Three-dimensional quantification of fibrosis and ossification after cochlear implantation via virtual re-sectioning: Potential implications for residual hearing.

Hearing preservation may be achieved initially in the majority of patients after cochlear implantation, however, a significant proportion of these patients experience delayed hearing loss months or years later. A prior histological report in a case of delayed hearing loss suggested a potential cochlear mechanical origin of this hearing loss due to tissue fibrosis, and older case series highlight the frequent findings of post-implantation fibrosis and neoosteogenesis though without a focus on the impact on residual hearing. Here we present the largest series (N = 20) of 3-dimensionally reconstructed cochleae based on digitally scanned histologic sections from patients who were implanted during their lifetime. All patients were implanted with multichannel electrodes via a cochleostomy or an extended round window insertion. A quantified analysis of intracochlear tissue formation was carried out via virtual re-sectioning orthogonal to the cochlear spiral. Intracochlear tissue formation was present in every case. On average 33% (SD 14%) of the total cochlear volume was occupied by new tissue formation, consisting of 26% (SD 12%) fibrous and 7% (SD 6%) bony tissue. The round window was completely covered by fibro-osseous tissue in 85% of cases and was associated with an obstruction of the cochlear aqueduct in 100%. The basal part of the basilar membrane was at least partially abutted by the electrode or new tissue formation in every case, while the apical region, corresponding with a characteristic frequency of < 500 Hz, appeared normal in 89%. This quantitative analysis shows that after cochlear implantation via extended round window or cochleostomy, intracochlear fibrosis and neoossification are present in all cases at anatomical locations that could impact normal inner ear mechanics.

Primary Neural Degeneration in the Human Cochlea: Evidence for Hidden Hearing Loss in the Aging Ear.

The noise-induced and age-related loss of synaptic connections between auditory-nerve fibers and cochlear hair cells is well-established from histopathology in several mammalian species; however, its prevalence in humans, as inferred from electrophysiological measures, remains controversial. Here we look for cochlear neuropathy in a temporal-bone study of "normal-aging" humans, using autopsy material from 20 subjects aged 0-89 yrs, with no history of otologic disease. Cochleas were immunostained to allow accurate quantification of surviving hair cells in the organ Corti and peripheral axons of auditory-nerve fibers. Mean loss of outer hair cells was 30-40% throughout the audiometric frequency range (0.25-8.0 kHz) in subjects over 60 yrs, with even greater losses at both apical (low-frequency) and basal (high-frequency) ends. In contrast, mean inner hair cell loss across audiometric frequencies was rarely >15%, at any age. Neural loss greatly exceeded inner hair cell loss, with 7/11 subjects over 60 yrs showing >60% loss of peripheral axons re the youngest subjects, and with the age-related slope of axonal loss outstripping the age-related loss of inner hair cells by almost 3:1. The results suggest that a large number of auditory neurons in the aging ear are disconnected from their hair cell targets. This primary neural degeneration would not affect the audiogram, but likely contributes to age-related hearing impairment, especially in noisy environments. Thus, therapies designed to regrow peripheral axons could provide clinically meaningful improvement in the aged ear.

Blast-induced cochlear synaptopathy in chinchillas.

When exposed to continuous high-level noise, cochlear neurons are more susceptible to damage than hair cells (HCs): exposures causing temporary threshold shifts (TTS) without permanent HC damage can destroy ribbon synapses, permanently silencing the cochlear neurons they formerly activated. While this "hidden hearing loss" has little effect on thresholds in quiet, the neural degeneration degrades hearing in noise and may be an important elicitor of tinnitus. Similar sensory pathologies are seen after blast injury, even if permanent threshold shift (PTS) is minimal. We hypothesized that, as for continuous-noise, blasts causing only TTS can also produce cochlear synaptopathy with minimal HC loss. To test this, we customized a shock tube design to generate explosive-like impulses, exposed anesthetized chinchillas to blasts with peak pressures from 160-175 dB SPL, and examined the resultant cochlear dysfunction and histopathology. We found exposures that cause large >40 dB TTS with minimal PTS or HC loss often cause synapse loss of 20-45%. While synaptopathic continuous-noise exposures can affect large areas of the cochlea, blast-induced synaptopathy was more focal, with localized damage foci in midcochlear and basal regions. These results clarify the pathology underlying blast-induced sensory dysfunction, and suggest possible links between blast injury, hidden hearing loss, and tinnitus.

