A Review on Potential Involvement of TRPV 1 Channels in Ischemia–Reperfusion Injury

Abstract

Besides functioning as thermosensors, transient receptor potential vanilloid 1 (TRPV₁) channels play a pivotal role in ischemia–reperfusion injury. Transient receptor potential vanilloid 1 channel activation attenuates ischemia–reperfusion-induced injury in various organs including the heart, lungs, kidneys, and the brain. Transient receptor potential vanilloid 1 channels are expressed on the sensory neurons innervating the myocardium, ventricles of the heart, epicardial surface of the heart, endothelial cells, and the vascular smooth muscle cells. During ischemic conditions, activation of TRPV₁ channels on the perivascular nerves stimulates the release of calcitonin gene-related peptide and substance P to produce cardioprotection. Furthermore, TRPV₁ channel activation reduces the generation of free radicals and inflammatory cytokines, inhibits neutrophil infiltration, and enhances the production of anti-inflammatory cytokines to reduce ischemia–reperfusion-induced tissue injury. The present review describes the potential involvement of TRPV₁ channels and the signaling cascade in attenuating ischemia–reperfusion injury in various organs.

Introduction

Transient receptor potential vanilloid 1 (TRPV₁) channels belong to the TRP family that displays dynamic ion selectivity for ions including H⁺, Na⁺, Ca²⁺, and Mg²⁺. ¹ The TRP family has been divided into 6 subfamilies, namely, vanilloid (TRPV), canonical (TRPC), melastatin (TRPM), polycystin (TRPP), mucoloipin (TRPML), and ankyrin (TRPA), based on the sequence of amino acids’ homology. ^2,3 Transient receptor potential vanilloid 1 channel possesses 6 transmembrane helices per subunit with cytoplasmic amino and carboxy termini, a loop localized between fifth and sixth transmembrane domain and it exists in the form of tetrameric assemblies. ⁴ Besides functioning as sensors of noxious heat, these channels are activated by various chemical agents including arachidonic acid metabolites, capsaicin, protons, and peptide toxins. ¹

Transient receptor potential vanilloid 1 channels are widely distributed in different organs including heart, liver, lungs, kidney, intestine, and brain. ^5,6 Although, within the cardiovascular system, TRPV₁ channels are expressed on the ventricles of the heart, ⁶ epicardial surface of the heart, endothelial cells, and vascular smooth muscle cells, ⁷ these are also abundantly expressed on the sensory neurons innervating the myocardium. ⁸ The activation of TRPV₁ channels located on the perivascular nerves enhances calcitonin gene-related peptide (CGRP) and substance P discharge to provide cardioprotective effects. ^9,10 Transient receptor potential vanilloid 1 channel activation has been implicated in attenuating ischemia–reperfusion-induced injury in several organs including heart, ^10,11 kidney, ^5,12 lungs, ¹³ and brain. ¹⁴ Moreover, activation of these channels is known to produce hypoxic preconditioning ¹⁵ and remote ischemic postconditioning in the isolated rat hearts. ¹⁶ The present review describes the potential involvement of TRPV₁ channels and the signaling cascade in attenuating ischemia–reperfusion injury in various organs.

Cardioprotective Effects of TRPV₁ channel Activation in Ischemia–Reperfusion Injury

Various researchers have explored the potential involvement of TRPV₁ channel activation in attenuating ischemia–reperfusion-induced myocardial injury. ^10,11,17,18 Wei and coauthors demonstrated that metabolic syndrome-dependent reduction of TRPV₁ channel expression and a decrease in CGRP release led to greater ischemia–reperfusion injury in the isolated mice hearts. The Langendorff perfused isolated hearts from diabetic mice exhibited greater reduction in the left ventricular developed pressure (LVDP), heart rate, coronary flow, and increased lactate dehydrogenase release in comparison with the hearts isolated from normal mice. Meanwhile, CGRP administration (10⁻⁷ mmol/L) for 5 minutes before 30-minute sustained ischemia–40-minute reperfusion-attenuated ischemia–reperfusion injury in both normal and diabetic mice hearts. However, capsaicin pretreatment (potent TRPV₁ channel agonist, 10⁻⁶ mol/L) for 5 minutes, before sustained ischemia, significantly abrogated ischemia–reperfusion injury in normal, but not in the diabetic mice hearts. The nonbeneficial effects of capsaicin in diabetic mice hearts may be partially due to the reduction in the number and functionality of myocardial TRPV₁ receptors in the diabetic hearts. Despite the reduced expression of TRPV1, the restoration of cardioprotective effects in the presence of exogenous CGRP suggests that diabetes does not disrupt the downstream pathway of the CGRP. ¹⁸

