A robust activity marking system for exploring active neuronal ensembles

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.

Modulation of dorsal raphe nucleus connectivity and serotonergic signalling to the insular cortex in the prosocial effects of chronic fluoxetine

Long-term exposure to fluoxetine and other selective serotonin reuptake inhibitors alters social and anxiety-related behaviours, including social withdrawal, which is a symptom of several neuropsychiatric disorders. Adaptive changes in serotonergic neurotransmission likely mediate this delayed effect, although the exact mechanisms are still unclear. Here we investigated the functional circuitry underlying the biphasic effects of fluoxetine on social approach-avoidance behaviour and explored the place of serotonergic dorsal raphe nucleus (DR) ensembles in this network, using c-Fos-immunoreactivity as a correlate of activity. Graph theory-based network analysis revealed changes in patterns of functional connectivity and identified neuronal populations in the insular cortex (IC) and serotonergic populations in the DR as central targets to the prosocial effects of chronic fluoxetine. To determine the role of serotonergic projections to the IC, a retrograde tracer was micro-injected in the IC prior to fluoxetine treatment and social behaviour testing. Chronic fluoxetine increased c-Fos immunoreactivity in insula-projecting neurons of the rostral, ventral part of the DR (DRV). Using a virally delivered Tet-Off platform for temporally-controlled marking of neuronal activation, we observed that chronic fluoxetine may affect social behaviour by influencing independent but interconnected populations of serotonergic DR ensembles. These findings suggest that sustained fluoxetine exposure causes adaptive changes in functional connectivity due to altered serotonergic neurotransmission in DR projection targets, and the increased serotonergic signalling to the IC likely mediates some of the therapeutic effects of fluoxetine on social behaviour.

Distinguishing neural ensembles in the infralimbic cortex that regulate stress vulnerability and coping behavior

Neural ensembles in the medial prefrontal cortex regulate several types of responses to stress. We used a Syrian hamster model to investigate the role of infralimbic (IL) neurons in coping with social defeat stress and vulnerability to subsequent anxiety-like behavior. We created social dominance relationships in male and female hamsters, used a robust activity marker (RAM) approach to label IL neural ensembles activated during social defeat stress, and employed light-dark (LD), social avoidance (SA), and conditioned defeat (CD) tests to assess anxiety-like behavior. We found that dominant animals were less anxious in LD tests compared to subordinate animals after achieving their higher status. Also, status-dependent differences in anxiety-like behavior were maintained following social defeat in males, but not females. Subordinate males showed greater RAM-mKate2 expression in IL parvalbumin (PV) cells during social defeat exposure compared to dominant males, and submissive behavior during CD testing was correlated with RAM/PV co-expression. In contrast, greater RAM-mKate2 expression in IL neurons was correlated with a longer latency to submit during social defeat in dominant females, although the correlation of RAM/PV co-expression and defeat-induced anxiety in females was mixed. Overall, these findings suggest that activation of IL PV cells during social defeat predicts the development stress vulnerability in males, whereas activation of IL neurons is associated with a proactive response to social defeat exposure in females. Understanding how social dominance generates plasticity in IL PV cells should improve our understanding of the mechanisms by which behavioral treatments prior to stress might promote stress resilience.

Regulation of PV interneuron plasticity by neuropeptide-encoding genes

Neuronal activity must be regulated in a narrow permissive band for the proper operation of neural networks. Changes in synaptic connectivity and network activity-for example, during learning-might disturb this balance, eliciting compensatory mechanisms to maintain network function1-3. In the neocortex, excitatory pyramidal cells and inhibitory interneurons exhibit robust forms of stabilizing plasticity. However, although neuronal plasticity has been thoroughly studied in pyramidal cells4-8, little is known about how interneurons adapt to persistent changes in their activity. Here we describe a critical cellular process through which cortical parvalbumin-expressing (PV+) interneurons adapt to changes in their activity levels. We found that changes in the activity of individual PV+ interneurons drive bidirectional compensatory adjustments of the number and strength of inhibitory synapses received by these cells, specifically from other PV+ interneurons. High-throughput profiling of ribosome-associated mRNA revealed that increasing the activity of a PV+ interneuron leads to upregulation of two genes encoding multiple secreted neuropeptides: Vgf and Scg2. Functional experiments demonstrated that VGF is critically required for the activity-dependent scaling of inhibitory PV+ synapses onto PV+ interneurons. Our findings reveal an instructive role for neuropeptide-encoding genes in regulating synaptic connections among PV+ interneurons in the adult mouse neocortex.

GHS-R1a signaling drives anxiety-related behavior by shaping excitability of ventromedial hypothalamic neurons

The neural substrates of anxiety are poorly understood, which hinders treatment of anxiety disorders. Here we found, αCaMKII+ neurons in the ventral medial hypothalamic nucleus (VMH) responded to stressors with increased activity in male mice, both under physiological conditions and after repeated restraint stress. Activation of VMH αCaMKII+ neurons were necessary and sufficient to ameliorate stress-induced anxiety. The peripheral metabolic hormone ghrelin and receptor GHS-R1a play a complex role in emotion regulation; however, the mechanism is uncertain. A delayed increase in GHS-R1a expression in VMH αCaMKII+ neurons coincided with the development of stress-induced enhancement of anxiety-related behavior. GHS-R1a expression in VMH αCaMKII+ neurons promoted anxiety-related behavior, whereas GHS-R1a knockdown had the opposite effect. GHS-R1a upregulation inhibited the excitability of VMH αCaMKII+ neurons. We conclude that GHSR1a signaling drives stress-induced anxiety by shaping the activity of VMH αCaMKII+ neurons. GHS-R1a may be a therapeutic target for treating anxiety disorders such as post-traumatic stress disorder.

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.

