Perforant Path-granule Cell Synapses Research Articles

Functional Coupling of BK Channels to NMDA Receptors in Hippocampal Dentate Gyrus

The large conductance calcium- and voltage-activated potassium channel (BK channels) is widely expressed in the central nervous system. The physiological roles of BK channels and the calcium sources of their activation in mammalian brain remain not well-understood. Our proteomic and biochemical analyses of BK channels and the calcium-permeable NMDA receptors and have found that they form protein complexes in whole brain and various brain regions, consisting of the obligatory BK channel alpha-subunit and GluN1 subunits and at least the regulatory GluN2A and GluN2B subunits. We examined the functional coupling of BK channels to NMDA receptors in mice hippocampal dentate gyrus. We observed that glutamate evoked outward currents of BK channels in dentate gyrus granule cell soma at potentials more positive than −20 mV which were sensitive to BK channel-specific blocker paxilline and abolished by NMDA receptor antagonist AP-5. We also found that blockade of BK channels by paxilline increased the amplitude of excitatory postsynaptic potential (EPSP) of granule cells evoked by stimulation in the perforant path-granule cell synapses, which was prevented by the presence of AP-5. We thus inferred that BK channels were activated by NMDA receptor-mediated Ca2+ influx in hippocampal dentate gyrus granule cells and the calcium-mediated functional coupling between these two types of channels plays a role in regulation of synaptic transmission.

Novel microRNA revealed by systematic analysis of the microRNA transcriptome in dentate gyrus granule cells

Post-transcriptional control of gene expression by microRNAs provides an important regulatory system within neurons, allowing co-ordinate and fine-tuned expression of plasticity-related proteins. Indeed, specific microRNAs have been shown to be regulated by synaptic activity in the dentate gyrus, and contribute to the regulated gene expression that underlies the persistence of long-term potentiation (LTP), a model of memory. To fully explore the contribution of microRNAs in synaptic plasticity, it is important to characterize the complete microRNA transcriptome in regions such as the dentate gyrus. Accordingly we used deep sequencing and miRDeep* analysis to search for novel microRNAs expressed in the dentate gyrus granule cell layer. Drawing on combined sequencing and bioinformatics analyses, including hairpin stability and patterns of precursor microRNA processing, we identified nine putative novel microRNAs. We did not find evidence of differential expression of any of these putative microRNAs following LTP at perforant path-granule cell synapses in awake rats (5 h post-tetanus; p > 0.05). Focusing on novel_miR-1, the most abundant novel miRNA, we showed that this sequence could be amplified from RNA extracted from dentate gyrus granule cells by reverse transcription-quantitative polymerase chain reaction. Further, by computationally predicting mRNA targets of this microRNA, we found that this novel microRNA likely contributes to the regulation of proteins that function at synapses.

Deletion of the amyloid precursor-like protein 1 (APLP1) enhances excitatory synaptic transmission, reduces network inhibition but does not impair synaptic plasticity in the mouse dentate gyrus.

Amyloid precursor-like protein 1 (APLP1) is a transmembrane synaptic protein belonging to the amyloid precursor protein (APP) gene family. Although the role of this gene family-in particular of APP-has been intensely studied in the context of Alzheimer's disease, the physiological roles of its family members remain poorly understood. In particular, the function of APLP1, which is predominantly expressed in the nervous system, has remained enigmatic. Since APP has been implicated in synaptic plasticity, we wondered whether APLP1 could play a similar role. First, using in situ hybridization and laser microdissection combined with reverse transcription-quantitative polymerase chain reaction (PCR) we observed that Aplp1 mRNA is highly expressed in dentate granule cells. Having this examined, we studied synaptic plasticity at the perforant path-granule cell synapses in the dentate gyrus of APLP1-deficient mice in vivo. Analysis of field excitatory postsynaptic potentials evoked by stimulation of perforant path fibers revealed increased excitatory transmission in APLP1-deficient mice. Moreover, we observed decreased paired-pulse inhibition of population spikes indicating a decrease in network inhibition upon deletion of APLP1. In contrast, short-term presynaptic plasticity (STP) as well as long-term synaptic plasticity (LTP) was unchanged in the absence of APLP1. Based on these results we conclude that APLP1 deficiency on its own does not lead to defects in synaptic plasticity, but affects synaptic transmission and network inhibition in the dentate gyrus.

