Cobalt-Induced Ototoxicity in Rat Postnatal Cochlear Organotypic Cultures

Cobalt (Co) is a required divalent metal used in the production of metal alloys, batteries, and pigments and is a component of vitamin B12. Excessive uptake of Co is neurotoxic causing temporary or permanent hearing loss; however, its ototoxic effects on the sensory hair cells, neurons, and support cells in the cochlea are poorly understood. Accordingly, we treated postnatal day 3 rat cochlear organotypic cultures with various doses and durations of CoCl₂ and quantified the damage to the hair cells, peripheral auditory nerve fibers, and spiral ganglion neurons (SGN). Five-day treatment with 250 μM CoCl₂ caused extensive damage to hair cells and neurons which increased with dose and treatment duration. CoCl₂ caused greater damage to outer hair cells than inner hair cells; damage was greatest in the base of the cochlea and decreased towards the base. CoCl₂ increased expression of superoxide radical in hair cells and SGNs and SGN loss was characterized by nuclear condensation and fragmentation, morphological features of apoptosis. CoCl₂ treatment increased the expression of caspase-3 indicative of caspase-mediated programmed cell death. These results identify hair cells and spiral ganglion neurons as the main targets of Co ototoxicity in vitro and implicate the superoxide radical as a trigger of caspase-mediated ototoxicity.