The effects of bacterial endotoxin on cochlear function and the blood labyrinth barrier

Abstract

Problem: Middle ear infection, one of the most common childhood diseases, has the potential to cause inner ear pathology, resulting in hearing loss or communication disorders. Specific mechanisms responsible for inner ear dysfunction due to otitis media are not clearly understood. It appears that inflammatory mediators and bacterial toxins pass through the round window membrane resulting in structural and biochemical changes in the inner ear leading to auditory dysfunction. This study was undertaken to examine auditory function and the blood labyrinthine barrier in the lateral wall of the cochlea, after bacterial endotoxin administration. Methods: Chinchillas weighing 400 to 500 g were injected intravenously with 1 ml of 10 mg/ml LPS (LPS, Sigma Co, derived from Escherichia coli serotype 055:B5). The same volume of normal saline was administered intravenously as control. Hearing threshold was measured by auditory brainstem response (ABR) before and after injection. Two hours after injection, 3.0 mL of 2% Evan’s blue was injected via the femoral vein and 3 hours later, the animals were euthanized by intracardiac perfusion of fixative. Temporal bones were removed and sections were coated to retain fluorescence of Evan’s blue. Fluorescence of Evan’s blue was detected by fluorescence microscopy and quantitative analysis of Evan’s blue extravasation in the lateral wall was measured by spectrophotometry. Results: There was leakage of Evan’s blue from vessels in the lateral wall after LPS administration and the hearing threshold change was 10–15 dB lower than controls. Conclusion: Administration of bacterial endotoxin caused a threshold change of ABR and breakdown of the blood labyrinthine barrier of the lateral wall, as evidenced by leakage of Evan’s blue. Extravasation of substances may interfere with ion transport, especially potassium, through the lateral wall and cause disturbance of the auditory transduction process leading to hearing dysfunction. Significance: The results of this study partially clarify mechanisms involved in inner ear disturbances caused by middle ear infection. Support: None reported.