Recently, Zheng et al also reported that metabolic disorder is associated with an extensive ischemia–reperfusion injury (after 30-minute ischemia–30-minute reperfusion), reduction in the expression of nerve growth factor, TRPV₁ channel expression, and release of CGRP and substance P in the isolated mice hearts. Adenovirus-mediated delivery of the nerve growth factor gene significantly improved post-ischemic recovery, increased the expression of TRPV₁ channels, and increased the release of CGRP, but not substance P, in the coronary effluent in normal and diabetic mice hearts. Meanwhile, administration of RP67580 (substance P receptor antagonist) did not modulate nerve growth factor–mediated cardioprotective effects, suggesting that substance P may not be involved in providing cardioprotection. However, nerve growth factor–dependent cardioprotective effects were significantly abolished in the presence of CGRP_8-37 (CGRP antagonist, 10⁻⁶ mol/L), emphasizing that nerve growth–dependent cardioprotective effects are mediated through CGRP release. Exogenous administration of low-dose capsaicin (10⁻⁶ mol/L), added in the perfusate 5 minutes before ischemia, significantly improved the post-ischemic functional recovery in diabetic heart, indicating the cardioprotective effects of TRPV₁ channel activation. Based on these, it has been hypothesized that nerve growth factor–dependent restoration of TRPV₁ channels and CGRP release may rescue diabetic mice heart from ischemia–reperfusion injury. ¹⁰

Zhong and Wang reported that N-oleoyldopamine (2 × 10⁻⁹ M, TRPV₁ agonist) administration in the Langendorff perfused hearts led to improvement in cardiac function in terms of increase in LVDP, coronary flow, and +dP/dt after exposure to sustained ischemia (> 30-minute ischemia)–reperfusion injury. Apparently, the protective effects of TRPV₁ agonist were absent in TRPV₁ ^−/− mice hearts, emphasizing the importance of TRPV₁ channels in cardioprotection. Administration of TRPV₁ agonist led to enhanced CGRP and substance P release in normal hearts in comparison with TRPV₁ ^−/− hearts. However, the enhanced neurotransmitter release and the protective effects of N-oleoyldopamine were significantly abolished in the presence of chelerythrine (5 × 10⁻⁶ M, Protein kinase C [PKC] antagonist), tetrabutylammonium (5 × 10⁻⁴ M, nonselective K⁺ channel antagonist), CGRP_8-37 (10⁻⁶ M), and RP67580 (10⁻⁶ M). This indicates that TRPV₁ channel activation possibly elicits cardioprotective effects via enhancing release of CGRP and substance P. These pharmacological modulators were added in the perfusate 5 minutes before agonist administration, and perfusion with these modulators was continued for 5 minutes after agonist administration. ⁹ The blockade of protective effects of TRPV₁ agonist in the presence of PKC inhibitor suggests the critical role of PKC in TRPV₁-induced cardioprotection. Studies have shown that PKC-mediated phosphorylation of TRPV₁ sensitizes the functioning of TRPV₁ channels, including an increase in SP and CGRP release. ^{19 –21} The blockade of TRPV₁ agonist–induced cardioprotective effects in the presence of K⁺ channel antagonist suggests the functional interrelationship between TRPV₁ and K⁺ channels. ⁹