Protocol to visualize three distinct neuronal ensembles encoding different events in the mouse brain using genetic and viral approaches

Activity tagging of neuronal ensembles has become an important tool in neuroscience. Here, we present a protocol for visualizing separate neuronal ensembles active during three distinct phases of a memory in transgenic mice. We describe steps to label active neurons using viral microinjection, inducing GFP expression under the robust activity marker (RAM) promoter, and transgenic mice, inducing tdTomato (TdT) expression, and immunohistochemical (IHC) visualization of endogenous cFos expression. We then detail procedures for preparation of tissue, imaging, and quantification of memory events. For complete details on the use and execution of this protocol, please refer to Lesuis et al.1.

Cutting-edge methodologies for tagging and tracing active neuronal coding in the brain

Decoding the neural substrates that underlie learning and behavior is a fundamental goal in neuroscience. Identifying "key players" at the molecular, cellular, and circuit levels has become possible with recent advancements in molecular technologies offering high spatiotemporal resolution. Immediate-early genes are effective markers of neural activity and plasticity, allowing for the identification of active cells involved in memory-based behavior. A calcium-dependent labeling system coupled with light or biochemical proximity labeling allows characterization of active cell ensembles and circuitry across broader brain regions within short time windows, particularly during transient behaviors. The integration of these systems expands the ability to address diverse research questions across behavioral paradigms. This review examines current molecular systems for activity-dependent labeling, highlighting their applications in identifying specific cell ensembles and circuits relevant to various scientific questions and further discuss their significance, along with future directions for the development of innovative methodologies.

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.

Engrams across diseases: Different pathologies - unifying mechanisms?

Memories are our reservoir of knowledge and thus, are crucial for guiding decisions and defining our self. The physical correlate of a memory in the brain is termed an engram and since decades helps researchers to elucidate the intricate nature of our imprinted experiences and knowledge. Given the importance that memories have for our lives, their impairment can present a tremendous burden. In this review we aim to discuss engram malfunctioning across diseases, covering dementia-associated pathologies, epilepsy, chronic pain and psychiatric disorders. Current neuroscientific tools allow to witness the emergence and fate of engram cells and enable their manipulation. We further suggest that specific mechanisms of mnemonic malfunction can be derived from engram cell readouts. While depicting the way diseases act on the mnemonic component - specifically, on the cellular engram - we emphasize a differentiation between forms of amnesia and hypermnesia. Finally, we highlight commonalities and distinctions of engram impairments on the cellular level across diseases independent of their pathogenic origins and discuss prospective therapeutic measures.

Neurobiological mechanisms of forgetting across timescales

Every species in the animal kingdom that learns, also forgets. Despite this balance between learning and forgetting, most neuroscientific explorations of memory have focused on how learning occurs, with recent studies identifying engrams as putative biological substrates for memory. Here we review an emerging literature that, in contrast, explores how our brains forget. These studies reveal that forgetting engages a broad collection of mechanisms that function to reduce engram accessibility. However, changes in accessibility emerge on vastly different timescales. At short timescales, forgetting is modulated by fluctuations in brain states that alter engram accessibility in a moment-to-moment fashion. In the intermediate- and long-term, forgetting depends, in part, on mechanisms that rewire engrams, rendering them gradually harder to access. Viewed this way, forgetting encompasses a family of plasticity mechanisms that modulate engram accessibility, perhaps in order to prioritize those memories that are most timely or relevant to the situation at hand.

Transcriptional determinants of goal-directed learning and representational drift in the parahippocampal cortex

Task learning involves learning associations between stimuli and outcomes and storing these relationships in memory. While this information can be reliably decoded from population activity, individual neurons encoding this representation can drift over time. The circuit or molecular mechanisms underlying this drift and its role in learning are unclear. We performed two-photon calcium imaging in the perirhinal cortex during task training. Using post hoc spatial transcriptomics, we measured immediate-early gene (IEG) expression and assigned monitored neurons to excitatory or inhibitory subtypes. We discovered an IEG-defined network spanning multiple subtypes that form stimulus-outcome associations. Targeted deletion of brain-derived neurotrophic factor in the perirhinal cortex disrupted IEG expression and impaired task learning. Representational drift slowed with prolonged training. Pre-existing representations were strengthened while stimulus-reward associations failed to form. Our findings reveal the cell types and molecules regulating long-term network stability that is permissive for task learning and memory allocation.

Hippocampal DNA Methylation Promotes Contextual Fear Memory Persistence by Facilitating Systems Consolidation and Cortical Engram Stabilization

BACKGROUND

Long-term fear memory storage involves gradual reorganization of supporting brain regions over time, a process termed systems consolidation. Memories initially rely on the hippocampus but gradually shift dependence to the neocortex. Although hippocampal activity drives this transfer, the molecular basis of systems consolidation is largely unknown. DNA methylation changes accompany persistent fear memory formation in the hippocampus and cortex, but its causal role in memory storage and systems consolidation remains unclear.

METHODS

We investigated the role of hippocampal DNA methylation in fear memory persistence through multiple approaches. Using recombinant adeno-associated virus (rAAV)-mediated gene transfer, we overexpressed or knocked down a DNA methyltransferase (DNMT3A2) in the dorsal hippocampus of mice and assessed its impact on fear memory duration. Engram tagging and manipulation tools were applied to study cortical fear engram stabilization. Finally, RNA sequencing analysis was used to identify transcriptional changes driven by DNMT3A2 overexpression.

RESULTS

Overexpression of hippocampal DNMT3A2 induced a persistent fear memory, while its knockdown impaired remote memory recall. RNA sequencing revealed that DNMT3A2 overexpression modified the expression of synaptic transmission regulatory genes. Furthermore, genetic engram tagging and manipulation revealed that hippocampal DNA methylation promoted the transfer of the fear memory trace from the hippocampus to the cortex and the stabilization of cortical fear memory traces.

