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

Decrease in the kainate-induced wet dog shake behavior in genetically epilepsy-prone rats: possible involvement of an impaired synaptic transmission to the 5-HT(2A) receptor

Genetically epilepsy-prone rats (GEPR-9s) were derived from Sprague-Dawley rats (SD). The number of kainate-induced wet dog shake behavior (WDS) responses was found to decrease significantly in GEPR-9s compared to SD. WDS responses were potentiated by 5-hydroxytryptophan or 2,5-dimethoxy-4-iodoamphetamine and antagonized by ritanserin. The antagonizing effect of ritanserin on WDS latency was more evident in GEPR-9s than in SD, and hippocampal expression of activity-regulated cytoskeleton-associated protein paralleled the severity of WDS. The results suggest that downstream serotonergic synaptic activation is less pronounced in GEPR-9s than in SD and that the serotonergic agent may directly activate postsynaptic 5-HT2A receptors in both strains.

The state of synapses in fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading genetic cause of autism. There is increasing evidence in both FXS and other forms of autism that alterations in synapse number, structure, and function are associated and contribute to these prevalent diseases. FXS is caused by loss of function of the Fmr1 gene, which encodes the RNA binding protein, fragile X mental retardation protein (FMRP). Therefore, FXS is a tractable model to understand synaptic dysfunction in cognitive disorders. FMRP is present at synapses where it associates with mRNA and polyribosomes. Accumulating evidence finds roles for FMRP in synapse development, elimination, and plasticity. Here, the authors review the synaptic changes observed in FXS and try to relate these changes to what is known about the molecular function of FMRP. Recent advances in the understanding of the molecular and synaptic function of FMRP, as well as the consequences of its loss, have led to the development of novel therapeutic strategies for FXS.

The effects of exercise and stress on the survival and maturation of adult-generated granule cells

Stress strongly inhibits proliferation of granule cell precursors in the adult dentate gyrus, whereas voluntary running has the opposite effect. Few studies, however, have examined the possible effects of these environmental manipulations on the maturation and survival of young granule cells. We examined the number of surviving granule cells and the proportion of young neurons that were functionally mature, as defined by seizure-induced immediate-early gene (IEG) expression, in 14- and 21-day-old granule cells in mice that were given access to a running wheel, restrained daily for 2 h, or given no treatment during this period. Treatments began 2 days after BrdU injection, to isolate effects on survival from those on cell proliferation. We found a large increase in granule cell survival in running mice when compared with controls at both time points. In addition, running increased the proportion of granule cells expressing the IEG Arc in response to seizures, suggesting that it speeds incorporation into circuits, i.e., functional maturation. Stressed mice showed no change in Arc expression, compared with control animals, but, surprisingly, showed a transient increase in survival of 14-day-old granule cells, which was gone 7 days later. Examination of cell proliferation, using the endogenous mitotic marker PCNA showed an increase in cell proliferation after 12 days of running but not after 19 days of running. The number of proliferating cells was unchanged 24 h after the 12th or 19th episode of daily restraint stress. These findings demonstrate that running has strong effects on survival and maturation of young granule cells as well as their birth and that stress can have positive but short-lived effects on granule cell survival. Published 2009 Wiley-Liss, Inc.

A transcription-dependent increase in miniature EPSC frequency accompanies late-phase plasticity in cultured hippocampal neurons

Background

The magnitude and longevity of synaptic activity-induced changes in synaptic efficacy is quantified by measuring evoked responses whose potentiation requires gene transcription to persist for more than 2-3 hours. While miniature EPSCs (mEPSCs) are also increased in amplitude and/or frequency during long-term potentiation (LTP), it is not known how long such changes persist or whether gene transcription is required.

Results

We use whole-cell patch clamp recordings from dissociated hippocampal cultures to characterise for the first time the persistence and transcription dependency of mEPSC upregulation during synaptic potentiation. The persistence of recurrent action potential bursting in these cultures is transcription-, translation- and NMDA receptor-dependent thus providing an accessible model for long-lasting plasticity. Blockade of GABA_A-receptors with bicuculline for 15 minutes induced action potential bursting in all neurons and was maintained in 50-60% of neurons for more than 6 hours. Throughout this period, the frequency but neither the amplitude of mEPSCs nor whole-cell AMPA currents was markedly increased. The transcription blocker actinomycin D abrogated, within 2 hours of burst induction, both action potential bursting and the increase in mEPSCs. Reversible blockade of action potentials during, but not after this 2 hour transcription period suppressed the increase in mEPSC frequency and the recovery of burst activity at a time point 6 hours after induction.

Conclusion

These results indicate that increased mEPSC frequency persists well beyond the 2 hour transcription-independent phase of plasticity in this model. This long-lasting mEPSC upregulation is transcription-dependent and requires ongoing action potential activity during the initial 2 hour period but not thereafter. Thus mEPSC upregulation may underlie the long term, transcription-dependent persistence of action potential bursting. This provides mechanistic insight to link gene candidates already identified by gene chip analysis to long lasting plasticity in this in vitro model.

Intrinsic neuronal excitability is reversibly altered by a single experience in fear conditioning

Learning is known to cause alterations in intrinsic cellular excitability but, to date, these changes have been seen only after multiple training trials. A powerful learning task that can be quickly acquired and extinguished with a single trial is fear conditioning. Rats were trained and extinguished on a hippocampus-dependent form of fear conditioning to determine whether learning-related changes in intrinsic excitability could be observed after a few training trials and a single extinction trial. Following fear training, hippocampal slices were made and intrinsic excitability was assayed via whole cell recordings from CA1 neurons. Alterations in intrinsic excitability, assayed by the postburst afterhyperpolarization and firing frequency accommodation, were observed after only three trials of contextual or trace-cued fear conditioning. Animals that had been trained in contextual and trace-cued fear were then extinguished. Context fear-conditioned animals extinguished in a single trial and the changes in intrinsic excitability were reversed. Trace-cue conditioned animals only partially extinguished in a single trial and reductions in excitability remained. Thus a single learning experience is sufficient to alter intrinsic excitability. This dramatically extends observations of learning-specific changes in intrinsic neuronal excitability previously observed in paradigms requiring many training trials, suggesting the excitability changes have a basic role in acquiring new information.

