Cytoskeletal Pathology Research Articles

Neurofibrillary pathology in transgenic mice overexpressing V717F beta-amyloid precursor protein.

Overexpression of mutated human amyloid precursor protein (hAPP717V-->F) under control of the platelet-derived growth factor promoter (PDAPP minigene) in transgenic (tg) mice results in plaque formation and astroglial activation similar to Alzheimer disease (AD). However, the extent of the neurofibrillary pathology in this model is less understood. In order to determine if these mice develop AD-like neurofibrillary pathology, vibratome sections from PDAPP tg mice (4- to 20-months-old) were immunolabeled with antibodies against phosphorylated tau (AT8) and phosphorylated neurofilaments (SMI 312, TA51), and analyzed by laser scanning confocal and electron microscopy. Phosphorylated neurofilament-immunoreactive dystrophic neurites in plaques were first seen in mice at 10 to 12 months of age, while phosphorylated tau-immunoreactive dystrophic neurites were observed after 14 months of age. Immunoelectron microscopic analysis revealed that phosphorylated neurofilament immunoreactivity was diffusely distributed along filamentous aggregates (12-15 nm in diameter) in the plaque dystrophic neurites, and occasionally in neuronal cell bodies. In contrast, phosphorylated tau immunoreactivity was observed as clusters distributed along filamentous structures accumulating in the dystrophic neurites and around neurotubules in the axons. However, no paired helical filaments were observed. Taken together, these studies indicate that the PDAPP tg model recapitulates early cytoskeletal pathology similar to that observed in AD.

Organization and functional roles of the cytoskeleton in oligodendrocytes

Mature oligodendrocytes are characterized by their numerous cytoplasmic extensions and flat membranous sheets. These sheets contain an extensive cytoskeletal network of microtubules (MTs) that maintain the cellular morphology, are specifically important for cellular sorting, and provide the rails for organelle trafficking. Mitochondria are localized in the primary and secondary processes and follow the tracks of the MTs in the cytoplasmic extensions. Oligodendrocytes express microtubule associated proteins (MAPs), specifically MAP2 and tau, which might be involved in the regulation and stabilization of the dynamic MT network in the myelin-containing cellular processes. Tau and MAP2 heterogeneity increases during oligodendroglia maturation, and in mature oligodendrocytes tau mRNA with four MT binding domains are more prominent than in progenitor cells. Filamentous cell inclusions are a unifying mechanism underlying a variety of late-onset neurodegenerative disorders and have mainly been viewed as neuron-specific. Recent evidence indicated that glial changes occur in CNS degenerative diseases and seem to be a more common feature than previously thought. Glial fibrillary tangles (GFTs) in oligodendrocytes were observed in familial multiple system tauopathy, and glial cytoplasmic inclusions (GCIs) and oligodendroglia degeneration are the histological hallmark of multiple system atrophy (MSA). GCIs are associated with MTs and contain stress proteins and MAPs. Thus, neurons and glial cells share common cytoskeletal pathologies. During health and disease, MAPs might be important regulators of the structural stability and plasticity of the oligodendroglia cytoskeleton.

Neurofilament-Immunoreactive Neurons Are Not Selectively Vulnerable in Alzheimer's Disease

Abnormal neurofilament protein distribution and phosphorylation contributes to the cytoskeletal pathology of Alzheimer's disease. Anatomical studies suggest that cortical neurons immunoreactive for nonphosphorylated 200-kDa neurofilament are most vulnerable. We repeated these studies in formalin-fixed temporal lobe tissue from five Alzheimer's disease cases with tissue volume loss compared to five controls without tissue loss. Immunohistochemistry for nonphosphorylated and phosphorylated forms of the neurofilament protein was counterstained for Nissl substance and immuno-positive and -negative pyramidal neurons quantified using areal fraction counts. Compared with controls, cases with Alzheimer's disease had similar numbers of neurons expressing the nonphosphorylated neurofilament protein, suggesting these neurons are largely spared by the disease process. In Alzheimer's disease there was a significant increase in neurons containing phosphorylated neurofilament and tau proteins and a decrease in neurons devoid of neurofilament protein. Our results challenge the theory that neurons containing 200 kDa neurofilament are selectively vulnerable in Alzheimer's disease.

Cell death mechanisms in neurodegeneration.

