Planting the Seed: What this Site is All AboutWe are a group of students enrolled in the Biology 181 Honors Section at the University of Arizona (Link to the The Student Biology Web Site at the University of Arizona). Our purpose is to create a Web site that discusses an article given to us, "The capsaicin receptor: a heat-activated ion channel in the pain pathway." The focus, Capsaicin, is a molecule found in the white "ribs" of hot peppers that is the root of our perception of heat from the peppers. It is assumed to be a defense mechanism in a variety of pepper plants. The capsaicin found in hot peppers work by binding to and stimulating capsaicin receptor proteins found in neuronal cells. The fibers of the neuronal cells then carry the stimulus from areas such as the tongue, to the roots of the spinal cord. Capsaicin also allows a deluge of calcium ions to enter the neuron. This is dangerous, because an extended exposure to calcium causes the fibers of the neuron to die. This page is designed to illustrate such topics as basic neurobiology, and to answer seemingly simple questions such as why peppers seem hot, or why water only makes the burning sensation worse. We will also discuss how some doses of capsaicin can be harmful, and how other doses can have a healing effect.
The Summary of "The capsaicin receptor: a heat-activated ion channel in the pain pathway"Capsaicin is a natural product of capsicum peppers, that is an active ingredient in many hot foods. When nociceptors- neurons that transmit information regarding tissue damage to pain-processing centers in the spinal cord and brain- come in contact with capsaicin, the neuron gets excited, and there is a perception of pain, and the a local release of inflammatory mediators. These nociceptors get excited by increasing permeability of plasma membrane to cations, but the molecular mechanism explaining this phenomenon is unclear. Capsaicin is being used in an analgesic agent in the treatment of painful disorders, causing long-term loss of responsiveness because it kills off the nociceptor, or it destroys the peripheral terminals. It was decide that the existence of a receptor site represents the most likely mechanism, because the capsaicin derivative showed structure-function relationships and evoked responses in a dose-dependent manner. A competitive capsaicin antagonist called capsazepine strengthened this model, along with discovering resiniferatoxin, an extremely potent capsaicin analogue for Euphorbia plants that mimics the cellular action of capsaicin. The cloning of a gene encoding a capsaicin receptor was decided to help develop more understanding of the molecular nature of capsaicin action and its relationship to endogenous pain signaling mechanisms. A cDNA clone that reconstitutes capsaicin responsiveness in non-neuronal cells was isolated. It was discovered that capsaicin gives off burning sensations through the activation of a heat-gated ion channel that is likely to contribute to the detection of painful thermal stimuli in vivo. Since the molecular structure of capsaicin receptors was not known, the experimenters adopted a functional screening strategy for isolating candidate cDNA clones. Because capsaicin has the ability to trigger robust calcium influx into sensory neurons in vitro, a cloning strategy was contrived. Since capsaicin responsiveness seemed to be confined to nociceptive neurons with cell bodies that resided within sensory ganglia, a cDNA library was constructed from dorsal root ganglion-derived messenger RNA. The pools consisted of 16,000 clones in each, and was transfected in human embryonic kidney-derived HEK193, and the transfected cells were filled with the fluorescent calcium-sensitive dye Fura-2 (emits light when in contact with Calcium), and examined for capsaicin-evoked changes in intracellular calcium levels. When a positive pool was found, it was reassayed, repeating the process over and over, until an individual clone containing a 3-kilobase cDNA insert was weeded out, which conferred capsaicin or resiniferatoxin. It was determined that this was the DNA-encoded aminoacid sequence of the protein that comprises the capsaicin receptor. Since the vanilloid group chemically represents capsaicin, it was called VR1. The scientists compared the pharmacological properties of the cloned receptor to those of native vanilloid sites in sensory ganglia. They examined the electrophysiological responses to a variety of vanilloid agonaists and antagonists. It was concluded that capsaicin and resiniferatoxin activate the current, while capsazepine, and synthetic antagonist, blocks the current. It was suggested that there is more that one binding site for the agonists molecule, which is true with vanilloid receptors. Capsaicin is able to bind to a channel form either the exterior or the interior of the plasma membrane. It was discovered that the cell lines made to express VR1 die after several hours of continuous exposure to capsaicin. The reason seems to be because of injury, and the researchers think it is induced by the continuous influx of ions. The reason for the death of cells when capsaicin is present in the cell, is because the capsaicin opens the ion channel, letting calcium in. When too much Calcium is let in, the cells die. It was suggested that vanilloid receptors serve as specific molecular markers for nociceptive neurons. Trigeminal and dorsal root sensory ganglia contain capsaicin-sensitive neurons. They were not found in the spinal cord or brain. Two other tissues that have been proposed to express capsaicin receptors are the nodose ganglion and the preoptic area of the hypothalamus. There was no detection of VR1 expression at these location, but vanilloid responsiveness here might be conferred by distinct VR1 subtypes. There was a hypothesis that VR1 is activated by noxious heat, and not innocuous heat. It was found that rapid increases in temperature evoke ion currents from expressed VR1 channels that closely mimic those induced by capsaicin. Therefore the hypothesis was supported by the results.