l-701324 and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic-acid

[3H]MK-801 binding in vivo was used to determine the occupancy of NMDA receptor ligands shown to allosterically modulate binding in vitro. ED(50) values (mg/kg) were obtained for the channel blockers (+)-5-methyl-10,11-dihydro-5,4-dibenzo[a,d]cyclohepten-5,10-imine maleate ((+)-MK-801, 0.2), 1-(1-phenylcyclohexyl)piperidine (phencyclidine, PCP, 1.7) and ketamine (4.4). Antagonists at the glutamate (DL-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (DL-CPP, 5.7)) and glycine site (7-Chloro-4-hydroxy-3-(3-phenoxy)-phenyl-2(H)quinolinone (L-701,324, 14.1), 3R(+)cis-4-methyl-pyrrollid-2-one (L-687,414, 15.1)) inhibited [3H]MK-801 binding in vivo to varying maximum levels (69%, 103% and 45%, respectively). NR2B subunit-selective compounds acting at the ifenprodil site inhibited [3H]MK-801 in vivo by a maximum of 52-72% and gave ED(50) values (mg/kg) of: (+/-)-(1S*, 2S*)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol ((+/-)CP-101,606), 1.9; (+/-)-(3R, 4S)-3-[4-(4-fluorophenyl)-4-hydroxypiperidin-1-yl]chroman-4,7-diol ((+/-)CP-283,097), 1.8; (+/-)-(R*, S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-piperidine propanol ((+/-)Ro 25-6981), 1.0; ifenprodil, 6.0. The glycine site agonist D-serine stimulated binding to 151% of control with an ED(50) of 1.7 mg/kg. Results show that [3H]MK-801 binding in vivo may be used to measure receptor occupancy of ligands acting not only within the ion channel but also at modulatory sites on the NMDA receptor complex.

Topics: Animals; Binding Sites; Binding, Competitive; Brain; Chromans; Dizocilpine Maleate; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Ketamine; Kinetics; Male; Membranes; Mice; Phencyclidine; Phenols; Piperazines; Piperidines; Pyrrolidinones; Quinolones; Radioligand Assay; Rats; Receptors, N-Methyl-D-Aspartate; Tritium

In order to investigate possible differences between NMDA receptor-coupled ion channels in the spinal cord and in the cerebral cortex, we have characterized [3H]MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine] binding and its regulation by glutamate and glycine in membrane preparations of the rat spinal cord and cerebral cortex. The K(D) value of [3H]MK-801 binding was higher in the spinal cord than in the cerebral cortex, mainly due to a lower association rate constant. When corrected for the concentrations of residual endogenous amino acids, the EC50 values for glycine were lower at spinal NMDA receptors compared to those in the cerebral cortex, whereas the EC50 values for glutamate were similar in both regions. The IC50 values of D-((3)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (D-CPP) were significantly lower in the spinal cord in the presence of saturating concentrations of glutamate. The IC50 values of 7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(H)-quinoline (L-701,324) were significantly lower in the spinal cord under all conditions. These results suggest that NMDA receptors in the spinal cord display low affinity for MK-801, which may correspond to a lower affinity of the voltage-dependent Mg2+ block. Furthermore, NMDA receptors in the spinal cord appear to display high sensitivity to glycine and to glutamate and glycine antagonists.

Topics: Animals; Binding, Competitive; Cerebral Cortex; Dizocilpine Maleate; Excitatory Amino Acid Antagonists; Glutamic Acid; Glycine; Male; Membranes; Piperazines; Quinolones; Rats; Rats, Sprague-Dawley; Spinal Cord; Tritium