A Novel Retrieval-Dependent Memory Process Revealed by the Arrest of ERK1/2 Activation in the Basolateral Amygdala

Inhibition of ERK1/2 or CRMP2 Disrupts Alcohol Memory Reconsolidation and Prevents Relapse in Rats

Relapse to alcohol abuse, often caused by cue-induced alcohol craving, is a major challenge in alcohol addiction treatment. Therefore, disrupting the cue-alcohol memories can suppress relapse. Upon retrieval, memories transiently destabilize before they reconsolidate in a process that requires protein synthesis. Evidence suggests that the mammalian target of rapamycin complex 1 (mTORC1), governing the translation of a subset of dendritic proteins, is crucial for memory reconsolidation. Here, we explored the involvement of two regulatory pathways of mTORC1, phosphoinositide 3-kinase (PI3K)-AKT and extracellular regulated kinase 1/2 (ERK1/2), in the reconsolidation process in a rat (Wistar) model of alcohol self-administration. We found that retrieval of alcohol memories using an odor-taste cue increased ERK1/2 activation in the amygdala, while the PI3K-AKT pathway remained unaffected. Importantly, ERK1/2 inhibition after alcohol memory retrieval impaired alcohol-memory reconsolidation and led to long-lasting relapse suppression. Attenuation of relapse was also induced by post-retrieval administration of lacosamide, an inhibitor of collapsin response mediator protein-2 (CRMP2)-a translational product of mTORC1. Together, our findings indicate the crucial role of ERK1/2 and CRMP2 in the reconsolidation of alcohol memories, with their inhibition as potential treatment targets for relapse prevention.

Interaction between reconsolidation and extinction of fear memory

Memory retrieval is not a passive process. When a memory is retrieved, it returns to a labile state and undergoes reconsolidation to be re-stored. The discovery of this memory reconsolidation has had a major impact on memory consolidation theory. In other words, it suggested that memory is more dynamic than expected and can be modified through reconsolidation. Conversely, a conditioned fear memory undergoes memory extinction after retrieval, and it is thought that extinction does not reflect its erasure, but rather new inhibitory learning of the original conditioned memory. We have investigated the relationship between memory reconsolidation and extinction by comparing their behavioral, cellular, and molecular mechanisms. Memory reconsolidation and extinction have opposite functions on contextual fear and inhibitory avoidance memories; reconsolidation maintains or strengthens fear memory, whereas extinction weakens it. Importantly, reconsolidation and extinction are contrasting memory processes not only at the behavioral level but also at cellular and molecular levels. Furthermore, our analysis revealed that reconsolidation and extinction are not independent processes, but interact with each other. Interestingly, we also found a "memory transition process" that switches the fear memory process from reconsolidation to extinction after retrieval. Investigating the mechanisms of reconsolidation and extinction will contribute to our understanding of the dynamic nature of memory.

Making Leaps and Hitting Boundaries in Reconsolidation: Overcoming Boundary Conditions to Increase Clinical Translatability of Reconsolidation-based Therapies

Reconsolidation results in the restabilisation, and thus persistence, of a memory made labile by retrieval, and interfering with this process is thought to enable modification or weakening of the original trace. As such, reconsolidation-blockade has been a focus of research aiming to target the maladaptive memories underlying mental health disorders, including post-traumatic stress disorder and drug addiction. Current first-line therapies are not effective for all patients, and a substantial proportion of those for whom therapies are effective later relapse. A reconsolidation-based intervention would be invaluable as an alternative treatment for these conditions. However, the translation of reconsolidation-based therapies to the clinic presents a number of challenges, with arguably the greatest being the overcoming of the boundary conditions governing the opening of the reconsolidation window. These include factors such as the age and strength of memory, and can broadly be divided into two categories: intrinsic features of the targeted memory itself, and parameters of the reactivation procedure used. With maladaptive memory characteristics inevitably varying amongst individuals, manipulation of the other limitations imposed by procedural variables have been explored to circumvent the boundary conditions on reconsolidation. Although several apparently discrepant results remain to be reconciled and these limitations yet to be truly defined, many studies have produced successful results which encouragingly demonstrate that boundary conditions may be overcome using various proposed strategies to enable translation of a reconsolidation-based intervention to clinical use.

