Ginseng saponins, major active components of ginseng root used by folk medicine in the treatment of various diseases, produce multiple pharmacological responses having many effects on the central and peripheral nervous system. Specifically, ginsenoside Rg 2 has been shown to block the nicotinic acetylcholine receptors in bovine chromaffin cells. We have studied the effect of Rg 2 on different types of human neuronal nicotinic acetylcholine receptors (nAChRs), both homomeric and heteromeric, expressed in Xenopus oocytes. Rg 2 did not affect the acetylcholine (ACh)-induced currents in ␣ 7 human receptors, however Rg 2 affected the peak currents, and mainly the desensitization of heteromeric receptors ␣ 3  4 , ␣ 3  2 , ␣ 4  4 , and ␣ 4  2 . Both effects, a diminution of peak current and an increase of desensitization, are dose-dependent and are very similar for all the receptors. The mechanism of action has been studied in more detail in ␣ 3  4 and ␣ 4  2 receptors where we found a negligible shift in the ACh dose-response curves and a persistence of the Rg 2 effects at high ACh concentrations, indicative of a noncompetitive antagonism. A lack of voltage dependence on the reduction of the peak currents induced by ACh also suggests that Rg 2 does not act as an open channel blocker of human nAChR. The results indicate that Rg 2 acts specifically on heteromeric human nAChRs modulating their desensitization and suggest a possible mechanism by which this saponin contributes to the multiple therapeutic effects of ginseng.
1 The signal transduction mechanism of ginsenosides, the active ingredients of ginseng, was studied in Xenopus oocytes using two-electrode voltage-clamp technique. Ginseng total saponin (GTS), i.e., an unfractionated mixture of ginsenosides produced a large outward current at membrane potentials more positive than 720 mV when it was applied to the exterior of oocytes, but not when injected intracellularly. The e ect of GTS was concentration-dependent (EC 50 : 4.4 mg ml 71 ) and reversible. 2 Certain fractionated ginsenosides (Rb 1 , Rb 2 , Rc, Rf, Rg 2 and Ro) also produced an outward current in a concentration-dependent manner with the order of potency of Rf4Ro4Rb 1 =Rb 2 4Rg 2 4Rc. Other ginsenosides (Rd, Re and Rg 1 ) had little or no e ect. 3 The GTS e ect was completely blocked by bath application of the Ca 2+ -activated Cl 7 channel blocker ni¯umic acid and by intracellular injection of the calcium chelator BAPTA or the IP 3 receptor antagonist heparin. Also, the e ect was partially blocked by bath-applied U-73122, a phospholipase C (PLC) inhibitor and by intracellularly injected GTPgS, a non-hydrolyzable GTP analogue. Whereas, it was not altered by pertussin toxin (PTX) pretreatment. 4 These results indicate that: (1) interaction of ginsenosides with membrane component(s) at the extracellular side leads to Ca 2+ -activated Cl 7 channel opening in Xenopus oocyte membrane; and (2) this process involves PLC activation, the release of Ca 2+ from the IP 3 -sensitive intracellular store and PTX-insensitive G protein activation.
We have shown that ginsenoside Rf (Rf) regulates voltagedependent Ca 2ϩ 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 ϩ (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 Ba 2ϩ , 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 Rb 1 and Rg 1 , 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-5Ј-(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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.