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M P Witter

Publications and source records attributed to M P Witter.

At least 19 recordsLinked to original sources
PubMed1997

GABAergic presubicular projections to the medial entorhinal cortex of the rat.

We characterized presubicular neurons giving rise to bilateral projections to the medial entorhinal cortex (MEA) of the rat. Retrograde labeling of presubiculo-entorhinal projections with horseradish peroxidase and subsequent GABA immunocytochemistry revealed that 20-30% of the ipsilaterally projecting neurons are GABAergic. No GABAergic projections to the contralateral MEA were observed. GABAergic projection neurons were observed only in the dorsal part of the presubiculum, which, when taking into account the topography of presubicular projections to MEA, indicates that only the dorsal part of MEA receives GABAergic input. The GABAergic projection neurons constitute approximately 30-40% of all GABAergic neurons present in the superficial layers of the dorsal presubiculum. Using double-label fluorescent retrograde tracing, we found that the ipsilateral and contralateral presubiculo-entorhinal projections originate from different populations of neurons. Anterograde labeling of presubiculo-entorhinal projections and electron microscopical analysis of labeled terminals substantiated the presence of a restricted GABAergic presubiculo-entorhinal projection. A small fraction of afferents to only ipsilateral dorsal MEA formed symmetrical synapses with dendritic shafts. No symmetrical synapses on spines were noted. Most afferents to the dorsal part of ipsilateral MEA, as well as all afferents to the remaining ipsilateral and contralateral MEA, formed asymmetrical synapses with both spines and dendritic shafts in an almost equal ratio. Thus, we conclude that the majority of the presubiculo-entorhinal projections exert an excitatory effect on both principal neurons and interneurons. The projections from the dorsal part of the presubiculum comprise a small inhibitory component that originates from GABAergic neurons and targets entorhinal interneurons.

Afferent Pathways↗
PubMed1996

Entorhinal-hippocampal interactions revealed by real-time imaging.

The entorhinal cortex provides the major cortical input to the hippocampus, and both structures have been implicated in memory processes. The dynamics of neuronal circuits in the entorhinal-hippocampal system were studied in slices by optical imaging with high spatial and temporal resolution. Reverberation of neural activity was detected in the entorhinal cortex and was more prominent when the inhibition due to gamma-aminobutyric acid was slightly suppressed. Neural activity was transferred in a frequency-dependent way from the entorhinal cortex to the hippocampus. The entorhinal neuronal circuit could contribute to memory processes by holding information and selectively gating the entry of information into the hippocampus.

Animals↗
PubMed1996

Projections from the nucleus reuniens thalami to the entorhinal cortex, hippocampal field CA1, and the subiculum in the rat arise from different populations of neurons.

The entorhinal cortex, CA1, and the subiculum receive a major input from the thalamic midline nucleus reuniens. At present, it is not known whether reuniens projections to these intimately interconnected regions are collateralized or arise from different cell populations. We employed the multiple fluorescent retrograde tracing technique with Fast Blue, Diamidino Yellow, and Fluoro-Gold to examine the possible collateralization of reuniens projections to the entorhinal cortex, CA1, and the subiculum. In addition, we studied the extent of collateralization within each target area. The results indicate that different, yet morphologically indistinguishable, populations of reuniens cells selectively innervate the entorhinal cortex, CA1, or subiculum. Within each of these areas, reuniens fibers display a locally restricted collateralization instead of distributing collaterals throughout the entire target structure. The rostal two-thirds of the nucleus reuniens is the major source of ipsilateral projections to CA1, subiculum, and entorhinal cortex. The perireuniens nucleus selectively projects to the perirhinal cortex. Reuniens projections to CA1 and medial entorhinal cortex originate in the dorsolateral part and throughout the medial one-half of the nucleus, respectively. For these two projections, no topography could be established. However, subicular afferents are topographically organized such that a dorsal-to-ventral gradient in the nucleus reuniens corresponds to a dorsal-to-ventral gradient along the subicular axis. Lateral entorhinal afferents display a subtle topography such that a lateral-to-medial shift of terminal fields in the lateral entorhinal cortex corresponds to a lateral-to-medial shift of projection neurons in the ventral nucleus reuniens.

