Two critical periods of protein and glycoprotein synthesis in memory consolidation for visual categorization learning in chicks

Computational analysis of memory consolidation following inhibitory avoidance (IA) training in adult and infant rats: Critical roles of CaMKIIα and MeCP2

Key features of long-term memory (LTM), such as its stability and persistence, are acquired during processes collectively referred to as consolidation. The dynamics of biological changes during consolidation are complex. In adult rodents, consolidation exhibits distinct periods during which the engram is more or less resistant to disruption. Moreover, the ability to consolidate memories differs during developmental periods. Although the molecular mechanisms underlying consolidation are poorly understood, the initial stages rely on interacting signaling pathways that regulate gene expression, including brain-derived neurotrophic factor (BDNF) and Ca2+/calmodulin-dependent protein kinase II α (CaMKIIα) dependent feedback loops. We investigated the ways in which these pathways may contribute to developmental and dynamical features of consolidation. A computational model of molecular processes underlying consolidation following inhibitory avoidance (IA) training in rats was developed. Differential equations described the actions of CaMKIIα, multiple feedback loops regulating BDNF expression, and several transcription factors including methyl-CpG binding protein 2 (MeCP2), histone deacetylase 2 (HDAC2), and SIN3 transcription regulator family member A (Sin3a). This model provides novel explanations for the (apparent) rapid forgetting of infantile memory and the temporal progression of memory consolidation in adults. Simulations predict that dual effects of MeCP2 on the expression of bdnf, and interaction between MeCP2 and CaMKIIα, play critical roles in the rapid forgetting of infantile memory and the progress of memory resistance to disruptions. These insights suggest new potential targets of therapy for memory impairment.

Spatiotemporally resolved protein synthesis as a molecular framework for memory consolidation

De novo protein synthesis is required for long-term memory consolidation. Dynamic regulation of protein synthesis occurs via a complex interplay of translation factors and modulators. Many components of the protein synthesis machinery have been targeted either pharmacologically or genetically to establish its requirement for memory. The combination of ligand/light-gating and genetic strategies, that is, chemogenetics and optogenetics, has begun to reveal the spatiotemporal resolution of protein synthesis in specific cell types during memory consolidation. This review summarizes current knowledge of the macroscopic and microscopic neural substrates for protein synthesis in memory consolidation. In addition, we highlight future directions for determining the localization and timing of de novo protein synthesis for memory consolidation with tools that permit unprecedented spatiotemporal precision.

Role of NMDA Receptors and Protein Synthesis in the Conditioned Aversion Learning in Young Chicks

The brain mechanisms underlying conditioned aversion learning in birds were studied using experimental model in young chicks. The learning consisted of a conditioning stimulus presentation followed by a delayed sickness-inducing treatment reinforcement. Intraventricular administration of an NMDA receptor antagonist MK-801, a protein synthesis inhibitor anisomycin, or an inhibitor of glycoprotein fucosylation 2-deoxygalactose just before presentation of the conditioning stimulus prevented aversion learning. Injections of the same chemicals before reinforcement did not affect learning. The obtained results show that the investigated mechanisms underlying aversion learning were critical at the early stage of memory formation. Later processes of association of the conditioning stimulus with the reinforcement appear to be independent of the NMDA receptors and protein synthesis/glycosylation, or alternatively to be located in other brain areas.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

In neurons, the specific spatial and temporal localization of protein synthesis is of great importance for function and survival. Here, we visualized tRNA and protein synthesis events in fixed and live mouse primary cortical culture using fluorescently-labeled tRNAs. We were able to characterize the distribution and transport of tRNAs in different neuronal sub-compartments and to study their association with the ribosome. We found that tRNA mobility in neural processes is lower than in somata and corresponds to patterns of slow transport mechanisms, and that larger tRNA puncta co-localize with translational machinery components and are likely the functional fraction. Furthermore, chemical induction of long-term potentiation (LTP) in culture revealed up-regulation of mRNA translation with a similar effect in dendrites and somata, which appeared to be GluR-dependent 6 h post-activation. Importantly, measurement of protein synthesis in neurons with high resolutions offers new insights into neuronal function in health and disease states.

