Specialists discover cerebrum circuit that goads tormenting in mice

Specialists discover cerebrum circuit that goads tormenting in mice

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Bullying — whether physical, emotional or social— should not be considered a normal rite of passage or “kids just being kids.” The effects of bullying can be serious, including depression, low self-esteem, health problems and even suicide.

In a 2013 nationwide survey, 20 percent of high school students reported being bullied on school property in the 12 months preceding the survey. An estimated 15 percent of high school students reported they were bullied electronically in the 12 months before the survey. During the 2012-2013 school year, 8 percent of public school students ages 12-18 reported being bullied on a weekly basis.

According to a study conducted at the Icahn School of Medicine at Mount Sinai, individual differences in the motivation to engage in or avoid aggressive social interaction are mediated by a specific circuit in the brain. Researchers discovered that this brain circuit — connecting the basal forebrain and lateral habenula — appears to control the motivation of a male mouse to be aggressive and subordinate another male mouse.

To study individual differences in aggressive behavior, the research team established a mouse behavioral 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 behavior (AGGs), while 30 percent of mice show no aggression at all (NONs).

Specifically, using electrophysiological and histological techniques, the research team found that when exposed to the opportunity to bully another individual, AGGs exhibit increased activity of the basal forebrain GABA projection neurons that reduce activity in the lateral habenula, an area of the brain that would normally encode an aversion to aggressive stimuli.

Conversely, they found NONs exhibit reduced basal forebrain activation and a subsequent increase in lateral habenula neuronal firing, which makes the aggression stimuli aversive. The significance of these findings is that the circuit seems to be telling an animal that subordinating — or “bullying” —another animal is a rewarding behavior.

Interestingly, the basal forebrain and lateral habenula have been previously shown to support conditioned place preference to drugs of abuse, such as nicotine and cocaine. This suggests similar neural processes mediate rewarding aspects of aggression and addictive substances.

According to Scott Russo, Ph.D., associate professor of neuroscience at the Icahn School of Medicine at Mount Sinai, this study is the first to demonstrate how bullying behavior activates a primary brain reward circuit that makes it pleasurable to a subset of individuals. In addition, the study shows manipulating activity in this circuit alters the activity of brain cells and, ultimately, aggressive behavior.

Identifying nerve cell communication between specific brain regions provides insight for the development of new therapeutic strategies and new information on possible motivation for bullying.


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