Excitatory amino acids and neurodegeneration: a hypothetical role of calcium precipitation

Emerging Applications for Quantitative Susceptibility Mapping in the Detection of Traumatic Brain Injury Pathology

Traumatic brain injury (TBI) is a common but heterogeneous injury underpinned by numerous complex and interrelated pathophysiological mechanisms. An essential trace element, iron is abundant within the brain and involved in many fundamental neurobiological processes, including oxygen transportation, oxidative phosphorylation, myelin production and maintenance, as well as neurotransmitter synthesis and metabolism. Excessive levels of iron are neurotoxic and thus iron homeostasis is tightly regulated in the brain, however, many details about the mechanisms by which this is achieved are yet to be elucidated. A key mediator of oxidative stress, mitochondrial dysfunction and neuroinflammatory response, iron dysregulation is an important contributor to secondary injury in TBI. Advances in neuroimaging that leverage magnetic susceptibility properties have enabled increasingly comprehensive investigations into the distribution and behaviour of iron in the brain amongst healthy individuals as well as disease states such as TBI. Quantitative Susceptibility Mapping (QSM) is an advanced neuroimaging technique that promises quantitative estimation of local magnetic susceptibility at the voxel level. In this review, we provide an overview of brain iron and its homeostasis, describe recent advances enabling applications of QSM within the context of TBI and summarise the current state of the literature. Although limited, the emergent research suggests that QSM is a promising neuroimaging technique that can be used to investigate a host of pathophysiological changes that are associated with TBI.

A Single Neonatal Exposure to BMAA in a Rat Model Produces Neuropathology Consistent with Neurodegenerative Diseases

Although cyanobacterial β-N-methylamino-l-alanine (BMAA) has been implicated in the development of Alzheimer's Disease (AD), Parkinson's Disease (PD) and Amyotrophic Lateral Sclerosis (ALS), no BMAA animal model has reproduced all the neuropathology typically associated with these neurodegenerative diseases. We present here a neonatal BMAA model that causes β-amyloid deposition, neurofibrillary tangles of hyper-phosphorylated tau, TDP-43 inclusions, Lewy bodies, microbleeds and microgliosis as well as severe neuronal loss in the hippocampus, striatum, substantia nigra pars compacta, and ventral horn of the spinal cord in rats following a single BMAA exposure. We also report here that BMAA exposure on particularly PND3, but also PND4 and 5, the critical period of neurogenesis in the rodent brain, is substantially more toxic than exposure to BMAA on G14, PND6, 7 and 10 which suggests that BMAA could potentially interfere with neonatal neurogenesis in rats. The observed selective toxicity of BMAA during neurogenesis and, in particular, the observed pattern of neuronal loss observed in BMAA-exposed rats suggest that BMAA elicits its effect by altering dopamine and/or serotonin signaling in rats.

Spatiotemporal Progression of Microcalcification in the Hippocampal CA1 Region following Transient Forebrain Ischemia in Rats: An Ultrastructural Study

Calcification in areas of neuronal degeneration is a common finding in several neuropathological disorders including ischemic insults. Here, we performed a detailed examination of the onset and spatiotemporal profile of calcification in the CA1 region of the hippocampus, where neuronal death has been observed after transient forebrain ischemia. Histopathological examinations showed very little alizarin red staining in the CA1 pyramidal cell layer until day 28 after reperfusion, while prominent alizarin red staining was detected in CA1 dendritic subfields, particularly in the stratum radiatum, by 14 days after reperfusion. Electron microscopy using the osmium/potassium dichromate method and electron probe microanalysis revealed selective calcium deposits within the mitochondria of degenerating dendrites at as early as 7 days after reperfusion, with subsequent complete mineralization occurring throughout the dendrites, which then coalesced to form larger mineral conglomerates with the adjacent calcifying neurites by 14 days after reperfusion. Large calcifying deposits were frequently observed at 28 days after reperfusion, when they were closely associated with or completely engulfed by astrocytes. In contrast, no prominent calcification was observed in the somata of CA1 pyramidal neurons showing the characteristic features of necrotic cell death after ischemia, although what appeared to be calcified mitochondria were noted in some degenerated neurons that became dark and condensed. Thus, our data indicate that intrahippocampal calcification after ischemic insults initially occurs within the mitochondria of degenerating dendrites, which leads to the extensive calcification that is associated with ischemic injuries. These findings suggest that in degenerating neurons, the calcified mitochondria in the dendrites, rather than in the somata, may serve as the nidus for further calcium precipitation in the ischemic hippocampus.

