Reconsolidation and the regulation of plasticity: moving beyond memory

Induction of long-term hyperexcitability by memory-related cAMP signaling in isolated nociceptor cell bodies

Persistent hyperactivity of nociceptors is known to contribute significantly to long-lasting sensitization and ongoing pain in many clinical conditions. It is often assumed that nociceptor hyperactivity is mainly driven by continuing stimulation from inflammatory mediators. We have tested an additional possibility: that persistent increases in excitability promoting hyperactivity can be induced by a prototypical cellular signaling pathway long known to induce late-phase long-term potentiation (LTP) of synapses in brain regions involved in memory formation. This cAMP-PKA-CREB-gene transcription-protein synthesis pathway was tested using whole-cell current clamp methods on small dissociated sensory neurons (primarily nociceptors) from dorsal root ganglia (DRGs) excised from previously uninjured ("naïve") rats. Six-hour treatment with the specific Gαs-coupled 5-HT4 receptor agonist, prucalopride, or with the adenylyl cyclase activator, forskolin, induced long-term hyperexcitability (LTH) in DRG neurons that manifested 12-24 hours later as action potential (AP) discharge (ongoing activity, OA) during artificial depolarization to -45 mV, a membrane potential that is normally subthreshold for AP generation. Prucalopride treatment also induced significant long-lasting depolarization of resting membrane potential (from -69 to -66 mV), enhanced depolarizing spontaneous fluctuations (DSFs) of membrane potential, and indications of reduced AP threshold and rheobase. LTH was prevented by co-treatment of prucalopride with inhibitors of PKA, CREB, gene transcription, and protein synthesis. As in the induction of synaptic memory, many other cellular signals are likely to be involved. However, the discovery that this prototypical memory induction pathway can induce nociceptor LTH, along with reports that cAMP signaling and CREB activity in DRGs can induce hyperalgesic priming, suggest that early, temporary, cAMP-induced transcriptional and translational mechanisms can induce nociceptor LTH that might last for long periods. An interesting possibility is that these mechanisms can also be reactivated by re-exposure to inflammatory mediators such as serotonin during subsequent challenges to bodily integrity, "reconsolidating" the cellular memory and thereby extending the duration of persistent nociceptor hyperexcitability.

Induction of long-term hyperexcitability by memory-related cAMP signaling in isolated nociceptor cell bodies

Persistent hyperactivity of nociceptors is known to contribute significantly to long-lasting sensitization and ongoing pain in many clinical conditions. It is often assumed that nociceptor hyperactivity is mainly driven by continuing stimulation from inflammatory mediators. We have tested an additional possibility: that persistent increases in excitability promoting hyperactivity can be induced by a prototypical cellular signaling pathway long known to induce late-phase long-term potentiation (LTP) of synapses in brain regions involved in memory formation. This cAMP-PKA-CREB-gene transcription-protein synthesis pathway was tested using whole-cell current clamp methods on small dissociated sensory neurons (primarily nociceptors) from dorsal root ganglia (DRGs) excised from previously uninjured ("naïve") male rats. Six-hour treatment with the specific Gαs-coupled 5-HT4 receptor agonist, prucalopride, or with the adenylyl cyclase activator forskolin induced long-term hyperexcitability (LTH) in DRG neurons that manifested 12-24 h later as action potential (AP) discharge (ongoing activity, OA) during artificial depolarization to -45 mV, a membrane potential that is normally subthreshold for AP generation. Prucalopride treatment also induced significant long-lasting depolarization of resting membrane potential (from -69 to -66 mV), enhanced depolarizing spontaneous fluctuations (DSFs) of membrane potential, and produced trends for reduced AP threshold and rheobase. LTH was prevented by co-treatment of prucalopride with inhibitors of PKA, CREB, gene transcription, or protein synthesis. As in the induction of synaptic memory, many other cellular signals are likely to be involved. However, the discovery that this prototypical memory induction pathway can induce nociceptor LTH, along with reports that cAMP signaling and CREB activity in DRGs can induce hyperalgesic priming, suggest that early, temporary, cAMP-induced transcriptional and translational mechanisms can induce nociceptor LTH that might last for long periods. The present results also raise the question of whether reactivation of primed signaling mechanisms by re-exposure to inflammatory mediators linked to cAMP synthesis during subsequent challenges to bodily integrity can "reconsolidate" nociceptor memory, extending the duration of persistent hyperexcitability.

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.

