Electrochemical properties of the non-excitable tissue stria vascularis of the mammalian cochlea are sensitive to sounds

Excitable cochlear hair cells convert the mechanical energy of sounds into the electrical signals necessary for neurotransmission. The key process is cellular depolarization via K⁺ entry from K⁺ -enriched endolymph through hair cells' mechanosensitive channels. Positive 80 mV potential in endolymph accelerates the K⁺ entry, thereby sensitizing hearing. This potential represents positive extracellular potential within the epithelial-like stria vascularis; the latter potential stems from K⁺ equilibrium potential (E_K ) across the strial membrane. Extra- and intracellular [K⁺ ] determining E_K are likely maintained by continuous unidirectional circulation of K⁺ through a putative K⁺ transport pathway containing hair cells and stria. Whether and how the non-excitable tissue stria vascularis responds to acoustic stimuli remains unclear. Therefore, we analysed a cochlear portion for the best frequency, 1 kHz, by theoretical and experimental approaches. We have previously developed a computational model that integrates ion channels and transporters in the stria and hair cells into a circuit and described a circulation current composed of K⁺ . Here, in this model, mimicking of hair cells' K⁺ flow induced by a 1 kHz sound modulated the circulation current and affected the strial ion transport mechanisms; the latter effect resulted in monotonically decreasing potential and increasing [K⁺ ] in the extracellular strial compartment. Similar results were obtained when the stria in acoustically stimulated animals was examined using microelectrodes detecting the potential and [K⁺ ]. Measured potential dynamics mirrored the E_K change. Collectively, because stria vascularis is electrically coupled to hair cells by the circulation current in vivo too, the strial electrochemical properties respond to sounds. KEY POINTS: A highly positive potential of +80 mV in K⁺ -enriched endolymph in the mammalian cochlea accelerates sound-induced K⁺ entry into excitable sensory hair cells, a process that triggers hearing. This unique endolymphatic potential represents an E_K -based battery for a non-excitable epithelial-like tissue, the stria vascularis. To examine whether and how the stria vascularis responds to sounds, we used our computational model, in which strial channels and transporters are serially connected to those hair cells in a closed-loop circuit, and found that mimicking hair cell excitation by acoustic stimuli resulted in increased extracellular [K⁺ ] and decreased the battery's potential within the stria. This observation was overall verified by electrophysiological experiments using live guinea pigs. The sensitivity of electrochemical properties of the stria to sounds indicates that this tissue is electrically coupled to hair cells by a radial ionic flow called a circulation current.

Combined effects of salicylic acid and furosemide and noise on hearing

Background

A major cause of the hearing loss following exposure to intense noise involves release of free radicals resulting from the elevated metabolism. The free radicals induce damage to several of the components of the cochlear amplifier including the outer hair cells and indirectly to the transduction currents. Salicylic acid induces a reversible hearing loss since it binds to the motor protein prestin in the outer hair cells, reducing electromotility. Furosemide also induces a reversible hearing loss since it reduces the endocochlear potential which is a major component of the cochlear transduction currents. On the other hand, each of these drugs also provides protection from a noise induced hearing loss if they are injected just before a noise exposure, probably as a result of the decreased metabolism induced in their presence, with release of lower levels of free radicals. In this study, both drugs were administered in order to assess whether their protective effects would be additive.

Methods

The study was conducted on normal hearing albino mice of the Sabra strain. They were injected with either salicylic acid alone (N = 11), or furosemide alone (N = 14), or both together (N = 14), or with saline control (N = 11) and exposed to broad band noise for 3.5 hours. An additional group of 9 mice was injected with both salicylic acid and furosemide, but not exposed to noise. The degree of the resulting hearing loss was assessed by recording thresholds of the auditory nerve brainstem evoked responses to broad band clicks before the injections and noise, and 7, 14 and 21 days after.

Results

The noise induced hearing loss in the mice injected with salicylic acid alone or furosemide alone was smaller than in those injected with saline, i.e. these drugs provided protection, as in previous studies in this laboratory. There was no threshold elevation after two weeks in the mice injected with both drugs without noise exposure, i.e. the effects of the two drugs given together was reversible. On the other hand, there was a significant hearing loss (i.e. threshold elevation) in the group which received both drugs and was also exposed to noise, with mean threshold elevations of 38.8 ± 19.0 dB and 28.3 ± 11.7 dB 7 days after noise exposure.