Ultrastructural investigation of microcalcification and the role of oxygen-glucose deprivation in cultured rat hippocampal slices

Intracellular calcium accumulation is associated with cell death in several neuropathological disorders including brain ischemia, but the exact mechanisms of calcification need to be clarified. We used organotypic hippocampal slice culture - cultures subjected to oxygen-glucose deprivation (OGD) mimicking the in vivo situation to investigate the events underlying ectopic calcification. Alizarin red staining indicating calcium deposition was observed in the cornu ammonis (CA)1 and dentate gyrus regions in control hippocampal slices despite no specific labeling for cell death markers. Electron microscopy using the osmium/potassium dichromate method revealed scattered degenerated cells throughout the normally appearing CA1 region. They contained electron-dense precipitates within mitochondria, and electron probe microanalysis confirmed that they were calcifying mitochondria. Selective calcium deposition was noted within, but not beyond, mitochondria in these mineralized cells. They showed ultrastructural features of non-necrotic, non-apoptotic cell death and retained their compact ultrastructure, even after the majority of mitochondria were calcified. Unexpectedly, no intracellular calcification was noted in necrotic CA1 pyramidal cells after OGD, and there was no progression of calcification in OGD-lesioned slices. In addition, mineralized cells in both control and OGD-lesioned slices were closely associated with or completely engulfed by astrocytes but not microglia. These astrocytes were laden with heterogeneous cytoplasmic inclusions that appeared to be related with their phagocytic activity. These data demonstrate that microcalcification specifically associated with mitochondria might lead to a novel type of cell death and suggest that astrocytes may be involved in the phagocytosis of these mineralized cells and possibly in the regulation of ectopic calcification.

Excitatory amino acid transporter 2 downregulation correlates with thalamic neuronal death following kainic acid-induced status epilepticus in rat

Recurrent seizures without interictal resumption (status epilepticus) have been reported to induce neuronal death in the midline thalamic region that has functional roles in memory and decision-making; however, the pathogenesis underlying status epilepticus-induced thalamic neuronal death is yet to be determined. We performed histological and immunohistochemical studies as well as cerebral blood flow measurement using 4.7 tesla magnetic resonance imaging spectrometer on midline thalamic region in Sprague-Dawley rats (n = 75, male, 7 weeks after birth, body weight 250-300 g) treated with intraperitoneal injection of kainic acid (10 mg/kg) to induce status epilepticus (n = 55) or normal saline solution (n = 20). Histological study using paraffin-embedded specimens revealed neuronal death showing ischemic-like changes and Fluoro-Jade C positivity with calcium deposition in the midline thalamic region of epileptic rats. The distribution of neuronal death was associated with focal loss of immunoreactivity for excitatory amino acid transporter 2 (EAAT2), stronger immunoreaction for glutamate and increase in number of Iba-1-positive microglial cells showing swollen cytoplasm and long processes. Double immunofluorescence study demonstrated co-expression of interleukin-1 beta (IL-1β) and inducible nitric oxide synthase (iNOS) within microglial cells, and loss of EAAT2 immunoreactivity in reactive astrocytes. These microglial alterations and astrocytic EAAT2 downregulation were also observed in tissue without obvious neuronal death in kainic acid-treated rats. These results suggest the possible role of glutamate excitotoxicity in neuronal death in the midline thalamic region following kainic acid-induced status epilepticus due to astrocytic EAAT2 downregulation following microglial activation showing upregulation of IL-1β and iNOS.

Intracellular fibril formation, calcification, and enrichment of chaperones, cytoskeletal, and intermediate filament proteins in the adult hippocampus CA1 following neonatal exposure to the nonprotein amino acid BMAA

The environmental neurotoxin β-N-methylamino-l-alanine (BMAA) has been implicated in the etiology of neurodegenerative disease, and recent studies indicate that BMAA can be misincorporated into proteins. BMAA is a developmental neurotoxicant that can induce long-term learning and memory deficits, as well as regionally restricted neuronal degeneration and mineralization in the hippocampal CA1. The aim of the study was to characterize long-term changes (2 weeks to 6 months) further in the brain of adult rats treated neonatally (postnatal days 9–10) with BMAA (460 mg/kg) using immunohistochemistry (IHC), transmission electron microscopy, and laser capture microdissection followed by LC-MS/MS for proteomic analysis. The histological examination demonstrated progressive neurodegenerative changes, astrogliosis, microglial activation, and calcification in the hippocampal CA1 3–6 months after exposure. The IHC showed an increased staining for α-synuclein and ubiquitin in the area. The ultrastructural examination revealed intracellular deposition of abundant bundles of closely packed parallel fibrils in neurons, axons, and astrocytes of the CA1. Proteomic analysis of the affected site demonstrated an enrichment of chaperones (e.g., clusterin, GRP-78), cytoskeletal and intermediate filament proteins, and proteins involved in the antioxidant defense system. Several of the most enriched proteins (plectin, glial fibrillar acidic protein, vimentin, Hsp 27, and ubiquitin) are known to form complex astrocytic inclusions, so-called Rosenthal fibers, in the neurodegenerative disorder Alexander disease. In addition, TDP-43 and the negative regulator of autophagy, GLIPR-2, were exclusively detected. The present study demonstrates that neonatal exposure to BMAA may offer a novel model for the study of hippocampal fibril formation in vivo.

