Arc/Arg3.1: Linking Gene Expression to Synaptic Plasticity and Memory

Comparative dynamics of MAPK/ERK signalling components and immediate early genes in the hippocampus and amygdala following contextual fear conditioning and retrieval

Over the past few years multiple studies have attempted to uncover molecular signatures of memory reconsolidation when compared to consolidation. In the present study we used immunocytochemical detection of the MAPK/ERK1/2 pathway, to track activated neuronal circuits in the hippocampus and amygdala recruited during the consolidation and reconsolidation of a contextual fear conditioning (CFC) memory. We report selective differences in magnitude and temporal dynamics of activated ERK1/2 signalling in different subregions of these two structures between the post-training and post-retrieval periods, except in the dentate gyrus, where the patterns of activation were similar. We then focused on this brain area to dissect out the patterns of downstream ERK1/2 signalling components, including the phosphorylation of MSK-1 and histone H3 on ser10, along with the induction of the Immediate Early Genes (IEGs) Arc/Arg3.1, c-Fos and Zif268/Egr1 following CFC training and retrieval. We found that the completion of the nucleosomal response as well as the induction of IEGs shorter during the reconsolidation period as compared to consolidation. Our results shed new light on the cellular mechanisms underlying the consolidation and reconsolidation processes engaged following CFC training and retrieval and further extend the notion that memory reconsolidation is not mechanistically a repetition of consolidation. In addition, we provide evidence that the strength of a previously established CFC memory is characterized by distinct patterns of ERK1/2 activation in different hippocampal and amygdalar subfields upon CFC memory recall. Our results emphasize the differences between consolidation and reconsolidation processes in relation to contextual fear memories.

The Geisler method: tracing activity-dependent cGMP plasticity changes upon double detection of mRNA and protein on brain slices

We recently demonstrated that an increase of guanosine 3',5'-cyclic monophosphate (cGMP) signaling could protect the inner ear from noise-induced hair cell damage. Noise exposure not only damages hair cells but also alters the central responsiveness to sound leading to plasticity changes. cGMP signaling has long been known to play a crucial role for plasticity changes and long-term potentiation (LTP). To get a first insight into the role of cGMP for noise-induced plasticity changes we aimed to co-trace the mRNA and protein of plasticity-related genes as, e.g., the immediate early gene Arc (activity-regulated cytoskeletal protein) with markers for the cGMP pathway. We developed a method that permits the simultaneous monitoring of mRNA and protein through light microscopy to visualize gene expression in neurons and synapses of its processes. Accordingly, different from previous fluorescence-based assays that detect, e.g., fluorochrome-labeled Arc antibodies and Arc mRNA, we describe here a methodology that allows the detection of mRNA and protein of synaptic genes using nonfluorescent stable tracers for high-resolution observation of activity-dependent plasticity changes using light microscopy even after weeks or months.

Harnessing plasticity to understand learning and treat disease

A large body of evidence suggests that neural plasticity contributes to learning and disease. Recent studies suggest that cortical map plasticity is typically a transient phase that improves learning by increasing the pool of task-relevant responses. Here, I discuss a new perspective on neural plasticity and suggest how plasticity might be targeted to reset dysfunctional circuits. Specifically, a new model is proposed in which map expansion provides a form of replication with variation that supports a Darwinian mechanism to select the most behaviorally useful circuits. Precisely targeted neural plasticity provides a new avenue for the treatment of neurological and psychiatric disorders and is a powerful tool to test the neural mechanisms of learning and memory.

Long-term decrease in immediate early gene expression after electroconvulsive seizures

Electroconvulsive therapy (ECT) is a well-established psychiatric treatment for severe depression. Despite its clinical utility, post-ECT memory deficits are a common side effect. Neuronal plasticity and memory consolidation are intimately related to the expression of immediate early genes (IEG), such as Egr1, Fos and Arc. Changes in IEG activation have been postulated to underlie long-term neuronal adaptations following electroconvulsive seizures (ECS), an animal model of ECT. To test this hypothesis, we used real-time PCR to examine the effect of acute and chronic ECS (8 sessions, one every other day) on the long-term (>24 h) expression of IEG Egr1, Fos and Arc in the hippocampus, a brain region implicated both in the pathophysiology of depression as well as in memory function. We observed a transient increase in Egr1 and Fos expression immediately after ECS, followed by a long-term decrease of IEG levels after both acute and chronic ECS. A separate group of animals, submitted to the same chronic ECS protocol and then subjected to open field or passive avoidance tasks, confirmed robust memory deficits 2 weeks after the last chronic ECS. The possible role of IEG downregulation on long-term learning deficits observed following ECS are discussed.

Cannabinoid mitigation of neuronal morphological change important to development and learning: insight from a zebra finch model of psychopharmacology

Normal CNS development proceeds through late-postnatal stages of adolescent development. The activity-dependence of this development underscores the significance of CNS-active drug exposure prior to completion of brain maturation. Exogenous modulation of signaling important in regulating normal development is of particular concern. This mini-review presents a summary of the accumulated behavioral, physiological and biochemical evidence supporting such a key regulatory role for endocannabinoid signaling during late-postnatal CNS development. Our focus is on the data obtained using a unique zebra finch model of developmental psychopharmacology. This animal has allowed investigation of neuronal morphological effects essential to establishment and maintenance of neural circuitry, including processes related to synaptogenesis and dendritic spine dynamics. Altered neurophysiology that follows exogenous cannabinoid exposure during adolescent development has the potential to persistently alter cognition, learning and memory.

Dysfunction of the ubiquitin ligase Ube3a may be associated with synaptic pathophysiology in a mouse model of Huntington disease

Huntington disease (HD) is a hereditary neurodegenerative disorder characterized by progressive cognitive, psychiatric, and motor symptoms. The disease is caused by abnormal expansion of CAG repeats in the gene encoding huntingtin, but how mutant huntingtin leads to early cognitive deficits in HD is poorly understood. Here, we demonstrate that the ubiquitin ligase Ube3a, which is implicated in synaptic plasticity and involved in the clearance of misfolded polyglutamine protein, is strongly recruited to the mutant huntingtin nuclear aggregates, resulting in significant loss of its functional pool in different regions of HD mouse brain. Interestingly, Arc, one of the substrates of Ube3a linked with synaptic plasticity, is also associated with nuclear aggregates, although its synaptic level is increased in the hippocampus and cortex of HD mouse brain. Different regions of HD mouse brain also exhibit decreased levels of AMPA receptors and various pre- and postsynaptic proteins, which could be due to the partial loss of function of Ube3a. Transient expression of mutant huntingtin in mouse primary cortical neurons further demonstrates recruitment of Ube3a into mutant huntingtin aggregates, increased accumulation of Arc, and decreased numbers of GluR1 puncta in the neuronal processes. Altogether, our results suggest that the loss of function of Ube3a might be associated with the synaptic abnormalities observed in HD.

