Outer hair cell driven reticular lamina mechanical distortion in living cochleae.

Cochlear distortions afford researchers and clinicians a glimpse into the conditions and properties of inner ear signal processing mechanisms. Until recently, our examination of these distortions has been limited to measuring the vibration of the basilar membrane or recording acoustic distortion output in the ear canal. Despite its importance, the generation mechanism of cochlear distortion remains a substantial task to understand. The ability to measure the vibration of the reticular lamina in rodent models is a recent experimental advance. Surprising mechanical properties have been revealed. These properties merit both discussion in context with our current understanding of distortion, and appraisal of the significance of new interpretations of cochlear mechanics. This review focusses on some of the recent data from our research groups and discusses the implications of these data on our understanding of vocalization processing in the periphery, and their influence upon future experimental directions. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.

Functional expression of P2X4 receptor in capillary endothelial cells of the cochlear spiral ligament and its role in regulating the capillary diameter.

The cochlear lateral wall generates the endocochlear potential (EP), which creates a driving force for the hair cell transduction current and is essential for normal hearing. Blood flow at the cochlear lateral wall is critically important for maintaining the EP. The vulnerability of the EP to hypoxia suggests that the blood flow in the cochlear lateral wall is dynamically and precisely regulated to meet the changing metabolic needs of the cochlear lateral wall. It has been reported that ATP, an important extracellular signaling molecule, plays an essential role in regulating cochlear blood flow. However, the cellular mechanism underlying ATP-induced regional blood flow changes has not been investigated. In the current study, we demonstrate that 1) the P2X4 receptor is expressed in endothelial cells (ECs) of spiral ligament (SL) capillaries. 2) ATP elicits a characteristic current through P2X4 on ECs in a dose-dependent manner (EC(50) = 0.16 mM). The ATP current has a reversal potential at ∼0 mV; is inhibited by 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD), LaCl(3), pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) tetrasodium salt hydrate (PPADS), and extracellular acidosis; and is less sensitive to α,ß-methyleneadenosine 5'-triphosphate (α,ß-MeATP) and 2'- and 3'-O-(4-benzoyl-benzoyl) adenosine 5'-triphosphate (BzATP). 3) ATP elicits a transient increase of intracellular Ca(2+) in ECs. 4) In accordance with the above in vitro findings, perilymphatic ATP (1 mM) caused dilation in SL capillaries in vivo by 11.5%. N(ω)-nitro-l-arginine methyl ester hydrochloride (l-NAME), a nonselective inhibitor of nitric oxide synthase, or 5-BDBD, the specific P2X4 inhibitor, significantly blocked the dilation. These findings support our hypothesis that extracellular ATP regulates cochlear lateral blood flow through P2X4 activation in ECs.

Effect of salicylate on KCNQ4 of the guinea pig outer hair cell.

Salicylate causes a moderate hearing loss and tinnitus in humans at high-dose levels. Salicylate-induced hearing loss has been attributed to impaired sound amplification by outer hair cells (OHCs) through its direct action on the OHC motility sensor and/or motor. However, there is a disparity of salicylate concentrations between the clinical and animal studies, i.e., extremely high extracellular concentrations of salicylate (from 1 to 10 mM) is required to produce a significant reduction of electromotility in animal studies. Such concentrations are above the clinical/physiological range for humans. Here, we showed that clinical/physiological concentration range of salicylate caused concentration-dependent and reversible reductions in I(K,n) (KCNQ4) and subsequent depolarization of OHCs. Salicylate reduced the maximal tail current of the activation curve of I(K,n) without altering the voltage-sensitivity (V(half)). The salicylate-induced reduction of I(K,n) was almost completely blocked by linopirdine (0.1 mM) and BaCl2 (10 mM). Consistent with the finding in OHCs, salicylate significantly reduced KCNQ4-mediated current expressed in Chinese hamster ovarian (CHO) cells by comparable amplitude to OHCs without significantly shifting V(half). Nonstationary fluctuation analysis shows that salicylate significantly reduced the estimated single-channel current amplitude and numbers. Intracellular Ca²+ elevation resulting from cytoplasmic acidosis also contributes to the current reduction of I(K,n) (KCNQ4) of OHCs. These results indicate a different model for the salicylate-induced hearing loss through the reduction of KCNQ4 and subsequent depolarization of OHCs, which reduces the driving force for transduction current and electromotility. The major mechanism underlying the reduction of I(K,n) (KCNQ4) is the direct blocking action of salicylate on KCNQ4.

