Michael J. Caterina

The capsaicin receptor : a heat-activated ion channel in the pain pathway

Capsaicin, the main pungent ingredient in ‘hot’ chilli peppers, elicits a sensation of burning pain by selectively activating sensory neurons that convey information about noxious stimuli to the central nervous system. We have used an expression cloning strategy based on calcium influx to isolate a functional cDNA encoding a capsaicin receptor from sensory neurons. This receptor is a non-selective cation channel that is structurally related to members of the TRP family of ion channels. The cloned capsaicin receptor is also activated by increases in temperature in the noxious range, suggesting that it functions as a transducer of painful thermal stimuli in vivo.

The Cloned Capsaicin Receptor Integrates Multiple Pain-Producing Stimuli

Capsaicin, the main pungent ingredient in hot chili peppers, elicits buming pain by activating specific (vanilloid) receptors on sensory nerve endings. The cloned vanilloid receptor (VR1) is a cation channel that is also activated by noxious heat. Here, analysis of heat-evoked single channel currents in excised membrane patches suggests that heat gates VR1 directly. We also show that protons decrease the temperature threshold for VR1 activation such that even moderately acidic conditions (pH < or = 5.9) activate VR1 at room temperature. VR1 can therefore be viewed as a molecular integrator of chemical and physical stimuli that elicit pain. Immunocytochemical analysis indicates that the receptor is located in a neurochemically heterogeneous population of small diameter primary afferent fibers. A role for VR1 in injury-induced hypersensitivity at the level of the sensory neuron is presented.

A capsaicin-receptor homologue with a high threshold for noxious heat.

Pain-producing heat is detected by several classes of nociceptive sensory neuron that differ in their thermal response thresholds. The cloned capsaicin receptor, also known as the vanilloid receptor subtype 1 (VR1), is a heat-gated ion channel that has been proposed to mediate responses of small-diameter sensory neurons to moderate (43u2009°C) thermal stimuli,. VR1 is also activated by protons, indicating that it may participate in the detection of noxious thermal and chemical stimuli in vivo. Here we identify a structurally related receptor, VRL-1, that does not respond to capsaicin, acid or moderate heat. Instead, VRL-1 is activated by high temperatures, with a threshold of ∼52u2009°C. Within sensory ganglia, VRL-1 is most prominently expressed by a subset of medium- to large-diameter neurons, making it a candidate receptor for transducing high-threshold heat responses in this class of cells. VRL-1 transcripts are not restricted to the sensory nervous system, indicating that this channel may be activated by stimuli other than heat. We propose that responses to noxious heat involve these related, but distinct, ion-channel subtypes that together detect a range of stimulus intensities.

Altered urinary bladder function in mice lacking the vanilloid receptor TRPV1

In the urinary bladder, the capsaicin-gated ion channel TRPV1 is expressed both within afferent nerve terminals and within the epithelial cells that line the bladder lumen. To determine the significance of this expression pattern, we analyzed bladder function in mice lacking TRPV1. Compared with wild-type littermates, trpv1−/− mice had a higher frequency of low-amplitude, non-voiding bladder contractions. This alteration was accompanied by reductions in both spinal cord signaling and reflex voiding during bladder filling (under anesthesia). In vitro, stretch-evoked ATP release and membrane capacitance changes were diminished in bladders excised from trpv1−/− mice, as was hypoosmolality-evoked ATP release from cultured trpv1−/− urothelial cells. These findings indicate that TRPV1 participates in normal bladder function and is essential for normal mechanically evoked purinergic signaling by the urothelium.

