Time-averaged holograms describing the sound-induced motion of the tympanic membrane (TM) in cadaveric preparations from three mammalian species and one live ear were measured using opto-electronic holography. This technique allows rapid measurements of the magnitude of motion of the tympanic membrane surface at frequencies as high as 25 kHz. The holograms measured in response to low and middle-frequency sound stimuli are similar to previously reported time-averaged holograms. However, at higher frequencies (f>4 kHz), our holograms reveal unique TM surface displacement patterns that consist of highly-ordered arrangements of multiple local displacement magnitude maxima, each of which is surrounded by nodal areas of low displacement magnitude. These patterns are similar to modal patterns (two-dimensional standing waves) produced by either the interaction of surface waves traveling in multiple directions or the uniform stimulation of modes of motion that are determined by the structural properties and boundary conditions of the TM. From the ratio of the displacement magnitude peaks to nodal valleys in these apparent surface waves, we estimate a Standing Wave Ratio of at least 4 that is consistent with energy reflection coefficients at the TM boundaries of at least 0.35. It is also consistent with small losses within the uniformly stimulated modal surface waves. We also estimate possible TM surface wave speeds that vary with frequency and species from 20 to 65 m/s, consistent with other estimates in the literature. The presence of standing wave or modal phenomena has previously been intuited from measurements of TM function, but is ignored in some models of tympanic membrane function. Whether these standing waves result either from the interactions of multiple surface waves that travel along the membrane, or by uniformly excited modal displacement patterns of the entire TM surface is still to be determined.
An octave band of noise (OBN) delivers fairly uniform acoustic energy over a specific range of frequencies. Above and below this range, energy is at least 30 dB SPL less than that within the OBN. When the ear is exposed to an OBN, hair-cell loss often occurs outside the octave band. The frequency location of hair-cell loss is evident when the percent distance from the apex of focal lesions is analyzed. Focal lesions involve substantial loss of outer hair cells (OHCs) only, inner hair cells (IHCs) only, or both OHCs and IHCs (i.e., combined lesions) in a specific region of the organ of Corti (OC). Data sets were assembled from our permanent collection of noise-exposed chinchillas as follows: (1) the sum of exposure duration and recovery time was less than or equal to 11 d; (2) the exposure level was less than or equal to 108 dB SPL; and (3) focal lesions were less than 1.5mm in length. The data sets included a variety of exposures ranging from high-level, short duration to moderate-level, moderate duration. The center of each focal lesion was expressed as percent distance from the OC apex. Means, standard deviations and medians were calculated for focal-lesion size resulting from exposure to a 4-kHz or a 0.5-kHz OBN. Histograms were then constructed from the percent-location data using 2.0% bins. For the 4-kHz OBN, 5% of the lesions were in the apical half of the OC and 95% were in the basal half. The mean lesion size was 1.68% of total OC length for OHC and combined focal lesions and 0.42% for IHC focal lesions. Most OHC and combined lesions occurred in the 5-7-kHz region, at and just above the upper edge of the OBN. Clusters of lesions were also found around 8 and 12 kHz. A cluster was present at and just below the lower edge of the OBN, as well as another in the 1.5-kHz region. For the 0.5-kHz OBN, 34% of the lesions were in the apical half of the OC and 66% were in the basal half. The mean lesion size was 0.93% for OHC and combined focal lesions and 0.32% for IHC focal lesions. OHC and combined focal-lesion distribution showed clusters at 0.25, 0.75 and 1.5 kHz in the apical half of the OC. In the basal half, the distribution of focal lesions was similar to that seen with the 4-kHz OBN (r=0.54). With both OBNs, most IHC focal lesions occurred in the basal half of the OC. High resolution power spectrum analysis of each OBN and non-invasive tests for harmonics and distortion products in a chinchilla were performed to look for exposure energy above and below the OBN. No energy was found that could explain the OC damage.
In a previous study, we examined the relation between total energy in a noise exposure and the percentage losses of outer (OHC) and inner (IHC) hair cells in the basal and apical halves of 607 chinchilla cochleae [Harding, G.W., Bohne, B.A., 2004a. Noise-induced hair-cell loss and total exposure energy: analysis of a large data set. J. Acoust. Soc. Am. 115, 2207-2220]. The animals had been exposed continuously to either a 4-kHz octave band of noise (OBN) at 47-108 dB SPL for 0.5h-36 d, or a 0.5-kHz OBN at 65-128 dB SPL for 3.5h-433 d. Interrupted exposures were also employed with both OBNs. Post-exposure recovery times ranged from 0 to 913 days. Cluster analysis was used to separate the data into three magnitudes of damage. The data were also separated into recovery times of 0 days (acute) and >0 days (chronic) and the apical and basal halves of the organ of Corti (OC). A substantial part of these hair-cell losses occurred in focal lesions (i.e., >or=50% loss of IHCs, OHCs or both over a distance of >or=0.03 mm). This aspect of the damage from noise was not included in the previous analysis. The present analysis describes, within the same three clusters, the apex-to-base distribution of 1820 focal lesions found in 468 of 660 (71%) noise-exposed cochleae. In these cochleae, OC length in mm was converted to percent distance from the apex. The lesion data were analyzed for location in percent distance from the apex and size (mm) of the lesions. In 55 of 140 (39%) non-noise-exposed, control OCs, there were 186 focal hair-cell lesions, the characteristics of which were also determined. Focal lesions with hair-cell loss >or=50% involved predominantly OHCs, IHCs only, or both OHCs and IHCs (i.e., combined OHC-IHC lesions). The predominantly OHC and combined lesions were pooled together for the analysis. The distributions of lesion location (in percent distance from the apex), weighted by lesion size (in percent of OC length) were tallied in 2%-distance bins. In controls, focal lesions were uniformly distributed from apex to base and 70% of them were pure IHC lesions. In cochleae exposed to the 4-kHz OBN, lesions were distributed throughout the basal half of the OC. In cochleae exposed to the 0.5-kHz OBN, lesions occurred in both halves of the OC. With continuous exposures, 74% of the lesions were predominantly OHC or combined lesions. With interrupted exposures, 52% of the lesions were OHC or combined lesions. Lesion size was generally larger in the chronic compared to acute cochleae with similar exposures. There was a minimum total energy at which focal lesions began to appear and slightly higher energies resulted in nearly all exposed cochleae having focal lesions.
