The master circadian clock, located in the mammalian suprachiasmatic nuclei (SCN), generates and coordinates circadian rhythmicity, i.e., internal organization of physiological and behavioral rhythms that cycle with a near 24-h period. Light is the most powerful synchronizer of the SCN. Although other nonphotic cues also have the potential to influence the circadian clock, their effects can be masked by photic cues. The purpose of this study was to investigate the ability of scheduled feeding to entrain the SCN in the absence of photic cues in four lines of house mouse (Mus domesticus). Mice were initially housed in 12:12-h light/dark cycle with ad libitum access to food for 6 h during the light period followed by 4 -6 mo of constant dark under the same feeding schedule. Wheel running behavior suggested and circadian PER2 protein expression profiles in the SCN confirmed entrainment of the master circadian clock to the onset of food availability in 100% (49/49) of the line 2 mice in contrast to only 4% (1/24) in line 3 mice. Mice from line 1 and line 4 showed intermediate levels of entrainment, 57% (8/14) and 39% (7/18), respectively. The predictability of entrainment vs. nonentrainment in line 2 and line 3 and the novel entrainment process provide a powerful tool with which to further elucidate mechanisms involved in entrainment of the SCN by scheduled feeding. nonphotic entrainment; scheduled feeding; suprachiasmatic; PER2; wheel running activity THE SUPRACHIASMATIC NUCLEI (SCN) of the anterior hypothalamus are the master circadian (near 24 h) clock in mammals, entraining clocks located in other brain regions and peripheral organs (26,33). The clock mechanism consists of molecular feedback loops containing positive and negative elements, i.e., clock gene products, that cycle with a near 24-h period in the absence of external signals (26). Light, the strongest natural zeitgeber or timing cue, shifts the clock so that it can entrain to the external light/dark (LD) cycle (28). Scheduled food availability is one of several nonphotic periodic signals (24, 25) that have been shown to entrain behavioral rhythms (17,19,22). Although entrainment of the master circadian clock was inferred in these studies (17,19,22), it was not confirmed at the level of the SCN (12).When feeding is dissociated from the normal activity period by allowing animals to eat only during their inactive period, two behavioral activity components result. One component entrains to and is in anticipation of onset of food availability, i.e., food anticipatory activity (FAA), which is controlled by an SCN-independent food-entrainable clock. The other is the animal's normal nocturnal or light-entrainable activity (LEA) component, which is controlled by the SCN when food access is not limited and has been presumed to be controlled by the SCN when access to food is temporally limited (8). The likelihood of the free-running LEA component being entrained by scheduled daily feeding in constant dark (DD) is species dependent. Hamsters typically show behavior...

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House mouse lines bidirectionally selected for nest-building behavior show a correlation between number of AVP cells in the suprachiasmatic nuclei (SCN), the master circadian clock in mammals, and level of nest-building behavior as well as a correlation between wheel-running activity and SCN AVP content. Similar genetic correlations between wheel-running activity and nest-building behavior have been reported in house mouse lines selected for increased voluntary wheel-running behavior. These similarities in genetic correlation structure in independently selected mouse lines allowed us to test whether AVP in the SCN and wheel running activity are truly correlated traits under identical testing procedures. In the mouse lines selected for voluntary wheel-running, no difference was found between the lines selected for high levels of voluntary wheel-running and randomly-bred control lines in the number of AVP immunoreactive neurons in the SCN (F 1,6 = 0.09, NS; replicate line effect: F 1,22 = 0.05, NS). This finding was confirmed at the level of individual variation, which revealed no relationship between number of AVP neurons in the SCN and total daily activity (R = −0.086, NS, n = 24), or circadian organization (i.e., the chi-squared periodogram waveform amplitude; R = −0.071, NS). Therefore our data do not support the hypothesis that differences in activity level and the circadian expression of activity in young adult mice are related to differences in the number of AVP-immunoreactive cells in the SCN.

In mammals, circadian rhythms controlled by the suprachiasmatic nuclei are entrained by photic stimuli. To investigate the molecular mechanism of photic entrainment, we examined lightinduced behavioral phase delays and associated changes in mPer1 and mPer2 gene expression in the suprachiasmatic nuclei of two mouse lines artificially selected for nest-building behavior. Big nest-builders show larger phase delays than small nest-builders. Light-induced mPer1 and mPer2 expression was examined in individual mice previously tested for phase shifting at circadian time 16. Light-induced mPer2 expression was significantly higher in big compared to small nestbuilders. No difference was found between lines in light-induced mPer1 expression. The results suggest a more important role for mPer2 than for mPer1 gene expression in behavioral phase delays.