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      Exogenous Glutamate Enhances Glutamate Receptor Subunit Expression during Selective Neuronal Injury in the Ventral Arcuate Nucleus of Postnatal Mice

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          Administration of high doses of glutamate (Glu) leads to selective neurodegeneration in discrete brain regions near circumventriclular organs of the early postnatal mouse. The arcuate nucleus-median eminence complex (ARC-ME) appears to be the most Glu-sensitive of these brain regions, perhaps because of the intimate relationships between its neurons and specialized astroglial tanycytes. To investigate the mechanism of Glu-induced neuronal loss, we administered graded doses of the sodium salt of glutamate (MSG) to postnatal mice, measured their plasma Glu concentrations, and performed microscopic analyses of the ARC-ME region 5 h after treatment. Nursing, 7-day-old mouse pups (CD1, Charles River, Hollister, Calif.) were injected subcutaneously with single doses of 0.1–0.5 or 1.0–4.0 mg of MSG per g BW, or with water vehicle alone. Mice were decapitated 5 h later and the brains immediately fixed by immersion in buffered aldehydes. Frontal vibratome tissue sections at comparable levels of the ARC-ME were examined by light microscopy. A dose of 4.0 mg MSG/g BW caused neurodegeneration throughout the ARC region, while 1.0 mg/g MSG resulted in less extensive damage. Injection of 0.2 mg MSG/g BW, which raised plasma Glu concentrations 17-fold after 15 min, was the minimum dose tested at which nuclear and cytoplasmic changes were observed in a small group of subependymal neurons near the lateral recesses of the third ventricle. Higher doses of 0.3–0.5 mg MSG caused injury to additional neurons situated farther laterally, but damage remained confined to the ventral region of the ARC nucleus. Ultrastructural examination showed some subependymal neurons with pyknotic nuclei, reduced cytoplasmic volume, and swollen subcellular organelles, while others had fragmented and condensed nuclear material. Immunostaining for tyrosine hydroxylase indicated that dopamine neurons were spared at the threshold dose, but suffered damage after higher doses of MSG. Immunostaining for Glu receptor subtypes revealed that 0.2 mg MSG/g BW enhanced neuronal expression of NMDAR1 and of GluR2/4, and that higher doses of MSG preferentially increased NMDAR1 expression in injured neurons. These results extend previous reports of Glu sensitivity in the ARC-ME region of 7-day postnatal mice. A dose of 0.2 mg MSG/g BW s.c. causes clear but discrete injury to specific subependymal neurons of undetermined phenotype near the base of the third ventricle. Slightly higher doses of MSG evoke damage of additional neurons confined to the ventral region of the ARC traversed by tanycytes. These same greater amounts of MSG promote dose-related increase in the expression of NMDAR1 more than of GluR2/4 in injured ARC neurons, suggesting that elevated Glu receptor levels may contribute to or be related to neuronal cell death. Taken together with previous findings, the data suggest that Glu responsitivity in the ARC-ME of the postnatal mouse may result from transient developmental conditions involving the numerical ratios and juxtaposition between tanycytes and neurons, expression of Glu receptors, and perhaps other ontogenetic factors which may not persist in the mature adult.

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          Most cited references 8

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          Glutamate-induced neuronal death: a succession of necrosis or apoptosis depending on mitochondrial function.

          During ischemic brain injury, glutamate accumulation leads to overstimulation of postsynaptic glutamate receptors with intracellular Ca2+ overload and neuronal cell death. Here we show that glutamate can induce either early necrosis or delayed apoptosis in cultures of cerebellar granule cells. During and shortly after exposure to glutamate, a subpopulation of neurons died by necrosis. In these cells, mitochondrial membrane potential collapsed, nuclei swelled, and intracellular debris were scattered in the incubation medium. Neurons surviving the early necrotic phase recovered mitochondrial potential and energy levels. Later, they underwent apoptosis, as shown by the formation of apoptotic nuclei and by chromatin degradation into high and low molecular weight fragments. These results suggest that mitochondrial function is a critical factor that determines the mode of neuronal death in excitotoxicity.
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            Primary structure and functional characterization of a high-affinity glutamate transporter.

             M Hediger,  Y. Kanai (1992)
            Glutamate transport across plasma membranes of neurons, glial cells and epithelial cells of the small intestine and kidney proceeds by high- and low-affinity transport systems. High-affinity (Km 2-50 microM) transport systems have been described that are dependent on Na+ but not Cl- ions and have a preference for L-glutamate and D- and L-aspartate. In neurons high-affinity glutamate transporters are essential for terminating the postsynaptic action of glutamate by rapidly removing released glutamate from the synaptic cleft. We have isolated a complementary DNA encoding an electrogenic Na(+)- but not Cl(-)-dependent high-affinity glutamate transporter (named EAAC1) from rabbit small intestine by expression in Xenopus oocytes. We find EAAC1 transcripts in specific neuronal structures in the central nervous system as well as in the small intestine, kidney, liver and heart. The function and pharmacology of the expressed protein are characteristic of the high-affinity glutamate transporter already identified in neuronal tissues. The abnormal glutamate transport that is associated with certain neurodegenerative diseases and which occurs during ischaemia and anoxia could be due to abnormalities in the function of this protein.
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              The TINS/TiPS Lecture. The molecular biology of mammalian glutamate receptor channels.

              In native brain membranes the principal excitatory neurotransmitter L-glutamate activates cation-conducting channels with distinct biophysical and pharmacological properties. Molecular cloning has revealed the existence of 16 channel subunits that can assemble in homomeric or heteromeric configurations in vitro to form receptor channels with disparate functional properties. This review describes the different channel types obtained by recombinant means and the genetic mechanisms controlling the expression of functionally important channel structures.

                Author and article information

                S. Karger AG
                August 1998
                21 August 1998
                : 68
                : 2
                : 77-88
                a Reproductive Endocrinology Center, Department of Ob/Gyn and Reproductive Sciences, University of California, San Francisco, Calif., and b Departments of Psychiatry, Pharmacology and Neuroscience, University of Pittsburgh School of Medicine, Pittsburgh, Pa., USA
                54353 Neuroendocrinology 1998;68:77–88
                © 1998 S. Karger AG, Basel

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                Page count
                Pages: 12
                Regulation of Hypothalamic Neurons


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