ABSTRACT
The absence of thyroid hormone (TH) during late gestation and early infancy can cause irreparable deafness in both humans and rodents. A variety of rodent models have been used in an effort to identify the underlying molecular mechanism. Here, we characterize a mouse model of secondary hypothyroidism, pituitary transcription factor 1 (Pit1(dw)), which has profound, congenital deafness that is rescued by oral TH replacement. These mutants have tectorial membrane abnormalities, including a prominent Hensen's stripe, elevated beta-tectorin composition, and disrupted striated-sheet matrix. They lack distortion product otoacoustic emissions and cochlear microphonic responses, and exhibit reduced endocochlear potentials, suggesting defects in outer hair cell function and potassium recycling. Auditory system and hair cell physiology, histology, and anatomy studies reveal novel defects of hormone deficiency related to deafness: (1) permanently impaired expression of KCNJ10 in the stria vascularis of Pit1(dw) mice, which likely contributes to the reduced endocochlear potential, (2) significant outer hair cell loss in the mutants, which may result from cellular stress induced by the lower KCNQ4 expression and current levels in Pit1(dw) mutant outer hair cells, and (3) sensory and strial cell deterioration, which may have implications for thyroid hormone dysregulation in age-related hearing impairment. In summary, we suggest that these defects in outer hair cell and strial cell function are important contributors to the hearing impairment in Pit1(dw) mice.
Subject(s)
Deafness/etiology , Gene Expression Regulation/genetics , Hypothyroidism/complications , KCNQ Potassium Channels/metabolism , Potassium Channels, Inwardly Rectifying/metabolism , Transcription Factor Pit-1/genetics , Age Factors , Animals , Animals, Newborn , Deafness/genetics , Deafness/pathology , Disease Models, Animal , Hair Cells, Auditory, Outer/diagnostic imaging , Hair Cells, Auditory, Outer/metabolism , Hair Cells, Auditory, Outer/pathology , Hair Cells, Auditory, Outer/ultrastructure , Hypothyroidism/genetics , KCNQ Potassium Channels/genetics , Mice , Mice, Mutant Strains , Microscopy, Electron, Transmission/methods , Molecular Motor Proteins/genetics , Molecular Motor Proteins/metabolism , Otoacoustic Emissions, Spontaneous/genetics , Potassium Channels, Inwardly Rectifying/genetics , Stria Vascularis/pathology , Synaptophysin/genetics , Synaptophysin/metabolism , Tectorial Membrane/pathology , Tectorial Membrane/ultrastructure , UltrasonographyABSTRACT
Single-shot transtympanic gentamicin therapy has become a popular treatment modality for Meniere's disease despite the known possible ototoxic properties of this drug. It was shown recently that NO production and iNOS were upregulated after gentamicin application, which was interpreted as a possible effect of ototoxicity. In this study we analyzed the expression of eNOS after gentamicin application to determine a possible correlation of this enzyme with gentamicin-induced ototoxicity. We compared eNOS expression in gentamicin-treated and non-treated guinea pigs in the second turn of the cochlea, an area corresponding to speech perception in humans. Gentamicin (4 mg) was injected intratympanically into the middle ear of guinea pigs ( n =3) and the reduction of the hearing threshold level was determined by recording acoustic-evoked potentials (AEP) before and 5 days after gentamicin application. Morphological alterations in the organ of Corti were analyzed by light and electron microscopy. Gold-labeled anti-eNOS antibodies were counted in eight different cell areas for quantification of eNOS expression. Seven animals were analyzed as controls. After gentamicin application, a deterioration of hearing level was observed varying from 10 to 30 dB. A high degree of vacuolization was identified in the third row of outer hair cells. At the subcellular level, the subsurface cisterns in outer hair cells were dissociated from the basolateral cell membrane, and the mitochondrial membranes were frequently damaged. Statistically significant upregulation of eNOS was observed in all cell types analyzed. Depending on the various cell types the amount of gold-labeled eNOS antibodies was 2.5 to 5.7 times higher after gentamicin application. We observed significant eNOS upregulation after gentamicin application in the cochlea, in conjunction with cellular damages and decreased hearing.
Subject(s)
Cochlea/enzymology , Gentamicins/toxicity , Nitric Oxide Synthase Type III/metabolism , Up-Regulation/drug effects , Animals , Anti-Bacterial Agents , Auditory Threshold , Cochlea/drug effects , Evoked Potentials, Auditory , Gentamicins/administration & dosage , Guinea Pigs , Hair Cells, Auditory, Outer/diagnostic imaging , Hair Cells, Auditory, Outer/drug effects , Injections , Organ of Corti/pathology , Tympanic Membrane , UltrasonographyABSTRACT
OBJECTIVES: Otic drops are commonly used not only for otitis externa, but also for otorrhea in the presence of tympanostomy tubes or tympanic membrane perforations. Many studies have demonstrated the ototoxicity of common otic preparations such as Cortisporin otic drops (Monarch Pharmaceuticals, Bristol, TN). The purpose of this study was to assess the relative ototoxicity of common otic preparations by direct exposure to isolated cochlear outer hair cells (OHCs). METHODS: OHCs from adult chinchilla cochlea were exposed to standard bathing solution (control), acetic acid, Acetasol HC (Alpharma USPD Inc., Baltimore, MD), Gentacidin (CIBA Vision Ophthalmics, Atlanta, GA), and Tobradex (Alcon, Fort Worth, TX). The cells were observed using an inverted microscope, and the images were recorded in digital still-frame and video, and analyzed on the Image Pro-Plus 3.0 program (Media Cybernetics, Silver Spring, MD). RESULTS AND CONCLUSIONS: As measured by time to cell death and change in morphology of OHCs, acetic acid with or without hydrocortisone was most toxic to OHCs. Cortisporin was more cytotoxic than gentamicin and Tobradex.
