88 D. NACHMANSOHN VOL. 4 (1950) 



tion, Calabro^^ had shown that, following prolonged stimulation of the rabbit vagus, 

 an acetylcholine-like substance is released from the cut end into the surrounding fluid. 

 BiNET AND MiNz" found, in 1934, that from the transsected surface of nerves a compound 

 is released which increases the sensitivity of the leech muscle to acetylcholine. Calabro's 

 findings were confirmed and extended by Bergami^^ and by Babski and Kisljuk^*' ^^. 

 In 1937 VON Muralt^^ described a diffeience of the acetylcholine content between 

 stimulated and unstimulated nerves. In view of the possibility of a very rapid disap- 

 pearance of the active ester, he developed a special technique by which he "shot" the 

 nerves into liquid air. Tested by bioassay after a short period of extraction the amount 

 of acetylcholine was 0.2 jug per gram in the stimulated as compared with 0.12 //g per 

 gram in the control nerve. In a large series of experiments the difference between 

 stimulated and control nerve was later found to be 0.09 //g per gram^-. However, the 

 difference between the two nerves disappears if extraction is continued for a longer 

 period of time. There is, therefore, some uncertainty as to the interpretation. It is 

 conceivable that the acetylcholine released from its complex is present in a free form 

 and therefore diffuses from the frozen tissue during extraction more rapidly than that 

 part of the acetylcholine which is bound to protein or lipoprotein. 



Even in sensory nerves release of acetylcholine has been demonstrated by Brecht 

 AND Corsten^^ from the cut end after stimulation. These investigators used a remarkably 

 sensitive method, the contraction of the frog lung in presence of eserine, and hereby 

 succeeded in detecting the ester released. The amounts are still smaller than those 

 released from motor nerves, but this difference appears consistent with the smaller rate 

 of metabolism indicated by the lower concentrations of acetylcholine-esterase and 

 choline acetylase". It is significant that the release of acetylchoHne has been demon- 

 strated in parasympathetic, motor and sensory nerve fibres. The situation is pertinent 

 in connection with the finding that the enzymes which form and hydrolyse acetylcholine 

 are present in all types of nerves and that the inactivation of acetylcholine-esterase 

 invariably leads to abolition of conduction. 



The facts described support the assumption that there is no difference in principle 

 between the release of acetylcholine at the synapse and in the axon, except that in the 

 latter case the ester cannot pass through the structural barrier. They make it appear 

 still more probable that this release is an intercellular process and that the appearance 

 outside the cell at the synapse must be attributed either to the poisoning of the enzymic 

 mechanism, normally preventing the leakage or to some other damage of the active 

 surface where it is least protected and most vulnerable. At the time when these findings 

 were described, acetylcholine was considered to be a chemical mediator and since 

 chemical transmission in the axon is inconceivable, it was difficult to integrate them 

 into the general picture. Little or no attention was consequently paid to these findings. 

 Von Muralt has been very cautious in his statements as to the possible significance 

 of the release of acetylcholine in the nerve fibre. He called the ester an "Aktions- 

 substanz", meaning that it may be important like many other substances for nerve 

 activity in the axon as well as at the synapse. This caution was well justified at a time 

 when nothing was known about the high speed of the reaction, the effects of acetyl- 

 choline-esterase inhibitors on conduction and the great variety of other factors 

 known today. These facts had to be established before it became possible to assume 

 a direct association of the ester with the generation of the electric currents which 

 propagate the impulse. In the light of recent developments, however, the situation 



References p. 93/9 5. 



