Differential brain area vulnerability to long-term subcortical excitotoxic lesions

Brain Pathology in Adult Rats Treated With Domoic Acid

Domoic acid (DA) is a neurotoxin reported to produce damage to the hippocampus, which plays an important role in memory. The authors inoculated rats intraperitoneally with an effective toxic dose of DA to study the distribution of the toxin in major internal organs by using immunohistochemistry, as well as to evaluate the induced pathology by means of histopathologic and immunohistochemical methods at different time points after toxin administration (6, 10, and 24 hours; 5 and 54 days). DA was detected by immunohistochemistry exclusively in pyramidal neurons of the hippocampus at 6 and 10 hours after dosing. Lesions induced by DA were prominent at 5 days following treatment in selected regions of the brain: hippocampus, amygdala, piriform and perirhinal cortices, olfactory tubercle, septal nuclei, and thalamus. The authors found 2 types of lesions: delayed death of selective neurons and large areas of necrosis, both accompanied by astrocytosis and microgliosis. At 54 days after DA exposure, the pathology was characterized by still-distinguishable dying neurons, calcified lesions in the thalamus, persistent astrocytosis, and pronounced microgliosis. The expression of nitric oxide synthases suggests a role for nitric oxide in the pathogenesis of neuronal degeneration and chronic inflammation induced by DA in the brain.

Loss of parvalbumin-positive neurons from the globus pallidus in animal models of Parkinson disease

The external segment of the globus pallidus (GPe) in humans and the equivalent structure in rodents, the globus pallidus (GP), influence signal processing in the basal ganglia under normal and pathological conditions. Parvalbumin (PV) immunoreactivity defines 2 main neuronal subpopulations in the GP/GPe: PV-immunopositive cells that project mainly to the subthalamic nucleus and the internal segment of the GP and PV-negative cells that mainly project to the striatum. We evaluated the number of neurons in the GP/GPe in animal models of Parkinson disease. In rats, dopaminergic denervation with 6-hydroxydopamine (6-OHDA) provoked a significant decrease in the number of GP neurons (12% ± 4%, p < 0.05), which specifically affected the PV subpopulation. A similar trend was observed in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. Markers of GABAergic activity (GAD65 and GAD67 mRNA) were not different from those of controls in 6-OHDA-lesioned rats. Taken together, these findings provide evidence for nondopaminergic neuronal cell loss in the basal ganglia of 6-OHDA-lesioned rats and suggest that a similar loss may occur in the MPTP monkey. These data suggest that in patients with Parkinson disease, the loss of GABAergic neurons projecting to the subthalamic nucleus may contribute to the hyperactivity of this nucleus despite the absence of gross alterations in GAD mRNA expression.

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.

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.

Similar calcification process in acute and chronic human brain pathologies

Cellular microcalcification observed in a diversity of human pathologies, such as vascular dementia, Alzheimer's disease, Parkinson's disease, astrogliomas, and posttraumatic epilepsy, also develops in rodent experimental models of central nervous system (CNS) neurodegeneration. Central to the neurodegenerative process is the inability of neurons to regulate intracellular calcium levels properly, and this is extensible to fine regulation of the CNS. This study provides evidence of a common pattern of brain calcification taking place in several human pathologies, and in the rat with glutamate-derived CNS lesions, regarding the chemical composition, physical characteristics, and histological environment of the precipitates. Furthermore, a common physical mechanism of deposit formation through nucleation, lineal growth, and aggregation is presented, under the modulation of protein deposition and elemental composition factors. Insofar as calcium precipitation reduces activity signals at no energy expense, the presence in human and rodent cerebral brain lesions of a common pattern of calcification may reflect an imbalance between cellular signals of activity and energy availability for its execution. If this is true, this new step of calcium homeostasis can be viewed as a general cellular adaptative mechanism to reduce further brain damage.

