Learned together, extinguished apart: reducing fear to complex stimuli

Reconsolidation and Fear Extinction: An Update

Fear memories can be updated behaviorally by delivering extinction trials during the reconsolidation window, which results in a persistent attenuation of fear memories (Monfils et al., Science 324:951-955, 2009). This safe and non-invasive paradigm, termed retrieval-extinction (or post-retrieval extinction), has also been found to be successful at preventing the return of fear in healthy fear conditioned humans (Schiller et al., Nature 463:49-53, 2010), and in the time since its discovery, there has been an explosion of research on the use of retrieval-extinction in fear memories in humans and other animals, some of which have found a long-term reduction in conditioned responding, and some who have not. These discrepant findings have raised concerns as to whether retrieval-extinction really results in updating of the original fear memory, or if it simply enhances extinction. We will first review the progress made on elucidating the cellular mechanisms underlying the fear attenuating effects of retrieval-extinction and how they differ from traditional extinction. Special attention will be paid to the molecular events necessary for retrieval-extinction to successfully occur and how these reconsolidated memories are represented in the brain. Next, we will examine the parameters that determine whether or not a memory will be updated via extinction during the reconsolidation window (also known as boundary conditions). These boundary conditions will also be discussed as possible explanations for discrepant findings of the retrieval-extinction effect. Then we will examine the factors that can determine whether an individual's fears will successfully be attenuated by retrieval-extinction. These individual differences include genetics, age, and psychopathology. Finally, we will discuss recent attempts to bring the retrieval-extinction paradigm from the bench to the bedside for the behavioral treatment of anxiety and trauma disorders.

A Case for Translation From the Clinic to the Laboratory

Laboratory procedures have been used for decades as analogues for clinical processes with the goal of improving our understanding of psychological treatments for emotional disorders and identifying strategies to make treatments more effective. This research has often focused on translation from the laboratory to the clinic. Although this approach has notable successes, it has not been seamless. There are many examples of strategies that work in the laboratory that fail to lead to improved outcomes when applied clinically. One possible reason for this gap between experimental and clinical research is a failure to focus on translation from the clinic to the laboratory. Here, we discuss potential benefits of translation from the clinic to the laboratory and provide examples of how this might be implemented. We first consider two well-established laboratory analogues (extinction and cognitive reappraisal), identify critical aspects of the related clinical procedures (exposure and cognitive restructuring) that are missing from these analogues, and propose variations to better capture the clinical process. Second, we discuss two clinical procedures that have more recently been brought into the laboratory (eye-movement desensitization and reprocessing and imagery rescripting). We conclude by highlighting potential implications of this proposed shift in focus for translational research.

Elemental and configural threat learning bias extinction generalization

Emotional experiences often contain a multitude of details that may be represented in memory as individual elements or integrated into a single representation. How details associated with a negative emotional event are represented in memory can have important implications for extinction strategies designed to reduce emotional responses. For example, is extinguishing one cue associated with an aversive outcome sufficient to reduce learned behavior to other cues present at the time of learning that were not directly extinguished? Here, we used a between-subjects multi-day threat conditioning and extinction task to assess whether participants generalize extinction from one cue to unextinguished cues. On Day 1, one group of participants learned that a compound conditioned stimulus, composed of a tone and colored square, predicted an uncomfortable shock to the wrist (Compound group). A second group learned that the tone and square separately predicted shock (Separate group). On Day 2, participants in both groups were exposed to the tone in the absence of shocks (cue extinction). On Day 3, we tested whether extinction generalized from the extinguished to the unextinguished cue, as well as to a compound composed of both cues. Results showed that configural and elemental learning had unique and opposite effects on extinction generalization. Subjects who initially learned that a compound cue predicted shock successfully generalized extinction learning from the tone to the square, but exhibited threat relapse to the compound cue. In contrast, subjects who initially learned that each cue individually predicted shock did not generalize extinction learning from the tone to the square, but threat responses to the compound were low. These results highlight the importance of whether details of an aversive event are represented as integrated or separated memories, as these representations affect the success or limits of extinction generalization.

