Distinct engrams control fear and extinction memory.

Memories are stored in engram cells, which are necessary and sufficient for memory recall. Recalling a memory might undergo reconsolidation or extinction. It has been suggested that the original memory engram is reactivated during reconsolidation so that memory can be updated. Conversely, during extinction training, a new memory is formed that suppresses the original engram. Nonetheless, it is unknown whether extinction creates a new engram or modifies the original fear engram. In this study, we utilized the Daun02 procedure, which uses c-Fos-lacZ rats to induce apoptosis of strongly activated neurons and examine whether a new memory trace emerges as a result of a short or long reactivation, or if these processes rely on modifications within the original engram located in the basolateral amygdala (BLA) and infralimbic (IL) cortex. By eliminating neurons activated during consolidation and reactivation, we observed significant impacts on fear memory, highlighting the importance of the BLA engram in these processes. Although we were unable to show any impact when removing the neurons activated after the test of a previously extinguished memory in the BLA, disrupting the IL extinction engram reactivated the aversive memory that was suppressed by the extinction memory. Thus, we demonstrated that the IL cortex plays a crucial role in the network involved in extinction, and disrupting this specific node alone is sufficient to impair extinction behavior. Additionally, our findings indicate that extinction memories rely on the formation of a new memory, supporting the theory that extinction memories rely on the formation of a new memory, whereas the reconsolidation process reactivates the same original memory trace.

Molecular mechanisms underpinning deconditioning-update in fear memory.

Traumatic experiences are closely associated with some psychiatric conditions such as post-traumatic stress disorder. Deconditioning-update promotes robust and long-lasting attenuation of aversive memories. The deconditioning protocol consists of applying weak/neutral footshocks during reactivations, so that the original tone-shock association is replaced by an innocuous stimulus that does not produce significant fear response. Here, we present the molecular bases that can support this mechanism. To this end, we used pharmacological tools to inhibit the activity of ionotropic glutamate receptors (NMDA-GluN2B and CP-AMPA), the activity of proteases (calpains), and the receptors that control intracellular calcium storage (IP3 receptors), as well as the endocannabinoid system (CB1). Our results indicate that blocking these molecular targets prevents fear memory update by deconditioning. Therefore, this study uncovered the molecular substrate of deconditioning-update strategy, and, broadly, shed new light on the traumatic memory destabilization mechanisms that might be used to break the boundaries regarding reconsolidation-based approaches to deal with maladaptive memories.

Characterization of deconditioning-update on fear memory attenuation.

Fear memory expression can be attenuated by updating the footshock perception during the plastic state induced by retrieval, from a strong unconditioned stimulus to a very weak one through deconditioning. In this process, the original fear association of the conditioned stimulus with the footshock is substituted by an innocuous stimulus and the animals no longer express a fear response. In the present study, we explore the boundaries of this deconditioning-update strategy by the characterization of this phenomenon. We found that there is an optimal mismatch between the footshock intensity delivered in the training and in the reactivation. Likewise, we characterized the temporal window that the protocol is efficient in hindering fear response. Our findings contribute to a better understanding of the limits in which deconditioning acts in attenuating fear memory, so that an optimized protocol using this strategy can be planned in order to deal with emotional disorders.

Effects of hippocampal IP3R inhibition on contextual fear memory consolidation, retrieval, reconsolidation and extinction.

Intracellular calcium stores (ICS) play a dynamic role in neuronal calcium (Ca2+) homeostasis both by buffering Ca2+ excess in the cytoplasm or providing an additional source of Ca2+ when concentration increase is needed. However, in spite of the large body of evidence showing Ca2+ as an essential second messenger in many signaling cascades underlying synaptic plasticity, the direct involvement of the intracellular Ca2+-release channels (ICRCs) in memory processing has been highly overlooked. Here we investigated the role of the ICRC inositol 1,4,5-trisphosphate receptor (IP3R) activity during different memory phases using pharmacological inhibition in the dorsal hippocampus during contextual fear conditioning. We first found that post-training administration of the IP3R antagonist 2-aminoethyl diphenylborinate (2-APB) impaired memory consolidation in a dose and time-dependent manner. Inhibiting IP3Rs also disrupted memory retrieval. Contextual fear memory reconsolidation or extinction, however, were not sensitive to IP3R blockade. Taken together, our results indicate that hippocampal IP3Rs play an important role in contextual fear memory consolidation and retrieval.

