Time sequence of auditory nerve and spiral ganglion cell degeneration following chronic kanamycin-induced deafness in the guinea pig.

We investigated the time sequence of morphological changes of the spiral ganglion cell (SGC) and auditory nerve (AN) following chronic kanamycin-induced deafness. Guinea pigs were treated with kanamycin by subcutaneous injection at 500 mg/kg per day for 7 days. Histological changes in hair cells, SGCs, Schwann cells and the area of the cross-sectional of the AN with vestibular ganglion (VG) in the internal acoustic meatus were quantified at 1, 7, 14, 28, 56 and 70 days after kanamycin treatment. Outer hair cells decreased at 7 and 14 days. Loss of inner hair cells occurred at 14 and 28 days. The cross-sectional area of the AN with VG increased at 1 day and decreased shortly following loss of SGCs and Schwann cells at 7, 14 and 28 days after deafening. There was a similar time course of morphological changes in the overall cochlea and the basal turn. Thus, the effects of kanamycin on hair cells, spiral ganglion and Schwann cells are progressive. Early degeneration of SGC and Schwann cell mainly results from the direct toxic effect of kanamycin. However, multiple factors such as loss of hair cell, degeneration of Schwann cell and the progressive damage of kanamycin, may participate in the late degeneration process of SGCs. The molecular mechanism of the degeneration of SGC and Schwann cell should be investigated in the future. Moreover, there is a different time sequence of cell degeneration between acute and chronic deafness by kanamycin.