CONCLUSIONS

Our findings demonstrate that hippocampal DNA methylation regulates the long-term storage of persistent fear memories by facilitating the transfer of memory traces from the hippocampus to the cortex and cortical stabilization. These results highlight DNA methylation as a key molecular mechanism underlying systems consolidation and long-term fear memory storage.

Shifting GnRH Neuron Ensembles Underlie Successive Preovulatory Luteinizing Hormone Surges

The gonadotropin-releasing hormone (GnRH) neurons operate as a neuronal ensemble exhibiting coordinated activity once every reproductive cycle to generate the preovulatory GnRH surge. Using GCaMP fiber photometry at the GnRH neuron distal dendrons to measure the output of this widely scattered population in female mice, we find that the onset, amplitude, and profile of GnRH neuron surge activity exhibits substantial variability from cycle to cycle both between and within individual mice. This was also evident when measuring successive proestrous luteinizing hormone surges. Studies combining short (c-Fos and c-Jun) and long (genetic robust activity marking) term indices of immediate early gene activation revealed that, while ∼50% of GnRH neurons were activated at the time of each surge, only half of these neurons had been active during the previous proestrous surge. These observations reveal marked inter- and intra-individual variability in the GnRH surge mechanism. Remarkably, different subpopulations of overlapping GnRH neurons are recruited to the ensemble each estrous cycle to generate the GnRH surge. While engendering variability in the surge mechanism itself, this likely provides substantial robustness to a key event underlying mammalian reproduction.

Stress disrupts engram ensembles in lateral amygdala to generalize threat memory in mice

Stress induces aversive memory overgeneralization, a hallmark of many psychiatric disorders. Memories are encoded by a sparse ensemble of neurons active during an event (an engram ensemble). We examined the molecular and circuit processes mediating stress-induced threat memory overgeneralization in mice. Stress, acting via corticosterone, increased the density of engram ensembles supporting a threat memory in lateral amygdala, and this engram ensemble was reactivated by both specific and non-specific retrieval cues (generalized threat memory). Furthermore, we identified a critical role for endocannabinoids, acting retrogradely on parvalbumin-positive (PV+) lateral amygdala interneurons in the formation of a less-sparse engram and memory generalization induced by stress. Glucocorticoid receptor antagonists, endocannabinoid synthesis inhibitors, increasing PV+ neuronal activity, and knocking down cannabinoid receptors in lateral amygdala PV+ neurons restored threat memory specificity and a sparse engram in stressed mice. These findings offer insights into stress-induced memory alterations, providing potential therapeutic avenues for stress-related disorders.

Boosting neuronal activity-driven mitochondrial DNA transcription improves cognition in aged mice

Deciphering the complex interplay between neuronal activity and mitochondrial function is pivotal in understanding brain aging, a multifaceted process marked by declines in synaptic function and mitochondrial performance. Here, we identified an age-dependent coupling between neuronal and synaptic excitation and mitochondrial DNA transcription (E-TCmito), which operates differently compared to classic excitation-transcription coupling in the nucleus (E-TCnuc). We demonstrated that E-TCmito repurposes molecules traditionally associated with E-TCnuc to regulate mitochondrial DNA expression in areas closely linked to synaptic activation. The effectiveness of E-TCmito weakens with age, contributing to age-related neurological deficits in mice. Boosting brain E-TCmito in aged animals ameliorated these impairments, offering a potential target to counteract age-related cognitive decline.

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.

Single-cell synaptome mapping: its technical basis and applications in critical period plasticity research

Our brain adapts to the environment by optimizing its function through experience-dependent cortical plasticity. This plasticity is transiently enhanced during a developmental stage, known as the "critical period," and subsequently maintained at lower levels throughout adulthood. Thus, understanding the mechanism underlying critical period plasticity is crucial for improving brain adaptability across the lifespan. Critical period plasticity relies on activity-dependent circuit remodeling through anatomical and functional changes at individual synapses. However, it remains challenging to identify the molecular signatures of synapses responsible for critical period plasticity and to understand how these plasticity-related synapses are spatiotemporally organized within a neuron. Recent advances in genetic tools and genome editing methodologies have enabled single-cell endogenous protein labeling in the brain, allowing for comprehensive molecular profiling of individual synapses within a neuron, namely "single-cell synaptome mapping." This promising approach can facilitate insights into the spatiotemporal organization of synapses that are sparse yet functionally important within single neurons. In this review, we introduce the basics of single-cell synaptome mapping and discuss its methodologies and applications to investigate the synaptic and cellular mechanisms underlying circuit remodeling during the critical period.

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.

Engineered serum markers for non-invasive monitoring of gene expression in the brain

Measurement of gene expression in the brain requires invasive analysis of brain tissue or non-invasive methods that are limited by low sensitivity. Here we introduce a method for non-invasive, multiplexed, site-specific monitoring of endogenous gene or transgene expression in the brain through engineered reporters called released markers of activity (RMAs). RMAs consist of an easily detectable reporter and a receptor-binding domain that enables transcytosis across the brain endothelium. RMAs are expressed in the brain but exit into the blood, where they can be easily measured. We show that expressing RMAs at a single mouse brain site representing approximately 1% of the brain volume provides up to a 100,000-fold signal increase over the baseline. Expression of RMAs in tens to hundreds of neurons is sufficient for their reliable detection. We demonstrate that chemogenetic activation of cells expressing Fos-responsive RMA increases serum RMA levels >6-fold compared to non-activated controls. RMAs provide a non-invasive method for repeatable, multiplexed monitoring of gene expression in the intact animal brain.