Augmenting exposure therapy with pre-extinction fear memory reactivation and deepened extinction: A randomized controlled trial

BACKGROUND

Pre-extinction fear memory reactivation (PE-FMR) and deepened extinction (DE) enhance long-term extinction of shock-conditioned fear, and may also enhance long-term extinction of naturally acquired fear. Preliminary data suggest that PE-FMR may additionally boost the speed of fear reduction during exposure therapy.

DESIGN

Randomized controlled trial, factorial design.

METHODS

Participants with elevated fears of either spiders or snakes were randomized to (1) exposure therapy alone (n = 41), (2) exposure therapy + PE-FMR (n = 42), (3) exposure therapy + DE (n = 41), or (4) exposure therapy + PE-FMR + DE (n = 42). Participants were assessed at baseline, post-treatment, and one-week follow-up on subjective and behavioral indices of phobia. Because treatment length was tailored to speed of fear reduction, survival analyses were used to examine the speed of fear reduction during treatment.

RESULTS

DE did not improve clinical outcomes at post-treatment or follow-up, whereas PE-FMR produced more rapid fear reduction and was able to achive equivalent outcomes even when the duration of exposure therapy (tailored to speed of fear reduction) was shortened by an average of 21%.

CONCLUSIONS

Data suggest that PE-FMR is a promising strategy for reducing the overall duration of exposure-based therapies.

CLINICAL TRIAL REGISTRATION

(clinicaltrials.gov)NCT02160470.

Early life sleep disruption is a risk factor for increased ethanol drinking after acute footshock stress in prairie voles

Early postnatal experiences are important for shaping the development of the stress response and may contribute to the later emergence of alcohol use disorders. We have previously found that early life sleep disruption impairs social development and alters GABA neurons in the brain of adult prairie voles, a socially monogamous rodent that displays natural ethanol preference in the laboratory. However, it is unclear whether these effects on social behavior are due, in part, to overall anhedonia and/or altered behavioral response to stress. To address this question, litters containing prairie vole pups were sleep disrupted by gentle cage agitation for 7 consecutive days from postnatal days (P) 14 to 21 (early life sleep disruption, or ELSD group) or allowed to sleep undisturbed (Control). Adult voles underwent a 2-bottle choice ethanol drinking procedure integrated with a single session of footshocks. Ethanol intake after footshock was measured as well as c-Fos immunoreactivity in the lateral and central amygdala. ELSD animals showed increased ethanol consumption and increased neural activity in these amygdala regions after footshock compared to control animals. There were no differences in baseline ethanol drinking prior to exposure to a stressor. These results suggest that early life sleep disruption in prairie voles does not produce anhedonia but can have long-lasting effects on stress reactivity. In addition to shaping species-typical social behavior, early life sleep may be important in the development of stress induced ethanol consumption and the activation of limbic pathways associated with stress. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

No persistent attenuation of fear memories in humans: A registered replication of the reactivation-extinction effect

It has been proposed that memory retrieval can destabilize consolidated memories, after which they need to be reconsolidated in order to be retained. The presentation of relevant information during memory reconsolidation could then result in the modification of a destabilized memory trace, by allowing the memory trace to be updated before being reconsolidated. In line with this idea, Schiller et al. (2010) have demonstrated that memory retrieval shortly before extinction training can prevent the later recovery of conditioned fear responding that is observed after regular extinction training. Those findings have been the subject of considerable controversy, due in part to theoretical reasons but also due to a number of failures to obtain similar results in conceptual replication attempts. Here, we report the results of a highly powered, direct, independent replication of the critical conditions of Schiller et al. (2010, Experiment 1). Due to misrepresentation of the exclusion criteria in the original Schiller et al. (2010) report, data collection was considerably delayed. When we eventually managed to attain our pre-registered sample size, we found that we could not observe any benefit of reactivation-extinction over regular extinction training in preventing recovery of conditioned fear. The results of the present study, along with the mixed findings in the literature and the misreporting in Schiller et al. (2010), give cause to question whether there is robust evidence that reactivation-extinction prevents the return of fear in humans.

