IMMUNOLOCALIZATION OF METABOTROPIC GLUTAMATE-RECEPTOR-7 IN THE RAT OLFACTORY-BULB

Metabotropic glutamate receptors (mGluRs) constitute a large family of G-protein-coupled receptors that are subdivided into three groups bas ed on sequence similarity, pharmacological profiles, and coupling to s econd messengers. Although mRNAs for seven of the eight mGluRs are exp ressed in the olfactory system, the localization and function of speci fic subtypes have not been fully characterized. Mitral cells of the ol factory bulb express mRNA for several mGluRs, including mGluR7, which has been suggested as a presynaptic glutamate autoreceptor. To investi gate the immunolocalization of mGluR7 in the olfactory system, we used a polyclonal antiserum specific for the carboxy terminus of the recep tor. Mitral cell somata and proximal dendrites were strongly labeled b y the mGluR7 antibody. Electron microscopic analysis revealed that mos t of the mitral cell somatic staining was cytoplasmic. In olfactory bu lb 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 termina ls. Distinct labeled fibers in all levels of layer I appeared to origi nate from labeled piriform cortex pyramidal cells in layers II and III . Our results indicate that mGluR7 is primarily presynaptic at olfacto ry bulb synapses. However, the postsynaptic localization of mGluR7 at selected synapses indicates that mGluR7 is not targeted exclusively to axonal compartments. (C) 1997 Wiley-Liss, Inc.