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.
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.