myoneural synapses. The initial highest central stimulation caused by 

 phenol appears to be connected with the stabilization of "physiological" 

 acetylcholine and its accumulation in cholinergic synapses. Central paraly- 

 sis, coming after the stimulation and caused by phenol, might be understood 

 as the result of the accumulation of acetylcholine in brain synapses in 

 ordinate pessimal concentrations which cause the inhibition. Certainly, 

 the acetylcholine accumulation in synapses might be caused by two methods-- 

 either due to cholinesterase inactivation or due to an increase in acetyl- 

 choline quanta isolation from nerve endings, but most likely both processes 

 take place. However, it is not of particular concern, since both methods 

 lead to acetylcholine accumulation in synapses in pessimal concentrations. 



Such are the basic results of the experimental studies into the mechan- 

 isms of phenol effect on fish, which we attempted in 'the early sixties. 

 They permitted us (4) to substantiate prospective use of the behavior- 

 reflex method during ichthyotoxicological experiments as the most sensitive 

 tests for determining the chronic effect of trace concentrations of various 

 homologues of the phenol series and for determining the MPC value of this 

 group of substances. Experimental data obtained by B.A. Flyorov and V.E. 

 Matey in their experiments on gold crucians and Lebistes groups (23-26) 

 fully supported this point of view. As should be expected, pathological 

 changes in behavior reflex activity in fish occur long before the appearance 

 of expressed phenol intoxication symptoms and are observed in concentrations 

 that are 5-8 times lower than acute toxic concentrations of this substance. 

 There are good reasons to believe that the behavior reflex method will hold 

 a fitting place among other methods for determining the MPC of various 

 groups of organic toxins with expressed activity on the central nervous 

 system which is characteristic for them. 



The growth of international cooperation in the field of water toxicology 

 and ichthyotoxicology brings our attention to the question of unifying 

 methods for estimating the degree of toxicity for various groups of harmful 

 substances, and standardizing experimental conditions and principles for 

 interpreting test data. At present, this is quite possible, thanks to the 

 accumulation of data concerning the dependence of the results from ichthyo- 

 toxicological experiments on many variables. In this case, two groups of 

 factors characterizing both fish habitat (chemical water state, oxygen con- 

 tent, pH value, water temperature, etc.) and the test object itself 

 (species, age, and sexual properties of fish sensitivity and stability as 

 well as initial functional state) play a dominant role. Therefore, in 

 order to develop an actual and potential toxicity for a harmful substance, 

 it is necessary to carry out experiments which allow for fluctuations in 

 physical-chemical parameters of the water medium (27), i.e., experiments 

 carried out at a relatively high temperature and a moderately low oxygen 

 content. Water hardness and pH value are selected in such a way as to 

 develop the highest possible toxicity for the substance under investigation. 

 The most sensitive species of fish of the ichthyofauna under investigation 

 (28) should be used as the test object. In this case, it is important to 

 consider the sensitivity of the test species at various stages of ontogene- 

 sis, and choose the most sensitive one (29, 3). Together with the physical - 

 chemical factors of the water environment and the biological properties of 



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