Comparison of L-[3H]glutamate, D-[3H]aspartate, DL-[3H]AP5 and [3H]NMDA as ligands for NMDA receptors in crude postsynaptic densities from rat brain.

L-[3H]Glutamate, D-[3H]aspartate, DL-[3H]2-amino-5-phosphonovalerate (AP5) and [3H]N-methyl-D-aspartate (NMDA) were evaluated as radioligands using postsynaptic density (PSD) preparations from rat brain. L[3H]Glutamate had the highest affinity and greatest percentage specific binding, followed by D-[3H]aspartate greater than DL-[3H]AP5 greater than [3H]NMDA. The pharmacological specificity of the binding of each radioligand indicated an interaction with NMDA-preferring receptors, the order of potency of displacing compounds tested being L-glutamate greater than D-aspartate greater than D-AP5 greater than DL-AP5 greater than ibotenate greater than NMDLA greater than quisqualate. For L-[3H]glutamate, the data revealed an interaction with two sites, the major one having NMDA receptor characteristics and the minor one resembling the quisqualate-preferring receptor. Against L-[3H]glutamate binding, quisqualate showed a two-component inhibition profile with an affinity of 25 microM at the NMDA site and 0.19 microM at the quisqualate site. Thus, by using several radioligands possessing activity at NMDA receptors, we confirm that an NMDA receptor binding site is present in crude PSDs. Although it is less selective than D-[3H]aspartate, DL-[3H]AP5 and [3H]NMDA, L-[3H]glutamate remains, by virtue of its high affinity, the ligand of choice for the study of NMDA receptors in preparations where such sites predominate.