BINDING OF THE RADIOLABELED GLYCINE SITE ANTAGONIST [H-3] MDL-105,519TO HOMOMERIC NMDA-NR1A RECEPTORS

We have characterized the binding of [H-3]MDL 105,519 arboxyethenyl)-4 ,6-dichloro-1H-indole-2-carboxylic acid), a NMDA receptor glycine reco gnition site antagonist, to homomeric NMDA subunit 1a (NR1a) receptors . Chinese hamster ovary cells (CHO-K1) were transfected with the rat N R1a gene and cell lines stably expressing the receptor were isolated f rom amongst clones resistant to the neomycin analog G418. Saturation a nalysis indicated that the radioligand bound to the homomeric receptor with a similar high affinity (K-d = 1.8 nM) to that reported for the native receptor. The binding capacity (B-max) was 370 fmol/mg protein reflecting approximately 110000 receptors per cell. The radioligand in teracted with a single class of binding sites as indicated by linear S catchard transformation of the saturation data and a unitary Hill slop e in competition experiments. Thus, the MDL 105,519 recognition site i s present on the NR1a subunit and has similar radioligand binding prop erties to the native brain-derived receptor. However, pharmacologic ch aracterization of [H-3]MDL 105,519 binding indicated that agonists wer e weaker competitors at the homomeric receptor relative to the native receptors. In contrast, representatives of three distinct chemical cla sses of glycine site antagonists exhibited similar potencies at both t ypes of binding sites.