Effects of pure tone on endocochlear potential and potassium ion concentration in the guinea pig cochlea

Physiological Basis of Noise-Induced Hearing Loss in a Tympanal Ear

Acoustic overexposure, such as listening to loud music too often, results in noise-induced hearing loss. The pathologies of this prevalent sensory disorder begin within the ear at synapses of the primary auditory receptors, their postsynaptic partners and their supporting cells. The extent of noise-induced damage, however, is determined by overstimulation of primary auditory receptors, upstream of where the pathologies manifest. A systematic characterization of the electrophysiological function of the upstream primary auditory receptors is warranted to understand how noise exposure impacts on downstream targets, where the pathologies of hearing loss begin. Here, we used the experimentally-accessible locust ear (male, Schistocerca gregaria) to characterize a decrease in the auditory receptor's ability to respond to sound after noise exposure. Surprisingly, after noise exposure, the electrophysiological properties of the auditory receptors remain unchanged, despite a decrease in the ability to transduce sound. This auditory deficit stems from changes in a specialized receptor lymph that bathes the auditory receptors, revealing striking parallels with the mammalian auditory system.SIGNIFICANCE STATEMENT Noise exposure is the largest preventable cause of hearing loss. It is the auditory receptors that bear the initial brunt of excessive acoustic stimulation, because they must convert excessive sound-induced movements into electrical signals, but remain functional afterward. Here we use the accessible ear of an invertebrate to, for the first time in any animal, characterize changes in auditory receptors after noise overexposure. We find that their decreased ability to transduce sound into electrical signals is, most probably, due to changes in supporting (scolopale) cells that maintain the ionic composition of the ear. An emerging doctrine in hearing research is that vertebrate primary auditory receptors are surprisingly robust, something that we show rings true for invertebrate ears too.

Physiological Basis of Noise-Induced Hearing Loss in a Tympanal Ear

Acoustic overexposure, such as listening to loud music too often, results in noise-induced hearing loss. The pathologies of this prevalent sensory disorder begin within the ear at synapses of the primary auditory receptors, their postsynaptic partners and their supporting cells. The extent of noise-induced damage, however, is determined by overstimulation of primary auditory receptors, upstream of where the pathologies manifest. A systematic characterization of the electrophysiological function of the upstream primary auditory receptors is warranted to understand how noise exposure impacts on downstream targets, where the pathologies of hearing loss begin. Here, we used the experimentally-accessible locust ear (male, Schistocerca gregaria) to characterize a decrease in the auditory receptor's ability to respond to sound after noise exposure. Surprisingly, after noise exposure, the electrophysiological properties of the auditory receptors remain unchanged, despite a decrease in the ability to transduce sound. This auditory deficit stems from changes in a specialized receptor lymph that bathes the auditory receptors, revealing striking parallels with the mammalian auditory system.SIGNIFICANCE STATEMENT Noise exposure is the largest preventable cause of hearing loss. It is the auditory receptors that bear the initial brunt of excessive acoustic stimulation, because they must convert excessive sound-induced movements into electrical signals, but remain functional afterward. Here we use the accessible ear of an invertebrate to, for the first time in any animal, characterize changes in auditory receptors after noise overexposure. We find that their decreased ability to transduce sound into electrical signals is, most probably, due to changes in supporting (scolopale) cells that maintain the ionic composition of the ear. An emerging doctrine in hearing research is that vertebrate primary auditory receptors are surprisingly robust, something that we show rings true for invertebrate ears too.

Onset kinetics of noise-induced purinergic adaptation of the 'cochlear amplifier'

A major component of slowly reversible hearing loss which develops with sustained exposure to noise has been attributed to release of ATP in the cochlea activating P2X2 receptor (P2X2R) type ATP-gated ion channels. This purinergic humoral adaptation is thought to enable the highly sensitive hearing organ to maintain function with loud sound, protecting the ear from acoustic overstimulation. In the study that established this hearing adaptation mechanism as reported by Housley et al. (Proc Natl Acad Sci U S A 110:7494-7499, 2013), the activation kinetics were determined in mice from auditory brainstem response (ABR) threshold shifts with sustained noise presentation at time points beyond 10 min. The present study was designed to achieve finer resolution of the onset kinetics of purinergic hearing adaptation, and included the use of cubic (2f1-f2) distortion product otoacoustic emissions (DPOAEs) to probe whether the active mechanical outer hair cell 'cochlear amplifier' contributed to this process. We show that the ABR and DPOAE threshold shifts were largely complete within the first 7.5 min of moderate broadband noise (85 dB SPL) in wildtype C57Bl/6J mice. The ABR and DPOAE adaptation rates were both best fitted by a single exponential function with ~ 3 min time constants. ABR and DPOAE threshold shifts with this noise were minimal in mice null for the P2rx2 gene encoding the P2X2R. The findings demonstrate a considerably faster purinergic hearing adaptation to noise than previously appreciated. Moreover, they strongly implicate the outer hair cell as the site of action, as the DPOAEs stem from active cochlear electromotility.

