Microglia in organotypic hippocampal slice culture and effects of hypoxia: ultrastructure and lipocortin-1 immunoreactivity

An In Vitro Brain Tumour Model in Organotypic Slice Cultures Displaying Epileptiform Activity

Brain tumours have significant impacts on patients' quality of life, and current treatments have limited effectiveness. To improve understanding of tumour development and explore new therapies, researchers rely on experimental models. However, reproducing tumour-associated epilepsy (TAE) in these models has been challenging. Existing models vary from cell lines to in vivo studies, but in vivo models are resource-intensive and often fail to mimic crucial features like seizures. In this study, we developed a technique in which normal rat organotypic brain tissue is implanted with an aggressive brain tumour. This method produces a focal invasive lesion that preserves neural responsiveness and exhibits epileptiform hyperexcitability. It allows for real-time imaging of tumour growth and invasion for up to four weeks and microvolume fluid sampling analysis of different regions, including the tumour, brain parenchyma, and peritumoral areas. The tumour cells expand and infiltrate the organotypic slice, resembling in vivo behaviour. Spontaneous seizure-like events occur in the tumour slice preparation and can be induced with stimulation or high extracellular potassium. Furthermore, we assess extracellular fluid composition in various regions of interest. This technique enables live cell confocal microscopy to record real-time tumour invasion properties, whilst maintaining neural excitability, generating field potentials, and epileptiform discharges, and provides a versatile preparation for the study of major clinical problems of tumour-associated epilepsy.

Effects of Ischemia on the Migratory Capacity of Microglia Along Collagen Microcontact Prints on Organotypic Mouse Cortex Brain Slices

Ischemic stroke is a severe insult in the brain causing cell death, inflammation, and activation of microglia. Microglia are the immune cells of the brain and play a role in any inflammatory process during neurodegeneration. Microglia are round ameboid and migrate to the lesion site, where they differentiate into ramified forms and activated phagocytic microglia. On the other hand, microglia can also release growth factors to repair degeneration. The aim of the present study is to explore the migratory capacity of microglia after ischemic insults. Organotypic brain slices of the mouse cortex (300 μm) were prepared. In order to study migration, the slices were connected to collagen-loaded microcontact prints (with or without monocyte chemoattractant protein-1, MCP-1) on the membranes. Slices were stimulated with lipopolysaccharide (LPS) for maximal microglial activation. Ischemic insults were simulated with oxygen-glucose deprivation (OGD) and acidosis (pH 6.5) for 3 days. After 3 weeks in culture, slices were fixed and immunohistochemically stained for the microglial markers Iba1, CD11b and macrophage-like antigen. Our data show that Iba1+ microglia migrated along the microcontact prints, differentiate and phagocyte 1.0 μm fluorescent microbeads. LPS significantly enhanced the number of round ameboid migrating microglia, while OGD and acidosis enhanced the number of ramified activated microglia. The effect was not visible on slices without any μCP and was most potent in μCP with MCP-1. We conclude that OGD and acidosis activate ramification and exhibit a similar mechanism, while LPS only activates round ameboid microglia. Collagen-loaded microcontact prints connected to mouse brain slices are a potent method to study activation and migration of microglia ex vivo.

Microglia Activation in the Midbrain of the Human Neonate: The Effect of Perinatal Hypoxic-Ischemic Injury

Perinatal hypoxia-ischemia (PHI) is a major risk factor for the development of neuropsychiatric deficits later in life. We previously reported that after prolonged PHI, the dopaminergic neurons of the human neonate showed a dramatic reduction of tyrosine hydroxylase (TH) in the substantia nigra, without important signs of neuronal degeneration despite the significant reduction in their cell size. Since microglia activation could precede neuronal death, we now investigated 2 microglia activation markers, ionized calcium-binding adapter molecule 1 (Iba1), and the phagocytosis marker Cd68. The highest Iba1 immunoreactivity was found in neonates with neuropathological lesions of severe/abrupt PHI, while the lowest in subjects with moderate/prolonged or older PHI. Subjects with very severe/prolonged or chronic PHI showed an increased Iba1 expression and very activated microglial morphology. Heavy attachment of microglia on TH neurons and remarkable expression of Cd68 were also observed indicating phagocytosis in this group. Females appear to express more Iba1 than males, suggesting a gender difference in microglia maturation and immune reactivity after PHI insult. PHI-induced microglial "priming" during the sensitive for brain development perinatal/neonatal period, in combination with genetic or other epigenetic factors, could predispose the survivors to neuropsychiatric disorders later in life, possibly through a sexually dimorphic way.

