- Open Access
Catching the engram: strategies to examine the memory trace
© Sakaguchi and Hayashi.; licensee BioMed Central Ltd. 2012
- Received: 18 July 2012
- Accepted: 18 September 2012
- Published: 21 September 2012
Memories are stored within neuronal ensembles in the brain. Modern genetic techniques can be used to not only visualize specific neuronal ensembles that encode memories (e.g., fear, craving) but also to selectively manipulate those neurons. These techniques are now being expanded for the study of various types of memory. In this review, we will summarize the genetic methods used to visualize and manipulate neurons involved in the representation of memory engrams. The methods will help clarify how memory is encoded, stored and processed in the brain. Furthermore, these approaches may contribute to our understanding of the pathological mechanisms associated with human memory disorders and, ultimately, may aid the development of therapeutic strategies to ameliorate these diseases.
- Conditioned Stimulus
- Unconditioned Stimulus
- Memory Trace
- cAMP Response Element Binding
- Fear Memory
One of the major aims of modern memory research is to locate the physical substrate of memory (also referred to as ‘memory trace’ or ‘neural substrates of memory’) in the brain. At the beginning of the 20th century, Richard Semon introduced the word, ‘engram’, to describe the memory trace, ‘Its result, namely, the enduring though primarily latent modification in the irritable substance produced by stimulus, I have called an Engram, ….’ . Later, Karl Lashley, a pioneer in the field of behavioral neuroscience, attempted to identify the location of the engram in rodents using a maze task . He systematically lesioned different parts of the brain and examined the behavioral consequences. Although he failed to locate a specific brain region where the memory trace exists, this approach has been and still is one of the most commonly used methods to understand the role of selected brain regions in memory.
Clinical studies of an epilepsy patient, known by the initials H.M., led to the accidental discovery of a milestone in memory research. To provide therapeutic relief for the epileptic seizures that H.M was experiencing, the hippocampi and amygdalae were surgically removed. Fortunately, with this treatment, H.M.’s epilepsy was cured. However, surgical intervention also produced severe anterograde and temporally-graded retrograde amnesia of autobiographical memory . Subsequent studies of H.M. revolutionized our view of memory by suggesting that there are particular locations in the brain that play an essential part in memory.
The association of elementary events has been proposed to play a central role in memory . In line with this notion, recent developments in memory research have focused on associative learning and memory . Pavlov was well known for implementing an experimental paradigm to quantify associative memory, using a so-called classical conditioning paradigm. In classical conditioning, subjects associate two different sensory stimuli, defined as the conditioned stimulus (CS) and the unconditioned stimulus (US) . The CS is a cue that is neutral but salient enough to be recognized by the subject, whereas the US is a cue that evokes an innate response in the subject, leading to an unconditioned response (UR). When the subject learns the association between the CS and the US, the subject may display a response similar to the UR upon exposure to the CS alone. This response is called a conditioned response (CR).
Many modern memory researchers have not only re-confirmed the earlier findings of Pavlov using different variants of classical conditioning paradigms, but these subsequent studies have provided many further insights into the neurobiology associated with memory (reviewed in ). Philips and LeDoux showed that distinct brain regions have different contributions to memory formation . Using a contextual fear conditioning paradigm, they showed that the hippocampus, in cooperation with the amygdala, plays an essential role in associative learning of a specific context (CS) and a foot-shock (US). This finding is in contrast to tone-fear conditioning, where subjects learn that a neutral tone (CS) can act as a predictor of a foot-shock (US). In this paradigm the amygdala played an essential role but the hippocampus was dispensable. While this study highlighted an important functional distinction between the hippocampus and amygdala in fear memory, it was still unclear at which stage of memory (e.g., acquisition, retention, recall) each brain region was important.
The obvious shortfall in the lesion studies is that surgical lesions not only destroy all locally existing structures including neuronal and glial cells but also incoming, outgoing, and even passing fibers. Also, the irreversibility of the procedure makes it difficult to determine the exact role of the lesioned brain regions at different stages of memory. Using drugs that selectively inactivate neuronal activity has some notable advantages over physical lesions and can help elucidate the relationship between biological phenomenon and memory. For example, Kandel et al. elucidated the molecular mechanisms of memory related behavior by applying drugs (e.g., cAMP or CRE oligonucleotides) onto target neurons of Aplysia . Morris showed a link between long term potentiation and spatial memory by applying a pharmacological reagent that blocks LTP (AP5, an antagonist of NMDA type glutamate receptors) directly into the brains of rats . Recent advances in genetic techniques (e.g. optogenetics) have enabled the manipulation of neural activity at an even higher level of sophistication and temporal control (e.g., inactivating targeted neurons in reversible manner at specific points in time [24, 25]). Using these techniques, it is now possible to visualize and control specific neuronal circuits that encode associative memory [26, 27].
Given these advances, in this review, we will provide an overview of recent studies that aim to allocate the memory engram at the circuit, cellular, and synaptic level and discuss the current debate, remaining questions and future perspectives.
Detection of IEG products
IEGs are a class of genes that are transcribed immediately after biological events (starting in less than a minute) without requiring the expression of other genes [28, 29]. Neural activity induces expression of various IEGs including proto-oncogene transcription factors such as c-fos and zif/268 (also named Egr1, NGFI-A, Krox 24) and genes that encode synaptic structural proteins such as Arc and Homer1a. Due to their property of activity dependent transcription, immunostaining or in situ hybridization of IEGs allows us to identify neurons that were active during a given memory paradigm.
Using the Arc catFISH method, Guzowski et al.  showed that when a rat is exposed to two different environments with an interval of 30 minutes, activation of Arc in response to the exposure to each environment occurs in separate neuronal populations in the CA1 (Figure 2B, lower row). In contrast, if a rat is exposed to the same environment twice, the same neurons that were activated during the first episode were reactivated during the second episode (Figure 2B, upper row). This result suggests that the environment specific memory trace is established in the CA1. Using an associative learning paradigm, Barot et al.  showed that individual neurons in the basolateral nucleus of the amygdala responded to both CS and US only after the subject had learned the association between CS and US.
