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Interest in the physiology and pharmacology of the dicarboxylic amino acids L-glutamate and L-aspartate originally stemmed from identification of the powerful excitatory actions of these amino acids in the motor cortex by Hayashi in 1954 (1) and in the spinal cord by Curtis and colleagues in 1960 (2). After a fairly prolonged period of doubt, excitatory amino acids (EAAs) have gradually become accepted as the major transmitter of fast excitatory signals in the mammalian central nervous system (eNS). The receptors mediating these actions have been classified into three subtypes based on the selective agonists N-methyl-D-aspartate (NMDA), quisqualate, and kainate (3-6). The recent rapid advances in this field have primarily re­ sulted from the identification and radiolabeling of potent and selective agonists and antagonists for elucidating the localization, density, ligand binding, and modulatory properties of these various subtypes of BAA re­ ceptors (7, 8). In particular, this approach has been very fruitful in the characterization of various sites associated with the NMDA receptor chan­ nel complex. This has complemented the powerful electrophysiological tech