The neuroprotective role of microglial cells against amyloid beta-mediated toxicity in organotypic hippocampal slice cultures

During Alzheimer's disease (AD) progression, microglial cells play complex roles and have potentially detrimental as well as beneficial effects. The use of appropriate model systems is essential for characterizing and understanding the roles of microglia in AD pathology. Here, we used organotypic hippocampal slice cultures (OHSCs) to investigate the impact of microglia on amyloid beta (Aβ)-mediated toxicity. Neurons in OHSCs containing microglia were not vulnerable to cell death after 7 days of repeated treatment with Aβ_1-42 oligomer-enriched preparations. However, when clodronate was used to remove microglia, treatment with Aβ_1-42 resulted in significant neuronal death. Further investigations indicated signs of endoplasmic reticulum stress and caspase activation after Aβ_1-42 challenge only when microglia were absent. Interestingly, microglia provided protection without displaying any classic signs of activation, such as an amoeboid morphology or the release of pro-inflammatory mediators (e.g., IL-6, TNF-α, NO). Furthermore, depleting microglia or inhibiting microglial uptake mechanisms resulted in significant more Aβ deposition compared to that observed in OHSCs containing functional microglia, suggesting that microglia efficiently cleared Aβ. Because inhibiting microglial uptake increased neuronal cell death, the ability of microglia to engulf Aβ is thought to contribute to its protective properties. Our study argues for a beneficial role of functional ramified microglia whereby they act against the accumulation of neurotoxic forms of Aβ and support neuronal resilience in an in situ model of AD pathology.

Conversion of Synthetic Aβ to In Vivo Active Seeds and Amyloid Plaque Formation in a Hippocampal Slice Culture Model

The aggregation of amyloid-β peptide (Aβ) in brain is an early event and hallmark of Alzheimer's disease (AD). We combined the advantages of in vitro and in vivo approaches to study cerebral β-amyloidosis by establishing a long-term hippocampal slice culture (HSC) model. While no Aβ deposition was noted in untreated HSCs of postnatal Aβ precursor protein transgenic (APP tg) mice, Aβ deposition emerged in HSCs when cultures were treated once with brain extract from aged APP tg mice and the culture medium was continuously supplemented with synthetic Aβ. Seeded Aβ deposition was also observed under the same conditions in HSCs derived from wild-type or App-null mice but in no comparable way when HSCs were fixed before cultivation. Both the nature of the brain extract and the synthetic Aβ species determined the conformational characteristics of HSC Aβ deposition. HSC Aβ deposits induced a microglia response, spine loss, and neuritic dystrophy but no obvious neuron loss. Remarkably, in contrast to in vitro aggregated synthetic Aβ, homogenates of Aβ deposits containing HSCs induced cerebral β-amyloidosis upon intracerebral inoculation into young APP tg mice. Our results demonstrate that a living cellular environment promotes the seeded conversion of synthetic Aβ into a potent in vivo seeding-active form.

SIGNIFICANCE STATEMENT

In this study, we report the seeded induction of Aβ aggregation and deposition in long-term hippocampal slice cultures. Remarkably, we find that the biological activities of the largely synthetic Aβ aggregates in the culture are very similar to those observed in vivo This observation is the first to show that potent in vivo seeding-active Aβ aggregates can be obtained by seeded conversion of synthetic Aβ in a living (wild-type) cellular environment.

Effects of blast overpressure on neurons and glial cells in rat organotypic hippocampal slice cultures