Bepridil decreases Aβ and calcium levels in the thalamus after middle cerebral artery occlusion in rats

Alzheimer's disease (AD) and cerebral ischaemia share similar features in terms of altered amyloid precursor protein (APP) processing and β-amyloid (Aβ) accumulation. We have previously shown that Aβ and calcium deposition, and β-secretase activity, are robustly increased in the ipsilateral thalamus after transient middle cerebral artery occlusion (MCAO) in rats. Here, we investigated whether the non-selective calcium channel blocker bepridil, which also inhibits β-secretase cleavage of APP, affects thalamic accumulation of Aβ and calcium and in turn influences functional recovery in rats subjected to MCAO. A 27-day bepridil treatment (50 mg/kg, p.o.) initiated 2 days after MCAO significantly decreased the levels of soluble Aβ40, Aβ42 and calcium in the ipsilateral thalamus, as compared with vehicle-treated MCAO rats. Expression of seladin-1/DHCR24 protein, which is a potential protective factor against neuronal damage, was decreased at both mRNA and protein levels in the ipsilateral thalamus of MCAO rats. Conversely, bepridil treatment restored seladin-1/DHCR24 expression in the ipsilateral thalamus. Bepridil treatment did not significantly affect heme oxygenase-1- or NAD(P)H quinone oxidoreductase-1-mediated oxidative stress or inflammatory responses in the ipsilateral thalamus of MCAO rats. Finally, bepridil treatment mitigated MCAO-induced alterations in APP processing in the ipsilateral thalamus and improved contralateral forelimb use in MCAO rats. These findings suggest that bepridil is a plausible therapeutic candidate in AD or stroke owing to its multifunctional role in key cellular events that are relevant for the pathogenesis of these diseases.

Lack of secondary pathology in the thalamus after focal cerebral ischemia in nonhuman primates

Remote regions such as the thalamus undergo secondary degeneration after cerebral ischemia. In rodents, the pathology in the thalamus is characterized by a robust inflammatory reaction, β-amyloid (Aβ) accumulation and calcification. Here we studied whether nonhuman primates subjected to middle cerebral artery occlusion (MCAO) display a similar pathology. Common marmosets (n=4) were subjected to transient MCAO for 3 h. Two sham-operated animals served as controls. All animals underwent MRI examination (T2) on postoperative day 7 to assess the location of the infarct. After a 45-day follow-up period, the animals were perfused for histology to evaluate β-amyloid and calcium load in the peri-infarct regions and the thalamus. There was no Aβ or calcium staining in the sham-operated marmosets. The contralateral hemisphere was devoid of Aβ and calcium staining in MCAO animals, except calcium staining in one animal. In the ipsilateral cortex, patchy groups of Aβ-positive cells were observed. Occasional calcium staining was observed in the peri-infarct regions, lesion core, and remote regions such as the substantia nigra. The most important, the thalamus was devoid of any sign of Aβ and calcium aggregation in MCAO animals. Staining for glial fibrillary acidic protein (GFAP) showed marked astrogliosis in the ipsilateral cortex and thalamus. In conclusion, our preliminary study in marmosets did not identify Aβ and calcium pathology in the thalamus following cerebral ischemia as shown in rodents.

Targeting microglial K(ATP) channels to treat neurodegenerative diseases: a mitochondrial issue

Neurodegeneration is a complex process involving different cell types and neurotransmitters. A common characteristic of neurodegenerative disorders is the occurrence of a neuroinflammatory reaction in which cellular processes involving glial cells, mainly microglia and astrocytes, are activated in response to neuronal death. Microglia do not constitute a unique cell population but rather present a range of phenotypes closely related to the evolution of neurodegeneration. In a dynamic equilibrium with the lesion microenvironment, microglia phenotypes cover from a proinflammatory activation state to a neurotrophic one directly involved in cell repair and extracellular matrix remodeling. At each moment, the microglial phenotype is likely to depend on the diversity of signals from the environment and of its response capacity. As a consequence, microglia present a high energy demand, for which the mitochondria activity determines the microglia participation in the neurodegenerative process. As such, modulation of microglia activity by controlling microglia mitochondrial activity constitutes an innovative approach to interfere in the neurodegenerative process. In this review, we discuss the mitochondrial K_ATP channel as a new target to control microglia activity, avoid its toxic phenotype, and facilitate a positive disease outcome.

