AaH II is a scorpion alpha-toxin derived from Androctonus australis Hector, recognized as the standard ligand for site 3 on mammalian voltage-gated sodium channels. It binds with exceptionally high affinity (K0.5 ~ 0.8–1.4 nM) to rat brain synaptosomes and skeletal muscle, inducing a dose-dependent prolongation of action potentials by selectively inhibiting fast inactivation. This maintained sodium current can reach up to 31.6% of the peak current amplitude in rat muscle fibres. Researchers commonly use this compound to probe sodium channel gating mechanisms, study toxin-channel interactions via recombinant mutant expression, and validate immunogen designs targeting channel-specific toxins.