Neurotensinergic augmentation of glutamate release at the perforant path-granule cell synapse in rat dentate gyrus: Roles of L-Type Ca2+ channels, calmodulin and myosin light-chain kinase

Neurotensin (NT) serves as a neuromodulator in the brain where it is involved in modulating a variety of physiological functions including nociception, temperature, blood pressure and cognition, and many neurological diseases such as Alzheimer's disease, schizophrenia and Parkinson's disease. Whereas there is compelling evidence demonstrating that NT facilitates cognitive processes, the underlying cellular and molecular mechanisms have not been fully determined. Because the dentate gyrus expresses high densities of NT and NT receptors, we examined the effects of NT on the synaptic transmission at the synapse formed between the perforant path (PP) and granule cells (GC) in the rats. Our results demonstrate that NT persistently increased the amplitude of the AMPA receptor-mediated EPSCs at the PP-GC synapse. NT-induced increases in AMPA EPSCs were mediated by presynaptic NTS1 receptors. NT reduced the coefficient of variation and paired-pulse ratio of AMPA EPSCs suggesting that NT facilitates presynaptic glutamate release. NT increased the release probability and the number of readily releasable vesicles with no effects on the rate of recovery from vesicle depletion. NT-mediated augmentation of glutamate release required the influx of Ca2+ via L-type Ca2+ channels and the functions of calmodulin and myosin light chain kinase. Our results provide a cellular and molecular mechanism to explain the roles of NT in the hippocampus.

Enhanced stability of hippocampal place representation caused by reduced magnesium block of NMDA receptors in the dentate gyrus.

BackgroundVoltage-dependent block of the NMDA receptor by Mg2+ is thought to be central to the unique involvement of this receptor in higher brain functions. However, the in vivo role of the Mg2+ block in the mammalian brain has not yet been investigated, because brain-wide loss of the Mg2+ block causes perinatal lethality. In this study, we used a brain-region specific knock-in mouse expressing an NMDA receptor that is defective for the Mg2+ block in order to test its role in neural information processing.ResultsWe devised a method to induce a single amino acid substitution (N595Q) in the GluN2A subunit of the NMDA receptor, specifically in the hippocampal dentate gyrus in mice. This mutation reduced the Mg2+ block at the medial perforant path–granule cell synapse and facilitated synaptic potentiation induced by high-frequency stimulation. The mutants had more stable hippocampal place fields in the CA1 than the controls did, and place representation showed lower sensitivity to visual differences. In addition, behavioral tests revealed that the mutants had a spatial working memory deficit.ConclusionsThese results suggest that the Mg2+ block in the dentate gyrus regulates hippocampal spatial information processing by attenuating activity-dependent synaptic potentiation in the dentate gyrus.

The Endocannabinoid 2-Arachidonoylglycerol Negatively Regulates Habituation by Suppressing Excitatory Recurrent Network Activity and Reducing Long-Term Potentiation in the Dentate Gyrus

