How do our brains retain a record of our past experience?

Transcript

Today, I’m going to address this issue, which is: can memories be erased?

Now I know that some of you in the audience are over 65, and you’re probably asking yourself, “Why would I be interested in that?” So your problem is not memory, it’s being able to remember. Your problem is being able to remember.

So you can’t remember, for example, where you parked your car when you came to this event. You might have trouble remembering what you had for breakfast this morning. Or in the worst case, you might be wondering who is that person that I woke up next to in bed this morning?

Okay, but there are people for whom memory persistence is a serious problem. Unfortunately, there are people who have had terrible traumatic experiences in their lives and they have difficulty shutting the memory of these experiences out of their minds.

So the question is, are there things that we can do to alleviate their suffering? And neuroscience has reached the point where we are actually at the beginning stages of being able to manipulate memory.

Now, in order to understand these advances, you first, we first have to go back and understand how memories are actually formed in our brains. And the first person to approach this topic was the man shown here who is the great Spanish neuroanatomist, Ramon y Cajal.

And Ramon y Cajal carried out pioneering investigations of the structure of the brain. And in the course of these investigations, he became convinced that learning causes changes in the connections between neurons in our brain.

So here you see one of Cajal’s micrographs and you’re looking at the human cortex and you see neurons here, and if you, the neurons have long processes that are called dendrites and those are the receiving parts of the neuron. And if you focus in on the area that’s depicted by the red square, what you see are little protrusions on those processes, and these are called spines.

And if you were to look in detail at this region, you would see that these spines are the sites of synaptic communication, so it’s where the sending part of a neuron called the axon ends and where the receiving part of the axon begins.

And so when an electrical impulse comes down the axon, it causes the release of a chemical called a neurotransmitter. That neurotransmitter crosses a region called the synaptic cleft and binds to receptors on the post-synaptic, or receiving part of the neuron.

Okay, so Cajal’s idea was that when we learn, there are new synapses that are formed in our brain. So there are physical changes in the brain. And this idea has been confirmed by modern neuroscience.

So what I’m going to show you here are some slides from an experiment that I carried out at Columbia University with Eric Kandel and Sam Schacher. And what you’re seeing here are axons of living neurons. And we, these neurons are growing together in a dish with other neurons, and we train these neurons and we subjected them to stimuli that would be identical to stimuli that would occur in the animal when the animal learn, and I’ll describe this technique in a little more detail later. But what we found was that, indeed, the result of the training was that there were new synaptic connections formed, and those are indicated by the arrows.

Okay, so other people have shown other things and other animals, and it’s generally accepted that when we learn there are new synapses formed in our brain. Okay. So if we, this raises the question then of what, how our memory is maintained.

So one idea is that the memories are maintained as molecular changes in the new synapses that were formed. Okay, so this idea has certain implications for the idea of manipulating memory. And what I’d like to talk about now is the idea that memories can actually be erased.

So in order to do that, I’m going to discuss for you a type of learning called fear conditioning. And in this, this is a sort of Pavlovian learning task. And as shown here, the way it works is you take a rat out of its home cage and you put it in a training chamber. And then after a certain period of time you give a sound to the rat, and then the sound’s on for 30 seconds or so, and then at the end of the sound the rat gets a foot shock.

Now, if you take the rat out of the training chamber and put it back into its home cage, and then 48 hours later you put it in a different chamber and sound the same tone, the rat’ll freeze. And that’s what rats do when they’re afraid. It’s very adaptive. The first thing they do is don’t move. And the amount of freezing is an indication of fear in the animal.

Now the memory goes through a transition, a transition during which, early on, it’s labile and subject to disruption. And this has been shown in experiments where you train the animal and then immediately after the training, you inject an inhibitor of protein synthesis into the animal’s brain. Now when you test them at 48 hours, the animal acts like it never learned. It doesn’t freeze to the tone. And by contrast, if you wait 24 hours and inject a protein synthesis inhibitor into the rat’s brain and test at 48 hours, the rat is afraid. It shows fear to the tone.

