The auditory sensory organs appear to be less damaged by exposure to high-level noise that is presented after exposure to non-traumatizing low-level noise. This phenomenon is known as the toughening or conditioning effect. Functionally, it is manifested by a reduced threshold shift, and morphologically by a reduced hair cell loss. However, it remains unclear whether prior exposure to toughening noise can mitigate the synaptic loss induced by exposure to damaging noise. Since the cochlear afferent synapse between the inner hair cells and primary auditory neurons has been identified as a novel site involved in noise-induced cochlear damage, we were interested in assessing whether this synapse can be toughened. In the present study, the synaptic loss was induced by a damaging noise exposure (106 dB SPL) and compared across Guinea pigs who had and had not been previously exposed to a toughening noise (85 dB SPL). Results revealed that the toughening noise heavily reduced the synaptic loss observed 1 day after exposure to the damaging noise. Although it was significant, the protective effect of the toughening noise on permanent synaptic loss was much smaller. Compared with cases in the control group without noise exposure, coding deficits were seen in both toughened groups, as reflected in the compound action potential (CAP) by signals with amplitude modulation. In general, the pre-exposure to the toughening noise resulted in a significantly reduced synaptic loss by the high-level noise. However, this morphological protection was not accompanied by a robust functional benefit.

中文翻译：

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预先暴露于较低级别的噪声可减轻由较高级别的噪声引起的耳蜗突触损失

听觉感觉器官在暴露于非创伤性​​低电平噪声后出现的高电平噪声似乎较少受到损害。这种现象被称为增韧或调理效应。在功能上，它表现为阈值偏移减少，形态上表现为毛细胞损失减少。然而，目前尚不清楚先前暴露于增韧噪声是否可以减轻因暴露于破坏性噪声而引起的突触损失。由于内毛细胞和初级听觉神经元之间的耳蜗传入突触已被确定为参与噪声诱导的耳蜗损伤的新位点，我们有兴趣评估该突触是否可以变硬。在目前的研究中，突触损失是由破坏性噪声暴露 (106 dB SPL) 引起的，并在之前暴露于和未暴露于强韧噪声 (85 dB SPL) 的豚鼠之间进行比较。结果表明，在暴露于破坏性噪音后 1 天，强化噪音大大减少了突触损失。虽然它很重要，但增韧噪声对永久性突触损失的保护作用要小得多。与没有噪声暴露的对照组病例相比，两个强化组都出现了编码缺陷，这反映在幅度调制信号的复合动作电位 (CAP) 中。一般来说，预先暴露于增韧噪声会导致高水平噪声显着减少突触损失。然而，这种形态保护并没有伴随着强大的功能优势。