Neurotrophins and the development of cochlear innervation

Brain-derived nerve growth factor in the cochlea - a reproducibility study

Objective

Brain-derived nerve growth factor (BDNF) plays an important role in cochlear development so it is plausible that it could restore hearing loss if delivered directly into the cochlea. We wished to confirm our previous report that a single intracochlear injection of brain-derived nerve growth factor (BDNF) was beneficial for hearing in guinea pigs. We wished to assess the reproducibility of our results and assess possible improved methods with a view to developing a clinical treatment for sensorineural hearing loss.

Methods

CDDP was used to create partial hearing loss in 25 guinea pigs. After 30 days the animals underwent ABR testing and unilateral BDNF injection through the round window in one ear and saline injection into the other ear. After allowing possible effects to stabilize, thirty days later, ABR threshold testing was repeated to assess change in threshold.

Results

Final ABR thresholds were 60–70 dB and were about 11 dB better in the ears treated with BDNF.

Conclusion

Our original finding that Intracochlear BDNF can improve hearing in guinea pigs was confirmed, but the improvement demonstrated by the methods in this paper is too small for clinical application.

Coding-in-Noise Deficits are Not Seen in Responses to Amplitude Modulation in Subjects with cochlear Synaptopathy Induced by a Single Noise Exposure

Since the first report of noise-induced synaptic damage in animals without permanent threshold shifts (PTSs), the concept of noise-induced hidden hearing loss (NIHHL) has been proposed to cover the functional deficits in hearing associated with noise-induced synaptopathy. Moreover, the potential functional deficit associated with the noise-induced synaptopathy has been largely attributed to the loss of auditory nerve fibers (ANFs) with a low spontaneous spike rate (SSR). As this group of ANFs is critical for coding at suprathreshold levels and in noisy background, coding-in-noise deficit (CIND) has been considered to be main consequence of the synaptopathy. However, such deficits have not been verified after a single, brief exposure to noise without PTS. In the present study, synaptopathy was generated by such noise exposure in both mice and guinea pigs. Responses to amplitude modulation (AM) were recorded at a high sound level in combination with masking to evaluate the existence of CINDs that might be associated with loss of low-SSR ANFs. An overall reduction in response amplitude was seen in AM-evoked compound action potential (CAP). However, no such reduction was seen in the scalp-recorded envelope following response (EFR), suggesting a compensation due to increased central gain. Moreover, there was no significant difference in masking effect between the control and noise groups. The results suggest that either there is no significant CIND after the synaptopathy we created, or the AM response tested with our protocol was not sufficiently sensitive to detect such a deficit; far-field EFR is not sensitive to cochlear pathology.

Noise-Induced Cochlear Synaptopathy and Ribbon Synapse Regeneration: Repair Process and Therapeutic Target

The synapse between the inner hair cells (IHCs) and the spiral ganglion neurons (SGNs) in mammalian cochleae is characterized as having presynaptic ribbons and therefore is called ribbon synapse. The special molecular organization is reviewed in this chapter in association with the functional feature of this synapse in signal processing. This is followed by the review on noise-induced damage to this synapse with a focus on recent reports in animal models in which the effect of brief noise exposures is observed without causing significant permanent threshold shift (PTS). In this regard, the potential mechanism of the synaptic damage by noise and the impact of this damage on hearing are summarized to clarify the concept of noise-induced hidden hearing loss, which is defined as the functional deficits in hearing without threshold elevation. A controversial issue is addressed in this review as whether the disrupted synapses can be regenerated. Moreover, the review summarizes the work of therapeutic research to protect the synapses or to promote the regeneration of the synapse after initial disruption. Lastly, several unresolved issues are raised for investigation in the future.

Noise-induced Cochlear Synaptopathy and Signal Processing Disorders

Noise-induced hidden hearing loss (NIHHL) has attracted great attention in hearing research and clinical audiology since the discovery of significant noise-induced synaptic damage in the absence of permanent threshold shifts (PTS) in animal models. Although the extant evidence for this damage is based on animal models, NIHHL likely occurs in humans as well. This review focuses on three issues concerning NIHHL that are somewhat controversial: (1) whether disrupted synapses can be re-established; (2) whether synaptic damage and repair are responsible for the initial temporal threshold shifts (TTS) and subsequent recovery; and (3) the relationship between the synaptic damage and repair processes and neural coding deficits. We conclude that, after a single, brief noise exposure, (1) the damaged and the totally destroyed synapses can be partially repaired, but the repaired synapses are functionally abnormal; (2) While deficits are observed in some aspects of neural responses related to temporal and intensity coding in the auditory nerve, we did not find strong evidence for hypothesized coding-in-noise deficits; (3) the sensitivity and the usefulness of the envelope following responses to amplitude modulation signals in detecting cochlear synaptopathy is questionable.