Whole-nerve responses to lowpass-filtered noise (2.5 kHz) and broadband click stimuli were recorded from the exposed intracranial portion of the eighth nerve in patients with normal hearing who were undergoing neurosurgical operations to relieve vascular compression of cranial nerves V, VII, and VIII. Cross-correlograms between the response and the noise showed a large degree of individual variation. When noise of 95 dB SPL was used, the correlograms in some patients had a large peak that represented a positive correlation between a negative nerve potential and the rarefaction phase of the sound and that peak could usually be identified at a delay that was close to the latency of the main negative peak in the response to high-intensity broadband click sounds (3.0-3.5 ms). The amplitude of these components decreased when the sound intensity was decreased, and, at stimulus intensities below 80 dB, the correlograms became dominated by components at longer delays. In other patients, peaks of a similar amplitude appeared in the cross-correlograms between delays of 2 and 12 ms in the entire range of sound intensities that were studied (65-105 dB SPL). In all patients the location of the peaks was little affected by the stimulus intensity in the range studied. All reproducible peaks that appeared at delays longer than 2.8 ms shifted towards longer delays when the recording electrode was moved from a location near the porus acusticus to a more central location (near the brainstem) on the exposed intracranial portion of the eighth nerve, indicating that components at longer delays than 2.8 ms result from propagated neural activity in the auditory nerve. It is assumed on the basis of the results that these correlograms are measures of phase-locking of neural activity to a complex stimulus sound (noise).
This study was carried out to determine the characteristics and incidence of hearing loss and vestibular disturbance in Behcet's syndrome with a large number of patients. Sixty-two patients with Behcet's syndrome were included in this study, 34 men and 28 women whose mean age was 33.7 (15-60). Sixty-two healthy normal control subjects (38 male and 24 female) were included. Patient and control groups were questioned about any history of audio-vestibular disturbance and underwent physical and ENT examination and the following audiologic tests: pure tone audiometric test (0.25, 0.5, 1, 2, 4, and 6 kHz), tympanogram, speech discrimination, short increment sensitivity index, tone-decay test, auditory brainstem response. Vestibular system was evaluated by videonistagmogram and caloric test. Cranial and brainstem magnetic resonance imagine (MRI) of patients who have vestibular disturbances were practiced to examine the central nervous system. Both the patient and the control groups were tested with the HLA-B51 antigen. Pure tone audiogram showed sensory-neural hearing loss (25 dB hearing level in at least two frequencies) in 20 of the 62 (32%) Behcet's patients while the control group were normal. There was a hearing loss involving high frequencies in the audiograms of Behcet's patients with hearing disturbances. The recruitment investigation tests and auditory brain stem response confirmed cochlear involvement in all 20 patients. Caloric stimulation tests revealed a normal vestibular function in all patient and control group. In electronystagmography, 21 (34%) patients had hypometric or hypermetric saccades and smooth pursuit tests showing that 4 (6%) patients had pathological changes while the control group was normal. HLA-B51 antigen was found positive in 15 of 20 Behcet's patient with hearing loss.
(1) The hearing and vestibular disturbances in Behcet's syndrome is more prevalent than previously recognized; (2) Hearing loss in high frequencies in Behcet's patients is an indicator of cochlear involvement in this disease; (3) There is a higher prevalence of central vestibular syndrome in Behcet's patients than it was thought before; (4) HLA-B51 antigen may be able to be a prognostic factor for sensorineural hearing loss in Behcet's patients.
Purinergic agonists regulate mucin secretion in the airway epithelial cells. This study examined the effects of the apical application of purinergic agonists on Ca(2+) influx ([Ca(2+)](i)), and mucin secretion along with their underlying signaling pathway in normal human middle ear epithelial (NHMEE) cells. The apical membrane of NHMEE cells were stimulated with various purinergic agonists, including UTP, and the [Ca(2+)](i) was measured using a miniature Ussing double perfusion chamber. P2Y(2) receptor in NHMEE cells was also localized by immunohistochemistry. UTP-induced mucin secretion was quantified by an immunoblotting assay. The order of the purinergic agonist potency with respect to [Ca(2+)](i) determined in this study was ATP = UTP > 2-MeSATP > UDP > adenosine which is consistent with that obtained from P2Y(2) receptor activation. The P2Y(2) receptor is expressed in the apical membrane of monolayered cultured NHMEE cells. Apical UTP-induced [Ca(2+)](i) was inhibited by 2-aminoethoxydiphenyl borate (2-APB) but not by ryanodine. UTP-induced mucin secretion was inhibited by a Ca(2+) chelating agent, BAPTA-AM, and was stimulated by ionomycin. UTP-induced mucin secretion was also suppressed by U73122 and 2-APB, while Calphostin C suppressed it to a small extent and PD98059 was ineffective. Caffeine also inhibited the UTP-induced [Ca(2+)](i) and mucin secretion. These results suggest that the P2Y(2) receptor is expressed in NHMEE cells, and plays a major role in modulating the [Ca(2+)](i) from the IP(3)-sensitive intracellular Ca(2+) store. UTP-induced mucin secretion in NHMEE cells is strongly dependent on Ca(2+)- and IP(3).
Endocochlear potential (EP) and cochlear microphonics (CM) were recorded during the perilymphatic perfusion with forskolin known as an adenylate cyclase stimulant. Forskolin produced a reversible EP elevation in a dose-dependent manner. Perfusion with 1,9-dideoxy-forskolin, an analogue of forskolin that does not stimulate adenylate cyclase, had no effect on EP, whereas perfusions with other agents that raise the cAMP-level (IBMX, a phosphodiesterase inhibitor, and dbcAMP) duplicated the effect of forskolin. The vigorous CM during the EP elevation and the large negative EP induced by anoxia superimposed on the elevated EP indicate that the K+ diffusion potential through the hair cell membrane cannot be altered by forskolin. The results suggest that the adenylate cyclase system in the stria vascularis and/or Reissner's membrane may modulate the generation of EP.
The f2/f1 frequency ratio of 1.3 in combination with stimulus levels of L1/L2 = 50/60 and 50/50 dB SPL produced a higher level of distortion product otoacoustic emissions (DPOAE) in the heterozygous (+/dn) mice than in the homozygous (+/+) mice. These results suggest that the dn gene carriers have a unique cochlear trait which may be related to the dn gene locus and expressed via a frequency- and intensity-dependent DPOAE function.
