6-Chloro-3 '-nitroflavone is a potent ligand for the benzodiazepine binding site of the GABA(A) receptor devoid of intrinsic activity

6-Chloro-3'-nitroflavone integrates a list of nearly 70 flavone derivatives synthesized in our laboratories. The effects of 6-chloro-3'-nitroflavone o n the benzodiazepine binding sites (BDZ-BSs) of the GABA(A) receptor were e xamined in vitro and in vivo. 6-Chloro-3'-nitroflavone inhibited the [H-3]f lunitrazepam [H-3]FNZ) binding to rat cerebral cortex membranes with a K-i of 6.68 nM and the addition of GABA to extensively washed membranes did not modify its affinity for the BDZ-BSs (GABA-shift = 1.16 +/- 0.12). The bind ing assays performed in rat striatal and cerebellar brain membranes showed that this compound has similar affinity to different populations of BDZ-BSs . Electrophysiological experiments revealed that 6-chloro-3'-nitroflavone d id not affect GABA(A)-receptors (GABA(A)-Rs) responses recorded in Xenopus oocytes expressing alpha(1)beta(2)gamma(2s) subunits, but blocked the poten tiation exerted by diazepam (DZ) on GABA-activated chloride currents. In vi vo experiments showed that 6-chloro-3'-nitroflavone did not possess anxioly tic, anticonvulsant, sedative, myorelaxant actions in mice or amnestic effe cts in rats; however, 6-chloro-3'-nitroflavone antagonized diazepam-induced antianxiety action, anticonvulsion, short-term, and long-term amnesia and motor incoordination. These biochemical, electrophysiological, and pharmaco logical results suggest that 6-chloro-3'-nitroflavone behaves as an antagon ist of the BDZ-BSs. (C) 2000 Elsevier Science Inc.