Brain Uptake of Bioactive Flavones in Scutellariae Radix and Its Relationship to Anxiolytic Effect in Mice

Scutellariae Radix (SR) and its bioactive flavones elicit a variety of effects in the brain. However, the brain uptake of individual SR flavones and its relationship to the elicited effects after SR administration remain unknown. Moreover, previous studies seldom measured pharmacokinetic and pharmacodynamic outcomes simultaneously. In the current study, the brain uptake of six major SR flavones and the anxiolytic behavior following oral administration of a SR extract at two clinically relevant doses (600 and 1200 mg/kg twice daily) were simultaneously investigated in mice (n = 18 per group). Brain and plasma concentrations of the flavones were measured by LC–MS/MS, while the anxiolytic effect was evaluated using the elevated plus maze. To further investigate the mechanism behind the differential brain uptake of the six SR flavones, these flavones were separately administered to mice at an equivalent molar oral dose (n = 6). The brain tissue bindings of the SR flavones were also measured with the in vitro brain slice method. Our results indicated that all six SR flavones including three aglycons (baicalein, wogonin, and oroxylin A) and three glucuronides (baicalin, wogonoside, and oroxyloside) could pass through the blood–brain barrier, with brain concentrations ranging from 7.9 to 224.0 pmol/g. It provided novel evidence that oroxylin A had the highest brain uptake among the six SR flavones regardless of its limited content in SR extract, in which 3.6–3.9% of the administered oroxylin A dose was present in the brain 6 h postdosing and with a brain-to-plasma ratio of 0.42–0.46. Although SR extract contains flavones that are positive modulators of the benzodiazepine binding site of GABAA receptors (baicalein, wogonin, and baicalin), our behavioral study for the first time indicated that SR extract (a mixture of six flavones) did not elicit significant anxiolytic effect at the studied doses. Oroxylin A also demonstrated the highest brain uptake when the six flavones were separately administered to mice, and the highest affinity to brain tissues in the in vitro tissue binding assay. The high brain uptake of oroxylin A, a GABAA antagonist which had been reported to antagonize diazepam-induced anxiolytic effect, might have suppressed the anxiolytic effects of the other flavones and account for the lack of overall anxiolytic effect of SR extract. The current study illustrates the importance of monitoring pharmacokinetics in a behavioral study, particularly for herbal medicines which consist of multiple components that might have different or even opposite pharmacological effects on the same target.