Detecting incipient inner-ear damage from impulse noise with otoacoustic emissions.

Audiometric thresholds and otoacoustic emissions (OAEs) were measured in 285 U.S. Marine Corps recruits before and three weeks after exposure to impulse-noise sources from weapons' fire and simulated artillery, and in 32 non-noise-exposed controls. At pre-test, audiometric thresholds for all ears were < or=25 dB HL from 0.5 to 3 kHz and < or=30 dB HL at 4 kHz. Ears with low-level or absent OAEs at pre-test were more likely to be classified with significant threshold shifts (STSs) at post-test. A subgroup of 60 noise-exposed volunteers with complete data sets for both ears showed significant decreases in OAE amplitude but no change in audiometric thresholds. STSs and significant emission shifts (SESs) between 2 and 4 kHz in individual ears were identified using criteria based on the standard error of measurement from the control group. There was essentially no association between the occurrence of STS and SES. There were more SESs than STSs, and the group of SES ears had more STS ears than the group of no-SES ears. The increased sensitivity of OAEs in comparison to audiometric thresholds was shown in all analyses, and low-level OAEs indicate an increased risk of future hearing loss by as much as ninefold.</p

Compound action potential (CAP) and cochlear microphonic (CM) threshold change as a function of test frequency over different time groups plotted in the same format as Fig 1.

<p>Each sampling group contained readings from six animals. The results show a decrease in both CAP and CM thresholds for HPN-07 treated animals relative to control animals beginning at 10 days after blast exposure. Significant differences were detected by two-way ANOVA analysis. The symbols of *; **; *** indicate significance levels of <i>p</i> < 0.05, 0.01 and 0.001, respectively.</p

Effects of Antioxidant Treatment on Blast-Induced Brain Injury

<div><p>Blast-induced traumatic brain injury has dramatically increased in combat troops in today’s military operations. We previously reported that antioxidant treatment can provide protection to the peripheral auditory end organ, the cochlea. In the present study, we examined biomarker expression in the brains of rats at different time points (3 hours to 21 days) after three successive 14 psi blast overpressure exposures to evaluate antioxidant treatment effects on blast-induced brain injury. Rats in the treatment groups received a combination of antioxidants (2,4-disulfonyl α-phenyl tertiary butyl nitrone and N-acetylcysteine) one hour after blast exposure and then twice a day for the following two days. The biomarkers examined included an oxidative stress marker (4-hydroxy-2-nonenal, 4-HNE), an immediate early gene (c-fos), a neural injury marker (glial fibrillary acidic protein, GFAP) and two axonal injury markers [amyloid beta (A4) precursor protein, APP, and 68 kDa neurofilament, NF-68]. The results demonstrate that blast exposure induced or up-regulated the following: 4-HNE production in the dorsal hippocampus commissure and the forceps major corpus callosum near the lateral ventricle; c-fos and GFAP expression in most regions of the brain, including the retrosplenial cortex, the hippocampus, the cochlear nucleus, and the inferior colliculus; and NF-68 and APP expression in the hippocampus, the auditory cortex, and the medial geniculate nucleus (MGN). Antioxidant treatment reduced the following: 4-HNE in the hippocampus and the forceps major corpus callosum, c-fos expression in the retrosplenial cortex, GFAP expression in the dorsal cochlear nucleus (DCN), and APP and NF-68 expression in the hippocampus, auditory cortex, and MGN. This preliminary study indicates that antioxidant treatment may provide therapeutic protection to the central auditory pathway (the DCN and MGN) and the non-auditory central nervous system (hippocampus and retrosplenial cortex), suggesting that these compounds have the potential to simultaneously treat blast-induced injuries in the brain and auditory system.</p> </div

Percent of outer hair cell (OHC) loss as a function of percent distance (and frequency-specific tonotopic position) from the OC apex in untreated and HPN-07-treated chinchilla at 3 days (d), 10 d, 21 d, and 180 d after noise exposure.

