Decreased heat sensitivity of lungfish TRPV1 revealed by the heterologous expression system

In this study, we focused on TRPV1 of African lungfish, Protopterus annectens. During drought at high temperature, African lungfish can survive by undergoing into aestivation in mud cocoons. Therefore, lungfish is considered to have some specialized heat-sensor, TRPV1, for heat tolerance. Further, lungfish which shares similarities with fishes and amphibians, is one of important species for investigating the fish-tetrapod transition. Since fish TRPV1 and tetrapod TRPV1 have some differences, character of lungfish TRPV1 attracts attention. Here, we first cloned TRPV1 paralogue from lungfish, Protopterus annectens (lfTRPV1) and determined the chemical and thermal sensitivities of lfTRPV1 by two-electrode voltage clamp method using frog oocytes. We detected activation of lfTRPV1 by acid and 2-APB, but capsaicin-induced activation was not observed. The sensitivity to acid of lfTRPV1 was similar to that of rat TRPV1 (rTRPV1), but the 2-APB sensitivity of lfTRPV1 was relatively weaker than rTRPV1. Heat stimulation up to 44 °C did not activate lfTRPV1 and the heat-activation was not detected even on acid condition of pH6. This dramatically decreased heat-sensitivity of TRPV1 may contribute the heat tolerance of African lungfish. Moreover, this might be the property of ancient tetrapod-type TRPV1 gene.

Enhancement of allyl isothiocyanate-evoked responses of mouse trigeminal ganglion cells by the kokumi substance γ-glutamyl-valyl-glycine (γ-EVG) through activation of the calcium-sensing receptor (CaSR)

Some γ-glutamyl peptides including glutathione (γ-Glu-Cys-Gly) and γ-glutamyl-valyl-glycine (γ-Glu-Val-Gly= γ-EVG) are reported to increase the intensity of basic tastes, such as salty, sweet, and umami, although they have no taste themselves at tested concentrations. The mechanism of action of γ-glutamyl peptides is not clearly understood, but the calcium sensing receptor (CaSR) may be involved. Glutathione and γ-EVG enhance the pungency of some spices, and the present study investigated the effects of γ-EVG on the responses of trigeminal ganglion (TG) cells to thermosensitiveTRP channel agonists. Single-cell RT-PCR revealed that most CaSR-expressing cells co-expressed TRPV1 (sensitive to capsaicin) and TRPA1 (sensitive to allyl isothiocyanate= AITC). Intracellular Ca²⁺ imaging showed that pretreatment with γ-EVG excited 7% of trigeminal ganglion (TG) cells and increased the amplitude of their responses to AITC, but not to capsaicin or menthol. The enhancing effect of γ-EVG was prevented by a CaSR inhibitor. The results indicate that γ-EVG increases AITC pungency by activating a subset of trigeminal ganglion cells that co-express CaSR and TRPA1.

Influence of topical capsaicin cream on thermoregulation and perception during acute exercise in the heat

PURPOSE

Determine if topical capsaicin, a transient receptor potential vanilloid heat thermoreceptor activator, alters thermoregulation and perception when applied topically prior to thermal exercise.

METHODS

Twelve subjects completed 2 treatments. Subjects walked (1.6 m s^-1, 5% grade) for 30 min in the heat (38 °C, 60% relative humidity) with either a capsaicin (0.025% capsaicin) or control cream applied to the upper (shoulder to wrist) and lower (mid-thigh to ankle) limbs covering ∼50% body surface area. Skin blood flow (SkBF), sweat (rate, composition), heart rate, temperature (skin, core), and perceived thermal sensation were measured prior to and during exercise.

RESULTS

The relative change in SkBF was not different between treatments at any time point (p = 0.284). There were no differences in sweat rate between the capsaicin (1.23 ± 0.37 L h^-1) and control (1.43 ± 0.43 L h^-1, p = 0.122). There were no differences in heart rate between the capsaicin (122 ± 38 beats·min^-1) and control (125 ± 39 beats·min^-1, p = 0.431). There were also no differences in weighted surface (p = 0.976) or body temperatures (p = 0.855) between the capsaicin (36.0 ± 1.7 °C, 37.0 ± 0.8 °C, respectively) and control (36.0 ± 1.6 °C, 36.9 ± 0.8 °C, respectively). The capsaicin treatment was not perceived as hotter than the control treatment until minute 30 of exercise (2.8 ± 0.4, 2.5 ± 0.5, respectively, p = 0.038) CONCLUSIONS: Topical capsaicin application does not alter whole-body thermoregulation during acute exercise in the heat despite perceiving the treatment as hotter late in exercise.

