Environmental enrichment alters plus maze, but not maternal defense performance in mice

Maternal defense behavior (or maternal aggression) is a highly conserved behavior for protecting offspring that is normally associated with decreased fear and anxiety in rodent dams. Environmental enrichment can decrease indices of fearfulness and anxiety (and elevate novelty seeking) in laboratory animals. This study examined whether enrichment could alter levels of maternal aggression and elevated plus maze performance in lactating mice. One group of female mice was exposed to a series of novel objects for 1.5 months while the other group was not. As expected, mice that had been exposed to an enriched environment showed significantly more entries to and time on the open arms of an elevated plus maze while they were lactating. Enriched mice showed similar maze performances when tested 3 months later in a non-lactating state, even though enrichment had been removed for 3 months. Further, offspring of enriched dams exhibited maze performance similar to that of their dams, suggesting possible epigenetic influences on maze performance. In contrast, no differences in measures of maternal aggression or pup retrieval were detected between enriched and control lactating mice. Together, these results indicate that environmental enrichment has long lasting effects on plus maze performance, but does not alter maternal aggression. The findings also suggest that whatever neural pathways are altered by enrichment do not strongly overlap or regulate those governing maternal aggression.