Different mechanisms and multiple stages of LTP

Synapses tagged, memories kept: synaptic tagging and capture hypothesis in brain health and disease

The synaptic tagging and capture (STC) hypothesis lays the framework on the synapse-specific mechanism of protein synthesis-dependent long-term plasticity upon synaptic induction. Activated synapses will display a transient tag that will capture plasticity-related products (PRPs). These two events, tag setting and PRP synthesis, can be teased apart and have been studied extensively-from their electrophysiological and pharmacological properties to the molecular events involved. Consequently, the hypothesis also permits interactions of synaptic populations that encode different memories within the same neuronal population-hence, it gives rise to the associativity of plasticity. In this review, the recent advances and progress since the experimental debut of the STC hypothesis will be shared. This includes the role of neuromodulation in PRP synthesis and tag integrity, behavioural correlates of the hypothesis and modelling in silico. STC, as a more sensitive assay for synaptic health, can also assess neuronal aberrations. We will also expound how synaptic plasticity and associativity are altered in ageing-related decline and pathological conditions such as juvenile stress, cancer, sleep deprivation and Alzheimer's disease. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

eIF4E phosphorylation recruits β-catenin to mRNA cap and promotes Wnt pathway translation in dentate gyrus LTP maintenance

The mRNA cap-binding protein, eukaryotic initiation factor 4E (eIF4E), is crucial for translation and regulated by Ser209 phosphorylation. However, the biochemical and physiological role of eIF4E phosphorylation in translational control of long-term synaptic plasticity is unknown. We demonstrate that phospho-ablated Eif4e^S209A Knockin mice are profoundly impaired in dentate gyrus LTP maintenance in vivo, whereas basal perforant path-evoked transmission and LTP induction are intact. mRNA cap-pulldown assays show that phosphorylation is required for synaptic activity-induced removal of translational repressors from eIF4E, allowing initiation complex formation. Using ribosome profiling, we identified selective, phospho-eIF4E-dependent translation of the Wnt signaling pathway in LTP. Surprisingly, the canonical Wnt effector, β-catenin, was massively recruited to the eIF4E cap complex following LTP induction in wild-type, but not Eif4e^S209A, mice. These results demonstrate a critical role for activity-evoked eIF4E phosphorylation in dentate gyrus LTP maintenance, remodeling of the mRNA cap-binding complex, and specific translation of the Wnt pathway.

A microRNA negative feedback loop downregulates vesicle transport and inhibits fear memory

The SNARE-mediated vesicular transport pathway plays major roles in synaptic remodeling associated with formation of long-term memories, but the mechanisms that regulate this pathway during memory acquisition are not fully understood. Here we identify miRNAs that are up-regulated in the rodent hippocampus upon contextual fear-conditioning and identify the vesicular transport and synaptogenesis pathways as the major targets of the fear-induced miRNAs. We demonstrate that miR-153, a member of this group, inhibits the expression of key components of the vesicular transport machinery, and down-regulates Glutamate receptor A1 trafficking and neurotransmitter release. MiR-153 expression is specifically induced during LTP induction in hippocampal slices and its knockdown in the hippocampus of adult mice results in enhanced fear memory. Our results suggest that miR-153, and possibly other fear-induced miRNAs, act as components of a negative feedback loop that blocks neuronal hyperactivity at least partly through the inhibition of the vesicular transport pathway.

DOI:http://dx.doi.org/10.7554/eLife.22467.001

Transcriptional regulation of long-term potentiation

Long-term potentiation (LTP), the persistent strengthening of synapses following high levels of stimulation, is a form of synaptic plasticity that has been studied extensively as a possible mechanism for learning and memory formation. The strengthening of the synapse that occurs during LTP requires cascades of complex molecular processes and the coordinated remodeling of pre-synaptic and post-synaptic neurons. Despite over four decades of research, our understanding of the transcriptional mechanisms and molecular processes underlying LTP remains incomplete. Identification of all the proteins and non-coding RNA transcripts expressed during LTP may provide greater insight into the molecular mechanisms involved in learning and memory formation.

HFS-Triggered AMPK Activation Phosphorylates GSK3β and Induces E-LTP in Rat Hippocampus In Vivo

BACKGROUND

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy and nutrient status, with substantial amount of cross talk with other signaling pathways, including its phosphorylation by Akt, PKA, and GSK3β.

AIMS

Various signaling pathways and energy-consuming transport of glutamate receptors subunits are required in synaptic plasticity. However, it is unknown which energy sensors integrate the signaling pathways in these processes. In this article, we elucidated the role of AMPK activation and GSK3β phosphorylation after HFS during the inducement of early-phase long-term potentiation (E-LTP).

METHODS

Synaptic LTP in vivo was induced by high-frequency stimulation (HFS at 200 Hz at a 5-s interval). In addition, phosphorylation of AMPK and glycogen synthase kinase 3β (GSK3β) were measured using Western blotting. The amount of hippocampal AMP, ADP and ATP was measured by HPLC.

RESULTS

We showed that the phosphorylation of AMPK and GSK3β was significantly increased by HFS in vivo. HFS-induced AMPK activation occurred via increased (AMP + ADP)/ATP ratio and activation of Ca(2+) /calmodulin-dependent kinase kinase beta (CaMKKβ). Pharmacological inhibition of AMPK by compound C (CC) prevented HFS-induced inhibitory phosphorylation of GSK3β and the induction of LTP in dentate gyrus (DG) area in vivo.

CONCLUSIONS

Our findings reveal that HFS-triggered AMPK activation phosphorylates GSK3β and induces E-LTP in vivo.

Phosphodiesterase 4D: an enzyme to remember

Cyclic adenosine monophosphate (cAMP) is one of the second messengers critically involved in the molecular mechanisms underlying memory formation. In the CNS, the availability of cAMP is tightly controlled by phosphodiesterase 4 (PDE4), a family of enzymes that degrades the cyclic nucleotide to inactive AMP. Among the different PDE4 isoforms, in the last few years PDE4D has been hogging the limelight due to accumulating evidence for its crucial role in cognitive processes, which makes this enzyme a promising target for therapeutic interventions in a variety of pathological conditions characterized by memory impairment, such as Alzheimer's disease. In this article, we review the role of the cAMP signal transduction pathway in memory formation with a particular focus on the recent progress in PDE4D research.

