Hops compounds modulatory effects and 6-prenylnaringenin dual mode of action on GABAA receptors

Hops (Humulus lupulus L.), a major component of beer, contain potentially neuroactive compounds that made it useful in traditional medicine as a sleeping aid. The present study aims to investigate the individual components in hops acting as allosteric modulators in GABA _A receptors and bring further insight into the mode of action behind the sedative properties of hops. GABA-potentiating effects were measured using [ ³H]ethynylbicycloorthobenzoate (EBOB) radioligand binding assay in native GABA _A receptors. Flumazenil sensitivity of GABA-potentiating effects, [ ³H]Ro 15–4513, and [ ³H]flunitrazepam binding assays were used to examine the binding to the classical benzodiazepines site. Humulone (alpha acid) and 6-prenylnaringenin (prenylflavonoid) were the most potent compounds displaying a modulatory activity at low micromolar concentrations. Humulone and 6-prenylnaringenin potentiated GABA-induced displacement of [ ³H]EBOB binding in a concentration-dependent manner where the IC ₅₀ values for this potentiation in native GABA _A receptors were 3.2 μM and 3.7 μM, respectively. Flumazenil had no significant effects on humulone- or 6-prenylnaringenin-induced displacement of [ ³H]EBOB binding. [ ³H]Ro 15–4513 and [ ³H]flunitrazepam displacements were only minor with humulone but surprisingly prominent with 6-prenylnaringenin despite its flumazenil-insensitive modulatory activity. Thus, we applied molecular docking methods to investigate putative binding sites and poses of 6-prenylnaringenin at the GABA _A receptor α1β2γ2 isoform. Radioligand binding and docking results suggest a dual mode of action by 6-prenylnaringenin on GABA _A receptors where it may act as a positive allosteric modulator at α+β- binding interface as well as a null modulator at the flumazenil-sensitive α+γ2- binding interface.