"Long-Term Potentiation"

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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 molecular basis of these changes is not known. Long-term doi: 10.1002/9780470015902.a0000165.pub2 potentiation (LTP) has been a focus of efforts to understand memory. The term refers to a long-lasting strengthening 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 mechanism. 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. Not only is Ca2+ elevation necessary for inducing postsynaptic depolarization. These connections will there- LTP, it also appears to be sufficient: LTP occurs when light fore undergo LTP. This strengthening makes it possible for releases Ca2+ from special light-sensitive buffers that have G cells to collectively fire the x cell. To see the utility of this been injected into the cell. See also: Calcium Signalling and change, consider a recall process in which the same scene is Regulation of Cell Function presented, but missing certain features, in particular the Fluorescent Ca2+ indicators and sensitive light detectors ones that would normally cause the x cell to fire. Cell x will have been used to directly measure the intracellular Ca2+ nevertheless fire because it receives strong inputs from the elevation that occurs during LTP. The actual sites of G cells. In this way, a complete memory can be recalled synaptic input into pyramidal cells are primarily on small when the network is cued with only a part, a fundamental (1 mm) protuberances from dendrites called spines. Dur- property of associative memory. See also: Learning and ing LTP, spines at which the Hebbian condition is met Memory undergo Ca2+ elevation to levels higher than 10 mmol L21.

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Remarkably, the Ca2+ level in the spine itself can be higher mentioned earlier has been used to induce LTP under than in the parent dendrite and nearby spines, even though conditions where the presynaptic axon is not functioning. the distances involved are very small (Connor et al., 1994). See also: AMPA Receptors The localization of Ca2+ may be responsible for the The evidence that LTP produces an increase in trans- synapse specificity of LTP. See also: Dendritic Spines; mitter release from the presynaptic cell comes from several Fluorescent Probes Used for Measuring Intracellular lines of experiments. An indirect class of evidence is the Calcium observation that synapses ‘fail’ less often after LTP induc- An important unanswered question about LTP is the tion. According to the classical interpretation of ‘failures’ identity and properties of the postsynaptic depolarizing derived from study of the neuromuscular junction, failures event or events that allow the NMDA receptor channels to occur because a presynaptic action potential sometimes open. One candidate is Na+ action potentials. It is known fails to produce the release of even a single synaptic vesicle. that synaptic stimuli strong enough to induce LTP produce Thus, a change in ‘failures’ after LTP induction is consist- action potentials. Many action potentials are initiated in ent with a presynaptic change in vesicle release. However, the axon hillock and actively backpropagate into the dend- there is now substantial evidence that transmission at cen- rites. If subthreshold synaptic stimulation too weak to tral synapses is different from that at the neuromuscular evoke LTP is repeatedly coupled with a spike produced by junction and that changes in ‘failures’ can sometimes arise somatic current injection, LTP occurs. However, the role of through a postsynaptic change in the following way. A Na+ action potentials during synaptically induced LTP has synapse might initially have NMDA receptor channels, but not been demonstrated. Indeed to the contrary, experi- no AMPA receptor channels (Liao et al., 1995). Because ments showed that if the back propagating action potential NMDA receptor channels