514 DOROTHEA STARBUCK MILLER 



Materials and Methods 



We have been working for several years with sound-induced convulsive 

 seizures in inbred mice, with particular reference to the mode of inheritance 

 and the physiolooic basis of the trait (Miller et al, 1952, 1955). Our 

 material consists of two pure lines of mice unusually sensitive to sound (DBA 

 lines 1 and 2) and two sublines of sound resistant mice (C57BL6 and 

 C57BL10) . The animals are tested for seizure susceptibility by being exposed 

 to a bell at 100 decibels for 2 minutes on four successive days, starting when 

 the mice are 30 days old. In DBA/1, more than half of the animals suffer 

 tonic-clonic seizures, usually fatal. In DBA/1, the frequency of seizures is 

 close to lOO'^r, mostly fatal. Seizures are rare in the C57 strain. 



The most obvious criterion of seizure susceptibility is the incidence of 

 seizures. Other measures helpful in determining the degree of su.sceptibility 

 are severity of seizures, latency, and percentage of seizures on the first trial. 



In DBA mice, the most severe and most common convulsive pattern is 

 the fatal tonic-clonic seizure. In a few cases, animals survive the tonic- 

 clonic seizure after respiratory arrest. The tonic seizure is rarely fatal, and 

 the least severe response, the standing spasm, is never fatal. 



Latency, or the number of seconds after the onset of sound stimulation 

 when the seizure occurs, is most commonly between 35 and 45 sec. Highly 

 susceptible mice tend to convulse early, and in less susceptible animals the 

 latency is longer. Premature seizines (before 16 sec.) are observed only in 

 very susceptible animals. Delayed seizures (after 60 sec.) are associated with 

 lower susceptibility. 



Very susceptible mice tend to convulse on the 1st exposure, while in less 

 susceptible mice, the 1st seizure may occur on the 3rd or 4th trial. There- 

 fore, in comparing groups of moderate susceptibility, the 1st trial data often 

 furnish a more accurate measure of relative susceptibility: actual differences 

 may be masked within the 4 trials. 



The Problem 



In July, 1952, our mouse colony was moxed from Whitman Laboratory 

 into a frame building behind the laboratory. At that time we were complet- 

 ing the genetic analysis of seizure susceptibility through crosses of both 

 sound-susceptible lines to both sound-resistant sublines. In each cross, a 

 fairly predictable incidence of seizures had been observed. Imniediately after 

 moving the colony to the new location, there was a striking increase in the 

 frequency of seizures in all groups. 



Table I summarizes the change in the percentage of fatal and total sei- 

 zures in those groups with adequate numbers for valid comparison. The 



