Short-term but not long-term exposure to an enriched environment reduces unconditioned fear responses but not conditioned fear responses

Environmental enrichment (EE) has been shown to produce beneficial effects in animal models of a wide variety of neurological and psychiatric disorders. EE exhibits antidepressant function; reduces anxiety, improves spatial learning and memory impairment. EE can reduce sensitivity to loss of reward by reducing frustration-like emotional states and facilitates the extinction of conditioned fear. However, some studies related to the emotional effects of EE present controversial results such as reduction or increase in anxiety. The time of exposure to an enriched environment seems to be an important factor in the behavioral responses presented by animals subjected to aversive stimuli. The present study compared the effects of two and four week exposure to EE with young adult Wistar rats under the same conditions and protocol on fear behavioral parameters in the face of footshock (unconditioned fear) and on re-exposure to an environment after electrical shock pairing (conditioned fear). We showed that the EE with a duration of two weeks reduced the freezing response of the animals in an unconditioned fear situation, that is, with the aversive stimulus present in the environment, however, did not influence the same behavior in a conditioned fear situation. In addition, the short-term EE developed the locomotor and exploratory activity, identified by the high rearing behavior, which may also suggest a low level of anxiety in these animals. We can conclude that EE changes the unconditioned fear responses of young adult rats. In addition, the duration of EE interferes differently, being two weeks of treatment with EE sufficient to cause improvement in coping with unconditioned aversive situations. We suggest that the emotional benefits resulting from the welfare provided by EE can be abolished by the longer duration of this treatment, due to the already known effect of tolerance to lasting or abundant rewards.