162 INVERTEBRATE PHYSIOLOGY 



Synthesis, Axonal Transport, Storage, and Release 

 OF Neurohormones 



As long ago as 1914, J. F. Gaskell observed the exciter action of adrena- 

 line on the pulsating blood vessels of the leech. Associating this effect with 

 the presence of chromaffin cells in the nervous system of the leech, Gaskell 

 said, "It is just possible that in the case of the leech the adrenaline passes 

 from the cell to the periphery by way of the motor nerve itself." While 

 this view of axonal transport of materials has since been expressed by a 

 number of workers, there has been a rather general reluctance to accept 

 the evidence as truly convincing. Studies of neurosecretory cells by Ernst 

 and Berta Scharrer and others (see Scharrer and Scharrer, 1954a,b), 

 provide evidence that synthesis of their products takes place largely in 

 the cell body. They are then carried inside the axon to the terminals, 

 which are often modified for storage, whence they are released on appro- 

 priate stimulation of the neurosecretory cell. Perhaps the most convincing 

 evidence for proximodistal movement of materials in axons of inverte- 

 brate neurons comes from cutting and ligation experiments such as those 

 of B. Scharrer (1952) on Lciicophaea and E. Thomsen (1954) on Calli- 

 phora. Other workers have done somewhat similar experiments in cer- 

 tain crustaceans (e.g.. Bliss and Welsh, 1952; Passano, 1953). When 

 neurosecretory tracts are cut or blocked, there is an accumulation of 

 secretory material proximal to the point of interruption. Normally, this 

 material with characteristic staining properties is found in far greater 

 concentration in the terminals of neurosecretory cells than elsewhere in 

 such neurons. 



Those who have been interested in neurohormones have given much 

 thought to the question of the state in which these highly active molecules 

 exist while yet within the neuron. Are they bound to a carrier protein 

 or lipoprotein? Do they exist as an inactive precursor homologue re- 

 quiring chemical transformation? Is the characteristic stainable material 

 of a neurosecretory cell a carrier substance or the neurohormone ? Potter 

 (1954) has described several tinctorial types of nerve endings in the so- 

 called sinus gland of the blue crab, Callinectes. Does this indicate that 

 each type contains a different neurohormone ? 



There is a recent and very interesting development that helps answer 

 some of these questions and that may prove to be one of the most im- 

 portant steps in our understanding of neurons. In the spring of 1954, 

 several groups of electron microscopists (De Robertis and Bennett, 

 Palade, Palay, Robertson) reported that axon terminals from a variety 

 of animals, including the crayfish and earthworm, contained collections 

 of mitochondria and small vesicles. De Robertis and Bennett (1955) de- 