Hippocampal HECT E3 ligase inhibition facilitates consolidation, retrieval, and reconsolidation, and inhibits extinction of contextual fear memory.

Ubiquitination is involved in synaptic plasticity and memory, but the involvement of HECT E3 ligases in these processes has not yet been established. Here, we bilaterally infused heclin, a specific inhibitor of some of these ligases, into the dorsal hippocampus of male Wistar rats that were trained in a contextual fear conditioning. Heclin improved short-term memory, consolidation, retrieval, and reconsolidation when administered immediately post training, prior to testing, or after memory reactivation, respectively. In addition, it impaired memory extinction when administered prior to a long reactivation session. Heclin infusion was also tested for locomotor activity and anxiety-like behavior in a circular arena, but no effect was seen. Taken together, these results indicate that HECT E3 ligases are involved in the modulation of fear memory.

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.

Calpain modulates fear memory consolidation, retrieval and reconsolidation in the hippocampus.

It has been proposed that long-lasting changes in dendritic spines provide a physical correlate for memory formation and maintenance. Spine size and shape are highly plastic, controlled by actin polymerization/depolymerization cycles. This actin dynamics are regulated by proteins such as calpain, a calcium-dependent cysteine protease that cleaves the structural cytoskeleton proteins and other targets involved in synaptic plasticity. Here, we tested whether the pharmacological inhibition of calpain in the dorsal hippocampus affects memory consolidation, retrieval and reconsolidation in rats trained in contextual fear conditioning. We first found that post-training infusion of the calpain inhibitor PD150606 impaired long-term memory consolidation, but not short-term memory. Next, we showed that pre-test infusion of the calpain inhibitor hindered memory retrieval. Finally, blocking calpain activity after memory reactivation disrupted reconsolidation. Taken together, our results show that calpain play an essential role in the hippocampus by enabling memory formation, expression and reconsolidation.

Synaptic consolidation as a temporally variable process: Uncovering the parameters modulating its time-course.

Memories are not instantly created in the brain, requiring a gradual stabilization process called consolidation to be stored and persist in a long-lasting manner. However, little is known whether this time-dependent process is dynamic or static, and the factors that might modulate it. Here, we hypothesized that the time-course of consolidation could be affected by specific learning parameters, changing the time window where memory is susceptible to retroactive interference. In the rodent contextual fear conditioning paradigm, we compared weak and strong training protocols and found that in the latter memory is susceptible to post-training hippocampal inactivation for a shorter period of time. The accelerated consolidation process triggered by the strong training was mediated by glucocorticoids, since this effect was blocked by pre-training administration of metyrapone. In addition, we found that pre-exposure to the training context also accelerates fear memory consolidation. Hence, our results demonstrate that the time window in which memory is susceptible to post-training interferences varies depending on fear conditioning intensity and contextual familiarity. We propose that the time-course of memory consolidation is dynamic, being directly affected by attributes of the learning experiences.

Hippocampal plasticity mechanisms mediating experience-dependent learning change over time.

The requirement of NMDA receptor (NMDAR) activity for memory formation is well described. However, the plasticity mechanisms for memory can be modified by experience, such that a future similar learning becomes independent of NMDARs. This effect has often been reported in learning events conducted with a few days interval. In this work, we asked whether the NMDAR-independency is permanent or the brain regions and plasticity mechanisms of experience-dependent learning may change over time. Considering that contextual memories undergo a gradual reorganization over time, becoming progressively independent from the hippocampus and dependent upon cortical regions, we investigated the brain regions mediating a new related learning conducted at a remote time-point, when the first memory was already cortically established. First, we demonstrated that anterior cingulate cortex was not able to support a learning subsequent to a previous systems-level consolidated memory; it did require at least one functional subregion of the hippocampus (ventral or dorsal). Moreover, after replicating findings showing that a few days interval between trainings induces a NMDAR-independent learning, we managed to show that a learning following a longer interval once again becomes dependent on NMDARs in the hippocampus. These findings suggest that while the previous memory grows independent from the hippocampus over time, an experience-dependent learning following a systems-consolidated memory once again engages the hippocampus and a NMDAR-dependent plasticity mechanism.