It is suggested that once memory is encoded, it is consolidated through off-line neuronal ensemble activity (e.g., activity during rest or sleep when there is no CS or US). Marrone et al.  compared neuronal ensemble activity in the hippocampus during periods of exploration and then the following rest period using the catFISH method. They found that the Arc expression pattern during the rest period partially recapitulated that of the exploration period. Hashikawa et al.  extended this idea by examining associative learning in the amygdala using a fear conditioning paradigm and showed that amygdala neurons that expressed Arc during conditioning also preferentially re-expressed Arc during the following rest period.
Use of IEG promoters
The induction of c-fos expression upon conditioning indicates that there is a correlation between neuronal activation and memory, but it does not necessarily prove that c-fos expressing neurons encode the memory. Koya et al.  provided evidence that c-fos expressing neuronal ensembles in the nucleus accumbens were involved in the memory trace that associated a specific context (CS) with cocaine administration (US). For this study, they utilized a combination of c-fos promoter-LacZ transgenic rats and Daun02, a prodrug that can be converted to Daunorubicin by LacZ. Daunorubicin inactivates neurons by reducing Ca2+ dependent action potentials [42–44]. Using this combinational approach, c-fos activated neurons (i.e., LacZ-positive neurons) could be selectively inactivated by Daun02 injection. In their study, learning of a context-drug administration memory induced c-fos expression in neuronal ensembles in the nucleus accumbens of rats. Subsequent administration of Daun02 reduced the context-specific cocaine-induced psychomotor sensitization, confirming that neurons that were activated during learning were also involved in recall of the memory that associated the context with cocaine administration.
Using the same c-fos-LacZ rat, Bossert et al. found neural ensembles that mediated a context-induced relapse to heroin addiction in the ventral medial prefrontal cortex . These results suggest that the c-fos promoter can be used to genetically tag the neuronal ensemble involved in memory encoding.
cAMP response elements (CRE) is a DNA sequence which can be found in the regulatory sequences of IEGs (e.g., c-fos , Arc ). In LTP, calcium and cAMP signals converge to activate cAMP response element binding protein (CREB) transcription activity by phosphorylating Serine residue 133 (CREBs133), which results in an increase in CRE mediated gene expression. Impey et al. made a transgenic mouse line, which carries tandem-repeat CRE sequences followed by a LacZ-reporter gene to monitor CRE mediated transcription activity upon LTP and memory formation . Indeed, LacZ expression was well correlated with the phosphorylation of CREBs133 and induction of L-LTP in the CA1 region of the hippocampus. Furthermore, the signaling pathway that induced L-LTP enhances CRE mediated transcription. Importantly, learning of contextual fear conditioning and passive avoidance tasks increased CRE dependent gene expression in the hippocampus . On the other hand, auditory fear conditioning, an amygdala dependent learning paradigm, only increased CRE dependent gene expression in the amygdala, suggesting that CRE dependent gene expression was memory type specific and that CRE up-regulation was involved not only in hippocampus-dependent but also in amygdala-dependent associative memories.
The studies described above underscore the importance of the CREB transcription factor and its downstream targets in modulating the cellular response to neuronal activity that takes place during learning and memory. CREB is an interface between neuronal activity and gene transcription by converting local and transient second messenger signaling into a persistent cell-wide transcriptional modification. This feature gave rise to the idea that CREB could be used as a tool to force a neuron to encode memory.
This feature of CREB allows us to selectively manipulate memory encoding cells by coexpressing CREB with functional molecules. Han et al. utilized a Cre recombinase (Cre)-inducible diphtheria toxin receptor (iDTR) transgenic mouse line that allowed the selective elimination of target cells (i.e., DTR expressing neurons) . They engineered a HSV vector that expresses both CREB and Cre to induce memory encoding preferentially to CREB positive neurons at the learning stage and to confer DT sensitivity to the same neurons (Figure 5B). After learning, DT was administered to selectivity ablate CREB expressing neurons. Interestingly, this process also resulted in the erasure of the newly acquired fear memory, suggesting that CREB positive neurons can selectively encode fear memory, by outcompeting other neurons in the amygdala .
Zhou et al. reported similar results utilizing an allatostatin receptor (AlstR)/ligand system, originally derived from insects . Binding of allatostatin to heterologously expressed AlstR activates endogenous mammalian G protein-coupled inwardly rectifying K+ (GIRK) channels, which causes membrane hyperpolarization, thereby decreasing neuronal excitability . The system allowed inducible silencing of target neurons (i.e., AlstR expressing neurons) in a reversible manner. Silencing of CREB overexpressing neurons by AlstR/ligand system resulted in a reduction in freezing during a tone-fear conditioning test, providing further evidence that CREB induces memory encoding in amygdala neurons.
The authors also examined the selectivity of memory induced by CREB. Conditioned taste aversion (CTA) memory is a type of memory known to depend on the amygdala . First, mice underwent tone-fear conditioning, then later, CREB and AlstR were coexpressed in amygdala neurons, and CTA training was performed. Using this paradigm, CREB was active only during CTA training, but not during tone-fear training. The subsequent infusion of allatostatin selectively disrupted the CTA memory but not the tone-fear memory, indicating that the specific memory encoding could be induced by CREB overexpression.
What is the mechanism that enables neurons overexpressing CREB to preferentially encode memory? Electrophysiological recordings of hippocampal neurons overexpressing a constitutively active form of CREB revealed larger N-methyl-D-aspartate type glutamate receptor (NMDAR) currents and a greater magnitude of LTP . A similar experiment performed on neurons from the nucleus accumbens indicates that CREB increases overall excitability of neurons by enhancing the Na+ current while suppressing the K+ current . Morphologically, neurons overexpressing a constitutively active form of CREB have a higher density of dendritic spines . Tone fear memory formation functionally strengthened thalamus-to-lateral amygdala synapses in CREB neurons but not neighboring neurons . These results suggest that enhanced neuronal excitability is one of the mechanisms by which CREB mediates the induction of memory encoding in amygdala dependent memories.
