Integrating TRPV1 Receptor Function with Capsaicin Psychophysics

Optical Control of TRPV1 Channels In Vitro with Tethered Photopharmacology

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that is important for nociception and inflammatory pain and is activated by a variety of nociceptive stimuli─including lipids such as capsaicin (CAP) and endocannabinoids. TRPV1's role in physiological systems is often studied by activating it with externally perfused ligands; however, this approach is plagued by poor spatiotemporal resolution. Lipid agonists are insoluble in physiological buffers and can permeate membranes to accumulate nonselectively inside cells, where they can have off-target effects. To increase the spatiotemporal precision with which we can activate lipids on cells and tissues, we previously developed optically cleavable targeted (OCT) ligands, which use protein tags (SNAP-tags) to localize a photocaged ligand on a target cellular membrane. After enrichment, the active ligand is released on a flash of light to activate nearby receptors. In our previous work, we developed an OCT-ligand to control a cannabinoid-sensitive GPCR. Here, we expand the scope of OCT-ligand technology to target TRPV1 ion channels. We synthesize a probe, OCT-CAP, that tethers to membrane-bound SNAP-tags and releases a TRPV1 agonist when triggered by UV-A irradiation. Using Ca2+ imaging and electrophysiology in HEK293T cells expressing TRPV1, we demonstrate that OCT-CAP uncaging activates TRPV1 with superior spatiotemporal precision when compared to standard diffusible ligands or photocages. This study is the first example of an OCT-ligand designed to manipulate an ion-channel target. We anticipate that these tools will find many applications in controlling lipid signaling pathways in various cells and tissues.

Primary Erythromelalgia Treated With 10% Capsaicin Cream: A Case Report and a 10-Year Follow-Up

In this case report, we describe the difficulty in finding a suitable treatment for a nine-year-old girl with erythromelalgia. Initially, she could only find pain relief through immersion of her hands and feet in buckets of cool water. Her pain did not respond to outpatient treatments, and she was ultimately admitted to the hospital for pain management. Many different medications and modalities were tried over the course of several weeks in the hospital. Finally, she received the most benefit from 10% compounded capsaicin cream administered under general anesthesia with regional analgesia for post-application pain. Over the course of several years, exacerbations of her pain were treated with additional applications of 10% capsaicin cream, with each application providing relief for an increased duration. Her severe pain flares eventually went into remission after several years. Today, after more than a decade following her initial presentation, she is a successful college student and is taking no medications for her erythromelalgia.

Sucrose alleviates capsaicin-induced tongue burning: An in vivo study

Background

Spicy foods are flavorful and stimulate salivation, which is beneficial for individuals with poor appetite. They are also ubiquitous in many regional cuisines, but the chemical compounds in such foods, especially capsaicin from chili peppers, can cause tissue inflammation and generate intolerable burning pain in the oral cavity.

Material and Methods

To identify a potential method to reduce capsaicin-induced burning pain without influencing food flavor, we tested the effects of mouth rinsing with various concentrations of sucrose. Inclusion criteria were good general and oral health, while exclusion criteria were poor baseline smell or taste, capsaicin allergy, and current orofacial pain complaints. To define an appropriate capsaicin dose, participants placed filter paper strips impregnated with 0.003%–0.3% capsaicin on the tip of the tongue and rated burning sensation by visual analog scale (VAS) score.

Results

A 0.1% capsaicin solution induced tongue burning in the midrange (VAS = 6.33 ± 0.52) and so was used for subsequent tests. We then examined the efficacy concentration of sucrose for reducing tongue burning by recording VAS scores at multiple time points following a 15-s oral rinse with various aqueous sucrose solutions (5%, 10%, and 20%), milk, or pure water (control) after 0.1% capsaicin application. Scores were compared at each time point by one-way ANOVA with post hoc Dunnett’s tests. A 15-s rinse with 20% sucrose significantly alleviated burning pain compared to water rinse at 45, 60, 120, and 180 s after capsaicin exposure.

