Effects of endotoxin on the cochlear electrophysiology and Na(+)-K(+)-ATPase activity

Hemophilus influenzae (type B) endotoxin 10 micrograms was put in the niche of the round window in guinea pigs. The CAP thresholds and N1 latencies were measured before and 12, 24 and 72 hours after experiments. The 10 kHz and 8 kHz CAP thresholds after experiments were significantly higher (P less than 0.01), the N1 latencies were prolonged (P less than 0.05). There were minute changes in 4kHz CAP thresholds and N1 latencies (P greater than 0.05). In all time groups, the Na(+)-K(+)-ATPase activity was outstandingly weakened. It suggested that: (1) endotoxin put in the niche of the round window is responsible for the cochlear function disturbances in high frequencies, and (2) the decrease of the Na(+)-K(+)-ATPase activity is an important factor for the electrophysiological disturbances in the inner ear.     

Threshold elevation at high frequencies of the auditory nerve action potential in acute versus chronic recordings in guinea pigs

Thresholds of the auditory nerve compound action potential (CAP) responses to filtered clicks from 0.5 to 40 kHz, defining a CAP frequency threshold curve, have been determined in the same guinea pigs: (1) in acute conditions, the animal still anesthetized at the end of the operation for permanent implantation of a round-window electrode, and (2) several days later in the implanted awake and unrestrained animal. The results show higher CAP thresholds for high frequencies in acute conditions as compared to chronic recordings. This difference appears above about 12.5 kHz and increases progressively to around 40 dB at 40 kHz. Similar effects, although somewhat less pronounced, were observed during anesthesia alone in already implanted guinea pigs. These observations could explain the discrepancies which appear between electrophysiological thresholds, acutely recorded either from single fibers or whole-nerve responses, and behavioral audiograms. Moreover, in the accurate evaluation of the cochlea, slight impairments may be masked by this phenomenon as illustrated in guinea pigs treated with low doses of ototoxic antibiotic.     

Suppression of the acoustically evoked auditory-nerve response by electrical stimulation in the cochlea of the guinea pig

There is increasing interest in the use of electro-acoustical stimulation in people with a cochlear implant that have residual low-frequency hearing in the implanted ear. This raises the issue of how electrical and acoustical stimulation interact in the cochlea. We have investigated the effect of electrical stimulation on the acoustically evoked compound action potential (CAP) in normal-hearing guinea pigs. CAPs were evoked by tone bursts, and electric stimuli were delivered at the base of the cochlea using extracochlear electrodes. CAPs could be suppressed by electrical stimulation under various conditions. The dependence of CAP suppression on several parameters was investigated, including frequency and level of the acoustic stimulus, current level of the electric stimulus and the interval between electric and acoustic stimulus (EAI). Most pronounced suppression was observed when CAPs were evoked with high-frequency tones of low level. Suppression increased with current level and at high currents low-frequency evoked CAPs could also be suppressed. Suppression was typically absent several milliseconds after the electric stimulus. Suppression mediated by direct neural responses and hair cell mediated (electrophonic) responses is discussed. We conclude that the high-frequency part of the cochlea can be stimulated electrically with little detrimental effects on CAPs evoked by low-frequency tones.     

多巴胺对豚鼠噪声性听力损失的保护作用

目的观察白噪声条件下多巴胺（dopamine,DA）对耳蜗内毛细胞的保护作用,为进一步探讨多巴胺对耳蜗传入通路的负反馈保护机制奠定基础。方法健康杂色豚鼠40只,随机分4组,行活体全耳蜗灌流：①单纯给予100dB白噪声组（以下同）;②灌流人工外淋巴液组;③灌流人工外淋巴液并给予白噪声组;④灌流1mmol/L多巴胺并给予白噪声组。在灌流第0、2h记录4kHz耳蜗微音电位（cochlear mirophonics,CM）幅值,并做相对幅度输入/输出函数曲线,和不同频率复合动作电位（compound action potential,CAP）阈值。结果给予噪声暴露的3组灌流后CM输入/输出曲线非线性特点均消失,相对幅度下降,差异有统计学意义（P〈0.05）。给予噪声暴露的3组2小时后各频率CAP阈值较前均上升,差异有统计学意义（P〈0.001）。但第4组较第1组除8kHzCAP阈移差异无统计学意义外,其余各频率CAP阈移明显减小（P〈0.05）。高频的阈移相差程度均较低频明显,其中16kHz阈移相差程度最为明显。结论白噪声暴露下多巴胺对耳蜗传入通路具有保护作用,并存在频率选择性,对高频纤维保护作用较低频更强。     

