Potentiation of capsaicin receptor activity by metabotropic ATP receptors as a possible mechanism for ATP-evoked pain and hyperalgesia

Discovery and isolation of novel capsaicinoids and their TRPV1-related activity

Chilis contain capsaicin and other structurally related molecules known as capsaicinoids. Capsaicin's target protein, the transient receptor potential cation channel subfamily V member 1 (TRPV1), has been linked to many post-activation effects, including changes in metabolism and pain sensation. Capsaicinoids also bind to TRPV1, but current studies often disregard non-capsaicin interactions. To fill in these gaps, we screened 40 different chili varieties derived from four Capsicum species by means of untargeted metabolomics and a rat TRPV1 (rTRPV1) calcium influx activation assay. The resulting capsaicinoid profiles were specific to each variety but only partially corresponded with species delimitations. Based on rTRPV1 activation elicited by crude chili extracts, capsaicinoids act in an additive manner and a capsaicinoid profile can serve as a gauge of this activation. In addition, we isolated eighteen capsaicinoids, including five previously unreported ones, and confirmed their structure by NMR and MS/MS. We then tested rTRPV1 activation by 23 capsaicinoids and three related compounds. This testing revealed that even slight deviations from the structure of capsaicin reduce the ability to activate the target, with a mere single hydroxylation on the acyl tail reducing potency towards rTRPV1 by more than 100-fold. In addition, we tested how rTRPV1 activity changes in the presence of capsaicin together with non-activating capsaicin analogs and weakly activating capsaicinoids and found both classes of molecules to positively modulate the effects of capsaicin. This demonstrates that even such compounds have measurable pharmacological effects, making a case for the use and study of natural chili extracts.

A review of the patent literature surrounding TRPV1 modulators

INTRODUCTION

TRPV1, a pivotal therapeutic target for chronic pain and pruritus, has been validated in the pathogenesis of several pathologies from diabetes to cancer. Despite the constellation of chemical structures and strategies, none of these molecules has yet been clinically developed as a new drug application due to safety concerns, particularly in thermoregulation. Thus, clinical development of TRPV1 modulators remains a challenge.

AREAS COVERED

This review covers the patent literature on TRPV1 modulators (2019-2024, PubMed, Google Patents, and Espacenet), from orthosteric ligands to innovative compounds of biotechnological origin such as interfering RNAs or antibodies, and dual modulators that can act on TRPV1 and associated proteins in different tissues.

EXPERT OPINION

Therapeutic strategies that preferentially act on dysfunctional TRPV1 channels appear essential, along with a superior understanding of the underlying mechanisms affecting changes in core body temperature (CBT). Recent findings describing differential receptor interactions of antagonists that do not affect CBT may pave the way to the next generation of orally active TRPV1 inhibitors. Although we have thus far experienced a bitter feeling in TRPV1 drug development, the recent progress in different disciplines, including human-based preclinical models, will set an interdisciplinary approach to design and develop clinically relevant TRPV1 modulators.

Adult Neurogenesis Is Regulated by the Endocannabinoid and Kisspeptin Systems

Neurogenesis is considered the most robust form of plasticity in the adult brain. To better decipher this process, we evaluated the potential crosstalk of Kisspeptin and Endocannabinoid Systems (KPS and ECS, respectively) on hippocampal neurogenesis. Male adolescent rats were exposed to kisspeptin-10 (KP10) and the endocannabinoid anandamide (AEA) administered alone or in combination with the type 1 cannabinoid receptor (CB1R) antagonist SR141716A. The expression of Kiss1 and Kisspeptin receptor (Kiss1R) has been characterized for the first time in rat hippocampus together with the expression of the CB1R and the Transient Receptor Potential Vanilloid 1 ion channel receptor (TRPV1). Results show that both systems inhibit neurogenesis by reducing the extracellular signal-regulated kinase (ERK) signaling. Despite little differences in the expression of Kiss1R and CB1R, TRPV1 is enhanced by both KP10 and AEA treatments, suggesting TRPV1 as a common thread. KP10 administration reduces CB1R expression in the dentate gyrus, while AEA does not. KPS, unlike ECS, promotes the expression of estrogen receptor α (ER-α) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), also upregulating sirtuin 1 (SIRT1), brain-derived-neurotrophic factor (BDNF), and c-Jun. These findings suggest that the interaction between ECS and KPS could be involved in the fine-tuning of neurogenesis, highlighting a novel role for KPS.

Interaction between thermosensitive TRP channels and anoctamin 1

Some thermosensitive transient receptor potential (TRP) channels form a protein complex with anoctamin 1 (ANO1, also called TMEM16A). TRP channels have high calcium permeability, and the calcium entering cells through TRP channel activation activates ANO1, a calcium-activated chloride channel, involved in many physiological and pathological conditions. The physiological significance of TRP channels is often mediated by their ability to activate ANO1, which controls chloride flux across the plasma membrane. This review summarizes the latest understanding on the interactions between ANO1 and thermosensitive TRP channels, including TRPV1, TRPV3, and TRPV4, which are involved in pain sensitization in primary sensory neurons, proliferation and migration of human keratinocytes, and fluid secretion such as sweat, respectively.

TRPV1 and thermosensitivity

The capsaicin receptor TRPV1 was identified as the first heat-activated ion channel in 1997. Since then, numerous studies have been performed on its physiological functions and structure-function relationship, and chemicals targeting TRPV1 have been developed. It has been more than 27 years since the initial cloning of the TRPV1 gene and more than 11 years since the clarification of its structure at the atomic level using cryo-EM. However, we still lack good chemical antagonists of TRPV1 as medicines. TRPV1 is involved in body temperature regulation, but how TRPV1 antagonists cause hyperthermia and how TRPV1 is involved in body temperature regulation are not yet clearly understood. More research is needed in the thermal biology field.

Transient Receptor Potential Vanilloid-1 Channels Facilitate Axonal Degeneration of Corneal Sensory Nerves in Dry Eye

Corneal nerve impairment contributes significantly to dry eye disease (DED) symptoms and is thought to be secondary to corneal epithelial damage. Transient receptor potential vanilloid-1 (TRPV1) channels abound in corneal nerve fibers and respond to inflammation-derived ligands, which increase in DED. TRPV1 overactivation promotes axonal degeneration in vitro, but whether it participates in DED-associated corneal nerve dysfunction is unknown. To explore this, DED was surgically induced in wild-type and TRPV1-knockout mice, which developed comparable corneal epithelial damage and reduced tear secretion. However, corneal mechanosensitivity decreased progressively only in wild-type DED mice. Sensitivity to capsaicin (TRPV1 agonist) increased in wild-type DED mice, and consistently, only this strain displayed DED-induced pain signs. Wild-type DED mice exhibited nerve degeneration throughout the corneal epithelium, whereas TRPV1-knockout DED mice only developed a reduction in the most superficial nerve endings that failed to propagate to the deeper subbasal corneal nerves. Pharmacologic TRPV1 blockade reproduced these findings in wild-type DED mice, whereas CD4+ T cells from both strains were equally pathogenic when transferred, ruling out a T-cell-mediated effect of TRPV1 deficiency. These data show that ocular desiccation triggers superficial corneal nerve damage in DED, but proximal propagation of axonal degeneration requires TRPV1 expression. Local inflammation sensitized TRPV1 channels, which increased ocular pain. Thus, ocular TRPV1 overactivation drives DED-associated corneal nerve impairment.

Oral treatment with Berberine reduces peripheral nociception: Possible interaction with different nociceptive pathways activated by different allogeneic substances

ETHNOPHARMACOLOGICAL RELEVANCE

Berberine was identified in extracts of Berberis ruscifolia Lam., a plant used in traditional medicine as an analgesic. Its presence may be involved in the reported pharmacological activity of this species. However, there is still a lack of scientific research concerning its analgesic activity in the peripheral nervous system.

AIM OF THE STUDY

To investigate Berb-induced antinociception in the formalin test and to evaluate several pathways related to its pharmacological antinociceptive effects in chemical models of nociception in mice.

