Abstract

[³H]LY341495 is a highly potent and selective antagonist for group II metabotropic glutamate (mGlu) receptors (mGlu2 and mGlu3), which has been used to label these receptors in cells expressing recombinant receptor subtypes. In this study, we characterized the kinetics, pharmacology, and distribution of [³H]LY341495 binding to mGlu receptors in rat brain tissue. Equilibrium experiments in the rat forebrain demonstrated binding to a single site that was saturable, reversible, and of high affinity (B_max, 3.9 ± 0.65 pmol/mg of protein,K_d, 0.84 ± 0.11 nM). The relative order of potencies for displacement of [³H]LY341495 by mGlu receptor ligands was LY341495 ≫ l-glutamic acid > LY354740 > (2S,1′S,2′S)-2-(carboxycyclopropyl)glycine > 4-(2R,4R)-aminopyrrolidine-2,4-dicarboxylate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid > (R,S)-α-methyl-4-phosphonophenylglycine > (R,S)3,5-dihydroxyphenylglycine > l-(+)-2-amino-4-phosphonobutyric acid. [³H]LY341495 was not displaced by the selective ionotropic glutamate receptor agonistsN-methyl-d-aspartic acid, (R,S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, or kainate at concentrations up to 1 mM. Comparison of [³H]LY341495 binding in rat brain with recombinant mGlu receptor subtypes demonstrated a very high correlation with mGlu3 receptor binding (r² = 0.957), a significant, but lower, correlation with mGlu2 receptor binding (r² = 0.869), but no significant correlation to mGlu8 receptor binding (r² = 0.284). Regional studies using autoradiography showed a similar distribution of [³H]LY341495 binding to that for group II mGlu receptors previously reported by others using immunocytochemical techniques. These studies indicate that [³H]LY341495 selectively labels group II (mGlu2/3) receptors, but under the conditions used, [³H]LY341495 may bind predominately to mGlu3 receptor populations in the rat forebrain.