Identification and Characterization of GABAA Receptor Modulatory Diterpenes from Biota orientalis That Decrease Locomotor Activity in Mice

An ethyl acetate extract of Biota orientalis leaves potentiated GABA-induced control current by 92.6% ± 22.5% when tested at 100 μg/mL in Xenopus laevis oocytes expressing GABAA receptors (α1β2γ2S subtype) in two-microelectrode voltage clamp measurements. HPLC-based activity profiling was used to identify isopimaric acid (4) and sandaracopimaric acid (5) as the compounds largely responsible for the activity. Sandaracopimaradienolal (3) was characterized as a new natural product. Compounds 4 and 5 were investigated for GABAA receptor subtype selectivity at the subtypes α1β1γ2S, α1β2γ2S, α1β3γ2S, α2β2γ2S, α3β2γ2S, and α5β2γ2S. Sandaracopimaric acid (5) was significantly more potent than isopimaric acid (4) at the GABAA receptor subtypes α1β1γ2S, α2β2γ2S, and α5β2γ2S (EC50 4: 289.5 ± 82.0, 364.8 ± 85.0, and 317.0 ± 83.7 μM vs EC50 5: 48.1 ± 13.4, 31.2 ± 4.8, and 40.7 ± 14.7 μM). The highest efficiency was reached by 4 and 5 on α2- and α3-containing receptor subtypes. In the open field test, ip administration of 5 induced a dose-dependent decrease of locomotor activity in a range of 3 to 30 mg/kg body weight in mice. No significant anxiolytic-like activity was observed in doses between 1 and 30 mg/kg body weight in mice.