Endocannabinoids are known to mediate retrograde suppression of synaptic transmission, modulate synaptic plasticity, and influence learning and memory. The 2-arachidonoylglycerol (2-AG) produced by diacylglycerol lipase α (DGLα) is regarded as the major endocannabinoid that causes retrograde synaptic suppression. To determine how 2-AG signaling influences learning and memory, we subjected DGLα knock-out mice to two learning tasks. We tested the mice using habituation and odor-guided transverse patterning tasks that are known to involve the dentate gyrus and the CA1, respectively, of the hippocampus. We found that DGLα knock-out mice showed significantly faster habituation to an odor and a new environment than wild-type littermates with normal performance in the transverse patterning task. In freely moving animals, long-term potentiation (LTP) induced by theta burst stimulation was significantly larger at perforant path-granule cell synapses in the dentate gyrus of DGLα knock-out mice. Importantly, prior induction of synaptic potentiation at this synapse caused a significant retardation of habituation in DGLα knock-out but not in wild-type littermates. The excitability of granule cells became higher in DGLα knock-out mice after they generated action potentials. Since no differences were found in intrinsic membrane properties and responses to odor stimuli in granule cells, the elevated excitability is considered to result from enhanced activity of an excitatory recurrent network composed of granule cells and mossy cells. These results suggest that retrograde 2-AG signaling negatively regulates habituation by suppressing excitatory recurrent network activity and reducing LTP in the dentate gyrus.

ZD7288 Enhances Long-Term Depression at Early Postnatal Medial Perforant Path-Granule Cell Synapses

Hyperpolarization-activated, cyclic nucleotide-gated nonselective (HCN) channels modulate both membrane potential and resistance and play a significant role in synaptic plasticity. We compared the influence of HCN channels on long-term depression (LTD) at the medial perforant path-granule cell synapse in early postnatal (P9–15) and adult (P30–60) rats. LTD was elicited in P9–15 slices using low-frequency stimulation (LFS, 900 pulses, 1 Hz; 80 ± 4% of baseline). Application of the specific HCN channel blocker ZD7288 (10 μM) before LFS significantly enhanced LTD (62 ± 4%; P < 0.01), showing HCN channels restrain LTD induction. However, when ZD7288 was applied after LFS, LTD was similar to control values and significantly different from the values obtained with ZD7288 application before LFS (81 ± 5%; P < 0.01), indicating that HCN channels do not modulate LTD expression. LTD in slices from adult rats were only marginally lower compared to those in P9–15 slices (85 ± 6%), but bath application of ZD7288 prior to LFS resulted in the same amount of LTD (85 ± 5%). HCN channels in adult tissue hence lose their modulatory effect. In conclusion, we found that HCN channels at the medial perforant path-granule cell synapse compromise LFS-associated induction, but not expression of LTD in early postnatal, but not in adult, rats.

Upregulation of presynaptic mGluR2, but not mGluR3 in the epileptic medial perforant path

Presynaptic metabotropic glutamate receptors (mGluRs) at glutamatergic synapses play a major role in governing release probability. Previous reports indicated a downregulation of group III mGluRs at the lateral perforant path-granule cell synapse in the chronically epileptic hippocampus. Here, we investigated the mGluR-dependent presynaptic inhibition at the medial perforant path-granule cell synapse in the pilocarpine-treated chronically epileptic rat. The specific group II mGluR agonist (2S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV, 10μM) significantly depressed medial perforant path-evoked responses in control slices, but significantly more so in epileptic tissue. This depression was accompanied by a significant increase of the paired-pulse ratio in both animal groups indicating a presynaptic mechanism. Moreover, we also found that this significantly enhanced DCG-IV effect in the medial perforant path recorded in slices from pilocarpine-treated rats was due to a significant increase of mGluR2, but not mGluR3 transcripts in the entorhinal cortex using quantitative real-time reverse transcriptase-PCR. Immunohistochemistry confirmed the increased expression of group II mGluRs in the epileptic medial molecular layer. These results demonstrate that chronic epilepsy not only causes downregulation of mGluRs in the hippocampus, but may also lead to enhanced expression of these receptors – at least in the medial perforant path.