Preventing fear return in humans: Music-based intervention during reactivation-extinction paradigm

In several research studies, the reactivation extinction paradigm did not effectively prevent the return of fear if administered without any intervention technique. Therefore, in this study, the authors hypothesized that playing music (high valence, low arousal) during the reconsolidation window may be a viable intervention technique for eliminating fear-related responses. A three-day auditory differential fear conditioning paradigm was used to establish fear conditioning. Participants were randomly assigned into three groups, i.e., one control group, standard extinction (SE), and two experimental groups, reactivation extinction Group (RE) and music reactivation extinction (MRE), of twenty participants in each group. Day 1 included the habituation and fear acquisition phases; on Day 2 (after 24 hours), the intervention was conducted, and re-extinction took place on Day 3. Skin conductance responses were used as the primary outcome measure. Results indicated that the MRE group was more effective in reducing fear response than the RE and SE groups in the re-extinction phase. Furthermore, there was no significant difference observed between SE and RE groups. This is the first study known to demonstrate the effectiveness of music intervention in preventing the return of fear in a healthy individual. Therefore, it might also be employed as an intervention strategy (non-pharmacological approach) for military veterans, in emotion regulation, those diagnosed with post-traumatic stress disorder, and those suffering from specific phobias.

Nociceptor activity induces nonionotropic NMDA receptor signaling to enable spinal reconsolidation and reverse pathological pain

Chronic, pathological pain is a highly debilitating condition that can arise and be maintained through central sensitization. Central sensitization shares mechanistic and phenotypic parallels with memory formation. In a sensory model of memory reconsolidation, plastic changes underlying pain hypersensitivity can be dynamically regulated and reversed following the reactivation of sensitized sensory pathways. However, the mechanisms by which synaptic reactivation induces destabilization of the spinal "pain engram" are unclear. We identified nonionotropic N-methyl-d-aspartate receptor (NI-NMDAR) signaling as necessary and sufficient for the reactive destabilization of dorsal horn long-term potentiation and the reversal of mechanical sensitization associated with central sensitization. NI-NMDAR signaling engaged directly or through the reactivation of sensitized sensory networks was associated with the degradation of excitatory postsynaptic proteins. Our findings identify NI-NMDAR signaling as a putative synaptic mechanism by which engrams are destabilized in reconsolidation and as a potential means of treating underlying causes of chronic pain.

Face your fears: attenuating remote fear memories by reconsolidation-updating

Traumatic events generate some of the most enduring memories, yet little is known about how long-lasting fear memories can be attenuated. In this review, we collect the surprisingly sparse evidence on remote fear memory attenuation from both animal and human research. What is becoming apparent is twofold: although remote fear memories are more resistant to change compared with recent ones, they can nevertheless be attenuated when interventions are targeted toward the period of memory malleability instigated by memory recall, the reconsolidation window. We describe the physiological mechanisms underlying remote reconsolidation-updating approaches and highlight how they can be enhanced through interventions promoting synaptic plasticity. By capitalizing on an intrinsically relevant phase of memory, reconsolidation-updating harbors the potential to permanently alter remote fear memories.

Memory loss at sleep onset

Every night, we pass through a transitory zone at the borderland between wakefulness and sleep, named the first stage of nonrapid eye movement sleep (N1). N1 sleep is associated with increased hippocampal activity and dream-like experiences that incorporate recent wake materials, suggesting that it may be associated with memory processing. Here, we investigated the specific contribution of N1 sleep in the processing of memory traces. Participants were asked to learn the precise locations of 48 objects on a grid and were then tested on their memory for these items before and after a 30-min rest during which participants either stayed fully awake or transitioned toward N1 or deeper (N2) sleep. We showed that memory recall was lower (10% forgetting) after a resting period, including only N1 sleep compared to N2 sleep. Furthermore, the ratio of alpha/theta power (an electroencephalography marker of the transition toward sleep) correlated negatively with the forgetting rate when taking into account all sleepers (N1 and N2 groups combined), suggesting a physiological index for memory loss that transcends sleep stages. Our findings suggest that interrupting sleep onset at N1 may alter sleep-dependent memory consolidation and promote forgetting.

Continuous theta-burst stimulation over the right dorsolateral prefrontal cortex disrupts fear memory reconsolidation in humans

Continuous theta-burst stimulation (cTBS), a non-invasive brain stimulation technique, can induce long-lasting changes in synaptic plasticity, vital for memory reconsolidation. For this study, a total of 170 participants completed four experiments by a randomized controlled design. Succeeding fear conditioning, the subjects received cTBS over the right dorsolateral prefrontal cortex (dlPFC) or vertex (control) with or without exposure to the conditioned stimulus to reactivate the original fear memory, and then underwent fear response tests. Compared with cTBS over the vertex and without memory reactivation, only cTBS over the right dlPFC after reactivation decreased the fear response for both recent and remote fear memories. This procedure was effective only during the reconsolidation window. The disruptive effect of cTBS over the right dlPFC on fear memory reconsolidation was delay-dependent. These findings demonstrate that cTBS time-dependently and delay-dependently prevents the return of fear and may have clinical potential for treating fear-related disorders.

