Acquisition of non-olfactory encoding improves odour discrimination in olfactory cortex.

Olfaction is influenced by contextual factors, past experiences, and the animal's internal state. Whether this information is integrated at the initial stages of cortical odour processing is not known, nor how these signals may influence odour encoding. Here we revealed multiple and diverse non-olfactory responses in the primary olfactory (piriform) cortex (PCx), which dynamically enhance PCx odour discrimination according to behavioural demands. We performed recordings of PCx neurons from mice trained in a virtual reality task to associate odours with visual contexts to obtain a reward. We found that learning shifts PCx activity from encoding solely odours to a regime in which positional, contextual, and associative responses emerge on odour-responsive neurons that become mixed-selective. The modulation of PCx activity by these non-olfactory signals was dynamic, improving odour decoding during task engagement and in rewarded contexts. This improvement relied on the acquired mixed-selectivity, demonstrating how integrating extra-sensory inputs in sensory cortices can enhance sensory processing while encoding the behavioural relevance of stimuli.

Cholinergic modulation of dentate gyrus processing through dynamic reconfiguration of inhibitory circuits.

The dentate gyrus (DG) of the hippocampus plays a key role in memory formation, and it is known to be modulated by septal projections. By performing electrophysiology and optogenetics, we evaluated the role of cholinergic modulation in the processing of afferent inputs in the DG. We show that mature granule cells (GCs), but not adult-born immature neurons, have increased responses to afferent perforant path stimuli upon cholinergic modulation. This is due to a highly precise reconfiguration of inhibitory circuits, differentially affecting Parvalbumin and Somatostatin interneurons, resulting in a nicotinic-dependent perisomatic disinhibition of GCs. This circuit reorganization provides a mechanism by which mature GCs could escape the strong inhibition they receive, creating a window of opportunity for plasticity. Indeed, coincident activation of perforant path inputs with optogenetic release of acetylcholine produces a long-term potentiated response in GCs, essential for memory formation.

Sustained CaMKII Delta Gene Expression Is Specifically Required for Long-Lasting Memories in Mice.

Although important information is available on the molecular mechanisms of long-term memory formation, little is known about the processes underlying memory persistence in the brain. Here, we report that persistent gene expression of CaMKIIδ isoform participates in object recognition long-lasting memory storage in mice hippocampus. We found that CaMKIIδ mRNA expression was sustained up to one week after training and paralleled memory retention. Antisense DNA infusion in the hippocampus during consolidation or even after consolidation impairs 7-day- but not 1-day-long memory, supporting a role of CaMKIIδ in memory persistence. CaMKIIδ gene expression was accompanied by long-lasting nucleosome occupancy changes at its promoter. This epigenetic mechanism is described for the first time in a memory process and offers a novel mechanism for persistent gene expression in neurons. CaMKIIδ protein is mainly present in nucleus and presynaptic terminals, suggesting a role in these subcellular compartments for memory persistence. All these results point to a key function of the sustained gene expression of this overlooked CaMKII isoform in long-lasting memories.

CaMKII Isoforms in Learning and Memory: Localization and Function.

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a key protein kinase in neural plasticity and memory, as have been shown in several studies since the first evidence in long-term potentiation (LTP) 30 years ago. However, most of the studies were focused mainly in one of the four isoforms of this protein kinase, the CaMKIIα. Here we review the characteristics and the role of each of the four isoforms in learning, memory and neural plasticity, considering the well known local role of α and ß isoforms in dendritic terminals as well as recent findings about the γ isoform as calcium signals transducers from synapse to nucleus and δ isoform as a kinase required for a more persistent memory trace.

Heterozygous Che-1 KO mice show deficiencies in object recognition memory persistence.

