mTOR inhibition impairs extinction memory reconsolidation

Rapamycin attenuates reconsolidation of a backwards-conditioned aversive stimuli in female mice

RATIONALE

The mammalian target of rapamycin (mTOR) kinase is known to mediate consolidation and reconsolidation of aversive memories. Most studies in this area use a forward conditioning paradigm in which the conditioned stimulus (CS) precedes the unconditioned stimulus (US). Little is known, however, about the neurobiological underpinnings of backwards (BW) conditioning paradigms, particularly in female mice. In BW conditioning, the CS does not become directly associated with the US; it instead evokes conditioned fear by reactivating a memory of the conditioning context and indirectly retrieving a memory of the aversive US.

OBJECTIVES

We sought to examine BW conditioned fear memory processes in female mice. First, we examined whether freezing to a BW CS is mediated by fear to the conditioning context. Second, we tested whether blocking consolidation of a BW CS attenuated memory of the CS and conditioning context. Finally, we tested whether blocking reconsolidation of a BW CS attenuated memory of the conditioning context.

RESULTS

We show that conditioned freezing to a BW CS is mediated by fear to the conditioning context. Furthermore, rapamycin-an mTOR inhibitor, when given immediately following BW conditioning, impairs consolidation of both cued and contextual fear memory. Similarly, rapamycin given following retrieval of a BW CS blocks context recall. Rapamycin is acting on reconsolidation as CS retrieval is necessary to see the effects of rapamycin on context memory recall.

CONCLUSIONS

Our study provides novel evidence that indirect retrieval cues are sensitive to rapamycin in female mice. The capacity to indirectly reactivate memories and render them susceptible to disruption is critical in the translation of reconsolidation-based approaches to the clinic.

Hippocampal CaMKII inhibition induces reactivation-dependent amnesia for extinction memory and causes fear relapse

Hippocampal GluN2B subunit-containing NMDAR (GluN2B-NMDAR) activation during recall destabilizes fear extinction memory, which must undergo brain-derived neurotrophic factor (BDNF)-dependent reconsolidation to persist. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a Ser/Thr protein kinase essential for hippocampus-dependent memory processing that acts downstream GluN2B-NMDAR and controls BDNF expression, but its participation in fear extinction memory reconsolidation has not yet been studied. Using a combination of pharmacological and behavioral tools, we found that in adult male Wistar rats, intra dorsal-CA1 administration of the CaMKII inhibitors autocamtide-2-related inhibitory peptide (AIP) and KN-93, but not of their inactive analogs scrambled AIP and KN-92, after fear extinction memory recall impaired extinction and caused GluN2B-NMDAR-dependent recovery of fear. Our results indicate that hippocampal CaMKII is necessary for fear extinction reconsolidation, and suggest that modulation of its activity around the time of recall controls the inhibition that extinction exerts on learned fear.

The Devastating Effects of Sleep Deprivation on Memory: Lessons from Rodent Models

In this narrative review article, we discuss the role of sleep deprivation (SD) in memory processing in rodent models. Numerous studies have examined the effects of SD on memory, with the majority showing that sleep disorders negatively affect memory. Currently, a consensus has not been established on which damage mechanism is the most appropriate. This critical issue in the neuroscience of sleep remains largely unknown. This review article aims to elucidate the mechanisms that underlie the damaging effects of SD on memory. It also proposes a scientific solution that might explain some findings. We have chosen to summarize literature that is both representative and comprehensive, as well as innovative in its approach. We examined the effects of SD on memory, including synaptic plasticity, neuritis, oxidative stress, and neurotransmitters. Results provide valuable insights into the mechanisms by which SD impairs memory function.

Reactivation of cocaine contextual memory engages mechanistic target of rapamycin/S6 kinase 1 signaling

Mechanistic target of rapamycin (mTOR) C1 and its downstream effectors have been implicated in synaptic plasticity and memory. Our prior work demonstrated that reactivation of cocaine memory engages a signaling pathway consisting of Akt, glycogen synthase kinase-3β (GSK3β), and mTORC1. The present study sought to identify other components of mTORC1 signaling involved in the reconsolidation of cocaine contextual memory, including eukaryotic translation initiation factor 4E (eIF4E)-eIF4G interactions, p70 S6 kinase polypeptide 1 (p70S6K, S6K1) activity, and activity-regulated cytoskeleton (Arc) expression. Cocaine contextual memory was established in adult CD-1 mice using conditioned place preference. After cocaine place preference was established, mice were briefly re-exposed to the cocaine-paired context to reactivate the cocaine memory and brains examined. Western blot analysis showed that phosphorylation of the mTORC1 target, p70S6K, in nucleus accumbens and hippocampus was enhanced 60 min following reactivation of cocaine memories. Inhibition of mTORC1 with systemic administration of rapamycin or inhibition of p70S6K with systemic PF-4708671 after reactivation of cocaine contextual memory abolished the established cocaine place preference. Immunoprecipitation assays showed that reactivation of cocaine memory did not affect eIF4E-eIF4G interactions in nucleus accumbens or hippocampus. Levels of Arc mRNA were significantly elevated 60 and 120 min after cocaine memory reactivation and returned to baseline 24 h later. These findings demonstrate that mTORC1 and p70S6K are required for reconsolidation of cocaine contextual memory.

