Inhibitory avoidance, pain reactivity, and plus-maze behavior in Wistar rats with high versus low rearing activity.

Substantial work has shown that rats although identical in strain, sex, age and housing conditions can differ considerably in terms of behavior and physiology. Such individual differences can be rather stable and may be detected by behavioral screening tests. Here, the degree of behavioral activation in a novel open-field situation has been shown to serve as a useful predictor to classify animals of a given population into sub-groups with high or low activity, based on measures like locomotion or rearing activity. We used such a screening test and assigned larger samples of male adult Wistar rats into those with high versus low rearing activity (HRA/LRA). They were then tested in the elevated plus-maze, in an inhibitory avoidance task, and in two tests of pain reactivity (hot-plate, tail-flick). In the open field, HRA rats not only showed more rearing behavior, but also more locomotor activity than LRA rats. In the plus-maze, HRA rats again showed more rearing behavior. Also, they spent less time in the open arms, and entered the closed arm more often than low responder rats, which is indicative of more anxiety-related behavior than in LRA rats. In the inhibitory avoidance test, HRA and LRA rats showed similar basal step-in latencies, whereas HRA rats had shorter retention scores than LRA rats after experience of footshock, especially when using a higher (0.5 mA) shock intensity. In contrast, repeated exposure to the avoidance apparatus without shock did not affect step-in latencies in either group. In the pain test, HRA and LRA rats behaved similarly, indicating that their differences in inhibitory avoidance behavior were probably not determined on the level of pain processing. The relevance of these findings is discussed in the context of previous work, especially with respect to the role of processing of aversive experiences.