Abstract

23-O-Acetylshengmanol 3-O-β-D-xylopyranoside (Ac-SM) isolated from Actaea racemosa L.—an herbal remedy for the treatment of mild menopausal disorders—has been recently identified as a novel efficacious modulator of GABA_A receptors composed of α₁-, β₂-, and γ_2S-subunits. In the present study, we analyzed a potential subunit-selective modulation of GABA-induced chloride currents (I_GABA) at GABA concentrations eliciting 3–8% of the maximal GABA response (EC_3–8) through nine GABA_A receptor isoforms expressed in Xenopus laevis oocytes by Ac-SM with two-microelectrode voltage clamp and behavioral effects 30 minutes after intraperitoneal application in a mouse model. Efficacy of I_GABA enhancement by Ac-SM displayed a mild α-subunit dependence with α₂β₂γ_2S (maximal I_GABA potentiation [E_max] = 1454 ± 97%) and α₅β₂γ_2S (E_max = 1408 ± 87%) receptors being most efficaciously modulated, followed by slightly weaker I_GABA enhancement through α₁β₂γ_2S (E_max = 1187 ± 166%)_, α₃β₂γ_2S (E_max = 1174 ± 218%), and α₆β₂γ_2S (E_max = 1171 ± 274%) receptors and less pronounced effects on receptors composed of α₄β₂γ_2S (E_max = 752 ± 53%) subunits, whereas potency was not affected by the subunit composition (EC₅₀ values ranging from α₁β₂γ_2S = 35.4 ± 12.3 µM to α₅β₂γ_2S = 50.9 ± 11.8 µM). Replacing β₂- with β₁- or β₃-subunits as well as omitting the γ_2S-subunit affected neither efficacy nor potency of I_GABA enhancement by Ac-SM. Ac-SM shifted the GABA concentration-response curve toward higher GABA sensitivity (about 3-fold) and significantly increased the maximal GABA response by 44 ± 13%, indicating a pharmacological profile distinct from a pure allosteric GABA_A receptor modulator. In mice, Ac-SM significantly reduced anxiety-related behavior in the elevated plus maze test at a dose of 0.6 mg/kg, total ambulation in the open field test at doses ≥6 mg/kg, stress-induced hyperthermia at doses ≥0.6 mg/kg, and significantly elevated seizure threshold at doses ≥20 mg/kg body weight. High efficacy and long biologic half-life of Ac-SM suggest that potential cumulative sedative side effects upon repetitive intake of A. racemosa L. preparations might not be negligible.