Gustatory responses to tetrodotoxin and saxitoxin in fishes

用电生理学方法研究了虹鱼(Salmo Gairdneri)、马哈鱼(Onclhychus Masou)、白点鲑鱼(Salvelinus Leucomaenis)和银鱼(Sebases Schlegeli)对河豚毒素(TTX)和岩藻毒素(STX)的味觉反应。TTX是一种高效的鲑鱼味觉刺激物质。TTX的阈值浓度为10^~(-7)~(-5)·m ol~(-1)，在10~(-5)~(-5)m ol~(-1)时引起的反应是该物种最强的氨基酸--L-脯氨酸的两倍。STX的阈值较轻，为10^~(-6)~(-6)mol/1，反应幅度一般较小。反之亦然，TTX在5×10~(-4)~(-4)~(-4)mol/1浓度时无效，而STX则有效，其阈值浓度为10~(-6)~(-6)~(-1)mol/1。在虹鱼中，交叉适应实验表明，TTX的IBER型受体与氨基酸、醋酸、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时相反应，即使在持续刺激下也迅速恢复到基线水平。用这些化学物质灌流小鼠的胃器官，没有抑制作用。行为学研究表明，鲑鱼是含有TTX的禽类食物。这些发现表明，鱼类中存在一种对TTX和STX敏感、特异的味觉受体系统，这表明在水生环境中对有毒猎物的防御机制。

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.