Microstimulation of auditory nerve for estimating cochlear place of single fibers in a deaf ear

Multielectrode cochlear prostheses seek to approximate the cochlea's normal frequency-place mapping through spatial segregation of stimulus currents. Various electrode configurations have been employed to achieve such segregation. Direct measurements of stimulation regions among single auditory nerve (AN) fibers has been possible only when normal hearing is preserved, such that each fiber's cochlear place can be inferred from its tuning curve. This precludes measurements in deafened ears, or ears compromised by implantation of the electrodes. Data presented here demonstrate that the cochlear place of an AN fiber can be estimated without acoustic sensitivity, using electrical microstimulation through a recording pipette in the AN bundle. The procedure exploits cochleotopic projection to isofrequency laminae within the contralateral inferior colliculus (IC). Microstimulation excites a small group of fibers neighboring the recorded fiber, generating centrally propagated volleys along a narrow frequency-specific pathway. Evoked potential recordings at varying depths are made to identify the ICC lamina where the response to AN microstimulation is greatest. Preliminary data are also presented for an alternative method of identifying the lamina using a frequency domain measure of binaural interactions within the IC.

R-phenylisopropyladenosine attenuates noise-induced hearing loss in the chinchilla

Reactive oxygen species, which are cytotoxic to living tissues, are thought to be partly responsible for noise-induced hearing loss. In this study R-phenylisopropyladenosine (R-PIA), a stable non-hydrolyzable adenosine analogue which has been found effective in upregulating antioxidant enzyme activity levels, was topologically applied to the round window of the right ears of chinchillas. Physiological saline was applied to the round window of the left ears (control). The animals were then exposed to a 4 kHz octave band noise at 105 dB SPL for 4 h. Inferior colliculus evoked potential thresholds and distortion product otoacoustic emissions (DPOAE) were measured and hair cell damage was documented. The mean threshold shifts immediately after the noise exposure were 70-90 dB at frequencies between 2 and 16 kHz. There were no significant differences in threshold shifts at this point between the R-PIA-treated and control ears. By 4 days after noise exposure, however, the R-PIA-treated ears showed 20-30 dB more recovery than saline-treated ears at frequencies between 4 and 16 kHz. More importantly, threshold measurements made 20 days after noise exposure showed 10-15 dB less permanent threshold shifts in R-PIA-treated ears. The amplitudes of DPOAE also recovered to a greater extent and outer hair cell losses were less severe in the R-PIA-treated ears. The results suggest that administration of R-PIA facilitates the recovery process of the outer hair cell after noise exposure.

Cochlear pathology induced by aminoglycoside ototoxicity during postnatal maturation in cats

Cochlear pathology resulting from neonatal administration of the aminoglycoside antibiotic, neomycin sulfate, was studied in young kittens at 15-24 days postnatal. Hearing thresholds showed severe to profound hearing loss in all but one animal. Scanning electron microscopy demonstrated that initial hair cell degeneration occurred in the extreme base (hook region) of the cochlea and sequentially progressed to the basal, middle, then the apical coil of the cochlea. The first row of outer hair cells degenerated first, followed by row 2, then row 3; the last cells to degenerate in a given region were the inner hair cells. This pattern of hair cell degeneration is similar to that seen in adults with neomycin ototoxicity. In contrast, the spiral ganglion exhibited a different pattern of degeneration with initial cell loss occurring in the middle of the cochlea, about 40-60% from the base (approximately 2.8-8 kHz). Thus, neuronal degeneration apparently is not secondary to sensory cell loss, but rather comprises an independent process in these neonatal animals. Taken together, the findings suggest that the spiral ganglion cell loss in the middle cochlear turn results from increased aminoglycoside sensitivity associated with an earlier initial onset of function in these neurons as compared to other cochlear regions.

