Arc/Arg3.1 mRNA global expression patterns elicited by memory recall in cerebral cortex differ for remote versus recent spatial memories

Synaptic plasticity during systems memory consolidation

After learning, memory is initially encoded in the hippocampus but subsequently stabilized in other brain regions such as the cortex for long-lasting storage. This process is known as systems memory consolidation, and its cellular mechanism has long been a fundamental question. Synaptic plasticity is the major cellular mechanism underlying learning and memory, and is therefore considered a key function in the process of systems memory consolidation. Therefore, many studies have aimed to establish a causal link between synaptic plasticity in the brain and memory-associated behaviors. In this review, I discuss the various lines of research showing the function of synaptic plasticity, mainly in the hippocampus and cortex during memory consolidation.

Functional neuroanatomy of allocentric remote spatial memory in rodents

Successful spatial cognition involves learning, consolidation, storage, and later retrieval of a spatial memory trace. The functional contributions of specific brain areas and their interactions during retrieval of past spatial events are unclear. This systematic review collects studies about allocentric remote spatial retrieval assessed at least two weeks post-acquisition in rodents. Results including non-invasive interventions, brain lesion and inactivation experiments, pharmacological treatments, chemical agent administration, and genetic manipulations revealed that there is a normal forgetting when time-periods are close to or exceed one month. Moreover, changes in the morphology and functionality of neocortical areas, hippocampus, and other subcortical structures, such as the thalamus, have been extensively observed as a result of spatial memory retrieval. In conclusion, apart from an increasingly neocortical recruitment in remote spatial retrieval, the hippocampus seems to participate in the retrieval of fine spatial details. These results help to better understand the timing of memory maintenance and normal forgetting, outlining the underlying brain areas implicated.

Retrosplenial cortex in spatial memory: focus on immediate early genes mapping

The ability to form, retrieve and update autobiographical memories is one of the most fascinating features of human behavior. Spatial memory, the ability to remember the layout of the external environment and to navigate within its boundaries, is closely related to the autobiographical memory domain. It is served by an overlapping brain circuit, centered around the hippocampus (HPC) where the cognitive map index is stored. Apart from the hippocampus, several cortical structures participate in this process. Their relative contribution is a subject of intense research in both humans and animal models. One of the most widely studied regions is the retrosplenial cortex (RSC), an area in the parietal lobe densely interconnected with the hippocampal formation. Several methodological approaches have been established over decades in order to investigate the cortical aspects of memory. One of the most successful techniques is based on the analysis of brain expression patterns of the immediate early genes (IEGs). The common feature of this diverse group of genes is fast upregulation of their mRNA translation upon physiologically relevant stimulus. In the central nervous system they are rapidly triggered by neuronal activity and plasticity during learning. There is a widely accepted consensus that their expression level corresponds to the engagement of individual neurons in the formation of memory trace. Imaging of the IEGs might therefore provide a picture of an emerging memory engram. In this review we present the overview of IEG mapping studies of retrosplenial cortex in rodent models. We begin with classical techniques, immunohistochemical detection of protein and fluorescent in situ hybridization of mRNA. We then proceed to advanced methods where fluorescent genetically encoded IEG reporters are chronically followed in vivo during memory formation. We end with a combination of genetic IEG labelling and optogenetic approach, where the activity of the entire engram is manipulated. We finally present a hypothesis that attempts to unify our current state of knowledge about the function of RSC.

mRNA Trafficking in the Nervous System: A Key Mechanism of the Involvement of Activity-Regulated Cytoskeleton-Associated Protein (Arc) in Synaptic Plasticity

Synaptic activity mediates information storage and memory consolidation in the brain and requires a fast de novo synthesis of mRNAs in the nucleus and proteins in synapses. Intracellular localization of a protein can be achieved by mRNA trafficking and localized translation. Activity-regulated cytoskeleton-associated protein (Arc) is a master regulator of synaptic plasticity and plays an important role in controlling large signaling networks implicated in learning, memory consolidation, and behavior. Transcription of the Arc gene may be induced by a short behavioral event, resulting in synaptic activation. Arc mRNA is exported into the cytoplasm and can be trafficked into the dendrite of an activated synapse where it is docked and translated. The structure of Arc is similar to the viral GAG (group-specific antigen) protein, and phylogenic analysis suggests that Arc may originate from the family of Ty3/Gypsy retrotransposons. Therefore, Arc might evolve through “domestication” of retroviruses. Arc can form a capsid-like structure that encapsulates a retrovirus-like sentence in the 3′-UTR (untranslated region) of Arc mRNA. Such complex can be loaded into extracellular vesicles and transported to other neurons or muscle cells carrying not only genetic information but also regulatory signals within neuronal networks. Therefore, Arc mRNA inter- and intramolecular trafficking is essential for the modulation of synaptic activity required for memory consolidation and cognitive functions. Recent studies with single-molecule imaging in live neurons confirmed and extended the role of Arc mRNA trafficking in synaptic plasticity.

