In Search of Memory: The Emergence of a New Science of Mind

by Eric R. Kandel

the learning capability of individual human beings and their historical memory have grown over the centuries through shared learning—that is, through the transmission of culture. Cultural evolution, a nonbiological mode of adaptation, acts in parallel with biological evolution as the means of transmitting knowledge of the past and adaptive behavior across generations.

Down’s syndrome, Alzheimer’s disease, and age-related memory loss are familiar examples of the many diseases that affect memory. We now know that defects in memory contribute to psychiatric disorders as well: schizophrenia, depression, and anxiety states carry with them the added burden of defective memory function.

We

now turned our attention to the synapses between the sensory and motor neurons. We observed that when we produced habituation by touching the skin repeatedly, the amplitude of the gill-withdrawal reflex decreased progressively. This learned change in behavior was paralleled by a progressive weakening of the synaptic connections. Conversely, when we produced sensitization by applying a shock to the animal’s tail or head, the enhanced gill-withdrawal reflex was accompanied by a strengthening of the synaptic connection. We concluded that during habituation an action potential in the sensory neuron gives rise to a weaker synaptic potential in the motor neuron, leading to less effective communication, while during sensitization it gives rise to a stronger synaptic potential in the motor neuron, leading to more effective communication.

We found that in classical conditioning, the neural signals from the innocuous (conditioned) and noxious (unconditioned) stimuli must occur in a precise sequence. That is, when the siphon is touched just before the tail is—thus predicting the shock to the tail—the sensory neurons will fire action potentials just before they receive signals from the tail. The precisely timed firing of action potentials in the sensory neurons, followed by the precisely timed arrival of the signals from the tail shock, leads to much greater strengthening of the synapse between the sensory and motor neurons than when signals from the siphon or the tail occur separately, as they do in sensitization.

These several results on habituation, sensitization, and classical conditioning led us irresistibly to think about how genetic and developmental processes interact with experience to determine the structure of mental activity. Genetic and developmental processes specify the connections among neurons—that is, which neurons form synaptic connections with which other neurons and when. But they do not specify the strength of those connections. Strength—the long-term effectiveness of synaptic connections—is regulated by experience.

Mecanismos de aprendizaje Genética más ambiente Notable!!

however, a creature’s environment and learning alter the effectiveness of the preexisting pathways, thereby leading to the expression of new patterns of behavior.

learning selects among a large repertoire of preexisting connections and alters the strength of a subset of those connections.

Cocepto clave!

[T]he data indicate that habituation and dishabituation (sensitization) both involve a change in the functional effectiveness of previously existing excitatory connections. Thus, at least in the simple cases,…[t]he capability for behavioral modification seems to be built directly into the neural architecture of the behavioral reflex. Finally, these studies strengthen the assumption…that a prerequisite for studying behavioral modification is the analysis of the wiring diagram underlying the behavior. We have, indeed, found that once the wiring diagram of the behavior is known, the analysis of its modification becomes greatly simplified. Thus, although this analysis pertains to only relatively simple and short-term behavioral modifications, a similar approach may perhaps also be applied to more complex as well as longer lasting learning processes.

THUS THE REDUCTIONIST APPROACH LED US TO DISCOVER several principles of the cell biology of learning and memory. First, we found that the changes in synaptic strength that underlie the learning of a behavior may be great enough to reconfigure a neural network and its information-processing ability. For example, one particular sensory cell in Aplysia communicates with eight different motor cells—five that produce movement of the gill and three that cause contraction of the ink gland and thus inking. Before training, activation of this sensory cell excited the five gill-innervating motor cells moderately, causing them to fire action potentials and thereby causing the gill to contract. Activation of this same sensory cell also excited the three ink gland-innervating motor neurons but only very weakly, not enough to produce action potentials or to elicit inking. Thus, before learning, gill withdrawal would take place in response to stimulation of the siphon but inking would not. After sensitization, however, synaptic communication between the

sensory cell and all eight motor cells is enhanced, causing the three ink gland-innervating motor neurons to fire action potentials as well. Thus, as a result of learning, when the siphon is stimulated, inking will occur along with more powerful gill withdrawal.

Second, consistent with my reformulation of Cajal’s theory and my earlier work with analogs, we found that a given set of synaptic connections between two neurons can be modified in opposite ways—strengthened or weakened—by different forms of learning. Thus, habituation weakens the synapse, whereas sensitization or classical conditioning strengthens it. These enduring changes in the strength of synaptic connections are the cellular mechanisms underlying learning and short-term memory. Moreover, because the changes occur at several sites in t...

Third, we found that in all three forms of learning, the duration of short-term memory storage depends on the length of time a synapse is weakened or strengthened.