Increased GHS-R1a expression in the hippocampus impairs memory encoding and contributes to AD-associated memory deficits

Growth hormone secretagogue receptor 1a (GHS-R1a), also known as the ghrelin receptor, is an important nutrient sensor and metabolic regulator in both humans and rodents. Increased GHS-R1a expression is observed in the hippocampus of both Alzheimer's disease (AD) patients and AD model mice. However, the causal relationship between GHS-R1a elevation in the hippocampus and AD memory deficits remains uncertain. Here, we find that increasing GHS-R1a expression in dCA1 pyramidal neurons impairs hippocampus-dependent memory formation, which is abolished by local administration of the endogenous antagonist LEAP2. GHS-R1a elevation in dCA1 pyramidal neurons suppresses excitability and blocks memory allocation in these neurons. Chemogenetic activation of those high GHS-R1a neurons during training rescues GHS-R1a overexpression-induced memory impairment. Moreover, we demonstrate that increasing GHS-R1a expression in dCA1 pyramidal neurons hampers these neurons' ability to encode spatial memory and reduces engram size in the dCA1 region. Finally, we show that GHS-R1a deletion mitigates spatial memory deficits in APP/PS1 mice with increased GHS-R1a expression in the hippocampus. Our findings reveal a negative, causal relationship between hippocampal GHS-R1a expression and memory encoding, and suggest that blocking the abnormal increase in GHS-R1a activity/expression may be a promising approach to improve memory and treat cognitive decline in AD.

Advances in the labelling and selective manipulation of synapses

Synapses are highly specialized neuronal structures that are essential for neurotransmission, and they are dynamically regulated throughout the lifetime. Although accumulating evidence indicates that these structures are crucial for information processing and storage in the brain, their precise roles beyond neurotransmission are yet to be fully appreciated. Genetically encoded fluorescent tools have deepened our understanding of synaptic structure and function, but developing an ideal methodology to selectively visualize, label and manipulate synapses remains challenging. Here, we provide an overview of currently available synapse labelling techniques and describe their extension to enable synapse manipulation. We categorize these approaches on the basis of their conceptual bases and target molecules, compare their advantages and limitations and propose potential modifications to improve their effectiveness. These methods have broad utility, particularly for investigating mechanisms of synaptic function and synaptopathy.

A distinct neuronal ensemble of prelimbic cortex mediates spontaneous pain in rats with peripheral inflammation

The absence of a comprehensive understanding of the neural basis of spontaneous pain limits the development of therapeutic strategies targeting this primary complaint of patients with chronic pain. Here we report a distinct neuronal ensemble within the prelimbic cortex which processes signals related to spontaneous pain in rats with chronic inflammatory pain. This neuronal ensemble specifically encodes spontaneous pain-related behaviors, independently of other locomotive and evoked behaviors. Activation of this neuronal ensemble elicits marked spontaneous pain-like behaviors and enhances nociceptive responses, whereas prolonged silencing of its activities alleviates spontaneous pain and promotes overall recovery from inflammatory pain. Notably, afferents from the primary somatosensory cortex and infralimbic cortex bidirectionally modulate the activities of the spontaneous pain-responsive prelimbic cortex neuronal ensemble and pain behaviors. These findings reveal the cortical basis of spontaneous pain at the neuronal level, highlighting a distinct neuronal ensemble within the prelimbic cortex and its associated pain-regulatory brain networks.

2P-NucTag: on-demand phototagging for molecular analysis of functionally identified cortical neurons

Neural circuits are characterized by genetically and functionally diverse cell types. A mechanistic understanding of circuit function is predicated on linking the genetic and physiological properties of individual neurons. However, it remains highly challenging to map the transcriptional properties to functionally heterogeneous neuronal subtypes in mammalian cortical circuits in vivo. Here, we introduce a high-throughput two-photon nuclear phototagging (2P-NucTag) approach optimized for on-demand and indelible labeling of single neurons via a photoactivatable red fluorescent protein following in vivo functional characterization in behaving mice. We demonstrate the utility of this function-forward pipeline by selectively labeling and transcriptionally profiling previously inaccessible 'place' and 'silent' cells in the mouse hippocampus. Our results reveal unexpected differences in gene expression between these hippocampal pyramidal neurons with distinct spatial coding properties. Thus, 2P-NucTag opens a new way to uncover the molecular principles that govern the functional organization of neural circuits.

Autism-associated CHD8 controls reactive gliosis and neuroinflammation via remodeling chromatin in astrocytes

Reactive changes of glial cells during neuroinflammation impact brain disorders and disease progression. Elucidating the mechanisms that control reactive gliosis may help us to understand brain pathophysiology and improve outcomes. Here, we report that adult ablation of autism spectrum disorder (ASD)-associated CHD8 in astrocytes attenuates reactive gliosis via remodeling chromatin accessibility, changing gene expression. Conditional Chd8 deletion in astrocytes, but not microglia, suppresses reactive gliosis by impeding astrocyte proliferation and morphological elaboration. Astrocyte Chd8 ablation alleviates lipopolysaccharide-induced neuroinflammation and septic-associated hypothermia in mice. Astrocytic CHD8 plays an important role in neuroinflammation by altering the chromatin landscape, regulating metabolic and lipid-associated pathways, and astrocyte-microglia crosstalk. Moreover, we show that reactive gliosis can be directly mitigated in vivo using an adeno-associated virus (AAV)-mediated Chd8 gene editing strategy. These findings uncover a role of ASD-associated CHD8 in the adult brain, which may warrant future exploration of targeting chromatin remodelers in reactive gliosis and neuroinflammation in injury and neurological diseases.

Acoustically targeted measurement of transgene expression in the brain

Gene expression is a critical component of brain physiology, but monitoring this expression in the living brain represents a major challenge. Here, we introduce a new paradigm called recovery of markers through insonation (REMIS) for noninvasive measurement of gene expression in the brain with cell type, spatial, and temporal specificity. Our approach relies on engineered protein markers that are produced in neurons but exit into the brain's interstitium. When ultrasound is applied to targeted brain regions, it opens the blood-brain barrier and releases these markers into the bloodstream. Once in blood, the markers can be readily detected using biochemical techniques. REMIS can noninvasively confirm gene delivery and measure endogenous signaling in specific brain sites through a simple insonation and a subsequent blood test. REMIS is reliable and demonstrated consistent improvement in recovery of markers from the brain into the blood. Overall, this work establishes a noninvasive, spatially specific method of monitoring gene delivery and endogenous signaling in the brain.

