Serotonin levels affect the brain’s response to anger
21 September 2011
Although reduced serotonin levels have previously been implicated in aggression, this is the first study to show how this chemical helps regulate behaviour in the brain, as well as why some individuals might be more prone to aggression.
For the study, serotonin levels in healthy volunteers were altered by manipulating their diet. On the serotonin depletion day, the volunteers were given a mixture of amino acids that lacked tryptophan, the building block for serotonin. On the placebo day, they were given the same mixture but with a normal amount of tryptophan.
Researchers at the University of Cambridge, who led the study, then scanned the volunteers' brains using functional magnetic resonance imaging (fMRI) as they viewed faces with angry, sad and neutral expressions. Using the fMRI, they were able to measure how different brain regions reacted and communicated with one another when the volunteers viewed angry faces, as opposed to sad or neutral faces.
The research revealed that when levels of serotonin in the brain are low, communications between the frontal lobes and the specific brain regions that are involved in processing emotion - including the amygdala - are weaker than those present under normal levels of serotonin. The findings suggest that low levels of serotonin may make it more difficult to control emotional responses to anger.
Using a personality questionnaire, the researchers also determined which individuals have a natural tendency to behave aggressively. In these individuals, the communications between the amygdala and the frontal lobes were even weaker when serotonin was depleted. As a result, those individuals who might be predisposed to aggression were the most sensitive to changes in serotonin depletion.
Dr Molly Crockett, who worked on the research as a PhD student at Cambridge's Behavioural and Clinical Neuroscience Institute, said: "We've known for decades that serotonin plays a key role in aggression, but it's only very recently that we've had the technology to look into the brain and examine just how serotonin helps us regulate our emotional impulses. By combining a long tradition in behavioral research with new technology, we were finally able to uncover a mechanism for how serotonin might influence aggression."
Dr Luca Passamonti, who worked on the research as a visiting scientist at the Cognition and Brain Sciences Unit of the Medical Research Council in Cambridge, said: "Although these results came from healthy volunteers, they are also relevant for a broad range of psychiatric disorders in which violence is a common problem.
"For example, these results may help to explain the brain mechanisms of a psychiatric disorder known as intermittent explosive disorder (IED). Individuals with IED typically show intense, extreme and uncontrollable outbursts of violence which may be triggered by cues of provocation such as a facial expression of anger.
"We are hopeful that our research will lead to improved diagnostics as well as better treatments for this and other conditions."
The findings were published in the journal 'Biological Psychiatry'.
Image: An angry face. Credit: teapics on Flickr.
Reference
Passamonti L et al. Effects of acute tryptophan depletion on prefrontal-amygdala connectivity while viewing facial signals of aggression. Biological Psychiatry 2011 (epub ahead of print).