Sleep and protein synthesis-dependent synaptic plasticity: impacts of sleep loss and stress

Sleep has been ascribed a critical role in cognitive functioning. Several lines of evidence implicate sleep in the consolidation of synaptic plasticity and long-term memory. Stress disrupts sleep while impairing synaptic plasticity and cognitive performance. Here, we discuss evidence linking sleep to mechanisms of protein synthesis-dependent synaptic plasticity and synaptic scaling. We then consider how disruption of sleep by acute and chronic stress may impair these mechanisms and degrade sleep function.

BDNF mechanisms in late LTP formation: A synthesis and breakdown

Unraveling the molecular mechanisms governing long-term synaptic plasticity is a key to understanding how the brain stores information in neural circuits and adapts to a changing environment. Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of stable, late phase long-term potentiation (L-LTP) at excitatory glutamatergic synapses in the adult brain. However, the mechanisms by which BDNF triggers L-LTP are controversial. Here, we distill and discuss the latest advances along three main lines: 1) TrkB receptor-coupled translational control underlying dendritic protein synthesis and L-LTP, 2) Mechanisms for BDNF-induced rescue of L-LTP when protein synthesis is blocked, and 3) BDNF-TrkB regulation of actin cytoskeletal dynamics in dendritic spines. Finally, we explore the inter-relationships between BDNF-regulated mechanisms, how these mechanisms contribute to different forms of L-LTP in the hippocampus and dentate gyrus, and outline outstanding issues for future research. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Differentiation in the protein synthesis-dependency of persistent synaptic plasticity in mossy fiber and associational/commissural CA3 synapses in vivo

Long-term potentiation (LTP) and long-term depression (LTD) are two mechanisms involved in the long-term storage of information in hippocampal synapses. In the hippocampal CA1 region, the late phases of LTP and LTD are protein-synthesis dependent. In the dentate gyrus, late-LTP but not LTD requires protein synthesis. The protein synthesis-dependency of persistent plasticity at CA3 synapses has not yet been characterized. Here, the roles of protein transcription and translation at mossy fiber (mf) and associational/commissural (AC)- synapses were studied in freely behaving rats. In control animals, low-frequency stimulation (LFS) evoked robust LTD (>24 h), whereas high-frequency stimulation (HFS) elicited robust LTP (>24 h) at both mf-CA3 and AC-CA3 synapses. Translation inhibitors prevented early and late phases of LTP and LTD at mf-CA3 synapses. In contrast, at AC–CA3 synapses, translation inhibitors prevented intermediate/late-LTP and late-LTD only. Transcription effects were also synapse-specific: whereas transcription inhibitors inhibited late-LTP and late-LTD (>3 h) at mf-CA3 synapses, at AC–CA3 synapses, protein transcription affected early-LTP and late-LTD. These results show that the AC-CA3 and mf-CA3 synapses display different properties in terms of their protein synthesis dependency, suggesting different roles in the processing of short- and long term synaptic plasticity.

Neurotoxic mechanisms of DNA damage: focus on transcriptional inhibition

Although DNA damage-induced neurotoxicity is implicated in various pathologies of the nervous system, its underlying mechanisms are not completely understood. Transcription is a DNA transaction that is highly active in the nervous system. In addition to its direct role in expression of the genetic information, transcription contributes to DNA damage detection and repair as well as chromatin organization including biogenesis of the nucleolus. Transcription is inhibited by DNA single-strand breaks and DNA adducts. Hence, transcription inhibition may be an important contributor to the neurotoxic consequences of such types of DNA damage. This review discusses the existing evidence in support of the latter hypothesis. The presented literature suggests that neuronal DNA damage interferes with the RNA-Polymerase-2-dependent transcription of genes encoding proteins with critical functions in neurotransmission and intracellular signaling. The latter category includes extracellular signal-regulated kinase-1/2 mitogen-activated protein kinase phosphatases whose lowered expression results in chronic activation of extracellular signal-regulated kinase-1/2 and its reduced responsiveness to physiological stimuli. Conversely, DNA damage-induced inhibition of RNA-Polymerase-1 and the subsequent disruption of the nucleolus induce p53-mediated apoptosis of developing neurons. Finally, decreasing nucleolar transcription may link DNA damage to chronic neurodegeneration in adults.

Protein degradation by the proteasome is required for synaptic tagging and the heterosynaptic stabilization of hippocampal late-phase long-term potentiation

Activity-dependent regulation of synaptic efficacy is believed to underlie learning and memory formation. Here we show that protein degradation by the proteasome is required for the induction of the protein synthesis-dependent late-phase of long-term potentiation (late-LTP) but not for its maintenance. Proteasome activity was also key to the polarity of heterosynaptic interactions between synapses expressing synaptic plasticity and newly activated synapses. In fact, proteasome activity was required for the consolidation of an otherwise transient potentiation (early-LTP) into late-LTP by strong tetanization of a separate afferent pathway both in the "weak-before-strong" and in the "strong-before-weak" two-pathway paradigms [Frey and Morris (1997) Nature 385:533-536; Frey and Morris (1998) Neuropharmacology 37:545-552], suggesting that proteasome activity plays a role in the synaptic tagging and capture of plasticity-related proteins at stimulated synapses. Additionally, proteasome inhibition abrogated immunity against heterosynaptic depotentiation of an established late-LTP when applied during weak tetanic stimulation in the "strong-before-weak" two-pathway paradigm. Such a heterosynaptic destabilizing effect of proteasome inhibition was abolished by concomitant inhibition of N-methyl-d-aspartate (NMDA) receptors, suggesting that it is an active process. Together, these results indicate that the proteasome plays important roles in the establishment of late-LTP and in the preservation of potentiated synapses when a subsequent synaptic plasticity is induced within the same neuronal population.