Recovering Spatial Information through Reactivation: Brain Oxidative Metabolism Involvement in Males and Females

Memory involves a complex network system of interconnected brain areas in which labile trace memories are transformed into enduring ones and reorganized in a time-dependant manner. Although it has been observed that remote memories are less prone to destabilizing, they can become fragile and lead to behavioural decline. We explored the behavioural outcomes of male and female rats in response to the reactivation of a previously acquired allocentric spatial reference memory, under conditions in which animals have shown a retrieval decay. In addition, we assessed their brain metabolic activity through cytochrome c oxidase (CCO) histochemistry. Our results show that a spatial memory amnesia-like behaviour with a time interval of 45 days can be recovered after re-exposure to the environmental configuration with the reinforced contingencies. Moreover, we observed that, following reactivation, male rats reveal a decrease in metabolic activity in septal nuclei and thalamic structures, whereas female rats add a metabolic reduction in the hippocampus, amygdala, mPFC, and retrosplenial, parietal and rhinal cortices, suggesting that they efficiently employ these brain areas when reactivation a memory that has suffered a decay with time. Finally, although male and female rats perform the behavioural task equally, we found sex differences at the brain metabolism level, revealing the differential contribution of brain limbic system energy demands by sex, even when their performance is similar. In conclusion, our work provides behavioural and brain data about remote spatial retrieval and memory reactivation processes.

Immediate Early Genes, Memory and Psychiatric Disorders: Focus on c-Fos, Egr1 and Arc

Many psychiatric disorders, despite their specific characteristics, share deficits in the cognitive domain including executive functions, emotional control and memory. However, memory deficits have been in many cases undervalued compared with other characteristics. The expression of Immediate Early Genes (IEGs) such as, c-fos, Egr1 and arc are selectively and promptly upregulated in learning and memory among neuronal subpopulations in regions associated with these processes. Changes in expression in these genes have been observed in recognition, working and fear related memories across the brain. Despite the enormous amount of data supporting changes in their expression during learning and memory and the importance of those cognitive processes in psychiatric conditions, there are very few studies analyzing the direct implication of the IEGs in mental illnesses. In this review, we discuss the role of some of the most relevant IEGs in relation with memory processes affected in psychiatric conditions.

Repeated anodal transcranial direct current stimulation induces neural plasticity-associated gene expression in the rat cortex and hippocampus

BACKGROUND

Anodal transcranial direct current stimulation (A-tDCS) induces a long-lasting increase in cortical excitability that can increase gene transcription in the brain.

OBJECTIVE

The purpose of this study was to evaluate the expression of genes related to activity-dependent neuronal plasticity in the sensorimotor cortex and hippocampus of young Sprague-Dawley rats following A-tDCS.

METHODS

We applied A-tDCS over the right sensorimotor cortex epicranially with a circular electrode (3 mm diameter) at 250 μA for 20 min per day for 7 consecutive days. Levels of mRNA for brain-derived neurotrophic factor (BDNF), cAMP response element-binding protein (CREB), synapsin I, Ca2+/calmodulin-dependent protein kinase II (CaMKII), activity-regulated cytoskeleton-associated protein (Arc), and c-Fos were analyzed using SYBR Green quantitative real-time polymerase chain reaction (PCR).

RESULTS

We found that 7 days of unilateral A-tDCS resulted in significant increases in transcription of all plasticity-related genes tested in the ipsilateral cortex. Daily A-tDCS also resulted in a significant increase in c-Fos mRNA in the ipsilateral hippocampus.

CONCLUSION

These results indicate that altered expression of plasticity-associated genes in the cortex and hippocampus is a molecular substrate of A-tDCS-induced neural plasticity.

