Karim Nader

Fear memories require protein synthesis in the amygdala for reconsolidation after retrieval.

‘New’ memories are initially labile and sensitive to disruption before being consolidated into stable long-term memories. Much evidence indicates that this consolidation involves the synthesis of new proteins in neurons. The lateral and basal nuclei of the amygdala (LBA) are believed to be a site of memory storage in fear learning. Infusion of the protein synthesis inhibitor anisomycin into the LBA shortly after training prevents consolidation of fear memories. Here we show that consolidated fear memories, when reactivated during retrieval, return to a labile state in which infusion of anisomycin shortly after memory reactivation produces amnesia on later tests, regardless of whether reactivation was performed 1 or 14 days after conditioning. The same treatment with anisomycin, in the absence of memory reactivation, left memory intact. Consistent with a time-limited role for protein synthesis production in consolidation, delay of the infusion until six hours after memory reactivation produced no amnesia. Our data show that consolidated fear memories, when reactivated, return to a labile state that requires de novo protein synthesis for reconsolidation. These findings are not predicted by traditional theories of memory consolidation.

Cellular and Systems Reconsolidation in the Hippocampus

Cellular theories of memory consolidation posit that new memories require new protein synthesis in order to be stored. Systems consolidation theories posit that the hippocampus has a time-limited role in memory storage, after which the memory is independent of the hippocampus. Here, we show that intra-hippocampal infusions of the protein synthesis inhibitor anisomycin caused amnesia for a consolidated hippocampal-dependent contextual fear memory, but only if the memory was reactivated prior to infusion. The effect occurred even if reactivation was delayed for 45 days after training, a time when contextual memory is independent of the hippocampus. Indeed, reactivation of a hippocampus-independent memory caused the trace to again become hippocampus dependent, but only for 2 days rather than for weeks. Thus, hippocampal memories can undergo reconsolidation at both the cellular and systems levels.

A single standard for memory: the case for reconsolidation

Consolidated memories can re-enter states of transient instability following reactivation, from which they must again stabilize in order to persist, contradicting the previously dominant view that memory and its associated plasticity mechanisms progressively and irreversibly decline with time. We witness exciting times, as neuroscience begins embracing a position, long-held in cognitive psychology, that recognizes memory as a principally dynamic process. In light of remaining controversy, we here establish that the same operational definitions and types of evidence underpin the deduction of both reconsolidation and consolidation, thus validating the extrapolation that post-retrieval memory plasticity reflects processes akin to those that stabilized the memory following acquisition.

Memory consolidation of Pavlovian fear conditioning: a cellular and molecular perspective

Pavlovian fear conditioning has emerged as a leading behavioral paradigm for studying the neurobiological basis of learning and memory. Although considerable progress has been made in understanding the neural substrates of fear conditioning at the systems level, until recently little has been learned about the underlying cellular and molecular mechanisms. The success of systems-level work aimed at defining the neuroanatomical pathways underlying fear conditioning, combined with the knowledge accumulated by studies of long-term potentiation (LTP), has recently given way to new insights into the cellular and molecular mechanisms that underlie acquisition and consolidation of fear memories. Collectively, these findings suggest that fear memory consolidation in the amygdala shares essential biochemical features with LTP, and hold promise for understanding the relationship between memory consolidation and synaptic plasticity in the mammalian brain.

Different lateral amygdala outputs mediate reactions and actions elicited by a fear-arousing stimulus.

Fear-arousing stimuli elicit innate reactions and can reinforce acquisition of new responses. We tested whether mechanisms mediating these conditioned stimulus (CS) properties were isomorphic or dissociable within the amygdala. Rats trained on a fear-conditioning task (CS paired with footshock) were then trained on an escape-from-fear task (EFF) in which the CS reinforced a locomotor response terminating the CS. Lateral nucleus (LA) lesions blocked acquisition of both conditioned freezing responses and the CSs reinforcement of a new response in the EFF task. Central nucleus (CE) lesions blocked conditioned freezing but not the EFF, whereas basal nucleus (B) lesions blocked the EFF but not conditioned freezing. Thus, activation of the LA by a CS seems to trigger conditioned reactions via CE and conditioned aversion via B activation, reduction of which reinforces new actions.

eIF2α phosphorylation bidirectionally regulates the switch from short to long-term synaptic plasticity and memory

The late phase of long-term potentiation (LTP) and memory (LTM) requires new gene expression, but the molecular mechanisms that underlie these processes are not fully understood. Phosphorylation of eIF2alpha inhibits general translation but selectively stimulates translation of ATF4, a repressor of CREB-mediated late-LTP (L-LTP) and LTM. We used a pharmacogenetic bidirectional approach to examine the role of eIF2alpha phosphorylation in synaptic plasticity and behavioral learning. We show that in eIF2alpha(+/S51A) mice, in which eIF2alpha phosphorylation is reduced, the threshold for eliciting L-LTP in hippocampal slices is lowered, and memory is enhanced. In contrast, only early-LTP is evoked by repeated tetanic stimulation and LTM is impaired, when eIF2alpha phosphorylation is increased by injecting into the hippocampus a small molecule, Sal003, which prevents the dephosphorylation of eIF2alpha. These findings highlight the importance of a single phosphorylation site in eIF2alpha as a key regulator of L-LTP and LTM formation.

