Memory Engram Cells Have Come of Age

Insights into the encoding of memories through the circuitry of fear

Associative learning induces physical changes to a network of cells, known as the memory engram. Fear is widely used as a model to understand the circuit motifs that underpin associative memories. Recent advances suggest that the distinct circuitry engaged by different conditioned stimuli (e.g. tone vs. context) can provide insights into what information is being encoded in the fear engram. Moreover, as the fear memory matures, the circuitry engaged indicates how information is remodelled after learning and hints at potential mechanisms for consolidation. Finally, we propose that the consolidation of fear memories involves plasticity of engram cells through coordinated activity between brain regions, and the inherent characteristics of the circuitry may mediate this process.

Inflammatory Factors Predicted the Resilient Phenotype in Social Defeat-induced Depression of Male Mouse

An absence of reward in chronic stress may impair the reward circuit in the brain, resulting in major depressive disorder (MDD). In a part of chronically stressed individuals, MDD is not present, i.e., there is resilience, implying endogenous anti-depressive mechanisms in the brain. We studied social defeat model mice and analyzed the mRNA maps of the hippocampus from a control group and social defeat (SD)-susceptible and SD-resilient mice using high-throughput sequencing techniques. It was found that the immune response was associated with depression. Existing studies have proven that microglia play an important role in the brain immune response, and their activation level increases after chronic social defeat stress (CSDS). In our study, minocycline inhibited the activation of microglia, thereby improving the depressive state of CSDS mice. In addition, minocycline combined with fluoxetine enhanced the efficacy of fluoxetine. Thus, our results propose the most probable mechanism underlying different responses to CSDS and indicate the potential of a combination of anti-inflammatory drugs and antidepressants in treating refractory depression.

Efficiency analysis by training sequence of high-fidelity simulation-based neonatal resuscitation program (NRP)

OBJECTIVE

This study was conducted to find a more effective education method in a neonatal resuscitation program (NRP) using a high-fidelity simulation that has recently shown positive results in terms of its effectiveness.

STUDY DESIGN

A single-blind prospective cohort study was performed. The high-fidelity simulation model was used in a formal NRP training course for trainees caring for neonatal patients. The trainees were divided into a group that conducted the scenario after the lecture (Group A) and a group that attended the lecture after the scenario (Group B) and they both took the test before, during, and after the training.

RESULTS

The increase in score after theory training was statistically significant in both groups, but the final score did not differ between the two groups. However, when compared by career, in Group A, trainees under 24 months tended to be more effective, and in Group B, trainees over 24 months tended to be more effective.

CONCLUSION

The difference in short-term memory of trainees according to the order of education identified by the test score was not prominent, but it was found that the degree of difference in test scores for the order of education tended to be different according to the career. It is thought that the effectiveness of the training might be increased by changing the education order according to the degree of experience of each trainee. More effective educational methods should be introduced by continuously developing lectures for repeated education of various trainees in the future.

The two tales of hippocampal sharp-wave ripple content: The rigid and the plastic

Sharp-wave ripples, prominently in the CA1 region of the hippocampus, are short oscillatory events accompanied by bursts of neural firing. Ripples and associated hippocampal place cell sequences communicate with cortical ensembles during slow-wave sleep, which has been shown to be critical for systems consolidation of episodic memories. This consolidation is not limited to a newly formed memory trace; instead, ripples appear to reactivate and consolidate memories spanning various experiences. Despite this broad spanning influence, ripples remain capable of producing precise memories. The underlying mechanisms that enable ripples to consolidate memories broadly and with specificity across experiences remain unknown. In this review, we discuss data that uncovers circuit-level processes that generate ripples and influence their characteristics during consolidation. Based on current knowledge, we propose that memory emerges from the integration of two parallel consolidation pathways in CA1: the rigid and plastic pathways. The rigid pathway generates ripples stochastically, providing a backbone upon which dynamic plastic pathway inputs carrying novel information are integrated.

Neocortical synaptic engrams for remote contextual memories

While initial encoding of contextual memories involves the strengthening of hippocampal circuits, these memories progressively mature to stabilized forms in neocortex and become less hippocampus dependent. Although it has been proposed that long-term storage of contextual memories may involve enduring synaptic changes in neocortical circuits, synaptic substrates of remote contextual memories have been elusive. Here we demonstrate that the consolidation of remote contextual fear memories in mice correlated with progressive strengthening of excitatory connections between prefrontal cortical (PFC) engram neurons active during learning and reactivated during remote memory recall, whereas the extinction of remote memories weakened those synapses. This synapse-specific plasticity was CREB-dependent and required sustained hippocampal signals, which the retrosplenial cortex could convey to PFC. Moreover, PFC engram neurons were strongly connected to other PFC neurons recruited during remote memory recall. Our study suggests that progressive and synapse-specific strengthening of PFC circuits can contribute to long-term storage of contextual memories.

Persistent representation of the environment in the hippocampus

In the hippocampus, environmental changes elicit rearrangement of active neuronal ensembles or remapping of place cells. However, it remains elusive how the brain ensures a consistent representation of a certain environment itself despite salient events occurring there. Here, we longitudinally tracked calcium dynamics of dorsal hippocampal CA1 neurons in mice subjected to contextual fear conditioning and extinction training. Overall population activities were significantly changed by fear conditioning and were responsive to footshocks and freezing. However, a small subset of neurons, termed environment cells, were consistently active in a specific environment irrespective of experiences. A decoder modeling study showed that these cells, but not place cells, were able to predict the environment to which the mouse was exposed. Environment cells might underlie the constancy of cognition for distinct environments across time and events. Additionally, our study highlights the functional heterogeneity of cells in the hippocampus.

