The effect of noise on ears with a hole in the vestibule

Several Mechanical Manipulations of the Wall of the Inner Ear Do Not Affect Air and Bone Conduction Auditory Thresholds

Objectives:

According to classic theories, auditory stimulation, whether air- or bone-conducted, has been thought to begin with sound-induced relative motion between the cochlear shell and the stapes footplate, producing a passive mechanical traveling wave along the basilar membrane. This study was designed to assess the effect of experimental mechanical manipulations of the cochlea on the auditory thresholds to air-conducted and bone-conducted stimulation.

Methods:

The left ear of Psammomys obesus (highest auditory sensitivity between 0.5 and 5.0 kHz) was initially ablated in all animals studied. After baseline recording of auditory nerve–brain stem evoked response (ABR) thresholds to air- and bone-conducted broadband click stimulation from the right ear, a hole was drilled in the vestibule of that ear in 3 animals. In 2 other animals, the round window of the animals was immobilized. In 3 additional animals, the round window was widely perforated. Repeat ABR thresholds were then determined.

Results:

There was no change in ABR thresholds to both air- and bone-conducted stimulation following these manipulations. The ABR wave latency also did not change.

Conclusions:

It is likely that an alternative mode of cochlear excitation is possible.

Reduced salicylic acid binding following noise: possible evidence for prestin disruption

To gain insight into the mechanism of noise induced permanent threshold shift (PTS), the magnitude of the auditory threshold elevation induced by injection of salicylic acid (which competitively binds with the motor protein prestin) to animals with a pre-existing PTS was compared to that in control animals (not exposed to noise). Normal mice were exposed to a noise intensity and duration which causes a small PTS. After determining the degree of the resulting PTS two weeks following the noise, salicylic acid was injected. The salicylic acid induced an additional threshold elevation and its magnitude was compared to that in control mice which had not been noise exposed. The mean noise induced PTS in the experimental (noise exposed) group was 25.5 dB. Following the administration of salicylic acid to these animals, there was an additional (salicylic acid induced) mean threshold elevation of 17.5 dB, and this was significantly smaller than that in control (not noise exposed) mice (36.8 dB). This may be evidence for a reduced number of salicylic acid binding sites on prestin and therefore the PTS may be due to disruption of prestin by the free radicals produced during the noise exposure.

Uniform comparison of several drugs which provide protection from noise induced hearing loss

Background

The ability of drugs to reduce noise induced hearing loss (NIHL) has been evaluated in diverse experimental conditions (animal species, noise intensities, durations, assessment techniques, etc), making it difficult to assess their relative efficacy. The present study was designed to provide more uniform comparisons and to allow to a better understanding of the mechanism of the NIHL. Methods: The drugs studied included furosemide (loop diuretic) and the antioxidants N Acetyl-L-Cysteine, vitamins A, C, E with the vasodilator magnesium. Mice were exposed to a continuous broadband noise (113 dB SPL for 3.5 hours) and the NIHL was assessed in all animals before noise exposure and 1 week after with auditory nerve brainstem evoked responses (ABR) to broadband clicks and to 8 kHz tone bursts.

Results

Each of the drugs alone and in combination led to similar reductions in NIHL.

Conclusions

The loop diuretic furosemide, by reducing the magnitude of the endocochlear potential in scala media, probably depressed active vibrations of the outer hair cells and basilar membrane, resulting in reduction of free radical formation during the noise exposure. The antioxidants N Acetyl-L-Cysteine and vitamins A, C, E with the vasodilator magnesium presumably counteract the free radicals. Thus, the administration of the antioxidants to animals in which free radical formation had already been reduced by previous injection of furosemide did not have an additional protective effect on the NIHL.