Effects of combined gentamicin and furosemide treatment on cochlear ribbon synapses.

It is well-established that aminoglycoside antibiotics are ototoxic, and the toxicity can be drastically enhanced by the addition of loop diuretics, resulting in rapid irreversible hair cell damage. Using both electrophysiologic and morphological approaches, we investigated whether this combined treatment affected the cochlea at the region of ribbon synapses, consequently resulting in auditory synaptopathy. A series of varied gentamicin and furosemide doses were applied to C57BL/6 mice, and auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) were measured to assess ototoxic damage within the cochlea. In brief, the treatment effectively induced cochlear damage and promoted a certain reorganization of synaptic ribbons, while a reduction of ribbon density only occurred after a substantial loss of outer hair cells. In addition, both the ABR wave I amplitude and the ribbon density were elevated in low-dose treatment conditions, but a correlation between the two events was not significant for individual cochleae. In sum, combined gentamicin and furosemide treatment, at titrated doses below those that produce hair cell damage, typically triggers synaptic plasticity rather than a permanent synaptic loss.

Forward and Reverse Middle Ear Transmission in Gerbil with a Normal or Spontaneously Healed Tympanic Membrane.

Tympanic membranes (TM) that have healed spontaneously after perforation present abnormalities in their structural and mechanical properties; i.e., they are thickened and abnormally dense. These changes result in a deterioration of middle ear (ME) sound transmission, which is clinically presented as a conductive hearing loss (CHL). To fully understand the ME sound transmission under TM pathological conditions, we created a gerbil model with a controlled 50% pars tensa perforation, which was left to heal spontaneously for up to 4 weeks (TM perforations had fully sealed after 2 weeks). After the recovery period, the ME sound transmission, both in the forward and reverse directions, was directly measured with two-tone stimulation. Measurements were performed at the input, the ossicular chain, and output of the ME system, i.e., at the TM, umbo, and scala vestibuli (SV) next to the stapes. We found that variations in ME transmission in forward and reverse directions were not symmetric. In the forward direction, the ME pressure gain decreased in a frequency-dependent manner, with smaller loss (within 10 dB) at low frequencies and more dramatic loss at high frequency regions. The loss pattern was mainly from the less efficient acoustical to mechanical coupling between the TM and umbo, with little changes along the ossicular chain. In the reverse direction, the variations in these ears are relatively smaller. Our results provide detailed functional observations that explain CHL seen in clinical patients with abnormal TM, e.g., caused by otitis media, that have healed spontaneously after perforation or post-tympanoplasty, especially at high frequencies. In addition, our data demonstrate that changes in distortion product otoacoustic emissions (DPOAEs) result from altered ME transmission in both the forward and reverse direction by a reduction of the effective stimulus levels and less efficient transfer of DPs from the ME into the ear canal. This confirms that DPOAEs can be used to assess both the health of the cochlea and the middle ear.

An intracochlear DP-gram: Proof of principle in noise-damaged rabbits.