Memory destabilization during reconsolidation: a consequence of homeostatic plasticity?

Remembering is not a static process: When retrieved, a memory can be destabilized and become prone to modifications. This phenomenon has been demonstrated in a number of brain regions, but the neuronal mechanisms that rule memory destabilization and its boundary conditions remain elusive. Using two distinct computational models that combine Hebbian plasticity and synaptic downscaling, we show that homeostatic plasticity can function as a destabilization mechanism, accounting for behavioral results of protein synthesis inhibition upon reactivation with different re-exposure times. Furthermore, by performing systematic reviews, we identify a series of overlapping molecular mechanisms between memory destabilization and synaptic downscaling, although direct experimental links between both phenomena remain scarce. In light of these results, we propose a theoretical framework where memory destabilization can emerge as an epiphenomenon of homeostatic adaptations prompted by memory retrieval.

Blockade of β-Adrenergic Receptors by Propranolol Disrupts Reconsolidation of Drug Memory and Attenuates Heroin Seeking

Persistent traces of drug reward memories contribute to intense craving and often trigger relapse. A number of pharmacological interventions on drug-associated memories have shown significant benefits in relapse prevention at a preclinical level but their translational potential is limited due to deleterious side effects. Propranolol, a non-specific β-adrenergic receptors antagonist, is known for its ability to erase maladaptive memories associated with nicotine or cocaine in rodents and humans. However, little is known about its effect on reconsolidation of heroin memory and heroin seeking. In the present study, rats with a history of intravenous heroin self-administration received the propranolol treatment (10 mg/kg; i.p.) at different time windows with or without CS (conditioned stimulus) exposure. Our results showed that propranolol, when administered immediately after CS exposure but not 6 h later, can significantly attenuate cue-induced and drug-primed reinstatement of heroin seeking, suggesting that propranolol has the ability to disrupt heroin memory and reduce relapse. The propranolol treatment without retrieval of drug memory had no effect on subsequent reinstatement of heroin seeking, suggesting that its interfering effects are retrieval-dependent. Importantly, the effects of propranolol were long lasting as rats showed diminished drug seeking even 28 days after the treatment. Altogether, our study suggests that propranolol can interfere with reconsolidation of heroin memory and reduce subsequent drug seeking, making it an attractive therapeutic candidate for the treatment of opioid addiction and relapse prevention.

Stress-induced resistance to fear memory destabilization is associated with an impairment of Lys-48-linked protein polyubiquitination in the Basolateral Amygdala: Influence of D-cycloserine

The destabilization/reconsolidation process can be triggered by memory recall, allowing consolidated memories to be modified. We have previously reported that stress prior to fear conditioning induces memories that exhibit resistance to the engagement of some molecular events associated with the destabilization/reconsolidation process. Here, we evaluated whether stress could affect the expression of Lys-48 polyubiquitinated proteins within the basolateral amygdala complex, a phenomenon crucially linked to memory destabilization. As expected, a post-recall increase of Lys-48 polyubiquitinated proteins in control animals was observed; however, this phenomenon was prevented by stress exposure before fear conditioning. On the other hand, pre-recall administration of D-cycloserine -a positive modulator of NMDA sites capable of reverting memory resistance to pharmacological interference-, facilitated the increase of Lys-48 polyubiquitinated proteins in stressed animals. In conclusion, the protein polyubiquitination-dependent destabilization is impaired after the recall of stress-induced resistant memories, with D-cycloserine restoring such molecular event. Hence, the present report contributes to further characterize the neurobiological events associated with stress-induced memory resistance as well as to corroborate the connection between glutamatergic signaling, protein degradation and memory destabilization in stress-induced resistant memories.

Contextual experience modifies functional connectome indices of topological strength and efficiency

Stimuli presented at short temporal delays before functional magnetic resonance imaging (fMRI) can have a robust impact on the organization of synchronous activity in resting state networks. This presents an opportunity to investigate how sensory, affective and cognitive stimuli alter functional connectivity in rodent models. In the present study we assessed the effect on functional connectivity of a familiar contextual stimulus presented 10 min prior to sedation for imaging. A subset of animals were co-presented with an unfamiliar social stimulus in the same environment to further investigate the effect of familiarity on network topology. Rats were imaged at 11.1 T and graph theory analysis was applied to matrices generated from seed-based functional connectivity data sets with 144 brain regions (nodes) and 10,152 pairwise correlations (after excluding 144 diagonal edges). Our results show substantial changes in network topology in response to the familiar (context). Presentation of the familiar context, both in the absence and presence of the social stimulus, strongly reduced network strength, global efficiency, and altered the location of the highest eigenvector centrality nodes from cortex to the hypothalamus. We did not observe changes in modular organization, nodal cartographic assignments, assortative mixing, rich club organization, and network resilience. We propose that experiential factors, perhaps involving associative or episodic memory, can exert a dramatic effect on functional network strength and efficiency when presented at a short temporal delay before imaging.