Continuous blockade of GABA-ergic inhibition induces novel forms of long-lasting plastic changes in apical dendrites of the hippocampal cornu ammonis 1 (CA1) in vitro

Hippocampal long-term potentiation (LTP) is considered as a fundamental mechanism for learning and memory formation. A role for GABA was reported for the induction and early but not late maintenance of LTP. We have now investigated whether GABA-receptor function is involved in the prolonged maintenance of LTP (>4 h) at afferent synapses at apical dendrites of cornu ammonis 1 (CA1)-pyramidal neurons in hippocampal slices in vitro. Our data demonstrate that GABA-receptor mediated events are not required for conventional, tetanically-induced early- or late-LTP in the hippocampal CA1-region in vitro. Inhibition of GABA-ergic transmission did not negatively influence either early- or late-LTP. In contrast, an additional facilitation was observed at time points corresponding to the establishment of late-LTP (after 3-4 h). Investigation of a second, non-tetanized control input to the same neuronal population revealed that the elevated potentiation of late-LTP in the tetanized input was not LTP-specific. Therefore, we have examined, whether continuous application of GABA-receptor inhibitors also affected the time course of the recorded potentials when a low-frequency stimulation protocol was used. Under these conditions two distinct forms of a late-onset potentiation occurred 5-6 h after drug application. Investigation of mechanisms responsible for this prolonged enhancement of potentials revealed that the higher form of potentiation (potentiation levels above 200%) was dependent on presynaptic activity and N-methyl-d-aspartate (NMDA)-receptor activation, whereas the lower form (potentiation less than 200%) did not require these mechanisms. However, the latter potentiation was prevented by nifedipine, an L-type voltage-dependent calcium channel inhibitor.

Interfering with the actin network and its effect on long-term potentiation and synaptic tagging in hippocampal CA1 neurons in slices in vitro

Long-term potentiation (LTP) is a cellular correlate for memory formation, which requires the dynamic changes of the actin cytoskeleton. As shown by others, the polymerization of the actin network is important for early stages of LTP. Here, we investigated the role of actin dynamics in synaptic tagging and particularly in the induction of protein synthesis-dependent late-LTP in the CA1 region in hippocampal slices in vitro. We found that the inhibition of actin polymerization affects protein synthesis-independent early-LTP, prevents late-LTP, and interferes with synaptic tagging in apical dendrites of hippocampal CA1. The transformation of early-LTP into late-LTP was blocked by the application of the structurally different actin polymerization inhibitors latrunculin A or cytochalasin D. We suggest that the actin network is required for early "housekeeping" processes to induce and maintain early-LTP. Furthermore, inhibition of actin dynamics negatively interacts with the setting of the synaptic tagging complex. We propose actin as a further tag-specific molecule in apical CA1 dendrites where it is directly involved in the tagging/capturing machinery. Consequently, inhibition of the actin network prevents the interaction of tagging complexes with plasticity-related proteins. This results in the prevention of late-LTP by inhibition of the actin network during LTP induction.

Distinct single but not necessarily repeated tetanization is required to induce hippocampal late-LTP in the rat CA1

The protein synthesis-dependent form of hippocampal long-term potentiation (late-LTP) is thought to underlie memory. Its induction requires a distinct stimulation strength, and the common opinion is that only repeated tetani result in late-LTP whereas as single tetanus only reveals a transient early-LTP. Properties of LTP induction were compared to learning processes where repetition is often the prerequisite for a long-lasting memory. However, also single events can lead to manifested memory. If LTP subserves processes of learning, similar results should be detectable for LTP. Here we show that a single tetanus is sufficient to induce late-LTP requiring dopaminergic co-transmission during induction.

'Synaptic tagging' and 'cross-tagging' and related associative reinforcement processes of functional plasticity as the cellular basis for memory formation

We focus on new properties of cellular and network processes of memory formation involving 'synaptic tagging' and 'cross-tagging' during long-term potentiation (LTP) and long-term depression (LTD) as well as associative heterosynaptic interactions, the latter of which are characterized by a time-window of about 1h. About 20 years ago we showed for the first time that the maintenance of LTP, like memory storage, depends on intact protein synthesis and thus consists of at least two temporal phases. Later, similar properties for LTD were shown by our own and other laboratories. Here we describe the requirements for the induction of the transient early-LTP/LTD and of the protein synthesis-dependent late-LTP/LTD. Late-LTP/LTD depend on the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory reinforcing inputs within specific, effective time-windows during their induction. The induction of late-LTP/LTD is characterized by novel, late-associative properties such as 'synaptic tagging', 'cross-tagging' and 'late-associative reinforcement'. All of these phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity, 'cross-tagging' determines the interaction between LTP and LTD in a neuron, and thus both are required for the specific processing of afferent signals for the establishment of late-LTP/LTD. 'Late-associative reinforcement' describes a process where early-LTP/LTD by the co-activation of modulatory inputs can be transformed into late-LTP/LTD in activated synapses where a tag is set. Recent experiments in the freely moving rat revealed a number of modulatory brain structures involved in the transformation of early-plasticity events into long-lasting ones. Further to this, we have characterized time-windows and activation patterns to be effective in the reinforcement process. Studies using a combined electrophysiological and behavioural approach revealed the physiological relevance of these reinforcement processes, which is also supported by fMRI studies in humans, which led to the hypothesis outlined here on cellular and system memory-formation by late-associative heterosynaptic interactions at the cellular level during functional plasticity events.

The role of protein synthesis in memory consolidation: progress amid decades of debate

A major component of consolidation theory holds that protein synthesis is required to produce the synaptic modification needed for long-term memory storage. Protein synthesis inhibitors have played a pivotal role in the development of this theory. However, these commonly used drugs have unintended effects that have prompted some to reevaluate the role of protein synthesis in memory consolidation. Here we review the role of protein synthesis in memory formation as proposed by consolidation theory calling special attention to the controversy involving the non-specific effects of a group of protein synthesis inhibitors commonly used to study memory formation in vivo. We argue that molecular and genetic approaches that were subsequently applied to the problem of memory formation confirm the results of less selective pharmacological studies. Thus, to a certain extent, the debate over the role of protein synthesis in memory based on interpretational difficulties inherent to the use of protein synthesis inhibitors may be somewhat moot. We conclude by presenting avenues of research we believe will best provide answers to both long-standing and more recent questions facing field of learning and memory.

