Transient receptor potential channels in pain and inflammation: therapeutic opportunities

The challenge of pharmacotherapy for musculoskeletal pain: an overview of unmet needs

Musculoskeletal disorders are characterized by several impairments, including pain, affecting muscles, bones, joints and adjacent connective tissue, resulting in temporary or permanent functional limitations and disability. Musculoskeletal pain is particularly prevalent worldwide and greatly impacts the quality of life, social participation and economic burden. To date, several issues persist about the classification of musculoskeletal pain and its management strategies and resources. The treatment of musculoskeletal pain conditions is complex and often requires a multimodal approach, including pharmacological and non-pharmacological therapy that might be ineffective in many cases, resulting in poor patient satisfaction and controversial expectations about the potential benefits of available interventions. This manuscript provides an overview of unmet needs in managing musculoskeletal pain, particularly focusing on pharmacotherapeutic pitfalls in this context.

Peripheral Neuroinflammation and Pain: How Acute Pain Becomes Chronic

The number of individuals suffering from severe chronic pain and its social and financial impact is staggering. Without significant advances in our understanding of how acute pain becomes chronic, effective treatments will remain out of reach. This mini review will briefly summarize how critical signaling pathways initiated during the early phases of peripheral nervous system inflammation/ neuroinflammation establish long-term modifications of sensory neuronal function. Together with the recruitment of non-neuronal cellular elements, nociceptive transduction is transformed into a pathophysiologic state sustaining chronic peripheral sensitization and pain. Inflammatory mediators, such as nerve growth factor (NGF), can lower activation thresholds of sensory neurons through posttranslational modification of the pain-transducing ion channels transient-receptor potential TRPV1 and TRPA1. Performing a dual role, NGF also drives increased expression of TRPV1 in sensory neurons through the recruitment of transcription factor Sp4. More broadly, Sp4 appears to modulate a nociceptive transcriptome including TRPA1 and other genes encoding components of pain transduction. Together, these findings suggest a model where acute pain evoked by peripheral injury-induced inflammation becomes persistent through repeated cycles of TRP channel modification, Sp4-dependent overexpression of TRP channels and ongoing production of inflammatory mediators.

Capsaicin and 1,25-Dihydroxyvitamin D3 Glycoside: Effects on the Reproductive Performance of Hyper-Prolific Sows

This study evaluated the effect of a natural source of vitamin D3 [1,25-(OH)2D3] and capsaicin (CAP) in the dietary supplementation of sows in the final phase (85-114 days) of gestation (Gest) and lactation (Lact) on the reproductive performance of the sows and health of piglets through two experiments (Exp I and II). In Exp I, 120 sows were subjected to four treatments: T1-control (without [1,25-(OH)2D3] and supplemental CAP); T2-3.5 µg 1,25-(OH)2D3/Gest/day and 7.0 µg Vit 1,25-(OH)2D3/Lact/day; T3-7.0 µg CAP/Gest/day and 14.0 µg CAP/Lact/day; T4-1.75 µg Vit 1,25-(OH)2D3 + 3.5 µg CAP/Gest/day and 3.5 µg 1,25-(OH)2D3 + 7.0 µg CAP/Lact/day. In Exp II, 200 sows were randomly blocked, factorial 2 × 2 (without or with Vit 1,25-(OH)2D3 and without or with CAP): T1-control (without Vit 1,25-(OH)2D3 and CAP); T2-3.5 µg Vit 1,25-(OH)2D3/Gest/day and 7.0 µg Vit 1,25-(OH)2D3/Lact/day; T3-7.0 µg CAP/Gest/day and 14.0 µg CAP/Lact/day; T4-3.5 µg Vit 1,25-(OH)2D3 + 7 µg CAP/Gest/day; and 7.0 µg Vit 1,25-(OH)2D3 + 14.0 µg CAP/Lact/day. The duration of delivery (3:48 vs. 4:57 h) and the percentage of stillbirths (5.37% vs. 7.61%) were improved (p < 0.05) in the group that received Vit 1,25-(OH)2D3 (Exp II) compared to the control group. Moreover, the dystocia rate decreased (p < 0.05) in Exp II, which received Vit 1,25-(OH)2D3 (4.21 vs. 27.63%), and in Exp I, which received the combination of Vit 1,25-(OH)2D3 + CAP (12 vs. 40%) compared to the respective control groups. Colostrum production was greater (p < 0.05) in sows that received Vit 1,25-(OH)2D3 supplementation compared to the control group, consequently resulting in higher colostrum intake (p < 0.05) of the piglets (330 vs. 258 g/piglet). The additives reduced the incidence of diarrhea (p < 0.05) in piglets (Exp I and II). Thus, the use of additives improved the reproductive performance of sows and contributed to litter growth.

Therapeutic activity of lipoxin A4 in TiO2-induced arthritis in mice: NF-κB and Nrf2 in synovial fluid leukocytes and neuronal TRPV1 mechanisms

Background

Lipoxin A4 (LXA4) has anti-inflammatory and pro-resolutive roles in inflammation. We evaluated the effects and mechanisms of action of LXA4 in titanium dioxide (TiO2) arthritis, a model of prosthesis-induced joint inflammation and pain.

Methods

Mice were stimulated with TiO2 (3mg) in the knee joint followed by LXA4 (0.1, 1, or 10ng/animal) or vehicle (ethanol 3.2% in saline) administration. Pain-like behavior, inflammation, and dosages were performed to assess the effects of LXA4 in vivo.

Results

LXA4 reduced mechanical and thermal hyperalgesia, histopathological damage, edema, and recruitment of leukocytes without liver, kidney, or stomach toxicity. LXA4 reduced leukocyte migration and modulated cytokine production. These effects were explained by reduced nuclear factor kappa B (NFκB) activation in recruited macrophages. LXA4 improved antioxidant parameters [reduced glutathione (GSH) and 2,2-azino-bis 3-ethylbenzothiazoline-6-sulfonate (ABTS) levels, nuclear factor erythroid 2-related factor 2 (Nrf2) mRNA and Nrf2 protein expression], reducing reactive oxygen species (ROS) fluorescent detection induced by TiO2 in synovial fluid leukocytes. We observed an increase of lipoxin receptor (ALX/FPR2) in transient receptor potential cation channel subfamily V member 1 (TRPV1)+ DRG nociceptive neurons upon TiO2 inflammation. LXA4 reduced TiO2-induced TRPV1 mRNA expression and protein detection, as well TRPV1 co-staining with p-NFκB, indicating reduction of neuronal activation. LXA4 down-modulated neuronal activation and response to capsaicin (a TRPV1 agonist) and AITC [a transient receptor potential ankyrin 1 (TRPA1) agonist] of DRG neurons.

Conclusion

LXA4 might target recruited leukocytes and primary afferent nociceptive neurons to exert analgesic and anti-inflammatory activities in a model resembling what is observed in patients with prosthesis inflammation.

Multiple Chemical Sensitivity: It's time to catch up to the science

Multiple chemical sensitivity (MCS) is a complex medical condition associated with low dose chemical exposures. MCS is characterized by diverse features and common comorbidities, including fibromyalgia, cough hypersensitivity, asthma, and migraine, and stress/anxiety, with which the syndrome shares numerous neurobiological processes and altered functioning within diverse brain regions. Predictive factors linked to MCS comprise genetic influences, gene-environment interactions, oxidative stress, systemic inflammation, cell dysfunction, and psychosocial influences. The development of MCS may be attributed to the sensitization of transient receptor potential (TRP) receptors, notably TRPV1 and TRPA1. Capsaicin inhalation challenge studies demonstrated that TRPV1 sensitization is manifested in MCS, and functional brain imaging studies revealed that TRPV1 and TRPA1 agonists promote brain-region specific neuronal variations. Unfortunately, MCS has often been inappropriately viewed as stemming exclusively from psychological disturbances, which has fostered patients being stigmatized and ostracized, and often being denied accommodation for their disability. Evidence-based education is essential to provide appropriate support and advocacy. Greater recognition of receptor-mediated biological mechanisms should be incorporated in laws, and regulation of environmental exposures.

Second-degree burn induced by high-concentration topical capsaicin with mobility sequelae: A case report

High-concentration topical capsaicin is used as a second-line treatment for neuropathic pain. Transient, mild burning sensation and erythema are expected adverse drug reactions. Here, we report the first case of second degree burn after the application of a high-concentration topical capsaicin patch with secondary mobility sequelae. Nine months after the application, neuropathic pain still remained and the patient described mobility difficulties in daily activities, preventing her from returning to work. This report aims to raise the question of the benefit/risk ratio of high concentration topical capsaicin.

