Characterization of [3H]quisqualate binding to recombinant rat metabotropic glutamate 1a and 5a receptors and to rat and human brain sections

Abstract

We have investigated the binding properties of [3H]quisqualate to rat metabotropic glutamate (mGlu) 1a and 5a receptors and to rat and human brain sections. Saturation isotherms gave K(D) values of 27 ± 4 and 81 ± 22 nM for mGlu1a and mGlu5a receptors, respectively. Several compounds inhibited the binding to mGlu1a and mGlu5a receptors concentration-dependently. (S)-4-Carboxyphenylglycine, (S)-4-carboxy-3-hydroxyphenylglycine, and (R, S)-1-aminoindan-1, 5-dicarboxylic acid, which completely inhibited [3H]quisqualate binding to the mGlu5a receptor, were inactive in a functional assay using this receptor. The distribution and abundance of binding sites in rat and human brain sections were studied by quantitative receptor radioautography and image analysis. Using 10 nM [3H]quisqualate, a high density of binding was detected in various brain regions with the following rank order of increasing levels: medulla, thalamus, olfactory bulb, cerebral cortex, spinal cord dorsal horn, olfactory tubercle, dentate gyrus molecular layer, CA1-3 oriens layer of hippocampus, striatum, and cerebellar molecular layer. The ionotropic component of this binding could be inhibited by 30 μM kainate, revealing the distribution of mGlu1+5 receptors. The latter were almost completely inhibited by the group I agonist (S)-3, 5-dihydroxyphenylglycine. The binding profile correlated well with the cellular sites of synthesis and regional expression of the respective group I receptor proteins revealed by in situ hybridization histochemistry and immunohistochemistry, respectively.