A new era for the design of TRPV1 antagonists and agonists with the use of structural information and molecular docking of capsaicin-like compounds

The design of TRPV1 antagonists and agonists has reached a new era since TRPV1 structures at near-atomic resolution are available. Today, the ligand-binding forms of several classical antagonists and agonists are known; therefore, the specific role of key TRPV1’s residues in binding of ligands can be elucidated. It is possible to place the well-defined pharmacophore of TRPV1 ligands, conformed by head, neck, and tail groups, in the right pocket regions of TRPV1. It will allow a more thorough use of molecular modelling methods to conduct more effective rational drug design protocols. In this work, important points about the interactions between TRPV1 and capsaicin-like compounds are spelled out, based on the known pharmacophore of the ligands and the already available TRPV1 structures. These points must be addressed to generate reliable poses of novel candidates and should be considered during the design of novel TRPV1 antagonists and agonists.

TRPV1 Ion Channel: Structural Features, Activity Modulators, and Therapeutic Potential

Although TRPV1 ion channel has been attracting researchers' attention for many years, its functions in animal organisms, the principles of regulation, and the involvement in pathological processes have not yet been fully clarified. Mutagenesis experiments and structural studies have identified the structural features of the channel and binding sites for its numerous ligands; however, these studies are far from conclusion. This review summarizes recent achievements in the TRPV1 research with special focus on structural and functional studies of the channel and on its ligands, which are extremely diverse in their nature and interaction specificity to TRPV1. Particular attention was given to the effects of numerous endogenous agonists and antagonists that can fine-tune the channel sensitivity to its usual activators, such as capsaicin, heat, acids, or their combination. In addition to the pain sensing not covered in this review, the TRPV1 channel was found to be involved in the regulation of many important physiological and pathological processes and, therefore, can be considered as a promising therapeutic target in the treatment of various diseases, such as pneumonia, ischemia, diabetes, epilepsy, schizophrenia, psoriasis, etc.

Hypofunctional TrkA Accounts for the Absence of Pain Sensitization in the African Naked Mole-Rat

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.

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TRPV1 ion channels in naked mole-rat nociceptors are not sensitized by NGF

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Naked mole-rat TRPV1 channels are sensitized by NGF in mouse nociceptors

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NGF activation of naked mole-rat TrkA receptors does not sensitize TRPV1

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One to three amino acids in the naked mole-rat TrkA receptors may render it hypofunctional

Blockade of transient receptor potential cation channel subfamily V member 1 promotes regeneration after sciatic nerve injury

The transient receptor potential cation channel subfamily V member 1 (TRPV1) provides the sensation of pain (nociception). However, it remains unknown whether TRPV1 is activated after peripheral nerve injury, or whether activation of TRPV1 affects neural regeneration. In the present study, we established rat models of unilateral sciatic nerve crush injury, with or without pretreatment with AMG517 (300 mg/kg), a TRPV1 antagonist, injected subcutaneously into the ipsilateral paw 60 minutes before injury. At 1 and 2 weeks after injury, we performed immunofluorescence staining of the sciatic nerve at the center of injury, at 0.3 cm proximal and distal to the injury site, and in the dorsal root ganglia. Our results showed that Wallerian degeneration occurred distal to the injury site, and neurite outgrowth and Schwann cell regeneration occurred proximal to the injury. The number of regenerating myelinated and unmyelinated nerve clusters was greater in the AMG517-pretreated rats than in the vehicle-treated group, most notably 2 weeks after injury. TRPV1 expression in the injured sciatic nerve and ipsilateral dorsal root ganglia was markedly greater than on the contralateral side. Pretreatment with AMG517 blocked this effect. These data indicate that TRPV1 is activated or overexpressed after sciatic nerve crush injury, and that blockade of TRPV1 may accelerate regeneration of the injured sciatic nerve.

Possible involvement of P2Y2 metabotropic receptors in ATP-induced transient receptor potential vanilloid receptor 1-mediated thermal hypersensitivity

The capsaicin receptor transient receptor potential V1 (TRPV1; also known as vanilloid receptor 1) is a sensory neuron-specific ion channel that serves as a polymodal detector of pain-producing chemical and physical stimuli. It has been reported that extracellular ATP potentiates the TRPV1 currents evoked by capsaicin or protons and reduces the temperature threshold for its activation through metabotropic P2Y receptors in a PKC-dependent pathway, suggesting that TRPV1 activation could trigger the sensation of pain at normal body temperature in the presence of ATP. Here, we show that ATP-induced thermal hyperalgesia was abolished in mice lacking TRPV1, suggesting the functional interaction between ATP and TRPV1 at a behavioral level. However, thermal hyperalgesia was preserved in P2Y1 receptor-deficient mice. Patch-clamp analyses using mouse dorsal root ganglion neurons indicated the involvement of P2Y2 rather than P2Y1 receptors. Coexpression of TRPV1 mRNA with P2Y2 mRNA, but not P2Y1 mRNA, was determined in the rat lumbar DRG using in situ hybridization histochemistry. These data indicate the importance of metabotropic P2Y2 receptors in nociception through TRPV1.

Intracellular ATP increases capsaicin-activated channel activity by interacting with nucleotide-binding domains

Capsaicin (CAP)-activated ion channel plays a key role in generating nociceptive neural signals in sensory neurons. Here we present evidence that intracellular ATP upregulates the activity of capsaicin receptor channel. In inside-out membrane patches isolated from sensory neurons, application of CAP activated a nonselective cation channel (i(cap)). Further addition of ATP to the bath caused a significant increase in i(cap), with a K(1/2) of 3.3 mm. Nonhydrolyzable analogs of ATP, adenylimidodiphosphate and adenosine 5'-O-(3-thio)-triphosphate, also increased i(cap). Neither Mg(2+)-free medium nor inhibitors of various kinases blocked the increase in i(cap) induced by ATP. The enhancing effect of ATP was also observed in inside-out patches of oocytes expressing vanilloid receptor 1, a cloned capsaicin receptor. Single point mutations (D178N, K735R) within the putative Walker type nucleotide-binding domains abolished the effect of ATP. These results show that ATP increases i(cap) in sensory neurons by direct interaction with the CAP channel without involvement of phosphorylation.

Capsaicin binds to the intracellular domain of the capsaicin-activated ion channel

Capsaicin (CAP) excites small sensory neurons, causing pain, neurogenic inflammation, and other visceral reflexes. These effects have been proposed to be the result of CAP activation of a nonselective cation current. It is generally assumed that CAP binds to an extracellular domain of the membrane receptor. However, the exact binding site is not known because of the lipophilic nature of CAP. To determine whether the binding domain is extracellular or intracellular, we tested the effect of a synthetic water-soluble CAP analog, DA-5018.HCl, on current activation. CAP activated the 45 pS (at -60 mV) nonselective cation channel from either side of the membrane. However, DA-5018.HCl, which had a greater potency and efficacy than CAP, activated the channels only from the cytosolic side of the patch membrane in a capsazepine, a CAP receptor antagonist, reversible manner. When applied extracellularly, DA-5018. HCl did not, but CAP did, activate whole-cell currents in sensory neurons, as well as in oocytes expressing vanilloid receptor 1, a recently cloned CAP receptor. Hydrogen ions, reported as a possible endogenous activator of cation current, failed to elicit any current when acidic medium (pH 5.0-6.0) was applied intracellularly, indicating that H+ does not mediate the CAP effect. These results indicate that CAP and its analog bind to the cytosolic domain of the CAP receptor and suggest that an endogenous CAP-like substance other than H+ may be present in the cell.