Department of Molecular Cell Biology, Leiden University Medical Centre, the Netherlands.
J Biol Rhythms. 2009 Dec;24(6):477-87. doi: 10.1177/0748730409349895.
The SCN of the hypothalamus contains a major pacemaker, which exhibits 24-h rhythms in electrical impulse frequency. Although it is known that SCN electrical activity is high during the day and low during the night, the precise relationship between electrical activity and behavioral rhythms is almost entirely unknown. The authors performed long-term recordings of SCN multiple unit activity with the aid of implanted microelectrodes in parallel with the drinking activity in freely moving mice. The animals were kept in a 12h:12h light-dark cycle (LD 12:12) and in short-day (LD 8:16) and long-day photoperiods (LD 16:8). Onsets and offsets of behavioral activity occurred when SCN discharge was around half-maximum value. Of the onsets 80%, and of the offsets 62%, occurred when SCN electrical activity differed less than 15% from the half-maximum electrical activity levels. Transitions between rest and activity could be described by a sigmoid shaped probability curve with Hill coefficients of 7.0 for onsets and 5.7 for offsets. The similarity in the onset and offset levels shows an absence of hysteresis in the control of behavioral activity by the SCN. Exposure to short- or long-day photoperiods induced significant alterations in the waveform of electrical activity but did not affect SCN electrical activity levels at which behavioral transitions occurred. In all photoperiods, the SCN signal was skewed with more rapid discharge changes during onsets (19% per hour) than offsets (11% per hour). The precision of the circadian system appears optimized, as transitions between behavioral activity and rest occur when the change in SCN electrical activity is maximal, both during the declining and rising phase. The authors conclude that transitions in behavioral state can be described by a probability function around half-maximum electrical activity levels and that the parameters of the SCN, predicting onset and offset of behavior, are remarkably insensitive to environmental conditions.
下丘脑的 SCN 包含一个主要的起搏器，其电冲动频率表现出 24 小时的节律。虽然已知 SCN 的电活动在白天较高，在夜间较低，但电活动与行为节律之间的确切关系几乎完全未知。作者在植入的微电极的帮助下，对 SCN 的多单位活动进行了长期记录，同时对自由活动的小鼠的饮水活动进行了记录。动物被放置在 12 小时光照：12 小时黑暗（LD 12:12）和短日（LD 8:16）和长日光照周期（LD 16:8）中。行为活动的开始和结束发生在 SCN 放电接近最大值的一半时。在 SCN 电活动与最大值电活动水平相差小于 15%的情况下，开始的 80%和结束的 62%发生。休息和活动之间的转换可以用具有 7.0 的 Hill 系数的 sigmoid 形状的概率曲线来描述，用于开始，而用于结束的 Hill 系数为 5.7。开始和结束水平的相似性表明，SCN 对行为活动的控制中没有滞后。暴露于短日或长日光照周期会导致电活动的波形发生显著变化，但不会影响发生行为转换的 SCN 电活动水平。在所有光照周期中，SCN 信号都偏向于开始时放电变化更快（每小时 19%）而不是结束时（每小时 11%）。昼夜节律系统的精度似乎得到了优化，因为在 SCN 电活动变化最大时，即下降和上升阶段，行为活动和休息之间会发生转换。作者得出结论，行为状态的转换可以用接近最大电活动水平的概率函数来描述，并且预测行为开始和结束的 SCN 参数对环境条件非常不敏感。
