Tinnitus behavior and hearing function correlate with the reciprocal expression patterns of BDNF and Arg3.1/arc in auditory neurons following acoustic trauma

The molecular changes following sensory trauma and the subsequent response of the CNS are poorly understood. We focused on finding a molecular tool for monitoring the features of excitability which occur following acoustic trauma to the auditory system. Of particular interest are genes that alter their expression pattern during activity-induced changes in synaptic efficacy and plasticity. The expression of brain-derived neurotrophic factor (BDNF), the activity-dependent cytoskeletal protein (Arg3.1/arc), and the immediate early gene c-Fos were monitored in the peripheral and central auditory system hours and days following a traumatic acoustic stimulus that induced not only hearing loss but also phantom auditory perception (tinnitus), as shown in rodent animal behavior models. A reciprocal responsiveness of activity-dependent genes became evident between the periphery and the primary auditory cortex (AI): as c-Fos and BDNF exon IV expression was increased in spiral ganglion neurons, Arg3.1/arc and (later on) BDNF exon IV expression was reduced in AI. In line with studies indicating increased spontaneous spike activity at the level of the inferior colliculus (IC), an increase in BDNF and GABA-positive neurons was seen in the IC. The data clearly indicate the usefulness of Arg3.1/arc and BDNF for monitoring trauma-induced activity changes and the associated putative plasticity responses in the auditory system.     

Detection and differentiation of sensorineural hearing loss in mice using auditory steady-state responses and transient auditory brainstem responses

Sensorineural hearing loss (SNHL) comprises hearing disorders with diverse pathologies of the inner ear and the auditory nerve. To date, an unambiguous phenotypical characterization of the specific pathologies in an affected individual remains impossible. Here, we evaluated the use of scalp-recorded auditory steady-state responses (ASSR) and transient auditory brainstem responses (ABR) for differentiating the disease mechanisms underlying sensorineural hearing loss in well-characterized mouse models. We first characterized the ASSR evoked by sinusoidally amplitude-modulated tones in wild-type mice. ASSR were robustly elicited within three ranges of modulation frequencies below 200 Hz, from 200 to 600 Hz and beyond 600 Hz in most recordings. Using phase information we estimated the apparent ASSR latency to be about 3 ms, suggesting generation in the auditory brainstem. Auditory thresholds obtained by automated and visual analysis of ASSR recordings were comparable to those found with tone-burst evoked ABR in the same mice. We then recorded ASSR and ABR from mouse mutants bearing defects of either outer hair cell amplification (KCNQ4-knockout) or inner hair cell synaptic transmission (Bassoon-mutant). Both mutants showed an increase of ASSR and ABR thresholds of approximately 40 dB versus wild-type when investigated at 8 weeks of age. Mice with defective amplification displayed a steep rise of ASSR and ABR amplitudes with increasing sound intensity, presumably reflecting a strong recruitment of synchronously activated neural elements beyond threshold. In contrast, the amplitudes of ASSR and ABR responses of mice with impaired synaptic transmission grew very little with sound intensity. In summary, ASSR allow for a rapid, objective and frequency-specific hearing assessment and together with ABR and otoacoustic emissions can contribute to the differential diagnosis of SNHL.     

CT多平面重建的耳蜗标准化测量在先天性感音神经性耳聋患儿中应用价值

目的探讨高分辨率CT(high-resolution CT, HRCT)多平面重建耳蜗标准方位图像在先天性感音神经性耳聋(sensorineural hearing loss, SNHL)中的临床意义。方法 1~7岁经颞部CT诊断内耳骨性结构正常的SNHL患儿50例(100耳)为SNHL组,听力正常儿童50例(100耳)为对照组,根据耳蜗标准坐标系统,采用HRCT多平面重建技术得到标准方位图像,测量耳蜗底转长径、耳蜗底转宽径、耳蜗高度、斜轴位蜗神经管宽度、斜矢位蜗神经管宽度,采用曲面重建测量耳蜗管长度,ROC曲线评价各径线对SNHL患儿是否存在微小内耳畸形的诊断效能。结果 SNHL组耳蜗底转长径[(8.55±0.22)mm]、耳蜗底转宽径[(6.09±0.22)mm]、斜轴位蜗神经管宽度[(1.82±0.20)mm]、斜矢位蜗神经管宽度[(1.90±0.29)mm]、耳蜗管长度[(34.99±2.10)mm]小于对照组[(8.75±0.21)、(6.21±0.24)、(1.91±0.18)、(2.00±0.27)、(35.70±1.98)mm],耳蜗高度[(3.60±0.22)mm]低于对照组[(3.71±0.19)mm](P<0.05);耳蜗底转长径≤8.65 mm时,诊断SNHL患儿存在微小内耳畸形的AUC为0.739(95%CI:0.691~0.787),灵敏度为68.5%,特异度为70.5%。结论儿童耳蜗的标准化径线测量可靠性良好,耳蜗底转长径对SNHL存在微小内耳畸形及人工耳蜗植入术的指导有提示意义。     

