Aging is associated with the progressive decline in function of multiple organ systems. In order to identify and quantify these functional deficits in mice, a standardized, methodical, and consistent process must be in place.46 Specialized phenotyping assays are necessary to distinguish phenotypic differences not easily discernible by gross observations, especially in aging cohorts. The specific assays to be used must be selected carefully to yield maximum amounts of information related to aging, while conserving resources of time, effort, and materials. Clinical and anatomical pathology assessments are standard measurements for comparing differences between aging cohorts and will cover a number of age-related conditions affecting multiple organ systems, including cardiovascular, kidney, pancreas, and skeletal muscle, as well as brain. Maximum life span can only be determined by allowing mice to live as long as they can. Euthanasia is necessary only if it is certain they would die without interference. The following reliable signs are used: sudden weight loss, failure to eat and drink, prominent appearing ribs and spine, and sunken hips; not responsive to being touched; slow or labored respiration; hunched up with matted fur; and cold to the touch. Mice up to 18 months of age are monitored 2 to 3 times per week, and mice older than 18 months, or mice with apparent health problems, are monitored daily, 7 days a week, or two to three times per day as needed.
Aging results in the progressive decline of the cardiovascular system, characterized in part by an increase in wall thickness of the ventricles. Aged rodents experience ventricular hypertrophy associated with an excess accumulation of collagen.47 Systemic mitochondrial dysfunction will frequently compromise muscle and cardiac function. Therefore, it is useful to evaluate mitochondrial physiology and cardiac function. Many neurological deficits associated with aging are subtle and not grossly observable, especially learning and memory deficits. The Morris water task is presently the most frequently used paradigm to evaluate learning and memory abilities in genetically engineered mice.22 Aged C57BL/6 mice show impairments in performance on this task.48 Additional neurological assessments for locomotor function include open field activity and rotarod procedures.49 Hearing is another neurosensory mechanism that exhibits an age-associated decline. The auditory-evoked brainstem response (ABR) is the most sensitive measure of auditory threshold that has been applied to mice.22 Using this procedure, age-related hearing loss has been demonstrated in the C57/BL6 mouse strain beginning as early as 2 months of age.50 The development of cataracts is an age-associated condition and can be readily evaluated in mice by slit lamp examination. The natural occurrence of age-related cataracts in mice and the protective effect of caloric restriction have recently been described.51 Bone loss is associated with aging in man and mouse,52 and bone scanning assays are frequently used to compare bone densities.
Caloric restriction (CR) is considered the "gold standard" against which other antiaging strategies are gauged.53 There are a number of specific protocols, but the basic feature consists of offering CR rodents 60% of what the control animals ingest.54 The amount of food consumed by the control group is measured on a weekly basis by weighing the food prior to offering it to the animals, and again at the end of the measuring period. Preweighed food allotments are provided three times a week, and adjusted each week according to the amount ingested by the control group. Open or closed diets are used. The NIH-31 Open Formula diet, which contains supplemental vitamins to provide the CR animals the same intake as that of the control animals, is a commonly used formula. The CR protocol for mice is started at 90% of the average control group beginning at 14 weeks of age, then reduced to 75% at 15 weeks of age, and further reduced to the full 60% at 16 weeks of age for the remainder of the study.
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