Mapping the spatial transcriptomic signature of the hippocampus during memory consolidation

Memory consolidation involves discrete patterns of transcriptional events in the hippocampus. Despite the emergence of single-cell transcriptomic profiling techniques, defining learning-responsive gene expression across subregions of the hippocampus has remained challenging. Here, we utilized unbiased spatial sequencing to elucidate transcriptome-wide changes in gene expression in the hippocampus following learning, enabling us to define molecular signatures unique to each hippocampal subregion. We find that each subregion of the hippocampus exhibits distinct yet overlapping transcriptomic signatures. Although the CA1 region exhibited increased expression of genes related to transcriptional regulation, the DG showed upregulation of genes associated with protein folding. We demonstrate the functional relevance of subregion-specific gene expression by genetic manipulation of a transcription factor selectively in the CA1 hippocampal subregion, leading to long-term memory deficits. This work demonstrates the power of using spatial molecular approaches to reveal transcriptional events during memory consolidation.

Defective engram allocation contributes to impaired fear memory performance in Down syndrome

Down syndrome (DS) is the most common genetic form of intellectual disability (ID). The cellular and molecular mechanisms contributing to ID in DS are not completely understood. Recent evidence indicates that a given memory is encoded by sparsely distributed neurons, highly activated during learning, the engram cells. Intriguingly, mechanisms that are of paramount importance for engram formation are impaired in DS. Here we explored engram formation in a DS mouse model, the Ts65Dn and we found a reduced number of engram cells in the dentate gyrus (DG), suggesting reduced neuronal allocation to engrams. We also show that trisomic engram cells present reduced number of mature spines than WT engram cells and their excitability is not enhanced during memory recall. In fact, activation of engram cells using a chemogenetic approach does not recover memory deficits in Ts65Dn. Altogether, our findings suggest that perturbations in engram neurons may play a significant role in memory alterations in DS.

The hippocampus in stress susceptibility and resilience: Reviewing molecular and functional markers

Understanding the individual variability that comes with the likelihood of developing stress-related psychopathologies is of paramount importance when addressing mechanisms of their neurobiology. This article focuses on the hippocampus as a region that is highly influenced by chronic stress exposure and that has strong ties to the development of related disorders, such as depression and post-traumatic stress disorder. We first outline three commonly used animal models that have been used to separate animals into susceptible and resilient cohorts. Next, we review molecular and functional hippocampal markers of susceptibility and resilience. We propose that the hippocampus plays a crucial role in the differences in the processing and storage of stress-related information in animals with different stress susceptibilities. These hippocampal markers not only help us attain a more comprehensive understanding of the various facets of stress-related pathophysiology, but also could be targeted for the development of new treatments.

Noradrenergic signaling mediates cortical early tagging and storage of remote memory

The neocortical prefrontal memory engram generated during initial learning is critical for remote episodic memory storage, however, the nature of early cortical tagging remains unknown. Here we found that in mice, increased norepinephrine (NE) release from the locus coeruleus (LC) to the medial prefrontal cortex (mPFC) during contextual fear conditioning (CFC) was critical for engram tagging and remote memory storage, which was regulated by the ventrolateral periaqueductal grey. β-Blocker infusion, or knockout of β1-adrenergic receptor (β1-AR) in the mPFC, impaired the storage of remote CFC memory, which could not be rescued by activation of LC-mPFC NE projection. Remote memory retrieval induced the activation of mPFC engram cells that were tagged during CFC. Inhibition of LC-mPFC NE projection or β1-AR knockout impaired mPFC engram tagging. Juvenile mice had fewer LC NE neurons than adults and showed deficiency in mPFC engram tagging and remote memory of CFC. Activation of β1-AR signaling promoted mPFC early tagging and remote memory storage in juvenile mice. Our data demonstrate that activation of LC NEergic signaling during CFC memory encoding mediates engram early tagging in the mPFC and systems consolidation of remote memory.

Exploiting the molecular diversity of the synapse to investigate neuronal communication: A guide through the current toolkit

Chemical synapses are tiny and overcrowded environments, deeply embedded inside brain tissue and enriched with thousands of protein species. Many efforts have been devoted to developing custom approaches for evaluating and modifying synaptic activity. Most of these methods are based on the engineering of one or more synaptic protein scaffolds used to target active moieties to the synaptic compartment or to manipulate synaptic functioning. In this review, we summarize the most recent methodological advances and provide a description of the involved proteins as well as the operation principle. Furthermore, we highlight their advantages and limitations in relation to studies of synaptic transmission in vitro and in vivo. Concerning the labelling methods, the most important challenge is how to extend the available approaches to the in vivo setting. On the other hand, for those methods that allow manipulation of synaptic function, this limit has been overcome using optogenetic approaches that can be more easily applied to the living brain. Finally, future applications of these methods to neuroscience, as well as new potential routes for development, are discussed.

Has the concept of systems consolidation outlived its usefulness? Identification and evaluation of premises underlying systems consolidation

Systems consolidation has mostly been treated as a neural construct defined by the time-dependent change in memory representation from the hippocampus (HPC) to other structures, primarily the neocortex. Here, we identify and evaluate the explicit and implicit premises that underlie traditional or standard models and theories of systems consolidation based on evidence from research on humans and other animals. We use the principle that changes in neural representation over time and experience are accompanied by corresponding changes in psychological representations, and vice versa, to argue that each of the premises underlying traditional or standard models and theories of systems consolidation is found wanting. One solution is to modify or abandon the premises or theories and models. This is reflected in moderated models of systems consolidation that emphasize the early role of the HPC in training neocortical memories until they stabilize. The fault, however, may lie in the very concept of systems consolidation and its defining feature. We propose that the concept be replaced by one of memory systems reorganization, which does not carry the theoretical baggage of systems consolidation and is flexible enough to capture the dynamic nature of memory from inception to very long-term retention and retrieval at a psychological and neural level. The term "memory system reorganization" implies that memory traces are not fixed, even after they are presumably consolidated. Memories can continue to change as a result of experience and interactions among memory systems across the lifetime. As will become clear, hippocampal training of neocortical memories is only one type of such interaction, and not always the most important one, even at inception. We end by suggesting some principles of memory reorganization that can help guide research on dynamic memory processes that capture corresponding changes in memory at the psychological and neural levels.

