Excitability mediates allocation of pre-configured ensembles to a hippocampal engram supporting contextual conditioned threat in mice

Systems consolidation reorganizes hippocampal engram circuitry

Episodic memories-high-fidelity memories for events that depend initially on the hippocampus-do not maintain their precision in perpetuity. One benefit of this time-dependent loss of precision is the emergence of event-linked gist memories that may be used to guide future behaviour in new but related situations (that is, generalization)1-3. Models of systems consolidation propose that memory reorganization accompanies this loss of memory precision1,4; however, the locus of this reorganization is unclear. Here we report that time-dependent reorganization of hippocampal engram circuitry is sufficient to explain shifts in memory precision associated with systems consolidation. Using engram labelling tools in mice, we demonstrate that the passage of time rewires hippocampal engram circuits, enabling hippocampal engram neurons to be promiscuously active and guide behaviour in related situations that do not match the original training conditions. Reorganization depends on hippocampal neurogenesis; eliminating hippocampal neurogenesis prevents reorganization and maintains precise, event memories. Conversely, promoting hippocampal neurogenesis accelerates memory reorganization and the emergence of event-linked gist memories in the hippocampus. Our results indicate that systems consolidation models require updating to account for within-hippocampus reorganization that leads to qualitative shifts in memory precision.

Prefrontal encoding of an internal model for emotional inference

A key function of brain systems mediating emotion is to learn to anticipate unpleasant experiences. Although organisms readily associate sensory stimuli with aversive outcomes, higher-order forms of emotional learning and memory require inference to extrapolate the circumstances surrounding directly experienced aversive events to other indirectly related sensory patterns that were not part of the original experience. This type of learning requires internal models of emotion, which flexibly track directly experienced and inferred aversive associations. Although the brain mechanisms of simple forms of aversive learning have been well studied in areas such as the amygdala1-4, whether and how the brain forms and represents internal models of emotionally relevant associations are not known5. Here we report that neurons in the rodent dorsomedial prefrontal cortex (dmPFC) encode a flexible internal model of emotion by linking sensory stimuli in the environment with aversive events, whether they were directly or indirectly associated with that experience. These representations form through a multi-step encoding mechanism involving recruitment and stabilization of dmPFC cells that support inference. Although dmPFC population activity encodes all salient associations, dmPFC neurons projecting to the amygdala specifically represent and are required to express inferred associations. Together, these findings reveal how internal models of emotion are encoded in the dmPFC to regulate subcortical systems for recall of inferred emotional memories.

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.

Parallel processing of past and future memories through reactivation and synaptic plasticity mechanisms during sleep

Every day, we experience new episodes and store new memories. Although memories are stored in corresponding engram cells, how different sets of engram cells are selected for current and next episodes, and how they create their memories, remains unclear. Here we show that in male mice, hippocampal CA1 neurons show an organized synchronous activity in prelearning home cage sleep that correlates with the learning ensembles only in engram cells, termed preconfigured ensembles. Moreover, after learning, a subset of nonengram cells develops population activity, which is constructed during postlearning offline periods, and then emerges to represent engram cells for new learning. Our model suggests a potential role of synaptic depression and scaling in the reorganization of the activity of nonengram cells. Together, our findings indicate that during offline periods there are two parallel processes occurring: conserving of past memories through reactivation, and preparation for upcoming ones through offline synaptic plasticity mechanisms.

Reelin marks cocaine-activated striatal neurons, promotes neuronal excitability, and regulates cocaine reward

Drugs of abuse activate defined neuronal populations in reward structures such as the nucleus accumbens (NAc), which promote the enduring synaptic, circuit, and behavioral consequences of drug exposure. While the molecular and cellular effects arising from experience with drugs like cocaine are increasingly well understood, mechanisms that dictate NAc neuronal recruitment remain unknown. Here, we leveraged unbiased single-nucleus transcriptional profiling and targeted in situ detection to identify Reln (encoding the secreted glycoprotein, Reelin) as a marker of cocaine-activated neuronal populations within the rat NAc. A CRISPR interference approach enabling selective Reln knockdown in the adult NAc altered expression of calcium signaling genes, promoted a transcriptional trajectory consistent with loss of cocaine sensitivity, and decreased MSN excitability. Behaviorally, Reln knockdown prevented cocaine locomotor sensitization, abolished cocaine place preference memory, and decreased cocaine self-administration behavior. These results identify Reelin as a critical mechanistic link between neuronal activation and cocaine-induced behavioral adaptations.

