Function of TRAAK current in the rat node of Ranvier

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We propose experiments to prove a new function of mechano-sensitive TRAAK channels in the rat node of Ranvier. In the mammalian node of Ranvier the action potential is brought about by Na+ current activation and inactivation, activation of voltage-dependent K channels does not contribute to repolarization. This is in contrast to the classical ion theory of the action potential where activation of voltage-dependent K current is important for repolarization. In the mammalian node outward current is provided by a relatively large voltage-independent leakage current (Schwarz and Eikhof, 1987; Schwarz et al., 1995). Recently, the human TRAAK channel was crystallized by R. MacKinnon (Brohawn et al., 2012). The MacKinnon lab also found that TRAAK channels are localized almost exclusively in the node of Ranvier, they do not occur elsewhere in the nervous system. In the proposed experiments we want to analyze the function of these newly discovered mechano-sensitive nodal TRAAK channels. An important prerequisite for this analysis is the detection of a TRAAK blocking compound (Su et al., 2016). We have done preliminary experiments in nodes of Ranvier of single myelinated rat nerve fibers by using the Nonner clamp (Nonner 1969). About 30% of the "leakage" current was blocked by the TRAAK channel blocker and the amplitude of the "leakage" current was almost doubled by 10 µM arachidonic acid and 10 mM trichlorethanol. Since these findings are characteristic for TRAAK channels they indicate the presence of TRAAK channels in the node of Ranvier. We have also shown that nodal TRAAK current is activated by stretching the axoplasma by superfusing the node under investigation with a hypotonic solution. In our grant application we propose experiments to test the hypothesis that the mechano-sensitive TRAAK channels are activated by the transient nodal swelling induced by the influx of Na+ and water during the upstroke of the action potential. The proposed function of TRAAK would be that the stretch-activated K+ current would accelerate action potential repolarization. We want to record action potentials before and after application of the TRAAK blocker and we want to carry out action potential clamp experiments with an action potential as potential template to see whether the TRAAK blocker broadens the inward current. Since TRAAK channels are very temperature-sensitive we want to do experiments at physiological temperature. We expect that the TRAAK current component of the leakage current increases considerably. If our hypothesis is correct then we would have discovered a new mechanism to stabilize the generation of the nodal action potential.