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.
 
In a study appearing in the journal Cell, MIT neuroscientists have 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.
 
Gillian Matthews first identified the loneliness neurons somewhat serendipitously, 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, there was a strengthening of connections in the DRN 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 sensitized 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 re-introduced to other mice.
 
The researchers also found that animals with a higher rank in the social hierarchy were more responsive to changes in DRN activity, suggesting that they may be more susceptible to feelings of loneliness following isolation.
 
“The social experience of every animal is not the same in a group,” Tye says. “If you’re the dominant mouse, maybe you love your social environment. And if you’re the subordinate mouse, and you’re being beat up every day, maybe it’s not so fun. Maybe you feel socially excluded already.”
 
The researchers are now studying whether these neurons actually detect loneliness or are responsible for driving the response to loneliness, and whether they might be part of a larger brain network that responds to social isolation. Another area to be explored is whether differences in these neurons can explain why some people prefer more social contact than others, and whether those differences are innate or formed by experience.

http://news.mit.edu/2016/neurons-loneliness-brain-0211