do not produce significant is blocked, the LTP induced by strong synaptic inputs still postsynaptic current near resting potential, the synapse occurs normally (Golding et al., 2002). Other depolarizing would appear to be ‘silent’ when synaptic transmission was events that could activate NMDA receptors include Ca2+ assayed under normal conditions (i.e. near resting poten- action potentials or the dendritic EPSP itself, some of tial). After LTP, postsynaptic modifications would lead to which is caused by current through the NMDA receptor the introduction of functional AMPA receptor channels at channel itself. We will return later to a discussion of the synapse and these would generate synaptic responses. It the biochemical processes triggered by Ca2+ elevation. follows that a decrease in ‘failures’ can be due to a postsy- See also: Action Potential: Ionic Mechanisms naptic modification. See also: Heterosynaptic Modulation of Synaptic Efficacy; Neurotransmitter Receptors in the Postsynaptic Neuron; Neurotransmitter Release from Presynaptic versus Postsynaptic Sites Presynaptic Terminals Because of this ambiguity, there has been the need for for Expression of Long-term more direct methods to determine whether there are Potentiation presynaptic changes during LTP. Direct evidence (Zakharenko et al., 2003) for such changes has been pro- The expression of potentiated transmission during LTP vided by experiments that use the dye FM1-43. The dye can could be due to changes that enhance the responsiveness of be loaded into synaptic vesicles. When the dye is released the postsynaptic cell to neurotransmitter. Alternatively, from the vesicle along with glutamate as a result of synaptic potentiation could also be due to presynaptic changes that stimulation, the release can be monitored by a change in result in more transmitter being released. Since LTP is in- fluorescence of the dye (the dye is responding to pH duced by postsynaptic events, if expression is presynaptic changes when the dye is released from the very acid interior there would have to be a ‘retrograde message’ that carried of the vesicle). This method shows that release is enhanced information from the postsynaptic cell back to the presy- after LTP. Importantly this enhancement develops slowly naptic cell. Determining the site of expression of LTP has (30 min) after LTP induction. By comparison, postsynaptic been surprisingly difficult, but there is now strong evidence effects develop within seconds. In addition to changes in the that postsynaptic modification occurs and specific mech- number of vesicles released, there are indications that a anisms for producing it have been established. There is also change in the mode of vesicle release may occur (Choi et al., increasing evidence for presynaptic modifications. 2003). In one mode, glutamate is released from the vesicle The strong evidence (Nicoll, 2003) for postsynaptic so slowly that AMPA channels are not activated. Such modification comes from several lines of experiments: (1) synapses are termed whispering synapses. After LTP in- AMPA receptor channels have enhanced conductance af- duction, release becomes fast enough to activate AMPA ter LTP; (2) Miniature synaptic current responses, the channels, thereby contributing to the potentiation of trans- amplitude of which is thought to be controlled postsynap- mission. See also: Nitric Oxide as a Neuronal Messenger tically, are increased after LTP; (3) By marking the GluR1 As LTP in the CA1 region is induced postsynaptically form of AMPA receptor channels with an electrophysio- as a result of activation of NMDA receptor channels, logically observable tag, it has been shown that GluR1 ‘retrograde’ messengers would seem to be required to receptors become incorporated into the synaptic mem- affect presynaptic release. The molecules nitric oxide brane after LTP; (4) The two-photon uncaging method (NO) and various metabolites of arachidonic acid

