Encoding of Discriminative Fear Memory by Input-Specific LTP in the Amygdala

Glutamatergic CYLD deletion leads to aberrant excitatory activity in the basolateral amygdala: association with enhanced cued fear expression

JOURNAL/nrgr/04.03/01300535-202511000-00029/figure1/v/2024-12-20T164640Z/r/image-tiff Neuronal activity, synaptic transmission, and molecular changes in the basolateral amygdala play critical roles in fear memory. Cylindromatosis (CYLD) is a deubiquitinase that negatively regulates the nuclear factor kappa-B pathway. CYLD is well studied in non-neuronal cells, yet under-investigated in the brain, where it is highly expressed. Emerging studies have shown involvement of CYLD in the remodeling of glutamatergic synapses, neuroinflammation, fear memory, and anxiety- and autism-like behaviors. However, the precise role of CYLD in glutamatergic neurons is largely unknown. Here, we first proposed involvement of CYLD in cued fear expression. We next constructed transgenic model mice with specific deletion of Cyld from glutamatergic neurons. Our results show that glutamatergic CYLD deficiency exaggerated the expression of cued fear in only male mice. Further, loss of CYLD in glutamatergic neurons resulted in enhanced neuronal activation, impaired excitatory synaptic transmission, and altered levels of glutamate receptors accompanied by over-activation of microglia in the basolateral amygdala of male mice. Altogether, our study suggests a critical role of glutamatergic CYLD in maintaining normal neuronal, synaptic, and microglial activation. This may contribute, at least in part, to cued fear expression.

Glutamatergic synaptic plasticity in medial vestibular nuclei during vestibular compensation

Vestibular compensation, the spontaneous recovery from vestibular dysfunction following unilateral vestibular loss, serves as a valuable model for investigating post-lesion plasticity in the adult central nervous system. Elucidating the mechanisms underlying vestibular compensation also offers promising therapeutic avenues for treating vestibular disorders. While most studies have focused on the dynamics of GABAergic synaptic plasticity and intrinsic cellular adaptations in the ipsilesional medial vestibular nucleus (MVN) after unilateral labyrinthectomy (UL), the role of glutamatergic synaptic plasticity in this process remains largely unexplored. Here, we employed Golgi staining, immunofluorescence, whole-cell patch-clamp recordings, and behavioral assessments to examine the structural and functional dynamics of glutamatergic synapses during vestibular compensation. Our results reveal rapid structural and functional plasticity of glutamatergic transmission in response to UL. Specifically, dendritic spine density and morphology in the ipsilesional MVN recovered to baseline levels within 6 to 24 h post-UL. Furthermore, UL-induced postsynaptic depression of glutamatergic synaptic strength, reflected by a reduced AMPA/NMDA ratio, was reversed within 24 h, likely due to an upregulation of Ca2+-permeable AMPA receptors. In contrast, presynaptic glutamate release probability, as indicated by a reduced frequency of spontaneous excitatory postsynaptic currents, was not fully compensated during this period. These results suggest that while presynaptic properties recover more slowly in ipsilesional MVN neurons following UL, postsynaptic glutamatergic transmission undergoes rapid structural and functional reorganization. The findings highlight glutamatergic synaptic plasticity as a critical driver for vestibular compensation and suggest that pharmacological interventions targeting these mechanisms may accelerate functional recovery, offering potential therapeutic avenues for vestibular disorders.

Fear conditioning: Insights into learning, memory and extinction and its relevance to clinical disorders

Fear, whether innate or learned, is an essential emotion required for survival. The learning, and subsequent memory, of fearful events enhances our ability to recognise and respond to threats, aiding adaptation to new, ever-changing environments. Considerable research has leveraged associative learning protocols such as contextual or auditory forms of fear conditioning in rodents, to understand fear learning, memory consolidation and extinction phases of memory. Such assays have led to detailed characterisation of the underlying neurocircuitry and neurobiology supporting fear learning processes. Given fear processing is conserved across rodents and humans, fear conditioning experiments provide translational insights into fundamental memory processes and fear-related pathologies. This review examines associative learning protocols used to measure fear learning, memory and extinction, before providing an overview on the underlying complex neurocircuitry including the amygdala, hippocampus and medial prefrontal cortex. This is followed by an in-depth commentary on the neurobiology, particularly synaptic plasticity mechanisms, which regulate fear learning, memory and extinction. Next, we consider how fear conditioning assays in rodents can inform our understanding of disrupted fear memory in human disorders such as post-traumatic stress disorder (PTSD), anxiety and psychiatric disorders including schizophrenia. Lastly, we critically evaluate fear conditioning protocols, highlighting some of the experimental and theoretical limitations and the considerations required when conducting such assays, alongside recent methodological advancements in the field. Overall, rodent-based fear conditioning assays remain central to making progress in uncovering fundamental memory phenomena and understanding the aetiological mechanisms that underpin fear associated disorders, alongside the development of effective therapeutic strategies.

A posterior insula to lateral amygdala pathway transmits US-offset information with a limited role in fear learning

During fear learning, associations between a sensory cue (conditioned stimulus, CS) and an aversive stimulus (unconditioned stimulus, US) are formed in specific brain circuits. The lateral amygdala (LA) is involved in CS-US integration; however, US pathways to the LA remain understudied. Here, we investigated whether the posterior insular cortex (pInsCx), a hub for aversive state signaling, transmits US information to the LA during fear learning. We find that the pInsCx makes a robust, glutamatergic projection specifically targeting the anterior LA. In vivo Ca2+ imaging reveals that neurons in the pInsCx and anterior LA display US-onset and US-offset responses; imaging combined with axon silencing shows that the pInsCx selectively transmits US-offset information to the anterior LA. Optogenetic silencing, however, does not show a role for US-driven activity in the anterior LA or its pInsCx afferents in fear memory formation. Thus, we describe a cortical projection that carries US-offset information to the amygdala with a limited role in fear learning.