A short-term memory trace persists for days in the mouse hippocampus

Active recall of short-term memory (STM) is known to last for a few hours, but whether STM has long-term functions is unknown. Here we show that STM can be optogenetically retrieved at a time point during which natural recall is not possible, uncovering the long-term existence of an STM engram. Moreover, re-training within 3 days led to natural long-term recall, indicating facilitated consolidation. Inhibiting offline CA1 activity during non-rapid eye movement (NREM) sleep, N-methyl-D-aspartate receptor (NMDAR) activity, or protein synthesis after first exposure to the STM-forming event impaired the future re-exposure-facilitated consolidation, which highlights a role of protein synthesis, NMDAR and NREM sleep in the long-term storage of an STM trace. These results provide evidence that STM is not completely lost within hours and demonstrates a possible two-step STM consolidation, first long-term storage as a behaviorally inactive engram, then transformation into an active state by recurrence within 3 days.

Short-term memory (STM) forms a protein synthesis-, NMDAR- and NREM sleep-dependent engram which lasts at least 3 days in the mouse hippocampus following a novel object location task, suggesting that STM is not completely lost within hours.

PDE2A Inhibition Enhances Axonal Sprouting, Functional Connectivity, and Recovery after Stroke

Phosphodiesterase (PDE) inhibitors have been safely and effectively used in the clinic and increase the concentration of intracellular cyclic nucleotides (cAMP/cGMP). These molecules activate downstream mediators, including the cAMP response element-binding protein (CREB), which controls neuronal excitability and growth responses. CREB gain of function enhances learning and allocates neurons into memory engrams. CREB also controls recovery after stroke. PDE inhibitors are linked to recovery from neural damage and to stroke recovery in specific sites within the brain. PDE2A is enriched in cortex. In the present study, we use a mouse cortical stroke model in young adult and aged male mice to test the effect of PDE2A inhibition on functional recovery, and on downstream mechanisms of axonal sprouting, tissue repair, and the functional connectivity of neurons in recovering cortex. Stroke causes deficits in use of the contralateral forelimb, loss of axonal projections in cortex adjacent to the infarct, and functional disconnection of neuronal networks. PDE2A inhibition enhances functional recovery, increases axonal projections in peri-infarct cortex, and, through two-photon in vivo imaging, enhances the functional connectivity of motor system excitatory neurons. PDE2A inhibition after stroke does not have an effect on other aspects of tissue repair, such as angiogenesis, gliogenesis, neurogenesis, and inflammatory responses. These data suggest that PDE2A inhibition is an effective therapeutic approach for stroke recovery in the rodent and that it simultaneously enhances connectivity in peri-infarct neuronal populations.SIGNIFICANCE STATEMENT Inhibition of PDE2A enhances motor recovery, axonal projections, and functional connectivity of neurons in peri-infarct tissue. This represents an avenue for a pharmacological therapy for stroke recovery.

Dose-dependent suppression of hippocampal contextual memory formation, place cells, and spatial engrams by the NMDAR antagonist (R)-CPP

We recently reported that the competitive NMDAR antagonist (R,S)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) does not suppress NMDAR-mediated field EPSPs (fEPSPNMDA) or long-term potentiation (LTP) in vitro at concentrations that block contextual conditioning in vivo. Here we tested one possible explanation for the mismatch - that the hippocampus is relatively resistant to CPP compared to other brain structures engaged in contextual fear conditioning. Using the context pre-exposure facilitation effect (CPFE) paradigm to separate the hippocampal and extra-hippocampal components of contextual learning, we found that the active enantiomer (R)-CPP suppressed the hippocampal component with an IC50 of 3.1 mg/kg, a dose that produces brain concentrations below those required to block fEPSPNMDA or LTP. Moreover, using in-vivo calcium imaging of place cells and spatial engrams to directly assess hippocampal spatial coding, we found that (R)-CPP dose-dependently reduced the development of place cells and interfered with the formation of stable spatial engrams when it was administered prior to exposing mice to a novel context. Both effects occurred at doses that interfered with freezing to context in CPFE experiments. We conclude that (R)-CPP blocks memory formation by interfering with hippocampal function, but that it does so by modulating NMDARs at sites that are not engaged in vitro in the same manner that they are in vivo - perhaps through interneuron circuits that do not contribute to fEPSPs and are not required to elicit LTP using standard induction protocols in vitro, but are essential for successful mnemonic function in vivo.

Personal factors and the role of memory in faculty refinding of stored information

Purpose

The study examined the personal information management (PIM) challenges encountered by faculty in six universities in Ghana, their information refinding experiences and the perceived role of memory. The study tested the hypothesis that faculty PIM performance will significantly differ when the differences in the influence of personal factors (age, gender and rank) on their memory are considered.

Design/methodology/approach

The study was guided by a sample survey design. A questionnaire designed based on themes extracted from earlier interviews was used to collect quantitative data from 235 faculty members from six universities in Ghana. Data analysis was undertaken with a discrete multivariate Generalized Linear Model to investigate how memory intermediates in the relationship between age, gender and rank, and, refinding of stored information.

Findings

The paper identified two subfunctions of refinding (Refinding 1 and Refinding 2) associated with self-confidence in information re-finding, and, memory (Memory 1 and Memory 2), associated with the use of complimentary frames to locate previously found and stored information. There were no significant multivariate effects for gender as a stand-alone variable. Males who were aged less than 39 could refind stored information irrespective of the memory class. Older faculty aged 40–49 who possess Memory 1 and senior lecturers who possess Memory 2 performed well in refinding information. There was a statistically significant effect of age and memory; and rank and memory.

Research limitations/implications

This study was limited to faculty in Ghana, whereas the study itself has implications for demographic differences in PIM.

Practical implications

Identifying how memory mediates the role of personal factors in faculty refinding of stored information will be necessary for the efforts to understand and design systems and technologies for enhancing faculty capacity to find/refind stored information.

Social implications

Understanding how human memory can be augmented by technology is a great PIM strategy, but understanding how human memory and personal factors interplay to affect PIM is more important.

