Does loud sound influence the intracochlear oxygen tension?

Two-photon microscopy allows imaging and characterization of cochlear microvasculature in vivo

Impairment of cochlear blood flow has been discussed as factor in the pathophysiology of various inner ear disorders. However, the microscopic study of cochlear microcirculation is limited due to small scale and anatomical constraints. Here, two-photon fluorescence microscopy is applied to visualize cochlear microvessels. Guinea pigs were injected with Fluorescein isothiocyanate- or Texas red-dextrane as plasma marker. Intravital microscopy was performed in four animals and explanted cochleae from four animals were studied. The vascular architecture of the cochlea was visualized up to a depth of 90.0±22.7 μm. Imaging yielded a mean contrast-to-noise ratio (CNR) of 3.3±1.7. Mean diameter in vivo was 16.5±6.0 μm for arterioles and 8.0±2.4 μm for capillaries. In explanted cochleae, the diameter of radiating arterioles and capillaries was measured with 12.2±1.6 μm and 6.6±1.0 μm, respectively. The difference between capillaries and arterioles was statistically significant in both experimental setups (P<0.001 and P=0.022, two-way ANOVA). Measured vessel diameters in vivo and ex vivo were in agreement with published data. We conclude that two-photon fluorescence microscopy allows the investigation of cochlear microvessels and is potentially a valuable tool for inner ear research.

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.

Volumetric Imaging of Blood Flow within Cochlea in Gerbil in vivo

Changes in blood flow to the inner ear are thought to influence a number of cochlear diseases, including noise-induced hearing loss, sudden hearing loss, and Meniere's disease. Advances have been made in the areas of vital microscopic studies of micro-circulation, and the laser Doppler flowmetry. But none of these techniques can provide in vivo three-dimensional (3-D) mapping of microvascular perfusion within the cochlea. To overcome this limitation we have developed and used a method of optical microangiography (OMAG) that can generate 3-D angiograms within millimeter of tissue depths by analyzing the endogenous optical scattering signal obtained from an illuminated sample. We used OMAG to visualize the cochlear microcirculation of adult living gerbil through the intact cochlea, which would be difficult, if not impossible, by use of any other current techniques.

An animal model for the analysis of cochlear blood flow [corrected] disturbance and hearing threshold in vivo

Impairment of cochlear blood flow (CBF) is considered to be important in inner ear pathology. However, direct measurement of CBF is difficult and has not been investigated in combination with hearing function. Six guinea pigs were used to show feasibility of an animal model for the analysis of cochlear microcirculation by intravital microscopy in combination with investigation of the hearing threshold by brainstem response audiometry (ABR). By the application of sodium nitroprusside (SNP), CBF was increased over 30 min. Reproducibility of measurements was shown by retest measurements. Mean baseline velocity of CBF was 109 +/- 19 mum/s. Vessel diameters had a mean value of 9.4 +/- 2.7 mum. Mean hearing threshold was 19 +/- 6 dB. In response to SNP, CBF velocity increased significantly to 161 +/- 26 mum/s. Mean arterial pressure decreased significantly to 36 +/- 11 mmHg. After the end of the application, CBF velocity recovered to a minimum of 123 +/- 17 microm/s. Within the retest, CBF velocity significantly increased to a maximum of 160 +/- 31 microm/s. Second recovery of CBF velocity was 125 +/- 14 mum/s. Within the second retest, CBF increased significantly to 157 +/- 25 microm/s. ABR thresholds did not change significantly. The increase in blood flow velocity occurred in spite of substantial hypotension as induced by a vasodilator. This may explain the fact that ABR threshold remained unchanged reflecting a maintained blood supply in this part of the brain. This technique can be used to evaluate effects of treatments aimed at cochlear microcirculation in inner ear pathologies.

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.

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.

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.

