438 ZEMAN, CURTIS, SCARPELLI, AND KLEINFELD 



received 8,000 rad with 5 mm beams at a rate of 1,600 rad per sec. At 72 

 hours after irradiation, numerous nerve cells showed cathepsin activity in the 

 cytoplasm. The active nerve cells were randomly distributed. 



The occurrence of cathepsin-like enzymes in the nerve cell cytoplasm after 

 irradiation is difficult to explain. Numerous controlled studies have shown 

 that, except for certain groups of neurons located in the hypothalamus and 

 the motor and reticular nuclei of the brain stem, the nerve cell bodies do not 

 contain cathepsin-like enzymes as demonstrated with the technique used in 

 this study. This would mean that ionizing- radiation can either activate the 

 precursor of such enzymes or hberate them from the lysosomes in which 

 they are locked up, according to de Duve (1959). That such submicroscopic 

 particles are abundantly present in nerve cell cytoplasm has been shown by 

 Scarpelli and Zeman (1960) in electron micrographs. Further studies to test 

 the latter hypothesis are underway. 



The occurrence of radiation-induced proteolytic activity in nerve cells 

 might help to explain radiation-induced central nervous system excitation 

 consistently being observed and described by Russian investigators (Stahl 

 1959). Evidence for the causal correlation of cellular excitation and pro- 

 teolytic activity has been amply provided by the work of Nassonow and his 

 school (discussed by Troschin, 1958). It could also be entertained that some 

 of the early clinical manifestations of radiation, such as shock and radiation 

 sickness, might be related to a release of proteolytic enzymes into the circu- 

 lating blood. 



In conclusion, it can be said that the present studies, although preliminary 

 and incomplete, might yield new knowledge in regard to certain phenomena 

 of radiation biology. 



Summary 



It has been shown that the direct radiation injury of nerve cells is asso- 

 ciated with the occurrence of proteolytic enzymes within the nerve cell 

 cytoplasm. 