Protease-activated receptors (PARs) comprise of 7 transmembrane domain G protein-coupled receptors that are activated by proteolytic cleavage of amino terminus of the receptor and act as sensors for extracellular proteases. ²² Protease-activated receptor 2 (PAR2) modulates inflammatory and injury response events and is activated by a broad array of serine proteases including trypsin, coagulation factors VIIa and Xa, tissue kallikreins, and mast cell tryptase. ^22,23 Amadesi et al reported that TRPV₁ channels are coexpressed with PAR2 in the cardiomyocytes, blood vessels, and the perivascular nerves. Administration of Synthetic peptide (SLIGRL) (10⁻⁷ M, PAR2 agonist) for 15 minutes after stabilization (25 minutes) significantly improved LVDP, +dP/dt, −dP/dt, and coronary flow rate, and these effects were abolished in the presence of CGRP_8-37 (10⁻⁶ M), R (10⁻⁷ M), PKC∊ V1-2 (10⁻⁴ M, PKC∊ inhibitor), and H-89 (5 × 10⁻⁶ M, protein kinase A, PKA inhibitor) that were added 5 minutes before agonist administration and continued for 5 minutes after agonist perfusion. This indicates the involvement of PKA, PKC signaling, and successive release of CGRP and substance P in mediating cardioprotection. Furthermore, the delivery of SLIGRL markedly enhanced the release of CGRP and substance P in the normal hearts that was absent in TRPV₁ ^−/− mice hearts. This indicates that PAR2 activation–mediated cardioprotective effects are possibly dependent on the presence of functional TRPV₁ channels. Previous study of Amadesi et al also indicated that PAR2 activation sensitizes TRPV₁ channel via PKC signaling. ²⁴ Thus, it may be proposed that PAR2 activation probably stimulates TRPV₁ channels via PKA/PKC signaling to enhance the release of CGRP and substance P to provide cardioprotective effects. ¹¹ Apart from this, scientists have shown that the inflammatory responses of G protein-coupled receptors are TRPV₁ dependent, and PKA ²⁵ - or PKC ²⁶ -mediated phosphorylation is required for TRPV₁-mediated activity.

Qin et al demonstrated that capsaicin delivery significantly improves myocardial performance in terms of LVDP and heart rate in in vivo model of ischemia–reperfusion injury. However, these protective effects markedly reduced in the presence of capsazepine and S-3144 (substance P receptor antagonist) that were delivered into the rat left ventricle via the right carotid artery, 5 minutes and 10 minutes before ischemia. This indicates that during ischemia, activation of TRPV₁ channels may enhance substance P release to reduce the deleterious effects of ischemia–reperfusion injury. ²⁷ The same group of researchers further reported that pretreatment with capsaicin also reduced the apoptotic index that was significantly reduced in the presence of capsazepine and S-3144 (delivered into the left ventricle via the right carotid artery at 10 minutes and 5 minutes before ischemia), indicating that the protective effects are mediated via activation of TRPV₁ channels. ²⁸

Wang and Wang found a predominant role of endogenous substance P, but not CGRP, in providing cardioprotection during ischemia–reperfusion injury. The authors reported that in isolated mice hearts, sustained ischemia–reperfusion-induced myocardial injury was more pronounced in TRPV₁ ^−/− mice hearts in comparison with the normal heart, indicating that the intact TRPV₁ channel tends to reduce ischemia–reperfusion-induced myocardial injury. Furthermore, pretreatment with capsazepine (10⁻⁶ mol/L), 5 minutes before ischemia, exaggerated ischemia–reperfusion-induced myocardial injury in the isolated normal hearts, emphasizing the involvement of TRPV₁ channels in reducing ischemia–reperfusion injury. Meanwhile, exogenous delivery of CGRP (10⁻⁷ mol/L) and substance P (10⁻⁶ mol/L), 5 minutes before ischemia, ensued significant improvement in the post-ischemic recovery in both isolated TRPV₁ ^−/− and normal mice hearts, suggesting that the cardioprotective effects of CGRP and substance P are independent of TRPV₁ channel activation. However, administration of RP67580 (10⁻⁶ mol/L), but not CGRP_8-37 (10⁻⁶ mol/L) into the perfusion fluid 5 minutes before ischemia, led to severe impairment in the functional recovery of normal mice hearts, emphasizing the involvement of endogenous substance P, but not CGRP, in attenuating ischemia–reperfusion injury. The authors further supported the critical role of substance P in ischemia–reperfusion injury by showing an increase in substance P release during ischemia–reperfusion that was significantly attenuated in the presence of capsazepine. Although substance P was also released during ischemia–reperfusion from TRPV₁ ^−/− hearts, the release was of smaller magnitude and capsazepine pretreatment did not attenuate its release. It probably suggests that TRPV₁ channels are not the sole source of substance P release, and there might exist TRPV₁-independent pathway for its release during ischemia–reperfusion injury (Figure 1). ¹⁷

