In pain and pain relief, mice can feel each other.
Research has shown that mice can “capture” the emotions of an injured or fearful partner. When some mice are injured, other healthy mice that live next to them behave as if they are in pain. Now, a study suggests that not only can pain be transmitted, but pain relief is also contagious.
Over the past decade, researchers have done a lot of work demonstrating that animals can capture and share the emotions of others, especially fear (SN: 20/05/19), says Monique Smith, a neuroscientist at Stanford University. She and colleagues published their new findings on pain and relief in the January 8 issue of Science magazine. Investigating these blocks of empathy in animals can help researchers understand human empathy, Smith says, and may one day lead to treatments for disorders that affect the ability to be sensitive to other people’s feelings and experiences.
“Pain is not just a physical experience,” Smith says. “It’s an emotional experience” too.
In experiments in pairs of mice, one mouse received an injection that caused arthritis-like inflammation in one hind leg while the other mouse was unharmed. After being together for an hour, “the viewer has it worse than the mouse that received the injection,” says Jeffrey Mogil, a neuroscientist at McGill University in Montreal who was not part of the work.
The injected mice acted as if one paw was in pain, as expected, showing extra sensitivity when bitten there with a plastic wire. His uninjured teammates also showed greater sensitivity and on both hind legs. Those mice act like they’re in the same amount of pain and in more places, Mogil says. "The behavior is amazing."
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In another set of experiments, both mice received the irritating injection, but one also received a dose of soothing morphine. For hours after these mice were mixed, the second mouse behaved as if it had the drug. “You actually relieved the pain in this animal by simply letting it pass with another animal whose pain was relieved,” says Robert Malenka, a neuroscientist also from Stanford University. In a control group where both partner mice experienced inflammation, the sensitivity of the animals did not change after their time together.
To understand how these mice collect the feelings of others, Smith, Malenka, and their neuroscientist colleague Naoyuki Asada observed that brain regions were active after the mice spent time together. The team saw that nerve cells or neurons fired into the anterior cingulate cortex, an important area in human empathy and part of the brain region responsible for memory and cognition.
The team found neurons that connect this area to other parts of the brain, including the nucleus accumbens, an area that deals with motivation and social behavior. When scientists disrupted that particular neural connection, “animals were no longer able to express empathy” for pain or pain relief, Malenka says.
The transfer of other emotions between mice may depend on different brain connections. The researchers also examined how mice feel the fear of others in experiments in which mice saw other mice receive an electric shock. The team found that the transfer of fear depended on connections from the cortex to part of the amygdala, a region known to respond to fear. This suggests that different processes in the brain are involved in different types of empathy. But the differences may also be related to how mice feel the emotions of their peers, Mogil says. In pain and pain relief experiments, mice spend time together smelling each other and odors may contain clues about the animal’s feelings. But in the tests on fear, the visual clues conveyed animal emotions.
“Not in vain, the circuits they are seeing are very similar to some of these processes in humans,” says Jules Panksepp, a social neuroscientist at the University of Wisconsin-Madison who was not part of the study. Both mice and humans share connection with their compatriots in emotional situations, he said, and the research points to a shared evolutionary basis for empathy.
If scientists can take advantage of neurochemicals that encourage empathic processes, Panksepp says, they may be able to design drugs to treat diseases, such as psychopathy or social personality disorders, that cause empathy to twist.