Picower Institute for Learning and Memory Inaugural Symposium: “The Future of the Brain," pt. 3
MODERATOR: Hello, there. This is your friendly voice at the front. Please have a seat. I hope your filet mignon was as good as mine. Want to welcome you all to the first afternoon session we're going to have. It's called Change your Mind, Memory and Disease, and it will focus on one theme and that is, unraveling the mechanisms that drive our capacity to remember and to learn, as well as related functions like perception, attention, and that bugaboo word that we've all avoided today-- consciousness.
Memory is the thread, really, that ties our next speakers together. And we're going to start talking about memory disorders as a health problem, in general. And then we're going to focus like a laser beam on two specific disorders-- Alzheimer's and post-traumatic stress disorder. So these will be great opportunities for your questions and answers because-- so after each of our speakers are finished here, we'll be moseying on over to those chairs over there and do our best to imitate Mr. Lipton as he sits down and talks to celebrities on his show. And we'll take questions from the audience as we do that.
Our first speaker is Dr. Thomas Insel. He is director of the National Institute of Mental Health at the NIH. He has a long history of public service, joining NIMH in 1979, then after 15 years, moving on to Emory University, where he worked on obsessive compulsive disorders. Now back at NIMH, he controls a $1.3 billion budget. And he will share with us, I'm hoping, his overview of the state of memory diseases and research, possible research to combat them. Please welcome Dr. Thomas Insel.
INSEL: Well, thank you, Ira, for the introduction, and thank you, Susumu, for the invitation. It's a real honor and pleasure to be here. Congratulations on the opening of this Institute. And for all of those who work here, I think it's an exciting beginning and a chance to do some of what we heard a little bit about this morning in terms of moving into the future. I guess I am the first of what we'll call the post-Nobel speakers today. I've been informed from my colleagues that they want this to be called the pre-Nobel part of the session.
And my comments are actually going to be quite focused on something that we call social cognition or social memory. There's so much we could talk about. But I thought it would be best, given that the morning was, shall we say, free ranging, that we'll try it in the afternoon to dig into a few stories where we get a little more of the detail, have a chance to chew a bit more on the intricacies of some very finite questions to get a sense of what the options are and what the tools are currently and where we're going in some of these areas that have to do with memory and may have implications for disease.
My brief is to talk about social memory, and we're going to put this in the framework of what we call social neuroscience. It's an exciting new area of neuroscience. There are really only three points that I want to make, that I want you to take from the next 20 minutes.
The first is that what we call social memory is different than all other forms of memory. It uses different pathways in the brain and probably responds to different kinds of signals. The second is that, though I'm mostly going to talk about mice, it turns out that humans also have very unique circuits, specialized circuits for social cognition. And finally, the growing evidence that the disease that we call globally autism-- which is probably many different disorders under one name-- all share a deficit in social cognition. It's really fundamental to this disorder.
Now, as you had a sense from this morning, a lot of what happens in science-- and it's no different in neuroscience-- is that we follow where the leads are, where the traction is. And in this field, we've been really fortunate in that there is a small group of neuropeptides, that were mentioned this morning, that nature seems to have selected through a long period of evolution to mediate many aspects of social behavior or social cognition. Who would have thought that that would have been one of the ways in which neuromodulators or neurotransmitters would have evolved?
But it does seem to be the case that, whether you're looking at worms or snails, reptiles, birds, fish, or even in mammals, this small group of neuropeptides has an important role either in social behavior generally or in sexual behavior, in particular. And in the case of mammals, there are really only two members of this neuropeptide family. One is called oxytocin, and the other is vasopressin.
If you've given birth in the United States, you've probably received oxytocin in the form of its synthetic version, which is called Pitocin, that's given usually during labor and delivery for most women who deliver in hospital in the US. We'll focus on this one oxytocin, and it's probably best known as a kind of maternal peptide because it's known in peripheral organ systems to be very important for labor and delivery. It causes the uterus to contract. It's also important for lactation. It causes mammary myoepithelium to contract to push milk out through the lactation ducts. And it's also important for the sexual response, and that's true in both males and females.
Now the key for this story today is that this is also found in the brain. And what does it do in the brain? It's not entirely clear what the peptide does in the brain, but the initial evidence is that what it does in the brain, in some ways, is very congruent with what it's doing in the periphery. And the evidence for that comes from studies, mostly in sheep and in rats, that suggests that in animals that otherwise would never become maternal, the injection of this peptide into select brain areas essentially turns on the whole maternal pathway.
That is, in a rat that may otherwise avoid newborn pups if she hasn't just gone through parturition, hasn't just delivered, if she's given oxytocin at the appropriate time, she will scurry around and build a nest. And if pups are anywhere nearby, she'll go over, retrieve them, and put them in the nest. And she'll crouch over them just as if her brain thinks that she's just delivered a litter. So it has an extraordinarily powerful effect within the brain to turn on maternal behavior, and we would suspect also maternal cognition in some way.
The last point about this is that this peptide seems to be also have different effects in different species, and that seems to depend on where the receptors for the peptide reside because they're somewhat different in the brains of mice, rats, voles, monkeys, and humans. What we do know is that in those species that pair bond, like the voles that Eric mentioned earlier, oxytocin seems to have a powerful effect on facilitating the pair bond, particularly in females.
Now we did what a lot of people do when they want to study a peptide. We had a knockout made. That is, we had a mouse that was genetically engineered to lack this peptide altogether. In the old days, meaning 10 years ago, we used to call that a highly selective genetic intervention. We now think of it as a sort of thermonuclear intervention because what you're doing is taking out this peptide or this gene all the way through development. So if you see a difference in behavior, it's hard to know what it means.
But here, we've got a very interesting observation because the mice that were made in this way, lacking oxytocin, actually look pretty normal. In fact, they don't lactate, but if they're given oxytocin, they can lactate. They don't show much of a deficit in maternal behavior or other aspects of maternal or social behavior initially. And you can imagine we were a little disappointed to find that. We later discovered why that was.
But they do have a specific deficit, which is around the social cognition. How do you study social cognition in a mouse? It's actually quite simple. This is what's called the social recognition test. In this case, we're testing this black mouse, who's in his own home cage. We put a white mouse in with them for five minutes, then take the white mouse out for 30 minutes. And we come back 30 minutes later and expose either to the white mouse the familiar animal or an unfamiliar animal.
And what you see here is that mice generally spend about, oh, 2 and 1/2 minutes in a kind of meet-and-greet ritual, which is a lot of anogenital and [INAUDIBLE] sniffing. They actually sniff wherever the scent glands are. So this is not something we'd recommend that any of you start doing in the near future, but this is one of the ways that, as Richard talked about this morning, that mice get to know each other very quickly.
And they develop this familiarity, and the way you can tell they're familiar-- interestingly, we're losing one of the bars here. Not sure why that is. Oh, there it is-- is that when you test them 30 minutes later, they spend a lot less time on investigation. And it's this difference between the 2 and 1/2 minutes here and the one minute here, about a 50% decrease in investigation time, that we believe reflects the fact that this has become a familiar mouse. Because if you put in a new mouse they've never seen before, they're back up to about two, 2 and 1/2 minutes.
So when you do this kind of a study, you can get some idea of whether mice are able to remember who they've been with. And in this case, we did this with the oxytocin knockout mice, as well as with controlled mice. They're called wild type mice. And we saw really no difference in this initial test. They spent about the same amount of time investigating initially. But when you look at them 30 minutes later, there is this actually quite interesting difference between the two groups.
The wild types, the normal mice, here they do a quick sniff say, oh, yes. You're probably the familiar mouse that I met 30 minutes ago, and walk away. But the knockout mice, while they first sniff in just the same way, they don't seem to be able to remember that this is a mouse that they've met before. In fact, they spend about the same amount of time investigating on the second trial as they did the first. And if they had a third and a fourth trial, you'd see much the same kind of behavior.