Figure 1.

Low pH or proteases activate PAR2 to stimulate TRPV₁ channels via PKA/PKC signaling. Furthermore, eicosanoids and low pH may also activate these channels and increase intracellular Ca²⁺ to enhance CGRP and substance P release from the synaptic vesicles. CGRP and substance P bind to the respective receptors to increase the release of anti-inflammatory cytokine, IL-10 which in turn reduces the TNF-α levels. Reduction in the TNF-α level may decrease ROS and neutrophil infiltration to attenuate ischemia-reperfusion injury in the respective tissues. CGRP indicates calcitonin gene–related peptide; IL-10, Interleukin 10; PAR2, protease-activated receptor 2; PKA, protein kinase A; PKC- Protein kinase C; ROS, Reactive oxygen species; TNF-α, tumor necrosis factor-alpha; TRPV₁, transient receptor potential vanilloid 1.

Cardioprotective Effects of TRPV₁ Channel Activation in Conditioning-Induced Cardioprotection

Recently, various researchers have shown that activation of TRPV₁ channels may be involved in conditioning-induced cardioprotective effects. ^15,16 Preconditioning stimuli activates arachidonate 12-lipoxygenase (ALOX) that metabolizes arachidonic acid to generate metabolites including 12(S)-hydroxyeicosatetraenoic acid and 12-hydroperoxyeicosatetraenoic acid (12[S]-HpETE), which may activate TRPV₁ channels to induce cardioprotection. ²⁹ Furthermore, it has also been shown that 12(S)-HpETE, 15-(S)- HpETE, 5- and 15-(S)-hydroeicosatetraenoic acid, and leukotriene B₄ activate TRPV₁ ion channels via G protein-coupled receptor signaling. ^26,30 Lu et al explored the role of activation of TRPV₁ channels in mediating hypoxic preconditioning (10% oxygen for 4 weeks)-induced cardioprotection in the isolated rat hearts. However, hypoxic preconditioning-induced cardioprotective effects were abolished in the presence of capsazepine (1 μM, TRPV₁ antagonist), indicating that hypoxic preconditioning-induced cardioprotection is mediated via activation of TRPV₁ channels. Furthermore, cinnamyl-3, 4-dihydroxy-α-cyanocinnamate (10 μM), and baicalein (10 μM; ALOX 12 inhibitors) abrogated hypoxic preconditioning-induced cardioprotection that was restored in the presence of capsaicin. Meanwhile, coadministration of capsazepine and baicalein abolished preconditioning-induced translocation of PKCα, PKCδ, and PKC∊ isoforms to the sarcolemmal membrane. ¹⁵ Thus, it may be proposed that preconditioning stimuli induces cardioprotective effects probably via augmenting myocardial ALOX12 expression, which in turn liberates arachidonic acid metabolites to activate TRPV₁ channels and enhance PKC translocation to the sarcolemmal membrane.