In the hippocampus, overexpression of CREB rescued a spatial memory deficit in a mouse model of Alzheimer’s disease  and also enhanced fear memory in CFC . It will be interesting to examine whether CREB overexpression can also induce memory encoding in the hippocampus or other brain regions .
Another powerful tool that has recently emerged in the field of memory research is the use of light-activated proteins to control neuronal activity. Boyden et al.  and Ishizuka et al.  were first to report the usefulness of channelrhodopsin, a blue light activated non-selective cation channel from green algae Chlamydomonas reinhardtii in enhancing spike generation. Further screening of this class of micro-organisms yielded halorhodopsin, a Cl- channel and Archaerhodopsin, a proton pump, which cause neuronal hyperpolarisation upon illumination with yellow or green light, respectively. Such light activated proteins make it possible to precisely control the temporal and spatial activity of neurons in vivo [25, 65, 67–71].
Johansen et al.  showed that optogenetic stimulation of pyramidal neurons in the lateral amygdala can replace the US in a tone fear conditioning paradigm. Choi et al.  succeeded in inducing neuronal ensemble activity in the piriform context to control memory related behaviors using optogenetics. They showed that the same neural ensembles could be trained to evoke both appetitive and aversive behavior interchangeably.
Goshen et al.  examined whether the hippocampus is still engaged in the remote memory using optogenetic approaches. In contrast to previous results, where inhibition of hippocampal activity at remote time points resulted in no apparent effect in fear memory retrieval [9, 12, 13], inhibition of the activity of CA1 αCaMKII-positive neurons using eNpHR3.1, an improved version of halorhodopsin, specifically during the memory retrieval test resulted in a reduction in freezing not only at recent but also at remote time points. Interestingly, inhibition of activity 30 minutes before the test abolished the above effect (i.e., reduction in freezing), which is in agreement with other reports that utilize the other methods (e.g., physical, pharmacological, and genetic lesions) [9, 12, 13]. These results showed that the hippocampus is still engaged after memory consolidation, and highlight the higher temporal resolution of optogenetic approaches over other more conventional approaches.
Hebb outlined a computational model proposing that memories may be encoded by the associative activity of connected neurons at synapses, the synaptic plasticity. The proposal was substantiated by the discovery of LTP; a prolonged strengthening of the efficiency at synapses. Although its molecular mechanisms are not fully understood and may differ amongst different regions of the brain, LTP is generally considered to involve two key phenomena. One is to increase the number of AMPARs, leading to an increase in the efficiency of transmission [74, 75]. For example, Rumpel et al. showed that blocking LTP by preventing synaptic trafficking of GluR1 AMPARs in neurons of the lateral amygdala can led to an impairment in memory encoding of cued fear conditioning in rats . The other is to increase the size of dendritic spines, where synapses reside [77, 78].
The synaptic engram can be visualized by detecting the underlying molecular mechanisms of synaptic plasticity. Ca2+ imaging offers a functional readout of synaptic responses in near real time. It revealed that coincidental synaptic input occurs on the scale of around 10 μm on a single dendrite [80, 81]. A study using a pH-sensitive GFP-tagged AMPA receptor showed that the synapses, in which AMPA receptors were newly inserted, formed clusters through a NMDA-R dependent mechanism, adding further support to the clustered plasticity model . Förster resonance energy transfer (FRET) is a sensitive method to determine if two fluorophores are within a small distance of each other. Sensor molecules, such as CaMKII (a major player in LTP) [82–85], and actin (a major synaptic structural protein) [78, 86], were engineered to visualize the synaptic engram using FRET.
The next question to address will be whether the clustering is related to the encoding of information that could impact animal’s behavior (e.g., CS and US), and ultimately, whether the clustering is necessary and essential for associative memory. Using transcranial two-photon microscopy, Fu et al. showed a correlation between the extent of motor learning and the clustering of new spines in the motor cortex. Motor learning facilitated clustered spine formation, whereas new spines tend to avoid regions with existing spines in control conditions . Lai et al.  examined the correlation between the amount of synapse turnover and behavioral changes during a fear conditioning paradigm. Fear learning eliminated spines in cortical neurons whereas fear extinction induced the formation of new spines on the same branch where the spine elimination took place. Importantly, re-conditioning after extinction resulted in the selective elimination of synapses that were newly formed during extinction, suggesting that the newly formed synapses could represent the process of extinction. It will be interesting to know whether such synaptic changes are necessary and/or sufficient to change fear memory .
If the synaptic engram is clustered, what is the underlying mechanism? Using the size of dendritic spines as an index for transmission efficiency, Harvey et al. demonstrated in hippocampal slices that a spine that received subthreshold stimulation which normally induces only a transient enlargement can be enlarged persistently by combining it with suprathreshold stimulation of a nearby spine . Moreover, the amplitude of excitatory postsynaptic potential (epsp) supralinearly sums up when stimuli are given to adjacent synapses . Importantly, the efficiency of the cross-talk between the synapses is governed by the distance between two synapses and the time interval between stimuli [70, 90].
These results suggest that there is signaling cross-talk between nearby dendritic spines. The imaging of movement of synaptic proteins using a photoactivatable GFP revealed that synaptic proteins are indeed shared between neighboring synapses [91, 92]. FRET imaging revealed that the activity of ras induced in a single spine by glutamate uncaging can spread to neighboring spines . These observations suggest that molecules activated at one synapse can spread to nearby synapses and such sharing may underlie the mechanisms of the cross-talk of synaptic plasticity between nearby synapses.
Researchers hope to clarify the mechanisms of learning and memory, and ultimately to apply the techniques and knowledge to treat memory related disorders in humans [94, 95]. Recent findings obtained from studies examining the mechanisms of reconsolidation in rodents could potentially be transferred to aid clinical applications in humans to attenuate/prevent the return of learnt fear [95, 96]. Such studies highlight the importance of understanding the basic mechanisms of memory to aid the establishment of viable strategies that can provide therapeutic relief to sufferers of memory disorders [95–97]. To clarify the mechanisms of learning and memory, we have to identify neuronal ensembles that encode the memory and to selectively manipulate them and observe its behavioral outcome. The main advantage of the methods discussed in this review is that they are able to selectively target memory-encoding neurons, whereas other conventional methods (such as pharmacological or surgical lesions, transcranial magnetic stimulation) cannot. At the same time certain technological advances need to be made to enable the efficient and safe delivery of genes to the human brain. The recent revival of virus based gene delivery methods [98, 99] and the establishment of a method to access the deeper regions of the intact human brain  could provide a foundation for the future development of therapeutic strategies for the treatment of human memory disorders by directly and selectively manipulating memory encoding neurons.
