A single episode of severe stress can be enough to kill off new nerve cells in the brain, research suggests. Rosalind Franklin University researchers believe their finding may give new insights into the development of depression. Working on rats, they found that cells were lost in the hippocampus, an area of the brain which processes learning, memory and emotion. The study features in the Journal of Neuroscience. The researchers found that in young rats, the stress of encountering aggressive, older rats did not stop the generation of new nerve cells in the hippocampus. However, it did prevent the cells from surviving - leaving fewer new neurons for processing feelings and emotions. The hippocampus is one of two regions of the brain that continues to develop new nerve cells throughout life, in both rats and humans. Treatment hope The researchers believe the loss of cells could be one cause of depression. However, their work also raises hope of possible treatments to stop acute stress from contributing to mood problems. They found that cells tended to die not immediately following a stressful situation, but after a delay of 24 hours or more. In principle, they argue it could eventually be possible to administer treatment during this time to prevent cells being lost. The researchers put each young rat in a cage with two older rats for 20 minutes. The older rats quickly pinned down, and in many cases, bit the intruder. The young rats had stress hormone levels six times as high as those who were not caged with older animals. Microscope analysis However, microscopic analysis of brain tissue showed that their ability to generate new cells in the hippocampus remained undimmed. This seemed to disprove a previous theory that stress hormones put a brake on the generation of new cells. A week after the encounter, however, only a third of the new cells had survived. Long-term survival of nerve cells was also compromised. In another part of their study, the researchers marked newborn cells in the hippocampus, and subjected rats to stress a week later. At the end of the month they counted a third fewer fully developed nerve cells. Lead researcher Dr Daniel Peterson said the next step was to understand how stress reduced cell survival. Mixed results Professor David Kendall, from the University of Nottingham, said previous research had shown that longer-term, unpredictable mild stress could depress nerve cell generation in the hippocampus. That study suggested the key seemed to be a reduction in production of a hormone that helps keep brain cells alive. However, Professor Kendall said there was also evidence to suggest that mild stress could be protective. "You might remember the issue of London cabbies allegedly having bigger hippocampi related to the stress of acquiring "The Knowledge". "The rule of thumb seems to be; a little stress is good for you but severe/unpredictable stress is bad."