Differential engagement of inhibitory and excitatory cardiopulmonary reflexes by capsaicin and phenylbiguanide in C57BL/6 mice

The Bezold-Jarisch reflex is a powerful inhibitory reflex initiated by activation of cardiopulmonary vagal nerves during myocardial ischemia, hemorrhage, and orthostatic stress leading to bradycardia, vasodilation, hypotension, and vasovagal syncope. This clinically relevant reflex has been studied by measuring heart rate (HR) and mean arterial pressure (MAP) responses to injections of a variety of chemical compounds. We hypothesized that reflex responses to different compounds vary due to differential activation of vagal afferent subtypes and/or variable coactivation of excitatory afferents. HR and MAP responses to intravenous injections of the transient receptor potential vanilloid-1 (TRPV1) agonist capsaicin and the serotonin 5-HT3 receptor agonist phenylbiguanide (PBG) were measured in anesthetized C57BL/6 mice before and after bilateral cervical vagotomy. Capsaicin and PBG evoked rapid dose-dependent decreases in HR and MAP followed by increases in HR and MAP above baseline. Bezold-Jarisch reflex responses were abolished after vagotomy, whereas the delayed tachycardic and pressor responses to capsaicin and PBG were differentially enhanced. The relative magnitude of bradycardic versus depressor responses (↓HR/↓MAP) in vagus-intact mice was greater with capsaicin. In contrast, after vagotomy, the magnitude of excitatory tachycardic versus pressor responses (↑HR/↑MAP) was greater with PBG. Although capsaicin-induced increases in MAP and HR postvagotomy were strongly attenuated or abolished after administration of the ganglionic blocker hexamethonium, PBG-induced increases in MAP and HR were mildly attenuated and unchanged, respectively. We conclude that responses to capsaicin and PBG differ in mice, with implications for delineating the role of endogenous agonists of TRPV1 and 5-HT3 receptors in evoking cardiopulmonary reflexes in pathophysiological states.

Capsaicin treatment in neuropathic pain: axon reflex vasodilatation after 4 weeks correlates with pain reduction

ABSTRACT

Capsaicin, an agonist at the transient receptor potential vanilloid 1, is used for the topical treatment of peripheral neuropathic pain. Reversible receptor defunctionalization and degeneration and subsequent regeneration of cutaneous nociceptors are discussed as its mechanism of action. Here, we hypothesize an accelerated functional recovery of a subclass of nociceptive afferents, the peptidergic vasoactive nociceptors, as the potential cause of capsaicin analgesia. In this noninterventional exploratory trial, 23 patients with peripheral neuropathic pain were treated with one topical high-concentration capsaicin application. Baseline pain ratings, comorbidities, and quality of life were assessed. Functional laser speckle contrast analysis (heat-evoked neurogenic vasodilatation to assess functional properties of peptidergic nociceptors) and quantitative sensory testing were performed in the affected skin. Four weeks after treatment, functional laser speckle contrast analysis and questionnaires were repeated. Telephone interviews were conducted at weeks 2, 10, and 12. Topical capsaicin treatment induced a significant reduction in pain intensity with a maximum at 4 weeks. At the same time, heat-evoked neurogenic vasodilatation was on average similar to pretreatment values. Half of the patients not only showed a functional recovery but also an improvement in vasodilatation, indicating regeneration of nerve fibers. Patients with improved heat-evoked neurogenic vasodilatation at week 4 showed a greater pain reduction than those with deterioration. The degree of vasodilatation significantly correlated with pain reduction. These findings suggest that (1) regeneration of peptidergic nociceptors may be the mechanism behind capsaicin-induced analgesia and (2) that a disease-modifying effect of capsaicin on these fibers already occurs 4 weeks after application.

ARMS/Kidins220 regulates nociception by controlling brain-derived neurotrophic factor secretion

ABSTRACT

Pain is an alarm mechanism to prevent body damage in response to noxious stimuli. The nerve growth factor (NGF)/TrkA axis plays an essential role as pain mediator, and several clinical trials using antibodies against NGF have yielded promising results, but side effects have precluded their clinical approval. A better understanding of the mechanism of NGF/TrkA-mediated nociception is needed. Here, we find that ARMS/Kidins220, a scaffold protein for Trk receptors, is a modulator of nociception. Male mice, with ARMS/Kidins220 reduction exclusively in TrkA-expressing cells, displayed hyperalgesia to heat, inflammatory, and capsaicin stimuli, but not to cold or mechanical stimuli. Simultaneous deletion of brain-derived neurotrophic factor (BDNF) reversed the effects of ARMS/Kidins220 knock down alone. Mechanistically, ARMS/Kidins220 levels are reduced in vitro and in vivo in response to capsaicin through calpains, and this reduction leads to enhanced regulated BDNF secretion from dorsal root ganglion. Altogether, these data indicate that ARMS/Kidins220 protein levels have a role as a pain modulator in the NGF/TrkA axis regulating BDNF secretion.