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(endocanabinoids, prostaglandins, eicosinoids) are can- forms of long-term depression and depotentiation (prop- didates. Early pharmacological studies suggested that osition 2) is reasonably good; this is reviewed in the related LTP could be blocked by interfering with NO, but sub- article on long-term depression. See also: Long-term sequent work showed that LTP in the CA1 hippocampal Depression and Depotentiation region could be induced even in the absence of neuronal CaMKII is a major component of the postsynaptic nitric oxide synthase, the isoform activated by synaptic density, a submembrane structure attached to the intra- activity. However, LTP under these conditions may have cellular side of the postsynaptic membrane. The kinase is been primarily postsynaptic. A clear demonstration of a thus strategically located to sense Ca2+ entry through presynaptic component of LTP dependent on NO activity NMDA receptor channels, the signal that we know trig- has been found in the barrel cortex of mice (Hardingham gers LTP. Furthermore, by being localized at the and Fox, 2006). In these mice the postsynaptic component synapse, CaM-kinase can regulate the proteins that con- of LTP dependent on the GluR1 receptor was eliminated trol synaptic strength (e.g. the AMPA receptor channels) genetically. The remaining LTP was dependent on nitric in a synapse-specific manner. Several strategies (Lisman oxide synthetase (NOS) activity and this LTP had the et al., 2002) have been used to test the role of CaMKII in properties of presynaptic changes in transmitter release. LTP. In early experiments, inhibitors of this kinase were injected into the postsynaptic cell and it was found that this blocked LTP induction. Subsequently, genetic methods were used to knockout the kinase and this also Molecular Mechanisms of Long-term blocked LTP induction. Of particular note is a set of Potentiation experiments in which a mutated form of CaMKII was knocked in, replacing the normal form (Giese et al., What kinds of biochemical processes could underlie bidi- 1998). This mutated form was altered in only one amino rectional synaptic modification and provide the basis for acid Thr286, a phosphorylation site crucial for the per- the stable storage of information (Lisman, 1989). No de- sistent activation of the kinase (see later). In this mutant, finitive answer is available yet, but a general framework LTP was blocked. Notably, behavioural experiments (Figure 1) is as follows: showed that memory was strongly reduced by this mu- tation. It has also been shown that introduction of active 1. The high Ca2+ elevation that occurs during LTP in- CaM-kinase into the postsynaptic cell can itself potenti- duction triggers enzymatic processes that strengthen ate synaptic transmission. This not only mimics LTP, but the synapse, whereas the more moderate Ca2+ eleva- prevents subsequent LTP induction by a tetanus, as if the tion that occurs during induction of LTD or depot- two methods for enhancing transmission share a com- entiation triggers processes that weaken the synapse. mon mechanism. 2. The high Ca2+ elevation activates a kinase, probably Progress has been made in understanding how CaM- CaMKII, whereas moderate Ca2+ elevation triggers a kinase can make synapses stronger. One mechanism is by phosphatase cascade. These produce reversible phos- the direct phosphorylation of AMPA receptor channel. phorylation and dephosphorylation, respectively, of A specific site (Ser831) on the GluR1 form of AMPA re- target proteins that control the strength of the synapse. ceptor channels can be phosphorylated by both CaM-kin- See also: Protein Phosphorylation and Long-term ase and C-kinase and enhances the function of the channels Synaptic Plasticity (Lee et al., 2000). In addition to increasing the conductance 3. The ‘memory’ of synaptic modification may arise from of AMPA channels by such modulation, CaMKII is likely the persistent nature of the kinase activity. In the case of to control the anchoring of these channels at the synapse. CaM-kinase, persistence arises from the maintained CaMKII is the most abundant protein in the postsynaptic autophosphorylation. According to this view, the kin- density (PSD), suggestive of a structural role. One way ase is a molecular switch that is turned on by Ca2+ and CaMKII can affect anchoring is by binding to SAP97, remains on until it is turned off by phosphatases acti- a protein that can bind AMPA channels. CaMKII is also vated during depotentiation. In the on-state, the switch bound indirectly to other AMPA binding proteins. During enhances synaptic transmission by phosphorylating LTP, CaMKII translocates to the synapse and remains AMPA receptor channels or stimulating their insertion there for a long period. This additional CaMKII may serve into the synapse. See also: Cellular Neuromodulation as structural seed for complexes that anchor additional AMPA channels at the synapse. There is also evidence to The role of Ca2+ in triggering LTP is well accepted, as is suggest that CaM-kinase has a role in L-LTP protein syn- the role of more moderate Ca2+ elevation in triggering thesis through phosphorylation of CPEB. Although most synaptic weakening and depotentiation. Proposition (1) research has focused on the role of postsynaptic CaMKII in above is thus on reasonably firm ground. The evidence for LTP, there is also evidence supporting a role for presynap- the role of CaMKII and perhaps other kinases in LTP is tic CaMKII (Lu and Hawkins, 2006); introduction of now strong, but their role in maintaining LTP (proposition CaM-kinase inhibitory peptide into the presynaptic cell (3)) is uncertain. These issues will be discussed in the next decreases a component of LTP. See also: Glutamatergic section. The evidence for the role of phosphatases in some Synapses: Molecular Organization