Distortion-product otoacoustic emissions (DPOAEs) can be used to assess cochlear damage and are often evaluated by generating a DP-gram in which 2f1-f2 DPOAE levels are plotted as a function of the higher-frequency primary at f2. DPOAEs are derived from the reverse propagation of distortion-product (DP) wavelets from their intracochlear sites of generation to emerge as measurable acoustic signals in the outer ear canal. However, at least, some of these same wavelets also propagate within the cochlea in the normal forward direction to the DP-frequency (fdp) place, where they appear as intracochlear distortion products (iDPs). Depending on several factors, especially, the extent to which DP wavelets add or cancel with each other in phase, one might expect iDPs to differ from DPOAEs in their ability to map the frequency pattern of cochlear damage. In the present study, the behavior of 2f1-f2 iDPs was inferred by interacting a probe tone (f3) with the iDP of interest to produce a 'secondary' DPOAE (i.e., DPOAE2ry), which was then used to infer the level of 2f1-f2 iDPs as a function of the f2-test frequency, thus, constituting a newly developed iDP-gram. To determine the feasibility of and potential applications for the iDP-gram procedure, noise-induced cochlear damage was assessed in two 'test' rabbits, one of which exhibited a well-defined punctate loss in their DP-gram, while the other exhibited a broader V-shaped loss. To validate the iDP-gram procedure, standard DP-grams were simultaneously collected and compared to their iDP-gram counterparts. Cochlear damage was independently assessed using auditory brainstem responses (ABRs) describing threshold-shift patterns to which both DP-gram types could be compared. Each DP-gram variety, to some extent, was able to detect a punctate loss in one rabbit and a broader V-shaped loss in the other. For the punctate-loss subject, the standard DP-gram showed a more generalized loss across test frequencies, while iDP-grams showed several localized notches superimposed on the generalized-loss pattern. In general, for the V-shaped loss pattern, both DP-gram types performed very well at detecting the large loss, with the lower primary-tone levels being most sensitive. At the narrow primary-tone ratios of f2/f1=1.05, standard DP-grams were unable to detect either loss pattern, while for the punctate loss, they paradoxically showed enhancement. Notably, the simultaneously collected iDP-grams performed favorably at the narrow-ratio setting, which is consistent with the notion that DPs travelling toward the 2f1-f2 fdp place are not subject to the cancellation of wavelets typical for narrow primary-ratio conditions that can confound measures of DPs moving towards the ear canal to emerge as DPOAEs.

Distortion product otoacoustic emissions: Sensitive measures of tympanic -membrane perforation and healing processes in a gerbil model.

Distortion product otoacoustic emissions (DPOAEs) evoked by two pure tones carry information about the mechanisms that generate and shape them. Thus, DPOAEs hold promise for providing powerful noninvasive diagnostic details of cochlear operations, middle ear (ME) transmission, and impairments. DPOAEs are sensitive to ME function because they are influenced by ME transmission twice, i.e., by the inward-going primary tones in the forward direction and the outward traveling DPOAEs in the reverse direction. However, the effects of ME injuries on DPOAEs have not been systematically characterized. The current study focused on exploring the utility of DPOAEs for examining ME function by methodically characterizing DPOAEs and ME transmission under pathological ME conditions, specifically under conditions of tympanic-membrane (TM) perforation and spontaneous healing. Results indicated that DPOAEs were measurable with TM perforations up to ∼50%, and DPOAE reductions increased with increasing size of the TM perforation. DPOAE reductions were approximately flat across test frequencies when the TM was perforated about 10% (<1/8 of pars tensa) or less. However, with perforations greater than 10%, DPOAEs decreased further with a low-pass filter shape, with ∼30 dB loss at frequencies below 10 kHz and a quick downward sloping pattern at higher frequencies. The reduction pattern of DPOAEs across frequencies was similar to but much greater than, the directly measured ME pressure gain in the forward direction, which suggested that reduction in the DPOAE was a summation of losses of ME ear transmission in both the forward and reverse directions. Following 50% TM perforations, DPOAEs recovered over a 4-week spontaneously healing interval, and these recoveries were confirmed by improvements in auditory brainstem response (ABR) thresholds. However, up to 4-week post-perforation, DPOAEs never fully recovered to the levels obtained with normal intact TM, consistent with the incomplete recovery of ABR thresholds and ME transmission, especially at high-frequency regions, which could be explained by an irregularly dense and thickened healed TM. Since TM perforations in patients are commonly caused by either trauma or infection, the present results contribute towards providing insight into understanding ME transmission under pathological conditions as well as promoting the application of DPOAEs in the evaluation and diagnosis of deficits in the ME-transmission system.

Effects of tympanic membrane perforation on middle ear transmission in gerbil.

