bluuebunny/crossref_metadata_2025_split huggingface dataset:
Astrocytic dye coupling in rat hippocampus: Topography, developmental onset, and modulation by protein kinase C
Authors: Konietzko Uwe , Müller Christian M. 
Published: May 2005

AbstractPrevious studies have shown that astrocytes constitute a functional syncytium whereas the cytoplasmata of individual cells are connected via gap junctions. Many studies have used cultured astrocytes and have examined electrical coupling with the help of double electrode techniques. Another approach has been the immunohistochemical detection of gap junction proteins in sections of brain tissue. From the results of these experiments it is difficult to infer the extent of astrocyticcoupling in situ. To get an impression of the distribution of coupled astrocytes we took advantage of the hippocampal slice preparation which leaves the topography of neurons and astrocytes intact. We performed injections of low molecular weight dyes into single electrophysiologically identified astrocyes. As these dyes can pass through gap junctions this leads to the staining of all connected cells in a certain area, limited by the diffusional spread of the dye. The results show that there is virtually no border to astrocytic coupling between the diverse hippocampal subdivisions. This widespread coupling could already be detected at postnatal day 4, the earliest age tested. Activation of protein kinase C with phorbol esters showed that it is possible to reduce gap junctional communication by some extent. We conclude that although no compartmentalization was seen with respect to astrocytic coupling, the intercellular communication of these glial cells has the capability of being regulated for example by neuronal signals which activate the phospholipase C pathway in astrocytes. © 1994 Wiley‐Liss, Inc.
Effects of medial septal or unilateral hippocampal inactivations on reference and working spatial memory in rats
Authors: Poucet B. , Buhot M.‐C. 
Published: May 2005

AbstractThe memory performances of rats receiving a reversible inactivation of either the medial septum or one side of the ventral hippocampus were compared in a radial arm maze task allowing the assessment of both working and reference memory. After pre‐surgery training, rats were chronically equipped with bilateral cannulae into the ventral hippocampus and a single cannula into the medial septum. Following post‐surgery retraining, animals received a series of test trials during which they received saline or lidocaine injections in either the medial septum or one side of the ventral hippocampus. Lidocaine injections in either structure resulted in both reference and working memory deficits. However, animals were more impaired after septal injections than after unilateral hippocampal injections. This result suggests that the septo‐hippocampal formation acts as a functionally homogeneous structure essential for spatial processing. © 1994 Wiley‐Liss, Inc.
Synaptic and intrinsic properties of neurons of origin of the perforant path in layer II of the rat entorhinal cortex in vitro
Authors: Jones Roland S. G. 
Published: May 2005

AbstractLayer II of the entorhinal cortex (EC) provides the first step in the hippocampal trisynaptic loop via the perforant path projection to the dentate gyrus. While a great deal is known about this projection and the properties of the dentate granule cells, much less information is available concerning the properties of and synaptic inputs to the cells of origin of the pathway in layer II. The present experiments have employed a slice preparation of the rat EC to study the intrinsic membrane properties and synaptic organization of layer II neurons. Two types of neurons could be identified electrophysiologically. The majority were designated type I and displayed a pronounced time‐dependent inward rectification in the hyperpolarizing direction. Type II displayed little evidence of this characteristic. However, morphological examination suggested that both types were spiny stellate neurons projecting via the perforant path. Synaptic responses of both types displayed evidence of excitatory inputs mediated by both N‐methyl‐D‐aspartate (NMDA) and non‐NMDA glutamate receptors. In general, however, at low frequencies the responses were dominated by inhibitory inputs mediated by both GABAA and GABAB receptors. At higher frequencies the bias was shifted much more toward excitation. The contribution of synaptic and intrinsic properties of layer II neurons to the processing capabilities of the EC is discussed. © 1994 Wiley‐Liss, Inc.
Computational analysis of the role of the hippocampus in memory
Authors: Treves Alessandro , Rolls Edmund T. 
Published: May 2005

