Biomedical subjects

M Frotscher

Publications and source records attributed to M Frotscher.

At least 73 records · Page 4Linked to original sources

PubMed1997

Dual role of Cajal-Retzius cells and reelin in cortical development.

Cajal-Retzius (CR) cells are transient neurons located in the marginal zones of the neocortex and hippocampus. Recent studies have shown that they synthesize and secrete the glycoprotein reelin. This extracellular matrix protein has sequence similarities with cell adhesion molecules and other extracellular matrix proteins, such as tenascin and laminin, suggesting a role in cell migration and process outgrowth. In reeler mutant mice lacking reelin, the orderly inside-out deposition of neocortical cells during development is disturbed, indicating that reelin is essential for normal cortical lamination. In the hippocampus, CR cells and reelin have recently been found to be important for the normal lamina-specific fiber ingrowth of afferents from the entorhinal cortex. These fibers are known to terminate in superficial layers of the hippocampus and dentate gyrus, regions in which CR cells are located. Both selective elimination of CR cells by local lesions and antibody blockade (CR-50 antibody) of an important epitope near the N-terminus of reelin result in severe alterations of the growth of entorhinal axons in co-cultures of the entorhinal cortex and hippocampus. It is hypothesized in this review that reelin functions as a stop signal for both migrating neurons and growing fibers in the developing central nervous system.

Animals↗

PubMed1997

Sprouting in the hippocampus is layer-specific.

Partial removal of layer-specific afferents of the hippocampus is said to induce sprouting of intact fibers from neighboring layers that invade the zone of the degenerating axons. However, recent in vivo and in vitro studies using sensitive anterograde tracers have failed to demonstrate sprouting across laminar boundaries. Sprouting does occur; but, it mainly involves unlesioned fiber systems terminating within the layer of fiber degeneration in addition to the degenerating afferents. These findings point to rigid laminar cues attracting certain fiber systems while repelling others in normal development and after partial deafferentation.

Animals↗

PubMed1997

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion.

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Animals↗

PubMed1997

Cholinergic sprouting in the rat fascia dentata after entorhinal lesion is not linked to early changes in neurotrophin messenger RNA expression.

After unilateral entorhinal cortex lesion cholinergic septohippocampal fibres sprout in the denervated fascia dentata. This process is dependent on neurotrophin changes following the lesion. Thus, there is an up-regulation of nerve growth factor and brain-derived neurotrophic factor messenger RNA expression in the denervated granule cells which is detectable 4 h postlesion and returns to control levels by 24 h. Here, using a competitive polymerase chain reaction and in situ hybridization, a transient neurotropin messenger RNA increase could be demonstrated bilaterally following unilateral electrolytic entorhinal cortex lesion. Treatment of the animals with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate blocked this messenger RNA increase, suggesting an involvement of this receptor type in the neurotrophin changes. However, in spite of this blockade, the typical cholinergic sprouting response as visualized with acetylcholinesterase histochemistry was present in animals four weeks after entorhinal cortex lesion. These data suggest that brief initial changes in neurotrophin messenger RNA expression in dentate granule cells are not responsible for the induction of the cholinergic sprouting. Changes in neurotrophin messenger RNA expression occurring immediately postlesion may be linked to glutamate release from entorhinal terminals resulting from the electrolytic lesion of the projection cells in the entorhinal cortex. We hypothesize that later changes in neurotrophin expression, for example in glial cells, are more likely to be related to the cholinergic sprouting process.

Acetylcholinesterase↗

PubMed1997

Basal expression, subcellular distribution, and up-regulation of the proto-oncogene c-JUN in the rat dentate gyrus after unilateral entorhinal cortex lesion.