Perforations of the tympanic membrane (TM) alter its structural and mechanical properties, thus resulting in a deterioration of sound transmission through the middle ear (ME), which presents itself clinically as a conductive hearing loss (CHL). The resulting CHL is proposed to be due to the loss of the pressure difference across the TM between the outer ear canal space and the ME cavity, a hypothesis which has been tested with both theoretical and experimental approaches. In the past, direct experimental observations had been either from the ME input (umbo) or the output of the stapes, and were focused mainly on the low frequency region. However, there was little documentation providing a thorough picture of the influence of systematically increasing sizes of TM perforations on ME sound transmission from the input (i.e., pressure at the TM or motion of the umbo) to the output (pressure produced by the motion of the stapes). Our study explored ME transmission in gerbil under conditions of a normal, intact TM followed by the placement of mechanically-induced TM perforations ranging from miniscule to complete removal of the pars tensa, leaving the other parts of ME intact. Testing up to 50 kHz, variations of ME transmission were characterized in simultaneously measured tone induced pressure responses at the TM (PTM), pressure responses in the scala vestibuli next to the stapes (PSV), and velocity measurements of the umbo (Vumbo), as well as by detailed descriptions of sound transmission from the TM to the stapes, i.e., the umbo transfer function (TF), the transfer of the sound stimulus along the ossicular chain as found from the ratio of cochlear pressure to umbo motion, and ME pressure gain (MEPG). Our results suggested that increasing the size of TM perforations led to a reduction in MEPG, which appeared to be primarily due to the reduction in the effective/initial mechanical drive to the umbo, with a relatively smaller decrease of sound transfer along the ossicular chain. Expansion of the perforation more than 25% appeared to drastically reduce sound transmission through the ME, especially for the higher frequencies.

Comparing Distortion Product Otoacoustic Emissions to Intracochlear Distortion Products Inferred from a Noninvasive Assay.

The behavior of intracochlear distortion products (iDPs) was inferred by interacting a probe tone (f3) with the iDP of interest to produce a "secondary" distortion product otoacoustic emission termed DPOAE(2ry). Measures of the DPOAE(2ry) were then used to deduce the properties of the iDP. This approach was used in alert rabbits and anesthetized gerbils to compare ear-canal 2f1-f2 and 2f2-f1 DPOAE f2/f1 ratio functions, level/phase (L/P) maps, and interference-response areas (IRAs) to their simultaneously collected DPOAE(2ry) counterparts. These same measures were also collected in a human volunteer to demonstrate similarities with their laboratory animal counterparts and their potential applicability to humans. Results showed that DPOAEs and inferred iDPs evidenced distinct behaviors and properties. That is, DPOAE ratio functions elicited by low-level primaries peaked around an f2/f1 = 1.21 or 1.25, depending on species, while the corresponding inferred iDP ratio functions peaked at f2/f1 ratios of ~1. Additionally, L/P maps showed rapid phase variation with DPOAE frequency (fdp) for the narrow-ratio 2f1-f2 and all 2f2-f1 DPOAEs, while the corresponding DPOAE(2ry) measures evidenced relatively constant phases. Common features of narrow-ratio DPOAE IRAs, such as large enhancements for interference tones (ITs) presented above f2, were not present in DPOAE(2ry) IRAs. Finally, based on prior experiments in gerbils, the behavior of the iDP directly measured in intracochlear pressure was compared to the iDP inferred from the DPOAE(2ry) and found to be similar. Together, these findings are consistent with the notion that under certain conditions, ear-canal DPOAEs provide poor representations of iDPs and thus support a "beamforming" hypothesis. According to this concept, distributed emission components directed toward the ear canal from the f2 and basal to f2 regions can be of differing phases and thus cancel, while these same components directed toward fdp add in phase.

Influence of sound-conditioning on noise-induced susceptibility of distortion-product otoacoustic emissions.

Cochlear damage caused by loud sounds can be attenuated by "sound-conditioning" methods. The amount of adaptation for distortion product otoacoustic emissions (DPOAEs) measured in alert rabbits previously predicted an ear's susceptibility to a subsequent noise exposure. The present study investigated if sound-conditioning influenced the robustness of such DPOAE adaptation, and if such conditioning elicited more protection by increasing the amount of DPOAE adaptation. Toward this end, rabbits were divided into two study groups: (1) experimental animals exposed to a sound-conditioning protocol, and (2) unconditioned control animals. After base-line measures, all rabbits were exposed to an overstimulation paradigm consisting of an octave band noise, and then re-assessed 3 weeks post-exposure to determine permanent changes in DPOAEs. A major result was that prior sound-conditioning protected reductions in DPOAE levels by an average of 10-15 dB. However, DPOAE adaptation decreased with sound-conditioning, so that such conditioning was no longer related to noise-induced reductions in DPOAEs. Together, these findings suggest that sound-conditioning affected neural pathways other than those that likely mediate DPOAE adaptation (e.g., medial olivocochlear efferent and/or middle-ear muscle reflexes).