Conclusions

Thus, periodic rinsing with 20% aqueous sucrose may help promote spicy food consumption among individuals with poor appetite.

Key words:Capsaicin, sucrose, burning sensation.

Ratio of Hydrophobic-Hydrophilic and Positive-Negative Residues at Lipid-Water-Interface Influences Surface Expression and Channel Gating of TRPV1

During evolution, TRPV1 has lost, retained or selected certain residues at Lipid-Water-Interface (LWI) and formed specific patterns there. The ratio of "hydrophobic-hydrophilic" and "positive-negative-charged" residues at the inner LWI remains conserved throughout vertebrate evolution and plays important role in regulating TRPV1 trafficking and localization. Arg575 is an important residue as Arg575Asp mutant has reduced surface expression, co-localization with lipid raft markers, cell area and increased cell lethality. This lethality is most likely due to the disruption of the ratio between positive-negative charges caused by the mutation. Such lethality can be rescued by either using TRPV1-specfic inhibitor 5'-IRTX or by restoring the positive-negative charge ratio at that position, i.e. by introducing Asp576Arg mutation in Arg575Asp backbone. We propose that Arg575Asp mutation confers TRPV1 in a "constitutive-open-like" condition. These findings have broader implication in understanding the molecular evolution of thermo-sensitive ion channels and the micro-environments involved in processes that goes erratic in different diseases. The segment of TRPV1 that is present at the inner lipid-water-interface (LWI) has a specific pattern of amino acid combinations. The overall ratio of +ve charge /-ve charge and the ratio of hydrophobicity/hydrophilicity remain constant throughout the vertebrate evolution (ca 450 million years). This specific pattern is not observed in the outer LWI region of TRPV1.

Antioxidant, Anti-Obesity, Nutritional and Other Beneficial Effects of Different Chili Pepper: A Review

Fruits and vegetables are important components of a healthy diet. They are rich sources of vitamins and minerals, dietary fibre and a host of beneficial non-nutrient substances including plant sterols, flavonoids and other antioxidants. It has been reported that reduced intake of fruits and vegetables may increase the risk of non-communicable diseases (NCDs). Chili pepper, is a common and important spice used to enhance taste and nutrition. Over the years, reports have shown its potential as antioxidant and an anti-obesity agent. Obesity is a serious health concern as it may initiate other common chronic diseases. Due to the side effects of synthetic antioxidants and anti-obesity drugs, scientists are now focusing on natural products which produce similar effects to synthetic chemicals. This up-to-date review addresses this research gap and presents, in an accessible format, the nutritional, antioxidant and anti-obesity properties of different chili peppers. This review article serves as a reference guide for use of chili peppers as anti-obesity agents.

Novel TRPV1 Modulators with Reduced Pungency Induce Analgesic Effects in Mice

Capsaicin, the compound in hot chili peppers responsible for their pungency and an agonist of the transient receptor potential cation channel, subfamily V, member 1 (TRPV1), has long been known to promote the desensitization of nociceptors at high concentrations. This has led to the utilization and implementation of topical capsaicin cream as an analgesic to treat acute and chronic pain. Critically, the application of capsaicin cream is limited due to capsaicin's high pungency, which is experienced prior to analgesia. To combat this issue, novel capsaicin analogues were developed to provide analgesia with reduced pungency. Analogues reported in this paper add to and show some differences from previous structure-activity relationship (SAR) studies of capsaicin-like molecules against TRPV1, including the necessity of phenol in the aromatic "A-region", the secondary amide in the "B-region", and modifications in the hydrophobic "C-region". This provided a new framework for de novo small-molecule design using capsaicin as the starting point. In this study, we describe the synthesis of capsaicin analogues, their in vitro activity in Ca2+ assays, and initial in vivo pungency and feasibility studies of capsaicin analogues YB-11 and YB-16 as analgesics. Our results demonstrate that male and female mice treated with YB capsaicin analogues showed diminished pain-associated behavior in the spontaneous formalin assay as well as reduced thermal sensitivity in the hotplate assay.