Dietary Capsaicin Reduces Chemically Induced Rat Urinary Bladder Carcinogenesis

Capsaicin (CAP) is the compound responsible for pungency in chili peppers, presenting several biological properties. But its general safety and effectiveness in the context of carcinogenesis has not been fully clarified. Thus, the present study evaluated whether dietary CAP modifies the development of urothelial lesions induced by the carcinogen N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in male Sprague-Dawley rats. Animals were randomly allocated into 6 groups: G1 - treated with 0.05% BBN in drinking water (weeks 1-12) and received a balanced diet (weeks 1-20); G2 and G3-treated with BBN (weeks 1-12) and received a balanced diet with 0.01 or 0.02% CAP (weeks 1-20), respectively; G4 and G5- only received a balanced diet with 0.01 or 0.02% CAP (weeks 1-20), respectively; G6 - only received a balanced diet (weeks 1-20). At the end of week 20, the incidence and types of urothelial lesions, proliferating cell nuclear antigen (PCNA) labeling index, and matrix metalloproteinases (MMP) 2 and 9 activities were analyzed. A significant reduction was observed in the incidence and multiplicity of simple (p = 0.020 and p = 0.011) and nodular/papillary (p = 0.030 and p = 0.003) hyperplasias and papillomas/carcinomas (p = 0.023 and p = 0.020), epithelial proliferation (p = 0.007) and in the activity of the intermediate form of MMP-2 (p < 0.001) and pro-MMP-9 activities (p < 0.002), in BBN + 0.02% CAP (G3) group in comparison to BBN (G1) group. Capsaicin intake per se did not alter body weight, liver and kidney weights, urothelial histology or serum biochemical parameters. Thus, dietary CAP was safe and showed a protective effect against rat BBN-induced urothelial carcinogenesis.

Drosophila pain sensitization and modulation unveiled by a novel pain model and analgesic drugs

In mammals, pain is regulated by the combination of an ascending stimulating and descending inhibitory pain pathway. It remains an intriguing question whether such pain pathways are of ancient origin and conserved in invertebrates. Here we report a new Drosophila pain model and use it to elucidate the pain pathways present in flies. The model employs transgenic flies expressing the human capsaicin receptor TRPV1 in sensory nociceptor neurons, which innervate the whole fly body, including the mouth. Upon capsaicin sipping, the flies abruptly displayed pain-related behaviors such as running away, scurrying around, rubbing vigorously, and pulling at their mouth parts, suggesting that capsaicin stimulated nociceptors in the mouth via activating TRPV1. When reared on capsaicin-containing food, the animals died of starvation, demonstrating the degree of pain experienced. This death rate was reduced by treatment both with NSAIDs and gabapentin, analgesics that inhibit the sensitized ascending pain pathway, and with antidepressants, GABAergic agonists, and morphine, analgesics that strengthen the descending inhibitory pathway. Our results suggest Drosophila to possess intricate pain sensitization and modulation mechanisms similar to mammals, and we propose that this simple, non-invasive feeding assay has utility for high-throughput evaluation and screening of analgesic compounds.

Cultured rat aortic vascular smooth muscle cells do not express a functional TRPV1

We showed previously that capsaicin, an active compound of chili peppers, can inhibit platelet-derived growth factor-induced proliferation in primary rat vascular smooth muscle cells (VSMCs). The inhibition of BrdU incorporation by capsaicin in these cells was revoked by BCTC, which might be explained by a role of TRPV1 in VSMCs proliferation. To further pursue the hypothesis of a TRPV1-dependent effect of capsaicin, we investigated TRPV1 expression and function. Commercially available antibodies against two different TRPV1 epitopes (N-terminus and C-terminus) were rendered invalid in detecting TRPV1, as shown: i) in western blot experiments using control lysates of TRPV1-expressing (PC-12 and hTRPV1 transfected HEK293T) and TRPV1-downregulated (CRISPR/Cas gene edited A10) cells, and ii) by substantial differences in staining patterns between the applied antibodies using fluorescence confocal microscopy. The TRPV1 agonists capsaicin, resiniferatoxin, piperine and evodiamine did not increase intracellular calcium levels in primary VSMCs and in A10 cells. Using RT qPCR, we could detect a rather low TRPV1 expression in VSMCs at the mRNA level (Cp value around 30), after validating the primer pair in NGF-stimulated PC-12 cells. We conclude that rat vascular smooth muscle cells do not possess canonical TRPV1 channel activity, which could explain the observed antiproliferative effect of capsaicin.

Capsaicin alleviates cisplatin-induced muscle loss and atrophy in vitro and in vivo

BACKGROUND

Cisplatin (CP) is a widely used chemotherapeutic drug with subsequent adverse effects on different organs and tissues including skeletal muscle loss and atrophy as the most common clinical symptoms. The molecular mechanism of cisplatin-induced muscle atrophy is not clearly understood. However, recent significant advances indicate that it is related to an imbalance in both the protein status and apoptosis. Capsaicin (CAP) is one of the major ingredients in chilli peppers. It is a valuable pharmacological agent with several therapeutic applications in controlling pain and inflammation with particular therapeutic potential in muscle atrophy. However, the mechanisms underlying its protective effects against cisplatin-induced muscle loss and atrophy remain largely unknown. This study aims to investigate capsaicin's beneficial effects on cisplatin-induced muscle loss and atrophy in vitro and in vivo.

