Current Medicinal Chemistry, 2010, 17, ????-???? 1 0929-8673/10 $55.00+.00 © 2010 Bentham Science Publishers Ltd. Oxidative Stress in the Cochlea: An Update A.L. Poirrier 1,2 , J. Pincemail 3 , P. Van Den Ackerveken 2 , P.P. Lefebvre 1 and B. Malgrange* ,2 1 Department of Otolaryngology, University Hospital of Liège, Av. de l'Hopital 1 (B35), 4000 Liège, Belgium 2 GIGA-Neurosciences, Developmental Neurobiology Unit, University of Liège, Av. de l'Hopital 1 (B36), 4000 Liège, Belgium 3 CREDEC, Department of Cardiovascular Surgery, CHU Sart-Tilman, Av. de l'Hopital 1 (B23), 4000 Liège, Belgium Abstract: This paper will focus on understanding the role and action of reactive oxygen species (ROS) and reactive nitro- gen species (RNS) in the molecular and biochemical pathways responsible for the regulation of the survival of hair cells and spiral ganglion neurons in the auditory portion of the inner ear. The pivotal role of ROS/RNS in ototoxicity makes them potentially valuable candidates for effective otoprotective strategies. In this review, we describe the major character- istics of ROS/RNS and the different oxidative processes observed during ototoxic cascades. At each step, we discuss their potential as therapeutic targets because an increasing number of compounds that modulate ROS/RNS processing or targets are being identified. Keywords: Reactive oxygen species, ototoxicity, aminoglycoside, cisplatin, noise, presbycusis, cochlea. Hearing functions require intricate interactions between specialised cells of the inner ear. The organ of Corti is a sen- sory epithelium composed of a highly ordered array of sen- sory hair cells and non-sensory supporting cells. Hair cells are directly connected to primary auditory neurons in the spiral ganglion. In mammals, mature hair cells and spiral ganglion neurons do not have the ability to regenerate, and their loss results in permanent hearing deficits, 1. ROLE OF ROS/RNS IN PHYSIOLOGICAL AND PATHOLOGICAL CONDITIONS Both aerobic and anaerobic metabolism are accompanied by the production of reactive oxygen (ROS) and/or reactive nitrogen species (RNS), and organisms ranging from pro- karyotes to mammals have evolved an elaborate and redun- dant complement of defences to confer protection against oxidative and nitrosative insults. Compelling data also indi- cate that ROS and RNS, at relatively low concentrations, are employed in physiological settings as signalling molecules in control of cell and tissue homeostasis, cell division, migra- tion, contraction, and mediator production [1-6]. These sig- nalling oxidants, which include nitric oxide (NO), superox- ide radical (O 2 .- ), peroxynitrite (ONOO - ), S-nitrosothiols (RSNOs) and hydrogen peroxide (H 2 O 2 ), are produced mainly by NADPH-dependent enzymes whose expression is tightly regulated. These enzymes function with the coen- zyme nicotinamide adenine dinucleotide phosphate (NADPH), an electron scavenger. ROS are species of oxy- gen, which are in a more reactive state than molecular oxy- gen, and in which the oxygen is reduced at varying degrees. During the normal course of metabolism, electrons carried by the electron transport chain in the mitochondrion, can leak out of the pathway and interact with O 2 , producingO 2 .- . Other sources of O 2 .- include endogenous enzymes, such as plasma NADPH oxidases or NOXs, cytoplasmic xanthine oxidase or cytochrome P-450 isozyme in the endoplasmic *Address correspondence to this author at the GIGA-Neurosciences, Developmental Neurobiology Unit, University of Liège, Av. de l'Hopital 1 (B36), 4000 Liège, Belgium; Tel: ?????????????; Fax: ?????????????; E-mail: bmalgrange@ulg.ac.be reticulum. NOX3 is specifically expressed in the sensory epithelium of the inner ear and in spiral ganglion neurons. NOX3 produces physiological amounts of O 2 .- essential for its development [7,8]. Further reduction of oxygen, from the dismutation of O 2 .- , produces H 2 O 2 spontaneously (especially at low pH) or catalysed by superoxide dismutase (SOD). Under physiological conditions, once O 2 .- is formed, the presence of H 2 O 2 becomes almost inevitable. Hydrogen per- oxide is then converted into water and oxygen by the cata- lase enzyme. However, further reactions may lead to the formation of hydroxyl radicals (OH • ), especially in the pres- ence of metal ions through the Fenton or Haber–Weiss reac- tions (Fig. 1). Hydroxyl radicals are extremely reactive, have a short half-life and will probably react with the first mole- cule they encounter. Another cellular source of ROS is xan- thine oxidase, which catalyses the synthesis of uric acid from purines, forming O 2 .- and H 2 O 2 . It thus appears that following the formation of O 2 .- a cas- cade of ROS production is initiated. Some of these ROS, especially H 2 O 2 , are key signalling molecules, while others appear to be extremely detrimental to biological systems, with effects that are dependent on the concentrations that are perceived by the cells [9]. Besides the enzymatic metabolism of oxygen-derived species, intracellular enzymes catalyse the production of RNS. Nitric oxide (NO), the major RNS, is formed endoge- nously from the oxidation of L-arginine to L-citrulline by a family of NADPH-dependent enzymes, the NO synthases (NOS) which exist in three isoforms: the neuronal NOS (nNOS) and the endothelial NOS (eNOS) forming the consti- tutive group, and the inducible NOS (iNOS). NO exists in different chemical forms (NO - , NO • and NO + ) and thus acts as an important oxidative biological signalling molecule in a large variety of physiological processes including neuro- transmission, blood pressure regulation and immune mecha- nisms [10]. NO can also combine with O 2 .- to form peroxyni- trite (ONOO - ), a highly reactive oxidant. Under physiologi- cal conditions, the constitutive NOS expressed in specific tissues produce NO for the cellular needs. The constitutive NOS are expressed in the inner ear by the hair cells and sen- sory neurons, and they produce small amounts of NO, which