Cytotoxicity and mitogenicity of adenosine triphosphate in the cochlea

Evidence is accumulating to indicate that extracellular adenosine 5'-triphosphate (ATP) may function as a neurotransmitter, neuromodulator, cytotoxin and mitogen. Many of the cells in the cochlea have ATP receptors, however, their function is unknown. The purpose of the present study was to test whether ATP may act as a cytotoxin in the cochlea. ATP was applied to acutely isolated outer hair cells (OHCs) and their shape changes monitored. In addition, ATP was applied into the cochlea by perfusion of the perilymph compartment for 2 h and the animals allowed to survive 3-4 weeks post drug application. At this time, sound-evoked cochlear potentials and distortion product otoacoustic emissions (DPOAEs) were monitored and the cochleas evaluated histologically. Results indicate that when applied to isolated OHCs, ATP (3-30 mM) induced a bleb formation in the infracuticular region of the cell that burst within a few minutes. Short OHCs were more sensitive to this effect of ATP than long OHCs. 3-4 weeks after the perilymph perfusion of ATP (60 mM; 2 h) cochlear potentials and DPOAEs were abolished, and histologically, cells in the organ of Corti and the stria vascularis were found to have been destroyed. In addition, there was loss of spiral ganglion cells and proliferating connective tissue filled varying proportions of the scala tympani and vestibuli. Application of sodium gluconate, a control, at the same concentrations had no effect either on the isolated OHCs or when applied in vivo. Results suggest that extracellular ATP or a metabolic product may act as a cytotoxin to some epithelial and neural elements in the cochlea and possibly as a mitogen to mesenchymal cells or fibrocytes.

An accurate, cost-effective approach for diagnosing retrocochlear lesions utilizing the T2-weighted, fast-spin echo magnetic resonance imaging scan

Recent data indicate that the auditory brainstem response (ABR) fails to identify a significant number of retrocochlear lesions. Although the magnetic resonance imaging (MRI) scan with paramagnetic enhancement is highly accurate at detecting these lesions, it is time consuming and expensive. We report on our prospective evaluation of a cohort of 155 patients who underwent T2-weighted, fast-spin echo MRI scans designed to screen for retrocochlear lesions. This imaging technique is rapidly performed and provides superb visualization of the relevant anatomic structures at a global cost of $475. Four tumors were identified with this technique. Cost analysis indicates that supplanting ABR with this limited MRI may well represent a cost-effective approach for evaluating patients with suspected retrocochlear lesions.

Nucleus double electrode array: a new approach for ossified cochleae

INTRODUCTION

The ossified cochlea is still a special surgical issue that requires a special surgical procedure. The current cochlear implants only have one electrode lead, which can be placed only partially in the drilled out basal turn. The small number of used active electrodes leads to worse performance as compared with patients with full insertion.

METHODS

To overcome this limitation, a special electrode was developed consisting of two arrays. One array with 11 active electrode rings is placed in the drilled out basal turn, the second array with 10 active electrodes in the opened second turn. The number of inserted electrodes can be significantly increased. The surgery is similar to that in nonossified cochleae. After the posterior tympanotomy, the bridge is removed and the incus is located. A cochleostomy is performed at the basal turn and the new-built tissue removed. A second cochleostomy is placed below the cochleariform process. In most cases, the second turn is not obliterated and the second electrode array can be fully inserted.

RESULTS

The surgical procedure was in all nine cases uneventful. Intraoperative stapedius reflex could be recorded with elevated thresholds. The wide variety of stimulation modes and sites allows an individual fitting to maximize the performance. All patients show a gap in the pitch scale between the apical and the basal array. The pitch variation is much smaller in the apical array. All patients have some benefit from the additional apical array and an improved performance.

CONCLUSION

The nucleus double electrode array is an advanced treatment option for patients with ossified cochleae. The receiver/stimulator is a regular nucleus cochlear implant.

Cochlear immunoglobulin in the C3H/lpr mouse model for autoimmune hearing loss

Immunoglobulin staining was conducted in cochlear tissue from the C3H/lpr autoimmune strain mouse to better understand the local immune processes underlying autoimmune inner ear disease. This mouse is a model for spontaneous systemic lupus erythematosus with coincident elevated cochlear thresholds. Cochleas were examined from C3H/lpr mice at 2 months of age, before disease onset, and at 8 months of age, when systemic disease and hearing loss are manifested. Sections of these cochleas, along with cochlear sections from age-matched C3H/HeJ nonautoimmune controls, were immunocytochemically stained for IgG and IgM to identify areas of abnormal immunoglobulin activity. IgM immunoreactivity was similar in control and autoimmune cochlear tissue and did not appear to vary with disease progression. Staining was limited to the inside of capillaries in the stria vascularis and other areas within the cochlea. Similar staining patterns were seen in control animals stained for IgG. However, C3H/lpr mice with autoimmune disease showed extensive IgG immunoreactivity spreading out from the stria vascularis capillaries into the extracapillary spaces. This increased permeability suggested that breakdown of the blood labyrinth barrier was coincident with systemic autoimmune disease.

