Serotonin Fluctuation Can Increase Levels of Aggression

A new study is the first to show how serotonin helps regulate behavior in the brain.

Reduced serotonin levels have been implicated in aggression, but a new study is the first to show how the chemical helps regulate behavior in the brain. It also explains why some individuals may be more prone to aggression when serotonin levels fluctuate.

The research from the University of Cambridge in the United Kingdom was published recently in Biological Psychiatry.

“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,” co-first author Molly Crockett, PhD, of the University of Zurich, said in a statement.

“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,” added Crockett, who conducted the research while a doctoral students at Cambridge’s Behavioural and Clinical Neuroscience Institute.

For the study, researchers altered the serotonin levels of healthy volunteers by manipulating their diet. On the serotonin depletion day, 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.

As the volunteers viewed faces with angry, sad, and neutral expressions, researchers scanned their brains using functional magnetic resonance imaging (fMRI). With 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 low brain serotonin made communications between specific brain regions of the amygdala and the frontal lobes weaker compared to those present under normal levels of serotonin. When serotonin levels are low, the findings suggest, it may be more difficult for the prefrontal cortex to control emotional responses to anger that are generated within the amygdala.

A personality questionnaire also allowed them to determine which individuals have a natural tendency to behave aggressively. It is more difficult for the prefrontal cortex to control the feelings of anger that are generated within the amygdala of these individuals when serotonin levels are low. Thus, individuals who might be predisposed to aggression were the most sensitive to changes in serotonin depletion.

“These results may help to explain the brain mechanisms of a psychiatric disorder known as intermittent explosive disorder [IED],” co-first author Luca Passamonti said in a statement. “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.”

SourcesSerotonin Levels Affect the Brain’s Response to Anger [Cambridge University]Effects of Acute Tryptophan Depletion on Prefrontal-Amygdala Connectivity While Viewing Facial Signals of Aggression [Biological Psychiatry]