RADIATION AND AUDIOGENIC SEIZURES IN MICE 529 



The time when radiation is applied appears to be important. If the mice 

 were exposed from birth throughout the 30 day period before being tested 

 for sound sensitivity, doses of gamma radiation as low as 4.8 mrad per day 

 (total dose 0.14 rad) increased the frequency and severity of seizures. When 

 the period of exposure was between days 23 and 30, a dosage of 350 mrad 

 per day (total 1.5 to 2 rad) was required to induce a significant rise in 

 seizure susceptibility. In one series of 150 Fi mice irradiated at Ys of this 

 dosage (220 mrad per day) , the incidence of seizures was elevated in females, 

 but not in males. In another group of 100 Fi mice, this dosage was without 

 efTect in either sex. 



Some fragmentary evidence suggests that radiation during the first few 

 days of life is especially effective in increasing seizure susceptibility; the first 

 1 1 days may be critical. Backcross litters exposed for only the first 3 days to 

 the remote Co*^" source in Room R showed a slight increase in seizure-inci- 

 dence. In the transition group of DBA/1 mice, born in Room R and moved 

 to Room G before being tested, those litters which had been in Room R for 

 7-10 days were as seizure prone as those which had been in the room for 

 17-19 days. From March to August, 1960, we had access to an insulated 

 cave in the hope that this would constitute a low background control en- 

 vironment. A population of Fi matings and litters was moved into the cave. 

 However, the incidence of seizures in this group was high rather than low. In 

 monitoring the room. Health Physics found a spot on the concrete floor 

 which emitted 7.5 mr per hour of mixed beta and gamma radiation: the 

 intensity dropped to 0.4 mr per hour at a distance of 6 in. The cave, there- 

 fore, served as an unexpected low level exposure situation, rather than the 

 reverse. Among the first litters tested in the cave, those that were older than 

 1 1 days when moved to the cave showed low seizure incidence. In the litters 

 that were 3-11 days old when introduced into the cave, a high proportion 

 suffered fatal seizures on the first trial. The fact that neonatal mice are small 

 and naked may explain why they are unusually radiosensitive. The influence 

 of exposure to radiation at various intervals in the life span has not been 

 tested systematically. 



An interesting contrast was observed in regard to the interaction of the 

 high fallout level with other sources of radiation in influencing seiziue sus- 

 ceptibility. In the backcross generation maintained in Room R, the effects of 

 increased background from fallout and of chronic low level gamma radiation 

 appeared to be additive (Table V). Both factors were operative throughout 

 the 30 day period before the sound test. In the Fi generation, 1.5-2 rad 

 sfiven after weanina; resulted in no greater incidence of seizures when the 

 mice were reared in a high background environment than at low background 

 (Table III). This difference is a further indication that \ei'y low doses of 