Neonatal exposure to the cyanobacterial toxin BMAA induces changes in protein expression and neurodegeneration in adult hippocampus

The cyanobacterial toxin β-N-methylamino-l-alanine (BMAA) has been proposed to contribute to neurodegenerative disease. We have previously reported a selective uptake of BMAA in the mouse neonatal hippocampus and that exposure during the neonatal period causes learning and memory impairments in adult rats. The aim of this study was to characterize effects in the brain of 6-month-old rats treated neonatally (postnatal days 9–10) with the glutamatergic BMAA. Protein changes were examined using the novel technique Matrix-Assisted Laser Desorption Ionization (MALDI) imaging mass spectrometry (IMS) for direct imaging of proteins in brain cryosections, and histological changes were examined using immunohistochemistry and histopathology. The results showed long-term changes including a decreased expression of proteins involved in energy metabolism and intracellular signaling in the adult hippocampus at a dose (150mg/kg) that gave no histopathological lesions in this brain region. Developmental exposure to a higher dose (460mg/kg) also induced changes in the expression of S100β, histones, calcium- and calmodulin-binding proteins, and guanine nucleotide-binding proteins. At this dose, severe lesions in the adult hippocampus including neuronal degeneration, cell loss, calcium deposits, and astrogliosis were evident. The data demonstrate subtle, sometimes dose-dependent, but permanent effects of a lower neonatal dose of BMAA in the adult hippocampus suggesting that BMAA could potentially disturb many processes during the development. The detection of BMAA in seafood stresses the importance of evaluating the magnitude of human exposure to this neurotoxin.

Increased frequency of distinct TSC2 allelic variants in focal cortical dysplasias with balloon cells and mineralization

Focal cortical dysplasias with balloon cells (FCD(IIb)) usually present with characteristic imaging and molecular features, that is, a transmantle sign on fluid-attenuated inversion recovery MRI and abundance of allelic variants of the tuberous sclerosis gene 1 (TSC1). Recently, we observed several mineralized lesions (n = 5) lacking this MRI pattern and which surprisingly turned out as FCD(IIb) upon neuropathological examination. These mineralized FCD(IIb) revealed an increased frequency of TSC2 allelic variants but not TSC1 (intron 31: 60% vs. 11% in controls; P = 0.0164, exon 41: 40% vs. 6.5% in controls; P = 0.0441). Mineralized FCD(IIb) have a favorable postsurgical outcome and need consideration in the presurgical differential diagnosis of calcified lesions associated with pharmacoresistant focal epilepsies.

Characterization of in vivo MRI detectable thalamic amyloid plaques from APP/PS1 mice

Amyloid deposits are one of the hallmarks of Alzheimer's disease. Recent studies, in transgenic mice modeling Alzheimer's disease showed that, using in vivo, contrast agent-free, MRI, thalamic amyloid plaques are more easily detected than other plaques of the brain. Our study evaluated the characteristics of these thalamic plaques in a large population of APP/PS1, PS1 and C57BL/6 mice. Thalamic spots were detected in all mice but with different frequency and magnitude. Hence, the prevalence and size of the lesions were higher in APP/PS1 mice. However, even in APP/PS1 mice, thalamic spots did not occur in all the old animals. In APP/PS1 mice, spots detection was related to high iron and calcium load within amyloid plaques and thus reflects the ability of such plaque to capture large amounts of minerals. Interestingly, calcium and iron was also detected in extra-thalamic plaques but with a lower intensity. Hypointense lesions in the thalamus were not associated with the iron load in the tissue surrounding the plaques, nor with micro-hemorrhages, inflammation, or a neurodegenerative context.

