JOURNAL OF BIOLOGICAL RHYTHMS / February 2001Kas, Edgar / SCHEDULED ACTIVITY IN OCTODON DEGUS Scheduled Voluntary Wheel Running Activity Modulates Free-Running Circadian Body Temperature Rhythms in Octodon degus

Abstract Entrainment of the circadian pacemaker to nonphotic stimuli, such as scheduled wheel-running activity, is well characterized in nocturnal rodents, but little is known about activity-dependent entrainment in diurnal or crepuscular species. In the present study, effects of scheduled voluntary wheel-running activ-ity on circadian timekeeping were investigated in Octodon degus, a hystricomorph rodent that exhibits robust crepuscular patterns of wakefulness. When housed in constant darkness, O. degus exhibited circadian rhythms in wheel-running activity and body temperature (Tb) with an average period length (τ) of 23.39 ± 0.11 h. When wheel running was restricted to a fixed 2-h schedule every 24 h, τ increased on average 0.39 ± 0.09 h but did not result in steady-state entrainment. Instead, relative coordination between the fixed run-ning schedule and circadian timing was observed. τ was greatest when sched-uled wheel running occurred at CT 20.5 (0.4 h greater than DD baseline τ). Sched-uled running activity also influenced Tb waveform symmetry, reflecting concomitant changes in the circadian activity-rest ratio (α:ρ). Aftereffects of th