Recently, Gao and coworkers revealed the role of TRPV₁ channel in mediating remote ischemic postconditioning-induced cardioprotection. The authors reported that remote ischemic postconditioning-induced cardioprotective effects were abolished in the presence of CGRP_8-37 (2 mg/kg, 2 minutes before reperfusion) and RP-67580 (5 mg/kg, 5 minutes before reperfusion), suggesting the involvement of CGRP and substance P in mediating remote ischemic postconditioning-induced cardioprotection. In addition, remote ischemic postconditioning remarkably augmented the levels of CGRP and substance P in the plasma and the heart that was abrogated in the presence of capsazepine (3 mg/kg, 10 minutes before reperfusion). Moreover, remote ischemic postconditioning also increased mRNA expression, along with an increase in CGRP and substance P levels in the dorsal root ganglion. This suggests that remote ischemic postconditioning stimulus possibly activates TRPV₁ channels and increases the synthesis and release of CGRP and substance P in the dorsal root ganglion, which may be released into the plasma and activate the corresponding myocardial receptors to produce cardioprotection. ¹⁶

Role of TRPV₁ Channel Activation During Ischemia–Reperfusion Injury in Other Organs

It is manifested that apart from myocardium, TRPV₁-expressing sensory nerves are widely distributed in various organs including kidney, and activation of these channels regulates the renal functioning under both physiological and pathophysiological conditions. ⁵ Chen et al observed that capsaicin administration (0.3 mg/kg) significantly reduced ischemia–reperfusion-induced acute kidney injury in terms of reduced creatinine levels, tubular damage, neutrophil gelatinase–associated lipocalin, and Ly-6B.2 positive polymorphonuclear inflammatory cell abundance in mice. However, TRPV₁ ablation and capsazepine (50 mg/kg) administration did not worsen renal functioning or alter histology after ischemia–reperfusion-induced acute kidney injury. This indicates that activation of these channels may potentially reduce ischemia–reperfusion-induced acute kidney injury, but endogenous inactivation of TRPV₁ channels is not involved in producing acute kidney injury. ⁵

Earlier, Ueda et al revealed that capsaicin and resiniferatoxin (highly potent capsaicin analog that is a specific TRPV₁ channel activator) pretreatment mitigate ischemia–reperfusion induced renal dysfunction in rats. Capsaicin (3, 10, and 30 mg/kg, orally) and resiniferatoxin (20 μg/kg, subcutaneous) administration significantly reduced renal neutrophil infiltration, superoxide generation, and tumor necrosis factor alpha (TNF-α) mRNA expression, and increased anti-inflammatory Interleukin- 10 (IL-10) expression. This indicates that TRPV₁ channel activation possibly induces renoprotective effects via reducing the gene expression of inflammatory mediators including TNF-α and enhancing the expression of anti-inflammatory cytokines, viz, IL-10. ¹²

Wang and coworkers demonstrated that left pulmonary hilus occlusion for 1 hour followed by 3-hour reperfusion produced lung ischemia–reperfusion injury in rabbits. Capsaicin (50 μg/kg, 5 minutes before ischemia) administration significantly improved gas exchange function, decreased lung wet/dry ratio, neutrophil infiltration in bronchoalveolar lavage fluid, lung malondialdehyde levels, myeloperoxidase activities, but increased superoxide dismutase activity and augmented CGRP level. Furthermore, in the presence of capsaicin, the pathological lesions were significantly reduced. However, the aforementioned changes were remarkably abolished in the presence of capsazepine (50 μg/kg, 5 minutes before ischemia). This indicates that TRPV₁ channel activation reduces ischemia–reperfusion-induced lung injury possibly via enhancing the release of CGRP and reducing inflammatory response and oxidative stress. ^13,31 In corroboration with the above studies, Zhao et al reported that lung ischemia–reperfusion injury also increases the expression of TRPV₁ channels, CGRP, and substance P receptors in the lungs. ³²

Furthermore, TRPV₁ channels are expressed in various areas of the brain and have the ability to detect variations in the temperature. ³³ Cao et al demonstrated that pharmacological activation of TRPV₁ channels induces hypothermia and produces neuroprotective effects against 1 hour ischemia–24 hours reperfusion–induced injury. Left middle cerebral artery and left common carotid artery occlusion-reperfusion produced significant focal cerebral ischemia–reperfusion injury in mice. However, dihydrocapsaicin (TRPV₁ agonist, 1.25 mg/kg/h.) administration at the onset of reperfusion induced hypothermia (33°C; after 90 minutes), reduced infarct volume by 87% and improved neurofunctional score in stroke-affected mice. However, hypothermic and neuroprotective effects were absent in TRPV₁ ^−/− mice, indicating that the neuroprotective benefit TRPV₁ channel activation in this study was through induction of mild hypothermia. ¹⁴