The influence of loud sound stress on expression of osmotic stress protein 94 in the murine inner ear.

Osmotic stress protein 94 (OSP94), a member of the heat shock protein 110/SSE subfamily, is expressed in certain organs such as the kidney, testis, and brain where it can act as a molecular chaperon. In general, its alteration in expression is in response to hyper-ionic and osmotic stress as well as heat shock stress. Since many cells in the inner ear are involved in active ion transportation and are constantly exposed to two ionic different environments, we hypothesize that OSP94 may be expressed in the inner ear and its expression may be influenced by loud sound stress (LSS). With immunohistochemistry combined with confocal microscopy, immunoblotting, and reverse transcription polymerase chain reaction techniques, we found that OSP94 was widely expressed in various cells in the murine cochlea including the stria vascularis, the organ of Corti, the interdental cells, spiral ganglion cells, the spiral ligament, and Reissner's membrane. Under the unstressed condition, the transcription and protein level of OSP94 expression in the inner ear was quantitatively similar to that of the kidney. Furthermore, its expression in the inner ear by LSS from broadband noise at 117 dB/SPL was upregulated, but remained unchanged in the kidney. In particular, the upregulation of OSP94 in the cochlear lateral wall tissue was slowly elicited in a LSS time-dependent manner compared with the response of two other HSPs; HSP25 and HSP70 are considered to play a cytoprotective role under stressful conditions. Our results show that OSP94 is expressed in the inner ear and indicate this may be necessary for cells in a special ionic and osmotic environment such as endo-perilymphatic ion compartments. The organ-specific upregulation of OSP94 by acoustic overstimulation reveals that OSP94 in the murine inner ear is potentially important for cellular functional adaptation to LSS.

Intracellular recordings from cochlear inner hair cells: effects of stimulation of the crossed olivocochlear efferents.

Intracellular recordings were obtained from inner hair cells located in the lower basal turn of the guinea pig cochlea. At low sound pressure levels the inner hair cells were highly frequency selective, producing receptor potentials only in response to sound frequencies between about 16 and 24 kilohertz. Electrical stimulation of efferent nerves in the crossed olivocochlear bundle markedly reduced these receptor potentials while causing little change in the resting membrane potential. At high sound levels, where cells responded to an increasingly wider range of sound frequencies, stimulation was less effective in reducing receptor potentials. Since the crossed olivocochlear bundle primarily innervates outer hair cells, these results support an outer hair cell contribution to the most sensitive response region of inner hair cells.

Direct evidence of trigeminal innervation of the cochlear blood vessels.

This paper provides the first detailed description of the trigeminal innervation of the inner ear vasculature. This system provides a newly discovered neural substrate for rapid vasodilatatory responses of the inner ear to high levels of activity and sensory input. Moreover, this discovery may provide an alternative mechanism for a set of clinical disturbances (imbalance, hearing loss, tinnitus and headache) for which a central neural basis has been speculated. Iontophoretic injections of biocytin were made via a glass microelectrode into the trigeminal ganglion in guinea-pigs. Tissue for histological sections was obtained 24 h later. Labeled fibers from the injection site were observed as bundles around the ipsilateral spiral modiolar blood vessels, as individual labeled fibers in the interscala septae, and in the ipsilateral stria vascularis. The dark cell region of the cristae ampullaris in the vestibular labyrinth was also intensively labeled. No labeled fibers were observed in the neuroepithelium of the cristae ampullaris or the semicircular canals. These results confirm and localize an earlier indirect observation of the trigeminal ganglion projection to the cochlea. This innervation may play a role in normal vascular tone and in some inner ear disturbances, e.g., sudden hearing loss may reflect an abnormal activity of trigeminal ganglion projections to the cochlear blood vessels.

