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.     

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.     

The quinoxalinediones DNOX, CNOX and two related congeners suppress hair cell-to-auditory nerve transmission

We tested 6,7-dinitroquinoxaline-2,3-dione (DNQX); 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); 6,7-dichloro-3-hydroxy-2-quinoxalinecarboxylic acid (DHQC); and 3-hydroxy-2-quinoxalinecarboxylic acid (3HQC), new kainate and quisqualate receptor antagonists, upon cochlear potentials in guinea pig. Perilymph spaces of guinea pig cochleae were perfused with artificial perilymph solutions containing up to 1000 microM concentrations of DHQC and 3HQC, and 500 microM concentrations of DNQX and CNQX, at a rate of 2.5 microliters/min for 10 min. Cochlear potentials evoked by 10 kHz tone bursts of varying intensity were recorded from the basal turn scala vestibuli. Cochlear perfusion of the four drugs resulted in a 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, and a decreased N1 latency at CAP threshold. None of the drugs had significant effects on cochlear microphonics (CM) or the summating potential (SP). EC50 values (concentrations causing a 50% reduction in CAP amplitude at 68 dB SPL) were 8 microM for DNQX, 30 microM for DHQC, 35 microM for CNQX, and 1 mM for 3HQC. Results support the hypothesis that kainate and quisqualate receptors are involved in neurotransmission between the hair cell and afferent nerve.     

Cochlear microphonic enhancement in two tone interactions

Two tone interaction functions of the cochlear microphonic (CM) were obtained from pigmented guinea pigs. First (basal) cochlear turn recording locations show optimally enhanced levels of CM when the interfering tone (F2) was positioned about 4 kHz above probe tones (F1) of 12 kHz and 20 kHz. Maximum enhancement occurred for equal level tones. No enhancement was seen for a probe tone of 4 kHz. When basal turn cochlear sensitivity was compromised, CM enhancement caused by the interfering tone was altered and only CM reduction was then seen. The CM reduction was the typical characteristic described by many earlier studies. Guinea pigs with various changes in cochlear sensitivity were studied, providing evidence in support of earlier reports that CM interference (both reductions and enhancements) depends on far field vector summation of the outputs of hair cells from a restricted area of the basilar membrane. No CM enhancement was seen in micropipette recordings from within the organ of Corti.     

豚鼠耳蜗顶部区域微循环障碍的实验研究

目的 研究豚鼠耳蜗顶部区域微循环障碍的听力损伤特点,建立以低上尖损伤为主的听力损伤模型,方法光化学法诱导豚鼠耳蜗顶部区域微循环障碍,常规火棉交切片观察耳蜗形态学变化,再用Madsen2250诱发电位系统记录各频率声刺激诱发的CAP和CM。结果 各频率CAP－N1潜伏期延长值、CAP阈移、CAP－N1和CM振幅下降示基听力损伤以低频较重,其血栓形成的 耳蜗顶回及第三回血管纹和螺旋韧带为主,结论 耳蜗顶部区域的局部微循环障碍可以导致低频范围为主的听力损伤。     

Intensity-related changes in cochlear blood flow in the guinea pig during and following acoustic exposure

Summary This study examined the effects of acoustic exposure at different intensities on cochlear blood flow (CBF) using laser Doppler flowmetry. CBF was measured in anesthetized guinea pigs exposed to either a 10 kHz pure tone at 125, 105, or 90 dB SPL, or wide-band noise at 85 dB SPL for 1 h. Mean arterial blood pressure and heart rate were recorded continuously. Arterial acid-base status, cochlear temperature, cochlear microphonics (CM), and compound action potentials (CAP) were measured before and after exposure. There was a small, but significant, steady decline in basal CBF after 40 min loud sound exposure (125 dB SPL), but no change in basal CBF occurred with the lower intensities (85–105 dB SPL). In contrast, there was a significant increase in apical CBF after 1 h exposure to either moderate wideband noise (85 dB SPL) or a 10 kHz tone at 105 dB SPL. These changes persisted during a 20-min post-exposure period. In most cases, the cochlear temperature and cardiorespiratory variables monitored remained unchanged during and after the exposures as compared to the controls. CM and CAP amplitudes showed extensive losses after acoustic overstimulation (125 dB SPL), but no permanent changes were found at the lower intensities used. The present findings confirm the occurrence of intensity-related effects of acoustic exposure on the cochlear microcirculation.Dedicated to Prof. H. J. Gerhardt on his 65th birthday Correspondence to: F. Scheibe     

Electrophysiological aspects of the middle ear muscle reflex in the rat: latency, rise time and effect on sound transmission

