Capsaicin modulates acetylcholine release at the myoneural junction

Localization of sensory nerve terminals containing calcitonin gene-related peptide (CGRP) on striated muscle fibers in the rat esophagus: Evidence for triple innervation via motor endplates

BACKGROUND

Many esophageal striated muscles of mammals are dually innervated by the vagal and enteric nerves. Recently, substance P (SP)-sensory nerve terminals with calcitonin gene-related peptide (CGRP) were found on a few striated muscle fibers in the rat esophagus, implying that these muscle fibers are triply innervated. In this study, we examined the localization and origin of CGRP-nerve endings in striated muscles to consider their possible roles in the esophagus regarding triple innervation.

METHODS

Wholemounts of the rat esophagus were immunolabeled to detect CGRP-nerve endings in striated muscles. Also, retrograde tracing was performed by injecting Fast Blue (FB) into the esophagus, and cryostat sections of the medulla oblongata, nodose ganglion (NG), and the tenth thoracic (T10) dorsal root ganglion (DRG) were immunostained to identify the origin of the CGRP-nerve endings.

RESULTS

CGRP-fine, varicose nerve endings were localized in motor endplates on a few esophageal striated muscle fibers (4 %), most of which received nitric oxide (NO) synthase nerve terminals, and most of the CGRP nerve endings were SP- and transient receptor potential vanilloid member 1 (TRPV1)-positive. Retrograde tracing showed many FB-labeled CGRP-neurons positive for SP and TRPV1 in the NG and T10 DGR.

CONCLUSIONS

This study suggests that the CGRP-varicose nerve endings containing SP and TRPV1 in motor endplates are sensory, and a few esophageal striated muscle fibers are triply innervated. The nerve endings may detect acetylcholine-derived acetic acid from the vagal motor nerve endings and NO from esophageal intrinsic nerve terminals in the motor endplates to regulate esophageal motility.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

Activation of TRPV1 Channels Inhibits the Release of Acetylcholine and Improves Muscle Contractility in Mice

TRPV1 represents a non-selective transient receptor potential cation channel found not only in sensory neurons, but also in motor nerve endings and in skeletal muscle fibers. However, the role of TRPV1 in the functioning of the neuromuscular junction has not yet been fully established. In this study, the Levator Auris Longus muscle preparations were used to assess the effect of pharmacological activation of TRPV1 channels on neuromuscular transmission. The presence of TRPV1 channels in the nerve terminal and in the muscle fiber was confirmed by immunohistochemistry. It was verified by electrophysiology that the TRPV1 channel agonist capsaicin inhibits the acetylcholine release, and this effect was completely absent after preliminary application of the TRPV1 channel blocker SB 366791. Nerve stimulation revealed an increase of amplitude of isometric tetanic contractions upon application of capsaicin which was also eliminated after preliminary application of SB 366791. Similar data were obtained during direct muscle stimulation. Thus, pharmacological activation of TRPV1 channels affects the functioning of both the pre- and postsynaptic compartment of the neuromuscular junction. A moderate decrease in the amount of acetylcholine released from the motor nerve allows to maintain a reserve pool of the mediator to ensure a longer signal transmission process, and an increase in the force of muscle contraction, in its turn, also implies more effective physiological muscle activity in response to prolonged stimulation. This assumption is supported by the fact that when muscle was indirect stimulated with a fatigue protocol, muscle fatigue was attenuated in the presence of capsaicin.

Recombinant Analogs of Sea Anemone Kunitz-Type Peptides Influence P2X7 Receptor Activity in Neuro-2a Cells

Purinergic P2X7 receptors (P2X7) have now been proven to play an important role and represent an important therapeutic target in many pathological conditions including neurodegeneration. Here, we investigated the impact of peptides on purinergic signaling in Neuro-2a cells through the P2X7 subtype in in vitro models. We have found that a number of recombinant peptides, analogs of sea anemone Kunitz-type peptides, are able to influence the action of high concentrations of ATP and thereby reduce the toxic effects of ATP. The influx of calcium, as well as the fluorescent dye YO-PRO-1, was significantly suppressed by the studied peptides. Immunofluorescence experiments confirmed that the peptides reduce the P2X7 expression level in neuronal Neuro-2a cells. Two selected active peptides, HCRG1 and HCGS1.10, were found to specifically interact with the extracellular domain of P2X7 and formed stable complexes with the receptor in surface plasmon resonance experiments. The molecular docking approach allowed us to establish the putative binding sites of the most active HCRG1 peptide on the extracellular domain of the P2X7 homotrimer and propose a mechanism for regulating its function. Thus, our work demonstrates the ability of the Kunitz-type peptides to prevent neuronal death by affecting signaling through the P2X7 receptor.

