Effect of aging on circadian activity in gray mouse lemurs

Microcebus murinus is a very photoperiod-dependent primate with a potential ly extended longevity (13 years). Reduction of artificial seasonal cycles a llows acceleration of the aging process. Under these conditions, age is def ined according to the number of seasonal cycles. We conducted experiments i n order to assess the effects of aging upon (1) the main parameters (period : tau; duration: alpha) of the circadian activity-rest rhythm; and (2) the plasticity of the response to light, which is the main entraining factor of the internal clock. We studied the evolution of tau and alpha through two types of experiments: a transverse one comparing 36 males of various ages ( 1-13 seasonal cycles) and a longitudinal one following 2 pairs of males fro m the same litter tone from each pair was maintained under natural cycle wh ile the other was submitted to a shortened cycle) over 54 months. Results f rom transverse experiments demonstrated no statistical difference in tau an d alpha with age except in 4 senescent (>10 cycles) subjects in which these two parameters were decreased. Longitudinal experiments confirmed this ten dency. The plasticity of responses to Eight, resynchronization after a shif t of the day-night cycle, or shift of activity onset after presentation of a light pulse at various circadian times was unaffected by aging. Taken tog ether, the data demonstrate that the parameters of the circadian activity-r est rhythm remain stable over a long span of life and/or that light remains a powerful entertaining parameter even in very old individuals.