Modulation of [3H]3-((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid ([3H]CPP) binding by ligands acting at the glycine and the polyamine sites of the rat brain NMDA receptor complex.

The competitive N-methyl-D-aspartate (NMDA) receptor antagonist [3H]3-((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid ([3H]CPP) interacts with two discrete binding sites, one of high- and the other of low-affinity, on rat forebrain synaptic plasma membranes. The high affinity site exhibited a Kd of 101.5 nM and a Bmax of 2.01 pmol/mg, while for the low affinity site the Kd was 4.11 microM with a Bmax of 19.7 pmol/mg. The glycine site antagonists 3-amino-1-hydroxy-2-pyrrolidone (HA-966), 1-aminocyclobutanecarboxylic acid (ACBC), the glycine site agonist 1-aminocyclopropanecarboxylic acid (ACC) and glycine itself (as well as the polyamines spermine and spermidine), enhanced [3H]CPP binding. When subjected to saturation analysis, this enhancement was found primarily to involve an increase in the affinity of the high affinity component of [3H]CPP binding. Neither of the parameters of the low affinity component of binding were affected. Although a similar enhancement was observed with the polyamines, the effects of these two classes of ligands were additive, consistent with their having actions at different recognition sites on the NMDA receptor complex.