Progressive cell loss in specific neuronal populations often associated with typical cytoskeletal protein aggregations is a pathological hallmark of neurodegenerative disorders, but the nature, time course and molecular causes of cell death and their relation to cytoskeletal pathologies are still unresolved. Apoptosis or alternative pathways of cell death have been discussed in Alzheimer's disease and other neurodegenerative disorders. Apoptotic DNA fragmentation in human brain as a sign of neuronal injury is found too frequent as to account for continuous neuron loss in these slowly progressive processes. Morphological studies revealed extremely rare apoptotic neuronal death in Alzheimer's disease but yielded mixed results for Parkinson's disease and other neurodegenerative disorders. Based on recent data in human brain, as well as in animal and cell culture models, a picture is beginning to emerge suggesting that, in addition to apoptosis, other forms of programmed cell death may participate in neurodegeneration. Better understanding of the molecular players will further elucidate the mechanisms of cell death in these disorders and their relations to cytoskeletal abnormalities. Susceptible cell populations in a proapoptotic environment show increased vulnerability towards multiple noxious factors discussed in the pathogenesis of neurodegeneration. In conclusion, although many in vivo and in vitro data are in favor of apoptosis involvement in neurodegenerative processes, there is considerable evidence that very complex events may contribute to neuronal death with possible repair mechanisms, the elucidation of which may prove useful for future prevention and therapy of neurodegenerative disorders.

The evolution of Alzheimer's disease-related cytoskeletal pathology in the human raphe nuclei.

The cross-sectional analyses currently available show that the Alzheimer's disease (AD)-related cytoskeletal alterations within the human brain affect variously susceptible areas of the cerebral cortex in a uniform sequence with very little interpatient variability. This sequence has been divided for research and comparative purposes into six stages (cortical NFT/NT-stages I-VI). Among the subcortical nuclei affected in AD are those belonging to the raphe system. Efforts were focused on the lesions present in these nuclei to see in which of the six stages the AD-related cytoskeletal anomalies begin and whether a correlation exists between the AD-related pathology developing within the cerebral cortex and the cytoskeletal damage that occurs in the nuclei of the raphe system. To this end, serial sections from the brainstems of 27 post-mortem cases with stages I-VI of cortical cytoskeletal lesions were examined. The cytoskeletal pathology was visualized using the modified silver iodide-Gallyas staining technique and the antibody AT8. The latter is directed specifically against the abnormally phosphorylated cytoskeletal protein tau. The dorsal raphe nucleus manifests the cytoskeletal lesions early on (stages I-II). The central and linear raphe nuclei, by contrast, do so initially in stages III-IV, and the caudal raphe nuclei register the first changes in stages V-VI. In stages V and VI, the dorsal raphe nucleus displays the most severe cytoskeletal pathology within the raphe system, followed by the central and linear raphe nuclei, whereas the cytoskeletal anomalies in the caudal raphe nuclei are slight. The developing damage within the nuclei of the raphe system correlates with the stages I-VI and, furthermore, progresses in the oral raphe nuclei in close connection with the evolution of the pathological process in cortical projection destinations of these nuclei. As the source of the ascending serotonergic system, the involvement of the oral raphe nuclei may be partially responsible for the early manifestation of the non-cognitive and emotional deficiencies possibly traceable to dysfunctions within the ascending serotonergic system.

Vulnerability of select neuronal types to Alzheimer's disease.

From a morphological perspective, Alzheimer's disease (AD) is primarily a degenerative disorder of the neuronal cytoskeleton involving lipofuscin-laden cortical projection neurons with long, thin, and sparsely myelinated axons. The neocortical primary fields, relatively small in extent but functionally sophisticated, exhibit an early and brief myelination cycle, whereas the much more expansive but relatively simply organized association areas undergo a late and prolonged myelination process. The greater the degree of myelination and the less intense the pigmentation, the more resistant a given projection neuron may be to oxidative stress as well as to the development of AD-related neurofibrillary changes and vice versa. The neurofibrillary pathology commences from those cortical areas that are less completely myelinated and gradually progresses to the most functionally developed cortical fields that display the highest degree of myelination, thereby reflecting a hierarchy in the susceptibility of diverse cortical areas to the evolution of the AD-associated cytoskeletal pathology.

Filamentous tau pathology in nerve cells, astrocytes, and oligodendrocytes of aged baboons.