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Long-term Potentiation

V -

Summed EPSPs L V and IPSPs K Calpain Negative α-Actinin R feedback calmodulin yR

High Ca2+ NDMA Moderate Ca2+ Presynaptic Vpost Positive SRC mGuR5 glutamate feedback PLC release PKC IP3R IP3 AA Neurogranin Calmodulin PLA release MAPK

Back propagating RAS + Na spikes NOS 2+ Ca spikes Syngap ACI cAMP PKA AMPA I1p I1 CPEB PP2b PP1 Binding CaMK CaMKp CaMK mRNA Synthesis NMDA PKM-ζ

AMPA CREB, MAPK channel Integrin Activity-dependent redistribution insertion NCAM Actin Snap N NSF -CAD HERIN

AMPA AMPA

Figure 1 Postsynaptic mechanisms underlying long-term potentiation (LTP), long-term depression (LTD) and depotentiation (adapted from (Lisman, 1994)). A dendritic spine is shown protruding from a small region of dendrite. The synaptic transmission (far right) is mediated by the neurotransmitter glutamate. The postsynaptic membrane contains AMPA and NMDA ionotropic glutamate channels and the metabotropic glutamate receptor, mGluR5. If the postsynaptic cell is strongly depolarized and if glutamate is being released presynaptically, then the NMDA receptor channel will open and LTP will be induced. The NMDA receptor channel itself is under complex control through positive and negative feedback loops. The final consequence of NMDA receptor channel opening is a high elevation of intracellular Ca+, which then triggers processes that lead to the upregulation of AMPA receptor channels. The upregulation occurs either by phosphorylation of existing AMPA receptor channels or by addition of new channels (see bottom of figure). During LTP induction the activity of CaM-kinase is enhanced and this produces the phosphorylation of AMPA receptor channels. CaM-kinase itself becomes phosphorylated and in this state, its phosphorylation is self-sustaining. Other protein kinases, PKA and PKC may also be involved in the phosphorylation of AMPA receptor channels. The controls on PKC appear to be complex and involve the synthesis of new forms (PKM-z) and the control by a positive feedback pathway involving arachidonic acid (AA), phospholipase A2 and RAS. The addition of new AMPA receptor channels depends on the movement of vesicles containing AMPA receptor channels into the spine during LTP induction and the fusion of vesicles containing AMPA receptor channels into the plasma membrane. Fusion involves two proteins, SNAP and NSF. The Ca2+ elevation that occurs during synaptic signalling may also depend on Ca2+ released from intracellular stores by IP3 receptors and ryanodine receptors and by Ca2+ entry through 2+ L-type voltage-dependent Ca channels located in the spines. If postsynaptic depolarization is not strong, NMDA receptor channels will be only moderately activated and this will lead to a moderate elevation of Ca2+ that induces synaptic weakening (LTD or depotentiation). One form of this weakening is controlled by activation of a phosphatase pathway involving phosphatase 2B (pp2b), which dephosphorylates Inhibitor 1 (I1), and leads to activation of phosphatase 1 (PP1). One role of PP1 is to dephosphorylate CaMK and this may lead to synaptic weakening. During LTP induction, when it is important for CaMK to become phosphorylated, it is undesirable to activate PP1. This is prevented by a pathway involving adenylate cyclase 1 (AC1), cAMP and PKA. This pathway acts to counteract the effect of pp2b on I1. Other molecules of potential significance for LTP include the cell adhesion factors shown on the bottom right and the activity- dependent mechanism that controls the translation of CaMK mRNA.

CaM-kinase is an interesting candidate as a memory information storage by such a process is spontaneous molecule because it has switch-like properties that make its phosphatase activity, which could dephosphorylate activity persist after calcium elevation returns to baseline. Thr286. Furthermore, all proteins undergo protein turn- This ‘memory’ occurs because Thr286 sites become auto- over, so information stored in any one molecule might be phosphorylated during the calcium elevation. Experiments slowly eliminated by this process. Theoretical work, how- show that this phosphorylation can persist for many hours ever, points to the possibility that ‘on’ CaMKII holoen- after LTP induction. What potentially limits long-term zyme (there are 12 similar subunits in a holoenzyme) could