METHODS

The anti-muscle-atrophic effect of capsaicin on cisplatin-induced muscle loss was investigated using in vivo and in vitro studies. By using the pretreatment model, pretreated capsaicin for 24 h and treated with cisplatin for 48 h, we utilized a C₂ C₁₂ myotube formation model where cell viability analysis, immunofluorescence, and protein expression were measured to investigate the effect of capsaicin in hampering cisplatin-induced muscle atrophy. C57BL/6 mice were administered capsaicin (10, 40 mg/kg BW) as a pretreatment for 5 weeks and cisplatin (3 mg/kg BW) for seven consecutively days to assess muscle atrophy in an animal model for protein and oxidative stress examination, and the grip strength was tested to evaluate the muscle strength.

RESULTS

Our study results indicated that cisplatin caused lower cell viability and showed a subset of hallmark signs typically recognized during atrophy, including severe reduction in the myotube diameter, repression of Akt, and mTOR protein expression. However, pretreatment with capsaicin could ameliorate cisplatin-induced muscle atrophy by up-regulating the protein synthesis in skeletal muscle as well as down-regulating the markers of protein degradation. Additionally, capsaicin was able to downregulate the protein expression of apoptosis-related markers, activated TRPV1 and autophagy progress modulation and the recovery of lysosome function. In vivo, capsaicin could relieve oxidative stress and cytokine secretion while modulating autophagy-related lysosome fusion, improving grip strength, and alleviating cisplatin-induced body weight loss and gastrocnemius atrophy.

CONCLUSIONS

These findings suggest that capsaicin can restore cisplatin-induced imbalance between protein synthesis and protein degradation pathways and it may have protective effects against cisplatin-induced muscle atrophy.

Capsaicin induces redox imbalance and ferroptosis through ACSL4/GPx4 signaling pathways in U87-MG and U251 glioblastoma cells

Glioblastoma is one of the deadliest malignant gliomas. Capsaicin is a homovanillic acid derivative that can show anti-cancer effects by regulating various signaling pathways. The aim of this study is to investigate the effects of capsaicin on cell proliferation via ferroptosis in human U87-MG and U251 glioblastoma cells. Firstly, effects of capsaicin treatment on cell viability were determined by MTT analysis. Next, cellular-proliferation and cytotoxicity assays were determined by analyzing bromodeoxyuridine (BrdU) and lactate dehydrogenase (LDH) activity, respectively. Following, acyl-CoA synthetase long chain family member 4 (ACSL4), glutathione peroxidase 4 (GPx4), 5-hydroxyeicosatetraenoic acid (5-HETE), total oxidant status (TOS), malondialdehyde (MDA), total antioxidant status (TAS) and reduced glutathione (GSH) levels were determined by ELISA. Additionally, ACSL4 and GPx4 mRNA and protein levels were analyzed. Capsaicin showed a concentration-dependent anti-proliferative effects in U87-MG and U251 cells. Cell viability was decreased in the both cell lines treated with capsaicin concentrations above 50 μM, while LDH activity increased. Treatment of 121.6, 188.5, and 237.2 μM capsaicin concentrations for 24 h indicated an increase in ACSL4, 5-HETE, TOS and MDA levels in U87-MG and U251 cells (p < 0.05). On the other hand, we found that capsaicin administration caused a decrease in BrdU, GPx4, TAS and GSH levels in U87-MG and U251 cells (p < 0.05). Besides, ACSL4 mRNA and protein levels were increased in the glioblastoma cells treated with capsaicin, whereas GPx4 mRNA and protein levels were decreased. Finally, capsaicin might be used as a potential anticancer agent with ferroptosis-induced anti-proliferative effects in the treatment of human glioblastoma.

Striving toward hyperthermia-free analgesia: lessons from loss-of-function mutations of human TRPV1

Transient receptor potential vanilloid 1 (TRPV1), a receptor for capsaicin and noxious heat, has been one of the most compelling targets for analgesics. However, systemic inhibition of TRPV1 is an impractical approach as a pain killer, since systemic antagonism induces hyperthermia. Two articles in this issue of the JCI report phenotypes from separate, rare missense mutations of human TRPV1. He, Zambelli, and colleagues investigated TRPV1K710N, which showed reduced functionality, while Katz, Zaguri, and co-authors reported on TRPV1N331K, which led to a complete functional knockout. The findings provide insights that will improve our understanding of the endogenous functions of TRPV1 in humans and may facilitate a rational TRPV1-targeting approach to achieve hyperthermia-free analgesia.

