Activity-regulated cytoskeletal-associated protein is localized to recently activated excitatory synapses

The activation of adenosine monophosphate-activated protein kinase in rat hippocampus contributes to the rapid antidepressant effect of ketamine

Recent studies have shown a rapid, robust, and lasting antidepressant effect of ketamine that makes ketamine a promising antidepressant drug. However, the mechanisms underlying this rapid antidepressant effect remain incompletely understood. The goal of the present study was to determine whether adenosine monophosphate-activated protein kinase (AMPK) was involved in ketamine's rapid antidepressant effect during the forced swimming test (FST). In the first stage of experiment, a lower level of phosphorylated form of AMPK (p-AMPK) in the hippocampus and a longer immobility time were observed in the depressed rats during FST; whereas ketamine reversed these changes at 30min after the administration. In the second stage of experiment, we observed that, ketamine up-regulated the levels of p-AMPK and brain-derived neurotrophic factor (BDNF) in the hippocampus of the depressed rats. Moreover, AMPK agonist strengthened the antidepressant effect of ketamine with an up-regulation of BDNF, while AMPK antagonist attenuated the antidepressant effect of ketamine with a down-regulation of BDNF. In conclusion, our results suggest that the activation of AMPK in rat hippocampus is involved in the procedure of ketamine exerting rapid antidepressant effect through the up-regulation of BDNF.

Emotional modulation of the synapse

Acute stress and emotional arousal can enhance the consolidation of long-term memories in a manner that is dependent on β -adrenoceptor activation in the basolateral complex of the amygdala (BLA). The BLA interacts with multiple memory systems in the brain to modulate a variety of classes of memory. However, the synaptic mechanisms of this interaction remain unresolved. This review describes the evidence of modulation of memory and synaptic plasticity produced by emotional arousal,stress hormones, and pharmacological or electrophysiological stimulation of the amygdala. The amygdala modulation of local translation and/or degradation of the synaptic plasticity-related proteins, activity-regulated cytoskeletal-associated protein and calcium/calmodulin dependent protein kinase II α , is offered as a potential mechanism for the rapid memory consolidation that is associated with emotionally arousing events. This model shares features with synaptic tagging and the emotional tagging hypotheses.

Arc in the nucleus regulates PML-dependent GluA1 transcription and homeostatic plasticity

The activity-regulated cytoskeletal protein Arc (also known as Arg3.1) is required for long-term memory formation and synaptic plasticity. Arc expression is robustly induced by activity, and Arc protein localizes to both active synapses and the nucleus. Whereas its synaptic function has been examined, it is not clear why or how Arc is localized to the nucleus. We found that murine Arc nuclear expression is regulated by synaptic activity in vivo and in vitro. We identified distinct regions of Arc that control its localization, including a nuclear localization signal, a nuclear retention domain and a nuclear export signal. Arc localization to the nucleus promotes an activity-induced increase in the expression of promyelocytic leukemia nuclear bodies, which decreases GluA1 (also called Gria1) transcription and synaptic strength. We further show that Arc nuclear localization regulates homeostatic plasticity. Thus, Arc mediates the homeostatic response to increased activity by translocating to the nucleus, increasing promyelocytic leukemia protein expression and decreasing GluA1 transcription, ultimately downscaling synaptic strength.

Human synaptic plasticity gene expression profile and dendritic spine density changes in HIV-infected human CNS cells: role in HIV-associated neurocognitive disorders (HAND)

HIV-associated neurocognitive disorders (HAND) is characterized by development of cognitive, behavioral and motor abnormalities, and occur in approximately 50% of HIV infected individuals. Our current understanding of HAND emanates mainly from HIV-1 subtype B (clade B), which is prevalent in USA and Western countries. However very little information is available on neuropathogenesis of HIV-1 subtype C (clade C) that exists in Sub-Saharan Africa and Asia. Therefore, studies to identify specific neuropathogenic mechanisms associated with HAND are worth pursuing to dissect the mechanisms underlying this modulation and to prevent HAND particularly in clade B infection. In this study, we have investigated 84 key human synaptic plasticity genes differential expression profile in clade B and clade C infected primary human astrocytes by using RT(2) Profile PCR Array human Synaptic Plasticity kit. Among these, 31 and 21 synaptic genes were significantly (≥3 fold) down-regulated and 5 genes were significantly (≥3 fold) up-regulated in clade B and clade C infected cells, respectively compared to the uninfected control astrocytes. In flow-cytometry analysis, down-regulation of postsynaptic density and dendrite spine morphology regulatory proteins (ARC, NMDAR1 and GRM1) was confirmed in both clade B and C infected primary human astrocytes and SK-N-MC neuroblastoma cells. Further, spine density and dendrite morphology changes by confocal microscopic analysis indicates significantly decreased spine density, loss of spines and decreased dendrite diameter, total dendrite and spine area in clade B infected SK-N-MC neuroblastoma cells compared to uninfected and clade C infected cells. We have also observed that, in clade B infected astrocytes, induction of apoptosis was significantly higher than in the clade C infected astrocytes. In conclusion, this study suggests that down-regulation of synaptic plasticity genes, decreased dendritic spine density and induction of apoptosis in astrocytes may contribute to the severe neuropathogenesis in clade B infection.

A new player in the "synaptopathy" of Alzheimer's disease - arc/arg 3.1

Alzheimer’s disease (AD) is increasingly referred to as a “synaptopathy.” This moniker reflects the loss or damage of synapses that occurs as the disease progresses, which in turn produces functional degeneration of specific neuronal circuits and consequent aberrant activity in neural networks. Accumulating evidence supports the functional importance of the early-expression activity-regulated cytoskeletal (Arc) gene in regulating memory consolidation. Interestingly, AD patients express anomalously high levels of Arc protein. Arc physically associates with presenilin1, a pivotal protease for the generation of Amyloid β (Aβ) peptides. Arc expression itself is disrupted in the vicinity of Aβ oligomers and plaques. Such alterations result in the interruption of neuronal network integration in vivo. It is not clear what the impacts of these alterations are on the functional neurophysiology of transgenic mouse models of AD-associated amyloidopathy. Our group and others have described alterations to neuronal excitability and thus intrinsic firing within these transgenic mice models. This brief review will emphasize the rising role of Arc and its involvement in neurophysiological alterations of current AD models.

KIF1Bβ transports dendritically localized mRNPs in neurons and is recruited to synapses in an activity-dependent manner

KIF1Bβ is a kinesin-like, microtubule-based molecular motor protein involved in anterograde axonal vesicular transport in vertebrate and invertebrate neurons. Certain KIF1Bβ isoforms have been implicated in different forms of human neurodegenerative disease, with characterization of their functional integration and regulation in the context of synaptic signaling still ongoing. Here, we characterize human KIF1Bβ (isoform NM015074), whose expression we show to be developmentally regulated and elevated in cortical areas of the CNS (including the motor cortex), in the hippocampus, and in spinal motor neurons. KIF1Bβ localizes to the cell body, axon, and dendrites, overlapping with synaptic-vesicle and postsynaptic-density structures. Correspondingly, in purified cortical synaptoneurosomes, KIF1Bβ is enriched in both pre- and postsynaptic structures, forming detergent-resistant complexes. Interestingly, KIF1Bβ forms RNA-protein complexes, containing the dendritically localized Arc and Calmodulin mRNAs, proteins previously shown to be part of RNA transport granules such as Purα, FMRP and FXR2P, and motor protein KIF3A, as well as Calmodulin. The interaction between KIF1Bβ and Calmodulin is Ca(+2)-dependent and takes place through a domain mapped at the carboxy-terminal tail of the motor. Live imaging of cortical neurons reveals active movement by KIF1Bβ at dendritic processes, suggesting that it mediates the transport of dendritically localized mRNAs. Finally, we show that synaptic recruitment of KIF1Bβ is activity-dependent and increased by stimulation of metabotropic or ionotropic glutamate receptors. The activity-dependent synaptic recruitment of KIF1Bβ, its interaction with Ca(2+) sensor Calmodulin, and its new role as a dendritic motor of ribonucleoprotein complexes provide a novel basis for understanding the concerted co-ordination of motor protein mobilization and synaptic signaling pathways.

