Motivation To Bully Others Is Regulated By Brain Reward Circuits

The researchers found that the motivation to engage in or to avoid the aggressive social interaction or the so called bullying is arbitrated by the basal forebrain and the lateral habenula circuit in the brain.

The study will be published in the journal Nature on June 30. It is conducted at the Icahn School of Medicine at Mount Sinai.

The research involved a mouse model. The researchers focus on identifying the mechanisms by which specific brain reward regions to interact to modulate the motivational or rewarding component of aggressive behavior.

Scott Russo, Ph.D., Associate Professor of Neuroscience at the Icahn School of Medicine at Mount Sinai said that their study is the first to demonstrate that bullying behavior activates a primary brain reward circuit that makes it pleasurable to a subset of individuals. He further said that they show that manipulating activity in this circuit alters the activity of brain cells and ultimately, aggression behavior.

In the study, the researchers established a mouse behavioral model that exposed those adult males to a younger subordinate mouse for three minutes every day for three consecutive days. They aimed to examine the individual differences in aggressive behavior. They discovered that 70 percent of mice exhibited aggressive behavior (AGGs) while 30 percent of mice display no aggression at all (NONs).

The researchers used the conditioned place preference, which is a technique used in animal studies to examine preferences for environmental stimuli that has been linked with a positive or negative reward. They discovered that AGGs mice bullied the subordinate mouse and subsequently developed a conditioned place preference for the intruder-paired context. This indicated that the aggressive found the ability to subordinate another mouse rewarding. On the other hand, the NONs mice did not bully the intruder mouse and subsequently developed a conditioned place aversion.

The researchers assessed the GABA projection neurons that can send long-range connections to prevent neurons in other brain regions. They discovered that when exposed to the opportunity to bully another individual, AGGs mice exhibit increased activity of the basal forebrain GABA projection neurons that lessen activity in the lateral nebula. This is an area of the brain that would normally encode an aversion to aggressive stimuli. On the other hand, they found NONs exhibit reduced basal forebrain activation and a subsequent increase in lateral habenula neuronal firing, which makes the aggression stimuli aversive.

Dr. Russo explained that when they artificially induced the rapid GABA neuron activation between the basal forebrain and lateral habenula, they watched in real time as the aggressive mice became tame and no longer showed bullying behavior. He further said that their study is unique and that they took information about the basal forebrain, lateral habenula projections and then went back and manipulated these connections within animals to show that the circuits bi-directionally control aggression behavior.

This study may provide useful information in the development of novel therapeutic drugs for treating aggression-related neuropsychiatric disorders.