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- Main article: Postactivation potentials
In neuroscience, long-term potentiation (LTP) is the long-lasting strengthening of the connection between two nerve cells. Experimentally, a series of short, high-frequency electric stimulations to a nerve cell synapse can strengthen, or potentiate, that synapse for minutes to hours. In living cells, LTP occurs naturally and can last from hours to days, months, and years.
The biological mechanisms of LTP, largely via the interplay of protein kinases, phosphatases, and gene expression, give rise to synaptic plasticity and provide the foundation for a highly adaptable nervous system. Most neuroscientific learning theories regard long-term potentiation and its opposing process, long-term depression, as the cellular basis of learning and memory.
LTP was discovered in the mammalian hippocampus by Terje Lømo in 1966 and has remained a popular subject of neuroscientific research since. Most modern LTP studies seek to better understand its biology, while other research aims to develop drugs that exploit these biological mechanisms to treat neurodegenerative diseases such as Parkinson's and Alzheimer's disease.
Early theories of learning
By about 1900, neurobiologists had good reason to believe that memories were generally not the product of new nerve cell growth. Scientists generally believed that the number of neurons in the adult brain (roughly 1011) did not increase significantly with age. With this realization came the need to explain how memories were created in the absence of new cell growth.
Among the first neuroscientists to suggest that learning was not the product of new cell growth was the Spanish anatomist Santiago Ramón y Cajal. In 1894 he proposed that memories might be formed by strengthening the connections between existing neurons to improve the effectiveness of their communication. Hebbian theory, introduced by Donald Hebb in 1949, echoed Ramón y Cajal's ideas, and further proposed that cells may grow new connections between each other to enhance their ability to communicate:
- Let us assume that the persistence or repetition of a reverberatory activity (or "trace") tends to induce lasting cellular changes that add to its stability.... When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased.
However, these theories of memory formation were foresighted as neuroscientists were not yet equipped with the neurophysiological techniques necessary for elucidating the biological underpinnings of learning in animals. These skills would not come until the latter half of the 20th century, at about the same time as the discovery of long-term potentiation.
Discovery of long-term potentiation
LTP was first observed by Terje Lømo in 1966 in the Oslo, Norway, laboratory of Per Andersen. There, Lømo conducted a series of neurophysiological experiments on anesthetized rabbits to explore the role of the hippocampus in short-term memory.
Isolating the connections between two parts of the hippocampus, the perforant pathway and dentate gyrus, Lømo observed the electrical changes in the dentate gyrus elicited by stimulation of the perforant pathway. As expected, a single pulse of electrical stimulation to the perforant pathway elicited an excitatory postsynaptic potential (EPSP) in the dentate gyrus. What Lømo did not expect was that a high-frequency train of stimulation produced larger, prolonged EPSPs compared to the responses evoked by a single stimulus. This phenomenon was soon dubbed "long-term potentiation".
Types of LTP
Since its original discovery in the rabbit hippocampus, LTP has been observed in a variety of other neural structures, including the cerebral cortex, cerebellum, amygdala, and many others. The underlying mechanisms of LTP are generally conserved across these different regions, but there are subtle differences in LTP's precise molecular machinery between sites. Very broadly, there are two types of LTP, associative and nonassociative. The mechanisms of the two types may or may not be the same.
Associative LTP is the molecular analog of associative learning (e.g. classical conditioning). When two or more synapses contribute to the firing (or repeated firings) of a neuron, then any resulting LTP affects all those "associated" synapses, regardless of each synapse's size of contribution. Conversely, synapses that are connected to the neuron but that do not contribute are not affected by LTP.
To detect the simultaneous activity of synapses on a post-synaptic neuron, associative LTP requires a coincidence detector. In many parts of the brain where associative LTP is observed, the NMDA receptor (NMDAR) fills the role of coincidence detector. At rest, the NMDAR's calcium channel is blocked by magnesium; the blockade is relieved only after strong postsynaptic depolarization. The calcium channel is also ligand-gated, so that it only opens when presynaptically-released glutamate binds the receptor. When the NMDAR opens, calcium floods the postsynaptic cell, triggering associative LTP.
NMDAR-dependent LTP has been demonstrated in the hippocampus, particularly in the Schaffer collaterals and perforant pathway, and several other brain regions including parts of the amygdala and cerebral cortex.
There are several types of associative LTP that do not depend on NMDA receptors. NMDAR-independent LTP has been observed, for example, in the amygdala, where it depends instead on voltage-gated calcium channels.
Nonassociative LTP is brought about by the repeated application of one stimulus (whereas in associative LTP there are at least two stimuli). At nonassociative synapses, such as those involved in habituation and sensitization, persistent stimulation of the synapse triggers an influx of calcium into the postsynaptic cell. As in associative LTP, calcium signals the beginning of long-term potentiation, but the precise mechanisms of nonassociative LTP are still unknown.