The influence of loud sound on red blood cell velocity and blood vessel diameter in the cochlea

Using intravital microscopy, we observed both decreases in red blood cell velocity and possible vasoconstriction in stria vascularis capillaries of the rat cochlea in response to loud sound (Quirk et al., 1991). However, our observation of vasoconstriction was subject to error in measurements from the two dimensional images obtained with our silicon intensified (SIT) camera due to the influence of focus causing image blur. The purpose of the current study was to apply an extended focus microscopy technique to obtain quantitative assessment of vessel diameter changes (Avinash et al., 1992), as well as to extend these studies to the guinea pig model. Broad-band sound stimulation at intensities of 84 dB SPL and 110 dB SPL were used. The results show that loud sound induces a sequence of changes in cochlear blood flow. Stimulation with 110 dB SPL resulted in a mean increase (maximum = 27%) in red blood cell velocity for the first 20 min of exposure followed by a gradual decrease below baseline (minimum = -12%) prior to termination of the signal. This velocity decrease and subsequent recovery were associated with significant changes in vessel diameters of selected and measured capillaries. In contrast, the 84 dB SPL stimulus caused an increase in red blood cell velocity (maximum = 20%) and vessel diameter (mean = 7.5) during the stimulation period. No recovery was observed during the 10 min observation period following sound. Several possible mechanisms responsible for these changes are discussed.

Measurements of perilymphatic oxygen tension in guinea pigs exposed to loud sound

Using different types of custom-made oxygen-sensitive microelectrodes, the perilymphatic oxygen partial pressure (PO2) was determined in anesthetized guinea pigs. Cochlear temperature, heart rate, and arterial blood pressure and acid-base status were monitored. The PO2 in the basal scala tympani perilymph (200 microns below the round window membrane) was found to be 53 +/- 17 mmHg (mean +/- SD) in 33 normal animals. In 11 guinea pigs exposed to loud sound for 15 min (10 kHz pure tone, 125 dB SPL) there was on average a continuous decline in the perilymphatic PO2, which was significant only 30 min post-exposure. A considerable variation in response was found in individual animals. Mean arterial blood pressures showed a slightly increasing time course, while heart rates did not change significantly during the whole period of the experiment. Arterial acid-base status and PO2 values remained within normal limits and did not change significantly. Cochlear microphonics and compound action potentials were substantially decreased after acoustic overstimulation. The results are discussed with due consideration of sources of error.

Noise-induced changes in red blood cell velocity in lateral wall vessels of the rat cochlea

The effects of loud sound on the microvasculature of the cochlea are not well characterized or understood. Morphological changes in the stria vascularis and changes in blood flow are known to occur during or following sound stimulation, however, the effects on cochlear blood flow appear to be complex. Studies have shown that noise exposure may produce increases in blood flow, decreases in blood flow, or no measureable change in blood flow. These inconsistent results probably reflect the various noise exposure parameters, the animal model used, and could be a function of the specific procedures utilized to assess blood flow changes. The purpose of the current study was to investigate the effects of one specific class of sound exposure (high intensity noise) on red blood cell velocity in the capillaries of the second turn of the rat cochlea using intravital microscopy. This class of sound exposure was selected in order to attempt a confirmation of previous findings of increased blood flow (Perlman and Kimura, 1962) using the quantitative technique of red blood cell velocity measurement. Following determination of pre-exposure red blood cell velocities in capillaries of the rat cochlea second turn, animals were exposed to 133 dB or 110 dB broad-band noise for ten minutes. The red blood cell velocity was recorded continuously during the exposure. Exposure to both sound intensities disrupted stable and orderly baseline flow patterns and resulted in overall intensity-dependent increases in red blood cell velocity.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoregulation of inner ear blood flow in normal and hydropic guinea pigs