Different roles of Rac1 in the acquisition and extinction of methamphetamine-associated contextual memory in the nucleus accumbens

Rationale: Repeated methamphetamine (METH) exposure induces long-term cognitive deficits and pathological drug-associated memory that can be disrupted by manipulating memory reconsolidation and extinction. The nucleus accumbens (NAc) is the key region of the brain reward system and predominantly consists of two subtypes of medium spiny neurons (MSNs) based on the expression of D1 or D2 dopamine receptors (D1-MSNs or D2-MSNs). Spine structural plasticity in the NAc is critical for the acquisition, reconsolidation and extinction of drug-associated memory. However, the molecular mechanisms underlying METH-associated memory and spine remodelling in each type of MSNs in the NAc remain unknown. Here, we explored whether Rac1 in the NAc mediates METH-associated contextual memory and spine remodelling.

Methods: Pharmacological and genetic manipulations of Rac1 were used to investigate its role during the acquisition, reconsolidation and extinction of METH-associated contextual memory. Recombinant adeno-associated viruses expressing mCherry under the control of the dopamine D1 receptor gene promoter (Drd1-mCherry) or dopamine D2 receptor gene promoter (Drd2-mCherry) were used to specifically label D1-MSNs or D2-MSNs.

Results: Using viral-mediated gene transfer, we demonstrated that decreased Rac1 activity was required for the acquisition of METH-associated contextual memory and the METH-induced increase in thin spine density, whereas increased Rac1 signalling was important for the extinction of METH-associated contextual memory and the related elimination of thin spines. Moreover, the increase of dendritic spines was both found in D1-MSNs and D2-MSNs during the acquisition process, but extinction training selectively decreased the spine density in D1-MSNs. Interestingly, Rac1 was responsible for METH-induced spine plasticity in D1-MSNs but not in D2-MSNs. Additionally, we found that microinjection of a Rac1 inhibitor or activator into the NAc was not sufficient to disrupt reconsolidation, and the pharmacological activation of Rac1 in the NAc facilitated the extinction of METH-associated contextual memory. Regarding cognitive memory, decreased Rac1 activity improved the METH-induced impairment in object recognition memory.

Conclusion: Our findings indicate that Rac1 plays opposing roles in the acquisition and extinction of METH-associated contextual memory and reveal the cell-specific role of Rac1 in METH-associated spine remodelling, suggesting that Rac1 is a potential therapeutic target for reducing relapse in METH addiction and remediating METH-induced recognition memory impairment.

β-adrenergic modulation of discrimination learning and memory in the auditory cortex

Despite vast literature on catecholaminergic neuromodulation of auditory cortex functioning in general, knowledge about its role for long‐term memory formation is scarce. Our previous pharmacological studies on cortex‐dependent frequency‐modulated tone‐sweep discrimination learning of Mongolian gerbils showed that auditory‐cortical D_1/5‐dopamine receptor activity facilitates memory consolidation and anterograde memory formation. Considering overlapping functions of D_1/5‐dopamine receptors and β‐adrenoceptors, we hypothesised a role of β‐adrenergic signalling in the auditory cortex for sweep discrimination learning and memory. Supporting this hypothesis, the β_1/2‐adrenoceptor antagonist propranolol bilaterally applied to the gerbil auditory cortex after task acquisition prevented the discrimination increment that was normally monitored 1 day later. The increment in the total number of hurdle crossings performed in response to the sweeps per se was normal. Propranolol infusion after the seventh training session suppressed the previously established sweep discrimination. The suppressive effect required antagonist injection in a narrow post‐session time window. When applied to the auditory cortex 1 day before initial conditioning, β₁‐adrenoceptor‐antagonising and β₁‐adrenoceptor‐stimulating agents retarded and facilitated, respectively, sweep discrimination learning, whereas β₂‐selective drugs were ineffective. In contrast, single‐sweep detection learning was normal after propranolol infusion. By immunohistochemistry, β₁‐ and β₂‐adrenoceptors were identified on the neuropil and somata of pyramidal and non‐pyramidal neurons of the gerbil auditory cortex. The present findings suggest that β‐adrenergic signalling in the auditory cortex has task‐related importance for discrimination learning of complex sounds: as previously shown for D_1/5‐dopamine receptor signalling, β‐adrenoceptor activity supports long‐term memory consolidation and reconsolidation; additionally, tonic input through β₁‐adrenoceptors may control mechanisms permissive for memory acquisition.