Nitric oxide (NO) production during hyposmotic stimulation in outer hair cells (OHCs) of the guinea pig cochlea was investigated using the NO sensitive dye DAF-2. Simultaneous measurement of the cell length and NO production showed rapid hyposmotic-induced cell swelling to precede NO production in OHCs. Hyposmotic stimulation failed to induce NO production in the Ca2+-free solution. L-NG-nitroarginine methyl ester (L-NAME), a non-specific NO synthase inhibitor and gadolinium, a stretch-activated channel blocker inhibited the hyposmotic stimulation-induced NO production whereas suramin, a P2 receptor antagonist did not. S-nitroso-N-acetylpenicillamine (SNAP), a NO donor inhibited the hyposmotic stimulation-induced increase in the intracellular Ca2+ concentrations ([Ca2+]i) while L-NAME enhanced it. 1H-[1,2,4]oxadiazole[4,3a]quinoxalin-1-one, an inhibitor of guanylate cyclase and KT5823, an inhibitor of cGMP-dependent protein kinase (PKG) mimicked effects of L-NAME on the Ca2+ response. Transient receptor potential vanilloid 4 (TRPV4), an osmo- and mechanosensitive channel was expressed in the OHCs by means of immunohistochemistry. 4alpha-phorbol 12,13-didecanoate, a TRPV4 synthetic activator, induced NO production in OHCs. These results suggest that hyposmotic stimulation can induce NO production by the [Ca2+]i increase, which is presumably mediated by the activation of TRPV4 in OHCs. NO conversely inhibits the Ca2+ response via the NO-cGMP-PKG pathway by a feedback mechanism.
A stimulus generalization paradigm was used with classical respiratory conditioning to study analytic listening in the goldfish. Animals were first conditioned to suppress respiration upon the presentation of a long-duration complex sound comprised of two sinusoidal components, 166 and 724 Hz. Conditioned animals were then presented with a set of eight novel test tones with frequencies between 95 and 1514 Hz, and including 166 and 724 Hz. Response magnitudes were greatest at the frequencies of the components making up the complex to which the animals were initially conditioned. This is a demonstration that the goldfish had acquired independent information about the frequencies of the individual sinusoidal components making up a complex sound, and thus had listened to the complex analytically. To my knowledge, this is the first demonstration of simultaneous frequency analysis and analytic listening by a nonhuman animal, and suggests that this fundamental aspect of human hearing may be a primitive character shared with the fishes and perhaps with all living vertebrates.
The hypothesis is tested that an open-canal hearing device, with a microphone in the ear canal, can be designed to provide amplification over a wide bandwidth and without acoustic feedback. In the design under consideration, a transducer consisting of a thin silicone platform with an embedded magnet is placed directly on the tympanic membrane. Sound picked up by a microphone in the ear canal, including sound-localization cues thought to be useful for speech perception in noisy environments, is processed and amplified, and then used to drive a coil near the tympanic-membrane transducer. The perception of sound results from the vibration of the transducer in response the electromagnetic field produced by the coil. Sixteen subjects (ranging from normal-hearing to moderately hearing-impaired) wore this transducer for up to a 10-month period, and were monitored for any adverse reactions. Three key functional characteristics were measured: (1) the maximum equivalent pressure output (MEPO) of the transducer; (2) the feedback gain margin (GM), which describes the maximum allowable gain before feedback occurs; and (3) the tympanic-membrane damping effect (D(TM)), which describes the change in hearing level due to placement of the transducer on the eardrum. Results indicate that the tympanic-membrane transducer remains in place and is well tolerated. The system can produce sufficient output to reach threshold for those with as much as 60 dBHL of hearing impairment for up to 8 kHz in 86% of the study population, and up to 11.2 kHz in 50% of the population. The feedback gain margin is on average 30 dB except at the ear-canal resonance frequencies of 3 and 9 kHz, where the average was reduced to 12 dB and 23 dB, respectively. The average value of D(TM) is close to 0 dB everywhere except in the 2-4 kHz range, where it peaks at 8dB. A new alternative system that uses photonic energy to transmit both the signal and power to a photodiode and micro-actuator on an EarLens platform is also described.
The common occurrence of hearing loss in both humans and mice, and the anatomical and functional similarities of their inner ears, attest to the potential of mice being used as models to study inherited hearing loss. A large-scale, auditory screening project is being undertaken at The Jackson Laboratory (TJL) to identify mice with inherited hearing disorders. To assess hearing sensitivity, at least five mice from each inbred strain had auditory brainstem response (ABR) thresholds determined. Thus far, we have screened 80 inbred strains of mice; 60 of them exhibited homogeneous ABR threshold values not significantly different from those of the control strain CBA/CaJ. This large database establishes a reliable reference for normal hearing mouse strains. The following 16 inbred strains exhibited significantly elevated ABR thresholds before the age of 3 months: 129/J, 129/ReJ, 129/SvJ, A/J, ALR/LtJ, ALS/LtJ, BUB/BnJ, C57BLKS/J, C57BR/cdJ, C57L/J, DBA/2J, I/LnJ, MA/MyJ, NOD/LtJ, NOR/LtJ, and SKH2/J. These hearing impaired strains may serve as models for some forms of human non-syndromic hearing loss and aid in the identification of the underlying genes.
Given recent interest in syllabic rates (∼2-5 Hz) for speech processing, we review the perception of "fluctuation" range (∼1-10 Hz) modulations during listening to speech and technical auditory stimuli (AM and FM tones and noises, and ripple sounds). We find evidence that the temporal modulation transfer function (TMTF) of human auditory perception is not simply low-pass in nature, but rather exhibits a peak in sensitivity in the syllabic range (∼2-5 Hz). We also address human and animal neurophysiological evidence, and argue that this bandpass tuning arises at the thalamocortical level and is more associated with non-primary regions than primary regions of cortex. The bandpass rather than low-pass TMTF has implications for modeling auditory central physiology and speech processing: this implicates temporal contrast rather than simple temporal integration, with contrast enhancement for dynamic stimuli in the fluctuation range. This article is part of a Special Issue entitled .
11β-hydroxysteroid dehydrogenase (11β-HSD) is an enzyme complex responsible for the conversion of hormonally active cortisol to inactive cortisone, and two isoforms of the enzyme (11β-HSD1 and 11β-HSD2) have been cloned and characterized. An immunohistochemical study was performed to determine the precise distribution of glucocorticoid receptors (GRs) and the isoforms of 11β-HSD in the rat (postnatal day 1, 4, 10, and adult). Immunoreactivity of GRs was detected in the stria vascularis (SV), the outer hair cells (OHCs), the inner hair cells (IHCs), the spiral ligament (SLig), the spiral limbus (SLib), the spiral ganglion cells (SGCs), Reissner's membrane (RM), the cochlear nerve (CN), the vestibular hair cells (VHCs), the dark cells (DCs), and the vestibular nerve (VN) in the rats. Immunostaining of 11β-HSD1 was observed in almost all the tissues in the cochlea and the vestibule except SLig, SLib, SGCs, CN, VHCs, and VN during all developmental stages, whereas, immunoreactivity of 11β-HSD2 was not detected in any of the inner ear tissues. A polymerase chain reaction (PCR) study was also performed on GRs, 11β-HSD1, and 11β-HSD2 in the OC, SV and vestibule of the postnatal rats, and revealed that mRNAs were detected in all those and tissues in all the developmental days of postnatal days 1, 4, and 10. This data indicates that expression of GRs and 11β-HSD isoforms in the inner ear is tissue and age-specific, and that different local steroid regulation by GRs and the isoforms of 11β-HSD is present in each part of the inner ear.
