Rapid phase resetting of a mammalian circadian rhythm by brief light pulses

The phase-shift (delta phi) responses of the circadian rhythm in the field mouse Mus booduga to brief light pulses (LPs) of 15 minutes duration and 1000 lux intensity were measured in 90 experiments. In each experiment, a resetting light pulse LP1 was administered at CT14 (CT, circadian time), and a scanning light pulse LP2 was then variously administered in separate experiments at CT16, CT20, and CT22 in the same and in the next circadian cycle. The delta phi obtained in all these two-pulse experiments did not differ significantly from theoretical values computed on the assumption that LP1 reset the phase response curve (PRC) rapidly. In each case, the steady-state delta phi observed after LP1 and LP2 differed significantly from the delta phi obtained at the same CT in determination of the single-pulse PRC (control) and also differed significantly from the values on the assumption of no delta phi in the PRC following LP1. These results indicate that the circadian pacemaker of M. booduga, as measured by its PRC, is substantially reset within 2h after a light pulse at CT14.