Scientists have shown that the chemical signal
dopamine plays an unexpected role in social
interactions. In mice, nerve cells in the brain that
release dopamine become particularly active in
animals kept on their own for a short time.
Dopamine is recognised as a messenger molecule in
the brain, and is known to play a role in memory,
learning, and associated disorders such as
addiction. It also helps to control our movements.
People with Parkinson’s disease develop reduced
mobility because their dopamine levels decrease.
Previous studies have hinted that it’s involved in
social behaviour, but little is known about this
aspect of its character.
Researchers know it’s released by nerve cells called
dopamine neurons, and that there are many different
groups of these in the brain. The study published in
Cell explores a group found in a particular area of
the brain called the dorsal raphe nucleus.
“We knew very little about this group of neurons.
Now we’ve shown that they have a fascinating and
surprising role in responding to social isolation,”
said Mark Ungless who leads the Neurophysiology
group at the MRC Clinical Sciences Centre based at
Imperial College London, and who was the co-senior
author of the paper. The study was an international
collaboration between Ungless and Kay Tye’s lab at
the Massachusetts Institute of Technology.
The researchers used optogenetics to show that when
this group of dopamine neurons is activated in mice
this prompts the animal to interact with others. The
finding could hold important clues about how
isolation affects people. “Social interactions are
no doubt important for our mental health and play a
role in conditions such as schizophrenia and
depression. Interestingly, these disorders are also
associated with dysfunction of dopamine neurons,”
said Ungless. “We’re less likely to be able to help
with such conditions if we continue to assume that
all dopamine neurons do the same thing.”
In the past, dopamine neurons have been thought to
respond to positive stimuli, or rewards, such as
food. In a previous study, Ungless showed that some
are in fact activated in response to negative
stimuli, such as pain. He says different groups of
dopamine neurons appear to do different things, and
that it’s increasingly clear that understanding the
diversity of these neurons will be key to
understanding the many disorders in which they’re
involved.
When the team stimulated this group of dopamine
neurons in the dorsal raphe of mice, they found that
these animals spent more time interacting with
others. When they inhibited the neurons, the mice
interacted less than usual. The suggestion is that
when an animal is on its own, these neurons are
sensitised, or primed, which indicates to the animal
that it’s alone – and encourages it to seek out
social interaction.
To explore this further, the researchers activated
the neurons only when the mice were in a particular
room. They saw that the mice learned to avoid this
room, which could suggest either that activating the
neurons induces a negative signal, or that the mice
have learned to positively seek a room where there’s
company over a room they know to be empty.
Intriguingly, mice that were kept on their own
overnight, and in which the dopamine neurons became
particularly active, showed no recognised signs of
anxiety. This suggests the dopamine signal is not
necessarily negative.
The team also found that the extent to which each
mouse altered its behaviour was linked to its social
rank. Dominant mice, which had experienced more
social exposure, were more sensitive than those
lower in the social ranks.
“Social interactions may be more rewarding in
dominant males because they have priority access to
food and mates, and tend to win territorial
conflicts,” Tye explained. “As a result, dominant
mice may experience a more pronounced
loneliness-like state, increasing their drive to
seek out social company after periods of isolation.”
“These findings provide a launch pad for
understanding the biological basis of the experience
of social isolation,” Tye says. “This could be
useful in determining novel targets for social
anxiety or autism spectrum disorders. We want to
explore ways that this population of neurons might
be selectively targeted in hopes of developing a
potential new therapy for social impairments.”
For more information
Cell
Dorsal Raphe Dopamine Neurons Represent the
Experience of Social Isolation
Link...
MRC Clinical Sciences Centre
Link...
Tye Lab at MIT - Massachusetts Institute of
Technology
Link...
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