Originality/value

PIM of faculty has been extensively examined in the literature, and limitations of memory has always been identified as a constraint. Human memory has been augmented with technology, although the outcome has been very minimal. This study shows that in addition to technology augmentation, personal factors interplay with human memory to affect PIM. Discrete multivariate Generalized Linear Model applied in this study is an innovative way of addressing the challenges of assimilating statistical methodologies in psychosocial disciplines.

Dynamic tripartite construct of interregional engram circuits underlies forgetting of extinction memory

Fear extinction allows for adaptive control of learned fear responses but often fails, resulting in a renewal or spontaneous recovery of the extinguished fear, i.e., forgetting of the extinction memory readily occurs. Using an activity-dependent neuronal labeling strategy, we demonstrate that engram neurons for fear extinction memory are dynamically positioned in the medial prefrontal cortex (mPFC), basolateral amygdala (BLA), and ventral hippocampus (vHPC), which constitute an engram construct in the term of directional engram synaptic connectivity from the BLA or vHPC to mPFC, but not that in the opposite direction, for retrieval of extinction memory. Fear renewal or spontaneous recovery switches the extinction engram construct from an accessible to inaccessible state, whereas additional extinction learning or optogenetic induction of long-term potentiation restores the directional engram connectivity and prevents the return of fear. Thus, the plasticity of engram construct underlies forgetting of extinction memory.

Control of fear by discrete prefrontal GABAergic populations encoding valence-specific information

Neurons activated by learning have been ascribed the unique potential to encode memory, but the functional contribution of discrete cell types remains poorly understood. In particular, it is unclear whether learning engages specific GABAergic interneurons and, if so, whether they differ functionally from interneurons recruited by other experiences. Here, we show that fear conditioning activates a heterogeneous neuronal population in the medial prefrontal cortex (mPFC) that is largely comprised of somatostatin-expressing interneurons (SST-INs). Using intersectional genetic approaches, we demonstrate that fear-learning-activated SST-INs exhibit distinct circuit properties and are selectively reactivated to mediate cue-evoked memory expression. In contrast, an orthogonal population of SST-INs activated by morphine experience exerts opposing control over fear and supports reward-like motivational effects. These results outline an important role for discrete subsets of GABAergic cells in emotional learning and point to an unappreciated capacity for functional specialization among SST-INs.

Amygdala-cortical collaboration in reward learning and decision making

Adaptive reward-related decision making requires accurate prospective consideration of the specific outcome of each option and its current desirability. These mental simulations are informed by stored memories of the associative relationships that exist within an environment. In this review, I discuss recent investigations of the function of circuitry between the basolateral amygdala (BLA) and lateral (lOFC) and medial (mOFC) orbitofrontal cortex in the learning and use of associative reward memories. I draw conclusions from data collected using sophisticated behavioral approaches to diagnose the content of appetitive memory in combination with modern circuit dissection tools. I propose that, via their direct bidirectional connections, the BLA and OFC collaborate to help us encode detailed, outcome-specific, state-dependent reward memories and to use those memories to enable the predictions and inferences that support adaptive decision making. Whereas lOFC→BLA projections mediate the encoding of outcome-specific reward memories, mOFC→BLA projections regulate the ability to use these memories to inform reward pursuit decisions. BLA projections to lOFC and mOFC both contribute to using reward memories to guide decision making. The BLA→lOFC pathway mediates the ability to represent the identity of a specific predicted reward and the BLA→mOFC pathway facilitates understanding of the value of predicted events. Thus, I outline a neuronal circuit architecture for reward learning and decision making and provide new testable hypotheses as well as implications for both adaptive and maladaptive decision making.

Functionally distinct NPAS4-expressing somatostatin interneuron ensembles critical for motor skill learning

During motor learning, dendritic spines on pyramidal neurons (PNs) in the primary motor cortex (M1) undergo reorganization. Intriguingly, the inhibition from local somatostatin-expressing inhibitory neurons (SST-INs) plays an important role in regulating the PN plasticity and thus new motor skill acquisition. However, the molecular mechanisms underlying this process remain unclear. Here, we identified that the early-response transcription factor, NPAS4, is selectively expressed in SST-INs during motor learning. By utilizing in vivo two-photon imaging in mice, we found that cell-type-specific deletion of Npas4 in M1 disrupted learning-induced spine reorganization among PNs and impaired motor learning. In addition, NPAS4-expressing SST-INs exhibited lower neuronal activity during task-related movements, and chemogenetically increasing the activity of NPAS4-expressing ensembles was sufficient to mimic the effects of Npas4 deletion. Together, our results reveal an instructive role of NPAS4-expressing SST-INs in modulating the inhibition to downstream task-related PNs to allow proper spine reorganization that is critical for motor learning.

Fos ensembles encode and shape stable spatial maps in the hippocampus

In the hippocampus, spatial maps are formed by place cells while contextual memories are thought to be encoded as engrams1-6. Engrams are typically identified by expression of the immediate early gene Fos, but little is known about the neural activity patterns that drive, and are shaped by, Fos expression in behaving animals7-10. Thus, it is unclear whether Fos-expressing hippocampal neurons also encode spatial maps and whether Fos expression correlates with and affects specific features of the place code11. Here we measured the activity of CA1 neurons with calcium imaging while monitoring Fos induction in mice performing a hippocampus-dependent spatial learning task in virtual reality. We find that neurons with high Fos induction form ensembles of cells with highly correlated activity, exhibit reliable place fields that evenly tile the environment and have more stable tuning across days than nearby non-Fos-induced cells. Comparing neighbouring cells with and without Fos function using a sparse genetic loss-of-function approach, we find that neurons with disrupted Fos function have less reliable activity, decreased spatial selectivity and lower across-day stability. Our results demonstrate that Fos-induced cells contribute to hippocampal place codes by encoding accurate, stable and spatially uniform maps and that Fos itself has a causal role in shaping these place codes. Fos ensembles may therefore link two key aspects of hippocampal function: engrams for contextual memories and place codes that underlie cognitive maps.