Neurological susceptibility to environmental exposures: pathophysiological mechanisms in neurodegeneration and multiple chemical sensitivity

The World Health Organization lists air pollution as one of the top five risks for developing chronic non-communicable disease, joining tobacco use, harmful use of alcohol, unhealthy diets and physical inactivity. This review focuses on how host defense mechanisms against adverse airborne exposures relate to the probable interacting and overlapping pathophysiological features of neurodegeneration and multiple chemical sensitivity. Significant long-term airborne exposures can contribute to oxidative stress, systemic inflammation, transient receptor subfamily vanilloid 1 (TRPV1) and subfamily ankyrin 1 (TRPA1) upregulation and sensitization, with impacts on olfactory and trigeminal nerve function, and eventual loss of brain mass. The potential for neurologic dysfunction, including decreased cognition, chronic pain and central sensitization related to airborne contaminants, can be magnified by genetic polymorphisms that result in less effective detoxification. Onset of neurodegenerative disorders is subtle, with early loss of brain mass and loss of sense of smell. Onset of MCS may be gradual following long-term low dose airborne exposures, or acute following a recognizable exposure. Upregulation of chemosensitive TRPV1 and TRPA1 polymodal receptors has been observed in patients with neurodegeneration, and chemically sensitive individuals with asthma, migraine and MCS. In people with chemical sensitivity, these receptors are also sensitized, which is defined as a reduction in the threshold and an increase in the magnitude of a response to noxious stimulation. There is likely damage to the olfactory system in neurodegeneration and trigeminal nerve hypersensitivity in MCS, with different effects on olfactory processing. The associations of low vitamin D levels and protein kinase activity seen in neurodegeneration have not been studied in MCS. Table 2 presents a summary of neurodegeneration and MCS, comparing 16 distinctive genetic, pathophysiological and clinical features associated with air pollution exposures. There is significant overlap, suggesting potential comorbidity. Canadian Health Measures Survey data indicates an overlap between neurodegeneration and MCS (p < 0.05) that suggests comorbidity, but the extent of increased susceptibility to the other condition is not established. Nevertheless, the pathways to the development of these conditions likely involve TRPV1 and TRPA1 receptors, and so it is hypothesized that manifestation of neurodegeneration and/or MCS and possibly why there is divergence may be influenced by polymorphisms of these receptors, among other factors.

ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

TRP channels and monoterpenes: Past and current leads on analgesic properties

The activation of the transient receptor potential (TRP) channels expressed by sensory neurons is essential to the transduction of thermal and mechanical sensory information. In the setting of chronic inflammatory conditions, the activation of the melastatin family member 8 (TRPM8), the TRP vanilloid 1 (TRPV1), and the TRP ankyrin 1 (TRPA1) is correlated with pain hypersensitivity reactions. Monoterpenes, among which pulegone and menthol, a major class of phytocompounds present in essential oils of medicinal plants, are known modulators of those TRP channels activity. In the present review, we correlate the monoterpene content of plants with their historical therapeutic properties. We then describe how monoterpenes exert their anti-inflammatory and antihyperalgesia effects through modulation of TRP channels activity. Finally, we discuss the importance and the potential of characterizing new plant extracts and reassessing studied plant extracts for the development of ethnopharmacology-based innovative treatments for chronic pain. This review suggests that monoterpene solutions, based on composition from traditional healing herbs, offer an interesting avenue for the development of new phytotherapeutic treatments to alleviate chronic inflammatory pain conditions.

Neurogenic inflammation as a novel treatment target for chronic pain syndromes

Chronic pain syndrome is a heterogeneous group of diseases characterized by several pathological mechanisms. One in five adults in Europe may experience chronic pain. In addition to the individual burden, chronic pain has a significant societal impact because of work and school absences, loss of work, early retirement, and high social and healthcare costs. Several anti-inflammatory treatments are available for patients with inflammatory or autoimmune diseases to control their symptoms, including pain. However, patients with degenerative chronic pain conditions, some with 10-fold or more elevated incidence relative to these manageable diseases, have few long-term pharmacological treatment options, limited mainly to non-steroidal anti-inflammatory drugs or opioids. For this review, we performed multiple PubMed searches using keywords such as "pain," "neurogenic inflammation," "NGF," "substance P," "nociception," "BDNF," "inflammation," "CGRP," "osteoarthritis," and "migraine." Many treatments, most with limited scientific evidence of efficacy, are available for the management of chronic pain through a trial-and-error approach. Although basic science and pre-clinical pain research have elucidated many biomolecular mechanisms of pain and identified promising novel targets, little of this work has translated into better clinical management of these conditions. This state-of-the-art review summarizes concepts of chronic pain syndromes and describes potential novel treatment strategies.

Piezo1 Channels as Force Sensors in Mechanical Force-Related Chronic Inflammation

Mechanical damage is one of the predisposing factors of inflammation, and it runs through the entire inflammatory pathological process. Repeated or persistent damaging mechanical irritation leads to chronic inflammatory diseases. The mechanism of how mechanical forces induce inflammation is not fully understood. Piezo1 is a newly discovered mechanically sensitive ion channel. The Piezo1 channel opens in response to mechanical stimuli, transducing mechanical signals into an inflammatory cascade in the cell leading to tissue inflammation. A large amount of evidence shows that Piezo1 plays a vital role in the occurrence and progression of chronic inflammatory diseases. This mini-review briefly presents new evidence that Piezo1 responds to different mechanical stresses to trigger inflammation in various tissues. The discovery of Piezo1 provides new insights for the treatment of chronic inflammatory diseases related to mechanical stress. Inhibiting the transduction of damaging mechanical signals into inflammatory signals can inhibit inflammation and improve the outcome of inflammation at an early stage. The pharmacology of Piezo1 has shown bright prospects. The development of tissue-specific Piezo1 drugs for clinical use may be a new target for treating chronic inflammation.

Inhibitory effect of polysulfide, an endogenous sulfur compound, on oxidative stress-induced TRPA1 activation

Polysulfide (PS), an endogenous sulfur compound, generated by oxidation of hydrogen sulfide, has a stimulatory action on the nociceptive TRPA1 channel. TRPA1 is also activated by reactive oxygen species such as hydrogen peroxide (H₂O₂) produced during inflammation. Here, we examined the effect of PS on H₂O₂-induced responses in native and heterologously expressed TRPA1 using a cell-based calcium assay. We also carried out behavioral experiments in vivo. In mouse sensory neurons, H₂O₂ elicited early TRPA1-dependent and late TRPA1-independent increases of [Ca²⁺]_i. The former was suppressed by the pretreatment with PS. In cells heterologously expressed TRPA1, PS suppressed [Ca²⁺]_i responses to H₂O₂. Simultaneous measurement of [Ca²⁺]_i and the intracellular PS level revealed that scavenging effect of PS was not related to the inhibitory effect. Removal of extracellular Ca²⁺, a calmodulin inhibitor and dithiothreitol attenuated the inhibitory effect of PS. Pretreatment with PS diminished nociceptive behaviors induced by H₂O₂. The present data suggest that PS suppresses oxidative stress-induced TRPA1 activation due to cysteine modification and Ca²⁺/calmodulin signaling. Thus, endogenous sulfurs may have regulatory roles in nociception via functional changes in TRPA1 under inflammatory conditions.

Analgesia for Sheep in Commercial Production: Where to Next?

Simple Summary

Increasing societal and customer pressure to provide animals with ‘a life worth living’ continues to apply pressure on industry to alleviate pain associated with husbandry practices, injury and illness. Although a number of analgesic solutions are now available for sheep, providing some amelioration of the acute pain responses, this review has highlighted a number of potential areas for further research.

Abstract

Increasing societal and customer pressure to provide animals with ‘a life worth living’ continues to apply pressure on livestock production industries to alleviate pain associated with husbandry practices, injury and illness. Over the past 15–20 years, there has been considerable research effort to understand and develop mitigation strategies for painful husbandry procedures in sheep, leading to the successful launch of analgesic approaches specific to sheep in a number of countries. However, even with multi-modal approaches to analgesia, using both local anaesthetic and non-steroidal anti-inflammatory drugs (NSAID), pain is not obliterated, and the challenge of pain mitigation and phasing out of painful husbandry practices remains. It is timely to review and reflect on progress to date in order to strategically focus on the most important challenges, and the avenues which offer the greatest potential to be incorporated into industry practice in a process of continuous improvement. A structured, systematic literature search was carried out, incorporating peer-reviewed scientific literature in the period 2000–2019. An enormous volume of research is underway, testament to the fact that we have not solved the pain and analgesia challenge for any species, including our own. This review has highlighted a number of potential areas for further research.

