Korean J Physiol Pharmacol.  2012 Apr;16(2):113-118. 10.4196/kjpp.2012.16.2.113.

Effects of Protopanaxatriol-Ginsenoside Metabolites on Rat N-Methyl-D-Aspartic Acid Receptor-Mediated Ion Currents

• Shin TJ
• Hwang SH
• Choi SH
• Lee BH
• Kang J
• Kim HJ
• Zukin RS
• Rhim H
• Nah SY

• KMID: 2071756
• DOI: http://doi.org/10.4196/kjpp.2012.16.2.113

Abstract

Ginsenosides are low molecular weight glycosides found in ginseng that exhibit neuroprotective effects through inhibition of N-methyl-D-aspartic acid (NMDA) receptor channel activity. Ginsenosides, like other natural compounds, are metabolized by gastric juices and intestinal microorganisms to produce ginsenoside metabolites. However, little is known about how ginsenoside metabolites regulate NMDA receptor channel activity. In the present study, we investigated the effects of ginsenoside metabolites, such as compound K (CK), protopanaxadiol (PPD), and protopanaxatriol (PPT), on oocytes that heterologously express the rat NMDA receptor. NMDA receptor-mediated ion current (INMDA) was measured using the 2-electrode voltage clamp technique. In oocytes injected with cRNAs encoding NMDA receptor subunits, PPT, but not CK or PPD, reversibly inhibited INMDA in a concentration-dependent manner. The IC50 for PPT on INMDA was 48.1+/-4.6 microM, was non-competitive with NMDA, and was independent of the membrane holding potential. These results demonstrate the possibility that PPT interacts with the NMDA receptor, although not at the NMDA binding site, and that the inhibitory effects of PPT on INMDA could be related to ginseng-mediated neuroprotection.

Keyword

Ginseng; Ginsenoside metabolites; N-methyl-D-aspartic acid receptor

MeSH Terms

Animals
Binding Sites
Gastric Juice
Ginsenosides
Glycosides
Inhibitory Concentration 50
Membranes
Molecular Weight
N-Methylaspartate
Neuroprotective Agents
Oocytes
Panax
Rats
RNA, Complementary
Sapogenins
Tuberculin
Ginsenosides
Glycosides
N-Methylaspartate
Neuroprotective Agents
RNA, Complementary
Sapogenins
Tuberculin

Figure

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  Fig. 1 Chemical Structure of Ginsenoside Rg3 and Ginsenoside Metabolites and the Effects of Ginsenoside Metabolites on INMDA. (A) Chemical structure of ginsenoside Rg3 and ginsenoside metabolites. (B) Effect of ginsenoside metabolites on INMDA in oocytes expressing the rat NMDA receptor. Application of NMDA (300 µM) and glycine (10 µM) elicited INMDA. Co-application of PPT (100 µM) but not CK (100 µM) or PPD (100 µM) with NMDA (300 µM) and glycine (10 µM) attenuated INMDA. The traces represent 6 separate oocytes.

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  Fig. 2 Concentration-dependent Effects of PPT Co-application with NMDA (300 µM) and Glycine (10 µM) on INMDA. PPT inhibited INMDA in a concentration-dependent manner. The traces represent 6 separate oocytes.

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  Fig. 3 Current-voltage Relationship of PPT-mediated INMDA Inhibition of the NMDA Receptor. Representative current-voltage relationships were obtained using voltage ramps of -100 to +50 mV for 1 s at a holding potential of -60 mV. Voltage steps were applied before and after application of 300 µM NMDA and 10 µM glycine in the presence or absence of 50 µM PPT. The reversal potential for the receptor was -14.70±0.9 mV and -13.82±0.9 mV in the presence and absence of PPT, respectively.

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  Fig. 4 Concentration-dependent Effects of NMDA on PPT-mediated Inhibition of INMDA. (A) The representative traces shown for the indicated concentrations of NMDA represent 6 separate oocytes from 3 different batches of frogs. (B) Concentration-response relationships for NMDA in NMDA receptors treated with NMDA (10~1000 µM; with 10 µM glycine) or with NMDA plus 50 µM PPT in oocytes expressing the rat NMDA receptor. The normalized INMDA of oocytes expressing the NMDA receptor was measured at the indicated concentrations of NMDA in the presence (○) or absence (□) of 50 µM PPT. Oocytes were exposed to NMDA alone or NMDA and PPT for 1 min prior to application. Oocytes were voltage-clamped at a holding potential of -60 mV.

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  Fig. 5 Voltage-independent PPT-mediated INMDA Inhibition of the NMDA Receptor. The effect of PPT (50 µM) on the response to NMDA (300 µM, with 10 µM glycine) was determined in oocytes expressing NR1b/NR2A. (A) Representative traces showing inhibition by 50 µM PPT. (B) Summary of percent inhibition induced by PPT at the indicated membrane holding potentials in oocytes expressing the NMDA receptor. Each point represents the mean±SEM. (n=6~8/group).

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