Postnatal Developmental Trends in Membrane Excitability and BK Channel Function in the Rodent Hippocampus

This dissertation outlines the postnatal development of excitability as well as expression and function of BK potassium channels in hippocampal neurons. I used patch clamp electrophysiology to measure how neuronal action potential waveforms and action potential firing frequencies change in early development, and how pharmacological blockade of BK channels affects these properties in hippocampal neurons. I also describe how the protein expression of the BK channel pore-forming α subunit and mRNA expression of different variants of the pore forming α subunit and auxiliary beta-4 subunit changes with development. I demonstrate in both cultured rat hippocampal neurons across the first seven postnatal days and in putative mouse hippocampal pyramidal neurons from postnatal day four to fifteen in acutely prepared slices that maturation brings changes in action potential kinetics and large increases in the capacity for high frequency firing. In the cultured neurons I demonstrate that during the first postnatal week, the contribution of BK channels to action potential repolarization decreases but the channel’s role in maintaining high-frequency firing increases. This change in the timing of BK channel activity in the action potential waveform is accompanied by a slowing of BK current onset measured by a decreasing effect of BK current blockade on potassium current rise time. Additionally, I demonstrated in pyramidal neurons in mouse brain slices that there is a loss of BK channel contribution to action potential repolarization between postnatal day four and fifteen. These changes in the BK channel’s role in neuronal action potential firing are accompanied by large increases in the expression of α subunit protein measured by Western blot and by large increases in mRNA transcript expression, measured by RT-qPCR, of both the α and beta-4 subunits in the hippocampus. The rise in beta-4 subunit expression can explain the apparent slowing of BK channel activation through development. I investigated whether BK channel blockade in neonatal brain slices could attenuate hyperexcitability in a model of seizure activity as BK channels in immature neurons share properties of BK channel variants implicated in epilepsy but found no evidence to support this hypothesis