Enrichment-dependent differences in novelty exploration in rats can be explained by habituation

In rats, exploratory activity and emotional reactivity towards novel stimuli reflect independent biological functions that are modulated differently by rearing experiences. Environmental enrichment is known to improve performance in exploratory tasks, while having inconsistent effects on emotionality. This study examined the effect of environmental enrichment on the behaviour of rats in two exploratory tasks. Male rats were reared under one of four conditions, differing in social and non-social complexity. At 9 weeks of age, exploration of a novel open field, and exploration of novel objects in the same open field following 24 h habituation, was assessed. Differences in social and non-social complexity of the rearing environment had inconsistent effects on exploration in the novel open field. In contrast, when rats were faced with novel objects in an otherwise familiar environment, exploration habituated faster with increasing stimulus complexity of the non-social environment. The social environment had no effect on this latter test. These findings indicate that environmental enrichment affects exploratory activity primarily through its effect on habituation to novelty. This effect depends on relative stimulus complexity of the rearing environment, but is independent of social factors. The present results further suggest that aversive tasks can obscure the expression of enrichment-dependent differences in habituation to novelty. (C) 2001 Elsevier Science B.V. All rights reserved.