Dynamic distribution of ototoxic gentamicin entry into inner hair cells of mice

Lower ototoxicity and absence of hidden hearing loss point to gentamicin C1a and apramycin as promising antibiotics for clinical use

Spread of antimicrobial resistance and shortage of novel antibiotics have led to an urgent need for new antibacterials. Although aminoglycoside antibiotics (AGs) are very potent anti-infectives, their use is largely restricted due to serious side-effects, mainly nephrotoxicity and ototoxicity. We evaluated the ototoxicity of various AGs selected from a larger set of AGs on the basis of their strong antibacterial activities against multidrug-resistant clinical isolates of the ESKAPE panel: gentamicin, gentamicin C1a, apramycin, paromomycin and neomycin. Following local round window application, dose-dependent effects of AGs on outer hair cell survival and compound action potentials showed gentamicin C1a and apramycin as the least toxic. Strikingly, although no changes were observed in compound action potential thresholds and outer hair cell survival following treatment with low concentrations of neomycin, gentamicin and paromomycin, the number of inner hair cell synaptic ribbons and the compound action potential amplitudes were reduced. This indication of hidden hearing loss was not observed with gentamicin C1a or apramycin at such concentrations. These findings identify the inner hair cells as the most vulnerable element to AG treatment, indicating that gentamicin C1a and apramycin are promising bases for the development of clinically useful antibiotics.

Spontaneous and Partial Repair of Ribbon Synapse in Cochlear Inner Hair Cells After Ototoxic Withdrawal

Ototoxicity is one of the major causes of sensorineural deafness. However, it remains unclear whether sensorineural deafness is reversible after ototoxic withdrawal. Here, we report that the ribbon synapses between the inner hair cells (IHCs) and spiral ganglion nerve (SGN) fibers can be restored after ototoxic trauma. This corresponds with hearing restoration after ototoxic withdrawal. In this study, adult mice were injected daily with a low dose of gentamicin for 14 consecutive days. Immunostaining for RIBEYE/CtBP2 was used to estimate the number and size of synaptic ribbons in the cochlea. Hearing thresholds were assessed using auditory brainstem responses. Auditory temporal processing between IHCs and SGNs was evaluated by compound action potentials. We found automatic hearing restoration after ototoxicity withdrawal, which corresponded to the number and size recovery of synaptic ribbons, although both hearing and synaptic recovery were not complete. Thus, our study indicates that sensorineural deafness in mice can be reversible after ototoxic withdrawal due to an intrinsic repair of ribbon synapse in the cochlea.