transient receptor potential vanilloid 1 (TRPV1) in knee-innervating neurons of the CFA-injected side. We also observed an increase in the co-expression of TRPV1 with tropomyosin receptor kinase A (TrkA), which is the receptor for nerve growth factor (NGF), suggesting that NGF partially induces the increased TRPV1 expression. Lastly, we found that systemic administration of the TRPV1 antagonist, A-425619, reversed the decrease in digging behavior induced by CFA injection, further confirming the role of TRPV1, expressed by knee neurons, in acute inflammatory joint pain.

•

Knee inflammation decreases digging behavior in mice.

•

Knee-innervating dorsal root ganglion neurons are hyperexcitable after inflammation.

•

NGF-mediated increase in TRPV1 expression is observed in knee-innervating neurons.

•

Systemic TRPV1 antagonist administration normalises digging behavior in mice.

TRPV1-Like Immunoreactivity in the Human Locus K, a Distinct Subregion of the Cuneate Nucleus

The presence of transient receptor potential vanilloid type-1 receptor (TRPV1)-like immunoreactivity (LI), in the form of nerve fibres and terminals, is shown in a set of discrete gray matter subregions placed in the territory of the human cuneate nucleus. We showed previously that those subregions share neurochemical and structural features with the protopathic nuclei and, after the ancient name of our town, collectively call them Locus Karalis, and briefly Locus K. TRPV1-LI in the Locus K is codistributed, though not perfectly overlapped, with that of the neuropeptides calcitonin gene-related peptide and substance P, the topography of the elements immunoreactive to the three markers, in relation to each other, reflecting that previously described in the caudal spinal trigeminal nucleus. Myelin stainings show that myelinated fibres, abundant in the cuneate, gracile and trigeminal magnocellular nuclei, are scarce in the Locus K as in the trigeminal substantia gelatinosa. Morphometric analysis shows that cell size and density of Locus K neurons are consistent with those of the trigeminal substantia gelatinosa and significantly different from those of the magnocellular trigeminal, solitary and dorsal column nuclei. We propose that Locus K is a special component of the human dorsal column nuclei. Its functional role remains to be determined, but TRPV1 appears to play a part in it.

Alleviation of Trigeminal Nociception Using p75 Neurotrophin Receptor Targeted Lentiviral Interference Therapy

Acute and chronic trigeminal (TG) neuropathies are the cause of considerable distress, with limited treatments available at present. Nociceptive neurons enriched with the vanilloid type 1 receptor (VR1) partake in pain sensation and sensitization in the TG system. While VR1 blockers with anti-nociceptive potential are of substantial medical interest, their use remains limited due to poor selectivity and lack of cell-targeting capabilities. This study describes a methodology for the alleviation of nociception via targeted depletion of VR1 in TG sensory neurons in rats. In cultured TG ganglion neurons, VR1 expression was virtually abolished by lentiviral short hairpin RNA (LV-VR1). By decorating GFP encoding LV (LV-GFP) and LV-VR1 with IgG192 for targeting TG sensory neurons enriched with the p75 neurotrophin receptor (p75NTR), transduction of a reporter GFP and VR1 depletion was achieved after injection of targeted vectors into the whisker pad. In IgG192/LV-VR1-injected rats, the behavioral response to capsaicin exposure as well as Erk 1/2 phosphorylation and VR1 current activation by capsaicin were significantly reduced. This pioneering investigation, thus, provides a proof of principle for a means of attenuating TG nociception, revealing therapeutic potential.

Identification of capsazepine as a novel inhibitor of system xc- and cancer-induced bone pain

The cystine/glutamate antiporter has been implicated in a variety of cancers as a major mediator of redox homeostasis. The excess glutamate secreted by this transporter in aggressive cancer cells has been associated with cancer-induced bone pain (CIBP) from distal breast cancer metastases. High-throughput screening of small molecule inhibitors of glutamate release from breast cancer cells identified several potential compounds. One such compound, capsazepine (CPZ), was confirmed to inhibit the functional unit of system x_c^- (xCT) through its ability to block uptake of its radiolabeled substrate, cystine. Blockade of this antiporter induced production of reactive oxygen species (ROS) within 4 hours and induced cell death within 48 hours at concentrations exceeding 25 μM. Furthermore, cell death and ROS production were significantly reduced by co-treatment with N-acetylcysteine, suggesting that CPZ toxicity is associated with ROS-induced cell death. These data suggest that CPZ can modulate system x_c^- activity in vitro and this translates into antinociception in an in vivo model of CIBP where systemic administration of CPZ successfully delayed the onset and reversed CIBP-induced nociceptive behaviors resulting from intrafemoral MDA-MB-231 tumors.