It is known that LTP requires both the release of glutamate as well as depolarisation of the postsynaptic cell.
Properties of LTP
NMDA receptor-dependent LTP classically exhibits four main properties: rapid induction, cooperativity, associativity, and input specificity.
LTP can be rapidly induced by applying one or more brief tetanic stimuli to a presynaptic cell. (A tetanic stimulus is a high-frequency sequence of individual stimulation.)
LTP can be induced either by strong tetanic stimulation of a single pathway to a synapse, or cooperatively via the weaker stimulation of many. It is explained by the presence of a stimulus threshold that must be reached in order to induce LTP.
When one pathway into a synapse is stimulated weakly, it produces insufficient postsynaptic depolarization to induce LTP. In contrast, when weak stimuli are applied to many pathways that converge on a single patch of the postsynaptic membrane, the individual postsynaptic depolarizations generated may collectively depolarize the postsynaptic cell enough to induce LTP cooperatively.
Associativity refers to the observation that when weak stimulation of a single pathway is insufficient for the induction of LTP, simultaneous strong stimulation of another pathway will induce LTP at both pathways. There is some evidence that associativity and cooperativity share the same underlying cellular mechanism (see Synaptic tagging).
Once induced, LTP at one synapse is not propagated to adjacent synapses; rather LTP is input specific.
Phases of LTP
LTP is often divided into two phases, an early, protein synthesis-independent phase (E-LTP) that lasts between one and five hours, and a late, protein synthesis-dependent phase (L-LTP) that lasts from days to months. Broadly, E-LTP produces a potentiation of a few hours duration. It does so by making the postsynaptic side of the synapse more sensitive to glutamate by adding additional AMPA receptors into the postsynaptic membrane.
Conversely, L-LTP results in a pronounced strengthening of the postsynaptic response largely through the synthesis of new proteins. These proteins include glutamate receptors (e.g. AMPAR), transcription factors, and structural proteins that enhance existing synapses and form new connections. There is also considerable evidence that late LTP prompts the postsynaptic synthesis of a retrograde messenger that diffuses to the presynaptic cell increasing the probability of neurotransmitter vesicle release on subsequent stimuli. All of this is largely hypothetical. The proposed mechanism of L-LTP are only weakly supported by existing data. Many investigators in the field doubt the very existence of L-LTP.
E-LTP can be induced experimentally by applying a few trains of tetanic stimulation to the connection between two neurons. Through normal synaptic transmission, this stimulation causes the release of neurotransmitters, particularly glutamate, from the presynaptic terminal onto the postsynaptic cell membrane, where they bind to neurotransmitter receptors embedded in the postsynaptic membrane. Though a single presentation of the stimulus is not sufficient to induce LTP, repeated presentations, if given at high-enough frequency, cause the postsynaptic cell to be progressively depolarized. This progressive depolarization is the result of EPSP summation. If each successive stimulus within a tetanic train reaches the postsynaptic cell before the previous EPSP can decay, successive EPSPs will add to the depolarization caused by the previous EPSPs. In synapses that exhibit NMDAR-dependent LTP, this progressive depolarization relieves the magnesium blockade of the NMDA receptor. When the magnesium block is removed, successive stimuli promote calcium entry through the NMDAR channel into the postsynaptic cell, rapidly increasing the intracellular concentration of calcium. It is this rapid rise in calcium concentration that induces E-LTP.
Beyond calcium's critical role in the induction of E-LTP, few downstream molecular events leading to the expression and maintenance of E-LTP are known with certainty. Yet there is considerable evidence that E-LTP induction depends upon the activity of several protein kinases, including calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase C (PKC) , protein kinase A (PKA), mitogen-activated protein kinase (MAPK), and tyrosine kinases.
Postsynaptically, the early phase of LTP is expressed primarily through the addition of new AMPA receptors to the postsynaptic membrane. In NMDA-dependent LTP in the CA1 hippocampus, the endogenous calcium chelator calmodulin rapidly binds calcium that is made available to it because it enters the cell through the NMDA receptor . The calcium-calmodulin complex directly activates CaMKII which 1) phosphorylates voltage-gated potassium channels increasing their excitability; 2) enhances the activity of existing AMPA receptors; and 3) phosphorylates intracellular AMPARs and activates Syn GAP (a Ras GTPase activating protein) and the MAPK cascade, facilitating the insertion of AMPARs into the postsynaptic membrane.
