[O-KS-09-3] Disruptive Effect of Simulated Microgravity on Circadian Activity Rhythm of Rats and Intensity-Dependent Counter Effect of Intermittent Centrifugation
Keywords:Weightlessness, Artificial Gravity, Circadian Rhythm
【Purpose】
Although weightlessness（e.g. bed rest, space flight）disrupts circadian rhythms, appropriate countermeasures are not yet developed. Artificial gravity using centrifugation could be a promising option. However, few studies have investigated artificial gravity as a countermeasure to the alteration of circadian rhythms. The purpose of this study was to examine whether and to what extent the intermittent gravitation attenuates the disruption of circadian activities.
【Methods】
Eight-week old male Wistar rats were hind-limb unloaded by their tail for 2 weeks（unloading period）followed by another 2-week free rearing（recovery period）. They were distributed into 5 groups of 8 individuals according to intensity of the gravitation during the unloading period：（1）Free Cage Control（Ctrl）；（2）Continuously Unloaded（UL）；（3）Reloaded for 1 hour a day（+1G）；（4）Centrifuged at 1.5G（+1.5G）；（5）Centrifuged at 2.5G（+2.5G）. Muscle weight of hindlimb extensors and Gross Motor Activity（GMA）using implanted activity logger were evaluated.
【Results】
Muscle weights of intervened groups were recovered to the Ctrl level（p＞0.05）eventually following transient decrease at the end of the unloading period（p＜0.01）. On the other hand, mean GMA exhibited distinct disruptions. After transient declines during the unloading period（p＜0.01 to Ctrl）, they were still lower than Ctrl except for +1.5G（p＞0.05 for +1.5G, p＜0.01 for others）. During the unloading period, diurnal activities（resting phase for rats）, were more affected（higher than Ctrl, p＜0.01）than nocturnal（p＞0.05）.
【Discussion】
Simulated microgravity by hindlimb unloading alters activity level of rats. Although gravitational interventions during unloading period could attenuate these disturbances, excess stimuli could impair the favorable effects. It implies reaction of gross activity to gravitational stimuli varies along the bell-shaped curve.
Although weightlessness（e.g. bed rest, space flight）disrupts circadian rhythms, appropriate countermeasures are not yet developed. Artificial gravity using centrifugation could be a promising option. However, few studies have investigated artificial gravity as a countermeasure to the alteration of circadian rhythms. The purpose of this study was to examine whether and to what extent the intermittent gravitation attenuates the disruption of circadian activities.
【Methods】
Eight-week old male Wistar rats were hind-limb unloaded by their tail for 2 weeks（unloading period）followed by another 2-week free rearing（recovery period）. They were distributed into 5 groups of 8 individuals according to intensity of the gravitation during the unloading period：（1）Free Cage Control（Ctrl）；（2）Continuously Unloaded（UL）；（3）Reloaded for 1 hour a day（+1G）；（4）Centrifuged at 1.5G（+1.5G）；（5）Centrifuged at 2.5G（+2.5G）. Muscle weight of hindlimb extensors and Gross Motor Activity（GMA）using implanted activity logger were evaluated.
【Results】
Muscle weights of intervened groups were recovered to the Ctrl level（p＞0.05）eventually following transient decrease at the end of the unloading period（p＜0.01）. On the other hand, mean GMA exhibited distinct disruptions. After transient declines during the unloading period（p＜0.01 to Ctrl）, they were still lower than Ctrl except for +1.5G（p＞0.05 for +1.5G, p＜0.01 for others）. During the unloading period, diurnal activities（resting phase for rats）, were more affected（higher than Ctrl, p＜0.01）than nocturnal（p＞0.05）.
【Discussion】
Simulated microgravity by hindlimb unloading alters activity level of rats. Although gravitational interventions during unloading period could attenuate these disturbances, excess stimuli could impair the favorable effects. It implies reaction of gross activity to gravitational stimuli varies along the bell-shaped curve.