In Vitro Cytotoxic Protective Effect of Alginate-Encapsulated Capsaicin Might Improve Skin Side Effects Associated with the Topical Application of Capsaicin

Chronic neuropathic pain, particularly peripheral pain, is a cause of great concern for diabetic patients. Current treatments include numerous agents such as capsaicinoids, a known deterrent of neuropathic pain despite the inconvenience associated with local side effects. In this context, the current work aims to elucidate the potential mechanisms involved in cytotoxicity by capsaicin and proposes an efficient formulation of capsaicin in alginate microcapsules, which significantly reduces side effects from capsaicin topical administration. For this, human dermal fibroblast cells were treated with alginate-microencapsulated capsaicin extracts and screened for potential cytotoxic effects produced by the treatment. Cell viability and morphology were examined, as well as oxidative stress status and anti-inflammatory potential. Our results show that the alginate encapsulated formulation of capsaicin exerted lower cytotoxic effects on human dermal fibroblasts as measured by cell viability and reactive oxygen species (ROS) production. Furthermore, the expression profiles of inflammatory cytokines were significantly altered by the treatment as compared with the control culture.

Gene expression profiles of transient receptor potential (TRP) channels in the peripheral blood mononuclear cells of psoriasis patients

Psoriasis is a chronic autoimmune disease in which peripheral blood mononuclear cells (PBMCs) are involved in the pathological process. Transient receptor potential (TRP) channels expressed in immune cells have been shown to be associated with inflammatory diseases. We aimed to evaluate mRNA expression levels of TRP channels in PBMCs of patients with psoriasis. 30 patients with plaque psoriasis and 30 healthy age- and gender-matched control subjects were included in this study. mRNA expression levels of TRP channels in psoriasis patients were determined by Real-time polymerase chain reaction. A decreased TRPM4, TRPM7, TRPV3, TRPV4, and TRPC6 genes expression levels were found in the patient group compared to controls, respectively (p = 0.045, p = 0.000, p = 0.000, p = 0.045, p = 0.009), whereas, an increased expression level was found in TRPM2 and TRPV1 genes in the patient group compared to controls (p = 0.001 and p = 0.028). This is the first study showing the TRP channel mRNA expressions in PBMCs of psoriasis patients. Different expression patterns of TRP channels may have a role in pathogenesis of psoriasis.

Antinociceptive effect of triterpene acetyl aleuritolic acid isolated from Croton zehntneri in adult zebrafish (Danio rerio)

Croton zehntneri is a plant known as canelinha de cunhã, prevalent in the northeast region of Brazil. Many constituents of the vegetable have already been studied, and their pharmacological properties have been proven, but this is the first study to analyze the antinociceptive effect in adult zebrafish (ZFa) of the triterpene acetyl aleuritolic acid (AAA) isolated from the stem bark. The animals (ZFa; n = 6/group) were treated intraperitoneally (ip; 20 μL) with AAA (0.1 or 0.3 or 1.0 mg/mL) or vehicle (0.9% saline; 20 μL), and submitted to the locomotor activity test, as well as 96 h acute toxicity. Other groups (n = 6/each) received the same treatments and underwent acute nociception tests (formalin, cinnamaldehyde, glutamate, acid saline, capsaicin, and hypertonic saline). Possible neuromodulation mechanisms were evaluated. AAA (0.1 or 0.3 or 1.0 mg/mL) reduced the nociceptive behavior induced by acid saline and capsaicin, as well as inhibited corneal nociception induced by hypertonic saline, both without altering the animals' locomotor system and without toxicity. These analgesic effects of AAA were significantly (p > 0.05) similar to those of morphine, used as a positive control. The antinociceptive effect of AAA was inhibited by methylene blue, ketamine, camphor, ruthenium red, amiloride, and mefenamic acid. The antinociceptive effect of AAA on the cornea of animals was inhibited by capsazepine. Therefore, AAA showed pharmacological potential for the treatment of acute pain, and this effect is modulated by cGMP, NMDA receptors, transient receptor potential channels (TRPs), ASICs and has pharmacological potential for the treatment of corneal pain modulated by the TRPV1 channel.

Fight fire with fire: Neurobiology of capsaicin-induced analgesia for chronic pain

Capsaicin, the pungent ingredient in chili peppers, produces intense burning pain in humans. Capsaicin selectively activates the transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptive primary afferents, and underpins the mechanism for capsaicin-induced burning pain. Paradoxically, capsaicin has long been used as an analgesic. The development of topical patches and injectable formulations containing capsaicin has led to application in clinical settings to treat chronic pain conditions, such as neuropathic pain and the potential to treat osteoarthritis. More detailed determination of the neurobiological mechanisms of capsaicin-induced analgesia should provide the logical rationale for capsaicin therapy and help to overcome the treatment's limitations, which include individual differences in treatment outcome and procedural discomfort. Low concentrations of capsaicin induce short-term defunctionalization of nociceptor terminals. This phenomenon is reversible within hours and, hence, likely does not account for the clinical benefit. By contrast, high concentrations of capsaicin lead to long-term defunctionalization mediated by the ablation of TRPV1-expressing afferent terminals, resulting in long-lasting analgesia persisting for several months. Recent studies have shown that capsaicin-induced Ca²⁺/calpain-mediated ablation of axonal terminals is necessary to produce long-lasting analgesia in a mouse model of neuropathic pain. In combination with calpain, axonal mitochondrial dysfunction and microtubule disorganization may also contribute to the longer-term effects of capsaicin. The analgesic effects subside over time in association with the regeneration of the ablated afferent terminals. Further determination of the neurobiological mechanisms of capsaicin-induced analgesia should lead to more efficacious non-opioidergic analgesic options with fewer adverse side effects.

Sustained laryngeal transient receptor potential vanilloid 1 activation inhibits mechanically induced swallowing in anesthetized rats

A major component of gastric acid is hydrochloric acid (HCl), which can activate transient receptor potential vanilloid 1 (TRPV1). In the present study, we investigated how sustained laryngeal TRPV1 activation affects the frequency of the swallowing reflex. Experiments were carried out on 85 male Sprague-Dawley rats. The effects of short and sustained application of chemicals (3 µl of 0.1 N HCl or capsaicin) on the frequency of swallowing and on time-dependent changes in the occurrence of swallowing evoked by supralaryngeal nerve stimulation were determined. To evaluate vascular permeability of the larynx, Evans blue dye was intravenously injected after 5 or 60 min of sustained TRPV1 activation. SB366791 (a TRPV1 inhibitor) and Cap/QX-314 (a TRPV1-expressed neuronal inhibitor) significantly inhibited HCl/capsaicin-induced swallowing, but air flow-induced swallowing was not affected. Although the number of air flow-induced swallows followed by capsaicin stimulation was not affected within 5 min, it was significantly reduced by 60-min capsaicin or HCl application. The swallowing threshold associated with supralaryngeal nerve stimulation did not significantly change throughout the recording period. Evans blue dye concentrations in the larynx were significantly higher at 60 min in the 10^-5 M capsaicin group than in the control group. Our results suggest that sustained TPRV1 activation not only desensitizes TRPV1 but also inactivates mechanoreceptors, which may be attributed to increases in vascular permeability and edema, as part of an inflammatory process.NEW & NOTEWORTHY Although a transient receptor potential vanilloid 1 (TRPV1) inhibitor or TRPV1-expressed neuronal inhibitor significantly inhibited HCl/capsaicin-evoked swallowing, air flow-induced swallowing was not affected. The number of air flow-induced swallows was significantly reduced within 60 min of TRPV1 activation. Evans blue dye concentration in the larynx increased 60 min after capsaicin application. TPRV1 activation not only desensitizes TRPV1 but also inactivates mechanoreceptors caused by increases in vascular permeability and edema.

Novel Agents in Neuropathic Pain, the Role of Capsaicin: Pharmacology, Efficacy, Side Effects, Different Preparations

PURPOSE OF REVIEW

Capsaicin is a natural substance used to treat neuropathic pain because of its ability to be used in a more direct form on patients and efficiently treat their pain without the amount of side effects seen in the use of oral medications.

RECENT FINDINGS

Currently, the treatments for neuropathic pain are, control of the underlying disease process, then focused on symptomatic relief with pharmacotherapy, topical analgesics, or other interventions. When all pharmacological agents fail to relieve the pain, interventional strategies can be considered, such as neural blocks, spinal cord stimulation, and intrathecal administered medications. The response to current treatment of neuropathic pain is only modest relief of symptoms. Multiple treatment options may be attempted, while ultimately leaving patients with refractory neuropathic pain. For these reasons, a better treatment approach to neuropathic pain is greatly needed. Overall, capsaicin has great potential for becoming a first- or second-line treatment for neuropathic pain, and for becoming a therapeutic option for many other neuropathic pain-related disease states.