Virally Mediated Overexpression of Glial-Derived Neurotrophic Factor Elicits Age- and Dose-Dependent Neuronal Toxicity and Hearing Loss

Contemporary cochlear implants (CI) are generally very effective for remediation of severe to profound sensorineural hearing loss, but outcomes are still highly variable. Auditory nerve survival is likely one of the major factors underlying this variability. Neurotrophin therapy therefore has been proposed for CI recipients, with the goal of improving outcomes by promoting improved survival of cochlear spiral ganglion neurons (SGN) and/or residual hair cells. Previous studies have shown that glial-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor, and neurotrophin-3 can rescue SGNs following insult. The current study was designed to determine whether adeno-associated virus vector serotype 5 (AAV-5) encoding either green fluorescent protein or GDNF can transduce cells in the mouse cochlea to express useful levels of neurotrophin and to approximate the optimum therapeutic dose(s) for transducing hair cells and SGN. The findings demonstrate that AAV-5 is a potentially useful gene therapy vector for the cochlea, resulting in extremely high levels of transgene expression in the cochlear inner hair cells and SGN. However, overexpression of human GDNF in newborn mice caused severe neurological symptoms and hearing loss, likely due to Purkinje cell loss and cochlear nucleus pathology. Thus, extremely high levels of transgene protein expression should be avoided, particularly for proteins that have neurological function in neonatal subjects.

Interaction of neurotrophin signaling with Bcl-2 localized to the mitochondria and endoplasmic reticulum on spiral ganglion neuron survival and neurite growth

Enhanced spiral ganglion neuron (SGN) survival and regeneration of peripheral axons following deafness will likely enhance the efficacy of cochlear implants. Overexpression of Bcl-2 prevents SGN death but inhibits neurite growth. Here we assessed the consequences of Bcl-2 targeted to either the mitochondria (GFP-Bcl-2-Maob) or the endoplasmic reticulum (ER, GFP-Bcl-2-Cb5) on cultured SGN survival and neurite growth. Transfection of wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, or GFP-Bcl-2-Maob increased SGN survival, with GFP-Bcl-2-Cb5 providing the most robust response. Paradoxically, expression of GFP-Bcl-2-Maob results in SGN death in the presence of neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF), neurotrophins that independently promote SGN survival via Trk receptors. This loss of SGNs is associated with cleavage of caspase 3 and appears to be specific for neurotrophin signaling, insofar as coexpression of constitutively active mitogen-activated kinase kinase (MEKDeltaEE) or phosphatidyl inositol-3 kinase (P110), but not other prosurvival stimuli (e.g., membrane depolarization), also results in the loss of SGNs expressing GFP-Bcl-2-Maob. MEKDeltaEE and P110 promote SGN survival, whereas P110 promotes neurite growth to a greater extent than NT-3 or MEKDeltaEE. However, wild-type GFP-Bcl-2, GFP-Bcl-2-Cb5, and GFP-Bcl-2-Maob inhibit neurite growth even in the presence of neurotrophins, MEKDeltaEE, or P110. Historically, Bcl-2 has been thought to act primarily at the mitochondria to prevent neuronal apoptosis. Nevertheless, our data show that Bcl-2 targeted to the ER is more effective at rescuing SGNs in the absence of trophic factors. Additionally, Bcl-2 targeted to the mitochondria results in SGN death in the presence of neurotrophins. (c) 2010 Wiley-Liss, Inc.

Brain-derived nerve growth factor in the treatment of sensorineural hearing loss

OBJECTIVES/HYPOTHESIS

A possible medical treatment for sensorineural hearing loss using brain-derived nerve growth factor (BDNF) was explored. The hypothesis is that direct intracochlear application of BDNF will result in improved hearing.

STUDY DESIGN

Animal research study.

METHODS

Significant hearing loss was created using cisplatin in 11 guinea pigs. One month later, bilateral cochleostomies were performed placing 0.05 microg of BDNF in one cochlea of each animal prior to plugging with connective tissue. The other cochlea served as a control. Auditory brain-stem response (ABR) testing was then carried out for three months at 6,000, 8,000, 12,000, and 24,000 Hz.

