Neuroprotective effects of ginsenoside Rg₃ against homocysteine-induced excitotoxicity in rat hippocampus

We previously demonstrated that ginsenoside Rg₃ (Rg₃), one of the active ingredients in Panax ginseng, attenuates NMDA receptor-mediated currents and NMDA-induced neurotoxicity (Kim, S., Kim, T., Ahn, K., Park, W.K., Nah, S.Y., Rhim, H., 2004. Ginsenoside Rg₃ antagonizes NMDA receptors through a glycine modulatory site in rat cultured hippocampal neurons. Biochem. Biophys. Res. Commun. 323, 416-424). Accumulating evidence suggests that homocysteine (HC), a metabolite of methionine, exerts its excitotoxicity through NMDA receptor activation. In the present study, we examined the neuroprotective effects of Rg₃ on HC-induced hippocampal excitotoxicity in vitro and in vivo. Our in vitro studies using rat cultured hippocampal neurons revealed that Rg₃ treatment significantly and dose-dependently inhibited HC-induced hippocampal cell death, with an EC₅₀ value of 28.7 ± 7.5 μM. Rg₃ treatment not only significantly reduced HC-induced DNA damage, but also dose-dependently attenuated HC-induced caspase-3 activity in vitro. Our in vivo studies revealed that intracerebroventricular (i.c.v.) pre-administration of Rg₃ significantly and dose-dependently reduced i.c.v. HC-induced hippocampal damage in rats. To examine the mechanisms underlying the in vitro and in vivo neuroprotective effects of Rg₃ against HC-induced hippocampal excitotoxicity, we examined the effect of Rg₃ on HC-induced intracellular Ca²⁺ elevations in cultured hippocampal cells and found that Rg₃ treatment dose-dependently inhibited HC-induced intracellular Ca²⁺ elevation, with an IC₅₀ value of 41.5 ± 17.5 μM. In addition, Rg₃ treatment dose-dependently inhibited HC-induced currents in Xenopus oocytes expressing the NMDA receptor, with an IC₅₀ of 47.3 ± 14.2 μM. These results collectively indicate that Rg₃-induced neuroprotection against HC in rat hippocampus might be achieved via inhibition of HC-mediated NMDA receptor activation.