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Williams JM, Guevremont D, Mockett B, Peppercorn K, Tate WP, Abraham WC. P4‐070: SECRETED AMYLOID PRECURSOR PROTEIN‐ALPHA REGULATES GLUTAMATE RECEPTOR TRAFFICKING IN THE HIPPOCAMPUS. Alzheimers Dement 2016. [DOI: 10.1016/j.jalz.2016.06.2160] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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Abstract
Memory is fundamentally important to everyday life, and memory loss has devastating consequences to individuals and society. Understanding the neurophysiological and cellular basis of memory paves the way for gaining insights into the molecular steps involved in memory formation, thereby revealing potential therapeutic targets for neurological diseases. For three decades, long-term potentiation (LTP) has been the gold standard synaptic model for mammalian memory mechanisms, in large part because of its long-lasting nature. Here, the authors summarize the characteristics of LTP persistence in the dentate gyrus of the hippocampus, comparing this with other hippocampal subregions and neocortex. They consider how long LTP can last and how its persistence is affected by subsequent behavioral experiences. Next, they review the molecular mechanisms known to contribute to LTP induction and persistence, in particular the role of new gene expression and protein synthesis and how they may be associated with potential structural reorganization of the synapse. A temporal schema for the processes important for consolidating LTP into a persistent form is presented. The parallels between the molecular aspects of LTP and memory strongly support the continuation with LTP as a model system for studying the mechanisms underlying long-term memory consolidation and retention.
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Abstract
Change in the synaptic communication between neurons—known as synaptic plasticity—plays a key role in learning and memory. It is not yet clear, however, whether the properties of synaptic plasticity are sufficient to account for long-term-memory maintenance. Recent studies have revealed that synaptic plasticity can indeed persist for weeks or months, as might be expected of a long-term-memory mechanism. However, memories encoded by neural systems are not static; they continue to evolve as new learning occurs. Furthermore, neural-network modeling has shown that synapses must be able to reconfigure their connection strengths during new learning if old information is to be preserved. Recent tests confirm that synapses, once modified, retain their capacity for further modification, indicating that they can indeed operate in the manner predicted to be necessary for memory maintenance in a dynamic learning network.
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Taylor CJ, Ohline SM, Moss T, Ulrich K, Abraham WC. The persistence of long-term potentiation in the projection from ventral hippocampus to medial prefrontal cortex in awake rats. Eur J Neurosci 2016; 43:811-22. [PMID: 26750170 DOI: 10.1111/ejn.13167] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2015] [Revised: 12/07/2015] [Accepted: 12/22/2015] [Indexed: 12/12/2022]
Abstract
A potentially vital pathway in the processing of spatial memory is the pathway from ventral hippocampus to medial prefrontal cortex (vHPC-mPFC). To assess long-term potentiation (LTP) induction and maintenance across days in this pathway, the effects of several induction paradigms were compared in awake, freely moving rats. Two different high-frequency stimulation (HFS) protocols generated LTP lasting no longer than 1 week. However, after delivering HFS on three consecutive days, LTP lasted an average of 20 days, due mainly to the greater initial induction. Thus the pathway does not require extensive multi-day stimulation to induce LTP, as for other intra-neocortical pathways, but also it does not exhibit the extremely long-lasting and stable LTP previously observed in area CA1 and the dentate gyrus. By using bilaterally placed stimulating and recording electrodes, we found that HFS in one vHPC generated responses and LTP in the contralateral mPFC, even when the ipsilateral mPFC was inactivated by CNQX. We attribute this crossed response to a polysynaptic pathway from the vHPC to the contralateral mPFC. Finally, we found that repeated overnight exposure to an enriched environment also potentiated the vHPC-mPFC response, but this too was a transient effect lasting < 9 days, declining to baseline even before the enriched environment treatment was completed. Overall, these findings are consistent with the view that potentiation of vHPC-mPFC pathway may play a key role in promoting the hippocampus-mPFC interplay that, over days, leads to long-term storage in the frontal cortex of memories that are independent of the hippocampus.
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Jedlicka P, Benuskova L, Abraham WC. Computational modeling of heterosynaptic plasticity in the hippocampus. BMC Neurosci 2015. [PMCID: PMC4697587 DOI: 10.1186/1471-2202-16-s1-p1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
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Jedlicka P, Benuskova L, Abraham WC. A Voltage-Based STDP Rule Combined with Fast BCM-Like Metaplasticity Accounts for LTP and Concurrent "Heterosynaptic" LTD in the Dentate Gyrus In Vivo. PLoS Comput Biol 2015; 11:e1004588. [PMID: 26544038 PMCID: PMC4636250 DOI: 10.1371/journal.pcbi.1004588] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/02/2015] [Accepted: 10/06/2015] [Indexed: 11/18/2022] Open
Abstract
Long-term potentiation (LTP) and long-term depression (LTD) are widely accepted to be synaptic mechanisms involved in learning and memory. It remains uncertain, however, which particular activity rules are utilized by hippocampal neurons to induce LTP and LTD in behaving animals. Recent experiments in the dentate gyrus of freely moving rats revealed an unexpected pattern of LTP and LTD from high-frequency perforant path stimulation. While 400 Hz theta-burst stimulation (400-TBS) and 400 Hz delta-burst stimulation (400-DBS) elicited substantial LTP of the tetanized medial path input and, concurrently, LTD of the non-tetanized lateral path input, 100 Hz theta-burst stimulation (100-TBS, a normally efficient LTP protocol for in vitro preparations) produced only weak LTP and concurrent LTD. Here we show in a biophysically realistic compartmental granule cell model that this pattern of results can be accounted for by a voltage-based spike-timing-dependent plasticity (STDP) rule combined with a relatively fast Bienenstock-Cooper-Munro (BCM)-like homeostatic metaplasticity rule, all on a background of ongoing spontaneous activity in the input fibers. Our results suggest that, at least for dentate granule cells, the interplay of STDP-BCM plasticity rules and ongoing pre- and postsynaptic background activity determines not only the degree of input-specific LTP elicited by various plasticity-inducing protocols, but also the degree of associated LTD in neighboring non-tetanized inputs, as generated by the ongoing constitutive activity at these synapses. The vast majority of computational studies that model synaptic plasticity neglect the fact that in vivo neurons exhibit an ongoing spontaneous spiking which affects the dynamics of synaptic changes. Here we study how key components of learning mechanisms in the brain, namely spike timing-dependent plasticity and metaplasticity, interact with spontaneous activity in the input pathways of the neuron. Using biologically realistic simulations we show that ongoing background activity is a key determinant of the degree of long-term potentiation and long-term depression of synaptic transmission between nerve cells in the hippocampus of freely moving animals. This work helps better understand the computational rules which drive synaptic plasticity in vivo.
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Cinco P, Jing Y, Waldvogel HJ, Curtis MA, Zhang H, Abraham WC, Faull RL, Liu P. P4‐017: Arginine decarboxylase and agmatinase immunoreactivity in Alzheimer's superior frontal gyrus. Alzheimers Dement 2015. [DOI: 10.1016/j.jalz.2015.06.1721] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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Parr-Brownlie LC, Bosch-Bouju C, Schoderboeck L, Sizemore RJ, Abraham WC, Hughes SM. Lentiviral vectors as tools to understand central nervous system biology in mammalian model organisms. Front Mol Neurosci 2015; 8:14. [PMID: 26041987 PMCID: PMC4434958 DOI: 10.3389/fnmol.2015.00014] [Citation(s) in RCA: 59] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/23/2015] [Accepted: 04/30/2015] [Indexed: 01/18/2023] Open
Abstract
Lentiviruses have been extensively used as gene delivery vectors since the mid-1990s. Usually derived from the human immunodeficiency virus genome, they mediate efficient gene transfer to non-dividing cells, including neurons and glia in the adult mammalian brain. In addition, integration of the recombinant lentiviral construct into the host genome provides permanent expression, including the progeny of dividing neural precursors. In this review, we describe targeted vectors with modified envelope glycoproteins and expression of transgenes under the regulation of cell-selective and inducible promoters. This technology has broad utility to address fundamental questions in neuroscience and we outline how this has been used in rodents and primates. Combining viral tract tracing with immunohistochemistry and confocal or electron microscopy, lentiviral vectors provide a tool to selectively label and trace specific neuronal populations at gross or ultrastructural levels. Additionally, new generation optogenetic technologies can be readily utilized to analyze neuronal circuit and gene functions in the mature mammalian brain. Examples of these applications, limitations of current systems and prospects for future developments to enhance neuroscience knowledge will be reviewed. Finally, we will discuss how these vectors may be translated from gene therapy trials into the clinical setting.
