The kinetics and morphological characteristics of the macrophage-microglial response to kainic acid-induced neuronal degeneration

Early onset of demyelination after N-methyl-D-aspartate lesions of the lateral hypothalamus

The function of neurons residing in a particular brain area is often assessed by injecting glutamatergic excitotoxins into that area and determining the consequences for the behavior of interest. However, injections of excitotoxins into the central nervous system not only kill local neurons but also demyelinate fibers of passage. Previous studies suggest that the myelin damage is triggered by a delayed inflammatory response to cell death mediated by monocytes of peripheral origin. If so, demyelination should commence only after recruitment of monocytes, their passage through the blood-brain barrier, and their metamorphosis into macrophages. This process is estimated to require at least 48 h. Using a hematoxylin (Weil) stain and immunohistochemistry for myelin basic protein, we looked for signs of demyelination at various times after injections of N-methyl-D-aspartate into the lateral hypothalamus. Demyelination was seen within 24 h after the lesion, sooner than predicted by the monocytic infiltration hypothesis. This finding has implications for interpreting effects of excitotoxic lesions and for developing means of improving their specificity.

Differential regulation of cytokine expression following pilocarpine-induced seizure

While the pathological changes that occur in the brain following seizure have been well characterized, the molecular mechanisms underlying these events are poorly understood. Cell death, reactive gliosis, and axonal sprouting are among the best studied alterations in the epileptic brain. Previous work in both the peripheral and the central nervous systems suggests that cytokines are capable of affecting each of these processes. To better understand the role of cytokines in seizures and their sequelae, we have characterized cytokine expression in an animal model of epilepsy. Using pilocarpine to chemically induce seizures, and RNase protection assays to assess mRNA levels, we have quantified changes in expression of several members of the neuropoietic cytokine family following a single, prolonged seizure. Levels of oncostatin M (OSM), leukemia inhibitory factor (LIF), cardiotrophin-1, and ciliary neurotrophic factor were all increased in the hippocampus after seizure, though to differing extents and with markedly different time courses. Cells expressing the most dramatically up-regulated cytokines, LIF and OSM, were identified by combined in situ hybridization and immunohistochemistry. The majority of LIF(+) cells in the hippocampus were glial fibrillary acidic protein(+) astrocytes, while the majority of OSM(+) cells had the morphology of interneurons and were occasionally colabeled with neurofilament markers. Both the time course and the localization of cytokine up-regulation following seizure suggest possible roles for these intercellular signaling molecules in epilepsy.

Microglial-neuronal interactions in synaptic damage and recovery

An understanding of the role of microglial cells in synaptic signaling is still elusive, but the neuron-microglia relationship may have important ramifications for brain plasticity and injury. This review summarizes current knowledge and theories concerning microglial-neuronal signaling, both in terms of neuron-to-microglia signals that cause activation and microglia-to-neuron signals that affect neuronal response to injury. Microglial activation in the brain involves a stereotypical pattern of changes including proliferation and migration to sites of neuronal activity or injury, increased or de novo expression of immunomodulators including cytokines and growth factors, and the full transformation into brain-resident phagocytes capable of clearing damaged cells and debris. The factors released from neurons that elicit such phenotypical and functional alterations are not well known but may include cytokines, oxidized lipids, and/or neurotransmitters. Once activated, microglia can promote neuronal injury through the release of low-molecular-weight neurotoxins and support neuronal recovery through the release of growth factors and the isolation/removal of damaged neurons and myelin debris. Because microglia respond quickly to neuronal damage and have robust effects on neurons, astrocytes, and oligodendrocytes, microglial cells could play potentially key roles in orchestrating the multicell cascade that follows synaptic plasticity and damage.

Thapsigargin induces microglial transformation from amoeboid to ramified type in vitro

Microglia generally display amoeboid morphology under prevalent culture conditions. We found that cultured microglia derived from rat cerebral cortex undergo a morphological transformation from amoeboid to process-bearing microglia upon treatment with thapsigargin (TG), a specific Ca2+-ATPase inhibitor of endoplasmic reticulum. Microglial transformation was further enhanced by exposure of amoeboid microglia to serum-free (N2) medium containing TG (TG/N2 treatment). TG/N2-treated microglia showed a marked reduction in the activity of phagocytosis and showed down-regulated expression of MRF-1 or F4/80, which are markers for activated microglia. Thus, both morphological and physiological criteria suggest that TG promotes the ramification of amoeboid microglia in vitro. This method would be helpful in characterization of ramified microglia in vitro.

Modulation of macrophage and microglial responses to axonal injury in the peripheral and central nervous systems

OBJECTIVE

After axonal injury, macrophages rapidly infiltrate and become activated in the mammalian peripheral nervous system (PNS) but not the central nervous system (CNS). We used the dorsal root pathway to study factors that modulate the response of macrophages to degenerating axons in both the PNS and the CNS.

METHODS

Lewis rats underwent transection of dorsal roots (Group 1), stab within the spinal cord (Group II), crush at the dorsal root entry zone (Group III), transection of dorsal roots combined with a CNS lesion (Group IV), or systemic administration of a known activator of macrophages, lipopolysaccharide, alone (Group V) or combined with transection of dorsal roots (Group VI). ED-1 antibody stained for macrophages and activated microglia at 7, 14, and 42 days postinjury.

RESULTS

At early time points, Group I demonstrated ED-1 cells in the PNS but not the CNS portion of the degenerating dorsal roots. Group II revealed ED-1 cells near the stab lesion. Group III demonstrated ED-1 cells adjacent to the dorsal root entry zone crush site. Group IV revealed ED-1 cells along both the PNS and the CNS portions of the degenerating dorsal roots when the CNS lesion was placed near the transected roots. Group V demonstrated few ED-1 cells in the PNS and the CNS, whereas Group VI revealed a marked ED-1 cellular response along both the PNS and the CNS portions of the transected dorsal roots.

CONCLUSION

Local CNS trauma and systemic administration of lipopolysaccharide can "prime" macrophages/microglia, resulting in an enhanced response to degenerating axons in the CNS. Such priming might prove useful in promoting axonal regeneration.

Microglial reactivity correlates to the density and the myelination of the anterogradely degenerating axons and terminals following perforant path denervation of the mouse fascia dentata

Transection of the entorhino-dentate perforant path is a well known model for lesion-induced axonal sprouting and glial reactions in the rat. In this study, we have characterized the microglial reaction in the dentate molecular layer of the SJL/J and C57Bl/6 mouse. The morphological transformation of the microglial cells and their densitometrically measured Mac-1 immunoreactivity were correlated with the density of silver-impregnated axonal and terminal degeneration and the myelination of the degenerating medial and lateral perforant pathways. Anterograde axonal and terminal degeneration leads to: (i) altered myelin basic protein immunoreactivity with the appearance of discrete myelin deposits preferentially in the denervated medial and significantly less so in the lateral perforant path zone from day 2 after lesioning; (ii) an increase in number and Mac-1 immunoreactivity of morphologically-changed microglial cells in the denervated perforant path zones with more pronounced morphological transformation of microglia in the medial than in the lateral perforant path zones at day 2 but not day 5 after lesioning; and (iii) a linear correlation between the density of microglial Mac-1 reactivity and axonal degeneration in the medial but not in the lateral perforant path zone at two days postlesion, and a linear correlation in both zones at five days postlesion. We propose that the differentiated microglial response is due to the different densities of axonal and terminal degeneration, as observed in the individual cases. The finding of a potentiated or accelerated microglial activation in the medial as compared to the lateral perforant path zone suggests different kinetics of microglial activation in areas with degenerating myelinated and unmyelinated fibers.

