Ototoxic effects of unilateral postauricular neomycin administration on the inner ears of Guinea pigs.

Backgrounds: Due to the therapeutic needs of inner ear diseases, a simple method of inner ear drug delivery has been under exploration. Postauricular administration, which involves injecting the drug through the middle of the retroauricular groove, is a potential novel approach, but its mechanisms remain unclear.

Objectives: To investigate threshold shifts in the auditory brainstem response (ABR), as well as the morphology and quantity of hair cells on the bilateral inner ear of guinea pigs after postauricular neomycin administration.

Material and methods: Thirty healthy guinea pigs were divided randomly into two groups and administered either neomycin 200 mg/kg or saline 1 ml via daily postauricular injection for 10 days. Auditory threshold-shifts were examined by tests of the ABR. The quantity and morphology of hair cells in the organ of Corti were assessed by confocal laser scanning microscopy.

Results: The ABR results showed that at a frequency of 16 kHz, the threshold shift was 21.00 ± 14.49 dB in the neomycin injection side, 10.00 ± 16.67 dB in the contralateral side, and 0.50 ± 3.69 dB in the saline group. At a frequency of 24 kHz, the threshold shift was 37.00 ± 16.36 dB in the neomycin injection side, 14.50 ± 9.26 dB in the contralateral side, and 1.00 ± 2.11 dB in the saline group. Histopathological analysis revealed that the outer hair cells (OHCs) in the neomycin injection side suffered significant morphological and numerical loss, while the OHC loss in the contralateral ear was less severe than in the injection side but greater than in the saline group.

Conclusions and significance: This study confirms that postauricular administration is a novel method of local drug delivery to the inner ear, which has practical value in future experimental modeling and the treatment of inner ear diseases.