Murinson Beth Brianna, Hoffman Paul Ned, Banihashemi Michael Reza, Meyer Richard Arthur, Griffin John Wesley
Department of Neurology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, USA.
J Comp Neurol. 2005 Apr 18;484(4):392-402. doi: 10.1002/cne.20506.
Unmyelinated nerve fibers (Remak bundles) in the rodent sciatic nerve typically contain multiple axons. This study asked whether C-fiber bundles contain axons arising from more than one type of neuron. Most small neurons of the lumbar dorsal root ganglion (DRG) are either glial cell line-derived neurotrophic factor dependent or nerve growth factor dependent, binding either isolectin B4 (IB4) or antibodies to calcitonin gene-related peptide (CGRP), respectively. Injection of IB4-conjugated horseradish peroxidase into a lumbar DRG resulted in intense labeling of IB4 axons, with very low background. Visualized by confocal fluorescence, IB4-binding and CGRP-positive nerve fibers originating from different DRG neurons came together and remained closely parallel over long distances, suggesting that these two types of axon occupy the same Remak bundle. With double-labeling immunogold electron microscopy (EM), we confirmed that IB4 and CGRP axons were distinct and were found together in single Remak bundles. Previous studies indicate that some DRG neurons express both CGRP and IB4 binding. To ensure that our immunogold results were not a consequence of coexpression, we studied large populations of unmyelinated axons by using quantitative single-label EM. Tetramethylbenzidine, a chromogen with strong intrinsic signal amplification of IB4-horseradish peroxidase, labeled as many as 52% of unmyelinated axons in the dorsal root. Concomitantly, 97% of the Remak bundles with more than one axon contained at least one IB4-labeled axon. Probabilistic modeling using binomial distribution functions rejected the hypothesis that IB4 axons segregate into IB4-specific bundles (P < 0.00001). We conclude that most Remak bundle Schwann cells simultaneously support diverse axon types with different growth factor dependences.
啮齿动物坐骨神经中的无髓神经纤维（雷马克束）通常包含多条轴突。本研究探讨了C纤维束是否包含源自不止一种类型神经元的轴突。腰段背根神经节（DRG）的大多数小神经元要么依赖胶质细胞系衍生的神经营养因子，要么依赖神经生长因子，分别结合异凝集素B4（IB4）或降钙素基因相关肽（CGRP）抗体。将IB4偶联的辣根过氧化物酶注入腰段DRG，导致IB4轴突强烈标记，背景非常低。通过共聚焦荧光观察，源自不同DRG神经元的IB4结合和CGRP阳性神经纤维聚集在一起，并在很长距离内保持紧密平行，表明这两种类型的轴突占据同一雷马克束。通过双标记免疫金电子显微镜（EM），我们证实IB4和CGRP轴突是不同的，并且在单个雷马克束中同时存在。先前的研究表明，一些DRG神经元同时表达CGRP和IB4结合。为确保我们的免疫金结果不是共表达的结果，我们通过使用定量单标记EM研究了大量无髓轴突。四甲基联苯胺是一种对IB4-辣根过氧化物酶具有强固有信号放大作用的显色剂，标记了背根中多达52%的无髓轴突。同时，97%的含有不止一条轴突的雷马克束至少包含一条IB4标记的轴突。使用二项分布函数的概率建模拒绝了IB4轴突分离到IB4特异性束中的假设（P < 0.00001）。我们得出结论，大多数雷马克束施万细胞同时支持具有不同生长因子依赖性的多种轴突类型。