Longitudinal monitoring of prefrontal cortical ensemble dynamics reveals new insights into stress habituation

The prefrontal cortex is highly susceptible to the detrimental effects of stress and has been implicated in the pathogenesis of stress-related psychiatric disorders. It is not well understood, however, how stress is represented at the neuronal level in the prefrontal cortical neuronal ensembles. Even less understood is how the representation of stress changes over time with repeated exposure. Here we show that the prelimbic prefrontal neuronal ensemble representation of foot shock stress exhibits rapid spatial drift within and between sessions. Despite this rapid spatial drift of the ensemble, the representation of the stressor itself stabilizes over days. Our results suggest that stress is represented by rapidly drifting ensembles and despite this rapid drift, important features of the neuronal representation are stabilized, suggesting a neural correlate of stress habituation is present within prefrontal cortical neuron populations.

A missense mutation in Kcnc3 causes hippocampal learning deficits in mice

Although a wide variety of genetic tools has been developed to study learning and memory, the molecular basis of memory encoding remains incompletely understood. Here, we undertook an unbiased approach to identify novel genes critical for memory encoding. From a large-scale, in vivo mutagenesis screen using contextual fear conditioning, we isolated in mice a mutant, named Clueless, with spatial learning deficits. A causative missense mutation (G434V) was found in the voltage-gated potassium channel, subfamily C member 3 (Kcnc3) gene in a region that encodes a transmembrane voltage sensor. Generation of a Kcnc3^G434V CRISPR mutant mouse confirmed this mutation as the cause of the learning defects. While G434V had no effect on transcription, translation, or trafficking of the channel, electrophysiological analysis of the G434V mutant channel revealed a complete loss of voltage-gated conductance, a broadening of the action potential, and decreased neuronal firing. Together, our findings have revealed a role for Kcnc3 in learning and memory.

Extrahippocampal seizure and memory circuits overlap

Seizures cause retrograde amnesia. We have previously demonstrated that seizures erode recently formed memories through shared ensembles and mechanisms in the CA1 region of the hippocampus. Here, we tested whether seizure circuits overlap spatial memory circuits outside of the CA. Spatial memory is consolidated by the hippocampal-cortical coupling that are connected via multiple pathways. We tested whether a seizure invades structures involved in memory consolidation by using the activity reporter TRAP2 mice. T-maze alternation learning activated neurons in the dentate gyrus, mediodorsal thalamus, retrosplenial cortex, and medial prefrontal cortex. This spatial memory relies on the plasticity of the AMPA receptor GluA1 subunit. GluA1 knockout/TRAP2 mice did not learn to alternate, and structures interposed between the hippocampus and the cortex were not active. A seizure prevented the recall of alternation memory and activated memory-labeled structures. There was a widespread overlap between learning-activated ensembles and seizure-activated neurons, which likely contributes to retrograde amnesia.Significance StatementWe propose that seizures cause retrograde amnesia by engaging the circuits that participate in memory consolidation.

Brain-wide screen of prelimbic cortex inputs reveals a functional shift during early fear memory consolidation

Memory formation and storage rely on multiple interconnected brain areas, the contribution of which varies during memory consolidation. The medial prefrontal cortex, in particular the prelimbic cortex (PL), was traditionally found to be involved in remote memory storage, but recent evidence points toward its implication in early consolidation as well. Nevertheless, the inputs to the PL governing these dynamics remain unknown. Here, we first performed a brain-wide, rabies-based retrograde tracing screen of PL engram cells activated during contextual fear memory formation in male mice to identify relevant PL input regions. Next, we assessed the specific activity pattern of these inputs across different phases of memory consolidation, from fear memory encoding to recent and remote memory recall. Using projection-specific chemogenetic inhibition, we then tested their functional role in memory consolidation, which revealed a hitherto unknown contribution of claustrum to PL inputs at encoding, and of insular cortex to PL inputs at recent memory recall. Both of these inputs further impacted how PL engram cells were reactivated at memory recall, testifying to their relevance for establishing a memory trace in the PL. Collectively, these data identify a spatiotemporal shift in PL inputs important for early memory consolidation, and thereby help to refine the working model of memory formation.

Propranolol Administration Modulates Neural Activity in the Hippocampal Hilus During Fear Retrieval

Altered fear learning is a strong behavioral component of anxiety disorders such as post-traumatic stress disorder (PTSD). Recent efforts have attempted to combine exposure therapies with drugs that target fear memory retrieval and memory reconsolidation, in order to improve treatment efficacy. The noradrenergic (NA) signaling system is of particular interest, due to its role in regulating the stress response and its involvement in fear and learning processes. Importantly, propranolol (P), a non-selective β-adrenergic antagonist, has shown the potential in decreasing exaggerated fear in both humans and animal models. In a previous study, we utilized an activity-dependent tagging murine model to determine the neural mechanisms by which propranolol attenuates learned fear. We found that propranolol acutely decreased memory trace reactivation specifically in the dorsal dentate gyrus (dDG), but not in CA3 or CA1. Here, we extended our previous study by investigating whether propranolol additionally altered activity in the hilus, a polymorphic layer that consists of neurons, mossy cells, and GABAergic interneurons. We found that propranolol acutely reduced overall hilar activity in both the dorsal and ventral hilus. Moreover, we report that propranolol significantly altered the activity of parvalbumin (PV)⁺ cells in the ventral (vDG), but not dorsal DG (dDG). Together, these results suggest that a β-adrenergic blockade may affect the activity of excitatory and inhibitory cell types in the hilar layer of the DG, and that these alterations may contribute to manipulating fear memory traces.

Long-term transverse imaging of the hippocampus with glass microperiscopes

The hippocampus consists of a stereotyped neuronal circuit repeated along the septal-temporal axis. This transverse circuit contains distinct subfields with stereotyped connectivity that support crucial cognitive processes, including episodic and spatial memory. However, comprehensive measurements across the transverse hippocampal circuit in vivo are intractable with existing techniques. Here, we developed an approach for two-photon imaging of the transverse hippocampal plane in awake mice via implanted glass microperiscopes, allowing optical access to the major hippocampal subfields and to the dendritic arbor of pyramidal neurons. Using this approach, we tracked dendritic morphological dynamics on CA1 apical dendrites and characterized spine turnover. We then used calcium imaging to quantify the prevalence of place and speed cells across subfields. Finally, we measured the anatomical distribution of spatial information, finding a non-uniform distribution of spatial selectivity along the DG-to-CA1 axis. This approach extends the existing toolbox for structural and functional measurements of hippocampal circuitry.

