Short-term environmental enrichment decreases the mnemonic response to estrogen in young, but not aged, female mice.

The present study was designed to examine if 4 weeks of exposure to an enriched housing environment affects the ability of estrogen to facilitate object recognition in young and aged female mice. Object recognition was tested using a novel object recognition task. Ovariectomized young and aged female mice were maintained in standard or enriched housing for 4 weeks prior to and then throughout object recognition testing. Immediately after training, mice were injected intraperitoneally with vehicle or 0.2 mg/kg 17 beta-estradiol and then were re-tested 24 and 48 h later. Among young females, estradiol alone improved object recognition at both delays relative to chance, an effect not present in enriched females treated with estradiol. Enrichment alone had no significant effect on object recognition in young females at either delay. In contrast, enrichment alone in aged females significantly enhanced both 24- and 48-h object recognition relative to chance, an effect not present in mice treated with both enrichment and estradiol. Estradiol alone had no effect on object recognition in aged females at either delay. Together, these data indicate that estradiol and enrichment alone differentially affect object recognition in young and aged females. However, the fact that the combination of estradiol and enrichment treatments did not affect object recognition at either age suggests that co-administration of both treatments is less effective than the most effective single treatment at each age.