Co-localization of the vanilloid capsaicin receptor and substance P in sensory nerve fibers innervating cochlear and vertebro-basilar arteries.

Evidence suggests that capsaicin-sensitive substance P (SP)-containing trigeminal ganglion neurons innervate the spiral modiolar artery (SMA), radiating arterioles, and the stria vascularis of the cochlea. Antidromic electrical or chemical stimulation of trigeminal sensory nerves results in neurogenic plasma extravasation in inner ear tissues. The primary aim of this study was to reveal the possible morphological basis of cochlear vascular changes mediated by capsaicin-sensitive sensory nerves. Therefore, the distribution of SP and capsaicin receptor (transient receptor potential vanilloid type 1-TRPV1) was investigated by double immunolabeling to demonstrate the anatomical relationships between the cochlear and vertebro-basilar blood vessels and the trigeminal sensory fiber system. Extensive TRPV1 and SP expression and co-localization were observed in axons within the adventitial layer of the basilar artery, the anterior inferior cerebellar artery, the SMA, and the radiating arterioles of the cochlea. There appears to be a functional relationship between the trigeminal ganglion and the cochlear blood vessels since electrical stimulation of the trigeminal ganglion induced significant plasma extravasation from the SMA and the radiating arterioles. The findings suggest that stimulation of paravascular afferent nerves may result in permeability changes in the basilar and cochlear vascular bed and may contribute to the mechanisms of vertebro-basilar type of headache through the release of SP and stimulation of TPVR1, respectively. We propose that vertigo, tinnitus, and hearing deficits associated with migraine may arise from perturbations of capsaicin-sensitive trigeminal sensory ganglion neurons projecting to the cochlea.

Capsaicin stimulation of the cochlea and electric stimulation of the trigeminal ganglion mediate vascular permeability in cochlear and vertebro-basilar arteries: a potential cause of inner ear dysfunction in headache.

Trigeminal neurogenic inflammation is one explanation for the development of vascular migraine. The triggers for this inflammation and pain are not well understood, but are probably vasoactive components acting on the blood vessel wall. Migraine-related inner ear symptoms like phonophobia, tinnitus, fluctuation in hearing perception and increased noise sensitivity provide indirect evidence that cochlear blood vessels are also affected by basilar artery migraine. The purpose of this investigation was to determine if a functional connection exists between the cochlea and the basilar artery. Neuronally mediated permeability changes in the cochlea and basilar artery were measured by colloidal silver and Evans Blue extravasation, following orthodromic and antidromic stimulation of the trigeminal ganglion innervating the cochlea. Capsaicin and electrical stimulation induced both dose- and time-dependent plasma extravasation of colloidal silver and Evans Blue from the basilar artery and anterior inferior cerebellar artery. Both orthodromic and antidromic activation of trigeminal sensory fibers also induced cochlear vascular permeability changes and significant quantitative differences between the treated and control groups in spectrophotometric assays. These results characterize a vasoactive connection between the cochlea and vertebro-basilar system through the trigeminal sensory neurons. We propose that vertigo, tinnitus and hearing deficits associated with basilar migraine could arise by excitation of the trigeminal nerve fibers in the cochlea, resulting in local plasma extravasation. In addition, cochlear "dysfunction" may also trigger basilar and cluster headache by afferent input to the trigeminal system.

Trigeminal ganglion innervation of the cochlea--a retrograde transport study.

