l-Glutamic acid, a neuromodulator of dopaminergic transmission in the rat corpus striatum.

A superfusion system was used to study the effects of neuroexcitatory amino acids upon spontaneous and depolarization-evoked release of exogenously taken up and newly synthesized [(3)H]dopamine by rat striatal slices. Neither l-glutamate nor other aminoacids such as l-aspartate and d-glutamate (5 x 10(?5) M) modified the spontaneous release of exogenous [(3)H]dopamine from rat striatal slices. In contrast, these neuroexcitatory aminoacids did potentiate spontaneous release of striatal [(3)H]dopamine newly synthesized from [(3)H]tyrosine. A different pattern of effects emerged when depolarization-evoked release of dopamine was studied. Only l-glutamate (5 x 10(?6)-1 x 10(?4) M) potentiated dopamine release under these experimental conditions in a rather specific and stereoselective manner. In addition, similar results were obtained regardless of whether depolarization-induced release of exogenous or newly synthesized [(3)H]dopamine was studied. The effect of l-glutamate on depolarization-induced release depended both upon the degree of neuronal depolarization and upon the presence of external Ca(2+) in the superfusion medium and it was blocked by l-glutamate diethylester. Furthermore, this effect of l-glutamate seemed quite specific with regard to regional localization within the brain as it was only demonstrated in slices from striatum and not in slices from olfactory tubercle or hippocampus. It is suggested that during depolarization a Ca(2+)-dependent event occurs at the striatal membrane level which changes the sensitivity of the dopamine release process to neuroexcitatory aminoacids in such a way as to render it relatively more specific and stereoselective towards l-glutamate stimulation. The findings reported have led us to propose that l-glutamic acid could play a role as a neuromodulator of dopaminergic transmission in the rat corpus striatum.