ENCYCLOPEDIA OF LIFE SCIENCES & 2007, John Wiley & Sons, Ltd. www.els.net 5 Long-term Potentiation retain the on-state indefinitely. Dephosphorylation of induced. Substances that reduce this inhibition can en- Thr286 sites could be counteracted by rapid rep- hance LTP induction. Perhaps less obvious is that the hosphorylation of these sites by neighbouring subunits. tetanic stimulation usually used to induce LTP places high Cooperative interactions between holoenzymes could also demands on the mechanisms that keep vesicles available for lead to phosphorylation of holoenyzmes newly inserted in release in the presynaptic terminal. Results suggest that the process of protein turnover. In this way, information brain-derived neurotrophic factor (BDNF) affects LTP in- could be stably stored by groups of CaMKII holoenzymes. duction in part by modulating such mechanisms, and the Although there are indications that CaMKII is responsible same may be true for agents that work through cGMP. for the maintenance of LTP, the evidence is not yet Other work shows that activation of MAP-kinase through conclusive. the insulin receptor can enhance the BK K+ conductance in Evidence is accumulating that another kinase, the PKM-z hippocampal neurons thereby reducing excitability. The isoform of C-kinase, has an important role in LTP main- resulting interference with synaptically induced depolari- tenance. The concentration of this enzyme is increased dur- zation may account for the reduction of LTP by these in- ing LTP and decreased during long-term depression. The hibitors. Also of clear importance in LTP are tyrosine increase of PKM-z after LTP induction is due to protein kinases, particularly Src. Results indicate that a major role synthesis. Importantly, the gene produces a protein that of Src is in the activity-dependent upregulation of the lacks a regulatory domain and is therefore constitutively NMDA receptor channel itself. ‘on’. Inhibition of PKM-z does not affect the early phase of LTP. Rather late phase LTP will not develop or will be reversed if inhibitor is applied after LTP is already established. Basal synaptic transmission is not affected. Interest- Structural Modification of Synapses ingly, a membrane-permeant peptide inhibitor of PKM-z, and Changes in Gene Expression ZIP, not only reverses LTP when applied in the slice preparation, but can irreversibly destroy a memory (place avoid- The structural basis of changes in synaptic strength is of ance), when applied in vivo after learning. (Pastalkova et al., increasing interest. When LTP is induced, spines get larger. 2006). See also: Arachidonic Acid Signalling in the Nervous Conversely, LTD makes spines become smaller. These System; Receptor Transduction Mechanisms changes in spine size involve changes in the cytoskeleton, An important area of progress has been in understanding notably changes in the amount of F-actin within the spine. the role of different AMPA receptor subunits in LTP and Only electron-microscopic (EM) methods have sufficient basal synaptic transmission (Malinow and Malenka, resolution to image the synapse itself. This form of mi- 2002). Different subunits appear to have different roles. croscopy requires fixation and sectioning, and it is there- Notably GluR1 is of particular importance in the earliest fore not possible to examine the same synapses before and phases of LTP. Knockout of GluR1 preferentially reduces after LTP induction. So far the only available approach has the early phase. Interestingly, this knockout totally elim- been to compare the average synapse structure in different inates extrasynaptic AMPA receptors and a phenomenon slices, before and after LTP induction. This type of work called distance-dependent scaling. There is increasing ev- strongly suggests that the synapse itself enlarges after LTP idence that extrasynaptic GluR1 is in a diffusional equi- induction (Harris et al., 2003). librium with binding sites at the synapse. GluR1 appears to Even nearby synapses on the same dendrite differ greatly be in a complex with a protein termed stargazin and it is the in structure. Interestingly, specializations on both sides of binding of stargazin to a synaptic anchoring protein, PSD- the synapse, the presynaptic grid and postsynaptic density, 95, that captures GluR1 at the synapse. The importance of are both highly variable in size, but have exactly the same PSD-95 is underscored by the finding that overexpression size at any given synapse. Indeed their edges are exactly in of this protein can enhance synaptic strength, while elim- register. This suggests that the synapse grows through a inating it and other proteins of the same function reduces structurally coordinated transsynaptic process. If such synaptic strength (El-Husseini Ael et al., 2002). One of the growth occurred during LTP, it would probably lead to difficulties in understanding transmission under basal con- both a change in the probability of release and an enhanced ditions is that different subunits and subunit combinations postsynaptic response. The biochemical factors involved in appear to have different roles, but the exact nature of these structural control are beginning to be elucidated. For in- roles remains to be elucidated. stance, LTP is inhibited by agents that interfere with ad- Scores of substances affect the induction of LTP, giving hesion molecules. These adhesion molecules, including L1, the impression that synaptic modification may be an ex- NCaM, integrins and cadherin, are found in synaptic junc- tremely complex process. However, many of these may not tions and may be particularly concentrated at the puncta- interfere with the biochemistry of synaptic modification adherin, a specialized subregion of the synaptic region. The itself, but rather act indirectly to affect postsynaptic depo- control mechanisms that regulate actin changes are also larization. Since this will affect the opening of NMDA re- being elucidated. One such mechanism involves cofilin, a ceptor channels, LTP will be secondarily affected. For powerful agent for actin reorganization through depolym- instance, in some regions the inhibition that is also evoked erization and cutting of actin filaments. Genetic removal of by the stimulating electrode is so strong the LTP cannot be LIM-kinase, which inactivates cofilin, leads to reduction of