In vivo co-ordinated interactions between inhibitory systems to control glutamate-mediated hippocampal excitability

We present an overview of the long-term adaptation of hippocampal neurotransmission to cholinergic and GABAergic deafferentation caused by excitotoxic lesion of the medial septum. Two months after septal microinjection of 2.7 nmol alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), a 220% increase of GABA(A) receptor labelling in the hippocampal CA3 and the hilus was shown, and also changes in hippocampal neurotransmission characterised by in vivo microdialysis and HPLC. Basal amino acid and purine extracellular levels were studied in control and lesioned rats. In vivo effects of 100 mm KCl perfusion and adenosine A(1) receptor blockade with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) on their release were also investigated. In lesioned animals GABA, glutamate and glutamine basal levels were decreased and taurine, adenosine and uric acid levels increased. A similar response to KCl infusion occurred in both groups except for GABA and glutamate, which release decreased in lesioned rats. Only in lesioned rats, DPCPX increased GABA basal level and KCl-induced glutamate release, and decreased glutamate turnover. Our results evidence that an excitotoxic septal lesion leads to increased hippocampal GABA(A) receptors and decreased glutamate neurotransmission. In this situation, a co-ordinated response of hippocampal retaliatory systems takes place to control neuron excitability.

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.

Mineralization of the basal ganglia: implications for neuropsychiatry, pathology and neuroimaging

This article examines the evidence for and against the existence of basal ganglia mineralization as a defined clinico-pathological entity. In reviewing the literature on basal ganglia mineralization, this article emphasizes evidence derived from different neuroimaging modalities, genetics, metabolic studies, postmortem series and their possible neuropsychiatric correlates. Relevant articles were collected through Medline and Index Medicus searches. Researchers have encountered multiple difficulties in accepting basal ganglia mineralization as a distinct entity. This syndrome lacks set clinical criteria or a unique etiology; not surprisingly, numerous articles have applied varied definitions. Because many of the reported cases have not been examined postmortem, both the extent and nature of their mineralization remains uncertain. Furthermore, researchers have considered small foci of basal ganglia mineralization a normal phenomenon of aging. However, when brain deposits are extensive, they are associated with a set of age-dependent, progressive clinical symptoms. They include cognitive impairment, extrapyramidal symptoms and psychosis. Most cases are related to abnormalities of calcium metabolism, but rare familial cases of idiopathic origin have been reported. Overabundant mineralization of the brain is judged pathological based on its amount, distribution and accompanying clinical symptoms. Although its relation with calcium dysregulation is well known, modern studies have emphasized abnormalities of iron and dopamine metabolism. The authors suggest that these metabolic abnormalities may link basal ganglia mineralization to psychotic symptomatology.

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.

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.

Differential DNA damage in response to the neonatal and adult excitotoxic hippocampal lesion in rats

We examined the developmental profile of excitotoxin-induced nuclear DNA fragmentation using the transferase dUTP nick-end labelling (TUNEL) technique, as a marker of DNA damage and cell death in rats with neonatal and adult excitotoxic lesions of the ventral hippocampus. We hypothesized that infusion of neurotoxin may result in a differential pattern of cell death in neonatally and adult lesioned rats, both in the infusion site and in remote brain regions presumably involved in mediating behavioural changes observed in these animals. Brains of rats lesioned at 7 days of age and in adulthood were collected at several survival times 1-21 days after the lesion. In the lesioned neonates 1-3 days postlesion, marked increases in TUNEL-positive cells occurred in the ventral hippocampus, the site of neurotoxin infusion, and in a wide surrounding area. Adult lesioned brains showed more positive cells than controls only at the infusion site. In the lesioned neonates, TUNEL-labelled cells were also present in the striatum and nucleus accumbens 1 day postlesion but not at later survival times. Our findings indicate that cell death in remote regions is more prominent in immature than adult brains, that it may lead to distinct alterations in development of these brain regions, and thus may be responsible for functional differences between neonatally and adult lesioned rats.

