Acoustic stimulation alters deoxyglucose uptake in the mouse cochlea and inferior colliculus

Impact of noise exposure on the circadian clock in the auditory system

Circadian rhythms control the timing of all bodily functions, and misalignment in the rhythms can cause various diseases. Moreover, circadian rhythms are highly conserved and are regulated by a transcriptional-translational feedback loop of circadian genes that has a periodicity of approximately 24 h. The cochlea and the inferior colliculus (IC) have been shown to possess an autonomous and self-sustained circadian system as demonstrated by recording, in real time, the bioluminescence from PERIOD2::LUCIFERASE (PER2::LUC) mice. The cochlea and IC both express the core clock genes, Per1, Per2, Bmal1, and Rev-Erbα, where RNA abundance is rhythmically distributed with a 24 h cycle. Noise exposure alters clock gene expression in the cochlea and the IC after noise stimulation, although in different ways. These findings highlight the importance of circadian responses in the cochlea and the IC and emphasize the importance of circadian mechanisms for understanding the differences in central and peripheral auditory function and the subsequent molecular changes that occur after daytime (inactive phase) or nighttime (active phase) noise trauma.

Evaluation and Management of Patients with Diabetes and Hearing Loss

Diabetes mellitus is a significant risk factor for acquired hearing loss and tinnitus. Persons with diabetes (PWD) may present with hearing loss symptoms earlier in life than those without diabetes. Furthermore, diabetes may exacerbate risk for hearing loss related to noise exposure and ototoxic drugs. The purpose of this article is to provide recommendations for the prevention, screening, evaluation, and management of hearing loss in PWD.

Impact of Activities OF NA+,K+-Atpase and CA2+-Atpase in the Cochlear Lateral Wall on Recovery from Noise-Induced Temporary Threshold Shift

The present study was designed to investigate the relationship between the noise-induced temporary threshold shift (TTS) and the specific activities of sodium potassium adenosine triphosphatase (Na+,K+-ATPase) and calcium adenosine triphosphatase (Ca2+-ATPase) in the cochlear lateral wall. The specific activities of these enzymes were quantified by microcolorimetric assay. Changes in auditory brain stem response (ABR) thresholds were compared with the quantitative alterations of the specific activities of Na+,K+-ATPase and Ca2+-ATPase in the cochlear lateral wall of guinea pigs with a noise-induced TTS. In the majority of those noise-exposed ears with complete recovery of ABR thresholds, the specific activities of both enzymes returned to at least 70% of the mean specific activity of the control group. Although other factors may be involved, reversible inactivation of Na+,K+-ATPase and Ca2+-ATPase in the cochlear lateral wall may be one component of the TTS. Our present findings could drive further studies on the molecular basis of noise-induced hearing loss.

No longer falling on deaf ears: mechanisms of degeneration and regeneration of cochlear ribbon synapses

Cochlear ribbon synapses are required for the rapid and precise neural transmission of acoustic signals from inner hair cells to the spiral ganglion neurons. Emerging evidence suggests that damage to these synapses represents an important form of cochlear neuropathy that might be highly prevalent in sensorineural hearing loss. In this review, we discuss our current knowledge on how ribbon synapses are damaged by noise and during aging, as well as potential strategies to promote ribbon synapse regeneration for hearing restoration.

Noise-induced time-dependent changes in oxidative stress in the mouse cochlea and attenuation by D-methionine

Oxidative stress in the cochlea is considered to play an important role in noise-induced hearing loss. This study determined changes in superoxide dismutase (SOD), catalase, lipid peroxidation (LPO) and the auditory brainstem response (ABR) in the cochlea of C57BL/6 mice prior to and immediately, 1, 3, 7, 10, 14 and 21 days after noise exposure (4 kHz octave band at the intensity of 110 dB SPL for 4 h). A significant increase in SOD activity immediately and on 1st day after noise exposure, without a concomitant increase in catalase activity suggested a difference in the time dependent changes in the scavenging enzymes, which facilitates the increase in LPO observed on day 7. The ABR indicated significant noise-induced functional deficits which stabilized in 2 weeks with a permanent threshold shift (PTS) of 15 dB at both 4 kHz and 8 kHz. The antioxidant D-methionine (D-Met) reversed the noise-induced changes in LPO levels and enzyme activities. It also significantly reduced the PTS observed on the 14th day from 15 dB to 5 dB for 4 kHz. In summary, the findings indicate that time-dependent alterations in scavenging enzymes facilitate the production of reactive oxygen species and that D-met effectively attenuates noise-induced oxidative stress and the associated functional loss in the mouse cochlea.

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.

