A macromolecular synthesis-dependent late phase of long-term potentiation requiring cAMP in the medial perforant pathway of rat hippocampal slices

Brain-derived neuerotrophic factor and related mechanisms that mediate and influence progesterone-induced neuroprotection

Historically, progesterone has been studied significantly within the context of reproductive biology. However, there is now an abundance of evidence for its role in regions of the central nervous system (CNS) associated with such non-reproductive functions that include cognition and affect. Here, we describe mechanisms of progesterone action that support its brain-protective effects, and focus particularly on the role of neurotrophins (such as brain-derived neurotrophic factor, BDNF), the receptors that are critical for their regulation, and the role of certain microRNA in influencing the brain-protective effects of progesterone. In addition, we describe evidence to support the particular importance of glia in mediating the neuroprotective effects of progesterone. Through this review of these mechanisms and our own prior published work, we offer insight into why the effects of a progestin on brain protection may be dependent on the type of progestin (e.g., progesterone versus the synthetic, medroxyprogesterone acetate) used, and age, and as such, we offer insight into the future clinical implication of progesterone treatment for such disorders that include Alzheimer's disease, stroke, and traumatic brain injury.

Burst firing is required for induction of Hebbian LTP at lateral perforant path to hippocampal granule cell synapses

High frequency burst firing is critical in summation of back-propagating action potentials (APs) in dendrites, which may greatly depolarize dendritic membrane potential. The physiological significance of burst firings of hippocampal dentate GCs in synaptic plasticity remains unknown. We found that GCs with low input resistance could be categorized into regular-spiking (RS) and burst-spiking (BS) cells based on their initial firing frequency (F_init) upon somatic rheobase current injection, and investigated how two types of GCs differ in long-term potentiation (LTP) induced by high-frequency lateral perforant pathway (LPP) inputs. Induction of Hebbian LTP at LPP synapses required at least three postsynaptic APs at F_init higher than 100 Hz, which was met in BS but not in RS cells. The synaptically evoked burst firing was critically dependent on persistent Na⁺ current, which was larger in BS than RS cells. The Ca²⁺ source for Hebbian LTP at LPP synapses was primarily provided by L-type calcium channels. In contrast, Hebbian LTP at medial PP synapses was mediated by T-type calcium channels, and could be induced regardless of cell types or F_init of postsynaptic APs. These results suggest that intrinsic firing properties affect synaptically driven firing patterns, and that bursting behavior differentially affects Hebbian LTP mechanisms depending on the synaptic input pathway.

eIF4E phosphorylation recruits β-catenin to mRNA cap and promotes Wnt pathway translation in dentate gyrus LTP maintenance

The mRNA cap-binding protein, eukaryotic initiation factor 4E (eIF4E), is crucial for translation and regulated by Ser209 phosphorylation. However, the biochemical and physiological role of eIF4E phosphorylation in translational control of long-term synaptic plasticity is unknown. We demonstrate that phospho-ablated Eif4e^S209A Knockin mice are profoundly impaired in dentate gyrus LTP maintenance in vivo, whereas basal perforant path-evoked transmission and LTP induction are intact. mRNA cap-pulldown assays show that phosphorylation is required for synaptic activity-induced removal of translational repressors from eIF4E, allowing initiation complex formation. Using ribosome profiling, we identified selective, phospho-eIF4E-dependent translation of the Wnt signaling pathway in LTP. Surprisingly, the canonical Wnt effector, β-catenin, was massively recruited to the eIF4E cap complex following LTP induction in wild-type, but not Eif4e^S209A, mice. These results demonstrate a critical role for activity-evoked eIF4E phosphorylation in dentate gyrus LTP maintenance, remodeling of the mRNA cap-binding complex, and specific translation of the Wnt pathway.

Aß Pathology and Neuron-Glia Interactions: A Synaptocentric View

Alzheimer's disease (AD) causes the majority of dementia cases worldwide. Early pathological hallmarks include the accumulation of amyloid-ß (Aß) and activation of both astrocytes and microglia. Neurons form the building blocks of the central nervous system, and astrocytes and microglia provide essential input for its healthy functioning. Their function integrates at the level of the synapse, which is therefore sometimes referred to as the "quad-partite synapse". Increasing evidence puts AD forward as a disease of the synapse, where pre- and postsynaptic processes, as well as astrocyte and microglia functioning progressively deteriorate. Here, we aim to review the current knowledge on how Aß accumulation functionally affects the individual components of the quad-partite synapse. We highlight a selection of processes that are essential to the healthy functioning of the neuronal synapse, including presynaptic neurotransmitter release and postsynaptic receptor functioning. We further discuss how Aß affects the astrocyte's capacity to recycle neurotransmitters, release gliotransmitters, and maintain ion homeostasis. We additionally review literature on how Aß changes the immunoprotective function of microglia during AD progression and conclude by summarizing our main findings and highlighting the challenges in current studies, as well as the need for further research.

Progression in Time of Dentate Gyrus Granule Cell Layer Widening due to Excitotoxicity Occurs along In Vivo LTP Reinstatement and Contextual Fear Memory Recovery

The dentate gyrus (DG) is the gateway of sensory information arriving from the perforant pathway (PP) to the hippocampus. The adequate integration of incoming information into the DG is paramount in the execution of hippocampal-dependent cognitive functions. An abnormal DG granule cell layer (GCL) widening due to granule cell dispersion has been reported under hyperexcitation conditions in animal models as well as in patients with mesial temporal lobe epilepsy, but also in patients with no apparent relation to epilepsy. Strikingly, it is unclear whether the presence and severity of GCL widening along time affect synaptic processing arising from the PP and alter the performance in hippocampal-mediated behaviors. To evaluate the above, we injected excitotoxic kainic acid (KA) unilaterally into the DG of mice and analyzed the evolution of GCL widening at 10 and 30 days post injection (dpi), while analyzing if KA-induced GCL widening affected in vivo long-term potentiation (LTP) in the PP-DG pathway, as well as the performance in learning and memory through contextual fear conditioning. Our results show that at 10 dpi, when a subtle GCL widening was observed, LTP induction, as well as contextual fear memory, were impaired. However, at 30 dpi when a pronounced increase in GCL widening was found, LTP induction and contextual fear memory were already reestablished. These results highlight the plastic potential of the DG to recover some of its functions despite a major structural alteration such as abnormal GCL widening.

Butyric Acid Precursor Tributyrin Modulates Hippocampal Synaptic Plasticity and Prevents Spatial Memory Deficits: Role of PPARγ and AMPK

BACKGROUND

Short chain fatty acids (SCFA), such as butyric acid (BA), derived from the intestinal fermentation of dietary fiber and contained in dairy products, are gaining interest in relation to their possible beneficial effects on neuropsychological disorders.

METHODS

C57BL/6J male mice were used to investigate the effect of tributyrin (TB), a prodrug of BA, on hippocampus (HIP)-dependent spatial memory, HIP synaptic transmission and plasticity mechanisms, and the expression of genes and proteins relevant to HIP glutamatergic transmission.