AbstractThe authors draw together the results of a series of detailed computational studies and show how they are contributing to the development of a theory of hippocampal function. A new part of the theory introduced here is a quantitative analysis of how backprojections from the hippocampus to the neocortex could lead to the recall of recent memories. The theory is then compared with other theories of hippocampal function. First, what is computed by the hippocampus is considered. The hypothesis the authors advocate, on the basis of the effects of damage to the hippocampus and neuronal activity recorded in it, is that it is involved in the formation of new memories by acting as an intermediate‐term buffer store for information about episodes, particularly for spatial, but probably also for some nonspatial, information. The authors analyze how the hippocampus could perform this function, by producing a computational theory of how it operates, based on neuroanatomical and neurophysiological information about the different neuronal systems contained within the hippocampus. Key hypotheses are that the CA3 pyramidal cells operate as a single autoassociation network to store new episodic information as it arrives via a number of specialized preprocessing stages from many association areas of the cerebral cortex, and that the dentate granule cell/mossy fiber system is important, particularly during learning, to help to produce a new pattern of firing in the CA3 cells for each episode. The computational analysis shows how many memories could be stored in the hippocampus and how quickly the CA3 autoassociation system would operate during recall. The analysis is then extended to show how the CA3 system could be used to recall a whole episodic memory when only a fragment of it is presented. It is shown how this recall could operate using modified synapses in backprojection pathways from the hippocampus to the cerebral neocortex, resulting in reinstatement of neuronal activity in association areas of the cerebral neocortex similar to that present during the original episode. The recalled information in the cerebral neocortex could then be used by the neocortex in the formation of long‐term memories. © 1994 Wiley‐Liss, Inc.
Non‐lamellar propagation of entorhinal influences in the hippocampal formation: Multiple electrode recordings in the isolated guinea pig brain in vitro
Authors: Paré Denis , Llinás Rodolfo 
Published: May 2005

AbstractExperiments were carried out to study the spatiotemporal organization of medial entorhinal inputs to the hippocampal system. They were performed in the isolated guinea pig brain in vitro preparation as it provides easy access to the medial entorhinal cortex (mEC) which is difficult to reach in vivo. Multiple simultaneous field potential recordings along the septotemporal extent of the dentate granular layer revealed that the mEC projection to the dentate gyrus (DG) is organized topographically. Thus, stimulation of the caudal regions of the mEC elicited population spikes (PSs) in the septal pole of the DG while successively more rostral stimulation sites activated progressively more temporal sectors of the DG. However, threshold mEC stimuli never elicited PSs over more than one‐third of the DG.In the CAl pyramidal layer, only trisynaptic PSs were evoked by the mEC stimulation (latency >20 ms at 30°C). However, PSs were widely distributed in the transverse and longitudinal axes of the hippocampus and, irrespective of the mEC stimulation site, the latency of CAl PSs gradually increased from the CA3/CAl border toward the subiculum. By contrast, in the longitudinal axis, each segment of the CAl region responded at a shorter latency to stimulation of a given rostrocaudal level of the mEC. Septal CAl levels responded at shorter latencies to caudal mEC stimulation sites while more temporal CAl levels responded at shorter latencies to rostral mEC stimulation sites. When stimulated at threshold stimulation intensity, the initial CAl response propagated to the rest of the CAl field with a conduction velocity of 0.5–0.9 m/s.These results suggest that the associative pathways linking CA3 and CA1 regions can transmit mEC‐elicited DG activities to large segments of the hippocampal formation while preserving the topographical relationship between the mEC and the hippocampus in the time domain. These findings are in contrast to the so‐called lamellar hypothesis. © 1994 Wiley‐Liss, Inc.
Model of local connectivity patterns in CA3 and CA1 areas of the hippocampus
Authors: Bernard Christophe , Wheal Howard V. 
Published: May 2005

