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

Bioactivity of a peptide derived from acetylcholinesterase: involvement of an ivermectin-sensitive site on the alpha 7 nicotinic receptor

A peptide fragment of 14 amino acids, derived from the C-terminus of acetylcholinesterase (AChE), might underlie the now well-established noncholinergic effects of the enzyme. This peptide is bioactive in a variety of systems including acute (brain slices) and chronic (organotypic culture) preparations of hippocampus, a pivotal area in Alzheimer's disease (AD); invariably, the action of the peptide is mediated specifically via an as yet unknown receptor. In this study, the allosteric alpha 7 agent, ivermectin (IVM), had a modest inhibitory effect, whilst that of the peptide was significantly more marked. However, ivermectin rendered ineffective the toxicity of high doses of the peptide, that is, when the two were co-applied, only the smaller effects of ivermectin were seen. Ivermectin, therefore, is presumably acting at a site that is identical to, or at least strongly interactive with, the normal binding site for AChE-peptide. This observation could have important implications for eventual therapeutic targeting of the action of AChE-peptide, in neurodegeneration.

Neuroprotective Effect of KR-31378 via KATP Channel Opening against Ischemic Insult

The opening of the adenosine triphosphate (ATP)-sensitive potassium (KATP) channel has been proposed as a therapeutic approach for ischemia. Here we examined the opening effect of KR-31378 on the KATP channel using patch clamp recording in neuroblastoma 2a (N2a) cells and investigated the neuroprotective effect of KR-31378 in organotypic hippocampal slice cultures exposed to oxygen/glucose deprivation. The treatment with KR-31378 (10 microM) to N2a cells seemed to induce KATP channel opening in a dose dependent manner. The opening effect of KR-31378 was more significant than that of other known KATP channel openers. Pretreatment with KR-31378 (10 microM) showed a neuroprotective effect in both CA1 and CA3 regions and its effect was attenuated by glibenclamide in a dose dependent manner in both areas. This remarkable neuroprotective effect of KR-31378 seemed to be mediated by the opening of the KATP channel. These results suggest that KR-31378 could be a possible neuroprotective agent against cerebral ischemia.

Osmotic regulation of neuronal activity: a new role for taurine and glial cells in a hypothalamic neuroendocrine structure

Maintenance of osmotic pressure is a primary regulatory process essential for normal cell function. The osmolarity of extracellular fluids is regulated by modifying the intake and excretion of salts and water. A major component of this regulatory process is the neuroendocrine hypothalamo-neurohypophysial system, which consists of neurons located in the paraventricular and supraoptic nuclei. These neurons synthesize the neurohormones vasopressin and oxytocin and release them in the blood circulation. We here review the mechanisms responsible for the osmoregulation of the activity of these neurons. Notably, the osmosensitivity of the supraoptic nucleus is described including the recent data that suggests an important participation of taurine in the transmission of the osmotic information. Taurine is an amino acid mainly known for its involvement in cell volume regulation, as it is one of the major inorganic osmolytes used by cells to compensate for changes in extracellular osmolarity. In the supraoptic nucleus, taurine is highly concentrated in astrocytes, and released in an osmodependent manner through volume-sensitive anion channels. Via its agonist action on neuronal glycine receptors, taurine is likely to contribute to the inhibition of neuronal activity induced by hypotonic stimuli. This inhibitory influence would complement the intrinsic osmosensitivity of supraoptic neurons, mediated by excitatory mechanoreceptors activated under hypertonic conditions. These observations extend the role of taurine from the regulation of cell volume to that of the whole body fluid balance. They also point to a new role of supraoptic glial cells as active components in a neuroendocrine regulatory loop.

Hippocampal synaptic density and glutamate immunoreactivity following transient cerebral ischaemia in the chick

A transient ischaemic episode of 10 min duration was induced in 1-day-old chicks. After a 1-week survival period, synapse density was assessed in the ventral hippocampus using the 'disector' technique. A significant decrease was observed in asymmetric synapses, markedly greater than that observed previously in the dorsal hippocampus. Because the effect occurred mainly on excitatory synapses, the distribution of glutamate in the ventral hippocampus was also assessed by a postembedding immunogold labelling technique. The density of gold particles was significantly greater in both boutons and neuropil in the ischaemic group compared to controls, lending support to the theory of excitotoxicity as an explanation for ischaemic neural degeneration.

