Synaptic Plasticity, Engrams, and Network Oscillations in Amygdala Circuits for Storage and Retrieval of Emotional Memories

Recent advances in the neurobiology of posttraumatic stress disorder: A review of possible mechanisms underlying an effective pharmacotherapy

Recent progress in the field of neurobiology supported by clinical evidence gradually reveals the mystery of human brain functioning. So far, many psychiatric disorders have been described in great detail, although there are still plenty of cases that are misunderstood. These include posttraumatic stress disorder (PTSD), which is a unique disease that combines a wide range of neurobiological changes, which involve disturbances of the hypothalamic-pituitary-adrenal gland axis, hyperactivation of the amygdala complex, and attenuation of some hippocampal and cortical functions. Such multiplicity results in differential symptomatology, including elevated anxiety, nightmares, fear retrieval episodes that may trigger delusions and hallucinations, sleep disturbances, and many others that strongly interfere with the quality of the patient's life. Because of widespread neurological changes and the disease manifestation, the pharmacotherapy of PTSD remains unclear and requires a multidimensional approach and involvement of polypharmacotherapy. Hopefully, more and more neuroscientists and clinicians will study PTSD, which will provide us with new information that would possibly accelerate establishment of well-tolerated and effective pharmacotherapy. In this review, we have focused on neurobiological changes regarding PTSD, addressing the most disturbed brain structures and neurotransmissions, as well as discussing in detail the recently taken and novel therapeutic paths.

Hippocampal network oscillations at the interplay between innate anxiety and learned fear

The hippocampus plays a central role as a hub for episodic memory and as an integrator of multimodal sensory information in time and space. Thereby, it critically determines contextual setting and specificity of episodic memories. It is also a key site for the control of innate anxiety states and involved in psychiatric diseases with heightened anxiety and generalized fear memory such as post-traumatic stress disorder (PTSD). Expression of both innate "unlearned" anxiety and "learned" fear requires contextual processing and engagement of a brain-wide network including the hippocampus together with the amygdala and medial prefrontal cortex. Strikingly, the hippocampus is also the site of emergence of oscillatory rhythms that coordinate information processing and filtering in this network. Here, we review data on how the hippocampal network oscillations and their coordination with amygdalar and prefrontal oscillations are engaged in innate threat evaluation. We further explore how such innate oscillatory communication might have an impact on contextualization and specificity of "learned" fear. We illustrate the partial overlap of fear and anxiety networks that are built by the hippocampus in conjunction with amygdala and prefrontal cortex. We further propose that (mal)-adaptive interplay via (dis)-balanced oscillatory communication between the anxiety network and the fear network may determine the strength of fear memories and their resistance to extinction.

Valence coding in amygdala circuits

The neural mechanisms underlying emotional valence are at the interface between perception and action, integrating inputs from the external environment with past experiences to guide the behavior of an organism. Depending on the positive or negative valence assigned to an environmental stimulus, the organism will approach or avoid the source of the stimulus. Multiple convergent studies have demonstrated that the amygdala complex is a critical node of the circuits assigning valence. Here we examine the current progress in identifying valence coding properties of neural populations in different nuclei of the amygdala, based on their activity, connectivity, and gene expression profile.

Do Brain Oscillations Orchestrate Memory?

Rhythmicity and oscillations are common features in nature, and can be seen in phenomena such as seasons, breathing, and brain activity. Despite the fact that a single neuron transmits its activity to its neighbor through a transient pulse, rhythmic activity emerges from large population-wide activity in the brain, and such rhythms are strongly coupled with the state and cognitive functions of the brain. However, it is still debated whether the oscillations of brain activity actually carry information. Here, we briefly introduce the biological findings of brain oscillations, and summarize the recent progress in understanding how oscillations mediate brain function. Finally, we examine the possible relationship between brain cognitive function and oscillation, focusing on how oscillation is related to memory, particularly with respect to state-dependent memory formation and memory retrieval under specific brain waves. We propose that oscillatory waves in the neocortex contribute to the synchronization and activation of specific memory trace ensembles in the neocortex by promoting long-range neural communication.