AbstractIn this study we describe a model of connectivity linking the different neurons in the CA3 and CA1 areas of the young male rat hippocampus. The anatomical and electrophysiological values of the parameters used in the model were selected from the available literature. Each type of synapse was characterized by its spatial location on the dendritic tree, its weight, its probability of activation, and the ionotropic receptors involved. We have shown that the degree of convergence and divergence of inputs is highly dependent upon the type of neuron and its spatial location. The different gradients of connectivity we describe support the lamellar hypothesis from a functional point of view, even if the anatomical patterns seem diffuse. The analysis of the proportion of common afferents to a class of neurons further confirmed this point. It is suggested that the circuitry creates local coherence in terms of processing of information by establishing restricted areas where information is preferentially treated. The functional consequences and limitations of these findings are also discussed. This model is the first step in the development of a network model of the hippocampus with realistic architecture. © 1994 Wiley‐Liss, Inc.
Dendritic Ca2+ accumulations and metabotropic glutamate receptor activation associated with an n‐methyl‐d‐aspartate receptor‐independent long‐term potentiation in hippocampal CA1 neurons
Authors: Petrozzino Jeffrey J. , Connor John A. 
Published: May 2005

AbstractBathing hippocampal slices in the potassium channel blocker tetraethylammonium (TEA), while stimulating the Schafer collaterals at a low frequency, induces Ca2+ ‐dependent, N‐methyl‐D‐aspartate (NMDA) receptor‐independent long‐term potentiation of synaptic transmission (LTPk) in CA1 neurons. We have combined ratio imaging of fura‐2 and mag‐fura‐5 in hippocampal CA1 neurons with intracellular and field recordings to evaluate postsynaptic Ca2+ changes that occur in the induction of LTPk. Test stimuli were applied at 0.05 Hz to stratum radiatum in the presence of the NMDA receptor antagonists D, L‐2‐amino‐5‐phosphonovaleric acid (100μM) or MK‐801 (10μM). During TEA exposure (15–25 mM; 10 min), cells fired prolonged action potentials both spontaneously and in response to test stimuli resulting in transient, micromolar Ca2+ accumulations in both somata and dendrites. The initial EPSP slope, measured 60 min after TEA wash‐out, was potentiated to approximately 200% of control. The Ca2+ channel blocker nimodipine (10 μM) greatly reduced Ca2+ transients in both magnitude and duration and prevented LTPk induction. Pretreatment of slices with compounds that block metabotropic glutamate receptor (mGluR)‐stimulated phosphoinositide hydrolysis, L‐2‐amino‐3‐phosphonopropionic acid (L‐AP3, 50‐200 μM) or L‐aspartate‐β‐hydroxamate (50–100 μM), as well as protein kinase C (PKC) inhibitors (sphingosine, 20 μM; RO‐31‐8220, 0.2 μM; or calphostin C, 2 μM) also blocked LTPk. Ca2+ transients were unaffected by L‐AP3 or RO‐31‐8220. These findings suggest that Ca2+ influx through voltage‐gated channels and co‐activation of PKC by mGluRs are both necessary for induction of LTPk. Activation of mGluRs must also occur in NMDA receptor‐dependent induction paradigms, but is possibly of lesser importance owing to the much greater gating of Ca2+ directly into the dendritic spines. © 1994 Wiley‐Liss, Inc.
“Short‐stops” in rats with fimbria‐fornix lesions: Evidence for change in the mobility gradient
Authors: Whishaw Ian Q. , Cassel Jean‐Christophe , Majchrzak Monique , Cassel Suzanne , Will Bruno 
Published: May 2005

AbstractRats with damage to the hippocampal formation and allied structures are hyperactive in many test situations but the cause of this hyperactivity is not known. Here the activity of control rats and rats with fimbria‐fornix lesions is documented in tests of overnight activity. Details of activity are then characterized from video recordings of behavior in an open field. Rats with fimbria‐fornix lesions make significantly more stops of shorter duration and thus more individual trips than control rats but they do not differ in the distance traveled on individual trips or in travel speed. It is suggested that the main difference between fimbria‐fornix rats and control rats is that when fimbria‐fornix rats stop they remain “still” for shorter durations than do control rats. This finding is discussed in relation to a theory of locomotor/exploratory behavior, and in relation to its implications with respect to the performance of fimbria‐fornix rats in studies of learning and memory. © 1994 Wiley‐Liss, Inc.
Correlation of anoxic neuronal responses and calbindin‐D28k localization in stratum pyramidale of rat hippocampus
Authors: Morris Mary E. , Baimbridge Kenneth G. , El‐Beheiry Hossam , Obrocea Gabriela V. , Rosen Andrei S. 
Published: May 2005