Early-life stress induces persistent astrocyte dysfunction associated with fear generalisation

Early-life stress can have lifelong consequences, enhancing stress susceptibility and resulting in behavioural and cognitive deficits. While the effects of early-life stress on neuronal function have been well-described, we still know very little about the contribution of non-neuronal brain cells. Investigating the complex interactions between distinct brain cell types is critical to fully understand how cellular changes manifest as behavioural deficits following early-life stress. Here, using male and female mice we report that early-life stress induces anxiety-like behaviour and fear generalisation in an amygdala-dependent learning and memory task. These behavioural changes were associated with impaired synaptic plasticity, increased neural excitability, and astrocyte hypofunction. Genetic perturbation of amygdala astrocyte function by either reducing astrocyte calcium activity or reducing astrocyte network function was sufficient to replicate cellular, synaptic, and fear memory generalisation associated with early-life stress. Our data reveal a role of astrocytes in tuning emotionally salient memory and provide mechanistic links between early-life stress, astrocyte hypofunction, and behavioural deficits.

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.

Sex Differences in Neural Circuits Underlying Fear Processing

Neural circuitry involved in anxiety and fear-related disorders exhibits strong sexual modulation. A limited number of studies integrating female and male data have revealed differences in neural networks, and distinct interconnectivity between these brain areas. Despite the efforts to incorporate female or mixed-sex data, there is compelling evidence that sex, as a biological variable, significantly influences fear processing. This chapter presents primary findings on sex differences in fear circuitry. It is imperative to consider this factor to ensure scientific research's integrity and understand how fear is processed in the central nervous system.

Dualism, Materialism, and the relationship between the brain and the mind in experiencing pain

Characterizing the relationship between the brain and the mind is essential, both for understanding how we experience sensations and for attempts to create machine-based artificial intelligence. Materialists argue that the brain and the mind are both physical/material in nature whereas Cartesian dualists posit that the brain is material, the mind is non-material, and that they are separate. Recent investigations into the mechanisms responsible for pain can resolve this issue. Pain from an injury requires both the induction of a long-term potentiation (LTP) in a subset of pyramidal neurons in the anterior cingulate cortex and the creation of electromagnetic waves in the surrounding area. The LTP sensitizes synaptic transmission and, by activating enzyme cascades, changes the phenotype of the pyramidal neurons. The changes sustain the generation of the waves and the pain. The waves rapidly disseminate information about the pain to distant areas of the brain and studies using Transcranial Stimulation show that EM waves can influence the induction of LTP. According to leading contemporary theories, the waves will communicate with the mind, which is where the painfulness is experienced. The material brain and immaterial mind are therefore separate and we can no longer attribute painfulness solely to the activities of the brain. This is a radical departure from the contemporary view of brain functions and supports Cartesian Dualism. Consequently, consciousness and higher mental functions cannot be duplicated by mimicking the activities of the brain.

The interaction between cannabinoids and long-term synaptic plasticity: A survey on memory formation and underlying mechanisms

Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), is an essential phenomenon in memory formation as well as maintenance along with many other cognitive functions, such as those needed for coping with external stimuli. Synaptic plasticity consists of gradual changes in the biochemistry and morphology of pre- and postsynaptic neurons, particularly in the hippocampus. Consuming marijuana as a primary source of exocannabinoids immediately impairs attention and working memory-related tasks. Evidence regarding the effects of cannabinoids on LTP and memory is contradictory. While cannabinoids can affect a variety of specific cannabinoid receptors (CBRs) and nonspecific receptors throughout the body and brain, they exert miscellaneous systemic and local cerebral effects. Given the increasing use of cannabis, mainly among the young population, plus its potential adverse long-term effects on learning and memory processes, it could be a future global health challenge. Indeed, the impact of cannabinoids on memory is multifactorial and depends on the dosage, timing, formula, and route of consumption, plus the background complex interaction of the endocannabinoids system with other cerebral networks. Herein, we review how exogenously administrated organic cannabinoids, CBRs agonists or antagonists, and endocannabinoids can affect LTP and synaptic plasticity through various receptors in interaction with other cerebral pathways and primary neurotransmitters.

Synaptic correlates of the corticocortical circuit in motor learning

Rodents actively learn new motor skills for survival in reaction to changing environments. Despite the classic view of the primary motor cortex (M1) as a simple muscle relay region, it is now known to play a significant role in motor skill acquisition. The secondary motor cortex (M2) is reported to be a crucial region for motor learning as well as for its role in motor execution and planning. Although these two regions are known for the part they play in motor learning, the role of direct connection and synaptic correlates between these two regions remains elusive. Here, we confirm M2 to M1 connectivity with a series of tracing experiments. We also show that the accelerating rotarod task successfully induces motor skill acquisition in mice. For mice that underwent rotarod training, learner mice showed increased synaptic density and spine head size for synapses between activated cell populations of M2 and M1. Non-learner mice did not show these synaptic changes. Collectively, these data suggest the potential importance of synaptic plasticity between activated cell populations as a potential mechanism of motor learning. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Non-Hebbian plasticity transforms transient experiences into lasting memories

The dominant models of learning and memory, such as Hebbian plasticity, propose that experiences are transformed into memories through input-specific synaptic plasticity at the time of learning. However, synaptic plasticity is neither strictly input-specific nor restricted to the time of its induction. The impact of such forms of non-Hebbian plasticity on memory has been difficult to test, and hence poorly understood. Here, we demonstrate that synaptic manipulations can deviate from the Hebbian model of learning, yet produce a lasting memory. First, we established a weak associative conditioning protocol in mice, where optogenetic stimulation of sensory thalamic input to the amygdala was paired with a footshock, but no detectable memory was formed. However, when the same input was potentiated minutes before or after, or even 24 hr later, the associative experience was converted into a lasting memory. Importantly, potentiating an independent input to the amygdala minutes but not 24 hr after the pairing produced a lasting memory. Thus, our findings suggest that the process of transformation of a transient experience into a memory is neither restricted to the time of the experience nor to the synapses triggered by it; instead, it can be influenced by past and future events.

