Effects of ammonia on glutamate transporter (GLAST) protein and mRNA in cultured rat cortical astrocytes

Ammonia is a neurotoxic substance which accumulates in brain in liver failu re and it has been suggested that ammonia plays a key role in contributing to the astrocytic dysfunction characteristic of hepatic encephalopathy. In particular, the effects of ammonia may be responsible for the reduced astro cytic uptake of neuronally-released glutamate and high extracellular glutam ate levels consistently seen in experimental models of hepatic encephalopat hy. To further address this issue, [H-3]-D-aspartate uptake was examined in primary rat cortical astrocyte cultures exposed to 5 mM ammonium chloride for a period of 7 days. In addition, reverse transcrirptase-polymerase chai n reaction (RT-PCR) and Western blot studies were performed to examine the mRNA and protein expression respectively of the glutamate transporter CLAST in ammonia-treated cells. Studies revealed a 57% (p < 0.05) decrease in [H -3]-D-aspartate uptake and a concomitant significant decrease in GLAST tran sporter protein (43%, p < 0.05) and mRNA (32%, p < 0.05) expression. The re duced capacity of astrocytes to reuptake glutamate following ammonia exposu re may result in compromised neuron-astrocyte trafficking of glutamate and could thus contribute to the pathogenesis of the cerebral dysfunction chara cteristic of hyperammonemic syndromes such as hepatic encephalopathy. (C) 2 000 Elsevier Science Ltd. All rights reserved.