Ginsenosides, active ingredients of
Panax ginseng, exist as stereoisomers depending on the position of the hydroxyl group on carbon-20;
i.e. 20(
R)-ginsenoside and 20(
S)-ginsenoside are epimers. We previously investigated the structure–activity relationship of the ginsenoside Rg
3 stereoisomers, 20-
R-protopanaxatriol-3-[
O-β-
D-glucopyranosyl (1→2)-β-glucopyranoside], (20(
R)-Rg
3) and 20-
S-protopanaxatriol-3-[
O-β-
D-glucopyranosyl (1→2)-β-glucopyranoside], (20(
S)-Rg
3) in regulating 5-HT
3A receptor-mediated ion currents (
I5-HT) expressed in
Xenopus oocytes and found that 20(
S)-Rg
3 rather than 20(
R)-Rg
3 was more stronger inhibitor of
I5-HT. In the present study, we further investigated the effects of 20(
R)-Rg
3 and 20(
S)-Rg
3 on mouse 5-HT
3A receptor channel activity after site-directed mutations of 5-HT
3A receptor facilitation site, which is located at pre-transmembrane domain I (pre-TM1). 5-HT
3A receptor was expressed in
Xenopus oocytes, and
I5-HT was measured using two-electrode voltage clamp technique. In wild-type, both 20(
R)-Rg
3 and 20(
S)-Rg
3 inhibited
I5-HT with concentration-dependent and reversible manner. Induction of 5-HT
3A receptor facilitation by point mutations of pre-TM1 amino acid residue R222 to R222A, R222D, R222E or R222T not only decreased EC
50 values for
I5-HT compared to wild-type but also abolished 20(
R)-Rg
3-induced inhibition of
I5-HT. Those mutations also shifted the IC
50 values by 20(
S)-Rg
3 into right direction by 2- to 4-folds compared with wild-type. These results indicate that 5-HT
3A receptor facilitation differentially affects 20(
R)-Rg
3- and 20(
S)-Rg
3-mediated 5-HT
3A receptor channel regulation.
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