Biomedical subjects

M Frotscher

Publications and source records attributed to M Frotscher.

At least 145 records · Page 8Linked to original sources

PubMed1992

Specificity of interneuronal connections.

A fundamental problem of neurobiological research is how specific connections between individual neurons are established and maintained. In this report different levels of neuronal specificity are described. Some neuronal populations display region specificity, but within the target region they establish synapses with a variety of neurons. A characteristic feature of the afferent innervation of hippocampal neurons is that many fibers terminate in a laminated fashion. Such a layer specificity is known for the afferents from the entorhinal cortex and for the mossy fibers. The entorhinal afferents terminate in the outer molecular layer of the fascia dentata and in the stratum lacunosum-moleculare of the hippocampus proper. The mossy fibers display both region specificity and layer specificity: they form numerous synapses in hippocampal region CA3 but never invade CA1; in CA3 they are restricted to stratum lucidum. An extremely high degree of neuronal specificity is observed in the case of the axo-axonic or chandelier cells. The axons of these neurons specifically terminate on the axon initial segments of projection neurons in the neocortex, hippocampus and fascia dentata. Thus, these cells not only display a target cell specificity but a selectivity for a distinct portion of the target cell's membrane. Some of the factors that contribute to these different levels of neuronal specificity are briefly discussed. Positional cues as well as diffusible molecules from the target region may guide the outgrowing growth cone to its target. Molecular interactions between pre- and postsynaptic membranes, the functional load of the synaptic contact, and the selective death of a number of neurons and synapses further determine the specificity of interneuronal connections.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent Pathways↗

PubMed1992

Cholinergic and GABAergic neurotransmission in the fascia dentata: electron microscopic immunocytochemical studies in rodents and primates.

This chapter summarizes immunocytochemical studies on the cholinergic and GABAergic innervation of dentate neurons. There are at least three types of neuron that give rise to the GABAergic innervation of dentate granule cells. First, there are the basket cells located in and directly underneath the granular layer. Their axons form a pericellular plexus around the cell bodies and proximal dendrites of the granule cells. Second are the dentate axo-axonic cells. These neurons are located in the innermost portion of the molecular layer and give rise to rows of boutons that impinge on the axon initial segments of the granule cells. Finally, there are GABAergic neurons in the septal region that are known to project to the hippocampus and fascia dentata. All types of GABAergic neurons establish symmetric synapses. Basket cells and axo-axonic cells are major inhibitory components of the fascia dentata. The septohippocampal GABAergic neurons selectively contact other GABAergic cells in the fascia dentata thereby serving disinhibition of the granule cells. The cholinergic fibers arising from the medial septum form a diffuse network in all layers of the fascia dentata. Electron microscopy reveals that both symmetric and asymmetric synapses are established. Cholinergic terminals contact granule cells as well as GABAergic and peptidergic neurons in the hilar region. The above data were obtained in rats. Preliminary studies in monkeys have shown that the types of cholinergic synapse are very similar in the rodent and primate fascia dentata. However, some differences were noted in the types of GABAergic synapse. We have thus observed numerous asymmetric synapses with spines in addition to the well-known symmetric synapses with dendritic shafts, cell bodies and axon initial segments.

Acetylcholine↗

PubMed1992

Synaptic connections of seizure-sensitive neurons in the dentate gyrus.

A selective loss of somatostatin- and neuropeptide Y-immunoreactive neurons has been reported in the dentate gyrus of rats with cerebral ischemia, following sustained electric stimulation, and in patients with non-tumor-related temporal lobe epilepsy. Three theoretical possibilities were tested that may explain why these neurons are more vulnerable than others, such as the cholecystokinin- and calcium-binding protein-containing cells: (1) the seizure-sensitive neurons are more involved in specific excitatory circuitry than are the seizure-resistant cells; (2) the somatostatin- and neuropeptide Y-immunoreactive neurons are less protected by inhibitory GABAergic inputs than cells immunoreactive for cholecystokinin; and (3) the seizure-sensitive neurons do not contain calcium-binding proteins. The present results of light and electron microscopic, single and double, immunostaining experiments and co-localization studies performed on the hippocampal formations of rats and non-human primates, support the idea that the calcium-binding protein content of a neuron defines its seizure sensitivity.

