The excitation of mammalian central neurones by amino acids.

1. The relative potencies of a number of analogues of L-glutamate as excitants of thalamic neurones in the rat have been compared. The most powerful compounds were kainate, ibotenate and (+/-)cis-1-amino-1,3-dicarboxycyclopentane. The D- and L-isomers of glutamate and aspartate were also compared. Whereas D-glutamate is approximately one-half as active as the L-form, D-aspartate is more potent than L-aspartate. 2. Computer analysis has indicated that ibotenate and cis-1-amino-1,3-dicarboxy-cyclopentane have relatively fixed and similar C alpha-N, Comega-N and C alpha-Comega interatomic distances which can also be achieved by glutamate in certain conformations of the molecule, but not by aspartate. 3. Parallel examination of the antagonists glutamate diethylester and D-alpha-aminoadipate has shown that the former preferentially reduces L-glutamate effects while the latter blocks the actions of other amino acid excitants more readily than those of L-glutamate. 4. The evidence is consistent with the hypothesis that at least two populations of neuronal receptors for the excitatory amino acids exist.