Long-term evolution of local, proximal and remote astrocyte responses after diverse nucleus basalis lesioning (an experimental Alzheimer model): GFAP immunocytochemical study

A study on long-term astrocytic responses (from 1 day to 20 months after lesioning in 4-month-old rats, and from 1 day to 6 months in 20-month-old rats) to diverse unilateral damage of the nucleus basalis (nbM) by injection of 40 nmol of ibotenic acid, or 50 or 100 nmols of quisqualic acid was performed using a histochemical method (immunoreactivity against the glial fibrillary acidic protein GFAP). Glial reactivity (i.e., isolated or clustered hypertrophic and/or hyper-reactive astrocytes) was evaluated in several ipsilateral and contralateral brain regions: the 'local response' within the damaged nbM region; the 'proximal response' (a new concept proposed by us) in the non-damaged structures neighbouring the nbM; and the 'remote response' in the ipsilateral brain cortex and in the contralateral cortex and nbM. In 4-month-old animals, the remote cortical glial responses, independent of the involution of cortical cholinergic activity and randomly located in layers I-V of motor and somatosensory cortical regions, were similar in appearance over a long period (13-20 months), with the highest reactivity 45 days after lesioning. The proximal response lasted from 1 day to 13 months and afterwards tended to disappear. Contralateral reactivity and ipsilateral cortical scars were observed. The local (nbM) glial response was maintained throughout the period studied. Subsets of astrocytes of different reactivities were observed, most of their elements being highly intermeshed. In 20-month-old animals, nbM lesions produced less positive, but similar, glial reactive patterns. This glial reactivity was superposed onto the glial reactivity of old age. All these results are discussed. The maintenance of reactive astrocytes many months after lesioning suggests the existence of cellular factors other than those produced by damaged nbM neurons. Taking into account the role of glial cells under pathological conditions, it is possible that these reactive astrocytes in humans could promote neurodegenerative processes, such as amyloid plaque formation and neurodegeneration (Alzheimer's disease). Along this line, nbM cholinergic involution could then originate cortical involution through induced reactive astrocytosis.

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

Activation of excitatory amino acid (EAA) receptors can induce neurodegeneration by two major mechanisms of excitotoxicity, one related to the influx of Na(+), Cl(-) and water, and the other to the increase in intracellular calcium concentration ([Ca(2+)](i)). Thus, acute microinjection of EAAs in several areas of the central nervous system (CNS) has been used to produce neurodegenerative models. We studied the excitotoxic pattern associated with acute microinjection of AMPA in rat hippocampus, medial septum-diagonal band of Broca (MS-DBB), prefrontal cortex and retina. In all cases progressive neuronal loss, glial reaction and development of intra- and extracellular calcium concretions were observed. However, a CNS-area differential vulnerability was revealed, as shown by the specific atrophy of MS-DBB and its limited calcification. Whether calcium deposits are a defensive mechanism against the massive increment of free cytoplasmatic calcium is discussed on the basis of ultrastructural data and previous results.

Characterization of striatal lesions produced by glutamate uptake alteration: cell death, reactive gliosis, and changes in GLT1 and GADD45 mRNA expression

This study investigated the time course of the striatal lesions produced by continuous local injection of the glutamate uptake inhibitor, L-trans-pyrrolidine-2,4-dicarboxylate (PDC) at the rate of 25 nmol/h in rats. The extent of the neurodegeneration area (defined as the lesion area) did not significantly vary with the duration of the PDC treatment between 3 and 14 days, but was markedly reduced 3 months after cessation of the 14-day treatment, probably reflecting striatal atrophy. After the 3-day treatment, the lesion zone showed calcium precipitates and marked microglial reaction contrasting with the reduction of astroglial labeling and loss of the glutamate transporter GLT1 mRNA expression; however reactive astrocytes were observed around the lesion. After the 14-day treatment, the lesion zone presented reactive astrocytes and microglia without calcification, and a partial recovery of GLT1 mRNA expression. Interestingly, the growth arrest DNA damage-inducible GADD45 mRNA expression was induced around the lesion after 3 days but inside the lesion after 14 days of treatment. Three months after the 14-day treatment, the astroglial reactivity persisted within the lesion whereas most of the other markers examined tended to normalize. These data suggest that defective glutamate transport induces primary death of neurons and dysfunction of astrocytes. They strongly implicate reactive astrocytes with GLT1 and GADD45 transcripts in preventing secondary neuronal death.

