Norepinephrine in cochlear microcirculation of guinea pigs

The blood flow in the radiating arteriole through a small cochlear fenestra was recorded with motion pictures in anesthetized guinea pigs, before and after norepinephrine injection into the ipsilateral carotid artery. The flow velocity was determined by measuring the displacement of plasma space running through the radiating arteriole per second. Norepinephrine injection of 0.01 and 0.15 mg/kg increased both flow velocity and arterial blood pressure. The flow velocity increase related directly to the increase in arterial blood pressure. However, a larger norepinephrine dose (1.2 and 2.5 mg/kg) led to transient cessation in flow, which was independent of the blood pressure increase. Dilatation of vessel diameter was always observed with the rise in blood pressure, irrespective of norepinephrine doses. When blood flow in the radiating arteriole stopped after a large norepinephrine injection, the arteriole's vascular lumen was completely obstructed by the aggregated red blood cells. These results suggested that cochlear microcirculation is disturbed by microemboli formed by norepinephrine-induced platelet hyperaggregation.     

Pattern of gentamicin-induced cochlear degeneration in the guinea pig

Summary Gentamicin-induced cochlear degeneration in the guinea pig was studied by complete hair-cell counting (cytocochleograms) and phase-contrast and interference microscopical examination of the stria vascularis and Reissner's membrane. Gentamicin (100 mg/kg/day) was administered over a period of 7–17 days. The first loss of hair cells (OHC) occurred in a region 6–8 mm from the round window. From this degeneration point, the loss of haircells progressed towards the round window (fast) and the apex (slowly). The stria vascularis showed no signs of degeneration. Reissner's membrane, on the other hand, showed intracellular vacuolization of the endolymphatic cells over the complete length of the cochlea after 12 or more days' intoxication. Hearing loss was measured by electrocochleography with skin electrodes. The histologic findings were compared with the objective audiograms.Supported by grants from the Heinsius Houbolt Foundation     

Patterns of degeneration in the human cochlear nerve

The patterns of neural degeneration of the spiral ganglion were studied in 12 human pathologic specimens and 2 normal neonatal specimens. Morphometric analysis of spiral ganglion cells included the maximum cross-sectional areas of both large (type 1) and small (type II) spiral ganglion cells. The organ of Corti in segments corresponding to the spiral ganglion, was evaluated for the presence or absence of inner (IHC) and outer (OHC) hair cells and supporting cells. The relationship between degeneration of spiral ganglion cells and degeneration in the organ of Corti, the age, sex, duration of deafness, cochlear location and delay between death and fixation was evaluated statistically.

Both primary and secondary degeneration of the spiral ganglion were more severe in the basal than apical half of the cochlea. Degeneration of the spiral ganglion was most severe when both IHCs and OHCs were absent in the organ of Corti. No survival advantage was identified for type II ganglion cells as has been previously reported. That is, there was no correlation between the degree of degeneration of the spiral ganglion and the prevalence of type II ganglion cells. In fact, there was more severe degeneration of type II cells when the corresponding organ of Corti was severely degenerated.

These findings in the human were compared with animal models of degeneration of the spiral ganglion, and the implications for cochlear implantation were discussed.     

Chlorpromazine inhibits cochlear function in guinea pigs

Outer hair cell (OHC) electromotility provides mechanical positive feedback that functions as the cochlear amplifier. In isolated OHCs, chlorpromazine shifts the electromotility voltage-displacement transfer function in a depolarizing direction without affecting its magnitude. This study sought to measure the effects of chlorpromazine on cochlear function in vivo. Salicylate, a drug that greatly reduces the magnitude of electromotility, was used for comparison. Perilymphatic perfusion of the guinea pig cochlea with chlorpromazine or salicylate increased the compound action potential (CAP) threshold across the frequency spectrum (1-20 kHz). Both drugs also increased distortion product otoacoustic emission (DPOAE) thresholds in the higher frequencies (10-20 kHz). Complete reversibility of these effects occurred after washout. Both drugs demonstrated concentration-dependent reductions in cochlear function that followed sigmoidal curves with similar fits to previously reported results in isolated OHCs. The endolymphatic potential was not affected by either of these drugs. Thus, chlorpromazine inhibits cochlear function in a manner consistent with what would be expected from data in isolated OHCs. This suggests that shifting the electromotility transfer function correspondingly reduces the gain of the cochlear amplifier.     

Histochemical evidence that cochlear efferents are carried with the inferior vestibular nerve in guinea pigs.

