Born D E, Rubel E W
Department of Otolaryngology, University of Virginia School of Medicine, Charlottesville 22908.
J Neurosci. 1988 Mar;8(3):901-19. doi: 10.1523/JNEUROSCI.08-03-00901.1988.
Studies of the avian auditory system indicate that neurons in nucleus magnocellularis (NM) and nucleus laminaris of young animals are dramatically altered by changes in the auditory receptor. We examined the role of presynaptic activity on these transneuronal regulatory events. TTX was used to block action potentials in the auditory nerve. TTX injections into the perilymph reliably blocked all neuronal activity in the cochlear nerve and NM. Far-field recordings of sound-evoked potentials revealed that responses returned within 6-12 hr after a single TTX injection. Changes in protein synthesis by NM neurons were measured by determining the incorporation of 3H-leucine using autoradiography. NM neurons on the side of the brain ipsilateral to the TTX injection were compared to normally active cells on the other side of the same tissue section. Grain counts over individual neurons revealed that a single injection of TTX produced a 40% decrease in grain density in ipsilateral NM neurons within 1.5 hr after the TTX injection. However, by 24 hr after a single TTX injection, grain densities were not different on the 2 sides of the brain. Continuous activity blockade for 6 hr caused the cessation of amino acid incorporation in a portion of NM neurons and a 15-20% decrease in the remaining neurons. These changes in amino acid incorporation are comparable to those following complete removal of the cochlea (Steward and Rubel, 1985). We also examined NM for neuron loss and soma shrinkage after blocking eighth nerve action potentials. TTX injected every 12 hr for 48 hr caused a 20% neuron loss and an 8% shrinkage of the remaining neurons. Similar reductions were found following cochlea removal (Born and Rubel, 1985). It is concluded that neuronal activity plays a major role in the maintenance of normal NM neurons. Furthermore, these results suggest that transneuronal morphological changes seen in neurons following deafferentation or alterations of sensory experience are a result of changes in the level of presynaptic activity.
对鸟类听觉系统的研究表明，幼小动物大细胞神经核（NM）和层状神经核中的神经元会因听觉感受器的变化而发生显著改变。我们研究了突触前活动在这些跨神经元调节事件中的作用。使用河豚毒素（TTX）阻断听神经中的动作电位。将TTX注入外淋巴可靠地阻断了耳蜗神经和NM中的所有神经元活动。声音诱发电位的远场记录显示，单次注射TTX后6 - 12小时内反应恢复。通过放射自显影测定3H - 亮氨酸的掺入来测量NM神经元蛋白质合成的变化。将注射TTX一侧大脑同侧的NM神经元与同一组织切片另一侧正常活动的细胞进行比较。对单个神经元的银粒计数显示，单次注射TTX后1.5小时内，同侧NM神经元的银粒密度降低了40%。然而，单次注射TTX后24小时，大脑两侧的银粒密度没有差异。持续6小时的活动阻断导致一部分NM神经元停止氨基酸掺入，其余神经元减少15 - 20%。这些氨基酸掺入的变化与完全切除耳蜗后的变化相当（Steward和Rubel，1985）。我们还研究了阻断第八神经动作电位后NM中的神经元丢失和胞体萎缩情况。每12小时注射一次TTX，持续48小时，导致20%的神经元丢失，其余神经元萎缩8%。在切除耳蜗后也发现了类似的减少情况（Born和Rubel，1985）。得出的结论是，神经元活动在维持正常NM神经元中起主要作用。此外，这些结果表明，在去传入或感觉经验改变后神经元中看到的跨神经元形态变化是突触前活动水平变化的结果。