The lateral neocortex is critical for contextual fear memory reconsolidation

Memories are a product of the concerted activity of many brain areas. Deregulation of consolidation and reprocessing of mnemonic traces that encode fearful experiences might result in fear-related psychopathologies. Here, we assessed how pre-established memories change with experience, particularly the labilization/reconsolidation of memory, using the whole-brain analysis technique of positron emission tomography in male mice. We found differences in glucose consumption in the lateral neocortex, hippocampus and amygdala in mice that underwent labilization/reconsolidation processes compared to animals that did not reactivate a fear memory. We used chemogenetics to obtain insight into the role of cortical areas in these phases of memory and found that the lateral neocortex is necessary for fear memory reconsolidation. Inhibition of lateral neocortex during reconsolidation altered glucose consumption levels in the amygdala. Using an optogenetic/neuronal recording-based strategy we observed that the lateral neocortex is functionally connected with the amygdala, which, along with retrograde labeling using fluorophore-conjugated cholera toxin subunit B, support a monosynaptic connection between these areas and poses this connection as a hot-spot in the circuits involved in reactivation of fear memories.

Histone acetylation determines transcription of atypical protein kinases in rat neurons

It is widely accepted that memory consolidation requires de-novo transcription of memory-related genes. Epigenetic modifications, particularly histone acetylation, may facilitate gene transcription, but their potential molecular targets are poorly characterized. In the current study, we addressed the question of epigenetic control of atypical protein kinases (aPKC) that are critically involved in memory consolidation and maintenance. We examined the patterns of expression of two aPKC genes (Prkci and Prkcz) in rat cultured cortical neurons treated with histone deacetylase inhibitors. Histone hyperacetylation in the promoter region of Prkci gene elicited direct activation of transcriptional machinery, resulting in increased production of PKCλ mRNA. In parallel, histone hyperacetylation in the upstream promoter of Prkcz gene led to appearance of the corresponding PKCζ transcripts that are almost absent in the brain in resting conditions. In contrast, histone hyperacetylation in the downstream promoter of Prkcz gene was accompanied by a decreased expression of the brain-specific PKMζ products. We showed that epigenetically-triggered differential expression of PKMζ and PKCζ mRNA depended on protein synthesis. Summarizing, our results suggest that genes, encoding memory-related aPKC, may represent the molecular targets for epigenetic regulation through posttranslational histone modifications.

Computational model of a positive BDNF feedback loop in hippocampal neurons following inhibitory avoidance training

Inhibitory avoidance (IA) training in rodents initiates a molecular cascade within hippocampal neurons. This cascade contributes to the transition of short- to long-term memory (i.e., consolidation). Here, a differential equation-based model was developed to describe a positive feedback loop within this molecular cascade. The feedback loop begins with an IA-induced release of brain-derived neurotrophic factor (BDNF), which in turn leads to rapid phosphorylation of the cAMP response element-binding protein (pCREB), and a subsequent increase in the level of the β isoform of the CCAAT/enhancer binding protein (C/EBPβ). Increased levels of C/EBPβ lead to increased bdnf expression. Simulations predicted that an empirically observed delay in the BDNF-pCREB-C/EBPβ feedback loop has a profound effect on the dynamics of consolidation. The model also predicted that at least two independent self-sustaining signaling pathways downstream from the BDNF-pCREB-C/EBPβ feedback loop contribute to consolidation. Currently, the nature of these downstream pathways is unknown.

Interaction of inhibitory and facilitatory effects of conditioning trials on long-term memory formation

Animals learn through experience and consolidate the memories into long-time storage. Conditioning parameters to induce protein synthesis-dependent long-term memory (LTM) have been the subject of extensive studies in many animals. Here we found a case in which a conditioning trial inhibits or facilitates LTM formation depending on the intervals from preceding trials. We studied the effects of conditioning parameters on LTM formation in olfactory conditioning of maxillary-palpi extension response with sucrose reward in the cockroach Periplaneta americana. We found, at first, that translation- and transcription-dependent LTM forms 1 h after training, the fastest so far reported in insects. Second, we observed that multiple-trial training with an intertrial interval (ITI) of 20 or 30 sec, often called massed training, is more effective than spaced training for LTM formation, an observation that differs from the results of most studies in other animals. Third, we found that a conditioning trial inhibits LTM formation when the intervals from preceding trials were in the range of 10–16 min. This inhibitory effect is pairing-specific and is not due to decreased motivation for learning (overtraining effect). To our knowledge, no similar inhibition of LTM formation by a conditioning trial has been reported in any animals. We propose a model to account for the effects of trial number and ITIs on LTM formation. Olfactory conditioning in cockroaches should provide pertinent materials in which to study neuronal and molecular mechanisms underlying the inhibitory and facilitatory processes for LTM formation.