The analysis of the auditory scene begins from the moment we hear sounds, making it possible for the infant to distinguish the mother's voice from other sounds in the environment. The purpose of the study was to determine, in two experiments, whether the frequency separation threshold, at which the perception of a mixture of sounds turns from being perceived as one stream to two streams, differs between two groups of school-aged children (ages 5-8 and 9-11 years) and adults. The results show a developmental course for the perception of auditory streams that is not simply dependent upon frequency discrimination. This suggests that maturation of the stream segregation process follows a longer developmental course than maturation of simple feature discrimination. The data indicate that the ability to hear distinct sound streams in the environment takes time to develop and becomes sharpened with experience and maturity.
The two most abundant proteins of the organ of Corti, OCP1 and OCP2, are acidic, cytosolic, low molecular weight proteins diffusely distributed within the cytoplasm of supporting cells. A recent study by Henzl et al. (2001) found first, that these two proteins co-localize with connexin 26 along the epithelial gap junction system and second, that OCP2 could participate with OCP1 in an organ of Corti-specific SCF complex (Skp1, cul1in, and Fbp), a ubiquitin ligase complex. Previous study has also implicated OCP2 in the recycling and regulation of intracellular K(+) efflux as well as pH homeostatic mechanisms. In the present study, we document the emergence and distribution features of OCP2 through various stages (weeks 11-28) of gestation in human fetal cochleae. Four fetal cochleae, the cochleae of a normal hearing human adult and a mature rat for positive control were fixed in 4% formalin within 2 h post mortem. Immunohistochemical studies were performed using a rabbit polyclonal antibody raised against a synthetic peptide corresponding to amino acids 3-16. Specimens were mounted in paraffin sections. Results show that OCP2 immunoreactivity is evident at a prenatal age of 11 weeks, peaks in expression at the onset of cochlear function at 20 weeks and achieves adult-like patterns of distribution just prior to histological maturation at 28 weeks. Though this protein could be associated with the development, maturation, and electrochemical maintenance of the cochlear gap junction system, the nature of this protein's function in the developing and mature human cochlea remains unclear.
The otoprotective peptide AM-111, a cell-permeable inhibitor of JNK mediated apoptosis, was tested for its efficacy as a rescue agent following impulse noise trauma. Single dose administrations of AM-111 at 1h or 4h post-impulse noise exposure (155 dB peak SPL) via systemic or local routes were evaluated with a total of 48 chinchillas. The animals received the compound either by IP injection or locally onto the round window membrane (hyaluronic acid gel formulation or osmotic mini-pump). Efficacy was determined by auditory brainstem responses (ABR) as well as cytocochleograms. Three weeks after impulse noise exposure, permanent threshold shifts (PTS) were significantly lower for AM-111 treated ears compared to controls, regardless of the drug administration route and the time point of drug delivery. Even the treatments which started 4h post-noise exposure, reduced hearing loss in the 2-8 kHz range compared to controls by up to 16-25 dB to a PTS as low as 6-17 dB, demonstrating significant protection against permanent hearing loss from impulse noise trauma. These findings suggest a key role for JNK mediated cochlear sensory cell death from oxidative stress.
This paper investigates the role of cholinergic mechanisms in auditory gating by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on behavioral and electrophysiological measures of consonant-vowel (CV) discrimination in quiet and in broadband noise (BBN). In a single-blind procedure, categorical boundaries and mismatch negativity (MMN) in two conditions (quiet, BBN) were obtained from 10 non-smokers and 4 smokers with normal hearing under two drug conditions (nicotine, placebo). After the nicotine sessions, plasma tests revealed a subject's nicotine concentration and subjects reported any symptoms. Larger MMN areas and steeper slopes at the boundary were interpreted as reflecting better electrophysiological and behavioral CV discrimination, respectively. Results indicate that, in non-smokers, the effects of nicotine on electrophysiological CV discrimination in quiet increase with an increase in severity of symptoms. Specifically, asymptomatic non-smokers (N = 5) demonstrate little improvement (and sometimes decrements) in performance while symptomatic non-smokers (N = 5) exhibit nicotine-enhanced discrimination, as do smokers. In noise, all subjects demonstrate nicotine-enhanced behavioral and electrophysiological discrimination. Additionally, in noise, smokers exhibit a larger number of measurable categorical boundaries as well as larger MMN areas than non-smokers in both placebo and nicotine sessions. Results are consistent with the hypothesis that nicotinic cholinergic mechanisms play a role in the gating of auditory stimuli.
Calpains, a family of calcium-activated proteases that breakdown proteins, kinases, phosphatases and transcription factors, can promote cell death. Since leupeptin, a calpain inhibitor, protected against hair cell loss from acoustic overstimulation, we hypothesized that it might protect cochlear and vestibular hair cells against gentamicin (GM) ototoxicity. To test this hypothesis, mouse organotypic cultures from the cochlea, maculae of the utricle and the crista of the semicircular canal (P1-P3) were treated with different doses of GM (0.1-3 mM) alone or in the presence of leupeptin (0.1-3 mM). The percentage of outer hair cells (OHCs) and inner hair cells (IHCs) decreased with increasing doses of GM between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced GM-induced damage to IHCs and OHCs; this protective effect was dose-dependent. GM also significantly reduced hair cell density in the crista and utricle in a dose-dependent manner between 0.1 and 3 mM. The addition of 1 mM of leupeptin significantly reduced hair cell loss in the crista and utricle for GM concentrations between 0.1 and 3 mM. These results suggest that one of the early steps in GM ototoxicity may involve calcium-activated proteases that lead to the demise of cochlear and vestibular hair cells.
The positive endocochlear potential (EP+) and high K+ concentration of the endolymph in the scala media of the mammalian cochlea are unusual. They have long been assumed to be due to a putative K-pump in the luminal membrane of the marginal cells of the stria vascularis, which were believed to have a negative internal potential. We show that the cell potential is more positive than the EP+, and that the ion pump is conventional Na,K-ATPase, probably in the basolateral membrane. The latter was determined from experiments in which the ionic environment of the strial cells was controlled by perfusion of the perilymphatic space of the cochlea, in the absence of vascular circulation. While the usual EP+ was maintained by normal perfusate, replacement of Na+ by choline resulted in a negative EP, showing that Na,K-ATPase is necessary for the production of EP+. Elimination of K+ as well as Na+ from the perfusate did not change the value of the negative EP, showing that no K-ATPase is involved.
