Park T J, Pollak G D
Department of Zoology, University of Texas at Austin 78712.
J Neurosci. 1993 Dec;13(12):5172-87. doi: 10.1523/JNEUROSCI.13-12-05172.1993.
Many neurons in the auditory forebrain of the mustache bat act as coincidence detectors for signals separated in time by up to 20 msec. Differences in path lengths cannot adequately explain how the nervous system delays one signal relative to the other to such a large degree. Several researchers have proposed that an inhibitory mechanism might account for long delays, but it has not been known where these delays are created. Previous studies, using a variety of mammals, have reported that the inferior colliculus contains some cells with much longer latencies than those of cells in lower auditory centers, suggesting that the inferior colliculus might be the site where long delays are generated. We characterized the latencies of cells in the 60 kHz contour of the mustache bat inferior colliculus and examined how GABAergic inhibition affected the latencies of those cells. Evaluations of the influence of GABA were made by documenting changes in response latency that occurred when GABAergic inputs were reversibly blocked by iontophoretic application of the GABAA antagonist bicuculline. Prior to bicuculline application, latencies varied over a wide range among the population of cells and we observed a pattern of latency changes with dorsoventral location. The pattern was that the population of neurons in the dorsal regions of the inferior colliculus had a wide range of latencies while the population in more ventral regions had progressively narrower latency ranges. Thus, while some cells at each depth had comparably short latencies, the average latency of the population at a given depth was long in the dorsal inferior colliculus and became progressively shorter ventrally. The same characteristic distribution of latencies and pattern of latency changes with depth were observed for cells that had different aural preferences, different rate-intensity functions, and different discharge patterns, suggesting that latency is an important organizational feature of the inferior colliculus. Bicuculline substantially shortened latency in about half of the cells studied, and it dramatically altered the pattern of latency changes with depth. These results suggest that GABA normally lengthens response latencies and creates a dorsoventral grading of delays in the inferior colliculus. This wide range of latencies could provide the large latency differences necessary for the coincidence detectors in the medial geniculate body tuned to signals separated by up to 20 msec.
许多髯蝠听觉前脑的神经元充当时间上相隔长达20毫秒的信号的重合探测器。路径长度的差异无法充分解释神经系统如何将一个信号相对于另一个信号延迟到如此大的程度。几位研究人员提出，一种抑制机制可能是造成长时间延迟的原因，但一直不清楚这些延迟是在哪里产生的。以前使用各种哺乳动物进行的研究报告称，下丘包含一些潜伏期比下听觉中枢细胞长得多的细胞，这表明下丘可能是产生长时间延迟的部位。我们对髯蝠下丘60千赫兹轮廓处细胞的潜伏期进行了表征，并研究了GABA能抑制如何影响这些细胞的潜伏期。通过记录当GABAA拮抗剂荷包牡丹碱通过离子电渗法可逆性阻断GABA能输入时发生的反应潜伏期变化，来评估GABA的影响。在应用荷包牡丹碱之前，潜伏期在细胞群体中变化范围很广，并且我们观察到潜伏期随背腹位置变化的模式。这种模式是，下丘背侧区域的神经元群体潜伏期范围很广，而更腹侧区域的群体潜伏期范围逐渐变窄。因此，虽然每个深度的一些细胞具有相当短的潜伏期，但给定深度处群体的平均潜伏期在背侧下丘较长，并且在腹侧逐渐变短。对于具有不同听觉偏好、不同速率-强度函数和不同放电模式的细胞，观察到相同的潜伏期特征分布和潜伏期随深度变化的模式，这表明潜伏期是下丘的一个重要组织特征。荷包牡丹碱在大约一半研究的细胞中显著缩短了潜伏期，并且它极大地改变了潜伏期随深度变化的模式。这些结果表明，GABA通常会延长反应潜伏期，并在下丘中产生背腹延迟分级。这种广泛的潜伏期范围可以为内侧膝状体中调谐到相隔长达20毫秒信号的重合探测器提供所需的大潜伏期差异。