The expression of the transcription factor c-JUN was investigated in the rat fascia dentata under normal conditions and after entorhinal cortex lesion. As shown by immunocytochemistry and in situ hybridization histochemistry c-JUN and its messenger RNA are present in the principal cell layers of the dentate gyrus and Ammon's horn (except hippocampal region CA2). Pre-embedding immunogold electron microscopy revealed an almost exclusive nuclear localization of c-JUN, where it is associated with chromatin. In addition, double immunolabelling for c-JUN and parvalbumin demonstrated that c-JUN immunoreactivity is primarily found in principal neurons since GABAergic parvalbumin-positive interneurons did not express c-JUN. After unilateral electrolytic lesion of the entorhinal cortex c-JUN was strongly up-regulated in the ipsilateral dentate gyrus within 2 h postlesion. This up-regulation was also present in the contralateral fascia dentata 12 h after entorhinal cortex lesion and returned to control levels on both sides 24 h postlesion. The cellular distribution of c-JUN did not change after entorhinal cortex lesion: parvalbumin-positive interneurons never contained c-JUN. These results point to a specific role of c-JUN in the granule cells of the fascia dentata in the normal animal and in rats with entorhinal cortex lesions. The selective induction of c-JUN after entorhinal lesion could be one of the first molecular steps that regulate transneuronal changes within granule cells after their denervation. A different mechanism has to be assumed for GABAergic interneurons known to receive an entorhinal innervation as well.

Animals↗

PubMed1997

Up-regulation of astrocyte-derived tenascin-C correlates with neurite outgrowth in the rat dentate gyrus after unilateral entorhinal cortex lesion.

The extracellular matrix protein tenascin-C has been implicated in the regulation of axonal growth. Using unilateral entorhinal cortex lesions, which induce a massive sprouting response in the denervated outer molecular layer of the rat fascia dentata, the role of tenascin-C for axonal growth was investigated in vivo. Monoclonal antibodies against the neurite outgrowth and anti-adhesive domains of the molecule were employed. Immunostaining was increased throughout the denervated outer molecular layer by day 2, reached a maximum around day 10, and was back to control levels by four weeks post lesion. Growth cone deflecting as well as neurite outgrowth promoting isoforms of tenascin-C were up-regulated after the lesion. Using electron microscopy, single intensely tenascin-C immunoreactive cells were identified as reactive astrocytes that phagocytose degenerated terminals. In situ hybridization histochemistry for tenascin-C messenger RNA revealed numerous cellular profiles in the denervated outer molecular layer of the ipsilateral and contralateral dentate gyrus two days post lesion. Tenascin-C messenger RNA-positive cells in the outer molecular layer were identified as astrocytes using double-labelling for tenascin-C messenger RNA and glial fibrillary acidic protein immunohistochemistry. Thus, a tenascin-C-rich substrate is present in the outer molecular layer during the time of sprouting and a sharp boundary is formed against the inner molecular layer. This pattern may contribute to the layer-specific sprouting response of surviving afferents after entorhinal lesion. Neurite outgrowth may be promoted within the denervated zone, whereas axons trying to grow into the denervated outer molecular layer, for example from the inner molecular layer, would be deflected by a tenascin-C-rich barrier.

Animals↗

PubMed1997

Submillisecond AMPA receptor-mediated signaling at a principal neuron-interneuron synapse.

Glutamatergic transmission at a principal neuron-interneuron synapse was investigated by dual whole-cell patch-clamp recording in rat hippocampal slices combined with morphological analysis. Evoked EPSPs with rapid time course (half duration = 4 ms; 34 degrees C) were generated at multiple synaptic contacts established on the interneuron dendrites close to the soma. The underlying postsynaptic conductance change showed a submillisecond rise and decay, due to the precise timing of glutamate release and the rapid deactivation of the postsynaptic AMPA receptors. Simulations based on a compartmental model of the interneuron indicated that the rapid postsynaptic conductance change determines the shape and the somatodendritic integration of EPSPs, thus enabling interneurons to detect synchronous principal neuron activity.

