INTERACTIONS BETWEEN N-ACETYLASPARTYLGLUTAMATE AND AMPA, KAINATE, ANDNMDA BINDING-SITES

The structure of N-acetylaspartylglutamate (NAAG) suggests this neuron al dipeptide as a candidate for interaction with discrete subclasses o f ionotropic and metabotropic acidic amino acid receptors. A substanti al difficulty in the assay of these interactions is posed by membrane- bound peptidase activity that converts the dipeptide to glutamate and N-acetylaspartate, molecules that will interfere with receptor assays. We have developed two sets of unique receptor assay conditions and ap plied one standard assay to measure the interactions, under equilibriu m binding conditions, of [H-3]kainate, [H-3]amino-3-hydroxy-5-methylis oxazole-4-propionic acid ([H-3]AMPA), and [H-3]CGS-19755 with the thre e classes (kainate, quisqualate, and N-methyl-D-aspartate) of ionotrop ic glutamate receptors, while inhibiting peptidase activity against NA AG. Under these conditions, NAAG exhibits apparent inhibition constant s (IC50) of 500, 790, and 8.8 mu M in the kainate, AMPA, and CGS-19755 receptor binding assays, respectively. Glutamate was substantially mo re effective and less specific in these competition assays, with inhib ition constants of 0.36, 1.1, and 0.37 mu M. These data support the hy pothesis that, relative to glutamate, NAAG functions as a specific, lo w potency agonist at N-methyl-D-aspartate subclass of ionotropic acidi c amino acid receptors, but the peptide is not likely to activate dire ctly the kainate or quisqualate subclasses of excitatory ionotropic re ceptors under physiologic conditions.