Impact of contextual cues in the expression of the memory associated with diazepam withdrawal: involvement of hippocampal PKMζ in vivo, and Arc expression and LTP in vitro

Hippocampal synaptic plasticity has been related to learning and adaptive processes developed during chronic drug administration, suggesting the existence of a common neurobiological mechanism mediating drug addiction and memory. Moreover, protein kinase M zeta (PKMζ) is critical for the maintenance of hippocampal long-term potentiation (LTP) and spatial conditioned long-term memories. Also, a link between activity-regulated cytoskeleton-associated protein (Arc), PKMζ and LTP has been proposed. Our previous results demonstrated that re-exposure to the withdrawal environment was able to evoke the memory acquired when the anxiety measured as a diazepam (DZ) withdrawal sign was experienced. In the present work we evaluated if the memory associated with DZ withdrawal could be affected by changes in the contextual cues presented during withdrawal and by intrahippocampal administration of a PKMζ inhibitor. We found that the context was relevant for the expression of withdrawal signs as changes in contextual cues prevented the expression of the anxiety-like behavior observed during plus-maze (PM) re-exposure, the associated enhanced synaptic plasticity and the increase in Arc expression. Furthermore, intrahippocampal administration of PKMζ inhibitor previous to re-exposure to the PM test also impaired expression of anxiety-like behavior and the facilitated LTP. These results support the relevance of the hippocampal synaptic plasticity in the maintenance of the memory trace during benzodiazepines withdrawal, adding new evidences for common mechanisms between memory and drug addiction that can be intervened for treatment or prevention of this pathology.

Memory traces compete under regimes of limited Arc protein synthesis: implications for memory interference

Recently encoded information can be lost in the presence of new information, a process called 'retrograde interference'. Retrograde interference has been extensively described for more than a century; however, little is known about its underlying mechanisms. Different approaches agree on the need of the synthesis of plasticity related proteins (PRPs) to consolidate a long-term memory (LTM). Our hypothesis is that when PRPs are limited, interference of a task over LTM formation of another may be due to the utilization of protein resources common to both tasks. Here, by combining the tasks of inhibitory avoidance (IA) and open field (OF) exploration in rats, we show that memory traces compete for their stabilization if PRPs are limited. As a result, LTM is formed for only one of the tasks with a consequent decrease in the memory for the other. Furthermore, infusing Arc antisense oligonucleotide into the dorsal hippocampus, we found that Arc is necessary for LTM formation of these two types of learning tasks and is one of the PRPs that can be shared between them when animals are trained in both OF and IA. In sum, these findings suggest that under conditions of reduced protein availability, a learning task interferes with LTM formation of another by using the available PRPs.

Contrasting brain activity patterns for item recognition memory and associative recognition memory: insights from immediate-early gene functional imaging

Recognition memory, the discrimination of a novel from a familiar event, can be classified into item recognition and associative recognition. Item recognition concerns the identification of novel individual stimuli, while associative recognition concerns the detection of novelty that arises when familiar items are reconfigured in a novel manner. Experiments in rodents that have mapped the expression of immediate-early genes, e.g., c-fos, highlight key differences between these two forms of recognition memory. Visual item novelty is consistently linked to increased c-fos activity in just two brain sites, the perirhinal cortex and the adjacent visual association area Te2. Typically there are no hippocampal c-fos changes. In contrast, visual associative recognition is consistently linked to c-fos activity changes in the hippocampus, but not the perirhinal cortex. The lack of a c-fos perirhinal change with associative recognition presumably reflects the fact that the individual items in an array remain familiar, even though their combinations are unique. Those exceptions, when item recognition is associated with hippocampal c-fos changes, occur when rats actively explore novel objects. The increased engagement with objects will involve multisensory stimulus processing and potentially create conditions in which rats can readily learn stimulus attributes such as object location or object order, i.e., attributes involved in associative recognition. Correlations based on levels of immediate-early gene expression in the temporal lobe indicate that actively exploring novel stimuli switches patterns of entorhinal-hippocampal functional connectivity to emphasise direct entorhinal-dentate gyrus processing. These gene activity findings help to distinguish models of medial temporal lobe function.

Epigenetic regulation of Arc and c-Fos in the hippocampus after acute electroconvulsive stimulation in the rat

Electroconvulsive stimulation (ECS) remains one of the most effective treatments of major depression. However, the underlying molecular changes still remain to be elucidated. Since ECS causes rapid and significant changes in gene expression we have looked at epigenetic regulation of two important immediate early genes that are both induced after ECS: c-Fos and Arc. We examined Arc and c-Fos protein expression and found Arc present over 4 h, in contrast to c-Fos presence lasting only 1 h. Both genes had returned to baseline expression at 24 h post-ECS. Histone H4 acetylation (H4Ac) is one of the important epigenetic marks associated with gene activation. We show increased H4Ac at the c-Fos promoter at 1 h post-ECS. Surprisingly, we also observed a significant increase in DNA methylation of the Arc gene promoter at 24 h post-ECS. DNA methylation, which is responsible for gene silencing, is a rather stable covalent modification. This suggests that Arc expression has been repressed and may consequently remain inhibited for a prolonged period post-ECS. Arc plays a critical role in the maintenance phase of long-term potentiation (LTP) and consolidation of memory in the rat brain. Thus, this study is one of the first to demonstrate DNA methylation as a regulator of ECS-induced gene expression and it provides a molecular link to the memory deficits observed after ECS.

Flavonoids as modulators of memory and learning: molecular interactions resulting in behavioural effects

There is considerable interest in the potential of a group of dietary-derived phytochemicals known as flavonoids in modulating neuronal function and thereby influencing memory, learning and cognitive function. The present review begins by detailing the molecular events that underlie the acquisition and consolidation of new memories in the brain in order to provide a critical background to understanding the impact of flavonoid-rich diets or pure flavonoids on memory. Data suggests that despite limited brain bioavailability, dietary supplementation with flavonoid-rich foods, such as blueberry, green tea and Ginkgo biloba lead to significant reversals of age-related deficits on spatial memory and learning. Furthermore, animal and cellular studies suggest that the mechanisms underpinning their ability to induce improvements in memory are linked to the potential of absorbed flavonoids and their metabolites to interact with and modulate critical signalling pathways, transcription factors and gene and/or protein expression which control memory and learning processes in the hippocampus; the brain structure where spatial learning occurs. Overall, current evidence suggests that human translation of these animal investigations are warranted, as are further studies, to better understand the precise cause-and-effect relationship between flavonoid intake and cognitive outputs.

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.

Aquaporin-4 promotes memory consolidation in Morris water maze

Aquaporin-4 (AQP4), the most abundant aquaporin in the brain, is polarized at the glial end-feet facing peri-synaptic areas. AQP4 has been hypothesized to modulate water and potassium fluxes associated with neuronal activity in pathophysiological states. However, the role of AQP4 in astroglial signaling under physiological conditions is unclear. Herein, AQP4 knockout mice and wild-type littermates were tested in the Morris water maze (MWM), which allows for investigating the role of AQP4 in long-term learning and memory. Compared with wild-type mice, AQP4 knockout mice appeared actually to find the platform more easy, but to forget more quickly, in the MWM, indicating that AQP4 knockout mice exhibited impaired memory consolidation in MWM. Moreover, the deficits of memory consolidations were associated with defects in theta-burst stimulation-induced long-term potentiation both in vivo and in vitro. Furthermore, AQP4 knockout mice were accompanied by a decrease in the incorporation of adult-generated granule cells into spatial memory networks. Taken together, our findings indicate that AQP4 plays a modulatory role in memory consolidation. Targeting glial AQP4 may be a new therapeutic strategy for neurodegenerative disorders and related memory impairment.

