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Alexander von Humboldt Foundation Conference «Technologies of the 21st century: biological, physical, informational and social aspects» Saint-Petersburg, Russia, September 27-29, 2005 PHYSIOLOGY OF SUBTHRESHOLD CURRENTS Jürgen R. Schwarz Institute of Applied Physiology, University of Hamburg Information processing within the brain involves the generation of action potentials which are responsible for fast communication between nerve cells. Action potentials have a short duration and are generated by a transient influx of Na+ and a delayed outflow of K+ through voltage-gated ion channels. In addition to these canonical ion channels, nerve cells are equipped with a large number of voltage-gated and Ca2+-gated ion channels which are able to modulate their excitability. These ion channels are activated within the subthreshold potential range. This property enables them not only to influence the threshold potential, they also change the action potential duration, induce various forms of afterhyperpolarizations and set the interspike interval. By modulating the shape of action potentials and spiking patterns they are able to change information processing in neurons, thus contributing to neuronal plasticity. The Ih and the M-current belong to the most important threshold currents. Ih is a pacemaker current, mediated by hyperpolarization-activated cyclic-nucleotide-gated cation channels (HCN). The M-current is mediated by members of the family of voltage-gated KCNQ channels. One important function of M-currents is the induction of frequency adaptation. The current mediated by ether-à-go-go-related gene K+ channels (erg) is predominantly known as a cardiac current which takes part in repolarizing the heart action potential. However, erg channels have recently been shown to also modulate neuronal electrical activity. Further members of subthreshold currents are the A-type current mediated by voltage-gated K+ channels of the Kv4 family and Ca2+-gated SK channels. Both types of current are involved in the modulation of the frequency of action potential firing, i.e. the precise timing of the interspike interval. In addition to the usage of slices for patch-clamping or of heterologous expression systems an overview of transgenic animal models used to study the properties of e.g. KCNQ and HCN channels will be given. Thereafter the molecular properties and physiology of the ion channels mediating the different types of subthreshold currents will be presented as well as an insight into their complex subunit composition. Special emphasis will be laid upon the description of a M-like current in the rat node of Ranvier as well as on the presentation of the physiology of erg K channels in various types of neurons and neuroendocrine cells. Prof. Dr. Jürgen R. Schwarz, M.D. Institut für Angewandte Physiologie Zentrum für experimentelle Medizin Universitätsklinikum Eppendorf Martinistraße 52, D-20246 Hamburg Tel.: 040-42803-2190; Fax: 040-42803-9127 E-mail: [email protected]