Noise and cochlear blood flow

Sympathetic Nervous System Regulation of Auditory Function

BACKGROUND

The auditory system processes how we hear and understand sounds within the environment. It comprises both peripheral and central structures. Sympathetic nervous system projections are present throughout the auditory system. The function of sympathetic fibers in the cochlea has not been studied extensively due to the limited number of direct projections in the auditory system. Nevertheless, research on adrenergic and noradrenergic regulation of the cochlea and central auditory system is growing. With the rapid development of neuroscience, auditory central regulation is an extant topic of focus in research on hearing.

SUMMARY

As such, understanding sympathetic nervous system regulation of auditory function is a growing topic of interest. Herein, we review the distribution and putative physiological and pathological roles of sympathetic nervous system projections in hearing. Key Messages: In the peripheral auditory system, the sympathetic nervous system regulates cochlear blood flow, modulates cochlear efferent fibers, affects hair cells, and influences the habenula region. In central auditory pathways, norepinephrine is essential for plasticity in the auditory cortex and affects auditory cortex activity. In pathological states, the sympathetic nervous system is associated with many hearing disorders. The mechanisms and pathways of sympathetic nervous system modulation of auditory function is still valuable for us to research and discuss.

Acoustic Trauma Modulates Cochlear Blood Flow and Vasoactive Factors in a Rodent Model of Noise-Induced Hearing Loss

Noise exposure affects the organ of Corti and the lateral wall of the cochlea, including the stria vascularis and spiral ligament. Although the inner ear vasculature and spiral ligament fibrocytes in the lateral wall consist of a significant proportion of cells in the cochlea, relatively little is known regarding their functional significance. In this study, 6-week-old male C57BL/6 mice were exposed to noise trauma to induce transient hearing threshold shift (TTS) or permanent hearing threshold shift (PTS). Compared to mice with TTS, mice with PTS exhibited lower cochlear blood flow and lower vessel diameter in the stria vascularis, accompanied by reduced expression levels of genes involved in vasodilation and increased expression levels of genes related to vasoconstriction. Ultrastructural analyses by transmission electron microscopy revealed that the stria vascularis and spiral ligament fibrocytes were more damaged by PTS than by TTS. Moreover, mice with PTS expressed significantly higher levels of proinflammatory cytokines in the cochlea (e.g., IL-1β, IL-6, and TNF-α). Overall, our findings suggest that cochlear microcirculation and lateral wall pathologies are differentially modulated by the severity of acoustic trauma and are associated with changes in vasoactive factors and inflammatory responses in the cochlea.

Low-level laser therapy for prevention of noise-induced hearing loss in rats

Noninvasive low-level laser therapy (LLLT) is neuroprotective, but the mechanism of this effect is not fully understood. In this study, the use of LLLT as a novel treatment for noise-induced hearing loss (NIHL) is investigated. Sprague-Dawley rats were exposed to intense noise and their right ears were irradiated with an 808nm diode laser at an output power density of 110 or 165mW/cm(2) for a 30min period for 5 consecutive days. Measurement of the auditory brainstem response revealed an accelerated recovery of auditory function in the groups treated with LLLT compared with the non-treatment group at days 2, 4, 7 and 14 after noise exposure. Morphological observations also revealed a significantly higher outer hair cell survival rate in the LLLT groups. Immunohistochemical analyses for inducible nitric oxide synthase (iNOS) and cleaved caspase-3 were used to examine oxidative stress and apoptosis. Strong immunoreactivities were observed in the inner ear tissues of the non-treatment group, whereas these signals were decreased in the LLLT group at 165mW/cm(2) power density. Our findings suggest that LLLT has cytoprotective effects against NIHL via the inhibition of iNOS expression and apoptosis.

