New research shows how bullying activates reward circuits.
Aggressive behavior is often a facet of psychiatric disorders. But it also readily occurs in people with no such condition.
Bullying has the potential to significantly reduce the victim's quality of life. As such, it is a topic well worthy of study.
According to the American Society for the Positive Care of Children, 28 percent of students aged 12-18 report being bullied at school.
There has been a great deal of study into the psychological and social reasons behind bullying. As neuroscience grows in strength, new findings are also adding to our knowledge about how and why bullying takes place.
Researchers now believe that aggressive behavior is associated with an inappropriate activation of the brain's reward system.
A recent study, conducted at Icahn School of Medicine at Mount Sinai, investigated the neural correlates of bullying-type behavior in mice.
The research, published in Nature and headed up by Scott Russo, Ph.D., associate professor of Neuroscience, took a new approach to areas of the brain known to be involved in aggressive behavior - the basal forebrain and the lateral habenula circuit.
Aggressive mouse model
Earlier work has implicated the basal forebrain as an important region for aggression-related behaviors. The present study went one step further; the investigators looked at how its connections to other brain areas affect aspects of aggression.
"Our study is the first to demonstrate that bullying behavior activates a primary brain reward circuit that makes it pleasurable to a subset of individuals. Furthermore, we show that manipulating activity in this circuit alters the activity of brain cells and ultimately, aggression behavior."
Scott Russo, Ph.D.
To study the activity of the basal forebrain in aggression, the team used a mouse model. This involved introducing a young, subordinate mouse to an adult male for 3 minutes each day for 3 consecutive days.
Under these conditions, 70 percent (resource no longer available at www.nature.com) of the adult mice acted aggressively toward the younger mouse (AGGs), and the remaining 30 percent showed no aggression (NONs).
Once the mice had been identified as AGGs or NONs, the team used a technique called conditioned place preference. This technique shows the preferences for environmental stimuli that have been associated with positive or negative experiences.
Conditioned place preference demonstrated that the AGGs mice, with a penchant for bullying, developed a preference for the area in which the confrontation took place; conversely, the less aggressive NONs developed an aversion to the situation.
In other words, the mice who bullied the subordinate mouse found it to be rewarding. The non-aggressive mice did not.
Gamma-aminobutyric acid and aggression
For the next phase of the study, the researchers investigated the importance of gamma-aminobutyric acid (GABA) projections. GABA is the primary inhibitory neurotransmitter in the mammalian brain. When it is released from synapses, it reduces the excitation of surrounding neurons, making them less active.
When AGGs were given the opportunity to bully the younger mouse, the team saw a spike in activity of the GABA projection neurons leading to the lateral habenula. This area of the brain is known to be involved in aversion to aggressive stimuli.
The NONs, however, showed the opposite response; the GABA pathways between the basal forebrain and the lateral habenula were weakened, and therefore, firing in the area increased.
Although the lateral habenula has been investigated in previous research, this study marks the first time that the neural mechanisms involved in regulating the motivation behind aggressive behaviors have been seen.
Artificially inducing aggression
In the final leg of the study, Russo and his colleagues used optogenetics to artificially manipulate the GABA pathways between the basal forebrain and the lateral habenula.
The results firmed up the findings from the earlier interventions:
"When we artificially induced the rapid GABA neuron activation between the basal forebrain and lateral habenula, we watched in real time as the aggressive mice became docile and no longer showed bullying behavior."
Scott Russo, Ph.D.
As Russo says, the research covers new ground: "Our study is unique in that we took information about the basal forebrain, lateral habenula projections and then actually went back and manipulated these connections within animals to conclusively show that the circuits bi-directionally control aggression behavior."
The results are fascinating and could eventually lead to novel treatments for psychiatric conditions in which aggression is a prominent feature.