Histological and functional outcomes after traumatic brain injury in mice null for the erythropoietin receptor in the central nervous system

Erythropoietin (EPO) and its receptor (EPOR), essential for erythropoiesis, are expressed in the nervous system. Recombinant human EPO treatment promotes functional outcome after traumatic brain injury (TBI) and stroke, suggesting that the endogenous EPO/EPOR system plays an important role in neuroprotection and neurorestoration. This study was designed to investigate effects of the EPOR on histological and functional outcomes after TBI. Experimental TBI was induced in adult EPOR-null and wild-type mice by controlled cortical impact. Neurological function was assessed using the modified Morris Water Maze and footfault tests. Animals were sacrificed 35 days after injury and brain sections stained for immunohistochemistry. As compared to the wild-type injured mice, EPOR-null mice did not exhibit higher susceptibility to TBI as exemplified by tissue loss in the cortex, cell loss in the dentate gyrus, impaired spatial learning, angiogenesis and cell proliferation. We observed that less cortical neurogenesis occurred and that sensorimotor function (i.e., footfault) was more impaired in the EPOR-null mice after TBI. Co-accumulation of amyloid precursor protein (axonal injury marker) and calcium was observed in the ipsilateral thalamus in both EPOR-null and wild-type mice after TBI with more calcium deposits present in the wild-type mice. This study demonstrates for the first time that EPOR null in the nervous system aggravates sensorimotor deficits, impairs cortical neurogenesis and reduces thalamic calcium precipitation after TBI.

Coaccumulation of calcium and beta-amyloid in the thalamus after transient middle cerebral artery occlusion in rats

Transient occlusion of the middle cerebral artery (MCAO) in rats leads to abnormal accumulation of beta-amyloid (Abeta) peptides in the thalamus. This study investigated the chemical composition of these deposits. Adult male human beta-amyloid precursor protein (APP) overexpressing (hAPP695) rats and their wild-type littermates were subjected to transient MCAO for 2 h or sham operation. After 26-week survival time, histological examination revealed an overlapping distribution pattern for rodent and human Abeta in the thalamus of hAPP695 rats subjected to MCAO. X-ray microanalysis showed that the deposits did not contain significant amount of iron, zinc, or copper typical to senile plaques. In contrast, the deposit both in hAPP695 and non-transgenic rats contained calcium and phosphorus in a ratio (1.28+/-0.15) characteristic to hydroxyapatites. Alizarin red staining confirmed that calcium coaccumulated in these Abeta deposits. It is suggested that APP expression is induced by ischemic insult in cortical neurons adjacent to infarct, which in turn is reflected as increased release of Abeta peptides by their corticothalamic axon endings. This together with insufficient clearance or atypical degradation of Abeta peptides lead to dysregulation of calcium homeostatis and coaccumulation in the thalamus.

Dissociation between hippocampal neuronal loss, astroglial and microglial reactivity after pharmacologically induced reverse glutamate transport

The inflammatory central nervous system response that involves activated microglia and reactive astrocytes may both heal and harm neurons, as inflammatory mediators lead to neuroprotection or excitation at one dose but to injury at a different concentration. To investigate these complex cellular interactions, L-trans-pyrrolidine-2,4-dicarboxylate (PDC), a selective substrate inhibitor of glutamate transport, was infused during 14 days in the rat hippocampus at three different rates: 5, 10 and 25 nmol/h. A microglial reaction appeared at the 5 nmol/h PDC rate in absence of astroglial reaction and neuronal loss. Microgliosis and neuronal death were observed at PDC 10 nmol/h in absence of astrogliosis and calcium precipitation, whereas all four aspects were present at the highest rate. This dissociation between neuronal loss and astroglial reactivity took place in presence of a permanent microglial reaction. These data suggest a specific response of microglia to PDC whose neuronal effects may differ with the infused dose.

Modification of dopaminergic markers expression in the striatum by neonatal exposure to glutamate during development

Monosodium l-glutamate (MSG) was administered subcutaneously to male neonatal rats, and the effect on developmental profile of tyrosine hydroxylase (TH), D1, D2 receptors, and dopamine (DA) transporter expression in the striatum was examined using Western blot. In addition, TH-immunopositive neurons at substantia nigra (SN) were also examined. MSG treatment (4mg/g of body weight, administered on postnatal days 1, 3, 5, and 7) resulted in a reduction of D1 and D2 receptor expression from 30 days of age and persisted to adulthood (120 days of age), while DA transporter expression was significantly reduced from 14 days of age to adulthood. TH immunopositive neurons at SN showed a significant reduction, as well as TH expression on postnatal days 10, 30, 60, and 120 at striatum was reduced. No changes of TH were observed at 14 days of age. Results indicate that an over-stimulation of the glutamatergic system by neonatal exposure to a high glutamate concentration induces a partial loss in TH-positive neurons in the SN and an important reduction in dopaminergic markers expression in the striatum, suggesting that early excitotoxicity could contribute to developmental alterations in the nigrostriatal pathway, which may be associated with various disorders of the basal ganglia.