Deleterious Effects of TRPV₁ Channel Activation

Besides describing the beneficial effects of TRPV₁ channel activation, a study has reported that activation of these channels can also be detrimental. ³⁴ Robertson et al showed that intratracheal instillation of diesel exhaust particulate followed by sustained ischemia–reperfusion injury led to a remarkable increase in systemic blood pressure, ventricular arrhythmia, and tissue edema in rats. Apparently, diesel exhaust instillation before myocardial ischemia increases oxidative stress, apoptosis, and necrosis in the myocardial tissue and ischemia–reperfusion injury was also significantly enhanced in the isolated buffer perfused hearts instilled in vivo with diesel particulate. The detrimental role of TRPV₁ channel activation has been investigated using AMG 9810, a selective TRPV₁ antagonist that blocks activation due to TRPV₁ known agonists including heat, protons, and endogenous ligands. Intratracheal delivery of AMG 9810 abrogated the sudden in vivo increase in the blood pressure and arrhythmia, whereas its addition into the perfusate reduced ex vivo myocardial reperfusion injury, suggesting the role of pulmonary TRPV₁ channels in the development of exhaust particulate-induced cardiac injury. ³⁴

Sun and coworkers reported that TRPV₁ channels are expressed in the H9C2 cells and are activated during hypoxia–reperfusion injury. Capsaicin administration (1-100 μM) increased intracellular Ca²⁺, superoxide generation, decreased mitochondrial membrane potential, and mitochondrial biogenesis to increase the number of apoptotic cells. These effects were abolished in the presence of capsazepine and TRPV₁ Small (or short) interfering RNA (siRNA). Furthermore, the cardiac function of TRPV₁ ^−/− hearts was improved compared to wild-type hearts in the presence of CGRP_8-37 and RP67580. Thus, the authors proposed that activation of TRPV₁ channels on the cardiac myocytes exacerbates hypoxia–reperfusion injury via enhancing Ca²⁺ accumulation and superoxide generation and increasing apoptosis in the H9C2 cells. ³⁵

Role of Other TRPV Channels and Acid Sensing Ion Channels in Ischemia–Reperfusion Injury

Apart from TRPV₁ channels, the other members of its family may also modulate ischemia–reperfusion injury. There have been inconsistent findings related to the role of TRPV₄ channels, as studies have shown both protective ³⁶ and detrimental effects during ischemia–reperfusion. ^37,38 Butenko et al reported that exposure to 15 minutes of cerebral ischemia markedly increased TRPV₄ channel expression in the hippocampal astrocytes, and administration of 4α-PDD (TRPV₄ agonist) in the cultured hippocampal astrocytes aggravated ischemic injury through increase in Ca²⁺ influx that was abolished after the removal of extracellular Ca²⁺ or in the presence of ruthenium red (nonspecific TRPV blocker) and RN1734 (specific TRPV₄ blocker). ³⁷ In concordance with the above study, Jie et al reported that intracerebroventricular injection of GSK1016790 (TRPV₄ agonist) aggravated cerebral ischemic injury that was abolished in the presence of HC-067047 (TRPV₄ antagonist). ³⁸ In contrast, Rath and coworkers have reported that activation of TRPV₄ channels led to hypoxic preconditioning-dependent endothelial relaxation. The authors reported that hypoxic preconditioning (3 cycles of hypoxia–reoxygenation) restored nitric oxide (NO)-mediated vascular relaxation and improved endothelium-derived hyperpolarizing factor (EDHF)-mediated endothelial relaxation in superior mesenteric arteries that was absent in TRPV₄ knockout mesenteric arteries. This indicated that TRPV₄ channels participate in NO- and EDHF-mediated vascular relaxation. ³⁶

