Kinzie J M, Shinohara M M, van den Pol A N, Westbrook G L, Segerson T P
Vollum Institute, Department of Medicine, Oregon Health Sciences University, Portland 97201-3098, USA.
J Comp Neurol. 1997 Sep 1;385(3):372-84.
Metabotropic glutamate receptors (mGluRs) constitute a large family of G-protein-coupled receptors that are subdivided into three groups based on sequence similarity, pharmacological profiles, and coupling to second messengers. Although mRNAs for seven of the eight mGluRs are expressed in the olfactory system, the localization and function of specific subtypes have not been fully characterized. Mitral cells of the olfactory bulb express mRNA for several mGluRs, including mGluR7, which has been suggested as a presynaptic glutamate autoreceptor. To investigate the immunolocalization of mGluR7 in the olfactory system, we used a polyclonal antiserum specific for the carboxy terminus of the receptor. Mitral cell somata and proximal dendrites were strongly labeled by the mGluR7 antibody. Electron microscopic analysis revealed that most of the mitral cell somatic staining was cytoplasmic. In olfactory bulb glomeruli, immunoreactivity was present in axons and dendrites. In the piriform cortex, diffuse staining was present in layer Ia that was markedly reduced following bulbectomy, consistent with expression of mGluR7 in mitral cell axon terminals. Electron microscopic analysis of this region confirmed the presence of mGluR7 in multiple axon terminals. Distinct labeled fibers in all levels of layer I appeared to originate from labeled piriform cortex pyramidal cells in layers II and III. Our results indicate that mGluR7 is primarily presynaptic at olfactory bulb synapses. However, the postsynaptic localization of mGluR7 at selected synapses indicates that mGluR7 is not targeted exclusively to axonal compartments.
代谢型谷氨酸受体（mGluRs）构成了一个庞大的G蛋白偶联受体家族，根据序列相似性、药理学特性以及与第二信使的偶联情况可分为三组。尽管8种mGluRs中的7种的mRNA在嗅觉系统中表达，但特定亚型的定位和功能尚未完全明确。嗅球的二尖瓣细胞表达多种mGluRs的mRNA，包括mGluR7，有人认为它是一种突触前谷氨酸自身受体。为了研究mGluR7在嗅觉系统中的免疫定位，我们使用了一种针对该受体羧基末端的多克隆抗血清。mGluR7抗体强烈标记了二尖瓣细胞的胞体和近端树突。电子显微镜分析显示，二尖瓣细胞胞体的大部分染色位于细胞质中。在嗅球小球中，轴突和树突存在免疫反应性。在梨状皮质中，Ia层存在弥漫性染色，在切除嗅球后明显减少，这与mGluR7在二尖瓣细胞轴突终末的表达一致。对该区域的电子显微镜分析证实了多个轴突终末中存在mGluR7。I层各水平中明显标记的纤维似乎起源于II层和III层中标记的梨状皮质锥体细胞。我们的结果表明，mGluR7在嗅球突触处主要位于突触前。然而，mGluR7在选定突触处的突触后定位表明，mGluR7并非仅靶向轴突区室。