Intracellular filamentous inclusions containing abnormally phosphorylated tau protein are hallmarks of several human neurodegenerative disorders. This study reveals tau-positive cytoskeletal abnormalities in neurons and glial cells of aged baboons. The brains of four baboons (Papio hamadryas, 20-30 yr of age) were examined using the Gallyas silver technique for neurofibrillary changes and phosphorylation-dependent anti-tau antibodies (AT8, AT100, AT270, PHF-1, TG-3). Conspicuous changes were noted in two animals, 26 and 30 yr of age. In both animals, a combination of neuronal and glial cytoskeletal pathology was seen preferentially affecting limbic brain areas, including the hippocampal formation. In the 30-yr-old animal, numerous tau-positive inclusions were seen in the granule cells of the fascia dentata. These cells even exhibited an accumulation of argyrophilic neurofibrillary tangles. The glial changes affected both astrocytes and oligodendrocytes. Tau-positive astrocytes were seen in perivascular, subpial, and subependymal locations. Tau-positive oligodendrocytes preferentially occurred in limbic fiber tracts including the entorhinal perforant path. Ultrastructurally, tau-positive straight filaments (10-14 nm) in both neurons and glial cells were revealed by anti-tau immunoelectron microscopy. This study thus indicates the potential usefulness of aged baboons for experimental investigation of neuronal and glial filamentous tau pathology. This nonhuman primate species may provide valuable information pertinent to the broad spectrum of human tauopathies.

Increased Cyclin G1 Immunoreactivity During Alzheimer's Disease.

Numerous proteins are alternatively expressed in neurons and glia during Alzheimer's disease (AD) and may contribute to the regulation of neuronal cell death or function in regenerative responses to neuronal injury. A recently described member of the cyclin gene family, cyclin G1, is expressed in post-mitotic neurons in the adult rat brain and is expressed at high levels after brain injury. In the current study we examined the expression and subcellular distribution of cyclin G1 in non-demented adult and AD brain. While low levels of cyclin G1 protein were observed in pyramidal neurons in control brain, abundant cyclin G1 immunoreactivity was present in the cytoplasm of pyramidal neurons in the neocortex and hippocampus of AD brain. Cyclin G1 immunoreactivity was not present in cells containing neurofibrillary pathology. Our results indicate that cyclin G1 is expressed in human adult brain and exhibits increased immunoreactivity in the cytoplasm of pyramidal neurons in AD. In addition, cyclin G1 immunoreactivity was not evident in cells containing cytoskeletal pathology.

Glycogen Synthase Kinase-3 Modulates Neurite Outgrowth in Cultured Neurons: Possible Implications for Neurite Pathology in Alzheimer's Disease.

Glycogen synthase kinase-3 (GSK-3) is thought to play an important role in the hyperphosphorylation of tau, and possibly other proteins, in Alzheimer's disease (AD). However, the effects of GSK-3 on neuronal metabolism are still largely unknown. Here we describe that a low concentration of lithium, which can partially inhibit endogenous GSK-3, favored the extension of neurites from developing neurons, whereas a high concentration of lithium impaired neurite growth. Furthermore, the overexpression of exogenous active GSK-3 in neurons by infection with a defective herpesviral vector blocked neurite growth, which was not affected by either expression of inactive GSK-3 or just the herpesviral vector infection. Neurite extension was restored when neurons overexpressing exogenous active GSK-3 were incubated with lithium. These results are consistent with a role for GSK-3 in the regulation of cytoskeletal dynamics during neurite growth. Accordingly, up-regulation of GSK-3 may contribute to cytoskeletal pathology within neurites in AD.

Re-entry into the cell cycle: a mechanism for neurodegeneration in Alzheimer disease

Several recent findings demonstrated increased expression of cell cycle-related proteins in the degenerating neurons found in Alzheimer disease. We hypothesize that this apparent attempt to re-enter the cell cycle is a neuronal response to external growth stimuli that leads to an abortive re-entry into the cell cycle. However, since neurons of adults apparently lack the capacity both to divide in vivo and in vitro, it is possible that they lack the components necessary to complete the cell division process. Nonetheless, the importance of these findings is that they provide an explanation for the increased phosphorylation of cytoskeletal proteins such as τ and neurofilaments that represent the most striking intracellular changes in the disease. Further, it is our contention that inappropriate re-entry into the cell cycle and interrupted mitotic processes are significant factors not only in the cytoskeletal pathology but also in the neuronal degeneration that characterizes the pathology of Alzheimer disease.

Cytoskeletal pathology in familial cerebral amyloid angiopathy (British type) with non-neuritic amyloid plaque formation.

