Voltage-gated sodium channels
(Na<sub>V</sub>s), large transmembrane protein complexes responsible for the
initiation and propagation of action potentials, are targets for a number of
acute poisons. Many of these agents act as allosteric modulators of channel
activity and serve as powerful chemical tools for understanding channel
function. Batrachotoxin (BTX) is a steroidal amine derivative most commonly
associated with poison dart frogs and is unique as a Na<sub>V</sub> ligand in
that it alters every property of the channel, including threshold potential of
activation, inactivation, ion selectivity, and ion conduction.
Structure-function studies with BTX are limited, however, by the inability to
access preparative quantities of this compound from natural sources. We have
addressed this problem through <i>de novo</i>
synthesis of BTX, which gives access to modified toxin structures. In this
report, we detail electrophysiology studies of three BTX C20-ester derivatives
against recombinant Na<sub>V</sub> subtypes (rat Na<sub>V</sub>1.4 and human Na<sub>V</sub>1.5).
Two of these compounds, BTX-B and BTX-<sup>c</sup>Hx, are functionally
equivalent to BTX, hyperpolarizing channel activation and blocking both fast
and slow inactivation. BTX-yne—a C20-<i>n</i>-heptynoate
ester—is a conspicuous outlier, eliminating fast but not slow inactivation. This
unique property qualifies BTX-yne as the first reported Na<sub>V</sub>
modulator that separates inactivation processes. These findings are supported
by functional studies with bacterial Na<sub>V</sub>s (BacNa<sub>V</sub>s) that lack
a fast inactivation gate. The availability of BTX-yne should advance future
efforts aimed at understanding Na<sub>V</sub> gating mechanisms and designing
allosteric regulators of Na<sub>V</sub> activity.