Noise-induced cochlear synaptopathy in rhesus monkeys (Macaca mulatta).

Cochlear synaptopathy can result from various insults, including acoustic trauma, aging, ototoxicity, or chronic conductive hearing loss. For example, moderate noise exposure in mice can destroy up to ∼50% of synapses between auditory nerve fibers (ANFs) and inner hair cells (IHCs) without affecting outer hair cells (OHCs) or thresholds, because the synaptopathy occurs first in high-threshold ANFs. However, the fiber loss likely impairs temporal processing and hearing-in-noise, a classic complaint of those with sensorineural hearing loss. Non-human primates appear to be less vulnerable to noise-induced hair-cell loss than rodents, but their susceptibility to synaptopathy has not been studied. Because establishing a non-human primate model may be important in the development of diagnostics and therapeutics, we examined cochlear innervation and the damaging effects of acoustic overexposure in young adult rhesus macaques. Anesthetized animals were exposed bilaterally to narrow-band noise centered at 2 kHz at various sound-pressure levels for 4 h. Cochlear function was assayed for up to 8 weeks following exposure via auditory brainstem responses (ABRs) and otoacoustic emissions (OAEs). A moderate loss of synaptic connections (mean of 12-27% in the basal half of the cochlea) followed temporary threshold shifts (TTS), despite minimal hair-cell loss. A dramatic loss of synapses (mean of 50-75% in the basal half of the cochlea) was seen on IHCs surviving noise exposures that produced permanent threshold shifts (PTS) and widespread hair-cell loss. Higher noise levels were required to produce PTS in macaques compared to rodents, suggesting that primates are less vulnerable to hair-cell loss. However, the phenomenon of noise-induced cochlear synaptopathy in primates is similar to that seen in rodents.

The spiral ganglion and Rosenthal's canal in beluga whales.

With the increase of human activity and corresponding increase in anthropogenic sounds in marine waters of the Arctic, it is necessary to understand its effect on the hearing of marine wildlife. We have conducted a baseline study on the spiral ganglion and Rosenthal's canal of the cochlea in beluga whales (Delphinapterus leucas) as an initial assessment of auditory anatomy and health. We present morphometric data on the length of the cochlea, number of whorls, neuron densities along its length, Rosenthal's canal length, and cross-sectional area, and show some histological results. In belugas, Rosenthal's canal is not a cylinder of equal cross-sectional area, but its cross-section is greatest near the apex of the basal whorl. We found systematic variation in the numbers of neurons along the length of the spiral ganglion, indicating that neurons are not dispersed evenly in Rosenthal's canal. These results provide data on functionally important structural parameters of the beluga ear. We observed no signs of acoustic trauma in our sample of beluga whales.

Deletion of SLC19A2, the high affinity thiamine transporter, causes selective inner hair cell loss and an auditory neuropathy phenotype.

Mutations in the gene coding for the high-affinity thiamine transporter Slc19a2 underlie the clinical syndrome known as thiamine-responsive megaloblastic anemia (TRMA) characterized by anemia, diabetes, and sensorineural hearing loss. To create a mouse model of this disease, a mutant line was created with targeted disruption of the gene. Cochlear function is normal in these mutants when maintained on a high-thiamine diet. When challenged with a low-thiamine diet, Slc19a2-null mice showed 40-60 dB threshold elevations by auditory brainstem response (ABR), but only 10-20 dB elevation by otoacoustic emission (OAE) measures. Wild-type mice retain normal hearing on either diet. Cochlear histological analysis showed a pattern uncommon for sensorineural hearing loss: selective loss of inner hair cells after 1-2 weeks on low thiamine and significantly greater inner than outer hair cell loss after longer low-thiamine challenges. Such a pattern is consistent with the observed discrepancy between ABR and OAE threshold shifts. The possible role of thiamine transport in other reported cases of selective inner hair cell loss is considered.

Otoacoustic emissions without somatic motility: can stereocilia mechanics drive the mammalian cochlea?