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.

Fluorescent Arc/Arg3.1 indicator mice: a versatile tool to study brain activity changes in vitro and in vivo

The brain-specific immediate early gene Arc/Arg3.1 is induced in response to a variety of stimuli, including sensory and behavior-linked neural activity. Here we report the generation of transgenic mice, termed TgArc/Arg3.1-d4EGFP, expressing a 4-h half-life form of enhanced green fluorescent protein (d4EGFP) under the control of the Arc/Arg3.1 promoter. We show that d4EGFP-mediated fluorescence faithfully reports Arc/Arg3.1 induction in response to physiological, pathological and pharmacological stimuli, and that this fluorescence permits electrical recording from activated neurons in the live mouse. Moreover, the fluorescent Arc/Arg3.1 indicator revealed activity changes in circumscribed brain areas in distinct modes of stress and in a mouse model of Alzheimer's disease. These findings identify the TgArc/Arg3.1-d4EGFP mouse as a versatile tool to monitor Arc/Arg3.1 induction in neural circuits, both in vitro and in vivo.

Amyloid beta-derived neuroplasticity in bone marrow-derived mesenchymal stem cells is mediated by NPY and 5-HT2B receptors via ERK1/2 signalling pathways

OBJECTIVE

In Alzheimer's disease, toxic soluble and insoluble forms of amyloid beta (Abeta) cause synaptic dysfunction and neuronal loss. Given its potential role in producing a toxic host microenvironment for transplanted donor stem cells, we investigated the interaction between Abeta and proliferation, survival, and differentiation of bone marrow-derived mesenchymal stem cells (BM-MSC) in culture.

MATERIALS AND METHODS

We used BM-MSC that had been isolated from mouse bone marrow and cultured, and we also assessed relevant reaction mechanisms using gene microarray, immunocytochemistry, and inhibitors of potential signalling molecules, such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK)1/2 and tyrosine protein kinase.

RESULTS AND CONCLUSIONS

Interestingly, we found that treatment with aggregated (1-40 or 1-42) and oligomeric (1-42) Abeta promoted neuronal-like differentiation of BM-MSC without toxic effects. This was not dependent on soluble factors released from BM-MSC progeny nor solely on formation of Abeta fibrils. The effect of Abeta is mediated by G-protein coupled receptors, neuropeptide Y1 (NPY1R) and serotonin (5-hydroxytryptamine) receptor 2B, via phosphatidylinositol-3-OH kinase-dependent activation of the MAPK/ERK1/2. Our results lend support to the idea that reciprocal donor stem cell-host interactions may promote a regenerative response that can be exploited by epigenetic modulation of NPY/serotonergic gene expression, for stem cell therapy, in Alzheimer's disease.

Differential regulation of psychostimulant-induced gene expression of brain derived neurotrophic factor and the immediate-early gene Arc in the juvenile and adult brain

Psychostimulant drugs are widely used in children for the treatment of attention-deficit/hyperactivity disorder. Recent animal studies have suggested that exposure to these agents in early life could be detrimental to brain development. Here, for the first time, the effect of methylphenidate (MPH) and D-amphetamine (AMPH) on the expression of two key genes for neuronal development and plasticity, brain-derived neurotrophic factor (bdnf) and the effector immediate early gene activity-regulated, cytoskeletal-associated protein (Arc), was examined in both juvenile and adult rats. Both MPH [2 mg/kg, intraperitoneal (i.p.)] and AMPH (0.5 mg/kg, i.p.) induced marked decreases of bdnf mRNA in hippocampal and cortical brain regions of juveniles, whereas effects in adults were significantly less (hippocampus) or opposite (frontal cortex). In comparison, Arc mRNA was decreased (hippocampus and parietal cortex), largely unaffected (frontal cortex) or increased (striatum) in juveniles, whereas in adults, Arc mRNA increased in most brain regions. MPH-induced locomotion was also measured, and showed a much smaller increase in juveniles than in adults. In summary, our data show that the effects of MPH and AMPH on expression of the neurodevelopmentally important genes, bdnf and Arc, differ markedly in juvenile and adult rats, with juveniles showing evidence of brain region-specific decreases in both genes. These age-dependent effects on gene expression may be linked with the reported long-term harmful effects of psychostimulants in animal models.

Global view of the mechanisms of improved learning and memory capability in mice with music-exposure by microarray

Music has been proved beneficial to improve learning and memory in many species including human in previous research work. Although some genes have been identified to contribute to the mechanisms, it is believed that the effect of music is manifold, behind which must concern a complex regulation network. To further understand the mechanisms, we exposed the mice to classical music for one month. The subsequent behavioral experiments showed improvement of spatial learning capability and elevation of fear-motivated memory in the mice with music-exposure as compared to the naïve mice. Meanwhile, we applied the microarray to compare the gene expression profiles of the hippocampus and cortex between the mice with music-exposure and the naïve mice. The results showed approximately 454 genes in cortex (200 genes up-regulated and 254 genes down-regulated) and 437 genes in hippocampus (256 genes up-regulated and 181 genes down-regulated) were significantly affected in music-exposing mice, which mainly involved in ion channel activity and/or synaptic transmission, cytoskeleton, development, transcription, hormone activity. Our work may provide some hints for better understanding the effects of music on learning and memory.

Single session of cocaine intravenous self-administration shapes goal-oriented behaviours and up-regulates Arc mRNA levels in rat medial prefrontal cortex

To separate the direct pharmacological effects of cocaine from those associated with active drug self-administration we employed a yoked control-operant paradigm and investigated the expression of well established markers of the rapid action of cocaine, i.e. the inducible early genes Arc and Zif268 and trophic factors, i.e. brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (FGF-2), in rats after a single intravenous (i.v.) cocaine self-administration session. Animals self-administering cocaine (SA, 0.25 mg/0.1 ml saline per infusion, 2-h session) did more active lever-presses than yoked-cocaine (YC) and yoked-vehicle (YV) animals. This goal-oriented behaviour was accompanied by a selective increase in Arc mRNA levels in the medial prefrontal cortex (mPFC). There were no changes in the expression of the other genes in this brain region. mRNA levels of Arc and Zif268 in striatum and Zif268 in the nucleus accumbens markedly increased both in SA and YC animals; but there was no change in the expression of FGF-2 and BDNF. No changes were observed in hippocampus, hypothalamus, frontal cortex, and midbrain in SA and YC animals compared to YV animals in any of the genes. These findings demonstrate that a single session of cocaine i.v. self-administration is sufficient to shape rat behaviour towards goal-directed behaviours and selectively up-regulate Arc expression in mPFC (of SA animals), providing the first evidence that the mPFC's function is already profoundly influenced by the first voluntary cocaine exposure.

