232 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



sponds to 0.2 to 0.5 Mg per gni- The difFercnce in 

 content suggests a specific function of the excess 

 acetylcholine in the postganglionic parasympathetic 

 fibers. The high content in these may be regarded as 

 strong support of the idea that these fibers act by 

 releasing acetylcholine. [For further details concern- 

 ing the occurrence and biosynthesis of acetylcholine 

 in cholinergic nerves see Burgen & Macintosh (15), 

 Gaddum (50) and Rosenblueth (113).] 



The method of estimating the amount of acetylcho- 

 line directly in the tissue cannot be used, however, to 

 estimate the amount of the cholinergic postganglionic 

 transmitter, since this will also occur in preganglionic 

 autonomic fibers and in motor nerves and possibly 

 also in small amounts in all kinds of nerves. 



The biosynthesis of the cholinergic transmitter 

 has been l.ir^c K elucidated by the studies of 

 Nachmansohn & Machado (102). These authors 

 were ai^le to show that an extract from rat brain con- 

 tained an enzyme system which could synthesize 

 acetylcholine in the presence of ATP as the source of 

 energy. This enzyme was called choline aceL)dase. It 

 was shown later that the acetylcholine synthesis occurs 

 in two steps. In a first reaction acetate is transformed 

 into active acetate, and in a second the active acetate 

 combines with choline to form acetylcholine (78). 

 The research work of Stern & Ochoa ( 1 20) and others 

 indicates that cholin e acetylase catalyzes the last step 

 in the acetylcholine formation and that the aciise 

 acetate is an acetyl coenzyme (coenzyme A). The 

 acetate used for the synthesis has to be activated by 

 inc. Ills of ATP, coenzyme and a transacetylase. The 

 active acetate thus formed is used for the final synthe- 

 sis of the acetylcholine. Choline acetylase has been 

 extracted from brain and from electric organs but is 

 also present in all nerve tissues. It has even been 

 demonstrated in tissues from various invertebrates, 

 such as annelids and flatworms. The presence of cho- 

 line acetylase in mitochondrial fractions in homoge- 

 nates of brain (60) suggests that this may be the case 

 al.so in the postganglionic neurons. 



As to the storage of the cholinergic transmitter it 

 appears likely that it is confined to structural elements 

 as demonstrated for adrenergic nerves. Some indirect 

 support for the opinion that acetylcholine is also in- 

 closed in separate particles may be found in the early 

 experiment by Loewi & Hellauer (86). Loewi (85) 

 points to the finding that when nerve tissue is ex- 

 tracted with Ringer's solution, the bulk of acetylcho- 

 line is found in the insoluble residue but that the use 

 of hypotonic .solution causes the greater part of the 

 acetylcholine to be released. This suggests that the 



acetylcholine is located in particles surrounded ijy a 

 membrane similar to mitochondria. When Ringer's 

 solution is used for extracting the acetylcholine in a 

 cholinergic nerve, such as the vagus, most of the 

 acetylcholine goes into solution, however. It is also 

 noteworthy that when acidified solutions are used, 

 the total amount of acetylcholine is extracted as is 

 also the case when extraction is made with acidified 

 alcohol. Some of the acetylcholine may be i^ound to 

 some lipid complex soluble in ether, which acetylcho- 

 line in itself is not (86). 



An analogous i^ehavior is shown by epinephrine 

 and norepinephrine and histamine. It therefore seems 

 possible that these amines form ether-soluble but 

 water-insoluble compounds in the particles. It is of 

 interest in this connection that Hillarp & Nilson (64) 

 found a high content of phosphatides in the supra- 

 renal medullary granules. 



Release in Organs 



Very little is known concerning the mechanism of 

 release of the cholinergic transmitter in the autonomic 

 neuromuscular junctions. By studying the release of 

 acetylcholine from the spontaneously beating or elec- 

 trically driven rabbit's heart, it has been possible to 

 show that the release is significantly higher at a 

 faster heart rate. Thus a spontaneously beating heart 

 with a mean rate of 56 per min. released 0.26 ± 

 0.08 ^g per heart in 40 min. while electrically driven 

 hearts with a mean rate of 210 per min. released 

 0.97 ± 0.36 Kg per heart in 40 min. (32). 



The relea.se of acetylcholine from an organ does 

 not necessarily mean that this substance originates 

 from nervous tissue since it is known that even nerve- 

 free tissue is able to synthesize and release acetylcho- 

 line (17). '^" 



Most of the knowledge on the action of acet\ Icho- 

 line and its release refers to the motor endplate which 

 has been studied in detail from a chemical point of 

 view as well as by electrophysiological techniques. 

 There is hardly any douijt, however, that the mecha- 

 nism of relea.se of the cholinergic transmitter from the 

 postganglionic cholinergic nerves is similar in kind to 

 that already outlined for the adrenergic transmitter. 

 We may thus assume that the active transmitter is 

 released at a terminal portion of the ner\e and acts 

 directly in a chemical manner on the smooth muscle 

 fibers. There is no reason to believe that the sijiooth 

 muscle cells are directly innervated by cholinergic 

 postganglionic fibers any more than they are by 

 adrenergic fibers. 



