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.

Cathodal transcranial direct current stimulation on the prefrontal cortex applied after reactivation attenuates fear memories and prevent reinstatement after extinction.

BACKGROUND: In the last decade, pharmacological strategies targeting reconsolidation after memory retrieval have shown promising efforts to attenuate persistent memories and overcome fear recovery. However, most reconsolidation inhibiting agents have not been approved for human testing. While non-invasive neuromodulation can be considered an alternative approach to pharmacological treatments, there is a lack of evidence about the efficacy of these technologies when modifying memory traces via reactivation/reconsolidation mechanism. OBJECTIVE: In this study, we evaluate the effect of cathodal (c-tDCS) and anodal (a-DCS) transcranial direct current stimulation applied after memory reactivation and extinction in rats. METHODS: Male Wistar rats were randomly assigned into three groups: one sham group, one anodal tDCS group, and one cathodal tDCS group (500 µA, 20 min). Reconsolidation and extinction of fear memories were evaluated using a contextual fear conditioning. RESULTS: Our results showed that c-tDCS and a-tDCS after memory reactivation can attenuate mild fear memories. However, only c-tDCS stimulation prevented both fear expression under strong fear learning and fear recovery after a reinstatement protocol without modification of learning rate or extinction retrieval. Nevertheless, the remote memories were resistant to modification through this type of neuromodulation. Our results are discussed considering the interaction between intrinsic excitability promoted by learning and memory retrieval and the electric field applied during tDCS. CONCLUSION: These results point out some of the boundary conditions influencing the efficacy of tDCS in fear attenuation and open new ways for the development of noninvasive interventions aimed to control fear-related disorders via reconsolidation.

Conflict Test Battery for Studying the Act of Facing Threats in Pursuit of Rewards.

Survival depends on the ability of animals to avoid threats and approach rewards. Traditionally, these two opposing motivational systems have been studied separately. In nature, however, they regularly compete for the control of behavior. When threat- and reward-eliciting stimuli (learned or unlearned) occur simultaneously, a motivational conflict emerges that challenges individuals to weigh available options and execute a single behavioral response (avoid or approach). Most previous animal models using approach/avoidance conflicts have often focused on the ability to avoid threats by forgoing or delaying the opportunity to obtain rewards. In contrast, behavioral tasks designed to capitalize on the ability to actively choose to execute approach behaviors despite threats are scarce. Thus, we developed a behavioral test battery composed of three conflict tasks to directly study rats confronting threats to obtain rewards guided by innate and conditioned cues. One conflict task involves crossing a potentially electrified grid to obtain food on the opposite end of a straight alley, the second task is based on the step-down threat avoidance paradigm, and the third one is a modified version of the open field test. We used diazepam to pharmacologically validate conflict behaviors in our tasks. We found that, regardless of whether competing stimuli were conditioned or innate, a low diazepam dose decreased risk assessment and facilitated taking action to obtain rewards in the face of threats during conflict, without affecting choice behavior when there was no conflict involved. Using this pharmacologically validated test battery of ethologically designed innate/learned conflict tasks could help understand the fundamental brain mechanisms underlying the ability to confront threats to achieve goals.

Chronic fluoxetine prevents fear memory generalization and enhances subsequent extinction by remodeling hippocampal dendritic spines and slowing down systems consolidation.

Fear memory overgeneralization contributes to the genesis and persistence of anxiety disorders and is a central hallmark in the pathophysiology of post-traumatic stress disorder (PTSD). Recent findings suggest that fear generalization is closely related to hippocampal dependency during retrieval. The selective serotonin reuptake inhibitor (SSRI) fluoxetine has been used as a first-line treatment for PTSD; however, how it exerts its therapeutic effect remains a matter of debate. Here, using contextual fear conditioning in rats, we show that chronic fluoxetine treatment prevents fear generalization and enhances subsequent extinction. Moreover, fluoxetine treatment after extinction prevents spontaneous recovery. The mechanism through which fluoxetine affects generalization and extinction seems to be through the postponement of systems consolidation, thereby maintaining hippocampal involvement during retrieval. Such an effect relies on a remodeling of dendritic spines in the hippocampus, as well as the number of mature, mushroom-type spines promoted by fluoxetine treatment. In order to further investigate whether fear generalization is a potential predictor of extinction effectiveness, we categorized a large naive population according to their generalization rate. We found that discriminator rats showed a better extinction profile compared to generalizers, suggesting that the generalization rate predicts extinction effectiveness. Hence, we propose that the therapeutic strategy of choice should take into account the extension of memory generalization, in which therapies based on extinction could induce a better outcome in patients who present less fear overgeneralization. These results open new avenues for the development of interventions that prevent fear generalization by maintaining memory dependency of the hippocampus.

Author Correction: Periodical reactivation under the effect of caffeine attenuates fear memory expression in rats.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Periodical reactivation under the effect of caffeine attenuates fear memory expression in rats.

