Positive allosteric modulation of native and recombinant GABAA receptors by hops prenylflavonoids

Hops are a major component of beer that is added during brewing. In addition to its wide range of bioactivity, it exhibits neuroactive properties as a sedative and sleeping aid. The compounds responsible for this activity are yet to be revealed and understood in terms of their pharmacological properties. Here we evaluated the potential of several hops flavonoids in modulating the GABAergic activity and assessed their selectivity to GABA(A) receptors subtypes. GABA-potentiating effects were measured using [H-3]ethynylbicycloorthobenzoate (EBOB) radioligand binding assay in native and recombinant alpha 1 beta 3 gamma 2, alpha 2 beta 3 gamma 2 and alpha 6 beta 3 delta receptors expressed in HEK293 cells. Flumazenil sensitivity of GABA-potentiating effects and [H-3]Ro 15-4513 binding assay were used to examine the flavonoids binding to benzodiazepine site. The prenylflavonoids xanthohumol (XN), isoxanthohumol (IXN) and 8-prenylnaringenin (8PN) potentiated GABA-induced displacement of [H-3]EBOB binding in a concentration-dependent manner. The IC50 for this potentiation in native GABA(A) receptors were 29.7 mu M, 11.6 mu M, 7.3 mu M, respectively. In recombinant receptors, the sensitivity to prenylflavonoid potentiation of GABA-induced displacement of [H-3]EBOB binding followed the order alpha 6 beta 3 delta > alpha 2 beta 3 gamma 2 > alpha 1 beta 3 gamma 2 with the strongest inhibition observed by 8PN in alpha 6 beta 3 delta (IC50 , = 3.6 mu M). Flumazenil had no significant effect on the prenylflavonoid-induced displacement of [H-3]EBOB binding and [H-3]Ro 15-4513 displacement from native GABA(A) receptors was only detected at high micromolar concentrations (100 mu M). We identified potent prenylflavonoids in hops that positively modulate GABA-induced responses in native and alpha beta gamma/delta recombinant GABA(A) receptors at low micromolar concentrations. These GABAergic modulatory effects were not mediated via the high-affinity benzodiazepine binding site.