Photobiomodulation at Different Wavelengths Boosts Mitochondrial Redox Metabolism and Hemoglobin Oxygenation: Lasers vs. Light-Emitting Diodes In Vivo

Our group previously examined 8 min photobiomodulation (PBM) by 1064 nm laser on the human forearm in vivo to determine its significant effects on vascular hemodynamics and cytochrome c oxidase redox activity. Since PBM uses a wide array of wavelengths, in this paper, we investigated (i) whether different wavelengths of lasers induced different PBM effects, and (ii) if a light-emitting diode (LED) at a similar wavelength to a laser could induce similar PBM effects. A broadband near-infrared spectroscopy (bbNIRS) system was utilized to assess concentration changes in oxygenated hemoglobin (Δ[HbO]) and oxidized cytochrome c oxidase (Δ[oxCCO]) during and after PBM with lasers at 800 nm, 850 nm, and 1064 nm, as well as a LED at 810 nm. Two groups of 10 healthy participants were measured before, during, and after active and sham PBM on their forearms. All results were tested for significance using repeated measures ANOVA. Our results showed that (i) lasers at all three wavelengths enabled significant increases in Δ[HbO] and Δ[oxCCO] of the human forearm while the 1064 nm laser sustained the increases longer, and that (ii) the 810-nm LED with a moderate irradiance (≈135 mW/cm²) induced measurable and significant rises in Δ[HbO] and Δ[oxCCO] with respect to the sham stimulation on the human forearm.

Topical Capsaicin for the Treatment of Sensory Neuropathic Cough

Objective

To evaluate a novel treatment for sensory neuropathic cough (SNC): topical capsaicin.

Study Design

Retrospective review.

Setting

Tertiary care laryngology clinic.

Methods

A retrospective review was performed on 201 consecutive patients treated for SNC with capsaicin 0.02% to 0.04% applied topically to the upper aerodigestive tract, typically after failure of standard medications. Patients were asked to use the spray 4 times daily for 2 weeks prior to assessment of benefit. Items assessed included the percentage reduction of coughing, type of benefit noted, and side effects.

Results

Of the 201 patients who used the spray, 36.3% noted no benefit, whereas 63.7% (n = 128) had benefit in terms of cough reduction: 30.8% (n = 62) reported ≥75% reduction; 17.4% (n = 35), 50%-74% reduction; 7.0% (n = 14), 25%-49% reduction; and 8.5% (n = 17), 1%-24% reduction. Of all patients, 78.3% reported no side effects or complications. Of the remaining 21.7%, 1 patient noted a nosebleed after a single administration, and 1 patient noted transient wheezing after administration. The others reported unpleasant local effects, including throat/ear discomfort, voice change, sneezing, reflexive vomiting, and headache.

Conclusion

In our group of 201 patients with SNC, most of which had failed to respond to standard treatments, 63.7% had some response to capsaicin spray, with 30.8% reporting ≥75% reduction. Minimal side effects of treatment were reported. Thus, we suggest that this therapy can be another treatment option for patients with SNC.

Bitter Taste Disrupts Spatial Discrimination of Piperine-Evoked Burning Sensations: A Pilot Study

Simple Summary

The chemical senses smell, taste, and trigeminal sense enable us to interact with the environment and play an essential role in protecting us from hazardous events. It is theorized that capsaicin and piperine not only elicit burning, but also bitter sensations through bitter taste-responding gustatory receptor cells that possess special channels. Similar psychophysiological responses to capsaicin and piperine suggest that bitter taste might also disrupt the spatial discrimination to piperine-induced burning sensations. Results showed that bitter taste disrupted the spatial discrimination of piperine-evoked burning sensations, providing further evidence for a qualitative similarity between burning and bitter sensations and the usefulness of chemical irritants in spatial discrimination tasks.