Mathematical modeling of sustainable synaptogenesis by repetitive stimuli suggests signaling mechanisms in vivo

The mechanisms of long-term synaptic maintenance are a key component to understanding the mechanism of long-term memory. From biological experiments, a hypothesis arose that repetitive stimuli with appropriate intervals are essential to maintain new synapses for periods of longer than a few days. We successfully reproduce the time-course of relative numbers of synapses with our mathematical model in the same conditions as biological experiments, which used Adenosine-3', 5'-cyclic monophosphorothioate, Sp-isomer (Sp-cAMPS) as external stimuli. We also reproduce synaptic maintenance responsiveness to intervals of Sp-cAMPS treatment accompanied by PKA activation. The model suggests a possible mechanism of sustainable synaptogenesis which consists of two steps. First, the signal transduction from an external stimulus triggers the synthesis of a new signaling protein. Second, the new signaling protein is required for the next signal transduction with the same stimuli. As a result, the network component is modified from the first network, and a different signal is transferred which triggers the synthesis of another new signaling molecule. We refer to this hypothetical mechanism as network succession. We build our model on the basis of two hypotheses: (1) a multi-step network succession induces downregulation of SSH and COFILIN gene expression, which triggers the production of stable F-actin; (2) the formation of a complex of stable F-actin with Drebrin at PSD is the critical mechanism to achieve long-term synaptic maintenance. Our simulation shows that a three-step network succession is sufficient to reproduce sustainable synapses for a period longer than 14 days. When we change the network structure to a single step network, the model fails to follow the exact condition of repetitive signals to reproduce a sufficient number of synapses. Another advantage of the three-step network succession is that this system indicates a greater tolerance of parameter changes than the single step network.

Retrieval of contextual memories increases activity-regulated cytoskeleton-associated protein in the amygdala and hippocampus

Activity-regulated cytoskeleton-associated protein (Arc) integrates information from multiple intracellular signaling cascades and, in turn, regulates cytoskeletal proteins involved in structural synaptic modifications. The purposes of the present study were: (1) to determine if the retrieval of contextual memories would induce Arc in hippocampal and amygdalar neurons; (2) use unbiased stereology at the ultrastructural level to quantify synapses contacting Arc-labeled (Arc+) and unlabeled (Arc-) postsynaptic structures in brain regions in which the amount of Arc integrated density (ID) correlated strongly with the degree of amphetamine conditioned place preference (AMPH CPP). The retrieval of contextual memories increased the Arc ID in the dentate gyrus, cornu ammonis (CA)1, and CA3 fields of the hippocampus and the basolateral, lateral, and central nuclei of the amygdala but not the primary auditory cortex, a control region. Stereological quantification of Arc+ and Arc- synapses in the basolateral nucleus of the amygdala (BLA) was undertaken because the strongest relationship between the amount of Arc ID and AMPH CPP was observed in the BLA. The retrieval of contextual memories increased the number and density of asymmetric (presumed excitatory) synapses contacting Arc+ spines and dendrites of BLA neurons, symmetric (presumed inhibitory or modulatory) synapses contacting Arc+ dendrites of BLA neurons, and multisynaptic boutons contacting Arc+ postsynaptic structures. Thus, the retrieval of contextual memories increases Arc in the amygdala and hippocampus, an effect that could be important for approach behavior to a drug-associated context.

Inverse synaptic tagging of inactive synapses via dynamic interaction of Arc/Arg3.1 with CaMKIIβ

The Arc/Arg3.1 gene product is rapidly upregulated by strong synaptic activity and critically contributes to weakening synapses by promoting AMPA-R endocytosis. However, how activity-induced Arc is redistributed and determines the synapses to be weakened remains unclear. Here, we show targeting of Arc to inactive synapses via a high-affinity interaction with CaMKIIβ that is not bound to calmodulin. Synaptic Arc accumulates in inactive synapses that previously experienced strong activation and correlates with removal of surface GluA1 from individual synapses. A lack of CaMKIIβ either in vitro or in vivo resulted in loss of Arc upregulation in the silenced synapses. The discovery of Arc's role in "inverse" synaptic tagging that is specific for weaker synapses and prevents undesired enhancement of weak synapses in potentiated neurons reconciles essential roles of Arc both for the late phase of long-term plasticity and for reduction of surface AMPA-Rs in stimulated neurons.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Memory-enhancing intra-basolateral amygdala infusions of clenbuterol increase Arc and CaMKIIα protein expression in the rostral anterior cingulate cortex

Activation of β-adrenoceptors in the basolateral complex of the amygdala (BLA) modulates memory through interactions with multiple memory systems. The cellular mechanisms for this interaction remain unresolved. Memory-modulating BLA manipulations influence expression of the protein product of the immediate early gene activity-regulated cytoskeletal-associated protein (Arc) in the dorsal hippocampus, and hippocampal expression of Arc protein is critically involved in memory consolidation and long-term potentiation. The present studies examined whether this influence of the BLA is specific to the hippocampus and to Arc protein. Like the hippocampus, the rostral portion of the anterior cingulate cortex (rACC) is involved in the consolidation of inhibitory avoidance (IA) memory, and IA training increases Arc protein in the rACC. Because the BLA interacts with the rACC in the consolidation of IA memory, the rACC is a potential candidate for further studies of BLA modulation of synaptic plasticity. The alpha isoform of the Calcium/Calmodulin-dependent protein kinase II (CaMKIIα) and the immediate early gene c-Fos are involved in long-term potentiation and memory. Both Arc and CaMKIIα proteins can be translated in isolated synapses, where the mRNA is localized, but c-Fos protein remains in the soma. To examine the influence of memory-modulating manipulations of the BLA on expression of these memory and plasticity-associated proteins in the rACC, male Sprague-Dawley rats were trained on an IA task and given intra-BLA infusions of either clenbuterol or lidocaine immediately after training. Findings suggest that noradrenergic stimulation of the BLA may modulate memory consolidation through effects on both synaptic proteins Arc and CaMKIIα, but not the somatic protein c-Fos. Furthermore, protein changes observed in the rACC following BLA manipulations suggest that the influence of the BLA on synaptic proteins is not limited to those in the dorsal hippocampus.

