Arg3.1/Arc mRNA induction by Ca2+ and cAMP requires protein kinase A and mitogen-activated protein kinase/extracellular regulated kinase activation

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.

Platelet-derived growth factor-mediated induction of the synaptic plasticity gene Arc/Arg3.1

Platelet-derived growth factor (PDGF) is a pleiotropic protein with critical roles in both developmental as well as pathogenic processes. In the central nervous system specifically, PDGF is critical for neuronal proliferation and differentiation and has also been implicated as a neuroprotective agent. Whether PDGF also plays a role in synaptic plasticity, however, remains poorly understood. In the present study we demonstrated that in the rat hippocampal neurons PDGF regulated the expression of Arc/Arg3.1 gene that has been implicated in both synapse plasticity and long term potentiation. Relevance of these findings was further confirmed in vivo by injecting mice with intracerebral inoculations of PDGF, which resulted in a rapid induction of Arc in the hippocampus of the injected mice. PDGF induced long term potentiation in rat hippocampal slices, which was abolished by PDGF receptor-tyrosine kinase inhibitor STI-571. We also present evidence that PDGF-mediated induction of Arc/Arg3.1 involved activation of the MAPK/ERK (MEK) pathway. Additionally, induction of Arc/Arg3.1 also involved the upstream release of intracellular calcium stores, an effect that could be blocked by thapsigargin but not by EGTA. Pharmacological approach using inhibitors specific for either MAPK/ERK phosphorylation or calcium release demonstrated that the two pathways converged downstream at a common point involving activation of the immediate early gene Egr-1. Chromatin immunoprecipitation assays demonstrated the binding of Egr-1, but not Egr-3, to the Arc promoter. These findings for the first time, thus, suggest an additional role of PDGF, that of induction of Arc.

Widespread transcription at neuronal activity-regulated enhancers

We used genome-wide sequencing methods to study stimulus-dependent enhancer function in neurons. We identified ∼12,000 neuronal activity-regulated enhancers that are bound by the general transcriptional co-activator CBP in an activity-dependent manner. A function of CBP at enhancers may be to recruit RNA polymerase II (RNAPII), as we also observed activity-regulated RNAPII binding to thousands of enhancers. Remarkably, RNAPII at enhancers transcribes bi-directionally a novel class of enhancer RNAs (eRNAs) within enhancer domains defined by the presence of histone H3 that is mono-methylated at lysine 4 (H3K4me1). The level of eRNA expression at neuronal enhancers positively correlates with the level of mRNA synthesis at nearby genes, suggesting that eRNA synthesis occurs specifically at enhancers that are actively engaged in promoting mRNA synthesis. These findings reveal that a widespread mechanism of enhancer activation involves RNAPII binding and eRNA synthesis.

Cellular and chemical neuroscience of mammalian sleep

Extraordinary strides have been made toward understanding the complexities and regulatory mechanisms of sleep over the past two decades thanks to the help of rapidly evolving technologies. At its most basic level, mammalian sleep is a restorative process of the brain and body. Beyond its primary restorative purpose, sleep is essential for a number of vital functions. Our primary research interest is to understand the cellular and molecular mechanisms underlying the regulation of sleep and its cognitive functions. Here I will reflect on our own research contributions to 50 years of extraordinary advances in the neurobiology of slow-wave sleep (SWS) and rapid eye movement (REM) sleep regulation. I conclude this review by suggesting some potential future directions to further our understanding of the neurobiology of sleep.

Loss of Arc renders the visual cortex impervious to the effects of sensory experience or deprivation

A myriad of mechanisms are suggested to account for the full richness of visual cortical plasticity. We report that visual cortex lacking Arc is impervious to the effects of deprivation or experience. Using intrinsic signal imaging and chronic visually evoked potential recordings, we find that Arc^−/− mice do not exhibit depression of deprived eye responses or a shift in ocular dominance after brief monocular deprivation. Extended deprivation also fails to elicit a shift in ocular dominance or open eye potentiation. Moreover, Arc^−/− mice lack stimulus–selective response potentiation. Although Arc^−/− mice exhibit normal visual acuity, baseline ocular dominance is abnormal and resembles that observed after dark–rearing. These data suggest that Arc is required for the experience–dependent processes that normally establish and modify synaptic connections in visual cortex.

Beyond antioxidants: the cellular and molecular interactions of flavonoids and how these underpin their actions on the brain

The consumption of flavonoid-rich foods and beverages has been suggested to limit the neurodegeneration associated with a variety of neurological disorders and to prevent or reverse normal or abnormal deteriorations in cognitive performance. Flavonoids mediate these effects via a number of routes, including a potential to protect neurons against injury induced by neurotoxins, an ability to suppress neuroinflammation and a potential to promote memory, learning and cognitive function. Originally, it was thought that such actions were mediated by the antioxidant capacity of flavonoids. However, their limited absorption and their low bioavailability in the brain suggest that this explanation is unlikely. Instead, this multiplicity of effects appears to be underpinned by three separate processes: first, through their interactions with important neuronal and glial signalling cascades in the brain, most notably the phosphatidylinositol 3-kinase/Akt and mitogen-activated protein kinase pathways that regulate pro-survival transcription factors and gene expression; second, through an ability to improve peripheral and cerebral blood flow and to trigger angiogenesis and neurogenesis in the hippocampus; third, by their capacity to directly react with and scavenge neurotoxic species and pro-inflammatory agents produced in the brain as a result of both normal and abnormal brain ageing. The present review explores the potential inhibitory or stimulatory actions of flavonoids within these three systems and describes how such interactions are likely to underlie neurological effects.