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Dalrymple-Alford JC, Harland B, Loukavenko EA, Perry B, Mercer S, Collings DA, Ulrich K, Abraham WC, McNaughton N, Wolff M. Anterior thalamic nuclei lesions and recovery of function: Relevance to cognitive thalamus. Neurosci Biobehav Rev 2015; 54:145-60. [PMID: 25637779 DOI: 10.1016/j.neubiorev.2014.12.007] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2014] [Revised: 11/02/2014] [Accepted: 12/04/2014] [Indexed: 12/01/2022]
Abstract
Injury to the anterior thalamic nuclei (ATN) and their neural connections is the most consistent neuropathology associated with diencephalic amnesia. ATN lesions in rats produce memory impairments that support a key role for this region within an extended hippocampal system of complex overlapping neural connections. Environmental enrichment is a therapeutic tool that produces substantial, although incomplete, recovery of memory function after ATN lesions, even after the lesion-induced deficit has become established. Similarly, the neurotrophic agent cerebrolysin, also counters the negative effects of ATN lesions. ATN lesions substantially reduce c-Fos expression and spine density in the retrosplenial cortex, and reduce spine density on CA1 neurons; only the latter is reversed by enrichment. We discuss the implications of this evidence for the cognitive thalamus, with a proposal that there are genuine interactions among different but allied thalamo-cortical systems that go beyond a simple summation of their separate effects.
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Ryan MM, Guévremont D, Luxmanan C, Abraham WC, Williams JM. Aging alters long-term potentiation--related gene networks and impairs synaptic protein synthesis in the rat hippocampus. Neurobiol Aging 2015; 36:1868-80. [PMID: 25716081 DOI: 10.1016/j.neurobiolaging.2015.01.012] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 01/12/2015] [Accepted: 01/14/2015] [Indexed: 10/24/2022]
Abstract
During aging, memory retention and persistence of long-term potentiation (LTP) are impaired, suggesting an aging-related deterioration in mechanisms regulating information storage. Late-phase LTP requires synthesis of proteins at synapses as well as integrated regulation of gene networks. Because aging diminishes the persistence of LTP, primarily by affecting the transition between early and late phases, we assessed whether this was reflected in perturbation of gene networks. Using DNA microarray analysis, we compared LTP-associated gene expression in young (5 months), middle-aged (15 months), and old (22 months) male Sprague-Dawley rats. As expected, we found no significant difference in LTP measured 20 minutes postinduction; however, we found that overall more genes were regulated in the young group. Bioinformatics predicted not only dysregulation of activator protein-1 and nuclear factor kB transcription factor activity and epigenetic modifications but also dysregulation of protein synthesis. Notably, we confirmed an age-related impairment in metabotropic and ionotropic receptor-mediated synaptic protein synthesis. Together, these results demonstrate that LTP-specific gene expression is altered with aging and suggest that dysregulation of synaptic protein synthesis also contributes to the age-dependent reduction in LTP persistence.
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Joilin G, Guévremont D, Ryan B, Claudianos C, Cristino AS, Abraham WC, Williams JM. Rapid regulation of microRNA following induction of long-term potentiation in vivo. Front Mol Neurosci 2014; 7:98. [PMID: 25538559 PMCID: PMC4260481 DOI: 10.3389/fnmol.2014.00098] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2014] [Accepted: 11/24/2014] [Indexed: 01/09/2023] Open
Abstract
Coordinated regulation of gene expression is essential for consolidation of the memory mechanism, long-term potentiation (LTP). Triggering of LTP by N-methyl-D-aspartate receptor (NMDAR) activation rapidly activates constitutive and inducible transcription factors, which promote expression of genes responsible for LTP maintenance. As microRNA (miRNA) coordinate expression of genes related through seed sites, we hypothesize that miRNA contribute to the regulation of the LTP-induced gene response. MiRNA function primarily as negative regulators of gene expression. As LTP induction promotes a generalized rapid up-regulation of gene expression, we predicted a complementary rapid down-regulation of miRNA levels. Accordingly, we carried out global miRNA expression profiling in the rat dentate gyrus 20 min post-LTP induction in vivo. Consistent with our hypothesis, we found a large number of differentially expressed miRNA, the majority down-regulated. Detailed analysis of miR-34a-5p and miR-132-3p revealed this down-regulation was transient and NMDAR-dependent, whereby block of NMDARs released an activity-associated inhibitory mechanism. Furthermore, down-regulation of mature miR-34a-5p and miR-132-3p occurred solely by post-transcriptional mechanisms, occurring despite an associated up-regulation of the pri-miR-132 transcript. To understand how down-regulation of miR-34a-5p and miR-132-3p intersects with the molecular events occurring following LTP, we used bioinformatics to identify potential targets. Previously validated targets included the key LTP-regulated genes Arc and glutamate receptor subunits. Predicted targets included the LTP-linked kinase, Mapk1, and neuropil-associated transcripts Hn1 and Klhl11, which were validated using luciferase reporter assays. Furthermore, we found that the level of p42-Mapk1, the protein encoded by the Mapk1 transcript, was up-regulated following LTP. Together, these data support the interpretation that miRNA, in particular miR-34a-5p and miR-132-3p, make a surprisingly rapid contribution to synaptic plasticity via dis-inhibition of translation of key plasticity-related molecules.
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Fisher SD, Gray JP, Black MJ, Davies JR, Bednark JG, Redgrave P, Franz EA, Abraham WC, Reynolds JNJ. A behavioral task for investigating action discovery, selection and switching: comparison between types of reinforcer. Front Behav Neurosci 2014; 8:398. [PMID: 25477795 PMCID: PMC4235381 DOI: 10.3389/fnbeh.2014.00398] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2014] [Accepted: 10/30/2014] [Indexed: 01/21/2023] Open
Abstract
Action discovery and selection are critical cognitive processes that are understudied at the cellular and systems neuroscience levels. Presented here is a new rodent joystick task suitable to test these processes due to the range of action possibilities that can be learnt while performing the task. Rats learned to manipulate a joystick while progressing through task milestones that required increasing degrees of movement accuracy. In a switching phase designed to measure action discovery, rats were repeatedly required to discover new target positions to meet changing task demands. Behavior was compared using both food and electrical brain stimulation reward (BSR) of the substantia nigra as reinforcement. Rats reinforced with food and those with BSR performed similarly overall, although BSR-treated rats exhibited greater vigor in responding. In the switching phase, rats learnt new actions to adapt to changing task demands, reflecting action discovery processes. Because subjects are required to learn different goal-directed actions, this task could be employed in further investigations of the cellular mechanisms of action discovery and selection. Additionally, this task could be used to assess the behavioral flexibility impairments seen in conditions such as Parkinson's disease and obsessive-compulsive disorder. The versatility of the task will enable cross-species investigations of these impairments.