A Unified Nomenclature for the Superfamily of TRP Cation Channels

The TRP superfamily includes a diversity of non-voltage-gated cation channels that vary significantly in their selectivity and mode of activation. Nevertheless, members of the TRP superfamily share significant sequence homology and predicted structural similarities. Currently, most of the genes and proteins that comprise the TRP superfamily have multiple names and, in at least one instance, two distinct genes belonging to separate subfamilies have the same name. Moreover, there are many cases in which highly related proteins that belong to the same subfamily have unrelated names. Therefore, to minimize confusion, we propose a unified nomenclature for the TRP superfamily.The current effort to unify the TRP nomenclature focuses on three subfamilies (TRPC, TRPV, and TRPM) that bear significant similarities to the founding member of this superfamily, Drosophila TRP, and which include highly related members in worms, flies, mice, and humans (Table 1)(Table 1). Members of the three subfamilies contain six transmembrane segments, a pore loop separating the final two transmembrane segments, and similarity in the lengths of the cytoplasmic and extracellular loops. In addition, the charged residues in the S4 segment that appear to contribute to the voltage sensor in voltage-gated ion channels are not conserved. The TRP-Canonical (TRPC) subfamily (formerly short-TRPs or STRPs) is comprised of those proteins that are the most highly related to Drosophila TRP. The TRPV subfamily (formerly OTRPC), is so named based on the original designation, Vanilloid Receptor 1 (VR1), for the first mammalian member of this subfamily (now TRPV1). The name for the TRPM subfamily (formerly long-TRPs or LTRPs) is derived from the first letter of Melastatin, the former name (now TRPM1) of the founding member of this third subfamily of TRP-related proteins. Based on amino acid homologies, the mammalian members of these three subfamilies can be subdivided into several groups each (Table 2Table 2 and Figure 1Figure 1) .Table 1Number of TRP Genes in Worms (C. elegans), Flies (Drosophila melanogaster), Mice, and HumansSubfamilyWormsFliesMiceHumansTRPC3376aaTRPV5255TRPM4188aTRPC2 is a pseudogene and is not counted.Table 2Nomenclature of the Mammalian TRP SuperfamilyNameGroupFormer NamesAccession NumbersTRPC11TRP1CAA61447, AAA93252TRPC1TRPC22TRP2X89067, AAD17195, AAD17196, AAG29950, AAG29951, AAD31453,TRPC2CAA06964TRPC33TRP3AAC51653TRPC3TRPC44TRP4CAA68125, BAA23599TRPC4TRPC54TRP5AAC13550, CAA06911, CAA06912TRPC5TRPC63TRP6NP_038866TRPC6TRPC73TRP7AAD42069, NP_065122TRPC7TRPV11VR1AAC53398OTRPC1TRPV21VRL-1AAD26363, AAD26364, BAA78478OTRPC2GRCTRPV3 (not assigned)TRPV42OTRPC4AAG17543, AAG16127, AAG28027, AAG28028, AAG28029,VR-OACCAC20703TRP12VRL-2TRPV53ECaC1CAB40138CaT2TRPV63CaT1AAD47636ECaC2CAC20416CaT-LCAC20417TRPM11MelastatinAAC13683, AAC80000TRPM22TRPC7BAA34700LTRPC2TRPM31KIAA1616AA038185LTRPC3TRPM43TRPM4H18835LTRPC4TRPM53MTR1AAF26288LTRPC5TRPM64Chak2AF350881TRPM74TRP-PLIKAAF73131Chak1LTRPC7TRPM82TRP-p8AC005538Indicated are the suggested gene and protein names, the groups within each subfamily, the former names, and accession numbers.Figure 1Phylogenetic Tree of the TRP SuperfamilyThe tree, which was adapted from Clapham et al., 2001 (Nat. Rev. Neurosci. 2, 387–396), was calculated using the neighbor-joining method and human, rat, and mouse sequences.View Large Image | View Hi-Res Image | Download PowerPoint SlideThe numbering system for the mammalian TRPC, TRPV, and TRPM proteins takes into account the order of their discovery and, in as many cases as possible, the number that has already been assigned to the genes and proteins (Table 2)(Table 2). In the case of the TRPV proteins, the numbering system is also based in part on the groupings of the TRPV proteins. New members of each subfamily will maintain the same root name and, with the exception of TRPV3, will be assigned the next number in the sequence. Currently, TRPV3 is unassigned to maintain the TRPV1/ TRPV2 and TRPV5/TRPV6 groupings and so that the former OTRPC4 could be renamed TRPV4. The next TRPV protein will be designated TRPV3.We hope this new nomenclature will add clarity to the field and simplify the naming of new members of the TRP superfamily. We recommend that accession numbers be used whenever it is necessary to unambiguously specify a given variant resulting from alternative mRNA splicing. Finally, this nomenclature has been approved by the HUGO Gene Nomenclature Committee and we recommend that this system be used in all future publications concerning TRPC, TRPV, and TRPM subfamily members.