Inner ear and brainstem blood circulation was measured by three different techniques: the laser Doppler (LD), hydrogen clearance (HC) and oxygen tension (PO2) measurements, while systemic blood pressure (BP) was modulated by norepinephrine infusion or removal of whole blood. Results were as follows: (i) The blood flow (BF) change determined by LD correlated well with that measured by HC and PO2 techniques; (ii) BF in the brainstem was maintained constant in the BP range of 35 to 80 mmHg; however, inner ear BF showed a poor autoregulatory function relative to the change of systemic BP; (iii) although the change of BF was similar for cochlea and semicircular canal the amount of PO2 decrease for lowered BP was significantly less in the cochlea than in the canal; (iv) in guinea pigs with unilaterally obliterated endolymphatic sac and duct, the decrease in cochlear BF was larger on the operated side than on the intact side. This suggests that the autoregulatory function for BF is impaired in the hydropic ear.

Noise and hypoxia induced temporary threshold shifts in rats studied by ABR

Rats were exposed for 2 h either to 115 dB SPL noise, to 5% oxygen in nitrogen (hypoxia) or to both hypoxia and noise. Auditory nerve-brainstem evoked responses (ABR) to 80 dB HL clicks and threshold were recorded prior to exposure, immediately (5-10 min) after the exposure, 2 h after and 2 weeks after the exposure. The findings in each experimental animal were compared to those in the control (non-exposed) group and to those in the other groups. Thresholds were elevated in each of the experimental groups, but these were temporary threshold shifts, since 2 weeks following the exposure, threshold had returned to normal. Latencies (wave I and the IV-I interpeak latency difference- (IPLD] were prolonged in the groups exposed to hypoxia (hypoxia alone and hypoxia + noise). These results are discussed in view of possible mechanisms of these noise and hypoxia induced temporary threshold shifts.

Measurement of cochlear blood flow: intravital fluorescence microscopy

A technique is described for directly observing in vivo cochlear microvasculature in the gerbil for physiologic and experimentally induced changes in vessel diameter and blood flow velocity. Measurements are made from computer processed video images of surgically exposed microvessels. These images are obtained using intravital fluorescence microscopy (IFM) with epi-illumination. The Mongolian gerbil is an ideal animal model for circulatory studies of the inner ear. It has a stable heart rate and blood pressure under urethane/alpha-chloralose anesthesia and its cochlea is surgically accessible. A window is created over the feeding artery (anterior inferior cerebellar artery) and over the stria vascularis of the second turn of the cochlea, atraumatically exposing radiating arterioles and strial capillaries. Our system of IFM provides images that are videorecorded, digitally analyzed with a computer image processor, and enhanced according to the type of measurement desired. Velocity measurements are obtained by tracking plasma gaps or single fluorescent labeled red blood cells through successive frames of the videorecorded images. This experimental technique allows us to analyze circulatory responsiveness to a variety of vasoactive drugs administered regionally to the cochlea in concentrations not affecting systemic circulation.

A comparison of laser Doppler and intravital microscopic measures of cochlear blood flow

Many inner ear disorders may be caused by alterations in cochlear blood flow (CBF). However, each measurement technique used to monitor CBF has limitations in examining the relationship between otopathologic states and blood flow. This study investigates laser Doppler flowmetry (LDF) and its fundamental drawback: the unknown relationship of LDF output to actual CBF. LDF readings are directly compared with concurrent intravital microscopy (IVM) measures of erythrocyte velocity in the lateral wall of the guinea pig cochlea. Positive end expiratory pressure, spontaneous respiration of 5% and 10% carbon dioxide, phenylephrine, and direct electrical stimulation of the cochlea were used to manipulate CBF. High, positive correlations were found between simultaneous LDF and IVM measurements of CBF. In addition, the study demonstrated that current microdissection techniques used to perform IVM do not cause changes in CBF. IVM measurements of CBF are a more sensitive indicator of CBF changes than are LDF measures. Despite the high correlation between measurement techniques within a single manipulation, simultaneous LDF and IVM measurements differed between manipulations. This may reflect regional changes in CBF affected by these manipulations and differences in the sampled vascular beds contributing to these two measures. It is unlikely that a single calibration factor can be defined that would allow the conversion of LDF output to actual units of blood flow across different manipulations used to alter CBF.