Abstract

This study aimed to investigate the perceptual similarity between piperine-induced burning sensations and bitter taste using piperine-impregnated taste strips (PTS). This pilot study included 42 healthy participants. PTS of six ascending concentrations (1 mg, 5 mg, 10 mg, 15 mg, 20 mg, and 25 mg piperine/dL 96% ethanol) were presented at the anterior tongue, and participants rated perceived intensity and duration. Then, participants performed a spatial discrimination task in which they had to report which of the two strips presented to the anterior tongue contained an irritating stimulus when one strip was always a PTS while the other strip was impregnated with either a single taste quality (sweet or bitter) or a blank strip. Repeated measures one-way ANOVA revealed that burning sensations of higher concentrated PTS were perceived more intense and more prolonged compared to lower concentrated PTS. McNemar’s test showed that PTS were identified correctly significantly less often when presented with bitter strips compared to when presented with blank (p = 0.002) or sweet strips (p = 0.017). Our results showed that bitter taste disrupts the spatial discrimination of piperine-evoked burning sensations. PTS might serve as a basis for further studies on disease-specific patterns in chemosensory disorders.

Comparative effects of capsaicin in chronic obstructive pulmonary disease and asthma (Review)

Chronic obstructive pulmonary disease (COPD) and asthma are chronic respiratory diseases with high prevalence and mortality that significantly alter the quality of life in affected patients. While the cellular and molecular mechanisms engaged in the development and evolution of these two conditions are different, COPD and asthma share a wide array of symptoms and clinical signs that may impede differential diagnosis. However, the distinct signaling pathways regulating cough and airway hyperresponsiveness employ the interaction of different cells, molecules, and receptors. Transient receptor potential cation channel subfamily V member 1 (TRPV1) plays a major role in cough and airway inflammation. Consequently, its agonist, capsaicin, is of substantial interest in exploring the cellular effects and regulatory pathways that mediate these respiratory conditions. Increasingly more studies emphasize the use of capsaicin for the inhalation cough challenge, yet the involvement of TRPV1 in cough, bronchoconstriction, and the initiation of inflammation has not been entirely revealed. This review outlines a comparative perspective on the effects of capsaicin and its receptor in the pathophysiology of COPD and asthma, underlying the complex entanglement of molecular signals that bridge the alteration of cellular function with the multitude of clinical effects.

Fabrication of Capsaicin Loaded Nanocrystals: Physical Characterizations and In Vivo Evaluation

Nano-crystallization is a new emerging strategy to promote the saturation solubility, dissolution rate and subsequent bioavailability of Biopharmaceutical Class II drugs. Capsaicin belongs to BCS class-II drugs having low water solubility and dissolution rate. Nano-crystals (NC) of pure Capsaicin was developed and optimized in order to increase its water solubility, dissolution and further to promote its adhesiveness to skin epidermis layer. NC formulations were subjected to stability studies, droplet size, surface charge, poly-dispensability index, drug content, entrapment efficiency, thermal analysis, surface morphology, crystalline studies, solubility profile, in vitro release and ex vivo permeation studies. In vivo anti-inflammatory assay (Carrageenan-induced paw edema) was performed in Sprague Dawley rats. Nanocrystals loaded with capsaicin showed particle size 120 ± 3.0 nm with surface charge of -20.7 ± 3.5 and PDI was 0.48 ± 1.5. Drug content and entrapment efficiency of T3 was 85% and 90 ± 1.9% respectively. Thermal studies predicted that melting peak of capsaicin was present in the formulation suggested that there was no interaction between active moieties and excipients in NC formulation. Surface morphology confirmed the presence of Nano-size crystals having rough crystalline surface. XRD proved that the capsaicin NC are successfully developed by using high speed homogenization. The solubility of capsaicin was found to be 12.0 ± 0.013 μg/mL in water. In vitro study revealed that 89.94 ± 1.9% of drug was released within 24 h. Similarly, drug permeation was 68.32 ± 1.83%, drug retained in skin was 16.13 ± 1.11% while drug retained on skin was 9.12 ± 0.14% after 12 h. The nanocrystals showed higher anti-inflammatory activity as compared to marketed product (Dicloran^®). The study concluded that improvement in dissolution rate of capsaicin may potentially provide the opportunities in the development of a much cost-effective dosage forms that will produce improved pharmacological effects, but at low dose as compared to the already available products.