Sound check, stage design and screen plot - how to increase the comparability of fear conditioning and fear extinction experiments

In the recent decade, fear conditioning has evolved as a standard procedure for testing cognitive abilities such as memory acquisition, consolidation, recall, reconsolidation, and extinction, preferentially in genetically modified mice. The reasons for the popularity of this powerful approach are its ease to perform, the short duration of training and testing, and its well-described neural basis. So why to bother about flaws in standardization of test procedures and analytical routines? Simplicity does not preclude the existence of fallacies. A short survey of the literature revealed an indifferent use of acoustic stimuli in terms of quality (i.e., white noise vs. sine wave), duration, and intensity. The same applies to the shock procedures. In the present article, I will provide evidence for the importance of qualitative and quantitative parameters of conditioned and unconditioned stimuli for the experimental outcome. Moreover, I will challenge frequently applied interpretations of short-term vs. long-term extinction and spontaneous recovery. On the basis of these concerns, I suggest a guideline for standardization of fear conditioning experiments in mice to improve the comparability of the experimental data.

Translational Control Mechanisms in Persistent Pain

Persistent pain, which is poorly treated and estimated to afflict one third of the world's population, is largely mediated by the sensitization of nociceptive neurons. This sensitization involves de novo gene expression to support biochemical and structural changes required to maintain amplified pain signaling that frequently persists even after injury to tissue resolves. While transcription-dependent changes in gene expression are important, recent work demonstrates that activity-dependent regulation of mRNA translation is key to controlling the cellular proteome and the development and maintenance of persistent pain. In this review, we highlight recent advances in translational regulation of gene expression in nociceptive circuits, with a focus on key signaling pathways and mRNA targets that may be tractable for the creation of next-generation pain therapeutics.

The Synaptic Theory of Memory: A Historical Survey and Reconciliation of Recent Opposition

Trettenbrein (2016) has argued that the concept of the synapse as the locus of memory is outdated and has made six critiques of this concept. In this article, we examine these six critiques and suggest that the current theories of the neurobiology of memory and the empirical data indicate that synaptic activation is the first step in a chain of cellular and biochemical events that lead to memories formed in cell assemblies and neural networks that rely on synaptic modification for their formation. These neural networks and their modified synaptic connections can account for the cognitive basis of learning and memory and for memory deterioration in neurological disorders. We first discuss Hebb's (1949) theory that synaptic change and the formation of cell assemblies and phase sequences can link neurophysiology to cognitive processes. We then examine each of Trettenbrein's (2016) critiques of the synaptic theory in light of Hebb's theories and recent empirical data. We examine the biochemical basis of memory formation and the necessity of synaptic modification to form the neural networks underlying learning and memory. We then examine the use of Hebb's theories of synaptic change and cell assemblies for integrating neurophysiological and cognitive conceptions of learning and memory. We conclude with an examination of the applications of the Hebb synapse and cell assembly theories to the study of the neuroscience of learning and memory, the development of computational models of memory and the construction of "intelligent" robots. We conclude that the synaptic theory of memory has not met its demise, but is essential to our understanding of the neural basis of memory, which has two components: synaptic plasticity and intrinsic plasticity.

Nitrous oxide may interfere with the reconsolidation of drinking memories in hazardous drinkers in a prediction-error-dependent manner

Weakening drinking-related reward memories by blocking their reconsolidation is a potential novel strategy for treating alcohol use disorders. However, few viable pharmacological options exist for reconsolidation interference in humans. We therefore examined whether the NMDA receptor antagonising gas, Nitrous Oxide (N₂O) could reduce drinking by preventing the post-retrieval restabilisation of alcohol memories in a group of hazardous drinkers. Critically, we focussed on whether prediction error (PE; a key determinant of reconsolidation) was experienced at retrieval. Sixty hazardous drinkers were randomised to one of three groups that retrieved alcohol memories either with negative PE (Retrieval + PE), no PE (Retrieval no PE) or non-alcohol memory retrieval with PE (No-retrieval +PE). All participants then inhaled 50% N₂O for 30 min. The primary outcome was change in beer consumption and alcohol cue-driven urge to drink from the week preceding manipulation (baseline) to the week following manipulation (test). The manipulation did not affect drinking following the intended retrieval+/- PE conditions However, a manipulation check, using a measure of subjective surprise, revealed that the group-level manipulation did not achieve the intended differences in PE at retrieval. Assessment of outcomes according to whether alcohol-relevant PE was actually experienced at retrieval, showed N₂O produced reductions in drinking in a retrieval and PE-dependent fashion. These preliminary findings highlight the importance of directly testing assumptions about memory reactivation procedures in reconsolidation research and suggest that N₂O should be further investigated as a potential reconsolidation-blocking agent.

