谷氨酸对豚鼠耳蜗传入神经元的兴奋毒性损害

Excitotoxic effect of glutamate on the afferent neurons in guinea pigs

目的 :观察不同剂量谷氨酸导致豚鼠耳蜗传入神经元兴奋性损伤性质及耳蜗形态、电生理改变。方法 :经耳蜗鼓阶灌注高浓度谷氨酸 (Glu H ,2 0mmol/L ,1 0 μl)、低浓度谷氨酸 (Glu L ,1 0mmol/L ,1 0 μl)后检测不同时点复合动作电位 (CAP)反应阈 ,并观察耳蜗显微和超微结构变化。结果 :①Glu L组 (≤ 1d)及Glu H组CAP反应阈与正常对照组比较明显提高 (P <0 .0 0 1 ) ;Glu L组 (≥ 1周 )CAP反应阈与正常对照组比较差异无统计学意义 (P >0 .0 5 )。②两给药组 (Glu H ,Glu L)蜗轴半薄切片Ⅰ型螺旋神经节细胞部分出现空泡样变化 ,胞浆变淡 ,胞核皱缩 ;正常对照组与给药组 (Glu H)之间螺旋神经节细胞病变率比较差异有统计学意义 (P <0 .0 5 )。③两给药组电镜下耳蜗传入神经树突末梢、有髓神经纤维、Ⅰ型螺旋神经节细胞有变性坏死 ,呈明显的剂量依赖性 ,Glu L组的神经元病变 4周内可基本恢复 ,而Glu H组进行性加重 ,由变性发展到坏死。结论 :谷氨酸鼓阶灌注可导致豚鼠耳蜗传入神经元形态和功能的损害。小剂量 (1 0mmol/L)的谷氨酸可导致耳蜗传入神经系统的可逆性损害 ,而大剂量 (2 0mmol/L)的则可导致不可逆的神经元损害。

Objective To compare Glu induced excitotoxic damage to the cochlear afferent neurons and changes of cochlear electrophysiology by directly injecting two different concentrations of glutamate into the inner ear of adult guinea pigs. Methods Guinea pigs were injected high doses glutamate ( Glu H ,20 mmol/L, 10 μl) and low doses of glutamate (Glu L, 10 mmol/L, 10 μl) into the perilymph of scala tympani.The compound action potential (CAP) was recorded and pathologic changes were evaluated with light and transmission electronic microscope at various time points after the application of glutamate.Results The thresholds of the CAP of Glu L group (10 mmol/L,less than one day) and Glu H group (20 mmol/L) were significantly increased compared with those of the normal group( P <0.001). But the thresholds of CAP of glutamate application(10 mmol/L,more than one week) weren′t significantly different from those of the normal group ( P >0.05). Some changes of the spiral ganglion cells of the cochlear axis were observed in the 2 drug groups by light microscope. There were some vacuoles in the cell body and condensation of nuclear chromatin, shallow cellar liquid. The ratios of pathological changes between the control group and Glu H group of different concentrations were notably different ( P <0.05). The observation under transmission electronic microscope found disvolution or necrosis of afferent nerve endings, myelinated nerve fibers, and model spiral ganglion cells. The pathological changes of the Glu H group were more serious than those of the Glu L group (10 mmol/L) . The pathological changes of the Glu L group would recover in 4 weeks, but those of the drug group would gradually deteriorate and degeneration would become necrosis.Conclusion The morphologic changes and functional losses of afferent neurons in guinea pigs were caused by glutamate in scala tympani. The low dose(10 mmol/L) had induced reversible damage to cochlear afferent neurons. The high dose (20 mmol/L) had triggered irreversible neurodegeneration.