Histochemical methods were employed to determine the course of the olivocochlear bundle (OCB) within the vestibular nerve of guinea pigs. Following transection of the inferior vestibular nerve, cholinesterase staining in the cochlea was greatly reduced. Transection of the superior vestibular nerve, however, yielded no detectable change in staining. It is concluded that the cochlear efferent innervation in guinea pigs is carried in the inferior vestibular nerve, at the point of entry into the medial bulla.     

Virus particles in the cochlear spiral ganglion of guinea pigs

All examined spiral ganglions of several guinea pig populations from different breeds showed intracytoplasmic viruses in some granular spiral ganglion cells. According to their localization and morphology we classify these viruses with the oncorna group. This is not in agreement with the classification of other authors. Apparently there is a world-wide latent viral infection in guinea pigs. Beside the virus particles we found an accumulation of lysosomes which indicate a local increased lysosomal activity of the infected ganglion cells. Further influences on the infected cells can neither be demonstrated nor denied.     

多巴胺对豚鼠听觉传入神经的抑制作用及其频率选择性

目的 观察多巴胺对听觉传入神经的抑制作用及其频率选择性,为进一步探讨多巴胺在内毛细胞下突触复合体中的调控作用奠定基础.方法 健康杂色豚鼠40只,随机分为4组,行全耳蜗灌流:①灌流人工外淋巴液组;②灌流10 mmol/L多巴胺组;③灌流30 mmol/L多巴胺组;④灌流50 mmol/L多巴胺组.灌流过程中分别在第0 h、1 h和2 h通过蜗窗记录不同频率刺激声(250、500、1000、2000、4000、8000及16 000 Hz)所引出的复合动作电位(CAP)阈值和4000 Hz刺激声所引出的耳蜗微音器电位(CM)的幅值.结果 灌流人工外淋巴液前后各频率CAP阈值差异均没有统计学意义(P值均>0.05);灌流多巴胺后大部分频率的CAP阈值提高,与灌流前相比差异具有统计学意义(P值均<0.05),而且随着多巴胺浓度的增加,其抑制作用也逐渐增强;灌流后各组内不同刺激声频率间的CAP阈移存在差异,灌流30 mmol/L多巴胺时最明显,该组中4000 Hz和8000 Hz处阈移最大.灌流人工外淋巴液和多巴胺对CM的非线性没有明显影响,灌流前后CM的相对幅值差异没有统计学意义(P值均>0.05).结论 多巴胺对豚鼠听觉传入神经具有抑制性作用,而对外毛细胞无影响;这种抑制作用具有频率选择性,对高频纤维的抑制作用较强,而对低频的抑制作用较弱.     

Auditory cortical projections to the cochlear nucleus in guinea pigs

We used anterograde tracing techniques to examine projections from auditory cortex to the cochlear nucleus in guinea pigs. Following injection of dextrans into the temporal cortex, labeled axons were present bilaterally in the cochlear nucleus. The distribution of boutons within the cochlear nucleus was similar on the two sides. The majority of boutons was usually located on the ipsilateral side. Most of the boutons were located in the granule cell areas, where many small boutons and a few larger, mossy-type endings were labeled. Additional small, labeled boutons were found in all layers of the dorsal cochlear nucleus, with the majority located in the fusiform cell layer. Labeled boutons were also present in the ventral cochlear nucleus, where they were located in the small cell cap as well as magnocellular parts of both posteroventral and anteroventral cochlear nucleus. Similar results were obtained with injections restricted to primary auditory cortex or to the dorsocaudal auditory field. The results illustrate direct cortical projections to the cochlear nucleus that are likely to modulate the activity in a number of ascending auditory pathways.     

Lack of short-term autoregulation in the cochlear microcirculation in guinea pigs

Summary To determine the relationship between the dynamics of mean arterial blood pressure (MABP) elevation and possible changes in the cochlear microcirculation the cochlear blood flow (CBF) was measured in guinea pigs by a laser Doppler method. The MABP was elevated at rates ranging from 0.02 mm Hg/s to 4 mm Hg/s by intravenous infusions of norepinephrine or epinephrine in various concentrations. A fall in MABP was induced by exsanguination of the animals. The purpose of the experiments was to record the time of onset and course of an expected autregulation in the cochlea in response to slow or rapid changes in MABP. The data suggest that there is no short-term autoregulation in the cochlear microcirculation reflecting the increase of the MABP, but a slight compensation occurs when the MABP declines. These latter changes could be attributed to the high CO₂ sensitivity of the cochlear blood vessels.     