The Role of C Fibers in Spinal Microglia Induction and Possible Relation with TRPV3 Expression During Chronic Inflammatory Arthritis in Rats

Introduction:

Stimulation of peptidergic fibers activates microglia in the dorsal horn. Microglia activation causes fractalkine (FKN) release, a neuron-glia signal, which enhances pain. The transient vanilloid receptor 1 (TRPV1) mediates the release of neuropeptides, which can subsequently activate glia. TRPV1 and TRPV2 are generally expressed on C and Aδ fibers, respectively. Expression of both proteins is upregulated during inflammation, but expression of TRPV3 after induction of inflammation is unclear.

Methods:

Adult male Wistar rats were used in all experiments. Arthritis was induced in them by single subcutaneous injection of complete Freund’s adjuvant (CFA) in their right hindpaws. Resiniferatoxin (RTX) was used to eliminate peptidergic fibers. We examined the relation between FKN and TRPV3 expression by administration of anti-FKN antibody.

Results:

Our study findings indicated that 1) spinal TRPV3 was mostly expressed on nonpeptidergic fibers, 2) expression of spinal TRPV3 increased following inflammation, 3) elimination of peptidergic fibers decreased spinal TRPV3 expression, 4) alteration of hyperalgesia was compatible with TRPV3 changes in RTX-treated rat, and 5) anti-FKN antibody reduced spinal TRPV3 expression.

Discussion:

It seems that the hyperalgesia variation during different phases of CFA-induced arthritis correlates with spinal TRPV3 expression variation on peptidergic fibers. Moreover, spinal microglial activation during CFA inflammation is involved in TRPV3 expression changes via FKN signaling.

Capsaicin: Current Understanding of Its Mechanisms and Therapy of Pain and Other Pre-Clinical and Clinical Uses

In this review, we discuss the importance of capsaicin to the current understanding of neuronal modulation of pain and explore the mechanisms of capsaicin-induced pain. We will focus on the analgesic effects of capsaicin and its clinical applicability in treating pain. Furthermore, we will draw attention to the rationale for other clinical therapeutic uses and implications of capsaicin in diseases such as obesity, diabetes, cardiovascular conditions, cancer, airway diseases, itch, gastric, and urological disorders.

Neurofibromatosis as a gateway to better treatment for a variety of malignancies

The neurofibromatoses (NF) are a group of rare genetic disorders that can affect all races equally at an incidence from 1:3000 (NF1) to a log unit lower for NF2 and schwannomatosis. Since the research community is reporting an increasing number of malignant cancers that carry mutations in the NF genes, the general interest of both the research and pharma community is increasing and the authors saw an opportunity to present a novel, fresh approach to drug discovery in NF. The aim of the paper is to challenge the current drug discovery approach to NF, whereby existing targeted therapies that are either in the clinic or on the market for other disease indications are repurposed for NF. We offer a suggestion for an alternative drug discovery approach. In the new approach, selective and tolerable targeted therapies would be developed for NF and later expanded to patients with more complex diseases such as malignant cancer in which the NF downstream pathways are deregulated. The Children's Tumor Foundation, together with some other major NF funders, is playing a key role in funding critical initiatives that will accelerate the development of better targeted therapies for NF patients, while these novel, innovative treatments could potentially be beneficial to molecularly characterized cancer patients in which NF mutations have been identified.

ARA 290 relieves pathophysiological pain by targeting TRPV1 channel: Integration between immune system and nociception

ARA 290 is an erythropoietin-derived polypeptide that possesses analgesic and tissue protective effect in many diseases such as diabetes and cancer. The analgesic effect of ARA 290 is mediated by its anti-inflammatory and immunomodulatory functions, or more specifically, by targeting the innate repair receptor (IRR) to down-regulate inflammation to alleviate neuropathic pain. However, whether other mechanisms or pathways are involved in ARA 290-mediated analgesic effect remains elusive. In this study, we are particularly interested in whether ARA 290 could directly target peripheral nociceptors by blocking or influencing receptors in pain sensation. Using calcium imaging, cell culture and behavioral tests, we demonstrated that ARA 290 was able to specifically inhibit TRPV1 channel activity, and relieve the mechanical hypersensitivity induced by capsaicin. Our study suggested that ARA 290 could potentially function as a novel antagonist for TRPV1 channel. This finding would not only contribute to the development of new pain treatment using ARA 290, but also help to improve our understanding of the integration between the immune system and the peripheral nervous system.