Comparison of Five Agents in Protecting the Cochlea Against the Ototoxic Effects of Cisplatin in the Hamster

The purpose of this investigation was to study the ameliorating effects of four agents on cis-platin-induced ototoxicity. Hamsters were given a series of five cisplatin injections either alone or in combination with sodium thiosulfate (STS), diethyldihydrothiocarbamate (DDTC), and S-2(3-aminopropylamino) ethylphosphorothiolc acid (WR-2721), or fosfomycin. Ototoxicity was assessed anatomically by quantifying the extent of cochlear damage with the scanning electron microscope and physiologically with measures of the auditory brain stem response. When administered alone, cisplatin induced widespread loss of outer hair cells (OHCs) along much of the cochlea in the hamster, especially in the basal and middle turns, with an average survival of only 56% of the OHC population. In contrast, inner hair cells resisted cisplatin ototoxicity in the hamster. Thus the ameliorative effects of the different test agents were assessed by counting the number of surviving OHCs in each treatment group and comparing with cisplatin-treated controls. STS provided the most effective protection against the ototoxic effects of cisplatin, yielding 91% survival of OHCs. DDTC also reduced the ototoxic effects of cisplatin, yielding 68% survival of OHCs. Cotreatment with WR-2721 and fosfomycin yielded 45% and 52% OHC survival, respectively, and thus did not provide any chemoprotection. The results closely paralleled those based on auditory brain stem response recordings in that the magnitude of threshold shift was proportional to the amount of OHC loss; also, the amount of threshold shift at each frequency was in good agreement with the pattern of hair cell loss along the cochlear spiral. Thus both histologic and physiologic results suggest that STS and DDTC hold promise for ameliorating the ototoxic effects of cisplatin chemotherapy.

Hearing loss in the RBF/DnJ mouse, a proposed animal model of Usher syndrome type IIa

The Usher syndromes (US) are a group of inherited disorders that feature autosomal recessive neurosensory hearing loss or deafness with retinitis pigmentosa (RP). Moderate to severe non-progressive high frequency hearing loss with RP and normal vestibular function describes Usher syndrome type IIa, which has been localized to 1q41. Severe retinal degeneration in the inbred mouse strain RBF/DnJ is caused by rd3, a recessive gene located on mouse chromosome 1 distal to akp1 in a region which is orthologous to human 1q32-q42. We evaluated rd3 as a candidate for orthology with USH2A by first reducing and refining the relatively broad region in which rd3 is thought to reside. DNA of offspring from an RBF/DnJ x MOLF/Ei backcross was genotyped with PCR markers closely flanking the predicted location of rd3. Our haplotype analysis re-positioned rd3 to a 3.6 cM region between markers D1Mit273 (cen) and D1Mit209 (tel), consistent with the expected position of an USH2A murine orthologue. Consequently, rd3 is a positional candidate for Usher type IIa. Next we assessed the rd3/rd3 audiological phenotype to see how closely it paralleled that of Usher IIa. Audiological evaluation of mice at various ages revealed evidence of high frequency progressive hearing loss, previously unreported in the RBF/DnJ strain. However, this newly discovered hearing deficit was observed to be inherited independently of rd3, establishing that a completely different gene is responsible.

Distortion product otoacoustic emissions in the C57BL/6J mouse model of age-related hearing loss

One of the earliest histopathological changes associated with age-related hearing loss appears to be the disruption of outer hair cells (OHCs). To evaluate age-related changes in OHC function, distortion product otoacoustic emissions (DPOAEs) were recorded in the young and aging C57BL/6J mouse. Starting in young adulthood, the C57 mouse displays age-related elevation of auditory brainstem response thresholds, beginning in the high frequencies and progressing toward lower frequencies. The 2f1-f2 DPOAEs of mice between 2 and 20 months of age were examined for f2s between 8 and 16 kHz. In this octave region, the features of 2f1-f2 DPOAEs in the 2-month-old C57 mouse were comparable to those described for non-murine rodents in the literature in terms of optimum f2/f1 ratio, optimum primary level difference, input/output (I/O) function features and microstructure. It was determined that f2/f1 = 1.2 and L1-L2 = 20 dB were optimal stimulus parameters for investigation of the effects of age on C57 DPOAEs. Age-related changes in DPOAE I/O functions consisted of a right shift (i.e. increased DPOAE detection thresholds), disappearance of 'notches' and shallowing of the slopes after 8 months of age. As DPOAE I/O functions continued to shift to the right and DPOAE levels decreased with age, the appearance of I/O functions became complex to include regions of steep or shallow slopes and plateaus. The present results suggest that the age-related elevation of auditory thresholds in the C57 mice is associated with substantial progressive changes in OHC function.