顺铂对豚鼠耳蜗毛细胞的损伤

目的观察顺铂中毒后豚鼠耳蜗毛细胞损伤特点,并探讨半胱胺酸蛋白酶蛋白-3(caspase-3)在耳蜗毛细胞凋亡过程中的作用机制。方法健康豚鼠随机分为实验组和对照组各10只,实验组顺铂2mg·kg~(-1)·d~(-1),腹腔注射,连续6天：对照组用生理盐水等量腹腔注射,连续6天。动物实验期间每天测体重以调整药量,用药前及用药后第7天检测DPOAE,第7天处死豚鼠。左耳冰冻连续切片,免疫组织化学方法检测细胞中Caspase-3的表达;右耳冰冻连续切片,DNA末端转移酶介导的缺口末端标记法检测毛细胞凋亡。结果实验组中毛细胞受损,在细胞内观察到Caspase-3免疫阳性反应,并见凋亡细胞;对照组中未见毛细胞损伤,未见Caspase-3表达,未见凋亡细胞。结论顺铂可以通过诱导凋亡造成耳蜗毛细胞损伤,同时Caspase-3的表达,提示Caspase家族参与了耳蜗毛细胞凋亡过程。     

噪声对豚鼠耳蜗电位及其超微结构的影响

目的观察噪声暴露对豚鼠耳蜗电位和超微结构的影响。方法选用健康杂色豚鼠10只,以右耳为噪声暴露耳,以左耳为对照耳,右耳持续给白噪声100dBSPL2小时。于噪声暴露前及噪声暴露后测量右耳耳蜗电位,并于噪声暴露后取噪声暴露耳和对照耳的耳蜗,应用透射电镜进行形态学的观察。结果噪声暴露后耳蜗微音电位幅度下降并且其非线性特点消失,听神经复合动作电位阈值明显升高;内毛细胞及其下方传入神经末梢空化,外毛细胞溶酶体增多、胞浆内出现空泡。结论噪声暴露不仅引起外毛细胞的损伤,还可以引起内毛细胞及传入神经纤维的损伤。     

Multi-frequency tympanometry in experimentally-induced cochlear lesions in chinchillas and guinea pigs

OBJECTIVES: To establish that susceptance-conductance tympanograms at a probe-tone frequency of 2 kHz reflects the status of the annular ligament (AL) and through it of the cochlea. METHODS: Experimental study in 5 chinchillas and 22 guinea pigs. Six validating experiments were used: blockages of the stapes and of the round window membrane (RWM), fistula of the RWM, fluid removal from the cochlea, injection of saline in the scala tympani (ST) and acoustic trauma (AT). Quantitative data (mean values of Y226, FR, Y2000, G2000 and B2000) and shape of the curves were analyzed before and immediately after lesions were done. RESULTS: Guinea pig was the most convenient provided bulla was vented and the same tip was used along the experiments. Only the shape of the curves are discriminant: 1/a supplementary sharp peak, centered around negative pressures, is observed in Y/G tympanograms in every case of RWM fistulas and in some case of AT. 2/injection of saline into ST induces immediate and reproducible Y2000, G2000, et B2000 curves modifications. 3/RWM and stapes blockages provoke foreseeable stiffening and sharpening of the tympanograms at 2 kHz. 4/on the contrary, fluid removal from the cochlea induces multiple peaks curves. CONCLUSIONS: Experimentally-induced modifications at the AL either direct (stapes blockage) or indirect by AT or decrease/increase of pressure load at the cochlear interface at the footplate result in noticeable, constant, reproducible changes of curves registered at 2 kHz. The stapes behaves both as the plotter of the curves and the interpreter of the inner ear pressure.     

Direct measurement flow resistance of cochlear aqueduct in guinea pigs

Objective The cochlear aqueduct connects the scala tympani to the subarachnoid space and is the main pressure equalization canal for the inner ear. Increases in inner ear volume and pressure are thought to cause clinical symptoms such as vertigo, tinnitus and fluctuating hearing loss. In this study the flow resistance of the cochlear aqueduct was determined and its relation with inner ear pressure was studied.

Material and Methods Inner ear pressure was measured in the scala tympani through the round window using a micropipette. Through a second micropipette, artificial perilymph was infused into, or withdrawn from, the scala tympani at various constant rates. From the infusion rate and the change in perilymphatic pressure during infusion the flow resistance of the cochlear aqueduct was calculated.

Results The flow resistance was found not to be constant but to depend on the position of the round window membrane and possibly on the magnitude and direction of fluid flow through the aqueduct. Measured flow resistance values were in the range 11–45 Pa s/nl. For very small flow values the flow resistance averaged over 6 animals was 21 Pa s/nl.

Conclusions The flow resistance of the cochlear aqueduct is not a constant value. The cochlear aqueduct is a canal with dynamic properties and may play a role in the complicated process of inner ear pressure regulation.     

