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The programmed cell death of stress-damaged auditory hair cells can occur through a variety of signal pathways, and therapeutic modalities that block pro-cell-death pathways are being developed and evaluated for hearing preservation. Because of their ability to have both anti-inflammatory and anti-apoptotic actions, corticosteroids have long been used to protect against several types of acute sensorineural hearing loss. Other anti-apoptotic drugs that target the mitogen-activated protein kinase (MAPK)/c-Jun-N terminal kinase (JNK) signal cascade, such as D-JNKI-1 (AM-111) and SP600125, have produced promising results both in vitro and in laboratory animal studies, with AM-111 showing promise in preliminary clinical trials. Antioxidant drugs, e.g. sodium thiosulfate, N-acetylcysteine, and D-methionine, have been shown in animal studies to attenuate permanent threshold shifts in hearing by reducing oxidative stress. In addition to reviewing selected therapeutic trends for the conservation of hearing, we review our experiences with dexamethasone and D-JNKI-1 and report results from our current research.

Objective: To test the hypothesis that bcl-2 prevents oxidative stress-induced apoptosis of auditory sensory cells in explants of the organ of Corti and dissociated cell cultures of the spiral ganglion. Methods: Organ of Corti explants and dissociated spiral ganglion cell cultures obtained from 3-day-old (P3) rats or adult spiral ganglion cell cultures from 28-day-old (P28) rats were transduced with vectors containing a human bcl-2 gene. Cultures were then exposed to neomycin, cisplatin or subjected to withdrawal of neurotrophin supplementation. Outcome measures included hair cell and neuron counts, mitochondrial membrane potential and a histological measure of apoptosis. Results: Expression of bcl-2 in the organ of Corti explants and neuronal cell cultures provided a significant level of protection against cell death. Bcl-2 expression in the organ of Corti explants also protected mitochondria from loss of membrane potential and blocked an early step in the commitment of hair cells to apoptosis. Conclusion: Expression of bcl-2 in cochlear tissues protects sensory cells from a variety of insults that have been demonstrated to damage the inner ear.

Oxidative stress insults such as neurotrophin withdrawal, sound trauma, hypoxia/ischemia, ototoxic antibiotics, and chemotherapeutic agents have been shown to induce apoptosis of both auditory hair cells and neurons. In this paper, we review some components of the apoptotic pathways leading to the death of hair cells and auditory induced by growth factor withdrawal or cisplatin intoxication: (1) reactive oxygen species and free radicals are formed as by-products of several metabolic pathways and these molecules can themselves cause cell damage by reacting with cellular proteins; (2) activation of caspases, and (3) activation of calpain. These mechanisms have several different points at which inhibitors could be targeted to protect cells from programmed cell death, including the prevention of oxidative stress-induced apoptosis and the activation of caspases and calpains.