Fluctuations¹（波动） of serotonin（血清素） levels in the brain, which often occur when someone hasn't eaten or is stressed, affects brain regions that enable people to regulate anger, new research from the University of Cambridge has shown. Although reduced serotonin levels have previously² been implicated³ in aggression⁴, this is the first study which has shown how this chemical helps regulate behaviour in the brain as well as why some individuals may be more prone⁵ to aggression. The research findings were published September 15, in the journal Biological Psychiatry⁶.

For the study, healthy volunteers' serotonin levels were altered by manipulating their diet. On the serotonin depletion⁷ day, they were given a mixture of amino acids that lacked tryptophan（色氨酸） , the building block for serotonin. On the placebo⁸ day, they were given the same mixture but with a normal amount of tryptophan.

The researchers then scanned the volunteers' brains using functional⁹ magnetic resonance¹⁰ 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 low brain serotonin made communications between specific brain regions of the emotional limbic system（边缘系统） of the brain (a structure called the amygdala) and the frontal lobes¹¹ weaker compared to those present under normal levels of serotonin. The findings suggest that when serotonin levels are low, it may be more difficult for the prefrontal cortex to control emotional responses to anger that are generated within the amygdala（扁桃体） .

Using a personality questionnaire, they also determined¹² which individuals have a natural tendency to behave aggressively. In these individuals, the communications between the amygdala and the prefrontal cortex was even weaker following serotonin depletion. 'Weak' communications means that it is more difficult for the prefrontal cortex to control the feelings of anger that are generated within the amygdala when the levels of serotonin are low. As a result, those individuals who might be predisposed to aggression were the most sensitive to changes in serotonin depletion.