Sequential learning during contextual fear conditioning guides the rate of systems consolidation: Implications for consolidation of multiple memory traces.

Systems consolidation has been described as a time-dependent reorganization process involving the neocortical and hippocampal networks underlying memory storage and retrieval. Previous studies of our lab were able to demonstrate that systems consolidation is a dynamic process, rather than a merely passive, time-dependent phenomenon. Here, we studied the influence of sequential learning in contextual fear conditioning (CFC) with different training intensities in the time-course of hippocampal dependency and contextual specificity. We found that sequential learning with high-intensity shocks during CFC induces generalization of the first learning (context A) and maintains contextual specificity of the second learning (context B) 15 days after acquisition. Moreover, subsequent experiences reorganize brain structures involved in retrieval, accelerating the involvement of cortical structures and diminishing the hippocampal participation. Exposure to original context before novelty seems to only induce context specificity in hippocampal-dependent memories. We propose that systems consolidation could be considered a potential biological mechanism for reducing possible interferences between similar memory traces. © 2017 Wiley Periodicals, Inc.

Reconsolidation-induced rescue of a remote fear memory blocked by an early cortical inhibition: Involvement of the anterior cingulate cortex and the mediation by the thalamic nucleus reuniens.

Systems consolidation is a time-dependent reorganization process involving neocortical and hippocampal networks underlying memory storage and retrieval. The involvement of the hippocampus during acquisition is well described; however we know much less about the concomitant contribution of cortical activity levels to the formation of stable remote memories. Here, after a reversible pharmacological inhibition of the anterior cingulate cortex (ACC) during the acquisition of a contextual fear conditioning, retrieval of both recent and remote memories were impaired, an effect that was reverted by a single memory reactivation session 48 h after training, through a destabilization-dependent mechanism interpreted as reconsolidation, that restored the normal course of systems consolidation in order to rescue a remote memory. Next we have shown that the integrity of both the anterior cingulate cortex and the thalamic nucleus reuniens (RE) were required for this reactivation-induced memory rescue. Because lidocaine infused into the RE inhibited LTP induction in the CA1-anterior cingulate cortex pathways, it seems that RE is a necessary component of the circuit underlying systems consolidation, mediating communication between dorsal hippocampus and cortical areas. To our notice, this is the first demonstration of the rescue of remote memories disrupted by ACC inhibition during acquisition, via a reconsolidation-driven mechanism. We have also shown the importance of RE to ensure the interconnection among brain areas that collectively seem to control the natural course of systems consolidation and allow the persistence of relevant emotional engrams. © 2017 Wiley Periodicals, Inc.

Novel learning accelerates systems consolidation of a contextual fear memory.

After initial encoding memories may undergo a time-dependent reorganization, becoming progressively independent from the hippocampus (HPC) and dependent on cortical regions such as the anterior cingulate cortex (ACC). Although the mechanisms underlying systems consolidation are somewhat known, the factors determining its temporal dynamics are still poorly understood. Here, we studied the influence of novel learning occurring between training and test sessions on the time-course of HPC- and ACC-dependency of contextual fear conditioning (CFC) memory expression. We found that muscimol was disruptive when infused into the HPC up to 35 days after training, while the ACC is vulnerable only after 45 days. However, when animals were subjected to a series of additional, distinct tasks to be learned within the first 3 weeks, muscimol became effective sooner. Muscimol had no effect in the HPC at 20 days after training, exactly when the ACC becomes responsive to this treatment. Thus, our data indicates that the encoding of new information generates a tight interplay between distinct memories, accelerating the reorganization of previously stored long term memories between the hippocampal and cortical areas. © 2016 Wiley Periodicals, Inc.

