Development of the hippocampus in vitro: cell types, synapses and receptors

Spontaneous pyramidal cell death in organotypic slice cultures from rat hippocampus is prevented by glutamate receptor antagonists

A predictable pattern of selective neuronal cell death occurs in organotypic slice cultures of neonatal rat hippocampus during the second and third weeks in vitro. We serially examined organotypic cultures at three, four, seven, 14, 21 and 28 days in vitro, using uptake of the fluorescent dye propidium iodide to identify degenerating cells. After seven days in vitro, the cell degeneration that accompanies the slicing procedure appears to have ended. However, at 14 days in vitro, degenerating neurons could be identified in area CA3. When many degenerating cells were present in a slice, they were distributed in the dentate hilus (CA4) and proximal portions of CA1 as well. Neuronal degeneration involving mainly CA1 pyramidal cells was still apparent at 21 days in vitro, but was much less marked than at 14 days. Study of fixed cultures with light and electron microscopy methods confirmed the presence of degenerating neurons with a pyknotic or vacuolated appearance. Spontaneous neuronal degeneration at 14 and at 21 days in vitro was almost entirely prevented by the addition of 10.5 mM Mg2+ or 3 mM kynurenic acid (a glutamate receptor antagonist), beginning at seven days in vitro. Cell death was markedly decreased by treatment with 100 microM DL-2-amino-5-phosphonovaleric acid (a selective antagonist of N-methyl-D-aspartate glutamate receptors). Removal of the blocking agents by returning cultures to control media at 28 days in vitro induced widespread neuronal degeneration, involving all the regions of the hippocampal slice cultures. The inhibition of spontaneous neuronal cell death by glutamate receptor antagonists and by blockade of glutamate release at synapses suggests that the mechanism of cell death involves glutamate receptors. The time course of degeneration suggests that the vulnerability to glutamate excitotoxicity is an aspect of developmentally regulated components of glutamatergic synapses acquired in the hippocampal organotypic cultures after the first week in vitro.

Adenosinergic modulation of CA1 neuronal tolerance to glucose deprivation in organotypic hippocampal cultures

Glucose deprivation produced neuronal degeneration of CA1 pyramidal neurons in hippocampal slice cultures. The effects of the adenosine agonist cyclohexyladenosine (CHA) and antagonist cyclopentylxanthine (CPX) on CA1 neuronal loss following hypoglycemia was examined using propidium iodide fluorescence as an indicator of cell death. The intensity of propidium iodide fluorescence in hippocampal area CA1 was quantified using Optimas image analysis software. Following 2 or 3 h of glucose deprivation, CPX significantly enhanced injury in the CA1 region while CHA provided significant protection. These results suggest that adenosine plays an important role in endogenous neuronal protection during hypoglycemic injury, and also supports a role for the use of adenosine agonists as neuroprotective agents.

Sprouting of CA3 pyramidal neurons to the dentate gyrus in rat hippocampal organotypic cultures

The understanding of the mechanisms of a functional synaptic plasticity in the hippocampus has been expanded greatly by the use of in vitro slice preparations. The question addressed in the present study was whether morphological plasticity observed in vivo can also be reproduced in hippocampal slices. In vivo, hippocampal commissural and association fibers are known to sprout and occupy synaptic sites vacated by deafferentation of the dentate gyrus (DG). In hippocampal slice preparations, the major input to the DG is eliminated, so that the DG is deafferented. Might intrinsic neurons sprout to the DG if the slice preparation is maintained for weeks? In this study hippocampal slices obtained from 6-day-old rats were cultured. Stimulation of the dentate stratum moleculare produced antidromic field potentials in the CA3 of the slices cultivated for more than 1 week. The antidromic response was not observed in CA1 pyramidal neurons. The CA3 to DG projection response was also observed in a CA3 mini-slice placed near a co-cultured whole hippocampal slice, when the DG in the latter was stimulated. Moreover, stimulation of the CA3 mini-slice induced synaptic responses in the DG of the whole-slice. The conclusion drawn is that deafferentation could induce axonal sprouting in a neuron-specific manner in hippocampal organotypic culture. This preparation would be potentially useful for the screening of chemical factors that influence sprouting of central neurons.

Neuroactive substances in the developing dorsomedial telencephalon of the pigeon (Columba livia): differential distribution and time course of maturation

The avian hippocampal formation has previously been shown to contain many of the same neurotransmitters and related enzymes that are found in mammals. In order to determine whether the relatively delayed development of the mammalian hippocampus is typical of other vertebrates, we investigated the maturation of a variety of neuroactive substances in the hippocampal formation of the homing pigeon. The distribution of two transmitter-related enzymes, choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH), the neurotransmitter GABA, and four neuropeptides (substance P, enkephalin, neuropeptide Y, and somatostatin) was studied by immunohistochemistry in the developing hippocampal complex. The pattern and/or the time course of changes in the distribution of immunoreactivity varied among the different neuroactive substances examined. Immunoreactivity to ChAT and TH was found exclusively in fibers and terminal-like processes, whereas GABA and peptide immunoreactivity was seen in cells and neuropil. Quantitative differences in the density, number, and size of stained cells were assessed by a computer-assisted image analyzer. For the majority of the substances, developmental patterns in the distribution of immunoreactivity differ between the hippocampus proper and the area parahippocampalis, the two major areas that together make up the avian hippocampal complex. The adult pattern of immunoreactivity was generally attained by 3 weeks after hatching. For many of the neuroactive substances found in cell bodies, there was a gradual decrease in the density of immunoreactive cells with a concomitant increase in the density of immunoreactive neuropil. The actual number of stained cells usually increased to a peak at 9 days posthatching and then declined until 3 weeks posthatching, when the adult value was reached. These results are discussed in relation to the advantages that the pigeon hippocampal complex may provide in the study of developmental processes. Parallels with the distribution of the same neuroactive substances in the mammalian hippocampus are used to suggest possible functional similarities between the avian and mammalian hippocampal regions.

Development of mossy fiber synapses in hippocampal slice culture

The mossy fiber synaptogenesis has been studied in hippocampal slice cultures. In vivo mossy fiber terminals contact the thorny excrescences of CA3 pyramidal neurons over a restricted portion, i.e. the proximal part of the apical dendrite. In organotypic cultures mossy fibers expand their terminal field and invade the infrapyramidal area of the CA3 region and the supragranular layer of the dentate gyrus. Newly formed mossy fiber synapses in CA3 region were examined, through electron microscopy, in cultures taken at various time intervals. The main events of the formation of newly formed mossy fiber synapses can be summarized as follows. During the first week following explantation mossy fiber axons contact the dendritic shaft of the pyramidal dendrite and establish both symmetrical and asymmetrical contacts. Subsequent modifications occur in the postsynaptic portion facing the mossy fiber bouton: (i) a massive accumulation of polyribosomes and coated vesicles in the subsynaptic cytoplasm; (ii) undulations of the plasma membrane; (iii) disappearance of neurotubules at postsynaptic sites and appearance of a fine network of filamentous material. Later on in culture, complex giant spines invaginate within the synaptic bouton. In conclusion this study shows that CA3 pyramidal neurons following deafferentation retain the capacity to form thorny excrescences, when contacted by mossy fibers. Moreover these results suggest a crucial role for mossy fibers to induce the formation of thorny excrescences in an heterotopic localization, i.e. over the basilar dendrites of CA3 pyramidal neurons.

