Role of potassium channels in regulating neuronal activity

Abstract

The firing behaviour of excitable cells is fundamental for the information processing in multicellular organisms, varying from single spikes to different forms of repetitive firing. Of the many regulators, voltage gated potassium channels play a major role.

In this thesis some aspects of the potassium channel regulation 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 model, (iii) the effect on the expression of potassium channels in general, and consequently on the firing, by 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 the spontaneous current events in hypothalamic neurons, shaping hypothalamic 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 sodium- and 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 on the neurons was, in addition to an enlarged size, a slight increase in firing frequency, compatible with a decreased density of potassium channels.

The study of the MCEPH expression in mceph/mceph mice, showed that it indeed was expressed, but completely retained in the ER. It was also found that it 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 hypothalamic current events was performed with medial preoptic area neurons and showed that the currents were due to calcium-activated potassium channels of the SK3 subtype, triggered by Ca2+ 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 information on the role of potassium channels in regulating neuronal firing at different levels. It suggests ways to understand pharmacological effects on firing patterns, presents a background for future studies on the trafficking of potassium channels, suggests a novel determinant involved in hERG kinetics and indicates a role for SK channels in neuronal firing.