Distortion product otoacoustic emissions (DPOAEs) evoked by low-level tones are a sensitive indicator of outer hair cell (OHC) function. High-level DPOAEs are less vulnerable to cochlear insult, and their dependence on the OHC function is more controversial. Here, the mechanism underlying high-level DPOAE generation is addressed using a mutant mouse line lacking prestin, the molecular motor driving OHC somatic motility, required for cochlear amplification. With prestin deletion, attenuated DPOAEs were measurable at high sound levels. DPOAE thresholds were shifted by approximately 50 dB, matching the loss of cochlear amplifier gain measured in compound action potentials. In contrast, at high sound levels, distortion products in the cochlear microphonic (CM) of mutants were not decreased re wildtypes (expressed re CM at the primaries). Distortion products in both CM and otoacoustic emissions disappeared rapidly after death. The results show that OHC somatic motility is not necessary for the production of DPOAEs at high SPLs. They also suggest that the small, physiologically vulnerable DPOAE that remains without prestin-based motility is due directly to the mechanical nonlinearity associated with stereociliary transduction, and that this stereocilia mechanical nonlinearity is robustly coupled to the motion of the cochlear partition to the extent that it can drive the middle ear.

Effects of anesthesia on efferent-mediated adaptation of the DPOAE.

Distortion product otoacoustic emissions (DPOAE) adapt after primary tone onset, with an approximately 100 ms time constant, due to feedback effects of medial olivocochlear (MOC) activity elicited by the primary tones. We tracked DPOAE postonset adaptation as a metric of MOC reflex strength, before during and after induction of anesthesia in guinea pigs. Reflex strength was significantly diminished by the barbiturate/neuroleptic anesthesia most commonly used in this species. The MOC reflex recovered more slowly than toe-pinch or startle reflexes, correlating better with recovery of general mobility. When individual anesthetic agents were assessed, the barbiturate (pentobarbital) significantly diminished MOC reflex strength, whereas fentanyl or droperidol did not. These results suggest that previous studies using anesthetized preparations may have underestimated the magnitude of sound-evoked responses in the OC pathway.

Effects of olivocochlear feedback on distortion product otoacoustic emissions in guinea pig.

Activation of ipsilaterally responsive olivocochlear (OC) neurons by sound produces rapid, post-onset alterations in the 2f1-f2 distortion product otoacoustic emission (DPOAE). The present study investigates the frequency and level dependence of this ipsilateral OC effect in the anesthetized guinea pig, compares its magnitude and sign to OC effects elicited by contralateral sound ("contralateral" OC effect), and characterizes the influence of such activity on steady-state DPOAE amplitude. DPOAEs were measured with fine time resolution in response to primary stimuli varied systematically in frequency and level. DPOAEs showed rapid and remarkably stereotyped post-onset amplitude alterations. These ipsilateral OC effects were greater for high (8-12 kHz) than for low (2-4 kHz) f2 primary frequencies and for higher primary levels (70-80 dB SPL). For any f2/f1 pair, the sign as well as the magnitude of the ipsilateral effects varied with. primary level ratio. For example, with L1 fixed at 75 and L2 varied in 1-dB steps from 60 to 75 dB SPL, DPOAE amplitude underwent a stereotyped progression from post-onset increases at the lowest levels of the f2 primary to post-onset decreases at the highest levels. At intermediate levels, near the region of sign change (L2 = 5-10 dB below L1), post-onset effects were often particularly large (as great as 20 dB). These large ipsilateral OC effects were always associated with "dips" in the DPOAE amplitude vs. level functions, and both disappeared after OC section. Although smaller in magnitude, contralateral OC effects were identical to ipsilateral effects in frequency and level dependence and in form.

Spiral ligament pathology: a major aspect of age-related cochlear degeneration in C57BL/6 mice.

