Is aggressive behavior something that sometimes happens just because we like it? As this ScienCentral news report explains, research done on mice suggests we enjoy it the same way we enjoy other addicting activities.
Researchers have found that in mice, the brain responds to aggression in the same way as other pleasures. Craig Kennedy, chairman of the Special Education Department at Vanderbilt University, says the work shows, "for mice aggression is rewarding and that dopamine's involved in that rewarding affect in the same areas of the brain that's rewarding for drugs, sex, can't say rock and roll with mice, but anything that seems to be pleasant."
Whenever we do something we like, brains in both animals and people release a drug called dopamine. When it attaches to receptors in the brain, it creates a pleasure circuit, sending out a signal of good feeling. Researchers are studying the role dopamine plays in a number of activities ranging from alcohol, drugs, and gambling, to food and even exercise.
As an educator interested in students with special needs, Kennedy has seen children with autism who show aggression towards other people. He's spent much of the past 25 years trying to understand what he calls the "neurobiology of aggression." He says, "that's led us to working with mice in the lab to try to ask some questions about what goes on in the brain during aggression."
He notes that aggression is present in "all kinds of animals, from flies to monkeys," and adds, "Being aggressive can be very adaptive (useful) in the wild. It allows you to gain access to food, maintain territory, access to mates, protect offspring; so it's a very adaptive set of behaviors." Kennedy emphasizes, however, that when too much aggression shows up in today's humans, it sometimes presents a problem.
Researchers have long suspected a relationship between dopamine and aggression – from experiments where they turned off dopamine in mice – but they were not able to prove it. Kennedy says, "Unfortunately, when you deliver something that makes dopamine less active in the brain, it also makes it very hard to move." He adds, "There was this puzzle of: Is it because the animal can't move that they're no longer aggressive or have they lost their motivation to be aggressive?"
Writing in the journal Psychopharmacology he and co-author Vanderbilt University doctoral candidate Maria Couppis, explained how they were able to deliver the anti-dopamine drug just to that tiny part of the mouse brain that's associated with reward and pleasure.
They then identified aggressive male mice. They did this by putting an intruder mouse on the other side of a door. If the test mouse learned to nose bump a target to open the door and get to the other mouse, the researchers knew they had an aggressive mouse.
They then put a chemical in the aggressive mouse's brain to counteract dopamine and repeated the intruder mouse experiment. Kennedy explains, "As we more and more deactivated dopamine in the brain, the mouse is less and less interested in earning access (to the intruder mouse)."
While the study was on mice and not people, because there are many similarities between mouse and human brains, Kennedy says it's possible that humans may have the same dopamine reaction with aggression. He adds, "It's likely that if aggression is adaptive for various animals in the wild, and human beings have evolved from those animals, it's also very rewarding for human beings, and has been very adaptive in the past for us."
Since dopamine is involved in many activities that are addictive in humans, Kennedy is now studying "the question of whether access to aggression is addictive and does it show the same type of addictive behaviors and properties and brain effects that drugs like cocaine and heroin produce in animals."
Kennedy is also testing humans by "imaging their brain activity" to see if the brain's pleasure centers react when volunteers are shown pictures displaying acts of aggression.
This research was published online the week of Jan.14, 2008 by the journal Psychopharmacology, and was funded by a Discovery Grant from Vanderbilt University.