Rapid regulation of microtubule-associated protein 2 in dendrites of nucleus laminaris of the chick following deprivation of afferent activity

Differential innervation of segregated dendritic domains in the chick nucleus laminaris (NL), composed of third-order auditory neurons, provides a unique model to study synaptic regulation of dendritic structure. Altering the synaptic input to one dendritic domain affects the structure and length of the manipulated dendrites while leaving the other set of unmanipulated dendrites largely unchanged. Little is known about the effects of neuronal input on the cytoskeletal structure of NL dendrites and whether changes in the cytoskeleton are responsible for dendritic remodeling following manipulations of synaptic input. In this study, we investigate changes in the immunoreactivity of high-molecular weight microtubule associated protein 2 (MAP2) in NL dendrites following two different manipulations of their afferent input. Unilateral cochlea removal eliminates excitatory synaptic input to the ventral dendrites of the contralateral NL and the dorsal dendrites of the ipsilateral NL. This manipulation produced a dramatic decrease in MAP2 immunoreactivity in the deafferented dendrites. This decrease was detected as early as 3 h following the surgery, well before any degeneration of afferent axons. A similar decrease in MAP2 immunoreactivity in deafferented NL dendrites was detected following a midline transection that silences the excitatory synaptic input to the ventral dendrites on both sides of the brain. These changes were most distinct in the caudal portion of the nucleus where individual deafferented dendritic branches contained less immunoreactivity than intact dendrites. Our results suggest that the cytoskeletal protein MAP2, which is distributed in dendrites, perikarya, and postsynaptic densities, may play a role in deafferentation-induced dendritic remodeling.

Hair cell regeneration in the avian inner ear

The postembryonic production of hair cells in fish and reptiles has been known for several decades. Until recently it was assumed that this capacity was absent in the more highly specialized inner ears of birds and mammals. Recent research has shown, however, that birds have the capacity to rebuild a damaged inner ear. Summarized here are studies conducted in our laboratory which address the following questions: (1) Which are the precursors of the regenerated hair cells? (2) Are the new hair cells functional? (3) What are the ultrastructural properties of regenerated hair cells? and (4) Can the level of proliferation be regulated? Both the auditory and the vestibular systems of the avian inner ear were studied. Our results provide some answers to these questions. The implications of the results are discussed.

FGFR3 expression during development and regeneration of the chick inner ear sensory epithelia

Several studies suggest fibroblast growth factor receptor 3 (FGFR3) plays a role in the development of the auditory epithelium in mammals. We undertook a study of FGFR3 in the developing and mature chicken inner ear and during regeneration of this epithelium to determine whether FGFR3 shows a similar pattern of expression in birds. FGFR3 mRNA is highly expressed in most support cells in the mature chick basilar papilla but not in vestibular organs of the chick. The gene is expressed early in the development of the basilar papilla. Gentamicin treatment sufficient to destroy hair cells in the basilar papilla causes a rapid, transient downregulation of FGFR3 mRNA in the region of damage. In the initial stages of hair cell regeneration, the support cells that reenter the mitotic cycle in the basilar papilla do not express detectable levels of FGFR3 mRNA. However, once the hair cells have regenerated in this region, the levels of FGFR3 mRNA and protein expression rapidly return to approximate those in the undamaged epithelium. These results indicate that FGFR3 expression changes after drug-induced hair cell damage to the basilar papilla in an opposite way to that found in the mammalian cochlea and may be involved in regulating the proliferation of support cells.

Accommodation enhances depolarizing inhibition in central neurons

Neurons in the avian cochlear nucleus are depolarized by GABAergic synaptic input. We recorded GABAergic synaptic currents using the gramicidin-perforated-patch method and found their reversal potential (V(rev)) to be depolarized relative to spike threshold, which is surprising given that these inputs are inhibitory. Depolarizing IPSPs (dIPSPs) are kept below spike generation threshold by the activation of a dendrotoxin-I-sensitive, voltage-gated K(+) conductance. We show experimentally that the polarity of IPSPs contributes to their efficacy; dIPSPs induce accommodation, the positive shift in spike threshold, and are therefore more strongly inhibitory than conventional, hyperpolarizing IPSPs in the same neurons. A similar inhibitory mechanism has been described in invertebrate sensory fibers and axons of dorsal root ganglion cells and may be a general means of amplifying the strength of inhibition in cases where the size of excitatory conductances greatly exceeds that of inhibitory conductances.

Tonotopic map of potassium currents in chick auditory hair cells using an intact basilar papilla

The avian basilar papilla is tonotopically organized such that hair cells along the sensory epithelium respond best to acoustic stimulation at differing frequencies. This specificity arises due to the mechanics of the cochlea itself and intrinsic electrical properties of the hair cells. Tall hair cells show membrane voltage oscillations in response to step current injection that may allow cells to act as electrical resonators, boosting the response at the resonant frequency. These oscillations and the underlying currents have been studied in enzymatically isolated cells. This study uses a whole chick (Gallus domesticus) basilar papilla preparation where the entire epithelium and its afferent connections are intact. With this preparation, a map of changes in potassium currents of tall hair cells was produced. All cells recorded from expressed two K+ currents, a calcium-activated K+ current, I(K(Ca)), and a voltage-activated K+ current, I(K). Also, apical cells expressed an inward rectifier K+ current, I(IR). The amplitude of total outward current increases in a gradient along the tonotopic axis. Pharmacological blockers were used to separate the outward K+ currents. These experiments showed that both currents individually increase in magnitude along a gradient from apex to base. Finally, measurements of oscillation frequency in response to current steps suggest a discontinuous change in the electrical resonances at about 33% from the apex. This study demonstrates a new preparation to study the electrical properties of hair cells in more detail along the tonotopic axis of the chick basilar papilla.

Visual influences on the development and recovery of the vestibuloocular reflex in the chicken

Whenever the head turns, the vestibuloocular reflex (VOR) produces compensatory eye movements to help stabilize the image of the visual world on the retina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal damage. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recovery of the VOR after hair cell loss and regeneration. In the first experiment, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal light. Strobe-rearing prevented the normal development of both h- and vVORs. After 8 wk of strobe-rearing, 3 days of exposure to normal light caused the VORs to recover partially but not to normal values. In the second experiment, 1-wk-old chicks were treated with streptomycin, which destroys most vestibular hair cells and reduces hVOR gain to zero. In birds, vestibular hair cells regenerate so that after 8 wk in normal illumination they appear normal and hVOR gain returns to values that are normal for birds of that age. The treated birds in this study recovered in either normal or stroboscopic illumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were measured and compared with those of untreated, age-matched controls at 8 wk posthatch, when hair cell regeneration is known to be complete. As in previous studies, the gain of the VOR decreased immediately to zero after streptomycin treatment. After 8 wk of recovery under normal light, the hVOR was normal, but vVOR gain was less than normal. After 8 wk of recovery under stroboscopic illumination, hVOR gain was less than normal at all frequencies. VCR recovery was not affected by the strobe environment. When streptomycin-treated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experiments suggest that continuous visual feedback can adjust VOR gain. In the absence of appropriate visual stimuli, however, there is a default VOR gain and phase to which birds recover or revert, regardless of age. Thus an 8-wk-old chicken raised in a strobe environment from hatch would have the same gain as a streptomycin-treated chicken that recovers in a strobe environment.