We thank Mr. Koichi Hashikawa for providing Arc-catFISH pictures and Dr Lily M.Y. Yu for critical comments on the manuscript. This work is partially supported by a RIKEN Special Postdoctoral Fellowship, the strategic programs for R&D (President’s discretionary fund) of RIKEN, RIKEN Fund for Seeds (Tane) of Collaborative Research, Uehara Memorial Foundation, Takeda Science Foundation, Research Foundation for Opto-science and Technology, and Grant-in-Aid for Young Researcher (B) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan (MEXT) to M.S., and by RIKEN, NIH grant R01DA17310, Grant-in-Aid for Scientific Research (A) and Grant-in-Aid for Scientific Research on Innovative Area ‘Foundation of Synapse and Neurocircuit Pathology’ from MEXT to YH. We apologies to authors whose work we were unable to include owing to space constraints.
- Semon R: The mneme. 1921, London: G. Allen & Unwin ltd.Google Scholar
- Lashely KS: In search of the engram. Symp Soc Exp Biol. 1950, 4: 454-Google Scholar
- Scoville WB, Milner B: Loss of recent memory after bilateral hippocampal lesions. J Neurol Neurosurg Psychiatry. 1957, 20: 11-21. 10.1136/jnnp.20.1.11.PubMed CentralView ArticlePubMedGoogle Scholar
- Sorabji R: Aristotle on memory. 2006, Chicago: University of Chicago Press, SecondGoogle Scholar
- Dudai Y: Memory from A to Z. 2002, Oxford: Oxford University PressGoogle Scholar
- Pavlov IP: Conditioned reflexes. An investigation of the physiological activity of the cerebral cortex. 1927, London: Oxford UniversityGoogle Scholar
- Crawley JN: What's wrong with my mouse?: Behavioral Phenotyping of Transgenic and Knockout Mice. 2007, Hoboken NJ: John Wiley & SonsView ArticleGoogle Scholar
- Phillips RG, LeDoux JE: Differential contribution of amygdala and hippocampus to cued and contextual fear conditioning. Behav Neurosci. 1992, 106: 274-285.View ArticlePubMedGoogle Scholar
- Kim JJ, Fanselow MS: Modality-specific retrograde amnesia of fear. Science. 1992, 256: 675-677. 10.1126/science.1585183.View ArticlePubMedGoogle Scholar
- Frankland PW, Bontempi B, Talton LE, Kaczmarek L, Silva AJ: The involvement of the anterior cingulate cortex in remote contextual fear memory. Science. 2004, 304: 881-883. 10.1126/science.1094804.View ArticlePubMedGoogle Scholar
- Maviel T, Durkin TP, Menzaghi F, Bontempi B: Sites of neocortical reorganization critical for remote spatial memory. Science. 2004, 305: 96-99. 10.1126/science.1098180.View ArticlePubMedGoogle Scholar
- Anagnostaras SG, Maren S, Fanselow MS: Temporally graded retrograde amnesia of contextual fear after hippocampal damage in rats: within-subjects examination. J Neurosci. 1999, 19: 1106-1114.PubMedGoogle Scholar
- Shimizu E, Tang YP, Rampon C, Tsien JZ: NMDA receptor-dependent synaptic reinforcement as a crucial process for memory consolidation. Science. 2000, 290: 1170-1174. 10.1126/science.290.5494.1170.View ArticlePubMedGoogle Scholar
- Riedel G, Micheau J, Lam AG, Roloff EL, Martin SJ, Bridge H, de Hoz L, Poeschel B, McCulloch J, Morris RG: Reversible neural inactivation reveals hippocampal participation in several memory processes. Nat Neurosci. 1999, 2: 898-905. 10.1038/13202.View ArticlePubMedGoogle Scholar
- Winocur G, Moscovitch M, Sekeres M: Memory consolidation or transformation: context manipulation and hippocampal representations of memory. Nat Neurosci. 2007, 10: 555-557. 10.1038/nn1880.View ArticlePubMedGoogle Scholar
- Nadel L, Moscovitch M: Memory consolidation, retrograde amnesia and the hippocampal complex. Curr Opin Neurobiol. 1997, 7: 217-227. 10.1016/S0959-4388(97)80010-4.View ArticlePubMedGoogle Scholar
- Wang SH, Teixeira CM, Wheeler AL, Frankland PW: The precision of remote context memories does not require the hippocampus. Nat Neurosci. 2009, 12: 253-255. 10.1038/nn.2263.View ArticlePubMedGoogle Scholar
- Nadel L, Samsonovich A, Ryan L, Moscovitch M: Multiple trace theory of human memory: computational, neuroimaging, and neuropsychological results. Hippocampus. 2000, 10: 352-368. 10.1002/1098-1063(2000)10:4<352::AID-HIPO2>3.0.CO;2-D.View ArticlePubMedGoogle Scholar
- Moscovitch M, Nadel L, Winocur G, Gilboa A, Rosenbaum RS: The cognitive neuroscience of remote episodic, semantic and spatial memory. Curr Opin Neurobiol. 2006, 16: 179-190. 10.1016/j.conb.2006.03.013.View ArticlePubMedGoogle Scholar
- Winocur G, Moscovitch M, Bontempi B: Memory formation and long-term retention in humans and animals: convergence towards a transformation account of hippocampal-neocortical interactions. Neuropsychologia. 2010, 48: 2339-2356. 10.1016/j.neuropsychologia.2010.04.016.View ArticlePubMedGoogle Scholar
- Sutherland RJ, Lehmann H: Alternative conceptions of memory consolidation and the role of the hippocampus at the systems level in rodents. Curr Opin Neurobiol. 2011, 21: 446-451. 10.1016/j.conb.2011.04.007.View ArticlePubMedGoogle Scholar