Changes in cochlear blood flow in mice due to loud sound exposure measured with Doppler optical microangiography and laser Doppler flowmetry

In this work we determined the contributions of loud sound exposure (LSE) on cochlear blood flow (CoBF) in an in vivo anesthetized mouse model. A broadband noise system (20 kHz bandwidth) with an intensity of 119 dB SPL, was used for a period of one hour to produce a loud sound stimulus. Two techniques were used to study the changes in blood flow, a Doppler optical microangiography (DOMAG) system; which can measure the blood flow within individual cochlear vessels, and a laser Doppler flowmetry (LDF) system; which averages the blood flow within a volume (a hemisphere of ~1.5 mm radius) of tissue. Both systems determined that the blood flow within the cochlea is reduced due to the LSE stimulation.

Monitoring hypoxia induced changes in cochlear blood flow and hemoglobin concentration using a combined dual-wavelength laser speckle contrast imaging and Doppler optical microangiography system

A synchronized dual-wavelength laser speckle contrast imaging (DWLSCI) system and a Doppler optical microangiography (DOMAG) system was developed to determine several ischemic parameters in the cochlea due to a systemic hypoxic challenge. DWLSCI can obtain two-dimensional data, and was used to determine the relative changes in cochlear blood flow, and change in the concentrations of oxyhemoglobin (HbO), deoxyhemoglobin (Hb) and total hemoglobin (HbT) in mice. DOMAG can obtain three-dimensional data, and was used to determine the changes in cochlear blood flow with single vessel resolution. It was demonstrated that during a hypoxic challenge there was an increase in the concentrations of Hb, a decrease in the concentrations of HbO and cochlear blood flow, and a slight decrease in the concentration of HbT. Also, the rate of change in the concentrations of Hb and HbO was quantified during and after the hypoxic challenge. The ability to simultaneously measure these ischemic parameters with high spatio-temporal resolution will allow the detailed quantitative analysis of several hearing disorders, and will be useful for diagnosing and developing treatments.

Involvement of nitric oxide generation in noise-induced temporary threshold shift in guinea pigs

The present study explored the role of endogenous nitric oxide (NO) in the temporary threshold shift caused by acoustic trauma. Guinea pigs were exposed to broadband white noise at a level of 105+/-2dB sound pressure level (SPL) for 10min, causing a temporary threshold shift (TTS). The guinea pigs were divided into six groups (N-1 to N-6) according to survival days after noise exposure (0, 1, 2, 3, 7, 28days). Auditory brainstem responses (ABR) were recorded before noise exposure, immediately after noise exposure and before sacrifice. Immediately after animals were sacrificed, the stria vascularis and the spiral ligament of the lateral wall of each individual cochlea were harvest as a unit and prepared for assay of NO. There was a significant correlation (P<0.001) between the NO concentration and final ABR threshold in the noise exposure groups. But the return of ABR threshold to pre-noise-exposed level is early than that of NO concentration. An average 16.2dB threshold shift was found immediately after noise exposure. The threshold returned to the pre-noise-exposed level on the second post-exposure day. Comparing to unexposed control animals, the NO concentration increased nearly threefold immediately following noise exposure and decreased to twofold when the hearing threshold had returned to the pre-noise-exposed level. On the seventh post-exposure day the NO concentration was not different from that in unexposed control animals. Those findings indicate that endogenous NO is generated in the noise-induced temporal threshold shift and its concentration is correlated with the hearing loss.

Effects of industrial noise exposure on distortion product otoacoustic emissions (DPOAEs) and hair cell loss of the cochlea--long term experiments in awake guinea pigs

Distortion product otoacoustic emissions (DPOAEs), a sensitive detector of outer hair cell (OHC) function, cochlear microphonics (CM), and hair cell loss have been monitored in 12 awake guinea pigs before and after 2 h exposure to specific, played-back industrial noise (105 dB SPL maximal intensity). All animals had stable DPOAE levels before noise exposure. In the first hours after noise exposure DPOAE levels were reduced significantly. In about 70% a partial recovery of the DPOAEs was found within 4 months after noise exposure. In 16% of the investigated ears no recovery of DPOAEs was observed. However, in a few ears increased DPOAEs were observed after noise exposure. Exposure to industrial noise caused both morphological changes in the middle turns of the cochlea and electrophysiological changes in the middle frequency range. A close correlation existed between reduced DPOAE levels, loss in CM potentials, and area of damaged or lost OHCs, but not with the numbers of damaged or lost OHCs in the cochlea. It can be concluded that continuous industrial noise causes a damage to OHCs which differs form the damage caused by impulse noise.