Low-frequency modulation of compound action potential in experimental perilymphatic fistula and endolymphatic hydrops.

We have tested the hypothesis that the cause of cochlear dysfunction associated with perilymphatic fistula (PLF) is closely related to endolymphatic hydrops (ELH). Using guinea pigs, we studied the tone-burst elicited compound action potential (CAP) and its modulation as caused by a 50 Hz biasing tone in experimental PLF. We compared these results with those of experimental ELH. Following perilymph aspiration through the perforated round window membrane, mild but significant elevations of CAP thresholds at tested frequencies were found. A reduction in the amplitude of cochlear microphonics (CM) for a 50 Hz sine wave appeared to correlate with these CAP threshold changes. However, there were no significant changes in the modulation effect of the 50 Hz biasing tone on the CAP elicited by an 8 kHz tone burst. This finding differed from that in ears with experimental ELH, in which significant reductions of both 50 Hz CM and the degree of CAP modulation were consistently observed. We concluded that it is unlikely that the underlying mechanisms of a modification to the low frequency response of the base of the cochlea following perilymph aspiration is linked to that of experimental ELH.     

The influence of moderate-intensity noise on the compound action potential evoked by tone bursts in the guinea pig, Cavia porcellus

Noise-induced changes in the compound action potential (CAP) evoked by tone bursts in the frequency range 0.5-24 kHz were studied in 15 pigmented guinea pigs by means of chronically implanted electrodes positioned near the round window. The animals were exposed for 120 h to continuous pink noise at the intensities 80, 90 and 100 dB SPL. During the exposure period, all the animals exhibited an exponential rise in CAP threshold, leveling out after 24-72 h (asymptotic threshold shift, ATS). The largest threshold shifts were recorded during exposure to 100 dB SPL, for frequencies in the range 8-12 kHz. In the recovery phase, after the end of noise exposure, the threshold to tones at all frequencies tested fell exponentially, reaching the original level in about 72 h in all cases.     

Evoked responses from inferior colliculus as an index of hearing thresholds in guinea pigs.

Bipolar electrodes were implanted in the inferior colliculus of 10 healthy guinea pigs, and visual detection level thresholds of auditory evoked responses were obtained at 17 frequencies between 0.5 and 20.0 kHz. One microvolt isopotential curves from the round window for the same test frequencies were obtained in these animals. A comparison between the evoked response threshold curves and previous reports of the behavioral audiogram reveals a favorable correlation. The cochlear potential curve, however, appears to be a less efficient predictor of the audibility curve than the evoked response thresholds. The utility of evoked response audiometry from the inferior colliculus for quickly assessing auditory thresholds in the guinea pig is discussed.     

Effects of salicylate on electrically evoked otoacoustic emissions in the guinea pig

Clinically, salicylates have been known to have a ototoxic side effect of reversible hearing loss and to reduce the voltage-dependent length change (electromotility) in experiments on isolated outer hair cells (OHC). In order to clarify how the reduction of OHC electromotility due to salicylates contributes to cochlear dysfunction in vivo, we observed compound action potentials (CAP) threshold as well as the outputs of the electrically evoked otoacoustic emissions (EEOAE) before and after intravenous administration of 500 mg/kg sodium salicylate in a guinea pig model. A silver ball electrode placed on the round window membrane of the animal was used for both recording of the CAP and stimulation to elicit the EEOAE. The CAP threshold to tone bursts with frequencies from 2 to 10 kHz elevated by 13 to 22 dB, and outputs of the EEOAE decreased approximately 4 dB for 5 kHz stimulation, and 12 dB for 8 kHz after salicylate administration. These results suggested that systemically administered salicylate also reduced the electromotility of the OHC, and caused the hearing impairment in vivo.     