So these are what the data look like when you quantify this. The orange bars are what you see in normal wild type mice. And as I mentioned before, there's about a 50% decrease in investigation time between the first trial and the subsequent trial 30 minutes later. And note that in the knockout mice, there's simply no difference at all. They spend as much time the second interval as they did the first. It's just like they've never met before.
Obviously, there are a lot of reasons why that could be. Maybe they can't smell, and maybe they can't learn. It's not that they spend much more time just investigating, because as you can see, they're actually the same at baseline. But there's certainly lots of other possibilities.
Jennifer Ferguson, who did this research as a graduate student, did a very simple follow-up study where she, instead of giving them a mouse to remember, she gave them just a cotton ball that had been soaked in lemon. And what she found was they'd spend about two minutes investigating the cotton ball, and then 30 minutes later, both the knockouts and the wild type were perfectly able to realize that this was familiar. I.e., they are able to smell. And in fact, they're able to learn. But there seems to be a deficit in learning something that is a social as opposed to a non-social cue.
She followed this up with what I think is a really interesting experiment where she said, rather than putting the lemon scent on a cotton ball, let's put it on a mouse and let's see how they perform in that case. And sure enough, if she, in this case, perfumes the mice with the lemon, they are able to recognize, that both the wild types and the knockouts seem to know 30 minutes later that this is indeed a familiar mouse.
But then, if you do the same experiment where you then 30 minutes later put the lemon-- you take a second mouse that they've never seen before, also scented with lemon, what you can see is the wild type mice very quickly say, this is a female I've never seen. She may be wearing the same perfume, but this is a novel female, whereas the knockouts can only follow the perfume. They can only follow the lemon, and they don't seem to be able to make this percept of this being a familiar individual, a familiar, conspecific female that I've known before.
Now, if we put this into human terms, we'd be talking about something that's called prosopagnosia, the inability to make social memories that are, in humans, visual instead of being olfactory. What would this mean? Let's use the example of the people that you've met this morning.
So this would be like having just met Eric Kandel this morning and then 30 minutes later, seeing him again. You'd probably have less reason to have the same conversation with him so you spend less time in discussion with him. But if he then did something quite odd and took his name tag off-- because you weren't able to remember his face but you can only follow his name tag-- and he put his name tag on Richard Axel, you would then greet Richard Axel very much as if he was Eric Kandel.
You wouldn't know. You probably should have been able to know this because only one would wear a bow tie, but in this case, that would be insufficient even for the mice. And you would be following now the semantic information and not the social information. So this is the kind of deficit that we're seeing here.
Now, there are two things about this that were intriguing to us. The first was that it's highly specific. So when you look at a whole range of other kinds of learning tasks, these mice look absolutely uninteresting. They're able to do spatial learning, habituation, other kinds of tasks without any deficits whatsoever. This seems to be unique to the social domain.
The second was that, for us, we really weren't interested in the genes. We were interested in the biology. And so for us, this just became a tool that we could use to now begin to interrogate, how does the brain normally make a social memory? How does the wild type mouse do it? And so we reasoned that we could use these knockout mice to help inform that because we could ask, what's not happening in the knockout mice that is happening in the wild type when they meet a new mouse, when they become familiar with a new mouse?
Now we know that the olfactory system for the mouse has been well-worked out by people like Richard and others. And there are essentially two parallel systems, one that's the main olfactory system that covers most of the olfactory world as we would know it, and then there's another one for non-volatile kinds of substances called the accessory olfactory system that picks up pheromones and a number of other compounds. And our assumption from the beginning was that they probably have a deficit in the way that they're processing information out in the olfactory bulb before it comes into the forebrain.
This is actually very analogous to the story that Richard Axel told this morning about Drosophila. So you can begin to either measure information processing at the nose, at the first synapse in the olfactory bulb, or here further up into the brain at the places where, whether there's a ghost in the machine or whether there's just some higher-order processing, that percept is then becoming something that has meaning.
And so we reasoned that somewhere along the lines the lights were not going on in the knockout mice when they were going on in the wild type. And so we tried to figure out how that was. We used a very simple method, which is to use Fos. It's an oncogene that turns on in cells when they're activated to do what a poor man's PET scan, I guess. And we found that indeed, all through this area, in both the olfactory bulb and in the amygdala, there was lots of activation when there was social interaction with a new mouse.
But in the knockout mice, though there was plenty of activation in the olfactory bulb-- both the accessory olfactory bulb and the main olfactory bulb-- when we got back to the medial amygdala and its projection sites, all the lights were off. So that told us that indeed it seemed to be that there was an area here in the medial amygdala that needed to have oxytocin to be able to make a social memory. And why that was intriguing to us is that we already knew from other studies we had done that an area right about here has the densest innervation or the densest expression of oxytocin receptors in the mouse forebrain, but there are only a few hundred cells that seem to be really critical at that point.
So the last study that Jennifer did as part of her thesis then was to say, if that's really true, we should be able to rescue social memory in the mice the way we rescued lactation by replacing oxytocin where it's missing. So she did one further experiment, and this is an example where it's a little difficult to follow perhaps. But let me just take you through it very quickly.
In this case, it was giving oxytocin directly into the brain, because you can't just inject it. It doesn't cross the blood-brain barrier. And giving it into the ventricle of the brain, she was able to do a dose response curve to find a dose that worked. Now the way these data are arrayed, it's each experiment is just shown as one bar. So this is the ratio of investigation on the second test versus the first test. So if you have social amnesia and you can't learn, you're spending as much time on the second test as you are on the first. And that's what you see in these bars. But if you are able to learn, like the wild type mice, you're showing about a 50% decrease.
So she took a dose that doesn't seem to work very well when given centrally, and she injected that directly into the amygdala in the knockout mice. And what you see is that when she injected specifically into this tiny area where the lights didn't seem to be going on with social interaction, the knockout mice were, in fact, able to learn. So very quickly, the black shows the control injection in knockouts.
So the control injection here is just CSF, cerebral spinal fluid. And that's failure to learn because they're spending as much time on the second test as the first test. This is when given into the olfactory bulb. Giving oxytocin into the bulb doesn't seem to help but giving it into this one area that's missing is really sufficient to rescue them and to now give them olfactory memory back.
Now the final experiment here is just the converse of this, to take the wild type animals and to say, if this is really the critical area, then if we just remove the peptide from that region, that should be really enough to give us the full knockout phenotype. And that's what you see. So again, if it's under 0.4 here, it means that they're perfectly able to learn. That's normal behavior. If you knock out oxytocin using an antagonist, in this case-- so doing it pharmacologically-- in the olfactory bulb, it has no effect whatsoever. But given into the medial amygdala-- again, this one spot where you've got just a few hundred, maybe a few thousand cells with the receptors-- seems to be really critical.
So to sum up, what I've told you is that at a behavioral level, these mice appear to have a selective deficit in social recognition. Other aspects of learning and memory seem to be retained. The cellular basis of this seems to be due to those receptor-containing cells in a small part of the media amygdala because you can rescue the deficit entirely by injecting into that area. And finally, at the molecular level, it appears that this gene is really critical for processing social information. But the take-home message here is that there's something really quite unique about social memory and social recognition that doesn't follow the rules, or at least doesn't follow the same circuitry and probably has different sets of mediators and different sets of underlying molecular substrates, than other forms of learning and memory. Now is this relevant to humans?
Well, as it turns out, in the human brain, as I mentioned at the outset, there are also very specific pathways for social information processing. But we are not olfactory mammals. We are really-- excuse me-- visual mammals. And so the pathways that we're dealing with here are in the visual system. And actually, Nancy Kanwisher, who works in this building, has probably done more than almost anyone to help to delineate these areas.