QTL Mapping of Endocochlear Potential Differences between C57BL/6J and BALB/cJ mice

We reported earlier that the endocochlear potential (EP) differs between C57BL/6J (B6) and BALB/cJ (BALB) mice, being lower in BALBs by about 10 mV (Ohlemiller et al. Hear Res 220: 10-26, 2006). This difference corresponds to strain differences with respect to the density of marginal cells in cochlear stria vascularis. After about 1 year of age, BALB mice also tend toward EP reduction that correlates with further marginal cell loss. We therefore suggested that early sub-clinical features of the BALB stria vascularis may predispose these mice to a condition modeling Schuknecht's strial presbycusis. We further reported (Ohlemiller et al. J Assoc Res Otolaryngol 12: 45-58, 2011) that the acute effects of a 2-h 110 dB SPL noise exposure differ between B6 and BALB mice, such that the EP remains unchanged in B6 mice, but is reduced by 40-50 mV in BALBs. In about 25 % of BALBs, the EP does not completely recover, so that permanent EP reduction may contribute to noise-induced permanent threshold shifts in BALBs. To identify genes and alleles that may promote natural EP variation as well as noise-related EP reduction in BALB mice, we have mapped related quantitative trait loci (QTLs) using 12 recombinant inbred (RI) strains formed from B6 and BALB (CxB1-CxB12). EP and strial marginal cell density were measured in B6 mice, BALB mice, their F1 hybrids, and RI mice without noise exposure, and 1-3 h after broadband noise (4-45 kHz, 110 dB SPL, 2 h). For unexposed mice, the strain distribution patterns for EP and marginal cell density were used to generate preliminary QTL maps for both EP and marginal cell density. Six QTL regions were at least statistically suggestive, including a significant QTL for marginal cell density on chromosome 12 that overlapped a weak QTL for EP variation. This region, termed Maced (Marginal cell density QTL) supports the notion of marginal cell density as a genetically influenced contributor to natural EP variation. Candidate genes for Maced notably include Foxg1, Foxa1, Akap6, Nkx2-1, and Pax9. Noise exposure produced significant EP reductions in two RI strains as well as significant EP increases in two RI strains. QTL mapping of the EP in noise-exposed RI mice yielded four suggestive regions. Two of these overlapped with QTL regions we previously identified for noise-related EP reduction in CBA/J mice (Ohlemiller et al. Hear Res 260: 47-53, 2010) on chromosomes 5 and 18 (Nirep). The present map may narrow the Nirep interval to a ~10-Mb region of proximal Chr. 18 that includes Zeb1, Arhgap12, Mpp7, and Gjd4. This study marks the first exploration of natural gene variants that modulate the EP. Their orthologs may underlie some human hearing loss that originates in the lateral wall.

Neuroprotective effects of cutamesine, a ligand of the sigma-1 receptor chaperone, against noise-induced hearing loss

The sigma-1 receptor, which is expressed throughout the brain, provides physiological benefits that include higher brain function. The sigma-1 receptor functions as a chaperone in the endoplasmic reticulum and may control cell death and regeneration within the central nervous system. Cutamesine (1-(3,4-dimethoxyphenethyl)-4-(3-phenylpropyl) piperazine dihydrochloride) is a ligand selective for this receptor and may mediate neuroprotective effects in the context of neurodegenerative disease. We therefore assessed whether cutamesine protects the inner ear from noise-induced or aging-associated hearing loss. Immunohistochemistry and Western blotting revealed that the sigma-1 receptor is present in adult cochlea. We treated mice with 0, 3, or 30 mg/kg cutamesine from 10 days before noise exposure until the end of the study. All subjects were exposed to a 120-dB, 4-kHz octave-band noise for 2 hr. We assessed auditory thresholds by measuring the auditory-evoked brainstem responses at 4, 8, and 16 kHz, prior to and 1 week, 1 month, or 3 months following noise exposure. For the aging study, measurements were made before treatment was initiated and after 3 or 9 months of cutamesine treatment. Damage to fibrocytes within the cochlear spiral limbus was assessed by quantitative histology. Cutamesine significantly reduced threshold shifts and cell death within the spiral limbus in response to intense noise. These effects were not dose or time dependent. Conversely, cutamesine did not prevent aging-associated hearing loss. These results suggest that cutamesine reduces noise-induced hearing loss and cochlear damage during the acute phase that follows exposure to an intense noise.