Is the Triggering of PD-L1 Dimerization a Potential Mechanism for Food-Derived Small Molecules in Cancer Immunotherapy? A Study by Molecular Dynamics

Using small molecules to inhibit the PD-1/PD-L1 pathway is an important approach in cancer immunotherapy. Natural compounds such as capsaicin, zucapsaicin, 6-gingerol and curcumin have been proposed to have anticancer immunologic functions by downregulating the PD-L1 expression. PD-L1 dimerization promoted by small molecules was recently reported to be a potential mechanism to inhibit the PD-1/PD-L1 pathway. To clarify the molecular mechanism of such compounds on PD-L1 dimerization, molecular docking and molecular dynamics simulations were performed. The results evidenced that these compounds could inhibit PD-1/PD-L1 interactions by directly targeting PD-L1 dimerization. Binding free energy calculations showed that capsaicin, zucapsaicin, 6-gingerol and curcumin have strong binding ability with the PD-L1 dimer, where the affinities of them follow the trend of zucapsaicin > capsaicin > 6-gingerol ≈ curcumin. Analysis by residue energy decomposition, contact numbers and nonbonded interactions revealed that these compounds have a tight interaction with the C-sheet, F-sheet and G-sheet fragments of the PD-L1 dimer, which were also involved in the interactions with PD-1. Moreover, non-polar interactions between these compounds and the key residues Ile54, Tyr56, Met115 and Ala121 play a key role in stabilizing the protein−ligand complexes in solution, in which the 4′-hydroxy-3′-methoxyphenyl group and the carbonyl group of zucapsaicin, capsaicin, 6-ginger and curcumin were significant for the complexation of small molecules with the PD-L1 dimer. The conformational variations of these complexes were further analyzed by free energy landscape (FEL) and principal component analysis (PCA) and showed that these small molecules could make the structure of dimers more stable. This work provides a mechanism insight for food-derived small molecules blocking the PD-1/PD-L1 pathway via directly targeting the PD-L1 dimerization and offers theoretical guidance to discover more effective small molecular drugs in cancer immunotherapy.

Gut microbiota modulates visceral sensitivity through calcitonin gene-related peptide (CGRP) production

Abdominal pain is common in patients with gastrointestinal disorders, but its pathophysiology is unclear, in part due to poor understanding of basic mechanisms underlying visceral sensitivity. Accumulating evidence suggests that gut microbiota is an important determinant of visceral sensitivity. Clinical and basic research studies also show that sex plays a role in pain perception, although the precise pathways are not elucidated. We investigated pain responses in germ-free and conventionally raised mice of both sexes, and assessed visceral sensitivity to colorectal distension, neuronal excitability of dorsal root ganglia (DRG) neurons and the production of substance P and calcitonin gene-related peptide (CGRP) in response to capsaicin or a mixture of G-protein coupled receptor (GPCR) agonists. Germ-free mice displayed greater in vivo responses to colonic distention than conventional mice, with no differences between males and females. Pretreatment with intracolonic capsaicin or GPCR agonists increased responses in conventional, but not in germ-free mice. In DRG neurons, gut microbiota and sex had no effect on neuronal activation by capsaicin or GPCR agonists. While stimulated production of substance P by DRG neurons was similar in germ-free and conventional mice, with no additional effect of sex, the CGRP production was higher in germ-free mice, mainly in females. Absence of gut microbiota increases visceral sensitivity to colorectal distention in both male and female mice. This is, at least in part, due to increased production of CGRP by DRG neurons, which is mainly evident in female mice. However, central mechanisms are also likely involved in this process.

Mitochondrial transplantation attenuates traumatic neuropathic pain, neuroinflammation, and apoptosis in rats with nerve root ligation

Traumatic neuropathic pain (TNP) is caused by traumatic damage to the somatosensory system and induces the presentation of allodynia and hyperalgesia. Mitochondrial dysfunction, neuroinflammation, and apoptosis are hallmarks in the pathogenesis of TNP. Recently, mitochondria-based therapy has emerged as a potential therapeutic intervention for diseases related to mitochondrial dysfunction. However, the therapeutic effectiveness of mitochondrial transplantation (MT) on TNP has rarely been investigated. Here, we validated the efficacy of MT in treating TNP. Both in vivo and in vitro TNP models by conducting an L5 spinal nerve ligation in rats and exposing the primary dorsal root ganglion (DRG) neurons to capsaicin, respectively, were applied in this study. The MT was operated by administrating 100 µg of soleus-derived allogeneic mitochondria into the ipsilateral L5 DRG in vivo and the culture medium in vitro. Results showed that the viable transplanted mitochondria migrated into the rats' spinal cord and sciatic nerve. MT alleviated the nerve ligation-induced mechanical and thermal pain hypersensitivity. The nerve ligation-induced glial activation and the expression of pro-inflammatory cytokines and apoptotic markers in the spinal cord were also repressed by MT. Consistently, exogenous mitochondria reversed the capsaicin-induced reduction of mitochondrial membrane potential and expression of pro-inflammatory cytokines and apoptotic markers in the primary DRG neurons in vitro. Our findings suggest that MT mitigates the spinal nerve ligation-induced apoptosis and neuroinflammation, potentially playing a role in providing neuroprotection against TNP.

