A brain circuit for loneliness.

Tye and colleagues find a neural circuit at the base of mouse brains that drives loneliness-like behaviors and drives the animals to seek company. From a review:

...connections between neurons in the circuit were stronger in mice that were separated from their cage mates than in those that were grouped together. Those neurons then fired more frequently when isolated mice were put in a cage with an unfamiliar mouse, compared with animals that had not been isolated. When the scientists inhibited the neurons with light, the isolated mice showed less interest in the stranger. Activating those neurons caused the animals to actively seek other mice.

Here is the abstract for the work:

Highlights

•Dorsal raphe nucleus (DRN) dopamine neurons are sensitive to acute social isolation 

•DRN dopamine neurons release dopamine and glutamate in downstream structures 

•Optical activation induces, whereas inhibition suppresses, a “loneliness-like” state 

•Social rank predicts the behavioral effect induced by optical manipulations

Summary

The motivation to seek social contact may arise from either positive or negative emotional states, as social interaction can be rewarding and social isolation can be aversive. While ventral tegmental area (VTA) dopamine (DA) neurons may mediate social reward, a cellular substrate for the negative affective state of loneliness has remained elusive. Here, we identify a functional role for DA neurons in the dorsal raphe nucleus (DRN), in which we observe synaptic changes following acute social isolation. DRN DA neurons show increased activity upon social contact following isolation, revealed by in vivo calcium imaging. Optogenetic activation of DRN DA neurons increases social preference but causes place avoidance. Furthermore, these neurons are necessary for promoting rebound sociability following an acute period of isolation. Finally, the degree to which these neurons modulate behavior is predicted by social rank, together supporting a role for DRN dopamine neurons in mediating a loneliness-like state.