Animals↗

PubMed1992

Parvalbumin in the medial septum/diagonal band complex of the African green monkey Cercopithecus aethiops.

The distribution of the calcium-binding protein Parvalbumin (PV), a marker for a subpopulation of GABAergic neurons, was studied in the septal complex of the African green monkey, Cercopithecus aethiops. PV-immunoreactive neurons were found in relatively small numbers in the medial septum and in large numbers in the nucleus of the diagonal band (BROCA). They were extremely rare in the dorsal and intermediate parts of the lateral septum. PV-immunoreactive neurons in the medial septum and the nucleus of the diagonal band were mostly fusiform when located close to the midline and round to polygonal in more lateral locations. Some perikarya of immunopositive as well as immunonegative cells were covered with immunostained boutons whereas others were not. Our present descriptive data suggest that there are no principal differences in PV-containing neurons in the septal region between non-human primates and rodents which have been analyzed in great detail in previous studies.

Animals↗

PubMed1992

Identification of neuronal connections by means of electron microscopic immunocytochemistry.

For many years neuroanatomical research has focused on the tracing of connections between populations of neurons in the central nervous system. With the development of immunocytochemical procedures employing antibodies against transmitters, transmitter-synthesizing enzymes and various neuropeptides, it has become possible to study the chemical characteristics of nerve cells. By combining tracer techniques and immunocytochemical approaches the transmitter identity of a projection can be determined. Moreover, electron microscopic double-labeling procedures allow the identification of the target cells of immunocytochemically characterized presynaptic terminals. This way, by the immunocytochemical identification of transmitters or neuropeptides in defined projection neurons, a more functional interpretation of neuroanatomical data has become possible. In the present report, these combinations of techniques are demonstrated by describing recent studies of cholinergic septohippocampal projection neurons.

Animals↗

PubMed1991

The mossy cells of the fascia dentata: a comparative study of their fine structure and synaptic connections in rodents and primates.

In this study the fine structure and synaptic connections of mossy cells in the rat and monkey fascia dentata were analyzed. In order to study commissural connections of identified mossy cells in the rat, hilar neurons were retrogradely labeled by horseradish peroxidase (HRP) or Fast Blue (FB) injections into the contralateral hippocampus. Vibratome sections containing retrogradely HRP-labeled hilar neurons were Golgi-impregnated and gold-toned. Hilar commissural neurons identified by contralateral FB injection were intracellularly labeled with Lucifer Yellow (LY). Lucifer Yellow staining was made electron-dense by photoconversion thereby allowing for an electron microscopic analysis of the retrogradely labeled and intracellularly stained neurons. With these two different approaches, we succeeded in identifying rat mossy cells projecting to the contralateral hippocampus. Mossy cells in the fascia dentata of primates (Papio anubis, Macaca mulatta, Saimiri sciureus) were, like mossy cells of rats, either Golgi-impregnated and gold-toned or intracellularly injected with LY. No major differences were found between mossy cells of rats and monkeys. The mossy cell dendrites originated from the two sides of an ovoid cell body and were mainly oriented parallel to the granule cell layer. In contrast to the rat, dendrites of mossy cells in the primate did not respect the granule cell layer and penetrated frequently into the molecular layer. The occurrence of excrescences on proximal dendrites was a characteristic feature of all mossy cells. These large spines were more complex in the primate than in the rat. In both rats and primates they formed numerous asymmetric synapses with large boutons of mossy fibers. Peripheral dendrites were covered with small, simple spines. Interestingly, these peripheral dendrites lacking excrescences also established asymmetric synapses with mossy fiber boutons as well as asymmetric and symmetric contacts with smaller terminals of unknown origin. These findings indicate that in both rats and primates the thorny excrescences are not the only target of the mossy terminals. While the proximal portions of the mossy cell dendrites appear to be exclusively contacted by the granule cells, a larger number of neuron types may converge on the distal dendrites. The axons of mossy cells, in both rats and primates, although incompletely stained with the present methods, were seen to ramify in the hilar region. Our results demonstrate that, despite minor species differences, the mossy cells of the fascia dentata represent a cell type that is preserved in phylogenetically distant species.