Differential vulnerability of hippocampus, basal ganglia, and prefrontal cortex to long-term NMDA excitotoxicity

In human brain, nonartherosclerotic calcification is associated with normal aging and several pathological conditions without any clear significance. In all situations, calcification appears predominantly in the basal ganglia, but is also frequent in the hippocampus and cerebral cortex. alpha-Amino-(3-hydroxi-5-methyl-4-isoxazol-4-il)-propionic acid-induced lesion of the globus pallidus is associated in rats with the formation of calcium deposits similar to those observed in the human brain. To determine whether direct neuronal activation may induce calcification, N-methyl-d-aspartate (NMDA) was microinjected in rat hippocampus, globus pallidus, and lateral prefrontal cortex. Two months later, neuronal death was associated with calcium deposits that were characterized in terms of distribution and size. A unique population of deposits was present in the hippocampus and prefrontal cortex, whereas in the globus pallidus two main groups could be differentiated. Calcification was always associated with a significant microglial reaction as shown by the peripheral benzodiazepine receptor autoradiography. Monoamine oxidase B autoradiography, reflecting the astroglial reaction, was also significantly increased. Our results provide evidence that acute NMDA neuronal activation leads with time to calcification associated with a glial reaction and indicate that nonartherosclerotic calcification in the human brain may develop from an acute NMDA receptor activation. A key role of the metabotropic mGluR1 receptor is also suggested.

Long-term effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione in the rat basal ganglia: calcification, changes in glutamate receptors and glial reactions

Previous data from our laboratory indicate that 25 mM ibotenic acid induces intracellular calcifications in the rat basal forebrain. Because of the lack of specificity of ibotenic acid for a glutamate receptor subtype, a dose-response study with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate was undertaken and calcified areas (identified with Alizarin Red staining) as well as astro- and microglial reactions (by autoradiography with [3H]lazabemide and [3H]Ro 5-4864) were quantified at one month post-lesion. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionate administered into the globus pallidus induced, in a dose-dependent manner, the formation of calcium deposits and the activation of both glial cells, the microglial reaction being particularly robust. From this study, a dose of 5.4 mM alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate was selected for further experiments. [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate, [3H]dizocilpine maleate and [3H]PN 200-110 binding in vitro were performed to assess autoradiographically whether the tissue damage was associated with changes in glutamate receptors and calcium channel binding sites. In the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-treated animals, the specific binding of [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate was significantly reduced by 28% in the lesioned ventral pallidum, whereas it was unchanged in the globus pallidus and substantia innominata. In these three nuclei, calcifications developed and an increase in both glial markers was measured. In contrast, the binding of [3H]PN 200-110 and [3H]dizocilpine maleate were unaffected. Co-injection of 15 mM 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione, a selective alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptor antagonist, prevented the formation of calcium concretions, the microglial reaction and the decrease in [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate binding but it failed to inhibit totally the astroglial reaction induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate. This may suggest that the microglial reaction and calcification take place through different mechanisms from the astrogliosis associated with the neuronal loss. In conclusion, acute administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate in the rat globus pallidus elicits a dose-dependent calcification process associated with a chronic reaction of astrocytes and microglia. alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-induced injury is accompanied by a slight reduction of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors in the ventral pallidum, whereas the binding of N-methyl-D-aspartate and L-type calcium channels receptors remains unchanged in any lesioned nucleus.

Effects of adenosine and gamma-aminobutyric acid A receptor antagonists on N-methyl-D-aspartate induced neurotoxicity in the rat hippocampus