The latency, the rise time and the influence of the acoustic reflex on sound transmission were investigated in the adult rat during ketamin anesthesia. This was done by recordings of the cochlear microphonics (CM) and electromyographic (EMG) recordings of the reflex responses of the tensor tympani muscle. The acoustic reflex was elicited by contralateral acoustic stimuli of which the intensity and frequency was varied. Ipsilaterally, the effect on sound transmission was determined by estimating the change in amplitude of the CM's of ipsilateral administered subliminal stimuli. It was shown that both the tensor tympani muscle and the stapedius muscle contribute in the reflex. The latency as well as the rise time of the reflex determined by CM recordings showed to be short (minimal values: 12 and 7 ms respectively). The mean latency of the tensor tympani muscle reflex, measured by EMG, was about 7 ms. The attenuation of 0.25-8 kHz tone bursts upto 115 dB SPL is limited to a mean maximum of 15 dB SPL. The maximal attenuation was shown to occur at 1 kHz. Frequencies above 2 kHz appeared to be the best elicitor of the middle ear muscle reflex.     

Experimental Study on the Effects of Gentamicin Injection on the Guinea-pig: Electrophysiological Studies

The inner ear distortion induced by gentamicin (GM), a type of aminoglycoside antibiotic, was examined in guinea-pigs. Previous studies which investigated the function of the eighth cranial nerve and organ of Corti using cochlear microphonics (CM) and compound action potential (CAP) reported ototoxicity following experimental exposure to GM. In this report, the effects of GM on the cochlea and the eighth cranial nerve were investigated systematically by measuring CM, CAP, summating potentials induced by 90 dB (SPL) tone burst, endocochlear potential (EP) and K⁺     

The effect of BAPTA and 4AP in scala media on transduction and cochlear gain

We have injected by iontophoresis 4-amino-pyridine, a K+ channel blocker and BAPTA, (a Ca++ chelator), into scala media of the first three turns of the guinea pig cochlea. We measured the reduction in outer hair cell (OHC) receptor current, as indicated by cochlear microphonic measured in scala media evoked by a 207 Hz tone, and compared this with the elevation of the cochlear action potential (CAP) threshold. We found that in the basal turn, for frequencies between 12 and 21 kHz, CAP threshold was elevated by about 30 dB, while in the second turn, at the 3 kHz place, the maximum elevation was 15 dB. In the third turn, iontophoresis of 4AP and BAPTA reduced CM by similar amounts to that in the basal and second turn, but caused negligible elevation of CAP threshold. We conclude that the gain of the cochlear amplifier is maximal for basal turn frequencies, is halved at 3 kHz, and is reduced to close to one for frequencies below 1 kHz (no active gain). The effect of 4AP and BAPTA on neural threshold and the receptor current represented by CM may be explained by their action on OHC transduction without the involvement of IHCs.     

Study of the effect of 350-Hz tone exposure on electrophysiological function of the inner ear of guinea pigs.

The effects of noise exposure were studied in fifty-nine 4- to 5-week-old albino Hartley guinea pigs with normal hearing (body weight 250-300 g). The following experiments and results were carried out: exposure to 350 Hz pure tones at 115 dB for 40 h and exposure to 350 Hz tones at 120 dB for 64 h. In order to investigate the effects of low-frequency tone exposures on the hearing of the guinea pigs, cochlear microphonics (CM), whole nerve action potentials (AP) and endocochlear potentials (EP) were measured. With a high-sound pressure, a decrease was observed in the CM maximum output voltage in the test frequencies of 2, 4 and 6 kHz while the CM threshold (pseudothreshold) of 6 kHz was elevated. Output voltage of the N1 potential of the AP using a 7-kHz tone burst decreased while the threshold of the N1 potential was elevated. An extension of latency and a decrease in the absolute value of the negative potential in EP were also observed.     

ATP-gamma-S shifts the operating point of outer hair cell transduction towards scala tympani