RNA binding proteins accumulate at the postsynaptic density with synaptic activity

Neuronal activity elicits changes in synaptic composition that play an important role in experience-dependent plasticity (Choquet and Triller, 2003; Lisman and Raghavachari, 2006; Bourne and Harris, 2008; Holtmaat and Svoboda, 2009). We used a modified version of stable isotope labeling by amino acids in cell culture to identify activity-dependent modifications in the composition of postsynaptic densities (PSDs) isolated from rat primary neuronal cultures. We found that synaptic activity altered ∼2% of the PSD proteome, which included an increase in diverse RNA binding proteins (RNABPs). Indeed, 12 of the 37 identified proteins whose levels changed with synaptic activity were RNABPs and included the heterogeneous nuclear ribonucleoproteins (hnRNPs) G, A2/B1, M, and D. Knockdown of hnRNPs M and G using shRNAs resulted in altered numbers of dendritic spines, suggesting a crucial role for these proteins in spine density. Synaptic activity also resulted in a concomitant increase in dendritic and synaptic poly(A) mRNA. However, this increase was not affected by knockdown of hnRNPs M or G. Our results suggest that hnRNP proteins regulate dendritic spine density and may play a role in synaptodendritic mRNA metabolism.

Interacting brain systems modulate memory consolidation

Emotional arousal influences the consolidation of long-term memory. This review discusses experimental approaches and relevant findings that provide the foundation for current understanding of coordinated interactions between arousal activated peripheral hormones and the brain processes that modulate memory formation. Rewarding or aversive experiences release the stress hormones epinephrine (adrenalin) and glucocorticoids from the adrenal glands into the bloodstream. The effect of these hormones on memory consolidation depends upon binding of norepinephrine to beta-adrenergic receptors in the basolateral complex of the amygdala (BLA). Much evidence indicates that the stress hormones influence release of norepinephrine in the BLA through peripheral actions on the vagus nerve which stimulates, through polysynaptic connections, cells of the locus coeruleus to release norepinephrine. The BLA influences memory storage by actions on synapses, distributed throughout the brain, that are engaged in sensory and cognitive processing at the time of amygdala activation. The implications of the activation of these stress-activated memory processes are discussed in relation to stress-related memory disorders.

Arc in synaptic plasticity: from gene to behavior

The activity-regulated cytoskeletal (Arc) gene encodes a protein that is critical for memory consolidation. Arc is one of the most tightly regulated molecules known: neuronal activity controls Arc mRNA induction, trafficking and accumulation, and Arc protein production, localization and stability. Arc regulates synaptic strength through multiple mechanisms and is involved in essentially every known form of synaptic plasticity. It also mediates memory formation and is implicated in multiple neurological diseases. In this review, we will discuss how Arc is regulated and used as a tool to study neuronal activity. We will also attempt to clarify how its molecular functions correspond to its requirement in various forms of plasticity, discuss Arc's role in behavior and disease, and highlight critical unresolved questions.

Ethanol-induced loss-of-righting response during ethanol withdrawal in male and female rats: associations with alterations in Arc labeling

BACKGROUND

There is increasing evidence for relevant sex differences in responses to ethanol. Several investigations have found differences in expression and recovery from ethanol withdrawal (EW) in people and across various animal models. We have found that female rats recover more quickly than male rats and show differential responses to various behavioral assessments and pharmacological challenges during withdrawal. The purpose of this study was to determine whether sex differences in EW behaviors extend to the hypnotic effects of acute ethanol administration.

METHODS

We used a repeated measures design to assess duration and latency for loss-of-righting reflex following an acute injection of ethanol (4.2 g/kg; 20% w/v) to pair-fed control or ethanol-withdrawn animals at 1 and 3 days EW in male, female, and ovariectomized female (OVX) rats. We determined protein levels of the activity-regulated cytoskeletal protein (Arc), used as a marker for synaptic activity in glutamatergic synapses, in the motor cortex and prefrontal cortex across these same treatment conditions.

RESULTS

Ethanol-withdrawn animals had a reduced ethanol-induced sleep time compared to controls at 1 day EW. Sleep time remained shortened at 3 days EW for males and OVX, but not females. Arc protein levels in motor cortex and preoptic nuclei significantly increased at 1 day EW across all sex conditions, suggestive of an association with the reduced ethanol-induced sleep times during EW. Arc levels increased further for males and OVX, but not females, at the 3 days EW time point.

CONCLUSIONS

These findings add further support to sex differences in effects of and responses to ethanol. They suggest that the more rapid recovery from EW for females than males also includes expression of tolerance to the hypnotic effects of ethanol. These sex differences may involve some differential neuroadaptations in glutamatergic signaling.

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.

Characterization of the plasticity-related gene, Arc, in the frog brain

In mammals, expression of the immediate early gene Arc/Arg3.1 in the brain is induced by exposure to novel environments, reception of sensory stimuli, and production of learned behaviors, suggesting a potentially important role in neural and behavioral plasticity. To date, Arc has only been characterized in a few species of mammals and birds, which limits our ability to understand its role in modifying behavior. To begin to address this gap, we identified Arc in two frog species, Xenopus tropicalis and Physalaemus pustulosus, and characterized its expression in the brain of P. pustulosus. We found that the predicted protein for frog Arc shared 60% sequence similarity with Arc in other vertebrates, and we observed high Arc expression in the forebrain, but not the midbrain or hindbrain, of female túngara frogs sacrificed at breeding ponds. We also examined the time-course of Arc induction in the medial pallium, the homologue of the mammalian hippocampus, in response to a recording of a P. pustulosus mating chorus and found that accumulation of Arc mRNA peaked 0.75 h following stimulus onset. We found that the mating chorus also induced Arc expression in the lateral and ventral pallia and the medial septum, but not in the striatum, hypothalamus, or auditory midbrain. Finally, we examined acoustically induced Arc expression in response to different types of mating calls and found that Arc expression levels in the pallium and septum did not vary with the biological relevance or acoustic complexity of the signal.

Arc regulates spine morphology and maintains network stability in vivo

Long-term memory relies on modulation of synaptic connections in response to experience. This plasticity involves trafficking of AMPA receptors (AMPAR) and alteration of spine morphology. Arc, a gene induced by synaptic activity, mediates the endocytosis of AMPA receptors and is required for both long-term and homeostatic plasticity. We found that Arc increases spine density and regulates spine morphology by increasing the proportion of thin spines. Furthermore, Arc specifically reduces surface GluR1 internalization at thin spines, and Arc mutants that fail to facilitate AMPAR endocytosis do not increase the proportion of thin spines, suggesting that Arc-mediated AMPAR endocytosis facilitates alterations in spine morphology. Thus, by linking spine morphology with AMPAR endocytosis, Arc balances synaptic downscaling with increased structural plasticity. Supporting this, loss of Arc in vivo leads to a significant decrease in the proportion of thin spines and an epileptic-like network hyperexcitability.

Synaptic loss and retention of different classic cadherins with LTP-associated synaptic structural remodeling in vivo

Cadherins are synaptic cell adhesion molecules that contribute to persistently enhanced synaptic strength characteristic of long-term potentiation (LTP). What is relatively unexplored is how synaptic activity of the kind that induces LTP-associated remodeling of synapse structure affects localization of cadherins, particularly in mature animals in vivo, details which could offer insight into how different cadherins contribute to synaptic plasticity. Here, we use a well-described in vivo LTP induction protocol that produces robust synaptic morphological remodeling in dentate gyrus of adult rats in combination with confocal and immunogold electron microscopy to localize cadherin-8 and N-cadherin at remodeled synapses. We find that the density and size of cadherin-8 puncta are significantly diminished in the potentiated middle molecular layer (MML) while concurrently, N-cadherin remains tightly clustered at remodeled synapses. These changes are specific to the potentiated MML, and occur without any change in density or size of synaptophysin puncta. Thus, the loss of cadherin-8 probably represents selective removal from synapses rather than overall loss of synaptic junctions. Together, these findings suggest that activity-regulated loss and retention of different synaptic cadherins could contribute to dual demands of both flexibility and stability in synapse structure that may be important for synaptic morphological remodeling that accompanies long-lasting plasticity.