Innervation patterns of sensory nerves from the trigeminal ganglion to the cochlear blood vessels were studied using retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. Guinea-pigs (n=7) were unilaterally implanted with an osmotic pump and a cannula for cochlear delivery of 2% or 20% wheat germ agglutinin horseradish peroxidase (Group 1), 2% wheat germ agglutinin-horseradish peroxidase followed by 100 micromol capsaicin (Group 2), or vehicle alone. Histological sections of the trigeminal ganglia, the C1 and C2 dorsal ganglia, the superior and inferior ganglia of the glossopharyngeal nerve bilaterally, the midbrain and the brainstem were obtained after 48 h of infusion. In Group 1, a large number of labeled nerve cell bodies were observed in the anteromedial portion of the trigeminal ganglion and at the origin of the ophthalmic nerve. Some labeled cells were also found on the lateral side of the ophthalmic nerve, as well as on the medial side of the maxillary nerve root. Capsaicin pretreatment significantly reduced the density of labeled neurons in the trigeminal ganglion. A few labeled neurons were also found in the trigeminal brainstem nucleus complex and in certain auditory brainstem nuclei. No wheat germ agglutinin horseradish peroxidase-positive cells were observed in the spinal C1 or C2 cervical ganglia or in the superior or inferior glossopharyngeal ganglia. In contrast, wheat germ agglutinin-horseradish peroxidase application to the middle ear resulted in labeled cells in the middle posterolateral portion of the trigeminal ganglia and in the superior ganglia of the glossopharyngeal nerve. These results provide the first direct evidence that the trigeminal ganglion sends projections to the cochlea.

Spontaneous basilar membrane oscillation and otoacoustic emission at 15 kHz in a guinea pig.

A spontaneous otoacoustic emission (SOAE) measured in the ear canal of a guinea pig was found to have a counterpart in spontaneous mechanical vibration of the basilar membrane (BM). A spontaneous 15-kHz BM velocity signal was measured from the 18-kHz tonotopic location and had a level close to that evoked by a 14-kHz, 15-dB SPL tone given to the ear. Lower-frequency pure-tone acoustic excitation was found to reduce the spontaneous BM oscillation (SBMO) while higher-frequency sound could entrain the SBMO. Octave-band noise centered near the emission frequency showed an increased narrow-band response in that frequency range. Applied pulses of current enhanced or suppressed the oscillation, depending on polarity of the current. The compound action potential (CAP) audiogram demonstrated a frequency-specific loss at 8 and 12 kHz in this animal. We conclude that a relatively high-frequency spontaneous oscillation of 15 kHz originated near the 15-kHz tonotopic place and appeared at the measured BM location as a mechanical oscillation. The oscillation gave rise to a SOAE in the ear canal. Electric current can modulate level and frequency of the otoacoustic emission in a pattern similar to that for the observed mechanical oscillation of the BM.

Dynamic response of cochlear blood flow to occlusion of anterior inferior cerebellar artery in guinea pigs.

In this study we investigated the autoregulation and hemodynamics of cochlear blood flow (CBF) as measured by laser-Doppler flowmetry. When the anterior inferior cerebellar artery was clamped, CBF decreased approximately 40% (not to "biological zero"), followed by a gradual increase. When the clamp was released, CBF quickly increased to as much as 167% of the baseline level and then slowly returned to baseline. We assume that the dynamic CBF response to anterior inferior cerebellar artery clamping reflects primarily a combination of passive elastic properties of the cochlear vessels and active autoregulatory mechanisms. The decrease portion of the negative phase and the increase portion of the positive phase reflect mainly passive behavior, static compliance, and resistance of vessels, whereas the slow exponential negative and positive changes indicate an active response of vessels: an autoregulatory mechanism based on compensatory vascular dilation and constriction. Our preliminary data show a very strong CBF autoregulatory response to a change in intravascular pressure. Sympathetic stimulation can enhance this autoregulation, and CO2 inhalation promotes compensatory dilation and inhibits compensatory vascular constriction.

Quantitative measure of multicomponents of otoacoustic emissions.

A method for quantitatively measuring measuring multicomponents of otoacoustic emissions (OAE) was developed in this study. The method is based on the rationale that, if the acoustic emission is a vector sum of multicomponents coming from different locations in the cochlea, each component will show a delay. The proposed method consists of the following steps: (1) the amplitude and phase of the emission is measured when the emission frequency is swept; (2) the real part of the spectrum is obtained based on the amplitude and phase spectra; and (3) the real part of the emission spectrum is then analyzed using a Fourier transform to extract the multiple components. The theoretical basis and practical procedure of this method are described, and in vitro and in vivo tests are used to demonstrate the validity of the method. Preliminary data demonstrate the multicomponents of the extracochlear electrically evoked otoacoustic emission (EEOAE).