Glutathione limits noise-induced hearing loss

The generation of reactive oxygen species (ROS) is thought to be part of the mechanism underlying noise-induced hearing loss (NIHL). Glutathione (GSH) is an important cellular antioxidant that limits cell damage by ROS. In this study, we investigated the effectiveness of a GSH supplement to protect GSH-deficient animals from NIHL. Pigmented guinea pigs were exposed to a 4 kHz octave band noise, 115 dB SPL, for 5 h. Group 1 had a normal diet, while groups 2, 3 and 4 were fed a 7% low protein diet (leading to lowered tissue levels of GSH) for 10 days prior to noise exposure. One hour before, immediately after and 5 h after noise exposure, subjects received either an intraperitoneal injection of 5 ml/kg body weight of 0.9% NaCl (groups 1 and 2), 0.4 M glutathione monoethyl ester (GSHE; group 3) or 0.8 M GSHE (group 4). Auditory thresholds were measured by evoked brain stem response at 2, 4, 8, 12, 16 and 20 kHz before and after noise exposure. Ten days post exposure, group 1 showed noise-induced threshold shifts of approximately 20 dB at 2, 16 and 20 kHz and 35 to 40 dB at other frequencies. Threshold shifts in group 2 were significantly greater than baseline at 2, 4, 16 and 20 kHz. GSHE supplementation in a dose-dependent fashion attenuated the threshold shifts in the low protein diet animals. Hair cell loss, as evaluated with cytocochleograms, was consistent with the auditory-evoked brainstem response results. Group 2 exhibited significantly more hair cell loss than any of the other groups; hair cell loss in group 3 was similar to that seen in group 1; group 4 showed less loss than group 1. These results indicate that GSH is a significant factor in limiting noise-induced cochlear damage. This is compatible with the notion that ROS generation plays a role in NIHL and that antioxidant treatment may be an effective prophylactic intervention.

Mechanisms of alterations in the microcirculation of the cochlea

Labyrinthine function is tightly coupled to proper homeostasis. This includes appropriate blood flow that is under strict autoregulatory control. Perturbations in labyrinthine microcirculation can lead to significant cochlear and vestibular dysfunction. The etiology of many otologic disorders, including sudden sensorineural hearing loss, presbyacusis, noise-induced hearing loss, and certain vestibulopathies, are suspected of being related to alterations in blood flow. Some of the mechanisms responsible for hypoperfusion and possibly ischemia, within the cochlea, are addressed, with emphasis on the possibility that both noise and age contribute to localized low blood-flow states and stasis. This reduction in blood supply to the cochlea is likely, in part, responsible for reduced auditory sensitivity associated with chronic noise exposure and aging.

Sarthran preserves cochlear microcirculation and reduces temporary threshold shifts after noise exposure

The cause of noise-induced hearing loss remains unclear despite years of both epidemiologic and experimental investigation. Among the many possible pathophysiologic mechanisms that may contribute to noise-induced temporary or permanent threshold shifts are insufficiencies in cochlear blood flow. Although the literature is inconsistent, several histologic and physiologic studies demonstrate signs of reduced circulation in the cochlea after noise exposure. Recent studies using computer-enhanced intravital microscopy complement these earlier findings. Evidence suggests that these microcirculatory events are mediated in part by several circulating factors, including the potent vasoactive peptide angiotensin. This study investigated this possibility by pretreating with the angiotensin receptor antagonist sarthran during noise exposure and examining both cochlear microcirculation and auditory sensitivity. The results of these experiments show noise-induced ischemia in the lateral wall of the cochlea and temporary threshold shifts. Treatment with sarthran prevented this noise-induced microcirculatory ischemia and preserved auditory sensitivity at the low frequencies tested. These findings support a role for the angiotensinergic system during noise exposure and suggest that preservation of cochlear blood flow is functionally related to auditory sensitivity.

