In vivo noise exposure alters the in vitro motility and viability of outer hair cells

Changes in purinoceptor distribution and intracellular calcium levels following noise exposure in the outer hair cells of the guinea pig

Among the cells of the inner ear, the outer hair cells (OHCs) are the most important targets of noise-induced effects, being the most sensitive cell types. The aim of this study was to examine the effects of noise (50 Hz-20 kHz, 80 dB sound pressure level, 14 days) on intracellular calcium levels and on the expression pattern of purinoceptors in the membrane of the OHCs of the guinea pig and to measure the stiffness changes of the lateral membrane of these cells. In noise-exposed animals, the resting intracellular calcium concentration increased compared to nontreated animals and was slightly higher in the cells of the basal (219 +/- 29 nM: ) than in the apical (181 +/- 24 nM: ) turns of the cochlea. After application of 180 muM: adenosine triphosphate, the intracellular calcium level rose by 60 +/- 22 nM: in cells from the apical and by 44 +/- 10 nM: in cells from the basal turns, significantly less than in nontreated animals. Expression of the P(2X1), P(2X2), P(2X4), P(2X7), P(2Y1) and P(2Y4) receptor subtypes was suppressed, while expression of the P(2Y2) subtype did not decrease in either of the two preparations. In parallel with the increase in intracellular calcium concentration, the stiffness of the lateral wall of the OHCs was increased. Noise-induced changes in intracellular calcium homeostasis and subsequently in the calcium-dependent regulatory mechanisms may modify OHC lateral wall stiffness and may lead to reduction of the efficacy of the cochlear amplifier.

Is there a close relationship between changes in amplitudes of distortion product otoacoustic emissions and hair cell damage after exposure to realistic industrial noise in guinea pigs?

In long-term experiments in awake guinea pigs (n = 12), distortion product otoacoustic emissions (DPOAEs) at various frequencies were measured repeatedly over 6-8 months. About 9 weeks after the first measurement, the animals were exposed to industrial noise (car industry, maximal intensity about 110 dB SPL) for 2 h. The amplitudes of DPOAE were measured prior to noise exposure and 10 min, 70 min, 1 day and 2 days after the noise exposure and then once every week. Three to four months after noise exposure, the animals were killed, and the cochleae were prepared for scanning electron microscopy. The row of inner hair cells (IHCs) was complete in all animals, while the rows of outer hair cells (OHCs) showed a considerable hair cell loss in some of the animals without a correlation to the change in amplitudes of DPOAE. However, a closer relationship between the decline of amplitudes of DPOAE and the number of missing and changed OHCs (fused stereocilia bundles, missing tip links) could be established. The number of lost OHC does not reflect the decline in DPOAE in all cases. This discrepancy must be considered when the degree of hearing loss needs to be established from changed DPOAE.

Effect of salicylate and short-term sound exposure on extracochlear electrically-evoked otoacoustic emissions

The effects of salicylate and acoustic overstimulation on the electromotility of the cochlear outer hair cells (OHCs) were assessed in vivo using electrically-evoked otoacoustic emissions (EEOAEs). Alternating currents to evoke the EEOAE were delivered by means of an extracochlear electrode on the round window, with which the compound action potentials (CAPs) were also monitored before and after the manipulations. The EEOAE outputs were a linear function of the injected currents between 52 and 267 microA rms. Administration of salicylate (500 mg/kg) reduced the EEOAE outputs significantly at 5 and 8 kHz (p < 0.005). while no change in EEOAEs was observed at any frequency after exposure to a 4 kHz pure tone at 100 dB SPL for 10 min or at 120 dB SPL, for 30 min. These results indicate that administration of salicylate reduces the electromotility of the OHCs, and thus produces losses in neural sensitivity of the cochlea. In contrast, the electromotility of OHCs appears to be protected against short-term intense sound exposure.

Calmodulin-dependent protein kinases mediate calcium-induced slow motility of mammalian outer hair cells

Cochlear outer hair cells in vitro respond to elevation of intracellular calcium with slow shape changes over seconds to minutes ('slow motility'). This process is blocked by general calmodulin antagonists suggesting the participation of calcium/calmodulin-dependent enzymatic reactions. The present study proposes a mechanism for these reactions. Length changes of outer hair cells isolated from the guinea pig cochlea were induced by exposure to the calcium ionophore ionomycin. ATP levels remained unaffected by this treatment ruling out depletion of ATP (by activation of calcium-dependent ATPases) as a cause of the observed shape changes. Involvement of protein kinases was suggested by the inhibition of shape changes by K252a, a broad-spectrum inhibitor of protein kinase activity. Furthermore, the inhibitors ML-7 and ML-9 blocked the shape changes at concentrations compatible with inhibition of myosin light chain kinase (MLCK). KN-62, an inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMKII), also attenuated the length changes. Inhibitors with selectivity for cyclic nucleotide-dependent protein kinases (H-89, staurosporine) were tested to assess potential additional contributions by such enzymes. The dose dependence of their action supported the notion that the most likely mechanism of slow motility involves phosphorylation reactions catalyzed by MLCK or CaMKII or both.

