Mapping the brain's aggression pathways
Maybe this sounds familiar… Some jerk pulls in front of you and cuts you off.  You’re already running late and…arghh!  You’re gonna explode!

Whether it’s road rage, an argument at work, or a spat with your spouse, it seems we are hard-wired, under certain circumstances, for aggression.

Kent State University’s Heather Caldwell studies the brain chemistry behind behaviors like aggression…in mice.

In Caldwell's lab, grad student Shannah Witchey examines an image of a section of the brain of a mouse who got in a fight with another mouse.

Witchey patiently counts the individual neurons that were part of the aggressive behavior. 

Caldwell says that the signalling for the fight impulse came from a part of the brain called the hypothalamus.

The data is helping the team understand the underlying chemistry of aggression and the role two hormones in particular play in its expression - vasopressin and oxytocin.

The roles of vasopressin and oxytocin 
These two hormones show-up in all animals, from earthworm to human, but Caldwell says the details of how they shape behavior is still a mystery.

“They don’t cause behavior," according to Caldwell, "what they do is increase or decrease the probability that a behavior will happen within a specific context.”

And the context can vary from a mother defending her pups to a male attacking a rival.

She says vasopressin and oxytocin are sister hormones that act differently in different parts of the body, and in the two sexes.

Oxytocin is known as the ‘love hormone’ because of its role in promoting bonding and trust in both sexes. Mothers are awash with it as they nurse their young. It also promotes uterine contractions during childbirth.

Vasopressin has roughly the opposite effect, prompting aggression, especially between rivals.  But it also promotes monogamy and parental care in males. Vasopressin also regulates the body’s retention of water. 


Brain hormones guide social behavior
Mapping aggression in the brain is a complicated process, says Michael Potegal of the University of Minnesota, because it comes in different flavors, at least in animals.

Potegal lists three types of aggression- predatory behavior in carnivores, fighting between animals of the same species, and defense against attack, either by predators or animals of the same species.

And Potegal says the neural circuitry is slightly different for each type of aggression.

Heather Caldwell says vasopressin and oxytocin not only influence behaviors like aggression and attraction, they help determine social relationships. 

"You have a lot of neural machinery set-up for aggressive behavior.” - Randy Nelson

Caldwell says, “who you are as a social individual changes the way that the brain hormones affect the system itself.”

For example, she says dominance behavior is reinforced because winning fights changes the brain chemistry.

“If you have had successful fights, you kind of feel like you’re a winner, like you can do it again," according to Caldwell, "and some of that is cellular changes in the brain.”

Aggression, according Randy Nelson, head the neuroscience department at Ohio State University, is part of who we are.

He says, when you want something, one way to get it is to fight, whether it’s mates or food or territory, and the victor wins, "so you have a lot of neural machinery set-up for aggressive behavior.”


Oxytocin and the roots of aggression
But Nelson says society’s way of treating violence with … violence, is not much better than our primitive forebears.

“And I just think we can do better than that frankly,” says Nelson.

And he says, part of the advances needed to treat anti-social behavior, is Caldwell’s work into the roots of aggression.

Caldwell this month received a grant from the National Science Foundation to study how oxytocin affects fetal neural development.

She’s finding that a lack of the nurturing hormone from the mother leads to increased aggression in offspring later in life, especially among males.

The study will be the first to look at the prenatal role for oxytocin in adult aggressive behavior.