Dendritic development of dentate granule cells in the absence of their specific extrinsic afferents

Dendrites and spines are postsynaptic structures that develop in association with presynaptic fibers. Recent studies have shown that granule cells of the fascia dentata survive in slice cultures and differentiate in a manner known from in situ studies. However, all extrinsic afferent fibers are absent under culture conditions. In the present study, we study whether dendrites and spines of granule cells in slice cultures differentiate normally, although they are not contacted by their normal layer-specific afferents. Slices of hippocampus were prepared from rat pups at the day of birth. After 5, 10, 15, and 20 days of incubation, granule cells in these cultures were Golgi impregnated. For comparison, perfusion-fixed hippocampal sections of 5-, 10-, 15-, and 20-day-old rats were impregnated the same way. Our results show that the total density of spines on granule cell dendrites in culture increased as in perfusion-fixed animals. However, after 20 days of incubation, the absolute number of dendritic spines on cultured neurons was reduced because of a reduction of peripheral dendrites. This reduction was accompanied by an increase in the number of stem dendrites originating from the perikaryon. The density of spines on these proximal dendrites was larger in cultured granule cells than in controls. Our results suggest that the lack of major extrinsic (entorhinal) afferents that normally terminate on peripheral granule cell dendrites causes retraction of these dendrites. At the same time, there is growth of proximal dendritic portions. Proximal dendrites are targets of associational fibers, which are known to sprout under these culture conditions.

Structural maturation, cell proliferation and bioelectric activity in long-term slice-cultures of immature rat hippocampus

Explants of transverse slices of the 6-day-old rat hippocampus were grown in a serum-free medium for 2-14 days. Histology performed after various culturing periods demonstrated that these slices maintain a high degree of 3-dimensional organotypy, while undergoing growth and differentiation of the main cellular elements similar to that seen in vivo. Histological indications of continuing cell proliferation were verified by autoradiography showing a labelling of neuroblasts in the dentate gyrus and of glioblasts at the sites of gliogenesis observed in vivo. Spontaneous bioelectric activity and evoked potentials were recorded, both indicating the development of impulse generation and neuronal connectivity within the explant. Silver impregnation and electron microscopic studies lent further support for the presence of neuronal networks intrinsic to the hippocampus. These findings suggest that within the period studied the hippocampal slice cultures mature in a fashion similar to that seen in situ.

Somatostatin-containing neurons in rat organotypic hippocampal slice cultures: light and electron microscopic immunocytochemistry

Light and electron microscopic immunocytochemical techniques were used to study the interneuron population staining for somatostatin (SRIF) in cultured slices of rat hippocampus. The SRIF immunoreactive somata were most dense in stratum oriens of areas CA1 and CA3, and in the dentate hilus. Somatostatin immunoreactive cells in areas CA1 and CA3 were characteristically fusiform in shape, with dendrites that extended both parallel to and into the alveus. The axonal plexus in areas CA1 and CA3 was most dense in stratum lacunosum-moleculare and in stratum pyramidale. Electron microscopic analysis of this area revealed that the largest number of symmetric synaptic contacts from SRIF immunoreactive axons were onto pyramidal cell somata and onto dendrites in stratum lacunosum-molecular. In the dentate gyrus, SRIF somata and dendrites were localized in the hilus. Hilar SRIF immunoreactive neurons were fusiform in shape and similar in size to those seen in CA1 and CA3. Axon collaterals coursed throughout the hilus, projected between the granule cells and into the outer molecular layer. The highest number of SRIF synaptic contacts in the dentate gyrus were seen on granule cell dendrites in the outer molecular layer. Synaptic contacts were also observed on hilar neurons and granular cell somata. SRIF synaptic profiles were seen on somata and dendrites of interneurons in all regions. The morphology and synaptic connectivity of SRIF neurons in hippocampal slice cultures appeared generally similar to intact hippocampus.

Organotypic slice culture of the rat suprachiasmatic nucleus: sustenance of cellular architecture and circadian rhythm

The suprachiasmatic nucleus of the mammalian brain is thought to be the anatomical locus of circadian rhythms. To examine the functional organization of the suprachiasmatic nucleus in vitro with intact intercellular connections for a prolonged period, we have established an organotypic slice culture system using a roller-tube technique. Brain slices (400 microns in thickness) containing the bilateral suprachiasmatic nuclei, were obtained from newborn rats at four to seven days old and were maintained in vitro for more than three weeks. During this three-week period, the slices flattened to one to three cell layers and two tightly packed neuronal cell-masses (neuronal zones), with diameters of about 1 mm were formed, which were surrounded by a peripheral glial cell-dispersed zone. In the neuronal cell zones, peptides and their messenger RNAs were found cytochemically with characteristic patterns similar to the suprachiasmatic nucleus in the brain. In situ hybridization and immunocytochemistry showed that vasoactive intestinal peptide messenger RNA expressing and vasoactive intestinal peptide-immunoreactive neurons were detected predominantly in the ventrolateral part of the neuronal zones in the suprachiasmatic nucleus slice culture. Vasopressin messenger RNA-expressing and vasopressin-immunoreactive cells were localized in the dorsomedial neuronal zones near the ependymal cell zone. The distribution of cell bodies and fibers containing these neuropeptides and their messenger RNAs in the neuronal zones of suprachiasmatic nucleus organotypic slice culture were similar to that of the suprachiasmatic nucleus in vivo. This suggests that the suprachiasmatic nucleus in these organotypic slice cultures retains the biological characteristics of these cells in vivo as the cells did develop, form compact neuronal masses and did establish connections. To examine the possibility that suprachiasmatic nucleus neurons in slice cultures show a persistent rhythmic activity, we also measured the amount of vasopressin released into the culture medium. Sampling at 4-h intervals combined with enzyme immunoassay revealed that vasopressin concentration in the medium embracing suprachiasmatic nucleus slice cultures fluctuated with a period of approximately 24 h. The present findings suggest that the intranuclear neuronal networks of the suprachiasmatic nucleus are maintained in vitro for a long duration and that organotypic cultures of the suprachiasmatic nucleus produce and release bioactive substances in an oscillating manner. The suprachiasmatic nucleus in slice cultures may be useful for future analysis of circadian rhythms in vitro.

Distribution and morphological characteristics of oligodendrocytes in the rat hippocampus in situ and in vitro: an immunocytochemical study with the monoclonal Rip antibody

Oligodendrocytes in the rat hippocampus in situ and in organotypic slice cultures were studied by light and electron microscopic immunocytochemistry using the monoclonal Rip antibody. Our results confirm that this antibody exclusively stains oligodendrocytes, while astrocytes and neurons are not labelled. In the light microscope, immunopositive cells had the appearance of myelinating oligodendrocytes with their characteristic tubular processes. In the electron microscope, stained cells showed intimate contacts with myelin sheaths but not with the basal laminae of endothelial cells. Rip-positive oligodendrocytes were unevenly distributed in the adult rat hippocampal formation. In general, they were abundant in layers known to contain many afferent and efferent fibres. In the hippocampus proper, there was a particularly strong immunolabelling of stratum radiatum of field CA2. In the fascia dentata, the hilar region displayed a high cell density, especially in the vicinity of the granule cell layer. A similar distribution of immunopositive cells was found in young animals (15-18 days old); however, the density of labelled cells was lower, particularly in the hilus. Immunolabelled cells in slice cultures of hippocampus displayed the characteristics of myelinating oligodendrocytes. Moreover, they showed an organotypic distribution, although afferent and efferent fibre projections normally myelinated by these cells were absent under these conditions.