MATERIALS AND METHODS

The antinociceptive activity of Berb was assessed by inducing the paw licking in mice with different allodynic agents. In the formalin test, the antiedematous and antithermal effect of Berb was evaluated simultaneously in the same experiment. Other nociceptive behavior produced by endogenous [prostaglandin E2 (PGE2), histamine (His), glutamate (Glu) or bradykinin (BK)] or exogenous [capsaicin (Caps) and cinnamaldehyde (Cin)] chemical stimuli, and activators as protein kinase A (PKA) and C (PKC), were also evaluated.The in vivo doses for p.o. were 3 and 30 mg/kg.

RESULTS

Berb, at 30 mg/kg p.o., showed a significant inhibition of the nociceptive action in formalin in both phases being stronger at the inflammatory phase (59 ± 9%) and more active than Asp (positive control) considering the doses evaluated. Moreover, Berb inhibited the edema (34 ± 10%), but not the temperature in the formalin test. Regarding the different nociceptive signaling pathways evaluated, the most relevant data were that the administration of p.o. of Berb, at 30 mg/kg, caused significant inhibition of nociception induced by endogenous [His (72 ± 11%), PGE2 (78 ± 4%), and BK (51 ± 7%)], exogenous [Cap (68 ± 4%) and Cinn (57 ± 5%)] compounds, and activators of the PKA [(FSK (86 ± 3%)] and PKC [(PMA(86 ± 6%)] signaling pathway. Berb did not inhibit the nociceptive effect produced by Glu.

CONCLUSION

The present study demonstrated, for the first time, the potential of Berb in several nociceptive tests, with the compound present in B. ruscifolia contributing to the analgesic effect reported for this species.

Activation of Transient Receptor Potential Vanilloid 1 Is Involved in Both Pain and Tumor Growth in a Mouse Model of Cancer Pain

Activation of calcitonin gene-related peptide (CGRP)-positive sensory neurons in the tumor microenvironment has been shown to be involved in tumor growth. However, how CGRP-positive sensory neurons are activated requires elucidation. In this study, we focused on transient receptor potential vanilloid 1 (TRPV1) and examined the contribution of TRPV1 to tumor growth and cancer pain in a mouse cancer model in which Lewis lung carcinoma was subcutaneously inoculated in the left plantar region. Tumor inoculation gradually increased the volumes of the hind paws of wild type (WT) mice over time, but those of both αCGRP knockout mice and TRPV1 knockout mice were significantly smaller than those of WT mice after tumor inoculation. Both TRPV1 and CGRP are therefore suggested to be involved in tumor growth. In an immunohistochemical study, the percentage of phosphorylated cyclic adenosine monophosphate response element-binding protein (p-CREB)-positive profiles in CGRP-positive dorsal root ganglion (DRG) neurons in WT mice was significantly increased after tumor inoculation. The percentage of p-CREB-positive profiles in CGRP-positive DRG neurons in TRPV1 knockout mice was also increased after tumor inoculation, but was significantly lower than that in WT mice, indicating the contribution of TRPV1 to activation of CGRP-positive DRG neurons. Cancer pain in TRPV1 knockout mice was significantly lower than that in WT mice. In conclusion, TRPV1 is involved in both tumor growth and cancer pain, potentially leading to a novel strategy for the treatment of cancer pain and cancer development. Cancer pain is also suggested to facilitate tumor growth.

TRPV1 Channel in Human Eosinophils: Functional Expression and Inflammatory Modulation

The transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel expressed on sensory neurons and immune cells. We hypothesize that TRPV1 plays a role in human eosinophil function and is modulated by inflammatory conditions. TRPV1 expression on human eosinophils was examined by qPCR, flow cytometry, and immunohistochemistry, respectively. TRPV1 functionality was analyzed by investigating calcium flux, apoptosis, modulation by cytokines and acidic pH, and CD69 externalization using flow cytometry. Activation of TRPV1 induced calcium influx and prolonged survival. Although eosinophils were not directly activated by TRPV1 agonists, activation by IL-3 or GM-CSF was mainly restricted to TRPV1-positive eosinophils. TRPV1 surface content was increased by acidic pH, IL-3, IL-31, IL-33, TSLP, TNF-α, BDNF, and NGF-β. Interestingly, TRPV1 was also expressed by eosinophils located in proximity to peripheral nerves in atopic dermatitis (AD) skin. In conclusion, eosinophils express functional TRPV1 channels which are increased by extracellular acidification and AD-related cytokines. Since eosinophils also express TRPV1 in AD skin, our results indicate an important role of TRPV1 for neuroimmune interaction mechanisms in itchy, inflammatory skin diseases, like AD.

Thermosensation and TRP Channels

Somatosensory neurons can sense external temperature by converting sensation of temperature information to neural activity via afferent input to the central nervous system. Various populations of somatosensory neurons have specialized gene expression, including expression of thermosensitive transient receptor potential (TRP) ion channels. Thermosensitive TRP channels are responsible for thermal transduction at the peripheral ends of somatosensory neurons and can sense a wide range of temperatures. Here we focus on several thermosensitive TRP channels including TRPV1, TRPV4, TRPM2, TRPM3, TRPM8, TRPC5, and TRPA1 in sensory neurons. TRPV3, TRPV4, and TRPC5 are also involved in somatosensation in nonneuronal cells and tissues. In particular, we discuss whether skin senses ambient temperatures through TRPV3 and TRPV4 activation in skin keratinocytes and the involvement of TRPM2 expressed by hypothalamic neurons in thermosensation in the brain.

Resensitization of TRPV1 channels after the P2 receptor activation in sensory neurons of spinal ganglia in rats

Introduction

TRPV1 channels are responsible for detecting noxious stimuli such as heat (>43°C), acid, and capsaicin. P2 receptors are involved in numerous functions of the nervous system, including its modulation and specific response to the application of ATP. In our experiments, we investigated the dynamics of calcium transients in DRG neurons associated with TRPV1 channel desensitization and the effect of activation of P2 receptors on this process.

Methods

We used DRG neurons from rats P7-8 after 1-2 days of culture to measure calcium transients by microfluorescence calcimetry using the fluorescent dye Fura-2 AM.

Results

We have shown that DRG neurons of small (d < 22 μm) and medium (d = 24-35 μm) sizes differ in TRPV1 expression. Thus, TRPV1 channels are mainly present in small nociceptive neurons (59% of the studied neurons). Short-term sequential application of the TRPV1 channel agonist capsaicin (100nM) leads to the desensitization of TRPV1 channels by the type of tachyphylaxis. We identified three types of sensory neurons based on responses to capsaicin: (1) desensitized 37.5%, (2) non-desensitized 34.4%, and (3) insensitive 23.4% to capsaicin. It has also been shown that P2 receptors are present in all types of neurons according to their size. So, the responses to ATP were different in different-sized neurons. Applying ATP (0.1 mM) to the intact cell membrane after the onset of tachyphylaxis caused recovery of calcium transients in response to the addition of capsaicin in these neurons. The amplitude of the capsaicin response after reconstitution with ATP was 161% of the previous minimal calcium transient in response to capsaicin.

Discussion

Significantly, the restoration of the amplitude of calcium transients under the ATP application is not associated with changes in the cytoplasmic pool of ATP because this molecule does not cross the intact cell membrane, thus, our results show the interaction between TRPV1 channels and P2 receptors. It is important to note that the restoration of the amplitude of calcium transients through TRPV1 channels after application of ATP was observed mainly in cells of 1-2 days of cultivation. Thus, the resensitization of capsaicin transients following P2 receptor activation may be associated with the regulation of the sensitivity of sensory neurons.

