And now we get to the third paper I could present for Journal Club. This one's on something that Sci doesn't really know as much about. This makes it both more exciting, and slightly more scary. But the science is elegant and the results are amazing. So it might very well be worth it.
Have you ever thought of what happens when you commit something to memory? If you're like Sci, you think of filing it away in little filing cabinets in your brain. But the way that memory encoding is actually done is still something of a mystery. We know that groups of neurons work together to serve as a physical representation of memory, otherwise known as the "memory trace". Groups of neurons acting together have been seen correlated with memories being encoded, and with the expression of memories. But we still don't know whether these disparate groups of neurons "are" memory, whether they are essential for a memory trace.
This group of researchers decided to find out whether specifically activated neurons were essential for memory learning and expression. As you might imagine, what they had to do was pretty insanely complicated, but what they found makes it very well worth it. Seriously. Complicated. My print-out of the paper (I had to print it out because I couldn't just read it, had to make notes), is COVERED IN INK. But I think I've got it, and they've got an awesome message. Now to pass it on!
Han, et al. "Selective erasure of a fear memory" Science, 2009.
To start with, it is know that certain neurons in an area of the brain known as the lateral amygdala are activated by auditory fear memory training and testing. The amygdala is a small, almond-shaped area of your brain, located right above and slightly in front of your hippocampus, and deep inside the temporal lobe.
The amygdala is an area involved in the processing and memory of strong emotional experiences, such as experiences involving fear. So it's a very good place to look when you're looking for how fear memories are processed and encoded. In this case, researchers used the paradigm of auditory fear learning in mice to look at activation of neurons in the amygdala.
Auditory fear learning is pretty simple. If you give a mouse an unhappy stimulus (usually a foot shock), and associate it with a tone, the mouse will learn the association. Pretty soon, if they hear the tone and remember that something unpleasant is about to happen, the mouse will freeze up and brace itself in preparation. This is a very simple learning task, and it's very easy to tell when a mouse has learned it, so it makes for a very good paradigm when you're testing something this complicated.
So they're looking at auditory fear learning, and they're looking at a subset of neurons in the lateral amygdala. This subset of neurons shows increased activity during memory training or testing. These neurons ALSO show an increase in a transcription factor known as CREB, which is a protein which play a role in increasing gene expression. The authors of this study were trying to find out whether the expression of CREB in these neurons was critical to the development of memory.
And this is where it gets interesting. Basically, they used a virus to inject CREB DNA (or control DNA which did nothing) into mouse neurons. These weren't ordinary mice. They were mice which contain something called DTR, which a diptheria toxin receptor. Activation of this receptor (by injecting diptheria toxin) will kill the cell. But the DTR isn't active. It's dormant, until you activate it with a sequence called 'cre'. Once the DTR is activated, you can then use diptheria toxin to kill off that cell.
So this virus they used to inject CREB or control ALSO had cre in it. It got into a specific, very small group of neurons, which are now susceptible to diptheria toxin. This means that you have cells that overexpress CREB, and you can knock them out and kill them whenever you want to, without ever affecting the other cells around them. It's complicated, but this is a GREAT model.
To the far left (in white) is your basic mouse which has DTR expressed. The DTR is normally inhibited (you can see the little "stop" in there). After injection of the virus, which has "cre", you can see the green mouse now expresses the DTR (it says "on" and he's got little receptors sticking out the sides). So he's now susceptible to diptheria toxin. Then you give diptheria toxin to the mouse (the blue triangles hitting the purple mouse), and the cells expressing DTR, and ONLY those cells, will die (the mouse in red).
So the researchers used this model to increase CREB expression in a small group of neurons in the lateral amygdala. They then trained the mice using fear conditioning, and looked to see which neurons showed more activity following training. It turned out that neurons which expressed high levels of CREB were much more likely to be activated than those that expressed normal levels of CREB or expressed no CREB at all. This implies that high levels of CREB expression influence which neurons are recruited in memory learning.
Next, they did the same task, on this time using a weak training for fear. Those mice which overexpressed CREB in some neurons learned better than those that did not, and when they gave diptheria toxin and knocked out the CREB, this effect reversed! This means that high expression of CREB can increase memory development, and that knocking down CREB can decrease the effect.
Not only that, since neurons with high expressions of CREB are activated most by memory learning, it seems that high CREB levels will determine which neurons will go into a forming memory trace and which will not. And these neurons with high CREB levels will then be the source of the memory. They found that, when they knocked out the CREB neurons in already trained mice, the mice...couldn't remember. They lost the memory, and it seemed to be permanent. BUT, they were still able to learn other tasks, presumably because other cells could pick up the slack.
What does it all MEAN?! It means that expression of CREB in neurons in the lateral amygdala is critical for the learning and expression of memory in mice (and possibly also in humans). Not only that, it means that the neurons themselves expressing the CREB, are essential for the learning and expression of memory. The neural networks hold the memories themselves, they don't just create an environment in which memories can form.
Mostly this is cool because it expands what we know about memory (and we still don't know a lot). And it's got some cool implications for problems of memory in the clinic. If we know how memories are formed, we are one step closer to finding out how they are lost. I can also see some implications in this for things like Post-traumatic stress disorder. If all memories (especially, in this case, fear-related ones) are formed with specific networks of neurons, then it may be possible to parse out and delete those specific neurons, deleting a traumatic memory that can have severe psychiatric consequences. Of course, I think this sort of thing would be a very long time coming, but it's always cool to think of the possibilities that can come from this basic knowledge of the brain.
Finally, the pros and cons:
This study is beautifully designed and executed. For every thing, there is a control. Gorgeous.
The finding is really novel and adds a lot to the knowledge on the topic.
The finding is not just a cool mouse, it's behavior, neuronal networks, AND a mechanism! AWESOME!
It's INSANELY COMPLICATED and very difficult to explain, especially given the time allotted.
...but it's SUCH a good paper...
Han, J., Kushner, S., Yiu, A., Hsiang, H., Buch, T., Waisman, A., Bontempi, B., Neve, R., Frankland, P., & Josselyn, S. (2009). Selective Erasure of a Fear Memory Science, 323 (5920), 1492-1496 DOI: 10.1126/science.1164139
EDIT: Based on the brilliant model proposed by Neuroskeptic, I made a model to show the main findings! And it's cute. So I'm putting it here:
This is where we start. A new memory can get encoded into any one of those boxes, each of which represents a neuronal network in the lateral amygdala. But it will preferentially go to ones that are red, the ones that are expressing CREB in high levels. So the scientists here painted one of the neuronal networks red, by causing it up over-express CREB:
Then they trained the mice and looked to see which neuronal network became activated, showing where the memory had encoded. Sure enough, the new memory preferentially encoded on the neuronal networks high in CREB:
And when they deleted the neuronal network with high levels of CREB, they ALSO deleted expression of the memory.
But there were still lots of neuronal networks left, so further memories could still be trained, and the animals could still encode them. I shall be suing this in my presentation, I think. Thanks, Neuroskeptic!!