Inner hair cell responses to the velocity of basilar membrane motion in the guinea pig.

Triangular wave acoustic stimulation at 200 Hz produced the expected square wave cochlear microphonic at the round window membrane and within the scala media. Intracellular recordings from inner hair cells (IHC) of the first cochlear turn showed a combination waveform having both spike impulse and square wave features. The IHC response suggests a sensitivity of these cells to both the displacement and to the velocity of basilar membrane motion.

Role of glutathione in protection against noise-induced hearing loss.

A potential mechanism of hearing loss due to acoustic overstimulation is the generation of reactive oxygen species (ROS). ROS not removed by antioxidant defenses could be expected to cause significant damage to the sensory cells of the cochlea. We studied the influence of the antioxidant glutathione (GSH) on noise-induced hearing loss by using l-buthionine-[S,R]-sulfoximine (BSO), an inhibitor of GSH synthesis, and 2-oxothiazolidine-4-carboxylate (OTC), a cysteine prodrug, which promotes rapid restoration of GSH when GSH is acutely depleted. Pigmented female guinea pigs were exposed to broadband noise (102 dB SPL, 3 h/day, 5 days) while receiving daily injections of BSO, OTC, or saline. By weeks 2 and 3 after noise exposure, BSO-treated animals showed significantly greater threshold shifts above 12 kHz than saline-treated subjects, whereas OTC-treated animals showed significantly smaller threshold shifts at 12 kHz than controls. Histologically assessed noise-induced damage to the organ of Corti, predominantly basal turn row 1 outer hair cells, was most pronounced in BSO-treated animals. High performance liquid chromatographic analysis showed that OTC significantly increased cysteine levels, but not GSH levels, in the cochlea. These findings show that GSH inhibition increases the susceptibility of the cochlea to noise-induced damage and that replenishing GSH, presumably by enhancing availability of cysteine, attenuates noise-induced cochlear damage.

Influence of intense sound exposure on glutathione synthesis in the cochlea.

Previous studies have shown that depletion of endogenous glutathione (GSH) potentiates noise-induced hearing loss (NIHL), whereas replenishment of GSH attenuates NIHL (Yamasoba et al., Brain Res. 784 (1998) 82-90). Since these findings indicate an important role of GSH in protection from NIHL, we assessed the influence of intense sound exposure (broadband noise, 105 dB SPL, 5 h) on GSH and cysteine levels in the guinea pig cochlea using high performance liquid chromatography. GSH levels were significantly increased in the lateral wall 2 and 4 h post-exposure and returned to normal 6 h post-exposure. GSH levels in the sensory epithelium and modiolus did not show significant changes following noise. Cysteine levels were unchanged in any of the cochlear segments. For the cochlea as a whole, intense sound exposure did not significantly change GSH or cysteine levels throughout the 6-h measurement period post-exposure. These results indicate that GSH synthesis is markedly upregulated selectively in the lateral wall by noise exposure, presumably in response to the robust consumption of GSH, as it is utilized in scavenging reactive oxygen species.

Matched filter estimation of serial blood vessel diameters from video images.

A method for making a contiguous series of blood vessel diameter estimates from digitized images is proposed. It makes use of a vessel intensity profile model based on the vessel geometry and the physics of the imaging process, providing estimates of far greater accuracy than previously obtained. A variety of techniques are used to reduce the computational demand. The method includes the generation of measurement estimation error, which is important in determining total vessel patency as well as providing a basic measure of diameter estimate accuracy.

Techniques for the observation and measurement of red blood cell velocity in vessels of the guinea pig cochlea.

Fluorescence techniques combined with intravital microscopy provide a powerful approach to the study of cochlear blood microcirculation. In the current study, fluorescein isothiocyanate conjugated to high molecular weight dextrans was added to plasma to enhance the visual contrast of flowing blood in microscopic images from the guinea pig cochlea. Photometric signals, obtained from video pictures of the blood vessels, provided a means to continuously measure red cell velocity by using crosscorrelation algorithms to extract the time delay for moving features of the image. Alternatively, a small amount of fluorescently-labeled red blood cells (RBCs) were added to the vascular volume to serve as natural indicators of whole blood flow. The speed of these cells was measured by video photometric detection of the time required for the cells to pass between two predetermined positions in the television image. RBCs can be made fluorescent by chemical bonding of a fluorochrome to the cell membrane or by internal loading of the cell with an inert fluorochrome. Labeled RBCs provide a means to determine blood velocity in capillaries having extremely poor optical contrast, a situation which is generally the case for relatively thick tissues such as the lateral wall of the membranous labyrinth.