Alterations in oxygenation of cochlear endolymph during loud sound exposure

The oxygen tension (pO2) of endolymph of the guinea pig cochlea was measured during exposure to loud sound (12 kHz or high-pass noise; 110 dB SPL up to 1 h duration). A small, but significant, steady decline in mean pO2 was observed after both pure tone and high-pass noise exposure. The extent of the change in pO2 varied from 0-50% in individual animals, compared with unexposed control animals. All exposed animals had an extensive loss of compound action potential (CAP) thresholds at frequencies of 8-30 kHz. However, there was no relationship between the extent of the change in pO2 of endolymph and CAP threshold loss.

In vivo capillary diameters in the stria vascularis and spiral ligament of the guinea pig cochlea

Blood microvessels in the membraneous lateral wall of the cochlea were examined using intravital microscopic techniques. A video analysis system made serial diameter measurements at 1 micron intervals along the length of selected vessel segments during four experimental conditions. For each vessel segment, the serial measurements were statistically converted into a single diameter estimate, such that the flow resistance in a uniform vessel of this diameter would equal the resistance of the real non-uniform vessel. Nominal vessel diameters found (spiral ligament: 9-12 micron; stria vascularis: 12-16 micron) were nearly double those reported earlier in histological observations (Axelsson, 1968). During stimulation the largest diameter change seen was a 3.7% dilation (about 0.5 micron) in response to breathing 5% CO2 in oxygen. Theoretically, this change could reduce vascular fluid resistance by 16%, nearly enough to explain the observed flow increase of 20%. No diameter changes occurred for 5% CO2 in air despite a 50% flow increase, nor for air pressure pulses applied at the tympanic membrane. Round window electrical stimulation of 50 microA also produced dilation (less than 2.5%), but higher current levels were ineffective. In general, blood flow increases seen in this study could not adequately be attributed to the small lateral wall vessel diameter increases nor systemic causes, suggesting that lateral wall blood flow in these instances is dependent on control within the modiolus.

Recent advances in cochlear blood flow measurements

Changes in blood flow to the inner ear have been thought to influence or underlie a number of cochlear diseases, including some forms of noise-induced hearing loss, sudden hearing loss, and Meniere's disease. Recently, important advances have been made in two technologies for the study of cochlear blood flow. The first is in the area of vital microscopic studies of cochlear microcirculation, and the second is based on the introduction of laser technology in the form of laser Doppler flowmetry. In this report, measurements are given of changes in cochlear circulation caused by carbon dioxide breathing, intravenous phenylephrine injection, systemic hemodilution, positive end expiratory pressure, and direct electrical stimulation of the cochlea. From these changes, we observe that cochlear blood circulation responds to systemic blood pressure alterations and is subject to local flow control mechanisms. Linearity and speed of response of the laser Doppler instrumentation also are shown. These advances show promise for contributing to our knowledge of control mechanisms of inner ear blood flow and for revealing the influence of various pharmacologic agents of potential clinical value.

Cochlear blood flow in noise-damaged ears

Cochlear blood flow was measured in rats with a known noise-induced hearing loss, using the microsphere technique. The animals were exposed to simulated industrial noise for 3 months. The rats were divided into four groups: young and old; normotensive (N) and spontaneously hypertensive (SH). The mean values of the cochlear blood flows were compared with those of rats from matched groups not exposed to noise. The mean cochlear blood flow in the noise-exposed groups was lower than in the non-exposed groups. The decrease in cochlear blood flow was significant for all groups except young SH rats and was more pronounced in the older age group. Cochleas from the groups of old N and SH rats were investigated in the scanning electron microscope. Extensive changes were far more common in SH than in N rats.

