Batrachotoxin-sensitive sodium channels in toxic birds challenge "target mutation" strategy of toxin autoresistance

Voltage-gated sodium channels (Na_Vs) are essential for muscle and nerve activity and are prime targets of natural toxins such as batrachotoxin (BTX). Poison dart frogs and multiple species of New Guinean toxic birds accumulate dietary BTX without self-poisoning. A recent report identified two Na_V1.4 substitutions under positive selection in toxic birds and proposed a "target mutation" autoresistance strategy for these passerines. Here, we test that hypothesis using structure-guided mapping and electrophysiology. The substitutions map outside known inner-cavity BTX receptor sites. Functionally, single and double mutants show wild-type activation and inactivation properties and remain sensitive to BTX, with comparable toxin-induced activation shifts and persistent current. These data argue that these Na_V1.4 substitutions do not provide BTX autoresistance. Together with prior work in BTX-carrying frogs and birds, our findings are consistent with a "sequestration" strategy using BTX-binding factors. Identifying these factors could enable antidote design and broaden strategies for neutralizing Na_V-targeting toxins.