Calcium and BK Potassium Channel Regulation of Circadian Rhythms in the Suprachiasmatic Nucleus

Abstract

Mammalian circadian rhythms are driven by a network of neurons in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN exhibits daily (24-hour) rhythms in spontaneous action potential (AP) firing rate that encodes a time-of-day signal that coordinates the timing of circadian physiological and behavioral processes. Large-conductance Ca2+-activated K+ (BK) channels have a major role in driving the diurnal patterns of spontaneous firing in SCN neurons. BK K+ currents are larger at night, correlating with reduced neuronal excitability. The diurnal variation in BK current in the SCN is required for setting the day-night difference in firing frequency. BK currents undergo multi-level regulation by genetic and posttranslational mechanisms as well as functional coupling to Ca2+ channels. Intracellular Ca2+ (Ca2+i) is required for BK channel activation and previous studies have shown BK current is predominantly coupled to two types of Ca2+ sources in the SCN: L-type Ca2+ channels (LTCCs), and ryanodine receptors (RyRs). Circadian rhythms in Ca2+i have also been identified in SCN neurons. However, the Ca2+ channels involved in generating both AP and Ca2+i rhythms have not been clearly identified. First, to determine which Ca2+ channels are involved in AP rhythms, this study measured the impact of Ca2+ channel agonists and antagonists on the circadian parameters of spontaneous AP activity from organotypic SCN slice cultures grown on multi-electrode arrays. Next, to determine which Ca2+ channels are involved in Ca2+i rhythms, this study tested the effects of the same Ca2+ channel pharmacology on the circadian parameters of Ca2+i measured from SCN slice cultures transfected with a fluorescent Ca2+ sensor. Lastly, this investigated a potential mechanism by which LTCCs contribute to firing rate in SCN neurons by examining their ability to activate BK channels under controlled conditions. This study provides insight into the roles of specific Ca2+ sources in neural coding of the circadian time signal in the SCN.