The challenge of memory destabilisation: From prediction error to prior expectations and biomarkers

The re-ignition of memory reconsolidation research sparked by Karim Nader in the early 2000s led to great excitement that 'reconsolidation-based' interventions might be developed for mental health disorders such as post-traumatic stress disorder and substance use disorder. Two decades on, it is clear that reconsolidation-based interventions have been more challenging to translate to the clinic than initially thought. We argue that this challenge could be addressed with a better understanding of how prior expectations interact with information presented in a putative memory reactivation / cue reminder session, and through the identification of non-invasive biomarkers for memory destabilisation that would allow reminder sessions to be 'tuned' to enhance memory lability in an ad hoc manner.

Effects of the retrieval-extinction paradigm with abstract reminders on fear memory extinction

Reactivated fear memories may enter an unstable state that could be interrupted by behavioral interventions such as the retrieval-extinction procedure. Studies of pharmacological interventions have shown that retrieval cues are not necessarily an exact reduplication of initial conditioned stimuli (CSs); they can instead be abstract word cues. However, the effectiveness of using abstract reminder cues in behavioral intervention procedures remains unknown, as do the conditions under which they are effective. The typicality of CS and the abstraction level of reminders are also likely to be important in the paradigm but have not been investigated to date. Here, in three experiments (n = 107), we manipulate the fear conditioning process, including single CS-unconditioned stimulus (US) and multiple CS-US connections, as well as the CS typicality and abstraction level of reminder cues, to explore the conditions for adopting retrieval-extinction with abstract reminders. The results of Experiments 1 and 2 show that the abstract cue is ineffective for use as a reminder in the procedure; this is the case for single exemplar category learning and perceptual discriminative learning, independent of the typicality of CS+ or the abstraction level of reminder cues. Only the outcome of Experiment 3, which contains multiple CS-US connections, shows no indication of fear return. The limitations of the study and possible interpretations of the results are discussed.

Reconsolidation: A paradigm shift

This commentary is part of a special issue honoring Karim Nader and his focal role in igniting the reconsolidation field. The commentary describes in broad strokes the evolution of the field, its branches, major challenges, and future endeavors.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

Orexin 1 Receptor Antagonism in the Basolateral Amygdala Shifts the Balance From Pro- to Antistress Signaling and Behavior

BACKGROUND

Stress produces differential behavioral responses through select molecular modifications to specific neurocircuitry elements. The orexin (Orx) system targets key components of this neurocircuitry in the basolateral amygdala (BLA).

METHODS

We assessed the contribution of intra-BLA Orx1 receptors (Orx1Rs) in the expression of stress-induced phenotypes of mice. Using the Stress Alternatives Model, a social stress paradigm that produces two behavioral phenotypes, we characterized the role of intra-BLA Orx1R using acute pharmacological inhibition (SB-674042) and genetic knockdown (AAV-U6-Orx1R-shRNA) strategies.

RESULTS

In the BLA, we observed that Orx1R (Hcrtr1) messenger RNA is predominantly expressed in CamKIIα+ glutamatergic neurons and rarely in GABAergic (gamma-aminobutyric acidergic) cells. While there is a slight overlap in Hcrtr1 and Orx2 receptor (Hcrtr2) messenger RNA expression in the BLA, we find that these receptors are most often expressed in separate cells. Antagonism of intra-BLA Orx1R after phenotype formation shifted behavioral expression from stress-sensitive (Stay) to stress-resilient (Escape) responses, an effect that was mimicked by genetic knockdown. Acute inhibition of Orx1R in the BLA also reduced contextual and cued fear freezing responses in Stay animals. This phenotype-specific behavioral change was accompanied by biased molecular transcription favoring Hcrtr2 over Hcrtr1 and Mapk3 over Plcb1 cell signaling cascades and enhanced Bdnf messenger RNA.

CONCLUSIONS

Functional reorganization of intra-BLA gene expression is produced by antagonism of Orx1R, which promotes elevated Hcrtr2, greater Mapk3, and increased Bdnf expression. Together, these results provide evidence for a receptor-driven mechanism that balances pro- and antistress responses within the BLA.