RESULTS

Ex vivo studies, carried out in HIP slices, revealed that TB can transform early-LTP into late-LTP (l-LTP) and to rescue LTP-inhibition induced by scopolamine. The facilitation of l-LTP induced by TB was blocked both by GW9662 (a PPARγ antagonist) and C-Compound (an AMPK inhibitor), suggesting the involvement of both PPARγ and AMPK on TB effects. Moreover, 48-hour intake of a diet containing 1% TB prevented, in adolescent but not in adult mice, scopolamine-induced impairment of HIP-dependent spatial memory. In the adolescent HIP, TB upregulated gene expression levels of Pparg, leptin, and adiponectin receptors, and that of the glutamate receptor subunits AMPA-2, NMDA-1, NMDA-2A, and NMDA-2B.

CONCLUSIONS

Our study shows that TB has a positive influence on LTP and HIP-dependent spatial memory, which suggests that BA may have beneficial effects on memory.

Investigating the effect of crocin on memory deficits induced by total sleep deprivation (TSD) with respect to the BDNF, TrkB and ERK levels in the hippocampus of male Wistar rats

BACKGROUND

Sleep deprivation (SD) induces cognitive impairments such as memory deficit. Brain-derived neurotrophic factor (BDNF) is considered as the most critical neurotrophin in the central nervous system that is involved in sleep and memory. The main receptor of BDNF, tropomyosin receptor kinase B (TrkB), is dramatically expressed in the hippocampus. Also, extracellular signal-regulated kinase (ERK) has a significant role in memory function. Crocin is a carotenoid chemical compound and the active component of the flower Crocus sativus L. (saffron) that improves memory function and increases the level of BDNF, TrkB and ERK.

AIMS

In this research, we aimed to investigate the effect of total SD (TSD, 24 h) and crocin on memory performance, and BDNF, TrkB and ERK hippocampal levels.

METHODS

Passive avoidance memory was assessed using step-through, and working memory was measured using Y-maze tasks. The level of proteins in both hemispheres of the hippocampus was evaluated using Western blotting. Crocin was injected intraperitoneally at doses of 1, 5 and 15 mg/kg.

RESULTS

Twenty-four-hour TSD impaired both types of memories and decreased the level of all proteins in both hemispheres of the hippocampus. Crocin at all doses restored TSD-induced memory deficits. Crocin (15 mg/kg) reversed the effect of TSD on levels of all proteins.

CONCLUSIONS

The adverse effect of TSD on the level of proteins in the hippocampus may disrupt synaptic plasticity and transmission, which induces memory impairment. Additionally, the restoration effect of crocin on the decrease in protein levels may be involved in its improvement effect on memory performance.

Hippocampal subfield-specific Homer1a expression is triggered by learning-facilitated long-term potentiation and long-term depression at medial perforant path synapses

Learning about general aspects, or content details, of space results in differentiated neuronal information encoding within the proximodistal axis of the hippocampus. These processes are tightly linked to long-term potentiation (LTP) and long-term depression (LTD). Here, we explored the precise sites of encoding of synaptic plasticity in the hippocampus that are mediated by information throughput from the perforant path. We assessed nuclear Homer1a-expression that was triggered by electrophysiological induction of short and long forms of hippocampal synaptic plasticity, and compared it to Homer1a-expression that was triggered by LTP and LTD enabled by different forms of spatial learning. Plasticity responses were induced by patterned stimulation of the perforant path and were recorded in the dentate gyrus (DG) of freely behaving rats. We used fluorescence in situ hybridization to detect experience-dependent nuclear encoding of Homer1a in proximodistal hippocampal subfields. Induction of neither STP nor STD resulted in immediate early gene (IEG) encoding. Electrophysiological induction of robust LTP, or LTD, resulted in highly significant and widespread induction of nuclear Homer1a in all hippocampal subfields. LTP that was facilitated by novel spatial exploration triggered similar widespread Homer1a-expression. The coupling of synaptic depression with the exploration of a novel configuration of landmarks resulted in localized IEG expression in the proximal CA3 region and the lower (infrapyramidal) blade of the DG. Our findings support that synaptic plasticity induction via perforant path inputs promotes widespread hippocampal information encoding. Furthermore, novel spatial exploration promotes the selection of a hippocampal neuronal network by means of LTP that is distributed in an experience-dependent manner across all hippocampus subfields. This network may be modified during spatial content learning by LTD in specific hippocampal subfields. Thus, long-term plasticity-inducing events result in IEG expression that supports establishment and/or restructuring of neuronal networks that are necessary for long-term information storage.

Atrx Deletion in Neurons Leads to Sexually Dimorphic Dysregulation of miR-137 and Spatial Learning and Memory Deficits

ATRX gene mutations have been identified in syndromic and non-syndromic intellectual disabilities in humans. ATRX is known to maintain genomic stability in neuroprogenitor cells, but its function in differentiated neurons and memory processes remains largely unresolved. Here, we show that the deletion of neuronal Atrx in mice leads to distinct hippocampal structural defects, fewer presynaptic vesicles, and an enlarged postsynaptic area at CA1 apical dendrite-axon junctions. We identify male-specific impairments in long-term contextual memory and in synaptic gene expression, linked to altered miR-137 levels. We show that ATRX directly binds to the miR-137 locus and that the enrichment of the suppressive histone mark H3K27me3 is significantly reduced upon the loss of ATRX. We conclude that the ablation of ATRX in excitatory forebrain neurons leads to sexually dimorphic effects on miR-137 expression and on spatial memory, identifying a potential therapeutic target for neurological defects caused by ATRX dysfunction.

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Loss of ATRX in neurons has sexually dimorphic effects on long-term spatial memory

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Targeted deletion of neuronal ATRX in mice causes ultrastructural synaptic defects

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ATRX null neurons show sex-specific changes in miR-137 and target synaptic transcripts

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ATRX directly binds and suppresses miR-137 in males via enrichment of H3K27me3

Epac: new emerging cAMP-binding protein

The well-known second messenger cyclic adenosine monophosphate (cAMP) regulates the morphology and physiology of neurons and thus higher cognitive brain functions. The discovery of exchange protein activated by cAMP (Epac) as a guanine nucleotide exchange factor for Rap GTPases has shed light on protein kinase A (PKA)-independent functions of cAMP signaling in neural tissues. Studies of cAMP-Epac-mediated signaling in neurons under normal and disease conditions also revealed its diverse contributions to neurodevelopment, synaptic remodeling, and neurotransmitter release, as well as learning, memory, and emotion. In this mini-review, the various roles of Epac isoforms, including Epac1 and Epac2, highly expressed in neural tissues are summarized, and controversies or issues are highlighted that need to be resolved to uncover the critical functions of Epac in neural tissues and the potential for a new therapeutic target of mental disorders.

