To investigate the characteristics of cochlear microphonic (CM), summating potential (SP) and compound action potential (CAP) and morphology changes of outer hair cells (OHCs) and inner hair cells (IHCs) after OHCs were selectively damaged by acute cochlear perfusion of cisplatin. Dynamic changes of CM, SP and CAP were recorded from the scala media of the third turn of the guinea pig’s cochlea during Cisplatin perfusion into the whole cochlea for 2 hours. The results indicated that after one hour of the perfusion, the amplitude of CM, SP and CAP decreased at stimulus intensity < 60 dB SPL, while the amplitude increased at stimulus intensity > 70 dB SPL, as compared to those examined before perfusion of Cisplatin (the average of CM increased by 3.6 mV at 90 dB SPL, average of SP increased by 1.6 mV at 120 dB SPL, average of CAP increased by 0.23 mV at 90dB SPL). After two hours of perfusion, the amplitude of CM, SP and CAP decreased in all stimulus intensities. The staining of Succinic Dehydrogenase (SDH) was weak in OHCs, while the stain ing of IHCs remained normal. transmission electron microscopic examination of organ of Corti showed varieties of morphological changes in OHCs, such as nucleus chromosome disappear, mitochondria denaturalization and number reduction, while the structure of IHC appeared normal. SDH staining showed normal staining pattern in both IHCs and OHCs when the cochlea was perfused with artificial perilymph for 2 hours. The abnormal augmentation phenomenon of CM and SP may be induced by the abnormal control of Ca++ in IHCs and OHCs. The abnormal augmentation of CAP suggests that the suppression effects of OHCs and efferent neurotransmitter on IHCs and afferent neurotransmitter may be reduced after OHCs damaged.
目的观察外源性谷氨酰胺合成酶对噪声暴露引起豚鼠听力损失的保护作用。方法应用豚鼠全耳蜗灌流技术,右耳耳蜗灌流人工外淋巴液和不同浓度的谷氨酰胺合成酶2小时,同时持续性给予右耳白噪声2小时,分别记录噪声暴露前和噪声暴露后的耳蜗微音电位(CM);并且应用透射电镜技术观察噪声暴露前后耳蜗形态学的变化。结果全耳蜗灌流人工外淋巴液同时给予100 dB SPL的白噪声2小时,CM幅度显著下降,并且其非线性特点消失,复合动作电位(CAP)阈值为79．5 dB SPL;全耳蜗灌流0．5μ/L和1μ/L的谷氨酰胺合成酶同时给予100 dB SPL的白噪声2小时,CM幅度下降减轻,但CM的非线性特点仍不存在,CAP阈值升高,其中灌流1μ/L的谷氨酰胺合成酶时CM幅度和CAP阈值的恢复更明显。形态学显示1μ/L谷氨酰胺合成酶+噪声组内毛细胞及其下方传入神经纤维基本正常,但是外毛细胞仍存在空化。结论谷氨酰胺合成酶通过摄取噪声暴露时过度释放的谷氨酸,对噪声性听力损失有部分保护作用。
To investigate the glutamate-like immunoreactivity (Glu-IR) changes in inner hair cells (IHCs) of guinea pigs cochlear after noise exposure. Guinea pigs were distributed into 6 groups including control group (gl), and groups of immediately (g2), 8 hours (g3), 1 day (g4), 3 days (g5) and 7days (g6) after noise exposure. The experimental groups were exposed to 120 dBLp 1/3 octave 4 kHz narrow band noise for 4 hours to ruin the organ of Corti. The first turn of the organ of Corti was dissected to make ultrathin sections and under went immunoelectron study. The density of gold particles of Glu-IR in IHCs was measured. Results showed that Glu-IR in IHCs after noise exposure changed dynamically. The density of gold particle in IHCs of g2 significantly increased (p < 0.001) compared with gl and decreased in g3 (p < 0.001), whereas there were no significant difference in the groups of g4, g5 and g6 (p > 0.05). The results suggest that there may be glutamate autoreceptors in the membrane of IHCs and the glutamate released from pre-synapse may act on these autoreceptors in a postitive feedback manner, which induced [Ca^2+] increased in IHCs immediately after noise exposure. Glutamate synthetase system was activated by Ca^2+ and more glutamic acid was produced. In the group of 8 hours after noise exposure, glutamic acid decreased in IHCs because of its over-releasing into synapses. Glu-IR's recovery to normal level indicated that the glutamateglutamine cycle may exist in guinea pig cochlea.
