The effect of 2G on mouse circadian rhythms

This study examined the effect of chronic 2G exposure on the regulation of body temperature (T(b)), activity (ACT), and circadian rhythms of mice. Five mice were implanted with biotelemetry units to record T(b) and ACT. The mice exhibited a stable daily mean of T(b) (37.1 +/- 2.1 degrees C) and ACT and robust circadian rhythms during the control 1G period. Mice exhibited a significant decline in T(b) (30.1 +/- 1.5 degrees C; t(4)=8.32, p<.01) and cessation of ACT within two hours following 2G onset. After 6 hours of continuous 2G exposure there was a recovery in T(b) (34.4 +/- 1.6 degrees C) that remained significantly below that of baseline (t(4)=3.66, p<.05). A similar pattern of recovery was seen following 12 hours of continuous 2G for ACT. A slower pattern of adaptation toward baseline levels occurred steadily over the next 6-13 days. Exposure to 2G also caused an immediate 4 day loss in circadian rhythm amplitude in both T(b) and ACT. Recovery to new steady state levels was achieved by 8 days and 13 days, respectively. These results demonstrate that under chronic 2G, the recovery time for the homeostatic steady-state values and circadian rhythms are shorter for the mouse than for the rat. These differences may be related to the scaling effects of 2G resulting from the mass difference between mice and rats.