Assessment of the role of MAP kinase in mediating activity-dependent transcriptional activation of the immediate early gene Arc/Arg3.1 in the dentate gyrus in vivo

Different physiological and behavioral events activate transcription of Arc/Arg3.1 in neurons in vivo, but the signal transduction pathways that mediate induction in particular situations remain to be defined. Here, we explore the relationships between induction of Arc/Arg3.1 transcription in dentate granule cells in vivo and activation of mitogen-activated protein (MAP) kinase as measured by extracellular-regulated kinase 1/2 (ERK1/2) phosphorylation. We show that ERK1/2 phosphorylation is strongly induced in dentate granule cells within minutes after induction of perforant path long-term potentiation (LTP). Phospho-ERK staining appears in nuclei within minutes after stimulation commences, and ERK phosphorylation returns to control levels within 60 min. Electroconvulsive seizures, which strongly induce prolonged Arc/Arg3.1 transcription in dentate granule cells, induced ERK1/2 phosphorylation in granule cells that returned to control levels within 30 min. Following 30, 60, and 120 min of exploration in a novel complex environment, Arc/Arg3.1 transcription was activated in many more granule cells than stained positively for p-ERK at all time points. Although Arc/Arg3.1 transcription was induced in most pyramidal neurons in CA1 following exploration, very few pyramidal neurons exhibited nuclear p-ERK1/2 staining. Local delivery of U0126 during the induction of perforant path LTP blocked transcriptional activation of Arc/Arg3.1 in a small region near the injection site and blocked Arc/Arg3.1 protein expression over a wider region. Our results indicate that activation of Arc/Arg3.1 transcription in dentate granule cells in vivo is mediated in part by MAP kinase activation, but other signaling pathways also contribute, especially in the case of Arc/Arg3.1 induction in response to experience.

N-methyl-D-aspartate-stimulated ERK1/2 signaling and the transcriptional up-regulation of plasticity-related genes are developmentally regulated following in vitro neuronal maturation

The general features of neuroplasticity are developmentally regulated. Although it has been hypothesized that the loss of plasticity in mature neurons may be due to synaptic saturation and functional reduction of N-methyl-D-aspartate receptors (NMDAR), the molecular mechanisms remain largely unknown. We examined the effects of NMDAR activation and KCl-mediated membrane depolarization on ERK1/2 signaling following in vitro maturation of cultured cortical neurons. Although NMDA stimulated a robust increase in intracellular calcium at both DIV (day in vitro) 3 and 14, the activation of ERK1/2 and cAMP responsive element-binding protein (CREB) was impaired at DIV 14. Specifically, the phosphorylation of ERK1/2 was stimulated by both NMDA and KCl at DIV 3. However, at DIV 14, NMDA- but not KCl-stimulated ERK1/2 and CREB phosphorylation was significantly diminished. Consistently, the NMDA-induced transcription of ERK/CREB-regulated genes Bdnf exon 4, Arc, and zif268 was significantly attenuated at DIV 14. Moreover, in comparison with 3 DIV neurons, the phosphorylated-ERK1/2 in 14 DIV neurons displayed a tremendous increase following maturation and was more susceptible to dephosphorylation. Blocking calcium channels by nifedipine or NMDAR by APV caused a more dramatic ERK dephosphorylation in 14 DIV neurons. We further demonstrate that the loss of plasticity-related signaling is unrelated to NMDA-induced cell death of the 14 DIV neurons. Taken together, these results suggest that the attenuation of certain aspects of neuroplasticity following maturation may be due to the reduction of NMDAR-mediated gene transcription and a saturation of ERK1/2 activity.

NMDA receptor-independent control of transcription factors and gene expression

Consolidation of synaptic plasticity seems to require transcription, but how the nucleus is informed in this context remains unknown. As NMDA receptor antagonists have been shown to interfere with action potential generation, the issue of whether or not a synaptically generated signal is required for nuclear signaling is currently unresolved. Here, we show that pharmacological maintenance of action potentials during NMDA receptor blockade allows for NMDA receptor-independent transcription factor binding and arc gene expression, both of which were previously thought to be NMDA receptor dependent. These data suggest that types of signaling in the nucleus previously attributed to NMDA-receptor-dependent synapse-to-nucleus signals can be initiated in the absence of NMDA receptor-dependent synaptic plasticity.

Flavonoids and cognition: the molecular mechanisms underlying their behavioural effects

Evidence suggests that a group of phytochemicals known as flavonoids are highly effective in reversing age-related declines in neuro-cognitive performance through their ability to interact with the cellular and molecular architecture of the brain responsible for memory and by reducing neuronal loss due to neurodegenerative processes. In particular, they may increase the number of, and strength of, connections between neurons, via their specific interactions with the ERK and Akt signalling pathways, leading to an increase in neurotrophins such as BDNF. Concurrently, their effects on the peripheral and cerebral vascular system may also lead to enhancements in cognitive performance through increased brain blood flow and an ability to initiate neurogenesis in the hippocampus. Finally, they have also been shown to reduce neuronal damage and losses induced by various neurotoxic species and neuroinflammation. Together, these processes act to maintain the number and quality of synaptic connections in the brain, a factor known to be essential for efficient LTP, synaptic plasticity and ultimately the efficient working of memory.

Novel translational control in Arc-dependent long term potentiation consolidation in vivo