Exacerbation of damage and altered NF-kappaB activation in mice lacking tumor necrosis factor receptors after traumatic brain injury

Tumor necrosis factor alpha (TNFalpha) is widely expressed in both neurons and glia and has been shown to be upregulated after traumatic brain injury (TBI). TNFalpha receptor activation results in activation of the transcription factor nuclear factor kappaB (NF-kappaB), which may serve an antiapoptotic role via the induction of target genes manganese superoxide dismutase (MnSOD) and/or calbindin. In the present study, we used a controlled cortical impact model of TBI with pertinent lines of transgenic mice to combine both morphological characterization and molecular analysis to elucidate the role of TNFalpha after TBI. Measurements of both the lesion volume and the blood-brain barrier breach indicated exacerbations in mice rendered genetically deficient in both the p55 and p75 TNFalpha receptors (TNFR-KO) compared with wild-type animals. Additionally, animals genetically altered to overexpress MnSOD showed a significant decrease in lesion volume compared with that of control littermates, whereas no alterations were observed in mice lacking the calcium-binding protein calbindin D28k. Analysis of NF-kappaB activation and relative levels of MnSOD revealed delayed responses in the injured cortex of TNFR-KO animals compared with wild-type animals, implying that endogenous TNFalpha may be neuroprotective after TBI.

Interleukin-1beta immunoreactivity and microglia are enhanced in the rat hippocampus by focal kainate application: functional evidence for enhancement of electrographic seizures

Using immunocytochemistry and ELISA, we investigated the production of interleukin (IL)-1beta in the rat hippocampus after focal application of kainic acid inducing electroencephalographic (EEG) seizures and CA3 neuronal cell loss. Next, we studied whether EEG seizures per se induced IL-1beta and microglia changes in the hippocampus using bicuculline as a nonexcitotoxic convulsant agent. Finally, to address the functional role of this cytokine, we measured the effect of human recombinant (hr)IL-1beta on seizure activity as one marker of the response to kainate. Three and 24 hr after unilateral intrahippocampal application of 0.19 nmol of kainate, IL-1beta immunoreactivity was enhanced in glia in the injected and the contralateral hippocampi. At 24 hr, IL-1beta concentration increased by 16-fold (p < 0.01) in the injected hippocampus. Reactive microglia was enhanced with a pattern similar to IL-1beta immunoreactivity. Intrahippocampal application of 0.77 nmol of bicuculline methiodide, which induces EEG seizures but not cell loss, enhanced IL-1beta immunoreactivity and microglia, although to a less extent and for a shorter time compared with kainate. One nanogram of (hr)IL-1beta intrahippocampally injected 10 min before kainate enhanced by 226% the time spent in seizures (p < 0.01). This effect was blocked by coinjection of 1 microgram (hr)IL-1beta receptor antagonist or 0.1 ng of 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonate, selective antagonists of IL-1beta and NMDA receptors, respectively. Thus, convulsant and/or excitotoxic stimuli increase the production of IL-1beta in microglia-like cells in the hippocampus. In addition, exogenous application of IL-1beta prolongs kainate-induced hippocampal EEG seizures by enhancing glutamatergic neurotransmission.

The role of microglia and macrophages in the pathophysiology of the CNS

Microglia are a major ghal component of the central nervous system (CNS) and are extremely sessile. Only a subtype, the perivascular microglia, are regularly replaced from the bone marrow in adult animals. Microglia respond to virtually any, even minor pathological events in the CNS. In most pathological settings microglia are aided by infiltrating hematogenous macrophages. Upon activation microglia and macrophages share most phenotypical markers and can exert similar effector functions. After transection of a CNS fibre tract microglia are insufficiently activated and hematogenous macrophages do not significantly enter the degenerating nerve stump. Thereby myelin debris that contains neurite outgrowth inhibiting activity persists for long time. This is in sharp contrast to the peripheral nervous system in which hematogenous macrophages are rapidly recruited in response to axotomy and clear myelin debris allowing regrowth of axons from the proximal stump. However, CNS lesion paradigms with breakdown of the blood-brain barrier such as cerebral ischemia, brain abscesses and stab wounds elicit prompt microglial activation, macrophage recruitment and debris clearance. There is increasing evidence that microglia play an active part in degenerative CNS diseases. In Alzheimer's disease activated microglia appear to be involved in plaque formation. In experimental globoid cell dystrophy T-cell independent induction of major histocompatibility complex class II molecules on microglia accelerates demyelination. In autoimmune diseases microglia probably have dual functions. Microglia present antigen to infiltrating T cells and exert effector functions thereby locally augmenting immune responses. On the other hand, microglia have the capacity to downregulate T cell responses. In the human acquired immunodeficiency syndrome (AIDS) virus infected macrophages probably introduce the virus to the CNS and in concert with microglia are involved in the pathophysiology of the AIDS dementia complex.

De novo expression of lipocortin-1 in reactive microglia and astrocytes in kainic acid lesioned rat cerebellum

An understanding of the role of reactive glia in the neurodegenerative/regenerative process requires a knowledge of the molecules synthesised by these cells following trauma. We investigated the cellular localisation of lipocortin-1 (LC-1), a putative neuroprotective agent, in cryostat sections of normal and kainic acid lesioned rat cerebellum. In the normal cerebellum lipocortin-1 immunoreactivity was detected in Purkinje cell bodies and molecular layer interneurons. Following kainic acid (1 microg) induced lesions, it was rapidly upregulated in activated microglia, from which it appeared to be secreted. At later time points it was detected in activated astrocytes. LC-1 protein levels were quantified by a sensitive and specific ELISA. Compared to control cerebellum, LC-1 levels were dramatically elevated following lesion, peaking at 3 days: 760% of basal (unlesioned) levels. In situ hybridisation studies revealed a marked upregulation of LC-1 mRNA at 1 and 3 days following the lesion, indicating the transient de novo synthesis of this protein, consistent with a localisation to microglia. In vitro studies, on cultured astrocytes and microglia, demonstrated high levels of intracellular LC-1 in both cell types. LC-1 was detected in microglial but not astrocytic, conditioned media, confirming the in vivo observations that activated microglia may secrete LC-1. Our data show that at early time points following excitotoxic lesion to the cerebellum, it is activated microglia that synthesise and possibly secrete this protein, suggesting an important role of this cell type in immunosuppression and neuroprotection following damage to the central nervous system.

