Fear conditioning and fear extinction are Pavlovian conditioning paradigms extensively used to study the mechanisms that underlie learning and memory formation. The neural circuits that mediate this ...learning are evolutionarily conserved, and seen in virtually all species from flies to humans. In mammals, the amygdala and medial prefrontal cortex are two structures that play a key role in the acquisition, consolidation and retrieval of fear memory, as well extinction of fear. These two regions have extensive bidirectional connections, and in recent years, the neural circuits that mediate fear learning and fear extinction are beginning to be elucidated. In this review, we provide an overview of our current understanding of the neural architecture within the amygdala and medial prefrontal cortex. We describe how sensory information is processed in these two structures and the neural circuits between them thought to mediate different aspects of fear learning. Finally, we discuss how changes in circuits within these structures may mediate fear responses following fear conditioning and extinction.
A major challenge in neurobiology in the 21st century is to understand how the brain adapts with experience. Activity-dependent gene expression is integral to the synaptic plasticity underlying ...learning and memory; however, this process cannot be explained by a simple linear trajectory of transcription to translation within a specific neuronal population. Many other regulatory mechanisms can influence RNA metabolism and the capacity of neurons to adapt. In particular, the RNA modification N6-methyladenosine (m6A) has recently been shown to regulate RNA processing through alternative splicing, RNA stability, and translation. Here, we discuss the emerging idea that m6A could also coordinate the transport, localization, and local translation of key mRNAs in learning and memory and expand on the notion of dynamic functional RNA states in the brain.
Epitranscriptomics or ‘RNA modifications’ contribute to RNA localization processes in the brain.N6-methyladenosine (m6A) is an RNA modification potentially critical for the localization of RNA.m6A not only has roles in the nuclear processing and export of RNA molecules but is also likely to contribute to the phase separation of RNA–protein complexes into RNA reservoirs e.g., RNA granules, stress granules, processing (P)-bodies and local translation at specific neuronal compartments.Many new techniques are capable of profiling m6A-modified transcripts at single-base resolution. However, these techniques need to be adapted to determine transcripts localized to various locations of the neuron including activated synapses, axons, and dendritic compartments.Investigations into the roles of m6A in RNA biology are rapidly expanding but require new techniques to fully understand m6A’s role in RNA localization in the brain.
Histone modifications contribute to the epigenetic regulation of gene expression, a process now recognized to be important for the consolidation of long-term memory. Valproic acid (VPA), used for ...many years as an anticonvulsant and a mood stabilizer, has effects on learning and memory and enhances the extinction of conditioned fear through its function as a histone deacetylase inhibitor (HDAC). Here we report that VPA enhances long-term memory for both acquisition and extinction of cued-fear. Interestingly, VPA enhances extinction, but also enhances renewal of the original conditioned fear when tested in a within-subjects design. This effect appears to be related to a reconsolidation-like process since a single CS reminder in the presence of VPA can enhance long-term memory for the original fear in the context in which fear conditioning takes place. We also show that by modifying the intertrial interval during extinction training, VPA can strengthen reconsolidation of the original fear memory or enhance long-term memory for extinction such that it becomes independent of context. These findings have important implications for the use of HDAC inhibitors as adjuncts to behavior therapy in the treatment of phobia and related anxiety disorders.
5-hydroxymethylcytosine (5-hmC) is a novel DNA modification that is highly enriched in the adult brain and dynamically regulated by neural activity. 5-hmC accumulates across the lifespan; however, ...the functional relevance of this change in 5-hmC and whether it is necessary for behavioral adaptation have not been fully elucidated. Moreover, although the ten-eleven translocation (Tet) family of enzymes is known to be essential for converting methylated DNA to 5-hmC, the role of individual Tet proteins in the adult cortex remains unclear. Using 5-hmC capture together with high-throughput DNA sequencing on individual mice, we show that fear extinction, an important form of reversal learning, leads to a dramatic genome-wide redistribution of 5-hmC within the infralimbic prefrontal cortex. Moreover, extinction learning-induced Tet3-mediated accumulation of 5-hmC is associated with the establishment of epigenetic states that promote gene expression and rapid behavioral adaptation.
