NEUROHORMONES OR TRANSMITTER AGENTS 



John H. Welsh 



Harvard University 



During the past two decades studies on invertebrate nervous systems 

 have contributed greatly to our understanding of the basic mode of op- 

 eration of nerve cells. Giant nerve fibers of the squid and crustacean leg 

 nerve fibers have provided axons of conveniently large size for detailed 

 study of the phenomena associated with conduction of the nerve impulse. 

 Various other invertebrate preparations have furnished highly suitable 

 systems for studies of the transmission process. Thus observations on the 

 isolated heart of Venus mercenaria have told us much concerning the 

 importance of molecular configuration in the reaction of acetylcholine 

 with its receptor substance and revealed for the first time the role of 

 5-hydroxytryptamine as a neurohormone. Insect and crustacean neuro- 

 secretory systems have shown how specialized certain neurons can be, 

 for purposes of producing quantities of transmitter agents. 



The results of these studies make it increasingly obvious that the most 

 characteristic feature of a neuron is its ability to release, at its termina- 

 tions, a substance which acts on an adjacent neuron or an effector cell, 

 or which is carried via the circulation to a distant part of the organism. 

 This paper will summarize a small portion of the evidence for the im- 

 portant role of neurohormones. 



It has been suggested elsewhere (Welsh, 1955) that the term neuro- 

 hormone be defined as an organic compound produced by neurons and 

 released at their endings to act as a chemical messenger or hormone, either 

 locally or at a distance. Included among the neurohormones would be the 

 neurohumors such as acetylcholine, nor-adrenaline, and 5-hydroxytrypta- 

 mine, which often act over rather short distances, and the neurosecretory 

 substances such as vertebrate oxytocin and vasopressin and the products 

 of the neurosecretory systems of insects and crustaceans (see Scharrer, 

 1955), which normally act at some distance from the point of 

 release. 



One purpose of this discussion will be to show how much the various 

 neurohormones have in common, especially in regard to their transport, 

 storage, and release. A second purpose wall be to review briefly our 

 knowledge of the chemical nature and distribution of the neurohormones. 

 One other aim will be to point out how incomplete is our understanding 

 of the basic mechanism of action of the individual neurohormones. 



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