Hair cell regeneration and recovery of auditory thresholds following aminoglycoside ototoxicity in Bengalese finches

Birds regenerate auditory hair cells when original hair cells are lost. Regenerated hair cells become innervated and restore hearing function. Functional recovery during hair cell regeneration is particularly interesting in animals that depend on hearing for vocal communication. Bengalese finches are songbirds that depend on auditory feedback for normal song learning and maintenance. We examined the structural and functional recovery of the Bengalese finch basilar papilla after aminoglycoside ototoxicity. Birds were treated with the ototoxic aminoglycoside, amikacin, daily for 1 week. Treatment resulted in hair cell loss across the basal half of the basilar papilla and corresponding high frequency hearing loss. Hair cell regeneration and recovery of auditory brainstem responses were compared in the same animals. Survival times following treatment were between 1 day and 12 weeks. Analysis of structural recovery at weekly intervals indicated that hair cells in the Bengalese finch papilla require a maximum of 1 week to regenerate and appear with immature morphology at the epithelial surface. An additional 6 days are required for adult-like morphology to develop. Repopulation of the damaged region was complete by 8 weeks. Recovery of auditory thresholds began 1 week after treatment and reached asymptote by 4 weeks. Slight residual threshold shifts at 2.0 kHz and above were observed up to 12 weeks after treatment. Direct comparison of structural and functional recovery indicates that auditory thresholds recover maximally before a full complement of hair cells has regenerated.

Patterns of cell death in mouse anteroventral cochlear nucleus neurons after unilateral cochlea removal

Developmental changes that influence the results of removal of afferent input on the survival of neurons of the anteroventral cochlear nucleus (AVCN) of mice were examined with the hope of providing a suitable model for understanding the cellular and molecular basis for these developmental changes in susceptibility. We performed unilateral cochlear ablation on wild-type mice at a variety of ages around the time of hearing onset to determine developmental changes in the sensitivity of AVCN neurons to afferent deprivation. In postnatal day 5 (P5) mice, cochlea removal resulted in 61% neuronal loss in the AVCN. By age P14, fewer than 1% of AVCN neurons were lost after this manipulation. This reveals a rather abrupt change in the sensitivity to disruption of afferent input, a critical period. We next investigated the temporal events associated with neuron loss after cochlea removal in susceptible animals. We demonstrate that significant cell loss occurs within 48 hours of cochlea removal in P7 animals. Furthermore, evidence of apoptosis was observed within 12 hours of cochlea removal, suggesting that the molecular events leading to cell loss after afferent deprivation begin to occur within hours of cochlea removal. Finally, we began to examine the role of the bcl-2 gene family in regulating afferent deprivation-induced cell death in the mouse AVCN. AVCN neurons in mature bcl-2 knockout mice demonstrate susceptibility to removal of afferent input comparable to neonatal sensitivity of wild-type controls. These data suggest that bcl-2 is one effector of cell survival as these cells switch from afferent-dependent to -independent survival mechanisms.

Cellular studies of auditory hair cell regeneration in birds

A decade ago it was discovered that mature birds are able to regenerate hair cells, the receptors for auditory perception. This surprising finding generated hope in the field of auditory neuroscience that new hair cells someday may be coaxed to form in another class of warm-blooded vertebrates, mammals. We have made considerable progress toward understanding some cellular and molecular events that lead to hair cell regeneration in birds. This review discusses our current understanding of avian hair cell regeneration, with some comparisons to other vertebrate classes and other regenerative systems.

Developmental regulation of EphA4 expression in the chick auditory brainstem

The avian auditory brainstem nuclei nucleus magnocellularis (NM) and nucleus laminaris (NL) display highly precise patterns of neuronal connectivity. NM projects tonotopically to the dorsal dendrites of ipsilateral NL neurons and to the ventral dendrites of contralateral NL neurons. The precision of this binaural segregation is evident at the earliest developmental stage at which connections can be observed. We have begun to examine the possibility that Eph receptor tyrosine kinase signaling is involved in establishing these spatially segregated connections. The expression of the EphA4 tyrosine kinase was examined at several developmental stages. EphA4 is expressed in rhombomere 5, which contains progenitors for both NM and NL. In this rhombomere, the labeling becomes striped during the time that precursor cells migrate to the auditory anlage. At the precise time when NM-NL projections are forming, EphA4 expression in NL is asymmetric, with markedly higher expression in the dorsal NL neuropil than in the ventral neuropil, suggesting a possible role in guiding growing axons to the appropriate region. At later embryonic ages EphA4 expression is symmetric around NL, and is absent in NM. As auditory function matures, EphA4 expression decreases so that by 4 days after hatch no EphA4 antibody labeling is evident in the auditory brainstem nuclei.

Embryonic origins of auditory brain-stem nuclei in the chick hindbrain

The auditory nuclei of the chick brain stem have distinct morphologies and highly specific synaptic connectivity. Nucleus magnocellularis (NM) and nucleus angularis receive tonotopically ordered cochlear input. NM in turn projects tonotopically to nucleus laminaris (NL), maintaining binaural specificity with projections to either dorsal or ventral NL dendrites. NM and NL arise from a common anlage, which differentiates as the cells migrate and acquire their mature morphologies. NM and NL cells are closely associated during embryogenesis and synapse formation. However, the morphologies of the nuclei and of the cells within the nuclei differ greatly between NM and NL. While later maturation of these nuclei has been described in considerable detail, relatively little is known about the early embryonic events that lead to the formation of these nuclei. We examined the embryonic origins of cells in brain-stem auditory nuclei with particular emphasis on NM and NL. Lipophilic dyes were injected into small regions of the embryonic hindbrain prior to the birth and migration of cells that contribute to these nuclei. We found that NM arises from rhombomeres r5, r6, and r7, and NL arises mostly from r5 with a few cells arising from r6. NM and NL thus have partially overlapping rhombomeres of origin. However, we found that the precursors for NM and NL are found in distinct regions within rhombomere 5, with NM precursors in medial regions and NL precursors in lateral regions. Our results do not support a lineage relationship between NM and NL cells and they suggest that NM and NL are specified prior to migration of precursors to the auditory anlage.

Tyrosine phosphatase SHP-1 immunoreactivity increases in a subset of astrocytes following deafferentation of the chicken auditory brainstem

Proliferation of astrocytes is a dramatic response of the central nervous system (CNS) to injury and disease. Such proliferation results in the formation of the neural/glial scar and the reconstitution of the glial limitans. However, not all astrocytes enter the proliferative cycle following injury, and for those that do, the period of cell division is limited. Little attention has focused on the events that regulate the duration and extent of astrocyte proliferation following damage, but clearly control mechanisms are in place as CNS injury does not result in the continuous astrocyte proliferation seen in glial tumorigenesis. Protein tyrosine phosphorylation has been implicated in both astrocyte proliferation and differentiation and plays an important role in the regulation of the cell cycle in a number of different systems. We have found a small subset of astrocytes in the chick auditory brainstem that are immunopositive for the protein tyrosine phosphatase SHP-1. SHP-1 appears to negatively regulate cellular division in the hematopoietic system and is involved in the mitogenic response to various growth factors. Following cochlea removal, there is a marked increase within the auditory brainstem nucleus, nucleus magnocellularis (NM), in both in the number of SHP-1-positive astrocytes and the length of their immunopositive fibers. Significantly, those animals showing the greatest increases in SHP-1 immunoreactivity do not exhibit large amounts of astrocyte proliferation. We hypothesize that the expression of SHP-1 plays a role in negatively regulating the mitotic behavior of astrocytes following deafferentation.

GABAergic inhibition in nucleus magnocellularis: implications for phase locking in the avian auditory brainstem

In the avian auditory brainstem, nucleus magnocellularis (NM) functions to relay phase-locked signals to nucleus laminaris for binaural coincidence detection. Although many studies have revealed that NM neurons exhibit intrinsic physiological and anatomical specializations for this purpose, the role of inhibition has not been fully explored. The present study characterizes the organization of GABAergic feedback to NM. Anterograde and retrograde labeling methods showed that NM receives a prominent projection from the ipsilateral superior olivary nucleus (SON). The functional features of this projection were explored in a brain slice preparation. Stimulating fibers from the SON evoked long-lasting, depolarizing responses in NM neurons that were blockable by bicuculline, a GABA(A) receptor antagonist. The slow time course of these responses allowed them to undergo temporal summation during repetitive stimulation. The summed GABAergic response was capable of blocking spikes generated in NM neurons by suprathreshold current injection. This inhibitory effect was attributable to a large reduction in input resistance caused by a combination of the opening of a GABAergic Cl(-) conductance and the recruitment of a low-voltage activated K(+) conductance. This large reduction of input resistance increased the amount of current necessary to drive NM neurons to threshold. The results lead us to propose that GABAergic inhibition enhances phase-locking fidelity of NM neurons, which is essential to binaural coincidence detection in nucleus laminaris.

