EXOGENOUS GLUTAMATE ENHANCES GLUTAMATE-RECEPTOR SUBUNIT EXPRESSION DURING SELECTIVE NEURONAL INJURY IN THE VENTRAL ARCUATE NUCLEUS OF POSTNATAL MICE

Administration of high doses of glutamate (GIu) leads to selective neu rodegeneration in discrete brain regions near circumventriclular organ s of the early postnatal mouse. The arcuate nucleus-median eminence co mplex (ARC-ME) appears to be the most Glu-sensitive of these brain reg ions, perhaps because of the intimate relationships between its neuron s and specialized astroglial tanycytes. To investigate the mechanism o f Glu-induced neuronal loss, we administered graded doses of the sodiu m salt of glutamate (MSG) to postnatal mice, measured their plasma Glu concentrations, and performed microscopic analyses of the ARC-ME regi on 5 h after treatment. Nursing, 7-day-old mouse pups (CD1, Charles Ri ver, 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 d ose of 4.0 mg MSG/g BW caused neurodegeneration throughout the ARC reg ion, while 1.0 mg/g MSG resulted in less extensive damage. Injection o f 0.2 mg MSG/g BW, which raised plasma Glu concentrations 17-fold afte r 15 min, was the minimum dose tested at which nuclear and cytoplasmic changes were observed in a small group of subependymal neurons near t he lateral recesses of the third ventricle. Higher doses of 0.3-0.5 mg MSG caused injury to additional neurons situated farther laterally, b ut damage remained confined to the ventral region of the ARC nucleus. Ultrastructural examination showed some subependymal neurons with pykn otic nuclei, reduced cytoplasmic volume, and swollen subcellular organ elles, while others had fragmented and condensed nuclear material. Imm unostaining for tyrosine hydroxylase indicated that dopamine neurons w ere spared at the threshold dose, but suffered damage after higher dos es 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 i n injured neurons. These results extend previous reports of Glu sensit ivity in the ARC-ME region of 7-day postnatal mice. A dose of 0.2 mg M SG/g BW s.c. causes clear but discrete injury to specific subependymal neurons of undetermined phenotype near the base of the third ventricl e. Slightly higher doses of MSG evoke damage of additional neurons con fined to the ventral region of the ARC traversed by tanycytes. These s ame greater amounts of MSG promote dose-related increase in the expres sion of NMDAR1 more than of GluR2/4 in injured ARC neurons, suggesting that elevated GIu receptor levels may contribute to or be related to neuronal cell death. Taken together with previous findings, the data s uggest that GIu responsitivity in the ARC-ME of the postnatal mouse ma y result from transient developmental conditions involving the numeric al ratios and juxtaposition between tanycytes and neurons, expression of Glu receptors, and perhaps other ontogenetic factors which may not persist in the mature adult.