Glutamate Release Induced by Activation of Glycine and GABA Transporters in Spinal Cord is Enhanced in a Mouse Model of Amyotrophic Lateral Sclerosis

Abstract

Amyotrophic lateral sclerosis is a progressive and fatal neurodegenerative disease, involving both upper and lower motor neurons, the cause of which is obscure, although glutamate (GLU)-induced excitotoxicity has been suggested to play a major role. We studied the release of [ 3H] d-aspartate ([ 3H] d-ASP) and endogenous glutamate evoked by glycine (GLY) or GABA from spinal cord synaptosomes in mice expressing a mutant form of human SOD1 with a Gly 93Ala substitution ([SOD1-G93A(+)]), a transgenic model of amyotrophic lateral sclerosis, in mice expressing the non-mutated form of human SOD1 [SOD1(+)], and in non-transgenic littermates [SOD1(−)/G93A(−)]. In parallel experiments, we also studied the release of [ 3H]GABA evoked by GLY and that of [ 3H]GLY evoked by GABA. Mutant mice were killed at advanced phase of pathology or during the pre-symptomatic period. In SOD1(−)/G93A(−) or SOD1(+) mice GLY evoked [ 3H] d-ASP and [ 3H]GABA release, while GABA caused [ 3H] d-ASP, but not [ 3H]GLY, release. The GLY-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]GLY, were more pronounced in SOD1-G93A(+) than in SOD1(+) or SOD1(−)/G93A(−) mice. Furthermore, the excessive potentiation of [ 3H] d-ASP by GLY or GABA was already present in asymptomatic 30–40 day-old SOD1-G93A(+) mice. The releases of endogenous glutamate and GABA also were enhanced by GLY and the GLY-evoked release of endogenous glutamate, but not of endogenous GABA, was higher in SOD1-G93A(+) than in control animals. Potentiation of the spontaneous amino acid release is likely to be mediated by activation of a GLY or a GABA transporter, since the effect of GLY was counteracted by the GLY transporter blocker glycyldodecylamide but not by the GLY receptor antagonists strychnine and 5,7-dichlorokynurenate while the effect of GABA was diminished by the GABA transporter blocker SKF89976-A but not by the GABA receptor antagonists SR9531 and CGP52432. It is concluded that the glutamate release machinery seems excessively functional in SOD1-G93A(+) animals.