Transcriptional regulation is a key process in the formation of long-term memories. Che-1 is a protein involved in the regulation of gene transcription that has recently been proved to bind the transcription factor NF-κB, which is known to be involved in many memory-related molecular events. This evidence prompted us to investigate the putative role of Che-1 in memory processes. For this study we newly generated a line of Che-1(+/-) heterozygous mice. Che-1 homozygous KO mouse is lethal during development, but Che-1(+/-) heterozygous mouse is normal in its general anatomical and physiological characteristics. We analyzed the behavioral characteristic and memory performance of Che-1(+/-) mice in two NF-κB dependent types of memory. We found that Che-1(+/-) mice show similar locomotor activity and thigmotactic behavior than wild type (WT) mice in an open field. In a similar way, no differences were found in anxiety-like behavior between Che-1(+/-) and WT mice in an elevated plus maze as well as in fear response in a contextual fear conditioning (CFC) and object exploration in a novel object recognition (NOR) task. No differences were found between WT and Che-1(+/-) mice performance in CFC training and when tested at 24h or 7days after training. Similar performance was found between groups in NOR task, both in training and 24h testing performance. However, we found that object recognition memory persistence at 7days was impaired in Che-1(+/-) heterozygous mice. This is the first evidence showing that Che-1 is involved in memory processes.

NF-κB transcription factor role in consolidation and reconsolidation of persistent memories.

Transcriptional regulation is an important molecular process required for long-term neural plasticity and long-term memory (LTM) formation. Thus, one main interest in molecular neuroscience in the last decades has been the identification of transcription factors that are involved in memory processes. Among them, the nuclear factor κB (NF-κB) family of transcription factors has gained interest due to a significant body of evidence that supports a key role of these proteins in synaptic plasticity and memory. In recent years, the interest was particularly reinforced because NF-κB was characterized as an important regulator of synaptogenesis. This function may be explained by its participation in synapse to nucleus communication, as well as a possible local role at the synapse. This review provides an overview of experimental work obtained in the last years, showing the essential role of this transcription factor in memory processes in different learning tasks in mammals. We focus the review on the consolidation and reconsolidation memory phases as well as on the regulation of immediate-early and late genes by epigenetic mechanisms that determine enduring forms of memories.

Nuclear factor kappa B-dependent Zif268 expression in hippocampus is required for recognition memory in mice.

Long-term memory formation requires gene expression after acquisition of new information. The first step in the regulation of gene expression is the participation of transcription factors (TFs) such as nuclear factor kappa B (NF-кB), which are present before the neuronal activity induced by training. It was proposed that the activation of these types of TFs allows a second step in gene regulation by induction of immediate-early genes (IEGs) whose protein products are, in turn, TFs. Between these IEGs, zif268 has been found to play a critical role in long-term memory formation and reprocessing after retrieval. Here we found in mice hippocampus that, on one hand, NF-кB was activated 45 min after training in a novel object recognition (NOR) task and that inhibiting NF-кB immediately after training by intrahippocampal administration of NF-кB Decoy DNA impaired NOR memory consolidation. On the other hand, Zif268 protein expression was induced 45 min after NOR training and the administration of DNA antisense to its mRNA post-training impaired recognition memory. Finally, we found that the inhibition of NF-кB by NF-кB Decoy DNA reduced significantly the training-induced Zif268 increment, indicating that NF-кB is involved in the regulation of Zif268 expression. Thus, the present results support the involvement of NF-кB activity-dependent Zif268 expression in the hippocampus during recognition memory consolidation.

Protein degradation by ubiquitin-proteasome system in formation and labilization of contextual conditioning memory.

The ubiquitin-proteasome system (UPS) of protein degradation has been evaluated in different forms of neural plasticity and memory. The role of UPS in such processes is controversial. Several results support the idea that the activation of this system in memory consolidation is necessary to overcome negative constrains for plasticity. In this case, the inhibition of the UPS during consolidation impairs memory. Similar results were reported for memory reconsolidation. However, in other cases, the inhibition of UPS had no effect on memory consolidation and reconsolidation but impedes the amnesic action of protein synthesis inhibition after retrieval. The last finding suggests a specific action of the UPS inhibitor on memory labilization. However, another interpretation is possible in terms of the synthesis/degradation balance of positive and negative elements in neural plasticity, as was found in the case of long-term potentiation. To evaluate these alternative interpretations, other reconsolidation-interfering drugs than translation inhibitors should be tested. Here we analyzed initially the UPS inhibitor effect in contextual conditioning in crabs. We found that UPS inhibition during consolidation impaired long-term memory. In contrast, UPS inhibition did not affect memory reconsolidation after contextual retrieval but, in fact, impeded memory labilization, blocking the action of drugs that does not affect directly the protein synthesis. To extend these finding to vertebrates, we performed similar experiments in contextual fear memory in mice. We found that the UPS inhibitor in hippocampus affected memory consolidation and blocked memory labilization after retrieval. These findings exclude alternative interpretations to the requirement of UPS in memory labilization and give evidence of this mechanism in both vertebrates and invertebrates.