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Knight RG, Tsui HSL, Abraham WC, Skeaff CM, McMahon JA, Cutfield NJ. Lack of effect of the apolipoprotein E ε4 genotype on cognition during healthy aging. J Clin Exp Neuropsychol 2014; 36:742-50. [DOI: 10.1080/13803395.2014.935706] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Liu P, Bergin D, Jing Y, Mockett B, Abraham WC, Zhang H. P2‐055: AGE‐RELATED PLASMA L‐ARGININE METABOLIC PROFILE CHANGES IN APP/PS1 MICE. Alzheimers Dement 2014. [DOI: 10.1016/j.jalz.2014.05.729] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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Harland BC, Collings DA, McNaughton N, Abraham WC, Dalrymple-Alford JC. Anterior thalamic lesions reduce spine density in both hippocampal CA1 and retrosplenial cortex, but enrichment rescues CA1 spines only. Hippocampus 2014; 24:1232-47. [PMID: 24862603 DOI: 10.1002/hipo.22309] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 05/22/2014] [Indexed: 12/14/2022]
Abstract
Injury to the anterior thalamic nuclei (ATN) may affect both hippocampus and retrosplenial cortex thus explaining some parallels between diencephalic and medial temporal lobe amnesias. We found that standard-housed rats with ATN lesions, compared with standard-housed controls, showed reduced spine density in hippocampal CA1 neurons (basal dendrites, -11.2%; apical dendrites, -9.6%) and in retrospenial granular b cortex (Rgb) neurons (apical dendrites, -20.1%) together with spatial memory deficits on cross maze and radial-arm maze tasks. Additional rats with ATN lesions were also shown to display a severe deficit on spatial working memory in the cross-maze, but subsequent enriched housing ameliorated their performance on both this task and the radial-arm maze. These enriched rats with ATN lesions also showed recovery of both basal and apical CA1 spine density to levels comparable to that of the standard-housed controls, but no recovery of Rgb spine density. Inspection of spine types in the CA1 neurons showed that ATN lesions reduced the density of thin spines and mushroom spines, but not stubby spines; while enrichment promoted recovery of thin spines. Comparison with enriched rats that received pseudo-training, which provided comparable task-related experience, but no explicit spatial memory training, suggested that basal CA1 spine density in particular was associated with spatial learning and memory performance. Distal pathology in terms of reduced integrity of hippocampal and retrosplenial microstructure provides clear support for the influence of the ATN lesions on the extended hippocampal system. The reversal by postoperative enrichment of this deficit in the hippocampus but not the retrosplenial cortex may indicate region-specific mechanisms of recovery after ATN injury.
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Liu P, Fleete MS, Jing Y, Collie ND, Curtis MA, Waldvogel HJ, Faull RLM, Abraham WC, Zhang H. Altered arginine metabolism in Alzheimer's disease brains. Neurobiol Aging 2014; 35:1992-2003. [PMID: 24746363 DOI: 10.1016/j.neurobiolaging.2014.03.013] [Citation(s) in RCA: 128] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2013] [Revised: 02/27/2014] [Accepted: 03/14/2014] [Indexed: 12/21/2022]
Abstract
L-arginine is a semi-essential amino acid with a number of bioactive metabolites. Accumulating evidence suggests the implication of altered arginine metabolism in the pathogenesis of Alzheimer's disease (AD). The present study systematically compared the metabolic profile of L-arginine in the superior frontal gyrus, hippocampus, and cerebellum from AD (mean age 80 years) and normal (mean age 80 or 60 years) cases. The activity and protein expression of nitric oxide synthase and arginase were altered with AD and age in a region-specific manner. There were also AD- and age-related changes in the tissue concentrations of L-arginine and its downstream metabolites (L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamate, γ-aminobutyric acid, and glutamine) in a metabolite- or region-specific manner. These findings demonstrate that arginine metabolism is dramatically altered in diverse regions of AD brains, thus meriting further investigation to understand its role in the pathogenesis and/or progression of the disease.
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Hulme SR, Jones OD, Raymond CR, Sah P, Abraham WC. Mechanisms of heterosynaptic metaplasticity. Philos Trans R Soc Lond B Biol Sci 2013; 369:20130148. [PMID: 24298150 DOI: 10.1098/rstb.2013.0148] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Synaptic plasticity is fundamental to the neural processes underlying learning and memory. Interestingly, synaptic plasticity itself can be dynamically regulated by prior activity, in a process termed 'metaplasticity', which can be expressed both homosynaptically and heterosynaptically. Here, we focus on heterosynaptic metaplasticity, particularly long-range interactions between synapses spread across dendritic compartments, and review evidence for intracellular versus intercellular signalling pathways leading to this effect. Of particular interest is our previously reported finding that priming stimulation in stratum oriens of area CA1 in the hippocampal slice heterosynaptically inhibits subsequent long-term potentiation and facilitates long-term depression in stratum radiatum. As we have excluded the most likely intracellular signalling pathways that might mediate this long-range heterosynaptic effect, we consider the hypothesis that intercellular communication may be critically involved. This hypothesis is supported by the finding that extracellular ATP hydrolysis, and activation of adenosine A2 receptors are required to induce the metaplastic state. Moreover, delivery of the priming stimulation in stratum oriens elicited astrocytic calcium responses in stratum radiatum. Both the astrocytic responses and the metaplasticity were blocked by gap junction inhibitors. Taken together, these findings support a novel intercellular communication system, possibly involving astrocytes, being required for this type of heterosynaptic metaplasticity.
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Savanthrapadian S, Wolff AR, Logan BJ, Eckert MJ, Bilkey DK, Abraham WC. Enhanced hippocampal neuronal excitability and LTP persistence associated with reduced behavioral flexibility in the maternal immune activation model of schizophrenia. Hippocampus 2013; 23:1395-409. [PMID: 23966340 DOI: 10.1002/hipo.22193] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 08/16/2013] [Indexed: 12/23/2022]
Abstract
Individuals with schizophrenia display a number of structural and cytoarchitectural alterations in the hippocampus, suggesting that other functions such as synaptic plasticity may also be modified. Altered hippocampal plasticity is likely to affect memory processing, and therefore any such pathology may contribute to the cognitive symptoms of schizophrenia, which includes prominent memory impairment. The current study tested whether prenatal exposure to infection, an environmental risk factor that has previously been associated with schizophrenia produced changes in hippocampal synaptic transmission or plasticity, using the maternal immune activation (MIA) animal model. We also assessed performance in hippocampus-dependent memory tasks to determine whether altered plasticity is associated with memory dysfunction. MIA did not alter basal synaptic transmission in either the dentate gyrus or CA1 of freely moving adult rats. It did, however, result in increased paired-pulse facilitation of the dentate gyrus population spike and an enhanced persistence of dentate long-term potentiation. MIA animals displayed slower learning of a reversed platform location in the water maze, and a similarly slowed learning during reversal in a spatial plus maze task. Together these findings are indicative of reduced behavioral flexibility in response to changes in task requirements. The results are consistent with the hypothesis that hippocampal plasticity is altered in schizophrenia, and that this change in plasticity mechanisms may underlie some aspects of cognitive dysfunction in this disorder.
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Eckert MJ, Guévremont D, Williams JM, Abraham WC. Rapid visual stimulation increases extrasynaptic glutamate receptor expression but not visual-evoked potentials in the adult rat primary visual cortex. Eur J Neurosci 2013; 37:400-6. [PMID: 23373691 DOI: 10.1111/ejn.12053] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/10/2011] [Revised: 09/28/2012] [Accepted: 10/10/2012] [Indexed: 11/29/2022]
Abstract
The model most used to study synaptic plasticity, long-term potentiation (LTP), typically employs electrical stimulation of afferent fibers to induce changes in synaptic strength. It would be beneficial for understanding the behavioral relevance of LTP if a model could be developed that used more naturalistic stimuli. Recent evidence suggests that the adult visual cortex, previously thought to have lost most of its plasticity once past the critical period, is in fact capable of LTP-like changes in synaptic strength in response to sensory manipulations alone. In a preliminary study, we used a photic tetanus (PT; flashing checkerboard stimulus) to induce an enhancement of the visual-evoked potential (VEP) in the primary visual cortex of anesthetised adult rats. In the present study, we sought to compare the mechanisms of this novel sensory LTP with those of traditional electrical LTP. Unexpectedly, we found that sensory LTP was not induced as reliably as we had observed previously, as manipulations of several parameters failed to lead to significant potentiation of the VEP. However, we did observe a significant increase in visual cortex glutamate receptor expression on the surface of isolated synapses following the PT. Both AMPA receptor expression and N-methyl-d-aspartate (NMDA) receptor subunit expression were increased, specifically in extrasynaptic regions of the membrane, in PT animals. These results provide biochemical confirmation of the lack of change in the VEP in response to PT, but suggest that PT may prime synapses for strengthening upon appropriate subsequent activation, through the trafficking of glutamate receptors to the cell surface.
