Specificity and potency of N-methyl-D-aspartate glycine site antagonists and of mephenesin on the rat spinal cord in vitro.

The potency, specificity and reversibility of various presumed glycine site N-methyl-D-aspartate (NMDA) antagonists was studied on neonatal rat spinal cord using the grease gap technique. 5,7-Dichlorokynurenate was the most potent and specific glycine site antagonist among the compounds tested. On the other hand mephenesin was a weak non-specific excitatory amino acid (EAA) antagonist; reduction of the response to NMDA was not reversed by D-serine. The EAA antagonist properties of mephenesin could explain its mode of action at the cellular level. The lack of effect of D-serine alone suggests that in our experimental conditions glycine sites on spinal neurones are occupied by an endogenous ligand.