Patterns of social-experience-related c-fos and Arc expression in the frontal cortices of rats exposed to saccharin or moderate levels of ethanol during prenatal brain development

Recent findings from our laboratory indicate that alterations in frontal cortex function, structural plasticity, and related social behaviors are persistent consequences of exposure to moderate levels of ethanol during prenatal brain development [24]. Fetal-ethanol-related reductions in the expression of the immediate early genes (IEGs) c-fos and Arc and alterations in dendritic spine density in ventrolateral and medial aspects of frontal cortex suggest a dissociation reminiscent of that described by Kolb et al. [38] in which these aspects of frontal cortex undergo reciprocal experience-dependent changes. In addition to providing a brief review of the available data on social behavior and frontal cortex function in fetal-ethanol-exposed rats, the present paper presents novel data on social-experience-related IEG expression in four regions of frontal cortex (Zilles LO, VLO, Fr1, Fr2) that are evaluated alongside our prior data from AID and Cg3. Social experience in normal rats was related to a distinct pattern of IEG expression in ventrolateral and medial aspects of frontal cortex, with generally greater expression observed in ventrolateral frontal cortex. In contrast, weaker expression was observed in all aspects of frontal cortex in ethanol-exposed rats, with the exception of an experience-related increase in the medial agranular cortex. Behaviors related to social investigation and wrestling/boxing were differentially correlated with patterns of activity-related IEG expression in the regions under investigation for saccharin- and ethanol-exposed rats. These observations suggest that recruitment and expression of IEGs in frontal cortex following social experience are potentially important for understanding the long-term consequences of moderate prenatal ethanol exposure on frontal cortex function, synaptic plasticity, and related behaviors.

Time-dependent expression of Arc and zif268 after acquisition of fear conditioning

Memory consolidation requires transcription and translation of new protein. Arc, an effector immediate early gene, and zif268, a regulatory transcription factor, have been implicated in synaptic plasticity underlying learning and memory. This study explored the temporal expression profiles of these proteins in the rat hippocampus following fear conditioning. We observed a time-dependent increase of Arc protein in the dorsal hippocampus 30-to-90-minute post training, returning to basal levels at 4 h. Zif268 protein levels, however, gradually increased at 30-minute post training before peaking in expression at 60 minute. The timing of hippocampal Arc and zif268 expression coincides with the critical period for protein synthesis-dependent memory consolidation following fear conditioning. However, the expression of Arc protein appears to be driven by context exploration, whereas, zif268 expression may be more specifically related to associative learning. These findings suggest that altered Arc and zif268 expression are related to neural plasticity during the formation of fear memory.

Sustained hippocampal IL-1beta overexpression impairs contextual and spatial memory in transgenic mice

Neuroinflammatory conditions such as traumatic brain injury, aging, Alzheimer's disease, and Down syndrome are often associated with cognitive dysfunction. Much research has targeted inflammation as a causative mediator of these deficits, although the diverse cellular and molecular changes that accompany these disorders obscure the link between inflammation and impaired memory. Therefore, we used a transgenic mouse model with a dormant human IL-1beta excisional activation transgene to direct overexpression of IL-1beta with temporal and regional control. Two weeks of hippocampal IL-1beta overexpression impaired long-term contextual and spatial memory in both male and female mice, while hippocampal-independent and short-term memory remained intact. Human IL-1beta overexpression activated glia, elevated murine IL-1beta protein and PGE(2) levels, and increased pro-inflammatory cytokine and chemokine mRNAs specifically within the hippocampus, while having no detectable effect on inflammatory mRNAs in the liver. Sustained neuroinflammation also reduced basal and conditioning-induced levels of the plasticity-related gene Arc.

The Arc of synaptic memory

The immediate early gene Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to synaptic plasticity, thereby optimizing information storage in the nervous system. Here, we attempt to overview the Arc system spanning from transcriptional regulation of the Arc gene, to dendritic transport, metabolism, and translation of Arc mRNA, to post-translational modification, localization, and degradation of Arc protein. Within this framework we discuss the function of Arc in regulation of actin cytoskeletal dynamics underlying consolidation of long-term potentiation (LTP) and regulation of AMPA-type glutamate receptor endocytosis underlying long-term depression (LTD) and homeostatic plasticity. Behaviorally, Arc has a key role in consolidation of explicit and implicit forms of memory, with recent work implicating Arc in adaptation to stress as well as maladaptive plasticity connected to drug addiction. Arc holds considerable promise as a “master regulator” of protein synthesis-dependent forms of synaptic plasticity, but the mechanisms that modulate and switch Arc function are only beginning to be elucidated.

Activity-dependent modulation of neural circuit synaptic connectivity

In many nervous systems, the establishment of neural circuits is known to proceed via a two-stage process; (1) early, activity-independent wiring to produce a rough map characterized by excessive synaptic connections, and (2) subsequent, use-dependent pruning to eliminate inappropriate connections and reinforce maintained synapses. In invertebrates, however, evidence of the activity-dependent phase of synaptic refinement has been elusive, and the dogma has long been that invertebrate circuits are “hard-wired” in a purely activity-independent manner. This conclusion has been challenged recently through the use of new transgenic tools employed in the powerful Drosophila system, which have allowed unprecedented temporal control and single neuron imaging resolution. These recent studies reveal that activity-dependent mechanisms are indeed required to refine circuit maps in Drosophila during precise, restricted windows of late-phase development. Such mechanisms of circuit refinement may be key to understanding a number of human neurological diseases, including developmental disorders such as Fragile X syndrome (FXS) and autism, which are hypothesized to result from defects in synaptic connectivity and activity-dependent circuit function. This review focuses on our current understanding of activity-dependent synaptic connectivity in Drosophila, primarily through analyzing the role of the fragile X mental retardation protein (FMRP) in the Drosophila FXS disease model. The particular emphasis of this review is on the expanding array of new genetically-encoded tools that are allowing cellular events and molecular players to be dissected with ever greater precision and detail.

Rapid activation of plasticity-associated gene transcription in hippocampal neurons provides a mechanism for encoding of one-trial experience

The hippocampus is hypothesized to support rapid encoding of ongoing experience. A critical prerequisite for such function is the ability to readily recruit enduring synaptic plasticity in hippocampal neurons. Hippocampal long-term potentiation (LTP) and memory consolidation require expression of the immediate-early gene (IEG) Arc. To determine whether Arc transcription could be driven by limited and controlled behavioral experience, we used a rectangular track paradigm. In past electrophysiological studies, pyramidal neurons recorded from rats running in one direction on similar tracks typically exhibited a single firing field. Using fluorescence in situ hybridization, we show that the behavioral activity associated with a single lap around the track was sufficient to trigger Arc transcription in complete CA3 neuronal ensembles, as predicted given the role of CA3 in one-trial learning. In contrast, Arc transcription in CA1 ensembles was recruited incrementally, with maximal activation achieved after four laps a day for 4 consecutive days. To test whether Arc transcription is linked to learning and plasticity, or merely elicited by location-specific firing, we inactivated the medial septum, a treatment that compromises hippocampus-dependent learning and LTP but spares location-specific firing in CA1 neurons. Septal inactivation abolished track training-induced Arc transcription in CA1 and CA3 neurons, showing that Arc transcription requires plasticity-inducing stimuli. Accordingly, LTP induction activated Arc transcription in CA1 neurons in vivo. These findings demonstrate for the first time that a single brief experience, equivalent to a single crossing of a firing field, can trigger IEG expression required for long-term plasticity in the hippocampus.