AbstractImmunohistochemical staining for the calcium‐binding protein calbindin‐D28k (CaBP) was combined with Lucifer Yellow (LY) identification and intracellular recording of changes in membrane parameters of pyramidal neurons in CA2, CA1, and the sebiculum of rat hippocampal slices during brief exposure (4.0 ± 0.19 min) to N2. Anoxia evoked either a depolarization or hyperpolarization of membrane potential (VM) (+21.5 ± 2.79 mV above VM = −70.5 ± 1.50 mV, n = 30 and −7.2 ± 0.72 mV below VM = −68.2 ± 1.34 mV, n = 24, respectively) and a fall in membrane resistance of =20%. Differences in the response could be correlated with the presence or absence of CaBP and the localization of neurons in different layers of stratum pyramidale and sectors of the hippocampus. For neurons immunopositive for calbindin (CaBP(+)), depolarization was observed more frequently (83%) than hyperpolarization (17%); in contrast, 44% of responses of calbindin‐negative (CaBP(−)) neurons were depolarizing and 56% were hyperpolarizing. Depolarizations of CaBP(+) neurons were more gradual in slope, and more rapidly reached a plateau in comparison with those recorded in CaBP(−) neurons. Responses of neurons in the superficial layer of stratum pyramidale (in which 79% of CaBP(+) pyramidal neurons were situated) were mainly depolarizing (91%), while for those in the deep layer (which contained 89% of the CaBP(−) cells) such responses were observed less often (45%). Depolarization was also more common than hyperpolarization for cells located in CA2/CA1c/CA1b (63%) than in the CA1a/subicular region (37%). The depolarizing response of the majority of pyramidal neurons which are CaBP(+), superficial, and closer to CA3 may reflect an efficient buffering of intracellular Ca2+, which maintains a low [Ca2+]i, steep gradient for Ca2+ influx and may facilitate the movement of Ca2+ away from points of entry. The neurons which are CaBP(−), deep, and closer to subiculum and in which N2 evokes hyperpolarization, on the other hand, may have a sustained elevation/accumulation of cytosolic Ca2+ which could activate K+ conductance, inhibit Ca2+ influx, and stabilize the membrane potential. These experiments provide a functional correlate for CaBP and suggest that it may have a significant role in Ca2+ homeostasis and the determination of selective neuronal vulnerability. © 1995 Wiley‐Liss, Inc.
Reduction of the threshold for long‐term potentiation by prior theta‐frequency synaptic activity
Authors: Christie Brain R. , Stellwagen David , Abraham Wickliffe C. 
Published: May 2005

AbstractActivation of the lateral perforant path input to the dentate gyrus with theta‐patterned conditioning trains produced LTP of synaptic efficacy that changed in magnitude as an inverted U‐shaped function of the number of trains. The LTP induction function was not fixed, however, and could be shifted to the left by administering 5 Hz “priming” stimulation to the lateral path 10 min prior to the conditioning trains. The priming effect was input specific and selective to a narrow window of stimulus frequencies. The shift to the left of the LTP induction function by priming stimulation was blocked by the muscarinic receptor antagonist atropine sulphate. Nimodipine, an antagonist of L‐type voltage‐sensitive calcium channels, did not mimic the priming effect but instead produced a general facilitation of LTP is induction. These data demonstrate that the degree to which LTP is induced in the lateral path is a non‐linear function of afferent activity, and that this function, including LTP threshold, can be shifted to the left by prior synaptic activity at hippocampal theta‐rhythm frequencies. © 1995 Wiley‐Liss, Inc.
Adenosine antagonists have differential effects on induction of long‐term potentiation in hippocampal slices
Authors: Forghani Reza , Krnjević K. 
Published: May 2005

