The transient receptor potential cation channel subfamily V member 1 (TrpV1), also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1gene. It was the first isolated member of the transient receptor potential vanilloid receptor proteins that in turn are a sub-family of the transient receptor potential protein group. This protein is a member of the TRPV group of transient receptor potential family of ion channels.
The function of TRPV1 is detection and regulation of body temperature. In addition, TRPV1 provides a sensation of scalding heat and pain (nociception).
The sensitivity of TRPV1 to noxious stimuli, such as high temperatures, is not static. Upon tissue damage and the consequent inflammation, a number of inflammatory mediators, such as various prostaglandins and bradykinin, are released. These agents increase the sensitivity of nociceptors to noxious stimuli. This manifests as an increased sensitivity to painful stimuli (hyperalgesia) or pain sensation in response to non-painful stimuli (allodynia). Most sensitizing pro-inflammatory agents activate the phospholipase C pathway. Phosphorylation of TRPV1 by protein kinase C have been shown to play a role in sensitization of TRPV1. The cleavage of PIP2 by PLC-beta can result in disinhibition of TRPV1 and, as a consequence, contribute to the sensitivity of TRPV1 to noxious stimuli.
Upon prolonged exposure to capsaicin, TRPV1 activity decreases, a phenomenon called desensitization. Extracellular calcium ions are required for this phenomenon, thus influx of calcium and the consequential increase of intracellular calcium mediate this effect. Various signaling pathways such as calmodulin and calcineurin, and the decrease of PIP2, have been implicated in desensitization of TRPV1. Desensitization of TRPV1 is thought to underlie the paradoxical analgesic effect of capsaicin.
Peripheral nervous system
Treatment of pain is an unmet medical need costing billions of dollars every year. As a result of its involvement in nociception, TRPV1 has been a prime target for the development of novel pain reducers (analgesics). Two major strategies have been used:
The major roadblock for the usefulness of these drugs is their effect on body temperature (hyperthermia).
The role of TRPV1 in the regulation of body temperature has emerged in the last few years. Based on a number of TRPV-selective antagonists' causing an increase in body temperature (hyperthermia), it was proposed that TRPV1 is tonically active in vivo and regulates body temperature by telling the body to "cool itself down". Without these signals, the body overheats. Likewise, this explains the propensity of capsaicin (a TRPV1 agonist) to cause sweating (i.e.: a signal to reduce body temperature). In a recent report, it was found that tonically active TRPV1 channels are present in the viscera and keep an ongoing suppressive effect on body temperature. Recently, it was proposed that predominant function of TRPV1 is body temperature maintenance  Experiments have shown that TRPV1 blockade increases body temperature in multiple species, including rodents and humans, suggesting that TRPV1 is involved in body temperature maintenance. Recently, AMG 517, a highly selective TRPV1 antagonist was dropped out of clinical trials due to the undesirable level of hyperthermia. A second molecule, SB-705498, was also evaluated in the clinic but its effect on body temperature was not reported. Recently, it was disclosed that clinical trials of two more TRPV1 antagonists, GRC 6211 and NGD 8243, have been stopped. Post translational modification of TRPV1 protein by its phosphorylation is critical for its functionality. Recent reports published from NIH suggest that Cdk5-mediated phosphorylation of TRPV1 is required for its ligand-induced channel opening.
TRPV1 is activated by numerous agonists from natural sources. Agonists such as capsaicin and resiniferatoxin activate TRPV1 and, upon prolonged application, cause TRPV1 activity to decrease (desensitization), leading to alleviation of pain via the subsequent decrease in the TRPV1 mediated release of inflammatory molecules following exposures to noxious stimuli. Agonists can be applied locally to the painful area in various forms, generally as a patch or an ointment. Numerous capsaicin-containing creams are available over the counter, containing low concentrations of capsaicin (0.025 - 0.075%). It is debated whether these preparations actually lead to TRPV1 desensitization; it is possible that they act via counter-irritation. Novel preparations containing higher capsaicin concentration (up to 10%) are under clinical trials. 8% capsaicin patches have recently become available for clinical use, with supporting evidence demonstrating that a 30-minute treatment can provide up to 3 months analgesia by causing regression of TRPV1-containing neurons in the skin. Currently, these treatments must be re-administered on a regular (albeit infrequent) schedule in order to maintain their analgesic effects.
