Characterization with [H-3]quisqualate of group I metabotropic glutamate receptor subtype in rat central and peripheral excitable tissues

Radioligand binding studies were performed to label metabotropic glutamate receptor (mGluR) in rat brain synaptic membranes using [H-3]quisqualic acid (QA) synthesized in our laboratory as a radioligand. In the presence of io notropic glutamate receptor (iGluR) agonists, including N-methyl-D-aspartic (NMDA), DL-alpha -amino-3-hydroxy-5-methylisoxasole-4-propionic (AMPA) and kainic acids (KA), at concentrations maximally effective in displacing eac h receptor binding, the agonists for group I mCluR subtype (+/-)-1-aminocyc lopentane-trans-1,3-dicarboxylic acid (trans-ACPD) and (S)-3,5-dihydroxyphe nylglycine ((S)-3,5-DHPG) more potently displaced [H-3]QA binding in a conc entration-dependent manner than their absence. The addition of these three iGluR agonists did not significantly affect potencies of (2S,2'R,3'R)-2-(2' ,3'-dicarboxycyclopropyl)glycine (DCG-IV) and L-(+)-2-amino-4-phosphonobuty ric acid (L-AP4) to displace [H-3]QA binding. Scatchard analysis revealed t hat [H-3]QA binding consisted of a single component with a maximal number o f binding sites (B-max) of 431.6 fmol/mg protein and a dissociation constan t (K-d) of 50.9 nM, in the presence of the three iGluR agonists. [H-3]QA bi nding was markedly inhibited by GTP and its analogues; but not by GDP, GMP and ATP, under these conditions. Inhibition by GTP was seen in all central structures examined, but [H-3]QA binding was not detectable in peripheral t issues, such as pituitary and adrenal glands. Neither reverses transcriptio n polymerase chain reaction nor immunoblotting analysis demonstrated the ex pression of mGluR1 and mGluR5 subunits in the aforementioned two peripheral tissues. These results suggest that [H-3]QA indeed labels group I mGluR su btype functionally coupled to CTP binding protein in rat brain synaptic mem branes under the experimental conditions employed. Group I. mGluR subtype s eems to be selectively distributed in central structures but not in pituita ry and adrenal glands. (C) 2001 Elsevier Science Ltd. All rights reserved.