Behavioral feedback regulation of circadian rhythm phase angle in light-dark entrained mice

Department of Psychology, Simon Fraser University, Burnaby British Columbia, Canada V5A 1S6 Induced and spontaneous wheel running can alter the phase and period ( ) of circadian rhythms in rodents. The relationship between spontaneous running and the phase angle ( ) of entrainment to 24-h light-dark (LD) cycles was evaluated in C57BL/6j mice. With a wheel freely available, was significantly correlated with the absolute ( r = 0.32) and relative ( r = 0.44) amount of activity during the first 2 h of the activity period. When wheels were locked during the first half of the night in LD and then unlocked in constant dark (DD), mice exhibited a delayed and lengthened compared with mice that had wheels locked during the second half of the night. In DD, correlated negatively with total daily activity. To evaluate if wheel running modulates the phase-resetting actions of LD, phase shifts to light pulses were measured at two time points in DD, when daily activity levels differed by 40%. Phase delays to light were 56% greater when activity levels were lower. However, in a counterbalanced follow-up experiment, phase advances and delays to light pulses were not affected by the availability of wheels, although an effect of time in DD was replicated. Spontaneous activity can regulate and without altering the response of the pacemaker to light. wheel running; entrainment; nonphotic zeitgeber; phase shifts; light pulses