A comparison of cochlear microphonics and N1 in audiogenic-seizure-susceptible and control rats

Although numerous studies have shown that cochlear impairment exists in audiogenic-seizure (AGS)-susceptible mice, there is only one report of cochlear potentials obtained from AGS-susceptible rats. To investigate the hypothesis that cochlear impairment also exists in AGS rats, cochlear microphonics (CM) and the primary afferent activity of the auditory division of the eighth cranial nerve (N1) were studied in AGS rats. AGS rats were obtained from the Veterans Administration Medical Center (Shreveport, La.) colony of Sprague Dawley derived animals, and control rats were obtained from Sprague Dawley, Inc. Two school bells ringing simultaneously were used to produce a sound of approximately 115 dB (AGS test stimulus). Exposure to the AGS test stimulus was once per week for three consecutive weeks. Chloramphenicol was used to treat the frequent otitis media found in the colony of AGS rats. Two categories of AGS-susceptible and control rats were studied: (1) rats exposed to the AGS test stimulus and chloramphenicol regimen; and (2) rats not receiving these treatments. All rats were anesthetized with i.p. Dial-Urethane and prepared for cochlear round window recording. Cochlear microphonics were recorded in response to a click stimulus. A significant decrease in cochlear sensitivity was seen in both groups of AGS rats when compared to appropriate controls as reflected by a 25-35 dB shift in all CM and N1 input-output functions. These results support the hypothesis that a functional cochlear impairment exists in the AGS rat.