Interactions between Chemesthesis and Taste: Role of TRPA1 and TRPV1

In addition to the sense of taste and olfaction, chemesthesis, the sensation of irritation, pungency, cooling, warmth, or burning elicited by spices and herbs, plays a central role in food consumption. Many plant-derived molecules demonstrate their chemesthetic properties via the opening of transient receptor potential ankyrin 1 (TRPA1) and transient receptor potential vanilloid 1 (TRPV1) channels. TRPA1 and TRPV1 are structurally related thermosensitive cation channels and are often co-expressed in sensory nerve endings. TRPA1 and TRPV1 can also indirectly influence some, but not all, primary taste qualities via the release of substance P and calcitonin gene-related peptide (CGRP) from trigeminal neurons and their subsequent effects on CGRP receptor expressed in Type III taste receptor cells. Here, we will review the effect of some chemesthetic agonists of TRPA1 and TRPV1 and their influence on bitter, sour, and salt taste qualities.

The Mysteries of Capsaicin-Sensitive Afferents

A fundamental subdivision of nociceptive sensory neurons is named after their unique sensitivity to capsaicin, the pungent ingredient in hot chili peppers: these are the capsaicin-sensitive afferents. The initial excitation by capsaicin of these neurons manifested as burning pain sensation is followed by a lasting refractory state, traditionally referred to as "capsaicin desensitization," during which the previously excited neurons are unresponsive not only to capsaicin but a variety of unrelated stimuli including noxious heat. The long sought-after capsaicin receptor, now known as TRPV1 (transient receptor potential cation channel, subfamily V member 1), was cloned more than two decades ago. The substantial reduction of the inflammatory phenotype of Trpv1 knockout mice has spurred extensive efforts in the pharmaceutical industry to develop small molecule TRPV1 antagonists. However, adverse effects, most importantly hyperthermia and burn injuries, have so far prevented any compounds from progressing beyond Phase 2. There is increasing evidence that these limitations can be at least partially overcome by approaches outside of the mainstream pharmaceutical development, providing novel therapeutic options through TRPV1. Although ablation of the whole TRPV1-expressing nerve population by high dose capsaicin, or more selectively by intersectional genetics, has allowed researchers to investigate the functions of capsaicin-sensitive afferents in health and disease, several "mysteries" remain unsolved to date, including the molecular underpinnings of "capsaicin desensitization," and the exact role these nerves play in thermoregulation and heat sensation. This review tries to shed some light on these capsaicin mechanisms.

Stability, permeation, and cytotoxicity reduction of capsicum extract nanoparticles loaded hydrogel containing wax gourd extract