Recent in vivo measurements of cochlear-partition motion indicate very high sensitivity and sharp mechanical tuning similar to the tuning of single cochlear nerve fibers. Our experience with mathematical models of the cochlea leads us to believe that this type of mechanical response requires the presence of active elements in the cochlea. We have developed an active cochlear model which incorporates negative damping components; this model produces partition displacement in good agreement with many of the mechanical and neural tuning characteristics which have been observed in vivo by other researchers. We suggest that the negative damping components of our model may represent an active mechanical behavior of the outer hair cells, functioning in the electromechanical environment of the normal cochlea.
Endolymph movements and endocochlear potential (EP) changes were measured during disturbances of perilymphatic pressure. induced by injecting artificial perilymph into scala tympani (ST) or scala vestibuli (SV) of the guinea pig cochlea. Injections were performed either with or without an outlet made in the opposite perilymphatic scala. Injections into ST without an outlet induced large pressure changes but virtually no endolymph movement or EP change. Injection at the same rate into ST with an outlet in SV produced smaller pressure changes which were accompanied by a basally-directed displacement of endolymph and significant EP changes. The magnitude of endolymph displacements and EP changes varied as a function of injection rate. Injections into SV, either with or without an outlet in ST, produced apically-directed endolymph displacement and EP changes. For the SV injections without an outlet, the cochlear aqueduct and round window are likely to provide an outlet and compliance, permitting flow along the perilymphatic scalae to occur even when no ST outlet was provided. We conclude that endolymph movements are not dependent on the absolute pressure of the perilymph, but instead occur when small, sustained pressure gradients are present across the cochlear partition, corresponding to times when perilymph flow is induced. This study demonstrates that in the normal. sealed cochlea, endolymph and EP are insensitive to fluid injections into ST, but are sensitive to fluid injections into SV. Endolymph movements are therefore unlikely to be generated by cerebrospinal fluid pressure fluctuations (such as those produced by respiration, posture changes, coughing, sneezing, etc) which are transmitted to ST by the cochlear aqueduct.
The purpose of this study was to explore possible mechanisms for the generation of the summating potential. Computer simulation was used to model the effects of potential hair cell nonlinearities on extracellular and intracellular d.c. potentials in the cochlea. No one nonlinearity can account for both extracellular and intracellular experimental data. However, a model which includes two nonlinearities (voltage-dependent cilia stiffness and nonlinear transducer channel resistance) produces extracellular and intracellular responses which match experimental data very well.
Cochlear endolymph is maintained at a potential of (+)80 mV by an active transport mechanism involving the stria vascularis (SV). This so-called endocochlear potential (EP) is integral to hair cell transduction. We compared the EP with changes in SV area and Na(+),K(+)-ATPase expression following a sensorineural hearing loss. Guinea pigs were deafened using kanamycin and a loop diuretic, and the EP was measured at two, 14, 56, 112 or 224 days following deafening. Auditory brainstem responses were used to confirm that each animal had a severe-profound hearing loss. There was a significant reduction in EP following two days of deafness (normal, 73.5 mV S.E.M.=2.4; deaf, 42.1 mV, S.E.M.=2.8; P<0.0001, t-test). In animals deafened for 14 days the EP had partially recovered (65.2 mV, S.E.M.=5.08), while animals deafened for longer periods exhibited a complete recovery (56 days 80.5 mV, S.E.M.=5.36; 112 days 75.7 mV, S.E.M.=2.71; 224 days 81.0 mV; S.E.M.=6.0). Despite this recovery, there was a systematic reduction in SV area with duration of deafness over the first 112 days of deafness. Significant reductions were localised to the basal turn in animals deafened for two days, but had extended to all turns in animals deafened for 112 days. While there was a significant reduction in strial area, the optical density of Na(+),K(+)-ATPase within the remaining SV was normal. Since the treated animals exhibited essentially a complete elimination of all hair cells, the total K(+) leakage current from the scala media would be expected to be significantly reduced. The large reduction in the extent of the SV after deafening suggests that a reduced strial volume is capable of maintaining a normal EP under conditions of reduced K(+) leakage current.
Because homologies between mice and human genomes are well established and hereditary abnormalities are similar in both, mice present a valuable animal model to study hereditary hearing disorders in humans. One of the manifestations of hereditary hearing disorders might be in the structure of cochlear elements, such as the gross morphology of the cochlea. Cochlear dimensions, however, are one factor that determines inner ear mechanics and thus hearing function. Therefore, gross cochlear dimension might be important when different strains of mice are compared regarding their hearing. Although several studies have examined mouse inner ear structures on a sub-cellular level, only few have studied cochlear gross morphology. Moreover, the sparse data available were acquired from fixed and dehydrated tissue. Dehydration, however, produces severe distortion of gel-like cochlear structures such as the tectorial membrane and the basilar membrane hyaline matrix. In this study, the hemicochlea technique, which allows fresh mouse cochlear material to be viewed from a radial perspective, was used to provide an itemized study of the dimensions of gross cochlear structures in four mouse strains (CBA/CaJ, 129/SvEv, 129/CD1 and C57BL/6J). Except for the CBA/CaJ, these strains are known to possess genes for age-related hearing loss. The measurements showed no major differences among the four strains. However, when compared with previous data, the thickness measures of the basilar membrane were up to 10 times larger. Such differences are likely to result from the different techniques used to process the material. The hemicochlea technique eliminates much of the distortion caused by dehydration, which was present in previous experiments.
20 kHz could not be evaluated. Permanent threshold shift (PTS) and hair cell losses, measured 1 week after high-intensity exposure to an 8-16 kHz noise band, were smaller in129/SvEv at all exposure levels and durations from 97 dB SPLx2 h to 106 dB SPLx8 h. Furthermore, PTS growth with increasing exposure energy was slower in 129/SvEv (<2 dB/dB) than CBA/CaJ (9 dB/dB). These data suggest that the vulnerability differences lie in the inner ear, not the middle ear. Several 129/Sv substrains show age-related hearing loss (AHL): 129/SvEv has not yet been evaluated (Zheng, Q.Y., Johnson, K. R., Erway, L.C., 1999. Assessment of hearing in 80 inbred strains of mice by ABR threshold analyses. Hear. Res. 130, 94-107). Thus, although other strains with AHL, e.g. C57Bl/6J, show increased vulnerability to noise-induced hearing loss (NIHL), pairing of AHL and NIHL vulnerabilities may not be obligatory.