- Kandel ER: The molecular biology of memory: cAMP, PKA, CRE, CREB-1, CREB-2, and CPEB. Mol Brain. 2012, 5: 14-10.1186/1756-6606-5-14.PubMed CentralView ArticlePubMedGoogle Scholar
- Morris RG: Long-term potentiation and memory. Philos Trans R Soc Lond B Biol Sci. 2003, 358: 643-647. 10.1098/rstb.2002.1230.PubMed CentralView ArticlePubMedGoogle Scholar
- Nakashiba T, Young JZ, McHugh TJ, Buhl DL, Tonegawa S: Transgenic inhibition of synaptic transmission reveals role of CA3 output in hippocampal learning. Science. 2008, 319: 1260-1264. 10.1126/science.1151120.View ArticlePubMedGoogle Scholar
- Goshen I, Brodsky M, Prakash R, Wallace J, Gradinaru V, Ramakrishnan C, Deisseroth K: Dynamics of retrieval strategies for remote memories. Cell. 2011, 147: 678-689. 10.1016/j.cell.2011.09.033.View ArticlePubMedGoogle Scholar
- Silva AJ, Zhou Y, Rogerson T, Shobe J, Balaji J: Molecular and cellular approaches to memory allocation in neural circuits. Science. 2009, 326: 391-395. 10.1126/science.1174519.PubMed CentralView ArticlePubMedGoogle Scholar
- Josselyn SA: Continuing the search for the engram: examining the mechanism of fear memories. J Psychiatry Neurosci. 2010, 35: 221-228. 10.1503/jpn.100015.PubMed CentralView ArticlePubMedGoogle Scholar
- Guzowski JF, McNaughton BL, Barnes CA, Worley PF: Environment-specific expression of the immediate-early gene Arc in hippocampal neuronal ensembles. Nat Neurosci. 1999, 2: 1120-1124. 10.1038/16046.View ArticlePubMedGoogle Scholar
- Hayashi Y, Okamoto M, Bosch M, Futai Y: Roles of neuronal activity-induced gene products in Hebbian and homeostatic synaptic plasticity, tagging and capture. Adv Exp Med Biol. 2012, 970: 335-354. 10.1007/978-3-7091-0932-8_15.View ArticlePubMedGoogle Scholar
- Guzowski JF, Worley PF: Cellular compartment analysis of temporal activity by fluorescence in situ hybridization (catFISH). Curr Protoc Neurosci. 2001, Chapter 1: 1-8.Google Scholar
- Barot SK, Kyono Y, Clark EW, Bernstein IL: Visualizing stimulus convergence in amygdala neurons during associative learning. Proc Natl Acad Sci USA. 2008, 105: 20959-20963. 10.1073/pnas.0808996106.PubMed CentralView ArticlePubMedGoogle Scholar
- Marrone DF, Schaner MJ, McNaughton BL, Worley PF, Barnes CA: Immediate-early gene expression at rest recapitulates recent experience. J Neurosci. 2008, 28: 1030-1033.View ArticlePubMedGoogle Scholar
- Hashikawa K, Matsuki N, Nomura H: Preferential Arc transcription at rest in the active ensemble during associative learning. Neurobiol Learn Mem. 2011, 95: 498-504. 10.1016/j.nlm.2011.02.013.View ArticlePubMedGoogle Scholar
- Reijmers LG, Perkins BL, Matsuo N, Mayford M: Localization of a stable neural correlate of associative memory. Science. 2007, 317: 1230-1233. 10.1126/science.1143839.View ArticlePubMedGoogle Scholar
- Matsuo N, Reijmers L, Mayford M: Spine-type-specific recruitment of newly synthesized AMPA receptors with learning. Science. 2008, 319: 1104-1107. 10.1126/science.1149967.PubMed CentralView ArticlePubMedGoogle Scholar
- Wang KH, Majewska A, Schummers J, Farley B, Hu C, Sur M, Tonegawa S: In vivo two-photon imaging reveals a role of arc in enhancing orientation specificity in visual cortex. Cell. 2006, 126: 389-402. 10.1016/j.cell.2006.06.038.View ArticlePubMedGoogle Scholar
- Clem RL, Celikel T, Barth AL: Ongoing in vivo experience triggers synaptic metaplasticity in the neocortex. Science. 2008, 319: 101-104. 10.1126/science.1143808.View ArticlePubMedGoogle Scholar
- Barth AL, Gerkin RC, Dean KL: Alteration of neuronal firing properties after in vivo experience in a FosGFP transgenic mouse. J Neurosci. 2004, 24: 6466-6475. 10.1523/JNEUROSCI.4737-03.2004.View ArticlePubMedGoogle Scholar
- Cifani C, Koya E, Navarre BM, Calu DJ, Baumann MH, Marchant NJ, Liu QR, Khuc T, Pickel J, Lupica CR, Shaham Y, Hope BT: Medial prefrontal cortex neuronal activation and synaptic alterations after stress-induced reinstatement of palatable food seeking: a study using c-fos-GFP transgenic female rats. J Neurosci. 2012, 32: 8480-8490. 10.1523/JNEUROSCI.5895-11.2012.PubMed CentralView ArticlePubMedGoogle Scholar
- Eguchi M, Yamaguchi S: In vivo and in vitro visualization of gene expression dynamics over extensive areas of the brain. NeuroImage. 2009, 44: 1274-1283. 10.1016/j.neuroimage.2008.10.046.View ArticlePubMedGoogle Scholar
- Grinevich V, Kolleker A, Eliava M, Takada N, Takuma H, Fukazawa Y, Shigemoto R, Kuhl D, Waters J, Seeburg PH, Osten P: Fluorescent Arc/Arg3.1 indicator mice: a versatile tool to study brain activity changes in vitro and in vivo. J Neurosci Methods. 2009, 184: 25-36. 10.1016/j.jneumeth.2009.07.015.PubMed CentralView ArticlePubMedGoogle Scholar