Leukocyte infiltration, neuronal degeneration, and neurite outgrowth after ablation of scar-forming, reactive astrocytes in adult transgenic mice

Reactive astrocytes adjacent to a forebrain stab injury were selectively ablated in adult mice expressing HSV-TK from the Gfap promoter by treatment with ganciclovir. Injured tissue that was depleted of GFAP-positive astrocytes exhibited (1) a prolonged 25-fold increase in infiltration of CD45-positive leukocytes, including ultrastructurally identified monocytes, macrophages, neutrophils, and lymphocytes, (2) failure of blood-brain barrier (BBB) repair, (3) substantial neuronal degeneration that could be attenuated by chronic glutamate receptor blockade, and (4) a pronounced increase in local neurite outgrowth. These findings show that genetic targeting can be used to ablate scar-forming astrocytes and demonstrate roles for astrocytes in regulating leukocyte trafficking, repairing the BBB, protecting neurons, and restricting nerve fiber growth after injury in the adult central nervous system.

Impaired inflammatory response to glial cell death in genetically metallothionein-I- and -II-deficient mice

Metallothionein I+II (MT-I+II) are acute-phase proteins which are upregulated during pathological conditions in the brain. To elucidate the neuropathological importance of MT-I+II, we have examined MT-I+II-deficient mice following ip injection with 6-aminonicotinamide (6-AN). 6-AN is antimetabolic and toxic for bone marrow cells and grey matter astrocytes. In MT+/+ mice, injection with 6-AN resulted in breakdown of the blood-brain barrier (BBB) and absence of GFAP-positive astrocytes in specific grey matter areas of the brain stem. Reactive astrocytosis encircled the damaged grey matter areas, which were heavily infiltrated by microglia/macrophages. The recruitment of hematogenous macrophages was accompanied by leakage of the BBB. The immunoreactivity (ir) of granulocyte-macrophage-colony-stimulating factor (GM-CSF) and the receptor for GM-CSF (GM-CSFrec) was significantly upregulated in astrocytes and microglia/macrophages, respectively. MT-I+IIir was also clearly increased in astrocytes surrounding the damaged areas, while that of the CNS-specific MT isoform, MT-III, was mildly increased in both astrocytes and microglia/macrophages. In MT-/- mice injected with 6-AN, the BBB remained almost intact. The damage to specific grey matter areas was similar to that observed in MT+/+ mice, but reactive astrocytosis, microglia/macrophages infiltration, and GM-CSFir and GM-CSFrecir were clearly reduced in MT-/- mice. In contrast, MT-IIIir was dramatically increased in MT-/- mice. Total zinc decreased and histochemically detectable zinc increased in the brain stem after 6-AN similarly in MT+/+ and MT-/- mice. Bone marrow myeloid monocytes and macrophages were increased as a reaction to 6-AN only in MT+/+ mice. The results demonstrate that the capability of MT-/- mice to mount a normal inflammatory response in the brain is severely attenuated, at least in part because of 6-AN-induced bone marrow affectation, involving MT-I+II for the first time as major factors during CNS tissue damage.

Upregulation of COX-2 and CGRP expression in resident cells of the Borna disease virus-infected brain is dependent upon inflammation

Infection of immunocompetent adult rats with Borna disease virus (BDV) causes severe encephalitis and neural dysfunction. The expression of COX-2 and CGRP, genes previously shown to be implicated in CNS disease and peripheral inflammation, was dramatically upregulated in the cortical neurons of acutely BDV-infected rats. Neuronal COX-2 and CGRP upregulation was predominantly seen in brain areas where ED1-positive macrophages/microglia accumulated. In addition, COX-2 expression was strongly induced in brain endothelial cells and the number of COX-2 immunoreactive microglial cells was increased. In contrast, despite increased expression of viral antigens, neither COX-2 nor CGRP expression was altered in the CNS of BDV-infected rats treated with dexamethasone, or tolerant to BDV. Thus, increased CGRP and COX-2 expression in the BDV-infected brain is the result of the inflammatory response and likely to be involved in the pathogenesis of virus-induced encephalitis.

The acute inflammatory response in CNS following injection of prion brain homogenate or normal brain homogenate

The neuropathological hallmarks of end-stage prion disease are vacuolation, neuronal loss, astrocytosis and deposition of PrPSc amyloid. We have also shown that there is an inflammatory response in the brains of scrapie-affected mice from 8 weeks post-injection. In this study we have investigated the acute CNS response to the intracerebral injection of scrapie-affected brain homogenate. The ME7 strain of scrapie (Neuropathogenesis Unit, Edinburgh) was used, and control mice were injected with brain homogenate derived from normal C57BL/6 J mice. One microlitre of 10% w/v ME7 (n = 33) and normal brain homogenate (n = 28) was injected stereotaxically into the right dorsal hippocampus. Cryostat sections of brains taken at 1, 2, 5, 7, 14 and 28 days post-injection were examined histologically for neuronal loss, and immunocytochemically to study the inflammatory response. This study shows that ME7 is not acutely neurotoxic in vivo. There is also no difference (ANOVA) in the inflammatory response, which peaked between 2 and 5 days and resolved by 4 weeks after intracerebral injection of either ME7 or normal brain homogenate. The well circumscribed inflammatory response seen previously at 8 weeks is therefore a consequence of a disease process rather than a surgical artefact. This disease process may be related to a localized accumulation of PrPSc sufficient to stimulate an inflammatory response which in turn may contribute to neuronal loss. The role of the inflammatory response in chronic neurodegeneration can be usefully studied using this mouse model of prion disease, and this will undoubtedly shed light on the pathogenic mechanisms underlying other chronic neurodegenerative diseases.

Response of microglial cells after a cryolesion in the peripheral proliferative retina of tench

We studied the glial response after inducing a lesion in the zone of the peripheral retina of tench, where there is proliferative neuroepithelium. In the retina and optic nerve, the microglial response was analysed with tomato lectin and the macroglial response with antibodies against GFAP and S-100. In lesioned retinas, there was a temporal-spatial distribution pattern of microglia. One day after lesion, primitive ramified cells appeared in the nerve fibre layer. These cells appeared progressively from the vitreal to the scleral layers until day 7 when cells appeared in all layers, with the exception of the outer plexiform layer. From this point, labelling decreased. In the optic nerve, 3 days after lesion, an increase in the number of microglial cells was observed, first in the nerve folds and from day 15 in specific areas of the optic nerve. In the central retina, in the optic nerve head and within the optic nerve itself, the appearance of microglial cells, after the lesion, near the blood vessels, could indicate a vascular origin of microglia, as has been proposed by many authors. However, we cannot discount the idea that some of the reactive microglial cells arise by proliferation of the microglia existing in the normal state. Using GFAP and S-100 antibodies, no important changes in the retina were observed, however in the optic nerve there was response to the lesion. Thus, the macroglial cells appeared to be involved in reorganisation of the optic nerve axons after lesion.

