次声对豚鼠听阈影响的实验研究

目的　探讨不同强度次声暴露下豚鼠听阈的变化。方法　采用面神经管长期置入电极方法 ,测试不同强度、不同时间的次声暴露下 2 0只清醒豚鼠的听阈变化 ,并将暂时阈移 (temporarythresholdshifting ,TTS)与永久阈移 (permanentthresholdshifting ,PTS)的指标进行系统分析。结果　 8Hz的次声在 130dBSPL暴露 30小时即可引起PTS ,而在 12 0dBSPL暴露 30小时对听阈的影响不大 ,可引起TTS。结论　在强度≥ 130dBSPL次声暴露 30小时可引起豚鼠耳蜗功能的永久性损害 ,造成PTS。     

强次声波对豚鼠Corti器超微结构的损伤

目的 观察强次声波对豚鼠Corti器超微结构的损伤情况。方法 将豚鼠置于频率8Hz、强度为135dBSPL的次声声场中连续暴露90min。应用扫描电镜分别观测强次声波暴露后即刻（2h内）、2天和7天时动物Corti器超微结构的变化，计算各组耳蜗的受损率，与对照组进行比较。结果 扫描电镜下见各实验组动物Corti器感觉毛细胞纤毛缺失、散乱、倒伏，表皮板等结构均有不同程度的损伤。在存活较长时期后，还可见到纤毛融合．部分细胞的表皮板破裂，以及支持细胞分离，细胞溶解，形成空洞。耳蜗受损率分别为70％～80％。结论 强次声波可引起豚鼠Corti器超微结构不同程度的损伤。     

Effects of acute infrasound exposure on vestibular and auditory functions and the ultrastructural changes of inner ear in the guinea pig]

OBJECTIVE: To define the effects of acute infrasound exposure on vestibular and auditory functions and the ultrastructural changes of inner ear in guinea pigs. METHODS: The animals involved in the study were exposed to 8 Hz infrasound at 135dB SPL for 90 minutes in a reverberant chamber. The sinusoidal pendular test (SPT), auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE) were respectively detected pre-exposure and at 0(within 2 hrs), 2 and 5 day after exposure. The ultrastructures of the inner ear were observed by scanning electron microscopy. RESULTS: The slow-phase velocity and the frequency of the vestibular nystagmus elicited by sinusoidal pendular test (SPT) declined slightly following infrasound exposure, but the changes were not significant (P > 0.05). No differences in the ABR thresholds, the latencies and the interval peak latencies of I, III, V waves were found between the normal and the experimental groups, and among experimental groups. The amplitudes of DPOAE at any frequency declined remarkably in all experimental groups. The ultrastructures of the inner ear were damaged to different extent. CONCLUSION: Infrasound could transiently depress the excitability of the vestibular end-organs, decrease the function of OHC in the organ of Corti and cause damage to the inner ear of guinea pigs.     

急性次声波暴露后豚鼠位听功能及内耳超微结构的变化

目的观察次声波对豚鼠位听功能和内耳超微结构的影响。方法将豚鼠置于频率8Hz、声压级135dBSPL的次声声场中连续暴露90min。应用正弦摆动试验(sinusoidalpendulartest,SPT)、听性脑干反应（auditorybrainstemresponse,ABR）和畸变产物耳声发射(distortionproductionotoacousticemission,DPOAE)评价次声波暴露前后豚鼠前庭功能和听功能的变化,扫描电镜观察豚鼠内耳各结构表面超微形态的变化。结果次声波暴露后不同时间正弦摆动诱发的豚鼠前庭性眼震的最大慢相速度(slow-phasevelocity,SPV)和频率较次声暴露前轻微降低,但无显著性意义(P>0.05)。次声波暴露后各组动物ABR阈值较正常时略有升高,亦无统计学差异(P>0.05),各组动物ABR各波潜伏期和波间期与次声暴露前比较差异均无显著性(P>0.05);DPOAE的幅度值在各个频率段均有明显的降低(P<0.01)。扫描电镜下见各实验组动物内耳半规管壶腹嵴两囊斑及Corti器感觉毛细胞纤毛缺失、散乱、倒伏及融合,表皮板等结构均有不同程度的损伤。结论次声波对豚鼠前庭末梢感受器兴奋性可能有一过性的轻微抑制作用,但SPT无有意义改变。次声波可引起豚鼠内耳毛细胞超微结构的损伤,可导致豚鼠耳蜗外毛细胞功能明显减退,这种功能减退尚不足以引起听力的明显改变。     

次声波对豚鼠畸变产物耳声发射幅度的影响

目的观察强次声波暴露后豚鼠畸变产物耳声发射(DPOAE)的变化情况.方法将15只豚鼠置于频率8Hz、强度为135dB SPL的次声声场中连续暴露90分钟.分别于强次声波暴露前及暴露后即刻(2h内)、2天和5天做畸变产物耳声发射测试.结果强次声波暴露后豚鼠DPOAE的幅度值在各个频率段与暴露前相比均有明显的降低(p＜0.01),随着时间的推移,各个频率的幅度虽有一定的恢复,但仍明显低于暴露前水平(p＜0.01).结论强次声波可导致豚鼠耳蜗外毛细胞功能明显减退.     