Half a century legacy of long-term potentiation

In 1973, two papers from Bliss and Lømo and from Bliss and Gardner-Medwin reported that high-frequency synaptic stimulation in the dentate gyrus of rabbits resulted in a long-lasting increase in synaptic strength. This form of synaptic plasticity, commonly referred to as long-term potentiation (LTP), was immediately considered as an attractive mechanism accounting for the ability of the brain to store information. In this historical piece looking back over the past 50 years, we discuss how these two landmark contributions directly motivated a colossal research effort and detail some of the resulting milestones that have shaped our evolving understanding of the molecular and cellular underpinnings of LTP. We highlight the main features of LTP, cover key experiments that defined its induction and expression mechanisms, and outline the evidence supporting a potential role of LTP in learning and memory. We also briefly explore some ramifications of LTP on network stability, consider current limitations of LTP as a model of associative memory, and entertain future research orientations.

Memory Trace for Fear Extinction: Fragile yet Reinforceable

Fear extinction is a biological process in which learned fear behavior diminishes without anticipated reinforcement, allowing the organism to re-adapt to ever-changing situations. Based on the behavioral hypothesis that extinction is new learning and forms an extinction memory, this new memory is more readily forgettable than the original fear memory. The brain's cellular and synaptic traces underpinning this inherently fragile yet reinforceable extinction memory remain unclear. Intriguing questions are about the whereabouts of the engram neurons that emerged during extinction learning and how they constitute a dynamically evolving functional construct that works in concert to store and express the extinction memory. In this review, we discuss recent advances in the engram circuits and their neural connectivity plasticity for fear extinction, aiming to establish a conceptual framework for understanding the dynamic competition between fear and extinction memories in adaptive control of conditioned fear responses.

Distinct prefrontal projection activity and transcriptional state conversely orchestrate social competition and hierarchy

Social animals compete for limited resources, resulting in a social hierarchy. Although different neuronal subpopulations in the medial prefrontal cortex (mPFC), which has been mechanistically implicated in social dominance behavior, encode distinct social competition behaviors, their identities and associated molecular underpinnings have not yet been identified. In this study, we found that mPFC neurons projecting to the nucleus accumbens (mPFC-NAc) encode social winning behavior, whereas mPFC neurons projecting to the ventral tegmental area (mPFC-VTA) encode social losing behavior. High-throughput single-cell transcriptomic analysis and projection-specific genetic manipulation revealed that the expression level of POU domain, class 3, transcription factor 1 (Pou3f1) in mPFC-VTA neurons controls social hierarchy. Optogenetic activation of mPFC-VTA neurons increases Pou3f1 expression and lowers social rank. Together, these data demonstrate that discrete activity and gene expression in separate mPFC projections oppositely orchestrate social competition and hierarchy.

Fear Learning: An Evolving Picture for Plasticity at Synaptic Afferents to the Amygdala

Unraveling the neuronal mechanisms of fear learning might allow neuroscientists to make links between a learned behavior and the underlying plasticity at specific synaptic connections. In fear learning, an innocuous sensory event such as a tone (called the conditioned stimulus, CS) acquires an emotional value when paired with an aversive outcome (unconditioned stimulus, US). Here, we review earlier studies that have shown that synaptic plasticity at thalamic and cortical afferents to the lateral amygdala (LA) is critical for the formation of auditory-cued fear memories. Despite the early progress, it has remained unclear whether there are separate synaptic inputs that carry US information to the LA to act as a teaching signal for plasticity at CS-coding synapses. Recent findings have begun to fill this gap by showing, first, that thalamic and cortical auditory afferents can also carry US information; second, that the release of neuromodulators contributes to US-driven teaching signals; and third, that synaptic plasticity additionally happens at connections up- and downstream of the LA. Together, a picture emerges in which coordinated synaptic plasticity in serial and parallel circuits enables the formation of a finely regulated fear memory.

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.

Optogenetics: Illuminating the Future of Hearing Restoration and Understanding Auditory Perception

Hearing loss is a prevalent sensory impairment significantly affecting communication and quality of life. Traditional approaches for hearing restoration, such as cochlear implants, have limitations in frequency resolution and spatial selectivity. Optogenetics, an emerging field utilizing light-sensitive proteins, offers a promising avenue for addressing these limitations and revolutionizing hearing rehabilitation. This review explores the methods of introducing Channelrhodopsin- 2 (ChR2), a key light-sensitive protein, into cochlear cells to enable optogenetic stimulation. Viral- mediated gene delivery is a widely employed technique in optogenetics. Selecting a suitable viral vector, such as adeno-associated viruses (AAV), is crucial in efficient gene delivery to cochlear cells. The ChR2 gene is inserted into the viral vector through molecular cloning techniques, and the resulting viral vector is introduced into cochlear cells via direct injection or round window membrane delivery. This allows for the expression of ChR2 and subsequent light sensitivity in targeted cells. Alternatively, direct cell transfection offers a non-viral approach for ChR2 delivery. The ChR2 gene is cloned into a plasmid vector, which is then combined with transfection agents like liposomes or nanoparticles. This mixture is applied to cochlear cells, facilitating the entry of the plasmid DNA into the target cells and enabling ChR2 expression. Optogenetic stimulation using ChR2 allows for precise and selective activation of specific neurons in response to light, potentially overcoming the limitations of current auditory prostheses. Moreover, optogenetics has broader implications in understanding the neural circuits involved in auditory processing and behavior. The combination of optogenetics and gene delivery techniques provides a promising avenue for improving hearing restoration strategies, offering the potential for enhanced frequency resolution, spatial selectivity, and improved auditory perception.

Engram cell connectivity as a mechanism for information encoding and memory function

Information derived from experiences is incorporated into the brain as changes to ensembles of cells, termed engram cells, which allow memory storage and recall. The mechanism by which those changes hold specific information is unclear. Here, we test the hypothesis that the specific synaptic wiring between engram cells is the substrate of information storage. First, we monitor how learning modifies the connectivity pattern between engram cells at a monosynaptic connection involving the hippocampal ventral CA1 (vCA1) region and the amygdala. Then, we assess the functional significance of these connectivity changes by artificially activating or inhibiting its presynaptic and postsynaptic components, respectively. Finally, we identify a synaptic plasticity mechanism mediated by postsynaptic density protein 95 (PSD-95), which impacts the connectivity pattern among engram cells and contributes to the long-term stability of the memory. These findings impact our theory of learning and memory by helping us explain the translation of specific information into engram cells and how these connections shape brain function.