Mitochondrial and metabolic features of salugenesis and the healing cycle

Pathogenesis and salugenesis are the first and second stages of the two-stage problem of disease production and health recovery. Salugenesis is the automatic, evolutionarily conserved, ontogenetic sequence of molecular, cellular, organ system, and behavioral changes that is used by living systems to heal. It is a whole-body process that begins with mitochondria and the cell. The stages of salugenesis define a circle that is energy- and resource-consuming, genetically programmed, and environmentally responsive. Energy and metabolic resources are provided by mitochondrial and metabolic transformations that drive the cell danger response (CDR) and create the three phases of the healing cycle: Phase 1-Inflammation, Phase 2-Proliferation, and Phase 3-Differentiation. Each phase requires a different mitochondrial phenotype. Without different mitochondria there can be no healing. The rise and fall of extracellular ATP (eATP) signaling is a key driver of the mitochondrial and metabolic reprogramming required to progress through the healing cycle. Sphingolipid and cholesterol-enriched membrane lipid rafts act as rheostats for tuning cellular sensitivity to purinergic signaling. Abnormal persistence of any phase of the CDR inhibits the healing cycle, creates dysfunctional cellular mosaics, causes the symptoms of chronic disease, and accelerates the process of aging. New research reframes the rising tide of chronic disease around the world as a systems problem caused by the combined action of pathogenic triggers and anthropogenic factors that interfere with the mitochondrial functions needed for healing. Once chronic pain, disability, or disease is established, salugenesis-based therapies will start where pathogenesis-based therapies end.

TRPV1 is involved in abdominal hyperalgesia in a mouse model of lipopolysaccharide-induced peritonitis and influences the immune response via peripheral noradrenergic neurons

AIMS

The transient receptor potential vanilloid subfamily 1 (TRPV1) not only plays a role as a nociceptor but also has some regulatory effects on the immune system. We investigated the effects of TRPV1 on abdominal pain and the immune system in lipopolysaccharide (LPS)-induced peritonitis and the association between TRPV1 and peripheral noradrenergic neurons.

MAIN METHODS

Experiments were performed in 8- to 14-week-old male wild-type (WT) and TRPV1 knockout (KO) mice. The mice were intraperitoneally injected with a non-lethal dose of LPS. Pain assessment and investigation of changes in the immune system were performed. Denervation of sympathetic nerves and the noradrenergic splenic nerve was induced by intraperitoneal administration of 6-hydroxydopamine.

KEY FINDINGS

The levels of serum cytokines were not significantly different in WT mice and TRPV1 KO mice. Abdominal mechanical hyperalgesia was greater in WT mice than in TRPV1 KO mice from 6 h to 3 days. The numbers of macrophages, neutrophils, dendritic cells, and CD4 T cells in the spleens of TRPV1 KO mice were significantly increased compared to those in WT mice 4 days after LPS administration. By noradrenergic denervation, the numbers of those cells in WT mice increased to levels comparable to those in TRPV1 KO mice.

SIGNIFICANCE

In LPS-induced peritonitis, abdominal inflammatory pain was transmitted via TRPV1. In addition, TRPV1 had an anti-inflammatory effect on the spleen in the late phase of peritonitis. This anti-inflammatory effect was thought to be mediated by activation of the sympathetic nervous system and/or noradrenergic splenic nerve induced by TRPV1 activation.

The role of adenosine receptor ligands on inflammatory pain: possible modulation of TRPV1 receptor function

Chronic pain has a debilitating consequences on health and lifestyle. The currently available analgesics are often ineffective and accompanied by undesirable adverse effects. Although adenosine receptors (AR) activation can affect nociceptive, inflammatory, and neuropathic pain states, the specific regulatory functions of its subtypes (A₁, A_2A, A_2B and A₃ ARs) are not fully understood. The aim of this study was to investigate the role of different AR ligands on inflammatory pain. The von Frey filament test was used to assess the anti-nociceptive effects of adenosine ligands on Complete Freund's Adjuvant (CFA)-induced mechanical allodynia in (180-220 g) adult male Sprague Dawley rats (expressed as paw withdrawal threshold, PWT). Neither the A_2AAR selective agonist CGS 21680 hydrochloride (0.1, 0.32 and 1 mg/kg) nor the A_2BAR selective agonist BAY 60-6583 (0.1, 0.32 and 1 mg/kg) produced any significant reversal of the PWT. However, the A₁AR selective agonist ( ±)-5'-Chloro-5'-deoxy-ENBA, the A₃AR selective agonist 2-Cl-IB-MECA, the A_2AAR selective antagonist ZM 241385 and the A_2BAR selective antagonist PSB 603 produced a significant reversal of the PWT at the highest dose of 1 mg/kg. Co-administration of the selective antagonists of A₁AR and A₃AR PSB36 (1 mg/ml) and MRS-3777 (1 mg/ml); respectively, significantly reversed the anti-nociceptive effects of their corresponding agonists. Furthermore, calcium imaging studies reveled that the effective AR ligands in the behavioral assay also significantly inhibit capsaicin-evoked calcium responses in cultured rat dorsal root ganglia (DRG) neurons. In conclusion, modulating the activity of the transient receptor potential vanilloid 1 (TRPV1) receptor by ARs ligands could explain their anti-nociceptive effects observed in vivo. Therefore, the cross talk between ARs and TRPV1 receptor may represent a promising targets for the treatment of inflammatory pain conditions.

Identification of a Partial and Selective TRPV1 Agonist CPIPC for Alleviation of Inflammatory Pain

Transient receptor potential vanilloid 1 (TRPV1) is a non-selective cation channel, predominantly expressed in a subset of peripheral sensory neurons for pain signaling. Topical application of agonist capsaicin for desensitizing TRPV1 currents has been approved for relief of chronic pain. However, the potent TRPV1 capsaicin is not ingestible and even topical capsaicin causes common side effects such as skin irritation, swelling, erythema and pruritus, suggesting that a mild TRPV1 agonist might be helpful for reducing side effects while reliving pain. In this study, we reported on a partial and selective TRPV1 agonist 4-(5-chloropyridin-2-yl)-N-(1H-indazol-6-yl)piperazine-1-carboxamide named CPIPC that was modified based on targeting the residue Arg557, important for conversion between the channel antagonism and agonism. Whole-cell patch clamp recordings indicated a concentration-dependent activation of TRPV1 currents by CPIPC with an EC₅₀ of 1.56 ± 0.13 μM. The maximum efficacy of CPIPC (30 μM) was about 60% of saturated capsaicin (10 μM). Repetitive additions of CPIPC caused TRPV1 current desensitization in both TRPV1-expressing HEK293 cells and dorsal root ganglion (DRG) sensory neurons. Oral administration of CPIPC dose-dependently alleviated inflammatory pain in mice. Further site-directed mutagenesis combined with molecular docking revealed that residue Arg557 is critical for TRPV1 activation by CPIPC. Taken together, we identified a novel partial and selective TRPV1 agonist CPIPC that exhibits antinociceptive activity in mice.

Peripheral mechanisms of chronic pain

Acutely, pain serves to protect us from potentially harmful stimuli, however damage to the somatosensory system can cause maladaptive changes in neurons leading to chronic pain. Although acute pain is fairly well controlled, chronic pain remains difficult to treat. Chronic pain is primarily a neuropathic condition, but studies examining the mechanisms underlying chronic pain are now looking beyond afferent nerve lesions and exploring new receptor targets, immune cells, and the role of the autonomic nervous system in contributing chronic pain conditions. The studies outlined in this review reveal how chronic pain is not only confined to alterations in the nervous system and presents findings on new treatment targets and for this debilitating disease.

The Role of Microglial Purinergic Receptors in Pain Signaling

Pain is an essential modality of sensation in the body. Purinergic signaling plays an important role in nociceptive pain transmission, under both physiological and pathophysiological conditions, and is important for communication between both neuronal and non-neuronal cells. Microglia and astrocytes express a variety of purinergic effectors, and a variety of receptors play critical roles in the pathogenesis of neuropathic pain. In this review, we discuss our current knowledge of purinergic signaling and of the compounds that modulate purinergic transmission, with the aim of highlighting the importance of purinergic pathways as targets for the treatment of persistent pain.

