Behavioral analysis of circadian rhythms: Entraining the circadian clock and determining the food-entrainable oscillator mechanism

Circadian rhythms are oscillations in behavior and physiological functions that are regulated by internal biological clocks. In mammals, the circadian rhythms can be best studied in the hypothalamic suprachiasmatic nucleus (SCN). The SCN has been considered as the master clock of circadian rhythms. SCN lesions abolish the circadian rhythms and transplantation of the SCN restores the behavioral rhythm; these findings confirm the role of the SCN as a circadian pacemaker. Recent discovery of molecular machineries of circadian oscillation enables us to examine genetically manipulated, molecular-clock-deficient mice, and use of these animal models has enhanced our understanding of biological clocks. It has been reported that prefeeding locomotor activity under a temporal restricted feeding condition exhibits the defining characteristic of a circadian rhythm. The prefeeding activity is driven by a distinct mechanism from the SCN. Despite extensive studies performed with the lesion strategy, the anatomic location of the food-entrainable oscillator has not been determined. Furthermore, recent studies using molecular-clock deficient mice reveal that the known circadian clock is dispensable for appearance of the prefeeding activity. Thus, examining the prefeeding activity alone is not sufficient and further examination for circadian oscillatory properties of the prefeeding activity rhythm is essential for understanding the food-entrainable circadian rhythms.