[3H]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 [3H]L-glutamate and [3H]3-[+/-)2-carboxypiperazin-4-yl)-propyl-1- phosphonic acid [( 3H]CPP) with Ki values of 230 and 5 nM, respectively. The radiolabeled compound [3H]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 Kd 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 [3H]N(1-[2-thienyl]cyclohexyl)3,4-piperidine [( 3H]TCP) binding in cortical membranes. These results suggest that [3H]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.

CGP 35348: a centrally active blocker of GABAB receptors

The biochemical, electrophysiological and pharmacological properties of the new GABAB receptor blocker CGP 35348 are described. In a variety of receptor binding assays CGP 35348 showed affinity for the GABAB receptor only. CGP 35348 had an IC50 of 34 microM at the GABAB receptor. The compound antagonized (100, 300, 1000 microM) the potentiating effect of L-baclofen on noradrenaline-induced stimulation of adenylate cyclase in rat cortex slices. In electrophysiological studies CGP 35348 (10, 100 microM) antagonized the effect of L-baclofen in the isolated rat spinal cord. In the hippocampal slice preparation CGP 35348 (10, 30, 100 microM) blocked the membrane hyperpolarization induced by D/L-baclofen (10 microM) and the late inhibitory postsynaptic potential. CGP 35348 appeared to be 10-30 times more potent than the GABAB receptor blocker phaclofen. Ionophoretic and behavioural experiments showed that GABAB receptors in the brain were blocked after i.p. administration of CGP 35348. This compound may be of considerable value in elucidating the roles of brain GABAB receptors.