The human synaptic vesicle protein, SV2A, functions as a galactose transporter in Saccharomyces cerevisiae

SV2A is a synaptic vesicle membrane protein expressed in neurons and endocrine cells and involved in the regulation of neurotransmitter release. Although the exact function of SV2A still remains elusive, it was identified as the specific binding site for levetiracetam, a second generation antiepileptic drug. Our sequence analysis demonstrates that SV2A has significant homology with several yeast transport proteins belonging to the major facilitator superfamily (MFS). Many of these transporters are involved in sugar transport into yeast cells. Here we present evidence showing, for the first time, that SV2A is a galactose transporter. We expressed human SV2A in hexose transport-deficient EBY.VW4000 yeast cells and demonstrated that these cells are able to grow on galactose-containing medium but not on other fermentable carbon sources. Furthermore, the addition of the SV2A-binding antiepileptic drug levetiracetam to the medium inhibited the galactose-dependent growth of hexose transport-deficient EBY.VW4000 yeast cells expressing human SV2A. Most importantly, direct measurement of galactose uptake in the same strain verified that SV2A is able to transport extracellular galactose inside the cells. The newly identified galactose transport capability of SV2A may have an important role in regulating/modulating synaptic function.

Molecular signatures and mechanisms of long-lasting memory consolidation and storage

A body of evidence emerged in the last decade regarding late posttraining memory processing. Most of this new information comes from aversively motivated learning tasks that mainly depend on hippocampus, amygdala and insular cortex, and points to the involvement of long-lasting changes in gene expression and protein synthesis in late stages of memory consolidation and storage. Here, we describe recent advances in this field and discuss how recurrent rounds of macromolecular synthesis and its regulation might impact long-term memory storage.

Chemoenzymatic probes for detecting and imaging fucose-α(1-2)-galactose glycan biomarkers

The disaccharide motif fucose-α(1-2)-galactose (Fucα(1-2)Gal) is involved in many important physiological processes, such as learning and memory, inflammation, asthma, and tumorigenesis. However, the size and structural complexity of Fucα(1-2)Gal-containing glycans have posed a significant challenge to their detection. We report a new chemoenzymatic strategy for the rapid, sensitive detection of Fucα(1-2)Gal glycans. We demonstrate that the approach is highly selective for the Fucα(1-2)Gal motif, detects a variety of complex glycans and glycoproteins, and can be used to profile the relative abundance of the motif on live cells, discriminating malignant from normal cells. This approach represents a new potential strategy for biomarker detection and expands the technologies available for understanding the roles of this important class of carbohydrates in physiology and disease.

Persistence of long-term memory storage: new insights into its molecular signatures in the hippocampus and related structures

Although much is known about long-term memory (LTM) consolidation, what puts the "long" in LTM is the exclusive feature of persisting over time. However, until recently the molecular mechanisms underneath memory persistence had never been properly studied. In rats, the protein translation inhibitor anisomycin impaired memory persistence when injected into the dorsal hippocampus 12 h after inhibitory avoidance (IA) training without affecting memory formation. Here, we also show learning-induced changes in hippocampal c-Fos, Homer 1a, Akt, CamKIIα, and ERK2 levels around 18-24 h after IA training. Thus, memory persistence is associated with a late phase of plasticity-related protein synthesis in the hippocampus.

Identification of the plasticity-relevant fucose-alpha(1-2)-galactose proteome from the mouse olfactory bulb

Fucose-alpha(1-2)-galactose [Fucalpha(1-2)Gal] sugars have been implicated in the molecular mechanisms that underlie neuronal development, learning, and memory. However, an understanding of their precise roles has been hampered by a lack of information regarding Fucalpha(1-2)Gal glycoproteins. Here, we report the first proteomic studies of this plasticity-relevant epitope. We identify five classes of putative Fucalpha(1-2)Gal glycoproteins: cell adhesion molecules, ion channels and solute carriers/transporters, ATP-binding proteins, synaptic vesicle-associated proteins, and mitochondrial proteins. In addition, we show that Fucalpha(1-2)Gal glycoproteins are enriched in the developing mouse olfactory bulb (OB) and exhibit a distinct spatiotemporal expression that is consistent with the presence of a "glycocode" to help direct olfactory sensory neuron (OSN) axonal pathfinding. We find that expression of Fucalpha(1-2)Gal sugars in the OB is regulated by the alpha(1-2)fucosyltransferase FUT1. FUT1-deficient mice exhibit developmental defects, including fewer and smaller glomeruli and a thinner olfactory nerve layer, suggesting that fucosylation contributes to OB development. Our findings significantly expand the number of Fucalpha(1-2)Gal glycoproteins and provide new insights into the molecular mechanisms by which fucosyl sugars contribute to neuronal processes.