Regulation of translation factor activity plays a major role in protein synthesis-dependent forms of synaptic plasticity. We examined translational control across the critical period of Arc synthesis underlying consolidation of long term potentiation (LTP) in the dentate gyrus of intact, anesthetized rats. LTP induction by high frequency stimulation (HFS) evoked phosphorylation of the cap-binding protein eukaryotic initiation factor 4E (eIF4E) and dephosphorylation of eIF2alpha on a protracted time course matching the time-window of Arc translation. Local infusion of the ERK inhibitor U0126 inhibited LTP maintenance and Arc protein expression, blocked changes in eIF4E and eIF2alpha phosphorylation state, and prevented initiation complex (eIF4F) formation. Surprisingly, inhibition of the mTOR protein complex 1 (mTORC1) with rapamycin did not impair LTP maintenance or Arc synthesis nor did it inhibit eIF4F formation or phosphorylation of eIF4E. Rapamycin nonetheless blocked mTOR signaling to p70 S6 kinase and ribosomal protein S6 and inhibited synthesis of components of the translational machinery. Using immunohistochemistry and in situ hybridization, we show that Arc protein expression depends on dual, ERK-dependent transcription and translation. Arc translation is selectively blocked by pharmacological inhibition of mitogen-activated protein kinase-interacting kinase (MNK), the kinase coupling ERK to eIF4E phosphorylation. Furthermore, MNK signaling was required for eIF4F formation. These results support a dominant role for ERK-MNK signaling in control of translational initiation and Arc synthesis during LTP consolidation in the dentate gyrus. In contrast, mTORC1 signaling is activated but nonessential for Arc synthesis and LTP. The work, thus, identifies translational control mechanisms uniquely tuned to Arc-dependent LTP consolidation in live rats.

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.

Functional versatility of transcription factors in the nervous system: the SRF paradigm

Individual transcription factors in the brain frequently display broad functional versatility, thereby controlling multiple cellular outputs. In accordance, neuron-restricted mutagenesis of the murine Srf gene, encoding the transcription factor serum response factor (SRF), revealed numerous SRF functions in the nervous system. First, SRF controls immediate early gene (IEG) activation associated with perception of synaptic activity, learning and memory. Second, processes linked to actin cytoskeletal dynamics are mediated by SRF, such as developmental neuronal migration, outgrowth and pathfinding of neurites, as well as synaptic targeting. Therefore, SRF seems to be instrumental in converting synaptic activity into plasticity-associated structural changes in neuronal connectivities. This highlights the decisive role of SRF in integrating cytoskeletal actin dynamics and nuclear gene expression. Finally, we relate SRF to the multi-functional transcription factor CREB and point out overlapping, distinct and concerted functions of these two transcriptional regulators in the brain.

Converging signal on ERK1/2 activity regulates group I mGluR-mediated Arc transcription

The expression of Arc is tightly coupled to synaptic activities. Recent studies suggested the functional relevance of Arc translation in group I metabotropic glutamate receptor (mGluR)-mediated long-term depression. The present study investigated the transcription-dependent changes of Arc in response to the activation of group I mGluR by (R,S)-3,5-dihydroxyphenylglycine (DHPG) in cultured cortical neurons. The increase in Arc mRNA did not require de novo protein synthesis, indicating that Arc is an immediate early gene upon DHPG stimulation. We further examined the major pathways involved in group I mGluR signaling, and found that DHPG-induced Arc up-regulation depended on CaMK, PLC, and ERK1/2 activity. Moreover, the activity of NMDA receptors, but not l-type voltage gated calcium channels (l-VGCC), was required for Arc transcription. Interestingly, blocking CaMK, PLC, and NMDAR, but not l-VGCC, suppressed DHPG-stimulated ERK1/2 activation. These data suggest the central role of ERK1/2 in group I mGluR-mediated Arc transcription.

A calcium-dependent switch in a CREST-BRG1 complex regulates activity-dependent gene expression

CREST plays a critical role in activity-dependent development, but its mechanism of action is not well understood. Here, we show that a CREST-BRG1 complex regulates promoter activation by orchestrating a calcium-dependent release of a repressor complex and a recruitment of an activator complex. In resting neurons, transcription of the c-fos promoter is inhibited by BRG1-dependent recruitment of a phospho-Rb-HDAC repressor complex. Upon calcium influx, Rb becomes dephosphorylated at serine 795 by calcineurin, which leads to release of the repressor complex. At the same time, there is increased recruitment of CBP to the promoter by a CREST-dependent mechanism, which leads to transcriptional activation. The CREST-BRG1 also binds to the NR2B promoter, and activity-dependent induction of NR2B expression involves a release of HDAC1 and recruitment of CBP, suggesting that this mechanism may be generally involved in regulating calcium-dependent transcription of neuronal genes.

Regulation of brain-derived neurotrophic factor-mediated transcription of the immediate early gene Arc by intracellular calcium and calmodulin

The induction of the immediate early gene Arc is strongly implicated in synaptic plasticity. Although the role of ERK has been demonstrated, the regulation of Arc expression is largely unknown. In this study, we investigated the major signaling pathways underlying brain-derived neurotrophic factor (BDNF)-mediated Arc transcription in cultured cortical neurons. The BDNF-stimulated Arc transcription was regulated solely by the Ras-Raf-MAPK signaling through ERK, but not by phosphoinositide 3-kinase (PI3K) and PLC-gamma activities. Although it was demonstrated that BDNF might promote calcium entry through calcium channels and NMDA receptors, chelating extracellular calcium with EGTA failed to block Arc transcription. In contrast, chelating intracellular calcium ([Ca(2+)](i)) by BAPTA-AM abolished BDNF-mediated Arc up-regulation. Surprisingly, BAPTA-AM did not block ERK activation, indicating that [Ca(2+)](i) and Ras-Raf-MAPK are not coupled, and the activation of ERK alone is not sufficient to up-regulate Arc transcription. Moreover, we found that inhibition of calmodulin (CaM) by W13 blocked both Arc transcription and ERK activation, revealing a Ca(2+)-independent function of CaM. These data suggested novel functions of [Ca(2+)](i) and CaM in BDNF signaling. Comparison of the Arc transcription profiles between Ca(2+)-stimulated and BDNF-stimulated neurons demonstrated that the regulatory mechanisms were distinctively tailored to the complex features of neuronal activity. Specifically, PI3K and CaM-dependent protein kinase (CaMK) activity were required for Ca(2+)-stimulated Arc transcription through regulating ERK signaling. Such cross-talks between PI3K, CaMK, and ERK was absent in BDNF-stimulated neurons.