Predicting extinction phenotype to optimize fear reduction

Fear conditioning is widely employed to study dysregulations of the fear system. The repeated presentation of a conditioned stimulus in the absence of a reinforcer leads to a decrease in fear responding-a phenomenon known as extinction. From a translational perspective, identifying whether an individual might respond well to extinction prior to intervention could prove important to treatment outcomes. Here, we test the hypothesis that CO₂ reactivity predicts extinction phenotype in rats, and that variability in CO₂ reactivity as well as extinction long-term memory (LTM) significantly predicts orexin activity in the lateral hypothalamus (LH). Our results validate a rat model of CO₂ reactivity and show that subcomponents of behavioral reactivity following acute CO₂ exposure explain a significant portion of the variance in extinction LTM. Furthermore, we show evidence that variability in CO₂ reactivity is also significantly predictive of orexin activity in the LH, and that orexin activity, in turn, significantly accounts for LTM variance. Our findings open the possibility that we may be able to use CO₂ reactivity as a screening tool to determine if individuals are good candidates for an extinction/exposure-based approach.

Memory boundaries: opening a window inspired by reconsolidation to treat anxiety, trauma-related, and addiction disorders

Pioneering research over the past two decades has shown that memories are far more malleable than we once thought, thereby highlighting the potential for new clinical avenues for treatment of psychopathology. We first briefly review the historical foundation of memory reconsolidation-a concept that refers to hypothetical processes that occur when a memory is retrieved and restored. Then, we provide an overview of the basic research on memory reconsolidation that has been done with humans and other animals, focusing on models of fear, anxiety-related disorders, and addiction, from the perspective that they all involve disorders of memory. This basic research has fuelled early stage developments of novel treatment techniques. More specifically, we consider behavioural interventions inspired by reconsolidation updating, namely retrieval-extinction techniques. We discuss the set of principles that would be needed for memory modifications within a putative reconsolidation time window, and review research that employs reconsolidation-based strategies with clinical populations. We conclude by highlighting current pitfalls and controversies surrounding the use of reconsolidation-based approaches, but end on an optimistic note for clinical research going forward. Despite the challenges, we believe that drawing on ideas from psychological science can help open up treatment innovation.

Multiple memory systems, multiple time points: how science can inform treatment to control the expression of unwanted emotional memories

Memories that have strong emotions associated with them are particularly resilient to forgetting. This is not necessarily problematic, however some aspects of memory can be. In particular, the involuntary expression of those memories, e.g. intrusive memories after trauma, are core to certain psychological disorders. Since the beginning of this century, research using animal models shows that it is possible to change the underlying memory, for example by interfering with its consolidation or reconsolidation. While the idea of targeting maladaptive memories is promising for the treatment of stress and anxiety disorders, a direct application of the procedures used in non-human animals to humans in clinical settings is not straightforward. In translational research, more attention needs to be paid to specifying what aspect of memory (i) can be modified and (ii) should be modified. This requires a clear conceptualization of what aspect of memory is being targeted, and how different memory expressions may map onto clinical symptoms. Furthermore, memory processes are dynamic, so procedural details concerning timing are crucial when implementing a treatment and when assessing its effectiveness. To target emotional memory in its full complexity, including its malleability, science cannot rely on a single method, species or paradigm. Rather, a constructive dialogue is needed between multiple levels of research, all the way 'from mice to mental health'.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Moderate Partially Reduplicated Conditioned Stimuli as Retrieval Cue Can Increase Effect on Preventing Relapse of Fear to Compound Stimuli

