In this study, we observed that the number of days until re-entrainment to a new LD cycle was age-dependent. The phase-advanced times for 12- and 24-month-old mice were lower than that for the 4-month-old mice (p < 0.05 and p < 0.01, ANOVA, respectively). Little can be said about the mechanisms responsible for the age-related reduction of re-entrainment. One possible explanation is that SCN neurons of aging rodents may exhibit weak responses to light exposure. Indeed, the NMDA-induced Ca2+ influx into the SCN and light-induced Fos production in the SCN were attenuated in aging mice and rats (our unpublished observation). These deteriorating changes may affect the light-induced re-entrainment. An alternative explanation is that aging extends the free-running period of locomotor activity (our unpublished observation), and this may delay the re-entrainment to 8-hr advanced LD cycles in aging animals.

Interestingly, the re-entrainment to 8-hr advanced LD cycles was faster in the SAMP8 than in the SAMR1 among the 4-month-old mice. Sanchez-Barcelo et al. (11) reported that re-entrainment to a 6-hr phase-advance was faster in SAMP8 than in

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