Effects of carbon monoxide on cochlear electrophysiology and blood flow

The belief that the cochlea is particularly vulnerable to a reduction in oxygen availability comes predominantly from studies reporting the disruption of electrophysiological measures, such as the compound action potential, endocochlear potential, inner hair cell intracellular potentials or afferent nerve fiber responses by asphyxiation. Because hypoxia has frequently been suggested as an underlying mechanism by which many ototoxic agents produce injury, and because such agents are not likely to completely disrupt oxygen delivery, we investigated the effects of graded hypoxia (using doses of carbon monoxide) on cochlear blood flow, the compound action potential (CAP) and the cochlear microphonic (CM). High doses of carbon monoxide injected intra-peritoneally yielded reversible loss of the CAP sensitivity for high frequency tone bursts, the extent of which was dose dependent. The loss was observed first at the highest frequency tested (50 kHz) and as carboxyhemoglobin levels increased, contiguous lower frequencies were influenced. Recovery progressed from low to high frequencies as carboxyhemoglobin levels declined. Carbon monoxide administration also produced a dose dependent elevation in the cochlear blood flow measured by a laser Doppler flow monitor. The data suggest that carbon monoxide administration disrupts cochlear function only under extremely severe exposure conditions. An elevation in cochlear blood flow may well serve as a protective mechanism which maintains cochlear function in the face of declining blood oxygen carrying capacity and delivery. While the site of action of carbon monoxide in the cochlea is uncertain, the data clearly indicate that elements involved in the generation of the CAP for high frequency tones are particularly vulnerable.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbon monoxide exposure potentiates high-frequency auditory threshold shifts induced by noise

Hypoxia has long been hypothesized to play a role in noise-induced hearing loss, and the disruption of auditory function by asphyxiation has been repeatedly demonstrated. Recent data, however, suggest that the cochlea is resistant to less extreme hypoxic events. The current report describes the combined effects of noise and hypoxia on a measure of auditory function, in an effort to clarify the role of hypoxia in hearing loss. Exposure to 1200 parts per million of carbon monoxide in air for 90 min preceding and 120 min concurrent with exposure to a broad-band noise at 110 dBA produced high-frequency threshold shifts of greater magnitude than those produced by exposure to noise alone. An equivalent carbon monoxide exposure in a 'quiet' environment did not produce any change in auditory detection thresholds. The potentiation of noise-induced threshold shifts by carbon monoxide provides additional support for the hypothesized role of metabolic exhaustion or bloodflow impairment in noise-induced hearing loss. It also suggests a possible interpretation of clinical findings of auditory impairment associated with carbon monoxide exposure.

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.

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.

The metabolic reaction of the cochlea to unphysiological noise exposure

Guinea pigs were exposed to continuous white noise of 120 dB SPL. The performance included a recording of CM, pO2, and metabolites in the perfusate of the perilymphatic spaces. The metabolic reactions observed following the exposure are limited. No evidence of an overstimulation or of an insufficiency of the energy metabolism in the cochlea could be found. The results suggest the metabolic processes to be secondary to the sound-dependent damage of the organ of Corti after irreversible mechanical destructions in the hair cells.

Reduced neuronal size and dendritic length in the medial superior olivary nucleus of albino rabbits

We have previously demonstrated that circumscribed structural and functional abnormalities exist in the brainstem auditory system of albino cats. Anomalies in the auditory brainstem evoked response of albino cats were correlated with anatomical defects in the medial superior olivary nucleus (MSO) of the same animals. To examine whether a similar syndrome is present in other albino mammals, we studied the MSO of albino and pigmented rabbits using both Nissl-stained and Golgi-impregnated material. Neurons in the MSO of the albinos were significantly smaller (24%) than those in the pigmented rabbits and there was no overlap in the size distributions between the two groups. Neurons in the abducens nucleus of the albinos were also 14% smaller than in the pigmented rabbits, but this difference was not statistically reliable. The broad overlap in the distributions of neuronal size in the abducens nucleus between groups indicated that not all cells in the albino brainstem are significantly smaller than normal. In the Golgi-impregnated material, the mean total dendritic length for the 'marginal' cell type in the MSO was 39% shorter in albinos than in the pigmented animals. The branching density of dendrites was also significantly reduced in the albinos. Mean total dendritic length for cerebellar granule cells was a statistically insignificant 6% longer in the albinos, demonstrating that dendritic structure is not uniformly affected in all regions of the albino brain. The demonstration of similar anatomical differences in albino rabbits and cats indicates that whatever process produces these effects is not species-specific and may be common to the albinos of other mammalian species. The evidence that the amount of cochlear melanin may be related to differences in auditory function further suggests that the differences in the MSO of the albinos may ultimately be related to absence of inner ear pigmentation and not to other gene effects.

