Age-Related Changes in Hamster Circadian Period, Entrainment, and Rhythm Splitting

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Abstract
Age has not been sufficiently explored as a variable that can influence circadian periodicity. The present experiments (1) re-evaluated changes in hamster circadian period with age while controlling for access to wheels; (2) studied whether the limits of entrainment differ between young and old animals; and (3) examined whether age influences the probability of rhythm splitting under prolonged constant light conditions. The circadian wheel-running period shortened with age, generally regardless of prior wheel access. Access to wheels was associated with increased weight gain, and animals with prolonged access to wheels died younger than animals without wheel access. These two measures may have been related to the housing conditions associated with wheel access. Although equal proportions of young and old hamsters entrained equally to all T-cycles used, loss of entrainment to long T-cycles was more rapid for old than for young animals. About 60% of young animals showed rhythm splitting, compared to 7% of old animals. The data are consistent with the observation that the circadian rhythm period of the male hamster shortens with age.