Expression of COX-2 by normal and reactive astrocytes in the adult rat central nervous system

We have used a previously characterized antiserum against cycloxygenase-2 (COX-2) together with cold methanol fixation to immunohistochemically locate the protein in astrocytes in rat brain. Although in cerebral cortex most enzyme was located in neuronal perikarya as previously described, a number of glial fibrillary acidic protein (GFAP)-positive astrocytes were also labeled. No COX-2-positive neurons were seen in the cerebellum, but here also a subset of GFAP+ astrocytes was present which contained the enzyme. The number of COX-2-positive astrocytes increased considerably after injection of the neurotoxin kainate into the cerebellum. These immunohistochemical data were supported by semiquantitative RT-PCR results, which were used to assess the levels of COX-2 mRNA relative to the housekeeping gene hypoxanthine phosphoribosyl transferase. PGE2 levels were measured in contralateral and lesioned cerebellum to correlate changes in COX-2 immunoreactivity and mRNA with physiological events. PGE2 levels increased by 230% in the lesioned cerebellar hemispheres in comparison to the contralateral ones. We discuss the possibility that the targets for astrocytic prostaglandins might include both autocrine effects and paracrine responses of neurons, lymphocytes and capillary endothelial cells.

Poly(ADP ribose) polymerase cleavage precedes neuronal death in the hippocampus and cerebellum following injury to the developing rat forebrain

Transient unilateral forebrain hypoxia-ischaemia (HI) in 14-day-old rats produces infarction and delayed neuronal death in the frontal cortex. Cell death can also be observed in regions distant from the primary injury, a phenomenon known as diaschisis. While apoptosis is involved in selective neuronal death, its role in infarction and diaschisis remains poorly understood. Here, we have investigated the proteolytic cleavage of poly(ADP ribose) polymerase (PARP) and the occurrence of apoptosis in the hippocampus and the cerebellum following either HI or traumatic brain injury. We demonstrate that: (i) in vitro, PARP is cleaved during apoptosis but not necrosis in cultured neuronal (N1E) cells and Swiss 3T3 fibroblasts; (ii) following HI, apoptotic cells can be detected by 4 h after injury in the hippocampus; (iii) in the ipsilateral hippocampus the appearance of cells with apoptotic morphology is preceded by a dramatic increase in PARP cleavage in the same region, starting immediately following HI and persisting for 24 h; (iv) HI also induces apoptosis in the cerebellum and, as in the hippocampus, the appearance of cells with apoptotic morphology is preceded by PARP cleavage that is greater on the side ipsilateral to forebrain injury; and (v) similarly, traumatic brain injury to the forebrain leads to PARP cleavage and apoptosis in the cerebellum. We conclude that HI injury or traumatic injury to the developing rat forebrain leads to PARP cleavage in directly affected areas and in sites distant from the primary injury that precedes the appearance of cells with apoptotic morphology. Our results are consistent with a role for apoptotic cell death in infarction and in diaschisis resulting from forebrain injury to the developing brain.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Differential blood-brain barrier breakdown and leucocyte recruitment following excitotoxic lesions in juvenile and adult rats

Acute neuronal degeneration can be induced by intracerebral injections of the glutamate receptor agonists kainic acid (KA) and NMDA (N-methyl-D-aspartate). It is accompanied by an inflammatory response that has not yet been fully investigated. We have previously demonstrated that the juvenile rat brain is more susceptible to an inflammatory challenge when compared to adult rat brain. This study set out to investigate whether this also applied to the inflammatory response associated with acute neuronal degeneration. NMDA and kainic acid were injected into the rat striatum and lesion size, leucocyte recruitment, and blood-brain barrier (BBB) breakdown were assessed after 4, 8, 12, 24, 72, and 168 h. Both NMDA and KA induced lesions of similar volume at either age and apoptotic and necrotic nuclei could be detected. NMDA induced cellular loss by 4 h, whereas KA-injected rats did not show signs of neuronal loss until 8-12 h. The inflammatory response was characterized by an infiltration of neutrophils followed by macrophages. Juvenile rats showed a greater susceptibility to leucocyte recruitment compared to adult rats. BBB breakdown in response to NMDA injection occurred in the absence of cellular recruitment at 4 h in juveniles and was significantly greater in juvenile compared to adult rats at 8 h. BBB breakdown was minimal in KA-injected animals while at 7 days an influx of serum IgG coincided with a loss of astrocytic GFAP staining within the lesion.

Kainic acid-induced neuronal loss and glial changes in the hippocampal CA3 of p53-deficient mouse

We examined kainic acid (KA)-induced neuronal death and changes in glial cells in p53-deficient (p53-/-) and wild-type (p53+/+) mice which were CBA and C57BL/6 background. The p53-/- mouse exhibited a KA-induced loss of CA3 pyramidal neurons similar to that in wild-type mouse. Before neuronal death, c-Jun protein was expressed, phosphorylated and translocated into several nuclei of CA3 pyramidal neurons. In p53-/- mouse, microglial activation was slightly faster and more continuous after 1-7 days than that in p53+/+ mouse. On the other hand, p53-/- astrocytes were relatively resistant to KA cytotoxicity, and marked astrocytosis also occurred after 7 days. These observations suggest that p53-null mutation may influence the activation and proliferation of glial cells rather than neuronal death.

A revised view of the central nervous system microenvironment and major histocompatibility complex class II antigen presentation

There are numerous observations reporting that phagocytes expressing major histocompatibility complex (MHC) Class II molecules are associated with the central nervous system (CNS) in normal and pathological conditions. Although MHC Class II expression is necessary for antigen presentation to CD4 + T-cells, it is not sufficient and co-stimulatory molecules are also required. We review here recent in vivo studies demonstrating that the microglia and perivascular macrophages are unable to initiate a primary immune response in the CNS microenvironment, but may support secondary immune responses. Although in vitro studies show that microglia do not support a primary immune response leading to T-cell proliferation, they do show that microglia may protect the CNS from the unwanted attentions of autoreactive T-cells by inducing their apoptosis. The lack of cells in the CNS parenchyma with the ability to initiate a primary immune response has a cost, namely that pathogens may persist in the CNS undetected by the immune system.

Macrophage and microglia-like cells in the avian inner ear

Recent studies suggest that macrophages may influence early stages of the process of hair cell regeneration in lateral line neuromasts; numbers of macrophages were observed to increase prior to increases in hair cell progenitor proliferation, and macrophages have the potential to secrete mitogenic growth factors. We examined whether increases in the number of leukocytes present in the in vivo avian inner ear precede the proliferation of hair cell precursors following aminoglycoside insult. Bromodeoxyuridine (BrdU) immunohistochemistry was used to identify proliferating cells in chicken auditory and vestibular sensory receptor epithelia. LT40, an antibody to the avian homologue of common leukocyte antigen CD45, was used to label leukocytes within the receptor epithelia. Macrophages and, surprisingly, microglia-like cells are present in normal auditory and vestibular sensory epithelia. After hair cell loss caused by treatment with aminoglycosides, numbers of macrophage and microglia-like cells increase in the sensory epithelium. The increase in macrophage and microglia-like cell numbers precedes a significant increase in sensory epithelial cell proliferation. The results suggest that macrophage and microglia-like cells may play a role in releasing early signals for cell cycle progression in damaged inner ear sensory epithelium.

