Excessive and precocious glutamate release in a mouse model of amyotrophic lateral sclerosis

The release of [3H]d-aspartate ([3H]d-ASP) or [3H]GABA evoked by glycine and that of [3H]d-ASP or [3H]glycine evoked by GABA from spinal cord synaptosomes were studied in SOD1-G93A(+) mice, a transgenic model of amyotrophic lateral sclerosis, SOD1(+) mice and SOD1(−)/G93A(−) animals. Mutant mice were killed at advanced phase of pathology or during the presymptomatic period. In SOD1(−)/G93A(−) or SOD1(+) mice glycine evoked [3H]d-ASP and [3H]GABA release, while GABA caused [3H]d-ASP, but not [3H]glycine, release. The glycine-evoked release of [3H]d-ASP, but not that of [3H]GABA, and the GABA-evoked [3H]d-ASP release, but not that of [3H]glycine, were more pronounced in SOD1-G93A(+) than in SOD1(+) mice. Furthermore, these potentiations were already present in asymptomatic 30- to 40-day-old mice. Basal [3H]d-ASP release was also higher in SOD1-G93A(+) than SOD1(+) or SOD1(−)/G93A(−) mice. The release of endogenous glutamate and GABA was also enhanced in asymptomatic animals; the glycine-evoked release of endogenous glutamate, but not of endogenous GABA, was higher in SOD1-G93A(+) than in SOD1(+) animals. The effects of glycine and GABA were insensitive to receptor blockers, but sensitive to transporter inhibitors, indicating coexistence of glutamate and glycine transporters and of glutamate and GABA transporters on glutamate-releasing terminals. The glutamate release machinery seems excessively functional in SOD1-G93A(+) animals.