Selective association of N-methyl aspartate and quisqualate types of L-glutamate receptor with brain postsynaptic densities.

Recognition sites for the excitatory neurotransmitter, L-glutamate, were studied in synaptic plasma membranes and postsynaptic densities (PSDs) isolated from rat brains. The results demonstrate (i) that L-glutamate binding sites may be resolved into three distinct subtypes (categories A1, A2, and A4), each corresponding to an electrophysiologically identified receptor class, and (ii) that the N-methyl aspartate (A1) and quisqualate (A2) receptor types are selectively associated with PSDs. L-[3H]Glutamate bound to an apparently homogeneous population of sites in PSDs with a Kd of 3.39 X 10(-7) M and a Bmax (maximum number of binding sites) of 6.1 pmol/mg of protein. Inhibition studies demonstrated that these sites could be resolved into two distinct subtypes. N-Methyl aspartate maximally inhibited 58% of PSD-located L-glutamate binding sites with a Ki of 7.2 X 10(-6) M (the A1 site), and quisqualate inhibited 42% with a Ki of 1.1 X 10(-6) M (the A2 site); the effects of both substances were additive. Experiments with a range of acidic amino acid analogues indicated that the ligand selectivities of these two binding sites conformed to those of the N-methyl D-aspartate and quisqualate receptor classes defined electrophysiologically. The Cl--dependent population of L-glutamate binding sites (the A4 site), which predominates in synaptic membranes, was absent from isolated PSDs.