Anatomic correlation of NMDA and 3H-TCP-labeled receptors in rat brain

Abstract

Using quantitative autoradiography, we have compared the regional distribution of N-methyl-D-aspartate (NMDA) receptors labeled with 3H-glutamate and dissociative anesthetic binding sites labeled with 3H-N-(1-[2-thienyl]cyclohexyl)3,4-piperidine (3H-TCP). Binding of both ligands was highest in the hippocampal formation, with high concentrations in a number of cortical and olfactory regions. Intermediate amounts of binding for both ligands were measured in several thalamic and basal telencephalic structures. Very little binding was observed in the hypothalamus, some deep forebrain regions, and most brain-stem structures. Linear-regression analysis comparing the binding at both sites revealed a marked concordance (R = 0.95; p < 0.001; Pearson product-moment). The granule cell layer of the cerebellum was the only region in which this concordance was not observed. Scatchard analysis of 3H-glutamate binding to NMDA receptors in stratum radiatum of hippocampal formation revealed an apparent single binding site with a B(max) of 9.78 ± 0.84 pmol/mg protein and K(D) of 158 ± 37 nM. 3H-TCP also bound to an apparent single site with a B(max) of 2.07 ± 0.16 pmol/mg protein and K(D) of 127 ± 30 nM. Our results are consistent with the hypothesis that the dissociative anesthetic binding site is linked to the NMDA receptor, and the data suggest that there are approximately 4-5 NMDA binding sites for each dissociative anesthetic binding site.