RESULTS

ABR thresholds were better in the treated ear for all frequencies. Threshold differences were statistically significantly better two months after treatment (general linear model, repeated measures P = .045).

CONCLUSIONS

Intracochlear application of BDNF may prevent hearing loss.

Reciprocal regulation of presynaptic and postsynaptic proteins in bipolar spiral ganglion neurons by neurotrophins

A unifying principle of sensory system organization is feature extraction by modality-specific neuronal maps in which arrays of neurons show systematically varied response properties and receptive fields. Only beginning to be understood, however, are the mechanisms by which these graded systems are established. In the peripheral auditory system, we have shown previously that the intrinsic firing features of spiral ganglion neurons are influenced by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). We now show that is but a part of a coordinated package of neurotrophin actions that also includes effects on presynaptic and postsynaptic proteins, thus encompassing the input, transmission, and output functions of the spiral ganglion neurons. Using immunocytochemical methods, we determined that proteins targeted to opposite ends of the neuron were organized and regulated in a reciprocal manner. AMPA receptor subunits GluR2 and GluR3 were enriched in base neurons compared with their apex counterparts. This distribution pattern was enhanced by exposure to BDNF but reduced by NT-3. SNAP-25 and synaptophysin were distributed and regulated in the mirror image: enriched in the apex, enhanced by NT-3 and reduced by BDNF. Moreover, we used a novel coculture to identify potential endogenous sources of neurotrophins by showing that sensory receptors from different cochlear regions were capable of altering presynaptic and postsynaptic protein levels in these neurons. From these studies, we suggest that BDNF and NT-3, which are systematically distributed in complementary gradients, are responsible for orchestrating a comprehensive set of electrophysiological specializations along the frequency contour of the cochlea.

In vitro and in vivo assessment of the ability of adeno-associated virus-brain-derived neurotrophic factor to enhance spiral ganglion cell survival following ototoxic insult

OBJECTIVES/HYPOTHESIS

Auditory dysfunction following ototoxic insult results from loss of cochlear hair cells. Secondary degeneration of auditory neurons ensues from withdrawal of neurotrophic support from hair cells and can be prevented with administration of neurotrophins. Administration of adeno-associated virus containing the gene for brain-derived neurotrophic factor will promote spiral ganglion neuron survival after the destruction of hair cells.

METHODS

Prevention of aminoglycoside-induced spiral ganglion neuron loss through the expression of brain-derived neurotrophic factor mediated by means of the adeno-associated virus was tested in vitro in cochlear explants and in vivo in mammalian cochlea.

RESULTS

Neuronal survival was significantly enhanced in adeno-associated virus-brain-derived neurotrophic factor transfected rat cochlear explants compared with control samples (30% vs. 19%, P <.05) following exposure to aminoglycoside. Following deafening with aminoglycoside and loop diuretic and introduction of adeno-associated virus-brain-derived neurotrophic factor through osmotic minipump, the experimental group of animals infused with adeno-associated virus-brain-derived neurotrophic factor displayed enhanced spiral ganglion neuron survival in the basal turn of the cochlea when compared with the control group infused with adeno-associated virus containing green fluorescent protein reporter gene.

CONCLUSIONS

Administration of adeno-associated virus-brain-derived neurotrophic factor enhances spiral ganglion neuron survival following ototoxic exposure in vitro and in vivo. These studies lay the groundwork for further exploration of its application as an adjunct therapy for patients undergoing cochlear implantation because the success of implantation depends directly on the population of neurons available for electrical stimulation.

Role of trophic factors in the development, survival and repair of primary auditory neurons

1. Neurotrophic factors have been identified as crucial for the development of the auditory system and have also been proven to be important for continued survival and maintenance of auditory neural connections. 2. In addition, both in vitro and in vivo studies have demonstrated that these trophic molecules can prevent the secondary wave of auditory neuron degeneration normally seen following the loss of hair cells. 3. Furthermore, neurotrophic factors have been reported to enhance neuronal excitation and to improve the efficacy of synaptic transmission. 4. As such, these molecules are strong candidates to be used as therapeutic agents in conjunction with the cochlear implant, or even to repair and/or regenerate damaged or lost auditory nerve and sensory cells.