Does Sleep Help Prevent Forgetting Rewarded Memory Representations in Children and Adults?

Sleep fosters the consolidation of rewarded memory representations in adults. However, sleep and its memory-supporting functions change through healthy development, and it is unclear whether sleep benefits the consolidation of rewarded memory representations in children as it does in adults. Based on previous findings, we expected sleep to benefit the consolidation of rewarded memory representations in children more than it does in adults. For that reason, 16 children (7–11 years) and 20 adults (21–29 years) participated in this experiment. During the encoding session, participants were asked to learn the location of 18 object pairs. Thereafter, one-half of the object locations were allocated to a high-rewarded condition and the other half to a low-rewarded condition. In the sleep condition, the encoding session took place in the evening (for children 7–8 pm, for adults 8–9 pm). After a fixed retention interval of 12 h the retrieval session was conducted the next morning (for children 7–8 am, for adults 8–9 am). In the wake condition, the time schedule was the same but reversed: the encoding session started in the morning (for children 7–8 am, for adults 8–9 am), and retrieval took place in the evening (for children 7–8 pm, for adults 8–9 pm). Sleep/wake had no impact on the memory performance regarding the low-rewarded memory items. In contrast, wakefulness in comparison to sleep reduced the memory performance on high-rewarded memory items. The interaction between sleep/wake and the degree of reward on memory performance was only significant in children. These results show that 12 h of wakefulness can deteriorate the memory performance for high-rewarded representations, whereas sleep can prevent the forgetting of these rewarded representations. It is discussed whether ontogenetic changes in sleep may play a role in conserving relevant but fragile memory representation.

β-Adrenergic signaling is required for the induction of a labile state during memory reconsolidation

Memory reconsolidation is the process by which previously consolidated memories reenter a labile state through reactivation of the memory trace and are actively consolidated through de novo protein synthesis. Although extensive studies have shown that β-adrenergic signaling plays a critical role in the restabilization of reactivated memory, its role in the destabilization of long-term memory is not well-studied. In this study, we found that membrane excitability increased in hippocampal CA1 neurons immediately after the retrieval of contextual fear memory. Interestingly, this increase in membrane excitability diminished after treatment with propranolol (a β-adrenergic receptor antagonist), an NMDA receptor antagonist, and a PKA inhibitor. In addition, we found that administration of propranolol prior to, but not after, the retrieval of fear memory ameliorated the memory impairment caused by anisomycin, indicating that inhibition of β-adrenergic signaling blocks the destabilization of contextual fear memory. Taken together, these results indicate that β-adrenergic signaling via NMDA receptors and PKA signaling pathway induces a labile state of long-term memory through increased neuronal membrane excitability.

Translation regulation in the spinal dorsal horn - A key mechanism for development of chronic pain

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Spinal sensitization shares molecular mechanisms with hippocampal LTP and memory.

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Changes in mRNA translation are observed in many chronic pain conditions.

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Targeting translational control mechanisms is a promising strategy to inhibit pain.

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Targeting spinal reconsolidation can reverse established hypersensitivity.

Chronic pain is a pathological condition characterized by long-lasting pain after damaged tissue has healed. Chronic pain can be caused and maintained by changes in various components of the pain pathway, including sensory neurons, spinal cord and higher brain centers. Exaggerated sensitivity and responsiveness of spinal nociceptive circuits, representing maladaptive plasticity, play key roles in the amplification of peripheral signals in chronic pain conditions. This spinal amplification mechanism profoundly contributes to the development and maintenance of chronic pain hypersensitivity in response to peripheral injury, and in some cases occurs independently of the peripheral stimulus. Long-lasting changes in the activity of spinal neurons are caused by alterations in their cellular proteome, which relies on de novo gene expression. Recent evidence indicates that translational control of gene expression plays a major role in determining protein levels, and is intricately involved in different forms of intrinsic and synaptic plasticity. In this review, we summarize findings supporting a key role for translational control in spinal cord-dependent mechanisms of chronic pain, and present recent approaches to reverse persistent pain by targeting these mechanisms.

