Intranasal gentamicin irrigation impacts number and morphology of auditory brainstem neurons

Gentamicin is an aminoglycoside antibiotic that broadly targets Gram-negative bacteria. Gentamicin exhibits ototoxic effects in both human subjects and animal models over several different routes of administration. While gentamicin is primarily vestibulotoxic, it causes sensorineural hearing loss attributed to hair cell damage at the base of the cochlea. Gentamicin can also be administered through intranasal irrigation to treat sinusitis in humans. While this route of delivery is believed to minimize ototoxic effects, we have shown gait ataxia, longer latency cervical vestibular-evoked myogenic potentials (cVEMPs) and fewer neurons in the vestibular brainstem nuclei, as well as elevated hearing thresholds and delayed auditory brainstem responses (ABRs) in rats. Since this route of delivery resulted in fewer brainstem neurons in vestibular nuclei, we hypothesized that threshold and ABR changes might be associated with fewer and smaller neurons in the auditory brainstem, as well as reduced expression of the activity dependent calcium binding protein calbindin (CB). We investigated this hypothesis in Sprague-Dawley rats that received intranasal irrigations of gentamicin or saline from postnatal day (P) 21-31. We used quantitative morphometrics and immunohistochemical labeling to examine total neuron number and cell body morphology in the spiral ganglion and auditory brainstem and examined CB immunolabeling in the medial nucleus of the trapezoid body (MNTB). We found significant changes in neuron morphology extending from the spiral ganglion to the central nucleus of the inferior colliculus, lower neuronal density in the spiral ganglion, and fewer neurons in the ventral cochlear nucleus, medial superior olive and MNTB. Additionally, we found that fewer MNTB neurons were CB immunopositive. Since gentamicin is known to be toxic to cochlear hair cells, these results indicate neuron loss and dysmorphology up to three synapses from the primary injury. These findings further characterize the toxic effects of gentamicin and highlight the need for auditory and vestibular screening after low dose gentamicin therapy.