A neurotoxin with potential analgesic activity. Tetrodotoxin binds to the pores of fast voltage-gated fast sodium channels in nerve cell membranes, inhibiting nerve action potentials and blocking nerve transmission. Although found in various species of fish (such as the pufferfish), newts, frogs, flatworms, and crabs, tetrodotoxin, for which there is no known antidote, is actually produced by bacteria such as Vibrio alginolyticus, Pseudoalteromonas tetraodonis, and other vibrio and pseudomonas bacterial species.