Gustatory responses to tetrodotoxin and saxitoxin in fishes

本文研究了虹鳟鱼（Salmo gairdneri）、马苏鲑鱼（Onchorhychus masou）、白斑鲑（Salvelinus leucomaenis）和jacobe （sebases schlegeli）从腭神经（第vii脑神经）上记录的河豚毒素（TTX）和蛤蚌毒素（STX）的味觉电生理反应。TTX对鲑科鱼类是非常有效的味觉刺激。在虹鳟鱼中，TTX的阈值浓度为10^<-7> mol/1，在10^<-5> mol/1时引起的反应是10^<-3> mol/1 l-脯氨酸的两倍，这是该物种最有效的氨基酸。STX的阈值较低，为10^<-6> mol/1，响应幅度一般较小。这种情况在jacover正好相反；TTX在5 × 10^<-4> mol/1时无效，但STX非常有效，阈值浓度为10^<-6> mol/1。在虹鳟鱼中，交叉适应实验表明，检测TTX的iber类型受体与检测氨基酸、乙酸、蔗糖和盐酸奎宁的受体不同。此外，对TTX或STX的综合味觉反应是一种快速适应的阶段性反应，即使在持续刺激下也能迅速恢复到基线。用这些化学物质灌注味觉器官没有抑制作用。行为研究表明，鲑科鱼类不吃含有TTX的食物。这些发现表明，鱼类对TTX和STX存在一个敏感的、特异性的味觉受体系统，这表明在水生环境中存在一种防御有毒猎物的机制。

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.