- Koya E, Golden SA, Harvey BK, Guez-Barber DH, Berkow A, Simmons DE, Bossert JM, Nair SG, Uejima JL, Marin MT, Mitchell TB, Farquhar D, Ghosh SC, Mattson BJ, Hope BT: Targeted disruption of cocaine-activated nucleus accumbens neurons prevents context-specific sensitization. Nat Neurosci. 2009, 12: 1069-1073. 10.1038/nn.2364.PubMed CentralView ArticlePubMedGoogle Scholar
- Farquhar D, Pan BF, Sakurai M, Ghosh A, Mullen CA, Nelson JA: Suicide gene therapy using E. coli beta-galactosidase. Cancer Chemother Pharmacol. 2002, 50: 65-70. 10.1007/s00280-002-0438-2.View ArticlePubMedGoogle Scholar
- Santone KS, Oakes SG, Taylor SR, Powis G: Anthracycline-induced inhibition of a calcium action potential in differentiated murine neuroblastoma cells. Cancer Res. 1986, 46: 2659-2664.PubMedGoogle Scholar
- Bossert JM, Stern AL, Theberge FR, Cifani C, Koya E, Hope BT, Shaham Y: Ventral medial prefrontal cortex neuronal ensembles mediate context-induced relapse to heroin. Nat Neurosci. 2011, 14: 420-422. 10.1038/nn.2758.PubMed CentralView ArticlePubMedGoogle Scholar
- Garner AR, Rowland DC, Hwang SY, Baumgaertel K, Roth BL, Kentros C, Mayford M: Generation of a synthetic memory trace. Science. 2012, 335: 1513-1516. 10.1126/science.1214985.PubMed CentralView ArticlePubMedGoogle Scholar
- Alexander GM, Rogan SC, Abbas AI, Armbruster BN, Pei Y, Allen JA, Nonneman RJ, Hartmann J, Moy SS, Nicolelis MA, McNamara JO, Roth BL: Remote control of neuronal activity in transgenic mice expressing evolved G protein-coupled receptors. Neuron. 2009, 63: 27-39. 10.1016/j.neuron.2009.06.014.PubMed CentralView ArticlePubMedGoogle Scholar
- Liu X, Ramirez S, Pang PT, Puryear CB, Govindarajan A, Deisseroth K, Tonegawa S: Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature. 2012, 484: 381-385.PubMed CentralView ArticlePubMedGoogle Scholar
- Sassone-Corsi P, Visvader J, Ferland L, Mellon PL, Verma IM: Induction of proto-oncogene fos transcription through the adenylate cyclase pathway: characterization of a cAMP-responsive element. Genes Dev. 1988, 2: 1529-1538. 10.1101/gad.2.12a.1529.View ArticlePubMedGoogle Scholar
- Kawashima T, Okuno H, Nonaka M, Adachi-Morishima A, Kyo N, Okamura M, Takemoto-Kimura S, Worley PF, Bito H: Synaptic activity-responsive element in the Arc/Arg3.1 promoter essential for synapse-to-nucleus signaling in activated neurons. Proc Natl Acad Sci USA. 2009, 106: 316-321. 10.1073/pnas.0806518106.PubMed CentralView ArticlePubMedGoogle Scholar
- Impey S, Mark M, Villacres EC, Poser S, Chavkin C, Storm DR: Induction of CRE-mediated gene expression by stimuli that generate long-lasting LTP in area CA1 of the hippocampus. Neuron. 1996, 16: 973-982. 10.1016/S0896-6273(00)80120-8.View ArticlePubMedGoogle Scholar
- Impey S, Smith DM, Obrietan K, Donahue R, Wade C, Storm DR: Stimulation of cAMP response element (CRE)-mediated transcription during contextual learning. Nat Neurosci. 1998, 1: 595-601. 10.1038/2830.View ArticlePubMedGoogle Scholar
- Han JH, Kushner SA, Yiu AP, Cole CJ, Matynia A, Brown RA, Neve RL, Guzowski JF, Silva AJ, Josselyn SA: Neuronal competition and selection during memory formation. Science. 2007, 316: 457-460. 10.1126/science.1139438.View ArticlePubMedGoogle Scholar
- Guan Z, Giustetto M, Lomvardas S, Kim JH, Miniaci MC, Schwartz JH, Thanos D, Kandel ER: Integration of long-term-memory-related synaptic plasticity involves bidirectional regulation of gene expression and chromatin structure. Cell. 2002, 111: 483-493. 10.1016/S0092-8674(02)01074-7.View ArticlePubMedGoogle Scholar
- Levenson JM, Sweatt JD: Epigenetic mechanisms in memory formation. Nat Rev Neurosci. 2005, 6: 108-118. 10.1038/nrn1604.View ArticlePubMedGoogle Scholar
- Buch T, Heppner FL, Tertilt C, Heinen TJ, Kremer M, Wunderlich FT, Jung S, Waisman A: A Cre-inducible diphtheria toxin receptor mediates cell lineage ablation after toxin administration. Nat Methods. 2005, 2: 419-426. 10.1038/nmeth762.View ArticlePubMedGoogle Scholar
- Han JH, Kushner SA, Yiu AP, Hsiang HL, Buch T, Waisman A, Bontempi B, Neve RL, Frankland PW, Josselyn SA: Selective erasure of a fear memory. Science. 2009, 323: 1492-1496. 10.1126/science.1164139.View ArticlePubMedGoogle Scholar
- Zhou Y, Won J, Karlsson MG, Zhou M, Rogerson T, Balaji J, Neve R, Poirazi P, Silva AJ: CREB regulates excitability and the allocation of memory to subsets of neurons in the amygdala. Nat Neurosci. 2009, 12: 1438-1443. 10.1038/nn.2405.PubMed CentralView ArticlePubMedGoogle Scholar
- Birgul N, Weise C, Kreienkamp HJ, Richter D: Reverse physiology in drosophila: identification of a novel allatostatin-like neuropeptide and its cognate receptor structurally related to the mammalian somatostatin/galanin/opioid receptor family. EMBO J. 1999, 18: 5892-5900. 10.1093/emboj/18.21.5892.PubMed CentralView ArticlePubMedGoogle Scholar