6 ENCYCLOPEDIA OF LIFE SCIENCES & 2007, John Wiley & Sons, Ltd. www.els.net Long-term Potentiation spine size and deficits in long-term potentiation. Comple- presynaptic cell during LTP induction, primarily as mentary experiments involving inhibition of the phospha- proto-BDNF, and is converted to BDNF through the ac- tase that activates phospho-cofilin, showed deficits in long- tion of plasmin, a protease that itself undergoes activity- term depression (Morishita et al., 2005). dependent secretion and activation by tpA and possibly The early phases of LTP develop with seconds after in- MMP-9. BDNF acts on presynaptic TrkB receptors to en- duction and thus must involve purely modulatory proc- hance the vesicle release process during high frequency esses. However, on a longer time scale changes in gene stimulation. It also acts postsynaptically to enhance depo- transcription and translation may be involved. There is larization directly and, through multiple kinase cascades, now clear evidence that later phases of LTP are inhibited by enhances protein synthesis. Among the proteins synthe- protein synthesis inhibitors. Moreover, the induction of sized is TrkB itself. The phase of LTP that occurs during the LTP triggers an increase in protein synthesis. For instance, first 30 min does not appear to require BDNF action, but it is known that a few forms of messenger ribonucleic acid later phases do. Notably, interfering with TrkB function at (mRNA) are translated locally in the dendrites and that 30 min after LTP induction produces a rapid decrease in local regulators of translation can themselves be controlled potentiation, suggesting that BDNF action is required for a by NMDA receptor channel activation. The utilization of considerable time after induction to maintain LTP. these proteins is affected by a process called synaptic tag- Experiments with dopamine antagonists or knockout of ging. This was shown by inducing LTP in one pathway and D1 receptors indicate that dopamine is required for late LTP. thereby activating protein synthesis. Then a protein syn- This raises the question of what causes the firing of the do- thesis inhibitor was applied and LTP induced in a second pamine cells in the ventral tegmental area (VTA) that pro- pathway. The important observation is that the LTP in this vides dopamine to the hippocampus. It now seems likely that second pathway had a normal late phase, despite the pres- the hippocampus itself computes the novelty of incoming ence of protein synthesis inhibitor, presumably because it information and that the resulting novelty signal is sent to the was able to use the proteins whose synthesis was triggered VTA. In this way, the transition to long-term information by the first pathway. Interestingly, this ability of one storage (late LTP) may be conditional upon the novelty of pathway to enhance another has a limited lifetime; if the the incoming information (Lisman and Grace, 2005). pathways were stimulated more than a few hours apart, there was no interaction. To explain these results, it was Prospects proposed (Frey and Morris, 1997) that LTP induction produces a transient synapse-specific biochemical modifi- There are many developing areas of research that will yield cation that functions as a tag to incorporate proteins whose new insight into the mechanisms of LTP and memory. synthesis is triggered by a process that is not synapse-spe- In the past, physiological studies have dealt almost exclu- cific. See also: Integrin Superfamily sively with the study of populations of synapses, but tech- LTP induction causes changes in the transcription of niques are now being developed to study individual many genes and triggers new protein synthesis. The early, synapses. Combined with imaging methods, it should be transient burst of protein synthesis in the postsynaptic possible to watch synapses as they undergo LTP and to dendrite utilizes pre-existing mRNA transcribed earlier obtain insight into structure–function issues. from what are known as intermediate-early genes (IEGs). Rapid progress is also being made in understanding the These genes encode proteins such as the transcription fac- molecular composition of the synapse. Once a molecule is tors zif/268, c-fos and jun; growth factors such as BDNF; identified, it is possible easily to screen for other molecules and enzymes such as CaMKII. The immediate early gene that bind to that molecule. In this way, a picture is begin- product, Arc/Arg3.1 interacts with dynamin and end- ning to emerge of the molecular complexes on both sides of ophilin to regulate AMPA channel receptor endocytosis the synapse. However, this effort is hindered by lack of (Tzingounis and Nicoll, 2006). These proteins form the direct structural information about the PSD. Refinements bridge between the temporary mechanistic changes com- of EM methods using tomography offers hope in this re- prising early LTP and the sustained late LTP structural and gard. See also: G Proteins; Synapses; Visual System functional changes. Experiments in which inhibitors of Development in Vertebrates transcription were found to block synaptic plasticity and Ultimately, the testing of hypotheses regarding the mo- the findings that some IEGs are transcription factors have lecular basis of memory requires the ability to assay mem- suggested a role for the nucleus in LTP. In support of this ory itself. There is increased understanding that there are hypothesis, in mutant mice lacking the transcription factor many forms of memory and that they may be mediated by CREB, LTP is nearly normal soon after induction but is different mechanisms. New behavioural tests are being de- reduced at late times. However, the accumulating evidence veloped that allow particular forms of memory to be as- about specific gene products and LTP mechanisms suggests sessed. The ability of new genetic methods to produce that local dendritic translation of pre-existing mRNA is defined molecular changes at precise times and precise lo- sufficient for synaptic plasticity. See also: Protein Synthesis cations in living animals will make it possible to test and Long-term Synaptic Plasticity whether particular forms of memory rely on particular BDNF has a major role in late LTP (Bramham and molecules. See also: Memory: Clinical Disorders; Alzhei- Messaoudi, 2005). The peptide is secreted from the mer Disease

