Scientists have discovered a neural mechanism that shapes our aversion to select individuals, opening the door to understanding how the brain drives our dislike of others.
A team including researchers from the University of Tokyo made these findings in a study on mice, and what happens in their brains when they encounter an individual they see as a threat.
“We have begun to understand the mechanism that rewrites the evaluation of like or dislike based on experience, while the memory of who the individual is remains intact,” says University of Tokyo Professor Teruhiro Okuyama, a behavioral neuroscience expert.
In the study, the team set out to see if specific behavior towards an individual would impact how the animal on the receiving end perceived future encounters.
After introducing two mice to one another and letting them interact, the scientists changed things up. One mouse was given a drug that made it more aggressive, resulting in it attacking another animal when introduced to it.
In future encounters, the mouse that had been attacked would avoid the aggressive individual, seemingly having formed a memory of previous unpleasant interactions.
When the researchers examined neurons in the attacked mouse's brain, they found altered circuitry suggesting that these negative social interactions had actually changed nerve cell pathways.
Connections between specific social memory cells in the hippocampus and fear-responsive cells in the amygdala had strengthened after the individual had experienced aggression from the other animal.
In the study, published in Science, the team notes that behavioral changes appeared to be driven by these changes in the brain.
"Social animals recognize familiar conspecifics and selectively avoid harmful ones," the researchers write. "As social relationships shift, continuous updating of social valence is essential, yet the underlying neural mechanisms remain unclear.
"Here, by artificially transforming a previously neutral conspecific into an aggressive one, we show that valence updating depends on enhanced synaptic connectivity and physiological changes within the hippocampal ventral CA1 (vCA1)-basolateral amygdala (BLA)-nucleus accumbens (NAc) circuit," they add.
"Following defeat, social memory engram neurons in the vCA1 strengthened their connections with BLA neurons carrying negative valence. The vCA1-BLA-NAc neural circuit flexibly regulates adaptive social behaviors."
The team focused on the hippocampus, a key player in memory processing, and the amygdala, which regulates our response to fear.
By weakening neuronal connections in these specific areas using optogenetics, the scientists were able to reverse that avoidance behavior seen in the attacked mice, which performed a sort of brain reset in the animal.
To demonstrate that this was more than memory of an interaction, the scientists stimulated the neurons they'd identified as creating this risk-averse behavior. By giving a mouse a brief electrical shock during an encounter with another individual, the mouse then began avoiding its non-aggressive "friend" even though there was no attack.
So while it's easy to say that yes, of course we avoid people we associate with unpleasant experiences, it may be tied to fundamental messaging pathways that go beyond memory alone.
The study is limited to mice and their brains, so whether the same neuronal shift occurs in humans is yet to be demonstrated. But it does hold the potential to advance treatment of mental health conditions such as social anxiety disorder and depression.
After all, mice are seen as an important gateway in developing therapeutics for human neurodevelopmental conditions.
The research was published in the journal Science.
Source: University of Tokyo via The Japan Times
Fact-checked by Mike McRae