The use of high dose topical capsaicin in the management of peripheral neuropathy: narrative review and local experience

Capsaicin, derived from the chilli pepper plant, is available in high concentration (8%) patches to provide topical therapy for neuropathic pain. Its analgesic effects relate to defunctionalisation and nerve terminal retraction of predominantly C fibres in the dermis and epidermis. Systematic reviews and meta-analysis support its use for the management of post-herpetic neuralgia and HIV neuropathy with some evidence for use in painful peripheral diabetic neuropathy. The article concludes with advice on the practicalities of running a topical 8% capsaicin clinic for peripheral neuropathic pain.

Mechanisms of pain in sickle cell disease

Objectives

The hallmark of sickle cell disease (SCD) is acute and chronic pain, and the pain dominates the clinical characteristics of SCD patients. Although pharmacological treatments of SCD targeting the disease mechanisms have been improved, many SCD patients suffer from pain. To overcome the pain of the disease, there have been renewed requirements to understand the novel molecular mechanisms of the pain in SCD.

Methods

We concisely summarized the molecular mechanisms of SCD-related acute and chronic pain, focusing on potential drug targets to treat pain.

Results

Acute pain of SCD is caused by vaso-occulusive crisis (VOC), impaired oxygen supply or infarction-reperfusion tissue injuries. In VOC, inflammatory cytokines include tryptase activate nociceptors and transient receptor potential vanilloid type 1. In tissue injury, the secondary inflammatory response is triggered and causes further tissue injuries. Tissue injury generates cytokines and pain mediators including bradykinin, and they activate nociceptive afferent nerves and trigger pain. The main causes of chronic pain are from extended hyperalgesia after a VOC and central sensitization. Neuropathic pain could be due to central or peripheral nerve injury, and protein kinase C might be associated with the pain. In central sensitization, neuroplasticity in the brain and the activation of glial cells may be related with the pain.

Discussion

In this review, we summarized the molecular mechanisms of SCD-related acute and chronic pain. The novel treatments targeting the disease mechanisms would interrupt complications of SCD and reduce the pain of the SCD patients.

An update on targets for treating osteoarthritis pain: NGF and TRPV1

Purpose of review

a)Osteoarthritis (OA) is the most common form of arthritis, and pain is the primary symptom of the disease, yet analgesic options for treating OA pain remain limited. In this review, we aimed to give an update on the current clinical and preclinical studies targeting two pathways that are being investigated for treating OA pain: the nerve growth factor (NGF) pathway and the transient receptor potential vanilloid-1 (TRPV1) pathway.

Recent findings

b)Antibodies against NGF, small molecule inhibitors of TrkA, TRPV1 agonists, and TRPV1 antagonists are all in different stages of clinical and pre-clinical testing for the treatment of OA pain. NGF antibodies have shown efficacy in the primary endpoints tested compared to placebo, however, rapidly progressive OA has been consistently observed in a subset of patients and the cause remains unclear. TRPV1 agonists have also demonstrated reduced pain with no serious adverse events - the most common adverse events include a burning or warming sensation upon administration.

Summary

c)Targeting the NGF and TRPV1 pathways appear effective for reducing OA pain, but further work is needed to better understand which patients may benefit most from these treatments. The anti-NGF antibody tanezumab and the TRPV1 agonist CNTX-4975 have both received fast-track designation from the FDA for the treatment of OA pain.

Simultaneous Fluorescence Imaging Reveals N-Methyl-d-aspartic Acid Receptor Dependent Zn2+/H+ Flux in the Brains of Mice with Depression

Depression is immensely attributed to the overactivation of N-methyl-d-aspartic acid (NMDA) receptor in the brains. As regulatory binding partners of NMDA receptor, both Zn²⁺ and H⁺ are intimately interrelated to NMDA receptor's activity. Therefore, exploring synergistic changes on the levels of Zn²⁺ and H⁺ in brains will promote the knowledge and treatment of depression. However, the lack of efficient, appropriate imaging tools limits simultaneously tracking Zn²⁺ and H⁺ in living mouse brains. Thus, a well-designed dual-color fluorescent probe (DNP) was fabricated for the simultaneous monitoring of Zn²⁺ and H⁺ in the brains of mice with depression. Encountering Zn²⁺, the probe evoked bright blue fluorescence at 460 nm. Meanwhile, the red fluorescence at 680 nm was decreased with H⁺ addition. With blue/red dual fluorescence signal of DNP, we observed the synchronous increased Zn²⁺ and H⁺ in PC12 cells under oxidative stress. Notably, in vivo imaging for the first time revealed the simultaneous reduction of Zn²⁺ and pH in brains of mice with depression-like behaviors. Further results implied that the NMDA receptor might be responsible for the coinstantaneous fluctuation of Zn²⁺ and H⁺ during depression. Altogether, this work is conducive to the knowledge of neural signal transduction mechanisms, advancing our understanding of the pathogenesis in depression.

The Pivotal Role of TRP Channels in Homeostasis and Diseases throughout the Gastrointestinal Tract

The transient receptor potential (TRP) channels superfamily are a large group of proteins that play crucial roles in cellular processes. For example, these cation channels act as sensors in the detection and transduction of stimuli of temperature, small molecules, voltage, pH, and mechanical constrains. Over the past decades, different members of the TRP channels have been identified in the human gastrointestinal (GI) tract playing multiple modulatory roles. Noteworthy, TRPs support critical functions related to the taste perception, mechanosensation, and pain. They also participate in the modulation of motility and secretions of the human gut. Last but not least, altered expression or activity and mutations in the TRP genes are often related to a wide range of disorders of the gut epithelium, including inflammatory bowel disease, fibrosis, visceral hyperalgesia, irritable bowel syndrome, and colorectal cancer. TRP channels could therefore be promising drug targets for the treatment of GI malignancies. This review aims at providing a comprehensive picture of the most recent advances highlighting the expression and function of TRP channels in the GI tract, and secondly, the description of the potential roles of TRPs in relevant disorders is discussed reporting our standpoint on GI tract–TRP channels interactions.

Expression and Activity of TRPA1 and TRPV1 in the Intervertebral Disc: Association with Inflammation and Matrix Remodeling

Transient receptor potential (TRP) channels have emerged as potential sensors and transducers of inflammatory pain. The aims of this study were to investigate (1) the expression of TRP channels in intervertebral disc (IVD) cells in normal and inflammatory conditions and (2) the function of Transient receptor potential ankyrin 1 (TRPA1) and Transient receptor potential vanilloid 1 (TRPV1) in IVD inflammation and matrix homeostasis. RT-qPCR was used to analyze human fetal, healthy, and degenerated IVD tissues for the gene expression of TRPA1 and TRPV1. The primary IVD cell cultures were stimulated with either interleukin-1 beta (IL-1β) or tumor necrosis factor alpha (TNF-α) alone or in combination with TRPA1/V1 agonist allyl isothiocyanate (AITC, 3 and 10 µM), followed by analysis of calcium flux and the expression of inflammation mediators (RT-qPCR/ELISA) and matrix constituents (RT-qPCR). The matrix structure and composition in caudal motion segments from TRPA1 and TRPV1 wild-type (WT) and knock-out (KO) mice was visualized by FAST staining. Gene expression of other TRP channels (A1, C1, C3, C6, V1, V2, V4, V6, M2, M7, M8) was also tested in cytokine-treated cells. TRPA1 was expressed in fetal IVD cells, 20% of degenerated IVDs, but not in healthy mature IVDs. TRPA1 expression was not detectable in untreated cells and it increased upon cytokine treatment, while TRPV1 was expressed and concomitantly reduced. In inflamed IVD cells, 10 µM AITC activated calcium flux, induced gene expression of IL-8, and reduced disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5) and collagen 1A1, possibly via upregulated TRPA1. TRPA1 KO in mice was associated with signs of degeneration in the nucleus pulposus and the vertebral growth plate, whereas TRPV1 KO did not show profound changes. Cytokine treatment also affected the gene expression of TRPV2 (increase), TRPV4 (increase), and TRPC6 (decrease). TRPA1 might be expressed in developing IVD, downregulated during its maturation, and upregulated again in degenerative disc disease, participating in matrix homeostasis. However, follow-up studies with larger sample sizes are needed to fully elucidate the role of TRPA1 and other TRP channels in degenerative disc disease.

