When faced with adversity, some people succumb to debilitating psychological diseases including post-traumatic stress disorder and depression, while others are able to remain remarkably optimistic.
Now, a new mouse study in the October 19 issue of the journal Cell, a publication of Cell Press, reveals that the difference may depend in part on the chemistry of the brains' reward circuits. The findings could point to new psychiatric drugs, and perhaps even new ways to encourage resilience for people in high-stress circumstances, including soldiers in combat, disaster relief workers, and disaster victims, according to the researchers.
They 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 behaviors. 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. They avoid their aggressors, but they also avoid all mice and even other fun things like sugar or sex.”
The observed distinction in the animals' susceptibility to stress was despite the fact that they were all virtually identical genetically and were raised in the same carefully controlled environment, Nestler noted. “There may still be environmental influences that affect the animals' ability to cope with stress later on, such as the dominance hierarchies within a litter,” he said. “But it's just as likely that random or stochastic events during development may play a role.”
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. While a vast literature describes the effects of several kinds of acute and chronic stress on an individual's physiology and behavior, much less has been known about the biological basis of individual differences in stress responses.