Sodium channel toxin-resistance mutations do not govern batrachotoxin (BTX) autoresistance in poison birds and frogs

Poisonous organisms carry small molecule toxins that alter voltage-gated sodium channel (Na{checkmark}) function. Among these, batrachotoxin (BTX) from Pitohui toxic birds and Phyllobates poison frogs, stands out because of its lethality and unusual effects on Nav function. How these toxin-bearing organisms avoid autointoxication remains poorly understood. In poison frogs, a Nav DIVS6 pore-forming helix N[->]T mutation has been proposed as the BTX resistance mechanism. Here, we show that this variant is absent from Pitohui and poison frog Navs, incurs a strong cost that compromises channel function, and fails to produce BTX-resistant channels when tested in the context of poison frog Navs. We further show that captive-raised poison frogs are BTX resistant, even though they bear BTX-sensitive Navs. Hence, our data refute the hypothesis that BTX autoresistance is rooted in Nav mutations and instead suggest that more generalizable mechanisms such as toxin sequestration act to protect BTX-bearing species from autointoxication.