Vol. 61, Issue 4, 928-935, April 2002

Functional Expression of a Novel Ginsenoside Rf Binding Protein from Rat Brain mRNA in Xenopus laevis Oocytes

Seok Choi, Se-Yeon Jung, Yoo-Seung Ko, Seong-Ryong Koh, Hyewhon Rhim, and Seung-Yeol Nah

National Research Laboratory for the Study of Ginseng Signal Transduction and Department of Physiology, College of Veterinary Medicine, Chonnam National University, Kwangju, Korea (S.C., S.-Y.J., Y.-S.K., S.-Y.N.); Korea Ginseng and Tobacco Research Institute, Daejon, Korea (S.-R.K.); and Biomedical Research Center, Korea Institute of Science and Technology, Seoul, Korea (H.R.)

We have shown that ginsenoside Rf (Rf) regulates voltage-dependent Ca²⁺ channels through pertussis toxin (PTX)-sensitive G proteins in rat sensory neurons. These results suggest that Rf can act through a novel G protein-linked receptor in the nervous system. In the present study, we further examined the effect of Rf on G protein-coupled inwardly rectifying K⁺ (GIRK) channels after coexpression with size-fractionated rat brain mRNA and GIRK1 and GIRK4 (GIRK1/4) channel cRNAs in Xenopus laevis oocytes using two-electrode voltage-clamp techniques. We found that Rf activated GIRK channel in a dose-dependent and reversible manner after coexpression with subfractions of rat brain mRNA and GIRK1/4 channel cRNAs. This Rf-evoked current was blocked by Ba²⁺, a potassium channel blocker. The size of rat brain mRNA responding to Rf was about 6 to 7 kilobases. However, Rf did not evoke GIRK current after injection with this subfraction of rat brain mRNA or GIRK1/4 channel cRNAs alone. Other ginsenosides, such as Rb₁ and Rg₁, evoked only slight induction of GIRK currents after coexpression with the subfraction of rat brain mRNA and GIRK1/4 channel cRNAs. Acetylcholine and serotonin almost did not induce GIRK currents after coexpression with the subfraction of rat brain mRNA and GIRK1/4 channel cRNAs. Rf-evoked GIRK currents were not altered by PTX pretreatment but were suppressed by intracellularly injected guanosine-5'-(2-O-thio) diphosphate, a nonhydrolyzable GDP analog. These results indicate that Rf activates GIRK channel through an unidentified G protein-coupled receptor in rat brain and that this receptor can be cloned by the expression method demonstrated here.