Neonatal capsaicin treatment in rats induces chronic hyperthermia resulting in infectious disease

Treatment of neonatal animals with capsaicin has previously been associated with long-lasting hyperthermia and severe cutaneous lesions. The present study analyzed the effects of capsaicin-induced hyperthermia on the occurrence of infectious disease and pruritic dermatitis in a rat model. Pregnant Sprague-Dawley (SD) rats were obtained 1 week prior to parturition. Pups from each litter were randomly assigned to the following experimental groups: Capsaicin-treated (cap-treated; n=10) or vehicle-treated (n=5). Capsaicin (50 mg/kg) or vehicle were systemically administered to the SD rat pups (age, 48 h), after which body temperature was measured using a biotelemetry system, and the effects of hyperthermia on the ability of the rat pups to resist bacterial infection were analyzed. Furthermore, pruritus-induced scratching behavior and dermatitis were assessed, and changes in interleukin (IL)-4- and IL-13-induced immunoglobulin E expression were measured. Treatment of neonatal rats with capsaicin resulted in chronic hyperthermia, which had negative effects on the host immune defense response. The expression levels of T-helper type 2 cell-associated cytokines were significantly increased (P<0.01) in the cap-treated rats following bacterial infection with Staphylococcus aureus or Streptococcus agalactiae. Furthermore, cap-treated rats exhibited pruritus-induced scratching behavior and dermatitis. The results of the present study suggested that treatment of neonatal rats with capsaicin induces chronic hyperthermia and decreases the effectiveness of the host defense system. Therefore, a cap-treated neonatal rat model may be considered useful when investigating the association between hyperthermia and infectious disease.

Neonatal capsaicin treatment in rats induces chronic hyperthermia resulting in infectious disease

Treatment of neonatal animals with capsaicin has previously been associated with long-lasting hyperthermia and severe cutaneous lesions. The present study analyzed the effects of capsaicin-induced hyperthermia on the occurrence of infectious disease and pruritic dermatitis in a rat model. Pregnant Sprague-Dawley (SD) rats were obtained 1 week prior to parturition. Pups from each litter were randomly assigned to the following experimental groups: Capsaicin-treated (cap-treated; n=10) or vehicle-treated (n=5). Capsaicin (50 mg/kg) or vehicle were systemically administered to the SD rat pups (age, 48 h), after which body temperature was measured using a biotelemetry system, and the effects of hyperthermia on the ability of the rat pups to resist bacterial infection were analyzed. Furthermore, pruritus-induced scratching behavior and dermatitis were assessed, and changes in interleukin (IL)-4- and IL-13-induced immunoglobulin E expression were measured. Treatment of neonatal rats with capsaicin resulted in chronic hyperthermia, which had negative effects on the host immune defense response. The expression levels of T-helper type 2 cell-associated cytokines were significantly increased (P<0.01) in the cap-treated rats following bacterial infection with Staphylococcus aureus or Streptococcus agalactiae. Furthermore, cap-treated rats exhibited pruritus-induced scratching behavior and dermatitis. The results of the present study suggested that treatment of neonatal rats with capsaicin induces chronic hyperthermia and decreases the effectiveness of the host defense system. Therefore, a cap-treated neonatal rat model may be considered useful when investigating the association between hyperthermia and infectious disease.

Trafficking of ThermoTRP Channels

ThermoTRP channels (thermoTRPs) define a subfamily of the transient receptor potential (TRP) channels that are activated by changes in the environmental temperature, from noxious cold to injurious heat. Acting as integrators of several stimuli and signalling pathways, dysfunction of these channels contributes to several pathological states. The surface expression of thermoTRPs is controlled by both, the constitutive and regulated vesicular trafficking. Modulation of receptor surface density during pathological processes is nowadays considered as an interesting therapeutic approach for management of diseases, such as chronic pain, in which an increased trafficking is associated with the pathological state. This review will focus on the recent advances trafficking of the thermoTRP channels, TRPV1, TRPV2, TRPV4, TRPM3, TRPM8 and TRPA1, into/from the plasma membrane. Particularly, regulated membrane insertion of thermoTRPs channels contributes to a fine tuning of final channel activity, and indeed, it has resulted in the development of novel therapeutic approaches with successful clinical results such as disruption of SNARE-dependent exocytosis by botulinum toxin or botulinomimetic peptides.