Fatty acid metabolites
Certain metabolites of polyunsaturated fatty acids have been shown to stimulate cells in a TRPV1-dependent fashion. The metabolites of linoleic acid, including 13(S)-hydroxy-9Z,11E-octadecadienoic acid (13(S)-HODE), 13(R)-hydroxy-9Z,11E-octadecadienoic acid (13(R)-HODE, 9(S)-hydroxy-10(E),12(Z)-octadecadienoic acid (9(S)-HODE), 9(R)-hydroxy-10(E),12(Z)-octadecadienoic acid (9(R)-HODE), and their respective keto analogs, 13-oxoODE and 9-oxoODE (see 13-HODE and 9-HODE sections on Direct actions), activate peripheral and central mouse pain sensing neurons. Reports disagree on the potencies of these metabolites with, for example, the most potent one, 9(S)-HODE, requiring at least 10 micromoles/liter. or a more physiological concentration of 10 nanomoles/liter to activate TRPV1 in rodent neurons. The TRPV1-dependency of these metabolites' activities appears to reflect their direct interaction with TPRV1. Although relatively weak agonists of TRPV1 in comparison to anandamide, these linoleate metabolites have been proposed to act through TRPV1 in mediating pain perception in rodents and to cause injury to airway epithelial cells and thereby to contribute to asthma disease in mice and therefore possibly humans. Certain arachidonic acid metabolites, including 20-hydroxy-5Z,8Z,11Z,14Z-eicosatetraenoic acid (see 20-Hydroxyeicosatetraenoic acid) and 12(S)-hydroperoxy-5Z,8Z,10E,12S,14Z-eicosatetraenoic acid (12(S)-HpETE), 12(S)-hydroxy-5Z,8Z,10E,12S,14Z-eicosatetraenoic acid (12(S)-HETE (see 12-HETE), hepoxilin A3 (i.e. 8R/S-hydroxy-11,12-oxido-5Z,9E,14Z-eicosatrienoic acid) and HxB3 (i.e. 10R/S-hydroxy-11,12-oxido-5Z,8Z,14Z-eicosatrienoic acid) likewise activate TRPV1 and may thereby contribute to tactile hyperalgesia and allodynia (see Hepoxilin#Pain perception).
Studies with mice, guinea pig, and human tissues and in guinea pigs indicate that another arachidonic acid metabolite, Prostaglandin E2, operates through its prostaglandin EP3G protein coupled receptor to trigger cough responses. Its mechanism of action involves activation and/or sensitization of TRPV1 (as well as TRPA1) receptors, presumably by an indirect mechanism. Genetic polymorphism in the EP3 receptor (rs11209716), has been associated with ACE inhibitor-induced cough in humans.
TRPV1 is also expressed at high levels in the central nervous system and has been proposed as a target for treatment not only of pain but also for other conditions such as anxiety.
Furthermore, TRPV1 appears to mediate long-term synaptic depression (LTD) in the hippocampus. LTD has been linked to a decrease in the ability to make new memories, unlike its opposite long-term potentiation (LTP), which aids in memory formation. A dynamic pattern of LTD and LTP occurring at many synapses provides a code for memory formation. Long-term depression and subsequent pruning of synapses with reduced activity is an important aspect of memory formation. In rat brain slices, activation of TRPV1 with heat or capsaicin induced LTD while capsazepine blocked capsaicin's ability to induce LTD. In the brainstem (solitary tract nucleus), TRPV1 controls the asynchronous and spontaneous release of glutamate from unmyelinated cranial visceral afferents - release processes that are active at normal temperatures and hence quite distinct from TRPV1 responses in painful heat. Hence, there may be therapeutic potential in modulating TRPV1 in the central nervous system, perhaps as a treatment for epilepsy (TRPV1 is already a target in the peripheral nervous system for pain relief).
The dorsal root ganglion (DRG) neurons of mammals were known to express a heat-sensitive ion channel that could be activated by capsaicin. The research group of David Julius, therefore, created a cDNA library of genes expressed in dorsal root ganglion neurons, expressed the clones in HEK 293 cells, and looked for cells that respond to capsaicin with calcium influx (which HEK-293 normally not do). After several rounds of screening and dividing the library, a single clone encoding the TRPV1 channel was finally identified in 1997. It was the first TRPV channel to be identified.
^ abcCaterina MJ, Schumacher MA, Tominaga M, Rosen TA, Levine JD, Julius D (October 1997). "The capsaicin receptor: a heat-activated ion channel in the pain pathway". Nature. 389 (6653): 816–24. doi:10.1038/39807. PMID9349813.
^Xue Q, Yu Y, Trilk SL, Jong BE, Schumacher MA (August 2001). "The genomic organization of the gene encoding the vanilloid receptor: evidence for multiple splice variants". Genomics. 76 (1–3): 14–20. doi:10.1006/geno.2001.6582. PMID11549313.
^Clapham DE, Julius D, Montell C, Schultz G (December 2005). "International Union of Pharmacology. XLIX. Nomenclature and structure-function relationships of transient receptor potential channels". Pharmacol. Rev. 57 (4): 427–50. doi:10.1124/pr.57.4.6. PMID16382100.
^Everaerts W, Gees M, Alpizar YA, Farre R, Leten C, Apetrei A, Dewachter I, van Leuven F, Vennekens R, De Ridder D, Nilius B, Voets T, Talavera K (February 2011). "The capsaicin receptor TRPV1 is a crucial mediator of the noxious effects of mustard oil". Curr. Biol. 21 (4): 316–21. doi:10.1016/j.cub.2011.01.031. PMID21315593.
^Cui M, Honore P, Zhong C, Gauvin D, Mikusa J, Hernandez G, Chandran P, Gomtsyan A, Brown B, Bayburt EK, Marsh K, Bianchi B, McDonald H, Niforatos W, Neelands TR, Moreland RB, Decker MW, Lee CH, Sullivan JP, Faltynek CR (2006). "TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists". J. Neurosci. 26 (37): 9385–93. doi:10.1523/JNEUROSCI.1246-06.2006. PMID16971522.
