Cisplatin ototoxicity in the guinea pig: vestibular and cochlear damage

Brain stem auditory potentials evoked by clicks in the presence of high-pass filtered noise in dogs

This study evaluates the effects of a high-frequency hearing loss simulated by the high-pass-noise masking method, on the click-evoked brain stem-evoked potentials (BAEP) characteristics in dogs. BAEP were obtained in response to rarefaction and condensation click stimuli from 60 dB normal hearing level (NHL, corresponding to 89 dB sound pressure level) to wave V threshold, using steps of 5 dB in eleven 58 to 80-day-old Beagle puppies. Responses were added, providing an equivalent to alternate polarity clicks, and subtracted, providing the rarefaction-condensation potential (RCDP). The procedure was repeated while constant level, high-pass filtered (HPF) noise was superposed to the click. Cut-off frequencies of the successively used filters were 8, 4, 2 and 1 kHz. For each condition, wave V and RCDP thresholds, and slope of the wave V latency-intensity curve (LIC) were collected. The intensity range at which RCDP could not be recorded (pre-RCDP range) was calculated. Compared with the no noise condition, the pre-RCDP range significantly diminished and the wave V threshold significantly increased when the superposed HPF noise reached the 4 kHz area. Wave V LIC slope became significantly steeper with the 2 kHz HPF noise. In this non-invasive model of high-frequency hearing loss, impaired hearing of frequencies from 8 kHz and above escaped detection through click BAEP study in dogs. Frequencies above 13 kHz were however not specifically addressed in this study.

Vascular endothelial growth factor (VEGF) expression in noise-induced hearing loss

Noise-induced hearing loss has been associated with alterations in cochlear blood flow. Our study analyzed the expression of Vascular Endothelial Growth Factor (VEGF) and its functional receptors, Flt-1 and Flk-1, in the cochlear structures of noise-exposed and unexposed guinea pigs. VEGF is a prototypical angiogenic agent, with multiple functions on vascular biology, ranging from vascular permeability to endothelial cell migration, proliferation, differentiation, and survival. Acoustic trauma was induced by a continuous pure tone of 6 kHz, at 120 dB SPL for 30 min. Auditory function was evaluated by electrocochleographic recordings at 2-20 kHz for 7 days. Noise-induced cochlear morphological changes were studied by immunohistochemistry and scanning electron microscopy. The expression of VEGF and its receptors was examined by immunohistochemistry and western blotting analysis. The hearing threshold shift reached a level of 60 dB SPL on day 1 after trauma and underwent a partial recovery over time, reaching a value of about 20 dB SPL on day 7. Outer hair cell loss was more prominent in the area located 14-16 mm from the apex. Increased cochlear VEGF expression was observed in noise-exposed animals, in particular at the level of stria vascularis, spiral ligament, and spiral ganglion cells. No changes were observed in the expression of VEGF-receptors. Our data suggest a role for VEGF in the regulation of the vascular network in the inner ear after acoustic trauma and during auditory recovery, with potentially important clinical and therapeutic implications.

Inhibition of the calcium- and voltage-dependent big conductance potassium channel ameliorates cisplatin-induced apoptosis in spiral ligament fibrocytes of the cochlea

The role of calcium- and voltage-dependent big conductance potassium channels in regulating apoptosis was investigated in cultured type I spiral ligament fibrocytes. Incubation of type I spiral ligament fibrocytes derived from gerbil cochlea with cisplatin induced dose- and time-dependent apoptosis as demonstrated by annexin V conjugated to fluorescein isothiocyanate/prodidium iodide assays. The average voltage activation threshold of whole cell current was sharply shifted to -40 mV in the cisplatin-treated cells as compared with a value of 40 mV in control cells. The average whole-cell current of cisplatin-treated cells induced by a depolarization voltage step from -80 to -10 mV was increased significantly to 1.2+/-0.4 nA as compared with 0.08+/-0.1 nA in control cells. Coincubation with tetraethylammonium and cisplatin retained the whole cell current in the normal range (0.12+/-0.2 nA). The increment of cisplatin-induced whole-cell current was inhibited (97+/-5%) by a specific calcium- and voltage-dependent big conductance potassium channel blocker iberiotoxin. Consistent with this, co-incubation with tetraethylammonium significantly attenuated cisplatin-induced apoptosis in type I spiral ligament fibrocytes by more than 50%. We conclude that the activation of BK channels is an early event associated with cisplatin-induced apoptosis in type I spiral ligament fibrocytes. These findings also point to the calcium- and voltage-dependent big conductance potassium channels as a potential pharmacological target for manipulating cisplatin ototoxicity.

NOX3, a superoxide-generating NADPH oxidase of the inner ear

Reactive oxygen species (ROS) play a major role in drug-, noise-, and age-dependent hearing loss, but the source of ROS in the inner ear remains largely unknown. Herein, we demonstrate that NADPH oxidase (NOX) 3, a member of the NOX/dual domain oxidase family of NADPH oxidases, is highly expressed in specific portions of the inner ear. As assessed by real-time PCR, NOX3 mRNA expression in the inner ear is at least 50-fold higher than in any other tissues where its expression has been observed (e.g. fetal kidney, brain, skull). Microdissection and in situ hybridization studies demonstrated that NOX3 is localized to the vestibular and cochlear sensory epithelia and to the spiral ganglions. Transfection of human embryonic kidney 293 cells with NOX3 revealed that it generates low levels of ROS on its own but produces high levels of ROS upon co-expression with cytoplasmic NOX subunits. NOX3-dependent superoxide production required a stimulus in the absence of subunits and upon co-expression with phagocyte NADPH oxidase subunits p47(phox) and p67(phox), but it was stimulus-independent upon co-expression with colon NADPH oxidase subunits NOX organizer 1 and NOX activator 1. Pre-incubation of NOX3-transfected human embryonic kidney 293 cells with the ototoxic drug cisplatin markedly enhanced superoxide production, in both the presence and the absence of subunits. Our data suggest that NOX3 is a relevant source of ROS generation in the cochlear and vestibular systems and that NOX3-dependent ROS generation might contribute to hearing loss and balance problems in response to ototoxic drugs.