The role of Clock in the plasticity of circadian entrainment

The mammalian circadian clock lying in suprachiasmatic nucleus (SCN) is synchronized to about 24h by the environmental light-dark cycle (LD). The circadian clock exhibits limits of entrainment above and below 24h, beyond which it will not entrain. Little is known about the mechanisms regulating the limits of entrainment. In this study, we show that wild-type mice entrain to only an LD 24h cycle, whereas Clock mutant mice can entrain to an LD 24, 28, and 32h except for LD 20h and LD 36h cycle. Under an LD 28h cycle, Clock mutant mice showed a clear rhythm in Per2 mRNA expression in the SCN and behavior. Light response was also increased. This is the first report to show that the Clock mutation makes it possible to adapt the circadian oscillator to a long period cycle and indicates that the clock gene may have an important role for the limits of entrainment of the SCN to LD cycle.