Activation of intracellular metabotropic glutamate receptor 5 in striatal neurons leads to up-regulation of genes associated with sustained synaptic transmission including Arc/Arg3.1 protein

The G-protein coupled receptor, metabotropic glutamate receptor 5 (mGluR5), is expressed on both cell surface and intracellular membranes in striatal neurons. Using pharmacological tools to differentiate membrane responses, we previously demonstrated that cell surface mGluR5 triggers rapid, transient cytoplasmic Ca(2+) rises, resulting in c-Jun N-terminal kinase, Ca(2+)/calmodulin-dependent protein kinase, and cyclic adenosine 3',5'-monophosphate-responsive element-binding protein (CREB) phosphorylation, whereas stimulation of intracellular mGluR5 induces long, sustained Ca(2+) responses leading to the phosphorylation of extracellular signal-regulated kinase (ERK1/2) and Elk-1 (Jong, Y. J., Kumar, V., and O'Malley, K. L. (2009) J. Biol. Chem. 284, 35827-35838). Using pharmacological, genetic, and bioinformatics approaches, the current findings show that both receptor populations up-regulate many immediate early genes involved in growth and differentiation. Activation of intracellular mGluR5 also up-regulates genes involved in synaptic plasticity including activity-regulated cytoskeletal-associated protein (Arc/Arg3.1). Mechanistically, intracellular mGluR5-mediated Arc induction is dependent upon extracellular and intracellular Ca(2+) and ERK1/2 as well as calmodulin-dependent kinases as known chelators, inhibitors, and a dominant negative Ca(2+)/calmodulin-dependent protein kinase II construct block Arc increases. Moreover, intracellular mGluR5-induced Arc expression requires the serum response transcription factor (SRF) as wild type but not SRF-deficient neurons show this response. Finally, increased Arc levels due to high K(+) depolarization is significantly reduced in response to a permeable but not an impermeable mGluR5 antagonist. Taken together, these data highlight the importance of intracellular mGluR5 in the cascade of events associated with sustained synaptic transmission.

Mode of action of agomelatine: synergy between melatonergic and 5-HT2C receptors

OBJECTIVES

The association between depression and circadian rhythm disturbances is well established and successful treatment of depressed patients is accompanied by restoration of circadian rhythms. The new antidepressant agomelatine is an agonist of melatonergic MT₁/MT₂ receptors as well as an antagonist of serotonergic 5-HT2C receptors. Animal studies showed that agomelatine resynchronizes disturbed circadian rhythms and reduces depression-like behaviour.

METHODS

This review analyzes results from different experimental studies.

RESULTS

Recent data on the effects of agomelatine on cellular processes involved in antidepressant mechanisms have shown that the drug is able to increase the expression of brain-derived neurotrophic factor in prefrontal cortex and hippocampus, as well as the expression of activity-regulated cytoskeleton associated protein (Arc) in the prefrontal cortex. In line with this, prolonged treatment with agomelatine increases neurogenesis within the hippocampus, particularly via enhancement of neuronal cell survival. Agomelatine attenuates stress-induced glutamate release in the prefrontal/frontal cortex. Treatment with 5-HT2C antagonists or melatonin alone failed to reproduce these effects.

CONCLUSIONS

The unique mode of action of agomelatine may improve the management of major depression by counteracting the pathogenesis of depression at cellular level, thereby relieving the symptoms of depression. These effects are suggested to be due to a synergistic action on MT₁/MT₂ and 5-HT2C receptors.

LTP- and LTD-inducing stimulations cause opposite changes in arc/arg3.1 mRNA level in hippocampal area CA1 in vivo

Immediate early genes (IEGs) typically are the first genetic responders to a variety of cellular activations. The IEG that encodes activity-regulated cytoskeleton-associated protein (arc/arg3.1) has attracted much interest because its mRNA is transported to and translated near activated synapses. Moreover, arc has been implicated in both long-term potentiation (LTP) and long-term depression (LTD). However, little is known about the time course of altered arc expression during LTP and LTD. Here we characterized arc mRNA levels in area CA1 of the adult rat hippocampus in vivo after LTP- and LTD-inducing stimulations that were identical, except for the temporal patterning of the stimulation pulses. We observed a persistent increase in arc mRNA level during LTP. In contrast, during LTD, arc mRNA level first was decreased and then transiently increased relative to control level. These findings demonstrate that arc mRNA is regulated differently during LTP and LTD, and they provide evidence for stimulation-induced downregulation of mRNA availability during LTD. Findings of abbreviated LTD when transcription was inhibited indicate that the prolonged maintenance of the type of N-methyl-D-aspartate receptor-dependent LTD studied here requires de novo transcription. Furthermore, lack of evidence for a LTD-associated change in the mRNA level of the IEG zif268 demonstrates that the decrease in arc mRNA during LTD is not a general genetic response. Thus, the regulation of arc expression not only differs between LTP and LTD, but also diverges from that of other IEGs implicated in activity-dependent synaptic plasticity.

A food predictive cue must be attributed with incentive salience for it to induce c-fos mRNA expression in cortico-striatal-thalamic brain regions

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS-US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of "connectivity" between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems-the cue must also be attributed with incentive salience.

Satb1 ablation alters temporal expression of immediate early genes and reduces dendritic spine density during postnatal brain development

Complex behaviors, such as learning and memory, are associated with rapid changes in gene expression of neurons and subsequent formation of new synaptic connections. However, how external signals are processed to drive specific changes in gene expression is largely unknown. We found that the genome organizer protein Satb1 is highly expressed in mature neurons, primarily in the cerebral cortex, dentate hilus, and amygdala. In Satb1-null mice, cortical layer morphology was normal. However, in postnatal Satb1-null cortical pyramidal neurons, we found a substantial decrease in the density of dendritic spines, which play critical roles in synaptic transmission and plasticity. Further, we found that in the cerebral cortex, Satb1 binds to genomic loci of multiple immediate early genes (IEGs) (Fos, Fosb, Egr1, Egr2, Arc, and Bdnf) and other key neuronal genes, many of which have been implicated in synaptic plasticity. Loss of Satb1 resulted in greatly alters timing and expression levels of these IEGs during early postnatal cerebral cortical development and also upon stimulation in cortical organotypic cultures. These data indicate that Satb1 is required for proper temporal dynamics of IEG expression. Based on these findings, we propose that Satb1 plays a critical role in cortical neurons to facilitate neuronal plasticity.