Selective neuronal vulnerability and specific glial reactions in hippocampal and neocortical organotypic cultures submitted to ischemia

Neurons from cerebral neocortex and hippocampus exhibit a striking difference in vulnerability to transient global ischemia. In order to study the contribution of neuronal connections and neuron-glia interactions to this variation in neuronal vulnerability, we used hippocampal and neocortical cultures submitted to various periods of histotoxic ischemia. Organotypic cultures were exposed at 37 degrees C for 0, 7, 30 and 60 min to a glucose-free NaCN-containing medium. Histological analysis using thionin staining and MAP2 immunostaining showed differences in the temporal profile of neuronal damage in hippocampal and neocortical structures, i.e., in decreasing order, CA1 (7 min) > CA3 and neocortical layers II, III, V, VI (30 min) > DG and neocortical layer IV (60 min). In parallel to the neurodegeneration study, the time course and the regional pattern of microglial and astroglial changes were also examined using GS-B4 isolectin and GFAP as immunohistochemical markers, respectively. The GS-B4 isolectin staining revealed an early (at 7 min for the hippocampus) and a specific microglial activation located in areas undergoing neuronal damage. For both organotypic cultures, astrogliosis occurred later (after 30 min of stress) with no specific regional distribution. Both hippocampal and neocortical cultures submitted to histotoxic ischemia allowed the replication of many of the cellular events observed in response to global ischemia in vivo. These findings support the hypothesis that neuron-neuron connections as well as interactions between neurons and glial cells are essential to reproduce in vitro the selective neuronal vulnerability described in vivo.

Tetanus toxin-enhanced GABA immunoreactivity in living neurons

Analysis of the connectivity between different neuronal cell types is dependent on an appreciation of their dendritic and axonal arborizations. A detailed study of the dendrites and axons of GABAergic neurons has been thwarted by the lack of a suitable technique for enhancing GABA immunoreactivity. This article describes a procedure using tetanus toxin which, when applied to organotypic hippocampal cultures, considerably enhances the immunoreactivity in the dendrites and axons of the GABA- and somatostatin-containing neurons and clearly demonstrates the co-localization of GABA and somatostatin immunoreactivities in the same neuron. Tetanus toxin was applied to the culture medium on Day 14 for a 24-hr period and the cultures were fixed at the end of Day 18. Tetanus toxin-treated cultures (n = 30) or untreated cultures (n = 40) were incubated for either GABA or somatostatin immunoreactivity. Tetanus toxin-treated cultures used for co-localization studies (n = 20) were incubated for both GABA and somatostatin immunoreactivity.

Trimethyltin (TMT) neurotoxicity in organotypic rat hippocampal slice cultures

The neurotoxic effects of trimethyltin (TMT) on the hippocampus have been extensively studied in vivo. In this study, we examined whether the toxicity of TMT to hippocampal neurons could be reproduced in organotypic brain slice cultures in order to test the potential of this model for neurotoxicological studies, including further studies of neurotoxic mechanisms of TMT. Four-week-old cultures, derived from 7-day-old donor rats and grown in serum-free medium, were exposed to TMT (0.5-100 microM) for 24 h followed by 24 h in normal medium. TMT-induced neurodegeneration was then monitored by (a) propidium iodide (PI) uptake, (b) lactate dehydrogenase (LDH) efflux into the culture medium, (c) cellular cobalt uptake as an index of calcium influx, (d) ordinary Nissl cell staining, and (e) immunohistochemical staining for microtubule-associated protein 2 (MAP-2). Cellular degeneration as assessed by densitometric measurements of PI uptake displayed a dose and time-dependent increase, with the following ranking of vulnerability of the hippocampal subfields: FD>CA4>/=CA3c>CA1>CA3ab. This differential neuronal vulnerability observed by PI uptake was confirmed by MAP-2 immunostaining and corresponded to in vivo cell stain observations of rats acutely exposed to TMT. The mean PI uptake of the cultures and the LDH efflux into the medium were highly correlated. The combined results obtained by the different markers indicate that the hippocampal slice culture method is a feasible model for further studies of TMT neurotoxicity.