Velocity of red blood cell flow in capillaries of the guinea pig cochlea.

Velocities of red blood cells (RBCs) in the lateral wall of the cochlea were determined in the anesthetized guinea pig from direct optical microscopic observations. Mean flow velocity of the blood was measured by timing the passage of a fluorescently-labeled subpopulation of RBCs across a measured capillary length. The optical observations were achieved using an intravital microscope equipped for epifluorescence and the measurements were derived from video images acquired with an image intensified television camera. In the third turn of the cochlea the velocity of RBCs differed significantly between two major classes of capillaries. The mean velocity in spiral ligament vessels was 0.12 mm/s while stria vascularis flow was slower (0.08 mm/s). In a typical animal, the range of velocities among different vessels of the ligament was 0.09-0.18 mm/s while it was 0.03-0.10 mm/s for stria vascularis vessels. Corresponding to this velocity difference, the apparent mean vessel diameters for the two types of vessels also differed. Spiral ligament capillaries were 9.3 micron while strial capillaries were 12.2 micron in diameter. Comparison of flow velocity in different turns of the cochlea indicated that the distribution of blood velocity throughout the cochlea lateral wall is constant.

Dynamic aspects of guinea pig inner hair cell receptor potentials with transient asphyxia.

DC and AC receptor potentials of cochlear inner hair cells in response to tone bursts of various frequencies and intensities were continuously measured during and following periods of transient asphyxia. The effects of asphyxia were most pronounced for low sound pressure level (SPL) acoustic stimuli near the characteristic frequency (CF) of the inner hair cell, leading to vulnerability of the 'tip' of the cell's frequency tuning curve (FTC). The resulting changes in the shape of the FTC are, first, a reduction in tip criterion sensitivity of 10-20 dB without significant loss in sharpness of tuning. Later, when the full effect of 30-45 s asphyxia occurs, tip sensitivity loss between 30 and 65 dB is accompanied by greatly broadened tuning and a shift downward in frequency of the CF by greater than 1/4 octave. The CF shift is due to a progressive loss of high frequency sensitivity. The linear segment of the input-output (intensity) function, plotted as log DC receptor potential versus SPL (at the original CF), becomes longer during the early phase asphyxia, and the slope of the segment declines by 50%. At high SPLs, for all frequencies, the time course of the receptor potential change was similar in shape to that exhibited by the endocochlear potential (EP). In particular, for high sound levels, the recovery of response matches the EP while for low level tip frequency sounds recovery is protracted. No difference between the decline of the AC and DC receptor potentials at CF was observed. Inner hair cell resting membrane potential (Em) hyperpolarized during asphyxia by 2-6 mV, correlating with the change in EP according to a ratio of 1/10 (Em/EP).

Cochlear microphonic enhancement in two tone interactions.

Two tone interaction functions of the cochlear microphonic (CM) were obtained from pigmented guinea pigs. First (basal) cochlear turn recording locations show optimally enhanced levels of CM when the interfering tone (F2) was positioned about 4 kHz above probe tones (F1) of 12 kHz and 20 kHz. Maximum enhancement occurred for equal level tones. No enhancement was seen for a probe tone of 4 kHz. When basal turn cochlear sensitivity was compromised, CM enhancement caused by the interfering tone was altered and only CM reduction was then seen. The CM reduction was the typical characteristic described by many earlier studies. Guinea pigs with various changes in cochlear sensitivity were studied, providing evidence in support of earlier reports that CM interference (both reductions and enhancements) depends on far field vector summation of the outputs of hair cells from a restricted area of the basilar membrane. No CM enhancement was seen in micropipette recordings from within the organ of Corti.