Data from systematic, light microscopic examination of cochlear histopathology in an age-graded series of C57BL/6 mice (1.5-15 months) were compared with threshold elevations (measured by auditory brain stem response) to elucidate the functionally important structural changes underlying age-related hearing loss in this inbred strain. In addition to quantifying the degree and extent of hair cell and neuronal loss, all structures of the cochlear duct were qualitatively evaluated and any degenerative changes were quantified. Hair cell and neuronal loss patterns suggested two degenerative processes. In the basal half of the cochlea, inner and outer hair cell loss proceeded from base to apex with increasing age, and loss of cochlear neurons was consistent with degeneration occurring secondary to inner hair cell loss. In the apical half of the cochlea with advancing age, there was selective loss of outer hair cells which increased from the middle to the extreme apex. A similar gradient of ganglion cell loss was noted, characterized by widespread somatic aggregation and demyelination. In addition to these changes in hair cells and their innervation, there was widespread degeneration of fibrocytes in the spiral ligament, especially among the type IV cell class. The cell loss in the ligament preceded the loss of hair cells and/or neurons in both space and time suggesting that fibrocyte pathology may be a primary cause of the hearing loss and ultimate sensory cell degeneration in this mouse strain.

Selective inner hair cell loss in premature infants and cochlea pathological patterns from neonatal intensive care unit autopsies.

BACKGROUND: Deafness and handicapping sensorineural hearing impairment occur frequently in neonatal intensive care unit survivors for unknown reasons. PATIENTS AND METHODS: Hearing was tested early and repeatedly in neonatal intensive care unit patients with an auditory brainstem response (ABR) screener. The temporal bones of 15 nonsurvivors (30 ears) were fixed promptly (average, 5 hours) after death for histological evaluation. RESULTS: Among these patients, 12 failed the ABR screen bilaterally, 1 passed unilaterally, and 2 passed bilaterally. Cochlear histopathologic conditions that could contribute to hearing loss included bilateral selective outer hair cell loss in 2 patients, bilateral selective inner hair cell loss in 3 (all premature), and a combination of both outer and inner hair cell loss in 2. Other hair cell abnormalities were noted; the 2 infants who had passed the ABR screen demonstrated normal histological features. Neuronal counts were normal. CONCLUSIONS: Auditory brainstem response failure among these neonatal intensive care unit infants who died was extremely common in part owing to an unexpected histological alteration, selective inner hair cell loss among premature newborns, that should be detectable uniquely by the ABR testing method. Additional histological patterns suggest more than one cause for neonatal intensive care unit hearing loss. Hair cell loss patterns seem frequently compatible with in utero damage.

Fast, but not slow, effects of olivocochlear activation are resistant to apamin.

Olivocochlear (OC) efferent suppression of auditory-nerve responses comprises a fast effect lasting tens of milliseconds and a slow effect building and decaying over tens of seconds. Both fast and slow effects are mediated by activation of the same alpha 9 nicotinic receptor. We have hypothesized that fast effects are generated at the OC synapse, but that slow effects reflect activation of calcium-activated potassium (K(Ca)) channels by calcium release from the subsurface cisternae on the basolateral wall of the hair cells. We measured in vivo effects of apamin, a blocker of small-conductance (SK) K(Ca) channels, and charybdotoxin, a blocker of large-conductance K(Ca) channels, perfused through scala tympani, on fast and slow effects evoked by electrical stimulation of the OC bundle in anesthetized guinea pigs. Apamin selectively and reversibly reduced slow-effect amplitude without altering fast effects or baseline amplitude of the auditory-nerve response, but only when perfused at concentrations of 100 microM. In contrast, the effects of charybdotoxin were noted at 30 nM, but were not specific, reducing both afferent and efferent responses. The very high concentrations of apamin needed to block efferent effects contrasts with the high sensitivity of isolated hair cells to apamin's block of acetylcholine's effects. The results suggest that in vivo fast OC effects are dominated by a conductance that is not apamin sensitive.

Morphometric analysis of age-related changes in the human basilar membrane.

