Morpho-Physiological recovery of frog vestibular hair cells following aminoglycoside toxicity

Aminoglycoside antibiotics, including gentamicin (GM), are known to induce severe degenerative effects in both cochlear and vestibular organs. The discovery that hair cells readily regenerate in fish, amphibians, reptiles and birds after antibiotic treatment has stimulated the study of the mechanisms of repair and regeneration of the hair cells in different inner ear organs. The present investigation was designed to elucidate the morphological and functional recovery of hair cells in frog semicircular canals. GM was administered intraotically close to the perilymphatic cisterna at the concentration of 5 mM (15 μl volume). Frogs were sacrificed at post-injection times ranging from 1 to 20 days. Degenerative changes at hair cell level started 1-2 days after GM treatment and were severe in the intermediate regions of the crista and then involved the central and peripheral regions. Hair cell degeneration in the three crista regions appeared complete after 6-8 days. Damaged hair cells showed stereocilia loss together with a swelling of cell bodies and nuclei. Partially extruded hair cells from the epithelium were also observed together with large epithelial holes. Regenerating hair cells were often seen 6 day after GM treatment. They were identified based on their small cell bodies and nuclei as well as their small immature hair bundles. From this stage, hair cells density increased and the sensory epithelium recovered a normal appearance within 15 days. The functional recovery of hair cells during the regenerative period was studied by using whole cell patch recordings in crista slice preparations up to 20 days from gentamicin treatment. Passive and active electrical properties of hair cells from control animals have been compared with those of regenerating hair cells. Regenerating cells showed patterns of responses qualitatively similar to those of normal hair cells. However, the magnitude of the ionic currents increased during recovery suggesting that new hair cells came from precursors which reacquired progressively their complement of ionic channels. We found that the complement of K+ channels were completely functional in regenerated hair cells at 15 days post-treatment with gentamicin. Moreover, the regenerated cells in each region of the crista neuroepithelium reacquired the same complement of channels of normal preparations.