The brain expression of genes involved in inflammatory response, the ribosome, and learning and memory is altered by centrally injected lipopolysaccharide in mice

Neuroinflammation plays a role in the progression of several neurodegenerative disorders. We used a lipopolysaccharide (LPS) model of neuroinflammation to characterize the gene expression changes underlying the inflammatory and behavioral effects of neuroinflammation. A single intracerebroventricular injection of LPS (5 microg) was administered into the lateral ventricle of mice and, 24 h later, we examined gene expression in the cerebral cortex and hippocampus using microarray technology. Gene Ontology (GO) terms for inflammation and the ribosome were significantly enriched by LPS, whereas GO terms associated with learning and memory had decreased expression. We detected 224 changed transcripts in the cerebral cortex and 170 in the hippocampus. Expression of Egr1 (also known as Zif268) and Arc, two genes associated with learning and memory, was significantly lower in the cortex, but not in the hippocampus, of LPS-treated animals. Overall, altered expression of these genes may underlie some of the inflammatory and behavioral effects of neuroinflammation.

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

Effects of repeated treatment of phencyclidine on cognition and gene expression in C57BL/6 mice

A number of studies indicate that glutamatergic N-methyl-D-aspartate (NMDA) neurotransmission is disturbed in schizophrenia partly based on the findings that NMDA receptor antagonists such as phencyclidine (PCP) can reproduce a schizophrenia-like syndrome in both humans and rodents. This study investigated whether repeated administration of low doses of PCP can induce cognitive dysfunctions in mice at doses which produce no sensorimotor disturbances. In addition, the effects on cognition were related to the expression of two genes, Arc and spinophilin, which have been related to neuronal plasticity and learning. Adult male C57Bl/6J mice received daily s.c. doses of PCP (0.5-2.0 mg/kg) or saline for 7 d. Testing was performed 24 h after the last day of treatment. Only the 2.0 mg/kg PCP dose produced a consistent impairment in spatial learning and working memory performed in the water-maze task without any apparent sensorimotor deficits. Importantly, the 2.0 mg/kg PCP dose produced no impairment in a non-spatial learning paradigm in the water-maze task. PCP treatment altered Arc mRNA levels in the hippocampus and retrosplenial agranular cortex while leaving the striatum and prefrontal cortex unaffected. The mRNA expression of spinophilin was down-regulated in striatum by repeated PCP treatment. These results demonstrate that repeated treatment with low doses of PCP in mice can produce specific cognitive deficits which are associated with alterations in gene expression in brain regions that appear to play a role in the pathophysiology of schizophrenia. These results suggest that the low-dose PCP model may have significant potential in characterizing the behavioural and molecular mechanisms underlying cognitive deficits seen in schizophrenia patients.

Expression pattern of neural synaptic plasticity marker-Arc in different brain regions induced by conditioned drug withdrawal from acute morphine-dependent rats

AIM

The immediate early gene Arc (activity-regulated cytoskeletal-associated protein) mRNA and protein are induced by strong synaptic activation and rapidly transported into dendrites, where they localize at active synaptic sites. Thus, the Arc mRNA and protein are proposed as a marker of neuronal reactivity to map the neural substrates that are recruited by various stimuli. In the present study, we examined the expression of Arc protein induced by conditioned naloxone-precipitated drug withdrawal in different brain regions of acute morphine-dependent rats. The objective of the present study was to address the specific neural circuits involved in conditioned place aversion (CPA) that has not yet been well characterized.

METHODS

Place aversion was elicited by conditioned naloxone-precipitated drug withdrawal following exposure to a single dose of morphine. An immunohistochemical method was employed to detect the expression of Arc, which was used as a plasticity marker to trace the brain areas that contribute to the formation of the place aversion.

RESULTS

Marked increases in Arc protein levels were found in the medial and lateral prefrontal cortex, the sensory cortex, the lateral striatum and the amygdala. This effect was more pronounced in the basolateral amygdala (BLA), the central nucleus of the amygdala (CeA), and the bed nucleus of the striatal terminals (BNST) when compared with the control group.

CONCLUSION

Our results suggest that these brain regions may play key roles in mediating the negative motivational component of opiate withdrawal.

De novo and long-term l-Dopa induce both common and distinct striatal gene profiles in the hemiparkinsonian rat

We compared for the first time the effects of de novo versus long-term l-Dopa treatment inducing abnormal involuntary movement on striatal gene profiles and related bio-associations in the 6-hydroxydopamine rat model of Parkinson's disease. We examined the pattern of striatal messenger RNA expression over 4854 genes in hemiparkinsonian rats treated acutely or chronically with l-Dopa, and subsequently verified some of the gene alterations by in situ hybridization or real-time quantitative PCR. We found that de novo and long-term l-Dopa share common gene regulation features involving phosphorylation, signal transduction, secretion, transcription, translation, homeostasis, exocytosis and synaptic transmission processes. We also found that the transcriptomic response is enhanced by long-term l-Dopa and that specific biological alterations are underlying abnormal motor behavior. Processes such as growth, synaptogenesis, neurogenesis and cell proliferation may be particularly relevant to the long-term action of l-Dopa.

Visualizing stimulus convergence in amygdala neurons during associative learning

A central feature of models of associative memory formation is the reliance on information convergence from pathways responsive to the conditioned stimulus (CS) and unconditioned stimulus (US). In particular, cells receiving coincident input are held to be critical for subsequent plasticity. Yet identification of neurons in the mammalian brain that respond to such coincident inputs during a learning event remains elusive. Here we use Arc cellular compartmental analysis of temporal gene transcription by fluorescence in situ hybridization (catFISH) to locate populations of neurons in the mammalian brain that respond to both the CS and US during training in a one-trial learning task, conditioned taste aversion (CTA). Individual neurons in the basolateral nucleus of the amygdala (BLA) responded to both the CS taste and US drug during conditioning. Coincident activation was not evident, however, when stimulus exposure was altered so as to be ineffective in promoting learning (backward conditioning, latent inhibition). Together, these data provide clear visualization of neurons in the mammalian brain receiving convergent information about the CS and US during acquisition of a learned association.