BDNF activates mTOR to regulate GluR1 expression required for memory formation

BACKGROUND

The mammalian target of Rapamycin (mTOR) kinase plays a key role in translational control of a subset of mRNAs through regulation of its initiation step. In neurons, mTOR is present at the synaptic region, where it modulates the activity-dependent expression of locally-translated proteins independently of mRNA synthesis. Indeed, mTOR is necessary for different forms of synaptic plasticity and long-term memory (LTM) formation. However, little is known about the time course of mTOR activation and the extracellular signals governing this process or the identity of the proteins whose translation is regulated by this kinase, during mnemonic processing.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that consolidation of inhibitory avoidance (IA) LTM entails mTOR activation in the dorsal hippocampus at the moment of and 3 h after training and is associated with a rapid and rapamycin-sensitive increase in AMPA receptor GluR1 subunit expression, which was also blocked by intra-hippocampal delivery of GluR1 antisense oligonucleotides (ASO). In addition, we found that pre- or post-training administration of function-blocking anti-BDNF antibodies into dorsal CA1 hampered IA LTM retention, abolished the learning-induced biphasic activation of mTOR and its readout, p70S6K and blocked GluR1 expression, indicating that BDNF is an upstream factor controlling mTOR signaling during fear-memory consolidation. Interestingly, BDNF ASO hindered LTM retention only when given into dorsal CA1 1 h after but not 2 h before training, suggesting that BDNF controls the biphasic requirement of mTOR during LTM consolidation through different mechanisms: an early one involving BDNF already available at the moment of training, and a late one, happening around 3 h post-training that needs de novo synthesis of this neurotrophin.

CONCLUSIONS/SIGNIFICANCE

IN CONCLUSION, OUR FINDINGS DEMONSTRATE THAT: 1) mTOR-mediated mRNA translation is required for memory consolidation during at least two restricted time windows; 2) this kinase acts downstream BDNF in the hippocampus and; 3) it controls the increase of synaptic GluR1 necessary for memory consolidation.

Persistence of long-term memory storage requires a late protein synthesis- and BDNF- dependent phase in the hippocampus

Persistence is the most characteristic attribute of long-term memory (LTM). To understand LTM, we must understand how memory traces persist over time despite the short-lived nature and rapid turnover of their molecular substrates. It is widely accepted that LTM formation is dependent upon hippocampal de novo protein synthesis and Brain-Derived Neurotrophic Factor (BDNF) signaling during or early after acquisition. Here we show that 12 hr after acquisition of a one-trial associative learning task, there is a novel protein synthesis and BDNF-dependent phase in the rat hippocampus that is critical for the persistence of LTM storage. Our findings indicate that a delayed stabilization phase is specifically required for maintenance, but not formation, of the memory trace. We propose that memory formation and memory persistence share some of the same molecular mechanisms and that recurrent rounds of consolidation-like events take place in the hippocampus for maintenance of the memory trace.

Protein synthesis-dependent reactivation of a contextual conditioned reflex in the common snail

We report here a study of the effects of blockade of protein synthesis with anisomycin during reactivation of a contextual conditioned reflex in the common snail. The amplitudes of the defensive reactions of snails to standard tactile stimulation before training were identical in two conditions: 1) testing of responses of snails fixed by the shell to a plastic ball floating in water and 2) on the surface of the terrarium glass. After applying electric shocks to the snails' skin for 5 days, a significant difference in responses reflecting the formation of a contextual conditioned reflex was seen in only one of the contexts. Placing trained snails in the same context (reminding) two days after training with simultaneous injection of anisomycin led to significant weakening of training, while control injections of physiological saline produced no such changes. These data suggest that the mechanisms of memory consolidation after training and reminding are not identical.