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.

Comparing behaviours induced by natural memory retrieval and optogenetic reactivation of an engram ensemble in mice

Memories are thought to be stored within sparse collections of neurons known as engram ensembles. Neurons active during a training episode are allocated to an engram ensemble ('engram neurons'). Memory retrieval is initiated by external sensory or internal cues present at the time of training reactivating engram neurons. Interestingly, optogenetic reactivation of engram ensemble neurons alone in the absence of external sensory cues is sufficient to induce behaviour consistent with memory retrieval in mice. However, there may exist differences between the behaviours induced by natural retrieval cues or artificial engram reactivation. Here, we compared two defensive behaviours (freezing and the syllable structure of ultrasonic vocalizations, USVs) induced by sensory cues present at training (natural memory retrieval) and optogenetic engram ensemble reactivation (artificial memory retrieval) in a threat conditioning paradigm in the same mice. During natural memory recall, we observed a strong positive correlation between freezing levels and distinct USV syllable features (characterized by an unsupervised algorithm, MUPET (Mouse Ultrasonic Profile ExTraction)). Moreover, we observed strikingly similar behavioural profiles in terms of freezing and USV characteristics between natural memory recall and artificial memory recall in the absence of sensory retrieval cues. Although our analysis focused on two behavioural measures of threat memory (freezing and USV characteristics), these results underscore the similarities between threat memory recall triggered naturally and through optogenetic reactivation of engram ensembles. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

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.

Multisite Assembly of Gateway Induced Clones (MAGIC): a flexible cloning toolbox with diverse applications in vertebrate model systems

Here we present the Multisite Assembly of Gateway Induced Clones (MAGIC) system, which harnesses site-specific recombination-based cloning via Gateway technology for rapid, modular assembly of between 1 and 3 "Entry" vector components, all into a fourth, standard high copy "Destination" plasmid backbone. The MAGIC toolkit spans a range of in vitro and in vivo uses, from directing tunable gene expression, to driving simultaneous expression of microRNAs and fluorescent reporters, to enabling site-specific recombinase-dependent gene expression. All MAGIC system components are directly compatible with existing multisite gateway Tol2 systems currently used in zebrafish, as well as existing eukaryotic cell culture expression Destination plasmids, and available mammalian lentiviral and adenoviral Destination vectors, allowing rapid cross-species experimentation. Moreover, herein we describe novel vectors with flanking piggyBac transposon elements for stable genomic integration in vitro or in vivo when used with piggyBac transposase. Collectively, the MAGIC system facilitates transgenesis in cultured mammalian cells, electroporated mouse and chick embryos, as well as in injected zebrafish embryos, enabling the rapid generation of innovative DNA constructs for biological research due to a shared, common plasmid platform.

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.

Destabilization of fear memory by Rac1-driven engram-microglia communication in hippocampus

Rac1 is a key regulator of the cytoskeleton and neuronal plasticity, and is known to play a critical role in psychological and cognitive brain disorders. To elucidate the engram specific Rac1 signaling in fear memory, a doxycycline (Dox)-dependent robust activity marking (RAM) system was used to label dorsal dentate gyrus (DG) engram cells in mice during contextual fear conditioning. Rac1 mRNA and protein levels in DG engram cells were peaked at 24 h (day 1) after fear conditioning and were more abundant in the fear engram cells than in the non-engram cells. Optogenetic activation of Rac1 in a temporal manner in DG engram cells before memory retrieval decreased the freezing level in the fear context. Optogenetic activation of Rac1 increased autophagy protein 7 (ATG7) expression in the DG engram cells and activated DG microglia. Microglia-specific transcriptomics and fluorescence in situ hybridization revealed that overexpression of ATG7 in the fear engram cells upregulated the mRNA of Toll-like receptor TLR2/4 in DG microglia. Knockdown of microglial TLR2/4 rescued fear memory destabilization induced by ATG7 overexpression or Rac1 activation in DG engram cells. These results indicate that Rac1-driven communications between engram cells and microglia contributes to contextual fear memory destabilization, and is mediated by ATG7 and TLR2/4, and suggest a novel mechanistic framework for the cytoskeletal regulator in fear memory interference.

Projection-TAGs enable multiplex projection tracing and multi-modal profiling of projection neurons

Single-cell multiomic techniques have sparked immense interest in developing a comprehensive multi-modal map of diverse neuronal cell types and their brain wide projections. However, investigating the spatial organization, transcriptional and epigenetic landscapes of brain wide projection neurons is hampered by the lack of efficient and easily adoptable tools. Here we introduce Projection-TAGs, a retrograde AAV platform that allows multiplex tagging of projection neurons using RNA barcodes. By using Projection-TAGs, we performed multiplex projection tracing of the mouse cortex and high-throughput single-cell profiling of the transcriptional and epigenetic landscapes of the cortical projection neurons. Projection-TAGs can be leveraged to obtain a snapshot of activity-dependent recruitment of distinct projection neurons and their molecular features in the context of a specific stimulus. Given its flexibility, usability, and compatibility, we envision that Projection-TAGs can be readily applied to build a comprehensive multi-modal map of brain neuronal cell types and their projections.

Distinct subpopulations of ventral pallidal cholinergic projection neurons encode valence of olfactory stimuli

To better understand the function of cholinergic projection neurons in the ventral pallidum (VP), we examined behavioral responses to appetitive (APP) and aversive (AV) odors that elicited approach or avoidance, respectively. Exposure to each odor increased cFos expression and calcium signaling in VP cholinergic neurons. Activity and Cre-dependent viral vectors selectively labeled VP cholinergic neurons that were activated and reactivated in response to either APP or AV odors, but not both, identifying two non-overlapping populations of VP cholinergic neurons differentially activated by the valence of olfactory stimuli. These two subpopulations showed differences in electrophysiological properties, morphology, and projections to the basolateral amygdala. Although VP neurons are engaged in both approach and avoidance behavioral responses, cholinergic signaling is only required for approach behavior. Thus, two distinct subpopulations of VP cholinergic neurons differentially encode valence of olfactory stimuli and play distinct roles in approach and avoidance behaviors.

