Creating a false memory in the hippocampus

DNA Methylation in Long-Term Memory

Understanding the neural mechanisms of memory has been one of the key questions in biology. Long-term memory, specifically, allows one to travel mentally without constraints of time and space. A long-term memory must have gone through a series of temporal processes: encoding, consolidation, storage, and retrieval. Decades of studies have revealed cellular and molecular mechanisms underlying each process. In this article, we first review the emerging concept of memory engrams and technologies of engram labeling, as these methods provide a new avenue to study the molecular mechanisms for memory. Then, we focus on DNA methylation and its role in long-term memory. Finally, we discuss some key remaining questions in this field and their implications in memory-related disease.

Not the same as it ever was: A review of memory modification, updating, and distortion in humans and rodents

Memory is a reconstructive and continuous process that enables existing information to be modified in response to a changing environment. Being able to dynamically update outdated memories is critical to an organism's survival. Memory modifications have been extensively studied in both rodents and humans, and prior work has revealed many regional, cellular, neurotransmitter, and subcellular molecular mechanisms underlying this process. However, these diverse bodies of literature have not yet been fully integrated into a comprehensive cross-species review. Integrating the finding across rodent and human work is important for furthering our understanding of memory modifications and the underlying neural mechanisms that support memory modification in both species. Here, we discuss advances in our understanding of adaptive and maladaptive memory modifications in terms of both underlying mechanisms (regional, cellular, and molecular) and behavioral outcomes. By emphasizing findings from both humans and rodents, the two major model systems in which memory modifications have been studied, we are able to highlight converging mechanisms and point to open questions in the field. Specifically, we discuss the major findings from several memory paradigms including declarative, aversive and procedural memory designs and highlight paradigms and models that have been readily translated between rodent and human models. Ultimately, this review identifies key parallels underlying memory updating across species, paradigms, tasks, and models.

Neuron-astrocyte associative memory

Astrocytes, the most abundant type of glial cell, play a fundamental role in memory. Despite most hippocampal synapses being contacted by an astrocyte, there are no current theories that explain how neurons, synapses, and astrocytes might collectively contribute to memory function. We demonstrate that fundamental aspects of astrocyte morphology and physiology naturally lead to a dynamic, high-capacity associative memory system. The neuron-astrocyte networks generated by our framework are closely related to popular machine learning architectures known as Dense Associative Memories. Adjusting the connectivity pattern, the model developed here leads to a family of associative memory networks that includes a Dense Associative Memory and a Transformer as two limiting cases. In the known biological implementations of Dense Associative Memories, the ratio of stored memories to the number of neurons remains constant, despite the growth of the network size. Our work demonstrates that neuron-astrocyte networks follow a superior memory scaling law, outperforming known biological implementations of Dense Associative Memory. Our model suggests an exciting and previously unnoticed possibility that memories could be stored, at least in part, within the network of astrocyte processes rather than solely in the synaptic weights between neurons.

The brain-body circuit mediates acute stress-induced antiinflammatory reflex in bacterial cystitis by suppressing ILC2 activation

Urinary tract infections (UTIs) are one of the most commonly encountered infections in clinical practice, in which psychological stress is a critical pathological contributor to modulate immune function. However, mechanistic pathways linking stress networks in the brain to bladder infection remain poorly understood. In this study, we discovered that acute stress treatment suppressed bladder inflammation in mice with UTIs, and a substantial number of neurons showing overlap between inflammation-associated markers and retrograde labeling were observed in the paraventricular nucleus (PVN) brain region of these mice. Activation of the PVN alleviated uropathogenic Escherichia coli-induced bladder inflammatory response. Moreover, a blocked hypothalamic-pituitary-adrenal axis reversed the antiinflammatory reflex mediated by acute stress, suggesting that glucocorticoids may modulate UTIs through the brain-body circuit. Single-cell RNA-Seq of bladder immune cells revealed that type 2 innate lymphoid (ILC2) cells expressed abundant levels of glucocorticoid receptor. The activation of the PVN effectively inhibited the expression of the pro-inflammatory cytokine colony-stimulating factor 2 by ILC2 cells through direct regulation of cell-intrinsic glucocorticoid signaling. Ultimately, our study has implications for the positioning of the brain-body circuit for UTI treatment.

Evolving Engrams Demand Changes in Effective Cues

A longstanding principle in episodic memory research, known as the encoding specificity hypothesis, holds that an effective retrieval cue should closely match the original encoding conditions. This principle assumes that a successful retrieval cue remains static over time. Despite the broad acceptance of this idea, it conflicts with one of the most well-established findings in memory research: The dynamic and ever-changing nature of episodic memories. In this article, we propose that the most effective retrieval cue should engage with the current state of the memory, which may have shifted significantly since encoding. By redefining the criteria for successful recall, we challenge a core principle of the field and open new avenues for exploring memory accessibility, offering fresh insights into both theoretical, and applied domains.

On the origin of memory neurons in the human hippocampus

The hippocampus is essential for episodic memory, yet its coding mechanism remains debated. In humans, two main theories have been proposed: one suggests that concept neurons represent specific elements of an episode, while another posits a conjunctive code, where index neurons code the entire episode. Here, we integrate new findings of index neurons in humans and other animals with the concept-specific memory framework, proposing that concept neurons evolve from index neurons through overlapping memories. This process is supported by engram literature, which posits that neurons are allocated to a memory trace based on excitability and that reactivation induces excitability. By integrating these insights, we connect two historically disparate fields of neuroscience: engram research and human single neuron episodic memory research.

EPSILON: a method for pulse-chase labeling to probe synaptic AMPAR exocytosis during memory formation

A tool to map changes in synaptic strength during a defined time window could provide powerful insights into the mechanisms of learning and memory. Here we developed a technique, Extracellular Protein Surface Labeling in Neurons (EPSILON), to map α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) exocytosis in vivo by sequential pulse-chase labeling of surface AMPARs with membrane-impermeable dyes. This approach yields synaptic-resolution maps of AMPAR exocytosis, a proxy for synaptic potentiation, in genetically targeted neurons during memory formation. In mice undergoing contextual fear conditioning, we investigated the relationship between synapse-level AMPAR exocytosis in CA1 pyramidal neurons and cell-level expression of the immediate early gene product cFos, a frequently used marker of engram neurons. We observed a strong correlation between AMPAR exocytosis and cFos expression, suggesting a synaptic mechanism for the association of cFos expression with memory engrams. The EPSILON technique is a useful tool for mapping synaptic plasticity and may be extended to investigate trafficking of other transmembrane proteins.