Kainic Acid Activates TRPV1 via a Phospholipase C/PIP2-Dependent Mechanism in Vitro

Kainic acid (KA) is an excitotoxic glutamate analogue produced by a marine seaweed. It elicits neuronal excitotoxicity leading to epilepsy in rodents. Activation of transient receptor potential vanilloid subfamily 1 (TRPV1), a nonselective cation channel protein, by capsaicin, prevents KA-induced seizures in a mouse model of temporal lobe epilepsy. However, the precise mechanism behind this protective effect of capsaicin remains unclear. In order to analyze the direct effect of KA on TRPV1, we evaluated the ability of KA to activate TRPV1 and analyzed its binding to TRPV1 using a molecular modeling approach. In vitro, KA activates a Ca2+ influx into TRPV1 expressing HEK293 cells but not in contsrol HEK293 cells. Pretreatment with either capsaicin (1 M) or capsazepine (10 M; TRPV1 antagonist) prevents the effect of KA. Pharmacological inhibition of phospholipase C (PLC) by U73122 or overexpression of phosphatidylinositol 5 phosphatase (Synaptojanin 1; Synj-1) counters the effect of KA. Further, KA treatment causes actin reorganization in HEKTRPV1 cells and PLC inhibition by U73122 prevents this. Molecular modeling data revealed that KA binds to TRPV1 and prebinding with capsaicin prevents the binding of KA to TRPV1. Consistently, the lack of effect of KA in activating chicken TRPV1, which is insensitive to capsaicin, suggests that there is a significant overlap between the sites of KA and capsaicin activation of TRPV1. However, PLC inhibition did not suppress TRPV1 activation by capsaicin. Collectively, our data suggest that KA binds to and activates TRPV1 and causes actin reorganization via PLC-dependent mechanism in vitro. We propose that KA mediates Ca2+ induced toxicity possibly by activating TRPV1. Therefore, inhibiting TRPV1 will be a beneficial strategy in abating Ca2+-induced neurotoxicity.

Ultra-low doses of the transient receptor potential vanilloid 1 agonist, resiniferatoxin, prevents vomiting evoked by diverse emetogens in the least shrew (Cryptotis parva)

Published studies have shown that the transient receptor potential vanilloid 1 (TRPV1) receptor agonist, resiniferatoxin (RTX), has pro and antiemetic effects. RTX can suppress vomiting evoked by a variety of nonselective emetogens such as copper sulfate and cisplatin in several vomit-competent species. In the least shrew, we have already demonstrated that combinations of ultra-low doses of RTX and low doses of the cannabinoid CB1/2 receptor agonist delta-9-tetrahydrocannabinol (Δ-THC) produce additive antiemetic effects against cisplatin-evoked vomiting. In the current study, we investigated the broad-spectrum antiemetic potential of very low nonemetic doses of RTX against a diverse group of specific emetogens including selective and nonselective agonists of serotonergic 5-hydroxytrptamine (5-HT3) receptor (5-HT and 2-Me-5-HT), dopaminergic D2 receptor (apomorphine and quinpirole), cholinergic M1 receptor (pilocarpine and McN-A-343), as well as the selective substance P neurokinin NK1 receptor agonist GR73632, the selective L-Type calcium channel agonist FPL64176, and the sarcoplasmic endoplasmic reticulum calcium ATPase (SERCA) inhibitor thapsigargin. When administered subcutaneously, ultra-low (0.01 µg/kg) to low (5.0 µg/kg) doses of RTX suppressed vomiting induced by the aforementioned emetogens in a dose-dependent fashion with 50% inhibitory dose values ranging from 0.01 to 1.26 µg/kg. This study is the first to demonstrate that low nanomolar nonemetic doses of RTX have the capacity to completely abolish vomiting caused by diverse receptor specific emetogens in the least shrew model of emesis.