Characterization of damage and regeneration in cultured avian utricles

Hair cell regeneration occurs spontaneously throughout life and following hair cell injury in the vestibular epithelia of mature birds and other nonmammalian vertebrates. We examined hair cell regeneration in post-hatch chick utricles that were cultured in media with or without the ototoxin, streptomycin, for various periods. The goal of our study was to characterize the dose- and time-dependent effects of streptomycin on hair cell loss and regeneration in vitro. Utricles that were cultured with streptomycin for 1 day displayed a dose-dependent loss of hair cells in spatial patterns and levels that were consistent with those observed in comparable experimental paradigms in vivo. Incorporation of the nucleotide analog bromodeoxyuridine (BrdU) demonstrated that supporting cell proliferation is decreased during the first day of culture in the presence of streptomycin, but it increases over time when cultures are subsequently placed in streptomycin-free media. Utricles cultured for 1 day with streptomycin followed by 2-4 more days without streptomycin had numerous bundles of immature stereocilia, suggesting that new hair cells were generated in vitro. We tested this hypothesis by culturing utricles with BrdU for 3 or 5 days and double-labeling them to detect BrdU and the hair cell-specific antigen, TuJ1. Numerous BrdU-positive/TuJ1-positive cells with phenotypic characteristics of immature hair cells were present in the cultures, and the number of such cells increased between 3 and 5 days in vitro, in a dose-dependent manner.

Temporal, spatial, and morphologic features of hair cell regeneration in the avian basilar papilla

Hair cell-selective antibodies were used in combination with the nucleotide bromode-oxyuridine (BrdU) to examine the temporal, spatial, and morphologic progression of auditory hair cell regeneration in chicks after a single gentamicin injection. New hair cells are first identifiable with an antibody to class III beta (beta) tubulin (TuJ1) by 14 hours after BrdU incorporation, but progenitor cells in S phase and M phase are TuJ1-negative. TuJ1 labeling reveals that new hair cells are first detected at 3 days after gentamicin, in the base, and the emergence and maturation of regenerating hair cells spreads apically over time. Differentiation of regenerating hair cells consists of a progressive series of morphologic changes. During early differentiation (14 hours to 1 day after BrdU), regenerating hair cells are round or fusiform and remain near the lumen, where they are generated. During intermediate differentiation (2-4 days after BrdU), regenerating hair cells resemble support cells; their somata are elongated, their nuclei are in the support cell layer, and they appear to contact both the lumenal surface and the basal lamina. The 275-kDa hair cell antigen is first expressed in regenerating hair cells during this period. During late differentiation (7 days after BrdU and later), TuJ1-positive cells acquire the globose shape of mature hair cells. Labeling with antibodies to hair cell antigen, calmodulin, and ribosomal RNA confirms this morphologic progression. Examination of sister cells born at 3 days post-gentamicin reveals that there is equal likelihood that they will assume the hair cell or support cell fate (i.e., both asymmetric and symmetric differentiation occur).

Dynamic studies of ototoxicity in mature avian auditory epithelium

Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside-induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose-dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage-gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.

Ontogenetic expression of trk neurotrophin receptors in the chick auditory system

Neurotrophins and their cognate receptors are critical to normal nervous system development. Trk receptors are high-affinity receptors for nerve-growth factor (trkA), brain-derived neurotrophic factor and neurotrophin-4/5 (trkB), and neurotrophin-3 (trkC). We examine the expression of these three neurotrophin tyrosine kinase receptors in the chick auditory system throughout most of development. Trks were localized in the auditory brainstem, the cochlear ganglion, and the basilar papilla of chicks from embryonic (E) day 5 to E21, by using antibodies and standard immunocytochemical methods. TrkB mRNA was localized in brainstem nuclei by in situ hybridization. TrkB and trkC are highly expressed in the embryonic auditory brainstem, and their patterns of expression are both spatially and temporally dynamic. During early brainstem development, trkB and trkC are localized in the neuronal cell bodies and in the surrounding neuropil of nucleus magnocellularis (NM) and nucleus laminaris (NL). During later development, trkC is expressed in the cell bodies of NM and NL, whereas trkB is expressed in the nerve calyces surrounding NM neurons and in the ventral, but not the dorsal, dendrites of NL. In the periphery, trkB and trkC are located in the cochlear ganglion neurons and in peripheral fibers innervating the basilar papilla and synapsing at the base of hair cells. The protracted expression of trks seen in our materials is consistent with the hypothesis that the neurotrophins/tyrosine kinase receptors play one or several roles in the development of auditory circuitry. In particular, the polarized expression of trkB in NL is coincident with refinement of NM terminal arborizations on NL.

Progenitor cell cycling during hair cell regeneration in the vestibular and auditory epithelia of the chick

We investigated nucleotide-labeling patterns during ongoing hair cell regeneration in the avian vestibular epithelium and during drug-induced regeneration in the avian auditory epithelium. For utricle experiments, post-hatch chicks received an injection of bromodeoxyuridine (BrdU) and were allowed to survive from 2 hours to 110 days after the injection. Utricles were fixed and immunoreacted to detect BrdU. The number of BrdU-labeled nuclei in the hair cell and support cell layers of the utricular sensory epithelium changes significantly between 2 hours and 110 days post-BrdU. At 2 hours, most labeled cells are isolated, while by 5-10 days, the majority of labeled cells are organized in pairs that are most frequently composed of a hair cell and a support cell. Pairs of labeled cells are seen as late as 110 days. Clusters of more than 3 labeled cells are uncommon at all time-points. The total number of labeled cells increases approximately 1.5-fold between 5 and 60 days post-BrdU. This increase is due primarily to a rise in the number of labeled support cells, and it is likely that it represents additional rounds of division by a subset of cells that were labeled at the time of the BrdU injection. There is a significant decrease in labeled nuclei in the hair cell layer between 60 and 110 days post-BrdU, suggesting that hair cells die during this period. To investigate support cell recycling in the drug-damaged auditory epithelium, we examined nucleotide double labeling after separate injections of BrdU and tritiated thymidine. A small number of support cells that incorporate BrdU administered at 3 days post-gentamicin treatment also label with tritiated thymidine administered between 17 and 38 hours later. We conclude that a small population of support cells recycles during regeneration in both the normal utricle and the drug-damaged basilar papilla.

Life and death in otolaryngology: mechanisms of apoptosis and its role in the pathology and treatment of disease

OBJECTIVES

To review recent advances in our understanding of programmed cell death, or apoptosis, and discuss implications of these basic science advances in our understanding of causes and potential treatments of a variety of diseases of the head and neck.

DATA SOURCES

Basic science literature relevant to the study of apoptosis and its clinical implications.

CONCLUSIONS

Apoptosis is now understood to be important in the normal development and survival of all multicellular organisms. Deregulation of this normally tightly controlled process underlies a variety of disease states, including neoplasia, autoimmune disease, and disorders of the central nervous system. A better understanding of this process and its regulation may help otolaryngologists better understand diseases relevant to this specialty and will lead to improved therapeutic interventions.