Effect of Calcium-Sensitive Receptor Agonist R568 on Gastric Motility and the Underlying Mechanism

INTRODUCTION

Calcium-sensitive receptor (CaSR) is expressed in the enteric nervous system of gastrointestinal tract. However, its role in the regulation of gastrointestinal motility has not yet been fully elucidated. We aimed to investigate the effect of the CaSR agonist - R568 on gastric motility and its potential mechanism.

METHODS

In vivo, R568 was given by gavage to explore gastric emptying with or without capsaicin which specifically blocks the function of vagal afferents; neurotransmitters synthetized in the myenteric plexus of the gastric corpus and antrum were analysed by ELISA and immunofluorescence staining; gastric muscle strips contraction recording and intracellular single unit firing recording were used to study the effect of R568 on muscle strips and myenteric interstitial cells of Cajal (ICCs) ex vitro.

RESULTS

Gastric emptying was inhibited by R568 in Kunming male mice, and capsaicin weakened this effect. The expression of c-fos-positive neurons increased in the nucleus tractus solitarius when R568 was treated. R568 decreased the expression of cholinergic neurons and reduced the synthesis of acetylcholine. Conversely, R568 increased the expression of nitrogenic neurons and enhanced the synthesis of nitric oxide in the myenteric plexus. Ex vitro results showed that R568 inhibited the contraction of the gastric antral muscle strip and suppressed the spontaneous firing activity of pacemaker ICCs.

CONCLUSION

Activation of the gastrointestinal CaSR inhibited gastric motility in vivo and ex vitro. Transmitting nutrient signals to the brain through the vagal afferent nerve, modulating the cholinergic and nitrergic system in the enteric nervous system, and inhibiting activity of pacemaker ICCs in the myenteric plexus are involved in the mechanism of CaSR in gastric motility suppression.

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

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

Dual effects of anandamide in the antiepileptic activity of diazepam in pentylenetetrazole-induced seizures in mice

The prototype endocannabinoid, anandamide activates both CB 1 and transient receptor potential vanilloid type 1 channels (TRPV1) receptor at different concentrations. At high concentrations, anandamide-mediated TRPV1 effects are opposite to its effects at low concentrations via CB 1 receptor. Thus, synaptic concentrations of anandamide govern the neuronal activity and consequently might affect the response of a drug. This study was undertaken to investigate the influence of high and low doses of anandamide on the anticonvulsant action of diazepam on the subcutaneous dose of pentylenetetrazole (PTZ) in Swiss mice weighing 20-25 g. Results revealed that intracerebroventricular administration of capsazepine (a TRPV1 antagonist: 1, 10, or 100 µg/mouse) and the low doses (10 µg/mouse) of anandamide, AM404 (anandamide transport inhibitor), or URB597 (fatty acid amide hydrolase inhibitor) augmented the anticonvulsant effect of diazepam. Conversely, higher dose of anandamide, AM404, URB597 (100 µg/mouse) as well as capsaicin (a TRPV1 agonist: 1, 10, or 100 µg/mouse) attenuated the protective effect of diazepam against PTZ-induced seizures. Thus, this study demonstrates that the effects of diazepam may be augmented by activating CB 1 receptors or dampened via TRPV1 receptors. The findings of the present study can be extrapolated to understand the use of TRPV1 blockers alone or in combination of benzodiazepines in the treatment of benzodiazepines-refractory status epilepticus, a condition associated with maladaptive trafficking of synaptic gamma-aminobutyric acid and glutamate receptors. However, potential clinical applications are needed to further support such preclinical studies.

Dietary Capsaicin: A Spicy Way to Improve Cardio-Metabolic Health?

Today's sedentary lifestyle with too much food and too little exercise has made metabolic syndrome a pandemic. Metabolic syndrome is a major risk factor for type-2 diabetes and cardiovascular disease. New knowledge of medical and nutraceutical intervention in the early stages of metabolic syndrome is central to prevent these deadly complications. People who eat chili pepper on a regular basis seem to stay healthier and live longer than those who do not. Animal experiments suggest a therapeutic potential for dietary capsaicin, the active principle in hot chili pepper, to reduce the risk of developing metabolic syndrome. This is an attractive theory since capsaicin has been a culinary staple for thousands of years, and is generally deemed safe when consumed in hedonically acceptable doses. The broad expression of the capsaicin receptor TRPV1 in metabolically active tissues lends experimental support to this theory. This review critically evaluates the available experimental and clinical evidence for and against dietary capsaicin being an effective dietary means to improve cardio-metabolic health. It comes to the conclusion that although a chili pepper-rich diet is associated with a reduced risk of dying due to cardiovascular disease, dietary capsaicin has no clear effect on blood glucose or lipid profiles. Therefore, the reduced mortality risk may reflect the beneficial action of digested capsaicin on gut microbiota.