Maximum length sequence (MLS) stimulation allows click evoked otoacoustic emissions (CEOAEs) to be averaged at very high stimulation rates. This enables a faster reduction of noise contamination of the response, and has been shown to improve the signal-to-noise ratio (SNR) of CEOAEs recorded from adult subjects. This study set out to investigate whether MLS averaging can enhance the SNR of CEOAEs recorded in newborns within the first day after birth, and so improve the pass rates for OAE screening in this period, when false alarm rates are very high. CEOAEs were recorded in a neonatal ward from 57 ears in 37 newborns ranging from 6 to 13h old, using both conventional (50/s) and high rate (5000/s) MLS averaging. SNR values and pass rates were compared for responses obtained within equal recording times at both rates. MLS averaging produced an SNR improvement of up to 3.8dB, with the greatest improvement found in higher frequency bands. This SNR advantage resulted in pass rate improvement between 5% and 10%, depending on pass criterion. A significant effect of age was found on both SNR and pass rate, with newborns between 6 and 10h old showing significantly lower values than those tested between 10 and 13h after birth, as well as a much greater improvement due to MLS averaging. The findings show that MLS averaging can reduce false alarm rates by up to 15% in very young neonates in a neonatal ward setting.
The purpose of this study was to investigate the effectiveness of 4-methylthiobenzoic acid (MTBA) as a protection agent against cisplatin (CDDP)-induced changes in organ of Corti surface structure, compared to electrophysiological changes. Electrophysiological change was assessed using auditory brainstem response (ABR) and morphological changes were assessed using scanning electron microscopy (SEM). Male Wistar rats underwent pre-treatment ABRs in response to clicks, and tone bursts at 2, 4, 8, 16, and 32 kHz. The three groups of rats were injected as follows: (1) MTBA (250 mg/kg, i.p.), (2) CDDP (16 mg/kg, i.p.), (3) CDDP+MTBA (16 mg/kg, i.p. + 250 mg/kg, i.p.). Post-treatment ABRs were performed 3 days after drug administration and rats were sacrificed. Their cochleae were harvested and SEM was used to examine the surface of the organ of Corti, specifically the number of inner hair cells (IHCs) and outer hair cells (OHCs) in the apical, middle and basal turns of the cochlea. Animal weight was measured on the first and final days. There was a good correlation between ABR threshold changes and hair cell loss in the high frequency region of the cochlea (basal turn), while threshold changes in the lower test frequencies (middle turn) appeared to be the result of more subtle changes in the cochlea. MTBA provided effective protection against cisplatin-induced ABR threshold changes at all test frequencies as well as hair cell loss. MTBA also protected against body weight loss.
Auditory processing in the cerebral cortex is comprised of an interconnected network of auditory and auditory-related areas distributed throughout the forebrain. The nexus of auditory activity is located in temporal cortex among several specialized areas, or fields, that receive dense inputs from the medial geniculate complex. These areas are collectively referred to as auditory cortex. Auditory activity is extended beyond auditory cortex via connections with auditory-related areas elsewhere in the cortex. Within this network, information flows between areas to and from countless targets, but in a manner that is characterized by orderly regional, areal and laminar patterns. These patterns reflect some of the structural constraints that passively govern the flow of information at all levels of the network. In addition, the exchange of information within these circuits is dynamically regulated by intrinsic neurochemical properties of projecting neurons and their targets. This article begins with an overview of the principal circuits and how each is related to information flow along major axes of the network. The discussion then turns to a description of neurochemical gradients along these axes, highlighting recent work on glutamate transporters in the thalamocortical projections to auditory cortex. The article concludes with a brief discussion of relevant neurophysiological findings as they relate to structural gradients in the network.
Amplitude modulation detection thresholds were obtained for pure-tone stimuli of 8, 10, 12 and 14 kHz at 5 dB intensity increments from 10 to 65 dB sensation level. Performance at 8 and 10 kHz was a non-monotonic function of sensation level for all four subjects with the largest difference limen measured near 30 dB sensation level and optimal performance at the highest sensation level (60 dB). Weber fractions at 12 and 14 kHz appear dependent on each subject's high frequency hearing profile; i.e., the difference limens remain high and either increase or remain essentially constant at high sensation levels only when the frequencies tested are near a particular subject's upper limit of hearing.
The current study evaluated changes in [14C]-2-deoxyglucose (2-DG) uptake along the auditory pathways of hamsters that were exposed unilaterally to intense sound. The measurement of the acoustically evoked auditory brainstem responses indicated that intense sound exposure caused asymmetrical hearing loss. The 2-DG results revealed some changes in metabolic activity in exposed animals, as compared to unexposed animals. Significant decreases in 2-DG uptake were found in the ipsilateral anteroventral and posteroventral cochlear nucleus, with respect to the exposed left ears. Exposed animals also showed significant increases in the ipsilateral nucleus of the lateral lemniscus, central nucleus of inferior colliculus and medial geniculate body. No significant changes in uptake were observed in the ipsilateral dorsal cochlear nucleus, superior olivary complex, auditory cortex and any contralateral structures. The mechanisms for the observed changes in 2-DG uptake are discussed.
There are endogenous intracellular mechanisms that provide cells with protection from stress, as well as repair from damage. These pathways often involve stress proteins and neurotrophic factors. The present study used Western blot analysis to examine changes in glial cell line-derived neurotrophic factor (GDNF) following noise overstimulation. A noise exposure was utilized which causes a temporary threshold shift and has been previously shown to upregulate heat shock protein 72 in the rat cochlea. This noise exposure also provides protection from a second noise exposure that would otherwise cause a permanent threshold shift. Experimental animals were assessed 2, 4, 8 and 12 h after cessation of noise exposure. Control animals received the same treatment except for the noise exposure and were assessed at the 8 h time point. A moderate expression of GDNF was observed in the normal cochlea. No significant change in GDNF levels was observed at 2 or 4 h following noise overstimulation. However, a significant increase was found at 8 h. At 12 h following noise overstimulation, GDNF levels were no longer significantly elevated from normal. These results suggest that GDNF is involved in the endogenous stress response in the cochlea and are consistent with the protection that exogenously applied GDNF has been shown to provide.
The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.
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.
A number of different qualitative and quantitative techniques have been used to measure inner ear blood flow and all have required that the animal be anesthetized. It is well known that anesthesia can cause a variety of circulatory as well as other systemic changes. In this study, we have employed a technique commonly used for quantifying brain blood flow, the iodo[(14)C]antipyrine technique ([(14)C]IAP). Unlike other techniques, [(14)C]IAP can be used in unanesthetized animals under conditions that are nearly normal, it is non-invasive, it can be used reliably in regions of low local blood flow, and data can be acquired from both the periphery and central nervous system. Results show that blood flow to the lateral wall of the basal turn of the cochlea (387 +/- 19 microl/g/min) is significantly higher (P<0.001) than that of the utricular macula (189 +/- 23 microl/g/min), horizontal (186 +/- 22 microl/g/min), superior (185 +/- 22 microl/g/min), or posterior canal crista (185 +/- 25 microl/g/min). Surprisingly, blood flow to all of the vestibular end-organs is remarkably similar. The use of this technique should allow pharmacological experimentation on inner ear blood flow without the unknown complications of anesthesia or invasive procedures.