This study investigated the modulatory actions of adenosine and gamma-aminobutyric acid (GABA) on several aspects of N-methyl-D-aspartate (NMDA)-induced neurotoxicity, including neuronal loss, atrophy, necrosis, and calcium accumulation in the hippocampus. For this purpose, we combined unilateral intrahippocampal injections of NMDA (24 nmoles) with acute injections of the selective A1 adenosine receptor antagonist DPCPX (0.03 pmoles), the selective adenosine A2a receptor antagonist CSC (1.5 pmoles), a combination of these two antagonists, and injections of the selective GABA A receptor antagonist bicuculline (60 pmoles). Fifteen days after NMDA injection, neuronal loss with preservation of architecture was observed in stratum oriens, pyramidale, radiatum, lacunosum-moleculare, and stratum moleculare of Ammon's horn, and in radial and granular layers of the dentate gyrus. NMDA plus vehicle also produced a small degree of brain tissue necrosis (holes in the structure) in four of five brains. Acute injections of CSC, but not DPCPX or bicuculline, significantly increased the extent of neuronal loss produced by NMDA plus vehicle. CSC in combination with NMDA induced significantly more necrosis than NMDA plus vehicle. A significant degree of atrophy was observed in the hippocampus after treatment with NMDA plus vehicle, and bicuculline significantly increased the magnitude of this atrophy. NMDA-induced calcium deposits were detected within the radiatum and lacunosum-moleculare layers of the hippocampus and in the hilus of the dentate, but not in the stratum oriens, stratum pyramidale, or in the granular layer of the dentate gyrus. However, treatment with the different antagonists did not significantly modify the magnitude of the NMDA-induced calcium deposits. These results reveal a selective vulnerability of certain areas of the hippocampus to the accumulation of calcium deposits, and a selective interaction between adenosine receptors and NMDA-induced neurotoxicity in the hippocampus.

Fimbria-fornix transection and excitotoxicity produce similar neurodegeneration in the septum

Fimbria-fornix transection produces neuronal injury in the septum. This cellular pathology is characterized by somatodendritic vacuolar abnormalities in neurons. Because these cellular changes are reminiscent of some of the morphological abnormalities seen with glutamate receptor-mediated excitoxicity, we tested whether excitotoxic injury to the septal complex of adult rats mimics the degeneration observed within the dorsolateral septal nucleus and medial septal nucleus following fimbria-fornix transection. The septal complex was evaluated at various time-points (6 h to 14 days) by light and electron microscopy following unilateral injection of the N-methyl-D-aspartate receptor agonist quinolinate or the non-N-methyl-D-aspartate receptor agonist kainate, and the morphological changes observed were compared to those abnormalities in the medial septal nucleus and dorsolateral septal nucleus at three to 14 days after fimbria-fornix transection. The patterns of cytoplasmic abnormalities and vacuolar injury were morphologically similar in the somatodendritic compartment of neurons following excitotoxicity and axotomy paradigms. These similarities were most evident when comparing the persistently injured neurons in the penumbral regions of the excitotoxic lesions at one to 14 days recovery to neurons in the medial septal nucleus and dorsolateral septal nucleus at seven and 14 days after fimbria-fornix transection. Pretreatment with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate prior to unilateral fimbria-fornix transection attenuated the somatodentritic vacuolar damage found within the ipsilateral dorsolateral and medial septal nuclei at 14 days recovery. Because glutamate is the principal transmitter of hippocampal efferents within the fimbria-fornix, we conclude that postsynaptic glutamate receptor activation participates in the evolution of septal neuron injury following fimbria-fornix transection. Thus, excitotoxicity is a possible mechanism for transneuronal degeneration following central nervous system axotomy.

Composition of ibotenic acid-induced calcifications in rat substantia nigra

Agonists of the excitatory neurotransmitter glutamate have neurotoxic properties and are, therefore, frequently used to place locally circumscript brain lesions. In certain vulnerable brain areas, especially the substantia nigra and globus pallidus, the ensuing neurodegeneration is accompanied by the formation of calcium deposits. In the present study, we investigated the structure and chemical composition of calcium deposits formed in rat substantia nigra upon local application of ibotenic acid. Using scanning and transmission electron microscopy in combination with X-ray analysis and analysis of the electron diffraction patterns, we demonstrate that the inorganic components of the calcifications consist of calcium and phosphate. The calcium phosphate is deposited in a polycrystalline manner in degenerating neurons and in a matrix surrounding the degenerated complexes. New matrix is continuously added around the enlarging calcium deposits. Content of inorganic material is always higher in the center of the deposits than in the margin, but in every case the diffraction pattern reveals that the calcium phosphates are present in the form of hydroxyapatite. Thus, organic and inorganic components of the calcifications are subject to a continuous process of growth and maturation. The ibotenic acid-induced calcium deposits in rat substantia nigra provide a reliable model system to study the pathogenesis of non-arteriosclerotic calcifications.

