Aging and biological rhythms in primates
Résumé
All living organisms exhibit rhythmic activities in a wide variety of endocrine and behavioural parameters. These biological rhythms are endogenously generated by a circadian clock, and they are entrained by cyclic variations of environmental factors called synchronizers. Aging is associated with changes in amplitude and temporal organization of many daily and seasonal rhythms. In humans, daily rhythms of sleep, thermoregulation and hormonal secretion are severely altered with aging. Except in humans, studies on primates are scarce. However, age-related effects on biological rhythms are relatively consistent among primate species studied to date, including humans. Therefore, non human primates are of valuable use for such investigations. Most studies have been performed on the Rhesus macaque (longevity 35-40 years) and on the gray mouse lemur (longevity 10-12 years). Like in humans, the rest-activity rhythm becomes fragmented in aged primates, and shows an increased activity during the resting period. Aging induces a decrease in amplitude of the body temperature rhythm and an increase in energy consumption. Various hormonal secretions exhibit a decrease with aging, but the rhythmic components of these declines have not always been depicted. Moreover, changes (amplitude or phase) in daily variations depended of the hormonal secretion tested. Taken together, these results suggest that the biological clock in the brain would be a primary target of aging. The main central clock is located in the suprachiasmatic nucleus of the hypothalamus whose endogenous oscillations are entrained by light. In this brain structure, cellular function and sensitivity to light show drastic changes with age in the mouse lemur. The precise knowledge of age-related alterations of biological rhythms in primates can have important consequences on the development of new treatments to maintain or restore biological rhythmicity in the elderly.