Integration of Parallel Opposing Memories Underlies Memory Extinction

Accurately predicting an outcome requires that animals learn supporting and conflicting evidence from sequential experience. In mammals and invertebrates, learned fear responses can be suppressed by experiencing predictive cues without punishment, a process called memory extinction. Here, we show that extinction of aversive memories in Drosophila requires specific dopaminergic neurons, which indicate that omission of punishment is remembered as a positive experience. Functional imaging revealed co-existence of intracellular calcium traces in different places in the mushroom body output neuron network for both the original aversive memory and a new appetitive extinction memory. Light and ultrastructural anatomy are consistent with parallel competing memories being combined within mushroom body output neurons that direct avoidance. Indeed, extinction-evoked plasticity in a pair of these neurons neutralizes the potentiated odor response imposed in the network by aversive learning. Therefore, flies track the accuracy of learned expectations by accumulating and integrating memories of conflicting events.

Specificity of Primate Amygdalar Pathways to Hippocampus

The amygdala projects to hippocampus in pathways through which affective or social stimuli may influence learning and memory. We investigated the still unknown amygdalar termination patterns and their postsynaptic targets in hippocampus from system to synapse in rhesus monkeys of both sexes. The amygdala robustly innervated the stratum lacunosum-moleculare layer of cornu ammonis fields and uncus anteriorly. Sparser terminations in posterior hippocampus innervated the radiatum and pyramidal layers at the prosubicular/CA1 juncture. The terminations, which were larger than other afferents in the surrounding neuropil, position the amygdala to influence hippocampal input anteriorly, and its output posteriorly. Most amygdalar boutons (76-80%) innervated spines of excitatory hippocampal neurons, and most of the remaining innervated presumed inhibitory neurons, identified by morphology and label with parvalbumin or calretinin, which distinguished nonoverlapping neurochemical classes of hippocampal inhibitory neurons. In CA1, amygdalar axons innervated some calretinin neurons, which disinhibit pyramidal neurons. By contrast, in CA3 the amygdala innervated both calretinin and parvalbumin neurons; the latter strongly inhibit nearby excitatory neurons. In CA3, amygdalar pathways also made closely spaced dual synapses on excitatory neurons. The strong excitatory synapses in CA3 may facilitate affective context representations and trigger sharp-wave ripples associated with memory consolidation. When the amygdala is excessively activated during traumatic events, the specialized innervation of excitatory neurons and the powerful parvalbumin inhibitory neurons in CA3 may allow the suppression of activity of nearby neurons that receive weaker nonamygdalar input, leading to biased passage of highly charged affective stimuli and generalized fear.SIGNIFICANCE STATEMENT Strong pathways from the amygdala targeted the anterior hippocampus, and more weakly its posterior sectors, positioned to influence a variety of emotional and cognitive functions. In hippocampal field CA1, the amygdala innervated some calretinin neurons, which disinhibit excitatory neurons. By contrast, in CA3 the amygdala innervated calretinin as well as some of the powerful parvalbumin inhibitory neurons and may help balance the activity of neural ensembles to allow social interactions, learning, and memory. These results suggest that when the amygdala is hyperactive during emotional upheaval, it strongly activates excitatory hippocampal neurons and parvalbumin inhibitory neurons in CA3, which can suppress nearby neurons that receive weaker input from other sources, biasing the passage of stimuli with high emotional import and leading to generalized fear.

Synaptic encoding of fear memories in the amygdala

Over the years Pavlovian fear conditioning has proved to be a powerful model to investigate the neural underpinnings of aversive associative memory formation. Although it is well appreciated that plasticity occurring at excitatory synapses within the basolateral complex of the amygdala (BLA) plays a critical role in associative memory formation, recent evidence suggests that plasticity within the amygdala is more distributed than previously appreciated. In particular, studies demonstrate that plasticity in the central nucleus (CeA) is critical for the acquisition of conditioned fear. In addition, a variety of interneuron populations within the amygdala, defined by unique neurochemical markers, contribute to distinct aspects of stimulus processing and memory formation during fear conditioning. Here, we will review and summarize recent advances in our understanding of amygdala networks and how unique players within this network contribute to synaptic plasticity associated with the acquisition of conditioned fear.

Rhythmogenesis evolves as a consequence of long-term plasticity of inhibitory synapses

Brain rhythms are widely believed to reflect numerous cognitive processes. Changes in rhythmicity have been associated with pathological states. However, the mechanism underlying these rhythms remains unknown. Here, we present a theoretical analysis of the evolvement of rhythm generating capabilities in neuronal circuits. We tested the hypothesis that brain rhythms can be acquired via an intrinsic unsupervised learning process of activity dependent plasticity. Specifically, we focused on spike timing dependent plasticity (STDP) of inhibitory synapses. We detail how rhythmicity can develop via STDP under certain conditions that serve as a natural prediction of the hypothesis. We show how global features of the STDP rule govern and stabilize the resultant rhythmic activity. Finally, we demonstrate how rhythmicity is retained even in the face of synaptic variability. This study suggests a role for inhibitory plasticity that is beyond homeostatic processes.

