Efferent effects elicited by electrical stimulation at the round window of the guinea pig

We report a technique for activating the efferent nerve fibres to the cochlea by electrical stimulation at the round window. Such electrical stimulation caused a reduction in the amplitude of the gross nerve response (N1) to a click presented after the electrical stimulus but did not alter the latency of the response. The reduction increased with increasing current strength above 200 microA and increasing rate of electrical pulses above 50 Hz. The effect was also dependent on the duration of the shock train and the pulse width. The reduction in N1 was most pronounced at low click intensities. Recovery of the N1 was almost complete about 80 ms after the end of the electrical stimulus. The effect of electrical stimulation in reducing the N1 amplitude could almost always be blocked by intraperitoneal injections of strychnine. Recovery from the strychnine block was observed when animals were maintained for periods of more than 60 min after the administration of strychnine. The ease of this technique allows it to be used to examine the effects of efferent stimulation on various aspects of cochlear function in the guinea pig.     

Modulation at the guinea pig round window of summating potentials and compound action potentials by low-frequency sound

Low-frequency sound was used to modulate responses to short single-frequency tone bursts at the guinea pig round window. Summating potentials (SP) increase (reach higher positive values) during the negative half-cycle of the low-frequency cochlear microphonic (LFCM) and decrease during the positive half-cycle of the LFCM. The compound action potential (AP) amplitude decreases during the negative half-cycle of the LFCM. The negative half-cycle of the LFCM can be identified with scala tympani displacement. SP modulation depth is defined as the difference between the highest and the lowest SP value found for tone burst stimulation at different phases of the low-frequency sound while the sound levels of the tone burst and the low-frequency bias are kept constant. When normalized with respect to the SP amplitude found without bias, the SP modulation depth is independent of the sound level of the tone burst in the range from 48 to 68 dB SPL. The normalized AP suppression tends to increase with decreasing tone burst sound level. A dynamic nonlinear mechanism which might explain these results is discussed. This mechanism is based on voltage-sensitive changes.     

Histological study of chronic electrode implantation through the round window of the guinea pig

A platinum-iridium electrode with a Teflon cover tube was chronically implanted through the round window into the scala tympani of the guinea pig. Animals were sacrificed at 30 to 40 weeks after electrode implantation, and the cochleae were observed by scanning electron microscopy or with light microscopy. The results revealed that the electrode was firmly enclosed with fibrous tissue on the inside of the round window and with epithelial tissue on the outside. These tissues were considered to act as a barrier to the entry of infection while preventing leakage of the perilymph.     

adenoviral-mediated hath1-egfp gene transfer into guinea pig cochlea through intact round window membrane

Objective To study expression of adenovira1-mediated Hathl-EGFP gene in the guinea pig cochlea after transfer through intact round window membrane (RWM), and to assess its effects on hearing. Methods Twenty adult guinea pigs were used, of which: 12 were surgically inoculated with AdHath1-EGFP in the bony groove of round window niche, and 8 with artificial perilymph. Auditory brainstem response(ABR) thresholds were determined in all animals before and 5 days after surgery. On post-surgery day 5 and day 14, animals were sacrificed and whole mounts of cochlea and fro zensections were examined. Results ABR tests showed no significant change of hearing after the surgery.Strong fluorescence staining in the cochleae was seen in Ad-Hathl-EGFP groups. The highest levels of gene expression were seen in the post-surgery day 5 group with tittle decrease on post-surgery day 14.The contralateral cochlea and those in the control groups were free of fluorescence staining. Conclusion The transgenic Hath1-EGFP can be effectively delivered into the inner ear through intact RWM, in an atraumatic manner.     

The reaction of the guinea pig cochlea to perforations of the round window membrane with and without perilymph aspiration

Summary The influence of simple opening of the round window (RW) membrane and the effect of aspiration of perilymph on the electrophysiological characteristics of the cochlea was tested in guinea pigs by measurement of the compound action potentials. We found that perforations of the RW membrane failed to lead to either shortterm or long-term damage in cochlear function. There was only a slight spontaneous escape of perilymph but without measurable functional loss. Additional aspiration of perilymph led to entry of air into the basal turn and to an immediate loss of function of the cochlea. This regressed within 4 weeks in the middle- and low-frequency ranges. Measurable long-term damage persisted only in the high-frequency ranges. We attribute contradictory results of other authors to methodological errors which we avoided by a specific selection of healthy animals and the development of standardized operation, recording and measurement procedures.     