The theory of memory reconsolidation argues that consolidated memory is not unchangeable. Once a memory is reactivated it may go back into an unstable state and need new protein synthesis to be consolidated again, which is called "memory reconsolidation". Boundary studies have shown that interfering with reconsolidation through pharmacologic or behavioral intervention can lead to the updating of the initial memory, for example, erasing undesired memories. Behavioral procedures based on memory reconsolidation interference have been shown to be an effective way to inhibit fear memory relapse after extinction. However, the effectiveness of retrieval-extinction differs by subtle differences in the protocol of the reactivation session. This represents a challenge with regard to finding an optimal operational model to facilitate its clinical use for patients suffering from pathogenic memories such as those associated with post-traumatic stress disorder. Most of the laboratory models for fear learning have used a single conditioned stimulus (CS) paired with an unconditioned stimulus (US). This has simplified the real situation of traumatic events to an excessive degree, and thus, limits the clinical application of the findings based on these models. Here, we used a basic visual compound CS model as the CS to ascertain whether partial repetition of the compound CSs in conditioning can reactivate memory into reconsolidation. The results showed that the no retrieval group or the 1/3 ratio retrieval group failed to open the memory reconsolidation time window. The 2/3 repetition retrieval group and the whole repetition retrieval group were able to prevent fear reinstatement, whereas only a 2/3 ratio repetition of the initial compound CS as a reminder could inhibit spontaneous recovery. We inferred that a retrieval-extinction paradigm was also effective in a more complex model of fear if a sufficient prediction error (PE) could be generated in the reactivation period. In addition, in order to achieve an optimal effect, a CS of moderate discrepancy should be used as a reminder.

A preregistered, direct replication attempt of the retrieval-extinction effect in cued fear conditioning in rats

In 2009, Monfils and colleagues proposed a behavioral procedure that was said to result in a permanent attenuation of a previously established fear memory, thereby precluding a possible return of fear after extinction (Monfils, Cowansage, Klann, & LeDoux, 2009). By presenting a single retrieval trial one hour before standard extinction training, they found an enduring reduction of fear. The retrieval-extinction procedure holds great clinical potential, particularly for anxiety patients, but the findings are not undisputed, and several conceptual replications have failed to reproduce the effect. These failures have largely been attributed to small procedural differences. This preregistered study is the first endeavor to exactly replicate three key experiments of the original report by Monfils et al. (2009), thereby gauging the robustness of their seminal findings. Despite adhering to the original procedures as closely as possible, we did not find any evidence for reduced return of fear with the retrieval-extinction procedure relative to regular extinction training, as assessed through spontaneous recovery, reinstatement and renewal. Behavior of animals in the control condition (extinction only) was comparable to that in the original studies and provided an adequate baseline to reveal differences with the retrieval-extinction condition. Our null findings indicate that the effect sizes in the original paper may have been inflated and question the legitimacy of previously proposed moderators of the retrieval-extinction effect. We argue that direct experimental evaluation of purported moderators of the retrieval-extinction effect will be key to shed more light on its nature and prerequisites.

Extinction after fear memory reactivation fails to eliminate renewal in rats

Retrieving fear memories just prior to extinction has been reported to effectively erase fear memories and prevent fear relapse. The current study examined whether the type of retrieval procedure influences the ability of extinction to impair fear renewal, a form of relapse in which responding to a conditional stimulus (CS) returns outside of the extinction context. Rats first underwent Pavlovian fear conditioning with an auditory CS and footshock unconditional stimulus (US); freezing behavior served as the index of conditioned fear. Twenty-four hours later, the rats underwent a retrieval-extinction procedure. Specifically, 1h prior to extinction (45 CS-alone trials; 44 for rats receiving a CS reminder), fear memory was retrieved by either a single exposure to the CS alone, the US alone, a CS paired with the US, or exposure to the conditioning context itself. Over the next few days, conditional freezing to the extinguished CS was tested in the extinction and conditioning context in that order (i.e., an ABBA design). In the extinction context, rats that received a CS+US trial before extinction exhibited higher levels of conditional freezing than animals in all other groups, which did not differ from one another. In the renewal context, all groups showed renewal, and none of the reactivation procedures reduced renewal relative to a control group that did not receive a reactivation procedure prior to extinction. These data suggest retrieval-extinction procedures may have limited efficacy in preventing fear renewal.

