The capsaicin (vanilloid) receptor VR1 is a cation channel expressed by primary sensory neurons of the "pain" pathway. Heterologously expressed VR1 can be activated by vanilloid compounds, protons, or heat (>43 degrees C), but whether this channel contributes to chemical or thermal sensitivity in vivo is not known. Here, we demonstrate that sensory neurons from mice lacking VR1 are severely deficient in their responses to each of these noxious stimuli. VR1-/- mice showed normal responses to noxious mechanical stimuli but exhibited no vanilloid-evoked pain behavior, were impaired in the detection of painful heat, and showed little thermal hypersensitivity in the setting of inflammation. Thus, VR1 is essential for selective modalities of pain sensation and for tissue injury-induced thermal hyperalgesia.

http://science.sciencemag.org/content/sci/288/5464/306.full.pdf

Impaired Nociception and Pain Sensation in Mice Lacking the Capsaicin Receptor

http://www.columbia.edu/cu/biology/courses/w3004/recitation7.pdf

The Cloned Capsaicin Receptor Integrates Multiple Pain-Producing Stimuli

The sensation of pain alerts us to real or impending injury and triggers appropriate protective responses. Unfortunately, pain often outlives its usefulness as a warning system and instead becomes chronic and debilitating. This transition to a chronic phase involves changes within the spinal cord and brain, but there is also remarkable modulation where pain messages are initiated - at the level of the primary sensory neuron. Efforts to determine how these neurons detect pain-producing stimuli of a thermal, mechanical or chemical nature have revealed new signalling mechanisms and brought us closer to understanding the molecular events that facilitate transitions from acute to persistent pain.

Molecular mechanisms of nociception

The amiloride-sensitive epithelial sodium channel constitutes the rate-limiting step for sodium reabsorption in epithelial cells that line the distal part of the renal tubule, the distal colon, the duct of several exocrine glands, and the lung. The activity of this channel is upregulated by vasopressin and aldosterone, hormones involved in the maintenance of sodium balance, blood volume and blood pressure. We have identified the primary structure of the alpha-subunit of the rat epithelial sodium channel by expression cloning in Xenopus laevis oocytes. An identical subunit has recently been reported. Here we identify two other subunits (beta and gamma) by functional complementation of the alpha-subunit of the rat epithelial Na+ channel. The ion-selective permeability, the gating properties and the pharmacological profile of the channel formed by coexpressing the three subunits in oocytes are similar to that of the native channel.

Amiloride-sensitive epithelial Na+ channel is made of three homologous subunits.

Natural products afford a window of opportunity to study important biology. If the natural product is used or abused by human beings, finding its biological target(s) is all the more significant. Hot pepper is eaten on a daily basis by an estimated one-quarter of the world’s population and

Vanilloid (Capsaicin) Receptors and Mechanisms

Acid-sensing is associated with both nociception1 and taste transduction2. Stimulation of sensory neurons by acid is of particular interest, because acidosis accompanies many painful inflammatory and ischaemic conditions. The pain caused by acids is thought to be mediated by H+-gated cation channels present in sensory neurons3–5. We have now cloned a H+-gated channel (ASIC, for acid-sensing ionic channel) that belongs to the amilor-ide-sensitive Na+ channel6–1 Vdegenerin12–14 family of ion channels. Heterologous expression of ASIC induces an amiloride-sensitive cation (Na+ ≥ Ca2+ ≥ K+) channel which is transiently activated by rapid extracellular acidification. The biophysical and pharmacological properties of the ASIC channel closely match the H+-gated cation channel described in sensory neurons3,15,16. ASIC is expressed in dorsal root ganglia and is also distributed widely throughout the brain. ASIC appears to be the simplest of ligand-gated channels.

A proton-gated cation channel involved in acid-sensing

Cystic fibrosis is associated with a defect in epithelial chloride ion transport which is caused by mutations in a membrane protein called CFTR (cystic fibrosis transmembrane conductance regulator). Heterologous expression of CFTR produces cyclicAMP-sensitive Cl(-)-channel activity. Deletion of phenylalanine at amino-acid position 508 in CFTR (delta F508 CFTR) is the most common mutation in cystic fibrosis. It has been proposed that this mutation prevents glycoprotein maturation and its transport to its normal cellular location. We have expressed both CFTR and delta F508 CFTR in Vero cells using recombinant vaccinia virus. Although far less delta F508 CFTR reached the plasma membrane than normal CFTR, sufficient delta F508 CFTR was expressed at the plasma membrane to permit functional analysis. delta F508 CFTR expression induced a reduced activity of the cAMP-activated Cl- channel, with conductance, anion selectivity and open-time kinetics similar to those of CFTR, but with much greater closed times, resulting in a large decrease of open probability. The delta F508 mutation thus seems to have two major consequences, an abnormal translocation of the CFTR protein which limits membrane insertion, and an abnormal function in mediating Cl- transport.

Altered chloride ion channel kinetics associated with the delta F508 cystic fibrosis mutation.

Molecular cloning of a non-inactivating proton-gated Na+ channel specific for sensory neurons.

http://www.jbc.org/content/275/33/25116.full.pdf

Isolation of a tarantula toxin specific for a class of proton-gated Na+ channels.

The peptide Phe-Met-Arg-Phe-NH2 (FMRFamide) and structurally related peptides are present both in invertebrate and vertebrate nervous systems. Although they constitute a major class of invertebrate peptide neurotransmitters, the molecular structure of their receptors has not yet been identified. In neurons of the snail Helix aspersa, as well as in Aplysia bursting and motor neurons, FMRFamide induces a fast excitatory depolarizing response due to direct activation of an amiloride-sensitive Na+ channel. We have now isolated a complementary DNA from Helix nervous tissue; when expressed in Xenopus oocytes, it encodes an FMRFamide-activated Na+ channel (FaNaCh) that can be blocked by amiloride. The corresponding protein shares a very low sequence identity with the previously cloned epithelial Na+ channel subunits and Caenorhabditis elegans degenerins, but it displays the same overall structural organization. To our knowledge, this is the first characterization of a peptide-gated ionotropic receptor.

Guy Champigny

Papers

A proton-gated cation channel involved in acid-sensing

Altered chloride ion channel kinetics associated with the delta F508 cystic fibrosis mutation.

Molecular cloning of a non-inactivating proton-gated Na+ channel specific for sensory neurons.

Isolation of a tarantula toxin specific for a class of proton-gated Na+ channels.

Cloning of the amiloride-sensitive FMRFamide peptide-gated sodium channel.