Loneliness is encoded in the brain in a way that closely resembles the neural architectures governing drives like thirst and hunger, researchers from the Dulac Lab report. A team led by postdoc Ding Liu has identified a population of neurons in the mouse brain that are activated when animals are socially isolated and encode “social need” or what humans would call the feeling of loneliness. They also found another population of neurons that inhibit the social-need neurons and encode the social satiety that occurs when animals are reunited after a period of isolation. While other brain regions are involved, the circuit formed by these two populations represents an important step forward for understanding how the brain generates the need for social interaction during episodes of social isolation.
“Neuroscientists already know a lot about the neural basis of physiological needs, like the needs for food, for water, and for sleep,” Liu says.”But very little is known about how social need is controlled and regulated in the brain.”
Liu and his colleagues were able to identify these neurons after they observed a behavior called “social rebound.” After mice are separated from their cage mates, later they compensate for the social isolation by spending more time interacting with other mice after they’re reunited. Scientists can measure the length and intensity of this social rebound as a proxy for the intensity of the isolated mouse’s social need.
By looking at a brain region called the medial preoptic nucleus in the hypothalamus and measuring neuronal activity, Liu and his colleagues found these two populations of neurons that respond to social isolation and social reunion respectively. To confirm each population’s role in encoding social need and social satiety respectively, they used a technique called optogenetics, where scientists artificially add a light-sensitive switch to neurons and activate or inhibit these neurons by light.
“One population we call social need neurons or loneliness neurons,” Liu says. “They are activated during [social] isolation, and if we use tools like optogenetics to artificially activate these neurons in grouped animals, we can mimic the isolation state with enhanced social motivation and aversive emotion.”
To mimic the aversive state of social isolation, the researchers conducted an experiment where mice had access to two chambers. In one chamber, they optogenetically stimulated the social need neurons and found that it caused the mice to avoid that chamber. This finding shows that activating the social need neurons creates an unpleasant feeling. The second population, which Liu terms “reunion” neurons, were activated during social rebound. When the mice had the option to sit in a chamber that optogenetically activated the reunion neurons, they chose to stay there, suggesting that the reunion neurons produce a contented feeling.
A third part of the study investigated what sensory information animals use to perceive that they are isolated or together and what sensory input contributes to fulfilling social needs. Liu and his colleagues devised a set up where the mice were separated from their cage mates by a divider with small openings that the mice could see, hear, and smell through. However, the mice separated by the divider still showed signs of loneliness and exhibited equally strong social rebound. What mattered for social satiation was touch, perhaps not surprisingly as we know how critical touch is in human social interactions.
In a follow-up experiment, Liu and his colleagues set up tunnels that were either made of bare plastic or wrapped in fuzzy cloth. Mice overwhelmingly preferred the cloth tunnels after social isolation, and spending time in the cloth tunnels reduced social rebound.
“We also have recordings of these neurons during touch, and we find these neurons have a very similar response mimicking social interaction,” Liu says,.”So these experiments provide an explanation of how touch can satisfy social need. It can change the activity of the neurons directly, just like social reunion.”
Liu says the study underscores the need for greater societal emphasis on caring for others. “People think, ‘it’s fine; I can stay alone.’ Even in some cultures, they see it as a [source of] shame, as an unnecessary feeling,” he says. ”However, in this study, we reveal the physiological basis of this feeling. It is really a physiological response. So we cannot deny that this feeling is important. For animals and for humans, it is an alarm signal that you need some social activities for your health.”
Dulac adds, “There is something very profound about finding similar circuit architectures to encode social need and physiological needs such as the needs for water, food and sleep. It directly underscores how fundamental social interactions are in animal and human lives.”