Basal and stress-induced modulation of activity-regulated cytoskeletal associated protein (Arc) in the rat brain following duloxetine treatment

RATIONALE

Therapeutic efficacy of antidepressant drugs appears to be related to their ability in producing neuroadaptive changes that restore normal brain function. Activity-regulated cytoskeletal associated protein (Arc) is an effector immediate early gene that plays a fundamental role in activity-dependent neural plasticity in corticolimbic brain regions and has been implicated in the modulation of several functions known to be profoundly perturbed in depressive states.

OBJECTIVE

In the present study, we investigated transcriptional and translational changes of Arc in response to acute or chronic treatment with the novel antidepressant duloxetine.

RESULTS

Although a limited increase of Arc messenger RNA (mRNA) levels was found in some structures after acute antidepressant administration, a marked up-regulation of its gene expression was found after chronic treatment, primarily at the level of frontal cortex. The changes observed after prolonged duloxetine administration strongly correlates with those previously reported on brain-derived neurotrophic factor mRNA levels Calabrese et al. (Neuropsychopharmacol 32:2351-2359, 2007). In addition, we found an anatomical-specific influence of chronic duloxetine on stress-dependent Arc modulation, which was limited to the frontal cortex.

CONCLUSIONS

We suggest that these neuroadaptive changes, among others, might contribute to the normalization of neuroplastic defects associated with mood disorders.

Midazolam reverses salicylate-induced changes in brain-derived neurotrophic factor and arg3.1 expression: implications for tinnitus perception and auditory plasticity

Tinnitus is a phantom auditory perception, which can be induced via application of concentrated sodium salicylate, and is known to be associated with hearing loss and altered neuronal excitability in peripheral and central auditory neurons. The molecular features of this excitability, however, has been poorly characterized to date. Brain-derived neurotrophic factor (BDNF), the activity-dependent cytoskeletal protein (Arg3.1, also known as Arc), and c-Fos are known to be affected by changes in excitability and plasticity. Using reverse transcription-polymerase chain reaction, in situ hybridization, and immunohistochemistry, the expression of these genes was monitored in the rat auditory system after local (cochlear) and systemic application of salicylate. Induction of tinnitus and hearing loss was verified in a behavioral model. Regardless of the mode of salicylate application, a common pattern became evident: 1) BDNF mRNA expression was increased in the spiral ganglion neurons of the cochlea; and 2) Arg3.1 expression was significantly reduced in the auditory cortex. Local application of the GABA(A) receptor modulator midazolam resulted in the reversal not only of salicylate-induced changes in cochlear BDNF expression, but also in cortical Arg3.1 expression, indicating that the tinnitus-associated changes in cochlear BDNF expression trigger the decline of cortical Arg3.1 expression. Furthermore, local midazolam application reduced tinnitus perception in the animal model. These findings support Arg3.1 and BDNF as markers for activity changes in the auditory system and suggest a role of GABAergic inhibition of cochlear neurons in the modulation of Arg3.1 plasticity changes in the auditory cortex and tinnitus perception.

Understanding the importance of mRNA transport in memory

RNA localization is an important mechanism to sort proteins to specific subcellular domains. In neurons, several mRNAs are localized in dendrites and their presence allows autonomous control of local translation in response to stimulation of specific synapses. Active constitutive and activity-induced mechanisms of mRNA transport have been described that represent critical steps in the establishment and maintenance of synaptic plasticity. In recent years, the molecular composition of different transporting units has been reported and the identification of proteins and mRNAs in these RNA granules contributes to our understanding of the key steps that regulate mRNA transport and translation. Although RNA granules are heterogeneous, several proteins are common to different RNA granule populations, suggesting that they play important roles in the formation of the granules and/or their regulation during transport and translation. About 1-4% of the neuron transcriptome is found in RNA granules and the characterization of bound mRNAs reveal that they encode proteins of the cytoskeleton, the translation machinery, vesicle trafficking, and/or proteins involved in synaptic plasticity. Non-coding RNAs and microRNAs are also found in dendrites and likely regulate RNA translation. These mechanisms of mRNA transport and local translation are critical for synaptic plasticity mediated by activity or experience and memory.

The ups and downs of translation-dependent plasticity

Neuroscientists have been looking for good examples linking neuronal activity to gene expression/regulation involved in synaptic plasticity and the formation of long-term memories. New findings from Park et al. and Waung et al. in this issue of Neuron show that fast dendritic translation of the immediate-early gene Arc/Arg3.1 is involved in hippocampal mGluR-LTD, a protein synthesis-dependent form of plasticity.

Blueberry-induced changes in spatial working memory correlate with changes in hippocampal CREB phosphorylation and brain-derived neurotrophic factor (BDNF) levels

Phytochemical-rich foods have been shown to be effective at reversing age-related deficits in memory in both animals and humans. We show that a supplementation with a blueberry diet (2% w/w) for 12 weeks improves the performance of aged animals in spatial working memory tasks. This improvement emerged within 3 weeks and persisted for the remainder of the testing period. Memory performance correlated well with the activation of cAMP-response element-binding protein (CREB) and increases in both pro- and mature levels of brain-derived neurotrophic factor (BDNF) in the hippocampus. Changes in CREB and BDNF in aged and blueberry-supplemented animals were accompanied by increases in the phosphorylation state of extracellular signal-related kinase (ERK1/2), rather than that of calcium calmodulin kinase (CaMKII and CaMKIV) or protein kinase A. Furthermore, age and blueberry supplementation were linked to changes in the activation state of Akt, mTOR, and the levels of Arc/Arg3.1 in the hippocampus, suggesting that pathways involved in de novo protein synthesis may be involved. Although causal relationships cannot be made among supplementation, behavior, and biochemical parameters, the measurement of anthocyanins and flavanols in the brain following blueberry supplementation may indicate that changes in spatial working memory in aged animals are linked to the effects of flavonoids on the ERK-CREB-BDNF pathway.