Cold memories control whole-body thermoregulatory responses

Environmental thermal challenges trigger the brain to coordinate both autonomic and behavioural responses to maintain optimal body temperature1-4. It is unknown how temperature information is precisely stored and retrieved in the brain and how it is converted into a physiological response. Here we investigated whether memories could control whole-body metabolism by training mice to remember a thermal challenge. Mice were conditioned to associate a context with a specific temperature by combining thermoregulatory Pavlovian conditioning with engram-labelling technology, optogenetics and chemogenetics. We report that if mice are returned to an environment in which they previously experienced a 4 °C cold challenge, they increase their metabolic rates regardless of the actual environmental temperature. Furthermore, we show that mice have increased hypothalamic activity when they are exposed to the cold, and that a specific network emerges between the hippocampus and the hypothalamus during the recall of a cold memory. Both natural retrieval and artificial reactivation of cold-sensitive memory engrams in the hippocampus mimic the physiological responses that are seen during a cold challenge. These ensembles are necessary for cold-memory retrieval. These findings show that retrieval of a cold memory causes whole-body autonomic and behavioural responses that enable mice to maintain thermal homeostasis.

Serotonin and neurotensin inputs in the vCA1 dictate opposing social valence

The ability to evaluate valence of a social agent based on social experience is essential for an animal's survival in its social group1. Although hippocampal circuits have been implicated in distinguishing novel and familiar conspecifics2-7, it remains unclear how social valence is constructed on the basis of social history and what mechanisms underlie the heightened valence versatility in dynamic relationships. Here we demonstrate that the ventral (v)CA1 integrates serotonin (5-HT) inputs from the dorsal raphe and neurotensin inputs from the paraventricular nucleus of the thalamus (PVT) to determine positive or negative valence of conspecific representations. Specifically, during an appetitive social interaction 5-HT is released into the vCA1 and disinhibits pyramidal neurons through 5-HT1B receptors, whereas neurotensin is released during an aversive social interaction and potentiates vCA1 neurons directly through NTR1s. Optogenetic silencing of dorsal raphe 5-HT and PVT neurotensin inputs into the vCA1 impairs positive and negative social valence, respectively, and excitation flexibly switches valence assignment. These results show how aversive and rewarding social experiences are linked to conspecific identity through converging dorsal raphe 5-HT and PVT neurotensin signals in the vCA1 that instruct opposing valence, and represent a synaptic switch for flexible social valence computation.

An improved FLARE system for recording and manipulating neuronal activity

To address the need for methods for tagging and manipulating neuronal ensembles underlying specific behaviors, we present an improved version of FLARE, termed cytoFLARE (cytosol-expressed FLARE). cytoFLARE incorporates cytosolic tethering of a transcription factor and expression of a more sensitive pair of calcium-sensing domains. We show that cytoFLARE captures more calcium- and light-dependent signals in HEK293T cells and higher signal-to-background ratios in neuronal cultures. We further establish cytoFLARE transgenic Drosophila models and apply cytoFLARE to label activated neurons upon sensory or optogenetic stimulation within a defined time window. Notably, through the cytoFLARE-driven expression of optogenetic actuators, we successfully reactivated and inhibited neurons involved in the larval nociceptive system. Our findings demonstrate the characterization and application of time-gated calcium integrators for both recording and manipulating neuronal activity in Drosophila larvae.

Somatostatin neurons detect stimulus-reward contingencies to reduce neocortical inhibition during learning

Learning involves the association of discrete events in the world to infer causality, likely through a cascade of changes at input- and target-specific synapses. Transient or sustained disinhibition may initiate cortical circuit plasticity important for association learning, but the cellular networks involved have not been well defined. Using recordings in acute brain slices, we show that whisker-dependent sensory association learning drives a durable, target-specific reduction in inhibition from somatostatin (SST)-expressing GABAergic neurons onto pyramidal (Pyr) neurons in superficial but not deep layers of mouse somatosensory cortex. Critically, SST output was not altered when stimuli and rewards were unpaired, indicating that these neurons are sensitive to stimulus-reward contingency. Depression of SST output onto Pyr neurons could be phenocopied by chemogenetic suppression of SST activity outside of the training context. Thus, neocortical SST neuron output can undergo long-lasting modifications to selectively disinhibit superficial layers of sensory neocortex during learning.

The molecular and cellular basis of memory engrams: Mechanisms of synaptic and systems consolidation

The capacity to record and store life experiences for periods ranging from days to a lifetime is what allows an individual to adapt and survive. Memory consolidation is the process that drives the stabilization and long-term storage of memory and takes place at two levels - synaptic and systems. Recently, several studies have provided insight into the processes that drive synaptic and systems consolidation through the characterization of the molecular, functional and structural changes of memory engram cells at distinct time points of the memory consolidation process. In this review we summarize and discuss these recent findings that have allowed a significant step forward in our understanding of how episodic memory is formed and stored in engram cells of the hippocampus and the medial prefrontal cortex.

Reactivation of memory-associated neurons induces downstream suppression of competing neuronal populations

Inducing apparent memory recall by tagging and optogenetically reactivating cells in the hippocampus was demonstrated over a decade ago. However, the hippocampal dynamics resulting from this reactivation remain largely unknown. While calcium imaging is commonly used as a measure of neuronal activity, GCaMP, the most common calcium indicator, cannot be used with optogenetic neuronal reactivation because both require blue light excitation. To resolve this overlap, we demonstrate optogenetic reactivation with a red-shifted opsin, ChrimsonR. We then conduct dual-color calcium imaging in CA1 during memory reactivation in DG. In addition to measuring population dynamics in CA1, CA1 cells tagged during the original experience were identified. In the fear-conditioned animals (FC+), nontagged cells in CA1 decreased their firing rate during stimulation, while tagged cells maintained their activity level. In the FC+ animals, as the behavioral effect of stimulation decreased across days, so did the changes in neural activity during stimulation. Our results both demonstrate the technical feasibility of calcium imaging during optogenetic reactivation of memory-associated neurons and advance our understanding of the dynamics underlying this reactivation.

Integrating Aversive Memories in the Basolateral Amygdala

Decades of research have established a critical role of the basolateral complex of the amygdala (BLA) in the encoding and storage of aversive memories. Much of this work has utilized Pavlovian fear conditioning procedures in which animals experience a single aversive event. Although this effort has produced great insight into the neural mechanisms that support fear memories for an isolated aversive experience, much less is known about how amygdala circuits encode and integrate multiple emotional experiences. The emergence of methods to label and record neuronal ensembles over days allows a deeper understanding of how amygdala neurons encode and integrate distinct aversive episodes over time. Here, we review evidence that the BLA is an essential site for the persistent storage of long-term fear memory. As a long-term storage site for fear memory, a challenge for encoding multiple fear memories is the mechanisms by which BLA neurons allocate, integrate, and discriminate distinct experiences from one another. In this review, we discuss the historical evidence supporting the BLA as a critical site for long-term memory storage, as well as new evidence that stems from technological advances that allow researchers to simultaneously study the encoding and storage of multiple memory traces, including recent versus remote experiences. We explore the possibility that dysfunction in ensemble coding schemes contributes to the pathophysiology of posttraumatic stress disorder and argue that future studies should place increased emphasis on potential subregional differences in memory coding schemes in the amygdala to deepen our understanding of both normal and pathological emotional memory.

