Scar formation after drug-induced cochlear insult

Structural and molecular changes in the guinea pig organ of Corti were studied using histochemistry and electron microscopy in the course of drug-induced hair cell degeneration. Actin filaments disappear from the cuticular plate and the stereocilia. An actin-rich bridge appears in the apical region of dying hair cells. Two supporting cells form a scar for a given hair cell. The supporting cells expand and invade the spaces of Nuel and then the region previously occupied by the hair cell. The scar region becomes cytokeratin-labeled. In this study, the apical domain of the hair cell is the last part of the cell to degenerate. Hair cell degeneration coincides temporally with scar formation. We define the resulting scar as a `type I' scar. The results provide preliminary information about the molecular composition of the type I scar and suggest a structural basis for the dynamics of scar formation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29474/1/0000560.pd

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A–C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A–C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed. Microsc. Res. Tech. 41:217–223, 1998. © 1998 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/35182/1/5_ftp.pd

Tyrosine hydroxylase immunoreactivity identifies possible catecholaminergic fibers in the organ of Corti

Antibodies to tyrosine hydroxylase. dopamine [beta]-hydroxylase and phenylethanolamine N-methyltransferase were used in an immunocytochemical examination of catecholamines in the cochlea. In cryostat sections, tyrosine hydroxylase and dopamine [beta]-hydroxylase-like immunoreactivities fibers were seen in the modiolus that did not extend to the organ of Corti. These corresponded to blood vessel-associated and non-blood vessel-associated fibers that have been previously described with histofluorescence. In surface preparations, tyrosine hydroxylase-like immunoreactivity was seen in the organ of Corti. in the inner and tunnel spiral bundles, suggesting an efferent component may be catecholaminergic.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26856/1/0000421.pd

Attenuation of gentamicin ototoxicity by glutathione in the guinea pig in vivo

The effect of glutathione co-therapy on the expression of gentamicin ototoxicity was tested in pigmented guinea pigs. The first group of animals was injected with gentamicin (100 mg/kg body weight/day) for two weeks followed by 10 weeks of rest. A second group received glutathione by gastric gavage immediately prior to each gentamicin injection. Two groups of controls were treated either with saline injections or glutathione gavage alone. Auditory brainstem responses, taken at 2-week intervals, revealed a progressive gentamicin-induced hearing loss reaching a 30 to 40 dB threshold shift at 2 kHz, approximately 60 dB at 8 kHz and 80 dB at 18 kHz. Glutathione co-therapy slowed the progression of hearing loss and significantly attenuated the final threshold shifts by 20 to 40 dB. Morphological evaluation confirmed hair cell loss after gentamicin treatment and protection by glutathione. Drug serum levels were assayed at 2 and 7 days of treatment. There were no differences between the gentamicin (mean = 183 [mu]g/ml; range, 90 to 300) and the gentamicin/glutathione group (mean = 164 [mu]g/ml; range, 80 to 320). Antimicrobial activity of gentamicin was tested against Staphylococcus aureus and Pseudomonas aeruginosa. A 30-fold molar excess of glutathione did not influence the efficacy of gentamicin. These studies suggest that glutathione protects against ototoxicity by interfering with the cytotoxic mechanism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31501/1/0000423.pd

Expression of NMDA-receptor mRNA in the rat cochlea

While there is considerable evidence that an excitatory amino acid and excitatory amino acid receptors are involved in the synapse between inner hair cells and the auditory nerve, evidence for the specific involvement of the N-methyl-D-aspartate (NMDA) receptor is more ambiguous. With the cloning of the NMDA receptor, probes are now available that can determine in which neurons the receptor is being expressed. In situ hybridization histochemical techniques were therefore utilized to examine the expression of NMDA receptor messenger ribonucleic acid (mRNA) in the rat cochlea. Expression of NMDA receptor mRNA was seen in spiral ganglion cells. These results suggest that NMDA receptor is a component of excitatory amino acid synapses in the cochlea.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30597/1/0000234.pd

Immunocytochemical localization of AMPA selective glutamate receptor subunits in the rat cochlea