Genetic disruption of protein kinase A anchoring reveals a role for compartmentalized kinase signaling in theta-burst long-term potentiation and spatial memory

Studies of hippocampal long-term potentiation (LTP), a cellular model of memory storage, implicate cAMP-dependent protein kinase (PKA) in presynaptic and postsynaptic mechanisms of LTP. The anchoring of PKA to AKAPs (A kinase-anchoring proteins) creates compartmentalized pools of PKA, but the roles of presynaptically and postsynaptically anchored forms of PKA in late-phase LTP are unclear. In this study, we have created genetically modified mice that conditionally express Ht31, an inhibitor of PKA anchoring, to probe the roles of anchored PKA in hippocampal LTP and spatial memory. Our findings show that at hippocampal Schaffer collateral CA3-CA1 synapses, theta-burst LTP requires presynaptically anchored PKA. In addition, a pool of anchored PKA in hippocampal area CA3 is required for spatial memory. These findings reveal a novel and significant role for anchored PKA signaling in cellular mechanisms underlying memory storage.

Cognitive and emotional information processing: protein synthesis and gene expression

Recent findings suggest that functional plasticity phenomena such as long-term potentiation (LTP) and long-term depression (LTD) - cellular processes underlying memory - are restricted to functional dendritic compartments. It was also shown, however, that a relatively strong activation of a synaptic input can abolish compartment restrictions. Our data support these findings and we present one cellular pathway responsible for uncompartmentalization of the normally localized plasticity processes by the action of rolipram, an inhibitor of type 4 phosphodiesterases. In contrast with compartment-restricted information processing, uncompartmentalization requires transcription. In the search for system relevance of compartmentalization versus uncompartmentalization we describe firstly data which show that more cognitive information processing in rats' behaviour may follow rules of compartmentalization, whereas stressful, more life-threatening, inputs abolish compartment-restricted information processing involving transcription. Our findings allow us to suggest that consolidation of processes which take place during the cognitive event most probably depend on local protein synthesis, whereas stress immediately induces gene expression in addition, resulting in a compartment-unspecific up-regulation of plasticity-related proteins (PRPs), providing the entire neuron with a higher level of 'reactiveness'. These data would provide a specific functional cellular mechanism to respond differentially and effectively to behaviourally weighted inputs.

The late maintenance of hippocampal LTP: Requirements, phases, ‘synaptic tagging’, ‘late-associativity’ and implications

Our review focuses on the mechanisms which enable the late maintenance of hippocampal long-term potentiation (LTP; >3h), a phenomenon which is thought to underlie prolonged memory. About 20 years ago we showed for the first time that the maintenance of LTP - like memory storage--depends on intact protein synthesis and thus, consists of at least two temporal phases. Here we concentrate on mechanisms required for the induction of the transient early-LTP and of the protein synthesis-dependent late-LTP. Our group has shown that the induction of late-LTP requires the associative activation of heterosynaptic inputs, i.e. the synergistic activation of glutamatergic and modulatory, reinforcing inputs within specific, effective time windows. The induction of late-LTP is characterized by novel, late-associative properties such as 'synaptic tagging' and 'late-associative reinforcement'. Both phenomena require the associative setting of synaptic tags as well as the availability of plasticity-related proteins (PRPs) and they are restricted to functional dendritic compartments, in general. 'Synaptic tagging' guarantees input specificity and thus the specific processing of afferent signals for the establishment of late-LTP. 'Late-associative reinforcement' describes a process where early-LTP by the co-activation of modulatory inputs can be transformed into late-LTP in activated synapses where a tag is set. Recent evidence from behavioral experiments, which studied processes of emotional and cognitive reinforcement of LTP, point to the physiological relevance of the above mechanisms during cellular and system's memory formation.

Amygdala modulation of memory-related processes in the hippocampus: potential relevance to PTSD

A key assumption in the study of stress-induced cognitive and neurobiological modifications is that alterations in hippocampal functioning after stress are due to an excessive activity exerted by the amygdala on the hippocampus. Research so far focused on stress-induced impairment of hippocampal plasticity and memory but an exposure to stress may simultaneously also result in strong emotional memories. In fact, under normal conditions emotionally charged events are better remembered compared with neutral ones. Results indicate that under these conditions there is an increase in activity within the amygdala that may lead to memory of a different quality. Studying the way emotionality activates the amygdala and the functional impact of this activation we found that the amygdala modulates memory-related processes in other brain areas, such as the hippocampus. However, this modulation is complex, involving both enhancing and suppressing effects, depending on the way the amygdala is activated and the hippocampal subregion examined. The current review summarizes our findings and attempts to put them in context with the impact of an exposure to a traumatic experience, in which there is a mixture of a strong memory of some aspects of the experience but impaired memory of other aspects of that experience. Toward that end, we have recently developed an animal model for the induction of predisposition to stress-related disorders, focusing on the consequences of exposure to stressors during juvenility on the ability to cope with stress in adulthood. Exposing juvenile-stressed rats to an additional stressful challenge in adulthood revealed their impairment to cope with stress and resulted in significant elevation of the amygdala. Interestingly, and similar to our electrophysiological findings, differential effects were observed between the impact of the emotional challenge on CA1 and dentate gyrus subregions of the hippocampus. Taken together, the results indicate that long-term alterations within the amygdala contribute to stress-related mnemonic symptoms and suggest that elucidating further these intra-amygdala alterations and their effects on modulating other brain regions is likely to be beneficial for the development of novel approaches to treat stress-related disorders.

