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Vol. 298, Issue 2, 453-460, August 2001
Lilly Research Laboratories, Eli Lilly and Company, Indianapolis,
Indiana
[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
(Bmax, 3.9 ± 0.65 pmol/mg of protein,
Kd, 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)-
-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 agonists
N-methyl-D-aspartic acid,
(R,S)-
-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.
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