TRPV1 in arteries enables a rapid myogenic tone

KEY POINTS

We explored the physiological role of TRPV1 in vascular smooth muscle. TRPV1 antagonists dilated skeletal muscle arterioles both ex vivo and in vivo, increased coronary perfusion and decreased systemic blood pressure. Stretch of arteriolar myocytes and increases in intraluminal pressure in arteries triggered rapid Ca²⁺ signaling and vasoconstriction respectively. Pharmacologic and/or genetic disruption of TRPV1 significantly inhibited the magnitude and rate of these responses. Furthermore, disrupting TRPV1 blunted the rapid vasodilation evoked by arterial constriction. Pharmacological experiments identified key roles for phospholipase C and protein kinase C, combined with temperature, in TRPV1-dependent arterial tone. These results show that TRPV1 in arteriolar myocytes dynamically regulates myogenic tone and blood flow in the heart and skeletal muscle.

ABSTRACT

Arterioles maintain blow flow by adjusting their diameter in response to changes in local blood pressure. In this process called the myogenic response, a vascular smooth muscle mechanosensor controls tone predominantly through altering the membrane potential. In general, myogenic responses occur slowly (minutes). In the heart and skeletal muscle, however, tone is activated rapidly (tens of seconds) and terminated by brief (100 ms) arterial constrictions. Previously, we identified extensive expression of TRPV1 in the smooth muscle of arterioles supplying skeletal muscle, heart and fat. Here we reveal a critical role for TRPV1 in the rapid myogenic tone of these tissues. TRPV1 antagonists dilated skeletal muscle arterioles in vitro and in vivo, increased coronary flow in isolated hearts, and transiently decreased blood pressure. All of these pharmacologic effects were abolished by genetic disruption of TRPV1. Stretch of isolated vascular smooth muscle cells or raised intravascular pressure in arteries triggered Ca²⁺ signaling and vasoconstriction. The majority of these stretch-responses were TRPV1-mediated, with the remaining tone being inhibited by the TRPM4 antagonist, 9-phenantrol. Notably, tone developed more quickly in arteries from wild-type compared with TRPV1-null mice. Furthermore, the immediate vasodilation following brief constriction of arterioles depended on TRPV1, consistent with a rapid deactivation of TRPV1. Pharmacologic experiments revealed that membrane stretch activates phospholipase C/protein kinase C signaling combined with heat to activate TRPV1, and in turn, L-type Ca²⁺ channels. These results suggest a critical role, for TRPV1 in the dynamic regulation of myogenic tone and blood flow in the heart and skeletal muscle. This article is protected by copyright. All rights reserved.

TRP Channels as Molecular Targets to Relieve Cancer Pain

Transient receptor potential (TRP) channels are critical receptors in the transduction of nociceptive stimuli. The microenvironment of diverse types of cancer releases substances, including growth factors, neurotransmitters, and inflammatory mediators, which modulate the activity of TRPs through the regulation of intracellular signaling pathways. The modulation of TRP channels is associated with the peripheral sensitization observed in patients with cancer, which results in mild noxious sensory stimuli being perceived as hyperalgesia and allodynia. Secondary metabolites derived from plant extracts can induce the activation, blocking, and desensitization of TRP channels. Thus, these compounds could act as potential therapeutic agents, as their antinociceptive properties could be beneficial in relieving cancer-derived pain. In this review, we will summarize the role of TRPV1 and TRPA1 in pain associated with cancer and discuss molecules that have been reported to modulate these channels, focusing particularly on the mechanisms of channel activation associated with molecules released in the tumor microenvironment.

20-HETE is a pivotal endogenous ligand for TRPV1-mediated neurogenic inflammation in the skin

BACKGROUND AND PURPOSE

Transient receptor potential cation channel subfamily V member 1 (TRPV1) is localised to sensory C-fibres and its opening leads to membrane depolarization, resulting in neuropeptide release and neurogenic inflammation. However, the identity of the endogenous activator of TRPV1 in this setting is unknown. The arachidonic acid (AA) metabolites 12-hydroperoxyeicosatetraenoyl acid (12-HpETE) and 20-hydroxyeicosatetraenoic acid (20-HETE) have emerged as potential endogenous activators of TRPV1 however, whether these lipids underlie TRPV1-mediated neurogenic inflammation remains unknown.

EXPERIMENTAL APPROACH

we analysed human cantharidin-induced blister samples and inflammatory responses in TRPV1 transgenic mice.

KEY RESULTS

In a human cantharidin-blister model the potent TRPV1 activators 20-HETE but not 12-HETE (stable metabolite of 12-HpETE) correlated with AA levels. Similarly, in mice levels of 20-HETE (but not 12-HETE) and AA were strongly positively correlated within the inflammatory milieu. Furthermore, LPS-induced oedema formation and neutrophil recruitment were substantially and significantly attenuated by pharmacological block or genetic deletion of TRPV1 channels, inhibition of 20-HETE formation or SP receptor neurokinin 1 (NK₁ ) blockade. LPS treatment also increased cytochrome-P450 ώ-hydroxylase gene expression, the enzyme responsible for 20-HETE production.

CONCLUSIONS AND IMPLICATIONS

Taken together, our findings suggest that endogenously generated 20-HETE activates TRPV1 causing C-fibre activation and consequent oedema formation. These findings identify a novel pathway that may be useful in the therapeutics of diseases/conditions characterized by a prominent neurogenic inflammation, as in several skin diseases.

A molecular perspective on identifying TRPV1 thermosensitive regions and disentangling polymodal activation

TRPV1 is a polymodal receptor ion channel that is best known to function as a molecular thermometer. It is activated in diverse ways, including by heat, protons (low pH), and vanilloid compounds, such as capsaicin. In this review, we summarize molecular studies of TRPV1 thermosensing, focusing on the cross-talk between heat and other activation modes. Additional insights from TRPV1 isoforms and non-rodent/non-human TRPV1 ortholog studies are also discussed in this context. While the molecular mechanism of heat activation is still emerging, it is clear that TRPV1 thermosensing is modulated allosterically, i.e., at a distance, with contributions from many distinct regions of the channel. Similarly, current studies identify cross-talk between heat and other TRPV1 activation modes, such as protons and capsaicin, and that these modes can generally be selectively disentangled. In aggregate, this suggests that future TRPV1 molecular studies should define allosteric pathways and provide mechanistic insight, thereby enabling mode-selective manipulation of the polymodal receptor. These advances are anticipated to have significant implications in both basic and applied biomedical sciences.

Photobiomodulation inhibits inflammation in the temporomandibular joint of rats

Photobiomodulation (PBM) has been applied as a non-invasive technique for treating temporomandibular joint symptoms, especially on painful condition's relief, however the anti-inflammatory mechanism underlying the effect of PBM remains uncertain. This study aims to evaluate the mechanisms of action of PBM (808 nm) in a carrageenan-induced inflammation on temporomandibular joint (TMJ) of rats. In this study male Wistar rats were pre-treated with irradiation of a low-power diode laser for 15 s on TMJ (infra-red 808 nm, 100 mW, 50 J/cm² and 1.5 J) 15 min prior an injection in the temporomandibular joint of carrageenan (100 μg/TMJ). 1 h after the TMJ treatments, the rats were terminally anesthetized for joint cavity wash and periarticular tissues collect. Samples analysis demonstrated that PBM inhibit leukocytes chemotaxis in the TMJ and significantly reduces amounts of TNF-α, IL-1β and CINC-1. In addition, Western blotting analysis demonstrated that PBM significantly decreased the protein levels of P2X3 and P2X7 receptors in the periarticular tissues. On the other hand, PBM was able to increase protein level of IL-10 (anti-inflammatory cytokine). In summary, it is possible to suggest that PBM inhibit inflammatory chemotaxis, modulation the balance of the pro- and anti-inflammatory characteristics of inflammatory cells.