Animals↗
PubMed1996

Afferents to the seizure-sensitive neurons in layer III of the medial entorhinal area: a tracing study in the rat.

Neurons in layer III of the medial entorhinal area (MEA) in the rat are extremely vulnerable to local injections of amino-oxyacetic acid and to experimentally induced limbic seizures. A comparable specific pathology has been noted in surgical specimens from patients with temporal lobe epilepsy. Efforts to understand this preferential neuronal vulnerability led us to study the neural input to this layer in the rat. Iontophoretic injection of the retrograde tracer fast blue, aimed at layer III of the MEA, resulted in retrogradely labeled neurons in the presubiculum in all the injected hemispheres. The nucleus reuniens thalami, the anteromedial thalamic nucleus, the ventral portion of the claustrum (endopiriform nucleus), the dorsomedial parts of the anteroventral thalamic nucleus, and the septum-diagonal band complex were labeled less frequently. In only one experiment, retrogradely labeled neurons were observed in the ventrolateral hypothalamus and in the brainstem nucleus raphe dorsalis. Since projections from claustrum to the entorhinal cortex has not been studied in the rat with modern sensitive anterograde tracing techniques, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were placed into the ventral portion of the claustrum. Anterogradely labeled fibers in the entorhinal area proved not to be confined to the MEA, since a prominent projection distributed to the lateral entorhinal area as well. In both areas, the densest terminal labeling was present in layers IV-VI, whereas layer III appeared to be only sparsely labeled. The present data indicate that of all potential afferents only those from the presubiculum distribute preferentially to layer III of the MEA. This, in turn, suggests a potentially important role of the presubiculum in the seizure-related degeneration of neurons in layer III of the MEA.

Afferent Pathways↗
PubMed1995

Quantitative morphological analysis of subicular terminals in the rat entorhinal cortex.

In the present report, we describe a morphological and quantitative analysis of subicular synapses in layer V of the lateral entorhinal cortex (LEA) of the rat. Projections from the dorsal subiculum were labeled anterogradely, and areas in LEA showing high terminal density were randomly selected for ultrathin sectioning. More than 400 terminals in LEA were photographed in the electron microscope, and synapse types and postsynaptic targets were identified and, subsequently, quantified with the unbiased disector method. Most subicular terminals appeared to form asymmetrical synapses. A majority of asymmetrical synapses terminated on spines (67.5%), whereas a smaller fraction of asymmetrical synapses (23.5%) terminated on dendritic shafts. A small fraction of the terminals (7%) had symmetrical features. These symmetrical synapses had an almost equal percentage of spines and dendritic shafts as postsynaptic elements. Labeled synapses on somata or axons were never observed. The findings of this study in conjunction with relevant electrophysiological observations (Jones [1987] Neurosci Left 81:209-214) leads to the conclusion that the subiculo-entorhinal pathway comprises a large excitatory and a smaller inhibitory projection, both making synaptic contacts with presumed principal neurons and interneurons in the entorhinal cortex.

Animals↗
PubMed1995

Parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat: localization, morphology, connectivity and ultrastructure.