Vanilloid receptor expression suggests a sensory role for urinary bladder epithelial cells

Edited by Louis J. Ignarro, University of California, Los Angeles School of Medicine, Los Angeles, CA, and approved August 27, 2001 (received for review May 16, 2001)

Mechanisms of sensory transduction in the skin

Sensory neurons innervating the skin encode the familiar sensations of temperature, touch and pain. An explosion of progress has revealed unanticipated cellular and molecular complexity in these senses. It is now clear that perception of a single stimulus, such as heat, requires several transduction mechanisms. Conversely, a given protein may contribute to multiple senses, such as heat and touch. Recent studies have also led to the surprising insight that skin cells might transduce temperature and touch. To break the code underlying somatosensation, we must therefore understand how the skins sensory functions are divided among signalling molecules and cell types.

Nociceptors lacking TRPV1 and TRPV2 have normal heat responses

Vanilloid receptor 1 (TRPV1) has been proposed to be the principal heat-responsive channel for nociceptive neurons. The skin of both rat and mouse receives major projections from primary sensory afferents that bind the plant lectin isolectin B4 (IB4). The majority of IB4-positive neurons are known to be heat-responsive nociceptors. Previous studies suggested that, unlike rat, mouse IB4-positive cutaneous afferents did not express TRPV1 immunoreactivity. Here, multiple antisera were used to confirm that mouse and rat have different distributions of TRPV1 and that TRPV1 immunoreactivity is absent in heat-sensitive nociceptors. Intracellular recording in TRPV1-/- mice was then used to confirm that TRPV1 was not required for detecting noxious heat. TRPV1-/- mice had more heat-sensitive neurons, and these neurons had normal temperature thresholds and response properties. Moreover, in TRPV1-/- mice, 82% of heat-responsive neurons did not express immunoreactivity for TRPV2, another putative noxious heat channel.

Altered Thermal Selection Behavior in Mice Lacking Transient Receptor Potential Vanilloid 4

Transient receptor potential vanilloid 4 (TRPV4), a cation channel responsive to hypotonicity, can also be activated by warm temperatures. Moreover, TRPV4-/- mice reportedly exhibit deficits in inflammation-induced thermal hyperalgesia. However, it is unknown whether TRPV4 or related transient receptor potential channels account for warmth perception under injury-free conditions. We therefore investigated the contribution of TRPV4 to thermosensation and thermoregulation in vivo. On a thermal gradient, TRPV4-/- mice selected warmer floor temperatures than wild-type littermates. In addition, whereas wild-type mice failed to discriminate between floor temperatures of 30 and 34°C, TRPV4-/- mice exhibited a strong preference for 34°C. TRPV4-/- mice also exhibited prolonged withdrawal latencies during acute tail heating. TRPV4-/- and wild-type mice exhibited similar changes in behavior on a thermal gradient after paw inflammation. Circadian body temperature fluctuations and thermoregulation in a warm environment were also indistinguishable between genotypes. These results demonstrate that TRPV4 is required for normal thermal responsiveness in vivo.

TRPV1 shows dynamic ionic selectivity during agonist stimulation.

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel that is gated by noxious heat, capsaicin and other diverse stimuli. It is a nonselective cation channel that prefers Ca2+ over Na+. These permeability characteristics, as in most channels, are widely presumed to be static. On the contrary, we found that activation of native or recombinant rat TRPV1 leads to time- and agonist concentration–dependent increases in relative permeability to large cations and changes in Ca2+ permeability. Using the substituted cysteine accessibility method, we saw that these changes were attributable to alterations in the TRPV1 selectivity filter. TRPV1 agonists showed different capabilities for evoking ionic selectivity changes. Furthermore, protein kinase C–dependent phosphorylation of Ser800 in the TRPV1 C terminus potentiated agonist-evoked ionic selectivity changes. Thus, the qualitative signaling properties of TRPV1 are dynamically modulated during channel activation, a process that probably shapes TRPV1 participation in pain, cytotoxicity and neurotransmitter release.