Locking and unlocking of running wheel affects circadian period stability differently in three inbred strains of rats.

Running-wheel access has been shown to shorten the circadian period length (tau) of various mammalian species. Due to the close correlation between tau and the level of activity, running wheel-induced changes of the activity level are thought to be responsible for the observed changes in tau. In the present study, the influence of the running wheel on tau and the activity level was examined in three inbred strains of rats (ACI, BH, LEW). Four animals of each strain had free access to their running wheels, while the wheels of the other 4 animals of each strain were mechanically locked. These conditions were changed twice, so that each animal encountered both kinds of changes, that is, from a locked to an unlocked running wheel and vice versa. During the whole study, overall activity was measured by infrared detectors. Running-wheel access resulted in a significant increase of overall activity in strains LEW and ACI. However, significant changes of tau were observed only in LEW rats. These rats showed a significant shortening of tau after the second change of the housing conditions regardless of whether the wheel was locked or unlocked. Consequently, no causal relationship was found between changes of tau and running wheel-induced changes of overall activity. Instead, the results suggest that subtle environmental influences like locking or unlocking the running wheel affect tau in a strain-dependent manner, whereas changes in the activity level are neither necessary nor sufficient to induce changes of tau.