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Ryan MM, Morris GP, Mockett BG, Bourne K, Abraham WC, Tate WP, Williams JM. Time-dependent changes in gene expression induced by secreted amyloid precursor protein-alpha in the rat hippocampus. BMC Genomics 2013; 14:376. [PMID: 23742273 PMCID: PMC3691674 DOI: 10.1186/1471-2164-14-376] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2012] [Accepted: 05/24/2013] [Indexed: 01/19/2023] Open
Abstract
Background Differential processing of the amyloid precursor protein liberates either amyloid-ß, a causative agent of Alzheimer’s disease, or secreted amyloid precursor protein-alpha (sAPPα), which promotes neuroprotection, neurotrophism, neurogenesis and synaptic plasticity. The underlying molecular mechanisms recruited by sAPPα that underpin these considerable cellular effects are not well elucidated. As these effects are enduring, we hypothesised that regulation of gene expression may be of importance and examined temporally specific gene networks and pathways induced by sAPPα in rat hippocampal organotypic slice cultures. Slices were exposed to 1 nM sAPPα or phosphate buffered saline for 15 min, 2 h or 24 h and sAPPα-associated gene expression profiles were produced for each time-point using Affymetrix Rat Gene 1.0 ST arrays (moderated t-test using Limma: p < 0.05, and fold change ± 1.15). Results Treatment of organotypic hippocampal slice cultures with 1 nM sAPPα induced temporally distinct gene expression profiles, including mRNA and microRNA associated with Alzheimer’s disease. Having demonstrated that treatment with human recombinant sAPPα was protective against N-methyl d-aspartate-induced toxicity, we next explored the sAPPα-induced gene expression profiles. Ingenuity Pathway Analysis predicted that short-term exposure to sAPPα elicited a multi-level transcriptional response, including upregulation of immediate early gene transcription factors (AP-1, Egr1), modulation of the chromatin environment, and apparent activation of the constitutive transcription factors CREB and NF-κB. Importantly, dynamic regulation of NF-κB appears to be integral to the transcriptional response across all time-points. In contrast, medium and long exposure to sAPPα resulted in an overall downregulation of gene expression. While these results suggest commonality between sAPPα and our previously reported analysis of plasticity-related gene expression, we found little crossover between these datasets. The gene networks formed following medium and long exposure to sAPPα were associated with inflammatory response, apoptosis, neurogenesis and cell survival; functions likely to be the basis of the neuroprotective effects of sAPPα. Conclusions Our results demonstrate that sAPPα rapidly and persistently regulates gene expression in rat hippocampus. This regulation is multi-level, temporally specific and is likely to underpin the neuroprotective effects of sAPPα.
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Jones OD, Hulme SR, Abraham WC. Purinergic receptor- and gap junction-mediated intercellular signalling as a mechanism of heterosynaptic metaplasticity. Neurobiol Learn Mem 2013; 105:31-9. [PMID: 23747410 DOI: 10.1016/j.nlm.2013.05.010] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2013] [Revised: 05/29/2013] [Accepted: 05/30/2013] [Indexed: 11/15/2022]
Abstract
Synaptic plasticity is subject to activity-dependent long-term modification (metaplasticity). We have recently described a novel form of heterosynaptic metaplasticity in hippocampal CA1, whereby 'priming' activity at one set of synapses confers a metaplastic state that inhibits subsequent LTP both within and between dendritic compartments. Here, we investigated the roles of purinergic signalling and gap junctions in mediating this long-distance communication between synapses. We found that the heterosynaptic metaplasticity requires the hydrolysis of extracellular ATP to adenosine, and activation of adenosine A2, but not A1 receptors. The metaplasticity was also blocked by the non-selective gap junction blockers carbenoxolone and meclofenamic acid, and by a connexin43-specific mimetic peptide. These results indicate that an intercellular signalling cascade underlies the long-distance communication required for this form of metaplasticity.
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Hulme SR, Jones OD, Abraham WC. Emerging roles of metaplasticity in behaviour and disease. Trends Neurosci 2013; 36:353-62. [PMID: 23602195 DOI: 10.1016/j.tins.2013.03.007] [Citation(s) in RCA: 137] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2012] [Revised: 03/19/2013] [Accepted: 03/19/2013] [Indexed: 12/01/2022]
Abstract
Since its initial conceptualisation, metaplasticity has come to encompass a wide variety of phenomena and mechanisms, creating the important challenge of understanding how they contribute to network function and behaviour. Here, we present a framework for considering potential roles of metaplasticity across three domains of function. First, metaplasticity appears ideally placed to prepare for subsequent learning by either enhancing learning ability generally or by preparing neuronal networks to encode specific content. Second, metaplasticity can homeostatically regulate synaptic plasticity, and this likely has important behavioural consequences by stabilising synaptic weights while ensuring the ongoing availability of synaptic plasticity. Finally, we discuss emerging evidence that metaplasticity mechanisms may play a role in disease causally and may serve as a potential therapeutic target.
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Eckert MJ, Abraham WC. Effects of environmental enrichment exposure on synaptic transmission and plasticity in the hippocampus. Curr Top Behav Neurosci 2013; 15:165-187. [PMID: 22798066 DOI: 10.1007/7854_2012_215] [Citation(s) in RCA: 51] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
Exposure to an enriched environment (EE) is beneficial to the structure and function of the brain. The added sensory, social, and spatial complexity of the EE also improves cognitive functions such as memory in both healthy brains and damaged or diseased brains, yet the underlying neural mechanisms of these cognitive improvements are poorly understood. In particular, studies that have examined the effects of EE on cellular function in the hippocampus, a structure critical for memory storage, have produced somewhat confusing results. Experiments performed in ex vivo hippocampal slices have reported a variety of EE effects on synaptic transmission and plasticity in both CA1 and the dentate gyrus. However, together with data from in vivo recordings made during and after the EE treatment, the overall results suggest an evolution of changes in neuronal function in the hippocampus, whereby there is an early transient increase in cell activity and plasticity that gives rise to more subtle long-term enhancements in cellular and network function that may contribute to enhanced hippocampus-dependent cognition.
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Li Q, Rothkegel M, Xiao ZC, Abraham WC, Korte M, Sajikumar S. Making synapses strong: metaplasticity prolongs associativity of long-term memory by switching synaptic tag mechanisms. ACTA ACUST UNITED AC 2012; 24:353-63. [PMID: 23048020 DOI: 10.1093/cercor/bhs315] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
One conceptual mechanism for the induction of associative long-term memory is that a synaptic tag, set by a weak event, can capture plasticity-related proteins from a nearby strong input, thus enabling associativity between the 2 (synaptic tagging and capture, STC). So far, STC has been observed for only a limited time of 60 min. Nevertheless, association of weak memory forms can occur beyond this period and its mechanism is not well understood. Here we report that metaplasticity induced by ryanodine receptor activation or synaptic activation of metabotropic glutamate receptors prolongs the durability of the synaptic tag, thus extending the time window for associative interactions mediating storage of long-term memory. We provide evidence that such metaplasticity alters the mechanisms of STC from a CaMKII-mediated (in non-primed STC) to a protein kinase Mzeta (PKMζ)-mediated process (in primed STC). Thus the association of weak synapses with strong synapses in the "late" stage of associative memory formation occurs only through metaplasticity. The results also reveal that the short-lived, CaMKII-mediated tag may contribute to a mechanism for a fragile form of memory while metaplasticity enables a PKMζ-mediated synaptic tag capable of prolonged interactions that induce a more stable form of memory that is resistant to reversal.
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Ryan MM, Ryan B, Kyrke-Smith M, Logan B, Tate WP, Abraham WC, Williams JM. Temporal profiling of gene networks associated with the late phase of long-term potentiation in vivo. PLoS One 2012; 7:e40538. [PMID: 22802965 PMCID: PMC3393663 DOI: 10.1371/journal.pone.0040538] [Citation(s) in RCA: 48] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2011] [Accepted: 06/08/2012] [Indexed: 01/02/2023] Open
Abstract
Long-term potentiation (LTP) is widely accepted as a cellular mechanism underlying memory processes. It is well established that LTP persistence is strongly dependent on activation of constitutive and inducible transcription factors, but there is limited information regarding the downstream gene networks and controlling elements that coalesce to stabilise LTP. To identify these gene networks, we used Affymetrix RAT230.2 microarrays to detect genes regulated 5 h and 24 h (n = 5) after LTP induction at perforant path synapses in the dentate gyrus of awake adult rats. The functional relationships of the differentially expressed genes were examined using DAVID and Ingenuity Pathway Analysis, and compared with our previous data derived 20 min post-LTP induction in vivo. This analysis showed that LTP-related genes are predominantly upregulated at 5 h but that there is pronounced downregulation of gene expression at 24 h after LTP induction. Analysis of the structure of the networks and canonical pathways predicted a regulation of calcium dynamics via G-protein coupled receptors, dendritogenesis and neurogenesis at the 5 h time-point. By 24 h neurotrophin-NFKB driven pathways of neuronal growth were identified. The temporal shift in gene expression appears to be mediated by regulation of protein synthesis, ubiquitination and time-dependent regulation of specific microRNA and histone deacetylase expression. Together this programme of genomic responses, marked by both homeostatic and growth pathways, is likely to be critical for the consolidation of LTP in vivo.