Excitatory subtypes of the lateral amygdala neurons are differentially involved in regulation of synaptic plasticity and excitation/inhibition balance in aversive learning in mice

The amygdala plays a crucial role in aversive learning. In Pavlovian fear conditioning, sensory information about an emotionally neutral conditioned stimulus (CS) and an innately aversive unconditioned stimulus is associated with the lateral amygdala (LA), and the CS acquires the ability to elicit conditioned responses. Aversive learning induces synaptic plasticity in LA excitatory neurons from CS pathways, such as the medial geniculate nucleus (MGN) of the thalamus. Although LA excitatory cells have traditionally been classified based on their firing patterns, the relationship between the subtypes and functional properties remains largely unknown. In this study, we classified excitatory cells into two subtypes based on whether the after-depolarized potential (ADP) amplitude is expressed in non-ADP cells and ADP cells. Their electrophysiological properties were significantly different. We examined subtype-specific synaptic plasticity in the MGN-LA pathway following aversive learning using optogenetics and found significant experience-dependent plasticity in feed-forward inhibitory responses in fear-conditioned mice compared with control mice. Following aversive learning, the inhibition/excitation (I/E) balance in ADP cells drastically changed, whereas that in non-ADP cells tended to change in the reverse direction. These results suggest that the two LA subtypes are differentially regulated in relation to synaptic plasticity and I/E balance during aversive learning.

Impact of volume and expression time in an AAV-delivered channelrhodopsin

Optogenetics has revolutionised neuroscience research, but at the same time has brought a plethora of new variables to consider when designing an experiment with AAV-based targeted gene delivery. Some concerns have been raised regarding the impact of AAV injection volume and expression time in relation to longitudinal experimental designs. In this study, we investigated the efficiency of optically evoked post-synaptic responses in connection to two variables: the volume of the injected virus and the expression time of the virus. For this purpose, we expressed the blue-shifted ChR2, oChIEF, employing a widely used AAV vector delivery strategy. We found that the volume of the injected virus has a minimal impact on the efficiency of optically-evoked postsynaptic population responses. The expression time, on the other hand, has a pronounced effect, with a gradual reduction in the population responses beyond 4 weeks of expression. We strongly advise to monitor time-dependent expression profiles when planning or conducting long-term experiments that depend on successful and stable channelrhodopsin expression.

Synergistic photoactivation of VTA-catecholaminergic and BLA-glutamatergic projections induces long-term potentiation in the insular cortex

The presentation of novel stimuli induces a reliable dopamine release in the insular cortex (IC) from the ventral tegmental area (VTA). The novel stimuli could be associated with motivational and emotional signals induced by cortical glutamate release from the basolateral amygdala (BLA). Dopamine and glutamate are essential for acquiring and maintaining behavioral tasks, including visual and taste recognition memories. In this study, we hypothesize that the simultaneous activation of dopaminergic and glutamatergic projections to the neocortex can underlie synaptic plasticity. High-frequency stimulation of the BLA-IC circuit has demonstrated a reliable long-term potentiation (LTP), a widely acknowledged synaptic plasticity that underlies memory consolidation. Therefore, the concurrent optogenetic stimulation of the insula's glutamatergic and dopaminergic terminal fibers would induce reliable LTP. Our results confirmed that combined photostimulation of the VTA and BLA projections to the IC induces a slow-onset LTP. We also found that optogenetically-induced LTP in the IC relies on both glutamatergic NMDA receptors and dopaminergic D1/D5 receptors, suggesting that the combined effects of these neurotransmitters can trigger synaptic plasticity in the neocortex. Overall, our findings provide compelling evidence supporting the essential role of both dopaminergic and glutamatergic projections in modulating synaptic plasticity within the IC. Furthermore, our results suggest that the synergistic actions of these projections have a pivotal influence on the formation of motivational memories.

The role of the hippocampus in the consolidation of emotional memories during sleep

Episodic memory relies on the hippocampus, a heterogeneous brain region with distinct functions. Spatial representations in the dorsal hippocampus (dHPC) are crucial for contextual memory, while the ventral hippocampus (vHPC) is more involved in emotional processing. Here, we review the literature in rodents highlighting the anatomical and functional properties of the hippocampus along its dorsoventral axis that underlie its role in contextual and emotional memory encoding, consolidation, and retrieval. We propose that the coordination between the dorsal and vHPC through theta oscillations during rapid eye movement (REM) sleep, and through sharp-wave ripples during non-REM (NREM) sleep, might facilitate the transfer of contextual information for integration with valence-related processing in other structures of the network. Further investigation into the physiology of the vHPC and its connections with other brain areas is needed to deepen the current understanding of emotional memory consolidation during sleep.

The Basolateral Amygdala: The Core of a Network for Threat Conditioning, Extinction, and Second-Order Threat Conditioning

Threat conditioning, extinction, and second-order threat conditioning studied in animal models provide insight into the brain-based mechanisms of fear- and anxiety-related disorders and their treatment. Much attention has been paid to the role of the basolateral amygdala (BLA) in such processes, an overview of which is presented in this review. More recent evidence suggests that the BLA serves as the core of a greater network of structures in these forms of learning, including associative and sensory cortices. The BLA is importantly regulated by hippocampal and prefrontal inputs, as well as by the catecholaminergic neuromodulators, norepinephrine and dopamine, that may provide important prediction-error or learning signals for these forms of learning. The sensory cortices may be required for the long-term storage of threat memories. As such, future research may further investigate the potential of the sensory cortices for the long-term storage of extinction and second-order conditioning memories.