A 3'UTR pumilio-binding element directs translational activation in olfactory sensory neurons

Prolonged stimulation leads to specific and stable changes in an animal's behavior. In interneurons, this plasticity requires spatial and temporal control of neuronal protein synthesis. Whether such translational control occurs in sensory neurons is not known. Adaptation of the AWC olfactory sensory neurons of C. elegans requires the cGMP-dependent protein kinase EGL-4. Here, we show that the RNA-binding PUF protein FBF-1 is required in the adult AWC for adaptation. In the odor-adapted animal, it increases translation via binding to the egl-4 3' UTR. Further, the PUF protein may localize translation near the sensory cilia and cell body. Although the RNA-binding PUF proteins have been shown to promote plasticity in development by temporally and spatially repressing translation, this work reveals that in the adult nervous system, they can work in a different way to promote experience-dependent plasticity by activating translation in response to environmental stimulation.

The immediate early gene arc/arg3.1: regulation, mechanisms, and function

In a manner unique among activity-regulated immediate early genes (IEGs), mRNA encoded by Arc (also known as Arg3.1) undergoes rapid transport to dendrites and local synaptic translation. Despite this intrinsic appeal, relatively little is known about the neuronal and behavioral functions of Arc or its molecular mechanisms of action. Here, we attempt to distill recent advances on Arc spanning its transcriptional and translational regulation, the functions of the Arc protein in multiple forms of neuronal plasticity [long-term potentiation (LTP), long-term depression (LTD), and homeostatic plasticity], and its broader role in neural networks of behaving animals. Worley and colleagues have shown that Arc interacts with endophilin and dynamin, creating a postsynaptic trafficking endosome that selectively modifies the expression of AMPA-type glutamate receptors at the excitatory synapses. Both LTD and homeostatic plasticity in the hippocampus are critically dependent on Arc-mediated endocytosis of AMPA receptors. LTD evoked by activation of metabotropic glutamate receptors depends on rapid Arc translation controlled by elongation factor 2. Bramham and colleagues have shown that sustained translation of newly induced Arc mRNA is necessary for cofilin phosphorylation and stable expansion of the F-actin cytoskeleton underlying LTP consolidation in the dentate gyrus of live rats. In addition to regulating F-actin, Arc synthesis maintains the activity of key translation factors during LTP consolidation. This process of Arc-dependent consolidation is activated by the secretory neurotrophin, BDNF. Moore and colleagues have shown that Arc mRNA is a natural target for nonsense-mediated mRNA decay (NMD) by virtue of its two conserved 3'-UTR introns. NMD and other related translation-dependent mRNA decay mechanisms may serve as critical brakes on protein expression that contribute to the fine spatial-temporal control of Arc synthesis. In studies in behaving rats, Guzowski and colleagues have shown that location-specific firing of CA3 and CA1 hippocampal neurons in the presence of theta rhythm provides the necessary stimuli for activation of Arc transcription. The impact of Arc transcription in memory processes may depend on the specific context of coexpressed IEGs, in addition to posttranscriptional regulation of Arc by neuromodulatory inputs from the amygdala and other brain regions. In sum, Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to diverse forms of synaptic plasticity, thereby optimizing information storage in active networks.

Object-place recognition learning triggers rapid induction of plasticity-related immediate early genes and synaptic proteins in the rat dentate gyrus

Long-term recognition memory requires protein synthesis, but little is known about the coordinate regulation of specific genes. Here, we examined expression of the plasticity-associated immediate early genes (Arc, Zif268, and Narp) in the dentate gyrus following long-term object-place recognition learning in rats. RT-PCR analysis from dentate gyrus tissue collected shortly after training did not reveal learning-specific changes in Arc mRNA expression. In situ hybridization and immunohistochemistry were therefore used to assess possible sparse effects on gene expression. Learning about objects increased the density of granule cells expressing Arc, and to a lesser extent Narp, specifically in the dorsal blade of the dentate gyrus, while Zif268 expression was elevated across both blades. Thus, object-place recognition triggers rapid, blade-specific upregulation of plasticity-associated immediate early genes. Furthermore, Western blot analysis of dentate gyrus homogenates demonstrated concomitant upregulation of three postsynaptic density proteins (Arc, PSD-95, and α-CaMKII) with key roles in long-term synaptic plasticity and long-term memory.

The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is required for memory consolidation of pavlovian fear conditioning in the lateral amygdala

The activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) is an immediate early gene that has been widely implicated in hippocampal-dependent learning and memory and is believed to play an integral role in synapse-specific plasticity. Here, we examined the role of Arc/Arg3.1 in amygdala-dependent Pavlovian fear conditioning. We first examined the regulation of Arc/Arg3.1 mRNA and protein after fear conditioning and LTP-inducing stimulation of thalamic inputs to the lateral amygdala (LA). Quantitative real-time PCR, in situ hybridization, Western blotting and immunohistochemistry revealed a significant upregulation of Arc/Arg3.1 mRNA and protein in the LA relative to controls. In behavioral experiments, intra-LA infusion of an Arc/Arg3.1 antisense oligodeoxynucleotide (ODN) was observed to be anatomically restricted to the LA, taken up by LA cells, and to promote significant knockdown of Arc/Arg3.1 protein. Rats given intra-LA infusions of multiple doses of the Arc/Arg3.1 ODN showed an impairment of LTM (tested approximately 24 later), but no deficit in STM (tested 3 h later) relative to controls infused with scrambled ODN. Finally, to determine whether upregulation of Arc/Arg3.1 occurs downstream of ERK/MAPK activation, we examined Arc/Arg3.1 expression in rats given intra-LA infusion of the MEK inhibitor U0126. Relative to vehicle controls, infusion of U0126 impaired training-induced increases in Arc/Arg3.1 expression. These findings suggest that Arc/Arg3.1 expression in the amygdala is required for fear memory consolidation, and further suggest that Arc/Arg3.1 regulation in the LA is downstream of the ERK/MAPK signaling pathway.

ARG3.1/ARC expression in hippocampal dentate gyrus astrocytes: ultrastructural evidence and co-localization with glial fibrillary acidic protein

Synaptic efficacy following long-term potentiation (LTP) and memory consolidation is associated with changes in the expression of immediate early genes (IEGs). These changes are often accompanied by increased expression of glial fibrillary acidic protein (GFAP). While the protein products of the majority of IEGs are mainly restricted to the cell body, Arg3.1/Arc product is rapidly delivered to dendrites, where it accumulates close to synaptic sites. Arg3.1/Arc protein was originally considered neurone specific; however, we have recently found Arg3.1/Arc immunoreactivity (Arg3.1/Arc-IR) within glial cells and demonstrated its increased expression after LTP in the hippocampal dentate gyrus (DG). Here, we have further investigated this novel finding, using electron microscopic immunocytochemistry to determine the localization and sub-cellular distribution of Arg3.1/Arc protein in GFAP positive glia (GFAP-IR) in the DG. Arg3.1/Arc labelling was seen prominently in GFAP-IR glial cell bodies and in large- and medium-sized glial filamentous processes. GFAP-labelled medium-small peri-synaptic glial profiles also displayed Arg3.1/Arc-IR; however, the very thin and distal glial filaments only displayed Arc-IR. Arc-IR was distributed throughout the cytoplasm, often associated with GFAP filaments, and along the plasma membrane of glial processes. Peri-synaptic glial Arg3.1/Arc-IR processes were apposed to pre- and/or post-synaptic profiles at asymmetric axospinous synapses. These data, taken with our earlier study which provided evidence for an increase in astrocytic Arg3.1/Arc-IR after the induction of LTP, suggest a role for glial Arg3.1/Arc in structural and synaptic plasticity which may be critical for the maintenance of cognitive functions.