Preventing the return of fear using reconsolidation updating and methylene blue is differentially dependent on extinction learning

Many factors account for how well individuals extinguish conditioned fears, such as genetic variability, learning capacity and conditions under which extinction training is administered. We predicted that memory-based interventions would be more effective to reduce the reinstatement of fear in subjects genetically predisposed to display more extinction learning. We tested this hypothesis in rats genetically selected for differences in fear extinction using two strategies: (1) attenuation of fear memory using post-retrieval extinction training, and (2) pharmacological enhancement of the extinction memory after extinction training by low-dose USP methylene blue (MB). Subjects selectively bred for divergent extinction phenotypes were fear conditioned to a tone stimulus and administered either standard extinction training or retrieval + extinction. Following extinction, subjects received injections of saline or MB. Both reconsolidation updating and MB administration showed beneficial effects in preventing fear reinstatement, but differed in the groups they targeted. Reconsolidation updating showed an overall effect in reducing fear reinstatement, whereas pharmacological memory enhancement using MB was an effective strategy, but only for individuals who were responsive to extinction.

The computational nature of memory modification

Retrieving a memory can modify its influence on subsequent behavior. We develop a computational theory of memory modification, according to which modification of a memory trace occurs through classical associative learning, but which memory trace is eligible for modification depends on a structure learning mechanism that discovers the units of association by segmenting the stream of experience into statistically distinct clusters (latent causes). New memories are formed when the structure learning mechanism infers that a new latent cause underlies current sensory observations. By the same token, old memories are modified when old and new sensory observations are inferred to have been generated by the same latent cause. We derive this framework from probabilistic principles, and present a computational implementation. Simulations demonstrate that our model can reproduce the major experimental findings from studies of memory modification in the Pavlovian conditioning literature.

DOI:http://dx.doi.org/10.7554/eLife.23763.001

Our memories contain our expectations about the world that we can retrieve to make predictions about the future. For example, most people would expect a chocolate bar to taste good, because they have previously learned to associate chocolate with pleasure. When a surprising event occurs, such as tasting an unpalatable chocolate bar, the brain therefore faces a dilemma. Should it update the existing memory and overwrite the association between chocolate and pleasure? Or should it create an additional memory? In the latter case, the brain would form a new association between chocolate and displeasure that competes with, but does not overwrite, the original one between chocolate and pleasure.

Previous studies have shown that surprising events tend to create new memories unless the existing memory is briefly reactivated before the surprising event occurs. In other words, retrieving old memories makes them more malleable. Gershman et al. have now developed a computational model for how the brain decides whether to update an old memory or create a new one. The idea at the heart of the model is that the brain will attempt to infer what caused the surprising event. The reason the chocolate bar tastes unpalatable, for example, might be because it was old and had spoiled. Every time the brain infers a new possible cause for a surprising event, it will create an additional memory to store this new set of expectations. In the future we will know that spoiled chocolate bars taste bad.

However, if the brain cannot infer a new cause for the surprising event – because, for example, there appears to be nothing unusual about the unpalatable chocolate bar – it will instead opt to update the existing memory. The next time we buy a chocolate bar, we will have slightly lower expectations about how good it will taste. The dilemma of whether to update an existing memory or create a new one thus boils down to the question: is the surprising event the consequence of a new cause or an old one? This theory implies that retrieving a memory nudges the brain to infer that its associated cause is once again active and, since this is an old cause, it means that the memory will be eligible for updating.

Many experiments have been performed on the topic of modifying memories, but this is the first computational model that offers a unifying explanation for the results. The next step is to work out how to apply the model, which is phrased in abstract terms, to networks of neurons that are more biologically realistic.