Transcription factor Sp4 is required for hyperalgesic state persistence

Understanding how painful hypersensitive states develop and persist beyond the initial hours to days is critically important in the effort to devise strategies to prevent and/or reverse chronic painful states. Changes in nociceptor transcription can alter the abundance of nociceptive signaling elements, resulting in longer-term change in nociceptor phenotype. As a result, sensitized nociceptive signaling can be further amplified and nocifensive behaviors sustained for weeks to months. Building on our previous finding that transcription factor Sp4 positively regulates the expression of the pain transducing channel TRPV1 in Dorsal Root Ganglion (DRG) neurons, we sought to determine if Sp4 serves a broader role in the development and persistence of hypersensitive states in mice. We observed that more than 90% of Sp4 staining DRG neurons were small to medium sized, primarily unmyelinated (NF200 neg) and the majority co-expressed nociceptor markers TRPV1 and/or isolectin B4 (IB4). Genetically modified mice (Sp4+/-) with a 50% reduction of Sp4 showed a reduction in DRG TRPV1 mRNA and neuronal responses to the TRPV1 agonist-capsaicin. Importantly, Sp4+/- mice failed to develop persistent inflammatory thermal hyperalgesia, showing a reversal to control values after 6 hours. Despite a reversal of inflammatory thermal hyperalgesia, there was no difference in CFA-induced hindpaw swelling between CFA Sp4+/- and CFA wild type mice. Similarly, Sp4+/- mice failed to develop persistent mechanical hypersensitivity to hind-paw injection of NGF. Although Sp4+/- mice developed hypersensitivity to traumatic nerve injury, Sp4+/- mice failed to develop persistent cold or mechanical hypersensitivity to the platinum-based chemotherapeutic agent oxaliplatin, a non-traumatic model of neuropathic pain. Overall, Sp4+/- mice displayed a remarkable ability to reverse the development of multiple models of persistent inflammatory and neuropathic hypersensitivity. This suggests that Sp4 functions as a critical control point for a network of genes that conspire in the persistence of painful hypersensitive states.

Mechanism for Regulation of Melanoma Cell Death via Activation of Thermo-TRPV4 and TRPV2

Background

Thermo-TRPs (temperature-sensitive transient receptor potential channels) belong to the TRP (transient receptor potential) channel superfamily. Emerging evidence implied that thermo-TRPs have been involved in regulation of cell fate in certain tumors. However, their distribution profiles and roles in melanoma remain incompletely understood.

Methods

Western blot and digital PCR approaches were performed to identify the distribution profiles of six thermo-TRPs. MTT assessment was employed to detect cell viability. Flow cytometry was applied to test cell cycle and apoptosis. Calcium imaging was used to determine the function of channels. Five cell lines, including one normal human primary epidermal melanocytes and two human malignant melanoma (A375, G361) and two human metastatic melanoma (A2058, SK-MEL-3) cell lines, were chosen for this research.

Results

In the present study, six thermo-TRPs including TRPV1/2/3/4, TRPA1, and TRPM8 were examined in human primary melanocytes and melanoma cells. We found that TRPV2/4, TRPA1, and TRPM8 exhibited ectopic distribution both in melanocytes and melanoma cells. Moreover, activation of TRPV2 and TRPV4 could lead to the decline of cell viability for melanoma A2058 and A375 cells. Subsequently, activation of TRPV2 by 2-APB (IC₅₀ = 150 μM) induced cell necrosis in A2058 cells, while activation of TRPV4 by GSK1016790A (IC₅₀ = 10 nM) enhanced apoptosis of A375 cells. Furthermore, TRPV4 mediated cell apoptosis of melanoma via phosphorylation of AKT and was involved in calcium regulation.

Conclusion

Overall, our studies revealed that TRPV4 and TRPV2 mediated melanoma cell death via channel activation and characterized the mechanism of functional TRPV4 ion channel in regulating AKT pathway driven antitumor process. Thus, they may serve as potential biomarkers for the prognosis and are targeted for the therapeutic use in human melanoma.

Diosmetin as a novel transient receptor potential vanilloid 1 antagonist with antinociceptive activity in mice

Diosmetin is an O‑methylated flavone found naturally in citrus fruit, and it was identified in Amphilophium crucigerum (L.), a plant popularly used as an analgesic. This compound had different pharmacological effects and presented a chemical structure like the flavonoid eriodyctiol that exhibited antinociceptive effects by TRPV1 antagonism. However, the possible antinociceptive effect of this compound was not well documented. Thus, the goal of the present study was to evaluate the antinociceptive effect of diosmetin and its mechanism of action. The diosmetin effect on different pain models and its possible adverse effects were assessed on adult Swiss male mice (25-30 g). Mice spinal cord samples were used on calcium influx and binding assays using TRPV1 agonists. First, it was observed that the diosmetin reduced calcium influx mediated by capsaicin in synaptosomes and displace the specific binding to [³H]-resiniferatoxin in membrane fractions from the spinal cord of mice. Diosmetin (0.15 to 1.5 mg/kg, intragastric, i.g.) presented antinociceptive and antiedematogenic effect in the capsaicin intraplantar test and induced antinociception in a noxious heat test (48 °C). Also, treatment with diosmetin reduced mechanical and heat hypersensitivity observed in a model of inflammatory or neuropathic pain. Acute diosmetin administration in mice did not induce locomotor or body temperature changes, or cause liver enzyme abnormalities or alter renal function. Moreover, there were no observed changes in gastrointestinal transit or induction of ulcerogenic activity after diosmetin administration. In conclusion, our results support the antinociceptive properties of diosmetin which seems to occur via TRPV1 antagonist in mice.

Biochemical and Pharmacological Characterization of a Mice Model of Complex Regional Pain Syndrome

BACKGROUND AND OBJECTIVES

Complex regional pain syndrome is a challenging disease to treat. Recently, a mouse fracture model of complex regional pain syndrome has been developed that has many signs of the clinical syndrome. However, many aspects of the sensory neuron biochemistry and behavioral and pharmacological characterization of this model remain to be clarified.

METHODS

Mice were randomly assigned to fracture/cast or control (naive) groups. Fracture/cast mice underwent a closed distal tibia facture, with hindlimb wrapped in casting tape for 3 weeks. After cast removal, mice were tested for mechanical allodynia, burrowing behavior, and motor ability over a 12-week period. Protein immunohistochemistry was performed for substance P, calcitonin gene-related peptide, tropomyosin receptor kinase A, nerve growth factor, Nav1.7, and transient receptor potential cation-channel V1, colocalized in neurons, in the ipsilateral lumbar dorsal root ganglia (DRGs). Analgesic drugs were tested for pain-relieving efficacy.

RESULTS

Mechanical allodynia was greater in the ipsilateral hindpaw (P = 0.0002) in the fracture/cast group versus the control group, over the 3- to 12-week period. The amount of burrowing material removed was decreased (P = 0.0026), and there were deficits in spontaneous motor-rearing behavior (P = 0.018). Immunostaining of substance P, calcitonin gene-related peptide, Trk A receptor, nerve growth factor, Nav1.7, and transient receptor potential cation-channel V1 all demonstrated up-regulation in the DRGs of fracture mice versus controls (all P < 0.05). Morphine, pregabalin, ketamine, acetaminophen, and dexamethasone transiently increased force withdrawal thresholds on the ipsilateral (fracture) side and improved burrowing activity after injection (all P < 0.05). Ketorolac improved only burrowing.

CONCLUSIONS

Persistent pain-related behavior was demonstrated in this mouse fracture/cast model with wide-scale DRG up-regulation of pain mediators. Antihyperalgesic drugs reduced mechanical allodynia and improved burrowing.

Atypical pharmacology of schistosome TRPA1-like ion channels

Parasitic flatworms of the genus Schistosoma cause schistosomiasis, a neglected tropical disease estimated to affect over 200 million people worldwide. Praziquantel is the only antischistosomal currently available for treatment, and there is an urgent need for new therapeutics. Ion channels play key roles in physiology and are targets for many anthelmintics, yet only a few representatives have been characterized in any detail in schistosomes and other parasitic helminths. The transient receptor potential (TRP) channel superfamily comprises a diverse family of non-selective cation channels that play key roles in sensory transduction and a wide range of other functions. TRP channels fall into several subfamilies. Members of both the TRPA and TRPV subfamilies transduce nociceptive and inflammatory signals in mammals, and often also respond to chemical and thermal signals. We previously showed that although schistosomes contain no genes predicted to encode TRPV channels, TRPV1-selective activators such as capsaicin and resiniferatoxin elicit dramatic hyperactivity in adult worms and schistosomula. Surprisingly, this response requires expression of a S. mansoni TRPA1-like orthologue (SmTRPA). Here, we show that capsaicin induces a rise in intracellular Ca²⁺ in mammalian cells expressing either SmTRPA or a S. haematobium TRPA1 orthologue (ShTRPA). We also test SmTRPA and ShTRPA responses to various TRPV1 and TRPA1 modulators. Interestingly, in contrast to SmTRPA, ShTRPA is not activated by the TRPA1 activator AITC (allyl isothiocyanate), nor do S. haematobium adult worms respond to this compound, a potentially intriguing species difference. Notably, 4-hydroxynonenal (4-HNE), a host-derived, inflammatory product that directly activates mammalian TRPA1, also activates both SmTRPA and ShTRPA. Our results point to parasite TRPA1-like channels which exhibit atypical, mixed TRPA1/TRPV1-like pharmacology, and which may also function to transduce endogenous host signals.