人工耳蜗植入术中圆窗耳蜗电图评价耳蜗残余听功能

目的应用术中圆窗耳蜗电图评估极重度感音神经性聋患者耳蜗残余听功能。方法20例患者全麻下人工耳蜗植入手术过程中,行术中圆窗耳蜗电图测试,测出的复合动作电位（CAP）阈值与术前纯音测听或其他听力测试之阈值进行比较。结果20例患者术中圆窗耳蜗电图测试测出的复合动作电位（CAP）阈值与术前1、2、4kHz听力测试阈值分别有较好的相关系数（0．20429,0．04076,0．38163）。结论术中圆窗耳蜗电图可以较准确客观地评估极重度感音神经性聋患者耳蜗残余听功能,且是人工耳蜗植入术前听力学评估方法的有意义的补充。     

血管内皮舒张因子对豚鼠耳蜗功能的影响

目的 探讨血管内皮性一氧化氮合酶(endothelial nitric oxide synthase,eNOS)对耳蜗电位的影响.方法 健康豚鼠100只,随机等分为10组:①人工外淋巴液组;②L-精氨酸组;③Ca2+-ATP酶抑制剂组;④Ca2+-ATP酶抑制剂+L-精氨酸组;⑤Ca2+-ATP酶抑制剂+环磷酸鸟苷(c...     

Effects of anoxia on the cochlear summating potential in the guinea pig

Summary The cochlear summating potential (SP) was recorded extracochlearly from the apex and the round window in 25 guinea pigs, applying anoxia during periods of 1–2 min. For 2 and 8 kHz tone pips at 80 and 60 dB SPL, respectively, the initially negative SP at the apex diminished and even became positive during oxygen cut-off. After restoration of oxygenation, the change in SP reversed direction and was followed by a large negative undershoot. At the round window, the initially positive SP evoked by 8 kHz, 60 dB SPL tone pips followed exactly the same time course, but with inverted polarity, while the majority of the recordings of the initially negative SP evoked by 2 kHz, 80 dB SPL tone pips followed a course similar to the apical negative SP. However, the negative round window SP showed only slight polarity reversal and no undershoot. A small number of the recordings at the round window for 2 kHz showed an increase of the negative SP during anoxia followed by a decline of the SP and recovery after anoxia. The similar time course of the initially negative SP at the apex and the initially positive SP at the round window failed to support the hypothesis of a multi-component SP. However, the polarity reversal during anoxia and the different reactions to anoxia of the 2 kHz round-window SP suggest that the SP may consist of several different components.     

Electrophysiological study of the ototoxicity of kanamycin during development in guinea pigs

In order to test the ototoxicity of antibiotics during development, pregnant guinea pigs were intoxicated with kanamycin at different stages of gestation. Eighty newborn guinea pigs were tested electrophysiologically by recording the cochlear microphonic potential and the compound action potential from the round window in response to tone bursts and filtered clicks of various frequencies. Thirty-one animals presented antibiotic injury and showed electrophysiological changes similar to those previously described in adult mammals after kanamycin intoxication.The results suggest a relationship between the ototoxicity of kanamycin and the onset of the auditory function: cochlear potentials were mostly affected when the intoxication was performed during the last 15 days of gestation.     