Direct measurement flow resistance of cochlear aqueduct in guinea pigs

OBJECTIVE: The cochlear aqueduct connects the scala tympani to the subarachnoid space and is the main pressure equalization canal for the inner ear. Increases in inner ear volume and pressure are thought to cause clinical symptoms such as vertigo, tinnitus and fluctuating hearing loss. In this study the flow resistance of the cochlear aqueduct was determined and its relation with inner ear pressure was studied. MATERIAL AND METHODS: Inner ear pressure was measured in the scala tympani through the round window using a micropipette. Through a second micropipette, artificial perilymph was infused into, or withdrawn from, the scala tympani at various constant rates. From the infusion rate and the change in perilymphatic pressure during infusion the flow resistance of the cochlear aqueduct was calculated. RESULTS: The flow resistance was found not to be constant but to depend on the position of the round window membrane and possibly on the magnitude and direction of fluid flow through the aqueduct. Measured flow resistance values were in the range 11-45 Pa s/nl. For very small flow values the flow resistance averaged over 6 animals was 21 Pa s/nl. CONCLUSIONS: The flow resistance of the cochlear aqueduct is not a constant value. The cochlear aqueduct is a canal with dynamic properties and may play a role in the complicated process of inner ear pressure regulation.     

Inhibition of caspases alleviates gentamicin-induced cochlear damage in guinea pigs

The efficacy of caspase inhibitors for protecting the cochlea was evaluated in an in vivo study using guinea pigs, as the animal model system. Gentamicin (12 mg/ml) was delivered via an osmotic pump into the cochlear perilymphatic space of guinea pigs at 0.5 microl/h for 14 days. Additional animals were given either z-Val-Ala-Asp (Ome)-fluoromethyl ketone (z-VAD-FMK) or z-Leu-Glu-His-Asp-FMK (z-LEHD-FMK), a general caspase inhibitor and a caspase 9 inhibitor, respectively, in addition to gentamicin. The elevation in auditory brain stem response thresholds, at 4, 7, and 14 days following gentamicin administration, were decreased in animals that received both z-VAD-FMK and z-LEHD-FMK. Cochlear sensory hair cells survived in greater numbers in animals that received caspase inhibitors in addition to gentamicin, whereas sensory hair cells in animals that received gentamicin only were severely damaged. These results suggest that auditory cell death induced by gentamicin is closely related to the activation of caspases in vivo.     

丁胺卡那霉素诱发豚鼠耳蜗外毛细胞凋亡的实验研究

目的 探讨氨基糖苷类抗生素作用于耳蜗时外毛细胞凋亡现象及其凋亡发生特点。方法 用一定剂量的丁胺卡那霉素对15只豚鼠行耳蜗造模后利用光镜、TUNEL标记技术（teminal-deoxynucl eotidy transferase mediated d-UTP nick-end labeling)原位检测凋亡发生。结果 肌注丁胺卡那霉素1d后即可诱发豚鼠蜗底转外毛细胞发生凋亡,随着用药时间延长,其余各转外毛细胞亦发生凋亡,甚至出现部分外毛细胞丢失现象。结论 氨基糖苷类抗生素作用于耳蜗系统时可引起外毛细胞凋亡,细胞凋亡是豚鼠耳蜗听毛细胞损伤的一条途径;氨基糖苷类抗生素耳毒性作用机制可能与其引起耳蜗毛细胞凋亡有关。     

Glutamate receptors in afferent cochlear neurotransmission in guinea pigs.

With the aid of microinotophoretic techniques we tested the action of the transmitter candidate glutamate (Glu) at the afferent synapses of inner hair cells (IHC) in guinea pigs. In order to determine the various types of glutamate receptors, further agonistic excitatory amino acids (EAA) as well as competitive EAA-antagonists were used. Applied perisynaptically, Glu, aspartate, N-methyl-D-aspartate (NMDA), quisqualate (Q) and kainate (K) activate the subsynaptic, phasic firing activity of the afferent dendrites. The NMDA-induced activation is augmented by simultaneous application of glycine. The firing rate induced by Glu and NMDA is blocked by the specific NMDA-antagonist D-2-amino-7-phosphonoheptanoate (AP-7). Furthermore, activity induced by Glu and Q decreases under the influence of the selective Q-antagonist glutamic acid diethylester (GDEE). These results are consistent with the hypothesis that Glu acts as a possible afferent neurotransmitter of the IHC. This neurotransmission is mediated by postsynaptic EAA-receptor subpopulations which are sensitive to NMDA, Q and K. The activity of the NMDA-receptors depends, however, on the amount of glycine available. Our data suggest that the afferent synapses of the IHC possess functional properties which are equivalent to the properties of glutamatergic NMDA-sensitive and NMDA-non-sensitive synapses in the central nervous system.