Is brain ageing inevitable?

Image credits: Baycrest

Image credits: Baycrest

I have a new gig over at Nature Publishing Group's Science of Learning Community site! Woot woot woot! I'll be able to geek out over studies about how we learn, what happens in the brain when we learn and how to learn better. Below is a recent post for the site. Keep up with me here, and I will also be reposting on this site. 

“Plastic” seems like a strange word to describe the mushy organ between our ears, but in neuroscience spheres, it means that the brain adapts and changes with learning.

As we age, the brain slowly loses its plasticity, resulting in a jump of unfortunate senior moments such as “where did I park my car again”? Not all brain regions are created equal, and the hippocampus—a seahorse-shaped area that’s crucial for certain types of learning and memory—is particularly vulnerable to the assault of age.

But there’s hope: recent studies show that this decline in plasticity isn’t inevitable, at least for the hippocampus—in fact, some interventions may slow memory loss in normal ageing, or even potentially rejuvenate the brain. Spoiler alert: vampires may have had the right idea all along.

If you’re feeling skeptical, good, you should be. Reversing brain ageing has long been the stuff of snake oil and late night infomercials. To really understand how we can give old brains a plasticity boost, we need to dig into why it tanks to begin with.

In the hippocampus, plasticity happens at multiple levels. Neurons, the brain’s basic computational unit, “talk” to each other through synapses that sit on mushroom-shaped protrusions dotted along their long processes. Learning tweaks the strength of these synapses, so that some neurons tend to activate together, while others don’t. This is called “synaptic plasticity”, and neuroscientists believe it’s the molecular basis of how we learn and remember.

But that’s not all. The hippocampus is one of the two brain regions that can give birth to new neurons—a process called neurogenesis. These new neurons integrate into existing neural circuits in the hippocampus, dramatically changing their wiring. Because this happens at the level of the cell, it’s dubbed “cellular plasticity”. New neurons seem to help us discriminate between similar scenarios when we learn, and they are a powerful way the hippocampus “refreshes” itself.

So what happens during ageing?

Because neurons can’t divide like other cells, they get older as we get older, and the molecular machinery that triggers synaptic plasticity starts breaking down. Scientists still haven’t quite figured out why this happens, but funky DNA expression and impaired metabolism seem to contribute.

Neurogenesis also decreases precipitously with ageing. Part of this is because the stem cells which eventually turn into neurons “senesce”, which is just a fancy way of saying “get old”. Ageing also puts the brain into a slightly inflamed state, causing the environment around the stem cells that normally helps their development to withdraw that support.

So there you have it, the triple whammy: ageing increases inflammation, decreases synaptic plasticity, and decreases neurogenesis, which all trip up normal brain function. The good news is that each one is a potential target to slow brain ageing down, and scientists have come up with some rather creative ideas.

One eyebrow-raising method, so far only proven effective in mice, is that molecules in the blood of young animals, when delivered to an old one, can cross the blood-brain barrier and rejuvenate the aged brain.

A few years ago, scientists carefully sewed together the blood circulation of a young and old mouse, and found that the old mouse had increased synaptic plasticity and neurogenesis. They also managed to find their way better in a challenging maze, suggesting that something in young blood is making old brains smarter.

Lucky for humans, the scientists saw similar improvements when they injected young blood into aged mice—the sewing part isn’t necessary! The treatment is now being tested in people with Alzheimer’s disease, and scientists are working hard to try to identify the “pro-youth” factors hiding in young blood.

But don’t go get a blood infusion just yet. There’s another tried-and-true way to boost synaptic plasticity, increase new neurons and lower inflammation in the brain: exercise. When challenged with spatial learning tasks, old mice given a running wheel learned faster and remembered longer than their couch potato siblings. Elderly humans also maintained their hippocampal volume and outshone sedentary controls on various cognitive tests after just a few weeks of light cardio. Exercise may not be glamorous, but it’s the closest we have (right now) to an elixir of youth.

These are just two quick examples of cutting edge research that’s helping us preserve our precious cognitive abilities well into old age. There are others: learning a new language, for example, seems to help the aged brain maintain its normal function when some processes deteriorate. More experimentally, zapping the brain with electricity or changing neuronal function with magnets also seem to boost learning in select scenarios, though these methods haven’t yet been tested on elderly humans.

Brain ageing was once thought to be an unstoppable slide into functional disarray. Obviously this isn’t the case. While tough, slowing cognitive loss isn’t an insurmountable challenge—there’s hope that we can keep ourselves mentally sharp well into old age.

Now go for a run – your brain will thank you.