Neural Coding of Appetitive Food Experiences in the Amygdala

Real-life experiences involve the consumption of various foods, yet it is unclear how the brain distinguishes and categorizes such food experiences. Despite the crucial roles of the basolateral amygdala (BLA) in appetitive behavior and emotion, how BLA pyramidal cells and interneurons encode food experiences has not yet been well characterized. Here we employ large-scale tetrode recording techniques to investigate the coding properties of pyramidal neurons vs. fast-spiking interneurons in the BLA as mice freely consumed a variety of foods, such as biscuits, rice, milk and water. We found that putative pyramidal cells conformed to the power-of-two-based permutation logic, as postulated by the Theory of Connectivity, to generate specific-to-general neural clique-coding patterns. Many pyramidal cells exhibited firing increases specific to a given food type, while some other pyramidal cells increased firings to various combinations of multiple foods. In contrast, fast-spiking interneurons can increase or decrease firings to given food types, and were more broadly tuned to various food experiences. We further show that a subset of pyramidal cells exhibited rapid desensitization to repeated eating of the same food, correlated with rapid behavioral habituation. Finally, we provide the intuitive visualization of BLA ensemble activation patterns using the dimensionality-reduction classification method to decode real-time appetitive stimulus identity on a moment-to-moment, single trial basis. Elucidation of the neural coding patterns in the BLA provides a key insight into how the brain's emotion and memory circuits performs the computational operation of pattern discrimination and categorization of natural food experiences.

Multiplexed oscillations and phase rate coding in the basal forebrain

Complex behaviors demand temporal coordination among functionally distinct brain regions. The basal forebrain's afferent and efferent structure suggests a capacity for mediating this coordination at a large scale. During performance of a spatial orientation task, synaptic activity in this region was dominated by four amplitude-independent oscillations temporally organized by the phase of the slowest, a theta-frequency rhythm. Oscillation amplitudes were also organized by task epoch and positively correlated to the task-related modulation of individual neuron firing rates. For many neurons, spiking was temporally organized through phase precession against theta band field potential oscillations. Theta phase precession advanced in parallel to task progression, rather than absolute spatial location or time. Together, the findings reveal a process by which associative brain regions can integrate independent oscillatory inputs and transform them into sequence-specific, rate-coded outputs that are adaptive to the pace with which organisms interact with their environment.

Synapse-specific representation of the identity of overlapping memory engrams

Memories are integrated into interconnected networks; nevertheless, each memory has its own identity. How the brain defines specific memory identity out of intermingled memories stored in a shared cell ensemble has remained elusive. We found that after complete retrograde amnesia of auditory fear conditioning in mice, optogenetic stimulation of the auditory inputs to the lateral amygdala failed to induce memory recall, implying that the memory engram no longer existed in that circuit. Complete amnesia of a given fear memory did not affect another linked fear memory encoded in the shared ensemble. Optogenetic potentiation or depotentiation of the plasticity at synapses specific to one memory affected the recall of only that memory. Thus, the sharing of engram cells underlies the linkage between memories, whereas synapse-specific plasticity guarantees the identity and storage of individual memories.

The involvement of CRF1 receptor within the basolateral amygdala and dentate gyrus in the naloxone-induced conditioned place aversion in morphine-dependent mice

