Nunzi M G, Russo M, Mugnaini E
Northwestern University Institute for Neuroscience, Searle Building, 5-474, 320 East Superior Street, Chicago, IL 60611, USA.
Neuroscience. 2003;122(2):359-71. doi: 10.1016/s0306-4522(03)00568-2.
Different isoforms of a vesicular glutamate transporter (VGLUT) mediate glutamate uptake into synaptic vesicles of excitatory neurons. There is agreement that the VGLUTs are differentially expressed in brain, and that two isoforms, VGLUT1 and VGLUT2, are localized to excitatory axon terminals in the cerebellar cortex. While granule cells express solely VGLUT1, there is no report about the VGLUT(s) of the unipolar brush cell (UBC), the second type of glutamatergic interneuron residing in the cerebellar granular layer. In the mouse, UBCs are particularly numerous in the uvula (lobule IX) and nodulus (lobule X). These folia contain two distinct subsets of UBCs: one kind expresses the calcium-binding protein calretinin (CR), and the other kind expresses the metabotropic glutamate receptor (mGluR) 1alpha. UBCs give rise to an extensive system of intrinsic mossy fibers (MF), whose terminals innervate granule cells and other UBCs, altogether similar to those formed by the extrinsic MFs. The presence of both extrinsic and intrinsic MFs in the vestibulocerebellum makes it difficult to determine which type of VGLUT is contained in MFs formed by the UBC axons. Hence, the nodulus was isolated from sagittal cerebellar slices from postnatal day 10 mice, and cultured for 15-20 days in vitro. Double immunofluorescence and confocal microscopy showed that mossy terminals of CR-positive (CR(+)) UBCs were immunoreactive for VGLUT1 and VGLUT2, while mossy terminals of mGluR1alpha-positive (mGluR1alpha(+)) UBCs were provided with VGLUT1 only. Moreover, CR(+) dendritic brushes were contacted by mossy terminals provided with both transporters, while mGluR1alpha(+) dendritic brushes were contacted by mossy terminals immunopositive for VGLUT1 and immunonegative for VGLUT2. These data indicate that the two UBC subsets use different modalities of vesicular glutamate storage and form separate networks. We consider it possible that expressions of CR with VGLUT1/VGLUT2 and mGluR1alpha(+) with VGLUT1 in the two subsets of vestibulocerebellar UBCs are determined by specific vestibular inputs, carried by groups of primary and/or secondary vestibular afferents.
囊泡谷氨酸转运体（VGLUT）的不同亚型介导谷氨酸摄取到兴奋性神经元的突触小泡中。人们一致认为，VGLUTs在大脑中存在差异表达，并且两种亚型VGLUT1和VGLUT2定位于小脑皮质的兴奋性轴突终末。虽然颗粒细胞仅表达VGLUT1，但关于单极刷细胞（UBC）的VGLUTs尚无报道，UBC是位于小脑颗粒层的第二类谷氨酸能中间神经元。在小鼠中，UBC在蚓垂（小叶IX）和小结（小叶X）中特别多。这些小叶包含两个不同的UBC亚群：一类表达钙结合蛋白钙视网膜蛋白（CR），另一类表达代谢型谷氨酸受体（mGluR）1α。UBC产生广泛的内源性苔藓纤维（MF）系统，其终末支配颗粒细胞和其他UBC，总体上类似于由外源性MF形成的那些。前庭小脑中外源性和内源性MF的存在使得难以确定UBC轴突形成的MF中包含哪种类型的VGLUT。因此，从小鼠出生后第10天的矢状小脑切片中分离出小结，并在体外培养15 - 20天。双重免疫荧光和共聚焦显微镜显示，CR阳性（CR(+)）UBC的苔藓终末对VGLUT1和VGLUT2呈免疫反应性，而mGluR1α阳性（mGluR1α(+)）UBC的苔藓终末仅具有VGLUT1。此外，CR(+)树突刷与同时具有两种转运体的苔藓终末接触，而mGluR1α(+)树突刷与对VGLUT1呈免疫阳性而对VGLUT2呈免疫阴性的苔藓终末接触。这些数据表明，两个UBC亚群使用不同的囊泡谷氨酸储存方式并形成独立的网络。我们认为，前庭小脑UBC的两个亚群中CR与VGLUT1/VGLUT2以及mGluR1α(+)与VGLUT1的表达可能由初级和/或次级前庭传入神经群体携带的特定前庭输入所决定。