Functional consequences of the lack of amyloid precursor protein in the mouse dentate gyrus in vivo

The amyloid precursor protein (APP) plays a crucial role in the pathogenesis of Alzheimer's disease. Here, we studied whether the lack of APP affects the synaptic properties in the dentate gyrus by measuring granule cell field potentials evoked by perforant path stimulation in anesthetized 9-11-month-old APP-deficient mice in vivo. We found decreased paired-pulse facilitation, indicating altered presynaptic short-term plasticity in the APP-deficient dentate gyrus. In contrast, excitatory synaptic strength and granule cell firing were unchanged in APP knockout mice. Likewise, long-term potentiation (LTP) induced by a theta-burst stimulation protocol was not impaired in the absence of APP. These findings suggest that the deletion of APP may affect presynaptic plasticity of synaptic transmission at the perforant path-granule cell synapse but leaves synaptic efficacy intact and LTP preserved, possibly due to functional redundancy within the APP gene family.

CD200 fusion protein decreases microglial activation in the hippocampus of aged rats

The glycoprotein, CD200, is primarily expressed on neurons and its cognate receptor CD200R is expressed principally on cells of the myeloid lineage, including microglia. The interaction of CD200 with its receptor plays a significant role in maintaining microglia in a quiescent state and therefore a decrease in CD200 expression in brain is associated with evidence of microglial activation. Conversely, activation of CD200R, for example using a CD200 fusion protein (CD200Fc), should result in a decrease in microglial activation. Here we assessed the effect of delivery of CD200Fc intrahippocampally on microglial activation and on long-term potentiation (LTP) in perforant path-granule cell synapses in young and aged rats. We hypothesized that the age-related changes in microglial activation would be attenuated by CD200Fc resulting in an improved ability of aged rats to sustain LTP. The data indicate that expression of markers of microglial activation including major histocompatibility complex Class II (MHCII) and CD40 mRNA, as well as MHCII immunoreactivity, were increased in hippocampus of aged, compared with young, rats and that these changes were associated with a deficit in LTP; these changes were attenuated in hippocampal tissue prepared from aged rats which received CD200Fc. Microglial activation and a deficit in LTP have also been reported in lipopolysaccharide (LPS)-treated rats and, here, we report that these changes were also attenuated in CD200Fc-treated animals. Thus the negative impact of microglial activation on the ability of aged and LPS-treated rats to sustain LTP is ameliorated when CD200R is activated by CD200Fc.

Facilitated Neurogenesis in the Developing Hippocampus After Intake of Theanine, an Amino Acid in Tea Leaves, and Object Recognition Memory

Theanine, γ-glutamylethylamide, is one of the major amino acid components in green tea. In this study, cognitive function and the related mechanism were examined in theanine-administered young rats. Newborn rats were fed theanine through dams, which were fed water containing 0.3% theanine, and then fed water containing 0.3% theanine after weaning. Theanine level in the brain was under the detectable limit 6 weeks after the start of theanine administration. Theanine administration did not influence locomotor activity in the open-field test. However, rearing behavior was significantly increased in theanine-administered rats, suggesting that exploratory activity is increased by theanine intake. Furthermore, object recognition memory was enhanced in theanine-administered rats. The increase in exploratory activity in the open-field test seems to be associated with the enhanced object recognition memory after theanine administration. On the other hand, long-term potentiation (LTP) induction at the perforant path-granule cell synapse was not changed by theanine administration. To check hippocampal neurogenesis, BrdU was injected into rats 3 weeks after the start of theanine administration, and brain-derived neurotropic factor (BDNF) level was significantly increased at this time. Theanine intake significantly increased the number of BrdU-, Ki67-, and DCX-labeled cells in the granule cell layer 6 weeks after the start of theanine administration. This study indicates that 0.3% theanine administration facilitates neurogenesis in the developing hippocampus followed by enhanced recognition memory. Theanine intake may be of benefit to the postnatal development of hippocampal function.