Does the human leukaemia differentiation factor fragment HLDF6 improve memory via brain DNA and protein synthesis?

The novel human differentiating factor peptide fragment HLDF6 (Thr-Gly-Glu-Asn-His-Arg) was synthesized and purified. HLDF6 (0.1mg/kg i.p. but not 1mg/kg i.p.) improved not only long-term (24h) memory in adult rats in the water maze behavioural paradigm but also performance in the delayed matching-to-position (DMTP) task (0.3 and 1.0 but not 0.1mg/kg i.p). Hence, HLDF6 not only enhanced allocentric spatial learning and reference memory (water maze) but also improved temporal, spatial and working memory processes in the DMTP behavioural paradigm. Immunoreactivity blotting analysis of HLDF (the protein precursor of HLDF6) was performed and the following rank order of visual intensities from brain structures was noted: hippocampus cerebral cortex cerebellum hypothalamus striatum. Subsequently, we found that the highest absolute levels of HLDF were expressed in the hippocampus and cerebral cortex as detected by ELISA. We also demonstrated that HLDF6 enhanced [(3)H]-thymidine and [(14)C]-leucine incorporation into whole brain and hippocampal homogenates (maxima occurring within the range 10 (-12)-10 (-6) M) suggesting that this hexapeptide promoted de novo DNA and protein biosynthesis. We discuss this data in terms of their implications for links with other integrative metabolic pathways involving immediate early gene activation which may underpin a potential application for HLDF6 in limiting memory impairments associated with neurodegenerative diseases.

A single time-window for protein synthesis-dependent long-term memory formation after one-trial appetitive conditioning

Protein synthesis is generally held to be essential for long-term memory formation. Often two periods of sensitivity to blockade of protein synthesis have been described, one immediately after training and another several hours later. We wished to relate the timing of protein synthesis-dependence of behavioural long-term memory (LTM) formation to an electrophysiological correlate of the LTM memory trace. We used the snail Lymnaea because one-trial appetitive conditioning of feeding using a chemical conditioned stimulus leads to a stable LTM trace that can be monitored behaviourally and then electrophysiologically in preparations made from the same animals. Anisomycin (an inhibitor of translation) injected 10 min after training blocked behavioural LTM formation. Actinomycin D (an inhibitor of transcription) was also effective at 10 min. When anisomycin, at doses shown to be effective in blocking central nervous system protein synthesis, was injected at 1, 2, 3, 4, 5 and 6 h after training there was no effect on recall. These results indicate that there is a single period of sensitivity to protein synthesis inhibition in Lymnaea lasting for between 10 min and 1 h after training with no evidence for a second window of sensitivity. An electrophysiological correlate of LTM was found to be sensitive to anisomycin injected 10 min after training. It is unusual to find only one period of protein synthesis-dependence in detailed time-course studies of LTM, and this suggests that the consolidation processes involving protein synthesis are relatively rapid in one-trial appetitive conditioning and complete within 1 h of training.

A role for fucose alpha(1-2) galactose carbohydrates in neuronal growth

We report a fucose alpha(1-2) galactose-mediated pathway for the modulation of neuronal growth and morphology. Our studies provide strong evidence for the presence of Fucalpha(1-2)Gal glycoproteins and lectin receptors in hippocampal neurons. Additionally, we show that manipulation of Fucalpha(1-2)Gal-associated proteins using small-molecule and lectin probes induces dramatic changes in neuronal morphology. These findings may provide a novel pathway to stimulate neuronal growth and regeneration.

The Ventral Hippocampus Supports a Memory Representation of Context and Contextual Fear Conditioning: Implications for a Unitary Function of the Hippocampus

The authors report that either inactivating the ventral hippocampus (VH) with muscimol prior to context preexposure or injecting anisomycin into the VH after preexposure significantly impaired rats' memory for context. Injecting anisomycin into the VH prior to contextual fear conditioning also greatly reduced long-term memory (48-hr retention test) but had no effect on short-term memory (1-hr retention test) for contextual fear. Together with other results, these data suggest that the memory for a novel context is distributed throughout the longitudinal extent of the hippocampus and that this representation helps to support contextual fear conditioning.