Apart from the TRPV channels, lactic acid, produced by anaerobic metabolism during myocardial ischemia, may activate acid sensing ion channels (ASICs) in the sensory neurons innervating the heart and mediate acidosis-associated pain. ^39,40 Acid sensing ion channels are Na⁺ permeable and pH-sensitive ion channels that are activated in the presence of extracellular protons. ^40,41 The exact role of ASIC in ischemia–reperfusion injury remains unclear, but few studies indicate that activation of these channels aggravates ischemia–reperfusion injury. ⁴² Zhang and coworkers reported that activation of these channels results in depolarizing Na⁺ currents, ⁴³ which results in membrane excitation and Ca²⁺ overload ^42,44 to induce ischemia–reperfusion injury. On the other hand, few authors report that subjection of brain to the conditioning stimuli upregulates the expression of ASIC2a, which in turn decreases Ca²⁺ influx and reduces ischemia–reperfusion injury. ^45,46

Summarized Findings and Discussion

Based on the above reports, it is conceivable that TRPV₁ channel activation plays an important role in attenuating ischemia–reperfusion injury in various organs including heart, lungs, kidneys, and brain. ^12,14,27,31 Activation of these channels enhances CGRP and substance P discharge from the perivascular sensory nerves innervating the myocardium to provide cardioprotective effects. ^9,10 The scientists have explored the involvement of TRPV₁ channels and subsequent CGRP and substance P release during ischemia–reperfusion injury using various pharmacological agents including N-oleoyldopamine, capsaicin, resiniferatoxin, CGRP_8-37, RP-67580, capsazepine, and TRPV₁ knockout mice. ^9,14 Scientists have also explored the signaling cascade involved in mediating PAR2-dependent TRPV₁ channel activation in the perivascular nerves. ¹¹ It was found that PAR2 activation possibly stimulates TRPV₁ channel via PKA/PKC signaling and subsequently, enhances the release of CGRP and substance P from the perivascular nerves. ¹¹ It has been well-documented that CGRP ⁴⁷ and substance P ⁴⁸ reduce ischemia–reperfusion injury. However, CGRP and substance P dependent cardioprotective signaling in association with TRPV₁ is unknown. Furthermore, researchers have shown that activation of TRPV₁ channels attenuates renal and lung ischemia–reperfusion injury via reducing TNF-α, cytokine-induced neutrophil chemoattractant-1 production, free radical formation and neutrophil infiltration, and increasing anti-inflammatory IL-10 generation. ¹² There have been reports that CGRP discharge possibly reduces free radical generation and neutrophil infiltration in the tissues to provide protection against ischemia–reperfusion injury. ^49,50 This indicates that activation of TRPV₁ channels induces CGRP discharge, which in turn reduces free radical formation, neutrophil infiltration, and production of other inflammatory mediators to increase the tissue tolerance against ischemia–reperfusion injury. Furthermore, scientists have also reported that TRPV₁ channel activation may induce vasodilation in isolated mesenteric arteries, ^51,52 dural, ^53,54 and cutaneous blood vessels. ^55,56 Accordingly, it may also be hypothesized that TRPV₁-mediated direct vasodilatory effects may counter the deleterious effects of ischemia to impart tissue protection. However, precise studies are required to explore the signaling cascade involved in TRPV₁ channel activation, CGRP and substance P release, reduced production of inflammatory cytokines, and the possible role of direct vasodilation in ischemia–reperfusion tissue injury.

Conclusion

Activation of TRPV₁ channels attenuates ischemia–reperfusion-induced injury in a variety of organs including heart, kidney, lungs, and brain possibly via enhancing the discharge of CGRP and substance P. In turn, CGRP may reduce the production of free radicals, neutrophil infiltration, and other inflammatory mediators to reduce ischemia–reperfusion injury. Nevertheless, further studies are warranted to explore the signaling cascade involved in TRPV₁ channel activation, CGRP and substance P release, and reduced generation of inflammatory cytokines during ischemia–reperfusion tissue injury.