Action potentials: to the nucleus and beyond

The neuronal nucleus is now widely accepted as playing a vital role in maintaining long-term changes in synaptic effectiveness. To act, however, the nucleus must be appropriately relayed with information regarding the latest round of synaptic plasticity. Several constraints of doing so in a neuron pertain to the often significant spatial distance of synapses from the nucleus and the number of synapses required for such a signal to reach functional levels in the nucleus. Largely based on the sensitivity of transcriptional responses to NMDA receptor antagonists, it has been postulated that the signals are physically relayed by biochemical messengers from the synapse to the nucleus. Alternatively, a second, less often considered but equally viable method of signal transduction may be initiated by action potentials generated proximal to the nucleus, wherefrom the signal can be relayed directly by calcium or indirectly by biochemical second messengers. We consider action potential-dependent signaling to the nucleus to have its own computational advantages over the synapse-to-nucleus signal for some functions. This minireview summarizes the logic and experimental support for these two modes of signaling and attempts to validate the action potential model as playing an important role in transcriptional regulation relating specifically to long-term synaptic plasticity.

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.

Structure-function relationships of pre-fibrillar protein assemblies in Alzheimer's disease and related disorders

Several neurodegenerative diseases, including Alzheimer's, Parkinson's, Huntington's and prion diseases, are characterized pathognomonically by the presence of intra- and/or extracellular lesions containing proteinaceous aggregates, and by extensive neuronal loss in selective brain regions. Related non-neuropathic systemic diseases, e.g., light-chain and senile systemic amyloidoses, and other organ-specific diseases, such as dialysis-related amyloidosis and type-2 diabetes mellitus, also are characterized by deposition of aberrantly folded, insoluble proteins. It is debated whether the hallmark pathologic lesions are causative. Substantial evidence suggests that these aggregates are the end state of aberrant protein folding whereas the actual culprits likely are transient, pre-fibrillar assemblies preceding the aggregates. In the context of neurodegenerative amyloidoses, the proteinaceous aggregates may eventuate as potentially neuroprotective sinks for the neurotoxic, oligomeric protein assemblies. The pre-fibrillar, oligomeric assemblies are believed to initiate the pathogenic mechanisms that lead to synaptic dysfunction, neuronal loss, and disease-specific regional brain atrophy. The amyloid beta-protein (Abeta), which is believed to cause Alzheimer's disease (AD), is considered an archetypal amyloidogenic protein. Intense studies have led to nominal, functional, and structural descriptions of oligomeric Abeta assemblies. However, the dynamic and metastable nature of Abeta oligomers renders their study difficult. Different results generated using different methodologies under different experimental settings further complicate this complex area of research and identification of the exact pathogenic assemblies in vivo seems daunting. Here we review structural, functional, and biological experiments used to produce and study pre-fibrillar Abeta assemblies, and highlight similar studies of proteins involved in related diseases. We discuss challenges that contemporary researchers are facing and future research prospects in this demanding yet highly important field.

Apolipoprotein E4 domain interaction: synaptic and cognitive deficits in mice

BACKGROUND

Apolipoprotein E4 (apoE4), the major genetic risk factor for Alzheimer's disease (AD) and other neurodegenerative diseases, has three structural and biophysical properties that distinguish it from the other isoforms-domain interaction, reduced stability, and lack of cysteine. Assessing their relative contributions to effects of apoE4-associated pathogenesis in AD is important from a mechanistic and therapeutic perspective, that is not possible using human apoE transgene or knock-in models.

METHODS

We analyzed Arg-61 apoE mice, a gene-targeted model that selectively displays domain interaction.

RESULTS

The mice displayed age-dependent loss of the synaptic protein synaptophysin in neocortex and hippocampus and had lower levels of the postsynaptic neuroligin-1. Activation of dentate gyrus granule neurons increased Arc expression 3.5-fold in wildtype mice but only 2.3-fold in Arg-61 mice. The losses of synaptic proteins caused a mild memory deficit in Arg-61 mice in the water-maze test. Since synaptic integrity requires efficient glutamate uptake, we measured astrocyte glutamate transporter 1 in the hippocampus. The level was reduced in Arg-61 mice, suggesting that inefficient glutamate uptake by astrocytes causes chronic excitotoxicity. Consistent with the reduced secretion of Arg-61 apoE by astrocytes in this model, cholesterol secretion was also reduced 34%. This reduction could also contribute to the synaptic deficits by limiting the availability of cholesterol for neuronal repair.

CONCLUSIONS

Domain interaction in the absence of other structural characteristics of apoE4 is sufficient to cause synaptic pathology and functional synaptic deficits, potentially associated with astrocyte dysfunction and impaired maintenance of neurons. Therapeutic targeting of domain interaction might blunt effects of apoE4 in neurodegenerative disease.

Dendritic spine loss and synaptic alterations in Alzheimer's disease

Dendritic spines are tiny protrusions along dendrites, which constitute major postsynaptic sites for excitatory synaptic transmission. These spines are highly motile and can undergo remodeling even in the adult nervous system. Spine remodeling and the formation of new synapses are activity-dependent processes that provide a basis for memory formation. A loss or alteration of these structures has been described in patients with neurodegenerative disorders such as Alzheimer's disease (AD), and in mouse models for these disorders. Such alteration is thought to be responsible for cognitive deficits long before or even in the absence of neuronal loss, but the underlying mechanisms are poorly understood. This review will describe recent findings and discoveries on the loss or alteration of dendritic spines induced by the amyloid beta (Abeta) peptide in the context of AD.

