Ashley Noah T, Ubuka Takayoshi, Schwabl Ingrid, Goymann Wolfgang, Salli Brady M, Bentley George E, Buck C Loren
Department of Biology, Western Kentucky University, Bowling Green, Kentucky, USA
Department of Biology, Waseda University, Shinjuku, Tokyo, Japan.
J Biol Rhythms. 2014 Dec;29(6):456-69. doi: 10.1177/0748730414552323. Epub 2014 Oct 17.
Most organisms in temperate or tropic regions employ the light-dark (LD) cycle as the primary Zeitgeber to synchronize circadian rhythms. At higher latitudes (>66°33'), continuous illumination during the summer presents a significant time-keeping dilemma for polar-adapted species. Lapland longspurs (Calcarius lapponicus), arctic-breeding migratory songbirds, are one of the few recorded species maintaining an intact diel rhythm in activity and plasma melatonin titers during polar summer. However, it is unknown whether rhythms are endogenous and entrain to low-amplitude polar Zeitgeber signals, such as daily variations in light intensity and the spectral composition of the sun (as measured by color temperature). Wild-caught male and female longspurs were brought into captivity, and locomotor activity was assessed using infrared detection. To examine if rhythms were endogenous, birds were exposed to constant bright light (LL; 1300 lux) or constant darkness (DD; 0.1 lux). All birds exhibited free-running activity rhythms in LL and DD, suggesting the presence of a functional circadian clock. Mean periods in LL (22.86 h) were significantly shorter than those in DD (23.5 h), in accordance with Aschoff's rule. No birds entrained to diel changes in light intensity, color temperature, or both. To examine endogenous molecular clock function, the Per2 gene was partially cloned in longspurs (llPer2) and transcripts were measured in hypothalamic tissue punches, eye, and liver using competitive polymerase chain reaction. Ocular llPer2 gene expression was periodic in LL and elevated at ZT24 (CT24) for LD or constant conditions (LL and DD), but llPer2 rhythmicity was not detected in hypothalamus or liver. Plasma melatonin was significantly lower in LL compared with LD or DD. In conclusion, rhythmic ocular Per2 expression and melatonin secretion may maintain the circadian activity rhythm across the polar day.
温带或热带地区的大多数生物利用光暗（LD）循环作为主要的授时因子来同步昼夜节律。在较高纬度地区（>66°33'），夏季的持续光照给适应极地环境的物种带来了重大的计时难题。拉普兰铁爪鹀（Calcarius lapponicus）是一种在北极繁殖的候鸟，是少数有记录的在极地夏季仍能保持完整的昼夜活动节律和血浆褪黑素水平的物种之一。然而，尚不清楚这些节律是否是内源性的，以及是否能与低振幅的极地授时信号同步，如光强度的每日变化和太阳的光谱组成（通过色温测量）。将野外捕获的雄性和雌性铁爪鹀圈养起来，利用红外探测评估其运动活动。为了检验节律是否是内源性的，将鸟类置于持续强光（LL；1300勒克斯）或持续黑暗（DD；0.1勒克斯）环境中。所有鸟类在LL和DD环境中均表现出自由运行的活动节律，表明存在功能性的昼夜节律钟。根据阿绍夫法则，LL环境中的平均周期（22.86小时）显著短于DD环境中的平均周期（23.5小时）。没有鸟类能与光强度、色温或两者的昼夜变化同步。为了研究内源性分子钟功能，在铁爪鹀中部分克隆了Per2基因（llPer2），并使用竞争性聚合酶链反应在下丘脑组织切片、眼睛和肝脏中测量转录本。在LL环境中，眼部llPer2基因表达呈周期性，在LD或恒定条件（LL和DD）下，在ZT24（CT24）时升高，但在下丘脑或肝脏中未检测到llPer2的节律性。与LD或DD相比，LL环境中的血浆褪黑素水平显著降低。总之，眼部Per2的节律性表达和褪黑素分泌可能在整个极昼期间维持昼夜活动节律。