A review of psychostimulant-induced neuroadaptation in developing animals

The effects of clinically relevant doses of commonly prescribed stimulants methylphenidate (MPH), d-amphetamine (d-AMPH), and dl-AMPH or mixed amphetamine salts (MAS) such as Adderall, on short- and long-term gene neuroadaptations in developing animals have not been widely investigated. In the present review, the effects of oral stimulant administration were compared with those of the subcutaneous or intra-peritoneal route. A selective set of studies between 1979 and 2010, which incorporated in their design developmental period, clinically relevant doses of stimulants, and repeated daily doses were reviewed. These studies indicate that neuroadaptation to chronic stimulants includes blunting of stimulated immediate early gene expression, sensitivity of younger (prepubertal) brain to smaller dosages of stimulants, and the persistence of some effects, especially behavioral neuroadaptations, into adulthood. In addition, oral amphetamines (MAS) have more profound effects than does oral MPH. Further animal developmental studies are required to understand potential long-term neuroadaptations to low, daily oral doses of stimulants. Implications for clinical practice were also discussed.

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.

Suppression of activity-regulated cytoskeleton-associated gene expression in the dorsal striatum attenuates extinction of cocaine-seeking

The caudate putamen (CPu) has been implicated in habit learning and neuroadaptive changes that mediate the compulsive nature of drug-seeking following chronic cocaine self-administration. Re-exposure to an operant chamber previously associated with cocaine, but not yoked-saline, increases activity-regulated cytoskeleton-associated (Arc) gene mRNA expression within the dorsolateral (dl) CPu following prolonged abstinence. In this study, we tested the hypothesis that antisense gene knockdown of Arc within the dlCPu would alter cocaine-seeking. Initial studies showed that a single infusion of Arc antisense oligodeoxynucleotide (ODN) into the dlCPu significantly attenuated the induction of Arc mRNA and Arc protein by a single cocaine exposure (20 mg/kg i.p.) compared to scrambled-ODN-infused controls. In cocaine self-administering rats, infusion of Arc antisense ODN into the dlCPu 3 h prior to a test of context-driven drug-seeking significantly attenuated Arc protein induction, but failed to alter responding during testing, suggesting striatal Arc does not facilitate context-induced drug-seeking following prolonged abstinence. However, Arc antisense ODN infusion blunted the decrease in responding during subsequent 1-h extinction tests 24 and 48 h later. Following re-exposure to a cocaine-paired context, surface expression of the AMPA-type glutamate receptor GluR1 was significantly reduced whereas GluR2 was significantly increased in the dlCPu, independent of Arc antisense ODN infusion. Together, these findings indicate an important role for Arc in neuroadaptations within brain regions responsible for drug-seeking after abstinence and direct attention to changes occurring within striatal circuitry that are necessary to break down the habitual behaviour that leads to relapse.

Overlapping role of dynamin isoforms in synaptic vesicle endocytosis

The existence of neuron-specific endocytic protein isoforms raises questions about their importance for specialized neuronal functions. Dynamin, a GTPase implicated in the fission reaction of endocytosis, is encoded by three genes, two of which, dynamin 1 and 3, are highly expressed in neurons. We show that dynamin 3, thought to play a predominantly postsynaptic role, has a major presynaptic function. Although lack of dynamin 3 does not produce an overt phenotype in mice, it worsens the dynamin 1 KO phenotype, leading to perinatal lethality and a more severe defect in activity-dependent synaptic vesicle endocytosis. Thus, dynamin 1 and 3, which together account for the overwhelming majority of brain dynamin, cooperate in supporting optimal rates of synaptic vesicle endocytosis. Persistence of synaptic transmission in their absence indicates that if dynamin plays essential functions in neurons, such functions can be achieved by the very low levels of dynamin 2.

Induction of c-Fos, Zif268, and Arc from acute bouts of voluntary wheel running in new and pre-existing adult mouse hippocampal granule neurons

The functional significance of newly formed granule neurons in the adult mammalian hippocampus remains a mystery. Recently, it was demonstrated that wheel running increases new neuron survival and c-Fos expression in new and pre-existing granule cells in an activity-dependent manner. It is currently unknown whether other immediate early genes (IEGs) become expressed in granule neurons from running. Further, it is unknown whether locomotor activity in home cages without wheels can influence neurogenesis and IEG expression similar to running. The purpose of this study was three-fold: (1) to determine if Arc and Zif268 expression are also induced from wheel running in both pre-existing and newly formed neurons (2) to determine if neurogenesis and IEG induction is related to horizontal distance traveled in home cages without wheels, and (3) to determine whether IEG induction is related to acute bouts of running or chronic effects. Adult C57BL/6J female mice were placed in cages with or without running wheels for 31 days. The first 10 days, mice received daily injections of 5-Bromo-2'-deoxyuridine (BrdU) to label dividing cells. On day 1, running and non-running animals were euthanized either 2 h after peak activity, or during a period of relative inactivity. Immunohistochemistry was performed on hippocampal sections with antibodies against BrdU, mature neuron marker NeuN, c-Fos, Arc, and Zif268. Results demonstrate that Arc, Zif268, and c-Fos are induced from wheel running but not movement in cages without wheels. All IEGs were expressed in new neurons from running. Further, IEGs were induced acutely by running, as increased expression did not continue into the light cycle, a period of relative inactivity. The results suggest that robust movements, like running, are necessary to stimulate IEG expression and neurogenesis. Moreover, results suggest new neurons from running may be processing information about running behavior itself.

Cortical map plasticity improves learning but is not necessary for improved performance

Cortical map plasticity is believed to be a key substrate of perceptual and skill learning. In the current study, we quantified changes in perceptual ability after pairing tones with stimulation of the cholinergic nucleus basalis to induce auditory cortex map plasticity outside of a behavioral context. Our results provide evidence that cortical map plasticity can enhance perceptual learning. However, auditory cortex map plasticity fades over weeks even though tone discrimination performance remains stable. This observation is consistent with recent reports that cortical map expansions associated with perceptual and motor learning are followed by a period of map renormalization without a decrement in performance. Our results indicate that cortical map plasticity enhances perceptual learning, but is not necessary to maintain improved discriminative ability.

Neuronal activity-regulated gene transcription in synapse development and cognitive function

Activity-dependent plasticity of vertebrate neurons allows the brain to respond to its environment. During brain development, both spontaneous and sensory-driven neural activity are essential for instructively guiding the process of synapse development. These effects of neuronal activity are transduced in part through the concerted regulation of a set of activity-dependent transcription factors that coordinate a program of gene expression required for the formation and maturation of synapses. Here we review the cellular signaling networks that regulate the activity of transcription factors during brain development and discuss the functional roles of specific activity-regulated transcription factors in specific stages of synapse formation, refinement, and maturation. Interestingly, a number of neurodevelopmental disorders have been linked to abnormalities in activity-regulated transcriptional pathways, indicating that these signaling networks are critical for cognitive development and function.