Class III beta-tubulin expression in sensory and nonsensory regions of the developing avian inner ear

A previous study showed that class III beta-tubulin, a widely used neuron-specific marker, is expressed in mature and regenerating hair cells but not the support cells of the avian inner ear. We investigated the expression of this marker in the developing avian inner ear. We found that class III beta-tubulin is not neuron-specific in the avian embryo, but appears to accumulate in neuronal cell types, including hair cells, about the time of their differentiation. In the developing inner ear, some degree of class III beta-tubulin immunoreactivity is found in all regions of the otic epithelium from its formation as the otic placode (stage 10 [embryonic day, E1.5]) until about stage 21 (E3.5), when the prospective tegmentum vasculosum begins to lose its staining. By stage 35 (E8-9), most of the nonsensory epithelia have lost their class III beta-tubulin staining, leaving distinct regions of staining between the morphological compartments of the inner ear. Concurrent with the loss of staining from nonsensory regions, the hair cells of the sensory epithelia accumulate class III beta-tubulin, whereas the supporting cells decrease their staining. We also observed a similar pattern of development in another hair cell organ, the paratympanic organ. Double labeling with the 275 kD hair cell antigen (HCA) indicated that the majority of hair cells identifiable with class III beta-tubulin are HCA-positive. Additionally, presumptive hair cells were identified which were not within defined sensory epithelia. Our findings show that class III beta-tubulin can be used as an early marker for hair cell differentiation in all hair cell sensory epithelia in the chicken.

Glutamate regulates IP3-type and CICR stores in the avian cochlear nucleus

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Gene disruption of p27(Kip1) allows cell proliferation in the postnatal and adult organ of corti

Hearing loss is most often the result of hair-cell degeneration due to genetic abnormalities or ototoxic and traumatic insults. In the postembryonic and adult mammalian auditory sensory epithelium, the organ of Corti, no hair-cell regeneration has ever been observed. However, nonmammalian hair-cell epithelia are capable of regenerating sensory hair cells as a consequence of nonsensory supporting-cell proliferation. The supporting cells of the organ of Corti are highly specialized, terminally differentiated cell types that apparently are incapable of proliferation. At the molecular level terminally differentiated cells have been shown to express high levels of cell-cycle inhibitors, in particular, cyclin-dependent kinase inhibitors [Parker, S. B., et al. (1995) Science 267, 1024-1027], which are thought to be responsible for preventing these cells from reentering the cell cycle. Here we report that the cyclin-dependent kinase inhibitor p27(Kip1) is selectively expressed in the supporting-cell population of the organ of Corti. Effects of p27(Kip1)-gene disruption include ongoing cell proliferation in postnatal and adult mouse organ of Corti at time points well after mitosis normally has ceased during embryonic development. This suggests that release from p27(Kip1)-induced cell-cycle arrest is sufficient to allow supporting-cell proliferation to occur. This finding may provide an important pathway for inducing hair-cell regeneration in the mammalian hearing organ.

The superior olivary nucleus and its influence on nucleus laminaris: a source of inhibitory feedback for coincidence detection in the avian auditory brainstem

Located in the ventrolateral region of the avian brainstem, the superior olivary nucleus (SON) receives inputs from nucleus angularis (NA) and nucleus laminaris (NL) and projects back to NA, NL, and nucleus magnocellularis (NM). The reciprocal connections between the SON and NL are of particular interest because they constitute a feedback circuit for coincidence detection. In the present study, the chick SON was investigated. In vivo tracing studies show that the SON projects predominantly to the ipsilateral NM, NL, and NA. In vitro whole-cell recording reveals single-cell morphology, firing properties, and postsynaptic responses. SON neurons are morphologically and physiologically suited for temporal integration; their firing patterns do not reflect the temporal structure of their excitatory inputs. Of most interest, direct stimulation of the SON evokes long-lasting inhibition in NL neurons. The inhibition blocks both intrinsic spike generation and orthodromically evoked activity in NL neurons and can be eliminated by bicuculline methiodide, a potent antagonist for GABAA receptor-mediated neurotransmission. These results strongly suggest that the SON provides GABAergic inhibitory feedback to laminaris neurons. We discuss a mechanism whereby SON-evoked GABAergic inhibition can influence the coding of interaural time differences for sound localization in the avian auditory brainstem.

Delta1 expression during avian hair cell regeneration

Postembryonic production of hair cells, the highly specialized receptors for hearing, balance and motion detection, occurs in a precisely controlled manner in select species, including avians. Notch1, Delta1 and Serrate1 mediate cell specification in several tissues and species. We examined expression of the chicken homologs of these genes in the normal and drug-damaged chick inner ear to determine if signaling through this pathway changes during hair cell regeneration. In untreated post-hatch chicks, Delta1 mRNA is abundant in a subpopulation of cells in the utricle, which undergoes continual postembryonic hair cell production, but it is absent from all cells in the basilar papilla, which is mitotically quiescent. By 3 days after drug-induced hair cell injury, Delta1 expression is highly upregulated in areas of cell proliferation in both the utricle and basilar papilla. Delta1 mRNA levels are elevated in progenitor cells during DNA synthesis and/or gap 2 phases of the cell cycle and expression is maintained in both daughter cells immediately after mitosis. Delta1 expression remains upregulated in cells that differentiate into hair cells and is downregulated in cells that do not acquire the hair cell fate. Delta1 mRNA levels return to normal by 10 days after hair cell injury. Serrate1 is expressed in both hair cells and support cells in the utricle and basilar papilla, and its expression does not change during the course of drug-induced hair cell regeneration. In contrast, Notch1 expression, which is limited to support cells in the quiescent epithelium, is increased in post-M-phase cell pairs during hair cell regeneration. This study provides initial evidence that Delta-Notch signaling may be involved in maintaining the correct cell types and patterns during postembryonic replacement of sensory epithelial cells in the chick inner ear.

Recovery of the vestibulocolic reflex after aminoglycoside ototoxicity in domestic chickens

Avian auditory and vestibular hair cells regenerate after damage by ototoxic drugs, but until recently there was little evidence that regenerated vestibular hair cells function normally. In an earlier study we showed that the vestibuloocular reflex (VOR) is eliminated with aminoglycoside antibiotic treatment and recovers as hair cells regenerate. The VOR, which stabilizes the eye in the head, is an open-loop system that is thought to depend largely on regularly firing afferents. Recovery of the VOR is highly correlated with the regeneration of type I hair cells. In contrast, the vestibulocolic reflex (VCR), which stabilizes the head in space, is a closed-loop, negative-feedback system that seems to depend more on irregularly firing afferent input and is thought to be subserved by different circuitry than the VOR. We examined whether this different reflex also of vestibular origin would show similar recovery after hair cell regeneration. Lesions of the vestibular hair cells of 10-day-old chicks were created by a 5-day course of streptomycin sulfate. One day after completion of streptomycin treatment there was no measurable VCR gain, and total hair cell density was approximately 35% of that in untreated, age-matched controls. At 2 wk postlesion there was significant recovery of the VCR; at this time two subjects showed VCR gains within the range of control chicks. At 3 wk postlesion all subjects showed VCR gains and phase shifts within the normal range. These data show that the VCR recovers before the VOR. Unlike VOR gain, recovering VCR gain correlates equally well with the density of regenerating type I and type II vestibular hair cells, except at high frequencies. Several factors other than hair cell regeneration, such as length of stereocilia, reafferentation of hair cells, and compensation involving central neural pathways, may be involved in behavioral recovery. Our data suggest that one or more of these factors differentially affect the recovery of these two vestibular reflexes.