So what this indicates is that memories require a period of consolidation in order to move from short-term to a long-term state. And during that period of consolidation, proteins must be synthesized. And presumably what those proteins are doing are building new synapses. So this is the standard neurobiological idea about how memories are formed in our brain.

Now this idea was upset by experiments that were carried out by this investigator, Karim Nader. And Karim did the same experiment that I just talked about, but he tweaked it slightly. He fear conditioned, gave a rat fear condition training, then at 24 hours he injected a protein synthesis inhibitor into the rat’s brain.

But immediately before he injected the protein synthesis inhibitor into the rat’s brain, he gave the rat the tone, the same tone that it had heard before the foot shock. And then when he tested the rat at 48 hours, the rat showed no memory of the tone. It didn’t freeze.

So Karim’s idea was that when you remember something, you remember an experience, the memory of that experience undergoes a new round of consolidation, and he called that re-consolidation. And if you disrupt that reconsolidation, then the memory will be lost forever.

Okay, so this has certain implications for how we manipulate memories. So I’ve told you that the standard idea is that when an animal learned something, there are new synapses that are formed and over time these become stable. So Karim’s data suggests that if you reactivate the memory and then block the reconciliation, those synapses go away. Now, is this possible? Is this what really happens?

Well, we wanted to address that issue, but we didn’t want to address it in mammals. Mammals are too complicated. We’ve got too many neurons, too many synapses. So we chose a simpler animal.

This is the animal we chose. Now, this is a marine snail. Its name is Aplysia californica. It lives in the ocean off Malibu, so it’s not that stupid. It gets free rent and all the seaweed you can eat.

But you might say, well, why would you want to study learning in this animal? Well, it has a very simple nervous system. So this is a part of the nervous system known as the abdominal ganglia. Now, Aplysia has only 20,000 neurons in its entire nervous system. Now, that sounds like a lot until you realize that we have a hundred billion, okay? So it’s still simpler.

Another advantage of Aplysia, and this is indicated here, is that the neurons are very big. So this neuron that I’ve shown here, R2, is a millimeter in diameter. You can see it without a microscope.

The other advantage is that most of the neurons have been mapped. We know the neurons, we know their physiological and behavioral functions, we know to a certain extent their synaptic connections and you can open up every Aplysia and look at, I can look at the nervous system, see these neurons, they’re my old friends, okay? They have names. So that makes life easier.

Now, none of this would be an advantage if they didn’t learn. But I’ll show you that they do learn. But first, I have to introduce you to the reflex that we study. This is a withdrawal reflex.

So this is, if you looked at a snail, one of these snails from the top down, what you’d see is the gill, with which the animal breathes; and then a mantle shelf, which is kind of a residual shell; and then an organ called the siphon. The siphon’s a fleshy spout-like organ that’s used to suck seawater out of the mantle cavity during respiration.

Now, as shown here, if you give the siphon a weak tactile stimulus, the siphon and gill will simultaneously contract. The gill will tuck itself underneath the mantle shelf, the siphon will dive down behind these flaps of skin shown here called parapodia. It’s a defensive withdrawal reflex.

Now this reflex can become modified by learning. So the type of learning that we study is a very simple non-associative form of learning called sensitization. And this is a ubiquitous type of learning. So if, God forbid, someone were to come in here and shoot off a gun, you would immediately be sensitized. You’d be alert to stimuli that you hadn’t noticed before. This is a non-associative form of learning, and as I said, an Aplysia exhibits this.

So how do you sensitize a snail? You give it an electrical shock to its tail. After that, the tail, the withdrawal reflex, will become enhanced. The more shocks you give, the longer the memory persists. So it can persist for up to a week.

So we use this to try to see, well, can we erase the memory in the snail? And the way we did that was we trained it, we gave it a series of electrical shocks, and then we gave it a reminder stimulus. And the reminder stimulus was a single electric shock.

Now, this only produces short-term memory in the snail, not long-term memory. But when we gave the reminder and then injected a protein synthesis inhibitor into the snail, the sensitization was eliminated. So this is similar to the studies of fear conditioning in a rat. We could have removed the memory.

