McNally Beth A, Plante Amber E, Meredith Andrea L
Department of Physiology, University of Maryland School of Medicine, Baltimore, MD, United States.
Front Physiol. 2021 Sep 28;12:737291. doi: 10.3389/fphys.2021.737291. eCollection 2021.
Daily regulation of Ca and voltage-activated BK K channel activity is required for action potential rhythmicity in the suprachiasmatic nucleus (SCN) of the hypothalamus, the brain's circadian clock. In SCN neurons, BK activation is dependent upon multiple types of Ca channels in a circadian manner. Daytime BK current predominantly requires Ca influx through L-type Ca channels (LTCCs), a time when BK channels are closely coupled with their Ca source. Here we show that daytime BK current is resistant to the Ca chelator BAPTA. However, at night when LTCCs contribute little to BK activation, BK current decreases by a third in BAPTA compared to control EGTA conditions. In phase with this time-of-day specific effect on BK current activation, LTCC current is larger during the day. The specific Ca channel subtypes underlying the LTCC current in SCN, as well as the subtypes contributing the Ca influx relevant for BK current activation, have not been identified. SCN neurons express two LTCC subtypes, Ca1.2 and Ca1.3. While a role for Ca1.2 channels has been identified during the night, Ca1.3 channel modulation has also been suggested to contribute to daytime SCN action potential activity, as well as subthreshold Ca oscillations. Here we characterize the role of Ca1.3 channels in LTCC and BK current activation in SCN neurons using a global deletion of CACNA1D in mouse (Ca1.3 KO). Ca1.3 KO SCN neurons had a 50% reduction in the daytime LTCC current, but not total Ca current, with no difference in Ca current levels at night. During the day, Ca1.3 KO neurons exhibited oscillations in membrane potential, and most neurons, although not all, also had BK currents. Changes in BK current activation were only detectable at the highest voltage tested. These data show that while Ca1.3 channels contribute to the daytime Ca current, this does not translate into a major effect on the daytime BK current. These data suggest that BK current activation does not absolutely require Ca1.3 channels and may therefore also depend on other LTCC subtypes, such as Ca1.2.
下丘脑视交叉上核（SCN）是大脑的生物钟，其动作电位节律需要对钙（Ca）和电压激活的大电导钙激活钾（BK）通道活性进行日常调节。在SCN神经元中，BK通道的激活以昼夜节律的方式依赖于多种类型的钙通道。白天的BK电流主要需要通过L型钙通道（LTCCs）的钙内流，此时BK通道与其钙源紧密耦合。在这里，我们表明白天的BK电流对钙螯合剂BAPTA具有抗性。然而，在夜间，当LTCCs对BK通道激活的贡献很小时，与对照EGTA条件相比，BK电流在BAPTA中降低了三分之一。与这种对BK电流激活的昼夜特异性效应相一致，白天的LTCC电流更大。SCN中LTCC电流的具体钙通道亚型，以及对BK电流激活相关的钙内流有贡献的亚型尚未确定。SCN神经元表达两种LTCC亚型，Ca1.2和Ca1.3。虽然已经确定Ca1.2通道在夜间起作用，但也有人提出Ca1.3通道的调节有助于白天SCN的动作电位活动以及阈下钙振荡。在这里，我们使用小鼠中CACNAI D的整体缺失（Ca1.3基因敲除）来表征Ca1.3通道在SCN神经元中LTCC和BK电流激活中的作用。Ca1.3基因敲除的SCN神经元白天的LTCC电流减少了50%，但总钙电流没有减少，夜间钙电流水平没有差异。在白天，Ca1.3基因敲除的神经元表现出膜电位振荡，并且大多数神经元（尽管不是全部）也有BK电流。BK电流激活的变化仅在测试的最高电压下可检测到。这些数据表明，虽然Ca1.3通道对白天的钙电流有贡献，但这并没有转化为对白天BK电流的主要影响。这些数据表明，BK电流激活不一定需要Ca1.3通道，因此也可能依赖于其他LTCC亚型，如Ca1.2。