Adaptation of distortion product otoacoustic emissions predicts susceptibility to acoustic over-exposure in alert rabbits.

A noninvasive test was developed in rabbits based on fast adaptation measures for 2f1-f2 distortion-product otoacoustic emissions (DPOAEs). The goal was to evaluate the effective reflex activation, i.e., "functional strength," of both the descending medial olivocochlear efferent reflex (MOC-R) and the middle-ear muscle reflex (MEM-R) through sound activation. Classically, it is assumed that both reflexes contribute toward protecting the inner ear from cochlear damage caused by noise exposure. The DP-gram method described here evaluated the MOC-R effect on DPOAE levels over a two-octave (oct) frequency range. To estimate the related activation of the middle-ear muscles (MEMs), the MEM-R was measured by monitoring the level of the f1-primary tone throughout its duration. Following baseline measures, rabbits were subjected to noise over-exposure. A main finding was that the measured adaptive activity was highly variable between rabbits but less so between the ears of the same animal. Also, together, the MOC-R and MEM-R tests showed that, on average, DPOAE adaptation consisted of a combined contribution from both systems. Despite this shared involvement, the amount of DPOAE adaptation measured for a particular animal's ear predicted that ear's subsequent susceptibility to the noise over-exposure for alert but not for deeply anesthetized rabbits.

Time-domain demonstration of distributed distortion-product otoacoustic emission components.

Distortion-product otoacoustic emissions (DPOAEs) were measured in rabbits as time waveforms by employing a phase-rotation technique to cancel all components in the final average, except the 2f1-f2 DPOAE. Subsequent filtering allowed the DPOAE waveform to be clearly visualized in the time domain. In most conditions, f2 was turned off for 6 ms, which produced a gap so that the DPOAE was no longer generated. These procedures allowed the DPOAE onset as well as the decay during the gap to be observed in the time domain. DPOAEs were collected with L1 = L2 = 65-dB sound pressure level primary-tone levels for f2/f1 ratios from 1.25 to 1.01 in 0.02 steps. Findings included the appearance of complex onsets and decays for the DPOAE time waveforms as the f2/f1 ratio was decreased and the DPOAE level was reduced. These complexities were unaffected by interference tones (ITs) near the DPOAE frequency place (fdp), but could be removed by ITs presented above f2, which also increased DPOAE levels. Similar outcomes were observed when DPOAEs were measured at a sharp notch in the DPOAE level as a function of the f2 primary tone frequency, i.e., DP-gram. Both findings were consistent with the hypothesis that the DPOAE-ratio function, and some notches in the DP-gram, are caused by interactions of distributed DPOAE components with unique phases.

Characterizing distortion-product otoacoustic emission components across four species.

Distortion-product otoacoustic emissions (DPOAEs) were measured as level/phase (L/P) maps in humans, rabbits, chinchillas, and rats with and without an interference tone (IT) placed either near the 2f(1)-f(2) DPOAE frequency place (f(dp)) or at one-third of an octave above the f(2) primary tone (1/3-oct IT). Vector differences between with and without IT conditions were computed to derive a residual composed of the DPOAE components removed by the IT. In humans, a DPOAE component could be extracted with the expected steep phase gradient indicative of reflection emissions by ITs near f(dp). In the laboratory species, ITs near f(dp) failed to produce any conclusive evidence for reflection components. For all species, 1/3-oct ITs extracted large DPOAE components presumably generated at or basal to the IT-frequency place that exhibited both distortion- and reflection-like phase properties. Together, these findings suggested that basal distortion components could assume reflection-like phase behavior when the assumptions of cochlear-scaling symmetry, the basis for shallow phase gradients for constant f(2)/f(1) ratio sweeps, are violated. The present results contradict the common belief that DPOAE components associated with steep or shallow phase slopes are unique signatures for reflection emissions arising from f(dp) or distortion emissions generated near f(2), respectively.