Schistosomes are parasitic flatworms that infect hundreds of millions of people worldwide. They cause schistosomiasis, a disease with major consequences for human health and economic development. There is only a single drug available for treatment and control of this highly prevalent disease, and there is an urgent need for development of new treatments. TRP ion channels play key roles in sensory (and other) functions. One type of TRP channel, TRPV1, is activated by capsaicin, the active ingredient in hot peppers. However, schistosomes do not have any TRPV-like channels. Nonetheless, we previously showed that capsaicin and similar compounds induce dramatic hyperactivity in schistosomes, and that this response is abolished by suppressing expression of SmTRPA, a schistosome TRPA1-like channel. Mammalian TRPA1 channels are not sensitive to capsaicin. Here, we show that the SmTRPA channel itself responds to capsaicin, resulting in an influx of Ca²⁺ into cells. ShTRPA, a TRPA1-like channel from another schistosome, S. haematobium, is also sensitive to capsaicin. Thus, the pharmacology of schistosome TRPA1 channels apparently differs from that of host mammalian channels, a characteristic that could indicate mixed TRPA/TRPV functionality and might be exploitable for development of new antischistosomal drugs. Furthermore, we show that schistosome TRPA1-like channels are activated by host-derived compounds, perhaps indicating a mechanism by which the parasite can respond to host signals.

Participation of transient receptor potential vanilloid 1 in paclitaxel-induced acute visceral and peripheral nociception in rodents

The clinical use of paclitaxel as a chemotherapeutic agent is limited by the severe acute and chronic hypersensitivity caused when it is administered via intraperitoneal or intravenous routes. Thus far, evidence has suggested that transient receptor potential vanilloid-1 (TRPV1) has a key role in the chronic neuropathy induced by paclitaxel. Despite this, the role of TRPV1 in paclitaxel -related acute nociception, especially the development of visceral nociception, has not been evaluated. Thus, the goal of this study was to evaluate the participation of TRPV1 in a model of acute nociception induced by paclitaxel in rats and mice. A single intraperitoneal (i.p.) paclitaxel administration (1 mg/kg, i.p.) produced an immediate visceral nociception response 1 h after administration, caused mechanical and heat hypersensitivity, and diminished burrowing behaviour 24 h after administration. These nociceptive responses were reduced by SB-366791 treatment (0.5 mg/kg, i.p., a TRPV1 antagonist). In addition, TRPV1-positive sensory fibre ablation (using resiniferatoxin, 200 µg/kg, s.c.) reduced visceral nociception and mechanical or heat hypersensitivity caused by paclitaxel injection. Similarly, TRPV1 deficient mice showed a pronounced reduction in mechanical allodynia to paclitaxel acute injection and did not develop heat hypersensitivity. Moreover, 24 h after its injection, paclitaxel induced chemical hypersensitivity to capsaicin (a TRPV1 agonist, 0.01 nmol/site) and increased TRPV1 immunoreactivity in the dorsal root ganglion and sciatic nerve. In conclusion, TRPV1 is involved in mechanical and heat hypersensitivity and spontaneous-pain behaviour induced 24 h after a single paclitaxel injection. This receptor is also involved in visceral nociception induced immediately after paclitaxel administration.

N-Arachidonoyl Dopamine Modulates Acute Systemic Inflammation via Nonhematopoietic TRPV1

N-Arachidonoyl dopamine (NADA) is an endogenous lipid that potently activates the transient receptor potential vanilloid 1 (TRPV1), which mediates pain and thermosensation. NADA is also an agonist of cannabinoid receptors 1 and 2. We have reported that NADA reduces the activation of cultured human endothelial cells by LPS and TNF-α. Thus far, in vivo studies using NADA have focused on its neurologic and behavioral roles. In this article, we show that NADA potently decreases in vivo systemic inflammatory responses and levels of the coagulation intermediary plasminogen activator inhibitor 1 in three mouse models of inflammation: LPS, bacterial lipopeptide, and polymicrobial intra-abdominal sepsis. We also found that the administration of NADA increases survival in endotoxemic mice. Additionally, NADA reduces blood levels of the neuropeptide calcitonin gene-related peptide but increases the neuropeptide substance P in LPS-treated mice. We demonstrate that the anti-inflammatory effects of NADA are mediated by TRPV1 expressed by nonhematopoietic cells and provide data suggesting that neuronal TRPV1 may mediate NADA's anti-inflammatory effects. These results indicate that NADA has novel TRPV1-dependent anti-inflammatory properties and suggest that the endovanilloid system might be targeted therapeutically in acute inflammation.

Transient receptor potential ion channel function in sensory transduction and cellular signaling cascades underlying visceral hypersensitivity

Visceral hypersensitivity is an important mechanism underlying increased abdominal pain perception in functional gastrointestinal disorders including functional dyspepsia, irritable bowel syndrome, and inflammatory bowel disease in remission. Although the exact pathophysiological mechanisms are poorly understood, recent studies described upregulation and altered functions of nociceptors and their signaling pathways in aberrant visceral nociception, in particular the transient receptor potential (TRP) channel family. A variety of TRP channels are present in the gastrointestinal tract (TRPV1, TRPV3, TRPV4, TRPA1, TRPM2, TRPM5, and TRPM8), and modulation of their function by increased activation or sensitization (decreased activation threshold) or altered expression in visceral afferents have been reported in visceral hypersensitivity. TRP channels directly detect or transduce osmotic, mechanical, thermal, and chemosensory stimuli. In addition, pro-inflammatory mediators released in tissue damage or inflammation can activate receptors of the G protein-coupled receptor superfamily leading to TRP channel sensitization and activation, which amplify pain and neurogenic inflammation. In this review, we highlight the present knowledge on the functional roles of neuronal TRP channels in visceral hypersensitivity and discuss the signaling pathways that underlie TRP channel modulation. We propose that a better understanding of TRP channels and their modulators may facilitate the development of more selective and effective therapies to treat visceral hypersensitivity.

Methylene Blue Effectiveness as Local Analgesic after Anorectal Surgery: A Literature Review

BACKGROUND

Methylene blue (MB) has been found to have unique analgesic property through temporary disruption of sensory nerve conduction. In anorectal surgery, MB is widely used as a biologic stain but the analgesic effect has never been studied. Thus, a literature review completed with critical appraisal is required to find out its efficacy.

METHODS

A review has been run to find out its efficacy. Literature search proceeded in database sites, namely, PubMed, EBSCO, Cochrane, Wiley, and ProQuest using the following keywords: "anorectal" OR "hemorrhoid" OR "anal fistula" OR "anal fissure" OR "anal abscess" OR "anal pruritus" AND "methylene blue" AND "analgesic"; then the critical appraisal and its implication were discussed.

RESULT

There were 491 articles in full text found, and four studies met the inclusion criteria. Two studies were focused on the evaluation of VAS in hemorrhoid surgery whereas the rest were focused on the evaluation of symptom score in anal pruritus.

CONCLUSIONS

A study with level of evidence 2 on VAS showed the efficacy. The rest showed insufficient evidence due to variations of anorectal surgery and the methods and techniques of MB application. A further prospective clinical study is required.