Macrophage as a Peripheral Pain Regulator

A neuroimmune crosstalk is involved in somatic and visceral pathological pain including inflammatory and neuropathic components. Apart from microglia essential for spinal and supraspinal pain processing, the interaction of bone marrow-derived infiltrating macrophages and/or tissue-resident macrophages with the primary afferent neurons regulates pain signals in the peripheral tissue. Recent studies have uncovered previously unknown characteristics of tissue-resident macrophages, such as their origins and association with regulation of pain signals. Peripheral nerve macrophages and intestinal resident macrophages, in addition to adult monocyte-derived infiltrating macrophages, secrete a variety of mediators, such as tumor necrosis factor-α, interleukin (IL)-1β, IL-6, high mobility group box 1 and bone morphogenic protein 2 (BMP2), that regulate the excitability of the primary afferents. Neuron-derived mediators including neuropeptides, ATP and macrophage-colony stimulating factor regulate the activity or polarization of diverse macrophages. Thus, macrophages have multitasks in homeostatic conditions and participate in somatic and visceral pathological pain by interacting with neurons.

TRPV1 Activation Promotes β-arrestin2 Interaction with the Ribosomal Biogenesis Machinery in the Nucleolus:Implications for p53 Regulation and Neurite Outgrowth

Transient receptor potential vanilloids (TRPV1) are non-selective cation channels that sense and transduce inflammatory pain signals. We previously reported that activation of TRPV1 induced the translocation of β-arrestin2 (ARRB2) from the cytoplasm to the nucleus, raising questions about the functional role of ARRB2 in the nucleus. Here, we determined the ARRB2 nuclear signalosome by conducting a quantitative proteomic analysis of the nucleus-sequestered L395Q ARRB2 mutant, compared to the cytosolic wild-type ARRB2 (WT ARRB2), in a heterologous expression system. We identified clusters of proteins that localize to the nucleolus and are involved in ribosomal biogenesis. Accordingly, L395Q ARRB2 or WT ARRB2 after capsaicin treatment were found to co-localize and interact with the nucleolar marker nucleophosmin (NPM1), treacle protein (TCOF1) and RNA polymerase I (POL I). We further investigated the role of nuclear ARRB2 signaling in regulating neuroplasticity. Using neuroblastoma (neuro2a) cells and dorsal root ganglia (DRG) neurons, we found that L395Q ARRB2 mutant increased POL I activity, inhibited the tumor suppressorp53 (p53) level and caused a decrease in the outgrowth of neurites. Together, our results suggest that the activation of TRPV1 promotes the ARRB2-mediated regulation of ribosomal biogenesis in the nucleolus. The ARRB2-TCOF1-p53 checkpoint signaling pathway might be involved in regulating neurite outgrowth associated with pathological pain conditions.

Beyond Neuronal Heat Sensing: Diversity of TRPV1 Heat-Capsaicin Receptor-Channel Functions

Transient receptor potential vanilloid 1 (TRPV1) is a calcium-permeable ion channel best known for its ability to be gated by the pungent constituent of red chili pepper, capsaicin, and related chemicals from the group of vanilloids as well as by noxious heat. As such, it is mostly expressed in sensory neurons to act as a detector of painful stimuli produced by pungent chemicals and high temperatures. Its activation is also sensitized by the numerous endogenous inflammatory mediators and second messengers, making it an important determinant of nociceptive signaling. Except for such signaling, though, neuronal TRPV1 activation may influence various organ functions by promoting the release of bioactive neuropeptides from sensory fiber innervation organs. However, TRPV1 is also found outside the sensory nervous system in which its activation and function is not that straightforward. Thus, TRPV1 expression is detected in skeletal muscle; in some types of smooth muscle; in epithelial and immune cells; and in adipocytes, where it can be activated by the combination of dietary vanilloids, endovanilloids, and pro-inflammatory factors while the intracellular calcium signaling that this initiates can regulate processes as diverse as muscle constriction, cell differentiation, and carcinogenesis. The purpose of the present review is to provide a clear-cut distinction between neurogenic TRPV1 effects in various tissues consequent to its activation in sensory nerve endings and non-neurogenic TRPV1 effects due to its expression in cell types other than sensory neurons.

Capsaicin: Plants of the Genus Capsicum and Positive Effect of Oriental Spice on Skin Health

BACKGROUND

Capsaicin, the main pungent ingredient in hot chili peppers, causes excitation of small sensory neurons. It also provides the basic pungent flavor in Capsicum fruits.

SUMMARY

Capsaicin plays a vital role as an agonist for the TRPV1 (transient receptor potential cation channel, subfamily V, member 1) receptor. TRPV1 is essential for the reduction of oxidative stress, pain sensations, and inflammation. Therefore, it has many pros related to health issue. Activation and positive impact of TRPV1 via capsaicin has been studied in various dermatological conditions and in other skin-related issues. Past studies documented that capsaicin plays a vital role in the prevention of atopic dermatitis as well as psoriasis. Moreover, TRPV1 is also very important for skin health because it acts as a capsaicin receptor. It is found in nociceptive nerve fibers and nonneural structures. It prompts the release of a compound that is involved in communicating pain between the spinal cord nerves and other parts of the body. Key Messages: Here, we summarize the growing evidence for the beneficial role of capsaicin and TRPV1 and how they help in the relief of skin diseases such as inflammation, permeation, dysfunction, atopic dermatitis, and psoriasis and in pain amplification syndrome.

Insulin-Like Growth Factor 1 Increases GABAergic Neurotransmission to GnRH Neurons via Suppressing the Retrograde Tonic Endocannabinoid Signaling Pathway in Mice

INTRODUCTION

Hypophysiotropic gonadotropin-releasing hormone (GnRH) neurons orchestrate various physiological events that control the onset of puberty. Previous studies showed that insulin-like growth factor 1 (IGF-1) induces the secretion of GnRH and accelerates the onset of puberty, suggesting a regulatory role of this hormone upon GnRH neurons.

METHODS

To reveal responsiveness of GnRH neurons to IGF-1 and elucidate molecular pathways acting downstream to the IGF-1 receptor (IGF-1R), in vitro electrophysiological experiments were carried out on GnRH-GFP neurons in acute brain slices from prepubertal (23-29 days) and pubertal (50 days) male mice.

RESULTS

Administration of IGF-1 (13 nM) significantly increased the firing rate and frequency of spontaneous postsynaptic currents and that of excitatory GABAergic miniature postsynaptic currents (mPSCs). No GABAergic mPSCs were induced by IGF-1 in the presence of the GABAA-R blocker picrotoxin. The increase in the mPSC frequency was prevented by the use of the IGF-1R antagonist, JB1 (1 µM), or the intracellularly applied PI3K blocker (LY294002, 50 µM), showing involvement of IGF-1R and PI3K in the mechanism. Blockade of the transient receptor potential vanilloid 1, an element of the tonic retrograde endocannabinoid machinery, by AMG9810 (10 µM) or antagonizing the cannabinoid receptor type-1 by AM251 (1 µM) abolished the effect.

DISCUSSION/CONCLUSION

These findings indicate that IGF-1 arrests the tonic retrograde endocannabinoid pathway in GnRH neurons, and this disinhibition increases the release of GABA from presynaptic terminals that, in turn, activates GnRH neurons leading to the fine-tuning of the hypothalamo-pituitary-gonadal axis.

Regulation of the cold-sensing TRPM8 channels by phosphoinositides and Gq-coupled receptors

The Transient Receptor Potential Melastatin 8 (TRPM8) ion channel is an important sensor of environmental cold temperatures. Cold- and menthol-induced activation of this channel requires the presence of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. This review discusses recent findings on the role of PI(4,5)P2 and G-proteins in the modulation of TRPM8 upon receptor activation. We will also summarize knowledge on the role of PI(4,5)P2 in Ca2+ dependent desensitization/adaptation of TRPM8 activity, and recent advances in the structural basis of how this lipid binds to TRPM8.

