Role of potassium channels in regulating neuronal activity

The firing behaviour of excitable cells is fundamental for the information processing in multicellular organisms. Of the many regulators of firing behaviour, voltage gated potassium channels play a major role. In this thesis, several aspects of how potassium channels regulate of firing are explored. (i) The role of the channel density per se is studied in an in silico model, (ii) the effect of a spontaneously mutated potassium channel is studied in hippocampal slices from a mouse, (iii) the effect on the expression of potassium channels in general, and consequently on the firing, of this spontaneous mutation is studied in Xenopus oocytes, (iv) the molecular mechanisms giving the hERG channel its specific regulatory role in cardiac firing are studied in Xenopus oocytes and (v) the mechanisms behind spontaneous current events in preoptic neurons, shaping neuronal firing patterns, are studied in mechanically isolated cells. The computational study was based on an analysis of a hippocampal interneuron and showed that varying the density of sodiumand potassium channels results in qualitatively different firing patterns and threshold dynamics, mathematically associated with different bifurcation types (saddle-node, Hopf and double-orbit). The study of the effects of a mutated potassium channel was performed on a megencephalic mouse model, having a truncated KV1.1 channel gene (mceph). A patch-clamp analysis of neurons in hippocampal slices showed that one effect of the truncation was, in addition to an enlarged neuronal size, a slight increase in firing frequency, compatible with a decreased density of potassium channels. The study of the MCEPH expression showed that MCEPH indeed was expressed, although retained in the ER. It was also found that MCEPH expression retained other KV1 channels in the ER, reducing their density in the plasma membrane. The study of the molecular mechanism underlying the specific features of hERG was performed by analysing Shaker channels with hERG-emulating substitutions. hERG is structurally characterized by aromatic residues in the internal vestibule. We introduced one of these, tyrosine, in Shaker, and found that it induced hERG-like features, suggesting that the tyrosine residue has a role in forming the specific hERG kinetics. In addition, the tyrosine substitution induced an inactivation component with inversed voltage dependence. The study of the spontaneous neuronal current events was performed by voltageand currentclamp recordings from neurons of the medial preoptic nucleus. They showed that the currents were due to calcium-activated potassium channels of the SK3 subtype, triggered by calcium release from intracellular stores via ryanodine receptor channels. Current clamp measurements showed that the spontaneous current events had a role in shaping the firing patterns of the medial preoptic neurons. In conclusion, this thesis work adds some pieces of new information about how potassium channels regulate neuronal firing. It suggests new ways to understand pharmacological effects on firing patterns, presents a background for future studies of the trafficking of potassium channels, suggests a novel determinant involved in hERG kinetics and indicates a role for SK channels in neuronal firing.