Environmental enrichment for adult rats: effects on trait and state anxiety.

Experimental evidence indicates that enriched environment (EE) induces neurobiological and behavioural alterations. EE in early life improves learning and memory and reduces trait and state anxiety. However, the effect of EE established in adulthood has rarely been investigated. Thus, the aim of this study was to evaluate the possibility of modifying the levels of trait and/or state anxiety of adult rats exposed to EE. Seventy adult Wistar male rats were first tested in the free-exploratory paradigm (FEP) and were categorized according to their levels of trait anxiety (high, medium and low). Subsequently, half of the animals from each category returned to their home cages (standard condition: SC) and the other half was transferred to an enriched environment (enriched condition: EC). After three weeks, all animals were again tested in FEP. Seven to 10 days later, fifty of the seventy animals were tested on the elevated plus-maze test (EPM). In FEP, EE reduced locomotor activity in the second exposition independently of the anxiety category and, it decreased the levels of trait anxiety of highly anxious rats. No effect of EE was observed on EPM. In conclusion, EE established in adulthood was able to reduce high trait anxiety, a major risk factor for anxiety disorders.