Traumatic brain injury modifies synaptic plasticity in newly-generated granule cells of the adult hippocampus

The hippocampus is vulnerable to traumatic brain injury (TBI), and hippocampal damage is associated with cognitive deficits that are often the hallmark of TBI. Recent studies have found that TBI induces enhanced neurogenesis in the dentate gyrus (DG) of the hippocampus, and this cellular response is related to innate cognitive recovery. However, cellular mechanisms of the role of DG neurogenesis in post-TBI recovery remain unclear. This study investigated changes in long-term potentiation (LTP) within the DG in relation to TBI-induced neurogenesis. Adult male rats received a moderate TBI or sham injury and were sacrificed for brain slice recordings at 30 or 60 days post-injury. Recordings were taken from the medial perforant path input to DG granule cells in the presence or absence of the GABAergic antagonist picrotoxin, reflecting activity of either all DG granule cells or predominately newborn granule cells, respectively. Measurements of LTP observed in the total granule cell population (with picrotoxin) showed a prolonged impairment which worsened between 30 and 60 days post-TBI. Under conditions which predominantly reflected the LTP elicited in newly born granule cells (no picrotoxin), a strikingly different pattern of post-TBI changes was observed, with a time-dependent cycle of functional impairment and recovery. At 30 days after injury this cell population showed little or no LTP, but by 60 days the capacity for LTP of the newly born granule cells was no different from that of sham controls. The time-frame of LTP improvements in the newborn cell population, comparable to that of behavioral recovery reported previously, suggests the unique functional properties of newborn granule cells enable them to contribute to restorative change following brain injury.

Optogenetic Manipulation of Postsynaptic cAMP Using a Novel Transgenic Mouse Line Enables Synaptic Plasticity and Enhances Depolarization Following Tetanic Stimulation in the Hippocampal Dentate Gyrus

cAMP is a positive regulator tightly involved in certain types of synaptic plasticity and related memory functions. However, its spatiotemporal roles at the synaptic and neural circuit levels remain elusive. Using a combination of a cAMP optogenetics approach and voltage-sensitive dye (VSD) imaging with electrophysiological recording, we define a novel capacity of postsynaptic cAMP in enabling dentate gyrus long-term potentiation (LTP) and depolarization in acutely prepared murine hippocampal slices. To manipulate cAMP levels at medial perforant path to granule neuron (MPP-DG) synapses by light, we generated transgenic (Tg) mice expressing photoactivatable adenylyl cyclase (PAC) in DG granule neurons. Using these Tg(CMV-Camk2a-RFP/bPAC)3Koka mice, we recorded field excitatory postsynaptic potentials (fEPSPs) from MPP-DG synapses and found that photoactivation of PAC during tetanic stimulation enabled synaptic potentiation that persisted for at least 30 min. This form of LTP was induced without the need for GABA receptor blockade that is typically required for inducing DG plasticity. The paired-pulse ratio (PPR) remained unchanged, indicating the cAMP-dependent LTP was likely postsynaptic. By employing fast fluorescent voltage-sensitive dye (VSD: di-4-ANEPPS) and fluorescence imaging, we found that photoactivation of the PAC actuator enhanced the intensity and extent of dentate gyrus depolarization triggered following tetanic stimulation. These results demonstrate that the elevation of cAMP in granule neurons is capable of rapidly enhancing synaptic strength and neuronal depolarization. The powerful actions of cAMP are consistent with this second messenger having a critical role in the regulation of synaptic function.

Neuromodulators and Long-Term Synaptic Plasticity in Learning and Memory: A Steered-Glutamatergic Perspective

The molecular pathways underlying the induction and maintenance of long-term synaptic plasticity have been extensively investigated revealing various mechanisms by which neurons control their synaptic strength. The dynamic nature of neuronal connections combined with plasticity-mediated long-lasting structural and functional alterations provide valuable insights into neuronal encoding processes as molecular substrates of not only learning and memory but potentially other sensory, motor and behavioural functions that reflect previous experience. However, one key element receiving little attention in the study of synaptic plasticity is the role of neuromodulators, which are known to orchestrate neuronal activity on brain-wide, network and synaptic scales. We aim to review current evidence on the mechanisms by which certain modulators, namely dopamine, acetylcholine, noradrenaline and serotonin, control synaptic plasticity induction through corresponding metabotropic receptors in a pathway-specific manner. Lastly, we propose that neuromodulators control plasticity outcomes through steering glutamatergic transmission, thereby gating its induction and maintenance.

A model of amygdala function following plastic changes at specific synapses during extinction

The synaptic networks in the amygdala have been the subject of intense interest in recent times, primarily because of the role of this structure in emotion. Fear and its extinction depend on the workings of these networks, with particular interest in extinction because of its potential to ameliorate adverse symptoms associated with post-traumatic stress disorder. Here we place emphasis on the extinction networks revealed by recent techniques, and on the probable plasticity properties of their synaptic connections. We use modules of neurons representing each of the principal components identified as involved in extinction. Each of these modules consists of neural networks, containing specific ratios of excitatory and specialized inhibitory neurons as well as synaptic plasticity mechanisms appropriate for the component of the amygdala they represent. While these models can produce dynamic output, here we concentrate on the equilibrium outputs and do not model the details of the plasticity mechanisms. Pavlovian fear conditioning generates a fear memory in the lateral amygdala module that leads to activation of neurons in the basal nucleus fear module but not in the basal nucleus extinction module. Extinction protocols excite infralimbic medial prefrontal cortex neurons (IL) which in turn excite so-called extinction neurons in the amygdala, leading to the release of endocannabinoids from them and an increase in efficacy of synapses formed by lateral amygdala neurons on them. The model simulations show how such a mechanism could explain experimental observations involving the role of IL as well as endocannabinoids in different temporal phases of extinction.

Calcium Release Mediated by Redox-Sensitive RyR2 Channels Has a Central Role in Hippocampal Structural Plasticity and Spatial Memory

AIMS

Previous studies indicate that hippocampal synaptic plasticity and spatial memory processes entail calcium release from intracellular stores mediated by ryanodine receptor (RyR) channels. In particular, RyR-mediated Ca²⁺ release is central for the dendritic spine remodeling induced by brain-derived neurotrophic factor (BDNF), a neurotrophin that stimulates complex signaling pathways leading to memory-associated protein synthesis and structural plasticity. To examine if upregulation of ryanodine receptor type-2 (RyR2) channels and the spine remodeling induced by BDNF entail reactive oxygen species (ROS) generation, and to test if RyR2 downregulation affects BDNF-induced spine remodeling and spatial memory.

RESULTS

Downregulation of RyR2 expression (short hairpin RNA [shRNA]) in primary hippocampal neurons, or inhibition of nitric oxide synthase (NOS) or NADPH oxidase, prevented agonist-mediated RyR-mediated Ca²⁺ release, whereas BDNF promoted cytoplasmic ROS generation. RyR2 downregulation or inhibitors of N-methyl-d-aspartate (NMDA) receptors, or NOS or of NADPH oxidase type-2 (NOX2) prevented RyR2 upregulation and the spine remodeling induced by BDNF, as did incubation with the antioxidant agent N-acetyl l-cysteine. In addition, intrahippocampal injection of RyR2-directed antisense oligodeoxynucleotides, which caused significant RyR2 downregulation, caused conspicuous defects in a memorized spatial memory task.

INNOVATION

The present novel results emphasize the key role of redox-sensitive Ca²⁺ release mediated by RyR2 channels for hippocampal structural plasticity and spatial memory.