Calcineurin phosphatase as a negative regulator of fear memory in hippocampus: control on nuclear factor-κB signaling in consolidation and reconsolidation.

Protein phosphatases are important regulators of neural plasticity and memory. Some studies support that the Ca(2+) /calmodulin-dependent phosphatase calcineurin (CaN) is, on the one hand, a negative regulator of memory formation and, on the other hand, a positive regulator of memory extinction and reversal learning. However, the signaling mechanisms by which CaN exerts its action in such processes are not well understood. Previous findings support that CaN negatively regulate the nuclear factor kappaB (NF-κB) signaling pathway during extinction. Here, we have studied the role of CaN in contextual fear memory consolidation and reconsolidation in the hippocampus. We investigated the CaN control on the NF-κB signaling pathway, a key mechanism that regulates gene expression in memory processes. We found that post-training intrahippocampal administration of the CaN inhibitor FK506 enhanced memory retention one day but not two weeks after training. Accordingly, the inhibition of CaN by FK506 increased NF-κB activity in dorsal hippocampus. The administration of the NF-κB signaling pathway inhibitor sulfasalazine (SSZ) impeded the enhancing effect of FK506. In line with our findings in consolidation, FK506 administration before memory reactivation enhanced memory reconsolidation when tested one day after re-exposure to the training context. Strikingly, memory was also enhanced two weeks after training, suggesting that reinforcement during reconsolidation is more persistent than during consolidation. The coadministration of SSZ and FK506 blocked the enhancement effect in reconsolidation, suggesting that this facilitation is also dependent on the NF-κB signaling pathway. In summary, our results support a novel mechanism by which memory formation and reprocessing can be controlled by CaN regulation on NF-κB activity.

Epigenetic mechanisms and memory strength: a comparative study.

Memory consolidation requires de novo mRNA and protein synthesis. Transcriptional activation is controlled by transcription factors, their cofactors and repressors. Cofactors and repressors regulate gene expression by interacting with basal transcription machinery, remodeling chromatin structure and/or chemically modifying histones. Acetylation is the most studied epigenetic mechanism of histones modifications related to gene expression. This process is regulated by histone acetylases (HATs) and histone deacetylases (HDACs). More than 5 years ago, we began a line of research about the role of histone acetylation during memory consolidation. Here we review our work, presenting evidence about the critical role of this epigenetic mechanism during consolidation of context-signal memory in the crab Neohelice granulata, as well as during consolidation of novel object recognition memory in the mouse Mus musculus. Our evidence demonstrates that histone acetylation is a key mechanism in memory consolidation, functioning as a distinctive molecular feature of strong memories. Furthermore, we found that the strength of a memory can be characterized by its persistence or its resistance to extinction. Besides, we found that the role of this epigenetic mechanism regulating gene expression only in the formation of strongest memories is evolutionarily conserved.

Nuclear factor κB-dependent histone acetylation is specifically involved in persistent forms of memory.

