Different effects of intensity and duration of locomotor activity on circadian period

An outstanding unresolved issue in chronobiology is how the level of locomotor activity influences length of the free-running, endogenous circadian period (tau). To address this issue, the authors studied a novel model, 4 replicate lines of laboratory house mice (Mus domesticus) that had been selectively bred for high wheel-running activity (S) and their 4 unselected control (C) lines. Previous work indicates that S mice run approximately twice as many revolutions/day and exhibit an altered dopaminergic function as compared with C mice. The authors report that S mice have a tau shorter by about 0.5 h as compared with C mice. The difference in tau was significant both under constant light (control lines: tau = 25.5 h; selected: tau = 24.9 h) and under constant dark (control lines: 23.7 h; selected: 23.4 h). Moreover, the difference remained statistically significant even when the effects of running speed and time spent running were controlled in ANCOVA. Thus, something more fundamental than just intensity or duration of wheel-running activity per se must underlie the difference in tau between the S and C lines. However, despite significant difference in total wheel-running activity between females and males, tau did not differ between the sexes. Similarly, among individuals within lines, tau was not correlated with wheel-running activity measured as total revolutions per day. Instead, tau tended to decrease with average running speed but increase with time spent running. Finally, within individuals, an increase in time spent running resulted in decreased tau in the next few days, but changes in running speed had no statistically significant effect. The distinctions between effects of duration versus intensity of an activity, as well as between the among- versus within-individual correlations, are critical to understanding the relation between locomotor activity and pace of the circadian clock.