Drug withdrawal-associated aversive memories trigger relapse to drug-seeking behavior. Corticotrophin-releasing factor (CRF) is an important mediator of the reinforcing properties of drugs of abuse. However, the involvement of CRF1 receptor (CRF1R) in aversive memory induced by opiate withdrawal has yet to be elucidated. We used the conditioned-place aversion (CPA) paradigm to evaluate the role of CRF1R on opiate withdrawal memory acquisition, along with plasticity-related processes that occur after CPA within the basolateral amygdala (BLA) and dentate gyrus (DG). Male mice were rendered dependent on morphine and injected acutely with naloxone before paired to confinement in a naloxone-associated compartment. The CPA scores as well as the number of TH-positive neurons (in the NTS-A2 noradrenergic cell group), and the expression of the transcription factors Arc and pCREB (in the BLA and DG) were measured with and without CRF1R blockade. Mice subjected to conditioned naloxone-induced morphine withdrawal robustly expressed CPA. Pre-treatment with the selective CRF1R antagonist CP-154,526 before naloxone conditioning session impaired morphine withdrawal-induced aversive memory acquisition. CP-154,526 also antagonized the enhanced number of TH-positive neurons in the NTS-A2 that was seen after CPA. Increased Arc expression and Arc-pCREB co-localization were seen in the BLA after CPA, which was not modified by CP-154,526. In the DG, CPA was accompanied by a decrease of Arc expression and no changes in Arc-pCREB co-localization, whereas pre-treatment with CP-154,526 induced an increase in both parameters. These results indicate that CRF-CRF1R pathway could be a critical factor governing opiate withdrawal memory storage and retrieval and might suggest a role for TH-NA pathway in the effects of withdrawal on memory. Our results might indicate that the blockade of CRF1R could represent a promising pharmacological treatment strategy approach for the attenuation of the relapse to drug-seeking/taking behavior triggered by opiate withdrawal-associated aversive memories.

Garth J. Thomas, Physiological Psychologist: An Appraisal of His Contributions to Memory Research

This note is a tribute to Garth J. Thomas (1916-2008), late Professor of Brain Research at the University of Rochester, New York. Thomas was an influential psychologist, albeit for his research in learning and memory, as much as for his work as editor of the Journal of Comparative and Physiological Psychology. In his studies, he combined experimental lesions with behavioral analyses. He introduced the terms "dispositional" and "representational" to describe the 2 different types of memory function, and emphasized that memory must be studied not only through behavioristic psychology, but also in a molecular and physiological context. Through his experimentation in rodents, Thomas concluded that distinct neural mechanisms underpin dispositional and representational memory. Prompted by Thomas' remarks on the future evolution of research techniques, we touch upon some ideas on the engram and the glial theory in a modern perspective.

Surviving threats: neural circuit and computational implications of a new taxonomy of defensive behaviour

Research on defensive behaviour in mammals has in recent years focused on elicited reactions; however, organisms also make active choices when responding to danger. We propose a hierarchical taxonomy of defensive behaviour on the basis of known psychological processes. Included are three categories of reactions (reflexes, fixed reactions and habits) and three categories of goal-directed actions (direct action-outcome behaviours and actions based on implicit or explicit forecasting of outcomes). We then use this taxonomy to guide a summary of findings regarding the underlying neural circuits.

Interaction of basolateral amygdala, ventral hippocampus and medial prefrontal cortex regulates the consolidation and extinction of social fear

Background

Following a social defeat, the balanced establishment and extinction of aversive information is a beneficial strategy for individual survival. Abnormal establishment or extinction is implicated in the development of mental disorders. This study investigated the time course of the establishment and extinction of aversive information from acute social defeat and the temporal responsiveness of the basolateral amygdala (BLA), ventral hippocampus (vHIP) and medial prefrontal cortex (mPFC) in this process.

Methods

Mouse models of acute social defeat were established by using the resident–intruder paradigm. To evaluate the engram of social defeat, the intruder mice were placed into the novel context at designated time to test the social behavior. Furthermore, responses of BLA, vHIP and mPFC were investigated by analyzing the expression of immediate early genes, such as zif268, arc, and c-fos.

Results

The results showed after an aggressive attack, aversive memory was maintained for approximately 7 days before gradually diminishing. The establishment and maintenance of aversive stimulation were consistently accompanied by BLA activity. By contrast, vHIP and mPFC response was inhibited from this process. Additionally, injecting muscimol (Mus), a GABA receptor agonist, into the BLA alleviated the freezing behavior and social fear and avoidance. Simultaneously, Mus treatment decreased the zif268 and arc expression in BLA, but it increased their expression in vHIP.

Conclusion

Our data support and extend earlier findings that implicate BLA, vHIP and mPFC in social defeat. The time courses of the establishment and extinction of social defeat are particularly consistent with the contrasting BLA and vHIP responses involved in this process.

Electronic supplementary material

The online version of this article (10.1186/s12993-018-0139-6) contains supplementary material, which is available to authorized users.

