Brown M C, Kujawa S G, Liberman M C
Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts 02114, USA.
J Neurophysiol. 1998 Jun;79(6):3088-97. doi: 10.1152/jn.1998.79.6.3088.
Previous studies have shown that daily, moderate-level sound exposure, or conditioning, can reduce injury from a subsequent high-level noise exposure. We tested the hypothesis that this conditioning produces an increased activity in the olivocochlear efferent reflex, a reflex known to provide protection to the cochlea. Guinea pigs were conditioned by a 10-day intermittent exposure to 2-4 kHz noise at 85 dB sound pressure level. This conditioning is known to reduce damage from a subsequent high-level exposure to the same noise band. Responses to monaural and binaural sound were recorded from single medial olivocochlear (MOC) efferent neurons, and data from conditioned animals were compared with those obtained from unexposed controls. MOC neurons were classified by their response to noise bursts in the ipsilateral or contralateral ears as ipsi units, contra units, or either-ear units. There were no significant differences in the distributions of these unit types between control and conditioned animals. There were also no differences in other responses to monaural stimuli, including the distribution of characteristic frequencies (CFs), the sharpness of tuning, or thresholds at the CF. For binaural sound at high levels, particularly relevant to sound-evoked activation of the MOC reflex during acoustic overstimulation, the firing rates of MOC neurons with CFs just above the conditioning band showed slight (but statistically significant) elevations relative to control animals. Frequency regions just above the conditioning band also demonstrated maximum conditioning-related protection; thus protection could be due, in part, to long-term changes in MOC discharge rates. For binaural sound at low levels, MOC firing rates in conditioned animals also were increased significantly relative to controls. Again, increases were largest for neurons with CFs just above the conditioning band. For equivalent monaural sound, rates were not significantly increased; thus, conditioning appears to increase binaural facilitation by opposite-ear sound. These data indicate that MOC neurons show long-term plasticity in acoustic responsiveness that is dependent on their acoustic history.
先前的研究表明，每日进行适度水平的声音暴露或预处理，可减轻后续高水平噪声暴露造成的损伤。我们检验了这样一个假设：这种预处理会使橄榄耳蜗传出反射的活动增强，该反射已知可对耳蜗起到保护作用。豚鼠通过在85分贝声压级下间歇性暴露于2 - 4千赫噪声10天进行预处理。已知这种预处理可减少后续高水平暴露于同一噪声频段所造成的损伤。从单个内侧橄榄耳蜗（MOC）传出神经元记录对单耳和双耳声音的反应，并将预处理动物的数据与未暴露对照组的数据进行比较。MOC神经元根据其对同侧或对侧耳噪声猝发的反应分为同侧单位、对侧单位或双耳单位。在对照组和预处理动物之间，这些单位类型的分布没有显著差异。对单耳刺激的其他反应，包括特征频率（CFs）的分布、调谐锐度或CF处的阈值，也没有差异。对于高水平的双耳声音，特别是与声学过度刺激期间MOC反射的声音诱发激活相关的声音，CFs略高于预处理频段的MOC神经元的放电率相对于对照动物略有升高（但具有统计学意义）。预处理频段上方的频率区域也表现出与预处理相关的最大保护作用；因此，保护作用可能部分归因于MOC放电率的长期变化。对于低水平的双耳声音，预处理动物中的MOC放电率相对于对照组也显著增加。同样，CFs略高于预处理频段的神经元增加幅度最大。对于等效的单耳声音，放电率没有显著增加；因此，预处理似乎增强了对侧耳声音的双耳易化作用。这些数据表明，MOC神经元在听觉反应性方面表现出长期可塑性，这取决于它们的听觉经历。
