86 D. NACHMANSOHN VOL. 4 (1950) 



of the barrier is not clear. The activity may be due to an unspecified esterase other than 

 acetylcholine-esterase or to the presence of small blood vessels, microscopic muscle 

 fibrils or cut nerve fibres where the surface may be reached by the ester. This is, how- 

 ever, entirely irrelevant for the major problem involved. 



The elucidation of the situation became possible by the fortunate circumstance that 

 so many different kinds of extremely potent inhibitors of acetylcholine-esterase were 

 available: reversible and irreversible types of inhibitors and in each of the two groups 

 compounds which penetrate and others which do not penetrate. This combination made 

 it possible to find a satisfactory answer to some of the most pertinent questions involved: 

 I. the necessity of acetylcholine-esterase for conduction; 2. the existence of a barrier 

 for methylated quaternary ammonium salts, and 3. the localization of the enzyme in 

 respect to the barrier. 



Even if a compound affects both axon and synapse, there may still be a great 

 difference as to the concentration required. Chemical substances may act upon the 

 apparently unprotected active surface of the post-synaptic membrane in concentrations 

 much smaller than those necessary for affecting the nerve or muscle fibre. An interesting 

 illustration is provided by the experiments of Roeder and his associates*®, who found 

 that DFP abolishes synaptic transmission in much lower concentrations than those 

 which affect conduction. DFP is very lipid soluble and may therefore accumulate in 

 the myeline sheath to a certain concentration before penetrating into the aqueous 

 interior of the fibre in concentrations sufficiently high to inactivate the enzyme and, 

 consequently, to abolish conduction. At the time when conduction disappears, the 

 concentration of DFP is small in the axoplasm compared with that in the outside 

 fluid^^. This finding supports the assumption that the concentration of DFP at the site 

 of action may be small and is consistent with the potency of the compound as inhibitor 

 of acetylcholine. The necessity of a high outside concentration may be attributed to 

 the relatively slow rate of penetration. In the case of eserine, the distribution between 

 inside and outside at the same period, i.e., at the time when the action potential has 

 disappeared, is very different. The rate of penetration will be determined by the pro- 

 perties of the various chemical compounds on the one hand and by the properties of 

 the various surface membranes. Additional factors may be of importance, such as the 

 affinity of the compound to the enzyme, its potency as inhibitor and the kinetics of the 

 inhibition. In view of the complexity of the process, it is not surprising that in applying 

 potent inhibitors of acetylcholine-esterase, the phenomena observed may differ sharply 

 in so many respects, although the underlying cause is the same chemical reaction. 



The action of procaine, one of the compounds marked on Fig. 2, requires comment. 

 The blocking of conduction by this and other similar anaesthetics cannot be explained 

 in terms of acetylcholine-esterase inhibition. These compounds are weak inhibitors of 

 acetylcholine-esterase, although other esterases may be affected more strongly^". 

 Thimann^^ has pointed out that these compounds have some resemblance in structure 

 to acetylcholine, but are tertiary amines. They will, therefore, easily penetrate into 

 the interior and they may act competitively with the ester on some proteins or lipo- 

 proteins of the membrane. Since apparently they do not depolarize the membrane^^^ 

 it is possible to assume that they form a complex but, in contrast to acetylcholine, they 

 do not change the condition of the protein. However, they may prevent the action 

 of the ester released and thereby block conduction, whereas otherwise the resting 

 condition may remain unchanged. This is consistent with the apparent failure of cocaine, 

 References p. 93I95. 