Key-value memory in the brain

Classical models of memory in psychology and neuroscience rely on similarity-based retrieval of stored patterns, where similarity is a function of retrieval cues and the stored patterns. Although parsimonious, these models do not allow distinct representations for storage and retrieval, despite their distinct computational demands. Key-value memory systems, in contrast, distinguish representations used for storage (values) and those used for retrieval (keys). This allows key-value memory systems to optimize simultaneously for fidelity in storage and discriminability in retrieval. We review the computational foundations of key-value memory, its role in modern machine-learning systems, related ideas from psychology and neuroscience, applications to a number of empirical puzzles, and possible biological implementations.

GluA1-containing AMPA receptors are necessary for sparse memory engram formation

Memory formation depends on the selective recruitment of neuronal ensembles into circuits known as engrams, which represent the physical substrate of memory. Sparse encoding of these ensembles is essential for memory specificity and efficiency. AMPA receptor (AMPAR) subunits, particularly GluA1, play a central role in synaptic plasticity, which underpins memory encoding. This study investigates how GluA1 expression influences the recruitment of neurons into memory engrams. Using global GluA1 knockout (GluA1KO) mice, localized knockout models, and contextual fear-conditioning paradigms, we evaluated the role of GluA1 in memory formation and engram sparsity. GluA1KO mice exhibited impaired short-term memory retention but preserved 24-hour contextual memory. Despite this, these mice displayed increased expression of the immediate early gene Arc in hippocampal neurons, indicative of a denser engram network. Electrophysiological analyses revealed reduced synaptic strength in GluA1-deficient neurons, irrespective of Arc expression. Localized GluA1 knockout in the hippocampus confirmed that GluA1 deficiency increases neuronal recruitment into engrams, disrupting the sparse encoding typically observed in wild-type mice. These findings demonstrate that GluA1-containing AMPARs constrain engram size, ensuring selective recruitment of neurons for efficient memory encoding. By regulating synaptic plasticity, GluA1 facilitates both the encoding and size of memory circuits. This study highlights the critical role of GluA1 in maintaining sparse engram formation and provides insight into mechanisms underlying memory deficits in conditions where synaptic composition is altered.

Brain-wide immunolabeling and tissue clearing applications for engram research

In recent years, there has been significant progress in memory research, driven by genetic and imaging technological advances that have given unprecedented access to individual memory traces or engrams. Although Karl Lashley argued since the 1930s that an engram is not confined to a particular area but rather distributed across the entire brain, most current studies have focused exclusively on a single or few brain regions. However, this compartmentalized approach overlooks the interactions between multiple brain regions, limiting our understanding of engram mechanisms. More recently, several studies have begun to investigate engrams across the brain, but research is still limited by a lack of standardized techniques capable of reconstructing multiple ensembles at single-cell resolution across the entire brain. In this review, we guide researchers through the latest technological advancements and discoveries in immediate early gene (IEG) techniques, tissue clearing methods, microscope modalities, and automated large-scale analysis. These innovations could propel the field forward in building brain-wide engram maps of normal and disease states, thus, providing unprecedented new insights. Ultimately, this review aims to bridge the gap between research focused on single brain regions and the need for a comprehensive understanding of whole-brain engrams, revealing new approaches for exploring the neuronal mechanisms underlying engrams.

Protein design accelerates the development and application of optogenetic tools

Optogenetics has substantially enhanced our understanding of biological processes by enabling high-precision tracking and manipulation of individual cells. It relies on photosensitive proteins to monitor and control cellular activities, thereby paving the way for significant advancements in complex system research. Photosensitive proteins play a vital role in the development of optogenetics, facilitating the establishment of cutting-edge methods. Recent breakthroughs in protein design have opened up opportunities to develop protein-based tools that can precisely manipulate and monitor cellular activities. These advancements will significantly accelerate the development and application of optogenetic tools. This article emphasizes the pivotal role of protein design in the development of optogenetic tools, offering insights into potential future directions. We begin by providing an introduction to the historical development and fundamental principles of optogenetics, followed by an exploration of the operational mechanisms of key photosensitive domains, which includes clarifying the conformational changes they undergo in response to light, such as allosteric modulation and dimerization processes. Building on this foundation, we reveal the development of protein design tools that will enable the creation of even more sophisticated optogenetic techniques.

An improved FLARE system for recording and manipulating neuronal activity

Recording and manipulating neuronal ensembles that underlie cognition and behavior in vivo is challenging. FLARE is a light- and calcium-gated transcriptional reporting system for labeling activated neurons on the order of minutes. However, FLARE is limited by its sensitivity to prolonged neuronal activities. Here, we present an improved version of FLARE, termed cytoFLARE. cytoFLARE incorporates cytosolic expression of the transcription factor and a more sensitive pair of calcium sensing domains. We showed that cytoFLARE provides more calcium- and light- dependent signals in HEK293T cells and higher signal-to-background ratios in neuronal cultures. We further established cytoFLARE transgenic Drosophila models and applied cytoFLARE to label activated neurons upon sensory or optogenetic stimulation within a defined time window. Notably, through cytoFLARE-driven expression of an optogenetic actuator, we successfully reactivated neurons involved in the larval nociceptive system. Our findings demonstrate the first characterization and application of time-gated calcium integrators in Drosophila.

Learning-associated astrocyte ensembles regulate memory recall

The physical manifestations of memory formation and recall are fundamental questions that remain unresolved1. At the cellular level, ensembles of neurons called engrams are activated by learning events and control memory recall1-5. Astrocytes are found in close proximity to neurons and engage in a range of activities that support neurotransmission and circuit plasticity6-10. Moreover, astrocytes exhibit experience-dependent plasticity11-13, although whether specific ensembles of astrocytes participate in memory recall remains obscure. Here we show that learning events induce c-Fos expression in a subset of hippocampal astrocytes, and that this subsequently regulates the function of the hippocampal circuit in mice. Intersectional labelling of astrocyte ensembles with c-Fos after learning events shows that they are closely affiliated with engram neurons, and reactivation of these astrocyte ensembles stimulates memory recall. At the molecular level, learning-associated astrocyte (LAA) ensembles exhibit elevated expression of nuclear factor I-A, and its selective deletion from this population suppresses memory recall. Taken together, our data identify LAA ensembles as a form of plasticity that is sufficient to provoke memory recall and indicate that astrocytes are an active component of the engram.

