Downregulation of N-methyl-D-aspartate receptor ζ1 subunit (GluN1) gene in inferior colliculus with aging.

Presbycusis is the impairment of auditory function associated with aging, which stems from peripheral cochlear lesions and degeneration of the central auditory process. The effect of age-induced peripheral hearing loss on the central auditory process is not fully understood. C57Bl/6 (C57) mice present accelerated peripheral hearing loss, which is well developed by middle-age and mimics the human presbycusis pattern. The aim of this study was to elucidate the molecular effects of peripheral hearing loss in the inferior colliculus (IC) with age between young and middle-aged C57 mice using cDNA microarray. Glutamate receptor ionotropic NMDA ζ1 (GluN1) exhibited the greatest decrease in the middle-aged group as determined using cDNA microarray and by further assessment using real-time PCR (qPCR). Histological assessment with in situ hybridization of GluN1 showed significantly decreased expression in all IC subdivisions of the middle-aged group. GluN1 is a receptor for excitatory neurotransmission, and significant downregulation of this gene may be subsequent to the decline of afferent input from the cochlea in aging C57 mice. Consequently, using the combination of microarray, qPCR, and in situ hybridization, we showed that the decline of GluN1 in the IC of aging animals might have a key role in the pathogenesis of presbycusis.

Gene transfer into guinea pig cochlea using adeno-associated virus vectors.

BACKGROUND: Several genes are candidates for treating inner ear diseases. For clinical applications, minimally invasive approaches to the inner ear are desirable along with minimal side-effects. METHODS: Adeno-associated virus (AAV) was used as a vector into the guinea pig inner ear. Six AAV-cytomegalovirus hybrids (AAV-2/1, -2/2, -2/5, -2/7, -2/8 and -2/9) were infused into perilymph of the cochlea basal turn, an approach that could be used in cochlear implant surgery. At 7 days after injection, distribution of gene expression, hearing and morphology were evaluated. Adenoviral vector was also used to compare distributions of gene expression. Moreover, distribution of cell surface receptors of AAV in the cochlea was examined using immunohistochemistry. RESULTS: Using the perilymphatic approach, adenovirus could be transferred to mesothelial cells lining the perilymph, but not sensory cells. Conversely, all AAV serotypes displayed tissue tropism to inner hair cells, with AAV-2/2 showing particularly efficient transfer to sensory cells. This tissue tropism of AAV could not be explained by the distribution of AAV receptors. Hearing and morphology were largely unaffected. CONCLUSIONS: Our results indicate that AAV vector can be safely applied to the inner ear and AAV-2/2 offers a good tool for transferring transgenes into sensory cells of the inner ear efficiently without toxicity.

Hearing results for ossicular reconstruction using a cartilage-connecting hydroxyapatite prosthesis with a spearhead.

OBJECTIVE: Assessment of the efficacy of ossicular reconstruction using a cartilage-connecting hydroxyapatite prosthesis designed with a spearhead to reduce extrusion and dislocation of the implant. PATIENTS: All patients undergoing ossicular reconstruction after chronic ear surgery, connecting the cartilage to the prosthesis, with a minimum of 1 year of postoperative follow-up. MAIN OUTCOME MEASURES: Postoperative change in pure-tone averages. Air-bone gap closures, and implant extrusion rates. RESULTS: Overall mean pure-tone averages improved by 12.2 dB (ranged between -40 and 60 dB). In total, 68.4% of the patients achieved an air-bone gap less than 20 dB. Gains in the mean air conduction thresholds were 9.5 dB in cases of partial ossicular reconstruction and 14.9 dB in cases with total ossicular reconstruction (p < 0.05). The overall extrusion rate was 4.21%. CONCLUSION: The cartilage-connecting hydroxyapatite prosthesis with a spearhead was found to restore hearing to a satisfactory level. The extrusion rate was relatively low. The cartilage-connecting hydroxyapatite prosthesis with a spearhead is an effective ossicular implant and offers an attractive alternative for ossicular reconstruction, particularly for total ossicular reconstructions.

Administration of amitriptyline attenuates noise-induced hearing loss via glial cell line-derived neurotrophic factor (GDNF) induction.

Antidepressant treatments have been described to induce neurotrophic factors (NTFs) and reverse the cell loss observed in rodent stress models. Amitriptyline (AT), a tricyclic antidepressant agent, has been reported in recent studies to induce glial cell line-derived neurotrophic factor (GDNF) synthesis and release in rat C6 glioblastoma cells. GDNF has shown protection against acoustic trauma in previous studies. Therefore, we investigated whether AT could induce GDNF synthesis in the cochlea and attenuate cochlea damage against acoustic trauma. We used Hartley guinea pigs and injected AT (30 mg/kg) or saline into the peritoneum. Subjects were exposed to 117 dB SPL octave band noise centered at 4 kHz for 24 h. Noise-induced hearing loss (NIHL) was assessed with auditory brain stem response (ABR) at 4, 8 and 16 kHz measured prior to the injection, 3 days and 7 days after noise exposure. For histological assessment, we observed the sensory epithelium using a surface preparation technique and assessed the quantitative hair cell (HC) damage. We evaluated GDNF synthesis with or without intense noise exposure at 3, 12 and 24 h after the administration of AT in the cochlea using Western blot analysis. GDNF expression was shown 3 h and 12 h after the injection without noise, whereas with noise the GDNF expression lasted for 24 h. The AT-administrated group showed significantly reduced ABR threshold shift and less HC damage than the saline-administrated group. These findings suggest that the administration of AT-induced GDNF levels in the cochlea and attenuated cochlea damage from NIHL.