In the last decade, several studies have shown that fear memories can be attenuated by interfering with reconsolidation. However, most of the pharmacological agents used in preclinical studies cannot be administered to humans. Caffeine is one of the world's most popular psychoactive drugs and its effects on cognitive and mood states are well documented. Nevertheless, the influence of caffeine administration on fear memory processing is not as clear. We employed contextual fear conditioning in rats and acute caffeine administration under a standard memory reconsolidation protocol or periodical memory reactivation. Additionally, potential rewarding/aversion and anxiety effects induced by caffeine were evaluated by conditioning place preference or open field, respectively. Caffeine administration was able to attenuate weak fear memories in a standard memory reconsolidation protocol; however, periodical memory reactivation under caffeine effect was necessary to attenuate strong and remote memories. Moreover, caffeine promoted conditioned place preference and anxiolytic-like behavior, suggesting that caffeine weakens the initial learning during reactivation through counterconditioning mechanisms. Thus, our study shows that rewarding and anxiolytic effects of caffeine during fear reactivation can change the emotional valence of fear memory. It brings a new promising pharmacological approach based on drugs widely used such as caffeine to treat fear-related disorders.

Enhancement of extinction memory by pharmacological and behavioral interventions targeted to its reactivation.

Extinction is a process that involves new learning that inhibits the expression of previously acquired memories. Although temporarily effective, extinction does not erase an original fear association. Since the extinction trace tends to fade over time, the original memory can resurge. On the other hand, strengthening effects have been described in several reconsolidation studies using different behavioral and pharmacological manipulations. In order to know whether an extinction memory can be strengthened by reactivation-based interventions in the contextual fear conditioning task, we began by replicating the classic phenomenon of spontaneous recovery to show that brief reexposure sessions can prevent the decay of the extinction trace over time in a long-lasting way. This fear attenuation was shown to depend both on L-type calcium channels and protein synthesis, which suggests a reconsolidation process behind the reactivation-induced strengthening effect. The extinction trace was also susceptible to enhancement by a post-reactivation infusion of a memory-enhancing drug (NaB), which was also able to prevent rapid fear reacquisition (savings). These findings point to new reactivation-based approaches able to strengthen an extinction memory to promote its persistence. The constructive interactions between extinction and reconsolidation may represent a promising novel approach in the realm of fear-related disorder treatments.

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.

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.

The dynamic nature of systems consolidation: Stress during learning as a switch guiding the rate of the hippocampal dependency and memory quality.

Memory fades over time, becoming more schematic or abstract. The loss of contextual detail in memory may reflect a time-dependent change in the brain structures supporting memory. It has been well established that contextual fear memory relies on the hippocampus for expression shortly after learning, but it becomes hippocampus-independent at a later time point, a process called systems consolidation. This time-dependent process correlates with the loss of memory precision. Here, we investigated whether training intensity predicts the gradual decay of hippocampal dependency to retrieve memory, and the quality of the contextual memory representation over time. We have found that training intensity modulates the progressive decay of hippocampal dependency and memory precision. Strong training intensity accelerates systems consolidation and memory generalization in a remarkable timeframe match. The mechanisms underpinning such process are triggered by glucocorticoid and noradrenaline released during training. These results suggest that the stress levels during emotional learning act as a switch, determining the fate of memory quality. Moderate stress will create a detailed memory, whereas a highly stressful training will develop a generic gist-like memory.

Medial orbitofrontal cortex lesion prevents facilitatory effects of d-cycloserine during fear extinction.

Animal models of fear extinction have an important clinical relevance to pharmacological and exposure-based therapies for anxiety disorders. Lesions of prefrontal structures impair fear extinction. On the other hand, d-cycloserine is able to enhance this process. We hypothesize that the integrity of cortical structures involved in inhibitory control of emotional responses is crucial for the facilitatory effects of d-cycloserine. Here, we showed that medial orbitofrontal cortex lesion prevents d-cycloserine enhancement of fear extinction. These preliminary results suggest that effects of pharmacological treatments could be dependent on cortical activity state to promote fear memory reduction.

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.

Reconsolidation allows fear memory to be updated to a less aversive level through the incorporation of appetitive information.

The capacity to adapt to new situations is one of the most important features of memory. When retrieved, memories may undergo a labile state that is sensitive to modification. This process, called reconsolidation, can lead to memory updating through the integration of new information into a previously consolidated memory background. Thus reconsolidation provides the opportunity to modify an undesired fear memory by updating its emotional valence to a less aversive level. Here we evaluated whether a fear memory can be reinterpreted by the concomitant presentation of an appetitive stimulus during its reactivation, hindering fear expression. We found that memory reactivation in the presence of appetitive stimuli resulted in the suppression of a fear response. In addition, fear expression was not amenable to reinstatement, spontaneous recovery, or rapid reacquisition. Such effect was prevented by either systemic injection of nimodipine or intra-hippocampal infusion of ifenprodil, indicating that memory updating was mediated by a reconsolidation mechanism relying on hippocampal neuronal plasticity. Taken together, this study shows that reconsolidation allows for a 're-signification' of unwanted fear memories through the incorporation of appetitive information. It brings a new promising cognitive approach to treat fear-related disorders.

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.

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.