Demethylation of CADM1 and SOCS1 using capsaicin in cervical cancer cell line

Cervical cancer is one of the leading causes of women's mortality in developing countries. The prevalence of cervical cancer is higher in developing countries like India and continents like Africa. Hyper-methylation of tumor suppressor genes through human papillomavirus (HPV) infection is known to be one of the major causes of cervical cancer. The promoter hypermethylation of the cell adhesion molecule 1 (CADM1) and suppressor of cytokine signalling (SOCS1) genes due to DNMT1 overexpression leads to their epigenetic silencing followed by gene repression causing cervical cancer. In silico study on the inhibition effect of capsaicin on DNMT1 was simulated by different servers. The binding energy was observed to be -7.8 kcal/mol. In vitro studies on the effect of capsaicin on aberrant methylation of CADM1 and SOCS1 were performed on the adenocarcinoma cervical cancer cell line, HeLa. The IC₅₀ of capsaicin was observed to be 160 μM through crystal violet assay. DNA methylation of the CADM1 and SOCS1 was analyzed by methylation-specific PCR along with their reversal using capsaicin (20 μM) by treating the cells for 72 h and 6 days. In silico results suggested that capsaicin has an inhibitory effect on DNMT1, which regulates DNA methylation leading to the hypermethylation of CADM1 and SOCS1 genes. The in vitro studies suggested that hypermethylation leads to the inhibition of CADM1 and SOCS1 expression, which could be reversed using capsaicin with visible changes in methylation-specific and unmethylation-specific bands in MS-PCR, respectively. The present study shows the reversal of methylation of CADM1 and SOCS1 after 72 h which showed a further increase in case of 6 days of treatment using 20 μM capsaicin, which makes capsaicin a potent candidate for causing demethylation of CADM1 and SOCS1 genes that may lead to the reactivation of the downregulated gene.

Capsaicin-Induced Endocytosis of Endogenous Presynaptic CaV2.2 in DRG-Spinal Cord Co-Cultures Inhibits Presynaptic Function

The N-type calcium channel, CaV2.2 is key to neurotransmission from the primary afferent terminals of dorsal root ganglion (DRG) neurons to their postsynaptic targets in the spinal cord. In this study, we have utilized CaV2.2_HA knock-in mice, because the exofacial epitope tag in CaV2.2_HA enables accurate detection and localization of endogenous CaV2.2. CaV2.2_HA knock-in mice were used as a source of DRGs to exclusively study the presynaptic expression of N-type calcium channels in co-cultures between DRG neurons and wild-type spinal cord neurons. CaV2.2_HA is strongly expressed on the cell surface, particularly in TRPV1-positive small and medium DRG neurons. Super-resolution images of the presynaptic terminals revealed an increase in CaV2.2_HA expression and increased association with the postsynaptic marker Homer over time in vitro. Brief application of the TRPV1 agonist, capsaicin, resulted in a significant down-regulation of cell surface CaV2.2_HA expression in DRG neuron somata. At their presynaptic terminals, capsaicin caused a reduction in CaV2.2_HA proximity to and co-localization with the active zone marker RIM 1/2, as well as a lower contribution of N-type channels to single action potential-mediated Ca2+ influx. The mechanism of this down-regulation of CaV2.2_HA involves a Rab11a-dependent trafficking process, since dominant-negative Rab11a (S25N) occludes the effect of capsaicin on presynaptic CaV2.2_HA expression, and also prevents the effect of capsaicin on action potential-induced Ca2+ influx. Taken together, these data suggest that capsaicin causes a decrease in cell surface CaV2.2_HA expression in DRG terminals via a Rab11a-dependent endosomal trafficking pathway.

Nitro Dihydrocapsaicin, a Non-Pungent Capsaicin Analogue, Inhibits Cellular Senescence of Lens Epithelial Cells via Upregulation of SIRT1

Diabetic cataracts are a common complication that can cause blindness among patients with diabetes mellitus. A novel nitro dihydrocapsaicin (NDHC), a capsaicin analog, was constructed to have a non-pungency effect. The objective of this research was to study the effect of NDHC on human lens epithelial (HLE) cells that lost function from hyperglycemia. HLE cells were pretreated with NDHC before an exposure to high glucose (HG) conditions. The results show that NDHC promoted a deacceleration of cellular senescence in HLE cells. This inhibition of cellular senescence was characterized by a delayed cell growth and lower production of reactive oxygen species (ROS) as well as decreased SA-β-galactosidase activity. Additionally, the expression of Sirt1 protein sharply increased, while the expression of p21 and phospho-p38 proteins decreased. These findings provide evidence that NDHC could exert a pharmacologically protective effect by inhibiting the senescence program of lens cells during diabetic cataracts.

