Honokiol and magnolol have been identified as modulators of the GABAA receptors in vitro. Our previous study suggested a possible selectivity of honokiol and magnolol on GABAA receptor subtypes. This possibility was examined in the current study by 3H-muscimol and 3H-flunitrazepam binding assays on various rat brain membrane preparations and human recombinant GABAA receptor subunit combinations expressed by the Sf-9/baculovirus system. Generally, honokiol and magnolol have a similar enhancing effect on 3H-muscimol binding to various membrane preparations in nonsaturation binding assays. Honokiol and magnolol preferentially increased 3H-muscimol binding to hippocampus compared to cortex and cerebellum (with a maximum enhancement of 400% of control). As for subunit combinations, honokiol and magnolol have a more potent enhancing effect on α2 subunit containing combinations (with a maximum enhancement of 400–450% of control). This action was independent of the γ subunit. In saturation binding assays, magnolol affected either the number of binding sites (ca. 4-fold on α2 containing combinations) or the binding affinity (on α1 containing combinations) of 3H-muscimol binding to various GABAA receptor subunit combinations. In contrast, honokiol increased only binding sites on α2β3γ2s and α2β3 combinations, but both the number of binding sites and the binding affinity on α1β2γ2S and α1β2 combinations. These results indicate that honokiol and magnolol have some selectivity on different GABAA receptor subtypes. The property of interacting with GABAA receptors and their selectivity could be responsible for the reported in vivo effects of these two compounds.

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