Quinolinic acid-induced inflammation in the striatum does not impair the survival of neural allografts in the rat

It has been suggested that inflammation related to intracerebral transplantation surgery can affect the survival of intrastriatal neural allografts. To test this hypothesis, we transplanted dissociated embryonic mesencephalic tissue from one of two rat strains, Lewis (allogeneic grafts) or Sprague-Dawley (syngeneic grafts), to the striatum of Sprague-Dawley rats. The target striatum was either intact or had received a local injection of quinolinic acid 9 days earlier, in order to induce a marked inflammation. At 6 or 12 weeks after transplantation, there was no significant difference between the different groups regarding the number of surviving grafted tyrosine hydroxylase immunoreactive neurons. However, the graft volume of both the syngeneic and allogeneic implants was significantly larger in the quinolinate-lesioned than in the intact striatum. There were dramatically increased levels of expression of major histocompatibility complex class I and II antigens, marked infiltrates of macrophages, activated microglia and astrocytes, and accumulation of large numbers of CD4 and CD8 positive T-lymphocytes in the quinolinate-lesioned striatum. In contrast, these immunological markers were much less abundant around both syngeneic and allogeneic grafts placed in intact striatum. We conclude that severe inflammation caused by quinolinic acid does not lead to rejection of intrastriatal neural allografts.

Kainic acid induced expression of interleukin-1 receptor antagonist mRNA in the rat brain

The endogenous interleukin-1 receptor antagonist (IL-1ra), a protein with partial homology with the proinflammatory cytokine interleukin-1beta (IL-1beta), prevents binding of IL-1beta to the signalling receptor. Exogenous IL-1ra has been shown to reduce the neuronal damage occurring after excitotoxic amino acid administration and ischemia. In the present study, in situ hybridization histochemistry was employed to investigate the regulation of endogenous IL-1ra mRNA expression in the rat brain after peripheral administration of kainic acid (10 mg/kg). IL-1ra mRNA expression was markedly induced in the hippocampus, thalamus, amygdala, piriform cortex, perirhinal cortex, entorhinal cortex, and to a lesser extent in the hypothalamus, and parietal and temporal cortex. The expression was first detected at 5 h after the kainic acid administration and it was markedly increased at 24 h. No signal was detected at 4 days after the injection. The majority of the cells expressing IL-1ra mRNA displayed the morphological characteristics of microglia. Expression of IL-1ra mRNA in neurons occurred mainly in the piriform and perirhinal cortex. The distribution pattern of IL-1ra mRNA expressing microglia-like cells was similar to that of cells labelled with ED1, a marker for activated microglia. The induction of IL-1ra mRNA expression may represent an endogenous response to balance IL-1 receptor mediated activity in the brain following kainic acid administration, conceivably to elicit neuroprotective and/or antiinflammatory effects.

The influence of plasma proteins on the distribution of leucocytes within the brain parenchyma in a murine model of stroke

Inflammatory responses are thought to play an important role in the exacerbation of neuronal loss following stroke. Leucocyte recruitment following cerebral ischaemia has been demonstrated in experimental animals, and procedures which reduce the entry of leucocytes into the brain reduce neuronal loss and improve aspects of functional recovery in these models. In this study we investigate whether leakage of plasma proteins into the central nervous system (CNS) following ischaemia influences leucocyte adhesion within the parenchyma. Using an in vitro adhesion assay, we demonstrate that the addition of exogenous serum proteins increases macrophage adhesion to CNS tissue. Following permanent middle cerebral artery occlusion (MCAO) in mice, plasma proteins leak into the apparently healthy cortex surrounding the infarcted area. We show that there is increased macrophage adhesion to sections in the border region where endogenous plasma proteins are present within the parenchyma. Using immunohistochemistry, we co-localize plasma protein distribution within the tissue with leucocyte recruitment following MCAO. We show that monocytes, not neutrophils, infiltrate the lesion border where plasma proteins are present in the parenchyma. This distribution is compatible with their contributing to neuropathology, whereas neutrophils are found in clusters in the lesion core. We conclude that leakage of plasma proteins into the brain could influence leucocyte adhesion within the parenchyma. Recruited monocytes may exacerbate neuropathology in situations such as permanent cerebral ischaemia, where disruption of the blood-brain barrier occurs.

Beta-actin immunoreactivity in rat microglial cells: developmental pattern and participation in microglial reaction after kainate injury

In the present study we investigated the developmental and post-injury pattern of beta-actin immunoreactivity in rat brain. Our data suggest that beta-actin is higher in microglia-macrophages than in any other central nervous system cell type, including neurons and astrocytes. We also show that beta-actin immunoreactivity is particularly high in ameboid-macrophagic cells, suggesting a role on the plastic changes that these cells experience during maturation or after activation.

Neuronal protection of oligodendrocytes from antibody-independent complement lysis

Cultured rat oligodendrocytes are lysed by complement via antibody-independent activation of the classical pathway. This susceptibility to complement lysis has been demonstrated to be due to lack of CD59, a complement regulatory protein which inhibits assembly of the membrane attack complex. In this study the effects of homologous and heterologous complement were examined in a co-culture system of rat oligodendrocytes and peripheral neurones, where axonal ensheathment was observed as early as 4 days after the addition of glial progenitors to the neurones. Following exposure to complement, ensheathing oligodendrocytes were markedly less sensitive to antibody-independent but not antibody-dependent complement lysis than were cells grown without neurones. Immunocytochemical data revealed that co-cultured oligodendrocytes remained CD59 negative, but in contrast to oligodendrocytes cultured alone, were negative for C3b when incubated with C7-deficient serum. Taken together these data indicate that the decreased sensitivity of co-cultured oligodendrocytes to complement lysis is not attributed to the increased expression of CD59, but rather in a failure to activate complement. Incubation of oligodendrocytes with neurone-conditioned medium afforded significant protection (68%), against antibody-independent complement attack, suggesting that soluble neuronal factors can protect oligodendrocytes from complement-mediated lysis.

Selective chemokine mRNA expression following brain injury

Injury in non-neuronal tissues stimulates chemokine expression leading to recruitment of inflammatory cells responsible for orchestration of repair processes. The signals involved in directing repair of damage to the brain are less well understood. We hypothesized that following brain injury, chemokines are expressed and regulate the rate and pattern of inflammatory cell accumulation. The two chemokine subfamilies are alpha(alpha)-chemokines, which primarily function as neutrophil chemoattractants, and the beta(beta)-chemokines, which function primarily as monocyte chemoattractants. We assessed alpha and beta chemokine mRNA expression patterns and leukocyte accumulation following a cerebral cortical lesion. Cortical lesions were produced with and without addition of endotoxin, Escherichia coli lipopolysaccharide (LPS), which stimulates cytokine expression. We studied the expression of the beta-chemokines: monocyte chemoattractant protein (gene product JE; MCP-1/JE), macrophage inflammatory protein-1 alpha and beta (MIP-1alpha and MIP-1beta), and the regulated upon activation normal T expressed and secreted chemokine (RANTES) as well as the alpha-chemokines: interferon-gamma-inducible protein (IP-10) and N51/KC (KC; a murine homologue of MIP-2). Changes in gene expression were analyzed by Northern analysis at different time points following injury. Leukocyte and macrophage densities were analyzed by immunohistochemistry at the same time intervals. All chemokines were elevated following cortical injury/endotoxin. MCP-1 and MIP-1alpha were elevated at 2 h and peaked 6 h, MIP-1beta peaked at 6 h, but declined more rapidly than MCP-1 or MIP-1alpha, and IP-10 peaked at 6 h and showed the most rapid decline. KC was elevated at 1 h, and peaked at 6 h following LPS. RANTES was elevated at 1 h and achieved a plateau level between 6 and 18 h, then declined. In contrast, sterile injuries produced in the absence of endotoxin only induced the mRNA of the beta-chemokine MCP-1, and its expression was delayed compared to the cortical injury/endotoxin group. The presence of chemokine message as early as 1 h indicates that expression of this class of molecules is an early response in the repair process following traumatic brain injury. Macrophage/microglia accumulation occurred more rapidly, activated microglia further from the lesion border, and more cells accumulated in cortical injury/endotoxin than in cortical lesions produced under sterile conditions. Thus, there was a positive correlation between beta-chemokine expression and the number of beta-chemokine responsive cells (i.e. microglia) accumulating in injury sites. This is the first comprehensive study using a panel of chemokine probes and specific marcophage/microglial markers to study in vivo activation of the brain following injury. Our data show that the brain is capable of expression of multiple chemokine genes upon appropriate stimulation (e.g. LPS-treatment). The gradient of microglial activation is consistent with physical damage stimulating release of chemokines that diffuse from the injury site. These data strongly suggest that chemokines are instrumental in the initiation of repair processes following brain injury.

