These findings, published in the scientific journal eLife (elifesciences.org/articles/46797), may help explain how long-term exposure to psychological stress and trauma increases the risk of mental illness and addiction.
"We already know that chronic psychosocial adversity can induce vulnerability to mental illnesses such as schizophrenia and depression," explains lead author Dr Michael Bloomfield, Excellence Fellow and leader of the Translational Psychiatry Research Group at University College London, UK. "What we're missing is a precise mechanistic understanding of how this risk is increased."
To address this question, Dr Bloomfield and his colleagues used an imaging technique called positron emission tomography (PET) to compare the production of dopamine in 34 volunteers exposed to an acute stress. Half of the participants had a high lifetime exposure to psychosocial stress, while the other half had low exposure. All of them undertook a task to make them feel stressed for a short period of time called the Montréal Imaging Stress Task, which involved receiving criticism as they tried to complete mental arithmetic.
Two hours after this task, the participants were injected with small amounts of a radioactive tracer. This acts a bit like a due to allow the scientists to measure dopamine production in their brains using PET scans. The scans revealed that in those with low exposure to chronic adversity, dopamine production was proportional to the degree of threat that the person perceived.
In people with high exposure to chronic adversity, however, the perception of threat was exaggerated whilst their production of dopamine was impaired. The researchers found that other physiological responses to stress were also dampened in this group. For example, their blood pressure and cortisol levels did not increase as much as in the low-adversity group in response to stress.
"This study can't prove that chronic psychosocial stress causes mental illness or substance abuse later in life by lowering dopamine levels," Dr Bloomfield cautions. "But we have provided a plausible mechanism for how chronic stress may increase the risk of mental illnesses by altering the brain's dopamine system."