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.

Pentoxifylline maintains cochlear microcirculation and attenuates temporary threshold shifts following acoustic overstimulation

The etiology of noise-induced hearing loss is poorly understood despite years of clinical experience and experimental investigations. One potential mechanism which may contribute to noise-induced temporary threshold shifts (TTS) are vascular pathologies in the microcirculation of the cochlea. Several studies have demonstrated histologic evidence of reduced cochlear blood flow following noise exposure. Recent studies utilizing intravital microscopy (IVM) complement these histologic studies and furthermore demonstrate localized ischemia during noise exposure. The purpose of the current study was to attempt to maintain cochlear blood flow during noise exposure by treating with pentoxifylline, a xanthine derivative which promotes blood flow in capillary beds. The possibility that preserved cochlear microcirculation with pentoxifylline treatment attenuates noise-induced TTS was also examined in this study. The results show treatment with pentoxifylline maintains cochlear microcirculation as assessed by continuous red blood cell movement through capillaries. Pentoxifylline treatment did not prevent vasoconstriction or increased permeability often observed in the cochlear microvasculature during noise. Treatment with this drug reduced noise-induced TTS.

The protective effects of allopurinol and superoxide dismutase on noise-induced cochlear damage

Several studies have demonstrated that noise exposure may result in local vasoconstriction of cochlear vessels. The subsequent decrease in cochlear blood flow may lead to hypoxia and predispose to the formation of free oxygen radicals (FORs). If hypoxia occurs in response to noise exposure, then drugs that scavenge or block the formation of FORs should protect the cochlea from damage resulting from hypoxic or ischemic events as well as noise trauma. Rats were exposed to 60 hours of continuous broad-band noise (90 dB SPL) and treated with superoxide dismutase-polyethylene glycol (SOD-PEG), allopurinol, or a control vehicle. Exposure to noise resulted in significant threshold shifts at each frequency tested (3, 8, 12, and 18 kHz) as measured by tone burst-evoked compound action potentials and cochlear microphonics recorded from the round window. Both of these thresholds in drug-treated animals were attenuated compared with animals exposed to noise alone. These findings show that SOD-PEG and allopurinol may preserve cochlear sensitivity associated with noise exposure. This suggests that noise-induced damage to the cochlea may be related to the activity of FORs.

Intensity-related changes in cochlear blood flow in the guinea pig during and following acoustic exposure

This study examined the effects of acoustic exposure at different intensities on cochlear blood flow (CBF) using laser Doppler flowmetry. CBF was measured in anesthetized guinea pigs exposed to either a 10 kHz pure tone at 125, 105, or 90 dB SPL, or wide-band noise at 85 dB SPL for 1 h. Mean arterial blood pressure and heart rate were recorded continuously. Arterial acid-base status, cochlear temperature, cochlear microphonics (CM), and compound action potentials (CAP) were measured before and after exposure. There was a small, but significant, steady decline in basal CBF after 40 min loud sound exposure (125 dB SPL), but no change in basal CBF occurred with the lower intensities (85-105 dB SPL). In contrast, there was a significant increase in apical CBF after 1 h exposure to either moderate wideband noise (85 dB SPL) or a 10 kHz tone at 105 dB SPL. These changes persisted during a 20-min post-exposure period. In most cases, the cochlear temperature and cardiorespiratory variables monitored remained unchanged during and after the exposures as compared to the controls. CM and CAP amplitudes showed extensive losses after acoustic overstimulation (125 dB SPL), but no permanent changes were found at the lower intensities used. The present findings confirm the occurrence of intensity-related effects of acoustic exposure on the cochlear microcirculation.