There are areas that are higher-order processing areas for visual information-- one that's called the fusiform gyrus being the main one. And the point of telling you this story is that in the human brain, information is parsed in a way that might be different than what we would imagine. And I think really part of the power of modern neuroscience is not just that we now know that mental life is neuro life, that you can find the evidence of mental activity in neuro activity, but you can begin to interrogate the brain to find out, how does the brain make sense of the world? How does it categorize the world? How is information actually processed?
And this is an interesting example of that because here, what you find is that almost all visual scenes are going to be processed through this fusiform area. But the brain separates out fairly early on animate from inanimate objects. And it doesn't matter actually whether you show a picture, whether you show the object itself, or even whether you just give this information semantically giving a card with the name hammer, saw, shovel on it. You're going to activate mostly medial regions of the fusiform. If you show another card with camel, dog, face, animate objects, you're going to be more lateral. So this is a so-called face area that's in the brain, and it's been now interrogated in the disease of autism to ask whether there's something abnormal in the way that autistic people or people with Asperger's syndrome-- part of the autistic spectrum-- whether they're processing facial information differently.
This is work from the Yale group led by Bob Schultz. It's published about three or four years ago. It's now been replicated a total of nine times using a fMRI study. And what was done here is people were shown faces, objects, or just matched visual activity. And what you see when you do that is that, indeed, this area of the brain, the fusiform gyrus on the bottom of the brain-- very important active for encoding face information-- is activated in healthy individuals but not in people who have Asperger's or who have autism. So in some way, the information simply isn't being processed where it should be processed in the brain of someone with autism.
Now the initial interpretation of that information was that there could be a lesion in this area, as if these people had had failure of a region to develop in the brain and therefore, weren't able to process the faces. It's actually much more interesting than that. It turns out that if you really look carefully at what's going on when people are in the scanner and shown pictures of faces, the problem is that they're not looking at the face. They're never encoding it because they're looking someplace else. And there was no control in any of these studies to make sure that people really were looking directly at the part of the face that they needed to to activate the fusiform gyrus.
And if you do that experiment and you ensure that people with autism are in fact encoding that part of the face with the eyes, that seems to be essential for activating this area, you can indeed activate the fusiform gyrus. But what you do at the same time is you increase the activity in the amygdala at a very high rate. So there is something about gaze avoidance and face avoidance that's really intrinsic to this disorder, something that we don't really understand very well. But it does tell us that there's something about when people with this disease do begin to look at the face and encode that information, they're able to do it. But they find it in some way aversive, or at least it's one of the things that seems to really activate the amygdala.
Now, a final story about this-- which isn't yet published but will be out very soon-- is that when you have people actually take oxytocin, to get back to the oxytocin story-- and this is now not with autism but with just healthy volunteers-- and you show them faces but now not just any face but threatening faces, you can greatly activate-- well, if you do this in any one of us, you'll activate the amygdala if the face is threatening in some way. It was any face might be that way for someone with autism. But if you take actual threatening faces and you have a person stare at those faces while they're in the fMRI scanner, you will turn on the amygdala big time. And that's what you see here, and you can see that again even with threatening scenes or looking at really catastrophic scenes.
What's interesting is that if you have healthy volunteers take oxytocin-- and it has to be done intranasally-- you very selectively turn off this activation in that particular part of the brain. So you see it both with faces and scenes, but it actually seems to be more true in this case-- this is the subtraction image-- more true with faces than with scenes, suggesting that indeed there is something quite specific, again in humans, about how oxytocin alters social information processing.
So to sum up, I think what we have here is a real opportunity to understand-- both through these genetic techniques and through neuroimaging techniques-- a lot about how social information and social cognition gets processed. We're beginning to identify the pathways in the brain that really count. And I think the comments that were made this morning were right on, that the real heart of the matter here is not going to be just the genes. It's going to be understanding the circuitry, but the genes help us to get to the circuitry. And that's what we found here.
So the information comes in to primary sensory pathways-- maybe olfactory, maybe visual maybe auditory, depending on the species-- and the question is answered. Is this social? At that point, subsequent pathways within the brain or subsequent weigh stations, like the amygdala, temporal cortex, prefrontal cortex, began to instantiate this then with value, to ask the question, who is this? Familiar? Novel? Is this somebody important to me, not important? Friend? Enemy? All of those kinds of issues that really require a lot more integration of information based on past experience and based on being able to decode what's coming in from a primary sensory pathway.
You go from there to a much more interesting area that Eric began to allude to this morning, to parts of the brain that have to do with reward, when this now becomes not only instantiated with meaning but with also hedonic value. Is this someone who is very important to me who I would die for, somebody who I'm pair bonded to, somebody who will take care of me? All of those issues, which happen, actually, in a different part of the brain altogether where we know if you have oxytocin and vasopressin receptors, you're more likely to show very high affiliative behavior and to again, infer that social information is extremely important.
And finally, there's a response axis that we haven't talked about. I think someone very affectionately called it the id this morning. But it's that piece that comes out of the hypothalamus that provides both the endocrine and the motor responses that are really important for the our interaction with the social world. So that is my social memory story. I thank you very much for your attention, and I look forward to a bit of time for questions. Forgot my water.
MODERATOR: Okay. We'll just spend some time over here answering questions if you have in the audience. I'll get the ball rolling while people make their way to the mic. One question I had was, are there other hormones that lend themselves to the same kind of investigation?
INSEL: Well, so there's a lot we don't know. And I think one of the things to remember is that we're at such an early phase in neuroscience in general. We know that there are 20,000 genes in the human genome, and we know that there are probably 500,000 proteins that are encoded by this genome. And almost everything that we've written about and talked about up until the last two or three years is based on probably less than 1% of what's there.
So we're really very much in a discovery era. I would be very sad if, in 2053, all we had to talk about were the very things that we had discovered in this first 1%. So I would assume that there is a lot more out there to work with. And I have to say I'm very envious of all the people up there who are going to have a lot longer to do that than most of us down here.
MODERATOR: You have a question up there? Yes?
AUDIENCE: A quick question for [INAUDIBLE]. Has oxytocin been tested in autism?
INSEL: Oxytocin been tested in autism. It has been tested giving it intravenously, which is underwhelming. I'm not sure how much of that would even get into the brain. It's not been given yet intranasally, which we know does get into the brain but not at a very high efficiency. But that does work. It hasn't been done.
AUDIENCE: A quick question is [INAUDIBLE] social recognition by overtraining the animals or [INAUDIBLE] between the training and the testing?
INSEL: Right. So can we overtrain by changing the interval. It's really remarkable. In the knockout mice, this is an extremely dense social amnesia. It's so dense that even if you have given them only a few seconds of time apart, they still show no evidence of recognition.
MODERATOR: Okay. We'll go to this side of the room. Well, while you're getting over there, let me ask you this question. Why do you think so little research has been going on in this field?
INSEL: Oh, why has so little--
MODERATOR: Is it an NIMH priority to--
INSEL: Well, we're interested in social neuroscience, generally, as is the drug abuse agency and others. I think that probably the-- as with any other area of science-- you go where the action is, where you have good candidates, where you have good models, and where you have good tools. And frankly, we have not had a lot of that for something as complex as social behavior. From my perspective, I think Cori Bargmann really got us started with the C. elegans work and her gene-- which, you'll appreciate this. The gene's called NPR, but not related to the radio.
MODERATOR: So he's looking for money.
INSEL: But that really was the first evidence I think that any of us had that you could attack what seemed to be complicated behavior with a rather simple genetic approach, that even a single gene could have fairly profound effects on a complicated behavior. And I think-- just to add one thing to that-- I think it helped us to realize that we're not even smart enough to know what is a complicated behavior, that we shouldn't be thinking complicated, simple we should possibly be thinking conserved, derived. If it's a conserved behavior, it may be more likely to have a relatively simple molecular basis. And I think her work got many of us going, thinking much more clearly about that.