We studied the distribution, morphology, ultrastructure and connectivity of parvalbumin-immunoreactive neurons in the entorhinal cortex of the rat. Immunoreactive cell bodies were found in all layers of the entorhinal cortex except layer I. The highest numbers were observed in layers II and III of the dorsal division of the lateral entorhinal area whereas the lowest numbers occurred in the ventral division of the lateral entorhinal area. Most such neurons displayed multipolar configurations with smooth dendrites. We distinguished a type with long dendrites and a type with short dendrites. We also observed pyramidal immunoreactive neurons. A dense plexus of immunoreactive dendrites and axons was prominent in layers II and III of the dorsal division of the lateral entorhinal area and the medial entorhinal area. None of the parvalbumin-immunoreactive cells became retrogradely labelled after injection of horseradish peroxidase into the hippocampal formation. By electron microscopy, immunoreactivity was observed in cell bodies, dendrites, myelinated and unmyelinated axons and axon terminals. Immunoreactive dendrites and axons occurred in all cortical layers. We noted many myelinated immunoreactive axons. Immunoreactive axon terminals were medium sized, contained pleomorphic synaptic vesicles, and established symmetrical synapses. Both horseradish peroxidase labelled and unlabelled immunonegative cell bodies often received synapses from immunopositive axon terminals arranged in baskets. Synapses between immunoreactive axon terminals and unlabelled dendritic shafts and spines were abundant. Synapses with initial axon segments occurred less frequently. In addition, synaptic contacts were present between immunopositive axon terminals and cell bodies and dendrites. Thus, the several types of parvalbumin-containing neuron in the entorhinal cortex are interneurons, connected to one another and to immunonegative neurons through a network of synaptic contacts. Immunonegative cells projecting to the hippocampal formation receive axo-somatic basket synapses from immunopositive terminals. This connectivity may form the morphological substrate underlying the reported strong inhibition of cells in layers II and III of the entorhinal cortex projecting to the hippocampal formation.

Animals↗
PubMed1994

Projections from the presubiculum and the parasubiculum to morphologically characterized entorhinal-hippocampal projection neurons in the rat.

The relations between the inputs from the presubiculum and the parasubiculum and the cells in the entorhinal cortex that give rise to the perforant pathway have been studied in the rat at the light microscopical level. Projections from the presubiculum and the parasubiculum were labeled anterogradely, and, in the same animal, cells in the entorhinal cortex that project to the hippocampal formation were labeled by retrograde tracing and subsequent intracellular filling with Lucifer Yellow. The distribution and the number of appositions between the afferent fibers and hippocampal-projection neurons in the various layers of the entorhinal cortex were analyzed. The results show that layers I-IV of the entorhinal cortex contain neurons that give rise to projections to the hippocampal formation. The morphology of these projection neurons is highly variable and afferents from the presubiculum and the parasubiculum do not show a preference for any specific morphological cell type. Both inputs preferentially innervate the dendrites of their target cells. However, presubicular and parasubicular projections differ with respect to the layer of entorhinal cortex they project to. The number of appositions of presubicular afferents with cells that have their cell bodies in layer III of the entorhinal cortex is 2-3 times higher than with cells in layer II. In contrast, afferents from the parasubiculum form at least 2-3 times as many synapses on the dendrites of cells located in layer II than on neurons that have their cell bodies in layer III. Cells in layers I and IV of the entorhinal cortex receive weak inputs from the presubiculum and parasubiculum. Not only is the presubiculum different from the parasubiculum with respect to the distribution of projections to the entorhinal cortex, they also differ in their afferent and efferent connections. In turn, cells in layer II of the entorhinal cortex differ in their electrophysiological characteristics from those in layer III. Moreover, layer II neurons give rise to the projections to the dentate gyrus and field CA3/CA2 of the hippocampus proper, and cells in layer III project to field CA1 and the subiculum. Therefore, we propose that the interactions of the entorhinal-hippocampal network with the presubiculum are different from those with the parasubiculum.

Animals↗
PubMed1994

Multiple anterograde tracing, combining Phaseolus vulgaris leucoagglutinin with rhodamine- and biotin-conjugated dextran amine.

The simultaneous use of different neuroanatomical anterograde tracers provides a potentially powerful method to study the convergence of afferent systems in a particular brain area. However, a simple routine procedure to apply multiple anterograde tracers in conjunction with their simultaneous visualization is still missing. We report an easy and straightforward application of three sensitive anterograde tracers: Phaseolus vulgaris leucoagglutinin (PHA-L), rhodamine-conjugated dextran amine (RDA) and biotin-conjugated dextran amine (BDA). These tracers can be visualized simultaneously and permanently through a triple-staining procedure with nickel-enhanced diaminobenzidine (DAB-Ni), DAB and 1-naphthol/Azur B as chromogens. Our test model comprised the projections from the nucleus reuniens thalami and entorhinal cortex. Both projection systems show a high degree of overlap in their terminal fields in the hippocampus. Two tracers were injected in the left and right entorhinal cortex, respectively; a third tracer was injected in the nucleus reuniens. This combination of injections provided a good opportunity to compare the three tracers in one and the same animal. PHA-L, RDA and BDA, injected in either of the injection sites, turned out to be equally sensitive and revealed the morphology of the involved projection systems in great detail. The triple-staining protocol yielded an excellent, simultaneous detectability of the three tracers with a remarkably low background level. Thus, the combination of the anterograde tracers PHA-L, RDA and BDA, in conjunction with the triple-staining procedure, offers a very attractive approach for neuroanatomical research.