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Ryan MM, Mason-Parker SE, Tate WP, Abraham WC, Williams JM. Rapidly induced gene networks following induction of long-term potentiation at perforant path synapses in vivo. Hippocampus 2012; 21:541-53. [PMID: 20108223 DOI: 10.1002/hipo.20770] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
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.
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Kameda M, Taylor CJ, Walker TL, Black DM, Abraham WC, Bartlett PF. Activation of latent precursors in the hippocampus is dependent on long-term potentiation. Transl Psychiatry 2012; 2:e72. [PMID: 22832734 PMCID: PMC3309542 DOI: 10.1038/tp.2011.70] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
The recent discovery of a large latent population of precursor cells in the dentate gyrus of adult mice led us to investigate whether activation of this population is regulated by synaptic activity, thereby explaining the observation that environmental signals can affect neurogenesis. Using a variety of stimulation protocols, we found that only a long-term potentiation (LTP)-inducing protocol activated the latent precursor pool, leading to increased neurogenesis in the dentate gyrus. LTP induced by high-frequency stimulation (HFS) of the perforant pathway in vivo produced a two-fold increase in the number of neurospheres cultured from the stimulated hippocampus, compared with the unstimulated hippocampus. No increase in neurosphere number or neurogenesis was observed when the HFS failed to induce LTP. These results show that LTP can activate latent neural precursor cells in the adult mouse dentate gyrus, thereby providing a direct mechanism for regulating activity-driven neurogenesis. In the future, it may be possible to utilize such learning- or stimulation-induced neurogenesis to overcome disorders characterized by neuronal loss.
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Baratchi S, Evans J, Tate WP, Abraham WC, Connor B. Secreted amyloid precursor proteins promote proliferation and glial differentiation of adult hippocampal neural progenitor cells. Hippocampus 2011; 22:1517-27. [PMID: 22147523 DOI: 10.1002/hipo.20988] [Citation(s) in RCA: 42] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 09/19/2011] [Indexed: 01/28/2023]
Abstract
Amyloid precursor protein (APP) is an integral membrane glycoprotein present at high levels in nerve cells. Two soluble secreted forms, sAPPα and sAPPβ, are processed from APP by two mutually exclusive proteolytic pathways. sAPPα shows a range of neuroprotective and growth factor properties, including reduction of neuronal injury and improvement in memory performance, in contrast to the generally less potent sAPPβ. In addition, sAPPα has been shown to increase the proliferation of both embryonic neural stem cells and neural progenitor cells (NPCs) derived from the subventricular zone (SVZ) of the adult brain. However, an effect of sAPPα (or sAPPβ) on adult hippocampal progenitor cell proliferation and differentiation has not previously been observed. In this study, we examined the effect of both the α- and β-cleaved ectodomains of sAPP on adult NPCs isolated from the subgranular zone (SGZ) of the rat hippocampus in the presence or absence of depolarizing conditions. Assays were performed to examine the effect of sAPPα and sAPPβ on SGZ-derived adult NPC proliferation in parallel with SVZ-derived cells and on differentiation with SGZ-derived cells. We observed both sAPPα and sAPPβ increased the proliferation of SGZ-derived NPCs in vitro. Further, treatment of SGZ-derived NPCs with either sAPPα or sAPPβ increased the number of cells expressing the astrocytic marker GFAP and promoted cell survival. The effect on differential fate was observed in both the presence and absence of depolarizing conditions. Thus, both sAPPα and sAPPβ exert a complex range of effects on SGZ-derived adult NPCs, including increasing NPC proliferation, maintaining cell viability, yet promoting glial over neuronal differentiation. These findings provide the first direct support for the secreted forms of APP regulating SGZ-derived NPCs, and raise the possibility some or all of the effects may have therapeutic benefit in models of neurological disease.
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Bowden JB, Abraham WC, Harris KM. Differential effects of strain, circadian cycle, and stimulation pattern on LTP and concurrent LTD in the dentate gyrus of freely moving rats. Hippocampus 2011; 22:1363-70. [PMID: 21853503 DOI: 10.1002/hipo.20972] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 07/14/2011] [Indexed: 01/09/2023]
Abstract
Because long-term potentiation (LTP) and long-term depression (LTD) are thought to be involved in learning and memory, it is important to delineate factors that modulate their induction and persistence, especially as studied in freely moving animals. Here, we investigated the effects of rat strain, circadian cycle, and high-frequency stimulation (HFS) pattern on LTP and concurrently induced LTD in the dentate gyrus (DG). Comparison of two commonly used rat strains revealed that medial perforant path field EPSP-population spike (E-S) coupling and LTP were greater in Long-Evans than Sprague-Dawley rats. Circadian cycle experiments conducted in Long-Evans rats revealed greater E-S coupling and enhanced LTP during the dark phase. Interestingly, concurrent LTD in the lateral perforant path did not significantly differ across strains or circadian cycle. Testing HFS protocols during the dark phase revealed that theta burst stimulation (100 Hz bursts at 5 Hz intervals) was ineffective in eliciting either LTP or concurrent LTD in DG, whereas 400 Hz bursts delivered at theta (5 Hz) or delta (1 Hz) frequencies produced substantial LTP and concurrent LTD. Thus, these natural and experimental factors regulate granule cell excitability, and differentially affect LTP and concurrent LTD in the DG of freely moving rats. © 2011 Wiley Periodicals, Inc.
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80
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Tenorio G, Connor SA, Guévremont D, Abraham WC, Williams J, O'Dell TJ, Nguyen PV. 'Silent' priming of translation-dependent LTP by ß-adrenergic receptors involves phosphorylation and recruitment of AMPA receptors. Learn Mem 2010; 17:627-38. [PMID: 21097606 DOI: 10.1101/lm.1974510] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The capacity for long-term changes in synaptic efficacy can be altered by prior synaptic activity, a process known as "metaplasticity." Activation of receptors for modulatory neurotransmitters can trigger downstream signaling cascades that persist beyond initial receptor activation and may thus have metaplastic effects. Because activation of β-adrenergic receptors (β-ARs) strongly enhances the induction of long-term potentiation (LTP) in the hippocampal CA1 region, we examined whether activation of these receptors also had metaplastic effects on LTP induction. Our results show that activation of β-ARs induces a protein synthesis-dependent form of metaplasticity that primes the future induction of late-phase LTP by a subthreshold stimulus. β-AR activation also induced a long-lasting increase in phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) GluA1 subunits at a protein kinase A (PKA) site (S845) and transiently activated extracellular signal-regulated kinase (ERK). Consistent with this, inhibitors of PKA and ERK blocked the metaplastic effects of β-AR activation. β-AR activation also induced a prolonged, translation-dependent increase in cell surface levels of GluA1 subunit-containing AMPA receptors. Our results indicate that β-ARs can modulate hippocampal synaptic plasticity by priming synapses for the future induction of late-phase LTP through up-regulation of translational processes, one consequence of which is the trafficking of AMPARs to the cell surface.
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81
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Eckert MJ, Abraham WC. Physiological effects of enriched environment exposure and LTP induction in the hippocampus in vivo do not transfer faithfully to in vitro slices. Learn Mem 2010; 17:480-4. [PMID: 20861169 DOI: 10.1101/lm.1822610] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
A number of experimental paradigms use in vitro brain slices to test for changes in synaptic transmission and plasticity following a behavioral manipulation. For example, a number of previous studies have reported a variety of effects of environmental enrichment (EE) exposure on field potential responses in hippocampal slices, but in no study was is it known what changes had been elicited in vivo. In the present study, we recorded from the hippocampus in vivo while rats underwent a brief period of EE. There was no detectable EE-induced change in synaptic efficacy in the dentate gyrus in vivo, but there was an increase in cellular excitability. In slices prepared from the same animals, we failed to observe any evidence of the excitability increase. We next tested whether LTP induction in vivo was better preserved in vitro. However, when slices from these rats were examined, there was no observable change in perforant path synaptic strength, although there was a modest increase in excitability that correlated with the increased excitability observed in vivo. These findings suggest that synaptic changes induced in vivo either are not preserved faithfully or are difficult to detect in hippocampal slices, while changes in cellular excitability are better preserved.