MODERATOR: Last question here.
AUDIENCE: Back in the dark ages of psychoanalysis, some of us maybe remember, there was, of course, the concept of cathexis. And you would cathect somebody if you developed strong emotions directly with that person. And one of the goals of analysis was to decathect. And of course, one wonders now whether the decathexis process might have had some reality to it and might have been related to changes in the numbers of oxytocin receptors. And in that vein, I wonder whether there is the possibility that oxytocin antagonists might be useful in treating panic disorders that are occasioned by individuals that the person sees.
INSEL: Well, I think before we jump into thinking about how to use this in people, we need to know a lot more about what it does. But one of the things that's so important about this system is the species' specificity of the peptides' effect. So what you see in mice is not quite what you see in rats, and what you see in rats is not what you see in voles. So I'm a little loathe to jump immediately from the mouse to the human. We've got the first human study, which isn't even yet published, on what the peptide seems to do in the brain. That will be out very soon.
But I have to say, that is promising. I think it does tell us that there are effects. I'm a little bit almost speechless about this. I've heard more about psychoanalysis here at MIT this morning than I have in three years at NIH. So my last thought is to refer your question to Dr. Kandel.
MODERATOR: Nothing personal. Thank you very much for taking the time to talk with us.
INSEL: Thank you.
MODERATOR: All right. Moving along. Our next speaker is looking at a different aspect of neurology. Let's talk about Alzheimer's disease for this period. It's one that is surely affecting more and more of us as the population ages, yours truly included. Our next speaker, Dr. Li-Huei Tsai of Harvard, has a laboratory that is trying to understand the underlying mechanisms of Alzheimer's. And if I read her research-- well, she'll explain it to you herself. Please welcome Dr. Li-Huei Tsai.
TSAI: So I feel extremely honored and humbled to be on this podium today to speak with you. So I just go right into my talk. Alzheimer's disease is a irreversible neurological disorder that progressively attenuates the cognitive abilities of those afflicted. I think many of us in this room have seen the effect of this disease in a elderly family member or a friend.
It was first documented almost exactly one century ago by this man, a German physician, Dr. Alois Alzheimer, describing a female patient, Auguste Deter, who was not able to care for herself, disoriented, with impaired memory. This disease really represent a great human tragedy, which can be appreciated by the following quote. "In Alzheimer's disease, the mind dies first-- names, dates, places-- the interior scrapbook of an entire life-- fade into mists of nonrecognition."
The disease is the most common among all human neurodegenerative disorders, with about 20 million people afflicted worldwide. And age is the single most significant risk factor for Alzheimer's. Currently, about 25% of those over 75 years old have the disease. So with life expectancies continue to rise, the victim of this disease is expected to triple by the year of 2050.
Alzheimer's patients who inherit the disease from family members are known as familial Alzheimer cases, and this is extremely rare, only representing less than 5% of all patients. So most of the patients are sporadic. Therefore, that means the disease is very complex. And to date, there is essentially no therapeutic strategy against this disease.
Brain is known to be severely reduced in size, with about 30% to 40% reduction of total brain volume in severe cases. But if you look more closely, there are two signature pathological features-- amyloid plaques and neurofibrillary tangles. The plaques are protein deposition in the brain composed of a-beta peptides with 39 to 42 amino acid in length. The tangles are intracellular aggregates of straight and paired helical filaments composed of hyperphosphorylated tau protein.
But one very important feature of Alzheimer's disease that sets it apart from most of the other neurodegenerative disorders is the specific brain areas that are affected by Alzheimer's pathology. Neurofibrillary tangle, as well as brain atrophy, always initiate in the hippocampal formation, part of the limbic system essential for learning and memory-- that you heard a lot about this morning. The pathology then spread into the adjacent cortical area, including the temporal lobe and subsequently to the prefrontal lobe-- important for other higher cognitive function. And eventually, in the terminal stage of the disease, the entire cortical area is affected.
And our initial understanding of the disease actually comes from the small percentage of the familial cases. The important role of the a-beta peptides, especially a-beta 42, in the early stage of the disease is supported by the familial Alzheimer disease mutations that preferentially elevated the production of a-beta 42. Therefore, the amyloid hypothesis predicts that accumulation of a-beta in brain, plaque formation, can trigger cascades of events eventually leading to dementia.
Recently, it is proposed that it is the oligomeric soluble form of a-beta that's toxic. In any case, elevated a-beta can change the cellular properties of neurons, induce neurofibrillary tangle formation, caused neuronal death, culminating in dementia. So the amyloid hypothesis nicely explain the familial cases of Alzheimer's disease.
However, the cause for the sporadic Alzheimer disease cases, which account for the greater than 95% of all patients, remain to be elucidated. And also, it is really not clear why only certain areas of the brain are selectively vulnerable to Alzheimer's and also, why age is the single most important factor. So I think it is reasonable to speculate that there must be other important factors contributing to the development of Alzheimer's disease.
So based on the pathogenic features of Alzheimer's, several years ago we predicted that the other factors contributing to Alzheimer's must fulfill the following criteria. We speculate that the factor must be abundantly present in the vulnerable area of the brain, and it should be able to cause neuronal death and brain atrophy, as well as amyloid and neurofibrillary tangle pathology, at least in an animal model. And finally, since the most vulnerable areas of the brain for Alzheimer's are exactly the same area essential for learning and memory, one can speculate that the factors might be normally involved in learning and memory but somehow can lead to altered learning and memory in the disease.
In the past several years, our research, and later on joined by other laboratories, provide compelling evidence that a small protein kinase, Cdk5, may fit the bill. Recently, polymorphisms and a splice variant of Cdk5 have been linked to Alzheimer's disease. As a protein kinase, Cdk5 is highly unusual in that it has to associate with a regulatory activator to become [INAUDIBLE] active.
And there are two such activators present for Cdk5, and we all have these two proteins-- p35 and p39. Cdk5, p35, and p39 are all specifically expressed in neurons. In fact, the highest levels of p35 and cdk5 expression are in the hippocampal formation and certain cortical areas. This is a in situ hybridization using p35 probe of a sagittal section of the mouse brain.
And similarly, the coronal section showed the same thing, that several areas in the hippocampal formation express high level of p35 as well as the piriform cortex, which is equivalent to the temporal lobe in human brain. Now it is important to know that p35 comes in two flavors. A protease known as calpain can chop of the end terminal one third of p35 and produce a smaller species, known as p25. P25 is more potent in activating cdk5, and it can cause hyperactivation of cdk5.
Interestingly enough, we found that p25 actually accumulates during aging. Here, I'm showing you p25 and p35 protein levels in a mouse brain in different ages. And you can see that while the p35 protein level remain more or less constant, p25 is much higher in a 28 months old mouse brain-- which is very, very old for a mouse-- than a seven-month-old mouse brain.
We have also previously shown that the p25, p35 ratio is generally elevated in post-mortem brain samples of Alzheimer's disease. And we recently obtained further evidence that this ratio is already elevated in the preclinical stage of Alzheimer's disease, known as mild cognitive impairment, and also in early stage of Alzheimer's disease. And the elevation is observed in the hippocampal formation and in the prefrontal cortex, areas susceptible to Alzheimer's disease. Therefore, p25 accumulation, leading to hyperactivation of cdk5, occurs during aging and in the development of Alzheimer's disease.
So if p25 contributes to the development of Alzheimer's disease, one will speculate that it should be able to induce Alzheimer disease pathology in an animal model. To this end, we created a p25 mouse model that specifically produces p25 in the vulnerable area of the brain. A more important feature of the mouse is the fact that we can actually turn on and off p25 expression in the brain just by feeding the mice with different kinds of diet, and this really gave us a lot of flexibility to test the potential role of p25 in neurodegeneration.
