The freerunning period () of the circadian pacemaker underlying the wheel-running activity rhythm of was found to be unaffected by the periods of environmental cycles (maternal and light/dark) under which the mice are raised. Mice born to mothers entrained to periods (T) of 28 or 20 h (ratio of light to dark of 14/10) and maintained on those cycle until beyond puberty showed only a temporary difference in freerunning period when placed into constant darkness. Such temporary 'after-effects ' of entrainment were shown, as had been previously, to occur in animals exposed to non-24-h cycles as adults only.After-effects on the ratio of activity to rest () were not even temporarily different in animals raised on T = 28 or T = 20.Rearing on T = 28 or T = 20 did not affect the abilities of animals to entrain to these cycles later in life.Measurements from young and old animals as well as remeasurement of the young animals later in their lives revealed several effects of age on the pacemaker: a) After-effects on freerunning period after T = 28 or T = 20 are not greater but last longer in older animals; b) Freerunning period is shorter in younger animals; and c) The ratio of activity to rest changes over time in constant darkness and is greater in young animals. Together these suggest that pacemaker 'plasticity' reflected in changes in and over time in constant darkness decreases with age.The length of gestation measured in 'real' time was the same in mice entrained to T = 28 or T = 20, demonstrating that gestation is not measured in circadian cycles.
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