Bullying in mice alters brain chemistry, leads to anxiety

Mice that are low on the totem pole have a tough life. It turns out that bullying has some measurable effects on the brain chemistry of mice.

And these mice have the potential to teach us even more about ourselves and how our brains work when dealing with stressful situations.


According to a Rockefeller University press release, researchers have found that mice who were bullied consistently by dominant males became abnormally nervous around new mice. Their change in behavior was paired with a heightened sensitivity to vasopressin, a hormone linked to an assortment of social behaviors. The results of the study suggest that bullying may contribute to continuing social anxiety at the molecular level.


“We found that chronic social stress affects neuroendocrine systems that are paramount for adaptive mammalian social behaviors such as courtship, pair-bonding and parental behaviors,” says Yoav Litvin, M. S. Stoffel Postdoctoral Fellow in Mind, Brain and Behavior.

“Changes in components of these systems have been implicated in human disorders, such as social phobias, depression, schizophrenia and autism,” he said.


Litvin and his colleagues in Donald Pfaff’s Laboratory of Neurobiology and Behavior arranged a tough school yard situation where a young mouse is put in a cage with a sequence of bigger older mice, a different mouse in each of the 10 days. The mice being territorial little bastards fight it out in a match that the new arrival always loses. After the 10 minute brawl, the mice were then separated in the same cage by a divider that keeps them separate but allows them to see, smell and hear each other; this is a stressful situation for loser mouse to be in.


The mice are given a day to rest and then the test mice are put in with nonthreatening mice of similar size and age. The most noticeable change in behavior was that the distressed mice were more unwilling to socialize with their fellow mice; they preferred to maintain a distance from others when compared to their counterparts who hadn’t been bullied. It was also more likely that the mice who had lost their battles would “freeze” in place for a longer period of time and show “risk assessment” behaviors towards their new cage buddies.


These are behaviors that have been shown to be effective indicators of fear and anxiety in humans. The scientists also gave a group of mice a drug that obstructed vasopressin receptors, which partially stopped some of the anxious behavior in the mice who are bullying victims.


The researchers then looked at the brains of the mice, especially sections in the center of the forebrain known to be linked with emotion and social behaviors.


This is the part of the mouse experiment that gets really scientific. Ok? Ok.


They found that mRNA manifestation for vasopressin receptors, specifically V1bRs, had enlarged in the bullied mice, making them more sensitive to the hormone, which is found in high levels in rats with inherent high anxiety. In furless humans, the hormone is related to aggression, stress and anxiety disorders. The outpouring of vasopressin receptors was particularly remarkable in the amygdala. Litvin and his colleagues reported these finding this month in Physiology & Behavior, an academic journal that most people cannot access (lame).


The length of these effects still remains a question that has not been answered. Different studies have found, for example, that prolonged stress can damage some cognitive tasks in rodents and people. Lucky for us and our rodent friends, our brains can bounce back; if they are given time to recover.


Many studies in rodents, primates and humans have displayed that early psychological trauma can have negative effects on health during life. Litvin says his study puts forward the idea that victims of bullying may have difficulty making new relationships, and it recognizes the possible role for an explicit vasopressin receptor.


“The identification of brain neuroendocrine systems that are affected by stress opens the door for possible pharmacological interventions,” Litvin says. “Additionally, studies have shown that the formation and maintenance of positive social relationships may heal some of the damage of bullying. These dynamic neuroendocrine systems may be involved.”


Most people consider rodents to be pests, but they cannot deny that they have taught us a lot about ourselves.

Information provided by: Rockefeller University