Trafficking of Systemic Fluorescent Gentamicin into the Cochlea and Hair Cells

Aminoglycosides enter inner ear hair cells across their apical membranes via endocytosis, or through the mechanoelectrical transduction channels in vitro, suggesting that these drugs enter cochlear hair cells from endolymph to exert their cytotoxic effect. We used zebrafish to determine if fluorescently tagged gentamicin (GTTR) also enters hair cells via apically located calcium-sensitive cation channels and the cytotoxicity of GTTR to hair cells. We then examined the serum kinetics of GTTR following systemic injection in mice and which murine cochlear sites preferentially loaded with systemically administered GTTR over time by confocal microscopy. GTTR is taken up by, and is toxic to, wild-type zebrafish neuromast hair cells. Neuromast hair cell uptake of GTTR is attenuated by high concentrations of extracellular calcium or unconjugated gentamicin and is blocked in mariner mutant zebrafish, suggestive of entry via the apical mechanotransduction channel. In murine cochleae, GTTR is preferentially taken up by the stria vascularis compared to the spiral ligament, peaking 3 h after intra-peritoneal injection, following GTTR kinetics in serum. Strial marginal cells display greater intensity of GTTR fluorescence compared to intermediate and basal cells. Immunofluorescent detection of gentamicin in the cochlea also revealed widespread cellular labeling throughout the cochlea, with preferential labeling of marginal cells. Only GTTR fluorescence displayed increasing cytoplasmic intensity with increasing concentration, unlike the cytoplasmic intensity of fluorescence from immunolabeled gentamicin. These data suggest that systemically administered aminoglycosides are trafficked from strial capillaries into marginal cells and clear into endolymph. If so, this will facilitate electrophoretically driven aminoglycoside entry into hair cells from endolymph. Trans-strial trafficking of aminoglycosides from strial capillaries to marginal cells will be dependent on as-yet-unidentified mechanisms that convey these drugs across the intra-strial electrical barrier and into marginal cells.     

A Comparison of Cochlear Microphonics and N1 in Audiogenic-Seizure-Susceptible and Control Rats1

Abstract

Although numerous studies have shown that cochlear impainnent exists in audiogenic-seizure (AGS)-susceptible mice, there is only one report of cochlear potentials obtained from AGS-susceptible rats. To investigate the hypothesis that cochlear impairment also exists in AGS rats, cochlear microphonics (CM) and the primary afferent activity of the auditory division of the eighth cranial nerve (N₁) were studied in AGS rats.

AGS rats were obtained from the Veterans Administration Medical Center (Shreveport, La.) colony of Sprague Dawley derived animals, and control rats were obtained from Sprague Dawley, Inc. Two school bells ringing simultaneously were used to produce a sound of approximately 115 dB (AGS test stimulus). Exposure to the AGS test stimulus was once per week for three consecutive weeks. Chloramphenicol was used to treat the frequent otitis media found in the colony of AGS rats.

Two categories of AGS-susceptible and control rats were studied: (1) rats exposed to the AGS test stimulus and chloramphenicol regimen; and (2) rats not receiving these treatments. All rats were anesthetized with i.p. Dial-Urethane and prepared for cochlear round window recording. Cochlear microphonics were recorded in response to pure tones of 4, 8, 12, and 15 KHz, and N₁ was recorded in response to a click stimulus. A significant decrease in cochlear sensitivity was seen in both groups of AGS rats when compared to appropriate controls as reflected by a 25-35 dB shift in all CM and N₁ input-output functions. These results support the hypothesis that a functional cochlear impairment exists in the AGS rat.