High-frequency auditory feedback is not required for adult song maintenance in Bengalese finches

Male Bengalese finches do not normally change their vocal patterns in adulthood; song is stereotyped and stable over time. Adult song maintenance requires auditory feedback. If adults are deafened, song will degrade within 1 week. We tested whether feedback of all sound frequencies is required for song maintenance. The avian basilar papilla is tonotopically organized; hair cells in the basal region encode high frequencies, and low frequencies are encoded in progressively apical regions. We restricted the spectral range of feedback available to a bird by killing either auditory hair cells encoding higher frequencies or those encoding both high and low frequencies and documented resultant changes in song. Birds were treated with either Amikacin alone to kill high-frequency hair cells or Amikacin and sound exposure to target hair cells across the entire papilla. During treatment, song was recorded from all birds weekly. After treatment and song recording, evoked-potential audiograms were evaluated on each bird, and papillas were evaluated by scanning electron microscopy. Results showed that hair cell damage over 46-63% of the basal papilla and the corresponding high-frequency hearing loss had no effect on song structure. In birds with hair cell damage extending further into the apical region of the papilla and corresponding low-frequency and high-frequency hearing loss, song degradation occurred within 1 week of beginning treatment and was comparable with degradation after surgical deafening. We conclude that either low-frequency spectral cues or temporal cues via feedback of the song amplitude envelope are sufficient for song maintenance in adult Bengalese finches.

Glutamatergic and GABAergic agonists increase [Ca2+]i in avian cochlear nucleus neurons

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are stimulated by glutamate, released from the auditory nerve, and GABA, released from both interneurons surrounding NM and from cells located in the superior olivary nucleus. In this study, the Ca2+ indicator dye Fura-2 was used to measure Ca2+ responses in NM stimulated by glutamate- and GABA-receptor agonists using a chicken brainstem slice preparation. Glutamatergically stimulated Ca2+ responses were evoked by kainic acid (KA), alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA), and N-methyl-D-aspartate (NMDA). KA- and AMPA-stimulated changes in [Ca2+]i were also produced in NM neurons stimulated in the presence of nifedipine, an L-type Ca2+ channel blocker, suggesting that KA- and AMPA-stimulated changes in [Ca2+]i were carried by Ca2(+)-permeable receptor channels. Significantly smaller changes in [Ca2+]i were produced by NMDA. When neurons were stimulated in an alkaline (pH 7.8) superfusate, NMDA responses were potentiated. KA- and AMPA-stimulated responses were not affected by pH. Several agents known to stimulate metabotropic receptors in other systems were tested on NM neurons bathed in a Ca2+ free-EGTA--buffered media, including L-cysteine sulfinic acid (L-CSA), trans-azetidine dicarboxylic acid (t-ADA), trans-aminocyclo-pentanedicarboxylic acid (t-ACPD), and homobromoibotenic acid (HBI). The only agent to reliably and dose-dependently increase [Ca2+]i was HBI, an analog of ibotenate. GABA also stimulated increases in [Ca2+]i in NM neurons. GABA-stimulated responses were reduced by agents that block voltage-operated channels and by agents that inhibit Ca2+ release from intracellular stores. Whereas GABA-A receptor agonist produced increases in [Ca2+]i GABA-B and GABA-C receptor agonists had no effect. There appear to be several ways for [Ca2+]i to increase in NM neurons. Presumably, each route represents a means by which Ca2+ can alter cellular processes.

Expression of novel potassium channels in the chick basilar papilla

Ionic currents are critical for the functioning of the inner ear auditory sensory epithelium. We set out to identify and molecularly clone the genes encoding the channels responsible for several currents in the chick basilar papilla. Here we describe an inward-rectifying K+ channel, cKir2.3, present in both hair cells and support cells in the apical end of the chick basilar papilla. The biophysical properties of the human ortholog, hKir2.3, are similar to those of an inward-rectifying channel found in the apical end of the chick basilar papilla, suggesting that this channel may contribute to the corresponding current. Additionally, we describe two new members of the Kv6 subfamily of putative regulatory voltage-gated K channels, cKv6.2 and cKv6.3. Both are expressed in hair cells in the apical end of the chick basilar papilla; cKv6.2 is also strongly expressed in support cells and in the brain.

Development of the base of the cochlea: place code shift in the gerbil

Distortion product otoacoustic emission measurements were made at 1/12 octave intervals before and after the injection of furosemide in gerbils aged 15 days after birth to adult, in order to obtain estimates of cochlear amplifier gain as a function of stimulus frequency. The frequency at which the gains went sharply to zero, defined as the 'base cutoff frequency', increased from about 20 kHz to over 50 kHz during development. This increase provides further confirmation of the hypothesis that the place code changes during development in the basal part of the cochlea. If the measured base cutoff frequency is identified with the characteristic frequency at the basal end of the cochlea, as defined by electrophysiological measures, then these emission data can be used to generate a frequency-place map as a function of age. The derived place code shift is consistent with published electrophysiological measures, and can be used to extend these measures. Near the base cutoff frequency, the observed cochlear amplifier gain typically dropped sharply from a relative maximum to zero, over a distance of about a half octave. Specifically, this distance appeared to exhibit a curvilinear variation with age, reaching a maximum of 3/4 of an octave at 19-21 days. After transforming from frequency to place using the map derived from emissions, however, the distance between the extreme base and the place associated with the peak gain decreased monotonically from about 1.2 mm at age 15-19 days to 0.6 mm at maturity. This distance is assumed to be characteristic of the length of the active amplification zone for the cochlear amplifier in the base region. Over the same time period, there was approximately a doubling of the rate of amplification with distance from the base, so that the cochlear amplifier gain at the peak actually changed very little from 15 days to adult.

Development of single- and two-tone responses of anteroventral cochlear nucleus neurons in gerbil

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.

Auditory perception following hair cell regeneration in European starling (Sturnus vulgaris): frequency and temporal resolution

Behavioral detection thresholds, auditory filter widths, and temporal modulation transfer functions were obtained from four starlings before, during and after 11 days of subcutaneous injections of kanamycin, an aminoglycoside antibiotic. Birds were operantly conditioned to respond to pure tones and amplitude modulated noises ranging in frequency from 0.25 to 7 kHz using adaptive staircase procedures and were tested daily for 92 days after the first injection of aminoglycoside. All birds had threshold shifts of at least -60 dB at frequencies above 4 kHz. Lower frequencies were affected in some birds, although none of the birds had hearing loss below 3 kHz. All four birds had wider auditory filters at 5 kHz immediately after the aminoglycoside series. Any changes in frequency resolution at frequencies below 5 kHz were slight, transitory, and rarely observed. Two of the four birds had permanently wider auditory filters at 5 kHz. Temporal modulation transfer functions were briefly affected in two birds during the time of greatest threshold shift. Recovery of detection thresholds began soon after the injections ceased and continued for approximately 60 days. Recovery in frequency resolution lagged behind auditory threshold by about 10 days. Normal temporal resolution was observed in the context of impaired intensity and frequency resolution. Changes in auditory threshold and frequency resolution were closely associated for all birds at 5 kHz, but were correlated with statistical significance in only two birds. Scanning electron microscopy was performed on all four birds after 90 days of recovery and confirmed that the extent of initial damage was consistent with the pattern of observed hearing loss.

Activity-dependent regulation of [Ca2+]i in avian cochlear nucleus neurons: roles of protein kinases A and C and relation to cell death