Animals↗

PubMed1997

192 IgG-saporin-induced loss of cholinergic neurons in the septum abolishes cholinergic sprouting after unilateral entorhinal lesion in the rat.

After unilateral lesion of the entorhinal cortex, cholinergic septohippocampal fibres are believed to sprout in the denervated outer molecular layer of the rat dentate gyrus. This cholinergic sprouting has been demonstrated by acetylcholinesterase (AChE) histochemistry, a method said selectively to label cholinergic septohippocampal fibres in the hippocampus. However, a recent report has questioned this concept, suggesting that AChE may not be an adequate marker to monitor cholinergic sprouting and that other, non-cholinergic axons sprouting after entorhinal cortex lesion cause the dense AChE-positive band in the denervated outer molecular layer. In order to determine the contribution of cholinergic septohippocampal fibres to the dense AChE band appearing after entorhinal cortex lesion, the neurotoxin 192 IgG-saporin, known to destroy cholinergic neurons in the basal forebrain selectively, was used. Rats received bilateral injections of 192 IgG-saporin into the lateral ventricles 3 weeks before entorhinal cortex lesion, simultaneously with entorhinal cortex lesion, or 8 weeks after entorhinal cortex lesion. Immunocytochemistry for choline acetyltransferase (ChAT) and in situ hybridization for ChAT mRNA demonstrated the loss of cholinergic neurons in the medial septum and diagonal band after 192 IgG-saporin treatment. The cholinergic sprouting response in the molecular layer, as visualized with AChE histochemistry, was abolished in all animals treated with immunotoxin. These data indicate that the dense AChE band forming after entorhinal cortex lesion represents the sprouting of cholinergic septohippocampal fibres.

Acetylcholinesterase↗

PubMed1997

Recovery of ChAT immunoreactivity in axotomized rat cholinergic septal neurons despite reduced NGF receptor expression.

Previous studies have suggested that target-derived nerve growth factor (NGF) is essential for the survival of cholinergic basal forebrain neurons. Thus, axotomy of septohippocampal neurons in adult rats resulting in the withdrawal of target-derived NGF caused a dramatic loss of choline acetyltransferase (ChAT)-immunoreactive neurons in the medial septum-diagonal band complex. We have recently shown that this loss of immunolabelled neurons does not indicate cell death, since many septohippocampal cholinergic neurons recover their immunoreactivity for ChAT after a long survival time despite disconnection from target-derived neurotrophins. One possibility would be that these surviving ChAT-immunoreactive neurons have gained access to other, probably local, NGF sources. Here we provide evidence that the recovery of ChAT immunoreactivity after axotomy is not accompanied by a similar recovery of NGF receptor expression in these neurons. In situ hybridization for p75NTR mRNA and trkA mRNA 6 months after bilateral fimbria-fornix transection revealed a substantial loss of labelled cells. In addition, there was a persisting loss of p75NTR-immunoreactive and NGF-immunoreactive medial septal neurons. Cholinergic neurons in controls did not express NGF mRNA, but were heavily immunostained for NGF protein due to receptor-mediated uptake. These data suggest that at least some cholinergic septohippocampal neurons re-express ChAT either independently of NGF or with a reduced need for NGF.

Animals↗

PubMed1997

Transient up-regulation of ciliary neurotrophic factor receptor-alpha mRNA in axotomized rat septal neurons.

Using non-radioactive in situ hybridization we investigated the effect of fimbria-fornix transection on the expression of ciliary neurotrophic factor receptor alpha (CNTFR alpha) mRNA in axotomized septohippocampal neurons of the rat septal complex. Whereas CNTFR alpha expression was undetectable in the medial septal nucleus/diagonal band complex (MSDB) of control animals, specific up-regulation was observed in MSDB neurons after fimbria-fornix transection. CNTFR alpha expression was maximal 7-10 days after the lesion and had returned to control levels after 3 weeks. Following unilateral fimbria-fornix transection, CNTFR alpha up-regulation was restricted to the MSDB ipsilateral to the lesion. When cholinergic septal neurons were selectively eliminated by immunolesioning with 192 IgG-saporin prior to fimbria-fornix transection, the lesion-induced expression of CNTFR alpha was still observed in many medial septal nucleus neurons. These results demonstrate that after fimbria-fornix transection CNTFR alpha expression is transiently induced in axotomized, non-cholinergic neurons of the medial septal nucleus, suggesting a postlesion function of locally supplied CNTF.