Pregnenolone sulfate enhances survival of adult-generated hippocampal granule cells via sustained presynaptic potentiation

Synaptic input activity affects the neurogenesis in adult hippocampal dentate gyrus (DG), while the input activity is potentiated by neurosteroid pregnenolone sulfate (PREGS). This study focused on exploring the effects of PREGS on the survival of newborn neurons in the DG of adult male mice. Proliferating cells were labeled with bromodeoxyuridine (BrdU) and injection (i.c.v.) of PREGS (3nmol for two successive days) was made on various paired days from 0–1 to 20–21 after BrdU-injection. PREGS given on day 11–12 or 15–16, roughly corresponding to the prophase of synaptogenesis, produced an approximately 2-fold increase in the number of 22-day-old BrdU+ cells. Hippocampal slices, which were prepared 60min after the in vivo PREGS-injection and followed by 60min perfusion with artificial cerebrospinal fluid (ACSF), showed a sustained (>60min) increase in the presynaptic glutamate release at perforant path-granule cell synapses. PREGS-inducted presynaptic potentiation required a co-activation of α7 nicotinic acetylcholine receptor (α7nAChR), NMDA receptor (NMDAr) and sigma-1 receptor (σ1R), while its maintenance after PREGS-washout depended only on NMDAr and neuronal NO synthase (nNOS) activation. PREGS enhanced the NMDAr-mediated nNOS expression to increase presynaptic glutamate release via a retrograde NO signaling. The sustained presynaptic potentiation was critical to keep the PREGS-enhanced survival of newborn neurons in an NMDAr-dependent manner. These results for the first time provide in vivo evidence that PREGS enhances the survival of newborn neurons in adult animals through the potentiation of synaptic input activity.

Behavioral deficits and subregion-specific suppression of LTP in mice expressing a population of mutant NMDA receptors throughout the hippocampus

The NMDA receptor (NMDAR) subunit GluN1 is an obligatory component of NMDARs without a known functional homolog and is expressed in almost every neuronal cell type. The NMDAR system is a coincidence detector with critical roles in spatial learning and synaptic plasticity. Its coincidence detection property is crucial for the induction of hippocampal long-term potentiation (LTP). We have generated a mutant mouse model expressing a hypomorph of the Grin1(N598R) allele, which leads to a minority (about 10%) of coincidence detection-impaired NMDARs. Surprisingly, these animals revealed specific functional changes in the dentate gyrus (DG) of the hippocampal formation. Early LTP was expressed normally in area CA1 in vivo, but was completely suppressed at perforant path-granule cell synapses in the DG. In addition, there was a pronounced reduction in the amplitude of the evoked population spike in the DG. These specific changes were accompanied by behavioral impairments in spatial recognition, spatial learning, reversal learning, and retention. Our data show that minor changes in GluN1-dependent NMDAR physiology can cause dramatic consequences in synaptic signaling in a subregion-specific fashion despite the nonredundant nature of the GluN1 gene and its global expression.

Reduced excitability in the dentate gyrus network of βIV‐spectrin mutant mice in vivo

The submembrane cytoskeletal meshwork of the axon contains the scaffolding protein betaIV-spectrin. It provides mechanical support for the axon and anchors membrane proteins. Quivering (qv(3j)) mice lack functional betaIV-spectrin and have reduced voltage-gated sodium channel (VGSC) immunoreactivity at the axon initial segment and nodes of Ranvier. Because VGSCs are critically involved in action potential generation and conduction, we hypothesized that qv(3j) mice should also show functional deficits at the network level. To test this hypothesis, we investigated granule cell function in the dentate gyrus of anesthetized qv(3j) mice after electrical stimulation of the perforant path in vivo. This revealed an impaired input-output relationship between stimulus intensity and granule cell population spikes and an enhanced paired-pulse inhibition of population spikes, indicating a reduced ability of granule cells to generate action potentials and decreased network excitability. In contrast, the input-output curve for evoked field excitatory postsynaptic potentials (fEPSPs) and paired-pulse facilitation of fEPSPs were unchanged, suggesting normal excitatory synaptic transmission at perforant path-granule cell synapses in qv(3j) mutants. To corroborate our findings, we analyzed the influence of VGSC density reduction on dentate network activity using an established computational model of the dentate gyrus network. This in silico approach confirmed that the loss of VGSCs is sufficient to explain the electrophysiological changes observed in qv(3j) mice. Taken together, our findings demonstrate that betaIV-spectrin is required for normal granule cell firing and for physiological levels of network excitability in the mouse dentate gyrus in vivo.

Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice

Event Abstract Back to Event Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice Peter Jedlicka1*, Raphael Winkels1, Felix K. Weise1, Christian Schultz2, Thomas Deller1 and Stephan W. Schwarzacher1 1 Goethe University, NeuroScience Center, Clinical Neuroanatomy, Germany 2 Justus Liebig University, Institute of Anatomy and Cell Biology, Germany The submembrane cytoskeletal meshwork of the axon contains the scaffolding protein betaIV-spectrin. It provides mechanical support for the axon and anchors membrane proteins. Quivering (qv3j) mice lack functional betaIV-spectrin and have reduced voltage-gated sodium channel (VGSC) immunoreactivity at the axon initial segment and nodes of Ranvier. Because VGSCs are critically involved in action potential generation and conduction, we hypothesized that qv3j mice should also show functional deficits at the network level. To test this hypothesis, we investigated granule cell function in the dentate gyrus of anesthetized qv3j mice after electrical stimulation of the perforant path in vivo. This revealed an impaired input-output (IO) relationship between stimulus intensity and granule cell population spikes and an enhanced paired-pulse inhibition (PPI) of population spikes, indicating a reduced ability of granule cells to generate action potentials and decreased network excitability. In contrast, the IO curve for evoked field excitatory postsynaptic potentials (fEPSPs) and paired-pulse facilitation of fEPSPs were unchanged, suggesting normal excitatory synaptic transmission at perforant path-granule cell synapses in qv3j mutants.To better understand the influence of betaIVspectrin and VGSC density changes on the dentate gyrus network activity, we employed computational modeling approach. We used a recently developed and highly detailed computational model of the dentate gyrus network (Santhakumar et al., J Neurophysiol 93:437–453, 2005). The network model is based on realistic morphological and electrophysiological data and consists of perforant path inputs and connections of granule, mossy, basket and hilar cells. The role of VGSCs in network excitability was analyzed by systematically varying their densities in axosomatic compartments. This in silico approach confirmed that the loss of VGSCs is sufficient to explain the electrophysiological changes observed in qv3j mice. Computer simulations of the IO and PPI test indicated that in the dentate circuit with altered VGSCs, network excitability decreases owing to impaired spike-generator properties of granule cells and subsequent relative increase of GABAergic inhibitory control over granule cell firing.Taken together, our in vivo and in silico data demonstrate that the destabilization of VGSC clustering in qv3j mice leads to a reduced spike-generating ability of granule cells and considerably decreased network excitability in the dentate circuit. This provides the first evidence that betaIV-spectrin is required for normal granule cell firing and for physiological levels of network excitability in the mouse dentate gyrus in vivo. Conference: Bernstein Conference on Computational Neuroscience, Frankfurt am Main, Germany, 30 Sep - 2 Oct, 2009. Presentation Type: Poster Presentation Topic: Information processing in neurons and networks Citation: Jedlicka P, Winkels R, Weise FK, Schultz C, Deller T and Schwarzacher SW (2009). Computational modeling of reduced excitability in the dentate gyrus of betaIV-spectrin mutant mice. Front. Comput. Neurosci. Conference Abstract: Bernstein Conference on Computational Neuroscience. doi: 10.3389/conf.neuro.10.2009.14.054 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 26 Aug 2009; Published Online: 26 Aug 2009. * Correspondence: Peter Jedlicka, Goethe University, NeuroScience Center, Clinical Neuroanatomy, Frankfurt, Germany, jedlicka@em.uni-frankfurt.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Google Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Google Scholar Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher PubMed Peter Jedlicka Raphael Winkels Felix K Weise Christian Schultz Thomas Deller Stephan W Schwarzacher Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Deficits in LTP and recognition memory in the genetically hypertensive rat are associated with decreased expression of neurotrophic factors and their receptors in the dentate gyrus