Dscam2 suppresses synaptic strength through a PI3K-dependent endosomal pathway

Dscam2 is a cell surface protein required for neuronal development in Drosophila; it can promote neural wiring through homophilic recognition that leads to either adhesion or repulsion between neurites. Here, we report that Dscam2 also plays a post-developmental role in suppressing synaptic strength. This function is dependent on one of two distinct extracellular isoforms of the protein and is autonomous to motor neurons. We link the PI3K enhancer, Centaurin gamma 1A, to the Dscam2-dependent regulation of synaptic strength and show that changes in phosphoinositide levels correlate with changes in endosomal compartments that have previously been associated with synaptic strength. Using transmission electron microscopy, we find an increase in synaptic vesicles at Dscam2 mutant active zones, providing a rationale for the increase in synaptic strength. Our study provides the first evidence that Dscam2 can regulate synaptic physiology and highlights how diverse roles of alternative protein isoforms can contribute to unique aspects of brain development and function.

α7 Nicotinic acetylcholine receptor contributes to the alleviation of lung ischemia-reperfusion injury by transient receptor potential vanilloid type 1 stimulation

BACKGROUND

Activation of transient receptor potential vanilloid type 1 (TRPV1) decreases lung ischemia-reperfusion injury (LIRI) in rabbits and rats. Stimulation of α7 nicotinic acetylcholine receptors (α7nAChRs) protects against lung injury. Here we examined whether α7nAChRs contribute to TRPV1-mediated protection against LIRI.

METHODS

Wild-type (WT) and TRPV1-knockout (KO) mice were subjected to 1-h lung ischemia by clamping left hilum, followed by 2-h reperfusion. WT or KO mice were pretreated with vehicle, TRPV1 agonist capsaicin, TRPV1 antagonist capsazepine, α7nAChR antagonist methyllycaconitine, or α7nAChR agonist PNU-282987. Arterial blood and lung tissues were obtained for blood gas, lung wet-to-dry weight ratio, interleukin (IL)1β, IL6, tumor necrosis factor-α (TNF-α), apoptosis-related proteins (caspases, Bax, Fas), and pathologic scoring.

RESULTS

Capsaicin pretreatment reduced wet-to-dry ratio, pathologic score, alveolar-arterial oxygen gradient (A-aDO₂), and IL1β, IL6, and TNFα levels in WT mice, with no effects in KO mice. This reduction was reversed by TRPV1 blockade. Furthermore, α7nAChR blockade before capsaicin exacerbated LIRI as evidenced by enhanced alveolar-arterial oxygen gradient, pathologic score, and IL1β, IL6, and TNFα levels, while α7nAChR agonist pretreatment under TRPV1 blockade showed opposite changes. Capsaicin also decreased cleaved caspase-3, caspase-3/9, and Bax protein expression, effects abolished by TRPV1 blockade. Similarly, α7nAChR blockade diminished capsaicin-induced downregulation of apoptotic proteins, and α7nAChR activation decreased expression levels even under TRPV1 blockade.

CONCLUSIONS

TRPV1 activation alleviates LIRI, partially dependent on α7nAChR activity. The α7nAChR stimulation with or without existence of TRPV1 alleviates LIRI. Thus, α7nAChR is involved in the pathway of TRPV1-mediated protection against LIRI and the specific mechanism remains to be revealed.

Ingestion of transient receptor potential channel agonists attenuates exercise-induced muscle cramps

INTRODUCTION

Exercise-associated muscle cramping (EAMC) is a poorly understood problem that is neuromuscular in origin. Ingestion of transient receptor potential (TRP) channel agonists has been efficacious in attenuating electrically induced muscle cramps. This study examines the effect of TRP agonist ingestion on voluntarily induced EAMC and motor function.

METHODS

Study 1: Thirty-nine participants completed 2 trials after ingesting TRP agonist-containing active treatment (A), or vehicle (V) control. Cramping in the triceps surae muscle was induced via voluntary isometric contraction. Study 2: After ingesting A or V, 31 participants performed kinematic and psychomotor tests of manual dexterity.

RESULTS

A increased precramp contraction duration (A, 36.9 ± 4.1 s; V, 27.8 ± 3.1 s), decreased cramp EMG area under the curve (A, 37.3 ± 7.7 %EMG_max ·s; V, 77.2 ± 17.7 %EMG_max ·s), increased contraction force to produce the cramp (A, 13.8 ± 1.8 kg; V, 9.9 ± 1.6 kg), and decreased postcramp soreness (A, 4.1 ± 0.3 arbitrary units (a.u.); V, 4.7 ± 0.3 a.u.). Kinematic and psychomotor tests were not affected.

DISCUSSION

TRP agonist ingestion attenuated EAMC characteristics without affecting motor function. Muscle Nerve 56: 379-385, 2017.