Rapid translation of Arc/Arg3.1 selectively mediates mGluR-dependent LTD through persistent increases in AMPAR endocytosis rate

Salient stimuli that modify behavior induce transcription of activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) and transport Arc mRNA into dendrites, suggesting that local Arc translation mediates synaptic plasticity that encodes such stimuli. Here, we demonstrate that long-term synaptic depression (LTD) in hippocampal neurons induced by group 1 metabotropic glutamate receptors (mGluRs) relies on rapid translation of Arc. mGluR-LTD induction causes long-term increases in AMPA receptor endocytosis rate and dendritic synthesis of Arc, a component of the AMPAR endocytosis machinery. Knockdown of Arc prevents mGluRs from triggering AMPAR endocytosis or LTD, and acute blockade of new Arc synthesis with antisense oligonucleotides blocks mGluR-LTD and AMPAR trafficking. In contrast, LTD induced by NMDA receptors does not persistently alter AMPAR endocytosis rate, induce Arc synthesis, or require Arc protein. These data demonstrate a role for local Arc synthesis specifically in mGluR-LTD and suggest that mGluR-LTD may be one consequence of Arc mRNA induction during experience.

Upregulation of Arc mRNA expression in the prefrontal cortex following cue-induced reinstatement of extinguished cocaine-seeking behavior

Cocaine-associated cues acquire incentive motivational effects that manifest as cue-elicited craving in humans and cocaine-seeking behavior in rats. Here we examine the hypothesis that neuronal processes associated with incentive motivational effects of cocaine cues involve increased expression of the plasticity-associated gene, Arc. Rats trained to self-administer cocaine subsequently underwent extinction training, during which cocaine-seeking behavior (i.e., responses without cocaine reinforcement) progressively decreased. Rats were then tested for cocaine-seeking behavior either with or without response-contingent presentations of light/tone cues that had been previously paired with cocaine infusions during self-administration training. Cues elicited reinstatement of cocaine-seeking behavior and were accompanied by increased Arc mRNA levels in the orbitofrontal, prelimbic, and anterior cingulate cortices, suggesting Arc involvement in conditioned plasticity associated with incentive motivational effects of cocaine cues. Additionally, rats with a history of cocaine self-administration and extinction exhibited upregulation of Arc expression in several limbic and cortical regions relative to saline-yoked controls regardless of cue exposure condition, suggesting persistent neuroadaptations involving Arc within these regions.

Synaptic plasticity, memory and the hippocampus: a neural network approach to causality

Two facts about the hippocampus have been common currency among neuroscientists for several decades. First, lesions of the hippocampus in humans prevent the acquisition of new episodic memories; second, activity-dependent synaptic plasticity is a prominent feature of hippocampal synapses. Given this background, the hypothesis that hippocampus-dependent memory is mediated, at least in part, by hippocampal synaptic plasticity has seemed as cogent in theory as it has been difficult to prove in practice. Here we argue that the recent development of transgenic molecular devices will encourage a shift from mechanistic investigations of synaptic plasticity in single neurons towards an analysis of how networks of neurons encode and represent memory, and we suggest ways in which this might be achieved. In the process, the hypothesis that synaptic plasticity is necessary and sufficient for information storage in the brain may finally be validated.

Sustained Arc/Arg3.1 synthesis controls long-term potentiation consolidation through regulation of local actin polymerization in the dentate gyrus in vivo

New gene expression is necessary for long-term potentiation (LTP) consolidation, yet roles for specific activity-induced mRNAs have not been defined. Here we probed the dynamic function of activity-induced Arc (activity-regulated cytoskeletal-associated protein)/Arg3.1 (activity-regulated gene 3.1 protein homolog) mRNA using brief, local infusions of antisense (AS) oligodeoxynucleotides at multiple time points during dentate gyrus LTP in vivo. Surprisingly, early Arc synthesis is necessary for early expression of LTP, whereas sustained synthesis is required to generate stably modified synapses. AS application 2 h after LTP induction results in a rapid and permanent reversal of LTP. This reversal is associated with rapid knockdown of upregulated Arc, dephosphorylation of actin depolymerization factor/cofilin, and loss of nascent filamentous actin (F-actin) at synaptic sites. Infusion of the F-actin stabilizing drug jasplakinolide during LTP maintenance blocks the ability of AS to reverse LTP. These results couple activity-induced expression of Arc to expansion of the actin cytoskeleton underlying enduring LTP. Furthermore, Arc synthesis is required for both the induction and consolidation of LTP elicited by local BDNF infusion, thus identifying Arc as a key molecular effector of BDNF in synaptic plasticity.

Networks of neurons, networks of genes: an integrated view of memory consolidation

Investigations into the mechanisms of memory formation have abided by the central tenet of the consolidation theory-that memory formation occurs in stages which differ in their requirement for protein synthesis. The current most widely accepted hypothesis posits that new memories are encoded as neural activity-induced changes in synaptic efficacy, and stabilization of these changes requires de novo protein synthesis. However, the basic assumptions of this view have been challenged by concerns regarding the specificity of the effects of the protein synthesis inhibitors used to support the claim. Studies on immediate-early genes (IEGs), in particular Arc, provide a distinct and independent perspective on the issue of the requirement of new protein synthesis in synaptic plasticity and memory consolidation. The IEG Arc and its protein are dynamically induced in response to neuronal activity, and are directly involved in synaptic plasticity and memory consolidation. Although we provide extensive data on Arc's properties to address the requirement of genomic and proteomic responses in memory formation, Arc is merely one element in a network of genes that interact in a coordinated fashion to serve memory consolidation. From gene expression and other studies, we propose the view that the stabilization of a memory trace is a continuous and ongoing process, which does not have a discrete endpoint and cannot be reduced to a single deterministic "molecular cascade". Rather, memory traces are maintained within metastable networks, which must integrate and update past traces with new ones. Such an updating process may well recruit and use many of the plasticity mechanisms necessary for the initial encoding of memory.

Dendritic mRNA: transport, translation and function

Many cellular functions require the synthesis of a specific protein or functional cohort of proteins at a specific time and place in the cell. Local protein synthesis in neuronal dendrites is essential for understanding how neural activity patterns are transduced into persistent changes in synaptic connectivity during cortical development, memory storage and other long-term adaptive brain responses. Regional and temporal changes in protein levels are commonly coordinated by an asymmetric distribution of mRNAs. This Review attempts to integrate current knowledge of dendritic mRNA transport, storage and translation, placing particular emphasis on the coordination of regulation and function during activity-dependent synaptic plasticity in the adult mammalian brain.