Lesion-induced neurite sprouting and synapse formation in hippocampal organotypic cultures

By sectioning, using a razor blade, one- and three-week-old rat hippocampal organotypic cultures, we have tested the possibility that neurite outgrowth and reactive synaptogenesis would take place even after several weeks in culture in this in vitro model. At the light-microscopic level, recovery from the section and formation of a thin scar were observed within six days following the lesion. Immunostainings using neurofilament antibodies showed the presence of numerous degenerative and regenerative images one day after the cut and many fibres crossing the section six days after the lesion. Electrophysiological recordings of synaptic responses elicited across the section indicated the formation of new functional synaptic contacts and complete recovery of transmission within three to six days. Interestingly, functional recovery in three-week-old cultures was found to be significantly slower than in one-week-old tissue. These findings were confirmed at the electron-microscopic level. Evidence was obtained for an effective cleaning of the lesion site by macrophages and astroglial cells, the existence of many degenerative and regenerative images one day after the cut and the presence of new dendrites, axonal fibres and synapses in the area of the section six days after the lesion. All these changes were slower in three- than in one-week-old cultures. These results indicate that organotypic cultures can be used as an interesting model for studies of reactive synaptogenesis.

Development of dopamine-beta-hydroxylase-positive fiber innervation in co-cultured hippocampus-locus coeruleus organotypic slices

Development of the noradrenergic innervation of the rat hippocampus by the nucleus locus coeruleus was examined immunohistochemically in the roller tube organotypic cultured slice preparation. Slices of rat hippocampus and locus coeruleus were co-cultured on glass coverslips for 2-6 weeks and evaluated for the presence of dopamine-beta-hydroxylase (DBH) and tyrosine hydroxylase (TH) immunoreactive cells and fibers. Large, multipolar DBH- and TH-positive cells were visible within the locus coeruleus; an occasional cell appeared near or just within co-cultured hippocampal tissue and in connecting fiber tracts. DBH-positive cells tended to concentrate near the edges of locus coeruleus tissue. Locus coeruleus slices cultured alone showed little indication of fiber outgrowth in any direction. In co-cultures, however, beaded DBH- and TH-positive fibers were directed toward the hippocampus. The majority of these fibers entered the hippocampus in the hilar/CA3 region and formed extensive collateral branches. Light microscopy suggests that DBH-positive fiber growth was densest at or near the pyramidal cell layer in CA3b and CA3c and in the infragranular region of the dentate hilus. This pattern of noradrenergic innervation of hippocampus by co-cultured locus coeruleus in vitro appears very similar to the pattern established in vivo (see Moudy et al., companion article, this issue).

Localized gene transfer into organotypic hippocampal slice cultures and acute hippocampal slices

Viral vectors derived from herpes simplex virus, type-1 (HSV), can transfer and express genes into fully differentiated, post-mitotic neurons. These vectors also transduce cells effectively in organotypic hippocampal slice cultures. Nanoliter quantities of a virus stock of HSVlac, an HSV vector that directs expression of E. coli beta-galactosidase (beta-gal), were microapplied into stratum pyramidale or stratum granulosum of slice cultures. Twenty-four hours later, a cluster of transduced cells expressing beta-gal was observed at the microapplication site. Gene transfer by microapplication was both effective and rapid. The titer of the HSVlac stocks was determined on NIH3T3 cells. Eighty-three percent of the beta-gal forming units successfully transduced beta-gal after microapplication to slice cultures. beta-Gal expression was detected as rapidly as 4 h after transduction into cultures of fibroblasts or hippocampal slices. The rapid expression of beta-gal by HSVlac allowed efficient transduction of acute hippocampal slices. Many genes have been transduced and expressed using HSV vectors; therefore, this microapplication method can be applied to many neurobiological questions.

Rapid decline in the ability of entorhinal axons to innervate the dentate gyrus with increasing time in organotypic co-culture

We have used the species-specific monoclonal antibodies OM1 and OM4 to identify the histiotypic pattern of projection from late embryonic rat entorhinal explants to the outer molecular layer of the dentate gyrus in organotypic cultures of 6-day postnatal mouse hippocampal slices. The presence of this entorhinal projection was detectable with the rat-specific OM1 and OM4 markers after 3-7 days in co-culture, and confirmed by use of the later-forming rat neuron-specific marker THy-1.1, which appeared during the second week. Hippocampal slices confronted with control explants of superior colliculus for 4 weeks in culture showed only sparse, non-specific growth of axons with no histiotypic pattern in the dentate gyrus. In order to assess whether the formation of specific entorhino-dentate projections in vitro is age-dependent, embryonic rat entorhinal cortical explants were cultured alone for periods of 1-5 weeks before cutting across the halo of axons radiating into the collagen matrix and presenting each with 6-day-old mouse hippocampal slices as targets to innervate. After allowing a 2 week period for fibre growth to take place, the density of immunostained axonal outgrowth was scored on a five-point scale for each weekly interval. The amount of new axon growth when the cuts were made after 1 week was slightly reduced compared to undamaged control cultures. However, outgrowth was greatly diminished when the cuts were made after 2 or 3 weeks, and essentially abolished if the interval was extended to > or = 4 weeks. Thus we demonstrate that, although hippocampal slices can survive in organotypic co-culture with entorhinal explants and maintain previously formed connections, the explants show an age-related failure in the ability to form new connections. Such a system provides a possible in vitro model for study of the factors influencing the failure of regeneration in the adult central nervous system.

Activation of Cl- channels by avermectin in rat cultured hippocampal neurons

The actions of the insecticide avermectin (AVM) were studied in rat cultured hippocampal neurons with patch-clamp techniques. Application of micromolar concentrations of AVM to voltage-clamped cells gave rise to whole-cell currents, which showed a slow time-course of activation in the order of 10 s, and wash-out periods of typically 20 min. Dose-response curves revealed a half-maximally activating AVM concentration (EC50) of 2.0 +/- 0.6 microM and a Hill coefficient of 1.5 +/- 0.9. The current activated by AVM was carried predominantly by Cl- ions, as demonstrated by ion-substitution experiments. The Cl- channel blocker picrotoxinin (100 microM) substantially but transiently reduced the AVM response. Outside-out patch recording showed that AVM opened Cl- channels with a conductance of 40 +/- 12 pS. The open-time distribution was characterized by two time constants of 11 ms and 259 ms. It is suggested that AVM directly activates Cl- channels in mammalian central neurons, which resemble the channels activated by the physiological transmitters GABA and glycine.

Presynaptic fibres of spiral neurons and reciprocal synapses in the organ of Corti in culture

Isolated segments of the newborn mouse organ of Corti were explanted together with the spiral ganglion components. Within the innervation provided by the spiral neurons, we observed presynaptic vesiculated nerve endings that form reciprocal ribbon-afferent/efferent synapses with inner hair cells. These intracochlear presynaptic fibres are characteristically located between adjoining inner hair cells, on the modiolar side, low and close to the supporting cells. The presynaptic fibres display different modes of synaptic connectivity, forming repetitive reciprocal synapses on single inner hair cells or on adjoining hair cells, or connecting adjoining inner hair cells through simultaneous efferent synapses. Many presynaptic fibres exhibit a distinctive ultrastructure: defined clusters of synaptic vesicles, dense core vesicles, coated vesicles, and mitochondria. These organelles occur focally at the synaptic sites; beyond the efferent synaptic specializations, the endings appear quite nondescript and afferent-like. We believe that the reciprocal synapses, although observed in cultures of the organ of Corti, represent real intracochlear synaptic arrangements providing a feedback mechanism between the primary sensory receptors and a special class of spiral ganglion cells that have yet to be recognized in the organ in situ.