TRPV1 activation counters diet-induced obesity through sirtuin-1 activation and PRDM-16 deacetylation in brown adipose tissue

Background/Objective

An imbalance between energy intake and expenditure leads to obesity. Increasing metabolism and thermogenesis in brown adipose tissue (BAT) can help in overcoming obesity. Here, we investigated the effect of activation of transient receptor potential vanilloid subfamily 1 (TRPV1) in the upregulation of thermogenic proteins in BAT to counter diet-induced obesity.

Subjects/Methods

We investigated the effect of dietary supplementation of capsaicin (TRPV1 agonist) on the expression of metabolically important thermogenic proteins in BAT of wild type and TRPV1^−/− mice that received either a normal chow or high fat (± capsaicin; TRPV1 activator) diet by immunoblotting. We measured the metabolic activity, respiratory quotient and BAT lipolysis.

Results

CAP antagonized high fat diet (HFD)-induced obesity without decreasing energy intake in mice. HFD suppressed TRPV1 expression and activity in BAT and CAP countered this effect. HFD feeding caused glucose intolerance, hypercholesterolemia and decreased the plasma concentration of glucagon like peptide-1 and CAP countered these effects. HFD suppressed the expression of metabolically important thermogenic genes, ucp-1, bmp8b, sirtuin 1, pgc-1α and prdm-16 in BAT and CAP prevented this effect. CAP increased the phosphorylation of sirtuin 1 and induced an interaction between PPARγ with PRDM-16. Further, CAP treatment, in vitro, decreased the acetylation of PRDM-16, which was antagonized by inhibition of TRPV1 by capsazepine, chelation of intracellular Ca²⁺ by cell permeable BAPTA-AM or the inhibition of SIRT-1 by EX 527. Further, CAP supplementation, post HFD, promoted weight loss and enhanced the respiratory exchange ratio. CAP did not have any effect in TRPV1^−/− mice.

Conclusions

Our data show that activation of TRPV1 in BAT enhances the expression of SIRT-1, which facilitates the deacetylation and interaction of PPARγ and PRDM-16. These data suggest that TRPV1 activation is a novel strategy to counter diet-induced obesity by enhancing metabolism and energy expenditure.

Capsaicin presynaptically inhibits glutamate release through the activation of TRPV1 and calcineurin in the hippocampus of rats

Capsaicin is the major ingredient in hot peppers of the plantCapsicum genuswith neuroprotective effects in several preclinical models; its effect on glutamate release has been investigated in the rat hippocampus using isolated nerve terminals (synaptosomes) and brain slices.

Protective effect of transient receptor potential vanilloid subtype 1 (TRPV1) modulator, against behavioral, biochemical and structural damage in experimental models of Alzheimer's disease

Alzheime's disease (AD) is an overwhelming neurodegenerative disorder, characterized by synaptic dysfunction, memory loss, neuro-inflammation and neural cell death. Very few treatments are in hand for the management of AD and they are only concentrating on peculiar aspects. Hence, an immense thrust is required to find utmost therapeutic targets to conquer this condition. This study investigates a potential role of vanillin, a selective agonist of transient receptor potential vanilloid subtype 1 (TRPV1) in the experimental models of AD viz. intracerebroventricular (i.c.v.) streptozotocin (STZ) and aluminum trichloride (AlCl3)+d-galactose induced AD in mice. The i.c.v. administration of STZ and intraperitoneally administration of AlCl3+d-galactose have significantly impaired learning-memory (Morris water maze and attentional set-shifting test), brain structure (hematoxylin, eosin and Congo red staining), enhanced brain oxidative stress (thiobarbituric acid reactive substance - TBARS and glutathione - GSH), nitrosative stress (nitrite/nitrate), acetylcholinesterase activity (AChE), inflammation (MPO), and calcium levels (Ca(++)). Treatment with vanillin in different doses and donepezil have significantly ameliorated i.c.v. STZ and AlCl3+d-galactose induced reduction in executive function, impaired reversal learning, cognition, memory and brain damage. Treatment with these drugs has also reduced the brain oxidative stress (TBARS and GSH), nitrosative stress (nitrite/nitrate), and AChE, MPO, and Ca(++) levels. These results indicate that vanillin, a selective agonist of TRPV1 and donepezil, a potent acetylcholine esterase inhibitor have attenuated i.c.v. STZ and AlCl3+d-galactose induced experimental AD. Hence, pharmacological positive modulation of TRPV1 channels may be a potential research target for mitigation of AD.