Disruption of ion-trafficking system in the cochlear spiral ligament prior to permanent hearing loss induced by exposure to intense noise: possible involvement of 4-hydroxy-2-nonenal as a mediator of oxidative stress

Noise-induced hearing loss is at least in part due to disruption of endocochlear potential, which is maintained by various K⁺ transport apparatuses including Na⁺, K⁺-ATPase and gap junction-mediated intercellular communication in the lateral wall structures. In this study, we examined the changes in the ion-trafficking-related proteins in the spiral ligament fibrocytes (SLFs) following in vivo acoustic overstimulation or in vitro exposure of cultured SLFs to 4-hydroxy-2-nonenal, which is a mediator of oxidative stress. Connexin (Cx)26 and Cx30 were ubiquitously expressed throughout the spiral ligament, whereas Na⁺, K⁺-ATPase α1 was predominantly detected in the stria vascularis and spiral prominence (type 2 SLFs). One-hour exposure of mice to 8 kHz octave band noise at a 110 dB sound pressure level produced an immediate and prolonged decrease in the Cx26 expression level and in Na⁺, K⁺-ATPase activity, as well as a delayed decrease in Cx30 expression in the SLFs. The noise-induced hearing loss and decrease in the Cx26 protein level and Na⁺, K⁺-ATPase activity were abolished by a systemic treatment with a free radical-scavenging agent, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl, or with a nitric oxide synthase inhibitor, N^ω-nitro-L-arginine methyl ester hydrochloride. In vitro exposure of SLFs in primary culture to 4-hydroxy-2-nonenal produced a decrease in the protein levels of Cx26 and Na⁺, K⁺-ATPase α1, as well as Na⁺, K⁺-ATPase activity, and also resulted in dysfunction of the intercellular communication between the SLFs. Taken together, our data suggest that disruption of the ion-trafficking system in the cochlear SLFs is caused by the decrease in Cxs level and Na⁺, K⁺-ATPase activity, and at least in part involved in permanent hearing loss induced by intense noise. Oxidative stress-mediated products might contribute to the decrease in Cxs content and Na⁺, K⁺-ATPase activity in the cochlear lateral wall structures.

Protection from noise-induced temporary threshold shift by D-methionine is associated with preservation of ATPase activities

OBJECTIVE

The present study was designed to test whether noise-induced temporary threshold shift (TTS) could be attenuated by D-methionine and its possible relation to the biochemical changes of cochlear lateral walls such as ATPase activities and oxidative stress in guinea pigs.

DESIGN

Thirty-two normal-hearing male guinea pigs were randomly divided into saline-treated and D-methionine-treated (300 mg/kg) experimental groups. One hour after treatment, they were exposed to a continuous broadband white noise at 105 +/- 2 dB sound pressure level for 10 min, causing TTS. Each group was then divided into four subgroups based on the number of survival days after noise exposure (0, 1, 2, and 7 days). Each subgroup had four animals and eight ears included. By means of click-evoked auditory brain stem responses (ABR), auditory thresholds of guinea pigs were measured before noise exposure, immediately after noise exposure, and before killing. After animals were killed, cochlear lateral walls were immediately harvested and assayed for enzyme-specific activities of Na+, K+-ATPase and Ca2+-ATPase, lipid peroxidation, and nitric oxide.

RESULTS

A 15.31 +/- 3.80 dB threshold shift was found immediately after noise exposure in saline-pretreated guinea pigs. In contrast, ABR threshold shift was significantly attenuated to 4.06 +/- 2.35 dB in D-methionine-treated animals. Furthermore, D-methionine enhanced the restoration of ABR threshold to baseline level by 1 day. In addition, noise significantly decreased Na+, K+-ATPase, and Ca2+-ATPase activities and increased lipid peroxidation and nitric oxide levels of the cochlear lateral walls. D-methionine significantly protected against all of these changes.

CONCLUSIONS

Noise not only induced TTS but also inhibited ATPase activities as well as increased oxidative stress in guinea-pig cochlear lateral walls; all of these changes could be attenuated by d-methionine through its antioxidative property. These results suggest the potential usefulness of d-methionine in protecting from noise-induced ototoxicity.

