摘要:

AbstractThe retrosplenial cortex is situated at the crossroads between the hippocampal formation and many areas of the neocortex, but few studies have examined the connections between the hippocampal formation and the retrosplenial cortex in detail. Each subdivision of the retrosplenial cortex projects to a discrete terminal field in the hippocampal formation. The retrosplenial dysgranular cortex (Rdg) projects to the postsubiculum, caudal parts of parasubiculum, caudal and lateral parts of the entorhinal cortex, and the perirhinal cortex. The retrosplenial granular b cortex (Rgb) projects only to the postsubiculum, but the retrosplenial granular a cortex (Rga) projects to the postsubiculum, rostral presubiculum, parasubiculum, and caudal medial entorhinal cortex. Reciprocating projections from the hippocampal formation to Rdg originate in septal parts of CA1, postsubiculum, and caudal parts of the entorhinal cortex, but these are only sparse projections. In contrast, Rgb and Rga receive dense projections from the hippocampal formation. The hippocampal projection to Rgb originates in area CA1, dorsal (septal) subiculum, and postsubiculum. Conversely, Rga is innervated by ventral (temporal) subiculum and postsubiculum. Further, the connections between the retrosplenial cortex and the hippocampal formation are topographically organized. Rostral retrosplenial cortex is connected primarily to the septal (rostrodorsal) hippocampal formation, while caudal parts of the retrosplenial cortex are connected with temporal (caudoventral) areas of the hippocampal formation. Together, the elaborate connections between the retrosplenial cortex and the hippocampal formation suggest that this projection provides an important pathway by which the hippocampus affects learning, memory, and emotional behavior.

ISSN:1050-9631    

DOI:10.1002/hipo.450020102

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe fluorescent retrograde tracer, fluorogold, was used to identify commissurally projecting neurons in the hippocampus and dentate gyrus. After injection of fluorogold into the hippocampus, the contralateral hippocampus was evaluated for fluorogold‐immunoreactive or fluorescent neurons. In addition to observing labeled hilar neurons and CA3 pyramidal cells that previously have been reported to send commissurally projecting axons to the contralateral hippocampus, the authors unexpectedly found a population of fluorogold‐labeled cells in the granule cell layer with the morphology and location of GABA‐immunoreactive basket cells. Immunocytochemical staining revealed that all fluorogold‐labeled cells of the granule cell layer were immunoreactive for parvalbumin. However, not all parvalbumin cells, shown previously to be a subset of GABA neurons, were fluorogold‐labeled. The association between fluorogold transport and parvalbumin immunoreactivity was unique for these cells of the granule cell layer. In the adjacent hilus, relatively few of the many fluorogold‐labeled cells were parvalbumin‐ or GABA‐immunoreactive. These results (1) identify a population of presumed inhibitory neurons that apparently form commissural projections; (2) document that all of these cells contain the calcium‐binding protein parvalbumin; and (3) indicate that the vast majority of commissurally projecting hilar neurons are neither parvalbumin‐ nor

ISSN:1050-9631    

DOI:10.1002/hipo.450020103

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

年代:1992

数据来源: WILEY

摘要:

AbstractResearch has demonstrated environmentally induced plasticity of hippocampal dentate gyrusevoked potentials. Other research has shown a role of the NMDA receptor in dentate gyrus long‐term potentiation (LTP). The authors tested the role of the NMDA receptor in one form of environmentally induced plasticity, in which transferring animals from their home cages to another environment results in significant excitatory postsynaptic potential (EPSP) enhancement and concomitant depression of the population spike. Rats were chronically implanted with stimulating electrodes in the perforant path and recording electrodes in the dentate gyrus bilaterally. Evoked potentials were recorded from freely behaving rats for four 20‐minute sessions (1/wk), which took place immediately following an environmental transfer. Rats received 0.00, 0.05, 0.08, or 0.10 mg/kg MK‐801 s. c. 30 minutes prior to recording sessions in either an ascending‐ or descending‐dose series. Results showed that MK‐801 produced a reduction of the EPSP enhancement, which takes place over the 20‐minute session. The effects of MK‐801 on spike depression varied as a function of dose series and time within a session, suggesting a long‐term effect of MK‐801 on spike depression. There was no detected effect of MK‐801 on behavior. Results suggest a role of the NMDA receptor in this form of environmentally induced plasticity with different effects of NMDA receptor antagonism on EPSP enhancement

ISSN:1050-9631    

DOI:10.1002/hipo.450020104

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

年代:1992

数据来源: WILEY

摘要:

