Abstract
Concurrent administration of a high dose of gentamicin (GM; 125mg/kg IM) and ethacrynic acid (EA; 40mg/kg IV) results in rapid destruction of virtually all cochlear hair cells; however, the cell death signaling pathways underlying this rapid form of hair-cell degeneration are unclear. To elucidate the mechanisms underlying GM/EA-mediated cell death, several key cell death markers were assessed in the chinchilla cochlea during the early stages of degeneration. In the middle and basal turns of the cochlea, massive hair-cell loss including destruction of the stereocilia and cuticular plate occurred 12h after GM/EA treatment. Condensation and fragmentation of outer hair-cell nuclei, morphological features of apoptosis, were first observed 5-6h post-treatment in the basal turn of the cochlea. Metabolic function, reflected by succinate dehydrogenase histochemistry and mitochondrial staining, decreased significantly in the basal turn 4h following GM/EA treatment; these early changes were accompanied by the release of cytochrome c from the mitochondria into the cytosol and intense expression of initiator caspase-9 and effector caspase-3. GM/EA failed to induce expression of extrinsic initiator caspase-8. These results suggest that the rapid loss of hair cells following GM/EA treatment involves cell death pathways mediated by mitochondrial dysfunction leading to the release of cytochrome c, activation of initiator caspase-9 and effector caspase-3.