AbstractHow adenosine leakage and tetanic release might affect long‐term potentiation (LTP) was investigated by applying adenosine antagonists 8(p‐sulfophenyl)theophylline (8SPT) or 8‐cyclopentyl‐3, 7‐dihydro‐1, 3‐dipropyl‐1H‐purine‐2, 6‐dione (DPCPX) to slices, while recording CA1 field EPSPs and population spikes. In the first series of experiments, we applied weak double tetani (at 100 Hz, for 1 s) that were subliminal for evoking LTP in initial control runs. In the presence of 8SPT—at concentrations (10–50 μM) which block both A1 and A2 receptors—the same tetani consistently evoked LTP of population spikes but not of excitatory postsynaptic potentials (EPSPs), whereas DPCPX (50 nM), which blocks only A1 receptors, facilitated LTP of both EPSPs and population spikes. These results are consistent with previous evidence that tetanic adenosine release on the one hand depresses LTP via A1 receptors but on the other facilitates LTP via A2 receptors.In a second set of experiments, 8SPT (50–100 μM) did not prevent the induction of LTP of both EPSPs and population spikes by stronger tetanic stimulation. Therefore A2 receptor activation is not essential for the induction of LTP when stronger tetani are applied. Overall, the main effect of endogenous adenosine release is to oppose LTP induction. © 1995 Wiley‐Liss, Inc.
Hippocampal long‐term potentiation does not affect either discrimination learning or reversal learning of the rabbit nictitating membrane response
Authors: Rioux George F. , Robinson Gilbert B. 
Published: May 2005

AbstractThe theoretical premise that the acquistion and storage of information occurs through the strengthening of synaptic connections has contributed to the popularity of long‐term potentiation (LTP) as a candidate neural mechanism for associative learning. However, whether experimentally induced LTP facilitates, disrupts, or has no effect on subsequent learning is a controversial issue. The present study examined the reported facilitative effect of LTP within hippocampal perforant path‐dentate gyrus synapses on subsequent discriminative conditioning of the rabbit nictitating membrane response. In addition, the effect of LTP on subsequent reversal learning of the initial discrimination was examined. LTP did not significantly affect acquisition of the initial discriminative response or subsequent reversal learning. Furthermore, the magnitude of LTP could not be used to predict the rate of acquisition of either task. The failure to find an effect of LTP on classical conditioning of the rabbit nictitating membrane response mirrors the recent failures to replicat the disruptive effect of LTP on spatial learning in the rat. Thus, the potential contribution of an LTP‐like mechanism to associative learning remains equivocal. © 1995 Wiley‐Liss, Inc.
Simulation of spatial learning in the Morris water maze by a neural network model of the hippocampal formation and nucleus accumbens
Authors: Brown Michael A. , Sharp Patricia E. 
Published: May 2005

AbstractCells in the hippocampal formation show spatial firing correlates thought to be critical to the role played by this structure in spatial learning. Place cells in the hippocampus proper show location‐specific activity, whereas cells in the postsubiculum fire as a function of momentary directional heading.One question which has received little attention is how these spatial signals are used by motor structures to actually guide spatial behavior.Here we present a model of how one kind of spatial behavior, instrumental learning in the Morris water maze, could be guided by the spatial information in the hippocampal formation. For this, we concentrate on the hippocampal projection to the nucleus accumbens, which is strongly implicated in instrumental learning.In the model, simulated firing patterns of place cells and head direction cells activate “motor” cells in the “accumbens.” Each motor cell causes a particular locomotor movement in a simulated rat. In this way, the “rat” locomotes through the simulated environment. Each step places the animal in a slightly different location and directional orientation, which, in turn, activates a different set of place and head direction cells, thus causing the next locomotor response, and so on. Connection strengths between cells are initially set randomly. When the animal encounters the reward location, however, connections are altered, so that recently active synapses are strengtheened. Thus, successful moves in a particular locational and directional context are “stamped in.”Simulated rats show rapid learning, similar in many ways to that of actual rats. In particular, they generate efficient routes to the goal after minimal experience, and can do so from somewhat novel starting positions.Consideration of the model architecture shows that (1) combined use of directional and place information is an example of a linearly inseparable problem and that (2) some types of novel route generation, often thought to require a “cognitive mapping” strategy, can be generated from the S‐R type model used here. © 1995 Wiley‐Liss, Inc.