Due to recent involvement in military conflicts, and an increase in the use of explosives, there has been an escalation in the incidence of blast-induced traumatic brain injury (bTBI) among US military personnel. Having a better understanding of the cellular and molecular cascade of events in bTBI is prerequisite for the development of an effective therapy that currently is unavailable. The present study utilized organotypic hippocampal slice cultures (OHCs) exposed to blast overpressures of 150 kPa (low) and 280 kPa (high) as an in vitro bTBI model. Using this model, we further characterized the cellular effects of the blast injury. Blast-evoked cell death was visualized by a propidium iodide (PI) uptake assay as early as 2 h post-injury. Quantification of PI staining in the cornu Ammonis 1 and 3 (CA1 and CA3) and the dentate gyrus regions of the hippocampus at 2, 24, 48, and 72 h following blast exposure revealed significant time dependent effects. OHCs exposed to 150 kPa demonstrated a slow increase in cell death plateauing between 24 and 48 h, while OHCs from the high-blast group exhibited a rapid increase in cell death already at 2 h, peaking at ~24 h post-injury. Measurements of lactate dehydrogenase release into the culture medium also revealed a significant increase in cell lysis in both low- and high-blast groups compared to sham controls. OHCs were fixed at 72 h post-injury and immunostained for markers against neurons, astrocytes, and microglia. Labeling OHCs with PI, neuronal, and glial markers revealed that the blast-evoked extensive neuronal death and to a lesser extent loss of glial cells. Furthermore, our data demonstrated activation of astrocytes and microglial cells in low- and high-blasted OHCs, which reached a statistically significant difference in the high-blast group. These data confirmed that our in vitro bTBI model is a useful tool for studying cellular and molecular changes after blast exposure.

Associated occurrence of p75 neurotrophin receptor expressing aldynoglia and microglia/macrophages in long term organotypic murine brain slice cultures

Growth-promoting aldynoglia, characterized by the expression of the prototype immature Schwann cell marker p75 neurotrophin receptor (NTR) have been shown to occur in some demyelinating diseases. However, the mechanisms determining the emergence and fate of such cells are largely unknown. This study aimed at the identification of such cells and potential triggering factors using an in vitro slice culture approach. Organotypic cerebrum and brain stem slices of adult mice were cultivated for up to 18 days in vitro. Immunohistochemistry for the detection of p75(NTR), CD107b, periaxin, growth associated protein (GAP)-43, and glial fibrillary acidic protein (GFAP) was performed. The results for p75(NTR) were substantiated by the use of in situ hybridization. Cultivation was associated with a progressively increasing spontaneous occurrence of bi- to multipolar p75(NTR)-positive, but periaxin-negative glia, indicative of aldynoglial Schwann cell like cells. Similar cells stained intensely positive for GAP-43, a marker for non-myelinating Schwann cells. The number of p75(NTR) positive glia did not correlate with GFAP expression, but showed a strong correlation with a remarkable spontaneous response of CD107b positive phagocytic microglia/macrophages. Moreover, aldynoglial p75(NTR) immunoreactivity negatively correlated to neuronal p75(NTR) expression, which was lost during culturing. The present results demonstrate that the cultivation of organotypic murine brain slices is accompanied by a spontaneous response of both microglia/macrophages and p75(NTR) positive cells, suggestive of Schwann cell like aldynoglia. The findings highlights the role of microglia/macrophages, which seem to be an important triggering factor, facilitating the occurrence of this unique type of macroglia.

Glia-related mechanisms in the anteroventral cochlear nucleus of the adult rat in response to unilateral conductive hearing loss

Conductive hearing loss causes a progressive decline in cochlear activity that may result in functional and structural modifications in auditory neurons. However, whether these activity-dependent changes are accompanied by a glial response involving microglia, astrocytes, or both has not yet been fully elucidated. Accordingly, the present study was designed to determine the involvement of glial related mechanisms in the anteroventral cochlear nucleus (AVCN) of adult rats at 1, 4, 7, and 15 d after removing middle ear ossicles. Quantitative immunohistochemistry analyses at light microscopy with specific markers of microglia or astroglia along with immunocytochemistry at the electron microscopy level were used. Also, in order to test whether trophic support by neurotrophins is modulated in glial cells by auditory activity, the expression and distribution of neurotrophin-3 (NT-3) and its colocalization with microglial or astroglial markers was investigated. Diminished cochlear activity after middle ear ossicle removal leads to a significant ipsilateral increase in the mean gray levels and stained area of microglial cells but not astrocytes in the AVCN at 1 and 4 d post-lesion as compared to the contralateral side and control animals. These results suggest that microglial cells but not astrocytes may act as dynamic modulators of synaptic transmission in the cochlear nucleus immediately following unilateral hearing loss. On the other hand, NT-3 immunostaining was localized mainly in neuronal cell bodies and axons and was upregulated at 1, 4 and 7 d post-lesion. Very few glial cells expressed this neurotrophin in both control and experimental rats, suggesting that NT-3 is primarily activated in neurons and not as much in glia after limiting auditory activity in the AVCN by conductive hearing loss.

