TRPV1 ion channels mediate the response to painful heat, extracellular acidosis, and capsaicin, the pungent extract from plants in the Capsicum family (hot chili peppers) (Szallasi, A., and P.M. Blumberg. 1999. Pharmacol. Rev. 51:159–212; Caterina, M.J., and D. Julius. 2001. Annu. Rev. Neurosci. 24:487–517). The convergence of these stimuli on TRPV1 channels expressed in peripheral sensory nerves underlies the common perceptual experience of pain due to hot temperatures, tissue damage and exposure to capsaicin. TRPV1 channels are nonselective cation channels (Caterina, M.J., M.A. Schumacher, M. Tominaga, T.A. Rosen, J.D. Levine, and D. Julius. 1997. Nature. 389:816–824). When activated, they produce depolarization through the influx of Na⁺, but their high Ca²⁺ permeability is also important for mediating the response to pain. In particular, Ca²⁺ influx is thought to be required for the desensitization to painful sensations over time (Cholewinski, A., G.M. Burgess, and S. Bevan. 1993. Neuroscience. 55:1015–1023; Koplas, P.A., R.L. Rosenberg, and G.S. Oxford. 1997. J. Neurosci. 17:3525–3537). Here we show that in inside-out excised patches from TRPV1 expressed in Xenopus oocytes and HEK 293 cells, Ca²⁺/calmodulin decreased the capsaicin-activated current. This inhibition was not mimicked by Mg²⁺, reflected a decrease in open probability, and was slowly reversible. Furthermore, increasing the calmodulin concentration in our patches by coexpression of wild-type calmodulin with TRPV1 produced inhibition by Ca²⁺ alone. In contrast, patches excised from cells coexpressing TRPV1 with a mutant calmodulin did not respond to Ca²⁺. Using an in vitro calmodulin-binding assay, we found that TRPV1 in oocyte lysates bound calmodulin, although in a Ca²⁺-independent manner. Experiments with GST-fusion proteins corresponding to regions of the channel NH₂-terminal domain demonstrated that a stretch of ∼30 amino acids adjacent to the first ankyrin repeat bound calmodulin in a Ca²⁺-dependent manner. The physiological response to pain involves an influx of Ca²⁺ through TRPV1. Our results indicate that this Ca²⁺ influx may feed back on the channels, inhibiting their gating. This type of feedback inhibition could play a role in the desensitization produced by capsaicin.

The Transient Receptor Potential Vanilloid 1 (TRPV1) channel is a polymodal protein with functions widely linked to the generation of pain. Several agonists of exogenous and endogenous nature have been described for this ion channel. Nonetheless, detailed mechanisms and description of binding sites have been resolved only for a few endogenous agonists. This review focuses on summarizing discoveries made in this particular field of study and highlighting the fact that studying the molecular details of activation of the channel by different agonists can shed light on biophysical traits that had not been previously demonstrated.

Transient Receptor Potential (TRP) channels are a family of ion channels whose members are distributed among all kinds of animals, from invertebrates to vertebrates. The importance of these molecules is exemplified by the variety of physiological roles they play. Perhaps, the most extensively studied member of this family is the TRPV1 ion channel; nonetheless, the activity of TRPV4 has been associated to several physio and pathophysiological processes, and its dysfunction can lead to severe consequences. Several lines of evidence derived from animal models and even clinical trials in humans highlight TRPV4 as a therapeutic target and as a protein that will receive even more attention in the near future, as will be reviewed here.

The TRPV1 ion channel serves as an integrator of noxious stimuli with its activation linked to pain and neurogenic inflammation. Cholesterol, a major component of cell membranes, modifies the function of several types of ion channels. Here, using measurements of capsaicin-activated currents in excised patches from TRPV1-expressing HEK cells, we show that enrichment with cholesterol, but not its diastereoisomer epicholesterol, markedly decreased wild-type rat TRPV1 currents. Substitutions in the S5 helix, rTRPV1-R579D, and rTRPV1-F582Q, decreased this cholesterol response and rTRPV1-L585I was insensitive to cholesterol addition. Two human TRPV1 variants, with different amino acids at position 585, had different responses to cholesterol with hTRPV1-Ile(585) being insensitive to this molecule. However, hTRPV1-I585L was inhibited by cholesterol addition similar to rTRPV1 with the same S5 sequence. In the absence of capsaicin, cholesterol enrichment also inhibited TRPV1 currents induced by elevated temperature and voltage. These data suggest that there is a cholesterol-binding site in TRPV1 and that the functions of TRPV1 depend on the genetic variant and membrane cholesterol content.