The histological features of familial cerebral amyloid angiopathy (British type) with non-neuritic amyloid plaque formation (FAB) include deposition of amyloid, (supposedly associated with the C-terminal fragments of both alpha- and beta-tubulin), in small cerebral and spinal arteries, hippocampal amyloid plaques and neurofibrillary tangles (NFTs) as well as ischaemic white matter changes. In the present study we report on the cytoskeletal pathology that occurs in association with FAB. Sections from the hippocampus and cerebellum of three cases from three unrelated families were stained with silver impregnation methods and antibodies to antigens including tau, neurofilaments, ubiquitin and glial fibrillary acidic protein. Electron microscopic examination of the hippocampus was carried out in one case. All hippocampal subregions contained large numbers of NFTs and neuropil threads (NT), which were stained with both phosphorylation-dependent and phosphorylation-independent tau antibodies and ultrastructurally were found to be composed of paired helical filaments (PHFs). Although the majority of the amyloid plaques were of the non-neuritic type, distended PHF-containing and tau-positive neurites were seen in close proximity of a minority of the hippocampal plaques. The perivascular amyloid deposits of the cerebellum contained numerous ubiquitin-positive granular elements similar to those seen in cerebellar A beta amyloid plaques in Alzheimer's disease. In FAB severe cytoskeletal pathology is present in areas most affected by amyloid plaque deposits, thus suggesting a localised neurotoxic effect of the poorly characterised amyloidogenic peptide characteristic of this condition.

Discrepancy between tau immunoreactivity and argyrophilia by the Bodian method in neocortical neurons of corticobasal degeneration.

To clarify different features of cytoskeletal pathology in neocortical neurons between corticobasal degeneration (CBD) and Alzheimer's disease (AD), the relationship between neurofibrillary tangles (NFTs), which are defined as a fibrillary structure stained by the Bodian method, and tau-immunopositive neurons in layers II-III of the premotor cortex was assessed by sequential staining for tau immunohistochemistry followed by the Bodian method on the same section. In AD brains, tau-like immunoreactivity in neurons was uniformly colocalized with Bodian-positive NFTs. In CBD brains, however, tau-immunopositive neurons could be classified into three different types: (1) those not stained at all by the Bodian method (diffuse cytoplasmic type), which represented the majority of these immunopositive neurons, (2) some partly stained by the Bodian method (mixed type), and (3) a few for which argyrophilia demonstrated by the Bodian method was completely colocalized with immunoreactivity (NFT type). This discrepancy between tau-like immunoreactivity and Bodian method is characteristic of CBD and suggests that the tau molecule is less liable to form argyrophilic fibrils in neocortical neurons of CBD.

Phosphorylation of Tau, Aβ-Formation, and Apoptosis After In Vivo Inhibition of PP-1 and PP-2A

Chronic inhibition of protein phosphatases 1 and 2A in vivo was induced by infusion of okadaic acid into lateral ventricles of rat brain for up to 4 months. Cytoskeletal pathology, alterations of the amyloid precursor protein, and apoptotic cell death induced by this treatment followed a certain sequence and spatial distribution. Changes in the expression, phosphorylation, and subcellular distribution of neurofilament proteins and tau, as well as first signs of apoptotic cell death, occurred already after about 2 weeks. The distribution of apoptotic cells, however, was different from those revealing a high accumulation of hyperphosphorylated tau, indicating that those cytoskeletal pathology had no obvious sequelae for the viability of these neurones. A continuation of treatment for longer than 2 weeks induced diffuse deposits of both hyperphosphorylated tau and Aβ-amyloid-immunoreactive material in white matter areas that increased in size and number over time. Because τ-phosphorylation is a regulator of the dynamic stability of microtubules, the pathology observed in the present experimental paradigm in the white matter might be viewed as an indication of a disturbed axonal transport. It is hypothesized that perturbations of the axonal transport might also be critically involved in the formation of paired helical filaments and amyloid deposits in Alzheimer’s disease.

Relationship between plaques, tangles, and loss of cortical cholinergic fibers in Alzheimer disease.

Recent observations in our laboratory have indicated substantial and systematic regional variations in the loss of cortical cholinergic fibers in Alzheimer disease (AD). Previous attempts to study the relationship between cortical cholinergic loss and the density of cortical pathological lesions have resulted in conflicting findings. Furthermore, most reports have correlated density of plaques and tangles with the residual level of cholinergic innervation rather than its loss. The purpose of the present study was to determine the relationship between loss of cholinergic axons and density of tangles and beta-amyloid (Abeta) deposits in various cortical areas of AD brains. Abeta deposits and tangles were observed throughout the cerebral cortex. Quantitative analysis revealed almost no correlation between loss of cholinergic fibers and the density of Abeta deposits. Qualitative observations revealed similar results when cored and neuritic plaques were considered separately. By contrast, cholinergic fiber loss displayed a significant correlation with the density of tangles (r = 0.52-0.79). However, in a few areas, such as the cingulate cortex, tangle density appeared to be unrelated to the loss of cholinergic fibers. These results indicate that cortical cholinergic denervation in AD is related to cytoskeletal pathology. However, the lack of a perfect relationship with cytoskeletal pathology implicates additional factors in the cholinergic pathology of AD.