Creatine and tempol attenuate noise-induced hearing loss

To define the role of free radical formation and potential energy depletion in noise induced hearing loss (NIHL), we measured the effectiveness of tempol (free radical scavenger) and creatine (enhances cellular energy storage) alone and in combination to attenuate NIHL. Guinea pigs were divided into four treatment groups: controls, 3% creatine diet (2 weeks prior to noise exposure), tempol (3 mM in drinking water 2 weeks prior to exposure), and creatine plus tempol and exposed to 120 dB SPL one-octave band noise centered at 4 kHz for 5 h. The noise-only control group showed frequency-dependent auditory threshold shifts (measured by auditory brainstem response, ABR) of up to 73 dB (16 kHz) on day 1, and up to 50 dB (8 kHz) on day 10. Creatine-treated subjects had significantly smaller ABR threshold shifts on day 1 and on day 10. Tempol alone significantly reduced ABR threshold shifts on day 10 but not on day 1. ABR shifts after combination treatment were similar to those in the creatine group. Hair cell loss on day 10 was equally attenuated by creatine and tempol alone or in combination. Our results indicate that the maintenance of ATP levels is important in attenuating both temporary and permanent NIHL, while the scavenging of free radicals provides protection from permanent NIHL.

Prevention of noise-induced hearing loss with Src-PTK inhibitors

Studies from our lab show that noise exposure initiates cell death by multiple pathways [Nicotera, T.M., Hu, B.H., Henderson, D., 2003. The caspase pathway in noise-induced apoptosis of the chinchilla cochlea. J. Assoc. Res. Otolaryngol. 4, 466-477] therefore, protection against noise may be most effective with a multifaceted approach. The Src protein tyrosine kinase (PTK) signaling cascade may be involved in both metabolic and mechanically induced initiation of apoptosis in sensory cells of the cochlea. The current study compares three Src-PTK inhibitors, KX1-004, KX1-005 and KX1-174 as potential protective drugs for NIHL. Chinchillas were used as subjects. A 30 microl drop of one of the Src inhibitors was placed on the round window membrane of the anesthetized chinchilla; the vehicle (DMSO and buffered saline) alone was placed on the other ear. After the drug application, the middle ear was sutured and the subjects were exposed to noise. Hearing was measured before and several times after the noise exposure and treatment using evoked responses. At 20 days post-exposure, the animals were anesthetized their cochleae extracted and cochleograms were constructed. All three Src inhibitors provided protection from a 4 h, 4 kHz octave band noise at 106 dB. The most effective drug, KX1-004 was further evaluated by repeating the exposure with different doses, as well as, substituting an impulse noise exposure. For all conditions, the results suggest a role for Src-PTK activation in noise-induced hearing loss (NIHL), and that therapeutic intervention with a Src-PTK inhibitor may offer a novel approach in the treatment of NIHL.

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.

Contributions of age, cochlear integrity, and auditory environment to avian cochlear nucleus metabolism

Most commercially raised broiler chickens display progressive cochlear degeneration with age [Hear. Res. 166 (2002) 82]. Recent work examining the effects of age and cochlear degeneration on avian cochlear nucleus (nucleus magnocellularis, NM) metabolism showed that changes in metabolic activity occur with age and cochlear damage [Hear. Res. 175 (2003) 101]. The auditory environment also differed between facilities housing young and adult birds. The relative contributions of age, cochlear degeneration, and auditory environment to these changes in NM metabolism are unknown. Using cytochrome oxidase (CO) histochemistry, NM neuron metabolism is examined in several age groups of birds under varying conditions. When normal cochlear integrity and auditory environment are held constant, CO staining is significantly decreased in adult vs. young birds. When age and auditory environment are held constant, CO staining is significantly decreased in birds with damaged vs. normal cochleae. When age and normal cochlear integrity are held constant, CO staining is significantly decreased in birds living in a quiet vs. noisy environment. All factors examined cause changes in CO staining, which is indicative of NM metabolic activity. Results are discussed in the context of mitochondrial aging, afferent regulation, and auditory deprivation and enrichment.

8-iso-prostaglandin F(2alpha), a product of noise exposure, reduces inner ear blood flow

Noise exposure induces the formation in the cochlea of 8-isoprostaglandin F(2alpha) (8-iso-PGF(2alpha)), a marker for reactive oxygen species [Ohinata et al., 2000a] and a potent vasoconstrictor, raising the possibility that 8-iso-PGF(2alpha) may be responsible for noise-induced reductions in cochlear blood flow (CBF). To test this hypothesis, CBF was assessed in the guinea pig in response to 'local' (via the anterior inferior cerebellar artery) and systemic (i.v.) delivery of 8-iso-PGF(2alpha) using laser Doppler flowmetry. Local 8-iso-PGF(2alpha) induced a clear reduction in CBF. With systemic infusion, vascular conductance (VC), the ratio of CBF to systemic blood pressure, decreased in a dose-dependent manner up to 30%, consistent with an 8-iso-PGF(2alpha)-induced constriction of the cochlear vasculature. Infusion of SQ29548, a specific antagonist of 8-iso-PGF(2alpha), appropriately blocked an 8-iso-PGF(2alpha)-induced CBF response. Similarly, noise-induced changes in CBF and VC were prevented by infusion of SQ29548 during noise exposure or by antioxidant treatment (glutathione monoethyl ester) prior to exposure. Prevention of isoprostane-mediated vasoconstriction may have clinical utility in the protection from noise-induced hearing loss.

