Time of day modulation of conditioned place preference in rats depends on the strain of rat used

In golden hamsters, the expression of a conditioned place preference (CPP) or avoidance (CPA) is regulated in a circadian pattern such that the preference and avoidance are exhibited strongly at the circadian time of prior training, but not at other circadian times. In the rat, reports are conflicting regarding whether time of day learning is evident. We investigated whether this conflict arises because different strains of rat have been used. In this experiment, Long Evans and Wistar rats were trained at a specific circadian time to discriminate between a context paired with food reward and an unpaired context. Animals were then tested for preference at the same or a different circadian time. Long Evans rats showed preference for the paired context at both times tested, whereas Wistar rats showed preference only when training and testing times matched. The results show that time of day learning can be generalized to rats using the Wistar strain. (C) 2004 Elsevier Inc. All rights reserved.