The aim of this study was to develop hydrogel loaded with capsicum extract nanoparticles and wax gourd extract for transdermal delivery of capsaicin. The addition of wax gourd extract was supposed to reduce cytotoxicity of capsaicin in capsicum extract against HaCaT keratinocyte cell line. Capsicum extract nanoparticles were prepared by solvent displacement method using hyaluronic acid as a stabilizer. The physical and chemical stability of capsicum extract nanoparticles were investigated by dynamic light scattering technique and UV-Visible spectrophotometry, respectively. Hydrogel loaded with capsicum extract nanoparticles and wax gourd fruit extract was then formulated by using Carbopol 940® as a gelling agent for transdermal delivery. The skin permeability of capsaicin from the hydrogel was evaluated by Franz diffusion cell approach. The cytotoxicity reduction of capsicum extract nanoparticles and capsicum extract nanoparticles by mixing with wax gourd extract was determined by MTT assay The results showed that capsicum extract nanoparticles exhibited an average diameter of 168.4 ± 5.3 nm with a polydispersity index and zeta potential value of 0.26 ± 0.01 and -45.7 ± 7.1 mV, respectively. After two month-storage, particle size, polydispersity index, and zeta potential values of capsicum extract nanoparticles stored at 4° C, 30° C, and 45 °C did not significantly change. The capsaicin content decreased to 78%, 71%, and 72% when stored at 4 °C, 30 °C, and 45 °C for three months, respectively. The pH values of hydrogel containing capsicum extract nanoparticles were found to be in the range of 5.58-6.05 indicating good stability. The hydrogel exhibited a pseudoplastic character. The rate of permeation flux of capsaicin from hydrogel was 7.96 µg/cm²/h. A significant increase in cell viability was observed when the cells were incubated with capsicum extract nanoparticles mixed with wax gourd, compared to capsicum extract nanoparticles alone. The wax gourd extract in the hydrogel protected HaCaT cells from capsaicin cytotoxicity, thus may provide a new approach for delivery of capsaicin to reduce cytotoxicity to skin cells.

Relationship between mustard pungency and allyl-isothiocyanate content: A comparison of sensory and chemical evaluations

The correlation of sensory and chemically evaluated pungency of mustard products was investigated via a time-intensity (TI) study and quantification of allyl isothiocyanate (AITC) contents using high-performance liquid chromatography (HPLC). Sweet, medium hot, hot, and extra hot commercial mustard products from different brands were examined. Notably, we found significant differences (p < 0.05) between the maximum perceived pungency intensity of various mustard products. The maximum perceived intensity (I_max ), the duration of the decreasing phase (DUR_Dec ), and the area under the curve (AUC) values increased proportionally to the increase in the sample AITC content and were also higher in products classified as hot than in sweet mustards. The AITC concentration varied greatly between products from different brands and also between different sensory evaluated pungency levels. Furthermore, sensory evaluations and analytical results were correlated using regression analysis. The best correlation (correlation coefficient 0.891) was observed between the AITC concentration and AUC, when compared to that between the AITC concentration and DUR_Dec (correlation coefficient 0.856) or the I_max value (correlation coefficient 0.803). The calculated regression model indicates that a higher AITC content induces an intensified trigeminal pungency sensation and that the sensory and chemical evaluations of mustard products were positively correlated. Therefore, by using this regression model, the sensory rating of mustard products may be predicted by chemical analysis of the AITC contents. PRACTICAL APPLICATION: This research paper provides a method to quantify the pungency inducing irritant allyl isothiocyanate in commercial mustard products and demonstrates a correlation between sensory and chemical data. Therefore, the amounts of sensory tests in product quality assurance can be reduced and replaced or at least supported by chemical quantification of pungent substances (especially AITC) in mustard products.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

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.

Development of a Regional Taste Test that uses Edible Circles for Stimulus Delivery

Introduction

Measurements of chemosensory function within specific regions of the tongue can yield important information about the sensitivity of lingual areas to chemosensory stimuli, and may identify possible nerve damage. A novel regional chemosensory test that uses thin edible circles was developed for human testing.

Methods

Edible circles placed at six different regions of the tongue were used to examine regional sensitivity to quinine for bitter taste, NaCl for salt taste, sucralose for sweet taste, and capsaicin for pungency. The six regions included the anterior tip of the tongue, the left and right lateral margins of the tongue (anterior and posterior), and the circumvallate region. Testing was completed with the mouth open, and the mouth closed.

Results

Intensity ratings at all sites were higher in the closed mouth condition for the three taste stimuli. Quinine intensity was highest at the circumvallate region with the mouth closed. NaCl and sucralose intensity were highest at the anterior tip and circumvallate regions. Capsaicin intensity was most highly perceived at the anterior tip of the tongue, but open and closed mouth intensity ratings showed no significant differences.

