Pessimists may be born to take a gloomy view, a study hints.
The study is trying to explore any link between genetics and depression.
To that end, researchers at the University of Michigan conducted a study of how the brain responds to stress and negative situations for 181 people.
They focused on a brain chemical called neuropeptide Y or NPY. People genetically predisposed to produce lower levels of NPY showed stronger responses to negatively charged words like "murderer" when scientists scanned their brain activity.
In the first part of the study, participants with low levels of NPY showed a stronger activation in their prefrontal cortex, which is involved in processing emotion.
In another experiment, Dr. Brian Mickey of the university's psychiatry department and his colleagues measured responses after subjects were injected with a salt solution in their jaw muscle. The injection caused moderate pain for about 20 minutes with no long-term damage.
Subjects with low NPY rated their emotional response to the stress more negatively, both when anticipating it and when they reflected on it afterwards, the study showed.
"This tells us that individuals with the risk-associated NPY gene variant tend to activate this key brain region more than other people, even in the absence of stress and before psychiatric symptoms are present," Mickey said in a release.
The findings expand scientists' understanding of the physiology of depression, said the study's senior author, Dr. Jon-Kar Zubieta, a professor of psychiatry and radiology and research professor, a colleague of Mickey's.
The final part of the study found a higher proportion of people with major depressive disorder also had low levels of NPY expression.
"Our findings may eventually have clinical implications," the study's authors concluded in the February issue of the Archives of General Psychiatry.
"The greatest potential for NPY-based biological markers may lie in guiding development of novel antidepressant agents for the many individuals who fail to respond to currently available treatments."
The study can't prove a cause-and-effect relationship, and the findings on research subjects may not apply to the general population, the researchers said.
The study was funded by the U.S. National Institute of Mental Health, the National Institute on Drug Abuse, the National Institute on Alcohol Abuse and Alcoholism, and the Phil F. Jenkins Research Fund.
Zubieta is a consultant for Eli Lilly and Co., an antidepressant maker.