Cochlear function after selective spiral ganglion cells degeneration induced by ouabain

Background Ouabain, a cardiac glycoside that specifically binds to Na/K-ATPase and inhibits its activity, was applied to gerbils to develop a method for studying auditory neuropathy. Methods Ouabain was applied to the round window of the cochlea in each gerbil by using a piece of gelfoam with 3 μl or 24 μl （1 mmol/L） ouabain solution. The changes of the threshold of auditory brainstem response, cochlear function round window electrocochleography, as well as the morphological changes of the spiral ganglion cells of the cochlea were observed after application of ouabain for 24 hours or 96 hours. Results In ouabain treated gerbils, auditory brainstem response and compound action potential thresholds showed either elevation or no response at all. However, the thresholds of cochlear microphonic and distortion product otoacoustic emissions were not affected. Degeneration and necrosis of some spiral ganglion cells in ears with applications of ouabain （24 hours, 3 μl, 1 mmol/L; 96 hours, 24 μl, 1 mmol/L ouabain）. The number of spiral ganglion cells was decreased （24 hours, 3 μl, 1 mmol/L ouabain） or near to a total loss （96 hours, 24 μl, 1 mmol/L ouabain）.Conclusions These results indicate a high degree of independence between the spiral ganglion ceils and the outer hair cell systems in the cochlear transduction mechanism. The method used in this study would provide a valuable tool for studying auditory neuropathy.

Gene transfection mediated by polyethyleneiminepolyethylene glycol nanocarrier prevents cisplatininduced spiral ganglion cell damage

Polyethyleneimine-polyethylene glycol （PEI-PEG）, a novel nanocarrier, has been used for trans- fection and gene therapy in a variety of cells. In our previous study, we successfully carried out PEI-PEG-mediated gene transfer in spiral ganglion cells. It remains unclear whether PEI-PEG could be used for gene therapy with X-linked inhibitor of apoptosis protein （XIAP） in the inner ear. In the present study, we performed PEI-PEG-mediated XIAP gene transfection in the cochlea of Sprague-Dawley rats, via scala tympani fenestration, before daily cisplatin injections. Audito- ry brainstem reflex tests demonstrated the protective effects of XIAP gene therapy on auditory function. Immunohistochemical staining revealed XIAP protein expression in the cytoplasm of cells in the spiral ganglion, the organ of Corti and the stria vascularis. Reverse transcription-PCR detected high levels of XIAP mRNA expression in the cochlea. The present findings suggest that PEI-PEG nanocarrier-mediated XIAP gene transfection results in XIAP expression in the cochlea, prevents damage to cochlear spiral ganglion cells, and protects hearing.

An efficient strategy for establishing a model of sensorineural deafness in rats

Ototoxic drugs can be used to produce a loss of cochlear hair cells to create animal models of deafness. However, to the best of our knowledge, there is no report on the establishment of a rat deafness model through the combined application of aminoglycosides and loop diuretics. The aim of this study was to use single or combined administration of furosemide and kanamycin sulfate to establish rat models of deafness. The rats received intravenous injections of different doses of furosemide and/or intramuscular injections of kanamycin sulfate. The auditory brainstem response was measured to determine the hearing threshold after drug application. Immunocytochemistry and confocal microscopy were performed to evaluate inner ear morphology. In the group receiving combined administration of furosemide and kanamycin, the auditory brainstem response threshold showed significant elevation 3 days after administration, higher than that produced by furosemide or kanamycin alone. The hair cells showed varying degrees of injury, from the apical turn to the basal turn of the cochlea and from the outer hair cells to the inner hair cells. The spiral ganglion cells maintained a normal morphology during the first week after the hair cells completely disappeared, and then gradually degenerated. After 2 months, the majority of spiral ganglion cells disappeared, but a few remained. These findings demonstrate that the combined administration of furosemide and kanamycin has a synergistic ototoxic effect, and that these drugs can produce hair cell loss and hearing loss in rats. These findings suggest that even in patients with severe deafness, electronic cochlear implants may partially restore hearing.

Lead neurotoxicity in rat cochlear organotypic cultures

Lead is a major environmental toxicant throughout the world.Lead can induce severe neurotoxicity including irreversible hearing impairment.Many in vivo studies have shown that lead damages the auditory nervous system,but has little or no effect on cochlear sensory hair cells.To gain insights on lead ototoxic and neurotoxic effects in vitro,lead acetate (LA) was applied to postnatal day 3-4 rat cochlear organotypic cultures for 24 or 72 h with doses of 0.1,0.5,1,2 or 4 mM.After 24 or 72 h treatment with lead acetate,nearly all of cochlear sensory hair cells were intact.However,after 72 h treatment,the peripheral auditory nerve fibers projecting to the hair cells and the spiral ganglion neurons (SGN) were damaged when lead concentration exceeded 2 mM.Our results indicated that 72 h treatment with only the high doses (> 2 mM) of lead actate damaged SGNs and peripheral nerve fibers;hair cells remained structurally intact even after 4 mM treatment.These results show that lead primarily damages cochlear nerve fibers andSGNratherthanhaircells.

Anatomy and physiology of peripheral auditory system and commen causes of hearing loss

For Otolaryngologist,it is the most important to know the principle of anatomy,physiology and common ototoxicity.Short but more concise summary has been sum up in the following review in order to help young ENT doctor to understand the importance of this basic knowledge.

Effects of delayed brain-derived neurotrophic factor application on cochlear pathology and auditory physiology in rats

Background The development and maintenance of spiral ganglion cells （SGCs） appear to be supported by neurotrophins Removal of this support leads to their gradual degeneration. Intracochlear infusion with neurotrophins can provide trophic support to SGCs in animal deafness models if given shortly after deafening. However, it is not known whether delayed intervention will provide similar protection, which might be clinically relevant. The present research was conducted to determine the effects of brain-derived neurotrophic factor （BDNF） administration on the capacity of the peripheral processes to resprout. Methods The left cochlea of 20 profoundly deafened rats, which were divided into 2 groups equally, was implanted with an electrode and drug-delivery system 30 days after deafening. Either BDNF or artificial perilymph （AP） was delivered continuously for 28 days. Electrically evoked auditory brainstem responses （EABRs） were recorded during the period. SGC body and peripheral process density were measured. Results The EABR thresholds of AP increase continually. Those of BDNF increase slowly at the beginning then decrease, and were significantly less than those of the AP group from day 14 to 28 （P 〈0.01）. In terms of SGC and peripheral process density, the difference between the treated and control ears of BDNF group was clearly significant （P 〈0.01）, but not in AP group （P 〉0.05）. Analysis of the left cochlea between the two groups demonstrated that SGC/peripheral process density of the BDNF group was significantly greater than that of the AP group. Finally, a functional formula was developed relating the last EABR threshold and SGC density and process density, which was as follows： T= 466.184-2.71 （F.B.L）. Conclusions Under the conditions of delayed intervention following 30 days after deafening in rats, it can be concluded that BDNF enhances SGC bodies and peripheral processes survival after differentiation and so improves auditory sensitivity. SGC peripheral processes influence the auditory sensitivity.