Transgenic SOD1 G93A mice develop reduced GLT-1 in spinal cord without alterations in cerebrospinal fluid glutamate levels

Glutamate-induced excitotoxicity is suggested to play a central role in the development of amyotrophic lateral sclerosis (ALS), although it is still u nclear whether it represents a primary cause in the cascade leading to moto r neurone death. We used western blotting, immunocytochemistry and in situ hybridization to examine the expression of GLT-1 in transgenic mice carryin g a mutated (G93A) human copper-zinc superoxide dismutase (TgSOD1 G93A), wh ich closely mimic the features of ALS. We observed a progressive decrease i n the immunoreactivity of the glial glutamate transporter (GLT-1) in the ve ntral, but not in the dorsal, horn of lumbar spinal cord. This effect was s pecifically found in 14- and 18-week-old mice that had motor function impai rment, motor neurone loss and reactive astrocytosis. No changes in GLT-1 we re observed at 8 weeks of age, before the appearance of clinical symptoms. Decreases in GLT-1 were accompanied by increased glial fibrillary acidic pr otein (GFAP) levels and no change in the levels of GLAST, another glial glu tamate transporter. The glutamate concentration in the cerebrospinal fluid (CSF) of TgSOD1 G93A mice was not modified at any of the time points examin ed, compared with age-matched controls. These findings indicate that the lo ss of GLT-1 protein in ALS mice selectively occurs in the areas affected by neurodegeneration and reactive astrocytosis and it is not associated with increases of glutamate levels in CSF. The lack of changes in GLT-1 at the p resymptomatic stage suggests that glial glutamate transporter reduction is not a primary event leading to motor neurone loss.