LIF is more potent than BDNF in promoting neurite outgrowth of mammalian auditory neurons in vitro

Neurotrophic factors are known to play a crucial role in the elongation and guidance of auditory nerve fibres to their targets within the organ of Corti. Maintenance of these neural connections following deafness would clearly influence the efficacy of therapies for hearing recovery. The growth factors leukaemia inhibitory factor (LIF), brain-derived neurotrophic factor (BDNF) and transforming growth factor-beta 5 (TGF-beta5) were tested for their efficacy in promoting neurite outgrowth from dissociated cultures of early postnatal rat auditory neurons. Our results indicate that while BDNF enhances neurite outgrowth in a strong fashion, LIF is more potent; moreover, the combined administration of both factors has even greater neuritogenic capacities. TGF-beta5, although neurotrophic, has no neuritogenic activity on cultured auditory neurons. LIF and BDNF may therefore be potential candidates when developing pharmacological therapies for hearing recovery.

Fibroblast growth factors (FGF-1, FGF-2) promote migration and neurite growth of mouse cochlear ganglion cells in vitro: immunohistochemistry and antibody perturbation

To study the effect of FGF in the early development of the sensory neurons of the auditory system, we established a culture preparation of ganglionic neuroblasts engaged in migration and process outgrowth. The presumed anlage of the cochlear ganglion was dissected from E11 otocysts, just as the neuronal precursors were migrating. The cultures were divided into 4 groups and supplemented for 7-9 days with either hrFGF-1 or hrFGF-2 or both or with defined medium only (control group). Measurements of the increase in explant growth, neuroblast migration, and neurite outgrowth were made by time-lapse imaging techniques in living cultures. Either FGF-1 or FGF-2 alone stimulated early migration and outgrowth of the ganglion cells by 5-10x. The effect of combining FGF-1 and FGF-2 was greater than either alone, but less than additive, consistent with a shared receptor. BrdU labeling confirmed that the effect was on migration, not on proliferation. Adding a neutralizing antibody for FGF-2 to the cultures inhibited migration and neurite outgrowth, suggesting an endogenous FGF-2 activity in these functions. Immunocytochemical observations in vitro and in situ with antibodies to FGF-1, FGF-2, or FGF receptor (R1) demonstrated immunopositive staining of the migrating ganglionic neuroblasts, their processes, and growth cones at corresponding stages (E13). Also non-neuronal cells, hair cells, and Schwann cells (in situ) expressed FGF-1 and FGF-2. Evidently both FGF-1 and FGF-2 play important roles in the migration and initial differentiation of cochlear ganglion neurons in the mouse.

Development of human cochlear active mechanism asymmetry: involvement of the medial olivocochlear system?

To study the functional development of the medial olivocochlear system, transient-evoked otoacoustic emission suppression experiments were conducted in 73 ears of 38 pre-term and 11 full-term neonates. The continuous contralateral stimulation was a broad band white noise, presented at 70 dB SPL. Efferent suppression was determined by subtracting the without-contralateral stimulation condition from the with-contralateral stimulation condition. Across this population, a mean suppression effect of contralateral stimulation on transient-evoked otoacoustic emissions was found, with most of the suppression effect observed after 8 ms. The amount of suppression is linearly, positively correlated with the conceptional age. In the subgroup of bilaterally tested neonates, the suppression of transient-evoked otoacoustic emissions is similar in the right ear and the left ear in subjects whose conceptional age is less than 36 weeks and significantly higher in the right ear than in the left ear in older neonates. This last observation was seen at frequencies where transient-evoked otoacoustic emission amplitudes became higher in the right ear than in the left ear as the conceptional age increased, a finding already reported in adults. This study shows that the functional adult pattern of the medial efferent system, probably involved in the detection of signals in noise such as speech sounds, seems to appear gradually in neonates and represents one of the several arguments in favor of functional auditory lateralization in humans, with a right ear advantage.

Hair cell protection from aminoglycoside ototoxicity by adenovirus-mediated overexpression of glial cell line-derived neurotrophic factor

Aminoglycosides are commonly used antimicrobial drugs that often have ototoxic side effects. The ototoxicity often involves permanent loss of cochlear hair cells (HCs). Neurotrophic factors have been shown to protect a variety of tissues, including HCs, from toxic trauma. To determine if glial cell line-derived neurotrophic factor (GDNF) can protect cochlear HCs from trauma, we inoculated an adenoviral vector encoding the human GDNF gene into guinea pig cochleae via the round window membrane 4 days prior to injection of aminoglycosides. Control groups showed little or no negative influence of the viral inoculation on cochlear structure and function. In contrast, ears that were inoculated with the GDNF vector had better hearing and fewer missing HCs after exposure to the ototoxins, as compared with controls. Our results demonstrate the feasibility of gene therapy for cochlear application and suggest that virus-mediated overexpression of GDNF may be developed as a valuable prevention against trauma-induced HC death.