Base de aprendizaje y memoria

One of the fundamental features of memory is that it is formed in stages. Short-term memory lasts minutes, while long-term memory lasts many days or even longer. Behavioral experiments suggest that short-term memory grades naturally into long-term memory and, moreover, that it does so through repetition. Practice does make perfect.

The first rigorous test of memory consolidation came in 1949, when the American psychologist C. P. Duncan applied electrical stimuli to the brain of animals during or immediately after training, resulting in convulsions that disrupted memory and caused retrograde amnesia. Producing seizures several hours after training had little or no effect on recall. Almost twenty years later, Louis Flexner at the University of Pennsylvania made the remarkable discovery that drugs that inhibit the synthesis of proteins in the brain disrupt long-term memory if given during and shortly after learning, but they do not disrupt short-term memory. This finding suggested that long-term memory storage requires the synthesis of new proteins. Together, the two sets of studies seemed to confirm the idea that memory storage takes place in at least two stages: a short-term memory lasting minutes

is converted—by a process of consolidation that requires the synthesis of new protein—into stable, long-term memory lasting days, weeks, or even longer.

Specifically, in long-term habituation the number of presynaptic connections among sensory neurons and motor neurons decreases, whereas in long-term sensitization sensory neurons grow new connections that persist as long as the memory is retained (figure 15–1). There is in each case a parallel set of changes in the motor cell.

Thus in Aplysia we could see for the first time that the number of synapses in the brain is not fixed—it changes with learning! Moreover, long-term memory persists for as long as the anatomical changes are maintained.

Short-term memory produces a change

in the function of the synapse, strengthening or weakening preexisting connections; long-term memory requires anatomical changes. Repeated sensitization training (practice) causes neurons to grow new terminals, giving rise to long-term memory, whereas habituation causes neurons to retract existing terminals. Thus, by producing profound structural changes, learning can make inactive synapses active or active synapses inactive.

Furthermore, musicians who began playing the instrument before age thirteen had larger representations of the fingers of their left hand than musicians who began playing after that age.

Desarrollo de habilidades antes de 13 años

Even identical twins with identical genes have different brains because of their different life experiences. Thus, a principle of cell biology that first emerged from the study of a simple snail turned out to be a profound contributor to the biological basis of human individuality.

We had learned that memory derives from changes in the synapses in a neural circuit: short-term memory from functional changes and long-term memory from structural changes.

We found that the change is quite one-sided: during short-term habituation lasting minutes, the sensory neuron releases less neurotransmitter, and during

short-term sensitization it releases more neurotransmitter. That neurotransmitter, we later discovered, is glutamate, also the major excitatory transmitter in the mammalian brain. By increasing the amount of glutamate a sensory cell sends to a motor cell, sensitization strengthens the synaptic potential elicited in the motor cell, thus making it easier for that neuron to fire an action potential and cause the gill to withdraw.

The synaptic potential between the sensory and motor neurons lasts only milliseconds, yet we had observed that a shock to Aplysia’s tail enhances glutamate release and synaptic transmission for many minutes. How does this come about? As my colleagues and I focused on the question, we noticed something curious. The strengthening of the synaptic connection between the sensory and motor neuron is accompanied by a very slow synaptic potential in the sensory cell, one that lasts for minutes rather than the milliseconds typical of synaptic potentials in the motor neuron. We soon found that the shock to Aplysia’s tail activates a second class of sensory neurons, one that receives information from the tail. These tail sensory neurons activate a group of interneurons that acts on the sensory neuron from the siphon. It is these interneurons that produce the remarkably slow synaptic potential. We th...

We found that the interneurons activated by a shock to Aplysia’s tail release a neurotransmitter called serotonin. Moreover, the interneu...

we could simulate the slow synaptic potential, the enhancement of synaptic strength, and the strengthening of the gill-withdrawal reflex simply by applying serotonin to the connections between the sensory and motor neurons.

We called these serotonin-releasing interneurons modulatory interneurons because they do not mediate behavior directly; rather, they modify the strength of the gill-withdrawal reflex by enhancing the strength of the connections between sensory and motor neurons.

These findings caused us to realize that there are two kinds of neural circuits important in behavior and learning: mediating circuits, which we had characterized earlier, and modulating circuits, which we were just beginning to characterize in detail (figure 16–1). Mediating circuits produce behavior directly and are therefore Kantian in nature. These are the genetically and developmentally determined neuronal components of the behavior, the neuronal architecture. The mediating circuit is made up of the sensory neurons that innervate the siphon, the interneurons, and the motor neurons that control the gill-withdrawal reflex. With learning, the mediating circuit becomes the student and acquires new knowledge. The modulating circuit is Lockean in nature; it serves as a teacher. It is not directly invo...

