A recent study1 sought to explore the impact of environment on brain development by raising a group of genetically identical inbred mice in a complex, enriched environment composed of multiple levels for exploration and activity. Via radio-frequency identification transponders and 20 antennas distributed over the cage space, the locations of the mice were monitored over a period of three months.
The experimenters’ specific hypothesis was that mice with a greater range of experience, as reflected by more exploratory behavior or an increased coverage of the enriched environment, would show higher levels of adult hippocampal neurogenesis. As the hippocampus is associated with learning and memory, the development of new neurons in that brain area are assumed to be a result of dealing with novelty and complexity.
When looking at the total number of antenna contacts – a proxy for sheer amount of locomotion – there was no difference in the number of new neurons that developed in the mice. But, when the researchers looked at the probability of a mouse being near to any one of the 20 antennas in the enclosure, a measure they called “roaming entropy,” they found a significant correlation with new neurons. Even though all the mice were genetically identical inbreds (C57Bl/6N females), there was a definite difference in the cumulative roaming entropy (what I like to refer to as “wanderlust”) that was related to adult neurogenesis. The more they wandered the more adult neurons they developed, in a linear relationship.
A slight tendency to wander more in some of the mice resulted in those mice developing more neurons, differential neurologic plasticity, and magnification of these differences in exploratory behavior over time. In other words, while the mice started out in their enriched environment with similar activity levels and similar numbers of neurons, some of the mice wandered more and developed more neurons. Over time, in the enriched environment the mice shared lost their genetic “sameness” and developed “mouse individuality,” much the same way that identical twins raised in the same home grow up to be different people. The authors conclude that this mouse model they developed can be used to study the “’mystery and controversy’ of the nonshared environment, or the ways in which living our lives makes us who we are.”
- Freund, J. et al. Emergence of Individuality in Genetically Identical Mice. Science 340, 756-759 (2013).