Ralston D D, Milroy A M
Department of Anatomy, University of California, San Francisco 94143-0452.
J Comp Neurol. 1992 Jun 1;320(1):97-109. doi: 10.1002/cne.903200107.
Rubrospinal neurons of the magnocellular division of the red nucleus of Macaca fascicularis were retrogradely labeled following spinal cord microinjections of wheat germ agglutinin-horseradish peroxidase, as demonstrated by the chromagen tetramethylbenzidine, identifying the mesencephalic cells of origin of this descending motor pathway. The tissue was processed for electron microscopy and subsequently tested on the electron microscope grid for immunoreactivity of gamma aminobutyric acid (GABA) in presumed local circuit neuronal somata, in dendrites, and in axonal terminals. Results demonstrate the presence of retrogradely labeled rubrospinal neurons of medium and large diameters (30-90 microns) and immunoreactive neurons of small size (less than 20 microns in diameter) within the nucleus. In addition, there are substantial numbers of GABAergic, presumably inhibitory, synaptic structures contacting somata and primary, medium, and small sized dendrites, as well as spineheads of rubrospinal neurons. The immunoreactive presynaptic profiles exhibit two different morphological appearances: one axonal and the other dendritic. Axonal terminals contain densely packed pleomorphic to flattened vesicles and form primarily symmetrical synapses with somata and all regions of the dendritic arbor. GABAergic profiles resembling presynaptic dendrites (PSDs) are also present. These profiles possess scattered flattened to pleomorphic synaptic vesicles in a translucent cytoplasm and are often postsynaptic to axonal terminals of unknown origin, or to GABAergic profiles. GABAergic local circuit neurons (LCNs), the neurites of which remain within the confines of the nucleus, appear to be contacted primarily by cortical and cerebellar afferents. These LCNs may or may not possess axons and thus may represent both the source of the GABAergic axonal terminals as well as that of the PSDs. Inhibitory afferents from other sources, such as the mesencephalic reticular formation, may also account for GABAergic terminals involved in this inhibition. We propose that the level of excitability of rubrospinal neurons and their subsequent activation of spinal motor neurons and interneurons is significantly regulated by the local circuit GABAergic inhibitory interneuronal population of the nucleus proper and probably by axons entering the nucleus from an extranuclear source.
食蟹猴红核大细胞部的红核脊髓神经元在脊髓微量注射小麦胚凝集素 - 辣根过氧化物酶后被逆行标记，用发色剂四甲基联苯胺证实，确定了这条下行运动通路的中脑起源细胞。对组织进行电子显微镜处理，随后在电子显微镜网格上检测假定的局部回路神经元胞体、树突和轴突终末中γ - 氨基丁酸（GABA）的免疫反应性。结果表明，在核内存在逆行标记的中、大直径（30 - 90微米）的红核脊髓神经元以及小尺寸（直径小于20微米）的免疫反应性神经元。此外，有大量的GABA能突触结构，推测为抑制性的，与红核脊髓神经元的胞体、初级、中级和小尺寸树突以及棘头相接触。免疫反应性突触前轮廓呈现两种不同的形态外观：一种是轴突的，另一种是树突的。轴突终末含有密集堆积的多形性至扁平小泡，主要与胞体和树突分支的所有区域形成对称突触。也存在类似突触前树突（PSD）的GABA能轮廓。这些轮廓在半透明的细胞质中具有分散的扁平至多形性突触小泡，并且常常是来自未知起源的轴突终末或GABA能轮廓的突触后结构。GABA能局部回路神经元（LCN），其神经突保留在核的范围内，似乎主要由皮质和小脑传入纤维接触。这些LCN可能有或没有轴突，因此可能代表GABA能轴突终末以及PSD的来源。来自其他来源的抑制性传入纤维，如中脑网状结构，也可能解释参与这种抑制的GABA能终末。我们提出，红核脊髓神经元的兴奋性水平及其随后对脊髓运动神经元和中间神经元的激活受到固有核内局部回路GABA能抑制性中间神经元群体以及可能从核外来源进入核内的轴突的显著调节。