CONCLUSION

Based on these combined results, we propose (i) that BDNF-induced RyR2-mediated Ca²⁺ release and ROS generation via NOS/NOX2 are strictly required for the dendritic spine remodeling and the RyR2 upregulation induced by BDNF, and (ii) that RyR2 channel expression is crucial for spatial memory processes. Antioxid. Redox Signal. 29, 1125-1146.

Delayed effects of combined stress and Aβ infusion on L-LTP of the dentate gyrus: Prevention by nicotine

Alzheimer's Disease (AD) is a progressive dementia hallmarked by the presence in the brain of extracellular beta-amyloid (Aβ) plaques and intraneuronal fibrillary tangles. Chronic stress is associated with heightened Aβ buildup and acceleration of development of AD, however, stress alone has no significant effect on synaptic plasticity in the dentate gyrus (DG) area. Previously, we have reported that the combination of stress and AD causes more severe inhibition of synaptic plasticity of hippocampal area CA1 than chronic stress or AD alone, and that chronic nicotine treatment prevents this impairment. To investigate the effect of stress and nicotine on synaptic plasticity in the relatively injury-resistant DG area, the present experiments analyzed the effect of chronic stress and the neuroprotective effect of nicotine on LTP in the DG area of a rat model of AD. Wistar rats were chronically stressed and treated with nicotine (1 mg/kg/twice daily; s.c.) for six weeks. Then, at weeks 5-6, AD model was generated by 14-day i.c.v osmotic pump infusion of Aβ peptides (300 pmol/day) into the brains of these rats. Field potential recordings from the DG area of anesthetized rats, revealed that while chronic stress did not accentuate Aβ-induced impairments of E-LTP, it markedly augmented Aβ effect on L-LTP that was only seen 100 min after multiple high frequency stimulation. This delayed action is likely to be due to impairment of process of de novo protein synthesis required for maintenance phase of L-LTP. Chronic nicotine treatment prevented stress-enhanced suppression of synaptic plasticity.

Immediate early gene expression related to learning and retention of a visual discrimination task in bamboo sharks (Chiloscyllium griseum)

Using the expression of the immediate early gene (IEG) egr-1 as a neuronal activity marker, brain regions potentially involved in learning and long-term memory functions in the grey bamboo shark were assessed with respect to selected visual discrimination abilities. Immunocytochemistry revealed a significant up-regulation of egr-1 expression levels in a small region of the telencephalon of all trained sharks (i.e., 'early' and 'late learners', 'recallers') when compared to three control groups (i.e., 'controls', 'undisturbed swimmers', 'constant movers'). There was also a well-defined difference in egr-1 expression patterns between the three control groups. Additionally, some staining was observed in diencephalic and mesencephalic sections; however, staining here was weak and occurred only irregularly within and between groups. Therefore, it could have either resulted from unintentional cognitive or non-cognitive inducements (i.e., relating to the mental processes of perception, learning, memory, and judgment, as contrasted with emotional and volitional processes) rather than being a training effect. Present findings emphasize a relationship between the training conditions and the corresponding egr-1 expression levels found in the telencephalon of Chiloscyllium griseum. Results suggest important similarities in the neuronal plasticity and activity-dependent IEG expression of the elasmobranch brain with other vertebrate groups. The presence of the egr-1 gene seems to be evolutionarily conserved and may therefore be particularly useful for identifying functional neural responses within this group.

Coupled feedback loops maintain synaptic long-term potentiation: A computational model of PKMzeta synthesis and AMPA receptor trafficking

In long-term potentiation (LTP), one of the most studied types of neural plasticity, synaptic strength is persistently increased in response to stimulation. Although a number of different proteins have been implicated in the sub-cellular molecular processes underlying induction and maintenance of LTP, the precise mechanisms remain unknown. A particular challenge is to demonstrate that a proposed molecular mechanism can provide the level of stability needed to maintain memories for months or longer, in spite of the fact that many of the participating molecules have much shorter life spans. Here we present a computational model that combines simulations of several biochemical reactions that have been suggested in the LTP literature and show that the resulting system does exhibit the required stability. At the core of the model are two interlinked feedback loops of molecular reactions, one involving the atypical protein kinase PKMζ and its messenger RNA, the other involving PKMζ and GluA2-containing AMPA receptors. We demonstrate that robust bistability-stable equilibria both in the synapse's potentiated and unpotentiated states-can arise from a set of simple molecular reactions. The model is able to account for a wide range of empirical results, including induction and maintenance of late-phase LTP, cellular memory reconsolidation and the effects of different pharmaceutical interventions.

A brief period of sleep deprivation causes spine loss in the dentate gyrus of mice

Sleep and sleep loss have a profound impact on hippocampal function, leading to memory impairments. Modifications in the strength of synaptic connections directly influences neuronal communication, which is vital for normal brain function, as well as the processing and storage of information. In a recently published study, we found that as little as five hours of sleep deprivation impaired hippocampus-dependent memory consolidation, which was accompanied by a reduction in dendritic spine numbers in hippocampal area CA1. Surprisingly, loss of sleep did not alter the spine density of CA3 neurons. Although sleep deprivation has been reported to affect the function of the dentate gyrus, it is unclear whether a brief period of sleep deprivation impacts spine density in this region. Here, we investigated the impact of a brief period of sleep deprivation on dendritic structure in the dentate gyrus of the dorsal hippocampus. We found that five hours of sleep loss reduces spine density in the dentate gyrus with a prominent effect on branched spines. Interestingly, the inferior blade of the dentate gyrus seems to be more vulnerable in terms of spine loss than the superior blade. This decrease in spine density predominantly in the inferior blade of the dentate gyrus may contribute to the memory deficits observed after sleep loss, as structural reorganization of synaptic networks in this subregion is fundamental for cognitive processes.

Synaptically driven phosphorylation of ribosomal protein S6 is differentially regulated at active synapses versus dendrites and cell bodies by MAPK and PI3K/mTOR signaling pathways

High-frequency stimulation of the medial perforant path triggers robust phosphorylation of ribosomal protein S6 (rpS6) in activated dendritic domains and granule cell bodies. Here we dissect the signaling pathways responsible for synaptically driven rpS6 phosphorylation in the dentate gyrus using pharmacological agents to inhibit PI3-kinase/mTOR and MAPK/ERK-dependent kinases. Using phospho-specific antibodies for rpS6 at different sites (ser235/236 versus ser240/244), we show that delivery of the PI3-kinase inhibitor, wortmannin, decreased rpS6 phosphorylation throughout the somatodendritic compartment (granule cell layer, inner molecular layer, outer molecular layer), especially in granule cell bodies while sparing phosphorylation at activated synapses (middle molecular layer). In contrast, delivery of U0126, an MEK inhibitor, attenuated rpS6 phosphorylation specifically in the dendritic laminae leaving phosphorylation in the granule cell bodies intact. Delivery of the mTOR inhibitor, rapamycin, abolished activation of rpS6 phosphorylation in granule cell bodies and dendrites, whereas delivery of a selective S6K1 inhibitor, PF4708671, or RSK inhibitor, SL0101-1, attenuated rpS6 phosphorylation throughout the postsynaptic cell. These results reveal that MAPK/ERK-dependent signaling is predominately responsible for the selective induction of rpS6 phosphorylation at active synapses. In contrast, PI3-kinase/mTOR-dependent signaling induces rpS6 phosphorylation throughout the somatodendritic compartment but plays a minimal role at active synapses. Collectively, these results suggest a potential mechanism by which PI3-kinase/mTOR and MAPK/ERK pathways regulate translation at specific subcellular compartments in response to synaptic activity.