Inhibition of hippocampal protein synthesis following recall disrupts expression of episodic-like memory in trace conditioning

Transition of short-term to long-term memory is referred to as consolidation and the process is dependent on protein synthesis. Recently, several studies have shown that expression of consolidated memory for simple forms of learning tasks (e.g., delay conditioning, contextual fear, inhibitory avoidance) becomes vulnerable to disruption by inhibition of protein synthesis when administered shortly after recall. In the present study, we address whether recall-induced dependence on protein synthesis is a fundamental property that can be applied to a form of memory requiring attentional awareness or is specific to memories for simple forms of conditioning. Trace fear conditioning is a form of learning that requires an active memory trace to associate a conditioned stimulus (CS) with an unconditioned stimulus (US) separated by time. Our data demonstrate that whether a CS-alone recall trial in a novel context acts as an extinction or reactivation trial depends on the strength of the original memory. Inhibition of protein synthesis following the recall trial in animals receiving one trace conditioning training session (that gives rise to weak memory) resulted in enhanced CS-elicited freezing compared with vehicle control, as a result of impaired extinction memory, but had no effect on contextual memory. However, inhibition of hippocampal protein synthesis following the recall trial in animals receiving two trace conditioning training sessions (that gives rise to stronger memory) resulted in impaired retention of both trace CS-US associative and contextual memory despite that the context-US association was not directly reactivated. This provides evidence that, for a robust memory, the CS-alone recall trial results in the reactivation of an episodic-like memory, including trace CS- and contextual-memory, and that hippocampal information storage for the memory as a whole is returned to a labile state requiring de novo protein synthesis. This and other studies are consistent with the role of the hippocampus in coordinating episodic memory retrieval.

Reminder effects: the molecular cascade following a reminder in young chicks does not recapitulate that following training on a passive avoidance task

Memory traces, once established, are no longer sensitive to disruption by metabolic inhibitors. However, memories reactivated by reminder are once again vulnerable, in a time-dependent manner, to amnestic treatment. To determine whether the metabolic events following a reminder recapitulate those following initial training we examined the temporal dynamics of amnesia induced by the protein synthesis inhibitor anisomycin and the glycosylation inhibitor 2-deoxygalactose. The effects of both were transient and dependent on time of reminder post-training and time of injection relative to reminder, and differed from those following initial training. 2-[(14)C]-deoxyglucose uptake increased in two brain regions, the intermediate medial hyperstriatum ventrale (IMHV) and lobus parolfactorius (LPO) following reminder as it did following training, but the increase was bilateral rather than confined to the left hemisphere and was more marked in LPO than IMHV. C-fos expression after reminder was increased only in the LPO, the chick brain region associated with a late phase of memory processing and recall. Thus although, like initial consolidation, memory processing after reminder is sensitive to inhibitors of protein synthesis and glycosylation, the temporal and pharmacological dynamics indicate differences between these two processes.

Consolidation of Long-Term Memory: Evidence and Alternatives

Memory loss in retrograde amnesia has long been held to be larger for recent periods than for remote periods, a pattern usually referred to as the Ribot gradient. One explanation for this gradient is consolidation of long-term memories. Several computational models of such a process have shown how consolidation can explain characteristics of amnesia, but they have not elucidated how consolidation must be envisaged. Here findings are reviewed that shed light on how consolidation may be implemented in the brain. Moreover, consolidation is contrasted with alternative theories of the Ribot gradient. Consolidation theory, multiple trace theory, and semantization can all handle some findings well but not others. Conclusive evidence for or against consolidation thus remains to be found.

Time-dependent effects of cycloheximide on long-term memory in the cuttlefish

When shown prawns in a glass tube, cuttlefish promptly learn to inhibit their predatory behavior and retain this ability for a long time. The cellular and molecular mechanisms of this long-term memory (LTM) are not yet known. In this study, we analyzed the dependency of LTM on de novo brain protein synthesis. Cycloheximide (CXM), a protein synthesis inhibitor, is injected intravenously immediately, 1 h, 3 h, 4 h or 6 h after the training. Retention is tested 24 h posttraining. The injections of CXM revealed one period of memory sensitivity to pharmacological intervention. CXM administered immediately or 6 h after training has no effect on LTM. Conversely, injections given between 1 and 4 h posttraining resulted in amnesia. Taken together, findings of this study establish for the first time in Sepia officinalis that de novo protein synthesis is an essential and time-dependent event for LTM formation of this form of associative learning.

