Modifications of aliphatic side chain of 20(S)-ginsenoside RG3 cause an enhancement or loss of brain Na+ channel current inhibitions.

A line of evidence has shown that ginsenoside Rg3 (Rg3) could be one of bioactive ligands in brain Na+ channel regulations. Rg3 exists as stereoisomer of 20(R)- or 20(S)-form. Rg3 consists of three different parts; steroid- like backbone structure, carbohydrate portion, and aliphatic side chain [-CH2CH2CH=C(CH3)2], which is coupled to the carbon-20 of backbone structure. In the previous report, we demonstrated that 20(S)- but not 20(R)-Rg3 and carbohydrate portion of Rg3 play important roles in rat brain NaV1.2 channel regulations. However, little is known about the role of aliphatic side chain coupled to the carbon-20 in brain Na+ channel regulations. In the present study, we prepared Rg3 derivatives by modifying the aliphatic side chain of Rg3, remaining with backbone structure and carbohydrate portion intact, and examined the effects of Rg3 derivatives on Na+ channel activity. We found that reduction of double bond in aliphatic side chain of Rg3 exhibited agonistic actions in Na+ channel current inhibitions by shifting concentration-response curve to leftward by three-fold, whereas deletion, hydroxylation, or oxygenation of aliphatic side chain caused an attenuation or loss of Na+ channel current inhibitions. These results provide evidences that the aliphatic side chain of Rg3 is also involved in Na+ channel regulations and further show a possibility that the aliphatic side chain of Rg3 could be the target of chemical modifications for abolishment or potentiation of Rg3 actions in Na+ channel regulations.