Animals↗
PubMed1994

Combined anterograde tracing with biotinylated dextran-amine, retrograde tracing with fast blue and intracellular filling of neurons with lucifer yellow: an electron microscopic method.

In order to determine the presence of synaptic connectivity between fibres originating from a specific source and neurones with a known morphology and known fibre projection, we have introduced a method for electron microscopy that combines three techniques: retrograde fluorescent tracing, anterograde tracing using biotinylated dextran-amine and intracellular injection of Lucifer Yellow (LY) in lightly fixed brain slices. Neurones in the rat entorhinal cortex that project to the infralimbic cortex and that might be in synaptic contact with fibres originating in the dorsal subiculum served as a model. After surgical application of the tracers and a survival period enabling transport, the brain was fixed and vibratome slices 300 microns thick were prepared in which retrogradely labelled cells were intracellularly injected with LY. This substance and the transported biotinylated dextran-amine were converted into different electron-dense labels. First, LY immunocytochemistry was conducted, then followed by silver-gold enhancement of the immunoprecipitate. Subsequently, the tissue sections were treated with an avidin-biotin-horseradish peroxidase complex and subjected to a diaminobenzidine-peroxide reaction. This protocol resulted in labelling of biotinylated dextran-amine-positive fibres and terminals that could easily be differentiated from the LY-positive neuronal elements and also showed well preserved ultrastructural detail.

Amidines↗
PubMed1993

Regional and laminar organization of projections from the presubiculum and parasubiculum to the entorhinal cortex: an anterograde tracing study in the rat.

The regional and laminar organization of the projections from the presubiculum and the parasubiculum to the entorhinal cortex was analyzed in the rat with the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). The projections from the presubiculum were bilateral and confined to layers III and I of the medial entorhinal area (MEA). Both the ispi- and the contralateral projections showed similar distributions and were almost of equal density. Projections to layer III of the entorhinal cortex arose predominantly from superficial layers of the presubiculum, whereas the fibers that reach layer I of the entorhinal cortex appear to originate preferentially from the deep layers of the presubiculum. These fibers also appeared to innervate weakly layer II of MEA. The parasubiculum distributed projections not only to MEA but also to the lateral entorhinal area (LEA), innervating layer II selectively. The innervation of LEA was quite dense and extensive. Very weak projections from the parasubiculum to the contralateral entorhinal cortex were observed in this study. The position of the terminal plexus in the entorhinal cortex was determined by the point of origin along both the dorsoventral and transverse or proximodistal axes of the presubiculum and parasubiculum. Projections from the presubiculum and parasubiculum entered the entorhinal cortex at the level of the injection, or slightly ventral to it, and the main terminal field was always present ventrally to the injection site. The dorsoventral axis of origin thus corresponded to a similarly oriented axis of termination in the entorhinal cortex. The distribution in relation to the origin along the transverse axis was more complex, and differences between the presubiculum and parasubiculum were present. The proximal presubiculum, i.e., the part closest to the subiculum, projected to the most lateral part of MEA and the central part of the presubiculum sent fibers to the most medial part of MEA. The distal part of the presubiculum, i.e., the part that borders the parasubiculum, projected to the central part of MEA. Projections from the portion of the parasubiculum directly adjacent to the presubiculum, the so-called proximal parasubiculum, reached medial parts of MEA, and those originating in the central part distributed preferentially to lateral parts of MEA and adjacent medial parts of LEA. The distal part of the parasubiculum that borders the entorhinal cortex projected mainly to almost the full mediolateral extent of LEA.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Restricted origin and distribution of projections from the lateral to the medial septal complex in rat and guinea pig.

