Hubert G W, Paquet M, Smith Y
Yerkes Regional Primate Research Center, Division of Neuroscience and Department of Neurology, Emory University, Atlanta, Georgia 30322, USA.
J Neurosci. 2001 Mar 15;21(6):1838-47. doi: 10.1523/JNEUROSCI.21-06-01838.2001.
Neurons in the rat substantia nigra (SN) are enriched in group I metabotropic glutamate receptor (mGluR) subtypes and respond to group I mGluR activation. To better understand the mechanisms by which mGluR1 and mGluR5 mediate these effects, the goal of this study was to elucidate the subsynaptic localization of these two receptor subtypes in the rat and monkey substantia nigra. At the light microscope level, neurons of the SN pars reticulata (SNr) displayed moderate to strong immunoreactivity for both mGluR1a and mGluR5 in rats and monkeys. However, mGluR1a labeling was much stronger in monkey than in rat SN pars compacta (SNc) neurons, whereas a moderate level of mGluR5 immunoreactivity was found in both species. At the electron microscope level, the immunoreactivity for both group I mGluR subtypes was primarily expressed postsynaptically, although light mGluR1a labeling was occasionally seen in axon terminals in the rat SNr. Immunogold studies revealed a striking difference in the subcellular distribution of mGluR1a and mGluR5 immunoreactivity in SNr and SNc neurons. Although the bulk of mGluR1a was attached to the plasma membrane, >80% of mGluR5 immunoreactivity was intracellular. Plasma membrane-bound immunoreactivity for group I mGluRs in both SNc and SNr neurons was mostly extrasynaptic or in the main body of symmetric, putative GABAergic synapses. On the other hand, asymmetric synapses either were nonimmunoreactive or displayed perisynaptic labeling. These data raise important questions about the trafficking, internalization, and potential functions of group I mGluRs at extrasynaptic sites or symmetric synapses in the substantia nigra.
大鼠黑质（SN）中的神经元富含I组代谢型谷氨酸受体（mGluR）亚型，并对I组mGluR激活产生反应。为了更好地理解mGluR1和mGluR5介导这些效应的机制，本研究的目的是阐明这两种受体亚型在大鼠和猴黑质中的突触下定位。在光学显微镜水平，大鼠和猴的黑质网状部（SNr）神经元对mGluR1a和mGluR5均显示出中度至强免疫反应性。然而，mGluR1a标记在猴中的强度远高于大鼠黑质致密部（SNc）神经元，而在两个物种中均发现mGluR5免疫反应性处于中等水平。在电子显微镜水平，I组mGluR亚型的免疫反应性主要在突触后表达，尽管在大鼠SNr的轴突终末偶尔可见轻度的mGluR1a标记。免疫金研究揭示了SNr和SNc神经元中mGluR1a和mGluR5免疫反应性在亚细胞分布上的显著差异。虽然大部分mGluR1a附着于质膜，但>80%的mGluR5免疫反应性位于细胞内。SNc和SNr神经元中I组mGluRs的质膜结合免疫反应性大多位于突触外或对称的、假定的GABA能突触主体中。另一方面，不对称突触要么无免疫反应性，要么显示突触周围标记。这些数据引发了关于I组mGluRs在黑质突触外位点或对称突触处的运输、内化和潜在功能的重要问题。