ATP receptor agonists and antagonists alter cochlear mechanics as measured by changes in distortion product otoacoustic emissions (DPOAE). Some of the effects on DPOAEs are consistent with the hypothesis that ATP affects mechano-electrical transduction and the operating point of the outer hair cells (OHCs). This hypothesis was tested by monitoring the effect of ATP-gamma-S on the operating point of the OHCs. Guinea pigs anesthetized with urethane and with sectioned middle ear muscles were used. The cochlear microphonic (CM) was recorded differentially (scala vestibuli referenced to scala tympani) across the basal turn before and after perfusion (20 min) of the perilymph compartment with artificial perilymph (AP) and ATP-gamma-S dissolved in AP. The operating point was derived from the cochlear microphonics (CM) recorded in response low frequency (200 Hz) tones at high level (106, 112 and 118 dB SPL). The analysis procedure used a Boltzmann function to simulate the CM waveform and the Boltzmann parameters were adjusted to best-fit the calculated waveform to the CM. Compared to the initial perfusion with AP, ATP-gamma-S (333 microM) enhanced peak clipping of the positive peak of the CM (that occurs during organ of Corti displacements towards scala tympani), which was in keeping with ATP-induced displacement of the transducer towards scala tympani. CM waveform analysis quantified the degree of displacement and showed that the changes were consistent with the stimulus being centered on a different region of the transducer curve. The change of operating point meant that the stimulus was applied to a region of the transducer curve where there was greater saturation of the output on excursions towards scala tympani and less saturation towards scala vestibuli. A significant degree of recovery of the operating point was observed after washing with AP. Dose response curves generated by perfusing ATP-gamma-S (333 microM) in a cumulative manner yielded an EC(50) of 19.8 microM. The ATP antagonist PPADS (0.1 mM) failed to block the effect of ATP-gamma-S on operating point, suggesting the response was due to activation of metabotropic and not ionotropic ATP receptors. Multiple perfusions of AP had no significant effect (118 and 112 dB) or moved the operating point slightly (106 dB) in the direction opposite of ATP-gamma-S. Results are consistent with an ATP-gamma-S induced transducer change comparable to a static movement of the organ of Corti or reticular lamina towards scala tympani.     

The effect of 6 kHz tone exposure on inner ear function of the guinea pig: relation to changes in cochlear microphonics,action potential,endocochlear potential and chemical potentials of K+-ions and Na+-ions,using a double-barrel glass electrode

Using 97 male albino guinea pigs and applying electrophysiological methods, the effects of a 6 kHz tone were investigated at a moderate sound pressure level to the inner ear during a 24-h exposure time. Following exposure to the 6 kHz tone at 90 dB, cochleas showed decreased maximum output voltage of cochlear microphonics (CM) and action potential (AP). In the endolymph, K⁺ ion and Na⁺ ion concentrations remained unchanged during 40 min anoxia and 90 dB tone exposure. At 80 dB sound exposure decreases in maximum output voltage of CM and AP and decreases in the absolute value of EP could not be detected. Endolymph K⁺-ion Na⁺-ion concentrations were also unchanged. These findings indicate that diffusion potentials are induced at the same time as decreases of maximum output voltage in CM induced by exposure to sound at 90 dB.     

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.     

Pilocarpine促内耳分泌功能的实验研究

目的 探讨Pilcoarpine是否具有促使内耳分泌细胞功能抗进作用,方法：皮下注射Pilocapine,用作电极记录栗鼠CM,CAP,SP,观察在用药前以及用药后30′,90′,120′,150′,180′,检测栗鼠500,1000,2000,4000和8000Hz的CAP听阈,CM,SP振幅变化,实验结束后将动物处死,透射电镜下观察内耳形态学改变,结果：皮下注射Pilocarpine后15′动物出现唾液分泌,排泄物增多的现象。30′时所有频率的SP振幅半大,以2000Hz最为显著,以后随着时间的延长,SP振幅逐渐增大。90′后逐渐下降,180′时部分频率的SP振幅已恢复到用药前水平,在用药后各个频率的CAP阈值的变化率呈正态分布,大多数动物无显著变化,只有少量动物的几个频率CAP振幅在10dB以内振荡,CM振幅无显著变化,透射电镜下内耳毛细胞形态正常,但在微纹区的暗细胞和内齿细胞（interdental cell,IDC)内有着色较深的颗粒,并且部分颗粒向外分泌,在基底膜与盖膜间的空腔间形成层状物。结论：pilocarpine能促进内耳分泌功能,内耳的某些细胞（内齿细胞）有Pilocarpine受体,于内淋巴容积微量增大而增加到SP振幅时,听阈不受影响。     

Phosphorothioate oligodeoxynucleotides can selectively alter neuronal activity in the cochlea.

