Ikeno Tomoko, Williams Cory T, Buck C Loren, Barnes Brian M, Yan Lily
Department of Psychology, Michigan State University, East Lansing, Michigan.
1. Department of Biological Sciences, Graduate School of Science, The University of Tokyo.
J Biol Rhythms. 2017 Jun;32(3):246-256. doi: 10.1177/0748730417702246. Epub 2017 Apr 28.
Most organisms have a circadian system, entrained to daily light-dark cycles, that regulates 24-h rhythms of physiology and behavior. It is unclear, however, how circadian systems function in animals that exhibit seasonal metabolic suppression, particularly when this coincides with the long-term absence of a day-night cycle. The arctic ground squirrel, Urocytellus parryii, is a medium-sized, semi-fossorial rodent that appears above-ground daily during its short active season in spring and summer before re-entering a constantly dark burrow for 6 to 9 months of hibernation. This hibernation consists of multiple week-long torpor bouts interrupted by short (< 20 h) arousal intervals when metabolism and body temperature (Tb) return to normal levels. Here, we used immunohistochemistry to measure the expression of daily or circadian rhythms of the protein products of 3 circadian clock genes, PER1, PER2, BMAL1, and the neural activity marker c-FOS in the suprachiasmatic nucleus (SCN) of arctic ground squirrels before, during, and after the first torpor bout of hibernation. Before torpor, while under 12:12-h light:dark conditions, animals showed significant daily rhythms in their Tb, as well as in protein expression levels of PER1 and PER2, but not BMAL1. Upon entering first torpor (Tb < 30°C), animals were moved into constant darkness. When sampled at 6-h intervals-beginning 24 h after the last light out, with Tb 3°C to 4°C-there were no circadian oscillations in PER1, PER2, or c-FOS expression. Sampling across 24 h during the first spontaneous arousal interval, c-FOS expression was elevated only when Tb reached 20°C and PER1 and PER2 expression did not show any Tb- or time-dependent changes. These results suggest that the central circadian clock might have stopped functioning during hibernation in this species, and the timing of arousal from torpor in arctic ground squirrels is unlikely to be controlled by the circadian clock within the SCN.
大多数生物都有一个生物钟系统，该系统与昼夜的明暗周期同步，调节着生理和行为的24小时节律。然而，目前尚不清楚生物钟系统在表现出季节性代谢抑制的动物中是如何发挥作用的，尤其是当这种情况与长期缺乏昼夜周期同时出现时。北极地松鼠（Urocytellus parryii）是一种中型半穴居啮齿动物，在春夏季短暂的活跃季节里，它每天都会出现在地面上，然后再进入持续黑暗的洞穴中进行6至9个月的冬眠。这种冬眠由多个为期一周的蛰伏期组成，期间会被短暂（<20小时）的苏醒间隔打断，苏醒时新陈代谢和体温（Tb）会恢复到正常水平。在此，我们使用免疫组织化学方法，测量了北极地松鼠在冬眠首次蛰伏期之前、期间和之后，视交叉上核（SCN）中3种生物钟基因（PER1、PER2、BMAL1）的蛋白质产物的每日或昼夜节律表达，以及神经活动标记物c-FOS的表达。在蛰伏前，处于12:12小时光照:黑暗条件下时，动物的Tb以及PER1和PER2的蛋白质表达水平呈现出显著的每日节律，但BMAL1没有。进入首次蛰伏（Tb < 30°C）后，动物被转移到持续黑暗环境中。从最后一次光照结束24小时后开始，每隔6小时进行一次采样，此时Tb为3°C至4°C，PER1、PER2或c-FOS的表达没有昼夜振荡。在首次自发苏醒间隔期间进行24小时采样，只有当Tb达到20°C时c-FOS表达才会升高，而PER1和PER2的表达没有显示出任何与Tb或时间相关的变化。这些结果表明，在该物种冬眠期间，中枢生物钟可能已经停止运作，北极地松鼠从蛰伏中苏醒的时间不太可能受SCN内生物钟的控制。