Lateral wall histopathology and endocochlear potential in the noise-damaged mouse cochlea

Noise exposure damages the stria and spiral ligament and may contribute to noise-induced threshold shift by altering the endocochlear potential (EP). The aim of this study was to correlate lateral wall histopathology with changes in EP and ABR thresholds. CBA/CaJ mice were exposed to octave band (8-16 kHz) noise for 2 h at intensities ranging from 94 to 116 dB SPL and evaluated 0 h to 8 weeks postexposure. EP in control mice averaged 86 and 101 mV in apical and basal turns, respectively. The 94 dB exposures caused a 40 dB temporary threshold shift (TTS), and there was with no corresponding change in EP. The 112 and 116 dB exposures caused >60 dB threshold shifts at 24 h, and EP was transiently decreased, e.g., to 21 and 27 mV in apical and basal turns after 116 dB. By 1 week postexposure, EP returned to control values in all exposure groups, although those exposed to 112 or 116 dB showed large permanent threshold shifts (PTS). Cochleas were plastic-embedded and serial-sectioned for light microscopic and ultrastructural analysis. Acute changes included degeneration of type II fibrocytes of the spiral ligament and strial edema. The strial swelling peaked at 24 h when significant EP recovery, had taken place, suggesting that these changes reflect compensatory volume changes. In the chronic state, massive loss of type II fibrocytes and degeneration of strial intermediate and marginal cells was observed with drastic reduction in membrane surface area. The results suggest that EP shifts do not occur with TTS and also do not add significantly to PTS in the steady state. However, EP loss could contribute to acute threshold shifts that resolve to a PTS. EP recovery despite significant strial degeneration may be partly due to decreased transduction current caused by hair cell damage.

Static material properties of the tectorial membrane: a summary

The tectorial membrane (TM) is a polyelectrolyte gel. Hence, its chemical, electrical, mechanical, and osmotic properties are inextricably linked. We review, integrate, and interpret recent findings on these properties in isolated TM preparations. The dimensions of the TM in alligator lizard, chick, and mouse are sensitive to bath ion concentrations of constituents normally present in the cochlear fluids - an increase in calcium concentration shrinks the TM, and an increase in sodium concentration swells the TM in a manner that depends competitively on the calcium concentration. The sodium-induced swelling is specific; it does not occur with other alkali metal cations. We interpret these findings as due to competitive binding of sodium and calcium to TM macromolecules which causes a change in their conformation that leads to a change in mechanical properties. In mouse TM, decreasing the bath pH below 6 or increasing it above 7 results in swelling of the TM. Electric potential measurements are consistent with the notion that the swelling is caused by a pH-driven increase in positive fixed charge at low pH and an increase in the magnitude of the negative fixed charge at high pH which is consistent with the known protonation pattern of TM macromolecules. Increasing the osmotic pressure of the bathing solution with polyethylene glycol shrinks the TM and decreasing the ionic strength of the bathing solution swells the TM. Both results are qualitatively consistent with predictions of a polyelectrolyte gel model of the TM.

An in vivo tracer study of noise-induced damage to the reticular lamina

An in vivo tracer was used to determine if the reticular lamina and/or the cell membranes abutting the endolymphatic space are temporarily disrupted after intense noise exposure (4-kHz OBN, 108-dB SPL, 1.75 h). Using a double-barreled micropipette, the endolymphatic potential (EP) was recorded and artificial endolymph containing 10% carbon particles was injected into the endolymphatic space either 0 days or 28 days post-exposure. The cochleae were fixed 30-45 min post-injection, then dehydrated, embedded in plastic and dissected as flat preparations. Damage in the organ of Corti (OC) was quantified, the location of carbon was determined, and some OC segments were then sectioned radially. EP averaged 72+/-5 mV in five controls. These cochleae had carbon tracer in the endolymphatic space only. Four of five noise-exposed chinchillas examined 3-4 h post-exposure had a low EP (30+/-6 mV). The cochleae from these 0-day animals had several focal lesions in which nearly all outer hair cells had just degenerated. At these lesions, carbon was attached to cell membranes and debris between the reticular lamina and basilar membrane. By transmission electron microscopy, discontinuities were found in the apical membranes of sensory and supporting cells. Carbon particles were found in the cytoplasm of these cells. Four of five animals examined at 28 days had an average EP of 70+/-11 mV. The cochleae from these animals had multiple lesions in the basal turn, all of which were healed by phalangeal scars or squamous epithelial cells. In these cochleae, no carbon was found within the OC. Acute disruption of the reticular lamina and the apical membranes of sensory and supporting cells from noise appears to be a major mechanism to account for degeneration in the cochlea that spreads or continues for days to weeks post-exposure.

