A recent publication in the journal Cell discusses the processes and changes in the brain that arise while fighting and watching intense fights.
Study: To see is to experience: Aggression neurons light up when witnessing a fight. Image Credit: Sergey Nivens / Shutterstock.com
Humans become excited when observing others fight, regardless of gender. A recent study provided the first insights into what happens in the brain and revealed that the ventrolateral part of the ventromedial hypothalamus (VMHvl) in a mouse was activated when the animal observed two other mice fight.
VMHvl has been established as the central region for aggression. VMHvl cells, particularly those expressing the progesterone receptor (PR) that overlap almost entirely with estrogen receptor alpha (Esr1), are necessary to drive aggression in both sexes. In addition, electrophysiological recordings and calcium imaging have identified elevated activity of VMHvlEsr1/PR cells during physical fights.
VMHvlPR cells activate when fighting and observing a fight
In the aforementioned study, VMHvlPR cells in mice were excited when observing a fight and during physical fighting. 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 VMHvlPR cell activation when demonstrator mice fought but not when their interactions were social/peaceful.
Furthermore, eliminating pheromone cues in mice caused no changes in the response of VMHvlPR cells when observing the demonstrators fight. Similarly, cell 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, VMHvlPR cell activation when observing a fight primarily depended on visual inputs, while chemical cues had minimal impact.
Additionally, miniscope fluorescent calcium imaging at single-cell resolution was used to determine if the cells excited when watching a fight were the same cells that were activated when fighting. More than 50% of cells were activated when fighting and observing the fight, thereby evoking a similar brain state.
Observation-activated cells modulate aggression
Next, the researchers investigated whether cells activated during fight observation could modulate fighting. This was tested by trapping observation-activated cells and expressing different ligand-gated inhibitors and activators.
When these trapped cells were inhibited, a significant decline in aggression was evident. Conversely, activating trapped cells caused the mice to attack more.
This effect was specific to the fight-observation-trapped cells. That is, inhibition of cells trapped when observing a peaceful interaction did not affect aggression, which suggests that fighting- and fight-observation-activated cells were highly overlapping. Manipulating the observation-activated cells was sufficient to influence aggression.
The functional significance of activating the same aggression-driving VMHvl cells when watching a fight remains elusive. Fighting is not only a motor response but is also associated with autonomic responses, including elevated heart rate, pupil dilation, and breathing.
Furthermore, winning a fight triggers dopamine release, a reward signal, possibly due to signals from the hypothalamus. Thus, activation of aggression cells in the hypothalamus may allow the observer to share the excitement of the fight experienced by the executor.