When a mouse with a painful foot injury encounters a companion in its cage, the social contact can be enough to transfer the painful feeling to the bystander mouse. The animals can share fear in the same way.
In a study published January 8 in Science, researchers now identify separate neural pathways in the mouse brain that are active during the social transfer of fear, pain and — surprisingly — pain relief.
A mouse soothed by morphine can convey that sense of relief to another mouse using the same pathway activated during social transfer of pain, according to Stanford University neuroscientist Robert Malenka and colleagues.
The paper is the first report of social transfer of analgesia in mice, the researchers said, and should prompt further studies of how social interaction might provide pain relief in people.
The findings also offer a closer look at the neural mechanisms that likely underlie the sense of empathy. In its simplest form, empathy is a person's ability to adopt another person's emotional and sensory states, as seems to be the case with the pain and fear-sharing mice.
Impaired empathy is found across a variety of psychiatric diseases from autism spectrum to personality disorders, said Malenka.
"If we can understand these mechanisms in the brain at a much more sophisticated level, maybe we can develop methodologies or interventions that would enhance empathy," he said. "Not only in individuals with brain disorders or pathologies, but to be honest, in the general population."
Science has a critical role to play in understanding and enhancing empathy and compassion, Malenka added. "I think it's obvious that if we leave it to our societal leaders, such as politicians, to change our society so that there's more empathy and compassion, we are doomed."
Malenka and Stanford researchers Monique L. Smith and Naoyuki Asada conducted a series of experiments to create social transfer of pain, pain relief and fear among mice. The transfer took place within an hour of being cagemates, and the shared feeling in the bystander mice lasted less than a day.
The scientists did not study exactly how the transfer takes place, but earlier studies in mice and other animals suggest that individuals use sight, sound, smell and touch cues to pick up on another individual's state.
Malenka and colleagues then traced brain activity related to social transfer using optogenetics, a technique that controls neurons that have been genetically altered to be sensitive to light.
They found that social transfer of pain and pain relief follows a neural circuit from a part of the brain called the anterior cingulate cortex to the nucleus accumbens. Socially transferred fear, on the other hand, uses a circuit that runs from the anterior cingulate cortex to another part of the brain known as the basolateral amygdala.
"Fear and pain, just intuitively, they're kind of both bad experiences," said Malenka. "So I did not expect the results we reported, that they would have such distinct separate pathways."
Undoubtedly the mechanisms are more complicated in humans, he noted. "What we're reporting is just the tip of the iceberg. There are going to be a lot of other circuits in the brain and areas involved in these complex phenomena."
[Credit for related image: Hy'Shqa/ Flickr]