Shedding light on circadian clock resetting by dark exposure: differential effects between diurnal and nocturnal rodents

The master circadian clock in mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, is entrained by light and behavioural stimulation. In addition, the SCN can be reset by dark pulses in nocturnal rodents under constant light conditions. Here, the shifting effects of a dark pulse on the SCN clock were detailed at both a behavioural and molecular level in a nocturnal rodent (Syrian hamster), and were compared to those of a diurnal rodent ( Arvicanthis ansorgei ). Four‐hour dark pulses led to phase advances in the circadian rhythm of locomotor activity from subjective midday to dusk in hamsters, but from subjective dusk to midnight in Arvicanthis . Moreover, dark pulses had no resetting effect during the middle of the subjective night in hamsters, while such a dead shifting zone occurred during most of the subjective day in Arvicanthis . The behavioural phase advances in both hamsters and Arvicanthis were most often accompanied by marked downregulation of the clock genes Per1 and/or Per2 in the SCN, and also by changes in the transforming growth factor‐α expression, a neuropeptide that suppresses daytime activity in nocturnal mammals. Despite that both hamsters and Arvicanthis showed dark‐induced phase advances at circadian time‐12, Per1 gene and its protein PER1 were downregulated in Arvicanthis but not in hamsters. Altogether these results show that dark resetting of the SCN is always associated with downregulation of Per1 and/or Per2 expression, and mostly occurs during resting. Thus, the circadian window of sensitivity to dark differs between nocturnal and diurnal rodents.