The origin and distribution of projections from the lateral to the medial septal complex were studied at the light- and electron-microscopical level in the rat and the guinea pig, with the use of sensitive anterograde tracing techniques. Injections in the lateral septal complex resulted in only weak to moderate labeling in the medial septal nucleus. In contrast, injections in all but the ventral subdivision of the lateral septal complex labeled restricted terminal arborizations in the angular portions of the vertical diagonal band. Analyses at the electron-microscopical level indicated that these fibers form both symmetrical and asymmetrical synapses with dendrites and somata, but not with axons. These findings are discussed in the light of a presumed functional circuit from the hippocampal formation, via the lateral septal complex, to cells in the medial septal complex that originate projections to the hippocampal formation.

Animals↗
PubMed1991

Distribution of hippocampal CA1 and subicular efferents in the prefrontal cortex of the rat studied by means of anterograde transport of Phaseolus vulgaris-leucoagglutinin.

Projections of the hippocampal formation to the prefrontal cortex were visualized in the rat by means of the anterograde tracer Phaseolus vulgaris-leucoagglutinin. These projections distribute only to the prelimbic and the medial orbital cortices and arise exclusively from restricted portions of field CA1 of the Ammon's horn and the subiculum. The most dorsal portion of CA1 does not contribute fibers to this projection. In the subiculum, its origin is restricted to the proximal half, i.e., the portion that directly borders field CA1. Fibers from field CA1 and the subiculum have comparable distribution patterns in the prelimbic and medial orbital cortices. The density and distribution in the prefrontal cortex of the projections from the proximal portion of the subiculum depends on the location of the injections along the dorsoventral axis of the hippocampal formation: the intermediate portion of the subiculum projects more densely and diffusely than its dorsal and ventral portions. In the prelimbic cortex, labeled fibers are present in all layers, showing marked morphological differences in deep versus superficial layers. In layers V and VI, most of the fibers are vertically oriented, while in layers II and III they are short and oriented towards the pial surface. Although no clear differences in terminal distribution were observed along the rostrocaudal extent of the prelimbic cortex, its dorsal and ventral portions show different innervation patterns. In the ventral portion of the prelimbic cortex, varicose fibers and terminal arborizations were present in all cortical layers, deep (V and VI) as well as superficial (II and III). In its dorsal part, the innervation was less dense and mostly present in the deep layers (V and VI). The fiber and terminal distribution in the medial orbital cortex was diffuse in all layers with a slight preference for layers deep to layer II.

Animals↗
PubMed1991

Entorhinal cortex of the monkey: V. Projections to the dentate gyrus, hippocampus, and subicular complex.

The topographic and laminar organization of entorhinal projections to the dentate gyrus, hippocampus, and subicular complex was investigated in the Macaca fascicularis monkey. Injections of 3H-amino acids were placed at various positions within the entorhinal cortex and the distribution of anterogradely labeled fibers and terminals within the other fields of the hippocampal formation was determined. Injections of the retrograde tracers Fast blue, Diamidino yellow, and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were also placed into the dentate gyrus, hippocampus, and subicular complex, and the distribution of retrogradely labeled cells in the entorhinal cortex was plotted using a computer-aided digitizing system. The entorhinal cortex gave rise to projections that terminated in the subiculum, in the CA1, CA2, and CA3 fields of the hippocampus, and in the dentate gyrus. Projections to the dentate gyrus, and fields CA3 and CA2 of the hippocampus, originated preferentially in layers II and VI of the entorhinal cortex whereas projections to CA1 and to the subiculum originated mainly in layers III and V. Anterograde tracing experiments demonstrated that all regions of the entorhinal cortex project to the outer two-thirds of the molecular layer of the dentate gyrus and to much of the radial extent of the stratum lacunosum-moleculare of CA3 and CA2. While the terminal distributions of entorhinal projections to the dentate gyrus, CA3, and CA2 were not as clearly laminated as in the rat, projections from rostral levels of the entorhinal cortex preferentially innervated the outer portion of the molecular layer and stratum lacunosum-moleculare, whereas more caudal levels of the entorhinal cortex projected relatively more heavily to the deeper portions of the entorhinal terminal zones. The entorhinal projection to the CA1 field of the hippocampus and to the subiculum followed a transverse rather than radial gradient of distribution. Rostral levels of the entorhinal cortex terminated most heavily at the border of CA1 and the subiculum. More caudal levels of the entorhinal cortex projected to progressively more distal portions of the subiculum (towards the presubiculum) and more proximal portions of CA1 (towards CA2). Lateral portions of the entorhinal cortex projected to caudal levels of the recipient fields and more medial parts of the entorhinal cortex projected to progressively more rostral portions of the fields.

