Mercuric chloride, but not methylmercury, inhibits glutamine synthetase activity in primary cultures of cortical astrocytes

Methylmercury (MeHg) is highly neurotoxic with an apparent dose-related lat ency period between time of exposure and the appearance of symptoms. Astroc ytes are known targets for MeHg toxicity and a site of mercury localization within the central nervous system (CNS). Glutamine synthetase (GS) is an e nzyme localized predominately within astrocytes. GS converts two potentiall y toxic molecules, glutamate and ammonia, to the relatively non-toxic amino acid,,glutamine. During prolonged exposure to MeHg, inorganic mercury (I-H g) accumulates within the brain, suggesting in situ demethylation of MeHg t o I-Hg. To determine if speciation of mercurials would differentially alter GS activity and expression, neonatal rat primary astrocyte cultures were e xposed to MeHg or mercuric chloride (HgCl2) for I or 6 h. MeHg produced no changes in GS activity, protein, or mRNA at any time or dose tested. In con trast, HgCl2 produced a dose dependent decrease in astrocytic GS activity a t both 1 and 6 h. There were no changes in GS protein or mRNA levels follow ing HgCl2 exposure. Additional studies were carried out to determine GS act ivity in cell lysates incubated with HgCl2 or MeHg. In cell lysates, HgCl2 was three-times more potent than MeHg in inhibiting GS activity. The inhibi tion of GS activity in cell lysates by HgCl2 was reversed by the addition o f dithiothreitol (DTT), while DTT did not restore GS activity following MeH g. These data suggest that astrocytic GS activity is not inhibited by physi ologically relevant concentrations of MeHg, but is inhibited by I-HX, which is present in CNS following chronic MeHg exposure. (C) 2001 Elsevier Scien ce B.V. All rights reserved.