Basal ganglia calcification in BB/E rats with diabetes

Human diabetes is associated with cognitive impairment and structural abnormalities in the brain such as cerebral atrophy. The aetiology of these abnormalities is not known. The BB/E rat is a well-established model of type 1 (insulin dependent) diabetes. A cohort of 34 BB/E rats with diabetes was divided into three sub-groups according to age (and duration of diabetes). Basal ganglia calcification (BGC) was present in the brains of more than 50% of diabetic animals, but not in any of 37 non-diabetic BB/E rats. BGC occurred more commonly in those animals which had the longest duration of diabetes (p=0.001), such that BGC was present in only 8% of animals with diabetes for 20 weeks, but in 100% of animals with diabetes for 60 weeks. There were no other significant light microscopic neuropathologic changes in diabetic animals. It will be important to investigate the mechanism of brain calcification, whether a similar process occurs in humans with diabetes, and its possible relationship to cognitive decline.

Putative glucosensing property in rat and human activated microglia

Microglial cells involved in the pathogenesis of many neurodegenerative diseases acquire the features of cytotoxic and phagocytic cells in response to certain pathogens and inflammatory signals. K(ATP) channels are energy sensors of ATP availability that link the cell's metabolic state to its membrane excitability. In pancreatic beta cells, they promote glucose-dependent insulin secretion, and in neurones, hyperpolarization that protects against hypoxic damage. This study analyses activated microglia in an in vivo rat neurodegenerative model based on acute hippocampal glutamate receptor overactivation and in postmortem samples from patients with Alzheimer's disease. We demonstrate that in activated microglia the K(ATP) channel components SUR-1 or SUR-2 are present together with glucokinase. Our results indicate that, according to glucose availability, these channels may modify microglia membrane potential. The functional relevance of these channels is seen as a new mechanism modulating the effects of external signals on microglia.

Heterogeneity between hippocampal and septal astroglia as a contributing factor to differential in vivo AMPA excitotoxicity

Astroglial participation in the regional differences of vulnerability to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced neurodegeneration was investigated in the rat hippocampus and medial septum using L-alpha-aminoadipate (alpha-AA) as a specific astroglial toxin. alpha-AA was microinjected in the hippocampus and the medial septum and a time-course study was carried out between 2 hr and 3 days. When compared to controls, microinjection of alpha-AA in the hippocampus induced within 3 days a reversible loss of glial fibrillary acidic protein (GFAP) immunostaining and a microglial reaction without any neuronal loss, whereas in the medial septum it caused no effects on astroglial, microglial, or neuronal populations. Differences in hippocampus and medial septum vulnerability were also evidenced when alpha-AA was co-injected with AMPA and neurodegeneration was assessed in terms of neuronal loss, glial reactions, calcification, and atrophy of the area. In the hippocampus, alpha-AA increased AMPA excitotoxicity with marked disorganization of all hippocampal subfields, increased neuronal loss, a more important astroglial reaction, a larger area of microgliosis, and a greater abundance of calcium deposits. By contrast, in the medial septum alpha-AA did not modify any parameter of the AMPA-induced lesion. In conclusion, the presence of different astroglial populations in hippocampus and medial septum results in a different participation to AMPA excitotoxicity that may determine, at least in part, the specific regional vulnerability to neurodegeneration.

In vivo neuroprotective adaptation of the glutamate/glutamine cycle to neuronal death

Synaptic increase of glutamate level, when not coupled to a heightened energy production, renders neurons susceptible to death. Astrocyte uptake and recycling of synaptic glutamate as glutamine is a major metabolic pathway dependent on energy metabolism, which inter-relationships are not fully understood and remain controversial. We examine how the glutamate-glutamine cycle and glucose metabolism are modified in two in vivo models of severe and mild brain injury. Graded reductions of glutaminase, the glutamate synthetic enzyme, were evidenced combined with increases in glutamine synthetase, the inactivating glutamate enzyme. Increased lactate dhydrogenase (LDH) activity was only present after a more severe injury. These results indicate an in vivo adaptation of the glutamate-glutamine cycle in order to increase the net glutamine output, reduce glutamate excitotoxicity, and avoid neuronal death. We conclude that the graded modification of the glutamate-glutamine correlation and neuronal lactate availability may be key factors in the apoptotic and necrotic neuronal demise, whose control may prove highly useful to potentiate neuronal survival.