What does engram encode?: Heterogeneous memory engrams for different aspects of experience

Long-lasting synaptic changes within the neuronal network mediate memory. Neurons bearing such physical traces of memory (memory engram cells) are often equated with neurons expressing immediate early genes (IEGs) during a specific experience. However, past studies observed the expression of different IEGs in non-overlapping neurons or synaptic plasticity in neurons that do not express a particular IEG. Importantly, recent studies revealed that distinct subsets of neurons expressing different IEGs or even IEG negative-(yet active) neurons support different aspects of memory or computation, suggesting a more complex nature of memory engram cells than previously thought. In this short review, we introduce studies revealing such heterogeneous composition of the memory engram and discuss how the memory system benefits from it.

Neuronal metabolism in learning and memory: The anticipatory activity perspective

Current research on the molecular mechanisms of learning and memory is based on the "stimulus-response" paradigm, in which the neural circuits connecting environmental events with behavioral responses are strengthened. By contrast, cognitive and systems neuroscience emphasize the intrinsic activity of the brain that integrates information, establishes anticipatory actions, executes adaptive actions, and assesses the outcome via regulatory feedback mechanisms. We believe that the difference in the perspectives of systems and molecular studies is a major roadblock to further progress toward understanding the mechanisms of learning and memory. Here, we briefly overview the current studies in molecular mechanisms of learning and memory and propose that studying the predictive properties of neuronal metabolism will significantly advance our knowledge of how intrinsic, predictive activity of neurons shapes a new learning event. We further suggest that predictive metabolic changes in the brain may also take place in non-neuronal cells, including those of peripheral tissues. Finally, we present a path forward toward more in-depth studies of the role of cell metabolism in learning and memory.

Technologies for large-scale mapping of functional neural circuits active during a user-defined time window

The mapping of neural circuits activated during behavior down to individual neurons is crucial for decoding how the brain processes information. Technologies allowing activity-dependent labeling of neurons during user-defined restricted time windows are rapidly developing. Precise marking of the time window with light, in addition to chemicals, is now possible. In these technologies, genetically encoded molecules integrate molecular events resulting from neuronal activity with light/drug-dependent events. The outputs are either changes in fluorescence or activation of gene expression. Molecular reporters allow labeling of activated neurons for visualization and cell-type identification. The transcriptional readout also allows further control of activated neuronal populations using optogenetic tools as reporters. Here we review the design of these technologies and discuss their demonstrated applications to reveal previously unknown connections in the mammalian brain. We also consider the strengths and weaknesses of the current approaches and provide a perspective for the future.

Understanding the physical basis of memory: Molecular mechanisms of the engram

Memory, defined as the storage and use of learned information in the brain, is necessary to modulate behavior and critical for animals to adapt to their environments and survive. Despite being a cornerstone of brain function, questions surrounding the molecular and cellular mechanisms of how information is encoded, stored, and recalled remain largely unanswered. One widely held theory is that an engram is formed by a group of neurons that are active during learning, which undergoes biochemical and physical changes to store information in a stable state, and that are later reactivated during recall of the memory. In the past decade, the development of engram labeling methodologies has proven useful to investigate the biology of memory at the molecular and cellular levels. Engram technology allows the study of individual memories associated with particular experiences and their evolution over time, with enough experimental resolution to discriminate between different memory processes: learning (encoding), consolidation (the passage from short-term to long-term memories), and storage (the maintenance of memory in the brain). Here, we review the current understanding of memory formation at a molecular and cellular level by focusing on insights provided using engram technology.

Excitability-Independent Memory Allocation for Repeated Event

How memory is organized in cell ensembles when an event is repeated is not well-understood. Recently, we found that retraining 24 h after the initial fear conditioning (FC) event induces turnover of neurons in the lateral amygdala (LA) that encodes fear memory. Excitability-dependent competition between eligible neurons has been suggested as a rule that governs memory allocation. However, it remains undetermined whether excitability is also involved in the allocation of a repeated event. By increasing excitability in a subset of neurons in the LA before FC, we confirmed that these neurons preferentially participated in encoding fear memory as previously reported. These neurons, however, became unnecessary for memory recall after retraining 24 h following initial FC. Consistently, the initial memory-encoding neurons became less likely to be reactivated during recall. This reorganization in cell ensembles, however, was not induced and memory was co-allocated when retraining occurred 6 h after the initial FC. In 24-h retraining condition, artificially increasing excitability right before retraining failed to drive memory co-allocation. These results suggest a distinct memory allocation mechanism for repeated events distantly separated in time.

Hippocampal-amygdala memory circuits govern experience-dependent observational fear

The empathic ability to vicariously experience the other's fearful situation, a process called observational fear (OF), is critical to survive in nature and function in society. OF can be facilitated by both prior similar fear experience in the observer and social familiarity with the demonstrator. However, the neural circuit mechanisms of experience-dependent OF (Exp OF) remain unknown. Here, we demonstrate that hippocampal-basolateral amygdala (HPC-BLA) circuits in mice without involving the anterior cingulate cortex, considered a center of OF, mediate Exp OF. Dorsal HPC neurons generate fear memory engram cells in BLA encoding prior similar fear experiences, which are essential for Exp OF. On the other hand, ventral HPC neurons respond to the familiar demonstrator's aversive situation during Exp OF, which reactivates the fear memory engram cells in BLA to elicit Exp OF. Our study provides new insights into the memory engram-dependent perception-action coupling that underlies empathic behaviors like Exp OF.