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82
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Zheng Y, Mason-Parker SE, Logan B, Darlington CL, Smith PF, Abraham WC. Hippocampal synaptic transmission and LTP in vivo are intact following bilateral vestibular deafferentation in the rat. Hippocampus 2010; 20:461-8. [PMID: 19533678 DOI: 10.1002/hipo.20645] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Numerous studies in animals and humans have shown that damage to the vestibular system in the inner ear results in spatial memory deficits, presumably because areas of the brain such as the hippocampus require vestibular input to accurately represent the spatial environment. Consistent with this hypothesis, studies in animals have demonstrated that complete bilateral vestibular deafferentation (BVD) causes a disruption of place cell firing as well as theta activity. The aim of this study was to investigate whether BVD in rats affects baseline field potentials (field excitatory postsynaptic potentials and population spikes) and long-term potentiation (LTP) in CA1 and the dentate gyrus (DG) of awake freely moving rats up to 43 days post-BVD and of anesthetized rats at 7 months post-BVD. Compared to sham controls, BVD had no significant effect on either baseline field potentials or LTP in either condition. These results suggest that although BVD interferes with the encoding, consolidation, and/or retrieval of spatial memories and the function of place cells, these changes are not related to detectable in vivo decrements in basal synaptic transmission or LTP, at least in the investigated pathways.
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83
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Eckert MJ, Bilkey DK, Abraham WC. Altered plasticity in hippocampal CA1, but not dentate gyrus, following long-term environmental enrichment. J Neurophysiol 2010; 103:3320-9. [PMID: 20393057 DOI: 10.1152/jn.01037.2009] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Exposure to an enriched environment can improve cognitive functioning in normal animals as well as in animal models of neurological disease and impairment. However, the physiological processes that mediate these changes are poorly understood. Previously we and others have found changes in hippocampal synaptic transmission and plasticity after 2-4 wk of enrichment although others have not observed effects. To determine whether long-term enrichment produces more robust changes, we housed rats continuously in an enriched environment for a minimum of 3 mo and then tested for effects on hippocampal physiology in vitro and in vivo. Enriched housing improved spatial learning compared with social and isolated housing, but surprisingly this was not accompanied by changes in basal synaptic transmission in either CA1 or the dentate gyrus as measured either in vitro or in vivo. This lack of change may reflect the operation of homeostatic mechanisms that keep global synaptic weights within a narrow range. In tests of synaptic plasticity, the induction of long-term potentiation was not changed in either CA1 or the dentate gyrus. However, in CA1 of enriched rats, there was less long-term depression in stratum radiatum, less depotentiation in stratum oriens, and altered paired-pulse inhibition of population spikes evoked in stratum oriens. These effects suggest that there are altered synaptic and network dynamics in hippocampal CA1 that contribute to the enrichment-related cognitive improvement.
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84
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Ireland DR, Abraham WC. Mechanisms of Group I mGluR-Dependent Long-Term Depression of NMDA Receptor–Mediated Transmission at Schaffer Collateral–CA1 Synapses. J Neurophysiol 2009; 101:1375-85. [DOI: 10.1152/jn.90643.2008] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The mechanisms underlying group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) of N-methyl-d-aspartate receptor (NMDAR)-mediated synaptic currents (EPSCsNMDAR) are poorly understood. Here we investigated the effects of ( R,S)-3,5-dihydroxyphenylglycine (DHPG), a selective agonist of group I mGluRs, on the EPSCsNMDAR in area CA1 of acute hippocampal slices from 6- to 8-wk Sprague-Dawley rats. DHPG acutely and persistently depressed the isolated EPSCNMDAR and transiently slowed its decay rate. Combined antagonism of mGluR1 and mGluR5 blocked the effects of DHPG. Strong calcium buffering with intracellular BAPTA did not reduce the acute depression or LTD, making the involvement of elevated postsynaptic calcium unlikely. The acute depression and LTD were not mediated by activation of tyrosine kinases or phosphatases, nor were they dependent on protein synthesis. However, the LTD was prevented by the intracellular actin-stabilizer jasplakinolide, raising the possibility that it was associated with a lateral movement of NMDARs. Supporting this hypothesis, when the effective spatial spread of synaptically released glutamate was increased using the glutamate transporter inhibitor TBOA, the resultant EPSCNMDAR did not undergo LTD in response to DHPG. Importantly, isolation of the extrasynaptic EPSCNMDAR by blockade of synaptic NMDARs with MK-801 showed that this was not due to a potentiation of the preexisting extrasynaptic component. These findings indicate that LTD of NMDAR-mediated synaptic transmission occurs via lateral movement of receptors away from the synapse.
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Sajikumar S, Li Q, Abraham WC, Xiao ZC. Priming of short-term potentiation and synaptic tagging/capture mechanisms by ryanodine receptor activation in rat hippocampal CA1. Learn Mem 2009; 16:178-86. [PMID: 19223601 DOI: 10.1101/lm.1255909] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
Activity-dependent changes in synaptic strength such as long-term potentiation (LTP) and long-term depression (LTD) are considered to be cellular mechanisms underlying learning and memory. Strengthening of a synapse for a few seconds or minutes is termed short-term potentiation (STP) and is normally unable to take part in the processes of synaptic tagging/capture due to its inability to set the "synaptic tags." Here, we report that priming of synapses with ryanodine receptor agonists such as ryanodine (10 microM) or caffeine (10 mM) facilitates subsequent synaptic tagging/capture, enabling an STP protocol to establish a late-LTP in response to strong tetanization of a heterosynaptic input. We identified calcium/calmodulin-dependent protein kinase II (CaMKII) as mediating the primed synaptic tag setting, which persisted for 1 h. We also identified protein kinase Mzeta (PKMzeta), presumably captured from the strongly tetanized heterosynaptic input, as a plasticity-related protein maintaining the LTP at the tagged synapses. In addition, synaptic tags in primed STP were erased or interfered with by delivering low-frequency depotentiating stimulation 5 or 10 min after its induction, thus preventing capture of newly synthesized proteins. These data reveal a novel form of metaplasticity, whereby ryanodine receptor activation lowers the threshold for subsequent synaptic tagging/capture, thus priming weakly activated synapses for heterosynaptic interactions that promote long-term functional plasticity.
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86
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Kennard JTT, Guévremont D, Mason-Parker SE, Abraham WC, Williams JM. Increased expression, but not postsynaptic localisation, of ionotropic glutamate receptors during the late-phase of long-term potentiation in the dentate gyrus in vivo. Neuropharmacology 2008; 56:66-72. [PMID: 18755203 DOI: 10.1016/j.neuropharm.2008.07.044] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2008] [Revised: 07/31/2008] [Accepted: 07/31/2008] [Indexed: 01/11/2023]
Abstract
Long-term potentiation (LTP) is extensively studied as a cellular mechanism of information storage in the brain. The induction and early expression mechanisms of LTP depend on activation and rapid modulation of ionotropic glutamate receptors. However, the mechanisms that underlie maintenance of LTP over the course of days or longer are poorly understood. Here, we have investigated the overall expression of AMPA- and NMDA-type glutamate receptors (AMPARs and NMDARs, respectively), as well as their levels at the synaptic surface membrane and in the postsynaptic density (PSD), in the dentate gyrus at 48h following the induction of LTP at perforant path synapses in awake rats. We found a high-frequency stimulation-dependent increase in the overall levels of AMPAR subunits GluA1 and GluA2, but not GluA3 in the dentate gyrus. The increases in GluA1 and GluA2 levels were partially NMDAR-dependent, but were not found in biochemically isolated synaptic surface membrane or PSD fractions. In contrast, we found that the core NMDAR subunit, GluN1, increased in the synaptic surface-membrane fraction but it also was not targeted to the PSD. The GluA1 and GluA2 expression and the surface localisation of GluN1 returned to baseline levels by 2 weeks post-LTP induction. These data suggest that the late-phase LTP is not mediated by an overt increase in the AMPAR content of perforant path synapses. The increase in surface expression NMDARs may influence thresholds for future plasticity events.