Focal delivery of fibroblast growth factor-1 by transfected cells induces spiral ganglion neurite targeting in vitro

Sensory cells in the cochlea of the rat transiently express acidic fibroblast growth factor (FGF-1) during the developmental period of terminal innervation in the sensory epithelium. To explore the potential role of FGF-1 in terminal innervation events, the response of cochlear ganglion neurons to FGF-1 was evaluated in culture. Explants from the spiral ganglion of postnatal day 5 rats were cultured in the presence of exogenous FGF-1, with or without heparin. FGF-1 in the culture medium produced a dose-dependent increase in the number and length of neurites produced by spiral ganglion neurons, a response that was enhanced by heparin. To assess the effects of FGF-1 produced by a focal, cellular source, additional explants were cocultured with 3T3 cell transfectants that secrete FGF-1. Neurites that came into contact with FGF-1 secreting cells branched, formed bouton-like terminal swellings on the surface of the transfectants, and stopped extending. The results suggest that FGF-1 may stimulate neurite extension into the sensory epithelium of the cochlea and that focal production of FGF-1 may contribute to the formation of contacts on sensory cells by developing neurites.

Programmed cell death in the mouse cochleovestibular ganglion during development

Programmed cell death (PCD) is as essential to development as is cell proliferation. Our objective was to elucidate the spatiotemporal occurrence of PCD during the development of the cochleovestibular ganglion. We performed a time-sequence study on the distribution of in situ PCD, apoptosis, during the development of the mouse cochleovestibular ganglion by using the TUNEL method to detect apoptosis histochemically. Apoptosis in the cochleovestibular ganglion was observed from the 11.5th gestational day (GD) to the 18. 5th GD. Apoptosis was seen most extensively in the vestibular ganglion cells at the 13.5th GD, while in the spiral ganglion cells apoptosis was maximal between the 15.5th and 16.5th GD. Because these times of peak apoptotic activity almost exactly corresponded to the events of innervation and terminal mitosis of the cochleovestibular ganglion cells, we infer that PCD is involved in the innervation and modulates the number of cochleovestibular ganglion cells overproduced by terminal mitosis.

Long-term effects of sectioning the olivocochlear bundle in neonatal cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

Differential damage to auditory neurons and hair cells by ototoxins and neuroprotection by specific neurotrophins in rat cochlear organotypic cultures

Therapeutic ototoxic drugs are one of the major causes of damage in the peripheral auditory system, leading to hearing loss. In this study, we have examined the toxic actions of three classes of ototoxins (sodium salicylate, gentamicin and cisplatin) in organotypic cultures of postnatal cochlear explants. In these cultures, afferent innervation of hair cells by primary auditory neurons remained intact. Double labelling with a monoclonal antibody against neurofilament protein and a phalloidin-fluorescein isothiocyanate conjugate revealed that the three types of drugs induced differential damage to auditory neurons and hair cells in the cochlea. While gentamicin preferentially caused hair cell death, sodium salicylate specifically induced degeneration of auditory neurons. In contrast, cisplatin resulted in destruction of both auditory neurons and hair cells. Neuronal degeneration was largely prevented by the addition of neurotrophin-4/5, brain-derived neurotrophic factor and neurotrophin-3 to the culture media together with the ototoxins, while nerve growth factor and other growth factors had no effect. In contrast, the hair cell loss caused by cisplatin or gentamicin was not attenuated by the presence of neurotrophins. These results suggest that ototoxic mechanisms of salicylates, aminoglycosides and chemotherapeutic agents are different. Auditory neuronal loss induced by ototoxins may be prevented by specific neurotrophins.