Cytoskeletal alterations in the aged human neurohypophysis

The hypophyses of 24 individuals, aged 79–89 years (mean age 83.5±3.3 years), were investigated for cytoskeletal changes associated with abnormally phosphorylated tau protein using the monoclonal antibodies AT8, PHF-1 and Alz-50. A previously unreported pattern of cytoskeletal changes was identified in the neurohypophysis consisting of axon-like fibers and large swellings resembling Herring bodies. The density of the cytoskeletal lesions was subject to notable variation among individuals. Marked neurohypophyseal alterations were also noted in cases even devoid of Alzheimer's disease-related cytoskeletal pathology in neocortical areas. Fully developed Alzheimer's disease is thus not a prerequisite for the presence of advanced neurohypophyseal alterations. In conclusion, the aged human neurohypophysis is revealed as a potential focus of abnormal cytoskeletal changes which may impair the neuroendocrine function of the hypothalamo-neurohypophyseal system.

Axonal cytoskeletal pathology in aged and diabetic human sympathetic autonomic ganglia

Prevertebral sympathetic ganglia develop markedly enlarged argyrophilic neurites as a function of age, gender and diabetes. Immunolocalization studies demonstrate their preferential labeling with antisera to highly phosphorylated 200 kDa neurofilament (NF-H) epitopes, NPY, peripherin and synapsin I, but not to hypophosphorylated NF-M and NF-H or MAP-2. The immunophenotype of dystrophic neurites in conjunction with the results of histochemical and ultrastructural studies are consistent with the terminal axonal and/or synaptic origin of neuritic dystrophy in the sympathetic ganglia of aged and diabetic human subjects.

The apolipoprotein E ϵ4 allele frequency is normal in fronto-temporal dementia, but correlates with age at onset of disease

The apolipoprotein (apoE) ϵ4 allele was studied in fronto-temporal dementia (FTD), a diagnostic category including the specific disorders Pick's disease and frontal lobe degeneration of non-Alzheimer type (FLD). These dementing diseases have neuronal and synaptic degeneration in common with Alzheimer's disease (AD), for which the presence of the apoE ϵ4 allele is a known risk factor, and lowers the age of onset of disease. Previous studies on the apoE ϵ4 allele frequency in FTD have been inconclusive. The structural hallmarks of AD, allegedly linked to apoE presentation, neuritic plaques (NP), primarily composed of aggregates of β-amyloid, and neurofibrillary tangles (NFT), primarily composed of hyperphosphorylated tau, are lacking in FTD. However, tau-positive cytoskeletal pathology is found in Pick's disease, but not in FLD. Resolving whether the ϵ4 frequency is increased in FTD or not may thus give clues to the pathogenetic mechanism of apoE in AD. We therefore studied apoE alleles in a well characterized material of FTD patients. The ϵ4 allele frequency was similar in 25 patients with FTD (14.0%) as compared with 26 healthy controls (13.5%). A post-mortem neuropathological examination was performed in 10 cases (nine had FLD and one Pick's disease). Our finding of a normal ϵ4 allele frequency in our group of FTD, principally consisting of FLD cases, support hypotheses involving differential binding of apoE to β-amyloid and/or tau, in the development of β-amyloid deposition and NP formation and/or tau hyperphosphorylation and NFT formation, for the pathogenetic role of apoE in AD. The age at onset was significantly lower ( P<0.01) in FTD patients possessing the ϵ4 allele (48.7±8.0 years) than in patients not possessing this allele (58.9±7.6 years). We conclude that, although the apoE ϵ4 allele frequency is not increase in FTD, the ϵ4 allele is not an etiological factor, but may rather be an accelerating factor in the degenerative process of FTD, thereby resulting in an earlier presentation of the disorder in individuals predisposed to develop FTD.

Expression of cell division markers in the hippocampus in Alzheimer's disease and other neurodegenerative conditions.