PINK1 Deficiency Enhances Inflammatory Cytokine Release from Acutely Prepared Brain Slices

Parkinson's disease (PD) is the second most common neurodegenerative motor disease caused by degeneration of dopaminergic neurons in the substantia nigra. Because brain inflammation has been considered a risk factor for PD, we analyzed whether PTEN induced putative kinase 1 (PINK1), an autosomal recessive familial PD gene, regulates brain inflammation during injury states. Using acutely prepared cortical slices to mimic injury, we analyzed expression of the pro-inflammatory cytokines tumor necrosis factor-α, interleukin (IL)-1β, and IL-6 at the mRNA and protein levels. Both mRNA and protein expression of these cytokines was higher at 6-24 h after slicing in PINK1 knockout (KO) slices compared to that in wild-type (WT) slices. In serial experiments to understand the signaling pathways that increase inflammatory responses in KO slices, we found that IκB degradation was enhanced but Akt phosphorylation decreased in KO slices compared to those in WT slices. In further experiments, an inhibitor of PI3K (LY294002) upstream of Akt increased expression of pro-inflammatory cytokines. Taken together, these results suggest that PINK1 deficiency enhance brain inflammation through reduced Akt activation and enhanced IκB degradation in response to brain injury.

Microglia activation by SIV-infected macrophages: alterations in morphology and cytokine secretion

HIV infection in the brain and the resultant encephalitis affect approximately one third of individuals infected with HIV, regardless of treatment with antiretroviral drugs. Microglia are the resident phagocytic cell type in the brain, serving as a "first responder" to neuroinvasion by pathogens. The early events of the microglial response to productively infected monocyte/macrophages entering the brain can best be investigated using in vitro techniques. We hypothesized that activation of microglia would be specific to the presence of simian immunodeficiency virus (SIV)-infected macrophages as opposed to responses to macrophages in general. Purified microglia were grown and stimulated with control or SIV-infected macrophages. After 6 h, aliquots of the supernatant were analyzed for 23 cytokines using Millipore nonhuman primate-specific kit. In parallel experiments, morphologic changes and cytokine expression by individual microglia were examined by immunofluorescence. Surprisingly, the presence of macrophages was more important to the microglial response rather than whether the macrophages were infected with SIV. None of the cytokines examined were unique to co-incubation with SIV-infected macrophages compared with control macrophages, or their supernatants. Media from SIV-infected macrophages, however, did induce secretion of higher levels of IL-6 and IL-8 than the other treatments. As resident macrophages in the brain, microglia would be expected to have a strong response to infiltrate innate immune cells such as monocyte/macrophages. This response is triggered by incubation with macrophages, irrespective of whether or not they are infected with SIV, indicating a rapid, generalized immune response when infiltrating macrophages entering the brain.

Neuroinflammation and microglia: considerations and approaches for neurotoxicity assessment

BACKGROUND

The impact of an inflammatory response, as well as interactions between the immune and nervous systems, are rapidly assuming major roles in neurodegenerative disease and injury. However, it is now appreciated that the exact nature of such responses can differ with each type of insult and interaction. More recently, neuroinflammation and the associated cellular response of microglia are being considered for their contribution to neurotoxicity of environmental agents; yet, so far, the inclusion of inflammatory end points into neurotoxicity assessment have relied primarily on relatively limited measures or driven by in vitro models of neurotoxicity.

OBJECTIVE

To present background information on relevant biological considerations of neuroinflammation and the microglia response demonstrating the complex integrative nature of these biological processes and raising concern with regards to translation of effects demonstrated in vitro to the in vivo situation. Specific points are addressed that would influence the design and interpretation of neuroinflammation with regards to neurotoxicology assessment.

CONCLUSION

There is a complex and dynamic response in the brain to regulate inflammatory processes and maintain a normal homeostatic level. The classification of such responses as beneficial or detrimental is an oversimplification. Neuroinflammation should be considered as a balanced network of processes in which subtle modifications can shift the cells toward disparate outcomes. The tendency to overinterpret data obtained in an isolated culture system should be discouraged. Rather, the use of cross-disciplinary approaches to evaluate several end points should be incorporated into the assessment of inflammatory contributions to the neurotoxicity of environmental exposures.