Defective HSV-1 vector expressing BDNF in auditory ganglia elicits neurite outgrowth: model for treatment of neuron loss following cochlear degeneration

The neurotrophins are a family of growth factors that play an important role in the development and maintenance of the nervous system. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family that appears to participate in the maturation and function of mammalian auditory neurons. Forms of deafness due to varied injurious stimuli that are amenable to treatment with implantable prosthetic devices require the survival of these BDNF-responsive auditory neurons for effective outcome. To evaluate the feasibility of developing a gene therapy for deafness that may be used in conjunction with a prosthetic device, we constructed replication-defective herpes simplex virus (HSV) amplicon vectors that carry the human BDNF cDNA. Using these vectors, HSVbdnf and HSVbdnflac (expresses BDNF and Escherichia coli beta-galactosidase), we evaluated the expression and biological activity in established cell lines and explant cultures prepared from spiral ganglia of the murine ear. Gene transfer with HSVbdnf resulted in the efficient expression of human BDNF mRNA in murine fibroblasts. Using two BDNF-responsive cell lines, PC12trkB and MG87trkB, we demonstrate efficient secretion of biologically active BDNF. Finally, transduction of explanted spiral ganglia with HSVbdnflac elicited robust neuritic process outgrowth comparable to exogenously added BDNF. Overall, these data demonstrate that HSV vectors can efficiently transfer and express the BDNF gene in many cell types, including auditory neurons. Moreover, they suggest that similar vectors may be used to express the neurotrophin in auditory neurons in vivo and perhaps as adjunctive gene therapy for deafness.

Cochlear inner hair cells exist transiently in the fetal Bronx Waltzer (bv/bv) mouse

In the cochlea of the adult Bronx waltzer (bv/bv) mouse, the majority of inner hair cells are missing or deformed. As a result, Bronx waltzer mice are severely hearing impaired or deaf. Previous studies determined that most inner hair cells in these mice are missing by the time of birth, but no studies have resolved whether the missing inner hair cells ever exist in the mutant cochlea. The present study used light and electron microscopy to locate inner hair cells in the mutant mouse before birth. Most, and possibly all, inner hair cells exist in the embryonic day (E) 17 mouse. The shapes of the cells vary from normal and elongated in the youngest animals, to round and protruding through the reticular lamina a few days later. The density of sensory cells in the inner hair cell region (inner hair cells/millimeter) decreases in the basal turn between E17 and birth, and in the apical turn between birth and the third postnatal day. The initial presence of the full complement of inner hair cells, taken together with the temporospatial pattern of degeneration, suggests that the cause of inner hair cell death in the Bronx waltzer mouse is related to a differentiation event subsequent to cell birth.

The role of adrenocortical steroid hormones in the development of hearing

Based on the findings that adrenocortical hormones are involved in the regulation of Na+, K(+)-ATPase in several tissues and the presence of receptors for these hormones in the ear during auditory development, it has been suggested that these hormones also induce Na+, K(+)-ATPase activity and the endocochlear potential in the ear, leading to auditory function in the fetus-neonate. In order to test this hypothesis, glucocorticoid and mineralocorticoid hormones were injected into rat pups and their auditory development, compared to control litter-mates, was tested by recording auditory nerve-brainstem evoked potentials (ABR). Those who received glucocorticoid hormones had elevated ABR thresholds on post-natal day (PND) 9, others on PND 11 and still others on PND 16, compared to control litter-mates. The ABR thresholds of those injected with mineralocorticoids were not different from those in controls. These results and additional considerations related to the time sequence of the natural appearance of these hormones in the plasma, of their receptors in the ear and the onset of hearing in rat pups makes it extremely unlikely that adrenocortical hormones are involved in the initiation of Na+, K(+)-ATPase pumps and thereby of the endocochlear potential in the inner ear. It is possible that these hormones and their receptors play a role in the later regulation of the number of pumps.

Distribution of non-NMDA glutamate receptor mRNAs in the developing rat cochlea

In situ hybridization was used to document the distribution of mRNA encoding six subunit isoforms of non-N-methyl D-aspartic acid (NMDA) glutamate receptors (GluR1, GluR2, GluR3, GluR4, GluR5 and GluR6) in the inner ears of embryonic, postnatal and adult rats. GluR2 and GluR3 expression in the spiral ganglion appeared well before birth, and reached adult levels several days before the onset of function in the cochlea. In the spiral limbus, expression of GluR2 and GluR3 mRNA reached very high levels at around the time of birth, then declined after a few days. Low levels of GluR1, GluR4 and GluR6 expression were detected in various tissues of the cochlea during development. In the adult cochlea, GluR expression was limited to GluR2 and GluR3 mRNAs in the spiral ganglion neurons and GluR2 mRNA in fibrocytes of the spiral limbus, a non-neural tissue. The ontogenetic expression of additional GluR subunit genes and their appearance in different cochlear tissues could reflect different roles for these genes during development, or less precise regulation of gene expression within the GluR family. In particular, the very high levels of GluR gene expression in the spiral limbus during the perinatal period support a non-neural function, perhaps as cell surface receptors during tissue differentiation.