168 INVERTEBRATE PHYSIOLOGY 



illustrate the type of action characterizing certain neurosecretory sub- 

 stances. Certain insects have groups of neurosecretory cells in the brain 

 whose axons end largely in the corpora cardiaca. These cells produce a 

 neurohormone that exerts a trophic influence on the thoracic glands. The 

 thoracic glands, in turn, produce a molting hormone that has recently 

 been isolated and crystallized, and its empirical formula determined 

 (Butenandt and Karlson, 1954). The molting hormone influences a num- 

 ber of physiological processes associated with molting and subsequent 

 growth. In this case we have a neurosecretory substance that may have to 

 exert an influence over a considerable period of time. Also it may act at 

 some distance from the point of release. 



The neurosecretory system of decapod crustaceans is anatomically 

 more complex than that of the insects (Bliss and Welsh, 1952; Bliss, 

 Durand, and Welsh, 1954). Likewise its physiological role appears more 

 involved. Products of this system are employed in bringing about color 

 changes, retinal pigment movements, gonad development, and molting 

 with its many attendant processes. Chromatophores and retinal pigments 

 are directly controlled by neurosecretory substances carried in the blood. 

 Molting in crustaceans appears to be controlled by a hormone from organ 

 Y, the production and release of which during the intermolt period are 

 probably prevented by a neurosecretory substance. Again we find the 

 neurosecretory substances acting at a distance and over considerable 

 periods of time. Until we know more concerning the chemical nature of 

 the invertebrate neurosecretory substances we cannot hope to under- 

 stand in full detail their mechanisms of action. It is possible that they act 

 in a manner similar to that of the neurohumors but are more stable and 

 tend to form a more lasting complex with cellular components. 



In conclusion, it may be said that the invertebrates are providing useful 

 information toward a better understanding of the neurohormones and 

 their modes of action. 



Summary 



The term neurohormone (or transmitter agent) is here used to designate 

 any organic compound that is released from neuronal endings and serves 

 to convey a message to other cells, tissues, or organs. One type of neuro- 

 hormone, which we might continue to call a neurohumor, is produced by 

 neurons that are in close association with other neurons or with efifectors. 

 Acetylcholine, adrenaline, nor-adrenaline, and 5-hydroxytryptamine are 

 neurohumors known to occur in certain invertebrates. A second type of 

 neurohormone consists of the neurosecretory materials released from 

 neurons which often end on blood spaces and which are sometimes highly 

 modified for the production, storage, and release of transmitter agents. 