Conclusions

This result is very surprising, if not paradoxical. Drugs which provide protection from a noise induced hearing loss when administered alone, not only do not provide protection when given together, but also induce a greater hearing loss when accompanied by noise. This observation may be related to the finding that the depression of the endocochlear potential normally caused by furosemide is reduced in the presence of salicylic acid, so that the protection usually provided by furosemide is not present when it is administered together with salicylic acid. Thus it seems that each drug may interfere with the protective action of the other when coupled with noise.

Leupeptin reduces impulse noise induced hearing loss

BACKGROUND

Exposure to continuous and impulse noise can induce a hearing loss. Leupeptin is an inhibitor of the calpains, a family of calcium-activated proteases which promote cell death. The objective of this study is to assess whether Leupeptin could reduce the hearing loss resulting from rifle impulse noise.

METHODS

A polyethelene tube was implanted into middle ear cavities of eight fat sand rats (16 ears). Following determination of auditory nerve brainstem evoked response (ABR) threshold in each ear, the animals were exposed to the noise of 10 M16 rifle shots. Immediately after the exposure, saline was then applied to one (control) ear and non-toxic concentrations of leupeptin determined in the first phase of the study were applied to the other ear, for four consecutive days.

RESULTS

Eight days after the exposure, the threshold shift (ABR) in the control ears was significantly greater (44 dB) than in the leupeptin ears (27 dB).

CONCLUSION

Leupeptin applied to the middle ear cavity can reduce the hearing loss resulting from exposure to impulse noise.

Effects of furosemide on the hearing loss induced by impulse noise

BACKGROUND

The permanent hearing loss following exposure to intense noise can be due either to mechanical structural damage (tearing) caused directly by the noise or to metabolic (biochemical) damage resulting from the elevated levels of free radicals released during transduction of the sound overstimulation. Drugs which depress active cochlear mechanics (e.g. furosemide and salicylic acid) or anti-oxidants (which counteract the free radicals) are effective in reducing the threshold shift (TS) following broadband continuous noise. This study was designed to determine whether furosemide can reduce the TS following exposure to impulse noise, similar to its action with continuous broadband noise.

METHODS

Shortly after furosemide injection, mice were exposed to simulated M16 rifle impulse noise produced by different loudspeakers and amplifiers in different exposure settings and, in other experiments, also to actual M16 rifle shots.

RESULTS

Depending on the paradigm, the simulated noises either did not produce a TS, or the TS was reduced by furosemide. The drug was not effective in reducing TS resulting from actual impulse noise.

CONCLUSION

Simulated M16 rifle impulse noise may not truly replicate the rapid rise time and very high intensity of actual rifle shots so that the TS following exposure to such noise can be reduced by these drugs. On the other hand, actual M16 impulse noise probably causes direct (frank) mechanical damage, which is not reduced by these drugs.

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.

How are the inner hair cells and auditory nerve fibers activated without the mediation of the outer hair cells and the cochlear amplifier?

The present study was designed to assess whether, in the presence of a depression of the cochlear amplifier i.e. a sensorineural hearing loss (SNHL), the inner hair cells (IHCs) require the presence of a normal endocochlear potential for transduction. An SNHL was induced by injecting salicylic acid (which binds to the motor protein prestin in the outer hair cells), and then furosemide (which depresses the endocochlear potential) was injected. Furosemide did not cause an additional elevation of the threshold of the auditory nerve brainstem evoked response (ABR) over that induced by the salicylic acid injection. Exposure to noise was also used to induce a SNHL in other mice, and then furosemide was injected. Here too furosemide did not cause an additional ABR threshold elevation over that induced by the noise. These results show that the IHCs (and the auditory nerve) can be excited in the presence of a SNHL (i.e. without the cochlear amplifier) and in the absence of an endocochlear potential. Possible mechanisms of excitation in such a state are discussed.