Amidines↗

PubMed1991

Maintenance of peripheral dendrites of GABAergic neurons requires specific input.

This study demonstrates that the removal of a major hippocampal afferent system, the fibers from the entorhinal cortex, results in transneuronal changes in postsynaptic GABAergic inhibitory neurons. Following swelling of their distal segments, the peripheral dendrites of these cells retract from the termination zones of degenerating entorhinal fibers in the outer molecular layer of the fascia dentata and in stratum lacunosum-moleculare of the hippocampus proper. These dendritic alterations are long-lasting and do not seem to be restored by sprouting of other intact afferents. Persisting transneuronal changes in GABAergic hippocampal neurons following the removal of their entorhinal afferents may play a role in Alzheimer's disease since there is a severe degeneration of the entorhino-hippocampal projection in this neurodegenerative disorder.

Afferent Pathways↗

PubMed1991

Astroglial processes around identified glutamatergic synapses contain glutamine synthetase: evidence for transmitter degradation.

Glutamate is the main excitatory transmitter in the cerebral cortex. The physiologically high spatial and temporal resolution in glutamatergic transmission requires effective transmitter removal. Thus, a close topochemical relation to the glutamatergic synapse is a prerequisite for an enzyme involved in glutamate transmitter degradation. Here we report that immunoreactivity against glutamine synthetase (GS), one of the glutamate metabolizing enzymes, is localized in the fine astrocytic processes associated with identified glutamatergic synapses in the rat hippocampus. We suggest that glutamate transmitter is rapidly taken up by these fine perisynaptic astrocytic processes and degraded by GS.

Animals↗

PubMed1991

Non-pyramidal cells in the CA3 region of the rat hippocampus: relationships of fine structure, synaptic input and chemical characteristics.

The non-pyramidal cells of the hippocampus are heterogeneous with respect to their morphology, peptide content, physiological properties, and postsynaptic targets. Here we demonstrate that the content of peptides (cholecystokinin, somatostatin) and calcium-binding proteins (parvalbumin and calbindin) of non-pyramidal cells is not related to a characteristic fine structure or synaptic input. Varying numbers of GABA-negative and GABA-positive input synapses of non-pyramidal cells indicate that these neurons are differently integrated in inhibitory and disinhibitory circuits.

Animals↗

PubMed1991

Differentiation of dentate granule cells in slice cultures of rat hippocampus: a Golgi/electron microscopic study.

The differentiation of granule cells in organotypic cultures of rat hippocampus was studied by means of the Golgi/electron microscopic (EM) technique. Like in vivo, the granule cells have a small round or avoid cell body which gives rise to apical dendrites densely covered with spines. However, the apical dendrites of the cultured granule cells are more horizontally oriented than in the normal fascia dentata where they form a cone-shaped dendritic arbor. Granule cells in slice cultures occasionally have basal dendrites invading the hilar region. Electron microscopic examination revealed many synaptic contacts on identified apical and basal dendrites of the gold-toned granule cells in culture. This suggests that a considerable synaptic reorganization takes place since all extrinsic afferents normally innervating the granule cells are lost. Several granule cells displayed deep infoldings of their nuclei which are known from in vivo studies to be a characteristic feature of non-granule cells in this region. i.e. basket cells. The presence of basal dendrites and nuclear infolding indicates an increased variability of this cell type which is situ displays a rather stereotyped morphology.

Animals↗

PubMed1991

Target cell specificity of synaptic connections in the hippocampus.

