Exercise may not erase old memories, as some studies in animals have previously suggested.
Running on an exercise wheel doesn’t make rats forget previous trips through an underwater maze, Ashok Shetty and colleagues report August 2 in the Journal of Neuroscience. Exercise or not, four weeks after learning how to find a hidden platform, rats seem to remember the location just fine, the team found.
The results conflict with two earlier papers that show that running triggers memory loss in some rodents by boosting the birth of new brain cells. Making new brain cells rejiggers memory circuits, and that can make it hard for animals to remember what they’ve learned, says Paul Frankland, a neuroscientist at the Hospital for Sick Children in Toronto. He has reported this phenomenon in mice, guinea pigs and degus (SN: 6/14/14, p. 7).
Maybe rats are the exception, he says, “but I’m not convinced.”
In 2014, Frankland and colleagues reported that brain cell genesis clears out fearful memories in three different kinds of rodents. Two years later, Frankland’s team found similar results with spatial memories. After exercising, mice had trouble remembering the location of a hidden platform in a water maze, the team reported in February in Nature Communications. Again, Frankland and colleagues pinned the memory wipeout on brain cell creation — like a chalkboard eraser that brushes away old information. The wipe seemed to clear the way for new memories to form.
Shetty, a neuroscientist at Texas A&M Health Science Center in Temple, wondered if the results held true in rats, too. “Rats are quite different from mice,” he says. “Their biology is similar to humans.”
Using a water maze similar to Frankland’s, Shetty’s team taught two groups of rats how to find a hidden platform in eight training sessions over eight days. Then rats in just one of the groups exercised on a running wheel. Four weeks later, rats in both groups performed the same in the maze test — despite the fact that running rats had 1.5 to 2 times more newly born brain cells in the hippocampus, a skinny strip of tissue that’s thought to help form new memories.
These results and other memory tests “clearly showed that exercise did not interfere with memory recall,” Shetty says. And it’s likely that exercise doesn’t harm human memories either, he says.
Frankland says it’s possible that Shetty’s rats just learned the water maze too well. Shetty’s team trained their rodents for longer than Frankland’s team did, perhaps etching memories more deeply in the brain.
“The stronger the memory is, the harder it is going to be to erase it,” Frankland says.
But he points out that erasing memories isn’t necessarily a bad thing. “People get hung up on this idea,” he says, but actually, clearing out old info from the brain — forgetting — is important. Without some sort of clearance process, “your memory is going to be full of junk.”