Mental Health

Aggressive Behavior Is Hardwired, And It May Lie In This Part Of The Brain

sneak attack
Researchers say they have pinpointed the area of the brain where violence is premeditated: the VMHv1 in the hypothalamus. Michael Bentley, CC by 2.0

Aggression is common to humans and animals. This behavior requires no learning, many scientists believe, due to an underlying neural circuit hardwired in the brain. A team of NYU Langone Medical Center researchers say they have located the area of the brain where violence is premeditated; a part of the hypothalamus commonly referred to as VMHv1 (ventrolateral part of the ventromedial hypothalamus). So far, however, the team has only studied this phenomenon in mice.

The hypothalamus controls body temperature, hunger, and sleep, according to the National Institutes of Health. Because it receives signals from all parts of the nervous system and then produces hormones via the pituitary gland, the hypothalamus is believed to be the essential link between the endocrine (hormones) and nervous systems. The ventromedial hypothalamus is a subunit of the medial hypothalamus, which has been implicated in behavioral control.

“The medial hypothalamus is a key component of the vertebrate social decision-making network and has a long-established role in inter-male aggression,” explained Dr. Dayu Lin, senior author, and her colleagues in the introduction to the new study.

Lin, an assistant professor at the Neuroscience Institute, and her team wondered which area of the hypothalamus drives aggressive behavior in situations where no provocation has occurred.

Angry Mice 

To answer this question, Lin and her co-authors began by training male mice to attack weaker male mice. Like many other animals, mice are territorial and will usually become aggressive when provoked, such as when they fear losing dominance of their space.

“In the wild, male mice exhibit stalking behavior if they suspect foreign encroachment,” explained Lin and her co-authors in the introduction to their study.

After training a set of commando mice, the team next placed each of these tiny aggressors into a controlled environment where aggression could be monitored. Specifically, the scientists measured the number of times each aggressive mouse poked his nose through a hole where another mouse might enter his space — in other words, these trained mice appeared to be itching for a fight. At the same time, because Lin’s past studies linked aggression to the VMHvl, the team also tracked brain activity transpiring in the hypothalamus of each mouse. Their probes tracked nerve activity occurring before, during, and after each mouse attack.

What happened was this: Nerve cell activity in the VMHvl peaked just before the mice began to poke their noses into the hole. The researchers explain this poking occurred before the aggressive mice could even smell or see an intended victim. Then, within moments after a weaker mouse appeared, nerve cell activity in the same brain region (of the aggressive mice) increased by as much as 10 times.

Exploring this brain reaction, Lin and her colleagues genetically manipulated the trained-for-violence mice in order to end VMHvl activity. At this point, nearly all aggressive actions ceased in the trained mice, though other learned behaviors, such as poking for a tasty treat, did not stop. While this reveals the VMHv1 is not essential to some reward activities, the researchers say the experiment does not “rule out a potential role for the VMHvl in other social or sexual seeking behaviors.”

Based on the overall results, Lin says her lab has pinpointed the brain circuits essential to aggressive motivations, which build up within animals as they premeditate and prepare to attack. Going forward, Lin plans to investigate which specific nerve cells and circuits in the VMHvl are involved in aggression.

Source: Faulkner A, Grosenick L, Davidson TJ, Deisseroth K, Lin D. Hypothalamic control of male aggression-seeking behavior. Nature Neuroscience. 2016.

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