Using Laser Capture Microdissection to Isolate Cortical Laminae in Nonhuman Primate Brain

Laser capture microdissection (LCM) is a technique that allows procurement of an enriched cell population from a heterogeneous tissue sample under direct microscopic visualization. Fundamentally, laser capture microdissection consists of three main steps: (1) visualizing the desired cell population by microscopy, (2) melting a thermolabile polymer onto the desired cell populations using infrared laser energy to form a polymer-cell composite (capture method) or photovolatizing a region of tissue using ultraviolet laser energy (cutting method), and (3) removing the desired cell population from the heterogeneous tissue. In this chapter, we discuss the infrared capture method only. LCM technology is compatible with a wide range of downstream applications such as mass spectrometry, DNA genotyping and RNA transcript profiling, cDNA library generation, proteomics discovery, and signal pathway mapping. This chapter profiles the ArcturusXT^™ laser capture microdissection instrument, using isolation of specific cortical lamina from nonhuman primate brain regions, and sample preparation methods for downstream proteomic applications.

Retrosplenial Cortex and Long-Term Memory: Molecules to Behavior

The retrosplenial cortex (RSC) is reciprocally connected with the hippocampus and various parahippocampal cortical regions, suggesting that RSC is well-positioned to contribute to hippocampal-dependent memory. Consistent with this, substantial behavioral evidence indicates that RSC is essential for consolidating and/or retrieving contextual and spatial memories. In addition, there is growing evidence that RSC neurons undergo activity-dependent plastic changes during memory formation and retrieval. In this paper we review both the behavioral and cellular/molecular data and posit that the RSC has a particularly important role in the storage and retrieval of spatial and contextual memories perhaps due its involvement in binding together multiple cues in the environment. We identify remaining questions and avenues for future research that take advantage of emerging methods to selectively manipulate RSC neurons both spatially and temporally and to image the RSC in awake, behaving animals.

NAc Shell Arc/Arg3.1 Protein Mediates Reconsolidation of Morphine CPP by Increased GluR1 Cell Surface Expression: Activation of ERK-Coupled CREB is Required

BACKGROUND

Relapse into drug abuse evoked by reexposure to the drug-associated context has been a primary problem in the treatment of drug addiction. Disrupting the reconsolidation of drug-related context memory would therefore limit the relapse susceptibility.

METHODS

Morphine conditioned place preference (CPP) was used to assess activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) and correlative molecule expression in the Nucleus accumbens (NAc) shell during the reconsolidation of morphine CPP. U0126 and Arc/Arg3.1 antisense oligodeoxynucleotide were adapted to evaluate the role and the underlying mechanism of Arc/Arg3.1 during the reconsolidation.

RESULTS

The retrieval of morphine CPP in rats specifically increased the Arc/Arg3.1 protein level in the NAc shell, accompanied simultaneously by increases in the phosphorylation of extracellular signal-regulated kinase1/2 (pERK1/2), the phosphorylation of Cyclic Adenosine monophosphate (cAMP) response element-binding (pCREB), and the up-regulation of the membrane α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors GluR1 subunit level. Intra-NAc shell infusion U0126, an inhibitor of the Mitogen-activated protein kinase kinase (MEK), prevented the retrieval-induced up-regulation of pERK1/2, pCREB, Arc/Arg3.1, and membrane GluR1 immediately after retrieval of morphine CPP. The effect of disrupting the reconsolidation of morphine CPP by U0126 could last for at least 14 days, and could not be evoked by a priming injection of morphine. Furthermore, the specific knockdown of Arc/Arg3.1 in the NAc shell decreased the membrane GluR1 level, and impaired both the reconsolidation and the reinstatement of morphine CPP.

CONCLUSIONS

Arc/Arg3.1 in the NAc shell mediates the reconsolidation of morphine-associated context memory via up-regulating the level of membrane of GluR1, for which the local activation of the ERK-CREB signal pathway, as an upstream mechanism of Arc/Arg3.1, is required.

Cues, context, and long-term memory: the role of the retrosplenial cortex in spatial cognition

Spatial navigation requires memory representations of landmarks and other navigation cues. The retrosplenial cortex (RSC) is anatomically positioned between limbic areas important for memory formation, such as the hippocampus (HPC) and the anterior thalamus, and cortical regions along the dorsal stream known to contribute importantly to long-term spatial representation, such as the posterior parietal cortex. Damage to the RSC severely impairs allocentric representations of the environment, including the ability to derive navigational information from landmarks. The specific deficits seen in tests of human and rodent navigation suggest that the RSC supports allocentric representation by processing the stable features of the environment and the spatial relationships among them. In addition to spatial cognition, the RSC plays a key role in contextual and episodic memory. The RSC also contributes importantly to the acquisition and consolidation of long-term spatial and contextual memory through its interactions with the HPC. Within this framework, the RSC plays a dual role as part of the feedforward network providing sensory and mnemonic input to the HPC and as a target of the hippocampal-dependent systems consolidation of long-term memory.