DOI:http://dx.doi.org/10.7554/eLife.23763.002

Use of a Brief Fear Memory Reactivation Procedure for Enhancing Exposure Therapy

We tested postretrieval extinction as an augmentation strategy for enhancing in vivo exposure therapy for naturally acquired pathological fear. Participants displaying marked phobic responding to either spiders or snakes were randomized to receive a standard one session in vivo exposure therapy protocol under one of two conditions. The experimental group (RFM-EXP) completed a 10-s fear reactivation procedure 30 min prior to initiating exposure therapy. Controls (EXP-RFM) completed the reactivation procedure after completing exposure therapy. Expected and peak fear during confrontation with live spiders or snakes were collected at pretreatment, posttreatment, and 1-month follow-up. RFM-EXP participants displayed significantly lower phobic responding at the 1-month follow-up relative to EXP-RFM controls. Unexpectedly, RFM-EXP participants showed more rapid fear attenuation during exposure relative to controls. Results provide preliminary support for further investigation of this exposure augmentation strategy across a wider range of anxiety-related disorders.

Post-retrieval extinction in adolescence prevents return of juvenile fear

Traumatic experiences early in life can contribute to the development of mood and anxiety disorders that manifest during adolescence and young adulthood. In young rats exposed to acute fear or stress, alterations in neural development can lead to enduring behavioral abnormalities. Here, we used a modified extinction intervention (retrieval+extinction) during late adolescence (post-natal day 45 [p45]), in rats, to target auditory Pavlovian fear associations acquired as juveniles (p17 and p25). The effects of adolescent intervention were examined by assessing freezing as adults during both fear reacquisition and social transmission of fear from a cagemate. Rats underwent testing or training at three time points across development: juvenile (p17 or p25), adolescent (p45), and adult (p100). Retrieval+extinction during late adolescence prevented social reinstatement and recovery over time of fears initially acquired as juveniles (p17 and p25, respectively). Adolescence was the only time point tested here where retrieval+extinction prevented fear recall of associations acquired 20+ days earlier.

Harnessing reconsolidation to weaken fear and appetitive memories: A meta-analysis of post-retrieval extinction effects

A new understanding of the mechanisms of memory retrieval and reconsolidation holds the potential for improving exposure-based treatments. Basic research indicates that following fear extinction, safety and fear memories may compete, raising the possibility of return of fear. One possible solution is to modify original fear memories through reconsolidation interference, reducing the likelihood of return of fear. Postretrieval extinction is a behavioral method of reconsolidation interference that has been explored in the context of conditioned fear and appetitive memory paradigms. This meta-analysis examines the magnitude of postretrieval extinction effects and potential moderators of these effects. A PubMed and PsycINFO search was conducted through June 2014. Sixty-three comparisons examining postretrieval extinction for preventing the return of fear or appetitive responses in animals or humans met inclusion criteria. Postretrieval extinction demonstrated a significant, small-to-moderate effect (g = .40) for further reducing the return of fear in humans and a significant, large effect (g = 0.89) for preventing the return of appetitive responses in animals relative to standard extinction. For fear outcomes in animals, effects were small (g = 0.21) and nonsignificant, but moderated by the number of animals housed together and the duration of time between postretrieval extinction/extinction and test. Across paradigms, these findings support the efficacy of this preclinical strategy for preventing the return of conditioned fear and appetitive responses. Overall, findings to date support the continued translation of postretrieval extinction research to human and clinical applications, with particular application to the treatment of anxiety, traumatic stress, and substance use disorders.