We have previously reported that a genetically hypertensive strain of Wistar rat (GH), is deficient in nerve growth factor (NGF) and Trk receptors in dentate gyrus and that these deficits are accompanied by impaired expression of long-term potentiation (LTP) in perforant path–granule cell synapses. Here we confirm this deficit in LTP and report that this strain of rat also displays impairments in long-term recognition memory when compared with normotensive controls. Further analysis of neurotrophin expression in dentate gyrus confirmed the previously-reported deficit in NGF and revealed a decrease in expression of brain-derived neurotrophic factor (BDNF), but not neurotrophin 3 (NT3) or neurotrophin 4 (NT4), in GH rats. These alterations in ligand expression were accompanied by changes in Trk receptor expression; specifically, a decrease in expression of TrkA and TrkB, but not TrkC, in the dentate gyrus of GH, compared with normotensive, rats. We conclude that the impairments in LTP and learning and memory observed in the GH strain are associated with aberrant expression of specific neurotrophic factors and their receptors in the dentate gyrus, adding weight to the evidence indicating a role for these proteins in several forms of synaptic plasticity.

Synaptic Commitment: Developmentally Regulated Reciprocal Changes in Hippocampal Granule Cell NMDA and AMPA Receptors Over the Lifespan

Synaptic transmission in hippocampal field CA1 is largely N-methyl-d-aspartate receptor (NMDA(R)) dependent during the early postnatal period. It becomes increasingly mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) receptors until an adult ratio of AMPA to NMDA receptors is achieved. It is shown here that increases in the AMPA receptor (AMPA(R))-mediated field potential response continue over the life span of the F-344 rat at the perforant path-granule cell synapse in the dentate gyrus. In contrast, the NMDA(R)-dependent component of the response decreases with age between 1 and 27 mo, leading to an increase of AMPA(R)/NMDA(R) ratio with age. One possible explanation of this age difference is that the AMPA(R)/NMDA(R) ratio can be modified by experience. To test the idea that the changed ratio is caused by the old rats' longer lives, an intensive 10-mo period of enrichment treatment was given to a group of animals, beginning at 3 mo of age. Compared with animals housed in standard cages, the enrichment treatment did not alter the glutamatergic response ratio measured with field potential recording methods. These data provide support for the conclusion that the observed change with age is developmentally regulated rather than experience dependent. Given the role of the NMDA(R) in synaptic plasticity, these changes suggest a progressive commitment of perforant path synapses to particular weights over the life span. One possible implication of this effect includes preservation of selected memories, ultimately at the expense of a reduced capacity to store new information.

Altered Dendritic Integration in Hippocampal Granule Cells of Spatial Learning-Impaired Aged Rats

Glutamatergic transmission at central synapses undergoes activity-dependent and developmental changes. In the hippocampal dentate gyrus, the non-N-methyl d-aspartate (NMDA) receptor component of field excitatory postsynaptic potentials (fEPSPs) increases with age in Fischer-344 rats. This effect may not depend on the animal's activity or experience but could be part of the developmental process. Age-dependent differences in synaptic transmission at the perforant path-granule cell synapse may be caused by changes in non-NMDA and NMDA receptor-mediated currents. To test this hypothesis, we compared whole cell excitatory postsynaptic currents (EPSCs) in dentate granule cells evoked by perforant path stimulation in young (3-4 mo) and aged (22-27 mo) Fischer-344 rats using a Cs+-based intracellular solution. Aged animals as a group showed spatial learning and memory deficits in the Morris water maze. Using whole cell recordings, slope conductances of both non-NMDA and NMDA EPSCs at holding potentials -10 to +50 mV were significantly reduced in aged animals and the non-NMDA/NMDA ratio in aged animals was found to be significantly smaller than in young animals. In contrast, we detected no differences in basic electrophysiological parameters, or absolute amplitudes of non-NMDA and NMDA EPSCs. Extracellular Cs+ increased the fEPSP in young slices to a greater degree than was found in the aged slices, while it increased population spikes to a greater degree in the aged rats. Our results not only provide evidence for reduced glutamatergic synaptic responses in Fischer-344 rats but also point to differential changes in Cs+-sensitive dendritic conductances, such as Ih or inwardly rectifying potassium currents, during aging.

