The mammalian circadian system is composed of a master clock in the suprachi-asmatic nucleus (SCN) of the hypothalamus, and peripheral clocks in organs and tissues. Light is the most dominant time cue which adjusts the intrinsic circadian period to a 24-h day-night cycle. The master clock in the SCN is the only clock which acts as an interface between our body and diurnal and seasonal cycles in the environment, while the peripheral clocks drive tissue-specific rhythms. The SCN is composed of a number of cell-autonomous oscillator neurons. Over the past decade, our understanding of the molecular clock mechanisms has advanced tremendously. Of special note is a bioluminescent cell-imaging technique which enables us to monitor each clock cell's ticking for days and weeks, using luciferase reporters of clock genes. By using bioluminescent imaging, we found the multiple oscillators in the SCN that encode seasonal changes in the photoperiod.

Key words Luciferase reporter ■ Clock gene ■ Circadian rhythm ■ Photoperiod ■ Bioluminescent imaging

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