Erasable hippocampal neural signatures predict memory discrimination

Memories involving the hippocampus can take several days to consolidate, challenging efforts to uncover the neuronal signatures underlying this process. Here, we use calcium imaging in freely moving mice to track the hippocampal dynamics underlying memory consolidation across a 10-day contextual fear conditioning task. We find two neural signatures that emerge following learning and predict memory performance: context-specific place field remapping and coordinated neural activity prior to memory recall (freezing). To test whether these signatures support memory consolidation, we pharmacologically induced amnesia in separate mice by administering anisomycin, a protein synthesis inhibitor, immediately following learning. We find that anisomycin paradoxically accelerates cell turnover. Anisomycin also arrests learning-related remapping and blocks coordinated activity predictive of memory-related freezing behavior, effects that are likewise absent in untreated mice that exhibit poor memory expression. We conclude that context-specific place field remapping and the development of coordinated ensemble activity underlie contextual memory consolidation.

Synaptic architecture of a memory engram in the mouse hippocampus

Memory engrams are formed through experience-dependent plasticity of neural circuits, but their detailed architectures remain unresolved. Using three-dimensional electron microscopy, we performed nanoscale reconstructions of the hippocampal CA3-CA1 pathway after chemogenetic labeling of cellular ensembles recruited during associative learning. Neurons with a remote history of activity coinciding with memory acquisition showed no strong preference for wiring with each other. Instead, their connectomes expanded through multisynaptic boutons independently of the coactivation state of postsynaptic partners. The rewiring of ensembles representing an initial engram was accompanied by input-specific, spatially restricted upscaling of individual synapses, as well as remodeling of mitochondria, smooth endoplasmic reticulum, and interactions with astrocytes. Our findings elucidate the physical hallmarks of long-term memory and offer a structural basis for the cellular flexibility of information coding.

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.

The Suprapyramidal and Infrapyramidal Blades of the Dentate Gyrus Exhibit Different GluN Subunit Content and Dissimilar Frequency-Dependent Synaptic Plasticity In Vivo

The entorhinal cortex sends afferent information to the hippocampus by means of the perforant path (PP). The PP input to the dentate gyrus (DG) terminates in the suprapyramidal (sDG) and infrapyramidal (iDG) blades. Different electrophysiological stimulation patterns of the PP can generate hippocampal synaptic plasticity. Whether frequency-dependent synaptic plasticity differs in the sDG and iDG is unclear. Here, we compared medial PP-DG responses in freely behaving adult rats and found that synaptic plasticity in the sDG is broadly frequency dependent, whereby long-term depression (LTD, > 24 h) is induced with stimulation at 1 Hz, short-term depression (< 2 h) is triggered by 5 or 10 Hz, and long-term potentiation (LTP) of increasing magnitudes is induced by 200 and 400 Hz stimulation, respectively. By contrast, although the iDG expresses STD following 5 or 10 Hz stimulation, LTD induced by 1 Hz is weaker, LTP is not induced by 200 Hz and LTP induced by 400 Hz stimulation is significantly smaller in magnitude than LTP induced in sDG. Furthermore, the stimulus-response relationship of iDG is suppressed compared to sDG. These differences may arise from differences in granule cell properties, or the complement of NMDA receptors. Patch clamp recordings, in vitro, revealed reduced firing frequencies in response to high currents, and different action potential thresholds in iDG compared to sDG. Assessment of the expression of GluN subunits revealed significantly lower expression levels of GluN1, GluN2A, and GluN2B in the middle molecular layer of iDG compared to sDG. Taken together, these data indicate that synaptic plasticity in the iDG is weaker, less persistent and less responsive to afferent frequencies than synaptic plasticity in sDG. Effects may be mediated by weaker NMDA receptor expression and differences in neuronal responses in iDG versus sDG. These characteristics may explain reported differences in experience-dependent information processing in the suprapyramidal and infrapyramidal blades of the DG.

The Possibility Space Concept in Neuroscience: Possibilities, Constraints, and Explanations

Although the brain is often characterized as a complex system, theoretical and philosophical frameworks often struggle to capture this. For example, mainstream mechanistic accounts model neural systems as fixed and static in ways that fail to capture their dynamic nature and large set of possible behaviors. In this paper, we provide a framework for capturing a common type of complex system in neuroscience, which involves two main aspects: (i) constraints on the system and (ii) the system's possibility space of available outcomes. Our analysis merges neuroscience examples with recent work in the philosophy of science to suggest that the possibility space concept involves two essential types of constraints, which we call hard and soft constraints. Our analysis focuses on a domain-general notion of possibility space that is present in manifold frameworks and representations, phase space diagrams in dynamical systems theory, and paradigmatic cases, such as Waddington's epigenetic landscape model. After building the framework with such cases, we apply it to three main examples in neuroscience: adaptability, resilience, and phenomenology. We explore how this framework supports a philosophical toolkit for neuroscience and how it helps advance recent work in the philosophy of science on constraints, scientific explanations, and impossibility explanations. We show how fruitful connections between neuroscience and philosophy can support conceptual clarity, theoretical advances, and the identification of similar systems across different domains in neuroscience.

