Receptor binding characteristics of [3H]NAD-299, a new selective 5-HT1A receptor antagonist.

In vitro receptor binding properties of the novel tritiated 5-hydroxytryptamine1A (5-HT1A) receptor antagonist (R)-3-N,N-dicyclobutylamino-8-fluoro-[6-3H]-3,4-dihydro-2H-1-benzo pyran-5-carboxamide ([3H]NAD-299, generic name robalzotan) were evaluated and compared with those of the agonist 8-hydroxy-2-[2,3-3H]di-n-(propylamino)tetralin ([3H]8-OH-DPAT). [3H]NAD-299 binding displayed a Kd value of 0.17 nM and a Bmax value of 26.7 pmol/g wet weight of rat hippocampus. Same binding affinity (Kd = 0.16 nM) was found to cloned human 5-HT1A receptors. Addition of the nonhydrolyzable GTP analog guanylylimidodiphosphate had no effect on the binding characteristics of [3H]NAD-299, while it significantly decreased both the affinity and density of receptors labeled with [3H]8-OH-DPAT. The rank order of potency of various compounds to inhibit [3H]NAD-299 binding is consistent with the labeling of 5-HT1A receptors. This newly developed high-affinity and selective antagonist radioligand provides a valuable tool for studies of 5-HT1A receptors both in vitro and in vivo.