PKA serves a role similar to that of CaMKII, but PKA's effects are more broad. PKA's activity is enhanced during LTP induction by elevated levels of cAMP as a result of calcium's activation of adenylyl cyclase-1. Like CaMKII, PKA phosphorylates voltage-dependent potassium channels and also calcium channels enhancing their excitability to future stimuli. Additionally, PKA phosphorylates intracellular AMPAR stores, facilitating their insertion postsynaptically. PKA may also enhance AMPAR delivery via activation of the MAPK cascade. However, the role of PKA, especially in early LTP is very controversial.
While LTP is induced postsynaptically, it is partially expressed presynaptically. One hypothesis of presynaptic facilitation is that enhanced CaMKII activity during early LTP gives rise to CaMKII autophosphorylation and constitutive activation. Persistent CaMKII activity then stimulates NO synthase, leading to the enhanced production of the putative retrograde messenger, NO. Since NO is a diffusable gas, it freely diffuses across the synaptic cleft to the presynaptic cell leading to a chain of molecular events that facilitate the presynaptic response to subsequent stimuli. (See Retrograde signaling for discussion about the identity of the retrograde messenger.)
Late LTP can be experimentally induced by a series of three or more trains of tetanic stimulation spaced roughly 10 minutes apart. Unlike early LTP, late LTP requires gene transcription and protein synthesis, making it an attractive candidate for the molecular analog of long-term memory.
The synthesis of gene products is driven by kinases which in turn activate transcription factors that mediate gene expression. cAMP response element binding protein-1 (CREB-1) is thought to be the primary transcription factor in the cascade of gene expression that leads to prolonged structural changes to the synapse enhancing its strength. CREB-1 is both necessary and sufficient for late LTP. It is active in its phosphorylated form and induces the transcription of so-called immediate-early genes, including c-fos and c-jun. Ultimately, the products of CREB-1-mediated transcription and protein synthesis give rise to new building materials for the synaptic connection between pre- and postsynaptic cell.
During L-LTP, constitutively active CaMKII activates a related kinase, CaMKIV. Additionally, enhanced Ca2+ levels during late LTP increase cAMP synthesis via adenylyl cyclase-1, further activating PKA and resulting in the phosphorylation and activation of MAPK. Facilitated by cAMP, both CaMKII and CaMKIV translocate to the cell nucleus along with PKA and MAPK (mediated by PKA), where they phosphorylate CREB-1.
There is also some evidence that L-LTP is mediated in part by nitric oxide (NO). In particular, NO may activate guanylyl cyclase, leading to the production of cyclic GMP and activation protein kinase G (PKG), which phosphorylates CREB-1. PKG may also cause the release of Ca2+ from ryanodine receptor-gated intracellular stores, increasing the Ca2+ concentration which activates other previously mentioned kinase cascades to further activate CREB-1.
Retrograde signalling is a theoretical concept that arises from the question: "If LTP is induced postsynaptically, but expressed presynaptically, how does the presynaptic terminal "know" that LTP has been induced?" The obvious answer is that there must be some communication "backwards" across the synapse, that is, in the retrograde direction from the postsynaptic to the presynaptic side. This concept led to a flurry of work in the early 1990's to demonstrate the existence of a retrograde messenger and also to identify such a messenger. A number of candidates were examined including carbon monoxide, platelet-activating factor, arachidonic acid, and nitric oxide.
Perhaps unfortunately for the retrograde signaling hypothesis, subsequent work has strongly established that LTP, at least early LTP, is expressed entirely postsynaptically (cf. Malenka and Bear, 2004). However, there is still life in the retrograde signalling hypothesis, since it has been demonstrated that induction of LTP may involve a retrograde messenger, since contrary to dogma, LTP induction does not appear to be entirely postsynaptic (Pavlidis, et al., 2000).
The gene expression and protein synthesis that mediate the long-term changes of LTP generally take place in the cell body, but LTP is synapse-specific; LTP induced at one synapse does not propagate to adjacent inactive synapses. Therefore, the cell is posed with the difficult problem of synthesizing plasticity-related proteins in the nucleus and cell body, but ensuring they only reach synapses that have received LTP-inducing stimuli.
The synthesis of a "synaptic tag" at a given synapse after LTP-inducing stimuli may serve to capture plasticity-related proteins shipped cell-wide from the nucleus. Studies of LTP in the marine snail Aplysia californica have implicated synaptic tagging as a mechanism for the input-specificity of LTP. There is some evidence that given two widely separated synapses, an LTP-inducing stimulus at one synapse drives several signaling cascades (described previously) that initiates gene expression in the cell nucleus. At the same synapse (but not the unstimulated synapse), local protein synthesis creates a short-lived (less than three hours) synaptic tag. The products of gene expression are shipped globally throughout the cell, but are only captured by synapses that express the synaptic tag. Thus only the input receiving LTP-inducing stimuli is potentiated, demonstrating LTP's input-specificity.
