Gustatory Responses to Tetrodotoxin and Saxitoxin in Fish: A Possible Mechanism for Avoiding Marine Toxins

The gustatory responses to tetrodotoxin (TTX) and saxitoxin (STX) recorded from the palatine nerve (VIIth cranial nerve) were studied electrophysiological in rainbow trout (Salmo gairdneri) and Arctic char (Salvelinus alpinus). Both toxins were highly effective gustatory stimuli in both species, in rainbow trout, TTX had a threshold concentration 2 × 10−7 mol/L and at 10−5 mol/L evoked a response four times that of 10−3 mol L-proline/L, the most potent amino acid for this species. The threshold for STX was Sower (10−8 mol/L), but unlike TTX the response magnitude reached a maximum at 10−6 mol/L. The reverse occurred in Arctic char; lower threshold for TTX (10−8 mol/L) than STX (10−7 mol/L) and the response magnitude never exceeded that of 10−3 mol L-proline/L. Cross-adaptation experiments indicated that the receptor(s) for TTX are distinct from those which detect amino acids and bile salts and that TTX and STX do not share the same receptor populations. Furthermore, the integrated response to TTX or STX was a fast-adapting, phasic response and rapidly returned to baseline even with continued stimulation. Perfusion of the gustatory organs with these toxins had little toxic effect. The sensitive, specific gustatory receptor system for the toxins suggests the existence of a mechanism for avoiding poisonous prey organisms that has adaptive advantage to the receiver (predator).