Reduced spine density in specific regions of CA1 pyramidal neurons in two transgenic mouse models of Alzheimer's disease

One major hallmark of Alzheimer's disease (AD) is the massive loss of synapses that occurs at an early clinical stage of the disease. In this study, we characterize alterations in spine density and the expression of synapse-associated immediate early gene Arc (activity-regulated cytoskeleton-associated protein) in the hippocampal CA1 regions of two different amyloid precursor protein (APP) transgenic mouse lines before plaque development and their connection to performance in hippocampus-dependent memory tests. The density of mushroom-type spines was reduced by 34% in the basal dendrites proximal to the soma of CA1 pyramidal neurons in 5.5-month-old Tg2576 mice, carrying the Swedish mutation, compared with wild-type littermates. A similar reduction of 42% was confirmed in the same region of 8-month-old APP/Lo mice, carrying the London mutation. In this strain, the reduction extended to the distal dendritic spines (28%), although no differences were found in apical dendrites in either transgenic mouse line. Both transgenic mice lines presented a significant increase in Arc protein expression in CA1 compared with controls, suggesting rather an overactivity and increased spine turnover that was supported by a significant decrease in number of somatostatin-immunopositive inhibitory interneurons in the stratum oriens of CA1. Behaviorally, the transgenic mice showed decrease freezing in the fear contextual conditioning test and impairment in spatial memory assessed by Morris water maze test. These data indicate that cognitive impairment in APP transgenic mice is correlated with impairment of synaptic connectivity in hippocampal CA1, probably attributable to loss of inhibitory interneurons and subsequent hyperactivity.

Effects of voluntary ethanol consumption on emotional state and stress responsiveness in socially isolated rats

Social isolation of rats immediately after weaning is thought to represent an animal model of anxiety-like disorders. This mildly stressful condition reduces the cerebrocortical and plasma concentrations of 3α-hydroxy-5α-pregnan-20-one (3α,5α-TH PROG) as well as increases the sensitivity of rats to the effects of acute ethanol administration on the concentrations of this neuroactive steroid. We further investigated the effects of voluntary consumption of ethanol at concentrations increasing from 2.5 to 10% over 4 weeks of isolation. Isolated rats showed a reduced ethanol preference compared with group-housed animals. Ethanol consumption did not affect the isolation-induced down-regulation of BDNF or Arc, but it attenuated the increase in the cerebrocortical concentration of 3α,5α-TH PROG induced by foot-shock stress in both isolated and group-housed animals as well as increased the percentage of number of entries made by socially isolated rats into the open arms in the elevated plus-maze test. Ethanol consumption did not affect expression of the α₄ subunit of the GABA(A) receptor in the hippocampus of group-housed or isolated rats, whereas it up-regulated the δ subunit throughout the hippocampus under both conditions. The results suggest that low consumption of ethanol may ameliorate some negative effects of social isolation on stress sensitivity and behavior.

Chronic methamphetamine exposure suppresses the striatal expression of members of multiple families of immediate early genes (IEGs) in the rat: normalization by an acute methamphetamine injection

RATIONALE

Repeated injections of cocaine cause blunted responses to acute cocaine challenge-induced increases in the expression of immediate early genes (IEGs).

OBJECTIVES

The aim of this study was to test if chronic methamphetamine (METH) exposure might cause similar blunting of acute METH-induced increases in IEG expression.

RESULTS

Repeated saline or METH injections were given to rats over 14 days. After 1 day of withdrawal, they received a single injection of saline or METH (5 mg/kg). Acute injection of METH increased c-fos, fosB, fra2, junB, Egr1-3, Nr4a1 (Nur77), and Nr4a3 (Nor-1) mRNA levels in the striatum of saline-pretreated rats. Chronic METH treatment alone reduced the expression of AP1, Erg1-3, and Nr4a1 transcription factors below control levels. Acute METH challenge normalized these values in METH-pretreated rats. Unexpectedly, acute METH challenge to METH-pretreated animals caused further decreases in Nr4a2 (Nurr1) mRNA levels. In contrast, the METH challenge caused significant but blunted increases in Nr4a3 and Arc expression in METH-pretreated rats. There were also chronic METH-associated decreases in the expression of cAMP responsive element binding protein (CREB) which modulates IEG expression via activation of the cAMP/PKA/CREB signal transduction pathway. Chronic METH exposure also caused significant decreases in preprotachykinin, but not in prodynorphin, mRNA levels.

CONCLUSIONS

These results support the accumulated evidence that chronic administration of psychostimulants is associated with blunting of their acute stimulatory effects on IEG expression. The METH-induced renormalization of the expression of several IEGs in rats chronically exposed to METH hints to a potential molecular explanation for the recurrent self-administration of the drug by human addicts.

Down-regulation of hippocampal BDNF and Arc associated with improvement in aversive spatial memory performance in socially isolated rats

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting changes in behavioral profile. Such isolation is thought to be anxiogenic for these normally gregarious animals, and the abnormal reactivity of isolated rats to environmental stimuli is thought to be a product of prolonged stress. We now show that isolation of rats at weaning reduced immobility time in the forced swim test, decreased sucrose intake and preference, and down-regulated both brain-derived neurotrophic factor (BDNF) and activity-regulated cytoskeletal associated protein (Arc) in the hippocampus. In the Morris water maze, isolated rats showed a reduced latency to reach the hidden platform during training, indicative of an improved learning performance, compared with group-housed rats. The cumulative search error during place training trials indicated a reliable difference between isolated and group-housed rats on days 4 and 5. The probe trial revealed a significant decrease of the average proximity to the target location in the isolated rats suggesting an improvement in spatial memory. Isolated rats also showed an increase in the plasma level of corticosterone on the 5th day of training and increased expression of BDNF and Arc in the hippocampus on both days 1 and 5. These results show that social isolation from weaning in rats results in development of depressive-like behavior but has a positive effect on spatial learning, supporting the existence of a facilitating effect of stress on cognitive function.

New views of Arc, a master regulator of synaptic plasticity

Many proteins have been implicated in synaptic and experience-dependent plasticity. However, few demonstrate the exquisite regulation of expression and breadth of functional importance as the immediate early gene product Arc. Here we review and attempt to synthesize the disparate views of Arc in neuronal function. The main conclusion garnered from this body of work is that Arc is a critical effector molecule downstream of many molecular signaling pathways and that dysregulation of Arc expression can have dire consequences for normal brain function.

Xenomitochondrial mice: investigation into mitochondrial compensatory mechanisms

Xenomitochondrial mice, harboring evolutionarily divergent Mus terricolor mitochondrial DNA (mtDNA) on a Mus musculus domesticus nuclear background (B6NTac(129S6)-mt(M. terricolor)/Capt; line D7), were subjected to molecular and phenotypic analyses. No overt in vivo phenotype was identified in contrast to in vitro xenomitochondrial cybrid studies. Microarray analyses revealed differentially expressed genes in xenomitochondrial mice, though none were directly involved in mitochondrial function. qRT-PCR revealed upregulation of mt-Co2 in xenomitochondrial mice. These results illustrate that cellular compensatory mechanisms for mild mitochondrial dysfunction alter mtDNA gene expression at a proteomic and/or translational level. Understanding these mechanisms will facilitate the development of therapeutics for mitochondrial disorders.