Effects of noise on cochlear potentials and endolymph potassium concentration recorded with potassium-selective electrodes

Guinea pig cochleas were exposed to either broad-band noise at intensities between 95 and 115 dBA or octave-band noise centered at 380 Hz or 4.2 kHz at intensities between 115 and 125 dB SPL. Cochlear microphonics (CM), summating potentials (SP) and action potentials (AP) were recorded from differential electrodes in the perilymphatic scalae between successive 20-min periods of noise exposure. The endocochlear potential (EP) and endolymph potassium concentration [Kendo+] were recorded continuously from scala media using double-barreled potassium-sensitive electrodes. It was found that the initial exposure to noise at 115 dBA produced considerable suppression of the CM and AP, while the EP and [Kendo+] were elevated above their normal values. When animals previously treated with kanamycin were subjected to the same level of noise exposure no systematic increase in either EP ro [Kendo+] was observed. After prolonged exposure to 380 Hz octave-band noise at 125 dB SPL, a slow decline of EP and [Kendo+] was observed. The relationships between the changes in EP, [Kendo+] and CM are discussed.     

Effect of infrasound on cochlear damage from exposure to a 4 kHz octave band of noise

Infrasound (i.e., <20 Hz for humans; <100 Hz for chinchillas) is not audible, but exposure to high-levels of infrasound will produce large movements of cochlear fluids. We speculated that high-level infrasound might bias the basilar membrane and perhaps be able to minimize noise-induced hearing loss. Chinchillas were simultaneously exposed to a 30 Hz tone at 100 dB SPL and a 4 kHz OBN at either 108 dB SPL for 1.75 h or 86 dB SPL for 24h. For each animal, the tympanic membrane (TM) in one ear was perforated ( approximately 1 mm(2)) prior to exposure to attenuate infrasound transmission to that cochlea by about 50 dB SPL. Controls included animals that were exposed to the infrasound only or the 4 kHz OBN only. ABR threshold shifts (TSs) and DPOAE level shifts (LSs) were determined pre- and post-TM-perforation and immediately post-exposure, just before cochlear fixation. The cochleae were dehydrated, embedded in plastic, and dissected into flat preparations of the organ of Corti (OC). Each dissected segment was evaluated for losses of inner hair cells (IHCs) and outer hair cells (OHCs). For each chinchilla, the magnitude and pattern of functional and hair cell losses were compared between their right and left cochleae. The TM perforation produced no ABR TS across frequency but did produce a 10-21 dB DPOAE LS from 0.6 to 2 kHz. The infrasound exposure alone resulted in a 10-20 dB ABR TS at and below 2 kHz, no DPOAE LS and no IHC or OHC losses. Exposure to the 4 kHz OBN alone at 108 dB produced a 10-50 dB ABR TS for 0.5-12 kHz, a 10-60 dB DPOAE LS for 0.6-16 kHz and severe OHC loss in the middle of the first turn. When infrasound was present during exposure to the 4 kHz OBN at 108 dB, the functional losses and OHC losses extended much further toward the apical and basal tips of the OC than in cochleae exposed to the 4 kHz OBN alone. Exposure to only the 4 kHz OBN at 86 dB produces a 10-40 dB ABR TS for 3-12 kHz and 10-30 dB DPOAE LS for 3-8 kHz but little or no OHC loss in the middle of the first turn. No differences were found in the functional and hair-cell losses from exposure to the 4 kHz OBN at 86 dB in the presence or absence of infrasound. We hypothesize that exposure to infrasound and an intense 4 kHz OBN increases cochlear damage because the large fluid movements from infrasound cause more intermixing of cochlear fluids through the damaged reticular lamina. Simultaneous infrasound and a moderate 4 kHz OBN did not increase cochlear damage because the reticular lamina rarely breaks down during this moderate level exposure.     

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.     

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.     

Effects of frequency and intensity of sound on cochlear blood flow.

