NEUROHORMONES 167 



tamine is excitatory. That nature seldom sticks to a set pattern, however, 

 is seen in the exception to this rule provided by the heart of Mytilus cali- 

 fornianiis. Here, both acetylcholine and 5-hydroxytryptamine are excit- 

 tory, as R. B. Wait (personal commvmication ) recently observed while 

 working at the Marine Field Laboratories of the University of Washing- 

 ton. 



Most moUuscan smooth muscle fibers are inconveniently small for use 

 of internal electrodes in recording membrane potentials. However, the 

 very long fibers of the anterior byssus retractor muscles of Mytilus edulis 

 permit measuring of a demarcation potential in a manner similar to that 

 long used in studying nerves. Taking advantage of this anatomical situa- 

 tion, Twarog (1954) finds that acetylcholine causes depolarization and a 

 tonic contraction of the byssus retractor muscle, while 5-hydroxytrypta- 

 mine relaxes tonic contractions. These observations suggest that this 

 muscle is doubly innervated and that opposing neurohumors mediate be- 

 tween nerves and muscle fibers. 



By a quantitative measure of the relative activities of a wide range of 

 acetylcholine analogues, Welsh and Taub (1948, 1950, 1951, 1953) were 

 able to show a relationship between molecular structvire and biological 

 activity on the Venus heart. Certain deductions could be made concerning 

 the so-called acetylcholine receptive substance. In many respects, the 

 patterns of pharmacological action of acetylcholine antagonists on the 

 Venus heart resemble those seen in vertebrate autonomic ganglia. How- 

 ever, important dififerences provide further evidence that acetylcholine 

 receptors which have a common basic configuration may nevertheless 

 differ in details. 



We now have considerable knowledge of the pharmacology of 5-hy- 

 droxytryptamine analogues on the Venus heart. The extraordinarily per- 

 sistent excitatory action of certain of the ergot alkaloids and of lysergic 

 acid diethylamide (LSD) appears due to the presence of the 5-hydroxy- 

 tryptamine structure in lysergic acid and the added stable nature and 

 stickiness of the ergot derivatives of lysergic acid (Welsh and Taub, 1948; 

 Welsh, unpublished). 



These examples of recent attempts to learn more concerning the struc- 

 ture-activity relations of neurohumors and their analogues, if discussed 

 in greater detail, would show how favorable certain invertebrate prepara- 

 tions can be in such studies. As certain invertebrate nerve fibers have been 

 useful in gaining further insight into the conduction process, so may other 

 properly chosen invertebrate preparations tell us much concerning the 

 details of the transmission process. 



Since we know more concerning neurosecretory systems and the action 

 of their products in insects and crustaceans, these groups will be used to 