Epigenetics in Learning and Memory

In animals, memory formation and recall are essential for their survival and for adaptations to a complex and often dynamically changing environment. During memory formation, experiences prompt the activation of a selected and sparse population of cells (engram cells) that undergo persistent physical and/or chemical changes allowing long-term memory formation, which can last for decades. Over the past few decades, important progress has been made on elucidating signaling mechanisms by which synaptic transmission leads to the induction of activity-dependent gene regulation programs during the different phases of learning (acquisition, consolidation, and recall). But what are the molecular mechanisms that govern the expression of immediate-early genes (IEGs; c-fos, Npas4) and plasticity-related genes (PRGs; Dlg4/PSD95 and Grin2b/NR2B) in memory ensemble? Studies in relatively simple in vitro and in vivo neuronal model systems have demonstrated that synaptic activity during development, or when induced by chemical stimuli (i.e., cLTP, KCl, picrotoxin), activates the NMDAR-Ca2+-CREB signaling pathway that upregulates gene expression through changes in the epigenetic landscape (i.e., histone marks and DNA methylation) and/or 3D chromatin organization. The data support a model in which epigenetic modifications in promoters and enhancers facilitate the priming and activation of these regulatory regions, hence leading to the formation of enhancer-promoter interactions (EPIs) through chromatin looping. The exploration of whether similar molecular mechanisms drive gene expression in learning and memory has presented notable challenges due to the distinct phases of learning and the activation of only sparse population of cells (the engram). Consequently, such studies demand precise temporal and spatial control. By combining activity-dependent engram tagging strategies (i.e., TRAP mice) with multi-omics analyses (i.e., RNA-seq, ChiP-seq, ATAC-seq, and Hi-C), it has been recently possible to associate changes in the epigenomic landscape and/or 3D genome architecture with transcriptional waves in engram cells of mice subjected to contextual fear conditioning (CFC), a relevant one-shot Pavlovian learning task. These studies support the role of specific epigenetic mechanisms and of the 3D chromatin organization during the control of gene transcription waves in engram cells. Advancements in our comprehension of the molecular mechanisms driving memory ensemble will undoubtedly play a crucial role in the development of better-targeted strategies to tackle cognitive diseases, including Alzheimer's disease and frontotemporal dementia, among other information-processing disorders.

Dysfunctional S1P/S1PR1 signaling in the dentate gyrus drives vulnerability of chronic pain-related memory impairment

Memory impairment in chronic pain patients is substantial and common, and few therapeutic strategies are available. Chronic pain-related memory impairment has susceptible and unsusceptible features. Therefore, exploring the underlying mechanisms of its vulnerability is essential for developing effective treatments. Here, combining two spatial memory tests (Y-maze test and Morris water maze), we segregated chronic pain mice into memory impairment-susceptible and -unsusceptible subpopulations in a chronic neuropathic pain model induced by chronic constrictive injury of the sciatic nerve. RNA-Seq analysis and gain/loss-of-function study revealed that S1P/S1PR1 signaling is a determinant for vulnerability to chronic pain-related memory impairment. Knockdown of the S1PR1 in the dentate gyrus (DG) promoted a susceptible phenotype and led to structural plasticity changes of reduced excitatory synapse formation and abnormal spine morphology as observed in susceptible mice, while overexpression of the S1PR1 and pharmacological administration of S1PR1 agonist in the DG promoted an unsusceptible phenotype and prevented the occurrence of memory impairment, and rescued the morphological abnormality. Finally, the Gene Ontology (GO) enrichment analysis and biochemical evidence indicated that downregulation of S1PR1 in susceptible mice may impair DG structural plasticity via interaction with actin cytoskeleton rearrangement-related signaling pathways including Itga2 and its downstream Rac1/Cdc42 signaling and Arp2/3 cascade. These results reveal a novel mechanism and provide a promising preventive and therapeutic molecular target for vulnerability to chronic pain-related memory impairment.

Abnormal c-Fos expression in TetTag mice containing fos-EGFP

Molecular and genetic techniques now allow selective tagging and manipulation of the population of neurons, often referred to as "engram cells," that were active during a specific experience. One common approach to labeling these cells is to use the fos-tTA transgenic mouse (TetTag). In addition to tagging cells active during learning, it is common to examine the reactivation of these cells using immediate early gene (IEG) expression as an index of neural activity. There are currently multiple TetTag lines available. The original line, cryopreserved at MMRRC, contains only the fos-tTA transgene, while Jackson Labs provides a version of the mouse that expresses both the fos-tTA and fos-shEGFP genes. In the current experiments, we examined IEG expression in these two mouse lines. Unexpectedly, we found that Jackson fos-tTA/fos-shEGFP mice express increased levels of c-Fos in the hippocampus compared to wild type animals when examined with immunohistochemistry (IHC). The expression of other IEGs, such as Arc and Egr-1, was not elevated in these mice, suggesting that the overexpression of c-Fos is not the result of increased excitability or broad changes in gene expression. qPCR revealed that Jackson fos-tTA/fos-shEGFP mice express mRNA corresponding to a c-Fos-Exon1-GFP fusion molecule, which may bind to C-Fos antibodies during IHC and inflate apparent c-Fos expression. Jackson fos-tTA/fos-shEGFP mice did not differ from their wild-type counterparts in fear expression or memory, indicating no behavioral effect of the presence of a c-Fos-GFP fusion protein. These results identify a major limitation inherent in the use of Jackson fos-tTA/fos-shEGFP mice.

NMDA Receptors Control Activity Hierarchy in Neural Network: Loss of Control in Hierarchy Leads to Learning Impairments, Dissociation, and Psychosis

While it is known that associative memory is preferentially encoded by memory-eligible "primed" neurons, in vivo neural activity hierarchy has not been quantified and little is known about how such a hierarchy is established. Leveraging in vivo calcium imaging of hippocampal neurons on freely behaving mice, we developed the first method to quantify real-time neural activity hierarchy in the CA1 region. Neurons at the top of activity hierarchy are identified as primed neurons. In cilia knockout mice that exhibit severe learning deficits, the percentage of primed neurons is drastically reduced. We developed a simplified neural network model that incorporates simulations of linear and non-linear weighted components, modeling the synaptic ionic conductance of AMPA and NMDA receptors, respectively. We found that moderate non-linear to linear conductance ratios naturally leads a small fraction of neurons to be primed in the simulated neural network. Removal of the non-linear component eliminates the existing activity hierarchy and reinstate it to the network stochastically primes a new pool of neurons. Blockade of NMDA receptors by ketamine not only decreases general neuronal activity causing learning impairments, but also disrupts neural activity hierarchy. Additionally, ketamine-induced super-synchronized slow oscillation during anesthesia can be simulated if the non-linear NMDAR component is removed to flatten activity hierarchy. Together, this study develops a unique method to measure neural activity hierarchy and identifies NMDA receptors as a key factor that controls the hierarchy. It presents the first evidence suggesting that hierarchy disruption by NMDAR blockade causes dissociation and psychosis.

Deconstruction of a memory engram reveals distinct ensembles recruited at learning

How are associative memories formed? Which cells represent a memory, and when are they engaged? By visualizing and tagging cells based on their calcium influx with unparalleled temporal precision, we identified non-overlapping dorsal CA1 neuronal ensembles that are differentially active during associative fear memory acquisition. We dissected the acquisition experience into periods during which salient stimuli were presented or certain mouse behaviors occurred and found that cells associated with specific acquisition periods are sufficient alone to drive memory expression and contribute to fear engram formation. This study delineated the different identities of the cell ensembles active during learning, and revealed, for the first time, which ones form the core engram and are essential for memory formation and recall.

