Aggression neurons fire when watching fights

A recent publication in the journal Cell discusses the processes and changes in the brain that occur during combat and observing intense combat.

Study: To see is to experience: the neurons of aggression light up when they witness a fight. Image Credit: Sergey Nivens / Shutterstock.com

Background

Humans get excited when they watch others fight, regardless of gender. A recent study provided the first insight into what happens in the brain and found that the ventrolateral part of the ventromedial hypothalamus (VMHvl) in a mouse was activated when the animal watched two other mice fight.

VMHvl was established as the core region of aggression. VMHv1 cells, particularly those expressing the progesterone receptor (PR) which almost entirely overlaps with the estrogen receptor alpha (Esr1), are required to induce aggression in both sexes. Additionally, electrophysiological recordings and calcium imaging identified elevated VMHvl activity.^Esr1/PR cells in physical combat.

VMHvl^PR cells activate when fighting and watching a fight

In the aforementioned study, VMHvl^PR mouse cells were excited when watching a fight and during a physical fight. Here, the authors used fiber photometry to record population calcium activity as the observer mouse watched two demonstrator mice interact through a perforated transparent barrier. This revealed VMHvl^PR cell activation when demonstrator mice were fighting but not when their interactions were social/peaceful.

Additionally, knocking out pheromone cues in mice caused no change in the VMHvl response.^PR cells watching the protesters fight. Similarly, cellular responses were unaffected when volatile signals were minimized.

There was no response when the experiment was performed under infrared light, which made the fight invisible. Thus, VMHvl^PR cellular activation during combat observation was primarily dependent on visual inputs, while chemical cues had minimal impact.

In addition, single-cell resolution miniscope fluorescent calcium imaging was used to determine whether cells excited during combat observation were the same cells that were activated during combat. More than 50% of the cells were activated when fighting and watching the fight, suggesting a similar brain state.

Observation-activated cells modulate aggression

Next, the researchers investigated whether cells activated during combat observation could modulate combat. This was tested by trapping observed-activated cells and expressing different ligand-dependent inhibitors and activators.

When these trapped cells were inhibited, a significant decrease in aggressiveness was evident. Conversely, activating the trapped cells caused the mice to attack more.

This effect was specific to cells trapped by combat observation. That is, inhibition of trapped cells when observing peaceful interaction did not affect aggression, suggesting that cells activated by fighting and observing fight overlapped strongly. Manipulation of observation-activated cells was sufficient to influence aggression.

Final remarks

The functional significance of the activation of the same aggression-generating VMHv1 cells during combat observation remains elusive. Fighting is not just a motor response, but is also associated with autonomic responses including elevated heart rate, pupil dilation, and respiration.

Additionally, winning a fight triggers the release of dopamine, a reward signal, possibly due to signals from the hypothalamus. Thus, the activation of aggression cells in the hypothalamus may allow the observer to share in the excitement of combat experienced by the enforcer.