Learning-induced arg 3.1/arc mRNA expression in the mouse brain

The effector immediate-early gene (IEG) arg 3.1, also called arc, encodes a protein interacting with the neuronal cytoskeleton. The selective localization of arg 3.1/arc mRNA in activated dendritic segments suggests that the arg 3.1/arc protein may be synthesized at activated post-synaptic sites and that arg 3.1/arc could participate in structural and functional modifications underlying cognitive processes like memory formation. To analyze whether learning itself is sufficient to trigger expression of arg 3.1/arc, we developed a one-trial learning paradigm in which mice learned to enter a dark compartment to escape from an aversively illuminated area. Arg 3.1/arc mRNA expression was analyzed by in situ hybridization in three groups of mice as follows: a control group with no access to the dark compartment, a learning group having access to the dark compartment for one trial, and a retrieval group having access to the dark compartment for two trials on consecutive days. All animals from the learning and retrieval groups escaped the illuminated area, and those tested 24 h later (retrieval group) showed a strongly reduced latency to enter the dark compartment, demonstrating the validity of our learning paradigm to induce long-term memory. Our results show that acquisition of a simple task results in a brain area-specific biphasic increase in arg 3.1/arc mRNA expression 15 min and 4.5 h post-training. This increase was detected specifically in the learning group but neither in the control nor in the retrieval groups. The pattern of arg 3.1/arc mRNA expression corresponds temporally to the two mRNA- and protein-synthesis-dependent periods of long-term memory formation. Our study provides the first unequivocal evidence that arg 3.1/arc expression is induced by a learning task and strongly suggests a role of arg 3.1/arc mRNA in the early and late cellular mechanisms underlying the stabilization of the memory trace.

Anisomycin treatment paradigm affects duration of long-term potentiation in slices of the amygdala

Anisomycin has been widely used as an inhibitor of protein synthesis in studies on learning and memory as well as synaptic plasticity. However, its mode of action is complicated. Besides the inhibition of translation, this drug displays other effects, most prominently on mitogen-activated protein kinases. In this report we analyze the effects of anisomycin on the late phase of long-term potentiation (LTP) in amygdala slices. The late phase of LTP was evoked by high-frequency repeated-train stimulation delivered to the external capsule and recorded in the lateral amygdaloid nucleus. In the no-drug condition, stimulation resulted in LTP lasting over 3 h without any sign of decay. Application of the drug 15 min before high-frequency stimulation (HFS) caused LTP decay to baseline within 1 h after induction. However, delivering the drug just after the first train of HFS resulted in LTP that returned to baseline level within 3 h since the onset of stimulation. These results show that the duration of the LTP in the amygdala depends on the anisomycin treatment paradigm and thus special caution should be exercised when interpreting the data obtained with this drug.

Two time periods of hippocampal mRNA synthesis are required for memory consolidation of fear-motivated learning

Information storage in the brain is a temporally graded process involving different memory types or phases. It has been assumed for over a century that one or more short-term memory (STM) processes are involved in processing new information while long-term memory (LTM) is being formed. It has been repeatedly reported that LTM requires de novo RNA synthesis around the time of training. Here we show that LTM formation of a one-trial inhibitory avoidance training in rats, a hippocampal-dependent form of contextual fear conditioning, depends on two consolidation periods requiring synthesis of new mRNAs. By injecting the RNA polymerase II inhibitors 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole or alpha-amanitin into the CA1 region of the dorsal hippocampus at various times before and after training, we found that hippocampal gene expression is critical in two time windows: around the time of training and 3-6 hr after training. Interestingly, these two periods of sensitivity to transcriptional inhibitors are similar to those observed using the protein synthesis inhibitor anisomycin. These findings underscore the parallel dependence of LTM formation of contextual fear on mRNA and protein synthesis in the hippocampus and suggest that the two time periods of anisomycin-induced amnesia depend at least in part on new mRNA synthesis.