A growing body of evidence indicates that extracellular adenosine triphosphate (ATP) may have a major role in cochlear function. Antagonists of ionotropic ATP receptors (P2X2) have significant effects on cochlear potentials and distortion product otoacoustic emissions (DPOAEs). We tested whether antisense oligodeoxynucleotides (ODNs) would mimic the functional deficiencies induced by the ATP antagonists through binding to P2X2 ATP receptor mRNA and thereby reduce the number of ATP receptors expressed in the membrane of the cells. Both a phosphorothioate ODN (S-ODN) antisense and a phosphodiester ODN (P-ODN) antisense to the P2X2 sequence and random sense ODNs containing 21 nucleotides were administered chronically (7 days) to the guinea pig cochlea via the perilymph compartment. Sound evoked cochlear potentials (cochlear microphonic; summating potential; compound action potential of the auditory nerve, CAP; latency of the first negative peak in the CAP, N1 latency) and DPOAEs were monitored to assess the effects of the ODNs. Results indicate that the phosphorothioate derivatives of both the antisense and random sense ODNs suppressed the CAP and prolonged the N1 latency with no significant effect on the other parameters. The P-ODNs had no effect. Since both the antisense and random sense S-ODNs had the same effect, we conclude that the S-ODNs affected neuronal function in a manner that did not involve binding to the ATP receptor mRNA.     

Thapsigargin suppresses cochlear potentials and DPOAEs and is toxic to hair cells

Thapsigargin, a drug that inhibits sarco-endoplasmic reticulum Ca(2+) ATPases (SERCAs), was infused into the perilymph compartment of the guinea pig cochlea in increasing concentrations (0.1-10 microM) while sound evoked cochlear potentials were monitored. Thapsigargin significantly suppressed the compound action potential of the auditory nerve, cochlear microphonics, and increased N(1) latency at low (56 dB SPL) and high intensity (92 dB SPL) levels of sound, suppressed low intensity sound evoked summating potential (SP) and greatly increased the magnitude of the high intensity sound evoked SP. At 10 microM, the drug suppressed the cubic distortion product otoacoustic emissions (2f(1)-f(2)=8 kHz, f(2)=12 kHz) evoked by both high and low intensity primaries (45, 60, 70 dB SPL). Thapsigargin (10 microM; 30 min) increased the endocochlear potential slightly (5 mV). In chronic animals, thapsigargin (10 microM; 60 min) destroyed many outer hair cells and some inner hair cells, especially in the basal turns. These effects are consistent with the hypothesis that the inhibition of the SERCAs affects the function of the cochlear amplifier and outer hair cells to a greater degree than it affects other functions of the cochlea.     

顺铂中毒后豚鼠耳蜗电位变化的特征及形态学实验观察

目的 观察顺铂对耳蜗微音电位（ＣＭ）、总和电位（ＳＰ）及复合动作电位（ＣＡＰ）的影响及毛细胞形态学改变。方法 用人工外淋巴液和顺铂灌流豚鼠耳蜗，分别记录耳蜗第三回中阶的ＣＭ、－ＳＰ及ＣＡＰ，琥珀酸脱氢酶染色观察毛细胞的数量变化，透射电镜观察外毛细胞结构。结果 顺铂灌流１ｈ：≤６０ｄＢ ＳＰＬ声强级刺激时ＣＭ、－ＳＰ和ＣＡＰ幅度均较灌流顺铂前略下降，≥７０ｄＢ ＳＰＬ声强级刺激时幅度比灌流顺铂前明显     

Electrophysiological determinations of the effects of 1 kHz noise exposure on the high-frequency hearing of guinea pigs

Summary The effects of noise were studied in 90 5-week-old albino Hartley guinea pigs with normal hearing. The following experimental setup was used: (1) exposure to 1 kHz tones at 100 dB for 4 h, 20 h and 40 h; (2) exposure to 1 kHz tones at 110 dB for 20 h and 40 h. In order to investigate the effects of the noise exposures on the hearing of the guinea pigs, cochlear microphonics (CM), whole nerve action potentials (AP) and endocochlear potentials (EP) were examined. We obtained the following results. With a high sound pressure, a decrease was observed in the CM maximum output voltage in the test frequencies from 2 kHz to 6 kHz while the CM threshold increased (pseudo threshold). Intensity function of the N₁ potential of the AP using a 7 kHz tone burst decreased; the threshold of the N₁ potential also increased considerably by exposure to chronic high sound pressure. An extension of latency and a decrease in the absolute value of the negative potential in EP were induced by chronic noise exposure.     

Effects of 250 and 500 Hz tone exposure on the inner ear of guinea pigs as determined by electrophysiological techniques

The electrophysiological study of cochlear microphonics (CM), whole nerve action potential (AP) and endocochlear potential (EP) were examined. (1) With the extension of the exposure time of 500 Hz tone, a decrease of CM maximum output voltage in test frequency from 2 to 6 kHz was observed. (2) N1 potential of AP decreased very significantly by 500 Hz tone exposure. (3) A very pronounced decrease of the absolute value of the negative potential of the EP in 500 Hz exposure was seen.