Reactivity and plasticity in the amygdala nuclei during opiate withdrawal conditioning: differential expression of c-fos and arc immediate early genes

Opiate withdrawal leads to the emergence of an aversive state that can be conditioned to a specific environment. Reactivation of these withdrawal memories has been suggested to be involved in relapse to drug-seeking of abstinent opiate addicts. Among the limbic areas that are likely to mediate these features of opiate dependence, amygdala nuclei represent critical neural substrates. Using a conditioned place aversion paradigm (CPA), we have previously shown specific opposite patterns of reactivity in the basolateral (BLA) and the central (CeA) amygdala, when comparing the experience of acute opiate withdrawal with the re-exposure to a withdrawal-paired environment. These data gave clues about the potential mechanisms by which amygdala nuclei may be involved in withdrawal memories. To extend these results, the present study aimed to assess the cellular reactivity and plasticity of amygdala nuclei during the opiate withdrawal conditioning process. For this, we have quantified c-fos and arc expression using in situ hybridization in rats, following each of the three conditioning sessions during CPA, and after re-exposure to the withdrawal-paired environment. BLA output neurons showed an increase in the expression of the plasticity-related arc gene during conditioning that was also increased by re-exposure to the withdrawal-paired environment. Interestingly, the CeA showed an opposite pattern of responding, and the intercalated cell masses (ITC), a possible inhibitory interface between the BLA and CeA, showed a persistent activation of c-fos and arc mRNA. We report here specific c-fos and arc patterns of reactivity in amygdala nuclei during withdrawal conditioning. These findings improve our understanding of the involvement of the amygdala network in the formation and retrieval of withdrawal memories. Plasticity processes within BLA output neurons during conditioning, may participate in increasing the BLA reactivity to conditioned stimuli, which could in turn (by the control of downstream nuclei) reinforce and drive the motivational properties of withdrawal over drug consumption.

Molecular and cellular mechanisms of memory allocation in neuronetworks

Determining how neuronal networks encode memories is a key goal of neuroscience. Although neuronal circuit processes involved in encoding, storing and retrieving memory have attracted a great deal of attention, the processes that allocate individual memories to specific neurons within a network have remained elusive. Recent findings unraveled the first insights into the processes that modulate memory allocation in neuronetworks. They showed that neurons in the lateral amygdala compete to take part in auditory fear conditioned memory traces and that the levels of the transcription factor CREB (cAMP-response element binding protein) can affect the probability of a neuron to be recruited into a given memory representation. CREB-mediated transcriptional regulation involves several signaling pathways, known to mediate nuclear responses to diverse behavioral stimuli, along with coordinated interactions with multiple other transcription activators, coactivators and repressors. Moreover, activation of CREB triggers an autoinhibitory feedback loop, a metaplastic process that could be used to allocate memories away from cells that have been recently involved in memory. Beyond CREB, there may be a host of other processes that dynamically modulate memory allocation in neuronetworks by shaping cooperation and competition among neurons.

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.

The immediate early gene Arc is associated with behavioral resilience to stress exposure in an animal model of posttraumatic stress disorder

Mechanisms involved in adaptative and maladaptive changes in neural plasticity and synaptic efficacy in various brain areas are pivotal to understanding the physiology of the response to stress and the pathophysiology of posttraumatic stress disorder (PTSD). Activity-regulated cytoskeletal-associated protein (Arc) is an effector immediate early gene (IEG) which has direct effects on intracellular homeostatic functions. Increased expression of Arc has been associated with increased neuronal activity and with consolidation of long-term memory. It may thus play an important role in mediating experience-induced reorganization and/or development of synaptic connections. This study sought to characterize the pattern of expression of mRNA for the Arc gene in selected brain areas of test subjects classified according to their individual pattern of behavioral response to a stressor, correlated with circulating levels of corticosterone (as a physiological marker of stress response). The hippocampal CA1 and CA3 subregions of individuals whose behavior was minimally or partially disrupted in response to predator scent stress demonstrated significantly increased levels of mRNA for Arc, compared to unexposed controls. The group whose behavior was severely disrupted demonstrated no such upregulation. Consistent with the hypothesis that the Arc gene has a promoting effect on neuronal function and/or structural changes, the lack of Arc expression in the behaviorally and physiologically more severely affected individuals raises the possibility that Arc may be associated with resilience and/or recovery after stress exposure.

Probing the role of AMPAR endocytosis and long-term depression in behavioural sensitization: relevance to treatment of brain disorders, including drug addiction

Modifying the function of postsynaptic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid subtype glutamate receptors (AMPARs) is one of the most important mechanisms by which the efficacy of synaptic transmission at excitatory glutamatergic synapses in the mammalian brain is regulated. Traditionally these types of modifications have been thought to be achieved mainly by altering the channel gating properties or conductance of the receptors. A large body of evidence accumulated from recent studies strongly suggests that AMPARs, like most integral plasma membrane proteins, are continuously recycled between the plasma membrane and the intracellular compartments via vesicle-mediated plasma membrane insertion and clathrin-dependent endocytosis. Regulation of either receptor insertion or endocytosis results in a rapid change in the number of these receptors expressed on the plasma membrane surface and in the receptor-mediated responses, thereby playing an important role in mediating certain forms of synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD). These studies have significantly advanced our understanding of the molecular mechanisms underlying LTP and LTD, and their potential contributions to learning and memory-related behaviours. Here I provide a brief summary of the current state of knowledge concerning clathrin-mediated AMPAR endocytosis and its relationship to the expression of certain forms of LTD in several brain areas. The potential impact of recent advancements on our efforts to probe the roles of synaptic plasticity in learning and memory-related behaviours, and their relevance to some brain disorders, particularly drug addiction, are also discussed.

Advances on the understanding of the origins of synaptic pathology in AD

Although Alzheimer's disease (AD) was first discovered a century ago, we are still facing a lack of definitive diagnosis during the patient's lifetime and are unable to prescribe a curative treatment. However, the past 10 years have seen a "revamping" of the main hypothesis about AD pathogenesis and the hope to foresee possible treatment. AD is no longer considered an irreversible disease. A major refinement of the classic beta-amyloid cascade describing amyloid fibrils as neurotoxins has been made to integrate the key scientific evidences demonstrating that the first pathological event occurring in AD early stages affects synaptic function and maintenance. A concept fully compatible with synapse loss being the best pathological correlate of AD rather than other described neuropathological hallmarks (amyloid plaques, neurofibrillary tangles or neuronal death). The notion that synaptic alterations might be reverted, thus offering a potential curability, was confirmed by immunotherapy experiments targeting beta-amyloid protein in transgenic AD mice in which cognitive functions were improved despite no reduction in the amyloid plaques burden. The updated amyloid cascade now integrates the synapse failure triggered by soluble Abeta-oligomers. Still no consensus has been reached on the most toxic Abeta conformations, neither on their site of production nor on their extra- versus intra-cellular actions. Evidence shows that soluble Abeta oligomers or ADDLs bind selectively to neurons at their synaptic loci, and trigger major changes in synapse composition and morphology, which ultimately leads to dendritic spine loss. However, the exact mechanism is not yet fully understood but is suspected to involve some membrane receptor(s).