Noise-induced changes in gene expression in the cochleae of mice differing in their susceptibility to noise damage.

The molecular mechanisms underlying the vast differences between individuals in their susceptibility to noise-induced hearing loss (NIHL) are unknown. The present study demonstrated that the effects of noise over-exposure on the expression of molecules likely to be important in the development of NIHL differ among inbred mouse strains having distinct susceptibilities to NIHL including B6 (B6.CAST) and 129 (129X1/SvJ and 129S1/SvImJ) mice. The noise-exposure protocol produced a loss of 40 dB in hearing sensitivity in susceptible B6 mice, but no loss for the two resistant 129 substrains. Analysis of gene expression in the membranous labyrinth 6 h following noise exposure revealed upregulation of transcription factors in both the susceptible and resistant strains. However, a significant induction of genes involved in cell-survival pathways such as the heat shock proteins HSP70 and HSP40, growth arrest and DNA-damage-inducible protein 45ß (GADD45ß), and CDK-interacting protein 1 (p21(Cip1)) was detected only in the resistant mice. Moreover, in 129 mice significant upregulation of HSP70, GADD45ß, and p21(Cip1) was confirmed at the protein level. Since the functions of these proteins include roles in potent anti-apoptotic cellular pathways, their upregulation may contribute to protection from NIHL in the resistant 129 mice.

Evidence for basal distortion-product otoacoustic emission components.

Distortion-product otoacoustic emissions (DPOAEs) were measured with traditional DP-grams and level/phase (L/P) maps in rabbits with either normal cochlear function or unique sound-induced cochlear losses that were characterized as either low-frequency or notched configurations. To demonstrate that emission generators distributed basal to the f(2) primary-tone contribute, in general, to DPOAE levels and phases, a high-frequency interference tone (IT) was presented at 1/3 of an octave (oct) above the f(2) primary-tone, and DPOAEs were re-measured as "augmented" DP-grams (ADP-grams) and L/P maps. The vector difference between the control and augmented functions was then computed to derive residual DP-grams (RDP-grams) and L/P maps. The resulting RDP-grams and L/P maps, which described the DPOAEs removed by the IT, supported the notion that basal DPOAE components routinely contribute to the generation of standard measures of DPOAEs. Separate experiments demonstrated that these components could not be attributed to the effects of the 1/3-oct IT on f(2), or DPOAEs generated by the addition of a third interfering tone. These basal components can "fill in" the lesion estimated by the commonly employed DP-gram. Thus, ADP-grams more accurately reveal the pattern of cochlear damage and may eventually lead to an improved DP-gram procedure.

Steep and shallow phase gradient distortion product otoacoustic emissions arising basal to the primary tones.

Distortion product otoacoustic emission (DPOAE) level/phase maps were collected in humans with and without an interference tone (IT) near the DPOAE frequency place (f(dp)) at primary-tone levels of 75 dB SPL. A DPOAE component with the expected steep phase gradient could be extracted at f(dp), however, considerable vertical-phase banding, presumably indicative of reflection emissions, remained. An IT placed 0.33 oct above f(2) removed most of this banding, revealing DPOAE components originating basal to the IT frequency place. These findings suggest that the commonly accepted two-source model of DPOAE generation may need to be qualified when higher primary-tone levels are utilized.

Sex differences in distortion-product and transient-evoked otoacoustic emissions compared.