Social experiences switch states of memory engrams through regulating hippocampal Rac1 activity

In pathological or artificial conditions, memory can be formed as silenced engrams that are unavailable for retrieval by presenting conditioned stimuli but can be artificially switched into the latent state so that natural recall is allowed. However, it remains unclear whether such different states of engrams bear any physiological significance and can be switched through physiological mechanisms. Here, we show that an acute social reward experience switches the silent memory engram into the latent state. Conversely, an acute social stress causes transient forgetting via turning a latent memory engram into a silent state. Such emotion-driven bidirectional switching between latent and silent states of engrams is mediated through regulation of Rac1 activity–dependent reversible forgetting in the hippocampus, as stress-activated Rac1 suppresses retrieval, while reward recovers silenced memory under amnesia by inhibiting Rac1. Thus, data presented reveal hippocampal Rac1 activity as the basis for emotion-mediated switching between latent and silent engrams to achieve emotion-driven behavioral flexibility.

Brain-wide mapping reveals that engrams for a single memory are distributed across multiple brain regions

Neuronal ensembles that hold specific memory (memory engrams) have been identified in the hippocampus, amygdala, or cortex. However, it has been hypothesized that engrams of a specific memory are distributed among multiple brain regions that are functionally connected, referred to as a unified engram complex. Here, we report a partial map of the engram complex for contextual fear conditioning memory by characterizing encoding activated neuronal ensembles in 247 regions using tissue phenotyping in mice. The mapping was aided by an engram index, which identified 117 cFos+ brain regions holding engrams with high probability, and brain-wide reactivation of these neuronal ensembles by recall. Optogenetic manipulation experiments revealed engram ensembles, many of which were functionally connected to hippocampal or amygdala engrams. Simultaneous chemogenetic reactivation of multiple engram ensembles conferred a greater level of memory recall than reactivation of a single engram ensemble, reflecting the natural memory recall process. Overall, our study supports the unified engram complex hypothesis for memory storage.

Acquiring new memories in neocortex of hippocampal-lesioned mice

The hippocampus interacts with the neocortical network for memory retrieval and consolidation. Here, we found the lateral entorhinal cortex (LEC) modulates learning-induced cortical long-range gamma synchrony (20–40 Hz) in a hippocampal-dependent manner. The long-range gamma synchrony, which was coupled to the theta (7–10 Hz) rhythm and enhanced upon learning and recall, was mediated by inter-cortical projections from layer 5 neurons of the LEC to layer 2 neurons of the sensory and association cortices. Artificially induced cortical gamma synchrony across cortical areas improved memory encoding in hippocampal lesioned mice for originally hippocampal-dependent tasks. Mechanistically, we found that activities of cortical c-Fos labeled neurons, which showed egocentric map properties, were modulated by LEC-mediated gamma synchrony during memory recall, implicating a role of cortical synchrony to generate an integrative memory representation from disperse features. Our findings reveal the hippocampal mediated organization of cortical memories and suggest brain-machine interface approaches to improve cognitive function.

Hippocampal lesioned mice form new memories. Here, the authors show the lateral entorhinal cortex modulates learning-induced cortical long-range gamma synchrony in a hippocampal-dependent manner and artificially induced cortical gamma synchrony across cortical areas improved memory encoding in hippocampal lesioned mice.

Brain-wide projection reconstruction of single functionally defined neurons

Reconstructing axonal projections of single neurons at the whole-brain level is currently a converging goal of the neuroscience community that is fundamental for understanding the logic of information flow in the brain. Thousands of single neurons from different brain regions have recently been morphologically reconstructed, but the corresponding physiological functional features of these reconstructed neurons are unclear. By combining two-photon Ca²⁺ imaging with targeted single-cell plasmid electroporation, we reconstruct the brain-wide morphologies of single neurons that are defined by a sound-evoked response map in the auditory cortices (AUDs) of awake mice. Long-range interhemispheric projections can be reliably labelled via co-injection with an adeno-associated virus, which enables enhanced expression of indicator protein in the targeted neurons. Here we show that this method avoids the randomness and ambiguity of conventional methods of neuronal morphological reconstruction, offering an avenue for developing a precise one-to-one map of neuronal projection patterns and physiological functional features.

Brain-wide axonal projections of single neurons have been extensively reconstructed without any functional characterization. The authors present a method that allows for developing a precise one-to-one map of both projection patterns and functional features of single neurons in mice.

De novo inter-regional coactivations of preconfigured local ensembles support memory

Neuronal ensembles in the amygdala, ventral hippocampus, and prefrontal cortex are involved in fear memory; however, how inter-regional ensemble interactions support memory remains elusive. Using multi-regional large-scale electrophysiology in the aforementioned structures of fear-conditioned rats, we found that the local ensembles activated during fear memory acquisition are inter-regionally coactivated during the subsequent sleep period, which relied on brief bouts of fast network oscillations. During memory retrieval, the coactivations reappeared, together with fast oscillations. Coactivation-participating-ensembles were configured prior to memory acquisition in the amygdala and prefrontal cortex but developed through experience in the hippocampus. Our findings suggest that elements of a given memory are instantly encoded within various brain regions in a preconfigured manner, whereas hippocampal ensembles and the network for inter-regional integration of the distributed information develop in an experience-dependent manner to form a new memory, which is consistent with the hippocampal memory index hypothesis.

The authors show that fear-memory-related cell-ensembles in the amygdala, hippocampus, and prefrontal cortex are inter-regionally co-activated in post-learning sleep. The co-activations are hosted by fast network oscillations and re-appear during recall.

Coordinated hippocampal-thalamic-cortical communication crucial for engram dynamics underneath systems consolidation

Systems consolidation refers to the time-dependent reorganization of memory representations or engrams across brain regions. Despite recent advancements in unravelling this process, the exact mechanisms behind engram dynamics and the role of associated pathways remain largely unknown. Here we propose a biologically-plausible computational model to address this knowledge gap. By coordinating synaptic plasticity timescales and incorporating a hippocampus-thalamus-cortex circuit, our model is able to couple engram reactivations across these regions and thereby reproduce key dynamics of cortical and hippocampal engram cells along with their interdependencies. Decoupling hippocampal-thalamic-cortical activity disrupts systems consolidation. Critically, our model yields testable predictions regarding hippocampal and thalamic engram cells, inhibitory engrams, thalamic inhibitory input, and the effect of thalamocortical synaptic coupling on retrograde amnesia induced by hippocampal lesions. Overall, our results suggest that systems consolidation emerges from coupled reactivations of engram cells in distributed brain regions enabled by coordinated synaptic plasticity timescales in multisynaptic subcortical-cortical circuits.

