Click on the image to watch hamsters on fluoxetine tussle in their cage.  Courtesy of Kereshmeh Taravosh-Lahn.  Image Credit: Marsha Miller

When hamsters tussle, the altercation might be one animal’s attempt to establish social dominance over the other. Or, maybe it’s the Prozac.

A new study indicates adolescent hamsters become more aggressive after receiving low doses of fluoxetine, the generic name for Prozac. Researchers say the findings could shed light on reported increases in aggression among young people who take antidepressants.

An increasing number of studies have suggested that some antidepressants can cause kids and teenagers to become aggressive, impulsive, and even suicidal. Since 2004, all antidepressants sold in the United States come with “black box” warnings about the potential dangers for juvenile patients.

To test the behavioral effects of fluoxetine, researchers at the University of Texas at Austin injected isolated adult and juvenile hamsters with high and low doses of the drug. Then, they added a second, same-sex hamster to the cage, creating a threatening situation for the resident hamster. The researchers taped the interactions between the two animals, recording the number of times the resident hamster attacked or pinned down the intruding hamster, how long the contact lasted, and how long the resident animal waited to attack the intruder.

Adult hamsters were calmed significantly by both high and low doses of fluoxetine. However, adolescent hamsters were only partially calmed by high doses of the drug. Low doses, on the other hand, actually made the adolescents more aggressive: these hamsters mounted more prolonged, more frequent, and more intense attacks on their intruders.

This increased aggressiveness is “completely contrary to what you would imagine,” said Kereshmeh Taravosh-Lahn, a doctoral student in the behavioral sciences at the University of Texis at Austin and the study’s lead author.

The findings, published in the October issue of the journal Behavioral Neuroscience, underscore the need to acknowledge a profound difference in adult and juvenile brains. According to Taravosh-Lahn, people often assume that kids and teenagers can take the same drugs as adults, just in smaller amounts. But because juvenile brains are different, she said, “this drug that we’re giving them is going to have different effects.”

One possible explanation for the differing reactions to fluoxetine is the different ratio of serotonin receptors in adult and adolescent brains, Taravosh-Lahn said. Prozac is a selective serotonin reuptake inhibitor, a type of antidepressant that works by prolonging the availability of serotonin—a neurotransmitter associated with depression—in the brain.

There are different types of serotonin receptors, two of which are serotonin 1A and serotonin 3. The former receptors are associated with the inhibition of aggression; the latter seem to enhance aggression. Serotonin 3 receptors are known to react faster than 1A receptors.

Taravosh-Lahn suggests that if juveniles have a higher proportion of serotonin 3 receptors than adults, fluoxetine could trigger them more quickly—explaining adolescents’ aggressive responses.

But researchers warn about inferring too much about human nature from hamster behavior.

“The increased aggression in juveniles, while that’s interesting, I think you have to be very careful before you try to extrapolate that to humans,” said Fritz Henn, a psychiatrist who now works at the Brookhaven National Laboratory in Long Island, New York.

To make a realistic link between humans and other animals, he said, “you really have to go up the phylum a little bit.”

Originally published October 24, 2006

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