AUDIOGENIC-SEIZURES IN UNILATERALLY SENSITIZED AND MONAURALLY STIMULATED WISTAR RATS

Results of previous studies (Pierson, M. and Swann, J., Epilepsia, 32 (1991) 1-9) have demonstrated that exposure of Wistar rats to noise on day 14 results in audiogenic seizure susceptibility. Experiments reported here examined whether unilateral susceptibility could be induced in rats by monaural restriction of this noise exposure. Behavioral attributes of seizures on day 28 were compared in groups that were: binaurally noise-exposed/binaurally tested, binaurally noise-exposed/monaurally tested, monaurally noise-exposed/binaurally tested and monaurally noise-exposed/monaurally tested. Effects of left- and right-ear exposures and tests were assessed separately. Unilateral susceptibility was evident since seizures could be elicited later only by stimulation of the originally noise-exposed ear. Seizures were behaviorally different in monaurally noise-exposed and binaurally noise-exposed animals. Convulsions, directional reversals during running episodes, and relatively short latencies occur only in binaurally noise-exposed rats. These behaviors occur with either monaural or binaural stimulation. Initial running direction was random in binaurally stimulated/binaurally noise-exposed rats, but was fixed in all other groups depending on which ear was exposed in either sensitization (day 14) or testing (day 28). Right- and left-ear sensitizations or tests resulted in left-directed and right-directed running onsets respectively. Previous studies of the effect of selective CNS lesions in instances of unilateral or bilateral susceptibility have led to the understanding that seizure initiation in unilaterally susceptible animals is mediated by the crossed ascending auditory pathway. From the present data it appears that initial running direction reflects which side of inferior colliculus (IC) mediates seizure initiation. Other data here suggest that both convulsions and two-directional running during seizures reflect bicollicular seizure activity. Since bidirectional seizures and convulsions occur in bilaterally sensitized rats even under conditions of monaural stimulation and are never observed in unilaterally sensitized rats, it may be that at least one pathway which contributes to seizures in bilaterally susceptible rats is uninvolved in seizures of unilaterally sensitized rats.