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.

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.

Memory engram synapse 3D molecular architecture visualized by cryoCLEM-guided cryoET

Memory is incorporated into the brain as physicochemical changes to engram cells. These are neuronal populations that form complex neuroanatomical circuits, are modified by experiences to store information, and allow for memory recall. At the molecular level, learning modifies synaptic communication to rewire engram circuits, a mechanism known as synaptic plasticity. However, despite its functional role on memory formation, the 3D molecular architecture of synapses within engram circuits is unknown. Here, we demonstrate the use of engram labelling technology and cryogenic correlated light and electron microscopy (cryoCLEM)-guided cryogenic electron tomography (cryoET) to visualize the in-tissue 3D molecular architecture of engram synapses of a contextual fear memory within the CA1 region of the mouse hippocampus. Engram cells exhibited structural diversity of macromolecular constituents and organelles in both pre- and postsynaptic compartments and within the synaptic cleft, including in clusters of membrane proteins, synaptic vesicle occupancy, and F-actin copy number. This 'engram to tomogram' approach, harnessing in vivo functional neuroscience and structural biology, provides a methodological framework for testing fundamental molecular plasticity mechanisms within engram circuits during memory encoding, storage and recall.

Sleep microstructure organizes memory replay

Recently acquired memories are reactivated in the hippocampus during sleep, an initial step for their consolidation1-3. This process is concomitant with the hippocampal reactivation of previous memories4-6, posing the problem of how to prevent interference between older and recent, initially labile, memory traces. Theoretical work has suggested that consolidating multiple memories while minimizing interference can be achieved by randomly interleaving their reactivation7-10. An alternative is that a temporal microstructure of sleep can promote the reactivation of different types of memories during specific substates. Here, to test these two hypotheses, we developed a method to simultaneously record large hippocampal ensembles and monitor sleep dynamics through pupillometry in naturally sleeping mice. Oscillatory pupil fluctuations revealed a previously unknown microstructure of non-REM sleep-associated memory processes. We found that memory replay of recent experiences dominated in sharp-wave ripples during contracted pupil substates of non-REM sleep, whereas replay of previous memories preferentially occurred during dilated pupil substates. Selective closed-loop disruption of sharp-wave ripples during contracted pupil non-REM sleep impaired the recall of recent memories, whereas the same manipulation during dilated pupil substates had no behavioural effect. Stronger extrinsic excitatory inputs characterized the contracted pupil substate, whereas higher recruitment of local inhibition was prominent during dilated pupil substates. Thus, the microstructure of non-REM sleep organizes memory replay, with previous versus new memories being temporally segregated in different substates and supported by local and input-driven mechanisms, respectively. Our results suggest that the brain can multiplex distinct cognitive processes during sleep to facilitate continuous learning without interference.

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.

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.

Memory engram stability and flexibility

Many studies have shown that memories are encoded in sparse neural ensembles distributed across the brain. During the post-encoding period, often during sleep, many of the cells that were active during encoding are reactivated, supporting consolidation of this memory. During memory recall, many of the same cells that were active during encoding and reactivated during consolidation are reactivated during recall. These ensembles of cells have been referred to as the memory engram cells, stably representing a specific memory. However, recent studies question the rigidity of the "stable memory engram." Here we review the past literature of how episodic-like memories are encoded, consolidated, and recalled. We also highlight more recent studies (as well as some older literature) that suggest that these stable memories and their representations are much more dynamic and flexible than previously thought. We highlight some of these processes, including memory updating, reconsolidation, forgetting, schema learning, memory-linking, and representational drift.

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.