Mechanism of mu-opioid receptor-mediated presynaptic inhibition in the rat hippocampus in vitro

1. The electrophysiological action of the mu-opioid receptor-preferring agonist D-Ala2, MePhe4, Met(O)5-ol-enkephalin (FK 33-824) on synaptic transmission has been studied in area CA3 of organotypic rat hippocampal slice cultures. 2. FK 33-824 (1 microM) had no effect on the amplitude of pharmacologically isolated N-methyl-D-aspartate (NMDA) or non-NMDA receptor-mediated EPSPs. 3. FK 33-824 (10 nM to 10 microM) reduced the amplitude of monosynaptic inhibitory postsynaptic potentials (IPSPs) that were elicited in pyramidal cells with local stimulation after pharmacological blockade of excitatory amino acid receptors. This effect was reversible, dose-dependent, and sensitive to naloxone and the mu-receptor antagonist Cys2,Tyr3,Orn5,Pen7-amide (CTOP). FK 33-824 at 1 microM caused a mean reduction in the amplitude of the monosynaptic IPSP of 70%. 4. Neither delta- nor kappa-receptor-preferring agonists had any effect on excitatory or inhibitory synaptic potentials. 5. The disinhibitory action of FK 33-824 was blocked by incubating the cultures with pertussis toxin (500 ng/ml for 48 h) or by stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu, 0.5 microM). 6. The depression of monosynaptic IPSPs by FK 33-824 was unaffected by extracellular application of the K+ channel blockers Ba2+ or Cs+ (1 mM each). 7. FK 33-824 produced a decrease in the frequency of miniature, action potential-independent, spontaneous inhibitory synaptic currents (mIPSCs) recorded with whole-cell voltage-clamp techniques, but did not change their mean amplitude. Application of the Ca2+ channel blocker Cd2+ (100 microM) or of nominally Ca(2+)-free solutions did not alter either the frequency and amplitude of mIPSCs or the reduction of mIPSC frequency induced by FK 33-824. 8. The effect of FK 33-824 on spontaneous mIPSCs was prevented by naloxone, and by incubation of cultures with pertussis toxin. 9. These results indicate that mu-opioid receptors decrease GABA release presynaptically by a G protein-mediated inhibition of the vesicular GABA release process, and not by changes in axon terminal K+ or Ca2+ conductances that are sensitive to extracellular Ba2+, Cs+ or Cd2+.

Functional heterogeneity of hippocampal GABAA receptors

gamma-Aminobutyric acid type A (GABAA) receptors were studied in cultured neurons taken from rat hippocampus at early postnatal stages. GABA-induced whole-cell currents showed a broad range of peak amplitudes and time-courses of desensitization. Dose-response curves of rapidly and slowly desensitizing cells revealed EC50 values of 8.5 and 37.3 microM GABA, respectively, with the Hill coefficient being greater than unity. The main-state conductance of GABAA receptor channels was 28-31 pS in all cells. GABA responses of low-affinity cells were more strongly affected by benzodiazepine receptor agonists (e.g. flunitrazepam, clonazepam) and inverse agonists (e.g. methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), as compared to cells exhibiting high-affinity GABA responses. Currents were also potentiated by zolpidem, but were little affected by Ro 15-4513 and Zn2+. These data suggest the presence of physiologically and pharmacologically distinct GABAA receptor isoforms in neurons of the early postnatal hippocampus, which may subserve different inhibitory control mechanisms in this brain region.

Developmental neurotrophin expression in slice cultures of rat hippocampus

Using reverse transcription in combination with the polymerase chain reaction, the developmental expression of neurotrophins in organotypic slice cultures of rat hippocampus was investigated. Nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNA levels after different time periods in vitro were compared with equivalent developmental stages in vivo. Our results show that neurotrophin expression occurs in hippocampal slice cultures with a similar time course as observed in the developing hippocampus in vivo. Thus, the development of neurotrophin expression in the hippocampus does not seem to be dependent on specific extrinsic afferents.

Excitatory and inhibitory pathways modulate kainate excitotoxicity in hippocampal slice cultures

In organotypic hippocampal slice cultures, kainate (KA) specifically induces cell loss in the CA3 region while N-methyl-D-aspartate induces cell loss in the CA1 region. The sensitivity of slice cultures to KA toxicity appears only after 2 weeks in vitro which parallels the appearance of mossy fibers. KA toxicity is potentiated by co-application with the GABA-A antagonist, picrotoxin. These data suggest that the excitotoxicity of KA in slice cultures is modulated by both excitatory and inhibitory synapses.

Cytoplasmic structure in organotypic cultures of rat hippocampus prepared by rapid freezing and freeze-substitution fixation

We have compared rapid freezing followed by freeze-substitution fixation with conventional aldehyde fixation as preparative methods for the electron microscopic study of organotypic cultures of neonatal rat hippocampus. Rapid freezing by contact with a copper block chilled by liquid helium was accomplished without mechanical distortion of superficial structures, and preserved structure to a depth of at least 20 microns without visible ice crystals. Freeze-substitution fixation in acetone/osmium tetroxide, followed by en bloc staining with tannic acid and uranyl acetate, provided satisfactory staining of cytoplasm and organelles. While both preparative techniques yielded generally satisfactory results, rapid freezing provided much better preservation of astrocytic lysosomal inclusions, and afforded new views of intermediate filament substructure. Rapid freezing and freeze-substitution fixation seemed especially well suited to the preservation of short filamentous proteins, such as those forming the membrane cytoskeleton of dendritic spines or those associated with synaptic vesicles. The combination of rapid freezing methods and organotypic culture provides an opportunity to examine cytoplasmic structure in tissue from deep regions of the brain which had previously been inaccessible to rapid freezing techniques.

Organotypic slice culture of the mammalian retina

Vertical slices of 6-day postnatal (P6) rat retina were cut at a thickness of 100 microns and cultured using the roller-tube technique. After 14-21 days in vitro there was significant distortion of normal retinal architecture, but localized areas of the slices showed the typical pattern of layering of mature retina. The following immunocytochemical markers were used to characterize the different retinal cell types: antibodies against protein kinase C (PKC), calcium binding protein (CabP 28kD), neurofilaments (NF), glia-specific antibodies (GFAP, vimentin), and transmitter-specific antibodies (GABA, TH). The expression of these markers was compared in P6 retina, adult retina, and slice culture. To further characterize the cultured cells, patch-clamp recordings were performed in combination with intracellular injection of Lucifer Yellow (LY). Transmitter- and voltage-gated membrane currents were recorded from morphologically identified neurons. The experiments show that a mammalian slice culture can be used to study differentiation and function of retinal cell types.

Loss of layer-specific astrocytic glutamine synthetase immunoreactivity in slice cultures of hippocampus

Glutamine synthetase (GS) supposedly inactivates the excitatory neurotransmitter glutamate. By using immunocytochemistry for GS, we recently demonstrated a layer-specific, perisynaptic distribution of GS-immunoreactive astrocytes and their processes in perfusion-fixed rat hippocampi. Highest levels of immunoreactivity were found in well defined termination zones of glutamatergic hippocampal afferents. In the present study we analysed the developmental aspect of this neuron-glia interaction by using hippocampal slice cultures lacking all extrinsic afferents. Under these conditions, no layer-specific distribution of astrocytic GS immunoreactivity could be demonstrated. This suggests that the laminated distribution of GS immunoreactivity is formed in parallel with the segregated termination of hippocampal afferents. Thus, there is no predetermined pattern of GS-containing astrocytes playing a role in the segregation of extrinsic fibres. The ultrastructural localization of GS immunoreactivity in fine astrocytic processes around asymmetric, probably glutamatergic excitatory spine synapses confirms earlier in situ findings, which suggests that this arrangement is a global phenomenon of glutamatergic systems.