Noise exposure alters the response of outer hair cells to ATP

The outer hair cells (OHCs) are one target of noise-induced effects. To date there are few studies which examine changes in the function of OHCs induced by noise exposure. There is increasing evidence that ATP may be a neuromodulator acting on OHCs. Therefore, we examined the possibility that the response to ATP may be altered by low-level noise exposure. ATP was tested on cation currents recorded from outer hair cells (OHCs) isolated from chronic noise-exposed guinea pigs and compared to currents recorded from normal control animals. The whole-cell variant of the patch-clamp technique was used. The incidence of response to 100 microM ATP was decreased in OHCs from noise-exposed animals as compared to controls when normal internal and external solutions were employed. When K+ was substituted by N-methyl-glucamine (NMG+) in the pipette solution, there were significant differences in the magnitudes of ATP-evoked currents between cells from noise-exposed and control animals. This was observed in both normal and 20 mM Ba2+ external solutions. In addition, the response to ATP exhibited a dependency on OHC length. In short OHCs (< 65 microns) from noise-exposed animals the magnitude of the response to ATP was significantly reduced. By contrast, the response in long OHCs (> 65 microns) from noise-exposed animals was increased. Results suggest that low-level noise exposure induces changes in OHCs which affect the response of the cell to ATP.

Hair cell mediated responses of the auditory nerve to sinusoidal electrical stimulation of the cochlea in the cat

Electrical stimulation of the cochlea elicits discharges of auditory nerve fibres which are mediated by the electrical or mechanical stimulation of inner hair cells (electrophonic responses). In order to isolate hair-cell mediated responses from those elicited by electrical stimulation of the nerve, the compound action potential (CAP) evoked by an acoustic probe was forward-masked by sinusoidal monopolar, or localized bipolar electrical stimulation of the base of the cochlea. The degree of masking of a given probe estimated the synaptically mediated response to the masker of the population of auditory nerve fibres innervating the cochlear location tuned to the probe. There was a peak of masking for probes close to the frequency of the electrical stimulus, suggesting a spatial tuning of the hair cell mediated response along the cochlea. This is consistent with excitation of the inner hair cells by a propagating mechanical response which is generated within the electrical field at the base of the cochlea. Furthermore, tuning curves for masking of a given probe were sharply tuned to electrical stimulation close to the probe frequency. This masking was not dependent upon the presence of functional outer hair cells close to the electrodes, suggesting an alternate transduction of electrical to mechanical energy.

Dissociation between the calcium-induced and voltage-driven motility in cochlear outer hair cells from the waltzing guinea pig

The waltzing guinea pig, possessing an hereditary progressive deafness, shows pathology to the actin-bearing structures within the hair cells of the organ of Corti. In particular, the affected structures include the stereocilia, the cuticular plate and, as shown in the present study, swollen and disorganized subsurface cisternae. To test whether this pathology affected outer hair cell motility, cells were isolated from waltzing guinea pigs and their age-matched controls and were subjected to either electrical or chemical stimulation. Visual detection thresholds and the magnitude of the electrically-induced length changes were equivalent for both groups. However, when intracellular calcium was increased with either the calcium ionophore, ionomycin or Ca2+/ATP (under permeabilized conditions with DMSO), length changes were significantly reduced for the outer hair cells from waltzing guinea pigs compared to the controls. The average percent length increase induced by 10 microM ionomycin for the outer hair cells from control animals was 2.3 +/- 1.7 whereas for postnatal day 4 waltzing guinea pigs it was 1.3 +/- 1.7. Postnatal day 7 and 10 waltzing guinea pigs responded with significantly smaller percent length changes. The intracellular concentration of ionic calcium increased similarly for both groups after the application of ionomycin as revealed with the indicator fluo-3. In the permeabilized cells in the presence of Ca2+/ATP, control cells responded with a percent length change of 3.5, whereas, age-matched waltzing outer hair cells responded with barely detectable length changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in size and shape of auditory hair cells in vivo during noise-induced temporary threshold shift

In this study we describe changes in the size and shape of auditory hair cells of the alligator lizard in vivo during noise-induced temporary threshold shift. These changes consist of a decrease in cell volume, a decrease in cell length and an increase in cell width. We speculate that these changes are due to relaxation of cytoskeletal contractile elements and osmotic loss of intracellular water. We also describe a decrease in the surface area of the hair cell plasmalemma, and speculate that it is related to the endocytosis and intracellular accumulation of cell membrane during synaptic vesicle recycling. Finally we describe an increase in the endolymphatic surface area of the hair cell, and speculate that this could alter the micromechanics of the stereociliary tuft to attenuate the effective stimulus.