**3. Conclusion**

a novel site of action. This suggests that there is a further flavonoid site on GABAA receptors

Quantitative structure-efficacy relationships have shown that flavone analogues differing only at position 6 show significantly different pharmacological properties at GABAA receptors [52]. This study clearly shows the importance of the 6-position as a determination of activity. However, further studies on 6-substitued flavones are needed to study the complex nature of the activation and modulation of GABAA receptor subtypes and to explore the unique thera-

Another interesting series of synthetic flavonoids are the flavan-3-ol esters, analogues of EGCG, a naturally occurring flavanol-3-ester. Fa131 (trans-(2S,3R)-3-acetoxy-4′-methoxyflavan, **Figure 3**) is a non-sedating anxiolytic and a selective positive modulator of α2-containing GABAA receptors, shown on the basis of efficacy [55, 56]. The efficacy of 2100% enhancement exceeds the highest efficacy previously recorded, which was 1250% by (+)-borneol at these

Fa173 (cis-(2S,3S)-3-acetoxy-3′,4′-dimethoxyflavan, **Figure 3**), a diastereo-isomeric flavan-3-ol ester with additional methoxy in the 3′ position, was shown to block the modulatory actions of Fa131 [58]. Additionally, Fa173 also blocked the enhancement of the GABA response by the anaesthetic etomidate, the sedative anticonvulsant loreclezole, and selectively blocked the low-affinity effect of diazepam (100 μM) at α1β2γ2L and α1β2 GABAA receptors, but not the high-affinity effect of diazepam (100 nM). Fa173 was found not to inhibit the positive modulation of GABA by the anaesthetic propofol, barbiturate thiopental, or neuroactive steroid allopregnanolone. This suggests that Fa131, etomidate, loreclezole and high (non-flumazenilsensitive) doses of benzodiazepine all exert their positive modulatory effects via a common or overlapping binding site that can be blocked by the neutralizing modulator Fa173. Of these agents, Fa131 alone shows selectivity for α2-containing GABAA recombinant receptors. Fa131 is the first positive modulator to distinguish between α2- and α3-containing GABAA receptors, highlighting the potential of targeting flumazenil-insensitive allosteric sites in the search

As flavonoids are significant constituents of our diet, it is important that we understand how natural flavonoids might influence brain function. Except when consumed as dietary supplements, flavonoids are generally consumed as a mixture of different flavonoids from one or more foodstuffs [59]. The effects of mixtures of flavonoids and other modulators on GABAA receptors need to be more thoroughly investigated. Synergies have been noted between indi-

Hesperidin, a glycosylated flavonone isolated from Valerian species, has shown synergistic effects in mice. The combination of hesperidin (2 mg/kg) with apigenin (1 mg/kg), 6,3′-dinitroflavone (0.02 mg/kg) or diazepam (0.3 mg/kg) enhanced the barbiturate-induced sleeping time in mice [27, 29]. Both hesperidin and diazepam administered separately showed a dose-dependent reduction in exploratory parameters (number of head dips,

vidual flavonoids [29, 60, 61], and between flavonoids and benzodiazepines [27, 28].

that mediate opening of the chloride channel in the absence of GABA.

peutic potential of these synthetic flavones.

344 Flavonoids - From Biosynthesis to Human Health

receptors [57].

for new anxio-selective drugs.

**2.6. Synergism between flavonoids**

Since flavonoids were first linked to benzodiazepine-binding sites on GABAA receptors many years ago, recent studies have clearly demonstrated that the actions of flavonoids on these receptors are far more complex than a single action at a single site. In addition to the now relatively well-characterized flumazenil-sensitive benzodiazepine-binding sites, there is significant interest in flumazenil-insensitive, non-benzodiazepine-binding sites for flavonoids. This overview has sought to highlight the action of representative flavonoids on GABAA receptors to illustrate the range of activities.

Recent studies have identified the presence of multiple sites on ionotropic GABA receptors at which flavonoids can act, modulating the effect of GABA. The sites may include ones that are insensitive to the classical benzodiazepine-binding site antagonist (neutralizing modulator) flumazenil and described as low-affinity benzodiazepine sites [15]. Perhaps, these would be more appropriately described as flavonoid sites as they appear to be activated by many naturally occurring and synthetic flavonoids.
