Characterization of specific, high-affinity binding sites for L-[3H]glutamic acid in rat brain membranes

L-[3H]Glutamic acid binds reversibly to rat brain membranes with high affinity. Specific binding is linear with tissue concentration and has a pH optimum at neutrality. Saturation isotherms reveal anomolous kinetics of specific binding with an high affinity site with a KD of 11 nM and a lower affinity site with a KD of 80 nM; the Scatchard plots intercept at a common bound/free ratio. Hill plots of the complete saturation isotherms have a slope of 1.0. There are marked regional differences in the distribution of binding sites in rat brain: parietal cortex, frontal cortex, hippocampus greater than striatum greater than thalamus greater than cerebellum, pons-medulla and hypothalamus. Except for a small amount of specific binding in heart, other peripheral tissues do not exhibit specific binding of L-[3H]glutamic acid. Several amino acids with neuroexcitatory effects inhibit the specific binding: L-glutamic acid greater than L-aspartic acid and D,L-homocysteic acid greater than D-glutamic acid and L-cysteine sulfinic acid; related amino acids without neuroexcitatory effects do not inhibit specific binding. Reputed antagonists of glutamate-induced neuronal depolarization block specific binding: alpha-aminoadipic acid greater than 2-amino,4-phosphonobutyric acid greater than glutamate diethylester. Prior kainate lesion of the neurons intrinsic to the striatum results in a 45% decrement in specific binding of L-[3H]glutamic acid whereas cortical ablation, which causes degeneration of a cortical-striatal glutamatergic projection and reduces striatal glutamate synaptosomal uptake, does not affect specific binding. These results are compatible with the interpretation that the binding of [3H]glutamic acid occurs at excitatory receptors on neurons.