As it turned out, production of p25 in mouse brain has a very severe deleterious effect. First of all, brain is severely reduced in size. I'm showing you a small animal MRI scan of a control mouse brain and a p25-producing mouse brain. This is 10 weeks after p25 production. And you can see the volume of the brain is massively reduced.
In general, we see a 30% to 35% reduction of the brain volume after production of p25. And this is clearly due to a very massive death of neurons, first in the hippocampal formation and later on spreading to the cortex. The upper panel here representing control normal mouse brain, and the bottom panel here are the brains producing p25. In addition, there is also massive infiltration of inflammatory cells in the afflicted area, another indication for neurodegeneration.
After prolonged p25 production, we see evidence for neurofibrillary tangle formation. Here, I'm showing you hyperphosphorylation of the tau protein using antibody staining and also filamentous tau protein formation using electron microscopy. And after about 30 weeks of p25 production, we can see evidence of neurofibrillary tangles with silver staining or with Thioflavin S staining.
Finally, another very revealing piece of information about p25 and Alzheimer's is the fact that p25 actually has a profound effect on the production of a-beta peptides. Here, I'm showing you some biochemical evidence for the level of a-beta peptides in a mouse brain with p25 production in green and without p25 production in red. So here, if you look at a-beta 42, the species of a-beta peptides particularly relevant to the etiology of familial Alzheimer disease is elevated by several fold.
A-beta 40 is also somewhat elevated now to a similar extent of a-beta 42. This elevated a-beta peptides accumulate inside neurons and readily form the aggregates looks filamentous in appearance. And this is accompanied by the appearance of condensed or pyknotic nuclei, indicative of sick or dying neurons. So, so far, I show you that cdk5 and its regulatory activator are abundantly present in the vulnerable areas of the brain for AD. And in an animal model, it can cause neuronal death and brain atrophy. It can also cause amyloid and neurofibrillary tangle pathology. And I haven't addressed the question, whether cdk5 is normally involved in learning and memory and whether hyperactivation of cdk5 can lead to altered learning and memory in Alzheimer's disease.
In fact, there is already a large body of literature addressing the role of cdk5 in regulating learning and memory. Especially in several animal models, it has been shown that reduction of cdk5 in mouse with pharmacological inhibitors can impair learning ability. Also, pathological function of p35 in mice through genetic manipulation can also result in impaired fear conditioning learning.
Recently, our lab produce and publish result indicating that inducible mice with cdk5 loss of function in the hippocampus show learning impairment in the Morris water maze test. So these observations suggest that cdk5 is normally involved in regulating learning and memory. So the Morris water maze test is a very well-established paradigm testing the spatial learning in mice. And actually, Dr. Richard Morris is in the audience today.
And briefly, mice are placed in a swimming pool filled with murky water and a platform is inserted in a swimming pool submerged from the water so the mouse cannot see it. There are also various visual cues placed in the room to facilitate orientation. So the mouse can only escape from swimming if they can manage to locate the hidden platform by learning where the platform is in relation to those cues. So it usually takes very intensive training for the mouse to learn, takes on average 10 to 14 days. And this is a typical learning curve for a normal mouse. With each day of training, it takes less time for the mouse to find the platform.
I also prepare a couple of movie just to give you some idea what it's like. This is a mouse first day in training. And this is the same mouse 10 days after training. And you can see that it behave very differently since know exactly where to go and quickly escaped. And one can also perform a so-called probe trial, which involves removal of the hidden platform from its usual place and then place the mouse back to the swimming pool and just follow a similar trajectory. If a mouse learned where the platform is, then it tends to spend a lot more time in the area where the platform used to be placed.
So when we applied this training paradigm to mouse expressing p25, if we just produce p25 for a very short period of time, we see something very intriguing, which is that mouse with p25 production actually for a few days actually locate the platform more quickly than the control animals. And when we perform the probe trial, we found that if the probe trial was tested six days after training, the transgenic animals actually already show a preference for the target quadrant. The control animals don't show such preference. But if we performed a probe test 10 days after training, then both the control and the transgenic animals both show preference for the target quadrant.
So this observation suggests that very acute p25 production actually facilitates learning and supports the notion that cdk5 normally positively regulates learning ability. Conversely, if we applied the animals after a prolonged p25 production to such a learning paradigm, then what we see is that the transgenic animals now show very impaired learning. This can also be shown with the probe trial after 10 days of training. While the control animals now clearly show a preference for the target quadrant, the transgenic animals do not show any preference. And they have severely impaired learning ability.
So with this observation, we can say that cdk5 is normally involved in learning and memory, but hyperactivation can lead to altered learning and memory, reminiscent in Alzheimer's disease. So in summary, today I show you that accumulation of p25 in mouse brain and therefore, hyperactivation of cdk5, can lead to pathological features reminiscent of Alzheimer's disease.
This include learning and memory impairment, neuronal loss and brain atrophy, neurofibrillary tangle pathology, and increase a-beta production and aggregation. And I think this is really quite remarkable. And it is probably the first animal model where expression of a single transgene can lead to the manifestation of all of the pathological hallmarks of Alzheimer's disease. And this observation suggests that cdk5 is causative for Alzheimer's disease.
So how can I reconcile this model with the amyloid hypothesis? We know that increase a-beta peptides can lead to Alzheimer's disease and dementia. And today, I also show you that p25 accumulation as a result of aging and other Alzheimer disease risk factors that I don't have time to tell you, leading to cdk5 hyperactivation can also lead to the development of all of the Alzheimer's disease pathology.
Importantly, p25 accumulation can lead to increased a-beta, especially a-beta 42 production. And conversely, we previously, and other labs, also showed that increased a-beta can lead to p25 accumulation. So here, while we have in terms of the pathogenic mechanism of Alzheimer's disease is really a feed-forward loop that amplifies itself until cells die.
So what kind of implication does a feed-forward mechanism for all disease have for therapeutic intervention? Currently, most effort in the pharmaceutical industry is to target a-beta peptide or to prevent its accumulation. But this is probably not sufficient as we know that there are other mechanisms that can lead to the development of Alzheimer's disease pathology as well. So effort should be made to target multiple components of the vicious cycle to efficiently combat Alzheimer's disease.
For instance, if there are drugs that can prevent p25 accumulation or inhibit cdk5 hyperactivation, such drugs should be very beneficial in addition to the a-beta drugs. But even after that we are successful in halting the deleterious process in the disease process, I think we still have a big task in front of us, which is with this Alzheimer disease patients with impaired learning and memory, how we can restore their learning ability and help them to retrieve their lost memory. And I think this really take a multidisciplinary approach combining different forces in neuroscience research to efficiently combat this devastating human illness.
So I would just end my talk with this very ancient quote. "Extreme remedies are very appropriate for extreme diseases." And these are two very famous victims of Alzheimer's disease. Thank you very much.
MODERATOR: We'll be taking questions now. Where do I sign up to get on that company that-- how close do you think you are to having solved the whole Alzheimer's riddle? I mean, you've got two different ways it happens. It seems logical. What do you need to do to prove this?
TSAI: Well, I actually think that among many other neurological disorders, we are making very good progress on Alzheimer's disease. And I think a lot of the mechanism are being actively studied by many, many people. And I think this is a very promising stage in the search for Alzheimer's.
MODERATOR: Would it be possible to move this to human trials in the future?
TSAI: I think that many of the remedies have been in the process for human trial. And I personally am quite optimistic about maybe having some beneficial therapeutic approaches in the next decade or so.
MODERATOR: Have you been approached by any drug companies in the audience? Any questions? Yes, right here. Let's go right here, and then we'll go in there.
