Natural products from ginseng inhibit [3H]batrachotoxinin A 20-α-benzoate binding to Na+ channels in mammalian brain

A [3H]batrachotoxinin A-20α-benzoate ([3H]BTX-B) binding assay was used to investigate the interaction of two ginseng aglycones (20(S)protopanaxadiol and 20(S)protopanaxatriol) and Rh2 (a monoglucoside of 20(S)protopanaxadiol) with voltage-gated sodium channels in mouse brain. All compounds inhibited the binding of [3H]BTX-B and IC50s were established at 42 μM (20(S)protopanaxadiol), 79 μM (20(S)protopanaxatriol) and 162 μM (Rh2). Scatchard analysis confirmed that 20(S)protopanaxadiol and Rh-2 reduced the Bmax of [3H]BTX-B binding while Rh2 also increased the Kd. At IC50 concentrations and above, 20(S)protopanaxadiol and Rh2 increased the dissociation of the [3H]BTX-B:sodium channel complex above that produced by a saturating concentration of veratridine, but failed to reduce the rate of association of [3H]BTX-B with sodium channels. Reversal of the inhibition of [3H]BTX-B binding by 20(S)protopanaxadiol and Rh2 occurred slowly. We conclude that the 20(S)protopanaxadiol and the less potent inhibitor Rh2 destabilize BTX-B-activated sodium channels through non-covalent allosteric modification of neurotoxin binding site 2.