It has been reported that animal behavior is influenced by both genetic and environmental factors. Although many studies focus on genetic factors associated with behaviors, it is an important issue to elucidate how environmental factors influence behaviors and cause individual difference. Studies by Levine, Denenberg and their colaborators have shown that repeated handling by humans during the pup period affect the stress response of the rat in adulthood. In two papers presented today, Meaney and colleagues tried to elucidate behavioral and molecular basis that influence changes in stress response in rats.

Please also read the following papers before reading this paper

Maternal care, hippocampal glucocorticoid receptors, and hypothalamic-pituitary-adrenal responses to stress (PDF)
Dong Liu, Josie Diorio, Beth Tannenbaum, Christian Caldji, Darlene Francis, Alison Freedman, Shakti Sharma, Deborah Pearson, Paul M. Plotsky, Michael J. Meaney
Science. 1997 Sep 12;277(5332):1659-62.