AUDIENCE: It was a very nice talk. I enjoyed it.
TSAI: Thank you, Richard.
AUDIENCE: Have you explored the impact of calpain inhibitors in any of your experiments?
AUDIENCE: That's the first question. And then the second is-- trying to think this through-- the implication of your argument particularly towards the end of your talk would be that if you were to cross your mice with APP null mice, you would still get some of the pathology. I'm wondering whether either of those thoughts have--
TSAI: I really love those questions. Yes. So first question, in terms of calpain, so those inhibitors, we have tried. In the culture system, they work. They are very beneficial. But apparently, they are very-- so far all the impeders, none of them can be a good drug pharmacokinetically. So I think it's still a ways for drug company to develop better inhibitors.
And your comment about crossing our mice to APP null or mouse now for one of the APP processing enzymes, I think it's going to be a very important experiment because, again, I believe that this is a vicious cycle. And I believe that a-beta clearly in our mouse model probably contribute to the pathology, but we have to know to what extent.
MODERATOR: Okay. There was a-- yes. Other gentleman.
AUDIENCE: Wonderful lecture. And I understand you're coming here. Is that correct?
AUDIENCE: Yes. With your knowledge of neurodevelopment as well as neurodegeneration, if one were to design studies in humans, at what point would you think it would be best to intervene? What kinds of ages, for instance? That's a first question. The second is, any implications of your research for post-traumatic stress disorder, which you know also is the hippocampus is implicated in?
TSAI: Yeah, thank you for those. So first of all, the first question is at what stage is the best window for therapeutics? You know, I think one reason why Alzheimer disease now is probably one of the most intensively invested program in all major drug companies is simply because there is this perspective of it's a matter of whether people live long enough to develop this disease. So I believe some of the drug companies have this philosophy that if they can come up with some sort of preventive medicine, then essentially everybody would take it. So the market potential is just unbelievably large.
AUDIENCE: Actually, I work for Harvard.
MODERATOR: You work for Harvard? Oh, sorry. Any other questions? Thank you very much. That was very informative. All right. Moving right along. Our next speaker says, "The main question I've been interested in is how the environment shapes the brain. How can we use hard science to understand the actual mechanisms of learning and memory as far as that shapes who we are?" Dr. Kerry Ressler wants to know how emotional events change your brain-- and my brain too, I imagine. As assistant professor of psychiatry at Emory University Center for Behavioral Neurosciences, Dr. Ressler works with victims of post-traumatic stress disorder and is trying to understand its mechanism. Please welcome Dr. Kerry Ressler. Where is-- oh, there he is.
RESSLER: Thank you very much. First of all, Michael Davis deeply regretted not being able to be here today. And it's a real honor and privilege for me to return to MIT to present our work. I'm going to be talking about learning memory and psychiatric disorders.
Many psychiatric disorders, particularly those involving fear, can be viewed as disorders of disregulated emotional learning. However, current medication approaches target the general symptoms of anxiety and fear but not the underlying causes that we talk today and are often inadequate for these disorders. And we would like to really ideally enhance the targeted learning that modulates specific memory pathways that underlie pathological emotional reactions.
I'm going to be talking about anxiety disorders and specifically fear disorders. The psychiatric disorders of anxiety can really be broken into three groups. The primarily anxiety disorders, primarily generalized anxiety disorder, are really a very dirty group and are highly overlapping with depression and pretty much clinically unable to distinguish from depression. Obsessive-compulsive disorder is really in a class by itself and probably uses different neurocircuitry and certainly has different types of symptoms.
We're focusing on the disorders of fear-- panic disorder, specific phobias, social phobias-- also called social anxiety disorder-- and post-traumatic stress disorder. These disorders are all very clinically similar in that they all have fear as a central theme. And if you do formal factor analysis of all the anxiety disorders, these all cluster together based on their fear symptomatology. And one way of thinking about the symptomatology of these disorders is the concept of a panic attack. And this is most associated with panic disorder but occurs in all of these disorders of fear.
And when patients with fear-based disorders have a either very subtle cue, as in panic attacks, or a very robust cue-- like in specific phobias. If you're afraid of spiders, it's quite clear what your cue is-- they have an increased heart rate and chest discomfort when they have this fear reflex-- chills, hot flashes, and sweating, GI symptoms of nausea and abdominal distress, and pulmonary symptoms of shortness of breath, faintness, lightheadedness. All of these really culminate in a fear reflex that occurs in the brain. And with the secondary cognitive symptoms of fear of dying and fear of losing control.
What's also interesting about the fear disorders is that they all involve the amygdala. And imaging studies have shown this, and years of neurobiology headed up by pioneers like Michael Davis, Joe LeDoux, Mike Fanselow, Steve Maren, all of these, have really shown that the amygdala's critically involved in fear learning. And the amygdala is on this medial temporal lobe. Tom Insel showed a nice picture of the amygdala activation with fear. And it's made up of multiple different sub nuclei.
And we are going to focus primarily on three of these sub nuclei-- lateral amygdala, basolateral amygdala, and central amygdala. And the lateral and basolateral regions are primarily involved in the learning of fear. If you take an animal or a person who's previously not afraid but you associate a previously neutral cue with an aversive response of pain, then learning occurs, such that in the future that same previously neutral cue will now predict a fear response, predict something to be afraid of, and that activates the central amygdala.
And what's so exciting about studying the fear disorders is that these years of work by prominent neurobiologists have shown that there's hardwired systems from the central amygdala to various brainstem regions-- can we back up one-- brainstem regions and hypothalamic areas that activate all of the symptoms that look like a panic disorder. Specifically, I'll be talking about startle response and freezing as two of the measures that we can quantitate in animals to study fear.
I'm going to talk about post-traumatic stress disorder as one example of fear-based disorders that's particularly relevant. It's a clinically important disorder. It occurs in 5% to 10% of the general population, up to 25-or-more percent of veterans depending on the war. My particular area of interest is in people who live in the inner city in very violent areas. And you can see rates of PTSD as high as 35% in those populations.
PTSD can be a very serious disorder, and people with severe PTSD, they have very high comorbid depression, psychotic disorders, alcoholism, substance abuse, often homelessness, inability to have relationships. It can be as devastating as the most devastating depression and psychotic disorders. What's interesting about it is that we know when it starts.
PTSD requires in its diagnosis an exposure to an extremely traumatic stress that revolves intense fear. And it has several different sets of symptoms that go along with it. First of all is the re-experiencing of the traumatic event, the inability to get these memories out of one's head. They haunt patients with PTSD over and over again for years.
Avoidance of stimuli associate with this trauma, and this is really avoidance of stimuli that will reactivate the fear system. And so it becomes avoidance of first people and specific things that remind them of the fear, and then it gets generalized to avoidance of everything and staying in one's house or something. Numbing of general responsiveness also can probably be thought of as emotional avoidance of emotions that remind them of the fear. And symptoms of increased arousal. People with PTSD often have a free-floating fear. They feel afraid all the time, never safe. This is a video of a woman who had been attacked and had many symptoms of PTSD. And now she's recovered in many ways but is talking about what it's like, the memories are like.
INTERVIEWEE: You can't get-- you cannot get those horrible thoughts-- it's just a repetitive wheel that keeps turning. And it interferes with your everything. And you're sitting there trying to feel happy or try to think yourself happy and just say, please, take those thoughts out of my head. And you constantly are trying to distract yourself or set up little different props just to try to get those thoughts out of your head.
RESSLER: She can't get the thoughts out of her head. Another nice thing about fear-based disorders is it's built upon a hundred years since the time of Pavlov of psychological understanding of fear. One of the most interesting things as well about fear-based disorders is not everybody gets it. If you take the case of two soldiers who go off to war-- both of them have horrible experiences, see awful things, come back. One of them is able to recover, carry on, have a productive and a happy life. Another one may come back and never be able to recover, never be able to work, have relationships.
