Humans, like all social animals, have a fundamental need for contact with others. This deeply ingrained instinct helps us to survive; it’s much easier to find food, shelter, and other necessities with a group than alone. Deprived of human contact, most people become lonely and emotionally distressed.
MIT neuroscientists recently identified a brain region that represents these feelings of loneliness. This cluster of cells, located near the back of the brain in an area called the dorsal raphe nucleus (DRN), is necessary for generating the increased sociability that normally occurs after a period of social isolation, the researchers found in a study of mice.
“To our knowledge, this is the first time anyone has pinned down a loneliness-like state to a cellular substrate. Now we have a starting point for really starting to study this,” says Kay Tye, an assistant professor of brain and cognitive sciences, a member of MIT’s Picower Institute for Learning and Memory, and one of the senior authors of the study.
Gillian Matthews, a postdoc at the Picower Institute, first identified the loneliness neurons while studying a completely different topic. As a PhD student at Imperial College London, she was investigating how drugs affect the brain, particularly dopamine neurons. She originally planned to study how drug abuse influences the DRN, a brain region that had not been studied very much.
As part of the experiment, each mouse was isolated for 24 hours, and Matthews noticed that in the control mice, which had not received any drugs, connections in the DRN got stronger following the isolation period.
Further studies, both at Imperial College London and then in Tye’s lab at MIT, revealed that these neurons were responding to the state of isolation. When animals are housed together, DRN neurons are not very active. However, during a period of isolation these neurons become especially receptive to social contact, and when the animals are reunited with other mice, DRN activity surges. At the same time, the mice become much more sociable than animals that had not been isolated.
When the researchers suppressed DRN neurons using optogenetics, a technique that allows them to control brain activity with light, they found that isolated mice did not show the same rebound in sociability when they were reintroduced to other mice.
“That suggested these neurons are important for the isolation-induced rebound in sociability,” Tye says. “When people are isolated for a long time and then they’re reunited with other people, they’re very excited—there’s a surge of social interaction. We think that this adaptive and evolutionarily conserved trait is what we are modeling in mice, and these neurons could play a role in that increased motivation to socialize.”