Neurotoxic responses by microglia elicited by excitotoxic injury in the mouse hippocampus

BACKGROUND

Injury to the brain induces dramatic local changes in gene expression, cellular morphology and behavior. Activation of microglial cells occurs as an early event after central nervous system (CNS) injury, but it has not been determined whether such activation plays a causal role in neuronal death. We have investigated this question using an excitotoxin-mediated brain injury model system, in conjunction with an endogenous peptide factor (macrophage/microglial inhibiting factor, MIF) that ablates microglial contribution to the cascade.

RESULTS

Using MIF, we inhibited the microglial activation that normally follows excitotoxic injury. In cell culture studies, we found that such inhibition blocked the rapid release of microglia-derived tissue plasminogen activator (tPA), an extracellular serine protease made by both neurons and microglia, which we had previously identified as mediating a critical step in excitotoxin-induced neuronal death. Finally, infusion of MIF into the mouse brain prior to excitotoxic insult resulted in the protection of neurons from cell death.

CONCLUSIONS

Our results demonstrate that microglia undertake a neurotoxic role when excitotoxic injury occurs in the CNS. They also suggest that the tPA released from microglia has a critical role in triggering neurodegeneration.

Neuronal death in the hippocampus is promoted by plasmin-catalyzed degradation of laminin

Excess excitatory amino acids can provoke neuronal death in the hippocampus, and the extracellular proteases tissue plasminogen activator (tPA) and plasmin (ogen) have been implicated in this death. To investigate substrates for plasmin that might influence neuronal degeneration, extracellular matrix (ECM) protein expression was examined. Laminin is expressed in the hippocampus and disappears after excitotoxin injection. Laminin disappearance precedes neuronal death, is spatially coincident with regions that exhibit neuronal loss, and is blocked by either tPA-deficiency or infusion of a plasmin inhibitor, both of which also block neuronal degeneration. Preventing neuron-laminin interaction by infusion of anti-laminin antibodies into tPA-deficient mice restores excitotoxic sensitivity to their hippocampal neurons. These results indicate that disruption of neuron-ECM interaction via tPA/plasmin catalyzed degradation of laminin sensitizes hippocampal neurons to cell death.

An extracellular proteolytic cascade promotes neuronal degeneration in the mouse hippocampus

Mice lacking the serine protease tissue plasminogen activator (tPA) are resistant to excitotoxin-mediated hippocampal neuronal degeneration. We have used genetic and cellular analyses to study the role of tPA in neuronal cell death. Mice deficient for the zymogen plasminogen, a known substrate for tPA, are also resistant to excitotoxins, implicating an extracellular proteolytic cascade in degeneration. The two known components of this cascade, tPA and plasminogen, are both synthesized in the mouse hippocampus. tPA mRNA and protein are present in neurons and microglia, whereas plasminogen mRNA and protein are found exclusively in neurons. tPA-deficient mice exhibit attenuated microglial activation as a reaction to neuronal injury. In contrast, the microglial response of plasminogen-deficient mice was comparable to that of wild-type mice, suggesting a tPA-mediated, plasminogen-independent pathway for activation of microglia. Infusion of inhibitors of the extracellular tPA/plasmin proteolytic cascade into the hippocampus protects neurons against excitotoxic injury, suggesting a novel strategy for intervening in neuronal degeneration.

Neuronal death in the central nervous system demonstrates a non-fibrin substrate for plasmin

Mice deficient for plasminogen exhibit a variety of pathologies, all of which examined to date are reversed when the animals are also made fibrin(ogen) deficient. These results suggested that the predominant, and perhaps exclusive, physiological role of plasminogen is clearance of fibrin. Plasminogen-deficient mice also display resistance to excitotoxin-induced neurodegeneration, in contrast with wild-type mice, which are sensitive. Based on the genetic interaction between plasminogen and fibrinogen, we investigated whether resistance to neuronal cell death in the plasminogen-deficient mice is dependent on fibrin(ogen). Unexpectedly, mice lacking both plasminogen and fibrinogen are resistant to neurodegeneration to levels comparable to plasminogen-deficient mice. Therefore, plasmin acts on substrates other than fibrin during experimental neuronal degeneration, and may function similarly in other pathological settings in the central nervous system.

The blood-brain barrier and the inflammatory response

The environment of the brain is controlled by a sophisticated endothelial barrier that prevents the free entry of solutes from the blood. It is commonly assumed that this blood-brain barrier (BBB) also prevents the entry of leukocytes into the central nervous system. However, recent evidence in animal models shows that this is not the case, and leukocytes can cross an intact BBB during health and disease. Indeed, in many neurological diseases, including Alzheimer's disease, prion diseases and AIDS-related dementia, leukocytes enter the brain parenchyma without concomitant BBB breakdown. Current research is concentrating on factors that control the integrity of the BBB and the mechanisms that leukocytes use to enter the brain.

Temporal profiles of the in vitro phosphorylation rate and immunocontent of glial fibrillary acidic protein (GFAP) after kainic acid-induced lesions in area CA1 of the rat hippocampus: demonstration of a novel phosphoprotein associated with gliosis

The in vitro phosphorylation rate and immunocontent of glial fibrillary acidic protein was studied in slices of area CA1 of the rat hippocampus after stereotaxic injection of 1 nmol of kainic acid. For controls the contralateral hippocampus was injected with saline. Hippocampal tissue was incubated with [32P]phosphate and analysed by two-dimensional electrophoresis for phosphorylation rate and by immunoblotting for immunocontent. Both these parameters decreased during the first 4 days after injection and then started to increase at 10 days and continued to increase until at least 84 days. Except for a small excess of phosphorylation rate at 28 days, the relationship between immunocontent and in vitro phosphorylation rate of glial fibrillary acidic protein remained constant, indicating that the reactive gliosis was not associated with hypo- or a major hyperphosphorylation of this protein. Histology showed a pronounced loss of CA1 pyramidal cells 1 day after injection. At 28 days after injection the pyramidal cells had disappeared and only a few abnormal neurones were present. In contrast, immunocytochemistry after 28 days showed a marked increase in astrocytes reacting positive to the antibody in the strata radiatum and lacunosum moleculare. Besides glial fibrillary acidic protein the expression of several other proteins was upregulated as a result of the injection of kainic acid. These included phosphovimentin and an unknown phosphoprotein designated pp25 which co-migrated on 2-D gels with a prominent phosphoprotein expressed in primary cultures of astrocytes. Pp25 was expressed in lesioned tissue more frequently than phosphovimentin and with a time course that started earlier. Of particular interest was the expression of pp25 in the contralateral saline-injected hippocampus 1 day after injection of kainic acid. It is possible that pp25 will prove to be a sensitive marker of gliosis.