It's thought now that up to 40% of the variance of who gets PTSD versus who is not given a trauma is mediated by the genes. And that's an interesting area for future research. Once the person experiences the traumatic event, they effectively learn the association of whatever they were experiencing at the time with a fear. And that's something we can model in animals-- the learning of fear.
After they've learned the fear and it's been consolidated, the person has cues that remind them of the event-- whether consciously or unconsciously-- and can elicit all the symptoms of fear. And again, in the pathological disorders, Khan lists the symptoms of PTSD. And this lasts for years and years.
Whether people get PTSD or able to recover depends on a number of different behavioral processes. But two of those that are in contradistinction to each other are sensitization versus extinction. The person with PTSD, as they reactivate these memories, the memories tend to get stronger. They avoid more. They're more afraid.
Other people are able to recover. Each time they have these memories, somehow they're able to extinguish slowly over time the fear reaction so that over time, they're able to separate the current context and situations from the ones that actually occurred with the original traumatic event. And we're going to be talking today just about extinction as one thing we can understand, because it turns out that extinction-- sorry. My animation's not quite working.
Extinction is a learning process in and of itself. Extinction is defined as the gradual reduction in fear reaction over time with repetitive exposure to cues that are not reinforced. It involves a new form of an inhibitory learning that's dependent upon the amygdala. It's been shown that it's not an erasure of the previous fear memory. Rather, it's an ongoing new safety memory that appears to occur in parallel with that fear memory. And we think it's the basis of exposure-based psychotherapy.
Fear extinction is dependent on many molecules that are important for learning, and I'm going to be talking about two examples today. Neurotrophins, specifically brain-derived neurotrophic factor, and glutamate, its activation on the NMDA receptor. We've talked a lot about the synapse today. It's basically the atom of connection between neurons. It's thought that learning and memory involves a change in the strength or the connectivity of synapses, and that occurs through a lot of different molecules.
And the two examples I'm going to talk about today are glutamate and BDNF. And their activation through the NMDA and TrkB receptor, respectively. Both of these pathways are able to, through the process of exposure to cues that are not reinforced, activate learning that then leads to extinction of fear. What we would like to do is be able to somehow increase the activation of these receptors and therefore enhance the extinction of fear in people. So the goal is if we can improve learning based on knowledge from basic science, we can then improve fear extinction in humans and thus improve the specific effects of psychotherapy in disease.
So how we study this? Well, one way to study fear in animals is the fear-potentiated startle reflex. And this was really pioneered by Mike Davis. And it's based on the idea that all mammals have a hardwired acoustic startle response. If I clap right now, we're kind of asleep. You're not really going to have much response. If you hear that same clap as you're walking down a dark alley tonight, you might jump out of your skin. And that's the fear-potentiated startle response, and it's conserved across mammals.
And what we can do is take a rodent, and we can pair a light with a shock or a tone or a noise. We can then test the animal in the presence or the absence of that cue and look at their startle response. And in the presence of the cue for which they're afraid, they're going to startle a lot higher. That's the fear-potentiated startle effect.
We can also measure freezing, which is a very conserved fear response across animals. Notice as these animals start walking, the one on the bottom is curious. They're both curious, exploring around. Tone comes on. That one's afraid. Scurries over to the side and freezes. And this one continues to explore. This one's been genetically modified to not be afraid.
So there's several different ways that we can study molecules involved in synaptic plasticity and their effects on learning of fear or extinction of fear. One way is genetic methods, and you've heard several talks today about using transgenic approaches. One thing that we do that's slightly different is genetically modified viruses to be able to over-express specific genes or proteins in neurons in specific brain regions. And this just shows a rat brain, which we've injected a virus that expresses green fluorescent protein within the amygdala. And we can see nice neuronal expression of that protein.
Because brain-derived neurotrophic factor doesn't have any good pharmacological agents that allow us to manipulate it, we can only really use genetic type methods to study this kind of synaptic plasticity molecule. And the top figure shows after either learning of fear or extinction fear, we see significant increases in brain-derived neurotrophic factor expression-- and this is a in situ hybridizatoin study-- in the amygdala in animals that have recently extinguished. They are learning a new connection, and that increases synaptic plasticity gene expression.
If we use this genetic approach to block BDNF, what happens? On the bottom, we've taken two groups of rats and trained-- them to be afraid. And then we give them a virus. One virus expresses this controlled GFP gene that just allows them to be green. Another virus is a dominant negative protein that turns off the TrkB receptor for BDNF. Then we extinguish the animals to fear and look at how afraid they are.
The control animals extinguish nicely. Their startle in the presence of the cue compared to the absence of cue is very similar. They're not very afraid. The TrkB blocker animals still have significant fear and are unable to extinguish. This is one example that synaptic plasticity molecules and synaptic plasticity within the amygdala is required for normal extinction of fear. What we'd really like is to have pharmacological approaches.
And one nice model for this is the NMDA receptor. There are many labs around here and around the country that study the NMDA receptor. It's one of the best understood molecules involved in synaptic plasticity and learning and memory. We are going to focus on one particular pharmaceutical agent, d-cycloserine, which acts at a particular site on the NMDA receptor and enhances the efficacy of the NMDA receptor. This figure here just shows there's plenty of NMDA receptor expression within the basolateral amygdala of the rodent and also the human.
What Mike Davis, Bill Falls-- who's now at Vermont-- and others showed in 1992 was that NMDA receptor activation is required for normal extinction of fear. These are rats that received fear training, and this is the measure of fear-potentiated startle. This red graph shows that before extinction, they're all quite afraid. They then receive 60 paired lights without shocks, the extinction paradigm, and the extinction extinguished very nicely. If however, you give an NMDA blocker, called AP5, within the amygdala, then-- if you could back up one-- then you block the extinction effect. NMDA is required in the amygdala for normal extinction.
Well, we wondered recently, then can we enhance the process of an NMDA function and thus enhance extinction? So to do that, we did a partial extinction paradigm in which, rather than 60 lights, we only gave 30 to only partially decrease the fear. And we gave this drug, d-cycloserine, in combination with those 30 lights. And sure enough, we found a significant decrease in fear in those animals.
Well, one possibility is that this drug just decreases their anxiety or their fear in general. So a control experiment that we did was to give the animals the drug alone after they'd been trained but not in the presence of the extinction learning paradigm. And we found that there was no enhancement of extinction. So to enhance extinction with d-cycloserine, it has to specifically be given at the time of the new learning event.
Dave Walker in the lab went on to show that this was specifically involved in amygdala-dependent learning and memory. They gave-- this throws, again, the amount of extinction with 30 lights or d-cycloserine given within the amygdala with, again, very significant enhancement of extinction. Using d-cycloserine to enhance extinction of fear, we've now replicated this in mice and multiple labs, notably Rick Richardson's lab in Australia has already replicated it several times in rats and with both startle and freezing. It's been replicated in other places and also using conditioned taste aversion as a model of social phobia.
One of the reasons that we were really interested in d-cycloserine one potential agonist for the NMDA receptor in the first place was that this is a drug that had been FDA approved for about 30 years for the treatment of tuberculosis, and it was found out in that late 1980s that it was a partial agonist. At much lower anti-tuberculosis doses, it was an agonist of the NMDA receptor. So we were able to rapidly move to human trials.
And to do that, as has been talked about several times today, we wanted to find a way that we could control the stimulus of psychotherapy. Well, that's obviously a very hard thing to control. So we approached Barbara Rothbaum, who's a professor of psychology at Emory and is one of the pioneers in using virtual reality for exposure-based psychotherapy. And what they had shown is that if you take people who have a significant fear of heights and have them go up in a virtual environment, up a virtual glass elevator, six to eight different therapy sessions, they'll get a significant reduction in their level of fear.