Brain-Derived Neurotrophic Factor Modulates Auditory Function in the Hearing Cochlea

Neurotrophins prevent spiral ganglion neuron (SGN) degeneration in animal models of ototoxin-induced deafness and may be used in the future to improve the hearing of cochlear implant patients. It is increasingly common for patients with residual hearing to undergo cochlear implantation. However, the effect of neurotrophin treatment on acoustic hearing is not known. In this study, brain-derived neurotrophic factor (BDNF) was applied to the round window membrane of adult guinea pigs for 4 weeks using a cannula attached to a mini-osmotic pump. SGN survival was first assessed in ototoxically deafened guinea pigs to establish that the delivery method was effective. Increased survival of SGNs was observed in the basal and middle cochlear turns of deafened guinea pigs treated with BDNF, confirming that delivery to the cochlea was successful. The effects of BDNF treatment in animals with normal hearing were then assessed using distortion product otoacoustic emissions (DPOAEs), pure tone, and click-evoked auditory brainstem responses (ABRs). DPOAE assessment indicated a mild deficit of 5 dB SPL in treated and control groups at 1 and 4 weeks after cannula placement. In contrast, ABR evaluation showed that BDNF lowered thresholds at specific frequencies (8 and 16 kHz) after 1 and 4 weeks posttreatment when compared to the control cohort receiving Ringer’s solution. Longer treatment for 4 weeks not only widened the range of frequencies ameliorated from 2 to 32 kHz but also lowered the threshold by at least 28 dB SPL at frequencies ≥16 kHz. BDNF treatment for 4 weeks also increased the amplitude of the ABR response when compared to either the control cohort or prior to treatment. We show that BDNF applied to the round window reduces auditory thresholds and could potentially be used clinically to protect residual hearing following cochlear implantation.     

Caffeine and ryanodine demonstrate a role for the ryanodine receptor in the organ of Corti

The hypothesis that the release of Ca(2+) from ryanodine receptor activated Ca(2+) stores in vivo can affect the function of the cochlea was tested by examining the effects of caffeine (1-10 mM) and ryanodine (1-333 microM), two drugs that release Ca(2+) from these intracellular stores. The drugs were infused into the perilymph compartment of the guinea pig cochlea while sound (10 kHz) evoked cochlear potentials and distortion product otoacoustic emissions (DPOAEs; 2f(1)-f(2)=8 kHz, f(2)=12 kHz) were monitored. Caffeine significantly suppressed the compound action potential of the auditory nerve (CAP) at low intensity (56 dB SPL; 3.3 and 10 mM) and high intensity (92 dB SPL; 10 mM), increased N1 latency at high and low intensity (3 and 10 mM) and suppressed low intensity summating potential (SP; 10 mM) without an effect on high intensity SP. Ryanodine significantly suppressed the CAP at low intensity (100 and 333 microM) and at high intensity (333 microM), increased N1 latency at low intensity (33, 100 and 333 microM) and at high intensity (333 microM) and suppressed low intensity SP (100 and 333 microM) and increased high intensity SP (333 microM). The cochlear microphonic (CM) evoked by 10 kHz tone bursts was not affected by caffeine at high or low intensity, and ryanodine had no effect on it at low intensity but decreased it at high intensity (10, 33, 100 and 333 microM). In contrast, caffeine (10 mM) and ryanodine (33 and 100 microM) significantly increased CM evoked by l kHz tone bursts and recorded from the round window. Caffeine (10 mM) and ryanodine (100 microM) reversibly suppressed the cubic DPOAEs evoked by low intensity primaries. Overall, low intensity evoked responses were more sensitive and were suppressed to a greater extent by both drugs. This is consistent with the hypothesis that release of Ca(2+) from ryanodine receptor Ca(2+) stores, possibly in outer hair cells and supporting cells, affects the function of the cochlear amplifier.     