A major question of neurobiological research is how precise connections between neurons are formed and maintained. In the hippocampus, afferent fiber systems are known to terminate in a laminated fashion. Previous studies have indicated that this lamination is largely due to spatiotemporal constraints during ontogenetic development. In this commentary, recent fine structural studies on the target cell specificity of the various hippocampal afferents are discussed. It becomes obvious that some afferent fibers establish synapses with all available target cells, whereas other afferents are restricted to distinct types of neurons. A high degree of neuronal specificity is found in the hippocampal and dentate axo-axonic cells, which are restricted not only to specific types of target cells (pyramidal neurons and granule cells, respectively) but also to distinct portions of the target cell's membrane (the axon initial segment). Altogether, these data indicate that there are different levels of target cell specificity in the hippocampus. It is suggested that specific molecular interactions between pre- and postsynaptic elements, in addition to spatial and temporal factors, play a role in the formation and stabilization of the various synaptic connections of the hippocampal formation.

Animals↗

PubMed1991

Area-specific morphological and neurochemical maturation of non-pyramidal neurons in the rat hippocampus as revealed by parvalbumin immunocytochemistry.

The time course of the morphological differentiation of non-pyramidal neurons in the rat hippocampus shows an area specificity. Thus, non-pyramidal neurons in CA3 appear more mature than in CA1 at early postnatal stages. Physiological data provide evidence for an earlier maturation of GABA-mediated inhibition in CA3 in comparison to CA1. As the calcium-binding protein parvalbumin (PARV) is thought to be a marker for highly active inhibitory neurons, we analyzed the area-specific appearance of PARV in GABAergic neurons during development. Employing combined light and electron microscopic immunocytochemistry, we revealed an area specificity in the time course of the neurochemical and morphological maturation of this functionally important subpopulation of non-pyramidal cells. The first appearance of PARV-immunoreactivity was observed at P7 and was exclusively located in cell bodies in CA3. At P8, neurons in CA3 exhibited PARV-immunoreactivity in cell bodies and dendrites, but very rarely in axon terminals. These neurons displayed the typical light and electron microscopic characteristics of GABAergic non-pyramidal cells. At P10, axon terminals formed typical baskets surrounding the pyramidal cells. The appearance of PARV-immunoreactivity in cell bodies, dendrites and axon terminals in CA1 was noticed about 1 to 2 days later. In the fascia dentata, non-granule cells displayed immunoreactivity not before P10. These data indicate a sequential neurochemical and morphological maturation of non-pyramidal neurons that may be related to differences in the maturation of inhibition during hippocampal development.

Animals↗

PubMed1991

Basket cells in the monkey fascia dentata: a Golgi/electron microscopic study.

This study describes non-granule cells in the fascia dentata of rhesus monkeys and baboons. Their cell bodies are located in the molecular layer and at the hilar border of the granular layer. They are called basket cells since their axons give rise to collaterals that branch in the close vicinity of the parent cell body and form symmetric synapses with dendrites and cell bodies of granule cells. These neurons are further classified with regard to the shape and location of their cell bodies and the orientation of their dendrites. Basket cells in the molecular layer are mainly bipolar with dendrites oriented perpendicular to the granular layer. These dendrites are densely innervated by presynaptic boutons forming asymmetric synapses. We have rarely observed molecular layer basket cells with dendrites traversing the granular layer and invading the hilus. We thus conclude that these cells are mainly activated by extrinsic afferents terminating in the molecular layer. Basket cells at the hilar border display pyramidal, fusiform or multipolar cell bodies that give rise to apical dendrites traversing the molecular layer and basal dendrites invading the hilar region. Large boutons establish asymmetric synapses with identified basal dendrites of these neurons. The dendrites of all types of basket cell are smooth, i.e. they had few or no spines. Many of them display varicosities. Cell counts in Cresyl Violet-stained sections revealed a ratio of basket cells to granule cells of 1:500. Essentially, the types of basket cell in the monkey fascia dentata are similar to those described previously for the rat. This contrasts sharply to our recent findings for pyramidal neurons and granule cells of the monkey hippocampus which showed an increased complexity and variability when compared with rodents. These data do not support the hypothesis that only local circuit neurons evolve in phylogeny.