Gliosis in the LP-BM5 murine leukemia virus-infected mouse: an animal model of retrovirus-induced dementia

Mice infected with the LP-BM5 murine leukemia virus (MuLV) mixture develop severe immunosuppression, neurotransmitter abnormalities and cognitive impairments in the absence of significant viral or macrophage invasion of the CNS. The time-course of the changes in glial activation have been characterized in an effort to understand the cellular basis of the neurobehavioral abnormalities observed in these mice. Glial activation was determined by measuring the relative changes in F4/80 protein and GFAP immunoreactivity using immunoblots. Augmented F4/80 expression preceded that of GFAP, with global elevations of 4-6-fold at 3 weeks, sustained for up to 12 weeks after inoculation. GFAP immunoreactivity increased 2-fold only in the cerebral cortex and striatum 5 weeks postinfection, declining to control levels by 12 weeks. Immunohistochemistry revealed significant increases in microglial size and staining intensity in the cortex, corpus callosum and striatum, with the development of a unique population of highly ramified, intensely stained microglia and microglial nodules in the corpus callosum and striatum. No evidence of ameboid microglia was found. Astrocyte size and degree of ramification was increased in the hippocampus, cortex, striatum and corpus callosum. Thus, microgliosis is an early event in LP-BM5 infection, preceding astrocytosis, neurotransmitter loss, and development of cognitive deficits. Activated microglia may secrete neurotoxins leading to the neurochemical alterations and cognitive deficits observed in these mice. Because gliosis and microglial nodule formation are hallmarks of HIV-1 encephalopathy, LP-BM5 MuLV-infected C57/B16 mice may afford insights into the mechanisms contributing to the early stages of this syndrome.

Alterations in apolipoprotein E expression during aging and neurodegeneration

Apolipoprotein E (apoE) is a 34 kDa protein that plays an important role in cholesterol transport, uptake and redistribution. Within the nervous system, apoE might be involved in maintaining synaptic integrity after injury and during aging. ApoE might help maintain the integrity of the synaptodendritic complex by several different mechanisms. Among them, recent studies have suggested that apoE: (1) stabilizes the neuronal cytoskeleton; (2) plays an important role in transporting esterified cholesterol to neurons undergoing reinnervation where it is taken up by the low density lipoprotein receptor-related protein pathway and used as a precursor for the synthesis of new synaptic terminals; (3) regulates interactions between neurons and the extracellular matrix (e.g. laminin); and (4) regulates levels of intracellular calcium. The main objective of the manuscript is to review the current progress in understanding the functions of apoE in the nervous system and how malfunctioning of this molecule might result in neurodegenerative disorders such as Alzheimer's disease.

Microglial activation by epileptic activities through the propagation pathway of kainic acid-induced hippocampal seizures in the rat

We studied the distribution of activated microglia in the brain during kainic acid-induced acute hippocampal seizures in rats. Although no microglial activation was observed 4 h after seizure induction, activation was detected in the primary focus and also in other selected structures in the limbic and non-limbic structures after 8 and 24 h, in the absence of any obvious morphological changes in the neurons. The structures with activated microglia were highly consistent with those included in the propagation pathways of the hippocampal seizures. These findings thus suggest that the microglial cells are activated through the propagation pathways by the seizure activities that propagate transynaptically from the primary focus, even without any apparent neuronal injury.

Recombinant human adenovirus with rat MIP-2 gene insertion causes prolonged PMN recruitment to the murine brain

Single injections of recombinant cytokines/chemokines into tissue have provided insights into their possible roles during the inflammatory response. Adenoviral technology may allow us to mimic the in vivo situation more closely, with protein generated in a continuous but transient fashion. Replication-deficient human type 5 adenovirus containing a rat macrophage inflammatory protein-2 (MIP-2) gene insertion and cytomegalovirus promoter was injected into the mouse brain to investigate the inflammatory response to continuous overproduction of MIP-2. Adenovirus with a LacZ gene insertion expressing beta-galactosidase was used as a control. At doses of 10(4) to 10(7) plaque-forming units, a minimal inflammatory response was detected to the LacZ virus, with leukocyte recruitment that was restricted to the injection site. A dose of 10(7) plaque-forming units of both the LacZ and the MIP-2 vector produced extensive transgene product expression that persisted for at least 7 days. Astrocytes, recognized by their morphology, were the predominant cell type expressing MIP-2 and beta-galactosidase. A dose of 10(7) plaque-forming units of MIP-2 vector caused dramatic polymorphonuclear leukocyte (PMN) recruitment to the brain parenchyma after 2 days. PMN recruitment was still observed after 4 and 7 days, but had become more localized to the injection site and was associated with numerous foam-like macrophages. At both 2 and 7 days the blood-brain barrier was breached in the region of leukocyte recruitment. Despite the extent of leukocyte recruitment there were no overt signs of neuronal degeneration or demyelination. Our findings demonstrate that continuous production of MIP-2 in the CNS results in persistent PMN recruitment to the brain parenchyma with no evidence of tachyphylaxis. The lack of PMN recruitment to the brain parenchyma following CNS injury may be a result of deficient production of PMN chemoattractants.

Evidence for an early inflammatory response in the central nervous system of mice with scrapie

In Alzheimer's disease, the most prevalent of the neurodegenerative diseases, inflammation of the CNS contributes to the pathology and is a target for therapy. In contrast, the group of neurodegenerative conditions known as the Prion Diseases have been widely reported as lacking any inflammatory elements despite the many similarities between the pathologies of Alzheimer's Disease and Prion Diseases We have found evidence for an inflammatory component in mouse scrapie, characterized by microglial activation and T-lymphocyte recruitment, which appears long before any clinical signs of the disease and spreads along well-defined anatomical pathways. These observations emphasize the potential value of murine scrapie as a model for studying the inflammatory pathology of other neurodegenerative diseases.

