RADIATION AND AUDIOGENIC SEIZURES IN MICE 521 



original level. In the litters irradiated at 1 .5-2 rad, the frequency of seizures 

 and of fatalities was significantly greater than in controls. Moreover, the 

 proportion of the fatal seizures occurring on the first trial was 75% in the 

 irradiated group, in contrast to 47% in the controls. 



This dosage of gamma radiation also influenced the latency of fatal sei- 

 zures. The graphs in Figs. 1 and 2 present the percentage of fatal seizures 

 occurring at each time interval in Fj males and females. The two curves at 

 the right in each graph compare the latency in the control and irradiated 

 mice during this period (12/56 to 4/57) . In the irradiated mice, the latency 

 was shorter, with an earlier peak and few delayed seizures. The three meas- 

 ures (increased seizure frequency, higher proportion of fatalities on the 1st 

 trial, and shorter latency of fatal seizures) all indicate heightened suscep- 

 tibility to audiogenic seizures following gamma irradiation at 1.5-2 rad. 



Backcross to C57BL6 



The Fi generation derived from new stocks of DBA/1 and C57BL6 was 

 crossed to the new C57BL6 line, and the resulting backcross was maintained 

 in Room G. After the seizure incidence in this low background environment 

 was established, some of the backcross matings were returned to Room R. 

 To control all other factors except radiation level in the room, part of the 

 backcross matings and their litters were placed behind lead and steel shields 

 in Room R. 



Table IV summarizes the percentage of seizures in the backcross genera- 

 tion under these difTerent conditions. In the 1954 group, there had been an 

 unsuspected genetic contribution to the high susceptibility: the C57BL6 

 parents were from our original stock, in which the susceptibility had risen 

 above the normal low level. The 1956-57 populations were derived from 

 the new, low incidence C57BL6 stock. The difTerence in response between 

 the unshielded and shielded groups in Room R constitutes strong evidence 

 for the influence on seizure susceptibility of this astonishingly low level of 

 chronic irradiation (0.2 mr per hour). 



Several additional series of backcross mice were maintained in Room R 

 during 1957 and 1958 while the 00*^° source in the adjacent laboratoiy was 

 in operation. Table V compares the seizure incidence in backcross litters 

 reared in Room R and in Room G. During 2 periods of se\cral months each, 

 we had an unexpected control on the influence of the radiation level in 

 Room R. In May, 1957, backcross matings were moved into the room, in the 

 belief that the source was to be up for the next 3 months. At the end of 3 

 months, we learned that the source had been up for only 3 days. The seizure 

 incidence was almost identical in the groups in Room R and Room G 

 (Table V). During the ensuing 3 months, while the source was up, the fre- 



