Characteristics of ginsenoside Rg(3)-mediated brain Na+ current inhibition

Abstract

We demonstrated previously that ginsenoside Rg(3) (Rg(3)), an active ingredient of Panax ginseng, inhibits brain-type Na+ channel activity. In this study, we sought to elucidate the molecular mechanisms underlying Rg(3)-induced Na+ channel inhibition. We used the two-microelectrode voltage-clamp technique to investigate the effect of Rg(3) on Na+ currents (I-Na) in Xenopus laevis oocytes expressing wild-type rat brain Na-V 1.2 alpha and beta 1 subunits, or mutants in the channel entrance, the pore region, the lidocaine/tetrodotoxin (TTX) binding sites, the S4 voltage sensor segments of domains I to IV, and the Ile-Phe-Met inactivation cluster. In oocytes expressing wild-type Na+ channels, Rg(3) induced tonic and use-dependent inhibitions of peak I-Na. The Rg(3)- induced tonic inhibition of I-Na was voltage-dependent, dose-dependent, and reversible, with an IC50 value of 32 +/- 6 mu M. Rg(3) treatment produced a 11.2 +/- 3.5 mV depolarizing shift in the activation voltage but did not alter the steady-state inactivation voltage. Mutations in the channel entrance, pore region, lidocaine/TTX binding sites, or voltage sensor segments did not affect Rg(3)-induced tonic blockade of peak I-Na. However, Rg(3) treatment inhibited the peak and plateau I-Na in the IFMQ3 mutant, indicating that Rg(3) inhibits both the resting and open states of Na+ channel. Neutralization of the positive charge at position 859 of voltage sensor segment domain II abolished the Rg(3)-induced activation voltage shift and use-dependent inhibition. These results reveal that Rg(3) is a novel Na+ channel inhibitor capable of acting on the resting and open states of Na+ channel via interactions with the S4 voltage-sensor segment of domain II.