Actin polymerization and ERK phosphorylation are required for Arc/Arg3.1 mRNA targeting to activated synaptic sites on dendrites

The mRNA for the immediate early gene Arc/Arg3.1 is induced by strong synaptic activation and is rapidly transported into dendrites, where it localizes at active synaptic sites. NMDA receptor activation is critical for mRNA localization at active synapses, but downstream events that mediate localization are not known. The patterns of synaptic activity that induce mRNA localization also trigger a dramatic polymerization of actin in the activated dendritic lamina and phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) throughout the postsynaptic cytoplasm. The local polymerization of actin in the activated dendritic lamina is of particular interest because it occurs in the same dendritic domains in which newly synthesized Arc/Arg3.1 mRNA localizes. Here, we explore the role of activity-induced alterations in the actin network and mitogen-activated protein (MAP) kinase activation in Arc/Arg3.1 mRNA localization. We show that actin polymerization induced by high-frequency stimulation is blocked by local inhibition of Rho kinase, and Arc/Arg3.1 mRNA localization is abrogated in the region of Rho kinase blockade. Local application of latrunculin B, which binds to actin monomers and inhibits actin polymerization, also blocked the targeting of Arc/Arg3.1 mRNA to activated synaptic sites. Local application of the MAP kinase kinase inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-amino-phenylthio]butadiene) blocked ERK phosphorylation, and also blocked Arc/Arg3.1 mRNA localization. Our results indicate that the reorganization of the actin cytoskeletal network in conjunction with MAP kinase activation is required for targeting newly synthesized Arc/Arg3.1 mRNA to activated synaptic sites.

The Drosophila ARC homolog regulates behavioral responses to starvation

The gene encoding dARC1, one of three Drosophila homologs of mammalian activity-regulated cytoskeleton-associated protein (ARC), is upregulated in both seizure and muscular hypercontraction mutants. In this study we generate a null mutant for dArc1 and show that this gene is not involved in synaptic plasticity at the larval neuromuscular junction or in formation or decay of short-term memory of courtship conditioning, but rather is a modifier of stress-induced behavior. dARC1 is expressed in a number of neurosecretory cells and mutants are starvation-resistant, exhibiting an increased time of survival in the absence of food. Starvation resistance is likely due to the fact that dArc1 mutants lack the normal hyperlocomotor response to starvation, which is almost universal in the animal kingdom. dARC1 acts in insulin-producing neurons of the pars intercerebralis to control this behavior, but does not appear to be a general regulator of insulin signaling. This suggests that there are multiple modes of communication between the pars and the ring gland that control starvation-induced behavioral responses.

Expression of FOS, EGR-1, and ARC in the amygdala and hippocampus of female rats during formation of the intromission mnemonic of pseudopregnancy

Pseudopregnancy (PSP) in the female rat is a neuroendocrine condition that is induced by repeated and intermittent vaginocervical stimulation received during mating and involves the expression of bicircadian prolactin surges and cessation of normal estrous cyclicity for 10-12 days postmating. The temporal patterning and number of intromissions received by the female are critical for PSP initiation, and thus, short-term encoding of VCS occurs during transduction of intromissions into PSP. In this experiment, we characterized and compared the mating-induced neural activation patterns within amygdalar and hippocampal regions using expression of the immediate early genes FOS, EGR-1, and ARC. Cycling female rats mated on proestrus received 15 or 5 intromissions under paced or nonpaced mating conditions. High numbers of intromissions during nonpaced mating or low numbers received during paced mating are sufficient to induce PSP, while five nonpaced intromissions and mounts without intromission are insufficient. Here we demonstrate that the CA1 region of the hippocampus was selectively sensitive to PSP-sufficient but not PSP-insufficient mating stimulation by showing significant effects of paced mating for all three IEGs. Paced mating also stimulated the expression of ARC within the basolateral, cortical, and central nuclei of the amygdala. The posterodorsal medial amygdala also showed selective EGR-1 responses to PSP-sufficient mating stimulation. There was no effect of hemisphere on IEG expression. The postmating expression profiles of these IEGs provide evidence that limbic areas involved in encoding and consolidation of memory are involved in initiating the neuroendocrine memory of PSP.

Bidirectional redistribution of AMPA but not NMDA receptors after perforant path simulation in the adult rat hippocampus in vivo

Long-term potentiation (LTP) in vitro reveals dynamic regulation of synaptic glutamate receptors. AMPA receptors may be inserted into synapses to increase neurotransmission, whereas NMDA receptors may redistribute within the synapse to alter the probability of subsequent plasticity. To date, the only evidence for these receptor dynamics in the hippocampus is from the studies of dissociated neurons and hippocampal slices taken from young animals. Although synaptic plasticity is induced easily, the extent of AMPA and NMDA receptor mobility after LTP is unknown in the adult, intact hippocampus. To test whether AMPA or NMDAR subunits undergo activity-dependent modifications in adult hippocampal synapses, we induced LTP at perforant path-dentate gyrus (DG) synapses in anesthetized adult rats, using high frequency stimulation (HFS), verified layer-specific Arc induction, and analyzed the distribution of postsynaptic AMPA and NMDAR subunits, using immunogold electron microscopy. The number of synapses with AMPA receptor labeling increased with LTP-inducing HFS in the stimulated region of the dendrite relative to the nonstimulated regions. The opposite trend was noted with low frequency stimulation (LFS). Moreover, HFS increased and LFS decreased the ratio of synaptic to extrasynaptic AMPA receptor labeling in the postsynaptic membrane. In contrast, HFS did not significantly alter NMDAR labeling. Thus, LTP in the adult hippocampus in vivo selectively enhanced AMPA but not NMDAR labeling specifically in synapses undergoing activity-dependent plasticity relative to the remainder of the dendritic tree. The results suggest a mechanism by which rapid adjustments in synaptic strength can occur through localized AMPA receptor mobility and that this process may be competitive across the dendritic tree.

Dynamics of immediate early gene and neuropeptide gene response to prolonged immobilization stress: evidence against a critical role of the termination of exposure to the stressor

Stress-induced expression of immediate early genes (IEGs) appears to be transient even if the exposure to the stressor persists. However, there are some exceptions which suggest that particular characteristics of stressors can affect the dynamics of IEG expression. We studied in selected telencephalic, diencephalic and brainstem regions the mRNA levels of two clearly distinct IEGs (c-fos and arc) during prolonged exposure to a severe stressor such as immobilization (IMO) and after releasing the rats from the situation. Although regional differences were observed with the two IEGs, overall, c-fos mRNA levels progressively declined over the course of 4 h of continuous exposure to IMO, whereas arc mRNA levels were maintained at high levels in the brain regions that express this gene under stress (telencephalon). Levels of CRF hnRNA in the hypothalamus paraventricular nucleus only slightly declined during prolonged exposure to IMO. Surprisingly, termination of exposure to IMO did not modify CRF gene expression in the paraventricular nucleus or the pattern of IEGs expression, with the exception of c-fos in the lateral septum. Thus, putative signals associated to the termination of exposure to IMO were unable to modify either IEG expression in most brain areas or CRF gene expression in the paraventricular nucleus.