Repetition dynamically and rapidly increases cortical, but not hippocampal, offline reactivation

No sooner is an experience over than its neural representation begins to be transformed through memory reactivation during offline periods. The lion's share of prior research has focused on understanding offline reactivation within the hippocampus. However, it is hypothesized that consolidation processes involve offline reactivation in cortical regions as well as coordinated reactivation in the hippocampus and cortex. Using fMRI, we presented novel and repeated paired associates to participants during encoding and measured offline memory reactivation for those events during an immediate post-encoding rest period. post-encoding reactivation frequency of repeated and once-presented events did not differ in the hippocampus. However, offline reactivation in widespread cortical regions and hippocampal-cortical coordinated reactivation were significantly enhanced for repeated events. These results provide evidence that repetition might facilitate the distribution of memory representations across cortical networks, a hallmark of systems-level consolidation. Interestingly, we found that offline reactivation frequency in both hippocampus and cortex explained variance in behavioral success on an immediate associative recognition test for the once-presented information, potentially indicating a role of offline reactivation in maintaining these novel, weaker, memories. Together, our findings highlight that endogenous offline reactivation can be robustly and significantly modulated by study repetition.

Higher-order interactions between hippocampal CA1 neurons are disrupted in amnestic mice

Across systems, higher-order interactions between components govern emergent dynamics. Here we tested whether contextual threat memory retrieval in mice relies on higher-order interactions between dorsal CA1 hippocampal neurons requiring learning-induced dendritic spine plasticity. We compared population-level Ca2+ transients as wild-type mice (with intact learning-induced spine plasticity and memory) and amnestic mice (TgCRND8 mice with high levels of amyloid-β and deficits in learning-induced spine plasticity and memory) were tested for memory. Using machine-learning classifiers with different capacities to use input data with complex interactions, our findings indicate complex neuronal interactions in the memory representation of wild-type, but not amnestic, mice. Moreover, a peptide that partially restored learning-induced spine plasticity also restored the statistical complexity of the memory representation and memory behavior in Tg mice. These findings provide a previously missing bridge between levels of analysis in memory research, linking receptors, spines, higher-order neuronal dynamics and behavior.

Divergent recruitment of developmentally defined neuronal ensembles supports memory dynamics

Memories are dynamic constructs whose properties change with time and experience. The biological mechanisms underpinning these dynamics remain elusive, particularly concerning how shifts in the composition of memory-encoding neuronal ensembles influence the evolution of a memory over time. By targeting developmentally distinct subpopulations of principal neurons, we discovered that memory encoding resulted in the concurrent establishment of multiple memory traces in the mouse hippocampus. Two of these traces were instantiated in subpopulations of early- and late-born neurons and followed distinct reactivation trajectories after encoding. The divergent recruitment of these subpopulations underpinned gradual reorganization of memory ensembles and modulated memory persistence and plasticity across multiple learning episodes. Thus, our findings reveal profound and intricate relationships between ensemble dynamics and the progression of memories over time.

Haploinsufficiency of GABAA Receptor-Associated Clptm1 Enhances Phasic and Tonic Inhibitory Neurotransmission, Suppresses Excitatory Synaptic Plasticity, and Impairs Memory

The mechanisms utilized by neurons to regulate the efficacy of phasic and tonic inhibition and their impacts on synaptic plasticity and behavior are incompletely understood. Cleft lip and palate transmembrane protein 1 (Clptm1) is a membrane-spanning protein that interacts with multiple γ-aminobutyric acid type A receptor (GABAAR) subunits, trapping them in the endoplasmic reticulum and Golgi network. Overexpression and knock-down studies suggest that Clptm1 modulates GABAAR-mediated phasic inhibition and tonic inhibition as well as activity-induced inhibitory synaptic homeostasis in cultured hippocampal neurons. To investigate the role of Clptm1 in the modulation of GABAARs in vivo, we generated Clptm1 knock-out (KO) mice. Here, we show that genetic KO of Clptm1 elevated phasic and tonic inhibitory transmission in both male and female heterozygous mice. Although basal excitatory synaptic transmission was not affected, Clptm1 haploinsufficiency significantly blocked high-frequency stimulation-induced long-term potentiation (LTP) in hippocampal CA3→CA1 synapses. In the hippocampus-dependent contextual fear-conditioning behavior task, both male and female Clptm1 heterozygous KO mice exhibited impairment in contextual fear memory. In addition, LTP and contextual fear memory were rescued by application of L-655,708, a negative allosteric modulator of the extrasynaptic GABAAR α5 subunit. These results suggest that haploinsufficiency of Clptm1 contributes to cognitive deficits through altered synaptic transmission and plasticity by elevation of inhibitory neurotransmission, with tonic inhibition playing a major role.

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.

Engrams

Frankland et al. provide a history of research on engrams and their relationship to memory processes, highlighting new technologies that have allowed careful dissection of engrams and their function.