The brainâs responses to exercise are particularly strong when animals are young, past experiments have found, because young brains are so reactive to all kinds of stimuli.
But it has not been clear whether these impacts are long lasting and beneficial for older brains, or if they sputter and vanish as we age, especially if we stop exercising.
So for the new experiment, which was published this month in eNeuro, researchers at the University of Toronto and other institutions gathered a group of adolescent rats.
They then divided the animals into two groups, one of which went to live in standard cages. The others were given cages with running wheels and allowed to exercise as much as they wished for six weeks. Rats seem to enjoy running, and these rats ran a few miles almost every day.
After six weeks, the wheels were removed and all of the animals became sedentary as they eased into adulthood.
When they were about 7 months old, which is middle-aged for rodents, the researchers injected the animals with a chemical that binds to newborn neurons in the brain and marks them. They then placed the rats in a specialized cage and lightly shocked them several times. This process, known as fear conditioning, creates strong memories in the hippocampus. When animals are reintroduced to that cage, they will typically freeze repeatedly as they remember their earlier experience.
The scientists waited two weeks before setting a number of the animals back into that cage. Some had been runners while young; others had not.
Another group of the rats, runners and not, were placed into a cage that was similar to but not precisely the same as the scary cage, while a final group was settled into a cage that was nothing like that earlier one.
The scientists noted how often each rat froze. Then they microscopically examined brain tissue, counting the total number of newborn neurons in each animalâs hippocampus.
The scientists also…