by Andy Coghlan
Dominant mice can be humbled and wimps made mighty by altering the strength of electrical connections in their brain.
The crucial connections dictating a mouse’s place in the social hierarchy appear to sit in the part of the brain called the medial prefrontal cortex (mPFC), responsible for emotion and decision-making.
To investigate the impact of the mPFC on social ranking, Hailan Hu of the Chinese Institute of Neuroscience in Shanghai and her colleagues first worked out the social hierarchy of mice through challenges between pairs of the animals in transparent tubes. When the mice came face to face, the subordinate animal would retreat and back out of the tube.
The team then injected a virus into some of the mice that inserts a gene called GluR4 into mPFC neurons. GluR4 amplifies transmission of electrical signals – a key step in strengthening connections.
Up the ladder
When the dominance tests were repeated, previously subordinate mice that had received the virus were propelled to the top of the social ladder.
“These mice also tended to gain more food in competition with their cage-mates, mark more territories and sing more courtship songs than their subordinate counterparts,” says Hu.
Hu’s team then took brain slices from the mice in order to measure the electrical currents produced from mPFC neurons. Mice that had received the virus had mPFC connections almost twice as strong as those in control mice.
And back down
Another virus that implanted a gene called R4Ct – that reduces transmission of electrical signals – into the mPFC reduced connections between neurons to 71 per cent of their strength in control mice. Previously dominant mice who received this treatment became subordinate.
Hu says that the strengthened connections probably enable mice to exert more control over anger, emotion and aggression. The mPFC is the “master region” dictating release of key neurotransmitters that regulate emotion, as well as hormones vital for fine-tuning dominance behaviours, including aggressiveness and responses to stress.
Hu’s team have begun experiments to identify circuits triggered by the mPFC. “We want to know how dominance rank is initiated and maintained by different neuronal activities in these circuits,” she says.
The work could throw new light on the brain mechanisms that dictate social standing in people and its effects on anxiety, motivation, addiction and relationships, says Hu. But she cautions that social rank in humans is much more complex than in mice. “In mice it’s mostly about temperament, whereas in humans it relies on factors such as education, wealth and heritage,” she says.
Results from humans show several areas of the prefrontal cortex are strongly involved in dominance and hierarchy, says Andreas Meyer-Lindenberg of the Central Institute of Mental Health in Mannheim, Germany. “So it’s good to see an approach to this social phenomenon pointing to shared neural mechanisms between humans and rodents.”
Journal reference: Science, DOI: 10.1126/science.1209951