Animals↗

PubMed1996

Regional variability and postsynaptic targets of chandelier cells in the hippocampal formation of the rat.

Chandelier cells are specialized cortical GABAergic neurons that establish synaptic contacts exclusively with the axon initial segments of principal neurons. They are found in all regions of the hippocampal formation. Here we describe their morphological features in the hilus and in regions CA1 and CA3 by using Golgi/electron microscopy. Attempts were also made to identify the target neurons of chandelier cells in the hilus and entorhinal cortex. Golgi-impregnated chandelier cells display a complex axonal arbor in CA1, with many collaterals forming strings of boutons. The axon plexuses of such cells are less developed in CA3, whereas those in the hilus cover the entire region, although single collaterals are rather simple, with only a few boutons. The dendrites of chandelier cells in CA1 and CA3 have an orientation similar to that of pyramidal cell dendrites and are thus likely to be activated by the same afferent fiber systems. The hilar chandelier cells do not give rise to dendrites invading the molecular layer. Thus, these cells may not receive a dense input from the entorhinal cortex but may be driven by the abundant mossy fiber collaterals in the hilar region. In the CA1 and CA3 regions, the axons of chandelier cells contact the axon initial segments of pyramidal cells. In the hilar region, gold-toned boutons were found to impinge on the initial segments of neurons displaying characteristics of mossy cells. This notion was substantiated by electron microscopic analysis of mossy cells identified by intracellular injection of Lucifer yellow. Those cells regularly showed numerous symmetric synapses on their axon initial segments. Entorhinohippocampal projection cells, identified by injection of horseradish peroxidase into the hippocampus, were found to be preferential targets of chandelier cells in the entorhinal cortex. Our data point to regional variations in chandelier cell morphology and connectivity and indicate that chandelier cells are a principal component of inhibitory mechanisms in all stations of the main excitatory pathway of the hippocampal formation.

Animals↗

PubMed1996

Differential survival of Cajal-Retzius cells in organotypic cultures of hippocampus and neocortex.

Cajal-Retzius (CR) cells are transient, pioneer neurons of layer I of the cortex that are believed to play essential roles in corticogenesis, e.g., in neuronal migration and synaptogenesis. Here we have used calretinin immunostaining to study the characteristics, survival, and fate of CR cells in single organotypic slice cultures of mouse neocortex and hippocampus deprived of their extrinsic afferents. In neocortical explants, CR cells were observed after 1-3 d in vitro (DIV), but they disappeared after 5-7 DIV, which is similar to their time of degeneration in vivo. The disappearance of CR cells in neocortical slices was prevented by incubation with tetrodotoxin and the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3,-dione but not by 2-amino-5-phosphonopentanoic acid, suggesting that neuronal activity and non-NMDA glutamate receptors may trigger CR cell death in the neocortex. In contrast to the situation in vivo, in which many hippocampal CR cells disappear at approximately the third postnatal week, CR cells survived in single hippocampal cultures after long incubation times (31 DIV), with their morphology essentially unaltered. In contrast, fewer CR cells were found when hippocampal slices were cocultured with explants from the entorhinal cortex. Because CR cells are transient synaptic targets for entorhinohippocampal afferents, these findings suggest a role for entorhinal afferents in the degeneration of CR cells in the hippocampus. In conclusion, this study shows different survival properties of CR cells in organotypic slice cultures of hippocampus and neocortex, and it suggests that different mechanisms are involved in the regulation of the process of naturally occurring CR cell death in the two cortical regions.

