Gustatory responses to tetrodotoxin and saxitoxin in fishes

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The gustatory responses to tetrodotoxin (TTX) and saxitoxin (STX) recorded from the palatine nerve (VIIth caranial nerve) were studied electrophysiologically in rainbow trout (Salmo gairdneri), masu salmon (Onchorhychus masou), whitespotted char (Salvelinus leucomaenis) and jacover (Sebastes schlegeli). TTX was highly effective gustatory stimulus in salmonid fishes. In rainbow trout TTX had a threshold concentration 10^<-7> mol/1 and at 10^<-5> mol-1 evoked a response two times that of 10^<-3> mol/1 L-proline, the most potent amino acid for this species. The threshold for STX was ligher, 10^<-6> mol/1, and the response magnitude was generally small. This situation was reversed in jacover; TTX was ineffective at 5x10^<-4> mol/1, but STX was highly effective, and had a threshold concentration 10^<-6> mol/1. In rainbow trout, cross-adaptation experiments indicated that receptors of iber types for TTX are distinct from those thatdetect amino acids, acetic acid, sucrose and quinine hydrochloride. Furthermore, the integrated gustatory responses to TTX or STX, a fast-adapting, phasic response, repidly returned to the baseline even with continuous stimulation. Perfusion of the gustabory organs with these chemicals had no inhibitory effect. Behavoioral studies demonstrated that salmonid fished aviod food containing TTX. These findings indicate the existence of a sensitive, specific gustatory receptor system for TTX and STX in fishes, suggesting a defense mechanism against poisonous prey in the aquatic environment.