Laser Doppler flowmetry demonstrates that loud sound induces a decrease of blood flow in the cochlea of the guinea pig. In this experiment, we observed the effects of frequency and intensity of sound on cochlear blood flow using 15 guinea pigs. In the first 5 guinea pigs, a Doppler probe was attached to the basal turn of the cochlea and sounds of 6, 7, 8, 9 and 10 kHz were delivered to the ear serially from lower to higher frequency, i.e. from 6 kHz to 10 kHz. In the next 5 guinea pigs, the sound was delivered from higher to lower frequency, i.e. from 10 kHz to 6 kHz. The sound intensity delivered to the ear was changed from lower to higher intensity (80 to 120 dB SPL by 10 dB width) at each frequency. In the last 5 guinea pigs, the blood flow in the basal, second, third, and fourth turns of the cochlea was measured at 120 dB SPL of 10 kHz. No change of blood flow was seen in the cochlear basal turn at 6 and 7 kHz up to 120 dB SPL, but a decrease of blood flow was found at 110 and 120 dB SPL at 8, 9, and 10 kHz. On the other hand, the sound of 120 dB SPL at 10 kHz induced a decrease of blood flow only in the basal turn of the cochlea. Our results suggest that there is a corresponding blood flow area which is sensitive to specific frequency in the cochlea.     

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     

Time lapse effects of impulse noise on cochlear microphonics in rabbits observed in chronic experiments

Cochlear microphonics (CM) were recorded from awake rabbits with chronically implanted electrodes. Test frequencies used were 1,250, 2,500, 5,000 and 10,000 Hz, with intensities increased from 40 to 100 dB SPL. The rabbits were exposed to ten noise impulses of 144 dB SPL, which were then followed by impulse intensities of 153 and 164 dB SPL for the same animal. Input-output functions before and after each noise exposure were recorded, and recovery processes of the CM were tracked. After 144 dB SPL impulses, complete recovery of CM occurred; the effects of 153 dB SPL impulses varied from restitution up to complete CM loss. Impulses of 164 dB SPL caused an irreversible CM loss in all cases. Despite inter-individual differences, the threshold for irreversible CM loss in rabbits may occur between impulse noise intensities of 153 and 164 dB SPL.     

Time lapse effects of impulse noise on cochlear microphonics in rabbits observed in chronic experiments

Summary Cochlear microphonics (CM) were recorded from awake rabbits with chronically implanted electrodes. Test frequencies used were 1,250, 2,500, 5,000 and 10,000 Hz, with intensities increased from 40 to 100 dB SPL. The rabbits were exposed to ten noise impulses of 144 dB SPL, which were then followed by impulse intensities of 153 and 164 dB SPL for the same animal. Input-output functions before and after each noise exposure were recorded, and recovery processes of the CM were tracked. After 144 dB SPL impulses, complete recovery of CM occurred; the effects of 153 dB SPL impulses varied from restitution up to complete CM loss. Impulses of 164 dB SPL caused an irreversible CM loss in all cases. Despite inter-individual differences, the threshold for irreversible CM loss in rabbits may occur between impulse noise intensities of 153 and 164 dB SPL.     

Threshold shift and inner ear pathology in guinea pigs exposed to octave bands of noise at 63 Hz and 4 kHz

So far there have been few studies on the effect of low frequency noise on auditory organs. Twenty-four Preyer's reflex normal guinea pigs were exposed to the octave bands of noise at 63 Hz and 4 kHz, 110 dB A (SPL). The durations of exposure were 4 and 8 hr. It was discovered that the permanent threshold shift for both frequencies was at 4-8 kHz. In guinea pigs exposed 4 hrs, the threshold shift for 63 Hz was smaller than that for 4 kHz, while the threshold shift for 63 Hz in guinea pigs exposed 8 hr was similar to that of 4 kHz. No change in inner ear morphology was observed in guinea pigs exposed 4 hrs to the octave band of noise at 63 Hz and 4 kHz, but ultrastructural abnormality was demonstrated. When the animal was exposed 8 hr, changes in inner ear morphology and ultrastructure were seen. The main pathological changes were at the second coil of the cochlea. The auditory changes suggest that an intensive low frequency noise may induce a high frequency hearing loss. The result also indicates that using A-weighted level to determine the damage risk criteria of noise is inappropriate.     

A comparative study on the effect of pure-tone exposure of the guinea pig cochlea

Electrophysiological methods were applied to 160 healthy adult male guinea pigs in order to investigate the effects of pure-tone exposure for 24 h on the inner ear. A reduction in cochlear microphonics (CM), action potential (AP) and endocochlear potential was observed following exposure to 110 dB at 100 Hz, 100 dB at 200 and 600 Hz and 95 dB at 2 kHz. The observed K⁺ endolymphatic concentration during 40 min anoxia remained unchanged. In contrast K+ decreased in control animals and following exposure to pure tones varying from 110 dB at 60 Hz to 85 dB at 2 kHz. These findings indicate that high frequency tones have a greater effect on inner ear functions than those of lower frequency, decreasing the maximum output voltage of CM and AP but not changing K⁺ endolymphatic concentration.     