Engram cell connectivity as a mechanism for information encoding and memory function

Information derived from experiences is incorporated into the brain as changes to ensembles of cells, termed engram cells, that allow memory storage and recall. The mechanism by which those changes hold specific information is unclear. Here we test the hypothesis that the specific synaptic wiring between engram cells is the substrate of information storage. First, we monitor how learning modifies the connectivity pattern between engram cells at a monosynaptic connection involving the hippocampal vCA1 region and the amygdala. Then, we assess the functional significance of these connectivity changes by artificially activating or inhibiting its presynaptic and postsynaptic components respectively. Finally, we identify a synaptic plasticity mechanism mediated by PSD-95, which impacts the connectivity pattern among engram cells and contributes to the long-term stability of the memory. These findings impact our theory of learning and memory by helping us explain the translation of specific information into engram cells and how these connections shape brain function.

How engram mediates learning, extinction, and relapse

Fear learning ensures survival through an expression of certain behavior as a conditioned fear response. Fear memory is processed and stored in a fear memory circuit, including the amygdala, hippocampus, and prefrontal cortex. A gradual decrease in conditioned fear response can be induced by fear extinction, which is mediated through the weakening of the original fear memory traces and the newly formed inhibition of those traces. Fear memory can also recover after extinction, which shows flexible control of the fear memory state. Here, we demonstrate how fear engram, which is a physical substrate of fear memory, changes during fear extinction and relapse by reviewing recent studies regarding engram.

Engram neurons: Encoding, consolidation, retrieval, and forgetting of memory

Tremendous strides have been made in our understanding of the neurobiological substrates of memory - the so-called memory "engram". Here, we integrate recent progress in the engram field to illustrate how engram neurons transform across the "lifespan" of a memory - from initial memory encoding, to consolidation and retrieval, and ultimately to forgetting. To do so, we first describe how cell-intrinsic properties shape the initial emergence of the engram at memory encoding. Second, we highlight how these encoding neurons preferentially participate in synaptic- and systems-level consolidation of memory. Third, we describe how these changes during encoding and consolidation guide neural reactivation during retrieval, and facilitate memory recall. Fourth, we describe neurobiological mechanisms of forgetting, and how these mechanisms can counteract engram properties established during memory encoding, consolidation, and retrieval. Motivated by recent experimental results across these four sections, we conclude by proposing some conceptual extensions to the traditional view of the engram, including broadening the view of cell-type participation within engrams and across memory stages. In collection, our review synthesizes general principles of the engram across memory stages, and describes future avenues to further understand the dynamic engram.

Npas4-mediated dopaminergic regulation of safety memory consolidation

Amygdala circuitry encodes associations between conditioned stimuli and aversive unconditioned stimuli and also controls fear expression. However, whether and how non-threatening information for unpaired conditioned stimuli (CS^-) is discretely processed remains unknown. The fear expression toward CS^- is robust immediately after fear conditioning but then becomes negligible after memory consolidation. The synaptic plasticity of the neural pathway from the lateral to the anterior basal amygdala gates the fear expression of CS^-, depending upon neuronal PAS domain protein 4 (Npas4)-mediated dopamine receptor D4 (Drd4) synthesis, which is precluded by stress exposure or corticosterone injection. Herein, we show cellular and molecular mechanisms that regulate the non-threatening (safety) memory consolidation, supporting the fear discrimination.

Silencing KCC2 in mouse dorsal hippocampus compromises spatial and contextual memory

Delayed upregulation of the neuronal chloride extruder KCC2 underlies the progressive shift in GABA signaling polarity during development. Conversely, KCC2 downregulation is observed in a variety of neurological and psychiatric disorders often associated with cognitive impairment. Reduced KCC2 expression and function in mature networks may disrupt GABA signaling and promote anomalous network activities underlying these disorders. However, the causal link between KCC2 downregulation, altered brain rhythmogenesis, and cognitive function remains elusive. Here, by combining behavioral exploration with in vivo electrophysiology we assessed the impact of chronic KCC2 downregulation in mouse dorsal hippocampus and showed it compromises both spatial and contextual memory. This was associated with altered hippocampal rhythmogenesis and neuronal hyperexcitability, with increased burst firing in CA1 neurons during non-REM sleep. Reducing neuronal excitability with terbinafine, a specific Task-3 leak potassium channel opener, occluded the impairment of contextual memory upon KCC2 knockdown. Our results establish a causal relationship between KCC2 expression and cognitive performance and suggest that non-epileptiform rhythmopathies and neuronal hyperexcitability are central to the deficits caused by KCC2 downregulation in the adult mouse brain.

A Sleep-Specific Midbrain Target for Sevoflurane Anesthesia

Sevoflurane has been the most widely used inhaled anesthetics with a favorable recovery profile; however, the precise mechanisms underlying its anesthetic action are still not completely understood. Here the authors show that sevoflurane activates a cluster of urocortin 1 (UCN1+ )/cocaine- and amphetamine-regulated transcript (CART+ ) neurons in the midbrain involved in its anesthesia. Furthermore, growth hormone secretagogue receptor (GHSR) is highly enriched in sevoflurane-activated UCN1+ /CART+ cells and is necessary for sleep induction. Blockade of GHSR abolishes the excitatory effect of sevoflurane on UCN1+ /CART+ neurons and attenuates its anesthetic effect. Collectively, their data suggest that anesthetic action of sevoflurane necessitates the GHSR activation in midbrain UCN1+ /CART+ neurons, which provides a novel target including the nucleus and receptor in the field of anesthesia.

