Hearing impairment in WBN/Kob rats with spontaneous diabetes mellitus

The inner ear of spontaneously diabetic WBN/Kob rats was functionally and morphologically examined in order to elucidate the relationship between diabetes mellitus and hearing impairment. At 3 months of age, WBN/Kob rats were non-diabetic, and their hearing function was normal. At 6-7 months of age, they showed decreased glucose tolerance and an increasing tendency toward urinary excretion of glucose without high plasma concentration of glucose, and were therefore judged to be pre-diabetic. They also displayed a significant elevation of hearing threshold in the auditory brainstem response, but showed little morphological and histochemical changes in the inner ear. At 12-13 months of age, they were spontaneously diabetic and showed a more apparent elevation of hearing threshold in auditory brainstem response than that in pre-diabetic animals. In addition, they displayed a marked decrease in the number of spiral ganglion cells and oedematous changes in the stria vascularis. The stria vascularis also showed a decrease in the intensity of staining with some lectins, i.e., wheat germ agglutinin, succinylated wheat germ agglutinin, Soranum tuberosum lectin, and concanavalin A. In conclusion, hearing impairment is induced by diabetes in the WBN/Kob rats first as an elevation of hearing threshold along with glucose intolerance; secondly, as a decrease in the number of spiral ganglion cells; and thirdly, as oedematous change of the stria vascularis with decreased intensity of lectin staining.