Trimethyltin-induced cochlear degeneration in rat

Trimethyltin(TMT) is an occupational and environmental health hazard behaving as a potent neurotoxin known to affect the central nervous system as well as the peripheral auditory system.However,the mechanisms underlying TMT-induced ototoxicity are poorly understood.To elucidate the effects of TMT on the cochlea,a single injection of 4 or 8 mg/kg TMT was administered intraperitoneally to adult rats.The compound action potential(CAP) threshold was used to assess the functional status of the cochlea and histological techniques were used to assess the condition of the hair cells and auditory nerve fibers.TMT at 4 mg/kg produced a temporary CAP threshold elevation of 25-60 dB that recovered by 28 d post-treatment.Although there was no hair cell loss with the 4 mg/kg dose,there was a noticeable loss of auditory nerve fibers particularly beneath the inner hair cells.TMT at 8 mg/kg produced a large permanent CAP threshold shift that was greatest at the high frequencies.The CAP threshold shift was associated with the loss of outer hair cells and inner hair cells in the basal,high-frequency region of the cochlea,considerable loss of auditory nerve fibers and a significant loss of spiral ganglion neurons in the basal turn.Spiral ganglion neurons showed evidence of soma shrinkage and nuclear condensation and fragmentation,morphological features of apoptotic cell death.TMT-induced damage was greatest in the high-frequency,basal region of the cochlea and the nerve fibers beneath the inner hair cells were the most vulnerable structures.

Pattern of hair cell loss and delayed peripheral neuron degeneration in inner ear by a high-dose intratympanic gentamicin

To gain insights into the ototoxic effects of aminoglycoside antibiotics(AmAn) and delayed peripheral ganglion neuron death in the inner ear.experimental animal models were widely used with several different approaches including AmAn systemic injections,combination treatment of AmAn and diuretics,or local application of AmAn.In these approaches,systemic AmAn treatment alone usually causes incomplete damage to hair cells in the inner ear.Co-administration of diuretic and AmAn can completely destroy the cochlear hair cells,but it is impossible to damage the vestibular system.Only the approach of AmAn local application can selectively eliminate most sensory hair cells in the inner ear.Therefore,AmAn local application is more suitable for studies for complete hair cell destructions in cochlear and vestibular system and the following delayed peripheral ganglion neuron death.In current studies,guinea pigs were unilaterally treated with a high concentration of gentamicin(GM,40 nig/ml) through the tympanic membrane into the middle ear cavity.Auditory functions and vestibular functions were measured before and after GM treatment.The loss of hair cells and delayed degeneration of ganglion neurons in both cochlear and vestibular system were quantified 30 days or 60 days after treatment.The results showed that both auditory and vestibular functions were completely abolished after GM treatment.The sensory hair cells were totally missing in the cochlea,and severely destroyed in vestibular end-organs.The delayed spiral ganglion neuron death 60 days after the deafening procedure was over 50%.However,no obvious pathological changes were observed in vestibular ganglion neurons 60 days post-treatment.These results indicated that a high concentration of gentamycin delivered to the middle ear cavity can destroy most sensory hair cells in the inner ear that subsequently causes the delayed spiral ganglion neuron degeneration.This model might be useful for studies of hair cell regenerations,delayed degeneration of peripheral auditory neurons,and/or vestibular compensation.In addition,a potential problem of ABR recording for unilateral deafness and issues about vestibular compensation are also discussed.

Standardization of experimental animals temporal bone sections

Preparation of the temporal bone for light microscopy is an important step in histological studies of the inner ear. Due to the complexity of structures of the inner ear, it is difficult to measure or compare structures of interest without a commonly accepted standardized measure of temporal bone sections. Therefore, standardization of temporal bone sections is very important for histological assessment of sensory hair cells and peripheral ganglion neurons in the cochlear and vestibular systems. The standardized temporal bone sectioning is oriented to a plane parallel to the outer and internal auditory canals. Sections are collected from the epitympanum to the hypotympanum to reveal layers in the order of the crista ampullaris of the superior and lateral semicircular canals, macula utriculi and macula sacculi, superior vestibular ganglion neurons, macula of saccule and inferior vestibular ganglion neurons, cochlear modiolus, endolymphatic duct and endolymphatic sac, and finally the crista ampullaris of the posterior semicircular canal. Moreover, technical details of preparing for temporal bone sectioning including fixation, decalcification, whole temporal bone staining, embedding penetration, and embedding orientation are also discussed.