ACx-projecting cholinergic neurons in the NB influence the BLA ensembles to modulate the discrimination of auditory fear memory

Animals need discriminating auditory fear memory (DAFM) to survive, but the related neural circuits of DAFM remain largely unknown. Our study shows that DAFM depends on acetylcholine (ACh) signal in the auditory cortex (ACx), which is projected from the nucleus basalis (NB). At the encoding stage, optogenetic inhibition of cholinergic projections of NB-ACx obfuscates distinct tone-responsive neurons of ACx recognizing from fear-paired tone to fear-unpaired tone signals, while simultaneously regulating the neuronal activity and reactivation of basal lateral amygdala (BLA) engram cells at the retrieval stage. This NB^ACh-ACx-BLA neural circuit for the modulation of DAFM is especially dependent on the nicotinic ACh receptor (nAChR). A nAChR antagonist reduces DAFM and diminishes the increased magnitude of ACx tone-responsive neuronal activity during the encoding stage. Our data suggest a critical role of NB^ACh-ACx-BLA neural circuit in DAFM: manipulation of the NB cholinergic projection to the ACx via nAChR during the encoding stage affects the activation of ACx tone-responsive neuron clusters and the BLA engram cells during the retrieval stage, thus modulating the DAFM.

Memory retrieval, reconsolidation, and extinction: Exploring the boundary conditions of post-conditioning cue exposure

Following fear conditioning, behavior can be reduced by giving many CS-alone presentations in a process known as extinction or by presenting a few CS-alone presentations and interfering with subsequent memory reconsolidation. While the two share procedural similarities, both the behavioral outcomes and the neurobiological underpinnings are distinct. Here we review the neural and behavioral mechanisms that produce these separate behavioral reductions, as well as some factors that determine whether or not a retrieval-dependent reconsolidation process or an extinction process will be in effect.

NMDAR-dependent synaptic potentiation via APPL1 signaling is required for the accessibility of a prefrontal neuronal assembly in retrieving fear extinction

BACKGROUND

The ventromedial prefrontal cortex (vmPFC) has been viewed as a locus to store and recall extinction memory. However, the synaptic and cellular mechanisms underlying this process remain elusive.

METHODS

We combined transgenic mice, electrophysiological recording, activity-dependent cell labeling, and chemogenetic manipulation to analyze the role of adaptor protein APPL1 in the vmPFC for fear extinction retrieval.

RESULTS

We found that both constitutive and conditional APPL1 knockout decreases NMDA receptor (NMDAR) function in the vmPFC and impairs fear extinction retrieval. Moreover, APPL1 undergoes nuclear translocation during extinction retrieval. Blocking APPL1 nucleocytoplasmic translocation reduces NMDAR currents and disrupts extinction retrieval. We further identified a prefrontal neuronal ensemble that is both necessary and sufficient for the storage of extinction memory. Inducible APPL1 knockout in this ensemble abolishes NMDAR-dependent synaptic potentiation and disrupts extinction retrieval, while simultaneously chemogenetic activation of this ensemble rescues the impaired behaviors.

CONCLUSIONS

Therefore, our results indicate that a prefrontal neuronal ensemble stores extinction memory, and APPL1 signaling supports these neurons to retrieve extinction memory via controlling NMDAR-dependent potentiation.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

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

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

Neural Oscillations in Aversively Motivated Behavior

Fear and anxiety-based disorders are highly debilitating and among the most prevalent psychiatric disorders. These disorders are associated with abnormal network oscillations in the brain, yet a comprehensive understanding of the role of network oscillations in the regulation of aversively motivated behavior is lacking. In this review, we examine the oscillatory correlates of fear and anxiety with a particular focus on rhythms in the theta and gamma-range. First, we describe neural oscillations and their link to neural function by detailing the role of well-studied theta and gamma rhythms to spatial and memory functions of the hippocampus. We then describe how theta and gamma oscillations act to synchronize brain structures to guide adaptive fear and anxiety-like behavior. In short, that hippocampal network oscillations act to integrate spatial information with motivationally salient information from the amygdala during states of anxiety before routing this information via theta oscillations to appropriate target regions, such as the prefrontal cortex. Moreover, theta and gamma oscillations develop in the amygdala and neocortical areas during the encoding of fear memories, and interregional synchronization reflects the retrieval of both recent and remotely encoded fear memories. Finally, we argue that the thalamic nucleus reuniens represents a key node synchronizing prefrontal-hippocampal theta dynamics for the retrieval of episodic extinction memories in the hippocampus.

Sex-Specific Neural Networks of Cued Threat Conditioning: A Pilot Study

Cued threat conditioning is the most common preclinical model for emotional memory, which is dysregulated in anxiety disorders and post-traumatic stress disorder. Though women are twice as likely as men to develop these disorders, current knowledge of threat conditioning networks was established by studies that excluded female subjects. For unbiased investigation of sex differences in these networks, we quantified the neural activity marker c-fos across 112 brain regions in adult male and female mice after cued threat conditioning compared to naïve controls. We found that trained females engaged prelimbic cortex, lateral amygdala, cortical amygdala, dorsal peduncular cortex, and subparafasicular nucleus more than, and subparaventricular zone less than, trained males. To explore how these sex differences in regional activity impact the global network, we generated interregional cross-correlations of c-fos expression to identify regions that were co-active during conditioning and performed hub analyses to identify regional control centers within each neural network. These exploratory graph theory-derived analyses revealed sex differences in the functional coordination of the threat conditioning network as well as distinct hub regions between trained males and females. Hub identification across multiple networks constructed by sequentially pruning the least reliable connections revealed globus pallidus and ventral lateral septum as the most robust hubs for trained males and females, respectively. While low sample size and lack of non-associative controls are major limitations, these findings provide preliminary evidence of sex differences in the individual circuit components and broader global networks of threat conditioning that may confer female vulnerability to fear-based psychiatric disease.

The Neural Circuit Architecture of Social Hierarchy in Rodents and Primates

Social status is recognized as a major determinant of social behavior and health among animals; however, the neural circuits supporting the formation and navigation of social hierarchies remain under extensive research. Available evidence suggests the prefrontal cortex is a keystone in this circuit, but upstream and downstream candidates are progressively emerging. In this review, we compare and integrate findings from rodent and primate studies to create a model of the neural and cellular networks supporting social hierarchies, both from a macro (i.e., circuits) to a micro-scale perspective (microcircuits and synapses). We start by summarizing the literature on the prefrontal cortex and other relevant brain regions to expand the current “prefrontal-centric” view of social hierarchy behaviors. Based on connectivity data we also discuss candidate regions that might inspire further investigation, as well as the caveats and strategies that have been used to further our understanding of the biological substrates underpinning social hierarchy and dominance.