The [14C]-2-deoxyglucose (2-DG) technique was used to study the frequency organization of the inferior colliculus (IC) of the guinea pig. Discrete regions of heightened 2-DG labelling were observed in the ICs of animals exposed to a variety of pure-tone stimuli. Regions associated with 1, 4, 10 and 19 kHz pure tones were described and displayed in three-dimensional representations. The IC of the guinea pig was found to be arranged as a series of sheet-like, iso-frequency planes that extend throughout the nucleus from its caudal to its rostral pole. Iso-frequency planes associated with low frequencies are located dorsolaterally in the nucleus and those associated with higher frequencies are located progressively more ventromedially. The predominant orientation, in the frontal plane, of all iso-frequency planes is oblique from dorsomedial to ventrolateral. Most planes, however, twist about their caudal-to-rostral axis in a caudal-to-rostral, horizontal-to-vertical direction. The extent to which each plane twists is frequency-dependent; planes associated with low frequencies twist most and those associated with high frequencies do not twist at all.
Measurements on human cadaver ears are reported that describe sound transmission through the middle ear. Four response variables were measured with acoustic stimulation at the tympanic membrane: stapes velocity, middle-ear cavity sound pressure, acoustic impedance at the tympanic membrane and acoustic impedance of the middle-ear cavity. Measurements of stapes velocity at different locations on the stapes suggest that stapes motion is predominantly 'piston-like', for frequencies up to at least 2000 Hz. The measurements are generally consistent with constraints of existing models. The measurements are used (1) to show how the cavity pressure and the impedance at the tympanic membrane are related, (2) to develop a measurement-based middle-ear cavity model, which shows that the middle-ear cavity has only small effects on the motion of the tympanic membrane and stapes in the normal ear, although it may play a more prominent role in pathological ears, and (3) to show that inter-ear variations in the impedance at the tympanic membrane and the stapes velocity are not well correlated.
Earlier we presented data (Scharf et al. (1994) Hear. Res. 75, 11-26) from a young patient (S.B.) who had undergone a vestibular neurotomy, during which the olivocochlear bundle (OCB) was severed. Those data are complemented by measurements on 15 other patients-some like S.B. with normal audiometric thresholds, none with a loss greater than 35 dB at experimental frequencies. Comparisons of performance for the same ear before and after surgery or between the operated and healthy ears do not provide evidence that the lack of OCB input impairs the following psychoacoustical functions: (1) detection of tonal signals, (2) intensity discrimination, (3) frequency selectivity, (4) loudness adaptation, (5) frequency discrimination within a tonal series, (6) in-head lateralization. Data on single-tone frequency discrimination are equivocal. These mostly negative results apply to listening both in the quiet and, where relevant, in noise. The only clear change in hearing after a vestibular neurotomy is that most patients detect signals at unexpected frequencies better than before. This change suggests an impaired ability to focus attention in the frequency domain. Although limited in scope, our finding that human hearing without OCB input is essentially normal agrees with much of the relevant literature on animal behavior and with the patients' self-reports.
The Tennessee Mouse Genome Consortium (TMGC) employed an N-ethyl-N-nitrosourea (ENU)-mutagenesis scheme to identify mouse recessive mutants with hearing phenotypes. We employed auditory brainstem responses (ABR) to click and 8, 16, and 32 kHz stimuli and screened 285 pedigrees (1819 mice of 8-11 weeks old in various mixed genetic backgrounds) each bred to carry a homozygous ENU-induced mutation. To define mutant pedigrees, we measured > or = 12 mice per pedigree in > or = 2 generations and used a criterion where the mean ABR threshold per pedigree was two standard deviations above the mean of all offspring from the same parental strain. We thus identified 17 mutant pedigrees (6%), all exhibiting hearing loss at high frequencies (> or = 16 kHz) with an average threshold elevation of 30-35 dB SPL. Interestingly, four mutants showed sex-biased hearing loss and six mutants displayed wide range frequency hearing loss. Temporal bone histology revealed that six of the first nine mutants displayed cochlear morphological defects: degeneration of spiral ganglia, spiral ligament fibrocytes or inner hair cells (but not outer hair cells) mostly in basal turns. In contrast to other ENU-mutagenesis auditory screens, our screen identified high-frequency, mild and sex-biased hearing defects. Further characterization of these 17 mouse models will advance our understanding of presbycusis and noise-induced hearing loss in humans.
SEM and CLSM studies were performed on the membranous labyrinth of Lampetra planeri, a threatened species of brook lamprey, spanning from the 1st to the 4th year of ammocoetes larval stages and on the adults. In all the examined stages, the entire membranous labyrinth does not show any morphologic differences, but only a progressive increase in size. SEM and CLSM observations show that the ciliated chamber is lined with numerous unsensorial multiciliated cells. In the early stages, the ciliary bundles were approximately 15 microm long, while in the late stages they reached 30 microm. In the crista sensory area, we observed two populations of hair cells. "Type II" cells are peculiar for this species and show both long stereocilia decreasing in length and a long kinocilium (10-12 microm). Two other types of ciliary bundles have been found on the sensory hair cells of the Macula communis: the first one has both kinocilium and stereocilia about 4-5 microm long; the second shows a long kinocilium (7-10 microm in length) and short stereocilia bundles with a gradual increase in length. In the early stages of development, the three macular areas show few and sparsely distributed hair cells. In the late developmental stages, hair cells become more numerous and densely populated.
