With good timing, experiences can rewire old brains

New experiences can rewire old brains — but the timing has to be just right.

Scientists have found that when adult mice spend time in cages with toys, tunnels and running wheels, a part of their brain creates many more connections between new and existing nerve cells than it does in mice kept in toy-free cages. But the boost in brain rewiring happens only from two to six weeks after the new brain cells are born, the researchers report in the Feb. 18 Neuron.

“It’s remarkable to think of the extent of this reorganization,” says Amar Sahay, a neuroscientist at Massachusetts General Hospital and Harvard University. It shows that the adult brain can change much more than scientists had thought, he says. The results may even help to explain why humans don’t all respond the same way to stress, and possibly how individual experiences shape behavior.

In the new study, Matteo Bergami, a neuroscientist at the University of Cologne in Germany, and his colleagues traced how and when new cells are integrated into existing nerve circuitry in the brains of adult mice.

Past studies have shown that adult brains make new nerve cells, also called neurons, in the dentate gyrus, part of the hippocampus — a seahorse-shaped brain region that sits above the brain stem and plays an important role in learning and memory. Experiments have also shown that genetically identical mice grow more neurons in the hippocampus and develop different personalities depending on how much they explore environments with toys and tunnels (SN: 6/29/2013, p. 13).

Bergami and colleagues found that in addition to growing new neurons, adult mice given access to toys and tunnels also develop about two to four times as many connections between those new neurons and existing neurons than mice in regular cages. Some of the connections were even long-range, linking new neurons in the dentate gyrus to other regions of the brain to form circuits that may play important roles in memory and navigation. But to make the connections, the neurons had to be just the right age. Toys and tunnels did not change the connections made by neurons younger than two weeks or older than nine weeks, the team reports.

Sahay says the results could help scientists recognize similar kinds of rewiring in human brains. Imaging new neuron growth in the hippocampi of adult humans has been challenging. High-resolution imaging might be able to pick up the reorganization of some the brain circuits revealed in the new study, he says.

Bergami says brain rewiring based on experience may play a role in anxiety and depression. Both conditions are linked to stress, which can negatively affect new neuron growth. If a person experiences something traumatic while new neurons are in that critical period of reorganizing brain connections, that rewiring could be dysfunctional, putting the individual in a constant state of anxiety. The team plans to test the idea in mice.

Brain rewiring based on experience could also play a role in humans’ diverse range of personalities. “As individuals, we have quite distinct experiences in our lives,” Bergami says. “The emergence of our individuality may be based on our individual set of brain connections.”