Glutamate uptake disguises neurotoxic potency of glutamate agonists in cerebral cortex in dissociated cell culture.
The pharmacological properties of glutamate agonists were compared in astrocyte-rich and astrocyte-poor cultures derived from embryonic rat cerebral cortex. The object of this investigation was to determine the extent to which glutamate uptake might influence the receptor-mediated neurotoxic actions of these compounds. In astrocyte-rich cultures, using 30 min exposures, we observed that the potencies of the poorly transported agonists NMDA (35 microM) and D-glutamate (89 microM) were higher than that of L-glutamate (205 microM). In astrocyte-poor cultures, L-glutamate was much more potent, with an EC50 of 5 +/- 4 microM (3-12 microM), for a 30 min exposure, whereas the potencies of NMDA and D-glutamate were essentially unchanged. L- and D-aspartate were also more effective in astrocyte-poor cultures, again with EC50 values of approximately 6-10 microM, as compared with 130 and 108 microM, respectively, in astrocyte-rich cultures. In other experiments, blocking sodium-dependent glutamate uptake in astrocyte-rich cultures, by using a sodium-free medium, made glutamate as potent an agonist as in astrocyte-poor cultures. Finally, we directly assessed the glutamate uptake system in astrocyte-rich and astrocyte-poor cultures and found that uptake was reduced approximately 25-fold in the astrocyte-poor cultures. These results show that in the presence of abundant astrocytes the neurotoxic potencies of L-glutamate, L-aspartate, and D-aspartate are substantially under-estimated.
Department of Neurology, Children's Hospital, Boston, Massachusetts 02115.
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Rosenberg
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Amin
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Leitner
D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism / Q000494:pharmacology; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001692:Biological Transport; D002470:Cell Survival / Q000187:drug effects*; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000196:embryology / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism* / Q000494:pharmacology / Q000633:toxicity; D018698:Glutamic Acid; D016202:N-Methylaspartate / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000494:pharmacology; D013237:Stereoisomerism
D005971:Glutamates; D001224:Aspartic Acid; D018698:Glutamic Acid; D016202:N-Methylaspartate; D012964:Sodium