We recently demonstrated a striking difference among inbred mouse strains in the effects of a single noise exposure, whereby CBA/J and CBA/CaJ (CBA) mice show moderate reversible reduction in the endocochlear potential (EP) while C57BL/6J (B6) mice do not (Ohlemiller, K.K., Gagnon, P.M., 2007. Genetic dependence of cochlear cells and structures injured by noise. Hear. Res. 224, 34-50). Acute EP reduction in CBA was reliably associated with characteristic pathology of the spiral ligament and stria vascularis, both immediately after noise and 8weeks later. Analysis of B6xCBA F1 hybrid mice indicated that EP reduction and its anatomic correlates are co-inherited in an autosomal dominant manner. Further analysis of N2 mice resulting from the backcross of F1 hybrids to B6 mice led us to suggest that the EP reduction phenotype principally reflects the influence of a small number of quantitative trait loci (QTLs). Here we report the results of QTL mapping of the EP reduction phenotype in CBA/J using 106 N2 mice from a (CBAxB6)xB6 backcross. Correlation of acute post-noise EP with 135 markers distributed throughout the genome revealed a single major effect QTL on chromosome 18 (12.5 cM, LOD 3.57) (Nirep, for noise-induced reduction in EP QTL), and two marginally significant QTLs on chromosomes 5 and 16 (LOD 1.43 and 1.73, respectively). Our results underscore that fact that different cochlear structures may possess different susceptibilities to noise through the influence of non-overlapping genes. While Nirep and similar-acting QTLs do not appear to influence the extent of permanent hearing loss from a single noise exposure, they could reduce the homeostatic 'reserve' of the lateral wall in protracted or continual exposures, and thereby influence long term threshold stability.
In 1956, Stevens 'commissioned' an experiment to equisect a pitch difference between two tones. Results appear to reveal a methodological flaw that would invalidate the Mel Scale (Stevens and Volkmann, 1940). Stevens sought to distinguish sensory continua, e.g., loudness and pitch, on various criteria. He expected that the pitch continuum would not exhibit 'hysteresis'; i.e., that subjects dividing a pitch difference (delta f) into equal-appearing parts would not set dividing frequencies higher when listening to notes in ascending order than in descending order. Seven subjects equisected a pitch difference, between tones of 400 and 7000 Hz, into equal-seeming parts by adjusting the frequencies of three intermediate tones. All seven exhibited hysteresis, contrary to expectation. This outcome bears on other issues. Years prior, Stevens suggested that equal pitch differences might correspond to equal cochlear distances, but not to equal frequency ratios nor to equal musical intervals (Stevens and Davis, 1938; Stevens and Volkmann, 1940). In 1960 (reported now), both the 1940 Mel Scale and the equal pitch differences of 1956 were compared to equal cochlear distances, using a frequency-position function that fitted Békésy's cochlear map (Greenwood, 1961, 1990). When ascending and descending settings were combined to contra-pose biases, equal pitch differences did coincide with equal distances--which the Mel Scale did not. Further, the biased ascending-order data coincided with the Mel Scale, suggesting the Mel Scale was similarly biased. Thus, the combined-order equal pitch differences of 1956--but not the Mel Scale--are consistent with equal cochlear distances. However, since the map between 400 and 7000 Hz is nearly logarithmic, equal frequency ratios also approximate equal distances. Ironically, above 400 Hz, Békésy's map and Stevens' equal-distance hypothesis jointly imply that musical intervals will nearly agree with equal pitch differences, which Stevens thought he had disconfirmed. However, given Békésy's map, only near the cochlear apex will equal distances not approximate equal frequency ratios; and Pratt's (Pratt, 1928) bisections of delta fs greater than an octave indicated that equal pitch differences, on average, did agree with equal distances. However, they did so for only two of four subjects and coincided instead with equal frequency ratios for one musical subject. Historical distinctions suggest that between the parts of equisected delta fs subjective equivalence may be of two kinds--one linked to musical intervals, leading to equal frequency ratios; a second linked to 'tone-height' and 'distance', leading to deviations from equal frequency ratios near the apex, though not appreciably if equisected delta fs are less than an octave (or if perhaps subjects are musicians). Data of other kinds suggest that, if pure-tone pitch height were a function of place, the place could be the apical excitation-pattern edge, in any case not a maximum, which in neural data shifts and disappears with tone level.
Several lines of evidence suggest that cochlear blood flow is under the control of the sympathetic nervous system and that this control is mediated via alpha-adrenergic receptors. The goal of the present study was to determine whether alpha-adrenergic receptors mediate vasoconstriction of the spiral modiolar artery and, if so, to determine which subtype dominates this response. Vascular diameter was measured with video microscopy in the isolated superfused spiral modiolar artery in vitro. The diameter of the spiral modiolar artery under control conditions was 61 +/- 2 microm (n = 60). Spontaneous vasomotion was observed in most specimens. Addition of norepinephrine to the superfusate caused a phasic vasoconstriction and an increase in the amplitude of vasomotion. These effects were limited to the vicinity of arteriolar branch points of the spiral modiolar artery. Norepinephrine-induced vasoconstriction occurred with EC50 of (1.9 +/- 0.4) x 10(-5) M (n = 44) and the vascular diameter was maximally reduced by a factor of 0.87 +/- 0.01 (n = 29). Neither the phasic nature nor the EC50 of the norepinephrine-induced vasoconstrictions was altered in the presence of the beta2-adrenergic receptor antagonist 10(-5) M ICI118551 or the nitric oxide synthase inhibitor 10(-4) M NOARG. In contrast, the alpha2-adrenergic receptor antagonist 10(-7) M yohimbine and the alpha1-adrenergic receptor antagonist 10(-9) and 10(-8) M prazosin caused a significant shift in the dose-response curve. The affinity constants (K(DB)) for yohimbine and prazosin were (5+/-2) x 10(-8) M (n=4) and (2.0+/-0.7) x 10(-10) M (n=18), respectively. The alpha1A-adrenergic receptor antagonist 10(-8) M 5-methyl urapidil and the alpha1D-adrenergic receptors antagonist 5 x 10(-6) M BMY7378 caused a significant shift in the dose-response curve. The K(DB) values for 5-methyl urapidil and for BMY7378 were (2.7 +/- 0.7) x 10(-10) M (n = 8) and (4.4 +/- 2.7) x 10(-7) M (n = 8), respectively. Further, total RNA was isolated from microdissected spiral modiolar arteries and the presence of transcripts for alpha1-adrenergic receptor subtypes was determined by reverse transcription polymerase chain reaction (RT-PCR). Primers specific for gerbil alpha1-adrenergic receptor subtypes were developed using RNA from rat and gerbil brain. Analysis of RNA extracted from the spiral modiolar artery revealed RT-PCR products of the appropriate size for the alpha1A-adrenergic receptor, however, no evidence for the alpha1B- and alpha1D-adrenergic receptor was found. Further, analysis of RNA extracted from blood, which was a contaminant of the microdissected spiral modiolar arteries, revealed no RT-PCR products. Sequence analysis of the RT-PCR product of the alpha1A-adrenergic receptor from the spiral modiolar artery confirmed its identity. Identity between the 175 nt gerbil sequence fragment and the known rat, mouse and human alpha1A-adrenergic receptor sequences was 90.9, 92.0 and 85.2%, respectively. These observations demonstrate that the spiral modiolar artery contains alpha1A-adrenergic receptors which mediate vasoconstriction at branch points.