The EJC factor eIF4AIII modulates synaptic strength and neuronal protein expression

Proper neuronal function and several forms of synaptic plasticity are highly dependent on precise control of mRNA translation, particularly in dendrites. We find that eIF4AIII, a core exon junction complex (EJC) component loaded onto mRNAs by pre-mRNA splicing, is associated with neuronal mRNA granules and dendritic mRNAs. eIF4AIII knockdown markedly increases both synaptic strength and GLUR1 AMPA receptor abundance at synapses. eIF4AIII depletion also increases ARC, a protein required for maintenance of long-term potentiation; arc mRNA, one of the most abundant in dendrites, is a natural target for nonsense-mediated decay (NMD). Numerous new NMD candidates, some with potential to affect synaptic activity, were also identified computationally. Two models are presented for how translation-dependent decay pathways such as NMD might advantageously function as critical brakes for protein synthesis in cells such as neurons that are highly dependent on spatially and temporally restricted protein expression.

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.

The immediate early gene early growth response gene 3 mediates adaptation to stress and novelty

Stress and exploration of novel environments induce neural expression of immediate early gene transcription factors (IEG-TFs). However, as yet no IEG-TF has been shown to be required for the normal biological or behavioral responses to these stimuli. Here we show that mice deficient for the IEG-TF early growth response gene (Egr) 3, display accentuated behavioral responses to the mild stress of handling paralleled by increased release of the stress hormone corticosterone. Egr3-/- mice also display abnormal responses to novelty, including heightened reactivity to novel environments and failure to habituate to social cues or startling acoustic stimuli. In a Y-maze spontaneous alternation task, they perform fewer sequential arm entries than controls, suggesting defects in immediate memory. Because stress and novelty stimulate hippocampal long-term depression (LTD), and because abnormalities in habituation to novelty and Y-maze performance have been associated with LTD deficits, we examined this form of synaptic plasticity in Egr3-/- mice. We found that Egr3-/- mice fail to establish hippocampal LTD in response to low frequency stimulation and exhibit dysfunction of an ifenprodil-sensitive (NR1/NR2B) N-methyl-d-aspartate receptor subclass. Long term potentiation induction was not altered. The NR2B-dependent dysfunction does not result from transcriptional regulation of this subunit by Egr3, because NR2B mRNA levels did not differ in the hippocampi of Egr3-/- and control mice. These findings are the first demonstration of the requirement for an IEG-TF in mediating the response to stress and novelty, and in the establishment of LTD.

Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease

Neural network dysfunction may play an important role in Alzheimer's disease (AD). Neuronal circuits vulnerable to AD are also affected in human amyloid precursor protein (hAPP) transgenic mice. hAPP mice with high levels of amyloid-beta peptides in the brain develop AD-like abnormalities, including cognitive deficits and depletions of calcium-related proteins in the dentate gyrus, a region critically involved in learning and memory. Here, we report that hAPP mice have spontaneous nonconvulsive seizure activity in cortical and hippocampal networks, which is associated with GABAergic sprouting, enhanced synaptic inhibition, and synaptic plasticity deficits in the dentate gyrus. Many Abeta-induced neuronal alterations could be simulated in nontransgenic mice by excitotoxin challenge and prevented in hAPP mice by blocking overexcitation. Aberrant increases in network excitability and compensatory inhibitory mechanisms in the hippocampus may contribute to Abeta-induced neurological deficits in hAPP mice and, possibly, also in humans with AD.

Abeta oligomer-mediated long-term potentiation impairment involves protein phosphatase 1-dependent mechanisms

Amyloid beta (Abeta) oligomers are derived from proteolytic cleavage of amyloid precursor protein (APP) and can impair memory and hippocampal long-term potentiation (LTP) in vivo and in vitro. They are recognized as the primary neurotoxic agents in Alzheimer's disease. The mechanisms underlying such toxicity on synaptic functions are complex and not fully understood. Here, we provide the first evidence that these mechanisms involve protein phosphatase 1 (PP1). Using a novel transgenic mouse model expressing human APP with the Swedish and Arctic mutations that render Abeta more prone to form oligomers (arcAbeta mice), we show that the LTP impairment induced by Abeta oligomers can be fully reversed by PP1 inhibition in vitro. We further demonstrate that the genetic inhibition of endogenous PP1 in vivo confers resistance to Abeta oligomer-mediated toxicity and preserves LTP. Overall, these results reveal that PP1 is a key player in the mechanisms of AD pathology.

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 Role of the GluR2 Subunit in AMPA Receptor Function and Synaptic Plasticity

The AMPA receptor (AMPAR) GluR2 subunit dictates the critical biophysical properties of the receptor, strongly influences receptor assembly and trafficking, and plays pivotal roles in a number of forms of long-term synaptic plasticity. Most neuronal AMPARs contain this critical subunit; however, in certain restricted neuronal populations and under certain physiological or pathological conditions, AMPARs that lack this subunit are expressed. There is a current surge of interest in such GluR2-lacking Ca2+-permeable AMPARs in how they affect the regulation of synaptic transmission. Here, we bring together recent data highlighting the novel and important roles of GluR2 in synaptic function and plasticity.

Homeostatic signaling: the positive side of negative feedback

Synaptic homeostasis provides a means for neurons and circuits to maintain stable function in the face of perturbations such as developmental or activity-dependent changes in synapse number or strength. These forms of plasticity are thought to utilize negative feedback signaling to sense some aspect of activity, compare this with an internal set point, and then adjust synaptic properties to keep activity close to this set point. However, the molecular identity of these signaling components has not been firmly established. Recent work suggests that there are likely to be multiple forms of synaptic homeostasis, mediated by distinct signaling pathways and with distinct expression mechanisms. These include presynaptic forms that depend on retrograde signaling to presynaptic Ca(2+) channels, and postsynaptic forms influenced by BDNF, TNFalpha and Arc signaling. Current challenges include matching signaling elements to their functions (i.e. as detectors of activity, as part of the set-point mechanism and/or as effectors of synaptic change), and fitting these molecular candidates into a unified view of the signaling pathways that underlie synaptic homeostasis.