Animals↗

PubMed1991

Proliferation and differentiation of glial fibrillary acidic protein-immunoreactive glial cells in organotypic slice cultures of rat hippocampus.

The present paper deals with the proliferation and differentiation of glial cells in organotypic slice cultures of the rat hippocampal formation. Transverse slices of hippocampus of newborn to five-day-old rats were cultivated using the roller tube technique. To study the development of glial cells under these conditions, the slice cultures were processed for immunostaining employing antibodies against the glial fibrillary acidic protein. The proliferation of glial cells was studied in double-labeling experiments employing glial fibrillary acidic protein-immunostaining and the bromodeoxyuridine technique. The three-dimensional glial scaffold in the cultures was analysed in semithin and ultrathin cross-sections through the slice cultures after varying periods following explanation. Our results can be summarized as follows: 1. At all intervals after explanation of the slices there are numerous glial fibrillary acidic protein-positive cells with morphological characteristics of astrocytes. 2. With some modifications, the differentiation of astrocytes and their processes follows similar rules as observed in the hippocampus in vivo. A radial glial scaffold is also formed in the cultures. However, in cultures, a regular pattern of radial fibers is more obvious in the hippocampus proper than in the dentate gyrus. This glial scaffold persists after 20 days in vitro whereas it is known to disappear after the first postnatal week in vivo. 3. Bromodeoxyuridine-positive nuclei of glial cells were found at all time periods after explanation. After short incubation periods, they were most frequent in the "ventricular" zones of the cultures. Following longer incubation periods after bromodeoxyuridine administration, proliferating cells were found throughout the cultures, covering and underlying the cultured tissue. A rim of laterally migrating astrocytes completely surrounds the cultures. Our results demonstrate that glial cells proliferate and differentiate under the present culture conditions. After three weeks of incubation the whole slice culture is surrounded by a glial cover which may play an important role for the survival and differentiation of the cultured hippocampal neurons.

Animals↗

PubMed1991

Selective innervation of embryonic hippocampal transplants by adult host dentate granule cell axons.

Fragments containing different cytoarchitectonic fields were dissected out of late embryonic rat hippocampal primordia and transplanted into the hippocampus or septum of adult syngeneic hosts. Field CA3 transplants contained clusters of large, angular (pyramidal) cell bodies surrounded by a radiating corona of dendrites. These cells stained selectively with our monoclonal antibody Py, and a proportion were labelled by [3H]thymidine administered on the 15th day of embryonic life. Field CA1 transplants contained smaller, angular, Py-negative cells, which formed elongated laminae rather than globular clusters. The ability of the host dentate granule cells to project to the transplants was examined by (1) the Timm stain for mossy fibres, (2) electron microscopy of Golgi-impregnated CA3 pyramidal neurons in the transplants, and (3) quantitative electron microscopic assessment of the proportions of large mossy fibre terminals in the synaptic population of the transplants. The Timm stain showed that CA3 transplants received a projection from host dentate granule cells when the transplants were placed in direct contact with the axons in the host mossy fibre pathway. As in the normal host field CA3, the ingrowing mossy fibres terminated selectively on the juxtacellular regions of the dendritic tree and ignored the major part of the dendrites in the radiating corona. The electron micrographs showed that within this territory the host mossy fibres formed synaptic terminals with all the complex features typical of normal mossy fibres, and were presynaptic to complex spines arising from the juxtacellular region of Golgi-impregnated donor CA3 pyramidal cells. The quantitative electron microscopic study demonstrated that the mossy fibre-innervated juxtacellular regions of the field CA3 transplants had up to 20% of the normal density of mossy fibre synapses found in the stratum lucidum of field CA3 in situ. CA3 transplants which were placed in the septum, remote from the host mossy fibres, had either trivial numbers of mossy fibre synapses or none. This confirmed that the abundant mossy fibre terminals in the intrahippocampal CA3 transplants were of host origin, and not due to donor dentate granule cells inadvertently included in the grafts. The selectivity of the host dentate projection for field CA3 transplants was demonstrated by the observation that CA1 transplants in the same locations received only slight mossy fibre projections in the Timm stain, and in electron micrographs their synaptic population had only insignificant numbers of large mossy fibre terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1990