Cortical gene transcription response patterns to water maze training in aged mice

Background

The hippocampus mediates the acquisition of spatial memory, but the memory trace is eventually transferred to the cortex. We have investigated transcriptional activation of pathways related to cognitive function in the cortex of the aged mouse by analyzing gene expression following water maze training.

Results

We identified genes that were differentially responsive in aged mice with accurate spatial performance during probe trials or repeated swimming sessions, relative to home cage conditions. Effective learners exhibited significantly greater activation of several pathways, such as the mitogen-activated protein kinase and insulin receptor signaling pathways, relative to swimmers. The genes encoding activity-related cytoskeletal protein (Arc) and brain-derived neurotrophic factor (BDNF) were upregulated in proficient learners, relative to swimmers and home cage controls, while the gene encoding Rho GTPase activating protein 32 (GRIT) was downregulated. We explored the regulation of Arc, BDNF, and GRIT expression in greater morphological detail using in situ hybridization. Recall during probe trials enhanced Arc expression across multiple cortical regions involved in the cognitive component of water maze learning, while BDNF expression was more homogeneously upregulated across cortical regions involved in the associational and sensorimotor aspects of water maze training. In contrast, levels of GRIT expression were uniformly reduced across all cortical regions examined.

Conclusions

These results suggest that cortical gene transcription is responsive to learning in aged mice that exhibit behavioral proficiency, and support a distributed hypothesis of memory storage across multiple cortical compartments.

Gene regulation in the rat prefrontal cortex after learning with or without cholinergic insult

The prefrontal cortex is essential for a wide variety of higher functions, including attention and memory. Cholinergic neurons are thought to be of prime importance in the modulation of these processes. Degeneration of forebrain cholinergic neurons has been linked to several neurological disorders. The present study was designed to identify genes and networks in rat prefrontal cortex that are associated with learning and cholinergic-loss-memory deficit. Affymetrix microarray technology was used to screen gene expression changes in rats submitted or not to 192 IgG-saporin immunolesion of cholinergic basal forebrain and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes, which were organized in several clusters of highly correlated genes and would be involved in biological processes such as intracellular signaling process, transcription regulation, and filament organization and axon guidance. Memory loss following cortical cholinergic deafferentation was associated with significant expression of genes belonging to only one clearly delineated cluster and would be involved in biological processes related to cytoskeleton organization and proliferation, and glial and vascular remodeling, i.e., in processes associated with brain repair after injury.

Arc/Arg3.1 mRNA expression reveals a subcellular trace of prior sound exposure in adult primary auditory cortex

Acquiring the behavioral significance of sound has repeatedly been shown to correlate with long term changes in response properties of neurons in the adult primary auditory cortex. However, the molecular and cellular basis for such changes is still poorly understood. To address this, we have begun examining the auditory cortical expression of an activity-dependent effector immediate early gene (IEG) with documented roles in synaptic plasticity and memory consolidation in the hippocampus: Arc/Arg3.1. For initial characterization, we applied a repeated 10 min (24 h separation) sound exposure paradigm to determine the strength and consistency of sound-evoked Arc/Arg3.1 mRNA expression in the absence of explicit behavioral contingencies for the sound. We used 3D surface reconstruction methods in conjunction with fluorescent in situ hybridization (FISH) to assess the layer-specific subcellular compartmental expression of Arc/Arg3.1 mRNA. We unexpectedly found that both the intranuclear and cytoplasmic patterns of expression depended on the prior history of sound stimulation. Specifically, the percentage of neurons with expression only in the cytoplasm increased for repeated versus singular sound exposure, while intranuclear expression decreased. In contrast, the total cellular expression did not differ, consistent with prior IEG studies of primary auditory cortex. Our results were specific for cortical layers 3-6, as there was virtually no sound driven Arc/Arg3.1 mRNA in layers 1-2 immediately after stimulation. Our results are consistent with the kinetics and/or detectability of cortical subcellular Arc/Arg3.1 mRNA expression being altered by the initial exposure to the sound, suggesting exposure-induced modifications in the cytoplasmic Arc/Arg3.1 mRNA pool.