TASTE FUNCTIONS IN FISH 223 



responses to various concentrations of salts resemble those of the " water 

 fibre " in the cat described by one of us (Zotterman, 1955), and thus it 

 seems likely that the comparatively weak response to water observed in 

 the fishes adapted to isotonic saline might be induced by the mechanism 

 similar to so-called water response found in cat, etc. It is particularly of 

 interest to note that in the fish adapted to water, the depressing action of 

 salt hardly occurs at the low concentration as shown in Fig. 7. 



It was previously stated that human saliva has a strong stimulating effect 

 on the chemoceptors in carp, especially in the Japanese carp. Single fibre 

 experiments revealed fibres responding only to saliva. This finding sug- 

 gests the possibility that the saliva would selectively stimulate receptors 

 which are sensitive specifically to an unknown substance in the saliva. 

 Experiments were undertaken in the hope of finding the gustatory active 

 substance in the saliva. The fact that human saliva is a powerful stimulant 

 of the fish palatal organ gives us of course a new aspect on the angler's 

 habit of spitting on the bait. 



The fact that saliva did not stimulate the '' salt fibres " excludes the 

 possibility that the electrolytes dissolved in the saliva may serve as the 

 eff'ective stimulating agent. In fact, salivary electrolytes such as KCl, 

 various phosphates, KCNS, KI and KBr could not give eff'ective responses 

 at concentrations similar to those found in saliva (Fig. 8). Salivary 

 enzymes, such as ptyalin, lipase and phosphatase, could not be regarded as 

 the eff'ective stimulant, as we found that saliva had the same stimulating 

 effect after heating (Fig. 8). Mucin in the saliva does not seem to be the 

 active substance, because mucin-free saliva had a strong effect, as will be 

 mentioned later. The cattle's submaxillary mucin* and also the mixed 

 solution of mucin and electrolytes produced only small responses (Fig. 8). 

 The composition of the submaxillary mucin of cattle is as follows : 



Sialic acid : ca. 30% Hexosamine : 13% Hexose : 3%. 

 The saliva contains many kinds of amino acids and vitamins at extremely 

 low concentration (Hinsberg and Schmid, 1953, and Kawamura, 1957). 

 A series of different amino acids and vitamins were tested, but all were 

 found to be inactive in low concentrations (Fig. 8). Neither alone nor 

 mixed could these amino acids or vitamins stimulate as strongly as natural 

 saliva. The substances tested were as follows : 



Amino acid : 1 -tryptophan, 1-arginine, 1 -valine, 1 -glutamic acid, 

 1-phenyl alanine, 1-threonine, 1-lysine, 1-glycine, 1-tyrosine, 1-isoleucine, 

 1-prohne, 1-leucine, 1-serine, 1-cystine, 1-histidine, 1-methionine, taurine. 



Vitamins : aneurin, pyridoxine, biotine, lactoflavin, pantothenic acid, 

 ascorbic acid, nicotinic acid, folic acid, vitamin K. 



It is interesting to note that 1-serine, which is said to be a powerful 

 olfactory stimulant for the salmon (Idler et al., 1956), although it is 



* For samples of pure mucin we are indebted to Prof. G. Blix, Uppsala. 