Functional and morphologic changes in inner ear after experimental rupture of the round window membrane in the guinea pig

The round window membrane of the guinea pig was perforated with a sharp instrument, and electrocochleography, vestibular function and histological changes of cochlea were studied in 1, 2, 4, 7, 14 and 28 days. It was found that the compound action potential (AP) threshold and the latency increased at 95 dB (spl) sound stimulus, and vestibular function decreased slightly, but histologic appearance of Corti's organ was normal in the early stage. Latency and threshold of AP and vestibular function gradually recovered to normal 2 weeks later. If no obvious histological change took place in inner ear, hearing loss would not appear. There was no significant difference of SP changes between the experimental and control ears.     

Ethanol infiltration into the stapedio-vestibular joint reduces low-frequency vibration of the ossicular chain and round window membrane in the guinea pig

Background: The synovial stapedio-vestibular joint (SVJ), which serves as a bridge between the stape and oval window, can be found in guinea pigs and most human adults. Unlike the fibrous SVJs in other animals, the contribution of the synovial SVJ to middle ear sound transmission remains unknown.

Aims/objectives: In this study, we investigate whether sclerosis of the synovial SVJ contributes to frequency-dependent vibration of the ossicular chain and round window membrane (RWM).

Materials and methods: A model of SVJ sclerosis model was established in the guinea pig using 75% ethanol. A laser Doppler vibrometer was then used to measure vibrations of the RWM and the long process of the incus (LPI) under pure tone sound stimulations of 0.25–16?kHz. The influence of SVJ sclerosis was analysed by comparing structural vibration displacement between the normal and sclerosis groups.

Results: Both LPI and RWM vibrations significantly decreased at low frequencies after infiltration of ethanol, which caused SVJ sclerosis.

Conclusions: SVJ sclerosis reduces low-frequency vibration of the ossicular chain and RWM in the guinea pig, which indicates that the synovial SVJ is vital to low-frequency sound transmission in the middle ear.

Significance: Providing useful data for further research regarding middle ear biomechanics.     

Regulation of inner ear fluid in the guinea pig cochlea after the application of saturated NaCl solution to the round window membrane

The regulation of K⁺ and Na⁺ in the inner ear fluid of the guinea pig was studied after the application of saturated NaCl solution to the round window membrane. K⁺ and Na⁺ activities in the scala tympani increased rapidly and then decreased. K⁺ activity in the scala media increased immediately, but Na⁺ activity continued to increase during the period of observation. K⁺ activity in the scala vestibuli continued to increase in the observation period. Na⁺ activity in the scala vestibuli increased and then decreased. The endocochlear potential decreased immediately to approximately 20% of its initial level. Total activities of K⁺ and Na⁺ increased immediately and then decreased in both the scala tympani and scala media. The total activity of K⁺ and Na⁺ increased slowly and showed no regulatory decrease in the scala vestibuli. Thus, changing patterns in the total activity of K⁺ and Na⁺ were similar for the scala tympani and scala media, but not for the scala media and the scala vestibuli. Different patterns of K⁺ and Na⁺ activities among the three scalae indicate that their mechanisms for regulating inner ear fluid differ.     

Transient focal cooling at the round window and cochlear nucleus shows round window CAP originates from cochlear neurones alone

We have measured the compound action potential (CAP) evoked by very brief high-frequency tone-bursts (20 kHz, 1/4 ms) at the round window (RW) and at the surface of the cochlear nucleus (CN) in guinea pigs before, during and after a localised chilling of either the cochlea or CN, with a non-toxic ‘freeze spray’. CN chilling almost abolished the negative-going component of the CAP measured in the CN (generated by the CN and here called the cochlear nucleus response or CNR), leaving a positive-going localised response from the cochlear neurones as they leave the internal auditory meatus. Within 3 min, the CNR recovered to control values. During that time, the N₁ component of the RW CAP was slightly increased and the P₁ was larger, even though the CNR was abolished, indicating that the P₁ was not due to electrotonic spread of current from the CN. The N₂ and successive peaks at the RW were also abolished, but returned after 30 s. When the cochlea was chilled, the RW CAP was initially reduced in amplitude, presumably due to a drop in the number of cochlear neurones spiking in response to sound, but recovered within 3 min to be larger than the control waveform, with a more prominent N₁ peak which was delayed slightly, making the CAP more monophasic. At the same time, the CNR was smaller, presumably due to fewer cochlear neurones responding, but overall the CN CAP was altered little in waveshape. These experiments indicate that that RW CAP is generated almost solely by cochlear neurones. We also suggest that some of the changes in the RW CAP during the chills were due to changes in the firing of the lateral olivo-cochlear system of efferent neurons.     