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.

Potential role of angiotensin II in noise-induced increases in inner ear blood flow

Guinea pigs were exposed to 120 dB white noise for 30 min and evidenced a four-fold elevation in plasma concentration of the potent vasoconstricting hormone angiotensin II (AII). Anesthetized animals received intra-arterial injections of AII at doses that approximated the endogenous levels measured following noise exposure. A marked decrease in skin blood flow was observed with a concomitant increase in cochlear blood flow as measured by laser Doppler flowmeters. Increased cochlear blood flow appeared to be secondary to the increases in systemic blood pressure induced by AII. These findings suggest that cochlear blood flow may increase during periods of intense noise exposure.

The laser Doppler: a non-invasive measure of cochlear blood flow

The present investigation demonstrates the utility of the laser Doppler flowmeter to provide a measure of cochlear blood flow dynamics. Cochlear and cutaneous blood flow were compared with arterial blood pressure during and following exposure to Angiotensin II, 5% carbon monoxide, 100% oxygen, mannitol, and saline. The observations indicate that: 1) cochlear blood flow generally parallels cutaneous blood flow; however, 2) when cutaneous beds vasoconstrict (e.g., AII, alpha-agonists), cochlear blood flow parallels blood pressure; and, 3) under the influence of agents that affect peripheral and central circulation (5% CO, 100% O2), cochlear blood flow may dissociate from cutaneous blood flow and blood pressure. The implications of these findings are discussed in terms of local control mechanisms that may be involved in the inner ear vasculature.

Laser Doppler measurements of cochlear blood flow

Blood flow to the inner ear was studied with a laser Doppler system in the acute guinea pig. Flow was measured through the lateral wall of the basal turn. Changes in simultaneous measures of cochlear and skin flow with rebreathing, epinephrine, phentolamine and terminal bleeding were studied. Cochlear blood flow followed skin blood flow in most cases; where it did not, the change was in an expected direction. The laser Doppler flowmeter appears to provide a direct, dynamic and linear measure of inner ear blood flow.

The effect of cervical sympathectomy on cochlear electrophysiology

The purpose of this study was to examine the electrophysiologic responses to sound from guinea pig cochleas, 3--6 days after a unilateral cervical sympathectomy. After recovery from surgery the guinea pigs developed a Horner's syndrome on the sympathectomized side. Some days later their bullas were opened and electrodes were placed bilaterally on the round windows. Most of the sympathectomized cochleas showed signs of decreased sensitivity to sound. They had a smaller dynamic range for the click- and tone-burst evoked compound action potential (CAP) compared with the non-sympathectomized cochleas. The threshold sound levels for the CAPs and the sound levels to produce 1 muV of cochlear microphonic potentials were unaffected. In other animals with chronic implanted electrodes, the same electrical responses were measured with only light sedation before and after sympathectomy and showed the same results. In additional animals the crossed olivocochlear bundle (COCB) was electrically stimulated and a similar inhibition of the CAP was registered on both the intact and the sympathectomized side. The results suggest that the sympathetic nerves to the cochlea, coming from the ipsilateral cervical ganglion and ending near the habenula perforata, may be important for the sound perception as they influence the CAP.