A small population of stress-responsive neurons in the hypothalamus-habenula circuit mediates development of depression-like behavior in mice

Accumulating evidence has shown that various brain functions are associated with experience-activated neuronal ensembles. However, whether such neuronal ensembles are engaged in the pathogenesis of stress-induced depression remains elusive. Utilizing activity-dependent viral strategies in mice, we identified a small population of stress-responsive neurons, primarily located in the middle part of the lateral hypothalamus (mLH) and the medial part of the lateral habenula (LHbM). These neurons serve as "starter cells" to transmit stress-related information and mediate the development of depression-like behaviors during chronic stress. Starter cells in the mLH and LHbM form dominant connections, which are selectively potentiated by chronic stress. Silencing these connections during chronic stress prevents the development of depression-like behaviors, whereas activating these connections directly elicits depression-like behaviors without stress experience. Collectively, our findings dissect a core functional unit within the LH-LHb circuit that mediates the development of depression-like behaviors in mice.

Update on "Emotion and autobiographical memory": 14 years of advances in understanding functions, constructions, and consequences

Holland and Kensinger (2010) reviewed the literature on "Emotion and autobiographical memory." They focused on two broad ways that emotions influence memory: (1) emotion during an event influences how the event is remembered, and (2) emotion and emotional goals during memory retrieval influence how past events are remembered. We begin by providing a brief update on the key points from that review. Holland and Kensinger (2010) also had noted a number of important avenues for future work. Here, we describe what has been learned about the functions of autobiographical memory and their reconstructive nature. Relatedly, we review more recent research on memory reconstruction in the context of visual perspective shifts, counterfactual thinking, nostalgia, and morality. This research has emphasized the reciprocal nature of the interactions between emotion and autobiographical memory: Not only do emotions influence memory, memories influence emotions. Next, we discuss advances that have been made in understanding the reciprocal relations between stress, mood, and autobiographical memory. Finally, we discuss the research that is situating emotional autobiographical memories within a social framework, providing a bedrock for collective memories. Despite the many advances of the past 14 years, many open questions remain; throughout the review we note domains in which we hope to see advances over the next decades.

Clustering of synaptic engram: Functional and structural basis of memory

Studies on memory engram have demonstrated how experience and learning can be allocated at a neuronal level for centuries. Recently emerging evidence narrowed down further to the synaptic connections and their patterned allocation on dendrites. Notably, groups of synapses within a specific range within dendrites known as 'synaptic clusters' have been revealed in association with learning and memory. Previous investigations have shown that a variety of factors mediated by both presynaptic inputs and postsynaptic dendrites contribute to clustering. Here, we review the neural mechanism of synaptic clustering and its correlation with memory. We highlight the recent findings about the clustering of synaptic engrams and memory formation and discuss future directions.

A contextual fear conditioning paradigm in head-fixed mice exploring virtual reality

Contextual fear conditioning is a classical laboratory task that tests associative memory formation and recall. Techniques such as multi-photon microscopy and holographic stimulation offer tremendous opportunities to understand the neural underpinnings of these memories. However, these techniques generally require animals to be head-fixed. There are few paradigms that test contextual fear conditioning in head-fixed mice, and none where the behavioral outcome following fear conditioning is freezing, the most common measure of fear in freely moving animals. To address this gap, we developed a contextual fear conditioning paradigm in head-fixed mice using virtual reality (VR) environments. We designed an apparatus to deliver tail shocks (unconditioned stimulus, US) while mice navigated a VR environment (conditioned stimulus, CS). The acquisition of contextual fear was tested when the mice were reintroduced to the shock-paired VR environment the following day. We tested three different variations of this paradigm and, in all of them, observed an increased conditioned fear response characterized by increased freezing behavior. This was especially prominent during the first trial in the shock-paired VR environment, compared to a neutral environment where the mice received no shocks. Our results demonstrate that head-fixed mice can be fear conditioned in VR, discriminate between a feared and neutral VR context, and display freezing as a conditioned response, similar to freely behaving animals. Furthermore, using a two-photon microscope, we imaged from large populations of hippocampal CA1 neurons before, during, and following contextual fear conditioning. Our findings reconfirmed those from the literature on freely moving animals, showing that CA1 place cells undergo remapping and show narrower place fields following fear conditioning. Our approach offers new opportunities to study the neural mechanisms underlying the formation, recall, and extinction of contextual fear memories. As the head-fixed preparation is compatible with multi-photon microscopy and holographic stimulation, it enables long-term tracking and manipulation of cells throughout distinct memory stages and provides subcellular resolution for investigating axonal, dendritic, and synaptic dynamics in real-time.

Natural forgetting reversibly modulates engram expression

Memories are stored as ensembles of engram neurons and their successful recall involves the reactivation of these cellular networks. However, significant gaps remain in connecting these cell ensembles with the process of forgetting. Here, we utilized a mouse model of object memory and investigated the conditions in which a memory could be preserved, retrieved, or forgotten. Direct modulation of engram activity via optogenetic stimulation or inhibition either facilitated or prevented the recall of an object memory. In addition, through behavioral and pharmacological interventions, we successfully prevented or accelerated forgetting of an object memory. Finally, we showed that these results can be explained by a computational model in which engrams that are subjectively less relevant for adaptive behavior are more likely to be forgotten. Together, these findings suggest that forgetting may be an adaptive form of engram plasticity which allows engrams to switch from an accessible state to an inaccessible state.

Extinction training suppresses activity of fear memory ensembles across the hippocampus and alters transcriptomes of fear-encoding cells

Contextual fear conditioning has been shown to activate a set of "fear ensemble" cells in the hippocampal dentate gyrus (DG) whose reactivation is necessary and sufficient for expression of contextual fear. We previously demonstrated that extinction learning suppresses reactivation of these fear ensemble cells and activates a competing set of DG cells-the "extinction ensemble." Here, we tested whether extinction was sufficient to suppress reactivation in other regions and used single nucleus RNA sequencing (snRNA-seq) of cells in the dorsal dentate gyrus to examine how extinction affects the transcriptomic activity of fear ensemble and fear recall-activated cells. Our results confirm the suppressive effects of extinction in the dorsal and ventral dentate gyrus and demonstrate that this same effect extends to fear ensemble cells located in the dorsal CA1. Interestingly, the extinction-induced suppression of fear ensemble activity was not detected in ventral CA1. Our snRNA-seq analysis demonstrates that extinction training markedly changes transcription patterns in fear ensemble cells and that cells activated during recall of fear and recall of extinction have distinct transcriptomic profiles. Together, our results indicate that extinction training suppresses a broad portion of the fear ensemble in the hippocampus, and this suppression is accompanied by changes in the transcriptomes of fear ensemble cells and the emergence of a transcriptionally unique extinction ensemble.

