60 YAMAZAKI, BENNETT AND CLEMENTE 



Arnold et al, 1954a,b,c; Davidoff it ai, 1938). Many adult rats receiving 

 1,000 r to the head alone appeared normal in every respect (Bennett, 1960) . 

 In young head-irradiated rats of unspecified age, it has been reported that 

 subependymal cells of the lateral \entricle were destroyed with 200 r. How- 

 ever, the associated neurologic findings were not mentioned (Hicks and 

 Montgomery, 1 952 ) . 



The time sequence dining which this radioresistance de\elops in the 

 neonatal period associated with the maturation of the brain has not been 

 as clearly demonstrated. In the fetal guinea pig, the neuronal differentiation 

 is closely correlated with biochemic, functional, and other morphologic 

 changes in the last trimester of gestation ( Flexner, 1953). The monograph 

 edited by Waelsch (1955) emphasizes that in the rat similar developments 

 occur during the first 10 postnatal days. Ionizing radiation inight well 

 alter these highly integrated events suggesting the possibility of a changing 

 pattern of response to irradiation with increasing age. Our study was under- 

 taken to systematically observe the immediate and long term behavioral and 

 histologic effects of single direct radiation graded doses to the brain of 

 neonatal and young rats. 



Method 



A total of 112 rats, aged 8 hours to 15 days, recei\ed x-radiation to the 

 head only. Single doses of radiation were given at four levels: 125, 300, 

 500, and 1,000 r. Observations were made on 48 control animals. From 48 

 to 72 hours after irradiation, 28 animals were autopsied. The remaining 84 

 animals were observed up to 14 months and sacrificed as described in pre- 

 vious reports (Yamazaki ct al., I960: C'lemente, ct ai, 1960). 



Observations 



The most interesting data in our study indicated that the severity ol the 

 nein'ologic and pathologic findings were dependent on two factors: the 

 intensity of the radiation dose and the stage of postnatal maturation of the 

 brain at the time of radiation. The most severe brain reactions were found 

 in rats radiated on the first 3 or 4 days postnatally. Animals irradiated on 

 the 5th postnatal day definitely showed fewer neurologic signs than rats 

 radiated on the first 4 days. In the 5-day rat, 500 r administered to the head 

 produced neurologic signs in a little less than half of the irradiated rats, 

 whereas all of the animals irradiated with 300 r and 500 r prior to this 

 time developed some evidence of central nervous system involvement, and 

 1,000 r often proved lethal during the first few days of life. Animals radiated 

 after the 5th postnatal day showed a marked decrease in the incidence and 



