Denenberg, V. H., Wehmer, F., Werboff, J. et al. 1969. Effects of postweaning enrichment and isolation upon emotionality and brain weight in the mouse. Physiology and Behavior 4, 403-406.

At weaning (21 days) 60 male C57BL/6J mice were placed into isolation cages while 60 others were grouped in enriched environments. Fifteen days later half of each group was shifted to the opposite housing condition while the other half continued to live in the same environments in which they had been placed at weaning. At the end of the second 15-day interval (51 days) one-third of the mice were killed immediately to obtain resting levels of plasma corticosterone while the remaining mice were placed into a novel environment, They were killed either 15 or 30 min later and their blood assayed for corticosterone; in addition, the number of boluses defecated while in the novel environment was recorded. Finally, whole brain weights, adrenal weights, and body weights were obtained on all animals. The groups did not differ on the corticosterone measure; enrichment during the second 15-day interval significantly increased adrenal weight; and enrichment immediately after weaning increased defecation while enrichment during the second phase of the study decreased defecation. The lack of relationship among these measures raises serious questions concerning the concept of emotionality in the mouse. Finally, animals which did not undergo a shift in housing environments had heavier brains than the two groups which did shift. This finding is not in agreement with data from the rat.