The regional vulnerability to blockade of action potentials in organotypic hippocampal culture

We investigated how blockade of spontaneous action potentials influenced the synaptogenesis by measuring the field population spike using hippocampal organotypic cultures. Although respective blockade of inhibitory and excitatory neurotransmission by picrotoxin and CNQX did not significantly induce cell death in all hippocampal area, sodium channel blocking drugs (tetrodotoxin or lidocaine) caused specific and severe damage and affected the formation of functional synapse in CA1 and the entorhinal cortex but not in CA3. It is suggested that the spontaneous action potentials would play a critical role during synaptogenesis.

Glycine causes increased excitability and neurotoxicity by activation of NMDA receptors in the hippocampus

Glycine is an inhibitory neurotransmitter in the spinal cord and also acts as a permissive cofactor required for activation of the N-methyl-D-aspartate (NMDA) receptor. We have found that high concentrations of glycine (10 mM) cause marked hyperexcitability and neurotoxicity in organotypic hippocampal slice cultures. The hyperexcitability, measured using intracellular recording in CA1 pyramidal neurons was completely blocked by the NMDA receptor antagonist MK-801 (10 microM), but not by the AMPA receptor antagonist DNQX (100 microM). The neurotoxicity caused by glycine occurred in all regions of hippocampal cultures but was most marked in area CA1. There was significant CA1 neuronal damage in cultures exposed to 10 mM glycine for 30 min or longer (P < 0.01) or those exposed to 4 mM glycine for 24 h compared to control cultures (P < 0.01). The NMDA antagonists MK-801 (10 microM) and APV (100 microM) significantly reduced glycine-induced neuronal damage in all hippocampal subfields (P < 0.01). The AMPA antagonists CNQX, DNQX, and NBQX (100 microM) had no effect on glycine-induced neuronal damage. High concentrations of glycine therefore appear to enhance the excitability of hippocampal slices in an NMDA receptor-dependent manner. The neurotoxic actions of glycine are also blocked by NMDA receptor antagonists.

Sensory gating in a computer model of the CA3 neural network of the hippocampus

We have developed a unique computer model of the CA3 region of the hippocampus that simulates the P50 auditory evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory processing deficit associated with schizophrenia. Our computer model of the CA3 hippocampal network includes recurrent activation from within the CA3 region as well as input from the entorhinal cortex and the medial septal nucleus. We used the model to help us determine if the cortical and septal inputs to the CA3 hippocampus alone are responsible for the gating of auditory evoked activity, or if the strong recurrent activity within the CA3 region contributes to this phenomenon. The model suggests that the medial septal input is critical for normal gating; however, to a large extent the activity of the medial septal input can be replaced by simulated stimulation of the hippocampal neurons by a nicotinic agonist. The model is thus consistent with experimental data that show that nicotine restores gating of the N40 evoked potential in fimbria-fornix lesioned rats and of the P50 evoked potential in schizophrenic patients.

Optical recording of rat entorhino-hippocampal system in organotypic culture

It is difficult to comprehend the entorhino-hippocampal information processing using acute transverse hippocampal slice, because the dorsally inclined connections of the entorhino-hippocampal projections can be damaged easily. Therefore, we investigated the spatial-temporal propagation in organotypic cultures of the hippocampus attaching to the entorhinal cortex using a real-time optical recording system with a voltage-sensitive dye and suitability as an in vitro model. Real-time imaging demonstrated that the stimulation of the perforant pathway induced excitatory propagation in trisynaptic pathway of the hippocampus and sequentially in the layer V from the medial to the lateral entorhinal cortex. The horizontal propagation from the lateral to the medial site was also seen after the stimulation of the lateral entorhinal cortex. The analysis of the entorhino-hippocampal organotypic culture would contribute to understanding of the mechanism of learning and memory.