Recent studies, showing that cell cycle-related nuclear proteins p105 and Ki-67 are associated with Alzheimer's disease (AD)-related cytoskeletal pathology, suggested that these proteins, in addition to their functions in regulating the cell cycle, may have more specialised functions in the adult nervous system. In order to test this hypothesis we studied the expression of the cell cycle-related proteins Ki-67, pCNA and p53 in the hippocampi of 33 subjects, including some with AD or other neurodegenerative disorders and some with no neurological disease. By immunohistochemistry we found nuclear expression of Ki-67 in all subregions of the hippocampus, with the highest levels in the dentate gyrus. Both neurons and glial cells expressed this protein. The proportion of cells positive for Ki-67 and the distribution pattern varied considerably depending on the pathological diagnosis. Neuronal nuclear expression of Ki-67 was increased in AD but was also elevated in young Down's syndrome subjects and in those with Pick's disease. Expression of this protein was therefore not AD-specific. We did not find nuclear pCNA or p53 expressed in our patient groups. Contrary to previous studies AD-type neurofibrillary tangles were not labelled with any of the cell cycle markers used. The presence of nuclear Ki-67 expression indicates that some hippocampal neurons are not in the quiescent G0 phase but have re-entered the cell cycle. The absence of nuclear pCNA or p53 suggests that the cycle is arrested in G1. The significance of our findings and their relationship to the production of neurodegenerative cell death via an apoptotic mechanism are discussed.

Can the mechanisms of aluminum neurotoxicity be integrated into a unified scheme?

Regardless of the host, the route of administration, or the speciation, aluminum is a potent neurotoxicant. In the young adult or developmentally mature host, the neuronal response to Al exposure can be dichotomized on morphological grounds. In one, intraneuronal neurofilamentous aggregates are formed, whereas in the other, significant neurochemical and neurophysiological perturbations are induced without neurofilamentous aggregate formation. Evidence is presented that the induction of neurofilamentous aggregates is a consequence of alterations in the posttranslational processing of neurofilament (NF), particularly with regard to phosphorylation state. Although Al has been reported to impact on gene expression, this does not appear to be critical to the induction of cytoskeletal pathology. In hosts responding to Al exposure without the induction of cytoskeletal pathology, impairments in glucose utilization, agonist-stimulated inositol phosphate accumulation, free radical-mediated cytotoxicity, lipid peroxidation, reduced cholinergic function, and altered protein phosphorylation have been described. The extent to which these neurochemical modifications correlate with the induction of a characteristic neurobehavioral state is unknown. In addition to these paradigms, Al is toxic in the immediate postnatal interval. Whether unique mechanisms of toxicity are involved during development remains to be determined. In this article, the mechanisms of Al neurotoxicity are reviewed and recommendations are put forth with regard to future research. Primary among these is the determination of the molecular site of Al toxicity, and whether this is based on Al substitution for divalent metals in a number of biological processes. Encompassed within this is the need to further understand the genesis of host- and developmental-specific responses.

Intracortical perfusion of glutamate in vivo induces alterations of tau and microtubule-associated protein 2 immunoreactivity in the rat.

Modifications of microtubule-associated proteins (MAP) have been reported in both acute and chronic degenerative conditions, such as cerebral ischaemia and Alzheimer's disease, and may be associated with cytoskeletal breakdown. Glutamate excitotoxicity has been implicated in the pathogenesis of both of these conditions and has been shown in some in vitro studies to induce changes in tau similar to those occurring in Alzheimer's disease. This study examines the effects of high extracellular glutamate concentrations on the distribution of tau and MAP2 in vivo in order to determine whether glutamate induces similar changes in tau to those previously reported in vitro in the intact, adult central nervous system. Monosodium glutamate was perfused into the rat parietal cortex for 90 min using in vivo microdialysis and at 4 h after the start of perfusion the distribution of tau and MAP2 was determined by immunohistochemistry. At the core of the glutamate-induced lesion tau immunostaining, as detected with the Tau 1 antibody, was decreased in axons and increased within perikarya compared to controls. Increased immunostaining was not apparent with polyclonal antibodies raised against full-length tau or towards the N or C termini of the protein. In contrast, increased tau immunoreactivity was detected, with all the antibodies used in this study, within oligodendrocytes following either glutamate or sodium chloride perfusion. MAP2 immunoreactivity was increased within perikarya at the core of the glutamate-induced lesion, while dendritic immunoreactivity was reduced. These results suggest that glutamate excitotoxicity in vivo may not be involved in neurofibrillary tangle formation but may be important in the progression of cytoskeletal pathology following cerebral ischaemia.