A Pivotal Role for Interleukin-4 in Atorvastatin-associated Neuroprotection in Rat Brain

Inflammatory changes, characterized by an increase in pro-inflammatory cytokine production and up-regulation of the corresponding signaling pathways, have been described in the brains of aged rats and rats treated with the potent immune modulatory molecule lipopolysaccharide (LPS). These changes have been coupled with a deficit in long-term potentiation (LTP) in hippocampus. The evidence suggests that anti-inflammatory agents, which attenuate the LPS-induced and age-associated increase in hippocampal interleukin-1beta (IL-1beta) concentration, lead to restoration of LTP. Here we report that atorvastatin, a member of the family of agents that act as inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, exerts powerful anti-inflammatory effects in brain and that these effects are mediated by IL-4 and independent of its cholesterol-lowering actions. Treatment of rats with atorvastatin increased IL-4 concentration in hippocampal tissue prepared from LPS-treated and aged rats and abrogated the age-related and LPS-induced increases in pro-inflammatory cytokines, interferon-gamma (IFNgamma) and IL-1beta, and the accompanying deficit in LTP. The effect of atorvastatin on the LPS-induced increases in IFNgamma and IL-1beta was absent in tissue prepared from IL-4(-/-) mice. The increase in IL-1beta in LPS-treated and aged rats is associated with increased microglial activation, assessed by analysis of major histocompatibility complex II expression, and the evidence suggests that IFNgamma may trigger this activation. We propose that the primary effect of atorvastatin is to increase IL-4, which antagonizes the effects of IFNgamma, the associated increase in microglial activation, and the subsequent cascade of events.

PREGS Induces LTP in the Hippocampal Dentate Gyrus of Adult Rats Via the Tyrosine Phosphorylation International Cooperative Research CREB Signaling

An acute application of neurosteroid pregnenolone sulfate (PREGS) to hippocampal slices from adult rats induced a long-lasting potentiation (LLP PREGS) at the perforant path-granule cell synapse. As a partial mechanism of the LLP PREGS, we previously revealed that PREGS transiently increases the probability of presynaptic glutamate release via a sensitization of alpha7-nicotinic acetylcholine receptor (alpha7nAChR). We herein demonstrate that the LLP PREGS could be separated into two independent processes: the above-mentioned early presynaptic-origin short-term potentiation (STP PREGS) and a delayed postsynaptic N-methyl-d-aspartate receptor (NMDAr)-dependent long-term potentiation termed LTP(PREGS). This study focused on the analysis of the signaling mechanism underlying the LTP PREGS. PREGS increased the tyrosine phosphorylation of NR2B, a subunit of NMDAr, and the NMDAr-mediated Ca2+ influx in the granule cells. The enhanced Ca2+ influx was largely attenuated by the NR2B subunit inhibitor ifenprodil and the Src kinase family inhibitor PP2. PREGS also triggered a persistent phosphorylation of extracellular signal-regulated kinase 2 (ERK2) followed by an ERK-dependent phosphorylation of cAMP-response element-binding protein (CREB), which was crucial for the LTP PREGS induction and was sensitive to ifenprodil. These results suggest that PREGS induces an acute increase in the NR2B tyrosine phosphorylation which enhances the Ca2+ influx through NMDAr, followed by an activation of the ERK/CREB signaling cascade that leads to LTP PREGS.