Hippocampal network oscillations as mediators of behavioural metaplasticity: Insights from emotional learning

Behavioural metaplasticity is evident in experience-dependent changes of network activity patterns in neuronal circuits that connect the hippocampus, amygdala and medial prefrontal cortex. These limbic regions are key structures of a brain-wide neural network that translates emotionally salient events into persistent and vivid memories. Communication in this network by-and-large depends on behavioural state-dependent rhythmic network activity patterns that are typically generated and/or relayed via the hippocampus. In fact, specific hippocampal network oscillations have been implicated to the acquisition, consolidation and retrieval, as well as the reconsolidation and extinction of emotional memories. The hippocampal circuits that contribute to these network activities, at the same time, are subject to both Hebbian and non-Hebbian forms of plasticity during memory formation. Further, it has become evident that adaptive changes in the hippocampus-dependent network activity patterns provide an important means of adjusting synaptic plasticity. We here summarise our current knowledge on how these processes in the hippocampus in interaction with amygdala and medial prefrontal cortex mediate the formation and persistence of emotional memories.

Removal of perineuronal nets disrupts recall of a remote fear memory

Throughout life animals learn to recognize cues that signal danger and instantaneously initiate an adequate threat response. Memories of such associations may last a lifetime and far outlast the intracellular molecules currently found to be important for memory processing. The memory engram may be supported by other more stable molecular components, such as the extracellular matrix structure of perineuronal nets (PNNs). Here, we show that recall of remote, but not recent, visual fear memories in rats depend on intact PNNs in the secondary visual cortex (V2L). Supporting our behavioral findings, increased synchronized theta oscillations between V2L and basolateral amygdala, a physiological correlate of successful recall, was absent in rats with degraded PNNs in V2L. Together, our findings suggest a role for PNNs in remote memory processing by stabilizing the neural network of the engram.

Fear Conditioning Downregulates Rac1 Activity in the Basolateral Amygdala Astrocytes to Facilitate the Formation of Fear Memory

Astrocytes are well known to scale synaptic structural and functional plasticity, while the role in learning and memory, such as conditioned fear memory, is poorly elucidated. Here, using pharmacological approach, we find that fluorocitrate (FC) significantly inhibits the acquisition of fear memory, suggesting that astrocyte activity is required for fear memory formation. We further demonstrate that fear conditioning downregulates astrocytic Rac1 activity in basolateral amygdala (BLA) in mice and promotes astrocyte structural plasticity. Ablation of astrocytic Rac1 in BLA promotes fear memory acquisition, while overexpression or constitutive activation of astrocytic Rac1 attenuates fear memory acquisition. Furthermore, temporal activation of Rac1 by photoactivatable Rac1 (Rac1-PA) induces structural alterations in astrocytes and in vivo activation of Rac1 in BLA astrocytes during fear conditioning attenuates the formation of fear memory. Taken together, our study demonstrates that fear conditioning-induced suppression of BLA astrocytic Rac1 activity, associated with astrocyte structural plasticity, is required for the formation of conditioned fear memory.

Plasticity in the auditory system

Over the last 30 years a wide range of manipulations of auditory input and experience have been shown to result in plasticity in auditory cortical and subcortical structures. The time course of plasticity ranges from very rapid stimulus-specific adaptation to longer-term changes associated with, for example, partial hearing loss or perceptual learning. Evidence for plasticity as a consequence of these and a range of other manipulations of auditory input and/or its significance is reviewed, with an emphasis on plasticity in adults and in the auditory cortex. The nature of the changes in auditory cortex associated with attention, memory and perceptual learning depend critically on task structure, reward contingencies, and learning strategy. Most forms of auditory system plasticity are adaptive, in that they serve to optimize auditory performance, prompting attempts to harness this plasticity for therapeutic purposes. However, plasticity associated with cochlear trauma and partial hearing loss appears to be maladaptive, and has been linked to tinnitus. Three important forms of human learning-related auditory system plasticity are those associated with language development, musical training, and improvement in performance with a cochlear implant. Almost all forms of plasticity involve changes in synaptic excitatory - inhibitory balance within existing patterns of connectivity. An attractive model applicable to a number of forms of learning-related plasticity is dynamic multiplexing by individual neurons, such that learning involving a particular stimulus attribute reflects a particular subset of the diverse inputs to a given neuron being gated by top-down influences. The plasticity evidence indicates that auditory cortex is a component of complex distributed networks that integrate the representation of auditory stimuli with attention, decision and reward processes.

Synapse-Specific Encoding of Fear Memory in the Amygdala

Input specificity is a fundamental property of long-term potentiation (LTP), but it is not known if learning is mediated by synapse-specific plasticity. Kim and Cho (2017) now show that fear conditioning is mediated by synapse-specific LTP in the amygdala, allowing animals to discriminate stimuli that predict threat from those that do not.