Neurons of the cochlear nucleus, nucleus magnocellularis (NM), of young chicks require excitatory afferent input from the eighth nerve for maintenance and survival. One of the earliest changes seen in NM neurons after deafferentation is an increase in intracellular calcium concentration ([Ca2+]i). This increase in [Ca2+]i is due to loss of activation of metabotropic glutamate receptors (mGluR) that activate second-messenger cascades involved in [Ca2+]i regulation. Because mGluRs are known to act via the phospholipase C and adenylate cyclase signal transduction pathways, the goal of this study was to determine the roles of protein kinases A (PKA) and C (PKC) activities in the regulation of NM neuron [Ca2+]i by eighth nerve stimulation. Additionally, we sought to determine the relationship between increased [Ca2+]i and cell death as measured by propidium iodide incorporation. [Ca2+]i of individual NM neurons in brain stem slices was monitored using fura-2 ratiometric fluorescence imaging. NM field potentials were monitored in experiments in which the eighth nerve was stimulated. Five hertz orthodromic stimulation maintained NM neuron [Ca2+]i at approximately 110 nM for 180 min. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to a mean of 265 nM by 120 min. This increase was attenuated by superfusion of PKC activators phorbol-12,13-myristate acetate (100 nM) or dioctanoylglycerol (50 microM) and by activators of PKA: 1 mM 8-bromoadenosine-3',5'-cyclophosphate sodium (8-Br-cAMP), 50 microM forskolin or 100 microM Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine. Inhibition of PKA (100 microM Rp-cAMPS) or PKC (50 nM bisindolymaleimide or 10 microM U73122) during continuous orthodromic stimulation resulted in an increase in NM neuron [Ca2+]i that exceeded 170 and 180 nM, respectively, by 120 min. Nonspecific kinase inhibition with 1 microM staurosporine during stimulation resulted in an [Ca2+]i increase that was greater in magnitude than that seen with either PKA or PKC inhibition alone, equal to that seen in the absence of stimulation, but much smaller than that seen with inhibition of mGluRs. In addition, manipulations that resulted in a [Ca2+]i increase >/=250 nM resulted in an increase in number and percentage of propidium iodide-labeled NM neurons. These results suggest that eighth nerve activity maintains [Ca2+]i of NM neurons at physiological levels in part via mGluR-mediated activation of PKA and PKC and that increases in [Ca2+]i due to activity deprivation or interruption of the PKA and PKC [Ca2+]i regulatory mechanisms are predictive of subsequent cell death.

Recent insights into regeneration of auditory and vestibular hair cells

Advances in hair cell regeneration are progressing at a rapid rate. This review will highlight and critique recent attempts to understand some of the cellular and molecular mechanisms underlying hair cell regeneration in non-mammalian vertebrates and efforts to induce regeneration in the mammalian inner ear sensory epithelium.

Rapid regulation of cytoskeletal proteins and their mRNAs following afferent deprivation in the avian cochlear nucleus

During development, removal of neuronal input can lead to profound changes in postsynaptic cells, including atrophy and cell death. In the chicken brainstem cochlear nucleus, the nucleus magnocellularis (NM), deprivation of auditory input via unilateral cochlea removal or silencing the eighth nerve with tetrodotoxin leads to a loss of 25-30% of the neurons and the atrophy of surviving neurons. One intracellular component that may be involved in both cell atrophy and cell death is the cytoskeleton. The degradation of the cytoskeleton following deafferentation could potentially lead to either atrophy or death of NM neurons. However, little is known regarding the role of neuronal input on the cytoskeletal structure of NM neurons and whether changes in the cytoskeleton are responsible for cell death following deafferentation. The present study examined whether changes in the cytoskeleton of NM neurons occurred following cochlea removal. Several components of the cytoskeleton were analyzed following unilateral afferent deprivation. Levels of immunostaining for tubulin, actin, and microtubule-associated protein 2 (MAP-2), and levels of beta-tubulin and beta-actin mRNAs were assessed in NM neurons following cochlea removal. Our results revealed that afferent deprivation results in a rapid decrease in immunostaining for all three cytoskeletal proteins examined. These decreases were observed as early as 3 hours after cochlea removal and persisted for up to 4 days. In addition, these changes occurred in all deafferented NM neurons at the early time points, indicating that both dying and surviving NM neurons undergo a similar change in their cytoskeletons. In contrast to the decreases in immunostaining, levels of beta-tubulin and beta-actin mRNAs were not noticeably altered by deafferentation. Our findings indicate that the cytoskeleton is altered or degraded following deafferentation but that this process is not regulated at the transcriptional level.

Mitochondrial regulation of calcium in the avian cochlear nucleus

The role of mitochondria and the endoplasmic reticulum in buffering [Ca2+]i in response to imposed calcium loads in neurons of the chick cochlear nucleus, nucleus magnocellularis (NM), was examined. Intracellular calcium concentrations were measured using fluorometric videomicroscopy. After depolarization with 125 mM KCl, NM neurons demonstrate an increase in [Ca2+]i that returns to near-basal levels within 6 min. Addition of the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP) dissipated the mitochondrial membrane potential, as evidenced by increased fluorescence when cells were loaded with rhodamine-123. Two micromolar CCCP had minimal effect on baseline [Ca2+]i. However, 2 or 10 microM CCCP interfered with the ability of NM cells to buffer [Ca2+]i in response to KCl depolarization without significantly affecting peak [Ca2+]i. Oligomycin also interfered with postdepolarization regulation of [Ca2+]i, but blocked late (7-8 min postdepolarization) increases in [Ca2+]i caused by CCCP. Thapsigargin had no effect on baseline, peak, or postdepolarization [Ca2+]i in NM cells. These results suggest that normal mitochondrial membrane potential and ATP synthesis play an important role in buffering [Ca2+]i in response to imposed calcium loads in NM neurons. Furthermore, the endoplasmic reticulum does not appear to play a significant role in either of these processes. Thus increases in mitochondrial number and function noted in NM cells after deafferentation may represent an adaptive response to an increased cytosolic calcium load.

Bengalese finches Lonchura Striata domestica depend upon auditory feedback for the maintenance of adult song

Male birds of age-limited song-learning species develop their full song repertoires in the first year of life. For this type of song learner, once song is stabilized in adulthood, it is highly stereotyped and stable over time. Traditionally, it has been believed that age-limited song learners do not depend on auditory feedback for the maintenance of adult song. A recent report, however, showed that adult song in zebra finches, age-limited learners, does change after long-term deafness. We report here that another species of age-limited learner, Bengalese finches, depends critically on auditory feedback for adult song maintenance. We surgically deafened adult males and recorded song for 12 weeks after surgery. Results show that song degraded significantly within 1 week of surgery and continued to degrade over the next 11 weeks. This represents a more rapid degradation of song than has been seen previously in age-limited species. Song deficits after deafening included a marked decrease in syllable sequence stereotypy, skewed syllable distribution within song bouts, degradation of syllable phonology, and dropped, combined, and new or unrecognizable syllables. Decreased sequence stereotypy and combined syllables appeared within 1 week of deafening and did not worsen over time. Skewed syllable distributions and syllable phonology changes appeared after 1 week and did worsen. Occurrences of dropped and new syllables appeared within 1 week and increased over time. Comparison with other species indicates that much variability exists among species in the extent to which auditory feedback is necessary for song maintenance.

Development of Cat-301 immunoreactivity in auditory brainstem nuclei of the gerbil

The developing brainstem auditory system has been studied in detail by using anatomical and physiological techniques. However, it is not known whether immature auditory neurons exhibit different molecular characteristics than those of physiologically mature neurons. To address this issue, we examined the distribution of Cat-301 immunoreactivity in the developing auditory brainstem of gerbils. Cat-301 is a monoclonal antibody that recognizes a 680-kD chondroitin sulfate proteoglycan similar to aggrecan, a high-molecular-weight chondroitin sulfate proteoglycan found in cartilage. In the central nervous system, Cat-301 immunoreactivity is localized to the extrasynaptic surface of neurons. It has been hypothesized by Hockfield and co-workers (Hockfield et al. [1990a]Cold Spring Harbor Symp. Quart. Biol. 55:504-514) that the Cat-301 proteoglycan is a molecular marker indicating that a neuron has acquired mature neuronal properties. In the current study, Cat-301 staining is first seen at 7 days after birth in the anterior ventral cochlear nucleus (AVCN), the posterior VCN (PVCN), and the medial nucleus of the trapezoid body (MNTB) shortly before the onset of sound-evoked activity. By 21 days after birth, neurons in the AVCN, the PVCN, and the lateral and medial superior olive have attained adult-like distributions of Cat-301 staining concomitant with the physiological maturation of these neurons. Neurons in MNTB attain adult-like distributions of Cat-301 immunoreactivity at 1 year. The maturation of Cat-301 immunoreactivity parallels the physiological maturation of gerbil auditory neurons, and the Cat-301 proteoglycan may play a role in the formation and/or stabilization of auditory synapses.