Molecular tools for recording and intervention of neuronal activity

Observing the activity of neural networks is critical for the identification of learning and memory processes, as well as abnormal activities of neural circuits in disease, particularly for the purpose of tracking disease progression. Methodologies for describing the activity history of neural networks using molecular biology techniques first utilized genes expressed by active neurons, followed by the application of recently developed techniques including optogenetics and incorporation of insights garnered from other disciplines, including chemistry and physics. In this review, we will discuss ways in which molecular biological techniques used to describe the activity of neural networks have evolved along with the potential for future development.

Phosphorylation of pyruvate dehydrogenase inversely associates with neuronal activity

For decades, the expression of immediate early genes (IEGs) such as FOS has been the most widely used molecular marker representing neuronal activation. However, to date, there is no equivalent surrogate available for the decrease of neuronal activity. Here, we developed an optogenetic-based biochemical screen in which population neural activities can be controlled by light with single action potential precision, followed by unbiased phosphoproteomic profiling. We identified that the phosphorylation of pyruvate dehydrogenase (pPDH) inversely correlated with the intensity of action potential firing in primary neurons. In in vivo mouse models, monoclonal antibody-based pPDH immunostaining detected activity decreases across the brain, which were induced by a wide range of factors including general anesthesia, chemogenetic inhibition, sensory experiences, and natural behaviors. Thus, as an inverse activity marker (IAM) in vivo, pPDH can be used together with IEGs or other cell-type markers to profile and identify bi-directional neural dynamics induced by experiences or behaviors.

Visuotactile integration facilitates mirror-induced self-directed behavior through activation of hippocampal neuronal ensembles in mice

Remembering the visual features of oneself is critical for self-recognition. However, the neural mechanisms of how the visual self-image is developed remain unknown because of the limited availability of behavioral paradigms in experimental animals. Here, we demonstrate a mirror-induced self-directed behavior (MSB) in mice, resembling visual self-recognition. Mice displayed increased mark-directed grooming to remove ink placed on their heads when an ink-induced visual-tactile stimulus contingency occurred. MSB required mirror habituation and social experience. The chemogenetic inhibition of dorsal or ventral hippocampal CA1 (vCA1) neurons attenuated MSB. Especially, a subset of vCA1 neurons activated during the mirror exposure was significantly reactivated during re-exposure to the mirror and was necessary for MSB. The self-responding vCA1 neurons were also reactivated when mice were exposed to a conspecific of the same strain. These results suggest that visual self-image may be developed through social experience and mirror habituation and stored in a subset of vCA1 neurons.

Inactivation of face-selective neurons alters eye movements when free viewing faces

During free viewing, faces attract gaze and induce specific fixation patterns corresponding to the facial features. This suggests that neurons encoding the facial features are in the causal chain that steers the eyes. However, there is no physiological evidence to support a mechanistic link between face-encoding neurons in high-level visual areas and the oculomotor system. In this study, we targeted the middle face patches of the inferior temporal (IT) cortex in two macaque monkeys using an functional magnetic resonance imaging (fMRI) localizer. We then utilized muscimol microinjection to unilaterally suppress IT neural activity inside and outside the face patches and recorded eye movements while the animals free viewing natural scenes. Inactivation of the face-selective neurons altered the pattern of eye movements on faces: The monkeys found faces in the scene but neglected the eye contralateral to the inactivation hemisphere. These findings reveal the causal contribution of the high-level visual cortex in eye movements.

Morphine- and foot shock-responsive neuronal ensembles in the VTA possess different connectivity and biased GPCR signaling pathway

Background: Neurons in the ventral tegmental area (VTA) are sensitive to stress and their maladaptation have been implicated in the psychiatric disorders such as anxiety and addiction, etc. The cellular properties of the VTA neurons in response to different stressors related to different emotional processing remain to be investigated. Methods: By combining immediate early gene (IEG)-dependent labeling, rabies virus tracing, ensemble-specific transcriptomic analysis and fiber photometry recording in the VTA of male mice, the spatial distribution, brain-wide connectivity and cellular signaling pathways in the VTA neuronal ensembles in response to morphine (Mor-Ens) or foot shock (Shock-Ens) stimuli were investigated. Results: Optogenetic activation of the Mor-Ens drove approach behavior, whereas chemogenetic activation of the Shock-Ens increased the anxiety level in mice. Mor-Ens were clustered and enriched in the ventral VTA, contained a higher proportion of dopaminergic neurons, received more inputs from the dorsal medial striatum and the medial hypothalamic zone, and exhibited greater axonal arborization in the zona incerta and ventral pallidum. Whereas Shock-Ens were more dispersed, contained a higher proportion of GABAergic neurons, and received more inputs from the ventral pallidum and the lateral hypothalamic area. The downstream targets of the G protein and β-arrestin pathways, PLCβ3 and phosphorylated AKT1Thr308, were relatively enriched in the Mor-Ens and Shock-Ens, respectively. Cariprazine, the G-protein-biased agonist for the dopamine D2 receptor, increased the response of Mor-Ens to sucrose water and decreased the anxiety-like behavior during morphine withdrawal, whereas the β-arrestin-biased agonist UNC9994 decreased the response of Shock-Ens to tail suspension. Conclusions: Taken together, these findings reveal the heterogeneous connectivity and signaling pathways of the VTA neurons in response to morphine and foot shock, providing new insights for development of specific interventions for psychiatric disorders caused by various stressors associated with different VTA neuronal functions.