Bilateral lesions of the subthalamic nucleus induce multiple deficits in an attentional task in rats

Lesioning the subthalamic nucleus (STN) has been suggested as possible therapy for the treatment of parkinsonism. Previous experiments investigating this hypothesis in rats confirmed that excitotoxic STN lesions alleviate the motor impairment induced by striatal dopamine depletion, which reproduced the degeneration observed in parkinsonism, but elicited presumed non-motor deficits such as premature responding, suggesting that the STN could be involved in other aspects of response control. The aim of the present study was to extend this analysis to choice paradigms. We thus investigated the behavioural effects of bilateral excitotoxic lesions of the STN in rats performing a five-choice test of divided and sustained visual attention, modelled on the human continuous performance task. This task required the animals to detect a brief visual stimulus presented in one of five possible locations and respond by a nose-poke in this illuminated hole within a fixed delay, for food reinforcement. Bilateral lesions of the STN severely impaired several aspects of performance, including discriminative accuracy, but also increased premature, anticipatory responding as well as perseverative panel pushes and nose-poke responses. While increasing the stimulus duration and reducing the waiting period for the stimulus partially alleviated the accuracy deficit and the premature responding deficit respectively, other deficits, such as perseverative panel pushes and nose-poke responses, were sustained under these conditions. Systemic injection of the mixed dopaminergic D1/D2 receptor antagonist, alpha-flupenthixol (0.03-0.18 mg/kg), reduced premature responses and perseverative panel pushing without affecting the perseverative nose-poke responses, suggesting that some of the deficits were independent of striatal dopaminergic transmission. These results suggest that STN lesions have multiple, dissociable effects on attentional performance, including discriminative deficits, impulsivity and perseverative behaviour. They are consistent in part with a hypothesized role of the STN in recent models of basal ganglia function in action selection and inhibition. The results also show that other aspects of behaviour should be monitored when examining the capacity of STN lesions to reverse the parkinsonian deficit induced by striatal dopamine depletion.

In vitro effects of arachidonic and L-glutamic acids on the high-affinity choline transport in rat hippocampus

A second messenger role for arachidonic acid (AA) in the regulation of the high-affinity choline uptake (HACU) was suggested. It was reported that micromolar concentrations of AA applied in vitro decreased the HACU values and increased the specific binding of [3H]hemicholinium-3 ([3H]HCh-3). It was published that L-glutamic acid (GA) applied in vivo produced a fall in the HACU values. In addition, GA liberates free AA. In this study, an ability of GA to influence in vitro the activity of presynaptic cholinergic nerve terminals via its effect on the release of AA is investigated in hippocampal synaptosomes of young Wistar rats. Millimolar concentrations of GA decrease both the high- and low-affinity choline uptake, the specific as well as nonspecific binding of [3H]HCh-3 and the activity of Na+, K(+)-ATPase. Kinetic analysis (Lineweaver-Burk and Scatchard plots) reveals a change in Vmax and Bmax, but not in KM and KD. It appears very likely that under normal conditions GA applied in vitro is not able to change markedly the choline transport via its effect on the release of AA. Results confirm the hypothesis about an indirect inhibitory role for glutamatergic receptors on cholinergic cells.