豚鼠耳蜗神经活动的频谱特征

目的了解清醒状态下豚鼠耳蜗神经电生理活动的平均功率谱（Ａｖｅｒａｇｅｓｐｅｃｔｒｕｍｏｆｅｌｅｃｔｒｏｐｈｙｓｉｏｌｏｇｉｃａｌｃｏｃｈｌｅｏｎｅｕｒａｌａｃｔｉｖｉｔｙ,ＡＳＥＣＡ）的正常特征及观察耳蜗不同状况下ＡＳＥＣＡ的变化。方法采用园窗慢性电极记录耳蜗神经电活动,通过ＦＦＴ变换获得ＡＳＥＣＡ。结果在安静条件清醒状态下豚鼠ＡＳＥＣＡ图为在低频１００Ｈｚ附近有一最大的能量峰,紧接着靠近５００Ｈｚ处的为一谱谷以及中心频率为１ｋＨｚ的宽谱峰（ＡＳＥＣＡ—１ｋＨｚ）。镇静剂、麻醉剂及电极植入手术均对ＡＳＥＣＡ产生影响。结论本研究表明ＡＳＥＣＡ１ｋＨｚ为ＡＳＥＣＡ的正常特征,且３６ｄＢ（ｒｅ１ｎＶ）为ＡＳＥＣＡ１ｋＨｚ幅值的正常低限值。     

Comparison of Response Properties of Neurons in the Inferior Colliculus of Guinea Pigs Under Different Anesthetics

Responses of the inferior colliculus (IC) neurons to acoustical stimulation were recorded in anesthetized guinea pigs. Three kinds of anesthesia were used: (1) urethane (8 ml/kg b.w. of 20 per cent solution i.p.); (2) ketamine-xylazine combination (1 ml/kg b.w. of mixture 2:1); and (3) pentobarbital (25 mg/kg i.p.) combined with intramuscular injection of fentanyl (0.5 ml) and droperidol (1 ml). The frequency tuning of neurons evaluated on the basis of Qio values and the composite neural audiogram represented by points of lowest thresholds of individual IC neurons were similar for guinea pigs treated with any of the anesthetics. The number of spontaneously active IC neurons was significantly larger with ketamine than with urethane or pentobarbital. The response latencies to tone bursts at characteristic frequency (CF) were shortest in animals anesthetized with pentobarbital. Whereas with ketamine and urethane many neurons were recorded in which response latencies were longer than 40 ms, in pentobarbital-anesthetized animals the latencies usually did not exceed 25 ms. The occurrence of neurons with an onset type of response was significantly larger with pentobarbital than with the other two anesthetics. In ketamine and urethane anesthesia, thresholds of units with sustained response weTe significantly lower than thresholds of units with onset response. Our results suggest that in experiments where the level of spontaneous activity, latency and type of responses are important parameters, the kind of anesthesia should be taken into account.     

Nimodipine, an L-channel Ca2+ antagonist, reverses the negative summating potential recorded from the guinea pig cochlea

Nimodipine, an L-type Ca2+ channel antagonist, was tested using sound-evoked cochlear potentials in guinea pigs to investigate whether these channels are involved in cochlear function. Perilymph spaces of guinea pig cochleae were perfused with artificial perilymph solutions containing 0.1-10 microM nimodipine at a rate of 2.5 microliters/min for 10 min. The cochlear potentials evoked by 10 kHz tone bursts of varying intensities were recorded from the basal turn of the scala vestibuli. Cochlear perfusion of nimodipine resulted in reversible, dose-related suppression of the compound action potential of the auditory nerve (CAP; N1-P1), a prolongation of N1 latency at suprathreshold levels, an elevated CAP threshold, a decrease in N1 latency at a constant amplitude measured at CAP threshold, a reduction in cochlear microphonics (CM), and a reduction of the negative summating potential (SP) to a point where it became positive (i.e., a reversal of SP). The endocochlear potential (EP) was not affected. These results support the hypothesis that L-type Ca2+ channels are directly involved in the operation of the organ of Corti. We speculate that L-type Ca2+ channels are integrally involved in generation of a negative summating potential and the dc motion of the cochlear partition described by others.     

