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 failure 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 astrocytic uptake of neuronally-released glutamate and high extracellular glutamate levels consistently seen in experimental models of hepatic encephalopathy. 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 chain 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 transporter protein (43%, p < 0.05) and mRNA (32%, p < 0.05) expression. The reduced capacity of astrocytes to reuptake glutamate following ammonia exposure may result in compromised neuron-astrocyte trafficking of glutamate and could thus contribute to the pathogenesis of the cerebral dysfunction characteristic of hyperammonemic syndromes such as hepatic encephalopathy. (C) 2000 Elsevier Science Ltd. All rights reserved.