The discoveries in Aplysia and Drosophila also reinforced an important biological principle: evolution does not require new, specialized molecules to produce a new adaptive mechanism. The cyclic AMP pathway is not unique to memory storage. As Sutherland had shown, it is not even unique to neurons: the gut, the kidney, and the liver all make use of the cyclic AMP pathway to produce persistent metabolic changes. In fact, of all the known second messengers, the cyclic AMP system is probably the most primitive. It is the most important, and in some cases the only second-messenger system found in single-celled organisms such as the bacterium E. coli, in which it signals hunger. Thus the biochemical actions underlying memory did not arise specifically to support memory. Rather, neurons simply recruited an efficient signaling system employed for other purposes in other cells and used it to produce the changes in synaptic strength required for memory storage.

In contrast to the engineer, evolution does not produce innovations from scratch. It works on what already exists, either transforming a system to give it a new function or combining several systems to produce a more complex one. If one wanted to use a comparison, however, one would have to say that this process resembles not engineering but tinkering, bricolage we say in French. While the engineer’s work relies on his having the raw materials and the tools that exactly fit his project, the tinkerer manages with odds and ends…. He uses whatever he finds around him, old cardboards, pieces of string, fragments of wood or metal, to make some kind of workable object. The tinkerer picks up an object that happens to be in his stock and gives it an unexpected function. Out of an old car wheel, he will make a fan; from a broken table a parasol.

18–4 Changes underlying short- and long-term memory in a single sensory and motor neuron.

THIS ADVENTURE BEGAN IN 1961 WHEN FRANÇOIS JACOB AND Jacques Monod of the Institut Pasteur in Paris published a paper entitled “Genetic Regulatory Mechanisms in the Synthesis of Protein.” Using bacteria as a model system, they made the remarkable discovery that genes can be regulated—that is, they can be switched on and off like a water faucet. Jacob and Monod inferred what we now know to be a fact: that even in a complex organism like a human being, almost every gene of the genome is present in every cell of the body. Every cell has in its nucleus all of the chromosomes of the organism and therefore all of the genes necessary to form the entire organism. This inference raised a serious question for biology: Why do not all genes function in the same way in every cell of the body? Jacob and Monod proposed what ultimately proved to be the case—namely, that a liver cell is a liver cell, and a brain cell is a brain cell because in each cell type only some of those genes are turned on, or expressed; all of the other genes are shut off, or repressed. Thus, each cell type contains a unique mix of proteins—a subpopulation of all the proteins available to the cell. This mix of proteins enables the cell to perform its specific biological functions. Genes are switched on and off as needed to achieve optimal functioning of the cell. Some genes are repressed for most of the lifetime of the organism; other genes, such as those involved in the production of energy, are always expressed b...

proteins they encode are essential for survival. But in every cell type some genes are expressed only at certain times, whereas others are turned on and off in response to signals from within the body or from the environment. This set of arguments caused a lightbulb to go on in my brain one night: What is learning but a set of sensory signals from the environment, with different forms of learning resulting from different types or patterns of sensory signals?

We summarized our views in “The Long and Short of Long-Term Memory,” a conceptual review published in 1986 in Nature. In this paper, we proposed that if gene expression was

entire process of long-term synaptic change! Dusan Bartsch, a creative and technically brilliant postdoctoral fellow, later found that simply injecting CREB that had been phosphorylated by protein kinase A into the nucleus of sensory neurons was sufficient to turn on the genes that produce long-term facilitation of these connections. Thus, even though

I had long been taught that the genes of the brain are the governors of behavior, the absolute masters of our fate, our work showed that, in the brain as in bacteria, genes also are servants of the environment.

Genes are the servents of the ambiance!!!!

They are guided by events in the outside world. An environmental stimulus—a shock to an animal’s tail—activates modulatory interneurons that release serotonin. The serotonin acts on the sensory neuron to increase cyclic AMP and to cause protein kinase A and MAP kinase to move to the nucleus and activate CREB. The activation of CREB, in turn, leads to the expression of genes that changes the function and the structure of the cell.

Indeed, CREB’s opposing regulatory actions provide a threshold for memory storage, presumably to ensure that only important, life-serving experiences are learned. Repeated shocks to the tail are a significant learning experience for an Aplysia, just as, say, practicing the piano or conjugating French verbs are to us: practice makes perfect, repetition is necessary for long-term memory. In principle, however, a highly emotional state, such as that brought about by a car crash, could bypass the normal restraints on long-term memory. In such a situation, enough MAP kinase molecules would be sent into the nucleus rapidly enough to inactivate all of the CREB-2 molecules, thereby making it easy for protein kinase A to activate CREB-1 and put the experience directly into long-term memory. This might account for so-called flashbulb memories, memories of emotionally charged events that are recalled in vivid detail—like my experience with Mitzi—as if a complete picture had

19–1 The molecular mechanisms of short- and long-term facilitation.