Unusual aspects of inflammation in the nervous system: Wallerian degeneration

Wallerian degeneration in the PNS is accompanied by the rapid recruitment of monocytes, but monocytes do not invade CNS fibre tracts undergoing Wallerian degeneration. In recent years it has become apparent that the acute inflammatory response to cell degeneration in the CNS is unlike that in other tissues. We have been interested to learn why Wallerian degeneration does not provoke a typical inflammatory response. We investigated whether the vascular endothelial cells express adhesion molecules during Wallerian degeneration in PNS and CNS. We found that in the degenerating sciatic nerve there was upregulation of ICAM-1 and VCAM-1 expression on endothelial cells in the distal stump of the injured nerve as well as at the site of the lesion. However, in the degenerating optic nerve, the endothelium failed to upregulated these molecules in the distal stump of the nerve and ICAM-1 expression was only increased in the crush site. The lack of adhesion molecule expression on CNS endothelium molecules may be an explanation for the poor leukocyte recruitment during Wallerian degeneration in CNS when compared with PNS.

Production of monocyte chemotactic protein-1 by rat brain macrophages

In the present study, we show that cultured rat brain macrophages release a soluble factor that stimulates the migration of bone marrow-derived macrophages, as determined by an in vitro chemotaxis assay. A checkerboard analysis indicated that most of this effect resulted from a polarized migration of the cells (chemotactic phenomenon), rather than in an increase in cell motility (chemokinesis). This activity was significantly decreased by an immune serum directed against the rat monocyte chemoattractant protein-1 (chemokine MCP-1). Northern blot analysis demonstrated expression of the MCP-1 gene in cultured brain macrophages, but its absence in unstimulated bone marrow-derived macrophages. Up-regulation of MCP-1 expression was observed when lipopolysaccharide was added to cultured brain macrophages, a peak occurring after a 6 h period of stimulation. Also, inflammatory cytokines such as interleukin (IL)-1 beta, colony stimulating factor-1, tumour necrosis factor-alpha and IL-6 individually increased the basal level of MCP-1 mRNA. Subsequently, we demonstrated the in vivo production of MCP-1 in the adult rat brain following injury induced by a local injection of kainic acid. MCP-1 synthesis was localized in both astrocytes and brain macrophages. These results suggest that the activation of resting microglial cells into brain macrophages and their subsequent secretion of chemokines could contribute to the mechanism(s), leading to the infiltration of the CNS by blood-derived monocytes, as observed in several pathologies.

Microglial response to N-methyl-D-aspartate-mediated excitotoxicity in the immature rat brain

The intracerebral injection of N-methyl-D-aspartate (NMDA) has been proposed as a model for hypoxic-ischemic insult in the immature brain. In this light, the aim of this study was to describe the time course of the microglial reaction in the areas undergoing primary degeneration at the site of intracortical NMDA injection as well as in areas undergoing secondary anterograde and/or retrograde degeneration. Fifty nanomoles of NMDA were injected in the sensorimotor cortex of 6-day-old rats. After survival times ranging from 10 hours to 28 days, cryostat sections were stained for routine histology and for the demonstration of microglial cells by means of tomato lectin histochemistry. The areas affected by primary degeneration caused by the intracortical injection of NMDA were the neocortex, the hippocampus, and the rostral thalamus. Secondary degeneration (retrograde and anterograde) was observed in the ventrobasal complex of the thalamus. The cortical lesion also caused Wallerian degeneration of the cortical descending efferents as observed in the basilar pons. Microglial reactivity in all these areas was present at 10 hours postinjection and was restricted to the areas undergoing neuronal or axonal degeneration. Reactive microglial cells were stained intensely and showed a round or pseudopodic morphology. At 3 days, an apparent increase in the number of tomato lectin-positive cells was observed in the areas undergoing neuronal death. By 7 days after the injection, the lesion became nonprogressive, and by 14 and 28 days, microglial cells showed moderate lectin binding and a more ramified morphology.

Activation and proliferation of murine microglia are insensitive to glucocorticoids in Wallerian degeneration

Activation and proliferation of microglia are commonly described in the central nervous system after a wide range of insults, but the mechanisms that regulate their phenotype in vivo are still poorly understood. We have studied the effect that adrenalectomy and dexamethasone treatment have on the proliferation and activation of microglia during Wallerian degeneration of the optic nerve in BALB/c mice. We found that the onset and rate of microglia proliferation is independent of glucocorticoids. There was an increase in F4/80-positive cells 3 days after optic nerve crush, with a peak at 7 days, both in the optic nerve and its target, the superior colliculus. The numbers of F4/80-positive cells remained high up to 3 weeks after crush, the longest time point examined. We also found that up-regulation of F4/80 and the complement receptor type 3 and expression of major histocompatibility complex class II antigens were not affected by adrenalectomy or dexamethasone treatment. These observations show that, unlike microglia in vitro or peripheral macrophages, microglia do not readily respond to glucocorticoids, which could indicate a lack of or reduced expression of glucocorticoid receptor in these cells.

The effects of interleukin-1 receptor antagonist protein (IRAP) infusion following spinal cord transection in rats

A laminectomy was performed at the T5-T6 vertebral level in adult, male, Sprague-Dawley rats and the spinal cord transected with a scalpel. A group of sham animals was subjected to the same surgery without the transection step. A group of unhandled control rats was also included. A subgroup of transected animals received a subcutaneous osmotic minipump that dispensed IL-1 receptor antagonist protein (IRAP) at the transection site for 7 consecutive days. Another transected subgroup received a minipump that infused the vehicle only. IRAP-treated rats displayed a significant reduction in body temperature (p < 0.05) compared with vehicle-treated rats. The IRAP-treated rats were also less active when assessed for locomotor behavior using an HVS computerized tracking system (p < 0.01). IRAP treatment had no effect on serum corticosterone, beta-endorphin levels, Con A, PHA, or LPS-induced splenocyte mitogenesis when compared with vehicle-treated animals. However, half of the IRAP-treated animals exhibited a substantive reduction in the number of reactive astrocytes near the transection site, suggesting a possible effect of IRAP on astrocyte activation.

Removal of cobalt-labeled neurons and nerve fibers by microglia from the frog's brain and spinal cord

We investigated the microglial reaction around cobalt-labeled degenerating neurons and nerve fibers in the frog central nervous system. The aim of these studies was to reveal the routes of migrating microglial cells during debris removal and the effect of seasonal changes on this process in a cold-blooded animal. Oculomotor and spinal motoneurons were filled with cobaltous-lysine complex through their axons. In the torus semicircularis and the isthmic nucleus, neurons were labeled with iontophoretically applied cobaltous-lysine complex through their injured dendrites and axons. The animals were left to survive for 1 to 50 days. During the summer, oculomotor neurons disintegrated by the seventh postoperative day. The debris from the neurons were phagocytosed by microglia-like cells identified by the presence of cobalt in their cytoplasm. Some of these cells were wedged between ependymoglial cells of the cerebral aqueduct, others appeared at the pial surface of the mesencephalon. The speed of this process was twice as fast during the summer as during the winter. Part of cobalt-labeled microglial cells in the torus semicircularis and the isthmic nucleus moved toward the ependyma of the optic ventricle and the cerebral aqueduct, respectively. Cobalt-loaded microglial cells did not move toward the surface in the spinal cord and the deep part of mesencephalic tegmentum, and left the brain probably via blood vessels. We conclude that microglial cells loaded with phagocytosed tissue debris may leave the brain tissue via three routes and their activity depends on the environmental temperature.