Exercise: Feeling the burn is good for your brain

Oct 20 2008 Published by under Uncategorized

I'm a big proponent of exercise. I'm a runner, and when you've had a stressful day, it's amazing what a good run can do to make you feel better. I've noticed for grad students (at least at my school), when the going gets tough, the tough play sports, or go the gym. The other half (perhaps they are tougher, perhaps not), when the going gets tough, go to the bar. Some of us do both (there is a LOT of stress here, thank you).
But exercise is not just for stress relief. There are several studies out there which imply that exercise can make you THINK better, too. And this paper goes one step further, and implicates exercise in helping young animals recover from brain cancer.
Naylor, et al. "Voluntary running rescues adult hippocampal neurogenesis after irradiation of the young mouse brain". PNAS, 2008.

There's a myth out there that you're born with all the brain cells that you'll ever have. Either that, or people believe that the final number of brain cells is established by adolescence. This was the prevailing view of neuroscience for many years, but recently (1996), it was found there are cells born in the hippocampus throughout life.
The hippocampus is a twisty little area (hippocampus comes from the Greek word for seahorse, with its curled tail) located near the underside of your brain, just in from your ear (you have one on each side, obviously). It's been known for a very long time that the hippocampus is involved in learning and memory. And scientists LOVE to study it. This is partially because of its obvious importance in our daily lives, but also because the hippocampus is quite lovely to behold. It's a little curled horn of layers (looks a little like one of those Swiss Rolls), each layer entirely distinct, and any cell in each layer almost the same as every other cell in the layer. This makes it relatively easy to tease out exactly which cells project where, where they receive projections from, and how they fire in response to stimulation.
So you know that your hippocampus is involed in learning and memory. It also appears that cells are born there throughout your life, and are very important in allowing you to keep your learning and memory abilities intact. But this system can be very easily messed up, and one of the easiest ways is with childhood cancers.
Childhood cancer is an incredibly scary thing. The good news is that 90% of childhood cancer patients will survive to adulthood. The bad news is that the stuff we use to treat cancer is nasty. One of the most important treatments out there right now is radiation therapy. this is the full body treatment that someone has to undergo before receiving a bone marrow transplant, and it is also used against many other types of cancer, sometimes as a primary treatment, and sometimes to help out while other treatments, such as chemotherapy or surgery, are ongoing.
Radiation therapy works very well for cancers (especially childhood cancers), but it has some problems, too. Radiation involves sending beams of ions at your cells, specifically to damage DNA. Your normal cells can handle this, they have mechanisms in place to repair DNA or prevent replication if the DNA is damaged. Cancer cells, however, cannot protect themselves very well, because their careful DNA regulation goes out the window when they start dividing like crazy and making tumors. This means that radiation therapy is effective against dividing cells preferentially.
So what does this mean for childhood cancer? The problem comes when you irradiate the head. Preferentially killing off dividing cells also kills off those cells that are dividing in your hippocampus. Thus, even though the child very often will recover from cancer, they often have reduced learning abilities compared to other children.
So why would exercise help? Well, we know that running in rodents increases cell proliferation in the hippocampus. We're not chasing the rodents around or anything. All you have to do is give them a wheel. A mouse will go on that thing all night (hamsters and gerbils will, too, which I'm sure any rodent owner has learned to their sleep-deprived chagrin). And rodents who exercise show improvement in cognitive function compared to lazy rodents. Wheel running also increased growth factors and other signals that make the hippocampus a nice spot for new neurons. So the authors of this study wanted to see if this improvement could work on mice that had received radiation common to cancer treatment.
What they found was that mice who had been irradiated during childhood showed cognitive impairment as adults as measured by locomotor activity patterns in an open field. I personally think this wasn't such a great test. I would have run them on something a little more determinate, such as a Morris Water Maze. But you work with what you have. They also found that irradiated mice showed low cell proliferation in the hippocampus compared to mice who had not beet irradiated. So we know that irradiation causes problems. How do we solve them?
The scientists gave half the mice running wheels. Control mice (who got no radiation) ran and ended up with cognitive improvements above and beyond their couch-potato friends. Mice that had received radiation also ran, and though they didn't show improvements quite as well as controls, by the time they were done they were at normal control mouse levels. They have cognitive improvement, and also showed increases in hippocampal cell proliferation. So the exercise made their hippocampi look like they hadn't been irradiated at all!
Obviously this has some really important implications for children who have had cancer. It might be a good idea to add some exercise to their therapy that they generally receive following radiation treatment. And really, as long as you're careful, what can it hurt? And for those of you who are feeling a little stupid today, go run around the block! Your hippocampus will thank you.
A. S. Naylor, C. Bull, M. K. L. Nilsson, C. Zhu, T. Bjork-Eriksson, P. S. Eriksson, K. Blomgren, H. G. Kuhn (2008). Voluntary running rescues adult hippocampal neurogenesis after irradiation of the young mouse brain Proceedings of the National Academy of Sciences, 105 (38), 14632-14637 DOI: 10.1073/pnas.0711128105

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