Dopamine-mediated formation of a memory module in the nucleus accumbens for goal-directed navigation

Spatial memories guide navigation efficiently toward desired destinations. However, the neuronal and circuit mechanisms underlying the encoding of goal locations and its translation into goal-directed navigation remain unclear. Here we demonstrate that mice rapidly form a spatial memory of a shelter during shelter experiences, guiding escape behavior toward the goal location-a shelter-when under threat. Dopaminergic neurons in the ventral tegmental area and their projection to the nucleus accumbens (NAc) encode safety signals associated with the shelter. Optogenetically induced phasic dopamine signals are sufficient to create a place memory that directs escape navigation. Converging dopaminergic and hippocampal glutamatergic inputs to the NAc mediate the formation of a goal-related memory within a subpopulation of NAc neurons during shelter experiences. Artificial co-activation of this goal-related NAc ensemble with neurons in the dorsal periaqueductal gray was sufficient to trigger memory-guided, rather than random, escape behavior. These findings provide causal evidence of cognitive circuit modules linking memory with goal-directed action.

Emerging many-to-one weighted mapping in hippocampus-amygdala network underlies memory formation

Memories are crucial for daily life, yet the network-level organizing principles governing neural representations of experiences remain unknown. Employing dual-site in vivo recording in freely behaving male mice, here we show that hippocampal dorsal CA1 (dCA1) and basolateral amygdala (BLA) utilize distinct coding strategies for novel experiences. A small assembly of BLA neurons emerged active during memory acquisition and persisted through consolidation, whereas most dCA1 neurons were engaged in both processes. Machine learning decoding revealed that dCA1 population spikes predicted BLA assembly firing rate, suggesting that most dCA1 neurons concurrently index an episodic event by rapidly establishing weighted communication with a specific BLA assembly - a process we term "many-to-one weighted mapping." We also found that dCA1 reactivations preceded BLA assembly activity preferably during elongated and enlarged dCA1 ripples. Using a closed-loop strategy, we demonstrated that suppressing BLA activity after large dCA1 ripples impaired memory. These findings highlight a many-to-one weighted mapping mechanism underlying both the acquisition and consolidation of new memories.

Involvement of a lateral entorhinal cortex engram in episodic-like memory recall

Episodic memory relies on the entorhinal cortex (EC), a crucial hub connecting the hippocampus and sensory processing regions. This study investigates the role of the lateral EC (LEC) in episodic-like memory in mice. Here, we employ the object-place-context-recognition task (OPCRT), a behavioral test used to study episodic-like memory in rodents. Electrophysiology in brain slices reveals that OPCRT specifically induces a shift in the threshold for the induction of synaptic plasticity in LEC superficial layer II. Additionally, a dual viral system is used to express chemogenetic receptors coupled to the c-Fos promoter in neurons recruited during the learning. We demonstrate that the inhibition of LEC neurons impairs the performance of the mice in the memory task, while their stimulation significantly facilitates memory recall. Our findings provide evidence for an episodic-like memory engram in the LEC and emphasize its role in memory processing within the broader network of episodic memory.

Ketamine ameliorates post-traumatic social avoidance by erasing the traumatic memory encoded in VTA-innervated BLA engram cells

Erasing traumatic memory during memory reconsolidation is a promising retrieval-extinction strategy for post-traumatic stress disorder (PTSD). Here, we developed an acute social defeat stress (SDS) mouse model with short-term and re-exposure-evoked long-term social avoidance. SDS-associated traumatic memories were identified to be stored in basolateral amygdala (BLA) engram cells. A single intraperitoneal administration of subanesthetic-dose ketamine within, but not beyond, the re-exposure time window significantly alleviates SDS-induced social avoidance, which reduces the activity and quantity of reactivated BLA engram cells. Furthermore, activation or inhibition of dopaminergic projections from the ventral tegmental area to the BLA effectively mimics or blocks the therapeutic effect of re-exposure with ketamine and is dopamine D2 receptor dependent. Single-cell RNA sequencing reveals that re-exposure with ketamine triggered significant changes in memory-related pathways in the BLA. Together, our research advances the understanding of how ketamine mitigates PTSD symptoms and offers promising avenues for developing more effective treatments for trauma-related disorders.

Reactivation-dependent transfer of fear memory between contexts requires M1 muscarinic receptor stimulation in dorsal hippocampus of male rats

Memory updating is essential for integrating new information into existing representations. However, this process could become maladaptive in conditions like post-traumatic stress disorder (PTSD), when fear memories generalize to neutral contexts. Previously, we have shown that contextual fear memory malleability in rats requires activation of M1 muscarinic acetylcholine receptors in the dorsal hippocampus. Here, we investigated the involvement of this mechanism in the transfer of contextual fear memories to other contexts using a novel fear memory updating paradigm. Following brief reexposure to a previously fear conditioned context, male rats (n = 8-10/group) were placed into a neutral context to evaluate the transfer of fear memory. We also infused the selective M1 receptor antagonist pirenzepine into the dorsal hippocampus before memory reactivation to try to block this effect. Results support the hypothesis that fear memory can be updated with novel contextual information, but only if rats are reexposed to the originally trained context relatively recently before the neutral context; evidence for transfer was not seen if the fear memory reactivation was omitted or if it occurred 6 h before neutral context exposure. The transferred fear persisted for 4 weeks, and the effect was blocked by M1 antagonism. These findings strongly suggest that fear transfer requires reactivation and destabilization of the original fear memory. The novel preclinical model introduced here, and its implication of muscarinic receptors in this process, could therefore inform therapeutic strategies for PTSD and similar conditions.

Neural correlates of learning and memory are altered by early-life stress

The ability to learn and remember, which is fundamental for behavioral adaptation, is susceptible to stressful experiences during the early postnatal period, such as abnormal levels of maternal care. The exact mechanisms underlying these effects still remain elusive. This study examined whether early life stress (ELS) alters memory and brain activation patterns in male mice. Therefore, we examined the expression of the immediate early genes (IEGs) c-Fos and Arc in the dentate gyrus (DG) and basolateral amygdala (BLA) after training and memory retrieval in a fear conditioning task. Furthermore, we examined the potential of RU38486 (RU486), a glucocorticoid receptor antagonist, to mitigate ELS-induced memory deficits by blocking stress signalling during adolescence. Arc::dVenus reporter mice, which allow investigating experience-dependent expression of the immediate early gene Arc also at more remote time points, were exposed to ELS by housing dams and offspring with limited bedding and nesting material (LBN) between postnatal days (PND) 2-9 and trained in a fear conditioning task at adult age. We found that ELS reduced both fear acquisition and contextual memory retrieval. RU486 did not prevent these effects. ELS reduced the number of Arc::dVenus+ cells in DG and BLA after training, while the number of c-Fos+ cells were left unaffected. After memory retrieval, ELS decreased c-Fos+ cells in the ventral DG and BLA. ELS also altered the colocalization of c-Fos+ cells with Arc::dVenus+ cells in the ventral DG, possibly indicating impaired engram allocation in the ventral DG after memory retrieval. In conclusion, this study shows that ELS alters neuronal activation patterns after fear acquisition and retrieval, which may provide mechanistic insights into enduring impact of ELS on the processing of fear memories, possibly via changes in cell (co-) activation and engram cell allocation.

