Distinct pharmacological properties of excitatory amino acid receptors in the rat striatum: study by Na+ efflux assay.

Specific 22Na+ efflux rates from preloaded rat striatal slices are increased in a dose-dependent manner by L-glutamate and other excitatory amino acids displaying the following order of efficiency: N-methyl-D-aspartate greater than DL-homocysteate greater than quisqualate greater than kainate greater than D-glutamate greater than L-glutamate greater than L-aspartate. Amino acid antagonists such as 2-amino-5-phosphonovalerate, gamma-D-glutamylglycine, DL-aminosuberate, DL-aminoadipate, and diethyl glutamate but not nonexcitatory amino acids such as gamma-aminobutyric acid inhibit the amino acid-induced increase in specific 22Na+ efflux rate. Increased K+ concentrations, in the presence of 2 mM Ca2+, increase the specific 22Na+ efflux. The latter and the response to N-methyl-D-aspartate, but not the responses to L-glutamate, L-aspartate, quisqualate, and kainate, are inhibited to similar extents by the same antagonists. These results suggest the release from striatal nerve terminals of a putative neurotransmitter with pharmacological properties different from those of L-glutamate or L-aspartate but similar to those of N-methyl-D-aspartate. The results of this study show that the stimulation of the 22Na+ efflux in brain slices by neuroactive amino acids and K+ ions is a valid and powerful tool for pharmacological investigations of excitatory amino acid receptors and their putative ligands.