Response of the primary auditory cortex to electrical stimulation of the auditory nerve in the congenitally deaf white cat

Neural activity plays an important role in the development and maintenance of sensory pathways. However, while there is considerable experience using cochlear implants in both congenitally deaf adults and children, little is known of the effects of a hearing loss on the development of the auditory cortex. In the present study, cortical evoked potentials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the functional organisation of the auditory cortex in the adult congenitally deaf white cat. The absence of click-evoked auditory brainstem responses during the first weeks of life demonstrated that these animals had no auditory experience. Under barbiturate anaesthesia, cortical potentials could be recorded from the contralateral auditory cortex in response to bipolar electrical stimulation of the cochlea in spite of total auditory deprivation. Threshold, morphology and latency of the evoked potentials varied with the location of the recording electrode, with response latency varying from 10 to 20 ms. There was evidence of threshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also varied with the configuration of the stimulating electrodes in accordance with changes previously observed in normal hearing animals. Single-unit recordings exhibited properties similar to the evoked potentials. Increasing stimulus intensity resulted in an increase in spike rate and a decrease in latency to a minimum of approximately 8 ms, consistent with latencies recorded in AI of previously normal animals (Raggio and Schreiner, 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording from auditory brainstem nuclei. Taken together, in these auditory deprived animals basic response properties of the auditory cortex of the congenitally deaf white cat appear similar to those reported in normal hearing animals in response to electrical stimulation of the auditory nerve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.

Severe sensory deficits but normal CNS development in newborn mice lacking TrkB and TrkC tyrosine protein kinase receptors

Analysis of mice carrying targeted mutations in genes encoding neurotrophins and their signalling Trk receptors has provided critical information regarding the role that these molecules play in the mammalian nervous system. In this study we generated mice defective in both TrkB and TrkC tyrosine kinase receptors to determine the biological effects of these receptors in the absence of compensatory mechanisms. trkB(-/-);trkC(-/-) double-mutant mice were born at the expected frequency, indicating that TrkB and TrkC signalling are not required for embryonic survival. However, these double-mutant mice had a significantly shorter lifespan and displayed more severe sensory defects than their single-mutant trkB(-/-) and trkC(-/-) littermates. The most dramatic sensory deficit observed in trkB(-/-);trkC(-/-) mutant mice was the absence of vestibular and cochlear ganglia. Interestingly, these mice developed inner ear sensory epithelia in spite of the complete absence of sensory innervation. Analysis of the CNS in trkB(-/-);trkC(-/-) mutant mice revealed a well formed hippocampus, cortex and thalamus. Moreover, the pattern of expression of several neuronal markers appeared normal in these animals. These observations suggest that neurotrophin signalling through TrkB and TrkC receptors is essential for the development of sensory ganglia; however, it does not play a major role in the differentiation and survival of CNS neurons during embryonic development.

Patterns of neural degeneration in the human cochlea and auditory nerve: implications for cochlear implantation

Although the identity of all the variables that may influence speech recognition after cochlear implantation is unknown, the degree of preservation of spiral ganglion cells is generally considered to be of primary importance. A series of experiments in our laboratories, directed at quantification of surviving spiral ganglion cells in the profoundly deaf, evaluation of the predictive value of a variety of clinical parameters, and the evaluation of the consequences of implantation in the inner ear, is summarized. Histologic study of the inner ears of patients who were deafened during life demonstrated that the cause of deafness accounted for 57% of the variability of spiral ganglion cell counts. Spiral ganglion cell counts were highest in individuals deafened by aminoglycoside toxicity or sudden idiopathic deafness and lowest in those deafened by postnatal viral labyrinthitis, congenital or genetic deafness, or bacterial meningitis. Study of the determinants of degeneration of the spiral ganglion revealed that degeneration is most severe in the basal compared with the apical turn and more severe when both inner and outer hair cells are absent. Unlike the findings in some experimental animal studies, no survival advantage of type II ganglion cells could be identified. There was a strong negative correlation between the degree of bony occlusion of the cochlea and the normality of the spiral ganglion cell count. However, even in specimens in which there was severe bony occlusion, significant numbers of spiral ganglion cells survived. A strong positive correlation between the diameter of the cochlear, vestibular, and eighth cranial nerves with the total spiral ganglion cell count (p < 0.001) was found. This would suggest that modern imaging techniques may be used to predict residual spiral ganglion cell population in cochlear implant candidates. Trauma from implantation of the electrode array was studied in both cadaveric human temporal bone models and temporal bones from individuals who received implants during life. A characteristic pattern of damage to the lateral cochlear wall and basilar membrane was identified in the upper basal turn. New bone formation and perielectrode fibrosis was common after cochlear implantation. Despite this significant trauma and reaction, there is no firm evidence that further degeneration of the spiral ganglion can be predicted as a consequence.