Intensity-dependent changes in oxygenation of cochlear perilymph during acoustic exposure

This study examined the effects of acoustic exposure at different intensities on local oxygenation of the cochlea. The oxygen partial pressure (pO2) of perilymph in the basal scala tympani was measured polarographically in anesthetized guinea pigs exposed to either wide-band noise at 85 dB SPL or a 10 kHz pure tone at 90, 105, or 125 dB SPL for 1 h. Cochlear temperature, heart rate, arterial blood pressure and acid-base status were monitored. The cochlear microphonics (CM) and compound action potentials (CAP) were recorded before and after exposure. There were clear intensity-dependent differences in the effect of acoustic exposure on perilymphatic oxygenation. Moderate exposure intensities (85-90 dB SPL) were found to increase the pO2 by an average of about 20% of the initial level. In contrast, high intensity acoustic exposure (125 dB SPL) resulted in a mean decrease of about 20%. These changes persisted within a subsequent 30-min post-exposure period. There was no significant change in cochlear temperature and cardiorespiratory variables during and after any of the exposures as compared to the controls. CM and CAP amplitudes showed an extensive loss after acoustic overstimulation (125 dB SPL), but no permanent change with lower exposure intensities. These findings suggest that intracochlear oxygenation plays an important role in inner ear physiology during acoustic stimulation.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

Laser Doppler measurements of cochlear blood flow during loud sound exposure in the guinea pig

This investigation examined the effects of loud sound of different frequencies and intensities on cochlear blood flow as measured by the laser Doppler flowmeter. Cochlear blood flow was measured in anesthetized guinea pigs during a 1 h exposure to either a 2, 4, or 12 kHz pure tone or high-pass noise (10-40 kHz) at 90, 103, or 110 dB SPL. Cochlear function was assessed using the compound action potential audiogram before and after exposure. There was no change in blood flow in the second turn with a 2, 4, or 12 kHz tone but there was a significant (P less than 0.05) decline in flow in the first cochlear turn at the end of either the 12 kHz tone or high-pass noise exposure at 103 and 110 dB SPL. There were elevations in the thresholds of the cochlear compound action potential after all but the 90 dB exposures to 12 kHz or high-pass noise. No such changes were observed in blood flow or electrophysiology in control animals. These findings demonstrate that there is a small but significant decline in cochlear blood flow with high intensity sound exposure. However, the relationship between this change in blood flow and the development of cochlear damage is unclear.

Effects of noise and quinine on the vessels of the stria vascularis: an image analysis study

Surface preparations of the stria vascularis from guinea pigs exposed to wide-band noise or intoxicated with quinine monohydrochloride dihydrate were studied by light microscopy and computerized image analysis in order to evaluate quantitatively the effects of these agents on two characteristics of the strial vasculature: vascular density and erythrocyte distribution. An image analyzer was used to measure the area of strial vessel lumen and erythrocyte distribution as a fraction of the total area of strial tissue under observation. The results demonstrate that changes in the strial vessels do occur in guinea pigs exposed to noise or given large doses of quinine. Localized vessel narrowings caused by swollen endothelial cells and possibly by contraction of pericytes were found in both experimental groups, but there was no apparent tonotopical relationship between these effects and the reduction in cochlear potentials. A significant reduction in the number of erythrocytes was found in all turns of the cochlea in both experimental groups. Although a significant difference in vascular density was found among turns of the cochlea in both experimental and control animals, there was no widespread change in vascular density as a result of either noise exposure or quinine treatment.

Control mechanisms of inner ear microcirculation

Some disease processes, drugs, and noise are said to produce vascular disorders of the inner ear, and in some instances changes in the capillaries have been observed during histologic examination of the temporal bones. Consequently the ability to control the circulation is important because of its therapeutic implications. This brief review points out the areas of conflicting experimental evidence and some of the factors that may be responsible for the variations.