14 DAY TRIAL // A new scientific study led by researchers at the Cold Spring Harbor Laboratory (CSHL) argues that the link between stress and social behaviors may be more complex than first estimated. In a series of experiments conducted on lab mice, the team determined that rodents exposed to stress early on in life exhibited disturbed social behaviors and psychoemotional states during adulthood.
A similar conclusion may also be valid for humans, the investigators argue in the March 25 online issue of the open-access, peer-reviewed journal PLoS ONE. The research group was led by CSHL associate professor Grigori Enikolopov, who says that different types of stress stimuli were used for this study.
The impact of chronic, early-life stress on adults has been an area of heavy interest for behavioral neuroscientists over the past few years, with numerous experiments conducted to assess the nature of this link. However, the picture that emerged following these efforts was that there is no clear-cut mechanism underlying this connection.
Other experts have demonstrated that this type of stress can adversely influence the neuroendocrine system, including the hypothalamic-pituitary-adrenal (HPA) axis. The latter is in charge of controlling corticotropin, gulocorticoid, and other stress hormones, as well as some endocrine glands. The new work confirms that hostile adolescence environments can have these effects in adulthood.
In the experiments, mice just 1 month old (adolescents by comparison to humans) were locked for 2 weeks in the same cage with a violent adult mouse. The two were only separated by a perforated, transparent obstacle, and the younger rodents were exposed to attacks from the more aggressive male every day for short periods of time. This type of stress is called social-defeat stress.
Using behavioral tests, the team then sought to determine the impact of such stress on the mice. “The tests assessed levels of anxiety, depression, and capacity to socialize and communicate with an unfamiliar partner,” Enikolopov explains. Test animals were later much less likely to interact with other mice socially, were more anxious and helpless, and were worse at communicating with other mice.
Neuroscientists have also determined that less neurogenesis (nerve cell growth) occurred in mice in the test group as opposed to their peers that were not stressed out. The worst-affected area was the subgranular zone in the dentate gyrus, a portion of the hippocampus that is also affected by depression in humans.
Interestingly, the team found that socially-stressed mice who were allowed to rest for a few weeks in a tranquil environment returned to normal in terms of social behaviors. Their neurogenesis levels also rebounded. “This shows that young mice, exposed to adult aggressors, were largely resilient biologically and behaviorally,” Enikolopov concludes.