Direct inner ear infusion of dexamethasone attenuates noise-induced trauma in guinea pig.

The protective effect of dexamethasone (DEX) against noise-induced trauma, as reflected in hair cell destruction and elevation in auditory brainstem response (ABR) sensitivity, was assessed in guinea pigs. The animals were administered DEX (1, 10, 100, and 1000 ng/ml) or artificial perilymph (AP) via a mini-osmotic pump directly into scala tympani and, on the fourth day after pump implantation, exposed to 120 dB SPL octave band noise, centered at 4 kHz, for 24 h. Animals receiving DEX demonstrated a dose-dependent reduction in noise-induced outer hair cell loss (significant at 1, 10 and 100 ng/ml DEX animals compared to AP control animals) and a similar attenuation of the noise-induced ABR threshold shifts, observed 7 days following exposure (significant at 100 ng/ml DEX animals compared to AP control animals). These physiological and morphological results indicate that direct infusion of DEX into the perilymphatic space has protective effects against noise-induced trauma in the guinea pig cochlea.

The N-methyl-D-aspartate (NMDA)-type glutamate receptor GluRepsilon2 is important for delay and trace eyeblink conditioning in mice.

It has been proposed that the N-methyl-d-aspartate (NMDA)-type glutamate receptor (GluR) plays an important role in synaptic plasticity, learning, and memory. The four GluRepsilon (NR2) subunits, which constitute NMDA receptors with a GluRzeta (NR1) subunit, differ both in their expression patterns in the brain and in their functional properties. In order to specify the distinct participation of each of these subunits, we focused on the GluRepsilon2 subunits, which are expressed mainly in the forebrain. We investigated delay and trace eyeblink conditioning in GluRepsilon2 heterozygous mutant mice whose content of GluRepsilon2 protein was decreased to about half of that in wild-type mice. GluRepsilon2 mutant mice exhibited severe impairment of the attained level of conditioned response (CR) in the delay paradigm, for which the cerebellum is essential and modulation by the forebrain has been suggested. Moreover, GluRepsilon2 mutant mice showed no trend toward CR acquisition in the trace paradigm with a trace interval of 500 ms, in which the forebrain is critically involved in successful learning. On the other hand, the reduction of GluRepsilon2 proteins did not disturb any basic sensory and motor functions which might have explained the observed impairment. These results are different from those obtained with GluRepsilon1 null mutant mice, which attain a normal level of the CR but at a slower rate in the delay paradigm, and showed a severe impairment in the trace paradigm. Therefore, the NMDA receptor GluRepsilon2 plays a more critical role than the GluRepsilon1 subunit in classical eyeblink conditioning.

Antioxidant gene therapy can protect hearing and hair cells from ototoxicity.

Aminoglycosides are commonly used antibiotics that often induce ototoxicity leading to permanent hair cell loss and hearing impairment. The ototoxic effects of aminoglycosides have been linked to oxidative stress. To determine the feasibility of antioxidant gene therapy for protecting the inner ear against aminoglycoside-induced oxidative stress, we used adenoviral vectors for overexpression of catalase, Cu/Zn superoxide dismutase (SOD1), and Mn superoxide dismutase (SOD2). We inoculated adenoviruses designated Ad.cat, Ad.SOD1, and Ad.SOD2 into the left guinea pig cochlea. Five days later, an ototoxic combination of kanamycin and ethacrynic acid was systemically administered. Artificial perilymph and adenovirus without a gene cassette (Ad.null) were used as controls. Biochemical analysis showed significant increase in catalase and a moderate elevation in SOD2 levels in tissues of the cochlea inoculated with the respective vectors. Auditory brain-stem responses were measured to monitor hearing thresholds. Animals were sacrificed 7 days after the ototoxic insult and their hair cells counted. Hair cells and hearing thresholds were significantly protected by Ad.cat and Ad.SOD2, while results with Ad.SOD1 were inconsistent. Control ears showed no significant protective effects. The results demonstrate that the expression of functional enzymes in the inner ear is feasible using adenoviral-mediated gene delivery. Furthermore, they confirm that reactive oxygen species contribute to aminoglycoside ototoxicity and suggest antioxidant gene therapy as a potential therapeutic strategy to reduce inner ear oxidative stress.

Intra-cochlear administration of dexamethasone attenuates aminoglycoside ototoxicity in the guinea pig.

This study demonstrates the attenuation of aminoglycoside ototoxicity by cochlear infusion of dexamethasone (Dex) using a microcannulation-osmotic pump delivery system. The results indicate that treating the cochlea with Dex both before and after kanamycin administration was more effective in preventing ototoxicity than Dex treatment only after kanamycin administration. A concentration of 1 ng/ml Dex showed the greatest protective effect on both kanamycin-induced threshold shift of the auditory brainstem response and outer hair cell survival. These results show that the Dex treatment attenuates both functional and structural damage of the inner ear from aminoglycoside toxicity.