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

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

N-acetylcysteine facilitates extinction of cued fear memory in rats via reestablishing basolateral amygdala glutathione homeostasis

Individual differences in the development of uncontrollable fear in response to traumatic stressors have been observed in clinic, but the underlying mechanisms remain unknown. In the present study we first conducted a meta-analysis of published clinical data and found that malondialdehyde, an oxidative stress biomarker, was significantly elevated in the blood of patients with fear-related anxiety disorders. We then carried out experimental study in rats subjected to fear conditioning. We showed that reestablishing redox homeostasis in basolateral amygdale (BLA) after exposure to fear stressors determined the capacity of learned fear inhibition. Intra-BLA infusion of buthionine sulfoximine (BSO) to deplete the most important endogenous antioxidant glutathione (GSH) blocked fear extinction, whereas intra-BLA infusion of dithiothreitol or N-acetylcysteine (a precursor of GSH) facilitated extinction. In electrophysiological studies conducted on transverse slices, we showed that fear stressors induced redox-dependent inhibition of NMDAR-mediated synaptic function, which was rescued by extinction learning or reducing agents. Our results reveal a novel pharmacological strategy for reversing impaired fear inhibition and highlight the role of GSH in the treatment of psychiatric disorders.

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

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

Regional synapse gain and loss accompany memory formation in larval zebrafish

Defining the structural and functional changes in the nervous system underlying learning and memory represents a major challenge for modern neuroscience. Although changes in neuronal activity following memory formation have been studied [B. F. Grewe et al., Nature 543, 670-675 (2017); M. T. Rogan, U. V. Stäubli, J. E. LeDoux, Nature 390, 604-607 (1997)], the underlying structural changes at the synapse level remain poorly understood. Here, we capture synaptic changes in the midlarval zebrafish brain that occur during associative memory formation by imaging excitatory synapses labeled with recombinant probes using selective plane illumination microscopy. Imaging the same subjects before and after classical conditioning at single-synapse resolution provides an unbiased mapping of synaptic changes accompanying memory formation. In control animals and animals that failed to learn the task, there were no significant changes in the spatial patterns of synapses in the pallium, which contains the equivalent of the mammalian amygdala and is essential for associative learning in teleost fish [M. Portavella, J. P. Vargas, B. Torres, C. Salas, Brain Res. Bull 57, 397-399 (2002)]. In zebrafish that formed memories, we saw a dramatic increase in the number of synapses in the ventrolateral pallium, which contains neurons active during memory formation and retrieval. Concurrently, synapse loss predominated in the dorsomedial pallium. Surprisingly, we did not observe significant changes in the intensity of synaptic labeling, a proxy for synaptic strength, with memory formation in any region of the pallium. Our results suggest that memory formation due to classical conditioning is associated with reciprocal changes in synapse numbers in the pallium.

A potential biomarker of preclinical Alzheimer's disease: The olfactory dysfunction and its pathogenesis-based neural circuitry impairments

The olfactory dysfunction can signal and act as a potential biomarker of preclinical AD. However, the precise regulatory mechanism of olfactory function on the neural pathogenesis of AD is still unclear. The impairment of neural networks in olfaction system has been shown to be tightly associated with AD. As key brain regions of the olfactory system, the olfactory bulb (OB) and the piriform cortex (PCx) have a profound influence on the olfactory function. Therefore, this review will explore the mechanism of olfactory dysfunction in preclinical AD in the perspective of abnormal neural networks in the OB and PCx and their associated brain regions, especially from two aspects of aberrant oscillations and synaptic plasticity damages, which help better understand the underlying mechanism of olfactory neural network damages related to AD.

The entorhinal cortex modulates trace fear memory formation and neuroplasticity in the mouse lateral amygdala via cholecystokinin

Although fear memory formation is essential for survival and fear-related mental disorders, the neural circuitry and mechanism are incompletely understood. Here, we utilized trace fear conditioning to study the formation of trace fear memory in mice. We identified the entorhinal cortex (EC) as a critical component of sensory signaling to the amygdala. We adopted both loss-of-function and gain-of-function experiments to demonstrate that release of the cholecystokinin (CCK) from the EC is required for trace fear memory formation. We discovered that CCK-positive neurons project from the EC to the lateral nuclei of the amygdala (LA), and inhibition of CCK-dependent signaling in the EC prevented long-term potentiation of the auditory response in the LA and formation of trace fear memory. In summary, high-frequency activation of EC neurons triggers the release of CCK in their projection terminals in the LA, potentiating auditory response in LA neurons. The neural plasticity in the LA leads to trace fear memory formation.

Regulation of learned fear expression through the MgN-amygdala pathway

Heightened fear responding is characteristic of fear- and anxiety-related disorders, including post-traumatic stress disorder. Neural plasticity in the amygdala is essential for both initial fear learning and fear expression, and strengthening of synaptic connections between the medial geniculate nucleus (MgN) and amygdala is critical for auditory fear learning. However, very little is known about what happens in the MgN-amygdala pathway during fear recall and extinction, in which conditional fear decreases with repeated presentations of the auditory stimulus alone. In the present study, we found that optogenetic inhibition of activity in the MgN-amygdala pathway during fear retrieval and extinction reduced expression of conditional fear. While this effect persisted for at least two weeks following pathway inhibition, it was specific to the context in which optogenetic inhibition occurred, linking MgN-BLA inhibition to facilitation of extinction-like processes. Reduced fear expression through inhibition of the MgN-amygdala pathway was further characterized by similar synaptic expression of GluA1 and GluA2 AMPA receptor subunits compared to what was seen in controls. Inhibition also decreased CREB phosphorylation in the amygdala, similar to what has been reported following auditory fear extinction. We then demonstrated that this effect was reduced by inhibition of GluN2B-containing NMDA receptors. These results demonstrate a new and important role for the MgN-amygdala pathway in extinction-like processes, and show that suppressing activity in this pathway results in a persistent decrease in fear behavior.