Afferent Pathways↗

PubMed1996

Development of cholinergic and GABAergic neurons in the rat medial septum: different onset of choline acetyltransferase and glutamate decarboxylase mRNA expression.

In the present study, we have investigated the developmental expression of the transmitter-synthesizing enzymes choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD) in rat medial septal neurons by using in situ hybridization histochemistry. In addition, we have employed immunostaining for ChAT and the calcium-binding protein parvalbumin, known to be contained in septohippocampal GABAergic neurons. A large number of GAD67 mRNA-expressing neurons were already observed in the septal complex on embryonic day (E) 17, the earliest time point studied. During later developmental stages, there was mainly an increase in the intensity of labeling. Neurons expressing ChAT mRNA were first recognized at E 20, and their number slowly increased during postnatal development of the septal region. The adult pattern of ChAT mRNA-expressing neurons was observed around postnatal day (P) 16. By using a monoclonal ChAT antibody, the first immunoreactive cells were not seen before P 8. Similarly, the first weakly parvalbumin-immunoreactive neurons were seen in the septal complex by the end of the 1st postnatal week. These results indicate that in situ hybridization histochemistry may be an adequate method to monitor the different development of transmitter biosynthesis in cholinergic and GABAergic septal neurons. Moreover, the late onset of ChAT mRNA expression would be compatible with a role of target-derived factors for the differentiation of the cholinergic phenotype.

Animals↗

PubMed1996

Multiple projections are unlikely to account for the survival of rat medial septal neurons after axotomy.

Previous studies have demonstrated that numerous septohippocampal neurons survive axotomy disconnecting them from target-derived neurotrophins. One reason for survival could be that these neurons have additional projections to other targets supplying them with neurotrophic factor(s). We show that anterograde tracing indeed labeled additional targets of the medial septum (MS) in controls as well as in experimental animals. However, only very few MS neurons could be double-labeled by retrograde tracer injections into the hippocampus and these targets. Thus, for the majority of septohippocampal neurons multiple projections cannot account for their survival following axotomy.

Animals↗

PubMed1996

Frequency and dendritic distribution of autapses established by layer 5 pyramidal neurons in the developing rat neocortex: comparison with synaptic innervation of adjacent neurons of the same class.

Synaptic contacts formed by the axon of a neuron on its own dendrites are known as autapses. Autaptic contacts occur frequently in cultured neurons and have been considered to be aberrant structures. We examined the regular occurrence, dendritic distribution, and fine structure of autapses established on layer 5 pyramidal neurons in the developing rat neocortex. Whole-cell recordings were made from single neurons and synaptically coupled pairs of pyramidal cells, which were filled with biocytin, morphologically reconstructed, and quantitatively analyzed. Autapses were found in most neurons (in 80% of all cells analyzed; n = 41). On average, 2.3 +/- 0.9 autapses per neuron were found, located primarily on basal dendrites (64%; 50-70 microns from the soma), to a lesser extent on apical oblique dendrites (31%; 130-200 microns from the soma), and rarely on the main apical dendrite (5% 480-540 microns from the soma). About three times more synaptic than autaptic contacts (ratio 2.4:1) were formed by a single adjacent synaptically coupled neuron of the same type. The dendritic locations of these synapses were remarkably similar to those of autapses. Electron microscopic examination of serial ultrathin sections confirmed the formation of autapses and synapses, respectively, and showed that both types of contacts were located either on dendritic spines or shafts. The similarities between autapses and synapses suggest that autaptic and synaptic circuits are governed by some common principles of synapse formation.

Animals↗

PubMed1996

A novel entorhinal projection to the rat dentate gyrus: direct innervation of proximal dendrites and cell bodies of granule cells and GABAergic neurons.

