Agarwood Essential Oil Displays Sedative-Hypnotic Effects through the GABAergic System

Although agarwood has been used as a tranquilizer in Asian countries for hundreds of years, the underlying pharmacological basis is still unclear. This study investigated the sedative-hypnotic effect of agarwood essential oil (AEO) using locomotor activity and pentobarbital-induced sleeping assays in mice. Single (1-day) and multiple (7- and 14-days) administrations of 60 mg/kg AEO generated significant sedative effect on inhibiting locomotor activity and hypnotic effect on pentobarbital-induced sleeping in mice. Interestingly, prolonged AEO treatment did not result in obvious desensitization. Concoitant measurement of the levels of brain neurotransmitters using ultrafast liquid chromatography-tandem mass spectrometry (UFLC-MS/MS) indicated that AEO had no significant effect on the levels of glutamic acid (Glu) and γ-aminobutyric acid (GABA) in the brain. However, the sedative-hypnotic effects were blocked by the type A GABA (GABA_A) receptor antagonists bicuculline and flumazenil. In addition, AEO significantly elevated the expression of GABA_A receptor subunits and subtypes in the cerebral cortex. Furthermore, AEO increased chlorine ion (Cl^-) influx through GABA_A receptors in human neuroblastoma cells. These results together demonstrate that AEO exerts its sedative-hypnotic effects through regulating gene expression of GABA_A receptors and potentiating GABA_A receptor function.