镫骨全切除和吸引噪声对豚鼠耳蜗功能与超微结构的影响

本实验应用耳蜗电图和扫描、透射电镜等技术观察了6组(44只)豚鼠镫骨全切除后,前庭窗开放时间的长短和听泡内使用吸引器对耳蜗功能和超微结构的影响程度.结果如下:①随前庭窗开放时间的延长、复合蜗神经动作电位(compound action potential.CAP)反应阈进行性提高,开放180min可导致底回部分外毛细胞变性或破坏;②镫骨全切除后,听泡内较长时间地(60min)应用吸引器可引起CAP明显阈移和耳蜗底回部分外毛细胞的不可逆病变;③镫骨未切除时,听泡内吸引60min,仅引起CAP阈移,但外毛细胞未见损伤.结果提示:术中长时间开放前庭窗和鼓室内应用吸引器可能是引起镫骨全切除术后感音神经性聋的因素之一,但鼓室成形术中鼓室内吸引噪声可能因其强度不够大,不足以引起术后感音神经性聋.     

The Auditory Nerve Overlapped Waveform (ANOW) Originates in the Cochlear Apex

Measurements of cochlear function with compound action potentials (CAPs), auditory brainstem responses, and otoacoustic emissions work well with high-frequency sounds but are problematic at low frequencies. We have recently shown that the auditory nerve overlapped waveform (ANOW) can objectively quantify low-frequency (<1 kHz) auditory sensitivity, as thresholds for ANOW at low frequencies and for CAP at high frequencies relate similarly to single auditory nerve fiber thresholds. This favorable relationship, however, does not necessarily mean that ANOW originates from auditory nerve fibers innervating low-frequency regions of the cochlear apex. In the present study, we recorded the cochlear response to tone bursts of low frequency (353, 500, and 707 Hz) and high frequency (2 to 16 kHz) during administration of tetrodotoxin (TTX) to block neural function. TTX was injected using a novel method of slow administration from a pipette sealed into the cochlear apex, allowing real-time measurements of systematic neural blocking from apex to base. The amplitude of phase-locked (ANOW) and onset (CAP) neural firing to moderate-level, low-frequency sounds were markedly suppressed before thresholds and responses to moderate-level, high-frequency sounds were affected. These results demonstrate that the ANOW originates from responses of auditory nerve fibers innervating cochlear apex, confirming that ANOW provides a valid physiological measure of low-frequency auditory nerve function.     

卡那霉素和速尿联合用药后豚鼠耳蜗听功能研究

目的 探讨卡那霉素和速尿联合用药对豚鼠耳蜗听功能的影响。方法实验组50只豚鼠肌肉注射卡那霉素500mg／kg，同时静脉注射速尿40mg／kg，空白对照组4只豚鼠。于药物注射3天（25只）和7天（25只）后行听觉脑干诱发电位（auditory brainstem response，ABR）．听神经复合动作电位（compound action potential，CAP）及耳蜗微音电位（cochlear microphonic，CM）检测，观察所有动物的中耳结构，对实验组和对照组豚鼠耳蜗听功能进行统计学分析。结果观察所有实验组豚鼠的中耳结构可见鼓膜完整．无充血或者穿孔．听骨链完整．听骨结构无破坏或者移位。豚鼠用药后听阈的个体差异较大，用药后同一频率CAP阈值低于ABR阈值，有的豚鼠用药后CAP阈值轻度提高，但是CM明显异常；将听阈〉95 dB SPL的豚鼠分为用药3天组和用药7天组，发现用药3天组各频率（2kHz，4kHz，8kHz和16kHz）比正常对照组同一频率的听力阈值明显提高（方差分析P〈0．01，差异有显著性），而用药3天组和用药7天组各频率（2kHz，4kHz，8kHz和16kHz）比较阈值没有差别（方差分析P〉0．05，差异没有显著性），用药后较高频率（8kHz，16kHz）的听力损失大于较低频率（2kHz，4kHz）。结论卡那霉素和速尿联合用药后3天可使豚鼠听功能严重受损，因此，卡那霉素和速尿联合用药是建立耳聋模型的一种快速而有效的方法。