Ion-channel degeneracy and heterogeneities in the emergence of signature physiological characteristics of dentate gyrus granule cells

Complex systems are neither fully determined nor completely random. Biological complex systems, including single neurons, manifest intermediate regimes of randomness that recruit integration of specific combinations of functionally specialized subsystems. Such emergence of biological function provides the substrate for the expression of degeneracy, the ability of disparate combinations of subsystems to yield similar function. Here, we present evidence for the expression of degeneracy in morphologically realistic models of dentate gyrus granule cells (GCs) through functional integration of disparate ion-channel combinations. We performed a 45-parameter randomized search spanning 16 active and passive ion channels, each biophysically constrained by their gating kinetics and localization profiles, to search for valid GC models. Valid models were those that satisfied 17 sub- and suprathreshold cellular-scale electrophysiological measurements from rat GCs. A vast majority (>99%) of the 15,000 random models were not electrophysiologically valid, demonstrating that arbitrarily random ion-channel combinations would not yield GC functions. The 141 valid models (0.94% of 15,000) manifested heterogeneities in and cross-dependencies across local and propagating electrophysiological measurements, which matched with their respective biological counterparts. Importantly, these valid models were widespread throughout the parametric space and manifested weak cross-dependencies across different parameters. These observations together showed that GC physiology could neither be obtained by entirely random ion-channel combinations nor is there an entirely determined single parametric combination that satisfied all constraints. The complexity, the heterogeneities in measurement and parametric spaces, and degeneracy associated with GC physiology should be rigorously accounted for while assessing GCs and their robustness under physiological and pathological conditions.NEW & NOTEWORTHY A recent study from our laboratory had demonstrated pronounced heterogeneities in a set of 17 electrophysiological measurements obtained from a large population of rat hippocampal granule cells. Here, we demonstrate the manifestation of ion-channel degeneracy in a heterogeneous population of morphologically realistic conductance-based granule cell models that were validated against these measurements and their cross-dependencies. Our analyses show that single neurons are complex entities whose functions emerge through intricate interactions among several functionally specialized subsystems.

Synapse device based neuromorphic system for biomedical applications

Despite holding valuable information, unstructured data pose challenges for efficient recognition due to the difficulties in feature extraction using traditional Von-Neumann architecture systems, which are limited by power and time bottlenecks. Although biological neural signals offer crucial insights, they require more effective recognition solutions due to inherent noise and the vast volumes of data. Inspired by the human brain, neuromorphic systems have emerged as promising alternatives because of their parallelism, low power consumption, and error tolerance. By leveraging deep neural networks (DNNs), these systems can recognize imprecise data through two key processes: learning (feature extraction) and testing (feature matching and recognition). During the learning phase, DNNs extract and store unique features such as weight changes in synapse units. In the testing phase, new data are compared with the stored features for recognition. The parallelization of the neuromorphic system enables the efficient processing of large, imprecise datasets with minimal energy consumption. Nevertheless, the hardware implementation is essential for determining the full potential of DNNs. This paper focuses on synapse devices, which are the core units for hardware DNN implementations, and presents a biomedical application example: a rat neural signal recognition system implemented using a synapse device-based neuromorphic system.

The lateral habenula integrates age and experience to promote social transitions in developing rats

Early caregiving adversity (ECA) is associated with social behavior deficits and later development of psychopathology. However, the infant neural substrates of ECA are poorly understood. The lateral habenula (LHb), a highly conserved brain region with consistent links to adult psychopathology, is understudied in development, when the brain is most vulnerable to environmental impacts. Here, we describe the structural and functional ontogeny of the LHb and its behavioral role in infant and juvenile rat pups. We show that the LHb promotes a developmental transition in social approach behavior under threat as typically reared infants mature. By contrast, we show that ECA disrupts habenular ontogeny, including volume, protein expression, firing properties, and corticohabenular connectivity. Furthermore, inhibiting a specific corticohabenular projection rescues infant social approach deficits following ECA. Together, these results identify immediate biomarkers of ECA in the LHb and highlight this region as a site of early social processing and behavior control.

False memories in cuttlefish

Episodic memory is a reconstructive process per se: during an event, the features composing it are encoded and stored separately in the brain, then reconstructed when the event's memory is retrieved. Even with source monitoring processes (e.g., did I see or did I smell it?), some mistakes can occur. These mnemonic mistakes happen especially when different events share several features, producing overlaps difficult to discriminate, leading to the creation of false memories. The common cuttlefish has the ability to remember specific events about what happened where and when, namely episodic-like memory. In order to investigate whether this memory, such as human episodic memory, is based on reconstructive processes, we elaborated a protocol promoting false memory formation. Our results suggest that cuttlefish do form visual false memories, but not olfactory false memories. These memory errors might be the first indication of the presence of reconstructive processes in the memory of cephalopods.

Optogenetic Brain-Computer Interfaces

The brain-computer interface (BCI) is one of the most powerful tools in neuroscience and generally includes a recording system, a processor system, and a stimulation system. Optogenetics has the advantages of bidirectional regulation, high spatiotemporal resolution, and cell-specific regulation, which expands the application scenarios of BCIs. In recent years, optogenetic BCIs have become widely used in the lab with the development of materials and software. The systems were designed to be more integrated, lightweight, biocompatible, and power efficient, as were the wireless transmission and chip-level embedded BCIs. The software is also constantly improving, with better real-time performance and accuracy and lower power consumption. On the other hand, as a cutting-edge technology spanning multidisciplinary fields including molecular biology, neuroscience, material engineering, and information processing, optogenetic BCIs have great application potential in neural decoding, enhancing brain function, and treating neural diseases. Here, we review the development and application of optogenetic BCIs. In the future, combined with other functional imaging techniques such as near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI), optogenetic BCIs can modulate the function of specific circuits, facilitate neurological rehabilitation, assist perception, establish a brain-to-brain interface, and be applied in wider application scenarios.

Dentate gyrus ensembles gate context-dependent neural states and memory retrieval

Memories of events are linked to the contexts in which they were encoded. This contextual linking ensures enhanced access to those memories that are most relevant to the context at hand, including specific associations that were previously learned in that context. This principle, referred to as encoding specificity, predicts that context-specific neural states should bias retrieval of particular associations over others, potentially allowing for the disambiguation of retrieval cues that may have multiple associations or meanings. Using a context-odor paired associate learning paradigm in mice, here, we show that chemogenetic manipulation of dentate gyrus ensembles corresponding to specific contexts reinstates context-specific neural states in downstream CA1 and biases retrieval toward context-specific associations.