Elements of a neurobiological theory of hippocampal function: the role of synaptic plasticity, synaptic tagging and schemas

The 2004 EJN Lecture was an attempt to lay out further aspects of a developing neurobiological theory of hippocampal function [Morris, R.G.M., Moser, E.I., Riedel, G., Martin, S.J., Sandin, J., Day, M. & O'Carroll, C. (2003) Phil. Trans. R. Soc. Lond. B Biol. Sci., 358, 773-786.] These are that (i) activity-dependent synaptic plasticity plays a key role in the automatic encoding and initial storage of attended experience; (ii) the persistence of hippocampal synaptic potentiation over time can be influenced by other independent neural events happening closely in time, an idea with behavioural implications for memory; and (iii) that systems-level consolidation of memory traces within neocortex is guided both by hippocampal traces that have been subject to cellular consolidation and by the presence of organized schema in neocortex into which relevant newly encoded information might be stored. Hippocampal memory is associative and, to study it more effectively than with previous paradigms, a new learning task is described which is unusual in requiring the incidental encoding of flavour-place paired associates, with the readout of successful storage being successful recall of a place given the flavour with which it was paired. NMDA receptor-dependent synaptic plasticity is shown to be critical for the encoding and intermediate storage of memory traces in this task, while AMPA receptor-mediated fast synaptic transmission is necessary for memory retrieval. Typically, these rapidly encoded traces decay quite rapidly over time. Synaptic potentiation also decays rapidly, but can be rendered more persistent by a process of cellular consolidation in which synaptic tagging and capture play a key part in determining whether or not it will be persistent. Synaptic tags set at the time of an event, even many trivial events, can capture the products of the synthesis of plasticity proteins set in train by events before, during or even after an event to be remembered. Tag-protein interactions stabilize synaptic potentiation and, by implication, memory. The behavioural implications of tagging are explored. Finally, using a different protocol for flavour-place paired associate learning, it is shown that rats can develop a spatial schema which represents the relative locations of several different flavours of food hidden at places within a familiar space. This schema is learned gradually but, once acquired, enables new paired associates to be encoded and stored in one trial. Their incorporation into the schema prevents rapid forgetting and suggests that schema play a key and hitherto unappreciated role in systems-level memory consolidation. The elements of what may eventually mature into a more formal neurobiological theory of hippocampal memory are laid out as specific propositions with detailed conceptual discussion and reference to recent data.

Search for a two-input model for future investigations of ‘synaptic tagging’ in freely moving animals in vivo

Processes of "synaptic tagging" guarantee synaptic input specificity after the induction of a protein synthesis-dependent late long-term potentiation (late-LTP). Distinct high-frequency stimulation can set a transient "synaptic tag" at the activated synapses, which captures plasticity-related proteins (PRPs) synthesized synapse-non-specifically in dendritic branches/compartments or the somata. Thus, only those synapses, which expressed a "tag", are also able to express late-LTP. Additionally, it was shown that the synthesis of PRPs is triggered by heterosynaptic, non-glutamatergic requirements during LTP-induction in tissue from adult animals. All these experiments were performed in hippocampal slices in vitro so far. Two questions now arise: first, is it possible to describe processes of 'synaptic tagging' in the intact, freely moving animal and second, is the stimulation of glutamatergic inputs sufficient to induce 'tagging' or is the co-activation of a modulatory-heterosynaptic input, also required for the process? We have first developed a technique, which allows us now to induce distinct forms of LTP at the ipsilateral CA1 site by specifically stimulating glutamatergic hippocampal structures at the contralateral site in the intact, freely moving rat. Thus, the used stimulation protocol allowed us to activate two separate synaptic inputs to the same neuronal stimulation, a pre-requisite for tagging-experiments to be investigated in vivo.

Mitogen-activated protein kinase-mediated reinforcement of hippocampal early long-term depression by the type IV-specific phosphodiesterase inhibitor rolipram and its effect on synaptic tagging

Rolipram, a selective inhibitor of cAMP-specific phosphodiesterase 4 (PDE4), has been shown to reinforce an early form of long-term potentiation (LTP) to a long-lasting LTP (late LTP). Furthermore, it was shown that the effects of rolipram-mediated reinforcement of LTP interacts with processes of synaptic tagging (Navakkode et al., 2004). Here we show in CA1 hippocampal slices from adult rats in vitro that rolipram also converted an early form of long-term depression (LTD) that normally decays within 2-3 h, to a long-lasting LTD (late LTD) if rolipram was applied during LTD-induction. Rolipram-reinforced LTD (RLTD) was NMDA receptor- and protein synthesis-dependent. Furthermore, it was dependent on the synergistic coactivation of dopaminergic D(1) and D(5) receptors. This let us speculate that RLTD resembles electrically induced, conventional CA1 late LTD, which is characterized by heterosynaptic processes and synaptic tagging. We therefore asked whether synaptic tagging occurs during RLTD. We found that early LTD in an S1 synaptic input was transformed into late LTD if early LTD was induced in a second independent S2 synaptic pathway during the inhibition of PDE by rolipram, supporting the interaction of processes of synaptic tagging during RLTD. Furthermore, application of PD 98059 (2'-amino-3'-methoxyflavone) or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), specific inhibitors of mitogen-activated protein kinases (MAPKs), prevented RLTD, suggesting a pivotal role of MAPK activation for RLTD. This MAPK activation was triggered during RLTD by the synergistic interaction of NMDA receptor- and D(1) and D(5) receptor-mediated Rap/B-Raf pathways, but not by the Ras/Raf-1 pathway in adult hippocampal CA1 neurons, as shown by the use of the pathway-specific inhibitors manumycin (Ras/Raf-1) and lethal toxin 82 (Rap/B-Raf).

Protein synthesis-dependent long-term functional plasticity: methods and techniques

There is growing interest in late-LTP and late-LTD, that is, distinct forms of functional plasticity that require somatic functions such as protein synthesis in addition to the transient synaptic processes that are required for short lasting forms. Interestingly, to date only these forms of lasting plastic events could be detected in healthy, freely moving animals and thus, they are considered as physiological cellular models of learning and memory formation. Late-LTP and -LTD are characterized by 'synaptic tagging' or 'capture' and 'synaptic cross-tagging', but there are only a few laboratories that can currently perform experiments studying these properties. In brain slice work, there are many different approaches to investigate these processes using different methodological variations: some allow slices to rest for long periods before the experiment starts, others do not; some run their experiments at near to physiological temperatures, others at lower temperatures; some stimulate frequently, others do not.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.

