Mice that are separated from their cagemates crave company after 24 hours.
Previous studies have looked at social reward but not the motivation to seek social contact.
Study co-authors Gillian Matthews and Mark Ungless, of Imperial College London in the UK, were using mice to study the effects of cocaine in some little-studied dopamine-releasing neurons in the dorsal raphe nucleus (DRN) of the brainstem.
They were surprised to observe that the properties of the neurons changed when mice were separated from their cagemates, regardless of cocaine exposure.
The findings led to a new research area: the neuroadaptations of acute social isolation.
The team housed mice either in groups or in social isolation for 24 hours, and then they measured the activity of DRN dopamine neurons.
When the isolated mice came out of solitude and met a young mouse, they experienced an increase in DRN dopamine neuron activity. The communally housed mice did not have the same reaction. This suggests that the DRN dopamine neurons respond to the effects of social contact after being isolated.
Activating or inhibiting neurons changes behavior
Next, a team led by Kay Tye, of the Massachusetts Institute of Technology, used optogenetics to find out whether the neurons actively motivate social behavior.
The scientists took some genetically modified neurons that would express the light-sensitive proteins that control neural excitability, to which they delivered light through an optic fiber: blue light to activate the cells and yellow light to inhibit them.
When they activated the DRN dopamine neurons, the mice chose to spend more time in company; when they inhibited the neurons, even mice that had been isolated for 24 hours appeared less inclined to spend time with other mice.
The results imply that DRN dopamine neurons are a key driver of social behavior following time spent in isolation.
Moreover, the extent to which the neurons change social behavior appears to reflect social rank.
In more dominant mice, stimulating DRN dopamine neurons was more likely to lead to social activity. Inhibiting the neurons, however, made dominant males even less likely to seek company after isolation.
Stronger urge to seek company among dominant males
Tye suggests that dominant males may find social interaction particularly rewarding, because they have easier access to food and mates, and are more likely to succeed in territorial conflicts than less dominant mice.
Because of this, says Tye, loneliness may be more profound for dominant mice, leading to a greater desire for social company after spending time alone.
The findings appear to reveal a neural circuit that affects how animals behave after being lonely, knowledge of which could enhance our understanding of social anxiety and autism spectrum disorders (ASD).
They also demonstrate that a group of neurons that previously have received little attention could be active in motivating behavior. Tye speculates that DRN dopamine neurons might "represent the subjective experience of a loneliness-like state," at least for mice.
However, she cautions: "We cannot assume that mice experience loneliness in the same way that humans do, and we can never assume to know the subjective emotional experience of a mouse. We can only look at the behavioral outputs."
When asked whether the theories about dominant mice could conceivably have implications for humans, Tye told Medical News Today:
"I speculate that dominant males (humans and mice) may enjoy their social environment more than subordinate mice. There is evidence that this is true in baboons from Robert Sapolsky's work as dominant males show lower basal levels of stress hormones than subordinates, which suggests they are more comfortable in their daily lives in their social environment. I think it is likely that there would be at least some shared characteristics between mice and humans in this regard."
The researchers hope to look further into the inputs and outputs of these neurons, the influence of social rank and whether the results extend to non-social mammals.
Ultimately, the findings could identify potential targets to help people with social impairments.
Gillian Matthews suggested to MNT that exploring how these neurons function in humans and comparing neurological activity between people with different social environments, states of loneliness or social impairments would be interesting.
Kay Tye told us that using this information to help people with social impairments could start by investigating whether similar neurons might also track a loneliness-like state in humans.
She also pointed out that the DRN, where the dopamine neurons are found, is in a deep brain structure that also houses serotonin neurons.
MNT reported last year that scientists had found the part of a rat's brains that is linked to anxiety.