Inverse regulation of plasticity-related immediate early genes by calcineurin in hippocampal neurons

In the mammalian hippocampus, changes in the expression of immediate early genes (IEGs) is thought to contribute to long term plastic changes in neurons brought about by learning tasks and high frequency stimulation of synapses. The phosphatase calcineurin has emerged as an important negative regulator of hippocampus-dependent learning and long term potentiation. Here we investigated the possibility that the constraining action of calcineurin on hippocampal plasticity is mediated in part by regulation of gene expression through negative control of transcription factors, such as cAMP-response element (CRE)-binding protein (CREB). We assessed the effect of calcineurin inhibitors on CREB activation by neuronal activity and show that calcineurin activity is in fact required for CREB-mediated gene expression. However, inhibition of calcineurin had disparate effects on the transcriptional induction of CREB-dependent IEGs. We find that the IEG c-fos is unaffected by suppression of calcineurin activity, the plasticity-related genes Egr1/Zif268 and Egr2/Krox-20 are up-regulated, and genes encoding the orphan nuclear hormone receptors Nor1 and Nur77 are down-regulated. We further show that the up-regulation of particular IEGs is probably due to the presence of serum response elements (SREs) in their promoters, because SRE-mediated gene expression is enhanced by calcineurin blockers. Moreover, expression of the c-fos gene, which is unaffected by calcineurin inhibitors, could be down-regulated by mutating the SRE. Conversely, SRE-mediated c-fos induction in the absence of a functional CRE was enhanced by calcineurin inhibitors. Our experiments thus implicate calcineurin as a negative regulator of SRE-dependent neuronal genes.

Estrogen stimulates activity-regulated cytoskeleton associated protein (Arc) expression via the MAPK- and PI-3K-dependent pathways in SH-SY5Y cells

Activity-regulated cytoskeleton associated protein (Arc) is known to be induced by synaptic plasticity following memory consolidation. Since estrogen has been shown to play an important role in synaptogenesis, a key aspect of the synaptic plasticity, we aimed to study the effects of estrogen on Arc expression in SH-SY5Y human neuroblastoma cells. Using quantitative real-time PCR, Western blot, and confocal immunocytochemistry techniques we found that estrogen markedly increased Arc mRNA and protein expression in SH-SY5Y cells. Estrogen-activated Arc expression was mediated via mitogen-activated protein kinase (MAPK) and phosphoinositide-3 kinase (PI-3K), but not protein kinase C (PKC) and Rho-associated kinase (ROCK), and in the estrogen receptor (ER)-dependent manner. Estrogen also significantly upregulated the dendritic spine scaffolding protein, postsynaptic density-95 (PSD-95), as well as expression of the presynaptic vesicle protein, synaptophysin. Our findings demonstrate the possible mechanisms of estrogen-induced synaptic plasticity, as well as memory consolidation.

Synaptic activity-responsive element in the Arc/Arg3.1 promoter essential for synapse-to-nucleus signaling in activated neurons

The neuronal immediate early gene Arc/Arg-3.1 is widely used as one of the most reliable molecular markers for intense synaptic activity in vivo. However, the cis-acting elements responsible for such stringent activity dependence have not been firmly identified. Here we combined luciferase reporter assays in cultured cortical neurons and comparative genome mapping to identify the critical synaptic activity-responsive elements (SARE) of the Arc/Arg-3.1 gene. A major SARE was found as a unique approximately 100-bp element located at >5 kb upstream of the Arc/Arg-3.1 transcription initiation site in the mouse genome. This single element, when positioned immediately upstream of a minimal promoter, was necessary and sufficient to replicate crucial properties of endogenous Arc/Arg-3.1's transcriptional regulation, including rapid onset of transcription triggered by synaptic activity and low basal expression during synaptic inactivity. We identified the major determinants of SARE as a unique cluster of neuronal activity-dependent cis-regulatory elements consisting of closely localized binding sites for CREB, MEF2, and SRF. Consistently, a SARE reporter could readily trace and mark an ensemble of cells that have experienced intense activity in the recent past in vivo. Taken together, our work uncovers a novel transcriptional mechanism by which a critical 100-bp element, SARE, mediates a predominant component of the synapse-to-nucleus signaling in ensembles of Arc/Arg-3.1-positive activated neurons.

Proteolysis of proBDNF is a key regulator in the formation of memory

It is essential to understand the molecular processes underlying long-term memory to provide therapeutic targets of aberrant memory that produce pathological behaviour in humans. Under conditions of recall, fully-consolidated memories can undergo reconsolidation or extinction. These retrieval-mediated memory processes may rely on distinct molecular processes. The cellular mechanisms initiating the signature molecular events are not known. Using infusions of protein synthesis inhibitors, antisense oligonucleotide targeting brain-derived neurotrophic factor (BDNF) mRNA or tPA-STOP (an inhibitor of the proteolysis of BDNF protein) into the hippocampus of the awake rat, we show that acquisition and extinction of contextual fear memory depended on the increased and decreased proteolysis of proBDNF (precursor BDNF) in the hippocampus, respectively. Conditions of retrieval that are known to initiate the reconsolidation of contextual fear memory, a BDNF-independent memory process, were not correlated with altered proBDNF cleavage. Thus, the processing of BDNF was associated with the acquisition of new information and the updating of information about a salient stimulus. Furthermore, the differential requirement for the processing of proBDNF by tPA in distinct memory processes suggest that the molecular events actively engaged to support the storage and/or the successful retrieval of memory depends on the integration of ongoing experience with past learning.

