Laboratory mice. Credit: unoL/ iStock/ Getty Images Plus.
Understanding how rodents feel and communicate emotions can shed light on the evolutionary origin and mechanisms of human empathy. With that goal, researchers at the Italian Institute of Technology in Genoa and University of Milan tried to understand the neural and social factors behind altruistic and selfish behaviours in mice1.
“There was already evidence in rats, whose helpful behaviour has been demonstrated2 a decade ago” explains Diego Scheggia from the University of Milan, first author of the study. “Now we investigated and demonstrated that social contagion is part of the behaviour of mice as well.”
The team used an experimental task based on game theory, the so-called dictator game — one of the most common methods in human social psychology and economics for measuring how behaviours are shared. The researchers adapted it to mice, to understand whether they can take actions that favour their co-specific mates at their own cost. The task offered animals the choice of sharing a reward with their peers or not. About 70% of them preferred sharing and were labelled as altruistic. After the first experiments, the scientists introduced a challenge for the altruistic animals, prompting them to sacrifice all the reward and leave it to their mate. Again, there was a difference in behaviours between mice, with some animals embracing the extra effort to benefit their peers.
Several factors influence these pro-social actions. “First of all, [sharing happens with] a familiar mate” explains Scheggia. “It is also important what the social hierarchy is in the cage, if there is a dominant mouse. The leader is usually the most altruistic.” And then there is an emotional contagion factor: the mice that felt their peers’ stress by observing them in difficulty, were the ones most dedicated to making sharing choices.
The team then investigated the neural circuitry starting from the basolateral amygdala, which is the oldest part of the brain in evolutionary terms and harbours emotional processes. By exploiting chemogenetics, a method to manipulate intracellular signalling pathways, they found that neurons in the basolateral amygdala were implicated in the pro-social process, together with their projections to the limbic part of the prefrontal cortex. When a subtype of neurons projecting from the prefrontal cortex to the amygdala was switched off, the altruistic behaviour was inhibited, and most mice behaved selfishly. Instead, by turning off the neurons projecting in the inverse direction – from the amygdala to the prefrontal cortex – the learning process was affected. The animals became confused and no longer understood the consequence of their reactions, with selfish and altruistic choices equally distributed. Brain activity measurements at the level of the amygdala were different in altruistic and selfish animals.
“Our data [show that] the emotional dimension of rodents is more complex than previously thought,” says Scheggia. “It could help understand pathological conditions such as antisocial behaviours and lack of empathy that occur in neuropsychiatric conditions and neurodegenerative diseases”.
