Collaborative Research: Persistence, entrainment, and function of circadian rhythms in arctic ground squirrels

地球的光-暗(LD)循环是记录昼夜节律的最强信号，被认为是内生钟出现和进化的主要驱动力。然而，在极地地区，光周期表现出极大的年度变化，最终导致太阳在较长时间内保持在地平线以上或以下。没有明确的LD周期，一些北极居民失去了日常行为和生理的组织，并假设驱动昼夜节律的分子发条可能在北极脊椎动物中很弱或不存在。这项研究测试了这样一种假设，即某些北极哺乳动物的昼夜节律组织的持久性是适应性的，因为它允许生物体将体温调节成本降至最低。具体地说，研究人员将检查北极地松鼠(AGS)在活动季节的连续白天和持续黑暗的冬眠期间的持续性、夹带性和昼夜节律的功能。四个研究目标从分子和神经生物学机制到生理、行为和生态适应：(1)确定在自由生活的AGS的年度周期中何时表现出昼夜节律，并确定春季节律的开始是否与首次暴露在光下一致，(2)通过测量大脑视交叉上核(SCN)中时钟基因的表达模式来确定AGS在冬眠期间的昼夜节律是否持续存在，(3)确定AGS是否将昼夜节律与光照质量或强度的每日变化联系在一起，以及(4)通过实验移相自由生活的地面松鼠和测量夜间活动的新陈代谢成本，研究北极夏季的再卷吸模式和节律的功能。在冬眠期间，昼夜节律是否会持续存在争议。转录和翻译被认为在深度昏迷期间受到全局抑制，唯一一项研究在多天昏迷期间检查SCN内时钟基因的表达没有发现振荡的证据。然而，在圈养地松鼠的一些研究中观察到了昏迷期间的昼夜体温节律，并假设唤醒的时间由生物钟控制。SCN中时钟基因表达的持续振荡将支持存在组织特有的机制，以翻译控制与冬眠期间生存相关的基因子集。相反，日常节律的表达可能与冬眠不相容；SCN功能和/或输出可能被抑制以延长麻木。这项研究通过(1)在实验室和野外对三所大学的博士后和学生进行生态生理学研究的培训，(2)通过参与阿拉斯加原住民科学和工程计划将阿拉斯加原住民纳入研究，以及(3)通过参与教师研究体验计划，增加K-12学生对研究的参与度，从而将研究和学习融为一体。研究结果将通过阿拉斯加大学媒体关系小组在当地的课堂演讲中传播，并在全国范围内传播。

The Earth's light-dark (LD) cycle is the strongest cue for entraining circadian rhythms and is considered the primary driver for the emergence and evolution of endogenous clocks. In polar regions, however, photoperiod exhibits extreme annual variation culminating with the sun remaining above or below the horizon for extended periods. Without a defined LD cycle, some arctic residents lose daily organization of behavior and physiology, and it has been hypothesized that the molecular clockwork that drives circadian rhythms may be weak or absent in arctic vertebrates. This study tests the hypothesis that persistence of circadian organization in some arctic mammals is adaptive because it allows organisms to minimize thermoregulatory costs. Specifically, the investigators will examine persistence, entrainment, and function of circadian rhythms in the arctic ground squirrel (AGS) during the continuous daylight of the active season and the continuous dark of hibernation. Four research objectives span from molecular and neurobiological mechanisms to physiological, behavioral and ecological adaptation: (1) determine when circadian rhythms are exhibited during the annual cycle of free-living AGS and establish if the onset of rhythmicity in spring coincides with first exposure to light, (2) determine if circadian rhythms persist in the master clock of AGS during hibernation by measuring patterns of clock gene expression in the suprachiasmatic nuclei (SCN) of the brain, (3) determine if AGS entrain circadian rhythms to daily changes in light quality or intensity, and (4) investigate patterns of re-entrainment and the function of rhythms in the arctic summer by experimentally phase-shifting free-living ground squirrels and measuring metabolic costs of nocturnal activity. Whether circadian rhythms persist during hibernation is contentious. Transcription and translation are thought to be globally suppressed during deep torpor and the only study to examine clock-gene expression within the SCN during multi-day torpor found no evidence of oscillations. However, circadian body temperature rhythms have been observed in some studies of captive ground squirrels during torpor and timing of arousals has been hypothesized to be controlled by a circadian clock. Persistence of oscillations in clock-gene expression in the SCN would support existence of tissue-specific mechanisms for translational control of a subset of genes relevant to survival during hibernation. In contrast, expression of daily rhythms may not be compatible with hibernation; SCN function and or output may be inhibited to prolong torpor. This study integrates research and learning by (1) training post-docs and students from three universities in ecophysiological studies in the laboratory and field, (2) inclusion of Alaska Natives in research through participation in the Alaska Native Science and Engineering Program, and (3) increasing K-12 students' engagement in research by participating in Teacher Research Experience programs. Results of research will be disseminated locally in classroom presentations and nationally through the University of Alaska media relations team.