α-Pinene, a Major Constituent of Pine Tree Oils, Enhances Non-Rapid Eye Movement Sleep in Mice through GABAA-benzodiazepine Receptors

α-Pinene is a major monoterpene of the pine tree essential oils. It has been reported that α-pinene shows anxiolytic and hypnotic effects upon inhaled administration. However, hypnotic effect by oral supplementation and the molecular mechanism of α-pinene have not been determined yet. By combining in vivo sleep behavior, ex vivo electrophysiological recording from brain slices, and in silico molecular modeling, we demonstrate that (-)-α-pinene shows sleep enhancing property through a direct binding to GABA_A-benzodiazepine (BZD) receptors by acting as a partial modulator at the BZD binding site. The effect of (-)-α-pinene on sleep-wake profiles was evaluated by recording electroencephalogram and electromyogram. The molecular mechanism of (-)-α-pinene was investigated by electrophysiology and molecular docking study. (-)-α-pinene significantly increased the duration of non-rapid eye movement sleep (NREMS) and reduced the sleep latency by oral administration without affecting duration of rapid eye movement sleep and delta activity. (-)-α-pinene potentiated the GABA_A receptor-mediated synaptic response by increasing the decay time constant of sIPSCs in hippocampal CA1 pyramidal neurons. These effects of (-)-α-pinene on sleep and inhibitory synaptic response were mimicked by zolpidem, acting as a modulator for GABA_A-BZD receptors, and fully antagonized by flumazenil, an antagonist for GABA_A-BZD receptor. (-)-α-pinene was found to bind to aromatic residues of α1- and -γ2 subunits of GABA_A-BZD receptors in the molecular model. We conclude that (-)-α-pinene enhances the quantity of NREMS without affecting the intensity of NREMS by prolonging GABAergic synaptic transmission, acting as a partial modulator of GABA_A-BZD receptors and directly binding to the BZD binding site of GABA_A receptor.