Prelimbic proBDNF Facilitates Retrieval-Dependent Fear Memory Destabilization by Regulation of Synaptic and Neural Functions in Juvenile Rats

Fear regulation changes as a function of the early life is a key developmental period for the continued maturation of fear neural circuitry. The mechanisms of fear retrieval-induced reconsolidation have been investigated but remain poorly understood. The involvement of prelimbic proBDNF in fear memory extinction and its mediated signaling have been reported previously. Specifically, blocking the proBDNF/p75^NTR pathway during the postnatal stage disrupts synaptic development and neuronal activity in adulthood. Given the inherent high expression of proBDNF during the juvenile period, we tested whether the prelimbic proBDNF regulated synaptic and neuronal functions allowing to influencing retrieval-dependent memory processing. By examining the freezing behavior of auditory fear-conditioned rats, we found the high level of the prelimbic proBDNF in juvenile rats enhanced the destabilization of the retrieval-dependent weak but not strong fear memory through activating p75^NTR-GluN2B signaling. This modification of fear memory traces was attributed to the increment in the proportion of thin-type spine and promotion in synaptic function, as evidenced by the facilitation of NMDA-mediated EPSCs and GluN2B-dependent synaptic depression at the prelimbic projection. Furthermore, the strong prelimbic theta- and gamma-oscillation coupling predicted the suppressive effect of juvenile proBDNF on the recall of postretrieval memory. Our results critically emphasize the importance of developmental proBDNF for modification of retrieval-dependent memory and provide a potential critical targeting to inhibit threaten memories associated with neurodevelopment disorders.

Persistence of spatial memory induced by spaced training involves a behavioral-tagging process

Spaced training, which involves long inter-trial intervals, has positive effects on memories. One of the main attributes of long-term memories (LTM) is persistence. Here, to identify the process that promotes LTM persistence by spaced learning, we used the spatial object recognition (SOR) task. The protocol consisted of a first strong training session that induced LTM formation (tested 1 day after training), but not LTM persistence (tested 7 or 14 days after training); and a second weak training session that promoted memory persistence when applied 1 day, but not 7 days, after the first training. We propose that the promotion of memory persistence is based on the Behavioral Tagging (BT) mechanism operating when the memory trace is retrieved. BT involves the setting of a tag induced by learning which gives rise to input selectivity, and the use of plasticity-related proteins (PRPs) to establish the mnemonic trace. We postulate that retraining will mainly retag the sites initially activated by the original learning, where the PRPs needed for memory expression and/or induced by retrieval would be used to maintain a persistent mnemonic trace. Our results suggest that the mechanism of memory expression, but not those of memory reinforcement or reconsolidation, is necessary to promote memory persistence after retraining. The molecular mechanisms involve ERKs1/2 activity to set the SOR learning tag, and the availability of GluA2-containing AMPA receptor. In conclusion, both the synthesis of PRPs and the setting of a learning tag are key processes triggered by retraining that allow SOR memory persistence.

Long-term behavioural rewriting of maladaptive drinking memories via reconsolidation-update mechanisms

BACKGROUND

Alcohol use disorders can be conceptualised as a learned pattern of maladaptive alcohol-consumption behaviours. The memories encoding these behaviours centrally contribute to long-term excessive alcohol consumption and are therefore an important therapeutic target. The transient period of memory instability sparked during memory reconsolidation offers a therapeutic window to directly rewrite these memories using targeted behavioural interventions. However, clinically-relevant demonstrations of the efficacy of this approach are few. We examined key retrieval parameters for destabilising naturalistic drinking memories and the ability of subsequent counterconditioning to effect long-term reductions in drinking.

METHODS

Hazardous/harmful beer-drinking volunteers (N = 120) were factorially randomised to retrieve (RET) or not retrieve (No RET) alcohol reward memories with (PE) or without (No PE) alcohol reward prediction error. All participants subsequently underwent disgust-based counterconditioning of drinking cues. Acute responses to alcohol were assessed pre- and post-manipulation and drinking levels were assessed up to 9 months.

RESULTS

Greater long-term reductions in drinking were found when counterconditioning was conducted following retrieval (with and without PE), despite a lack of short-term group differences in motivational responding to acute alcohol. Large variability in acute levels of learning during counterconditioning was noted. 'Responsiveness' to counterconditioning predicted subsequent responses to acute alcohol in RET + PE only, consistent with reconsolidation-update mechanisms.

CONCLUSIONS

The longevity of behavioural interventions designed to reduce problematic drinking levels may be enhanced by leveraging reconsolidation-update mechanisms to rewrite maladaptive memory. However, inter-individual variability in levels of corrective learning is likely to determine the efficacy of reconsolidation-updating interventions and should be considered when designing and assessing interventions.