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We are who we are largely because of our accumulated Philosophical Transactions of the Royal Society of London, Se- experience stored in memory. As more is learned about ries B: Biological Sciences 358(1432): 745–748. LTP, it will make possible the manipulation of memory and Lee HK, Barbarosie M, Kameyama K, Bear MF and Huganir RL provide solutions to memory problems. It would clearly be (2000) Regulation of distinct AMPA receptor phosphorylation undesirable to wipe our memory clear. But as we come to sites during bidirectional synaptic plasticity. Nature 405(6789): understand the biochemical basis of memory, it may be- 955–959. come possible to affect specific memories. This may help in Liao D, Hessler NA and Malinow R (1995) Activation of the treatment of unwanted memories, such as those in- postsynaptically silent synapses during pairing-induced LTP volved in posttraumatic stress disorders. The implications in CA1 region of hippocampal slice. 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Further Reading Roberson ED, English JD and Sweatt JD (1996) A biochemist’s view of long-term potentiation. Learning and Memory Kelleher RJ 3rd, Govindarajan A and Tonegawa S (2004) Trans- 3(1): 1–24. lational regulatory mechanisms in persistent forms of synaptic Sheng M and Hoogenraad CC (2006) The postsynaptic architecture plasticity. Neuron 44(1): 59–73. of excitatory synapses: a more quantitative view. Annual Review of Malenka RC and Bear MF (2004) LTP and LTD: an embarrass- Biochemistry 76 EPUB: http://arjournals.annualreviews.org/doi/ ment of riches. Neuron 44(1): 5–21. abs/10.1146/annurev.biochem.76.060805.160029 Milner B, Squire LR and Kandel ER (1998) Cognitive neuroscience and the study of memory. Neuron 20: 445–468.

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