TRPV1-Targeted Drugs in Development for Human Pain Conditions

The transient receptor potential vanilloid-1 (TRPV1) is a non-specific cation channel known for its sensitivity to pungent vanilloid compound (i.e. capsaicin) and noxious stimuli, including heat, low pH or inflammatory mediators. TRPV1 is found in the somatosensory system, particularly primary afferent neurons that respond to damaging or potentially damaging stimuli (nociceptors). Stimulation of TRPV1 evokes a burning sensation, reflecting a central role of the channel in pain. Pharmacological and genetic studies have validated TRPV1 as a therapeutic target in several preclinical models of chronic pain, including cancer, neuropathic, postoperative and musculoskeletal pain. While antagonists of TRPV1 were found to be a valuable addition to the pain therapeutic toolbox, their clinical use has been limited by detrimental side effects, such as hyperthermia. In contrast, capsaicin induces a prolonged defunctionalisation of nociceptors and thus opened the door to the development of a new class of therapeutics with long-lasting pain-relieving effects. Here we review the list of TRPV1 agonists undergoing clinical trials for chronic pain management, and discuss new indications, formulations or combination therapies being explored for capsaicin. While the analgesic pharmacopeia for chronic pain patients is ancient and poorly effective, modern TRPV1-targeted drugs could rapidly become available as the next generation of analgesics for a broad spectrum of pain conditions.

Acoustic Trauma Increases Ribbon Number and Size in Outer Hair Cells of the Mouse Cochlea

Outer hair cells (OHCs) in the mouse cochlea are contacted by up to three type II afferent boutons. On average, only half of these are postsynaptic to presynaptic ribbons. Mice of both sexes were subjected to acoustic trauma that produced a threshold shift of 44.2 ± 9.1 dB 7 days after exposure. Ribbon synapses of OHCs were quantified in post-trauma and littermate controls using immunolabeling of CtBP2. Visualization with virtual reality was used to determine 3-D cytoplasmic localization of CtBP2 puncta to the synaptic pole of OHCs. Acoustic trauma was associated with a statistically significant increase in the number of synaptic ribbons per OHC. Serial section TEM was carried out on similarly treated mice. This also showed a significant increase in the number of ribbons in post-trauma OHCs, as well as a significant increase in ribbon volume compared to ribbons in control OHCs. An increase in OHC ribbon synapses after acoustic trauma is a novel observation that has implications for OHC:type II afferent signaling. A mathematical model showed that the observed increase in OHC ribbons considered alone could produce a significant increase in action potentials among type II afferent neurons during strong acoustic stimulation.

Involvement of Neural Transient Receptor Potential Channels in Peripheral Inflammation

Transient receptor potential (TRP) channels are a superfamily of non-selective cation channels that act as polymodal sensors in many tissues throughout mammalian organisms. In the context of ion channels, they are unique for their broad diversity of activation mechanisms and their cation selectivity. TRP channels are involved in a diverse range of physiological processes including chemical sensing, nociception, and mediating cytokine release. They also play an important role in the regulation of inflammation through sensory function and the release of neuropeptides. In this review, we discuss the functional contribution of a subset of TRP channels (TRPV1, TRPV4, TRPM3, TRPM8, and TRPA1) that are involved in the body’s immune responses, particularly in relation to inflammation. We focus on these five TRP channels because, in addition to being expressed in many somatic cell types, these channels are also expressed on peripheral ganglia and nerves that innervate visceral organs and tissues throughout the body. Activation of these neural TRP channels enables crosstalk between neurons, immune cells, and epithelial cells to regulate a wide range of inflammatory actions. TRP channels act either through direct effects on cation levels or through indirect modulation of intracellular pathways to trigger pro- or anti-inflammatory mechanisms, depending on the inflammatory disease context. The expression of TRP channels on both neural and immune cells has made them an attractive drug target in diseases involving inflammation. Future work in this domain will likely yield important new pathways and therapies for the treatment of a broad range of disorders including colitis, dermatitis, sepsis, asthma, and pain.

Genetic variants and expression changes in urgency urinary incontinence: A systematic review

Aim

To perform a systematic review summarizing the knowledge of genetic variants, gene, and protein expression changes in humans and animals associated with urgency urinary incontinence (UUI) and to provide an overview of the known molecular mechanisms related to UUI.

Methods

A systematic search was performed on March 2, 2020, in PubMed, Embase, Web of Science, and the Cochrane library. Retrieved studies were screened for eligibility. The risk of bias was assessed using the ROBINS‐I (human) and SYRCLE (animal) tool. Data were presented in a structured manner and in the case of greater than five studies on a homogeneous outcome, a meta‐analysis was performed.

Results

Altogether, a total of 10,785 records were screened of which 37 studies met the inclusion criteria. Notably, 24/37 studies scored medium‐high to high on risk of bias, affecting the value of the included studies. The analysis of 70 unique genes and proteins and three genome‐wide association studies showed that specific signal transduction pathways and inflammation are associated with UUI. A meta‐analysis on the predictive value of urinary nerve growth factor (NGF) levels showed that increased urinary NGF levels correlate with UUI.

Conclusion

The collective evidence showed the involvement of two molecular mechanisms (signal transduction and inflammation) and NGF in UUI, enhancing our understanding of the pathophysiology of UUI. Unfortunately, the risk of bias was medium‐high to high for most studies and the value of many observations remains unclear. Future studies should focus on elucidating how deficits in the two identified molecular mechanisms contribute to UUI and should avoid bias.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Negative Modulation of TRPM8 Channel Function by Protein Kinase C in Trigeminal Cold Thermoreceptor Neurons

TRPM8 is the main molecular entity responsible for cold sensing. This polymodal ion channel is activated by cold, cooling compounds such as menthol, voltage, and rises in osmolality. In corneal cold thermoreceptor neurons (CTNs), TRPM8 expression determines not only their sensitivity to cold, but also their role as neural detectors of ocular surface wetness. Several reports suggest that Protein Kinase C (PKC) activation impacts on TRPM8 function; however, the molecular bases of this functional modulation are still poorly understood. We explored PKC-dependent regulation of TRPM8 using Phorbol 12-Myristate 13-Acetate to activate this kinase. Consistently, recombinant TRPM8 channels, cultured trigeminal neurons, and free nerve endings of corneal CTNs revealed a robust reduction of TRPM8-dependent responses under PKC activation. In corneal CTNs, PKC activation decreased ongoing activity, a key parameter in the role of TRPM8-expressing neurons as humidity detectors, and also the maximal cold-evoked response, which were validated by mathematical modeling. Biophysical analysis indicated that PKC-dependent downregulation of TRPM8 is mainly due to a decreased maximal conductance value, and complementary noise analysis revealed a reduced number of functional channels at the cell surface, providing important clues to understanding the molecular mechanisms of how PKC activity modulates TRPM8 channels in CTNs.

TRPV1 is crucial for thermal homeostasis in the mouse by heat loss behaviors under warm ambient temperature

Thermal homeostasis in mammalians is a self-regulating process by which biological systems maintain an internal thermal stability, even under different temperature conditions; however, the molecular mechanisms involved under warm ambient temperature remain unclear. Here, we aimed to clarify functional significance of transient receptor potential vanilloid receptor 1 (TRPV1) under warm ambient temperature. TRPV1 KO mice exhibited transient hyperthermia when exposed to 30.0 and 32.5 °C, whereas wild-type (WT) mice did not. TRPV1 KO mice exhibited prolonged and prominent hyperthermia upon exposure to 35.0 °C, whereas WT mice showed transient hyperthermia. Hyperthermia also occurs in WT mice that received intracerebroventricular injection of TRPV1 antagonist AMG9810 upon exposure to 35.0 °C. Heat loss behaviors, sleeping and body licking, were deficient in TRPV1 KO mice exposed to warm temperatures. Therefore, the present results indicate that central TRPV1 is crucial for maintaining a constant body temperature via the initiation of heat loss behaviors under warm ambient temperature.