Reminder effects - reconsolidation or retrieval deficit? Pharmacological dissection with protein synthesis inhibitors following reminder for a passive-avoidance task in young chicks

It is generally accepted that memory formation involves an irreversible passage via labile phases to the stable form of 'long-term memory' impervious to amnestic agents such as protein synthesis inhibitors. However, recent experiments demonstrate that reactivation of memory by way of a reminder renders it labile to such inhibitors, suggesting that such retrieval is followed by a so-called reconsolidation process similar or identical in its cellular and molecular correlates to that occurring during the initial consolidation. We compared the effects of the protein synthesis inhibitor anisomycin and the glycoprotein synthesis inhibitor 2-deoxygalactose on the temporal dynamics and pharmacological sensitivity of initial consolidation and memory expression following a reminder in a one-trial passive-avoidance task in day-old chicks. This comparison revealed three differences between the action of the inhibitors on newly formed compared with reactivated memory. First, the recall deficit after the reminder was temporary, whilst the amnesia following inhibitor treatment during training was stable. Second, the sensitive period for the effect of anisomycin was shorter in the reminder than in the training situation. Third, the effective dose for either inhibitor for reminder-associated amnesia was several times lower than for amnesia developing after training. Thus though like initial consolidation, memory expression at delayed periods following reminder depends on protein and glycoprotein synthesis, the differences between the temporal and pharmacological dynamics in the two situations point to the distinct character of the molecular processes involved in postreminder effects.

Identification of genes expressed in the amygdala during the formation of fear memory

In this study we describe changes of gene expression that occur in the basolateral complex of the mouse amygdala (BLA) during the formation of fear memory. Through the combination of a behavioral training scheme with polymerase chain reaction-based expression analysis (subtractive hybridization and virtual Northern analysis) we were able to identify various gene products that are increased in expression after Pavlovian fear conditioning and are of potential significance for neural plasticity and information storage in the amygdala. In particular, a key enzyme of monoamine metabolism, aldehyde reductase, and the protein sorting and ubiquitination factor Praja1, showed pronounced and learning-specific induction six hours after fear conditioning training. Aldehyde reductase and Praja1, including a novel alternatively spliced isoform termed Praja1a, were induced in the BLA depending on the emotional stimulus presented and showed different expression levels in response to associative conditioning, training stress, and experience of conditioned fear. Stress and fear were further found to induce various signal transduction factors (transthyretin, phosphodiesterase1, protein kinase inhibitor-alpha) and structural reorganization factors (e.g., E2-ubiquitin conjugating enzyme, neuroligin1, actin, UDP-galactose transporter) during training. Our results show that the formation of Pavlovian fear memory is associated with changes of gene expression in the BLA, which may contribute to neural plasticity and the processing of information about both conditioned and unconditioned fear stimuli.

Fornix-dependent induction of hippocampal CCAAT enhancer-binding protein [beta] and [delta] Co-localizes with phosphorylated cAMP response element-binding protein and accompanies long-term memory consolidation

The cAMP response element-binding protein (CREB) is an evolutionarily conserved transcription regulator essential for long-term memory formation. It is not known, however, whether the molecular events downstream of CREB activation are also conserved. An early, cAMP-dependent event necessary for learning-related long-term synaptic plasticity in the invertebrate Aplysia californica is the induction of the transcription factor CCAAT enhancer-binding protein (C/EBP). Here we show that two homologs in the rat, C/EBPbeta and C/EBPdelta, are induced at discrete times after inhibitory avoidance learning and co-localize with phosphorylated CREB in the hippocampus. This induction is blocked by fornix lesions, which are known to disrupt activation of CREB in the hippocampus and to impair memory consolidation. These results indicate that C/EBPs are evolutionarily conserved components of the CREB-dependent gene cascade activated in long-term memory.

God's organism? The chick as a model system for memory studies

The young chick is a powerful model system in which to study the biochemical and morphological processes underlying memory formation. Training chicks on a one trial passive avoidance task results in a molecular cascade in a specific brain region, the intermediate medial hyperstriatum ventrale. This cascade is initiated by glutamate release and engages a series of synaptic transients including increased calcium flux, up-regulation of NMDA-glutamate receptors, membrane protein phosphorylations, and the retrograde messenger NO. Expression of immediate early genes c-fos and c-jun precedes the synthesis, glycosylation, and redistribution, >4 hr downstream, of a number of synaptic membrane proteins, notably NCAM and L1. Other membrane proteins required in the early phase of memory formation include the amyloid precursor protein (APP) and apolipoprotein E. There are concomitant increases in dendritic spine number and changes in synaptic structure. Nonsynaptic factors, including corticosterone and BDNF, can modulate retention of the avoidance response, enhancing the salience of otherwise weakly retained memory. These results are discussed in relation to general concepts of memory formation and the spatio-temporal distribution of the putative memory trace.