MicroRNAs, miR-23a-3p and miR-151-3p, Are Regulated in Dentate Gyrus Neuropil following Induction of Long-Term Potentiation In Vivo

Translation of synaptic mRNA contributes to alterations in the proteome necessary to consolidate long-term potentiation (LTP), a model of memory processes. Yet, how this process is controlled is not fully resolved. MicroRNAs are non-coding RNAs that negatively regulate gene expression by suppressing translation or promoting mRNA degradation. As specific microRNAs are synaptically located, we hypothesized that they are ideally suited to couple synaptic activation, translational regulation, and LTP persistence. The aim of this study was to identify LTP-regulated microRNAs at or near synapses. Accordingly, LTP was induced unilaterally at perforant path-dentate gyrus synapses in awake adult Sprague-Dawley rats. Five hours later, dentate gyrus middle molecular layer neuropil, containing potentiated synapses, was laser-microdissected. MicroRNA expression profiling, using TaqMan Low Density MicroRNA Microarrays (n = 4), identified eight regulated microRNAs. Subsequent individual TaqMan assays confirmed upregulation of miR-23a-3p (1.30 ± 0.10; p = 0.015) and miR-151-3p (1.17 ± 0.19; p = 0.045) in a second cohort (n = 7). Interestingly, bioinformatic analysis indicated that miR-151-3p and miR-23a-3p regulate synaptic reorganisation and transcription, respectively. In summary, we have demonstrated for the first time that microRNAs are regulated in isolated neuropil following LTP induction in vivo, supporting the hypothesis that synaptic, LTP-responsive microRNAs contribute to LTP persistence via regulation of the synaptic proteome.

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

Glutamate Neurotransmission in Rodent Models of Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability in people younger than 45 and is a significant public health concern. In addition to primary mechanical damage to cells and tissue, TBI involves additional molecular mechanisms of injury, termed secondary injury, that continue to evolve over hours, days, weeks, and beyond. The trajectory of recovery after TBI is highly unpredictable and in many cases results in chronic cognitive and behavioral changes. Acutely after TBI, there is an unregulated release of glutamate that cannot be buffered or cleared effectively, resulting in damaging levels of glutamate in the extracellular space. This initial loss of glutamate homeostasis may initiate additional changes in glutamate regulation. The excitatory amino acid transporters (EAATs) are expressed on both neurons and glia and are the principal mechanism for maintaining extracellular glutamate levels. Diffusion of glutamate outside the synapse due to impaired uptake may lead to increased extrasynaptic glutamate signaling, secondary injury through activation of cell death pathways, and loss of fidelity and specificity of synaptic transmission. Coordination of glutamate release and uptake is critical to regulating synaptic strength, long-term potentiation and depression, and cognitive processes. In this review, we will discuss dysregulation of extracellular glutamate and glutamate uptake in the acute stage of TBI and how failure to resolve acute disruptions in glutamate homeostatic mechanisms may play a causal role in chronic cognitive symptoms after TBI.

Synaptic activation of ribosomal protein S6 phosphorylation occurs locally in activated dendritic domains

Previous studies have shown that induction of long-term potentiation (LTP) induces phosphorylation of ribosomal protein S6 (rpS6) in postsynaptic neurons, but the functional significance of rpS6 phosphorylation is poorly understood. Here, we show that synaptic stimulation that induces perforant path LTP triggers phosphorylation of rpS6 (p-rpS6) locally near active synapses. Using antibodies specific for phosphorylation at different sites (ser235/236 versus ser240/244), we show that strong synaptic activation led to dramatic increases in immunostaining throughout postsynaptic neurons with selectively higher staining for p-ser235/236 in the activated dendritic lamina. Following LTP induction, phosphorylation at ser235/236 was detectable by 5 min, peaked at 30 min, and was maintained for hours. Phosphorylation at both sites was completely blocked by local infusion of the NMDA receptor antagonist, APV. Despite robust induction of p-rpS6 following high frequency stimulation, assessment of protein synthesis by autoradiography revealed no detectable increases. Exploration of a novel environment led to increases in the number of p-rpS6-positive neurons throughout the forebrain in a pattern reminiscent of immediate early gene induction and many individual neurons that were p-rpS6-positive coexpressed Arc protein. Our results constrain hypotheses about the possible role of rpS6 phosphorylation in regulating postsynaptic protein synthesis during induction of synaptic plasticity.

EPO induces changes in synaptic transmission and plasticity in the dentate gyrus of rats

Erythropoietin has shown wide physiological effects on the central nervous system in animal models of disease, and in healthy animals. We have recently shown that systemic EPO administration 15 min, but not 5 h, after daily training in a water maze is able to induce the recovery of spatial memory in fimbria-fornix chronic-lesioned animals, suggesting that acute EPO triggers mechanisms which can modulate the active neural plasticity mechanism involved in spatial memory acquisition in lesioned animals. Additionally, this EPO effect is accompanied by the up-regulation of plasticity-related early genes. More remarkably, this time-dependent effects on learning recovery could signify that EPO in nerve system modulate specific living-cellular processes. In the present article, we focus on the question if EPO could modulate the induction of long-term synaptic plasticity like LTP and LTD, which presumably could support our previous published data. Our results show that acute EPO peripheral administration 15 min before the induction of synaptic plasticity is able to increase the magnitude of the LTP (more prominent in PSA than fEPSP-Slope) to facilitate the induction of LTD, and to protect LTP from depotentiation. These findings showing that EPO modulates in vivo synaptic plasticity sustain the assumption that EPO can act not only as a neuroprotective substance, but is also able to modulate transient neural plasticity mechanisms and therefore to promote the recovery of nerve function after an established chronic brain lesion. According to these results, EPO could be use as a molecular tool for neurorestaurative treatments.

cAMP-dependent protein kinase inhibits α7 nicotinic receptor activity in layer 1 cortical interneurons through activation of D1/D5 dopamine receptors

KEY POINTS

Protein kinases can modify the function of many proteins including ion channels. However, the role of protein kinase A in modifying nicotinic receptors in the CNS has never been investigated. We showed through whole-cell recordings of layer 1 prefrontal cortical interneurons that α7 nicotinic responses are negatively modulated by protein kinase A. Furthermore, we show that stimulation of dopamine receptors can similarly attenuate α7 nicotinic responses through the activation of protein kinase A. These results suggest how the interaction of the cholinergic and dopaminergic systems may influence neuronal excitability in the brain.