Capsaicin Decreases Kidney Iron Deposits and Increases Hepcidin Levels in Diabetic Rats with Iron Overload: A Preliminary Study

Iron overload (IOL) increases the risk of diabetes mellitus (DM). Capsaicin (CAP), an agonist of transient receptor potential vanilloid-1 (TRPV1), reduces the effects of IOL. We evaluated the effects of chronic CAP administration on hepcidin expression, kidney iron deposits, and urinary biomarkers in a male Wistar rat model with IOL and DM (DM-IOL). IOL was induced with oral administration of iron for 12 weeks and DM was induced with streptozotocin. Four groups were studied: Healthy, DM, DM-IOL, and DM-IOL + CAP (1 mg·kg^-1·day^-1 for 12 weeks). Iron deposits were visualized with Perls tissue staining and a colorimetric assay. Serum hepcidin levels were measured with an enzyme-linked immunosorbent assay. Kidney biomarkers were assayed in 24 h urine samples. In the DM-IOL + CAP group, the total area of iron deposits and the total iron content in kidneys were smaller than those observed in both untreated DM groups. CAP administration significantly increased hepcidin levels in the DM-IOL group. Urinary levels of albumin, cystatin C, and beta-2-microglobulin were similar in all three experimental groups. In conclusion, we showed that in a DM-IOL animal model, CAP reduced renal iron deposits and increased the level of circulating hepcidin.

TRPA1, but not TRPV1, is involved in the increase of the non-adrenergic non-cholinergic outflow induced by hydrogen sulfide in pithed rats

Hydrogen sulfide (H₂S) is a gasotransmitter that modulates the peripheral transmission regulating the vascular tone. In vitro studies have suggested that H₂S induces vasodilation by stimulating capsaicin-sensitive sensory neurons. This study was designed to determine the effects of H₂S on the non-adrenergic/non-cholinergic (NANC) outflow in the pithed rat, and the underlying mechanisms. For that purpose, 72 male Wistar rats were anesthetized, pithed and the carotid, femoral and jugular veins were cannulated and then divided into two main sets. The first set of animals (n = 48) was used to determine the effect of NaHS (H₂S donor) on the vasodepressor responses induced by: 1) NANC outflow electrical stimulation (n = 24); and 2) i.v. bolus of α-CGRP (n = 24) and subdivided into 4 groups (n = 6 each): 1) control group (without infusion); continuous infusion of: 2) PBS (vehicle; 0.02 ml/kg·min); 3) NaHS 10 μg/kg·min; and 4) NaHS 18 μg/kg·min. The second set of animals (n = 24) received an i.v. bolus of either (1) HC 030031 (TRPA1 channel antagonist; 18 μg/kg; n = 12) or (2) capsazepine (TRPV1 channel antagonist; 100 μg/kg; n = 12) in presence and absence of 18 µg/kg·min NaHS i.v. continuous infusion to determine the underlying mechanism of the NaHS effect on the NANC outflow. Our results show that NaHS infusion increased the vasodepressor responses induced by electrical stimulation, but not by α-CGRP, effect that was abolished by HC030031 and remained unaffected after capsazepine. These data suggest that activation of TRPA1 channels, but no TRPV1, is responsible for the NaHS-induced NANC neurotransmission stimulation.

Activation of Meningeal Afferents Relevant to Trigeminal Headache Pain after Photothrombotic Stroke Lesion: A Pilot Study in Mice

Stroke can be followed by immediate severe headaches. As headaches are initiated by the activation of trigeminal meningeal afferents, we assessed changes in the activity of meningeal afferents in mice subjected to cortical photothrombosis. Cortical photothrombosis induced ipsilateral lesions of variable sizes that were associated with contralateral sensorimotor impairment. Nociceptive firing of mechanosensitive Piezo1 channels, activated by the agonist Yoda1, was increased in meningeal afferents in the ischemic hemispheres. These meningeal afferents also had a higher maximal spike frequency at baseline and during activation of the mechanosensitive Piezo1 channel by Yoda1. Moreover, in these meningeal afferents, nociceptive firing was active during the entire induction of transient receptor potential vanilloid 1 (TRPV1) channels by capsaicin. No such activation was observed on the contralateral hemi-skulls of the same group of mice or in control mice. Our data suggest the involvement of mechanosensitive Piezo1 channels capable of maintaining high-frequency spiking activity and of nociceptive TRPV1 channels in trigeminal headache pain responses after experimental ischemic stroke in mice.