摘要:

AbstractTo characterize excitatory inputs to dentate basket cells from dentate granule cells and the perforant path, the whole‐cell recording technique was used in neonatal rat hippocampal slices. Spontaneous excitatory input to basket cells was also examined and compared to that of other interneurons in the dentate gyrus. Basket cells were separable from other neurons in the dentate gyrus based on morphology and location, as determined by biocytin staining following recording, and by resting membrane potential, propensity to fire action potentials spontaneously, and miniature excitatory postsynaptic current (EPSC) characteristics. Minimal electrical stimulation of the granule cell layer evoked in basket cells short latency EPSCs that were composed of bothN‐methyl‐D‐aspartate (NMDA) and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazole‐propionate (AMPA) components as judged by their time course, voltage dependence, and blockade by selective antagonists. Perforant path EPSCs exhibited slower kinetics than EPSCs evoked by granule cell stimulation. Like granule cell evoked EPSCs, however, perforant path EPSCs were composed of both NMDA and AMPA components. Minimal electrical stimulation of the granule cell layer and perforant path evoked monosynaptic EPSCs in only 67% and 62% of the trials, respectively, suggesting that these inputs are as unreliable as previously determined inputs from CA3 pyramidal cells (48%).Tetrodotoxin‐insensitive spontaneous miniature EPSCs were frequent in basket cells and non‐basket interneurons residing either at the border between the granule cell layer and the hilus or deep within the hilus. Miniature EPSCs recorded from all cells held at −70 mV were blocked completely by 3 μSM 6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione (CNQX). Though a component of the miniature EPSCs recorded from border and deep hilar interneurons at +40 mV was blocked by the NMDA receptor antagonistD‐2‐amino‐phosphonovaleric acid (D‐APV), miniature EPSCs in basket cells were insensitive to D‐APV.We conclude that input from granule cells and the perforant path results in activation of basket cells via glutamatergic synapses that employ both NMDA and AMPA receptors. These inputs to basket cells likely contribute to feedback and feedforward inhibition of granule cells. The absence of an NMDA receptor component in spontaneous miniature EPSCs of dentate basket cells implies a difference in organization of excitatory synapses made onto basket cells compared

ISSN:1050-9631    

DOI:10.1002/hipo.450050302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

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‐L

ISSN:1050-9631    

DOI:10.1002/hipo.450050303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

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.

ISSN:1050-9631    

DOI:10.1002/hipo.450050304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractWhite and McDonald (1993, Behav Brain Res 55:269–281) previously reported that animals with amygdala lesions failed to acquire a conditioned‐cue preference (CCP) based on spatial cues, but that animals with fornix lesions exhibited larger CCPs of this type than normal animals. The present experments focused on the hippocampal interference with amygdala‐based CCP learning inferred from this finding. In experiment 1 we tested the hypothesis that this interference was due to the acquistion of information about the maze and its environment during a 10 min period of free exploration of the maze before the start of CCP training, hitherto given to all animals in these experiments. Normal animals that were not preexposed to the maze and animals that were preexposed to similar maze in a different room both exhibited larger CCPs than animals that were preexposed to the same maze in the same room as CCP training and testing. This suggests that normal animals acquire context‐specific information during the preexposure period and that this may be the cause of the hippocampus‐based interference with the amygdala‐mediated CCP. In experiment 2 we attempted to determine if the information thought to be acquired by the hippocampal memory system interferes with acquistion or expression of the CCP. As previously demonstrated, animals that received fornix lesions before preexposure (i.e., before the start of the experiment) exhibited larger than normal CCPs. Animals that received fornix lesions after preexposure but before CCP training and animals that received fornix lesions after CCP training but before testing both exhibited normal CCPs. These findings suggest that a hippocampally mediated process acts via a pathway that does not involve the fornix to inhibit either acquistion or expression, or both, of the CCP. These results imply the existence of some form of competition for behavioral control among neural systems that may be specialized for acquiring different types of information about the same situations. © 1995 Wil

ISSN:1050-9631    

DOI:10.1002/hipo.450050305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThe object exploration task allows the measure of changes in locomotor and exploratory activities, habituation, and reaction to a spatial change and to novelty. The effects of intrahippocampal (dorsal CA1 field) microinjections of serotonin 1 receptor (5‐HT1) agonists on these behavioral components were evaluated in the rat. 8‐Hydroxy‐2‐(din‐propylamino)‐tetralin (8‐OH‐DPAT,5 μg/μl) was used as a 5‐HT1A agonist, 3‐(1,2,5,6‐tetrahydropyrid‐4‐yl) pyrrolo[3,2‐b] pyrid‐5‐one (CP 93,129, 16 μg/μl) as a 5‐HT1B agonist, and scopolamine (10 μg/μl) as a muscarinic cholinergic antagonist. Scopolamine induced a long‐lasting increase in locomotor activity and a lack of reaction to spatial change; both these results are in agreement with the known crucial influence of the septo‐hippocampal cholinergic system in hippocampal functioning. Stimulation of 5‐HT1A and 5‐HT1B receptors induced a decrease in object exploration and habituation without affecting the retrieval of spatial information. But stimulation of hippocampal 5‐HT1B receptors induced a selective change in the animal's emotional state, i.e., an initial decrease in locomotor activity and a neophobic reaction in response to a new object; such effects did not occur following stimulation of 5HT1A receptors. These results have to be considered in the light of the anxiogenic propety of 5‐HT1B agonists. On the whole, they support the hypothesis of the involvement of the serotonergic system, via 5HT1A and 5‐HT1B receptors, in the modula