The histopathologic correlates of presbycusis suggest several categories, including degeneration of sensory cells, neurons, or the stria vascularis. Lack of clear-cut histopathologic changes in some cases has suggested an "indeterminate category"; however, several studies have suggested that a disorder of the basilar membrane (BM) may underlie indeterminate presbycusis. The objective of the present study was to quantify age-related changes in the human BM and correlate them with audiometric patterns. Under high-resolution light microscopy, BM thickness was calculated, and the number of tympanic mesothelial cells (TMCs) lining the BM was counted, at 4 cochlear locations in 92 temporal bones. The control group (n = 80) included subjects from 10 decades of age with normal hearing and/or histopathologic findings. The indeterminate group (n = 12) consisted of elderly patients (ages 64 to 91 years) with hearing loss and no apparent histopathologic changes. Age-related BM thickening was seen in both groups, but only in the most basal cochlear region. The BM thickness in the indeterminate group was not significantly different from that of age-matched controls. Counts of TMCs showed age-related decreases in all cochlear regions in both groups; however, TMC counts in the indeterminate group were not different from those of age-matched controls. The results suggest that BM histopathology is not a common cause of presbycusis. Although age-related BM thickening, seen in both groups, could contribute to hearing loss, the extreme basal region, to which the thickening was confined, is not tested in routine audiometry.

Sound conditioning reduces noise-induced permanent threshold shift in mice.

The phenomenon of conditioning-related protection, whereby prior exposure to moderate-level, non-traumatic, sound protects the ear from subsequent traumatic exposure, has been documented in a number of mammalian species. To probe the molecular mechanisms underlying this effect, the mouse would be a useful model; however, a previous study reported no conditioning effects in this species (Fowler et al. , 1995). In our study, mice (CBA/CaJ) were exposed to a traumatic octave-band noise (8-16 kHz at 100 dB SPL for 2 h) with, or without, prior exposure to a sound-conditioning protocol consisting of exposure to the same noise band at lower sound pressure levels. Two conditioning protocols were investigated: one (81 dB SPL for 1 week) was analogous to those used in other conditioning studies in mammals; the second was significantly shorter (89 dB SPL for 15 min). Noise-induced permanent threshold shift (PTS) was assessed in a terminal experiment, after the traumatic exposure, via compound action potentials. Neither conditioning protocol elevated threshold, indeed both protocols increased amplitudes of distortion product otoacoustic emissions when animals were conditioned but not traumatized. Both conditioning exposures significantly reduced PTS from the subsequent traumatic exposure, compared to groups exposed without prior conditioning. Protective effects of 15-min conditioning were maximal when the condition-trauma interval was 24 h; protection disappeared when the traumatic exposure was presented 48 h after conditioning. These data are consistent with the view that protein synthesis is required for expression of the protective effect. The enhancement of distortion products in the condition-only state suggests that conditioning changes outer hair cell function.

Afferent innervation of outer and inner hair cells is normal in neonatally de-efferented cats.

It has been hypothesized that normal pruning of exuberant branching of afferent neurons in the developing cochlea is caused by the arrival of the olivocochlear efferent neurons and the resulting competition for synaptic sites on hair cells. This hypothesis was supported by a report that afferent innervation density on mature outer hair cells (OHCs) is elevated in animals deefferented at birth, before the olivocochlear system reaches the outer hair cell area (Pujol and Carlier [1982] Dev. Brain Res. 3:151-154). In the current study, this claim was evaluated quantitatively at the electron microscopic level in four cats that were de-efferented at birth and allowed to survive for 6-11 months. A semiserial section analysis of 156 OHCs from de-efferented and normal ears showed that, although de-efferentation essentially was complete in all four cases, the number and distribution of afferent terminals on OHCs was indistinguishable from normal, and the morphology of afferent synapses was normal in both the inner hair cell area and the OHC area. Thus, the postnatal presence of an efferent system is not required for the normal development of cochlear afferent innervation, and the synaptic competition hypothesis is not supported.

Predicting vulnerability to acoustic injury with a noninvasive assay of olivocochlear reflex strength.

Permanent noise-induced damage to the inner ear is a major cause of hearing impairment, arising from exposures occurring during both work- and pleasure-related activities. Vulnerability to noise-induced hearing loss is highly variable: some have tough, whereas others have tender ears. This report documents, in an animal model, the efficacy of a simple nontraumatic assay of normal ear function in predicting vulnerability to acoustic injury. The assay measures the strength of a sound-evoked neuronal feedback pathway to the inner ear, the olivocochlear efferents, by examining otoacoustic emissions created by the normal ear, which can be measured with a microphone in the external ear. Reflex strength was inversely correlated with the degree of hearing loss after subsequent noise exposure. These data suggest that one function of the olivocochlear efferent system is to protect the ear from acoustic injury. This assay, or a simple modification of it, could be applied to human populations to screen for individuals most at risk in noisy environments.