Conclusions

Regional differences in chemosensory perception were observed on the tongue, and these differences were dependent on the chemosensory stimulus, tongue region, and tasting mode.

Implications

Edible circles show minimal diffusion with saliva, can be used to examine both taste and irritation, and may be used to identify regional papillae counts on the tongue. Finally, edible circles should be invaluable for examining damage to the oral cavity.

Transient receptor potential ankyrin 1 protects against sepsis-induced kidney injury by modulating mitochondrial biogenesis and mitophagy

This study was undertaken to investigate the cytoprotective role of transient receptor potential ankyrin 1 (TRPA1) in sepsis-induced kidney injury. The Cecal ligation and puncture (CLP) was employed to induce septic kidney injury in C57BL/6 mice. Six hours before CLP or a sham procedure, mice were injected intraperitoneally with 10 mg/kg hemin or 30 mg/kg of the TRPA1 antagonist A-967079. Our study showed that mice treated with A-967079 exhibited less sepsis-induced mortality and kidney injury compared with those in the sham group. Moreover, A-967079 prevented multiple organ dysfunction, pathological changes, and increased secretion of in proinflammatory cytokines. In addition, A-967079 decreased the levels of mitochondrial lipid peroxidation and mitochondrial dysfunction in kidney tissues. The protein levels of mitochondrial biogenesis markers, including Sirt1, nuclear respiratory factor 1, and mitochondrial transcription factor A, were decreased in the A-967079 treatment group. Additionally, A-967079 treatment attenuated mitochondrial mitophagy. The levels of PTEN-induced putative kinase 1 increased and parkin levels decreased compared to the untreated CLP group. Our findings suggest that TRPA1 prevents septic injury by modulating mitochondrial biogenesis and mitophagy.

Orexin-1 receptors in the rostral ventromedial medulla are involved in the modulation of capsaicin evoked pulpal nociception and impairment of learning and memory

AIM

To investigate the role of rostral ventromedial medulla orexin-1 receptors in the modulation of orofacial nociception as well as nociception-induced learning and memory impairment in adult male rats.

METHODOLOGY

Pulpal nociception was induced by intradental application of capsaicin (100 μg) into the incisors of rats. Orexin-1 receptors agonist (orexin-A, 10, 25 and 50 pmol L^-1  rat^-1 ) and antagonist (SB-334867-A, 40 and 80 nmol L^-1  rat^-1 ) were microinjected into the rostral ventromedial medulla prior to capsaicin administration. Total time spent on nocifensive behaviour was recorded by direct visualization of freely moving rats whilst learning and memory were evaluated by the Morris water maze test. One-way analysis of variance and repeated-measures were used for the statistical analysis.

RESULTS

Capsaicin-treated rats had a significant increase of nocifensive behaviours (P < 0.001), as well as learning and memory impairment (P < 0.001). However, intraventromedial medulla prior micro-injection of orexin-A (50 pmol L^-1  rat^-1 ) significantly reduced the nociceptive behaviour (P < 0.001). This effect was blocked by pre-treatment with SB334867-A (80 nmol L^-1  rat^-1 ). Orexin-A (50 pmol L^-1  rat^-1 ) also inhibited nociception-induced learning and memory deficits. Moreover, administration of SB-334867-A (80 nmol L^-1  rat^-1 ) plus orexin-A (50 pmol L^-1  rat^-1 ) had no effect on learning and memory deficits induced by capsaicin.

CONCLUSIONS

The data suggest that rostral ventromedial medulla orexin-A receptors are involved in pulpal nociceptive modulation and improvement of learning and memory deficits induced by intradental application of capsaicin.

