Scheduled voluntary wheel running activity modulates free-running circadian body temperature rhythms in Octodon degus

Entrainment of the circadian pacemaker to nonphotic stimuli, such as schedu led wheel-running activity, is well characterized in nocturnal rodents, but little is known about activity-dependent entrainment in diurnal or crepusc ular species. In the present study, effects of scheduled voluntary wheel-ru nning activity on circadian timekeeping were investigated in Octodon degus, a hystricomorph rodent that exhibits robust crepuscular patterns of wakefu lness. When housed in constant darkness, O. degus exhibited circadian rhyth ms in wheel-running activity and body temperature (Tb) with an average peri od length (tau) of 23.39 +/- 0.11 h. When wheel running was restricted to a fixed 2-h schedule every 24 h, tau increased on average 0.39 +/- 0.09 h bu t did not result in steady-state entrainment. Instead, relative coordinatio n between the fixed running schedule and circadian timing was observed. tau was greatest when scheduled wheel running occurred at CT 20.5 (0.4 h great er than DD baseline tau). Scheduled running activity also influenced Tb wav eform symmetry, reflecting concomitant changes in the circadian activity-re st ratio (alpha:rho). Aftereffects of the scheduled wheel-running paradigm were also observed. In 2 animals, tau lengthened from 23.20 and 23.80 h to 24.14 and 24.15 h, respectively, and remained relatively stable for approxi mately 1 month during the wheel schedule. Although behavioral activity appe ars to be a weak zeitgeber in this species, these data suggest that nonphot ic stimuli can phase delay the circadian pacemaker in O. degus at similar t imes of the day as in nocturnal hamsters and mice, and in humans.