Increased mGlu5 mRNA expression in BLA glutamate neurons facilitates resilience to the long-term effects of a single predator scent stress exposure

Post-traumatic stress disorder (PTSD) develops in a subset of individuals exposed to a trauma with core features being increased anxiety and impaired fear extinction. To model the heterogeneity of PTSD behavioral responses, we exposed male Sprague-Dawley rats to predator scent stress once for 10 min and then assessed anxiety-like behavior 7 days later using the elevated plus maze and acoustic startle response. Rats displaying anxiety-like behavior in both tasks were classified as stress Susceptible, and rats exhibiting behavior no different from un-exposed Controls were classified as stress Resilient. In Resilient rats, we previously found increased mRNA expression of mGlu5 in the amygdala and prefrontal cortex (PFC) and CB1 in the amygdala. Here, we performed fluorescent in situ hybridization (FISH) to determine the subregion and cell-type-specific expression of these genes in Resilient rats 3 weeks after TMT exposure. Resilient rats displayed increased mGlu5 mRNA expression in the basolateral amygdala (BLA) and the infralimbic and prelimbic regions of the PFC and increased BLA CB1 mRNA. These increases were limited to glutamatergic cells. To test the necessity of mGlu5 for attenuating TMT-conditioned contextual fear 3 weeks after TMT conditioning, intra-BLA infusions of the mGlu5 negative allosteric modulator MTEP were administered prior to context re-exposure. In TMT-exposed Resilient rats, but not Controls, MTEP increased freezing on the day of administration, which extinguished over two additional un-drugged sessions. These results suggest that increased mGlu5 expression in BLA glutamate neurons contributes to the behavioral flexibility observed in stress-Resilient animals by facilitating a capacity for extinguishing contextual fear associations.

GABAergic microcircuitry of fear memory encoding

The paradigm of fear conditioning is largely responsible for our current understanding of how memories are encoded at the cellular level. Its most fundamental underlying mechanism is considered to be plasticity of synaptic connections between excitatory projection neurons (PNs). However, recent studies suggest that while PNs execute critical memory functions, their activity at key stages of learning and recall is extensively orchestrated by a diverse array of GABAergic interneurons (INs). Here we review the contributions of genetically-defined INs to processing of threat-related stimuli in fear conditioning, with a particular focus on how synaptic interactions within interconnected networks of INs modulates PN activity through both inhibition and disinhibition. Furthermore, we discuss accumulating evidence that GABAergic microcircuits are an important locus for synaptic plasticity during fear learning and therefore a viable substrate for long-term memory. These findings suggest that further investigation of INs could unlock unique conceptual insights into the organization and function of fear memory networks.

Synaptic correlates of associative fear memory in the lateral amygdala

Successful adaptation to the environment requires an accurate response to external threats by recalling specific memories. Memory formation and recall require engram cell activity and synaptic strengthening among activated neuronal ensembles. However, elucidation of the underlying neural substrates of associative fear memory has remained limited without a direct interrogation of extinction-induced changes of specific synapses that encode a specific auditory fear memory. Using dual-eGRASP (enhanced green fluorescent protein reconstitution across synaptic partners), we found that synapses among activated neuronal ensembles or activated synaptic ensembles showed a significantly larger spine morphology at auditory cortex (AC)-to-lateral amygdala (LA) projections after auditory fear conditioning in mice. Fear extinction reversed these enhanced synaptic ensemble spines, whereas re-conditioning with the same tone and shock restored the spine size of the synaptic ensemble. We suggest that synaptic ensembles encode and represent different fear memory states.

SRC3 acetylates calmodulin in the mouse brain to regulate synaptic plasticity and fear learning

Protein acetylation is a reversible posttranslational modification, which is regulated by lysine acetyltransferase (KAT) and lysine deacetyltransferase (KDAC). Although protein acetylation has been shown to regulate synaptic plasticity, this was mainly for histone protein acetylation. The function and regulation of nonhistone protein acetylation in synaptic plasticity and learning remain largely unknown. Calmodulin (CaM), a ubiquitous Ca²⁺ sensor, plays critical roles in synaptic plasticity such as long-term potentiation (LTP). During LTP induction, activation of NMDA receptor triggers Ca²⁺ influx, and the Ca²⁺ binds with CaM and activates calcium/calmodulin-dependent protein kinase IIα (CaMKIIα). In our previous study, we demonstrated that acetylation of CaM was important for synaptic plasticity and fear learning in mice. However, the KAT responsible for CaM acetylation is currently unknown. Here, following an HEK293 cell-based screen of candidate KATs, steroid receptor coactivator 3 (SRC3) is identified as the most active KAT for CaM. We further demonstrate that SRC3 interacts with and acetylates CaM in a Ca²⁺ and NMDA receptor-dependent manner. We also show that pharmacological inhibition or genetic downregulation of SRC3 impairs CaM acetylation, synaptic plasticity, and contextual fear learning in mice. Moreover, the effects of SRC3 inhibition on synaptic plasticity and fear learning could be rescued by 3KQ-CaM, a mutant form of CaM, which mimics acetylation. Together, these observations demonstrate that SRC3 acetylates CaM and regulates synaptic plasticity and learning in mice.

Brain-wide mapping of presynaptic inputs to basolateral amygdala neurons

The basolateral amygdala (BLA), a region critical for emotional processing, is the limbic hub that is connected with various brain regions. BLA neurons are classified into different subtypes that exhibit differential projection patterns and mediate distinct emotional behaviors; however, little is known about their presynaptic input patterns. In this study, we employed projection-specific monosynaptic rabies virus tracing to identify the direct monosynaptic inputs to BLA subtypes. We found that each neuronal subtype receives long-range projection input from specific brain regions. In contrast to their specific axonal projection patterns, all BLA neuronal subtypes exhibited relatively similar input patterns. This anatomical organization supports the idea that the BLA is a central integrator that associates sensory information in different modalities with valence and sends associative information to behaviorally relevant brain regions.