Migration of cochlear lateral wall cells

The role of apoptosis and proliferation in maintenance of cochlear lateral wall cells was examined. The methods employed for detection of apoptosis were the Hoechst fluorescence stain and TUNEL (TdT-mediated dUTP-biotin nick-end-labeling) assay, and proliferations were 5-bromo-2'-deoxyuridine (BrdU) incorporation and presence of the proliferating cell nuclear antigen. The incidence of apoptosis in the strial marginal cell was 50% greater (32.9+/-3.7%) than strial intermediate and basal cells but similar to spiral ligament cells. Although division of marginal strial cells was rarely detected, a significant number of proliferating cells in the remaining stria vascularis and spiral ligament were observed. These data implied that replacement of marginal cells arose elsewhere and could be followed by a BrdU-deoxythymidine pulse-chase study. At 2 h post injection, nuclear BrdU in marginal cells was not detected; however, by 24 h post injection, 20-25% of marginal cell nuclei were BrdU-positive. These observations are consistent with the hypothesis that marginal cells were replaced by underlying cells. Cell migration appears to be an important mechanism for preserving the function and structure of the stria vascularis.

Alteration of 6-phosphofructo-1-kinase subunits during neonatal maturation of the rat cochlear cells

During postnatal development of rat cochlear cells and the onset of hearing (10-23 days), the increasing endocochlear potential and energy requirements are largely provided by increased glucose utilization. It is well established that the ability of maturing rat tissues to use glucose is directly related to alteration of 6-phosphofructo-1-kinase (PFK) subunits. To gain insight into the alteration of PFK subunit levels in the cochlea from 6 to 60 days of age, PFK subunit types were measured in sections of paraffin-embedded temporal bone using IgG specific for each type of PFK subunit and quantified by computer image analysis. Although the L-type and C-type subunits did not exhibit statistically significant changes in the cochlear structures during maturation, the levels of M-type subunit in the stria vascularis cells, spiral ligament cell types I, II, and III, outer hair cells, inner hair cells, and support cells significantly increased. Also, the type IV and V spiral ligament fibrocytes during this period did not exhibit significant alterations of the M-type subunit. These data suggest that during neonatal development of the cochlear, the elevated levels of the M-type subunit are associated with increased glucose utilization and the onset of hearing.

Intense noise induces formation of vasoactive lipid peroxidation products in the cochlea

This study investigates the correlation between the formation of reactive oxygen species (ROS) and auditory damage in noise-induced hearing loss. The noise exposure (4-kHz octave band, 115 dB SPL, 5 h) created permanent threshold shifts at frequencies from 2 to 20 kHz. The lipid peroxidation product, 8-isoprostane, was determined biochemically and histochemically as an indicator of ROS. Noise exposure increased 8-isoprostane levels in the cochlea in a time-dependent manner. After 5 h of exposure, 8-isoprostane levels were more than 30-fold greater than baseline, and decreased rapidly after the termination of noise. The immunoreactivity to 8-isoprostane was increased in the stria vascularis, spiral ganglion cells and the organ of Corti. In the organ of Corti, immunostaining was restricted to the second turn in a region 10-12 mm from the apex. This region sustained most of the permanent hair cell damage as revealed in surface preparations. Outer hair cells were more heavily immunostained than inner hair cells while Hensen's cells showed still less immunostain. These data are consistent with the view that ROS are involved in noise-induced damage. However, the relationship between ROS formation and tissue damage appears complex. In the organ of Corti, the pattern of noise-induced lipid peroxidation correlates well with subsequent morphological damage. The stria vascularis, however, does not sustain permanent damage despite intense lipid peroxidation. Differences in endogenous antioxidant levels and commitment to different apoptotic or survival pathways may underlie such differential responses.

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.

Detection of glutamate decarboxylase isoforms in rat inferior colliculus following acoustic exposure

The inferior colliculus is a central auditory structure which serves as a site for the integration of ascending and descending auditory information. Changes in central auditory structures may occur with acoustic exposure, which cannot be explained by alterations in cochlear function alone. Rats were exposed to a 10-kHz tone at 100 dB SPL for 9 h. Auditory brainstem response measures showed an initial 25-30-dB threshold shift across all tested frequencies. By 30 days post-exposure, thresholds for clicks and most frequencies returned to near control levels; however, thresholds remained elevated at 10 and 20 kHz. Inner hair cell loss was confined to apical and basal ends of the cochlea, and did not exceed 20%. Inferior colliculus levels of the two isoforms of the GABA synthetic enzyme glutamate decarboxylase (65,000 and 67,000 mol. wt forms) were measured immediately post-exposure (0 h) and at two and 30 days post-exposure using quantitative immunocytochemical and western blotting techniques. Zero-hour measures revealed a significant increase in the level of glutamate decarboxylase (mol. wt 67,000) protein (118%), as well as in the optical density (35%) of immunolabeled cells. By 30 days post-exposure, inferior colliculus protein levels of both glutamate decarboxylase isoforms were significantly below unexposed controls (39% and 21% for the 65,000 and 67,000 mol. wt forms, respectively). These studies describe increased markers for GABA immediately following acoustic exposure, followed by a decline to below control levels from two to 30 days post-exposure. It remains to be determined whether noise trauma-induced changes in glutamate decarboxylase levels in the inferior colliculus reflect protective up-regulation in response to intense stimulation, followed by the establishment of new neurotransmitter equilibrium levels.