The synaptic tag hypothesis may also give rise to LTP's associativity. Associativity (see above) is observed when one synapse is excited with LTP-inducing stimulation while a separate synapse is only weakly stimulated. Whereas one might expect only the strongly stimulated synapse to undergo LTP (since weak stimulation alone is insufficient to induce LTP at either synapse), both synapses will in fact undergo LTP. While weak stimuli are unable to induce gene expression in the cell nucleus, they appear to prompt the synthesis of a synaptic tag. Simultaneous strong stimulation of a separate pathway, capable of inducing nuclear gene expression, then prompts the production of plasticity-related proteins, which are shipped cell-wide. With both synapses expressing the synaptic tag, both capture the protein products resulting in the induction of LTP in both the strongly stimulated and weakly stimulated pathways.
LTP's cooperativity may also be explained by synaptic tagging. While weak stimulation of a single pathway is insufficient to induce LTP, the simultaneous weak stimulation of two pathways is sufficient. As noted previously, weak stimulation initiates the synthesis of a synaptic tag, but is insufficient to trigger late LTP and thus CREB-1-mediated gene expression. But simultaneous weak input converges on kinases that sufficiently activate CREB-1 thereby inducing the synthesis of plasticity-related proteins, which are shipped out cell-wide as described previously. Since a synaptic tag has been synthesized at both synapses, both capture the products of gene expression and both are subsequently potentiated.
|DA receptors||cAMP, MAPK amplification|
|β-adrenergic receptors||cAMP, MAPK amplification|
|mGluR||PKC, MAPK amplification|
|NO synthase||Guanylyl cyclase, PKG, NMDAR|
In addition to the signalling pathways described above, hippocampal LTP can be modulated by a variety of molecules. For example, the steroid hormone estradiol is one of several molecules that enhances LTP by driving CREB-1 phosphorylation and subsequent dendritic spine growth (9920677). Additionally, β-adrenergic receptor agonists such as norepinephrine contribute to the protein synthesis-dependent late phase of LTP (12770561). Nitric oxide synthase also plays an important role, leading to the up-regulation of nitric oxide and subsequent activation of guanylyl cyclase and PKG, as described previously (10575022). Similarly, activation of dopamine receptors enhances LTP via the cAMP/PKA signaling pathway (1833673)(8922403).
LTP and behavioral memory
The mere fact that cultured synapses can undergo long-term potentiation when stimulated by electrodes says little about LTP's relation to memory. Several studies have provided some insight as to whether LTP is a requirement for memory.
Richard Morris provided some of the first evidence that LTP was indeed required for the formation of memories . He tested the spatial memory of two groups of rats, one whose hippocampi were bathed in the NMDA receptor blocker APV, and the other acting as a control group. (The hippocampus, where LTP was originally observed, is required for some forms of spatial learning). Both groups were then subjected to the Morris water maze, in which rats were placed into a circular pool of murky water (often made opaque with milk powder or white paint) and tested on how quickly they could locate a platform hidden just beneath the water's surface.
Rats in the control group were able to locate the platform and escape from the pool, whereas the ability of APV-treated rats to complete the task was significantly impaired. Moreover, when slices of the hippocampus were taken from both groups of rats, LTP was easily induced in controls, but could not be induced in the brains of APV-treated rats. This provided some evidence that the NMDA receptor — and thus LTP — was somehow involved with at least some types of learning and memory.
Similarly, Susumu Tonegawa has demonstrated that a specific region of the hippocampus, namely CA1, is crucial to the formation of spatial memories . So-called place cells located in this region fire when the rat is in a particular location in the environment. Since a large group of these cells will have place fields evenly distributed throughout the environment, one interpretation is that these cells form a sort of map. The accuracy of these maps determines how well a rat learns about its environment, and thus how well it can navigate about it.
Tonegawa found that by impairing the NMDA receptor, specifically by genetically removing the NMDAR1 subunit in the CA1 region, the place fields generated were substantially less specific than those of controls. That is, rats produced faulty spatial maps when their NMDA receptors were impaired. As expected, these rats performed very poorly on spatial tasks compared to controls, providing more support to the notion that LTP is the underlying mechanism of spatial learning.
Enhanced NMDA receptor activity in the hippocampus has also been shown to produce enhanced LTP and an overall improvement in spatial learning. Joe Tsien produced a line of mice with enhanced NMDA receptor function by overexpressing the NR2B subunit in the hippocampus . These smart mice, nicknamed "Doogie mice" after the prodigious doctor Doogie Howser, had larger long-term potentiation and excelled at spatial learning tasks, once again suggesting LTP's involvement in the formation of hippocampal-dependent memories .
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