ABSTRACT

Phosphorylation of ion channels, including nicotinic acetylcholine receptors (nAChRs), by protein kinases plays a key role in the modification of synaptic transmission and neuronal excitability. α7 nAChRs are the second most prevalent nAChR subtype in the CNS following α4β2. Serine 365 in the M3-M4 cytoplasmic loop of the α7 nAChR is a phosphorylation site for protein kinase A (PKA). D1/D5 dopamine receptors signal through the adenylate cyclase-PKA pathway and play a key role in working memory and attention in the prefrontal cortex. Thus, we examined whether the dopaminergic system, mediated through PKA, functionally interacts with the α7-dependent cholinergic neurotransmission. In layer 1 interneurons of mouse prefrontal cortex, α7 nicotinic currents were decreased upon stimulation with 8-Br-cAMP, a PKA activator. In HEK 293T cells, dominant negative PKA abolished 8-Br-cAMP's effect of diminishing α7 nicotinic currents, while a constitutively active PKA catalytic subunit decreased α7 currents. In brain slices, the PKA inhibitor KT-5720 nullified 8-Br-cAMP's effect of attenuating α7 nicotinic responses, while applying a PKA catalytic subunit in the pipette solution decreased α7 currents. 8-Br-cAMP stimulation reduced surface expression of α7 nAChRs, but there was no change in single-channel conductance. The D1/D5 dopamine receptor agonist SKF 83822 similarly attenuated α7 nicotinic currents from layer 1 interneurons and this attenuation of nicotinic current was prevented by KT-5720. These results demonstrate that dopamine receptor-mediated activation of PKA negatively modulates nicotinic neurotransmission in prefrontal cortical interneurons, which may be a contributing mechanism of dopamine modulation of cognitive behaviours such as attention or working memory.

Plasticity-related microRNA and their potential contribution to the maintenance of long-term potentiation

Long-term potentiation (LTP) is a form of synaptic plasticity that is an excellent model for the molecular mechanisms that underlie memory. LTP, like memory, is persistent, and both are widely believed to be maintained by a coordinated genomic response. Recently, a novel class of non-coding RNA, microRNA, has been implicated in the regulation of LTP. MicroRNA negatively regulate protein synthesis by binding to specific messenger RNA response elements. The aim of this review is to summarize experimental evidence for the proposal that microRNA play a major role in the regulation of LTP. We discuss a growing body of research which indicates that specific microRNA regulate synaptic proteins relevant to LTP maintenance, as well as studies that have reported differential expression of microRNA in response to LTP induction. We conclude that microRNA are ideally suited to contribute to the regulation of LTP-related gene expression; microRNA are pleiotropic, synaptically located, tightly regulated, and function in response to synaptic activity. The potential impact of microRNA on LTP maintenance as regulators of gene expression is enormous.

Variation in the persistence of memory: An interplay between actin dynamics and AMPA receptors

William James noted that memories could persist from minutes to weeks. This essay attempts to explain this variation by situating the explanation in the biochemistry of dendritic spines. Two outcomes are critical to generate the synaptic basis of memory: (1) the actin cytoskeleton in the spine must be degraded to permit (2) additional AMPA receptors (GluA1s) to enter new "hot spots" in the postsynaptic density. These initial outcomes can support short-lasting memories. The threshold for these events is low but the underlying synaptic changes cannot resist the endocytic processes that remove the added AMPA receptors. For the memory to persist the degraded actin cytoskeleton must be rebuilt and the vacated "hot spots" refilled with GluA2 receptors. A primary claim is that it is the stabilization of an enlarged actin cytoskeleton that is the target outcome that consolidates the synaptic basis of memory (see Lynch et al., 2007). The stabilized actin cytoskeleton has properties that enable it to garner the synaptic proteins it needs to self sustain the potentiated state and to benefit from activation of memory modulation systems. This article is part of a Special Issue entitled Brain and Memory.

Life extension factor klotho enhances cognition

Aging is the primary risk factor for cognitive decline, an emerging health threat to aging societies worldwide. Whether anti-aging factors such as klotho can counteract cognitive decline is unknown. We show that a lifespan-extending variant of the human KLOTHO gene, KL-VS, is associated with enhanced cognition in heterozygous carriers. Because this allele increased klotho levels in serum, we analyzed transgenic mice with systemic overexpression of klotho. They performed better than controls in multiple tests of learning and memory. Elevating klotho in mice also enhanced long-term potentiation, a form of synaptic plasticity, and enriched synaptic GluN2B, an N-methyl-D-aspartate receptor (NMDAR) subunit with key functions in learning and memory. Blockade of GluN2B abolished klotho-mediated effects. Surprisingly, klotho effects were evident also in young mice and did not correlate with age in humans, suggesting independence from the aging process. Augmenting klotho or its effects may enhance cognition and counteract cognitive deficits at different life stages.

Neurological effects of honey: current and future prospects

Honey is the only insect-derived natural product with therapeutic, traditional, spiritual, nutritional, cosmetic, and industrial value. In addition to having excellent nutritional value, honey is a good source of physiologically active natural compounds, such as polyphenols. Unfortunately, there are very few current research projects investigating the nootropic and neuropharmacological effects of honey, and these are still in their early stages. Raw honey possesses nootropic effects, such as memory-enhancing effects, as well as neuropharmacological activities, such as anxiolytic, antinociceptive, anticonvulsant, and antidepressant activities. Research suggests that the polyphenol constituents of honey can quench biological reactive oxygen species and counter oxidative stress while restoring the cellular antioxidant defense system. Honey polyphenols are also directly involved in apoptotic activities while attenuating microglia-induced neuroinflammation. Honey polyphenols are useful in improving memory deficits and can act at the molecular level. Therefore, the ultimate biochemical impact of honey on specific neurodegenerative diseases, apoptosis, necrosis, neuroinflammation, synaptic plasticity, and behavior-modulating neural circuitry should be evaluated with appropriate mechanistic approaches using biochemical and molecular tools.

Cocaine facilitates protein synthesis-dependent LTP: the role of metabotropic glutamate receptors

Cocaine addiction alters synaptic plasticity in many brain areas involved in learning and memory processes, including the hippocampus. Long-term potentiation (LTP) is one of the best studied examples of hippocampal synaptic plasticity and it is considered as one of the molecular basis of learning and memory. We previously demonstrated that in the presence of cocaine, a long lasting form of hippocampal LTP is induced by a single pulse of high frequency stimulation, which in normal conditions evokes only an early form of LTP. In this study, we further explore the molecular basis of this modulation of synaptic plasticity by cocaine. By performing pharmacological experiments on hippocampal slices, we were able to show that cocaine converts early LTP to a form of LTP dependent on protein synthesis, probably through the cAMP-dependent protein kinase and extracellular signal-regulated kinase signaling cascades. We also found that metabotropic glutamate receptors are involved in this phenomenon. These studies further clarify the molecular machinery used by cocaine to alter synaptic plasticity and modulate learning and memory processes.