Exploring the memory: existing activity-dependent tools to tag and manipulate engram cells

The theory of engrams, proposed several years ago, is highly crucial to understanding the progress of memory. Although it significantly contributes to identifying new treatments for cognitive disorders, it is limited by a lack of technology. Several scientists have attempted to validate this theory but failed. With the increasing availability of activity-dependent tools, several researchers have found traces of engram cells. Activity-dependent tools are based on the mechanisms underlying neuronal activity and use a combination of emerging molecular biological and genetic technology. Scientists have used these tools to tag and manipulate engram neurons and identified numerous internal connections between engram neurons and memory. In this review, we provide the background, principles, and selected examples of applications of existing activity-dependent tools. Using a combination of traditional definitions and concepts of engram cells, we discuss the applications and limitations of these tools and propose certain developmental directions to further explore the functions of engram cells.

Spatiotemporal Mapping and Molecular Basis of Whole-brain Circuit Maturation

Brain development is highly dynamic and asynchronous, marked by the sequential maturation of functional circuits across the brain. The timing and mechanisms driving circuit maturation remain elusive due to an inability to identify and map maturing neuronal populations. Here we create DevATLAS (Developmental Activation Timing-based Longitudinal Acquisition System) to overcome this obstacle. We develop whole-brain mapping methods to construct the first longitudinal, spatiotemporal map of circuit maturation in early postnatal mouse brains. Moreover, we uncover dramatic impairments within the deep cortical layers in a neurodevelopmental disorders (NDDs) model, demonstrating the utility of this resource to pinpoint when and where circuit maturation is disrupted. Using DevATLAS, we reveal that early experiences accelerate the development of hippocampus-dependent learning by increasing the synaptically mature granule cell population in the dentate gyrus. Finally, DevATLAS enables the discovery of molecular mechanisms driving activity-dependent circuit maturation.

Synaptic Engram

The concept of the engram refers to structural and/or physiological changes that underlie memory associations during learning. However, the precise biological basis of the engram remains elusive, with ongoing controversy regarding whether it resides at the cellular level or within the synaptic connections between activated cells. Here, we briefly review the studies investigating the cellular engram and the challenges they encounter. Subsequently, we delve into the biological basis of the engram within synaptic connections. In this regard, we introduce the history of synaptic engrams and discuss recent findings suggesting that synaptic plasticity serves as a substrate for memory. Additionally, we provide an overview of key technologies utilized in the study of synaptic plasticity.

All IEGs Are Not Created Equal-Molecular Sorting Within the Memory Engram

When neurons are recruited to form the memory engram, they are driven to activate the expression of a series of immediate-early genes (IEGs). While these IEGs have been used relatively indiscriminately to identify the so-called engram neurons, recent research has demonstrated that different IEG ensembles can be physically and functionally distinct within the memory engram. This inherent heterogeneity of the memory engram is driven by the diversity in the functions and distributions of different IEGs. This process, which we call molecular sorting, is analogous to sorting the entire population of engram neurons into different sub-engrams molecularly defined by different IEGs. In this chapter, we will describe the molecular sorting process by systematically reviewing published work on engram ensemble cells defined by the following four major IEGs: Fos, Npas4, Arc, and Egr1. By comparing and contrasting these likely different components of the memory engram, we hope to gain a better understanding of the logic and significance behind the molecular sorting process for memory functions.

Understanding Others' Distress Through Past Experiences: The Role of Memory Engram Cells in Observational Fear

For individuals to survive and function in society, it is essential that they recognize, interact with, and learn from other conspecifics. Observational fear (OF) is the well-conserved empathic ability of individuals to understand the other's aversive situation. While it is widely known that factors such as prior similar aversive experience and social familiarity with the demonstrator facilitate OF, the neural circuit mechanisms that explicitly regulate experience-dependent OF (Exp OF) were unclear. In this review, we examine the neural circuit mechanisms that regulate OF, with an emphasis on rodent models, and then discuss emerging evidence for the role of fear memory engram cells in the regulation of Exp OF. First, we examine the neural circuit mechanisms that underlie Naive OF, which is when an observer lacks prior experiences relevant to OF. In particular, the anterior cingulate cortex to basolateral amygdala (BLA) neural circuit is essential for Naive OF. Next, we discuss a recent study that developed a behavioral paradigm in mice to examine the neural circuit mechanisms that underlie Exp OF. This study found that fear memory engram cells in the BLA of observers, which form during a prior similar aversive experience with shock, are reactivated by ventral hippocampal neurons in response to shock delivery to the familiar demonstrator to elicit Exp OF. Finally, we discuss the implications of fear memory engram cells in Exp OF and directions of future research that are of both translational and basic interest.

Examining memory linking and generalization using scFLARE2, a temporally precise neuronal activity tagging system

How memories are organized in the brain influences whether they are remembered discretely versus linked with other experiences or whether generalized information is applied to entirely novel situations. Here, we used scFLARE2 (single-chain fast light- and activity-regulated expression 2), a temporally precise tagging system, to manipulate mouse lateral amygdala neurons active during one of two 3 min threat experiences occurring close (3 h) or further apart (27 h) in time. Silencing scFLARE2-tagged neurons showed that two threat experiences occurring at distal times are dis-allocated to orthogonal engram ensembles and remembered discretely, whereas the same two threat experiences occurring in close temporal proximity are linked via co-allocation to overlapping engram ensembles. Moreover, we found that co-allocation mediates memory generalization applied to a completely novel stimulus. These results indicate that endogenous temporal evolution of engram ensemble neuronal excitability determines how memories are organized and remembered and that this would not be possible using conventional immediate-early gene-based tagging methods.

