Aggression Avenue: Tracing How the Brain Is Wired for Violence

By Gaidos, Susan | Science News, March 21, 2015 | Go to article overview

Aggression Avenue: Tracing How the Brain Is Wired for Violence


Gaidos, Susan, Science News


Male mice in David Anderson's Caltech laboratory are typical rodents. Mellow most of the time, they'll defend their cages if provoked--chasing male intruders away. The mice will lunge and nip until one admits, paws up, social defeat. Rarely do they actually hurt each other.

But with the flick of a switch, Anderson's team can convert an ordinary lab mouse into a vicious brute that won't back down. Like Bruce Banner morphing into the Hulk, the mouse seems to have no choice but to let the monster spring forth, inflicting bite after bite on its cowering victim.

To draw out the animal's natural aggression, scientists activate a small group of nerve cells, or neurons, identified by Anderson's group and others, that act as a control center for aggressive behavior. Turning on those brain cells instantly increases a mouse's appetite to fight.

During these confrontations, the scientists map the physical and chemical connections among the cells to track the neural roots of violent behavior. Such rodent scuffles may seem a far cry from, say, the aggression of a school bully or cold-blooded killer. But researchers think some of the same circuits in the brain maybe involved.

The brain's attack neurons reside in an area called the ventromedial hypothalamus, a region long linked to aggression in animals. Scientists began closing in on those neurons in 2011 in the process of discovering a way to transform docile mice into angry goons.

Since then, researchers have narrowed the list of neurons involved. Last May in Nature, Anderson's team described a small group of neurons that specifically escalate aggression. Nirao Shah's team at the University of California, San Francisco independently identified the same neurons.

"In 2011, we knew the neighborhood where these neurons lived, but we didn't know the exact houses or streets in that neighborhood," Anderson says. "We now know that."

This aggression hub in the hypothalamus doesn't work alone. It is part of a network of brain structures. From this neighborhood, where attack behavior is organized, each neuron connects to others throughout the brain to produce the features associated with fighting: a racing heart, increased metabolism and the muscle movements involved in charging or biting. By studying nerve signals coming into the hub and tracking those moving out, scientists aim to build a circuit diagram, showing how the brain puts the entire body into a fight state--or holds aggressive impulses in check.

Anderson calls this hub "our beachhead into the brain for studying aggression."

Details of the map are still emerging. In some areas, neurons related to aggression differ in males and females. And neurons clustered in a second brain region of the mouse also induce an attack when activated, scientists reported February 3 in Cell Reports. These cells incite aggressive behavior in males and females alike.

Experiments in flies and birds reveal attack-promoting neurons in brain regions similar to those found in mice. Such studies of attack centers in animals promise a better understanding of how aggression is organized and produced in the brain: Is it distributed diffusely or controlled by a few key nodes? Because the human brain is wired pretty much the same as the mouse's, the studies may lend insight into human aggression as well.

Bad neighborhoods

Occasional outbursts of aggression are common, even normal, in animals from birds to mice to humans. Faced with a genuine threat, most creatures will raise their voice or take physical action to fend off an attack.

Neurons that elicit attack behaviors sit in the hypothalamus--a region in the center of the brain. Formed by dozens of small clusters of cells, the hypothalamus governs many basic functions such as feeding, metabolism, body temperature, thirst, fatigue and sleep. It also regulates basic drives, such as mating and aggression. …

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