stress

” A single, socially stressful situation can kill off new nerve cells in the brain region that processes learning, memory, and emotion, and possibly contribute to depression, new animal research shows.

Researchers found that in young rats, the stress of encountering aggressive, older rats did not stop the generation of new nerve cells-the first step in the process of neurogenesis. But stress did prevent the cells, located in the hippocampus, from surviving, leaving fewer new neurons for processing feelings and emotions.”
http://www.sfn.org

Stress and Neurogenesis in the Rat
Rosanne M. Thomas, Gregory Hotsenpiller, and Daniel A. Peterson
The Journal of Neuroscience, March 14, 2007, 27(11)
(see pages 2734–2743)