Diacylglycerol kinases regulate TRPV1 channel activity

The transient receptor potential vanilloid 1 (TRPV1) channel is activated by heat and by capsaicin, the pungent compound in chili peppers. Calcium influx through TRPV1 has been shown to activate a calcium-sensitive phospholipase C (PLC) enzyme and to lead to a robust decrease in phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] levels, which is a major contributor to channel desensitization. Diacylglycerol (DAG), the product of the PLC-catalyzed PI(4,5)P₂ hydrolysis, activates protein kinase C (PKC). PKC is known to potentiate TRPV1 activity during activation of G protein-coupled receptors, but it is not known whether DAG modulates TRPV1 during desensitization. We found here that inhibition of diacylglycerol kinase (DAGK) enzymes reduces desensitization of native TRPV1 in dorsal root ganglion neurons as well as of recombinant TRPV1 expressed in HEK293 cells. The effect of DAGK inhibition was eliminated by mutating two PKC-targeted phosphorylation sites, Ser-502 and Ser-800, indicating involvement of PKC. TRPV1 activation induced only a small and transient increase in DAG levels, unlike the robust and more sustained increase induced by muscarinic receptor activation. DAGK inhibition substantially increased the DAG signal evoked by TRPV1 activation but not that evoked by M1 muscarinic receptor activation. Our results show that Ca²⁺ influx through TRPV1 activates PLC and DAGK enzymes and that the latter limits formation of DAG and negatively regulates TRPV1 channel activity. Our findings uncover a role of DAGK in ion channel regulation.

Lifting the veil on the keratinocyte contribution to cutaneous nociception

Cutaneous nociception is essential to prevent individuals from sustaining injuries. According to the conventional point of view, the responses to noxious stimuli are thought to be exclusively initiated by sensory neurons, whose activity would be at most modulated by keratinocytes. However recent studies have demonstrated that epidermal keratinocytes can also act as primary nociceptive transducers as a supplement to sensory neurons. To enlighten our understanding of cutaneous nociception, this review highlights recent and relevant findings on the cellular and molecular elements that underlie the contribution of epidermal keratinocytes as nociceptive modulators and noxious sensors, both under healthy and pathological conditions.

A novel approach for detection of functional expression of TRPV1 channels on regenerated neurons following nerve injury

Transient receptor potential vanilloid 1 (TRPV1) is a polymodal receptor channel, which plays an important role in pain transduction. It is important to understand the functional expression of this channel under neuropathic pain (NP) conditions. A novel method was used to investigate the dynamics of functional expression of this channel on regenerated neurons under NP conditions following trigeminal nerve injury using a combination of a permanently charged sodium channel blocker (QX-314) and a TRPV1 agonist (capsaicin; QX-CAP). The combination was originally introduced as a local anesthetic. Synchronization between the local anesthetic effect of QX-CAP and TRPV1 expression on regenerated neurons was observed following the nerve injury. QX-CAP had no local anesthetic effect under NP conditions 2 weeks after the injury when TRPV1 expression on regenerated neurons was low. However, this combination was effective under NP conditions 3 and 4 weeks following injury when TRPV1 expression in regenerated neurons was moderate to high. The current review, discusses the potential of QX-314 as a local anesthetic and a novel approach of using QX-CAP to reveal the dynamics of functional expression of TRPV1 on regenerated neurons following trigeminal nerve injury.

TRP Channels in the Focus of Trigeminal Nociceptor Sensitization Contributing to Primary Headaches

Pain in trigeminal areas is driven by nociceptive trigeminal afferents. Transduction molecules, among them the nonspecific cation channels transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), which are activated by endogenous and exogenous ligands, are expressed by a significant population of trigeminal nociceptors innervating meningeal tissues. Many of these nociceptors also contain vasoactive neuropeptides such as calcitonin gene-related peptide (CGRP) and substance P. Release of neuropeptides and other functional properties are frequently examined using the cell bodies of trigeminal neurons as models of their sensory endings. Pathophysiological conditions cause phosphorylation, increased expression and trafficking of transient receptor potential (TRP) channels, neuropeptides and other mediators, which accelerate activation of nociceptive pathways. Since nociceptor activation may be a significant pathophysiological mechanism involved in both peripheral and central sensitization of the trigeminal nociceptive pathway, its contribution to the pathophysiology of primary headaches is more than likely. Metabolic disorders and medication-induced painful states are frequently associated with TRP receptor activation and may increase the risk for primary headaches.

Role of opioid receptors in modulation of P2X receptor-mediated cardiac sympathoexcitatory reflex response

Myocardial ischemia evokes powerful reflex responses through activation of vagal and sympathetic afferents in the heart through the release of ischemic metabolites. We have demonstrated that extracellular ATP stimulates cardiac sympathetic afferents through P2 receptor-mediated mechanism, and that opioid peptides suppress these afferents' activity. However, the roles of both P2 receptor and endogenous opioids in cardiac sympathoexcitatory reflex (CSR) responses remain unclear. We therefore hypothesized that activation of cardiac P2 receptor evokes CSR responses by stimulating cardiac sympathetic afferents and these CSR responses are modulated by endogenous opioids. We observed that intrapericardial injection of α,β-methylene ATP (α,β-meATP, P2X receptor agonist), but not ADP (P2Y receptor agonist), caused a graded increase in mean arterial pressure in rats with sinoaortic denervation and vagotomy. This effect of α,β-meATP was abolished by blockade of cardiac neural transmission with intrapericardial procaine treatment and eliminated by intrapericardial A-317491, a selective P2X_2/3 and P2X₃ receptor antagonist. Intrapericardial α,β-meATP also evoked CSR response in vagus-intact rats. Furthermore, the P2X receptor-mediated CSR responses were enhanced by intrapericardial naloxone, a specific opioid receptor antagonist. These data suggest that stimulation of cardiac P2X_2/3 and P2X₃, but not P2Y receptors, powerfully evokes CSR responses through activation of cardiac spinal afferents, and that endogenous opioids suppress the P2X receptor-mediated CSR responses.

Capsaicin: Physicochemical properties, cutaneous reactions and potential applications in painful and inflammatory conditions

Capsaicin is a natural protoalkaloid recognized as the main pungent component in hot peppers (Capsicum annuum L.). The capsaicin receptor is highly expressed in the unmyelinated type C nerve fibers originating from small diameter sensory neurons in dorsal root ganglia and cranial nerve ganglia correspondents. Capsaicin and related vanilloids have a variety of effects on primary sensory neurons function, from sensory neuron excitation characterized by local burning sensation and neurogenic inflammation, followed by conduction blockage accompanied by reversible ultrastructural changes of peripheral nociceptive endings (desensitization), going as far as irreversible degenerative changes (neurotoxicity). The main role in capsaicin-induced neurogenic inflammation relies on the capsaicin sensitive, small diameter primary sensory neurons, therefore its evaluation could be used as a diagnostic instrument in functional alterations of cutaneous sensory nerve fibers. Moreover, capsaicin-induced desensitization and neurotoxicity explain the analgesic/anti-nociceptive and anti-inflammatory effects of topical capsaicin and its potential use in the management of painful and inflammatory conditions. In this study, we describe the effects of capsaicin on neurogenic inflammation and nociception, as well as its potential diagnostic value and therapeutic impact in various conditions involving impairment of sensory nerve fibers.

Involvement of presynaptic TRPV1 channels in prostaglandin E2-induced facilitation of spontaneous synaptic transmission in the rat spinal trigeminal subnucleus caudalis

Prostaglandin E₂ (PGE₂) synthesized in the central nervous system influences various physiological functions including nociception. Recently, we have demonstrated that PGE₂ facilitates spontaneous synaptic transmission through presynaptic EP1 receptors in the spinal trigeminal subnucleus caudalis (Vc) neurons that receive nociceptive signals from the orofacial area. Increasing evidence suggests that the action of PGE₂ is related to activation of transient receptor potential vanilloid 1 (TRPV1) channels. The present study investigated whether TRPV1 channels contribute to the facilitatory effect of PGE₂ on synaptic transmission in the Vc neurons. Spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were recorded from Vc neurons in the rat brainstem slice by whole-cell patch-clamp mode. Superfusion of capsaicin (0.3, 1.0 μM) concentration-dependently increased the frequency of both sEPSCs and sIPSCs without any significant effect on their amplitude. The effect of capsaicin was completely abolished by a TRPV1 channel blocker AMG9810 (0.1 μM). PGE₂ (5.0 μM) increased the frequency of sEPSCs and sIPSCs. This facilitatory effect of PGE₂ was attenuated by AMG9810 and in neurons desensitized by repeated application of capsaicin. While a low concentration of either PGE₂ (1.0 μM) or capsaicin (0.1 μM) had an insignificant effect on the sEPSCs and sIPSCs, co-application of these drugs increased their frequency. The present study demonstrated involvement of the presynaptic TRPV1 channels in PGE₂-induced facilitation of spontaneous synaptic transmissions and suggests interaction of PGE₂ with TRPV1 channels in modification of nociceptive signals from the orofacial area to the Vc neurons.