Entorhinal axons project to dentate gyrus in organotypic slice co-culture

We have demonstrated the formation of entorhinodentate projections by axons arising from explants of embryonic mouse entorhinal cortex or slices of postnatal rat entorhinal area co-cultured in contact with slices of postnatal rat hippocampus in roller tube and static culture. Species-specific markers (Thy-1 alleles and M6) showed that the most dense part of the projection was to the outer part of the molecular layer of the dentate gyrus (i.e. excluding the commissural-association zone). Retrograde axonal transport of fluorescent tracers placed in the dentate gyrus labelled a densely packed superficial layer of stellate cells in the entorhinal cortex. Anterograde axonal transport of biocytin placed in the entorhinal cortex showed that the entorhinodentate fibres formed typical parallel bundles oriented at right angles to the dentate granule cell dendrites and had short-stalked boutons. The formation of entorhinodentate synapses was confirmed in the electron microscope by electron-dense degeneration after cutting the previously formed connection between the co-cultures. Synaptic transmission was demonstrated by extracellular recording of postsynaptic field potentials after entorhinal stimulation. The entorhinal fibres also projected to the hippocampal stratum lacunosum-moleculare of fields CA1 and CA3, and were present in the outer part of the stratum oriens of the subiculum; in some cases they perforated the pyramidal cell layer of the subiculum. We conclude that the necessary molecular and tissue organizational signals for the formation of an entorhinodentate projection are present in tissues maintained in organotypic slice co-culture, and remain effective in the cross-species mouse-to-rat situation.

Formation of the septohippocampal projection in vitro: an electron microscopic immunocytochemical study of cholinergic synapses

Cholinergic neurons in the medial septum/diagonal band complex project to the hippocampus and fascia dentata and establish characteristic types of synapses on a variety of target neurons. At present we do not know the principles that underlie the development of this projection and the formation of the cholinergic synapses. Here we have used co-cultured slices of septum and hippocampus of one- to six-day-old rat pups to study the development of the septohippocampal pathway and the formation of cholinergic synapses on hippocampal target neurons in vitro. Slices of septum and hippocampus were incubated together for 10-46 days applying the roller-tube technique. The fluorescent dye dioctadecyltetramethylindocarbocyanine perchlorate and histochemical staining for acetylcholinesterase labeled many fibers connecting both explants. Combined light- and electron-microscopic immunocytochemistry for choline acetyltransferase, the acetylcholine-synthesizing enzyme, revealed multipolar immunopositive neurons with long aspiny dendrites in the septal culture. Numerous varicose immunoreactive, supposedly cholinergic fibers could be followed from the septal to the hippocampal culture where they ramified and formed a three-dimensional network. As in situ, cholinergic terminals formed characteristic symmetric synapses on cell bodies, spines and, most often, on dendritic shafts of the hippocampal target neurons. No immunoreactive fibers and synapses were observed in single cultures of hippocampus. These results demonstrate that the cholinergic septohippocampal projection develops in vitro and that similar types of cholinergic synapses are established on co-cultured hippocampal target neurons as observed in situ.

Structural modifications associated with synaptic development in area CA1 of rat hippocampal organotypic cultures

Using morphological techniques, we characterized the developmental reorganization that takes place during the first weeks after explanation in area CA1 of organotypic hippocampal cultures maintained at the interface between medium and a CO2-enriched atmosphere. Pyramidal neurones redistributed from a vertical into an horizontal cell layer in the middle of a three-dimensional culture, with apical dendrites running above the pyramidal layer. Glial cells redistributed into a thin layer at the bottom of the culture, forming an interface between tissue and culture medium. Astrocytes were identified as the most numerous non neuronal cells. No sign of glial proliferation could be observed, except for a transient increase during the first days after explanation. The density of synaptic contacts in the stratum radiatum decreased immediately after explanation and then increased by about 20-fold to reach values in the proximal part of the apical layer after 4 weeks in culture which were only slightly smaller than those measured in 1-month-old rats. The synaptic density in the most distal part of the dendritic layer which receives connections extrinsic to the hippocampus remained significantly lower than in vivo. The ratio of spine to shaft contacts was comparable to that found in vivo. These results indicate that interface type of organotypic cultures can be used as an interesting model for studies of synaptic development in vitro.

Time course of synaptic development in hippocampal organotypic cultures

Using electrophysiological recordings of field potentials, we investigated the time course of synapse formation and maturation in organotypic cultures prepared from neonate animals of different ages. Following explanation, the size of the maximal synaptic responses elicited in area CA1 by stimulation of a small group of CA3 neurons increased progressively during the first three weeks in culture in a way that corresponded to the changes observed in synaptic contact density. Growth of synaptic responses was found to occur much more rapidly in cultures prepared from 8-day-old as compared with 2-day-old rats. Development of synaptic connections between CA3 and CA1 neurones was also faster than between granule cells and CA3 neurones. Acquisition of mature synaptic properties occurred in vitro as indicated by changes in degree of paired-pulse facilitation and the onset of long-term potentiation (LTP) after a few days in culture. The onset of LTP was much faster in cultures prepared from 8-day-old as compared with 2-day-old neonates and corresponded approximately to the 12-14th postnatal day. It is concluded that development proceeds in the cultures with a time course that resembles the in situ situation.

Pharmacology of GABA receptor Cl- channels in rat retinal bipolar cells

gamma-Aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian nervous system, is known to operate bicuculline-sensitive Cl- channels through GABAA receptors and bicuculline-insensitive cation channels through GABAB receptors. Recent observations indicate that the retina may contain GABA receptors with unusual pharmacological properties. Here we report that GABA gates bicuculline-insensitive Cl- channels in rod bipolar cells of the rat retina, which were not modulated by flunitrazepam, pentobarbital and alphaxalone and were only slightly blocked by picrotoxinin. Moreover, the GABAB receptor agonist baclofen, and the antagonist 2-hydroxysaclofen had no effect. The underlying single-channel conductance was 7 pS and the open time 150 ms. These values are clearly different from those obtained for GABAA receptor channels recorded in other neurons of the same preparation, and in other parts of the brain. The bicuculline- and baclofen-insensitive GABA receptors were activated selectively by the GABA analogue cis-4-aminocrotonic acid (CACA). Hence they may be similar to those receptors termed GABAC receptors.

Reversible loss of dendritic spines and altered excitability after chronic epilepsy in hippocampal slice cultures

The morphological and functional consequences of epileptic activity were investigated by applying the convulsants bicuculline and/or picrotoxin to mature rat hippocampal slice cultures. After 3 days, some cells in all hippocampal subfields showed signs of degeneration, including swollen somata, vacuolation, and dendritic deformities, whereas others displayed only a massive reduction in the number of their dendritic spines. Intracellular recordings from CA3 pyramidal cells revealed a decrease in the amplitude of evoked excitatory synaptic potentials. gamma-Aminobutyric acid-releasing interneurons and inhibitory synaptic potentials were unaffected. Seven days after withdrawal of convulsants, remaining cells possessed a normal number of dendritic spines, thus demonstrating a considerable capacity for recovery. The pathological changes induced by convulsants are similar to those found in the hippocampi of human epileptics, suggesting that they are a consequence, rather than a cause, of epilepsy.

Presynaptic inhibition of miniature excitatory synaptic currents by baclofen and adenosine in the hippocampus

Presynaptic inhibition of neurotransmitter release is thought to be mediated by a reduction of axon terminal Ca2+ current. We have compared the actions of several known inhibitors of evoked glutamate release with the actions of the Ca2+ channel antagonist Cd2+ on action potential-independent synaptic currents recorded from CA3 neurons in hippocampal slice cultures. Baclofen and adenosine decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) without affecting the distribution of their amplitudes. Cd2+ blocked evoked synaptic transmission, but had no effect on the frequency or amplitude of either mEPSCs or inhibitory postsynaptic currents (IPSCs). Inhibition of presynaptic Ca2+ current therefore appears not to be required for the inhibition of glutamate release by adenosine and baclofen. Baclofen had no effect on the frequency of miniature IPSCs, indicating that gamma-aminobutyric acid B-type receptors exert distinct presynaptic actions at excitatory and inhibitory synapses.

