Characterization of the binding of two novel glycine site antagonists to cloned NMDA receptors: evidence for two pharmacological classes of antagonists

1 The potency of two novel glycine site antagonists, GV150,526A and GV196,7 71A, was assessed by their ability to inhibit the binding of [H-3]-MDL105,5 19 to cell homogenates prepared from mammalian cells transfected with eithe r NR1-1a, NR1-2a, NR1-1a/NR2A, NR1-1a/NR2B, NR1-1a/NR2C or NR1-1a/NR2D NMDA receptor clones. 2 The inhibition constants (K(i)s) for GV150,526A displacement of [H-3]-MDL 105,519 binding to either NR1-1a or NR1-2a expressed alone were not signifi cantly different and were best fit by a one-site binding model. GV150,526A inhibition to NR1-1a/NR2 combinations was best fit by a two-site model with the NR1-1a/NR2C having an approximate 2-4 fold lower affinity compared to other NR1-1a/NR2 receptors. 3 The K(i)s for GV196,771A displacement of [H-3]-MDL105,519 binding to NR1- 1a, NR1-2a and all NR1-1a/NR2 combinations was best fit by a two-site bindi ng model. There was no significant difference between the K(i)s for the bin ding to NR1-1a and NR1-2a; NR1-1a/NR2A receptors had an approximate 4 fold lower affinity for GV196,771A compared to other NR1-1a/NR2 combinations. 4 The K(i)s for both GV150,526A and GV196,771A for the inhibition of [H-3]- MDL105,519 binding to membranes prepared from adult rat forebrain were dete rmined and compared to the values obtained for binding to cloned NMDA recep tors. 5 The K(i)s for a series of glycine site ligands with diverse chemical stru ctures were also determined for the inhibition of [H-3]-MDL105,519 binding to NR1-1a/NR2A receptors. L689,560 displayed similar binding characteristic s to GV150,526A. 6 It is suggested that glycine site antagonists may be divided into two cla sses based on their ability to distinguish between NR1 and NR1/NR2 receptor s with respect to binding curve characteristics.