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87
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Abraham WC. Metaplasticity: tuning synapses and networks for plasticity. Nat Rev Neurosci 2008; 9:387. [PMID: 18401345 DOI: 10.1038/nrn2356] [Citation(s) in RCA: 655] [Impact Index Per Article: 40.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Synaptic plasticity is a key component of the learning machinery in the brain. It is vital that such plasticity be tightly regulated so that it occurs to the proper extent at the proper time. Activity-dependent mechanisms that have been collectively termed metaplasticity have evolved to help implement these essential computational constraints. Various intercellular signalling molecules can trigger lasting changes in the ability of synapses to express plasticity; their mechanisms of action are reviewed here, along with a consideration of how metaplasticity might affect learning and clinical conditions.
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88
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Taylor CJ, Ireland DR, Ballagh I, Bourne K, Marechal NM, Turner PR, Bilkey DK, Tate WP, Abraham WC. Endogenous secreted amyloid precursor protein-alpha regulates hippocampal NMDA receptor function, long-term potentiation and spatial memory. Neurobiol Dis 2008; 31:250-60. [PMID: 18585048 DOI: 10.1016/j.nbd.2008.04.011] [Citation(s) in RCA: 147] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2007] [Revised: 04/23/2008] [Accepted: 04/30/2008] [Indexed: 11/16/2022] Open
Abstract
Secreted amyloid precursor protein-alpha (sAPP alpha) levels are reduced during the pathogenesis of Alzheimer's disease, but the significance of this for neural function is not well understood. Here, we show that intrahippocampal infusion of antibodies targeted to endogenous sAPP alpha reduced long-term potentiation (LTP) in the dentate gyrus of adult rats by approximately 50%. Conversely, infusion of recombinant sAPP alpha dose-dependently increased LTP and facilitated in vitro tetanically evoked NMDA receptor-mediated currents. Pharmacological inhibition of alpha-secretase and other a-disintegrin-and-metalloproteases by TAPI-1 reduced both LTP and tetanus-evoked NMDA receptor-mediated currents in dentate granule cells. Both effects were prevented by co-application of exogenous recombinant sAPP alpha. Similarly, spatial memory was inhibited by intrahippocampal TAPI-1, an effect that was prevented by co-application of recombinant sAPP alpha. Together these findings indicate that endogenous sAPP alpha is a key contributor to synaptic plasticity and spatial memory. Its reduced production in Alzheimer's disease may thus contribute to the clinical memory deficits.
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89
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Abraham WC, Williams JM. LTP maintenance and its protein synthesis-dependence. Neurobiol Learn Mem 2008; 89:260-8. [DOI: 10.1016/j.nlm.2007.10.001] [Citation(s) in RCA: 122] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2007] [Revised: 10/01/2007] [Accepted: 10/01/2007] [Indexed: 12/22/2022]
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90
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Abraham WC, Logan B, Wolff A, Benuskova L. "Heterosynaptic" LTD in the dentate gyrus of anesthetized rat requires homosynaptic activity. J Neurophysiol 2007; 98:1048-51. [PMID: 17537906 DOI: 10.1152/jn.00250.2007] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Heterosynaptic long-term depression (LTD) is conventionally defined as occurring at synapses that are inactive during a time when neighboring synapses are activated by high-frequency stimulation. A new model that combines computational properties of both the Bienenstock, Cooper and Munro model and spike timing-dependent plasticity, however, suggests that such LTD actually may require presynaptic activity in the depressed pathway. We tested experimentally whether presynaptic activity is in fact necessary for previously described heterosynaptic LTD in lateral perforant path synapses in the dentate gyrus of urethane-anesthetized rats. As predicted by the model, procaine infusion into the lateral path fibers, sufficient to transiently block neural activity in this pathway, prevented the induction of LTD in the lateral path following medial path high-frequency stimulation. These data indicate that the previously described heterosynaptic LTD in the dentate gyrus in vivo is actually a form of homosynaptic LTD, requiring presynaptic activity in the depressed pathway.
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91
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Turner PR, Bourne K, Garama D, Carne A, Abraham WC, Tate WP. Production, purification and functional validation of human secreted amyloid precursor proteins for use as neuropharmacological reagents. J Neurosci Methods 2007; 164:68-74. [PMID: 17537517 DOI: 10.1016/j.jneumeth.2007.04.001] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2006] [Revised: 03/30/2007] [Accepted: 04/02/2007] [Indexed: 11/21/2022]
Abstract
The secreted fragment of the amyloid precursor protein (sAPPalpha) generated following cleavage by alpha-secretase is an important mediator of cell function and is both neurotrophic and neuroprotective. HEK 293T cells have been stably integrated with a fragment of the APP gene to produce and secrete either sAPPalpha, or the alternative cleavage product sAPPbeta. Heparin binding domains on the proteins have been utilised to develop a one-step fast-performance-liquid-chromatography (FPLC) purification of sAPPs from the conditioned media. Immunoblotting analyses with a sAPP specific antibody coupled with highly sensitive silver staining techniques have validated the expression and purification strategy. Functional activity of the purified fragments was demonstrated by their ability to protect COS-7 and SH-SY5Y (neuroblastoma) cells against the adverse effects of glucose deprivation in a cell viability assay. The purified sAPPs also activated the NFkappaB transcription factor in COS-7 cells transfected with a luciferase reporter plasmid, with sAPPalpha the more potent activator as expected. The simple protocol to produce these mammalian expressed proteins will facilitate their use as potential neuropharmacological reagents in the elucidation of biochemical pathways modulated by sAPPs, and in the study of Alzheimer's disease mechanisms in general.
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Mockett BG, Guévremont D, Williams JM, Abraham WC. Dopamine D1/D5 receptor activation reverses NMDA receptor-dependent long-term depression in rat hippocampus. J Neurosci 2007; 27:2918-26. [PMID: 17360914 PMCID: PMC6672564 DOI: 10.1523/jneurosci.0838-06.2007] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Activation of dopamine D1/D5 receptors (D1/D5Rs) in area CA1 of the rat hippocampus modulates the expression of synaptic plasticity in a manner that is dependent on the timing of the D1/D5R activation. Here, we measured field EPSPs in rat hippocampal slices to examine the modulation of long-term depression (LTD) in CA1 by D1/D5Rs when activated immediately after the induction of LTD by low-frequency stimulation (LFS) or bath application of NMDA or the metabotropic glutamate receptor agonist DHPG [(RS)-3,5-dihydroxyphenylglycine]. Activation of D1/D5Rs by SKF 38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide] completely reversed a moderate LFS-induced LTD in a time-dependent manner, presumably through an adenylate cyclase/cAMP cascade. In support of this, general adenylate cyclase activation by forskolin ([3R-(3 alpha,4a beta,5 beta,6 beta,6a alpha,10 alpha,10a beta,10b alpha)]-5-(acetyloxy)-3-ethenyldodecahydro-6,10,10b-trihydroxy-3,4a,7,7,10a-pentamenthyl-1H-naphtho[2,1-b]pyran-1-one) immediately, but not 60 min, after LFS also reversed the LTD. Beta-adrenergic receptor activation by isoproterenol failed to reverse the LTD, indicating that reversal is specific to D1/D5R-mediated increased cAMP production. SKF 38393 only partially reversed a more robust LFS-induced LTD, indicating that some components of consolidated LTD are resistant to reversal. LTD induced by bath application of NMDA, but not DHPG, was also reversed by SKF 38393. Western blot analysis of postsynaptic density fractions after NMDA-induced LTD revealed that the LTD was attributable to dephosphorylation of the AMPA receptor subunit glutamate receptor 1 (GluR1) at serine 845, without a change in total GluR content. Reversal of the LTD by SKF 38393 was associated with rephosphorylation of this same residue. Together, these findings demonstrate a new role for dopamine in the neuromodulation of hippocampal LTD.
