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

地球的光暗循环（LD）是昼夜节律的最强提示，被认为是内源性时钟出现和进化的主要驱动力。然而，在极地地区，光周期表现出极端的年度变化，太阳在较长时间内停留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组织，并且已经假设驱动昼夜节律的分子时钟可能在北极脊椎动物中很弱或没有。这项研究验证了一种假设，即一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。具体来说，研究人员将研究北极地松鼠（AGS）在活动季节的连续白昼和冬眠的连续黑暗期间的昼夜节律的持久性、携带性和功能。四个研究目标涵盖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的年周期中何时表现出昼夜节律，并确定春季节律性的开始是否与第一次暴露于光的时间一致；(2)通过测量大脑视交叉上核（SCN）的时钟基因表达模式，确定冬眠期间AGS的主时钟是否持续存在昼夜节律；(3)确定AGS是否将昼夜节律与每日光质量或强度的变化联系起来。(4)通过对自由生活的地松鼠进行相移实验，并测量其夜间活动的代谢成本，研究北极夏季的复带模式和节律功能。昼夜节律在冬眠期间是否持续存在是有争议的。转录和翻译被认为在深度睡眠期间受到全局抑制，唯一一项在多日睡眠期间检查SCN内时钟基因表达的研究没有发现振荡的证据。然而，在圈养地松鼠冬眠期间的一些研究中，已经观察到昼夜体温节律，并且已经假设觉醒的时间是由生物钟控制的。SCN中时钟基因表达的持续振荡将支持存在组织特异性机制，用于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功能和输出可能被抑制以延长麻木。本研究将研究与学习结合起来，通过(1)在实验室和实地培训来自三所大学的博士后和学生进行生态生理学研究，(2)通过参与阿拉斯加原住民科学与工程项目将阿拉斯加原住民纳入研究，以及(3)通过参与教师研究经验项目提高K-12学生对研究的参与度。研究结果将通过课堂报告在当地传播，并通过阿拉斯加大学媒体关系小组在全国传播。

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.