Pharyngula

TRPV1 Ion Channels

I noticed that PZ posted one of our take-home exam questions on Pharyngula and so I decided to make an entry with my answer (I okayed this with PZ first although he did warn me of the certainty of harsh reader criticism). The question referred us to a recent article in Nature about TRPV1 ion channels and asked us to describe TRPV1 ion channels and the testing that was done on them.

The transient receptor potential cation channel (TRPV1), also referred to as vanilloid receptor subtype 1, is a ligand-gated cation channel (2). This means that the channel contains organic molecules that can form covalent bonds with positive ions and is thus operated chemically. TRPV1 ion channels are non-specific and can be found on TRPV1 nociceptor (pain sensing) neurons in the central nervous system and peripheral nervous system. It may be related to thermal hyperalgesia (abnormally increased sense of pain) in both regions (2). The TRVP1 ion channel The opening in the TRPV1 ion channel was determined experimentally to be large enough to pass a 452 Da (1 Dalton = 1.657×10-24 g) dye molecule through (1). The TRPV1 ion channel, when opened by the proper agnostic, can allow anesthetic molecules to be introduced into nociceptic neurons, making it an important channel some regional anesthesics.

Most anesthetics are hydrophobic, cell membrane permeable, and function by blocking sodium ion channels on the inside of the cell. This blocks sensory nerves as well as motor and autonomic nerves (1). The main idea behind Binshtok, Bean, and Woolf’s experiment was to formulate an anesthetic that blocks the pain of sensory nerves but not motor and autonomic functions. They sited multiple sources stating that when QX-314, a charged derivative of lidocane, is introduced to the inside of a nerve cell it can inhibit sodium channels and produce analgesia. QX-314 was experimentally found to have no significant effect on nociceptor neurons externally. QX-314 is impermeable to nerve cell membranes but with a mass of 263Da is small enough to fit through TRPV1 ion channels. Capsaicin is a TRPV1 agonist, meaning it has an affinity for TRPV1 channel receptors and can affect them physiologically, in this case causing them to open.

Binshtok, Bean, and Woolf observed the membrane potential changes of rat dorsal root ganglia of various diameters exposed to QX-314 (an anesthetic), capsaicin (a TRPV1 agonist), and a mixture of the two. The voltage clamp method was used to determine whether or not the neuronic sodium nerve channels were inhibited and also which nerves channels were inhibited on. The voltage clamp method involves two wires placed in the axoplasm of a nerve cell. The first wire measures potential across the membrane and the second wire propagates electrical current (3). The voltage across the membrane is controlled while the ionic current is measured. Using this method, they found that QX-314 and capsaicin applied together could block the generation of action potentials (1). This effect can be attributed to neuronic sodium channel inhibition. If sodium channels are blocked then the depolarization phase of the action potential cannot take place and a wave of depolarization, a nervous signal, cannot be propagated along the neuron (3). The voltage clamp method used in this experiment involved blocking potassium and calcium ion currents so that the sodium ion current could be recorded by itself.

Binshtok, Bean, and Woolf concluded that neither QX-314 nor capsaicin produced significant effects on nociceptor neurons when applied individually but almost entirely block nociceptor sodium channel function when applied together. The brilliant idea behind all of this is that, if QX-314, an anesthetic, is introduced along with capsaicin, a TRPV1 agnostic, it will only travel through the TRPV1 ion channels of nociceptor neurons. Sodium channels in nociceptor neurons will be blocked while other neurons that lack TRPV1 will remain unaffected. The end result is an anesthetic that blocks painful sensation but does not compromise autonomic and motor nerve function.

References:
1. Alexander M. Binshtok, Bruce P. Bean, & Clifford J. Woolf. Inhibition of nociceptors by TRPV1-mediated entry of impermeant sodium channel blockers. Nature. 4 October 2007. Vol449 pp607-610.

2. M.Cui, P.Honore, C.Zhong, D.Gauvin, J.Mikusa, G.Hernandez, P.Chandran, A.Gomtsyan, B.Brown, E.K.Bayburt, K.Marsh, B.Bianchi, H.McDonald, W.Niforatos, T.R.Neelands, R.B.Moreland, M.W.Decker, C.H.Lee, J.P.Sullivan, C.R.Faltynek. TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists. 13 September 2006. Neuroscience Research. Global Pharmaceutical Research and Development. Abbott Laboratories.

3. Elaine N. Marieb. Human Anatomy and Physiology. Sixth Edition. Pearson, Benjamin, Cummings. 2004.

Comments

  1. #1 PZ Myers
    October 16, 2007

    I hope it is agonists rather than agnostics that are opening those channels. Agnostics would rather just straddle the membrane and do nothing.

  2. #2 Dustin
    October 16, 2007

    Ishtar damn him, he beat me to it.

  3. #3 Christian Burnham
    October 16, 2007

    I’d be as rude about this post as all the others if I could understand a single word of it.

  4. #4 David Marjanovi?
    October 16, 2007

    There may not be an omnibenevolent god, but spellcheckers are pretty good evidence for an immense force of evil…

  5. #5 David Marjanovi?
    October 16, 2007

    There may not be an omnibenevolent god, but spellcheckers are pretty good evidence for an immense force of evil…

  6. #6 speedwell
    October 16, 2007

    That’s fascinating. I am a chili-head who eats hot peppers at practically every meal, so I’ve always been interested in the medicinal uses of capsaicin. Through personal experience, I’ve found that it often works as an effective pain reliever on my headaches, cramps, and (surprisingly) bouts of medium-upset tummy. The mechanisms for this may be quite different from the anesthetic-helping mechanism you’re describing here, but it makes me wonder if there is some chemical made naturally by the body that works in the same way as the anesthetic does here.

