Serotonin Keeps Aggression in Check

by Scott P. Edwards

September, 2008

Many people think of serotonin as nature’s “feel-good” chemical. The most widely distributed—and most widely studied—neurotransmitter in the brain, serotonin regulates a vast range of psychological and biological functions, including mood, sleep, arousal and appetite.

Now, researchers at Cambridge University and UCLA have found that serotonin also plays a critical role in regulating emotions such as impulsive aggression during social decision making. Impulsive aggression is the tendency to respond with hostility or aggression when faced with serious frustration.

The researchers believe their results suggest that serotonin plays a critical role in social decision making by normally keeping aggressive social responses in check.

By manipulating diet, the researchers were able to lower serotonin levels in the brains of healthy volunteers. Tryptophan, the essential amino acid necessary for the body to produce serotonin, can be obtained only through diet. Twenty healthy volunteers fasted overnight and were then given a protein drink, receiving drinks with tryptophan one day and without it the next.

The Ultimatum Game

On both days, the study participants played the “Ultimatum Game,” in which one player poses a way to split a sum of money with a partner. If the partner accepts the offer, both players are paid accordingly. If the offer is rejected, however, neither player is paid. Some of the offers were considered “fair” (45 percent of the cash), while others were considered “unfair” (18 percent of the cash). When the players’ serotonin levels were low they showed increased aggression toward the offers they perceived to be unfair.

The findings highlight why some people become aggressive or act impulsively when they have not eaten, says lead researcher Molly Crockett of the University of Cambridge Behavioral and Clinical Neuroscience Institute. Because the body needs tryptophan to make serotonin, levels of the neurotransmitter naturally fluctuate throughout the day, depending on food intake. Foods such as chocolate, oats, bananas and poultry, especially turkey, are rich in tryptophan.

“Tryptophan is an essential amino acid, and you have to get it in your diet, which is not hard to do,” says psychiatrist Emil Coccaro, an expert on impulsive aggression and mood disorders at the University of Chicago. He notes that when someone is given a drink containing amino acids other than tryptophan, the liver pulls tryptophan from the blood to make the protein with the new amino acid the person was just given. This process causes a drop in blood tryptophan levels to about 10 percent to 20 percent of what they are normally, leading to a reduction in the amount of newly made serotonin and, thus, a decrease in serotonin activity.

Under normal circumstances, serotonin works in the frontal areas of the brain to inhibit the firing of the amygdala, the almond-shaped structure that controls fear, anger and other emotional responses.

“If there is less serotonin in the frontal brain areas, there will be less inhibition of the amygdala,” says Coccaro. “When the amygdala is stimulated by outside novel and potentially threatening events, it will become more active, driving the person to act on their impulses.”

People who suffer from mood disorders or impulsiveness caused by low serotonin levels can be treated with a class of drugs called selective serotonin reuptake inhibitors, or SSRIs, which enhance neurotransmission and make more serotonin available to nerve cells in the brain.

Although Crockett’s study examined low serotonin levels in healthy people, she says she is not certain what the results of a similar study would be in people with disorders in which low serotonin levels are implicated, such as depression or obsessive compulsive disorder.

“It’s possible that lowering serotonin would have a similar or perhaps even more extreme effect, since the system is probably more vulnerable in these individuals,” she said in an e-mail. “On the other hand, it’s equally plausible that tryptophan depletion would have a negligible effect since serotonin is already low and lowering it more may not change much in terms of behavioral output.”