Phase-shifting effects of acute increases in activity on circadian locomotor rhythms in hamsters

Experiments were conducted in golden hamsters to examine the relationship between induced acute increases in locomotor activity and phase shifts in the circadian clock underlying the rhythm of activity. Injections of the short-acting benzodiazepine triazolam (TZ) 6 h before the onset of activity resulted in an acute increase in activity and a phase advance in the rhythm of activity; injections of TZ induced larger phase shifts in animals housed without running wheels than in those housed with wheels. Transfer to a cage with access to a running wheel for 1 h at different circadian times induced large phase advances (mean of 2 h) and small phase delays depending on the circadian time of transfer. Maximal mean phase advances resulted when animals were transferred to a cage with wheel 3 h before activity onset, and at this circadian time there was a significant correlation between the magnitude of the phase shift and the amount of increase over baseline activity for the first hour after transfer. These results indicate that access to a running wheel in animals housed without wheels can be a significant phase-shifting stimulus to the circadian clock and that the phase shifts induced by injection of TZ or transfer to a new cage with wheel are related to the activity state of the animal or to the amount of locomotor activity that is induced at particular times.