Purpose

It has been well established in mammals that circadian behavior as well as the molecular clockwork can be synchronized to the light-dark (LD) cycle via the suprachiasmatic nucleus of the hypothalamus (SCN). In addition to light, it has been demonstrated that non-photic time cues, such as restricting the time of food availability, can alter circadian behavior and clock gene expression in selected peripheral tissues such as 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 hours of light followed by 12 hours of darkness (12L: 12D) then exposed to 2 hours of voluntary or involuntary exercise during the light phase for 4 weeks. 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 hours per day for 2 weeks) in order to compare the effects of two non-photic cues on PER2∷ LUC bioluminescence. Skeletal muscles, lung and SCN tissue explants were cultured for 5-6 days 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 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.