So we next asked, “Well okay, If we remove the memory, are the synapses gone?” So in order to do that experiment, we had to look at, we had to study synapses in culture. So another advantage of Aplysia is that you can take out the neurons that mediate the withdrawal reflex. You can take out the sensory neuron, you can take out the motor neuron, put them together in cell culture, they reform their synaptic connections, and now you have a little micro circuit in the dish that you can train.

How do you train the micro circuit? Well, you give it the same transmitter that is released when you give the animal a shock. And this transmitter is known as serotonin. So what we did then was train this culture and look to see if we would get new synapses.

Now, what you’re seeing with the arrowheads are little things, little fluorescent things that we call puncta. And those are the sites of synaptic contact. So we counted all the little dots, the little green dots, and then we train the culture and then we recount the dots and we look to see, well, were there more synapses after learning?

And the answer is yes, and here are some examples of some data from one of our experiments. If you look after the five treatments, five pulses of serotonin, you can see new little fluorescent dots, new puncta, as indicated by the red arrowheads.

Now, when we induced re-consolidation of the synaptic memory — and the way we did that was just give a single pulse of the, of the serotonin and then applied a protein synthesis inhibitor to the dish — the synapses were reversed, the growth was reversed. And this implies then that we were able to reverse the memory.

So we were pretty excited about this. We thought, “Okay well, you know, we’re actually reversing memory.” But we made the mistake of doing another experiment. And that’s always what scientists do. They they always do another experiment and it gets them into trouble. And that’s what we did.

So we trained the animal, we did everything the same. So we gave the animal a lot of electrical shocks and then we induced reconsolidation by giving it a reminder, and then we block protein synthesis and the memory went away.

But then we gave the animal weak additional training. So we gave it three shocks, which by itself induces short-term sensitization, but not long-term. And when we did that, we found that the memory came back. It was back as strong as it was before.

So what that implies then is that even though by every neurobiological criterion, the behavior looked like it was gone, the synapses appeared to be erased, the memory is still there. So this means that the memory can’t be at the synapses.

So where is it? Where is the memory being stored?

Well we, recent experiments that we’ve done suggest that the memory is stored in the cell nucleus and it is stored as changes in chromatin. So our chromosomes are made up of chromatin fibers, and chromatin fibers consist of DNA and protein called histone, and enzymes can modify the structure of the chromatin. And this has certain implications for understanding memory. It means that it’s going to be much more difficult to alter the memory than we thought before.

All right, so to summarize then, what are, what the data show is that memories, to a certain extent, are manipulable. But so far, it looks like we’re not able to erase memories.

The other thing that we’ve shown is that Cajal was half-right. He was right that memory is due to the growth of new synaptic connections in the brain, but he was wrong to think that the memories were actually stored in those synaptic connections.

Instead, we think the memories are stored in the nucleus, and this has certain implications. First of all, it means that memories are going to be tougher to change than we thought. But the other implication, if you look at it y’know, positively, scientists are always — you know, in my experience, scientists tend to be positive people. If you look at it positively, it means that there is a possibility that we never really forget. That as long as the cell is there, the nucleus is there, the memory may be there.

So I’d like to end the talk with this slide. This shows the future king of England. This is little Prince George and he’s together with his father, Prince William, and his mother, the Duchess Kate. And they’re on a trip to New Zealand and he is being shown a wombat. Okay, that’s this animal here. All right, it’s the first time he’s ever seen a wombat. When he gets to be his father’s age, he’s not going to have any memory of this experience.

All of us have had experiences before we were two years old of which we have no memory. And what I’m suggesting to you is that maybe those memories are still there. They’re still locked somewhere in our brains waiting to be reactivated. And that, I believe, is a real possibility.

And that gives us hope for treatment of diseases like Alzheimer’s. It may be possible to reactivate memories. It may be that certainly in the early stages of some of these Alzheimer’s, diseases like Alzheimer’s, the memory may be there and we just may not have the right stimulus to reactivate it.

But the more we understand about how the brain stores memories, the closer we’re going to be able to get for treatment for some of these terrible diseases. Thank you very much.