α7 nicotinic receptor agonism mitigates phencyclidine-induced changes in synaptophysin and Arc gene expression in the mouse prefrontal cortex

Repeated phencyclidine (PCP) administration in mice reproduces several histopathological features of schizophrenia, such as reduced synaptophysin and parvalbumin mRNA expression in the frontal cortex. These changes can be prevented by co-administering the α7 nicotinic acetylcholine receptor (nAChR) agonist SSR180711 with PCP, but it is not known to what extent PCP-induced changes can be normalized once they have already occurred. Here we use semi-quantitative in situ hybridization to show that repeated administration of SSR180711 (3 mg/kg b.i.d. for 5 days) subsequent to repeated PCP administration (10 mg/kg/day for 10 days) is able to mitigate the reduction of synaptophysin mRNA expression induced by PCP in two prefrontal cortical regions, the medial prefrontal cortex (mPFC) and the ventrolateral orbitofrontal cortex (VLO). This effect is accompanied by a normalization of the PCP-induced increase in Arc mRNA expression in the same regions. In contrast, subsequent administration of SSR180711 does not affect PCP-induced decreases in parvalbumin mRNA in the mPFC, and glutamate decarboxylase 67 mRNA in the mPFC or VLO. These data demonstrate that it is possible to restore some, but not all, of the molecular dysregulations induced by repeated PCP administration with an α7 nAChR agonist. They also suggest that the previously demonstrated cognitive improvement with SSR180711 subsequent to PCP treatment does not require normalization of parvalbumin expression, but may instead be related to a restoration of synaptophysin and/or Arc levels in the frontal cortex. These data lend support to the potential for development of α7 nAChR agonists for the treatment of cognitive deficits in schizophrenia.

Context-driven cocaine-seeking in abstinent rats increases activity-regulated gene expression in the basolateral amygdala and dorsal hippocampus differentially following short and long periods of abstinence

In this study, the expression patterns of zif268 and activity-regulated cytoskeleton-associated gene (arc) were investigated in the basolateral amygdala (BLA) and dorsal hippocampal (dHPC) subregions during context-induced drug-seeking following 22 h or 15 d abstinence from cocaine self-administration. Arc and zif/268 mRNA in BLA and dHPC increased after re-exposure to the cocaine-paired chamber at both timepoints; however, only the BLA increases (with one exception-see below) were differentially affected by the presence or absence of the cocaine-paired lever in the chamber. Following 22 h of abstinence, arc mRNA was significantly increased in the BLA of cocaine-treated rats re-exposed to the chamber only with levers extended, whereas following 15 d of abstinence, arc mRNA in the BLA was increased in cocaine-treated rats returned to the chamber with or without levers extended. In contrast, zif268 mRNA in the BLA was greater in cocaine-treated rats returned to the chamber with levers extended vs. levers retracted only after 15 d of abstinence. In the dentate gyrus (DG) following 22 h of abstinence, zif268 mRNA was greater in rats returned to the chamber where levers were absent regardless of drug treatment whereas arc mRNA was increased in CA1 (cell bodies and dendrites) and CA3 only in cocaine-treated groups. Following 15 d of abstinence, arc mRNA was significantly greater in CA1 and CA3 of both cocaine-treated groups returned to the chamber than in those placed into a familiar, non-salient alternate environment; however, only in CA1 cell bodies the cocaine context-induced increases significantly greater than in yoked-saline controls. In contrast, zif/268 mRNA in all dHPC regions was significantly greater in both cocaine-treated groups returned to the cocaine context than in the cocaine-treated group returned to an alternative environment or saline-treated groups. These data suggest that the temporal dynamics of arc and zif268 gene expression in the BLA and dHPC encode different key elements of drug context-induced cocaine-seeking.

Comparative analysis of cis-regulation following stroke and seizures in subspaces of conserved eigensystems

BACKGROUND

It is often desirable to separate effects of different regulators on gene expression, or to identify effects of the same regulator across several systems. Here, we focus on the rat brain following stroke or seizures, and demonstrate how the two tasks can be approached simultaneously.

RESULTS

We applied SVD to time-series gene expression datasets from the rat experimental models of stroke and seizures. We demonstrate conservation of two eigensystems, reflecting inflammation and/or apoptosis (eigensystem 2) and neuronal synaptic activity (eigensystem 3), between the stroke and seizures. We analyzed cis-regulation of gene expression in the subspaces of the conserved eigensystems. Bayesian networks analysis was performed separately for either experimental model, with cross-system validation of the highest-ranking features. In this way, we correctly re-discovered the role of AP1 in the regulation of apoptosis, and the involvement of Creb and Egr in the regulation of synaptic activity-related genes. We identified a novel antagonistic effect of the motif recognized by the nuclear matrix attachment region-binding protein Satb1 on AP1-driven transcriptional activation, suggesting a link between chromatin loop structure and gene activation by AP1. The effects of motifs binding Satb1 and Creb on gene expression in brain conform to the assumption of the linear response model of gene regulation. Our data also suggest that numerous enhancers of neuronal-specific genes are important for their responsiveness to the synaptic activity.

CONCLUSION

Eigensystems conserved between stroke and seizures separate effects of inflammation/apoptosis and neuronal synaptic activity, exerted by different transcription factors, on gene expression in rat brain.

Regulation of markers of synaptic function in mouse models of depression: chronic mild stress and decreased expression of VGLUT1

Depression has been linked to failure in synaptic plasticity originating from environmental and/or genetic risk factors. The chronic mild stress model regulates the expression of synaptic markers of neurotransmitter function and associated depressive-like behaviour. Moreover, mice heterozygous for the synaptic vesicle protein vesicular glutamate transporter 1 (VGLUT1), have been proposed as a genetic model of deficient glutamate function linked to depressive-like behaviour. Here, we aimed to identify, in these two experimental models, mechanisms of failure in synaptic plasticity, common to stress and impaired glutamate function. First, we show that chronic mild stress induced a transient decrease of different plasticity markers (VGLUT1, synapsin 1, sinaptophysin, rab3A and activity regulated cytoskeletal protein - Arc) but a long-lasting decrease of the brain derived neurotrophic factor as well as depressive-like behaviour. The immediate early gene Arc was also down-regulated in VGLUT1+/- heterozygous mice. In contrast, an opposite regulation of synapsin 1 was observed. Finally, both models showed a marked increase of cortical Arc response to novelty. Increased Arc response to novelty could be suggested as a molecular mechanism underlying failure to adapt to environmental changes, common to chronic stress and altered glutamate function. Further studies should investigate whether these changes are associated to depressive-like behaviour both in animal models and in depressed patients.

