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

Abstract

Radioligand binding studies were performed to label metabotropic glutamate receptor (mGluR) in rat brain synaptic membranes using [ 3H]quisqualic acid (QA) synthesized in our laboratory as a radioligand. In the presence of ionotropic glutamate receptor (iGluR) agonists, including N-methyl- d-aspartic (NMDA), DL-α-amino-3-hydroxy-5-methylisoxasole-4-propionic (AMPA) and kainic acids (KA), at concentrations maximally effective in displacing each receptor binding, the agonists for group I mGluR subtype (±)-1-aminocyclopentane- trans-1,3-dicarboxylic acid ( trans-ACPD) and ( S)-3,5-dihydroxyphenylglycine (( S)-3,5-DHPG) more potently displaced [ 3H]QA binding in a concentration-dependent manner than their absence. The addition of these three iGluR agonists did not significantly affect potencies of (2 S,2′ R,3′ R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG-IV) and l-(+)-2-amino-4-phosphonobutyric acid (L-AP4) to displace [ 3H]QA binding. Scatchard analysis revealed that [ 3H]QA binding consisted of a single component with a maximal number of binding sites ( B max) of 431.6 fmol/mg protein and a dissociation constant ( K d) of 50.9 nM, in the presence of the three iGluR agonists. [ 3H]QA binding 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 [ 3H]QA binding was not detectable in peripheral tissues, such as pituitary and adrenal glands. Neither reverses transcription polymerase chain reaction nor immunoblotting analysis demonstrated the expression of mGluR1 and mGluR5 subunits in the aforementioned two peripheral tissues. These results suggest that [ 3H]QA indeed labels group I mGluR subtype functionally coupled to GTP binding protein in rat brain synaptic membranes under the experimental conditions employed. Group I mGluR subtype seems to be selectively distributed in central structures but not in pituitary and adrenal glands.