This just in from the journal Cell: Your hippocampus, a key region for memory in your brain, makes a few hundred new neurons every day.
Does this mean you can now drink Tequila shots with impunity because you can more than make up for the brain cells you damage? Nope, no reason to think so. But the findings in Cell could have implications for future research in areas from antidepressants to Alzheimer’s disease.
Mainly, the new study helps cement the long-controversial claim that new neurons keep a-borning in the human brain all through life. And it does so in a creative new way, using carbon-14 left in humans by above-ground nuclear tests in the mid-20th century to measure the ages of brain cells.
I asked Prof. Joshua Sanes, director of Harvard University’s Center for Brain Science, to explain what the study could mean — why it matters whether our hippocampi keep making new neurons or not. His reply, lightly edited:
The basic dogma of neurobiology has been that you’re born with all the neurons you’re ever going to get, and then everything goes downhill from there.
But there was heated debate about this, and eventually, it was found in experimental animals that you do actually get new neurons throughout life — but weirdly, only in a few places. Where would depend on the species, but for mammals like us, it’s your olfactory bulb — what the heck that is about, nobody has any idea — and the other place is the hippocampus.
The hippocampus has proven to be critical to memory, and I’m not sure whether you’d say memories are stored there, but they certainly seem to be made there. You probably know about the famous patient HM: When he lost his hippocampus, he lost his ability to make memories.
So the idea arose that maybe if you’re making new neurons in the hippocampus, that’s to help you make new memories. In mice, there’s some evidence that favors that idea. I think nobody thinks it’s going to be as simple as that — that every time you need a new memory, you make a new neuron — but there are lots of experiments where they prevent the making of new neurons and somehow degrade memory in mice. And it seems that a lot of the things that a mouse does can affect how many new neurons are made, or at least how many of the new neurons that are made wire up.
One of those things is exercise: if you exercise more, you make, or keep, more new neurons. If you suffer a lot of stress, you make fewer neurons. Depression has been implicated; nobody knows how but there’s some idea that antidepressants can help you make new neurons, and if you’re depressed, you make fewer neurons.
So people have been interested in these new neurons, but nobody knew whether they were made in the human hippocampus, and this new study tells you that they are.
But, I asked, was it really the first study to show that?
There have been a couple of studies — one study did show in a kind of a one-off way that there were some new neurons in the hippocampus, and it was a completely reliable study, it’s not like anybody doubted it. But it was not known: Are they made throughout life? Is it just a few of them or a lot? There were no counts, no different ages, and only a few people were in the study for complicated reasons.
The new neurons were detected using a method normally used in mice, and for just a short period of time it was used in people diagnostically, and those people who happened to get this diagnosis later died and their brains were examined. So it happened only once. There was no reason to doubt it, and I’d say if you took a poll of neurobiologists, 95 percent would have said there will be new neurons in the hippocampus, but as we always say, ‘No fair guessing.’
So what might this new study promise?
I think what it speaks to is the value of animal experimentation. Everyone’s hope, including mine — I work in mice — is that what we find out in experimental animals will translate into people, and in this case, obviously, the translations are important.
The hippocampus is involved in Alzheimer’s disease, memory, depression. Suppose that people were testing out antidepressants in mice and they said, ‘This could be a fantastic antidepressant because it really helps with new neurons,’ and then we found out that humans don’t have any new neurons anyway. So what the study does is firmly secure an exciting area of animal research to the human condition.
So how about, I asked, at the level of the trivial? Will it now creep into our daily banter that, say, ‘I don’t think my brain’s making as many new neurons as usual today’?
When I was little, people always used to say that once you’re 40, you lose many neurons a day, and that turns out not to be true. It’s still no fun to get old, but the problem wasn’t that you were losing your neurons. That was an old wives’ tale that is still around. There’s a lot of evidence that in most parts of the brain, humans or animals don’t really lose many neurons with old age.
But sure, the people who said ‘Well, I just had too many drinks, I killed 3,000 neurons,” — maybe now they’ll say, ‘I just drank a little too much — I think I made 230 neurons today instead of 700…”