Application of the Golgi/electron microscopy technique for cell identification in immunocytochemical, retrograde labeling, and developmental studies of hippocampal neurons

In this study the Golgi/electron microscopy (EM) technique has been used for an analysis of the fine structure, specific synaptic connections, and differentiation of neurons in the hippocampus and fascia dentata of rodents. In a first series of experiments the specific synaptic contacts formed between cholinergic terminals and identified hippocampal neurons were studied. By means of a variant of the section Golgi impregnation procedure, Vibratome sections immunostained for choline acetyltransferase, the acetylcholine-synthesizing enzyme, were Golgi-impregnated in order to identify the target neurons of cholinergic terminals in the hippocampus. It could be shown with this combined approach that cholinergic septohippocampal fibers form a variety of synapses with different target structures of the Golgi-impregnated and gold-toned hippocampal neurons. In this report cholinergic synapses on the heads of small spines, the necks of large complex spines, dendritic shafts, and cell bodies of identified dentate granule cells are described. The variety of cholinergic synapses suggests that cholinergic transmission in the fascia dentata is a complex event. Next, the Golgi/EM technique was applied to Vibratome sections that contained retrogradely labeled neurons in the hilar region of the fascia dentata following horseradish peroxidase (HRP) injection into the contralateral hippocampus. With this combined approach some of the hilar cells projecting to the contralateral side were identified as mossy cells by the presence of retrogradely transported HRP in thin sections through these Golgi-impregnated and gold-toned neurons. Our findings suggest that the mossy cells are part of the commissural/associational system terminating in the inner molecular layer of the fascia dentata. They are mainly driven by hilar collaterals of granule cell axons that form giant synapses on their dendrites. Finally, the Golgi/EM procedure was used to study the differentiation and developmental plasticity of hippocampal and dentate neurons in transplants and slice cultures of hippocampus. Under both experimental conditions, the differentiating neurons are deprived of their normal laminated afferent innervation but develop their major cell-specific characteristics including a large number of postsynaptic structures (spines). As revealed in thin sections of gold-toned identified cells, all these spines formed synapses with presynaptic boutons suggesting sprouting of the transplanted and cultured neurons, respectively. Altogether, the present report demonstrates the usefulness of the Golgi/EM technique, particularly of the section impregnation procedure, for a variety of studies requiring the identification of individual neurons at the ultrastructural level.

Soluble and membrane-bound forms of brain acetylcholinesterase in Alzheimer's disease

In order to determine the effect of Alzheimer's disease on the relative distribution of soluble and membrane-bound molecular forms of acetylcholinesterase (AChE) in the brain, postmortem samples (delay interval less than 12 h) were obtained from parietal cortex (Brodmann area 40) and hippocampus as well as the areas containing their respective projection nuclei, i.e., substantia innominata and septal nucleus, in 9 patients with Alzheimer's disease (AD) and 4 normal controls. The monomer (G1), dimer (G2), and tetramer (G4) forms of AChE were examined. In AD compared to controls, significant changes occurred in area 40 and hippocampus but not in the areas containing projection nuclei, and included loss of mean total AChE activity, decrease in the relative percentage of membrane-bound G4, and increase in the relative percentage of soluble G1-G2. Percent of soluble G4 was unaffected in AD brain. In area 40 but not hippocampus a large increase in percent membrane-bound G1-G2 occurred. Thus, these results emphasize that the selective decrease in membrane-bound G4 accounts for the decrease in total G4 activity in AD brain.

Effect of beta amyloid peptides on neurons in hippocampal slice cultures

Several investigators have described the neurotrophic and neurotoxic effects of beta amyloid peptide fragments on dissociated hippocampal neurons in culture. In these prior studies, the peptides were added to dissociated cultures between day 0 and day 4 in vitro, before hippocampal neurons are fully mature. We have analyzed the neurotrophic and neurotoxic effects of beta amyloid fragments beta 1-28, beta 25-35 and beta 1-40 on hippocampal slice cultures, whose physiology and morphology resembles the intact hippocampus. Addition of beta 1-28 or beta 25-35 to the growth medium did not produce significant changes in dendritic length or number of branches. Nerve growth factor, previously reported to enhance the neurotoxic effects of beta 1-40 on dissociated hippocampal neurons in culture, did not significantly enhance the neurotrophic effects of beta 1-28. To achieve high local concentrations of peptides and to avoid potential access problems in the cultures, we injected beta 1-28, beta 25-35, and beta 1-40 directly into the cultures. Amyloid-mediated neurotoxicity was not observed for beta 1-28 or beta 25-35, but beta 1-40 appeared to produce neurodegeneration around the site of injection.

Parvalbumin-containing nonpyramidal neurons in intracortical transplants of rat hippocampal and neocortical tissue: a light and electron microscopic immunocytochemical study

Previous immunocytochemical studies have shown that GABAergic nonpyramidal neurons of the rat hippocampus survive in intracerebral transplants. However, information is still lacking about the dendritic organization and the input synapses of these cells as well as their capacity to express the calcium-binding protein parvalbumin (PARV) under transplant conditions. In the present study, a monoclonal antibody against PARV was used to examine the dendritic morphology and the synaptic organization of parvalbumin-containing GABAergic neurons in hippocampal and dentate transplants. In addition, parvalbumin-containing nonpyramidal neurons were studied in neocortical transplants to compare the differentiation of grafted allocortical and neocortical nonpyramidal neurons. Tissue blocks of hippocampus and fascia dentata and of the parietal neocortex were taken from late embryonic rats (E 21 and E 16, respectively) and were transplanted into a cavity in the somatosensory cortex of young adult rats. After 3.5 or 7 months survival, the recipient brains were fixed by perfusion and immunostained for PARV. As in the hippocampal formation in situ, PARV-containing neurons in the hippocampal transplants were observed within and in the vicinity of the pyramidal and granule cell layer. In neocortical transplants, PARV-immunoreactive cells were distributed in all parts of the transplant with dendrites extending in various directions. In both hippocampal and neocortical transplants, immunoreactive dendrites were smooth and displayed the characteristic regular varicosities known from in situ studies of these cells. Numerous unlabeled terminals as well as a few immunoreactive boutons established synapses on the immunoreactive dendrites. PARV-positive terminals formed the typical pericellular baskets around the immunonegative cell bodies of pyramidal neurons and granule cells in the transplants. They established symmetric synapses with cell bodies and proximal dendrites. Synapses on axon initial segments were absent or rare. Our results demonstrate that allocortical as well as neocortical nonpyramidal neurons transplanted to the neocortex of adult recipients survive transplantation, express the calcium-binding protein parvalbumin, and develop a cell-specific morphology.