Arc/Arg3.1 interacts with the endocytic machinery to regulate AMPA receptor trafficking

Arc/Arg3.1 is an immediate-early gene whose mRNA is rapidly transcribed and targeted to dendrites of neurons as they engage in information processing and storage. Moreover, Arc/Arg3.1 is known to be required for durable forms of synaptic plasticity and learning. Despite these intriguing links to plasticity, Arc/Arg3.1's molecular function remains enigmatic. Here, we demonstrate that Arc/Arg3.1 protein interacts with dynamin and specific isoforms of endophilin to enhance receptor endocytosis. Arc/Arg3.1 selectively modulates trafficking of AMPA-type glutamate receptors (AMPARs) in neurons by accelerating endocytosis and reducing surface expression. The Arc/Arg3.1-endocytosis pathway appears to regulate basal AMPAR levels since Arc/Arg3.1 KO neurons exhibit markedly reduced endocytosis and increased steady-state surface levels. These findings reveal a novel molecular pathway that is regulated by Arc/Arg3.1 and likely contributes to late-phase synaptic plasticity and memory consolidation.

Control of synaptic consolidation in the dentate gyrus: mechanisms, functions, and therapeutic implications

Synaptic consolidation refers to the development and stabilization of protein synthesis-dependent modifications of synaptic strength as observed during long-term potentiation (LTP) and long-term depression (LTD). Activity-dependent changes in synaptic strength are thought to underlie memory storage and other adaptive responses of the nervous systems of importance in mood stability, reward behavior, and pain control. This chapter focuses on the mechanisms and functions of synaptic consolidation in the dentate gyrus, a critical structure not only in hippocampal memory function, but also in regulation of stress responses and cognitive aspects of depression. Recent evidence suggests that synaptic consolidation at excitatory medial perforant path-granule cell synapses requires brain-derived neurotrophic factor (BDNF) signaling and induction of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is strongly induced and transported to dendritic processes following high-frequency stimulation (HFS) that induces LTP in the rat dentate gyrus in vivo. Sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Furthermore, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has revealed a panel of BDNF-regulated genes that may cooperate with Arc during synaptic consolidation. In addition to regulating gene expression, BDNF signaling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively-expressed mRNA in dendrites, BDNF may effectively control the window of synaptic consolidation. Dysregulation of BDNF synthesis and Arc function, specifically within the dentate gyrus, is linked to behavioral symptoms and cognitive deficits in animal models of depression and Alzheimer's disease. Therapeutics strategies targeting synaptic consolidation hold promise for the future.

Increased expression of the immediate-early gene arc/arg3.1 reduces AMPA receptor-mediated synaptic transmission

Arc/Arg3.1 is an immediate-early gene whose expression levels are increased by strong synaptic activation, including synapse-strengthening activity patterns. Arc/Arg3.1 mRNA is transported to activated dendritic regions, conferring the distribution of Arc/Arg3.1 protein both temporal correlation with the inducing stimulus and spatial specificity. Here, we investigate the effect of increased Arc/Arg3.1 levels on synaptic transmission. Surprisingly, Arc/Arg3.1 reduces the amplitude of synaptic currents mediated by AMPA-type glutamate receptors (AMPARs). This effect is prevented by RNAi knockdown of Arc/Arg3.1, by deleting a region of Arc/Arg3.1 known to interact with endophilin 3 or by blocking clathrin-coated endocytosis of AMPARs. In the hippocampal slice, Arc/Arg3.1 results in removal of AMPARs composed of GluR2 and GluR3 subunits (GluR2/3). Finally, Arc/Arg3.1 expression occludes NMDAR-dependent long-term depression. Our results demonstrate that Arc/Arg3.1 reduces the number of GluR2/3 receptors leading to a decrease in AMPAR-mediated synaptic currents, consistent with a role in the homeostatic regulation of synaptic strength.

Spatial exploration induces ARC, a plasticity-related immediate-early gene, only in calcium/calmodulin-dependent protein kinase II-positive principal excitatory and inhibitory neurons of the rat forebrain

Active behavior, such as exploring a novel environment, induces the expression of the immediate-early gene Arc (activity-regulated cytoskeletal associated protein, or Arg 3.1) in many brain regions, including the hippocampus, neocortex, and striatum. Arc messenger ribonucleic acid and protein are localized in activated dendrites, and Arc protein is required for the maintenance of long-term potentiation and memory consolidation. Although previous evidence suggests that Arc is expressed in neurons, there is no direct demonstration that only neurons can express Arc. Furthermore, there is no characterization of the main neuronal types that express Arc. The data reported here show that behavior- or seizure-induced Arc expression in the hippocampus, primary somatosensory cortex, and dorsal striatum of rats colocalizes only with neuronal (NeuN-positive) and not with glial (GFAP-positive) cells. Furthermore, Arc was found exclusively in non-GABAergic alpha-CaMKII-positive hippocampal and neocortical neurons of rats that had explored a novel environment. Some GAD65/67-positive neurons in these regions were observed to express Arc, but only after a very strong stimulus (electroconvulsive seizure). In the dorsal striatum, spatial exploration induced Arc only in GABAergic and alpha-CaMKII-positive neurons. Combined, these results show that although a very strong stimulus (seizure) can induce Arc in a variety of neurons, behavior induces Arc in the CaMKII-positive principal neurons of the hippocampus, neocortex, and dorsal striatum. These results, coupled with recent in vitro findings of interactions between Arc and CaMKII, are consistent with the hypothesis that Arc and CaMKII act as plasticity partners to promote functional and/or structural synaptic modifications that accompany learning.

Brain-derived neurotrophic factor and control of synaptic consolidation in the adult brain

Interest in BDNF (brain-derived neurotrophic factor) as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF and its receptor tyrosine kinase TrkB (tropomyosin receptor kinase B) to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and synaptic plasticity in the adult brain. Development of stable LTP (long-term potentiation) in response to high-frequency stimulation requires new gene expression and protein synthesis, a process referred to as synaptic consolidation. Several lines of evidence have implicated endogenous BDNF-TrkB signalling in synaptic consolidation. This mini-review emphasizes new insights into the molecular mechanisms underlying this process. The immediate early gene Arc (activity-regulated cytoskeleton-associated protein) is strongly induced and transported to dendritic processes after LTP induction in the dentate gyrus in live rats. Recent work suggests that sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin-depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Moreover, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has also revealed a panel of BDNF-regulated genes that may co-operate with Arc during LTP maintenance. In addition to regulating gene expression, BDNF signalling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively expressed mRNA in neuronal dendrites, BDNF may effectively control the window of synaptic consolidation. These findings have implications for mechanisms of memory storage and mood control.

In vivo two-photon imaging reveals a role of arc in enhancing orientation specificity in visual cortex

Cortical representations of visual information are modified by an animal's visual experience. To investigate the mechanisms in mice, we replaced the coding part of the neural activity-regulated immediate early gene Arc with a GFP gene and repeatedly monitored visual experience-induced GFP expression in adult primary visual cortex by in vivo two-photon microscopy. In Arc-positive GFP heterozygous mice, the pattern of GFP-positive cells exhibited orientation specificity. Daily presentations of the same stimulus led to the reactivation of a progressively smaller population with greater reactivation reliability. This adaptation process was not affected by the lack of Arc in GFP homozygous mice. However, the number of GFP-positive cells with low orientation specificity was greater, and the average spike tuning curve was broader in the adult homozygous compared to heterozygous or wild-type mice. These results suggest a physiological function of Arc in enhancing the overall orientation specificity of visual cortical neurons during the post-eye-opening life of an animal.