Involvement of the infralimbic cortex and CA1 hippocampal area in reconsolidation of a contextual fear memory through CB1 receptors: Effects of CP55,940.

The endocannabinoid system (ECS) has a pivotal role in different cognitive functions such as learning and memory. Recent evidence confirm the involvement of the hippocampal CB1 receptors in the modulation of both memory extinction and reconsolidation processes in different brain areas, but few studies focused on the infralimbic cortex, another important cognitive area. Here, we infused the cannabinoid agonist CP55,940 either into the infralimbic cortex (IL) or the CA1 area of the dorsal hippocampus (HPC) of adult male Wistar rats immediately after a short (3min) reactivation session, known to labilize a previously consolidated memory trace in order to allow its reconsolidation with some modification. In both structures, the treatment was able to disrupt reconsolidation in a relatively long lasting way, reducing the freezing response. To our notice, this is the first demonstration of ECS involvement in reconsolidation in the Infralimbic Cortex. Despite poorly discriminative between CB1 and CB2 receptors, CP55,940 is a potent agent, and these results suggest that a similar CB1-dependent circuitry is at work both in HPC and in the IL during memory reconsolidation.

The cannabinoid system in the retrosplenial cortex modulates fear memory consolidation, reconsolidation, and extinction.

Despite the fact that the cannabinoid receptor type 1 (CB1R) plays a pivotal role in emotional memory processing in different regions of the brain, its function in the retrosplenial cortex (RSC) remains unknown. Here, using contextual fear conditioning in rats, we showed that a post-training intra-RSC infusion of the CB1R antagonist AM251 impaired, and the agonist CP55940 improved, long-term memory consolidation. Additionally, a post-reactivation infusion of AM251 enhanced memory reconsolidation, while CP55940 had the opposite effect. Finally, AM251 blocked extinction, whereas CP55940 facilitated it and maintained memory extinguished over time. Altogether, our data strongly suggest that the cannabinoid system of the RSC modulates emotional memory.

Reconsolidation may incorporate state-dependency into previously consolidated memories.

Some memories enter into a labile state after retrieval, requiring reconsolidation in order to persist. One functional role of memory reconsolidation is the updating of existing memories. There are reports suggesting that reconsolidation can be modulated by a particular endogenous process taking place concomitantly to its natural course, such as water or sleep deprivation. Here, we investigated whether an endogenous process activated during a natural/physiological experience, or a pharmacological intervention, can also contribute to memory content updating. Using the contextual fear conditioning paradigm in rats, we found that the endogenous content of an aversive memory can be updated during its reconsolidation incorporating consequences of natural events such as water deprivation, transforming a previously stored memory into a state-dependent one. This updating seems to be mediated by the activation of angiotensin AT1 receptors in the dorsal hippocampus and local infusion of human angiotensin II (ANGII) was shown to mimic the water deprivation effects on memory reconsolidation. Systemic morphine injection was also able to turn a previously acquired experience into a state-dependent memory, reproducing the very same effects obtained by water deprivation or local angiotensin II infusion, and suggesting that other state-dependent-inducing protocols would also be able to contribute to memory updating. These findings trigger new insights about the influence of ordinary daily life events upon memory in its continuing reconstruction, adding the realm of reconsolidation to the classical view of endogenous modulation of consolidation.

Social interaction-induced fear memory reduction: exploring the influence of dopamine and oxytocin receptors on memory updating.

It has been well established that a consolidated memory can be updated during the plastic state induced by reactivation. This updating process opens the possibility to modify maladaptive memory. In the present study, we evaluated whether fear memory could be updated to less-aversive level by incorporating hedonic information during reactivation. Thus, male rats were fear conditioned and, during retrieval, a female was presented as a social rewarding stimulus. We found that memory reactivation with a female (but not a male) reduces fear expression within-session and in the test, without presenting reinstatement or spontaneous recovery. Interestingly, this intervention impaired extinction. Finally, we demonstrated that this emotional remodeling to eliminate fear expression requires the activation of dopamine and oxytocin receptors during retrieval. Hence, these results shed new lights on the memory updating process and suggests that the exposure to natural rewarding information such as a female during retrieval reduces a previously consolidated fear memory.