Effects of stimulus frequency and intensity on c-fos mRNA expression in the adult rat auditory brainstem

Induction of the cellular fos gene (c-fos) is one of the earliest transcriptional changes observed following neuronal excitation. Although not an activity marker in the strict electrophysiological sense, many neurons in the central nervous system increase their c-fos expression after periods of sustained stimulation at physiological levels of intensity. In the present study, induction of c-fos mRNA expression was examined in the auditory brainstem after 1 hour of continuous free-field acoustic stimulation. Sprague-Dawley rats were exposed to pure tones of 2, 8, 16, or 32 kHz or half-octave noise bands centered on 2, 8, or 32 kHz at 80-120 dB SPL. Stimulation-induced c-fos mRNA expression was evident at all levels of the auditory brainstem, and this expression was intensity dependent. In some brain areas, induced expression manifested a clear tonotopic organization, i.e., in dorsal, posteroventral, and anteroventral cochlear nuclei, and in the medial nucleus of the trapezoid body. The inferior colliculus exhibited multiple tonotopic representations. The dorsal nucleus of the lateral lemniscus had a crude tonotopy. Although expression was present, tonotopy was not evident in periolivary nuclei or in the ventral or intermediate nuclei of the lateral lemniscus. Free-field diotic stimulation did not induce c-fos mRNA expression in the medial or lateral superior olivary nuclei. Expression was induced in the lateral superior olive by dichotic stimulation (after a unilateral cochlear ablation), and that expression was tonotopically organized. The results suggest that stimulation-induced c-fos mRNA expression can be an effective way of mapping neuronal activity in the central auditory system under both normal and pathological conditions.

Changes in dorsal cochlear nucleus blood flow during noise exposure

Changes in dorsal cochlear nucleus (DCN) blood flow during noise exposure were assessed using both intravital microscopy (IVM) and laser Doppler flowmetry (LDF). Mature Syrian golden hamsters were anesthetized, tracheotomized and implanted with a carotid artery catheter for monitoring blood pressure and infusion of fluorescent dye. An occipital craniectomy was performed and the cerebellum partly aspirated for access to the DCN. Fluorescent dye was infused to enhance observations of the DCN surface using a customized IVM. Red blood cell velocity, vessel diameter and blood pressure were continuously monitored. Subjects were presented with 110 dB SPL broad-band noise for 15 min. A second group was presented with the same protocol for laser Doppler assessment of blood-flow changes. Control groups included animals not exposed to noise using both IVM and laser Doppler measures. Additional control measurements were obtained for noise-exposed and control groups with laser Doppler measures obtained from the obex, a brainstem structure with no known auditory function. Arterioles of the DCN showed a low-frequency oscillatory pattern of red blood cell velocity in control animals and in baseline conditions of the noise-exposure group. Presentation of noise abolished these velocity oscillations. Additionally, significant decreases in both red blood cell velocity and vessel diameter were measured during stimulation. These microvascular measures recovered slightly during the post-stimulus period. Laser Doppler measures of the overall blood flow in the nucleus were consistent with IVM findings. Measurements from DCN and obex in groups not exposed to noise, and animals exposed to noise and measured from the obex, showed stable oscillatory flow. These data show exposure to noise eliminates oscillatory patterns of blood flow and induces decreases in vascular perfusion. Furthermore, these changes appear specific to the auditory structure studied.

Intensity and frequency functions of [14C]2-deoxyglucose labelling in the central nucleus of the inferior colliculus in the cat

The frequency organization of the central nucleus of the inferior colliculus (ICC) in the anesthetised cat was quantitatively mapped using [14C]2-deoxyglucose. From a standardised rostrocaudal region of the ICC, the position of peak selective labelling along the tonotopic axis closely conformed to the reported tonotopic organization of this nucleus. The position of the peak was found not to significantly change its position along the tonotopic axis with increasing stimulus intensity. However, the amplitude of peak uptake and width of selective labelling were shown to monotonically increase with increase in stimulus intensity. The increase in width of selective labelling, about the position of peak uptake, showed a slight asymmetry toward the high-frequency regions of the ICC. A 2-DG frequency-position function for the ICC, similar to that for the cochlea, enabled the width of 2-DG bands to be expressed in terms of their frequency spread along the tonotopic axis. This inturn enabled 2-DG tuning curves to be plotted which, when compared to electrophysiologically determined tuning curves, showed marked similarities. The minimum threshold and width (Q10) of these 2-DG tuning curves fell within the range reported for single units in the cat auditory pathway.