Somatostatin and neuropeptide Y in organotypic slice cultures of the rat hippocampus: An immunocytochemical and in situ hybridization study

The neuronal distributions of somatostatin and neuropeptide Y and their respective mRNAs in hippocampal slice cultures were examined by immunohistochemical staining and in situ hybridization. For the in situ hybridization we used an alkaline phosphatase-labelled oligodeoxynucleotide probe for somatostatin mRNA and an 35S-labelled oligodeoxynucleotide probe for neuropeptide Y mRNA. For both neuropeptides the immunostained and hybridized neurons displayed a comparable, organotypic distribution. Most labelled neurons were located in the dentate hilus and stratum oriens of CA3 and CA1. Additional neurons were found in stratum radiatum and pyramidale of CA3, but very few in the corresponding layers of CA1. In all locations the density of somatostatin- and neuropeptide Y-reactive cells exceeded that observed in vivo. Also, the hybridization signal of the individual neurons appeared enhanced in the slice cultures. Methodologically it was noted that the non-radioactive alkaline phosphatase-labelled oligodeoxynucleotide probe gave excellent in situ hybridization results with detailed cellular resolution and no apparent problems of tissue penetration, even when used on whole-mount explants. These results demonstrate that somatostatin and neuropeptide Y-immunoreactive and mRNA containing neurons retain their organotypic distribution and basic morphological characteristics in the slice cultures. The supernormal density of these neurons and their hybridization signals indicate that a transient developmental increase in neuropeptide expression may persist in vitro.

Neuroactive amino acids in organotypic slice cultures of the rat hippocampus: An immunocytochemical study of the distribution of GABA, glutamate, glutamine and taurine

Antisera raised against protein-glutaraldehyde-amino acid conjugates were used to study the light and electron microscopic distribution of GABA, glutamate, glutamine and taurine in organotypic slice cultures of rat hippocampi. In the stratum oriens and radiatum, glutamate-like immunoreactivity was particularly concentrated in nerve endings establishing asymmetric junctions with dendritic spines. Mossy fiber terminals in CA3 and the dentate hilus were also strongly labeled. A quantitative immunogold analysis of the glutamate-immunolabelled profiles showed a pattern that was highly reminiscent of that previously observed in perfusion-fixed hippocampi, including a correspondingly sparse labeling of glial processes and of presynaptic elements in symmetric synapses. GABA-like immunoreactivity was localized predominantly in interneurons and in presynaptic terminals contacting dendritic shafts and neuronal cell bodies, while immunoreactivities for glutamine and taurine were found mainly in astroglial cells and pyramidal cells, respectively. Our data indicate that the major intrinsic fiber systems of the cultured hippocampi have retained their normal transmitter phenotypes.

Comparison of the actions of baclofen at pre- and postsynaptic receptors in the rat hippocampus in vitro

1. Intracellular microelectrode recordings were used to study the cellular location, pharmacology, and mechanism of action of gamma-aminobutyric acidB (GABAB) receptors on pyramidal cells and presynaptic axonal endings in area CA3 of organotypic hippocampal slice cultures. 2. Baclofen (bath applied at 10 microM) caused a 10-15 mV hyperpolarization of CA3 cells and a 75-100% decrease in the amplitude of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). Baclofen reduced the amplitude of monosynaptic IPSPs elicited in the presence of excitatory amino acid receptor antagonists, as well as the amplitude of EPSPs elicited after blocking GABAA receptors and reducing subsequent epileptic bursts with excitatory amino acid receptor antagonists. These data indicate that GABAB receptors are located on both excitatory and inhibitory presynaptic elements. 3. The GABAB receptor antagonist CGP 35 348 blocked the postsynaptic action of baclofen, the late IPSP, and the reduction of EPSPs and monosynaptic IPSPs by baclofen. 3-Aminopropylphosphinic acid (3-APA) mimicked all the pre- and postsynaptic actions of baclofen, and its effects were fully antagonized by CGP 35 348. 4. Incubation of cultures with pertussis toxin (500 ng/ml for 48 h) prevented both the postsynaptic hyperpolarization and the block of monosynaptic IPSPs induced by baclofen. The action of baclofen on isolated EPSPs, however, was not affected by pertussis toxin treatment. Stimulation of protein kinase C with phorbol ester (phorbol 12, 13 dibutyrate, 1 microM for 10 min) reduced all pre- and postsynaptic effects of GABAB receptor activation. 5. Barium (bath applied at 1 mM) prevented both the baclofen-induced hyperpolarization of pyramidal cells and the block of monosynaptic IPSPs by baclofen. In the presence of barium, however, baclofen was fully capable of blocking EPSPs. 6. We conclude that pre- and postsynaptic GABAB receptors are pharmacologically indistinguishable, at present, and that all actions of GABAB receptors are inhibited by stimulation of protein kinase C. Both the postsynaptic action of baclofen and the block of GABA release from interneurons are mediated by pertussis toxin-sensitive G proteins which can be inactivated by stimulation of protein kinase C. Baclofen acts at postsynaptic sites and on the axon terminals of inhibitory interneurons by activating the same barium-sensitive K+ conductance. GABAB receptors on excitatory axons must, however, work through some other mechanism.

Paroxysmal inhibitory potentials mediated by GABAB receptors in partially disinhibited rat hippocampal slice cultures

1. Intracellular recording techniques were used to study synaptic potentials in CA3 pyramidal cells elicited with mossy fibre stimulation in partially disinhibited hippocampal slice cultures. Two experimental protocols were used: (1) high concentrations (20-40 microM) of the A-type gamma-aminobutyric acid (GABAA) receptor antagonist bicuculline plus low concentrations (2-4 microM) of the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or (2) low concentrations (1-2.5 microM) of bicuculline alone. 2. Under the first condition, stimulation of mossy fibre afferents evoked epileptic bursts alternating with a response consisting of an excitatory postsynaptic potential (EPSP) followed by an unusually large and long-lasting hyperpolarizing potential with a maximal amplitude in the range of -30 mV from the resting membrane potential. 3. This paroxysmal inhibitory potential (PIP) had a reversal potential near that of potassium. The amplitude of the PIP was not dependent on action potentials superimposed on the preceding EPSP, and was present in cells recorded with microelectrodes containing the Ca2+ chelator EGTA. These data suggest that the PIP is not a Ca(2+)-activated K+ potential. 4. The PIP was prolonged by the GABA-uptake blocker nipecotic acid, was reduced by hyperpolarizing interneurons with the opioid agonist FK 33-824, and was abolished by the GABAB-receptor antagonist CGP 35 348. These data indicate that the PIP is mediated by the activation of GABAB receptors following GABA release from interneurons. 5. The NMDA-receptor antagonist D-2-amino-5-phosphonovalerate (D-APV) strongly reduced the amplitude of the PIP, but had no effect on the GABAB receptor-mediated inhibitory postsynaptic potential (IPSP) under control conditions. 6. Under the first condition, regular stimulation elicited a cyclical pattern of evoked responses. There was either an alternation between an epileptic burst and a PIP or, at shorter interstimulus intervals, a sequence of gradually increasing PIPs followed by an epileptic burst, which then reset the cycle. 7. Under the second condition, in low concentrations of bicuculline alone, the early GABAA-mediated IPSP was little affected, but the late GABAB-mediated IPSP was greatly enhanced. These enhanced late IPSPs were comparable in amplitude and duration to the PIPs seen under the first conditions, could exhibit cyclical behaviour, and were reduced by D-APV. 8. Application of CGP 35 348 abolished the late IPSP under control conditions, but had no effect on hippocampal excitability. In contrast, CGP 35 348 blocked the PIP elicited in low bicuculline, and consequently led to intense epileptic discharge.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological organization of rat hippocampal slice cultures

Using various histological methods, we investigated the cellular and morphological organization of rat hippocampal slice cultures. Many of the typical features of the hippocampus were retained in vitro over a long period of time. The principal cell types of the hippocampus and dentate gyrus, the pyramidal cells and granule cells, were well preserved and matured in vitro. Nonpyramidal cells and gamma-aminobutyric-acid (GABA) cells were also present in slice cultures and exhibited a strikingly similar dendritic appearance at the light microscopic level. Moreover, GABA-immunoreactive cell bodies and presynaptic terminals could be identified at the electron microscopic level; they expressed typical symmetric synaptic contacts with cell bodies and dendrites. The course of the intrinsic hippocampal fiber pathways--the mossy fibers, Schaffer collaterals, and alveus--was generally retained in vitro. Additional aberrant fiber projections could be identified. Finally, three types of nonneuronal cells could be distinguished on the basis of immunocytochemical methods.