Extracellular matrix protein SC1/hevin in the hippocampus following pilocarpine-induced status epilepticus

Pilocarpine-induced status epilepticus (SE) mimics many features of temporal lobe epilepsy and is a useful model to study neural changes that result from prolonged seizure activity. In this study, distribution of the anti-adhesive extracellular matrix protein SC1 was examined in the rat hippocampus following SE. Western blotting showed decreased levels of SC1 protein in the week following SE. Immunohistochemistry demonstrated that the decrease in overall SC1 protein levels was reflected by a reduction of SC1 signal in granule cells of the dentate gyrus. Interestingly, levels of SC1 protein in neurons of the seizure-resistant CA2 sector of the hippocampus did not change throughout the seizure time course. However, at 1 day post-SE, a subset of neurons of the hippocampal CA1, CA3, and hilar regions, which are noted for extensive neuronal degeneration after SE, exhibited a transient increase in SC1 signal. Neurons exhibiting enhanced SC1 signal were not detected at 7 days post-SE. The cellular stress response was also examined. A prominent induction of heat-shock protein (Hsp70) and Hsp27 was detected following SE, while levels of constitutively expressed Hsp40, Hsp90, Hsp110, and Hsc70 showed little change at the time points examined. The subset of neurons that demonstrated a transient increase in SC1 colocalized with the cellular stress marker Hsp70, the degeneration marker Fluoro-Jade B, and the neuron activity marker activity-regulated cytoskeleton-associated protein (Arc). Taken together, these findings suggest that SC1 may be a component of the 'matrix response' involved in remodeling events associated with neuronal degeneration following neural injury.

Food for thought: the role of dietary flavonoids in enhancing human memory, learning and neuro-cognitive performance

Emerging evidence suggests that dietary-derived flavonoids have the potential to improve human memory and neuro-cognitive performance via their ability to protect vulnerable neurons, enhance existing neuronal function and stimulate neuronal regeneration. Long-term potentiation (LTP) is widely considered to be one of the major mechanisms underlying memory acquisition, consolidation and storage in the brain and is known to be controlled at the molecular level by the activation of a number of neuronal signalling pathways. These pathways include the phosphatidylinositol-3 kinase/protein kinase B/Akt (Akt), protein kinase C, protein kinase A, Ca-calmodulin kinase and mitogen-activated protein kinase pathways. Growing evidence suggests that flavonoids exert effects on LTP, and consequently memory and cognitive performance, through their interactions with these signalling pathways. Of particular interest is the ability of flavonoids to activate the extracellular signal-regulated kinase and the Akt signalling pathways leading to the activation of the cAMP-response element-binding protein, a transcription factor responsible for increasing the expression of a number of neurotrophins important in LTP and long-term memory. One such neurotrophin is brain-derived neurotrophic factor, which is known to be crucial in controlling synapse growth, in promoting an increase in dendritic spine density and in enhancing synaptic receptor density. The present review explores the potential of flavonoids and their metabolite forms to promote memory and learning through their interactions with neuronal signalling pathways pivotal in controlling LTP and memory in human subjects.

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.

Flavonoids: modulators of brain function?

Emerging evidence suggests that dietary phytochemicals, in particular flavonoids, may exert beneficial effects on the central nervous system by protecting neurons against stress-induced injury, by suppressing neuroinflammation and by improving cognitive function. It is likely that flavonoids exert such effects, through selective actions on different components of a number of protein kinase and lipid kinase signalling cascades, such as the phosphatidylinositol-3 kinase (PI3K)/Akt, protein kinase C and mitogen-activated protein kinase (MAPK) pathways. This review explores the potential inhibitory or stimulatory actions of flavonoids within these pathways, and describes how such interactions are likely to underlie neurological effects through their ability to affect the activation state of target molecules and/or by modulating gene expression. Future research directions are outlined in relation to the precise site(s) of action of flavonoids within signalling pathways and the sequence of events that allow them to regulate neuronal function.

Activation of phasic pontine-wave generator in the rat: a mechanism for expression of plasticity-related genes and proteins in the dorsal hippocampus and amygdala

A number of behavioral studies have emphasized the importance of interactions between the pontine-wave (P-wave) generator and the dorsal hippocampus (DH) in two-way active avoidance (TWAA) memory processing; however, the direct involvement of the P-wave generator in the TWAA training trial-induced molecular events in the DH and amygdala has not been systematically evaluated. Here we demonstrate that the TWAA learning training trials activate P-wave generator, and increase phosphorylation of CREB (pCREB) and expression of activity-regulated cytoskeletal-associated (Arc) protein, as well as messenger ribonucleic acid (mRNAs) of Arc, brain-derived nerve growth factor (BDNF) and early growth response-1 (Egr-1) in the DH and amygdala. Selective elimination of P-wave-generating cells abolished P-wave activity and suppressed TWAA learning training trial-induced expression of pCREB and Arc proteins and Arc, BDNF and Egr-1 mRNAs in the DH and amygdala. Following a session of TWAA training, all rats were equal in terms of time spent in wakefulness, slow-wave sleep and rapid eye movement (REM) sleep irrespective of P-wave lesions. The second set of experiments demonstrated that localized cholinergic stimulation of the P-wave generator increased expression of Arc, BDNF and Egr-1 mRNAs in the DH. Together, these findings provide the first direct evidence that activation of P-wave-generating cells is critically involved in the TWAA training trial-induced expression of plasticity-related genes in the DH and amygdala. These findings are discussed in relation to the role of P-wave generator activation for the REM sleep-dependent development and cognitive functions of the brain.