Characterization of Fear Memory Reconsolidation

Reactivation of consolidated memories returns them to a protein synthesis-dependent state. One interpretation of these findings is that the memory reconsolidates after use. Two alternative interpretations are that protein synthesis inhibition facilitates extinction and that postreactivation protein synthesis inhibition leads to an inability to retrieve the consolidated memory. First, using two different approaches, we report that reconsolidation cannot be reduced down to facilitated extinction. We show that the reconsolidation deficit does not show renewal after a contextual shift, whereas an extinguished auditory fear memory does under the same conditions and the deficit occurs regardless of whether the memory is reactivated with an extinction [conditioned stimulus (CS) alone] or a reinforced trial (CS-unconditioned stimulus). To address the issue of whether postreactivation anisomycin leads to an inability to retrieve the consolidated memory, we used two traditional assays for retrieval deficits. First, we demonstrate that the amnesia induced by blockade of reconsolidation does not show any spontaneous recovery. Second, we show that application of reminder shock does not result in the reinstatement of the memory. These findings support the idea that reactivation of consolidated memories initiates a second time-dependent memory formation process.

Translational control of hippocampal synaptic plasticity and memory by the eIF2α kinase GCN2

Studies on various forms of synaptic plasticity have shown a link between messenger RNA translation, learning and memory. Like memory, synaptic plasticity includes an early phase that depends on modification of pre-existing proteins, and a late phase that requires transcription and synthesis of new proteins. Activation of postsynaptic targets seems to trigger the transcription of plasticity-related genes. The new mRNAs are either translated in the soma or transported to synapses before translation. GCN2, a key protein kinase, regulates the initiation of translation. Here we report a unique feature of hippocampal slices from GCN2-/- mice: in CA1, a single 100-Hz train induces a strong and sustained long-term potentiation (late LTP or L-LTP), which is dependent on transcription and translation. In contrast, stimulation that elicits L-LTP in wild-type slices, such as four 100-Hz trains or forskolin, fails to evoke L-LTP in GCN2-/- slices. This aberrant synaptic plasticity is mirrored in the behaviour of GCN2-/- mice in the Morris water maze: after weak training, their spatial memory is enhanced, but it is impaired after more intense training. Activated GCN2 stimulates mRNA translation of ATF4, an antagonist of cyclic-AMP-response-element-binding protein (CREB). Thus, in the hippocampus of GCN2-/- mice, the expression of ATF4 is reduced and CREB activity is increased. Our study provides genetic, physiological, behavioural and molecular evidence that GCN2 regulates synaptic plasticity, as well as learning and memory, through modulation of the ATF4/CREB pathway.

Autism-related deficits via dysregulated eIF4E-dependent translational control

Hyperconnectivity of neuronal circuits due to increased synaptic protein synthesis is thought to cause autism spectrum disorders (ASDs). The mammalian target of rapamycin (mTOR) is strongly implicated in ASDs by means of upstream signalling; however, downstream regulatory mechanisms are ill-defined. Here we show that knockout of the eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2)—an eIF4E repressor downstream of mTOR—or eIF4E overexpression leads to increased translation of neuroligins, which are postsynaptic proteins that are causally linked to ASDs. Mice that have the gene encoding 4E-BP2 (Eif4ebp2) knocked out exhibit an increased ratio of excitatory to inhibitory synaptic inputs and autistic-like behaviours (that is, social interaction deficits, altered communication and repetitive/stereotyped behaviours). Pharmacological inhibition of eIF4E activity or normalization of neuroligin 1, but not neuroligin 2, protein levels restores the normal excitation/inhibition ratio and rectifies the social behaviour deficits. Thus, translational control by eIF4E regulates the synthesis of neuroligins, maintaining the excitation-to-inhibition balance, and its dysregulation engenders ASD-like phenotypes.

NMDA receptors are critical for unleashing consolidated auditory fear memories.

Memories are dynamic and can change when recalled. The process that returns memories to a labile state during remembering is unclear. We found that the presence of NMDA, but not AMPA, receptor antagonists in the amygdala prior to recall prevented the consolidated fear memory from returning to a labile state. These findings suggest that NMDA receptors in the amygdala are critical for transforming a memory from a fixed to a labile state.