Activation of spinal d1/d5 receptors induces late-phase LTP of C-fiber-evoked field potentials in rat spinal dorsal horn

Long-term potentiation (LTP) of C-fiber-evoked field potentials in spinal dorsal horn may be relevant to pathological pain. Our previous work has shown that the late phase of the spinal LTP is protein synthesis-dependent. Considerable evidence has accumulated that dopamine D1/D5 receptors are important for late-phase LTP in hippocampus. In this study, the role of D1/D5 receptors in LTP of C-fiber-evoked field potentials in spinal dorsal horn was evaluated in urethan-anesthetized Sprague-Dawley rats. We found the following. 1) Spinal application of SKF 38393, a D1/D5 receptor agonist, induced a slowly developed LTP of C-fiber-evoked field potentials, lasting for >10 h, and the effect was blocked by the D1/D5 antagonist SCH 23390, whereas a D2 receptor agonist (quinpirole) induced depression of C-fiber responses, lasting for 2 h. 2) The potentiation produced by D1/D5 receptor agonist occluded the late phase but not the early phase of the spinal LTP produced by tetanic stimulation. 3) SCH 23390 selectively depressed the late-phase LTP, when applied 40 min before tetanic stimulation. 4) The D1/D5 agonist-induced potentiation is blocked by the protein synthesis inhibitor anisomycin. 5) Activation of protein kinase A by spinal application of 8-Br-cAMP also induced spinal LTP, and the action occluded the potentiation induced by the D1/D5 receptor agonist. These results suggest that the spinal D1/D5 receptors participate in the protein synthesis-dependent late-phase LTP of C-fiber-evoked field potentials in spinal dorsal horn through the cAMP signaling pathway.

Regulation of the phosphodiesterase PDE4B3-isotype during long-term potentiation in the area dentata in vivo

Hippocampal long-term potentiation (LTP) is the most prominent cellular model underlying learning and memory formation. However, which cellular processes are involved in maintaining LTP remains largely unknown. We have previously detailed temporal modulations of cyclic adenosine monophosphate (cAMP) and a cAMP-specific phosphodiesterase, PDE4B3, after LTP-induction and its maintenance in hippocampal area CA1 in vitro. To test whether other hippocampal sub-structures are characterised by similar mechanisms, tissue from the area dentata of freely moving rats was analysed at different LTP-time points. The tissue was fractionated into three components, where PDE4B-levels and cAMP-concentrations were measured. In contrast with data obtained in area CA1, we now detail an LTP-specific translational, but not transcriptional regulation of PDE4B3 within the first 8 h after tetanization and present spatio-temporal changes of PDE4B proteins and cAMP that is LTP-specific.

The amygdala, the hippocampus, and emotional modulation of memory

There are two views regarding the role of the amygdala in emotional memory formation. According to one view, the amygdala modulates memory-related processes in other brain regions, such as the hippocampus. According to the other, the amygdala is a site for some aspects of emotional memory. Here the authors adduce behavioral, electrophysiological, and biochemical evidence in support of an integrative view, assuming both roles for the amygdala. This integrative view, however, suggests a level of complexity not referred to before: the assumption that emotional conditions induce long-term neural plasticity in the amygdala suggests that the interrelations between the amygdala and brain regions, such as the hippocampus, may not be static but dynamic. The way the amygdala will affect memory-related processes in the hippocampus may thus largely depend on the previous history of the individual.

Emotional tagging of memory formation--in the search for neural mechanisms

Memory-related areas, such as the hippocampus, should be able to sort out the more significant from the less relevant aspects of an experience in order to transform only the earlier into long-term memory. We have recently suggested the Emotional Tagging concept, according to which the activation of the amygdala in emotionally arousing events mark the experience as important and aids in enhancing synaptic plasticity in other brain regions. Here, we review evidence from both human and animal studies that lend support to the Emotional Tagging hypothesis and to the central role the amygdala may play in its formation. We further speculate on potential neural mechanisms that may underlie emotional tagging. Long-term memory formation is considered to involve lasting alterations in synaptic efficacy, known as synaptic plasticity. It has been suggested that two factors are crucial for obtaining a synapse-specific long-term plasticity: (a) the successful activation of a synapse-specific, protein synthesis-independent tag, and (b) the activation of synapse-non-specific protein synthesis. The activation of protein synthesis can then induce lasting plasticity only in those synapses marked by a tag. Interestingly and relevant to the Emotional Tagging hypothesis, it has been recently shown that the activation of the amygdala could transform transient into long-lasting plasticity. These recent findings seem to fit well with the Emotional Tagging hypothesis. It seems reasonable to assume that the activation of the amygdala triggers neuromodulatory systems, which in turn reduce the threshold for the activation of the synaptic tag, and by this facilitate the transformation of early- into late-phase memory.

LTP in cultured hippocampal-entorhinal cortex slices from young adult (P25-30) rats

Cultured hippocampal neurons and immature organotypic slice cultures overcome temporal limitations of acute hippocampal slices and have been useful for investigating long-lasting plasticity. Difficulties with culturing adult neurons have restricted such studies to preparations from embryonic, perinatal, and juvenile tissue. By improving the methods for culturing and maintaining hippocampal-entorhinal cortex slices obtained from mature rats (P25-30), we show that their use in long-term electrophysiological investigations is feasible. Our cultured slices maintained an intact and functional trisynaptic cascade, normal synaptic function, and reliable long-term recording stability for at least 14 days in vitro. The electrophysiological properties and, in particular, the induction of long-term potentiation (LTP) in our mature organotypic slices were highly sensitive to dissection and tissue culture techniques. We present data describing the extracellular stimulation requirements for LTP-induction and its long-lasting maintenance (>4 h) at the Schaffer-collateral-CA1 synapse, and show that such changes in synaptic efficiency are NMDA receptor dependent. Our hippocampal-entorhinal cortex cultures from mature tissue can retain the electrophysiological properties required for long-term plasticity for several weeks in vitro.

