Numerous studies have shown that the brain molecule neuropeptide Y (NPY) helps to restore calm after stressful events.
The scientists hope the research will eventually help with early diagnosis and intervention for depression and other psychiatric illnesses, and in the development of therapies that can be tailored to individuals based on their genetic profiles.
â€œThis is what we mean when we talk about â€˜personalized medicine,â€™ â€ says the studyâ€™s lead author, Brian Mickey, an assistant professor of psychiatry at the University of Michigan Medical School and researcher at the U-M Molecular and Behavioral Neurosciences Institute.
â€œThese are genetic features that can be measured in any person. We hope they can guide us toward assessing an individualâ€™s risk for developing depression and anxiety.â€
The findings also help fill in new areas on the genetic map of depression, says the studyâ€™s senior author Jon-Kar Zubieta, a professor of psychiatry and radiology and research professor at the Molecular and Behavioral Neurosciences Institute.
â€œWeâ€™ve identified a biomarker - in this case genetic variation - that is linked with increased risk of major depression,â€ Mr Zubieta says. â€œThis appears to be another mechanism, independent of previous targets in depression research, such as serotonin, dopamine and norepinephrine.â€
Using three separate approaches, researchers found that individuals with the genotype that produces lower amounts of NPY had measurably stronger brain responses to negative stimuli and psychological responses to physical pain. They were also overrepresented in a population diagnosed with a major depressive disorder.
Using three separate approaches, each with a varying number of research subjects ranging from 58 to 152, U-M researchers and their partners studied the link between NPY gene expression and emotional processing.
U-M researchers recruited and characterized participants, performed neuroimaging, and conducted the pain challenge. Their partners at the Laboratory of Neurogenetics at the National Institute on Alcohol Abuse and Alcoholism in Bethesda, Maryland performed the genotyping.
The research subjects were classified according to whether they had low, medium or high NPY gene expression.
Using functional magnetic resonance imaging, the scientists observed the brain activity of each subject as he or she viewed neutral words (such as material) negatively charged words (like murderer), and positively charge words (like hopeful).
In response to negative words, subjects in the low NPY group showed strong activation in the prefrontal cortex, which is involved with processing emotion, while subjects with high NPY demonstrated a much smaller response.
â€œThis 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,â€ says Mr Mickey.
In a second test, healthy subjects reported their emotional experiences during a stress challenge. Saline solution was injected into the jaw muscle, which produces moderate pain for 20 minutes, but no lasting harm. The level of pain was adjusted for each person to until it was, for them, a four on a scale of one to 10.
These subjects rated the positivity or negativity of their feelings both before and after the pain challenge. Those in the low NPY group were more negative both before and after the pain - meaning they were more emotionally affected while anticipating the pain and while reflecting on their experience immediately afterward.
Lastly, scientists compared the NPY genotypes of subjects with major depressive disorder with control subjects to see if there was an association between the condition and low expression of NPY.
Subjects with low-expression NPY genotypes were overrepresented in the group with depression.
â€œWeâ€™re not just associating a particular gene with a particular illness,â€ Mr Zubieta says. â€œWeâ€™re expanding the understanding of the physiology of depression.â€
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