Individual differences in the motivation to engage in or to avoid aggressive social interaction (bullying) are mediated by the basal forebrain, lateral habenula circuit in the brain, researchers said.

For the study, scientists from Icahn School of Medicine at Mount Sinai in the US focused on identifying the mechanisms by which specific brain reward regions interact to modulate the motivational or rewarding component of aggressive behaviour using a mouse model.

Maladaptive aggressive behaviour is associated with a number of psychiatric disorders and is thought to partly result from inappropriate activation of brain reward systems in response to aggressive or violent social stimuli, researchers said.

"Our study is the first to demonstrate that bullying behaviour activates a primary brain reward circuit that makes it pleasurable to a subset of individuals," said Scott Russo from Icahn School.

"Furthermore, we show that manipulating activity in this circuit alters the activity of brain cells and ultimately, aggression behaviour," said Russo.

To study individual differences in aggressive behaviour, researchers established a mouse behavioural model that exposed adult males to a younger subordinate mouse for three minutes each day for three consecutive days, and found that 70 percent of mice exhibited aggressive behaviour (AGGs) while 30 percent of mice showed no aggression at all (NONs).

Using conditioned place preference, a technique commonly used in animal studies to evaluate preferences for environmental stimuli that have been associated with a positive or negative reward, researchers found that AGGs mice bullied/attacked the subordinate mouse and subsequently developed a conditioned place preference for the intruder-paired context, suggesting that the aggressive mice found the ability to subordinate another mouse rewarding.

Conversely, NONs mice did not bully/attack the intruder mouse and subsequently developed a conditioned place aversion, researchers said.

Using electrophysiological and histological techniques, researchers found that when exposed to the opportunity to bully another individual, AGGs mice exhibit increased activity of the basal forebrain gamma aminobutyric acid (GABA) projection neurons that reduce activity in the lateral habenula, an area of the brain that would normally encode an aversion to aggressive stimuli.

Found throughout the brain and produced by neurons, GABA is an inhibitory neurotransmitter that binds to GABA receptors, making the neighbouring neuron less excitable.

Conversely, researchers found NONs exhibit reduced basal forebrain activation and a subsequent increase in lateral habenula neuronal firing, which makes the aggression stimuli aversive.

The findings were published in the journal Nature.

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