Souroubea sympetala (Marcgraviaceae): a medicinal plant that exerts anxiolysis through interaction with the GABA_A benzodiazepine receptor

The mode of action of the anxiolytic medicinal plant Souroubea sympetala was investigated to test the hypothesis that extracts and the active principle act at the pharmacologically important GABA_A–benzodiazepine (GABA_A–BZD) receptor. Leaf extracts prepared by ethyl acetate extraction or supercritical extraction, previously determined to have 5.54 mg/g and 6.78 mg/g of the active principle, betulinic acid, respectively, reduced behavioural parameters associated with anxiety in a rat model. When animals were pretreated with the GABA_A–BZD receptor antagonist flumazenil, followed by the plant extracts, or a more soluble derivative of the active principle, the methyl ester of betulinic acid (MeBA), flumazenil eliminated the anxiety-reducing effect of plant extracts and MeBA, demonstrating that S. sympetala acts via an agonist action on the GABA_A–BZD receptor. An in vitro GABA_A–BZD competitive receptor binding assay also demonstrated that S. sympetala extracts have an affinity for the GABA_A–BZD receptor, with an EC₅₀ value of 123 μg/mL (EtOAc leaf extract) and 154 μg/mL (supercritical CO₂ extract). These experiments indicate that S. sympetala acts at the GABA_A–BZD receptor to elicit anxiolysis.