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.

A simple method for organotypic cultures of nervous tissue

Hippocampal slices prepared from 2-23-day-old neonates were maintained in culture at the interface between air and a culture medium. They were placed on a sterile, transparent and porous membrane and kept in petri dishes in an incubator. No plasma clot or roller drum were used. This method yields thin slices which remain 1-4 cell layers thick and are characterized by a well preserved organotypic organization. Pyramidal neurons labelled by extra- and intracellular application of horse radish peroxidase resemble by the organization and complexity of their dendritic processes those observed in situ at a comparable developmental stage. Excitatory and inhibitory synaptic potentials can easily be analysed using extra- or intracellular recording techniques. After a few days in culture, long-term potentiation of synaptic responses can reproducibly be induced. Evidence for a sprouting response during the first days in culture or following sections is illustrated. This technique may represent an interesting alternative to roller tube cultures for studies of the developmental changes occurring during the first days or weeks in culture.

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.

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.

Glutamate-mediated selective vulnerability to ischemia is present in organotypic cultures of hippocampus

Ischemic damage to the brain, whether induced experimentally or observed clinically, often produces a pattern of delayed selective cell death in subfield CA1 of hippocampus which has been associated with significant neurologic deficits. The present study demonstrates that this selective vulnerability of CA1 neurons to ischemia, with relative preservation of their neighbors, is expressed in organotypic tissue culture and is prevented by the N-methyl-D-aspartate (NMDA) receptor blocker, MK-801. These data provide conclusive evidence that this selective cell death does not have a vascular etiology but is mediated by factors intrinsic to the hippocampal neurons and/or local circuitry. This model system provides an opportunity both to examine mechanisms of ischemic cell death in an avascular environment and to study methods of prevention in the absence of systemic variables.

A chemically-defined medium for organotypic slice cultures

Organotypic slice cultures provide an excellent system for the analysis of study of the molecular mechanisms of this development necessitates the use of a chemically defined culture medium. We report here the development of a medium, EOL1 defined medium, designed specifically for this purpose. Cultures of both cerebral cortex and basal forebrain demonstrate that this defined medium allows a high degree of cytoarchitectural maintenance while promoting neural metabolism and process outgrowth.

Plasticity of identified neurons in slice cultures of hippocampus: a combined Golgi/electron microscopic and immunocytochemical study

The combined Golgi/electron microscope (EM) technique and immunocytochemistry for glutamate decarboxylase (GAD) were used to study the differentiation of pyramidal neurons and GABAergic inhibitory non-pyramidal cells in slice cultures of rat and mouse hippocampus. Golgi-impregnated and gold-toned cultures showed the characteristic curved structure of the Ammon's horn. Hippocampal regions CA1, CA3 and fascia dentata could easily be recognized. Pyramidal neurons in CA1 displayed all characteristics of this cell type known from Golgi studies in situ. A triangular cell body gives rise to a main apical dendritic shaft which gives off several side branches. Basal dendrites and the axon originate at the basal pole of the cell body. Apical and basal dendrites are densely covered with spines. As a characteristic feature of the cultured pyramidal cells, numerous spines were observed on the cell body. Most likely due to flattening of the slice during incubation, the pyramidal neurons in CA1 are no longer arranged in a densely packed layer. This results in more space between cell bodies which is filled in by numerous horizontal and basal dendrites originating from the pyramidal cell perikaryon. CA1 pyramidal neurons in slice cultures of the rat or mouse thus resemble the pyramidal neurons in the CA1 region of the primate hippocampus where a similar loose distribution of cell bodies is found. In the electron microscope, cell bodies and dendritic shafts of the gold-toned pyramidal cells formed symmetric synaptic contacts with presynaptic terminals. Numerous boutons were observed that established asymmetric synaptic contacts on gold-toned spines of peripheral pyramidal cell dendrites. This suggests that considerable synaptic reorganization takes place because in situ spines on peripheral dendritic segments are contacted mainly by extrinsic afferents. Like in situ, at least some of the terminals that establish symmetric synaptic contacts are GABAergic. In our immunocytochemical study we observed numerous GAD-positive terminals that formed a dense pericellular plexus around immunonegative cell bodies of pyramidal neurons. In the electron microscope these structures were identified as presynaptic boutons which formed symmetric synaptic contacts on cell bodies and dendritic shafts. They most likely originated from the GAD-positive neurons scattered in all layers of the slice culture. Our results have shown that the main cell types in the hippocampus, pyramidal neurons and GABAergic inhibitory non-pyramidal cells, survive and differentiate under the present culture conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic plasticity in rat hippocampal slice cultures: local "Hebbian" conjunction of pre- and postsynaptic stimulation leads to distributed synaptic enhancement

A central theme in neurobiology is the search for the mechanisms underlying learning and memory. Since the seminal work, first of Cajal and later of Hebb, the synapse is thought to be the basic "storing unit." Hebb proposed that information is stored by correlation: synapses between neurons, which are often coactive, are enhanced. Several recent findings suggest that such a mechanism is indeed operative in the central nervous system. Pairing of activity on presynaptic fibers with strong postsynaptic depolarization results in synaptic enhancement. While there is substantial evidence in favor of a postsynaptic locus for detection of the synchronous pre- and postsynaptic event and subsequent initiation of synaptic enhancement, the locus of this enhancement and its ensuing persistence is still disputed: both pre- and postsynaptic contributions have been suggested. In all previous studies, the enhancement was presumed to be specific to the synapses where synchronous pre- and postsynaptic stimulation was applied. We report here that two recording techniques--optical recording, using voltage-sensitive dyes, and double intracellular recordings--reveal that synaptic enhancement is not restricted to the stimulated cell. Although we paired single afferent volleys with intracellular stimulation confined to one postsynaptic cell, we found that strengthening also occurred on synapses between the stimulated presynaptic fibers and neighboring cells. This suggests that synaptic enhancement by the "paired-stimulation paradigm" is not local on the presynaptic axons and that, in fact, the synapses of many neighboring postsynaptic cells are enhanced.

Electrophysiological characteristics of neurons in neocortical explant cultures

We examined the electrophysiological and morphological properties of neocortical neurons maintained in explant cultures prepared from the parietal cortex of newborn Sprague-Dawley rats. After 3-6 weeks in vitro, cultures showed regional differences in cellular density reminiscent of cortical layering, and an abundance of axonal processes. Pyramidal-shaped neurons with spinous dendrites were the dominant elements revealed by Lucifer yellow injections. Intracellular recordings revealed that many electrophysiological properties of neurons in the explants resembled those of neocortical neurons in vivo and in slice preparations. In response to depolarizing current injection, neurons in the explants showed the same three patterns of repetitive firing described in neocortical slices, as well as a similar array of responses. Spontaneous synaptic potentials were recorded from all neurons and complex PSPs were evoked in response to focal extracellular stimulation. GABAa receptors mediated a significant component of the evoked responses. Fifteen of sixty neurons generated action potentials that arose spontaneously from resting potentials. Neurons in many slices generated large, prolonged depolarizing potentials that reflected coordinated synaptic activity within the explants. These results underscore the usefulness of the neocortical explant as a valuable model for studying aspects of the behavior of circuits of cortical neurons.