<p>The data are plotted as mean ± SEM and “n” represents the number of cochleae. Vertical lines at right of figures show a significant difference of mean values for noise only "Control" and Treated" groups as determined by two- way ANOVA followed by Bonferroni post-tests: *, <i>p</i> < 0.05; **<i>p</i> < 0.01; ***<i>p</i> < 0.001. Shaded area demarcates the range of noise exposure centered at 4 kHz. Significantly less OHC loss was observed in the basal turn of the OC in the HPN-07 "Treated" group (<i>p</i> < 0.05, 0.01 or 0.001).</p

Representative confocal images of connexin 26 (red) and 30 (green) labeling collected from the spiral ligament.

<p>Nuclei of cells were stained by DAPI (blue). Merged images are shown in the column 3 (A3-I3, merged). Positive Connexin 26 labeling (red) is observed in fibrocytes in the spiral ligament of normal (naïve) controls (A1). Decreased connexin 26 labeling (column 1) is observed in the spiral ligament of chinchilla all time points after noise exposure (B-H) except in the treated group at 6 months after noise exposure(I). No or very low CX-30 labeling is observed in fibrocytes of the spiral ligament of chinchilla while the root cells show positive Cx-30 labeling (A2-I2). Scale bar = 20 μm in I3 for A1-I3.</p

HPN-07, a free radical spin trapping agent, protects against functional, cellular and electrophysiological changes in the cochlea induced by acute acoustic trauma - Fig 8

<p>Representative photomicrographs of cochlear hair cells after phalloidin staining at about 70% of the distance from the apex in normal (naïve) (A), and control (untreated) (B) and treated chinchilla at 21 days after the blast exposure. Arrows show examples of missing outer hair cells (OHC) that have been replaced by scars. Scale bar = 50 μm.</p

Percent of inner hair cell (IHC) loss as a function of percent distance (and tonotopic position in kHz) from the OC apex in untreated and HPN-07-treated chinchilla at 3 days, 10 days, 21 days and 6 months after noise exposure.

<p>Significantly less IHC loss was observed in the basal turn of the OC in the HPN-07 treatment group (<i>p</i> < 0.05, 0.01 or 0.001). The data are plotted as mean ± SEM, and “n”represents the number of cochleae. Vertical lines at right of figures show a significant difference of mean values for control (untreated) and HPN-07-treated" groups as determined by two-way ANOVA followed by Bonferroni post-tests: *<i>p</i> <0.05; **<i>p</i> < 0.01; ***<i>p</i> < 0.001. Shaded area demarcates the range of noise exposure centered at 4 kHz.</p

Distortion-product otoacoustic emission (DPOAE) amplitude shifts plotted in the same format as Fig 1.

<p>The number of ears at each time point is the same as ABR data. The symbols with vertical bars represent the means and SEM, respectively of DPOAE amplitude shifts at individual stimulus frequencies. Two-way ANOVA and Bonferroni Post Hoc test were applied. The symbols of *** indicate significance levels of <i>p</i> < 0.001.</p

Connexin 26 relative fluorescence intensity in the OC was measured and statistically analyzed.

<p>Both noise exposed treated and control (untreated) groups showed a similar significant decrease in Cx-26 expression at 21 days and 6 months after noise exposure compared to the normal (naïve) animals (<i>p</i> < 0.05).</p

Examples of APP immunolabeling in the hippocampus of the NC (A), 24H-B (B) and 24H-B/T (C) groups.

<p>No positive APP staining was observed in the hippocampus of normal controls (A). Strong positive APP labeling was observed in the hippocampus of the 24H-B group (arrows in B). Decreased APP expression was observed in the hippocampus of the 24H-B/T group relative to the 24H-B group (arrows in C). APP-positive labeling in the hippocampus was quantified and statistically analyzed (D). Two to three hippocampal sections from each rat brain (6 rats in each group) were used in these analyses. Significantly increased APP expression was observed in the hippocampus of the 24H-B group compared to the NC group (<i>p</i> < 0.001). An antioxidant treatment effect was found at 24 hours after blast exposure (<i>p</i> < 0.001), however no significant difference was observed between the treated and untreated groups 7 days after blast exposure (7D-B v.s. 7D-B/T, all <i>p</i> > 0.05). Error bars represent standard error of the means. Scale bar = 10 µm in C for A-C. *** indicate <i>p</i> < 0.001.</p