Requirement of beta-adrenergic receptor activation and protein synthesis for LTP-reinforcement by novelty in rat dentate gyrus

Long-term potentiation (LTP) is supposed to be a cellular mechanism involved in memory formation. Similar to distinct types of memory formation, LTP can be separated into a protein synthesis-independent early phase (early-LTP) and a protein synthesis-dependent late phase (late-LTP). An important question is whether the transformation from early- into late-LTP can be elicited by behavioural conditions such as the attention to novel events. Therefore, we investigated the effect of exploration of a novel environment (novelty-exploration) on subsequently induced early-LTP in the dentate gyrus of freely moving rats. While a delay of 60 min between exploration onset and LTP induction had no effect, intervals of 30 or 15 min led to a reinforcement of early- to late-LTP. Exploration of a familiar environment failed to prolong LTP maintenance. The novelty-induced LTP reinforcement was blocked when the translation inhibitor anisomycin or the beta-adrenergic antagonist propranolol were applied intracerebroventricularly before exploration onset. These findings support the hypothesis that the synergistic interplay of novelty-triggered noradrenergic activity and weak tetanic stimulation promotes the synthesis of certain proteins that are required for late-LTP. Such a cellular mechanism may underlie novelty-dependent enhancement of memory formation.

Stress-related modulation of hippocampal long-term potentiation in rats: Involvement of adrenal steroid receptors

Stress is usually correlated with an increased release of glucocorticoids from the adrenal glands. Within the hippocampus, a structure long known to be involved in spatial learning, two corticosterone-binding receptors are identified: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Activation of these receptors impairs or facilitates hippocampal long-term potentiation (LTP), respectively. Stress elicited by behavioral manipulations may interfere with cognitive modulations of LTP during learning experiments. Here, we explore the influence of two stress-inducing procedures, handling and swimming, on the maintenance of dentate gyrus LTP in the rat induced by a weak tetanization of the perforant path. Manipulations started 15 min after tetanization. Handling alone resulted in a complete reversal of LTP. Handling followed by a 2 min swim in a water tank elicited prolonged protein synthesis but not beta-adrenergic-dependent LTP compared with either control or handled animals. Blockade of the GRs but not of the MRs prevented the reversal of LTP by handling. Inactivation of the MRs but not of the GRs hindered LTP prolongation by swimming. Because the activated receptor complexes act as transcription factors, MR- and GR-related proteins may play a role in the maintenance of LTP. The data suggest a complex interplay of corticosterone-binding receptors on modulations of hippocampal LTP and thus, of stress on learning and functional plasticity in general.

Involvement of beta-adrenergic receptors in protein synthesis-dependent late long-term potentiation (LTP) in the dentate gyrus of freely moving rats: the critical role of the LTP induction strength

We have investigated the requirement of beta-adrenergic receptor activation and protein synthesis for the induction and specifically for the maintenance of long-term potentiation (LTP) in the dentate gyrus of freely moving rats in dependency on different LTP-induction procedures. Three tetanization paradigms were used: a relatively weak protocol A (10 bursts of 15 biphasic pulses at 200 Hz; 10-s interburst interval; 0.2-ms pulse width per phase), a stronger protocol B (as protocol A but 20 bursts and 0.25-ms pulse width) and, as the strongest condition, protocol C (2 times protocol B; inter-tetanus interval: 5 min). All protocols led to robust late-LTP in control animals. Late- but not early-LTP was protein synthesis-dependent under all tetanization conditions as indicated by the absence of long-lasting LTP when the protein synthesis inhibitor anisomycin was applied before tetanization. Application of the beta-adrenergic receptor antagonist propranolol before LTP induction prevented late-LTP when either protocol A or B but not when protocol C was used. Thus, repeated strong tetanization can compensate for the loss of beta-adrenergic receptor activation. We suggest that the results could provide a link to cellular mechanisms of memory consolidation in respect to the strength and relevance of the incoming sensory information during learning.

Expression of the specific type IV phosphodiesterase gene PDE4B3 during different phases of long-term potentiation in single hippocampal slices of rats in vitro

Hippocampal long-term potentiation (LTP), the most prominent cellular model for learning and memory formation, consists of phases: early-LTP (<4 h) and late-LTP (>4 h), with the latter dependent upon protein translation and transcription. To explore the molecular processes that might be specifically regulated during late-LTP, we have modified standard electrophysiological and molecular biological methods, which allowed the cloning of activated genes and their products from single hippocampal slices in vitro 8 h after LTP induction. From one such screen we identified a specific type IV phosphodiesterase gene, PDE4B3, the first cAMP-specific phosphodiesterase to be associated with LTP. Previous studies documented an integral role for the cAMP-PKA system in late-LTP and recently, inhibition of cAMP degradation facilitates LTP and ameliorates mnemonic deficits. We now report that PDE4B3 is modulated during LTP phases. Its activation is NMDA-receptor dependent and its transcription is transiently up-regulated 2 h after tetanization. Protein expression peaks 6 h after LTP induction and is rapidly down-regulated at 8 h, whereas cAMP levels decrease during LTP phases. Immunohistochemical studies identified that the majority of type IV phosphodiesterase protein staining is localized to the cell bodies and dendrites of neurones in hippocampal CA1.

Brain-derived neurotrophic factor triggers transcription-dependent, late phase long-term potentiation in vivo

Acute intrahippocampal infusion of brain-derived neurotrophic factor (BDNF) leads to long-term potentiation (BDNF-LTP) of synaptic transmission at medial perforant path-->granule cell synapses in the rat dentate gyrus. Endogenous BDNF is implicated in the maintenance of high-frequency stimulation-induced LTP (HFS-LTP). However, the relationship between exogenous BDNF-LTP and HFS-LTP is unclear. First, we found that BDNF-LTP, like HFS-LTP, is associated with enhancement in both synaptic strength and granule cell excitability (EPSP-spike coupling). Second, treatment with a competitive NMDA receptor (NMDAR) antagonist blocked HFS-LTP but had no effect on the development or magnitude of BDNF-LTP. Thus, NMDAR activation is not required for the induction or expression of BDNF-LTP. Formation of stable, late phase HFS-LTP requires mRNA synthesis and is coupled to upregulation of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Local infusion of the transcription inhibitor actinomycin D (ACD) 1 hr before or immediately before BDNF infusion inhibited BDNF-LTP and upregulation of Arc protein expression. ACD applied 2 hr after BDNF infusion had no effect, defining a critical time window of transcription-dependent synaptic strengthening. Finally, the functional role of BDNF-LTP was assessed in occlusion experiments with HFS-LTP. HFS-LTP was induced, and BDNF was infused at time points corresponding to early phase (1 hr) or late phase (4 hr) HFS-LTP. BDNF applied during the early phase led to normal BDNF-LTP. In contrast, BDNF-LTP was completely occluded during the late phase. The results strongly support a role for BDNF in triggering transcription-dependent, late phase LTP in the intact adult brain.