Induction of cell proliferation in avian inner ear sensory epithelia by insulin-like growth factor-I and insulin

Postembryonic production of inner-ear hair cells occurs both normally and after insult in lower vertebrates and avians. To determine how this proliferation is controlled, several growth factors were tested for effects on progenitor-cell division in cultured avian vestibular sensory epithelium. Mitogenic effects of bombesin, epidermal growth factor, insulin-like growth factor-I (IGF-I), insulin, and transforming growth factor-alpha were assayed in organo-typic cultures of utricles from the mature, undamaged (normal) chicken inner ear. Tritiated thymidine and autoradiographic techniques and 5-bromo-2'-deoxyuridine (BrdU) immunocytochemistry were used to identify cells synthesizing DNA. IGF-I stimulated DNA synthesis in the vestibular sensory receptor epithelium in a dose-dependent manner. DNA synthesis was also stimulated by insulin. These results suggest that stimulation of the IGF-I receptors by IGF-I or insulin binding stimulates cell proliferation in the mature avian vestibular sensory epithelium.

Functional MR spectroscopy of the auditory cortex in healthy subjects and patients with sudden hearing loss

PURPOSE

To use MR spectroscopy to study the biochemical changes produced by auditory stimuli in patients with sudden sensorineural hearing loss and to compare these findings with the biochemical changes seen in healthy volunteers.

METHODS

Single-voxel MR spectroscopy was used to study biochemical changes in the auditory cortex in 11 control subjects and 19 patients with sudden sensorineural hearing loss. MR spectroscopic signals were measured during three different sound conditions (scanner noise, music, and sirens).

RESULTS

A lower MR spectroscopic lactate signal was observed in control subjects during the music stimulus than during the other sound conditions. This music-induced lactate change was not observed in patients with hearing loss. The other proton metabolites (choline, creatine, N-acetylaspartate [NAA]) remained stable during the different auditory stimuli. However, the NAA/creatine ratio was higher in the auditory cortex of patients than in the control subjects, and was not dependent on the sound condition.

CONCLUSION

The detection of stimulus-induced and stable biochemical MR spectroscopic changes in patients with hearing loss may be useful in assessing disease activity.

Neurofilament proteins in avian auditory hair cells

The distribution of middle-weight neurofilament protein (NF-M), an intermediate filament of neurons, was examined in the developing and mature avian inner ear by using immunocytochemical techniques. NF-M was detected in auditory hair cells and VIIIth cranial nerve neurons. NF-M-positive hair cells are first detected at embryonic day 11 (E11) in superior hair cells in the mid-proximal (mid-frequency) region of the chicken basilar papilla. With time, increasing numbers of hair cells express NF-M. Two developmental gradients occur: 1) a radial gradient, in which superior hair cells are labeled first, and progressively more inferiorly located hair cells are labeled during ontogeny, and 2) a longitudinal gradient, in which hair cells in the mid-proximal region are labeled first, and then progressively more distal (low-frequency) hair cells are labeled. There is also a small proximally directed progression of NF-M expression. By E19, NF-M-positive hair cells are found throughout the distal and mid-proximal regions, and this expression is maintained through 3 weeks posthatching. By 22 weeks posthatching, NF-M staining in hair cells is markedly diminished; staining is seen in only a few tall hair cells in the distal one-fourth of the papilla and in short hair cells in the distal one-half of the papilla. NF-M is never expressed by hair cells at the proximal (high-frequency) end of the papilla at any time examined. These findings suggest that some cell types that have traditionally been classified as nonneural may express neurofilament and that the basilar papilla of the neonatal chicken is not morphologically mature.

Reactive oxygen species in chick hair cells after gentamicin exposure in vitro

Reactive oxygen species have been invoked as a causative agent of cell death in many different developmental and pathological states. The presence of free radicals and their importance of hair cell death due to aminoglycosides is suggested by a number of studies that have demonstrated a protective effect of antioxidants. By using dichlorofluorescin (DCFH) a fluorescent compound that is a reporter of reactive oxygen species, we have shown that free radicals are rapidly produced by avian hair cells in vitro after exposure to gentamicin. In addition, free radical scavengers, catalase and glutathione, were tested with DCFH fluorescent imaging for their ability to quench the production of reactive oxygen species in hair cells after drug exposure. Both free radical scavengers were very effective in suppressing drug-induced production of free radicals. Next, we investigated the ability of these antioxidants to preserve the structural integrity of hair cells after exposure to gentamicin. We were not able to detect any attenuation of the hair cell loss using antioxidants in conjunction with gentamicin. This result must be qualified by the fact that the antioxidants used were not effective over long-term gentamicin exposure. Therefore, methodological constraints prevented adequately testing possible protective effects of the free radical scavengers in this model system.

Development of distortion product emissions in the gerbil: "filter" response and signal delay

Amplitude and phase responses of distortion product otoacoustic emissions as a function of stimulus frequency ratio were measured for frequencies between 2 and 48 kHz, in Mongolian gerbils (Meriones unguiculates) aged 15 to 30 days after birth. After baseline measurements, furosemide was administered to distinguish active from passive emissions. At all ages, structure in the form of multiple peaks was observed in the amplitude responses of specific odd-order emissions. This structure depended on the emission frequency, not the stimulus frequency ratio, and did not generally depend on the stimulus amplitude. Nor was it dependent on the functioning of the cochlear amplifier: At moderate stimulus levels, the observed emission distribution simply shifted to lower amplitudes when the cochlear amplifier was made temporarily dysfunctional by furosemide injection. The center frequencies and widths of the peaks in the amplitude response did not generally change with age, except that the relative amplitudes of the higher-frequency peaks were increased in younger animals. At 2 kHz, however, the distribution showed other evidence of maturation, with the frequency of maximum emission moving downward with age. The phase responses yielded estimates of the round trip signal (group or traveling wave) delay. At a given frequency, the active signal delay typically decreased substantially with increasing stimulus level. However, there was a rapid variation in delay as the stimulus level passed the normal active-passive crossover level. At stimulus levels measured relative to the active-passive crossover level, i.e., either 20 or 30 dB lower, the active signal delay decreased only slightly with age. Overall, both filter response and signal delay characteristics were found to be essentially mature near the onset of hearing.

Eighth nerve activity regulates intracellular calcium concentration of avian cochlear nucleus neurons via a metabotropic glutamate receptor

1. Neurons in the cochlear nucleus, nucleus magnocellularis (NM), of embryonic and neonatal chicks are dependent on eighth nerve activity for their maintenance and survival. Removing this input results in the death of 20-40% of the NM neurons and profound changes in the morphology and metabolism of surviving neurons. 2. One of the first changes in NM neurons after an in vivo cochlea removal is an increase in intracellular calcium concentration ([Ca2+]i). Increased [Ca2+]i has been implicated in a number of neuropathologic conditions. 3. In this study, we orthodromically and antidromically stimulated NM neurons in an in vitro brain stem slice preparation and monitored NM field potentials while simultaneously assessing the [Ca2+]i of NM neurons using fura-2. 4. During continuous orthodromic stimulation, [Ca2+]i of NM neurons remained constant at 80 nM. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to 230 nM by 90 min. Antidromic and contralateral stimulation produced a [Ca2+]i increase in NM neurons that was similar in magnitude but slightly more rapid than that observed in the absence of stimulation. 5. Addition of the metabotropic glutamate receptor (mGluR) antagonists (R,S)-alpha-methyl-4-carboxyphenylglycine or 2-amino-3-phosphonopropionic acid to the superfusate during continued orthodromic stimulation resulted in a dose-dependent, rapid, and dramatic increase in NM neuron [Ca2+]i without affecting the postsynaptic field potentials recorded from NM. 6. The ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovalerate eliminated NM field potentials during continued orthodromic stimulation but did not result in an increase in [Ca2+]i. 7. Continuous superfusion of trans-(+/-)-aminocyclopentane dicarboxylate, but not glutamate, prevented the increase in [Ca2+]i in the absence of stimulation. 8. These results suggest that NM neurons rely on eighth nerve activity-dependent activation of a mGluR to maintain physiological [Ca2+]i. Removal of this mGluR activation results in an increase in [Ca2+]i that may contribute to the early stages of degeneration and eventual death of these neurons.