^ abGavva NR, Bannon AW, Surapaneni S, Hovland DN, Lehto SG, Gore A, Juan T, Deng H, Han B, Klionsky L, Kuang R, Le A, Tamir R, Wang J, Youngblood B, Zhu D, Norman MH, Magal E, Treanor JJ, Louis JC (March 2007). "The vanilloid receptor TRPV1 is tonically activated in vivo and involved in body temperature regulation". J. Neurosci. 27 (13): 3366–74. doi:10.1523/JNEUROSCI.4833-06.2007. PMID17392452.
^Steiner AA, Turek VF, Almeida MC, Burmeister JJ, Oliveira DL, Roberts JL, Bannon AW, Norman MH, Louis JC, Treanor JJ, Gavva NR, Romanovsky AA (July 2007). "Nonthermal activation of transient receptor potential vanilloid-1 channels in abdominal viscera tonically inhibits autonomic cold-defense effectors". J. Neurosci. 27 (28): 7459–68. doi:10.1523/JNEUROSCI.1483-07.2007. PMID17626206.
^Gavva NR, Treanor JJ, Garami A, Fang L, Surapaneni S, Akrami A, Alvarez F, Bak A, Darling M, Gore A, Jang GR, Kesslak JP, Ni L, Norman MH, Palluconi G, Rose MJ, Salfi M, Tan E, Romanovsky AA, Banfield C, Davar G (May 2008). "Pharmacological blockade of the vanilloid receptor TRPV1 elicits marked hyperthermia in humans". Pain. 136 (1–2): 202–10. doi:10.1016/j.pain.2008.01.024. PMID18337008.
^Chizh BA, O'Donnell MB, Napolitano A, Wang J, Brooke AC, Aylott MC, Bullman JN, Gray EJ, Lai RY, Williams PM, Appleby JM (November 2007). "The effects of the TRPV1 antagonist SB-705498 on TRPV1 receptor-mediated activity and inflammatory hyperalgesia in humans". Pain. 132 (1–2): 132–41. doi:10.1016/j.pain.2007.06.006. PMID17659837.
^Boonen, Brett; Startek, Justyna B.; Talavera, Karel (2016-01-01). Chemical Activation of Sensory TRP Channels. Topics in Medicinal Chemistry. Springer Berlin Heidelberg. pp. 1–41.  doi:10.1007/7355_2015_98.
^Grilo A, Sáez-Rosas MP, Santos-Morano J, Sánchez E, Moreno-Rey C, Real LM, Ramírez-Lorca R, Sáez ME (2011). "Identification of genetic factors associated with susceptibility to angiotensin-converting enzyme inhibitors-induced cough". Pharmacogenetics and Genomics. 21 (1): 10–7. doi:10.1097/FPC.0b013e328341041c. PMID21052031.
^Qu Q, Xuan W, Fan GH (2015). "Roles of resolvins in the resolution of acute inflammation". Cell Biology International. 39 (1): 3–22. doi:10.1002/cbin.10345. PMID25052386.
^Högestätt ED, Jönsson BA, Ermund A, Andersson DA, Björk H, Alexander JP, Cravatt BF, Basbaum AI, Zygmunt PM (September 2005). "Conversion of acetaminophen to the bioactive N-acylphenolamine AM404 via fatty acid amide hydrolase-dependent arachidonic acid conjugation in the nervous system". J. Biol. Chem. 280 (36): 31405–12. doi:10.1074/jbc.M501489200. PMID15987694.
^Ligresti A, Moriello AS, Starowicz K, Matias I, Pisanti S, De Petrocellis L, Laezza C, Portella G, Bifulco M, Di Marzo V (September 2006). "Antitumor activity of plant cannabinoids with emphasis on the effect of cannabidiol on human breast carcinoma". J. Pharmacol. Exp. Ther. 318 (3): 1375–87. doi:10.1124/jpet.106.105247. PMID16728591.
^ abMorenilla-Palao C, Planells-Cases R, García-Sanz N, Ferrer-Montiel A (2004). "Regulated exocytosis contributes to protein kinase C potentiation of vanilloid receptor activity". J. Biol. Chem. 279 (24): 25665–72. doi:10.1074/jbc.M311515200. PMID15066994.
^ abFonseca, B. M.; Correia-da-Silva, G.; Teixeira, N. A. (2018-02-13). "Cannabinoid-induced cell death in endometrial cancer cells: involvement of TRPV1 receptors in apoptosis". Journal of Physiology and Biochemistry. 74 (2): 261–272. doi:10.1007/s13105-018-0611-7. ISSN1138-7548. PMID29441458.
Clapham DE, Julius D, Montell C, Schultz G (2005). "International Union of Pharmacology. XLIX. Nomenclature and structure-function relationships of transient receptor potential channels". Pharmacol. Rev. 57 (4): 427–50. doi:10.1124/pr.57.4.6. PMID16382100.