Sealing of the round window reversibly affects auditory brainstem response latencies

The round window (RW) niche was sealed in 12 adult female albino Sprague-Dawley rats. Electrophysiological auditory function was measured with a computerized auditory brainstem response (ABR) recording technique, using 1- and 6 kHz stimuli. ABR measurements were performed prior to closure of the niche, immediately after the sealing, and 4 h, 24 h and 7 days later. After sealing, the latencies elicited by a 6 kHz stimulus increased, but returned to normal between 24 h and 7 days after closure. Stimulation with 1 kHz did not influence the latencies in such a clear-cut manner as when using 6 kHz. It is conceivable that sealing of the RW deprives a minor fraction of oxygen supplied to the basal part of the cochlea through the RW membrane, a procedure which by us is monitored as transient electrophysiological changes.     

Surgery of the round window

Tympanotomy is routinely performed in our institution to control the round window membrane for patients with recurrent sudden deafness, with Meniere-like attacks, and those with Meniere's disease. In a large number of these patients, especially those with vertigo, perilymph fistula has been diagnosed. Most patients are free of attacks after the fistula has been closed, and in those with a short history of illness, hearing can become normal. In the future, closure of the round window membrane should be considered an important procedure in the treatment of inner ear dysfunctions.     

Diffusion of amikacin at the membrane of the round window. Histologic and electrophysiologic study in newborn guinea pigs]

The aim of our study was to evaluate the permeability of the round window membrane for amikacin during postnatal development. The morphologic and electrophysiologic changes in the cochlea were dependent on the applied concentration of amikacin. When giving high doses a complete degeneration of hair cells and deafness could be found. In these cases an influence on the contralateral cochlea could be proved.     

Immunodefense of the round window

A systematic analysis using serial sectioning of the round window membrane (RWM) in the cynomolgus monkey was performed. Light and transmission electron microscopy (LM and TEM) revealed that the RWM rim may be endowed with gland-like structures with glyco-protein material secernated into the window niche. This was detected in one third of the specimens. The secreted material displayed waste material and scavenger cells. There was also a rich network of capillaries, lymph channels, and sinusoidal veins containing leukocytes. Their abluminal surfaces displayed mature plasma cells and monocytes. These findings suggest that in certain primates the middle ear may have developed specific immunoprotective means for disposal of foreign and noxious substances before they reach the inner ear.     

明胶海绵和透明质酸在地塞米松圆窗灌注中对豚鼠圆窗膜形态及功能的影响

目的圆窗分别放置明胶海绵与透明质酸,观察地塞米松圆窗灌注对豚鼠圆窗膜形态及功能影响。方法①36只豚鼠随机分为3组,Ⅰ组圆窗龛放置明胶海绵并在圆窗置管;Ⅱ组圆窗龛放置含2%透明质酸明胶海绵置管,Ⅲ组圆窗龛放置明胶海绵置管,Ⅱ组和Ⅲ组经圆窗置管向圆窗灌注地塞米松7d后。用光学显微镜和扫描电镜观察圆窗膜变化。36只豚鼠中随机取6只左耳作为正常对照（Ⅳ组）。②另取18只豚鼠,按给药时间分为1、4、7d3组,每组各取3只分为实验组和对照组,实验组圆窗放置透明质酸明胶海绵,对照组圆窗放置明胶海绵。各组经圆窗置管圆窗灌注地塞米松后,用LC-6A高效液相色谱仪测定外淋巴液中地塞米松浓度。结果光镜和扫描电镜检查发现Ⅰ、Ⅱ和Ⅲ组圆窗膜厚度和形态与正常对照Ⅳ组比较无明显差异（P〉0.05）;圆窗放置透明质酸明胶海绵后豚鼠外淋巴液中地塞米松含量明显高于明胶海绵组（P〈0.01）。结论①圆窗放置明胶海绵和透明质酸对豚鼠圆窗膜形态无明显影响。②透明质酸明胶海绵能提高地塞米松的圆窗膜通透性。