Glutamate Receptor Interacting Protein 1 in the Dorsal CA1 Drives Alpha-amino-3-hydroxy-5-methyl-4-Isoxazolepropionic Acid Receptor Endocytosis and Exocytosis Bidirectionally and Mediates Forgetting, Exploratory, and Anxiety-like Behavior

Endocytosis of GluA2-containing alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in CA1 of the hippocampus regulates forgetting; deficits in forgetting nociceptive memory can induce serious stress disorders. As a transporter of GluA2-containing AMPAR, the functions of glutamate receptor interacting protein 1 (GRIP1) in forgetting and possible stress responses remain unclear. Lentivirus-mediated interference of GRIP1 expression or function in the dorsal CA1 of the hippocampus in mice indicated that GRIP1 overexpression enhanced spatial memory, impaired forgetting in a Barnes maze, and induced anxiety-like behavior in the open field and elevated plus-maze test. In contrast, GRIP1 knockdown impaired learning capacity. Furthermore, inhibition of the PDZ2 and PDZ4/5 domains of GRIP1 and GluA2-dn enhanced learning capacity, whereas GluA2-dn impaired spatial memory; inhibition of the PDZ2 and PDZ4/5 domains of GRIP1 also decreased forgetting capacity to some extent. Importantly, inhibition of both the PDZ2 and PDZ4/5 domains of GRIP1 induced anxiety-like behavior but not GluA2-dn. Furthermore, optogenetic control of both GluA1 and GluA2 insertion into the postsynaptic membrane impaired memory and induced anxiety-like behavior. In vitro experiments showed that GRIP1-ov and -dn, inhibition of PDZ2 and PDZ4/5 domains of GRIP1, and GluA2-dn decreased glycine-induced GluA1 and GluA2 exocytosis; meanwhile, GRIP1-ov and -dn, and interference of PDZ2 and PDZ4/5 domains of GRIP1 blocked AMPA- and NMDA-induced GluA1 and GluA2 endocytosis. Overall, these results suggest that GRIP1 drives AMPA receptor endocytosis and exocytosis bidirectionally; furthermore, GRIP1-induced stabilization of anchoring postsynaptic GluA1 and GluA2 impairs forgetting and induces anxiety-like behavior. GRIP1 may provide a potential therapeutic target in posttraumatic syndromes and anxiety disorders.

Anshen Dingzhi prescription in the treatment of PTSD in mice: Investigation of the underlying mechanism from the perspective of hippocampal synaptic function

BACKGROUND

Anshen Dingzhi prescription (ADP) is an important prescription for the treatment of mental diseases in traditional Chinese medicine and is widely used to treat neuropsychiatric disorders.

PURPOSE

To explore the ameliorative effect of ADP on post-traumatic stress disorder (PTSD)-like behaviors in mice and determine the underlying mechanism.

METHODS

The constituents of ADP were analyzed by UPLC-Q-TOF/MS. The PTSD-like behaviors of mice subjected to single prolonged stress (SPS) were evaluated using behavioral tests. Potential pathological changes in the hippocampus were assessed by hematoxylin and eosin (H&E) staining. Western blotting and immunohistochemistry (IHC) were employed to detect the expression of proteins involved in relevant signaling pathways.

RESULTS

Five quality control markers (ginsenoside Rg1, ginsenoside Rb1, tenuifolin, poricoic acid B, and α-asarone) were detected in the ADP solution. The ginsenoside Rg1 content in ADP was found to be 0.114 mg/g. Mice subjected to SPS showed obvious fear generalization and anxiety-like behaviors. ADP treatment prevented the behavioral changes caused by exposure to SPS. Compared with control animals, the number of normal pyramidal cells in the hippocampal CA1 region of mice exposed to SPS was decreased and the number of degenerating pyramidal cells was increased; however, ADP administration could counteract these effects. Furthermore, the protein expression of BDNF, p-TrkB, μ-calpain, PSD95, GluN2A, GluA1, p-AKT, p-mTOR, and ARC was decreased, while that of PTEN and GluN2B was increased in the hippocampus of mice subjected to SPS compared with that in control animals; however, these changes in protein expression were reversed following ADP treatment. Importantly, the ameliorative effect of ADP on PTSD-like behaviors and synaptic protein expression were inhibited by rapamycin administration.

CONCLUSIONS

ADP administration improves PTSD-like behaviors in mice and this effect may be mediated through an mTOR-dependent improvement in synaptic function in the hippocampus.

mTOR regulates cocaine-induced behavioural sensitization through the SynDIG1-GluA2 interaction in the nucleus accumbens

Behavioral sensitization is a progressive increase in locomotor or stereotypic behaviours in response to drugs. It is believed to contribute to the reinforcing properties of drugs and to play an important role in relapse after cessation of drug abuse. However, the mechanism underlying this behaviour remains poorly understood. In this study, we showed that mTOR signaling was activated during the expression of behavioral sensitization to cocaine and that intraperitoneal or intra-nucleus accumbens (NAc) treatment with rapamycin, a specific mTOR inhibitor, attenuated cocaine-induced behavioural sensitization. Cocaine significantly modified brain lipid profiles in the NAc of cocaine-sensitized mice and markedly elevated the levels of phosphatidylinositol-4-monophosphates (PIPs), including PIP, PIP_2, and PIP₃. The behavioural effect of cocaine was attenuated by intra-NAc administration of LY294002, an AKT-specific inhibitor, suggesting that PIPs may contribute to mTOR activation in response to cocaine. An RNA-sequencing analysis of the downstream effectors of mTOR signalling revealed that cocaine significantly decreased the expression of SynDIG1, a known substrate of mTOR signalling, and decreased the surface expression of GluA2. In contrast, AAV-mediated SynDIG1 overexpression in NAc attenuated intracellular GluA2 internalization by promoting the SynDIG1-GluA2 interaction, thus maintaining GluA2 surface expression and repressing cocaine-induced behaviours. In conclusion, NAc SynDIG1 may play a negative regulatory role in cocaine-induced behavioural sensitization by regulating synaptic surface expression of GluA2.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.