Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats
Postnatal refinement of auditory nerve projections to the cochlear nucleus in cats
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DOI：
10.1002/cne.10176
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发表时间：
2002-06-17
影响因子：
2.5
通讯作者：
Hradek, GT
中科院分区：
文献类型：
--
作者：
Leake, PA;Snyder, RL;Hradek, GT
对视觉系统发育的研究表明，活动驱动的竞争对于将最初的局部弥散的神经元投射细化为精确的成年模式至关重要。这导致了一种断言，即这一过程可能会影响整个神经系统的地形连接的发展。由于猫的听觉系统在出生时非常不成熟，听觉神经细胞最初表现出非常低的自发活动或没有自发活动，我们假设听觉神经纤维最初可能在耳蜗核(CN)内形成地形上广泛的投射，后来在成人样的频率选择性发展时变得在地形上精确。在这项研究中，我们进行了神经生物素的限制性注射，标记了耳蜗螺旋神经节的小区段(300-500微米)，以研究代表窄带频率的听神经纤维的投射。结果表明，在新生儿中，在功能性听力开始之前的许多天，甚至在听神经自发活动发展之前，从基础耳蜗到CN的投射就已经在音调上组织起来了。然而，结果也表明，听神经初级传入轴突的地形图特异性显著细化发生在妊娠晚期或出生后早期。在出生时或出生前，对CN的所有三个亚区的投射都显示出明显的强直组织，但围产期幼猫对频带板层纤维的地形限制明显低于成年猫。在耳蜗内间隔大于或等于2 mm的两次注射导致前腹侧CN内的投射轴突的标记带比预期的要宽53%，如果它们与成年动物的轴突成比例，并且在所研究的最年轻的动物中，这两个投射不完全分离。新生儿的后腹侧核(PVCN)投影(按CN大小归一化)比成人宽36%，最年轻的受试者两次注射的投影在PVCN中几乎融合。正常情况下，新生儿CN背侧部的投射比成人宽32%，但背侧CN投射总是离散的，即使在研究的最早年龄也是如此。
Studies of visual system development have suggested that competition driven by activity is essential for refinement of initial topographically diffuse neuronal projections into their precise adult patterns. This has led to the assertion that this process may shape development of topographic connections throughout the nervous system. Because the cat auditory system is very immature at birth, with auditory nerve neurons initially exhibiting very low or no spontaneous activity, we hypothesized that the auditory nerve fibers might initially form topographically broad projections within the cochlear nuclei (CN), which later would become topographically precise at the time when adult-like frequency selectivity develops. In this study, we made restricted injections of Neurobiotin, which labeled small sectors (300-500 mum) of the cochlear spiral ganglion, to study the projections of auditory nerve fibers representing a narrow band of frequencies. Results showed that projections from the basal cochlea to the CN are tonotopically organized in neonates, many days before the onset of functional hearing and even prior to the development of spontaneous activity in the auditory nerve. However, results also demonstrated that significant refinement of the topographic specificity of the primary afferent axons of the auditory nerve occurs in late gestation or early postnatal development. Projections to all three subdivisions of the CN exhibit clear tonotopic organization at or before birth, but the topographic restriction of fibers into frequency band laminae is significantly less precise in perinatal kittens than in adult cats. Two injections spaced greater than or equal to2 mm apart in the cochlea resulted in labeled bands of projecting axons in the anteroventral CN that were 53% broader than would be expected if they were proportional to those in adults, and the two projections were incompletely segregated in the youngest animals studied. Posteroventral CN (PVCN) projections (normalized for CN size) were 36% broader in neonates than in adults, and projections from double injections in the youngest subjects were nearly fused in the PVCN. Projections to the dorsal division of the CN were 32% broader in neonates than in adults when normalized, but the dorsal CN projections were always discrete, even at the earliest ages studied.