ABSTRACT
Evidence has been amassed over the last 15 years implicating reactive oxygen species
(ROS) in cochlear injury due to ischemia, noise, and ototoxicants. Because ROS appear
to be broadly involved in most cellular injury processes, it might be expected that
antioxidants applied with optimal timing, dose, and route should completely prevent
cochlear injury. This expectation has remained unmet, however. Recent experiments
involving noise injury in knockout mice for key antioxidant enzymes also have yielded
surprisingly modest, even paradoxical, results. Research in the area of oxidative
stress and deafness is moving into a more mature phase, wherein simplistic models
and hypotheses are being modified to include more of the emerging complexity of reduction-oxidation
(redox) biochemistry. Sensory cell injury and death probably includes parallel ROS-dependent
and ROS-independent pathways. In addition, ROS-related processes are complex and include
myriad checks and balances, such that the manipulation of a single component can produce
unexpected results. Varied cochlear cell types and epithelia may differ in antioxidant
capacity or sustain injury through different ROS-mediated cascades. Finally, some
ROS serve as messengers, both under normal circumstances and as cells strive to maintain
homeostasis after stress. Antioxidant therapy in some form nevertheless retains promise
for protecting the cochlea from acute and chronic stress. Consideration of ROS within
an appropriately broad cell biological context may favor combined pharmacological
remedies.
KEYWORDS
Cochlea - noise-induced hearing loss - ototoxicants - reactive oxygen species - free
radicals - antioxidants - mitochondria