Reinnervation of stratum lucidum by hippocampal mossy fibers is developmentally regulated

Mossy fibers from dentate gyrus granule cells establish synapses on CA3 pyramidal neurons during the first 3 postnatal weeks in the rat. Mossy fiber synapses are primarily restricted to the stratum lucidum. When examined by Timm stain after 10-14 days in vitro, cultured hippocampal slices from postnatal day 4 rat pups show a similar mossy fiber termination pattern in stratum lucidum. Thus, axon guidance cues used by mossy fibers in vivo appear to be preserved in these cultured slices. Three experimental manipulations were performed on hippocampal slice cultures to examine whether the axon guidance cues used by mossy fibers are developmentally regulated. First, mossy fibers were transected on the day of culture or day 7 in vitro. Mossy fibers transected on either day were able to reestablish their synaptic pattern in stratum lucidum of CA3. Second, dentates and hippocampi of same age or different age were co-cultured. Same age co-cultures (P4 dentates to P4 hippocampi or P11 dentates to P11 hippocampi) showed good mossy fiber reinnervation of stratum lucidum, as did different age co-cultures from P4 dentates to P11 hippocampi. However, P11 dentates to P4 hippocampi co-cultures showed little mossy fiber reinnervation of stratum lucidum. Third, new P4 or P11 dentates were co-cultured onto hippocampal slices in which mossy fibers had been allowed to degenerate. New mossy fibers reinnervated these hippocampi, but did not reestablish their normal synaptic pattern in stratum lucidum. These three experimental manipulations suggest that mossy fiber axon guidance mechanisms are developmentally regulated, and that existing mossy fibers play a role in directing mossy fiber reinnervation of stratum lucidum.

Pre-exposure to subtoxic levels prevents kainic acid lesions in organotypic hippocampal slice cultures: effects of kainic acid on parvalbumin-immunoreactive neurons and expression of heat shock protein 72 following the induction of tolerance

The effects of kainic acid on the survival of principal neurons and parvalbumin-immunoreactive (PARV-IR) neurons, and on the expression of heat shock protein 72 immunoreactivity (HSP72-IR) were investigated in organotypic hippocampal slice cultures. Untreated cultures displayed an organotypic organization and the development and morphology of PARV-IR neurons in the hippocampus paralleled that reported to occur in vivo, with the exception of the hilar region of the dentate gyrus which exhibited a marked lack of PARV-IR neurons. No constitutive expression of HSP72 was found in untreated cultures. The lesion of CA3 neurons and the reduction in numbers of PARV-IR neurons in both CA3 and CA1 after chronic exposure to 5 microM kainic acid were similar to those reported to occur in vivo. Exposure to 1 microM doses of kainic acid resulted in a widespread appearance of HSP72-IR and the induction of tolerance to a previously toxic dose of kainic acid. These results suggest the presence of endogenous neuroprotective mechanisms, activated by a stress response which induces HSP72, and is reminiscent of the induced tolerance reported to occur after a mild ischaemic insult.

Factors influencing mossy fiber collateral sprouting in organotypic slice cultures of neonatal mouse hippocampus

Collateral sprouting of dentate granule cell axons, the mossy fibers, occurs in response to denervation, kindling, or excitotoxic damage to the hippocampus. Organotypic slice culture of rodent hippocampal tissue is a model system for the controlled study of collateral sprouting in vitro. Organotypic roller-tube cultures were prepared from hippocampal slices derived from postnatal day 7 mice. The Timm heavy metal stain and densitometry were used to assay the degree of mossy fiber collateral sprouting in the molecular layer of the hippocampal dentate gyrus. Factors influencing mossy fiber collateral sprouting were time in culture, positional origin of the slice culture along the septotemporal axis of the hippocampus, and presence of attached subicular-entorhinal cortical tissues. Collateral sprouting in the molecular layer was first detected after 6 days in culture and increased steadily thereafter. By 2 weeks considerable sprouting was apparent, and at 3 weeks intense sprouting was observed within the molecular layer. An intrinsic septal-to-temporal gradient of collateral sprouting was apparent at 14 days in culture. To determine whether differential damage to the mossy fibers was the basis for the differences in collateral sprouting along the septotemporal axis, we made complete transections of the mossy fiber projection as it exited the dentate hilus at various levels along the septotemporal axis; no differences were found on subsequent collateral sprouting in the dentate molecular layer. Timm-stained hippocampal cultures with an attached entorhinal cortex, a major source of afferent innervation to the dentate granule cells, displayed significantly less collateral sprouting at 10 days in culture compared to that in cultures from adjacent sections without attached subicular-entorhinal tissues present. Thus, time in culture, position along the septotemporal axis, and presence of afferent cortical tissues influence aberrant neurite collateral sprouting in organotypic slice cultures of neonatal mouse hippocampus.