19–3 Two mechanisms of long-term change. New proteins are sent to all of the synapses (above), but only synapses stimulated with serotonin use them to initiate the growth of new axon terminals. Proteins synthesized locally (below) are needed to sustain the growth initiated by gene expression.

As Kausik looked carefully at the amino acid sequence of the novel CPEB, he noticed something very peculiar. One end of the protein had all the characteristics of a prion. Prions are probably the weirdest proteins known to modern biology. They were first discovered by Stanley Prusiner of the University of California, San Francisco as the causal agents of several mysterious neurodegenerative diseases, such as mad cow disease (bovine spongiform encephalopathy) in cattle and Creutzfeldt-Jakob disease in people (this is the disease that tragically killed Irving Kupfermann in 2002, at the prime of his scientific career). Prions differ from other proteins in that they can fold into two functionally distinct shapes, or conformations; one of these is dominant, the other recessive. The genes that encode prions give rise to the recessive form, but the recessive form can be converted to the dominant form, either by chance alone, as may have happened with Irving, or by eating food that contains an active form of the protein. In the dominant form, prions can be lethal to other cells. The second way in which prions differ from other proteins is that the dominant form is self-perpetuating; it causes the recessive conformation to change

I remember it was a beautiful New York afternoon in the spring of 2001, the bright sunlight rippling off the Hudson River outside my office windows, when Kausik walked into my office and asked, “What would you say if I told you that the CPEB has prion-like properties?” A wild idea! But if true, it could explain how long-term memory is maintained in synapses indefinitely, despite constant protein degradation and turnover. Clearly, a self-perpetuating molecule could remain at a synapse indefinitely, regulating the local protein synthesis needed to maintain newly grown synaptic terminals.

19–4 Long-term memory and the prion-like CPEB protein. As a result of a prior stimulus, the sensory cell’s nucleus has sent dormant messenger RNA (mRNA) to all axon terminals (1). Five pulses of serotonin at one terminal convert a prion-like protein (CPEB) that is present at all synapses into a dominant, self-perpetuating form (2). Dominant CPEB can convert recessive CPEBs to the dominant form (3). Dominant CPEB activates dormant messenger RNA (4). The activated messenger RNA regulates protein synthesis at the new synaptic terminal, stabilizes the synapse, and perpetuates the memory.

Beyond discovering the new prion’s relevance to the persistence of memory or even to the functioning of the brain, Kausik and I had found two new biological features of prions. First, a normal physiological signal—serotonin—is critical for converting CPEB from one form to another. Second, CPEB is the first self-propagating form of a prion known to serve a physiological function—in this case, perpetuation of synaptic facilitation and memory storage. In all other cases previously studied, the self-propagating form either causes disease and death by killing nerve cells or, more rarely, is inactive.

IN RETROSPECT, OUR WORK ON LONG-TERM SENSITIZATION AND the discovery of the prionlike mechanism brought to the forefront three new principles that relate not only to Aplysia but to memory storage in all animals, including people. First, activating long-term memory requires the switching on of genes. Second, there is a biological constraint on what experiences get stored in memory. To switch on the genes for long-term memory, CREB-1 proteins must be activated and CREB-2 proteins, which suppress the memory-enhancing genes, must be inactivated. Since people do not remember everything they have learned—nor would anyone want to—it is clear that the genes that encode suppressor proteins set a high threshold for converting short-term to long-term memory. It is for this reason that we remember only certain events and experiences for the long run. Most things we simply forget. Removing that biological constraint triggers the switch to long-term memory. The genes activated by CREB-1 are required for new synaptic growth. The fact that a gene must be switched on to form long-term memory shows clearly that genes are not simply determinants of behavior but are also responsive to environmental stimulation, such as learning. Finally, the growth and maintenance of new synaptic terminals makes memory persist. Thus, if you remember anything of this book, it will be because your brain is slightly different after you have finished reading it. This ability to grow new synaptic connections as a ...

surface are subject to constant modification in response to changing input from sensory pathways.

colleagues and I examined slices of the hippocampus taken from genetically modified mice and found that in each of the three major pathways of the hippocampus, long-term potentiation has two phases similar to those of long-term facilitation in Aplysia. A single train of electrical stimuli produces a transient, early phase of long-term potentiation that lasts only one to three hours and does not require the synthesis of new protein. The reaction of neurons to those stimuli was just as Roger Nicoll had described: NMDA receptors in the postsynaptic cell are activated, leading to the flow of calcium ions into the postsynaptic cell. Here calcium acts as a second messenger; it triggers long-term potentiation by enhancing the existing AMPA receptors’ response to glutamate and by stimulating the insertion of new AMPA receptors into the membrane of the postsynaptic cell. In response to certain patterns of stimulation, the postsynaptic cell also sends a signal back to the presynaptic cell, calling for more glutamate.