ISSN:1050-9631    

DOI:10.1002/hipo.450050306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractThis study extends previous investigations into the effect of adenosine on bicuculline‐resistant paired‐pulse inhibition between field potentials evoked 300 ms apart in the CA1 area of the rat hippocampal slice. A direct assessment of the effect of adenosine on paired‐pulse inhibition is complicated by the facts that adenosine directly depresses evoked potentials and bicuculline‐resistant paired‐pulse inhibition is greather between pairs of small potentials than between pairs of larger potentials. Adenosine increased biculline‐resistant paired‐pulse inhibition when stimulus strength was constant between adenosine and control but paired‐pulse inhibition of responses in adenosine was markedly less than paired‐pulse inhibition of control responses of the same size. Futhermore, adenosine decreased the size of conditioned potentials to a significantly lesser extent than unparired potentials of the same initial size. Taken together the results indicate that adenosine can decrease bicuculline‐resistant paired‐pulse inhibition in the hippocampus. A possible mechanism for this effect is that adenosine is suppressing transmission at excitatory terminals onto interneurones which would suggest that these receptors are more sensitive to adenosine than those on the Schaffer collateral/CA1 pyramidal cell synapses. In this case adenosin should reduce paired‐pulse inhibition at lower concentrations than are required for depression of single evoked potentials. A comparison of the concentration‐response relationships for the effects of adenosine on paired‐pulse inhibition and on single evoked potentials ruled out greater sensitivity of adenosine receptors at excitatory terminals onto interneurones as an explanation for adenosine's action on bicuculline‐resistant paired‐pulse inhibition. Adenosine was less effective at reducing inhibition evoked by large supramaximal conditioning stimuli than by submaximal for evoked potential size, although control paired‐pulse inhibiton is larger in the later case. This finding is consistent with adenosine reducing bicuculline‐resistant paired‐pulse inhibition by causing an increase in simultaneous paired‐pul

ISSN:1050-9631    

DOI:10.1002/hipo.450050307

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractPrevious studies described the postnatal development of CA3 pyramidal neurons and their afferents in the rat. However, the postnatal development of the primate hippocampus was not previously studied. Thus, pyramidal neurons of the CA3 area of the monkey hippocampus were analyzed postnatally in the present study. At birth, a few thorny excrescences, the complex spines postsynaptic to mossy fibers, were found on the proximal segments of both apical and basal dendrites, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number until the third month. A continuous increase in the number of spines per unit length along the distal dendrites was observed during the first 12 months. The ultrastructural features of somata and dendrites of pyramidal cells in newborn monkeys were similar to those of adults.The analysis of the afferents to the CA3 pyramidal neurons was limited to the development of mossy fibers, the axons of granule cells, and myelinated axons in the alveus, stratum oriens, and stratum lacunosum‐moleculare. At birth, most mossy fiber terminals were densely packed with synaptic vesicles and formed mainly axospinous synapses with CA3 pyramidal cells. By 1 month of age, the number of mitochondria and embedded spines increased to mature amounts. In the first postnatal month, degenerating axons and axon terminals were frequently observed in the mossy fiber bundles in stratum lucidum. The proportion of myelinated axons increased simultaneously in all three examined layers. At birth most axons were unmyelinated, whereas at 7 months of age the proportion of myelinated axons was similar to that found in adults.The present study indicates that most pyramidal neurons of the CA3 region in monkeys are in an advanced stage of development at the time of birth. Thus, mossy fibers from granule cells in the dentate gyrus have established mature‐looking synapses, and the thorny excrescences of pyramidal cells that are postsynaptic to mossy fibers are also adult‐like. Nevertheless, several of the adult features, such as the spine density of distal dendrites of pyramidal neurons and the myelination of afferent axons, develop during an extended period of time in the first year. The significance of this early anatomical maturation in a brain region involved in memory function is consistent with recent behavioral data that show a rapid postnatal maturation of limbic‐dependent recognition memory in rhesus monkeys. © 1995 Wiley

ISSN:1050-9631    

DOI:10.1002/hipo.450050308

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1050-9631    

DOI:10.1002/hipo.450050309

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1050-9631    

DOI:10.1002/hipo.450050310

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:1050-9631    

DOI:10.1002/hipo.450050311

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY