Brain Growth May Cause Differences in Identical Twin Mice

Identical mice reared together in the same environment developed differences in brain structure and behavior as they aged, according to a study that could help explain why traits of identical human twins can be poles apart.

The mice that interacted most with the environment generated the most new neurons while adults, the study of 40 rodents found. Scientists monitored the animals’ movements and brain activity for three months through implanted microchips, according to the results published today in the journal Science.

The growth of new neurons occurred mostly in the hippocampus, a seahorse-shaped area of the brain that plays important roles in memory formation and emotional life, the study led by Julia Freund, a biologist at the Center for Regenerative Disease Therapies Dresden, found. The findings show that the malleable brain is shaped by experiences that promote individual traits even in identical individuals, researchers said in the study.

“Understanding how adult neurogenesis influences brain plasticity may teach us, as Freund et al. explain, how living our lives makes us who we are,” wrote Olaf Bergman and Jonas Frisen in a related editorial.

The differences between the mice twins increased over time, the researchers found.

The study also showed that the more complex an environment is, the more pronounced individual differences are, researchers said. One group of mice was kept in a boring enclosure. These mice showed the least amount of growth of new brain cells.

Depression Clues

Knowing more about the growth of new nerves also may help depression researchers, as limited growth of brain cells have been linked to the mental disorder. Some scientists have proposed that the formation of new neurons, called neurogenesis, may effectively treat depression.

An estimated 9 percent of American adults are depressed, according to the U.S. Centers for Disease Control and Prevention.

It’s not clear how current antidepressants including selective serotonin reuptake inhibitors, or SSRIs, like Eli Lilly & Co.’s Prozac, work in the brain. SSRIs are designed to block the reabsorption of the brain chemical serotonin and they boost the creation of new neurons in the brain.

Previous research attempting to show serotonin is solely responsible for depression has been unsuccessful. The alternative theory about the role of neurogenesis developed in response.

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