Long-term hippocampal slices: a model system for investigating synaptic mechanisms and pathologic processes

Organotypic cultures provide a unique strategy with which to examine many aspects of brain physiology and pathology. Long-term slice cultures from the hippocampus, a region involved in memory encoding and one that exhibits early degeneration in Alzheimer's disease and ischemia, are particularly valuable in this regard due to their expression of synaptic plasticity mechanisms (e.g., long-term potentiation) and responsiveness to pathological insults (e.g., excitotoxicity). Long-term slices can be prepared from hippocampi at the second or third postnatal week of development and thus incorporate a number of relatively mature features; further signs of maturation and the preservation of adult-like characteristics occur over succeeding weeks. The stability of the cultured slice renders it an appropriate model for studying 1) prolonged regulation/stabilization events linked to synaptogenesis and certain forms of plasticity, 2) temporal patterns of cellular atrophy associated with pathogenic conditions such as ischemia and epilepsia, and 3) slow processes associated with aging and age-related pathologies.

Beta amyloid is neurotoxic in hippocampal slice cultures

We examined the neurotoxicity of the 40 amino acid fragment of beta amyloid peptide (A beta 1-40) in cultured hippocampal slices. When injected into area CA3, A beta 1-40 produced widespread neuronal damage. Injection of the reverse sequence peptide, A beta 40-1, or vehicle alone produced little damage. The distribution A beta 1-40 was highly correlated with the area of neuronal damage. Thioflavine S and electron microscopic analysis confirmed that injected A beta 1-40 formed 7-9 nm AD type amyloid fibrils in the cultures. A beta 1-40 also altered the number of GFAP immunoreactive astrocytes and ED-1 immunoreactive microglia/macrophages within and around the A beta 1-40 deposit. The observed neurotoxicity of A beta 1-40 in hippocampal slice cultures provides evidence that this peptide may be responsible for the neurodegeneration observed in AD.

Protection from neuronal damage induced by combined oxygen and glucose deprivation in organotypic hippocampal cultures by glutamate receptor antagonists

Organotypic hippocampal cultures were exposed to defined periods (30 and 60 min) of combined oxygen and glucose deprivation, mimicking transient ischemic conditions. The involvement of different glutamate receptors in individual hippocampal subfields (CA1, CA3 and dentate gyrus) was studied using antagonists of NMDA (dizocilpine) and AMPA/kainate receptors (CNQX and GYKI 52466). Staining with the fluorescent dye propidium iodide (PI) allowed detection of damaged cells. For quantitative determination of neuronal damage, fluorescence intensity was measured after a 22 h recovery period and was related to maximal fluorescence intensity measured after fixation and PI restaining of the cultures at the end of the experiment. Dizocilpine (10 microM), CNQX (100 microM) and GYKI 52466 (100 microM) provided complete protection in CA1, CA3 and dentate gyrus following the moderate ischemic insult, when the antagonists were present permanently. This indicates that none of the ionotropic glutamate receptor subtypes dominated toxicity in the most sensitive subpopulation of neurons. When applied only during the recovery period protection with dizocilpine (10 microM) or CNQX (100 microM) was drastically reduced by about 60% in the most sensitive area (CA1), but only slightly by 15% in CA3. Therefore the onset of irreversible damage seems to occur earlier in CA1 than in CA3. Blockade of AMPA/kainate receptors by GYKI 52466 (100 microM) offered no neuroprotection if the compound was applied only during the recovery period.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative measurement of neuronal degeneration in organotypic hippocampal cultures after combined oxygen/glucose deprivation