Bilateral superior cervical sympathectomy and noise-induced, permanent threshold shift in guinea pigs.

The rich sympathetic innervation to the cochlea suggests its potential control of cochlear blood flow and activity during noise exposure, as part of the general and local stress sympathetic reaction evoked by noise. In a previous study, superior cervical sympathectomy prior to sound exposure in guinea pigs in an awake state, resulted in reduced temporary threshold shift. The present study was conducted to explore whether this potential protection would also be manifested in conditions producing permanent threshold shift (PTS). Thirty-six guinea pigs, divided into four groups of nine guinea pigs each, were sound exposed for 2 h in an awake state. Eighteen guinea pigs underwent superior cervical sympathectomy prior to sound exposure. Auditory brainstem thresholds were recorded prior to sound exposure, and then at 24 h, 1 and 6 weeks post-exposure. Results indicated a reduced PTS at 122 dB sound pressure level (SPL) exposure, suggesting a protective effect of the sympathectomy. However, at 125 dB SPL exposure, the protective effect was reduced.     

Optimum primary tone level setting for measuring high amplitude DPOAEs in guinea pigs

A special stimulus paradigm needs to be applied when distortion product otoacoustic emission (DPOAE) input/output functions are used to investigate the loss of sensitivity and loss of compression of outer hair cell cochlear amplifiers during noise exposure, drug treatment, etc. This stimulus paradigm should be able to reflect cochlear non-linear compressive sound processing known from direct basilar membrane measurements. Such a paradigm has already been established for humans that accounts for the different compression of the primary tones at the DPOAE generation site at f2 with L1 = 0.4 L2 + 39 dB SPL ('scissor paradigm'). The purpose of the present study was to develop an equivalent parameter setting for guinea pigs. Ninety-six different L1 - L2 combinations were presented to 24 ears in 18 pigmented guinea pigs at seven f2 frequencies between 2 and 16 kHz. L2 ranged from 20 to 60 dB SPL, L1 from 20 to 65 dB SPL. An extreme value analysis was performed to achieve the maximum DPOAE level for L1 in relation to L2. Linear regression analysis yielded a scissor paradigm specific to guinea pigs (L1 = 0.46 L2 + 41 dB SPL) which is similar to that of humans.     

Feinstrukturelle Befunde am Cortischen Organ nach experimenteller Beschallung mit Sinustönen

Summary 28 guinea pigs were exposed to the pure tone stimulus of 3000 Hz with different intensity of 85–130 dB SPL (15 min, 130 dB up to 126 h, 85 dB SPL). The inner ears were examined by means of scanning and transmission electron microscopy. The location of hair cell lesions were found accordingly to the frequency at the end of the basal coil to the 1st coil from base. The complete loss of all cell structures was observed either after a long term stimulation of low intensity or after short time sound exposure of high intensity (15 min, 130 dB). The fine structural alterations of the same specimen could be examined by means of scanning and of transmission electron microscopy due to a special embedding procedure.The tonotopical damage of the organ of Corti after pure tone stimulation suggest a mainly mechanical overstimulation of these areas.     

Effect of loud sound exposure on the cochlear blood flow

The effect of loud sound exposure on cochlear blood flow was studied in the guinea pig by the laser Doppler method. Fourteen guinea pigs with normal Preyer reflex were anesthetized and tracheotomy was performed. A tracheotomy tube was connected to a ventilator and the experiment was performed with artificial ventilation. After exposure of the tympanic bulla and complete removal of the mucosa, a probe of a laser Doppler flowmeter was attached to the lateral wall of the basal turn of the cochlea. A specially-designed ear piece connected with a speaker was inserted into the external ear canal and loud sound (10 kHz at 120 dB SPL) was delivered to the ear for 30 min. Thirteen of the 14 exposed guinea pigs showed a prompt decrease in cochlear blood flow at the onset of the sound exposure and 12 of the 13 guinea pigs showed a prompt recovery of the cochlear blood flow after the cessation of the sound exposure.     

Variability in the depression of cochlear microphonic responses after noise exposure

20 guinea pigs were exposed to a broad band noise (120-140 dB SPL) for short durations. The conditions of stimulation were strictly identical for all animals. Cochlear microphonic responses (CM) were recorded with conventional differential electrodes after each exposure, from the first turn of the cochlea. Variations were observed in the degree of CM depression and also in the alteration of the CM transfer function. The origin of this variability is discussed.