Bioactivity-Guided Isolation of GABAA Receptor Modulating Constituents from the Rhizomes of Actaea racemosa

Black cohosh (Actaea racemosa) is a frequently used herbal remedy for the treatment of mild climacteric symptoms. In the present study, the modulation of gamma-aminobutryic acid (GABA)-induced chloride currents (I-GABA) through GABA type A (GABA(A)) receptors by black cohosh extracts and isolated compounds was investigated. GABA(A) receptors, consisting of alpha(1), beta(2), and gamma(2S) subunits, were expressed in Xenopus laevis oocytes, and potentiation of I-GABA was measured using the two-microelectrode voltage clamp technique. In a bioactivity-guided isolation procedure the positive modulation of I-GABA could be restricted to the plant terpenoid fractions, resulting in the isolation of 11 cycloartane glycosides, of which four significantly (p < 0.05) enhanced I-GABA. The most efficient effect was observed for 23-O-acetylshengmanol 3-O-beta-D-xylopyranoside (4, 100 mu M), enhancing I-GABA by 1692 +/- 201%, while actein (1), cimigenol 3-O-beta-D-xylopyranoside (6), and 25-O-acetylcimigenol 3-O-alpha-L-arabinopyranoside (8) were significantly less active. In the absence of GABA, only 4 induced small (not exceeding 1% of IGABA-max) chloride inward currents through GABA(A) receptors. It is hypothesized that the established positive allosteric modulation of GABA(A) receptors may contribute to beneficial effects of black cohosh extracts in the treatment of climacteric symptoms.