[H-3] CGP-39653 - A NEW N-METHYL-D-ASPARTATE ANTAGONIST RADIOLIGAND WITH LOW NANOMOLAR AFFINITY IN RAT-BRAIN

CGP 39653 (D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid) was initially discovered to inhibit the binding of [H-3]L-glutamate and [H-3]3-((+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid ([H-3]CPP) with K(i) values of 230 and 5 nM, respectively. The radiolabeled compound [H-3]CGP 39653 binds to rat frontal cortical membranes in a saturable and reversible manner. Analysis of saturation experiments revealed that the ligand labels one binding site with a K(d) value of 6 nM. Competition experiments indicated that the order of potency of a number of competitive excitatory amino acid agonist and antagonist compounds was similar to that found previously for other N-methyl-D-aspartate (NMDA) receptor ligands. In contrast to these competitive inhibitors, which produced steep inhibition curves, glycine inhibited binding in a complex manner. When the functional activity of the unlabeled compound was explored, CGP 39653 blocked NMDA-evoked depolarizations in the rat cortical wedge in vitro and inhibited L-glutamate stimulated [H-3]N(1-[2-thienyl]cyclohexyl)3,4-piperidine ([H-3]TCP) binding in cortical membranes. These results suggest that [H-3]CGP 39653 selectively binds to the NMDA receptor as an antagonist with high affinity and is currently the ligand of choice for labeling the NMDA receptor.