N-methyl-D-aspartate receptors in the retina: 3-[(+/-)-2-carboxypiperazin- 4-yl]-propyl-1-phosphonic acid (CPP) binding studies.

3-[(RS)-2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) has been known for some years as one of the most selective antagonists at the N-methyl-D-aspartate (NMDA) receptor in the brain. The characteristics of the binding of [3H]CPP to chick retinal membranes were studied from the biochemical and pharmacological point of view. Magnesium induced a dose-dependent increase in the binding of [3H]CPP (EC50 = 4 microM). In the absence of this ion, a single population of receptors was found (KB = 431 nM; Bmax = 9.5 pmol/mg protein), which was not modified by the addition of 1 mM MgCl2. An additional, high affinity site (KB = 59 nM; Bmax = 2.2 pmol/mg protein) became evident in the latter condition. Saturation curves of the binding of [3H]CPP, using 1 mM AMPA [(RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate] or L-aspartate, as displacers in the presence of Mg2+, showed a KB = 200 and 395 nM, respectively. The relative potency of some analogues of excitatory amino acids, for displacing bound CPP in the absence of Mg2+, was AMPA = APH greater than L-glutamate = CPP; Mg2+ significantly increased the potency of AMPA, APH and L-glutamate. These results showed that CPP bound to high affinity (Mg(2+)-dependent) and low affinity sites and that AMPA and L-aspartate compete with this compound only at the low affinity sites. These findings suggest that either the binding of CPP in the retina shows different properties from those described in the brain or alternatively, that AMPA is not as specific for the quisqualate receptor in this organ.