Running wheel size influences circadian rhythm period and its phase shift in mice

Running wheels are widely used in studies on biological rhythms. In mice wh eel diameters have ranged from 11 cm to 23 cm. We provided mice with runnin g wheels of two different sizes: 15 cm diameter and 11 cm diameter. The amo unt of running in the 12-h light:12-h dark condition and the endogenous per iod of wheel running in constant darkness was determined over 40 days. On t he Ist day in constant darkness all animals were exposed to a 15-min light pulse at circadian time 13. The animals in the small wheel ran significantl y less both in 12 h light:12 h dark and constant darkness, and showed a lon ger endogenous period in constant darkness compared to animals in the large wheel. Moreover, after the light pulse at circadian time 13, mice in the s mall wheel showed a significantly smaller phase delay in running wheel acti vity than mice in the larger wheels. The data suggest that the magnitude of a photic phase shift depends on the amount and timing of activity the anim als display in relation to this stimulus. It can be concluded that technica l features of the running wheel can influence the circadian period of wheel running.