Plasticity of neuronal dynamics in the lateral habenula for cue-punishment associative learning

The brain's ability to associate threats with external stimuli is vital to execute essential behaviours including avoidance. Disruption of this process contributes instead to the emergence of pathological traits which are common in addiction and depression. However, the mechanisms and neural dynamics at the single-cell resolution underlying the encoding of associative learning remain elusive. Here, employing a Pavlovian discrimination task in mice we investigate how neuronal populations in the lateral habenula (LHb), a subcortical nucleus whose excitation underlies negative affect, encode the association between conditioned stimuli and a punishment (unconditioned stimulus). Large population single-unit recordings in the LHb reveal both excitatory and inhibitory responses to aversive stimuli. Additionally, local optical inhibition prevents the formation of cue discrimination during associative learning, demonstrating a critical role of LHb activity in this process. Accordingly, longitudinal in vivo two-photon imaging tracking LHb calcium neuronal dynamics during conditioning reveals an upward or downward shift of individual neurons' CS-evoked responses. While recordings in acute slices indicate strengthening of synaptic excitation after conditioning, support vector machine algorithms suggest that postsynaptic dynamics to punishment-predictive cues represent behavioral cue discrimination. To examine the presynaptic signaling in LHb participating in learning we monitored neurotransmitter dynamics with genetically-encoded indicators in behaving mice. While glutamate, GABA, and serotonin release in LHb remain stable across associative learning, we observe enhanced acetylcholine signaling developing throughout conditioning. In summary, converging presynaptic and postsynaptic mechanisms in the LHb underlie the transformation of neutral cues in valued signals supporting cue discrimination during learning.

Distinct subpopulations of ventral pallidal cholinergic projection neurons encode valence of olfactory stimuli

The ventral pallidum (VP) mediates motivated behaviors largely via the action of VP GABA and glutamatergic neurons. In addition to these neuronal subtypes, there is a population of cholinergic projection neurons in the VP, whose functional significance remains unclear. To understand the functional role of VP cholinergic neurons, we first examined behavioral responses to an appetitive (APP) odor that elicited approach, and an aversive (AV) odor that led to avoidance. To examine how VP cholinergic neurons were engaged in APP vs. AV responses, we used an immediate early gene marker and in-vivo fiber photometry, examining the activation profile of VP cholinergic neurons in response to each odor. Exposure to each odor led to an increase in the number of cFos counts and increased calcium signaling of VP cholinergic neurons. Activity and cre-dependent viral vectors were designed to label engaged VP cholinergic neurons in two distinct contexts: (1) exposure to the APP odor, (2) followed by subsequent exposure to the AV odor, and vice versa. These studies revealed two distinct, non-overlapping subpopulations of VP cholinergic neurons: one activated in response to the APP odor, and a second distinct population activated in response to the AV odor. These two subpopulations of VP cholinergic neurons are spatially intermingled within the VP, but show differences in electrophysiological properties, neuronal morphology, and projections to the basolateral amygdala. Although VP cholinergic neurons are engaged in behavioral responses to each odor, VP cholinergic signaling is only required for approach behavior. Indeed, inhibition of VP cholinergic neurons not only blocks approach to the APP odor, but reverses the behavior, leading to active avoidance. Our results highlight the functional heterogeneity of cholinergic projection neurons within the VP. These two subpopulations of VP cholinergic neurons differentially encode valence of olfactory stimuli and play unique roles in approach and avoidance behaviors.

EpiPro, a Novel, Synthetic, Activity-Regulated Promoter That Targets Hyperactive Neurons in Epilepsy for Gene Therapy Applications

Epileptogenesis is characterized by intrinsic changes in neuronal firing, resulting in hyperactive neurons and the subsequent generation of seizure activity. These alterations are accompanied by changes in gene transcription networks, first with the activation of early-immediate genes and later with the long-term activation of genes involved in memory. Our objective was to engineer a promoter containing binding sites for activity-dependent transcription factors upregulated in chronic epilepsy (EpiPro) and validate it in multiple rodent models of epilepsy. First, we assessed the activity dependence of EpiPro: initial electrophysiology studies found that EpiPro-driven GFP expression was associated with increased firing rates when compared with unlabeled neurons, and the assessment of EpiPro-driven GFP expression revealed that GFP expression was increased ~150× after status epilepticus. Following this, we compared EpiPro-driven GFP expression in two rodent models of epilepsy, rat lithium/pilocarpine and mouse electrical kindling. In rodents with chronic epilepsy, GFP expression was increased in most neurons, but particularly in dentate granule cells, providing in vivo evidence to support the "breakdown of the dentate gate" hypothesis of limbic epileptogenesis. Finally, we assessed the time course of EpiPro activation and found that it was rapidly induced after seizures, with inactivation following over weeks, confirming EpiPro's potential utility as a gene therapy driver for epilepsy.

Inactivation of face selective neurons alters eye movements when free viewing faces

During free viewing, faces attract gaze and induce specific fixation patterns corresponding to the facial features. This suggests that neurons encoding the facial features are in the causal chain that steers the eyes. However, there is no physiological evidence to support a mechanistic link between face encoding neurons in high-level visual areas and the oculomotor system. In this study, we targeted the middle face patches of inferior temporal (IT) cortex in two macaque monkeys using an fMRI localizer. We then utilized muscimol microinjection to unilaterally suppress IT neural activity inside and outside the face patches and recorded eye movements while the animals free viewing natural scenes. Inactivation of the face selective neurons altered the pattern of eye movements on faces: the monkeys found faces in the scene but neglected the eye contralateral to the inactivation hemisphere. These findings reveal the causal contribution of the high-level visual cortex in eye movements.

Significance

It has been shown, for more than half a century, that eye movements follow distinctive patterns when free viewing faces. This suggests causal involvement of the face-encoding visual neurons in the eye movements. However, the literature is scant of evidence for this possibility and has focused mostly on the link between low-level image saliency and eye movements. Here, for the first time, we bring causal evidence showing how face-selective neurons in inferior temporal cortex inform and steer eye movements when free viewing faces.