D1/D5 receptors and histone deacetylation mediate the Gateway Effect of LTP in hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus is critical for spatial memory and is also thought to be involved in the formation of drug-related associative memory. Here, we attempt to test an aspect of the Gateway Hypothesis, by studying the effect of consecutive exposure to nicotine and cocaine on long-term synaptic potentiation (LTP) in the DG. We find that a single injection of cocaine does not alter LTP. However, pretreatment with nicotine followed by a single injection of cocaine causes a substantial enhancement of LTP. This priming effect of nicotine is unidirectional: There is no enhancement of LTP if cocaine is administrated prior to nicotine. The facilitation induced by nicotine and cocaine can be blocked by oral administration of the dopamine D1/D5 receptor antagonist (SKF 83566) and enhanced by the D1/D5 agonist (SKF 38393). Application of the histone deacetylation inhibitor suberoylanilide hydroxamic acid (SAHA) simulates the priming effect of nicotine on cocaine. By contrast, the priming effect of nicotine on cocaine is blocked in genetically modified mice that are haploinsufficient for the CREB-binding protein (CBP) and possess only one functional CBP allele and therefore exhibit a reduction in histone acetylation. These results demonstrate that the DG of the hippocampus is an important brain region contributing to the priming effect of nicotine on cocaine. Moreover, both activation of dopamine-D1 receptor/PKA signaling pathway and histone deacetylation/CBP mediated transcription are required for the nicotine priming effect in the DG.

Mechanisms of translation control underlying long-lasting synaptic plasticity and the consolidation of long-term memory

The complexity of memory formation and its persistence is a phenomenon that has been studied intensely for centuries. Memory exists in many forms and is stored in various brain regions. Generally speaking, memories are reorganized into broadly distributed cortical networks over time through systems level consolidation. At the cellular level, storage of information is believed to initially occur via altered synaptic strength by processes such as long-term potentiation. New protein synthesis is required for long-lasting synaptic plasticity as well as for the formation of long-term memory. The mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of cap-dependent protein synthesis and is required for numerous forms of long-lasting synaptic plasticity and long-term memory. As such, the study of mTORC1 and protein factors that control translation initiation and elongation has enhanced our understanding of how the process of protein synthesis is regulated during memory formation. Herein we discuss the molecular mechanisms that regulate protein synthesis as well as pharmacological and genetic manipulations that demonstrate the requirement for proper translational control in long-lasting synaptic plasticity and long-term memory formation.

Mechanisms underlying early odor preference learning in rats

Early odor preference training in rat pups produces behavioral preferences that last from hours to lifetimes. Here, we discuss the molecular and circuitry changes we have observed in the olfactory bulb (OB) and in the anterior piriform cortex (aPC) following odor training. For normal preference learning, both structures are necessary, but learned behavior can be initiated by initiating local circuit change in either structure. Our evidence relates dynamic molecular and circuit changes to memory duration and storage localization. Results using this developmental model are consistent with biological memory theories implicating N-methyl-D-aspartate (NMDA) receptors and β-adrenoceptors, and their associated cascades, in memory induction and consolidation. Finally, our examination of the odor preference model reveals a primary role for increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor synaptic strength, and in network strength, in the creation and maintenance of preference memory in both olfactory structures.

Licking microstructure reveals rapid attenuation of neophobia

Many animals hesitate when initially consuming a novel food and increase their consumption of that food between the first and second sessions of access-a process termed attenuation of neophobia (AN). AN has received attention as a model of learning and memory; it has been suggested that plasticity resulting from an association of the novel tastant with "safe outcome" results in a change in the neural response to the tastant during the second session, such that consumption increases. Most studies have reported that AN emerges only an hour or more after the end of the first exposure to the tastant, consistent with what is known of learning-related plasticity. But these studies have typically measured consumption, rather than real-time behavior, and thus the possibility exists that a more rapidly developing AN remains to be discovered. Here, we tested this possibility, examining both consumption and individual lick times in a novel variant of a brief-access task (BAT). When quantified in terms of consumption, data from the BAT accorded well with the results of a classic one-bottle task-both revealed neophobia/AN specific to higher concentrations (for instance, 28mM) of saccharin. An analysis of licking microstructure, however, additionally revealed a real-time correlate of neophobia-an explicit tendency, similarly specific for 28-mM saccharin, to cut short the initial bout of licks in a single trial (compared with water). This relative hesitancy (i.e., the shortness of the first lick bout to 28-mM saccharin compared with water) that constitutes neophobia not only disappeared between sessions but also gradually declined in magnitude across session 1. These data demonstrate that the BAT accurately measures AN, and that aspects of AN-and the processes underlying familiarization-begin within minutes of the very first taste.

Long-term potentiation of synaptic transmission in the adult mouse insular cortex: multielectrode array recordings

The insular cortex (IC) is widely believed to be an important forebrain structure involved in cognitive and sensory processes such as memory and pain. However, little work has been performed at the cellular level to investigate the synaptic basis of IC-related brain functions. To bridge the gap, the present study was designed to characterize the basic synaptic mechanisms for insular long-term potentiation (LTP). Using a 64-channel recording system, we found that an enduring form of late-phase LTP (L-LTP) could be reliably recorded for at least 3 h in different layers of IC slices after theta burst stimulation. The induction of insular LTP is protein synthesis dependent and requires activation of both GluN2A and GluN2B subunits of the NMDA receptor, L-type voltage-gated calcium channels, and metabotropic glutamate receptor 1. The paired-pulse facilitation ratio was unaffected by insular L-LTP induction, and expression of insular L-LTP required the recruitment of postsynaptic calcium-permeable AMPA receptors. Our results provide the first in vitro report of long-term multichannel recordings of L-LTP in the IC in adult mice and suggest its potential important roles in insula-related memory and chronic pain.

A cAMP analog reverses contextual and tone memory deficits induced by a PKA inhibitor in Pavlovian fear conditioning

Despite extensive investigations, molecular mechanisms underlying the formation of fear memory are not completely understood. We have previously investigated the role of protein kinase AII (PKAII) in spatial acquisition and memory retention using Morris water maze. In the current study, we have further analyzed the role of PKA in memory consolidation using a fear conditioning response model. Experiments were performed using intrahippocampal infusions of dibutyryl cyclic AMP (bucladesine), a cell permeable analog of cAMP and H89, as a selective PKAII inhibitor, in male rats. The animals were trained for one session which included 4 consecutive trials of tone-shock pairing on the training day. Fear to the context and tone was evaluated by measuring freezing behavior 24h and 48h after training respectively. Bilateral infusion of 100 and 300μM/side of bucladesine immediately after training increased freezing percentage in rats, which indicated an improvement in memory consolidation. Administration of 10μM/side (but not 5μM/side) of H-89 impaired contextual and cued fear conditioning significantly compared to the control animals. Moreover, administration of bucladesine (100 and 300μM/side), 5min after H-89 infusion (10μM/side) reversed the impairment of contextual and auditory-cued fear conditioning in rats and freezing percentage increased to those of control animals. Altogether, these results indicate that PKA inhibitor-induced memory deficit was attenuated by a cAMP analog and that the cAMP/PKA signaling pathway has an important role for memory consolidation in fear conditioning response model.