Alzheimer amyloid beta-peptide inhibits the late phase of long-term potentiation through calcineurin-dependent mechanisms in the hippocampal dentate gyrus

The perforant path projecting from the entorhinal cortex to the hippocampal dentate gyrus is a particularly vulnerable target to the early deposition of amyloid beta (Abeta) peptides in Alzheimer's brain. The authors previously showed that brief applications of Abeta at subneurotoxic concentrations suppressed the early-phase long-term potentiation (E-LTP) in rat dentate gyrus. The current study further examines the effect of Abeta on the late-phase LTP (L-LTP) in this area. Using multiple high-frequency stimulus trains, a stable L-LTP lasting for at least 3 h was induced in the medial perforant path of rat hippocampal slices. Bath application of Abeta(1-42) (0.2-1.0 microM) during the induction trains attenuated both the initial and late stages of L-LTP. On the other hand, Abeta(1-42) perfusion within the first hour following the induction primarily impaired the late stage of L-LTP, which resembled the action of the protein synthesis inhibitor emetine. Blockade of calcineurin activity with FK506 or cyclosporin A completely prevented Abeta-induced L-LTP deficits. These results suggest that Abeta(1-42) impaired both the induction and maintenance phase of dentate L-LTP through calcineurin-dependent mechanisms. In the concentration range effective for inhibiting L-LTP, Abeta(1-42) also reduced the amplitude of NMDA receptor-mediated synaptic currents in dentate granule cells via a postsynaptic mechanism. In addition, concurrent applications of Abeta(1-42) with the protein synthesis inhibitor caused no additive reduction of L-LTP, indicating a common mechanism underlying the action of both. Thus, inhibition of NMDA receptor channels and disruption of protein synthesis were two possible mechanisms contributing to Abeta-induced L-LTP impairment.

Forced swimming evokes a biphasic response in CREB phosphorylation in extrahypothalamic limbic and neocortical brain structures in the rat

The transcription factor cAMP response element-binding protein (CREB) plays a critical role in plasticity processes underlying learning and memory. We investigated the phosphorylation of CREB in rat brain after forced swimming, a stressor known to impact on higher limbic and neocortical brain areas. As shown by immunohistochemistry, forced swimming increased phosphorylated CREB (P-CREB) levels in the dentate gyrus, all neocortical areas, the medial, lateral and basolateral nuclei of the amygdala, cerebellum but not in the hypothalamic paraventricular nucleus. Distinct differences in the P-CREB pattern were observed in the deeper vs. superficial layers of the neocortex. The response in P-CREB was stressor type-specific because exposure to either ether or a cold environment was ineffective. The forced swimming-induced changes in P-CREB levels showed a biphasic time-course: an early peak detected at 15 min was followed by a marked drop at 60 min; a second rise starting after 1-2 h, reached maximal values between 6 and 8 h, and remained elevated for at least 48 h. Examination of the neuroanatomical induction pattern of the CRE-inducible immediate early gene product c-fos revealed that it was only partly overlapping with that of P-CREB. Western analyses showed that only the 43-kDa CREB protein (an enhancer of CRE-containing promotors) was phosphorylated after forced swimming, while other members of the CREB/ATF family (CREM, ATF-1 and ATF-2) remained unaffected. The NF-kappaB pathway was not activated, indicating that forced swimming does not unspecifically evoke transcription factor activation. Thus, in contrast to physical stressors, such as ether or cold exposure, forced swimming, a stressor with a strong psychological component, elicits the recruitment of the CREB pathway in a widespread manner in the limbic system and neocortex; brain regions known to be implicated in various forms of (stress-related) learning and memory.

Aminoguanidine prevented the impairment of learning behavior and hippocampal long-term potentiation following transient cerebral ischemia

The present study was performed to investigate whether increases in nitric oxide (NO) production via inducible nitric oxide synthase (iNOS) were involved in impairment of learning behavior and hippocampal long-term potentiation (LTP) following transient ischemia. Rats with four-vessel occlusion (4-VO) were used as an ischemic model. One week after permanent occlusion of the vertebral arteries, the common carotid arteries were clamped for 10 min under halothane anesthesia. Aminoguanidine (10 mg/kg i.p.), a relatively selective iNOS inhibitor, or saline was administered 30 min before common carotid arteries occlusion, and every 24-h for 4 days. We investigated whether hippocampal NO production was increased by ischemic insult using microdialysis on days 1, 4 and 7 after 4-VO. On days 1 and 4 after 4-VO, increases in NO production were observed, and this effect was inhibited by aminoguanidine. Four days after 4-VO, rats were subjected to the Y-maze test and contextual fear conditioning. Ischemic insults impaired learning behavior, and aminoguanidine ameliorated the impairment induced by 4-VO. Four days after 4-VO, the changes in the population spike amplitude were recorded as an index of LTP in Schaffer collateral-CA1 (carotid artery 1), the mossy fiber-CA3 and the perforant path-dentate gyrus synapses. LTP was significantly inhibited by 4-VO, except in mossy fiber-CA3 synapses. Pretreatment with aminoguanidine prevented the reduction of LTP in perforant path-dentate gyrus, but not in Schaffer collateral-CA1 synapses. These results suggest that post-ischemic increase in NO production via iNOS impaired the learning behavior. There is an association between behavioral performance and LTP formation in perforant path-dentate gyrus synapses, but neither in Schaffer collateral-CA1 nor in mossy fiber-CA3 synapses.