Mechanisms of microglia-mediated neurotoxicity in a new model of the stroke penumbra

After an ischemic stroke, neurons in the core are rapidly committed to die, whereas neuron death in the slowly developing penumbra is more amenable to therapeutic intervention. Microglia activation contributes to delayed inflammation, but because neurotoxic mechanisms in the penumbra are not well understood, we developed an in vitro model of microglia activation and propagated neuron killing. To recapitulate inflammatory triggers in the core, microglia were exposed to oxygen glucose-deprived neurons and astrocytes. To model the developing penumbra, the microglia were washed and allowed to interact with healthy naive neurons and astrocytes. We found that oxygen-glucose deprivation (OGD)-stressed neurons released glutamate, which activated microglia through their group II metabotropic glutamate receptors (mGluRs). Microglia activation involved nuclear factor kappaB (NF-kappaB), a transcription factor that promotes their proinflammatory functions. The activated microglia became neurotoxic, killing naive neurons through an apoptotic mechanism that was mediated by tumor necrosis factor-alpha (TNF-alpha), and involved activation of both caspase-8 and caspase-3. In contrast to some earlier models (e.g., microglia activation by lipopolysaccharide), neurotoxicity was not decreased by an inducible nitric oxide synthase (iNOS) inhibitor (S-methylisothiourea) or a peroxynitrite scavenger [5,10,15,20-tetrakis(N-methyl-4'-pyridyl)porphinato iron (III) chloride], and did not require p38 mitogen-activated protein kinase (MAPK) activation. The same microglia neurotoxic behavior was evoked without exposure to OGD-stressed neurons, by directly activating microglial group II mGluRs with (2S,2'R,3'R)-2-(2'3'-dicarboxycyclopropyl) glycine or glutamate, which stimulated production of TNF-alpha (not nitric oxide) and mediated TNF-alpha-dependent neurotoxicity through activation of NF-kappaB (not p38 MAPK). Together, these results support potential therapeutic strategies that target microglial group II mGluRs, TNFalpha overproduction, and NF-kappaB activation to reduce neuron death in the ischemic penumbra.

Glial influence on nerve fiber formation from rat ventral mesencephalic organotypic tissue cultures

Rat fetal ventral mesencephalic organotypic cultures have demonstrated two morphologically different dopamine nerve fiber growth patterns, in which the initial nerve fibers are formed in the absence of astrocytes and the second wave is guided by astrocytes. In this study, the presence of subpopulations of dopamine neurons, other neuronal populations, and glial cells was determined. We used "roller-drum" organotypic cultures, and the results revealed that beta-tubulin-positive/tyrosine hydroxylase (TH)-negative nerve fibers were present as early as 1 day in vitro (DIV). A similar growth pattern produced by TH-positive neurons was present from 2 DIV. These neurites grew to reach distances over 4 mm and over time appeared to be degenerating. Thin, vimentin-positive processes were found among these nerve fibers. As the first growth was retracted, a second outgrowth was initiated and formed on migrating astrocytes. TH- and aldehyde dehydrogenase-1 (ALDH1)-positive nerve fibers formed both the nonglia-associated and the glia-associated outgrowth. In cultures with membrane inserts, only the glia-associated outgrowth was found. Vimentin-positive cells preceded migration of NG2-positive oligodendrocytes and Iba-1-positive microglia. Oligodendrocytes appeared not to be involved in guiding neuritic growth, but microglia was absent over areas dense with TH-positive neurons. In conclusion, in "roller-drum" cultures, nerve fibers are generally formed in two sequences. The early-formed nerve fibers grow in the presence of thin, vimentin-positive processes. The second nerve fiber outgrowth is formed on astroglia, with no correlation to the presence of oligodendrocytes or microglia. ALDH1-positive nerve fibers, presumably derived from A9 dopamine neurons, participate in formation of both sequences of outgrowth.

The mammalian central nervous synaptic cleft contains a high density of periodically organized complexes

Cryo-electron microscopy of vitreous section makes it possible to observe cells and tissues at high resolution in a close-to-native state. The specimen remains hydrated; chemical fixation and staining are fully avoided. There is minimal molecular aggregation and the density observed in the image corresponds to the density in the object. Accordingly, organotypic hippocampal rat slices were vitrified under high pressure and controlled cryoprotection conditions, cryosectioned at a final thickness of approximately 70 nm and observed below -170 degrees C in a transmission electron microscope. The general aspect of the tissue compares with previous electron microscopy observations. The detailed analysis of the synapse reveals that the density of material in the synaptic cleft is high, even higher than in the cytoplasm, and that it is organized in 8.2-nm periodic transcleft complexes. Previously undescribed structures of presynaptic and postsynaptic elements are also described.

