Photic and nonphotic effects on the circadian activity rhythm in the diurnal rodent Arvicanthis ansorgei

The main purpose of the study was to compare behavioural properties of entrainment to photic (30 min; 200 lx) and nonphotic (melatonin: 1 h; 100 μg) stimuli in the diurnal rodent Arvicanthis ansorgei. Male animals (n = 38) were used, and running wheel activity was recorded. Following entrainment to 12:12 h LD the animals were transferred to DD (dim red light) to freerun before treatment started. A phase response curve (PRC) to light was determined showing a phase delay region in the early subjective night (CT 8-16) and a phase advance region in the late subjective night (CT 18-4). Activity onset defined CT = 0. Entrainment to daily phase advance and phase delay light pulses occurred at circadian phases corresponding to the respective phase shift regions of the PRC. Similarly, also entrainment to daily melatonin pulses occurred in two narrow time windows located near the beginning (CT 0) and the end of the subjective day (CT 10), but where light had a phase advance effect melatonin had a phase delay effect and vice versa. These results are consistent with the neurobiological model of Hastings et al. (Chronobiol Int 1998;15:425-445) on the differential effects of photic and nonphotic resetting cues on the circadian pacemaker.