Glucose utilization of the rat vestibular end organs: a quantitative 2-deoxyglucose study

The local metabolic rate of glucose utilization (LMRglc) for the rat vestibular end organs was determined with a modification of the [14C]deoxyglucose method. Data are expressed as micromoles per 100 g per minute +/- SEM. Results indicate that the LMRglc is similar within the utricle (40.3 +/- 3.2) and saccule (41.2 +/- 5.5) and significantly higher than that for the superior (20.1 +/- 2.9), posterior (25.4 +/- 2.0), or lateral canal (22.0 +/- 2.6) ampullae. These differences in LMRglc may be related to differences in the ratios of sensory to nonsensory cells, dark cell distributions, response to acoustic stimulation, or activity levels during the experimental period. Given the high blood flow rates reported for the vestibular end organs by Lyon and coworkers, a much higher LMRglc was expected. Together, these data would suggest that delivery of metabolites is not a primary regulating force for vestibular blood flow. Instead, the primary reason for a high blood flow rate may be waste removal, the maintenance of pH, ion balance, and/or temperature.

Dorsal Cochlear Nucleus Blood Flow During Acoustic Stimulation

Dynamic in vivo changes in dorsal cochlear nucleus blood flow during pure-tone stimulation were assessed with intravital microscopy. Subjects were stimulated with 5-, 10-, or 15-kHz pure tones at 70, 80, and 90 dB sound pressure level. Measurements in red blood cell velocity and vessel diameter were made in capillaries overlying the 10-kHz isofrequency band of the dorsal cochlear nucleus. Stimulation with 10 kHz induced intensity-dependent increases in local blood flow in the 10-kHz isofrequency band of the dorsal cochlear nucleus. Stimulation with 5 kHz and 15 kHz, frequencies represented in remote locations on the dorsal cochlear nucleus surface, did not significantly alter blood flow in the defined 10-kHz isofrequency band. These data demonstrate a direct relationship between spectral and intensity-dependent pure-tone stimulation of the dorsal cochlear nucleus and increases in local blood flow. These findings suggest that tonal stimulation of the dorsal cochlear nucleus induces an increase in local metabolic demands with resultant rapid blood flow increases.

The effect of noise trauma following training exposures in the mouse

The effect of moderate level acoustic stimulation, or 'training', on a subsequent high intensity noise exposure was studied in CBA/Ca mice. Eight groups of mice were exposed to a variety of training paradigms as well as different intensity traumatic exposures. We sought a combination which would result in the maximum protective effect from acoustic trauma as measured by the auditory brainstem responses. Using a narrow band noise centered at 4.5 kHz, we investigated the effects of a 10-day 'interval' training regimen, allowing a rest period between successive training exposures, as well as several continuous training exposures. These training paradigms were followed by a 24 h traumatic noise exposure (also centered at 4.5 kHz) at one of three intensities, 107, 110, or 117 dB SPL which induce a temporary, a moderate, or a severe permanent threshold shift, respectively. In none of these trained groups was a protective effect demonstrated at any time up to one month following a subsequent traumatic noise exposure. Several groups demonstrated higher compound threshold shifts after the traumatic noise exposure compared to controls. After a recovery period of 4 weeks nearly all trained groups demonstrated a tendency toward higher permanent threshold shifts than the control, untrained, animals. While no protective effect was demonstrated, examination of the threshold shifts following the training periods and after the traumatic noise exposures raised interesting questions for future investigation regarding the inherent resistance to noise induced threshold shifts in the mouse.

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.

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.

Effects of frequency and intensity of sound on cochlear blood flow

Laser Doppler flowmetry demonstrates that loud sound induces a decrease of blood flow in the cochlea of the guinea pig. In this experiment, we observed the effects of frequency and intensity of sound on cochlear blood flow using 15 guinea pigs. In the first 5 guinea pigs, a Doppler probe was attached to the basal turn of the cochlea and sounds of 6, 7, 8, 9 and 10 kHz were delivered to the ear serially from lower to higher frequency, i.e. from 6 kHz to 10 kHz. In the next 5 guinea pigs, the sound was delivered from higher to lower frequency, i.e. from 10 kHz to 6 kHz. The sound intensity delivered to the ear was changed from lower to higher intensity (80 to 120 dB SPL by 10 dB width) at each frequency. In the last 5 guinea pigs, the blood flow in the basal, second, third, and fourth turns of the cochlea was measured at 120 dB SPL of 10 kHz. No change of blood flow was seen in the cochlear basal turn at 6 and 7 kHz up to 120 dB SPL, but a decrease of blood flow was found at 110 and 120 dB SPL at 8, 9, and 10 kHz. On the other hand, the sound of 120 dB SPL at 10 kHz induced a decrease of blood flow only in the basal turn of the cochlea. Our results suggest that there is a corresponding blood flow area which is sensitive to specific frequency in the cochlea.