A critical appraisal of the role of auditory brain stem response and magnetic resonance imaging in acoustic neuroma diagnosis

Auditory brain stem response (ABR) is the most sensitive and specific audiometric test for acoustic neuroma (AN) detection. The ABR sensitivity rate increases with tumor size; consequently, small tumors have notably low ABR sensitivity for detection. However, larger tumors also can fail recognition by ABR testing. The ABR must be viewed on its own merits compared to magnetic resonance imaging studies for the diagnosis of AN. Where does ABR fit into the screening process for detecting AN today?

Cochlear changes following destruction of semicircular canal in healthy and previously toxin-exposed rats. An electrophysiological and morphological investigation

One group of Sprague-Dawley rats (group A, n = 6) was treated by instilling Pseudomonas aeruginosa exotoxin A (PaExoA), and another (group B, n = 6) treated similarly with Haemophilus influenzae type b endotoxin (HiBEndo). In group A a 20 dB hearing loss was observed, predominantly in the high-frequency region, which was reversible within 1 month. In group B no significant hearing impairment was noted. Between 1 and 6 months later, the lateral and posterior semicircular canals (SCCs) were ablated unilaterally. Control rats (group C, n = 8) were subjected to ablation only. All rats were cochleotomized contralaterally prior to labyrinthine surgery. Frequency-specific evoked potential testing at 2-31.5 kHz tone bursts was performed before and directly after surgery, 6, 24 and 48 hours and 1, 4 and 16 weeks postoperatively. After surgery in 18 rats, thresholds rose immediately, predominantly at 2, 4 and 6 kHz, followed by varying degrees of recovery. Greatest immediate postoperative hearing loss was observed in group A; no rat recovered completely and two rats showed severe permanent threshold elevation. All group B rats recovered completely, except one showing moderate threshold impairment. No permanent hearing loss was observed in group C. This study shows that destruction of SCCs in rats does not necessarily cause permanent hearing loss, even if the fluid spaces are not sealed off. However, previous exposure of the middle ear to PaExoA (but not HiBEndo) renders the cochlea more vulnerable and can result in persistent hearing loss.

Age-related decline of auditory function in the chinchilla (Chinchilla laniger)

The aim of this study was to examine the functional consequences of aging in the chinchilla, a rodent with a relatively long life span and a range of hearing similar to that of humans. Subjects were 21 chinchillas aged 10-15 years, and 23 young controls. Thresholds were determined from auditory evoked potentials (EVPs), and outer hair cell (OHC) functioning was assessed by measuring 2f1-f2 distortion product otoacoustic emissions (DPOAEs). Six cochleas from 11-12-year-old animals were examined for hair cell loss and gross strial pathology. The results show that the chinchilla exhibits a small but significant decline of auditory sensitivity and OHC functioning between 3 and 15 years of age, with high-frequency losses exceeding and growing more rapidly than low-frequency losses. Compared to rodents with shorter life spans, the chinchilla has a rate of loss that is more similar to that of humans, which could make it a valuable model for understanding the etiology of human presbycusis.

Combined effects of a simultaneous exposure to noise and toluene on hearing function

To study the combined effects of noise and toluene on auditory function, three experimental groups of Long-Evans adult rats were used. The first group was exposed to toluene (2000 ppm, 6 h/day, 5 days/week, 4 weeks), the second group to an octave band of noise centered at 8 kHz (92 dB SPL), and the last group to a simultaneous exposure to toluene and noise. Auditory function was tested by recording brainstem (inferior colliculus) auditory-evoked potentials. The auditory deficit induced by the combined exposure exceeded the summated losses caused by toluene alone and by noise alone within the range (2-32 kHz) of test frequencies. The nature of the cochlear damage induced by noise alone (injured stereocilia) or by toluene alone (outer hair cells loss) is different.

