The researchers found that mice who are more susceptible to social defeat show increased levels of a growth factor known as brain-derived neurotrophic factor (BDNF) in a portion of the brain integral to reward- and emotion-related behaviours. Mice that seem to cope better with the same stressful circumstances don´t show the same chemical rise. BDNF promotes plasticity in the brain, presumably enabling new connections between neurons, the researchers explained, a process which is considered the cellular basis for learning and memory.
“The increase in BDNF may have an adaptive role normally, allowing an animal to learn that a situation is bad and avoid it in the future,” said Eric Nestler of The University of Texas Southwestern Medical Center. “But under conditions of extreme social stress, susceptible animals may be `over-learning´ this principle and generalizing it to other situations.”
An individual´s emotional response to acute stresses, such as terrorist acts, or to more prolonged chronic stress, such as a divorce, is determined by genetic and environmental elements that interact in complex and poorly understood ways, Nestler said. To shed some light on the biological basis of individual differences in stress responses the researchers repeatedly forced small mice into aggressive interactions with larger mice.
Nestler´s group revealed that all of the defeated animals had signs of anxiety, but “some of the mice show a syndrome with features that are similar to post traumatic stress disorder or depression. Others don´t,” Nestler said. In addition to their lonely lifestyle, the susceptible mice showed significant weight loss and less interest in sugar, both of which are consistent with a depression-like state, they reported. They traced the differences in stress response to BDNF, revealing that the susceptible animals had a 90 percent greater concentration of the growth factor in the brain´s reward circuits than normal mice did. Levels of BDNF in more resilient mice remained steady.
MEDICA.de; Source: Cell Press