- The peptide conotoxin TxVIA is selectively toxic to molluscs and slows sodium current inactivation in mollusc neurons. Here we show that TxVIA binds with high affinity to new sites on sodium channels in both mollusc and rat central nervous systems, despite its lack of toxicity to vertebrates. Furthermore, TxVIA protects from the toxic effects of Conus striatus toxin in rat brain. The TxVIA binding site differs from other neurotoxin receptor sites affecting sodium channel inactivation in that binding is not voltage-dependent and undergoes negative allosteric modulation by veratridine. TxVIA therefore represents a novel category of sodium channel probes, designated delta-conotoxins. TxVIA is shown to discriminate between sodium channels in different phyla by activity but not by binding, thus providing a lead for the study of structural elements affecting gating modes of sodium channels.