Feasibility of Topical Applications of Natural High-Concentration Capsaicinoid Solutions in Patients with Peripheral Neuropathic Pain: A Retrospective Analysis

Background. Capsaicin, one of several capsaicinoid compounds, is a potent TRPV1 agonist. Topical application at high concentration (high concentration, >1%) induces a reversible disappearance of epidermal free nerve endings and is used to treat peripheral neuropathic pain (PNP). While the benefit of low-concentration capsaicin remains controversial, the 8%-capsaicin patch (Qutenza®, 2010, Astellas, Netherlands) has shown its effectiveness. This patch is, however, costly and natural high-concentration capsaicinoid solutions may represent a cheaper alternative to pure capsaicin. Methods. In this retrospective study, 149 patients were screened, 132 were included with a diagnosis of neuropathic pain, and eighty-four were retained in the final analyses (median age: 57.5 years [IQR25–75: 44.7–67.1], male/female: 30/54) with PNP who were treated with topical applications of natural high-concentration capsaicinoid solutions (total number of applications: 137). Indications were postsurgical PNP (85.7%) and nonsurgical PNP (14.3%) (posttraumatic, HIV-related, postherpetic, and radicular PNP). Objectives. To assess the feasibility of topical applications of natural high-concentration capsaicinoid solutions for the treatment of PNP. Results. The median treated area was 250 cm² [IQR25–75: 144–531]. The median amount of capsaicinoids was 55.1 mg [IQR25–75: 28.7–76.5] per plaster and the median concentration was 172.3 μg/cm² [IQR25–75: 127.6–255.2]. Most patients had local adverse effects on the day of treatment, such as mild to moderate burning pain and erythema. 13.6–19.4% of the patients experienced severe pain or erythema. Following treatment, 62.5% of patients reported a lower pain intensity or a smaller pain surface, and 35% reported a sustained pain relief lasting for at least 4 weeks. Conclusion. Analgesic topical treatment with natural high-concentration capsaicinoid is feasible and may represent a low cost alternative to alleviate PNP in clinical practice.

Kunitz-Type Peptide HCRG21 from the Sea Anemone Heteractis crispa Is a Full Antagonist of the TRPV1 Receptor

Sea anemone venoms comprise multifarious peptides modulating biological targets such as ion channels or receptors. The sequence of a new Kunitz-type peptide, HCRG21, belonging to the Heteractis crispa RG (HCRG) peptide subfamily was deduced on the basis of the gene sequence obtained from the Heteractis crispa cDNA. HCRG21 shares high structural homology with Kunitz-type peptides APHC1-APHC3 from H. crispa, and clusters with the peptides from so named "analgesic cluster" of the HCGS peptide subfamily but forms a separate branch on the NJ-phylogenetic tree. Three unique point substitutions at the N-terminus of the molecule, Arg1, Gly2, and Ser5, distinguish HCRG21 from other peptides of this cluster. The trypsin inhibitory activity of recombinant HCRG21 (rHCRG21) was comparable with the activity of peptides from the same cluster. Inhibition constants for trypsin and α-chymotrypsin were 1.0 × 10-7 and 7.0 × 10-7 M, respectively. Electrophysiological experiments revealed that rHCRG21 inhibits 95% of the capsaicin-induced current through transient receptor potential family member vanilloid 1 (TRPV1) and has a half-maximal inhibitory concentration of 6.9 ± 0.4 μM. Moreover, rHCRG21 is the first full peptide TRPV1 inhibitor, although displaying lower affinity for its receptor in comparison with other known ligands. Macromolecular docking and full atom Molecular Dynamics (MD) simulations of the rHCRG21-TRPV1 complex allow hypothesizing the existence of two feasible, intra- and extracellular, molecular mechanisms of blocking. These data provide valuable insights in the structural and functional relationships and pharmacological potential of bifunctional Kunitz-type peptides.

Contemporary views on inflammatory pain mechanisms: TRPing over innate and microglial pathways

Tissue injury, whether by trauma, surgical intervention, metabolic dysfunction, ischemia, or infection, evokes a complex cellular response (inflammation) that is associated with painful hyperalgesic states. Although in the acute stages it is necessary for protective reflexes and wound healing, inflammation may persist well beyond the need for tissue repair or survival. Prolonged inflammation may well represent the greatest challenge mammalian organisms face, as it can lead to chronic painful conditions, organ dysfunction, morbidity, and death. The complexity of the inflammatory response reflects not only the inciting event (infection, trauma, surgery, cancer, or autoimmune) but also the involvement of heterogeneous cell types including neuronal (primary afferents, sensory ganglion, and spinal cord), non-neuronal (endothelial, keratinocytes, epithelial, and fibroblasts), and immune cells. In this commentary, we will examine 1.) the expression and regulation of two members of the transient receptor potential family in primary afferent nociceptors and their activation/regulation by products of inflammation, 2.) the role of innate immune pathways that drive inflammation, and 3.) the central nervous system’s response to injury with a focus on the activation of spinal microglia driving painful hyperalgesic states.

Thermal nociception using a modified Hargreaves method in primates and humans

Nociception is an important protective mechanism. The Hargreaves method, which involves measuring withdrawal latency following thermal stimulation to Thermal nociception using a modified Hargreaves method in primates and humans the paw, is commonly used to measure pain thresholds in rodents. We modified this technique to measure pain thresholds in monkeys and humans. The modified Hargreaves method was used to quantitate pain sensitivity in eight normal rhesus monkeys, 55 human volunteers, and 12 patients with spinal cord or cauda equina lesions. Thermal stimulation was delivered at 80% of maximum output, and the duration of the stimulation was set at a maximum of 10 seconds to avoid skin injury. The following withdrawal latencies were recorded: 2.7 ± 0.12 seconds in volunteers and 3.4 ± 0.35 seconds in neurologically intact monkeys (p>0.05). Patients with spinal cord or cauda equina lesions showed significantly increased latencies (p<0.001). The modified Hargreaves technique is a safe and reliable method that can provide a validated measure of physiological pain sensation.

Role of transient receptor potential vanilloid subetype 1 in the increase of thermal pain threshold by moxibustion

OBTECTIVE: To explore the role of transient receptor potential vanilloid subetype 1 (TRPV1) in the increase of the thermal pain threshold by moxibustion.

METHODS

Forty Kunming mice (20 ± 2) g were randomized into control group, capsaicin group, capsazepine group, moxibustion group and moxibustion + capsazepine (MC) group with 8 mice in each, and 16 C57BL/6 wild-type mice (18 ± 2) g were randomized into wild-type (WT) control group and WT moxibustion group with 8 mice in each, and 14 TRPV1 knockout mice (18 ± 2) g were randomized into knockout (KO) control group and KO moxibustion-group with 7 in each. Each mouse in the capsaicin group was subcutaneously injected with the amount of 0.1 mL/10 g into L5 and L6 spinal cords; each mouse in the capsazepine group was intraperitoneally injected with the amount of 0.1 mL/10 g. Similarly, each mouse in the moxibustion group was given a suspended moxibustion with specially-made moxa-stick for 20 min on L5 and L6 spinal cords. Each mouse in MC group was intraperitoneally injected with the amount of 0.1 mL/1 0 g first, then after 15 min was given a suspended moxibustion for 20 min on L5 and L6 spinal cords. Each mouse in WT moxibustion group and KO moxibustion group was given a suspended moxibustion with specially-made moxa-stick for 20 min on L5 and L6 spinal cords. The control group, WT control group and KO control group were of no treatment in any way. After all treatments were completed, the digital-display measurement instrument for thermal pain was used to measure the threshold of thermal pain in each group respectively.

RESULTS

Compared with the control group, the thresholds of thermal pain in the moxibustion group and MC group were significantly increased (P <0.01); no significant changes in the thresholds in the capsaicin group and the capsazepine group (P > 0.05); compared with moxibustion group, he threshold of thermal in MC group was obviously decreased (P < 0.01). Compared with WT control group, the threshold of thermal pain in WT moxibustion group was significantly increased (P < 0.01); compared with KO control group, no changes in the threshold in KO moxibustion group (P > 0.05).

CONCLUSION

TRPV1 participated in the process of increasing the threshold of thermal pain by stimulating L5 and L6 of mice spinal cord with burning mosa-stick.

Mother root of Aconitum carmichaelii Debeaux exerts antinociceptive effect in Complet Freund's Adjuvant-induced mice: roles of dynorpin/kappa-opioid system and transient receptor potential vanilloid type-1 ion channel

BACKGROUND

Processed Chuanwu (PCW), the mother root of Aconitum carmichaelii Debeauxv, has been widely used as a classic Traditional Chinese Medicine for pain relieve for over two millennia clinically. However, its action on chronic inflammatory pain has not been clarified. Here, we investigated the antinociceptive effect of PCW in complete freund's adjuvant (CFA)-induced mice and its possible mechanisms associated with opioid system and TRPV1 ion channel.

METHODS

Male ICR mice were intraplantarly injected with CFA. PCW (0.34, 0.68 and 1.35 g/kg) was orally given to mice once a day for 7 days. Von frey hairs and planter test were assessed to evaluate the antinociceptive effect of PCW. To investigate the participation of dynorphin/opioid system in PCW antinociception, subtype-specific opioid receptor antagonists or anti-dynorphin A antiserum were used. To eliminate other central mechanisms that contribute to PCW antinociception, hot plate (50 °C) test were performed. Further, involvements of TRPV1 in PCW antinociception were evaluated in CFA-induced TRPV1(-/-) and TRPV1(+/+) C57BL/6 male mice, and in capsaicin-induced nociception ICR naive mice pretreated with nor-BNI. Meanwhile, calcium imaging was performed in HEK293T-TRPV1 cells. Finally, rotarod, open-field tests and body temperature measurement were carried out to assess side effects of PCW.