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.

Neuronal Circuits Associated with Fear Memory: Potential Therapeutic Targets for Posttraumatic Stress Disorder

Posttraumatic stress disorder (PTSD) is a psychiatric disorder that is associated with long-lasting memories of traumatic experiences. Extinction and discrimination of fear memory have become therapeutic targets for PTSD. Newly developed optogenetics and advanced in vivo imaging techniques have provided unprecedented spatiotemporal tools to characterize the activity, connectivity, and functionality of specific cell types in complicated neuronal circuits. The use of such tools has offered mechanistic insights into the exquisite organization of the circuitry underlying the extinction and discrimination of fear memory. This review focuses on the acquisition of more detailed, comprehensive, and integrated neural circuits to understand how the brain regulates the extinction and discrimination of fear memory. A future challenge is to translate these researches into effective therapeutic treatment for PTSD from the perspective of precise regulation of the neural circuits associated with the extinction and discrimination of fear memories.

Forgetting as a form of adaptive engram cell plasticity

One leading hypothesis suggests that memories are stored in ensembles of neurons (or 'engram cells') and that successful recall involves reactivation of these ensembles. A logical extension of this idea is that forgetting occurs when engram cells cannot be reactivated. Forms of 'natural forgetting' vary considerably in terms of their underlying mechanisms, time course and reversibility. However, we suggest that all forms of forgetting involve circuit remodelling that switches engram cells from an accessible state (where they can be reactivated by natural recall cues) to an inaccessible state (where they cannot). In many cases, forgetting rates are modulated by environmental conditions and we therefore propose that forgetting is a form of neuroplasticity that alters engram cell accessibility in a manner that is sensitive to mismatches between expectations and the environment. Moreover, we hypothesize that disease states associated with forgetting may hijack natural forgetting mechanisms, resulting in reduced engram cell accessibility and memory loss.

The use of rodent models to better characterize the relationship among epilepsy, sleep, and memory

Epilepsy, a neurological disorder characterized by recurrent seizures, is known to be associated with impaired sleep and memory. Although the specific mechanisms underlying these impairments are uncertain, the known role of sleep in memory consolidation suggests a potential relationship may exist between seizure activity, disrupted sleep, and memory impairment. A possible mediator in this relationship is the sleep spindle, the characteristic electroencephalographic (EEG) feature of non-rapid-eye-movement (NREM) sleep in humans and other mammals. Growing evidence supports the idea that sleep spindles, having thalamic origin, may mediate the process of long-term memory storage and plasticity by generating neuronal conditions that favor these processes. To study this potential relationship, a single model in which memory, sleep, and epilepsy can be simultaneously observed is of necessity. Rodent models of epilepsy appear to fulfill this requirement. Not only do rodents express both sleep spindles and seizure-induced sleep disruptions, but they also allow researchers to invasively study neurobiological processes both pre- and post- epileptic onset via the artificial induction of epilepsy (a practice that cannot be carried out in human subjects). However, the degree to which sleep architecture differs between rodents and humans makes direct comparisons between the two challenging. This review addresses these challenges and concludes that rodent sleep studies are useful in observing the functional roles of sleep and how they are affected by epilepsy.

The limbic memory circuit and the neural basis of contextual memory

The hippocampus, retrosplenial cortex and anterior thalamus are key components of a neural circuit known to be involved in a variety of memory functions, including spatial, contextual and episodic memory. In this review, we focus on the role of this circuit in contextual memory processes. The background environment, or context, is a powerful cue for memory retrieval, and neural representations of the context provide a mechanism for efficiently retrieving relevant memories while avoiding interference from memories that belong to other contexts. Data from experimental lesions and neural manipulation techniques indicate that each of these regions is critical for contextual memory. Neurophysiological evidence from the hippocampus and retrosplenial cortex suggest that contextual information is represented within this circuit by population-level neural firing patterns that reliably differentiate each context a subject encounters. These findings indicate that encoding contextual information to support context-dependent memory retrieval is a key function of this circuit.

PKMζ maintains remote contextual fear memory by inhibiting GluA2-dependent AMPA receptor endocytosis in the prelimbic cortex

Fear memories allow animals to recognize and adequately respond to dangerous situations. The prelimbic cortex (PrL) is a crucial node in the circuitry that encodes contextual fear memory, and its activity is central for fear memory expression over time. However, while PrL has been implicated in contextual fear memory storage, the molecular mechanisms underlying its maintenance remain unclear. Protein kinase M zeta (PKMζ) is a persistently active enzyme which has been shown to maintain many forms of memories by inhibiting the endocytosis of GluA2-containing AMPA receptors. Therefore, we hypothesized that PKMζ action upon GluA2-containing AMPARs could be a mechanism for contextual fear memory maintenance in the PrL. To test this hypothesis, we trained rats in a contextual fear conditioning (CFC) paradigm and administered intra-PrL infusions of the PKMζ inhibitor ZIP, the GluA2-dependent endocytosis inhibitor GluA2_3Y or the inactive peptide GluA2_3Y(s), either two or twenty days after conditioning, and assessed long-term memory retention twenty-four hours later. We found that acute inhibition of GluA2-dependent AMPAR endocytosis in the PrL does not affect recent or remote contextual fear memory maintenance. Also, PKMζ inhibition in the PrL does not impair the maintenance of recent contextual fear memory. However, we found that inhibition of prelimbic PKMζ at a remote time point disrupted contextual fear memory maintenance, and that blocking GluA2-dependent removal of AMPARs prevents this impairment. Our results confirm the central role of PrL in fear memory and identify PKMζ-induced inhibition of GluA2-containing AMPAR endocytosis as a key mechanism governing remote contextual fear memory maintenance.