Acoustic injury in mice: 129/SvEv is exceptionally resistant to noise-induced hearing loss.

129/SvEv is an inbred mouse strain popular for use in genetic knockout studies. Here, we compare normal auditory function and vulnerability to acoustic injury in wild-type mice of the 129/SvEv vs. CBA/CaJ strains. Compound action potentials (CAPs) and distortion product otoacoustic emissions (DPOAEs) showed slightly higher thresholds for 129/SvEv re CBA/CaJ, especially at frequencies >20 kHz. Middle-ear motion (i.e. umbo velocity) was similar in the two strains; although frequencies >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.

Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs.

The medial olivocochlear (MOC) efferent system, which innervates cochlear outer hair cells, suppresses cochlear responses. MOC-mediated suppression includes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs suggest that both slow and fast effects on cochlear responses require an initial acetylcholine activation of alpha-9 nicotinic receptors on outer hair cells and that slow effects require additional intracellular events downstream from those mediating fast effects. Gentamicin, an aminoglycoside antibiotic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without changing slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by either intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in unanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from effects of the lateral OC system.

Heat stress and protection from permanent acoustic injury in mice.

The inner ear can be permanently damaged by overexposure to high-level noise; however, damage can be decreased by previous exposure to moderate level, nontraumatic noise (). The mechanism of this "protective" effect is unclear, but a role for heat shock proteins has been suggested. The aim of the present study was to directly test protective effects of heat stress in the ear. For physiological experiments, CBA/CaJ mice were exposed to an intense octave band of noise (8-16 kHz) at 100 dB SPL for 2 hr, either with or without previous whole-body heat stress (rectal temperature to 41. 5 degrees C for 15 min). The interval between heat stress and sound exposure varied in different groups from 6 to 96 hr. One week later, inner ear function was assessed in each animal via comparison of compound action potential thresholds to mean values from unexposed controls. Permanent threshold shifts (PTSs) were approximately 40 dB in the group sound-exposed without previous heat stress. Heat-stressed animals were protected from acoustic injury: mean PTS in the group with 6 hr heat-stress-trauma interval was reduced to approximately 10 dB. This heat stress protection disappeared when the treatment-trauma interval surpassed 24 hr. A parallel set of quantitative PCR experiments measured heat-shock protein mRNA in the cochlea and showed 100- to 200-fold increase over control 30 min after heat treatment, with levels returning to baseline at 6 hr after treatment. Results are consistent with the idea that upregulation of heat shock proteins protects the ear from acoustic injury.

Divergent roles for thyroid hormone receptor beta isoforms in the endocrine axis and auditory system.

Thyroid hormone receptors (TRs) modulate various physiological functions in many organ systems. The TR alpha and TR beta isoforms are products of 2 distinct genes, and the beta 1 and beta 2 isoforms are splice variants of the same gene. Whereas TR alpha 1 and TR beta 1 are widely expressed, expression of the TR beta 2 isoform is mainly limited to the pituitary, triiodothyronine-responsive TRH neurons, the developing inner ear, and the retina. Mice with targeted disruption of the entire TR beta locus (TR beta-null) exhibit elevated thyroid hormone levels as a result of abnormal central regulation of thyrotropin, and also develop profound hearing loss. To clarify the contribution of the TR beta 2 isoform to the function of the endocrine and auditory systems in vivo, we have generated mice with targeted disruption of the TR beta 2 isoform. TR beta 2-null mice have preserved expression of the TR alpha and TR beta 1 isoforms. They develop a similar degree of central resistance to thyroid hormone as TR beta-null mice, indicating the important role of TR beta 2 in the regulation of the hypothalamic-pituitary-thyroid axis. Growth hormone gene expression is marginally reduced. In contrast, TR beta 2-null mice exhibit no evidence of hearing impairment, indicating that TR beta 1 and TR beta 2 subserve divergent roles in the regulation of auditory function.