Repeated administration of alpha7 nicotinic acetylcholine receptor (nAChR) agonists, but not positive allosteric modulators, increases alpha7 nAChR levels in the brain

The alpha7 nicotinic acetylcholine receptor (nAChR) is an important target for treatment of cognitive deficits in schizophrenia and Alzheimer's disease. However, the receptor desensitizes rapidly in vitro, which has led to concern regarding its applicability as a clinically relevant drug target. Here we investigate the effects of repeated agonism on alpha7 nAChR receptor levels and responsiveness in vivo in rats. Using [(125)I]-alpha-bungarotoxin (BTX) autoradiography we show that acute or repeated administration with the selective alpha7 nAChR agonist A-582941 increases the number of alpha7 nAChR binding sites in several brain regions, particularly in the prefrontal cortex. The alpha7 nAChR agonists SSR180711 and PNU-282987 also increase [(125)I]-BTX binding, suggesting that this is a general consequence of alpha7 nAChR agonism. Interestingly, the alpha7 nAChR positive allosteric modulators PNU-120596 and NS1738 do not increase [(125)I]-BTX binding. Furthermore, A-582941-induced increase in Arc and c-fos mRNA expression in the prefrontal cortex is enhanced and unaltered, respectively, after repeated administration, demonstrating that the alpha7 nAChRs remain responsive. Contrarily, A-582941-induced phosphorylation of Erk2 in the prefrontal cortex occurs following acute, but not repeated administration. Our results demonstrate that repeated agonist administration increases the number of alpha7 nAChRs in the brain, and leads to coupling versus uncoupling of specific intracellular signaling pathways. Additionally, our data suggest a fundamental difference between the sequelae of repeated administration with agonists and allosteric modulators of the alpha7 nAChR.

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.

Unraveling mechanisms of homeostatic synaptic plasticity

Homeostatic synaptic plasticity is a negative feedback mechanism that neurons use to offset excessive excitation or inhibition by adjusting their synaptic strengths. Recent findings reveal a complex web of signaling processes involved in this compensatory form of synaptic strength regulation, and in contrast to the popular view of homeostatic plasticity as a slow, global phenomenon, neurons may also rapidly tune the efficacy of individual synapses on demand. Here we review our current understanding of cellular and molecular mechanisms of homeostatic synaptic plasticity.

Depression-prone mice with reduced glucocorticoid receptor expression display an altered stress-dependent regulation of brain-derived neurotrophic factor and activity-regulated cytoskeleton-associated protein

Increasing evidence suggests that depression is characterised by impaired brain plasticity that might originate from the interaction between genetic and environmental risk factors. Hence, the aim of this study was to investigate changes in neuroplasticity following exposure to stress, an environmental condition highly relevant to psychiatric disorders, in glucocorticoid receptor-deficient mice (GR(+/-)), a genetic model of predisposition to depression. Specifically, we have analysed the neurotrophin brain-derived neurotrophic factor (BDNF) and the immediate-early gene activity-regulated cytoskeletal-associated protein (Arc), two closely related molecules that can contribute to neuroplastic and morphological changes observed in depression. We found a region-specific influence of the GR-genotype on BDNF levels both under basal and stress conditions. Steady-state levels of BDNF mRNA were unchanged in hippocampus while up-regulated in frontal lobe of GR(+/-) mice. Following exposure to an acute stress, increased processing from pro- to mature BDNF was observed in hippocampal synaptosomes of wild-type mice, but not in GR mutants. Furthermore, the stress-dependent modulation of Arc was impaired in the hippocampus of GR(+/-) mice. These results indicate that GR(+/-) mice show overt differences in the stress-induced modulation of neuroplastic proteins, which may contribute to pathologic conditions that may originate following gene x environment interaction.

RNA localization in neurite morphogenesis and synaptic regulation: current evidence and novel approaches

It is now generally accepted that RNA localization in the central nervous system conveys important roles both during development and in the adult brain. Of special interest is protein synthesis located at the synapse, as this potentially confers selective synaptic modification and has been implicated in the establishment of memories. However, the underlying molecular events are largely unknown. In this review, we will first discuss novel findings that highlight the role of RNA localization in neurons. We will focus on the role of RNA localization in neurotrophin signaling, axon outgrowth, dendrite and dendritic spine morphogenesis as well as in synaptic plasticity. Second, we will briefly present recent work on the role of microRNAs in translational control in dendrites and its implications for learning and memory. Finally, we discuss recent approaches to visualize RNAs in living cells and their employment for studying RNA trafficking in neurons.

Gene expression changes in the medial prefrontal cortex and nucleus accumbens following abstinence from cocaine self-administration

Background

Many studies of cocaine-responsive gene expression have focused on changes occurring during cocaine exposure, but few studies have examined the persistence of these changes with cocaine abstinence. Persistent changes in gene expression, as well as alterations induced during abstinence may underlie long-lasting drug craving and relapse liability.

Results

Whole-genome expression analysis was conducted on a rat cocaine binge-abstinence model that has previously been demonstrated to engender increased drug seeking and taking with abstinence. Gene expression changes in two mesolimbic terminal fields (mPFC and NAc) were identified in a comparison of cocaine-naïve rats with rats after 10 days of cocaine self-administration followed by 1, 10, or 100 days of enforced abstinence (n = 6-11 per group). A total of 1,461 genes in the mPFC and 414 genes in the NAc were altered between at least two time points (ANOVA, p < 0.05; ± 1.4 fold-change). These genes can be subdivided into: 1) changes with cocaine self-administration that do not persist into periods of abstinence, 2) changes with cocaine self-administration that persist with abstinence, 3) and those not changed with cocaine self-administration, but changed during enforced abstinence. qPCR analysis was conducted to confirm gene expression changes observed in the microarray analysis.

Conclusions

Together, these changes help to illuminate processes and networks involved in abstinence-induced behaviors, including synaptic plasticity, MAPK signaling, and TNF signaling.

IL-1RA blocks E. coli-induced suppression of Arc and long-term memory in aged F344xBN F1 rats

In normal aging, a peripheral immune challenge induces a sensitized and protracted neuroinflammatory response in parallel with long-term memory (LTM) impairments. Pro-inflammatory mediators of neuroinflammation impair LTM, synaptic plasticity and LTP. The immediate early gene Arc is considered a critical protein regulating LTM and synaptic plasticity. The present investigation examined whether (1) a peripheral Escherichia coli infection suppresses hippocampal Arc expression, and (2) central pro-inflammatory cytokines (IL-1beta and IL-6) mediate the effects of peripheral E. coli infection on Arc and LTM. In 24 months F344xBN F1 rats, E. coli infection suppressed basal Arc gene expression as well as contextual fear conditioning-induced Arc expression. E. coli treatment failed to alter either basal or conditioning-induced c-Fos expression. At 24h post-infection, intra-cisterna magna (ICM) treatment with the anti-inflammatory cytokine IL-1RA blocked the E. coli-induced suppression of hippocampal Arc and increases in IL-6 protein. At 4-day post-infection, IL-1RA blocked the E. coli-induced LTM impairments and increases in IL-6 protein. The present results suggest that central pro-inflammatory cytokines play a salient role in the suppression of Arc and impairments of LTM by a peripheral immune challenge in older animals.