The protective effects of allopurinol and superoxide dismutase-polyethylene glycol on ischemic and reperfusion-induced cochlear damage

The purpose of this study was to assess the protective effects of allopurinol, a blocker of free oxygen radical (FOR) formation, and superoxide dismutase-polyethylene glycol (SOD-PEG), a scavenger of FORs, on ischemic and reperfusion-induced cochlear damage. Fifteen Wistar Kyoto rats (WKY) were randomly assigned to three groups: (1) a control group (5 animals) that was exposed to 15 minutes of cochlear ischemia by clamping the anterior inferior cerebellar artery (AICA), followed by 15 minutes of reperfusion as documented by laser Doppler flowmetry; (2) a drug-treated group (5 animals) that received allopurinol before ischemia/reperfusion; and (3) a drug-treated group (5 animals) that received SOD-PEG before ischemia/reperfusion. In the control group, the tone burst-evoked compound action potential (CAP) recorded from the round window (RW) of the cochlea was abolished, and the cochlear microphonic (CM) was reduced after ischemia. In contrast, both allopurinol and SOD-PEG-treated animals showed post-reperfusion sensitivity in CAP and CM measures. We interpret these results to indicate that damage to the cochlear from ischemia and subsequent reperfusion can be attenuated by pretreatment with allopurinol or SOD-PEG. This provides indirect evidence that FORs may be partially responsible for cochlear damage resulting from ischemic conditions.

Influence of topically applied adrenergic agents on cochlear blood flow

This study was designed to assess the role of adrenergic receptors in the control of cochlear blood flow. Laser Doppler flowmetry was used to determine the effects of adrenergic drugs topically applied to the round window membrane of the cochlea. The relative influence of the various receptor types (alpha 1, alpha 2, beta 1, and beta 2) was examined by a selection of agonists and antagonists. The agonists norepinephrine and epinephrine, which have mixed alpha- and beta-receptor effects, and phenylephrine, a strong alpha 1-agonist, all induced a dose-dependent reduction in cochlear blood flow. The agonists isoproterenol (beta-active), salbutamol (alpha 2-active) had no effect on cochlear blood flow. Of the antagonists, when tested alone, only the selective alpha 1-antagonist prazosin had a direct effect on cochlear blood flow, demonstrating an increase in cochlear blood flow. The selective alpha 2-antagonist idazoxan, the beta-antagonist propranolol, and the unselective alpha-antagonist phentolamine had no effect on cochlear blood flow. Interaction studies of agonists and antagonists were performed to specifically define the receptor subclasses responsible for the cochlear blood flow increases with norepinephrine and epinephrine. The results are consistent with the presence of an alpha 1-adrenergic sympathetic control of cochlear blood flow.

Insulin stimulates protein synthesis and phospholipid signaling systems but does not regulate glucose uptake in the inner ear

High-affinity insulin receptors exist in the organ of Corti (Kd = 1.1 +/- 0.5 nM) and in the lateral wall (stria vascularis and ligamentum spirale; Kd = 1.1 +/- 0.4 nM) of the inner ear of the guinea pig as determined by the binding of radiolabeled porcine or bovine insulin in vitro. Carrier-mediated transport of glucose (defined as the cytochalasin B-sensitive part of total uptake) was measured in vitro with 2-deoxy-D-glucose as the substrate. Its Km was 188 microM in the lateral wall (r = 0.99 and 0.94, respectively). Neither the Km nor the rates of transport (0.20 +/- 0.10 pmol/micrograms protein/hr in the organ of Corti, and 0.56 +/- 0.34 pmol/micrograms protein/hr in the lateral was) were affected by insulin. In contrast, 0.1 mM ouabain decreased deoxyglucose uptake in the organ or Corti by 37% and in the lateral wall tissues by 45% indicating the presence of an active, Na(+)-dependent transporter in these tissues. Insulin influenced both protein and lipid metabolism in the inner ear. Proteins and lipids were labeled in situ by perfusion of the perilymphatic space of the cochlea with [3]-leucine or [32P]-orthophosphate and [3H]-glycerol, respectively. Thirty nM insulin stimulated the incorporation of [3H]-leucine into protein of the organ of Corti from 39 to 56 pmol/mg protein but was ineffective in the tissues of the lateral wall. In the organ of Corti, [32P]-orthophosphate was incorporated into the phosphoinositides and phosphatidate, and 30 nM insulin increased this incorporation by 101 to 149%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cochlear ablation in deafness mutant mice: 2-deoxyglucose analysis suggests no spontaneous activity of cochlear origin

Deafness mutant mice show no stimulus-related cochlear potentials as well as abnormal electrically-evoked responses recorded from the inferior colliculus. Abnormal spontaneous activity in the auditory periphery could result in abnormal development and/or maintenance of the central auditory pathways. We therefore assessed spontaneous activity of cochlear origin in the central nuclei of the mutants by ablating one cochlea and subsequently using the 2-deoxyglucose (2DG) technique to study metabolic activity. Any asymmetries in labeling in a given nucleus should be due to spontaneous activity in the cochlear nerve on the unoperated side. In control animals (+/dn mice undergoing unilateral cochlea ablation), statistically significant decreased 2DG labeling was observed in the ipsilateral PVCN and AVCN, and contralateral MNTB and IC; all receive primary excitatory input from the ablated ear. No significant differences in labeling between right and left sides were observed in any of the nuclei studied in the mutant animals. These findings suggest that there is no spontaneous activity of cochlear origin in these mutants, even though many cochlear nerve fibers and spiral ganglion cells survive.

