VOL. 4 (1950) PERMEABILITY AND NERVE FUNCTION, I 87 



described in the following paper, to produce a significant change in permeability. 



b) Release of acetylcholine during activity. In view of the permeability studies 

 described, the limitation of the action of acetylcholine to the synapse, if the ester is 

 applied externally, cannot be used as an indication for a special role at this junction, 

 as was proposed by the theory of chemical transmission. For the same reason, the second 

 fact on which the hypothesis was built has to be reconsidered. The appearance of acetyl- 

 choline in the perfusion fluid of the synapse following nerve stimulation must be attrib- 

 uted to the absence of an insulating membrane. If acetylcholine cannot pass through 

 the structural barrier into the interior, it will not be able to leak from the inside to the 

 outside in stimulated nerve and muscle fibres. The only site where such leakage will 

 be possible is the postsynaptic membrane. However, even at the synaptic junction the 

 ester does not appear under physiological conditions. Dale and his associates have 

 repeatedly emphasized that the ester appears in their experiments only if the normal 

 mechanism responsible for the rapid removal of the ester, viz., acetylcholine-esterase, is 

 largely inactivated by the presence of eserine. Even in presence of the drug, the amounts 

 leaking out were extremely small, about one hundred-thousandth of that required to 

 set up a stimulus. On the basis of more recent experiments, in which acetylcholine was 

 applied directly to the motor end plate, the difference was of the same order of magni- 

 tude. Such a difference is not easily explained in terms of chemical mediation. It is 

 true that in Loewi's original observations on the frog heart, no eserine was present. 

 However, considerable difficulties were encountered by him as well as other investigators 

 when they tried to reproduce the appearance of the ester. For this reason, Loewi's 

 theory was repeatedly criticized^^' ^*. When a heart preparation has been perfused for 

 a certain period of time with Ringer's solution, the post-synaptic membrane may not 

 be in a completely normal condition and may therefore permit leakage of the compound, 

 which under physiological conditions may be rapidly inactivated. The condition of the 

 membrane may depend upon a variety of factors, such as the length of the perfusion 

 period, the composition of the perfusion fluid, the condition and the species of the frog 

 used, etc. Variations of these factors may explain the difficulties encountered by a 

 number of investigators who tried to reproduce this observation. The same consideration 

 may be applied to the finding of Kibjakow^^, who in 1932 described the appearance of 

 acetylcholine in the perfusion fluid of the synaptic ganglion in absence of eserine. His 

 observations were questioned by Dale's school, but it is conceivable that with the less 

 perfect perfusion technique in Kibjakow's experiments, the active membrane suffered 

 more damage and thus permitted the leakage of traces just in the measurable range. 

 So far there is no conclusive evidence that the appearance of the ester outside the cell 

 is a physiological event. 



It is an interesting psychological phenomenon, encountered frequently in the pro- 

 gress of science as well as in the work of individual investigators, that certain observa- 

 tions are neglected or even discarded because they are inconvenient, puzzling and do 

 not fit into preconceived ideas. Later, when the views have changed, the facts may 

 suddenly gain significance and it becomes possible to integrate them into the general 

 picture. The release of acetylcholine at the synapse assumes a new aspect if considered 

 in connection with other pertinent observations which at the time of their presentation 

 did not find sufficient attention. 



In 1933, simultaneously with or even prior to the finding of Dale that acetylcholine 

 appears in the perfusion fluid of the sympathetic ganglion or of the neurc muscular junc- 

 References p. 93I95. 