Lack of Effect of Propranolol on the Reconsolidation of Conditioned Fear Memory due to a Failure to Engage Memory Destabilisation

The prospect of exploiting memory reconsolidation to treat mental health disorders has received great research interest, particularly following demonstrations that the β-adrenergic receptor antagonist propranolol, which is safe for use in humans, can disrupt the reconsolidation of pavlovian conditioned fear memories. However, recent studies have failed to replicate the effects of propranolol on fear memory reconsolidation, and have questioned whether treatments based upon reconsolidation blockade would be robust enough for clinical translation. It remains possible, though, that studies reporting no effect of propranolol on memory reconsolidation could be due to a failure to engage the memory destabilisation process, which is necessary for the memory to become susceptible to disruption with amnestic agents. Demonstrating that memory destabilisation has not been engaged is challenging when only using behavioural measures, but there are molecular correlates of memory destabilisation that can be used to determine whether memory lability has been induced. Here, we attempted to replicate the classic finding that systemic administration of propranolol disrupts the reconsolidation of a pavlovian auditory fear memory. Following a failure to replicate, we manipulated the parameters of the memory reactivation session to enhance prediction error in an attempt to overcome the boundary conditions of reconsolidation. On finding no disruption of memory despite these manipulations, we examined the expression of the post-synaptic density protein Shank in the basolateral amygdala. Degradation of Shank has been shown to correlate with the induction of memory lability, but we found no effect on Shank expression, consistent with the lack of observed behavioural effects.

Genistein enhances expression of extracellular regulated kinases (ERK) 1/2, and learning and memory of mouse

Genistein (GEN) is a well known phytoestrogen. It acts through estrogen receptor (ER) and performs plethora of functions in the brain. ERK1/2 is an activated kinase which involves in neuron differentiation, adult neurogenesis and several brain functions including learning and memory. However, GEN dependent expression of ERK1/2 and its effect in learning and memory of mice are unknown. In this study, Swiss albino male mice of 25weeks weighing 30 g were used for the experiments. Mice were placed in two groups- control (C) and genistein treated (GEN). Treated group received GEN dissolved in sesame oil (1 mg/kg/day) whereas the control group received sesame oil only. To study the effects of GEN on learning and memory, open-field (OF) test and novel object recognition (NOR) test were performed. Moreover, immunoblotting (IB) was performed to check the expression of ERK1/2 in the mouse brain of both groups. In the OF test, no significant change was observed in motor activity and anxiety in GEN treated mice as compared to control. Moreover, NOR test suggested that entry towards the dissimilar object was higher in case of GEN treated mice as compared to control. These findings suggest higher learning and memory of GEN treated mice than of control. IB showed that the expression of ERK1/2 was significantly high in GEN treated mouse brain as compared to control. Such study may be helpful to understand GEN mediated learning and memory involving ERK1/2.

Understanding the dynamic and destiny of memories

Memory formation enables the retention of life experiences overtime. Based on previously acquired information, organisms can anticipate future events and adjust their behaviors to maximize survival. However, in an ever-changing environment, a memory needs to be malleable to maintain its relevance. In fact, substantial evidence suggests that a consolidated memory can become labile and susceptible to modifications after being reactivated, a process termed reconsolidation. When an extinction process takes place, a memory can also be temporarily inhibited by a second memory that carries information with opposite meaning. In addition, a memory can fade and lose its significance in a process known as forgetting. Thus, following retrieval, new life experiences can be integrated with the original memory trace to maintain its predictive value. In this review, we explore the determining factors that regulate the fate of a memory after its reactivation. We focus on three post-retrieval memory destinies (reconsolidation, extinction, and forgetting) and discuss recent rodent studies investigating the biological functions and neural mechanisms underlying each of these processes.

Two spaced training trials induce associative ERK-dependent long term memory in Neohelice granulata

Memory formation depends upon several parametric training conditions. Among them, trial number and inter-trial interval (ITI) are key factors to induce long-term retention. However, it is still unclear how individual training trials contribute to mechanisms underlying memory formation and stabilization. Contextual conditioning in Neohelice granulata has traditionally elicited associative long-term memory (LTM) after 15 spaced (ITI = 3 min) trials. Here, we show that LTM in crabs can be induced after only two training trials by increasing the ITI to 45 min (2t-LTM) and maintaining the same training duration as in traditional protocols. This newly observed LTM was preserved for at least 96 h, exhibiting protein synthesis dependence during consolidation and reconsolidation as well as context-specificity. Moreover, we demonstrate that 2t-LTM depends on inter-trial and post-training ERK activation showing a faster phosphorylation after the second trial compared to the first one. In summary, we present a new training protocol in crabs through a reduced number of trials showing associative features similar to traditional spaced training. This novel protocol allows for intra-training manipulation and the assessment of individual trial contribution to LTM formation.