The α7 nicotinic receptor agonist SSR180711 increases activity regulated cytoskeleton protein (Arc) gene expression in the prefrontal cortex of the rat

Nicotinic alpha7 acetylcholine receptors (alpha7 nAChR) have been shown to enhance attentional function and aspects of memory function in experimental models and in man. The protein Arc encoded by the effector immediate early gene arc or arg3.1 has been shown to be strongly implicated in long-term memory function. We have sought to determine if alpha7 nAChR mediate the stimulation of arc gene expression, and if so, where in the brain such activation may occur using semi-quantitative in situ hybridisation. Administration of the novel and selective alpha7 nAChR agonist, SSR180711 (1, 3 and 10 mg/kg) to adolescent rats, produced a dose- and time-dependent increase in the expression of Arc mRNA in the prefrontal cortex and the ventral orbital cortex. By contrast, no change in mRNA levels was detected in the parietal cortex and the CA1 of the hippocampus. These data show that alpha7 nAChR activates a subset of neurons in the rat prefrontal cortex and this activation likely is important for the attentional effects of this new class of drugs.

Nandrolone-induced hippocampal phosphorylation of NMDA receptor subunits and ERKs

The age-related decline in gonadal steroids is associated with changes in mood and memory function. It appears that normal physiological concentrations of the steroids are required for adequate synaptic plasticity. However, the effects of high levels of androgens subsequent to misuse of anabolic androgenic steroids (AAS) are largely unknown. In this study, rats were given i.m. nandrolone as a single dose or daily for 14 days and the effects on synaptic components in hippocampal synaptoneurosomes were measured 24h after the last injection. Western blot analysis revealed that a single injection of AAS increased phosphorylation of the NMDA receptor subunits NR2A and NR2B and ERK1/2, while the levels of phosphorylated CaMKIIalpha were unaltered. No changes were seen in other synaptic proteins tested, i.e., BDNF, Arc, TUC-4, and beta-tubulin III. Daily administration of nandrolone for 2 weeks did not affect the content of any of the proteins tested. From this in vivo study, it is concluded that important synaptic components respond to a single high dose of nandrolone, an effect that may influence synapse function.

NMDA and AMPA receptors: old channels, new tricks

Learning and memory depend on persistent changes in synaptic strength that require neuronal gene expression. An unresolved question concerns the mechanisms by which activity at synapses is transduced into a nuclear transcriptional response. In the prevailing view, N-methyl-D-aspartate (NMDA)- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors have distinct roles in controlling synaptic strength: AMPA receptors effect short-term changes in synaptic strength, whereas NMDA receptors regulate genes that are required for the long-term maintenance of these changes. Here, we review recent data on the roles of these two types of receptor in activity-dependent gene expression. We discuss evidence that signals from NMDA receptors and AMPA receptors are integrated to specify transcriptional responses for particular plasticity related genes.

Reelin depletion in the entorhinal cortex of human amyloid precursor protein transgenic mice and humans with Alzheimer's disease

Reelin regulates nervous system development and modulates synaptic plasticity in the adult brain. Several findings suggest that alterations in Reelin signaling may contribute to neuronal dysfunction associated with Alzheimer's disease (AD). Cell surface receptors for Reelin, including integrins and very-low-density lipoprotein receptor/apolipoprotein E2 receptor, may be targets of amyloid-beta (Abeta) peptides presumed to play key roles in the pathogenesis of AD. Reelin also regulates the extent of tau phosphorylation. Finally, increased amounts of Reelin fragments have been found in CSF from AD patients, suggesting altered processing of Reelin. We therefore hypothesized that Reelin levels might be altered in the brains of human amyloid precursor protein (hAPP) transgenic mice, particularly in brain regions vulnerable to AD such as hippocampus and entorhinal cortex. Compared with nontransgenic controls, hAPP mice had significantly fewer Reelin-expressing pyramidal cells in the entorhinal cortex, the major population of glutamatergic neurons expressing Reelin in the brain. Western blot analysis of the hippocampus, which receives projections from the entorhinal cortex, revealed significant reductions in Reelin levels. In contrast, the number of Reelin-expressing GABAergic interneurons was not altered in either the entorhinal cortex or the hippocampus. Thus, neuronal expression of hAPP/Abeta is sufficient to reduce Reelin expression in a specific population of entorhinal cortical pyramidal neurons in vivo. Underscoring the relevance of these findings, we found qualitatively similar reductions of Reelin-expressing pyramidal neurons in the entorhinal cortex of AD brains. We conclude that alterations in Reelin processing or signaling may be involved in AD-related neuronal dysfunction.

Rapid and long-term induction of effector immediate early genes (BDNF, Neuritin and Arc) in peri-infarct cortex and dentate gyrus after ischemic injury in rat brain

The genomic response following brain ischemia is very complex and involves activation of both protective and detrimental signaling pathways. Immediate early genes (IEGs) represent the first wave of gene expression following ischemia and are induced in extensive regions of the ischemic brain including cerebral cortex and hippocampus. Brain-derived neurotrophic factor (BDNF), Neuritin and Activity-regulated cytoskeleton-associated protein (Arc) belong to a subgroup of immediate early genes implicated in synaptic plasticity known as effector immediate early genes. Here, we investigated the spatial and temporal activation pattern for these genes during the first 24 h of reperfusion following 2-h occlusion of the middle cerebral artery. Neuritin showed a persistent activation in frontal-cingulate cortex while Arc displayed a biphasic response. Also, in dentate gyrus, activation was observed at 0-6 h of reperfusion for Neuritin and 0-12 h of reperfusion for Arc while BDNF was induced 0-9 h of reperfusion. Our study demonstrates a rapid and long-term activation of effector immediate early genes in distinct brain areas following ischemic injury in rat. Effector gene activation may be part of long-term synaptic responses of ischemic brain tissue.