[Vestibular function: a prerequisite for normal language development?]

BACKGROUND

First language acquisition depends on intermodal perception, especially auditive, tactile-kinesthetic, and partly visual in addition to sensomotoric integration. The influence of the vestibular function for a physiological language development is still unknown.

PATIENT

A case history of a child with bilateral aplasia of all semicircular canals, normacusis in the right ear, and severe sensorineural hearing loss in the left (hearing aid supplied), without mental deficiency, allows us to draw initial conclusions.

RESULTS

Logopedic diagnostics revealed only a mild articulation disorder (interdental sigmatism, inconstant gammacism) as a possible consequence of recurrent middle ear effusions since early childhood. Language development diagnostics did not demonstrate any significant norm deviation.

CONCLUSIONS

In contrast to the statomotoric disorder, no evidence for a specific language impairment was found. Our report elucidates the importance of a selected diagnostic imaging (spiral CT and MIP MRI) in the phoniatric-ped-audiological field.

Pathology of the cochlea following a spontaneous mutation in DBA/2 mice

The DBA/2 strain of mice usually presents with noise-induced epileptic seizures and hearing disorders. After a spontaneous mutation a strain with early hearing loss and circling behaviour was produced. This strain presents with clinical symptoms found in diseases connected to inner ear disorders. These animals do not suffer from periodical disorders, however, but have functional disturbances continuously and can therefore serve as an animal model for diseases originating from both parts of the inner ear. The genetic inheritance appears to be autosomal recessive. Offspring showed circling behaviour and severe pathology in the vestibular part of the inner ear. In the present study pathology of the cochlear part of the inner ear was visualized using conventional microscopical techniques. The content of actin and fodrin was labelled immunohistochemically, and hearing was assessed with auditory brainstem recordings. After 1 month the animals showed deterioration of the cochlear part of the inner ear. At 6 months no organ of Corti remained and the animals were deaf. Transmission and scanning electron microscopy revealed severe apical hair cell changes. The content of alpha-actinin and fodrin in the DBA/2 mouse was already fainter than that in age-matched CBA control mice at the age of 1 month. Labelling of antibodies against fodrin increased in the supporting cells of the older animals, probably owing to the replacement of hair cells.

Total ablation of cochlear haircells by perilymphatic perfusion with water

Inner and outer haircells are destroyed within one day following perfusion of the perilymphatic spaces with water, otherwise the structural integrity of the cochlea and the organ of Corti are preserved. This technique is a reliable method to create an acute unilateral deafness model for investigations of auditory function.

Processing and analyzing the mouse temporal bone to identify gross, cellular and subcellular pathology

A technique has been developed for preparing the mouse temporal bone for histopathological examination: first, as a whole mount to detect any gross malformations of the bony or membranous labyrinths; second, in dissected segments to localize damage in the different sensory organs and to quantify sensory- and supporting-cell losses; and finally, in semi-thick and thin sections to identify and characterize subcellular pathology. Examples are given of the successful application of this technique to mice with very different inner-ear problems, including those with an abnormally short cochlear spiral, a defective lateral semicircular canal, abnormal otoliths over the saccular macula, an increased susceptibility to noise damage and those which lack fibroblast growth factor receptor 3.

Implication of NMDA type glutamate receptors in neural regeneration and neoformation of synapses after excitotoxic injury in the guinea pig cochlea

In the adult mammalian cochlea, the ability of nerve fibres to regenerate has been observed following disruption of the organ of Corti by various means, or transsection of the cochlear nerve in the internal auditory meatus. Based upon the implication of glutamate as a neurotransmitter at synapses between sensory hair cells and terminal dendrites of the auditory nerve in the mammalian cochlea, we have developed, in a previous study, an in vivo model of neural regeneration and formation of synapses after the destruction of the afferent nerve endings by local application of the glutamate agonist alpha-amino-3-hydroxy-5-methyl-isoxazol-propionic acid (AMPA). In situ hybridization experiments performed during the re-innervation process revealed an overexpression of mRNA coding for NR1 subunit of N-methyl-D-aspartate (NMDA) receptors in the spiral ganglion neurons, suggesting that these receptors are implicated in neural regenerative processes. The present study has been designed to study the functional implication of NMDA receptors in the regrowth and synaptic repair of auditory dendrites in the guinea pig cochlea, by blocking the NMDA receptors during the period of normal functional recovery. In a first set of experiments, we recorded compound action potential after acute perilymphatic perfusion of cumulative doses (0.03-10mM) of DL 2-amino-5-phosphonovalerate (D-AP5), a NMDA antagonist, to determine the efficiency of the drug. In a second set of experiments, the auditory dendrites were destroyed by local application of the glutamate agonist AMPA. The blockage of NMDA by the antagonist D-AP5 applied with an osmotic micropump delayed the functional recovery and the regrowth of auditory dendrites. The findings of our study support the hypothesis that, in addition to acting as a fast transmitter, glutamate has a neurotrophic role via the activation of NMDA receptors.

