HA-966 antagonizes N-methyl-D-aspartate receptors through a selective interaction with the glycine modulatory site

3-Amino-1-hydroxypyrrolid-2-one (HA-966) has been known for several years as an excitatory amino acid antagonist, acting principally at the N-methyl-D-aspartate (NMDA) receptor subtype. We report here that HA-966 blocks NMDA responses through a selective interaction with the glycine modulatory site present within the receptor complex. In radioligand binding experiments, HA-966 inhibited strychnine-insensitive 3H-glycine binding to rat cerebral cortex synaptic plasma membranes with an IC50 of 17.5 microM. At concentrations up to 1 mM, HA-966 caused minimal inhibition of radioligand binding to the transmitter recognition sites of the NMDA, quisqualate, or kainate receptor subtypes and was similarly inactive against the binding of 3H-strychnine to rat spinal cord/brain stem membranes. In electrophysiological experiments, HA-966 produced a selective block of NMDA responses in a rat cortical slice preparation. The degree of antagonism caused by HA-966 was maximal at 250 microM and was not increased further by raising the HA-966 concentration. Both glycine (1 mM) and D-serine (100 microM) reversed the antagonism of NMDA responses caused by HA-966. In patch-clamp experiments using rat cortical neurons in culture, HA-966 blocked the potentiation of NMDA responses by glycine but had little effect on basal NMDA responses themselves. This profile of antagonism differs from that observed with 7-chlorokynurenate, another recently discovered antagonist of the glycine site on the NMDA receptor (Kemp et al., 1988) and may indicate that glycine antagonists of differing efficacies can exist. Previous experiments with HA-966 may now be interpreted to suggest that activation of the glycine site on the NMDA receptor occurs in vivo and is important for the participation of NMDA receptors in synaptic transmission.