Trinucleotide-repeat expansions and neurodegenerative disease: a mechanism of pathogenesis

1. Studies of a number of hereditary neurodegenerative diseases, the most common of which is Huntington's disease, have identified the expansion of trinucleotide repeats as a common causative mutation. 2. The diseases are caused by expansions of CAG repeats, encoding polyglutamine tracts, within the coding regions of a variety of unrelated genes. The mechanism whereby this specific genetic instability leads to selective neurodegeneration is currently unknown. 3. Our current understanding of these polyglutamine expansion neurodegenerative diseases is outlined. A potential mechanism is discussed whereby subtle alterations in glutamine, and consequently glutamate levels, may induce chronic excitotoxicity and slow cell death in neuronal populations possessing specific glutamate receptors. The potential role of glutamate receptor-mediated changes to intracellular calcium levels and energy metabolism in the neurodegenerative pathway is also addressed.

Ibotenic acid in the medial septum increased brain-derived neurotrophic factor mRNA levels in the dorsal rat hippocampal formation

Excitotoxic lesion of the medial septum with ibotenic acid leading to partial disappearance of the septal cholinergic nerve cells was used to investigate the role of cholinergic mechanisms in the control of trophic factors for hippocampal plasticity, namely brain-derived neurotrophic factor (BDNF) and glucocorticoid receptor (GR). Their mRNA levels were tested by in situ hybridization 13 days after the lesion. A persistent and widespread increase of BDNF mRNA was found in all parts of the dorsal hippocampal formation that was not accompanied by a significant modification in GR expression. The present data suggest that subcortical excitotoxic lesions at the septal level have long-term consequences for the adaptive trophic responses occurring in the dorsal hippocampus.

Nucleus reticularis thalami and neocortical paroxysms in the rat

The role of the nucleus reticularis thalami in spike-wave discharges in rats with genetic absence epilepsy has already been demonstrated. This study further investigated the role of the nucleus reticularis thalami in paroxysmal synchronizations in Sprague-Dawley rats; this strain shows no propensity to epileptic activity. Electroencephalographic patterns were followed in chronically implanted, unrestrained rats. After both electrolytic and chemical unilateral lesions, stereotaxically placed in the anterolateral sectors of this nucleus (verified post mortem), abnormal electroencephalographic rhythms (high-voltage polyspikes and spike-wave complexes) were recorded from the frontoparietal cortex, primarily in the contralateral hemisphere. Stereotyped discharges at 3 Hz developed progressively from multiple spikes within the alpha frequency range through the lengthening of the wave component. The excessive synchronized activity recorded from the intact hemisphere was of greater amplitude and occurred slightly earlier than from the lesioned hemisphere. These EEG patterns were associated with behavioural manifestations closely resembling those seen during absence seizures in humans. Bilateral lesions did not induce paroxysmal activity, both hemispheres being characterized by dominant delta/theta activity without signs of EEG-synchronized sleep. The seizures may thus have been due to disinhibition of the contralateral reticularis nucleus, recently shown to project to the reticularis nucleus of the other side in rats. This working hypothesis is supported by callosal cuts. The results indicate that the reticular neurons exert a control over neocortical paroxysmal activity even in animals which do not present genetic absence epilepsy.

Calcium deposit formation and glial reaction in rat brain after ibotenic acid-induced basal forebrain lesion

The mechanisms underlying amino acid neurotoxicity may involve a rise in the intracellular concentration of calcium. Some neurons appear to die as a consequence of increased intracellular calcium levels induced by excitatory amino acids. One month after injection of ibotenic acid in the rat basal forebrain, the induced formation of calcium deposits and concomitant glial reaction were studied. Alizarine Red-positive calcium deposits were observed after ibotenic acid injection in the ventral part of the globus pallidus, but not in the medial septum. These deposits were present in the globus pallidus, ventral pallidum, substantia innominata, zona incerta, lateral hypothalamic area, entopeduncular nucleus, medial amygdaloid nucleus and several thalamic nuclei. Three types of round shaped deposit were identified morphologically. Differential astroglial and microglial reactions, studied autoradiographically with the monoamine oxidase-B marker [3H]Ro19-6327 and the peripheral benzodiazepine receptor marker [3H]Ro5-4864 respectively, were observed after both lesions. Our data suggest that excitotoxic lesions in the globus pallidus and medial septum lead with time to different neurodegenerative consequences and glial reactions. This differential sensitivity is discussed on the basis of the presence of different glutamate receptor subtypes and calcium-binding proteins.