Purpose: It has been well established in mammals that circadian behavior as well as the molecular clockwork can be synchronized to the light-dark cycle via the suprachiasmatic nucleus of the hypothalamus (SCN). In addition to light, it has been demonstrated that nonphotic time cues, such as restricting the time of food availability, can alter circadian behavior and clock gene expression in selected peripheral tissues such as the liver. Studies have also suggested that scheduled physical activity (exercise) can alter circadian rhythms in behavior and clock gene expression; however, currently, the effects of exercise alone are largely unknown and have not been explored in skeletal muscle.
Methods: Period2::Luciferase (Per2::Luc) mice were maintained under 12 h of light followed by 12 h of darkness then exposed to 2 h of voluntary or involuntary exercise during the light phase for 4 wk. Control mice were left in home cages or moved to the exercise environment (sham). A second group of mice had restricted access to food (4 h · d(-1) for 2 wk) to compare the effects of two nonphotic cues on PER2::LUC bioluminescence. Skeletal muscle, lung, and SCN tissue explants were cultured for 5-6 d to study molecular rhythms.
Results: In the exercised mice, the phase of peak PER2::LUC bioluminescence was shifted in the skeletal muscle and lung explants but not in the SCN suggesting a specific synchronizing effect of exercise on the molecular clockwork in peripheral tissues.
Conclusions: These data provide evidence that the molecular circadian clock in peripheral tissues can respond to the time of exercise suggesting that physical activity contributes important timing information for synchronization of circadian clocks throughout the body.