Fight or flight? Researchers uncover the neural basis of aggression

  • Scientists have been trying to understand the body's stress response and have studied the fight or flight response for many years
  • A new study by researchers in Rome looked at the role of the VMH in mice when placed in this situation
  • The findings, the authors say, could contribute towards a better understanding of how emotions are regulated 

“Fight or flight”, described as an automatic physiological reaction to a stressful or frightening event, is understood to be driven by our sympathetic nervous system.

According to the latest research on the topic, the ventromedial hypothalamus (VMH) plays a key role in this type of scenario.

The VMH is a central node in the brain that receives sensory input from the amygdala (responsible for emotions and emotional behaviour) and sends outputs to motor areas of the brainstem.

Triggering defensive responses

The researchers, from the Gross group at the European Molecular Biology Laboratory in Rome, suggest that this position midway between sensory input and motor output makes the VMH the ideal focus to understand how threats drive behaviour.

The team therefore investigated how the VMH may be involved in the decision between attacking and escaping a threat. To do this, they measured the activation of neurons in mice exposed to more aggressive mice.

When the mice were under threat, the researchers found that the activity of a large class of neurons increased relative to the intensity of the threat, confirming their hypothesis that the VMH may encode an internal state of threat necessary to trigger defensive responses.

"These results can contribute to understanding how emotions like fear and aggression are regulated, especially in the context of territory," Piotr Krzywkowski – who conducted the research as a PhD student in the Gross group – said in a statement by EMBL.

The study was published in the journal eLife

Additional discoveries

In addition to the above discovery, the team also observed the activation of the same neurons when the mice returned to explore the place where they had been threatened, despite the current absence of threat.

A second set of neurons also became activated when the mice returned to their home cage, they wrote.

By simply looking at the firing of neurons in their VMH, the scientists could predict precisely if the mice were located in the "threat cage" or home cage.

This demonstrates that the VMH encodes spatial context, they wrote, adding that this function has never been previously attributed to the hypothalamus.

Exposure to a more aggressive mouse was also found to dramatically increase the ability of the VMH to promote flight.

When a mouse’s VMH was artificially activated in this situation, the animal rapidly ran away from a threat. However, this didn’t occur when the VMH was activated before this situation, showing that social experience can change the VMH.

The researchers are currently trying to understand what mechanisms might be involved in this transformation.

Adapting survival behaviours

"This finding has important implications for the field, because previous work had argued that the VMH is hardwired to respond to threats," said study leader Cornelius Gross.

"Our view holds that the VMH is dedicated to controlling both attack and flight, and that this choice is driven by its encoding of social space. When an animal is in its own territory, it favours attack, but when it is in the territory of another animal it favours flight."

Most importantly, the team’s findings suggest a novel role of the hypothalamus in behaviour where, “rather than being viewed as an innate behavioural response region, the hypothalamus should be seen as a region that integrates present and past sensory and contextual information, processing the level of threat and adapting survival behaviours to a changing environment".

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