Using a range of tests we have studied alterations in behavior with advancing age in female C57BL/6 (of Jackson origin), the golden standard on which most genetically engineered mice are back-crossed. In parallel, growth and survival data were collected. In a protected environment the 90% and 75% cohort survival age was 20 and 25 months, respectively, and the 50% cohort survival was 32 months. In mice, body weight increases continuously until 15-20 months of age, while in advanced age whole body weight drops. The body mass loss in senescence is associated with emergence of other aged phenotype features such as kyphosis, balding and loss of fur-color. Our behavioral data show that aging modulates certain aspects of basic behavior in a continuous manner, like explorative and locomotor activities. Advanced age associates with an acceleration of behavioral impairments evident in most of the tests used, including motor skill acquisition and memory consolidation. However, certain domains of mouse behavior were well preserved also in advanced age such as thermal noxious threshold and working memory as assessed by an object recognition task. The decreased drive to explore is suggested to be a key factor underlying many aspects of reduced performance including cognitive capacity during aging. Behavioral aging affects genetically closely related individuals housed under strictly standardized conditions differentially (Collier, T.J., Coleman, P.D., 1991. Divergence of biological and chronological aging: evidence from rodent studies. Neurobiol. Aging, 12, 685-693; Ingram, D.K., 1988. Motor performance variability during aging in rodents. Assessment of reliability and validity of individual differences. Ann. N.Y. Acad. Sci., 515, 70-96). Consistent with this a subpopulation of the 28-month-old mice showed an explorative activity similar to young-adult mice and a significantly stronger preference for a novel object than aged mice with a less explorative behavior. Thus, subtle environmental factors and epigenetic modifications may be important modulators of aging.
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