Mechanisms of Diurnal Regulation of the BK current in the Brain's Central Clock

Abstract

The large conductance Ca2+- and voltage-activated potassium (BK) channel is activated via two important cellular signaling mechanisms: increases in intracellular Ca2+ and membrane depolarization. Though the BK channel is encoded via a single gene, Kcnma1, currents produced by this channel exhibit a large degree of phenotypic variability across different cell types, indicating regulation of the BK channel can influence BK current properties. Furthermore, regulation of current properties, as opposed to changing expression of the channel, may act as a mechanism through which excitability can be modulated in various neurons. In order to determine whether regulation of BK current properties can influence neuronal excitability, we utilized the suprachiasmatic nucleus (SCN), as a model system. The SCN is the brain's central circadian oscillator, encoding time-of-day information through changes in neuronal excitability. Increased excitability is found during the day, promoting faster firing rates, while at night excitability is dampened, slowing firing rate. This diurnal change in firing rate is due in part to an increase in BK channel protein found in the SCN at night. While regulation of expression of the BK channel gene can account for part of the diurnal difference in excitability of SCN neurons, we hypothesized that daily regulation of BK current properties in the SCN via β-subunit association, alternative splicing, and control of Ca2+ sources limits a role for BK currents in setting firing frequency during the day. Using patch-clamp electrophysiolgy we demonstrate that BK current properties in the SCN are diurnally regulated, with smaller BK currents during the day found to be due to BK channel inactivation, Ca2+ source regulation and alternative splicing, rather than simply a decrease in BK channel expression. We also found loss of these mechanisms produces an increase in BK current and results in a decrease in firing rate during the day in SCN neurons. These results support the hypothesis that BK current regulation is just as important as regulation of BK protein expression in setting a diurnal difference in firing rate in the SCN. Furthermore, these results provide a framework through which modulation of current properties may impact firing in other pacemaking neurons.