Neonatal representation of odour objects: distinct memories of the whole and its parts

Extraction of relevant information from highly complex environments is a prerequisite to survival. Within odour mixtures, such information is contained in the odours of specific elements or in the mixture configuration perceived as a whole unique odour. For instance, an AB mixture of the element A (ethyl isobutyrate) and the element B (ethyl maltol) generates a configural AB percept in humans and apparently in another species, the rabbit. Here, we examined whether the memory of such a configuration is distinct from the memory of the individual odorants. Taking advantage of the newborn rabbit's ability to learn odour mixtures, we combined behavioural and pharmacological tools to specifically eliminate elemental memory of A and B after conditioning to the AB mixture and evaluate consequences on configural memory of AB. The amnesic treatment suppressed responsiveness to A and B but not to AB. Two other experiments confirmed the specific perception and particular memory of the AB mixture. These data demonstrate the existence of configurations in certain odour mixtures and their representation as unique objects: after learning, animals form a configural memory of these mixtures, which coexists with, but is relatively dissociated from, memory of their elements. This capability emerges very early in life.

Relapse of extinguished fear after exposure to a dangerous context is mitigated by testing in a safe context

Aversive events can trigger relapse of extinguished fear memories, presenting a major challenge to the long-term efficacy of therapeutic interventions. Here, we examined factors regulating the relapse of extinguished fear after exposure of rats to a dangerous context. Rats received unsignaled shock in a distinct context ("dangerous" context) 24 h prior to auditory fear conditioning in another context. Fear to the auditory conditioned stimulus (CS) was subsequently extinguished either in the conditioning context ("ambiguous" context) or in a third novel context ("safe" context). Exposure to the dangerous context 30 min before a CS retention test caused relapse to the CS in the ambiguous and safe test contexts relative to nonextinguished controls. When rats were tested 24 h later (with or without short-term testing), rats tested in the ambiguous context continued to exhibit relapse, whereas rats tested in the safe context did not. Additionally, exposure of rats to the conditioning context--in place of the unsignaled shock context--did not result in relapse of fear to the CS in the safe testing context. Our work highlights the vulnerabilities of extinction recall to interference, and demonstrates the importance of context associations in the relapse of fear after extinction.

Epigenetic mechanisms of memory formation and reconsolidation

Memory consolidation involves transcriptional control of genes in neurons to stabilize a newly formed memory. Following retrieval, a once consolidated memory destabilizes and again requires gene transcription changes in order to restabilize, a process referred to as reconsolidation. Understanding the molecular mechanisms of gene transcription during the consolidation and reconsolidation processes could provide crucial insights into normal memory formation and memory dysfunction associated with psychiatric disorders. In the past decade, modifications of epigenetic markers such as DNA methylation and posttranslational modifications of histone proteins have emerged as critical transcriptional regulators of gene expression during initial memory formation and after retrieval. In light of the rapidly growing literature in this exciting area of research, we here examine the most recent and latest evidence demonstrating how memory acquisition and retrieval trigger epigenetic changes during the consolidation and reconsolidation phases to impact behavior. In particular we focus on the reconsolidation process, where we discuss the already identified epigenetic regulators of gene transcription during memory reconsolidation, while exploring other potential epigenetic modifications that may also be involved, and expand on how these epigenetic modifications may be precisely and temporally controlled by important signaling cascades critical to the reconsolidation process. Finally, we explore the possibility that epigenetic mechanisms may serve to regulate a system or circuit level reconsolidation process and may be involved in retrieval-dependent memory updating. Hence, we propose that epigenetic mechanisms coordinate changes in neuronal gene transcription, not only during the initial memory consolidation phase, but are triggered by retrieval to regulate molecular and cellular processes during memory reconsolidation.

Looking beyond Fear and Extinction Learning: Considering Novel Treatment Targets for Anxiety

Fear conditioning studies provide valuable insight into how fears are learned and extinguished. Previous work focuses on fear and extinction learning to understand and treat anxiety disorders. However, a cascade of cognitive processes that extend beyond learning may also yield therapeutic targets for anxiety disorders. Throughout this review, we will discuss recent findings of fear generalization, memory consolidation, and reconsolidation. Factors related to effectiveness, efficiency and durability of extinction-based treatments will be addressed. Moreover, adolescence may be a key developmental stage when threat-related perturbations emerge; therefore, targeting interventions during adolescence when these nascent processes are more malleable may alter the trajectory of anxiety disorders.