DNA methylation was once considered to be a static epigenetic modification whose primary function was restricted to directing the development of cellular phenotype. However, it is now evident that ...the methylome is dynamically regulated across the lifespan: during development as a putative mechanism by which early experience leaves a lasting signature on the genome and during adulthood as a function of behavioral adaptation. Here, we propose that experience-dependent variations in DNA methylation, particularly within the context of learning and memory, represent a form of genomic metaplasticity that serves to prime the transcriptional response to later learning-related stimuli and neuronal reactivation.
The RNA modification N(6)-methyladenosine (m(6)A) influences mRNA stability and cell-type-specific developmental programming, and is highly abundant in the adult brain. However, it has not been ...determined whether m(6)A is dynamically regulated by experience. Based on transcriptome-wide profiling of m(6)A, we report that the level of m(6)A increases in the medial prefrontal cortex (mPFC) of mice in response to behavioral experience. The modulation was enriched near the stop codon of mRNAs, including genes related to neuronal plasticity. In primary cortical neurons, in vitro, modulation of m(6)A by the RNA demethylase FTO influenced the degradation profiles of a subset of transcripts with modulated sites. In vivo, the expression of Fto and the m(6)A methyltransferase, Mettl3 correlated with the observed increase in m(6)A levels post-training. Furthermore, targeted knockdown of FTO in the mPFC led to enhanced consolidation of cued fear memory. Thus, together with its role in early development, the dynamic regulation of m(6)A in the adult brain serves as an important epitranscriptomic mechanism associated with behavioral adaptation.
N(6)-methyladenosine (m(6)A) is the most prevalent internal modification on RNA, however, its cellular dynamics in vivo remains elusive. Here we provide the first demonstration of m(6)A upregulation in the mouse medial prefrontal cortex (mPFC) following behavioral training. Knocking down the m(6)A demethylase FTO in the mPFC, which increases total m(6)A level, results in enhanced consolidation of fear memory. Our findings suggest that m(6)A is regulated in an activity-dependent manner in the adult brain, and may function to fine-tune mRNA turnover during memory-related processes.
There are significant sex differences in vulnerability to develop fear-related anxiety disorders. Females exhibit twice the rate of post-traumatic stress disorder (PTSD) as males and sex differences ...have been observed in fear extinction learning in both humans and rodents, with a failure to inhibit fear emerging as a precipitating factor in the development of PTSD. Here we report that female mice are resistant to fear extinction, and exhibit increased DNA methylation of "Bdnf" exon IV and a concomitant decrease in mRNA expression within the medial prefrontal cortex. Activation of BDNF signaling by the trkB agonist 7,8-dihydroxyflavone blocks the return of fear in female mice after extinction training, and thus represents a novel approach to treating fear-related anxiety disorders that are characterized by a resistance to extinction and increased propensity for renewal.
Non‐coding RNAs (ncRNAs) are highly plastic RNA molecules that can sequester cellular proteins and other RNAs, serve as transporters of cellular cargo and provide spatiotemporal feedback to the ...genome. Mounting evidence indicates that ncRNAs are central to biology, and are critical for neuronal development, metabolism and intra‐ and intercellular communication in the brain. Their plasticity arises from state‐dependent dynamic structure states that can be influenced by cell type and subcellular environment, which can subsequently enable the same ncRNA with discrete functions in different contexts. Here, we highlight different classes of brain‐enriched ncRNAs, including microRNA, long non‐coding RNA and other enigmatic ncRNAs, that are functionally important for both learning and memory and adaptive immunity, and describe how they may promote cross‐talk between these two evolutionarily ancient biological systems.
We suggest that learning and the brain's adaptive immune system are functionally linked by a plethora of non‐coding RNA. That is, the same ncRNA can be repurposed through temporary modifications to its flexible structure, and used to transmit and encode information throughout and between the two systems. This theory comes from a multitude of studies showing that the same ncRNA species function in both systems and affecting the cells of one system affects the other. Consequentially, future academic and clinical research involving these systems should consider their intertwined nature.
Higher-order organisms possess information processing capabilities that are only made possible by their biological complexity. Emerging evidence indicates a critical role for regulatory RNAs in ...coordinating many aspects of cellular function that are directly involved in experience-dependent neural plasticity. Here, we focus on a structurally distinct class of RNAs known as circular RNAs. These closed loop, single-stranded RNA molecules are highly stable, enriched in the brain, and functionally active in both healthy and disease conditions. Current evidence implicating this ancient class of RNA as a contributor toward higher-order functions such as cognition and memory is discussed.
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