Neural circuits in the central circadian clock and their regulation of sleep and wakefulness in mammals

Circadian rhythms are defined as approximately 24-hour oscillations in physiology and behavior. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is known as the central circadian clock. Based on current understanding, circadian rhythms are believed to be generated by transcription-translation feedback loops (TTFL) involving several clock genes and their protein products. However, several studies have shown that circadian oscillation in single SCN cells is still detectable in several clock gene deficient mice. These results suggest that there might be some oscillatory mechanisms without TTFL in mammalian cells. Other important aspects of circadian rhythms include neuronal circuits in the brain that regulate timing of physiological func-tions. Especially, functional output pathways from the SCN that regulate sleep and wakefulness have not been identified. In this review, I describe recent findings on circadian rhythm in the SCN, and of neuronal mechanisms that control circadian clock regulated sleep and wakefulness in mice.

Automated summary

Circadian rhythms are 24-hour oscillations in physiology and behavior, regulated by the suprachiasmatic nucleus (SCN) as the central circadian clock. Studies have shown that circadian oscillations can still occur in mice lacking certain clock genes, suggesting the presence of alternative oscillatory mechanisms. The neuronal circuits responsible for regulating sleep and wakefulness from the SCN have not been fully identified.