The Effects Of Different Light-dark Cycles On Growth Rate And Food Intake Of Mice

Objective: The adaptation of the endogenous rhythm of an organism to external cycles may be influence the development of physiological processes on animals. This study was designed to investigate effects of different light/dark (LD) cycle on growth rate, food intake, the level of plasma corticosterone and growth hormone of mice, and the relationship among them. Method: 2-week-old male mice were raised under different period length of LD cycle including LD5/5 (light: 5h; dark: 5h), LD12/12 (light: 12h; dark: 12h) and LD16/16 (light: 16h; dark: 16h) for four weeks and measured the growth rate and food intake every week. Plasma growth hormone and growth hormone levels were measured by standard radioimmunoassay. Results: Mice in LD5/5 and LD16/16 groups manifested higher locomotor activity, level of plasma corticosterone and growth hormone concentrations, and growth rate than that in LD12/12 group. The locomotor activity of the animals of groups LD5/5 and LD16/16, expressed as number of counts per hour, was higher in those animals corresponding to group LD12/12 (P < 0.001). The level of plasma corticosterone in LD12/12 group was lower than that in LD5/5 and LD16/16 groups (P = 0.022). The level of plasma growth hormone in LD12/12 group was lower than that in LD5/5 and LD16/16 groups (P=0.010). The increase of body weight in animals was lowest in LD12/12 group (P < 0.001). Group LD12/12 had the lowest food efficiency level (P=0.003). Conclusions: The results suggest that different LD cycle may affect many physiological processes including growth rate and food intake, and the change of growth rate in different LD cycle may be related to the level of corticosterone and growth hormone concentrations; food efficiency seems to be related to the animal's endogenous period and the period of external LD cycle, indicating the importance of the circadian system in influencing growth rate, food intake and, perhaps, metabolism.