  7. #7 Dustin
    October 16, 2007

    There may not be an omnibenevolent god, but spellcheckers are pretty good evidence for an immense force of evil…

    The auto-correction code was written by Aristide Torchia under the direction of Satan himself and hidden in the engravings of three different copies of “The Nine Gates of the Kingdom of Shadows”.

  8. #8 Jeb, FCD
    October 16, 2007

    See, PZ (#1). You godless, liiberal, professoriness is rubbing off on your poor innocent, babes-in-the-woods students.

  9. #9 cm
    October 16, 2007

    Yeah, the agnostic thing is just a brilliant mistake.

    Christian, here’s my nutshell translation into plain English of this stuff …(and written as though I were a 15 year old cheerleader)

    These TRPV1 receptors are doorways into nerve cells (neurons), and they open the door when they sense heat, acid, or the chemical found in hot peppers (capsaicin). When they open the door, tons o’ charged sodium and calcium atoms fly on through, charge up the neurons’ voltage, the neuron gets all excited, and passes the message to the brain. The message basically says “OWW!! The tissue around me is burning!!”. Hot peppers therefore trick out this system and that’s why they are perceived as “hot”. Interestingly, from what I’ve heard, birds don’t have this system, and hot peppers don’t effect them (but they HATE the chemical taste of “grape” used in candy).

    The guys from Harvard took advantage of this system to do a neat thing. They figured these TRPV1 channels–pretty good sized doorways into the nerve cells–could be big enough to fit this drug called QX-314. QX-314 is a sort of cousin to the anesthetics dentists give you, like novocaine. Its job is to prevent cells from getting all excited and hyper by preventing sodium to come into neurons. So it can be used as an anesthetic. Problem is, it is hard to get into cells because it doesn’t cross the cell’s membrane. It needs to either be introduced via a glass pipette by a frustrated grad student, or has to sneak in a side door, like some mongo channel. Aha! The TRPV1 channel is such a channel.

    …But the damn door is normally closed! DAMNIT! If only there was a “key” to open that baby. Wait, I know! HOT PEPPERS! The capsaicin in them will open the door of the TRPV1 channel, the Qx-314, which has been introduced into the general neighborhood by the scientist, will find its way through the open door, quiet down the cell, and then the cell will not be able to communicate its pain message to the brain. No message, no pain. Nice. Hey they don’t even need to use hot peppers, just buy pure capsaicin from Sigma.

    So, here capsaicin is acting like a confederate to “open the door and let in” the “quieting agent” of Qx-314. Even more coolly–as if it could GET any cooler!–the TRPV1 receptors are mainly found on–get this–just the very sensory fibers which are responsible for communicating pain messages! Other neurons, which are just trying to do the job of moving muscles and stuff, don’t have ’em, so they are left alone by Qx-314. So this is targeted right to where it ought to go.

    Others may feel free to correct inaccuraces, but perhaps this helps get the story across?

  10. #10 Stephen
    October 16, 2007

    Spellcheckers are good when used as the first stage of checking ones work, bad when used as the last stage and evil when used as the only stage.

  11. #11 Christian Burnham
    October 16, 2007

    Thank-you CM. That was a great translation (though maybe you’re having me on).

  12. #12 lithopithecus
    October 16, 2007

    uh, just a style point:
    get hip to the et. al.
    as in, Binshtok et. al.
    it’s not a big deal, but the, “Binshtok, Bean, and Woolf…”
    got a little repetitive.

    -cheers.

  13. #13 sailor
    October 16, 2007

    CM, That was great! Maybe you should become science journalist/writer.

  14. #14 Mooser
    October 16, 2007

    Yeah, that “agnostic” got me, too. WTH? I thought.

  15. #15 Peter Ashby
    October 16, 2007

    I thought the line about the TRPV1 being nonspecific was unclear. I knew what you meant, but I wouldn’t give a mark for it since you didn’t communicate that you knew what it meant.

    Just my point from my marking experience 😉

  16. #16 cm
    October 16, 2007

    (though maybe you’re having me on).

    No, no put on here, just glad to be of service!

  17. #17 Dale Hoyt
    October 16, 2007

    Just a correction to comment #11 by lithopithecus — it is et al. There is no period after “et”, which is Latin for “and” and, thus, not an abbreviation. You should write: “Smith et al.” and not “Smith et. al.”

  18. #18 Apu Illapu
    October 17, 2007

    [/lurk]
    Blue_Expo, I must second Peter Ashby here and point at some confusion about the meaning of these concepts: ‘ligand-gated ion channel’, ‘cation channel’ and ‘non-selective ion channel’. Look them up, they are easy to grasp and they will make a huge difference to your understanding of the topic.
    You tried to explain ‘thermal hyperalgesia’, but your definition was that of hyperalgesia, period. For this one, think warm shower on top of sunburn.
    And a small quibble: yes, TRPV1 does look like it’s ligand-gated (and temperature gated, and pH-gated). But a closer look reveals it to really be a voltage-gated channel. All known agonists seem to act by shifting the channel’s voltage dependence towards more negative potentials, closer to the physiological range, thereby increasing the open probability.
    [lurk]

  19. #19 lithopithecus
    October 17, 2007

    D’oh!
    you’re right…
    my bad.

  20. #20 efrique
    October 18, 2007

    Interesting. More please.

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