Effects of metabotropic glutamate receptor agonists and antagonists on D-aspartate release from mouse cerebral cortical and striatal slices

The cytosolic release of L-glutamate has been held to be responsible for the increase in extracellular glutamate to toxic levels in the brain. The mechanism and regulation of this release was now studied in cerebral cortical and striatal slices with D-[3H]aspartate, a non-metabolized analogue of L-glutamate and a poor substrate for vesicular uptake. L-Glutamate and D-aspartate strongly stimulated the release in a concentration-dependent manner. Of the ionotropic glutamate receptor agonists, only kainate enhanced the basal release in the striatum. Of the metabotropic glutamate receptor ligands, the group I agonist (S)-3,5-dihydroxyphenylglycine (S-DHPG) failed to affect the basal release but inhibited the D-aspartate-evoked release in the striatum. The group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) had no effect on the basal release in either preparation but enhanced the L-glutamate-evoked release and inhibited the D-aspartate-evoked release in the striatum, not however in the cerebral cortex. The group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and the group II antagonist (2S)-2-ethylglutamate (EGLU) were without effect on the basal, D-aspartate- and L-glutamate-evoked releases of D-[3H]aspartate in either preparation. The group III agonist L-serine-O-phosphate (L-SOP) failed to affect the basal release but reduced the D-aspartate-evoked release in the striatum. The group III antagonist (RS)alpha-methylserine-O-phosphate (MSOP) failed to affect the basal release but increased the glutamate-evoked release and inhibited the D-aspartate-evoked release in the striatum. Both L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) and (2S,1'S,2'R)-2-carboxycyclopropyl)glycine (L-CCG-III), transportable inhibitors of the high-affinity glutamate uptake, enhanced the basal release, more strongly in the striatum than in the cerebral cortex. L-CCG-III also increased the L-glutamate-evoked release in the striatum. Nontransportable dihydrokainate enhanced the basal release much less and failed to affect the glutamate-evoked release. The results indicate that the release of glutamate from cytosolic pools is carrier-mediated via homoexchange. This process is regulated in the striatum by metabotropic group I and group III receptors in a manner different from the regulation of the vesicular release of glutamate from presynaptic terminals.