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.

Differential brain area vulnerability to long-term subcortical excitotoxic lesions.

To investigate the long-term effects of excitatory amino acid microinjections into the basal forebrain and its correlation with a possible Ca2+ imbalance associated with the excitotoxic process, ibotenic acid, mainly an N-methyl-D-aspartate receptor agonist, and quisqualic acid, an agonist of non-N-methyl-D-aspartate receptors, were injected into two regions rich in cholinergic neurons, namely the medial septal nucleus and the ventral globus pallidus. Within the globus pallidus but not within the medial septal nucleus, 13 days and one year postlesion, nerve cell death was associated with the appearance of calcium deposits within the large putative GABAergic pallidal neurons, being more pronounced in ibotenic acid than quisqualic acid-lesioned rats. An intermediate two month post-lesion study with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and ibotenic acid microinjections in globus pallidus demonstrated that the AMPA subtype of glutamate receptor may also be involved in this Ca2+ imbalance, together with the N-methyl-D-aspartate and metabotropic subtype receptors. Quisqualic acid lesions in globus pallidus and medial septum were associated with a substantial disappearance of cholinergic cell bodies and their nerve terminal networks within the cerebral cortex and hippocampal formation respectively, as assessed by choline acetyltransferase and acetylcholine esterase immunocytochemistry. Ibotenic acid lesions resulted in a lower reduction of cholinergic markers. One year after septal lesions induced either by ibotenic or quisqualic acid, a marked atrophy of the entire dorsolateral septal nucleus was observed. Our results support the hypothesis that brief and intense glutamate exposure can induce long-term neurodegenerative processes and give evidence that long-term excitotoxic lesions of the two areas studied result in marked differences in neuronal damage, including intracellular calcium deposits which do not correlate with the cholinergic deficits produced by multiple glutamate receptor subtypes.

Time-related cortical amino acid changes after basal forebrain lesion: a microdialysis study.

Cholinergic basal forebrain (BF) lesions in experimental animals have been used as a potential model for cholinergic deficits in cortex and hippocampus that occur in normal aging and Alzheimer's disease (AD). Glutamatergic cortical neurons are also affected in AD and could be part of the neurodegenerative process. In the present study, the effect of bilateral BF lesion with ibotenic acid microinjection on cortical extracellular amino acid levels was determined. Samples were collected every 20 min with microdialysis probes in awake, freely moving rats under basal and potassium stimulation conditions and measured by HPLC with fluorescence detection. Microdialysis experiments were performed 13 days, 21 days, and 30 days after BF lesion. The effectiveness of the lesion was shown by a significant 30% depletion in acetyl-CoA:choline O-acetyltransferase (EC 2.3.1.6) activity in the frontal cortex. Under basal conditions at 13 days only extracellular levels of taurine (Tau) and Glu were significantly reduced. Tau and Glu levels were recovered after 21 days and 30 days, respectively. In contrast, increase in Gly levels reaches its significance only at 30 days after lesion. Significant increases of Gln levels were observed at 21 days and 30 days. Asp and Ser levels remained constant throughout the period studied. Potassium stimulation led to increased Asp, Glu, Gly, and Tau levels, whereas Gln content decreased and Ser remained unaltered. As Ser is not believed to be a neurotransmitter, its lack of variation in any of the experimental conditions studied supports specific neuronal changes of the other amino acids. Results are discussed with reference to data observed in AD patients and possible mechanisms underlying the changes are suggested.

Decreased cortical octopamine level in basal forebrain lesioned rats: a microdialysis study.

Ibotenic acid-induced lesion of the basal forebrain resulted after 13 days in a 90% reduction of octopamine (OA) in the frontoparietal cortex of adult rats, whereas dihydroxyphenylacetic (DOPAC), homovanillic (HVA) and 5-hydroxyindoleacetic (5-HIAA) acids were not modified as measured by microdialysis and high-performance liquid chromatography (HPLC) with electrochemical detection. At this time, cortical choline acetyltransferase (ChAT) activity was decreased by 34%. The results are discussed with respect to possible octopamine involvement in reduced age-associated performance in neurodegenerative processes.

Neuronal changes in the nigrostriatal pathway of 1-methyl-4-phenylpyridine-treated mice.

Thirty young-adult mice were treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) 30 mg/kg/day for 2 days and sacrificed 24 hours later in order to determine striatal catecholamines and to study morphological changes in the nigrostriatal pathway. Immunohistological techniques were also used with polyclonal antibodies for glial fibrillary acidic protein (GFAP) and tyrosine hydroxylase (TH), and monoclonal antibodies for two subunits of neurofilaments. Silver impregnation demonstrated conspicuous neuronal changes affecting cellular processes from substantia nigra in all treated mice. Terminal and axonal degeneration were also observed in striata. These changes were associated with a moderate to marked gliosis. The TH immunoreactivity was normal in cell bodies of substantia nigra but was decreased in striata from MPTP-treated mice. These data indicate that in mice the deterioration of dendritic and axonal neuropil may constitute a significant causal factor of MPTP neurotoxicity.

Widespread neuronal degeneration after ibotenic acid lesioning of cholinergic neurons in the nucleus basalis revealed by in situ hybridization.

In efforts to test the cholinergic hypothesis for Alzheimer's disease and to create an animal model for this disease, ibotenic acid has been used to lesion cholinergic neurons in the basal forebrain. In this study we have used in situ hybridization with oligonucleotide probes specific for mRNAs encoding choline acetyltransferase and glutamic acid decarboxylase, respectively, to study the effects of such a lesion. Our results show that lesion paradigms normally used to induce neuronal degeneration in nucleus basalis by ibotenic acid not only lesion the cholinergic neurons within this nucleus, but in addition, a major fraction of gamma-aminobutyric acid (GABA) neurons in nucleus basalis, substantia innominata, globus pallidus and ventral pallidum.