Stephen M. Taubenfeld

The consolidation of new but not reactivated memory requires hippocampal C/EBPβ

Long-term memory formation consists of multiple phases. A new memory is initially labile and sensitive to disruption by a variety of interfering events or agents. To become stable, this new memory undergoes a process known as consolidation, which, in the case of declarative memories, occurs within the medial temporal lobes and requires gene expression. When recalled, memories re-enter a new phase of vulnerability and seem to require a reconsolidation process in order to be maintained. Here we show that consolidation but not reconsolidation of inhibitory avoidance memory requires the expression of the transcription factor CCAAT enhancer binding protein β (C/EBPβ) in the hippocampus. Furthermore, in the same region, de novo protein synthesis is not essential for memory reconsolidation. C/EBPβ is an evolutionarily conserved genetic marker that has a selective role in the consolidation of new but not reactivated memories in the hippocampus.

PKMζ maintains spatial, instrumental, and classically conditioned long-term memories

How long-term memories are stored is a fundamental question in neuroscience. The first molecular mechanism for long-term memory storage in the brain was recently identified as the persistent action of protein kinase Mzeta (PKMζ), an autonomously active atypical protein kinase C (PKC) isoform critical for the maintenance of long-term potentiation (LTP). PKMζ maintains aversively conditioned associations, but what general form of information the kinase encodes in the brain is unknown. We first confirmed the specificity of the action of zeta inhibitory peptide (ZIP) by disrupting long-term memory for active place avoidance with chelerythrine, a second inhibitor of PKMζ activity. We then examined, using ZIP, the effect of PKMζ inhibition in dorsal hippocampus (DH) and basolateral amygdala (BLA) on retention of 1-d-old information acquired in the radial arm maze, water maze, inhibitory avoidance, and contextual and cued fear conditioning paradigms. In the DH, PKMζ inhibition selectively disrupted retention of information for spatial reference, but not spatial working memory in the radial arm maze, and precise, but not coarse spatial information in the water maze. Thus retention of accurate spatial, but not procedural and contextual information required PKMζ activity. Similarly, PKMζ inhibition in the hippocampus did not affect contextual information after fear conditioning. In contrast, PKMζ inhibition in the BLA impaired retention of classical conditioned stimulus–unconditioned stimulus (CS-US) associations for both contextual and auditory fear, as well as instrumentally conditioned inhibitory avoidance. PKMζ inhibition had no effect on postshock freezing, indicating fear expression mediated by the BLA remained intact. Thus, persistent PKMζ activity is a general mechanism for both appetitively and aversively motivated retention of specific, accurate learned information, but is not required for processing contextual, imprecise, or procedural information.

Cell Shape and Negative Links in Regulatory Motifs Together Control Spatial Information Flow in Signaling Networks

The role of cell size and shape in controlling local intracellular signaling reactions, and how this spatial information originates and is propagated, is not well understood. We have used partial differential equations to model the flow of spatial information from the beta-adrenergic receptor to MAPK1,2 through the cAMP/PKA/B-Raf/MAPK1,2 network in neurons using real geometries. The numerical simulations indicated that cell shape controls the dynamics of local biochemical activity of signal-modulated negative regulators, such as phosphodiesterases and protein phosphatases within regulatory loops to determine the size of microdomains of activated signaling components. The model prediction that negative regulators control the flow of spatial information to downstream components was verified experimentally in rat hippocampal slices. These results suggest a mechanism by which cellular geometry, the presence of regulatory loops with negative regulators, and key reaction rates all together control spatial information transfer and microdomain characteristics within cells.

A molecular correlate of memory and amnesia in the hippocampus

Memory consolidation in humans and other species is profoundly disrupted by lesions of either the medial temporal lobes or regions of the thalamus. It has been proposed that these structures regulate the neuronal gene expression necessary for long-term memory. Evidence suggests that long-term memory formation requires the activity of members of the cAMP response element (CRE) binding protein (CREB) transcription factor family, and that CRE-regulated genes are expressed in the hippocampus in response to inhibitory avoidance training. Here we show that lesions of the fornix, a massive fiber bundle connecting the hippocampus with the septum and hypothalamus, specifically disrupt both consolidation of inhibitory avoidance memory and CREB-mediated responses in the hippocampus. We propose that inputs passing through the fornix regulate this memory consolidation by regulating CREB-mediated gene expression in hippocampal neurons.

