6‐Methylflavone, a benzodiazepine receptor ligand with antagonistic properties on rat brain and human recombinant GABA A receptors in vitro

Seventeen flavonoids were found to have inhibitory activity on the central nervous system GABA A /benzodiazepine (BZD) receptors with IC 50 values ranging from 0.12 to 8 μM. 6‐Methylflavone, the most potent, was pharmacologically characterized by radioligand binding assays on rat brain membranes in vitro and human recombinant GABA A /BZD receptors expressed in Sf‐9 insect cells, as well as electrophysiologically by the whole‐cell patch clamp technique. Scatchard plot analysis showed that 6‐methylflavone was a competitive inhibitor of [ 3 H]‐Ro 15‐1788, binding to rat brain cortical membranes. The GABA ratio of 1.06 for [ 3 H]‐diazepam binding to cortex and 1.23 for cerebellum indicated an antagonistic or a weak partial agonistic profile of 6‐methylflavone on the rat BZD 1 receptors, while the GABA ratio of 0.76 on hippocampus indicated an antagonistic or partial‐inverse agonistic profile on the BZD 2 receptors. In Sf‐9 insects cells, the GABA ratios showed a weak partial agonistic profile on the α 1 β 2 γ 2S (GABA ratio 1.29) subtype combination, an antagonistic profile on the α 2 β 2 γ 2S (1.13) and α 3 β 2 γ 2S (1.03), and a partial inverse agonistic profile on the α 5 β 2 γ 2S (0.79) subtype combination. The modulation of GABA‐induced chloride currents by 6‐methylflavone suggests that the compound is an antagonist at human GABA A receptor subtypes. Based on our data of GABA A /BZD receptor active as well as inactive flavonoids, some general structure–activity relationships are discussed. Drug Dev. Res. 41:99–106, 1997. © 1997 Wiley‐Liss, Inc.