Active Transition of Fear Memory Phase from Reconsolidation to Extinction through ERK-Mediated Prevention of Reconsolidation

The retrieval of fear memory induces two opposite memory process, i.e., reconsolidation and extinction. Brief retrieval induces reconsolidation to maintain or enhance fear memory, while prolonged retrieval extinguishes this memory. Although the mechanisms of reconsolidation and extinction have been investigated, it remains unknown how fear memory phases are switched from reconsolidation to extinction during memory retrieval. Here, we show that an extracellular signal-regulated kinase (ERK)-dependent memory transition process after retrieval regulates the switch of memory phases from reconsolidation to extinction by preventing induction of reconsolidation in an inhibitory avoidance (IA) task in male mice. First, the transition memory phase, which cancels the induction of reconsolidation, but is insufficient for the acquisition of extinction, was identified after reconsolidation, but before extinction phases. Second, the reconsolidation, transition, and extinction phases after memory retrieval showed distinct molecular and cellular signatures through cAMP responsive element binding protein (CREB) and ERK phosphorylation in the amygdala, hippocampus, and medial prefrontal cortex (mPFC). The reconsolidation phase showed increased CREB phosphorylation, while the extinction phase displayed several neural populations with various combinations of CREB and/or ERK phosphorylation, in these brain regions. Interestingly, the three memory phases, including the transition phase, showed transient ERK activation immediately after retrieval. Most importantly, the blockade of ERK in the amygdala, hippocampus, or mPFC at the transition memory phase disinhibited reconsolidation-induced enhancement of IA memory. These observations suggest that the ERK-signaling pathway actively regulates the transition of memory phase from reconsolidation to extinction and this process functions as a switch that cancels reconsolidation of fear memory.SIGNIFICANCE STATEMENT Retrieval of fear memory induces two opposite memory process; reconsolidation and extinction. Reconsolidation maintains/enhances fear memory, while extinction weakens fear memory. It remains unknown how memory phases are switched from reconsolidation to extinction during retrieval. Here, we identified an active memory transition process functioning as a switch that inhibits reconsolidation. This memory transition phase showed a transient increase of extracellular signal-regulated kinase (ERK) phosphorylation in the amygdala, hippocampus and medial prefrontal cortex (mPFC). Interestingly, inhibition of ERK in these regions at the transition phase disinhibited the reconsolidation-mediated enhancement of inhibitory avoidance (IA) memory. These findings suggest that the transition memory process actively regulates the switch of fear memory phases of fear memory by preventing induction of reconsolidation through the activation of the ERK-signaling pathway.

Retrieval-Dependent Mechanisms Affecting Emotional Memory Persistence: Reconsolidation, Extinction, and the Space in Between

Maladaptive emotional memories contribute to the persistence of many mental health disorders, and therefore the prospect of disrupting these memories to produce long-term reductions in relapse is of great clinical appeal. Reducing the impact of maladaptive emotional memories on behaviour could be achieved by two retrieval-dependent manipulations that engage separate mnemonic processes: "reconsolidation disruption" and "extinction enhancement." Extinction occurs during a prolonged re-exposure session in the absence of the expected emotional outcome and is widely accepted as reflecting the formation of a new, inhibitory memory that prevents behavioural expression of the original trace. Reconsolidation, by contrast, involves the destabilisation of the original memory, allowing for subsequent updating and restabilisation in specific brain regions, unless the re-stabilization process is prevented through specific pharmacological or behavioural interventions. Both destabilisation of the original memory and memory extinction require that re-exposure induces prediction error-a mismatch between what is expected and what actually occurs-but the parameters that allow reconsolidation and extinction to occur, and control the transition between them, have not been well-characterised. Here, we review what is known about the induction of memory destabilisation and extinction, and the transition period that separates these mnemonic processes, drawing on preclinical and clinical examples. A deeper understanding of the processes that determine the alternative routes to memory persistence or inhibition is critical for designing new and more reliable clinical treatments targeting maladaptive emotional memories.