Low sensitivity of retina to AMPA-induced calcification

Glutamate is involved in most CNS neurodegenerative diseases. In particular, retinal diseases such as retinal ischemia, retinitis pigmentosa, and diabetic retinopathy are associated with an excessive synaptic concentration of this neurotransmitter. To gain more insight into retinal excitotoxicity, we carried out a dose-response study in adult rats using alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), a glutamate analogue. AMPA intraocular injections (between 0.27 and 10.8 nmol) caused no morphologic modification, but a 10.8 + 21 nmol double injection in a 10-day interval produced a lesion characterized by discrete neuronal loss, astroglial and microglial reactions, and calcium precipitation. Abundant calcium deposits similar to those present in rat and human brain excitotoxicity or hypoxia-ischemia neurodegeneration were detected by alizarin red staining within the retinal surface and the optic nerve. Glial reactivity, associated normally with astrocytes in the nerve fiber, was assessed in Müller cells. GABA immunoreactivity was detected not only in neuronal elements but also in Müller cells. In contrast to the high vulnerability of the brain to excitotoxin microinjection, AMPA-induced retinal neurodegeneration may provide a useful model of low central nervous system sensitivity to excitotoxicity.

Excitotoxic lesioning of the rat basal forebrain with S-AMPA: consequent mineralization and associated glial response

Regional depositions of calcium within the basal ganglia, cortex, cerebellum, and white matter and at perivascular sites have been observed in several pathological conditions. These generally indicate signs of ongoing apoptosis or necrotic processes, whereby the activation of glutamate receptors causes a rise in intracellular calcium levels leading to mineralization of neurons, and ultimately to cell death. The selective degeneration of cholinergic neurons in the basal forebrain is a major neuropathological component of Alzheimer's disease, and may result in abnormal deposition of calcium. In experimental models, selective lesions of the basal forebrain can be induced by intraparenchymal infusions of excito- or immunotoxins targeting cholinergic neurons. Excitotoxic lesions are often accompanied by calcium deposition within affected areas. In a previous study we also noted the presence of unusual deposition in areas close to the site of injections following unilateral S-AMPA-induced lesions of the basal forebrain (T. Perry, H. Hodges, and J. A. Gray, 2001, Brain Res. Bull. 54, 29-48). In this paper, we have characterized these deposits histologically and evaluated the microglial (CD11b) and astrocytic (GFAP) responses at 8 and 16 weeks following lesioning of the nucleus basalis magnocellularis with S-AMPA. The resulting deposits were heterogeneous in morphology and composed primarily of calcium. Small granular deposits were detected around blood vessels, whereas larger calcospherites were situated within the parenchyma. These deposits were more widely dispersed at 16 weeks postlesioning, affected neighboring nuclei, and displayed a progressive increase in size and frequency of occurrence. However, calcification within these regions was differentially associated with microglial and astrocytic reactivity at the two time points. Both microglial and astrocytic responses were pronounced at 8 weeks, whereas at 16 weeks, astrocytic reactivity prevailed and the microglial response was markedly attenuated. Importantly, the pattern of reactivity for microglia detected at 8 weeks was specifically localized to vulnerable nucleated areas prior to their substantial accumulation of calcium deposits, which was clearly evident by 16 weeks. We suggest that the initial microglial response could be used as a selective predictor of tissue necrosis and subsequent calcification, and that astrocytes, which form a glial scar in the affected tissues, may contribute toward the buildup of calcium deposits. The functional relevance of these findings is discussed.

Calcium precipitation in acute and chronic brain diseases

In rat brain, calcification associated with excitotoxicity has been proposed to play a protective role, whereas in human brain, nonartherosclerotic calcification is present in several pathological conditions without any clear significance. To determine if calcification can be viewed as a protective step of calcium homeostasis during chronic and acute neuronal suffering, cerebral cortex and hippocampus of patients with Alzheimer's disease, vascular dementia and neonatal hypoxia-ischemia were investigated. To investigate the human specificity, these two areas were also studied in dogs with established cognitive deficits. In all groups, calcium precipitates were observed in the cerebral parenchyma associated with neuronal damage. The cerebral cortex presented a higher degree of calcification than the hippocampus. The neonatal hypoxia-ischemia group was characterised by a higher degree of calcification, whereas the groups with lowest calcification were the Alzheimer's patients and dogs. As shown by X-ray microanalysis, in the precipitates, calcium is mainly associated with phosphorus in a form that resembles hydroxyapatites. Thus, intracellular calcium concentration associated with neuronal suffering may reduce the energy extrusion. We propose that, to help overcome excitotoxicity, calcium precipitation acts in CNS of vertebrates as a new compartment of the calcium homeostasis in which free cytoplasmic calcium ions are inactivated by phosphate ones.