Discharge properties of pigeon single auditory nerve fibers after recovery from severe acoustic trauma

The time course of recovery of compound action potential (CAP) thresholds was observed in individual adult pigeons after severe acoustic trauma. Each bird had electrodes implanted on the round window of both ears. One ear was exposed to a tone of 0.7 kHz at 136-142 dB SPL for 1 hr under general anesthesia. Recovery of CAP audiograms was monitored twice a week after trauma. Single unit recordings from auditory nerve fibers were made after 3 weeks and after 4 or more months of the exposure. The CAP was abolished immediately after overstimulation in all animals. Based on the temporal patterns of functional recovery of the CAP three groups of animals were identified. The first group was characterized by fast functional recovery starting immediately after trauma followed by a return to pre-exposure values within 3 weeks. In the second group, slow functional recovery of threshold started 1-2 weeks after trauma followed by a return to pre-exposure values by 4-5 weeks. A mean residual hearing loss of 26.3 dB at 2 kHz remained. The third group consisted of animals that did not recover after trauma. Three weeks after the exposure, tuning curves of single auditory nerve fibers were very broad and sometimes irregular in shape. Their thresholds hovered around 120 dB SPL. Spontaneous firing rate and driven rate were much reduced. Four or more months after exposure, the thresholds and sharpness of tuning of many single units were almost completely recovered. Spontaneous firing rate and driven rate were comparable to those of control animals. In the slow recovery group neuronal tuning properties showed less recovery, especially at frequencies above the exposure frequency. Thresholds and sharpness of tuning were normal at frequencies below the exposure frequency, but were much poorer at frequencies above the exposure. Spontaneous firing rate was much reduced in fibers with high characteristic frequencies. In fast recovering animals, the papilla was repopulated with hair cells after 4 months. In slow recovering animals, short (abneural) hair cells were still missing over large parts of the papilla after 4 months of recovery. Residual short (abneural) hair cell loss was largest at two areas, one more basal and the other more apical to the characteristic place of the traumatizing frequency. The results show that, in adult birds, functional recovery from severe damage to both short (abneural) and tall (neural) hair cells occurs. However, the onset of recovery is delayed and the time course is slower than after destruction of short (abneural) hair cells alone. Also, recovery is incomplete, both functionally and morphologically. There is residual permanent hearing loss, and regeneration of short (abneural) hair cells is incomplete.

Progression of cochlear and retinal degeneration in the tubby (rd5) mouse

Mice homozygous for a defect of the tub (rd5) gene exhibit cochlear and retinal degeneration combined with obesity, and resemble certain human autosomal recessive sensory deficit syndromes. To establish the progressive nature of sensory cell loss associated with the tub gene, and to differentiate tub-related losses from those associated with the C57 background on which tub arose, we evaluated cochleas and retinas from tub/tub, tub/+, and +/+ mice, aged 2 weeks to 1 year by light and electron microscopy. Cochleas from mice of all three genotypes show progressive inner (IHC) and outer hair cell (OHC) loss. Relative to tub/+ and +/+ animals, however, tub homozygotes show accelerated OHC loss, affecting the extreme cochlear base (hook region) by 1 month, and the apex by 6 months. IHC loss in tub/tub animals is accelerated in the basal half of the cochlea, affecting the hook region by 6 months. Spiral ganglion cell losses were observed only in tub/tub mice, and only in the cochlear base. Retinas of tub/tub mice are abnormal at maturity, exhibiting shortened photoreceptor outer segments by 2 weeks, and progressive photoreceptor loss thereafter. Because the tub mutation causes degeneration of sensory cells in the ear and eye but has no other neurological effects, tubby mice hold unique promise for the study of human syndromic sensory loss.