Emerging Perspectives on Pain Management by Modulation of TRP Channels and ANO1

Receptor-type ion channels are critical for detection of noxious stimuli in primary sensory neurons. Transient receptor potential (TRP) channels mediate pain sensations and promote a variety of neuronal signals that elicit secondary neural functions (such as calcitonin gene-related peptide [CGRP] secretion), which are important for physiological functions throughout the body. In this review, we focus on the involvement of TRP channels in sensing acute pain, inflammatory pain, headache, migraine, pain due to fungal infections, and osteo-inflammation. Furthermore, action potentials mediated via interactions between TRP channels and the chloride channel, anoctamin 1 (ANO1), can also generate strong pain sensations in primary sensory neurons. Thus, we also discuss mechanisms that enhance neuronal excitation and are dependent on ANO1, and consider modulation of pain sensation from the perspective of both cation and anion dynamics.

Capsaicin: Effects on the Pathogenesis of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most frequent cancers, and to date, there have been very few drugs available that can improve survival, the most well-known being sorafenib. The pathogenesis of HCC is complex, involving multiple processes including abnormal cell and tissue regeneration, angiogenesis, genomic instability, cellular proliferation, and signaling pathway alterations. Capsaicin is a substance that holds increasingly high interest and is studied as a therapeutic option in a wide array of diseases. Several studies have investigated capsaicin roles in various stages of HCC oncogenesis. This paper aims to thoroughly detail the available information on the individual effects of capsaicin on the cellular mechanisms and pathways involved in HCC development, as well as investigate their possible cooperation and interferences. The synergistic antitumor effects of capsaicin and sorafenib are also addressed.

G αq Sensitizes TRPM8 to Inhibition by PI(4,5)P2 Depletion upon Receptor Activation

The cold- and menthol-sensitive transient receptor potential melastatin 8 (TRPM8) channel is important for both physiological temperature detection and cold allodynia. Activation of G-protein-coupled receptors (GPCRs) by proinflammatory mediators inhibits these channels. It was proposed that this inhibition proceeds via direct binding of G _αq to the channel. TRPM8 requires the plasma membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂ or PIP₂] for activity. However, it was claimed that a decrease in cellular levels of this lipid upon receptor activation does not contribute to channel inhibition. Here, we show that supplementing the whole-cell patch pipette with PI(4,5)P₂ reduced inhibition of TRPM8 by activation of G_αq-coupled receptors in mouse dorsal root ganglion (DRG) neurons isolated from both sexes. Stimulating the same receptors activated phospholipase C (PLC) and decreased plasma membrane PI(4,5)P₂ levels in these neurons. PI(4,5)P₂ also reduced inhibition of TRPM8 by activation of heterologously expressed muscarinic M1 receptors. Coexpression of a constitutively active G _αq protein that does not couple to PLC inhibited TRPM8 activity, and in cells expressing this protein, decreasing PI(4,5)P₂ levels using a voltage-sensitive 5'-phosphatase induced a stronger inhibition of TRPM8 activity than in control cells. Our data indicate that, upon GPCR activation, G _αq binding reduces the apparent affinity of TRPM8 for PI(4,5)P₂ and thus sensitizes the channel to inhibition induced by decreasing PI(4,5)P₂ levels.SIGNIFICANCE STATEMENT Increased sensitivity to heat in inflammation is partially mediated by inhibition of the cold- and menthol-sensitive transient receptor potential melastatin 8 (TRPM8) ion channels. Most inflammatory mediators act via G-protein-coupled receptors that activate the phospholipase C pathway, leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂]. How receptor activation by inflammatory mediators leads to TRPM8 inhibition is not well understood. Here, we propose that direct binding of G _αq both reduces TRPM8 activity and sensitizes the channel to inhibition by decreased levels of its cofactor, PI(4,5)P₂ Our data demonstrate the convergence of two downstream effectors of receptor activation, G _αq and PI(4,5)P₂ hydrolysis, in the regulation of TRPM8.

Antinociceptive effects of new pyrazoles compounds mediated by the ASIC-1α channel, TRPV-1 and μMOR receptors

Pyrazoles are potent medicinal scaffolds and exhibit a wide spectrum of biological activities, such as analgesic, anti-inflammatory and antipyretic. In this paper we report on research we have performed with the aim of continuing the biological evaluation of the regio-isomeric pyrazole compounds, LQFM-020 (fluorine, para position), LQFM-021 (fluorine, meta position), and LQFM-039 (fluorine, ortho position) in models of pain induced by acidified saline, capsaicin, and formalin. We also investigated the mechanisms of action of these compounds via electrophysiological analyses using the two-electrode voltage-clamp technique and heterologous expression in Xenopus laevis oocytes. This enabled us to study different potassium channel subtypes: the ASIC-1α channel, TRPV-1, and μMOR receptors. Our results indicate that LQFM-020, LQFM-021, and LQFM-039 (15, 30 or 60 mg.kg^-1) compounds inhibited the nociceptive response induced by acidified saline in a dose-dependent manner. The dose of 30 mg.kg^-1 inhibited the nociceptive response induced by capsaicin by 53.3%, 51.4%, and 52.1%, respectively. In addition, we found that naloxone reverses the antinociceptive effect produced by the compounds in both phases of the formalin test. In electrophysiological analyses, we observed that the LQFM-020, LQFM-021, and LQFM-039 compounds did not modulate voltage-gated K + channel subtypes. In contrast, all the compounds tested inhibited the ASIC-1α channel at pH 4.5, with IC50-values of 96.1, 91.6, and 235.2 μM, respectively. All compounds also inhibited the TRPV-1 channel with IC50-values of 139.1, 212.5, and 159.1 μM, respectively. In contrast to the ASIC-1α and TRPV-1 targets, all compounds showed agonist activity on the μMOR receptor with an EC50-value of 117.4, 98.9, and 86.3 μM, respectively. We thus conclude that the ASIC-1α, TRPV-1, and μMOR channels are targets that are directly involved in the antinociceptive effect of LQFM-020, LQFM-021, and LQFM-039. Furthermore, the modifications of the fluorine positions in the phenyl analogs do not change the analgesic effect. However, LQFM-039 showed lower interaction with ASIC-1α channel.

The Role of Transient Receptor Potential (TRP) Channels in the Transduction of Dental Pain

Dental pain is a common health problem that negatively impacts the activities of daily living. Dentine hypersensitivity and pulpitis-associated pain are among the most common types of dental pain. Patients with these conditions feel pain upon exposure of the affected tooth to various external stimuli. However, the molecular mechanisms underlying dental pain, especially the transduction of external stimuli to electrical signals in the nerve, remain unclear. Numerous ion channels and receptors localized in the dental primary afferent neurons (DPAs) and odontoblasts have been implicated in the transduction of dental pain, and functional expression of various polymodal transient receptor potential (TRP) channels has been detected in DPAs and odontoblasts. External stimuli-induced dentinal tubular fluid movement can activate TRP channels on DPAs and odontoblasts. The odontoblasts can in turn activate the DPAs by paracrine signaling through ATP and glutamate release. In pulpitis, inflammatory mediators may sensitize the DPAs. They could also induce post-translational modifications of TRP channels, increase trafficking of these channels to nerve terminals, and increase the sensitivity of these channels to stimuli. Additionally, in caries-induced pulpitis, bacterial products can directly activate TRP channels on DPAs. In this review, we provide an overview of the TRP channels expressed in the various tooth structures, and we discuss their involvement in the development of dental pain.