Preclinical Assessment for Selectively Disrupting a Traumatic Memory via Postretrieval Inhibition of Glucocorticoid Receptors

BACKGROUND Traumatic experiences may lead to debilitating psychiatric disorders including acute stress disorder and posttraumatic stress disorder. Current treatments for these conditions are largely ineffective, and novel therapies are needed. A cardinal symptom of these pathologies is the reexperiencing of the trauma through intrusive memories and nightmares. Studies in animal models indicate that memories can be weakened by interfering with the postretrieval restabilization process known as memory reconsolidation. We previously reported that, in rats, intraamygdala injection of the glucocorticoid receptor antagonist RU38486 disrupts the reconsolidation of a traumatic memory. Here we tested parameters important for designing novel clinical protocols targeting the reconsolidation of a traumatic memory with RU38486. METHODS Using rat inhibitory avoidance, we tested the efficacy of postretrieval systemic administration of RU38486 on subsequent memory retention and evaluated several key preclinical parameters. RESULTS Systemic administration of RU38486 before or after retrieval persistently weakens inhibitory avoidance memory retention in a dose-dependent manner, and memory does not reemerge following a footshock reminder. The efficacy of treatment is a function of the intensity of the initial trauma, and intense traumatic memories can be disrupted by changing the time and number of interventions. Furthermore, one or two treatments are sufficient to disrupt the memory maximally. The treatment selectively targets the reactivated memory without interfering with the retention of another nonreactivated memory. CONCLUSIONS RU38486 is a potential novel treatment for psychiatric disorders linked to traumatic memories. Our data provide the parameters for designing promising clinical trials for the treatment of flashback-type symptoms of PTSD.

Profound molecular changes following hippocampal slice preparation: loss of AMPA receptor subunits and uncoupled mRNA/protein expression

The acute hippocampal slice preparation is a convenient, in vitro model widely used to study the biological basis of synaptic plasticity. Although slices may preserve their electrophysiological properties for several hours, profound molecular changes in response to the injury caused by the slicing procedure are likely to occur. To determine the magnitude and duration of these changes we examined the post‐slicing expression kinetics of three classes of genes known to be implicated in long‐term synaptic plasticity: glutamate AMPA receptors (GluR), transcription factors and neurotrophins. Slicing resulted in a striking loss of GluR1 and GluR3, but not of GluR2 proteins suggesting that rapid changes in the composition of major neurotransmitter receptors may occur. Slicing caused a significant induction of the transcription factors c‐fos, zif268, CCAAT enhancer binding protein (C/EBP) β and δ mRNAs and of the neurotrophin brain‐derived neurothophic factor (BDNF) mRNA. In contrast, there was no augmentation, and sometimes a decline, in the levels of the corresponding proteins. These data reveal that significant discrepancies exist between the slice preparation and the intact hippocampus in terms of the metabolism of molecular components known to be involved in synaptic plasticity.

Disrupting the memory of places induced by drugs of abuse weakens motivational withdrawal in a context-dependent manner

Addicts repeatedly relapse to drug seeking even after years of abstinence, and this behavior is frequently induced by the recall of memories of the rewarding effects of the drug. Established memories, including those induced by drugs of abuse, can become transiently fragile if reactivated, and during this labile phase, known as reconsolidation, can be persistently disrupted. Here we show that, in rats, a morphine-induced place preference (mCPP) memory is linked to context-dependent withdrawal as disrupting the reconsolidation of the memory leads to a significant reduction of withdrawal evoked in the same context. Moreover, the hippocampus plays a critical role in linking the place preference memory with the context-conditioned withdrawal, as disrupting hippocampal protein synthesis and cAMP-dependent-protein kinase A after the reactivation of mCPP significantly weakens the withdrawal. Hence, targeting memories induced by drugs may represent an important strategy for attenuating context-conditioned withdrawal and therefore subsequent relapse in opiate addicts.