Capsaicin in Metabolic Syndrome

Capsaicin, the major active constituent of chilli, is an agonist on transient receptor potential vanilloid channel 1 (TRPV1). TRPV1 is present on many metabolically active tissues, making it a potentially relevant target for metabolic interventions. Insulin resistance and obesity, being the major components of metabolic syndrome, increase the risk for the development of cardiovascular disease, type 2 diabetes, and non-alcoholic fatty liver disease. In vitro and pre-clinical studies have established the effectiveness of low-dose dietary capsaicin in attenuating metabolic disorders. These responses of capsaicin are mediated through activation of TRPV1, which can then modulate processes such as browning of adipocytes, and activation of metabolic modulators including AMP-activated protein kinase (AMPK), peroxisome proliferator-activated receptor α (PPARα), uncoupling protein 1 (UCP1), and glucagon-like peptide 1 (GLP-1). Modulation of these pathways by capsaicin can increase fat oxidation, improve insulin sensitivity, decrease body fat, and improve heart and liver function. Identifying suitable ways of administering capsaicin at an effective dose would warrant its clinical use through the activation of TRPV1. This review highlights the mechanistic options to improve metabolic syndrome with capsaicin.

Capsaicin: Friend or Foe in Skin Cancer and Other Related Malignancies?

Capsaicin is the main pungent in chili peppers, one of the most commonly used spices in the world; its analgesic and anti-inflammatory properties have been proven in various cultures for centuries. It is a lipophilic substance belonging to the class of vanilloids and an agonist of the transient receptor potential vanilloid 1 receptor. Taking into consideration the complex neuro-immune impact of capsaicin and the potential link between inflammation and carcinogenesis, the effect of capsaicin on muco-cutaneous cancer has aroused a growing interest. The aim of this review is to look over the most recent data regarding the connection between capsaicin and muco-cutaneous cancers, with emphasis on melanoma and muco-cutaneous squamous cell carcinoma.

Medicinal Chemistry, Pharmacology, and Clinical Implications of TRPV1 Receptor Antagonists

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel expressed on sensory neurons triggering an influx of cations. TRPV1 receptors function as homotetramers responsive to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Its phosphorylation increases sensitivity to both chemical and thermal stimuli, while desensitization involves a calcium-dependent mechanism resulting in receptor dephosphorylation. TRPV1 functions as a sensor of noxious stimuli and may represent a target to avoid pain and injury. TRPV1 activation has been associated to chronic inflammatory pain and peripheral neuropathy. Its expression is also detected in nonneuronal areas such as bladder, lungs, and cochlea where TRPV1 activation is responsible for pathology development of cystitis, asthma, and hearing loss. This review offers a comprehensive overview about TRPV1 receptor in the pathophysiology of chronic pain, epilepsy, cough, bladder disorders, diabetes, obesity, and hearing loss, highlighting how drug development targeting this channel could have a clinical therapeutic potential. Furthermore, it summarizes the advances of medicinal chemistry research leading to the identification of highly selective TRPV1 antagonists and their analysis of structure-activity relationships (SARs) focusing on new strategies to target this channel.

Harnessing the Therapeutic Potential of Capsaicin and Its Analogues in Pain and Other Diseases

Capsaicin is the most predominant and naturally occurring alkamide found in Capsicum fruits. Since its discovery in the 19th century, the therapeutic roles of capsaicin have been well characterized. The potential applications of capsaicin range from food flavorings to therapeutics. Indeed, capsaicin and few of its analogues have featured in clinical research covered by more than a thousand patents. Previous records suggest pleiotropic pharmacological activities of capsaicin such as an analgesic, anti-obesity, anti-pruritic, anti-inflammatory, anti-apoptotic, anti-cancer, anti-oxidant, and neuro-protective functions. Moreover, emerging data indicate its clinical significance in treating vascular-related diseases, metabolic syndrome, and gastro-protective effects. The dearth of potent drugs for management of such disorders necessitates the urge for further research into the pharmacological aspects of capsaicin. This review summarizes the historical background, source, structure and analogues of capsaicin, and capsaicin-triggered TRPV1 signaling and desensitization processes. In particular, we will focus on the therapeutic roles of capsaicin and its analogues in both normal and pathophysiological conditions.