Diño M R, Schuerger R J, Liu Y, Slater N T, Mugnaini E
Northwestern University Institute for Neuroscience (NUIN), Chicago, IL 60611, USA.
Neuroscience. 2000;98(4):625-36. doi: 10.1016/s0306-4522(00)00123-8.
Unipolar brush cells are a class of interneurons in the granular layer of the mammalian cerebellum that receives excitatory mossy fiber synaptic input in the form of a giant glutamatergic synapse. Previously, it was shown that the unipolar brush cell axon branches within the granular layer, giving rise to large terminals. Single mossy fiber stimuli evoke a prolonged burst of firing in unipolar brush cells, which would be distributed to postsynaptic targets within the granular layer. Knowledge of the ultrastructure of the unipolar brush cell terminals and of the cellular identity of its postsynaptic targets is required to understand how unipolar brush cells contribute to information processing in the cerebellar circuit. To investigate the unipolar brush cell axon and its targets, unipolar brush cells were patch-clamped in fresh parasagittal slices from rat cerebellar vermis with electrodes filled with Lucifer Yellow and Biocytin, and examined by confocal fluorescence and electron microscopy. Biocytin was localized with diaminobenzidine chromogen or gold-conjugated, silver-intensified avidin. Light microscopic examination revealed a single thin axon emanating from the unipolar brush cell soma that gave rise to 2-3 axon collaterals terminating in mossy fiber-like rosettes in the granular layer, typically within a few hundred microm of the soma. In some cases, axon collaterals crossed the white matter within the same folium before terminating in the adjacent granular layer. Electron microscopic examination of serial ultrathin sections revealed that proximal unipolar brush cell axons and axon collaterals were unmyelinated and devoid of synaptic contacts. However, the rosette-shaped enlargements of each collateral formed the central component of glomeruli where they were surrounded by dendrites of granule cells and/or other unipolar brush cells, with which they formed asymmetric synaptic contacts. A long-latency repetitive burst of polysynaptic activity was observed in granule cells in this cerebellar region following white matter stimulation. The unipolar brush cell axons, therefore, form a system of cortex-intrinsic mossy fibers. The results indicate that synaptic excitation of unipolar brush cells by mossy fibers will drive a large population of granule cells, and thus will contribute a powerful form of distributed excitation within the basic circuit of the cerebellar cortex.
单极刷状细胞是哺乳动物小脑颗粒层中的一类中间神经元，它通过巨大的谷氨酸能突触接收兴奋性苔藓纤维突触输入。此前研究表明，单极刷状细胞轴突在颗粒层内分支，形成大的终末。单个苔藓纤维刺激可在单极刷状细胞中引发长时间的放电爆发，这种爆发会分布到颗粒层内的突触后靶点。为了理解单极刷状细胞如何在小脑回路中参与信息处理，需要了解单极刷状细胞终末的超微结构及其突触后靶点的细胞特性。为了研究单极刷状细胞轴突及其靶点，用充满路西法黄和生物胞素的电极在大鼠小脑蚓部新鲜矢状切片中对单极刷状细胞进行膜片钳记录，并通过共聚焦荧光显微镜和电子显微镜进行检查。生物胞素用二氨基联苯胺显色剂或金标记、银增强抗生物素蛋白进行定位。光学显微镜检查显示，从单极刷状细胞胞体发出一条单一的细轴突，该轴突产生2 - 3个轴突侧支，终止于颗粒层中苔藓纤维样的玫瑰花结，通常在胞体几百微米范围内。在某些情况下，轴突侧支在终止于相邻颗粒层之前会穿过同一小叶内的白质。对连续超薄切片的电子显微镜检查显示，单极刷状细胞近端轴突和轴突侧支无髓鞘且无突触接触。然而，每个侧支的玫瑰花结样膨大形成了小球的中央成分，它们被颗粒细胞和/或其他单极刷状细胞的树突所包围，并与之形成不对称突触接触。在该小脑区域的白质刺激后，在颗粒细胞中观察到了长时间延迟的多突触活动重复爆发。因此，单极刷状细胞轴突形成了一个皮质内源性苔藓纤维系统。结果表明，苔藓纤维对单极刷状细胞的突触兴奋将驱动大量颗粒细胞，从而在小脑皮质的基本回路中贡献一种强大的分布式兴奋形式。