Changes in NMDA-receptor gene expression are associated with neurotoxicity induced neonatally by glutamate in the rat brain

The N-methyl-D-aspartate receptor (NMDA-R) is fully functional in the rat early in embryogenesis, and diverse neuronal plasticity events are regulated through its activation later in postnatal development. On the other hand, systemic administration of glutamate (Glu) to rats at birth induces neuronal degeneration in glutamatergic central nervous system regions via Glu receptor activation. However, it is not known whether an increase in neonatal Glu levels modifies the gene expression of NMDA-R subunits, or if these putative changes are related to gamma-aminobutyric acid-mediated (GABAergic) neurotransmission. We measured, by means of semi-quantitative reverse transcriptase polymerase chain reaction, changes in gene expression of the NMDA-R subunits: NMDA-R1, NMDA-R 2A and NMDA-R 2B in cerebral cortex (CC), striatum (ST) and hippocampus (HP) in the brains of rats treated neonatally with monosodium L-glutamate (MSG). These studies were supported by histological and quantitative analysis of the glia. Our results showed histological evidence of neuronal damage, and increased glial cell number and activity were detected. This was seen mainly in the ST and HP of MSG-treated animals. Significant increases in NMDA-R1, 2A and 2B subunits gene expression was also observed in ST and HP but not in CC, where only NMDA-R 2B was increased in MSG-treated rats. Our data suggest that increases in Glu levels and activation of Glu-receptors after neonatal administration of MSG induce an increase in glial cell reactivity and important changes in NMDA-R molecular composition, with signs of neuronal damage.

Age-related resistance to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid-induced hippocampal lesion

This study compares the effects of acute alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) administration in the hippocampus in adult (3 months) and middle-aged (15 months) rats at 15 days postinjection. Injection of 1 and 2.7 mM AMPA produced dose-dependent neurodegeneration, assessed by Nissl staining; a glial reaction shown by glial fibrillary acidic protein immunocytochemistry; and calcification, revealed by alizarin red staining. Furthermore, at both doses, these alterations were significantly greater in 3-month-old rats. Finally, at AMPA 2.7 mM, no significant changes in the density of hippocampal parvalbumin- or calbindin-immunoreactive neurons or in choline acetyltransferase, glutamate uptake, or GABA uptake activities were found in 15-month-old animals, whereas significant reductions in parvalbumin (-76%) and calbindin (-32%) immunostaining and in GABA uptake (-27%) were observed in 3-month-old animals compared to the respective sham-operated or control animals. In conclusion, this study clearly demonstrates that in rats the vulnerability of hippocampal neurons and the glial and calcification reactions to AMPA-induced injury decreased with age between 3 and 15 months. Our results also indicate that hippocampal cholinergic, glutamatergic, and GABAergic systems show an adaptive response to excitotoxic damage in both adult and middle-aged animals.

Perinatal human hypoxia-ischemia vulnerability correlates with brain calcification

Deregulation of intracellular calcium homeostasis is widely considered as one of the underlying pathophysiological mechanisms of hypoxic-ischemic brain injury. Whether this alteration can result in cerebral calcification was investigated in basal ganglia, cerebral cortex, and hippocampus of human premature and term neonates together with glial reaction. In all samples nonarteriosclerotic calcifications were observed, their number and size were area-specific and increased in term neonates. Basal ganglia always presented the highest degree of calcification and hippocampus the lowest, located mainly in the CA1 subfield. In all cases, neuronal damage was associated with astroglial reaction and calcium precipitates, with microglial reaction only in basal ganglia and cerebral cortex, and argues for the participation of excitatory amino acid receptors in hypoxia-ischemia damage. These data correlate with hypoxia-ischemia vulnerability in the perinatal period. The clinical relevance of these precipitates and the neuroprotective interest of non-NMDA receptor manipulation are discussed in the light of our results.

Neonatal exposure to monosodium glutamate induces cell death and dendritic hypotrophy in rat prefrontocortical pyramidal neurons

Monosodium glutamate was administered subcutaneously to male neonate rats, and the effects on cell number and cytoarchitecture of third-layer pyramidal neurons from the prefrontal cerebral cortex were studied in the adult. Monosodium glutamate treatment (4 mg/g of body weight, on post-natal days 1, 3, 5 and 7) resulted in fewer neurons, and shorter and less ramified dendritic processes, than those observed in control animals. Both density and proportional shapes of dendritic spines were not modified. We propose a dual effect of neonatal exposure to glutamate: an excitotoxic effect leading to cell death, and; a secondary neuroprotective effect, arising from the proliferation of glial cells and their subsequent uptake of glutamate, that favors the survival of the remaining neurons, and leads to a further hypotrophic effect on their dendritic processes.