Constitutive activation of the G-protein subunit Galphas within forebrain neurons causes PKA-dependent alterations in fear conditioning and cortical Arc mRNA expression

Memory formation requires cAMP signaling; thus, this cascade has been of great interest in the search for cognitive enhancers. Given that medications are administered long-term, we determined the effects of chronically increasing cAMP synthesis in the brain by expressing a constitutively active isoform of the G-protein subunit Galphas (Galphas*) in postnatal forebrain neurons of mice. Previously, we showed that Galphas* mice exhibit increased adenylyl cyclase activity but decreased cAMP levels in cortex and hippocampus due to a PKA-dependent increase in total cAMP phosphodiesterase (PDE) activity. Here, we extend previous findings by determining if Galphas* mice show increased activity of specific PDE families that are regulated by PKA, if Galphas* mice show PKA-dependent deficits in fear memory, and if these memory deficits are associated with PKA-dependent alterations in neuronal activity as mapped by Arc mRNA expression. Consistent with previous findings, we show here that Galphas* mice exhibit a significant compensatory increase in cAMP PDE1 activity and a trend toward increased cAMP PDE4 activity. Further, inhibiting the presumably elevated PKA activity in Galphas* mice fully rescues short- and long-term memory deficits in a fear-conditioning task, while extending the training session from one to four CS-US pairings partially rescues these deficits. Mapping of Arc mRNA levels suggests these PKA-dependent memory deficits may be related to decreased neuronal activity specifically within the cortex. Galphas* mice show decreased Arc mRNA expression in CA1, orbital cortex, and cortical regions surrounding the hippocampus; however, only the deficits in cortical regions surrounding the hippocampus are PKA dependent. Our results imply that chronically stimulating targets upstream of cAMP may detrimentally affect cognition.

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.

Arc/Arg3.1 translation is controlled by convergent N-methyl-D-aspartate and Gs-coupled receptor signaling pathways

Arc/Arg3.1 is an immediate early gene whose expression is necessary for the late-phase of long-term potentiation (LTP) and memory consolidation. Whereas pathways regulating Arc transcription have been extensively investigated, less is known about the role of post-transcriptional mechanisms in Arc expression. Fluorescence microscopy experiments in cultured hippocampal neurons revealed that Arc protein level was dramatically increased by activation of the cAMP-dependent protein kinase (PKA) pathway, which is implicated in long-term memory. A PKA-dependent increase in Arc protein level was observed after pharmacological or synaptic activation of N-methyl-D-aspartate (NMDA) receptors, which play a critical role in both LTP induction and learning. Arc protein was also up-regulated by activation of PKA through G(s)-coupled dopamine and beta-adrenergic receptors, which regulate the late-phase of LTP and memory. When agonists for the NMDA and G(s)-coupled receptors were co-applied, they had an additive effect on Arc protein expression. Interestingly, G(s)-coupled receptor stimulation was ineffective in the presence of an NMDA receptor antagonist, suggesting calcium influx through the NMDA receptor plays a gating role in this pathway. Stimulation of the cAMP/PKA pathway did not affect Arc mRNA level or protein stability, identifying translational efficacy as the main determinant of Arc protein expression level. It is concluded that efficient Arc translation requires NMDA receptor activity, whereas a further enhancement can be achieved with activation of G(s)-coupled receptors. These experiments have, therefore, revealed remarkable similarities in the signaling pathways that control Arc expression and those that regulate LTP, learning, and memory.

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.

Acute onset by 5-HT6-receptor activation on rat brain brain-derived neurotrophic factor and activity-regulated cytoskeletal-associated protein mRNA expression

A number of previous studies have shown that chronic but not acute treatment with antidepressant drugs targeting the central 5-HT system, enhances mRNA expression for a number of genes including, brain-derived neurotrophic factor (BDNF) and the effector immediate early gene (IEG), activity-regulated, cytoskeletal-associated protein (Arc). The present study investigated the effects of 5-HT(6)-receptor activation on hippocampal and cortical levels of mRNA expression of BDNF and Arc in the rat. The selective 5-HT(6)-receptor agonist LY-586713 was administered acutely (0.1-10 mg/kg, s.c.) and mRNA levels of BDNF and Arc were measured 18 h later. Administration of LY-586713 caused a bell-shaped dose response on hippocampal BDNF mRNA expression, having no effect at 0.1 mg/kg, a significant up-regulation at 1 mg/kg and no effect at 10 mg/kg. The up-regulation in BDNF expression observed at 1 mg/kg was completely blocked by pre-treatment with the selective 5-HT(6)-receptor antagonist SB-271046 (10 mg/kg, s.c.). The effective dose (1 mg/kg) of LY-586713 on the induction of BDNF expression was also tested on Arc expression. Acute administration of LY-586713 at this dose caused marked increases of the Arc mRNA levels in cortical and hippocampal regions. These increases were also attenuated by SB-271046 (10 mg/kg) in all regions of the hippocampus, as well as the parietal cortex. However, in frontal cortical regions there was no attenuation by the antagonist. Moreover, SB-271046 alone increased Arc expression in these regions. The results presented here provide the first evidence for the involvement of the 5-HT(6) receptor in regulating BDNF and Arc mRNA expression, suggesting that LY-586713 has potential effects on neuronal plasticity. Overall, these findings suggest that, as opposed to more general 5-HT receptor activation by, for example, antidepressants, direct 5-HT(6)-receptor activation results in a more rapid rise in BDNF and Arc mRNA expression which does not require repeated administration.

A form of perforant path LTP can occur without ERK1/2 phosphorylation or immediate early gene induction

Stimulation paradigms that induce perforant path long-term potentiation (LTP) initiate phosphorylation of ERK1/2 and induce expression of a variety of immediate early genes (IEGs). These events are thought to be critical components of the mechanism for establishing the changes in synaptic efficacy that endure for hours or longer. Here we show that in mice, perforant path LTP can be induced using a standard protocol (repeated trains at 250 Hz), without accompanying increases in immunostaining for p-ERK1/2 or increased in expression of representative IEGs (Arc and c-fos). Signaling pathways capable of inducing ERK phosphorylation and IEG transcription are intact in mice because ERK phosphorylation differs strikingly in awake versus anesthetized mice, and IEG expression is strongly induced by electroconvulsive seizures. In pursuing the reasons for the lack of induction with LTP, we found that in rats, one of the stimulation paradigms used to induce perforant path LTP (trains at 250 Hz) also does not activate MAP kinase or induce IEG expression, despite the fact that the LTP induced by 250 Hz stimulation requires NMDA receptor activation and persists for hours. These findings indicate that there are different forms of perforant path LTP, one of which does not require MAP kinase activation or IEG induction. Moreover, these data demonstrate that different LTP induction paradigms do not have identical molecular consequences, which may account for certain discrepancies between previous studies.