A refined in vitro model to study inflammatory responses in organotypic membrane culture of postnatal rat hippocampal slices

Background

Propagated tissue degeneration, especially during aging, has been shown to be enhanced through potentiation of innate immune responses. Neurodegenerative diseases and a wide variety of inflammatory conditions are linked together and several anti-inflammatory compounds considered as having therapeutic potential for example in Alzheimer's disease (AD). In vitro brain slice techniques have been widely used to unravel the complexity of neuroinflammation, but rarely, has the power of the model itself been reported. Our aim was to gain a more detailed insight and understanding of the behaviour of hippocampus tissue slices in serum-free, interface culture per se and after exposure to different pro- and anti-inflammatory compounds.

Methods

The responses of the slices to pro- and anti-inflammatory stimuli were monitored at various time points by measuring the leakage of lactate dehydrogenase (LDH) and the release of cytokines interleukin 6 (IL-6) and tumour necrosis factor alpha (TNF-α) and nitric oxide (NO) from the culture media. Histological methods were applied to reveal the morphological status after exposure to stimuli and during the time course of the culture period. Statistical power analysis were made with nQuery Advisor^®, version 5.0, (Statistical Solutions, Saugus, MA) computer program for Wilcoxon (Mann-Whitney) rank-sum test.

Results

By using the interface membrane culture technique, the hippocampal slices largely recover from the trauma caused by cutting after 4–5 days in vitro. Furthermore, the cultures remain stable and retain their responsiveness to inflammatory stimuli for at least 3 weeks. During this time period, cultures are susceptible to modification by inflammatory stimuli as assessed by quantitative biochemical assays and morphological characterizations.

Conclusion

The present report outlines the techniques for studying immune responses using a serum-free slice culture model. Statistically powerful data under controlled culture conditions and with ethically justified use of animals can be obtained as soon as after 4–5 DIV. The model is most probably suitable also for studies of chronic inflammation.

Identification and characterization of two neurogenic zones in interface organotypic hippocampal slice cultures

Neurogenesis plays a role in many physiological (memory formation) and pathological (stroke, depression) processes. However the mechanisms of postnatal stem cell proliferation and neurogenesis are still poorly understood. We characterized early neurogenesis in vitro in rat organotypic hippocampal slice cultures. Proliferation was assessed by bromodeoxyuridine incorporation, neurogenesis by bromodeoxyuridine-double labeling with doublecortin or beta-III tubulin. We showed for the first time that in addition to the dentate gyrus organotypic hippocampal slice cultures include a second neurogenic zone: the posterior periventricle, which is a part of the lateral ventricle wall. This structure lining the stratum oriens contained Nestin+ precursors. We could identify morphological and functional differences between dentate gyrus and posterior periventricle precursor populations. Our data demonstrate that basic fibroblast growth factor treatment induced a fast but short-lasting neurogenic response in the dentate gyrus while the posterior periventricle showed a more pronounced and long lasting neurogenic effect of basic fibroblast growth factor. Thus two neurogenic zones with different neurogenic properties were identified in organotypic hippocampal slice cultures.

Pathological consequences of inducible nitric oxide synthase expression in hippocampal slice cultures

The generation of toxic concentrations of nitric oxide by the inducible nitric oxide synthase expressed in microglia and other brain cell types is frequently invoked as a causative factor in neurodegeneration. Experiments were carried out on slice cultures of rat hippocampus to test this hypothesis. Exposure of the slices to bacterial lipopolysaccharide plus interferon-gamma led to a time-dependent expression of functional inducible nitric oxide synthase that was found only in microglia. Microglial activation by other means, such as physical damage, was not associated with inducible nitric oxide synthase expression. Damage and cell death in slices expressing inducible nitric oxide synthase was evaluated over a period of 6 days, but none was found. Consistent with this result, cGMP measurements indicated that the average local nitric oxide concentration remained in the low nanomolar range. When the microglial population was expanded to a density three-fold above normal by applying granulocyte-macrophage colony stimulating factor, however, lipopolysaccharide plus interferon-gamma provoked neurodegeneration that could be blocked by an inducible nitric oxide synthase inhibitor. The associated nitric oxide concentration in the slices was saturating for guanylyl cyclase-coupled nitric oxide receptors, signifying at least 10 nM. It is concluded that inducible nitric oxide synthase is expressed in microglia only in response to specific stimuli involving the innate immune system, and that the resulting level of nitric oxide in intact brain tissue is normally too low to inflict damage directly. Quantities of nitric oxide sufficient to contribute directly or indirectly to pathology could be produced should the density of microglia become high enough, although caution must be exercised in extrapolating this finding to the human brain in vivo.

