TRPV1


The transient receptor potential cation channel subfamily V member 1, also known as the capsaicin receptor and the vanilloid receptor 1, is a protein that, in humans, is encoded by the TRPV1 gene. 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. In primary afferent sensory neurons, it cooperates with TRPA1 to mediate the detection of noxious environmental stimuli

Function

TRPV1 is a nonselective cation channel that may be activated by a wide variety of exogenous and endogenous physical and chemical stimuli. The best-known activators of TRPV1 are: temperature greater than ; acidic conditions; capsaicin ; and allyl isothiocyanate, the pungent compound in mustard and wasabi. The activation of TRPV1 leads to a painful, burning sensation. Its endogenous activators include: low pH, the endocannabinoid anandamide, N-oleyl-dopamine, and N-arachidonoyl-dopamine. TRPV1 receptors are found mainly in the nociceptive neurons of the peripheral nervous system, but they have also been described in many other tissues, including the central nervous system. TRPV1 is involved in the transmission and modulation of pain, as well as the integration of diverse painful stimuli.

Sensitization

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 or pain sensation in response to non-painful stimuli. 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.

Desensitization

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.

Clinical significance

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. Three major strategies have been used:

TRPV1 Use

The TRPV1 receptor is useful to be able to measure how an organism can sense temperature change. In the lab the receptor may be removed from mice giving them the inability to detect differences in ambient temperature. In the pharmaceutical field this allows for the blocking of heat receptors giving patients with inflammatory disorders or severe burning pains a chance to heal without the pain. The lack of the TRPV1 receptor gives a glimpse into the developing brain as heat can kill most organisms in large enough doses, so this removal process shows researchers how the inability to sense heat may be detrimental to the survivability of an organism and then translate this to human heat disorders.

Antagonists

block TRPV1 activity, thus reducing pain. Identified antagonists include the competitive antagonist capsazepine and the non-competitive antagonist ruthenium red. These agents could be useful when applied systemically. Numerous TRPV1 antagonists have been developed by pharmaceutical companies. TRPV1 antagonists have shown efficacy in reducing nociception from inflammatory and neuropathic pain models in rats. This provides evidence that TRPV1 is capsaicin's sole receptor In humans, drugs acting at TRPV1 receptors could be used to treat neuropathic pain associated with multiple sclerosis, chemotherapy, or amputation, as well as pain associated with the inflammatory response of damaged tissue, such as in osteoarthritis.
These drugs can affect body temperature which is a challenge to therapeutic application. For example, a transient temperature gain was measured in rats with the application of TRPV1 antagonist AMG9810. 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 a mild increase in body temperature, 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 to cause sweating. 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. In 2008, AMG 517, a highly selective TRPV1 antagonist was dropped out of clinical trials due to the causation of hyperthermia is agonist-mediated desensitization then the hyperthermic effects of effects of antagonists may not be relevant. Secondarily in applications such as TRPV1 antagonism for the treatment of severe conditions such as heart failure, then there may be an acceptable trade-off with mild hyperthermia, although no hyperthermia was observed in rodent models of heart failure treated with BCTC, SB366791 or AMG9810. Post translational modification of TRPV1 protein by its phosphorylation is critical for its functionality. Rent reports published from NIH suggest that Cdk5-mediated phosphorylation of TRPV1 is required for its ligand-induced channel opening.

Agonists

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, 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. 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 are under clinical trials. Eight percent 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 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-hydroxy-9Z,11E-octadecadienoic acid, 13-hydroxy-9Z,11E-octadecadienoic acid -HODE, 9-hydroxy-10,12-octadecadienoic acid, 9-hydroxy-10,12-octadecadienoic acid, and their respective keto analogs, 13-oxoODE and 9-oxoODE, activate peripheral and central mouse pain sensing neurons. Reports disagree on the potencies of these metabolites with, for example, the most potent one, 9-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 and 12-hydroperoxy-5Z,8Z,10E,12S,14Z-eicosatetraenoic acid, 12-hydroxy-5Z,8Z,10E,12S,14Z-eicosatetraenoic acid -HETE, hepoxilin A3 and HxB3 likewise activate TRPV1 and may thereby contribute to tactile hyperalgesia and allodynia.
Studies with mice, guinea pig, and human tissues and in guinea pigs indicate that another arachidonic acid metabolite, Prostaglandin E2, operates through its prostaglandin EP3 G protein coupled receptor to trigger cough responses. Its mechanism of action involves activation and/or sensitization of TRPV1 receptors, presumably by an indirect mechanism. Genetic polymorphism in the EP3 receptor, has been associated with ACE inhibitor-induced cough in humans.
Resolvin E1, RvD2, neuroprotectin D1, and maresin 1 are metabolites of the omega 3 fatty acids, eicosapentaenoic acid or docosahexaenoic acid. These metabolites are members of the specialized proresolving mediators class of metabolites that function to resolve diverse inflammatory reactions and diseases in animal models and, it is proposed, humans. These SPMs also dampen pain perception arising from various inflammation-based causes in animal models. The mechanism behind their pain-dampening effects involves the inhibition of TRPV1, probably by an indirect effect wherein they activate other receptors located on the neurons or nearby microglia or astrocytes. CMKLR1, GPR32, FPR2, and NMDA receptors have been proposed to be the receptors through which these SPMs operate to down-regulate TRPV1 and thereby pain perception.

Fatty acid conjugates

, an endocannabinoid found in the human CNS, structurally similar to capsaicin, activates the TRPV1 channel with an EC50 of approximately of 50 nM.
N-Oleyl-dopamine, another endogenous agonist, binds to human VR1 with an Ki of 36 Nm.
Another endocannabinoid anandamide has also been shown to act on TRPV1 receptors.
AM404—an active metabolite of paracetamol —that serves as an anandamide reuptake inhibitor and COX inhibitor also serves as a potent TRPV1 agonist.
The plant-biosynthesized cannabinoid cannabidiol also shows "either direct or indirect activation" of TRPV1 receptors. TRPV1 colocalizes with CB1 receptors and CB2 receptors in sensory and brain neurons respectively, and other plant-cannabinoids like CBN, CBG, CBC, THCV, and CBDV are also agonists of this ion channel. There is also evidence that non cannabinoid components of the Cannabis secondary metabolome such as Myrcene activate TRPV1.

Central nervous system

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 in the hippocampus. LTD has been linked to a decrease in the ability to make new memories, unlike its opposite long-term potentiation, 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, 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.

Interactions

TRPV1 has been shown to interact with:
The dorsal root ganglion 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. 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.