Acute physical exercise in humans enhances reconsolidation of emotional memories

Increasing evidence suggests that when a memory is reactivated through retrieval, it becomes temporarily vulnerable to environmental or pharmacological manipulation, which can consequently update or strengthen the memory. Physical exercise has been shown to modulate the maintenance of fear memories in animals following memory reactivation. This study investigated the effect of intense exercise in modulating the reconsolidation of trauma memories. Fifty-four undergraduate students watched a trauma film depicting the aftermath of a highway car crash. Two days later, participants engaged in either (a) 20-25min of incremental cycling following a memory reactivation induction (Reactivation/Exercise), (b) 20-25min of mild cycling (Reactivation/No Exercise) following memory reactivation, or (c) 20-25min of incremental cycling but no memory reactivation (No Reactivation/Exercise). Saliva samples were collected to index salivary amylase and cortisol at baseline and post activity. Participants completed memory questionnaires relating to declarative and intrusive memory recall two days after memory reactivation. Reactivation/Exercise participants recalled more central details of the trauma film relative to other participants. Increased cortisol predicted better total memory recall in the Reactivation/Exercise, but not in the other conditions. These findings suggest that intense exercise during the period of memory reactivation enhances subsequent trauma memory, and provides human evidence consistent with recent findings of exercise-induced fear reconsolidation in animals.

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.

Cyclin Y-mediated transcript profiling reveals several important functional pathways regulated by Cyclin Y in hippocampal neurons

Cyclin Y (CCNY), which is a cyclin protein known to play a role in cell division, is unexpectedly and thus interestingly expressed in non-proliferating neuronal cells. There have been only a few studies reporting the neuronal functions of CCNY in synapse remodeling and hippocampal long-term potentiation. Therefore, we here provide global and comprehensive information on the putative functions of CCNY in biological and functional pathways in neuronal systems. We adopted high-throughput RNA-sequencing technology for analyzing transcriptomes regulated by CCNY and utilized bioinformatics for identifying putative molecules, biological processes, and functional pathways that are possibly connected to CCNY functions in hippocampal neuronal cells of rats. We revealed that several enriched annotation terms and pathways associated with CCNY expression within neurons, including apoptosis, learning or memory, synaptic plasticity, actin cytoskeleton, focal adhesion, extracellular matrix-receptor interaction and chemokine signaling pathway are targeted by CCNY. In addition, the mRNA levels of some genes enriched for those annotation terms and pathways or genes reported to be altered in Alzheimer's disease mouse model were further validated by quantitative real-time PCR in hippocampal neuronal cells. The present study provides an excellent resource for future investigations of CCNY functions in neuronal systems.

Reactivation of seizure-related changes to interictal spike shape and synchrony during postseizure sleep in patients

OBJECTIVE

Local field potentials (LFPs) arise from synchronous activation of millions of neurons, producing seemingly consistent waveform shapes and relative synchrony across electrodes. Interictal spikes (IISs) are LFPs associated with epilepsy that are commonly used to guide surgical resection. Recently, changes in neuronal firing patterns observed in the minutes preceding seizure onset were found to be reactivated during postseizure sleep, a process called seizure-related consolidation (SRC), due to similarities with learning-related consolidation. Because IISs arise from summed neural activity, we hypothesized that changes in IIS shape and relative synchrony would be observed in the minutes preceding seizure onset and would be reactivated preferentially during postseizure slow-wave sleep (SWS).

METHODS

Scalp and intracranial recordings were obtained continuously across multiple days from clinical macroelectrodes implanted in patients undergoing treatment for intractable epilepsy. Data from scalp electrodes were used to stage sleep. Data from intracranial electrodes were used to detect IISs using a previously established algorithm. Partial correlations were computed for sleep and wake periods before and after seizures as a function of correlations observed in the minutes preceding seizures. Magnetic resonance imaging (MRI) and computed tomography (CT) scans were co-registered with electroencephalography (EEG) to determine the location of the seizure-onset zone (SOZ).

RESULTS

Changes in IIS shape and relative synchrony were observed on a subset of macroelectrodes minutes before seizure onset, and these changes were reactivated preferentially during postseizure SWS. Changes in synchrony were greatest for pairs of electrodes where at least one electrode was located in the SOZ.

SIGNIFICANCE

These data suggest preseizure changes in neural activity and their subsequent reactivation occur across a broad spatiotemporal scale: from single neurons to LFPs, both within and outside the SOZ. The preferential reactivation of seizure-related changes in IISs during postseizure SWS adds to a growing body of literature suggesting that pathologic neural processes may utilize physiologic mechanisms of synaptic plasticity.

β-arrestin-2 regulates NMDA receptor function in spinal lamina II neurons and duration of persistent pain

Mechanisms of acute pain transition to chronic pain are not fully understood. Here we demonstrate an active role of β-arrestin 2 (Arrb2) in regulating spinal cord NMDA receptor (NMDAR) function and the duration of pain. Intrathecal injection of the mu-opioid receptor agonist [D-Ala², NMe-Phe⁴, Gly-ol⁵]-enkephalin produces paradoxical behavioural responses: early-phase analgesia and late-phase mechanical allodynia which requires NMDAR; both phases are prolonged in Arrb2 knockout (KO) mice. Spinal administration of NMDA induces GluN2B-dependent mechanical allodynia, which is prolonged in Arrb2-KO mice and conditional KO mice lacking Arrb2 in presynaptic terminals expressing Nav1.8. Loss of Arrb2 also results in prolongation of inflammatory pain and neuropathic pain and enhancement of GluN2B-mediated NMDA currents in spinal lamina IIo not lamina I neurons. Finally, spinal over-expression of Arrb2 reverses chronic neuropathic pain after nerve injury. Thus, spinal Arrb2 may serve as an intracellular gate for acute to chronic pain transition via desensitization of NMDAR.