Activity-regulated cytoskeleton-associated protein Arc/Arg3.1 binds to spectrin and associates with nuclear promyelocytic leukemia (PML) bodies

Activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is an immediate early gene, whose expression in the central nervous system is induced by specific patterns of synaptic activity. Arc is required for the late-phase of long-term potentiation (LTP) and memory consolidation, and has been implicated in AMPA receptor trafficking. Since Arc's molecular function remains incompletely understood, we have determined its subcellular localization in cultured hippocampal neurons and HEK 293T cells. Fluorescence microscopy experiments revealed that both endogenous and exogenous Arc protein was primarily found in the nucleus, where it concentrated in puncta associated with promyelocytic leukemia (PML) bodies, proposed sites of transcriptional regulation. Arc co-localized and interacted with the betaIV spectrin splice variant betaSpIVSigma5, a nuclear spectrin isoform associated with PML bodies and the nuclear matrix. A small region of Arc containing the coiled-coil domain is also restricted to beta-spectrin-positive puncta, while the isolated spectrin homology domain is diffusely localized. Finally, Arc and betaSpIVSigma5 synergistically increased the number of PML bodies. These results suggest that Arc functions as a spectrin-binding protein, forming a complex that may provide a role at sites of transcriptional regulation within the nucleus.

Neuronal competition and selection during memory formation

Competition between neurons is necessary for refining neural circuits during development and may be important for selecting the neurons that participate in encoding memories in the adult brain. To examine neuronal competition during memory formation, we conducted experiments with mice in which we manipulated the function of CREB (adenosine 3',5'-monophosphate response element-binding protein) in subsets of neurons. Changes in CREB function influenced the probability that individual lateral amygdala neurons were recruited into a fear memory trace. Our results suggest a competitive model underlying memory formation, in which eligible neurons are selected to participate in amemorytrace as a function of their relative CREB activity at the time of learning.

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.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Tinnitus behavior and hearing function correlate with the reciprocal expression patterns of BDNF and Arg3.1/arc in auditory neurons following acoustic trauma

The molecular changes following sensory trauma and the subsequent response of the CNS are poorly understood. We focused on finding a molecular tool for monitoring the features of excitability which occur following acoustic trauma to the auditory system. Of particular interest are genes that alter their expression pattern during activity-induced changes in synaptic efficacy and plasticity. The expression of brain-derived neurotrophic factor (BDNF), the activity-dependent cytoskeletal protein (Arg3.1/arc), and the immediate early gene c-Fos were monitored in the peripheral and central auditory system hours and days following a traumatic acoustic stimulus that induced not only hearing loss but also phantom auditory perception (tinnitus), as shown in rodent animal behavior models. A reciprocal responsiveness of activity-dependent genes became evident between the periphery and the primary auditory cortex (AI): as c-Fos and BDNF exon IV expression was increased in spiral ganglion neurons, Arg3.1/arc and (later on) BDNF exon IV expression was reduced in AI. In line with studies indicating increased spontaneous spike activity at the level of the inferior colliculus (IC), an increase in BDNF and GABA-positive neurons was seen in the IC. The data clearly indicate the usefulness of Arg3.1/arc and BDNF for monitoring trauma-induced activity changes and the associated putative plasticity responses in the auditory system.

Arc/Arg3.1 is essential for the consolidation of synaptic plasticity and memories

Arc/Arg3.1 is robustly induced by plasticity-producing stimulation and specifically targeted to stimulated synaptic areas. To investigate the role of Arc/Arg3.1 in synaptic plasticity and learning and memory, we generated Arc/Arg3.1 knockout mice. These animals fail to form long-lasting memories for implicit and explicit learning tasks, despite intact short-term memory. Moreover, they exhibit a biphasic alteration of hippocampal long-term potentiation in the dentate gyrus and area CA1 with an enhanced early and absent late phase. In addition, long-term depression is significantly impaired. Together, these results demonstrate a critical role for Arc/Arg3.1 in the consolidation of enduring synaptic plasticity and memory storage.

A role for eukaryotic translation initiation factor 2B (eIF2B) in taste memory consolidation and in thermal control establishment during the critical period for sensory development

All species exhibit critical periods for sensory development, yet very little is known about the molecules involved in the changes in the network wiring that underlies this process. Here the role of transcription regulation of the translation machinery was determined by evaluating the expression of eIF2Bepsilon, an essential component of translation initiation, in both taste-preference development and thermal control establishment in chicks. Analysis of the expression pattern of this gene after passive-avoidance training revealed clear induction of eIF2Bepsilon in both the mesopallium intermediomediale (IMM) and in the striatum mediale (StM). In addition, a correlation was found between the concentration of methylanthranilate (MeA), which was the malaise substrate in the passive-avoidance training procedure, the duration of memory, and the expression level of eIF2Bepsilon. Training chicks on a low concentration of MeA induced short-term memory and low expression level of eIF2Bepsilon, whereas a high concentration of MeA induced long-term memory and a high expression level of eIF2Bepsilon in both the IMM and StM. Furthermore, eIF2Bepsilon-antisense "knock-down" not only reduced the amount of eIF2Bepsilon but also attenuated taste memory formation. In order to determine whether induction of eIF2Bepsilon is a general feature of neuronal plasticity, we checked whether it was induced in other forms of neuronal plasticity, with particular attention to its role in temperature control establishment, which represents hypothalamic-related plasticity. It was established that eIF2Bepsilon-mRNA was induced in the preopotic anterior hypothalamus during heat conditioning. Taken together, these results correlate eIF2Bepsilon with sensory development.