- Yamamoto T, Shimura T, Sako N, Yasoshima Y, Sakai N: Neural substrates for conditioned taste aversion in the rat. Behav Brain Res. 1994, 65: 123-137. 10.1016/0166-4328(94)90097-3.View ArticlePubMedGoogle Scholar
- Marie H, Morishita W, Yu X, Calakos N, Malenka RC: Generation of silent synapses by acute in vivo expression of CaMKIV and CREB. Neuron. 2005, 45: 741-752. 10.1016/j.neuron.2005.01.039.View ArticlePubMedGoogle Scholar
- Dong Y, Green T, Saal D, Marie H, Neve R, Nestler EJ, Malenka RC: CREB modulates excitability of nucleus accumbens neurons. Nat Neurosci. 2006, 9: 475-477. 10.1038/nn1661.View ArticlePubMedGoogle Scholar
- Yiu AP, Rashid AJ, Josselyn SA: Increasing CREB function in the CA1 region of dorsal hippocampus rescues the spatial memory deficits in a mouse model of Alzheimer's disease. Neuropsychopharmacology. 2011, 36: 2169-2186. 10.1038/npp.2011.107.PubMed CentralView ArticlePubMedGoogle Scholar
- Restivo L, Tafi E, Ammassari-Teule M, Marie H: Viral-mediated expression of a constitutively active form of CREB in hippocampal neurons increases memory. Hippocampus. 2009, 19: 228-234. 10.1002/hipo.20527.View ArticlePubMedGoogle Scholar
- Boyden ES, Zhang F, Bamberg E, Nagel G, Deisseroth K: Millisecond-timescale, genetically targeted optical control of neural activity. Nat Neurosci. 2005, 8: 1263-1268. 10.1038/nn1525.View ArticlePubMedGoogle Scholar
- Ishizuka T, Kakuda M, Araki R, Yawo H: Kinetic evaluation of photosensitivity in genetically engineered neurons expressing green algae light-gated channels. Neurosci Res. 2006, 54: 85-94. 10.1016/j.neures.2005.10.009.View ArticlePubMedGoogle Scholar
- Johansen JP, Wolff SB, Luthi A, Ledoux JE: Controlling the Elements: An Optogenetic Approach to Understanding the Neural Circuits of Fear. Biol Psychiatry. 2012, 71 (12): 1053-1060. 10.1016/j.biopsych.2011.10.023.PubMed CentralView ArticlePubMedGoogle Scholar
- Ciocchi S, Herry C, Grenier F, Wolff SB, Letzkus JJ, Vlachos I, Ehrlich I, Sprengel R, Deisseroth K, Stadler MB, Muller C, Luthi A: Encoding of conditioned fear in central amygdala inhibitory circuits. Nature. 2010, 468: 277-282. 10.1038/nature09559.View ArticlePubMedGoogle Scholar
- Haubensak W, Kunwar PS, Cai H, Ciocchi S, Wall NR, Ponnusamy R, Biag J, Dong HW, Deisseroth K, Callaway EM, Fanselow MS, Luthi A, Anderson DJ: Genetic dissection of an amygdala microcircuit that gates conditioned fear. Nature. 2010, 468: 270-276. 10.1038/nature09553.PubMed CentralView ArticlePubMedGoogle Scholar
- Harvey CD, Svoboda K: Locally dynamic synaptic learning rules in pyramidal neuron dendrites. Nature. 2007, 450: 1195-1200. 10.1038/nature06416.PubMed CentralView ArticlePubMedGoogle Scholar
- Letzkus JJ, Wolff SB, Meyer EM, Tovote P, Courtin J, Herry C, Luthi A: A disinhibitory microcircuit for associative fear learning in the auditory cortex. Nature. 2011, 480: 331-335. 10.1038/nature10674.View ArticlePubMedGoogle Scholar
- Johansen JP, Hamanaka H, Monfils MH, Behnia R, Deisseroth K, Blair HT, LeDoux JE: Optical activation of lateral amygdala pyramidal cells instructs associative fear learning. Proc Natl Acad Sci USA. 2010, 107: 12692-12697. 10.1073/pnas.1002418107.PubMed CentralView ArticlePubMedGoogle Scholar
- Choi GB, Stettler DD, Kallman BR, Bhaskar ST, Fleischmann A, Axel R: Driving opposing behaviors with ensembles of piriform neurons. Cell. 2011, 146: 1004-1015. 10.1016/j.cell.2011.07.041.PubMed CentralView ArticlePubMedGoogle Scholar
- Shi SH, Hayashi Y, Petralia RS, Zaman SH, Wenthold RJ, Svoboda K, Malinow R: Rapid spine delivery and redistribution of AMPA receptors after synaptic NMDA receptor activation. Science. 1999, 284: 1811-1816. 10.1126/science.284.5421.1811.View ArticlePubMedGoogle Scholar
- Hayashi Y, Shi SH, Esteban JA, Piccini A, Poncer JC, Malinow R: Driving AMPA receptors into synapses by LTP and CaMKII: requirement for GluR1 and PDZ domain interaction. Science. 2000, 287: 2262-2267. 10.1126/science.287.5461.2262.View ArticlePubMedGoogle Scholar
- Rumpel S, LeDoux J, Zador A, Malinow R: Postsynaptic receptor trafficking underlying a form of associative learning. Science. 2005, 308: 83-88. 10.1126/science.1103944.View ArticlePubMedGoogle Scholar
- Matsuzaki M, Honkura N, Ellis-Davies GC, Kasai H: Structural basis of long-term potentiation in single dendritic spines. Nature. 2004, 429: 761-766. 10.1038/nature02617.PubMed CentralView ArticlePubMedGoogle Scholar
- Okamoto K, Nagai T, Miyawaki A, Hayashi Y: Rapid and persistent modulation of actin dynamics regulates postsynaptic reorganization underlying bidirectional plasticity. Nat Neurosci. 2004, 7: 1104-1112. 10.1038/nn1311.View ArticlePubMedGoogle Scholar