Naturalistic housing condition promotes behavioral flexibility and increases resilience to stress in rats.

Behavioral flexibility is an indispensable cognitive ability that allows the adjustment of behavioral responses to different situations, while resilience refers to the capability to deal effectively with stress. On one hand, standard laboratory housing provides impoverished cognitive, sensory, and physical stimulation compared to the conditions found in nature. Conversely, enriched and naturalistic housing conditions offer a broadening in the behavioral repertoire that can be depicted by the animals in their home cages, in addition to enabling a better management of possible stressors. Here, we investigated the effects of environmental enrichment and naturalistic housing compared to the standard laboratory housing on different behavioral tasks, including Morris water maze, open field, object location, and fear conditioning. This allowed us to evaluate how different housing conditions modulate behavioral flexibility and resilience to stress, in addition to spatial memory, in adult male rats. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Memory reconsolidation allows the consolidation of a concomitant weak learning through a synaptic tagging and capture mechanism.

Motivated by the synaptic tagging and capture (STC) hypothesis, it was recently shown that a weak learning, only able to produce short-term memory (STM), can succeed in establishing long-term memory (LTM) with a concomitant, stronger experience. This is consistent with the capture, by the first-tagged event, of the so-called plasticity-related proteins (PRPs) provided by the second one. Here, we describe how a concomitant session of reactivation/reconsolidation of a stronger, contextual fear conditioning (CFC) memory, allowed LTM to result from a weak spatial object recognition (wSOR) training. Consistent with an STC process, the effect was observed only during a critical time window and was dependent on the CFC reconsolidation-related protein synthesis. Retrieval by itself (without reconsolidation) did not have the same promoting effect. We also found that the inactivation of the NMDA receptor by AP5 prevented wSOR training to receive this support of CFC reconsolidation (supposedly through the production of PRPs), which may be the equivalent of blocking the setting of a learning tag in the dorsal CA1 region for that task. Furthermore, either a Water Maze reconsolidation, or a CFC extinction session, allowed the formation of wSOR-LTM. These results suggest for the first time that a reconsolidation session can promote the consolidation of a concomitant weak learning through a probable STC mechanism. These findings allow new insights concerning the influence of reconsolidation in the acquisition of memories of otherwise unrelated events during daily life situations.

Role of TRPV1 in consolidation of fear memories depends on the averseness of the conditioning procedure.

Despite the fact that TRPV1 receptors are widely expressed in brain structures such as the hippocampus, its functions remain largely unknown. In the present study, we have investigated the possible modulatory role of the hippocampal endovanilloid system upon memory consolidation of two different behavioral tasks in rats. Post-training infusion of the TRPV1 antagonist capsazepine disrupted memory consolidation with a strong training protocol, but not with a weak one in the contextual fear conditioning or in the step-down inhibitory avoidance task. These results provide evidence that the modulation of the hippocampal memory consolidation through TRPV1 receptors takes place only in presence of a strong emotional experience, suggesting that a certain aversiveness level is required in order to recruit endovanilloids to exert this function. A possible synergic role of hippocampal endovanilloid and endocannabinoid system on memory consolidation is discussed.

Systems consolidation and fear memory generalisation as a potential target for trauma-related disorders.

Fear memory generalisation is a central hallmark in the broad range of anxiety and trauma-related disorders. Recent findings suggest that fear generalisation is closely related to hippocampal dependency during retrieval. In this review, we describe the current understanding about memory generalisation and its potential influence in fear attenuation through pharmacological and behavioural interventions. In light of systems consolidation framework, we propose that keeping memory precision could be a key step to enhance therapeutic outcomes.