Long-term Potentiation Advanced article Article Contents John Lisman, Brandeis University, Massachusetts, USA . Introduction . Synapse Specificity and Associative Nature of Long-term Long-term potentiation is an activity-dependent strengthening of synapses that is Potentiation thought to underlie memory. Long-term Potentiation as a Mechanism for Memory Storage . Role of Calcium Entry through NMDA Receptor Channels . Presynaptic versus Postsynaptic Sites for Expression of Introduction Long-term Potentiation . Molecular Mechanisms of Long-term Potentiation One of the major unsolved problems in neuroscience is . Structural Modification of Synapses and Changes in Gene the mechanism by which memory is stored in the brain. Expression Some property of our neurons must change when we . Prospects learn, but what parts of the neuron change and the mo- lecular basis of these changes is not known. Long-term doi: 10.1002/9780470015902.a0000165.pub2 potentiation (LTP) has been a focus of efforts to under- stand memory. The term refers to a long-lasting strength- ening of synapses that can be triggered by particular brief patterns of synaptic stimulation. Other patterns can pro- Synapse Specificity and Associative duce a long-term weakening of synapses; this result is termed as long-term depression (LTD) or depotentiation. Nature of Long-term Potentiation These findings show that synapses have a defined strength and that this strength can be bidirectionally There are two properties of LTP that have made it a par- modified. The evidence that such changes in synaptic ticularly attractive candidate for memory storage. The first strength are actually involved in memory has strength- is the finding that synapses can be independently modified. ened in recent years. One class of experiments involved The evidence for this ‘synapse specificity’ comes from ex- the selective genetic removal of N-methyl-D-aspartate periments in which two sets of synapses on to the same (NMDA) receptor channels from CA1 cells (Tsien et al., neuron are stimulated. If a tetanus is given to one set, these 1996). These mice lack the ability to induce LTP. synapses will be strengthened, but the synapses in the other Behavioural studies showed that the animals are defec- set will not. A more recent experiment used two-photon tive in learning and memory tasks. In a complementary uncaging of glutamate in the direct vicinity of an individual approach, two groups of experimenters (Gruart et al., synapse (Matsuzaki et al., 2004). Such stimulation could be 2006; Whitlock et al., 2006) have demonstrated the con- used to induce LTP at that synapse in much the way LTP is verse correlation, namely that learning tasks induce LTP induced by presynaptically released glutamate. The re- in CA1 cells. markable finding was that other nonstimulated synapses Several fundamentally different forms of LTP have only microns away did not undergo LTP. These results been discovered. This article will focus on the most stud- indicate that each synapse can be used to store information. ied form that found at the glutamatergic synapses of the Because most neurons have over 10 000 excitatory CA1 region of the hippocampus. A typical experiment synapses, the potential for information storage is vast. starts by measuring the strength of a group of synapses. See also: Neurons This is done by firing a single action potential in some of The second attractive property of LTP is that it is asso- the axons that enter this region. These axons make ciative. According to nerve network theory (see next sec- synapses with pyramidal cells and generate a graded ex- tion), it is this property that enables small changes in many citatory postsynaptic potential (EPSP). The strength of synapses to produce distributed storage of a complex the synapses is defined by the magnitude of the EPSP. memory in a nerve network. What associativity means is LTP is then induced by stimulating the axons to fire at that LTP cannot be triggered by activity in a single input high frequency (typically 100 Hz for 1 s), a stimulus re- axon, but can be triggered if that activity is associated with ferred to as a tetanus. The remarkable finding is that this other active excitatory inputs. The basis of this ass- brief tetanus causes a long-lasting potentiation of the ociativity requirement is that LTP induction requires the strength of the synapses. The size of the EPSP typically membrane voltage of the pyramidal cell to become very increases by 50–100% but can increase by as much as depolarized (a positive deflection from resting potential) by 800%. In the brain-slice preparation used for most stud- an amount much larger than that produced by a single ies, potentiation persists until the slice is no longer viable excitatory synaptic input (500 mV). This requirement has (5–12 h). LTP can also be induced in living animals, and been demonstrated by artificially depolarizing the cell with there