Although several studies have documented the existence of sex differences in spontaneous otoacoustic emissions (SOAEs) and transient-evoked OAEs (TEOAEs) in humans, less has been published about sex differences in distortion-product OAEs (DPOAEs). Estimates of sex and ear differences were extracted from a data set of OAE measurements previously collected for other purposes. In accord with past findings, the sex differences for TEOAEs were substantial for both narrowband and wideband measures. By contrast, the sex differences for DPOAEs were about half the size of those for TEOAEs. In this sample, the ear differences were small for TEOAEs in both sexes and absent for DPOAEs. One implication is that the cochlear mechanisms underlying DPOAEs appear to be less susceptible to whatever influences are responsible for producing sex differences in TEOAEs and SOAEs in humans. We discuss the possibility that differences in the effective level of the stimuli may contribute to these outcomes.

Noise-induced hearing loss in mice treated with antiretroviral drugs.

The results reported here for CBA/CaJ mice describe the effects of regular dosing with a common antiretroviral drug combination on outer hair cell (OHC) function using measures of 2f1-f2 distortion product otoacoustic emissions (DPOAEs) and auditory brainstem responses (ABRs). Specifically, experimental mice were treated daily over a 3-mo period with the nucleoside reverse transcriptase inhibitors (NRTIs), zidovudine (ZDV) and lamivudine (3TC), dissolved in their drinking water, while their control counterparts received untreated water. DPOAE levels and ABR detection thresholds prior to and after 12 wk of NRTI treatment did not differ between experimental and control groups. To assess whether NRTI treatment potentiates the adverse effects of noise over-exposure on OHC function, both experimental and control mice were exposed 1 wk later, while still on the drug regimen, to a 10-kHz octave-band noise (OBN) at 105-dB SPL for 1h. A major outcome of the sound over-exposure episode was that the NRTI-pretreated mice showed significantly greater permanent OBN-induced reductions in DPOAE levels at 2 wk postexposure than were observed for the untreated control animals. These findings support the notion that a synergistic relationship exists between certain NRTIs and intense sounds in that such preexposure drug treatments produced greater noise-induced decreases in DPOAE activity than did noise exposure alone. This drug/noise interaction is consistent with the known harmful effects of NRTIs on cellular mitochondrial activity.

Comparison of distortion product otoacoustic emissions in 28 inbred strains of mice.

Cochlear function was evaluated in a longitudinal study of 28 inbred strains of mice at 3 and 5 mo of age using measures of distortion product otoacoustic emissions (DPOAEs) in response to a federal initiative to develop rapid mouse phenotyping methodologies. DP-grams at f(2) frequencies ranging from 6.3 to 54.2kHz were obtained in about 3min/ear by eliciting 2f(1)-f(2) DPOAEs in 0.1-octave steps of f(2) with primary tones at L(1)=L(2) =55, 65, and 75dB SPL. CBA/CaJ mice exhibited average levels of approximately 26dB SPL and this strain was selected as the normal reference strain against which the others were compared. Based upon the configurations of their DP-grams, the 28 mouse strains could be categorized into four distinct groups. That is, nine of the strains including the CBA were designated as the CBA-like group because these mice displayed robust DPOAE levels across frequency. In contrast, the remaining three groups all exhibited irregular DP-gram patterns. Specifically, eight of the remaining 19 strains showed a progressive high- to low-frequency reduction in DPOAE levels that was typical of age-related hearing loss (AHL) associated with mouse strains homozygous for the ahl allele and were labeled as AHL-like strains. Seven strains demonstrating relatively even patterns of reduced DPOAE levels across the frequency-test range were designated as Flat-loss strains. Finally, the remaining four strains exhibited no measurable DPOAEs at either 3 or 5 mo of age and thus were classified as Absent strains. Extending the f(2) test frequencies up to approximately 54kHz led to the detection of very early-onset reductions in cochlear function in non-CBA-like groups so that all strains could be categorized by 3 mo of age. Predictably, the AHL-like strains showed more pronounced DPOAE losses at 5 mo than at 3 mo. A similar deterioration in DPOAE levels was not apparent for the Flat-loss strains. Both the AHL-like and Flat-loss strains showed considerably more variability in DPOAE levels than did the CBA-like strains. Together, these findings indicate that DP-grams adequately reveal both frequency-specific loss patterns and details of inbred strain variability.