The localization of subunits of the [alpha]-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) selective glutamate receptor, termed Glutamate receptor (GluR) was examined in the rat cochlea using affinity purified polyclonal antibody to GluR subunits (GluR 1-4). GluR 2/3 and GluR 4 immunoreactive (IR) staining was observed in rat spiral ganglion cells, while GluR 1 IR was not. GluR 4 IR staining was also seen in puncta at inner and outer hair cell bases. These results suggest that GluR 2/3 and GluR 4 are components of excitatory amino acid synapses in the rat cochlea.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31208/1/0000110.pd

An Evaluation of Otopathology in the MOV-13 Transgenic Mutant Mouse a

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72482/1/j.1749-6632.1991.tb19595.x.pd

Putative neurotransmitters in the rat cochlea at several ages

We have performed a longitudinal study of the content of the putative neurotransmitter substances, enkephalin, dynorphin, and acetylcholine (ACh), in the cochlea of the Fischer 344 rat. It is the first study of transmitters in the rat cochlea over an extended time span. This study also provides biochemical verification of the presence of ACh in cochlear tissues. No change was seen in the cochlear content of these transmitter candidates up to 24 months of age.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27136/1/0000129.pd

Neonatal deafening causes changes in Fos protein induced by cochlear electrical stimulation

The influence of neonatal deafness on cochlear electrically evoked Fos expression in the auditory brainstem was examined. Newborn rats were deafened by systemic injection of kanamycin, 1 mg/g daily for 12 days. At 4, 5, 6 or 8 weeks of age, these animals received cochlear electrical stimulation with a basal monopolar electrode for 90 minutes. Age-matched untreated control animals received similar stimulation. Experimental and control animals were assessed for spiral ganglion cell densities and Fos immunoreactive staining in the central nucleus of the inferior colliculus. Spiral ganglion cell assessments showed significant decreases in spiral ganglion cell densities in deafened rats compared to age-matched controls, at 5 weeks of age in lower turns and 6 and 8 weeks in all turns. Cochlear electrical stimulation induced Fos immunoreactive staining in the nucleus of auditory brain stem neurons in treatment and control groups. A significantly greater number of Fos immunoreactive neurons was found in the contralateral central nucleus of inferior colliculus in 5, 6 and 8 week old deafened animals compared to age-matched controls. The increases were larger with a longer duration of deafness. These results suggest that there are changes in auditory processing as a consequence of neonatal deafness.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47464/1/11068_2004_Article_5253210.pd

Deafferentiation-associated changes in afferent and efferent processes in the guinea pig cochlea and afferent regeneration with chronic intrascalar brain-derived neurotrophic factor and acidic fibroblast growth factor

Deafferentation of the auditory nerve from loss of sensory cells is associated with degeneration of nerve fibers and spiral ganglion neurons (SGN). SGN survival following deafferentation can be enhanced by application of neurotrophic factors (NTF), and NTF can induce the regrowth of SGN peripheral processes. Cochlear prostheses could provide targets for regrowth of afferent peripheral processes, enhancing neural integration of the implant, decreasing stimulation thresholds, and increasing specificity of stimulation. The present study analyzed distribution of afferent and efferent nerve fibers following deafness in guinea pigs using specific markers (parvalbumin for afferents, synaptophysin for efferent fibers) and the effect of brain derived neurotrophic factor (BDNF) in combination with acidic fibroblast growth factor (aFGF). Immediate treatment following deafness was compared with 3-week-delayed NTF treatment. Histology of the cochlea with immunohistochemical techniques allowed quantitative analysis of neuron and axonal changes. Effects of NTF were assessed at the light and electron microscopic levels. Chronic BDNF/aFGF resulted in a significantly increased number of afferent peripheral processes in both immediate- and delayed-treatment groups. Outgrowth of afferent nerve fibers into the scala tympani were observed, and SGN densities were found to be higher than in normal hearing animals. These new SGN might have developed from endogenous progenitor/stem cells, recently reported in human and mouse cochlea, under these experimental conditions of deafferentation-induced stress and NTF treatment. NTF treatment provided no enhanced maintenance of efferent fibers, although some synaptophysin-positive fibers were detected at atypical sites, suggesting some sprouting of efferent fibers. J. Comp. Neurol. 507:1602–1621, 2008. © 2008 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58023/1/21619_ftp.pd