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.

Cochlear blood flow increases after systemic hemodilution: comparison of simultaneous laser Doppler flowmetry and radioactive microsphere measurements

Guinea pig cochlear blood flow was measured before and after systemic normovolemic hemodilution with high molecular weight dextran. Absolute determinations of blood flow (in the cochlea, brain, kidney and lung) were accomplished by use of radioactive-labeled (85Sr or 141Ce) microspheres. Relative measurements of the cochlear blood flow changes were made simultaneously by the use of a laser Doppler flowmeter. The flowmeter probe was placed on the first cochlear turn. Hemodilution to an average systemic hematocrit of 20% increased cochlear blood flow by 250% as measured with microspheres. The laser Doppler instrument significantly underestimated the actual flow increase giving an indication of 148%. Furthermore, the data, when analyzed on an individual trial basis, showed a very poor correlation between the two methods. The theoretical basis for these findings in relation to the use of the laser Doppler instrument is discussed.

Changes in cochlear blood flow during acoustic stimulation as determined by 14C-iodoantipyrine autoradiography

Local blood flow was measured in the tissues of the cochlea using the [14C]iodoantipyrine autoradiographic technique. Flow observed without acoustic stimulation was compared with that seen during exposure to wide-band noise at 85 or 105 dB SPL. Compared with the unexposed cochlea, substantial increases in blood flow were observed during exposure to 85 dB SPL noise in the spiral ganglion, VIII nerve and spiral lamina. Little or no change was noted in external wall structures. These results are consistent with changes in cochlear metabolism which have been reported previously using similar techniques, suggesting that increases in blood flow may be linked to increases in local metabolism. No changes in blood flow were measured during exposure to 105 dB SPL noise. This result is similar to those of other investigators using potentially damaging intensities of acoustic stimulation.

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.

The effects of noise on histological measures of the cochlear vasculature and red blood cells: a review

This paper compiles the results from seven experiments which have investigated noise exposure effects on histological measures of the cochlear vasculature and red blood cells. Two of these studies included at least two experimental conditions and all evaluated numerous histological parameters in several cochlear vessels. The combined results suggest that noise has a consistent general effect of reducing apparent cochlear blood flow as indicated by decreased RBC density, increased aggregation of RBCs, increased variability in RBC density, decreased number of RBC columns, increased vessel lumen irregularity and encroachment of perivascular cells upon the lumen wall. When considered at the level of the individual vessels, however, inconsistent results were observed. Fewer effects were noted in experiments which permitted animals to survive after the noise exposure than in those which sacrificed the animals immediately. Impulse noise resulted in more frequent vascular sequelae than did continuous noise. Further, impulse noise more frequently influenced vessels of the external wall than did continuous noise.

Local cerebral glucose utilization in fetal sheep exposed to noise

Rate of local cerebral glucose utilization in fetal sheep was measured in conditions of normal laboratory ambient sound (65 to 70 dB sound pressure levels) and broadband noise (105 and 120 dB sound pressure levels) delivered by two earphones situated on the abdominal surface of the ewe. Broadband noise had neither a visible effect on the ewe nor specific effects on the fetal arterial pH, PaO2, PaCO2, and plasma glucose concentration. However, glucose utilization of many cerebral structures was elevated in noise-exposed fetuses.

The postnatal development of stimulated deoxyglucose uptake into the mouse cochlea and the inferior colliculus

The effect of acoustic stimulation on the postnatal development of deoxyglucose uptake into the mouse cochlea and the inferior colliculus was evaluated. Animals between postnatal day 4 and 20 were separated into four different groups depending on their age. Tritiated deoxyglucose was injected intraperitoneally into each animal and tracer uptake was quantitated by microdissection of the tissues and scintillation counting. Acoustic stimulation at a noise level of 100 dB (A) resulted in supra-normal levels of deoxyglucose uptake for all auditory tissues during postnatal days 13, 14, and 15. The lateral wall tissues, which are non-sensory and non-neuronal, also increased deoxyglucose uptake following acoustic stimulation in a manner that paralleled the uptake by the sensory structures. Serum radioactivity and glucose levels remained unchanged during postnatal development, with these parameters remaining stable with acoustic stimulation.

