Dissociation of the circadian rhythm of locomotor activity in a 22 h light-dark cycle impairs passive avoidance but not object recognition memory in rats.

We analyzed the effect of dissociation of the circadian rhythm of locomotor activity on the performance in two memory tasks in rats. One group of animals was maintained in a normal 24 h light-dark cycle of 12:12 (T24 group, control). A second group was housed in a 22 h cycle of 11:11 (T22 group, experimental), a condition which is known to produce dissociation of the circadian rhythm of locomotor activity in two components. Both groups were tested on two memory tasks: passive avoidance and object recognition. An additional control group, kept under constant darkness (DD group), was used for a passive avoidance task. Testing occurred 30 min (short-term memory--STM) and 24 h (T24 and DD group) or 22 h (T22 group) (long-term memory--LTM) after training. The T22 group showed impairment on the passive avoidance task (STM and LTM) compared with the T24 and DD groups. On the object recognition task, the T22 and T24 groups performed similarly in all the sessions. In conclusion, circadian rhythm dissociation induced a performance deficit in the passive avoidance task but had no effect on the object recognition task. We suggest that dissociation of the circadian rhythm of locomotor activity may selectively affect some emotional component related to fear and risk evaluation.