RESULTS

PCW dose-dependently attenuated mechanical and heat hypersensitivities with no tolerance, which could be partially attenuated by coadministration of k-opioid receptor antagonist nor-binaltorphimine (nor-BNI) or anti-dynorphin A (1-13) antiserum. And PCW antinociception was totally erased by pretreatment with nor-BNI in the hot plate test. In addition, PCW antinociception was decreased in TRPV1(-/-) mice compared to TRPV1(+/+) group. And PCW still manifested inhibitory effects in capsaicin-induced nociception with nor-BNI pretreatment. PCW significantly inhibited capsaicin-induced calcium influx in HEK293T-TRPV1 cells. Finally, no detectable side effects were found in naive mice treated with PCW.

CONCLUSIONS

This study shows PCW's potent antinociceptive effect in inflammatory conditions without obvious side effects. This effect may result from the activation of κ-opioid receptor via dynorpin release and the inhibition of TRPV1. These findings indicate that PCW might be a potential agent for the management of chronic inflammatory pain.

Wu-tou decoction inhibits chronic inflammatory pain in mice: participation of TRPV1 and TRPA1 ion channels

Wu-tou decoction (WTD) is a classic traditional Chinese medicine formula and has been used effectively to treat joint diseases clinically. Previous reports indicated that WTD possesses anti-inflammatory activity; however, its actions on pain have not been clarified. Here, we investigated the antinociceptive activity of WTD in CFA-induced mice, and its possible mechanism of the action associated with transient receptor potential (TRP) ion channels was also explored. Our results showed that 1.58, 3.15, and 6.30 g/kg WTD significantly attenuated mechanical, cold, and heat hypersensitivities. Moreover, WTD effectively inhibited spontaneous nociceptive responses to intraplantar injections of capsaicin and cinnamaldehyde, respectively. WTD also effectively suppressed jumping and wet-dog-shake behaviors to intraperitoneal injection of icilin. Additionally, WTD significantly reduced protein expression of TRPV1 and TRPA1 in dorsal root ganglia and skins of injured paw. Collectively, our data demonstrate firstly that WTD exerts antinociceptive activity in inflammatory conditions by attenuating mechanical, cold, and heat hypersensitivities. This antinociceptive effect may result in part from inhibiting the activities of TRPV1, TRPA1, and TRPM8, and the suppression of TRPV1 and TRPA1 protein by WTD was also highly effective. These findings suggest that WTD might be an attractive and suitable therapeutic agent for the management of chronic inflammatory pain.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

Magnesium sulfate with lidocaine for preventing propofol injection pain: a randomized, double-blind, placebo-controlled trial

PURPOSE

Propofol injection pain, despite various strategies, remains common and troublesome. This study aimed to test the hypothesis that pretreatment with the combination of intravenous lidocaine and magnesium would have an additive effect on reducing propofol injection pain.

METHODS

After institutional review board (IRB) approval and informed consent, we performed a prospective, double-blind, placebo-controlled, randomized trial. Subjects were randomly assigned to pretreatment with either lidocaine (50 mg), magnesium sulfate (0.25 mg), lidocaine (50 mg) plus magnesium sulfate (0.25 mg), or 0.9 % sodium chloride. Following pretreatment, propofol (50 mg) was administered, and subjects were questioned regarding injection site pain and observed for behavioral signs of pain.

RESULTS

Two hundred subjects were enrolled and 158 subjects (39 placebo, 38 lidocaine, 44 magnesium sulfate, and 37 lidocaine plus magnesium sulfate) received their assigned pretreatment intervention. Intergroup baseline characteristics were similar. The proportion of subjects reporting propofol injection pain was highest in those pretreated with magnesium sulfate (57 %), followed by those pretreated with placebo (46 %), lidocaine plus magnesium sulfate (41 %), and then lidocaine (29 %; p = 0.011). When adjusted for age, gender, diabetes mellitus, chronic pain, tobacco use, and selective-serotonin reuptake inhibitor use, the pain response scale scores were significantly reduced by lidocaine pretreatment compared to magnesium sulfate and placebo (p = 0.031 and p = 0.0003, respectively).

CONCLUSIONS

In this double-blind, placebo-controlled, randomized trial, the combination of intravenous magnesium sulfate and lidocaine offered no additional benefit for the relief of propofol injection pain compared to intravenous lidocaine alone. An improved, receptor-based understanding of the mechanism of propofol injection pain is still needed.

The anticancer antibiotic mithramycin-A inhibits TRPV1 expression in dorsal root ganglion neurons

Activation of peripheral nociceptors by products of inflammation has been shown to be dependent on specific sensory transducing elements such as the capsaicin receptor, TRPV1. The development of high-affinity antagonists to TRPV1 as well as to other receptors capable of detecting noxious stimuli has now become a major focus in analgesic development. Another critical feature of nociception is the relative abundance of a particular pain transducing receptor under normal or pathophysiologic conditions. Increases in expression and/or changes in distribution of nociceptive receptors such as TRPV1 have been correlated with progression of tissue injury and persistence of pain behaviors. Although some details are emerging as to what regulates nociceptor-specific gene expression, compounds that could potentially be used to block or reverse over-expression of nociceptive gene expression are essentially absent. In our efforts to better understand the transcriptional regulation of TRPV1 in sensory neurons, we identified an anticancer agent, mithramycin-A, that decreased TRPV1 expression in primary rat dorsal root ganglion (DRG) neurons. Mithramycin-A dose-dependently (10-50 nM) decreased endogenous TRPV1 mRNA content and appeared to decrease TRPV1-like protein expression in DRG neurons. We also observed that mithramycin-A directed a decrease in the number of capsaicin-responsive DRG neurons without a significant change in the capsaicin-response magnitudes. Interestingly, mithramycin-A also reduced the mRNA encoding Sp1 and Sp4 in DRG neurons, transcription factors previously found to positively regulate TRPV1 expression in sensory neurons. Taken together, we propose that mithramycin-A directs an inhibitory effect on a subpopulation of capsaicin-responsive DRG neurons that utilize Sp1-like factors for TRPV1 expression. Given the therapeutic correlate of mithramycin-A effectiveness in the treatment of certain cancers, small molecule transcriptional inhibitors such as mithramycin-A may serve as useful tools of discovery in pain transduction and possibly future analgesic development.

Capsaicinoids in the treatment of neuropathic pain: a review

The treatment of neuropathic pain is difficult. Oral pharmaceuticals have significant side effects, and treatment efficacy tends to be modest. The use of topical analgesics reduces the potential for systemic side effects and allows direct application of medications to the area of pain. The natural spicy substance, capsaicin, has historically been known for its topical use. Capsaicin, once applied to the skin, causes a brief initial sensitization followed by a prolonged desensitization of the local pain nerves. This occurs through stimulation of the transient receptor potential vanilloid-1 (TRPV1) expressing pain nerve fibers. While low-dose capsaicin has not resulted in good efficacy, the larger dose 8% topical capsaicin has had some of the best data currently available in the treatment of post-herpetic neuralgia (PHN) and other neuropathic conditions. This paper discusses the data currently existing for capsaicin 8% in the treatment of PHN. It further reviews data for the low-dose capsaicin products and the current status in the development of other capsaicinoids, e.g. resiniferotoxin, and high-concentration liquid capsaicin.

TRPV1 in the central nervous system: synaptic plasticity, function, and pharmacological implications

The function of TRPV1 in the peripheral nervous system is increasingly being investigated for its anti-inflammatory and antinociceptive properties in an effort to find a novel target to fight pain that is nonaddictive. However, in recent years, it was discovered that TRPV1 is also associated with a wide array of functions and behaviors in the central nervous system, such as fear, anxiety, stress, thermoregulation, pain, and, more recently, synaptic plasticity, the cellular mechanism that allows the brain to adapt to its environment. This suggests a new role for brain TRPV1 in areas such as learning and memory, reward and addiction, and development. This wide array of functional aspects of TRPV1 in the central nervous system (CNS) is in part due to its multimodal form of activation and highlights the potential pharmacological implications of TRPV1 in the brain. As humans also express a TRPV1 homologue, it is likely that animal research will be translational to humans and therefore worthy of exploration. This review outlines the basic expression patterns of TRPV1 in the CNS along with what is known regarding its signaling mechanisms and its role in the aforementioned brain functions. As TRPV1 involvement in synaptic plasticity has never been fully reviewed elsewhere, it will be a focus of this review. The chapter concludes with some of the potential pharmaceutical implications of further TRPV1 research.