it can persist for at least a year. positive current. Under these conditions, even stimulation ENCYCLOPEDIA OF LIFE SCIENCES & 2007, John Wiley & Sons, Ltd. www.els.net 1 Long-term Potentiation of a single input can produce LTP. Conversely, if many Role of Calcium Entry through NMDA inputs are stimulated, but the cell is hyperpolarized (made more negative) by current injection, LTP does not occur. Receptor Channels Thus, a synapse will be strengthened if two conditions are met: the synapse is activated by presynaptic activity and A major discovery is that the properties of a single receptor, there is substantial postsynaptic depolarization due to the NMDA receptor, can account for the Hebbian prop- other excitatory inputs (and not too much inhibitory in- erty of LTP. The NMDA receptor channel is one of the two put). This dual requirement for strengthening is often re- major subtypes of ionotropic receptors for the neurotrans- ferred to as the ‘Hebb rule’. See also: Membrane Potential mitter glutamate, the neurotransmitter at most excitatory One of the elegant aspects of the Hebb rule is that it can synapses in the central nervous system. The other major be executed at each synapse using information that is lo- subtype of ionotropic receptor is the a-amino-3-hydroxy- cally available. A synapse can ‘know’ that its presynaptic 5-methyl-4-isoxazolepropionate (AMPA) receptor. These input is active because its receptors will be activated by the receptors are also ion channels and they produce the bulk neurotransmitter released. The synapse can also ‘know’ of the excitatory postsynaptic potential. The function of about the overall depolarization of the neuron because NMDA receptors was unclear until it was shown that they this spreads fairly uniformly over the cell membrane; to a have a special role in synaptic plasticity. This was demon- first approximation, the voltage at any point reflects the strated by showing that if NMDA receptor channels are combined influence of all the excitatory and inhibitory in- pharmacologically blocked, LTP cannot be induced (Bliss puts on to a cell. Membrane ion channels in the postsy- and Collingridge, 1993). Subsequent work used genetic naptic membrane have the capability of sensing this methods to eliminate the NMDA receptor channel came to voltage. As we will see later, an ion channel called the the same conclusion. See also: Glutamate as a Neuro- (NMDA) receptor can in fact execute the Hebb rule by transmitter; NMDA Receptors sensing both presynaptic glutamate release and membrane An unusual property of NMDA receptor channels ex- depolarization. plains how these channels trigger LTP in accord with the Hebb rule. Most transmitter-activated channels are opened by transmitter alone; their opening does not depend on membrane voltage. In contrast, NMDA receptor channels Long-term Potentiation as a open only if there is also substantial postsynaptic depo- Mechanism for Memory Storage larization. This voltage dependence has a simple mecha- nism. If the postsynaptic neuron is near resting potential, The theory of nerve networks provides us with a rough glutamate cannot activate the channel because the pore of outline of how synapses in a network of neurons might the channel is blocked by external magnesium. However, if store a memory in a distributed way using a Hebbian form the postsynaptic neuron is depolarized, Mg2+ is electrically of LTP. Let us consider how a visual scene might be stored. driven out of the pore, and the channel can open. These The scene would first be processed by low-level networks in dual requirements for opening the NMDA receptor chan- which neurons responded to elementary visual features. nel are exactly the requirements for synaptic modification These features would then be synaptically associated in a according to the Hebb rule. See also: Neural Activity and higher-level network specialized for memory storage. In an the Development of Brain Circuits idealized version of such a memory network, each neuron But what is it about the opening of the NMDA receptor makes a synaptic connection with every other neuron and it channels that leads to LTP induction? It is now clear that is at these ‘recurrent’ connections that memory can be NMDA receptor channels are much more permeable to stored by the Hebbian modification. Let us say that when Ca2+ than are most AMPA receptor channels and that it is the visual scene is present, a subset of cells in the memory the influx of Ca2+ through the NMDA receptors that trig- network will fire, specifically cell x and a group of other gers LTP. A key finding was that LTP could be blocked by cells, G. Focusing on the connection of the G cells to the x intracellular Ca2+ buffers (Lynch et al., 1983), molecules cell, we can see that the Hebb rule will be satisfied at all that bind Ca2+ and prevent elevations in Ca2+ concentra- these synapses because there is both presynaptic input and tion.