Sequential steps underlying neuronal plasticity induced by a transient exposure to gabazine

Periods of intense electrical activity can initiate neuronal plasticity leading to long lasting changes of network properties. By combining multielectrode extracellular recordings with DNA microarrays, we have investigated in rat hippocampal cultures the temporal sequence of events of neuronal plasticity triggered by a transient exposure to the GABA(A) receptor antagonist gabazine (GabT). GabT induced a synchronous bursting pattern of activity. The analysis of electrical activity identified three main phases during neuronal plasticity induced by GabT: (i) immediately after termination of GabT, an early synchronization (E-Sync) of the spontaneous electrical activity appears that progressively decay after 3-6 h. E-Sync is abolished by inhibitors of the ERK1/2 pathway but not by inhibitors of gene transcription; (ii) the evoked response (induced by a single pulse of extracellular electrical stimulation) was maximally potentiated 3-10 h after GabT (M-LTP); and (iii) at 24 h the spontaneous electrical activity became more synchronous (L-Sync). The genome-wide analysis identified three clusters of genes: (i) an early rise of transcription factors (Cluster 1), primarily composed by members of the EGR and Nr4a families, maximally up-regulated 1.5 h after GabT; (ii) a successive up-regulation of some hundred genes, many of which known to be involved in LTP (Cluster 2), 3 h after GabT likely underlying M-LTP. Moreover, in Cluster 2 several genes coding for K(+) channels are down-regulated at 24 h. (iii) Genes in Cluster 3 are up-regulated at 24 h and are involved in cellular homeostasis. This approach allows relating different steps of neuronal plasticity to specific transcriptional profiles.

Altered expression and modulation of activity-regulated cytoskeletal associated protein (Arc) in serotonin transporter knockout rats

A gene variant in the human serotonin transporter (SERT) can increase the vulnerability to mood disorders. SERT knockout animals show similarities to the human condition and represent an important tool to investigate the mechanisms underlying the pathologic condition in humans. Along this line of thinking, we used SERT KO rats (SERT(+/-) and SERT(-/-)) to investigate abnormalities in the expression and function of the activity-regulated gene Arc (Activity-regulated cytoskeletal associated protein) and the early inducible gene Zif-268, (zinc finger binding protein clone 268), which are important players in neuronal plasticity. We found lower basal Arc mRNA levels in hippocampus and prefrontal cortex of mutant rats in comparison with wild-type animals. Moreover SERT mutant rats show altered stress responsiveness. Indeed an acute swim stress significantly up-regulated the levels of Arc mRNA in hippocampus and prefrontal cortex, as well as of Zif-268 in frontal cortex, only in SERT(+/-) and SERT(-/-) rats. These alterations may be associated to behavioral traits linked to SERT and may contribute to the neuroplastic and morphological changes observed in depression.

The function of activity-regulated genes in the nervous system

The mammalian brain is plastic in the sense that it shows a remarkable capacity for change throughout life. The contribution of neuronal activity to brain plasticity was first recognized in relation to critical periods of development, when manipulating the sensory environment was found to profoundly affect neuronal morphology and receptive field properties. Since then, a growing body of evidence has established that brain plasticity extends beyond development and is an inherent feature of adult brain function, spanning multiple domains, from learning and memory to adaptability of primary sensory maps. Here we discuss evolution of the current view that plasticity of the adult brain derives from dynamic tuning of transcriptional control mechanisms at the neuronal level, in response to external and internal stimuli. We then review the identification of "plasticity genes" regulated by changes in the levels of electrical activity, and how elucidating their cellular functions has revealed the intimate role transcriptional regulation plays in fundamental aspects of synaptic transmission and circuit plasticity that occur in the brain on an every day basis.

CSF biomarkers: pinpointing Alzheimer pathogenesis

Intense research during the last decades has resulted in an unprecedented accumulation of knowledge regarding the pathogenesis of Alzheimer's disease. Primarily, the focus has been directed toward amyloid and tau pathology and their relations to synaptic and neuronal loss. However, as the complexity of the disease becomes increasingly evident, the importance of other factors, such as inflammation, oxidative stress, and mitochondrial dysfunction, grow apparent. Here, we review available CSF biomarkers for these pathological processes. We also consider their usability in clinical practice and in clinical trials.

Opposite effect of phencyclidine on activity-regulated cytoskeleton-associated protein (Arc) in juvenile and adult limbic rat brain regions

The psychotomimetic effect of NMDA antagonists such as phencyclidine (PCP) in humans spurred the hypoglutamatergic theory of schizophrenia. This theory is supported by animal studies demonstrating schizophrenia-like behavioral and molecular changes following PCP administration to adult or neonatal animals. However, schizophrenia is believed to develop in part due to neurodevelopmental dysfunction during adolescence. Therefore, the effects of PCP in juvenile animals may better reflect the pathophysiology of schizophrenia. Here, we compare the effect of PCP (5mg/kg/day for 5 days) on activity-regulated cytoskeleton-associated protein (Arc) and parvalbumin mRNA expression in juvenile and adult rats. Arc is a marker for excitatory neurotransmission. Parvalbumin is a marker for GABAergic neurotransmission, known to be reduced in postmortem brains of schizophrenics. PCP reduced parvalbumin mRNA expression in the medial prefrontal cortex (mPFC), ventrolateral orbitofrontal cortex (VLO) and shell of the nucleus accumbens (ACCshell) in both juvenile and adult rats. Contrarily, PCP produced opposite effects on Arc mRNA expression in the mPFC, VLO and ACCshell, leading to decreased expression in juvenile and increased expression in adult rats. The differential effect of PCP in juvenile and adult rats may be caused by the immature functional state of the prefrontal cortex in juvenile rats. These results demonstrate differences between the effects of PCP in juvenile and adult rats. The decrease in Arc mRNA in juvenile rats corresponds best with the proposed "hypofrontality" in schizophrenia, suggesting the merits of using PCP in juvenile animals as a model for schizophrenia, as this would relate better to the typical onset and clinical features of schizophrenia.

Temporal gene expression profile of the hippocampus following trace fear conditioning

In this paper we report the results of gene expression profiling of C57Bl/6N mice hippocampus after trace fear conditioning (TFC), and the identification of genes regulated at early and late steps after conditioning. Several of the genes regulated at early steps following TFC appeared common to many training protocols. At later stages (2 and 6 h), most of the genes identified were different from those identified following other learning paradigms resulting in memory consolidation. At 6 h after training, few genes were upregulated in respect to the naïve condition, suggesting that many gene products have eventually to be downregulated to achieve stable synapses modification and memory formation. In conclusion, the results presented highlight a number of genes whose expression is specifically modified in the mouse hippocampus following TFC and demonstrate the specificity associated to different forms of conditioning.

Molecular and cellular approaches to memory allocation in neural circuits

Although memory allocation is a subject of active research in computer science, little is known about how the brain allocates information within neural circuits. There is an extensive literature on how specific types of memory engage different parts of the brain, and how neurons in these regions process and store information. Until recently, however, the mechanisms that determine how specific cells and synapses within a neural circuit (and not their neighbors) are recruited during learning have received little attention. Recent findings suggest that memory allocation is not random, but rather specific mechanisms regulate where information is stored within a neural circuit. New methods that allow tagging, imaging, activation, and inactivation of neurons in behaving animals promise to revolutionize studies of brain circuits, including memory allocation. Results from these studies are likely to have a considerable impact on computer science, as well as on the understanding of memory and its disorders.