The Mammalian Circadian Timing System: Organization and Coordination of Central and Peripheral Clocks

Most physiology and behavior of mammalian organisms follow daily oscillations. These rhythmic processes are governed by environmental cues (e.g., fluctuations in light intensity and temperature), an internal circadian timing system, and the interaction between this timekeeping system and environmental signals. In mammals, the circadian timekeeping system has a complex architecture, composed of a central pacemaker in the brain's suprachiasmatic nuclei (SCN) and subsidiary clocks in nearly every body cell. The central clock is synchronized to geophysical time mainly via photic cues perceived by the retina and transmitted by electrical signals to SCN neurons. In turn, the SCN influences circadian physiology and behavior via neuronal and humoral cues and via the synchronization of local oscillators that are operative in the cells of most organs and tissues. Thus, some of the SCN output pathways serve as input pathways for peripheral tissues. Here we discuss knowledge acquired during the past few years on the complex structure and function of the mammalian circadian timing system.

哺乳动物的大多数生理机能和行为都呈现每日的节律性波动。这些节律性过程受环境因素（例如光照强度和温度的波动）、体内的生物钟计时系统以及该计时系统与环境信号之间的相互作用所调控。在哺乳动物中，生物钟计时系统具有复杂的结构，由大脑视交叉上核（SCN）中的一个中枢起搏器以及几乎每个身体细胞中的子钟组成。中枢时钟主要通过视网膜感知的光信号并由电信号传递给SCN神经元来与地球物理时间同步。反过来，SCN通过神经和体液信号以及通过使大多数器官和组织细胞中起作用的局部振荡器同步来影响昼夜节律的生理机能和行为。因此，一些SCN输出通路充当了外周组织的输入通路。在此，我们讨论在过去几年中所获得的有关哺乳动物生物钟计时系统复杂结构和功能的知识。