Activities of Na(+),K(+)-ATPase and Ca(2+)-ATPase in cochlear lateral wall after acoustic trauma

Na(+),K(+)-ATPase and Ca(2+)-ATPase are well known participants in the active transport of ions in the inner ear. These two enzymes play an important role in maintaining cochlear function. Although changes in these enzymes' activities in the cochlea have been implicated in noise-induced hearing loss, no evidence of quantitative alteration of Na(+),K(+)-ATPase or Ca(2+)-ATPase activities has ever been shown. The present study was undertaken to determine the quantitative alterations of their activities by microcolorimetric assay in the cochlear lateral wall after acoustic trauma. Adult albino guinea pigs were exposed to white noise at 105+/-2 dB A for 10 min or 40 h. The age-matched control animals were not exposed to noise. Noise exposure resulted in a significant threshold shift of the auditory brainstem response (P<0.001). Significant decreases in activities of Na(+),K(+)-ATPase and Ca(2+)-ATPase were found in the cochlear lateral wall after noise exposure (P<0.001). Statistical analysis indicated that a good correlation held not only between the decline of these enzyme activities and noise-induced hearing loss, but also between the gradual partial recovery of these parameters during the first 10-day recovery period. The present findings suggest that metabolic damage and ionic disturbance may contribute, at least partially, to noise-induced hearing threshold shift.

The osmotic response of the isolated, unfixed mouse tectorial membrane to isosmotic solutions: effect of Na+, K+, and Ca2+ concentration

Changes in the size, shape, and structure of the isolated tectorial membrane (TM) of the mouse were measured in response to isosmotic changes in the ionic composition of the bathing solution. Substitution of artificial perilymph (AP) for artificial endolymph (AE) caused a small (approximately 1%) shrinkage of the TM's thickness. This substitution alters not only the predominate cation (from K+ to Na+) but also the Ca2+ concentration (from 20 mumol/l to 2 mmol/l). When the predominate cation was changed from K+ to Na+, while holding Ca2+ concentration constant, results depended on Ca2+ concentration: there was a small (approximately 1%) swelling for 20 mumol/l Ca2+, larger (approximately 14%) swelling for lower (< 7 mumol/l) concentrations of Ca2+, and little response for 2 mmol/l Ca2+ or for solutions containing the Ca2+ chelator EGTA. Addition of Ca2+ while holding the predominate cation constant caused shrinkage of the TM; both removal of Ca2+ and addition of the Ca2+ chelator EGTA caused swelling. Swelling responses were largely reversible if the magnitude of the swelling was small. Responses greater than a few percent were only partially reversible and caused long-lasting changes. Changes in ionic composition of the bath affected not only the thickness of the TM but also its other dimensions. Solution changes that increase TM thickness tend to cause radial shearing motions of the surfaces of the TM, which are accompanied by small decreases in width. Little change in length was observed. Although the responses were non-isotropic, increases in thickness were highly correlated with increases in volume. Swelling of the TM was also accompanied by a reduction in prominence of its radially oriented fibrillar structure. These results for the isolated TM of the mouse are qualitatively similar to those obtained previously for the isolated chick TM (Freeman et al., 1994) but different from those obtained for the in vitro mouse TM (Kronester-Frei, 1979a).

Ionic changes in cochlear endolymph of the guinea pig induced by acoustic injury

The effects of acoustic overstimulation on the endocochlear potential (EP) and on concentrations of ions (K+, Na+, Cl-, H+, HCO3-, and Ca2+) in endolymph were investigated using ion-selective microelectrodes. A slight but significant elevation of the EP and alkalinization of the endolymph were induced by acoustic overstimulation, whereas there was little change in the K+, Na+, Cl-, and HCO3- concentrations. The changes in H+ and HCO3- concentrations implied a depression of PCO2, suggesting an increase in blood flow to the cochlea. On the other hand, the Ca2+ concentration increased abruptly to 48 times the pre-exposure value. In contrast, no significant change in the Ca2+ concentration was observed in cochleae with damaged hair cells. We discuss the mechanism of the tone-induced Ca2+ elevation in endolymph and its effect on hearing acuity.

Ionic environment of cochlear hair cells

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.