- Govindarajan A, Kelleher RJ, Tonegawa S: A clustered plasticity model of long-term memory engrams. Nat Rev Neurosci. 2006, 7: 575-583. 10.1038/nrn1937.View ArticlePubMedGoogle Scholar
- Takahashi N, Kitamura K, Matsuo N, Mayford M, Kano M, Matsuki N, Ikegaya Y: Locally synchronized synaptic inputs. Science. 2012, 335: 353-356.PubMedGoogle Scholar
- Kleindienst T, Winnubst J, Roth-Alpermann C, Bonhoeffer T, Lohmann C: Activity-dependent clustering of functional synaptic inputs on developing hippocampal dendrites. Neuron. 2011, 72: 1012-1024. 10.1016/j.neuron.2011.10.015.View ArticlePubMedGoogle Scholar
- Takao K, Okamoto K, Nakagawa T, Neve RL, Nagai T, Miyawaki A, Hashikawa T, Kobayashi S, Hayashi Y: Visualization of synaptic Ca2+ /calmodulin-dependent protein kinase II activity in living neurons. J Neurosci. 2005, 25: 3107-3112. 10.1523/JNEUROSCI.0085-05.2005.View ArticlePubMedGoogle Scholar
- Lee SJ, Escobedo-Lozoya Y, Szatmari EM, Yasuda R: Activation of CaMKII in single dendritic spines during long-term potentiation. Nature. 2009, 458: 299-304. 10.1038/nature07842.PubMed CentralView ArticlePubMedGoogle Scholar
- Kwok S, Lee C, Sanchez SA, Hazlett TL, Gratton E, Hayashi Y: Genetically encoded probe for fluorescence lifetime imaging of CaMKII activity. Biochem Biophys Res Commun. 2008, 369: 519-525. 10.1016/j.bbrc.2008.02.070.PubMed CentralView ArticlePubMedGoogle Scholar
- Mower AF, Kwok S, Yu H, Majewska AK, Okamoto K, Hayashi Y, Sur M: Experience-dependent regulation of CaMKII activity within single visual cortex synapses in vivo. Proc Natl Acad Sci USA. 2011, 108: 21241-21246. 10.1073/pnas.1108261109.PubMed CentralView ArticlePubMedGoogle Scholar
- Okamoto K, Hayashi Y: Visualization of F-actin and G-actin equilibrium using fluorescence resonance energy transfer (FRET) in cultured cells and neurons in slices. Nat Protoc. 2006, 1: 911-919. 10.1038/nprot.2006.122.View ArticlePubMedGoogle Scholar
- Fu M, Yu X, Lu J, Zuo Y: Repetitive motor learning induces coordinated formation of clustered dendritic spines in vivo. Nature. 2012, 483: 92-95. 10.1038/nature10844.PubMed CentralView ArticlePubMedGoogle Scholar
- Lai CS, Franke TF, Gan WB: Opposite effects of fear conditioning and extinction on dendritic spine remodelling. Nature. 2012, 483: 87-91. 10.1038/nature10792.View ArticlePubMedGoogle Scholar
- Losonczy A, Makara JK, Magee JC: Compartmentalized dendritic plasticity and input feature storage in neurons. Nature. 2008, 452: 436-441. 10.1038/nature06725.View ArticlePubMedGoogle Scholar
- Govindarajan A, Israely I, Huang SY, Tonegawa S: The dendritic branch is the preferred integrative unit for protein synthesis-dependent LTP. Neuron. 2011, 69: 132-146. 10.1016/j.neuron.2010.12.008.PubMed CentralView ArticlePubMedGoogle Scholar
- Gray NW, Weimer RM, Bureau I, Svoboda K: Rapid redistribution of synaptic PSD-95 in the neocortex in vivo. PLoS Biol. 2006, 4: e370-10.1371/journal.pbio.0040370.PubMed CentralView ArticlePubMedGoogle Scholar
- Tsuriel S, Geva R, Zamorano P, Dresbach T, Boeckers T, Gundelfinger ED, Garner CC, Ziv NE: Local sharing as a predominant determinant of synaptic matrix molecular dynamics. PLoS Biol. 2006, 4: e271-10.1371/journal.pbio.0040271.PubMed CentralView ArticlePubMedGoogle Scholar
- Harvey CD, Yasuda R, Zhong H, Svoboda K: The spread of Ras activity triggered by activation of a single dendritic spine. Science. 2008, 321: 136-140. 10.1126/science.1159675.PubMed CentralView ArticlePubMedGoogle Scholar
- Debiec J, Ledoux JE: Disruption of reconsolidation but not consolidation of auditory fear conditioning by noradrenergic blockade in the amygdala. Neuroscience. 2004, 129: 267-272. 10.1016/j.neuroscience.2004.08.018.View ArticlePubMedGoogle Scholar
- Schiller D, Monfils MH, Raio CM, Johnson DC, Ledoux JE, Phelps EA: Preventing the return of fear in humans using reconsolidation update mechanisms. Nature. 2010, 463: 49-53. 10.1038/nature08637.PubMed CentralView ArticlePubMedGoogle Scholar
- Monfils MH, Cowansage KK, Klann E, LeDoux JE: Extinction-reconsolidation boundaries: key to persistent attenuation of fear memories. Science. 2009, 324: 951-955. 10.1126/science.1167975.PubMed CentralView ArticlePubMedGoogle Scholar
- Foa EB: Prolonged exposure therapy: past, present, and future. Depress Anxiety. 2011, 28 (12): 1043-1047. 10.1002/da.20907.View ArticlePubMedGoogle Scholar
- Kohn DB, Candotti F: Gene therapy fulfilling its promise. N Engl J Med. 2009, 360: 518-521. 10.1056/NEJMe0809614.View ArticlePubMedGoogle Scholar
- Gene therapy deserves a fresh chance. Nature. 2009, 461: 1173-1174.Google Scholar
- Kringelbach ML, Jenkinson N, Owen SL, Aziz TZ: Translational principles of deep brain stimulation. Nat Rev Neurosci. 2007, 8: 623-635. 10.1038/nrn2196.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.