Organotypic hippocampal cultures were used to study cell degeneration during the recovery period after defined periods (30 and 60 min) of combined oxygen/glucose deprivation mimicking transient ischemic conditions. Staining with the fluorescent dye propidium iodide allowed detection of damaged cells. Fluorescence intensity was measured by an image analysis system and used to quantify cell damage at different time points during the recovery period (up to 22 h). At 30 min of oxygen/glucose deprivation cells in the CA1 area were relatively more sensitive compared to CA3 and dentate gyrus cells, with respect to the time course of degeneration and the percentage of affected cells. Expanding the oxygen/glucose deprivation period from 30 to 60 min drastically increased the percentage of cells dying in all hippocampal areas. Still, however, cells in CA1 degenerated faster compared to those in the CA3 area and dentate gyrus. A histological analysis of toluidine blue as well as MAP2-immunostained sections revealed that almost all neurons degenerated in all hippocampal areas following the 60-min deprivation period, whereas GFAP-stained astrocytes appeared to be unaffected. Therefore, neuronal degeneration could be quantified by taking the fluorescence intensity values 22 h after 60 min of oxygen/glucose deprivation as 100% neuronal damage. The possibility to quantify neuronal damage in organotypic cultures offers a useful tool for detailed studies on mechanisms of neuronal cell death in a cell culture system which is closer to in situ conditions than monolayer cell cultures.

Glycine site NMDA receptor antagonists provide protection against ischemia-induced neuronal damage in hippocampal slice cultures

Ischemia-induced neuronal injury can be reduced by glutamate antagonists acting at the N-methyl-D-aspartate (NMDA) receptor. 7-Chlorokynurenic acid and the recently synthesized compound Acea 1021 block NMDA receptors by acting at the strychnine-insensitive glycine site. The anti-ischemic properties of these compounds were tested by evaluating their ability to reduce CA1 neuronal damage in hippocampal slice cultures deprived of oxygen and glucose. Acea 1021 and 7-chlorokynurenic acid significantly reduced CA1 injury produced by oxygen and glucose deprivation in a dose-dependent manner. The neuroprotective effect of these compounds was reversed by the addition of glycine. The phencyclidine site NMDA antagonist MK-801 also provided significant protection to CA1 neurons against the same insult, and this protection was not affected by the addition of glycine. These results indicate that Acea 1021 and 7-chlorokynurenic acid can provide protection to CA1 neurons against ischemia-induced injury by a glycine-sensitive mechanism.

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.

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.

A developmental study of lactate dehydrogenase isozyme and aspartate aminotransferase activity in organotypic rat hippocampal slice cultures and primary cultures of mouse neocortical and cerebellar neurons

The development of enzyme activity and isozyme distribution of lactate dehydrogenase (LDH) was studied in murine organotypic hippocampal slice cultures and dissociated cultures of neocortical neurons and cerebellar granule cells and compared with that of the respective brain regions in vivo. In the hippocampal slice cultures and the hippocampus in vivo, the activity of aspartate aminotransferase (AAT) was also measured. During development in culture the specific activity of LDH increased in all types of cultures reaching values similar to that found in the corresponding brain areas in vivo. However, significant differences in the isozyme distribution were observed between the preparations in vitro and in vivo. During development in vivo, the LDH isozyme pattern changed from a preferential M-subunit composition to a preferential H-subunit composition regardless of the brain area. This shift was not observed in the respective cultures where the M4-isozyme prevailed at all culture periods examined accounting for 30-45% of the total LDH activity. The cultured cerebellar granule cells did not express the H4-isozyme at all, while in the hippocampal slice cultures and the cultured neo-cortical neurons this isozyme accounted for about 5% of the total LDH activity. The activity of AAT in the hippocampal organotypic slice cultures increased considerably during the culture period in parallel with the increase in AAT activity during postnatal development of hippocampus in vivo. The activity of AAT in the slice cultures was, however, consistently lower than the corresponding activity in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

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.

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.