The cellular mechanisms underlying acute pain transitions to chronic pain are poorly understood. Here the authors show that the scaffolding protein β-arrestin 2 contributes to these processes via desensitization of NMDA receptors in spinal neurons.

Synaptic plasticity in the anterior cingulate cortex in acute and chronic pain

The anterior cingulate cortex (ACC) is activated in both acute and chronic pain. In this Review, we discuss increasing evidence from rodent studies that ACC activation contributes to chronic pain states and describe several forms of synaptic plasticity that may underlie this effect. In particular, one form of long-term potentiation (LTP) in the ACC, which is triggered by the activation of NMDA receptors and expressed by an increase in AMPA-receptor function, sustains the affective component of the pain state. Another form of LTP in the ACC, which is triggered by the activation of kainate receptors and expressed by an increase in glutamate release, may contribute to pain-related anxiety.

NMDAR-dependent proteasome activity in the gustatory cortex is necessary for conditioned taste aversion

Taste information is processed in different brain structures in the mammalian brain, including the gustatory cortex (GC), which resides within the insular cortex. N-methyl-d-aspartate receptor (NMDAR) activity in the GC is necessary for the acquisition of conditioned taste aversion (CTA) but not positive novel taste learning. Previous studies have shown that taste memory consolidation requires intact protein synthesis in the GC. In addition, the direct involvement of translation initiation and elongation factors was documented in the GC during taste learning. However, protein expression is defined by protein synthesis, degradation, and localization. Protein degradation is critical for the consolidation and reconsolidation of other forms of learning, such as fear learning and addiction behavior, but its role in cortical-dependent learning is not clear. Here, we show for the first time that proteasome activity is specifically increased in the GC 4h following experiencing of a novel taste. This increase in proteasome activity was abolished by local administration to the GC of the NMDA antagonist, APV, as well as a CaMKII inhibitor, at the time of acquisition. In addition, local application of lactacystin, a proteasome inhibitor, resulted in impaired CTA, but not novel taste learning. These results suggest that NMDAR-dependent proteasome activity in the GC participates in the association process between novel taste experience and negative visceral sensation.

Differential Susceptibility of the Developing Brain to Contextual Adversity and Stress

A swiftly growing volume of literature, comprising both human and animal studies and employing both observational and experimental designs, has documented striking individual differences in neurobiological sensitivities to environmental circumstances within subgroups of study samples. This differential susceptibility to social and physical environments operates bidirectionally, in both adverse and beneficial contexts, and results in a minority subpopulation with remarkably poor or unusually positive trajectories of health and development, contingent upon the character of environmental conditions. Differences in contextual susceptibility appear to emerge in early development, as the interactive and adaptive product of genetic and environmental attributes. This paper surveys what is currently known of the mechanisms or mediators of differential susceptibility, at the levels of temperament and behavior, physiological systems, brain circuitry and neuronal function, and genetic and epigenetic variation. It concludes with the assertion that differential susceptibility is inherently grounded within processes of biological moderation, the complexities of which are at present only partially understood.

Auditory feedback blocks memory benefits of cueing during sleep

It is now widely accepted that re-exposure to memory cues during sleep reactivates memories and can improve later recall. However, the underlying mechanisms are still unknown. As reactivation during wakefulness renders memories sensitive to updating, it remains an intriguing question whether reactivated memories during sleep also become susceptible to incorporating further information after the cue. Here we show that the memory benefits of cueing Dutch vocabulary during sleep are in fact completely blocked when memory cues are directly followed by either correct or conflicting auditory feedback, or a pure tone. In addition, immediate (but not delayed) auditory stimulation abolishes the characteristic increases in oscillatory theta and spindle activity typically associated with successful reactivation during sleep as revealed by high-density electroencephalography. We conclude that plastic processes associated with theta and spindle oscillations occurring during a sensitive period immediately after the cue are necessary for stabilizing reactivated memory traces during sleep.

Exposure to memory cues during sleep improves subsequent memory recall. Here the authors demonstrate that presenting an additional auditory stimulus during a critical time window following the memory cue abolishes the memory benefit of cueing and its oscillatory correlates during sleep in humans.