Differential activation of extracellular signal-regulated kinase 1 and a related complex in neuronal nuclei

The extracellular signal-regulated kinases 1 and 2 (ERKs 1/2) are known to participate in regulating transcription in response to moderate depolarization, such as synaptic stimulation, but how the same active enzyme can differentially regulate distinct transcriptional programs induced with abnormal depolarization (high potassium) is unknown. We hypothesized that ERK1 or 2 accomplishes this differential nuclear response through close association with other proteins in stable complexes. In support of this hypothesis, we have found that immunoreactivity for an apparent high molecular weight complex containing phospho-ERK1 increased in response to synaptic stimulation, but decreased in response to high potassium; p-ERK immunoreactivity at 44/42 kDa increased in both cases. Evidence supporting the conclusion that the band of interest contained ERK1 in a complex, as opposed to it being an unrelated protein crossreacting with antibodies against p-ERK, is that ERK1 (p44 MAPK) and 14-3-3 protein were electroeluted from the 160-kDa band cut from a gel. We also found the nuclear complexes to be exceptionally durable, suggesting a role for the crosslinking enzyme, transglutaminase, in its stabilization. In addition, we found other components of the ERK pathway, including MEK, ERK2, p90RSK, and Elk-1, migrating at higher-than-expected weights in brain nuclei. These results describe a novel stable complex of ERK1 in neuronal nuclei that responds differentially to synaptic and depolarizing stimulation, and thus may be capable of mediating gene transcription in a way distinct from the monomeric protein.

Memantine protects against LPS-induced neuroinflammation, restores behaviorally-induced gene expression and spatial learning in the rat

Neuroinflammation is reliably associated with the pathogenesis of a number of neurodegenerative diseases, and can be detected by the presence of activated microglia. Neuroinflammation can be induced by chronic lipopolysaccharide (LPS) infusion into the 4th ventricle of the rat resulting in region-selective microglia activation and impaired hippocampal-dependent memory. Furthermore, this treatment results in altered behaviorally-induced expression of the immediate early gene Arc, indicating altered network activity. LPS is known to activate microglia directly, leading to increased glutamate release, and in enhanced N-methyl-d-aspartate (NMDA) -dependent signaling. Taken together, the foregoing suggests that decreasing NMDA receptor activation during early stages of chronic neuroinflammation should reduce a) microglia activation, b) overexpression of Arc, and c) spatial memory deficits. Memantine, a low to moderate affinity open channel uncompetitive NMDA receptor antagonist, at low doses was used here to test these hypotheses. Rats were chronically infused into the 4th ventricle for 28 days with LPS alone, vehicle alone (via osmotic minipump) or LPS and memantine (10 mg/kg/day memantine s.c.). The results reported here demonstrate that memantine reduces OX6-immunolabeling for activated microglia, spares resident microglia, returns Arc (activity-regulated cytoskeletal associated protein, protein) -expressing neuronal populations to control levels (as revealed by Arc immunolabeling and fluorescence in situ hybridization), and ameliorates the spatial memory impairments produced by LPS alone. These data indicate that memantine therapy at low doses, recreating plasma levels similar to those of therapeutic doses in human, acts in part through its ability to reduce the effects of neuroinflammation, resulting in normal gene expression patterns and spatial learning. Combined, these findings suggest that low, therapeutically relevant doses of memantine delivered early in the development of neuroinflammation-influenced diseases may confer neural and cognitive protection.

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

Arc/Arg3.1 is an effector immediate-early gene implicated in the consolidation of memories. Although cloned a decade ago, the physiological role of Arc/Arg3.1 in the brain has remained elusive. Four papers in this issue of Neuron address this function. These studies show that Arc/Arg3.1 regulates endophilin 3 and dynamin 2, two components of the endocytosis machinery. Genetic ablation of Arc/Arg3.1 in mice or overexpression in culture suggest that Arc/Arg3.1 regulates AMPA receptor trafficking and synaptic plasticity. Finally, Arc/Arg3.1 knockout mice show memory retention deficits. These recent developments provide new insights into the function of this popular activity-dependent neuronal marker.

Scopolamine attenuates tinnitus-related plasticity in the auditory cortex

High doses of salicylate induce a temporary hearing deficit and a temporary subjective tinnitus in humans and animals. In animals, salicylate suppresses activity in the ascending auditory system but generates activity and plasticity in the auditory cortex and central amygdala. In addition to changes in the auditory system evoked by hearing impairment, enhanced stress may be crucial for the salicylate-induced alterations in the auditory cortex. The central amygdala responds to stress, and may influence cortical excitability and plasticity through cholinergic mechanisms. We investigated whether salicylate-induced plasticity in the auditory cortex is prevented by blocking cortical cholinergic receptors with the muscarinic antagonist scopolamine. Scopolamine suppresses salicylate-induced plasticity in the auditory cortex and, therefore, may be effective in suppressing the tinnitus sensation.

Different signal transduction cascades are activated simultaneously in the rat insular cortex and hippocampus following novel taste learning

Novel taste learning is a robust one-trial incidental learning process, dependent on functional activity of the insular (taste) cortex. In contrast to that of the cortex, the role of the hippocampus in taste learning is controversial. We set out to identify the time courses of the activation of mitogen-associated protein kinase (MAPK), transcription factor cAMP-response element-binding protein (CREB) and Akt/PKB (protein kinase B) in the insular cortex and hippocampus of rats subsequent to novel taste learning. Following taste learning, an early response (20 min) occurred at the same time in the insular cortex and the hippocampus. However, whereas MAPK was activated specifically in the insular cortex, CREB and Akt were phosphorylated in the hippocampus but not in the cortex. In addition, the immediate early gene, CCAAT/enhancer binding protein (C/EBPbeta) was induced in both the hippocampus and the insular cortex 18 h following taste learning. The results demonstrate, for the first time, correlative activation and gene expression in the hippocampus following novel taste learning. Moreover, the results suggest that different signal transduction cascades necessary for taste learning are activated in concert in different brain structures, to enable taste learning and consolidation.