L-glutamate receptor heterogeneity: labeling of distinct receptor sub-types using radioligand binding techniques

This study demonstrates (1) that L-[3H]glutamate labels 3 distinct binding sites (types A1, A2 and A4) in isolated rat brain membranes and (2) that only the N-methyl-aspartate (A1) and quisqualate (A2) receptor classes are associated with the postsynaptic density (PSD). L-[3H]glutamate bound to PSDs with Kd 339 nM and Bmax 6 X 1 pmol/mg protein. These sites were resolved into 2 distinct sub-types on the basis of inhibition studies. N-Methyl-aspartate maximally inhibited 57% of PSD-located L-glutamate binding sites (the A1 site) and quisqualate 43% (the A2 site); the effects of both substances were additive. The ligand selectivities of these 2 sites indicated their identity with the N-methyl-D-aspartate and quisqualate receptor classes defined electrophysiologically. The C1--dependent population of L-glutamate binding sites (the A4 site) which predominates in synaptic membranes was absent from PSDs.