Different modes of action of 3-amino-1-hydroxy-2-pyrrolidone (HA-966) and 7-chlorokynurenic acid in the modulation of N-methyl-D-aspartate-sensitive glutamate receptors

The N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors are known to be inhibited by 3-amino-1-hydroxy-2-pyrrolidone (HA-966) and 7-chlorokynurenic acid (Cl-KYN), which act at the glycine-regulated allosteric modulatory center. In this work we show that, in synaptic membranes prepared from rat brain, Cl-KYN and HA-966 inhibit the binding of [3H]glycine. Moreover, Cl-KYN can also completely inhibit the binding of [3H]glutamate to the primary transmitter recognition site for the NMDA receptor, whereas HA-966 only partially reduces this binding. Cl-KYN also abolishes the binding of the NMDA receptor antagonist [3H]3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). In contrast, HA-966 increases [3H]CPP binding, affecting the affinity but not the maximal number of binding sites. This increase is inhibited by glycine and Cl-KYN. The binding of [3H] (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801), used as an index of NMDA receptor activation, is completely inhibited by Cl-KYN but only partially by HA-966. In addition, HA-966, but not Cl-KYN, increases the potency of CPP in inhibiting [3H]MK-801 binding. Our results demonstrate that Cl-KYN and HA-966 differ in their ability to modulate the NMDA receptor, perhaps acting at distinct but overlapping recognition sites. Furthermore, our results suggest that agonist and antagonist recognition sites of the NMDA receptor may be independently regulated by glycine and HA-966, which would result, respectively, in a positive and negative allosteric modulation of the NMDA receptor complex.