Effect of noise on auditory evoked responses in awake guinea pigs

Changes in the auditory nerve action potential (AP), evoked responses from the inferior colliculus (IC-ER) and auditory cortex (AC-ER) were assessed after exposure to white noise of 120 dB SPL for 1 h in awake guinea pigs. Auditory thresholds were estimated with the aid of averaged AP, IC-ER and AC-ER, besides the threshold shifts also the changes in amplitude-intensity functions were evaluated. Auditory thresholds for tone pips and clicks increased by 20-30 dB 1 h after exposure and were similar in all the three investigated structures. The maximum threshold shifts for tone pips were observed at 8 kHz and were 33.2 +/- 12.9 dB for AP, 30.4 +/- 12.7 dB for IC-ER and 30.8 +/- 13.0 dB for AC-ER (n = 20). The thresholds recovered to preexposure levels within one week. Reduction in AP and IC-ER amplitudes 1 h after exposure was similar, the amplitude-intensity functions were shifted by 20-40 dB. In contrast, the amplitude-intensity functions in the auditory cortex 1 h after exposure were steeper than before exposure and this amplitude enhancement was present for 24 h after exposure. The enhancement of the AC-ER which resembles recruitment and which may be a sign of hypersensitivity of the animal to auditory stimuli was present only when the animals exposed to noise were awake. The noise exposure in animals anaesthetized with urethane reduced the amplitude-intensity functions of all three recorded potentials.

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.

Resting and high-frequency evoked 2-deoxyglucose uptake in the rat inferior colliculus: developmental changes and effects of short-term conduction blockade

Development of resting and high-frequency-evoked metabolic patterns in the central nucleus of the inferior colliculus (ICC) in rat was investigated using the 2-deoxyglucose (2-DG) method. The effects of early short-term acoustic deprivation were studied by unilateral blockade of the external auditory meatus on postnatal day 12. Quantitative and qualitative analyses were performed on 2-DG uptake in 5 regional sectors of ICC. Adult resting ICC uptake is robust and exhibits regional differentiation with the ventromedial region preferentially labelled. At 17 days of age, resting uptake is substantially lower and regionally more uniform than in the adult. This age-dependent change is likely a function of the immature stage of neuronal and cochlear development at 17 days. Unilateral 50 kHz stimulation in the adult results in bilaterally enhanced 2-DG incorporation in ventromedial ICC, with a discrete band of elevated uptake along the contralateral ventromedial border. 50 kHz stimulation evokes ventromedial banding in the contralateral, but not ipsilateral, ICC of undeprived 17-day-olds, though at a much reduced level relative to the adult. Following monaural conduction blockade from 12 to 17 days of age, stimulation through the reopened deprived ear elicits a band of uptake along the lateral ICC border, oblique to the normal tonotopic gradient. Cochlear mechanisms may account for modified patterning of high-frequency-evoked 2-DG uptake in ICC subsequent to conductive hearing loss during a period of elevated susceptibility to experiential factors.

Overview of mechanical damage to the inner ear: noise as a tool to probe cochlear function

The majority of experiments causing mechanical damage to the cochlea involve the use of sound pressure waves to cause overstimulation. This presentation is an overview of the research during the past years on the structural damage produced by noise. The effect of noise on the cochlea depends on the type of noise exposure-impulse or continuous. Experiments have been conducted to determine the effect of increasing intensity, the effect of increasing duration, and the effect of equal energy presented over varying periods of time. The initial mechanism of damage, the progression of damage over time, and the ability of hair cells to recover are discussed. Noise has been used as a tool to probe cochlear function by selectively damaging regions along the length of the sensory epithelium and by selectively damaging one of the two types of hair cells. Results obtained from these types of experiments have given us information on cochlear mechanics, as well as of stereocilia micromechanics and transduction. Information on susceptibility of hair cells to noise confirms previous results, suggesting the presence of structural and metabolic gradients both longitudinally and radially within the sensory epithelium. Moreover, noise lesions have been used to map the afferent innervation pattern to the cochlear nucleus, and noise studies show correlation of hair cell damage with efferent innervation pattern.

Combined electrophysiological and autoradiographic delimitation of retrocochlear dysfunction in a mouse mutant

The hereditary retrocochlear dysfunction in the quivering (qv) mouse was investigated with autoradiography and single unit recordings. Whilst the cochlea appears to function normally, earlier studies had indicated some single unit dysfunction detectable at the level of the cochlear nucleus (CN) and abnormality of auditory evoked potentials recorded at the inferior colliculus (IC). The present study investigated the possibility of progressive deterioration of function at successive higher levels in the auditory system. The 2-deoxyglucose technique illustrated auditory activity in the CN of quivering mice similar to that seen in normally-hearing control animals. There was only a slight increment in metabolic activity detectable at the level of the IC. Electrophysiology demonstrated that this minimal IC activity was the result of abnormally raised thresholds associated with all single units recorded, rather than of activity in a few normally responding cells. There was no evidence from autoradiography for any enhanced auditorily evoked metabolic activity in either the superior olivary complex or the lateral lemniscus. This study suggests that the retrocochlear dysfunction in quivering mice is due to a specific abnormality at a low stage in the auditory pathway rather than being non-specific and cumulative over stages.

Autoradiographic demonstration of deoxyglucose uptake in guinea pig cochlea

The 2-deoxyglucose autoradiographic method was applied to whole-body cryosectioning to include the cochlea. The highest levels of 2-deoxyglucose uptake were observed in the vascular stria, spiral ligament and spiral prominence. The cochlear nerve showed the next highest level of uptake, while the organ of Corti and the spiral ganglion showed low levels. The functional significance of the results was briefly discussed.