Distortion-product otoacoustic emission suppression growth in normal and noise-exposed rabbits.

This study investigated noise-induced changes in suppression growth (SG) of distortion product otoacoustic emissions (DPOAEs). Detailed measurements of SG were obtained in rabbits as a function of f2 frequencies at four primary-tone levels. SG measures were produced by using suppressor tones (STs) presented at two fixed distances from f2. The magnitude of suppression was calculated for each ST level and depicted as contour plots showing the amount of suppression as a function of the f2 frequency. At each f2, SG indices included slope, suppression threshold, and an estimate of the tip-to-tail value. All suppression measures were obtained before and after producing a cochlear dysfunction using a monaural exposure to a 2-h, 110-dB SPL octave-band noise centered at 2 kHz. The noise exposure produced varying amounts of cochlear damage as revealed by changes in DP-grams and auditory brainstem responses. However, average measures of SG slopes, suppression thresholds, and tip-to-tail values failed to mirror the mean DP-gram loss patterns. When suppression-based parameters were correlated with the amount of DPOAE loss, small but significant correlations were observed for some measures. Overall, the findings suggest that measures derived from DPOAE SG are limited in their ability to detect noise-induced cochlear damage.

Assessment of cochlear function in mice: distortion-product otoacoustic emissions.

Distortion-product otoacoustic emissions (DPOAEs) can be measured in the ear canal following the presentation of two tones. These emissions are generated by the outer hair cells (OHCs) of the inner ear and they are reduced or absent when the OHCs are damaged by, for example, exposure to excessive noise or ototoxic drugs. Consequently, DPOAEs provide a powerful and noninvasive means to assess the robustness of OHC function. A detailed method is described for measuring DPOAEs to assess cochlear function in mice. Recommendations are given for the required equipment and instructions are presented for setting up a DPOAE system. Also, a protocol is outlined for measuring DPOAEs in mice and troubleshooting tips are provided. Examples of data analysis procedures following noise exposure in mice are included, as well. These methods are not only applicable to mice, but can be performed using essentially all small laboratory animals.

Human efferent adaptation of DPOAEs in the L1,L2 space.

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2f(1)-f2 were investigated in 12 ears of normally hearing adults aged 18-30 years using long-lasting 1-s primary-tone on-times. In this manner, DPOAE adaptation at a single f2 of 1.55 kHz (f2/f1=1.21) was evaluated as a function of the levels of the primary tones in a matrix of L1, L2 settings, which varied from 45 to 80 dB SPL, in 5-dB steps. DPOAEs were elicited under both monaural and binaural stimulus-presentation conditions. Adaptation was defined as the difference in DPOAE levels between the initial 92-ms baseline measure using a standard protocol and one obtained during the final 92 ms of the prolonged 1-s primary-tones. These differences were averaged across subjects to create contour plots of mean adaptation in the L1,L2 space. The 2f(1)-f2 DPOAE revealed consistent regions of suppression (-0.5 dB difference) or enhancement (+0.5 dB difference) with respect to baseline measures within the L(1),L(2) matrix for both acoustic-stimulation conditions. Specifically, 2f(1)-f2 DPOAE suppressions of 1-2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L1>L2. In contrast, larger 2f(1)-f2 DPOAE enhancements of 3-4 dB occurred for only the binaural condition, at primary-tone level combinations where L1

Cochlear function in mice following inhalation of brevetoxin-3.

Brevetoxin-3 was shown previously to adversely affect central auditory function in goldfish. The present study evaluated the effects of exposure to this agent on cochlear function in mice using the 2f(1)-f(2) distortion-product otoacoustic emission (DPOAE). Towards this end, inbred CBA/CaJ mice were exposed to a relatively high concentration of brevetoxin-3 (approximately=400 microg/m(3)) by nose-only inhalation for a 2-h period. Further, a subset of these mice received a second exposure a day later that lasted for an additional 4 h. Mice exposed only once for 2 h did not exhibit any notable cochlear effects. Similarly, mice exposed two times, for a cumulative dose of 6 h, exhibited essentially no change in DPOAE levels.