Multiple components of eIF4F are required for protein synthesis-dependent hippocampal long-term potentiation

Persistent forms of synaptic plasticity are widely thought to require the synthesis of new proteins. This feature of long-lasting forms of plasticity largely has been demonstrated using inhibitors of general protein synthesis, such as either anisomycin or emetine. However, these drugs, which inhibit elongation, cannot address detailed questions about the regulation of translation initiation, where the majority of translational control occurs. Moreover, general protein synthesis inhibitors cannot distinguish between cap-dependent and cap-independent modes of translation initiation. In the present study, we took advantage of two novel compounds, 4EGI-1 and hippuristanol, each of which targets a different component of the eukaryotic initiation factor (eIF)4F initiation complex, and investigated their effects on long-term potentiation (LTP) at CA3-CA1 synapses in the hippocampus. We found that 4EGI-1 and hippuristanol both attenuated long-lasting late-phase LTP induced by two different stimulation paradigms. We also found that 4EGI-1 and hippuristanol each were capable of blocking the expression of newly synthesized proteins immediately after the induction of late-phase LTP. These new pharmacological tools allow for a more precise dissection of the role played by translational control pathways in synaptic plasticity and demonstrate the importance of multiple aspects of eIF4F in processes underlying hippocampal LTP, laying the foundation for future studies investigating the role of eIF4F in hippocampus-dependent memory processes.

Progesterone, brain-derived neurotrophic factor and neuroprotection

While the effects of progesterone in the CNS, like those of estrogen, have generally been considered within the context of reproductive function, growing evidence supports its importance in regulating non-reproductive functions including cognition and affect. In addition, progesterone has well-described protective effects against numerous insults in a variety of cell models, animal models and in humans. While ongoing research in several laboratories continues to shed light on the mechanism(s) by which progesterone and its related progestins exert their effects in the CNS, our understanding is still incomplete. Among the key mediators of progesterone's beneficial effects is the family of growth factors called neurotrophins. Here, we review the mechanisms by which progesterone regulates one important member of the neurotrophin family, brain-derived neurotrophic factor (BDNF), and provides support for its pivotal role in the protective program elicited by progesterone in the brain.

Role of signal transduction crosstalk between adenylyl cyclase and MAP kinase in hippocampus-dependent memory

One of the intriguing questions in neurobiology is how long-term memory (LTM) traces are established and maintained in the brain. Memory can be divided into at least two temporally and mechanistically distinct forms. Short-term memory (STM) lasts no longer than several hours, while LTM persists for days or longer. A crucial step in the generation of LTM is consolidation, a process in which STM is converted to LTM. Hippocampus-dependent LTM depends on activation of Ca(2+), Erk/MAP kinase (MAPK), and cAMP signaling pathways, as well as de novo gene expression and translation. One of the transcriptional pathways strongly implicated in LTM is the CREB/CRE (calcium, cAMP response element) transcriptional pathway. Interestingly, this transcriptional pathway may also contribute to other forms of neuroplasticity including adaptive responses to drugs. Evidence discussed in this review indicates that activation of the Erk1/2 MAP Kinase (MAPK)/CRE transcriptional pathway during the formation of hippocampus-dependent memory depends on calmodulin (CaM)-stimulated adenylyl cyclases.

Aging-related changes in neuroimmune-endocrine function: implications for hippocampal-dependent cognition

Healthy aged individuals are more likely to suffer profound memory impairments following a challenging life event such as a severe bacterial infection, surgery, or an intense psychological stressor, than are younger adults. Importantly, these peripheral challenges are capable of producing a neuroinflammatory response, (e.g., increased pro-inflammatory cytokines). In this review we will present the literature demonstrating that in the healthy aged brain this response is exaggerated and prolonged. Normal aging primes or sensitizes microglia and this appears to be the source of this amplified response. We will review the growing literature suggesting that a dysregulated neuroendocrine response in the aged organism is skewed toward higher brain CORT levels, and that this may play a critical role in priming microglia. Among the outcomes of an exaggerated neuroinflammatory response are impairments in synaptic plasticity, and reductions in key downstream mediators such as Arc and BDNF. We will show that each of these mechanisms is important for long-term memory formation, and is compromised by elevated pro-inflammatory cytokines.

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

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

Rapidly induced gene networks following induction of long-term potentiation at perforant path synapses in vivo

The canonical view of the maintenance of long-term potentiation (LTP), a widely accepted experimental model for memory processes, is that new gene transcription contributes to its consolidation; however, the gene networks involved are unknown. To address this issue, we have used high-density Rat 230.2 Affymetrix arrays to establish a set of genes induced 20-min post-LTP, and using Ingenuity Pathway network analysis tools we have investigated how these early responding genes are interrelated. This analysis identified LTP-induced regulatory networks in which the transcription factors (TFs) nuclear factor-KB and serum response factor, which, to date, have not been widely recognized as coordinating the early gene response, play a key role alongside the more well-known TFs cyclic AMP response element-binding protein, and early growth response 1. Analysis of gene-regulatory promoter sites and chromosomal locations of the genes within the dataset reinforced the importance of these molecules in the early gene response and predicted that the coordinated action might arise from gene clustering on particular chromosomes. We have also identified a transcription-based response that affects mitogen-activated protein kinase signaling pathways and protein synthesis during the stabilization of the LTP response. Furthermore, evidence from biological function, networks, and regulatory analyses showed convergence on genes related to development, proliferation, and neurogenesis, suggesting that these functions are regulated early following LTP induction. This raises the interesting possibility that LTP-related gene expression plays a role in both synaptic reorganization and neurogenesis.

Transient increases in dendritic spine density contribute to dentate gyrus long-term potentiation

Dendritic spines are the primary sites for excitatory neurotransmission in the adult brain and exhibit changes in their number and morphology with experience. The relationship between spine formation and synaptic activity has been best characterized along the apical dendrites of pyramidal neurons in the hippocampal CA1 subfield. However, less is known about the structural mechanisms at the spine that mediate plasticity in other hippocampal subfields. The dentate gyrus is the predominant point of entry for synaptic input to the hippocampus, and dentate granule cells differ from CA1 pyramidal neurons in terms of their morphology and biophysical properties. In order to understand the structural mechanisms for plasticity in the dentate gyrus, we measured dendritic spine density in hippocampal slice preparations at different intervals following synaptic stimulation. We observed that transient increases in dendritic spine density are detectable 30 min after induction of long-term potentiation (LTP). By 60 min poststimulation, dendritic spine density has returned to basal levels. Both early LTP and enhancements in dendritic spine density could be blocked by destabilizing actin filaments, but not by inhibitors of transcription or protein synthesis. These results indicate that spine formation is a transient event that is required for dentate gyrus LTP.

Hippocampal long term potentiation in rats under different regimens of vitamin D: an in vivo study

Evidence indicates that vitamin D involves in development of brain as well as its function. This study assesses occurrence of long term potentiation (LTP), as an experimental form of synaptic plasticity, in adult rats under the normal regimen (CON), and the regimens without vitamin D (CON-D) or with a supplement of 1,25(OH)2D3 (CON+D). Stimulating the Schaffer collaterals pre- and post-tetanus excitatory postsynaptic potentials (EPSPs) were recorded in the CA1 area of hippocampus in anesthetized animals. Amplitude change of the EPSPs was considered for comparisons. Our results indicated that the basic EPSPs were similar in the three groups. Tetanization elicited a considerable LTP in both the CON and CON+D rats but a moderate potentiation in the CON-D group. We concluded that optimal level of vitamin D is required for induction of LTP.