Neuroprotective activity of N-acetylaspartylglutamate in cultured cortical cells

The endogenous dipeptide, α-N-acetylaspartylglutamate behaves as a partial agonist of N-methyl-D-aspartate receptors, but can also activate metabotropic glutamate receptors, with a high degree of selectivity for the metabotropic glutamate receptor 3 subtype. Knowing that agonists of group-II metabotropic glutamate receptors (i.e. of mGlu2 and -3 receptors) are neuroprotective, we have examined the neuroprotective activity of α-N- acetylaspartylglutamate in mixed cultures of mouse cortical cells exposed to a toxic pulse with N-methyl-D-aspartate. α-N-acetylaspartylglutamate co- applied with N-methyl-D-aspartate was neuroprotective, but its action was insensitive to the selective group-II metabotropic glutamate receptor antagonist, ethylglutamate. Protection was instead antagonized by ethylglutamate when α-N-acetylaspartylglulamate was applied to the cultures immediately after the N-methyl-D-aspartate pulse, a condition in which there was no direct competition between α-N-acetylaspartylglutamate and N-methyl- D-aspartate at the level of N-methyl-D-aspartate receptors. α-N- acetylaspartylglutamate was highly neuroprotective when transiently applied to pure cultures of cortical astrocytes and the conditioned medium, collected 20 h later, was transferred to sister mixed cultures challenged with N- methyl-D-aspartate. This particular form of neuroprotection was attenuated or abolished when astrocytes where exposed to α-N-acetylaspartylglutamate in the presence of the group-II metabotropic glutamate receptor antagonists ethylglutamate or (2S, 1'S,2'S,3'R)-2-(2'-carboxy-3'- phenylcyclopropyl)glycine, but not in the presence of the N-methyl-D- aspartate receptor antagonist, D-2-amino-5-phosphonopentanoate. These results indicate that (α-N-acetylaspartylglutamate induces neuroprotective effects in culture, which are mediated, at least in part, by the activation of glial metabotropic glutamate receptor 3 receptors.