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Aggression on the brain
24 JANUARY 2008

Super Bowl Super Speculation: Is aggression hard-wired in the brain?
Players in heavy padding launch into action on a green field watched my thousands of fansScientists don't always deliver the results we want, and here's a case in point. For decades, they have said that dopamine -- a feel-good signaling molecule in the brain -- is a big cheese in the drive for sex, mind-bending drugs and food.

Those activities are "rewarding," in neuro-talk, and dopamine carries the reward signal.

Think there's some strange satisfaction to be derived from the meat-grinder called a football game? New research on aggression suggests where it originates. Photo: Jeff Miller, UW-Madison

There were also suggestions that dopamine helped deliver a similar reward from aggression, and now comes proof that it does -- in mice. Psychologists have long studied the aggressive response of a male mouse to an intruder, says Craig Kennedy, a professor of special education at Vanderbilt University. "The 'resident' mouse will usually turn sideways, rattle its tail as a threat response, run toward the other ['intruder'] animal, wave its paws, and box, wrestle or bite."

In the past, when experimenters gave mice a drug that blocks the dopamine receptor (and thus prevents the brain from understanding the dopamine signal), the mice responded less aggressively to an intruder.

But these drugs reached the whole brain, and they also made the mice more sluggish. Was the lowered level of aggression due to blocking dopamine, or to generalized indolence?

Pink globule hovers over flat surface with blue cylinders poking above and below surface
Dopamine is a "feel-good" neurotransmitter. After release, dopamine can bind to dopamine receptors on the nearby neuron, carrying a rewarding signal that's linked to food, sex, drugs and perhaps aggression. When you block a neurotransmitter, you block its signal. Graphic: NIDA

This is your brain on drugs
To isolate the role of dopamine in the reward system, Kennedy's graduate student Maria Couppis installed a tube to the nucleus accumbens, a brain region where dopamine signals make mice want to eat, have sex, or take drugs like heroin or cocaine. Dropping in the dopamine-blocker acted like a switch, Kennedy said, adding that the experimental setup "allowed us to separate the motor effects from the rewarding effects. When the animal had dopamine suppression in its reward pathways, it had no motivation to earn aggression as a reward."

You read that right. Getting to intimidate and attack a male intruder was considered a "reward." If that sounds odd, you must not understand the "pleasures" of war, football, or pro wrestling.

Woman holds two strawberry ice cream cones, luxuriously licking cone in left hand The researchers started the experiment by teaching the manly mice to punch a button with the nose, causing the intruder to appear, so the resident could have the pleasure of intimidating and attacking it. The mice learned this lesson in just two days, only a bit slower than they need to learn that nose-poking can produce chow.

Considering the well-established role that food plays in survival, "We were actually quite surprised" by how effectively aggression could change mouse behavior, Kennedy says.

The neural rewards for food occur in the same place as the rewards for sex. Need we say more? Photo by Jeff Miller, UW-Madison

Shall we box?
In the new aggression study, the attack only happened when the dopamine was working; the behavior disappeared when the dopamine-blocking chemical was present.

So the dopamine system is essential to the rewards of aggression: No dopamine signal, no reward. We shudder to imagine the box office if pro football had to survive in a dopamine-free universe...

And no matter how blighted the reputation of aggression, it's a trait that can be useful far beyond the sports arena. "You see aggression from flies all the way to human beings," Kennedy says, "probably because it serves a very adaptive function" in protecting and gaining access to resources such as mates, food and territory.

On human aggression
Although there is no proof at present that dopamine rewards aggression in people as it does in mice, the dopamine signal is immensely powerful in humans, Kennedy says. "Dopamine is highly involved in rewarding for sex, drugs of abuse and food. The same neurotransmitters seem to be involved, the same regions of the brain ... but nobody has looked at whether aggression in humans triggers the same region" as it does in mice.

A dozen men in red and white shirts and shorts, dive into a pile after a ball
USA vs. England in a rugby match. Are we talking aggression with a capital "A"? Photo: USA Rugby

Kennedy will approach the "where's it at?" question by studying college guys as they watch football. If the nucleus accumbens is involved, as it is in mice, the next step would be to look directly at the role of dopamine in human aggression. "What drives our mouse research is trying to understand some of the neurobiological regulation of human aggression," says Kennedy, who studies aggressive behavior in autistics.

And when one surveys the non-stop carnage of the 20th century, it's unnerving -- but explanatory -- to think that aggression could be as rewarding as sex or drugs. But that may not be the end of the story, Kennedy adds. "Maybe humans retain some kind of animal vestiges of wanting to be aggressive, but because society does not sanction that, humans have come up with sports and other activities, like watching football or boxing, that tap into the same kind of mechanism, but are vicariously rewarding."

It's a nice thought, and it does give us one more excuse to waste the weekend watching helmeted fellows crash into each other in the name of victory: "My dopamine made me do it!"

Although aggression is often associated with the "fight or flight hormone" adrenaline, we could soon need to make room for dopamine, the ancient, adaptive Dr. Feelgood of the nervous system. "Maybe it's not an adrenaline rush," says Kennedy, "but more of dopamine rush, or both, that rewards aggression."

- David Tenenbaum

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• The rewarding effect of aggression is reduced by nucleus accumbens dopamine receptor antagonism in mice, Maria H. Couppis & Craig H. Kennedy, Psychopharmacology, DOI 10.1007/s00213-007-1054-y.

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