Memory consolidation requires gene expression regulation by transcription factors, which eventually may induce chromatin modifications as histone acetylation. This mechanism is regulated by histone acetylases and deacetylases. It is not yet clear whether memory consolidation always recruits histone acetylation or it is only engaged in more persistent memories. To address this question, we used different strength of training for novel object recognition task in mice. Only strong training induced a long-lasting memory and an increase in hippocampal histone H3 acetylation. Histone acetylase inhibition in the hippocampus during consolidation impaired memory persistence, whereas histone deacetylase inhibition caused weak memory to persist. Nuclear factor κB (NF-κB) transcription factor inhibition impaired memory persistence and, concomitantly, reduced the general level of H3 acetylation. Accordingly, we found an important increase in H3 acetylation at a specific NF-κB-regulated promoter region of the Camk2d gene, which was reversed by NF-kB inhibition. These results show for the first time that histone acetylation is a specific molecular signature of enduring memories.

Characterization of the beta amyloid precursor protein-like gene in the central nervous system of the crab Chasmagnathus. Expression during memory consolidation.

BACKGROUND: Human ß-amyloid, the main component in the neuritic plaques found in patients with Alzheimer's disease, is generated by cleavage of the ß-amyloid precursor protein. Beyond the role in pathology, members of this protein family are synaptic proteins and have been associated with synaptogenesis, neuronal plasticity and memory, both in vertebrates and in invertebrates. Consolidation is necessary to convert a short-term labile memory to a long-term and stable form. During consolidation, gene expression and de novo protein synthesis are regulated in order to produce key proteins for the maintenance of plastic changes produced during the acquisition of new information. RESULTS: Here we partially cloned and sequenced the beta-amyloid precursor protein like gene homologue in the crab Chasmagnathus (cappl), showing a 37% of identity with the fruit fly Drosophila melanogaster homologue and 23% with Homo sapiens but with much higher degree of sequence similarity in certain regions. We observed a wide distribution of cappl mRNA in the nervous system as well as in muscle and gills. The protein localized in all tissues analyzed with the exception of muscle. Immunofluorescence revealed localization of cAPPL in associative and sensory brain areas. We studied gene and protein expression during long-term memory consolidation using a well characterized memory model: the context-signal associative memory in this crab species. mRNA levels varied at different time points during long-term memory consolidation and correlated with cAPPL protein levels CONCLUSIONS: cAPPL mRNA and protein is widely distributed in the central nervous system of the crab and the time course of expression suggests a role of cAPPL during long-term memory formation.

Histone acetylation is recruited in consolidation as a molecular feature of stronger memories.

Gene expression is a key process for memory consolidation. Recently, the participation of epigenetic mechanisms like histone acetylation was evidenced in long-term memories. However, until now the training strength required and the persistence of the chromatin acetylation recruited are not well characterized. Here we studied whether histone acetylation is involved in consolidation in invertebrates, whether it depends on the training strength, and whether it is a permanent or transient mechanism. We used a well-characterized memory model in invertebrates, the context-signal memory in crabs. Our results show no changes in histone 3 (H3) acetylation during consolidation of a standard training protocol. However, strong training induced a significant increase in H3 acetylation 1-h post-training, returning to basal levels afterward. Accordingly, the administration of histone deacetylase inhibitors sodium butyrate (NaB) and trichostatin A allowed a weak training to induce long-term memory. NaB enhanced memory in two phases during consolidation. These findings support that H3 acetylation (1) is involved in consolidation, (2) occurs only after strong training, (3) is a transient process, and (4) memory is enhanced in two phases. The coincidence of these phases with other mechanisms of gene expression is discussed.

Lessons from a crab: molecular mechanisms in different memory phases of Chasmagnathus.

Consolidation of long-term memory requires the activation of several transduction pathways that lead to post-translational modifications of synaptic proteins and to regulation of gene expression, both of which promote stabilization of specific changes in the activated circuits. In search of the molecular mechanisms involved in such processes, we used the context-signal associative learning paradigm of the crab Chasmagnathus. In this model, we studied the role of some molecular mechanisms, namely cAMP-dependent protein kinase (PKA), extracellular-signal-regulated kinase (ERK), the nuclear factor kappa B (NF-kappaB) transcription factor, and the role of synaptic proteins such as amyloid beta precursor protein, with the object of describing key mechanisms involved in memory processing. In this article we review the most salient results obtained over a decade of research in this memory model.