Animals↗
PubMed1990

Projection from the nucleus reuniens thalami to the hippocampal region: light and electron microscopic tracing study in the rat with the anterograde tracer Phaseolus vulgaris-leucoagglutinin.

In order to study the morphological substrate of possible thalamic influence on the cells of origin and area of termination of the projection from the entorhinal cortex to the hippocampal formation, we examined the pathways, terminal distribution, and ultrastructure of the innervation of the hippocampal formation and parahippocampal region by the nucleus reuniens of the thalamus (NRT). We employed anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L). Injections of PHA-L in the NRT produce fiber and terminal labeling in the stratum lacunosum-moleculare of field CA1 of the hippocampus, the molecular layer of the subiculum, layers I and III/IV of the dorsal subdivision of the lateral entorhinal area (DLEA), and layers I and III-VI of the ventral lateral (VLEA) and medial (MEA) divisions of the entorhinal cortex. Terminal labeling is most dense in the stratum lacunosum-moleculare of field CA1, the molecular layer of the ventral part of the subiculum, MEA, and layer I of the perirhinal cortex. In layer I of the caudal part of DLEA and in MEA, terminal labeling is present in clusters. Injections in the rostral half of the NRT produce the same distribution in the hippocampal region as those in the caudal half of the NRT, although the projections from the rostral half of the NRT are much stronger. A topographical organization is present in the projections from the head of the NRT, so that the dorsal part projects predominantly to dorsal parts of field CA1 and the subiculum and to lateral parts of the entorhinal cortex, whereas the ventral part projects in greatest volume to ventral parts of field CA1 and the subiculum and to medial parts of the entorhinal cortex. The distribution of the reuniens fibers coursing in the cingulate bundle was determined by comparing cases with and without transections of this bundle. The fibers carried by the cingulate bundle exclusively innervate field CA1 of the hippocampus, the dorsal part of the subiculum, and the presubiculum and parasubiculum. They participate in the innervation of the ventral part of the subiculum and MEA. Electron microscopy was used to visualize the axon terminals of PHA-L-labeled reuniens fibers. These terminals possess spherical synaptic vesicles and form asymmetric synaptic contacts with dendritic spines or with thin shafts of spinous dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

The subiculum: cytoarchitectonically a simple structure, but hodologically complex.

The subiculum gives rise to the majority of hippocampal projections to various telencephalic and diencephalic structures. Previously, these projections have been described using anterograde tracing with radioactively labeled amino acids. As part of an ongoing detailed analysis of the connectivity of the hippocampal region in the rat, we studied the projections of the subiculum by means of the recently introduced sensitive anterograde tracer Phaseolus vulgaris-leukoagglutinin (PHA-L) and double-labeling protocols with retrogradely transported fluorescent tracers. Within the subiculum, populations of neurons can be differentiated that each give rise to projections to a unique set of target structures. These populations of neurons, characterized according to common efferent connectivity, are differentially positioned along the transverse axis of the subiculum. Thus, subicular cells near the border with the CA1 field project to targets different from those reached by projections from subicular cells situated close to the border with the presubiculum. We further observed that major afferents of the subiculum, i.e. those arising from field CA1 and from the entorhinal cortex, are also organized along the transverse axis of the subiculum. We suggest that within the subiculum, that appears homogeneous with respect to both cytoarchitectonic and chemoarchitectonic characteristics, a differentiation can be made with respect to its major connections. Whether this differentiation takes the form of a "columnar organization" as known for the neocortex, or a "compartmentation" as shown for the striatum is not yet clear.

Animals↗