Diverse microglial motility behaviors during clearance of dead cells in hippocampal slices

We used two-channel three-dimensional time-lapse fluorescence confocal imaging in live rat hippocampal slice cultures (1-7 days in vitro) to determine the motility behaviors of activated microglia as they engage dead and dying cells following traumatic brain tissue injury. Live microglia were labeled with a fluorescently conjugated lectin (IB(4)), and dead neurons were labeled with a membrane-impermeant fluorescent DNA-binding dye (Sytox Orange or To-Pro-3). Tissue injury during the slicing procedure induced neuronal death and microglial activation, but the density of dead cells diminished approximately 10-fold by 7 days in vitro as resident microglia cleared dead cells. In time-lapse movies (4-20 h long), activated microglia exhibited varying levels of motile and locomotory activity. The motility of microglia could change abruptly following contact by other microglia or death of nearby cells. When neighboring cells died, some microglia rapidly moved toward or extended a process to engulf the dead cell, consistent with a chemotactic signaling response. Dead cell nuclei usually were engulfed and carried along by highly motile and locomoting microglia. The mean time to engulfment was approximately 5 times faster for newly deceased cells (33 min) than for extant dead cells (160 min), suggesting that the efficacy of microglial phagocytosis in situ might vary with time after cell death or mode of cell death. These observations demonstrate that activated microglia are heterogeneous with respect to motile activity following traumatic tissue injury and further indicate that cell motility in situ is temporally regulated at the single cell level, possibly by direct cell-cell contact and by diffusible substances emanating from nearby dead cells.

Bismuth tracing in organotypic cultures of rat hippocampus

Bismuth is known to have neurotoxic side effects in humans and animals. In the 1970s France experienced about a thousand cases of patients suffering from bismuth-induced encephalopathy. Studies suggest that bismuth may provoke a selective degeneration of CA1 pyramidal cells in the organotypic cultures of rat hippocampus. A currently established technique for the histochemical visualization of bismuth was applied on hippocampal tissue cultures allowing the tracing of bismuth in concentrations hitherto not possible. The accumulation and subcellular localization of bismuth is demonstrated in the tissue cultures of rat hippocampus. CA1 pyramidal cells in the rat hippocampus exhibit the highest uptake of bismuth. High bismuth citrate concentrations (10 microM) are able to totally destroy the cytoarchitecture of the hippocampus. At ultrastructural levels bismuth was found to be located exclusively in lysosome-like organelles.

Astrocytic factors protect neuronal integrity and reduce microglial activation in an in vitro model of N-methyl-D-aspartate-induced excitotoxic injury in organotypic hippocampal slice cultures

Acute CNS lesions lead to neuronal injury and a parallel glial activation that is accompanied by the release of neurotoxic substances. The extent of the original neuronal damage can therefore be potentiated in a process called secondary damage. As astrocytes are known to secrete immunomodulatory and neuroprotective substances, we investigated whether astrocytic factors can attenuate the amount of neuronal injury as well as the degree of microglial activation in a model of excitotoxic neurodegeneration. Treatment of organotypic hippocampal slice cultures with N-methyl-D-aspartate (NMDA) resulted in a reproducible loss of viable granule cells, partial destruction of the regular hippocampal cytoarchitecture and a concomitant accumulation of amoeboid microglial cells at sites of neuronal damage. Astrocyte-conditioned media reduced the amount of NMDA-induced neuronal injury by 45.3%, diminished the degree of microglial activation and resulted in an improved preservation of the hippocampal cytoarchitecture. Transforming growth factor (TGF)-beta failed to act as a neuroprotectant and even enhanced the amount of neuronal injury by 52.5%. Direct effects of astrocytic factors on isolated microglial cells consisted of increased microglial ramification and down-regulated expression of intercellular adhesion molecule-1, whereas incubation with TGF-beta had no such effects. In summary, our findings show that hitherto unidentified astrocyte-derived factors that are probably not identical with TGF-beta can substantially enhance neuronal survival, either by eliciting direct neuroprotective effects or by modulating the microglial response to neuronal injury.