Blunted cortisol rhythm is associated with learning impairment in aged hamsters

In mammals, the cognitive decline that accompanies unsuccessful aging is associated with circadian rhythm dysfunction and increased levels of circulating glucocorticoids. The possible causal relations among these factors are not known. To test for primary effects of circadian clock dysfunction versus increased glucocorticoid levels as contributors to age-related learning impairment, we measured cortisol and wheel-running rhythms along with context learning in aged hamsters (15-18 months). At this age, locomotor rhythms of learners and nonlearners were found to be indistinguishable. However, plasma cortisol levels were lower, and the amplitude of the cortisol rhythm was reduced in the impaired animals. These data suggest that age-related cognitive decline may be related more to a loss of hormone rhythmicity than to a loss of behavioral rhythmicity or an increase in hormone level.