AbstractA modified medium containing an AMPA receptor antagonist and low concentrations of magnesium was used to investigate the factors governing the potentiation of synaptic responses mediated by NMDA receptors. When long‐term potentiation (LTP) was induced in standard medium and NMDA responses were analyzed by changing to the modified medium, no statistically significant differences were observed between potentiated and control pathways. Returning the slices to the standard medium showed that LTP was still present, indicating that the potentiation effect was not reversed by the modified medium. High‐frequency stimulation applied in the modified medium produced an enhancement of synaptic responses, but this was not occluded by prior potentiation in standard medium. The degree of potentiation induced in the modified medium and expressed by NMDA responses was larger in the presence than in the absence of inhibition and, unlike LTP, was proportionately larger when recorded in the stratum pyramidale than in the stratum radiatum. These results indicate that the potentiation of NMDA receptor‐mediated responses triggered by high‐frequency stimulation applied in modified medium differs in several respects from the LTP induced in standard conditions. They confirm that LTP is expressed to a markedly different degree by NMDA and non‐NMDA receptors and suggest that events that do not necessarily accompany LTP affect the potentiation of NMDA receptordependent synaptic

ISSN:1050-9631    

DOI:10.1002/hipo.450020105

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe hypothesis that the maintenance or decay of an associative memory trace after an extended retention interval is a function of the residual strength of the synapses originally strengthened during learning was examined in a classical conditioning paradigm in which high‐frequency stimulation of a hippocampal input—the medial perforant path—served as a conditioned stimulus. Rats received perforant path stimulus‐foot shock pairings while engaged in a previously acquired food‐motivated lever‐pressing task. Conditioned suppression of lever pressing was the behavioral measure of learning and retention of the association. Stimulus trains to the perforant path at an intensity above the threshold for eliciting a population spike induced long‐term potentiation of synaptic transmission in the dentate gyrus. Synaptic potentials recorded extracellularly in the dentate gyrus were subsequently monitored for 31 days to examine quantitatively the decay of synaptic potentiation, a period after which retention of the learned association was assessed. All rats learned the association to a similar extent and displayed equivalent amounts of long‐term potentiation by the end of conditioning. A slowly decaying function of synaptic potentiation was observed in remembering rats, i. e., rats with high retention performance after the 31‐day learning‐to‐retention interval, while forgetting was associated with a rapid decay of long‐term potentiation. Behavioral performance at the long‐term memory test was linearly correlated with the amplitude of long‐term potentiation maintained just prior to the retention test. The results favor the hypothesis that long‐term associative memory depends, at least in part, on the maintenance of elevated synaptic strengths in the pathway activated during learning and suggest a role for the lasting component of long‐term potentiati

ISSN:1050-9631    

DOI:10.1002/hipo.450020106

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe hypothesis that long‐term potentiation (LTP) involves receptor modifications was tested with aniracetam, a nootropic drug that selectively increases currents mediated by the AMPA subclass of glutamate receptors. Aniracetam had different effects on the waveform of synaptic potentials in hippocampus before and after induction of LTP: (1) the drug caused a slight reduction (or delay) of the initial segment of the response after LTP; and (2) the facilitatory effects of aniracetam occurred at a later time point in the response after LTP than before. The interactions between LTP and aniracetam were still present when synaptic responses were greatly reduced by partial blockade of postsynaptic receptors and were not reproduced by increasing release or the number of stimulated synapses. A mathematical treatment of synaptic currents produced the following results: (1) if aniracetam facilitates AMPA receptor currents simply by reducing desensitization, then its complex interaction with LTP emerges when potentiation changes the kinetic and conductance properties of receptor channels; (2) if aniracetam also significantly increases conductance, then the experimental data can be reproduced by modeling LTP as an increase in channel conductance alon

ISSN:1050-9631    

DOI:10.1002/hipo.450020107

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

年代:1992

数据来源: WILEY

摘要:

AbstractPrevious studies have shown that either norepinephrine (NE) or isoproterenol (ISO) enhances the slope of the field excitatory postsynaptic potential (EPSP) in the dentate gyrus of the rat hippocampal formation. In contrast, NE and ISO cause no increase in excitatory transmission in area CA1 of the hippocampus. The molecular mechanism underlying this brain region‐specific increase in synaptic transmission is not known. The phosphorylation of synapsin I and synapsin II, two homologous presynaptic vesicle‐associated proteins, is thought to promote neurotransmitter release. The authors have observed previously NE‐ and ISO‐enhanced phosphorylation of synapsins I and II in the dentate gyrus. The purpose of this study was to determine whether ISO‐stimulated phosphorylation also occurs in the CA1, where ISO has no effect on excitatory neurotransmission. These studies were correlated with electrophysiological studies inin vitrohippocampal slices. Superfusion of slices with ISO resulted in an increase in EPSP slope in the dentate but not in area CA1. The enhanced dentate EPSP returned to baseline levels within 30 minutes of washout of the drug. Isoproterenol produced corresponding increases in the phosphorylation of the synapsins in dentate slices but had no effect on these proteins in CA1 slices. Moreover, in dentate slices exposed to a 30‐minute wash following incubation with ISO, phosphorylation of the synapsins returned to control levels. This close temporal and brain regional correlation between ISO stimulation of both synapsin phosphorylation and synaptic transmission suggests that the synapsin proteins may play a role in the synaptic potentiation produced by ISO in