Identification of a Novel Retrieval-dependent Memory Process in the Crab Neohelice granulata

Fully consolidated associative memories may be altered by alternative retrieval dependent memory processes. While a brief exposure to the conditioned stimulus (CS) can trigger reconsolidation of the original memory, a prolonged CS exposure will trigger memory extinction. The conditioned response is maintained after reconsolidation, but is inhibited after extinction, presumably by the formation of a new inhibitory memory trace. In rats and humans, it has been shown that CS exposure of intermediate duration leave the memory in an insensitive or limbo state. Limbo is characterised by the absence of reconsolidation or extinction. Here we investigated the evolutionary conserved nature of limbo using a contextual Pavlovian conditioning (CPC) memory paradigm in the crab Neohelice granulata. In animals with fully consolidated CPC memory, systemic administration of the protein synthesis inhibitor cycloheximide after 1 CS presentation disrupted the memory, presumably by interfering with memory reconsolidation. The same intervention given after 320 CSs prevented CPC memory extinction. Cycloheximide had no behavioural effect when administered after 80 CS presentations, a protocol that failed to extinguish CPC memory. Also, we observed that a stronger CPC memory engaged reconsolidation after 80 CS instead of limbo, indicating that memory strength affects the parametrical conditions to engage either reconsolidation or limbo. Altogether, these results indicate that limbo is an evolutionary conserved memory process segregating reconsolidation from extinction in the number of CSs space. Limbo appears as an intrinsic component of retrieval dependent memory processing, with a key function in the transition from memory maintenance to inhibition.

Heat shock disrupts expression of excitatory and extinction memories in planaria: Interaction with amount of exposure

In planarians, as seen in rodents, natural reinforcers (sucrose) and drugs of abuse support Conditioned Place Preference (CPP), which is a form of Pavlovian learning to examine the rewarding effects of natural reinforcers and drugs of abuse. Using this preparation, we have previously observed acquisition, extinction and reinstatement of sucrose CPP. In the present experiments, we used planaria to investigate the amnestic effects of Heat Shock (HS, a known stressor in planaria) following different amounts of CPP extinction sessions. Experiment 1 showed that planarians developed a CPP response to a sucrose-paired surface. Heat shock, when given in conjunction with exposure to the sucrose-paired surface, produced amnesia as assessed by a subsequent sucrose reinstatement test. We interpreted that the amnesic effect of HS was due to HS affecting the dominant excitatory memory at the time of HS exposure. Thus, we hypothesized that after extensive extinction training (10 exposures), HS would lead to recovery from extinction (when the new inhibitory memory is dominant at the time of HS exposure). Experiment 2 explored this possibility and showed that given HS following 10 extinction sessions had no amnestic effect on the excitatory CPP response. In Experiment 3, we hypothesized that 16 extinction sessions would produce a stronger (and hence dominant) extinction inhibitory trace, which then would be vulnerable to HS. We observed that HS impaired the expression of the extinction memory following 16 exposures. These results reveal different effects of HS on CPP memories depending on the amount of extinction, and are fully consistent with the literature using rodents and humans. In addition, they suggest that planaria is a promising pre-clinical model to assess fundamental memory processes.

LIMK, Cofilin 1 and actin dynamics involvement in fear memory processing

Long-term memory has been associated with morphological changes in the brain, which in turn tightly correlate with changes in synaptic efficacy. Such plasticity is proposed to rely on dendritic spines as a neuronal canvas on which these changes can occur. Given the key role of actin cytoskeleton dynamics in spine morphology, major regulating factors of this process such as Cofilin 1 (Cfl1) and LIM kinase (LIMK), an inhibitor of Cfl1 activity, are prime molecular targets that may regulate dendritic plasticity. Using a contextual fear conditioning paradigm in mice, we found that pharmacological induction of depolymerization of actin filaments through the inhibition of LIMK causes an impairment in memory reconsolidation, as well as in memory consolidation. On top of that, Cfl1 activity is inhibited and its mRNA is downregulated in CA1 neuropil after re-exposure to the training context. Moreover, by pharmacological disruption of actin cytoskeleton dynamics, the process of memory extinction can either be facilitated or impaired. Our results lead to a better understanding of the role of LIMK, Cfl1 and actin cytoskeleton dynamics in the morphological and functional changes underlying the synaptic plasticity of the memory trace.

Retrieval-Extinction and Relapse Prevention: Rewriting Maladaptive Drug Memories?

Addicted individuals are highly susceptible to relapse when exposed to drug-associated conditioned stimuli (CSs; "drug cues") even after extensive periods of abstinence. Until recently, these maladaptive emotional drug memories were believed to be permanent and resistant to change. The rediscovery of the phenomenon of memory reconsolidation-by which retrieval of the memory can, under certain conditions, destabilize the previously stable memory before it restabilizes in its new, updated form-has led to the hypothesis that it may be possible to disrupt the strong maladaptive drug-memories that trigger a relapse. Furthermore, recent work has suggested that extinction training "within the reconsolidation window" may lead to a long-term reduction in relapse without the requirement for pharmacological amnestic agents. However, this so-called "retrieval-extinction" effect has been inconsistently observed in the literature, leading some to speculate that rather than reflecting memory updating, it may be the product of facilitation of extinction. In this mini review article, we will focus on factors that might be responsible for the retrieval-extinction effects on preventing drug-seeking relapse and how inter-individual differences may influence this therapeutically promising effect. A better understanding of the psychological and neurobiological mechanisms underpinning the "retrieval-extinction" paradigm, and individual differences in boundary conditions, should provide insights with the potential to optimize the translation of "retrieval-extinction" to clinical populations.

Fear not: recent advances in understanding the neural basis of fear memories and implications for treatment development

Fear is a highly adaptive emotion that has evolved to promote survival and reproductive fitness. However, maladaptive expression of fear can lead to debilitating stressor-related and anxiety disorders such as post-traumatic stress disorder. Although the neural basis of fear has been extensively researched for several decades, recent technological advances in pharmacogenetics and optogenetics have allowed greater resolution in understanding the neural circuits that underlie fear. Alongside conceptual advances in the understanding of fear memory, this increased knowledge has clarified mechanisms for some currently available therapies for post-traumatic stress disorder and has identified new potential treatment targets.

Limited replicability of drug-induced amnesia after contextual fear memory retrieval in rats

With the ultimate goal of investigating boundary conditions for post-reactivation amnesia, we set out to replicate studies in which systemic, post-reactivation administration of midazolam, propranolol, or cycloheximide resulted in amnesia for contextual fear memories. Our experiments involved conceptual as well as exact replications of previously published studies. In most of our experiments, we adopted a procedure that conformed to the standard 3-day protocol typically used in the literature, with contextual fear conditioning on day 1, unreinforced re-exposure to the conditioning context followed by systemic injection of the amnestic drug on day 2, and a memory retention test on day 3. Given the plethora of successful studies with large effects sizes and the absence of any failed replications in the literature, we were surprised to find that we were generally unable to replicate those findings. Our results suggest that post-reactivation amnesia by systemic drug administration in rats is more difficult to obtain than what would be expected based on published empirical reports. At present, it remains unclear which conditions determine the success of this procedure.

Relevance of ERK1/2 Post-retrieval Participation on Memory Processes: Insights in Their Particular Role on Reconsolidation and Persistence of Memories

Back in 1968, Misanin and his group posited that reactivation of consolidated memories could support changes in that trace, similar to what might happen during the consolidation process. Not until 2000, when Nader et al. (2000) studied the behavioral effect of a protein synthesis inhibitor on retrieved memories, could this previous statement be taken under consideration once again; suggesting that consolidated memories can become labile after reactivation. The process of strengthening after memory labilization was named memory reconsolidation. In recent years, many studies pointed towards a critical participation of the extracellular signal-regulated kinase (ERK)/mitogen activated protein kinases (MAPKs) pathway in different memory processes (e.g., consolidation, extinction, reconsolidation, among others). In this review article, we will focus on how this system might be modulating the processes triggered after retrieval of well-consolidated memories in mice.

