Synthetic Batrachotoxin Derivatives as Molecular Probes of Voltage-Gated Sodium Ion Channel Function

Abstract

Batrachotoxin (BTX) is a potent and selective agonist of voltage-gated sodium channels (NaVs). Toxin binding to the inner pore of the channel (Site II) elicits a multitude of effects, including a change in activation potential, inhibition of inactivation at all potentials, and a loss of ion selectivity. De novo synthesis of BTX has enabled structure-activity studies with toxin derivatives to develop a molecular picture of how BTX alters NaV function. Substitution of the naturally occurring C20-pyrrole ester with other acyl groups has been explored. Replacement of the rigid ring with a conformationally flexible C20-heptynoate ester results in a compound that demonstrates unusual effects on channel gating, most markedly on steady-state inactivation. Channels altered by BTX fail to inactivate over a range of depolarizing potentials. By contrast, the heptynoate derivative does not completely abrogate channel inactivation at weakly depolarizing potentials; however, at membrane potentials >-70 mV, channel inactivation is inhibited. To our knowledge, this is the first report of a BTX derivative that decouples the effects on threshold activation and steady-state inactivation, two hallmarks of BTX binding to NaVs. A channel state model to explain the observed differences is proposed.