And this generalizes to the real world. They're able to drive over bridges they couldn't before, go up buildings they couldn't before. So we did a double-blind, placebo-controlled study where we had only a suboptimal treatment-- again, only two treatment sessions-- and they received either placebo or drug. The patients, the therapists, and assessors were all blind to the treatment. And for the treatment, they only got two pills for the entire study-- one prior to each of two sessions. This just shows that the groups had similar levels of fear prior to treatment. This is subjective levels of fear and what floor they're on. And this actually is the data from the first treatment trial showing that the drug onboard alone does not have any effect on anxiety.
We then looked after two treatment sessions to see if there was any improvement. The group that got placebo had no significant improvement, and we expected that. They only had two treatment sessions compared to the six or eight that we knew from previous studies are required. However, the group that got d-cycloserine showed significant reductions in fear. This was at one-week follow up. We asked them to go away and come back three months later. And if we could have the next one.
And we found that this difference was maintained. So what this suggested is that their subjective measure of fear after only two treatment trials was significantly decreased when the exposure extinction trial was done in combination with d-cycloserine. In addition to these subjective measures, we also did measured galvanic skin response and showed that that was significantly less in these patients.
One other measure that we asked was, was there any real meaning in their lives? Were they able to do things that they weren't able to do before? And one way to measure this, you just say, how many times did you go up buildings and bridges since we saw you last? And those who received the d-cycloserine in combination with the two treatment trials had significantly increased ability to go out in the world and not avoid heights.
So this was a very well-controlled, basic study of phobias. We now have ongoing studies looking at post-traumatic stress disorder that we don't have the data for at this point. There are also studies of panic disorder that the blinds are going to be broken on soon. And Mike Otto and Mark Pollack at Mass General and now at BU recently completed a study that's now impressive that looking at social anxiety disorder or social phobia. And what they did was to give patients-- again, d-cycloserine-- just a few treatments of public speaking exposure. And again, those with d-cycloserine had significant improvement, significant more reductions in their anxiety than those without. So we think that these data suggest that d-cycloserine may enhance extinction of fear in humans.
One other thing-- I've talked about so far this morning is just fear, but there's also-- the concept of extinction is used a lot in the substance abuse literature because people who are drug addicts, who are addicted to drugs, crave-- and very small cues that remind them of their experience induced craving and often induce relapse. And so extinction of this craving has been studied a lot in the drug abuse world. And the question was, would d-cycloserine also enhance extinction of drug craving?
One of the models for this is condition-placed preference, and Jane Stewart in Canada presented some new data at Society for Neuroscience this year and that looks as follows. They did what's called conditioned-place preference where they take an animal and they, over a couple days of training, put them in two different boxes. And in one box, they get cocaine and in one box saline. It doesn't take the animal long to realize he'd rather be where the cocaine was. And then they bring it back and test them for several days.
Normally, it takes several days for the animal to realize he's not getting cocaine anymore and to extinguish so they soon spend equal amounts of time in both places. However, the animals that received d-cycloserine instead of saline-- If you could go back, please. Where is the bottom figure? There's supposed to be a bottom figure here. Well, you'll have to take my word for it, I guess. Anything you can do about that? Oh well. On that bottom figure they extinguished after only three days that light looked like that can send you the poster if you're interested
Okay, so in summary, molecular mediators of synaptic plasticity are required for the extinction of fear memories. Enhancement of an NMDA function with d-cycloserine enhances extinction in mice and rats in a variety of paradigms. D-cycloserine appears to enhance extinction in humans. And thus the basic science knowledge of learning and memory mechanisms may directly lead to novel approaches to improve clinically-relevant learning via perhaps through psychotherapy and its effects on psychiatric disease.
And I would just like to thank the National Institutes of Mental Health, specifically NSF, NARSAD, Pfizer, and Anxiety Disorder Association for funding. All of this was done in collaboration with Mike Davis and colleagues Dave Walker and Kwok-Tung Lu, and the behavioral results were done in collaboration with Barbara Rothbaum and her colleagues. Thank you very much.
MODERATOR: Very interesting. I've got so many questions, but I'll leave it to the-- let me ask my first question while you're getting ready. Can this be prescribed as an off-label drug now for fear, with doctors' permission?
RESSLER: I don't know if I'm allowed to say that, but it's generically available drug. I'll defer to Tom about what the rules are about off label prescriptions but--
MODERATOR: Any side effects that this drug has?
RESSLER: In about 20 years of use with tuberculosis, it has very few severe side effects. And those that do occur usually are with much higher dosing for chronic treatment. And whether this is going to be the drug or not, I think for us is exciting about the idea that you can take a-- as opposed, for the most part, all of our drugs in psychiatry and most medical disorders, we give chronically from the idea of, this is a chronic problem. You take this, it's going to fix it.
And I think what's exciting about pharmaceutical approaches to learning and memory is the real answer may be to take it in a very acute format and then it may help it. So it may require much less dosing. And in fact, if you give d-cycloserine daily, you lose the effect. You get rapid down regulation of the NMDA receptors.
MODERATOR: So you'd only take it in a fearful moment?
RESSLER: Right. So these are only two pills taken in that study.
MODERATOR: Because there are other drugs that take six weeks to take pharmacological effect on you.
RESSLER: Yeah, obviously very different mechanisms.
MODERATOR: Yeah. This takes effect immediately.
RESSLER: The idea is that there are many pharmacological agents that have acute effects, and we rarely take advantage of those in medicine.
MODERATOR: And for other-- you mentioned panic disorders. Other fear is being studied-- studies being fear now. Okay. Yes. Over here. Interesting.
AUDIENCE: Yeah, I just had a technical question. Are you inducing a new memory with the extinction, or you're erasing the old memory of the fear-inducing traumatic event?
RESSLER: Well, it's a simple question, but nobody knows the answer yet. That's the real question of what is extinction? There's a lot of behavioral data for decades that suggests that the original fear memory is still there and that what extinction does is overlay a new inhibitory memory on top of it. And if you want, later I can tell you the various experiments that lead to that, but it's still somewhat unclear.
AUDIENCE: --of the extinction to get back their-- I mean, you wouldn't want to do this clinically, but--
RESSLER: Right. In animal models, yeah, you can do things called reinstatement. You give the animal another unconditioned stimulus, a shock, and the fear comes back. There's a lot of context specificity and things like that show that the original trace of memory still seems to be there.
AUDIENCE: So Kerry, what makes this drug specific to extinction learning as opposed to other regular learning?
RESSLER: Well, we certainly-- this was originally identified in spatial maze tasks. So I think there's every reason to think that if it were done in the right way that it might work in hippocampal and other spatial memory tasks as well. Another interesting side question there is, how do we know that we're not going to make the fear worse? Is it possible if you take this and are afraid that you're going to increase the fear? It doesn't appear to happen experimentally.
And one theoretical possibility for why that might be is it's a partial agonist. If the glycine side is already fully activated with endogenous serine, you may not see an effect. It may only be in partial-- so it may only work in weak forms of memory.
MODERATOR: Any other questions? Where do you go from here with this then?
RESSLER: Well, for now, we're kind of waiting and seeing what the rest of the human studies will look like. And if they work, I wish somebody well going with them. And we're going back to the lab and trying to find what's the next way of thinking about it.
MODERATOR: One more question over here. Yes.
AUDIENCE: Yeah. There are some sex differences in the acquisition of the fear response, and I'm wondering if you're seeing pharmacological correlates to that sex difference.
RESSLER: In this particular study, which was an n of 30, we put the post hoc analysis for gender and didn't find an effect. The animal studies have all been done in males. So it's an interesting question. We don't know the answer.
MODERATOR: Thank you very much. Very interesting.