Detection of Memory Engrams in Mammalian Neuronal Circuits

It has long been assumed that activity patterns persist in neuronal circuits after they are first experienced, as part of the process of information processing and storage by the brain. However, these "reverberations" of current activity have not been directly observed on a single-neuron level in a mammalian system. Here we demonstrate that specific induced activity patterns are retained in mature cultured hippocampal neuronal networks. Neurons within the network are induced to fire at a single frequency or in a more complex pattern containing two distinct frequencies. After the stimulation was stopped, the subsequent neuronal activity of hundreds of neurons in the network was monitored. In the case of single-frequency stimulation, it was observed that many of the neurons continue to fire at the same frequency that they were stimulated to fire at. Using a recurrent neural network trained to detect specific, more complex patterns, we found that the multiple-frequency stimulation patterns were also retained within the neuronal network. Moreover, it appears that the component frequencies of the more complex patterns are stored in different populations of neurons and neuron subtypes.

Molecular tools to capture active neural circuits

To understand how neurons and neural circuits function during behaviors, it is essential to record neuronal activity in the brain in vivo. Among the various technologies developed for recording neuronal activity, molecular tools that induce gene expression in an activity-dependent manner have attracted particular attention for their ability to clarify the causal relationships between neuronal activity and behavior. In this review, we summarize recently developed activity-dependent gene expression tools and their potential contributions to the study of neural circuits.

Reactivation of encoding ensembles in the prelimbic cortex supports temporal associations

Fear conditioning is encoded by strengthening synaptic connections between the neurons activated by a conditioned stimulus (CS) and those activated by an unconditioned stimulus (US), forming a memory engram, which is reactivated during memory retrieval. In temporal associations, activity within the prelimbic cortex (PL) plays a role in sustaining a short-term, transient memory of the CS, which is associated with the US after a temporal gap. However, it is unknown whether the PL has only a temporary role, transiently representing the CS, or is part of the neuronal ensembles that support the retrieval, i.e., whether PL neurons support both transient, short-term memories and stable, long-term memories. We investigated neuronal ensembles underlying temporal associations using fear conditioning with a 5-s interval between the CS and US (CFC-5s). Controls were trained in contextual fear conditioning (CFC), in which the CS-US overlaps. We used Robust Activity Marking (RAM) to selectively manipulate PL neurons activated by CFC-5s learning and Targeted Recombination in Active Populations (TRAP2) mice to label neurons activated by CFC-5s learning and reactivated by memory retrieval in the amygdala, medial prefrontal cortex, hippocampus, perirhinal cortices (PER) and subiculum. We also computed their co-reactivation to generate correlation-based networks. The optogenetic reactivation or silencing of PL encoding ensembles either promoted or impaired the retrieval of CFC-5s but not CFC. CFC-5s retrieval reactivated encoding ensembles in the PL, PER, and basolateral amygdala. The engram network of CFC-5s had higher amygdala and PER centralities and interconnectivity. The same PL neurons support learning and stable associative memories.

The Pattern of CA1 c-Fos Expressing Place Cells Correlates with Learning Progression

Spatial memory supported by the hippocampus is a critical cognitive function in daily life. It helps us to navigate to salient goal locations. Hippocampal place cells stably represent the spatial location, while c-Fos-expressing cells (engram cells) contain rich information about the environment, contributing to the encoding and retrieving of memories. However, the role of cFos as a gateway to a more profound comprehension of the mechanisms underlying the process of goal-oriented spatial memory remained poorly understood. In this study, we labeled rats' c-Fos+ neurons by Channelrhodopsin-2 during memory encoding of a specific goal location. A subset of place cells whether they expressed c-Fos was identified optogenetically. As the rats learned the new goal locations, the spatial correlation between task periods remained stable for the c-Fos- place cell, while increased for the c-Fos+ place cell. This finding shed light on the processing of goal-oriented spatial memories by the hippocampal network involved in c-Fos. C-Fos- place cells consistently maintained a stable representation of the locations, while c-Fos+ place cells exhibited significant flexibility of the goal.

Retrieval of contextual memory can be predicted by CA3 remapping and is differentially influenced by NMDAR activity in rat hippocampus subregions

Episodic memory is essential to navigate in a changing environment by recalling past events, creating new memories, and updating stored information from experience. Although the mechanisms for acquisition and consolidation have been profoundly studied, much less is known about memory retrieval. Hippocampal spatial representations are key for retrieval of contextually guided episodic memories. Indeed, hippocampal place cells exhibit stable location-specific activity which is thought to support contextual memory, but can also undergo remapping in response to environmental changes. It is unclear if remapping is directly related to the expression of different episodic memories. Here, using an incidental memory recognition task in rats, we showed that retrieval of a contextually guided memory is reflected by the levels of CA3 remapping, demonstrating a clear link between external cues, hippocampal remapping, and episodic memory retrieval that guides behavior. Furthermore, we describe NMDARs as key players in regulating the balance between retrieval and memory differentiation processes by controlling the reactivation of specific memory traces. While an increase in CA3 NMDAR activity boosts memory retrieval, dentate gyrus NMDAR activity enhances memory differentiation. Our results contribute to understanding how the hippocampal circuit sustains a flexible balance between memory formation and retrieval depending on the environmental cues and the internal representations of the individual. They also provide new insights into the molecular mechanisms underlying the contributions of hippocampal subregions to generate this balance.

Differential Activation States of Direct Pathway Striatal Output Neurons during l-DOPA-Induced Dyskinesia Development

l-DOPA-induced dyskinesia (LID) is a debilitating motor side effect arising from chronic dopamine (DA) replacement therapy with l-DOPA for the treatment of Parkinson's disease. LID is associated with supersensitivity of striatal dopaminergic signaling and fluctuations in synaptic DA following each l-DOPA dose, shrinking the therapeutic window. The heterogeneous composition of the striatum, including subpopulations of medium spiny output neurons (MSNs), interneurons, and supporting cells, complicates the identification of cell(s) underlying LID. We used single-nucleus RNA sequencing (snRNA-seq) to establish a comprehensive striatal transcriptional profile during LID development. Male hemiparkinsonian mice were treated with vehicle or l-DOPA for 1, 5, or 10 d, and striatal nuclei were processed for snRNA-seq. Analyses indicated a limited population of DA D1 receptor-expressing MSNs (D1-MSNs) formed three subclusters in response to l-DOPA treatment and expressed cellular markers of activation. These activated D1-MSNs display similar transcriptional changes previously associated with LID; however, their prevalence and transcriptional behavior were differentially influenced by l-DOPA experience. Differentially expressed genes indicated acute upregulation of plasticity-related transcription factors and mitogen-activated protein kinase signaling, while repeated l-DOPA-induced synaptic remodeling, learning and memory, and transforming growth factor-β (TGF-β) signaling genes. Notably, repeated l-DOPA sensitized Inhba, an activin subunit of the TGF-β superfamily, in activated D1-MSNs, and its pharmacological inhibition impaired LID development, suggesting that activin signaling may play an essential role in LID. These data suggest distinct subsets of D1-MSNs become differentially l-DOPA-responsive due to aberrant induction of molecular mechanisms necessary for neuronal entrainment, similar to processes underlying hippocampal learning and memory.