Kinase and Phosphatase Engagement Is Dissociated Between Memory Formation and Extinction

Associative long-term memories (LTMs) support long-lasting behavioral changes resulting from sensory experiences. Retrieval of a stable LTM by means of a large number of conditioned stimulus (CS) alone presentations produces inhibition of the original memory through extinction. Currently, there are two opposing hypotheses to account for the neural mechanisms supporting extinction. The unlearning hypothesis posits that extinction affects the original memory trace by reverting the synaptic changes supporting LTM. On the contrary, the new learning hypothesis proposes that extinction is simply the formation of a new associative memory that inhibits the expression of the original one. We propose that detailed analysis of extinction-associated molecular mechanisms could help distinguish between these hypotheses. Here we will review experimental evidence regarding the role of protein kinases and phosphatases (K&P) on LTM formation and extinction. Even though K&P regulate both memory processes, their participation appears to be dissociated. LTM formation recruits kinases, but is constrained by phosphatases. Memory extinction presents a more diverse molecular landscape, requiring phosphatases and some kinases, but also being constrained by kinase activity. Based on the available evidence, we propose a new theoretical model for memory extinction: a neuronal segregation of K&P supports a combination of time-dependent reversible inhibition of the original memory [CS-unconditioned stimulus (US)], with establishment of a new associative memory trace (CS-noUS).

Neurochemical and molecular mechanisms underlying the retrieval-extinction effect

Extinction within the reconsolidation window, or 'retrieval-extinction', has received much research interest as a possible technique for targeting the reconsolidation of maladaptive memories with a behavioural intervention. However, it remains to be determined whether the retrieval-extinction effect-a long-term reduction in fear behaviour, which appears resistant to spontaneous recovery, renewal and reinstatement-depends specifically on destabilisation of the original memory (the 'reconsolidation-update' account) or represents facilitation of an extinction memory (the 'extinction-facilitation' account). We propose that comparing the neurotransmitter systems, receptors and intracellular signalling pathways recruited by reconsolidation, extinction and retrieval-extinction will provide a way of distinguishing between these accounts.

Post-weaning housing conditions influence freezing during contextual fear conditioning in adult rats

The present study aimed to investigate the influence of housing conditions on contextual fear memory malleability. Male Wistar rats were housed in enriched, standard, or impoverished conditions after weaning and remained in these conditions throughout the entire experiment. After six weeks into those housing conditions, all animals underwent a 3-day protocol including contextual fear conditioning (day 1), memory reactivation followed by systemic administration of midazolam or vehicle (day 2), and a retention test (day 3). Percentage freezing was used as a behavioral measure of contextual fear. There was no evidence for an effect of housing conditions on the sensitivity of contextual fear memory to amnestic effects of post-reactivation midazolam administration, and no indication for amnestic effects of post-reactivation midazolam overall (including in the standard group). The inability to replicate previous demonstrations of post-reactivation amnesia using the same protocol underscores the subtle nature of post-reactivation pharmacological memory interference. Notably, impoverished housing resulted in a decrease in contextual freezing during contextual fear conditioning, reactivation and retention testing, compared to enriched and standard housing conditions. This observation warrants caution when interpreting the results from experiments regarding effects of housing on fear memory processes, particularly when freezing is used as a measure of fear.

ERK1/2: A Key Cellular Component for the Formation, Retrieval, Reconsolidation and Persistence of Memory

Extracellular regulated kinase 1/2 (ERK1/2) has been strongly implicated in several cellular processes. In the brain ERK1/2 activity has been primarily involved in long-term memory (LTM) formation and expression. Here, we review earlier evidence and describe recent developments of ERK1/2 signaling in memory processing focusing the attention on the role of ERK1/2 in memory retrieval and reconsolidation, and in the maintenance of the memory trace including mechanisms involving the protection of labile memories. In addition, relearning requires ERK1/2 activity in selected brain regions. Its involvement in distinct memory stages points at ERK1/2 as a core element in memory processing and as one likely target to treat memory impairments associated with neurological disorders.

Reconsolidation blockade for the treatment of addiction: challenges, new targets, and opportunities

Addiction is a chronic, relapsing disorder. The progression to pathological drug-seeking is thought to be driven by maladaptive learning processes which store and maintain associative memory, linking drug highs with cues and actions in the environment. These memories can encode Pavlovian associations which link predictive stimuli (e.g., people, places, and paraphernalia) with a hedonic drug high, as well as instrumental learning about the actions required to obtain drug-associated incentives. Learned memories are not permanent however, and much recent interest has been generated in exploiting the process of reconsolidation to erase or significantly weaken maladaptive memories to treat several mental health disorders, including addictions. Normally reconsolidation serves to update and maintain the adaptive relevance of memories, however administration of amnestic agents within the critical "reconsolidation window" can weaken or even erase maladaptive memories. Here we discuss recent advances in the field, including ongoing efforts to translate preclinical reconsolidation research in animal models into clinical practice.