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93
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Mellentin C, Jahnsen H, Abraham WC. Priming of long-term potentiation mediated by ryanodine receptor activation in rat hippocampal slices. Neuropharmacology 2007; 52:118-25. [PMID: 16905161 DOI: 10.1016/j.neuropharm.2006.07.009] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2006] [Revised: 07/04/2006] [Accepted: 07/05/2006] [Indexed: 11/19/2022]
Abstract
Administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) facilitates ("primes") subsequent long-term potentiation (LTP) through a phospholipase C signaling cascade that may involve release of Ca2+ from the endoplasmic reticulum (ER). We investigated the intracellular calcium pathways involved in this priming effect, recording field potentials from area CA1 of rat hippocampal slices before and after high-frequency stimulation. The priming of LTP by DHPG was prevented by co-administration of cyclopiazonic acid, which depletes ER Ca2+ stores. The priming effect was also blocked by the ryanodine receptor (RYR) antagonist ryanodine (RYA, 100 microM). In contrast, a low dose of RYA (10 microM) which opens the RYR channel, by itself primed LTP. In addition to RYR activation, entry of extracellular calcium through store-operated channels appears necessary for priming, since diverse treatments known to impede store-operated channel activity completely blocked both RYA and DHPG priming effects. Thus, RYR activation plays a critical role in the priming of LTP by Group 1 mGluRs, and this effect is coupled to the entry of extracellular calcium, probably through store-operated calcium channels.
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Benuskova L, Abraham WC. STDP rule endowed with the BCM sliding threshold accounts for hippocampal heterosynaptic plasticity. J Comput Neurosci 2006; 22:129-33. [PMID: 17053995 DOI: 10.1007/s10827-006-0002-x] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2006] [Revised: 07/13/2006] [Accepted: 07/14/2006] [Indexed: 11/25/2022]
Abstract
We have combined the nearest neighbour additive spike-timing-dependent plasticity (STDP) rule with the Bienenstock, Cooper and Munro (BCM) sliding modification threshold in a computational model of heterosynaptic plasticity in the hippocampal dentate gyrus. As a result we can reproduce (1) homosynaptic long-term potentiation of the tetanized input, and (2) heterosynaptic long-term depression of the untetanized input, as observed in real experiments.
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Taylor CJ, Bourne K, Tate WP, Abraham WC. P3–437: Secreted amyloid precursor protein–alpha regulates the induction of LTP in anesthetized rats. Alzheimers Dement 2006. [DOI: 10.1016/j.jalz.2006.05.1708] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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96
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Clapp WC, Eckert MJ, Teyler TJ, Abraham WC. Rapid visual stimulation induces N-methyl-D-aspartate receptor-dependent sensory long-term potentiation in the rat cortex. Neuroreport 2006; 17:511-5. [PMID: 16543816 DOI: 10.1097/01.wnr.0000209004.63352.10] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Previously we have demonstrated that rapidly presented sensory stimulation (visual or auditory) can induce long-lasting increases in sensory evoked potentials recorded from the human cortex. Long-term potentiation was suggested as the underlying mechanism of these increases. In the present experiment, we applied the same visual paradigm to anesthetized rats to investigate the properties and mechanisms of this effect. Our results indicated that visual evoked responses were significantly enhanced for at least 1 h and, when followed, up to 5 h after the presentation of a 'photic tetanus.' Furthermore, the potentiation was N-methyl-D-aspartate receptor-dependent and cortically generated. This type of sensory long-term potentiation may underlie perceptual learning, and serves as a model system for investigating sensory-evoked plasticity.
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Irvine GI, Logan B, Eckert M, Abraham WC. Enriched environment exposure regulates excitability, synaptic transmission, and LTP in the dentate gyrus of freely moving rats. Hippocampus 2006; 16:149-60. [PMID: 16261558 DOI: 10.1002/hipo.20142] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Performance in hippocampus-dependent and other tasks can be improved by exposure to an enriched environment (EE), but the physiological changes in neural function that may mediate these effects are poorly understood. To date, there have been conflicting reports regarding potential mechanisms, such as an increase in basal synaptic transmission, an increase in cell excitability, or altered synaptic plasticity. Here, we reexamined in freely moving animals the conditions under which varying degrees of EE exposure might lead to increases in synaptic or neural function in the dentate gyrus of the hippocampus. Adult male Sprague-Dawley rats were chronically implanted with stimulating and recording electrodes in the perforant path and dentate gyrus, respectively, and housed singly in standard cages. After stable recordings were established for field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs), the effects of various degrees of periodic novel environment exposure for 19 days were assessed. Exposure to an EE increased fEPSPs, but only when animals were kept in nominally low-stress housing conditions. An increase in granule-cell excitability, as evidenced by PS increases, was induced by all environmental treatments with the greatest effect being induced by overnight EE exposure. EE exposure did not change the level of long-term potentiation (LTP) induced by a moderate high-frequency tetanus, but continued EE exposure post-tetanus produced a significantly faster decay of LTP relative to control animals. These results suggest that, in adult animals, EE exposure may augment hippocampal information processing, but may also speed turnover of information in the hippocampus during the maintenance period.
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98
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Abraham WC, Mason-Parker SE, Irvine GI, Logan B, Gill AI. Induction and activity-dependent reversal of persistent LTP and LTD in lateral perforant path synapses in vivo. Neurobiol Learn Mem 2006; 86:82-90. [PMID: 16458543 DOI: 10.1016/j.nlm.2005.12.007] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2005] [Revised: 12/12/2005] [Accepted: 12/13/2005] [Indexed: 11/20/2022]
Abstract
The reversibility of long-term potentiation (LTP) and heterosynaptic long-term depression (LTD) lasting weeks was examined in the lateral perforant path of freely moving adult Sprague-Dawley rats. LTP lasting weeks was rapidly reversed within minutes by high-frequency heterosynaptic stimulation of the medial perforant path, in an N-methyl-D-aspartate receptor-dependent manner. LTP reversal also occurred, albeit more slowly and to a lesser extent, when animals were given 1-3 weeks of overnight exposure to an enriched environment (EE). LTD likewise was reversed upon repeated EE exposure. A covert similarity between the degrees of LTP and LTD reversal was revealed when the small potentiation effect of EE treatment by itself on lateral path responses was taken into account. Despite its ability to reverse previously acquired synaptic plasticity, two weeks of EE treatment had no effect on animals' retention of the platform location in a spatial watermaze task, although it did facilitate new learning. These data are in agreement with the hypothesis that hippocampal synapses retain the capacity for rapid synaptic change even when otherwise relatively stable plasticity has previously been induced. Slow reversal of such plasticity did not correlate with a loss of memory retention, possibly because either slow changes permit reorganization of representations such that both old and new information can be accommodated, or else the new information is synaptically represented in orthogonal fashion to the old information.
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Irvine GI, Abraham WC. Enriched environment exposure alters the input-output dynamics of synaptic transmission in area CA1 of freely moving rats. Neurosci Lett 2005; 391:32-7. [PMID: 16154267 DOI: 10.1016/j.neulet.2005.08.031] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2005] [Revised: 08/14/2005] [Accepted: 08/15/2005] [Indexed: 11/30/2022]
Abstract
We examined in freely moving animals whether EE exposure directly alters synaptic transmission in CA1 of the hippocampus. Adult male Sprague-Dawley rats were chronically implanted with recording and stimulating electrodes in CA1 and housed singly in standard cages. After stable recordings were established for field excitatory postsynaptic potentials (fEPSPs), the effects of three levels of environmental enrichment for 19 days were assessed. A change in fEPSPs was observed only after repeated overnight enriched environment exposure, such that response potentiation occurred at high stimulus intensities but a reduced response was evident at low stimulus intensities. Behavioural tests confirmed that EE exposure was sufficient to facilitate performance on spatial working memory tasks. These results suggest that overnight EE exposure refines the dynamics of CA1 synaptic transmission in a way that may contribute to augmented information processing and thus performance on hippocampus-dependent tasks.
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Abstract
Memory maintenance is widely believed to involve long-term retention of the synaptic weights that are set within relevant neural circuits during learning. However, despite recent exciting technical advances, it has not yet proved possible to confirm experimentally this intuitively appealing hypothesis. Artificial neural networks offer an alternative methodology as they permit continuous monitoring of individual connection weights during learning and retention. In such models, ongoing alterations in connection weights are required if a network is to retain previously stored material while learning new information. Thus, the duration of synaptic change does not necessarily define the persistence of a memory; rather, it is likely that a regulated balance of synaptic stability and synaptic plasticity is required for optimal memory retention in real neuronal circuits.
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