ISSN:1050-9631    

DOI:10.1002/hipo.450020108

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

年代:1992

数据来源: WILEY

摘要:

AbstractPrevious studies have demonstrated that the dentate granule and the CA3 pyramidal cells of the rat hippocampal formation are neuronal populations vulnerable to the toxic effects of ethanol. It also has been shown that the resulting alterations do not end after withdrawal from ethanol. As the neurons in the dentate hilus are heavily interconnected with the dentate granule cells, the authors decided to examine the fate of the hilar neurons after chronic alcohol consumption and withdrawal, inasmuch as the hilar somatostatin‐immunoreactive (SS‐I) neurons were found to be sensitive to cerebral ischemia and to seizures.The following groups of adult rats were studied: (1) alcohol‐fed for 6 and 12 months; (2) alcoholfed for 6 months and then switched to water for a further 6 months; (3) pair‐fed controls; and (4) controls fedad libitum. The authors determined the numerical density of hilar neurons and the number of its SS‐I subpopulation. These were found to be significantly reduced in both the alcohol‐fed and withdrawal groups when compared with the respective age‐matched controls. The consequent loss of the integrative action of the hilar neurons, including the SS‐Is, could explain some of the alcohol‐related functional deficits as well as their persistence

ISSN:1050-9631    

DOI:10.1002/hipo.450020109

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

年代:1992

数据来源: WILEY

摘要:

AbstractBoth the hippocampus and the entorhinal cortex are known to be crucial for spatial learning, but the contribution of the pathway linking the two structures, the perforant path (PP), has never been tested in a spatial learning paradigm. The present study examined the role of the PP in spatial learning using the Morris water maze. Seven days after bilateral transection of the PP with a fine‐bladed knife, rats were habituated to the pool, then trained to swim from varying start locations to a platform submerged in a fixed location. After 28 training trials over 5 days, probe trials (without any platform present) were given to assess spatial memory for the location. Compared to sham‐operated controls, lesioned rats showed slower learning and poorer asymptotic performance in terms of both swim path distance and escape latency, and less preference for the correct quadrant during probe trials. When the platform location was “reversed” to the opposite quadrant, the lesioned rats again showed poorer learning, poorer asymptotic performance, and reduced preference for the correct quadrant on the probe trial. When tested with a visible platform whose position varied from trial to trial, lesioned rats performed as well as controls. These results are congruent with previous analyses of the contributions of the entorhinal cortex and hippocampus to spatial learning and suggest that for spatial learning, the PP is a critical functional link between these two str

ISSN:1050-9631    

DOI:10.1002/hipo.450020110

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

年代:1992

数据来源: WILEY

摘要:

AbstractThis study was undertaken to compare the effect of hippocampal neurotoxic lesions in rats on two behavioral tasks, one a test of spatial learning, and the other an operant discrimination task that is acquired by forming nonspatial configural associations. Lesions of the hippocampus were made with microinjections of ibotenic acid. After postoperative recovery, rats were trained initially to locate a camouflaged escape platform in a water maze using distal spatial cues. Rats also were trained in the maze apparatus with a visible escape platform under conditions in which spatial information was made irrelevant to performance, i. e., cue learning. In an operant task, the same rats were then trained on a discrimination that included simultaneous feature positive and feature negative components (trial types XA+, A−, XB−, B+). After completion of this nonspatial configural learning task, rats received additional training in the water maze using a new platform location for spatial learning. To the extent that proficient performance in both the maze and operant tasks depends on a common function of the hippocampus, i. e., configural learning, the expectation was that hippocampal lesions would prove equally detrimental to performance in both tasks. Contrary to this expectation, lesioned rats were severely impaired in spatial learning but readily acquired the operant discrimination, even exhibiting some evidence of enhanced performance on this nonspatial configural learning task. Performance of the lesioned rats during cue training in the water maze was also enhanced relative to the control group. These results demonstrate that a spatial learning deficit in rats with hippocampal damage does not necessarily occur in the setting of a general configural learning impairment, as originally suggested by Sutherland and Rudy (1989, Psychobiology 17:129

ISSN:1050-9631    

DOI:10.1002/hipo.450020111

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

年代:1992

数据来源: WILEY