Hair cell regeneration and recovery of the vestibuloocular reflex in the avian vestibular system

1. Although auditory and vestibular hair cells are known to regenerate after aminoglycoside intoxication in birds, there is only sparse evidence that the regenerated hair cells are functional. To address this issue, we examined the relation of hair cell regeneration to recovery of the vestibuloocular reflex (VOR), whose afferent signal originates at hair cells in the vestibular epithelium. Hair cell damage was produced by treating white Leghorn chicks (Gallus domesticus, 4-8 days posthatch) with streptomycin sulfate in normal saline (1,200 mg.kg-1.day-1 im) for 5 days. 2. In the 1st wk after treatment, the VOR gain was essentially 0, and hair cell density as assessed by light microscopy was approximately 40% of normal. Between the 1st and 3rd wk after treatment, the VOR was present. Although VOR gain varied considerably from one chick to another, it increased, on average, between the 1st and 3rd wk, as did the average hair cell density. At the end of 8-9 wk, the gain and phase of the VOR had returned to normal values, as had the average density of hair cells. 3. Therefore, despite the catastrophic initial effect of hair cell loss on the VOR, recovered hair cells appeared to restore the VOR completely. Average hair cell density increased with average VOR gain. VOR gain correlated better with recovery of type 1 hair cells than with recovery of type II hair cells. 4. In contrast to hair cell density, the appearance of the vestibular epithelia as assessed by hair cell stereocilia in scanning electron micrographs was a poor indicator of VOR gain. In both treated and control birds, epithelia with the same appearance could have quite different VOR gains, suggesting a variation in the functional viability of the hair cells. 5. This observation suggests that several factors, such as the repair of stereocilia, the efficacy of hair cell synapses on afferent fibers, and the extent of compensation by central vestibular pathways, may affect the recovery of VOR gain. However, our data suggest that hair cell regeneration plays an important role in this recovery.

Hair cell differentiation in chick cochlear epithelium after aminoglycoside toxicity: in vivo and in vitro observations

Inner ear epithelia of mature birds regenerate hair cells after ototoxic or acoustic insult. The lack of markers that selectively label cells in regenerating epithelia and of culture systems composed primarily of progenitor cells has hampered the identification of cellular and molecular interactions that regulate hair cell regeneration. In control basilar papillae, we identified two markers that selectively label hair cells (calmodulin and TUJ1 beta tubulin antibodies) and one marker unique for support cells (cytokeratin antibodies). Examination of regenerating epithelia demonstrated that calmodulin and beta tubulin are also expressed in early differentiating hair cells, and cytokeratins are retained in proliferative support cells. Enzymatic and mechanical methods were used to isolate sensory epithelia from mature chick basilar papillae, and epithelia were cultured in different conditions. In control cultures, hair cells are morphologically stable for up to 6 d, because calmodulin immunoreactivity and phalloidin labeling of filamentous actin are retained. The addition of an ototoxic antibiotic to cultures, however, causes complete hair cell loss by 2 d in vitro and generates cultures composed of calmodulin-negative, cytokeratin-positive support cells. These cells are highly proliferative for the first 2-7 d after plating, but stop dividing by 9 d. Calmodulin- or TUJ1-positive cells reemerge in cultures treated with antibiotic for 5 d and maintained for an additional 5 d without antibiotic. A subset of calmodulin-positive cells was also labeled with BrdU when it was continuously present in cultures, suggesting that some cells generated in culture begin to differentiate into hair cells.

Influence of mitochondrial protein synthesis inhibition on deafferentation-induced ultrastructural changes in nucleus magnocellularis of developing chicks

Following cochlea removal in developing chicks, about 30% of the neurons in the ipsilateral second-order auditory nucleus, nucleus magnocellularis, undergo cell death. Administration of chloramphenicol, a mitochondrial protein synthesis inhibitor, results in a pronounced increase in deafferentation-induced cell death. In this study, we examined whether the chloramphenicol enhancement of deafferentation-induced cell death reveals the same ultrastructural characteristics that are seen in degenerating nucleus magnocellularis neurons after cochlea removal alone. Unilateral cochlea removal was performed on anaesthetized posthatch chicks. One group of animals was simultaneously treated with chloramphenicol. Six, twelve, or twenty-four hours following cochlea removal, n. magnocellularis neurons were studied by routine transmission electron microscopy. Particular attention was paid to the integrity of the polyribosomes and rough endoplasmic reticulum. Two ultrastructurally different types of neuronal degeneration were observed in the deafferented nucleus magnocellularis neurons: an early onset electron-lucent type that always involved ribosomal dissociation and a late-onset electron-dense type displaying nuclear pyknosis and severely damaged mitochondria. The percentage of nucleus magnocellularis neurons displaying ribosomal disintegration following cochlea removal was found to be markedly increased after chloramphenicol treatment. This finding suggests that mitochondrial function is important for the maintenance of a functional protein synthesis apparatus following deafferentation.

Glutamate modulates intracellular Ca2+ stores in brain stem auditory neurons

1. Fura-2 imaging was used to measure the effects of glutamate on caffeine-sensitive Ca2+ stores in neurons of the avian cochlear nucleus, n. magnocellularis (NM). 2. On average, 100-mM caffeine stimulated a 250-nM increase in intracellular calcium ion concentration {[Ca2+]i} in Ca(2+)-free media; 1-mM glutamate significantly attenuated caffeine-stimulated Ca2+ responses. 3. The metabotropic glutamate receptor agonist, ACPD, also inhibited the caffeine-stimulated rise in [Ca2+]i. 4. Glutamate has an important role in regulating Ca2+ stores in NM neurons. Glutamate-deprivation (viz. cochlear removal) results in a rise in [Ca2+]i that may, in part, be the result of release from Ca2+ stores. We hypothesize that Ca(2+)-induced Ca2+ release stores (CICRs) may be involved in deprivation-induced cell death.

Development of the cochlear amplifier

The development of the cochlear amplifier was examined in gerbils aged 14 days after birth (dab) to adult, for stimulus frequencies from 1 to 48 kHz. Distortion product otoacoustic emissions (DPOAEs) were employed to determine the characteristics of active emissions associated with cochlear amplifier operation. DPOAEs were also used to determine the characteristics of "passive" emissions remaining when the cochlear amplifier operation was interrupted by acute furosmide intoxication. The input-output functions of the passive emissions, and of the active emissions at low stimulus levels, could be approximated by parallel straight lines. The horizontal distance between these parallel lines, i.e., the increase in stimulus level required to obtain a passive emission amplitude equal to the active emission, is an estimate of the gain of the cochlear amplifier. Further, the lowest stimulus level at which active and passive emissions become approximately equal defines a passive threshold level. At 14 dab, the cochlear amplifier gain was already at adult levels for the midfrequencies (4-8 kHz), but no emissions were detected at the extremes (at 1 kHz, and 24 kHz and above). During the period over which the endocochlear potential (EP) is known to increase most sharply (14 to 18 dab), the gain at all frequencies increased. At low frequencies there was little or no gain in the youngest age group, but matured by 23 dab. The gain at the middle frequencies subsequently decreased, resulting by 30 dab in a gain that was remarkably flat across frequency from 1 to 32 kHz. The passive thresholds generally improved with age at all frequencies, but most dramatically at the high frequencies. Results are consistent with the view that the elements of the cochlear amplifier are functional in the base of the cochlea at all ages, but that auditory function is primarily limited by the lower passive base cutoff frequency at younger ages. The proposed increase in passive base cutoff frequency with age accounts for the known place code shift.