Entorhinal fibers to the fascia dentata originating from layer II stellate neurons are known to terminate exclusively in the outer two thirds of the molecular layer, where they innervate distal dendritic portions of dentate neurons. Using anterograde tracing with Phaseolus vulgaris leucoagglutinin, we unraveled a previously unknown entorhinal projection that directly innervates proximal dendritic portions and somata of granule cells and GABAergic neurons. This projection originates from neurons located in entorhinal layers IV-VI of the medial entorhinal area. These fibers enter the outer two thirds of the molecular layer, traverse the inner molecular layer (IML) and granule cell layer, where they form numerous boutons, and finally arborize subjacent to the granule cells. Correlated light and electron microscopy revealed that the boutons formed by these fibers establish asymmetric synapses on dendrites in the IML, on spines and somata of granule cells, and on spineless dendrites subjacent to the granule cell layer. Postembedding immunogold staining indicates that this entorhino-dentate projection is not GABAergic and that it also terminates on GABAergic inhibitory neurons. These data demonstrate that not all entorhino-dentate fibers display a similar high laminar specificity for the outer molecular layer (OML). Although fibers from the superficial layers of the entorhinal cortex terminate exclusively in the OML, entorhinal fibers arising from deeper layers are not confined to laminar boundaries. Finally, the possibility that these supposedly excitatory entorhinal afferents may monosynaptically activate proximal dendrites and somata of dentate neurons needs to be incorporated into contemporary concepts of the hippocampal network.

Animals↗

PubMed1996

Selective expression of the immediate early gene c-jun in axotomized rat medial septal neurons is not related to neuronal degeneration.

In the present study, we use the anatomically well defined septohippocampal projection to study the molecular events involved in the reaction of neurons to axotomy. The expression of three immediate early genes (c-fos, c-jun, and jun B) was investigated in rat septohippocampal neurons after axotomy by bilateral fimbria-fornix transection (FFT). Moreover, the extent of retrograde degeneration in the septal complex was assessed by analyzing DNA fragmentation. In a postoperative time course analysis, a strong increase of c-jun immunoreactivity (IR) was observed in the nuclei of neurons in the medial septum/diagonal band complex (MSDB) 2 and 6 d postaxotomy, which was followed by a decline after 12 d and 3 weeks, respectively. Nine weeks after FFT, c-jun IR had disappeared. The c-jun-positive MS neurons were identified as former septohippocampal projection cells by double-labeling with the retrogradely transported tracer Fluoro-Gold injected into the hippocampus before axotomy. In line with the immunocytochemical data, there was a massive induction of c-jun mRNA in the axotomized MS neurons as visualized by in situ hybridization histochemistry. c-fos mRNA and c-fos or jun B IR were not detectable in either unoperated or lesioned medial septal neurons. Experiments using the TdT-mediated deoxyuridine triphosphate nick-end-labeling technique, designed to detect nuclear DNA fragmentation in degenerating neurons, complemented this study. During the postoperative time range studied, MS neurons did not exhibit DNA fragmentation. We conclude that MSDB neurons survive axotomy by FFT and display characteristic changes in gene expression.

Acetylcholine↗

PubMed1996

Activity of choline acetyltransferase in the rat medial septal nucleus following fimbria-fornix transection or selective immunolesioning with 192 IgG-saporin.

Changes in the biochemical activity of choline acetyltransferase in the medial septal nucleus, diagonal band and hippocampus were determined following bilateral fimbria-fornix transection or selective immunolesioning of cholinergic septohippocampal neurons with 192 IgG-saporin. Following axotomy, choline acetyltransferase activity in the medial septal nucleus not only persisted but increased much above control values 6 months postlesion, confirming that many cholinergic neurons survive the transection of their axons. In contrast, immunolesioning led to a significant decrease in enzyme activity in the medial septal nucleus corresponding to the selective loss of septal cholinergic neurons in this lesion paradigm.

Animals↗