When a group of genetically identical mice lived in the same complex enclosure for 3 months, individuals that explored the environment more grew more new neurons than less adventurous mice, according to a study published on May 9 in Science. This link between exploratory behavior and adult neurogenesis shows that brain plasticity can be shaped by experience and suggests that the process may promote individuality, even among genetically identical organisms.
Scientists have often tried to tackle the question of how individual differences in behavior and personality develop in terms of the interactions between genes and environment. “But there is next to nothing [known] about the neurobiological mechanisms underlying individuality,” said Gerd Kempermann of the German Center for Neurodegenerative Diseases in Dresden.
One logical way to study this phenomenon is to look at how the brain’s structure and function change over time. Plasticity is hard to study because it mostly takes place at the synaptic level, so Kempermann and his colleagues chose to look at the growth of new neurons in the adult hippocampus. Earlier studies have demonstrated that both physical and cognitive activity increases adult neurogenesis in genetically identical mice, but there were differences between individuals in the amount of neuron growth.
To understand more, Kempermann and his colleagues housed 40 genetically identical female inbred mice in a complex 5-square-meter, 5-level enclosure filled with all kinds of objects designed to encourage activity and exploration. The mice were tagged with radio-frequency infer-red transponders which tracked their every movement. After 3 months, the researchers assessed adult neurogenesis in the mice by counting proliferating precursor cells, which had been labeled before the study began.
The researchers found that individual differences in exploratory behavior correlated with individual differences in the numbers of new neurons generated. “To our knowledge, it’s the first example of a direct link between individual behavior and brain plasticity,” Kempermann said.
The findings could help explain why human identical twins raised in the same family end up with different personalities.
One caution, though: although the mice in the study were genetically identical, it may be that they were not behaviorally identical to begin with: some variation may have occurred at a very early stage that made individuals more or less likely to explore.
The research provides a novel approach to studying the role of the environment in shaping personalities. “We’ve come up with an animal model to help address the ways living our lives make us who we are,” said Kempermann. “So we have touched on an approach that allows us start to get the small part of an answer to a very big question.”
J. Freund et al., “Emergence of individuality in genetically identical mice,” Science, 340:756-59, 2013.