Late appearance of parvalbumin-immunoreactivity in the development of GABAergic neurons in the rat hippocampus.

The calcium-binding protein parvalbumin (PARV) is supposed to have a protective function under conditions of experimental seizure and hypoxia in a subgroup of GABAergic inhibitory neurons in the adult rat hippocampus. Here we studied the appearance of PARV immunoreactivity in rat hippocampal non-pyramidal cells during postnatal development in comparison to glutamate decarboxylase (GAD) immunoreactivity. PARV-immunoreactive neurons were not observed before postnatal day 7 whereas GAD-positive neurons and terminal-like puncta were present at postnatal day 2 (P2) and were frequent around P5. From other studies it is known that all GABAergic neurons are formed prenatally. Our data thus indicate that in the early postnatal period GABAergic non-pyramidal cells are poorly protected by calcium-binding proteins against a pathological calcium influx.

Animals↗

PubMed1990

Most somatostatin-immunoreactive neurons in the rat fascia dentata do not contain the calcium-binding protein parvalbumin.

A selective loss of somatostatin (SS)-containing neurons in the hilar region has been reported in patients suffering from temporal lobe epilepsy. Conversely, neurons containing calcium-binding proteins such as parvalbumin (PARV) are known to be very resistant under experimental seizure conditions. In this study, we analyzed the coexistence of SS and PARV in neurons of the rat fascia dentata by using serial semi-thin cryostat sections for pre-embedding immunocytochemistry. Our results show that only 5.7% of the SS-immunoreactive hilar neurons contain PARV. The data suggest that SS-containing hilar neurons are less protected against seizure-induced calcium overload than other neurons containing calcium-binding proteins.

Animals↗

PubMed1990

Afferent and efferent synaptic connections of somatostatin-immunoreactive neurons in the rat fascia dentata.

The aim of this study was to determine whether somatostatin (SS)-immunoreactive neurons of the rat fascia dentata are involved in specific excitatory circuitries that may result in their selective damage in models of epilepsy. Synaptic connections of SS-immunoreactive neurons were determined at the electron microscopic level by using normal and colchicine pretreated rats. Vibratome sections prepared from both fascia dentata of control animals and from rats that had received an ipsilateral lesion of the entorhinal cortex 30-36 hours before sacrifice were immunostained for SS by using a monoclonal antibody (SS8). Correlated light and electron microscopic analysis demonstrated that many SS-immunoreactive neurons in the hilus send dendritic processes into the outer molecular layer of the fascia dentata, and dendrites of the same neurons occupy broad areas in the dentate hilar area. The majority of SS-immunoreactive axon terminals form symmetric synapses with the granule cell dendrites in the outer molecular layer and also innervate deep hilar neurons. Via their dendrites in the outer molecular layer, the SS-immunoreactive neurons receive synaptic inputs from perforant pathway axons which were identified by their anterograde degeneration following entorhinal lesions. The axons from the entorhinal cortex are the first segment of the main hippocampal excitatory loop. The hilar dendrites of the same SS-immunoreactive cells establish synapses with the mossy axon collaterals which represent the second member in this excitatory neuronal chain. These observations suggest that SS-immunoreactive neurons in the dentate hilar area may be driven directly by their perforant path synapses and via the granule cells which are known to receive a dense innervation from the entorhinal cortex. These observations demonstrate that SS-immunoreactive neurons in the hilar region are integrated in the main excitatory impulse flow of the hippocampal formation.

Animals↗